1

CHAPTER NO. 1

INTRODUCTION

2

1. INTRODUCTION

1.1 Since the first carbon fiber (CF), which was prepared by carbonizing

cotton and bamboo, was used as the filament of a light bulb in 1879 by Thomas

Edison. CNF was first discovered in 1991 by S.Lijima1. Carbon Nanofibers

(CNF’s) are sp2- based linear, non-continuous filaments that are different from

continuous, several micrometer diameter carbon fibers. It is sheets of graphite

rolled into a cylinder. CNF as one of the most important members of carbon

fibers, has been investigated in both fundamental practical and scientific research

applications. CNF composites can be applied in many fields like electrical

devices, electrode materials for batteries and super capacitors and as sensors

depending upon the diameter, length and size. CNF’s have been synthesized

from a variety of materials including pitch2, rayon, polyacrylonitrile3, solid

carbon materials, acetone, hydrocarbon gases, by employing

electrospinning/carbonization, chemical vapor deposition (CVD)4 and

arc/plasma5 techniques.

Recent interests are focusing on renewable feedstock like lignin and cellulose6,

rather than the conventional chemicals from petroleum industry. The

combination of high specific area, flexibility, and high mechanical strength

allow nanofibers to be used in our daily life as well as in fabricating composites

for vehicles and aerospace. They should be distinguished from conventional

carbon fibers and vapor grown carbon fibers in their small diameter.

Conventional carbon fibers and vapor- grown carbon fibers have several

micrometer-sized diameters. Carbon nanofibers could be grown by passing

carbon feedstock over nano-sized metal particles7 at high temperature. Carbon

fibers represent an important class of graphite related materials that are closely

related to carbon nanofibers with regard to structure and properties. The

3

temperature and pressure are necessary to prepare a carbon fiber. CNFs are

mainly prepared by two methods, namely a) Catalytic Thermal chemical vapor

deposition growth8,9 b) Electrospinning10,11 followed by Heat treatment. Two

types of CNF can be prepared by catalytic thermal vapor deposition, namely the

cup-stacked CNF and platelet CNF. The cup-stacked CNF12 are also known as

conical CNF. Electrospinning is another widely used method for the preparation

of CNFs. The CNFs prepared by electrospinning and carbonization was

according to their structure and properties. The CNFs is fabricated by the

electrospinning method, the polymer nanofibers are required. Once the polymer

nanofibers have been successfully prepared, a heat treatment will be applied to

carbonize the polymer nanofibers to form CNFs. The diameter, purity,

crystallinity, and morphology are governed by the parameters of the heat

treatment process, such as atmosphere and temperature.

Herein we synthesized carbon nanofibers form carbon dioxide via a simple

thermos-electrochemical method. Electrolytic conversion of carbon dioxide

dissolved in molten carbonate is carried out.

4

1.2 Carbon dioxide is a colorless and odorless gas vital to life on Earth. It is

composed of a carbon atom covalently double bonded to two oxygen atoms and

concentration of about 0.04 percent by volume in atmosphere. Carbon dioxide is

a potential carbon resource abundant on earth. It is the most notorious

greenhouse gas with a rapidly increasing atmospheric concentration. It is also a

necessary material for the growth of all earth’s plants and for many industrial

processes. In an ideal scenario, the CO2 produced on Earth should be balanced

with what is consumed, so that the level of CO2 remains constant to maintain

environmental stability. Unfortunately, with the intensification of human

industrial activities, this balance has gradually been disrupted, leading to more

CO2 production and making global warming13 a pressing issue.

Therefore, reducing CO2 production and converting CO2 into useful

materials seems to be necessary, indeed critical, for environmental protection,

and various governments worldwide have signaled their concern by increasing

their investment in research to address the CO2 issue. The different proposed

technologies follow one of two major approaches: to capture and geologically

sequestrate CO214 or to convert CO2 into useful low-carbon fuels. Chemical

fixation of CO2 is an attractive technique for utilization of carbon resources, as

well as for the reduction of the atmospheric concentration of CO2 and it is most

stable among carbon based substances under the environmental conditions. CO2

can be electrochemically reduced to useful products under mild conditions .We

synthesize a valuable commodity, CNF’s directly from CO2 in a one -pot

synthesis15. CO2 can contribute to lower greenhouse gases for example by

consuming, rather than emitting CO2 and by providing a carbon composite

material that can be used as an alternative to steel, aluminum, and cement whose

productions are associated with massive CO2 emissions.

5

1.3 We also report the fabrication of an electrochemical detector of

dopamine based on the as synthesized CNF and graphene composite. Detection

of dopamine has great medical relevance16. Dopamine (DA), as one of the

important neurotransmitter, plays a vital role in mammalian central nervous

system. The abnormal levels of DA may cause neurological disorders such as

Schizophrenia and Parkinson’s diseases. Therefore, monitoring concentration of

DA is of great importance. Electrochemical techniques for detection of DA17

have received considerable interest over the past two decades because of their

simplicity, high sensitivity and low cost. In present work, we fabricate

graphene/CNF modified electrode and report its application for dopamine

sensing.

Dopamine

6

CHAPTER NO. 2

MATERIALS AND METHODOLOGY

7

2.0 MATERIALS AND METHODOLOGY

2.1 MATERIALS

Lithium carbonate (Alfa Aesar 99%, mp 730 OC) and Cesium carbonate

(Alfa Aesar 99%, mp 610 OC) were procured and used as electrolyte without

any further purification. Ni mesh and galvanized stainless steel wire (1.1 mm

diameter) were used as anode and cathode respectively. The following

equipments were used in the studies: a) Furnace (1000 OC), b) Aplab L1602 –

Regulated DC supply and c) Zahner IM6- Germany.

2.2 SET UP

Figure 1. Set up for the thermos electrochemical reduction of carbon dioxide to

carbon nanofibers

8

1. Compressed Carbon dioxide Cylinder

The cylinder helps to maintain a continuous flow rate of 1.5 Pascals per

square inch (psi).

2. Aplab L1602 – Regulated DC supply

Figure 2a- Aplab regulated DC supply

3. Alumina crucible and the electrodes

It can tolerate the temperature up to 8000c for molten carbonate.

Figure 2b- Alumina crucible (1000 oC)

9

Anode: Nickel (Ni) mesh as anode

Figure 2c- Ni mesh

Cathode: Galvanized steel as a cathode.

Figure 2d- galvanized stainless steel electrode

10

4. Furnace

Figure 2e

Figure 2f

The furnace maintains the temperature for the carbonate to remain

molten.

11

2.3 PROCESS DESCRIPTION

The setup is designed as shown in Figure 1. Carbon dioxide is passed via a

stainless steel tube of diameter 4 mm, which takes the carbon dioxide supply

from the compressed gas cylinder. The furnace is initially preheated with the

crucible in it at 300 oC to remove all the impurities. Then weighed amounts of

the carbonates (Lithium carbonate and/or Cesium Carbonate) are taken

accordingly. The electrodes are then fixed onto the crucible making sure that

they are intact and then connect to the positive and negative terminals of the

rectifier. Ni mesh is used as the anode and galvanized stainless steel wire,

coiled spirally, is used as the cathode. The carbonate/s are then melted in the

furnace by ramping up the temperature to 730 oC. The reaction initiates in the

molten carbon electrolyte.

During electrolysis, the carbon product accumulates at the cathode but falls off

when the cathode is extracted, cooled, and uncoiled. The product is washed

with either Deionized water (or) 6 m HCL (both yield similar product, but the

later solution accelerates the washing process) and separated from the washing

solution is separated either by paper filtration or centrifugation.

12

Figure 3. Carbon nanofibers deposited on the electrode.

The process was carried out by varying the conditions so as to optimize the

product formation. Process parameters of different trials are as mentioned

below:

13

2.3.1 Trial 1 (Intermittent supply of CO2 and higher initial current density)

Process Parameters

Trial 1

Cathodic (Galvanized steel) surface

area

23.38 cm2

Anodic (Ni mesh) Surface area

4.5 cm2

Lithium carbonate

50 g

Initial current density

15 mA cm-2

Rate of CO2 supplied

3 psi (Intermittent, every 15

seconds)

Table 1a

14

Table 1b

In Trial 1, intermittent (every 15 seconds) supply of Carbon dioxide at 3 psi

was provided. And an initial current density of 15 mA cm-2 was applied for the

first 20 minutes and gradually increasing up to 68 mA cm-2.

S.NO VOLTAGE

(V)

CURRENT

(A)

Time

(min)

1 1.2 0.35 0

2 1.3 0.50 20

3 1.4 0.70 40

4 1.5 1.0 90

5 1.8 1.6 120

15

2.3.2 Trial 2 (Continuous supply of CO2 at a lower flow rate and lower initial

current density)

Process Parameters

Trial 2

Cathodic (Galvanized steel) surface

area

20.22cm2

Anodic (Ni mesh) Surface area

4.5 cm2

Lithium carbonate

50 g

Initial current density

5 mA cm-2

Rate of CO2 supplied

1.5 psi (continuous)

Table 2a

16

S.NO VOLTAGE

(V)

CURRENT

(A)

TIME

(MIN)

1 0.5 0.10 0

2 1.4 0.13 15

3 1.7 0.21 30

4 1.9 0.33 60

5 2.0 0.46 90

6 2.3 0.63 120

Table 2b

In Trial 2, carbon dioxide was supplied continuously at 1.5 psi so as to reduce

the shearing force experienced by the electrolyte. Further, the initial current

density was maintained at 5 mA cm2.

17

2.3.3 Trial 3 (Mixture of carbonates)

Process Parameters

Trial 3

Cathodic (Galvanized steel) surface

area

22.04 cm2

Anodic (Ni mesh) Surface area

4.5 cm2

Lithium Carbonate

Cesium Carbonate

45 g

20 g

Initial current density

5 mA cm-2

Rate of CO2 supplied

1.5 psi (continuous)

Table 3a

18

S.No Voltage

(V)

Current

(A)

Time

(SEC)

1 1.4 0.11 0

2 1.6 0.23 60

3 1.8 0.32 95

4 2.0 0.41 125

5 2.9 1.0 185

Table 3b

Here in, apart from maintaining the already optimized process parameters, a

mixture of carbonates are used to see the effect of composition of the

electrolyte on the carbon nanofibers formation.

Besides all the trials, multiple sets of sub-trials were carried out to assure the

reproducibility of the process.

19

2.4 DOPAMINE DETECTION

The final washed product of Trial 3 was used for electrochemical detection of

dopamine. A composite of the as prepared carbon nanofiber with graphene was

prepared by taking 1:1 of graphene and CNF in de-ionized water at 60 oC and

agitating the mixture for 12 hours on a magnetic stirrer. The product was then

collected by centrifugation and then re-dispersed in DMF by ultra-sonication.

Thus prepared composite was then drop casted onto glassy carbon electrode to

form the graphene / CNF modified GC electrode. And the modified GC

electrode was electrochemically tested by cyclic voltammetry on Zahner IM6.

20

CHAPTER NO. 3

CHARACTERIZATION

21

3.1 CHARACTERIZATION

3.1.1 Scanning electron microscopy

The scanning electron microscope (SEM)18 uses a focused beam of high-energy

electrons to generate a variety of signals at the surface of solid specimens. The

signals that derive from electron-sample interactions reveal information about

the sample including external morphology (texture), chemical composition, and

crystalline structure and orientation of materials making up the sample. In most

applications, data are collected over a selected area of the surface of the sample,

and a 2-dimensional image is generated that displays spatial variations in these

properties. Areas ranging from approximately 1 cm to 5 microns in width can

be imaged in a scanning mode using conventional SEM techniques

(magnification ranging from 20X to approximately 30,000X, spatial resolution

of 50 to 100 nm). The SEM is also capable of performing analyses of selected

point locations on the sample; this approach is especially useful in qualitatively

or semi-quantitatively determining chemical compositions (using EDS),

crystalline structure, and crystal orientations (using EBSD). The design and

function of the SEM is very similar to the EPMA and considerable overlap in

capabilities exists between the two instruments.

Working Principle

Accelerated electrons in an SEM carry significant amounts of kinetic energy,

and this energy is dissipated as a variety of signals produced by electron-sample

interactions when the incident electrons are decelerated in the solid sample.

These signals include secondary electrons (that produce SEM images),

backscattered electrons (BSE), diffracted backscattered electrons (EBSD that

are used to determine crystal structures and orientations of minerals), photons

(characteristic X-rays that are used for elemental analysis and continuum X-

rays), visible light (cathodoluminescence–CL), and heat. Secondary electrons

and backscattered electrons are commonly used for imaging samples: secondary

22

electrons are most valuable for showing morphology and topography on

samples and backscattered electrons are most valuable for illustrating contrasts

in composition in multiphase samples (i.e. for rapid phase discrimination). X-

ray generation is produced by inelastic collisions of the incident electrons with

electrons in discrete ortitals (shells) of atoms in the sample. As the excited

electrons return to lower energy states, they yield X-rays that are of a fixed

wavelength (that is related to the difference in energy levels of electrons in

different shells for a given element). Thus, characteristic X-rays are produced

for each element in a mineral that is "excited" by the electron beam. SEM

analysis is considered to be "non-destructive"; that is, x-rays generated by

electron interactions do not lead to volume loss of the sample, so it is possible

to analyze the same materials repeatedly.

23

3.1.2 Transmission electron microscopy

The transmission electron microscope (TEM)19 operates on the same basic

principles as the light microscope but uses electrons instead of light. What you

can see with a light microscope is limited by the wavelength of light. TEMs

use electrons as "light source" and their much lower wavelength makes it

possible to get a resolution a thousand times better than with a light

microscope.

You can see objects to the order of a few angstrom (10-10 m). For example, you

can study small details in the cell or different materials down to near atomic

levels. The possibility for high magnifications has made the TEM a valuable

tool in both medical, biological and materials research.

Magnetic Lenses Guide the Electrons

A "light source" at the top of the microscope emits the electrons that travel

through vacuum in the column of the microscope. Instead of glass lenses

focusing the light in the light microscope, the TEM uses electromagnetic

lenses to focus the electrons into a very thin beam. The electron beam then

travels through the specimen you want to study. Depending on the density of

the material present, some of the electrons are scattered and disappear from the

beam. At the bottom of the microscope the unscattered electrons hit a

fluorescent screen, which gives rise to a "shadow image" of the specimen with

its different parts displayed in varied darkness according to their density. The

image can be studied directly or photographed with a camera

High resolution transmission electron microscopy (HRTEM): The contrast

formation in high resolution TEM (HRTEM) can only be explained by the

wave nature of electrons. In HRTEM a virtually planar electron wave transmits

a thin specimen (thickness < 20 nm), in most cases a crystal. During

transmission the incident electron wave is scattered (or diffracted in the case of

24

a crystal) at the potentials of the atoms, and thereby the phase of the electron

wave is changed. At the exit surface of the specimen the object wave is

formed, which carries direct and highly resolved information on the object.

The object wave is magnified in the electron microscope and during this

process the wave suffers additional phase shifts due to imperfect lenses

(aberrations). Finally, the image recorded on film plates or digital cameras is

an interference pattern of the image wave, which itself and it contains

essentially phase contrast with all the microscopic aberrations included. A

single recorded image in HRTEM consists of electron intensities only - the

phase of the wave and hence an important information on the object is lost.

In conventional HRTEM, image interpretation is performed by an iterative

procedure by comparing numerically simulated images with images acquired

at the electron microscope. The computer-simulated images are based on

atomic model structures, including all imaging parameters that need to be

known as precisely as possible. The resolution limit of the structure analysis is

determined by the point resolution of the microscope which is the optical

resolution of the objective lens.

25

3.1.2 X-Ray Diffraction

X-ray powder diffraction (XRD) is a rapid analytical technique primarily used

for phase identification of a crystalline material and can provide information

on unit cell dimensions. The analyzed material is finely ground, homogenized,

and average bulk composition is determined.

Max von Laue, in 1912, discovered that crystalline substances act as three-

dimensional diffraction gratings for X-ray wavelengths similar to the spacing

of planes in a crystal lattice. X-ray diffraction is now a common technique for

the study of crystal structures and atomic spacing.

X-ray diffraction is based on constructive interference of monochromatic X-

rays and a crystalline sample. These X-rays are generated by a cathode ray

tube, filtered to produce monochromatic radiation, collimated to concentrate,

and directed toward the sample. The interaction of the incident rays with the

sample produces constructive interference (and a diffracted ray) when

conditions satisfy Bragg's Law (nλ=2d sin θ). This law relates the wavelength

of electromagnetic radiation to the diffraction angle and the lattice spacing in a

crystalline sample. These diffracted X-rays are then detected, processed and

counted. By scanning the sample through a range of 2θangles, all possible

diffraction directions of the lattice should be attained due to the random

orientation of the powdered material. Conversion of the diffraction peaks to d-

spacings allows identification of the mineral because each mineral has a set of

unique d-spacings. Typically, this is achieved by comparison of d-spacings

with standard reference patterns.

All diffraction methods are based on generation of X-rays in an X-ray tube.

These X-rays are directed at the sample, and the diffracted rays are collected.

A key component of all diffraction is the angle between the incident and

diffracted rays. Powder and single crystal diffraction vary in instrumentation

beyond this.

26

CHAPTER NO. 4

RESULT AND DISCUSSION

27

4.1 RESULT AND DISCUSSION

We have demonstrated the synthesis of Carbon nanofibers from CO2. The

synthesis is carried out using very simple setup and cheap and easily available

electrodes. A simple molten carbon electrolysis was used as the driving force

for the above reaction. Initially the carbonate triggers the formation of the CNF

and once it gets completely used up, then CO2 supplied externally carries

forward the reaction. The concentration of CO2 in air is 0.04%, which is only

1.7 x 10-5 mol of tetravalent carbon per liter. Molten carbonate contains

approximately 20 mol of reducible tetravalent carbon per liter. Hence, the need

of concentrating CO2 via a separate process is eliminated by using electrolytic

conversion of CO2 to carbon nanofibers. Carbonate’s higher concentration of

active, reducible tetravalent carbon sites logarithmically decreases the

electrolysis potential and can facilitate charge transfer at low electrolysis

potentials. CO2 is bubbled into the molten carbonate, and during electrolysis,

oxygen is evolved at the anode, while a thick solid carbon builds at the cathode

as shown in the Figure 2, each mole of solid carbon product formed depends on

four moles of electrons);. Electrolysis above 800 oC was not carried out because

at higher temperatures the product gradually shifts towards carbon monoxide

(two electron reduction) as major product rather than the four electron reduction

to carbon.

The four electron process can be represented as:

Li2CO3(molten) C(solid) + Li2O(dissolved) + O2(gas)

and

CO2(cylinder) C(solid) + O2(gas)

28

Spirally coiled galvanized steel was used as the cathode and an oxygen

generating nickel anode was used as the anode in a molten Li2CO3. A low

current of 5 mA cm-2 was used initially and was maintained for at least 15

minutes. Then the current density was increased to 40 mA cm2 for the rest of

the electrolysis. The cooled product consists of fibers mixed with solidified

electrolyte. The fibers fall of the cathode when it is uncoiled. The product was

then carefully recovered after washing. The washed products were further

characterized by XRD, SEM, FESEM and HRTEM

The Ni anode undergoes corrosion in the lithium carbonate electrolyte. The

corrosion is helpful as these Ni nanoparticles deposited at the cathode acts as

the nucleation site for the carbon nanofibers to grow on. Although, Ni

undergoes only a minor corrosion initially and then it is stable

The characteristic CNFs are observed when the initial current is kept low and

then gradually increased to around 40 mA cm-2. However if the electrolysis is

started directly at higher current densities ( 40 mA cm-2), the product was found

to be predominantly amorphous which was undesirable. Due to its low

solubility and lower reduction potential, nickel (in this case originating from the

anode) is preferentially deposited at low applied electrolysis currents (5 or 10

mA cm−2). Hence lower currents aids in Ni nucleation and the higher currents

helps in carbonate reduction.

As noted, we observe that electrolysis initiated at a high current generated a

profusion of amorphous graphite, and a variety of carbon nanostructures rather

than a high yield of nanofibers, while electrolysis initiated with a low current

step prior to the high current can generate a high yield of uniform CNFs. It will

be of relevance for the future studies to refine and isolate these different

structures formed by varying the electrolytic conditions and by changing the

29

electrode materials.

The structure of the CNFs synthesized was also seen to be effected by the flow

rate of carbon dioxide gas passed. When the flow rate was kept at 3 psi, more

amorphous carbon was obtained, which could possibly be due to the high shear

force in the liquid electrolyte by the bubbling carbon dioxide gas. The force was

disabling the initiation of CNF formation. Hence, with a disrupted growth

phase, was leading to formation of amorphous carbon. Further optimization

resulted in maintaining the flow rate at 1.5 psi.

Trail 1

Figure 4- Photograph and Scanning electron micrograph of carbon formed in

Trial 1.

Trail 1, where in a higher initial current density was maintained, along with a

higher flow rate of carbon dioxide, the product formed was very less. As shown

in the Figure 4, carbon is sparsely deposited onto the steel coil. The scanning

electron micrograph also shows that most of the carbon formed is amorphous in

nature and just few traces of CNF are formed. Some graphitic sheets are also

observed.

30

Trial 2

Figure 5- Photograph and Scanning electron micrograph of carbon formed in

Trial 2.

The second trial which was carried out at a much lower initial current density,

eventually resulted in more of rod shaped carbon nanofibers and also some rods

and tubes. Much less amorphous carbon was observed. Figure 5 shows the

photograph of the electrode deposited with carbon nanofibers. Evidently, there

is more deposition than Trial 2. The scanning electron micrograph clearly

shows the tubular rods and fibers formed.

31

Figure 6- The energy dispersive X-ray spectrum of the CNFs prepared in trial 2

The Energy dispersive X-ray spectrum of the sample, as shown in the Figure 6

also indicates that most of the product is predominantly carbon.

2 4 6 8 10 12 14keV

0

2

4

6

8

10

12

14

16

18

20

22

24

cps/eV

C

32

Figure 7- A field emission scanning electron micrograph of the linear carbon

nanofibers and nano rods

The field emission scanning electron micrograph of the sample, as shown in the

Figure 7, gives a better insight in to the nanostructure of the CNF formed. Rod

shaped nanofibers which are under 100 nm diameter are seen. Ni nanoparticles,

which came out into the solution after anodic corrosion are also seen to be

dispersed in the image. This clearly shows that, by maintain a lower initial

current density, the activation of the nucleation site formation is favored.

33

Trial 3

Figure 8- Photograph showing the deposited carbon nanofibers (Trial 3)

Most of the product formed in Trial 3 is predominantly carbon nanofibers. The

photographic image, as shown in the Figure 8 shows that the electrode is almost

completely covered with CNFs. The addition of Cesium Carbonate had effect

on the types of CNFs formed.

34

Figure 9- High resolution transmission electron micrograph showing the carbon

nanotubes

Trial 3 was carried out to investigate the effect of an additional carbonate in the

electrolyte on the structure and the formation of the CNFs. The High resolution

Transmission electron micrograph of the thus formed CNFs, identifies a

variation in the nanostructure of the CNFs. At lower current densities the

formation of carbon is predominantly in the form of carbon nanotubes (Figure

9) with internal diameter of 15 nm and the external diameter of 25 nm. This

could be due to slow rate of formation of carbon.

35

Figure 10- HRTEM showing the tendril like CNF formed in Trial 3

Whereas the Figure 10, shows a tendril-like CNFs. Tendril-like CNFs are

formed at higher current densities. The diameter of such nanofibers are around

150 nm. Evidently, high concentration of oxides, localized where the nanofibers

are formed would result torsional effect on the CNFs, which in turn would

result in entanglement of the nanofibers. Such tendril-like nanofibers may

possess linear elastic properties, opening up some very interesting domain of

research on the elastic materials of the nano regime. Further studies are in

progress to elucidate the effect of mixture of molten carbonates being used as

the electrolyte.

36

XRD

Figure 11

The XRD diffraction peaks in Figure 11 at 26° and 43° are assigned to the

hexagonal graphite (002) and diffraction planes (JCPDS card files no. 41-1487)

within the CNF (specifically, the stacking of parallel graphene layers and the

size of graphene layer, respectively)22. The resolved XRD peaks at 43° (100

plane) and 44° (101 plane) is evidence of homogeneity of the synthesized

CNFs.

37

4.2 Electrocatalytic activity of Graphene/CNF modified electrode towards the

Sensitivity of dopamine: Figure 12 shows the electrochemical response of the

bare Glassy Carbon electrode (GC), Graphene and Graphene/CNF modified GC

(A) without dopamine and (B) with 118.5 × 10-6 mol L-1 dopamine in pH 7.4

Phosphate buffer solution at scan rate 50 mV s-1. In Figure 12 (A), the back

ground current of Graphene/CNF modified electrode was higher than the

Graphene modified GC and GC, demonstrates that the Graphene/CNF modified

electrode was highly catalytic active because of large surface area of composite.

Figure 12 (B) shows the electrocatalytic activity of three electrodes towards the

dopamine. In this, the Graphene/CNF modified GC shows the high oxidation

current of dopamine between the -0.1 to 0.3 V vs. SCE region than the remaining

modified electrodes, which explains that the surface modified with

Graphene/CNF is highly active. Figure 12 C displays the CV curves of the

Graphene/CNF modified GC in pH 7.4 PBS containing different concentrations

of dopamine. A well-defined redox peak was formed between the region of -0.1

to 0.3 V vs. SCE and peak current was increased with dopamine concentration. It

was confirmed by the linear relationship between oxidation peak current and

dopamine concentration expressed as Ipa = -8.09 + 0.59 CD (µM) with R2 =

0.99, as shown in Figure 12 D.

38

Figure 12. CVs of GC, Graphene, Graphene/CNF modifed electrode (A)

without dopamine, (B) with 118.57 × 10-6 mol L-1 dopamine, (C) CVs of 19.96 ×

10-6 mol L-1, 39.84 × 10-6 mol L-1, 49.75 × 10-6 mol L-1, 59.64 × 10-6 mol L-1,

69.5 5 × 10-6 mol L-1, 79.36 × 10-6 mol L-1, 89.19 × 10-6 mol L-1, 99.00 × 10-6

mol L-1,108.80 × 10-6 mol L-1, 118.57 × 10-6 mol L-1 dopamine at

Graphene/CNF modified GC, Scan rate 50 mV s-1, (D) show the relation

between Ipa and concentration of dopamine.

39

5. CONCLUSIONS

Carbon nanofibers from carbon dioxide was prepared electrochemically under

molten electrolysis condition. Hence, exploring a way to reduce the harmful,

green house gas, carbon dioxide and to convert it to economically important

carbon nanofibers via a facile synthetic route is the prime aim of this study. The

prepared carbon nanofibers were characterised with analytical techniques like

SEM, EDX, FESEM, HRTEM and XRD. The diameter and structural aspects of

the fibers were predominantly effected by the initial current density and the flow

rate of carbon dioxide passed. Varying structures ranging from CNTs to tendril

like CNFs were obtained. By further optimizing the experimental parameters, we

could isolate homogenous products which can then be applied in various fields

like electrocatalysis, stronger carbon based composites etc. The CNF thus

prepared was also studied for its electrocatalytic sensing of dopamine.

40

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