Theoretical Studied On Non-metal Element Doped β-Ga₂O₃

Photocatalytic hydrogen generation, using solar energy to produce H2, especially from water splitting, is considered to be a promising technology to address environmental crises and energy shortage. In this field, one of the most important issues is the development of suitable semiconductors as photocatalysts. During the past decades, a large number of semiconductors have been reported to be active for water splitting. However, most of them only function under UV irradiation owing to their wide band gaps. Among the developed photocatalysts, semiconductors with a d10 electron configuration exhibit superior photocatalytic activities, mainly because their conduction bands are formed by hybridized sp orbitals with large dispersion able to generate photoexcited electrons with large mobility. Gallium oxide（Ga₂O₃） is a representative of such d10 metal oxides, exhibiting high activity of overall water splitting and degradation of organic pollutants. However, since β-Ga₂O₃ is a wide band gap semiconductor, its practical application is limited. In order to overcome the limitation of the majority of oxides that they are only UV responsive for water splitting, many methods have been developed to extend their light absorption into the visible region. Among them, anion doping is an efficient strategy to shift the optical response of oxides into the visible spectral range.In this work, we presented a systematic study using density functional theory on the geometric structure, energetic stability and electronic properties of X-doped β-Ga₂O₃ bulk and surface（X= C, N, F, Si, P, S, Cl, Se, Br and I）. We analyzed the effect of doping on the band gap and photocatalytic oxidation/reduction ability of bulk β-Ga₂O₃ and β-Ga₂O₃（100） surface. Our results lead to the following conclusions:（1） In order to rationally simulate bulk β-Ga₂O₃, we firstly chose the supercells including 40 atoms and 80 atoms to calculate the formation energy of each impurity case, respectively. We found that the small supercell and larger one produced the same trend on the geometrical structures and relative stablility for every doping system. Therefore, we will use the supercell including 40 atoms as our research objection.（2） Based on the calculated formation energy, local geometrical structure, density of states and the differences of charge distribution, we summed up:（a） The doping is energetically more favorable under Ga-rich growth conditions than under O-rich growth conditions;（b） The comparison of formation energies by doping at different oxygen sites suggests that it is the most difficult to replace the fourfold coordinated O atom with non-metal atoms;（c） For X-doped Ga₂O₃（X=N, S, Se,Cl, Br, I）, clean and narrowed band gaps are obtained. Meanwhile, photocatalytic reduction or/and oxidation abilities are improved with respect to pure Ga₂O₃;（d） Se-doped and I-doped β-Ga₂O₃ are promising candidates for visible-light photocatalysts owing to the largest band gap narrowing and the increase of photocatalytic redox ability.（3） As to the calculation on X-doped β-Ga₂O₃（100）, we found that（a） the doping is energetically more favorable under Ga-rich growth conditions than under O-rich growth conditions, which is similar to the results in bulk β-Ga₂O₃;（b） due to the similar coordination, replacing O1 and O2 sites on the surface is energetically almost equal;（c） Se-doped β-Ga₂O₃（100） presents the largest band gap narrowing and the remarkable increase of photocatalytic redox ability, which is a promising candidate for visible-light photocatalysts.