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Synchrotron Photoemission Spectroscopy Study of p‑GaInP2(100) Electrodes Emersed from Aqueous HCl Solution under Cathodic Conditions Mikhail V. Lebedev,*,† Wolfram Calvet,‡ Bernhard Kaiser,§ and Wolfram Jaegermann§ †

Ioffe Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia Helmholtz-Zentrum Berlin (HZB), Albert-Einstein-Str. 15, 12489 Berlin, Germany § Institute of Material Science, Darmstadt University of Technology, Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany ‡

S Supporting Information *

ABSTRACT: (Photo)electrochemical processes occurring under cathodic polarization at the p-GaInP2(100)/1 M HClaq solution interface were investigated in detail by highresolution surface sensitive synchrotron-radiation photoemission spectroscopy. It was found that on application of the cathodic bias in the dark to the p-GaInP2(100)/1 M HClaq solution interface the electrochemical processes are started at a bias of about −1.0 V vs reversible hydrogen electrode (RHE), where cathodic current passing through the semiconductor/ electrolyte interface starts to rise. Under higher cathodic bias applied in the dark, hydroxyl groups and metallic gallium are accumulated at the surface, which is accompanied by a decrease in work function of the semiconductor. Accumulation of hydroxyl groups can be related only to splitting of water molecules at the semiconductor/electrolyte interface, since the aqueous HCl solution contains no hydroxyl groups intrinsically. Accumulation of hydroxyl groups and metallic gallium is accelerated under visible light illumination, which indicates participation of photogenerated electrons in the surface electrochemical reactions. The formation of the metallic gallium without simultaneous metallic indium formation testifies that the In−P bonds of the GaInP2 compound are more stable against cathodic corrosion than the Ga−P bonds.

1. INTRODUCTION

surface, as well as the interfacial composition of the electrolyte.15 Photoemission spectroscopy may provide detailed information on the chemical composition and electronic structure of the solid/electrolyte phase boundary. However, ultrahigh vacuum (UHV) is required, which is not compatible with the liquid contacts. In the past, a number of approaches have already been suggested for the study of semiconductor/ electrolyte interfaces using photoemission spectroscopy, e.g., freezing-in a thin electrolyte layer on the semiconductor surface,16 as well as step-by-step coadsorption of electrolyte components onto cooled sample surfaces.17,18 Also the application of near-ambient pressure X-ray photoelectron spectroscopy (NP-XPS)19,20 can enable the detailed study of water interaction with semiconductor surfaces at different temperatures. First experiments using operando ambientpressure X-ray photoelectron spectroscopy (AP-XPS),21 as well as in situ electrochemical X-ray photoelectron spectroscopy,22 have been performed to analyze semiconductor/liquid junctions at room temperature under real-time electrochemical

Gallium indium phosphide (GaInP) is a ternary semiconductor compound applicable in heterojunction electronic devices1−4 and nanostructures.5,6 In addition, GaInP2 is widely used as the largest bandgap top cell in efficient single and multijunction solar cells,7−10 and it has been employed for photoelectrochemical water splitting,11,12 as its band gap of 1.8−1.9 eV is large enough to cover the difference between H2/H2O and O2/H2O redox potentials. However, the GaInP2 surface tends to react with aqueous electrolyte solutions, which often leads to changes at the semiconductor/electrolyte interface. So far, the GaInP2/ aqueous electrolyte contact was mainly investigated by electrochemical techniques, such as cyclic voltammetry and impedance measurements.13,14 However, the photoelectrochemical surface processes as ion adsorption, formation and dissolution of surface species, as well the charge transfer from the semiconductor band edges to electrolyte components have not been studied on an atomic level so far. Thus, a more detailed view of contact formation and subsequent (photo)electrochemical charge transfer reactions requires a precise analysis of the phase boundary structure including the atomic composition and electronic structure of the semiconductor © XXXX American Chemical Society

Received: February 10, 2017 Revised: April 4, 2017 Published: April 5, 2017 A

DOI: 10.1021/acs.jpcc.7b01343 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

wire was used as counter electrode. No rinsing was done after each experimental step in order to detect also weakly bound reaction products of the electrochemical double layer. Photoelectrochemical experiments were carried out in the “dark”, i.e., at low background light intensity of the BESSY experimental hall, and under white light illumination produced by a halogen lamp with a power density of about 100 mW/cm2, which was directed to the semiconductor/electrolyte interface using a glass pipe as waveguide. The experimental procedure was described in detail elsewhere.27 Two different samples from the same wafer were used. Each sample was treated with the 1 M aqueous HCl solution for 5 min for native oxide removal. With GaAs we have observed that this procedure was able to remove most of the native oxide.27 Photoemission was used to check whether the samples had identical chemical compositions after surface etching. Thereafter each sample was brought into contact to the electrolyte again, maintained at zero-current potential (ZCP) for 30 min, and subsequently was analyzed by photoemission. ZCP was determined from the current−voltage characteristics of the p-GaInP2(100) in 1 M aqueous HCl solution. The idea of this pretreatment step is to obtain reproducible starting conditions at the semiconductor/electrolyte interface for subsequent (photo)electrochemical studies. Afterward, the samples were immersed again into the solution and different cathodic potentials were applied either in the dark (sample 1) or under illumination (sample 2) for about 30 min. At the end of the contact time, the solution meniscus was blown off with an Ar jet (emersion) under applied voltage, before transferring the sample into the spectrometer chamber. To ensure that the semiconductor/electrolyte contact is maintained under similar (photo)electrochemical conditions during the 30 min exposure time, the current flowing through the semiconductor/electrolyte interface was monitored continuously (Figure S1). The photoemission studies have been performed at the undulator beamline U49/2 of the BESSY II storage ring. Photoemission spectra were recorded using different excitation energies to characterize surface chemistry with different surface sensitivity. All spectra were measured at normal emission. Fitting of the core level spectra was performed with Voigt functions after Shirley background subtraction by using IGOR Pro software (WaveMetrics, Inc.). The spectra were obtained using the Phoibos 150 (SPECS) energy analyzer of the SoLiAS system.43 For calibration of the binding energy scale, the Fermi level of a polycrystalline Ag foil cleaned by Ar-ion sputtering was determined for each excitation energy. The work function of the samples was determined from the low kinetic energy cutoff of the secondary electron background (“secondary edge”) in the valence band spectra.44 A bias of −6 V was applied to the sample to distinguish between the analyzer and the sample cutoff.

control. Nevertheless, so far the most photoemission studies of semiconductor/electrolyte interfaces were carried out on emersed samples, which can provide detailed information on the surface chemical composition after chemical and electrochemical treatment.23−27 Whereas GaAs surfaces have been actively studied in the past,16,18,23−27 there are only a few investigations published for InP28−32 and GaP,33−37 and only very few analytical studies on GaInP2 using XPS.38,39 In this study we analyze the chemical reactions induced at the interface of p-GaInP2(100) and aqueous HCl solution by applying different cathodic potentials to the semiconductor electrode without and with visible light illumination. This study is motivated by the application of p-GaInP2(100) as a photoelectrode for light induced hydrogen formation from aqueous solutions,11−13,40 which, however, shows the instability of the electrode surface in the electrolyte. Aqueous HCl solutions are used very often in different etching procedures and in (photo)electrochemical experiments.29−32,38 In particular, they can stabilize and passivate the p-InP surface with respect to cathodic decomposition, as opposed to aqueous H2SO4 solutions.29 We have utilized highly surface-sensitive synchrotron-radiation photoemission spectroscopy on emersed photoelectrode surfaces to investigate the interface reactions and species formed on p-GaInP2(100) photoelectrodes under varying operation conditions.

2. EXPERIMENTAL SECTION For our investigations we have used p-Ga0.52In0.48P(100) samples with the doping level of about 1 × 1018 cm−3 (henceforth, p-GaInP2(100)) grown as epitaxial layer 2 μm thick by metalorganic vapor phase epitaxy (MOVPE) on a highly doped p-GaAs(100) substrate (1 × 1018 cm−3) misoriented to the (111) direction by 6°.41 To provide rear ohmic contact, the sample was joined to the stainless-steel sample holder with silver paste. Prior to joining, the backside of the sample was mechanically scratched to remove the native oxide layer and to roughen the surface for providing better electrical contact. Furthermore, the electrode surface was cleaned ultrasonically with acetone and 2-propanol. As electrolyte solution, a 1 M aqueous HCl solution was used (reactant-grade concentrated HCl (37%) mixed with Milli-Q water). The etching rate of the semiconductor with such diluted solution should be negligible,42 and thus only the interaction with the surface native oxide layer is expected. Prior to the experiment, the electrolyte solution was bubbled with Ar gas for 30 min to remove diluted air. The electrochemical experiments were carried out using a conventional three-electrode setup in the glass chamber of the integrated system SoLiAS (Solid/Liquid Analysis System)43 permanently operated at BESSY. This glass chamber is purged with inert, dry, carbon-free Ar gas and is directly attached to the UHV system via a special buffer chamber. With this arrangement, it is possible to transfer the sample from the electrolyte solution into UHV without reoxidation and contamination caused by exposure to ambient atmosphere. The glass electrochemistry chamber contains a special cell, which allows contacting the sample with a meniscus of the solution in a three-electrode electrochemical setup.27 The sample size was a square 1 × 1 cm2, whereas the real area of the semiconductor/electrolyte contact was a circle of about 0.5 cm in diameter. The electrode potential in the three-electrode arrangement was measured with respect to the Ag/AgCl reference electrode (in saturated KCl aqueous solution); a Pt

3. RESULTS 3.1. p-GaInP2(100) Surface Exposed to 1 M Aqueous HCl Solution. For defining the starting conditions of the pGaInP2(100) surfaces we show in Figure 1 a comparison of the O 1s, In 3d, P 2p, and Ga 3d/In 4d core level spectra measured on p-GaInP2(100) as covered with the native oxide, after etching with 1 M HClaq solution for 5 min, and after exposure to the electrolyte under ZCP conditions for another 30 min. The XPS results on etching have been described in detail elsewhere.45 B


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Figure 1. Spectra of the O 1s (a), In 3d5/2 (b), P 2p (c), and Ga 3d/In 4d core levels for the p-GaInP2(100) surface covered with the native oxide, etched with 1 M aqueous HCl solution for 5 min, and finally exposed to 1 M aqueous HCl solution for another 30 min under ZCP conditions.

After etching of the p-GaInP2(100) surface covered by its native oxide layer with 1 M HClaq solution the oxygen content is reduced by a factor of 3.5 (Figure 1a). Besides, we observed a Cl 2p core level line, which appears at 198 eV (Figure S2), whereas the carbon C 1s photoemission intensity remains mostly unaffected after 1 M HClaq solution treatment. The constant binding energy of the In 3d5/2, P 2p, and Ga 3d lines indicates that the etching process has very little influence on the surface band bending (Figure 1). In particular, the energy separation between Fermi level and valence-band photoemission onset remains 0.5 eV.45 After the first etching step (removal of the native-oxide layer with 1 M HClaq solution and recording of photoemission spectra), the sample was again brought into contact with 1 M HClaq solution for further photoelectrochemical and photoemission studies. The current−voltage curves of the polarization measurements in the dark (background light of BESSY experimental hall) and under illumination are shown in Figure 2. The zero-current potential (ZCP) value was determined to be about +0.25 ± 0.05 V vs reversible hydrogen electrode (RHE) both under dark and under illumination (Figure 2). The low photovoltage indicates that the remaining surface oxides lead to Fermi level pinning at the GaInP2(100)/electrolyte contact. The exposure of the p-GaInP2(100) surface to 1 M HClaq solution under ZCP conditions for 30 min had very little influence on the intensity of the O 1s photoemission, as well as on the In 3d, P 2p, and Ga 3d/In 4d core levels features (Figure 1). Work function of the p-GaInP2(100) surface was measured at 4.65 ± 0.05 eV after such an exposure (Figure S3). It should be noted that due to close binding energy values for In 3d and Ga 3d core levels the inelastic mean free paths46 of

Figure 2. Current−voltage characteristics of p-GaInP2(100) in 1 M HClaq solution measured in “dark” conditions, i.e., at low background light intensity of BESSY experimental hall, and under conditions of illumination by white light of halogen lamp with a power density of about 100 mW/cm2 directed to the semiconductor/electrolyte interface through a glass waveguide. Scan rate 50 mV/s.

the respective photoelectrons are almost identical, which enables a quantitative analysis of the In and Ga atomic depth distribution by varying the excitation energy and considering their corresponding photoionization cross sections.47 The depth distribution of the ratio of Ga and In atomic concentrations obtained from Ga 3d/In 4d spectra measured at four different excitation energies is presented in Figure 3. Obviously, on the initial native oxide covered surface the numbers of gallium and indium atoms are similar to good accuracy throughout the whole analyzed depth. After etching and exposure to 1 M aqueous HCl solution under ZCP the Ga/ C


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In ratio decreases at the very top near-surface layer. The gallium depletion in the near-surface region can be explained by segregation due to different etching rates of the InP and GaP constituents and/or different solubility of the formed hydroxides/phosphates. In summary, the prolonged exposure of the p-GaInP2(100) surface to 1 M HClaq solution for 30 min under ZCP conditions has very little influence on the shape of the main core levels, such as O 1s, In 3d, P 2p, and Ga 3d/In 4d, in comparison to the surface etched preliminary with the same solution (Figure 1). It means that after rather fast stage of initial etching removing most of the native oxide layer the chemical composition of the surface stabilizes and a stable junction pGaInP2(100)/1 M HClaq solution with a thin layer of residual indium and gallium chlorides/oxides/hydroxides and phosphates is formed, which thickness can be estimated as about one monolayer. 3.2. p-GaInP2(100) Surface Composition after Cathodic Polarization in 1 M HClaq Solution in the Dark. It is obvious from the polarization curve that with application of a cathodic bias of about −1.0 V vs RHE under dark conditions

Figure 3. Ga/In atomic ratio as a function of depth for the pGaInP2(100) surface covered with native oxide, as well as for this surface etched with 1 M HCl aqueous solution for 5 min and exposed to 1 M HCl aqueous solution for another 30 min under zero-current potential. The dependence of the Ga 3d/In 4d ratio corrected by the relative photoionization cross sections is shown as a function of the electron inelastic mean free path. The numbers indicate the excitation energies for which the electron inelastic mean-free path values46 have been calculated.

Figure 4. (a) Current−voltage characteristics of p-GaInP2(100) in 1 M HClaq solution measured in “dark” conditions, and fitting of the O 1s core level spectra measured after exposure of the p-GaInP2(100) surface to 1 M HClaq solution under ZCP and cathodic voltages of −0.7 and −1.8 V vs RHE in dark, as well as corresponding difference spectra. Corresponding In 3d5/2 (b), P 2p (c), and Ga 3d/In 4d (d) spectra for identical emersion potentials as for the O 1s spectra with their difference spectra. D


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GaInP2(100) surface to the 1 M HClaq solution (Figure S2), though the In2O3-related component has similar chemical shift.32,48 Obviously, the application of the cathodic bias in the dark to the p-GaInP2(100)/1 M HClaq solution interface has very little effect on the shape of the In 3d spectrum. As can be seen from the difference spectra, only a decrease of the metallic indium signal is observed after exposure to the solution under −0.7 V, whereas after exposure under −1.8 V in dark the In 3d spectrum remains essentially unchanged (Figure 4b). Fitting of the P 2p core level spectrum of the p-GaInP2(100) surface emersed from 1 M HClaq solution under ZCP conditions was performed with 5 components using the spin−orbit splitting energy of 0.8 eV, branching ratio of 2.0, Gaussian width of 0.65 ± 0.02 eV, and Lorentzian width varied to obtain the best deconvolution (Figure 4c). Besides the main component with the binding energy of about 128.5 eV caused by bulk P−Ga and P−In bonds there are 3 components shifted to higher binding energy by 1.0, 3.8, and 4.8 eV, as well as the component shifted to low binding energy by −0.4 eV. The latter one was observed at the InP(100) surface32 and assigned as a surface-related component. The component with ΔBE = 1.0 eV can be assigned to elemental phosphor,32,48,51 which after exposure to HCl solution contains some contribution from the P−Cl bonds.50,52 The component with ΔBE of 3.8 eV is associated with Inx(HPO4)y32 and/or related gallium phosphates. The species with ΔBE of 4.8 eV can be assigned to indium phosphate InPO4.32 The application of the cathodic bias of −0.7 V in the dark to the p-GaInP2(100)/1 M HClaq solution interface causes a slight increase in the phosphate components, as is evident from the difference spectrum, whereas the application of the bias of −1.8 V results in a slight reduction of the elemental phosphorus (Figure 4c). The binding energy region containing the Ga 3d and In 4d core levels measured with 250 eV excitation energy after emersion of the p-GaInP2(100) sample from the 1 M HClaq solution in the dark under ZCP, as well as under cathodic bias of −0.7 and −1.8 V vs RHE, is shown in Figure 4d. These core levels have very similar binding energies, which complicated curve fitting, since two different spin−orbital splitting energies and their branching ratios must be considered. We fixed the binding energies difference between indium and gallium bulk peaks at 1.8 eV, in agreement with the value of 1.82 eV reported for a pristine InGaAs(100) surface.53 Rough fitting of the spectra reveals components related to gallium and indium (hydro)oxides/chlorides (Figure 4d). The broad component related to gallium (hydro)oxides/chlorides shows a chemical shift of about 0.9 eV from the bulk Ga−P photoemission. It should be noted that the assignment of this component is very complex since both gallium monochloride GaCl,16 as well as gallium hydroxide GaOH,24 gallium suboxides26 and even gallium hydrate Ga−H54 fall in this range. The depth distribution of the ratio of Ga and In atomic concentration obtained from Ga 3d/In 4d spectra measured at four different excitation energies is presented in Figure 5. After exposure to 1 M aqueous HCl solution under cathodic polarization the Ga/In ratio increases in the near-surface region of GaInP2, while the topmost layer of semiconductors remains depleted of gallium. In summary, application of the cathodic bias in the dark to the p-GaInP2(100)/1 M HClaq solution interface has very little effect on both the Ga 3d and the In 4d core level spectra (Figure 4d). To reveal fine changes in detail, the surface sensitive difference spectra within the same binding energy range can be considered (Figure 4d). After exposure of the p-

the cathodic current through the semiconductor/electrolyte interface starts to increase (Figure 2) indicating the onset of an electrochemical process. Under illumination, the cathodic current increases rapidly starting from the ZCP (Figure 2). The O 1s core level spectrum (excitation energy of 650 eV) obtained after emersion from 1 M HClaq solution under ZCP conditions (Figure 1a) can be fitted with two main components with the full width at half-maximum (fwhm) parameter of 1.8 ± 0.2 eV (Figure 4a). The largest component with the binding energy of about 531 eV can be assigned to residuals of gallium, indium, and phosphorus oxides remaining on the surface after contact with the 1 M HClaq solution (etching and further exposure under ZCP conditions). The second component with the binding energy of about 533 eV can be associated with OHgroups bound to surface atoms. The O 1s core level spectrum of the p-GaInP2(100) surface emersed from 1 M HClaq solution under cathodic polarization of −0.7 V in the dark looks very similar to the spectrum measured from the surface emersed under ZCP conditions (Figure 4a). This polarization corresponds to a low current plateau in the I−V polarization curve. From the difference spectrum (Figure 4a), it is clear that the only process taking place under cathodic polarization of −0.7 V in dark is the slight etching of the surface oxides. The amount of OH-groups at the surface remains unchanged. If a cathodic bias of −1.8 V vs RHE is applied to the pGaInP2(100)/1 M HClaq solution interface in the dark, the current passing through the semiconductor/electrolyte interface increases approximately by 1 order of magnitude (Figure 4a), which indicates the occurrence of the electrochemical process. At the same time, the OH-group-related component in the O 1s core level spectrum increases indicating accumulation of OH-groups on the surface, while the oxide-related component of the O 1s core level spectrum continues to decrease (Figure 4a). The surface sensitive In 3d5/2 and P 2p core-level spectra measured after emersion the p-GaInP2(100) sample from 1 M HClaq solution in the dark under ZCP, as well as under cathodic bias of −0.7 and −1.8 V vs RHE, are shown in Figure 4b,c, respectively. The In 3d5/2 and In 3d3/2 peaks of the In 3d doublet were fitted simultaneously with the spin−orbit splitting energy of 7.6 eV, the branching ratio for the doublet peaks of 1.45, Gaussian width of 0.64 ± 0.02 eV, and Lorentzian width varied to obtain the best deconvolution. For clarity only the fitting of the In 3d5/2 peak is presented in Figure 4b. The In 3d5/2 core level measured with the excitation energy of 650 eV can be fitted well using five components. Besides the In−P bulk photoemission component with the binding energy of 444.1 eV, additional components of lower binding energy (ΔBE = −0.4 eV) and at higher binding energy (ΔBE = 0.4, 0.7, and 1.3 eV) are clearly visible (Figure 4b). The component with the chemical shift of −0.4 eV is assigned to metallic indium.32,48 We expect that this component is formed during the MOVPE growth of the GaInP2 layer.49 The components, which are at ΔBE of 0.7 and 1.3 eV, are assigned to indium hydroxide In(OH)3 and indium phosphate InPO4, respectively.32,48 This indium hydroxide component explains the presence of the OHgroups component in the initial O 1s core level spectrum measured on the p-GaInP2(100) surface emersed from the electrolyte solution under ZCP conditions (Figure 4a). The component with ΔBE = 0.4 eV is assigned to InCl29,50 as the Cl 2p photoemission is always visible after exposure of the pE


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As the shape of the In 3d photoemission spectrum changes very little with the change of the cathodic bias from −0.7 to −1.8 V (Figure 4b), the increase in the difference photoemission signal (Figure 4d) can solely be related to a change of the bonding of the gallium. Typically, the Ga 2p3/2 core level can be examined to separate In and Ga photoemission signals. However, due to the high binding energy (of about 1110 eV), it is difficult to tune the excitation energy of the synchrotron source in such a wide range to obtain a depth distribution of Ga-related chemical bonds. Therefore, in this study the Ga 3p core level was analyzed to study the gallium bonding distribution over the sample depth. Figure 6a shows the Ga 3p core level spectra (excitation energy of 250 eV) after emersion of the p-GaInP2(100) sample from 1 M HClaq solution in the dark under ZCP, as well as under the cathodic bias of −0.7 and −1.8 V vs RHE. The spectra obtained after exposure of the p-GaInP2(100) surface to the 1 M HClaq solution in the dark under ZCP and under cathodic bias of −0.7 V look very similar, whereas the spectrum taken after application of a cathodic bias of −1.8 V exhibits a new feature with a binding energy of about 102 eV. To reveal the origin of this feature the Ga 3p core level spectra were measured with varied excitation energies. Spectra of the p-GaInP2(100) surface exposured to 1 M HClaq solution in the dark under a cathodic bias of −0.7 V show a very similar shape irrespective of the excitation energy (Figure 6b). Only a slight increase of the spectral half-width is visible with the decrease in the excitation energy. This is due to the increasing number of the gallium (hydro)oxides/chlorides toward the surface. However, the Ga 3p spectra of the p-GaInP2(100) surface exposed to 1 M HClaq solution in the dark under a cathodic bias of −1.8 V measured with various excitation energies look essentially different (Figure 6c). At first, the Ga 3p spectrum obtained with the excitation energy of 650 eV is similar to the Ga 3p spectra measured after emersion of the p-GaInP2(100) sample from 1 M HClaq solution in the dark under ZCP and

Figure 5. Ga/In atomic ratio as a function of depth for the pGaInP2(100) surface exposured to 1 M HCl aqueous solution under ZCP and cathodic bias voltages of −0.7 and −1.8 V vs RHE. The Ga 3d/In 4d ratio is corrected by the cross sections and plotted vs the electron inelastic mean free path. Numbers indicate the excitation energies used to determine the electron inelastic mean-free path46 values.

GaInP2(100) surface to the 1 M HClaq solution under cathodic bias of −0.7 V in the dark, the Ga 3d photoemission signal in the binding energy range from about 22 to 18.5 eV increases with respect to the signal obtained after emersion under ZCP conditions, in agreement with the increase in the Ga/In ratio (Figure 5). Increase of the photoemission signal in the binding energy range from 22 to 20 eV (Figure 4d) can be related to the increase in the number of Ga−Cl bonds at the surface as the Cl 2p photoemission increases (Figure S2), though GaOH and even Ga(OH)3 components can be also in this range.24 On the other hand, the OH-related component of the O 1s spectrum does not change on application of −0.7 V vs RHE in dark (Figure 4a). A portion of this signal can be also related to some gallium phosphates, since the increase in the phosphate component is observed in the P 2p spectrum, while the indium phosphate component of the In 3d spectrum remains essentially unchanged (Figure 4b).

Figure 6. (a) Surface-sensitive Ga 3p core-level spectra of p-GaInP2(100) surface emersed from 1 M HClaq solution after exposure under ZCP and after cathodic polarization at −0.7 and −1.8 V vs RHE under dark conditions (excitation energy of 250 eV). Ga 3p core-level spectra of the pGaInP2(100) surface emersed from 1 M HClaq solution after cathodic polarization at −0.7 V (b) and −1.8 V (c) vs RHE in dark conditions measured with different excitation energies. F


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Figure 7. (a) Current−voltage characteristics of p-GaInP2(100) in 1 M HClaq solution measured under illumination and fitting of the O 1s core level spectra measured after exposure the p-GaInP2(100) surface to 1 M HClaq solution under ZCP and cathodic voltages of −0.7 and −1.8 V vs RHE under illumination, as well as corresponding difference O 1s spectra. Corresponding In 3d5/2 (b), P 2p (c), and Ga 3d/In 4d (d) spectra with difference spectra.

under a cathodic bias of −0.7 V (Figure 6a,b). At this excitation energy, the electron inelastic mean-free path λ of the excited electrons is about 18 Å.46 Therefore, the information depth for these Ga 3p photoelectrons can be roughly estimated as d650eV ≈ 3λ ≈ 50−55 Å, which approximates the depth resolution of standard XPS. With the decrease of the excitation energy, which coincides with a decrease in the information depth, a new feature with the binding energy of about 102 eV starts to emerge. First, it is just visible as a shoulder (at the excitation energy of 350 eV), and finally, this feature becomes dominant in the whole Ga 3p core level spectrum (in the most surface sensitive spectrum obtained with the excitation energy of 150 eV, Figure 6c). So, this feature is directly related to the semiconductor surface, since its intensity decreases clearly with the increase of excitation energy. As the binding energy of this feature is lower than the binding energy of the Ga 3p bulk photoemission for GaInP2, it can be assigned to metallic gallium Ga0. It should be noted that the low-binding energy part of the Ga 3d spectrum with the metallic gallium photoemission is overlapped with the indium (hydro)oxidechloride components of the In 4d spectra (Figure 4) and thus can hardly be resolved there.

Correspondingly, the work function of the p-GaInP2(100) surface decreases slightly after exposure to the solution under cathodic bias in the dark from 4.65 ± 0.05 eV (ZCP) to 4.60 ± 0.05 eV (−0.7 V) and 4.50 ± 0.05 eV (−1.8 V) (Figure S4a). Slight decrease of the work function after exposure of the surface to the solution under cathodic bias of −0.7 V can be related to further decrease in the oxides content at the surface (Figure 4), while the decrease of the work function after exposure under higher cathodic bias of −1.8 V can be in addition associated with the accumulation of the metallic gallium, as its work function is equal to 4.2 eV,55 though the accumulation of the atomic hydrogen at the surface cannot be ruled out in this context as well.56,57 3.3. p-GaInP2(100) Surface Composition after Cathodic Polarization in 1 M HClaq Ssolution under Visible Light Illumination. Once the semiconductor is illuminated under cathodic bias, the current passing through the pGaInP2(100)/1 M HClaq solution interface increases essentially (Figure 2), as it would be expected for a depletion layer formed at the p-semiconductor/electrolyte contact. The O 1s core level spectrum of the p-GaInP2(100) surface emersed from the 1 M HClaq solution under cathodic polarization of −0.7 V under G


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Figure 8. (a) Surface-sensitive Ga 3p core-level spectra of p-GaInP2(100) surface emersed from 1 M HClaq solution after exposure under ZCP and after cathodic polarization at −0.7 and −1.8 V vs RHE in dark conditions measured with the excitation energy of 250 eV. Ga 3p core-level spectra of p-GaInP2(100) surface emersed from 1 M HClaq solution after cathodic polarization at −0.7 V (b) and −1.8 V (c) vs RHE in dark conditions measured with different excitation energies. (d) Corresponding difference spectra.

Particularly, with application of −0.7 V bias the indium hydroxide, as well as different phosphate components decrease essentially, which is in agreement with the decrease in the oxide-related component of the O 1s photoemission (Figure 7a). At the same time, the metallic indium component slightly decreases as well, as is evident from the difference spectrum. However, after application of −1.8 V bias under illumination an increased semiconductor oxidation is found (Figure 7a), and the In 3d, P 2p, and Ga 3d/In 4d spectra change accordingly (Figure 7b−d). The Ga 3p core level spectra measured with an excitation energy of 250 eV after emersion the p-GaInP2(100) sample from 1 M HClaq solution under ZCP, as well as under cathodic bias of −0.7 and −1.8 V vs RHE under illumination, are shown in Figure 8a. In the Ga 3p core level spectra of p-GaInP2(100) surfaces treated at the cathodic biases of −0.7 and −1.8 V under illumination the component assigned previously to metallic gallium Ga0 is clearly visible (Figure 8a). The intensity of this component is higher in the spectrum of the pGaInP2(100) surface emersed under the cathodic bias of −1.8 V. This component is hardly visible in the spectra measured with the excitation energy of 650 eV and dominates in the most surface sensitive spectra measured using the excitation energy of 150 eV (Figure 8b,c). It should be noted that the change in

illumination differs from the spectrum measured from the surface emersed under ZCP conditions (Figure 7a). In particular, the OH-group-related shoulder at the binding energy of about 533 eV becomes more prominent. This is much more visible from the corresponding difference spectrum, illustrating the change in the O 1s photoemission signal from exposure in solution under ZCP condition to the exposure in the solution under application of a cathodic bias of −0.7 V with illumination (Figure 7a). However, once a cathodic bias of −1.8 V vs RHE is applied to the illuminated p-GaInP2(100)/1 M HClaq solution contact, the current passing through the semiconductor/electrolyte interface is changed only slightly with respect to the application of a cathodic bias of −0.7 V under illumination (Figures 2 and 7a). Nevertheless, the shape of the O 1s core level spectrum changes largely (Figure 7a). In particular, both the OH and oxide related components are strongly increased, and thus, not only the accumulation of the hydroxyl-groups but also the oxidation of the semiconductor surface takes place, which is surprising on a first glance as a reduction current (electron transfer) is induced by illumination. Application of the cathodic bias to the p-GaInP2(100)/1 M HClaq solution interface under illumination also causes strong changes in the In 3d and P 2p core level spectra (Figure 7b,c). H


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dissolution has been observed in the (photo)electrochemical processes at the p-GaInP2(100)/1 M HClaq solution interface (Figures 4b and 7b). Therefore, the formation of the metallic gallium cannot be related to simple electrochemical reduction at the semiconductor/electrolyte interface. The 1 M HClaq solution is an acidic solution and thus contains no hydroxyl ions intrinsically. Therefore, all hydroxylgroups accumulated at the surface can only arise from a decomposition of the water molecules. Density functional theory and ab initio molecular dynamics simulations have shown that the presence of a submonolayer oxide layer on InP(100) or GaP(100) surfaces promotes the dissociation of water molecules making it strongly exothermic and barrierless.60 From the photoemission spectra (Figures 4 and 7), the thickness of the oxide layer at the surface remaining after exposure to 1 M HClaq solution under ZCP can be estimated to be about one monolayer and thus water dissociation can proceed very easy on this surface. Besides, the p-GaInP2(100) surface contains initially some amount of indium hydroxides In(OH)3 (Figure 4b) and, accordingly, a small OH-related component is visible in the O 1s core level spectrum of the initial p-GaInP2(100) surface obtained after emersion of the semiconductor from the solution under ZCP conditions (Figure 4a). On the other hand, after (photo)electrochemical interaction with the 1 M HClaq solution the amount of indium hydroxide decreases (Figures 4b and 7b). Therefore, the accumulation of the OH-groups at the semiconductor surface after decomposition of water molecules at the p-GaInP2(100)/ 1 M HClaq solution interface should proceed mostly at the gallium surface atoms. Submonolayer surface oxide layer at the GaInP2 surface can act as proton acceptor promoting dissociative adsorption of water molecules and thus adsorption of additional oxygen and hydroxyl.60 As a result, a continuous exchange of protons, hydroxyl groups and water molecules can take place between surface atoms and solution.61 Once the large enough cathodic bias is applied to the semiconductor/electrolyte interface, the electrons are transferred from the semiconductor to the adsorbed protons and hydrogen molecules are formed. This process should be accompanied by accumulation of the OHgroups at the surface. Molecular dynamics simulations showed that InP and GaP surfaces interact with water in qualitatively different manner due to different rigidity of the hydrogen-bond network at the semiconductor/water interface.60,61 Above all, this results in different corrosion processes at the InP/water and GaP/water interfaces. In particular, the fluid hydrogenbond network at the interface of water with the InP surface covered with thin oxide layer promotes surface hydrogen transport on InP(100) and thus the surface dangling bonds can be easily passivated.61 On the other hand, the GaP/water interface is characterized by rigid hydrogen-bond network and a similar mechanism as for InP will not be observed since the surface hydrogen transfer is kinetically limited.61 Besides, the oxygen atoms bridge-bonded to gallium atoms can be possible nucleation sites for photocorrosion.62 One of the possible mechanisms of such corrosion in the cathodic process can be the release of phosphine and agglomeration of the metallic gallium at the surface.63 The lack of evidence for the metallic indium formation (Figures 4b and 7b) with clear indication of the metallic gallium formation (Figures 6 and 8) under (photo)electrochemical cathodic conditions testifies that the In−P bonds of the GaInP2 compound are more stable against corrosion than the Ga−P

the Ga 3p photoemission under application of cathodic biases and under illumination is related mainly to accumulation of the elemental gallium at the surface, as is evident from the difference spectra (Figure 8d). The work function of the p-GaInP2(100) surface is decreased to 4.45 ± 0.10 eV after exposure to the solution at −0.7 V under illumination (Figure S3b), which again can be associated with the accumulation of the metallic gallium and atomic hydrogen at the surface.55−57 On the other hand, the strong oxidation of the p-GaInP2(100) surface observed under the applied bias of −1.8 V under illumination is accompanied by a drastic increase in the work function of the semiconductor to 5.0 ± 0.05 eV (Figure S4b). It should be noted that the work function of the native-oxide-covered p-GaInP2(100) surface was 4.9 ± 0.05 eV (Figure S3). Therefore, the drastic increase in the work function can be associated with formation of oxides at the surface. The results can evidently be summarized as follows. Due to the cathodic reduction current the metallic gallium is formed, though the density of the metallic indium present initially at the surface is somewhat reduced. Subsequently the gallium surface species, which are in contact with the 1 M aqueous HCl electrolyte solution, will be oxidized to form gallium oxide/ hydroxide surface oxidation products. Thus, the composition of the surface layer is the consequence of different competitive rates of water molecules dissociation, H+ reduction to H2, formation of metallic gallium, as well as surface reactions and etching/dissolution of the formed oxides/hydroxides species.

4. DISCUSSION The results presented in this paper give some additional information about the origin of the p-GaInP2(100) surface reactivity in aqueous acidic environment. In the analysis of the presented photoemission spectra one should consider that the spectra have been measured with the highest surface sensitivity on the electrodes emersed from the electrolyte under applied potential. Therefore, the surface composition is a result of main surface reaction products and minor contributions of intermediates in the multistep surface reactions. The (photo)electrochemical processes at the pGaInP2(100)/1 M HClaq solution interface lead to the accumulation of hydroxyl groups and metallic gallium at the semiconductor surface (Figures 4 and 6−8). If the cathodic current passing through the p-GaInP2(100)/1 M HClaq solution interface is about zero as in the case of the zero current potential or rather small as in the case of the cathodic bias of −0.7 V vs RHE applied in the dark, no metallic gallium is observed in the core level spectra measured with high surface sensitivity. At the same time, no additional OH-groups are observed at the surface (Figure 4a). However, additional OH groups and the formation of metallic gallium is observed at the p-GaInP2(100) surface after emersion from the solution under bias of −1.8 V in the dark, as well as under biases of −0.7 and −1.8 V applied under illumination (Figures 4 and 6−8), i.e., when an essential cathodic current has passed through the semiconductor/electrolyte interface. Therefore, one can conclude that the accumulation of the OH-groups at the pGaInP2(100) surface under (photo)electrochemical interaction with the 1 M HClaq solution is accompanied by transformation of the surface gallium ions at the GaInP2(100) surface to metallic gallium. On the other hand, the reduction potential of In3+/In is lower than that of Ga3+/Ga (−0.34 V58 vs −0.56 V59), while no metallic indium accumulation, but even its I


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The Journal of Physical Chemistry C bonds, in agreement with theoretical considerations.61 Selfpassivation of the In−P bonds with the aqueous HCl solution under cathodic polarization is also in agreement with previously reported results.29 Under the bias of −1.8 V applied to the p-GaInP2(100)/1 M HClaq solution interface under illumination the oxidation of the surface takes place (Figure 7). Simultaneously the indium phosphate component of the In 3d spectrum, the gallium oxide component of the Ga 3d spectrum, and the corresponding indium phosphate components of the P 2p spectrum are increased (Figure 7). It should be noted that the formation of gallium phosphates can be ruled out since the corresponding component in the P 2p core level spectrum should have much larger chemical shift (around 5.5 eV35). A mechanism for the formation of indium phosphates at the interface between p-InP and aqueous HCl solution was proposed in ref 31. In accordance with this mechanism, injection of holes into the semiconductor is needed for indium phosphate formation. However, under cathodic conditions the hole injection into the p-doped semiconductor is not possible. Therefore, the mechanism of phosphate formation under large cathodic bias on the illuminated p-GaInP2(100)/1 M HClaq solution interface is not clear yet. These observations clearly show the importance of the here presented experiments as a first step to elucidate the processes taking place at the semiconductor/ electrolyte interfaces during hydrogen evolution reaction. However, further and complementary investigations are necessary in the future to get a complete insight into the chemical reactions taking place at the solid/liquid boundary. Due to the interface oxide layer and related surface states, Fermi level pinning is assumed evidenced by the unchanged core level binding energy values and a persistent position of the valence band onset around 0.5 eV, as well as the missing photovoltage or onset of photocurrent at the identical ZCP values in dark and under illumination (Figure 2). The origin of possible defect states can hardly be deduced from the experiments presented here and will need further fundamental surface science experiments. However, as InP shows reasonable photoconversion efficiency also with oxide/hydroxide/phosphate surface oxidation layers,64,65 the defect states that cause the Fermi level pinning can be assigned to the Ga3+-related surface defects as Ga2O3/Ga(OH)3.66 On the other hand, lower gallium suboxides create no states within the band gap.66,67 In addition, it was found that the density of Ga−O bonds correlates with the density of the near-midgap states at the In0.53Ga0.47As surface, as opposite to In−In and In−O bonds.68 It is clear from these data that InP, GaInP2, or related semiconductors with larger band gap expected to be of interest for photoelectrochemical water splitting applications need more sophisticated surface etching/passivation treatments (deposition of dielectric and chemical passivation layers40,69−73) in order to shift the photocurrent onset to more positive potentials for their possible use in efficient photoelectrochemical devices.

that the 1 M aqueous HCl solution reduces essentially the amount of oxides left on the surface, though it cannot remove them completely. Second, the p-GaInP2(100) surface at the interface with 1 M aqueous HCl solution is covered mainly with indium chlorides, hydroxides, and phosphates, as well as with gallium chlorides and (hydro)oxides. On application of the cathodic bias in the dark to the p-GaInP2(100)/1 M HClaq solution interface the electrochemical processes are started at the bias of about −1.0 V vs RHE, where cathodic current passing through the semiconductor/electrolyte interface begins to rise. Under higher cathodic bias applied in the dark the hydroxyl groups and metallic gallium are accumulated at the surface, while no evidence for the metallic indium accumulation has been observed. As the aqueous HCl solution contains no hydroxyl groups intrinsically, the accumulation of OH groups can be related only to splitting of water molecules at the semiconductor/electrolyte interface. These processes are accelerated under illumination with visible light, which clearly illustrates the participation of photogenerated electrons in the water splitting. The formation of the metallic gallium without simultaneous metallic indium formation testifies that the In−P bonds of the GaInP2 compound are more stable against corrosion than the Ga−P bonds. Such corrosion can be caused by release of the phosphine from the surface. The variation of the composition of the p-GaInP2(100) electrode under different (photo)electrochemical conditions causes changes in the surface electronic structure. Compared to ZCP values, the work function and ionization energy values decrease when accumulation of the metallic gallium and hydroxyl groups is observed. However, at large cathodic bias under light illumination, where surface oxidation takes place in addition, the work function and the ionization energy of the semiconductor essentially increase by 0.5 eV. The behavior of GaInP2 is dominated by Fermi level pinning probably due to surface defects related to gallium oxide/hydroxide species. Therefore, a possible use of such Ga-containing wide band gap semiconductors for water splitting needs more sophisticated etching/passivation techniques to produce efficient buried junction photoelectrochemical cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01343. Variation of current passing through the semiconductor/ electrolyte interface during the exposure of the p-GaInP2 sample to 1 M HClaq solution under bias applied without or with illumination (Figure S1). Surface-sensitive Cl 2p core-level spectra of p-GaInP2(100) surface emersed from 1 M HClaq solution after exposure under ZCP and after cathodic polarization at −0.7 and −1.8 V vs RHE in dark conditions (Figure S2a) and under visible light illumination (Figure S2b) measured with the excitation energy of 250 eV. Example of the fitting of the Cl 2p core-level spectrum of p-GaInP2(100) surface emersed from 1 M HClaq solution after exposure under ZCP (Figure S2c). Secondary electron cutoff region for the initial GaInP2 surface covered with native oxide, surface etched with 1 M HCl aqueous solution for 5 min, and the same surface exposed to 1 M HCl aqueous solution under ZCP for another 30 min (Figure S3). Secondary electron cutoff region for the GaInP2 emersed from 1 M

5. CONCLUSION High-resolution surface sensitive synchrotron-radiation photoemission spectroscopy was used for the detailed investigations of the (photo)electrochemical processes occurring at the interface between p-GaInP2(100) and 1 M HClaq solution at different cathodic bias voltages. With this method many reaction products could be identified, which were formed during the (photo)electrochemical processes. First, it was found J


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HClaq solution after exposure under ZCP and after cathodic polarization at −0.7 and −1.8 V vs RHE in dark conditions and under illumination with visible light (Figure S4). Valence band spectra measured with the excitation energy of 90 eV on the GaInP2 surfaces emersed from 1 M HClaq solution after exposure under ZCP and after cathodic polarization at −0.7 and −1.8 V vs RHE in dark conditions and under illumination with visible light (Figure S5). (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]ffe.ru. ORCID

Mikhail V. Lebedev: 0000-0002-0249-5161 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the German science foundation (DFG, JA 859/30-1) and by the Russian Foundation for Basic Research (Project No. 14-02-91342). Furthermore, partial support by the DFG Excellency graduate school for “Energy Science and Engineering” (GSC 1070) is gratefully acknowledged by B.K. and W.J.

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