The Tunable HER Activity Of Single Layered TiO₂ Nanomaterials From First Principles Calculations

It is much urgent to search for renewable energy due to the serious environmental pollution caused by overuse of fossil fuels.In recent years,hydrogen energy is widely considered as one candidate of clean,abundant,and endlessly renewable energy resources.Moreover,photocatalytic water splitting is the cleanest and most ideal routes for hydrogen production until now.And as a most promising candidate in photocatalysis,single-layered titanium dioxide(LNS-TiO2)nanosheets have attracted more and more attentions in photocatalytic water splitting because of its unique physical and chemical properties.In this paper,with first-principles calculations based on the density functional theory,the tunable hydrogen evolution reaction(HER)performance of LNSTiO2 nanosheets was systematically investigated by transition metal doping and strain engineering.Furthermore,their micro-mechanism and origin of electronic structure were explored and discussed in deep.The main conclusions are listed below:(1)As for the in-plane transition metal doped LNS-TiO2 nanosheets,the hydrogen adsorption free energy of Cr-doped system was reduced to almost 0 e V,indicating that it will be a highly efficient HER photocatalyst.The tunable hydrogen adsorption ability was linearly correlated with the O-2pz band center level of reaction sites.In addition,with upshifting the O-2pz band center level,biaxial strains could decrease the hydrogen adsorption free energy of V doped LNS-TiO2 nanosheet even close to 0 e V with 7%tensile strain.(2)Taking into account the preferred oxygen vacancy defect in oxide compounds,the first-principles calculations were performed to investigate the HER activity of transition metal doped LNS-TiO2 nanosheets with oxygen vacancies.When the adsorbed hydrogen occupies the site of oxygen vacancy,an optimal HER activity(?GH*=-0.12 e V)was obtained in Fe doped LNS-TiO2 nanosheet.The variation of hydrogen adsorption free energy was well understood in light of the d-band center level of dopants.Moreover,HER performance can be further modified by imposing external biaxial strain,which is also dominated by the d-band center.(3)For the transition metal edge-doped LNS-TiO2 nanoribbons,both oxygen atoms on surface and those from inner sites can be activated by Cr doping,which results in the improved HER activity.When hydrogen atom adsorbs to the inner O atom at the edge sites,the tunable hydrogen adsorption ability is strongly dependent on the O-2py+z band center level.In addition,the structure of nanoribbon is seriously distorted under compressive strain,leading to the invalid of conventional band center model.Then,the deformation energy contribution was proposed to explain the dependence of the amendatory hydrogen adsorption free energy on the band center level in the distorted nanoribbon structures.