Preparation Of Cobalt-based Bimetallic Oxide（Sulfide） Compounds/g-C₃N₄ Heterojunction And Study On Their Photocatalytic Hydrogen Performance

Photocatalytic hydrogen（H₂）production is an ideal way for solar energy conversion and storage,which is an effective strategy to solve the depletion of fossil fuels and its environmental problems.Graphitic carbon nitride（g-C₃N₄）has attracted wide attention owing to its low cost,physical and chemical stability,non-toxic and so on.However,the application of bulk g-C₃N₄is limited because of poor light absorption,low specific surface area and rapid recombination of photogenerated carriers.At present,the construction of heterojunction photocatalyst is an effective way to solve the problem of light absorption and charge recombination.In recent years,cobalt based bimetallic oxygen/sulfide shows more excellent photoelectric properties than single metal,in which the multivalent metal ions can provide abundant redox sites and facilitate the H₂evolution.Based on the above analysis,Zn Co₂O₄/g-C₃N₄and ZnCo₂S₄/g-C₃N₄heterojunctions were constructed.Meanwhile,the structural composition,micro morphology and photoelectric properties of the samples were analyzed.In addition,the photocatalytic H₂production performance and the mechanism of the samples were investigated.The specific contents and conclusions are as follows:（1）Zn Co₂O₄was loaded on the surface of two-dimensional g-C₃N₄nanosheets by refluxing-calcination method.The results shows that the introduction of Zn Co₂O₄nanoparticles not only increases the specific surface area and enhances the light absorption performance,but also significantly accelerates the separation and transfer of carriers.Meanwhile,the composite sample has good structural stability.Zn Co₂O₄/g-C₃N₄system follows Z-scheme charge transfer mechanism,the electrons（e^–）in the conduction band of g-C₃N₄were combined with holes（h⁺）in the valence band of Zn Co₂O₄due to a strong internal electric field,retaining the original high redox properties of g-C₃N₄and Zn Co₂O₄.Hence,the system shows excellent H₂production ability.Using 20 vol.%triethanolamine（TEOA）as the sacrificial agent,the H₂production rate of 10%-Zn Co₂O₄/g-C₃N₄can reach1530μmol·h^-1·g^-1under 300 W Xe lamps irradation（350-780 nm）,which is 10 times higher than that of pure g-C₃N₄(150μmol·h^-1·g^-1),and is also higher than that of Zn O/g-C₃N₄(402μmol·h^-1·g^-1)and Co₃O₄/g-C₃N₄(552μmol·h^-1·g^-1).（2）ZnCo₂S₄nanoparticles was prepared by a simple hydrothermal method,and then attached to the surface of g-C₃N₄nanosheets by solvent evaporation method to obtain ZnCo₂S₄/g-C₃N₄composites.The results shows that the introduction of ZnCo₂S₄not only broadens the optical absorption range,but also generates more carriers,leading to low recombination rate.Further analysis demonstrates that the Z-scheme charge transfer of ZnCo₂S₄/g-C₃N₄heterojunction can effectively retain the strong redox ability of electrons and holes in the valence band of g-C₃N₄and conduction band of ZnCo₂S₄,thus promoting the rapid consumption of h⁺and the reduction of H⁺to produce H₂.Using 20 vol.%triethanolamine（TEOA）as the sacrificial agent,the H₂production rate of25%-ZnCo₂S₄/g-C₃N₄sample can reach 6619μmol·h^-1·g^-1under 300 W Xe lamps irradation（350-780 nm）,which is 77 and 58 times of that of pure ZnCo₂S₄(84μmol·h^-1·g^-1)and g-C₃N₄(120μmol·h^-1·g^-1).