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Photoelectrochemical Properties of GaN Photoanodes with Cobalt Phosphate Catalyst for Solar Water Splitting in Neutral Electrolyte Jumpei Kamimura, Peter Bogdanoff, Fatwa Abdi, Jonas Lähnemann, Roel van de Krol, Henning Riechert, and Lutz Geelhaar J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on June 1, 2017
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The Journal of Physical Chemistry
Photoelectrochemical Properties of GaN Photoanodes with Cobalt Phosphate Catalyst for Solar Water Splitting in Neutral Electrolyte Jumpei Kamimura,*,† Peter Bogdanoff,‡ Fatwa F. Abdi,‡ Jonas Lähnemann,† Roel van de Krol,‡ Henning Riechert,† and Lutz Geelhaar† †
Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany
‡
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Hahn-
Meitner-Platz 1, 14109 Berlin, Germany
Abstract Cyclic voltammetry measurements are carried out in neutral phosphate buffered electrolyte using n-type Ga-polar GaN thin film photoelectrodes with and without cobalt phosphate (Co-Pi) modification. Without Co-Pi, the variation of the photocurrent with the bias potential exhibits a two-step behavior, and under chopped illumination spikes occur at low bias potential. Thus, in this regime surface recombination is dominant. Co-Pi modification suppresses surface recombination and significantly increases the photocurrent, especially for low bias potentials. At the same time, stability tests reveal that Co-Pi does not protect GaN against photocorrosion. Experiments using H2O2 imply that this photocorrosion is a reductive process, and probably related to the presence of charged surface defects.
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INTRODUCTION Photoelectrochemical (PEC) water splitting is one of the most interesting methods to produce hydrogen and oxygen using sunlight without emitting CO2. Research in this field was triggered by the first demonstration using TiO2 photoelectrodes.1 Ever since, many researchers have tried to explore stable materials that enable highly efficient solar water splitting. GaN, which has been developed for blue light emitting diodes and lasers,2,3 is one of the promising candidates. The bandgap can be tuned from the ultraviolet to the near infrared region by alloying with In, which allows to cover the entire solar spectrum. In addition, the conduction and valence band edges have been calculated to straddle the H+/H2 and O2/H2O redox potentials for In content up to 50%.4 The PEC properties of GaN have been investigated in several electrolytes.5-13 Among them, mild pH conditions are interesting from an ecological point of view,9-11 but as far as we know all the corresponding studies employed NaCl as an electrolyte. However, this choice of electrolyte may be problematic since the oxidation of Cl- can compete with the oxidation of water.11-13 This competition may limit the water splitting efficiency and hinder a detailed analysis of the oxygen evolution reaction (OER) on GaN photoanodes. In this work, we study the charge carrier kinetics of the OER on GaN films in phosphate buffered electrolyte with pH≈7. Furthermore, we systematically investigate how the OER is affected by the deposition of a catalyst on GaN. In particular, we employ the earth-abundant material cobalt phosphate (Co-Pi) as catalyst,14 which has widely been applied to enhance the performance of oxide photoanodes such as Fe2O3,15 WO3,16 BiVO4,17 TiO2,18 under neutral pH conditions. Our study is of general relevance since GaN is single crystalline and can be used as a model system for water-splitting photoelectrodes.19
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The Journal of Physical Chemistry
EXPERIMENTAL SECTION We used nominally undoped n-type Ga-polar GaN templates grown by hydride vapour phase epitaxy (≈5 µm GaN on AlN/sapphire) purchased from Kyma Technologies Inc. The net ionized donor density of the GaN templates was determined to be 5×1017 cm-3 by Mott-Schottky measurements.20 Cyclic voltammetry measurements were performed with a scan rate of 30 mV/s using a standard three-electrode PEC cell with a platinum wire as the counter electrode, a Ag/AgCl electrode as the reference electrode, a neutral solution of 0.1 M KPi (pH≈7) as an electrolyte, and a Xe lamp (intensity ≈100 mW/cm2) as a light source. In other experiments, H2O2 was added in the electrolyte. In both cases, a circular area with a diameter of 6 mm on the GaN surface was exposed to the electrolyte and illuminated. Potentials vs. the Ag/AgCl electrode were converted to the reversible hydrogen electrode (RHE) scale by the equation VRHE = VAg/AgCl + 197 mV + (59 mV×pH), based on the fact that GaN follows the Nernstian pH response.8,21 For the electrical contact, a Ti/Al/Ti layer was deposited by electron beam evaporation on part of the GaN surface, and this contact was not exposed to the electrolyte. For the modification of GaN with Co-Pi, we followed the photoelectrodeposition procedure developed by Nocera et al.14 We used a 0.1 M KPi + 0.5 mM Co(NO3)2 electrolyte solution and fabricated a series of samples in which the amount of charge passed between the electrodes at 0.2 VRHE was varied to 6, 18, 54, and 162 mC/cm2. Scanning electron microscopy (SEM) revealed for the sample with the largest deposited amount of Co-Pi that the GaN film was covered by an almost continuous layer of Co-Pi with a thickness of around 18 nm (see Figure 1). We note that the existence of cobalt and phosphorus in the sample was confirmed by energy dispersive X-ray (EDX) spectroscopy, consistent with previous reports on Co-Pi catalysts (data shown in Supporting Information, Figure S1).14,18 All other Co-Pi film thicknesses were deduced
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by linear interpolation; the charge densities of 6, 18, and 54 mC/cm2 correspond thus to the thicknesses of ≈0.7, 2, and 6 nm, respectively.
Figure 1. Cross-sectional (a) and top-view (b) SEM images of Co-Pi modified GaN. The amount of deposited Co-Pi corresponds to a passed charge of 162 mC/cm2. RESULTS AND DISCUSSION First, we analyzed the GaN film as a reference without any catalyst. Figure 2(a) shows the corresponding current-voltage (J-V) curves acquired in the dark, under illumination, and under chopped illumination. The dark current is negligible. Under illumination, the J-V curve shows a pronounced two-plateau behavior with the transition occurring at ~0.7 VRHE. This behavior has not been reported before. Under chopped light conditions, transient photocurrent spikes are observed at low bias potential (
