Preparation And Photocatalytic Hydrogen Production Of WS₂ And Its Composite With MoS₂

In recent decades,as a promising way to alleviate the pressure on energy supply,solar hydrogen production by photocatalytic water splitting has received extensive attention from researchers at home and abroad.Up to now,many kinds of photocatalytic materials have been developed,but the development of low-cost,high-performance,stable and environment-friendly photocatalysts is still one of the key issues that limit the practical application of this technology.Two-dimensional transition metal chalcogenide compounds(TMDs),especially MoS2 and WS2,have been widely studied in the field of photocatalysis for their unique graphene-like layered structure,adjustable band gap,good visible light absorption ability and excellent catalytic activity.In comparison,researches on the photocatalytic hydrogen production of MoS2 are relatively in-depth,but the researches on WS2 with similar structure and performance to MoS2 are still very limited.In addition,among the various preparation methods of WS2,the liquid-phase precipitation method is more favorable for large-scale industrial preparation.However,in the preparation process,impurity WO3 is easily introduced.Therefore,in this paper,based on preparation of WS3 by precipitation method,a certain amount of thiourea was introduced in the subsequent calcination process to convert WO3 into WS2.Furthermore,considering that the heterojunction can realize effective separation of photogenerated carriers,which is more beneficial for the photocatalyst to obtain higher catalytic performance,simple acidification precipitation combined with the calcination method was used to prepare WS2/MoS2 composite photocatalyst,and its hydrogen production performance was studied.The detailed research contents are as follows:(1)WS3 was first prepared by acidifying the mixture of Na2WO4 and Na2S with HCl.Then the obtained WS3 was mixed with different amounts of thiourea and calcined under N2 atmosphere to obtain a series of photocatalysts denoted as x-WS2.The composition,micro-morphology and element valence of the photocatalysts were analyzed by a series of characterization methods such as XRD,SEM,TEM,XPS and UV-Vis DRS.Finally,using 30 W LED lamp as light source,Erythrosine B sodium salt(ErB)as photosensitizer and triethanolamine(TEOA)as sacrificial agent,the effect of thiourea addition amount on photocatalytic hydrogen production was investigated.It was found that the introduction of thiourea can successfully realize the in-situ vulcanization of WO3 to WS2.When 0.3 g of thiourea was added into WS3(0.5 g),the photocatalytic hydrogen production activity was 1686.3 μmol h-1g-1,which was comparable to that of 0-WS2(1637.8 μmol h-1 g-1)without thiourea(composed of WO3/WS2 heterojunction).When the amount of thiourea continues to be increased,g-C3N4 formed.An effective WS2/g-C3N4 heterojunction was constructed when the amount of thiourea was 2.0 g and the catalytic performance was the highest among all the catalysts(2428.7 μmol h-1 g-1).Secondly,the effects of ErB concentration and types of dyes on the performance of photocatalytic hydrogen production were also explored.The results showed that 3.0 mmol L-1 ErB dye exhibited the best sensitization effect.Furthermore,the effect of preparation method of WS2 on photocatalytic hydrogen production performance was also investigated and the results showed that the performance of WS2 prepared by this method was superior to those of WS2 prepared by other methods(solid-state sintering with Na2WO4 and thiourea as raw materials,and direct pyrolysis of ammonium tetrathiomolybdate(ATM)).In addition,cyclic hydrogen production experiments on 0.3-WS2 were carried out and the results showed that its cyclic stability was not ideal,which was mainly caused by the photodegradation of dye sensitizer.Finally,based on the fluorescence quenching of x-WS2 photocatalyst on ErB and photoelectrochemical results,the hydrogen production mechanisms of WS2,WO3/WS2 and WS2/g-C3N4 were proposed.(2)Using the method similar to that of WS2 described above,WS3/MoS3 was first prepared by acidifying ammonium tetrathiotungstate(ATT)and ATM with 1 mol L-1 HCl at room temperature.Then the obtained WS3/MoS3 was thermally decomposed under N2 atmosphere to obtain WS2/MoS2 composite photocatalyst.The obtained samples were characterized by XRD,SEM,TEM,XPS and Raman spectroscopy techniques.The effects of different preparation methods and the ratio of W/Mo on the hydrogen production performance of WS2/MoS2 were investigated.The results showed that the photocatalytic hydrogen production performances of WS2/MoS2 with different W/Mo ratios were better than that of pure MoS2,but lower than that of WS2.In addition,the change in W/Mo ratio had no significant effect on the catalytic activity.Finally,the reason for this phenomenon was speculated.