Investigation On Behaviors Of Photogenerated Charges And Photocatalytic Activity Of Optimized ZnO Materials

Semiconductor-based photocatalysts, which can convert solar energy intochemical energy, are promising solution for many environment-related issues. Amongnumerous semiconductor photocatalysts, as a typical II–VI semiconductors, ZnO hasbecome a prospective material in degradation of environmental organic pollutants,because of its excellent photoelectrical properties in the filed of solar energy research.In addition, ZnO also has some advantages such as simple in preparation, morphologydiversity, nontoxicity, photochemical stability and low-cost, and shows a similarphotodegradation mechanism with that of TiO₂, which is one of the best alternativefor substituting the conventional TiO₂photocatalyst. Nevertheless, some drawbackslimit its experiment investigation and practical application in the photocatalyst region,which include low efficiency of light utilization due to the wide band gap and poorquantum efficiency of it. In recent years, it is reported that loading or doping withtransition metal （TM） has been considered as one of the most effective way tooptimize the properties of ZnO and achieve the desirable photoelectrical abbility.However, few reports expound the effect of modified elements on the behaviors ofphotogenerated charges （including the separation, transportion and recombition, etc.）of ZnO photocatalyst materials. While, the behaviors of photogenerated chargesclosely relate to the field of solar energy conversion and photocatalysis, and it is theforefront topic at surface and interface science research field. Considering what mentioned above, in this thesis, Ag, Co, Mn and Pt&Comodified one-dimensional ZnO-based photocatalysts were prepared. Their optimizedphotophysical properties were investigated with the help of surfacephotovoltage/photocurrent technology （surface photovoltage spectroscopy （SPS）,transient photovoltaic technique （TPV）, surface photocurrent （SPC）） to reveal theeffect of different elements on the photogenerated charges‘behaviors and also therelationship between surface photovoltaic properties and photocatalytic activity ofmodified photocatalysts is discussed, from which we obtained some novel results:1. Silver-loaded ZnO （Ag/ZnO） chain-like photocatalysts were prepared by afacile semi–gel method. Meanwhile, the properties of photogenerated charges wereinvestigated by SPS and TPV techniques. The conclusions are as follows: Ag acted aselectron acceptors and effectively inhibited the charge recombination in ZnO with UVlight illumination, while under the illumination of visible light a redshift of theabsorption threshold was observed due to the appearance of a Plasmon-inducedabsorption resulting from the formation of Ag nanoparticle clusters. Thephotocatalytic activity tests were also carried out. Compared to the pure ZnO, asignificant enhancement in the photocatalytic activity of Ag/ZnO was observed underboth UV light and white light （UV+visible light） irradiation, respectively. Theinformation providing here should be beneficial for the design of high performancephotocatalysts and the understanding of the behaviors of the photogenerated charges.2. A series of cobalt-doped ZnO （Co:ZnO） nanorods have been prepared by afacile solvothermal process. It is confirmed that the doping ions substitute for some ofthe lattice zinc ions, and Co²⁺and Co³⁺ions coexist. The Co doped samples have anextended light absorption range compared with the pure ZnO and shows highefficiency of photocatalytic activity under visible light irradiation （λ>420nm）. Thephotophysical mechanism of the enhanced visible photocatalytic activity wasinvestigated with the help of surface photovoltage technique. The results indicatedthat the superior photo-response performance of Co:ZnO was mainly attributed to theaction of charge transfer transition between Co ions and ZnO, and thus theincorporation of Co promoted the charge separation and enhanced the charge transfer ability, and at the same time, effectively inhibited the recombination ofphotogenerated electrons and holes. Also, the results may provide a theoretic evidencefor the design of the novel visible-light drive photocatalysts.3. A ZnO photocatalyst system in the presence of multivalent forms of Mn ions(Mn³⁺/Mn²⁺)（Mn:ZnO） was prepared via a simple solvothermal approach. Theabsorption edge of ZnO nanorods shifted to low energy after Mn doping. The Mndoped system exhibited a higher photo-response than the pure ZnO. Based on theinvestigation of the separation and transfer behavior of the photogenerated charges inthe visible region by means of SPS, SPC and TPV techniques, it is demonstrated thatthe incorporation of multivalent Mn in ZnO promoted the separation and inhibited therecombination of photogenerated charges. Thus prolonged the charges lifetime andresulted in a highly effcient photocatalytic activity. Evidence shows that there is astrong electronic interaction occurred between multivalent Mn ions and ZnO with theconcentration of Mn³⁺increasing, and Mn³⁺acted as surface active sites that presentedin doping system. These results provide a new point of view for the understanding ofthe photocatalysis mechanism of the multivalent ions doped photocatalyst.4. Base on the preparation of Co:ZnO nanorods, Pt loaded Co:ZnO hybridphotocatalysts （Pt/Co:ZnO） were prepared with hexachloroplatinic acid （H₂PtCl₆） asplatinum source. The hybrids displayed a strong photo-response in the UV and visibleregion and exhibited low recombination rate of photogenerated electron and hole. Bymean of SPS, TPV techniques, it was experimentally demonstrated the existence ofthe synergetic effect of Pt loading and Co doping effect, which significantly promotedthe separation of photogenerated charges, and inhibited the recombination of betweenthem. Thus effectively prolongated the charges lifetime and led an enhanced photo-activity and photovoltaic property. The synergetic effect exist in Pt/Co:ZnO induced a10-fold enhancement in the photo–activity of hybrids compared to the precursorCo:ZnO. The whole researches provided a basic understanding in experimental andtheoretical for designing optimized, novel, high performance ZnO-basedphotocatalysts.