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J. Phys. Chem. B 1999, 103, 5280-5288
In-Situ Infrared Monitoring of Surface Chemistry and Free-Carrier Concentration Correlated with Voltammetry: Germanium, a Model Electrode F. Maroun, F. Ozanam,* and J.-N. Chazalviel Laboratoire de Physique de la Matie` re Condense´ e, CNRS-EÄ cole Polytechnique, 91128 Palaiseau, France ReceiVed: January 8, 1999; In Final Form: April 20, 1999
Peaks in the voltammograms of germanium in acidic electrolyte are traditionally ascribed to a back-and-forth change between hydrogenated and hydroxylated surfaces. We report in-situ infrared spectroscopy measurements confirming this prediction and identify GeH2 species at the hydrogenated surface, as well as GeH species corresponding to (111) terrace sites, step (or facet boundary) sites, and sterically hindered or buried sites. Step sites are hydrogenated and reoxidized at higher potentials than terrace sites. The potential dependence of electron and hole surface concentrations indicates that the hydrogenation of the surface is accompanied by a negative shift of the flatband potential. Infrared absorption of ionic species demonstrates that this shift is due to a change in surface charge balanced by ions in solution, rather than to the dipolar change between GeH and GeOH. This is in contrast to previous reports. The surface roughness on the atomic scale accounts for the dependence of absorption intensities on infrared polarization and crystal orientation. Calculation of the electric-field map at a microfaceted surface/electrolyte interface shows that significant enhancements in IR sensitivity may be expected from (111) microfaceting of a (100) surface.
I. Introduction Most electrode surfaces are thought to evolve from an oxidized state at positive potentials to a hydrogenated state at negative potentials, the adsorption/desorption of hydrogen and hydroxyl groups being inferred from cyclic voltammetry. Platinum is a celebrated example of such a behavior.1 However, the voltammetric signature may be ambiguous, as multiple species may be responsible for the voltammetric peaks. Furthermore, the voltammograms may be affected by surface structural transitions or other effects not directly related to adsorption/desorption phenomena. A direct (e.g., spectroscopic) monitoring of the adsorbed species is therefore highly desirable. Adsorbed species have been monitored through in-situ infrared (IR) spectroscopy2 or related techniques.3 Also, there are several reports on the evidence of hydrogen bound to various electrode surfaces,4-12 and most of these experimental results are in contrast with previous expectations. To our knowledge, there is no report of spectroscopic monitoring of the adsorption/ desorption of hydrogen clearly correlated with voltammetric peaks. Specifically, the correlation of the reversible loops of hydrogen adsorption/desorption on Pt, with specific spectroscopic signature of a Pt-H bonding, is still under debate.4-8 Monitoring adsorbed hydrogen is difficult, due to the weak polarity of the bonding and the resulting poor sensitivity of the techniques. In a recent letter, we reported preliminary results of an insitu IR study of germanium electrodes in acidic electrolytes.13 Germanium is an interesting material; its gap is large enough for use as an IR multiple-internal-reflection prism (providing high IR sensitivity) and small enough that the semiconductor/ electrolyte interface does not exhibit a rectifying behavior at low current densities. The voltammograms of such electrodes have long been known to exhibit peaks, which were ascribed to a back-and-forth change of the surface between a hydroge* Author to whom correspondence should be addressed.
nated and a hydroxylated state.14,15 Herein, we report the use of IR spectroscopy as a tool for the identification of the species associated with the different voltammetric peaks. After description of the experiments (Section II) and the results (Section III), the data are discussed in Section IV, where we examine information gained from cyclic voltammetry (subsection A), from vibrational infrared absorption (subsection B-D), and from infrared background absorption (subsection E-F). II. Experimental Section Low-doped n-type (2-40 Ω cm) Ge samples were cut from a 0.5 mm thick (100)- or (111)-oriented polished wafer, shaped as 15 × 15 mm2 platelets, and 45° beveled for use as multipletotal-internal-reflection prisms. The platelets were provided with ohmic contacts using 400 °C evaporation of a (Au + 1% Sb) alloy and indium soldering on the gold pads. After thorough cleaning with sulfochromic acid/HF cycles, they were pressed against a 10 mm diameter aperture in the wall of a cell with a fluoroelastomer O-ring seal. This geometry provided ∼10 reflections of the IR beam at the electrochemical interface. The electrolyte was 1 M HClO4 in ultrapure (18.2 MΩ cm) water, and was deaerated by bubbling pure nitrogen. The cell was equipped with a carbon counter electrode and a saturated calomel reference electrode, the latter being separated from the main cell compartment by a 30 mm long KCl salt bridge. This arrangement prevents contamination of the electrolyte by metal ions, which plate easily on the Ge surface and alter the voltammograms dramatically. Voltammograms and IR spectra were recorded simultaneously, at scanning rates of 10 mV/s. The IR spectra were taken with a Bomem MB100 FTIR spectrometer. A spectrum was taken every 50 mV, the cyclic voltammogram was repeated 30 times, and the spectra corresponding to the same potential were coadded. The results are displayed either as absorbances relative to a common reference, taken at the positive bound of the
10.1021/jp9901186 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/06/1999
Germanium, a Model Electrode
Figure 1. (a) Typical voltammogram of n-Ge (100) and (111) in 1M HClO4, at a scan rate of 10 mV/s. (b) Voltammograms of n-Ge (111) at 20 mV/s, showing various scans superimposed, with different lower and upper potential bounds.
voltammogram [ln(I(Vmax)/I(Vn)), where I(Vn) is IR intensity for potential Vn], or as differential absorbances between two consecutive potentials [ln(I(Vn)/I(Vn+1))]. The spectra were recorded for both an s-polarized beam (IR electric field parallel to the Ge/electrolyte interface) and a p-polarized beam (IR electric field in the incidence plane). III. Results A. Voltammetry. Figure 1a shows typical voltammograms for a (100)- and a (111)-oriented electrode. These voltammograms are similar to those in the literature.14,15 The latter exhibit three cathodic peaks labeled R, β, and γ and one anodic peak labeled δ.15 Here, we observe two main cathodic peaks around -0.5 and -0.7 VSCE , preceding the large current increase associated with hydrogen evolution. Despite differences in peak positions, due to our reduced scanning rates, these peaks can
J. Phys. Chem. B, Vol. 103, No. 25, 1999 5281 be identified to peaks R and β of ref 15. Near the hydrogen evolution region, extra current on the negative scan is detected, probably associated with the γ peak of ref 15. The narrow anodic peak, in the region -0.1 to 0 VSCE preceding the oxidation current, appears composed of a sharp component δ and a weaker feature δ′ on the negative side. This splitting was not resolved in previous studies at large potential scanning rates.14,15 These voltammograms undergo changes upon long cycling but keep the same features provided the current is kept below 50 µA/cm2. Changes at the (111) Ge surface are characterized by a negative shift of the anodic peaks accompanied by a negative shift of the onset of the oxidation current. Changes at the (100) Ge surface are characterized by a slight negative shift of the onsets of both oxidation current and of hydrogen evolution. The onset potential of the oxidation current is ∼150 mV more positive on (111) than on (100), and the potential interval between the onsets of oxidation current and hydrogen evolution is ∼150 mV higher on (111) than on (100). The R peak has the same amplitude for (111) and (100), whereas the β and γ peaks are smaller on (111) as compared to (100). The total area below the R-β-γ peaks amounts to ∼250 µC/cm2 for the (111) surface and ∼375 µC/cm2 for (100). These Coulombic charges do not depend on scan rate, confirming that the peaks are associated with a surface phenomenon and not with redox species diffusing in the electrolyte. We have performed systematic studies of the voltammograms when the positive and negative potential bounds are varied (Figure 1b). As the positive bound decreases, the cathodic peaks occur at the same potentials (as for a complete voltammogram) but their intensity decreases by different amounts. Specifically, when the positive bound of the potential cycle is limited to the δ′ peak, the decrease in height of the R peak is much more pronounced than that of the β and γ peaks (see Figure 1b). On the other hand, as the negative bound is limited to the region of the R peak (curves 1-3), a single anodic peak is observed at -0.15/-0.1 VSCE, which shifts toward positive potentials when the negative bound decreases. When the negative bound decreases through the β and γ region (curves 4-7), the voltammogram exhibits another anodic peak and both anodic peaks continue shifting toward positive potential. The Coulombic charge below the peaks when the negative part of the scan is limited to the R-β region (oxidation and hydrogen evolution currents being subtracted) is about the same for both anodic and cathodic components. However, when the negative scan includes the γ-peak region, the anodic charge appears somewhat lower than the cathodic charge. Finally, electrochemical quartz microbalance measurements on an evaporated Ge film indicate no net change in mass after 10 potential cycles during which Ge oxidation and H evolution were avoided. This result clearly proves that the electrochemical processes associated with the voltammetric peaks are not related to Ge surface dissolution. B. Infrared Spectra. Figure 2 shows typical infrared spectra as a function of potential. Here the results are displayed as absorbances relative to the spectrum taken at the positive bound of the voltammogram (0.075 VSCE). Figure 3, parts a and b, shows two typical spectra of Figure 2 taken at -1.025 VSCE and at -0.225 VSCE (on the positive scan). They exhibit two different baselines, ascribed to free-electron and free-hole absorption, respectively.16 The vibrational band near 2000 cm-1 corresponds to νGeH. The broad band centered at 3200 cm-1 may be partially accounted for by a change in water absorption, as indicated by the weak peak at 1640 cm-1 (water scissor mode). However, up to one-third of the νOH band may originate
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Figure 2. IR spectra as a function of potential. (100) surface, p-polarization. (a) cathodic scan and (b) anodic scan. The reference is taken at the upper potential bound.
from a change in water absorption, in view of the magnitude of the scissor peak. Moreover, the center of the band (3200 cm-1) is compatible with its attribution to surface hydroxyl groups in hydrogen-bond interactions with one another or with water. The potential variations of the νGeH and νOH bands (Figure 2) bring conspicuous evidence that the cathodic (respectively anodic) peaks are associated with hydrogenation (respectively hydroxylation) of the surface. This is the first direct spectroscopic proof of the interpretation of the voltammograms. Vibrational absorptions in the region 900-1900 cm-1 (Figure 3c) exhibit a weak feature at 1710 cm-1 and a W-shaped band in the region of ClO4- absorption (1000-1200 cm-1). The νOH band may be modeled as a single broad Gaussian curve. The νGeH band, however, exhibits two distinct line shapes for s- and p-polarized light (Figure 4) and cannot be analyzed as a single component. A reasonable fit is obtained as the sum of two Gaussian curves of rms width 30-40 cm-1, composed of a weak line centered at 2020 cm-1 and a stronger one centered between 1950 and 1990 cm-1. The second line shifts to lower energy with increasing intensity. The intensities of the νOH and of the two νGeH components are shown in Figure 5a-c as a function of potential, together with the voltammogram (Ge oxidation and H2 evolution currents being subtracted), for a (100) surface. Here the data have been analyzed in differential form: each point represents the change in intensity during a potential interval of 50 mV, allowing the IR-intensity curves to be compared to the voltammogram. Since the relevant currents to be subtracted near the positive and negative bounds of the voltammograms are difficult to determine, one should be cautious when comparing voltammograms in Figure 5a with IR intensities in these regions. On the negative scan, the growth of the νGeH band is found to occur below -0.45 VSCE, with prominent increases around -0.5, -0.7, and -0.95 VSCE. These are in near coincidence with the voltammetric peaks R, β, and γ. The growth of the 2020 cm-1 component occurs mostly in the R region. The wavenumber of the low-frequency component of νGeH shifts negatively in the R region and has a constant value in the β and γ regions. On the positive scan, the disappearance of the νGeH band coincides with the anodic peak, and the 2020 cm-1 band disappears at the end of the peak. The wavenumber of the
Figure 3. Typical IR spectra of (100) Ge, for p-polarized light, taken at -1.025 VSCE (a) and -0.225 VSCE (positive scan) (b) exhibiting two different baselines corresponding to free-carrier absorption. The reference is taken at the upper potential bound. (c) Perchlorate and water absorption region at -0.575 VSCE (positive scan) after subtraction of the background. Notice the change in scale as compared to (a) and (b).
low-frequency component of νGeH increases slightly before decreasing. The intensity variations of the νOH band appear about complementary to those of the νGeH band. IV. Discussion A. Voltammetry. The changes in cyclic voltammetry when decreasing the positive bound suggest that the δ peak is related to the presence of the R peak, and that the δ′ peak is related to the presence of both β and γ peaks. This points to the existence of at least two kinds of species, and suggests that those that are less easily reduced are also more readily reoxidized. This nicely agrees with the infrared observation that the 2020 cm-1 absorbance appearing in the R-peak range disappears at the end of the anodic peak. When the positive bound of the cycle varies with a fixed negative bound, the excess cathodic charge (as compared to the anodic charge) indicates that reduced species produced in the γ range either (i) are oxidized chemically during the positive scan, or (ii) recombine to form hydrogen molecules. Finally, the shift of the anodic peak when the negative bound
Germanium, a Model Electrode
Figure 4. Analysis of the νGeH spectrum, for (100) (a) and (111) (b) Ge, for s- and p-polarizations, at -0.825 VSCE. The data exhibit evidence for two contributions, with distinct polarization dependences. The dotted curves show the analysis made here.
Figure 5. Comparison of the current density (where hydrogenevolution and oxidation currents have been subtracted), with νOH, νGeH2 (2020 cm-1), νGeH (1990-1960 cm-1) unpolarized IR differential intensities, and νGeH wavenumber as a function of potential, for (100) Ge.
of the cycle is varied indicates that the oxidation of species reduced in the R range is easier when the surface is partly
J. Phys. Chem. B, Vol. 103, No. 25, 1999 5283 oxidized. Interestingly, a symmetric behavior is not found when the positive potential bound is varied. B. Attribution of the GeH Components to Specific Surface Sites. A key challenge is to ascribe the two νGeH components and to unravel their relation with surface orientation and morphology. According to UHV adsorption experiments,17 the expected vibrational energy of a (111) surface germanium monohydride is ∼1970 cm-1. This is in agreement with the position of the main, low-frequency νGeH component (19501990 cm-1). The 2020 cm-1 component most plausibly originates from GeH2 groups17 (its frequency of 2020 cm-1 appears too small to be accounted for by an oxygen first neighbor of the Ge site,18 and too large for oxygen second neighbors). In striking contrast to the case of silicon,16 at no time is there evidence for species of the type GeOxHy , which would appear at markedly higher energies.18 It may be inferred that such species are less stable than their silicon counterparts. Interestingly, no analogue of siloxenes [SiOxHy] appears to have been reported in the chemistry of germanium. Another major difference with the case of silicon is the clear absence of trihydride species. C. νOH Band and Electrolyte Bands. We have interpreted the large νOH band as evidence for hydroxylation of the surface. This ascription is supported by the fact that, in aqueous medium, Ge(0) exhibits no range of thermodynamic stability in the pH/ potential plane, hence hydrogen desorption is expected to be accompanied by oxidation or hydroxylation of the surface. The stable surface species to be expected are then ≡GeOGe≡, ≡GeOH, ≡GeO-, and ≡GeOH2+. If the thermodynamic data of germanic acid are taken as a guideline ([H+][A-]/[HA] ≈ 10-9, [H+][HA]/[H2A+] ≈ 10-0.7),19,20 ≡GeOH and ≡GeOH2+ appear as the most plausible forms of a hydroxyl group in acidic medium.21 This conclusion is indeed in agreement with the large increase in νOH absorption at positive potentials. ≡GeOGe≡ whose absorption peaks are at 858 and 970 cm-1,22 may also be a stable form of surface species, even though its solubility in HClO4 is much higher than that of its silicon analogue. As no IR absorption corresponding to ≡GeOGe≡ has been detected, we conclude that the surface concentration of this species is small (less than 1013 cm-2) or remains unchanged in the potential range studied here. The magnitude of the δOH2 mode at 1640 cm-1, of the order of 5 × 10-5 per reflection, may arise from water molecules or surface ≡GeOH2+ groups. It is equivalent to a decrease of the water/electrode distance by an amount of ∼1 Å, which may be due partly to the change in the hydrophilicity of the surface. It is also equivalent to doubling of the IR cross section of the water molecules in the Helmholtz layer, which may arise from hydrogen bonding with the hydroxylated surface. Although all these mechanisms may coexist, the last two plausibly play the major role, since the surface concentration of ≡GeOH2+ groups needed to account for the IR absorption at 1640 cm-1 yields an unplausibly large surface charge. Finally, the broad peak centered at 1710 cm-1, clearly distinct from the water δOH2 mode, and the band in the 1000-1200 cm-1 region (see Figure 3c) are ascribed to ions in the electrolyte. We ascribe the 1710 cm-1 peak to H3O+ species, and the W-shaped 1000-1200 cm-1 band to perchlorate absorption whose width changes with applied potential. The IRactive mode of a ClO4- ion is indeed degenerate and is expected to split in the presence of a large electric field, as the one present in the region of the outer Helmholtz plane. The distribution of the electric-field induced splitting would lead to a broadening of the band, in qualitative agreement with observation. Adsorption of ClO4- ion on silver has been reported in the literature,23
5284 J. Phys. Chem. B, Vol. 103, No. 25, 1999 and similar IR observations on platinum in HClO4 have been attributed to ClO4- adsorption.24 D. GeH Intensities: Effect of Crystallographic Orientation. The above attribution of the GeH and GeH2 components leads us to deduce that the R peak bears a dominant GeH2 character, a rather surprising conclusion since it is the most prominent peak on a (111) electrode, where ideally only monohydrides should be present. This leads us to closer examination of the band intensity dependence as a function of IR polarization and surface orientation. Comparison with the hydrogenated HFrinsed silicon surface is especially helpful.25,26 (i) On the (111) Ge surface, in striking similarity with the case of silicon,25 the GeH band in p-polarization is twice as large as in s-polarization, whereas the GeH2 band is insensitive to polarization. On the (100) surface, the p/s ratio falls to 1.45 for GeH and remains close to 1 for GeH2. Though this behavior quantitatively deviates from that on Si (100),26 a similar attenuated polarization effect is observed for both materials. (ii) The overall GeHx intensity for (100) is ∼3 times as large as for (111), again in typical agreement with the case of Si.25,26 However, the Coulometric charge needed for surface hydrogenation is only 1.5 times larger on (100) than on (111). The same ratio of 3 is also observed for the νOH intensities. (iii) The GeH to GeH2 ratio is between 3 and 4 for both faces. These observations stand as evidence that the surfaces are not ideal, but are rough on the atomic scale, as previously reported for Si. As will be shown below, faceting can account for these various observations through local IR field enhancement effects and the presence of specific surface sites on the boundary of (111) microfacets. IR Electric Field Enhancement. Surface microroughness may affect IR intensities through enhancement of the local field, due to the different refractive indexes of the electrode and the electrolyte. This may lead to strong enhancements of the IR sensitivity and dramatic changes of the polarization dependence. To quantitatively assess such effects, we have calculated the IR electric field E in the vicinity of a rippled surface, which may represent a (001) surface consisting of (111) and (1h1h1) microfacets. For facet sizes much smaller than the IR wavelength, the electric field can be regarded as deriving from an electrostatic potential, which verifies the Laplace equation with the classical boundary conditions at the interface (continuity of E| and E⊥). The result of the calculation is shown in Figure 6 for two characteristic directions of the incident IR electric field. In this geometry, polarization effects are found to almost disappear as shown in Figure 6c: p/s intensity ratios averaged along a facet fall to 1 and 1.3 for the electric-field components parallel and perpendicular to the facet, respectively. Moreover, even if the total amount of hydrogen is assumed unchanged upon faceting,27 the increase in the electric field at some regions of the surface [(111)-like facets] results in a factor of ∼3 increase of the mean (i.e., surface averaged) sensitivity compared to a flat surface. Such a sensitivity increase, associated with a (111) faceting of both surfaces, would explain the discrepancy between the changes in the νGeH IR intensities corresponding to different voltammetric peaks, and their relative coulometric charge. Faceting is likely to occur in our experiments, since the positive bound of the potential scans lies in the germanium dissolution range and it has indeed been observed on a larger scale by SEM for electrodes cycled for a long time (see Figure 7a). The factor of 2 in the overall GeHx IR intensity between (100) and (111) (after scaling to identical hydrogen areal concentration) would then be due to the more pronounced roughness of the (100) Ge surface as compared to (111), or a
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Figure 6. Electromagnetic effects near a rippled surface (at a scale much smaller than IR wavelengths). Map of constant-potential lines for a macroscopic IR field E0 (a) parallel and (b) perpendicular to the surface. E1 and E2 respectively refer to the microscopic-field components parallel and perpendicular to the local surface. (c) Enhancement of the local IR intensity in directions parallel and perpendicular to the local surface as a function of the position on the facet, for s- and p-polarized light [in our ATR geometry, s-polarization corresponds to the case depicted in (a), whereas p-polarization corresponds to the “sum” of the cases depicted in (a) and (b)]. Average enhancement factors E12/ E02 and E22/E02 for ripples with random orientation in the surface plane are 0.79 and 2.77 for p-polarized light, and 0.77 and 2.17 for s-polarized light.
roughness more favorable to enhancement on (100) than on (111). Different Surface Sites. A rough (100) surface with (111) faceting can be achieved by randomly removing surface atoms (Figure 7b), provided that no overhangs are created upon removal. On such a surface, (111), (1h11), (11h1), and (1h1h1) microfacets appear as well as different surface sites. The junctions between two such facets are of four types: the “ridges” (R1) or “valleys” (V1) between two facets differing by a single index [e.g., (111) and (1h11)] do not exhibit dihydride sites. On the other hand, the “ridges” (R2) between two facets differing by two indexes [e.g., (111) and (1h1h1)] are terminated by dihydrides, while the “valleys” (V2) between two such facets make steric hindrance problems appear (hydrogen-hydrogen distance of 1.5 Å, much smaller than 2.4 Å, the sum of their two van der Waals radii). A rough (111) surface achieved with the same rules exhibits (1h11), (11h1), and (111h) facets, whose intersections lead also to R1, V1, R2, and V2 sites. All kinds of sites may then be expected on both (111) and (100) surfaces, and the proportion of different sites will depend on the degree of the roughness of the surface.
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J. Phys. Chem. B, Vol. 103, No. 25, 1999 5285
Figure 8. (a) (100)-Ge voltammogram obtained after prolonged negative polarization (-1 VSCE). (b) differential spectrum associated with the anodic peak at -0.5 VSCE (shown reversed). The dotted line shows the shape of the spectrum associated with the R-β-γ peaks.
Figure 7. Rough Ge surface: (a) (001) SEM view after prolonged anodic treatment, (b) rough (001) model. The small black dots stand for hydrogen. (b) The four types of surface monodentate Ge sites, corresponding to (111), (1h11), (11h1), and (1h1h1) facets, have been painted in different gray tones. The bidentate Ge sites are shown in white. Notice the four types of intersection between facets (R1, V1, R2, V2) and especially the occurrence of steric hindrance problems for V2type boundaries. The concentration of such sites will certainly be minimized in the physical system, through a different topological arrangement or chemical state (e.g., GeOGe “bridges”).
The actual state of the sterically hindered sites (V2 sites) is not clear. At the hydroxylated surface, they can accommodate a single oxygen atom (GeOGe bridge), but as discussed above, the change in GeOGe concentration should be lower than 1013 cm-2. At the hydrogenated surface, they may accommodate two hydrogen atoms with Ge back-bond distortion. They may also dimerize, or a bond may be left unsatisfied, leading to the presence of an electronic surface state in the band gap. The larger expected proportion of V2 sites at a rough (111) surface may then be responsible for a larger surface-state density, as suggested by the data on free-carrier absorption. On the other hand, sites along V1 can accommodate two hydrogens, but only one hydroxyl group with or without a hydrogen on the other site. Correlation with Voltammetry. From the above microscopic study, it appears that three different types of Ge-sites are present on rough (100) and (111) surfaces with (111) faceting: terrace monohydride (GeHT) corresponding to a (111) microfacet site, R1 monohydride (GeHS) and R2 dihydride (GeH2) corresponding to step or facet-boundary sites. GeHS sites differ from GeHT sites by being bonded to another Ge surface atom. When a first
neighbor of a GeHS site is hydroxylated, the νGeHS absorption wavenumber is expected to be higher than that of a GeHT and to shift to lower wavenumbers during surface hydrogenation. Such a downward shift of the monohydride vibration is indeed observed in coincidence with the R peak, whereas the monohydride wavenumber is invariable for the β and γ peaks. The potential dependence of the coverage, intensity, and wavenumber of the νGeH vibrations, as monitored by IR, and their correlation with the voltammetric peaks (position and amplitude) yield the following picture: the R peak is associated with GeH2 and GeHS species, and the β peak is associated with GeHT. The attribution of the γ peak is less straightforward. The increase of νGeHT and νGeH2 IR intensities in the γ peak region does not appear to be fully compensated by a decrease of the νOH IR intensity, which is specially the case on the (111) surface.28 Therefore, the γ peak appears partly associated with Ge-Ge bond breaking. This is further supported by in-situ STM studies29 which show the formation of a disordered Ge layer in this potential range. To get further insight on the origin of the γ peak, a polarization of -1 VSCE has been applied to the sample for a few minutes. Upon scanning back the potential, a sharp anodic peak was observed at -0.5 VSCE (Figure 8a), associated with a weak decrease of the GeHx intensity. This behavior is observed for both (111) and (100) Ge surfaces. The loss of hydrogen corresponds to a broad GeH spectrum centered at ∼1940-1950 cm-1 (Figure 8b). This spectrum may be regarded as the superimposition of the usual components (partial loss of surface hydride) and of a broader component at ∼1900 cm-1, corresponding to hydrogen buried in a perturbed Ge layer 30,31 (the νGeH band in amorphous hydrogenated germanium lies around 1900 cm-1 30). Penetration of hydrogen may be initiated by adsorption in “bays” or kinks. The low-frequency GeH species appear more fragile than the other GeH bonds, since they are created at more negative potentials and are readily reoxidized.
5286 J. Phys. Chem. B, Vol. 103, No. 25, 1999 Then, these sites may be candidates as intermediates for a number of reactions, e.g., for hydrogen evolution or oxygen reduction on Ge.15 Finally, on the positive scan, the δ peak is associated with GeH2 and GeHS oxidation and the δ′ peak with GeHT oxidation. Interestingly, the R peak increases upon cycling (i.e., roughening) for both (111) and (100) surfaces, which supports its attribution to intersections between facets. Moreover, from cyclic voltammetry (Figure 1b), it seems that GeH2 and GeHS species are more easily oxidized when the surface is weakly hydrogenated. This can be due to oxidized first neighbors which weaken the GeH bond. This neighboring effect should be weaker for GeHT species, which have no surface-Ge first neighbor, a fact which is observed indeed. E. Free-Carrier Absorption. At a semiconductor-electrolyte interface in typical conditions, the Helmholtz capacitance is much larger than the space-charge capacitance and a variation in applied potential results in a change of the semiconductor space charge. Such a change can be conveniently monitored using infrared spectroscopy. In potential-difference spectra, it appears as a smooth baseline, which increases at low wavenumbers (Drude-like absorption).16 From the known infraredabsorption cross sections of the free carriers, the baseline intensity can be converted into a change in free-carrier concentration per unit area. In cases where the changes in applied potential are partially taken by the Helmholtz layer (this may be the case under strong accumulation or inversion conditions, or when surface states are present at the interface), the quantitative analysis of such data is helpful for telling how the applied potential is shared between the Helmholtz layer and the semiconductor space-charge layer. For electrons in germanium, the absorption coefficient is written Α ) bn(σ/σ0)2, where n is electron concentration, σ is the IR wavenumber, b ) 4 × 10-17 cm2, and 1/σ0 ) 9 µm;32 for holes, the absorption coefficient is proportional to hole concentration p, but the spectrum is more complex, due to intravalence-band optical transitions.33 This makes the absorption spectra of electrons and holes markedly different (see Figure 3a-b), and gives an unambiguous means of identification of their respective contributions.34 The baseline in our spectra has been fitted as a combination of electron- and hole-absorption. Figure 9a,b shows the deduced changes in surface concentrations of electrons and holes, for a (100) and a (111) Ge surface. As expected, the surface concentration of electrons becomes large at the most negative potentials. The equivalent increase of hole surface concentration for the most positive potentials is not seen because the upper bound of the potential range is not sufficiently positive. Between, there is a wide potential range where both contributions should stay negligible. However, preceding each voltammetric peak, a change in free-carrier concentration is discernible (hole concentration for the anodic peaks, electron concentration for the cathodic peaks). The concentration changes are seen to be of the order of 1012 cm-2, which represents local volumic concentrations at the surface much larger than the bulk carrier concentrations (accumulation or inversion: n,p . ND, where ND is donor concentration). However, the electron density is conveniently related to the local electrostatic potential using Boltzmann statistics, as far as n,NC and p,NV, (where NC is effective density of states of the conduction band and NV effective density of states of the valence band). Under these approximations, integration of the Poisson equation from the Ge bulk to the surface leads to a relation between the measured areal carrier concentration change and band bending.35 Noting nS the
Maroun et al.
Figure 9. Free-carrier absorption: experimental results for (a) (100) and (b) (111) orientation. (c) Predicted change of free-hole absorption near an H-desorption peak (R ) 0.7, see model in the text).
measured areal electron concentration, the bottom of the conduction band at the surface is predicted to lie at an energy EF - 2kT ln[nS/(NCλC)], where EF is Fermi level, k Boltzmann’s constant, T temperature, e elementary charge, and λC ) [20kT/ (NCe2)]1/2 a characteristic screening length ( is the Ge dielectric constant). From the measured changes in areal electron concentration, one can then determine the position of the conduction-band edge on the potential scale: VC ) V + (2kT/e) ln[nS/ (NCλC)]. In the same way, from the areal hole concentration pS, and introducing the corresponding quantities NV and λV, one deduces the position of the valence band on the potential scale: VV ) V - (2kT/e) ln[pS/(NVλV)]. We first discuss the shape of free-carrier concentration changes in the vicinity of a voltammetric peak. The volumic concentration of holes at the surface p(0) is of the type p(0) ≈ NV exp[e(V - VV)/(kT)]. Neglecting bulk hole concentration, the change in IR absorption is proportional to p(0)λSC, where λSC ) λV exp[-e(V - VV)/(2kT)] is the thickness of the spacecharge layer. p(0)λSC is expected to increase as V is scanned positive. However, the observed shape is reminiscent of that of the voltammetric peak itself, with the maximum in carrier accumulation occurring significantly before the current maximum. This behavior may be understood if one keeps in mind that the flatband potential shifts positive with hydroxyl surface coverage.14,15 Such a shift corresponds to a variation in the Helmholtz potential, whose origin will be discussed in Section IV, subsection F. At a semiconductor electrode, reaction rates are largely determined by the surface concentration of free carriers.36 If we assume that the rate of hydroxyl adsorption is proportional to p(0) and that VV depends linearly on hydroxyl
Germanium, a Model Electrode
Figure 10. (a) Position of the conduction band on the potential scale, as deduced from the data in Figure 9a,b. Filled symbols are deduced from free-electron absorption, and open symbols from free-hole absorption with VC )VV - Eg/e. (b) νGeH IR absorption intensity (referred to positive potential bound) as a function of potential. (c) H3O+ and ClO4- IR absorption intensities as a function of potential, for (100) and (111) orientations.
coverage θOH, the law of variation for θOH and VV would be given by: dVV/dt (∝ dθOH/dt) ≈ K (1 - θOH) exp[e(V - VV)/ (kT)]. The resulting increase in VV will in turn tend to decrease p(0). A numerical solution of this system of coupled equations, assuming that the reaction rate K is a function of the Helmholtz potential drop K ) K0 exp[Re∆VV/(kT)], shows that variations of p(0)λSC similar to the voltammetric peaks may be expected. Figure 9c shows the result of such a simulation, which is qualitatively quite similar to the oxidation wave on the (100) surface. We now discuss the correlation between surface chemistry and changes in free-carrier concentration. Using the procedure described above, the data of Figure 9a,b have been converted into values of VC and VV , and replotted in Figure 10a as VC and VV - Eg/e (with Eg ) 0.665 eV) versus V. For the (100) orientation, areal electron and hole concentrations were both measurable. The resulting values of VC and VV - Eg/e match nicely, providing a good check of the validity of the procedure. In agreement with previous reports,14,15 VC changes by as much as 0.5 V during the voltammetric cycle. The VC(V) cycle exhibits a shape very similar to that of the hydrogen coverage as a function of V (see Figure 10a,b), providing a further check that the band-edge shift is associated with the change in surface chemistry. However, the correlation is not so good for the (111) orientation: (i)VC starts decreasing from its maximum value [∼0.25 VSCE more positive than for (100)] significantly before the beginning
J. Phys. Chem. B, Vol. 103, No. 25, 1999 5287 of surface-hydroxide reduction; (ii) it reincreases as soon as the potential sweep is reversed at the negative potential bound, although the surface appears to stay hydrogenated. Unfortunately, no significant hole accumulation is ever observed on (111) because the conduction-band potential is at least 200 mV more positive than for (100). Such a dependence of the flatband potential upon crystallographic orientation, though not reported for germanium, is typical of what may be observed at semiconductor electrodes.37 Also, it may be paralleled with the distinct behaviors obtained for the work function of germanium in the presence of adsorbed hydrogen or oxygen for various surface orientations.38 However, our IR data lead us to reconsidering the problem of the origin of the shift in flatband potential associated with the change in chemical state of the Ge surface. F. Origin of the Band Edge Shifts. From the analogy between electrochemical and vacuum experiments,38 it was inferred that the change in flatband potential is primarily due to a change in the surface dipole from GeH to GeOH, although surface charges are certainly needed in order to account for the pH dependence of the flatband potential.39 However, it is surprising that no similar effect occurs for silicon (actually, the flatband potential of n-Si in nonaqueous medium has been reported to shift negatiVely by ∼150 mV when the surface is changed from hydrogenated to oxidized40). Furthermore, the analogy with vacuum experiments may be misleading since the vacuum experiments report on changes in work function, and the insensitivity of the work function to temperature and doping38 shows that the Fermi level is pinned in the band gap. Hence, by no means can work function changes be regarded as changes in electron affinity. The existence of various ionic surface species has been suggested (GeO-, Ge-) in order to account for the pHdependence of flatband potential.39 Unfortunately, none of them can be easily characterized by IR spectroscopy (the wavenumber of the GeO- IR absorption band would be lower than that of ≡GeOGe≡ as it is the case for SiO-).41 Another possible partner at the hydroxylated surface may be a proton adsorbed on a GeOH group (GeOH2+ species), although no clear signature of this species can be found in the infrared spectra. The pKa for such an association is higher for Ge than for Si,19,20 which would account for the different behavior of the two materials. Infrared data may be helpful in determining the origin of the band-edge shift. If the origin were purely dipolar (Ge+OH- versus Ge-′H+′), the ionic concentrations in the double layer should exactly balance the free-carrier concentrations at the electrode surface (e.g., increase in H3O+ concentration and decrease in ClO4- concentration proportional to electron accumulation). If one assumes that the IR cross section of the observed ClO4- is equal to that of bulk species, the amount of ions determined from the magnitude of the ClO4- signal is of the order of a few 1013 cm-2. As changes in free-carrier concentrations are only on the order of a few 1012 cm-2, the observed ClO4- IR signal has clearly another origin (though changes in ClO4- IR absorption at the negative bound of the voltammogram can partly be correlated to free-electron accumulation). Moreover, the dependence of H3O+ and ClO4IR signals on potential is essentially associated with the change from GeOH to GeH (see Figure 10c) 42 suggesting that the Ge surface holds a charge, which becomes more negative (or less positive) when the surface changes from GeOH to GeH. Using a typical value CH ) 10 µF/cm2 for the Helmholtz capacitance, the measured changes in ionic concentration ∆n of a few 1013 cm-2 are just consistent with a band-edge shift e∆n/CH of a
5288 J. Phys. Chem. B, Vol. 103, No. 25, 1999 few hundred millivolts. This picture may be further complicated by the presence of electronic surface states, as indicated by slight differences in the IR absorption of ionic species between (111) and (100) [ClO4- shape more perturbed on (100) than on (111)]. These differences are plausibly related to the larger density of surface states on (111) than on (100), which is consistent with previous reports.15 In conclusion, we think that the observed change in ionic concentrations associated with the change in surface chemistry, of the right sign and correct order of magnitude, strongly supports that the origin of the band-edge shift is essentially in terms of a change in surface charge, and that the dipole contribution favored in the literature may not be essential. V. Conclusion Germanium provides us with the first clear-cut in-situ spectroscopic evidence of the back-and-forth hydrogenation/ hydroxylation of an electrode surface associated with a welldefined voltammetric cycle. In agreement with the literature, the change in surface chemistry is found to be associated with a change in flatband potential, as directly monitored by the variations of free-carrier absorption as a function of electrode potential. However, the origin of this band-edge shift appears dominated by a change in surface charge balanced by an ionic charge in the electrolyte, rather than by a change of surfaceattached dipole. In acidic electrolyte, the (100) and (111) Ge faces appear atomically rough, in close similarity with the HFrinsed Si surface. The data for the (100) face bear evidence of (111) microfaceting yielding sensitivity enhancement to hydrogen adsorbed on (111)-like facets and reduced polarization effects. The β and R voltammetric peaks appear associated with hydrogenation of the microfacets and steps (or facet edges), respectively, hence generating GeH and GeH2 species. When a completely hydrogenated Ge surface is kept under strong cathodic polarization for a few minutes, excess hydrogen is adsorbed on sites exhibiting a νGeH mode at a lower wavenumber. This probably occurs through breaking of fragile Ge-Ge bonds or GeOGe bridges, and offers a path for hydrogen evolution as well as for hydrogen incorporation into the Ge lattice. The behavior of germanium stands in marked contrast to that of silicon, on which hydrogen can be attached only chemically, upon oxide dissolution,16,25 or to that of GaAs, on which electrochemical removal of hydrogen cannot be separated from anodic dissolution of the electrode.12 Hence, despite its covalent character, germanium can be regarded as an electrode material on which hydrogen adsorption/desorption is suggestive of a metallic behavior. Results reported herein provide a starting point for spectroscopic studies of underpotential hydrogen adsorption on various electrode materials, and for investigating the change in hydrogen interaction with the surface as the electrode material is varied from a covalent solid to a freeelectron like material. Acknowledgment. Dr. B. H. Erne´ is gratefully acknowledged for very useful discussions and for pointing key references to our attention. Dr. D. Lincot brought access and invaluable assistance for the quartz microbalance measurements. References and Notes (1) See, e.g., Clavilier, J.; Durand, R.; Guinet, G.; Faure, R. J. Electroanal. Chem. 1981, 127, 281.
Maroun et al. (2) Ashley, K.; Pons, S. Chem. ReV. 1988, 88, 673. (3) Guyot-Sionnest, P.; Tadjeddine, A. Chem. Phys. Lett. 1990, 172, 341. (4) Bewick, A.; Russell, J. W. J. Electroanal. Chem. 1982, 132, 329. (5) Maeda, T.; Sasaki, C.; Horie, C.; Osawa, M. J. Electron Spectrosc. 1993, 64/65, 381. (6) Nichols, R. J.; Bewick, A. J. Electroanal. Chem. 1988, 243, 445. (7) Ogasawara, H.; Ito, M. Chem. Phys. Lett. 1994, 221, 213. (8) Peremans, A.; Tadjeddine, A. Phys. ReV. Lett. 1994, 73, 3010. (9) Ren, B.; Huang, Q. J.; Cai, W. B.; Mao, B. W.; Liu, F. M.; Tian, Z. Q. J. Electroanal. Chem. 1996, 415, 175. (10) Tian, Z. Q.; Gao, J. S.; Li, X. Q.; Ren, B.; Huang, Q. J.; Cai, W. B.; Liu, F. M.; Mao, B. W. J. Raman Spectrosc. 1998, 29, 703. (11) Chazalviel, J.-N.; Ozanam, F. J. Appl. Phys. 1997, 81, 7684 and references therein. (12) Erne´, B. H.; Ozanam, F.; Chazalviel, J.-N. J. Electrochem. Soc. 1998, 145, 447; Phys. ReV. Lett. 1998, 80, 4337. (13) Maroun, F.; Ozanam, F.; Chazalviel, J.-N. Chem. Phys. Lett. 1998, 292, 493. (14) Gerischer, H.; Mauerer, A.; Mindt, W. Surf. Sci. 1966, 4, 431. (15) Memming, R.; Neumann, G. J. Electroanal. Chem. 1969, 21, 295. (16) Venkateswara Rao, A.; Chazalviel, J.-N.; Ozanam, F. J. Appl. Phys. 1986, 60, 696. (17) Crowell, J. E.; Lu, G. J. Electron Spectrosc. 1990, 54/55, 1045. (18) Schaefer, J. A.; Broughton, J. Q.; Bean, J. C.; Farrel, H. H. J. Electron Spectrosc. 1986, 39, 127. (19) CRC Handbook of Chemistry and Physics, 78th edition; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1997; pp 8.43-8.44. (20) Nazarenko, V. A.; Flyantikova, G. V. Zh. Neorg. Khim. 1968, 13, 1855. (21) We have not found data for the surface of GeO2. However, at the surface of SiO2, the pKa of a SiOH group is ≈ 7 (Hair, M. L.; Hertl, W. J. Phys. Chem. 1970, 74, 91) as compared to the first pKa of silicic acid ≈ 9.9 (ref 19). This suggests that using the data from germanic acid may lead to an overestimate of the pKa of a surface GeOH group. Despite this possible overestimate, and of the fact that a positive applied potential will tend to favor deprotonation, we regard protonated forms as more plausible than deprotonated forms in 1M HClO4 medium. (22) Galeener, F. L.; Lucovsky, G. Phy. ReV. Lett. 1976, 37, 1474. (23) Diesing, D.; Winker, H.; Otto, A. Phys. Status Solidi A 1997, 159, 243. (24) Sawatari, Y.; Inukai, J.; Ito, M. J. Electron Spectrosc. 1993, 64/ 65, 515. (25) Burrows, V. A.; Chabal, Y. J.; Higashi, G. S.; Raghavachari, K.; Christman, S. B. Appl. Phys. Lett. 1988, 53, 998. (26) Chabal, Y. J.; Higashi, G. S.; Raghavachari, K.; Burrows, V. A. J. Vac. Sci. Technol., A 1989, 7, 2104. (27) This is indeed the case if one assumes one hydrogen atom per surface (111) site, and 2 hydrogen atoms per surface (100) site. (28) The analysis of the νOH IR intensity is more difficult in the γ-peak potential range, because the electrolyte absorption shows up in the spectra, due to a “cross-effect” with free-carrier absorption (Chazalviel, J.-N.; Ozanam, F. J. Electron Spectrosc. 1990 54/55, 1229). (29) Kepler, K. D.; Gewirth, A. A. Surf. Sci. 1994, 303, 101. (30) Cardona, M. Phys. Status Solidi B 1983, 118, 463. (31) Mandal, K. C.; Ozanam, F.; Chazalviel, J.-N. Appl. Phys. Lett. 1990, 57, 2788. (32) Fan, H. Y.; Spitzer, W.; Collins, R. J. Phys. ReV. 1956, 101, 566. (33) Kaiser, W.; Collins, R. J.; Fan, H. Y. Phys. ReV. 1953, 91, 1380. (34) Gobrecht, H.; de Haan, A.; Thull, R. Ber. Bunsen-Ges. Phys. Chem. 1972, 76, 602. (35) Chazalviel, J.-N. Coulomb Screening by Mobile Charges; Birkha¨user: Boston, 1999; pp 49-52. (36) Gerischer, H. Electrochim. Acta 1990, 35, 1677. (37) See, for example, Belaı¨di, A.; Fotouhi, B.; Cachet, H.; Kerbache, T.; Chari, A.; Ozanam, F.; Chazalviel, J.-N.; Gorochov, O.; Etman, M. J. Electroanal. Chem. 1998, 455, 191. (38) Dillon, J. A., Jr.; Farnsworth, H. E. J. Appl. Phys. 1957, 28, 174. (39) Gerischer, H.; Hoffmann-Perez, M.; Mindt, W. Ber. Bunsen-Ges. Phys. Chem. 1965, 69, 130. (40) Chazalviel, J.-N. J. Electroanal. Chem. 1987, 233, 37. (41) da Fonseca, C.; Ozanam, F.; Chazalviel, J.-N. Surf. Sci. 1996, 365, 1. (42) From Figure 10c, one might infer the presence of changes in H3O+ and ClO4- absorptions occurring about in coincidence with hole accumulation. However, these changes have the same sign as for electron accumulation. This indicates that they are rather artifacts of the background subtraction procedure, and not real physical effects.
