Palladium Nanoparticles and Bimetal Palladium Layers for Enhanced

Hydrogenation Properties

by

Manika Khanuja Depa比ment of Physics

Submitted

in fulfillment of the requirement of the Doctor of Philosophy to the

INDIAN INSTITUTE OF TECHNOLOGY DELHI NEW DELHI-110016, INDIA

February 2009

I would like to dedicate my thesis to

Sameer, myfamily and my in-laws

Certifficate

We approve that the thesis of M面ka 助如可a entitled "Palladium Nanoparticles

and B血etal Palladium L習ers for Enhanced Hydrogenation Prope市es" is

worthy of consideration for the award of the degree of Doctor of Philosop畑加d is a

record of original and bonaffide research work carried out by her under our

supervision. OEe results contained in it have not been submitted in part or full to any

other university or institute for awards of any degree/diploma.

Date: c ・え・ユ。。へ

Prof. B. R Mehta

OEmn Film Laboratory

Department of P取sics

Indian Institute of Technology Delhi

New Delhi-i 10016

India

Prof. S. M. Shivaprasad

Chemistry&Physics of Materials Unit (CPMU),

Jawaharlal Nehru Centre for Advance Scientiffic

Research (JNCASR), Jakkur-P.O.

Bangalore-560064

India

Acknowledgement This research started in November 2003 and this thesis is the result of ffive years of

work whereby I have been accompanied and supported by many people. It is a

pleasant aspect that I have now the opportunity to express my gratitude for all of

them.

Prof B. R. Mehta, my supervisor. I would like to express my gratitude for

his continuous support. His enthusiastic and integral view on research and his

mission for undertaking 'only high-quality work and not less', has made a deep

impression on me. I owe him lots of gratitude for initiating me into this fleld of

化se町ch.

Prof. S. M. Shiv叩rasad, my supervisor for being supportive and encouraging in

my research work. His valuable inputs always helped me, both in XPS analysis

and paper publication. His wisdom, knowledge and commitment always inspired

and motivated me.

Dr F. E. Kruis, Institute for Nano Structures Technique 但ST), University of

Duisburg-Essen, Duisburg Campus, Germany, for inviting me at Duisburg

University as a Guest Scientist and supporting me to cany out part of my research

work at Germany.

Dr. D. K. Awasthi, Senior Scientist, Inter-University Accelerator Center (IUAC),

New Delhi, India and Mr. P. K. Kulriya for fruitful collaboration in carrying out

experiments related to in-situ X-ray diffraction measurements

CSIR (Council of Scientiffic and Industrial Research) India for their ffinancial

support in the form of "Junior Research Fellowship" and "Senior Research

Fellowship".

'''

. Dr. Deepak Varandani and Dr. I. Aruna for their continuous guidance, support and

motivation during my experimental and thesis work.

. Dr. V. N. Singh for providing support by carrying out TEM&HRTEM

measurements.

. Special thanks to Shubhra Kala for her support in Synthesis of Size selected

nanoparticles.

. Pragya Agar, Dr. Vandana, Mandeep Singh, Himani Shanna, Rupali Nagar and

Mukesh for being supportive d面ng my research work and thesis writing.

・ K. Gopinathan, Kanwal Preet Bhatti, Mandeep Singh, Sangeeta Handuja, and

Cham Dube for lending their support during XRD analysis. Mr. Vinod Khanna for

TEM measurements.

National Physical Laboratory 国PL) for providing me inifiastructure where I have

done XPS studies and Dr. Govind, Dr. Amish Joshi, Dr. Mahesh Kum叫 Dr.

Him面 Sharma, Praveen, Manoj and Jithesh for backing me up in numerous

ways.

Mr. Balraj Khatri, Mr. Nagendra Chaudhary and members ofIDDC, lIT Delhi, for

providing technical help during modiffication ofthe experimental set up.

. My school and college teachers Ms. Poonam Tandon, Ms. U. Chugh and Dr．斑ta

Chandra for providing me their valuable guidance and motivation to pursue higher

studies.

Dr. Sujeet chaudhary for allowing me to use conductivity measurement set up.

Suchitra, Sudesh and K. Gopinathan for helping me carrying out these

measuremenis.

I would like to appreciate support provided 句 my lab-mates Suneet Arora,

Mandeep Singh, Mukesh Kumar, Ajay Kumar Mann, Sanju Rani, Jaswinder Kaur,

Iv

Sudesh, Sangeeta Handuja, Priyanka Gupta, Bhawna Pandey, Ranga Rao and

Sandeep Chokker.

Finally, I need to thanks my partner, Sameer, my family and my in-laws for so

much patience over the last ffive years and making sure that I could focus on my

research.

Manika Khanuja

v

Abstract

In the present study, two approaches (i) nanoparticle route and (ii) bimetal

layer route have been employed to enhance the Pd-H interaction. Pd nanoparticles are

synthesized by two different techniques of inert gas evaporation and gas phase

synthesis. Pd thin flims have been deposited using vacuum evaporation technique. Pd

nanoparticle layers have been prepared by inert gas evaporation technique. Size-

selected, monodisperse Pd nanoparticles having well-deffined size distributions have

been synthesized by using an integrated set up based on gas phase synthesis. OEe

hydrogen sensing properties of Pd nanoparticle layers have been compared with that

of the Pd thin flim. In Pd nanoparticles, a unique pulsed like hydrogen sensing

response has been obtained in which electrical resistance increases due to electronic

effect (EE) sharply followed by a sudden decrease due to geometric eLect (GE), when

the hydrogen is switched on. Electronic effect 面ses due to the hydrogen acting as

scattering centres in Pd lattice that is responsible for increase in resistance on Pd-H

interaction. Geometric effect arises due to the filing of interparticle gaps during

lattice expansion on hydride formation due to which electrical resistance decreases.

Due to well-connected crystallite formation in thin flim samples, decrease in electrical

resistance due to filing of the interparticle gaps is not signifficant. In comparison, Pd

thin flims exhibit a slow and subdued sensing response because of the overlap of the

above two opposing effects and hydrogen induced lattice strain.

OEe hydrogen sensing response is studied at different H2 concentrations and

measurement temperatures, in size-selected and monodisperse Pd nanoparticles. Two

types of sensing behaviours are observed as a function of H2 concentration (He): (i) a

normally observed 'saturated' response and (ii) a 'pulsed' response. At 20OC, pulsed

VI

response is observed at H2 concentration と 2.5% and at H2 concentration 5 2.0%, the

saturated response is observed. OEe geometric effect is absent at H2 concentration 5

2.0% as at such a low H2 concentration negligible or the small change in the

nanoparticle dimensions occurred that is insuffficient for causing signifficant changes

in the interparticle gaps. Thus, occurrence of both the EE and GE is observed to be a

pre-requisite for observing pulsed like sensing response. At 20OC, pulsed response is

observed at 取drogen concentration 但C）と2.5% and at H。5 2.0%, the saturated

response is observed. This shows that at 20OC, HC 5 2%is insuffficient to cause

signifficant change in nanoparticle dimensions required for observing GE. At 80OC,

saturated response is observed at all Hc fflom 0.25 to 5%. This is due to a smaller

amount of hydrogen getting incorporated into Pd due to reduced p珂sical-adsorption.

It was observed that the threshold H2 concentration is a strong function of

measurement temperature. OEus, measurement of the temperature, at which the

saturated response changes to pulsed response, can give important information about

the H2 concentration level. This type of concentration-speciffic sensor can be quite

useful in a variety of applications, like fuel cell, nuclear reactors, and Ni-H batteries

for detecting the H2 concentration level. The present stu町 shows that the hydrogen

sensing response of Pd nanoparticle due to EE is much faster (about 7-8 times) than

that of GE, whereas, the sensitivity of GE is larger (about 5 times for HC=4%) in

comparison to EE. Thus, the concentration-speciffic Pd nanoparticle based H sensor

has the useful features of faster response of initial EE and higher sensitivity of the GE.

In-situ XRD measurements have been carried out on Pd nanoparticle layers

and thin flims to directly observe the changes in lattice structure during hydrogen

loading and reversibility of Pd-H interaction at structural level d面ng hydrogen

VII

deloading. It is observed that change in lattice constant is a strong function of

measurement temperature and hydrogen concentration.

In the second approach of using bimetal layer, properties of 加o elements are

combined in a synergic manner to yield a surface which is more reactive than either of

the two. Cu and Ag thin flims have been deposited onto Pd layers and 取drogenation

properties of Cu (thin ffilm)/Pd (thin ffilm) and Ag (thin ffilm)/Pd (thin ffilm) bimetal

layers have been studied. Stable and reversible hydrogen sensing response is observed

over a number of hydrogen loading and deloading cycles. OEe si師fficant sensing

signal is observed in CulPd bimetal layer at 取drogen concentration up to 0.5%.

Percentage change in resistance on 取drogen loading was observed to decrease till on

annealing up to 300OC. On annealing at 350OC, 400OC and 450OC, percentage change

in resistance was found to increase. To explain the 取drogen sensing response in

terms of changes that occur during annealing, surface sensitive X-ray photoelectron

spectroscopy (XPS) and glancing angle X-ray di缶action (GAXRD) studies have been

done. OEe variation of the Pd to Cu core line intensity m面fests the compositional

changes and its temperature dependence. XPS results show that with increasing

annealing temperature, intermixing occurs between Cu and Pd, and a surface alloy is

formed at the intermediate temperature region of 267OC to 534OC. Pd (3d5 and 3d3n)

core level peaks exhibit two components, one at higher binding energy of 337. 1 eV

corresponding to alloyed component and other at lower binding energy of 335.6 eV

corresponding to metallic component. Formation of new states in the valence band,

that are different fflom the pure Cu and Pd features, conffirms the surface alloy

formation. GAXRD studies also show that d spacing values lie in between those of

v川

pure palladium and copper in case of annealed samples co面rming the surface alloy

formation.

In as-deposited Cu/Pd bimetal layer, two factors are important. First is 'ligand'

effect that arises due to the presence of different kinds of the Pd atoms. Pd-H

interaction decreases in as-deposited Cu/Pd bimetal layer system due to electron flow

from Pd to Cu. Second is ' lattice-mismatch' effect, due to 7. 1 % lattice-mismatch

between Cu and Pd, compressive stress occurs that suppress the catalytic activity of

Pd towards 比． Due to surface alloy formation on annealing, the influence of both the

effects on catalytic interaction of Pd with H2 decreases. On annealing, Cu and Pd

forms common valence band whose catalytic activities are completely different from

those of the pure Pd and Cu. Thus, catalytic interaction of Pd towards hydrogen

increases on annealed Cu/Pd bimetal layer samples.

In Ag/Pd b加eta! layer system, the eLect of converting the metal over layer

fflom thin fllm (TF) to metal nanoparticle 伽P) layers has been investigated. Two

加es of samples have been studied (i) Ag(TF)/Pd(TF) and (ii) Ag(NP)/Pd(TF).OEe

observed hydrogen sensing response is stable and reversible over a number of

取drogen loading and deloading cycles in both bimetallic systems. OEe GAXRD and

xPS studies carried on AgPd bimetal layer sample shows that alloying between Ag

and Pd is suppressed in Ag国P)/Pd(TF) bimetal layers as compared to

Ag(TF)/Pd(TF). On 取drogen loading, resistance change of about 2% and 5% is

observed in 300OC annealed Ag(TF)/Pd(TF) and Ag(NP)/Pd(TF) samples. Along with

low sensitivity, large response time has been observed in Ag(TF)/Pd(TF) bimetal

layers as compared to Ag倒P)/Pd(TF). Hence alloying, suppresses the 珂drogen

sensing response in Ag(TF)/Pd(TF) bimetal layer system. This is due to the d-hand

Ix

centroid positions of Pd and Ag. The d-band centroids of these metals are quite far

apart. On annealing, there is no common valence band formation as occurs in case of

Cu/Pd bimetal layer system. Density of states near the Fermi level also reduces on

alloy formation between Ag and Pd. As a result, interaction of Pd 面th 取drogen gets

reduced on annealing due to alloy formation between Ag and Pd in Ag(TF)/Pd(TF)

bimetal layers. This effect can be reduced by using Ag nanoparticles in place of Ag

thin ffilms.

X

Table of Contents

Certificate II

Acknowledgement Ill Abstract VI Table of Contents XI List of Figures XIV List of Tables XXIII Nomenclature XXIV

Chapter 1.Introduction..............．■．...............................................-..........................................I

1.1 Palladium .............................................................................................................................. i

i.2 Hydrogen .............................................................................................................................. i i .3 Palladium-Hydrogen .................．。． ......................................................................................... 2

1.3.1 Palladium-Hydrogen Interaction ........................................................................... 2

1.3.2 Phase Diagram ...............................,..................................................................... 5

1.3.3 Influence of Hydrogen on the Pd Crystal Structure.............,................................ 7

1.3.4 Hysteresis ............................................................................................................. 8 1.3.5 Hydrogen Sensors .................,.............................................................................. 9

i .3. 5. 1 Optical Hyd rogen Sensors ......................................................,..................... lo

1.3.5.2 Pd-MOS Based Hydrogen Sensors .....................................,........................ lo i.3.5.3 Resistive Hydrogen Sensors.........................................................................10

I .3.5.4 SAW (surface acoustical wave) Hydrogen Sensors ..................................... li

1 .3.5.5 Piezoelectric Based Hydrogen Sensor..........................................................i i 1.3.5.6 Pyroelectric Sensor....................................................................................... 11

I .4 Nanostructured Materials....................................................................................................11 1.5 Low Dimensional Materials.................................................................................................i4

i.5.i Quantum Well................................................................................................... 14 1.5.2 Quantum Wire.....................................................................................................14

1.5.3 Quantum Dot ................................................,..................................................... 14 1.6 Quantum Confinement Effect ............................................................................................. 14

1.7 Enhanced Surface Effect....................................................................................................16 1.8 Synthesis Method ............................................................................................................... 18

1.8.1 Electrochemical Method . .. . . .. . . . .. . . . ... .. . .. ... ... . .. .. ... .. .. . .. .. .. ... .. .. .. .. . ... . . .. .. .. . . .. .. . .. .. .. . 18

1 .8.2 Inert Gas Deposition Technique ......................................................................... 18 1 .8.3 Gas Phase Synthesis ......................................................................................... 18

I .9 Pd Nanoparticles ................................................................................................................ 19

1 . 10 M/Pd (M= Cu, Ag, Nり BimetalLayers...............................................................................20

xl

1.10.1 Cu/Pd Bimetal Layer .........................．。． ............................................................. 21

1.10.2 Ag/Pd Bimetal Layer ......................................................................................... 21 1.10.3 Ni/Pd Bimetal Layer .......................................................................................... 22

1.10.4 Au/Pd Bimetal Layer ................................................................'........................ 22

i . 1 1 Background and Objective of Thesis Work ........................．。． ........................................... 22

1.12 Thesis Plan ....................................................................................................................... 24

Chapter 2. Deposition and Characterization Techniques..．・・・・． .............．・．．・・ー・・・・・・・． .............. 26

2.1 Introduction.........................................................................................................................26

2.2 Inert Gas Deposition Technique ......................................................................................... 26

2.3 Gas Phase Synthesis Technique ....................................................................................... 28 2.4 Substrate Cleaning ............................................................................................................. 34 2.5 Sensing Set up .............................................................,..................................................... 35

2.6 Resistivity Measurement.....................................................................................................36 2.7 Glancing Angle X-ray Diffraction (GAXRD)........................................................................ 37

2.8 X-ray Photoelectron Spectroscopy (XPS)........................'................................................. 40

2.9 Scanning Microscopy..........................................................................................................43

2.9.1 Scanning Tunneling Microscopy (STM)..............................................................43 2.9.2 Atomic Force Microscopy (AFM)........................................................................ 44

2.10 Transmission Electron Microscopy (TEM)........................................................................45 2. 1 1 Thickness Measurement...................................................................................................46

Chapter 3. Palladium Nanoparticle Layers and Thin Films..............．・．・・・ー・．．ー・ーー・． ...．、． ..．…48

3.1 Introduction ..................'...................................................................................................... 48

3.2 Scanning Tunneling Microscopy Studies............................................................................49 3.3 Glancing Angle X- Ray Diffraction Studies ......................................................................... 50

3.4 Hydrogen Sensing Response.............................................................................................51 3.5 Conductivity-Temperature Measurements .. ...... ... .. .... .. .. ..... ..'.'.. .... .................... ....... ....... 60

3.6 Discussion...........................................................................................................................63 3.7 Conclusions . .... ... . .. .. .. .. .. ................. ... .. ......... .... .. .. .. .. ............... .. ....... ...... ..... .. .. ... .. ....... .... ... 64

Chapter4. Monodisperse Palladium Nanoparticles............................................................65

4.1 Introduction ......................................................................................................................... 65

4.2 Transmission Electron Microscopy Studies........................................................................66 4.3 Hydrogen Sensing Response.............................................................................................69

4.3.1 Effect of Hydrogen Concentration ...................................................................... 69

4.3.2 Effect of Nanoparticle Size ................................................................................. 73 4.3.3 Effect of Temperature.........................................................................................74

4.4 Resistivity-Temperature Measurements.............................................................................77

4.5 Conclusions ........................................................................................................................ 81

XII

Chapter 5. StructuraI Properties of Palladium Nanoparticle Layers and Thin Films ....... 83 5.1 Introduction ......................................................................................................................... 83 5.2 Hydrogenation of Pd Thin Film and Nanoparticle Layers...................................................83 5.3 Effect of Temperature ...........,............................................................................................. 86 5.4 Effect of Hydrogen Partial Pressure (PH)............................................................................94 5.5 Conclusions. ....................................................................................................................... 99

Chapter 6. Cu!Pd Bimetal Layers .............．■ー． .............-................................................,...... I 00 6.1 Introduction ................,...................................................................................................... ico 6.2 Atomic Force Microscopy Studies .................................................................................... ico 6.3 Hydrogen Sensing Response........................................,.................................................. I 02 6.4 X-ray Photoelectron Spectroscopy studies.......................................................................I 05 6.5 Glancing Angle X-ray Diffraction Studies ......................................................................... 113 6.6 Discussion.........................................................................................................................115 6.7 Conclusions ...................................................................................................................... 116

Chapter 7. Ag/Pd Bimetal Layers ........．■． ....．■ー． ..-..-.-..-..............-....．■． -.........．、． ................ 117 7.1 Introduction ....................................................................................................................... 117 7.2 Atomic Force Microscopy Studies .................'.................................................................. 118 7.3 Hydrogen Sensing Response...........................................................................................120 7.4 X-ray Photoelectron Spectroscopy Studies......................................................................i 22 7.5 Glancing Angle X-ray Diifiraction Studies ....................................................................'.... I 26 7.6 Discussion.........................................................................................................................129 7.7 Conclusion ...........,............................................................................................................ 130

Chapter 8. Conclusions and Scope for further study .....．・．・・……■． .........．・、’ '．・・・・・・ー・・・．．・・・…… 1 31 8.1 Conclusions of the present research work ....................................................................... I 31 8.2 Scope for further study .................................................................,................................... 135

References.............................................................................................................................136

Listof Publications....................................................................．ーー． .............．ーー． .................. 144

Biodata..................................................................................................................................147

XIII