Manipulation of Pt∧Ag Nanostructures for Advanced Electrocatalyst

1242

J. Phys. Chem. C 2009, 113, 1242–1250

Manipulation of Pt∧Ag Nanostructures for Advanced Electrocatalyst Dan Zhao, Yuan-Hao Wang, Bing Yan, and Bo-Qing Xu* InnoVatiVe Catalysis Program, Key Laboratory of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing, 100084, China ReceiVed: July 14, 2008; ReVised Manuscript ReceiVed: NoVember 19, 2008

Attempts are made in this study to manipulate nanostructures of Pt-flecks on Ag nanoparticles (Ptm∧Ag) for advanced electrocatalysts by reflux citrate reduction of Pt from PtIICl42- or PtIVCl62- ions in solution containing Ag colloids at different atomic Pt/Ag ratio (m). Characterizations with UV-vis, SERS, XPS, and XRD showed a gradual Pt covering of the Ag colloids with increasing m when PtIICl42- was the precursor of Pt (Ptm∧Ag-A samples). However, due to an involvement of the galvanic replacement reaction between PtIVCl62- and the metallic Ag colloids during the citrate reduction of PtIVCl62- ions, a distinct alloying of Pt with the underlying Ag particles took place at the surface region of the colloidal Ag particles when PtIVCl62- was the precursor of Pt (Ptm∧Ag-B samples). Cyclic voltammetry (CV) measurement of the electrochemically active surface area (EAS) showed that the Pt utilization (UPt) in Ptm∧Ag-A increased with the decrease in m. The massspecific activity (MSA) of Pt for the electrooxidation of either methanol or formic acid increased linearly with UPt in Ptm∧Ag-A, but was enhanced significantly with proper Pt-Ag alloying in Ptm∧Ag-B catalysts. Fine-tuning the extent of Pt-Ag alloying resulted in optimized Ptm∧Ag-B catalyst at 0.47 e m e 0.53, whose activity by MSA of Pt was 1 order of magnitude higher in methanol electrooxidation and six times higher in formic acid electrooxidation than its Ptm∧Ag-A counterpart of similar UPt. 1. Introduction Polymer electrolyte membrane fuel cells (PEMFC) are known to be viable low-temperature green power sources for future communication, transportation, and residence.1 Up to now, platinum has been the major component of the anode and cathode catalysts in PEMFC technology. The insufficient activity and costly use of Pt in electrocatalysts, however, have been the major obstacles to practical applications of PEMFC.2-5 Among many attempts to overcome the obstacles, deliberate construction of Pt-skin or Pt-rich deposits on surfaces of relatively inexpensive metal substrates (symbolized as Pt∧M, M refers to the other metal) has recently been proposed as a promising approach.6-13 Unusual atom arrangement or aggregates at the surface and nearsurface regions of such “bimetallic” systems could make special electronic structure6-8,11,12 or higher Pt utilization9-13 (UPt, which is defined as the exposed percentage of Pt atoms9,10 and is often known as Pt dispersion in heterogeneous catalysis), which could remarkably enhance the mass specific activity of Pt in comparison with conventional monometallic Pt electrocatalyst. Since Au surface is well resistant to corruption from the electrooxidation reactions in acid electrolytes14 and favors the reductive-deposition of Pt,15,16 Au nanoparticles were often used as the substrate for Pt deposition in the construction of nanostuctured Pt∧Au electrocatalysts.7-13,17,18 Earlier works from this laboratory prepared two series of Ptm∧Au nanostructures (m being the atomic Pt/Au ratio) by depositing Pt on Au colloids in two size ranges (10.0 ( 1.2 and 3.0 ( 0.6 nm), in which the dispersive state of Pt deposits on Au gradually changed from shell-like overlayers to two-dimensional rafts or very small flecks of Pt by reducing m.9-11 Such changes in the Pt dispersive state led to UPt (or Pt dispersion) increase up to 100%, which in turn induced dramatic activity enhancement of Pt for both * Corresponding author: [email protected].

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methanol and formic acid electrooxidation reactions.9-13 Similar results were also obtained on Pt-modified Au nanoparticles with UPt higher than >60% in ref 7, 17, and 18. These works suggested that a higher UPt in Pt∧M nanostuctures would be a key to the Pt-based electrocatalytic activity. On the other hand, it would be expected that the interaction between the two metals in a bimetallic catalyst would always significantly affect the properties (including catalysis) of the bimetallic material.6,19,20 It would thus be desirable to investigate potential advantages of other metal colloids for the substrate of Pt electrocatalyst in such bimetallic systems. Colloidal Ag could be a first alternative of Au since it resembles Au in resistance to electrochemical corruption in acidic electrolytes, but could have some “liveliness” to effect more or less Pt-Ag interaction especially if Pt salts of varying redox potentials such as PtIVCl62- and PtIICl42- would be used as the precursors of Pt deposits. In this work, we make attempts to manipulate the nanostructure of Pt∧Ag by citrate reduction of Pt from PtIICl42- or PtIVCl62- ions in solutions containing Ag colloids. As the redox potential of [PtIVCl62-|Pt] (1.44 V versus SCE) is significantly higher than that of [Ag+|Ag] (0.80 V versus SCE), the deposition of Pt on Ag surface by the citrate reduction of PtIVCl62- would easily involve a galvanic replacement reaction between PtIVCl62- and metallic Ag:21,22

4Ag + [PtCl6]2- f Pt + 4Ag+ + 6Cl-

(1)

However, reaction 1 would not occur when PtIICl42- is used as the Pt precursor since the redox potential of [PtIICl42-|Pt] (0.76 V versus SCE) is lower than that of [Ag+|Ag]. We demonstrate that nanostructured Ptm∧Ag-A prepared from PtIICl42- are inferior to Ptm∧Ag-B from PtIVCl62- for the electrooxidation of either methanol or formic acid. The galvanic replacement reaction 1 involved during the preparation of Ptm∧Ag-B (from PtIVCl62-) can induce Pt-Ag alloying at the Pt/Ag interface, which is crucial for the activity enhancement of Pt in Ptm∧Ag

10.1021/jp806190w CCC: $40.75  2009 American Chemical Society Published on Web 01/06/2009

Manipulation of Pt∧Ag Nanostructures

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1243

TABLE 1: Composition, XPS Binding Energies (Ag03d and Pt04f) and XRD Angels (2θ) of Pt∧Ag/C, Ag/C, Pt0.5Ag/C, and Pt/C Catalysts metal loadinga (wt %) catalysts

XPS BE (eV) atomic Pt/Ag ratioa

Pt

Ag

Pt0.6∧Ag-A/C Pt0.5∧Ag-A/C Pt0.4∧Ag-A/C Pt0.2∧Ag-A/C Pt0.1∧Ag-A/C Pt0.05∧Ag-A/C Pt0.03∧Ag-A/C Pt0.01∧Ag-A/C

5.2 4.5 3.1 1.7 0.72 0.34 0.16 0.06

4.5 4.6 4.3 4.6 4.4 4.3 4.3 4.2

0.64 0.54 0.40 0.21 0.089 0.043 0.020 0.008

Pt0.6∧Ag-B/C Pt0.55∧Ag-B/C Pt0.5∧Ag-B/Cb Pt0.5∧Ag-B/C Pt0.45∧Ag-B/C Pt0.4∧Ag-B/C Pt0.2∧Ag-B/C Pt0.1∧Ag-B/C Pt0.05∧Ag-B/C

4.8 4.5 3.9 4.1 3.5 3.3 1.8 0.9 0.36

4.1 4.3 4.4 4.2 4.2 4.1 4.4 4.4 4.3

0.64 0.58 0.49 0.53 0.46 0.44 0.22 0.11 0.046

Ag/C Pt0.5Ag/C Pt/C

4.5 4.6

4.4 4.4

0.56

a

XRD 2θ (deg)

Ag3d5/2

Ag3d3/2

Pt4f7/2

Pt4f5/2

368.5

374.5

70.9

74.3

368.5

374.5

70.8

368.5

374.5

368.2

[Pt/Ag]XPS

[111]

[200]

[220]

0.82

38.1

44.2

64.4

74.3

0.26

38.1

44.2

64.4

71.0

74.3

0.08

374.2

71.5

74.8

0.76

38.3

44.4

64.6

368.2

374.2

71.5

74.8

0.71

38.4

44.4

64.8

368.3 368.3

374.3 374.4

71.3 71.2

74.6 74.5

0.46 0.19

38.3 38.2

44.2 44.2

64.5 64.4

368.4

374.3

71.2

74.4

0.06

368.5 368.2

374.5 374.2

71.1 70.9

74.5 74.3

0.38

38.1 38.9 39.8

44.2 44.8 46.2

64.4 65.9 67.4

Obtained by ICP-AES measurement. b A reprepared Pt0.5∧Ag-B/C sample.

for the electrooxidation reactions. Without the Pt-Ag alloying, the mass-specific activity of Pt in Ptm∧Ag (Ptm∧Ag-A) increases with UPt as in Pt∧Au samples.11,12 But fine-tuning the extent of Pt-Ag alloying in Ptm∧Ag-B nanostructures by adjustment of m can increase the activity by up to 1 order of magnitude at similar UPt. 2. Experimental Section 2.1. Chemicals. All of the aqueous solutions were prepared with deionized water from a Milli-Q system (Millipore). Rhodamine B, trisodium citrate dihydrate NaBH4, AgNO3, K2PtCl4, and K2PtCl6 were purchased from Beijing Chemical Reagent Company. Five weight percent Nafion solution was a product of Aldrich Company. These chemicals were in the analytical grade and were used as received. 2.2. Preparation of Ag and Pt∧Ag Particles and Pt∧Ag/C Catalysts. An aqueous solution of Ag colloids (6.3 ( 3.9 nm by TEM, see Figure 5a) was prepared by reducing AgNO3 with NaBH4 at room temperature according to the procedure of Yang et al.23 In brief, 5 mL of 1.0 × 10-2 M aqueous AgNO3 solution was mixed with 5 mL of 3.0 × 10-2 M aqueous sodium citrate solution and diluted to 50 mL with deionized water. Five milliliters of 0.1 M fresh aqueous NaBH4 solution was then added dropwise under vigorous stirring, giving rise to the Ag colloids. The Ptm∧Ag-A samples were prepared by citrate reduction of K2PtIICl4 in Ag colloidal solution under reflux, with also a similar procedure of Yang et al.23 In brief, 50 mL of aqueous solution containing a desired amount of K2PtCl4 was aged for 1 h under reflux (98 °C); the as-prepared Ag colloids (4.9 mg of Ag in 50 mL of solution) and 3.0 × 10-2 M sodium citrate solution (10 mL) were then added. The mixture was kept refluxing for 2 h to ensure complete Pt deposition. Adjustment of the atomic Pt/Ag ratio (m) was made by controlling the amount of K2PtCl4 since the amount of colloidal Ag was kept constant in all the syntheses. The Ptm∧Ag-B samples were prepared by a similar procedure by using K2PtCl6, instead of K2PtCl4, for the Pt precursor. For

reference, monometallic Pt nanoparticles were also prepared by citrate reduction of K2PtCl6 under reflux and a “fully alloyed” Pt0.5Ag sample by room temperature coreduction of Ag+ and PtCl62- with NaBH4 in the presence of sodium citrate. The Ptm∧Ag particles were loaded on Vulcan XC-72 carbon black (BET surface area: 250 m2/g) to prepare catalysts for electrochemical measurements as in our previous works.9,10 The metal loadings and atomic Pt/Ag ratios in these electrocatalysts, as determined by ICP-AES, were found very close to their theoretic values in the preparations (Table 1). 2.3. Characterizations. UV-vis spectra of colloidal Ag and Ptm∧Ag solutions were recorded on a Shimadzu UV-2100S dual beam spectrometer operated at a resolution of 0.5 nm. The colloidal solutions were filled in a quartz cell of 1 cm lightpath length and the light absorption spectra were given in reference to deionized water. Measurements of the surface enhanced Raman spectra (SERS) of Rhodamine B adsorbed on Ag and Ptm∧Ag partciles were carried out on a RM2000 microscopic confocal Raman spectrometer (Renishaw PLC.) employing an air-cooled He-Ne laser operating at 632.8 nm and focused on a spot size of approximately 5 µm by a 20× long working distance objective. The samples were first made in thin films by air-drying an equal amount of the colloidal solutions on a glass substrate; 1 mL of 1.0 × 10-6 M aqueous Rhodamine B was dropped on every film and then dried in air for 30 min before the spectra were recorded. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a PHI 5300 ESCA1610 SAM instrument. The Ag3d and Pt4f core level spectra were recorded with unmonochromatized Mg KR radiation. The resolution in binding energy was higher than 0.1 eV. X-ray diffraction (XRD) patterns were measured with a D8Advance Bruker diffractometer at a scan rate of 0.02 deg/s in the angle (2θ) range of 5° to 90°. The wavelength of the incident radiation was 1.5406 Å (Cu KR). The morphology, size, and size distribution of the colloidal particles and their carbon-supported catalysts were characterized

1244 J. Phys. Chem. C, Vol. 113, No. 4, 2009

Zhao et al.

Figure 1. UV-vis spectra of Ptm∧Ag samples.

by transmission electron microscopy (TEM), using a JEM-2010 system operating at 200 kV. The colloid particles were placed on a 3 mm Cu grid by applying a drop of the colloidal solution, followed by drying under ambient conditions. TEM images of the electrocatalysts were measured at Pacific Northwest National Laboratory (PNNL, USA) with the kind help of Dr. Jun Liu. 2.4. Electrochemical Measurements. Cyclic voltammetry (CV) measurements were carried out at 298 K in a threeelectrode electrochemical cell, in which a saturated calomel electrode (SCE) was used as the reference electrode and a 1.0 cm × 1.0 cm Pt foil was used as the counter electrode. The working electrode was prepared by pasting and drying the catalyst-ink (catalyst powder dispersed in Nafion solution) on a commercial carbon paper, as described in detail in our previous work.9-11 The potentials were given with respect to SCE. The electrochemically active surface area (EAS) and UPt data were obtained according to the charge associated with the electrooxidation of adsorbed hydrogen on the CV curves in 0.5 M H2SO4; the details were given previously.9-11 In brief, the charge consumed under the H desorption peaks on the CV curves, QH, was converted to the number of adsorbed H atoms on surface Pt sites in the electrocatalyst. This later number was taken as the number of surface Pt sites or exposed surface Pt atoms (Ns) in the catalyst since the stoichiometry for H adsorption at the Pt surface has been well established as H/Pts ) 1:1. Then, a calculation of the ratio Ns/Nt (Nt is the total number of Pt atoms) gives UPt or dispersion of Pt in the working electrocatalyst.9,10 For the methanol and formic acid electrooxidation studies, the working electrode was activated by performing repeatedly CV scans from -0.2 to 1.0 V with a scanning rate at 20 mV/s in a solution of 0.5 M H2SO4 + 2 M CH3OH or 0.5 M H2SO4 + 2 M HCOOH until a stable voltammgram was obtained.9-13 Then, CV curves of methanol or formic acid electrooxidation were formally collected. 3. Results and Discussion 3.1. Formation and Structure of Pt Deposits on Ag Colloids (Pt∧Ag). The metal loadings in the electrocatalysts were determined by ICP-AES and are listed in Table 1; the actual numbers of m (atomic Pt/Ag ratio) in Ptm∧Ag particles and in the final Ptm∧Ag/C electrocatalysts were found the same and very close to their theoretic values used in the preparations (Table 1), indicating a quantitative Pt deposition onto the surface of Ag colloids during the reduction of PtIICl42- and PtIVCl62- ions. Figure 1 shows the UV-vis spectra of Ag particles before and after the deposition of Pt from the PtIICl42- and PtIVCl62-

Figure 2. SERS spectra of Rhodamine B on Ag and Ptm∧Ag samples: (a) Ag; (b) Pt0.05∧Ag-A; (c) Pt0.1∧Ag-A; (d) Pt0.2∧Ag-A; (e) Pt0.5∧Ag-A; (f) Pt0.05∧Ag-B; (g) Pt0.1∧Ag-B; (h) Pt0.2∧Ag-B; (i) Pt0.5∧Ag-B. Note: The spectra for Ptmˆ∧Ag-B samples were magnified by a factor of 10.

precursors, repectively. A continued weakening of the Ag plasmon adsorption at around 390 nm23 with increasing m was observed on both Ptm∧Ag-A and -B samples, indicating a gradual covering of Pt deposits on the surface of Ag colloids. In comparison with the Ptm∧Ag-A samples (Figure 1A), a faster weakening of the plasmon adsorption peak on Ptm∧Ag-B samples (Figure 1B) would suggest a stronger Pt-Ag interaction or more severe disturbance of Pt to the surface of underlying Ag colloids.24,25 Considering that the surface of Ag colloids is efficient in inducing SERS and the enhancement factor is very sensitive to the nature of the Ag surface,26 the adsorption of Rhodamine B (a well-documented indicator) on colloidal Ag and representative Ptm∧Ag samples was characterized by SERS, as shown in Figure 2. Rhodamine B adsorbed on the “bare” Ag colloids showed typical SERS bands (Figure 2a): the band at around 1600 cm-1 can be assigned to the aromatic CdC stretching, those at 1330-1560 cm-1 to the aromatic CsC stretchings, and those at 1000-1300 cm-1 to the aromatic CsH stretching and bridged CsC stretching; the band at around 610 cm-1 is attributed to an aromatic bending mode.27-29 The signal intensity on Ptm∧Ag-A samples declined only by 10% at m ) 0.05 but became an order of magnitude lower when m was increased to 0.2 (Figure 2b-d) and then invisible at m ) 0.5 (Pt0.5∧Ag-A, Figure 2e), indicating a gradual covering of the Ag surface by Pt deposits. In comparison with the spectra on Ptm∧Ag-A samples, a more dramatic loss in the SERS signals was observed with the Ptm∧Ag-B samples; the intensity was reduced by factors of 11 at m ) 0.05 and 50 at m ) 0.1 (Figure 2f,g), and then disappeared completely at m g 0.2 (Figure 2h,i). It was proposed that electron migration from the d-states of SERSsilent metal (Pt or Pd) to those of SERS-active substrate (Au or Ag) can be responsible for the quenching of the surface plasmon excitations necessary for the sensitivity enhancement in Raman spectroscopy.30,31 Therefore, the much more rapid loss of the SERS signals on Ptm∧Ag-B than on their Ptm∧Ag-A counterparts would be due to a stronger Pt-Ag interaction or more severe disturbance of Pt deposition to the surface property of the underlying Ag in Ptm∧Ag-B samples. Figure 3 presents the XPS spectra of the representative samples, and the corresponding binding energy data are listed

Manipulation of Pt∧Ag Nanostructures

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1245

Figure 3. XPS spectra of representative samples: (a) Pt0.05∧Ag-A; (b) Pt0.2∧Ag-A; (c) Pt0.5∧Ag-A; (d) Pt0.05∧Ag-B; (e) Pt0.2∧Ag-B; (f) Pt0.4∧Ag-B; (g) Pt0.5∧Ag-B; (h) Pt0.6∧Ag-B; (i) Pt0.5Ag.

Figure 4. XRD patterns of representative samples: (a) Ag; (b) Pt0.2∧Ag-A; (c) Pt0.5∧Ag-A; (d) Pt0.2Ag-B; (e) Pt0.4∧Ag-B; (f) Pt0.5∧Ag-B; (g) Pt0.6∧Ag-B; (h) Pt0.5Ag; (i) Pt.

in Table 1. The deposition of Pt on the surface of Ag colloids on increasing m in both series of Ptm∧Ag samples was characterized by a continued intensification of the Pt04f signals (ca. 70.8 and 74.3 eV)32,33 relative to the Ag03d ones (ca. 368.5 and 374.5 eV).34,35 However, the Ag03d signals were observed clearly in every sample even when m was increased to as high as 0.5, suggesting that the Ag surface was not fully covered by Pt for all Ptm∧Ag samples and the Pt deposits were dispersed as Pt-flecks on the Ag particles. The XPS binding energies (BEs) of Ag and Pt (Table 1) in Ptm∧Ag-A were very close to those of “pure” Ag and Pt, which would suggest that any interaction between Pt-Ag in the Ptm∧Ag-A samples was insignificant by XPS. A similar situation was identified in our earlier XPS measurements of Pt∧Au samples.9-11 In contrast, the BEs of Ag shifted lower and those of Pt higher for Ptm∧Ag-B, and the shifts increased sensitively with m up to m ) 0.5 and remained unchanged thereafter. With reference to the signature of the “fully alloyed” Pt0.5Ag sample (Figure 3i, Table 1), these data would suggest that Pt-Ag alloying had taken place in Ptm∧Ag-B. Therefore, the control of m can lead to effective manipulation of the degree of Pt-Ag alloying/interaction in the Ptm∧Ag-B sample. Evidence for the absence and presence of surface Pt-Ag alloying in Pt∧Ag samples can also be extracted in the XRD patterns, as shown in Figure 4 and Table 1. The Ptm∧Ag-A samples (e.g., Figure 4b,c) showed only diffractions (2θ ) 38.1°, 44.2°, and 64.4°) that well matched those for (111), (200), and (220) planes of purely metallic Ag particles (Figure 4a),34,36 indicating no occurrence of Pt-Ag alloying. In contrast, the Ptm∧Ag-B samples showed diffractions at 2θ angles significantly higher (e.g., Figure 4e-g, Table 1) than those of Ag, but still distinctly lower than those of the “fully alloyed” Pt0.5Ag reference (Figure 4h). These observations would suggest that the formation of Pt-Ag “alloy” in the Ptm∧Ag-B samples is most probably limited to a “shallow surface region” of the underlying Ag colloids, which can be easily understood when the galvanic replacement reaction 1 between Ag surface and PtIVCl62- was the main cause for the “alloy” formation. Interestingly, we observed that the degree of Pt-Ag alloying in such a shallow surface region was sensitive to m since the maximum shift in the XRD 2θ angles was detected at m ) 0.5. Under otherwise fixed conditions in the preparation of Ptm∧Ag-B, the involvement of the galvanic replacement reaction 1 would increase with increasing the concentration or number of PtIVCl62- ions in the solution, which would then lead

to increased Pt-Ag alloying at the surface of Ag colloids. On the other hand, the availability of Ag surface for this galvanic replacement reaction would be limited when the concentration of PtIVCl62- becomes too high, leading to increased Pt deposition by the direct citrate reduction channel and decreased Pt-Ag alloying. This is most probably why the degree of Pt-Ag alloying in Ptm∧Ag-B increased with m up to ca. m ) 0.5. It is thus that the nature of Pt precursor (PtIICl42- or PtIVCl62-) determines Pt-Ag alloying at the surface of Ag nanoparticles. The absence of Pt-Ag alloying in Ptm∧Ag-A could be due to an impossibility of having a galvanic replacement reaction between PtIICl42- and Ag. One would wonder if Ag+ ions, products of the galvanic replacement reaction 1, would precipitate as AgCl in the preparation. This chemistry was excluded by our experimental observation that AgCl added to the solution of sodium citrate was even reduced under reflux to Ag colloids showing distinct plasmon adsorption (UV-vis). The loss of Ag in Ptm∧Ag-B products, as checked by ICP-AES analysis, was always insignificant ( 0.4), the Ptm∧Ag-B/C catalysts showed even slightly higher UPt than Ptm∧Ag-A/C. This could be due to an inhibitory effect of the Pt-Ag alloying on the growth of Pt-flecks, as demonstrated by the TEM images (Figure 5). All of the electrocatalysts were further employed for the electrooxidation of CH3OH and HCOOH. Figure 7 shows the CH3OH electrooxidation CV curves on Ag/C, Pt/C, and both series of Ptm∧Ag/C catalysts, the currents were normalized according to the Pt loadings. Note that the features at around 0.2 V were associated with the reversible electrooxidation and reduction of exposed Ag,39 as was observed in Figure 6. On both series of Ptm∧Ag/C catalysts, the electrooxidation currents of CH3OH were clearly detected in the potential range of 0.4-0.8 V with a peak appearing at ca. 0.65 V on the positivegoing scans and in 0.2-0.6 V with a peak at ca. 0.40 V on the negative-going ones, which resembles the behavior of the reference Pt/C (Figure 7g) and Ptm∧Au/C catalysts in our previous study.9-12

Manipulation of Pt∧Ag Nanostructures

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1247

TABLE 2: EAS, UPt, and Catalytic Activity of Ptm∧Ag/C, Pt0.5Ag/C, and Pt/C Catalysts

catalysts

catalytic activity MSA-CH3OH a MSA-HCOOH b EAS (mA/mg Pt) (mA/mg Pt) (m2/gPt) UPt (%)

Pt0.6∧Ag-A/C Pt0.5∧Ag-A/C Pt0.4∧Ag-A/C Pt0.2∧Ag-A/C Pt0.1∧Ag-A/C Pt0.05∧Ag-A/C Pt0.03∧Ag-A/C Pt0.01∧Ag-A/C

37.4 43.2 62.0 80.2 138.7 186.5 197.8 212.8

15.8 18.2 26.1 33.8 58.5 78.7 83.5 89.8

17.6 19.1 23.8 36.4 59.2 90.6 102.5 116.9

18.2 22.6 29.8 37.1 68.6 94.3 107.3 123.6

Pt0.6∧Ag-B/C Pt0.55∧Ag-B/C Pt0.5∧Ag-B/C Pt0.5∧Ag-B/Cc Pt0.45∧Ag-B/C Pt0.4∧Ag-B/C Pt0.2∧Ag-B/C Pt0.1∧Ag-B/C

45.7 52.6 51.1 50.5 47.2 44.9 52.9 117.6

19.4 22.2 21.7 21.2 19.9 19.1 22.5 49.9

46.8 63.2 185.2 179.7 87.6 27.5 34.4 76.8

112.7 121.3 142.7 147.2 130.6 105.1 90.5 109.7

39.8 48.0

16.8 20.4

71.1 26.4

79.2 56.9

Pt0.5Ag/C Pt/C

a Mass specific activity (MSA) for CH3OH electrooxidation at 0.65 V. b MSA for HCOOH electrooxidation at 0.75 V. c A reprepared Pt0.5∧Ag-B/C sample.

Figure 8 shows the HCOOH electrooxidation CV curves on Ptm∧Ag/C catalysts. On either Ptm∧Ag-A/C or Ptm∧Ag-B/C catalysts, the main electrooxidation currents of HCOOH were detected in the potential range of 0.6-0.9 V with a peak at ca. 0.75 V on the positive-going scans and in 0-0.6 V with a broad peak at ca. 0.40 V on the negative-going ones, which also resembles the behavior of Pt/C catalyst.13 It is worthy of noting that the electrooxidation of HCOOH would occur according to the so-called “dual pathways”:40,41 a direct reaction pathway involving a fast dehydrogenation of HCOOH to CO2 in low potential range (-0.2 to 0.6 V), and an indirect pathway involving a slow oxidation of poisonous intermediates (e.g., adsorbed CO species) formed from the dehydration of HCOOH in high potential range (0.6-0.9 V). In our previous study of HCOOH electrooxidation on Pt∧Au/C catalysts,11,13 the main current peak of HCOOH electrooxidation on the positive-going half-curve was found to shift from 0.6-0.9 V (high potential range) to -0.2 to 0.6 V (low potential range) when UPt or Pt dispersion on Au colloids increased up to higher than 40%, which demonstrated that the direct reaction pathway of HCOOH electrooxidation prevailed on those Ptm∧Au catalysts containing highly dispersed Pt-flecks (i.e., UPt > 40%). In this present work, the Pt-deposits in Ptm∧Ag/C catalysts also existed at m e 0.1 as Pt-flecks with UPt higher than 40% (Table 2). Surprisingly, the main current peak of HCOOH electrooxidation was detected only at above 0.6 V over these highly dispersed Pt-flecks but not in the lower potentials (Figure 8), hinting that the indirect reaction pathway of HCOOH electrooxidation was also prevalent over the highly dispersed Pt-flecks in Ptm∧Ag/C catalysts. Independent of the UPt, a minor and anomalous electrooxidation feature was also observed in the low potential range of -0.2 to 0.6 V on the positive-going scans for all of the Ptm∧Ag/C catalysts. It should be mentioned that the electrooxidation of exposed Ag could also produce a current peak at ca. 0.2 V,39 as observed on the CV curves in H2SO4 with or without CH3OH (Figures 6 and 7). Thus, the unusual shape of this feature could arise more or less from a kind of overlapping electrooxidation of exposed Ag and HCOOH by the direct reaction pathway. These observations suggested that the nature of substrate metals

Figure 7. CV curves for CH3OH electrooxidation over Ag/C, Ptm∧Ag/ C, and Pt/C catalysts in (0.5 M H2SO4 + 2 M CH3OH) at a scan rate of 20 mV/s: (a) Ag/C; (b) Pt0.1∧Ag-A/C; (c) Pt0.2∧Ag-A/C; (d) Pt0.4∧Ag-A/C; (e) Pt0.5∧Ag-A/C; (f) Pt0.6∧Ag-A/C; (g) Pt/C; (h) Pt0.1∧Ag-B/C; (i) Pt0.2∧Ag-B/C; (j) Pt0.4∧Ag-B/C; (k) Pt0.5∧Ag-B/ C; (l) Pt0.6∧Ag-B/C.

in Pt∧M nanostructures can indeed affect significantly the electrocatalytic property of the Pt deposits. Considering that the anodic or positive-going scans would allow poisonous intermediate formed in the electrochemical reaction to accumulate on the catalyst surface,42,43 which is also closer to the real working situation compared to the negativegoing scans, the peak currents on the positive-going scan curves of CH3OH and HCOOH electrooxidations were taken to compare the catalyst performance. Figure 9 shows the positivegoing scan CV curves of CH3OH electrooxidation on the two series of Ptm∧Ag/C catalysts. As it was seen on Ptm∧Au/C catalysts in our earlier work,9-12 the current increased remarkably with decreasing m in the Ptm∧Ag-A/C catalysts (Figure 9A). However, the dependence of current on m in the Ptm∧Ag-B/C catalysts was not that straightforward (Figure 9B); the current obtained over the Pt0.5∧Ag-B/C catalyst was far beyond those on other Ptm∧Ag-B/C (m * 0.5) catalysts. Figure 10 shows the positive-going scan CV curves of HCOOH electrooxidation in the potential range of 0.4-1.0 V on the two series of Ptm∧Ag/C catalysts. In qualitative agreement

1248 J. Phys. Chem. C, Vol. 113, No. 4, 2009

Zhao et al.

Figure 9. Normalized CV curves for CH3OH electrooxidation on Ptm∧Ag-A/C (A) and Ptm∧Ag-B/C (B) catalysts: As(a) Pt0.01∧Ag-A/ C; (b) Pt0.05∧Ag-A/C; (c) Pt0.1∧Ag-A/C; (d) Pt0.2∧Ag-A/C; (e) Pt0.4∧Ag-A/C; (f) Pt0.5∧Ag-A/C; (g) Pt0.6∧Ag-A/C; Bs(a) Pt0.1∧Ag-B/ C; (b) Pt0.2∧Ag-B/C; (c) Pt0.4∧Ag-B/C; (d) Pt0.45∧Ag-B/C; (e) Pt0.5∧Ag-B/C; (f) Pt0.5∧Ag-B/C (reprepared); (g) Pt0.55∧Ag-B/C; (h) Pt0.6∧Ag-B/C; (i) Pt0.5Ag/C.

Figure 8. CV curves for HCOOH electrooxidation over Ptm∧Ag/C and Pt/C catalysts in (0.5 M H2SO4 + 2 M HCOOH) at a scan rate of 20 mV/s: (a) Pt0.05∧Ag-A/C; (b) Pt0.1∧Ag-A/C; (c) Pt0.2∧Ag-A/C; (d) Pt0.4∧Ag-A/C; (e) Pt0.5∧Ag-A/C; (f) Pt0.6∧Ag-A/C; (g) Pt/C; (h) Pt0.1∧Ag-B/C; (i) Pt0.2∧Ag-B/C; (j) Pt0.4∧Ag-B/C; (k) Pt0.5∧Ag-B/ C; (l) Pt0.6∧Ag-B/C.

with the observation on CH3OH electrooxidation (Figure 9), the current of HCOOH electrooxidation also increased remarkably with decreasing m in the Ptm∧Ag-A/C catalysts (Figure 10A), while the Pt0.5∧Ag-B/C catalyst effected the maximum current among all of the other Ptm∧Ag-B/C catalysts (Figure 10B). Table 2 compares the maximum oxidative currents normalized to a milligram of Pt in Ptm∧Ag/C as well as reference Pt/C and Pt0.5Ag/C catalysts. These currents were expressed as the massspecific activity (MSA) of Pt, which was measured as the peak currents of CH3OH electrooxidation at 0.65 V (Figure 9) or HCOOH electrooxidation at 0.75 V (Figure 10).11,12 The MSA data are correlated with UPt in Figure 11 for both series of Ptm∧Ag/C catalysts. The MSAs of Ptm∧Ag-A/C for both the electrooxidation reactions of CH3OH and HCOOH increased linearly with UPt (dotted lines), which resembles the trends observed on Ptm∧Au/C catalysts.9–11 These results further demonstrated that the enhancement in MSA by redcucing the size of Pt deposits in Pt∧M nanostuctures involving no significant Pt-M interaction could be directly correlated with the increase in EAS or UPt of Pt deposits.9-13 In addition, it should be noted that the slope of the MSA-UPt relationship for CH3OH

electrooxidation on Ptm∧Ag-A/C is lower than that on Ptm∧Au/C catalysts,9,10 which indicated that the activity of Pt deposits on the two colloidal metals (Au and Ag) is quite different even at the same UPt or EAS. These results also demonstrate that although the structural feature of Ptm∧Ag-A (unalloyed) was macroscopically similar to that of Ptm∧Au samples, some kind subtle difference in the electronic property could exist between the Pt-Ag and Pt-Au interfaces. In contrast, distinct volcano relationships for both CH3OH and HCOOH electrooxidation reactions were seen on Ptm∧Ag-B/C catalysts (solid lines); the maximum activities appeared on Pt0.5∧Ag-B/C for both reactions. It is very interesting that Pt0.4∧Ag-B/C and Pt0.6∧Ag-B/C, both of which showed somewhat different degrees of Pt-Ag alloying, were 2-6 times less active than the Pt0.5∧Ag-B/C catalyst. For CH3OH electrooxidation, MSA of Pt0.5∧Ag-B/C (UPt ) 22%) was almost an order of magnitude higher than those of Ptm∧Ag-A/C catalysts with similar UPt; for reference, MSAs of the “monometallic” Pt/C (UPt ) 20%) and “fully alloyed” Pt0.5Ag/C (UPt ) 18%) catalysts were only one-seventh and twofifths that of Pt0.5∧Ag-B/C (Table 2). For HCOOH electrooxidation, MSA of Pt0.5∧Ag-B/C was 5-7 times higher than those of Ptm∧Ag-A/C catalysts with similar UPt; MSAs of Pt/C and Pt0.5Ag/C catalysts were one-third and a one-half that of Pt0.5∧Ag-B/C. Therefore, the “monometallic” Pt, “non-alloyed” Ptm∧Ag-A and “fully-alloyed” Pt0.5Ag samples with similar UPt were all inferior to Pt0.5∧Ag-B/C catalyst for the electrooxidation of both CH3OH and HCOOH. To identify an accurate composition window or the most suitable range of m for the formation of most efficient Ptm∧Ag-B/C catalyst for the electrooxidation reactions, the atomic Pt/Ag ratio (m) was carefully changed in the range of 0.45 e m e 0.55 (Table 2),

Manipulation of Pt∧Ag Nanostructures

Figure 10. Normalized CV curves for HCOOH electrooxidation on Ptm∧Ag-A/C (A) and Ptm∧Ag-B/C (B) catalysts: As(a) Pt0.01∧Ag-A/ C; (b) Pt0.05∧Ag-A/C; (c) Pt0.1∧Ag-A/C; (d) Pt0.2∧Ag-A/C; (e) Pt0.4∧Ag-A/C; (f) Pt0.5∧Ag-A/C; (g) Pt0.6∧Ag-A/C; Bs(a) Pt0.1∧Ag-B/ C; (b) Pt0.2∧Ag-B/C; (c) Pt0.4∧Ag-B/C; (d) Pt0.45∧Ag-B/C; (e) Pt0.5∧Ag-B/C; (f) Pt0.5∧Ag-B/C (reprepared); (g) Pt0.55∧Ag-B/C; (h) Pt0.6∧Ag-B/C; (i) Pt0.5Ag/C.

which narrowed the composition window to 0.47 e m e 0.53. The existence of such a narrow composition would indicate that the most beneficial interaction and Pt-Ag alloying in Ptm∧Ag-B materials for the oxidative electrocatalysis occurs only near a very specific composition (m). Interestingly, the catalyst activity for CH3OH electrooxidation appeared to be extremely sensitive to such a type of alloying interaction. Since the accumulation of intermediate poisons like CO on Pt sites is the major kinetic obstacle for the electrooxidation of CH3OH and HCOOH,45,46 the electrocatalytic activity of a given catalyst for these oxidation reactions would depend on its antipoison property or capability of removing the poisons from the Pt sites. Addition of a second metal to Pt would effect composition-dependent metal-metal interaction or alloy formation with Pt, leading to manipulation of the catalytic property of Pt sites according to the composition or ratio of the two metals. The activity enhancement of Pt by the presence of a second metal in Pt-based bimetallic materials was usually accounted for either by a bifunctional mechanism or an electronic (ligand) effect mechanism.47,48 In the bifunctional mechanism, the second metal like Ru and Sn serves to promote water dissociation to form adsorbed OH species, which then react with the poisonous reaction intermediates (e.g., CO) adsorbed on Pt sites to generate CO2.49,50 Bimetallic Pt-Ru alloy nanoparticles with optimized composition by an atomic Ru/Pt ratio of 1:1 were rationalized as an excellent catalyst for CH3OH electrooxidation by the bifuntional mechanism.51,52 In the ligand effect mechanism, the second metal like Fe can hardly promote water dissociation but its electronic interaction with Pt (including alloy formation) can change the d-band center position of Pt, and significantly weaken the bonding (chemisorption) and reduce the coverage of poisonous reaction intermediates on Pt

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1249

Figure 11. Dependence of MSAs on UPt over Ptm∧Ag/C catalysts for CH3OH electrooxidation (A) and HCOOH electrooxidation (B).

sites.53 Very recently, an alloyed PtFe catalyst with optimized composition at Fe/Pt ) 2:3 (atomic ratio) was identified as a highly efficient catalyst for HCOOH electrooxidation.54 Also, a controlled Pt alloying with Ni near the surface regions (Nirich in the second atomic layer) was shown very effective for the activity enhancement of Pt cathode catalyst; for the oxygen reduction reaction (ORR) the activity of the alloyed catalyst appeared 90 times higher than the reference Pt catalyst without Pt-Ni alloying.6 A volcano relationship between the anode activity of Pd and degree of Pd-Co alloying was also observed for alloyed bimetallic Pd-Co cathode catalyst, and the maximum activity appeared on an alloyed Pd11Co with Co in the subsurface layers.44 Thus, the second metal (Ni or Co) in those reported anode Pt-Ni6 and Pd-Co44 catalysts also effected electronically in promoting the catalytic anode reaction. The bifunctional mechanism would not prevail in the electrooxidation of CH3OH and HCOOH on the present Ptm∧Ag-B catalysts since metallic Ag is even much less efficient for water dissociation than a monometallic Pt. As was shown by the BE shifts of Pt in the XPS data (Figure 3, Table 1), the Pt-Ag alloying in Ptm∧Ag-B did effect significant changes in the electronic structure of Pt according to m, which could be a clue to explain the enhanced activity of Ptm∧Ag-B compared to their Ptm∧Ag-A counterparts with no Pt-Ag alloying. Thus, the very positive effect of Pt-Ag alloying on the activity (MSA) of Pt in Ptm∧Ag-B at around m ) 0.5 could mainly arise from an electronic (ligand) effect of Ag on Pt. The requirement of an optimized electronic structure (e.g., d-band center position) at the active metal for chemical interaction (chemisorption) of reactants was considered as the

1250 J. Phys. Chem. C, Vol. 113, No. 4, 2009 scientific basis for tuning the alloying degree to maximize activity of most bimetallic electrocatalysts.6,44,54,55 The right degree of Pt-Ag alloying in maximizing the activity of Pt in Ptm∧Ag-B nanostructures could not develop outside the narrow composition range of 0.47 e m e 0.53 (Table 2, Figures 10 and 11). This fact may imply that a well-defined “alloy structure” was generated in Pt0.5∧Ag-B, whose Pt sites could have a specific electronic structure suitable for the electrochemical activation of CH3OH and HCOOH. Further understanding of such Pt-Ag interaction and electronic properties of the Pt sites would require detailed atomic-scale characterizations of the Ptm∧Ag nanostructures. 4. Conclusions We showed that citrate reduction of Pt under reflux from PtIICl42- or PtIVCl62- ions in solutions containing Ag colloids was viable to prepare catalytic Ptm∧Ag nanostructures for electrochemical reactions. No signature of Pt-Ag interaction was detected in Ptm∧Ag-A nanostructures prepared from PtIICl42-, and the utilization/dispersion and mass-specific activity of Pt in these Ptm∧Ag-A samples increased with decreasing m (or the atomic Pt/Ag ratio). Due to a concurrence of the galvanic replacement reaction between metallic Ag and PtIVCl62- ions during the preparation, significant Pt-Ag alloying at the surface region of Ag colloids was detected in Ptm∧Ag-B nanostructures prepared from PtIVCl62-. Fine-tuning the composition by m resulted in properly alloyed specific Pt0.5∧Ag-B catalyst with remarkably higher activity for the electrooxidation reactions of CH3OH and HCOOH. These results show that proper alloying of Pt with the underlying metal surface in Ptm nanostructures is of vital importance for the activity enhancement of Pt for the electrochemical reactions. Acknowledgment. We thank Dr. Jun Liu (PNNL, USA) for his help in TEM measurements, and Mrs. Feng-En Chen and Prof. Gao-Quan Shi (Department of Chemistry, Tsinghua University) for their help in Raman measurements. This work is supported by NSF 20590362 & 20773074 and MOST (2006AA03Z225) of China. References and Notes (1) Carrette, L.; Friedrich, K. A.; Stimming, U. ChemPhysChem 2000, 1, 162. (2) Costamagna, P.; Srinivasan, S. J. Power Sources 2001, 102, 242. (3) Preli, F. Fuel Cells 2005, 2, 5. (4) Dillon, R.; Srinivasan, S.; Arico`, A. S.; Antonucci, V. J. Power Sources 2004, 127, 112. (5) Cameron, D. S. Platinum Met. ReV. 2007, 1, 27. (6) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493. (7) Park, I. S.; Lee, K. S.; Jung, D. S.; Park, H. Y.; Sung, Y. E. Electrochim. Acta 2007, 52, 5599. (8) Stamenkovic, V. R.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Nørskov, J. K. Angew. Chem., Int. Ed. 2006, 45, 2897. (9) Zhao, D.; Xu, B. Q. Angew. Chem., Int. Ed. 2006, 45, 4955. (10) Zhao, D.; Xu, B. Q. Phys. Chem. Chem. Phys. 2006, 8, 5106. (11) Zhao, D. Ph. D. Thesis, Tsinghua University, June 2007. (12) Zhao, D.; Bing, Y.; Xu, B. Q. Electrochem. Commun. 2008, 10, 884. (13) Wang, Y. H.; Zhao, D.; Xu, B. Q. Chin. J. Catal. 2008, 29, 297.

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