Electrospinning of Polymeric and Ceramic Fibers ... · Electrospinning of Polymeric and Ceramic...

Electrospinning of Polymeric and Ceramic

Fibers: Understanding of the Morphological

Control and Its Application

by

Qing Du

Submitted in Partial Fulfillment of the

Requirements for the Degree

Doctor of Philosophy

Supervised by

Professor Hong Yang

Department of Chemical Engineering

Arts, Science and Engineering

Edmund A. Hajim School of Engineering and Applied

Sciences

University of Rochester

Rochester, New York

2013

ii

Biographical Sketch

The author was born and grew up in her beloved hometown of Wuhan, Hubei

Province, China. In the fall of 2001, she joined the Class of National Base of Chemistry,

Wuhan University and earned her Bachelor of Science degree in Chemistry in 2005. In

the same year, she joined Changchun Institute of Applied Chemistry, Chinese Academy

of Sciences to study Polymer Chemistry and Physics and obtained her Master of Science

degree in 2008. Immediately after that, she came to the United States and began her

doctoral studies in the Department of Chemical Engineering at the University of

Rochester. Following a period of study on copper indium gallium selenium solar cells,

she started her research in the area of electrospinning of polymeric and ceramic fibers -

understanding of the morphological control and its application, under the direction of

Professor Hong Yang. She was awarded Horton Fellowships in 2010, 2011 and 2012. She

received her second Master of Science degree in Chemical Engineering from the

University of Rochester in 2012.

Part of Qing’s research work during doctoral study is reflected from the following

publication list:

1. Qing Du, Hong Yang, David Harding, “Magnetic-Field-Assisted Electrospinning

of Twisted Peanut-shaped Poly (Vinyl Pyrrolidone) Ribbons.” Polymer, under

review.

iii

2. Qing Du, Jianbo Wu, Hong Yang, “Pt@Nb-TiO2 Catalyst Membranes Fabricated

by Electrospinning and Atomic Layer Deposition.” ACS Catalysis, to be

submitted.

3. Qing Du, Hong Yang, David Harding, Fabrication of SiC Fibers by

Electrospinning and UV Cure.” In preparation.

4. Hao Lin, Qing Du, Wei Xia, Hsiang Ning Wu, Ching W. Tang, “MoOx as a metal

diffusion barrier to stabilize CdS/CdTe thin film solar cells.” In preparation.

iv

Acknowledgements

First, I would like to express my deepest and sincerest gratitude to my thesis

advisor, Professor Hong Yang for his consistent guidance and help in the past five years

of my Ph.D. research. He created many opportunities to support my thesis work. He

helped me to broaden my horizon and patiently guided me throughout the whole research

project. Without his supervision and support, this thesis would not have been

accomplished.

Second, I would like to thank Professor David R. Harding and Professor Ching W.

Tang for their great comments and suggestions. I sincerely appreciate their willingness to

share the facilities in their labs, which facilitated the progress of my research. In addition,

they provided a lot of guidance on my research shown and not shown in this dissertation.

I would also like to thank another committee member, Professor James C.M. Li, for his

tremendous help.

Third, I sincerely appreciate the friendship and assistance from all the fellow

students in my lab. Hopefully I can cover all the names here: Dr. Jianbo Wu, Dr.

Zhenmeng Peng, Miao Shi, Xi Yin, Samantha S. Lyu, Elliot Yu, Steven A. Warren and

Yung-Tin Pan. My sincere gratitude is also extended to Joe Madathil, Dr. Wei Xia,

Sunny Wu and Felipe Angel in Professor Ching W. Tang’s group for their help and

friendship.

v

Forth, I would like to thank Mr. Brian McIntyre for sharing the electron

microscopes and atomic layer deposition system, and thank Ms. Christine Pratt for

providing help with the X-ray diffractometer. I would like to acknowledge the kind

assistance from our department staff, especially from Ms. Sandra Willison, Ms. Gina

Eagan and Mr. Larry Kuntz.

Last but not the least, my gratefulness goes to my family. No words can express

my love and thanks for the unconditional supports and encouragement from my beloved

parents in China. I especially thank my husband, Hao Lin, for his wholehearted support

not only in life but also in my research study. He always encourages me and helps me to

head out again when I feel depressed. He is the morning star in my life.

vi

Abstract

In this dissertation, polymeric fibers, poly(vinyl pyrrolidone) (PVP), and two

kinds of ceramic fibers, silicon carbide (SiC) and niobium-doped titania (Nb-TiO2), were

successfully fabricated by the electrospinning technique. The morphology of electrospun

fibers was studied by adjusting the parameters during the electrospinning process,

especially the humidity. The application of Nb-TiO2 fibers was explored.

Firstly, helical peanut-shaped ribbons of poly(vinyl pyrrolidone) were prepared by

a magnetic field-assisted electrospinning method. The formation mechanism of helical

ribbons was attributed to the synergistic interaction of bending instability and rigidity of

the polymers during the electrospinning process. The relative humidity was found to

significantly influence the morphology of PVP fibers. When the relative humidity of the

environment decreased, the morphology changed from straight rods to helical ribbons,

and the percentage of helical ribbons increased dramatically. At the same time, the cross-

section of electrospun fibers changed from circle, to ellipse, and to peanut shape. These

fibers had different mechanical properties, and the helical ribbons were shown to have the

largest ultimate elongation.

Secondly, SiC fibers were synthesized by electrospinning from preceramic

polymer without the help of guiding polymer, UV Cure and pyrolysis processes. The

morphology and diameter of the electrospun preceramic fibers is controlled by varying

mixtures of solvents. UV cure was used to crosslink and strengthen the preceramic fibers.

As a result, the morphology of SiC fibers remained intact during the pyrolysis process.

vii

Lastly, a facile method to fabricate niobium-doped titania supported platinum

(Pt@Nb-TiO2) catalyst membrane for oxygen-reduction reaction (ORR) in polymer

electrolyte membrane fuel cell (PEMFC) was developed by a new two-step approach,

electrospinning and atomic layer deposition (ALD). The 30000-cycle accelerated-

stability test on the Pt@Nb-TiO2 catalyst membrane showed that it had high stability. The

effect of fiber morphology, doping amount of niobium, number of ALD cycle, post-

treatment temperature and crystal phase of Nb-TiO2 support on the catalytic activity of

Pt@Nb-TiO2 membrane were discussed. The area-specific ORR activity of Pt@Nb-TiO2

catalyst membrane was increased by about 20 folds over the Pt@TiO2 fiber membrane, if

10 at% of Nb was incorporated into the ceramic fibers. The area-specific activity also

increased with the number of ALD cycles. The ORR activity further increased to 0.28

mA/cm2

Pt at 0.9 V after post-treatment of the catalyst membrane at high temperature in

H2–containing atmosphere, due to conductivity improvement of Nb-TiO2 fiber membrane

and better crystalinity of Pt NPs. The area-specific activity of Pt@Nb-TiO2 catalyst with

rutile phase of support was higher than the one with anatase phase of support due to the

improvement of support conductivity.

viii

Contributors and Funding Sources

This work was supervised by a dissertation committee consisting of Professor

Hong Yang (advisor) and Professor Ching W. Tang of the Department of Chemical

Engineering, Professor David R. Harding of the Laboratory for Laser Energetics,

Professor James C.M. Li and Professor John Lambropoulos (chair) of the Department of

Materials Science. XRD spectra were recorded by Christine Pratt of Mechanical

Engineering. Dr. Jianbo Wu of the Department of Chemical Engineering provided some

suggestions on the analysis in Chapter 4. All the other work conducted for the

dissertation was completed by the author independently. Graduate study was sponsored

by a Horton Fellowship from the Laboratory of Laser Energetics, University of Rochester.

ix

Table of Contents

Chapter 1 Introduction ........................................................................................................ 1

1.1 History of Electrospinning .........................................................................1

1.2 Theory of Electrospinning ..........................................................................4

1.2.1 Apparatus of Electrospinning ..................................................................4

1.2.2 Electrospinning Process ..........................................................................4

1.2.3 Mechanism of Electrospinning ...............................................................5

1.2.4 Important Parameters of Electrospinning................................................7

1.3 Applications of Electrospinning .................................................................9

1.4 Objective and Overview of Thesis ...........................................................10

1.5 References ................................................................................................12

Chapter 2 Helical Peanut-Shaped Poly(Vinyl Pyrrolidone) Ribbons Generated by

Electrospinning ................................................................................................................. 18

2.1 Introduction ..............................................................................................18

2.2 Experimental Section ...............................................................................19

2.3 Results and Discussion .............................................................................20

2.3.1 Formation of Helical Ribbons ...............................................................20

2.3.2 Effect of the Relative Humidity on the Fiber Morphology ...................24

2.3.3 Effect of the Relative Humidity on the Cross-section Shape ................27

2.3.4 Effect of Solvents on the Fiber Morphology.........................................33

2.3.5 Effect of Concentration on the Fiber Morphology ................................35

x

2.3.6 Ultimate Elongation of Fibers Electrospun at Different Relative

Humidity ...............................................................................................36

2.4 Conclusion ................................................................................................39

2.5 References ................................................................................................40

Chapter 3 Fabrication of SiC Fibers by Electrospinning and UV Cure ............................ 43

3.1 Introduction ..............................................................................................43



3.3.1 Morphology and Diameter Control of Preceramic Fibers.....................46

3.3.2 Characterization of Preceramic Polymer ..............................................49

3.3.3 Influence of UV Cure on Morphology of SiC Fibers ...........................53

3.4 Conclusion ................................................................................................60

3.5 References ................................................................................................60

Chapter 4 Catalyst Membrane of Pt@Nb-TiO2 Fabricated by Electrospinning and Atomic

Layer Deposition ............................................................................................................... 63

4.1 Introduction ..............................................................................................63



4.3.1 Experimental Process ............................................................................68

4.3.2 Effect of Fiber Morphology on Electrocatalytic Activity .....................72

4.3.3 Effect of Nb-doped Percentage on Electrocatalytic Activity ................75

xi

4.3.4 Effect of ALD Cycles and Post-treatment on Electrocatalytic Activity81

4.3.5 Accelerated-stability Test......................................................................85

4.4 Conclusion ................................................................................................89

4.5 References ................................................................................................90

Chapter 5 Synthesis of Rutile TiO2 by Electrospinning and Its Supported Pt

Electrocatalysts for the Oxygen Reduction Reaction ....................................................... 95

5.1 Introduction ..............................................................................................95



5.3.1 Synthesis and Characterization of r-TiO2 Fibers ..................................99

5.3.2 Synthesis and Characterization of r-Nb-TiO2 Fibers ..........................102

5.3.3 ORR Activity of Pt@r-Nb-TiO2 Catalysts ..........................................106

5.4 Conclusion ..............................................................................................109

5.5 References ..............................................................................................109

Chapter 6 Conclusions and Future Research .................................................................. 112

6.1 Conclusions ............................................................................................112

6.2 Future Research ......................................................................................115

6.2.1 Future Research of Helical Ribbons ...................................................115

6.2.2 Future Research of Electrospun Nb-TiO2 Fibers ................................115

6.3 References ..............................................................................................116

xii

Appendices ...................................................................................................................... 119

xiii

List of Figures

Figure 1.1. The number of publications searched in Web of Science by the topic of

“electrospinning” or “electrospraying”. .............................................................................. 3

Figure 2.1. (A, B) SEM images and (C, D) TEM micrographs of PVP fibers made at a

relative humidity of 16%. ................................................................................................. 21

Figure 2.2. SEM images of helical fibers collected on the top of magnets. .................... 23

Figure 2.3. SEM images of PVP fibers made at different relative humidity of the

environment: (A) 31-32%, (B) 28-29%, (C) 26-27%, (D) 23-24%, (E) 22%, (F) 21%, (G)

18%, and (H) 16%, respectively. ...................................................................................... 25

Figure 2.4. Dependence of the percentage of helical ribbons on the relative humidity.

The electrospinning was carried out with an 18-Gauge and 20-Gauge needle. ................ 27

Figure 2.5. SEM images of the cross-section or side-view of PVP fibers generated at

different relative humidity of the environment: (A) 28%, (B) 23%, (C) 22%, and (D) 16%,

respectively. ...................................................................................................................... 29

Figure 2.6. SEM images of PVP fibers generated at the EtOH/H2O volumetric ratio of (A)

3:1, (B) 2:1, (C) 1:1, and (D) 1:2, respectively. The relative humidity of the environment

was 16%. ........................................................................................................................... 35

Figure 2.7. SEM images of PVP fibers generated at the concentration of electrospun

solution of (A) 11%, (B) 9% and (C) 7% (w/v), respectively. The relative humidity of the

environment was 16%. ...................................................................................................... 36

Figure 2.8. Dependence of ultimate elongation of PVP fibers and percentage of helical

ribbons on the relative humidity. ...................................................................................... 37

Figure 3.1. SEM images of preceramic fibers using mixture of toluene and

dichloromethane as solvent. The weight percentage of toluene was (A) 30%, (B) 20%, (C)

10% and (D) 0%, respectively. ......................................................................................... 47

Figure 3.2. SEM images of preceramic fibers using mixture of NMP and

dichloromethane as solvent. The weight percentage of NMP was (A) 20% and (B) 10%.

........................................................................................................................................... 48

Figure 3.3. 1H NMR spectra of RD material. The structural formula of RD was sketched

on the top........................................................................................................................... 50

Figure 3.4. ATR-IR spectrum of RD material. ................................................................ 51

xiv

Figure 3.5. TGA curve of RD material. ........................................................................... 52

Figure 3.6. XRD patterns of RD pyrolyzed in Ar atmosphere at 1350 °C. XRD patterns

of β-SiC are shown by the red lines as a reference based on the JCPDS database (No. 74-

2307). ................................................................................................................................ 53

Figure 3.7. SEM images of SiC fibers which were cured by UV light at different

durations and then pyrolyzed in Ar atmosphere at 1100°C. The periods of UV cure were

(A) 15min, (B) 30 min, (C) 1 hr, and (D) 2 hr, respectively............................................. 54

Figure 3.8. ATR-IR spectra of as-spun preceramic fibers which were cured by UV light

at different durations. ........................................................................................................ 56

Figure 3.9. Solution of RD in dichloromethane after RD was cured by UV light for

different durations. ............................................................................................................ 59

Figure 4.1. (A) TEM and (B) STEM micrographs of Pt@Nb-TiO2 catalyst. (C) HAADF-

STEM study showing the distribution of Pt (blue), Ti (red) and Nb (green) elements. The

area for HAADF-STEM study is the marked region in (B).............................................. 71

Figure 4.2. SEM images of (A1 and B1) as-spun polymeric fibers, (A2 and B2) ceramic

fibers of Nb-TiO2 with 10 at% of Nb after hydrolysis and (A3 and B3) sintering and

catalysts of Pt@Nb-TiO2 with 10 at% of Nb after 40-cycle ALD of Pt NPs. The

morphologies of the fibers were: (A) straight fiber and (B) beaded fiber. ....................... 73

Figure 4.3. SEM images of (A1, B1, C1 and D1) as-spun polymeric fibers, (A2, B2, C2

and D2) Nb-TiO2 fibers after hydrolysis and sintering and (A3, B3, C3 and D3) Pt@Nb-

TiO2 catalysts after 40-cycle ALD of Pt NPs. The amount of Nb was (A) 0 at%, (B) 5

at%, (C) 10 at%, and (D) 25 at%, respectively. ................................................................ 77

Figure 4.4. TEM micrograph of TiO2 fiber. ..................................................................... 78

Figure 4.5. XRD patterns of Nb-TiO2 fibers with different amount of Nb. XRD patterns

of anatase TiO2 are shown by the black lines as a reference based on the JCPDS database

(No. 84-1286). ................................................................................................................... 79

Figure 4.6. Plot showing the dependence of area-specific ORR activities of Pt@Nb-TiO2

catalyst membrane at 0.9 V (versus RHE) on Nb-doped percentage in the ceramic fibers.

........................................................................................................................................... 81

Figure 4.7. SEM images of Pt@Nb-TiO2 catalysts with 10 at% of Nb obtained by

different numbers of ALD cycle and different temperature of post-treatment in 5% H2 w/

Ar atmosphere. (A-D) were treated at 200 °C. (E-H) were treated at 500 °C. The numbers

of ALD cycle were as following: (A, E) 10, (B, F) 20, (C, G) 40, and (D, H) 80. ........... 83

xv


with 10 at% of Nb at 0.9 V (versus RHE) on the number of ALD cycle for making Pt NPs,

and the temperature of post-treatment in 5% H2 w/ Ar atmosphere. ................................ 84

Figure 4.9. Accelerated-stability test of Pt@Nb-TiO2 catalyst with 10 at% Nb: (A) CV

curves and (B) ORR polarization curves measured at 1600 rpm...................................... 86

Figure 4.10. TEM micrographs of Pt@Nb-TiO2 catalyst with 10 at% of Nb (A) before

and (B) after the accelerated-stability test. ....................................................................... 88

Figure 5.1. SEM images of (A) as-spun preceramic fibers and (B) TiO2 fibers after

hydrolysis and sintering. ................................................................................................. 100

Figure 5.2. TEM micrograph of TiO2 fibers sintered at 900 °C. ................................... 101

Figure 5.3. XRD patterns of TiO2 fibers sintered at 900 °C. XRD patterns of bulk rutile

TiO2 are shown by the red lines as a reference based on the JCPDS database (No. 86-

0147). .............................................................................................................................. 102

Figure 5.4. SEM images of (A) as-spun polymeric fibers and (B) Nb-TiO2 fibers after

hydrolysis and sintering at 900 °C. ................................................................................. 103

Figure 5.5. TEM micrograph of Nb-TiO2 fibers sintered at 900 °C. ............................. 104

Figure 5.6. XRD patterns of Nb-TiO2 fibers sintered at 900 °C. XRD patterns of rutile

TiO2 are shown by the red lines (No. 86-0147), anatase TiO2 by the blue lines (No. 84-

1286), and TiNb2O7 by the green lines (No. 77-1374) as references based on the JCPDS

database. .......................................................................................................................... 105

Figure 5.7. SEM image of Pt@ r-Nb-TiO2 catalysts. .................................................... 107

Figure 5.8. Electrocatalytic property of Pt@r-Nb-TiO2 catalyst: (A) CV curves and (B)

ORR polarization curves measured at 1600 rpm. ........................................................... 108

xvi

List of Schemes

Scheme 1.1. Schematic illustration of formation of spiral coils. ........................................ 7

Scheme 2.1. Schematic illustration of formation process of helical ribbons. .................. 23

Scheme 2.2. Schematic illustration of the formation of electrospun polymeric fibers with

different shapes in cross-section. ...................................................................................... 31

Scheme 2.3. Schematic illustration of the deformation of (A) a helical ribbon and (B) a

straight round fiber under extension forces F. .................................................................. 38

Scheme 3.1. Schematic illustration of crosslinking reactions of as-spun preceramic fibers

during the treatment of UV cure. ...................................................................................... 59

Scheme 4.1. Schematic illustration of the process for the production of Pt@Nb-TiO2

catalyst membrane. (A) Step 1: preparation of Nb-TiO2 support membrane by

electrospinning technique and sintering. (B) Step 2: preparation of catalyst membrane of

Pt@Nb-TiO2 by the ALD technique. The SEM image shows a final membrane made of

Pt@Nb-TiO2 fibers. ........................................................................................................... 70

xvii

List of Tables

Table 2.1. Summary of morphology and diameter (nm) of electrospun PVP fibers made

using 18-G and 20-G needles under different relative humidity. ..................................... 25

Table 4.1. Summary of the diameter of fibers and size of beads of as-spun polymeric and

Nb-TiO2 fibers at different Nb-doped amount (at%). ....................................................... 77

Table 4.2. Electrochemically active surface area (ECSA), kinetic current density (ik) and

area-specific current density (is) at 0.9 V (versus RHE) of Pt@Nb-TiO2 catalyst

membranes with 10 at% of Nb after a given number of potential cycles during

accelerated-stability test; relative percentages are given in parentheses. ......................... 87

Table A1. Physical properties of solvents. ..................................................................... 119

1

Chapter 1

Chapter 1 Introduction

One-dimensional (1D) nanostructures have gained tremendous attention in recent

years because of their unique chemical and physical properties and wide applications.1-6

A number of methods have been demonstrated to fabricate 1D nanostructures, especially

fibers. These methods include vapor-liquid-solid (VLS) growth,7-10

template synthesis,11-

13 conventional fiber spinning (wet spinning,

14, 15 dry spinning,

16-18 melt spinning

19-21 and

gel spinning5, 22

), and electrospinning.23-34

Electrospinning has obvious advantages

superior to other methods because it is a cost-effective, simple and direct method with

universality. Compared to conventional spinning, it can generate fibers with much

smaller diameter. Overall, electrospinning is a versatile method which can process almost

any soluble or fusible polymers or preceramic polymers into polymeric or ceramic fibers

with diameters ranging from nanometers to micrometers.

1.1 History of Electrospinning

Just as Newton discovered the laws of universal gravitation, the first observation

of electrospinning, at that time it was called electrospraying, is full of occasionlity. In the

late 1500s, William Gilbert found when a suitably charged piece of amber was brought

near a droplet of water, it would form a cone shape and small droplets would be ejected

from the tip of the cone.35

It is the beginning of the story of eletrospinning.

2

Chapter 1

Later, the development of electrospinning went into a long quiet period. The

process of electrospinning was patented by J.F. Cooley in May 190036

and February

190237

and by W.J. Morton in July 1902.38

In these patents, the conditions to form fibers

rather than droplets by electrospinning are pointed out. (i) Solution must have sufficient

viscosity. (ii) Solvent is volatile enough to allow generation of the solid polymer. (iii)

The intensity of the electric field must be within a certain range.

After that, the development of electrospinning accelerated. In 1914 John Zeleny

began the attempt to mathematically model the behavior of fluids under electrostatic

forces.39-41

In 1934, the electrospinning of polymeric materials was first patented by

Anton Formhals.42

Later, he filed several other patents to produce continuous fine fibers

for use on standard textile machinery.43-46

In 1936, electrospinning from a melt rather

than a solution was patented by C.L. Norton.47

In 1938 Nathalie D. Rozenblum and Igor

V. Petryanov-Sokolov generated electrospun fibers, which were developed into filter

materials known as "Petryanov filters".48

By 1939, this work had led to the establishment

of a factory in Tver' for the manufacture of electrospun smoke filter elements for gas

masks.

Between 1964 and 1969, Geoffrey I. Taylor worked on electrostatics and

contributed to electrospinning by mathematically modeling the shape of the cone formed

by the fluid droplet under the effect of an electric field.49-51

He concluded that the conical

interface between air and the fluid was stable at the semi-angle conical angle of 49.3°,

which is a universal value for conductive fluids. This characteristic droplet shape is now

3

Chapter 1

known as the Taylor cone, which is important as it defines the onset of the

electrospinning process.

In the early 1990s, Reneker and Rutledge popularized the name electrospinning

for the process and demonstrated that many organic polymers could be electrospun into

nanofibers.8 Since then, the number of publications about electrospinning has been

increasing exponentially every year. Figure 1.1 shows the number of publications

searched in Web of Science by the topic of “electrospinning” or “electrospraying”.

Figure 1.1. The number of publications searched in Web of Science by the topic of

“electrospinning” or “electrospraying”.

4

Chapter 1

The theoretical mechanism of the electrospinning process has also been developed

since 1995. Yarin described the shape of the Taylor cone and the subsequent ejection of a

fluid jet.52, 53

Reneker visualized the real jet trajectory with the help of a high speed

camera resulting in a major breakthrough in the development of the electrospinning

process.54, 55

It offered direct evidence of the mechanisms of electrospinning. Hohman

described the most important instability to the electrospinning process, the bending

(whipping) instability.56

1.2 Theory of Electrospinning

1.2.1 Apparatus of Electrospinning

The standard setup of electrospinning consists of a spinneret, a high-voltage DC

power supply and a grounded collector. Two bar magnets are introduced at the collector

region to obtain aligned fibers by an additional radial Lorenz force in our group, which is

called Magnetic-Field-Assisted Electrospinning.57

1.2.2 Electrospinning Process

When the solution or the melt is injected by the syringe pump at a constant rate,

sessile and pendant droplets would hang on the end of spinneret due to surface tension. If

an electric field is added between the spinneret and the collector, the shape of droplet will

be deformed according to the equilibrium of electric forces and surface tension. When the

5

Chapter 1

intensity of electric field is increased, a conical shape, the so-called Taylor cone, will

form at a critical potential.58

Then a charged fluid jet will be ejected from the tip of the

Taylor cone towards the collector if the electric field continues to be increased. During

the travel process in the air between the spinneret and the collector, the fluid jet is

elongated by electrostatic forces and dried as the solvent is evaporated. Finally, solidified

fibers are obtained at the collector.

1.2.3 Mechanism of Electrospinning

Electrospinning is basically a drawing process based on electrostatic interactions.

The hardest part to understand electrospinning is why a droplet with size of millimeter

could be transferred into a fiber with diameter of nanometer. When an electric field about

is applied to the droplet, charges from the electrode flowed onto the surface of the liquid

droplet and three kinds of forces are generated within the droplet. The first one is the

electrical force from the external field, pulling the droplet toward the collector. The

second one is the Coulomb repulsive forces between the charges, which try to decrease

the density of charges by increasing the surface area per unit mass fluid. The last one is

cohesive force (surface tension), the intermolecular attraction between molecules within

the fluid, which prevents the deformation of droplet. When the intensity of the electric

field increases, the balance of these forces is disturbed. The droplet is deformed into a

conical shape which is called the Taylor cone by the new balance of three forces. And a

fluid jet is ejected from the Taylor cone towards collect, which is formed by the

6

Chapter 1

combination of the first and second forces. The surface area is increased and electrical

charges are carried away from the droplet by the flowing jet. As long as there are enough

fluid and charges supplied to the droplet, the fluid jet is maintained.

The real jet trajectory offered by a high speed camera shows that the fluid jet is a

straight path at first, but after a short distance, around several millimeters, the straight

path grows into a spiral coil.54, 55

It is called bending instability. And after several turns, a

new electrical bending instability (second bending instability) formed a smaller coil on

the basis of the larger coil. The coils are formed due to the balance of these forces. As the

intensity of charges on the surface of the fluid jet are so high that the coulomb repulsive

force is very large. Take the charge on Scheme 1.1 as an example, the charge on the top

will pull it downward (Fd) heavily and the charge on the bottom will push it upward (Fu)

heavily. It is not a stable status. As a result, the fluid jet bends. Then the force from the

above charge becomes downward and outward (Fdo); and the force from the below charge

becomes upward and outward (Fuo). The direction of result force (Fout) is outward.

Cohesive force prevents the deformation due to the high viscosity of solution and

generates an inward force (Fin). External electrical force pulls it downward, resulting a

downward force (Fdown). The combination of Fout, Fin and Fdown generates a centrifugal

force. So the bend part grows into a three-dimensional coil (Scheme 1.1). The shape

occupied by the spiral path is an envelope cone. As diameter of fluid jet becomes smaller

and smaller due to coulomb repulsive force. Cohesive force decreases and the centrifugal

force increases. So the radius of coils grows larger. The size of the fluid jet shrinks

7

Chapter 1

continuously from the spinneret to the collector until it solidifies. The fiber may be

elongated by 10,000 times from the initial length of the fluid segment.

Scheme 1.1. Schematic illustration of formation of spiral coils.

1.2.4 Important Parameters of Electrospinning

Electrospinning appears to be straightforward, but it is a rather complicated

process which depends on material, technical, and ambient parameters. Only a proper

combination of these parameters would successfully result in uniform fibers.59, 60

Otherwise, a range of other morphologies such as dots,61, 62

break-up, beads63, 64

and

branching65, 66

will show up.

8

Chapter 1

Material parameters include physical properties of target polymer or preceramic

polymer (molecular weight, melting point and glass transition temperature), physical

properties of solvents (boiling point, vapor pressure, surface tension and dielectric

constant), concentration of electrospun materials, surfactant usage and so on. At first, a

target polymer or preceramic polymer with high molecular weight needs to be chosen to

enable enough chain entanglements. If the material is electrospun in the form of melt, its

melting temperature and glass transition temperature will be optimized to obtain uniform

fibers.67, 68

If it is electrospun in the form of solution, a good solvent, either single or

mixed, should be carefully selected.69-71

The solvent should have suitable surface tension

and dielectric constant so that enough ions could be generated on the surface of fluid jet.

In addition, the volatility of solvent should be high to dry the fluid jet during the travel

path, which means the solution should have a low boiling point and a high vapor pressure

at room temperature. The concentration of solution is related to the viscosity of the fluid

jet. The influences of these parameters on the morphology and diameter of electrospun

fibers vary greatly according to the target material. However, some common rules exit.

Usually, the diameter of electrospun fibers would increase if the molecular weight of

target material is enlarged, or if the concentration of the electrospun solution increases.

Adding surfactants or salts to the electrospun solution would eliminate beads and shrink

the size of fibers.72

Technical parameters are consisted of the intensity of the electric field, the

distance between the spinneret and the collector, feeding rate and size of the spinneret.

The most important factor is the intensity of the electric field. Only when it is larger than

9

Chapter 1

the critical potential, fiber can be electrospun. Further increasing the intensity decreases

the diameter of the fiber. In some case, the critical potential is so large that the

conductivity of the electrospun melt or the solution needs to be increased by adding

surfactants or using solvents with high dielectric constant.73, 74

The distance between the

spinneret and the collector should be long enough to dry the fluid jet in the air. Otherwise

wet fibers may fuse together on the collector. In a suitable range, increasing feeding rate

will increase the diameter of the electrospun fiber but the influence is trivial. Excessively

increasing the feeding rate may destroy the equilibrium of electrospinning, leading to

beaded fibers.75-77

Increasing the size of spinneret slightly increase the diameter of

fibers.78, 79

Ambient parameters include temperature, humidity and air velocity. All these

parameters can affect the drying rate of fluid jet, affecting the morphology and size of the

electrospun fibers. Humidity is especially important when water is used as the solvent.

But the influence of humidity is rarely studied. Besides drying rate, temperature may also

affect the crystallinity of fibers.

1.3 Applications of Electrospinning

Electrospun fibers (also called nonwovens) have various applications, such as

template, filler and textile, catalysis, nanofiber reinforcement and medical application.

Electrospun polymeric fibers can be used as templates for the preparation of hollow

fibers.80, 81

Pore structure of nonwovens controls its properties and functions in filter and

10

Chapter 1

textile applications. Nonwovens composed of nanofibers can greatly enhance airflow

resistance of textile.82

As electrospun fibers have high surface area, it is applied to load

monometallic or bimetallic nanoparticles to increase catalyst activity.83, 84

Electrospun

glass fibers, carbon fibers, and poly(p-phenylene terephthalamide) fibers are used for the

reinforcement of synthetic materials in many technical products.85, 86

Electrospun fibers

have also been extensively studied in the medical applications including tissue

engineering,87, 88

transport and release of drugs.89, 90

1.4 Objective and Overview of Thesis

The objective of this dissertation is to fabricate polymeric fiber (poly(vinyl

pyrrolidone)) and ceramic fibers (silicon carbide and niobium-doped titania) by

electrospinning technique. It will study the effect of parameters during electrospinning

process, especially the humidity, on the morphology of electrospun fibers. Some

applications of the electrospun polymeric and ceramic fibers are explored.

Chapter 2 is focused on the preparation of helical peanut-shaped ribbons of

poly(vinyl pyrrolidone) (PVP) and understanding their formation mechanism. The

influences of the relative humidity on the morphology, shape of cross-section and the

ultimate elongation of the electrospun fibers are studied.

Chapter 3 is concentrated on the fabrication of silicon carbide (SiC) fibers by

electrospinning from preceramic polymer without the help of guiding polymer. The

importance of UV Cure on morphology preservation is pointed out.

11

Chapter 1

Chapter 4 and 5 describe a facile method to fabricate niobium-doped titania

supported platinum (Pt@Nb-TiO2) catalyst membrane for oxygen reducing reaction by a

new two-step approach, including electrospinning and atomic layer deposition. Chapter 4

is focus on electrospinning of anatase Nb-TiO2 fiber support and discussing the effect of

fiber morphology, doping amount of niobium, number of ALD cycle and the temperature

of post-treatment on the catalyst activity of Pt@Nb-TiO2. Chapter 5 is focused on the

formation of rutile phase Nb-TiO2 fibers by electrospinning.

12

Chapter 1

1.5 References

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13

Chapter 1


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21. Li, J. Y.;Zhang, Y. F.;Zhong, X. H.;Yang, K. Y.;Meng, J.;Cao, X. Q. Single-

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keratinocytes and fibroblasts in vitro. Biomaterials 2004, 25, 1289-1297.

26. Zhang, Y. Z.;Venugopal, J.;Huang, Z. M.;Lim, C. T.;Ramakrishna, S.

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27. Megelski, S.;Stephens, J. S.;Chase, D. B.;Rabolt, J. F. Micro- and nanostructured

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28. Badami, A. S.;Kreke, M. R.;Thompson, M. S.;Riffle, J. S.;Goldstein, A. S. Effect

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electrospun poly(lactic acid) substrates. Biomaterials 2006, 27, 596-606.

14

Chapter 1

29. Goldberg, M.;Langer, R.;Jia, X. Q. Nanostructured materials for applications in

drug delivery and tissue engineering. J. Biomater. Sci.-Polym. Ed. 2007, 18, 241-268.

30. Agarwal, S.;Wendorff, J. H.;Greiner, A. Use of electrospinning technique for

biomedical applications. Polymer 2008, 49, 5603-5621.

31. Xie, J. W.;Liu, W. Y.;MacEwan, M. R.;Yeh, Y. C.;Thomopoulos, S.;Xia, Y. N.

Nanofiber Membranes with Controllable Microwells and Structural Cues and Their Use

in Forming Cell Microarrays and Neuronal Networks. Small 2011, 7, 293-297.

32. Liu, Y. Y.;Sun, Y.;Yan, H.;Liu, X. Y.;Zhang, W.;Wang, Z.;Jiang, X. Y.

Electrospun Fiber Template for Replica Molding of Microtopographical Neural Growth

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33. Yang, F.;Murugan, R.;Wang, S.;Ramakrishna, S. Electrospinning of nano/micro

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35. Gilbert, W. De Magnete Magnetcisque Corporibus, et de Magno Magnete Tellure

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37. Cooley, J. F. Apparatus for electrically dispersing fluids. US 692631, 1902.

38. Morton, W. J. Method of dispersing fluids. US 0705691, 1902.

39. Zeleny, J. The electrical discharge from liquid points, and a hydrostatic method of

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40. Zeleny, J. Electrical discharges from pointed conductors. Phys. Rev. 1920, 16,

102-125.

41. Zeleny, J. The role of surface instability in electrical discharges from drops of

alcohol and water in air at atmospheric pressure. J. Frankl. Inst. 1935, 219, 659-675.

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45. Formhals, A. Production of Artificial Fibres From Fiber Forming Liquids. US

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46. Formhals, A. Method and Apparatus for Spinning. US 2349950, 1944.

47. Norton, C. L. Method and apparatus for producing fibrous or filamentary material.

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50. Taylor, G. FORCE EXERTED BY AN ELECTRIC FIELD ON A LONG

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51. Taylor, G. ELECTRICALLY DRIVEN JETS. Proceedings of the Royal Society

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56. Hohman, M. M.;Shin, M.;Rutledge, G.;Brenner, M. P. Electrospinning and

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18

Chapter 2

Chapter 2 Helical Peanut-Shaped Poly(Vinyl Pyrrolidone)

Ribbons Generated by Electrospinning

2.1 Introduction

Electrospinning is a facile and versatile method to fabricate nanometer (nm)- and

submicron-sized fibers for a range of applications.1-5

Electrospun fibers have been shown

to be useful for tissue engineering, especially in those areas where guided growth of cells

is important.6-10

The morphology may play important roles in determining the function of

materials made of electrospun fibers.11-16

Straight round-shaped fibers are the most

commonly observed, with a few exception that helical fibers with different shapes in

cross-section are made, although detailed studies on the factors that affect fiber

morphologies have hardly been reported.

A rare example for the formation of fibers with non-circular cross-section is based

on thin polymer skins.17

Circular tube made of thin polymer layers collapsed under

atmospheric pressure to form elliptical or flat shape as the solvent evaporated. For helical

ribbons, however, there are even fewer examples and no clear understanding on the mode

of formation to date. It was observed that this morphology was the result of particular

types of polymers and solvents;18-19

or formed in a narrow range of polymer

concentration.20

The spirals were attributed to the difference in the azimuthal angles of

those planes of collapsing segments.17

Different solvent evaporation rates in the mixture

were also important to the formation of spirals observed in poly(4-methyl-1-pentene)

19

Chapter 2

fibers.21-23

The lack of a convincing mechanism for the formation of helical ribbons in

part is due to the fact that there is no reliable method to control the degree of spirals in

the spun fibers. In this article, we describe the preparation of helical peanut-shaped PVP

ribbons by magnetic-field-assisted electrospinning (MFAE) method. We show that extent

of the spiral and shape of the fiber cross-section can be well controlled through adjusting

the relative humidity of the environments. We offer our understanding on the effect of

relative humidity on the fiber morphology, which is an important but often overlooked

factor in the electrospinning process.

2.2 Experimental Section

Electrospinning of Poly(Vinyl Pyrrolidone) (PVP) Fibers. The magnetic-field-

assisted electrospinning (MFAE) method was used to prepare PVP fibers.24

Typically,

PVP (molecular weight: 1300 kg mol-1

, Sigma-Aldrich) was dissolved in a mixture of

anhydrous ethyl alcohol and de-ionized (DI) water to form a polymer solution at various

concentration. Normally the concentration of 13% (w/v) was used. The volumetric ratio

of ethyl alcohol to water was kept at 4:1 unless indicated otherwise. The polymer solution

was loaded into a plastic syringe with an 18-Gauge (inner diameter: 0.033 inch) stainless-

steel needle if not indicated specifically. The sharp tip of the needle was cut and sanded

into smooth circular, flat opening to ensure the proper direction of fluid jets. The needle

was connected to a high-voltage power supply and the collector was grounded and 8 cm

below the needle tip. The voltage between the needle and the collector was maintained at

20

Chapter 2

15 kV. The feeding rate was controlled by a syringe pump at a constant rate of 1 mL/h.

The humidity of the environment was adjusted by a humidifier in an enclosed space and

monitored by a digital hygrometer during the electrospinning process.

Characterization. The morphology of the fibers was studied by Field Emission

Scanning Electron Microscopy (FE-SEM, Zeiss-Leo DSM982) and Field Emission

Transmission Electron Microscopy (FE-TEM, FEI TECNAI F-20). Average diameter

was measured by Image J (National Institutes of Health, USA). Thermal Mechanical

Analyzer (TMA, Pyris Diamond, Perkin Elmer) was used to study the mechanical

property of PVP fibers made under different relative humidity. A bundle of aligned PVP

fibers were fixed by two small chucks and then loaded onto TMA. An extension force

was exerted on the two chucks at a constant rate of 15 mN/min until the fibers were

fractured. Load and fiber extension were recorded and used to calculate the ultimate

elongation.

2.3 Results and Discussion

2.3.1 Formation of Helical Ribbons

Morphology of electrospun polymer fibers was observed to be depended heavily

on humidity of the environment. When the relative humidity of the environment was

~16%, helical ribbons were obtained (Figure 2.1). These fibers were made of PVP from

its ethyl alcohol and de-ionized (DI) water solution. The fibers were uniform; and their

cross-section had a peanut shape with a typical width of 1.4 µm and a height of 430 nm

21

Chapter 2

(Figure 2.1B). Noticeably, such fiber had a thin skin layer of 30 - 100 nm in thickness.

Because of this unique morphology, the fiber formed spirals along its longitude direction

(Figure 2.1C). Overall, the surface of these helical ribbons was fairly smooth.

Figure 2.1. (A, B) SEM images and (C, D) TEM micrographs of PVP fibers made at a

relative humidity of 16%.

The formation of helical ribbons should be closely related to the electrospinning

process (Scheme 2.1). When an electric field is applied between a spinneret and a

22

Chapter 2

collector, charges are generated on the surface of the polymer solution droplet, which

hang on the tip of the spinneret due to surface tension. If the static force surpasses the

surface tension, a Taylor cone forms and a fluid jet is ejected and travels to the collector.

The longitudinal stress caused by the external electric field stabilizes the straight jet to a

certain distance. Then the internal repulsive forces between the adjacent segments, which

carry the same charges, generate the bending instability and turn the straight path into a

spiral caused by the Coulomb repulsive forces.25-28

The subsequent bending instability

generates new and size-reduced spirals on the existing ones until the fluid jet reaches the

collector and dries.25, 29

Previously, buckled fibers were made based on this principle.30

Similar helical fibers were observed in this polymer system (Figure 2.2), suggesting small

spirals were formed due to the second or third bending instability during the

electrospinning process. With the MFAES method, aligned fibers could be formed and

suspended over the gap between two magnets under Lorentz force.24, 31

This additional

force could alter the direction of the fluid jet and pull the two ends of small spirals to

align them over the gap. If the polymer has enough rigidity, the jet cannot fully relax but

retain the spiral feature while being pulled by the Lorentz force, leading to the formation

of helical ribbons (Scheme 2.1). In another word, the uniformity of helical ribbons is

attributed to such interaction. In the followings, we further discuss the effect of humidity

on the morphology of helical ribbons, which could not completely be explained based on

the existing theory.17

23

Chapter 2

Scheme 2.1. Schematic illustration of formation process of helical ribbons.

Figure 2.2. SEM images of helical fibers collected on the top of magnets.

24

Chapter 2

2.3.2 Effect of the Relative Humidity on the Fiber Morphology

The humidity played an important role in controlling the morphology of the

electrospun PVP fibers. When the relative humidity of the environment changed from

32% to 16%, the morphology of spun PVP fibers changed from straight round fibers to

helical ribbons, and the percentage of helical ribbons also increased (Figure 2.3). The

electrospun jets of PVP solution formed straight round fibers or flat ribbons when the

relative humidity of the environment was in the range between 26% and 32% (Figure

2.3.A-C). At the relative humidity of 23-24%, helical ribbons began to show up in the

samples made using an 18-Gauge needle (inner diameter: 0.033 inch) (Figure 2.3.D).

When relative humidity further decreased, percentage of helical ribbons increased (Figure

2.3.E-H). The observed changes of fiber morphology at different humidity and the size of

fibers were summarized in Table 2.1. The same trend of morphological transformation

was observed even when the inner diameter of the needle decreased from 0.033 to 0.024

inch (20-Gauge). However, the transition from one kind of morphology to another tended

to be at lower relative humidity for 20-G needle (Table 2.1).

25

Chapter 2

Figure 2.3. SEM images of PVP fibers made at different relative humidity of the

environment: (A) 31-32%, (B) 28-29%, (C) 26-27%, (D) 23-24%, (E) 22%, (F) 21%, (G)

18%, and (H) 16%, respectively.

Table 2.1. Summary of morphology and diameter (nm) of electrospun PVP fibers made

using 18-G and 20-G needles under different relative humidity.

Humidity

Needle

Size

31-32% 28-29% 26-27% 23-24% 22% 21% 18% 16%

18 Gauge □/○

887

□/○

1322

□/○

1387 ※□ ※ ※※ ※※※ ※※※

20 Gauge □/○

768

□/○

933

□/○

1266

□

1495 ※□ ※ ※※※ ※※※

※※※: strongly helical peanut-shaped ribbons, the percentage of helical ribbons > 90%

※※: mildly helical peanut-shaped ribbons, the percentage of helical ribbons ~60%

※: weakly helical peanut-shaped ribbons, the percentage of helical ribbons ~30%

※□: helical peanut-shaped ribbons and flat ribbons, the percentage of helical ribbons ~10%

□/○: flat ribbons or straight round fibers

□: flat ribbons

○: straight round fibers

26

Chapter 2

There were two possible reasons for this observed trend. First, small spirals

formed due to the existence of higher order bending instability, when the relative

humidity decreased. When the relative humidity decreased, fibers dried relatively faster.

The characteristic charge relaxation time, that is the period required for the excess

charged toward to the equilibrium position on the surface of a fluid jet, increased because

charges travelled at a reduced rate in solid than in liquid.29

If the characteristic charge

relaxation time increased, there should be additional charges on the surface of a fluid jet.

Therefore, the bending instability occurred readily. In the case where the distance

between the spinneret and the collector was kept constant, characteristic charge relaxation

time increased, resulting in a shortened straight path and increased subsequent bending

instability. Hence, small spirals formed easily when the relative humidity was low.

Second, a fluid jet became rigid if the relative humidity was lowered. Under fast drying

rate of the fluid jet, fibers turned rigid and resisted to further relaxation. As a result, fibers

tent to maintain the spiral morphology under the influence of Lorentz force. Collectively,

these factors caused the ribbons to spiral when the relative humidity was low. Thus, the

percentage of the helical ribbons increased from 0% to 99%, when the relative humidity

decreased from 32% to 16% (Figure 2.4). When the size of spinneret decreased from

0.033 (18-G) to 0.024 inch (20-G), the size of the droplet hang on the tip of spinneret

decreased, which was exposed to the environment, resulting in a slower drying rate of the

27

Chapter 2

fluid jet. As a result, the transition from straight round fiber to helical ribbon became

slower.

Figure 2.4. Dependence of the percentage of helical ribbons on the relative humidity.

The electrospinning was carried out with an 18-Gauge and 20-Gauge needle.

2.3.3 Effect of the Relative Humidity on the Cross-section Shape

The cross-section of the fibers changed continuously from circle, to ellipse, and to

peanut shape when relative humidity of the environment decreased from 32% to 16%

(Figure 2.5). At the relative humidity between 23% and 32%, the cross-section had a

round or elliptical shape. When the relative humidity decreased to 22%, rectangular and

28

Chapter 2

peanut-shaped cross-sections formed. PVP fibers obtained at a relative humidity of 28%

had an elliptical cross-section with the transverse diameter of around 570 nm and the

conjugate diameter of around 315 nm (Figure 2.5A). PVP fibers with a circular cross-

section could still be observed at a relative humidity of 23%, but their diameters were

relatively large and ~2 µm (Figure 2.5B). A rectangular cross-section appeared at a

relative humidity of 22% as shown in the SEM image of the side view of a fiber (Figure

2.5C). The width of rectangle for this particular fiber was ~460 nm. Peanut-shaped cross-

section could be observed for fibers made at a relative humidity of 16% (Figure 2.5D).

29

Chapter 2

Figure 2.5. SEM images of the cross-section or side-view of PVP fibers generated at

different relative humidity of the environment: (A) 28%, (B) 23%, (C) 22%, and (D) 16%,

respectively.

Fluid jet typically has a circular cross-section to minimize the surface tension.

During its travel from a spinneret to a collector, the fluid jet however dries gradually and

the cross-section changes accordingly because of the force associated with drying. This

drying force is generally influenced by two factors. One is the decreasing rate of solvent

mass due to evaporation. This rate is determined by the following equation:

Equation 2.1

where

is the mass decreasing rate of solvent, Cs,eq(T) is the saturation vapor

concentration of a solvent at temperature of T, Cs∞ is the vapor concentration in

atmosphere far from the jet, ρ is the density of solvent, hm is the mass transfer coefficient

for evaporation, is the cross-sectional radius, λ is the geometrical stretching ratio, and s

is Lagrangian parameter.28, 32

The other factor is the diffusion rate of solvent from the core to surface region of

the fluid jet due to the concentration gradient within the jet. The diffusion rate is

calculated based on the relationship below:

Equation 2.2

where D is the diffusion coefficient and is the density of the diffusing material at

location r and time t.

The solvents used in this system were ethyl alcohol and DI water. During the

electrospinning process, the solvents of fluid jet evaporated quickly because they had low

boiling points and high vapor pressure at room temperature (Table A1). Cs,eq(T) of ethyl

30

Chapter 2

alcohol and water remained unchanged because all the experiments were carried out at

the same temperature. Cs∞ of water was proportional to the relative humidity of the

environment. Its value decreased as relative humidity decreased. As a result, the mass

decreasing rate of water,

, increased according to the Equation 2.1 when the relative

humidity decreased. Assuming the diffusion rates of the solvents were the same, the

decreasing rate of solvent mass should be smaller than or almost the same as the diffusion

rate when relative humidity was high, as illustrated in Scheme 2.2. Thus, solvent in the

fluid jet evaporated in all directions to the environment evenly and the circular cross-

section maintained upon the drying of the fluid jet (Scheme 2.2A). If the relative

humidity was low, the decreasing rate of solvent mass was larger than the diffusion rate.

The outer shell of the fluid jet was dried firstly to form a thin polymer skin (Scheme

2.2B).17

As the solvent continuously evaporated and diffused outwards, the inner pressure

inside the polymer shell decreased and could not be balanced by the outward one. As a

result, the pressure was imposed on the polymer skin in all directions. If the round fluid

jet collapsed along any given direction, the charges were no longer distributed evenly but

flowed to the surface regions with high curvature due to the point effect.17

The pressure

from the environment continued to push the polymer skin until the fluid jet was

completely dried. Under such situation, an elliptical or rectangular cross-section was

obtained (Scheme 2.2B).

31

Chapter 2

Scheme 2.2. Schematic illustration of the formation of electrospun polymeric fibers with

different shapes in cross-section.

32

Chapter 2

If the relative humidity dropped below 22%, the decreasing rate of solvent mass

became fast, resulting in various pores in the wall of the semi-dried fluid jet due to rapid

drying, which generated a capillary force. When a liquid is transported in a porous media,

the cumulative volume V of absorbed liquid after a time t is , where A is cross-

sectional area of the pores and S is the sorptivity of the medium. The transportation rate is

related to the sorptivity of the porous media, which is a constant. When the humidity

decreased, the density of pores increased due to the quick shrinking of polymers, which

caused by the increase of the mass decreasing rate. Then the diffusion rate increased due

to the capillary force, and it increased with the density of pores. As a consequence, the

overall drying rate of the fluid jet greatly increased with the decrease of the relative

humidity. Once the fluid jet was dried, the fibers became rigid and could no longer be

deformed by the atmospheric pressure. Since the charges on the surface of fluid jet

concentrated at the two side regions with high curvature, two circular shapes formed

around those regions due to the charge repulsion. As a result, a peanut-shaped cross-

section formed (Scheme 2.2C).

The diameter of the polymeric fibers or width of the ribbons could be controlled

by changing the relative humidity. As the relative humidity decreased, the diameter or

width increased (data are summarized in Table 2.1). Two main factors could attribute to

this observed trend. Firstly, when the relative humidity was high (in the situation of

Scheme 2.2A), the fluid jet dried uniformly and slowly. If the relative humidity increased,

33

Chapter 2

fluid jet dried slower. Coulomb repulsive force could cause the elongation of the fibers

during a relative longer period, thus reduced the size in fiber diameter. Secondly, when

the relative humidity was low (in the situation of Scheme 2.2B), the size of fibers were

controlled by the synergistic interaction between the mass decreasing rate and diffusion

rate of the solvents. If the mass decreasing rate was slightly larger than the diffusion rate

and the fluid jet collapsed slightly before it was dried completely, resulting in narrow

width. If the humidity was further decreased, the decreasing rate was much larger than

the diffusion rate. The fluid jet spread outwards further before it was totally dried. As a

result, the width increased.

2.3.4 Effect of Solvents on the Fiber Morphology

To further explore this concept, the volumetric ratio of ethyl alcohol (EtOH) to DI

water (H2O) was varied from 3:1, 2:1, 1:1, to 1:2, while keeping the relative humidity at

16%. When EtOH/H2O volumetric ratio was at 4:1 or 3:1, the morphology of electrospun

PVP fibers was helical peanut-shaped ribbon (Figure 2.6A). Percentage of the helical

ribbons decreased from 98% to 85% when EtOH/H2O ratio changed from 4:1 (v/v) to 3:1

(v/v). At the volumetric ratio of 2:1 or 1:1, the morphology changed to flat ribbon or

straight round fiber (Figure 2.6B and 2.6C). The width of flat ribbons or diameter of

straight round fibers decreased from 1.2 µm to 770 nm when EtOH/H2O ratio changed

from 2:1 (v/v) to 1:1 (v/v). Thin round fibers with beads formed when the ratio reached

1:2 (v/v) (Figure 2.6D).

34

Chapter 2

As shown in Table A1, Boiling point of DI water is higher than ethyl alcohol,

while the vapor pressure is lower at room temperature. Thus the mass decreasing rate of

water is smaller than that of ethyl alcohol under the experimental conditions. As

volumetric percentage of water increases, decreasing rate of solvent mass should decrease.

This effect should hold the same as relative humidity of the environment increases. As a

result, the morphology of electrospun PVP fibers changed from helical peanut-shaped

ribbons to flat ribbons and eventually to straight round fibers while the volumetric

percentage of water increased. Meanwhile, percentage of helical ribbons and the width of

flat ribbons decreased. In additional, the surface tension and dielectric constant of water

are much higher than those of ethyl alcohol. Coulomb forces and the surface tension

could not be balanced in the fluid jet if other conditions retain the same. The fluid jet

became unstable and beads began to form (Figure 2.6C and D).33

35

Chapter 2

Figure 2.6. SEM images of PVP fibers generated at the EtOH/H2O volumetric ratio of

(A) 3:1, (B) 2:1, (C) 1:1, and (D) 1:2, respectively. The relative humidity of the

environment was 16%.

2.3.5 Effect of Concentration on the Fiber Morphology

When the concentration of electrospun solution was varied from 11%, 9% to 7%

(w/v) while other conditions were maintained the same, the morphology of PVP fibers

changed from helical peanut-shaped ribbon to flat ribbon or straight round fiber, and at

last to thin fiber with beads (Figure 2.7). The percentage of helical ribbon decreased from

83% to 26% and at last to 0%. The width of flat ribbons or diameter of straight round

36

Chapter 2

fibers decreased from 489 nm to 346 nm when the concentration of solution changed

from 9% (w/v) to 7% (w/v). Thin round fibers with beads formed when the concentration

decreased to 7% (w/v).

Figure 2.7. SEM images of PVP fibers generated at the concentration of electrospun

solution of (A) 11%, (B) 9% and (C) 7% (w/v), respectively. The relative humidity of the

environment was 16%.

Concentration of polymer solution affects on morphology of the electrospun fiber,

the bead formation, bead density and diameter of the fiber.34-35

As the concentration of

polymer solution decreased, the viscosity of fluid jet decreased accordingly, leading to a

lower drying rate. So the same trend was observed just as the volume percentage of DI-

water increased or as the humidity of environment increased. While the concentration

was as low as 7%, the viscosity was so lower that cohesive force decreased. The surface

tension was dominant which tended to form beads. Then the fluid jet became unstable

and beads formed.

2.3.6 Ultimate Elongation of Fibers Electrospun at Different Relative Humidity

37

Chapter 2

Thermal mechanical analyzer was used to characterize the elasticity of the

electrospun fibers. Figure 2.8 shows the ultimate elongation of PVP fibers with different

morphologies and percentage of helical ribbons electrospun at a range of relative

humidity. The ultimate elongation decreased when the relative humidity increased.

Figure 2.8. Dependence of ultimate elongation of PVP fibers and percentage of helical

ribbons on the relative humidity.

It is because the ultimate elongation of helical ribbons is larger than that of

straight round fibers. As illustrated in Scheme 2.3, if the extension force is applied on the

two ends of helical ribbons, the degree of spiral decreases continuously until almost no

spiral exists. Subsequently, the width of the ribbons decreases until the ribbons breaks.

38

Chapter 2

However, if the extension force is exerted on the two ends of the straight round fibers, the

diameter of the fibers decreases until the fibers are fractured. The percentage of helical

ribbons decreased when the relative humidity increased as mentioned in Section 2.3.2.

Therefore, if the relative humidity increases, the ultimate elongation decreases.

Scheme 2.3. Schematic illustration of the deformation of (A) a helical ribbon and (B) a

straight round fiber under extension forces F.

39

Chapter 2

2.4 Conclusion

This chapter describes the preparation and understanding of helical peanut-shaped

ribbons of poly(vinyl pyrrolidone) (PVP) by a magnetic field-assisted electrospinning

method. The relative humidity was found to significantly influence the morphology of

PVP fibers. When the relative humidity of the environment decreased from 32% to 16%,

the morphology of the electrospun PVP fibers changed from straight fiber to helical

ribbon, and the percentage of helical ribbons increased to 99%. The cross-section of

electrospun fibers changed from circle, to ellipse, and to peanut shape. The ultimate

elongation was 33% for straight round fibers made at relative humidity of 32% and 43%

for helical ribbons made at relative humidity of 16%. The formation of helical ribbons

was explained based on the synergistic interaction of bending instability of electrospun

jet, and the rigidity of the polymer materials during the MFAE process, involving the

mass decreasing rate and the diffusion rate of the solvents. This work provides insights

on the effect of relative humidity on the extent of spirals and the shape of the cross-

section of the electrospun polymer fibers. Additionally, the influence of solvent property

and concentration on the morphology of electrospun fibers were also discussed.

Controllable morphology of electrospun fibers may be applied to guide the growth of

cells, thus the technique described in this work is potentially useful for making

biomaterials in tissue engineering. This technique also offers a way to control the

mechanical property of electrospun fiber scaffolds.

40

Chapter 2

2.5 References

1. Agarwal, S.;Wendorff, J. H.;Greiner, A. Use of electrospinning technique for

biomedical applications. Polymer 2008, 49, 5603-5621.

2. Bhardwaj, N.;Kundu, S. C. Electrospinning: A fascinating fiber fabrication

technique. Biotechnol. Adv. 2010, 28, 325-347.

3. Doshi, J.;Reneker, D. H. ELECTROSPINNING PROCESS AND

APPLICATIONS OF ELECTROSPUN FIBERS. J. Electrost. 1995, 35, 151-160.

4. Huang, Z. M.;Zhang, Y. Z.;Kotaki, M.;Ramakrishna, S. A review on polymer

nanofibers by electrospinning and their applications in nanocomposites. Compos. Sci.

Technol. 2003, 63, 2223-2253.

5. Lu, X. F.;Wang, C.;Wei, Y. One-Dimensional Composite Nanomaterials:

Synthesis by Electrospinning and Their Applications. Small 2009, 5, 2349-2370.



7. Sill, T. J.;von Recum, H. A. Electro spinning: Applications in drug delivery and

tissue engineering. Biomaterials 2008, 29, 1989-2006.






10. Tay, C. Y.;Irvine, S. A.;Boey, F. Y. C.;Tan, L. P.;Venkatraman, S. Micro-/Nano-

engineered Cellular Responses for Soft Tissue Engineering and Biomedical Applications.

Small 2011, 7, 1361-1378.

11. Badami, A. S.;Kreke, M. R.;Thompson, M. S.;Riffle, J. S.;Goldstein, A. S. Effect

of fiber diameter on spreading, proliferation, and differentiation of osteoblastic cells on

electrospun poly(lactic acid) substrates. Biomaterials 2006, 27, 596-606.

12. Min, B. M.;Lee, G.;Kim, S. H.;Nam, Y. S.;Lee, T. S.;Park, W. H. Electrospinning

of silk fibroin nanofibers and its effect on the adhesion and spreading of normal human

keratinocytes and fibroblasts in vitro. Biomaterials 2004, 25, 1289-1297.

13. Yang, F.;Murugan, R.;Wang, S.;Ramakrishna, S. Electrospinning of nano/micro

scale poly(L-lactic acid) aligned fibers and their potential in neural tissue engineering.

Biomaterials 2005, 26, 2603-2610.

14. Xie, J. W.;Liu, W. Y.;MacEwan, M. R.;Yeh, Y. C.;Thomopoulos, S.;Xia, Y. N.

Nanofiber Membranes with Controllable Microwells and Structural Cues and Their Use

in Forming Cell Microarrays and Neuronal Networks. Small 2011, 7, 293-297.

15. Liu, Y. Y.;Sun, Y.;Yan, H.;Liu, X. Y.;Zhang, W.;Wang, Z.;Jiang, X. Y.

Electrospun Fiber Template for Replica Molding of Microtopographical Neural Growth

Guidance. Small 2012, 8, 676-681.

16. Nain, A. S.;Phillippi, J. A.;Sitti, M.;MacKrell, J.;Campbell, P. G.;Amon, C.

Control of cell behavior by aligned micro/nanofibrous biomaterial scaffolds fabricated by

41

Chapter 2

spinneret-based tunable engineered parameters (STEP) technique. Small 2008, 4, 1153-

1159.

17. Koombhongse, S.;Liu, W. X.;Reneker, D. H. Flat polymer ribbons and other

shapes by electrospinning. J. Polym. Sci. Pt. B-Polym. Phys. 2001, 39, 2598-2606.

18. Rojas, O. J.;Montero, G. A.;Habibi, Y. Electrospun Nanocomposites from

Polystyrene Loaded with Cellulose Nanowhiskers. J. Appl. Polym. Sci. 2009, 113, 927-

935.

19. Duan, H. W.;Wang, Y.;Zhang, Y. J.;Geng, L. Characterization of ODPA-ODA

polyimide non-woven membranes prepared by electrospinning. J. Polym. Eng. 2008, 28,

33-42.

20. Holzmelster, A.;Rudisile, M.;Greiner, A.;Wendorff, J. H. Structurally and

chemically heterogeneous nanofibrous nonwovens via electrospinning. Eur. Polym. J.

2007, 43, 4859-4867.

21. Lee, K. H.;Givens, S. R.;Snively, C. M.;Chase, B.;Rabolt, J. F. Crystallization

behavior of electrospun PB/PMP blend fibrous membranes. Macromolecules 2008, 41,

3144-3148.

22. Lee, K. H.;Snively, C. M.;Givens, S.;Chase, D. B.;Rabolt, J. F. Time-dependent

transformation of an electrospun isotactic poly(1-butene) fibrous membrane.

Macromolecules 2007, 40, 2590-2595.

23. Lee, K. H.;Givens, S.;Chase, D. B.;Rabolt, J. F. Electrostatic polymer processing

of isotactic poly(4-methyl-1-pentene) fibrous membrane. Polymer 2006, 47, 8013-8018.

24. Liu, Y. Q.;Zhang, X. P.;Xia, Y. N.;Yang, H. Magnetic-Field-Assisted

Electrospinning of Aligned Straight and Wavy Polymeric Nanofibers. Adv. Mater. 2010,

22, 2454-2457.

25. Reneker, D. H.;Yarin, A. L.;Fong, H.;Koombhongse, S. Bending instability of

electrically charged liquid jets of polymer solutions in electrospinning. J. Appl. Phys.

2000, 87, 4531-4547.


electrically forced jets. I. Stability theory. Phys. Fluids 2001, 13, 2201-2220.


electrically forced jets. II. Applications. Phys. Fluids 2001, 13, 2221-2236.

28. Yarin, A. L.;Koombhongse, S.;Reneker, D. H. Bending instability in

electrospinning of nanofibers. J. Appl. Phys. 2001, 89, 3018-3026.

29. Reneker, D. H.;Yarin, A. L. Electrospinning jets and polymer nanofibers.

Polymer 2008, 49, 2387-2425.

30. Han, T.;Reneker, D. H.;Yarin, A. L. Buckling of jets in electrospinning. Polymer

2007, 48, 6064-6076.

31. Zhang, C. L.;Lv, K. P.;Hu, N. Y.;Yu, L.;Ren, X. F.;Liu, S. L.;Yu, S. H.

Macroscopic-Scale Alignment of Ultralong Ag Nanowires in Polymer Nanofiber Mat and

Their Hierarchical Structures by Magnetic-Field-Assisted Electrospinning. Small 2012, 8,

2936-2940.

42

Chapter 2

32. Reneker, D. H.;Yarin, A. L.;Zussman, E.;Xu, H. Advances in Applied Mechanics.

In Electrospinning of Nanofibers from Polymer Solutions and Melts; Aref, H.,van der

Giessen, E., Eds.; Elsevier Academic Press: San Diego, USA, 2007; Vol. 41, pp 43-195.

33. CRC Handbook of Chemistry and Physics. In Fluid Properties [Online];93rd

Edition (Internet Version 2013); Haynes, W. M., Eds.; CRC Press / Taylor and Francis:

Boca Raton, USA, 2013; Section 6. https://z.rochester.edu/zimbra/#3 (accessed January 1,

2013).



404-411.

35. Ding, B.;Kim, C. K.;Kim, H. Y.;Se, M. K.;Park, S. J. Titanium dioxide nanoribers

prepared by using electrospinning method. Fiber. Polym. 2004, 5, 105-109.

36. Kim, C. H.;Jung, Y. H.;Kim, H. Y.;Lee, D. R.;Dharmaraj, N.;Choi, K. E. Effect

of collector temperature on the porous structure of electrospun fibers. Macromol. Res.

2006, 14, 59-65.

43

Chapter 3

Chapter 3 Fabrication of SiC Fibers by Electrospinning and

UV Cure

3.1 Introduction

Silicon carbide (SiC) has various desirable properties including high tensile

strength, high elastic modulus, excellent thermal stability, outstanding oxidation

resistance and remarkable electronic properties.1-4

As a result, it is a good reinforcement

material and a good candidate for high-speed electronics.

Fabrication of one dimensional (1D) SiC catches a lot of attentions. Catalyst

assisted pyrolysis method is cheap and fast.5-8

But the low yield and the involvement of

metallic elements limit the practical utilization of final 1D materials. Template modulated

method necessitates the use of template and etchant, which increases the fabrication cost

and increases the chance to impair the fibers.9-10

Additionally, the length of 1D materials

fabricated by catalyst assisted and template modulated methods is limited. Conventional

spinning method, although good for producing SiC fibers with large aspect ratio, is only

useful for making SiC with the diameter of larger than 10µm.10-11

Compared to these

methods, electrospinning is a much superior method, because it can fabricate 1D fibers

with the size ranging from nanometer to micrometer facilely. For example, SiO2 fibers

are fabricated by electrospinning from preceramic precursor tetraethoxysilane (TEOS)

with the help of guiding polymer polyvinyl alcohol (PVA) or polyacrylonitrile (PAN) at

first; then they are carbonized by graphite to obtain SiC fibers.12-14

SiC fibers were also

44

Chapter 3

obtained from preceramic polymer, polycarbosilane, with the help of guiding polymer by

emulsion electrospinning15

and concentric electrospinning.16

In addition, to maintain the

morphology of preceramic fibers when converting it to SiC fibers by pyrolysis, electron

beam (e-beam) irradiation up to 10 MGy in an inert gas atmosphere was used.17-19

In this chapter, preceramic fibers could be obtained directly by magnetic-field-

assisted electrospinning (MFAE) method from the preceramic polymer without any

guiding polymer. Later, SiC fibers were produced by UV Cure and post-pyrolysis

treatment of the preceramic fibers. Compared to e-beam irradiation, the UV Cure method

is a much energy-conserving technique and easy to handle.


Materials. Allylhydridophenylpolycarbosilane (RD, Mn=1340, Mw=10300) was

purchased from Starfire System. Toluene (anhydrous, 99.8%), dichloromethane

(anhydrous, ≥99.8%), n-methyl-2-pyrrolidone (NMP, anhydrous, 99.5%) were purchased

from Aldrich. All chemicals were used as received without further purification.

Electrospinning of Preceramic Fibers. Aligned preceramic fibers was fabricated

by magnetic-field-assisted electrospinning method from preceramic polymer RD directly.

RD was dissolved into various mixtures of solvents while the concentration of solution

was kept at 50 wt%. The polymer solution was loaded into a plastic syringe with a 22-

Gauge (inner diameter: 0.028 inch) stainless-steel needle. The sharp tip of the needle was

cut and sanded into smooth circular, flat opening to ensure the proper direction of fluid

45

Chapter 3

jets. The needle was connected to a high-voltage power supply and a collector was

grounded and 15 cm below the needle tip. The voltage between the spinneret and the

collector was maintained at 23 kV. The collector was a U-type bracket wrapped by foil

and placed on the top of the two magnets. The space between the two arms of the U-type

bracket was the same as the distance between two magnets. So the electrospun

preceramic fibers could hang on the arms of the bracket. The feeding rate was controlled

by a syringe pump at a constant rate of 1.5 mL/h.

Fabrication of SiC Fibers. The as-spun preceramic fibers fixed by U-type

bracket were cured by a UV lamp (254nm, PSD-UVT, Novascan Technologies Inc.) in

Ar atmosphere at room temperature under various durations. After UV Cure, the fibers

became robust and could be cut from the bracket integrally. Later, the fibers were

pyrolyzed at 1100 °C in Ar atmosphere to form SiC fibers.

Characterization. The morphology of the preceramic fibers and SiC fibers were

studied by Field Emission Scanning Electron Microscope (FE-SEM, Zeiss-Leo DSM982).

Average diameter was measured by Image J (National Institutes of Health, USA). 1H

NMR spectra were recorded in CDCl3 on Brucker Avance 400-2 NMR Spectrometer at

400 MHz. Attenuated total reflectance infrared spectra (ATR-IR) were collected on a

Shimadzu FTIR-8400S spectrophotometer with a MIRacle ATR accessory. Powder X-ray

diffraction (PXRD) patterns were recorded using a Philips MPD diffractometer with a Cu

Kα X-ray source (λ = 1.5405 Å). The thermal gravimetric analysis (TGA) was conducted

on an SDT-Q600 TA instrument. In a standard procedure, ~5 mg samples were

transferred into a platinum pan, which was subsequently placed onto the dual beam

46

Chapter 3

sample holder. The samples were heated from room temperature to 1000 °C with a

heating rate of 2 °C/min. The flow rate of N2 was set at 50 mL/min.


3.3.1 Morphology and Diameter Control of Preceramic Fibers

The morphology and diameter of the electrospun preceramic fibers can be

adjusted by varying mixtures of solvents. When the mixture of toluene and

dichloromethane was used as solvent, the morphology of preceramic fibers changed

continuously from beaded fiber to straight fiber by varying the weight percentage of

toluene (

) from 30%, to 20%, to 10%, and to 0% (Figure 3.1). The

density of beads dwindled and the diameter of fibers increased from 1.5µm with 30% of

toluene in solvent mixture, to 2.1µm with 20%, to 2.3µm with 10%, and to 3.4µm with

no toluene.

47

Chapter 3

Figure 3.1. SEM images of preceramic fibers using mixture of toluene and

dichloromethane as solvent. The weight percentage of toluene was (A) 30%, (B) 20%,

(C) 10% and (D) 0%, respectively.

The diameter of preceramic fibers could be tuned by using mixture of n-methyl-

2-pyrrolidone (NMP) and dichloromethane as solvent (Figure 3.2). The diameter was 600

nm when the weight percentage of NMP (

) in solvent was 20% and 1

µm when it was 10%. The morphology of the fibers was flat ribbon.

48

Chapter 3

Figure 3.2. SEM images of preceramic fibers using mixture of NMP and

dichloromethane as solvent. The weight percentage of NMP was (A) 20% and (B) 10%.

Physical properties of solvent are vital to the morphology of electrospun fibers,

including boiling point, vapor pressure at room temperature, surface tension and

dielectric constant.20-23

Boiling point and vapor pressure at room temperature determine

the drying rate and viscosity of the fluid jet, which influence the viscoelastic stress.

Dielectric constant determines the density of the charge generated in the fluid jet and

therefore affect the Coulomb repulsive force.23

As a result, different mixtures of solvents

lead to different balances of Coulomb force and surface tension, which would yield fibers

with various morphologies and diameters.

The morphology and the size of electrospun fibers could be modified by changing

the physical properties of solvent through mixing different solvents. Fluid jet with high

surface tension and low charge density per unit area (low dielectric constant) tends to

form beads. The dielectric constant of toluene is much lower than that of

dichloromethane, though they have similar surface tension (Table A1). As a result, by

49

Chapter 3

using mixture of toluene and dichloromethane as solvent, fiber with beads was obtained.

By decreasing the weight percentage of toluene, the density of beads decreased until the

straight fibers were obtained. Normally higher bead density would generate thinner fibers,

and lower bead density would generate thicker fibers. So the diameter of fibers also

increased.

Although the surface tension of NMP is higher than that of dichloromethane, its

dielectric constant is much higher. As a result, Coulomb repulsive force became stronger

by mixing NMP into dichloromethane. In addition, the boiling point of NMP is higher

and vapor pressure at room temperature is lower than that of dichloromethane. The

drying rate of fluid jet is much slower when NMP is used as the solvent. Therefore the

fiber could be elongated further before it solidified. So by increasing the weight

percentage of NMP, fiber diameter decreased. In the following sections, pure

dichloromethane was chosen as the solvent.

3.3.2 Characterization of Preceramic Polymer

Silicon carbide (SiC) fibers could be obtained by pyrolysis of as-spun preceramic

fibers in Ar atmosphere at elevated temperature. But RD materials melt above 100 °C. It

is a great problem to maintain the morphology of fibers while pyrolyzing. To solve this

problem, the properties of RD are characterized at first.

RD was characterized by 1H NMR and ATR-IR to determine its structural

formula. In the 1H NMR spectrum (Figure 3.3), resonances corresponding to benzene,

50

Chapter 3

CH2=CH, SiH, SiCH2 and SiCH3 are observed in the regions of 7.1-7.7 (peak a), 4.8-5.7

(peak b), 3.1-4.4 (peak c), 1-1.6 (peak d) and 0-0.6 (peak e) ppm respectively. Peak f is

the resonances of solution CDCl3. Based on these data; the structural formula of RD is

deducted and sketched on the top of 1H NMR spectrum. ATR-IR spectrum (Figure 3.4)

further confirms its structure. Characteristic vibrations of SiH, C=C, Si-C (phenyl) and

Si-C (alkyl) are observed at 2112, 1589, 1430 and 1110, and 1250 cm−1

respectively.

Figure 3.3. 1H NMR spectra of RD material. The structural formula of RD was sketched

on the top.

51

Chapter 3

Figure 3.4. ATR-IR spectrum of RD material.

Figure 3.5 shows the TGA curve of RD material. From the TGA curve, it is

noticed that the weight loss took place in three stages. A weight loss of 10% observed

below 250 °C is due to the loss of low molecular weight oligomers. During this stage, RD

melted and formed a gel. The major weight loss occurs in the region 250 – 550 °C (17

wt% loss), which can be attributed to the loss of phenyl and hydrogen from the polymeric

chain resulting in onset of cross-linking. The third stage of weight loss (∼6%) noticed in

the region 550 – 700 °C is attributed to the loss of the CH4.25-27

During the second and

third stage, the color of RD materials darkened; the gel solidified gradually and finally

formed a black solid. The weight loss of RD finishes at 750 °C, but higher temperature

52

Chapter 3

treatment (larger than 1200 °C) is needed to transfer amorphous SiC to crystalline SiC.

The net ceramic residue of the RD is found to be 65 wt% at 1350 °C in argon atmosphere.

Figure 3.5. TGA curve of RD material.

XRD pattern of RD pyrolyzed at 1350 °C is given in Figure 3.6. Diffraction lines

corresponding to (111), (200), (220) and (311) planes of β-SiC are detected. It verifies

that SiC could be obtained by pyrolysis of preceramic polymer RD.

53

Chapter 3

Figure 3.6. XRD patterns of RD pyrolyzed in Ar atmosphere at 1350 °C. XRD patterns

of β-SiC are shown by the red lines as a reference based on the JCPDS database (No. 74-

2307).

3.3.3 Influence of UV Cure on Morphology of SiC Fibers

According to the result of TGA, RD material melted and formed a gel at first

when the temperature of pyrolysis was elevated from room temperature due to its low

molecular weight and glass transition point. Subsequently, cross-linking reaction

happened at high temperature and the gel solidified gradually due to the growth of

molecular weight. To solve the problem of morphology deformation of preceramic fibers

during the pyrolysis process, as-spun fibers need to be pre-cured to improve its molecular

weight and glass transition point. In this study, UV cure is chosen, whose wavelength is

254 nm.

54

Chapter 3

SEM images of SiC fibers, which were cured by UV light at different durations

followed by pyrolyzing in Ar atmosphere at 1100 °C, are shown in Figure 3.7. Fibers

cured by UV light for only 15 min totally fused together after the pyrolysis treatment.

Fibers cured for 30 min also fused together but the outline of the fibers could be seen. For

the batches which were cured for 1 hr, only a few fibers fused together. For the batches

which were cured for 2 hr, the morphology of as-spun fibers was integrally maintained.

Figure 3.7. SEM images of SiC fibers which were cured by UV light at different

durations and then pyrolyzed in Ar atmosphere at 1100°C. The periods of UV cure were

(A) 15min, (B) 30 min, (C) 1 hr, and (D) 2 hr, respectively.

55

Chapter 3

UV light can break and reunite some bonds of polymer.19, 28

So UV cure

crosslinks the precursor polymer RD, resulting in the increase of its molecular weight and

glass transition point. Therefore, as-spun preceramic fibers after UV cure become stable

enough for the post-pyrolysis treatment. ATR-IR spectra of as-spun preceramic fibers

cured by UV light at different durations are shown in Figure 3.8. The progress of the

crosslinking reaction is monitored by the absorbance of Si-H stretching vibration (2112

cm-1

), Si-Phenyl group (1430 and 1110 cm-1

) and Si-CH3 group (1250 cm-1

) in the IR

spectra. Overall, all the intensity of these absorbance decreased as the period of UV Cure

increased, indicating that the amount of these groups decreased with progress of cross-

linking reaction. Particularly, in the region of characteristic absorbance of Si-H stretching

vibration (red square in Figure 3.8), the intensity of the peak at 2112 cm-1

decreased

while the ones at 2025 and 1974 cm-1

increased when the duration of UV cure was

prolonged. It is due to the differences of the adjoining group to Si-H, such as R group in

R-Si-H. According to the book of Aldrich Library of FT-IR, the peak of Si-H shifts from

2120 to 2090 cm-1

if R group changes from methyl, ethyl, to isopropyl group; and it also

shifts from 2120 to 2110 cm-1

if R group varies from phenyl group to hexane group.29

In

the region of the characteristic absorbance of Si-phenyl group (green square in Figure

3.8), the intensity of peak at 1430 cm-1

decreased while the one at 1408 cm-1

increased

when the duration was extended. Indeed the absorbance peak at 1430 cm-1

is C-H (in Si-

phenyl) bending vibration peak, and the one at 1408 cm-1

is C-H (in Si-CH=CH2) )

56

Chapter 3

bending vibration peak.29

It indicates that the benzene ring of phenyl group in Si-phenyl

was broken into ethylene group gradually by UV exposure.

Figure 3.8. ATR-IR spectra of as-spun preceramic fibers which were cured by UV light

at different durations.

According to the results of ATR-IR spectra, crosslinking reactions are proposed

(Scheme 3.1). When RD material is irradiated by UV light, double bonds break and free

radicals are generated. The free radicals may attack CH2 or Si-H group as illustrated in

the proposed reaction. Then the amount of Si-H, Si-phenyl and Si-CH3 group decrease

57

Chapter 3

with progress of crosslinking reaction. R group in R-Si-H changes from methyl to

isopropyl group, resulting in the variation of peaks of Si-H stretching vibration in ATR-

IR spectra. Si-phenyl group is transferred to Si-vinyl group leading to the change of

peaks of Si-phenyl group in ATR-IR spectra. All these are consistent with the results of

IR spectra. It supports our proposition that UV cure can crosslink the precursor polymer

RD and increase its molecular weight. Therefore, as-spun preceramic fibers after UV cure

become stable and the morphology could be maintained during the post-pyrolysis

treatment.

58

Chapter 3

59

Chapter 3

Scheme 3.1. Schematic illustration of crosslinking reactions of as-spun preceramic fibers

during the treatment of UV cure.

Additional evidence of crosslinking reaction is that the solubility of RD material

in dichloromethane decreases when the duration of UV cure is increased. As shown in

Figure 3.9, RD material without UV cure could be dissolved completely in

dichloromethane. A few part of the material could not be dissolved when it was cured by

UV light for 1 hr. And the amount of insoluble part increased when the duration was

extended to 2 hr.

Figure 3.9. Solution of RD in dichloromethane after RD was cured by UV light for

different durations.

60

Chapter 3

3.4 Conclusion

In this chapter, SiC fibers were synthesized by electrospinning from preceramic

polymer, UV Cure and pyrolysis processes. The morphology and diameter of the

electrospun preceramic fibers could be adjusted by using various mixtures of solvents.

UV cure can crosslink and strengthen the preceramic fibers, leading to the preservation of

morphology during pyrolysis process at high temperature. The morphology of preceramic

fibers could be maintained if they were cured by UV light for duration of longer than 2 hr.

The mechanism of UV cure was proposed. UV light could break double bonds of RD

material and generate free radicals. The free radicals attacked CH2 or Si-H group, leading

to the crosslink reaction and stability of preceramic fibers.

3.5 References

1. Yajima, S.;Hayashi, J.;Omori, M.;Okamura, K. DEVELOPMENT OF A

SILICON-CARBIDE FIBER WITH HIGH-TENSILE STRENGTH. Nature 1976, 261,

683-685.

2. Bourenane, K.;Keffous, A.;Nezzal, G.;Bourenane, A.;Boukennous,

Y.;Boukezzata, A. Influence of thickness and porous structure of SiC layers on the

electrical properties of Pt/SiC-pSi and Pd/SiC-pSi Schottky diodes for gas sensing

purposes. Sens. Actuator B-Chem. 2008, 129, 612-620.

3. Eddy, C. R.;Gaskill, D. K. Silicon Carbide as a Platform for Power Electronics.

Science 2009, 324, 1398-1400.

4. Vanhaecke, E.;Ivanova, S.;Deneuve, A.;Ersen, O.;Edouard, D.;Wine, G.;Nguyen,

P.;Pham, C.;Pham-Huu, C. 1D SiC decoration of SiC macroscopic shapes for filtration

devices. J. Mater. Chem. 2008, 18, 4654-4662.

5. Zhou, J. Y.;Chen, Z. Y.;Zhou, M.;Gao, X. P.;Xie, E. Q. SiC Nanorods Grown on

Electrospun Nanofibers Using Tb as Catalyst: Fabrication, Characterization, and

Photoluminescence Properties. Nanoscale Res. Lett. 2009, 4, 814-819.

6. Vakifahmetoglu, C.;Colombo, P.;Carturan, S. M.;Pippel, E.;Woltersdorf, J.

Growth of One-Dimensional Nanostructures in Porous Polymer-Derived Ceramics by

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Chapter 3

Catalyst-Assisted Pyrolysis. Part II: Cobalt Catalyst. J. Am. Ceram. Soc. 2010, 93, 3709-

3719.

7. Vakifahmetoglu, C.;Pippel, E.;Woltersdorf, J.;Colombo, P. Growth of One-

Dimensional Nanostructures in Porous Polymer-Derived Ceramics by Catalyst-Assisted

Pyrolysis. Part I: Iron Catalyst. J. Am. Ceram. Soc. 2010, 93, 959-968.

8. Yang, W. Y.;Gao, F. M.;Fan, Y.;An, L. N. Al-Doped Single-Crystalline SiC

Nanowires Synthesized by Pyrolysis of Polymer Precursors. J. Nanosci. Nanotechnol.

2010, 10, 4729-4732.

9. Elyassi, B.;Kim, T. W.;Sahimi, M.;Tsotsis, T. T. Effect of polystyrene on the

morphology and physical properties of silicon carbide nanofibers. Mater. Chem. Phys.

2009, 118, 259-263.

10. Vakifahmetoglu, C. Fabrication and properties of ceramic 1D nanostructures from

preceramic polymers: a review. Adv. Appl. Ceram. 2011, 110, 188-204.

11. Colombo, P.;Mera, G.;Riedel, R.;Sorarù, G. D. Polymer-Derived Ceramics: 40

Years of Research and Innovation in Advanced Ceramics. Journal of the American

Ceramic Society 2010, 93, 1805-1837.

12. Li, J. Y.;Zhang, Y. F.;Zhong, X. H.;Yang, K. Y.;Meng, J.;Cao, X. Q. Single-

crystalline nanowires of SiC synthesized by carbothermal reduction of electrospun

PVP/TEOS composite fibres. Nanotechnology 2007, 18, 1-4.

13. Delavari, H.;Kokabi, M. SILICON CARBIDE NANOWIRES FROM

POLYVINYL ALCOHOL/SILICA ELECTROSPUN NANOFIBERS. Nano 2011, 6, 41-

45.

14. Kim, B. H.;Kim, C. H.;Yang, K. S.;Kim, K. Y.;Lee, Y. J. SiC/SiO2 coating for

improving the oxidation resistive property of carbon nanofiber. Appl. Surf. Sci. 2010, 257,

1607-1611.

15. Choi, S. H.;Youn, D. Y.;Jo, S. M.;Oh, S. G.;Kim, I. D. Micelle-Mediated

Synthesis of Single-Crystalline beta(3C)-SiC Fibers via Emulsion Electrospinning. ACS

Appl. Mater. Interfaces 2011, 3, 1385-1389.

16. Liu, H. A.;Balkus, K. J. Electrospinning of beta silicon carbide nanofibers. Mater.

Lett. 2009, 63, 2361-2364.

17. Kang, P. H.;Jeun, J. P.;Seo, D. K.;Nho, Y. C. Fabrication of SiC mat by radiation

processing. Radiat. Phys. Chem. 2009, 78, 493-495.

18. Takeda, M.;Imai, Y.;Ichikawa, H.;Kasai, N.;Seguchi, T.;Okamura, K. Thermal

stability of SiC fiber prepared by an irradiation-curing process. Compos. Sci. Technol.

1999, 59, 793-799.

19. Eick, B.;Youngblood, J. SiC nanofibers by pyrolysis of electrospun preceramic

polymers. J. Mater. Sci. 2009, 44, 160-165.

20. Tungprapa, S.;Puangparn, T.;Weerasombut, M.;Jangchud, I.;Fakum,

P.;Semongkhol, S.;Meechaisue, C.;Supaphol, P. Electrospun cellulose acetate fibers:

effect of solvent system on morphology and fiber diameter. Cellulose 2007, 14, 563-575.

21. Zhao, J. H.;Xu, A. H.;Yuan, W. Z.;Gao, J. B.;Tang, J. K.;Wang, L.;Ai, F.;Zhang,

Y. M. Evaluation of electrospun nanofiber formation of perfluorosulfonic acid and poly

(N-vinylpyrrolidone) through solution rheology. J. Mater. Sci. 2011, 46, 7501-7510.

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Chapter 3

22. Luo, C. J.;Nangrejo, M.;Edirisinghe, M. A novel method of selecting solvents for

polymer electrospinning. Polymer 2010, 51, 1654-1662.

23. Uyar, T.;Besenbacher, F. Electrospinning of uniform polystyrene fibers: The

effect of solvent conductivity. Polymer 2008, 49, 5336-5343.

24. CRC Handbook of Chemistry & Physics. In Eds.; 92 ed.; CRC net Base Product:

2011-2012.

25. Berbon, M.;Calabrese, M. Effect of 1600 degrees C heat treatment on C/SiC

composites fabricated by polymer infiltration and pyrolysis with

allylhydridopolycarbosilane. J. Am. Ceram. Soc. 2002, 85, 1891-1893.

26. Interrante, L. V.;Jacobs, J. M.;Sherwood, W.;Whitmarsh, C. W., Fuentes,

M.,MartinezEsnaola, J. M.,Daniel, A. M., Eds. Trans Tech Publications: Clausthal

Zellerfe, 1997; Vol. 127-3, pp 271-278.

27. Sreeja, R.;Swaminathan, B.;Painuly, A.;Sebastian, T. V.;Packirisamy, S.

Allylhydridopolycarbosilane (AHPCS) as matrix resin for C/SiC ceramic matrix

composites. Mater. Sci. Eng. B-Adv. Funct. Solid-State Mater. 2010, 168, 204-207.

28. Shanmuganathan, K.;Sankhagowit, R. K.;Iyer, P.;Ellison, C. J. Thiol-Ene

Chemistry: A Greener Approach to Making Chemically and Thermally Stable Fibers.

Chem. Mat. 2011, 23, 4726-4732.

29. Aldrich Library of FT-IR Spectra. In Eds.; Second ed.; Sigma-Aldrich Co.: 1997;

Vol. 3.

63

Chapter 4

Chapter 4 Catalyst Membrane of Pt@Nb-TiO2 Fabricated by

Electrospinning and Atomic Layer Deposition

4.1 Introduction

Polymer electrolyte membrane fuel cell (PEMFC) is considered to be a green

energy conversion technology for the future.1-5

Among the PEMFCs, supported Pt and Pt

alloys have been extensively explored as electrocatalysts in oxygen-reduction reaction

(ORR), which is the major challenge in the system.6-9

Carbon black is the typical support

material for catalysts because of its large surface area, high electrical conductivity, and

well-developed pore structure.10

However, during the ORR cycles, carbon tends to be

corroded under the harsh electrochemical reaction conditions.11-14

The corrosion of

carbon support causes the agglomeration and sintering of Pt catalyst particles, resulting in

a decreased electrochemical surface area (ECSA) and activity of the catalysts. Carbon

corrosion also leads isolated Pt particles to detach from the support. Moreover, it has

been found that Pt can accelerate the corrosion rate of carbon support.15-16

These effects

strongly affect the durability of PEMFCs. Therefore, non-carbon supports increasingly

attract a great deal of attention.17-23

TiO2 becomes an alternative candidate for the support

material, because it has excellent stability in both acidic and oxidative environments.24

To use TiO2 as the electrocatalyst support, one has to develop novel approach to

the deposition of highly active catalysts on TiO2. The support also has to be conductive,

64

Chapter 4

because the conductivity of TiO2 is several orders of magnitude lower than that of carbon

black (Vulcan XC-72). In this context, electrospinning and atomic layer deposition

(ALD) are both simple and controllable techniques and the combination of these two may

be suitable for production of TiO2-supported Pt catalysts. Mats consisted of one-

dimensional TiO2 fibers with high surface area are made before by electrospinning.17, 25-28

ALD technique is able to deposit uniform nanoparticles (NPs) without capping ligands on

granular and porous materials, thus avoiding surface contaminations by organic capping

molecules. In addition, high degree of control over size of NPs is possible using ALD

technique due to its ability to precisely control the delivery of chemical reagents and to

the self-limiting nature of the surface-saturated reactions mechanism.29-32

Compared with

normal Pt/C catalysts, Pt NPs are anchored on the surface of supports by ALD technique

instead of simple mixing, which hinders the agglomeration and surface diffusion of NPs

and increases the stability of catalysts.

A typical method for improving the conductivity of TiO2 is through the

incorporation of n-type dopants, such as vanadium (V),33-34

niobium (Nb),20, 35

and

tantalum (Ta).36-37

Another approach is heat treatment at high temperatures under

reducing atmosphere in order to create oxygen vacancies in TiO2.38

When dopant method

is used, Nb is often preferred, because Nb5+

(r = 0.70 Å) has similar ionic radii as Ti4+

(r

= 0.68 Å), there exists negligible lattice mismatch between TiO2 and Nb2O5. Thus, Nb2O5

forms a complete range of solid solutions with TiO2.35, 39-40

In this work, we present an innovative approach for making catalyst membranes

of Pt NPs on Nb-doped titania (Pt@Nb-TiO2) fiber membranes. This approach is

65

Chapter 4

developed based upon the recent success in two independent areas, namely, the

fabrication of TiO2 fibers by electrospinning25-28

and the deposition of Pt NPs on supports,

such as WC,30

Al2O3,41

SrTiO331, 42

and carbon nanotube by ALD.43-44

These catalyst

membranes could potentially be directly integrated in membrane electrode assembly

(MEA) as the catalyst layer, avoiding complicated process of making catalyst inks and

then spaying or painting them onto a gas diffusion layer, because the catalyst layer is

already in a membrane form. The effects of Nb amount, conditions for ALD and post-

treatment on electrocatalytic activity and stability of the membranes are studied.


Materials. Polyvinyl acetate (PVAc, Mw 500,000), titanium (IV) isopropoxide

(Ti(OiPr)4, 99.999%) and niobium ethoxide (Nb(OEt)5, 99.95%) were purchased from

Sigma Aldrich. N,N-dimethyl formamide (DMF, anhydrous) was obtained from

Honeywell Burdick and Jackson. Acetic acid (glacial, 99.7%) was from Alfa Aesar.

Nafion 117 solution was from Fluka. All chemicals were used as received without further

purification.

Synthesis of Membrane Supports Made of Nb-TiO2 Support. Typically, 0.123

g PVAc was dissolved in 1 mL of DMF in glove box.35

After complete dissolution,

Ti(OiPr)4, Nb(OEt)5 and 0.119 g acetic acid were added into the solution in glove box.

The atomic ratio of Ti(OiPr)4 and Nb(OEt)5 was varied according to the predetermined

Nb percentage, while the total amount of Ti and Nb was kept at 1.67 mmol. The resulting

66

Chapter 4

solution was mixed well and electrospun at an accelerating voltage of 20 kV and at a flow

rate of 1 mL/h. The membrane of polymeric fibers was collected on a grounded

aluminum foil placed 15 cm below the spinneret. The relative humidity of environment

was kept at above 40% if not mentioned particularly. The membrane was then peeled off

from foil using tweezers and left in the air for ~10 h to let the hydrolysis reaction go to

completion. Then the membrane was sintered in air at 500 °C for 24 h to remove PVAc

and allow nucleation and growth of niobium-doped titania (Nb-TiO2) particles in the fiber

structure.

Synthesis of Membrane of Pt@Nb-TiO2 Catalysts. Atomic layer deposition

(ALD) is utilized to deposit Pt NPs onto the membrane of Nb-TiO2 fibers. The membrane

of Nb-TiO2 fibers was fixed in a chuck and hung in the air inside the chamber of ALD

system (Savannah 100 & 200, Cambridge Nano Tech Inc.) to ensure both side of the

membrane to be exposed to the reactants. The chamber was kept at 270 °C. Platinum

precursor, Trimethyl(methylcyclopentadienyl) platinum(IV) ((MeCp)Pt(CH3)3), was kept

at 70 °C and gas lines were held at 150 °C to avoid precursor condensation. High-purity

O2 was used as the co-reactant, and high-purity N2 was used as both the carrier and purge

gas. The flow rate of N2 was maintained at 5 sccm. A typical ALD cycle was as follows:

2 s of Pt precursor pulse, 5 s of N2 purge, 2 s of O2 pulse and 28 s of N2 purge. Pt NPs

with different sizes were obtained by changing the total number of ALD growth cycle.

For electrochemical measurement, catalyst membrane of Pt@Nb-TiO2 was treated at

either 200 °C or 500 °C in 5% H2 in Ar atmosphere for 2 h.

67

Chapter 4

Electrochemical Measurement. A three-electrode cell was used to perform the

electrochemical measurements. The working electrode was a glassy-carbon rotating-disk

electrode (RDE; area: 0.196 cm2). Pt foil (1 cm × 1 cm) was used as the counter electrode.

A reversible hydrogen electrode (RHE) was used as the reference electrode, which was

placed in a separate compartment. The electrolyte for all of the measurements was 0.1 M

perchloric acid, which was diluted with de-ionized water from a 70% doubly distilled

stock solution (GFS Chemicals, USA). To prepare the working electrode, the Pt@Nb-

TiO2 catalyst was first dispersed in ethanol and the mixture was sonicated for 30 min. 5

µL of suspension was added onto RDE by a pipette and dried under air. It was repeated

until the surface of RDE was covered by a thin-uniform layer of the catalysts. Then 10

µL of 0.0125% (wt%) Nafion-ethanol solution was dropped on the surface of the thin

film to prevent the catalysts from detaching by the flow of testing solution. The

measurements of the total electrochemically active surface area (ECSA) were performed

by integrating the hydrogen adsorption from the cyclic voltammetry (CV) data, which

was recorded at room temperature in argon saturated solution of 0.1 M HClO4. The

potential scan rate was 50 mVs-1

for the CV measurements. The ORR measurements

were conducted at a rotation rate of 1600 rpm in 0.1 M HClO4, which was purged with

oxygen for 30 min prior to and during the measurements. The scan rate for the ORR

measurements was 10 mVs-1

in the positive direction. The accelerated-stability test was

conducted at ambient temperature in an Ar-protected 0.1 M solution of aqueous HClO4.

The potential was cycled 30000 times between 0.6 and 1.0 V (versus RHE) at a scan rate

of 100 mVs-1

and the polarization curves were recorded after every 10000 cycles. The

68

Chapter 4

area-specific activity was obtained by normalizing the kinetic current density against the

ECSA values, and the kinetic current density was obtained from the Koutecky-Levich

plot.

Characterization. The morphology of the fibers and Pt@Nb-TiO2 catalyst

membrane were studied by field emission scanning electron microscope (FE-SEM, Zeiss-

Leo DSM982) and field emission transmission electron microscopy (FE-TEM, FEI

TECNAI F-20). Scanning transmission electron microscopy (STEM) and elemental maps

were carried out using the high-angle annular dark field (HAADF) mode on FE-TEM.

Energy dispersive X-ray (EDX) analysis of catalysts was carried out on FE-SEM

equipped with an EDAX detector. Average diameter was measured by Image J (National

Institutes of Health, USA). Powder X-ray diffraction (PXRD) patterns were recorded on a

Philips MPD diffractometer with a Cu Kα X-ray source (λ = 1.5405 Å).


4.3.1 Experimental Process

Scheme 1 illustrates the two-step process for the fabrication of catalyst membrane

by combining the electrospinning and ALD techniques. In the first step, support

membrane of Nb-TiO2 was produced by electrospinning technique and sintering. At first,

membrane of polymeric fibers was obtained by electrospinning from ceramic precursors

(Ti(OiPr)4 and Nb(OEt)5) and guiding polymer PVAc. The Nb-doped percentage was

varied by controlling the atomic ratio between Ti(OiPr)4 and Nb(OEt)5. Then the

69

Chapter 4

membrane of Nb-TiO2 was obtained by hydrolysis and sintering of the as-spun membrane

in the air. In the second step, Pt NPs were deposited onto the support membrane by ALD

technique to form catalyst membrane of Pt@Nb-TiO2. Typically, an ALD cycle began

with a organometallic precursor ((MeCp)Pt(CH3)3) pulse. (MeCp)Pt(CH3)3 reacted with

oxygen atoms on the surface of Nb-TiO2 membranes, generating PtCxHy-containing

compounds and byproducts of CH4, CO2 and H2O. The excess (MeCp)Pt(CH3)3 and

byproducts were cleared by N2 purging gas, followed by the O2 pulse. O2 reacted with

PtCxHy segment to form Pt–O and byproduct, CO2 and H2O. The O atom generated could

be used to react with (MeCp)Pt(CH3)3 in the subsequent ALD cycle. The ALD cycle was

finished by another N2 purge to eliminate excess O2 and byproducts. By repeating the

ALD growth cycle, different size of Pt NPs could be deposited onto the Nb-TiO2

membrane. Because the reaction streams of (MeCp)Pt(CH3)3 and O2 were separated by a

purge gas of N2. During each reactant pulse, only one reactant reacted with substrate

surface, through a self-limiting process that was capable of precisely controlling the

growth of Pt NPs.

70

Chapter 4

Scheme 4.1. Schematic illustration of the process for the production of Pt@Nb-TiO2

catalyst membrane. (A) Step 1: preparation of Nb-TiO2 support membrane by

electrospinning technique and sintering. (B) Step 2: preparation of catalyst membrane of

Pt@Nb-TiO2 by the ALD technique. The SEM image shows a final membrane made of

Pt@Nb-TiO2 fibers.

Figure 4.1 shows the TEM, STEM and HAADF-STEM micrographs of Pt NPs

dispersed on a Nb-TiO2 fiber made after 40 ALD cycles. The patchy, uneven contrast

shown in the TEM micrograph indicates the surface of Nb-TiO2 fiber was not smooth.

Both TEM and STEM micrographs show that the Pt NPs were uniformly dispersed on the

71

Chapter 4

ceramic fibers with an average diameter of 14 nm. Several studies previously show the

dispersion of Pt particles increased with increasing the number of oxygen-containing

surface species in the support.45-47

In this system, Nb-TiO2 fibers contained abundant

oxygen surface groups. As a result, the deposition of Pt NPs was uniform. The HAADF-

STEM micrograph indicates that Pt, Ti and Nb elements were distributed homogeneously

throughout the Nb-TiO2 fiber. These results indicate metal nanoparticles with high

uniformity in size could be deposited onto metal oxide fiber membranes by

electrospinning and ALD techniques. This two-step process is easy to operate and

scalable, because the size of the membrane generated by electrospinning and ALD could

be very large depending on the scale of the equipment. In our experiment, the size of a

typical membrane was 1.5 cm × 2.2 cm and limited by the operating area of the ALD

system and the tube furnace (inserted photo in Scheme 4.1B).

Figure 4.1. (A) TEM and (B) STEM micrographs of Pt@Nb-TiO2 catalyst. (C) HAADF-

STEM study showing the distribution of Pt (blue), Ti (red) and Nb (green) elements. The

area for HAADF-STEM study is the marked region in (B).

72

Chapter 4

4.3.2 Effect of Fiber Morphology on Electrocatalytic Activity

Two kinds of morphology of Nb-TiO2 fibers can be produced by electrospinning.

Figure 4.2 shows SEM images of as-spun polymeric fibers (left column), ceramic fibers

of Nb-TiO2 with 10 at% of Nb (middle column) and catalysts of Pt@Nb-TiO2 with 10

at% of Nb (right column) with these two kinds of morphology. One was straight fiber

which could be obtained when the relative humidity was lower than 40% during

electrospinning process (Figure 4.2A). The diameters of as-spun polymeric fibers and

Nb-TiO2 fibers after hydrolysis and sintering with morphology of straight fiber were 181

nm and 164 nm, respectively. The other was beaded fiber which could be obtained when

the relative humidity was kept higher than 40% (Figure 4.2B). The diameters of fibers

before and after sintering were 183 nm and 170 nm, respectively. The sizes of the beads

were 368 nm and 343 nm, respectively.

73

Chapter 4

Figure 4.2. SEM images of (A1 and B1) as-spun polymeric fibers, (A2 and B2) ceramic

fibers of Nb-TiO2 with 10 at% of Nb after hydrolysis and (A3 and B3) sintering and

catalysts of Pt@Nb-TiO2 with 10 at% of Nb after 40-cycle ALD of Pt NPs. The

morphologies of the fibers were: (A) straight fiber and (B) beaded fiber.

During the electrospinning process, ceramic precursors contacted with the

moisture in the air and were hydrolyzed partially. Meanwhile, the condensation reaction

happened.48-49

The rate of hydrolysis and condensation reactions increased when the

relative humidity of the environment increased. The molecular weight of ceramic

precursors grew as the hydrolysis and condensation reactions continued, leading the

increase of the viscosity of electrospun solution. In summary, the viscosity increased with

the relative humidity. Then the balance of the Coulomb repulsive force and the surface

74

Chapter 4

tension, which had close relationship with viscosity, was destroyed. As a result,

electrospun jet became unstable and beads began to form when the relative humidity was

higher than 40%.50

In the following, Pt NPs were deposited onto the support membrane of Nb-TiO2

with 10 at% of Nb with these two kinds of morphology by 40-cycle ALD respectively.

Although the morphology and size of the Nb-TiO2 supports were different, the diameter

of Pt NPs were all around 14 nm (Figure 4.2.A3 and B3). Thus, the size of Pt NPs

apparently depended only on the operating temperatures of the Pt precursors and ALD

chamber, and the number of ALD growth cycles.51-52

Then electrochemical measurement

was carried out to study the area-specific ORR activity (is) of the catalyst membranes

with these morphologies. The area-specific activity of catalyst membrane with

morphology of straight fiber was 0.079 ± 0.008 mA/cm2

Pt at 0.9 V (versus RHE), and the

one with beaded fiber was 0.16 ± 0.059 mA/cm2

Pt. The activity of the catalyst membrane

with the morphology of straight fiber was much lower than the one with beaded fiber due

to the difference of porosity. The porosity of the support membrane with morphology of

beaded fiber was supposed to be higher due to the existence of beads, leading facilely

deposition of Pt NPs into the inner holes of membrane during ALD process. So the

activity of catalyst membrane with morphology of straight fibers was lower.

75

Chapter 4

4.3.3 Effect of Nb-doped Percentage on Electrocatalytic Activity

Different amount of niobium (5 at%, 10 at% and 25 at%) were doped into TiO2 to

form Nb-TiO2 support membranes by varying the Nb(OEt)5/Ti(OiPr)4 atomic ratio from

5 : 95, to 10 : 90 and to 25 : 75 in the electrospun solution to improve the conductivity of

the supports while other parameters were maintained the same. Pt NPs were loaded onto

these membranes by 40-cycle ALD. Figure 4.3 shows SEM images of as-spun polymeric

fibers (left column), Nb-TiO2 fibers (middle column) and Pt@Nb-TiO2 catalysts (right

column). Table 4.1 summarizes the diameter of the fibers and size of the beads of as-spun

polymeric and ceramic Nb-TiO2 fibers with different doping amount of Nb. The diameter

of Nb-TiO2 fibers did not change much, when compared with the as-spun polymeric

fibers. Void spaces were generated after the pyrolysis in air due to the removal of PVAc

and to the homogeneous nature of the preceramic mixture of PVAc and metal alkoxide

precursors (Figure 4.3, middle column). The ceramic fibers were continuous and made of

small, crystal domains without preferred orientation (Figure 4.4). Although the

morphology of fibers were different at the different compositions of Nb-TiO2 ceramic,

the diameter of Pt NPs deposited by 40-cycle ALD were all around 14 nm as mentioned

in Section 4.3.2.

76

Chapter 4

77

Chapter 4

Figure 4.3. SEM images of (A1, B1, C1 and D1) as-spun polymeric fibers, (A2, B2, C2

and D2) Nb-TiO2 fibers after hydrolysis and sintering and (A3, B3, C3 and D3) Pt@Nb-

TiO2 catalysts after 40-cycle ALD of Pt NPs. The amount of Nb was (A) 0 at%, (B) 5

at%, (C) 10 at%, and (D) 25 at%, respectively.

Table 4.1. Summary of the diameter of fibers and size of beads of as-spun polymeric and

Nb-TiO2 fibers at different Nb-doped amount (at%).

Nb amount (at%) as-spun polymeric fiber Nb-TiO2 fiber

fiber (nm) bead (nm) fiber (nm) bead (nm)

0 193 405 152 321

5 212 426 157 356

10 183 368 170 343

25 270 - 220 -

78

Chapter 4

Figure 4.4. TEM micrograph of TiO2 fiber.

XRD patterns of the support membranes only show the diffraction peaks, which

could be assigned to anatase phase of TiO2 (Figure 4.5). No diffraction peaks from

niobium oxide were observed, which indicates that Nb was incorporated into the TiO2

crystal structure. The formation of mixed oxide ceramic could be attributed to the similar

ionic radii between the two metal ions. Increased Nb amount in the ceramic resulted in

large lattice strain and the shift in the lattice parameters.53-54

The XRD peaks of Nb-TiO2

shifted to the low 2θ angles gradually when the amount of Nb increased, primarily

because of relative large radius of Nb.

79

Chapter 4

Figure 4.5. XRD patterns of Nb-TiO2 fibers with different amount of Nb. XRD patterns

of anatase TiO2 are shown by the black lines as a reference based on the JCPDS database

(No. 84-1286).

Figure 4.6 shows the dependence of area-specific ORR activities (is) of Pt@Nb-

TiO2 catalyst membranes on the amount of Nb. The area-specific activity changed from

0.0084 mA/cm2

Pt for Pt@TiO2, to 0.086 mA/cm2

Pt for Pt@Nb-TiO2 with 5 at% of Nb,

and to 0.16 mA/cm2

Pt for Pt@Nb-TiO2 with 10 at% of Nb at 0.9 V (versus RHE). The

catalyst membrane of Pt@Nb-TiO2 with 10 at% of Nb showed a ~20-fold enhancement

in ORR activity over the Pt@TiO2 membrane. Increasing the Nb amount further to 25%

did not result in further increase in ORR activity. Since there was no major difference in

size and shape of the Pt NPs, the change in catalytic activity could largely attributed to

80

Chapter 4

the Nb-TiO2 fiber membranes. It is known that the conductivity of Nb-doped TiO2

increases with the relative amount of Nb in thin film55

and powders.56-57

Wang et al.

reported that the conductivity of Nb-doped titania powders changed from 4.9 × 10-8

S·cm-

1, to

9.6 × 10

-7 S·cm

-1, to 7.9 × 10

-5 S·cm

-1 and to 1.2 × 10

-3 S·cm

-1 when the amount of

Nb increased from 0 at%, to 5 at%, to 10 at% and to 20 at%.57

Thus, the area-specific

ORR activities of Pt@Nb-TiO2 catalyst membrane increased when the percentage of Nb

increased from 0 at% to 10 at% due to improvement of conductivity of the ceramic

support. However, when the Nb-doped amount increased to 25 at%, the morphology of

support was straight fiber while others were beaded fiber (Figure 4.3). According to

Section 4.3.2, the area-specific activity of Pt@Nb-TiO2 catalysts would be higher if the

morphology of support was beaded fiber. This kind of morphology could be obtained if

the relative humidity of environment was maintained higher than 40% when Nb-doped

percentage was between 0 at% and 10 at%. If the Nb amount increased to 25 at%, only

straight fibers were obtained. Due to the differences of morphology, the activity of Pt@

Pt@Nb-TiO2 with 25 at% of Nb decreased.

81

Chapter 4


catalyst membrane at 0.9 V (versus RHE) on Nb-doped percentage in the ceramic fibers.

4.3.4 Effect of ALD Cycles and Post-treatment on Electrocatalytic Activity

ALD is typically used to deposit thin films of oxides on flat substrate through a

self-limiting process. When ALD was used to grow Pt on substrates, however, discrete Pt

NPs formed and dispersed uniformly over the supports (Figure 4.7). As the number of

ALD cycles increased, the size of Pt NPs grew up and the distance between isolated Pt

NPs decreased (Figure 4.7A-C) until the Pt nanocrystals became to connect with each

other (Figure 4.7D). Normally the reactivity difference between substrate and Pt active

sites led to a preference for Pt NPs to grow up rather than to nucleate new particles on the

substrate. But sometimes, new particles also nucleated with the growth of large Pt NPs

showing obvious small particles (Figure 4.7C). The average size of Pt NPs on Nb-TiO2

82

Chapter 4

with 10 at% of Nb was 4.5 nm after the growth of 10 ALD cycles, 9.2 nm after 20 cycles

and 14 nm after 40 cycles at the post-treatment temperature of 200 °C. If the number of

ALD cycles increased to 80, Pt particles became to connect with each other.

As the ALD process of Pt ended with an oxygen pulse, the outer layer of Pt NPs

was covered with PtO (Scheme 4.1B).52

Thus the catalyst membranes of Pt@Nb-TiO2

were post-treated at 200 °C in 5% H2 w/ Ar atmosphere to reduce PtO to Pt. This

procedure did not affect the size and morphology of Pt NPs. To check the effect of

temperature on ORR activity, the membranes were treated at 500 °C in 5% H2 w/ Ar. At

this temperature, the size and morphology of Pt NPs changed. Pt NPs would migrate and

coalesce on the surface of support at high temperature according to Smoluchowski

model; and atoms from small Pt NPs can evaporate and condense on larger ones in a

process similar to the Ostwald ripening of dispersed precipitates in solution.58-59

So Pt

NPs grew up when post-treated at 500 °C.60

The sizes of Pt NPs increased to 7.5 nm for

sample made after 10 ALD cycles, 12.6 nm after 20 cycles, and 18.7 nm after 40 cycles

(Figure 4.7 E-G). The Pt particles also became connected if the number of ALD cycle

increased to 80 (Figure 4.7H). In addition, the surface of Pt NPs was not smooth; edges

and faces were noticed obviously because of the crystallite growth of Pt NPs at high

temperature (Figure 4.7G and H).

83

Chapter 4

Figure 4.7. SEM images of Pt@Nb-TiO2 catalysts with 10 at% of Nb obtained by

different numbers of ALD cycle and different temperature of post-treatment in 5% H2 w/

Ar atmosphere. (A-D) were treated at 200 °C. (E-H) were treated at 500 °C. The numbers

of ALD cycle were as following: (A, E) 10, (B, F) 20, (C, G) 40, and (D, H) 80.

The area-specific ORR activities of catalyst membrane of Pt@Nb-TiO2 with 10

at% of Nb increased with the number of ALD cycle (Figure 4.8). Weight percentage of Pt

in the catalyst increased from 49% for catalyst membrane with 10 ALD cycles, to 65%

with 20 cycles, to 80% with 40 cycles, and to 87% with 80 cycles, based on the energy

dispersive X-ray analysis. The increase in weight percentage of Pt NPs could also help to

improve the conductivity of catalyst membrane.

84

Chapter 4


with 10 at% of Nb at 0.9 V (versus RHE) on the number of ALD cycle for making Pt NPs,

and the temperature of post-treatment in 5% H2 w/ Ar atmosphere.

Comparing area-specific activities of the Pt@Nb-TiO2 catalyst membranes with

the Pt NPs deposited by the same number of ALD cycle, the one post-treated at 500 °C

had higher activities. One reason was the increase of crystallinity of Pt NPs, which was

improved after the post-treatment at high temperature. In addition, hydrogen treatment at

high temperature could result in hydrogen doping into Nb-TiO2 support.38

The n-type

TiO2 formed by introducing hydrogen interstitial or substitution sites.61

Thus the

conductivity of the support was further increased. As a result, the area-specific activity of

catalyst membrane treated at 500 °C in H2 atmosphere was higher than those treated at

200 °C.

85

Chapter 4

4.3.5 Accelerated-stability Test

Accelerated-stability test was conducted to examine the electrochemical stability

of the Pt@Nb-TiO2 catalyst with 10 at% of Nb. The sample was pretreated by 40 cycles

of cyclic voltammetry (CV) measurement between 0.05 and 1.0 V (versus RHE) before

the test to active the catalyst. Subsequently, the first (1st) cycle of ORR and CV

measurements were tested. For the stability test, the potential was cycled 30000 times

between 0.6 and 1.0 V (versus RHE), while the ORR and CV measurements were

performed every 10000 cycles (Figure 4.9). Table 4.2 summarizes the electrochemically

active surface area (ECSA), kinetic-current density (ik) and area-specific activity (is) at

0.9 V (versus RHE). The ORR polarization curves barely changed after the 30000-cycle

accelerated-stability test, indicating that the catalyst membranes were very stable. Kinetic

current density decreased only within 10% after 30000 cycle accelerated-stability test

although area-specific activity decreased 25% (Table 4.2). The big differences between

kinetic current density and area-specific activity were due to the problem of ECSA

measurement.

86

Chapter 4

Figure 4.9. Accelerated-stability test of Pt@Nb-TiO2 catalyst with 10 at% Nb: (A) CV

curves and (B) ORR polarization curves measured at 1600 rpm.

87

Chapter 4

Table 4.2. Electrochemically active surface area (ECSA), kinetic current density (ik) and

area-specific current density (is) at 0.9 V (versus RHE) of Pt@Nb-TiO2 catalyst

membranes with 10 at% of Nb after a given number of potential cycles during

accelerated-stability test; relative percentages are given in parentheses.

# of Cycle ECA (cm2) ik (mA/cm

2) is (mA/cm

2Pt)

1st 14.74 (100%) 15.87 (100%) 0.21 (100%)

10000th

15.80 (107%) 16.25 (102%) 0.20 (96%)

20000th

16.61 (113%) 15.13 (95%) 0.18 (85%)

30000th

17.44 (118%) 14.15 (89%) 0.16 (75%)

The total ECSA increased slightly after the stability study. This result is

uncommon, because the ECSA value of the catalysts typically decreased after the

accelerated-stability test because of the corrosion of catalyst support, agglomeration and

sintering of Pt particles, and the detachment of Pt particles from the supports.11-12, 14

It

indicated that the catalyst membranes were very stable. The increase was attributed to

activation of the tiny holes in the fibers, which might only allow small H2 molecules to

get in and adsorb on the surface but not O2. So these types of H2 adsorbed sites could not

contribute the ORR. TEM micrographs of Pt@Nb-TiO2 catalyst with 10 at% of Nb before

and after 30000-cycle accelerated-stability test shows that the catalysts were kept largely

intact and the size of the Pt NPs did not have obvious change (Figure 4.10).

88

Chapter 4

Figure 4.10. TEM micrographs of Pt@Nb-TiO2 catalyst with 10 at% of Nb (A) before

and (B) after the accelerated-stability test.

89

Chapter 4

4.4 Conclusion

A facile method to fabricate niobium-doped titania supported platinum (Pt@Nb-

TiO2) catalyst membrane for oxygen-reduction reaction in polymer electrolyte membrane

fuel cell was developed by a new two-step approach, electrospinning and atomic layer

deposition. The process started with making niobium-doped titania fiber membrane by

electrospinning, followed by the deposition of Pt NPs using atomic layer deposition

technique. The area-specific activity of the catalyst membrane with support morphology

of beaded fiber was higher than the one with morphology of straight fiber. The area-

specific ORR activity of Pt@Nb-TiO2 catalyst membrane was increased by about 20

folds over the Pt@TiO2 fiber membrane, if 10 at% of Nb was incorporated into the

ceramic fibers. The area-specific activity also increased with the number of ALD cycles.

The ORR activity further increased to 0.28 mA/cm2

Pt at 0.9 V after post-treatment of the

catalyst membrane at high temperature in H2–containing atmosphere, due to conductivity

improvement of Nb-TiO2 fiber membrane and better crystalinity of Pt NPs. The results of

accelerated-stability test showed that the Pt@Nb-TiO2 catalyst membrane was highly

stable and lost only 10% of its initial activity after 30000 potential cycles (0.6 to 1.0 V vs.

RHE) under a strong acidic condition. This method has the advantages of simplicity in

making uniform ORR catalysts on new types of oxide supports with tunable conductivity

and without the need for surface organic ligand through a controllable two-step approach.

90

Chapter 4

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Electrospinning. J. Am. Ceram. Soc. 2010, 93, 4096-4102.

36. Liu, J.;Yang, H. T.;Tan, W. W.;Zhou, X. W.;Lin, Y. A. Photovoltaic performance

improvement of dye-sensitized solar cells based on tantalum-doped TiO2 thin films.

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37. Niishiro, R.;Kato, H.;Kudo, A. Nickel and either tantalum or niobium-codoped

TiO2 and SrTiO3 photocatalysts with visible-light response for H-2 or O-2 evolution

from aqueous solutions. Phys. Chem. Chem. Phys. 2005, 7, 2241-2245.

38. Sheppard, L. R.;Bak, T.;Nowotny, J.;Nowotny, M. K. Titanium dioxide for solar-

hydrogen V. Metallic-type conduction of Nb-doped TiO2. Int. J. Hydrog. Energy 2007,

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39. Arbiol, J.;Cerda, J.;Dezanneau, G.;Cirera, A.;Peiro, F.;Cornet, A.;Morante, J. R.

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40. Bernasik, A.;Radecka, M.;Rekas, M.;Sloma, M. ELECTRICAL-PROPERTIES

OF CR-DOPED AND NB-DOPED TIO2 THIN-FILMS. Appl. Surf. Sci. 1993, 65-6,

240-245.

41. Paredis, K.;Ono, L. K.;Mostafa, S.;Li, L.;Zhang, Z. F.;Yang, J. C.;Barrio,

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42. Feng, Z. X.;Christensen, S. T.;Elam, J. W.;Lee, B.;Hersam, M. C.;Bedzyk, M. J.

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45. de Miguel, S. R.;Scelza, O. A.;Roman-Martinez, M. C.;de Lecea, C. S.

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46. Suh, D. J.;Park, T. J.;Ihm, S. K. EFFECT OF SURFACE OXYGEN GROUPS

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pre-treatment on the properties and performance for nitrobenzene hydrogenation of Pt/C

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Doped Anatase TiO2 by First-Principles Calculations. Rare Metal Mat. Eng. 2011, 40,

475-477.

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S.;Shigesato, Y. Thermophysical Properties of Transparent Conductive Nb-Doped TiO2

Films. Jpn. J. Appl. Phys. 2012, 51.

56. Jung, S. Y.;Ha, T. J.;Seo, W. S.;Lim, Y. S.;Shin, S.;Cho, H. H.;Park, H. H.

Thermoelectric Properties of Nb-Doped Ordered Mesoporous TiO2. J. Electron. Mater.

2011, 40, 652-656.

57. Nguyen, S. T.;Yang, Y. H.;Wang, X. Ethanol electro-oxidation activity of Nb-

doped-TiO2 supported PdAg catalysts in alkaline media. Appl. Catal. B-Environ. 2012,

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PEM fuel cell catalysts for oxygen reduction reaction. J. Power Sources 2007, 173, 891-

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60. Han, K. S.;Moon, Y. S.;Han, O. H.;Hwang, K. J.;Kim, I.;Kim, H. Heat treatment

and potential cycling effects on surface morphology, particle size, and catalytic activity

of Pt/C catalysts studied by C-13 NMR, TEM, XRD and CV. Electrochem. Commun.

2007, 9, 317-324.

61. Pan, H.;Zhang, Y. W.;Shenoy, V. B.;Gao, H. J. Effects of H-, N-, and (H, N)-

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12231.

95

Chapter 5

Chapter 5 Synthesis of Rutile TiO2 by Electrospinning and Its

Supported Pt Electrocatalysts for the Oxygen Reduction

Reaction

5.1 Introduction

Titanium dioxide (TiO2) is a good candidate as catalyst support because of its

large surface area, high mechanical resistance and excellent stability in acidic and

oxidative environments.1-2

However, the low conductivity of TiO2, which is only eight

orders of magnitude lower than that of carbon black (Vulcan XC-72), is a great deficit to

its application. Normally, there exist two crystal structures of TiO2; one is the anatase

phase, and the other is the rutile phase. At room temperature, the rutile TiO2 has a band

gap of 3.0 eV while the anatase has a band gap of 3.2 eV. Due to this difference in their

band gaps, the conductivity of the rutile TiO2 is higher than that of the anatase phase.3-4

It

was found that the conductivity of the rutile phase (610 µS·cm-1

) was almost three orders

of magnitude higher than that of the anatase phase (0.12 µS·cm-1

).5 The highest reported

conductivity of the rutile phase was 1.11 S·cm-1

by Brank et al., which was comparable

with carbon black.6

TiO2 nanomaterials can be prepared by many techniques including sol-gel

process7-8

, pyrolysis9, reactive sputtering

10-11, electron beam evaporation

12-13, plasma-

enhanced chemical vapor deposition14-16

, atomic-layer deposition17-18

, molecular beam

epitaxy19-21

and hydrothermal technique22

. All these methods only generated titanium

96

Chapter 5

dioxide powders or films. In contrast, electrospinning as a novel and direct technique is

able to make fibers with size ranging from nanometer to micrometer. TiO2 nanofibers

with high surface area could be obtained by electrospinning.23-27

In this work, rutile TiO2 fibers were successfully prepared by electrospinning and

pyrolysis. The morphology and crystal structure were discussed. Pt nanoparticles (NPs)

were deposited on these rutile TiO2 fibers by atomic layer deposition (ALD) technique.

The electrocatalytic activity of rutile-titania supported platinum (Pt@r-TiO2) for oxygen-

reduction reaction was studied.


Materials. Polyvinyl acetate (PVAc, Mw 500,000), titanium (IV) isopropoxide

(Ti(OiPr)4, 99.999%) and niobium ethoxide (Nb(OEt)5, 99.95%) were purchased from

Sigma Aldrich. N,N-dimethyl formamide (DMF, anhydrous) was obtained from

Honeywell Burdick and Jackson. Acetic acid (glacial, 99.7%) was from Alfa Aesar.

Nafion 117 solution was from Fluka. All chemicals were used as received without further

purification.

Synthesis of Rutile TiO2 Fibers. Typically, 0.123 g PVAc was dissolved in 1 mL

DMF in glove box.28

After complete dissolution, 0.475 g Ti(OiPr)4 and 0.119 g acetic

acid were added into the solution in glove box. The resulting solution was mixed well and

electrospun at an accelerating voltage of 20 kV and at a flow rate of 1 mL/h. The

membrane of polymeric fibers was collected on a grounded aluminum foil placed 15 cm

below the spinneret. The membrane of polymeric fibers was then peeled off from foil

97

Chapter 5

using tweezers and left in the air for ~10 h to let the hydrolysis reaction go to completion.

Then the membrane was sintered in the air at 900 °C for 24 h to remove PVAc and allow

nucleation and growth of titania (TiO2) particles in the fiber structure. Titania doped with

10 at% niobium (Nb) fibers (Nb-TiO2) were fabricated with similar method as TiO2. In

particular, the mixture of Ti(OiPr)4 and Nb(OEt)5 was used instead of solo Ti(O

iPr)4. The

atomic ratio of Ti(OiPr)4 and Nb(OEt)5 was 9 : 1 and the total amount of Ti and Nb was

kept at 1.67 mmol.

Synthesis of Pt@r-Nb-TiO2 Catalysts. Atomic layer deposition (ALD) is

utilized to deposit Pt NPs onto the membrane of rutile Nb-TiO2 (r-Nb-TiO2) fibers. The

membrane of r-Nb-TiO2 fibers was fixed in a chuck and hung in the air inside the

chamber of ALD system (Savannah 100 & 200, Cambridge Nano Tech Inc.) to ensure

both side of the membrane to be exposed to the reactants. The chamber was kept at

270 °C. Platinum precursor, Trimethyl(methylcyclopentadienyl) platinum(IV)

((MeCp)Pt(CH3)3), was kept at 70 °C and gas lines were held at 150 °C to avoid

precursor condensation. High-purity O2 was used as the co-reactant, and high-purity N2

was used as both the carrier and purge gas. The flow rate of N2 was maintained at 5 sccm.

A typical ALD cycle was as follows: 2 s of Pt precursor pulse, 5 s of N2 purge, 2 s of O2

pulse and 28 s of N2 purge. Pt NPs were deposited by 40 ALD cycles. Before

electrochemical measurement, catalyst membrane of Pt loaded r-Nb-TiO2 (Pt@r-Nb-

TiO2) was treated at 200 °C in 5% H2 w/ Ar atmosphere for 2 hr.

Electrochemical Measurement. A three-electrode cell was used to perform the

electrochemical measurements. The working electrode was a glassy-carbon rotating-disk

98

Chapter 5

electrode (RDE; area: 0.196 cm2). Pt foil (1 cm × 1 cm) was used as the counter electrode.

A reversible hydrogen electrode (RHE) was used as the reference electrode, which was

placed in a separate compartment. The electrolyte for all of the measurements was 0.1 M

perchloric acid, which was diluted with de-ionized water from a 70% doubly distilled

stock solution (GFS Chemicals, USA). To prepare the working electrode, the Pt@Nb-

TiO2 catalyst was first dispersed in ethanol and the mixture was sonicated for 30 min. 5

µL of suspension was added onto RDE by a pipette and dried under air. It was repeated

until the surface of RDE was covered by a thin-uniform layer of the catalysts. Then 10

µL of 0.0125% (wt%) Nafion-ethanol solution was dropped on the surface of the thin

film to prevent the catalysts from detaching by the flow of testing solution. The

measurements of the total electrochemically active surface area (ECSA) were performed

by integrating the hydrogen adsorption from the cyclic voltammetry (CV) data, which

was recorded at room temperature in argon saturated solution of 0.1 M HClO4. The

potential scan rate was 50 mVs-1

for the CV measurements. The ORR measurements

were conducted at a rotation rate of 1600 rpm in 0.1 M HClO4, which was purged with

oxygen for 30 min prior to and during the measurements. The scan rate for the ORR

measurements was 10 mVs-1

in the positive direction. The area-specific activity was

obtained by normalizing the kinetic-current density against the ECSA values, and the

kinetic-current density was obtained from the Koutecky-Levich plot.

Characterization. The morphology of the fibers and Pt@r-Nb-TiO2 catalyst

membrane were studied by field emission scanning electron microscope (FE-SEM, Zeiss-

Leo DSM982) and field emission transmission electron microscopy (FE-TEM, FEI

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Chapter 5

TECNAI F-20). Average diameter was measured by Image J (National Institutes of

Health, USA). Powder X-ray diffraction (PXRD) patterns were recorded on a Philips

MPD diffractometer with a Cu Kα X-ray source (λ = 1.5405 Å).


5.3.1 Synthesis and Characterization of r-TiO2 Fibers

Figure 5.1 shows the SEM images of as-spun preceramic fibers and TiO2 fibers

after pyrolysis and sintering. The morphology of preceramic fibers was straight fiber with

a diameter of 263 nm. After pyrolysis and sintering at 900 °C, the morphology of TiO2

changed to chains which consisted of pearl-like grains with a smooth grain surface. The

average diameter of the fibers was 191 nm. The size of the grains was not uniform,

ranging from 157 nm to 609 nm.

100

Chapter 5

Figure 5.1. SEM images of (A) as-spun preceramic fibers and (B) TiO2 fibers after

hydrolysis and sintering.

TEM micrograph shows that the chains of TiO2 fibers sintered at 900 °C were

consisted of grains which linked together loosely (Figure 5.2). Most grains only contacted

with two other grains. In other words, the end of one grain connected with the head of the

second grain; and the end of the second grain connected with the head of the third one,

and so on. Some adjacent grains just had edges contacted, and some had a small area

overlapped together. As a result, the chains of TiO2 were very fragile. Compared with

Figure 4.4, TiO2 fibers sintered at 500 °C were consisted with small crystal domains

without preferred orientation. It proved that it was polycrystalline structure. However,

TiO2 fibers sintered at 900 °C were consisted of directive grains, indicating that its

crystallinity was greatly increased. The morphology transformation was related to a

dramatic change in crystalline structure. When titania fibers were calcined at 900 °C, the

crystallite structure of titania changed into rutile phase. During crystallization, the crystal

domains aggregated together, which favored the round shape to minimize the surface

area.29-31

101

Chapter 5

Figure 5.2. TEM micrograph of TiO2 fibers sintered at 900 °C.

XRD pattern of TiO2 fibers sintered at 900 °C is shown in Figure 5.3. Strong

diffraction peaks were appeared, suggesting the formation of well crystalline phases after

the high temperature sintering. The diffraction peaks at 2θ of 27.4°, 36.1°, 39.2°, 41.2°,

44°, 54.3°, 56.6°, 62.8°, 64°, 69.0° and 69.8° could be indexed well to lattice planes of

rutile TiO2 (JCPDS, No.: 86-0147), suggesting rutile TiO2 was successfully formed.

102

Chapter 5

Figure 5.3. XRD patterns of TiO2 fibers sintered at 900 °C. XRD patterns of bulk rutile

TiO2 are shown by the red lines as a reference based on the JCPDS database (No. 86-

0147).

5.3.2 Synthesis and Characterization of r-Nb-TiO2 Fibers

10 at% of Nb(OEt)5 was mixed with Ti(OiPr)4 in the ceramic precursors to obtain

fibers of Nb-doped titania. Figure 5.4 shows the SEM images of as-spun preceramic

fibers and Nb-TiO2 fibers after pyrolysis and sintering at 900 °C. The morphology of

preceramic fibers was straight fiber with a diameter of 207 nm. After pyrolysis and

sintering, the morphology of Nb-TiO2 was chains consisted of stone-like grains with

sharp edge and smooth surface. The average diameter of the fibers was 183 nm.

103

Chapter 5

Figure 5.4. SEM images of (A) as-spun polymeric fibers and (B) Nb-TiO2 fibers after

hydrolysis and sintering at 900 °C.

TEM micrograph of Nb-TiO2 fibers sintered at 900 °C shows that the chains were

consisted of grains which linked tightly (Figure 5.5). Compared with Figure 5.2, the size

of grains was much smaller, and several grains connected with each other compactly. The

contact area between grains was much larger. As a result, the chains of Nb-TiO2 were

robust.

104

Chapter 5

Figure 5.5. TEM micrograph of Nb-TiO2 fibers sintered at 900 °C.

Figure 5.6 shows XRD pattern of Nb0.1Ti0.9O2 fibers sintered at 900 °C, which

indicated a mixture of several different phases. The main diffraction peaks at 2θ of 27.4°,

36.1°, 39.2°, 41.2°, 44°, 54.3°, 56.6°, 62.8°, 64° 69.0° and 69.8° were indexed to lattice

planes of rutile TiO2 (JCPDS, No. 86-0147), suggesting rutile TiO2 was formed. The

diffraction peaks at 2θ of 25.3°, 37.8°, 48.1°, 54°, 55.1°, 62.8° and 75.1° were consisted

with the lattice planes of anatase TiO2 (JCPDS, No. 84-1286), indicating the existence of

anatase phase. In addition, the weak diffraction peaks at 2θ of 23.9°, 26°, 26.3°, 32.8°,

44.1°, 47.8°, 51.2°, 55.2° and 62.8° could be indexed to the lattice planes of TiNb2O7

(JCPDS, No. 77-1374), showing that a small amount of TiNb2O7 existed. In conclusion,

105

Chapter 5

the crystal structure of Nb-TiO2 was a mixture of three phases: the majority was rutile

phase of TiO2; minority was anatase phase of TiO2 and a little amount of TiNb2O7 phase.

Figure 5.6. XRD patterns of Nb-TiO2 fibers sintered at 900 °C. XRD patterns of rutile

TiO2 are shown by the red lines (No. 86-0147), anatase TiO2 by the blue lines (No. 84-

1286), and TiNb2O7 by the green lines (No. 77-1374) as references based on the JCPDS

database.

As mentioned in Chapter 4, Nb2O5 and TiO2 can form a complete range of solid

solutions NbxTi(1-x)O2+y due to the similar ionic radii of Nb5+

and Ti4+

.28, 32-33

The

solubility limit of Nb into titania is greater than 0.1 for the anatase phase (x > 0.1) and

0.06 for the rutile phase (x = 0.06).34

If the preceramic fibers were calcinated at 500 °C,

only anantase phase of TiO2 formed as shown in Chapter 4. When they were sintered at

900 °C, once the solubility limit of Nb atoms was surpassed, a ternary phase ascribed to

106

Chapter 5

TiNb2O7 showed up. In addition, high concentration of Nb helps to stabilize the anatase

phase, which prevents grain coarsening and phase transformation.34-35

As a result, mixed

phases of rutile and anatase TiO2 and TiNb2O7 formed. In addition, the size of grains was

smaller than the one of pure rutile phase.

5.3.3 ORR Activity of Pt@r-Nb-TiO2 Catalysts

Because r-TiO2 fibers were very fragile, the membrane of r-TiO2 always broke

when Pt nanoparticles were deposited on it by ALD technique. Then Pt NPs were only

deposited onto the surface of r-Nb-TiO2 fibers by 40-cycle of ALD. Figure 5.7 shows

SEM image of Pt@r-Nb-TiO2 catalysts. The size of Pt NPs was 14 nm.

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Chapter 5

Figure 5.7. SEM image of Pt@ r-Nb-TiO2 catalysts.

Electrochemical measurement was conducted to examine the electrocatalytic

property of the Pt@r-Nb-TiO2 catalyst (Figure 5.8). The total electrochemically active

surface area (ECSA) was calculated by integrating the hydrogen adsorption from the

cyclic voltammetry data, which was 8.72 cm2. The area-specific activity (is) at 0.9 V

(versus RHE) was 0.25 ± 0.03 mA/cm2

Pt, which was much higher than the one of

Pt@anatase-Nb-TiO2 catalyst (is = 0.16 ± 0.06 mA/cm2

Pt) characterized in Chapter 4.3.3

due to the higher conductivity of rutile phase.

108

Chapter 5

Figure 5.8. Electrocatalytic property of Pt@r-Nb-TiO2 catalyst: (A) CV curves and (B)

ORR polarization curves measured at 1600 rpm.

109

Chapter 5

5.4 Conclusion

Rutile phase titania were successfully fabricated by electrospinning, hydrolysis

and sintering at 900 °C. The morphology of r-TiO2 was chains consisted of pearl-like

grains with a smooth grain surface. Rutile phase titania doped with 10 at% niobium was

also synthesized under the same process. XRD pattern showed that the crystal structure of

r-Nb-TiO2 was actually a mixed phase of rutile and anatase TiO2 and TiNb2O7, although

rutile phase occupied the major part. The morphology of r-Nb0.1Ti0.9O2 was chains

consisted of stone-like grains with sharp edge and smooth surface. The area-specific

activity of Pt@r-Nb-TiO2 catalyst at 0.9 V (versus RHE) increased to 0.25 ± 0.03

mA/cm2

Pt due to the improvement of the conductivity of the support, compared with the

one of anatase r-Nb-TiO2 support.

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2008, 8, 668-672.



26. Madhugiri, S.;Sun, B.;Smirniotis, P. G.;Ferraris, J. P.;Balkus, K. J. Electrospun

mesoporous titanium dioxide fibers. Microporous Mesoporous Mat. 2004, 69, 77-83.

27. Onozuka, K.;Ding, B.;Tsuge, Y.;Naka, T.;Yamazaki, M.;Sugi, S.;Ohno,

S.;Yoshikawa, M.;Shiratori, S. Electrospinning processed nanofibrous TiO2 membranes

for photovoltaic applications. Nanotechnology 2006, 17, 1026-1031.

28. Archana, P. S.;Jose, R.;Jin, T. M.;Vijila, C.;Yusoff, M. M.;Ramakrishna, S.

Structural and Electrical Properties of Nb-Doped Anatase TiO2 Nanowires by

Electrospinning. J. Am. Ceram. Soc. 2010, 93, 4096-4102.

29. Park, J. Y.;Lee, I. H. Characterization and Morphology of Prepared Titanium

Dioxide Nanofibers by Electrospinning. J. Nanosci. Nanotechnol. 2010, 10, 3402-3405.

30. Son, W. K.;Cho, D.;Park, W. H. Direct electrospinning of ultrafine titania fibres

in the absence of polymer additives and formation of pure anatase titania fibres at low

temperature. Nanotechnology 2006, 17, 439-443.

31. Zhao, B. T.;Cai, R.;Jiang, S. M.;Sha, Y. J.;Shao, Z. P. Highly flexible self-

standing film electrode composed of mesoporous rutile TiO2/C nanofibers for lithium-ion

batteries. Electrochim. Acta 2012, 85, 636-643.

32. Arbiol, J.;Cerda, J.;Dezanneau, G.;Cirera, A.;Peiro, F.;Cornet, A.;Morante, J. R.

Effects of Nb doping on the TiO2 anatase-to-rutile phase transition. J. Appl. Phys. 2002,

92, 853-861.

33. Bernasik, A.;Radecka, M.;Rekas, M.;Sloma, M. ELECTRICAL-PROPERTIES

OF CR-DOPED AND NB-DOPED TIO2 THIN-FILMS. Appl. Surf. Sci. 1993, 65-6,

240-245.

34. Ruiz, A. M.;Dezanneau, G.;Arbiol, J.;Cornet, A.;Morante, J. R. Insights into the

structural and chemical modifications of Nb additive on TiO2 nanoparticles. Chem. Mat.

2004, 16, 862-871.

35. Ahmad, A.;Buzby, S.;Ni, C.;Shah, S. I. Effect of Nb and Sc doping on the phase

transformation of sol-gel processed TiO2 nanoparticles. J. Nanosci. Nanotechnol. 2008, 8,

2410-2418.

112

Chapter 6

Chapter 6 Conclusions and Future Research

6.1 Conclusions

The objective of this dissertation is to fabricate polymeric and ceramic fibers by

electrospinning technique, to control the morphology of fibers by adjusting the

parameters during electrospinning process, especially the humidity, and to explore the

application of the electrospun polymeric and ceramic fibers.

Chapter 2 is focused on preparation of polymeric fibers, poly(vinyl pyrrolidone)

(PVP) and understanding the influence of the relative humidity on the morphology, shape

of cross-section and the ultimate elongation of the electrospun fibers. In this chapter, PVP

fibers were successfully fabricated by a magnetic field-assisted electrospinning method;

especially helical peanut-shaped PVP ribbons were obtained at a particular condition. The

morphology of the electrospun fibers was significantly influenced by the relative

humidity. When the relative humidity of the environment decreased from 32% to 16%,

the morphology of the electrospun PVP fibers changed from straight fiber to helical

ribbon, and the percentage of helical ribbons increased to 99%. At the same time, the

cross-section of electrospun fibers changed from circle, to ellipse, and to peanut shape. In

addition, the ultimate elongation of electrospun PVP fibers was also affected by the

relative humidity. It was 33% for straight round fibers made at a relative humidity of 32%

and 43% for helical ribbons made at a relative humidity of 16%. The formation

mechanism of helical ribbons was discussed. It was explained based on the synergistic

interaction of bending instability of electrospun jet, and the rigidity of the polymer

113

Chapter 6

materials during the MFAE process, involving the mass decreasing rate and the diffusion

rate of the solvents. In addition, the effect of solvent property and concentration on the

morphology of electrospun PVP fibers were also discussed in this chapter.

Chapter 3-5 is concentrated on the fabrication of ceramic fibers by

electrospinning technique. Normally, this process is divided into two steps. At first,

preceramic fibers are electrospun from ceramic precursor with the help of guiding

polymer. Secondly, the preceramic fibers are transformed into ceramic fibers by the

pyrolysis process at high temperature.

In Chapter 3, silicon carbide (SiC) fibers were synthesized by electrospinning

from preceramic polymer without the help of guiding polymer, UV cure and pyrolysis

processes. The morphology and diameter of the electrospun preceramic fibers could be

controlled by varying mixtures of solvents. The importance of UV cure on morphology

preservation of SiC fibers was discussed. UV light could break double bonds of RD

material and generate free radicals. The free radicals attacked CH2 or Si-H group, leading

to the crosslink reaction and stability of preceramic fibers. As a result, the morphology

was preserved when transferring preceramic fiber into SiC fibers during pyrolysis process

at high temperature.

Chapter 4 and 5 describe a facile method to fabricate niobium-doped titania

supported platinum (Pt@Nb-TiO2) catalyst membrane by a new two-step approach,

electrospinning and atomic layer deposition, and explore the application of Pt@Nb-TiO2

catalyst in oxygen reducing reaction (ORR) for polymer electrolyte membrane fuel cell.

Chapter 4 is focus on electrospinning of anatase Nb-TiO2 fiber support and discussing the

114

Chapter 6

effect of fiber morphology, doping amount of niobium, number of ALD cycle and post-

treatment temperature on the catalyst activity of Pt@Nb-TiO2 membrane. The area-

specific activities of the catalysts membrane of Pt@Nb-TiO2 increased from 0.0084

mA/cm2

Pt, to 0.086 mA/cm2

Pt and to 0.16 mA/cm2

Pt at 0.9 V (versus RHE) when the

niobium atomic percentage increased from 0%, to 5% and to 10% due to the

improvement of conductivity. The area-specific activity also increased with the number

of atomic layer deposition cycles due to the increase of Pt weight percentage in the

catalysts. The activity could further increased to 0.28 mA/cm2

Pt at 0.9 V (versus RHE) by

post-treatment of the catalyst membrane at high temperature in H2 atmosphere, which

resulted in conductivity improvement of Nb-TiO2 fiber membrane and significant

crystallite growth of Pt. The 30000-cycle accelerated-stability test on the Pt@Nb-TiO2

catalyst membrane showed that it had high stability.

Chapter 5 is focused on the formation of rutile phase Nb-TiO2 (r-Nb-TiO2) fibers

by electrospinning. Rutile phase of titania (r-TiO2) fibers were successfully fabricated by

electrospinning, hydrolysis and sintering at 900 °C. The morphology of r-TiO2 was

chains consisted of pearl-like grains with a smooth grain surface. Rutile phase titania

doped with 10 at% niobium (r-Nb-TiO2) fibers was also synthesized under the same

process. But the crystal structure was a mixed phase of rutile and anatase TiO2 and

TiNb2O7, while rutile phase TiO2 occupied the major part. The morphology of r-Nb-TiO2

was chains consisted of stone-like grains with sharp edge and smooth surface. The area-

specific activity of Pt@Nb-TiO2 catalyst at 0.9 V (versus RHE) increased to 0.25 ± 0.03

115

Chapter 6

mA/cm2

Pt due to the improvement of the conductivity of the support, compared with the

one of anatase Nb-TiO2 support.

6.2 Future Research

6.2.1 Future Research of Helical Ribbons

Controllable morphology of electrospun fibers has been applied to guide the

growth of cells.1-5

As mentioned in Chapter 2, helical PVP ribbons could be obtained by

controlling the relative humidity of environment. This technique described in this work is

potentially useful for making biomaterials in tissue engineering. In addition, because the

ultimate elongation of helical ribbons is larger, this technique also offers a way to control

the mechanical property of electrospun fiber scaffolds.

6.2.2 Future Research of Electrospun Nb-TiO2 Fibers

(1) Cell-Polarization Studies

Chapter 4 and 5 mentioned the catalyst membranes of Pt@Nb-TiO2 can be

applied in polymer electrolyte membrane fuel cell (PEMFC). Normally, membrane

electrode assembly (MEA) in PEMFC is composed of a backing layer, a gas diffusion

layer and a catalyst layer.6-8

Our catalyst membrane could be used as the catalyst layer

directly to avoid complicated process of making catalyst inks and then spaying or

painting them onto a gas diffusion layer, because the catalyst layer is already in a

membrane form. But in these chapters, we only tested the oxygen-reduction reaction

116

Chapter 6

(ORR) activity of the catalyst membranes. It is just half-cell characterization. In the

future, full-cell characterization will be studied.

(2) Photoluminescence Studies

Reports show that rutile and anatase TiO2 have very different photoluminescence

spectrum.9-10

We have fabricated many different kinds of Nb-TiO2 fibers; for example,

Nb-TiO2 with different morphologies, niobium contents and crystal structures (pure

anatase phase, pure rutile phase and a mixed phase of rutile, anatase TiO2 and TiNb2O7).

The photoluminescence of these fibers will be tested in the future.

(3) Application to Water Gas Shift Reaction

A number of metal catalysts such as Pt,11-13

Au,14-15

Ru,16-17

and Re12, 18

loaded on

oxide and mixed oxide supports have been reported for the application to water gas shift

(WGS) reaction. As mentioned in Chapter 4 and 5, we have developed a facile method to

fabricate niobium-doped titania supported platinum catalyst membrane by a new two-step

approach, electrospinning and atomic layer deposition. These catalyst membranes have

high stability. In the future, we will try to apply it to WGS reaction.

6.3 References



2. Sill, T. J.;von Recum, H. A. Electro spinning: Applications in drug delivery and

tissue engineering. Biomaterials 2008, 29, 1989-2006.






117

Chapter 6

5. Tay, C. Y.;Irvine, S. A.;Boey, F. Y. C.;Tan, L. P.;Venkatraman, S. Micro-/Nano-

engineered Cellular Responses for Soft Tissue Engineering and Biomedical Applications.

Small 2011, 7, 1361-1378.

6. Bauer, A.;Chevallier, L.;Hui, R.;Cavaliere, S.;Zhang, J. J.;Jones, D.;Roziere, J.

Synthesis and characterization of Nb-TiO2 mesoporous microsphere and nanofiber

supported Pt catalysts for high temperature PEM fuel cells. Electrochim. Acta 2012, 77,

1-7.

7. Selvarani, G.;Maheswari, S.;Sridhar, P.;Pitchumani, S.;Shukla, A. K. A PEFC

With Pt-TiO2/C as Oxygen-Reduction Catalyst. J. Fuel Cell Sci. Technol. 2011, 8.

8. Thepkaew, J.;Therdthianwong, A.;Therdthianwong, S. Key parameters of active

layers affecting proton exchange membrane (PEM) fuel cell performance. Energy 2008,

33, 1794-1800.

9. Jia, C. W.;Zhao, J. G.;Duan, H. G.;Xie, E. Q. Visible photoluminescence from

Er3+-doped TiO2 nanofibres by electrospinning. Mater. Lett. 2007, 61, 4389-4392.

10. Zhao, J. G.;Jia, C. W.;Duan, H. G.;Li, H.;Xie, E. Q. Structural properties and

photoluminescence of TiO2 nanofibers were fabricated by electrospinning. J. Alloy.

Compd. 2008, 461, 447-450.

11. Iida, H.;Igarashi, A. Structure characterization of Pt-Re/TiO2 (rutile) and Pt-

Re/ZrO2 catalysts for water gas shift reaction at low-temperature. Appl. Catal. A-Gen.

2006, 303, 192-198.

12. Iida, H.;Igarashi, A. Difference in the reaction behavior between Pt-Re/TiO2

(Rutile) and Pt-Re/ZrO2 catalysts for low-temperature water gas shift reactions. Appl.

Catal. A-Gen. 2006, 303, 48-55.

13. Subramanian, V.;Potdar, H. S.;Jeong, D. W.;Shim, J. O.;Jang, W. J.;Roh, H.

S.;Jung, U. H.;Yoon, W. L. Synthesis of a Novel Nano-Sized Pt/ZnO Catalyst for Water

Gas Shift Reaction in Medium Temperature Application. Catal. Lett. 2012, 142, 1075-

1081.

14. Daly, H.;Goguet, A.;Hardacre, C.;Meunier, F. C.;Pilasombat, R.;Thompsett, D.

The effect of reaction conditions on the stability of Au/CeZrO4 catalysts in the low-

temperature water-gas shift reaction. J. Catal. 2010, 273, 257-265.

15. Zhao, X. E.;Ma, S. G.;Hrbek, J.;Rodriguez, J. A. Reaction of water with Ce-

Au(111) and CeOx/Au(111) surfaces: Photoemission and STM studies. Surf. Sci. 2007,

601, 2445-2452.

16. Utaka, T.;Okanishi, T.;Takeguchi, T.;Kikuchi, R.;Eguchi, K. Water gas shift

reaction of reformed fuel over supported Ru catalysts. Appl. Catal. A-Gen. 2003, 245,

343-351.

17. Xu, W. Q.;Si, R.;Senanayake, S. D.;Llorca, J.;Idriss, H.;Stacchiola, D.;Hanson, J.

C.;Rodriguez, J. A. In situ studies of CeO2-supported Pt, Ru, and Pt-Ru alloy catalysts

for the water-gas shift reaction: Active phases and reaction intermediates. J. Catal. 2012,

291, 117-126.

18. Sato, Y.;Terada, K.;Hasegawa, S.;Miyao, T.;Naito, S. Mechanistic study of water-

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2005, 296, 80-89.

118

Chapter 6

119

Appendices

Appendices

Table A1. Physical properties of solvents.

Boiling Point

(℃)

Vapor Pressure

@ 20 °C

(mmHg)

Surface Tension

@ 25 °C

(Dynes/cm)

Dielectric

Constant

Ethanol 78 44.6 21.97 25.3

Water 100 17.5 71.99 80.1

Toluene 110-111 22 28.4 2.379

NMP 202 0.29 40.79 32.55

Dichloromethane 39.8-40 353 27.2 8.93

Citing from CRC Handbook of Chemistry and Physics. In Fluid Properties [Online]; 93rd

Edition; Haynes, W. M., Eds.; CRC Press / Taylor and Francis: Boca Raton, USA, 2013

(accessed January 1, 2013).

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