Band Alignment at n-GaN/Electrolyte Interface Explored by Photo-Induced Offset of Open-Circuit Potential for Efficient Water Splitting
Authors: Imazeki Yuuki , Iwai Yohei , Nakamura Akihiro , Koike Kayo , Sato Shin-ichiro , Ohshima Takeshi , Fujii Katsushi , Sugiyama Masakazu , Nakano Yoshiaki 
Published: February 2020

For semiconductor photoelectrochemistry to split water upon sunlight irradiation, alignment of semiconductor band edges with respect to redox potentials for the evolution of hydrogen and oxygen from water is of crucial importance. Two essential factors are involved in the alignment: 1) the position of semiconductor band edges with respect to the standard electrode potential and 2) the band bending inside the depletion region of the semiconductor adjacent to an electrolyte. Both of these are related to the atomistic nature at the semiconductor/electrolyte interface: the former is affected by the electrical double layer at the interface and the latter is determined by the energy level of mid-gap states originating from the chemical bonds of the semiconductor exposed to the electrolyte. These may be affected by a lot of factors such as pH of the electrolyte, an oxidized layer and atomistic disorder on the semiconductor surface, and crystal defects inside the semiconductor. Due to such complexity, it is not straightforward to characterize the band alignment at the interface.
We here propose a method for clarifying such band alignment at the semiconductor/electrolyte interface. For a n-type GaN electrode, open-circuit potential (OCP) moves to the negative direction with increased light intensity incident on its surface. For an ideal situation like a Schottky junction between a metal and a semiconductor, the relationship between OCP and the logarithm of light intensity will follow a linear relationship owing to the accumulation of light-induced carriers inside the semiconductor and the reduction of band bending, approaching a situation with nearly flat band under a sufficiently-large light intensity. For a n-type GaN electrode in a pH=14 electrolyte with NaOH, a similar behavior was observed: OCP with respect to a Ag/AgCl reference electrode moved to the negative direction with a logarithmic increase in the intensity (from 10-5 to 102 mW/cm2) of a Xe lamp irradiated on the GaN surface. For a n-type GaN electrode with its surface reduced in an electrolyte by cathodic current, OCP stayed at approximately 1.0 eV below the flat-band potential (FBP) under a light intensity range from 10-5 to 10-3 mW/cm2. Above this intensity range, OCP increased drastically and exhibited an almost linear behavior with the logarithm of light intensity from 0.1 mW/cm2 until it reached FBP with 102 mW/cm2. Here, the FBP value of -1.4 V was measured by Mott-Schottky plot under dark in a different experiment. Such a behavior would indicate that the native defect states in n-GaN existed at 1.0 eV below the conduction band edge and these states pinned the quasi-Fermi level (qFL) under the light intensity of 10-3 mW/cm2.
We have extended the analysis for n-GaN electrodes with a variety of surface modification: anodic oxidation of the surface in an electrolyte, irradiation with Ar atomic beam and high energy electron beam. Such treatments changed the value of OCP under low light intensity, indicating that such surface modification created surface states depending on the treatment and those states pinned qFL until the states are filled with a sufficient amount of light-induced carriers, i.e., unpinning of qFL. Above a threshold light intensity which was dependent on the surface modification, OCP started to approach to FBP with an almost linear manner with the logarithm of light intensity. The tendency suggests that OCP of n-GaN was strongly affected by the surface states which was dependent on the method of surface modification, and the intense light irradiation anyhow make the band alignment close to flat-band.
When the surface of n-GaN was covered by a continuous layer such as transparent conducting oxide (TCO), the saturation value of OCP under a large light intensity was quite apart from the FBP for a naked n-GaN electrode, indicating that the TCO modified the band alignment at the n-GaN/electrolyte interface completely by both the electrical double layer on the TCO surface and the band offset between TCO and n-GaN.
Such behavior is not only of interest from a scientific point viewpoint but also a suggestion that we can modify the alignment between the semiconductor band edges and redox potentials in order to facilitate an photoelectrochemical reaction with an appropriate surface modification on the semiconductor surface.