Modification of carbon nanotube on carbon cloth…

MWNTs on carbon cloth are functionalized by several chemical solutions in the sample tube with sand bath. The parameters of chemical modification are chemical solutions (HNO3, H2SO4, and KOH), temperature (80⁰C, 90⁰C, and 100⁰C), concentration (2M and 14M), and time (0 hr, 6 hr, 12 hr, 18 hr, 24 hr, and 48 hr). Then, the sample is collected after immersing with deionized water until the filtrate pH became nearly the pristine pH. Fig. 3.3 shows schematic diagram of the chemical modification

Fig. 3.3 Schematic diagram of the chemical modification

3.3 Dispersion of Pt on prepared carbon cloth

The prepared carbon cloth is immersed in a chemical solution containing H2PtCl6·6H2O, PVP-4000, and ethylene glycol mixture diluted with acetone. Polymer (PVP) is the protection agent to limit metal particle growth spacing. Pt is deposited by synthesis from H2PtCl6·6H2O in ethylene glycol solution under 160⁰C for 3 hr, followed by filtration with acetone for 8 times and sintered for 1 hr at 250⁰C. This method, polyol process, is expected a uniformly homogeneous nucleation and growth mechanism of nanoparticle. The reaction dominates by temperature control and solvent plays as reductant as well. Fig 3.4 shows the experimental procedures of polyol process.

Fig. 3.4 The experimental procedures of polyol process

3.4 Analysis Instruments

3.4.1 Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) is used to observe the surface morphology of wide range kinds of objects. There are many advantages including of easy sample preparation, high image resolution, large depth of field, and high magnification. [33]

The SEM image is that signals (secondary electrons and backscattered electrons) emit from the sample surface as the sample is bombarded by the high energy incident electrons. The fabricating CNT morphology and the dispersing Pt on CNT morphology could be observed by JEOL JSM6500F in NCTU with field emission electron source and 15kV accelerate voltage.

Fig.3.5 Diagram of a Scanning Electron Microscopy [http://www.le.imm.cnr.it/sito/laboratories/jsm6500f.html]

3.4.2 Transmission Electron Microscopy (TEM)

In a typical TEM a static beam of electrons at 100-400kV accelerating voltage illuminate a region of an electron transparent specimen which is immersed in the objective lens of the microscope. The transmitted and diffracted electrons are recombined by the objective lens to form a diffraction patter in the back focal plane of that lens and a magnified image of the sample in its image plane. [33]

The raw MWNTs and the modified MWNTs morphology could be compared by TEM using

a JEOL JEM 4000 system in NCTU operating at 200kV. And the particle morphology, size and size distribution of Pt nanoparticles dispersed on the surface of MWNTs are also characterized by TEM.

3.4.3 X-ray Photoelectron Spectroscopy (XPS)

The phenomenon is based on the photoelectric effect. The concept of the photon was used to describe the ejection of electrons from a surface when photons impinge upon it. The XPS technique is highly surface specific (< 5nm) due to the short range of the photoelectrons that are excited from the solid. The energy of the photoelectrons leaving the sample is determined using a Spherical Capacitor Analyzer (SCA) this gives a spectrum with a series of photoelectron peaks.

The binding energy of the peaks is characteristic of each element. The peak areas can be used (with appropriate sensitivity factors) to determine the composition of the materials surface. [33]

XPS could determine the difference of the raw CNT and the modified CNT due to the element C chemical shifts. Furthermore, it may ensure if Pt is reductive by the same way. In this study, XPS analysis is carried out on a ESCA PHI 1600 using an Mg Kα X-ray source in NTHU.

Fig.3.6 Diagram of a X-ray Photoelectron Spectroscopy [http://www.nscric.nthu.edu.tw/Other/augeresca/auesca.html]

3.4.4 Fourier Transform Infrared Spectrometer (FTIR)

Fourier Transform Infrared Spectroscopy (FTIR) is an analytical technique used to identify organic and inorganic materials. This technique measures the absorption of various infrared light wavelengths by the material of interest. These infrared absorption bands identify specific molecular components and structures. [33]

The functional groups on the surface of MWNTs modified by chemical solution could be determined by FTIR. The FTIR measurements are performed on a PROTEGE 460 series FTIR apparatus by transmission spectroscopy in NCTU.

Fig.3.7 Diagram of a Fourier Transform Infrared Spectrometer [http://www.forumsci.co.il/HPLC/FTIR_page.html]

3.4.5 Cyclic Voltammetry (CV) Potentiostat

A potentiostat is an electronic device that controls the voltage difference between a working electrode and a reference electrode. Both electrodes are contained in an electrochemical cell.

The potentiostat implements this control by injecting current into the cell through an auxiliary, or counter, electrode. In almost all applications, the potentiostat measures the current flow between the working and auxiliary electrodes. The controlled variable in a potentiostat is the cell potential

and the measured variable is the cell current.

The CHI Version 5.01 system in the potentiostat in NCTU is used to measure the electrochemical specific surface area of the dispersive Pt on the surface of MWNTs for the fuel-cell electrodes. From the CV, the charge equivalent to the area under the hydrogen desorption region is evaluated and the electrochemical specific surface area is calculated assuming that the charge is required for the adsorption-desorption of a monolayer of atomic hydrogen on the surface.

Fig.3.8 Schematic of a Cyclic Voltammetry (CV) Potentiostat [http://www.gamry.com/App_Notes/Potentiostat_Primer.htm#Workking]

3.4.6 Energy Dispersive X-ray (EDX)

It is a technique used for identifying the elemental composition of the specimen, or an area of interest thereof. The EDX analysis system works as an integrated feature of SEM (JEOL JSM6500F) in NCTU. An EDX spectrum plot not only identifies the element corresponding to each of its peaks, but the type of X-ray to which it corresponds as well. For example, a peak

corresponding to the amount of energy possessed by X-rays emitted by an electron in the L-shell going down to the K-shell is identified as a K-Alpha peak. The peak corresponding to X-rays emitted by M-shell electrons going to the K-shell is identified as a K-Beta peak. [33]

EDX measurements show the element on the MWNTs and the content of Pt/MWNTs. It appears that the difference content of Pt is on the raw MWNTs and the modified MWNTs.

Fig.3.9 Elements in an EDX spectrum are identified based on the energy content of the X-rays

[http://www.semiconfareast.com/edxwdx.htm]

3.4.7 X-Ray Diffraction (XRD)

X-ray Diffraction (XRD) is one of the primary techniques used by solid state chemists to characterize materials. XRD can provide information about crystalline structure and particle size in a sample even when the crystallite size is too small for single crystal x-ray diffraction. [33]

An X-ray beam hits a sample and is diffracted. We can observe the diffraction peaks when the distances between the planes of the atoms apply to Bragg's Law. Bragg's Law is:

nλ =2dsinθ (1) Where the integer n is the order of the diffracted beam, is the wavelength of the incident X-ray beam, d is the distance between adjacent planes of atoms (the d-spacings), and is the angle of incidence of the X-ray beam.

The broader diffraction peaks for the catalyst led to smaller average particle size as calculated by the Scherrer equation.

θB

λ

θ α

cos B L K

2 1

= K (2)

Where L is the average particle size, K is the constant, λ K^α1 is the X-ray wavelength

(1.54056 Ǻ for Cu Kα1 radiation), B2θ is the peak broadening, and θB is the angle corresponding to the peak maximum.

In this study, the particle size calculated by XRD is compared with the data measured by TEM. It is determined how the functional group (COOH) on the MWNTs affects the Pt particle size forming.

Fig.3.10 Schematic of X-Ray Diffraction

[http://pubs.usgs.gov/info/diffraction/html/index.html]

Chapter 4 Results and Discussion

4.1 Morphology of carbon nanotube on carbon cloth

In this study, the purpose is that the longer MWNTs are fabricated vertically by MPECVD. The use of a support, well-aligned MWNTs, for catalyst has many advantages including more the anchoring sites of available catalyst, producing a lot of functional groups to attract catalyst at the caps and the sidewalls of MWNTs easily by modified the surface structure, using inner specific surface area of tube by opening end of MWNTs.

Fig 4.1 shows that a piece of carbon cloth without any treatment. It is composed of textured carbon fibers which have a smooth surface. Fig 4.2 shows that the images of long MWNTs fabricated on carbon cloth densely and vertically. Then, MWNTs are analyzed in the higher magnification. The length of MWNTs is about 20 μm and its diameter is about 20 nm in Fig 4.3. As shown in Fig. 4.4, the TEM images show that MWNTs display hollow tubes with amorphous and crystalline layers. On the other hand, the growth model of MWNTs could be found including tip growth model and base growth model.

The density of the vertical MWNTs may not be controlled dispersedly so the spraying of Pt particle may just stop on the top surface. Therefore, even the high specific surface area of MWNTs still can not be utilized totally until Pt is deposited uniformly by using polyol process. However, the raw surface of MWNTs is relatively inert and difficult to support particles homogeneously, which often results in the agglomeration of nanoparticles. Thus, it is important to improve the adhesion through the following surface chemical modification techniques.

Fig. 4.1 SEM images of pristine carbon cloth with different magnification

Fig 4.2 SEM images of MWNTs on carbon cloth with different magnification

Fig 4.3 The length of MWNTs is about 20 μm and its diameter is about 20 nm.

Base growth model Tip growth model

20 μm 20 nm

20 nm

20 nm

4.2. Multi-wall carbon nanotubes are modified by HNO

₃

, H

₂

SO

₄

, and KOH

Surface chemical modification is a common method and is essential for the deposition of catalysts and other species onto carbon nanotube surfaces for nanocatalytic applications [34]. Oxidation, with popular oxidants including HNO₃, KMnO4, HNO3 / H2SO4, is an often attempted approach to activate the surface of CNTs [35, 36]. However, there is little analysis about functional groups after surface chemical modification of CNTs. In this study, MWNTs are modified by HNO3, H2SO4, and KOH at 80⁰C in Table 4.1 and may produce different functional groups to attract more Pt ions nucleating uniformly and densely. Thus, after the following experiments, it would be found that which chemical solution is better for surface chemical modification of MWNTs.

Table 4.1 MWNTs are modified by HNO3, H2SO4, and KOH

Solution HNO3 H2SO4 KOH

Concentration 16M 18M 6M

Temperature 80⁰C 80⁰C 80⁰C

Time 6 hr 6 hr 6 hr

Hypothetical functional groups

-COOH -OH

-SO3H -OH

4.2.1 Analysis of functional groups

The information on the surface chemistry of the modified nanotubes has been provided by the FTIR. Table 4.2 indicates the absorption spectra ranges of several functional groups. Fig 4.5 shows the absorption spectra of MWNTs modified by HNO₃, H2SO4, and KOH at 80⁰C for 6 hr. In fact, one broad intensive band observing at 1375 cm^-1 is assigned to the alcoholic hydroxyl groups (-OH) or the carboxylic acids (-COOH) and the other observing at 1574 cm^-1 is assigned to the carboxylic acids (-COOH).

However, the functional group, sulfonic acid (-SO₃H), can not be observed. Therefore, the reaction pathways for forming the functional group most probably involve the following reaction. Hydration of the olefinic C=C moieties is released from the conjugation network by decarbonization of the nanotube sidewalls due to HNO3, H2SO4, and KOH oxidation so the alcoholic hydroxyl groups (-OH) and carboxylic acids (-COOH) could be formed [32].

Indeed, we try to assay sulfonic acid (-SO₃H) groups with FTIR. However, the detection is too difficult because of their weak response in the IR mode. For the reason, we used XPS to directly detect the sulfur atom [37]. No S spectra of various MWNTs, given in Fig. 4.6, show that there is no sulfonic acid (-SO3H). On the other hand, the carboxyl groups would be detected in Fig. 4.7(a) (b) (c). The C 1s spectrum appears to be composed of C=C (~284.6 eV), C-C (~285.85 eV), CO (287.46 eV), COO-(289.24 eV), OCOO- (~291.5 eV) functional groups [38]. The observed chemical shift following the chemical treatment is about 1 eV. Therefore, the result from the FTIR and XPS analysis indicates that MWNTs merely forms the alcoholic hydroxyl groups (-OH) and the carboxylic acids (-COOH) with chemical modification of HNO3, H2SO4, and KOH at 80⁰C for 6 hr.

Table 4.2 The absorption spectra ranges of several functional groups

Fig 4.5 The absorption spectra of MWNTs modified by HNO₃, H₂SO₄, and KOH Functional group

Wave number (cm^-1) -OH alcoholic hydroxyl groups 1410-1260

-SO₃H sulfonic acid 1150-1250

-COOH carboxylic acids 1610-1550 1420-1300 -C=O carboxyl groups 1690-1760

4000 3000 2000 1000

-COOH -OH -COOH

80⁰C 16M HNO

3 06hr 80⁰C 18M H

2SO

4 06hr 80⁰C 6M KOH 06hr w/o

absorptions (a.u.)

wave number (cm ^-1 )

Fig. 4.6 XPS survey spectra of MWNTs modified by HNO3, H2SO4, and KOH

Fig.4.7 (a) The C 1s spectrum of HNO₃-MWNTs

1200 1000 800 600 400 200 0

O (1s)

C (1s)

Binding Energy (eV)

Intesity (a.u.)

HNO₃ C (1s) (285.2 eV) O (1s) (532 eV) H₂SO₄ C (1s) (285.1 eV) O (1s) (533 eV) KOH C (1s) (285 eV) O (1s) (531 eV)

294 292 290 288 286 284 282 280

OCOO^- COO^- C-O C-C

C=C

Binding Energy (eV)

Intesity (a.u.)

C (1s)

sp² C=C (284.6) sp³ C-C (285.85) C-O (287.46) COO- (289.24) OCOO- (291.5)

Fig.4.7 (b) The C 1s spectrum of H2SO4-MWNTs

Fig.4.7 (c) The C 1s spectrum of KOH-MWNTs

292 290 288 286 284 282 280

OCOO^- COO

300 295 290 285 280

Intesity (a.u.)

4.2.2 Qualitative analysis of Pt on MWNTs

Pt particles are deposited on MWNTs by using polyol process after samples are modified chemically. Then, we take EDX and XPS of qualitative analysis for samples in order to be sure that polyol process is successful. Fig. 4.8 shows the mapping of EDX for HNO3-MWNTs. It indicates that the dispersive nanoparticles on MWNTs are Pt. On the other hand, Fig. 4.9 also shows the XPS survey spectrum of HNO₃-MWNTs after the Pt reduction. Notably, a very strong Pt4f peak indicates that there are Pt nanoparticles on MWNTs because XPS is a surface-sensitive tool.

The formation process of PVP-protected Pt nanoparticles synthesized in ethylene glycol is presented that the H₂PtCl₆ may be completely reduced according to the eq (1) and the eq (2).

Pt⁴⁺(aq) + 6Cl^-(aq) + 2H⁺(aq) ⇌ H2PtCl6 (aq) (1)

H2PtCl6(aq)+ CH2OHCH2OH(aq) ⇌ Pt⁰ + 6HCl(aq)+ 2HCHO(aq) (2)

As mentioned above, the Pt precursors are reduced to Pt⁰ atoms at 160⁰C for 3hr [39]. Fig. 4.10 shows that the dispersive Pt nanoparticles on MWNTs are Pt⁰ because there is no significant chemical shift of binding energy in Pt4f_7/2 and Pt4f_5/2 with Ar⁺ etching [40]. Table 4.3 shows that the binding energy of Pt4f7/2 and Pt4f5/2 are developed by Ar⁺ etching (25mA and 3kV) with time. Finally, the Pt⁰ nanoparticles are stabilized on MWNTs by using ethylene glycol.

Fig. 4.8 The mapping of EDX for HNO3-MWNTs

Fig. 4.9 The XPS survey spectrum of HNO₃-MWNTs after reduction of Pt

1200 1000 800 600 400 200 0

Intesity (a.u.)

Pt C

Binding Energy (eV)

C (285.13eV) Pt (70.5eV)

Fig. 4.10 Chemical shift of binding energy in Pt4f7/2 and Pt4f5/2 with Ar⁺ etching

Table 4.3 The development of the binding energy of Pt4f7/2 and Pt4f5/2 with time

Ar⁺ (25mA, 3kV) 4f_5/2 4f_7/2

0 min 75.04 eV 71.77 eV

3 min 74.99 eV 71.76 eV

6 min 75.03 eV 71.77 eV

9 min 75.07 eV 71.80 eV

88 86 84 82 80 78 76 74 72 70 68

Intesity (a.u.)

4f_7/2 4f5/2

Binding Energy (eV)

00 min sputter 03 min sputter 06 min sputter 09 min sputter

4.2.3 Analysis of dispersive Pt on MWNTs

Pt nanoparticles are actually deposited onto the surface of MWNTs from the last section. Then, by comparing Pt on the raw and modified MWNTs, we would like to understand the influence of the functional groups on Pt dispersion on MWCTNs. Fig.

4.11 illustrates many Pt nanoparticles are agglomerated on some areas like a larger nanoparticle. Fig. 4.12 illustrates many Pt nanoparticles disperse uniformly on HNO3-MWNTs due to presence of the carboxylic acids (-COOH). If Pt nanoparticles were agglomerated, the total effective activating catalyst area would decrease due to the clustered shelter of Pt nanoparticles. Therefore, MWNTs have to be modified chemically in order to disperse Pt nanoparticles on MWNTs uniformly.

4.2.4 EDX analysis of Pt on MWNTs

Pt loading on carbon nanotube would be increased because more functional groups, the carboxylic acids (-COOH), act nucleation sites and make active catalyst areas large.

The energy dispersive analysis in Fig. 4.13, Fig. 4.14, Fig. 4.15, and Fig. 4.16 show that the amount of Pt loaded on MWNTs with reference to carbon can be evaluated qualitatively as 16.88 wt%, 21.23 wt%, 22.84 wt%, and 26.08 wt% after the different chemical modification. The most Pt loading on MWNTs modified by HNO₃ (strong oxidizing agent) in Fig. 4.17 may be suggested that the most functional groups would be formed on the surface of MWNTs at the same time with different chemical solutions.

Fig. 4.11 Pt nanoparticles are agglomerated like a larger nanoparticle

Fig. 4.12 Pt nanoparticles disperse uniformly on HNO₃-MWNTs

100.00

Fig. 4.13 The amount of Pt loading on raw MWNTs is 16.88 wt%.

Fig. 4.14 The amount of Pt loading on KOH -MWNTs is 21.23 wt%.

Fig. 4.15 The amount of Pt loading on H2SO4 -MWNTs is 22.84 wt%.

Fig. 4.16 The amount of Pt loading on HNO3-MWNTs is 26.08 wt%.

Fig. 4.17 Pt loading on MWNTs with different chemical modification 100.00

Totals

2.13 26.08

Pt

97.87 73.92

C

Atomic%

Weight%

Element

WO KOH H2SO4 HNO3

0 10 20 30

40

Pt EDX

Pt weight %

4.2.5 Half-cell test

The activity of catalyst is practically important in the study on fuel cell, which is usually evaluated by the current peaks with the dispersion and the amount of catalyst.

The electrocatalytic activity of Pt nanoparticles is obtained from the CV measurements performed in 1M methanol and 1M sulfuric acid electrolyte. Fig. 4.18 shows that the electrocatalytic activity of Pt nanoparticles is evaluated by the current peaks of methanol oxidation in eq. (3) and eq. (4)

Pt-CO+Pt-OH 2Pt+CO²+H⁺+e-

(3)

Pt-(CH³OH)^ads Pt-(CO)^ads+4H⁺+4e-

(4)

As well known, it can be observed that the electrocatalytic activity of Pt nanoparticles on HNO3-MWNTs is higher than the others due to more amounts of dispersive Pt nanoparticles.

4.2.6 Summary

1. Various solvents, eg. HNO3, H2SO4, and KOH (80⁰C, 6 hr), modified MWNTs can form the same functional group -COOH and –OH.

2. Well-dispersed functional groups on MWNTs could improve the efficiency of half-cell test.

3. The functional groups on MWNTs increase anchoring sites of Pt precursor to increase the efficiency of half-cell test.

4. The amount of functional groups in order to attract Pt on HNO3-MWNTs is more than on H₂SO₄-MWNTs and on KOH-MWNTs due to a strong oxidizing agent HNO3.

W/O KOH H2SO4 HNO3

-Fig. 4.18 Electrocatalytic activity is evaluated by the current peaks

W/O KOH H2SO4 HNO3

(1) Pt-CO+Pt-OH -> 2Pt+ CO

2+H⁺+e

4.3. Multi-wall carbon nanotubes are modified by HNO

₃

with Temperature (T), time (t), and concentration (conc.).

MWNTs are modified by HNO3, H2SO4, and KOH at high temperature in Section 4.2 and may produce the same functional groups, the alcoholic hydroxyl groups (-OH) and carboxylic acids (-COOH), to attract more Pt ions nucleating uniformly and densely.

Moreover, the amount of functional groups in order to attract Pt on HNO3-MWNTs is more than on H2SO4-MWNTs and on KOH-MWNTs due to a strong oxidizing agent HNO3. However, there are the other parameters, temperature, time, and concentration, in the function of chemical modification for MWNTs. In this work, we would find the best temperature (T) for HNO3-MWNTs may produce a lot of functional groups to increase Pt anchoring sites in Table. 4.4. Furthermore, for 2M HNO3, the highest temperature would be lower 100⁰C to maintain the concentration.

Table 4.4 HNO₃-MWNTs with different T

Temperature (T) 80⁰C 90⁰C 100⁰C 2M HNO3

(b.p. ~105⁰C)

24 hr 24 hr 24 hr

14M HNO3

(b.p. ~122⁰C)

24 hr 24 hr 24 hr

4.3.1.1 Analysis of MWNTs morphology with T

Fig. 4.19 and Fig. 4.20 show that there is little damage for 14 M HNO3-MWNTs at 80⁰C and 90⁰C for 24 hr. On the contrary, Fig. 4.21 shows that there is a serious damage for 14 M HNO3-MWNTs at 100⁰C for 24 hr. The total loading amount of Pt would decrease if the amount of MWCNT as support were broken. Therefore, the temperature of chemical modification would be lower than 100⁰C.

4.3.1.2 Analysis of functional groups with T

We try to assay the functional groups with FTIR after MWNTs are modified by 2 M or by 14 M HNO3 with temperature. Fig. 4.22(a) (b) show that oxidization with HNO3 at 90⁰C and 100⁰C successfully introducedcarboxylic acids (-COOH), carboxyl groups (-C=O), and alcoholic hydroxyl groups (-OH) on MWNTs surfaces. However, the functional groups of 2M and 14M HNO3-MWNTs at 80⁰C would be difficult to be detected because of their weak response in the IR mode. It is suggested that temperature is a main factor if the oxidization of chemical reaction could proceed. Finally, 90⁰C is the optimum temperature for 2M and 14M HNO3-MWNTs to form a lot of functional groups.

Fig. 4.19 Little damage for 14M HNO3-MWNTs at 80⁰C

Fig. 4.20 Little damage for 14M HNO₃-MWNTs at 90⁰C

Fig. 4.21 A serious damage for 14M HNO3-MWNTs at 100⁰C

Fig. 4.22 (a) FTIR of 2M HNO3-MWNTs with different T

Fig. 4.22 (b) FTIR of 14M HNO₃-MWNTs with different T

4000 3000 2000 1000

From Section 4.3.1, the modified MWNTs at 90⁰C form a lot of functional groups for 2 M HNO3 and14 M HNO3. In the followed Section 4.3.2 and Section 4.3.3, we would find the other best parameters time (t) with high (14M) and low (2M) concentration (conc.) for HNO3-MWNTs may produce the most functional groups to increase Pt anchoring sites in Table. 4.5

Table 4.5 MWNTs are modified by HNO₃ at 90⁰C for 2M and 14M with t

90⁰C Time (hr)

2M HNO₃ 00 06 12 18 24 48

14M HNO3 00 06 12 18 24 48

4.3.2.1 FTIR of 2M HNO

3

-MWNTs with t

The number of carboxylic acids (-COOH) would be expected by its intensity in the

The number of carboxylic acids (-COOH) would be expected by its intensity in the

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