Preparation Of G-C₃N₄-molybdenum Oxide Composite Photocatalysts And Their Photocatalytic Performance

At present,energy consumption is serious,and new clean energy is urgently needed to replace it.In addition,the degree of environmental pollution is gradually increasing with the progress of science and technology.The conversion of solar energy into easily stored hydrogen energy through photocatalytic water splitting technique can balance this contradiction,and represent the possible trend and direction of emerging energy in the future.g-C₃N₄ possesses a narrow bandgap with visible-light response,high chemical stability,and high conduction band position,which endows it with excellent reducing ability.Moreover,g-C₃N₄ can be exfoliated into a special two-dimensional layered structure through a simple route.However,due to the high recombination rate of photogenerated electron-hole pairs and limited light absorption,the photocatalytic performance of g-C₃N₄ is still limited,and not reaches the expectation.Therefore,the compounding of other semiconductors and g-C₃N₄ to construct Z-scheme heterostructures can efficiently enhance the charge separation and transferring.It can facilitate the diversity of photocatalysts and ultimately promote the enhancement of photocatalytic hydrogen evolution activity.As an important transition metal oxide,molybdenum oxide（MoO₃,MoO₂）has unique physical and chemical properties.The band structure of MoO₃is matching well with that of g-C₃N₄ with the band gap of 2.7-3.2 e V,and the high valence band potential has good oxidation activity.MoO₃ and MoO₂ have good optoelectronic properties with significantly improved light absorption capacity in the visible and near-infrared regions caused by oxygen vacancies.Therefore,the extended light adsorption can be obtained by their composites with g-C₃N₄.At present,there are few reports on the facile methods to control the content of oxygen vacancies,and the morphology and size of molybdenum oxide materials,which significantly affected the photocatalytic activity.Based on the above descriptions,this dissertation aims to develop molybdenum oxide materials containing oxygen vacancies with regulated morphology and size.Furthermore,the Z-scheme composite of 2D g-C₃N₄-MoO_x with greatly improved hydrogen evolution activity can be prepared through a facile ultrasonic exfoliation and compounding process.1.MoO_3-xnanorods with rich oxygen vacancies were synthesized by a solvothermal method under a trace amount of oleylamine.It is electrostatically self-assembled with g-C₃N₄nanosheets to form a direct Z-scheme composite photocatalyst.The composite material exhibits strong absorption in the visible and infrared regions.When the loading amount of MoO_3-x is 19 mg,the hydrogen evolution activity of CNM19 reaches 4.183 mmol·g^-1·h^-1,and the apparent quantum efficiency is 4.4%irradiated at 365 nm,which is 4.4 times of g-C₃N₄.The content of small-sized MoO_3-x in the composite and the number of oxygen vacancies have important effects on the photocatalytic activity.In addition,the generated rates for H₂ and O₂from the water splitting of CNM19 were 0.755 and 0.368μmol·g^-1·h^-1.2.By controlling the ratio of oleylamine and oleic acid,MoO₂ nanosheets with different morphologies and sizes were prepared through a solvothermal synthesis route.Among them,the Z-scheme two-dimensional composite nanophotocatalysts of g-C₃N₄and MoO₂ nanosheets with a diameter of 30 nm were produced by a liquid-phase two-phase interfacial compounding process.The existence of oxygen vacancies in MoO₂is verified by various analyses.Compared with pure g-C₃N₄,the hydrogen evolution activities of three composites were significantly improved.Among them,CNMo2-53.5 presented the highest hydrogen evolution activity of9.185 mmol·g^-1·h^-1,which was 5.36 times that of g-C₃N₄.The great improvement of catalytic performance is resulted from oxygen vacancies in MoO₂,which improves the visible light absorption capacity of the material.Secondly,the two-dimensional structure of MoO₂nanosheets and g-C₃N₄ nanosheets have a large bonding area,which improves more active sites.Furthermore,MoO₂has good conductivity with the strong H⁺adsorption capability.The synergistic effect of these three aspects further accelerates the charge transfer between Z-scheme interfaces.3.Ultrafine MoO₃ nanowires were synthesized by a hydrothermal method,and a ternary Z-scheme heterojunction of MoO₃,carboxylated graphene oxide（GO-COOH）and g-C₃N₄was constructed under ultrasound procedure.The ternary composite exhibited better hydrogen evolution activity of 4.771 mmol·g^-1·h^-1 than g-C₃N₄ and binary materials,which was 2.79times that of g-C₃N₄.The improvement of photocatalytic performance lies in the promoted light absorption capacity of the material with the introduction of MoO₃,a larger specific surface area and more active sites caused by the small size of MoO₃,and the accelerated transfer of charges between the interfaces with GO-COOH conductive medium.