X-ray Photoelectron Spectroscopic Study of the Activation of

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X-ray Photoelectron Spectroscopic Study of the Activation of Molecularly-Linked Gold Nanoparticle Catalysts Mathew M. Maye,† Jin Luo,† Yuehe Lin,‡ Mark H. Engelhard,‡ Maria Hepel,§ and Chuan-Jian Zhong*,† Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902, Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, and Department of Chemistry, State University of New York at Potsdam, Potsdam, New York 13676 Received August 15, 2002. In Final Form: October 20, 2002 This paper reports the results of a study on the activation of core-shell assembled gold nanoparticle catalysts using X-ray photoelectron spectroscopy (XPS). The goal is to determine the surface reconstitution of the nanostructured catalysts upon electrochemical activation for the electrocatalytic oxidation of methanol. The decanethiolate-capped gold nanoparticles of 2∼5 nm core sizes were assembled as catalyst thin films on electrode surfaces using 1,9-nonanedithiol and 11-mercaptoundecanoic acid as model molecular linkers. The XPS results have provided two important insights into the surface reconstitution of the activated nanostructure. First, the capping/linking thiolates or dithiolates are partially removed to produce the catalytic access, with the degree of removal being dependent on the nature of the molecular linker. Second, oxygenated species are detected on the activated gold nanocrystals, demonstrating the formation of surface gold oxide and its participation in the electrocatalytic oxidation of methanol. The findings are also correlated with results from studies of surface microscopic morphology and interfacial mass flux and provide further insights into issues related to the design and preparation of highly active nanostructured gold catalysts.

Introduction Nanostructured gold catalysts have attracted increasing interest1-3 since the pioneer work of Haruta4 which demonstrated unusually high catalytic activities for CO and hydrocarbon oxidation when the nanoparticles were made less than ∼10 nm diameter in size and supported on oxides. While it is known that a combination of factors is responsible for the high catalytic activity of nanosized gold, including size, support, and preparative route, the understanding of the mechanistic aspects in such a restricted size range is an active topic of current research.1 We have recently demonstrated an approach to the preparation of nanoscale metal catalysts, that is, coreshell assembled nanoparticles that consist of metal or alloy nanocrystal cores and organic monolayer shells with molecular wiring or linkage to define the interparticle spatial property.5-6 This approach derives its motivation from diverse attributes of such core-shell nanoparticles, including size controllability, monodispersity, processi* To whom correspondence should be addressed. Phone: 607777-4605. E-mail: [email protected]. † State University of New York at Binghamton. ‡ Pacific Northwest National Laboratory. § State University of New York at Potsdam. (1) (a) Bond, G. C.; Thompson, D. T. Catal. Rev. 1999, 41, 319. (b) Bond, G. C.; Thompson, D. T. Gold Bull. 2000, 33, 41; and references therein. (2) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (3) (a) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W. D.; Hakkinen, H.; Barnett, R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 9573. (b) Heiz, U.; Sanchez, A.; Abbet, S.; Schneider, W. D. Chem. Phys. 2000, 262, 189. (c) Schmid, G.; Emde, S.; Maihack, V.; Meyer-Zaika, W.; Peschel, S. J. Mol. Catal. A 1996, 107, 95. (4) (a) Haruta, M. Catal. Today 1997, 36, 153, and references therein. (b) Haruta, M.; Date, M. Appl. Catal., A 2001, 222, 427. (5) (a) Maye, M. M.; Lou, Y. B.; Zhong, C. J. Langmuir 2000, 16, 7520. (b) Lou, Y. B.; Maye, M. M.; Han, L.; Luo, J.; Zhong, C. J. Chem. Commun. 2001, 473. (c) Luo, J.; Lou, Y. B.; Maye, M. M.; Zhong, C. J.; Hepel, M. Electrochem. Commun. 2001, 3, 172. (6) Zhong, C. J.; Maye, M. M. Adv. Mater. 2001, 13, 1507.

bility, and stability.7-11 The nanoscale building blocks can be viewed as a new class of catalyst candidates with sizetunable and aggregation- or poison-resistant catalytic properties. To find effective means to activate the catalytic activity toward electrooxidation of CO and methanol in alkaline electrolytes, which is of fundamental importance to fuel cell catalysis,12 we recently demonstrated that it is possible to activate the catalytic activity by applying an anodic polarization potential.5 The understanding of how the core-shell surface components reconstitute in such a catalytic activation process will have important implications to the design and fabrication of nanostructured catalysts. In this paper, we report the results of a study of the activation of the core-shell assembled gold nanoparticle catalysts using X-ray photoelectron spectroscopy (XPS). (7) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Comm. 1994, 801. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Comm. 1995, 1655. (8) (a) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212. (b) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (c) Hostetler, M. J.; Zhong, C. J.; Yen, B. K. H.; Anderegg, J.; Gross, S. M.; Evans, N. D.; Porter, M. D.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 9396. (d) Shon, Y. S.; Gross, S. M.; Dawson, B.; Porter, M.; Murray, R. W. Langmuir 2000, 16, 6555. (9) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (10) (a) Leibowitz, F. L.; Zheng, W. X.; Maye, M. M.; Zhong, C. J. Anal. Chem. 1999, 71, 5076. (b) Zheng, W. X.; Maye, M. M.; Leibowitz, F. L.; Zhong, C. J. Anal. Chem. 2000, 72, 2190. (c) Han, L.; Maye, M. M.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J. J. Mater. Chem. 2001, 11, 1258. (11) (a) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137. (b) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499. (c) Templeton, A. C.; Zamborini, F. P.; Wuelfing, W. P.; Murray, R. W. Langmuir 2000, 16, 6682. (12) (a) Acres, G. J. K.; Frost, J. C.; Hards, G. A.; Potter, R. J.; Ralph, T. R.; Thompsett, D.; Burstein, G. T.; Hutchings, G. J. Catal. Today 1997, 38, 393. (b) Wasmus, S.; Kuver, A. J. Electroanal. Chem. 1999, 461, 14.

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We note that XPS studies of as-prepared gold and alloy nanoclusters with alkanethiolate monolayer protection have previously been reported.8b-d,13 The focus in this work is to probe the surface reconstitution of the nanostructures upon catalytic activation for the electrocatalytic oxidation of methanol. The decanethiolate-capped gold nanoparticles of 2-nm and 5-nm core sizes assembled on planar substrates using 1,9-nonanedithiol and 11-mercaptoundecanoic acid as molecular linkers were studied as a model system. XPS is employed to detect the identity of surface species and to analyze the elemental composition or oxidation states of the nanomaterials, from which we derive structural information about the surface reconstitution of the core-shell nanostructured catalysts. Experimental Section Chemicals. Major chemicals included decanethiol (DT, 96%), 1,9-nonanedithiol (NDT, 95%), 11,-mercaptoundecanoic acid (MUA, 98%), hydrogen tetracholoroaurate (HAuCl4, 99%), tetraoctylammonium bromide (TOABr, 99%), sodium borohydride (NaBH4, 99%), toluene (99.8%), hexane (99.9%), and methanol (99.9%). All chemicals were purchased from Aldrich and used as received. Water was purified with a Millipore Milli-Q water system. Synthesis. The synthesis of DT-capped gold nanoparticles of ∼2 nm core size followed the standard two-phase protocol.7a,8b Briefly, AuCl4- was first transferred from aqueous HAuCl4 solution (10 mM) to toluene solution by phase transfer reagent TOABr (36 mM). Thiols, for example, DT, were then added to the solution (21 mM) at a DT/Au mole ratio of 2:1. An excess of aqueous reducing agent (NaBH4) was slowly added into the solution. The reaction was allowed to proceed under stirring at room temperature for 4 h, producing a dark-brown solution of DT-encapsulated nanoparticles (1.9 ( 0.7 nm), which were cleaned using ethanol and acetone. DT-capped gold nanoparticles of a larger core size were also studied, which were produced from the 2-nm particles by a thermally activated processing route.14 The size of the processed gold nanoparticles is 5.3 ( 0.3 nm. Preparation. Details of the preparation of the molecularly linked gold nanoparticle films were described in previous reports,10 which involves an exchange-cross-linking-precipitation route. Briefly, NDT or MUA was used as the molecular linker agent. A substrate electrode (glassy carbon, HOPG, or graphite) was immersed into a hexane solution of DT-capped nanoparticles (∼5 µM) and molecular linker (NDT ∼ 25 µM or MUA ∼ 0.3 µM) for 5-10 h. The thickness of the resulting nanoparticle thin films on the substrate, that is, NDT-Au2-nm or MUA-Au2-nm, was controlled by immersion time. The films were thoroughly rinsed with the solvent and dried under argon before characterizations. For the XPS analysis, the samples were removed at open circuit potential after the anodic sweeping. On the basis of previous studies on the exposure effect for SAM monolayers on gold, we believe that the exposure to Al Ka X-ray source, as we used in this work, has an insignificant damage effect on the surface species. Several types of substrates were used for the thin film preparation. Glassy carbon (GC) disks (geometric area: 0.5 cm2), polished with 0.03 µm Al2O3 powders, were mainly used for electrochemical and AFM characterizations. Sheets of graphite and highly oriented pyrolytic graphite (HOPG) and graphite disks were utilized for electrochemical and XPS characterizations. Instrumentation. Electrochemical Measurement. Electrochemical measurements were performed using an EG&G Model 273A potentiostat. A three-electrode cell was employed, with a platinum coil as the auxiliary electrode and a standard calomel electrode (SCE) as the reference electrode; all potentials are given with respect to this reference. All electrolytic solutions were deaerated with high-purity argon before the measurement. An Electrochemical Quartz Crystal Nanobalance setup was employed (13) Bourg, M. C.; Badia, A.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 6562. (14) (a) Maye, M. M.; Zheng, W. X.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J. Langmuir 2000, 16, 490. (b) Maye, M. M.; Zhong, C. J. J. Mater. Chem. 2000, 10, 1895.

Maye et al. for measurements of mass changes in the voltammetric experiment. It was composed of a microcomputer-controlled potentiostat (Model PS-1705, ELCHEMA) and EQCN-900 instrument (ELCHEMA). The error limit of the mass detection is less than (0.1 ng. XPS Analysis. The XPS measurements were made using a Physical Electronics Quantum 2000 Scanning ESCA Microprobe. This system uses a focused monochromatic Al KR X-ray (1486.7 eV) source for excitation and a spherical section analyzer. The instrument has a 16 element multichannel detection system. The X-ray beam used was a 98 W beam with a 107-µm diameter that is rastered over a 1.4 mm by 0.2 mm rectangle on the sample. The X-ray beam is incident normal to the sample and the X-ray detector is at 45° away from the normal. The collected data were referenced to an energy scale with binding energies for Cu (2p3/2) at 932.67 ( 0.05 eV and Au (4f7/2) at 84.0 ( 0.05 eV. The percentages of individual element detected were determined from the relative composition analysis of the peak areas of the bands. It is based on the relative peak areas and their corresponding sensitivity factors to provide relative compositions. In the deconvolution of XPS spectra, we used curve fitting with a Lorentzian-type profile. For S(2p), the constraint is a doublet of peaks that have an equal half-height width (0.90 ( 0.05 eV).15 This doublet consists of a spin-orbital coupling splitting of S(2p) with 2p3/2 at 162.0 eV and 2p1/2 at 163.2 eV with an intensity ratio of 2:1 (i.e., ∝ 2J + 1, where J is the coupling quantum number). In comparison with the corresponding precursors (∼163.3 and ∼164.5 eV15), the chemical shift of S(2p) is in agreement with previous data assigned to thiolate species.16 A Multimode NanoScope IIIa (Digital Instruments), equipped with an E scanner, was utilized for imaging. The capability of TappingMode (TM)-AFM allows imaging at minimum disruption of the nanostructures. Standard silicon cantilevers (Nanoprobes) were used.

Results and Discussion In the following sections, we first outline the electrochemical data for the characterization of the electrochemical activation and catalytic activity. We then present XPS data, which reveal insights into surface binding and composition of the nanostructured catalysts after the catalytic activation. This section is followed by discussing implications of our findings to the understanding of the nanostructured gold catalysis. 1. Electrochemical Activation. Figure 1 shows a typical set of cyclic voltammetric (CV) data to demonstrate the electrocatalytic activity toward methanol oxidation at gold nanoparticles assembled by two types of molecular linkers, NDT and MUA. The NDT linkage involves Authiolate bonding at both ends of NDT, whereas the MUAlinking involves an Au-thiolate bonding at the -SH end and a head-to-head hydrogen bonding at the -CO2H terminal group.10b Using as-prepared thin films, the voltammetric curves for both molecular linkers are basically featureless (a). Upon catalytic activation (b), which is achieved by either the application of a constant anodic potential (+800 mV) or the continuous cycling of the potential between -400 and +800 mV, a large oxidation wave is evident (b). The activity is dependent on the nature of the molecular linkers, which is reflected by the difference between Figure 1A and B in terms of peak current and potential. For the electrooxidation of MeOH at NDT-Au2nm film of ∼5 layers (15) Lindberg, B. J.; Hamrin, K.; Johansson, G.; Gelius, U.; Fahlman, A.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 1, 286. (16) (a) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (b) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (c) Schonherr, H.; Ringsdorf, H. Langmuir 1996, 12, 3891. (d) Trevor, J. L.; Lykke, K. R.; Pellin, M. J.; Hanley, L. Langmuir 1998, 14, 1664. (e) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (f) Zhong, C. J.; Zak, J.; Porter, M. D. J. Electroanal. Chem. 1997, 421, 9.

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Figure 1. Cyclic voltammetric curves for the electrocatalytic oxidation of methanol (3M) in 0.5 M KOH electrolyte at a glassy carbon electrode coated with NDT-Au2-nm film (A) and MUA-Au2-nm film (B). Film thickness: ∼5 equivalent layers. Before activation: dashed line (a). After activation: solid line (b). A ) 0.5 cm2, v ) 50 mV/s.

on glassy carbon electrode (Figure 1), the film is activated after continuously cycling of the potential between -400 and +800 mV for about 20 times. The large anodic wave observed at +300 mV (b) is attributed to the oxidation wave of methanol.5b The control experiment, as reported in our recent work5b (and later in Figure 4), did not show this wave. This wave is superimposed with the gold oxidation wave. The cathodic wave is attributed to the reduction of gold oxides. A detailed peak assignment has been reported in our previous work.5 In comparison, the similarly activated MUA-Au2-nm film on glassy carbon electrode is found to display a higher electrocatalytic activity (Figure 1B). The thickness of the film is also ∼5 layers, which has a surface coverage very close to the previous NDT-Au2-nm film. The peak potential of the oxidation wave for the activated MUA-Au2-nm (+280 mV) is slightly lower than that for the activated NDT-Au2-nm film. The peak current is larger by a factor of ∼2. The MUA-linked film is relatively easy to be activated, as evidenced by the fact that a less number of potential cycling (∼10 cycles) between -400 and +800 mV was needed to activate the film. The MUA film was completely activated by only 10 cycles, after which the number of cycles had no effect. Similar voltammetric characteristics have also been observed for thin films assembled from 5-nm gold nanoparticles, though subtle differences in peak potential and current can be identified. The observation that the MUA-linked film was more active than the NDT-linked film was reproducible on the basis of at least 10 sets of experiments with different film thicknesses. The ready catalytic activation of the MUAlinked nanoparticles by the potential polarization is intriguing because it demonstrates the influence of the linker molecular structure on the catalytic activity. It is previously shown that the electronic conductivity of NDTlinked gold nanoparticles is higher than the MUA-linked nanoparticles by 2∼3 orders of magnitude. The electronic effect does not, however, explain the observed difference in the catalytic activation of these two types of films. We thus believe that the electronic effect had a minimum impact when the film was very thin (∼5 layers). There are two explanations. First, it is very likely that the presence of a large open framework and an easy access of electrolytes are operative for accounting for the ready catalytic activation of the MUA-linked nanoparticle film. Second, the formation of carboxylate-terminated thiolates in the electrolyte is likely to enable a better solubility of MUA than NDT. This phenomenon was known from early

reductive desorption studies of self-assembled alkanethiolate monolayers on planar gold surfaces.16e,f A more indepth understanding of how the molecular linker affects the catalytic activity of the activated catalysts requires, however, the knowledge about the surface properties of the activated nanostructures, which is probed next using XPS technique. 2. XPS Characterization. In this section, the description of the results of the XPS analysis of the nanostructured catalysts before and after the electrochemical activation is mainly focused on the 2-nm particle system. The discussion is divided in two parts, (A) XPS spectral characteristics and (B) surface composition analysis. Similar voltammetric characteristics have been observed at various carbon-based electrode materials, such as glassy carbon, graphite, and HOPG. A. Spectral Characteristics. Figures 2 and 3 show two representative sets of XPS spectra for the NDT-Au2-nm and MUA-Au2-nm films, respectively. The spectra are shown for the regions of S(2p) (A), O(1s) (B), and Au(4f) (C). The C(1s) feature at 284.6 eV is not included because it is largely from the carbon substrate. The as-prepared film (a) and the electrochemically activated film (b) are compared in each spectral region. The S(2p) region (Figure 2A) is characterized by a doublet that arises from spin-orbit coupling (2p3/2 and 2p1/2). For most neat thiols, this region is generally defined by the more intense 2p3/2 band which lies between 163 and 165 eV. In contrast, the binding energy (BE) observed for monolayers derived from thiols in which the sulfur species interact strongly with the surface Au(I) are ∼1 eV lower, that is, ∼162 eV for the 2p3/2 band8b-c,13,17. The deconvolution of this doublet using 2p3/2 to 2p1/2 intensity ratio of ∼2:1 as a constraint (dashed lines under the S(2p) envelope) reveals a small fraction of doublet component at 163.5 and 164.5 eV, which we believe is likely due to free -SH group from one end of the NDT linker molecule. This component is more significant in the S(2p) spectra for NDT-Au5-nm film (not shown). After the catalytic activation, the S(2p) band intensity is significantly reduced, but still detectable, indicating a partial removal of the thiolates species. A close examination of the envelope of the remaining (S2p) band (b) in comparison with the as-prepared film (a) is also suggestive of the presence of two components, which is believed to be due to the presence (17) Zhong, C. J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518.

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Figure 2. XPS spectra in regions of S(2p) (A), O(1s) (B), and Au(4f) (C) for a NDT-Au2-nm film before (a) and after (b) the electrochemical activation.

Figure 3. XPS spectra in regions of S(2p) (A), O(1s) (B), and Au(4f) (C) for a MUA-Au2-nm film before (a) and after (b) the electrochemical activation.

of unremoved thiolates and weakly bound -SH group from one end of NDT. Furthermore, the detection of the small band at 169.2 eV is indicative of the presence of sulfonate species (-SO3-).18 This finding implies the possibility that the desorption of the capping/linking agent involves oxidation of thiolate to sulfonate species. This mechanism is qualitatively consistent with earlier findings in studies of electrochemical oxidative desorption of self-assembled alkanethiolate monolayers on planar gold surfaces.19 In the MUA-Au2-nm film (Figure 3A), a similar S(2p) spectral feature is evident for the as-prepared film, demonstrating the similarity of the Au-thiolate bonding. The deconvolution of this doublet reveals effectively the absence of the higher-BE component as seen in Figure 2A. This finding is consistent with the fact that only one thiolate is present in the MUA molecule, in contrast to the NDT case. After catalytic activation, it is evident that the removal of thiolates is much more effective than that in the case of the NDT-linked nanoparticles. The S(2p) bands corresponding to thiolate and sulfonate species are (18) (a) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer: Eden Prairie, 1978. (b) Briggs, D.; Seah, M. P. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; John Wiley & Sons: New York, 1983. (c) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657. (19) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335.

barely detectable. While thiolates are partially removed, the gold particles were found to stay on the electrode surfaces because of their insolubility in the electrolyte and particle-particle or particle-substrate interactions. This was evidenced by AFM images to be shown later. There may be a small fraction of nanoparticles being removed as a result of the dithiolate removal, which should not be a significant factor on the basis of our studies of thickness effect, in which the catalytic activity was mainly from the particles accessible by methanol, and analysis of the particles in the electrolyte. By prolonged electrolysis, some loss of the catalytic activity was however detected. In the O(1s) region (Figure 2B), the O(1s) feature is detected at 532.3 eV for the as-prepared film (a), which is slightly above the noise level of the background. The presence of this trace of oxygen species in the film is either due to impurity on the nanocrystal surfaces or simply surface species from the substrate. The latter assessment is supported by a control experiment. After activation (b), a significant increase of the O(1s) band is detected at 532.4 eV. The peak position is quite close to those observed for surface oxide species.18 We attribute it to the formation of oxygenated species (oxides) on the surface of gold nanocrystals. The detected O(1s) band also contains a contribution of sulfonate species after the activation. While the sulfonate is present, its contribution to the total O(1s) should be relatively small on the basis of the relative

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Table 1. Relative Compositions Based on the XPS Analysis of the Nanostructured Gold Catalyst Films before (As-Prepared) and after Electrochemical Activation (E-Activated) composition thin film

treatment

sulfur % oxygen % oxygen gold %

NDT-Au2-nm as-prepared e-activated

8.2 2.1

0.3 10.7

17.2 7.0

as-prepared e-activated

2.7 0.7

3.7 7.6

10.4 9.2

MUA-Au2-nm

intensity ratios of the S(2p) peaks to the O(1s) peak. The formation of gold oxides is consistent with the condition of the electrochemical experiment which involves the application of an anodic potential (> +600 mV) and the use of an alkaline electrolyte. For the MUA-Au2-nm film (Figure 3B), the O(1s) band is detected at 532.6 eV. This band is largely due to carboxylic acid groups of the MUA linkers. After activation (b), the O(1s) band is increased but to a less degree in comparison with the NDT-Au2-nm film. The band also shows a slight shift to 532.0 eV. The detected oxygen species is produced by the activation, which is similar to the case for the activation of NDT-Au2-nm film. In the Au(4f) region (Figure 2C), a close examination of the band shape and position reveals two major differences after the catalytic activation. First, the overall peak position is shifted to a lower BE by ∼0.4 eV. Second, there are two overlapping components under the band envelope. By spectral deconvolution, we have identified two sets of Au(4f) bands, that is, 84.1 and 84.9 eV for Au(4f7/2), respectively (dashed lines). The lower BE component is attributed to a combination of Au(0) and Au(I) after the partial removal of thiolates, whereas the higher BE component is likely indicative of the presence of both Au(I) and Au(III) oxides (e.g., Au2O3 or Au(OH)x). The assessment about the presence of gold oxides is supported by the detection of the oxygen species. The subtle difference in the intensity of the Au(4f) band is likely due to a combination of the surface reconstitution and attenuation effects. For the MUA-Au2-nm film (Figure 3C), the general spectral feature for the Au(4f) band before and after the catalytic activation is similar to that for the NDT-Au2-nm film. However, a subtle difference is reflected by the deconvolution result of the activated samples. The higher BE component (∼84.9 eV) has much less intensity in comparison with the activated NDT-Au2-nm film. The difference is not completely understood at this time, but one possible implication is the presence of less gold oxides. This possibility is qualitatively consistent with the fact that less oxygenated species are detected for the MUAAu2-nm film than for the NDT-Au2-nm film. B. Composition Analysis. In Table 1, the relative elemental composition of the nanostructured catalysts before and after the electrochemical activation is further compared. The changes of the sulfur and the oxygen percentages provide quantitative information to assess the surface reconstitution. For the NDT-Au2-nm film, we have found that the surface relative compositions for both sulfur and oxygen species are changed after the catalytic activation. The net change of sulfur percentage (from 8.2% to 2.1%) corresponds to a 75% removal of the thiolate capping/linking monolayers on the gold nanocrystals by the electrochemical activation. The sulfonate species detected is ∼20% of the remaining sulfur-containing species. Accompanying this change,

there is a significant increase of oxygen species (from 0.3% to 10.7%). The ratio of oxygen to sulfur (∼5) suggests that there are relatively more oxygen species than sulfur on the activated gold nanocrystals. In MUA-Au2-nm thin film, the removal of sulfur species upon the electrochemical activation seemed to be more effective. While the net change of the sulfur percentage (from 2.7% to 0.7%), ∼74%, appears similar to the NDT case, the detected sulfur species is