The Synthesis, Structure And Photocatalytic Study Of Visible-light Photocatalysts

This dissertation has three chapters:the first chapter reviews the related backgrounds of the combinatorial materials science and semiconductor photocatalysis. The following two chapters describe our work on the research of the new visible-light-driven photocatalysts,including the synthesis,structrue and characterization of superior photocatalytic activity of CaIn₂O₄ in the second chapter, the combinatorial screening of new photocatalysts YInO₃ and YAlO₃ and their photocatalytic performances in the third chapter.Chapter one is the introduction of the related background knowledge.At first,the mechanism of semiconductor photocatalysis and principle of photocatalytic water splitting are summarized based on energy band theory for semiconductor,and the latest research progress in visible light responsible photocatalysts is detailed reviewed. Secondly,the basic concept,content and history of the combinatorial materials science are introduced,and the latest progresses in parallel synthesis and high-throughput characterization for combinatorial library are reviewed.Finally,the research outline of this dissertation is summarized.In chapter two,the synthesis and characterization of superior photocatalytic activity of CaIn₂O₄ are described.The CaIn₂O₄ is synthesized with different kinds of fuel using a solution combustion method.When glycine is used as the fuel,the pure orthorhombic phase of CaIn₂O₄ is formed after the combustion reaction.The XRD data shows the impurities and structural imperfections are reduced by the following high-temperature postannealing. While urea is used as the fuel,the resulting CaIn₂O₄ is impure,and a small amount of CaO and In₂O₃ still remaining after calcination.The microstructure of the samples is investigated using the TEM technique.The precursor synthesized with the fuel of glycine shows a fine ellipsoidal shape.The average diameter and length is about 60 nm and 120 nm.After the calcination at 1100℃for 12 h,the nanometer grains self-assemble into regular rods.The average diameter and length of the rods is about 300 nm and 2μm.Further investigation indicates that the nanometric grains of the precursors self-assemble into nano-capsules first,then the nano-capsules link end to end to form the final rods during the high-temperature postannealing.However,this peculiar structure is not observed in the sample synthesized with the fuel of urea. The photocatalytic performances of the three Caln₂O₄(including the SSR, as-combusted,and rods) for Methylene blue(MB) degradation,toluene oxidation and water decomposition under visible-light irradiation(300 W Xe arc lamp,the output power from 390 nm to 770 nm is 19.6 W,λ＞400 nm,the diameter of the facula is 65 mm) are investigated.Among them,the rods show the highest activity,which can not only effectively mineralize MB and toluene into small inorganic molecules,but also exhibit excellent activity on water splitting.The reason can be attributed to the joint effect of the relatively large surface area and high crystallization degree of the rods.Firstly,in the performance of MB degradation,0.36 g rods take only 90 min to mineralize 120 mL MB solution with the initial concentration of 20.1μmol/L.The activity is about four times higher than that of the as-combusted and seven times higher than that of the SSR.The synthesis conditions(different calcination temperatures and times) which influence on the activity of the rods are studied. Furthermore,the reaction kinetics of MB degradation is also studied.It can be explained by the L-H model.The reaction is first-order.Secondly,in the performance of toluene oxidation,under the conditions of 0.35 g catalyst and 23%relative humidity,the removal ratios of toluene with the initial concentration of 175 ppmV are 17%,58%,74%,and the mineralization ratios are 15%,23%,66%respectively for the SSR,as-combusted and rods after 6 h visible photocatalytic reaction.The activity of the rods is more than four times higher than that of the SSR derived sample.Finally,in the performance of water splitting,under the conditions of 0.35 g catalyst and 120 mL deionized water,the formation rate of H₂ evolution are 0.06,0.53 and 1.22μmol·h^-1·g^-1 with the dispersion of 0.5 wt%Pt.The presence of methanol (20%(v/v))further increases the H₂ production rate up to 0.73,2.56 and 6.92μmol·h^-1·g^-1 respectively.The apparent quantum efficiency for H₂ evolution over the 0.5 wt% Pt/Caln₂O₄ rods is estimated to be about 0.24%.In chapter three,the combinatorial screening of new photocatalysts YInO₃ and YAlO₃ and their photocatalytic performances are described.The materials library containing ABO₃-type(A=Y,La,Nd,Sm,Eu,Gd,Dy,Yb,B=Al and In) semiconductors is synthesized by a parallel solution combustion synthesis technique according to the library design.Among these compounds,two novel photocatalysts YInO₃ and YAlO₃ are identified rapidly,which show high activities for MB degradation under sunlight irradiation.The activity of YInO₃ is better than that of YAlO₃.The influences on the performance of ABO₃,including electronegativity and ion radius,are discussed.The pure orthorhombic phase of YAlO₃ and cubic phase of YInO₃ are synthesized at a relatively low temperature using solution combustion method by controlling the molar ratio of reducer to oxidant(Φ_e).In the synthesis of YAlO₃,a mixture of Y₃Al₅O₁₂,Y₄Al₂O₉ and YAlO₃ is obtained under the condition of fuel-rich(Φ_e=1.2) or fuel-stoichiometric(Φ_e=1),while only pure phase of orthorhombic YAlO₃ is detected under the condition of fuel-lean(Φ_e=0.8).In the synthesis of YInO₃,the pure phase of cubic YInO₃ is achieved under the condition of fuel-stoichiometric.The microstructures of YInO₃ and YAlO₃ are both the porous structure characteristics of solution combustion reaction.They both show a fine ellipsoidal shape.The average diameter and length is about 100 nm and 200 nm.The surface area is 7.64 and 2.21m²/g,respectively.The photocatalytic performances of YInO₃ and YAlO₃ on toluene oxidation and water splitting under visible-light irradiation are studied.The same optical system mentioned in the second chapter is employed.In the performance of toluene oxidation, under the conditions of 0.35 g catalyst,175 ppmV intial concentration and 23% relative humidity,the toluene removal ratio and mineralization ratio are 71%and 41% for YInO₃ after 6 h visible photocatalytic reaction,which are higher than 51%and 24%of YAlO₃.In the performance of water splitting,under the conditions of 0.35 g catalyst,120 mL deionized water and the hole scavengers(Na₂S and Na₂SO₃),YInO₃ exhibits significant activity with the H₂ evolution rate of 18.89μmol·h^-1·g^-1,while the formation rate of H₂ evolution is only 0.54μmol·h^-1·g^-1 for YAlO₃.The apparent quantum efficiency for H₂ evolution is estimated to be about 0.65%and 0.02%, respectively.The influence of the kind of sacrificial reagents on water splitting for YInO₃ is studied.The result indicates that the formation of H₂ evolution in the solution of Na₂S and Na₂SO₃ is much higher than that in methanol or ethanol solution, which may be ascribed to their oxidation potentials.The YInO₃,with significant photocatalytic activity,has weak light absorption ability in visible region.The reason may be attributed to the joint effect of the large surface area and high crystallization degree.