Thy First-principles Study On Properties Of TiO₂ Hybrized With Layered Graphene-liked Materials

Recently,the environmental and energy crisis are becoming a serious challenge for human survival and social development.Photocatalytic technology has been recognized as an effective means to environmental contaminant control and remediation.Due to its chemical stability,low cost and strong oxidizing characteristics,TiO₂ has been widely used as photocatalyst over the past several decades.But unfortunately,TiO₂ can utilize no more than 5%of the total solar energy due to its wide band gap（3.0-3.2 eV）,which limits its application in the industrial application of photocatalytic technology.On the other hand,the fast recombination of photo-generated electrons（e^-）and hole（h⁺）pairs results in low photocatalytic activity.Many strategies,such as doping with elements,surface modification and coupling with other semiconductors,have been made to improve its photocatalytic performance.Two-dimensional layered materials have the advantages of large specific surface area,strong mechanical strength and high exposure to surface atoms for its unique structure,which make them in photoelectric,catalytic,chemical and biological sensing,solar cells and other fields have a broad application prospects,and the composite with TiO₂ can effectively make up for the shortcomings of Ti O₂,effectively enhancing the photocatalytic performance of TiO₂.In this work,the first-principles based density functional theory（DFT）were performed to simulate photocatalytic properties of TiO₂ based semiconductor nanocomposites.The grapheme-like material（like g-C₃N₄,MoS₂）was combined with TiO₂ and try to clarify the mechanism of enhanced photocatalytic performance.The study was summaried simply as follows:（1）It indicated that（TiO₂）_n nanocluster was a suitable structural model to explore its structure and electronic properties.To reduce timeconsuming calculations,we select the（TiO₂）₃ cluster in our study.First-principles calculation based on density functional theory（DFT）was used to explore the enhanced photocatalytic mechanism of TiO₂ by combined with both pristine and defective monolayer MoS₂.It was demonstrated that the combination of TiO₂ with MoS₂ was favorable thermodynamically.The charge densities between the interface of TiO₂ and MoS₂were investigated to clarify the improved property of TiO₂.Electrons migrated from TiO₂ to MoS₂ across the interface under irradiation,which caused the electrons accumulating on the MoS₂ side and electrons depleting on the other TiO₂ side.There appeared a built-in electric field in the interface between TiO₂ and monolayer MoS₂.And due to its presence,electrons and holes recombination was effectively suppressed,contributing to the enhanced photocatalytic performance of TiO₂/MoS₂.The electrons dispersed from highest occupied molecular orbital（HOMO）to lowest unoccupied molecular orbital（LUMO）,promoting the separation of the electrons and holes.The efficient separation of photoexcited electrons and holes prolonged the lifetime of photoexcited carriers,which effectively improve the photocatalytic activity of TiO₂.The theoretical study could provide credible evidence to understand the mechanism of enhanced photocatalytic activity of TiO₂/MoS₂.（2）First-principles calculation based on DFT was used to study the enhanced photocatalytic mechanism of TiO₂ hybridized with pristine and defective g-C₃N₄ systematically.The theoretical investigations on both geometry structure and electronic properties,involving band structure,density of states,charge population and density difference,were carried out to characterize the improved property of TiO₂.It was found that the combination TiO₂ with g-C₃N₄ could be verified for high thermodynamic stability.The interaction between TiO₂ and g-C₃N₄ led to form a built-in electric field at the interface,which facilitated the separation of electron-hole pairs and restrained photo-generated carrier recombination.Furthermore,electrons transiting from the highest occupied molecular orbital（HOMO）to the lowest unoccupied molecular orbital（LUMO）promoted the separation of electrons and holes.The effective separation of electron-hole pairs prolonged the lifetime of carries and enhanced the photocatalytic activity of TiO₂/g-C₃N₄.The theoretical investigations could verify the experimental observation results[Chem.Sci.,2014,5,3946-3951,Phys.Chem.Chem.Phys.,2015,17,17406-17412]and illustrate the mechanisms of photocatalytic enhancement of TiO₂/g-C₃N₄ composite photocatalysts.Furthermore,the calculated optical absorption curves demonstrated that the absorption edges of TiO₂/g-C₃N₄composites shifted to visible-light region and its photocatalytic performance improved under visible-light irradiation.The theoretical investigation might provide valuable and reference information for understanding the observed enhanced photocatalytic mechanism in experiments.（3）Since（001）facet was the most active surface of anatase TiO₂,we used anatase TiO₂（001）surface combined with the pristine and defective g-C₃N₄（001）surface to construct a TiO₂/g-C₃N₄ heterostructure.The details of the electronic structure,interfacial interaction and photogenerated acrriers are important for explaining photocatalytic properties of a heterostructure,so the density of states,charge density difference and Mulliken charge population of monolayer heterojunction were systematically investigated in our work.The theoretical results indicated that TiO₂/g-C₃N₄ heterojunction was a van der Waals heterostructure.Furthermore,it illustrated that the electrons might migrate from g-C₃N₄ monolayer to TiO₂ according to results of charge density difference and work function,which led to the formation of a built-in electronic field at the interface.Under illumination,the built-in electric field accelerates the transfer of photoexcited electrons,thus resulting in the photoexcited carriers effective separation.The separation of photoexcited electrons and holes prolonged the lifetime of photoexcited carries,and resulted in more photogenerated electrons and holes participate in the photocatalytic reaction,which effectively improved the photocatalytic activity of the TiO₂/g-C₃N₄heterostructure.These theoretical conclusions provided insights to the related experimental discovers and were helpful to further develop effective photocatalyst based on TiO₂.