Modifying The Nanostructured ZnO And TiO₂ By Graphene Oxide Leading To Improved Photocatalysis

Semiconductors photocatalysis is one of the most important research area with exciting developments in semiconductor material properties study and degradation of organic pollutants,bacteria inactivation,water splitting and CO₂ reduction.It provides possible solution for energy shortage with environmental friendly approach for harvesting light energy into chemical energy.The semiconductors photocatalysis includes a broad range of materials such as metal-oxides,nitride-or sulfide-based semiconductors,silicon,III-V semiconductors,conducting polymers,graphene-based materials and the emerging layered compounds.This thesis work focus on the semiconductor defect states to modify those defects by graphene oxide hybridization and its effect on semiconductor photocatalysis.Firstly,ZnO nanoparticles?ZnO-NPs?have been combined with graphene oxide?GO?with different contents?10%,20%and 30%?via microwave processing.The procedure provided well-dispersed ZnO-NPs between and onto the rGO layers?GZCs?.The annealing temperature and graphene oxide contents affected the UV-Vis absorption,PL emission,defect-states of the ZnO,EPR signals,photo-electrochemical response and charge transfer properties.The HRTEM microscopy images of the GZCs showed interpenetrating structures and clearly visible vacancy defects.The results indicate that the defect sites?Zn interstitials,oxygen vacancy,ionized zinc vacancy and oxygen interstitials?significantly decreased after hybridization with GO.The photo-conversion efficiency of the GZC-10%(?=13.1?10^-3%),is 13 times higher than the ZnO-NPs(?=1.02?10^-3%)illustrating higher exciton production and separation efficiency of the GZCs under photo-excitation.The GZC-10%has lower?8-15??charge transfer resistance(R_ct)compared to all the GZCs under same experimental conditions,therefore an important reason of better performance of the GZC 10%.The EPR spectra showed presence of radicals in all the samples with GZC 10%most intense signal among the different GZCs.Secondly,photocatalysis by visible light is important for a wide range of applications.In this work,graphene oxide?GO?based hybrid photocatalyst ZnO/?Fe₃O₄@SiO₂?/rGO?GZF?composites were prepared via microwave synthesis by introducing ZnO nanoparticles?ZnO NPs?and Fe₃O₄@SiO₂ onto the surface of reduced GO?rGO?.GZF composites possess magnetism therefore easy to separate from the solution simply by a hand-held magnet.They exhibit excellent photo-catalysis for the oxidative degradation of methylene blue and BPA,which is 3.6 and 3.8 times higher under visible light when compared to the ZnO NPs respectively.The surface defects in the ZnO nanoparticles before and after graphene hybridization were studied.The optical results show that the visible emission consists of violet,blue and green regions,attributed to the zinc interstitial,zinc vacancy and oxygen vacancy defects.After hybridization with the graphene the vacancy sites in the ZnO nanoparticles were significantly decreased,oxygen vacancies were healed,albeit oxygen interstitial defect?O_i?levels appeared as a new defect.The superoxide and hydroxyl radicals are responsible for the photocatalytic reaction.The present results are important for exploring graphene based magnetic photocatalysts for various applications.Thirdly,the composition of exhausts from petroleum and paint industry involves xylenes and other components mixed with water.In this work,the stepwise growth of ZnO particles to flowers was achieved through a solution-based method,and then the graphene oxide was hybridized with ZnO flowers?GZ-hybrids?via microwave-irradiation.Core level Zn 3d was employed to align the valence band off set?VBO?and the change in band gaps were measured by the X-ray photoelectron spectroscopy.The crystal defects in ZnO were significantly reduced after the graphene oxide hybridization.The origin of better performance was explored by the cyclic voltammetry?CV?,photo-electrochemical and Electrochemical Impedance Spectroscopy?EIS?indicating that GZ-15%composite has 4.1 times higher photo-conversion efficiency attributed to the higher photo-current and lower charge transfer resistance(R_ct).The comparison of o-xylene photo-degradation by the ZnO flowers and the GZ-hybrid showed that o-xylene has3.8 times higher rate of photo-catalytic degradation under UV-light when GZ-15%composite was used as the photo-catalyst.This study provides necessary photo-catalytic oxidation?PCO?degradation details of o-xylene in water.Additionally,the increasing carbon dioxide?CO₂?emission is one of the serious environmental problem,therefore its reduction and sustainable conversion to fuels by using solar energy is highly desirable.Herein,for the 1st time the TiO₂/rGO?TG?and TiO₂-?Fe₃O₄@SiO₂?/rGO?TFSG?nanocomposites have been successfully synthesized by coupling the anatase TiO₂ hollow nano boxes,Fe₃O₄@SiO₂ and graphene oxide?GO?.XPS results indicated that Ti⁺³ defects were healed out and oxygen vacancies remarkably decreased after GO hybridization.The effect of surface defect densities before and after graphene loading on CO₂ conversion to CH₄ showed intriguing results.The TG composites showed highest photocatalytic activity after GO coupling(49 mol·g^-1·h^-1).The 28.6 times higher photocurrent provided much higher quantum efficiency?3.17%@400 nm?when compared to the TiO₂nanoboxes.The mechanism of enhanced photo-catalytic CO₂ conversion to CH₄ elucidated through electrochemical reduction and photo-catalytic experiments with traceable isotope containing carbon dioxide(¹³CO₂).The Hydrogen radicals were the dominant intermediates responsible for the conversion of CO₂ to CO and CH₄.This work explains the application of highly efficient graphene nanocomposites?photo-catalyst?for the direct photo-conversion of CO₂ into chemical fuels.