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Structural changes at various Pt single crystal surfaces with potential cycles in acidic and alkaline solutions
Journal of Electroanalytical Chemistry 467 (1999) 85 – 91
Structural changes at various Pt single crystal surfaces with potential cycles in acidic and alkaline solutions Nagakazu Furuya *, Masami Shibata Department of Applied Chemistry, Faculty of Engineering, Yamanashi Uni6ersity, 4 -3 Takeda, Kofu 400 -8511, Japan Received 24 August 1998; received in revised form 23 January 1999; accepted 9 February 1999
Abstract Structural changes at various Pt single crystal surfaces of higher Miller index before and after many potential cycles were examined with in situ STM and cyclic voltammetry in acidic and alkaline solutions. STM images at Pt(111), Pt(100) and Pt(210) after 100 potential cycles in 0.05 M H2SO4 solution were observed. Many islands of platinum atoms on the terraces of Pt(111) and Pt(100) were found after the potential cycles, but not on Pt(210). The Pt(210) surface is changed with more difficulty with potential cycling than the Pt(111) and Pt(100) surfaces. Voltammograms for surfaces of higher Miller index after 500 potential cycles were examined. The sharp peaks of the hydrogen desorption wave become wider after the potential cycles. After the potential cycles, the shape of the voltammograms at Pt(111) and Pt(110) groups change into that at Pt(320) or Pt(530). The shapes of the voltammograms at Pt(100) groups change into that at Pt(310) after the potential cycles. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Pt single crystal; Higher Miller index; Electrochemical activation; In situ STM; Structural change
1. Introduction It is 18 years since the vivid feature of the hydrogen wave for the Pt(111) face in a voltammogram was reported by Clavilier et al. [1,2]. Recently, welldefined single crystal surfaces are necessities for the investigation of electrochemical behavior [1 – 11], electrocatalysis [12–21] and STM measurements [22– 24,29,31]. Electrocatalysis on the oxidation and reduction of simple organic compounds has been studied at Pt single crystals. It is known that the surface structure of Pt single crystals affects the electrocatalytic activity [13 – 15,25 – 27].
Dedicated to Jean Clavilier on the occasion of his retirement from LEI CNRS and in recognition of his contribution to Interfacial Electrochemistry. * Corresponding author. Tel.: +81-552-208559; fax: + 81-552208560. E-mail address: [email protected] (N. Furuya)
Potential cycles between hydrogen and oxygen evolution are usually applied to a polycrystalline Pt electrode after etching with aqua regia when the electrocatalytic activity is examined. The electrocatalytic activity at the Pt electrode is enhanced with this potential cycling, i.e. the ‘electrochemical activation’, and becomes reproducible. Voltammograms at the Pt electrode after etching with aqua regia become reproducible by means of the electrochemical activation. Structural changes at the Pt(111) surface during electrochemical activation have been investigated with in situ STM [28]. Randomly oriented islands, whose height corresponds to a few atomic steps, have been observed on the Pt(111) terrace after the electrochemical activation [28]. Moreover a few studies on a Pt(100) surface during electrochemical activation were observed with in situ STM [29–31]. In contrast to the existing papers, we report new systematic results on the structural changes at various Pt single crystal surfaces of higher Miller index in acidic and alkaline solutions.
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2. Experimental A bead single crystal was prepared by the flame float-zone method, being similar to Dash’s technique [32] for growing dislocation-free silicon single crystals. The center of a Pt wire (diameter= 1 mm, length = 150 mm, 99.99% purity) was melted locally in an oxygenrich hydrogen flame. The melted float-zone was moved gradually toward the end of the wire. Then the melted float-zone, being a single crystal with a high quality, was grown to 15–20 mm length. A large single crystal was grown using this single crystal as a seed. The size of the single crystal for the float-zone method was about 4 mm in diameter. The crystal was oriented by the laser beam method and then cut to expose various planes, and the surface of the exposed plane was polished with diamond paste. Then the surface was annealed at 1500°C for 3 h in a flame. For electrochemical characterization of the Pt surface, the single crystal was annealed at 1500°C for 5 s, again in a flame, cooled immediately in a stream of helium gas with 0.1% hydrogen [33], and then immersed in ultrapure water saturated with hydrogen. The electrode mounted in its all-glass holder was transferred to an electrochemical cell [33], while protecting the single crystal surface with a drop of water [1]. Measurements of the linear sweep voltammograms were conducted by the dipping method [1]. Sulfuric acid solution (0.05 mol dm − 3 = M) and sodium hydroxide solution (0.1 M) were prepared by dissolving reagent grade H2SO4 and NaOH in twice distilled water, respectively. All potentials were referenced to a reversible hydrogen electrode (RHE). Potential cycles (sweep rate: 0.5 V s − 1) between 0.05 and 1.5 V were applied to the Pt single crystal electrode for the electrochemical activation. For STM observation, the single crystal bead was annealed at 1500°C for 3 s again in a flame and was cooled in a stream of helium gas with 0.1% hydrogen [33]. STM images were observed at the single crystal surface at 0.6 V. In situ STM was carried out with a Seiko STE-330 (Seiko Instruments). The STM probe was a tungsten wire covered with resin in such a manner that only the tip-point was exposed.
(a). STM images were observed at 1.2 V in the positive scan of the first cycle and at 0.6 V in the negative scan of the first cycle. Terraces with monatomic steps were found at the Pt(111) face after the first cycle of oxygen adsorption and desorption. Fig. 1 (b) shows an STM image after 100 potential cycles in 0.1 M NaOH solution. A rough surface having flat islands was observed after this electrochemical activation. In 0.05 M H2SO4 solution, structural changes at the Pt(111) surface during the electrochemical activation have been investigated with in situ STM [28]. Many islands have been observed on the Pt(111) terrace after 10 potential cycles [28]. It has been reported that the diameter and the average height of each island are ca. 2–3 nm and ca. 0.4–0.7 nm, respectively [28]. We examined an STM image after 100 potential cycles in 0.05 M H2SO4 solution. As the number of potential cycles is increased, the size of these islands becomes large. The distance between two neighboring islands is ca. 6.4 nm. The height of these islands is ca. 1.0 nm. The oriented islands with electrochemical acti-
3. Results and discussion
3.1. STM obser6ations of Pt(111) before and after electrochemical acti6ation in acidic and alkaline solutions STM images of the Pt(111) face were observed in 0.1 M NaOH solution. Wide atomically flat terraces with monatomic steps, intersecting one another at angles of 60°, were found at the Pt(111) face, as shown in Fig. 1
Fig. 1. STM images of a Pt(111) face at 0.6 V in a 0.1 M NaOH solution. (a) before electrochemical activation. (b) after 100 potential cycles.
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vation are formed more easily in 0.05 M H2SO4 solution than that in 0.1 M NaOH solution.
3.2. STM obser6ations of Pt(100) after electrochemical acti6ation in an acidic solution We found wide atomically flat terraces with monatomic steps (at a height of ca. 0.23 nm), intersecting one another at angles of 90°, at the Pt(100) face [31], as shown in the existing literature [30]. Many islands of platinum atoms on the Pt(100) terrace were found after 100 potential cycles. The distance between two neighboring islands is ca. 6 nm. The height of islands is ca. 0.7 nm. As the potential cycles are increased, the size of these islands becomes large. The islands are oriented in a regular array and are formed in squares after 1000 potential cycles. The shape of these islands seems to be a rectangular pyramid.
3.3. STM obser6ations of Pt(210) after electrochemical acti6ation in acidic and alkaline solutions STM images of a Pt(210) face were observed in 0.05 M H2SO4 and 0.1 M NaOH solution. Wide atomically flat surfaces were found at the Pt(210) face in 0.05 M H2SO4 and 0.1 M NaOH solution, as shown in Fig. 2(a). STM images were observed after 100 potential cycles in 0.05 M H2SO4 and 0.1 M NaOH solution. Islands of Pt atoms on the Pt(210) face were not found after the electrochemical activation, as shown in Fig. 2(b). The Pt(210) surface is more difficult to change with potential cycling than the Pt(111) and Pt(100) surfaces. The difference between the structural changes with the electrochemical activation at Pt(111) and that at Pt(210) can be explained using Fig. 3. Surface atoms at Pt(111) have larger coordination numbers. Moreover, it is difficult to adsorb oxygen species or atoms on the surface atoms of Pt(111) [1,27]. Therefore, the oxygen atoms have to invade the surface lattice at more positive potentials [27], replacing surface Pt atoms resulting in Pt adatoms on the surface [33], as shown in Fig. 3 (b). In the reduced state of the Pt surface, the Pt adatoms formed can move to more stable positions on the surface, but do not always return to their original position, as shown in Fig. 3 (c). Therefore, the islands can grow in size with many potential cycles. On the other hand, surface atoms at Pt(210) have smaller coordination numbers than those at Pt(111), and adsorb the oxygen species or atoms at more negative potentials [34]. The oxygen species or atoms can be adsorbed at the surface without replacing many surface Pt atoms, as shown in Fig. 3 (e). If some Pt adatoms are formed at the surface, they would move easily to their original position in the reduced state of the Pt surface. Therefore, it would be difficult for the Pt(210) surface to roughen with potential cycling.
Fig. 2. STM images of a Pt(210) face at 0.6 V in a 0.1 M NaOH solution. (a) before electrochemical activation. (b) after 100 potential cycles.
3.4. Voltammograms at 6arious Pt single crystal surfaces after the electrochemical acti6ation in an acidic solution Voltammograms for various Pt single-crystal surfaces of higher Miller index, which consist of terraces and steps, have been shown in our previous work [34]. The 0.05–0.7 V region corresponds to hydrogen adsorption and the 0.8–1.5 V region to adsorption of oxygen. The electrode surfaces involve planes with (100), (110), (111) indices and intermediate ones. It has been found that the hydrogen desorption wave varies systematically with the change of index. Fig. 4 shows voltammograms for the surfaces of higher Miller index after 500 potential cycles in 0.05 M H2SO4 solution. The peaks of hydrogen desorption and oxygen adsorption change with the potential cycles. The sharp peaks of hydrogen desorption seen before the potential cycles [34] become wider as the number of potential cycles increases. It can be seen that the hydrogen desorption wave varies systematically with chang-
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ing index. The shape of the voltammogram at indices between (530) and (110) and between (110) and (211) with the exception of (221) and (331) in the positive scan of the 500th cycle is almost identical to that of Pt(530) or (320) shown in our previous paper [34]. The shape of the voltammogram at indices between (511) and (310) in the positive scan of the 500th cycle is almost identical to that of Pt(310) shown in our previous paper [34].
3.5. Voltammograms at 6arious Pt single crystal surfaces before and after electrochemical acti6ation in an alkaline solution Fig. 5 shows voltammograms for various Pt single crystal surfaces of higher Miller index before potential cycling in 0.1 M NaOH solution. The 0.05 – 0.65 V region corresponds to hydrogen adsorption and the 0.65 –1.5 V region to adsorption of oxygen. It can be seen that the hydrogen desorption wave varies systematically with changing index. The voltammograms at indices between (310) and (110) and between (110) and (332) have obvious peaks at 0.25 V. The voltammograms at indices between (655) and (911) and between (910) and (310) have obvious peaks at 0.4 V. Then the voltammograms at indices between (17,1,1) and (12,1,0) have obvious peaks at potentials more positive than 0.45 V. In order to exhibit the relation between the atomic arrangements and the peaks in the hydrogen waves, the coordination numbers of atoms and their proportions in the respective surfaces have been investigated in our previous paper [34]. It has been found that the positions and the heights of the peaks change according to the
Fig. 3. Schematic diagrams for Pt(111) and Pt(210) surfaces at oxygen adsorption and desorption.
Fig. 4. Voltammograms (sweep rate: 0.05 V s − 1) for various Pt single crystals after 500 potential cycles in a 0.1 M H2SO4 solution.
variation of the coordination numbers in 0.05 M H2SO4 solution [34]. This relation can apply in the case of 0.1 M NaOH solution, because the hydrogen desorption wave in 0.1 M NaOH solution varies systematically with changing index, and is similar to that in 0.05 M H2SO4 solution. Fig. 6 shows voltammograms for the surfaces of higher Miller index after 500 potential cycles in 0.1 M NaOH solution. Waves of hydrogen desorption and oxygen adsorption change with the potential cycles. The hydrogen-desorption peaks at 0.25 and around 0.4 V become wider after 500 potential cycles. Moreover the peaks at potentials more positive than 0.45 V disappear after the potential cycling. The shape of the voltammogram at indices (551) and between (210) and (110) in the positive scan of the 500th cycle is almost identical to that of Pt(530) or (320) before potential cycling. The shape of the voltammogram for indices between (331) and (310) in the positive scan of the 500th cycle is almost identical to that of Pt(310) before potential cycling. It can be seen that the hydrogen desorption wave varies systematically with changing index.
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3.6. Surface structure after a great number of potential cycles The changes of voltammograms described in Section 3.4 are shown schematically in Fig. 7. The voltammograms after 500 potential cycles are classified roughly into two groups, i.e. Pt(111) – (110) and Pt(100) groups. After the potential cycles, the shape of the voltammograms for the Pt(111) and Pt(110) groups change into that for Pt(320) or Pt(530). The shapes of the voltammograms for Pt(100) groups change into that for Pt(310) after potential cycling. Namely, it is suggested that various Pt single crystal faces would be changed into Pt(320), Pt(530) or Pt(310) faces with a great number of potential cycles. However the Pt surfaces do not have the wide flat terraces of Pt(320), Pt(530) or Pt(310), because many islands of platinum atoms are formed on the surfaces after the potential cycles, as shown in Fig. 1 (b) and in the literature [28,31]. It is suggested that inclined planes of the islands may be composed of Pt(320), Pt(530) or Pt(310) faces. STM topview images and a cross-sectional view at the Pt(100) surface were observed after 1000 potential cycles [31]. The islands are oriented in a regular array and are formed in squares. The shape of these islands seems to be a rectangular pyramid ca. 1.3 nm in height and the Fig. 6. Voltammograms (sweep rate: 0.05 V s − 1) for various Pt single crystals after 500 potential cycles in a 0.1 M NaOH solution.
Fig. 5. Voltammograms (sweep rate: 0.05 V s − 1) for various Pt single crystals before electrochemical activation in a 0.1 M NaOH solution.
base ca. 10 nm in length. The gradient of the inclined plane of the pyramid can be calculated from these values. The mean of the tangent of a plane angle is ca. 0.269. Moreover, the mean of the plane angle is ca. 15°. We can evaluate the Miller index for the inclined plane of the pyramid from the slope. The tangent of a plane angle at the Pt(310) face is 0.283, the plane angle of Pt(310) is ca. 15°. The gradient of the inclined plane of an island is similar to the Pt(310) face. The (310) plane seems to be formed as the most stable face with electrochemical activation for the Pt(100) surface. STM topview images and a cross-sectional view at the Pt(111) surface were observed after 1000 potential cycles. The islands are likely to be oriented in a regular array and are formed in triangles. The shape of the islands may be a hexagonal pyramid. The mean of the plane angle is ca. 35 9 3°, which is measured from the STM cross-sectional view. We can evaluate the Miller index for the inclined plane from the slope. The gradient of the inclined plane of an island is similar to that of the Pt(530) face. The (530) plane seems to be formed as the most stable face with electrochemical activation of the Pt(111) surface. In order to prove the Miller index for the inclined plane of islands with STM measurement alone, STM images must be measured with atomic resolution.
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Fig. 7. Unit triangle of stereographic projection, showing location of various Pt surfaces before and after electrochemical activation.
In acidic solutions, the shape of the voltammograms at the (221) and (331) hardly changes after potential cycling, as shown in Fig. 4 and in our previous paper [34]. On the other hand, the results for (221) and (331) in alkaline solutions seem to fit into the Pt(111) –(110) group, as shown in Fig. 6. The difference of the potential cycling results between those in acidic and those in alkaline solutions is interesting. One explanation for the difference would be to take into account the adsorption of anions. The adsorption of oxygen species can be suppressed by sulfate or bisulfate in acidic solutions. On the other hand, the adsorption of oxygen species in alkaline solutions occurs at potentials more negative than that in acidic solutions, as shown in Fig. 5. Therefore surface restructuring in alkaline solution can take place more easily than that in acidic solutions.
References [1] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [2] J. Clavilier, J. Electroanal. Chem. 107 (1980) 211. [3] J. Clavilier, R. Durand, G. Guinet, R. Faure, J. Electroanal. Chem. 127 (1981) 281. [4] J. Clavilier, D. Armand, B.L. Wu, J. Electroanal. Chem. 135 (1982) 159.
[5] J. Clavilier, D. Armand, J. Electroanal. Chem. 199 (1986) 187. [6] J. Clavilier, D. Armand, S.G. Sun, J. Electroanal. Chem. 205 (1986) 267. [7] D. Armand, J. Clavilier, J. Electroanal. Chem. 263 (1989) 109. [8] D. Armand, J. Clavilier, J. Electroanal. Chem. 270 (1989) 331. [9] J. Clavilier, K. El Achi, A. Rodes, J. Electroanal. Chem. 272 (1989) 253. [10] A. Rodes, K. El Achi, M.A. Zamakhchari, J. Clavilier, J. Electroanal. Chem. 284 (1990) 245. [11] A. Rodes, M.A. Zamakhchari, K. El Achi, J. Clavilier, J. Electroanal. Chem. 305 (1991) 115. [12] J. Clavilier, C. Lamy, J.M. Leger, J. Electroanal. Chem. 125 (1981) 249. [13] J. Clavilier, R. Parsons, R. Durand, C. Lamy, J.M. Leger, J. Electroanal. Chem. 124 (1981) 321. [14] C. Lamy, J.M. Leger, J. Clavilier, J. Electroanal. Chem. 135 (1982) 321. [15] C. Lamy, J.M. Leger, J. Clavilier, R. Parsons, J. Electroanal. Chem. 150 (1983) 71. [16] S.G. Sung, J. Clavilier, A. Bewick, J. Electroanal. Chem. 210 (1988) 147. [17] J. Clavilier, A. Fernandez-Vega, J.M. Feliu, A. Aldaz, J. Electroanal. Chem. 258 (1989) 89. [18] A. Fernandez-Vega, J.M. Feliu, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 258 (1989) 101. [19] J. Clavilier, A. Fernandez-Vega, J.M. Feliu, A. Aldaz, J. Electroanal. Chem. 261 (1989) 113. [20] S. Sun, J. Clavilier, J. Electroanal. Chem. 263 (1989) 109. [21] J. Clavilier, J.M. Orts, R. Gomez, J.M. Feliu, A. Aldaz, J. Electroanal. Chem. 404 (1996) 281. [22] N. Kimizuka, K. Itaya, Faraday Discuss 94 (1992) 117.
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N. Kimizuka, T. Abe, K. Itaya, Denki Kagaku 61 (1993) 796. M.S. Zei, N. Batina, D.M. Kolb, Surf. Sci. 306 (1994) L519. J. Clavilier, J.M. Orts, J.M. Feliu, J. Phys. IV (1994) C1-303. N. Furuya, M. Ichinose and M. Shibata, J. Electroanal. Chem. 460 (1999) 251. [32] W. Dash, J. Appl. Phys. 29 (1958) 736. [33] S. Motoo, N. Furuya, J. Electroanal. Chem. 172 (1984) 339. [34] N. Furuya, S. Koide, Surf. Sci. 220 (1989) 18.
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Structural changes at various Pt single crystal surfaces with potential cycles in acidic and alkaline solutions Nagakazu Furuya *, Masami Shibata Department of Applied Chemistry, Faculty of Engineering, Yamanashi Uni6ersity, 4 -3 Takeda, Kofu 400 -8511, Japan Received 24 August 1998; received in revised form 23 January 1999; accepted 9 February 1999
Abstract Structural changes at various Pt single crystal surfaces of higher Miller index before and after many potential cycles were examined with in situ STM and cyclic voltammetry in acidic and alkaline solutions. STM images at Pt(111), Pt(100) and Pt(210) after 100 potential cycles in 0.05 M H2SO4 solution were observed. Many islands of platinum atoms on the terraces of Pt(111) and Pt(100) were found after the potential cycles, but not on Pt(210). The Pt(210) surface is changed with more difficulty with potential cycling than the Pt(111) and Pt(100) surfaces. Voltammograms for surfaces of higher Miller index after 500 potential cycles were examined. The sharp peaks of the hydrogen desorption wave become wider after the potential cycles. After the potential cycles, the shape of the voltammograms at Pt(111) and Pt(110) groups change into that at Pt(320) or Pt(530). The shapes of the voltammograms at Pt(100) groups change into that at Pt(310) after the potential cycles. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Pt single crystal; Higher Miller index; Electrochemical activation; In situ STM; Structural change
1. Introduction It is 18 years since the vivid feature of the hydrogen wave for the Pt(111) face in a voltammogram was reported by Clavilier et al. [1,2]. Recently, welldefined single crystal surfaces are necessities for the investigation of electrochemical behavior [1 – 11], electrocatalysis [12–21] and STM measurements [22– 24,29,31]. Electrocatalysis on the oxidation and reduction of simple organic compounds has been studied at Pt single crystals. It is known that the surface structure of Pt single crystals affects the electrocatalytic activity [13 – 15,25 – 27].
Dedicated to Jean Clavilier on the occasion of his retirement from LEI CNRS and in recognition of his contribution to Interfacial Electrochemistry. * Corresponding author. Tel.: +81-552-208559; fax: + 81-552208560. E-mail address: [email protected] (N. Furuya)
Potential cycles between hydrogen and oxygen evolution are usually applied to a polycrystalline Pt electrode after etching with aqua regia when the electrocatalytic activity is examined. The electrocatalytic activity at the Pt electrode is enhanced with this potential cycling, i.e. the ‘electrochemical activation’, and becomes reproducible. Voltammograms at the Pt electrode after etching with aqua regia become reproducible by means of the electrochemical activation. Structural changes at the Pt(111) surface during electrochemical activation have been investigated with in situ STM [28]. Randomly oriented islands, whose height corresponds to a few atomic steps, have been observed on the Pt(111) terrace after the electrochemical activation [28]. Moreover a few studies on a Pt(100) surface during electrochemical activation were observed with in situ STM [29–31]. In contrast to the existing papers, we report new systematic results on the structural changes at various Pt single crystal surfaces of higher Miller index in acidic and alkaline solutions.
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2. Experimental A bead single crystal was prepared by the flame float-zone method, being similar to Dash’s technique [32] for growing dislocation-free silicon single crystals. The center of a Pt wire (diameter= 1 mm, length = 150 mm, 99.99% purity) was melted locally in an oxygenrich hydrogen flame. The melted float-zone was moved gradually toward the end of the wire. Then the melted float-zone, being a single crystal with a high quality, was grown to 15–20 mm length. A large single crystal was grown using this single crystal as a seed. The size of the single crystal for the float-zone method was about 4 mm in diameter. The crystal was oriented by the laser beam method and then cut to expose various planes, and the surface of the exposed plane was polished with diamond paste. Then the surface was annealed at 1500°C for 3 h in a flame. For electrochemical characterization of the Pt surface, the single crystal was annealed at 1500°C for 5 s, again in a flame, cooled immediately in a stream of helium gas with 0.1% hydrogen [33], and then immersed in ultrapure water saturated with hydrogen. The electrode mounted in its all-glass holder was transferred to an electrochemical cell [33], while protecting the single crystal surface with a drop of water [1]. Measurements of the linear sweep voltammograms were conducted by the dipping method [1]. Sulfuric acid solution (0.05 mol dm − 3 = M) and sodium hydroxide solution (0.1 M) were prepared by dissolving reagent grade H2SO4 and NaOH in twice distilled water, respectively. All potentials were referenced to a reversible hydrogen electrode (RHE). Potential cycles (sweep rate: 0.5 V s − 1) between 0.05 and 1.5 V were applied to the Pt single crystal electrode for the electrochemical activation. For STM observation, the single crystal bead was annealed at 1500°C for 3 s again in a flame and was cooled in a stream of helium gas with 0.1% hydrogen [33]. STM images were observed at the single crystal surface at 0.6 V. In situ STM was carried out with a Seiko STE-330 (Seiko Instruments). The STM probe was a tungsten wire covered with resin in such a manner that only the tip-point was exposed.
(a). STM images were observed at 1.2 V in the positive scan of the first cycle and at 0.6 V in the negative scan of the first cycle. Terraces with monatomic steps were found at the Pt(111) face after the first cycle of oxygen adsorption and desorption. Fig. 1 (b) shows an STM image after 100 potential cycles in 0.1 M NaOH solution. A rough surface having flat islands was observed after this electrochemical activation. In 0.05 M H2SO4 solution, structural changes at the Pt(111) surface during the electrochemical activation have been investigated with in situ STM [28]. Many islands have been observed on the Pt(111) terrace after 10 potential cycles [28]. It has been reported that the diameter and the average height of each island are ca. 2–3 nm and ca. 0.4–0.7 nm, respectively [28]. We examined an STM image after 100 potential cycles in 0.05 M H2SO4 solution. As the number of potential cycles is increased, the size of these islands becomes large. The distance between two neighboring islands is ca. 6.4 nm. The height of these islands is ca. 1.0 nm. The oriented islands with electrochemical acti-
3. Results and discussion
3.1. STM obser6ations of Pt(111) before and after electrochemical acti6ation in acidic and alkaline solutions STM images of the Pt(111) face were observed in 0.1 M NaOH solution. Wide atomically flat terraces with monatomic steps, intersecting one another at angles of 60°, were found at the Pt(111) face, as shown in Fig. 1
Fig. 1. STM images of a Pt(111) face at 0.6 V in a 0.1 M NaOH solution. (a) before electrochemical activation. (b) after 100 potential cycles.
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vation are formed more easily in 0.05 M H2SO4 solution than that in 0.1 M NaOH solution.
3.2. STM obser6ations of Pt(100) after electrochemical acti6ation in an acidic solution We found wide atomically flat terraces with monatomic steps (at a height of ca. 0.23 nm), intersecting one another at angles of 90°, at the Pt(100) face [31], as shown in the existing literature [30]. Many islands of platinum atoms on the Pt(100) terrace were found after 100 potential cycles. The distance between two neighboring islands is ca. 6 nm. The height of islands is ca. 0.7 nm. As the potential cycles are increased, the size of these islands becomes large. The islands are oriented in a regular array and are formed in squares after 1000 potential cycles. The shape of these islands seems to be a rectangular pyramid.
3.3. STM obser6ations of Pt(210) after electrochemical acti6ation in acidic and alkaline solutions STM images of a Pt(210) face were observed in 0.05 M H2SO4 and 0.1 M NaOH solution. Wide atomically flat surfaces were found at the Pt(210) face in 0.05 M H2SO4 and 0.1 M NaOH solution, as shown in Fig. 2(a). STM images were observed after 100 potential cycles in 0.05 M H2SO4 and 0.1 M NaOH solution. Islands of Pt atoms on the Pt(210) face were not found after the electrochemical activation, as shown in Fig. 2(b). The Pt(210) surface is more difficult to change with potential cycling than the Pt(111) and Pt(100) surfaces. The difference between the structural changes with the electrochemical activation at Pt(111) and that at Pt(210) can be explained using Fig. 3. Surface atoms at Pt(111) have larger coordination numbers. Moreover, it is difficult to adsorb oxygen species or atoms on the surface atoms of Pt(111) [1,27]. Therefore, the oxygen atoms have to invade the surface lattice at more positive potentials [27], replacing surface Pt atoms resulting in Pt adatoms on the surface [33], as shown in Fig. 3 (b). In the reduced state of the Pt surface, the Pt adatoms formed can move to more stable positions on the surface, but do not always return to their original position, as shown in Fig. 3 (c). Therefore, the islands can grow in size with many potential cycles. On the other hand, surface atoms at Pt(210) have smaller coordination numbers than those at Pt(111), and adsorb the oxygen species or atoms at more negative potentials [34]. The oxygen species or atoms can be adsorbed at the surface without replacing many surface Pt atoms, as shown in Fig. 3 (e). If some Pt adatoms are formed at the surface, they would move easily to their original position in the reduced state of the Pt surface. Therefore, it would be difficult for the Pt(210) surface to roughen with potential cycling.
Fig. 2. STM images of a Pt(210) face at 0.6 V in a 0.1 M NaOH solution. (a) before electrochemical activation. (b) after 100 potential cycles.
3.4. Voltammograms at 6arious Pt single crystal surfaces after the electrochemical acti6ation in an acidic solution Voltammograms for various Pt single-crystal surfaces of higher Miller index, which consist of terraces and steps, have been shown in our previous work [34]. The 0.05–0.7 V region corresponds to hydrogen adsorption and the 0.8–1.5 V region to adsorption of oxygen. The electrode surfaces involve planes with (100), (110), (111) indices and intermediate ones. It has been found that the hydrogen desorption wave varies systematically with the change of index. Fig. 4 shows voltammograms for the surfaces of higher Miller index after 500 potential cycles in 0.05 M H2SO4 solution. The peaks of hydrogen desorption and oxygen adsorption change with the potential cycles. The sharp peaks of hydrogen desorption seen before the potential cycles [34] become wider as the number of potential cycles increases. It can be seen that the hydrogen desorption wave varies systematically with chang-
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ing index. The shape of the voltammogram at indices between (530) and (110) and between (110) and (211) with the exception of (221) and (331) in the positive scan of the 500th cycle is almost identical to that of Pt(530) or (320) shown in our previous paper [34]. The shape of the voltammogram at indices between (511) and (310) in the positive scan of the 500th cycle is almost identical to that of Pt(310) shown in our previous paper [34].
3.5. Voltammograms at 6arious Pt single crystal surfaces before and after electrochemical acti6ation in an alkaline solution Fig. 5 shows voltammograms for various Pt single crystal surfaces of higher Miller index before potential cycling in 0.1 M NaOH solution. The 0.05 – 0.65 V region corresponds to hydrogen adsorption and the 0.65 –1.5 V region to adsorption of oxygen. It can be seen that the hydrogen desorption wave varies systematically with changing index. The voltammograms at indices between (310) and (110) and between (110) and (332) have obvious peaks at 0.25 V. The voltammograms at indices between (655) and (911) and between (910) and (310) have obvious peaks at 0.4 V. Then the voltammograms at indices between (17,1,1) and (12,1,0) have obvious peaks at potentials more positive than 0.45 V. In order to exhibit the relation between the atomic arrangements and the peaks in the hydrogen waves, the coordination numbers of atoms and their proportions in the respective surfaces have been investigated in our previous paper [34]. It has been found that the positions and the heights of the peaks change according to the
Fig. 3. Schematic diagrams for Pt(111) and Pt(210) surfaces at oxygen adsorption and desorption.
Fig. 4. Voltammograms (sweep rate: 0.05 V s − 1) for various Pt single crystals after 500 potential cycles in a 0.1 M H2SO4 solution.
variation of the coordination numbers in 0.05 M H2SO4 solution [34]. This relation can apply in the case of 0.1 M NaOH solution, because the hydrogen desorption wave in 0.1 M NaOH solution varies systematically with changing index, and is similar to that in 0.05 M H2SO4 solution. Fig. 6 shows voltammograms for the surfaces of higher Miller index after 500 potential cycles in 0.1 M NaOH solution. Waves of hydrogen desorption and oxygen adsorption change with the potential cycles. The hydrogen-desorption peaks at 0.25 and around 0.4 V become wider after 500 potential cycles. Moreover the peaks at potentials more positive than 0.45 V disappear after the potential cycling. The shape of the voltammogram at indices (551) and between (210) and (110) in the positive scan of the 500th cycle is almost identical to that of Pt(530) or (320) before potential cycling. The shape of the voltammogram for indices between (331) and (310) in the positive scan of the 500th cycle is almost identical to that of Pt(310) before potential cycling. It can be seen that the hydrogen desorption wave varies systematically with changing index.
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3.6. Surface structure after a great number of potential cycles The changes of voltammograms described in Section 3.4 are shown schematically in Fig. 7. The voltammograms after 500 potential cycles are classified roughly into two groups, i.e. Pt(111) – (110) and Pt(100) groups. After the potential cycles, the shape of the voltammograms for the Pt(111) and Pt(110) groups change into that for Pt(320) or Pt(530). The shapes of the voltammograms for Pt(100) groups change into that for Pt(310) after potential cycling. Namely, it is suggested that various Pt single crystal faces would be changed into Pt(320), Pt(530) or Pt(310) faces with a great number of potential cycles. However the Pt surfaces do not have the wide flat terraces of Pt(320), Pt(530) or Pt(310), because many islands of platinum atoms are formed on the surfaces after the potential cycles, as shown in Fig. 1 (b) and in the literature [28,31]. It is suggested that inclined planes of the islands may be composed of Pt(320), Pt(530) or Pt(310) faces. STM topview images and a cross-sectional view at the Pt(100) surface were observed after 1000 potential cycles [31]. The islands are oriented in a regular array and are formed in squares. The shape of these islands seems to be a rectangular pyramid ca. 1.3 nm in height and the Fig. 6. Voltammograms (sweep rate: 0.05 V s − 1) for various Pt single crystals after 500 potential cycles in a 0.1 M NaOH solution.
Fig. 5. Voltammograms (sweep rate: 0.05 V s − 1) for various Pt single crystals before electrochemical activation in a 0.1 M NaOH solution.
base ca. 10 nm in length. The gradient of the inclined plane of the pyramid can be calculated from these values. The mean of the tangent of a plane angle is ca. 0.269. Moreover, the mean of the plane angle is ca. 15°. We can evaluate the Miller index for the inclined plane of the pyramid from the slope. The tangent of a plane angle at the Pt(310) face is 0.283, the plane angle of Pt(310) is ca. 15°. The gradient of the inclined plane of an island is similar to the Pt(310) face. The (310) plane seems to be formed as the most stable face with electrochemical activation for the Pt(100) surface. STM topview images and a cross-sectional view at the Pt(111) surface were observed after 1000 potential cycles. The islands are likely to be oriented in a regular array and are formed in triangles. The shape of the islands may be a hexagonal pyramid. The mean of the plane angle is ca. 35 9 3°, which is measured from the STM cross-sectional view. We can evaluate the Miller index for the inclined plane from the slope. The gradient of the inclined plane of an island is similar to that of the Pt(530) face. The (530) plane seems to be formed as the most stable face with electrochemical activation of the Pt(111) surface. In order to prove the Miller index for the inclined plane of islands with STM measurement alone, STM images must be measured with atomic resolution.
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Fig. 7. Unit triangle of stereographic projection, showing location of various Pt surfaces before and after electrochemical activation.
In acidic solutions, the shape of the voltammograms at the (221) and (331) hardly changes after potential cycling, as shown in Fig. 4 and in our previous paper [34]. On the other hand, the results for (221) and (331) in alkaline solutions seem to fit into the Pt(111) –(110) group, as shown in Fig. 6. The difference of the potential cycling results between those in acidic and those in alkaline solutions is interesting. One explanation for the difference would be to take into account the adsorption of anions. The adsorption of oxygen species can be suppressed by sulfate or bisulfate in acidic solutions. On the other hand, the adsorption of oxygen species in alkaline solutions occurs at potentials more negative than that in acidic solutions, as shown in Fig. 5. Therefore surface restructuring in alkaline solution can take place more easily than that in acidic solutions.
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