Phase-controllable Synthesis Of Maghemite-carbonaceous Hybrid For Highly Efficient Photocatalytic H₂ Production

Photocatalytic water splitting to gaseous oxygen and hydrogen is of great research value,as clean and renewable energy source like solar fuel could release the reliance on fossil fuel and reduce the emission of greenhouse gases.Eversince the debut of TiO₂ for water photolysis proposed by Honda and Fujishima,the exploitation of efficient photocatalytic water splitting systems is currently experiencing an ever-increasing situation.Some transition metal oxides?e.g.,Fe2O3,Cu2O and CoO?have suitable band gaps?Eg<3 eV?to absorb visible light;however,most of their band positions are improper for overall water-splitting.For instance,Fe₂O₃ and WO₃ possess valence bands?VB?with positions sufficiently low for water oxidation half-reaction,but the conduction bands?CB?positions determine their unfavored characteristics in the application in the reduction half-reaction to produce H₂.To make full use of such visible-light absorbability and overcome the drawbacks of their undesired band position edges in redox reactions,tremendous efforts have been made including the construction of nanocomposites containing those metal oxides as one component.In this thesis,we report a facile,cost-effective and controllable strategy for the synthesis of different phases of iron oxide?FeOx?and their carbonaceous hybrids by employing a Fe-containing metal-organic framework?MOF?and graphene oxide as the sacrificial template.The prepared composite material is then applied to hydrogen production from water splitting under visible light.The main results of the research are as follows:?1?After 8 h of ultrasonic pretreatment of metal salt solution and graphene oxide,an obvious change in morphology from spindle to octahedron and a significantly reduced size of MOF particles.The use of infrared spectroscopy shows that pre-ultrasonics have a certain effect on the anchoring of metal ions on graphene oxide sheets and the regulation of particle morphology;?2?Through thermolysis of the as-synthesized MOF-GO composite under designed conditions,iron oxide-carbonaceous hybrid with different phases,i.e.,??Fe₂O₃,?-Fe₂O₃,and Fe₃O₄,could be obtained,which were then throughoutly analysed by XRD,XPS,Raman,SEM/TEM and so on.Uniform and monodisperse octahedral nanostructure of?-Fe₂O₃embedded on reduced graphene oxide?rGO?nanosheets were achieved,and the advantages of such structural features would be favorable to the maintenance of maghemite phase at large temperature gradients;?3?We performed a series of photoelectronic analysis including Mott-Schottky,electrochemical impedance spectroscopy and transient photocurrent responses experiments to prove that upon incorporation of graphene with iron oxide,the conduction band of?-Fe₂O₃-rGO nanocomposite has been elevated toward photocatalytic water reduction to produce H₂,and the recombination of photogenerated electrons and holes could be effectively retarded.Consequently,the H2 evolution rate of the as-formed maghemite-carbonaceous hybrid could be optimized to 318.0?mol/g/h in the absence of cocatalysts under the irradiation of a Xe lamp.