Theoretical Study Of TiO₂-based Heterostructure Photocatalytic Materials

Titanium dioxide（TiO₂）semiconductors are widely used to solve environment al pollution and energy shortages due to their excellent redox capability,stable optical properties,high catalytic activity,low cost and non-toxicity.However,the wide application of TiO₂ semiconductor photocatalysts is subject to certain limitations.First,because of its wide band gap（Anatase:³.2 eV,Rutile:³.0 eV）,TiO₂ can only absorb ultraviolet light part of the sunlight,while the ultraviolet light part only accounts for5%of the full spectrum solar energy.Second,the recombination rate of photogener ated electron-hole pairs is high.Therefore,we need to modify the TiO₂ to improve its photocatalytic performance.The use of modification methods to constr uct semiconductor heterostructures to improve photocatalytic performance is a hot topic in recent years,because it not only broadens the response range of TiO₂ to visible light,but also choose the semiconductor coupling match to the TiO₂ band which can improve the separation and transmission efficiency of photogenerated electron-hole pairs.However,most of studies based on TiO₂ constructed heterostructures were carried out through experiment.The electronic properties,charge transfer and interfaci al interactions of TiO₂-based heterostructure composite photocatalytic materials have not been systematically and comprehensively studied,and the intrinsic mechanisms of their photocatalytic performance enhancement remain unclear.In this paper,TiO₂photocatalytic materials were used as substrates,and different types of typical materials were selected to construct heterostructures with TiO₂.The first-principl es calculation method is used to systematically and study the interface microstruct ure and properties of TiO₂-based heterostructure composite photocatalytic materials in detail.The mechanism of the influence of interface structure and properties on photocatalytic performance is deeply analyzed.The main research contents and result s of this paper are as follows:（1）Heterostructure with the same crystal structure:The heterostructure is constructed by selecting SnO₂ having the same crystal form as TiO₂.The interact ion and properties of the abrupt and graded interfaces of different crystal planes were studied.The calculated results showed that the four heterogeneous structures belong to the type II heterostructure.Photogenerated electrons and photogenerated holes will transfer in opposite directions under light irradiation conditions,thereby ensuring that the photogenerated electron-hole pairs in the TiO₂/SnO₂ heterostructure can be effectively separated and transferred.（2）Homostructure:The microstructure,electronic properties,band offs et,homostructure effect and the possible directions of phase transitions in homostruct ure of TiO₂ composed of anatase and rutile are studied.The calculated results showed that the phase transition of TiO₂ from anatase to rutile is mainly determined by the connection mode of[TiO₆]octahedron at the interface of the homostructure,and the most likely phase transition direction is A（110）/R（011）.However,since the direct ion of the built-in electric field and the relative offset of the energy band in this homostructure are opposite,it is not conducive to the separation of photogener ated electron-hole pairs.Therefore,the photocatalytic performance will be enhanced only after the formation of other homostructure which are slightly higher in the interfaci al energy and facilitate the separation of photogenerated electron-hole pairs.This is one of the most significant reasons for the enhanced photocatalytic performance of P25with 70%anatase and 30%rutile.（3）Two-dimensional-three-dimensional heterostructure:The interface struct ure and properties of two-dimensional-three-dimensional（2D-3D）composi te heterostructures constructed by graphite-like monolayer of g-C₃N₄ in different ways（horizontal and vertical）and TiO₂ were studied.When g-C₃N₄ is combine with TiO₂ in a horizontal manner,a van der Waals interface is formed between the two,although an effective photogenerated electron-hole pair separation is not produced.However,due to the regulation of van der Waals,the electronic structures of g-C₃N₄ and TiO₂ have been modified to enhance their photocatalytic properties.When g-C₃N₄ is combine with TiO₂ in a vertical manner,the formed type II band alignment facilitates the separation of photogenerated electron-hole pairs.At the same time,the interface state exhibits metallicity,which is very advantageous for the rapid transmission of photogenerated carriers.（4）Wide-narrow bandgap semiconductor coupled heterostructure:CuGaO₂ has a special quasi-two-dimensional layered superlattice structure with a band gap of².0eV,so it was selected as the research object and combined with TiO₂ to form a wide-narrow bandgap semiconductor coupled heterostructure.The calculation results show that the heterostructures constructed by CuGaO₂ and TiO₂ belong to the type II heterostructure.Relatively speaking,the heterostructure formed by CuGaO₂ with O as the cut-off surface combined with TiO₂ is more stable,and the formed type II heterostructure is more favorable for the separation and transmission of photogenerated electron-hole pairs.In this paper,based on TiO₂ photocatalytic materials,four types of heterostruct ure composite photocatalytic materials were systematically analyze by theoreti cal calculation methods.Summarize the above calculation results,it can be found that the enhancement of photocatalytic performance through heterostructure construct ion requires attention to the following points:1)The edge position of the heterostruct ure component is the basis for determining the band alignment,which is the primary condition for component selection.2)The generation of the interface state is determined by the formation of the heterostructure,and the the same crystal struct ure does not produce the interface state.Since the atoms at the interface are able to obtain a near-ideal coordination environment,the interface state is generally not form in the forbidden band.The metallic interface state in the（quasi）two-dimensional component facilitates the rapid transport of photogenerated carriers.3)The electronic struct ure of the components on both sides of the interface can also enhance the photocatalyt ic performance to some extent if it can be controlled by the interface interaction.The research system in this paper illustrates the intrinsic physical mechanism of the enhanced photocatalytic performance of heterostructures.The summarized rules and principles can provide a certain theoretical basis for the development of high-efficiency composite photocatalytic materials in the future,and have import ant guiding significance.