Hydrogen generation through direct photoelectrolysis of water was studied using photoelectrochemical (PEC) cells made of Mn-doped GaN photoelectrodes. In addition to its absorption of the ultraviolet spectrum, Mn-doped GaN photoelectrodes could absorb photons in the visible spectrum. The photocurrents measured from PEC cells made of Mn-doped GaN were at least one order higher than those measured from PEC cells made of undoped GaN-working electrodes. Under the visible light illumination and a bias voltage below 1.2 V, the Mn-doped GaN photoelectrodes could drive the water splitting reaction for hydrogen generation. However, hydrogen generation could not be achieved under the same condition wherein undoped GaN photoelectrodes were used. According to the results of the spectral responses and transmission spectra obtained from the experimental photoelectrodes, the enhanced photocurrent in the Mn-doped GaN photoelectrodes, compared with the undoped GaN photoelectrodes, was attributable to the Mn-related intermediate band within the band gap of GaN that resulted in further photon absorption.

In this study, we demonstrated photoelectrochemical (PEC) hydrogen generation using p-GaN photoelectrodes associated with immersed finger-type indium tin oxide (IF-ITO) ohmic contacts. The IF-ITO/p-GaN photoelectrode scheme exhibits higher photocurrent and gas generation rate compared with p-GaN photoelectrodes without IF-ITO ohmic contacts. In addition, the critical external bias for detectable hydrogen generation can be effectively reduced by the use of IF-ITO ohmic contacts. This finding can be attributed to the greatly uniform distribution of the IF-ITO/p-GaN photoelectrode applied fields over the whole working area. As a result, the collection efficiency of photo-generated holes by electrode contacts is higher than that of p-GaN photoelectrodes without IF-ITO contacts. Microscopy revealed a tiny change on the p-GaN surfaces before and after hydrogen generation. In contrast, photoelectrodes composed of n-GaN have a short lifetime due to n-GaN corrosion during hydrogen generation. Findings of this study indicate that the ITO finger contacts on p-GaN layer is a potential candidate as photoelectrodes for PEC hydrogen generation.

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To reduce light reflection and enlarge the effective reaction surface area at the n-GaN/electrolyte interface, n-GaN epitaxial layers with naturally textured surface are utilized. The layers are combined with Cr/Au ohmic contacts on n-GaN to form working electrodes that generate hydrogen by direct photoelectrolysis of water. Although the surface reflection on the naturally textured n-GaN samples is lower than that on n-GaN epitaxial layers with flat surface, our results reveal that the photocurrent ͑I ph ͒ and gas generation rates ͑R gas ͒ obtained from the naturally rough n-GaN samples are lower than those from the flat samples. The results can be attributed to the fact that the rough n-GaN surface caused by dense surface pits leads to significant recombination of photogenerated carriers with charged defects; this occurs before carriers reach the ohmic contacts, thereby resulting in lower I ph and R gas. Related analyses have been performed and presented in this paper to initially explain the possible mechanism.

An n-GaN photoelectrochemical (PEC) cell with immersed finger-type indium tin oxide (ITO) ohmic contacts was demonstrated in the present study to enhance the hydrogen generation rate. The finger-type ITO ohmic contacts were covered with SiO₂ layers to prevent the PEC cell from generating leakage current. Using a 1M NaCl electrolyte and external biases, the typical photocurrent density and gas generation rate of the n-GaN working electrodes with ITO finger contacts were found to be higher than those with Cr/Au finger contacts. The enhancement in photocurrent density or gas generation rate can be attributed to the transparent ITO contacts which allowed the introduction of relatively more photons into the GaN layer. No significant corrosion was observed in the ITO layer after the PEC process compared with the Cr/Au finger contacts which were significantly peeled from the GaN layer. These results indicate that the use of n-GaN working electrodes with finger-type ITO ohmic contacts is a promising approach for PEC cells.