First-Principles Study The Related Process Of Surface Modification Of Anatase Titanium Dioxide

Semiconductor photocatalytic technology based on TiO₂ has been widely studied and applied in the field of manufacturing clean energy and eliminating environmental pollutants.However,the narrow optical absorption range and high carrier recombination rate lead to low catalytic activity of TiO₂ and,consequently,limit its application and development.Therefore,it is an important subject to adjust its energy band,decrease the carrier recombination rate and improve the photocatalytic activity of TiO₂.Surface modification could affect the surface structure and electronic structure of TiO₂,improving the photocatalytic efficiency.Based on this strategy,many experimental and theoretical efforts have been payed,but the related reaction processes are very complex and many mechanisms remain unclear.Experimentally,it was found that surface Ti-peroxo complexes??Ti?IV?-OOH?form,when H₂O₂react with anatase TiO₂ under relatively lower temperature??90??,leading to visible-light sensitivity,but which didn't improve the photocatalytic activity.However,the underlying mechanism of these confusing results are still unclear.To better understand these experiments,clarifying the atomic model of Ti-peroxo complex is necessary.Possible configurations were proposed in a theoretical paper,but the related electronic structure has not been clarified.Hence,it is still unclear that why the formation of surface Ti-peroxo complexes detrimental to the photocatalytic activity.Recently,it was found that oxygen-rich A-TiO₂ could be formed through H₂O₂ treating under 400?condition,revealing the coorperation of O atom of H₂O₂ could enter into A-TiO₂ lattice.Contrasting to the negative effect of surface Ti-peroxo complexes,the O-rich sample showed much enhanced visible light responsive activity.However,to the best of our knowledge,the related mechanism has not been studied theoretically.Based on first-principles calculations,a new stable dissociative configuration OOH/H is found when H₂O₂ dissociates on A-TiO₂surface.More importantly,we find that an oxygen atom of this new OOH/H configuration could enter into A-TiO₂ lattice with forming surface?O₂?o.The calculated reaction barriers based on CI-NEB method indicates that the formations of the new OOH/H and surface?O₂?o species need to cross energy barriers0.30 and 1.28 eV,respectively,explaining previous experimental results that the formations of Ti-peroxo complexes and?O₂?o species under lower and higher temperature,respectively.Electronic structure analysis show that both the new OOH/H and surface?O₂?o species could reduce band gap,but the former and latter species increase and decrease the carrier recombination probability,respectively,revealing the origin of different photocatalytic performance of A-TiO₂ treated by H₂O₂ under different temperature.These results could well explain experimental results and deepen the understanding of the role of H₂O₂ in the photocatalytic reaction of anatase TiO₂.Loading noble metal Pt on TiO₂ surface is a common strategy to improve the photocatalytic activity of TiO₂.More interestingly,the co-workers from experimental group found that photocatalytic activity of Pt-loaded anatase TiO₂ with oxygen vacancy defects is much higher than that of the sample without such defects.Hence,we can deduce the existence of synergistic effect between Pt and oxygen vacancy may enhance the photocatalytic.Based on first-principles calculations,the Pt₄ small clusters loaded on A-TiO₂?101?surface without and with oxygen vacancy defects were investigated.And,the presence of oxygen vacancies on the surface,subsurface,and deeper layers were considered.It was found that the Pt₄cluster could significantly reduce the formation energy of surface/subsurface oxygen vacancies,while the oxygen vacancies on deeper TiO₂ monolayers are little effected,indicating the surface Pt₄ clusters could more favor the existence of oxygen vacancy on surface region rather in bulk.Electronic structure calculations indicate that the presence of subsurface oxygen vacancies enhances the hybridization of Pt-5d orbitals with Ti-3d and O-2p orbitals,increasing the charge density around the surface Pt,which may reduce the probability of carrier recombination and thus improve catalytic activity.These results explain the experimental results and deepen the understanding of the role of oxygen vacancy defects on the photocatalytic activity of TiO₂ with Pt loading.Experimentally,it was found that hydrogenation could change surface morphology of anatase TiO₂?e.g.the color changing of anatase TiO₂ from white to black and the emergence of disordered surface layers?and electronic structure?effectively reducing band gap and the decreasing the carrier recombination rate?,consequently,the photocatalytic efficiency.Previous theoretical studies focused on the adsorption and diffusion of hydrogen on the anatase TiO₂ surface,the distortant of surface lattice and various defects caused by hydrogenation.However,no theoretical study systematically investigated the complexity of the hydrogenated TiO₂ surface microstructures,such as surface distortion,oxygen vacancies/Ti³⁺ions,surface hydroxyl groups and Ti-H bonds.Traditional experimental researches can only detect the finite thermodynamic conditions and the standard DFT calculations depend on an initial structural model,leading to an incomplete understanding to the rich microstructure of black TiO₂ surface as well as it's structure-activity relationship.As well known,the formation of materials determined by kinetic and thermodynamic factors.However,best to our knowledge,no one has studied the distortion mechanism of anatase TiO₂ surface induced by hydrogenation from the dynamic point of view,because the lack of clear surface structures.In this paper,the interplay of between hydrogen and anatase TiO₂?101?surface are systematically investigated by using crystal structure prediction software package?USPEX?based on Ab initio evolutionary algorithm,first-principles calculation package?VASP?and CI-NEB method.Different coverage of hydrogen on the surface are considered.However,this work is in progress and not yet completed.