- Email: [email protected]
electrolyte interface under potential control
Applied Surface Science 130–132 Ž1998. 506–511
Enhanced diffusion of surface atoms at metalrelectrolyte interface under potential control Nobumitsu Hirai ) , Hiroaki Tanaka, Shigeta Hara Department of Materials Science and Processing, Faculty of Engineering, Osaka UniÕersity, Yamadaoka 2-1, Suita 565, Japan Received 1 September 1997; accepted 24 December 1997
Abstract We have investigated the surface self-diffusion coefficient Ž D S . on AgŽ100. in aqueous 50 mM H 2 SO4 solution using electrochemical atomic force microscopy ŽECAFM.. The results were compared with those observed on AuŽ100.. The D S values of AgŽ100. increased exponentially with potential between y50 and 350 mV Žvs. normal hydrogen electrode, NHE.. From the D S –E relationship, we propose that the activation energy of surface diffusion decreases because of the surface excess charge. We also found that the D S values on AgŽ100. were a hundred times larger than those on AuŽ100. within the potential range from 150 to 350 mV Žvs. NHE.. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Metal–electrolyte interface; Silver; Solid–liquid interface; Surface diffusion; Surface structure
1. Introduction Since Binnig et al. invented the scanning tunnelling microscope ŽSTM. and the atomic force microscope ŽAFM., much attention has been paid to direct observations of many surface processes with atomic resolution. These microscopes can work in various environments, such as gas, vacuum, and solution w1–3x. In our experiments, using some electrochemical processes at the electroderelectrolyte interface, such as the underpotential deposition ŽUPD. process in acid solution w4x, we found that Ag atoms electrodeposited on AuŽ100. electrode made the surface atomically flat w5x. On the other hand, we have found that the self-diffusion of surface atoms on AuŽ100. is enhanced when a suitable potential is )
Corresponding author. Tel.: q81-6-879-7467; fax: q81-6879-7467; e-mail: [email protected].
applied at the AurH 2 SO4 solution interface w6x. In this paper, we present a study of the surface self-diffusion process on AgŽ100. in aqueous 50 mM H 2 SO4 in comparison with that observed on AuŽ100.. 2. Experimental The AgŽ100. samples were prepared from an Ag single crystal rod Ž12 mm diameter., which was grown by the Bridgman method. After being annealed at 700 K for 30 min in H 2 gas, Ag was deposited onto AgŽ100. in aqueous 50 mM H 2 SO4 q 1 mM Ag 2 SO4 solution at 300 mV for about 30 min in order to make the surface atomically flat on a larger scale. The Ag deposition rate was about three monolayers per minute. The microscope used is a Nanoscope E ŽDigital Instruments. equipped with a potentiostat. All potentials reported here are relative to the normal hydrogen electrode ŽNHE., although a
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 0 6 1 - 0
N. Hirai et al.r Applied Surface Science 130–132 (1998) 506–511
HgrHg 2 SO4 reference electrode Ž0.68 V vs. NHE. was actually used. The D S values on AgŽ100. were measured using the following procedure. By scanning continuously on a small area of the sample with the AFM tip in aqueous 50 mM H 2 SO4 for a few minutes, a small hole can be easily created at 350 mV. For making a hole, the force between the tip and the surface is in the order of 10y7 N, otherwise, for imaging, it is in
507
the order of 10y9 N. Subsequently, the potential was jumped to a different potential of interest for surface self-diffusion measurements in the double layer region Žbetween y50 and 350 mV. and the scan area was enlarged. The D S values were calculated from successive AFM images that showed the refilling process of the hole. More details of this procedure in the case of AuŽ100. are presented in the earlier paper w6x.
Fig. 1. AFM images of AgŽ100. surface obtained at 150 mV in aqueous 50 mM H 2 SO4 q 1 mM Ag 2 SO4 solution after Ag deposition. Ža. Large area image Ž800 nm = 800 nm, height mode image.. Žb. High-resolution AFM image Ž8 nm = 8 nm., showing an unreconstructed structure. The w011x direction is indicated by an arrow.
508
N. Hirai et al.r Applied Surface Science 130–132 (1998) 506–511
3. Results and discussion
Fig. 2. Cyclic voltammogram for AgŽ100. in aqueous 50 mM H 2 SO4 solution. The scan rate is 50 mVrs.
Fig. 1a shows an AFM image Žheight mode. of AgŽ100. surface at 150 mV in aqueous 50 mM H 2 SO4 q 1 mM Ag 2 SO4 solution after Ag deposition. On the flat terrace of AgŽ100. surface, we can observe an unreconstructed AgŽ100.-Ž1 = 1. structure at 150 mV, ŽFig. 1b.. Fig. 2 shows a typical cyclic voltammogram ŽCV. for AgŽ100. in aqueous 50 mM H 2 SO4 . The sweep potential was from y350 to 550 mV and the scan
Fig. 3. Series of AFM images Ž150 nm = 150 nm, height mode image. together with the cross-section views obtained at 250 mV showing the refilling process of the hole created on the terrace. Image Ža. was taken immediately after the hole was made. The subsequent images were obtained 50 s Žb. and 134 s Žc. after image Ža..
N. Hirai et al.r Applied Surface Science 130–132 (1998) 506–511
Fig. 4. Relationship between log D S and electrode potential. The dots are the experimental D S values of AgŽ100. and AuŽ100. and the dotted lines are the D S values calculated from Eq. Ž4., where the constant ab is assumed 0.4. The broken lines are the extrapolated D S values of Ag and Au under vapor phase, reported by Gjostein w7x.
rate was 50 mVrs. As can be seen from Fig. 2, the CV is characterized by a double-layer region between the anodic peak of Ag dissolution at 550 mV and the cathodic peak of H 2 evolution at y350 mV. After making a hole on an atomically flat AgŽ100. surface, we observed the refilling process of the hole at various potentials. Fig. 3 shows a series of AFM images Žheight mode. of the refilling process at 250 mV along with the cross-section views. Fig. 3a is an image taken immediately the hole was made. Usually, the depth of the hole was about 3 nm. Around the hole, a hill was observed. As shown in Fig. 3b and c, the disappearance of the hole could be observed. Since the depth of the hole decreased linearly with time, we calculated the D S values from the following equation w7x, D S s d 2r2 t ,
Ž 1.
where d is the horizontal distance between the top of the hill and the bottom of the hole, and this the average refilling time per atomic layer. Fig. 4 shows the relationship between D S on AgŽ100. and potential. For comparison, the D S values observed on AuŽ100. w6x are also plotted. The D S values on Ag and Au in vacuum is extrapolated from the following equation w7x: 2
D S s D S 0 exp Ž yQSrRT . cm rs ,
at
D S 0 s 0.014, QS s 13Tm Ž calrmole. s 54.6Tm Ž Jrmole.
Ž 2.
509
are also shown in Fig. 4, where QS is the activation energy for surface diffusion, D S0 is a preexponential factor, Tm is the melting temperature ŽTm s 1234 K for Ag, Tm s 1336 K for Au. and T is the experimental temperature ŽT s 298 K.. The volume diffusion constant Ž D V . can be ignored, because it is several orders of magnitude smaller than D S w3,8x. In the case of AuŽ100., the minimum value of D S in aqueous 50 mM H 2 SO4 solution was located at EPZC of AuŽ100. Ž6270 mV w9x. and it agrees well with the one extrapolated from Eq. Ž2. w7x. In the case of AgŽ100., however, the extrapolated D S value at y450 mV, which is the E PZC of AgŽ100. in 10 mM Na 2 SO4 w10x, is much smaller than that calculated from Eq. Ž2.. It should be discussed whether D S0 and QS in Eq. Ž2. are valid or not for AgŽ100., because on one hand, Eq. Ž2. was derived from experimental values at temperatures more than 700 K and on the other hand, the D S values of Ag derived from Eq. Ž2. were a hundred times larger than the experimental ones even at 880 K w7x. Another question arises from whether the E PZC of y450 mV in 10 mM Na 2 SO4 can be used or not for the experiments performed in 50 mM H 2 SO4 solution. From Fig. 4, it can also be seen that the D S of AgŽ100. increased exponentially with potential within the range from y50 to 350 mV. The schematic diagram for the surface self-diffusion process of AgŽ100. and AuŽ100. under the presence of applied potential Ž E ., shown in Fig. 5, is used for deriving the D S –E relationship. The QS0 is the activation energy of surface diffusion when the applied potential is E PZC and is equal to the QS derived from Eq. Ž2.. At E ) E PZC , the reduction in the activation
Fig. 5. Schematic diagram of the activation energy of surface diffusion of Ag Žor Au. adatom under the presence of applied potential Ž E ., if E G E PZ C . The QS0 is the activation energy of surface diffusion at E PZ C . The DQS is the reduction in the activation energy of surface diffusion at E in comparison with at E PZ C .
N. Hirai et al.r Applied Surface Science 130–132 (1998) 506–511
510
energy for surface diffusion Ž DQS . can be obtained as DQS
s q M dV s CVdV s Ž 1r2 . CV 2
H
H
s Ž 1r2 . q M V s Ž 1r2 . a NA < e < b Ž E y E PZC . ,
Ž 3.
where q M Žs CV s a NA < e <. is the surface excess charge, V Žs b Ž E y E PZC .. is the applied voltage between inner Helmholtz layer and the surface, a and b are constants Žabsolute numbers., NA is Avogadro’s constant, and e is the charge of an electron. Taking into account the consideration mentioned above, DS
Fig. 6. Schematic diagrams for Ža. the diffusion of Ag on AuŽ100. and Žb. the self-diffusion on AgŽ100..
s D S0 exp Ž yQSrRT . s D S0 exp y Ž QS0 y DQS . rRT 4 s D S0 exp w yQS0
Ž 4.
q Ž 1r2 . ab NA < e < Ž E y EPZC . rRT 4 , log D S s Ž ab NA < e
Ž 4X . are derived from Eqs. Ž2. and Ž3.. Eqs. Ž4. and Ž4X . proposed by us take into consideration the effect of the applied potential on the surface self-diffusion. The dotted lines in Fig. 4 are the D S of Ag and Au calculated from Eq. Ž4.. The QS of Ag was modified in Eq. Ž4. in order to fit the D S value at E PZC of AgŽ100. predicted from experimental values with that extrapolated from Eq. Ž2.. If E1 6 E2 4 EEPZ and if we assume a and b at E s E1 to be nearly equal to those at E s E2 , log D SEs E1 y log D SEs E 2 s Ž ab NA < e
Ž 4Y .
can be obtained from Eq. Ž4X .. From Eq. Ž4Y . and D S –E relationship, we can calculate ab . The value of 0.4 for ab of both Ag and AuŽ100. at E 4 EPZC was obtained from experimental values plotted in Fig. 4. From Fig. 4, we also clearly see that the D S values on AgŽ100. were a hundred times larger than those on AuŽ100. within the measured potential range, i.e., from 150 to 350 mV. In our previous experiment w5x, we have found that Ag films were
formed on Ž1 = 1.-Ag UPD adlayer on AuŽ100. in aqueous 50 mM H 2 SO4 q 1 mM Ag 2 SO4 solution by an ideal layer-by-layer growth mode even at a high deposition rate ŽF 7.2 MLrmin. and the flatness of Ag films was improved as the deposition proceeded. Fig. 6 shows the schematic diagram for the diffusion of Ag on AuŽ100. and the self-diffusion on AgŽ100.. Because adatoms were mainly attracted by the neighboring atoms, the D S values of Ag adatoms on Ž1 = 1.-Ag UPD adlayer on AuŽ100. Žs D SAD . must be similar to that on AgŽ100. Žs D SSUB .. On the other hand, the D S values of UPD Ag atoms on AuŽ100. Žs D SUPD . must be smaller than D SSUB because the UPD atoms were attracted by AuŽ100. surface atoms. From the considerations mentioned above, D SAD must be larger than D SUPD , i.e.: D SAD 6 D SSUB ) D SUPD ,
Ž 5.
so the Ag adatoms can diffuse without too much moving of the Ag UPD layer. In conclusion, it has been found that the D S values of both AgŽ100. and AuŽ100. in aqueous 50 mM H 2 SO4 solution increase exponentially with the applied potential. From the D S –E relationship, we propose that the activation energy of surface diffusion decrease because of the surface excess charge. We have also found that the D S values on AgŽ100. in aqueous 50 mM H 2 SO4 solution were a hundred times larger than those on AuŽ100. within the potential range from 150 to 350 mV.
N. Hirai et al.r Applied Surface Science 130–132 (1998) 506–511
Acknowledgements This work was partially supported by Grant-in-Aid for Scientific Research on Priority Area of ‘Electrochemistry of Ordered Interfaces’ ŽNo. 09237237. from the Ministry of Education, Science, Sports and Culture, Japan.
References w1x R. Sonnenfeld, P.K. Hansma, Science 232 Ž1986. 211. w2x X. Gao, G.J. Edens, A. Hamelin, M.J. Weaver, Surf. Sci. 296 Ž1993. 333.
511
w3x H. Gobel, P.V. Blanckenhagen, Surf. Sci. 331 Ž1995. 885. w4x N. Ikemiya, K. Yamada, S. Hara, J. Vac. Sci. Technol. B 14 Ž1996. 1369. w5x N. Ikemiya, K. Yamada, S. Hara, Surf. Sci. 348 Ž1996. 253. w6x N. Ikemiya, M. Nishide, S. Hara, Surf. Sci. 340 Ž1995. L965. w7x N.A. Gjostein, in: Surfaces and Interfaces 1, Chap. 11, p. 271. w8x W. Gust, I. Kaur, in: Handbook of Grain and Interphase Boundary Diffusion Data, Vols. 1–2, Ziegler Press, Stuttgart, 1989. w9x J. Schneider, D.M. Kolb, Surf. Sci. 193 Ž1988. 579. w10x T. Vitanov, A. Popov, Dokl. Akad. Nauk. S.S.S.R., Surf. Sci. 226 Ž1976. 373.
Enhanced diffusion of surface atoms at metalrelectrolyte interface under potential control Nobumitsu Hirai ) , Hiroaki Tanaka, Shigeta Hara Department of Materials Science and Processing, Faculty of Engineering, Osaka UniÕersity, Yamadaoka 2-1, Suita 565, Japan Received 1 September 1997; accepted 24 December 1997
Abstract We have investigated the surface self-diffusion coefficient Ž D S . on AgŽ100. in aqueous 50 mM H 2 SO4 solution using electrochemical atomic force microscopy ŽECAFM.. The results were compared with those observed on AuŽ100.. The D S values of AgŽ100. increased exponentially with potential between y50 and 350 mV Žvs. normal hydrogen electrode, NHE.. From the D S –E relationship, we propose that the activation energy of surface diffusion decreases because of the surface excess charge. We also found that the D S values on AgŽ100. were a hundred times larger than those on AuŽ100. within the potential range from 150 to 350 mV Žvs. NHE.. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Metal–electrolyte interface; Silver; Solid–liquid interface; Surface diffusion; Surface structure
1. Introduction Since Binnig et al. invented the scanning tunnelling microscope ŽSTM. and the atomic force microscope ŽAFM., much attention has been paid to direct observations of many surface processes with atomic resolution. These microscopes can work in various environments, such as gas, vacuum, and solution w1–3x. In our experiments, using some electrochemical processes at the electroderelectrolyte interface, such as the underpotential deposition ŽUPD. process in acid solution w4x, we found that Ag atoms electrodeposited on AuŽ100. electrode made the surface atomically flat w5x. On the other hand, we have found that the self-diffusion of surface atoms on AuŽ100. is enhanced when a suitable potential is )
Corresponding author. Tel.: q81-6-879-7467; fax: q81-6879-7467; e-mail: [email protected].
applied at the AurH 2 SO4 solution interface w6x. In this paper, we present a study of the surface self-diffusion process on AgŽ100. in aqueous 50 mM H 2 SO4 in comparison with that observed on AuŽ100.. 2. Experimental The AgŽ100. samples were prepared from an Ag single crystal rod Ž12 mm diameter., which was grown by the Bridgman method. After being annealed at 700 K for 30 min in H 2 gas, Ag was deposited onto AgŽ100. in aqueous 50 mM H 2 SO4 q 1 mM Ag 2 SO4 solution at 300 mV for about 30 min in order to make the surface atomically flat on a larger scale. The Ag deposition rate was about three monolayers per minute. The microscope used is a Nanoscope E ŽDigital Instruments. equipped with a potentiostat. All potentials reported here are relative to the normal hydrogen electrode ŽNHE., although a
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 0 6 1 - 0
N. Hirai et al.r Applied Surface Science 130–132 (1998) 506–511
HgrHg 2 SO4 reference electrode Ž0.68 V vs. NHE. was actually used. The D S values on AgŽ100. were measured using the following procedure. By scanning continuously on a small area of the sample with the AFM tip in aqueous 50 mM H 2 SO4 for a few minutes, a small hole can be easily created at 350 mV. For making a hole, the force between the tip and the surface is in the order of 10y7 N, otherwise, for imaging, it is in
507
the order of 10y9 N. Subsequently, the potential was jumped to a different potential of interest for surface self-diffusion measurements in the double layer region Žbetween y50 and 350 mV. and the scan area was enlarged. The D S values were calculated from successive AFM images that showed the refilling process of the hole. More details of this procedure in the case of AuŽ100. are presented in the earlier paper w6x.
Fig. 1. AFM images of AgŽ100. surface obtained at 150 mV in aqueous 50 mM H 2 SO4 q 1 mM Ag 2 SO4 solution after Ag deposition. Ža. Large area image Ž800 nm = 800 nm, height mode image.. Žb. High-resolution AFM image Ž8 nm = 8 nm., showing an unreconstructed structure. The w011x direction is indicated by an arrow.
508
N. Hirai et al.r Applied Surface Science 130–132 (1998) 506–511
3. Results and discussion
Fig. 2. Cyclic voltammogram for AgŽ100. in aqueous 50 mM H 2 SO4 solution. The scan rate is 50 mVrs.
Fig. 1a shows an AFM image Žheight mode. of AgŽ100. surface at 150 mV in aqueous 50 mM H 2 SO4 q 1 mM Ag 2 SO4 solution after Ag deposition. On the flat terrace of AgŽ100. surface, we can observe an unreconstructed AgŽ100.-Ž1 = 1. structure at 150 mV, ŽFig. 1b.. Fig. 2 shows a typical cyclic voltammogram ŽCV. for AgŽ100. in aqueous 50 mM H 2 SO4 . The sweep potential was from y350 to 550 mV and the scan
Fig. 3. Series of AFM images Ž150 nm = 150 nm, height mode image. together with the cross-section views obtained at 250 mV showing the refilling process of the hole created on the terrace. Image Ža. was taken immediately after the hole was made. The subsequent images were obtained 50 s Žb. and 134 s Žc. after image Ža..
N. Hirai et al.r Applied Surface Science 130–132 (1998) 506–511
Fig. 4. Relationship between log D S and electrode potential. The dots are the experimental D S values of AgŽ100. and AuŽ100. and the dotted lines are the D S values calculated from Eq. Ž4., where the constant ab is assumed 0.4. The broken lines are the extrapolated D S values of Ag and Au under vapor phase, reported by Gjostein w7x.
rate was 50 mVrs. As can be seen from Fig. 2, the CV is characterized by a double-layer region between the anodic peak of Ag dissolution at 550 mV and the cathodic peak of H 2 evolution at y350 mV. After making a hole on an atomically flat AgŽ100. surface, we observed the refilling process of the hole at various potentials. Fig. 3 shows a series of AFM images Žheight mode. of the refilling process at 250 mV along with the cross-section views. Fig. 3a is an image taken immediately the hole was made. Usually, the depth of the hole was about 3 nm. Around the hole, a hill was observed. As shown in Fig. 3b and c, the disappearance of the hole could be observed. Since the depth of the hole decreased linearly with time, we calculated the D S values from the following equation w7x, D S s d 2r2 t ,
Ž 1.
where d is the horizontal distance between the top of the hill and the bottom of the hole, and this the average refilling time per atomic layer. Fig. 4 shows the relationship between D S on AgŽ100. and potential. For comparison, the D S values observed on AuŽ100. w6x are also plotted. The D S values on Ag and Au in vacuum is extrapolated from the following equation w7x: 2
D S s D S 0 exp Ž yQSrRT . cm rs ,
at
D S 0 s 0.014, QS s 13Tm Ž calrmole. s 54.6Tm Ž Jrmole.
Ž 2.
509
are also shown in Fig. 4, where QS is the activation energy for surface diffusion, D S0 is a preexponential factor, Tm is the melting temperature ŽTm s 1234 K for Ag, Tm s 1336 K for Au. and T is the experimental temperature ŽT s 298 K.. The volume diffusion constant Ž D V . can be ignored, because it is several orders of magnitude smaller than D S w3,8x. In the case of AuŽ100., the minimum value of D S in aqueous 50 mM H 2 SO4 solution was located at EPZC of AuŽ100. Ž6270 mV w9x. and it agrees well with the one extrapolated from Eq. Ž2. w7x. In the case of AgŽ100., however, the extrapolated D S value at y450 mV, which is the E PZC of AgŽ100. in 10 mM Na 2 SO4 w10x, is much smaller than that calculated from Eq. Ž2.. It should be discussed whether D S0 and QS in Eq. Ž2. are valid or not for AgŽ100., because on one hand, Eq. Ž2. was derived from experimental values at temperatures more than 700 K and on the other hand, the D S values of Ag derived from Eq. Ž2. were a hundred times larger than the experimental ones even at 880 K w7x. Another question arises from whether the E PZC of y450 mV in 10 mM Na 2 SO4 can be used or not for the experiments performed in 50 mM H 2 SO4 solution. From Fig. 4, it can also be seen that the D S of AgŽ100. increased exponentially with potential within the range from y50 to 350 mV. The schematic diagram for the surface self-diffusion process of AgŽ100. and AuŽ100. under the presence of applied potential Ž E ., shown in Fig. 5, is used for deriving the D S –E relationship. The QS0 is the activation energy of surface diffusion when the applied potential is E PZC and is equal to the QS derived from Eq. Ž2.. At E ) E PZC , the reduction in the activation
Fig. 5. Schematic diagram of the activation energy of surface diffusion of Ag Žor Au. adatom under the presence of applied potential Ž E ., if E G E PZ C . The QS0 is the activation energy of surface diffusion at E PZ C . The DQS is the reduction in the activation energy of surface diffusion at E in comparison with at E PZ C .
N. Hirai et al.r Applied Surface Science 130–132 (1998) 506–511
510
energy for surface diffusion Ž DQS . can be obtained as DQS
s q M dV s CVdV s Ž 1r2 . CV 2
H
H
s Ž 1r2 . q M V s Ž 1r2 . a NA < e < b Ž E y E PZC . ,
Ž 3.
where q M Žs CV s a NA < e <. is the surface excess charge, V Žs b Ž E y E PZC .. is the applied voltage between inner Helmholtz layer and the surface, a and b are constants Žabsolute numbers., NA is Avogadro’s constant, and e is the charge of an electron. Taking into account the consideration mentioned above, DS
Fig. 6. Schematic diagrams for Ža. the diffusion of Ag on AuŽ100. and Žb. the self-diffusion on AgŽ100..
s D S0 exp Ž yQSrRT . s D S0 exp y Ž QS0 y DQS . rRT 4 s D S0 exp w yQS0
Ž 4.
q Ž 1r2 . ab NA < e < Ž E y EPZC . rRT 4 , log D S s Ž ab NA < e
Ž 4X . are derived from Eqs. Ž2. and Ž3.. Eqs. Ž4. and Ž4X . proposed by us take into consideration the effect of the applied potential on the surface self-diffusion. The dotted lines in Fig. 4 are the D S of Ag and Au calculated from Eq. Ž4.. The QS of Ag was modified in Eq. Ž4. in order to fit the D S value at E PZC of AgŽ100. predicted from experimental values with that extrapolated from Eq. Ž2.. If E1 6 E2 4 EEPZ and if we assume a and b at E s E1 to be nearly equal to those at E s E2 , log D SEs E1 y log D SEs E 2 s Ž ab NA < e
Ž 4Y .
can be obtained from Eq. Ž4X .. From Eq. Ž4Y . and D S –E relationship, we can calculate ab . The value of 0.4 for ab of both Ag and AuŽ100. at E 4 EPZC was obtained from experimental values plotted in Fig. 4. From Fig. 4, we also clearly see that the D S values on AgŽ100. were a hundred times larger than those on AuŽ100. within the measured potential range, i.e., from 150 to 350 mV. In our previous experiment w5x, we have found that Ag films were
formed on Ž1 = 1.-Ag UPD adlayer on AuŽ100. in aqueous 50 mM H 2 SO4 q 1 mM Ag 2 SO4 solution by an ideal layer-by-layer growth mode even at a high deposition rate ŽF 7.2 MLrmin. and the flatness of Ag films was improved as the deposition proceeded. Fig. 6 shows the schematic diagram for the diffusion of Ag on AuŽ100. and the self-diffusion on AgŽ100.. Because adatoms were mainly attracted by the neighboring atoms, the D S values of Ag adatoms on Ž1 = 1.-Ag UPD adlayer on AuŽ100. Žs D SAD . must be similar to that on AgŽ100. Žs D SSUB .. On the other hand, the D S values of UPD Ag atoms on AuŽ100. Žs D SUPD . must be smaller than D SSUB because the UPD atoms were attracted by AuŽ100. surface atoms. From the considerations mentioned above, D SAD must be larger than D SUPD , i.e.: D SAD 6 D SSUB ) D SUPD ,
Ž 5.
so the Ag adatoms can diffuse without too much moving of the Ag UPD layer. In conclusion, it has been found that the D S values of both AgŽ100. and AuŽ100. in aqueous 50 mM H 2 SO4 solution increase exponentially with the applied potential. From the D S –E relationship, we propose that the activation energy of surface diffusion decrease because of the surface excess charge. We have also found that the D S values on AgŽ100. in aqueous 50 mM H 2 SO4 solution were a hundred times larger than those on AuŽ100. within the potential range from 150 to 350 mV.
N. Hirai et al.r Applied Surface Science 130–132 (1998) 506–511
Acknowledgements This work was partially supported by Grant-in-Aid for Scientific Research on Priority Area of ‘Electrochemistry of Ordered Interfaces’ ŽNo. 09237237. from the Ministry of Education, Science, Sports and Culture, Japan.
References w1x R. Sonnenfeld, P.K. Hansma, Science 232 Ž1986. 211. w2x X. Gao, G.J. Edens, A. Hamelin, M.J. Weaver, Surf. Sci. 296 Ž1993. 333.
511
w3x H. Gobel, P.V. Blanckenhagen, Surf. Sci. 331 Ž1995. 885. w4x N. Ikemiya, K. Yamada, S. Hara, J. Vac. Sci. Technol. B 14 Ž1996. 1369. w5x N. Ikemiya, K. Yamada, S. Hara, Surf. Sci. 348 Ž1996. 253. w6x N. Ikemiya, M. Nishide, S. Hara, Surf. Sci. 340 Ž1995. L965. w7x N.A. Gjostein, in: Surfaces and Interfaces 1, Chap. 11, p. 271. w8x W. Gust, I. Kaur, in: Handbook of Grain and Interphase Boundary Diffusion Data, Vols. 1–2, Ziegler Press, Stuttgart, 1989. w9x J. Schneider, D.M. Kolb, Surf. Sci. 193 Ž1988. 579. w10x T. Vitanov, A. Popov, Dokl. Akad. Nauk. S.S.S.R., Surf. Sci. 226 Ž1976. 373.