First-principles modeling of GaN(0001)/water interface: Effect of surface charging

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.