Many-Body Green's Function Theory Studies On Water Photolysis And Titanium Dioxide Photocatalysis

Owing to the shortages of fossil fuel and environmental pollution issue,people have devoted to search for a clean and rich energy source.Solar energy has become the priority choice since it is inexhaustible and clean.The efficient conversion of solar energy to a storable energy has been a worldwide target recently.Photocatalytic technology not only achieves the conversion,but also degrades pollutants.It starts from the discovery of photocatalytic hydrogen production on titanium dioxide(TiO2),the principle of which is that the electron-hole pair created through the absorption of photons by catalyst separates into an independent electron and hole to perform the reduction and oxidation reactions,respectively.To date,photocatalytic reaction has become one of the most interesting scientific researches.Because of its excellent features such as stability,nontoxicity,low cost and interesting electronic and optical properties,TiO2 has become one of the most widely used photocatalytic materials.Generally,researches on TiO2 photocatalysis mainly can be classified into two aspects,i.e.the photocatalytic reaction mechanism and the properties of photocatalysts.Clarification of reaction mechanism and the properties of materials is helpful to people for finding the breakthrough point of improving the photocatalytic efficiency and for making the photocatalytic technology have better applications.People have focused on the photocatalytic water splitting by TiO2 for decades as it is one of the most representative photocatalytic reactions.So far,although a large amount of experimental and theoretical studies have been carried out in order to clarify the mechanism of oxygen evolution reaction,no consensus has been reached yet.Since irradiating liquid water by ultraviolet light can create hydrogen and oxygen as well even when there is no photocatalysts involved,research on pure water photolysis could give us some hints on photocatalytic water splitting reaction.Besides the photocatalytic water splitting reaction,the photolysis of condensed-phase water is also an interesting subject because it plays an important role in the fields like bioscience,energy production and atmospheric sciences.Methanol(CH3OH)is often employed as hole scavenger to prevent the recombination of electron and hole so as to improve the hydrogen production efficiency.Besides,the photocatalytic oxidation of CH3OH has been applied in the production of chemicals and the degradation of organic pollutants.Therefore,CH3OH photocatalysis on TiO2 has attracted much attention in recent years,especially for the optical properties and the oxidation mechanism.Similarly,a common strategy to improve the photocatalytic efficiency is to tune intrinsic defects in TiO2 since the photocatalytic activity is closely related to the defects.Many researches have been performed on the defects in three-dimensional crystals,including both rutile and anatase phases.However,the properties of defects in two-dimensional nanosheets,which show excellent photocatalytic ability recently,have not been studied yet.Many-body Green's function theory(MBGFT)is a state-of-the-art first-principles method for studying the electronic structure and excited-state properties of materials.Scientific researches have shown the high accuracy of this method in calculating electronic level and excitation energy.By this method,we carry out four studies according to the research needs of condensed-phase water photolysis and TiO2 photocatalysis.We systematically investigate the excited-state behaviors of condensed-phase water,TiO2/water interface,CH3OH/TiO2 interface and defected nanosheet,explaining some phenomena and debates in experiments.Contents and conclusions are as follows:(1)Photolysis of the condensed-phase water under ultraviolet(UV)irradiation is an important reaction on the Earth and in the interstellar space.Experiments have observed the water photolysis by 157 nm and 193 nm irradiation.As a key photoproduct,OH radical is actively involved in subsequent chemical and biological reactions with other organic and inorganic species.To date,water photodissociation in the condensed phase has been investigated in both experiment and theory for a long time,many controversies still remain on the mechanism for OH radical production.In this work,we study the excited-state dynamics of water photolysis in the condensed phase for the first time by the MBGFT.We choose the representative hexagonal ice as model system to simulate the hydrogen-bond environment in condensed-phase water.We first solve the problems in optical absorption.We find that the Urbach tail in the optical absorption of condensed-phase water can be ascribed to the inherent hydroxide ions or transient structures formed in the autoionization process.Second,we find that OH radical is created via quite different mechanisms for the long-wavelength(-200 nm)UV and short-wavelength(?150 nm)UV irradiation.OH radical is created by directly irradiating hydroxide ion in the long-wavelength region.Irradiation by the light at the short-wavelength UV excites an electron out of the intact water molecule with the transfer of the proton.Water photodissociation is also shown to be a barrierless process.(2)Photocatalytic splitting of water on TiO2(110)surface is an important chemical process in solar-to-chemical energy conversion.The mechanism behind it has received intensive attention,especially for the oxygen evolution reaction(OER)which is the rate-determining step.The nature of surface active sites in OER has not reached consensus.In this work,we investigate the behavior of electron-hole pair at some possible active sites of TiO2/water interfaces using the state-of-the-art ab initio many-body Green's function theory.GW results show that the valence band edges of TiO2 and water are elevated by their interaction.We find that the typical water species.including Ti-OH,OH-and H2O which are generally believed to be the active sites by researchers,cannot be the trapping sites of hole under low-energy light irradiation.With higher-energy ultraviolet light,hole may be initially captured on Ti-OH and OH-groups,however,the hole might eventually decay onto three-coordinated lattice O atoms of TiO2 if it cannot participate OER in time.Our calculations suggest that there must be other hole trapping site at TiO2/water interface to promote the OER.(3)As a prototype in heterogeneous photocatalysis,photo-oxidation of CH3OH on the TiO2 surface has been extensively studied experimentally and theoretically.However,some fundamental issues in this reaction are still elusive and disputed,including for example whether CH3OH is photoactive or not,whether the dissociation from CH3OH to CH3O is assisted by photogenerated holes,etc.Unraveling the behavior of the photogenerated hole is a prerequisite to solve these issues.In this work,for the first time we employ the two-particle Green's function theory,which simultaneously consider the photogenerated electron and hole,to systematically investigate the optical properties of CH3OH on TiO2 surface and excited-state behavior for O-H dissociation Our GW+BSE results uncover the extremely low photoactivity of CH3OH molecule adsorbed on TiO2 surface.With the high-energy UV light irradiation on the CH3OH/TiO2 interface,the population of hole on CH3OH is insignificant,but the hole trapped by CH3O group can be localized.The CH3O group own higher photoactivity.In the oxidation of CH3OH to CH3O,the hole remains within TiO2 in the whole process and this reaction is essentially thermal-driven.(4)Due to the unique low-dimensional properties of two-dimensional materials,TiO2 nanosheets have been widely used in catalysis and electrochemical energy storage.In this work,we apply the MBGFT to systematically investigate the electronic levels and excited-state properties of the lepidocrocite phase TiO2(L-TiO2)nanosheets with and without defects.Our results show that the ultraviolet absorption edge of the nanosheet with Ti vacancies lies at-4.0 eV,which is in line with the optical band gap(?3.84 eV)in experiment.It verifies the fact that there are Ti vacancies in the nanosheets.The exciton binding energy in perfect L-TiO2 nanosheet is so large that electrons and holes can not easily separate.The vacancy defects in nanosheets not only enlarge the range of optical absorption,but also reduce the exciton binding energy to prevent the recombination of electrons and holes,especially for nanosheets with Ti vacancies whose exciton binding energy can be as low as?0.1 eV.