Modification Mechanism Of TiO₂ And BiVO₄ Based Visible-light Photocatalysts And Their Activities For Degrading Pollutants

Visible light photocatalysis is regarded as an ideal solution to energy crisis and environment pollution. Among the photocatalysts investigated, Ti O2 and Bi VO4 are concerned especially due to their extensive source and chemical stability. How ever, they are still suffered from the wide bandgap(for Ti O2 is 3.2 e V) and low conduction band level(for Bi VO4 is ~ 0 e V vs SHE), which leads to low efficiency for solar energy utilization and high recombination rate of charge carrier. As a result, they usually exhibit weak photocatalytic activities under visible irradiation. Based on that, we carry out this work, in which the visible light activities o f Ti O2 and Bi VO4 are improved by doping with nonmetal ions, phosphate modification and constructing nanocomposite, while the influences of them on charge carrier properties and activities are investigated in detail.To the problem that Ti O2 has a wide bandgap so that it can only be excited by the UV light, on the basis of combining the advantages of hydrolysis process and solvothermal process, we develop a phase-separated hydrolysis-solvothermal process(HST) to synthesize N-doped Ti O2 with high doping content and good dispersibility. The results show that the N doping enhances the visible light absorption of Ti O2 effectively so as to improve its activities for degrading pollutants and splitting water to produce H2 under visible irradiation. It is noticed that the N-doped Ti O2 prepared by HST process shows higher activity than that prepared by the high-temperature nitrogenization process, that is mainly attributed to the high doping content and good dispersibility. The results of the atmosphere-controlled(AC-SPS) and time-resolved(TR-SPV) surface photovoltage measurement show that the surface state induced by N doping could capture the photogenerated hole so as to prolong the lifetime of charge carrier and improve the separation, which is the another important reason for the improved visible-light activity. In addition, N, S-codoped Ti O2 is prepared by HST process and show good activity, indicating HST process is a universal method for preparing non-metal doped Ti O2.Photogenerated charge carrier property is an very important factor in determining the activity of photocatalyst. Hence, we try to improve photogenerated charge carrier property of Bi VO4 by phosphate modification so as to improve the activity. The results show that phosphate modification increases the amount of surface-carried negative charge of Bi VO4 in water and improve its activities for degrading pollutants and water oxidation under visible irradiation. Based on the results of zeta potential, TR-SPV and a series of experiments designed, it is suggested that the improved activity of Bi VO4 is mainly due to the enhanced surface negative electrostatic field, which could induce the holes to transfer to the surface so as to improve the separation of charge carrier.To the problem that the conduction band bottom level of Bi VO4 is low so that the photogenerated electron on it exhibits low reduction activity, which usually leads to weak charge carrier separation, on the considering of the band structure and exciting process, we develop a strategy to improve the charge carrier properties of Bi VO4 by achieving the energetic electron transfer. The results show that coupling with a proper amount of Ti O2 could prolong the lifetime of charge carrier of Bi VO4 and improve the separation, which are mainly attributed to the visible photogenerated energetic electron transfer from Bi VO4 to Ti O2, by which the reduction activity of the electron of Bi VO4 is kept and the charge carriers are separated spatially. The improved charge carrier properties lead to remarkable promoted photocatalytic activities of Bi VO4 for degrading pollutants and H2 evolution(up to 2.2 μmol h-1 at the aid of Pt as cocatalyst) under visible irradiation,.To the problem that the charge transfer efficiency between Bi VO4 and Ti O2 is low, which is attributed to the low lattice matching degrees, the phosphate bridge is introduced into the Ti O2/Bi VO4 nanocomposite to improve the connection situation between Bi VO4 and Ti O2 and then promote the energetic electron transfer. The results show that, the introduction of phosphate bridge remarkable prolong s the lifetime of charge carrier of nanocomposite, leading to greatly enhanced photocatalytic activities of Bi VO4 for degrading pollutants and H2 evolution, about 3 times higher than the bare one. The results of electrochemical impedance spectra(EIS), ultra-low temperature electron spin resonance spectra(ESR) and a series of experiments designed show that the improved charge carrier properties of Ti O2/Bi VO4 nanocomposite are mainly attributed to that the energetic electron transfer from Bi VO4 to Ti O2 is promoted at the aid of introduced phosphate bridge.By means of these strategies above, the problems of Ti O2 and Bi VO4 mentioned above are solved well, leading to improved photocatalytic activit ies for degrading pollutants and splitting water to H2 under visible irradiation, and the corresponding mechanism research provides references for designing visible-response photocatalysts with high activities.