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Langmuir 1997, 13, 594-596
Adsorption of Hexachloroplatinate Complex on Au(111) Electrode. An in Situ Scanning Tunneling Microscopy and Electrochemical Quartz Microbalance Study
Introduction Modification of surface properties by the adsorption of foreign atoms and molecules is attracting many scientists working in very wide areas of chemistry including heterogeneous catalysis, electrochemistry, and sensors. Platinum is one of the most important elements to be used in catalysis,1 and the platinum adatom is often used to improve the surface electrocatalytic properties.2-4 To understand the role of adsorbate for the catalytic action, it is essential to monitor the surface structure in situ with atomic/molecular resolution. Although many reports are available for in situ observation of organic molecules5-7 and so-called underpotentially deposited (UPD) metal adlayer such as copper,8-12 silver,13-15 and lead,16 rather limited evidence has been given for the adsorption of the metal ions including metal ion complexes.17,18 Here we investigated the surface structure of an Au(111) electrode in HClO4 solution containing H2PtCl6 by electrochemical scanning tunneling microscope (STM) at potentials where neither cathodic nor anodic current flowed and found an ordered but rather complicated structure with the lattice constant larger than Au(111), suggesting the formation of a PtCl62- adlayer. A model was proposed for the adlayer structure, and the amount of adsorbate estimated from the model was in good agreement with that determined by the electrochemical quartz microbalance (EQCM) measurement. After the electrode was rinsed by HClO4 solution, this adlayer structure disappeared and a well-defined surface structure of Au(111)-(1×1) was observed in HClO4 solution, showing the adsorption of PtCl62- was weak.
trochemical STM cell which can accommodate a single crystal bead electrode. The STM tip was a simply cut Pt wire (0.3 mm) insulated with nail polish. STM observations were carried out at an atomically flat (111) facet formed on the surface of a gold single crystal bead which was prepared by Clavilier’s method.19 The electrochemical and EQCM characteristics were obtained at a gold electrode prepared by vacuum evaporation of a 3 nm thick layer of titanium followed by a 150 nm thick layer of gold onto a 5 MHz AT-cut quartz crystal plate (diameter 13 mm) at 300 °C with an evaporation rate of 0.01 nm/s. Standard surface treatments including annealing with gas/O2 flame and quenching in pure water were used to obtain the reproducible surface states of the substrate. The formation of the highly ordered Au(111) phase with long range step-terrace structure by these procedures on various substrates has been well documented20-22 and was confirmed in this study. The resonant frequency of quartz crystal electrode, which was oscillated by a home-made oscillation circuit, was monitored simultaneously with an electrode potential and current by a frequency counter (Hewlett-Packard, HP53131A). The counter was controlled by a personal computer (NEC PC8801MH) through a GPIB interface. The mass change was estimated from the resonant frequency shift by the Sauerbrey equation.23,24 The mass sensitivity of the 5 MHz AT-cut quartz crystal was determined by measuring the frequency changes during silver deposition from a 0.1 M HClO4 solution containing 1 mM AgNO3 and lead deposition from 0.1 M HClO4 solution containing 5 mM PbO2. Both measurements gave similar values (the former -19.5 ng/(Hz cm2) and the latter -19.1 ng/(Hz cm2)) and -19.3 ng/(Hz cm2) was used for the calculation of the mass change in this study. Electrode potential was controlled by a potentiostat (Hokuto Denko, HA-151), and external potential modulation was provided by a function generator (Hokuto Denko, HB-111). The potential sweep rate for cyclic voltammetry (CV) was 20 mV/s unless otherwise stated. The frequency stability of the EQCM system was better than 0.1 Hz for a sampling gate time of 0.1 s. Details of the EQCM system used in this study were similar to those described previously.25 Electrolyte solutions were prepared by using Suprapure HClO4 (Wako Pure Chemicals), hydrogen hexachloroplatinate(IV) hexahydrate, H2PtCl6‚6H2O (Kanto Chemicals), and Milli-Q water. A quasi-reversible hydrogen electrode and a platinum wire (0.5 mm) were used as a reference and a counter electrode, respectively. Potential is presented with respect to the reversible hydrogen electrode (RHE) in this paper. All measurements were carried out after the solution was deaerated by passing purified N2 gas through the solution for at least 20 min.
Experimental Section
Results and Discussion
Electrochemical STM measurements were carried out by using a NanoScope E (Digital Instrument) with a home-made elec-
Before the addition of H2PtCl6, in situ STM measurement was carried out in HClO4 solution to make sure the atomically ordered Au(111) surface was exposed. A wide scan image shows flat terraces and monoatomic steps crossed each other with an angle of ca. 60°, and a welldefined atomic arrangement of hexagonal symmetry with a measured periodic spacing of ca. 0.29 nm, which is in agreement with the atomic distance of Au(111) as reported previously,8-11 was observed in a high-resolution image, confirming that a clean Au(111) surface was exposed in HClO4 solution in the present study. Figure 1 shows the potential dependence of current and frequency shift observed at the gold electrode deposited on a quartz crystal measured in 50 mM HClO4 solution containing 0.6 mM H2PtCl6. When the potential was scanned in anodic direction from +1.0 to +1.7 V (solid
Kohei Uosaki,* Shen Ye, Yasuhiro Oda, Toshio Haba, and Kei-ichi Hamada Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060, Japan Received July 23, 1996. In Final Form: October 28, 1996
(1) Somoriai, G. A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca and London, 1981. (2) Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1973, 44, 83. (3) Furuya, N.; Motoo, S. J. Electroanal. Chem. 1978, 88, 151. (4) Leung, L. H.; Weaver, M. J. J. Am. Chem. Soc. 1987, 109, 5113. (5) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337. (6) Ogaki, K.; Batina, N.; Kunitake, M.; Itaya, K. J. Phys. Chem. 1996, 100, 7185. (7) Batina, N.; Kunitake, M.; Itaya, K. J. Electroanal. Chem. 1996, 405, 245. (8) Magnussen, O. M.; Hotlos, J.; Nichols, R. J.; Kolb, D. M.; Behm, R. J. Phys. Rev. Lett. 1990, 64, 2929. (9) Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183. (10) Hachiya, T.; Honbo, H.; Itaya, K. J. Electroanal. Chem. 1991, 315, 275. (11) Batina, N.; Will, T.; Kolb, D. M. Faraday Discuss. 1992, 94, 93. (12) Sashikata, K.; Furuya, N.; Itaya, K. J. Electroanal. Chem. 1991, 316, 361. (13) Kimizuka, N.; Itaya, K. Faraday Discuss. 1992, 94, 117. (14) Chen, C. H.; Vesecky, S. M.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 451. (15) Ogaki, K.; Itaya, K. Electrochim. Acta 1995, 40, 1249. (16) Tao, N. J.; Pan, J.; Li, Y.; Oden, P. I.; DeRose, J. A.; Lindsay, S. M. Surf. Sci. Lett. 1992, 271, L338. (17) Ge, M.; Zhong, B.; Klemperer, W. G.; Gewerth, A. A. J. Am. Chem. Soc. 1996, 118, 5812. (18) Sawaguchi, T.; Yamada, T.; Okinaka, Y.; Itaya, K. J. Phys. Chem. 1995, 99, 14149.
(19) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (20) Chidsay, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45. (21) Zei, M. S.; Nakai, Y.; Lehmpfuhl, G.; Kolb, D. M. J. Electroanal. Chem. 1983, 150, 201. (22) Uosaki, K.; Ye, S.; Kondo, T. J. Phys. Chem. 1995, 99, 14117. (23) Sauerbrey, G. Z. Z. Phys. 1959, 155, 206. (24) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355. (25) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385.
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Figure 2. Time course of the frequency of the gold/QCM electrode immersed in 50 mM HClO4 solution (50 mL volume). The arrows A and B indicate the addition of 0.5 mL of 50 mM HClO4 solution and of 50 mM HClO4 solution containing 60 mM of H2PtCl6, respectively.
Figure 1. Potential dependence of current (top panel) and frequency shift (bottom panel) of the gold electrode deposited on a quartz crystal measured in a 50 mM HClO4 solution containing 0.6 mM H2PtCl6: solid line, first scan from +1.0 V f +1.7 V f +0.6 V f +1.0 V; broken line, second scan from +1.0 V f +1.7 V f +1.0 V.
line), anodic current started to flow around +1.3 V. The increase of the frequency during the anodic current flow clearly showed that this current was mainly associated with the dissolution of the electrode. This is in contrast to the result obtained in HClO4 solution where the frequency was decreased with the anodic current flow and the frequency change was much smaller, showing the anodic current was due to the formation of gold oxide. Dissolution of the gold electrode surface may be related to the existence of the small amount of free Cl- in solution. In the reverse scan, cathodic peaks were observed at +1.2 and +1.1 V. The frequency was increased and decreased during the first and second cathodic peaks, respectively. These results suggest the first peak was due to the reduction of gold oxide and the second peak may be due to the reduction, i.e., redeposition of AuCl42- (cf. standard redox potential of AuCl42-/Au is +1.0 V vs NHE26). Cathodic current increased significantly and the frequency of the EQCM decreased as the potential became more negative than +0.70 V. This current should be related to the platinum bulk deposition onto the gold surface as the equilibrium potential of PtCl62-/Pt is +0.75 V.27-29 The potential-current relation in the second scan (broken line) was different from that of the first scan because the surface was covered with a Pt layer. The details of the electrochemical characteristics of gold in this solution are now under detailed investigation. The adsorption of PtCl62- was investigated by in situ EQCM measurement while potential was kept at +1.0 V where neither cathodic nor anodic current flowed. Figure 2 shows the time course of the frequency of the gold/QCM electrode immersed in 50 mM HClO4 solution (50 mL volume). After the constant frequency was attained, 0.5 mL of 50 mM HClO4 solution was added into the cell (arrow A). Except for the transient perturbation for a short period, no net change of the frequency was observed. On (26) Schmid, G. M.; Curley-Fiorino, M. E. Encyclopedia of the Electrochemistry of the Elements, IV; Bard, A. J., Ed.; Marcel Dekker, Inc.: New York, 1975; pp 87-178. (27) Goldberg, R. N.; Hepler, L. G. Chem. Rev. 1968, 68, 229-252. (28) Ginstrup, O. Acta Chem. Scand. 1972, 26, 1527-1541. (29) Llopis, J. F.; Colom, F. Encyclopedia of the Electrochemistry of the Elements, VI; Bard, A. J., Ed.; Marcel Dekker, Inc.: New York, 1976; pp 169-219.
the other hand, the frequency decreased steeply when 0.5 mL of 50 mM HClO4 solution containing 60 mM of H2PtCl6 was added into the solution (arrow B) so that the final concentration of H2PtCl6 in the solution became 0.6 mM, showing the adsorption of some form of the platinum complex. The amount of adsorbate calculated from the total frequency change (-6 Hz) and the mass sensitivity of the quartz crystal (-19.3 ng/(Hz cm2)) is 115.8 ng/cm2, which is equivalent to 2.8 × 10-10 mol/cm2 if one assumes that the adsorbate is PtCl62-. Figure 3 shows typical STM images obtained at a (111) facet of the gold single crystal bead electrode in 50 mM HClO4 solution containing 0.6 mM H2PtCl6 at +0.9 V where no current flowed. In a region of 20 × 20 nm2, a number of domains with an ordered surface adlayer structure was observed (Figure 3a). A high-resolution STM image of 5 × 5 nm2 shows a rather complicated adlayer structure of spots with different brightness and contrast (Figure 3b). The spots of the same height showed a hexagonal symmetry with a nearest neighbor distance of ca. 0.76 nm, which is 2.64 (nearly equal to x7) times longer than that of the unit length of the Au(111) lattice. The nearest neighbor distance between the brightest and second brightest spots is ca. 0.33 nm. The highly ordered image was obtained as far as the potential was kept more negative than +1.3 V where anodic current started to flow. This should be due to the fact that the anodic current is associated with the dissolution of the surface as mentioned before. After the STM images shown in Figure 3 were obtained, the electrolyte solution in the STM cell was replaced by a 50 mM HClO4 solution several times while the electrode potential was kept at +0.9 V so that the electrode was thoroughly rinsed and, then, the STM observation of the same facet was carried out in a solution containing only 50 mM HClO4. The adlayer structure shown in Figure 3 disappeared, and a new atomically ordered structure was observed as shown in Figure 4. The nearest neighbor atomic distance was ca. 0.29 nm, which is equal to that of the Au(111)(1 × 1) lattice. If the PtCl62- complex was introduced into the STM cell again, STM images similar to those shown in Figure 3 were observed. These results suggest that the STM images shown in Figure 3 should be related to some species which adsorbed weakly on the Au(111) electrode surface. Since the equilibrium potentials of PtCl62-/Pt(0.75 V) and PtCl62-/PtCl42- (0.77 V)27-30 are more negative than +0.9 V at which the STM observation was carried out, the most probable adsorbed species is the PtCl62- itself. (30) Yamamoto, H.; Tanaka, S.; Takashi, N.; Takeshi, T. Denki Kagaku 1964, 32, 43.
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Figure 4. STM image of a (111) facet of the gold single crystal electrode in 50 mM HClO4 at +0.9 V after the electrode was rinsed by 50 mM HClO4 at the same potential after Figure 3 was obtained.
Figure 5. A proposed model for the surface structure of the PtCl62- adlayer on the Au(111) surface with an adlattice structure of (x7 × x7)R19.1°. A molecular model drawn from its respective atom-atom distances is also shown. Figure 3. STM images of a (111) facet of the gold single crystal electrode in regions of (a) 20 × 20 nm2 and (b) 5 × 5 nm2 obtained in 50 mM HClO4 solution containing 0.6 mM H2PtCl6 at +0.9 V where no current flowed. The brightest and second brightest spots are indicated by triangles and pluses, respectively. The distances between these two spots are also shown.
The above results can be well explained by a model shown in Figure 5 in which we assumed that the adsorbate is PtCl62- and the surface structure of the PtCl62- adlayer on the Au(111) surface is (x7 × x7)R19.1°. A molecular model of PtCl62- drawn from its respective atom-atom distances is also shown in the figure. The Pt4+ center cation is much smaller than the Cl- ion, and it is difficult to see it from the surface of the model. Since PtCl62- is expected to adsorb through three of its six chloride atoms onto the Pt(111) surface and several adsorption sites are available on the Pt(111) surface, the vertical positions for these chloride atoms should be different. The highest and the second highest chloride atoms shown in Figure
5 correspond to the brightest and second brightest spots in Figure 3b, respectively. The other weaker spots observed in Figure 3b should be related to the chloride atoms at lower positions shown in the model. The amount of adsorbate based on this model is 3.3 × 10-10 mol/cm2, which is in good agreement with the value determined from the EQCM measurement with the assumption that adsorbate is PtCl62- (2.8 × 10-10 mol/ cm2). This result supports the model. In conclusion, we demonstrated that the highly ordered but weakly adsorbed PtCl62- layer was formed on Au(111) surface in solution containing H2PtCl6 and proposed a model for the adlayer structure. Acknowledgment. This work was partially supported by Yamada Science Foundation. We thank Dr. M. Koinuma for drawing Figure 5. LA960728M
