| More and more serious environmental pollution problems and burning of fossil fuels urge us to discover for uncontaminated and renewable new energy resources.Hydrogen has adopted extensive notice regard as a promising candidate energy to fossil fuel. So it is very necessary to find a safe and uncontaminated methods to produce hydrogen. The search for visible light active photocatalyst material is very concerned about the problem for the production of hydrogen. The semiconductor based photocatalyst for water splitting using solar energy is one of the most effective approaches to produce hydrogen from water.The layered perovskite structure based photocatalyst, Na Nb O3 was reported extensively the reason is able to split water into oxygen and hydrogen in the ultraviolet region. However, the band gap of the Na Nb O3 is too wide（3.29 e V）,which can absorb ultraviolet light that accounts for only 4% of the sunlight. In general, to make efficient use of the visible light（43%） for hydrogen generation by water splitting and improve the photocatalytic efficiency, it is desirable that the forbidden gap of the semiconductor would be about 2.0 e V for the efficient absorption of sunlight, and the valence band maximum（VBM） must be lower than the oxidation potential of O2 /H2O（1.23 V）,while the conduction band minimum（CBM） must be higher than the reduction potential of H+/H2（0 V）. Therefore, to improve Na Nb O3 response to visible light（390-750 nm）, great efforts have been devoted to tuning its band structure. In this work,we discuss that how to reduce the band gap by doping, thus improve the efficiency of water splitting to hydrogen.Based on the first-principles calculations of density functional theory（DFT）, we investigate the electronic and optical properties of（N, P, C, S） monodoped and（N+N,P+P, N+P, C+S） codoped Na Nb O3 systems. Due to the VBM position is 1.39 e V lower than the oxidation potential of O2/H2 O, in order to reduce the band gap of the Na Nb O3,it make the VBM position may be changed upwards, So we have chosen the non-metallic elements（N, P, C, S） monodoping because they exist higher p orbital energy in comparison to the 2p orbital energy of oxygen. The（N, P, C） monodoped Na Nb O3 systems appear unfilled impurity states in middle of the forbidden gap, which will reduce the efficiency of the photocatalysis for water splitting. For the S doped Na Nb O3 system, the band gap reduction is not sufficient for visible light photocatalysis.The hole-hole mediated coupling of（N+N, N+P, C+S） codoped Na Nb O3 systems not only induce appreciate gap narrowing, but also remove the impurity states meaningfully.The defect formation energy shows that the codoping is energetically more favorable than the corresponding monodoping and is easier to form in O-poor conditions. The relative stabilities between the codoped（anion-anion） and the monodoped（anion）Na Nb O3 systems have been accounted for by the binding energy calculations. The band edge positions state that the（N+N, C+S） codoped Na Nb O3 are very good candidates for the photocatalysis of water for hydrogen production. The optical absorption curves obviously reveal that the（N+N, C+S） codoped Na Nb O3 systems indicate high photocatalytic activity under the visible light irradiation. |
PDF Full Text Request