16966
J. Phys. Chem. 1996, 100, 16966-16977
Synchrotron-Induced Photoelectron Spectroscopy of Semiconductor/Electrolyte Model Interfaces: Coadsorption of Br2 and H2O on WSe2(0001) T. Mayer, C. Pettenkofer, and W. Jaegermann* Bereich Physikalische Chemie, Abteilung Grenzfla¨ chen, Hahn-Meitner-Institut, Glienicker Strasse 100, 14109 Berlin, Germany ReceiVed: April 17, 1996; In Final Form: June 13, 1996X
Semiconductor/electrolyte model interfaces are prepared in UHV by coadsorbing the redox active electrolyte species Br2 and the solvent H2O onto chemically inert van der Waals (0001) surfaces of WSe2. Only a small fraction of the adsorbed Br2 is ionosorbed due to charge exchange with the semiconductor bulk; most of it and H2O are molecularly adsorbed. Thus the observed changes of band bending and work function are related to charge transfer processes at the interface to achieve electronic equilibrium. The value of band bending obtained for the model electrolyte is in reasonable agreement to the equivalent Volta potential of the electrolyte contact. For the Br2/H2O coadsorption system it is only determined by Br2 and not affected by coadsorbed H2O. In addition, the relative density of state distribution of the semiconductor and the model electrolyte can directly be determined from the electron distribution curve of the valence band region and compared to theoretical expectations of electrolyte interfaces. We observe for Br2 and H2O adsorbates remarkable discrepancies to established electrochemical theories, which in part is attributed to the multielectron nature of the redox couples.
1. Introduction The thermodynamic equilibrium properties of solid/electrolyte junctions as e.g. the electric potential distribution depend on the geometric and electronic structure of the interface. Also the nonequilibrium properties as e.g. the rates of competing electrochemical reactions are determined by interface properties. For a complete understanding of interfacial electrochemistry, the chemical composition and structure of the solid electrode, the distribution of ions, molecules, and solvent molecules in the different double layers of the electrolyte, as well as the nature of specific surface interactions, must be known in molecular detail. The limited molecular information available from classical electrochemical methods, measuring potentials and currents, has motivated the development of a variety of interfacesensitive spectroscopies in order to complement standard electrochemical techniques.1-4 Especially, synchrotron radiation based techniques offer specific sensitivity for the in-situ investigation of the electrochemical interface as is reported in ref 2. But also ex-situ ultrahigh-vacuum (UHV) investigations may contribute to the analysis of electrolyte induced chemical changes. In addition, the details of bonding of atomic and molecular species or ions and the electronic structure of the interface can be investigated in great detail.5-10 Thus, various UHV surface science techniques and especially photoelectron spectroscopy have been applied for the ex-situ analysis of electrochemical interfaces. The related problem of emersion and transfer into UHV has been discussed with respect to obtainable results and limitations. But, to our knowledge, synchrotron radiation as photoemission source has not yet been used for ex-situ experiments. Evidently, the more severe restrictions of obtaining uncontaminated surfaces, because of the increased surface sensitivity, have not been solved up to now. Here we report on an alternative approach of investigating the properties of solid/electrolyte interfaces. It is based on the * Corresponding author. Fax: +49 30 8062 2434. X Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01116-1 CCC: $12.00
pioneering work of Sass and co-workers, who started to model the electrochemical interface in UHV by adsorbing electrolyte components onto defined metal surfaces.11,12 Thus, all modern techniques of surface science can be combined to investigate nearly all aspects of chemical, structural, and electronic surface interaction. It was shown that a close correspondence could be obtained between the work function change in such model experiments and the change in the point of zero charge (PZC) within the electrolyte.13 Evidently, the measured surface potentials in the model experiments are significant compared to electrolyte contacts. For this reason, many more scientists started to investigate the interaction of model electrolytes on inert metal substrates (see refs 14-17 and references therein). Unfortunately, comparable studies with semiconductor/electrolyte interfaces have hardly been performed besides our experiments investigating the interaction of halogens and H2O on layered chalcogenides,18-20 FeS2,21 and CuInSe2.22 Thus, it has to be stated that the semiconductor/electrolyte interface is even less understood than metal/electrolyte interfaces. On the other hand, there exist numerous studies on semiconductor/solid interfaces using surface science techniques to elucidate the fundamental mechanism of barrier formation in semiconductor devices (see e.g. refs 23 and 24). In these experiments, the interface is prepared stepwise by in-situ deposition of the contact material onto UHV-prepared semiconductor surfaces. We are convinced that similar experiments are also urgently needed to investigate fundamental aspects of semiconductor/electrolyte interactions. But, as such model experiments are performed at low sample temperatures and low pressures, the results must be interpreted with care and cannot be used for the state conditions of the fluid electrolyte without modification. It should be noted at this stage that the investigations of semiconductor/electrolyte model interfaces exhibit two principal advantages compared to metals: (i) The effect of charge transfer, which leads to a change of the Volta potential, can be discerned from dipole changes by the formation of an extended space charge layer.25,26 (ii) Also, electrochemical © 1996 American Chemical Society
Spectra of Semiconductor/Electrolyte Model Interfaces reactions may be studied by inducing charge transfer processes with illumination.19,27 As our previous results suggest that there is indeed a reasonable agreement between the redox potentials of the halogens and the induced electronic potentials at the semiconductor/adsorbate interfaces,18,22 we have started a systematic approach toward a more complex UHV modeling of the semiconductor/electrolyte interface containing anions, cations, and the solvent. In principle, it can be expected that such model experiments give information on (1) the geometrical and electronic structure of the semiconductor surface and possible changes due to the adsorbates, (2) the structure and steric position of solvent and redox active species and thus on the electric double layer potential distributions, and (3) the density of state distribution of the interface region and the “bulk” electrolyte and the influence of solvation.25,26 In order to restrict the complications we started our model experiments on chemically inert layered metal chalcogenides. In a first step, Br2,18,19 Na,28 and H2O20 were adsorbed separately on different layered chalcogenides. In an attempt to vary the concentration of the oxidized [Ox] and the reduced [Red] species, we also investigated the coadsorption of Na and Br2 on WSe2(0001).29 This work reports on the coadsorption of a redox active species (Br2) and the solvent H2O on WSe2(0001) to investigate the influence of intermolecular interactions of the adsorbates (solvation?) on contact formation. The surface properties of the substrate WSe2 may be found in a review on surface studies of layered chalcogenides.30 2. Experimental Section Photoemission spectra were taken in a commercial multichamber UHV apparatus (analyzer chamber, two preparation chambers) with an angle resolving electron spectrometer (VG ADES 500) in normal emission. A scheme of the apparatus is given in ref 26. As excitation source monochromatic synchrotron light (TGM 7) of the BESSY storage ring was used. Excitation energies between 15 and 120 eV are available and the overall resolution (fwhm) used in the experiments was better than 0.3 eV. In order to reach the highest surface sensitivity, the excitation energy was chosen to give electron kinetic energies close to the minimum of the escape depth curve. If not stated otherwise the emission line positions are given in binding energy referred to the Fermi level of the metallic sample holder (EFB ) 0 eV). In the experiments reported here we used WSe2 single crystals, which were prepared by chemical vapor transport with excess selenium for p-type and bromine for n-type doping, reaching typical doping densities of p ) 1017 cm-3 and n ) 1016 cm-3. The crystals were mounted to a Cu sample holder by Ag epoxy to ensure a back contact with reasonable conductivity. An important feature of the layered semiconductors like WSe2 is their inert van der Waals cleavage plane. As no bonds are broken across this plane, clean, mirrorlike (0001) surfaces with a very low density of surface states are easily obtained by cleaving in UHV.30 The base pressure of the analyzer chamber was below 10-10 mbar. Br2 and H2O cleaned by several destillation cycles were adsorbed via leak valves in an adsorption chamber separated from the analyzer. The dosage is given in langmuirs determined by the uncorrected pressure rise in the deposition chamber. During an adsorption experiment the samples were permanently cooled by thermal contact to a liquid nitrogen reservoir, as at room temperature neither Br2 nor H2O sticks to the (0001) cleavage plane. The standard adsorption temperature of about 100 K could be raised by counterheating the sample holder. In
J. Phys. Chem., Vol. 100, No. 42, 1996 16967 a typical “flashing experiment” the temperature was raised within 5 min to about 150 K and recooled immediately within 10 min to 100 K. The binding energies were measured with respect to the Fermi level of the metallic sample holder cleaned by Ar+ ion sputtering prior to the cleave. In a great number of experiments with differently doped WSe2 single crystals (strongly p-doped: p ) 1019 cm-3; strongly n-doped: n ) 5 × 1018 cm-3) we observed surface binding energies of the core levels W 4f7/2 and Se 3d5/2 ranging from 31.9 to 33.14 eV and from 54.00 to 55.24 eV, respectively. The emission of the highest state of the valence band, which is mainly due to W dz2 orbitals, was found between 1.04 (p-doped) and 2.28 eV (n-doped).31 Thus the surface Fermi level may be found in the full span of the energy gap of 1.24 eV at 100 K. This is in agreement to the bulk doping determined by Hall experiments, which indicates flatband position after cleavage. In the absence of surface reactions, the surface position of the Fermi level in the energy gap is directly monitored by the actually measured binding energy of the substrate emission peaks. We are well aware of the problem of surface photovoltages (SPV) induced by the excitation source.28,32-34 Therefore, a Kelvin probe was installed to measure the SPV induced by white bias light illumination and to compare it to the SPV monitored by photoemission. In contrast to the observations made with adsorbed metals we found no significant source-induced SPV in the adsorption experiments we report here, although the SPV induced by bias light was very large and full back bending to flatband situation may be induced by light intensities of about 100 mW/cm2 (W/halogen lamp). The cutoff energy of the secondary electron emission was used to monitor the work function of the adsorption system. In order to separate the secondary emission of the sample from those of the spectrometer, a bias voltage of -5 eV was applied. With the measured value of the work function Φ and the position of EF vs the valence band (given by the energy difference EF ) EBF ) 0 eV to the valence band maximum EVBM), the ionization potential and thus with the bandgap EG the electron affinity χ is determined.26,30 3. Experimental Results Based on our previous Br2 adsorption experiments on WSe2 and MoSe2,18,19 it is known that adsorbed Br2 behaves as an electron acceptor inducing strong band bending on n-type substrates. H2O, on the other hand, acts as electron donor inducing strong band bending on p-type substrates as observed for MoS210 and WSe2.20 To investigate the interaction between the solvent (H2O) and the redox active species Br2 and its combined effect on the contact potential (space charge layer), we performed two coadsorption sequences: (1) coadsorption of H2O to preadsorbed Br2 on n-type and (2) coadsorption of Br2 to preadsorbed H2O on p-type WSe2(0001). With this combination we expect to investigate the influence of the coadsorbate on the space charge layer induced by the first adsorbate in each case. To have both adsorption series is also helpful to discriminate effects induced by the substrate from effects related to the interaction of the coadsorbates, since for each adsorbate the “monoadsorbed” state is given. We will present the coadsorption system n-WSe2/ Br2/H2O first, which should be closer to the interface situation with the liquid bulk electrolyte, since bromide tends to adsorb specifically. In the second part we will present experimental results of the coadsorption system p-WSe2/H2O/Br2. Besides the valence band spectra shown below we have also measured the substrate core levels (W(4f) and Se(3d)) in all cases and have used them to extract the band bending induced by the adsorbates. They will not be presented here in detail as no
16968 J. Phys. Chem., Vol. 100, No. 42, 1996
Figure 1. Valence band photoemission spectra (hν ) 21 eV) of the adsorption system n-WSe2/Br2/H2O. The emission bands, which are clearly related to molecular Br2 and H2O, are labeled as molecular orbitals.
chemically shifted extra lines appear. The WSe2(0001) surface is completely inert to chemical reactions with the coadsorbates, as already observed before for their separate adsorption.19,20 3.1. n-WSe2/Br2/H2O. The valence band photoelectron spectra (hν ) 21 eV) are displayed in Figure 1 with increasing amounts of adsorbed Br2 and coadsorbed H2O on the freshly cleaved n-WSe2(0001) surface. Due to adsorption of Br2, three extra emission peaks appear, which are the typical valence band features of molecularly adsorbed bromine, labeled by the molecular orbitals σg, πu, and πg.35,36 The measured binding energies versus the Fermi level, i.e., EFB values, change with coverage and are finally measured at 3.0, 5.2, and 6.4 eV. The shift of the substrate valence band features to lower binding energies due to band bending (eVbb ) 0.9 eV) indicates the buildup of the depletion layer, which is due to electron transfer from the n-doped semiconductor to the adsorbate. The resulting potential distribution corresponds to an inversion situation and agrees to previously obtained results.19 The charge transfer is followed by the partial dissociation of Br2 to Br-, which can be observed as a small shoulder on the low binding energy side of the bromine 3d core level line (Figure 3b). This assignment was proven by coadsorption experiments with Na published elsewhere,29 which led to the formation of NaBr. In addition, a shift of the secondary cutoff due to an overall increase of the work function Φ by 2.0 eV is observed. With the known value of band bending, the increase of electron affinity ∆χ is calculated as 1.1 eV (∆Φ ) eVbb + ∆χ). The saturation value of ∆χ found in our previous Br2 adsorption experiments was 1.4 eV.19 When H2O is adsorbed to the Br2 precovered substrate, three main additional emission features induced by the coadsorbate can clearly be identified as the molecular orbitals of H2O labeled 1b1, 3a1, and 1b2 (Figures 1 and 2). The H2O emission pattern is assigned to the adsorption of a H2O ice layer with H-bonded H2O molecules based on the comparison to literature data.37,38 Coadsorption of H2O does not change the band bending significantly (Table 1 and Figure 8a) but the electron affinity
Mayer et al.
Figure 2. Difference valence band spectra (hν ) 30 eV) of the adsorption system n-WSe2/Br2/H2O. The spectra are obtained by subtracting the 1.3 langmuir WSe2/Br2 monoadsorbate spectrum from the coadsorbate spectra. The emission bands B and D are not observed for the monoadsorption of either Br2 or H2O.
(secondary cutoff) is decreased by 0.3 eV after exposure to 5 langmuirs of H2O and by 0.7 eV after flashing the sample to 140 K (Table 1 and Figure 7a). This is the typical electron affinity change observed for adsorbed H2O on different layered semiconductors.20 After flashing, also the emissions from the adsorbates are intensified while the substrate emissions are weakened. This indicates the tendency to form three-dimensional clusters during original deposition and a smoother coverage of the surface at elevated temperatures. The tendency to form three-dimensional clusters of nonreacting adsorbates is well-known on the layered chalcogenide van der Waals planes30,34,39 and is related to the weak van der Waals interaction on the (0001) surface. However, the changes with temperature observed here are surprising at first glance, as usually the formation of clusters is enhanced at higher sample temperatures. The contrasting behavior of the Br2/H2O coadsorption system may be taken as clear evidence of the intermolecular interaction of the adsorbates. The coverage of the adsorbate on the substrate is evidently changed to a more even distribution by a thermally activated diffusion. The binding energies EFB of the water orbitals increase parallel to the work function leading to a nearly constant ionization energy Evac B (see Figure 1, Table 1). Adsorbed H2O has a constant ionization energy in all our experiments even on different substrate materials31 and in different adsorption/ coadsorption situations (compare Tables 1 and 2). In these tables the binding energy of the H2O orbitals (1b1, 3a1, 1b2) is given with respect to the Fermi level EFB, as measured, and with respect to the vacuum level Evac B equal to the ionisation F potential (Evac B ) EB + Φad). The difference spectra (Figure 2) of the above given adsorption series taken with hν ) 30 eV shows more clearly two additional emission maxima labeled B and D at EFB ) 4.0 and 8.2 eV induced by the coadsorbed H2O. The difference spectra were obtained by subtracting the 1.3 langmuirs Br2 spectrum from the succeeding spectra after normalizing to the intensity of the Br2 πu orbital. The identification of these emissions cannot be given right away based on literature survey. They are due to either one or two additional species. They may be
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J. Phys. Chem., Vol. 100, No. 42, 1996 16969
TABLE 1: Characteristic Surface Potentials and Peak Level Positions of the Interface n-WSe2/Br2/H2Oa EFb
EFb
Evac b
system
eVbb
Φad
∆χ
Br2(3d5/2)
Br-(3d5/2)
1b1
1b2
2a1
1b1
1b2
2a1
n-WSe2/Br2 n-WSe2/Br2/H2O (low) n-WSe2/Br2/H2O (high) 140 K flash
+0.9 +0.95 +0.9 +0.9
5.7 5.7 5.4 5.1
+1.1 +1.05 +0.8 +0.5
69.0 69.0 68.8 68.8
67.2 67.2 67.4 67.6
5.5 5.9 6.3
11.9 12.2 12.7
25.0 25.5 25.8
11.2 11.3 11.4
17.6 17.6 17.8
30.7 31.0 30.9
a
Binding energies and ionization energies are given in eV. The changes of electron affinity ∆χ are referred to the freshly cleaved sample.
TABLE 2: Characteristic Surface Potentials and Peak Level Positions of the Interface p-WSe2/H2O/Br2a EFb system
eVbb
Φad
∆χ
p-WSe2/H2O p-WSe2/H2O/Br2 (low) p-WSe2/H2O/Br2 (high) 160 K flash
-0.65 -0.1 +0.1 +0.1
3.4 4.1 4.9 5.2
-0.25 -0.05 +0.5 +0.8
a
Br2(3d5/2) 69.3 69.1 69.0
EFb
Evac b
Br-(3d5/2)
1b1
1b2
2a1
1b1
1b2
2a1
68.2 68.4 68.2
7.9 7.3 6.6 6.3
14.4 13.5 13.0 12.5
27.2 26.8 26.4 26.0
11.3 11.4 11.5 11.5
17.8 17.6 17.9 17.7
30.7 30.9 31.3 31.2
Binding energies and ionization energies are given in eV. The changes of electron affinity ∆χ are referred to the freshly cleaved sample.
attributed to the molecular orbitals 1π and 3σ of OH-, with binding energies of 5-7 eV and 9-11 eV as found on metals.37 Alternatively, the emission B may be assigned to the p-orbital of an increased concentration of adsorbed Br-35,36,40 and D to a modified H2O species. In this case the additional adsorption state D would be related to H2O bound by H-bridges to Br(corresponding to the solvation of anions in the liquid bulk electrolyte). The binding energy of the p-orbital of chemisorbed Br- obtained after an X-ray induced reaction of molecular adsorbed bromine with the layered chalcogenide InSe was found to be 4.2 eV,41 very close to the binding energy of emission B in the case reported here. Additional information on the species formed can be derived from SXP core level spectra. The O(2s) core level (the H2O 2a1 molecular orbital, Figure 3a) shows a broad emission band. It has evidently at least two components at low H2O concentrations, as is deduced from a comparison to monoadsorbed H2O (Figure 6a) and only one component remains visible for large H2O dosages. The binding energy of its maximum EFB changes like the valence band features of H2O parallel to the work function (leading to a nearly constant ionization energy Evac B ; see Table 1). The ionization energy of the main maximum at high coverages is identical to the ionization energy measured for monoadsorbed H2O (Figure 6a). We therefore relate this emission to molecular H2O undisturbed by the coadsorbate. The second component (low H2O concentration, labeled E in Figure 3) is shifted by 1.5 eV to lower binding energies and is due to the interaction with bromine. The formation of this component due to coadsorbed bromine can more clearly be derived from the reversed coadsorption sequence (section 3.2, Figure 6a). The lowered binding energy of the E component suggests the presence of either a negatively charged dissociated species (OH-) or a H2O species with increased negative charge as would be expected for the interaction of H2O by a H bridge to a negatively charged Br- ion. As the pendant to the OH- species, a partially positively charged bromine species should appear, as the WSe2 substrate is not involved in any chemical surface reaction. The core level spectra of the Br 3d orbital (Figure 3b) give no hint for such a species. We only observe an enhancement of the relative intensity of the Br- component by coadsorbed H2O. Thus the emission feature E can be assigned to H2O molecules interacting with Br-. It is interesting to note that the difference in binding energy between the Br2 molecule and the Br- component is lowered by 0.4 eV with 5 langmuirs of coadsorbed H2O and by 0.6 eV after flashing to 140 K. The increased binding energy of the Br- ion in the presence of coadsorbed H2O fits well to the above given interadsorbate
interaction (solvation) model, where the negative ion is surrounded by H2O molecules. With increasing amounts of coadsorbed water molecules the intensity of the Br2 3d emission is lowered (Figure 3b) while the intensity of the O 2s (Figure 3a) emission increases in agreement to the observations in the valence band spectra (see above). After flashing to 140 K the O 2s emission is even more intensified while the Br2 3d emission is only slightly lowered and the substrate emission is strongly suppressed. Therefore, we draw the conclusion that Br2 molecules act as nucleation centers during deposition, on top of which water adsorbs in three-dimensional clusters. At higher temperatures the clusters “melt” and the water molecules form a more continuous overlayer covering a larger part of the substrate area. A closer inspection of the intensities of the bromine 3d level during H2O coadsorption shows, in addition, the relative increase of the Brversus the Br2 component, while the absolute intensity of Bris not increased. The SXP spectra of the substrate core levels W 4f and Se 3d (not shown here) give no evidence for any reaction with the substrate. They only show the binding energy shift of 0.9 eV due to the formation of the inversion layer by Br2 adsorption. The coadsorption of H2O does not considerably change their EFB values (see Figure 8a). Since, in addition, the coadsorbate system is completely desorbed at about 250 K and the core level and valence band spectra obtained after desorption are identical to those of the freshly cleaved surface, a reaction with the substrate can surely be excluded. 3.2. p-WSe2/H2O/Br2. The reverse adsorption series was investigated with p-doped samples, which allows to monitor the effect of the coadsorbed Br2 on the electron injection and band bending by initially adsorbed H2O. The valence band spectra (hν ) 30 eV) of the sequence with the first adsorbate being H2O and the coadsorbate bromine are displayed in Figure 4. The spectra obtained with hν ) 21 eV are not as clear due to enhanced substrate emission and will not be shown for this reason. After adsorption of 4 langmuirs of H2O the typical emission features of ice-like molecular water grow in37,38 (EFB in Table 2). In agreement with our previous adsorption studies with H2O on different layered materials,10,20 the bands of the substrate are bent downward (to higher binding energies) by 0.65 eV due to water adsorption. Induced by the coadsorption of Br2 the emissions B and D show up as in the reversed coadsorption sequence (discussed in section 3.1). These emissions are more evident in the difference spectra (Figure 5). The emissions labeled A and C
16970 J. Phys. Chem., Vol. 100, No. 42, 1996
Mayer et al.
Figure 4. Valence band photoemission spectra (hν ) 30 eV) of the adsorption system p-WSe2/H2O/Br2. Coadsorbed Br2 induces the emissions labeled A, B, C, and D. The assignment to adsorbed species is given in the text.
Figure 3. (a, top) SXP spectra of the O 2s core level (hν ) 80 eV) for the system n-WSe2/Br2/H2O. The shoulder E is not observed for monoadsorbed H2O. The shift of the binding energy equals the change of the work function. (b, bottom) SXP spectra of the Br 3d core level (hν ) 100 eV) for the system n-WSe2/Br2/H2O. The doublet 3d5/2 and 3d3/2 is marked for molecularly adsorbed Br2 and ionosorbed Br-.
correspond to the molecular orbitals πg and πu of Br2 (see Figure 1). For obtaining the difference spectra, the valence band spectra with coadsorbed Br2 (Figure 4) were energetically referred to the 1b1 orbital of the spectrum obtained after 4 langmuirs of H2O exposure to account for band-bending effects. The spectra were subtracted, after being normalized in intensity to the 1b1 emission in each case. Since the binding energies of the water orbitals change strongly with increasing exposure to the coadsorbate Br2 running from (1b1) 7.9 to 6.6 eV and after flashing to 160 K to 6.3 eV, while the substrate shifts by 0.75 eV only, an artificial peak occurs in the difference spectra caused by the substrate valence band (W(dz2)) as indicated in Figure 5. As in the reversed coadsorption sequence (section 3.1) the variation of the H2O binding energies follows the change of the work function (Table 2). With increasing amounts of Br2 the band bending induced by H2O is removed and even reversed by 0.1 eV (compare Figure 8b) as also indicated by the shifts
Figure 5. Difference valence band spectra (hν ) 30 eV) of the adsorption system p-WSe2/H2O/Br2. The spectra are obtained by subtracting the 4 langmuir WSe2/H2O monoadsorbate spectrum from the coadsorbate spectra. The negative bands are artificial due to the shift of H2O emission bands relative to substrate emission bands induced by Br2 coadsorption.
of the substrate emission line. The electron affinity χ (see Table 2, Figure 7b) is lowered by 0.25 eV only after adsorption of 4 langmuirs of H2O, indicating that saturation is not reached (the saturation value is 0.65 eV20). After coadsorption of 4.5 langmuirs of Br2, χ is increased by 0.5 eV relative to the cleaved substrate, i.e., by 0.75 eV relative to the H2O-covered interface. This value is raised to 1.05 eV after flashing to 160 K, a value close to the saturation value of 1.3 eV observed for Br2 adsorption on WSe2.19 The O(2s) spectra (Figure 6a) initially show the emissions typical for undisturbed H2O (one broad symmetric line), which shift to lower binding energies upon coadsorption of Br2. The shift ∆EFB does not quite follow the change in work function leading to an increase of the ionization energy from 30.7 eV for monoadsorbed H2O to 31.3 eV in the presence of Br2. Clearly the coadsorption of bromine creates a shoulder on the low binding energy side of the O2s level marked E in Figure 6a. The shoulder E as well as the valence emissions B and D
Spectra of Semiconductor/Electrolyte Model Interfaces
J. Phys. Chem., Vol. 100, No. 42, 1996 16971
Figure 7. Change of electron affinity ∆χ in the course of Br2 adsorption and H2O coadsorption onto n-WSe2 (a, top) and of H2O adsorption and Br2 coadsorption onto p-WSe2 (b, bottom).
Figure 6. (a, top) SXP spectra of the O 2s core level (hν ) 80 eV) for the system p-WSe2/H2O/Br2. Br2 coadsorption induces the shoulder E. (b, bottom) SXP spectra of the Br 3d core level (hν ) 100 eV) for the system p-WSe2/H2O/Br2.
are created independently of the coadsorption sequence (compare section 3.1) and therefore have to be taken as characteristic features of the interaction between the coadsorbed H2O and Br2. As for the system n-WSe2/Br2/H2O, we observe two components of the Br 3d doublet (Figure 6b). The binding energies of the 3d5/2 lines are 69.3 and 68.2 eV, respectively. We relate these to the Br2 molecules and the Br- ions, created by electron transfer from the substrate space charge layer to the adsorbate. The intensity of the Br- relative to the Br2 component agrees well with the relative intensities found in the reversed coadsorption sequence. As for the system p-WSe2/H2O/Br2, we found no evidence for a reaction with the substrate based on substrate core level spectra (not shown here). The substrate core level spectra just reflect EFB shifts due to band bending induced by charge transfer between the space charge layers and the adsorbing species (Figure 8b). As for the reversed sequence the adsorbate system desorbes completely during heating to room temperature.
3.3. Summary of the Experimental Results. Neither the adsorbates Br2 and H2O nor the coadsorbate systems show a reaction with the WSe2(0001) surface. There is also no evidence for an irreversible chemical reaction between the coadsorbates. But the emission features of the coadsorbate system cannot be explained as a simple superposition of the emission features of the pure adsorbates. Independent of the adsorption sequence we observe additional features in the valence band spectra as well as in the adsorbate SXP core level spectra. These are the emissions B and D at EFB ) 4 and 8 eV, a shoulder E on the low binding energy side of the O 2s level, and an intensified component on the low-energy side of the Br 3d emission doublet due to Br-. While for the bromine monoadsorbate the 3d core level of the Br- component is shifted by 1.8 eV to lower binding energies relative to the molecular component, this difference is lowered in the presence of water to 1.2 eV for the Br2/H2O and to 0.8 eV for the H2O/Br2 coadsorption series. The change in electron affinity ∆χ ) 0.3 eV by the coadsorption of H2O to the bromine precovered surface (Figure 7a) meets the values obtained for H2O adsorption on the clean surface ∆χ ) 0.25 eV (Figure 7b). Similarly, bromine changes the electron affinity of the H2O precovered (Figure 7b; ∆χ ) 0.75 eV before to ∆χ ) 1.05 eV after flashing) and the clean surface by a similar amount (Figure 7a; ∆χ ) 1.1 eV). While the binding energies EFB of H2O emissions differ drastically in both sequences and change during adsorption/coadsorption, the are very small variations of their ionization energies Evac B (Tables 1 and 2). On the other hand, the binding energy of the 3d level of the molecular bromine component stays nearly constant during H2O coadsorption, as does the band bending. The most striking feature of the two adsorption sequences is the different behavior of the position of the Fermi level at the surface during coadsorption as displayed in Figure 8a,b. The shift of the Fermi level is deduced from the shift of the substrate valence band maximum. Identical shifts are measured for the substrate W 4f and Se 3d core level lines. In both cases the position of the surface Fermi level in the band gap of the substrate is governed by bromine: The band bending obtained by adsorption of Br2 on n-WSe2 is not changed by coadsorbed H2O while the band bending obtained by adsorption of H2O on p-WSe2 is removed and even pushed to the opposite side by coadsorbed Br2.
16972 J. Phys. Chem., Vol. 100, No. 42, 1996
Figure 8. (a, top) Surface Fermi level in the course of Br2 adsorption and H2O coadsorption onto n-WSe2. The band bending obtained by Br2 adsorption is not significantly changed by H2O coadsorption. (b, bottom) Surface Fermi level in the course of H2O adsorption and Br2 coadsorption onto p-WSe2. The band bending obtained by H2O adsorption is completely reversed by H2O coadsorption.
For comparison of experimental data we have listed the surface potential values of the semiconductor/coadsorbate interface and the binding energies EFB of those species which are also observed in the monoadsorption experiments in Tables 1 and 2. For H2O the ionization energies Evac B are listed as well. The change in electron affinity ∆χ is related to the freshly cleaved substrate in each case. 4. Discussion 4.1. Adsorbed Species and Interatomic Interactions. The separated adsorption of H2O and Br2 reported here leads to the same additional emission bands as observed in our previous adsorption studies of H2O and Br2 on WSe2.19,20 The assignment to H2O and Br2 molecular orbitals and to Br- states is given accordingly and is in good correspondence to literature data on different substrates.35-38 Here we will concentrate on the interpretation of the characteristic features of the coadsorption system. Independent of the coadsorption sequence the relative intensity of the Br- component is increased on the surface in the presence of H2O as clearly observed in the SXP spectra of the Br 3d core level (Figures 3b and 6b). We relate the additional emission B in the valence band to this component, since it shows a comparable intensity dependence and the binding energy agrees well with the binding energy of the p-orbital of chemisorbed Br- on InSe and other materials.35,36,40 The shift of the Br- core level to higher binding energies due to the coadsorbate H2O gives a clear indication for a local interaction between the bromide ions and water molecules. The low binding energy shoulder E of the O 2s (H2O 2a1) level, which is not present for adsorbed H2O only, may be assigned to those H2O molecules interacting with Br- ions. The chemical shift to lower binding energy of this core-like orbital results
Mayer et al. from the partial transfer of positive charge from the water molecule toward the bromide ion in a H-bridge type bonding. The extra emission D in the valence band may then be assigned to the 3a1 orbital of water interacting with Br- thereby being shifted to lower binding energy as the O 2s core level line E. The alternative assignment of the extra emissions B and D to a dissociated species of H2O according to the reaction Br2 + H2O f HBr + BrOH does not correlate with the measured binding energies, which are too low for being assigned to the molecular orbitals σ and π of OH-.37,38 In addition, one expects an increase of ∆χ for adsorbed OH- in contrast to the observed decrease of ∆χ in our H2O coadsorption experiments. Also, the assignment of the emission D to an atomic or molecular oxygen species can be excluded since the binding energies of the O2s level are well above the values observed for oxygen on Pt(111) (20 eV) and Ge(001) (22 eV). Our assignments given above are supported by other coadsorption studies of halogens with H2O on metal surfaces.43-46 Also in these studies an interadsorbate interaction via the H atoms of the adsorbed H2O molecules is found and H2O dissociation is excluded (with the exception of F/H2O coadsorption46). Unfortunately, only limited information based on photoemission spectra is available for these cases,42,43 which excludes any detailed comparison. The local interaction of H2O and Br2 in the coadsorption experiment may approximate solvation effects in solutions. It is clear from our experiments that there is indeed a strong intercoadsorbate interaction by two different mechanisms. On the one hand, the evident surface composition is changed, i.e., the concentration of the reduced species Br- is increased in the presence of H2O. This effect may be in part due to the rearrangement of the Br2, Br-, and H2O molecules on the surface, as is also suggested by the annealing step to about 150 K. The size and distribution of the adsorbate clusters are evidently changed, which lead to different intensities in the photoemission spectra. Since we observe a relative increase of the Br- component independently of the coadsorption sequence, only geometrical argument for the intensity changes, i.e., H2O coverage of the molecules only but not of the ions, seems to be unlikely. Therefore, we attribute the increased intensity of Br- to the enhanced dissociation of Br2 molecules in the presence of water. Since the emissions of the H2O molecules, which interact with Br- shift to lower binding energies, these species can be excluded as the direct source of the negative charge needed for the reversible dissociation of Br2 to 2Br-. Therefore, we postulate an increased electron transfer from the substrate to adsorbed Br2 mediated by H2Osubstrate interaction (surface molecule formation) and promoted by the H2O-Br- interaction (solvation), as is discussed in more detail in the next section. As a second effect we observe an energetic stabilization of the Br- ions by surrounding H2O. This stabilization leads to an increase of the measured binding energy of the Br- 3d orbitals in relation to the Br2 3d states by ∆EFB ) 0.6 eV. Also, lower EFB values are found for those H2O molecules directly interacting with Br2. Both findings may be attributed to a transfer of positive charge from H2O toward Br- in a H bridge like interaction. Similar bonding interactions have also been proposed for halogen/H2O coadsorption systems on metal surfaces.42-46 However, a quantitative comparison of the experimentally observed solvation effects must be handled with care. On metals the halogens are dissociatively adsorbed and form an ordered overlayer, the coadsorption of H2O is considered as model for specific adsorption to simulate the structure of the inner Helmholtz layer. In our case no specific adsorption with the substrate occurs and a defined solvent structure and
Spectra of Semiconductor/Electrolyte Model Interfaces hydration number cannot be deduced (LEED experiments of the adsorbate covered surfaces only show a diffuse background). One may expect that the Br- solvation on the weakly interacting van der Waals planes of WSe2 is similar to the interaction of such ions with gaseous and liquid H2O.48-51 However, for such a comparison the relative contribution of final state effects in photoemission (hole screening) must be discerned from the solvent stabilization by Br2 molecules (initial state effects). It is qualitatively expected that the final state effects are considerably larger in the gas phase as compared to the adsorbate stage as also observed experimentally. The binding energy shifts due to solvent stabilization as given in ref 50 for gas phase studies (∆EB ) 3.34 eV for Br-/H2O) are considerably larger than our results (∆EFB ) 0.6 eV). In solvents the range of reorganization energies of Br- has been determined between 0.15 for weakly interacting solvents and 1.3 eV for H2O.51 One may therefore expect that solvation effects on the electronic states of redox couples may be better investigated on weakly interacting substrates. However, more detailed investigations are needed, which based on the results presented above may give direct information on the reorganizational stabilisation energy of redox couples by the solvation shell. With these results the density of state distribution of redox couples in the electrolyte may be deduced (see ref 52 and section 4.3 for a more detailed discussion of this topic). 4.2. Charge Transfer, Semiconductor Band Bending, and Interface Potentials. In this paper we mainly wanted to address the formation of electronic equilibrium at the semiconductor/ adsorbate interface of charge transfer and to assign the occupied and empty orbitals of the participating species. We have noticed that Br2 acts as electron acceptor, as is indicated by the induced band bending on the clean n-type and backbending on the H2O precovered p-type sample, respectively (Figure 8a,b). For an electron transfer from the n-type semiconductor bulk (space charge layer) to the adsorbate-covered surface, unoccupied (acceptor) electron states should be available below the Fermi level of the semiconductor substrate. These may result from mostly unperturbed states of the adsorbate or may result from surface molecule states via substrate/adsorbate bonding. The LUMO of Br2 is the 2σu antibonding level. Its energetic position can be estimated to be 3.0 eV above the HOMO (πg) based on electron energy loss data of molecular Br2 adsorbed on Br covered Fe.47 From the measured position of the πg orbital at higher Br2 coverages (EFB ) 3.0 and 2.8 eV for n-WSe2 and p-WSe2/H2O, respectively), we conclude that a direct charge transfer from the semiconductor to 2σu is possible (Figure 9a) until the semiconductor Fermi level coincides with this level. H2O acts as electron donor on p-type WSe2, as is deduced from the shift of the Fermi level, but does not significantly change the position of the surface Fermi level on n-WSe2/Br2 (Figure 8a,b). For a direct electron transfer the HOMO of the adsorbate should be expected to lie above the Fermi level of the substrate. The energetic positions of the adsorbed H2O states on WSe2 surfaces can directly be deduced from the experimental results as has been discussed in detail elsewhere.25 As the measured position of the HOMO i.e., the 1b1 orbital is far below the Fermi level (EB ) 5-8 eV; Table 1,2), any direct electron transfer into the semiconductor can be excluded. Therefore, we suggest an indirect charge transfer from a surface molecule formed by interaction of H2O with the semiconductor surface. The injecting orbital is the antibonding combination of the HOMO (1b1) of H2O and occupied states at the valence band edge, which are derived from W(dz2) orbitals (for details see ref 25). A schematic LCAO diagram for the formation of this extrinsic donor surface state at p-WSe2/H2O interfaces is given
J. Phys. Chem., Vol. 100, No. 42, 1996 16973
Figure 9. (a, top) Schematic representation of the charge transfer process to adsorbed Br2 leading to Br- formation. The energy scale is given from the experimental results (see text). (b, bottom) LCAO diagram for the formation of an extrinsic donor surface state at p-WSe2/ H2O interfaces. The energy scale is given from the experimental results.
in Figure 9b. This donor state, labeled “dz2”, is mainly derived from the occupied W(dz2) semiconductor valence band states which interact with the occupied lone pair orbitals of adsorbed H2O. This surface molecule is stable only, when the electrons of this antibonding combination can be donated to the space charge region of p-WSe2, which explains the weak bonding interaction of the adsorbed H2O. Similar weak bonding interactions are also found on metal surfaces as discussed, e.g. by Hoffmann.53 The more evident mechanism assuming a corrosive interaction of H2O with surface steps,54,55 which also may in principle lead to electron injection, was excluded for several reasons: (1) The H2O adsorption and also the induced band bending are found to be completely reversible; when H2O is desorbed above 150-170 K, flat band conditions are established again. (2) Stepped surfaces lead to inhomogeneous broadening effects and pinning already for the clean surfaces,56 which was not observed in the experiments reported here. (3) H2O adsorption on stepped surfaces (produced by cutting instead of sputtering) gives no dissociative adsorption.56 The energy diagrams for the different adsorbate interfaces as determined in our experiments are summarized in Figure 10. The clean surfaces show the expected shifts in work function as determined by the bulk doping (p vs n-doped material), which indicates flatband position after cleavage. The monoadsorption of H2O and Br2 leads to strong band bending on p- and n-doped WSe2 substrates, respectively, which indicates that H2O acts as electron donor and Br2 as electron acceptor as explained above.
16974 J. Phys. Chem., Vol. 100, No. 42, 1996
Mayer et al.
Figure 10. Schematic energy diagram for the investigated WSe2/H2O/Br2 (co)adsorption systems as determined with photoemission spectroscopy in comparison to the standard redox potential of the Br2/Br- couple. Indicated are the valence band maximum and the conduction band minimum, the position of the surface Fermi level, and the change of the work function ∆Φ and electron affinity ∆χ with respect to the freshly cleaved sample. Also indicated are the energetic positions of the HOMO of Br2 (πg), Br- (pxyz), and H2O (1b1) as determined in this investigation and the LUMO of Br2 (σu) as derived from energy loss experiments.47
However, in the coadsorption experiments with H2O on Br2precovered n-WSe2 as well as Br2 on H2O-precovered p-WSe2, the band bending (the position of the Fermi level) is determined only by the adsorbed Br2 (Figures 10 and 8). Evidently, the strength of donor and acceptor character is different for H2O and Br2, respectively. The electrochemical potential of adsorbates as well as of electrolytes is determined by the energetic position, distribution and occupation of the highest occupied molecular orbital (HOMO) and lowest unoccupied MO (LUMO) of the contact phase. We observe a similar position of the semiconductor Fermi level for adsorbed Br2 and Br2/H2O coadsorption. Also the EFB value of the πg level of Br2 of about 3.0 eV is identical in these cases, which implies that the LUMO of Br2 coincides with the Fermi level of the semiconductor substrate. The energy positions of the adsorbate levels are also included in Figure 10. Based on these results it may be concluded that electronic equilibrium of the interfaces prepared by Br2 adsorption as well as by Br2/H2O coadsorption is determined by the energetic position of the 2σu level of Br2. Electronic equilibrium is evidently governed by adsorbed Br2/ Br- alone and is not influenced by coadsorbed H2O in correspondence to the contact formation of semiconductors to Br2/Br- redox couples in aqueous electrolytes. Evidently, Br2 is a stronger electron acceptor than H2O is an electron donor. This finding agrees well with the different models of charge transfer developed for the monoadsorbates (see above and Figure 9): While Br2 may directly accept electrons into its LUMO 2σu there are no HOMO states of H2O available above the Fermi level. As a reactive interaction of H2O can be excluded, H2O leads to electron donation only indirectly through the weak interaction of the 1b1 level with the occupied W(dz2) substrate states. As the amount of charge transfer from the H2O-induced surface molecular state “dz2” depends on the position of EF, it is expected that a lowering of EF by adsorbed Br2 leads to increased electron injection. Turning this argument around, the amount of ionosorbed Br- will increase due to coadsorbed H2O, as an electron donation from adsorbed H2O to adsorbed Br2 via the semiconductor is expected as observed experimentally (see section 4.1). In the next step we may now try to correlate the energetic diagram as determined in our adsorption experiments to semiconductor/electrolyte interfaces (Figure 10). Thermodynamic equilibrium between the semicoductor and the adsorbate (electrolyte) is reached when the electrochemical potentials of
the contact phases coincide. The electrochemical potential of the semiconductor/adsorbate interface is given by the position of the Fermi level experimentally determined by measuring the work function Φ. The electrochemical potential of the Br2/ Br- redox couple in electrolytes is given by the Nernst equation:
Ered/ox(Br2/Br-) ) E0(Br2/Br-) +
aBr2 RT ln 2F (a -)2
(1)
Br
where the logarithmic part describes the dependence on the Red/ Ox concentration (activity) ratio. The value of the standard potential E0(Br2/Br-) is 1.1 eV vs NHE.57 This corresponds approximately to a work function of 5.5-5.8 eV vs vacuum as follows from the difference of vacuum vs electrochemical scale given by 4.4-4.8 eV.58,59 The experimentally observed presence of the oxidized (Br2) and reduced species (Br-) on the surface may be formally treated equivalent to a surface redox couple. For the low temperature of the adsorption experiment the logarithmic part of the Nernst equation can be neglected for reasonable concentration ratios. Therefore, the absolute redox potential may now be compared to the experimentally determined work function Φ of the adsorbate interface which is about 6.2 eV for the (saturated) n-WSe2/Br2 system19,29 and 5.2 and 5.1 eV respectively for the coadsorption system n-WSe2/ Br2/H2O and p-WSe2/H2O/Br2 (as found in this study). We consider the agreement between the observed work functions Φ and electrochemical potentials of the adsorbate and coadsorbate interfaces with the redox potential in electrolyte solution as quite reasonable. The value obtained without H2O coadsorption shows a closer agreement, which we relate to the fact that the coadsorbed H2O leads to an additional dipole toward vacuum, as suggested by the change of electron affinity (Figure 7, and discussion below). The dipole contribution ∆χ at the interface shall be considered in more detail. The overall work function change ∆Φ at the semiconductor/adsorbate (electrolyte) interface contains a part ∆EFSC due to charge transfer shifting EF in the semiconductor bandgap (equivalent to band bending) and a surface dipole ∆χ shifting the semiconductor band edges vs vacuum:25,26
∆Φ ) ∆EFSC + ∆χ ) eVbb + ∆χ
(2)
Spectra of Semiconductor/Electrolyte Model Interfaces
J. Phys. Chem., Vol. 100, No. 42, 1996 16975
The first part has been discussed above in detail. Now we will concentrate on the second part, the change of electron affinity. As is evident from a comparison of Figures 7 and 8, band bending is dominated by adsorbed Br2 whereas the electron affinity also depends on adsorbed H2O. The observed shift of electron affinity is qualitatively similar to the coadsorption systems halogen/H2O on metals42-46 where Br2 induces an increase and H2O a decrease of electron affinity (for metals changes of ∆χ correspond to changes of work function ∆Φ). As on metal substrates, only the overall work function is usually measured the changes induced by different adsorbates and at different positions normal to the surface cannot be easily discriminated. As is discussed elsewhere in detail,25,26 the overall electron affinity change ∆χ may be formally divided into several contributions, which have different physical origins and are situated at different positions at the phase boundary: ss ∆χ ) ∆χsc + ∆χads + ∆χ
(3)
Besides, dipolar effects at the surface boundary of the substrate ss and the contri∆χsc, the occupation of surface states ∆χads bution of adsorbed dipoles ∆χ, which may be situated at different positions above the surface and on each other, must be considered. For the nonreactive layered semiconductors, ss can be neglected, as no surface states are formed. As is ∆χads evident from our investigations, the monoadsorbates Br2 and H2O show a different behavior regarding their binding energy shifts during the formation of the space charge layer. While the energetic distance of the Br2 states is constant with respect to the substrate states, the energetic distance of H2O states change with respect to the substrate states but is nearly constant with respect to the vacuum level (Tables 1 and 2). Following arguments for the interpretation of adsorbate core level shifts collected e.g. in ref 60 (and neglecting final state effects) we relate this difference to a different position of the adsorbate induced dipole. Br2 induces a dipole at the adsorbate/vacuum interface ∆χ. Thus the ionization energies of the substrate and of the adsorbate is changed by the same amount. Also the substrate and adsorbate emissions shift parallel to band bending. H2O, on the other hand, induces a dipole at the substrate/ adsorbate interface ∆χsc. In this case the ionization energy of the adsorbate remains nearly constant while the substrate ionization energy is changed. In addition, the adsorbate emissions do not show the same binding energy shifts as the substrate emissions following band bending. Unfortunately, most of the investigation of metal/electrolyte model interfaces use Kelvin probe measurements to determine work function changes, which only accounts for the overall change of the work function. In such measurements the relative shifts of different adsorbates cannot be discriminated. Therefore, a more detailed evaluation of the spatial distribution of the different dipolar potential drops, which would account for the different electrochemical double layers, has to be addressed in future work. 4.3. Adsorbate vs Electrolyte Density of State Distributions. Finally, the density of states of the adsorbate as identified in our model experiments shall be discussed in relation to the expected density of states of electrolyte solutions DEI containing redox couples. For reversible weakly interacting one-electron redox couples DEI is given by the electron eigenstate of the redox species involved in charge transfer, which is broadened by solvent interaction into a distribution of the reduced Dred and oxidized state Dox (DEI ) Dred + Dox). Usually DEI is represented in the solvent-fluctuation model52,61 derived from the Marcus theory of electron transfer,62 which has been adapted to semiconductor electrochemistry by Gerischer.52 It is based
on the thermally activated reorganization of the solvation shell around the redox active species. Thus DEI depends on the analytical form assumed for the reorganization energy λ and the strength of solvation (see section 4.2). With this model two Gaussian type distribution functions are given for the reduced, Dred(E), and oxidized, Dox(E) component. The important parameters are the energetic position of maximum probability for the reduced and oxidized component Ered and Eox and the energetic position expressing half-occupation probability E1/2 ≡ Ered/ox ≡ EF. The half-width of the distribution functions for the reduced and oxidized component is given by their reorganization energies λ. In general, different values for the reduced (λred) and oxidized form (λox) of the redox couple have to be taken into account. They arise from their different solvation shells and the nonequal reorganizational stabilization. But very often one common value is approximated (as also assumed by Morrison61). Then the energetic distance between Ered and Eox is 2λ, and Ered (Eox) to EF one λ with a typical number in solutions of 2λ ≈ 2 eV. It is expected that the density of state distribution of adsorbates Dads is different in our model experiments of the electrolyte. The lower temperature as well as the changed solvation without H2O coadsorption will lead to a reduced splitting of the reduced Dred(E) and oxidized Dox(E) component. In addition, the larger concentration of adsorbed Br2 as compared to its reduced counterpart Br- should lead to an asymmetry. But one would still expect a distribution of occupied and unoccupied states around the Fermi level. Knowing this density of states is a central ingredient of understanding charge transfer processes as was recently discussed in a theoretical paper calculating the interfacial electronic structure.63 Model experiments of the kind presented here may give additional experimental evidence for the assumptions used in such electronic density of states calculations. But in the adsorption and coadsorption experiments presented here there are no indications of occupied electron states around the Fermi level (EFB ) 0), which can clearly be assigned to the adsorbate (model electrolyte) as expected from theoretical considerations. This is surprising as the measured band-bending effects involve adsorbate electron states, which are related to spectroscopically identified adsorbate species (see sections 4.1 and 4.2). There are several reasons which may account for this discrepancy. First of all, the substrate valence band photoemission intensity is very large in the energy range of interest. Therefore, small emission intensity, which may be related to the one-electronreduced Br2 component (Br2-), cannot be identified. We did not succeed in obtaining the difference spectra which clearly allows to assign adsorbate induced states around the Fermi level. It may also be possible that the energetic positions of the adsorbate states are shifted relative to the substrates band edge states due to different final state relaxation effects after photoemission. But this relative shift due to a different screening may be expected to be small as the typical and considerably larger relaxation energy shift between gas phase to adsorbed species are below 2 eV.60 In order to check for such effects, model experiments have to be performed with oneelectron redox species preferable on wide band gap semiconductors of low valence band photoemission intensity in the important energy range. The only clearly identified occupied adsorbate state is the Br- p level at around 4 eV below EF. Br- is evidently formed by charge transfer from the semiconductor and should therefore be the counterpart (reduced component) of the oxidized component (adsorbed Br2) as is shown in Figure 9a. Evidently, the assumption of only one-electron state (the Br2 HOMO level
16976 J. Phys. Chem., Vol. 100, No. 42, 1996 2σu) being involved in electronic equilibrium is not fulfilled for the Br2/Br- redox couple. After charge transfer and the formation of Br2- as first reduced component this species dissociates and forms the spectroscopically identified Brspecies. For this species the position of the HOMO is wellknown and is also found in our adsorption experiments (Figure 9a). Therefore, we conclude that the simple argumentation of the reorganizational broadening of the one-electron state (the LUMO 2σu of Br2) cannot be used for multielectron redox couples as e.g. Br2/Br- and the density of state distribution must account for subsequent dissociation reactions, which form new electronic states (the Br- p state). In such cases the electrolyte density of states must be modified in correspondence to Figure 9a. This is of course even more true for the H2O/O2 redox couples, for which the overall transfer of four electrons must be considered. The occupied H2O HOMO 1b1 level, which in a simple-minded view corresponds to Dred, is found far below EF (see Figure 9b). It is therefore also far below the equilibrium redox potential E0(H2O/O2), which is derived from thermodynamic considerations. A direct hole transfer to this state cannot be expected for most small band gap semiconductors. This would imply a completely different mechanism of photoelectrochemical H2O oxidation. We would like to speculate that the initial hole transfer from the semiconductor valence band will involve surface molecule states, as suggested here to explain the H2O-induced donator character observed in our experiments (Figure 9a). Unfortunately, these surface molecule donor states, which are implicated by the experimentally observed band bending, have not yet been observed in our photoemission results. A hole charge transfer mediated via surface molecular states is also expected from the catalytic effect needed for efficient H2O oxidation, which involves a strong interaction of the reactants with the solid surface. 5. Summary and Conclusion The presented results show that the ansatz to simulate the semiconductor/electrolyte interfaces in (co)adsorption experiments in UHV may provide many information on the semiconductor/adsorbate interactions which are important for a microscopic understanding of electrochemistry. Photoelectron spectroscopy allows the identification of surface reactions, the formation of surface states, and their consequences on interface energy diagrams and especially band bending.26 Nonreactive charge transfer processes may be identified as demonstrated for Br2 and H2O adsorption and coadsorption and for Br2/Na coadsorption in a previous study.29 The investigation of such (co)adsorbates on semiconductor substrates offers a specific advantage compared to metal substrates. Charge transfer processes changing the Volta potential are accompanied by the formation of an extended space charge layer. This is monitored by the shift of substrate binding energies reflecting the movement of the surface Fermi level. The additionally measured value of ∆Φ allows the determination of surface dipole changes ∆χ. To a certain degree it is even possible to locate the position of the induced dipole ∆χ from the relative binding energy shifts of substrate and adsorbate states.26,56 The coadsorption system of Br2 and H2O shows that the electrochemical potential of the coadsorbate phase is actually determined by the redox active species Br2/Br-. Its value corresponds qualitatively to the standard redox potential. The energetic position of the LUMO of adsorbed Br2 is found to determine the equilibrium formation (surface Fermi level position of the semiconductor). Br2 is found to be a strong acceptor whereas H2O is only a weak donor. The difference can be explained by different mechanism of charge exchange.
Mayer et al. One may also expect to determine the density of state distribution (DOS) of the electrolyte directly from such model experiments. By the use of photoemission spectroscopy the DOS of the (co)adsorbate occupied electron states are directly given by the additional photoemission bands (neglecting final state effects). In our experiments we did not get clear evidence for a high concentration of occupied (co)adsorbate electron states around the Fermi level. We do not expect that the low sample temperature and the observed clustering of the investigated adsorbates lead to strong deviations in the DOS in relation to the electrolyte. We attribute the deviation of the observed adsorbate DOS to the expectations for the electrolyte DOS to the fact that halogens (and also H2O) are no easy redox couples since two equivalent redox steps and a dissociation reaction are involved. In addition, we observed binding energy shifts in the coadsorbate system which are due to interadsorbate interaction (solvation). H2O is found to be bound to adsorbed Br- via H bridges. A more detailed investigation of solvation on weakly interacting surfaces such as the layered chalcogenides seem to be possible. They must be specifically addressed in future coadsorption experimetns like those presented here but with the intermittent use of annealing steps to enhance the formation of equilibrium solvation shells. Acknowledgment. This work was supported by a grant of the BMBF. Experimental help of J. Lehmann and the support by the BESSY staff is gratefully acknowledged. References and Notes (1) Abruna, H. D., Ed. Electrochemical Interfaces: Modern Techniques for in-situ Interface Characterization; Verlag Chemie: New York, 1991. (2) Melendres, C. A., Todjeddine, A., Eds. Synchrotron Techniques in Interfacial Electrochemistry; NATO ASI Series, Vol. 432; Kluwer: Dordrecht, 1994. (3) Trasatti, S., Wandelt, K., Surf. Sci. 1995, 335. (4) Gutierrez, C., Melendres, C., Eds. Spectroscopic and Diffraction Techniques in Interfacial Electrochemistry; NATO ASI Series C320; Kluwer: Dordrecht, 1990. (5) Hubbard, A. T. Chem. ReV. 1988, 88, 633. (6) Ross, P. N.; Wagner, F. T. In AdVances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; John Wiley: New York, 1984; Vol. 13. (7) Kolb, D. M. Z. Phys. Chem. 1987, 154, 179. (8) Ko¨tz, R. in ref 4. (9) Soriega, M. P., Ed. Electrochemical Surface Science; American Chemical Society: Washington, DC, 1988. (10) Jaegermann, W.; Tributsch, H. Prog. Surf. Sci. 1988, 29, 1. (11) Sass, J. K. Vacuum 1983, 33, 741. (12) Stuve, E. M.; Bange, K.; Sass, J. K. In Trends in Interfacial Electrochemistry; Silva, A. F., Ed.; D. Reidel: Dordrecht, 1986. (13) Bange, K.; Straehler, B.; Sass, J. K.; Parsons, R. J. Electroanal. Chem. 1987, 229, 87. (14) Baumann, P.; Pirug, G.; Reuter, D.; Bonzel, H. P. in ref 3. (15) Wagner, R. T. In Structure of Electrified Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1993. (16) Pirug, G.; Bonzel, H. P. In ref 15. (17) Stuve, E. M., In ref 3. (18) Jaegermann, W. Chem. Phys. Lett. 1986, 126, 339. Jaegermann, W. Ber. Bunsenges. Phys. Chem. 1988, 92, 537. (19) Mayer, T.; Pettenkofer, C.; Jaegermann, W. J. Phys. Condens. Matter 1991, 3, 161. (20) Mayer, T.; Klein, A.; Lang, O.; Pettenkofer, C.; Jaegermann, W. Surf. Sci. 1992, 269/270, 909. (21) Pettenkofer, C.; Jaegermann, W.; Bronold, M. Ber. Bunsenges. Phys. Chem. 1991, 95, 560. (22) Sander, M.; Jaegermann, W.; Lewerenz, H. J. J. Phys. Chem. 1992, 96, 782. (23) Mo¨nch, W. Semiconductor Surfaces and Interfaces, 2nd ed.; Springer: Berlin, 1995. (24) King, D. A., Woodruff, D. P., Eds. The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; Surface Properties of Electronic Materials; Elsevier: Amsterdam, 1988; Vol. 5. (25) Mayer, T.; Jaegermann, W. In ref 2, p 451; ref 3, p 343.
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