Direct photoelectrolysis of water to generate hydrogen was performed using n-type GaN films with Cr/Au ohmic contacts to serve as working electrodes. To enhance the efficiency of electron collection in the GaN working electrode, meshed Cr/Au contacts with a SiO 2 protection layer were immersed in the NaCl electrolyte. With an external bias of 1 V, the typical photocurrent densities ͑gas generation rate͒ of the n-GaN working electrodes with and without the immersed ohmic contact layer were approximately 19.6 ͑9.4͒ and 9.9 A/cm 2 ͑3.6 mL/h͒, respectively, which corresponded to an enhancement in the photocurrent density ͑gas generation rate͒ of around 98% ͑160%͒. The marked enhancement in the gas generation rate could be attributed to the fact that the distance between the neighbor Cr/Au ohmic contacts is small enough to reduce the recombination probability of photogenerated electrons with holes or charged defects in the GaN layer before the electrons reach the ohmic contacts. In other words, the photogenerated electrons can be effectively collected by the Cr/Au ohmic contacts and thereby reach the Pt counter electrode to lead the generation of hydrogen.Using solar power to direct photoelectrolysis is a promising method of generating hydrogen from aqueous water. 1,2 There are many kinds of oxides serving as the photoelectrodes ͑working electrodes͒ that drive electrochemical reactions. 3-5 However, using these oxides as the working electrodes for water splitting has an insufficient solar-to-hydrogen conversion efficiency for practical application because of the limitation of the usable solar spectrum. 1 There are also many kinds of lower bandgap semiconductor materials that can absorb solar light more effectively, such as InP, GaAs, and CdSe. It has been reported, however, that these materials were easily corroded in acidic or alkaline solution. 6,7 Besides, to split aqueous water by using a photoelectrochemical cell, which is made of semiconductor materials, the conduction bandedge potential of the semiconductor material must be lower than that of the cathode-reduction half-reaction; furthermore, its valence band-edge potential must be higher than that of the anode-oxidation half-reaction. 1 Therefore, InGaN-based materials are promising candidates for direct photoelectrolysis of water not only because its band-edge potential could satisfy the condition to split water 8,9 but also because it is potentially resistant to the aqueous solutions. 10 Furthermore, the bandgap can vary from 3.4 to 0.7 eV by changing the content of In, making it possible for us to produce a photoelectrochemical cell that could fit the solar spectrum to enhance the light absorption. 10 Waki et al. reported that the patterned n-GaN epitaxial layer formed by the selective area regrowth with metal stripes was used to achieve the direct photoelectrolysis of water. 8 The metal stripes functioned to eliminate current crowding, thereby improving the photocurrent. However, the metal stripes they used were directly immersed in the electrolyte. The met...

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