The Preparation, Modification And Photocatalytic Water Splitting Properties Of ZrW₂O₈

Nowadays, it is very urgent for human beings to develop clean, non-polluting and sustainable alternative energy resources, since the environmental problems caused by the large-scale exploitation and use of fossil energy, as well as the energy crisis caused by the incoming depletion of fossil fules are becoming more and more serious. Hydrogen energy has many advantages, such as: it could be produced from many hydrogen-containing resources and utilized in various forms; it has excellent combustion performance and water is the only combustion product; moreover, the water produced from the burning of hydrogen can be reused to generate hydrogen. Considering above merits, hydrogen energy has been regarded as the most promising alternative energy resource in 21st century by the majority of scholars around the world. Among various hydrogen production paths, the heterogeneous photocatalytic water splitting technique developed from"Fujishima-Honda"effect is increasingly under the spotlight, due to the fact that the system is simple and sustainable, and solar energy could be utilized directly.However, the low photocatalytic efficiency is still the bottleneck which constrains the further development of photocatalytic water splitting technique currently. In order to improve the efficiency, key strategies used by researchers at this stage includes: (1) to develop visible light responsive photocatalysts, so as to utilize the visible light in the solar irradiation which accounts for 43% of energy; (2) to improve the performance of existing photocatalysts by varieties of modification methods; (3) to develop novel photocatalysts with excellent catalytic performance.In this paper, based on the materials design concept a new method for developing novel photocatalysts was put forward. Firstly, design a possible target material based on the relationship of electronic structure-photocatalytic property. Secondly, calculate the electronic structure of the target material by using the first-principles calculation, and predict whether the target material possesses the photocatalytic water splitting ability. Finally, conduct experimental verification and further follow-up study of the target material which has passed the screening step. According to the above idea, a new type of Zr-W-based catalyst ZrW2O8 for photocatalytic water splitting was successfully developed in this paper. The preparation, characterization, photocatalytic water splitting properties and the visible light sensitization of ZrW2O8 were studied in detail by using XRD, DRS, XPS, SEM, TEM, BET, TG-DTA and elemental analysis techniques. The study also found that, as the precursor for preparing ZrW2O8 by hydrothermal reaction method, ZrW2O7(OH)2(H2O)2 possessed the photocatalytic water splitting ability as well.The content of this study and important conclusions were summarized as follows:(1) ZrW2O8 was selected as the target material based on the idea for designing and developing novel photocatalysts proposed in this paper, and its electronic structure was calculated by using the first-principles method. It was found ZrW2O8 possessed the characteristic band structure of general metal-oxide semiconductors, that the conduction band was mainly constituted by Zr4d and W5d hybrid orbitals, while the valence band was mainly constituted by O2p orbitals. The band structure of ZrW2O8 was predicted based on the theoretical calculations and the actual band structure of ZrO2 and WO3. It was predicted ZrW2O8 had suitable band structure for photocatalytic water splitting, and was quite possible to be developed as a water splitting photocatalyst.(2) ZrW2O8 samples were prepared by hydrothermal reaction method, and the effect of Zr:W mole ratio, HCl concentration, hydrothermal temperature and hydrothermal time on the structure and physicochemical properties were studied in detail. It was found the four parameters mainly affected the crystalline phase purity and crystallinity of synthesized samples. When excessive Zr was used (Zr:W is higher than 1:2), HCl concentration was higher than 4 mol/L, hydrothermal temperature was higher than 120°C and hydrothermal time was longer than 3 h, ZrW2O8 samples with single phase and good crystallinity could be synthesized. The absorption edges of synthesized samples were in the range of 280-360 nm, showing slight differences between samples prepared under different synthesis conditions. The four parameters didn't show any significant effect on the specific surface area of synthesized samples, which were in the range of 1-10 m2/g. (3) The crystal structure, photon absorption property, surface chemical state, morphology, and specific surface area of ZrW2O8 as well as its photocatalytic water splitting properties were examined in detail. It was shown the prepared ZrW2O8 was crystallized in cubic phase (P213), with the crystalline grain shape similar as a bamboo-leaf. The absorption edge was 300 nm and the surface area was 3.58 m2/g. Under the UV irradiation (260nm<λ<390nm), the average rate of H2 evolution over 0.3wt%Pt/ZrW2O8(0.5 g) in the presence of CH3OH as electron donor (ED) was 23.4μmol/h, and the average rate of O2 evolution over ZrW2O8(0.5 g) in the presence of AgNO3 as electron scavenger (ES) was 9.8μmol/h. Moreover, H2 was evolved over 0.3wt%Pt/ZrW2O8(0.5 g) from pure water splitting at a rate of 5.2μmol/h. Oxygen evolution was not detected. The experimental results confirmed the prediction by first-principle calculation: the band structure of ZrW2O8 was suitable for reducing H+ to H2 and oxidizing H2O to O2, and was a new type of photocatalyst for water splitting.(4) The effects of co-catalyst loading on the photocatalytic properties of ZrW2O8 were studied. When several common nobel metal co-catalysts (Pt, Au, Ru, Rh, Pd) were used for improving UV responsive acitivity of ZrW2O8, Pt and Au loaded by in situ photochemical deposition showed better modification effect. When Pt was used as co-catalyst, the in-situ photochemical deposition method was superior to the impregnation method, and the optimal loading amount of Pt was 0.3wt%. When several common oxide co-catalysts (RuO2, Ni-NiO, Pt-RuO2) were used, RuO2 and Pt-RuO2 showed relative better modification effect.(5) The effects of preparation conditions on the photocatalytic properties of ZrW2O8 were studied. Under the combined effects of crystallinity, specific surface area and crystal phase purity, samples prepared under different conditions showed slight differences in performance. ZWO-(6 h) sample was found to show the optimal properties. Under the UV irradiation (260nm<λ<390nm), the average rate of H2 evolution over 0.3wt%Pt/ZrW2O8(0.5 g) in the presence of CH3OH as electron donor (ED) reached 61.2μmol/h, and the average rate of O2 evolution over ZrW2O8(0.5 g) in the presence of AgNO3 as electron scavenger (ES) reached 53.1μmol/h.(6) In order to expand the light absorption range of ZrW2O8, Bi-doping, N-doping and S-doping techniques were attempted. The crystal structure, photon absorption property, surface chemical state of doped ZrW2O8 samples as well as their photocatalytic water splitting properties were examined in detail. Also the effect of preparation conditions (calcination temperature and the ratio of raw materials) on the structure and properties of synthesized S-doped ZrW2O8 samples were studied. Through Bi-doping, the absorption edge of ZrW2O8 could be extended to 450 nm. However, ZrW2O8 was found to decompose into ZrO2 and WO3, indicating that introducing Bi into to the lattice of ZrW2O8 by impregnation method was not achieved in present study. When N-doped ZrW2O8 was prepared by the high-temperature ammonia decomposition method, the alkaline property of NH3 would destroy the crystal structure of ZrW2O8, thus the purpose of visible light sensitization of ZrW2O8 by N-doping could not be achieved. The S-doped ZrW2O8 samples could be successfully prepared by burning the mixture of ZrW2O8 and thiourea under Ar atmosphere. The crystal structure of ZrW2O8 could be maintained, and S was able to be doped into the O2- position in the state of S2-. S-doping could significantly extend the absorption properties of ZrW2O8, and the maximum absorption wavelength could reach up to 600 nm. After S-doping, the sample maintained similar photocatalytic water splitting activity with that of un-doped one under the full arc irradiation. Morever, S-doped sample could utilize light with longer wavelength for photocatalytic water splitting. Specifically, S-doped ZrW2O8 sample can use light with wavelength up to 360 nm to produce hydrogen, and can use light with wavelength up to 510 nm to produce oxygen.(7) It was found ZrW2O7(OH)2(H2O)2 showed similar absorption property with that of ZrW2O8, that is, it possessed a steep absorption edge in the UV light region, suggesting ZrW2O7(OH)2(H2O)2 might also possess the ability of photocatalytic water splitting. Therefore, the thermal decomposition property, crystal structure, photon absorption property and specific surface area as well as the photocatalytic water splitting properties of ZrW2O7(OH)2(H2O)2 were studied in detail. The results showed that ZrW2O7(OH)2(H2O)2 was crystallized well in tetragonal phase, with absorption edge of 310 nm, band gap energy of 3.9 eV, and specific surface area of 5.9 m2/g. Under the UV irradiation (260nm<λ<390nm), the average rate of H2 evolution over 0.3wt%Pt/ ZrW2O7(OH)2(H2O)2(0.5 g) in the presence of CH3OH as electron donor (ED) was 3.7μmol/h, and the average rate of O2 evolution over ZrW2O(0.5 g) in the presence of AgNO3 as electron scavenger (ES) was 27.8μmol/h. It was concluded that the hydroxy group containing ZrW2O7(OH)2(H2O)2 had suitable band structure and possessed the photocatalytic ability to split water. Under the same experimental conditions, the photocatalytic water splitting properties of ZrW2O7(OH)2(H2O)2 was lower than that of ZrW2O8. The differences in crystal structure (crystal field and crystal packing factor) might cause the differences in performance.In this paper, a new method for designing and developing novel photocatalysts was attempted. The idea proposed in this paper was expected to provide a new approach for the development of novel photocatalysts in future. At the same time, the study of Zr-W-based photocatalysts not only enriched the existing photocatalytic water splitting material systems, but also provided important references for the follow-up study which may focus on improving the performance of ZrW2O8 photocatalyst.