2, RIKEN, Saitama, , Japan
First principles calculation has become a powerful tool for materials design in many fields [1,2]. However, theoretical design of photocatalysts and photoelectrodes has met with limited success, due to the lack of adequate modeling criteria. Under illumination, in the case of usual n-type semiconductor photoelectrode, owing to the space charge region formed in the vicinity of the semiconductor surface, photogenerated (excess) electrons drift to the bulk of the semiconductor whereas excess holes accumulate at the semiconductor surface. However, to the best of our knowledge, there are no computational studies that systematically examine the effect of “surface charging” on photocatalytic activities. This is presumably because, in the periodic boundary conditions, excess surface charge cannot be handled with the standard first-principles computational approaches in a straightforward manner.
In this work we study the effect of “surface charging” on the electronic and geometrical structures of the photoelectrode/electrolyte interface, with the aid of first-principles density functional theory (DFT) calculations. The effective screening medium (ESM) method , which can handle non-integer charge, is utilized in order to methodically vary the concentration of excess holes at the semiconductor surface. Well studied polar GaN(0001)/water interface is chosen as a model system. The bare surfaces, terminated with H atom and hydroxyl group, are considered. In addition, in order to account for the Helmholtz layer which is formed within the electrolyte, we adopt the recently developed ESM-reference interaction site method (RISM) , which can treat non-integer counter ions in the solvent.
The computational results show that the surface state originating from Ga dangling bonds is located roughly 1 eV below the conduction band minimum (CBM) and has a dispersion larger than 1 eV. The surface states originating from Ga dangling bonds diminished with increasing coverage of H atoms and hydroxyl groups, resulting in the decrease in the band bending and the strength of the Fermi level pinning induced by the surface states. Moreover, we find that the adsorption energy of H and hydroxyl group is strongly dependent not only on the adsorbate coverage, but also on the amount of excess charge at the surface. We will also report the effect of water molecule alignment and ion distribution on the electronic and geometrical structures of the interface. These results demonstrate that our surface-charge-sensitive modeling approach can provide critical insight into photocatalytic activities.
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