Density Functional Theory Calculation On Highly Efficient Perovskite-type Photocatalysts

Photocatalytic water splitting based on semiconductors represents a promising strategy for production of H₂ by utilizing solar energy.There are three crutial steps for the photocatalytic water splitting reaction:solar light harvesting,charge separation and the catalytic H₂ and O₂ evolution reaction.Based on the three steps mentioned above,multifarious strategies have been applied to modulate the physicochemical properties of semiconductor photocatalysts with the aim of improving the light absorption efficiency,suppressing carrier recombination and accelerating the kinetics of surface reactions.These strategies include doping element,constructing semiconductor heterostructures,loading cocatalysts and so on.Non-metal doping is one of the major strategies to reduce the wide band gap of semiconductors into the visible light region.In this thesis,first-principles calculations based on density functional theory?DFT?have been performed to investigate the effect of non-metal dopants X?X=C,Si,N,P,S,Se,F,Cl,Br and I?with X@O and X@Ti on the geometric and electronic structures,stability,and photocatalytic property of perovskite-type BaTiO₃ with a wide band gap.Our calculations provide reasonable explanations on experimental observations of the narrow band gap for N-doped BaTiO₃.It is found that the preferred site of dopant X depends on the ionic size of dopant with respect to that of O^2-or Ti⁴⁺.The complexity of doping depends on the distortion of local structure and electronegativity of dopants.C-and I-doped BaTiO₃ with X@O induce the extension of absorption edge to visible light range with improved abilities of photocatalytic water splitting.The replacement of lattice O/Ti with S or Se not only leads to the band gap narrowing but also enhances the photo-oxidation and photo-reduction capabilities of semiconductor.Further experimental studies are highly demanded to explore the promising application of these four systems for the photocatalytic field.Perovskite-type NaTaO₃ loaded with NiO cocatalyst is one of the few photocatalysts for overall water splitting in UV region,which has attracted considerable attention.However,the role of NiO particles in the photocatalytic system is not fully understood.First-principles calculations based on DFT have been carried out to reveal the origin of the remarkable enhancement of photocatalytic activity achieved by loading NiO on NaTaO₃ surface.The interfacial geometries,electronic structures,charge transport,optical absorptions and band offsets of NiO?001?/NaTaO₃?001?slab model have been investigated in detail.The formations of Ta-O and O-Ni interfacial bonds are thermodynamically stable,showing a covalent interaction between two components of interface.We find that the structural rearrangement around the interfacial region has a negligible effect on the light absorption.The analyses on density of states and band offsets show that NiO?001?/NaTaO₃?001?heterostructure possesses a Type-II band alignment with the valence band maximum and conduction band minimum of NiO above those of NaTaO₃,which results in the accumulation of photogenerated electrons on NaTaO₃ side and photogenerated holes on NiO side.The calculations have confirmed recent experimental observations on NiO as a hole trap.The difference of electrostatic potentials around the interface as a driving force boosts the migration of electrons and holes to different domains of the interface,which is beneficial to extend the lifetime of photoinduced carriers and improve the photocatalytic activity of NaTaO₃ system.Our results provide new insights into understanding the influence of loading NiO cocatalyst on the photocatalytic performance of NaTaO₃,which will help in designing new semiconductor composities for photocatalytic applications.SrTiO₃ is a promising candidate photocatalyst for overall water splitting.Loading suitable cocatalysts,such as NiO_x,the mixture of Ni and NiO,remarkably improve the photocatalytic activity.However,spatial locations and functions of components in NiO_x/SrTiO₃ are under debate.Here,using first-principles DFT calculations we investigate the initial growth of Ni_n?n=1–4?and?NiO?_n?n=1,2 and 4?clusters on stoichiometric?100?surfaces of SrTiO₃,and explore interfacial and electronic structures of composite photocatalysts.It is found that Ni_n clusters are easier to undergo aggregation on SrO-termination than on TiO₂-termination.The adsorption of Ni_n cluster on?100?surfaces elevates the Fermi level towards the conduction band,which may benefit the occurrence of hydrogen evolution reaction.The structural similarity between?NiO?_n cluster and surface has an essential effect on the most stable adsorption configuration.For?NiO?_n/SrTiO₃ systems,the occupied states of?NiO?_n cluster well overlap with those of?100?surfaces in the valence band maximum,which is in favor of the separation of photogenerated electrons and holes to SrTiO₃ support and?NiO?_n cluster,respectively.The detailed DFT analysis provides important insights into the growth of NiO_x on surfaces of SrTiO₃ and presents an explanation on the different models of NiO_x/SrTiO₃ photocatalyst proposed by experimental groups.Our calculations build a basis for further investigations on the mechanism of photocatalytic water-splitting reaction in NiO_x/SrTiO₃ composite system.