Preparation Of Porous ZnO Films And Their Photocatalytic Performance

ZnO is aâ…¡-â…¥group compound and one of the most promising semiconductor materials for its wide band gap(E_g=3.37 eV) and large excitonic binding energy(60 meV) at room temperature.It may be applied in fields of luminescence,gas sensors, super hydrophobic materials,dye-sensitized solar cells and photocatalytic degradating organic wastewater and so on.In this thesis,micro/nano-structured porous Zn films were obtained on the substrates of 304 stainless steel foils and grids by electrodopositing,using hydrogen bubbles as templates at higher cathodic polarization.Then the metallic films were oxidated to form the porous ZnO films in air atmosphere at 350℃for 1 h and 450℃for 2 h.Ag/ZnO and F/ZnO films were also prepared by photoreduction of AgNO₃ and anodic potentiostatic fluorination in NaF solution on the porous ZnO films,respectively.The strucutures and morphologies of the samples were characterized by scanning electron microscopy(SEM) combined with energy dispersive X-ray spectroscopy(EDXS),FTIR spectrum(FIIR),UV-vis diffuse reflectance spectroscopy(UVDRS).The photocatalytic activities of the porous ZnO films were tested using photodegradation performance of rhodamine B(RhB,10mL,3～7 mg·dm^-3) in aqueous solution,with fluorescent mercury lamp(125W) as the light source.The effects of preparation parameters on morphologies, structures and photocatalytic performance were investigated in detail.The main works are as follow:1.The porous ZnO films were obtained on stainless steel foils substrates by heating porous Zn films.The influences of deposition parameters,including current density, concentration of ZnSO₄ and NH₄Cl,and temperature of the electrolyte on the morphologies and structures of the porous Zn films had been systematically studied.The results show that the size of the pores and the mophologies of the pore walls can be tuned by changing the electrodeposition parameters.The Zn films with the major pores of 50～100μm and dentric crystallines made up of particles of 100～300 nm in the pore walls can be obtained in the solution of pH 5 containing ZnSO₄(0.05 mol·dm^-3),NH₄Cl(2.0 mol·dm^-3) at 30℃and current densities of 0.3 to 0.8 A·cm(-2).The porous ZnO films obtained from heated the porous Zn films electrodeposited at 0.3 A·cm^-2 show the best performance for photocatalytic degradation of RhB.The degradation ratio is above 90% after illumination 6 h.2.The porous Zn films were obtained on stainless steel grid substrates by the same method.The size of the pores and the mophologies of the pore walls can also be tuned by controlling the current density and electrodeposition time.The results show that 0.6 A·cm^-2 and 60s is optimal electrodeposition condition.The porous Zn films transformed into the porous ZnO films at 350℃for 1 hour and 450℃for 2 hours,which has the best photocatalytic performance for RhB degradation.After irradiation for 4 h,the ratio of degradation is above 90%,and kept about 85%during 10 recycles.3.The Ag modified ZnO(Ag/ZnO) films were prepared by depositing metallic silver on the surface of porous ZnO films with a method of AgNO₃ photoreduction.Their crystal structure,optical properties and photocatalytic performance had been investigated. The results show that,after Ag photodeposited,the surface morphologies is changed, absorption edge is red-shift and the photocatalytic activities is improved.After irradiation for 2 h,the ratio of degradation is above 90%,and remains above 80%during 20 recycles.4.Porous F/ZnO films were first prepared with the method of anodic potentiostatic fluorination using ZnO films as working electrode at 0.2～1.4 V(vs SCE) in solution of 0.5 mol·dm^-3 Na₂SO₄ and NaF.The effects of the preparation parameters including the fluorination potential,concentration of NaF and fluorination time were investigated.The results show that the crystallic structure of the films are unchanged.while the morphologies alter and contents of oxygen species on the surface decrease after the fluorination processes.The films prepared in 30 mmol·dm^-3 NaF,at 0.8 V for 10 min exhibit the highest photocatalytic performance.The degradation ratio of RhB is 94% after photocatalysising for 2 h.And the ratio of degradation remained above 90%during 10 cycles.