First-principle calculation on electronic structures and optical properties of hybrid graphene and BiOI nanosheets

Photocatalytic technology has wide potential applications in the fields of energy generation and pollutant purification due to its advantages of low cost and environmental friendliness. Besides traditional photocatalysts of TiO2 and ZnO, the developing of new photocatalyst with novel properties of strong oxidation, reduction ability, and visible light response has received more attention. Bismuth compounds, such as BiOX (X=Cl, Br, I), exhibit highly efficient photocatalytic activity because of its layered structure and electronic characteristics. The special layered structure, resulting in built-in-field, is favorable for the separation and migration of photogenerated electrons and holes. Among BiOX compounds, BiOI has the best optical absorption characteristics in the range of visible light, and also has the best photocatalytic activity for the degradation of organic pollutants under visible light irradiation. Graphene is an ideal two-dimensional crystal with zero band gap and a high specific surface area. Many researches have shown that graphene can effectively reduce recombination probability of hole and electron because of its unique electron transport property, and it can improve the photocatalytic activity and light stability of the composite catalytic materials.In this paper, by constructing BiOI nanosheets and hybrid graphene/BiOI, the nanocomposite photocatalytic materials each with a high specific surface area and good photocatalytic activity are obtained. First-principle calculation based on density functional theory is used to investigate the electronic and optical properties of single/double layer BiOI nanosheets and their nanocomposites with graphene. Three kinds of vacancy defects, such as Bi, O and I in BiOI, are also considered. The calculated results show that the spontaneous charge transfer from graphene to BiOI takes place, forming electron-hole puddle because of the interface interaction between graphene and BiOI. Additionally, the hybrid graphene/BiOI complex displays an enhanced optical absorption behavior in the visible light region, improving its photocatalytic activity. The calculated results about the vacancy defects show that the Bi vacancy enhances the charge transfer between BiOI and graphene and forms more electron-hole puddles. In contrast, O and I defects restrain the charge separation between two layers and reduce the formation of electron-hole puddles.