Ethylenediamine-modified multiwall carbon nanotubes as a Pt catalyst support

Materials Chemistry and Physics 130 (2011) 657–664

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Ethylenediamine-modified multiwall carbon nanotubes as a Pt catalyst support Goran D. Vukovic´ a , Maja D. Obradovic´ b , Aleksandar D. Marinkovic´ a , Jelena R. Rogan a , Petar S. Uskokovic´ a , Velimir R. Radmilovic´ a,c , Sneˇzana Lj. Gojkovic´ a,∗ a b c

Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoˇseva 12, 11001 Belgrade, Serbia National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA

a r t i c l e

i n f o

Article history: Received 27 August 2010 Received in revised form 22 November 2010 Accepted 14 July 2011 Keywords: Nanostructures Electrochemical properties DTA TEM

a b s t r a c t Multi-walled carbon nanotubes (MWCNTs) were used as a support for Pt nanoparticles prepared by the microwave-assisted polyol method. The MWCNTs were pretreated by chemical oxidation (o-MWCNTs) followed by modification by ethylenediamine (eda-MWCNTs). Characterization of both oxidized and eda-modified materials by UV-spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy revealed that the modification by eda leads to (i) agglomeration of the MWCNTs, (ii) a decrease in the capacitance of the material and (iii) reduced rate of electron transfer between the MWCNTs and solution species. However, the Pt loading of Pt/o-MWCNTs was only 2 mass% while the loading of Pt/edaMWCNTs was 20 mass%. Much higher efficiency of Pt deposition on eda-MWCNTs than on o-MWCNTs was ascribed to the shift in pHpzc value of the MWCNT surface from 2.43 to 5.91 upon modification by eda. Transmission electron microscopy revealed that the mean diameter of the Pt particles in Pt/edaMWCNTs is 2.5 ± 0.5 nm and that their distribution on the support is homogenous with no evidence of pronounced particle agglomeration. Cyclic voltammetry of a Pt/eda-MWCNT thin film indicated a clean Pt surface with well-resolved peaks characteristic of polycrystalline Pt. Its electrocatalytic activity for oxygen reduction was examined and the results corresponded to the commercial Pt nanocatalyst. This study shows that modification of o-MWCNTs by eda helps to achieve homogenous distribution of small Pt nanoparticles and does not impede its electrocatalytic activity. © 2011 Elsevier B.V. All rights reserved.

1. Introduction High area carbon materials [1] are used extensively in a variety of electrochemical systems: as supporting material for nanocatalysts in fuel cells [2], as active material for electrochemical supercapacitors [3], as well as in biosensing [4,5]. In this class of material, carbon nanotubes (CNTs) are especially attractive because of their unique tubular structure, excellent chemical and thermal stability, mechanical properties and, in some cases, even metallic conductivity [6,7]. However, the strong ␲–␲ interaction between tubes leads to formation of tangled bundles [6], which makes them difficult to disperse. Several methods have been proposed to overcome this problem: oxidative treatment [8–14], polymer wrapping [15–17] and sidewall functionalization [18]. Common oxidative procedure is acid treatment in HNO3 [8–11] or in mixture of HNO3 and H2 SO4 [12–14], upon which the nanotubes are shortened, less tangled, their ends become open [10,13], traces of metal catalysts used for the MWCNT synthesis are removed [10,11] and oxygen-

∗ Corresponding author. Tel.: +381 11 3303 753; fax: +381 11 3370 387. ´ E-mail address: [email protected] (S.Lj. Gojkovic). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.07.046

containing functional groups are produced on the surface [11]. Functionalities such as carboxyl groups, lactones, phenol and lactol groups render acidic character of the CNT surface, while carbonyl groups, diketone, chromene- and pyrone-like structures contribute to the basic character [19–24]. For the application of MWCNTs as a catalyst support it is crucial that metal nanoparticles are uniformly distributed on their surface. Metal nanoparticles can be successfully deposited on oxidized MWCNTs [25,26] with oxygen containing groups acting as anchor sites. However, several authors have reported that the dispersion of metal nanoparticles is improved if previously oxidized MWCNT surface is modified by ethylenediamine (eda) [11,27], 1,4-benzendiamine [28], 4,4-dypyridine [29], 2-aminophenoxazin3-one [30] or by introducing thiol groups [31]. Modification of MWCNTs by eda was found to increase charge transfer rate, probed by the [Fe(CN)6 ]3− /[Fe(CN)6 ]4− redox couple, in comparison to mildly oxidized MWCNTs [4]. Several studies of the electrocatalytic activity of Pt nanoparticles supported on MWCNTs have shown that they are more active for methanol oxidation than the same nanoparticles supported on other high area carbons [14,32,33]. This can be attributed to the appropriate Pt nanoparticle size, good distribution on the support

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as well as to the better electrical conductance of the MWCNTs compared with the classical carbonaceous supports [14]. However, it was also reported that the carbon support has no influence on the electrocatalysts activity for methanol oxidation and oxygen reduction [34]. Evaluation of nanocatalysts supported on MWCNTs modified by various amines show that the presence of organic groups on the surface does not impede their electrochemical characteristics [11,27–29,35] and that in some cases even enhances the catalyst activity, as shown for methanol [11] and ethanol [27] oxidation on Pt–Ru nanoparticles supported on MWCNTs modified by eda. Our previous investigation has demonstrated that MWCNTs, after being exposed to the mixture of concentrated H2 SO4 and HNO3 under the ultrasonic agitation, posses charge transfer properties as good as those of bulk Au electrode [36,37]. Therefore, we supposed that MWCNTs oxidized by this procedure (o-MWCNTs) might enhance the electrocatalytic activity of Pt nanoparticles deposited on it. Since deposition of Pt nanoparticles onto oMWCNTs turned out to be low-efficient process, o-MWCNTs was modified by eda (eda-MWCNTs) and used as a catalyst support. The eda molecule was selected because it can be successfully attached to o-MWCNTs [38] and its beneficial effect to the electrocatalytic activity of Pt–Ru catalysts has recently been reported [11,27]. In the present paper the influence of eda modification of o-MWCNTs on their electrochemical behavior and the suitability for a catalyst supporting material has been studied. The electrocatalytic activity of the synthesized catalyst Pt/eda-MWCNTs toward oxygen reduction reaction has been examined and compared to the commercial catalyst. 2. Experimental 2.1. Oxidation and modification of MWCNTs manufactured by Sigma–Aldrich (o.d. × i.d. × length: MWCNTs 20–30 nm × 5–10 nm × 0.5–200 ␮m) were used in the study. Oxidation of MWCNTs was performed in concentrated H2 SO4 + HNO3 mixture (volume ratio 3:1) in an ultrasonic bath for 3 h. This oxidative treatment was shown to produce highly functionalized surface with excellent charge transfer properties [36,37]. o-MWCNTs were modified by eda applying sonification in the presence of coupling agent N-HATU, which was proved to be fast and efficient method for attaching various amines to the o-MWCNT surface [36,38]. Elemental analyses and Fourier Transform Infrared (FTIR) spectroscopy of carbon powders before and after the modification by eda verify that eda is covalently attached to the surface of MWCNTs [38].

2.2. Acid–base properties of o-MWCNT and eda-MWCNT surfaces The acidic and basic site concentrations of o-MWCNT and eda-MWCNT surfaces were determined using the Boehm titration method [19,38]. The number of acidic sites of o-MWCNTs was found to be higher than the number of basic sites, implying acidic characteristics of the surface. Modification of o-MWCNTs by eda decreased the number of acidic sites and significantly increased the number of basic sites, while total amount of all groups remained constant. The surface content of the terminal amino functions present on eda-MWCNTs is predicted with the aid of the quantitative Kaiser test [39] and found to be 0.65 mmol g−1 of free amino groups. This value is close to the increase in quantity of total basic sites upon modification. The net surface charge on a MWCNTs depends on the pH of the solution. The particle may adsorb H+ or OH− ions. Also, the functional groups populating carbon surface can be protonated at lower pH values generating positive charge to the surface, or deprotonated at higher pH values generating negative charge to the surface. The pH at which the surface has zero net charge is known as pHpzc . The pHpzc of o-MWCNTs and eda-MWCNTs were determined by the pH drift method [40,41] and values of 2.43–5.91 were found, respectively [38].

2.4. Synthesis of Pt/o-MWCNT and Pt/eda-MWCNT Pt nanoparticles have been prepared by heating of H2 PtCl6 solution (10% solution, Alfa Aesar) in ethylene glycol (Alfa Aesar) in the microwave oven (Samsung, 2450 MHz, 700 W for 60 s) [42,43]. The electromagnetic waves heat the substrate uniformly, leading to a more homogeneous nucleation and shorter aggregation time. pH of the prepared suspension of colloidal Pt nanoparticles was ∼8. At first, o-MWCNTs were added in the suspension of colloidal Pt nanoparticles and ultrasonicated over 1 h. Then the suspension was vacuum-filtered, the powder was washed with water and dried in a vacuum oven at 80 ◦ C over 4 h. The deposition of Pt on eda-MWCNTs was performed following the same procedure. In both cases the mass ratio of the support and Pt colloid was 80:20. 2.5. Thermogravimetric analysis Thermal degradation of o-MWCNTs and eda-MWCNTs as well as Pt loading of the Pt/o-MWCNT and Pt/eda-MWCNT samples were investigated by thermogravimetric analysis (TGA) [44] using a SDT Q600 TGA/DSC instrument (TA Instruments). Several milligrams of the sample were heated to 800 ◦ C at the heating rate of 20 ◦ C min−1 . The furnace atmosphere consisted of air at a flow rate of 100 cm3 min−1 . 2.6. TEM characterization High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) were used to characterize the morphology, size and distribution of Pt nanocatalyst particles along the MWCNTs. The measurements were performed at the National Center for Electron Microscopy using the TEAM 0.5 TEM/STEM double-aberration-corrected electron microscope at 80 kV equipped with the Gatan 2k × 2k CCD cameras. Particle size distribution was determined from images of, on average, 20 different regions of the catalyst; each region contained 15–50 particles. The particle shapes were determined by real space crystallography with the use of high-resolution images taken from particles attached to the MWCNTs. 2.7. Electrochemical characterization For electrochemical characterization, the MWCNT or Pt/MWCNT powders were applied on a gold substrate in the form of a thin-film [45]. A suspension was made by mixing of 2.0 mg of the powder with 1 cm3 of high purity water (Millipore, 18 M cm resistivity) and 50 ␮L of the Nafion® solution (5 wt.%, 1100 E.W., Aldrich). After 1 h of agitation in an ultrasonic bath, 10 ␮L of the suspension was placed onto the Au electrode (Tacussel rotating disk electrode, 5 mm in diameter) and left to dry overnight. This procedure of film preparation gave 0.10 mg of powder per cm2 of the Au surface. A three-compartment electrochemical glass cell was used with a Pt wire as the counter electrode and a saturated calomel electrode as the reference electrode. All the potentials reported in the paper are expressed on the scale of the reversible hydrogen electrode (RHE). The cyclic voltammetry was carried out in 0.1 M H2 SO4 (Merck), in 1 M KCl (Lach-ner) and in 1 M KCl with the addition of 5 mM K4 [Fe(CN)6 ] (Merck). Upon immersion into the electrolyte, a thin-film carbon electrode was preconditioned by the potential cycling between 0.3 and 1.15 V for 10 cycles at 0.1 V s−1 [46]. In the case of Pt nanocatalyst, the electrode was pre-conditioned by the potential cycling between 0.03 and 1.25 V at 0.1 V s−1 till steady state cyclic voltammogram was obtained. The electrolytes were deaerated by the N2 bubbling. All the experiments were conducted at 298 ± 0.5 K. A Pine RDE4 potentiostat and Philips PM 8143 X-Y recorder were employed. Electrochemical impedance spectroscopy (EIS) experiments were performed in 1 M KCl at the potential of 1.0 V. This potential was chosen because it corresponds to the minimal pseudocapacitance current, as indicated by cyclic voltammmetry. Instrumentation involved Gamry Instruments Femtostat, model FAS32, guided by Gamry Framework software. The working electrode responded to the potential input sinusoidal signal of ±10 mV (rms) amplitude in the frequency range from 0.1 Hz to 150 kHz. Prior to starting the frequency scan, the electrode was pre-conditioned by the potential cycling and then held at 1.0 V until the current attained a stationary value. This was usually below 0.1 ␮A. Electrocatalytic properties of Pt/MWCNTs were tested by examining oxygen reduction in 0.1 M H2 SO4 solution saturated by O2 . After reaching steady-state voltammogram in N2 saturated solution, O2 was started to bubble through solution while the electrode potential was cycled continuously from 0.03 to 1.25 V. Upon saturation of the electrolyte by O2 , polarization curve for O2 reduction was recorded with the scan rate of 20 mV s−1 . The same experiment was performed with Pt/XC-72R (20 mass% of Pt) manufactured by E-Tek, which served as a reference catalyst.

3. Results and discussion 2.3. UV spectroscopy of MWCNT surface Stability of water suspensions of carbon powders was investigated by UV spectroscopy. The suspensions of the concentration of 100 ␮g cm−3 were prepared by ultrasonic agitation over 1 h. Four hours later the spectra of the suspensions were recorded on a Shimadzu UV1700 spectrometer.

3.1. UV spectroscopy of MWCNT suspensions Optical spectrum of the suspension in visible and UV region can be used for the comparison of the relative size of the particles or

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Fig. 1. TEM images of (a) o-MWCNT and (b) eda-MWCNT samples.

agglomerates in the suspensions of solid materials, where higher absorbance means smaller particles [47,48]. We used this technique to assess possible agglomeration of o-MWCNTs after the modification by eda. The absorbance of the water suspensions taken at 600 nm were 1.022 and 0.567 for o-MWCNTs and eda-MWCNTs, respectively. A decreased absorbance for eda-MWCNT suspension indicates formation of the agglomerates caused by the interactions between amine and/or oxygen containing functionalities, either from the same or from the different nanotubes. Besides, o-MWCNTs bear a negative charge due to dissociation of some oxygen containing groups introduced during the oxidative treatment. Modification of o-MWCNTs by eda neutralizes this charge, diminishing repulsion between eda-MWCNTs and favoring their agglomeration.

3.2. TEM characterization of o-MWCNTs and eda-MWCNTs In order to get further insight into agglomeration of o-MWCNTs after modification by eda, low magnification TEM images of oMWCNT and eda-MWCNT samples were recorded. Comparison of Fig. 1a and b shows that eda-MWCNTs are more entangled than o-MWCNTs, as indicated by the absorbance of the corresponding suspensions. However, the agglomeration provoked by the attachment of eda on o-MWCNT surface is not severe, thus eda-MWCNTs can presumably be used as a catalyst support.

3.3. Cyclic voltammetry and EIS of MWCNTs Cyclic voltammetry of o-MWCNTs and eda-MWCNTs was performed in 0.1 M H2 SO4 solution. By dividing the voltammetric currents with the potential scan rate and the mass of carbon placed onto the electrode, the specific capacitance values of the materials were obtained. Therefore, steady-state cyclic voltammograms of o-MWCNTs and eda-MWCNTs are presented in Fig. 2 as specific capacitance vs. electrode potential. The voltammogram for oMWCNTs is characterized by broad quasi-reversible peaks around 0.6 V, commonly attributed to a surface redox reaction of oxygen containing groups. As previous FTIR measurements revealed, a wealth of oxygen containing functional groups was introduced onto the MWCNT surface by chemical oxidation [37]. Although many of them can be electrochemically active, Fuente et al. [20] and Andreas and Conway [24] assigned the peaks at ∼0.6 V to the

Fig. 2. Cyclic voltammograms in the form of specific capacitance vs. electrode potential of o-MWCNTs and eda-MWCNTs recorded in deaerated 0.1 M H2 SO4 at the sweep rate of 100 mV s−1 .

redox reaction of quinones attached to the edges of graphene layers, i.e. pyrone-like groups:

(1) After modification of o-MWCNTs by eda, the pair of quasi-reversible peaks disappeared from the voltammogram (eda-MWCNT sample in Fig. 1). This implies that eda is attached to o-MWCNTs via oxygen containing group contributing to the peaks, i.e. via pyrone groups. A possible mechanism is a nucleophilic attack of amine to the electrophilic C C sites attached to carbonyl group at the periphery of the graphene layers. The reaction was assigned to Michael-like reactions and can be presented by the scheme [49–51]:

(2) where Z represents electron withdrawing groups on the carbon surface such as carboxyl or carbonyl. Reaction (2) shows that

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modification by amine leads to reduction of electrophilic C C sites making carbonyl group arranged at the periphery of the graphene layers electrochemically inactive. Another reaction being proposed to explain anchoring of amines to the oxidized carbon surface is the reaction with –COOH group [52]:

(3) Suppression of the redox peaks on the potentiodynamic curve for o-MWCNTs after modification by amine suggests that these peaks are due to the reaction

(4) Although electrochemical oxidation of carboxylic group, with the formation of physisorbed CO2 , certainly occurs [1], it is not reversible reaction, thus it cannot produce a pair of reversible voltammetric peaks. This supports the assumption about the involvement of the double bond (Eq. (2)) in eda modification of o-MWCNTs. The presence of terminal amino group has been confirmed on the eda-MWCNT surface by FTIR and Kaiser test [36,38]. This functionality is reported to be electrochemically active undergoing oxidation reaction [53]. However, cyclic voltammogram of edaMWCNTs is featureless, meaning that amino group is not oxidized within the potential limits employed in our experiments. This is probably caused by the protonation of amino group in acid media. Being protonated, amino group is not susceptible to the electrochemical oxidation in 0.1 M H2 SO4 . Amine oxidation could lead to the polymerization and formation of C–N [49] or C–C [53] bond. However, 10 successive cyclic voltammograms of eda-MWCNTs in the potential range from 0.05 to 1.15 V (Fig. 1) were stable implying that the polymerization reaction does not occur. The influence of eda modification on the capacitance of MWCNTs was also examined by the EIS. In Fig. 3 the EIS spectra of o-MWCNTs and eda-MWCNTs in 1 M KCl are presented in the form of the Nyquist and Bode plots. The Nyquist plot shows that both of the carbon samples display similar high-frequency loops, which can roughly be ascribed to the electrical characteristics of the film as a whole [37], while at lower frequencies the increase of imaginary part of impedance is slower for eda-MWCNTs than for o-MWCNTs. From the imaginary part of the complex impedance displayed in Fig. 3, the apparent capacitance was calculated according to: C=−

1 ω · Im(Z)

(5)

and plotted as a function of frequency in inset of Fig. 3. The diagram shows that the apparent capacitance of o-MWCNTs, which tends to reach a constant value at the frequencies below 50 Hz, is decreased by eda modification. This effect was also registered by cyclic voltammetry (Fig. 2), but in less extent. Cyclic voltammetry reflects primarily the behavior of the surface of the MWCNT thin film facing the electrolyte, while the capacitance determined from EIS is influenced by the properties of the internal parts of the film as well [37]. Therefore, lower capacitance of the eda-MWCNT film is to be attributed to the limited electrolyte access to the internal parts of the thin film induced by agglomeration of MWCNTs upon eda modification. The reduced capacitance of carbon fibers after electrochemical modification by eda has already been reported [49].Kinetics of Fe(CN)6 3− /Fe(CN)6 4− on MWCNTs

Fig. 3. Nyquist plots for the thin layer of o-MWCNTs and eda-MWCNTs in deaerated 1 M KCl. The solution resistance is subtracted. Inset: Bode plot – apparent capacitance as the function of frequency.

The influence of eda modification on the electron-transfer properties of MWCNTs was probed by simple redox transition of Fe(CN)6 3− /Fe(CN)6 4− couple in 1 M KCl + 5.0 mM K4 [Fe(CN)6 ]. Instead of well-defined redox peaks of the oxygen containing groups on the voltammogram of o-MWCNTs recorded in 0.1 M H2 SO4 (Fig. 2), in 1 M KCl (pH ∼5.5) these processes are manifested as the shoulders at the potentials around 0.6 V (Fig. 4b). The difference between the voltammograms in the acid and near-neutral solutions is caused by the insufficient quantity of protons available at pH 5.5 for the reaction presented by Eq. (1). Modification of oMWCNTs by eda also leads to a decrease in the capacitance, as it was already observed in 0.1 M H2 SO4 (Figs. 3 and 4). As a test of the purity of the system, the voltammograms of bare Au substrate in 1 M KCl and in 1 M KCl with the addition of 5 mM K4 [Fe(CN)6 ] were recorded (Fig. 4a). The voltammetric profiles show a reversible behavior of the Fe(CN)6 3− /Fe(CN)6 4− process on Au of almost ideal peak separation, Ep , of 67 mV. The voltammograms of Fe(CN)6 3− /Fe(CN)6 4− on the thin film of o-MWCNTs (Fig. 4b) show reversible currents of Fe(CN)6 3− /Fe(CN)6 4− transition (Ep = 66 mV) superposed to the high capacitance currents. After modification of o-MWCNTs by eda (Fig. 4c), a decrease in the peak current and an increase in the peak separation to 90 mV is observed, indicating slower electron transfer process to the solution species. This confirms that eda molecules are attached to the o-MWCNT surface via oxygen containing groups, because these groups are responsible for the fast electron transfer on o-MWCNTs [37]. In the previous work [36] we investigated Fe(CN)6 3− /Fe(CN)6 4− redox process on MWCNTs modified by diethylenetriamine, triethylenetetramine and 1,6-hexanediamine and observed even more hindered electron transfer than on eda-MWCNTs. Such a behavior is most likely related to the structure of amine and its reactivity. Eda molecules have the smallest size and the smallest number of the amine functionalities, so the blockage of the oxygen containing groups is lower and interconnection between the tubes modified by eda is less pronounced than in the case of modification with more complex amines. It should be noted that Tang et al. [4] found that the kinetics of Fe(CN)6 3− /Fe(CN)6 4− redox couple on oMWCNTs were enhanced after the modification by eda. However, the oxidation medium was diluted HNO3 without H2 SO4 , which

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Fig. 4. Cyclic voltammograms of (a) bare Au, (b) o-MWCNTs and (c) eda-MWCNTs. Dashed lines present voltammograms in deaerated 1 M KCl and solid lines present voltammograms after the addition of 5 mM K4 [Fe(CN)6 ].

renders incomplete activation of MWCNTs [37], thus the charge transfer properties of o-MWCNTs used as the reference in the work by Tang et al. [4] were poor. 3.5. Thermogravimetric analysis of MWCNTs and Pt/MWCNTs Thermogravimetric analysis gives useful information about thermal stability of functional groups present on MWCNT surface and their susceptibility toward oxidative processes, but also enables determination of Pt content in Pt/MWCNT powder. TGA weight-loss curves obtained upon heating o-MWCNTs, edaMWCNTs, Pt/o-MWCNTs and Pt/eda-MWCNTs in air are displayed in Fig. 5. As seen in Fig. 5a, the weight loss curves of o-MWCNTs and eda-MWCNTs overlap below 300 ◦ C. Absorbed water and traces of the reactants used in the oxidative or modification processes are desorbed below 120 ◦ C [54,55]. These processes are followed by the decarboxylation of the carboxyl groups on the MWCNT walls occurring between 150 and 350 ◦ C [56]. Other oxygen containing groups, such as lactone and phenolic group, remain relatively unchanged and decompose at higher temperature, which explains further weight loss of o-MWCNTs [54]. At temperatures over 300 ◦ C the curve for eda-MWCNTs becomes steeper indicating that amino groups have lower thermal stability [55] than oxygen-containing groups, which are the only functionalities present on o-MWCNTs. Both MWCNT samples are completely decomposed at about 700 ◦ C. The TGA curves of Pt/o-MWCNTs and Pt/eda-MWCNTs are given in Fig. 5b. After measuring of Pt weight in the residual ash obtained at 563 ◦ C for Pt/o-MWCNTs and at 700 ◦ C for Pt/eda-MWCNTs, it was calculated that Pt/o-MWCNTs contained only 2.0 mass% of Pt, while the Pt content in Pt/eda-MWCNTs was 20 mass%.

661

Fig. 5. TGA curves for (a) o-MWCNTs and eda-MWCNTs and (b) Pt/o-MWCNTs and Pt/eda-MWCNTs, recorded in air.

Low Pt content of the Pt/o-MWCNTs is probably caused by the negative charge of Pt nanoparticles prepared in chloride solution, so the particles cannot be efficiently adsorbed on highly negatively charged o-MWCNT surface (pHpzc 2.43) [38]. The attachment of eda to the o-MWCNT shifts pHpzc to 5.91, enabling quantitative adsorption of Pt colloid particles on eda-MWCNT surface. The TGA curves in Fig. 5b show no difference in the thermal degradation of Pt/o-MWCNTs and Pt/eda-MWCNTs below 400 ◦ C. However, at higher temperatures Pt/o-MWCNTs decompose faster than Pt/eda-MWCNTs and also faster than both samples without Pt (Fig. 5a). This result points out that Pt nanoparticles facilitate the thermal decomposition of MWCNTs, but the effect is dependent on the composition of the supporting material. Although Pt/edaMWCNT sample contains 10 times more Pt than Pt/o-MWCNT sample, the catalytic effect of Pt on MWCNTs decomposition is much more exhibited in the case of Pt/o-MWCNTs. This can be explained by the thermal degradation of amines, which starts at ∼300 ◦ C, i.e. before the onset of the MWCNT oxidation catalyzed by Pt. Degradation residuals, produced at temperatures over 300 ◦ C, might adsorb on the Pt particles diminishing their catalytic effect on the MWCNT thermal degradation. 3.6. TEM characterization of Pt/eda-MWCNTs Representative HRTEM and high angle annular dark field (HAADF) images of the Pt/eda-MWCNT sample are shown in Fig. 6a and b, respectively. As seen, the Pt nanoparticles are homogenously distributed over eda-MWCNTs, with no evidence of pronounced particle agglomeration. Platinum nanoparticles in Fig. 6b appear brighter than the lower Z structure of MWCNTs and carbon film support visible in the background. The Pt particle size distribution, given in Fig. 6c, is of monomodal type with the mean Pt particle

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Fig. 6. (a) HRTEM micrograph of Pt/eda-MWCNTs with digital diffractograms of the Pt nanoparticles in corresponding white squares, showing their orientation along several zone axes, (b) HAADF image and (c) histogram of the particle size distribution.

diameter of 2.5 ± 0.5 nm. Digital diffractograms of the Pt nanoparticles showing their orientations along several zone axes are given in Fig. 6a. The presence of many different crystallographic orientations indicates that there is no preferential orientation-relationship between MWCNTs and Pt nanoparticles. The suitability of eda-MWCNTs prepared from the highly activated o-MWCNTs as the catalyst support can be demonstrated through the comparison of Pt particle size obtained for Pt/edaMWCNT sample to the similar composite materials described in the literature. The diameter of Pt nanoparticles supported on MWCNTs modified by 1,4-benzendiamine is between 2 and 5 nm [28], the PtRu nanoparticles supported on eda-MWCNTs from 10 to 15 nm large clusters with the single metal particles between 2 and 4 nm [11], while the PtRu nanoparticles supported on MWCNTs modified by 2-aminophenoxazin-3-one have diameter between 1.6 and 2.8 nm [30]. The real surface area (rsa) of the Pt nanoparticles can be determined, assuming that all of them are spherical, homogeneous and non-agglomerated, using the equation: rsa =

6 ·d

(6)

where  is the density of Pt (21.45 g cm−3 ) and d is mean diameter of the Pt nanoparticles. The rsa of Pt particles in Pt/eda-MWCNTs having 2.5 ± 0.5 nm in diameter (Fig. 6c), is calculated to be 112 ± 24 m2 g−1 . 3.7. Cyclic voltammetry of Pt/MWCNTs Cyclic voltammograms of o-MWCNTs and eda-MWCNTs before and after adsorption of Pt nanoparticles were recorded in H2 SO4 solution and the results are presented in Fig. 7. As expected from the Pt loading of these two samples determined by TGA measurements, the contribution of Pt to the voltammetric currents of Pt/o-MWCNTs is rather small (Fig. 7a), while the voltammogram

Fig. 7. Cyclic voltammograms of (a) o-MWCNTs and Pt/o-MWCNTs and (b) edaMWCNTs and Pt/eda-MWCNTs recorded in deaerated 0.1 M H2 SO4 at 100 mV s−1 .

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of Pt/eda-MWCNTs clearly shows the presence of Pt (Fig. 7b). Although the supporting material in Pt/eda-MWCNTs contains amine grafted to the surface of MWCNTs, no significant contamination of the Pt surface is indicated by the voltammogram. The peaks for hydrogen adsorption/desorption are well resolved and formation and reduction of Pt oxides are clearly seen. The electrochemically active surface area (ecsa) of Pt in Pt/edaMWCNT sample was determined from the hydrogen desorption charge after the correction for the double-layer charging, Q(HUPD ), and assuming 210 ␮C cm−2 for the monolayer of adsorbed hydrogen. Hence, using the equation: ecsa =

Q (HUPD ) 210 ␮C cm−2 · w(Pt) · mc

(7)

where w(Pt) is the Pt loading of Pt/eda-MWCNTs and mc is mass of the Pt/eda-MWCNTs on the electrode, the ecsa of Pt in Pt/edaMWCNTs was calculated to be 52 m2 g−1 . The ratio of esca and rsa, termed as Pt utilization efficiency (Pt ), reveals which portion of Pt surface is available for the electrochemical reaction. The results for Pt/eda-MWCNTs show that Pt is 0.46 ± 0.10. This is lower than 0.61 reported for the commercial Pt/XC-72 of the same mean Pt particle size [57], but the difference is reasonable having in mind the presence of organic adsorbates on the eda-MWCNT surface. The similar effect of the MWCNT modifiers on the Pt has been recently reported in the literature. Pt nanoparticles of 3.2 nm in diameter supported on MWCNTs wrapped by pyridine-containing polybenzimidazole exhibited the ecsa of 51.6 m2 g−1 and Pt of 0.59 [58]. Surprisingly high Pt was determined for Pt supported on MWCNTs wrapped by polybenzimidazole [59]. The Pt particle diameter in this material was ∼4 nm and Pt of 0.74 was reported, which is even higher than Pt for Pt/XC-72. According to these literature data, it seems that larger Pt nanoparticles have higher Pt , even in the presence of the modifiers of the supporting MWCNTs. 3.8. Oxygen reduction on Pt/eda-MWCNTs Electrocatalytic activity of Pt/eda-MWCNTs was probed by the oxygen reduction reaction and the results were compared to the commercial Pt/XC-72R catalyst. The polarization curves were recorded in 0.1 M H2 SO4 by linear sweep of 20 mV s−1 going from the limiting current plateau toward the open circuit potential. After subtraction of the background current determined in N2 saturated solution, the currents were corrected for the diffusion effects using the equation: Ikin =

I · IL IL − I

(8)

where I, IL and Ikin are measured current, limiting current, and kinetic currents. Then the kinetic currents were normalized per electrochemically active Pt surface area and plotted vs. potential in a form of Tafel diagram presented in Fig. 8. As seen, the oxygen reduction commences at ∼1.02 V on both catalyst and exhibits the Tafel slopes of ∼60 mV dec−1 at low and ∼120 mV dec−1 at high current densities, which is typical for this reaction at Pt surfaces [60]. Although the activity of Pt/eda-MWCNTs is slightly lower than that of the commercial catalyst, the current densities in both sets of data are within the values given as a benchmark of Pt activity for oxygen reduction in the review paper by Gasteiger et al. [61]. Namely, the authors quoted 0.19–0.22 mA cm−2 at 0.90 V measured on several different Pt/C catalysts at 60 ◦ C. The current densities determined in our experiments at the same potential but at 25 ◦ C are 0.082 and 0.10 mA cm−2 for Pt/eda-MWCNTs and Pt/XC-72R, respectively. Taking 21 kJ mol−1 as the activation energy [62], the current density at 60 ◦ C would be 0.19 mA cm−2 on Pt/eda-MWCNTs and 0.23 mA cm−2 on Pt/XC-72R. Besides oxygen reduction reaction, we

Fig. 8. Polarization curves for oxygen reduction on Pt/eda-MWCNTs and Pt/XC recorded in 0.10 M H2 SO4 by sweep rate 20 mV s−1 at the RDE rotating at 1600 rpm. Data corrected for diffusion effects and normalized per Pt surface area.

recorded polarization curves for methanol oxidation and formic acid oxidation on the same catalysts (not shown) and difference between them were within the experimental error. It can be concluded that the electrocatalytic activity of Pt/edaMWCNTs is high and similar to the commercial nanocatalysts supported on high area carbon. The modification of o-MWCNTs by eda does not improve the activity of Pt nanoparticles for the electrochemical reaction, but this procedure is necessary for achieving high loading of Pt on the o-MWCNT support. 4. Conclusions The comparative study of chemically oxidized MWCNTs, before and after modification by eda, revealed that the modification leads to (i) agglomeration of the MWCNTs, (ii) decrease in the capacitance of the material, (iii) reduced rate of electron transfer between the MWCNTs and solution species and (iv) shift of the pHpzc value from 2.43 to 5.91. Although the first three effects of modification impair the characteristics of MWCNTs as an electrode material, the increase in pHpzc value enables its application as a support of Pt nanoparticles. The amount of Pt nanoparticles, synthesized by the microwave-assisted polyol method, was found to be 10 times higher when supported on eda-MWCNTs than on oMWCNTs. The investigation of Pt/eda-MWCNTs by TEM revealed that the mean diameter of the Pt particles is 2.5 ± 0.5 nm and that their distribution on eda-MWCNTs is homogenous with no evidence of pronounced particles agglomeration. Cyclic voltammetry of Pt/eda-MWCNT indicated a clean Pt surface with well-resolved peaks characteristic for polycrystalline Pt. Electrochemically active surface area of Pt in Pt/eda-MWCNTs was determined to be 52 m2 g−1 Pt, corresponding to the Pt utilization efficiency of 0.46 ± 0.10. The Pt/eda-MWCNT was tested as an electrocatalyst for oxygen reduction and its activity matches the commercial catalysts. Acknowledgements This work was financially supported by the Ministry of Science of Republic of Serbia, Contract No. ON172054 and III45019. G.D.V. and V.R.R. also acknowledge support of FP7 NANOTECH FTM – 245916. Electron Microscopy characterization was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Contract No. DE-AC0205CH11231.

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