Effects Of Defects On Photocatalytic Materials

It is urgen to develope high-performance photocatalysts for producing the green energy and realizing the waste treatment to solve the problems of energy crisis and environment pollution Defects inevitablely exist in catalysts,thus it is rather vital to investigate the effects of defects on structures and properties of photocatalysts.Excess electrons and holes induced by defects may interact with lattice phonons to form polarons,thus more attentions should be paid to investigate the effects of polarons in the catalysts.In this work,we mainly analyze the distrubition and effects of polarons by investigating the polarons on TiO2 surface.On the other side,it is believed that the performance of photocatalysts could be improved through the moudulation of defects.To this end,the effects of heterostructure interface defects were considered,and the charge transfer of the materials were carefully investigated by the first-principles calcualtions.The main contents of this work are as follow:(1)The physical properties of polaron induced by the H in anatase TiO2(101)and its effect on water splitting.Excess electrons and holes donated by defects can interact with lattice phonons to form polarons,which in turn may affect the system's structure as well as properties.We analyze the distribution of the polaron,the effects of the polaron on the structure,the interactin between the polaron and the proton,and the interaction bewteen the polaron and the water adsorbed on the surface by taking the anatase TiO2(101)surface as an example.First,we find that the H-induced polaron on anatase TiO2(101)surface prefers to stay close to the H.and the bond elongation induced by the H is determined by the trapping site of the polaron rather than the adsorption site for the H atom.Next,we investigate the coupling of the polaron and the proton on the surface,and we find that the proton would increase the energy consumed to distort the lattice and the energy gained by the localization of the electron.With the increase of the distance between the polaron and the proton,the energy consumed to distort the lattice would dencrease,and the energy term induced by electron would decrease until the localization of the polaron is almost unaffected by the proton.Then,we investigate the effect of the adsorption of water on the polaron.We find that the water adsorption on the surface could drag the polaron from interior region to surface,and the water adsorption also decreases the diffusion energy barrier of proton within the layer because of the decoupling between the polaron and proton induced by water adsorption.At last,we investigate the effects of the surface polaron on the water splitting.The results show that the polaron could enhance the adsorption of the water at the polaronic site,the polaron has the tendency to promote the decomposition of the water.(2)The physical properties of polaron induced by the oxygen vacancy in the crystalline/amorphous TiO2 heterostructure and its effect on water splitting.Amorphous structures have the characteristic of short range order and long range disorder,which may affect the properties of the material,such as the light adsorption.First,we investigate the structure and electronic structure of the amorphous TiO2.The band gap of the amorphous TiO2 is smaller than that of the anatase TiO2.Besides,the top of the valence band and the bottom of the conduction band are confined on several O atoms and Ti atoms,respectively.Then we analyze the structure of the crystalline/amorphous TiO2 heterostructure,and the results show that the oxygen vancacy tends to form at the interface.Subsequently,we investigate the distribution of polarons in the crystalline/amorphous TiO2 heterostructure.We find that the excess hole induced by the surface adsorbed OH tends to gather in the amorphous layer,while the excess electrons induced by the oxygen vacancy at the interface tend to gather in the amorphous region of the interface.We investigate the effects of the surface adsorbed water on the polaron through the first-principles molecule dynamics.We find that the water would promote the migation of excess electrons induced by the oxygen vacancy at the amorphous region of the interface to the amorphous surface.Finally,we investigate the effects of the polarons on the state of water adsorption.We find that the water molecules adsorbed on the amorphous region with oxygen vacancy would decompose faster than that with crystal region with oxygen vacancy.However,no matter whether the oxygen vacancy is in the amorphous or the crystal region of the interface,the water molecules adsorbed on the crystal surface would decompose slower than the water molecules adsorbed on the amorphous surface.(3)The physical properties and its effect on the charge carrier transfer mechanism of the excess electrons induced by the interface defects in the heterostructure.The spectrum absorption range of catalytic materials can be broadened by combining a wide-band-gap semiconductor with another narrow-band-gap semiconductor into a Z-scheme heterostructure.However,this kind of combination always leads to the formation of Type ?junctions.Although the Type ? junctions could also promote the separation of photogenerated electrons and holes,this kind of junctions may affect the redox potentials of the system.Therefore,we try to construct Z-scheme heterostructures by controlling the interface defects and the ZnO/BiVO4 and Cu2O/TiO2 heterostructures are taken as examples to explore the effects of interface defects.We find that the ZnO/BiVO4 is a Type ? junction,however the charge transfer could be turned into Z-scheme after the formation of interface defects.Similarly,Cu2O/TiO2 could also be changed into Z-scheme when there exists defects at the interface.Meanwhile,the defects could also inhibit the oxidation of Cu1+,thus enhancing the stability of Cu2O.