The Theoretical Study On The Photocatalytic Activity Of Doped Cu₂O

Cu2O is a common and important semiconductor which has received great application in the fields of renewable energy, environment protection and protective oral nasal mask et al due to a considerable absorption coefficient, abundant and cheap component elements in the Earth’s crust. The band gap of pure Cu2O is about 2.17 eV (experiment value), which is in great agreement with the ideal band gap (about 2.0 eV) of photocatalists, thus Cu2O should have considerable photocatalytic activity and can absorb a large proportion of visible light. However, experimental studies show that the photocatalytic activity of Cu2O is very low, then, theoretical physicists had proved that although the band gap of Cu2O is very suitable for photocatalytic reaction, its band edges are dominated by the 3d and 4s electronic states of Cu, which doesn’t meet the electronic transition of angular momentum selection rules (A/=±1), and this has large influence on the photocatalytic activity of Cu2O. In addition, the photocatalytic reaction takes place on the surface of the photocatalists, the photo-production electron-hole pairs recombine very fast and easily before they arrive at the surface of Cu2O due to the relatively little band gap, this is the another reason for low photocatalytic activity of Cu2O. To improve the photocatalytic activity of Cu2O in the whole optical band, we must solve the problems as mentioned above. As for changing the electronic structure of semiconductor, doping is one of the most effective and important ways.In recent years, people have tried all kinds of element mono-doping to change the electronic structure of Cu2O, and found that N doping can improve the electrical conductivity of Cu2O, respectively, and make the absorption coefficient of Cu2O meet the dipole forbidden rule.While, doping with Ag results in the reduction of the band gap. Thus, mono-doping can facilitate the absorption of visible light of Cu2O in a certain extent. However, researchers have also found that mono-doping would induce some energy levels in the forbidden band of Cu2O and they could act as effective recombination center of the carriers, thus the improvement of the visible light absorption of Cu2O via mono-doping is in a limited field. In 2009 the cationic-anionic passivated codoping method was first proposed and published in <Physical Review Letters> by Wei Suhuai et al. They found that cationic-anionic passivated codoping such as (Mo+C) would effectively reduce the band gap of TiO2, but without inducing impurity energy levels in the forbidden band. Then, this method was found can also solve other semiconductor with wide band gap. However, for Cu2O, the main consideration is not the reduction of the band gap, but the modification of the d character of either CBM or VBM as p character, whether this passivated codoping method is still effective or not is under discussion. Up to now, there is no report about passivated codoping about Cu2O, thus, it is of great significance to solve the problem of Cu2O via this method.In this paper, we have studied the electronic structure, stability and optical property of the non-metal (B,C,N,S) doped, metal (Ag,Ga,Ge,In,Sn) doped and metal-nonmetal passivated codoped Cu2O via density functional theory. We found that anion (B,C,N) doping results in inducing large 2p density of state of electron in the VBM, thus the dipole forbidden rule get satisfied in these systems. Cation (Ga, In) doping results in the increase in the band gap, while Ge and Sn doping possesses a typical n-type semiconductor behavior and the dominated states at the CBM have changed from d character to p character. All of the above mentioned elements induces some impurity energy levels in the forbidden band of Cu2O, while they did not appear in the Ag or S doped Cu2O, this is because Ag and S have the same valence electrons as Cu and O. There is no impurity energy level in the forbidden band of codoped Cu2O, either, and their band gaps get reduced, thus, they could enlarge the absorption region of solar light, in the (Ge+B) and (Sn+B) doped systems, the CBM of Cu2O are dominated by p electronic state, while the VBM keeps no change, thus the dipole forbidden is disabled. The calculation of the binding energies for the mono- and co-doping systems indicates that codoping is more stable than monodoping. To check whether the codoping approach can really improve the photocatalytic activity or not, we have calculated the optical property spectrum, the results show that all the codoping could enlarge the absorption region to visible light, and (Sn+B) codoped system exhibits the largest absorption region, thus (Sn+B) codoped Cu2O is expected to a good candidate of photocatalysts.We believe that our findings will encourage the experimentalists to investigate the (Sn+B) codoped Cu2O as the high efficient visible light photocatalyst.