Preparation And Properties Of MX₂ Chalcogenides （Mo Se₂）

Molybdenum diselenide is a member of the transition metal chalcogenide family with the advantage of a narrow bandgap（1.33-1.72 eV）.Therefore,it is considered as a potential photocatalyst and has a great development prospect in the field of clean energy that degrades water pollution.In spite of this,the photocatalytic activity of molybdenum diselenide is still limited by some drawbacks,such as the tendency for the layered material to accumulate,the low light absorption efficiency,and the high combined probability of photogenerated electron-hole pairs.In response to the above problems,many researchers try to use such effective means to solve as surface modification,doping,semiconductor compounding,and structural adjustment.These methods can overcome the above drawbacks and greatly improve the photocatalytic activity of the sample.In this paper,the molybdenum selenide materials were modified by adjusting the crystal form to obtain more active sites and prepare heterostructures.The amorphous MoSe_x materials and MoSe₂/g-C₃N₄ composites with excellent photocatalytic properties were prepared by simple low-temperature hydrothermal method and solvothermal method.A series of structural characterization,performance tests and important photocatalytic mechanism analysis were performed on these two materials.The main research contents of this paper are as follows:1.A novel nanostructure of amorphous MoSe_x was produced by a facile low-temperature hydrothermal method.Better adsorption and photodegradation of rhodamine B（RhB）and methylene blue（MB）were achieved with amorphous MoSe_x than with crystalline MoSe_x under visible light irradiation.More importantly,the catalytic mechanisms of the amorphous and crystalline samples were compared by free radical-trapping experiments.The results showed that the main active substance that increased the photocatalytic activity in amorphous MoSe_x was superoxide radicals（·O₂^-）,whereas the main active substances for crystalline MoSe_x were holes（h⁺）and hydroxyl radicals（·OH）.In addition,amorphous MoSe_x had more unsaturated atoms and a greater specific surface area because of its unique amorphous structure,thus enhancing its photocatalytic performance and adsorption capacity.The study of amorphous molybdenum selenide provides a new idea for improving the photocatalytic activity of two-dimensional materials,which is of great significance.2.MoSe₂/g-C₃N₄ composites are successfully prepared use a simple hydrothermal method.Transmission electron microscopy showed that the MoSe₂/g-C₃N₄heterostructure has the coating structure of MoSe₂ nanoplates perpendicularly intercalated on g-C₃N₄.Finally,its photocatalytic activity was investigated by UV-Vis absorption spectroscopy and electrochemical impedance spectroscopy.The optimized MoSe₂/g-C₃N₄（1:2）heterostructure achieved 100%degradation rate of rhodamine B（RhB）at 60 min and 80%reduction rate of Cr（VI）at 120 min under visible light irradiation,respectively.And the photocatalytic rate constants of MoSe₂/g-C₃N₄（1:2）for degradation RhB and photoreduced Cr（VI）are much higher than pure MoSe₂ and g-C₃N₄.The possible photocatalytic mechanism of degradation organic dyes and reduction Cr（VI）is discussed in detail.Outstanding photocatalytic activity of MoSe₂/g-C₃N₄（1:2）is considered as strong light absorption,high carrier density,and inhibition of recombination of charge carriers.The photocatalytic mechanism reported here would have great significance for the research and development of heterogeneous photocatalysis.