Photocatalytic water splitting takes place at the semiconductor/electrolyte interface. Although the reactions are strongly affected by the subtle changes in the interface structure, little is known about the interface from an atomistic point of view. In this study, we investigate the GaN(0001)/water interface structure by combining first-principles calculation and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In particular, the relationship between the geometric and electronic structure of the interface is revealed. First, the evolution of the GaN/water interface structure upon water adsorption is predicted from firstprinciples calculations. Computational results indicate that (1) at low coverage (below 3/4 monolayer), the Fermi level is pinned to the surface states originating from surface Ga atom dangling bonds, and water adsorbs dissociatively, forming oxygen atoms as well as hydroxyl groups, and (2) at higher coverage (above 3/4 monolayer), the Fermi level becomes free from the pinning, and adsorption of intact water becomes dominant. AP-XPS measurements were carried out for the water coverage ranging from submonolayer (low coverage) to several monolayers (high coverage). The core-level binding energies calculated from first-principles were used successfully to assign the adsorbate species to experimental O 1s peaks. Both the electronic and geometric structures predicted by the first-principles calculation explain well the experimental spectra obtained by the AP-XPS measurements. The results demonstrate that the combined spectroscopic and first-principles computational approach offers a detailed atomic level understanding of the solid/liquid interface structures.

The accumulation properties of photogenerated carriers at the semiconductor surface determine the performance of photoelectrodes. However, to the best of our knowledge, there are no computational studies that methodically examine the effect of “surface charging” on photocatalytic activities. In this work, the effect of excess carriers at the semiconductor surface on the geometric and electronic structures of the semiconductor/electrolyte interface is studied systematically with the aid of first-principles calculations. We found that the number of water molecules that can be dissociated follows the “extended” electron counting rule; the dissociation limit is smaller than that predicted by the standard electron counting rule (0.375 ML) by the number of excess holes at the interface. When the geometric structure of the GaN/water interface obeys the extended electron counting rule, the Ga-originated surface states are removed from the bandgap due to the excess holes and adsorbates, and correspondingly, the Fermi level becomes free from pinning. Clearly, the excess charge has a great impact on the interface structure and most likely on the chemical reactions. This study serves as a basis for further studies on the semiconductor/electrolyte interface under working conditions.

To improve the performance of semiconductor photoelectrodes for water splitting, the amount of band bending in the depletion layer of a semiconductor should be accurately ascertained, since it determines the splitting efficiency of photogenerated carriers. Band bending has been determined by X-ray photoelectron spectroscopy (XPS) from the valence band maximum (VBM), which has been calculated from the Ga 3d peak using the energy difference between VBM and Ga 3d (ΔE VBM‑3d). This work validates several values for ΔE VBM‑3d which have been reported previously, by analyzing the spectrum around the VBM and its distance from Ga 3d for the n-GaN(0001) surface under both ultrahigh vacuum (UHV) and ambient H2O. ΔE VBM‑3d is estimated to be between 17.36 and 17.55 eV. By adopting 17.5 eV as ΔE VBM‑3d, the amounts of band-bending were 0.5 eV under UHV and 0.1 eV under a relative humidity of 46%, respectively. For the latter condition, a surface photovoltage of 20 meV was observed upon Xe lamp irradiation, confirming the existence of band bending even with H2O adsorption on the surface. The origin of such band bending seems to be Fermi level pinning to the subsurface states which cannot be compensated by H2O.

The behavior of photoexcited carriers in a GaN photoanode was investigated by using photoluminescence (PL) in an electrolyte. The near-band-edge (NBE) emission observed in the electrolyte at around the flat-band potential (E FB ), −0.47 V RHE , was stronger than that observed in air because of the disappearance of band bending that existed for the GaN surface in air. A broad PL related to the surface states was observed between the NBE emission and yellow luminescence (YL) and decreased rapidly with an increase of potential above a significant onset of anodic photocurrent. This quenching reflects a deactivation of surface states through the suppressed diffusion of electrons to the surface because of the significant charge separation by band bending. The relative intensity of the YL increased above the significant onset of anodic photocurrent, indicating that the deep levels were partially filled with holes, and photoexcited holes were utilized for oxygen evolution reaction from water.

GaN is an excellent candidate for photocatalytic, optoelectronic, and high-power devices, and the interaction between the GaN surface and ambient species, especially H2O and O2, has drawn exceptional attention. In this study, the evolution of the n-GaN(0001) surface geometric structure and the corresponding band bending (a key parameter that describes the surface electronic structure of a semiconductor) during H2O and O2 exposure is predicted from first-principles calculations and confirmed by ambient pressure X-ray photoemission spectroscopy (AP-XPS) measurements. Overall, the AP-XPS results are in good agreement with the predictions, and we discuss the possible origin of the difference in the band bending of H2O- and O2-adsorbed surfaces. In the case of the O2-exposed surface, upward band bending is observed above the effective coverage of 3/4 ML (3/8 ML of O atoms) because the Fermi level becomes pinned to the N-2p-originated surface states, which is formed through Ga–N bond scission by O atom adsorption and insertion into the slab. As for the H2O-exposed surface, the saturated band bending depends on the H2O supply rate: When the supply rate is high, half dissociation of H2O is dominant and the band bending approaches the flat-band condition due to the termination of surface Ga dangling bonds by H and OH; when the supply rate is low, the saturated band bending matches that of the O2-adsorbed surface, presumably due to the O atoms that are formed by full dissociation of H2O.