- Email: [email protected]
Arrangement of bismuth on a ruthenium electrode
SHORT ARRANGEMENT
COMMUNICATION
OF BISMUTH ELECTRODE
M. A. QUIROZ*,
ON A RUTHENIUM
L. SALGADO and Y. MEAS
Universidad Aut6noma Metropolitana-Unidad Iztapalapa, Departamento de Quimica, Area de Electroquimica, Apdo. Postal 55-534.09340 M&co, DF Abstract-The deposit of Bi adatoms by UPD on Ru, was systematically studied as function of the potential and time of deposition. The UPD Bi was characterized by a monolayer peak at 0.60 V and with an UPD shift A,?, = 0.36 V. The electrochemical behaviour of the Bi adsorbed layer, the stoichiometry for the Ru/Bi system and a possible rearrangement of Bi, are discussed.
The deposit and oxidation of Bi on mono and polycrystalline Au electrodes[14] as well as on polycrystalline Pt electrodes[5,6], have been studied using HCIO, and/or HCl as supporting electrolyte. In these works it has been shown that the deposition of Bi produces layers with an ordered structure upon the substrate surface. Also, it has been observed that a complete Bi monolayer is deposited before the bulk deposit can occur. However, the Bi layers that are deposited after the formation of the monolayer, are of a complex nature on both Au and Pt. In the case of Ru electrodes, no studies have been reported concerning the UPD Bi, probably due to the experimental inaccessibility of the true area by the classical methods of hydrogen and oxygen electrosorptioncurrently applied for metals which belong to the Pt group[7]. Nevertheless we have recently shown that by using UPD Cu, it is possible to obtain a reasonable measure of the surface area of Ru electrodes[8,9]. On the basis of the previous arguments, this work is devoted to study the electrochemical characteristics of UPD Bi upon a polycrystalline Ru electrode, as well as to determine its electroadsorption stoichiometry. The experiments were performed in 1 N H,SO, electrolyte, prepared from 96”/, H,SO, (suprapur Merck) and triply distilled water, since if a chlorinated medium is used (es, HCI, HCIO,, NaCIO,) the Ru dissolves quickly. The electrochemical cell, as well as the preparation of the Ru electrode and the electronics have been described elsewhere[l, 91. All the potentials have been reported on the RHE scale. The potentiodynamic profile for the Ru electrode used here, is similar to that reported in the literature[8-121. The electrodeposition of Bi on Ru, from a 1 x 10e4M Bis+ solution, was achieved by holding the electrolysis potential in the range between 0.22-0.65 V, during the time required to reach the adsorption-desorption equilibrium. Since our objective was to establish the monolayer characteristics of Bi, the limit of the Bi deposit was set up at two monolayers. This prevents the bulk Bi deposition (whose behaviour may affect the electrochemical characteristics of the monolayer) and restricts the analysis of the deposit just to the formation of a monolayer.
Figure la shows the effect of the Bi adatoms upon the potentiodynamic profile of Ru (positive sweep only). Two peaks are observed during the anodic potential sweep, at 0.27 V (peak II) and 0.57 V (peak I). Peak II is related to the oxidation of the Bi deposited after the monolayer is completed and grows with the depositing time. Peak I is associated with the oxidation of Bi adatoms. With the purpose of analyzing the behaviour of Bi deposited in the monolayer and submonolayer levels on Ru, the corresponding oxidation curves as function of the deposition potential were obtained. Such oxidation profiles are shown in Fig. 1b. As can be observed, the process of the oxidation of Bi is characterized by a “monolayer peak” at 0.60 V and AE, = 0.36 V. This peak increases with the degree of surface coverage (0). When the Bi monolayer has been completely formed, the peak height grows no further. The fact that the electrooxidation curves (Fig. lb) show only one oxidation peak suggests a homogeneity of adsorption sites on the Ru surface. However, this is not likely to happen, since this electrode is polycrystalline Ru. A more correct interpretation may be given if we assume that, due to the strong Ru-Bi interaction (known from the difference in work functions, A+ = 0.51 eV)[13, 141, the adsorption of Bi is not influenced by the existence of adsorption sites of different energies on the electrode surface. Moreover, Bruckenstein[6] has shown that the UPD Bi upon polycrystalline Pt does not have a preference for the occupation of sites for the weak and strong adsorption of hydrogen, and that if a mechanism by sites were possible, it could only occur at 8 < 0.3. On the other hand, the half width of the oxidation peak (Fig. lb), 64 = 120 mV, could suggest in the first instance the presence of important repulsive interactions among the Bi adatoms. However, we do not think that the peak broadening in our system (Ru/Bi), is produced by repulsive interactions, due to the following facts: (i) The difference in electronegativity between Ru and Bi, Ax = 0.25 (14), fulfills satisfactorily the preestablished condition of Ax < 0.5 (15) for the formation of covalent bonds between substrate-adsorbate and a total charge transfer stoichiometry, y = z.
43.5
h-i. A. Qumoz et al.
436 ‘:
’ n
E
=;
b II
i ._
I’
69-
II II II II
55 -
‘I 11
I
I
r
I
’
0
0
i
’I
il-
I
0.2
CWW
4”
‘I
41-
1
N E-
0.4
1 2
0
0.8 E/V 1.0
I
0.45 0.40
n7 ; :.
0.35
! 6!
0.4
I
b
Q.65 0.46
5
0.2
I
Ed/V
3 4
0.6
I
0.6
O.eE/V
1.0
Fig. 1. (a) Anodic potentiodynamic i-E curveat 20 mV s- ’ obtained after holding the potential at 0.19 V in a 1 N HzSO+ + 1.10m4M Bi(NO& solution for 300 s. First potential sweep (broken line): stripping of Bi following potential sweep (solid line): bare Ru surface. (b) Electrooxidation curves for Bi adatoms
adatoms;
electrodeposited
on Ru for various
(ii) The position of the oxidation peak of Bi remains essentialy constant in the whole surface coverage range (0 -z 6(Bi) d 1). Conway et al. have shown, by using simulation methods, that the existence of lateral repulsions among adsorbed species would shift the desorption peak towards more positive potentials as more adsorbate is desorbed[lb]. Such effect is not observed by us. Therefore, it is more likely that the oxidation peak broadening is due to geometric factors originated by the large difference between the atomic radii (rBi/rRu = 1.32), rather than to lateral repulsive interactions among the adsorbed particles. Such difference may lead, as in the case of Pt/Pb’+ system (17), to a blockage of neighbour adsorption sites by the large Bi atoms. The sudden decrease of hydrogen adsorption on Ru (Fig. lb), reinforces this interpretation. The isotherm corresponding to the Ru/Bi system was constructed with the maximum amounts of Bi deposited as a function of the deposition potential (Fig. 2). As can be observed, the amount of UPD Bi reaches a plateau at about 0.2W.25 V, which indicates that the monolayer has been completed. The total number of Bi adatoms in the monolayer [N(Bih+,,_ = 6.9.10r5 at.Bi] has been calculated by the total charge involved in the monolayer formation [Q(Bi)Mr = 228 &XXII-~]. UPD Cu experiences were performed as described elsewhere (8,9), in order to calculate the number of Ru atoms exposed in the same Ru surface. This value is found to be: N(Ru)s = 1.6 x 1Or6 at.Ru; then, the Ru/Bi stoichiometric ratio (number of Ru sites occupied by each Bi adatom deposited) is given by: SR(Ru/Bi)
= N(Ru)JN(Bi)ML
= 2.3 at.Ru/at.Bi
Such result indicates that for every Bi adatom deposited at underpotential, 2.3 surface Ru atoms must be blocked by comparison with the Cu electroadsorption process, where SR(Ru/Cu) = 1[8]. Since the maximum coverage is a function of the atomic radius of the
deposition
potentials.
414
276
138
Fig. 2. Adsorption isotherm for UPD Bi on Ru vs potential of adsorption (obtqined by integration of the curves given in Fig. lb).
adsorbate[l, 183, and also, being rBi(0.178 nm) more than loo/, larger than rRu(0.134 nm), it is expected that the Bi monolayer on the Ru surface will not be epitaxial. Now, since the positions of the Bi atoms will differ from those of the Ru atoms, it is likely that the monolayer will be independent of the substrate orientation[l, 19,201. Figure 3 shows a structural model for the Bi monolayer on a Ru surface, where the preferentially exposed plane corresponds to the basal plane (0001). The stoichiometric ratio calculated from this model is SR(Ru/Bi), = 2.1, and adjusts well with the experimental value SR(Ru/Ri) = 2.3, obtained from the previous equation. If the stoichiometry found for the Ru/Bi system is correct, then the M/Bi ratio must decrease when the atomic radius of the substrate (M) increases. In other words, the smaller the atomic radius of the substrate,
Arrangement
of bismuth
on a ruthenium
electrode
437
Acknowledgements-The authors are indebted to Dr Jorge Ibatiez for his valuable help in the translation to English of this manuscript and thank PRONAES for financial support.
REFERENCES
Fig. 3. Structural model for the Bi monolayer on a Ru surface, where the preferentially exposed plane corresponds to the basal plane (0001). rRu = 0.134nm and rBi = 0.178 nm; calculated stoichiometric ratio: SR(Ru/Bi)c = 2.1.
the larger the number of sites that will be blocked by the adsorbate, leading to a larger substrate-adsorbate stoichiometric ratio. Bowles[5] and Bruckenstein[6] have shown for Pt, that for each Bi atom electroadsorbed the adsorption of 1.99 and 1.9 hydrogen atoms, respectively, is inhibited. Since the adsorption ratio Pt/H has an accepted value of 1, the stoichiometric ratio between Pt and Bi would then be SR(Pt/Bi) = 2. From the results published by Bruckenstein[4] for the Au/Bi system, we have calculated that the stoichiometric ratio for the monolayer of Bi on Au is SR(Au/Bi) = 1.4. Then, the predicted sequence is adequatly followed by these systems: rRu(0.134 with
nm) < rPt(0.139
SR(Ru/Bi)
> SR(Pt/Bi)
nm) < rAu(0.144 > SR(Au/Bi).
nm),
1. J. W. Schultze and D. Dickertmann, Surj Sci. 54, 489 (1976). 2. D. Dickertmann and J. W. Schultze, Electrochim. Acto 22, 117 (1977). and D. Dickertmann, Farady Symp. Chem. 3. J. W.‘Sch&e Sot. 12. 36 (19771. &J S: Bruckenstein, J. electrochem. Sot. 119. 4. S. H. C&e 1166 (1972). 5. B. J. Bowles, Electrochim. Acta IS, 737 (1970). Anal. Chem. 44, 1993 6. S. H. Cadle and S. Bruckenstein, (1972). Chemistry, Vol. 9, 7. R. Woods, A. J. Bard Electroanalytical Marcel Dekker, New York (1976). 8. M. A. Quiroz, Y. Meas, E. Lamy-Pitara and J. Barbier, J. electraanal. Chem. 157, 165 (1983). and J. Barbier, 9. M. A. Quiroz, Y. Meas, E. Lamy-Pitara Electrochim. Acta 31, 503 (1986). 10. D. Michell, D. A. J. Rand and R. Woods, J. electronnal. Chem. 89. 11 (19781. 11. R. Lezna,.N. R‘. de Tacconi and A. J. Arvia, J. electraanaL Chem. 151, 193 (1983). H. Angerstein-Kozlowska and B. E. 12. S. Hadzi-Jordanov, Conway, J. electrochem. S&., 125, 1471 (1978). 13. D. M. Kolb, M. Przasnyski and H. Gerisher, J. electroanal. Chem. 54, 125 (1974). 14. S. Trasatti, J. electroannl. Chem. 33, 351 (1971). 15. J. W. Schultze and F. D. Koppitz, Electrochim. Acto, 21,
327 (1976). 16. H. Angerstein-Kozlowska,
J. electroanal. 17. K. Engelsmann,
J. Klinger
and B. E. Conway,
Chem. 75, 45 (1977). W. J. Lorenz
and E. Schmidt,
J. elec-
tronnaL Chem. 114, 1 (1980). 18. R. L. Gerlach and T. N. Rhodin. Surl: Sci. 17, 32 (1969). 19. N. Furuya and S. Motto, J. electroonal. Gem. 98, 189 (1979). 20. C. W. Tucker, Surf: Sci. 31, 172 (1972).
COMMUNICATION
OF BISMUTH ELECTRODE
M. A. QUIROZ*,
ON A RUTHENIUM
L. SALGADO and Y. MEAS
Universidad Aut6noma Metropolitana-Unidad Iztapalapa, Departamento de Quimica, Area de Electroquimica, Apdo. Postal 55-534.09340 M&co, DF Abstract-The deposit of Bi adatoms by UPD on Ru, was systematically studied as function of the potential and time of deposition. The UPD Bi was characterized by a monolayer peak at 0.60 V and with an UPD shift A,?, = 0.36 V. The electrochemical behaviour of the Bi adsorbed layer, the stoichiometry for the Ru/Bi system and a possible rearrangement of Bi, are discussed.
The deposit and oxidation of Bi on mono and polycrystalline Au electrodes[14] as well as on polycrystalline Pt electrodes[5,6], have been studied using HCIO, and/or HCl as supporting electrolyte. In these works it has been shown that the deposition of Bi produces layers with an ordered structure upon the substrate surface. Also, it has been observed that a complete Bi monolayer is deposited before the bulk deposit can occur. However, the Bi layers that are deposited after the formation of the monolayer, are of a complex nature on both Au and Pt. In the case of Ru electrodes, no studies have been reported concerning the UPD Bi, probably due to the experimental inaccessibility of the true area by the classical methods of hydrogen and oxygen electrosorptioncurrently applied for metals which belong to the Pt group[7]. Nevertheless we have recently shown that by using UPD Cu, it is possible to obtain a reasonable measure of the surface area of Ru electrodes[8,9]. On the basis of the previous arguments, this work is devoted to study the electrochemical characteristics of UPD Bi upon a polycrystalline Ru electrode, as well as to determine its electroadsorption stoichiometry. The experiments were performed in 1 N H,SO, electrolyte, prepared from 96”/, H,SO, (suprapur Merck) and triply distilled water, since if a chlorinated medium is used (es, HCI, HCIO,, NaCIO,) the Ru dissolves quickly. The electrochemical cell, as well as the preparation of the Ru electrode and the electronics have been described elsewhere[l, 91. All the potentials have been reported on the RHE scale. The potentiodynamic profile for the Ru electrode used here, is similar to that reported in the literature[8-121. The electrodeposition of Bi on Ru, from a 1 x 10e4M Bis+ solution, was achieved by holding the electrolysis potential in the range between 0.22-0.65 V, during the time required to reach the adsorption-desorption equilibrium. Since our objective was to establish the monolayer characteristics of Bi, the limit of the Bi deposit was set up at two monolayers. This prevents the bulk Bi deposition (whose behaviour may affect the electrochemical characteristics of the monolayer) and restricts the analysis of the deposit just to the formation of a monolayer.
Figure la shows the effect of the Bi adatoms upon the potentiodynamic profile of Ru (positive sweep only). Two peaks are observed during the anodic potential sweep, at 0.27 V (peak II) and 0.57 V (peak I). Peak II is related to the oxidation of the Bi deposited after the monolayer is completed and grows with the depositing time. Peak I is associated with the oxidation of Bi adatoms. With the purpose of analyzing the behaviour of Bi deposited in the monolayer and submonolayer levels on Ru, the corresponding oxidation curves as function of the deposition potential were obtained. Such oxidation profiles are shown in Fig. 1b. As can be observed, the process of the oxidation of Bi is characterized by a “monolayer peak” at 0.60 V and AE, = 0.36 V. This peak increases with the degree of surface coverage (0). When the Bi monolayer has been completely formed, the peak height grows no further. The fact that the electrooxidation curves (Fig. lb) show only one oxidation peak suggests a homogeneity of adsorption sites on the Ru surface. However, this is not likely to happen, since this electrode is polycrystalline Ru. A more correct interpretation may be given if we assume that, due to the strong Ru-Bi interaction (known from the difference in work functions, A+ = 0.51 eV)[13, 141, the adsorption of Bi is not influenced by the existence of adsorption sites of different energies on the electrode surface. Moreover, Bruckenstein[6] has shown that the UPD Bi upon polycrystalline Pt does not have a preference for the occupation of sites for the weak and strong adsorption of hydrogen, and that if a mechanism by sites were possible, it could only occur at 8 < 0.3. On the other hand, the half width of the oxidation peak (Fig. lb), 64 = 120 mV, could suggest in the first instance the presence of important repulsive interactions among the Bi adatoms. However, we do not think that the peak broadening in our system (Ru/Bi), is produced by repulsive interactions, due to the following facts: (i) The difference in electronegativity between Ru and Bi, Ax = 0.25 (14), fulfills satisfactorily the preestablished condition of Ax < 0.5 (15) for the formation of covalent bonds between substrate-adsorbate and a total charge transfer stoichiometry, y = z.
43.5
h-i. A. Qumoz et al.
436 ‘:
’ n
E
=;
b II
i ._
I’
69-
II II II II
55 -
‘I 11
I
I
r
I
’
0
0
i
’I
il-
I
0.2
CWW
4”
‘I
41-
1
N E-
0.4
1 2
0
0.8 E/V 1.0
I
0.45 0.40
n7 ; :.
0.35
! 6!
0.4
I
b
Q.65 0.46
5
0.2
I
Ed/V
3 4
0.6
I
0.6
O.eE/V
1.0
Fig. 1. (a) Anodic potentiodynamic i-E curveat 20 mV s- ’ obtained after holding the potential at 0.19 V in a 1 N HzSO+ + 1.10m4M Bi(NO& solution for 300 s. First potential sweep (broken line): stripping of Bi following potential sweep (solid line): bare Ru surface. (b) Electrooxidation curves for Bi adatoms
adatoms;
electrodeposited
on Ru for various
(ii) The position of the oxidation peak of Bi remains essentialy constant in the whole surface coverage range (0 -z 6(Bi) d 1). Conway et al. have shown, by using simulation methods, that the existence of lateral repulsions among adsorbed species would shift the desorption peak towards more positive potentials as more adsorbate is desorbed[lb]. Such effect is not observed by us. Therefore, it is more likely that the oxidation peak broadening is due to geometric factors originated by the large difference between the atomic radii (rBi/rRu = 1.32), rather than to lateral repulsive interactions among the adsorbed particles. Such difference may lead, as in the case of Pt/Pb’+ system (17), to a blockage of neighbour adsorption sites by the large Bi atoms. The sudden decrease of hydrogen adsorption on Ru (Fig. lb), reinforces this interpretation. The isotherm corresponding to the Ru/Bi system was constructed with the maximum amounts of Bi deposited as a function of the deposition potential (Fig. 2). As can be observed, the amount of UPD Bi reaches a plateau at about 0.2W.25 V, which indicates that the monolayer has been completed. The total number of Bi adatoms in the monolayer [N(Bih+,,_ = 6.9.10r5 at.Bi] has been calculated by the total charge involved in the monolayer formation [Q(Bi)Mr = 228 &XXII-~]. UPD Cu experiences were performed as described elsewhere (8,9), in order to calculate the number of Ru atoms exposed in the same Ru surface. This value is found to be: N(Ru)s = 1.6 x 1Or6 at.Ru; then, the Ru/Bi stoichiometric ratio (number of Ru sites occupied by each Bi adatom deposited) is given by: SR(Ru/Bi)
= N(Ru)JN(Bi)ML
= 2.3 at.Ru/at.Bi
Such result indicates that for every Bi adatom deposited at underpotential, 2.3 surface Ru atoms must be blocked by comparison with the Cu electroadsorption process, where SR(Ru/Cu) = 1[8]. Since the maximum coverage is a function of the atomic radius of the
deposition
potentials.
414
276
138
Fig. 2. Adsorption isotherm for UPD Bi on Ru vs potential of adsorption (obtqined by integration of the curves given in Fig. lb).
adsorbate[l, 183, and also, being rBi(0.178 nm) more than loo/, larger than rRu(0.134 nm), it is expected that the Bi monolayer on the Ru surface will not be epitaxial. Now, since the positions of the Bi atoms will differ from those of the Ru atoms, it is likely that the monolayer will be independent of the substrate orientation[l, 19,201. Figure 3 shows a structural model for the Bi monolayer on a Ru surface, where the preferentially exposed plane corresponds to the basal plane (0001). The stoichiometric ratio calculated from this model is SR(Ru/Bi), = 2.1, and adjusts well with the experimental value SR(Ru/Ri) = 2.3, obtained from the previous equation. If the stoichiometry found for the Ru/Bi system is correct, then the M/Bi ratio must decrease when the atomic radius of the substrate (M) increases. In other words, the smaller the atomic radius of the substrate,
Arrangement
of bismuth
on a ruthenium
electrode
437
Acknowledgements-The authors are indebted to Dr Jorge Ibatiez for his valuable help in the translation to English of this manuscript and thank PRONAES for financial support.
REFERENCES
Fig. 3. Structural model for the Bi monolayer on a Ru surface, where the preferentially exposed plane corresponds to the basal plane (0001). rRu = 0.134nm and rBi = 0.178 nm; calculated stoichiometric ratio: SR(Ru/Bi)c = 2.1.
the larger the number of sites that will be blocked by the adsorbate, leading to a larger substrate-adsorbate stoichiometric ratio. Bowles[5] and Bruckenstein[6] have shown for Pt, that for each Bi atom electroadsorbed the adsorption of 1.99 and 1.9 hydrogen atoms, respectively, is inhibited. Since the adsorption ratio Pt/H has an accepted value of 1, the stoichiometric ratio between Pt and Bi would then be SR(Pt/Bi) = 2. From the results published by Bruckenstein[4] for the Au/Bi system, we have calculated that the stoichiometric ratio for the monolayer of Bi on Au is SR(Au/Bi) = 1.4. Then, the predicted sequence is adequatly followed by these systems: rRu(0.134 with
nm) < rPt(0.139
SR(Ru/Bi)
> SR(Pt/Bi)
nm) < rAu(0.144 > SR(Au/Bi).
nm),
1. J. W. Schultze and D. Dickertmann, Surj Sci. 54, 489 (1976). 2. D. Dickertmann and J. W. Schultze, Electrochim. Acto 22, 117 (1977). and D. Dickertmann, Farady Symp. Chem. 3. J. W.‘Sch&e Sot. 12. 36 (19771. &J S: Bruckenstein, J. electrochem. Sot. 119. 4. S. H. C&e 1166 (1972). 5. B. J. Bowles, Electrochim. Acta IS, 737 (1970). Anal. Chem. 44, 1993 6. S. H. Cadle and S. Bruckenstein, (1972). Chemistry, Vol. 9, 7. R. Woods, A. J. Bard Electroanalytical Marcel Dekker, New York (1976). 8. M. A. Quiroz, Y. Meas, E. Lamy-Pitara and J. Barbier, J. electraanal. Chem. 157, 165 (1983). and J. Barbier, 9. M. A. Quiroz, Y. Meas, E. Lamy-Pitara Electrochim. Acta 31, 503 (1986). 10. D. Michell, D. A. J. Rand and R. Woods, J. electronnal. Chem. 89. 11 (19781. 11. R. Lezna,.N. R‘. de Tacconi and A. J. Arvia, J. electraanaL Chem. 151, 193 (1983). H. Angerstein-Kozlowska and B. E. 12. S. Hadzi-Jordanov, Conway, J. electrochem. S&., 125, 1471 (1978). 13. D. M. Kolb, M. Przasnyski and H. Gerisher, J. electroanal. Chem. 54, 125 (1974). 14. S. Trasatti, J. electroannl. Chem. 33, 351 (1971). 15. J. W. Schultze and F. D. Koppitz, Electrochim. Acto, 21,
327 (1976). 16. H. Angerstein-Kozlowska,
J. electroanal. 17. K. Engelsmann,
J. Klinger
and B. E. Conway,
Chem. 75, 45 (1977). W. J. Lorenz
and E. Schmidt,
J. elec-
tronnaL Chem. 114, 1 (1980). 18. R. L. Gerlach and T. N. Rhodin. Surl: Sci. 17, 32 (1969). 19. N. Furuya and S. Motto, J. electroonal. Gem. 98, 189 (1979). 20. C. W. Tucker, Surf: Sci. 31, 172 (1972).