J. Chew Soc., Faraduy Trans. I, 1989, 85(6), 1351-1356 In situ Scanning Tunnelling Microscopy of a Platinum { 11 l} Surface in Aqueous Sulphuric Acid Solution Shizuo Sugawara and Kingo Itaya" Department of Engineering Science, Faculty of Engineering, Tohoku University, Sendai 980, Japan An in situ scanning tunnelling microscope was applied, for the first time, to a single-crystal platinum { 1 1 l} surface both before and after electrochemical potential cycling in aqueous sulphuric acid solution. The single-crystal Pt{ 11 l} was annealed in a flame near 1100 "C for 1 min and then quickly brought into contact with pure water. It was not possible to see any particular structures on the fire-annealed single crystal. It is shown that the fire-annealing procedure can produce an almost completely atomically flat surface on a single crystal.The flat { 11 l} surface of the Pt crystal was roughened by the electrochemical potential cycling. Semi-spherical domains have predominantly been observed on the single crystal. These domains seem to be randomly distributed over the surface. The diameter and heigbt of the semi-spherical domains were in the ranges 20-30 and 5-10A, respectively. The electrochemical properties of platinum, one of the most important electrode materials, have long been discussed by many workers.' Great efforts have been made to characterize the role of surface structure in electrocatalysis, using single crystals with different orientations as well as polycrystalline Pt. 1-5 The characterization of the adsorption and desorption of hydrogen on clean Pt surfaces is a central issue in efforts to establish the effect of surface structure.To define accurately a Pt surface at the atomic level, low-energy electron diffraction (LEED) electrochemical systems have recently been employed.6-'2 In these systems, Pt surfaces have been prepared and examined by electron spectroscopies such as LEED and Auger in ultra-high vacuum (UHV). Such systems, however, cannot be applied directly to in situ solid-liquid interfaces. Recent papers describing scanning tunnelling microscopy (STM) for samples immersed in aqueous solutions have given persuasive evidence that STM is a powerful new method for in situ electrode surface characterization with atomic resolution. 13-217 24 There have been successful demonstrations of the applicability of STM in electro- chemical cells for the observation of Ag,16 Au15 and Pt20 islands deposited electrochemically on a highly ordered pyrolytic graphite (HOPG).This new method has also been employed in various environments with Pt e1ectr0des.l~~ 21-22 Vazquez et al. described ex situ STM in air to determine the topography of electrochemically highly activated Pt surfaces.22T23 Fan and Bard have recently examined the topography of different Pt surfaces obtained with various pretreatments in air and water.19 In our previous papers, in situ STM of polycrystalline Pt electrodes was carried out, for the first time, both before and after moderate electrochemical activation in an aqueous solution of sulphuric acid.21* 24 Large changes in the surface structure were induced by the electrochemical activation procedure.The appearance of a very regular parallel-terrace structure suggested that the migration of adatoms of Pt produced by the oxidation and reduction cycles would occur in a particular crystallographic direction. 21* 24 Semi-spherical domains were also observed at different positions in the polycrystalline Pt. The appearance of such different morphologies at different points was possibly 13511352 STM of a Platinum (1 11) Surface caused by the different orientations of the individual single crystals of which the polycrystalline Pt sample was composed. In this paper, we report the results of in situ observation of a Pt(ll1) surface as well as the effects of potential cycling for the electrochemical activation procedure in a 0.05 mol dm-3 H,SO, solution.Experimental A single Pt crystal was prepared by the method of Clavilier et ~ 1 . ~ The single crystal Pt bead obtained after melting was ca. 2 mm in diameter. The crystallographic axes of the crystal were determined by X-ray diffraction. The crystal was cut with a diamond cutting wheel to give the { 11 1) surface. The Pt{ 11 1) surface was mechanically polished with successively finer grades of alumina down to 0.05 pm. The mechanically polished single crystal Pt(ll1) surface was annealed in a gas + oxygen or hydrogen + oxygen flame at 1200-1 400 "C for 1 h. A final treatment for STM and electrochemical measurements was performed according to the methods of Clavilier et ~ 1 . ~ and Motoo and F u r ~ y a .~ The single-crystal Pt, annealed in a gas + oxygen or hydrogen + oxygen flame near 1 100 "C for 1 min, was quickly brought into contact with ultra-pure water (Millipore-Q) saturated with hydrogen. Exactly the same method was employed in our previous work for polycrystalline Pt.,l The STM apparatus used was the same as that detailed p r e v i o u ~ l y . ~ ~ , ~ ~ A glass-coated Pt electrode was used as a tunnelling tip. Although the details are described elsewhere,,, it should be mentioned that the electrochemical residual current for well sealed tips (ca. 0.1 nA) was only a few percent of the total tunnelling current (ca. 5 nA) in a 0.05 mol dm-3 H,SO, solution. Results and Discussion Fig. 1 shows typical cyclic voltammograms of the single-crystal Pt{ 1 1 1) electrode obtained in a 0.05 mol dm-3 H,SO, solution after the final treatment.Fig. 1 (a) shows a voltammogram for hydrogen adsorption4esorption, in which positive potential was limited in the double-layer region. The range of potential cycling employed here avoids the electrochemical oxidation of Pt surfaces. The shape of curve (a) is quite similar to those reported by Clavilier et ~ 1 . ~ and Motoo and F ~ r u y a . ~ The total charge for the hydrogen adsorption was calculated as 260 pC ern-,, which is in agreement with the calculated value, 243 pC ern-,, obtained by assuming a coverage of one hydrogen atom per Pt surface atom of an ideal Pt(ll1) surface (1 x l).3 The above result strongly indicates that the final surface treatment described above could give atomically flat surfaces.Fig. l(b) was recorded after the completion of 50 potential cycles between the potentials, -0.25 and 1.05 V versus a standard calomel electrode, SCE, at a scan rate of 0.2 V s-l. Two peaks at ca. -0.2 and -0.05 V us. SCE were gradually formed during successive potential cycles and the shape of the voltammogram approached that for the polycrystalline Pt. This behaviour is essentially the same as that reported previ~usly.~.~ It is interesting to note that the total charge for the hydrogen adsorption was increased by ca. 20 % after 50 potential cycles. This might be due to an increase in the surface area caused by the oxidation-reduction cycling of a Pt oxide layer. Based on the above electrochemical result, in situ observation of a Pt{ 1 1 1 ) surface was carried out before and after the electrochemical potential cycling in the 0.05 mol dm-3 H,SO, solution described above.Fig. 2 shows in situ STM images obtained with different magnifications on a virgin Pt{ 1 1 1) surface (unactivated) in a 0.05 mol dmP3 H,SO, solution. The same final treatment of the Pt crystal, described earlier, was applied for the STM study. The tunnelling current and the tip bias voltage were 4 nA and 50 mV, respectively.S. Sugawara and K. Itaya 1353 I I I I I I l l 1 I l l 1.0 0.5 0 Vvs. SCE Fig. 1. Cyclic voltammograms of a single-crystal Pt{ 11 1) surface obtained in a 0.05 mol dm-3 H,SO, solution. The scan rate was 50 mV s-l. (a) The electrode potential was scanned between 0.4 and -0.23 V us. SCE. (b) after 50 potential cycles between 1.05 and -0.25 V us.SCE at a scan rate of 200 mV s-'. - 50 8, - 20 A Fig. 2. STM images of a virgin Pt{ 1 1 I } surface obtained in a 0.05 mol dm-3 H,SO, solution. The tunnelling current and the tip potential were 4 nA and 50 mV, respectively. As can be seen in fig. 2, the virgin Pt{ 1 1 1) surface is microscopically smooth and nearly atomically flat. In the Fase of well crystallized polycrystalline Pt, a few shallow multiatomic steps (1-20 A in height) were found in our previous study.21.24 However, it was not possible to see any particular structures on the single crystal even when views were taken from different positions. The tunnelling current was reasonably stable during the scan of the tip in the x direction. The above result is the first clear evidence that a final annealing procedure can produce an almost completely or completely atomically flat surface on a single crystal.Wagner and Ross have recently shown that a well defined reconstructed surface of Pt prepared in UHV is changed to a ( 1 x 1) structure on contact with an aqueous solution.'' The lack of surface structures as shown in fig. 2 seems to be an1354 STM of a Platinum { 11 l} Surface 50 8, 50 ‘A Fig. 3. STM images of a single-crystal Pt{ 1 1 l } surface after 50 potential cycles between 1.05 and -0.25 V us. SCE. The tunnelling current was 4 nA and tip potential was 50 mV. indication that the surface is atomically flat; this surface is designated the Pt{ 11 1)-( 1 x 1) surface.’l However, it is noteworthy that the final treatment in a hydrogen+oxygen flame needs to be carried out under certain conditions, both the temperature of the flame and the transfer of the Pt to the hydrogen-saturated water seem to be critical.We have sometimes observed an appreciable degree of roughening for a ‘virgin’ Pt{ 1 1 l} surface prepared by cooling in an atmosphere containing oxygen. The effect of annealing and subsequent cooling on the voltammogram for hydrogen adsorption-desorption has been investigated in detail by Motoo and F u r ~ y a . ~ They reported a quite different voltammogram from that shown in fig. 1 (a) for a Pt{ 11 l} surface cooled in an oxygen atmosphere. The above result strongly suggests that a contact with oxygen must be avoided in the transfer of the Pt to the hydrogen-saturated water in order to obtain an atomically smooth, clean surface.After the observation of the unactivated Pt(l1 l} face by STM as shown in fig. 2, the Pt electrode potential was cycled for 50 times at a scan rate of 0.2 V s-l, (the same procedure used in the electrochemical study shown in fig. 1). During the potential cycling, the tunnelling tip was retracted from the surface and disconnected from the z piezo feedback loop. Fig. 3 shows typical STM images obtained after the electrochemical potential cycles. Fig. 3(a) and 3(b) were obtained at different positions of the same sample with the same magnification. In fig. 3(a) it is clearly seen that nearly all the area of the almost completely atomically flat surface has been roughened by tbe electrochemical treatment employed here.However, there were a few areas (ca. 200 A in diameter) which seemed to be still atomically flat even after 50 potential cycles, as shown in fig. 3(b). Nevertheless, the percentage of these flat regions was estimated from many STM images obtained in this study to be < 10% of the total surface area. As shown in fig. 3, semi-spherical domains have predominantly been observed. TheseS. Sugawara and K. Itaya 1355 domains seem to be randomly distributed over the surface, with no particular direction of orientation preferred. Fig. 3(c) is a typical image obtained on a roughened surface with a larger magnification. Tbe diameter and height of the semi-spherical domains are in the ranges 20-30 and 5-10 A, respectively. Note that within a short range there seems to be an ordering of the semi-spherical domains.The electrochemically activated Pt{ 1 1 l} surface seems to consist of hexagonal cells, each of which is semi-spherical in shape, as is shown in fig. 3(c). Many saw-toothed lines hape also been observed, as shown in fig. 3 (c). The height of each tooth is in the range 3-6 A, which seems to correspond to mono- or di-atomic steps. These sharp features might be attributed to the existence of adatoms or kinks on the surface. We have observed similar semi-spherical domains, as well as regular parallel- terrace structures, on polycrystalline Pt after applying the same electrochemical 24 The result shown in fig. 3 suggests that the crystallographic orientation of each of the small single crystals, which comprised the polycrystalline sample examined previously, were such that Pt{ 11 l} surfaces were exposed where the semi-spherical domains have predominantly been observed.It will now be of particular interest to observe different single-crystal surfaces such as Pt{ 100) and Pt{ 1 lo). Finally, it is noteworthy that structures formed by similar potential cycling for Pt( loo} and Pt{ 1 1 11 surfaces have recently been reported in detail by Wagner and ROSS." Using LEED spot-profile analysis, they found that excursions to higher anodic potentials induced the formation of broadly distributed mono-atomic-height up-and-down steps, stating that such a 'randomly stepped surface' was perhaps better visualized as a ' random mesa structure '.'I As pointed out by these authors, a unique transformation from reciprocal-space to real-space structure in the LEED analysis is impossible in the case of the randomly stepped surface.However, the result shown in fig. 3 is a real-space physical structure caused by the oxidation-reduction cycling. We believe that the semi- spherical domains discussed above must correspond to the random mesa structure proposed by Wagner and Ross. A more detailed study changing both the number of potential cycles and the upper potential limits is now under investigation. The authors thank Professor N. Furuya (Yamanashi University) for his guidance in the preparation and treatment of Pt single crystals. References 1 B. E. Conway, Progress in Surface Science (Pergamon Press, 1984), vol. 16, pp. 1-138, and references 2 F.G. Will, J. Electrochem. Soc., 1965, 112, 451. 3 J. Clavilier, D. Armand and B. L. Wu, J . Electroanal. Chem., 1982, 135, 159. 4 C. L. Scortichini and C. N. Reilley, J. Electroanal. Chem., 1982, 139, 247. 5 S. Motoo and N. Furuya, J . Electroanal. Chem., 1984, 172, 339. 6 E. Yeager, J. Electrochem. Soc., 1981, 128, 160C. 7 A. S . Homa, E. Yeager and B. D. Cahan, J. Electroanal. Chem., 1983, 150, 181. 8 A. T. Hubbard, Acc. Chem. Res., 1980, 13, 177. 9 J. L. Stickney, S. D. Rosasco, G. N. Salaita and A. T. Hubbard, Langmuir, 1985, 1, 66. cited therein. 10 P. N. Ross Jr and F. T. Wagner, Advances in Electrochemistry and Electrochemical Engineering (John 11 F. T. Wagner and P. N. Ross Jr, Surf. Sci., 1985, 160, 305. 12 K. Yamamoto, D. M. Kolb, R. Kotz and G. Lehmpfuhl, J. Electroanal. Chem., 1979, 96, 233. 13 R. Sonnenfeld and P. K. Hansma, Science, 1986, 232, 21 1. 14 J. Schneir, R. Sonnenfeld, P. K. Hansma and J. Tersoff, Phys. Rev. B, 1986, 34, 4979. 15 B. Drake, R. Sonnenfeld, J. Schneir and P. K. Hansma, Surf. Sci., 1987, 181, 92. 16 R. Sonnenfeld and B. C. Schardt, Appl. Phys. Lett., 1986, 49, 1172. 17 H. Y. Liu, F. F. Fan, C. W. Liu and A. J. Bard, J . Am. Chem. SOC., 1986, 108, 3838. 18 C. W. Liu, F. F. Fan and A. J. Bard, J . Electrochem. SOC., 1987, 134, 1038. Wiley, Chichester, 1984), vol. 13, pp. 69-1 12.1356 STM of a Platinum ( 1 1 1 ) Surface 19 F. F. Fan and A. J. Bard, Anal. Chem., 1988, 60, 751. 20 K. Itaya and S. Sugawara, Chem. Lett., 1987, 1927. 21 K. Itaya, K. Higaki and S. Sugawara, Chem. Lett., 1988, 421. 22 L. Vazquez, J. Gbmez, A. M. Barb, N. Garcia, M. L. Marcos, J. G. Velasco, J. M. Vara, A. J. Arvia, 23 J. Gbmez, L. Vazquez, A. M. Bard, N. Garcia, C . L. Perdriel, W. E. Triaca and A. J. Ariva, Nature 24 K. Itaya, S. Sugawara and K. Higaki, J. Phys. Chem., 1988, 92, 6714. J. Presa, A. Garcia and M. Aguilar, J. Am. Chem. Soc., 1987, 109, 1730. (London), 1986, 323, 612. Paper 8/02146E; Received 31st May, 1988