Charge transfer of n-type GaN photoelectrolysis in HCl solution for H 2 gas generation at a counterelectrode
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Charge transfer of n-type GaN photoelectrolysis in HCl solution for H2 gas generation at a counterelectrode Katsushi Fujii 1, Masato Ono2, Takashi Ito2, Kazuhiro Ohkawa 1,2 1 Nakamura Inhomogeneous Crystal Project, Exploratory Research for Advanced Technology, Japan Science and Technology Agency, 1-3 Kagurazaka, Shinjyuku, Tokyo 162-8601, JAPAN 2 Department of Applied Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjyuku, Tokyo 162-8601, JAPAN ABSTRACT In order to clarify the charge transfer characteristics for H2 generation, photoelectrochemical properties of n-type GaN in HCl solution were investigated. The flatband potential under illumination and the onset voltages of photocurrent located approximately the same position. From the result, we concluded that the positively charged surface by hole capture is the main reason of the extra voltage requirement for H2 generation. The carrier concentration in n-type GaN also affects the photocurrent. INTRODUCTION Photoelectrolysis using a semiconductor by solar power is a promising method of producing H2 gas from aqueous solution [1]. Many III-V and II-VI semiconductors are expected to be effective materials for photoelectrolysis [2]. However, many of them are easily undergo oxidation and corrosion. Dissolution of a semiconductor electrode is a serious problem in view of stable photoelectrolysis. GaN shows a considerable resistance to corrosion in aqueous solutions [3-6]. Photoelectrochemical properties of nitride semiconductors have been investigated [7-10]. We have succeeded in the first H2 generation from counterelectrode by photo-assisted electrolysis using GaN and KOH aqueous solution [7]. The n-type GaN electrode, however, dissolves in KOH electrolyte under illumination. HCl solution is promising for H2 gas generation. The electrochemical reactions in moderately concentrated HCl solution were H2O reduction and Cl- oxidation instead of H2O electrolysis [3,11]. Therefore, the photoanodic dissolution of n-type GaN was reported to compete for a very small percentage with the oxidation of Cl- [10,11]. In this paper, we report the photoelectrochemical properties of n-type GaN as a working photoelectrode in HCl solution, and discuss the improvement of H2 generation by photoelectrochemical reactions. EXPERIMENTAL DETAILS The working electrodes used were GaN layers on a (0001) sapphire substrate grown by
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metalorganic vapor-phase epitaxy. A Si-doped n-type GaN layer (1.5 to 4.0 µm, n = 4.0×1016 to 7.4×1018 cm-3 at room temperature) was grown followed by the growth of a low-temperature GaN buffer layer for each sample. A Ti (10nm) /Au (50nm) ohmic contact was formed on the surface at the periphery of the GaN samples. The annealing of the ohmic contact formed was performed by heating at 400 ºC for 10 min in N2 atmosphere. The aqueous contact area of the GaN working electrodes was identical with the illuminated area, and was 10 mm in diameter. The counterelectrode and reference electrode were Pt and Ag/AgCl/NaCl (sodium-chloride-saturate
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