First-principle Studies On The Structures And Properties Of Titanium Oxide Compounds And Zinc Dioxide Under High Pressure

TiO₂ can use photocatalysis to decompose water to produce hydrogen,and can also degrade organic pollutants in the air and water.It provides treatment methods for environmental pollution and energy crisis.However,the band gap of TiO₂?3.2 eV?is much larger than the ideal photoelectrode material band gap of 2.0 eV,which makes it impossible to apply to the visible light region.Therefore,the regulation of TiO₂ band gap has become a research hotspot.The main method is to reduce the band gap by doping,but the doping method will also have problems such as introduction,which is not conducive to the application of the material.The extreme condition of high pressure can change the distance and interaction force between atoms,and change the microstructure and electronic properties of materials.Therefore,it is necessary to study the structure and properties of titanium oxide under high pressure.We used the USPEX software package based on genetic evolution algorithm to conduct an extensive structure search for Ti-O compounds in the high pressure range of 0-20 GPa,and studied the stable crystal structure and its electronic properties using first-principles calculation methods.We found that the structures of stable oxygen-rich compounds are:?P-4m2?Ti₂O₃,?P-62m?TiO and?C2/m?Ti₄O₇.Ti₂O₃,TiO and Ti₄O₇ are P-4m2,P-62m and C2/m phases in the studied pressure range,respectively.The stable structure of titanium-rich compounds is consistent with the results of previous studies.It can be seen from the electronic energy band structure that the titanium-rich compounds are metallic at normal pressure and the high pressure range studied.The oxygen-rich titanium compounds we proposed are also metallic at normal pressure and high pressure.Zinc oxide is widely used in the fields of chemical sensors and solar cells due to its unique photoelectric properties.Because of the excellent performance of ZnO,the zinc-oxygen system has attracted attention.Among them,ZnO₂ is favored by many researchers.It is a wide band gap semiconductor with a band gap of 4.1-4.5 eV under normal pressure.We use the USPEX software package to conduct a structural search for different ratios of Zn-O at a pressure of 20-300 GPa.The results show that in the Zn-O system,there are only two stable structural ratios of ZnO and ZnO₂.ZnO₂ found three structures under high pressure:Pa-3,P3₁2₁ and I4/mcm.Among them,Pa-3 and P3₁2₁ are thermodynamically unstable,which is considered to be a metastable phase,while the I4/mcm crystal structure can stably exist in 137.5-300 GPa,in line with previous research.In the calculation of electronic characteristics,the HSE method is used to find that the I4/mcm structure is a semiconductor in the corresponding stable high-pressure interval and its band gap is positively correlated with pressure.