| Over the past twenty years, photocatalysis for eliminating organic pollutions has become one of the active research fields. The photocatalytic degradation method is based on semiconductor as a catalyst, which could generate electron-hole pairs under irradiation of light and form oxidation-reduction reaction system on the surface of the semiconductor, to react with external substances, causing reductions and oxidations. Preparation and properties of the photocatalyst is one of the key assist of photocatalysis. Recently spinel-type oxide AB2O4(A=Zn2+, Ni2+, Fe2+, B=Fe3+, Cr3+) oxides photocatalysts have been interested because of its narrow band gap, and high photocatalytic efficiency. In this paper, we prepared the ZnO/spinel-type AB2O4composite oxide photocatalysts by a novel and simple method, electrodepositing zinc on a304stainless steel substrate and followed by thermal oxidation in air. The effects of preparation conditions including electrochemical and thermal oxidation parameters on the structure, morphology and visible-photocatalytic activity of the composite oxide films were studied. Also the formation mechanism of ZnO/spinel-type AB2O4composite oxide films was preliminary investigated. Details are as following:(1) The effects of electrochemical parameters on the formation of the ZnO/spinel-type AB2O4composite oxide films were investigated. Zn films were electrodeposited on304stainless steel substrate in the aqueous solution containing0.05mol/L ZnSO4and2mol/L NH4Cl at0.03A/cm2for30s,60s,120s,600s and900s, respectively. Then the Zn films on304stainless steel substrate were heated to form the oxide films in air atmosphere at350℃for1h firstly and following450℃for2h. The structure, composition and morphologies of the prepared samples were characterized by X-ray diffractometry (XRD), Raman spectrometer and scanning electron microscopy (SEM). The results showed that ZnO/spinel-type AB2O4composite oxide films were prepared by electrodepositing Zn for120s,600s and900s, then heating oxidation at350℃for1h firstly and following450℃for2h.(2)The influence of the oxidation temperature on the formation of the ZnO/spinel-type AB2O4composite oxide films were studied. The time of electrodepositing Zn was fixed. The prepared Zn films on the304stainless steel surface were first oxided at150℃,250℃,350℃or450℃for1h respectively, and then oxided at450℃for2h in air. The results illustrated that the electrodeposition time of Zn for120s on304stainless steel substrate, could just form ZnO/spinel-type AB2O4composite oxide films heated at350℃for1h and followed by thermal oxidation at450℃for2h.(3) The visible-photocatalytic performance of the ZnO/spinel-type AB2O4composite oxide films were investigated by the photodegradation of3mg/L rhodamine B(RhB) in aqueous solution under visible light irradiation(λ>400nm) for1h. The results revealed that the ZnO/spinel-type AB2O4composite oxide films prepared by electrodepositing Zn for120s,600s and900s, then heating oxidation at350℃for1h firstly and following450℃for2h can availably degradate RhB with photo-degradation ratio up to72.1%,87.2%,82.4%, respectively. The results showed that the visible light photocatalytic activities of ZnO could be enhanced by compositing spinel-type AB2O4.(4) The formation mechanism of the ZnO/spinel-type AB2O4composite oxide films were preliminary studied. The structures of oxide films, prepared by oxidizing electrodeposition Zn films using different substrate (conductive glass, copper sheet and304stainless steels, respectively) and by oxidizing zinc powder adhered on the304stainless steel surface mechanically, and the ZnO/AB2O4composite oxide films were analysed with XRD. Based on the comprehensive analysis of the experimental results, the mechanism of the formation of the ZnO/AB2O4composite oxide films is suggested as followed:during the heated, the zinc could diffuse into304stainless steel driven by the energy of heat and contact electrical field between Zn-ZnO interface. When the amounts of the Zn diffused on the surface of stainless steels was enough, the reaction of zinc, iron, nickel and chrome of stainless steel with oxygen in air could contemporarily occur and the stable spinel-type oxide AB2O4could form in the interface of the coat and304stainless steel. Therefore the ZnO/AB2O4composite oxide films consisting of ZnO outer layer and AB2O4inner layer were formed. |
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