Comparative Study On Dye-sensitized ZnO And TiO₂ For H₂ Evolution By Water Splitting,and Its Application In The Degradation Of Formaldehyde

A promising system for H₂ production is the photosensitized reduction of water over the dye- sensitized semiconductor, which is a simple, low cost technology with none- pollution, high photoactivitytic activity and stability, It was applied for not only conversion of solar cells, hydrogen evolution under visible light but also degradation of organic pollutants recently.Compared to the terms of TiO₂, ZnO has a similar band structure,low cost. Accordingly, TiO₂ and ZnO are often used as semiconductor catalysts in the photocatalytic technology to achieve high efficiency under visible light. While as an amphoteric compand, ZnO is limited for acid and alkali environment. Also it is unstable in photocatalytic process. In order to clarify the pH value, alcohols as donors etc have the influence on electron transfer and hydrogen producing, comparative study on dye-sensitized ZnO and TiO₂ for H₂ evolution by water splitting, and its application in the degradation of formaldehyde in visible light were discussed in the experiment.This research was divided into three parts:?1? In the photocatalytic experiment, Pt as co-catalyst, TCPP as a sensitizer, alcohol?methanol, ethanol, n-propanol, isopropanol was used respectively? as donor in TCPP/ZnO/Pt system for H₂ evolution by water splitting under visible light. To explore the influence of the pH value and alcohol on H₂ evolution by water splitting, the same amount of TCPP/ZnO/Pt nanoparticles as photocatalyst were used in the same volume alcohol aqueous solution with different pH value, respectively. For further discuss the charge transfer influence mechanism in photocatalytic process, such as absorption spectrum, the adsorption quantity, fluorescence quenching and electrochemical experiment were done. The experiments results shown that for TCPP/ZnO/Pt system, the hydrogen yield for pH = 7 methanol, n-propanol and isopropanol as donor was significantly higher than others, but the optimal environment for TCPP/ZnO/Pt system is for ethanolsolution in pH = 9. The reason was that ZnO own gender characteristics and pH value effecting on the TCPP molecular structure, the linking strength at the ZnO interfaces, ZnO conduction band potential, etc, resulting in its higher adsorption amount of TCPP on the surface of the ZnO, but lower visible absorption ability, lower electronic injection,separation and movement efficiency between excited states of dye molecular and ZnO conduction band.?2? Comparative study on dye- sensitized ZnO system and TiO₂ system for H₂ evolution by photocatalytic water splitting shown that the TCPP/ZnO/Pt system alcohols as donors can produce hydrogen under visible light, but TCPP/TiO₂/Pt for largest H₂ evolution yield were significantly higher than that of TCPP/ZnO/Pt system. It is related to some factors, such as the amphoteric characters of ZnO, influence of pH on TCPP molecule structure, dye existing status in solution, combination mode with ZnO, potential of ZnO conduction band, and so on. To compare with TCPP/TiO₂/Pt system, although TCPP/ ZnO/Pt system has higher dye adsorption amount, lower visible absorption ability, lower electronic injection and charge transfer efficiency, which result in lower H₂ production amount by using TCPP/ ZnO /Pt system.?3?The applied research on influence factors of degrading gaseous formaldehyde is also discussed by dye-sensitized photocatalytic TiO₂ thin films under visible light. And highly photocatalytic activity of TiO₂ is used in this part study. In order to achieving degradation of formaldehyde efficiently in different concentrations of formaldehyde gas, experiments were conducted to reactor design, and dye sensitizer?TCPP, Rhodamine B, Eosin Y?,formaldehyde initial concentration and catalyst film work area on formaldehyde degradation rate of orthogonal test. The experiment results shown that the largest degradation efficiency has achieved 85%, and degradation rate more than 5% h-1in the optimal sensitizer of TCPP after 12 h under the visible light in the reactor. Increasing the illumination time, the efficiency of degrading gaseous formaldehyde was improved. In the other hand, it shown that the photocatalytic system has highly stability and efficiency for degrading gaseous formaldehyde. The technology of dye- sensitized TiO₂ photocatalytic degradation of formaldehyde gas is sultable for different concentrations of formaldehyde, and the rule of formaldehyde initial concentration on its degradation efficiency follows the kinetic model ofLangmuir-Hinshelwood. Increasing the catalyst of working area of the film is helpful to improve the degradation efficiency of formaldehyde gas in the further.