pubs.acs.org/Langmuir © 2009 American Chemical Society
Improvement of the Platinum Nanoparticles-Carbon Substrate Interaction by Insertion of a Thiophenol Molecular Bridge Patrick Urchaga,† Martin Weissmann,† Steve Baranton,*,† Thierry Girardeau,‡ and Christophe Coutanceau† †
Laboratoire de Catalyse en Chimie Organique, Equipe Electrocatalyse, UMR 6503 CNRS, 40 avenue du recteur Pineau, F-86022 Poitiers Cedex, France, and ‡Laboratoire de M etallurgie Physique, Universit e de Poitiers, SP2MI, Bd M. et P. Curie, BP 30179, 86962 Futuroscope Chasseneuil Cedex, France Received January 9, 2009. Revised Manuscript Received March 27, 2009
The effect of thiophenol layer grafted on carbon for platinum catalyst stabilization was studied. The grafted layer was prepared by reduction of 4-thiophenoldiazonium ions in the presence of Vulcan XC72 substrate. The grafted layer was characterized by elemental analysis, thermogravimetric analysis coupled with mass spectrometry, and X-ray photoelectron spectroscopy. Platinum nanoparticles prepared by the “water in oil” microemulsion method were then deposited on modified substrates and bare Vulcan XC72. The platinum stability improvement was characterized by in situ X-ray diffraction and electrochemical aging. These experiments enabled to evidence a lower crystallite growth during heat treatment under hydrogen atmosphere and a lower active surface area loss for platinum particles deposited on modified substrates compared to those deposited on bare Vulcan XC72. This stability improvement can be attributed to a better interaction between platinum particles and carbon substrate due to the thiophenol molecular bridge.
1. Introduction Because of excellent electrical properties, good chemical and electrochemical stability, and a wide range of designs, high surface carbon substrates (powders or nanotubes) are widely used as catalyst supports, particularly for fuel cell electrocatalyst dispersion.1-4 However, the mobility of catalyst nanoparticles on carbon substrates leads to a decrease of their activity and the long-term stability of this catalyst-substrate system is not guaranteed.5-7 In order to improve carbon substrate properties, several surface modification methods were developed, like carbon substrate oxidation by nitric acid,8 grafting of a molecular layer by oxidation of amine compounds,9 or reduction of diazonium cations.10-13 This last modification method enables to obtain phenyl-substituted groups layers covalently grafted to carbon substrate. Moreover, since it is a reduction reaction, it is a less destructive method for the substrate. The nature of the interaction between support and aryl groups grafted layer was discussed in *To whom correspondence should be addressed. E-mail: steve.baranton@ univ-poitiers.fr. (1) Coutanceau, C.; Brimaud, S.; Lamy, C.; Leger, J. M.; Dubau, L.; Rousseau, S.; Vigier, F. Electrochim. Acta 2008, 53, 6865–6880. (2) Antolini, E. Appl. Catal., B 2008, . (3) Charreteur, F.; Ruggeri, S.; Jaouen, F.; Dodelet, J. P. Electrochim. Acta 2008, 53, 6881–6889. (4) Yano, H.; Kataoka, M.; Yamashita, H.; Uchida, H.; Watanabe, M. Langmuir 2007, 23, 6438–6445. (5) Wilson, M. S.; Garzon, F. H.; Sickafus, K. E.; Gottesfeld, S. J. Electrochem. Soc. 1993, 140, 2872–2877. (6) Taniguchi, A.; Akita, T.; Yasuda, K.; Miyazaki, Y. J. Power Sources 2004, 130, 42–49. (7) Shao, Y.; Yin, G.; Gao, Y. J. Power Sources 2007, 171, 558–566. (8) Guha, A.; Lu, W.; Zawodzinski, T. A.; Schiraldi, D. A. Carbon 2007, 45, 1506–1517. (9) Barbier, B.; Pinson, J.; Desarot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137, 1757–1764. (10) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883–5884. (11) Downard, A. J. Langmuir 2000, 16, 9680–9682. (12) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534–6540. (13) Liu, Y. C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254–11259.
Langmuir 2009, 25(11), 6543–6550
several articles.14-17 A grafting mechanism of aryl groups involving a π adsorption of molecules was discarded by AFM study on modified HOPG14 which evidenced that the derivatization occurs preferentially on edges and defects of the basal plane. Furthermore, XPS analysis on modified iron substrates15,16 has established the presence of carbide component at 283.3 eV. Experiments led on carbon nanotubes by Raman and NIR spectroscopy17 have shown a loss of the sp2 structure of nanotubes during spontaneous modification, which indicated an opening of the π system to generate a bond between the substrate and aryl grafted groups. These different observations led to conclude to a covalent attachment of grafted groups at the substrate surface. Among the various molecules available for carbon surface modification via the reduction of diazonium cations, thiophenol is of particular interest in order to limit metal nanoparticle mobility on carbon surface due to the covalent bond between substrate and grafted layer18 and the strong interaction between thiol groups and metal.19 Hence, the interaction between the substrate and nanoparticles is strongly improved. The system chosen for this study is platinum nanoparticles dispersed on Vulcan XC72 substrate. This catalyst-substrate system is widely used for low temperature fuel cell electrodes. The modification of Vulcan XC72 was performed by reduction of a diazonium cation via the mechanism described on Scheme 1. The reduction is carried out in the diazonium synthesis medium. This method is referred to as an in situ modification.20 (14) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947–5951. (15) Adenier, A.; Bernard, M. C.; Chehimi, M. M.; Cabet-Deliry, E. Desbat, B.; Fagebaume, O.; Pinson, J.; Podvorica, F. J. Am. Chem. Soc. 2001, 123, 4541–4549. (16) Boukerma, K.; Chehimi, M. M.; Pinson, J.; Blomfield, C. Langmuir 2003, 19, 6333–6335. (17) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3823–3824. (18) Allongue, P.; Delamar, M.; Desbat, D.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1997, 119, 201–207. (19) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (20) Baranton, S.; Belanger, D. J. Phys. Chem. B 2005, 109, 24401–24410.
Published on Web 4/21/2009
DOI: 10.1021/la9000973
6543
Article
Urchaga et al. Scheme 1. Carbon Surface Derivatization by Reduction of a Diazonium Cation
This mechanism is well-known for electrochemical derivatization of carbon and metal surfaces15,21 and was recently adapted to carbon powder or metal substrate modification via a so-called “spontaneous reduction” of the diazonium cation:22-24 the substrate derivatization was performed without electrochemical induction. The reduction of diazonium cations was performed by the carbon substrate, and the resulting radical led to the formation of the grafted layer on its surface. The spontaneous reduction of diazonium cation was studied on metal substrates23 and carbon substrates.24 Diazonium reduction reaction investigated on metal surfaces (copper, zinc, iron, and nickel) by FT-IRRAS23 demonstrated the effective loss of nitrogen during the modification process without electrochemical induction. Studies carried out on carbon powder substrate evidenced that the use of a reducing agent (H2PO3) to generate aryl-substituted radical led to the same surface coverage than a modification carried out without reducing agent.24 This result indicates that in both cases the available sites for grafting reaction are saturated and that carbon can be used as a reducing agent to generate radicals. In order to explain the mechanism of spontaneous grafting reaction on carbon, the possibility of diazonium ions to be reduced in the presence of a nucleophile such as Vulcan XC72 electron-donating π-electron systems was pointed out.24 The characterization of this modification led to the conclusion that the grafted layer is generally thinner than that obtained by electrochemically controlled reduction of diazonium (typically, a fraction of a monolayer is obtained)22 which may avoid the typical increase of the charge transfer resistance observed for substrates with a grafted layer.10 The grafted layer was evidenced and characterized by thermogravimetric analysis coupled with mass spectrometry, X-ray photoelectron spectroscopy, and elemental analysis. The stabilization of platinum nanoparticles was characterized by in situ X-ray diffraction measurements under a hydrogen atmosphere at 200 °C and by electrochemical aging (accelerated durability test).25
2. Experimental Section 2.1. Support Modification. The Vulcan XC72 carbon powder was modified with a thiophenol grafted layer in aqueous medium by spontaneous reduction of the corresponding in situ generated diazonium cation. First modification was performed in order to obtain a surface coverage expressed in weight percent of the substrate of 4 wt %. The surface coverage was controlled by adjusting the amount of reactants in the synthesis medium. 200 mg of carbon Vulcan XC72 was placed in 50 mL of a 1 M HCl solution containing 4-aminothiophenol (97%, Sigma-Aldrich) at 1.4 mM. A second substrate was prepared with a large excess of in situ generated diazonium cations (200 mg of carbon Vulcan XC72 in 50 mL of 1 M HCl solution containing 16 mmol L-1 4-aminothiophenol). The solution was vigorously stirred for (21) Datsenko, S.; Ignat’ev, N.; Barthen, P.; Frohn, H. J.; Scholten, T.; Schroer, T.; Welting, D. Z. Anorg. Allg. Chem. 1998, 624, 1669–1673. (22) Toupin, M.; Belanger, D. J. Phys. Chem. C 2007, 111, 5394–5401. (23) Adenier, A.; Cabet-Deliry, E.; Chausse, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Chem. Mater. 2005, 17, 491–501. (24) Toupin, M.; Belanger, D. Langmuir 2008, 24, 1910–1917. (25) Grolleau, C.; Coutanceau, C.; Pierre, F.; Leger, J. M. Electrochim. Acta 2008, 53, 7157–7165.
6544
DOI: 10.1021/la9000973
30 min before sodium nitrite addition. NaNO2 (ACS reagent, Sigma-Aldrich) was added to reach a concentration twice that of 4-aminothiophenol in order to ensure a total transformation of the amine into diazonium in spite of the nitrogen oxide gas release. The role of nitrite species on the reduction reaction of diazonium ions and the obtained grafted layer was previously studied.20,26 The electrochemically controlled reduction of diazonium was carried out in solution containing isolated diazonium salt or with in situ generated diazonium ions with an excess of nitrite (sodium nitrite in aqueous medium and tert-butyl nitrite in organic medium). Both electrochemical measurements and XPS analysis on these modified substrates indicated that the reduction potential of diazonium ions and the resulting grafted layer are not influenced by the presence of nitrite species. The influence of nitrites on grafted layers formed by spontaneous reduction was also studied on carbon powder substrates modified by in situ generated diazonium ions and isolated diazonium salts.24 These two different modification methods led to similar surface coverage, indicating that there is no influence of the presence of nitrite for carbon surface modification by spontaneous reduction of diazonium ions. The solution was stirred and left to react at room temperature for 24 h. The carbon was then filtered and abundantly rinsed with water, methanol, dimethylformamide, acetonitrile, methanol, and water, successively. The resulting modified carbon powder was dried under air at 75 °C overnight. Modified substrates will be called “XC72-SH-n”, where n corresponds to the thiophenol surface coverage, expressed in wt %. Unmodified Vulcan XC72 substrate is called “XC72”. 2.2. Catalytic Powder Synthesis. Platinum catalysts were synthesized by a “water in oil” microemulsion method. The microemulsion was composed by 0.8 mL of 0.2 M H2PtCl6 aqueous solution in 25 mL of n-heptane with 10 mL of surfactant, Brij 30 (Sigma-Aldrich). After having obtained a homogeneous microemulsion by a slow stirring for a few seconds, 76 mg of NaBH4 was added to the microemulsion, leading to the reduction of platinum salt into nanoparticles of platinum. After having sonicated the colloidal solution for 30 min, the appropriate amount of carbon substrate was added to reach a Pt/C catalytic powder with a metal loading of about 40 wt %. The powder was then filtered and abundantly washed with acetone, a mixture of water and acetone 50% v/v, and water. This rinsing step was repeated three times in order to completely remove the surfactant from platinum nanoparticles. The resulting Pt/C powder was then dried under air at 75 °C overnight.
2.3. Thermogravimetric Analysis Coupled with Mass Spectrometry (TGA-MS). TGA-MS experiments were performed on a TGA instrument SDT-Q600 from TA Instruments coupled with a mass spectrometer TCP 015 from Pfeiffer vacuum. 10 mg of sample was disposed in a platinum crucible and heated from 25 to 900 °C (with a 10 °C min-1 temperature slope) under an air atmosphere (50 mL min-1 flow rate). The mass spectroscopy was performed at m/z = 64, corresponding to the SO2 signal, and recorded during the thermogravimetric analysis. 2.4. Elemental Analysis. Modified carbon substrates and bare substrate were prepared for elemental analysis by flash combustion. The elements analyzed are carbon, nitrogen, and sulfur. Results are given in weight percent of the sample. 2.5. X-ray Photoelectron Spectroscopy (XPS). XPS experiments were carried out by using an ISA-RIBER setup (26) Baranton, S.; Belanger, D. Electrochim. Acta 2008, 53, 6961–6967.
Langmuir 2009, 25(11), 6543–6550
Urchaga et al.
Article
equipped with a MAC 2 detector. An Mg anode was used as source operating (hν = 1253.6 eV). The pressure in the analytical chamber of the spectrometer was kept below 10-10 Torr. For each sample, a survey scan was performed, followed by a core level spectrum of the C 1s and S 2p region. The binding energies were lined up with respect to the C 1s peak at 284.6 eV. 2.6. Transmission Electronic Microscopy (TEM). TEM measurements were carried out using a JEOL (200 keV) transmission electronic microscope with a resolution of 0.35 nm. ImageJ free software was used to determine nanoparticle size on a total of 500 counted particles.
Table 1. Substrate Compositions Determined by Elemental Analysis sample
carbon (wt %)
sulfur (wt %)
thiophenol (wt %)
XC72 XC72-SH-4 XC72-SH-8
95.2 95.1 88.8
0.0 1.1 2.3
0.0 3.5 7.8
2.7. In Situ X-ray Diffraction Measurements (XRD). Powder diffraction patterns were recorded on a Bruker D8 Bragg-Brentano diffractometer operated with a copper tube powered at 40 kV and 40 mA. The optical setup includes a Kβ filter to keep the Cu KR emission (Cu KR1 = 1.540 60 A˚, Cu KR2 = 1.544 43 A˚, intensity ratio 0.5). Platinum nanoparticle aging experiments were carried out at 200 °C with the diffractometer equipped with an Anton PAAR HTK16 high-temperature chamber operating under H2 3%/He atmosphere (30 mL min-1 flow rate). The catalytic powder (10 mg) was deposited on a Kanthal foil, and the experiments were carried out in the 2θ range from 20° to 60°. The acquisition of the whole pattern is performed for 183 s. The crystallite size was determined with the Scherrer’s equation using the full width at half-maximum. Lv ¼
Kλ β2θ cosðθÞ
β2θ is the full width at half-maximum of 111 diffraction peak in radian, λ is the wavelength of the X-ray beam in angstroms (λ = 1.5406 A˚), θ is the half-angle value of the diffraction peak and Lv is the crystallite size in angstroms. The K value is a constant which is dependent on the method of determination of the breadth (0.89 < K < 1). The value of K was taken equal to 0.94.27 2.8. Electrochemical Aging. The electrochemical setup consisted of a three-electrode cell with a reversible hydrogen electrode (RHE) as reference electrode, a 3 mm diameter glassy carbon disk as working electrode, and a glassy carbon plate as counter electrode. The cell was maintained under a nitrogen atmosphere during electrochemical measurements. Cyclic voltammetry was performed with a computer-controlled Voltalab PGZ 402 potentiostat. The experiments were performed in 0.1 M HClO4 electrolyte (ultrapure, Merck) in ultrapure water (produced by a Milli-Q Millipore System, 18.2 MΩ cm) deaerated by bubbling N2 (U quality from L’Air Liquide). A 3 μL drop of an aqueous ink containing 25 mg of catalytic powder (platinum nanoparticles and substrate), 0.5 mL of a 5 wt % Nafion solution in aliphatic alcohol (Sigma-Aldrich), and 2.5 mL of ultrapure water was deposited on the working electrode. The electrode was then dried under nitrogen before being introduced in the electrochemical cell.
3. Results and Discussion 3.1. Elemental Analysis. In a first step, the carbon surface coverage by thiophenol was evaluated by elemental analysis for two modified substrates and a bare Vulcan XC72 substrate. Results are presented in Table 1. The total elemental composition does not reach 100% since oxygen atoms are not taken into account. The thiophenol content expressed in weight percent is calculated from the sulfur content, assuming that sulfur atoms originate from thiophenol molecules only. This hypothesis is in agreement with the result obtained for unmodified substrate since (27) Warren, B. E. X-Ray Diffraction; Dover Publications: New York, 1990; pp 251-254.
Langmuir 2009, 25(11), 6543–6550
Figure 1. (a) TGA curves of carbon substrates with (b) the resulting MS signal for m/z = 64: XC72 (solid curve), XC72-SH-4 (dotted curve), XC72-SH-8 (dashed curve).
no sulfur was observed on this sample. The 3.5 wt % obtained for the XC72-SH-4 sample is in good agreement with the nominal value (4 wt %). Considering the large excess of 4-aminothiophenol for the synthesis of modified Vulcan XC72 leading to a measured surface coverage of 7.8 wt %, it seems that this value may be close to the maximum surface coverage reachable by this modification method. This sample will be called XC72-SH-8. Considering the different cleaning steps performed on the modified substrates (successive rinsing with water, methanol, dimethylformamide, acetonitrile) which would have eliminated almost all molecules weakly bonded to the carbon substrate (π-stacking or adsorption interactions), the thiophenol surface coverage measured on cleaned modified substrates indicates that diazonium reduction led to a covalent bonding of grafted molecules. The result obtained for the unmodified substrate (absence of sulfur) is also interesting with regard with the result obtained for this sample by MS-TGA (solid line, Figure 1b), where it seems that some SO2 are formed at high temperature (between 600 and 700 °C). This will be discussed later. 3.2. Thermogravimetric Analysis Coupled with Mass Spectrometry. For the derivatization of Vulcan XC72 with thiophenol molecules, it was attempted to graft thiophenol molecules with two different surface coverage. These modified carbon powders were then characterized under air by thermogravimetric analysis coupled with mass spectrometry. The TGA results are presented in Figure 1a. The carbon combustion under DOI: 10.1021/la9000973
6545
Article
Urchaga et al.
air is easily identified from the unmodified sample (solid curve) and takes place between 550 and 700 °C. In the case of thiophenolmodified samples, the mass loss observed before this temperature range may be due to the grafted layer degradation. As expected, the more important the nominal surface coverage is, the more important is the mass loss before 550 °C. Another effect of the modification can be noticed: the Vulcan XC72 combustion profile is slightly altered by the derivatization. The mass decrease for modified carbons is slower in the 550-700 °C region, and the combustion can end after 750 °C. Since this difference for the substrate combustion is quite significant, it was not possible to estimate the mass of the grafted layer by TGA results. However, the mass spectrometry coupled with TGA enabled an accurate characterization of the presence of the grafted layer. Figure 1b presents the MS signal for the unmodified Vulcan XC72 (solid curve) and for the modified substrates. The MS signal was measured for m/z = 64, which corresponds to SO2 species, in order to characterize the grafted layer. These species were chosen as a probe for grafted thiophenol molecules assuming that their combustion under air will lead to the formation of SOx and that the signal for m/z = 64 will not interfere with other species coming from the substrate combustion. On the curve corresponding to Vulcan XC72 substrate, a small peak appears on the MS signal curve between 600 and 700 °C during the carbon combustion. This increase of the MS signal during the substrate combustion may not be due to small sulfur or sulfur oxides occlusions in the substrate as confirmed by the elemental analysis (Table 1) but to an artifact during the measurement. The combustion of the substrate, which is a fast phenomenon, may lead to an increase of the local pressure on top of the sample. This pressure increase probably causes the presence of the peak on MS measurement. Another explanation to the presence of this signal for m/z = 64 is the presence of fragments of molecules coming from carbon oxidation. This small peak is also present on the modified substrates but is preceded by two other peaks: the first one in the 200-450 °C temperature range and the second one in the 450-600 °C temperature range. Those two peaks increase with the nominal thiophenol surface coverage of the samples and are totally absent from the unmodified sample. This observation is in accordance with the fact that SO2 is a probe molecule for the degradation of the grafted layer. Furthermore, since the signal increases regularly with the nominal surface coverage, it confirms that the control of the amount of thiophenol on the surface is achieved. However, it is only a qualitative measurement, and it does not lead to an experimental value for the amount of molecules on the surface of Vulcan XC72. 3.3. XPS Measurements. XPS measurements were carried out for substrates with thiophenol grafted layers: XC72-SH-4 and XC72-SH-8 samples. The unmodified Vulcan XC72 was not analyzed since sulfur species were not reported on its surface in previous studies.22 The S 2p core level spectra for both samples are presented in Figure 2a,b. For thiophenol grafted layer, only one S 2p peak at a binding energy close to 163 eV was expected,28 but both spectra present two peaks: a peak at 163 eV and another one around 168 eV. Both peaks increase with the increase of thiophenol surface coverage, which indicates that the signal at 168 eV is due to the grafted layer and not to the substrate. Indeed, when the grafted layer growths, the XPS signal of the substrate surface decreases, which does not correspond to the phenomenon observed for the S 2p peak at 168 eV. In order to assign this peak to (28) Wagner, C. D.; Riggs, W. M.; Davis, M. E.; Moulder, S. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer: Eden Prairie, MN, 1979.
6546
DOI: 10.1021/la9000973
Figure 2. S 2p core level spectra recorded by XPS for (a) XC72SH-4 and (b) XC72-SH-8 substrates. Table 2. Atomic Ratio of Surface Sulfur Oxide Groups versus Thiophenol Groups Measured on S 2p XPS Core Level Spectra sample
S 2p at 163 eV (-SH, at. %)
S 2p at 168 eV (-SOx, at. %)
-SOx/-SH
XC72-SH-4 XC72-SH-8
65 49
35 51
0.54 1.04
sulfur species in the film, a fit with doublet of S 2p1/2 and S 2p3/2 peaks was attempted. To perform this fit, a standard 1.2 eV difference between the two peaks of the doublet was considered. The signal located at 163 eV was perfectly fitted; however, the signal at 168 eV required at least two S 2p doublets to be fitted. This approach combined to the observation that the full width at half-maximum of the peak located at 168 eV is larger than that at 163 eV led to the hypothesis that the XPS signal around 168 eV may be due to mixed sulfur oxides,28 namely -SOx groups. In order to understand the origin of these groups, a comparison of the S 2p peak area at 168 eV versus the S 2p peak area at 163 eV is presented on Table 2. The increase of the thiophenol surface coverage from 4 to 8 wt % leads to an increase of the -SOx/-SH ratio. This behavior may be explained by the derivatization conditions: the synthesis of 4-thiophenoldiazonium ions from the corresponding amine is performed in acid medium with sodium nitrite (leading to the formation of nitrosonium ions). This reaction is fast and total20 and involves the reaction of one nitrosonium ion with one 4-aminothiophenol molecule to form a diazonium ion. For this synthesis step, sodium nitrite was introduced in excess (NaNO2 concentration is set twice higher than that of 4-aminothiophenol), which led to a long exposition of 4-thiophenoldiazonium ion and 4-thiophenol groups in the grafted layer to the oxidizing agent. Considering the synthesis conditions, the XC72-SH-8 sample was prepared in the most oxidizing medium and presents a ratio Langmuir 2009, 25(11), 6543–6550
Urchaga et al.
Article
Figure 3. Particle size distribution measured from TEM experiment (500 counted particles) for platinum nanoparticles dispersed on XC72 substrate (a), XC72-SH-4 substrate (b), and XC72-SH-8 substrate (c). A TEM image associated with the size distribution is presented for each sample: platinum dispersed on XC72 substrate (d), on XC72-SH-4 substrate (e), and on XC72-SH-8 substrate (f ).
-SOx/-SH twice higher than that of XC72-SH-4 sample, the excess of sodium nitrite introduced for the synthesis may be the cause of the formation of SOx species on the grafted layer. 3.4. Platinum Catalysts Dispersed on Modified Carbon Substrates. In order to measure the effect of thiol groups grafted on carbon substrates, platinum nanoparticles were synthesized by the microemulsion method. This synthesis method enable to obtain nanosized platinum particles, and their cleaning does not require heat treatment. This point is important in order to study the effect of a thiophenol grafted layer since its temperature stability is unknown. The particle size distribution measured from TEM images after dispersion on unmodified carbon substrate is presented in Langmuir 2009, 25(11), 6543–6550
Figure 3. More than 75% of the particles present a size included in the 1-5 nm range, which confirms that platinum particles are nanostructured. Furthermore, the particles with a diameter over 5 nm are mainly particle agglomerates which cannot be accurately dissociated in several smaller particles. This limitation is the main factor which discarded TEM measurements to study the phenomenon of platinum particles growth. A more objective measurement method is then required, like in situ XRD measurements, which allow determining platinum crystal size. 3.5. Thermogravimetric Analysis Coupled with Mass Spectrometry. In order to determine the loading of platinum dispersed on carbon substrates, TGA measurements were carried DOI: 10.1021/la9000973
6547
Article
Urchaga et al.
out. The thermal stability of thiophenol grafted layers under air conditions was also evaluated from mass spectrometry coupled with TGA. The TGA results are presented together with MS signals in Figure 4. From the final weight of samples (reached over 500 °C), the platinum loading measured for all samples is 36 wt %, which indicates a good reproducibility of platinum deposition and dispersion on carbon substrate from microemulsion colloidal solution. Carbon combustion is observed for the three samples between 250 and 500 °C. The combustion is considerably catalyzed by platinum particles and starts at temperatures 300 °C lower than that observed for carbon substrates alone (Figure 1a). Furthermore, as noticed for TGA performed on modified substrates without platinum, the presence of the grafted layer has an effect on the carbon substrate combustion. The combustion of carbon substrate for the sample with the highest nominal surface coverage (XC72-SH-8, dashed curve) occurs at a temperature 100 °C lower than that of unmodified carbon substrate. The sample with an intermediate nominal surface coverage (XC72-SH-4, dotted curve) presents an intermediate behavior. The MS signal corresponding to m/z = 64 was recorded simultaneously with the TGA for the three samples. The unmodified carbon substrate (XC72) did not present any signal for sulfur dioxide formation at m/z = 64, which confirms that the signal observed for this substrate in Figure 2 may have been an artifact. For both modified substrates, the grafted layer degradation occurs for temperatures over 230 °C, with two distinct peaks, which increase when the surface coverage increases. This information validates the use of modified substrates for PEMFCs electrodes since those systems are designed to work under 200 °C. Furthermore, such modified substrates could be used with catalysts nanoparticles synthesized by a method which requires a heat treatment below 200 °C. 3.6. Particles Growth under High Temperature. The thiophenol layer grafted on surface of Vulcan XC72 aims at improving the substrate-particle interaction via a covalent bound with the carbonaceous substrate and a strong interaction between the thiol groups and platinum particles. The characterization of this stronger substrate-particle interaction was first carried out by in situ XRD measurements during a heat treatment at 200 °C of the catalytic powder under reductive atmosphere (H2 3%/He). This temperature was chosen for its compatibility with the grafted layer thermal stability and its good concordance with the targeted working conditions of high-temperature proton exchange membrane fuel cell (PEMFC).29,30 The measurements were carried out under a 3% H2 in helium atmosphere in order to mimic PEMFC anode conditions. The presence of hydrogen enhances the mobility of platinum particles on the substrate and their sintering kinetics.31-33 At the cathode, the phenomena responsible for a catalytic surface loss may be more complex than a coalescence of platinum nanoparticles. Notably, platinum dissolution is a factor which has to be taken into account in the cathodic catalytic surface loss. Figure 5 presents the evolution of the size factor versus time for three samples: the unmodified Vulcan XC72 substrate and two substrates modified with a nominal surface coverage of 4 and 8 wt %. The initial crystallite size of platinum nanoparticles was 3.1 nm for platinum deposited on unmodified substrate and 3.5 nm for platinum catalyst supported on both modified sub(29) Li, Q.; He, R.; Gao, J.; Jensen, J. O.; Bjerrum, N. J. J. Electrochem. Soc. 2003, 1502, A1599–A1605. (30) Mallant, R. K. A. M. J. Power Sources 2003, 118, 424–429. (31) Baird, T.; Paal, Z.; Thomson, S. J. J. Chem. Soc., Faraday Trans. 1 1973, 69, 50–55. (32) Baird, T.; Paal, Z.; Thomson, S. J. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1237–1241. (33) Vleeming, J. H.; Kuster, B. F. M.; Marin, G. B.; Oudet, F.; Courtine, P. J. Catal. 1997, 166, 148–159.
6548
DOI: 10.1021/la9000973
Figure 4. TGA-MS curves of platinum particles dispersed on carbon substrates (36 wt % Pt/C) for three different substrates: XC72 (solid curve), XC72-SH-4 (dotted curve), XC72-SH-8 (dashed curve).
Figure 5. Platinum nanoparticles crystallite size evolution under 3% H2/He atmosphere at 200 °C. For platinum catalyst dispersed on the three substrates: XC72 (solid curve), XC72- SH-4 (dotted curve), XC72-SH-8 (dashed curve). The crystallite size is related to a size factor corresponding to the ratio of the measured crystallite size over the initial crystallite size (3.1 nm for platinum supported on XC72 substrate and 3.5 nm for platinum dispersed on modified substrates). Crystallite sizes were determined with Scherrer’s equation.
strates. The size factor is the crystallite diameter measured during the experiment normalized with respect to the initial crystallite diameter. The change of the crystallite diameter is represented for 5 h after the temperature was set at 200 °C. The curves recorded for the three samples present the same typical shape for crystallite growth:34 the crystallite size remains constant for the first 50 min, and then a drastic increase of the diameter occurs in few minutes to reach its final value. Considering that the particles diffusion on the substrate does not lead to a crystallite growth but to the agglomeration of particles, this phenomenon occurs before the crystallite diameter increase. The size of crystallites measured after 5 h results from the size of agglomerates formed by platinum particles diffusion and gives an indirect measurement of this phenomenon. Even if the diffusion is indirectly characterized, this method enables an explicit evidence of the efficiency of the thiophenol grafted layer to avoid catalytic surface loss. After 5 h at 200 °C, the platinum particles dispersed on the unmodified substrate present a 35% increase of the crystallite diameter (solid curve). On the other hand, the crystallite diameter (34) Manninger, I. J. Catal. 1984, 89, 164–167.
Langmuir 2009, 25(11), 6543–6550
Urchaga et al.
of platinum particles dispersed on modified substrate with a 4 and 8 wt % surface coverage increased by 25% and 15%, respectively. This result clearly shows the improvements of the substrateparticles interaction and its effect on the nanoparticles stability under a hydrogen atmosphere. 3.7. Active Surface Area Loss. The platinum active surface area is an important parameter for fuel cell catalysts. The XRD experiments gave a first estimation of the active surface area loss via the crystallite size measurement. In order to obtain a direct characterization of the catalytic surface loss, an electrochemical aging was performed. It consists in cycling the electrode potential at 50 mV s-1 in perchloric acid medium (0.1 M solution) to obtain an accelerated degradation of platinum nanoparticles under potential control. Three aging experiments were performed at 25 °C during 2000 cycles with three different upper potential limits: from 0.05 V vs RHE to 0.8 V vs RHE, from 0.05 V vs RHE to 1.0 V vs RHE, and from 0.05 V vs RHE to 1.2 V vs RHE. Those three upper potential limits were chosen to evidence the influence of platinum dissolution in the potential domain of oxideshydroxides monolayer formation.35-37 They were carried out on platinum catalysts dispersed on three different substrates: the unmodified Vulcan XC72 (XC72), Vulcan XC72 with 4 wt % thiophenol groups (XC72-SH-4), and Vulcan XC72 with 8 wt % thiophenol groups (XC72-SH-8). The modification of carbon substrate with thiophenol groups may not avoid the active surface area loss due to the platinum dissolution process in aqueous medium since this reaction is not dependent on the substrate nature. The active surface area is measured from the coulometry of the hydrogen desorption signal between 0.05 V vs RHE and 0.45 V vs RHE (considering that the specific faradaic charge for hydrogen desorption is 210 μC per platinum active surface area in square centimeters) and normalized with respect to the initial surface area. The initial active surface areas were determined from three experiments for each samples. Platinum catalyst deposited on XC72 presented an active surface area of about 43 m2 gpt-1 and platinum on modified substrates have an active surface area of 45 m2 gpt-1. The three samples studied presented a similar active surface area which indicates that these samples can be easily compared in term of particle mobility. Furthermore, the active surface areas obtained for the three samples confirms that thiophenol grafted layer is not a polluting agent for platinum nanoparticles and may not decrease the catalytic activity of platinum deposited on modified substrates. The inset in Figure 6 presents the voltammograms recorded at the beginning of the experiment and after 2000 cycles between 0.05 V vs RHE and 1.0 V vs RHE. At first, the voltammograms obtained are typical for platinum nanoparticles in sulfuric acid medium, which indicates that the thiophenol grafted layer is thin and does not affect significantly the charge transfer.10 Furthermore, the decrease of the hydrogen adsorption and desorption current densities after 2000 cycles appears clearly, indicating the decrease of the platinum active surface area. Figure 6 presents the evolution of the platinum active surface area during 2000 cycles between 0.05 V vs RHE and 1.0 V vs RHE for the three substrates. The rate of surface loss is higher at the beginning of the aging experiment than after 2000 cycles, indicating that this limit of 2000 cycles enables a good evaluation of the degradation process. During the whole experiment, both modified substrates presented (35) Yasuda, K.; Taniguchi, A.; Akita, T.; Ioroi, T.; Siroma, Z. Phys. Chem. Chem. Phys. 2006, 8, 746–752. (36) Yoda, T.; Uchida, H.; Watanabe, M. Electrochim. Acta 2007, 52, 5997– 6005. (37) Mitsushima, S.; Koizumi, Y.; Uzuka, S.; Ota, K. I. Electrochim. Acta 2008, 54, 455–460.
Langmuir 2009, 25(11), 6543–6550
Article
Figure 6. Evolution of the platinum active surface area with the number of electrochemical cycles between 0.05 V vs RHE and 1.0 V vs RHE. For platinum catalyst dispersed on carbon substrates: XC72 (solid curve), XC72-SH-4 (dotted curve), XC72-SH-8 (dashed curve). Inset: cyclic voltammetry recorded at 50 mV s-1 for XC72-SH-8 substrate at the beginning of the aging experiment and after 2000 cycles. Table 3. Active Surface Area Loss (in %) of Platinum Catalysts Dipersed on Thiophenol-Modified Substrate and Bare Vulcan XC72 after 2000 Cycles
XC72 XC72-SH-4 XC72-SH-8
active surface area loss (0.05-0.8 V vs RHE)
active surface area loss (0.05-1.0 V vs RHE)
active surface area loss (0.05-1.2 V vs RHE)
14 11 5
35 26 24
56 54 57
a higher remaining platinum active surface area than that of platinum dispersed on unmodified Vulcan XC72. The conservation of the platinum active surface area by thiophenol groups present on modified substrates is noticeable after 500 cycles, and this behavior is maintained for the complete aging experiment. Furthermore, after 2000 cycles, the surface loss rate observed in Figure 6 is slower for both modified substrate compared with the bare Vulcan XC72 substrate, indicating that the difference of active surface area loss between modified substrates and bare carbon will be amplified above 2000 cycles. The active surface area loss observed after 2000 cycles for the two thiophenol-modified substrates and the bare Vulcan XC72 with different anodic potential limits are reported in Table 3. With an upper potential limit at 0.8 V vs RHE, the three samples described the same behavior during the measurement, leading to a total decrease of the active surface area close to 15% after 2000 cycles. In spite of an intensive cleaning of the electrochemical setup, it was not possible to avoid this small active surface area decrease due to polluting molecules adsorption. Indeed, for a 24 h measurement, even under inert gas flow in order to maintain a fresh and clean atmosphere on top of the electrolytic solution, it is not possible to keep the cell perfectly clean for the total duration of the experiment. This effect only appeared with an upper potential limit at 0.8 V vs RHE since these experimental conditions avoid the adsorption of oxygenated species on platinum and, consequently, avoid the cleaning of platinum surface by these adsorbed oxygenated species. In order to restore the polluted platinum active surface area via the adsorption-desorption of oxygenated species, 5 cycles were recorded between 0.05 V vs RHE and 1.2 V vs RHE immediately at the end of the aging experiment, and the final active surface area was measured on the DOI: 10.1021/la9000973
6549
Article
Urchaga et al.
last of these 5 cycles. This effect was reproduced three times on the three samples, which ensure a reproducibility of the phenomenon. After this surface cleaning step, the active surface area of platinum dispersed on bare Vulcan XC72 substrate presented a total decrease of 14% after 2000 cycles, whereas only 5% active surface area loss is observed for platinum dispersed on XC72-SH-8 substrate. The platinum dispersed on XC72-SH-4 substrate displays an intermediate behavior. This experiment illustrates well the role of thiophenol groups in avoiding the platinum agglomeration by diffusion on the substrate. The durability of the catalyst under electrochemical cycling conditions is severely improved, and the active surface area loss is up to 3 times lowered in the presence of the thiophenol grafted layer. With an anodic limit at 1.2 V vs RHE, the three substrates present the same behavior with an active surface area loss close to 55%. When the upper potential limit is decreased to 1 V vs RHE (Table 3 and Figure 6), the surface loss is 10% lower for thiophenol-modified substrates than for bare Vulcan XC72, but the dissolution phenomenon still occurs and a surface loss is observed for the three electrodes. These aging conditions point out the inefficiency of surface thiophenol groups to avoid platinum active surface area loss due to platinum dissolution reaction in aqueous medium. Indeed, in the potential domain over 0.8 V vs RHE, the platinum dissolution reaction occurs via the formation of surface oxides.35-37
4. Conclusion The modification of carbon substrate with thiophenol groups was performed by a spontaneous diazonium cations reduction reaction. The control of the thiophenol surface coverage (expressed as a mass percentage of the substrate) was qualitatively demonstrated by TGA with substrates bearing 4 wt % thiophenol groups compared to substrate prepared with a large excess of thiophenol diazonium ions and was quantified by elemental
6550
DOI: 10.1021/la9000973
analysis. However, the diazonium synthesis and grafting method have to be improved since XPS experiments established that 3050% of the grafted molecules are sulfur oxides. The modification of the diazonium synthesis parameters, such as sodium nitrite concentration, by adding this reactant dropwisely for a long period may enable to decrease the sulfur oxidation reaction. Thermogravimetric analysis of modified substrates indicates good agreement between the nominal loading of 40 wt % Pt and the real one (36 wt % Pt) for all samples. Furthermore, the grafted layer combustion occurs after 200 °C, which enables the use of modified Vulcan XC72 for various application including fuel cells. This combustion temperature allows the use of nanoparticles produced by synthesis way, requiring a heat treatment under 200 °C after dispersion on carbon like ethylene glycol synthesis.38 XRD measurements performed under a hydrogen atmosphere at 200 °C presented a lower agglomeration and crystallite growth for platinum dispersed on modified substrates, indicating a lower mobility of particles due to thiophenol grafted layer. In electrochemical aging experiments with an upper potential limit over 0.8 V, the platinum dissolution reaction occurs and the thiophenol groups efficiency is limited. In aging experiments with potential cycling below 0.8 V, for which the platinum dissolution reaction is avoided, the modified substrates presented a good ability for reducing the platinum active surface area loss process. In conclusion, the active surface area loss due to platinum particle mobility on the carbon substrate is clearly reduced by thiophenol grafted layers. This reduced mobility of platinum particles, due to an improvement of the substrate-particle interaction, led to an improved stability of platinum catalyst under different conditions: on high temperature aging under hydrogen atmosphere and by accelerated electrochemical durability test in aqueous acidic medium. (38) Zhou, Z.; Wang, S.; Zhou, W.; Wang, G.; Jiang, L.; Li, W.; Song, S.; Liu, J.; Sun, G.; Xin, Q. Chem. Commun. 2003, 394–395.
Langmuir 2009, 25(11), 6543–6550
