Theoretical Study On The Mechanism Of Semiconductor Facet Induced Charge Spatial Separation

Semiconductor crystal facet effect can induce photo-excited electrons and holes to migrate to different crystal facets,reduce the chance of recombination,and improve the efficiency of charge separation.Understanding the process of crystal facet induced charge separation can help to design the photocatalyst with high catalytic activity,but there is a great controversy over the explanation of the mechanism of the charge separation process.Using density functional theory calculation,this paper selected three representative systems TiO2,Cu2WS4 and SrTiO3 to study the relationship between the characteristics of different crystal facets and charge separation,and tried to find out the relation between some explanations for charge separation mechanisms.The main results obtained are as follows:(1)The surface energy calculation results show that the surface energy of different crystal facets have significant differences,and the surface energy is positively correlated with the number of surface dangling bonds.Combining the experimental results of charge spatial separation,it was further found that holes tend to accumulate on surfaces with high surface energy and electrons tend to accumulate on the surface with low surface energy.(2)The projected density of states of each layer in each crystal facet indicates that different crystal facets have different band bending,and this band bending leads to different energy levels of different valence band maximum and conduction band minimum for different crystal facets.This difference in energy level is indeed conducive to charge separation and is consistent with the experimental results,but here the range of band bending is only a few layers of atoms(about 1 nm),so it may have limited effects for charge separation.The surface states can be found by comparing the energy band of the surface with the projected bulk energy band.The atomic orbital distribution calculations can further confirm that the surface states are only contributed by the surface atoms.(3)The projected density of states of the crystal facets after the adsorption of water molecules shows that the adsorption of water molecules has little effect on the surface heterojunction and does not change the band bending of atoms in several surface layers.(4)The calculation of the effective mass shows that the anisotropy of the effective mass along different crystal plane directions cannot explain the charge separation phenomenon in experiments.From the energy anisotropy of the bulk energy band,it is found that the energy anisotropy of bulk energy band along different crystal facet directions may be an important intrinsic factor of charge separation.Therefore,both the surface heterojunction and the bulk band anisotropy lead to the charge space separation on different crystal facets.