Design And Preparation Of TiO₂ And G-C₃N₄Based Nano-composite Film For Their Efficient Photoelectrochemical Water Splitting Hydrogen Production Properties

Abstract:In recent years,the fossil fuel shortage and its severe environmental impact have threatened the sustainable development and even survival of human society.Therefore,it is anurgent task to explore a new type of green renewable energy.Thus,it is highly desirable to convert the rich solar energy intoclean environment-friendly hydrogennenergy to satisfy the current and future demand of human for energy by semiconductor photocatalyst.In this thesis,three kinds of photocatalytic composite films were prepared by using TiO₂ and g-C₃N₄to improve their photocatalysis/photoelectrochemical water splitting hydrogen production properties.Three kinds of photocatalytic composite films were fabricated on the FTO conductive glass,including CdS/H-3D-TiO₂,CoO/TiO₂ and g-C₃N₄/CdS.The crystal form,morphology and optical properties of the samples were characterized byX-ray diffraction?XRD?,transmission electron microscopy?TEM?,scanning electron microscopy?SEM?,UV-visspectrophotometer?UV-vis?etal.The photoelectrochemical tests,including linearsweep voltammetry?LSV?,current-time curve?IT?,photoelectrochemicalwater splitting hydrogen productionwere measured using an electrochemical workstation.The main contents are as follows:?1?CdS/H-3D-TiO₂was prepared by immobilizing CdS nanoparticles?NPs?on hydrogenated branched TiO₂ nanorod arrays?H-3D-TiO₂?using sequential chemical bath depositionmethod.Furthermore,a Pt wire was proposed to build a“highway”forphotogenerated electrons and an effective catalytic hydrogengenerator in CdS/H-3D-TiO₂/Pt-wire photocatalysis system forthe first time.Without the application ofpotential bias,the designed photocatalysis system offers ahydrogen production rate of 18.42?mol cm^-2 h^-1 under visiblelight irradiatio n,which is 11.2timesthat of CdS/H-3D-TiO₂ without Pt.The highly efficient photocatalystactivity of the samp was contributed to the hydrogenationtreatment and Pt wire as“highway”of the photogenerated electrons and cocatalyst.Hydrogenation reduces the resistance of3D-TiO₂ and increased the binding force between CdS and 3D-TiO₂.The Pt wire acts as an electron super highway between the FTOsubstrate and H⁺ions to evacuate the generated electrons to H⁺ions and catalyze the photocatalytic hydrogen productionreduction reaction and consequently generate H₂ gas.?2?The p-type CoO nanoparticles were successfully deposited on the n-type three-dimensional branched TiO₂ nanorod arrays?3D-TiO₂?by photochemical deposition and thermal decomposition method to for m a novel p-n heterojunction nanocomposites?CoO/3D-TiO₂?.The band gap of CoO nanoparticles is about 2.4 eV,which exhibited an excellent visible light absorption performance,and its deposition amount has an important effect on the photoelectrochemistry?PEC?performance of the sampe.When the deposition cyclic number was 5,the maximum hydrogen production was 0.54 mL h^-1 cm^-2?at a potential of 0.0 V vs Ag/AgC l?.In addition,the saturated photocurrent of CoO/3D-TiO₂-5 under visible light was 1.68 mA cm^-2,which is about 2.5 times that of pure 3D-TiO₂.Due to the p-n heterojunction structure of CoO/3D-TiO₂,forming an internal electric field with the directionfrom n-type TiO₂to p-type CoO,promoted the effective charge separation and transferand,resulted in a high efficiency andstable PEC activity.?3?The unique structure ofg-C₃N₄with white fungus-like structure disperses evenly on the blank FTO substrate by an electrophoretic deposition method at first and then in situ growth of CdS by chemical bath deposition?CBD?method,which forms a heterojunction structure nanocomposite.The results showed that the as-synthesized optimal g-C₃N₄/CdS obtains the highest photocurrent density?5.4 mA cm²?,which was ca.1.8 times that of pure CdS.This is contribute to the coupling CdS and g-C₃N₄to synthesis g-C₃N₄/CdS heterostructure composites not only improved photocatalytic performance,but also enhanced the photostability of CdS under visible light irradiation.