Synthesis and characterization of Pt nanoparticles on sulfur-modified carbon nanotubes for methanol oxidation

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 7 2 7 5 e7 2 8 3

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Synthesis and characterization of Pt nanoparticles on sulfur-modified carbon nanotubes for methanol oxidation Rezgar Ahmadi, Mohammad K. Amini* Chemistry Department, University of Isfahan, Hezar Jareeb St, Isfahan 81744-73441, Iran

article info

abstract

Article history:

Pt nanoparticles supported on carbon nanotubes (Pt/CNTs) have been synthesized from

Received 6 November 2010

sulfur-modified CNTs impregnated with H2PtCl6 as Pt precursor. The dispersion and size of

Received in revised form

Pt nanoparticles in the synthesized Pt/CNT nanocomposites are remarkably affected by the

27 February 2011

amount of sulfur modifier (S/CNT ratio). The results of X-ray diffraction and transmission

Accepted 5 March 2011

electron microscopy indicate that an S/CNT ratio of 0.3 affords well dispersed Pt nano-

Available online 8 April 2011

particles on CNTs with an average particle size of less than 3 nm and a narrow size distribution. Among different catalysts, the Pt/CNT nanocomposite synthesized at S/CNT

Keywords:

ratio of 0.3 showed highest electrochemically active surface area (88.4 m2 g
1) and highest

Carbon nanotubes

catalytic activity for methanol oxidation reaction. The mass-normalized methanol oxida-

Pt nanoparticles

tion peak current observed for this catalyst (862.8 A g
1) was w 6.5 folds of that for Pt

Catalyst support

deposited on pristine CNTs (133.2 A g
1) and w 2.3 folds of a commercial Pt/C (381.2 A g
1).

Methanol oxidation reaction

The results clearly demonstrate the effectiveness of a relatively simple route for prepa-

Sulfur

ration of sulfur-modified CNTs as a precursor for the synthesis of Pt/CNTs, without the

Fuel cell

need for tedious pretreatment procedures to modify CNTs or complex equipments to achieve high dispersion of Pt nanoparticles on the support. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Among different types of polymer electrolyte fuel cells, direct methanol fuel cells (DMFCs) are promising as highly efficient and clean energy sources for electric vehicles and portable electronic devices, which are widely seen as the first major commercial application for these sources [1]. However, one of the major challenges towards commercialization of DMFCs is high cost of Pt currently used as the state-of-the-art electrocatalyst in these fuel cells. Decreasing the amount of Pt catalyst used in fuel cells via increasing its utilization efficiency has been one of the major concerns during the past decade [2]. The utilization efficiency of catalysts is strongly related to their particle size, distribution, and the type of

support. Highly distributed catalyst nanoparticles with small size and narrow size distribution are ideal for high electrocatalyst activity owing to their large surface-to-volume ratio [3]. It is well known that the physicochemical properties of the support can greatly influence the dispersion and stability of electrocatalyst nanoparticles and consequently their electrochemical behavior [4]. CNTs because of their exceptional surface structures, high electrical conductivities, relatively large surface areas, and high mechanical, chemical and electrochemical stabilities, have been utilized as novel supports for metal catalysts [5,6]. There are many ingenious methods for immobilizing metal nanoparticles onto CNT substrates, each offering varying degrees of control of particle size and distribution along the

* Corresponding author. Tel.: þ98 311 7932708; fax: þ98 311 6736055. E-mail address: [email protected] (M.K. Amini). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.013

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CNT [7]. These methods have been grouped into two general categories [8]: (a) in situ methods based on formation of metal nanoparticles in the presence of pristine or functionalized CNTs [9,10] employing various methods such as electrochemical and electroless deposition [7,11], and gas phase deposition [12]; (b) ex situ methods in which metal nanoparticles are pre-formed and then immobilized onto CNT surface [3,13]. However, due to their chemical inertness, and hydrophobic nature, pristine CNTs do not provide enough anchoring sites for high dispersion of precious metal nanoparticles [14e18]. Therefore, it is important to modify the surface of CNTs by proper techniques, because the distribution and density of nanoparticles on the substrate is controlled, at least in part, by the type of pretreatment employed [19,20]. In general, the surface of CNTs can be modified by covalent attachment of functional groups or non-covalent adsorption of various functional molecules onto their surface. Chemical modification is the most common and widely used way to introduce the linkers, as well as to improve solubility or dispersibility of CNTs, which is also important for the efficient and uniform deposition of nanoparticles [16,21e23] (see also references therein). Among various functionalities, sulfurbased functional groups have attracted much attention because of their strong affinity towards noble metals. Such affinity which arose from soft acid-soft base interactions can be successfully employed for anchoring metal nanoparticles to CNTs [24e26]. Sulfur-modified CNTs, can be prepared by covalent and non-covalent methods [27,28]. Various functionalization schemes have been devised to covalently modify CNTs with sulfur-containing linkers [29,30]. Generally, sulfur modifiers with small molecules are preferred when the modified CNTs are to be used for immobilization of uniformly dispersed metal nanoparticles [30,31]. Long and flexible alkyl chains do not anchor the particles in a specific orientation and result in a large contact resistance between metal nanoparticles and the CNTs [31]. However, most of these functionalizations are based on harsh reaction conditions including corrosive reactants and tedious work up. Moreover, covalent functionalization generally change the hybridization of carbon atoms from sp2 to sp3 which disrupts the band-to-band transitions of p electrons and causes loss of the novel properties of CNTs, such as their high conductivity and remarkable mechanical properties [23]. In contrast, non-covalent functionalizations, which are mainly based on van der Waals interactions, hydrogen bonding, electrostatic forces and other attractive forces, are generally conducted under relatively mild reaction conditions, and thus would not destroy the electronic network of CNTs [21,32]. Various bifunctional compounds can non-covalently bound with the surface of CNTs, with the capability to act as interlinkers between the nanotube surface and nanoparticles [33]. For instance, G.W. Yang et al. [32] and D.Q. Yang et al. [34] found that non-covalently functionalized CNT surfaces interact strongly with Pt nanoparticles through the formation of PteS bonds, resulting in very high Pt nanoparticle loadings, with both good dispersion and a narrow size distribution. In spite of all these advancements, developing simpler and cheaper alternatives for modification of CNTs that provide

well-dispersed Pt nanoparticles without destroying their structure are still desirable. In this regard, Plank et al. in an effort to directly attach sulfur atoms to CNTs, reported on direct thiolation of single-walled CNTs by physically mixing elemental sulfur with CNTs and exposing the mixture to Ar/H2 plasma [35]. Yuan et al. have recently shown that elemental sulfur-CNT composites with core-shell structure can be prepared through capillarity between elemental sulfur and CNTs [36]. They used the synthesized composite as cathode of lithium-sulfur batteries. Nazar and coworkers [37] have also used a similar method to modify ordered mesoporous carbon (OMC) with sulfur for the same type of application. Using the sulfur-coated OMCs, they also introduced a method for synthesizing size-controlled noble metal or bimetallic nanocrystallites embedded within the porous structure of OMC for direct formic acid fuel cell anodes [38]. Here, we report the use of sulfur-modified CNTs for the synthesis of highly dispersed Pt nanoparticles with narrow size distribution on CNTs. The modification is based on sulfur impregnation of CNTs from toluene solution followed by a melt-coat step. Pt nanoparticles are supported on the CNTs by the wet impregnation-thermal reduction method. Several techniques including transmission electron microscopy (TEM) and X-ray diffraction (XRD) are used to investigate the effect of S/CNT ratios on the particle size and distribution of Pt nanoparticles on the CNTs. The electrochemical performance of the synthesized Pt/CNT electrocatalysts for methanol oxidation reaction is evaluated by cyclic voltammetry. To the best of our knowledge there is no report on the use of elemental sulfur-modified CNTs for preparation of highly dispersed Pt nanoparticles for methanol oxidation.

2.

Experimental

2.1.

Materials

Hexachloroplatinic acid (40 wt.% Pt), sulfur and methanol were obtained from Merck. MWCNTs from Green Nanotech Co., Tehran Iran (purity  95%, 10e15 nm in diameters, and 10e30 mm in length) were used as support. Nafion solution (5 wt.% in lower aliphatic alcohols and water) was supplied by Aldrich. All other chemicals used in this investigation were of analytical grade from Merck. Deionized water was used for preparation of all solutions.

2.2.

Synthesis of Pt/CNT catalysts

The sulfur-modified CNTs with different S/CNT wt. ratios (0.1, 0.2, 0.3, 0.5 and 0.7) were prepared by a solvent impregnation and melt-coat method. For S/CNT ratio of 0.3, typically, 48.0 mg of elemental sulfur was ultrasonically dissolved in 20 mL toluene and then 160 mg of CNT was added. The mixture was sonicated for 1 h to break aggregates and disperse the CNTs. Then, the solvent was evaporated at room temperature and the sample was ground thoroughly in an agate mortar. The mixture in a sealed vial was heated to 155  C in an oven for 6 h. The modified CNTs were impregnated with Pt(IV) by ultrasonically dispersing the sample in 15 mL acetone solution containing 0.205 mmol H2PtCl6 (corresponding to

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 7 2 7 5 e7 2 8 3

40 mg Pt to afford 20 wt.% Pt on CNTs), and then the solvent was evaporated at room temperature. The resulting mixture in a quartz tube was heated under a flow of H2/Ar (10% H2) to reduce the Pt ions and to remove its sulfur content. The heating was carried out by ramping from room temperature to 350  C with a heating rate of 5  C min
1 and holding at 350  C for 3 h, followed by quenching under Ar during a period of 1 h. The resulting electrocatalysts prepared at different S/CNT wt. ratios are denoted as Pt/CNT-x, where x stands for the ratio used in the syntheses.

2.3.

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was formed on the electrode surface. Then, the electrode was subjected to potential cycling in deaerated 0.5 M H2SO4 solution between
0.25 and þ1.2 V at a sweep rate of 50 mV s
1 until a stable voltammogram was observed. Adsorption/desorption of underpotentially deposited hydrogen was used to evaluate the active surface area of the Pt catalyst, assuming a value of 2100 mC m
2 of polycrystalline Pt. Cyclic voltammograms (CVs) for methanol oxidation were recorded from 0.0 to 1.0 V at a scan rate of 20 mV s
1 in deaerated 1.0 M CH3OH þ 0.5 M H2SO4 solution.

Physicochemical characterization

The morphologies and size of the catalyst nanoparticles were characterized by TEM on Philips/FEI CM200 operating at 200 kV. Sample preparation for TEM examination involved the ultrasonic dispersion of the sample in ethanol and placing a drop of the suspension on a copper grid covered with perforated carbon film. The mean particle size and distribution were obtained from randomly chosen areas in the TEM images containing about 200e300 particles. High resolution TEM (HRTEM) images were recorded with a JEOL JEM 3010 instrument operating at an accelerating voltage of 300 kV. The XRD analysis of the samples was performed with a Bruker D8 Advance powder diffractometer using Ni filtered ˚ ). X-ray photoelectron spectrosCu-Ka radiation (l ¼ 1.54056 A copy (XPS) analysis of the Pt/CNT-0.3 catalyst was carried out on a VG Microtech Twin anode XR3E2 X-ray source and a concentric hemispherical analyzer operated at a base pressure of 5  10
10 mbar using Al Ka (hn ¼ 1486.6 eV). The experimental data were curve fitted with GaussianeLorentzian combination peaks after subtracting a linear background. Binding energies were referenced to C 1s peak at 284.5 eV. Lowresolution survey spectra, as well as higher resolution spectra for C, S, and Pt were collected. The exact Pt loading of the Pt/CNT catalyst prepared at S/ CNT wt. ratio of 0.3 was determined by inductively coupled plasma optical emission spectrometry (ICP-OES; Optima 7300DV, PerkineElmer) after ashing the catalyst at 850  C and then dissolving the ash in hot aqua regia. The residual sulfur content of the catalyst was determined using an elemental analyzer (CHNS-932, LECO). Electrochemical measurements were performed on an Autolab PGSTAT-101 instrument (Eco Chemie BV, the Netherland) using NOVA 1.5 software in a conventional onecompartment three electrode glass cell with a glassy carbon working electrode (0.385 cm2), a Pt gauze counter electrode and a double-junction Ag/AgCl (sat’d KCl) reference electrode. The salt bridge was daily filled with 0.5 M H2SO4 solution. The GC disk electrode was polished to a mirror finish with 0.5 and 0.05 mm alumina pastes, respectively, and sonicated in acetone/water (50/50, v/v) for 5 min. In a typical preparation of the catalyst ink, 5 mg Pt/CNTs, 25 mg Nafion solution, 800 mL absolute ethanol and 200 mL water were added to a vial, and sonicated for 1 h. For comparison, a catalyst ink was also prepared with a commercially available 20 wt.% Pt/C (AlfaAesar). Then, 20 mL of the catalyst mixture was dripped onto the GC surface to yield a nominal Pt loading of 20 mg on the working electrode (corresponding to 52 mg cm
2 geometric electrode area). After drying the ink under infrared lamp, a smooth film

3.

Results and discussion

3.1.

Physicochemical characterization

Fig. 1 shows a typical XRD pattern for the Pt/CNT-0.3. The peak around 26 corresponds to the (002) planes of graphitized carbon. Diffraction peaks located at 2q values of 39.6, 46.3, 67.4 and 81.4 are assigned to (111), (200), (220), and (311) planes, respectively, characteristic of face-centered-cubic (fcc) crystallographic structure typical of Pt. There are no characteristic peaks of elemental sulfur (2q values of 31.5, 34.5, 37.5, and 51.5 ) or PtxSy (normally located at 47.5, 52.0 and 61.8 ). However, these may be present in small amounts below the detection limit of the instrument or even in amorphous forms. The exact Pt loading of the catalyst, as determined by ICP, was 22.4 wt.%. This catalyst also showed a residual sulfur concentration of 1.66 wt.%, as determined by the CHNS analyzer, indicating an S/ CNT wt. ratio of 0.022 compared to the initial value of 0.3, i.e., ca. 93% of the sulfur has been removed during the heat treatment step under hydrogen. The average crystallite size of Pt in Pt/CNT-x electrocatalysts synthesized with different S/CNT wt. ratios was estimated from full width at half maximum (FWHM) of Pt (220) diffraction peak at 2q ¼ 67.4 , which is completely isolated from the CNT as well as other Pt diffraction peaks, Table 1. The broad peaks observed for the Pt/CNT-x samples in Fig. 2 indicate that the diffracting phases are nanocrystalline in nature. Furthermore, it can be seen that the line broadening of the Pt (220) peak in different Pt/CNT-x samples change with the S/CNT ratio. The average crystallite sizes of Pt in Pt/CNT-x

Fig. 1 e XRD spectrum of Pt/CNT catalyst synthesized on sulfur-modified CNTs at S/CNT wt. ratio of 0.3.

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Table 1 e Parameters obtained for Pt/CNT catalysts synthesized with sulfur-modified CNTs at different S/CNT wt. ratios, and commercial Pt/C. Pt/CNTs catalysts

Crystallite Size of Pt (nm)

QH (mC)

EASA (m2 g
1)

Specific activity (mA cm
2)

Mass activity (A g
1)

8.6 3.1 3.0 2.8 3.3 3.6 3.8a

0.41 3.21 3.44 4.18 2.48 2.30 2.41

8.6 67.9 72.8 88.4 52.4 48.8 57.5

7.8 35.9 45.9 50.4 30.7 27.0 19.8

133.2 614.7 784.9 862.8 524.6 461.3 381.2

Pt/CNT-0 Pt/CNT-0.1 Pt/CNT-0.2 Pt/CNT-0.3 Pt/CNT-0.5 Pt/CNT-0.7 Pt/C-Alfa Aeser a From Ref. [53].

catalysts synthesized with S/CNT ratios of 0, 0.1, 0.2, 0.3, 0.5, and 0.7, were 8.6, 3.1, 3.0, 2.8, 3.3, and 3.6 nm, respectively, as calculated from Scherrer formula, Eq. (1): d¼

0:9lKa1 B2q cosqmax

(1)

where d is the average size of Pt crystallites, lka1 is the X-ray ˚ ), qmax is the Bragg angle at the (220) wavelength (1.54056 A peak maximum in radians and B2q is the FWHM of the diffraction peak. Comparison of the crystallite size of Pt in Pt/CNT0 (synthesized in the absence of sulfur) with other Pt/CNT-x catalysts clearly indicate that the presence of sulfur significantly affects the crystallite size of Pt on CNTs. The crystallite size of Pt decreases by increasing S/CNT ratio up to a ratio of 0.3 and increases again at higher ratios. Fig. 3 shows typical TEM and HRTEM images of the Pt/CNTs synthesized at S/CNT ratios of 0, 0.3 and 0.7. For the catalyst synthesized on pristine CNTs, Fig. 3a, Pt nanoparticles are apparently agglomerated and not well dispersed on the CNT surfaces. The significant agglomeration and formation of Pt clusters in composites prepared with pristine CNTs can be related to the hydrophobic and chemically inert nature of the nanotubes arose from their relatively perfect graphene sheet structure, which hindered the high dispersion of Pt with uniform size distribution using the conventional impregnation method. It is worthwhile to point out that pristine CNTs do not possess a high number of functional groups on their surface and

Intensity

a b c d e f 62

64

66

68

70

72

2q / degree

Fig. 2 e XRD pattern of Pt (220) peak of Pt/CNT catalysts synthesized at S/CNT wt. ratios of (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.5 and (f) 0.7.

mainly surface defects such as vacancies or pentagoneheptagon pairs (Stone-Wales defects) formed during their synthesis, which results in locally enhanced chemical reactivity of the graphitic nanostructures, can serve as anchoring sites for metals [14,20,23,39]. This causes Pt to have a very low nucleation density. The presence of relatively large Pt particles and aggregates certainly resulted from aggregation of smaller Pt nanoparticles during the heat treatment of the composite under H2/ Ar atmosphere, most likely because of a low concentration of widely separated nucleation sites and instability of Pt nanoparticles on untreated CNTs. These results signify that modifying the surface of CNTs is an essential prerequisite for creating surface active sites and better dispersing of metal nanoparticles. A significantly enhanced Pt nucleation density is observed for the Pt/CNTs synthesized on sulfur-modified CNTs, where Pt nanoparticles have smaller size and more uniform dispersion, as shown by the TEM images in Fig. 3b and (d) for S/CNT ratio of 0.3. It appears that sulfur generates Pt anchoring sites on the inert surface of CNTs and enables high dispersion of Pt nanoparticles. Comparison of Fig. 3b and (c), corresponding to S/CNT ratios of 0.3 and 0.7, respectively, reveals that high S/CNT ratio also leads to some aggregation of Pt nanoparticles on the substrate. These observations are in good agreement with the XRD patterns shown in Fig. 2 and the corresponding crystallite sizes obtained from the line broadenings. It seems that a thin layer of sulfur on CNTs is adequate to act as a mediator between the metal nanoparticles and CNTs, and to restrict their particle growth during reduction. At high sulfur loading, where the distance between Pt species and CNTs increases, Pt nanoparticles may not readily approach CNTs during the heat treatment step, and increased metalemetal interactions results in partial agglomeration of Pt nanoparticles. This behavior can be seen in Fig. 3c for Pt/CNT-0.7. The histograms of Pt particle size distribution based on measuring 200e300 randomly chosen particles in the magnified TEM images showed that both the mean and the standard deviation of the observed diameters decrease with increasing S/ CNT ratio from 0 to 0.3, and increase again at higher ratios. Fig. 4 shows typical histograms for Pt/CNTs with S/CNT ratios of 0.3, 0.7 and 0.0. The average particle sizes for these catalysts were 2.7  0.7, 4.0  0.9, and 8.0  1.7 nm, respectively, which are in good agreement with those of XRD (see Section 3.1 and Table 1). The XPS analysis has been performed on Pt/CNT-0.3 catalyst to obtain information about the oxidation state of platinum nanoparticles and their interaction with residual sulfur. The main peaks observed in the survey scan (Fig. 5a) are Pt 4f,

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Fig. 3 e TEM images of the Pt/CNT catalysts synthesized at S/CNT ratios of (a) 0.0, (b) 0.3 and (c) 0.7. Fig. 3(d) shows HRTEM images of the catalyst with S/CNT ratio of 0.3 at two different magnifications.

Fig. 4 e Histograms of particle size distribution of Pt nanoparticles for the catalysts synthesized at S/CNT ratios of 0.0, 0.3 and 0.7.

S 2p, C 1s, and O 1s peaks. The Pt 4f spectrum (Fig. 5b) exhibits a broad band that can be curve-fitted into two pairs of peaks, both of which have a spin-orbit splitting of 4f7/2 and 4f5/2 states of ca. 3.3 eV and relative peak area close to the expected ratio of 4:3. The majority is present at binding energy (BE) of 71.4 eV (76.5%) together with the 4f5/2 component at 74.7 eV, correspond to Pt in the zero-valent state [34]. This Pt 4f peak shifted toward a higher BE compared with bulk Pt (71.1e71.2 eV [40,41]), which can be elucidated in terms of final state relaxation (paticle size effect) [17,42]. The 71.4 eV peak is assigned to Pt nanoparticles directly attached to the CNTs [34]. The minor peak at 72.6 eV (23.5%), with the 4f5/2 component at 75.9 eV, is attributed to Ptdþ species and can be assigned to the Pt bonded to S, in agreement with previous reports [34,43,44]. We eliminate the possibility that the 72.6 eV peak originated from Pt oxide species, because the observed BE is lower than the values reported for Pt oxides (PtO and PtO2) on partially oxidized nanoparticles [43]. Fig. 5c shows the high-resolution XPS spectrum of S 2p, which was fitted for S 2p3/2 and S 2p1/2 doublet with spin-orbit splitting of 1.2 eV and the intensity ratio of 2:1 [35]. The S 2p3/2

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Fig. 5 e XPS spectra of Pt/CNT-0.3 catalyst. (a) Survey scan of the spectral region from 0 to 1000 eV; (b) Pt 4f region; (C) S 2p region; (d) C 1s region. The spectra were obtained by calibration based on C 1s peak at 284.5 eV.

peak is located at 163.2 eV, revealing a shift of w0.8 eV with respect to the BE reported for elemental sulfur (w164 eV) [40,41]. A negative shift of about 1.5 eV has been reported for the S 2p peak in self-assembled monolayers (SAMs) of thiols on platinum, where one electron is transferred from the metal to S [45,46]. The observed S 2p peak in Fig. 5c indicates that there is relatively less charge transfer than in the case of the PteS SAMs, suggesting the formation of partial chemical bond between the residual sulfur and some of the Pt atoms. Therefore, the positive and negative shifts of BEs for Pt 4f and S 2p, respectively, can be related to the charge transfer from Pt nanoparticles to S atoms to form Ptdþ-Sd- chemical bond in a PteS-CNT arrangement. Although sulfur is generally recognized to be a surface poison that decreases electrocatalytic activity of Pt catalyst, as will be seen in Section 3.2, no such effect is noticed for the synthesized catalysts in methanol oxidation experiments. As reported by Kim and Mitani [17], sulfur does not exert poisonous effect when there is strong adhesion of the sulfur-containing moiety to the CNT. Moreover, as reported by Swider and Rolison [47], sulfur in the immediate environment of Pt is catalytically oxidized to sulfate during the standard preparation of the electrodes in fuel cells and, thus, does not poison the Pt electrocatalyst. Fig. 5d shows high-resolution C 1s XPS spectrum around 285 eV. The principal deconvoluted component at 284.5 eV (74.6%) is unambiguously assigned to the C 1s of graphitic

carbon, corresponding to non-functionalized CNT support. Besides the main peak, the peak with BE of 285.8 eV (15.3%) can be attributed to oxygen atom bound to the carbon by single bond and the two peaks at BEs of 287.1 and 288.8 eV (7.6 and 2.4%, respectively) are a consequence of functional groups wherein the oxygen is attached to the carbon by double bond [48,49]. The BE of carbons associated with residual sulfur can not be distinguished from those of carbonecarbon skeletons because of the similarity in electronegativity between sulfur and carbon [50].

3.2.

Electrochemical characterization

The electrochemically active surface area (EASA) of the fuel cell catalysts could be considered as a measure of their electrocatalytic activity towards such reactions as oxidation of hydrogen, methanol or other small organic molecules, and reduction of oxygen [51]. The EASA is directly related to the particle size and degree of dispersion of the catalyst. Aggregation of the nanoparticles certainly results in a decrease of the EASA, and thus smaller mass electrocatalytic activity than otherwise anticipated. For obtaining the EASAs of different Pt/ CNTs, CVs of the catalysts were recorded in deaerated 0.5 M H2SO4 solution, and the results are shown in Fig. 6. The CV features are characteristics of polycrystalline Pt catalysts in acidic electrolytes. A couple of broad oxidation-reduction

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2

I / mA

1 0 Pt/CNT-0 Pt/C com. Pt/CNT-0.7 Pt/CNT-0.5 Pt/CNT-0.1 Pt/CNT-0.2 Pt/CNT-0.3

-1 -2

0.0

0.4

0.8

1.2

E/V

Fig. 6 e Cyclic voltammograms of Pt/CNT catalysts synthesized at different S/CNT wt. ratios between 0.0 and 0.7, and commercial Pt/C in deaerated 0.5 M H2SO4 solution. Scan rate: 50 mV sL1.

peaks appeared between 0.1 and
0.25 V, which originate from underpotential adsorption and desorption of a monatomic hydrogen layer on Pt surfaces, followed by sharp cathodic and anodic peaks for massive hydrogen evolution and oxidation. Thus, the charge associated with hydrogen adsorption on the Pt surface was used to estimate EASA of the Pt catalysts, using Eq. (2): EASA m2 g
1




¼

QH LPt  2100

(2)

where, LPt represents the platinum loading (g) in the electrode, QH is the charge associated with hydrogen monolayer adsorption (mC), and the value of 2100 (mC m
2) is the charge required to oxidize a monolayer of atomic hydrogen on a unit surface area of polycrystalline Pt [52]. The calculated EASA values for Pt/CNT-x catalysts, prepared with different S/CNT ratios, together with other parameters are given in Table 1. The EASA was 8.6 m2 g
1 for Pt/CNT-0, and 52.4e88.4 m2 g
1 for other Pt/CNT-x catalysts, among which Pt/CNT-0.3 represented the highest EASA value. These observations, in line with the XRD and TEM results, clearly indicate the beneficial effect of

sulfur-modified nanotubes for the preparation of Pt nanoparticles on CNTs with more uniform distribution and smaller particle size. The electrocatalytic activity of the Pt/CNTs for methanol oxidation reaction was investigated by recording CVs in deaerated 0.5 M H2SO4 þ 1 M CH3OH solution, Fig. 7. The CV features are typical of methanol oxidation on Pt/C electrocatalysts in acidic media with two anodic peaks [15], one peak in the forward sweep around 0.7 V corresponding to methanol oxidation and another one in the reverse sweep around 0.5 V due to oxidative removal of incompletely oxidized intermediates formed in the forward sweep. As can be seen in Fig. 7 and Table 1, the methanol oxidation activities of the Pt/CNT-x, increase by increasing the S/CNT ratio of the support precursor from 0.1 to 0.3, but decrease at higher ratios. The results, expressed as specific and mass activities, are very well consistent with the order of EASA values according to the data given in Table 1. It can be seen that the mass activity (A g
1 Pt) of the catalyst for methanol electrooxidation is strictly related to its degree of dispersion. Here again, Pt/CNT-0.3 shows the best performance among different catalysts. The massnormalized methanol oxidation peak current observed for this catalyst (862.8 A g
1) is w6.5 folds of that of Pt/CNT-0 (133.2 A g
1), Table 1. The corresponding values for other Pt/CNT-x catalysts are lower than that of Pt/CNT-0.3, although they are all significantly higher than that of Pt/CNT-0. For comparison, the activity of the electrode prepared with commercial Pt/C electrocatalyst was also tested and included in Fig. 7. It can be seen that the mass-normalized methanol oxidation activities of all the Pt/CNTs synthesized on sulfur-coated CNTs are significantly higher than that of Pt/C. This enhanced activity is attributed to the high dispersion of Pt nanoparticles on CNTs, and indicate the beneficial effect of using sulfur-modified CNTs as a precursor for the synthesis of Pt/CNTs Long-term stability of Pt/CNT-0.3 electrocatalyst was investigated in 1.0 M CH3OH þ 0.5 M H2SO4 solution using cyclic voltammetry method. For reference, the commercial Pt/ C electrocatalyst was also chosen in this experiment. From the results shown in Fig. 8, it can be observed that the peak currents for both electrocatalysts decrease gradually with successive scans, but the activity of Pt/CNT remains superior to that of the commercial one even after 500 scans.

Peak current / mA mg-1 Pt

1600

1200 a 800 b

400

0 0

Fig. 7 e Cyclic voltammograms of Pt/CNTs catalysts synthesized at different S/CNT wt. ratios and commercial Pt/C in deaerated 0.5 M H2SO4 D 1 M CH3OH solution. Scan rate: 20 mV sL1.

100

200

300

400

500

Cycle number

Fig. 8 e Long-term electrochemical stability test in deaerated 0.5 M H2SO4 D 1.0 M CH3OH; (a) Pt/CNT-0.3 and (b) Pt/C (Alfa Aesar). Scan rate: 50 mV sL1.

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Conclusions

The sulfur-modified CNTs generate Pt anchoring sites on the inert surface of the CNT and enables high Pt dispersion on the support surface, without damaging the intrinsic conductivity of the nanotubes that otherwise limits the overall electrocatalytic activity. Pt/CNTs with uniform distribution of Pt nanoparticles and particle size of less than 3 nm were synthesized by thermal treatment of the Pt-impregnated sulfur-modified CNTs. The dispersion and size of Pt nanoparticles in Pt/CNT nanocomposites are remarkably affected by the S/CNT ratio. Both TEM and XRD results indicate that an S/CNT ratio of 0.3 affords well dispersed Pt nanoparticles on CNTs. The Pt/CNT nanocomposite synthesized at S/CNT ratio of 0.3 shows highest electrochemically active area (EASA) and catalytic activity toward methanol oxidation. The above results clearly demonstrated the effectiveness of a relatively simple route for preparation of sulfur-modified CNTs as a precursor for the synthesis of Pt/CNTs, without the need for tedious pretreatment procedures to modify CNTs or complex equipments to achieve high dispersion of Pt nanoparticles on the support. The improved electrochemical response and facile synthesis of the Pt/CNTs make the present method promising for the preparation of fuel cell catalysts with well dispersed Pt nanoparticles.

Acknowledgements This work was financially supported by the Research Council and Center of Excellence for Catalyst and Fuel Cell of the University of Isfahan, and Iranian Nanotechnology Initiative Council.

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