Metal Oxide-based Composite For Overall Water Splitting

Metal oxide-based semiconductors have recived more and more attention in photocataiytic overall water splitting due to their suitable band gap structures,excellent physical and chemical properties and earth-abundant feature.Overall water splitting under solar irradiation with a semiconductor has been long investigated as an attractive pathway to convert solar energy into chemical energy in the form of H2.However,using only single semiconductor as photocatalyst remains a great challenge.Designing hetero-junction systems is a most common method used for photocatalysis.In this paper,three metal oxide-based nanocomposites were synthesized,including Cobalt(II)oxide/graphitic carbon nitride(Co O/g-C3N4)composite,Z-Scheme Co3(PO4)2/?-Fe2O3 structure and Cu2+1O/NCQDs nanocomposite.The photocatalytic activity and corresponding catalytic mechanism in these systems were also explored.The detailed works included the following parts:(1)Co O/g-C3N4 composites were synthesized by one-step thermal decomposition method,which exhibit enhanced photocatalytic efficiency for overall water splitting without sacrificial reagents.The 10%Co O/g-C3N4 exhibits optimal photocatalytic performance,where H2 and O2 evolution rates are respectively 0.46 ?mol/h and 0.21 ?mol/h under visible light irradiation without any sacrificial reagents.It is worth mentioning that the H2 and O2 production almost approach the stoichiometric ratio 2:1.The enhanced photocatalytic activity of Co O/g-C3N4 may be ascribed to the efficient charge separation and fast decomposition of H2O2 by Co O.(2)A Z-Scheme Co3(PO4)2/?-Fe2O3 structure was designed as efficient photocatalyst for overall water splitting under visible light.The as-prepared Co3(PO4)2/?-Fe2O3 photocatalyst shows excellent improvement in visible light absorption and photocatalytic efficiency for overall water splitting,in which Co3(PO4)2 and Fe2O3 act as H2-evolving and O2-evolving photocatalysts,respectively.By varying the amount of ?-Fe2O3 in the photocatalysts,the results indicated that the Co3(PO4)2/?-Fe2O3-1.6% exhibited the optimal photocatalytic activity in hydrogen production(0.63 ?mol/h)and oxygen evolution(0.32 ?mol/h),which is almost 35-fold of pure Co3(PO4)2 catalyst.In addition,the chemical stoichiometric ratio of H2 and O2 in the photocatalysis process is almost 2:1.The enhanced photocatalytic activity was attributed to the synergetic effects of Co3(PO4)2 and ?-Fe2O3 in Z-Scheme structure,which result in highly efficient separation of photogenerated charge carriers.(3)Cu2+1O/NCQDs nanocomposites were obtained by stirring a mixture of ammonia treated CQDs and Cu2+1O for photocatalytic water spiltting.Photocatalytic tests show that Cu2+1O can split H2 O into H2 and O2 via a four-electron pathway.The introduction of NCQD into the bulk improves the separation efficiency of charges.The results indicate that the Cu2+1O/NCQDs-1.5% has the optimal performance under visible light without any cocatalyst and sacrificial reagents.The hydrogen production is 0.14 ?mol/h and oxygen evolution is 0.07 ?mol/h,which was much better than the pure Cu2+1O.In addition,the composites perform enhanced photostability,which can be attributed to the fast separation of charges after the introduction of NCQDs.