Determination of (111) Ordered Domains on Platinum Electrodes by

Anal. Chem. 2005, 77, 5317-5323

Determination of (111) Ordered Domains on Platinum Electrodes by Irreversible Adsorption of Bismuth Paramaconi Rodrı´guez, Jose´ Solla-Gulloń, Francisco J. Vidal-Iglesias, Enrique Herrero,* Antonio Aldaz, and Juan M. Feliu

Departamento de Quı´mica Fı´sica and Instituto de Electroquı´mica, Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain

Irreversible adsorbed bismuth can be used to determine the fraction of (111) domains on a given platinum sample. On Pt(111) electrodes, the surface redox process of adsorbed bismuth takes place at 0.63 V in a well-defined peak. The behavior of this redox process on the Pt(111) vicinal surfaces indicates that the bismuth atoms involved in the redox process are only those deposited on the (111) terrace sites and that the charge under the peak at 0.63 V is directly proportional to the number of sites on (111) ordered domains (terraces). The good linear relationship obtained between the charge for the bismuth redox process and the number of (111) terrace sites on the vicinal surfaces allows construction of a calibration curve. This calibration curve has been used to directly estimate the amount of (111) ordered domain terrace sites on polycrystalline platinum samples with different surface ordered domains. The results agree with what we would expect from our knowledge of these surfaces. The use of platinum single crystals in electrochemistry has allowed understanding of the role of the surface structure in the electrochemical reaction. Well-defined surfaces are necessary to rationalize the experimental results that are observed in fundamental electrochemical processes, since many of them are structure-sensitive reactions. Most of the studies with platinum single crystals use the basal planes, that is, Pt(111), Pt(100), and Pt(110), in which all the surface sites have the same symmetry.1,2 However, it is difficult to extrapolate the results obtained with the basal plane electrodes to the surfaces of the electrodes employed in practical applications, since the latter have surface structures that are much more complex. In the case of precious metals, the practical electrodes are composed of nanoparticles supported on a conducting material. The surface of the nanoparticles can normally be described as a series of (111) and (100) small domains connected by atoms having a low coordination number. The behavior of such surfaces can be approached by the use of stepped surfaces, which consists of a regular succession of terraces with a given symmetry separated by a monatomic steps. By changing the terrace width and the symmetry of the step, the contributions of the different domains (with different * Corresponding author. E-mail: [email protected]. (1) Herrero, E.; Buller, L. J.; Abrun ã, H. D. Chem. Rev. 2001, 101, 1897. (2) Markovic, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 117. 10.1021/ac050347q CCC: $30.25 Published on Web 07/13/2005

© 2005 American Chemical Society

symmetry and size) can be analyzed and linked to the results of the real surfaces. To carry out a detailed fundamental approach to the search for more active electrocatalysts and to improve our understanding of the electrocatalytic behavior of nanoparticles, it would be of interest to know the type and distribution of the catalytic sites with a given geometry on the catalyst surface. For this reason, it is necessary to develop in situ methods to measure the distribution of the different sites on the nanoparticle surface. Within the available techniques for characterizing nanoparticles, only ex situ HRTEM can provide, after some simulation, direct information on the surface structure of the nanoparticle.3 However, because the technique is time-consuming and the number of nanoparticles examined in a given sample is small, the results obtained may not be representative of the “average” surface structure of the sample. To determine the presence of different symmetry sites on a given platinum sample and, therefore, to characterize the surface structure of the sample, we propose the use of the irreversible adsorption of a foreign adatom that occurs on the surface of a metal substrate.4 For this process, the adatom remains adsorbed on the surface of the host metal in a wide potential range, despite the fact that the solution does not contain ions of the adatom. The most interesting situation is found when the adatom is fully blocking the surface and undergoes a surface reaction in a potential range in which contributions from other surface orientations are negligible.5 This reaction might be used to test the presence of a characteristic type of surface site on a surface. If the reaction is selective enough, it could be used to test and monitor the presence of sites with this particular symmetry on the surface of polycrystalline samples. It was shown that irreversibly adsorbed bismuth on Pt(111) shows a well-defined process at 0.63 V.6 Contributions from the other basal planes are negligible in this potential range. In addition, the adsorbed adatoms remain on the surface of the corresponding platinum electrodes, provided the upper potential limit is controlled. In this way, bismuth adsorbed on platinum electrodes could be a good probe to test the presence of ordered (111) (3) Wang, Z. L. J. Phys. Chem. 2000, 104, 1153. (4) Feliu, J. M.; Fernańdez-Vega, A.; Orts, J. M.; Aldaz, A.; J. Chim. Phys. 1991, 88, 1493. (5) Orts, J. M.; Rodes, A.; Feliu, J. M. J. Electroanal. Chem. 1997, 434, 121. (6) Clavilier, J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1998, 243, 419.

Analytical Chemistry, Vol. 77, No. 16, August 15, 2005 5317

domains in a given sample, in a similar way to the determination of (100) sites by the use of irreversibly adsorbed germanium.7 The aim of this paper is to study irreversible adsorption of bismuth on platinum electrode surfaces having terraces with (111) symmetry to test whether in situ determination of such surface sites is possible by using a calibration curve. EXPERIMENTAL SECTION Platinum single-crystal electrodes vicinal to the (111) pole were oriented, cut and polished from small single-crystal platinum beads (2.5-mm diameter) following the procedure described by Clavilier and co-workers.8 The surfaces used were Pt(111) itself and those belonging to the series of Pt(S)[n(111) × (111)] having Miller indices Pt(n, n, n - 2) and Pt(S)[n(111) × (100)] that has Miller indices Pt(n + 1, n - 1, n - 1). As usual, n represents the number of terrace atoms. Other platinum stepped surfaces belonging to other combinations of terrace and step sites have been also used for the sake of comparison and will be described below. The electrodes were cleaned by flame annealing, cooled in H2/Ar, and protected with water in equilibrium with this gas mixture to prevent contamination before immersion in the electrochemical cell, as described elsewhere.9 It has been shown that this procedure leads to surface topographies reasonably close to the nominal ones.10,11 The nominal step density of these electrodes is defined by the following equations,

N(111) × (111) )

2

(

x3d n -

2 3

)

(1)

for the Pt(n, n, n - 1) surfaces and

N(111) × (100) )

2

(

x3d n -

1 3

)

(2)

for the Pt(n + 1, n - 1, n - 1) surfaces. In these equations, d is the platinum atomic diameter and is equal to 0.278 nm. A platinum single crystal bead fully immersed in the cell solution was also used as a reproducible form of a polyoriented platinum electrode having a distribution of all surface sites (i.e., a polycrystalline platinum electrode). Irreversible adsorption of bismuth was performed as described previously by putting the electrode surface in contact with a solution of Bi2O3 (10-5- 10-4 M) in 0.5 M sulfuric acid for a short period of time (5-60 s).6 The modified electrode was then rinsed with water and immersed again in the electrochemical cell. Experiments were carried out at room temperature, 20 °C, in classical two-compartment electrochemical cells deaerated by using Ar (N50, Air Liquide in all gases used), including a platinum counter electrode and a reversible hydrogen (N50) electrode (7) Rodriguez, P.; Herrero, E.; Solla-Gulloń, J.; Vidal-Iglesias, F. J.; Aldaz, A.; Feliu, J. M. Electrochim. Acta 2005, 50, 3111. (8) Clavilier, J.; Armand, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267. (9) Rodes, A.; El Achi, K.; Zamakhchari, M. A.; Clavilier, J. J. Electroanal. Chem. 1990, 284, 245. (10) Herrero, E.; Orts, J. M.; Aldaz, A.; Feliu, J. M. Surf. Sci. 1999, 444, 259. (11) Garcıá-Araéz, N.; Cliement, V.; Herrero, E.; Feliu, J. M. Surf. Sci. 2004, 560, 269.

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Analytical Chemistry, Vol. 77, No. 16, August 15, 2005

(RHE) as reference. Solutions were prepared from sulfuric acid (doubly distilled, Aldrich) and ultrapure water from Elga. The cleanliness of the solutions was tested by the stability of the characteristic voltammetric features of well-defined Pt(111) electrodes. The purity of other chemicals was the same as in previous work.6 RESULTS AND DISCUSSION Electrochemical Behavior of Irreversibly Adsorbed Bi on Pt Surfaces Vicinal to the (111) Pole. The main purpose of this work is to develop a method to evaluate the fraction of (111) ordered domains present on a platinum surface. For that, a structure-sensitive reaction, such as that of irreversibly adsorbed bismuth on a platinum surface, has to be used. Irreversibly adsorbed bismuth on platinum electrodes has a surface redox process that shows well-defined peaks at different potentials for the three basal planes; i.e., the redox process has a peak at 0.63 V on Pt(111) electrodes, whereas the peak potential on Pt(100) and Pt(110) electrodes is 0.825 and 0.915 V, respectively.6 For the Pt(110) electrode, the potential for the bismuth redox peak overlaps with the surface oxidation, and for Pt(100), the redox peak is close to the onset of surface oxidation. Moreover, this onset of surface oxidation for surfaces vicinal to the Pt(100) is shifted toward negative potentials as the step density increases.12 Since the surface oxidation process of platinum electrodes may lead to the surface modification with the creation of defects and the reduction of long-range order, only the peak for the Pt(111) electrode can be used safely for the characterization. The appearance of a peak at 0.63 V when bismuth is adsorbed on a given platinum electrode may be then used to deduce the presence of (111) ordered domains. Additionally, the charge of the redox process may also serve to quantify the fraction of the surface with (111) ordered domains. The charge underneath this peak corresponds to the formal oxidation/reduction process of surface bismuth from Bi(0) to an oxygenated Bi(II) compound;6 therefore, the number of bismuth adatoms adsorbed on the (111) domain can be measured. However, a direct estimation of this value could be not reliable, since real surfaces, such as those present in nanoparticle electrodes, do not have wide and wellordered (111) domains. To determine whether this surface redox process on the (111) domains can be used to characterize the surface structure of a real electrode, stepped surfaces have been used as controlled model electrodes. Stepped surfaces are used to bridge the gap between single crystal basal planes and real surfaces, since they have a well-defined surface structure (and therefore, the study of surface sensitive processes can be easily rationalized) in which the size and nature of the ordered domains can be controlled by changing the terrace size, the terrace symmetry, and the step symmetry to simulate those present on real surfaces. Figure 1 shows the voltammetric profiles obtained for the stepped electrodes vicinal to the (111) pole after the irreversible adsorption of bismuth in 0.5 M sulfuric acid solution when the surface is completely blocked by the adatom. It should be mentioned that higher coverage values can be obtained for Pt(111) electrodes;13 however, those coverages are not stable upon cycling on the selected potential limits and evolve to the situation (12) Domke, K.; Herrero, E.; Rodes, A.; Feliu, J. M. J. Electroanal. Chem. 2003, 552, 115.

Figure 1. Voltammetric profiles of the Pt(n, n, n - 2) and Pt(n + 1, n - 1, n - 1) electrodes covered with irreversibly adsorbed bismuth in 0.5 M H2SO4. Scan rates: 50 mV s-1.

presented in Figure 1. Two potential regions can be observed in these voltammetric profiles. In the first one, between 0.06 and 0.56 V, a small and almost constant current is recorded in this potential window and can be associated with double layer charge/ discharge process. The absence of any other currents in this potential region indicates that hydrogen adsorption has been completely blocked; i.e., all platinum sites are covered by bismuth. In the upper potential region, a pair of reversible peaks can be observed. These peaks are associated with the oxidationreduction of the adsorbed bismuth. Since the potential is very close to that observed on Pt(111) electrodes, these peaks can be initially associated with the redox process of bismuth on the (111) terrace sites. The redox peak charge and morphology is very dependent on the terrace size. Thus, the peak becomes smaller (and comprises less charge) as the terrace length diminishes. The peak is also broader and more complex, since two or more peaks can be observed for the narrower terraces. This situation is similar to that found for compressed bismuth adlayers on Pt(111).6,14,15 For this latter electrode, a single peak is observed for a coverage value of 0.33 (see Figure 1), and for higher coverage values, which imply compression in the adlayer, redox peak splits, showing characteristics similar to those observed with (111) narrower terraces. (13) Dollard, L.; Evans, R. W.; Attard, G. A. J. Electroanal. Chem. 1993, 354, 205. (14) Evans, R. W.; Attard, G. A. J. Electroanal. Chem. 1993, 345, 337. (15) Smith, S. P. E.; Abrun ã, H. D. J. Phys. Chem. B 1998, 102, 3506.

The main characterization magnitude in this study is related to charge density measurements when the surface is fully blocked by bismuth. Uncorrected electric charges are the same in both sweeps, and the voltammetric profiles remain constant in successive cycles. This ensures the stability of the system and the absence of any reaction that could disturb the surface processes. To establish the theoretical equations for the process, the number of electrons transferred in the bismuth redox process per Pt need to be known. For bismuth adatoms on Pt(111), it has been proposed that the adatom blocks three sites (i.e., the maximum coverage is 0.33) and transfers 2 electrons6,14,16 according to the following reaction.

Pt3Biads + 2H2O h Pt3Bi(OH)2,ads + 2H+ +2e

(3)

Therefore, there will be an effective transfer of 0.67e per platinum atom when the surface is fully blocked by bismuth. Since the peaks in sulfuric and perchloric acid solutions appear at the same potential and have the same morphology, a significant implication of the anions in the redox process of bismuth can be discarded.15 Anion adsorption, however, should not modify the characterization of the adlayers, since it is based on pure experimental results. It should be mentioned that the exact chemical state of the species involved in the bismuth redox process is still not clear, but the (16) Blais, S.; Jerkiewicz, G.; Herrero, E.; Feliu. J. M. J. Electroanal. Chem. 2002, 519, 111.


5319

Figure 2. Charge density values of the bismuth redox peak vs (1/n - (1/3)) cos R for the Pt(n + 1, n - 1, n - 1) electrodes or (1/n - (2/3)) cos R for the Pt(n, n, n - 2) electrodes.

results obtained here do not depend on that, since they are based on experimental data. To obtain the equation that links the bismuth redox charge and the (111) terrace sites, it will be assumed that the oxidation peaks are only due to the oxidation of bismuth on the (111) terrace sites. For these surfaces, it is known that bismuth is able to decorate completely the step site before deposition of the adatom starts on the terrace sites.17 When bismuth is only adsorbed on the step sites, no peak is observed at ∼0.63 V. This peak only appears when bismuth is present on the terrace site. Therefore, it can be proposed that the oxidation peaks are only due to the oxidation of bismuth on the (111) terrace sites. The charge for a redox process exchanging one electron per platinum terrace atom, qt, can be calculated using the hard sphere model. This charge for the Pt(n, n, n - 2) electrodes is18,19

t q(111) × (111)

(

)(

1 q 3 Pt(111) 80 ) qPt(111) ) 241 µC cm-2 2 2 nn3 3 (4)

)

where qPt(111) is the charge density measured for a process taking place on an ideal Pt(111) electrode transferring 1 e per platinum site, and it is equal to 241 µC cm-2. For the surfaces of the Pt(n + 1, n - 1, n - 1) series, the corresponding equation is20

t q(111) × (100)

(

)(

2 q 3 Pt(111) 161 ) qPt(111) ) 241 µC cm-2 1 1 nn3 3 (5)

)

The charge densities defined in the previous equations are calculated using the area of the surface projected on the plane of the terrace, that is, the (111) plane.18-20 However, the current densities in the voltammograms are referred to the geometric area of the electrode surface. To compare with the values given by the hard sphere model, eqs 4 and 5 should be corrected by multiplying by the cosine of the angle (R) between the surface 5320


and the (111) plane, which can be calculated as

cos R )

3n - 2

x9n

2

(6)

- 12n + 12

for the Pt(n, n, n - 2) electrodes and

cos R )

3n - 1

x9n2 - 6n + 3

(7)

for the Pt(n + 1, n - 1, n - 1) electrodes. Therefore, if the experimental charges are only due to processes taking place on the (111) terraces, the representation of the charge for the bismuth redox peak vs (1/(n - 2/3)) cos R or (1/(n - 1/3)) cos R should be linear. As shown in Figure 2, a linear relationship is obtained in both series, indicating that the charge measured is proportional to the total number of platinum sites on the terrace, provided that the stoichiometry of the bismuth reaction does not depend on the width of the terraces; i.e., two electrons are exchanged per bismuth atom, which blocks 3 (111) terrace sites. The extrapolations of the results also indicate that no bismuth charge should be measured for two atom-wide terraces. This fact was, indeed, corroborated experimentally, and for these electrodes (Pt(331) and Pt(311)), no visible redox peak was measured in this region. To explain the absence of the bismuth redox peak on these surfaces, the sizes of the step site and bismuth have to be compared. Figure 3 shows the side view of the stepped surfaces with a bismuth adatom deposited on the step. The atoms are drawn to scale using the interatomic distances of platinum and bismuth obtained in the metal, 2.77 and 3.07 nm, respectively.21 As can be seen, the step site on both series is smaller than the bismuth atom, and therefore, when bismuth is deposited on the (17) Herrero, E.; Climent, V.; Feliu, J. M. Electrochem. Comm. 2000, 2, 636. (18) Clavilier, J.; El Achi, K.; Rodes, A. Chem. Phys. 1990, 141, 1. (19) Clavilier, J.; El Achi, K.; Rodes, A. J. Electroanal. Chem. 1989, 272, 253. (20) Rodes, A.; El Achi, K.; Zamakhchari, M. A.; Clavilier, J. J. Electroanal. Chem. 1990, 284, 245.

t-1 q(111)×(100)

(

)

5 q 3 Pt(111) ) qPt(111) cos R ) 1 n3 402 241 cos R µC cm-2 (9) 1 n3

(

)

4 qPt(111) 3 t-1 q(111)×(111) ) qPt(111) cos R ) 2 n3 321 241 cos R µC cm-2 (8) 2 n3

for the Pt(n + 1, n - 1, n - 1) series. Figure 4 shows the bismuth redox charge at full coverage vs the charge calculated by eqs 8 and 9. Two comments could be made for this plot. First of all, the positive slope shows that the bismuth contribution comes from the (111) ordered domains present on the electrode surface. Second, the lines cross the origin, thus validating the assumptions made to estimate the bismuth-related charge densities. Finally, the stoichiometry of the surface electrochemical reaction 3 is reasonably fulfilled in all the Pt(111) terrace sites, since the slope obtained in Figure 3 (0.64 ( 0.03) is very close to the value of 0.67 e per platinum site obtained for the Pt(111) electrodes. It should be stressed that qt-1 is directly proportional to the number of Pt sites present in the ordered domains per area unit, and therefore, its value can be used to estimate the fraction of (111) ordered domains on a given sample. Several other stepped surfaces with (100) and (110) terrace sites were used to verify the absence of a peak at 0.63 V. Regardless of the symmetry of the step site, (100) or (111), the surfaces that do not have (111) terrace sites do not show any voltammetric signal between 0.56 and 0.7 V, confirming the assignation of this peak to the bismuth redox process on the (111) terrace sites. Surface Characterization of Polycrystalline Platinum. The results obtained from well-defined surfaces indicate that the bismuth redox peak can be used to evaluate the ratio of (111) ordered domains (being more than two atoms wide) in any sample. This can be checked in polycrystalline samples. As a model surface, a polyoriented single-crystal bead, as those used to prepare the well-ordered surfaces, was used. This electrode shows large facets corresponding to the (111) poles of the fcc crystal and also smaller facets that correspond to the (100) faces. Two different treatments were used prior to bismuth adsorption: first of all, the electrode was flame-annealed and cooled in H2Ar, and the corresponding electrode exhibited well-defined (111) domains, as shown by the presence of a small spike at 0.45 V, reminiscent of that found in the voltammetric profile of Pt(111) electrodes in 0.5 M sulfuric acid. Additionally, there are also contributions from wide (100) terraces, as shown by the presence of marked contributions around 0.37 V. Other contributions from (100) sites, either step or terrace border sites, are responsible for the 0.27 V adsorption state and the (110) steps sites at 0.125 V. In a second case, the flame-annealed electrode was successively cycled up to 1.4 V, enabling oxygen adsorption. It is known that this latter procedure implies the breaking of the long range order of the electrode. The (100) and (111) domains initially present on the surface disappear as the contributions above 0.3 V diminish, and the contributions of the step sites increase. As a first step, the surface area of the electrode should be normalized. This was performed in the traditional way by

for the Pt(n, n, n - 2) series and

(21) Martienssen, W.; Warlimont, H. Springer Handbook of Condensed Matter and Materials Data; Springer: Heidelberg, 2005.

Figure 3. Side view of the (111) and (100) step site blocked by a bismuth adatom on the Pt(111) vicinal surfaces.

step sites, the adatom is also blocking some terraces sites. In these cases, for both series of stepped surfaces, the presence of bismuth on the step sites is also blocking the terrace sites on the atomic row neighboring the step sites. It should be mentioned that the Pt(n, n, n - 2) surfaces can be denoted as Pt(S)[n(111) × (111)] or Pt(S)[(n - 1)(111) × (110)]; i.e., the symmetry of the step can be considered as (111) or (110) (the preferred notation is Pt(S)[n(111) × (111)]). When the (110) symmetry for the step is considered, the terrace has one fewer atom than in the other symmetry, which now belongs to the step site. Therefore, the (110) step site then is larger than the (111) step site and can accommodate perfectly the bismuth atom, Additionally, bismuth redox process on Pt(110) electrodes has an oxidation peak at 0.915 V,6 a potential much higher than that used in this study for the voltammetric profiles, thus justifying the absence of a redox peak for the adsorbed adatoms on the steps in the voltammograms. All these facts suggest that the step site behaves as a (110) site for bismuth. A similar situation is found for hydrogen adsorption on these step surfaces, for which the step site also behaves as a (110) site.18,19 For the other series of stepped surfaces, those having (100) sites, the situation can be regarded as similar, although there is no other possible notation for the step symmetry. However, in the studies of adsorption of anions on (100) step sites, the adsorbed species on the steps also blocks some additional terrace sites, also associating additional sites to the step.11,12 Figure 2 demonstrates that the bismuth redox process is probing (n - 1) atoms on the (111) terrace, and therefore, the charge of the redox process of the irreversibly adsorbed bismuth can be used to estimate the number of (111) terrace sites present in the surface for domains in which n > 2. For that purpose, a calibration curve can be constructing plotting the bismuth redox charge, measured at full coverage, vs that of the platinum sites on the (111) terraces, eliminating the row of atoms close to the step. These Pt(111) terrace sites can be easily calculated from eqs 4 and 5 by subtracting a row of atoms and multiplying by cos R, that is,

(

(

)

)


5321

Figure 4. Charge density values of the bismuth redox peak vs the charge associated with the (111) sites on the terrace, calculated using eqs 8 and 9. (9) Pt(n, n, n - 2) electrodes, (O) Pt(n + 1, n - 1, n 1) electrodes.

Figure 5. (A) Voltammetric profile of a flame-annealed polycrystalline electrode (s) and a polycrystalline electrode after several cycles up to 1.4 V (- -) in 0.5 M H2SO4. (B) Voltammetric profile of the electrodes of panel A after adsorption of bismuth.

considering that the electrical charge measured by integration of the voltammogram in the low potential region (Figure 5A), corrected from a double layer contribution, corresponds to 220 mC cm-2. It is confirmed that in the disordering process, the electrical charge remains practically constant, meaning that the disordering step affects only the microroughness of the surface. 5322 Analytical Chemistry, Vol. 77, No. 16, August 15, 2005

It is clearly seen in Figure 5A that the disordering step leads to an increase in the charge associated with the peaks at 0.12 V, related to adsorption on Pt(110) sites. The diminution of the signals at 0.37 V also points out that the overall number of (100) terrace sites initially present on the surface has diminished as a consequence of the disordering step. It is well-documented that the complex voltammetric profiles of Figure 5A can be decomposed into a series of contributions that can be linked to the different surface sites.22,23 This deconvolution of the voltammogram can use the information gained from stepped surfaces but has a serious drawback in relation to the position of the zero-current level. This arises because not only does some double layer current density level have to be assumed, but also the contribution from (111) terrace sites has to be considered. This latter contribution is almost constant in this potential range, resulting in a constant value of current density in all the potential range of the voltammogram.23 The lack of definition of these contributions makes it difficult to analyze quantitatively the blank voltammogram. On the other hand, the use of a specific probe enabling the evaluation of a particular type of site on the surface is much more convenient. This can be made by using the contribution of adsorbed bismuth on these surfaces. Figure 5B shows the voltammetric profiles of these two polycrystalline samples after treatment with the bismuth-containing solution, as done for the stepped surfaces. The main features reported for the well-ordered surfaces can also be identified on these electrodes, that is, the featureless region below 0.5 V and the redox process associated with the presence of bismuth on the surface at higher potentials. In this latter potential region, it can be seen that the charge involved in the disordered sample is smaller, thus reflecting the diminution of the area of the terraces having (111) symmetry. Moreover, the bismuth redox peak for the unperturbed surface is well-defined, whereas the peak for the perturbed surfaces resembles that obtained for the stepped surfaces with narrow terraces. These two features also agree with the disorder of the surfaces and its influence in the corresponding voltammetric profiles as a consequence of electrochemical oxygen adsorption, a process that is well-documented by in situ STM.24,25 The well-defined peak at 0.63 V is quantitatively linked to the presence of wide (111) terraces on the polycrystalline sample, and the overall charge diminution of (111) sites in favor of sites with (110) symmetry can be determined. In this case, the knowledge of the charge density related to the irreversibly adsorbed bismuth and the calibration plot of Figure 3 supplies a reasonable estimation of the number of (111) terrace sites present on these two platinum surfaces. The charge density for the (111) domains obtained form the calibration plot is then normalized to the total charge density associated with the platinum electrode, that is, 220 µC cm-2, to obtain the fraction of (111) ordered domains on the surface. The result indicates that the surface of the flame-annealed electrode contains 15% (111) terrace sites, including long and short domains, whereas in the surface after electrochemical oxygen adsorption, this fraction has (22) Armand, D.; Clavilier, J. J. Electroanal. Chem. 1987, 225, 205. (23) Armand, D.; Clavilier, J. J. Electroanal. Chem. 1987, 233, 251. (24) Itaya, K.; Sugawara, S.; Sashikata, K.; Fururya, N. J. Vac. Sci. Technol., A 1990, 8, 515. (25) Feliu, J. M.; Rodes, A.; Orts, J. M.; Clavilier, J. Pol. J. Chem. 1994, 68, 1575.

diminished to 9%, large domains being practically absent from the surface. The charge ratio between the bismuth and platinum has to be considered as a whole in the present case, and long-term deconvolution of the voltammetric profiles should be made to reach deeper conclusions. It should be mentioned that the values obtained for the number of the (111) sites in the polycrystalline sample used here cannot be considered as a general value for any polycrystalline sample. The number of (111) sites on a given polyoriented electrode will depend on their own surface, a result of the preparation of the platinum sample. CONCLUSIONS Irreversible adsorbed bismuth can be used to determine the fraction of (111) domains on a given platinum sample. On Pt(111) electrodes, the surface redox process of adsorbed bismuth takes place at 0.63 V in a well-defined peak. The behavior of this redox process on the Pt(111) vicinal surfaces indicates that the bismuth atoms involved in the redox process are only those deposited on the (111) terrace sites and that the charge under the peak at 0.63 V is directly proportional to the number of sites on (111) ordered

domains (terraces). The good linear relationship obtained between the charge for the bismuth redox process and the number of (111) terrace sites on the vicinal surfaces allows construction of a calibration curve. This calibration curve has been used to directly estimate the number of (111) ordered domains terrace sites on polycrystalline platinum samples with different surface-ordered domains. The results agree with what could be expected from our knowledge of these surfaces. ACKNOWLEDGMENT This work has been performed in the framework of projects BQU2003-03877, BQU2003-4029 from McyT (Spain), GRUPOS03/ 126 and GRUPOS03/208 from GV (Spain). J. Solla-Gulloń, and F. J. Vidal-Iglesias are also grateful to the Ministerio de Ciencia y Tecnologıá for their research grants.

Received for review February 25, 2005. Accepted May 17, 2005. AC050347Q


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Determination of (111) Ordered Domains on Platinum Electrodes by

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