First-principles Study On Photocatalysis Of C/Si/Ge/Sn Doped TiO₂ And Phase Transition Of CH₃NH₃PbCl₃

The electronic structures and absorption spectra have been studied with density functional theory plus U for isoelectronic impurities C/Si/Ge/Sn-doped anatase TiO₂.Our theoretical results show that silicon atoms can attract electrons from the surrounding oxygen atoms due to their greater electronegativity than that of titanium atoms which leads to increasing the energy of 2p orbitals of the surrounding oxygen atoms and then forming new acceptor impurity level composed of oxygen 2p orbitals near the top of TiO₂ valence band.This new acceptor level can easily bind the photoexcited holes for a period of time,thus effectively preventing the recombination of photo-excited electron-hole pairs,which play a key role in achieving higher light photocatalytic activity than pure TiO₂ for degradation of contaminants in water.We also systematically described the electronic structure and optical properties of the germanium doped system and then explained experimental observations of absorption spectra with red-shift at a suitable Ge dopant concentration.Especially,compared with pure TiO₂,Sn-doped TiO₂ has better photocatalytic activity because of the synergistic effect of Sn atoms and oxygen vacancies.Our theoretical calculations not only explained the key role of isoelectronic defect in preventing the recombination of photo-excited electron-hole pairs but also provided a new way for enhance of photocatalytic activity.Order-disorder phase transitions for CH₃NH₃PbCl₃ were studied with density functional theory.Our calculations show that there are two kinds of disorders in the cubic phase,namely,the disorder of orientation and rotation of organic groups.Organic groups of[CH₃]and[NH₃]in cubic crystals can easily rotate around its C₃ axis.At the same time,[CH₃NH₃]⁺organic groups can also quickly orient along different spatial directions due to the weak interactions between organic group and inorganic frame.Our results show that its possible phase transition path starts from the deviation of organic groups from the C₃crystal axis.The disordered high temperature cubic phase can be expressed as a statistical average of substructures we rebuilt.The possible phase transition paths begin with the fact that the organic groups can spontaneously deviate from the C₃ axis and choose different orientations,and eventually form new symmetry under dipole interaction.In the process of decreasing the symmetry,the structure of the tetragonal structure is changed from the disordered cubic phase for organic group to the disordered tetragonal structure for hydrogen group.