Study On Visible-Light-Driven Photocatalysts For Water Splitting

Nowadays, the world is facing serious environmental problem. Many environmental problems, such as green house effect, acid rain, photochemical pollution and so on, occurred in the produce and usage of fossil energy resources. So it is very urgent to develop a clean and sustainable energy system to solve such problems. Solar energy can be transformed to hydrogen energy by photocatalysis for water splitting in one step. Water is the only product in using hydrogen energy. This is a clean and sustainable energy system. Heterogeneous photocatalysis is becoming one of highlights for water splitting to produce hydrogen. The core of heterogeneous photocatalysis for water splitting is to develop solid photocatalysts. A lot of photocatalysts were discovered by researchers but many of them only work under UV light, which occupy only 4% of solar energy. In order to take advantage of much more solar energy, visible-light-driven photocatalysts becomes more and more important. However, few photocatalysts can stoichiometrically split water into hydrogen and oxygen under visible light up to date. Many visible-light-driven photocatalysts produced either hydrogen or oxygen from sacrificial reagent solution due to unsuitable conduction band or valence band, for example BiVO₄ and Bi₂WO₆.In this thesis, several kinds of photocatalysts, BiYWO6, I41/amd-type Bi_xY_1-xVO₄, I4/a-type Bi_xY_1-xVO₄ and I41/amd Bi_0.5Ln_0.5VO₄(Ln=La, Ce) were prepared by solid state reaction and ball milling. They were discovered to have photocatalytic activity to split water into H₂ and O₂ under full arc and visible light irradiation. The physical and chemical properties of them were characterized by XRD, DRS, XPS, SEM, BET and so on. The effects of cocatalysts and pH on the activity of photocatalysts were investigated. The relationship between the structure and photocatalytic performance was discussed .The detailed researches were summarized as follows:1. As we knew, sacrificial reagents (SRs) are extensively used in a variety of photocatalysis systems to prove the hydrogen-producing or oxygen-producing ability of photocatalysts. It was discovered that these SRs such as Na₂SO₃,Na₂S,KI,EDTA-Na and CH₃OH have the abilities to reduce H₂O to H₂, and AgNO₃ and Fe(NO₃)₃ have the abilities to oxidize H₂O to O₂, even without the presence of photocatalysts. The water reduction and oxidation is attributable to the photochemical property of SRs acted as electron donors (EDs) and as electron scavengers (ESs) respectively under irradiation. The influence of photochemical reactions of SRs on photocatalytic evolution of H₂ and O₂ could be avoided by using a 300 nm cutoff filter. Comparing the amounts of H₂ or O₂ evolved from the SRs aqueous solution without Pt/TiO₂ to those with Pt/TiO₂, CH3OH and Fe(NO3)3 were the most excellent ED and ES respectively for photocatalytic reduction and oxidation of water. It was proposed that photochemistry between SRs and H₂O should be considered when evaluating the activity of photocatalysts for photocatalytic reduction and oxidation of water into H₂ and O₂.2. To adjust the conduction band of Bi₂WO₆, BiYWO (BYW) oxide solid solution was prepared by solid state reaction to act as photocatalyst for overall water splitting under visible light. The band gap of BYW was 2.71 eV and it absorbed visible light up to 470 nm. When loading of cocatalysts, BYW can split water under visible light due to incorperation of Y, which improve the conduction band of BYW. RuO₂ was the best cocatalyst comparing with other cocatalysts such as Cr2O3-Pt, Pt and Au. Under irradiation ofλ> 420 nm light, the amounts of the produced hydrogen and oxygen were about 12.3μmol and 5.6μmol in 3 h respectively. This study indicated that the formation of solid solution was the feasible method to adjust the conduction band and valence band to obtain visible-light-driven photocatalyst. 3. As same as BiYWO6, Bi_0.5Y_0.5VO₄(BYV(0.5)), the solid solution of I41/amd BiVO₄ and YVO₄, was prepared by solid state reaction to adjust the conduction band of BiVO₄. BYV(0.5) can absorb the visible light up to 520 nm so that its band gap was estimated to be 2.38 eV. In the absorption region, there are two absorption mechanisms. Whenλ<420 nm, electron was excited to conduction band from valence band consisting of O₂p. Whenλ>420 nm, however electron was excited from valence band consisting of Bi6s. BYV(0.5) was discovered to split water stoichiometrically under full arc and visible light irradiation up to 510nm. The quantum efficiency was 0.47% under 420 nm visible light irradiation., BYV(0.5) shows best result when pH=9. Under full arc irradiation, the coloading of 1 wt% Rh and 1wt % Fe2O3 make BYV(0.5) to produce more H₂ and O₂ than other cocatalyst. However, coloading of 1 wt% Rh and 1 wt% CuO was best method for BYV(0.5) under visible light irradiation. This difference, we suggest, was due ascribed to two absorption mechanism of BYV(0.5). The results of XRD and XPS proved the stability of BYV(0.5) in the photocatalytic reaction.4. The solid solution Bi_xY_1-xVO₄ (BYV, 0≤x≤1)were prepared by solid state solution to study the effect of ratio of component on the photocatalytic activity. With increasing of Bi in BYV, its crystal structure will transform from I41/a to I41/amd. When x is in the region of I41/amd, unit cell parameters was increasing as x, which agreed with Vegard's Law. When 0.1252 and O₂ under full arc and visible light irradiation with cocatalysts. When x=0.375, BYV has the best photocatalytic activity.5. Besides Y, Lanthanide also can substitute Bi because of their same charge and similar ion radia.Bi_0.5M_0.5VO₄ (M=Y, La, Ce) were prepared by solid state reaction. The band gap of Bi_0.5La_0.5VO₄ is 2.40 eV, however, Bi_0.5Ce_0.5VO₄ doesn't have absorption edge. Bi_0.5M_0.5VO₄(M=La, Ce) also have the photocalytic activity to split water under visible light. Comparing with the results of Bi_0.5M_0.5VO₄ (M=Y, La, Ce) , 4f electron has adverse effect on photocatalytic activity.6. Novel I41/a-type Bi-Y-V solid solution(BYV) was discovered by ball milling. When 0.625≤x≤0.75, Bi_xY_1-xVO₄ is in single phrase and belongs to I41/a space group. No report about this type solid solution was found. I41/a-type BYV can reciprocally transform with I41/amd-type BYV by ball milling and heat treatment respectivelly. I41/a-type BYV can absorb visible light up to 550 nm and split water into H₂ and O₂ under full arc and visible light. An interesting phenomenon was found in this study that I41/amd BYVs prepared by solid state reaction or hear treatment from I41/a-type BYV has same crystal structure but different absorption property and photocalytic activities. It was probably due to the different structure of surface of samples. So, we think that the modification of surface of photocalysts can effectively change their photocatalytic property.