Preparation Of Photoelectrode Materials In Photoelectrochemical Cell And Their Photocatalytic Water Splitting Properties

Recently, with the development of society, energy shortages and environmental issues have become increasingly prominent and thus seeking for clean and sustainable energy is an urgent task. Solar energy attracts much attention and has been considered as one of the major strategies for solving these problems. Converting solar energy to hydrogen by photocatalytic water splitting seems to be an effective way. Since water splitting system based on Ti O2 electrode was discovered by Fujishima in 1972, semiconductor-based photocatalytic system in photoelectrochemical cells gains its popularity. However, the intrinsic drawbacks such as fast chargerecombinationrate and low visible-light absorption limit itspractical application. Thus, many research groups have beenfocusing on developing novel semiconductors with improved photocatalytic efficiencies, such as doping cocatalysis and surface sensitization. The discovery of graphene has opened up a new way to enhance the photocatalytic performance due to its remarkable conductivity and superior electron mobility. In this master’s thesis, we prepared novel photoelectrode materials, using grahene as an excellent electron accepter and mediator. The photocatalytic performance of the materials was carried out in semiconductor photocatalytic system and photoelectrochemical system. The transformation of photoelectrons and the proposed mechanism were studied. The main points were shown as follows:(1) A novel ternary nanocomposite comprised of mesoporous WO3, Mn3O4 nanoparticles and N-doped graphene was prepared by a one-potdeposition method. The nanocomposite was characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The results demonstrated that the Mn3O4 nanoparticles had been successfully hybridized with the mesoporous WO3 and the WO3/Mn3O4 hybrid was well dispersed on the surface of N-doped graphene with superior interactions. The nanocomposite exhibits higher photocatalytic activity for water oxidation than the individualmesoporous WO3 and WO3/Mn3O4 catalysts. The amount of oxygen evolution from the optimized heterostructural photocatalyst(1.5wt% Mn3O4 and 2wt% N-doped graphene) was 294 mmol g-1, which was about 3.6 times as high as that from m-WO3. The heterostructure formed between Mn3O4 and m-WO3 enhances photogenerated electron/hole transfer and restrains the recombination of charges greatly. N-doped graphene in the nanocompositeacting as an excellent electron accepter and mediator also contributes to the increase of photocatalytic performance by promoting the separation and transfer of photogenerated charges. Last we offer the mechanism of oxygen production over WO3/Mn3O4/NGR photocatalyst.(2) A p-n type photoelectrochemical tandem cell based on p-type Cu doped Zn0.3Cd0.7S/graphene(Zn0.3Cd0.7S(Cu)/GR) photocathode and n-type WO3/graphene(WO3/GR) photoanode was successfully fabricated. Through examination of the optoelectronic and photoelectrochemicalproperties of Zn0.3Cd0.7S(Cu)/GR and WO3/GR photoelectrodes, we evaluate the feasibility of the tandem cell for overall water splitting under UV-vis(and visible) light irradiation. The Mott-Schottky analysis suggests that Cu-doped Zn0.3Cd0.7S becomes a p-type semiconductor. Moreover, the Cu dopant could enhance the photogenerated electrons transfer greatly during photocatalytic process. The optimal Cu doping in Zn0.3Cd0.7S photocathode concentration was found to be 6%. As is expected, WO3 photoelectrode worked as a photoanode to produce oxygen from water. The amounts of hydrogen and oxygen evolved from this tandem cell with the optimal electrodes were 459.0 and 86.2 mmol g-1, respectively. Last we also advise the mechanism of water splitting in the PEC cell.(3) A novel reactor based on photoelectrochemical cell was fabricated for overall water splitting by using Ti Si2 film as H2-photocatalyst, and WO3 film as O2-photocatalyst. The two films were characterized by scanning electron microscopy(SEM), atomic force microscopy(AFM), X-ray diffraction(XRD), UV-vis diffuse reflectance spectra, photoelectrical response and electrochemical impedance spectra. The system demonstrated nice photocatalytic activity and stability for water splitting. The amount of H2 and O2 evolved after 7 h irradiation was 171 mmol g-1 and 58 mmol g-1, respectively. Last we also advise the mechanism of water splitting in the PEC cell.