Photoelectrochemical Behavior of n-Type GaAs(100) Electrodes

Mar 10, 2016 - Small 2017 13 (21), 1603574. III?V Semiconductor Photoelectrodes. Georges Siddiqi , Zhenhua Pan , Shu Hu. 2017,81-138. Related Content:...
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Photoelectrochemical Behavior of n‑Type GaAs(100) Electrodes Coated by a Single Layer of Graphene Fan Yang, Adam C. Nielander, Ronald L. Grimm,† and Nathan S. Lewis* Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States S Supporting Information *

ABSTRACT: The photoelectrochemical behavior of n-type GaAs(100) electrodes coated with a single layer of graphene was compared with the behavior of bare, freshly etched n-type GaAs(100) electrodes, both for electrodes in contact with an aqueous solution containing K3[Fe(CN)6]/K4[Fe(CN)6] and for electrodes in contact with nonaqueous solutions containing a series of one-electron redox couples selected such that the Nernstian solution potentials spanned a range greater than 1 V. Under simulated 1 Sun illumination, the graphene-coated electrodes produced a short-circuit photocurrent density of 20 mA cm−2 for up to 8 h of continuous operation in nonaqueous electrolytes (H2O concentration 0.1%, v/v), while, under the same conditions, the unprotected n-GaAs electrodes showed a rapid decay of the photocurrent density within ∼400 s. Although the graphene monolayers enhanced the stability of n-GaAs photoanodes in nonaqueous electrolytes, the graphene did not fully protect photoanodes operated in contact with Fe(CN)63−/4−(aq) from corrosion. The dependence of the open-circuit voltage measured for graphene-coated n-GaAs photoanodes on the Nernstian potential of the solution was effectively identical to that of freshly etched n-GaAs photoanodes, indicating that addition of the graphene layer did not introduce significant pinning of the Fermi level of GaAs beyond the Fermi-level pinning attributable to mid-gap and solution-derived charge-carrier trap states previously observed at GaAs/liquid junctions.

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demonstrated the ability to protect illuminated np+-GaAs16 and GaAs/InGaP tandem devices17 for tens of hours of operation as oxygen-evolving photoanodes in water-splitting devices that use 1 M KOH(aq) as an electrolyte. Graphene, a two-dimensional film that is made of a single layer of sp2-bonded carbon atoms, is optically transparent18 and electrically conductive.19 Graphene can be readily prepared on large (∼0.6 m2) scales20 and has a density of states near its Fermi level that is low relative to other conductive materials.21 These optical and electronic properties make graphene a nearly ideal candidate for use as a general protection layer on semiconductor photoelectrodes. Graphene has been shown to attenuate the oxidation of copper in air as well as in aqueous solutions.22,23 Graphene has also enhanced the stability of silicon in contact with Fe(CN)63−/4−(aq) while yielding desirable photoelectrochemical performance.24 We report herein the behavior of n-GaAs photoanodes covered with a single layer of graphene. The photoelectrochemical behavior of graphene-covered n-GaAs (nGaAs/Gr) photoanodes has been evaluated in contact with a series of one-electron, outer-sphere redox species in CH3CN, ranging from ferrocenium/ferrocene (Fc+/0) to cobaltocenium/ cobaltocene (CoCp2+/0), to assess the fundamental energetic

INTRODUCTION Gallium arsenide (GaAs) is an important semiconducting material for high-efficiency solar cells. However, when operated in contact with an aqueous solution in a photoelectrochemical cell, GaAs undergoes rapid photocorrosion processes that result either in formation of an insulating oxide in near-neutral pH electrolytes (pH ∼ 4−10) or in dissolution of GaAs electrodes in acidic or alkaline electrolytes.1 Multiple strategies have been developed to increase the stability of GaAs photoanodes in aqueous solutions. Thin noble metals such as Pt, Au, and Rh have been applied on GaAs, and although these metals improve the anodic stability of GaAs, they also form a Schottky barrier that pins the Fermi level, leading to nonoptimal photovoltages.2−5 Polymer films such as polypyrrole,6,7 polystyrene,8 and polythiophene9,10 have been coated on GaAs, but the stability of these systems is limited by peeling of the film, and the unusually high photovoltages exhibited by such electrodes are indicative of active photocorrosion. Another method that has been used to stabilize the n-GaAs/H2O interface in regenerative photoelectrochemical cells involves enhancing kinetic competition for photogenerated holes by using electrolytes containing redox couples, such as Se − / Se2−(aq)11−13 or I−/I3−(aq),14,15 at high concentrations, and by adding metal ions that chemisorb to the electrode to increase the rate of transport of photogenerated holes away from the GaAs surface. Furthermore, thick (∼60−120 nm) amorphous TiO2 films grown by atomic-layer deposition have © XXXX American Chemical Society

Received: January 8, 2016 Revised: March 8, 2016

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from the intervening space between the graphene and the GaAs. The PMMA/graphene/GaAs stack was then heated on a hot plate at 80 °C for 10 min, followed by immersion for 30 min in an acetone bath to remove the PMMA layer. The resulting GaAs/Graphene (GaAs/Gr) stack was annealed for ∼12 h at 300 °C under forming gas (95:5 v:v N2:H2). Electrodes were fabricated as described previously,26 with typical exposed electrode areas of 0.02−0.1 cm2 for electrochemical measurements and 0.8−1.2 cm2 for Mott−Schottky measurements. All areas were determined by digitizing magnified scanned images of the electrodes. The GaAs/Gr electrodes were kept in a N2(g)-purged glass desiccator for the first 6 h to allow the epoxy to cure, and the electrodes were then maintained under vacuum overnight before being introduced into a glovebox for use in nonaqueous electrochemical measurements. Instrumentation. A standard three-electrode potentiostatic setup with either a Princeton Applied Research Model 2273 or a Gamry Reference 600 potentiostat was used for all photoelectrochemical measurements, which included a Pt wire quasi-reference electrode (0.5 mm dia., 99.99% trace metals basis, Sigma-Aldrich) and a Pt mesh counter electrode (100 mesh, 99.9% trace metals basis, Sigma-Aldrich). The open-circuit voltage, Voc, was determined using the forward scan (from negative to positive bias) and was defined as the potential at which no current was observed. Voc values extracted using this method were checked against values obtained by directly measuring the voltage at open circuit, and the two methods consistently yielded values within 10 mV of each other. For each redox couple, the Nernstian potential E(A/A−) and formal potential Eo′(A/A−) were determined by cyclic voltammetry of the Pt wire electrode. The measured potential was then calibrated relative to the formal potential of ferrocene+/0, Eo′(Fc+/0), by adding ferrocene+/0 to the solution. The effective solution potentials (Figure S1), Eeff(A/A−), were converted relative to the potential of a saturated calomel electrode, SCE, as adjusted by the experimentally determined conversion, Eo′(Fc+/0) = +0.311 V vs SCE for ferrocene+/0 in CH3CN−1.0 M LiClO4, with details described in the Supporting Information. The aqueous solution (K3[Fe(CN)6], 50 mM; K4[Fe(CN)6], 350 mM) contained 1 M Na2SO4 as supporting electrolyte and had a pH of 6.7. The solution was rapidly stirred with a small, Teflon-covered stirring bar. Illumination was provided from an ELH-type tungsten-halogen lamp. The light intensity was adjusted to 1 Sun, as determined through the concurrent use of a Si photodiode (Thor Laboratories) that was calibrated relative to a secondary standard photodetector that was NIST-traceable and calibrated at 100 mW cm−2 of AM1.5 illumination. Nonaqueous electrochemistry was performed in an Ar(g)-filled glovebox. Aqueous electrochemistry was performed in air. Spectral response measurements were performed on the electrode in a photoelectrochemical cell that contained an electrolyte consisting of 0.5 mM Fc+, 100 mM Fc0, and 1.0 M LiClO4 in CH3CN. Light from a 150 W Xe lamp (Oriel) was passed through a monochromator to generate wavelengths between 400 and 1000 nm and was then chopped at a frequency of 13 Hz. The intensity of the light was determined using a calibrated Si photodiode that was placed at the same position as the working electrode. A Gamry Reference 600 potentiostat was used to measure the photocurrent at zero bias voltage. A lock-in amplifier (EG&G Princeton Applied Research) was used to collect signals from both the reference

properties of the n-GaAs/Gr/liquid interface. Additionally, the n-GaAs/Gr photoelectrodes have been characterized in contact with aqueous solutions of Fe(CN)63−/4−, to assess the ability of a monolayer of graphene to suppress corrosion processes while n-GaAs is operated under photoanodic conditions.

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EXPERIMENTAL METHODS Materials and Chemicals. Si-doped, n-type, singlecrystalline (100)-oriented GaAs wafers that had been grown by the vertical gradient-freeze method were purchased from MTI Corporation. The 5.08 cm diameter × 0.5 mm thick wafers were polished on one side, had a carrier concentration of (4.2−6.7) × 1016 cm−3, and a resistivity of (2.0−4.0) × 10−3 Ω·cm. Ohmic contacts were prepared by thermal evaporation of a 100 nm layer of 12% Ge and 88% Au, followed by annealing in forming gas at 475 °C for 5 min. Water with a resistivity ≥ 18.0 M Ω·cm was obtained from a Barnstead Nanopure system. Copper Etch Type CE − 100 (FeCl3-based, Transene Company, Inc., Danvers, MA), acetone (HPLC grade, Sigma-Aldrich), and hydrochloric acid (ACS reagent, Sigma-Aldrich) were used as received. Acetonitrile (99.8% anhydrous, Sigma-Aldrich) was dried over Al2O3 and stored over activated 3 Å sieves. The dry acetonitrile was used for nonaqueous electrochemical experiments other than stability tests. Since the stability of GaAs in CH3CN depends strongly on the water content, water was added to the CH3CN to produce a controlled H2O concentration of 0.1% by volume for the electrochemical stability measurements. Petri dishes (Falcon Optilux) were cleaned with water 3 times before use. All other chemicals were used as received unless otherwise noted. Graphene Growth by Chemical Vapor Deposition. A monolayer film of graphene was grown by low-pressure chemical vapor deposition (CVD) following the same procedure as reported previously.25 The graphene was grown on a 25 μm thick Cu foil (99.999%, Alfa Aesar) that was first annealed at 1030 °C under a 2 sccm flow of H2(g) for 15 min in a tube furnace. Graphene was subsequently synthesized on the surface of the Cu foil by flowing CH4(g) (1 sccm) and H2(g) (2 sccm) for 90 min at 1000 °C and 12 mTorr over the substrate. After growth of the graphene, the gas flow rates and chamber pressure were maintained, and the Cu foil was rapidly cooled to room temperature by using an external fan to blow air onto the tube furnace. Two applications of PMMA (495 K A4 poly(methyl methacrylate), MicroChem) were spin-coated on the Cu/graphene surface at 2000 rpm (500 rpm/s acceleration) for 60 s, followed by a 10 min bake at 185 °C. Electrode Fabrication. The Cu was removed by etching the sample for 30 min with a 40% FeCl3/1% HCl(aq) solution (Transene). To remove any etchant residue, the resultant PMMA/graphene stack was transferred consecutively four times to 18 MΩ·cm resistivity water in a Petri dish. The transfer was executed using a freshly piranha-cleaned (3:1 v:v H2SO4:H2O2, 1 s) SiO2-coated Si wafer to collect the PMMA/ Gr stack from one bath and release the stack into a fresh water bath. The PMMA/Gr stack was then laid on a 0.01 M HCl solution in a Petri dish. n-type GaAs samples were washed consecutively with water, methanol, acetone, methanol, and water and were then submerged in a diluted hydrochloric acid (1:10 concentrated HCl:H2O) solution for 30 min. The cleaned PMMA/graphene stack was transferred from the 0.01 M HCl solution onto the cleaned, freshly etched GaAs(100) surface, and a gentle steam of N2(g) was used to remove water B

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Figure 1. (a) Current density versus potential (J−E) behavior for freshly etched n-GaAs (dotted red curve) and n-GaAs/Gr (green dashed-dotted curve) photoanodes in contact with a dry acetonitrile solution containing 5 mM Fc+, 100 mM Fc0, and 1.0 M LiClO4 while under 1.5AM illumination or in the dark (black and dark red dashed curves). The scan rate was 50 mV s−1. (b) Quantum yield versus wavelength of incident light for n-GaAs (black, filled circles) and n-GaAs/Gr (green, open circles) electrodes in contact with a dry acetonitrile solution containing 0.5 mM Fc+, 100 mM Fc0, and 1.0 M LiClO4.

graphene-transfer process and observed using XPS (Figure S4). The ideal regenerative cell energy-conversion efficiencies,28 ηIRC, of n-GaAs and n-GaAs/Gr photoanodes were 9.0 ± 0.7% and 5.32 ± 0.7%, respectively, with no correction for residual uncompensated resistance losses, concentration overpotentials, or optical reflection losses at the various interfaces in the device, and with at least 3 electrodes of each type tested. Figure 1b presents the spectral response data for n-GaAs and n-GaAs/Gr photoanodes in contact with a dry acetonitrile solution containing 0.5 mM Fc+, 100 mM Fc0, and 1.0 M LiClO4. The response exhibited a sharp absorption edge at 870 nm which corresponds to the GaAs band-gap energy, Eg = 1.42 eV. The decreases in external quantum yield at 650 and 450 nm are due to absorption by ferrocenium and ferrocene, respectively. The external quantum yield showed a limiting plateau value of ∼0.70 over most of the visible region. The optical reflection loss at the smooth GaAs electrode surfaces accounts for ∼30% of the incident photons at each wavelength. The predicted Jsc values were calculated by integration of the spectral response data of Figure 1b with respect to the known spectral irradiance properties of the ELH source. The value predicted under AM1.5 spectral illumination was 20.0 mA cm−2 for n-GaAs and 18.9 mA cm−2 for n-GaAs/Gr. The measured Jsc values of 20.0 ± 1.0 mA cm−2 for n-GaAs and 19.0 ± 1.0 mA cm−2 for n-GaAs/Gr under ELH-type illumination were in excellent agreement with these expectations. Figure 2 displays the area-corrected Mott−Schottky (A2Cdiff−2−E) behavior of n-GaAs (black circle) and n-GaAs/ Gr (green square) anodes in the dark and in contact with 5 mM Fc+, 100 mM Fc0, and 1.0 M LiClO4 in dry acetonitrile, CH3CN-Fc+/0. Three independent measurements of n-GaAs/ CH3CN-Fc+/0 junctions yielded an average flat-band potential, Vfb, of −1.073 ± 0.023 V, which was in excellent agreement with the value reported previously.29 The averaged slope returned a value for the doping density that was in the agreement with that specified by the manufacturer of the sample (calculated Nd = 6.6 × 1016 cm−3; specified Nd = (4.2− 6.7) × 10 16 cm −3 ). For the n-GaAs/Gr/CH 3 CN-Fc +/0 junctions, an average Vfb = −1.003 ± 0.025 V and dopant density Nd = 2.6 × 1017 cm−3 were obtained. The difference between Vfb for n-GaAs and Vfb for n-GaAs/Gr was 70 mV, which was in good agreement with the observed change in Voc

Si photodiode channel and the working electrode. No correction was made for any optical absorption or transmission losses in the cell, so the reported values are true external quantum yields for the system under study. Raman spectra of monolayer graphene on 300 nm SiO2 substrates were obtained with a Renishaw inVia Raman microprobe (Renishaw, Wotton Under Edge, United Kingdom). The single atomic layer structure of the graphene synthesized from CVD was confirmed by the G/2D = 1/2 ratio, as shown in Figure S2. The amplitude of the D peak indicates the density of defects in the graphene.27 Electrochemical impedance spectroscopy data were acquired with a Gamry Reference 600 potentiostat. A sinusoidal, 10 mVRMS AC signal between 10−1 and 106 Hz in frequency was superimposed on each DC potential. The DC potentials were stepped in a sequence from 0 to 0.5 V with a step of 0.05 V. All measurements were performed in stirred solutions in the absence of illumination. Larger electrodes (0.8−1.2 cm2) were used for impedance spectroscopy than for other electrochemical measurements (0.02−0.1 cm2), to reduce the magnitude of any edge effects due to the epoxy. Data were fit using the Randles circuit model (Figure S3), and the flat-band potential was calculated using the Mott−Schottky equation. More detailed data analysis is provided in the Supporting Information.

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RESULTS AND DISCUSSION Figure 1a depicts the current density versus potential (J−E) behavior in the presence and absence of AM1.5 illumination, respectively, for n-GaAs and n-GaAs/Gr electrodes in contact with a dry acetonitrile solution containing 5 mM Fc+, 100 mM Fc0, and 1.0 M LiClO4. The open-circuit photovoltage, Voc, was −750 ± 15 mV for n-GaAs surfaces and was −700 ± 25 mV for the n-GaAs/Gr electrodes. The short-circuit current density, Jsc, was 20.0 ± 1.0 mA cm−2 for n-GaAs surfaces and was 19.0 ± 1.0 mA cm−2 for the n-GaAs/Gr electrodes. The n-GaAs/Gr electrodes showed lower fill factors (f f) than the n-GaAs electrodes (0.40 ± 0.05 vs 0.60 ± 0.02), indicating the presence of a resistance at the n-GaAs/Gr/CH3CN contact that was not present for the n-GaAs/CH3CN contact. The additional resistance observed for graphene-coated electrodes was likely due to the presence of surface oxides formed during the C

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the wide range of solution potentials that was investigated. Partial Fermi-level pinning of GaAs/CH3CN contacts has been observed previously in cyclic voltammetric and/or impedance studies.30−36 The observation that GaAs electrodes can produce a photovoltage even at the most negative cell potential has been attributed to the pinning of the Fermi level by surface states. Even when a single layer of graphene was inserted between the GaAs and CH3CN, the n-GaAs/Gr contact exhibited exactly the same effects of partial Fermi-level pinning as the n-GaAs/ CH3CN contacts (Figure 3). The observed Fermi-level pinning at the GaAs/Gr/liquid interface was indistinguishable from that for the GaAs/liquid interface, showing no effects of the graphene layer on the interfacial energetics, and indicating that the principal sources of the pinning likely were the same for both the graphene-free and graphene-containing cases. Figure 4 displays the stability toward photopassivation of the illuminated n-GaAs/Gr surface relative to the n-GaAs surface in contact with the Fc+/0 redox couple in 1.0 M LiClO4/CH3CN containing a trace amount of water (0.1%, v/v). Trace amounts of water in the nonaqueous solvent induce the formation of an oxide layer on the GaAs that protects the semiconductor from further rapid corrosion but also prevents charge transfer between the photoelectrode and electrolyte.37 Both the nGaAs/Gr and the n-GaAs electrodes were illuminated by an AM1.5 ELH lamp while the short-circuit photocurrent was measured. Figure 4a shows that a stable photocurrent density of 20 mA cm−2 was maintained by the n-GaAs/Gr electrode for >8 h, whereas, in contrast, the photocurrent density of the nGaAs electrode decayed from 20 to 2 mA cm−2 within ∼400 s, as shown in Figure 4b, indicating that the branching ratio for Faradaic charge transfer relative to photopassivation was ∼100fold larger for the n-GaAs/Gr electrode than for the n-GaAs electrode. Figure 5 presents the photoelectrochemical behavior of the n-GaAs/Gr and n-GaAs electrodes in contact with an aqueous solution containing 50 mM Fe(CN)63− and 350 mM Fe(CN)64−. Figure 5a shows 12 consecutive potential sweeps obtained under illumination for a freshly etched n-GaAs electrode as well as for an n-GaAs/Gr electrode. The freshly etched n-GaAs electrode (black) showed a very low photocurrent (∼2 mA cm−2) and Voc of ∼ −900 mV, which is a combination of a photovoltage and a corrosion voltage. In contrast, the n-GaAs/Gr electrode (green) showed Voc = −700 mV and Jsc = 25.8 mA cm−2 for the first J−E cycle. After each cyclic voltammogram (CV) was collected, the short-circuit current was monitored for 2 h before collection of the next CV. Therefore, the 12 CV scans shown in Figure 5a were collected over a period of 24 h. The photocurrent decreased on each successive cycle while |Voc| slightly increased over the same time period. Figure 5b shows Jsc (E = 0 vs E(A/A−)) measured as a function of time during the 12 consecutive 2 h intervals between each J−E cycle shown in Figure 5a. The n-GaAs electrode (black) exhibited Jsc = 2 mA cm−2 and did not change substantially over 24 h, indicating a predominant passivation process as expected in the near-neutral pH aqueous solution. Although the n-GaAs/Gr electrode (green) showed an initial Jsc = 25.8 mA cm−2, Jsc rapidly decreased to 12.1 mA cm−2 after 4 h then slowly decreased further, to 3 mA cm−2, after 24 h of continuous operation. This behavior is consistent with the hypothesis that, although graphene protects n-GaAs from corrosion in aqueous solution, corrosion at the defects and grain boundaries (Figure S5) leads to a decrease of photocurrent indicative of a predominant surface passivation process.

Figure 2. Area-corrected Mott−Schottky (A2Cdiff−2−E) plots of the inverse of the differential capacitance of the electrode vs the Nernstian potential of the solution (E(A/A−)) for the n-GaAs (black circles) and n-GaAs/Gr (green squares) in contact with CH3CN-Fc+/0 in the dark.

from −750 mV for n-GaAs to −700 mV for n-GaAs/Gr (Figure 1a). The apparent increase of dopant density for n-GaAs/Gr relative to n-GaAs can be attributed to the Randles circuit model, which does not account for potential drops across the graphene and Helmholtz layers and, therefore, leads to an overestimated dopant density (see the Supporting Information). Figure 3 presents the dependence of Voc on the effective solution potential Eeff(A/A−), for n-GaAs (black, open circles)

Figure 3. Open-circuit voltage, Voc, vs the effective solution redox potential, Eeff(A/A−), in 1.0 M LiClO4/CH3CN for n-GaAs (black, open circles) and n-GaAs/Gr (green, filled circles). The corresponding lines serve as guides to indicate the observed trends in the different regions of Voc, vs Eeff(A/A−).

and n-GaAs/Gr (green, filled circles) in contact with dry acetonitrile containing 1.0 M LiClO4 (1.0 M LiClO4/CH3CN, and also annotated simply as CH3CN where junctions or contacts are being described) and a series of one-electron redox couples under AM1.5 illumination. Each point contains measurement results from at least 2 different electrodes with average values and standard deviations. For n-GaAs/CH3CN contacts, the open-circuit voltage changed only by ∼300 mV, in spite of a change of over 1.0 V in Eeff(A/A−) between the nGaAs/CH3CN−Co(Cp)2+/0 and the n-GaAs/CH3CN−Fc+/0 contacts. For ideal semiconductor/liquid junctions, Voc should change linearly with Eeff(A/A−) with a slope of unity. Instead, as displayed in Figure 3, a much weaker dependence of Voc was observed as Eeff(A/A−) was varied. These data clearly indicate that partial Fermi-level pinning is present in this system over D

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Figure 4. (a) Comparison of the J−t behavior of potentiostatically controlled n-GaAs (black) and n-GaAs/Gr (green) electrodes (E = 0 V vs solution) in contact with a CH3CN-Fc+/0 system containing a trace amount of water (H2O concentration 0.1%, v/v) under AM1.5 illumination. (b) J−t data shown in (a) on an expanded abscissa.

Figure 5. Stability test of n-GaAs and n-GaAs/Gr in contact with Fe(CN)63−/4−(aq) under AM1.5 illumination. (a) J−E behavior (12 cycles at 50 mV s−1) of the n-GaAs (black) and n-GaAs/Gr (green) electrodes in Fe(CN)63−/4−(aq) under AM1.5 illumination. Between each of the 12 cycles, the short-circuit current density was monitored potentiostatically for 2 h in between each of the 12 cycles; thus the J−E data were obtained over a period of 24 h. (b) J−t behavior of n-GaAs (black) and n-GaAs/Gr (green) electrodes in Fe(CN)63−/4−(aq) under AM1.5 illumination over 24 h (E = 0 V vs the solution), as collected in the intervals between the J−E scans shown in (a).

However, the eventual increase in |Voc| to >800 mV and continued observation of 3 mA cm−2 of current toward the end of the experiment are consistent with incomplete surface passivation of the n-GaAs/Gr that allows for some continued corrosion of the electrode.

graphene-free and the graphene-containing cases. Further studies of the protection of graphene-coated p-GaAs and other III−V and II−IV semiconductors will advance the understanding of the extent of the graphene-imparted stability. The influence of chemical doping of graphene on the photoelectrochemical behavior of such systems is also currently under investigation.

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CONCLUSIONS We have demonstrated that monolayer graphene substantially increases the stability of GaAs toward passivation in nonaqueous solutions containing a trace amount of water. However, graphene provides only limited protection against corrosion to GaAs photoanodes operated in contact with aqueous solutions. Whereas freshly etched n-GaAs photoanodes immediately showed effects of corrosion when operated in aqueous solutions and did not produce current densities > 2 mA cm−2, introduction of the graphene layer allowed initial operation at a short-circuit current density of 25.8 mA cm−2 with a steady decrease in current density over several hours to