Synthesis Of Molybdenum Disulfide Matrix Composites And Their Photocatalytic Performance

With the acceleration of global industrialization and modernization,environmental pollution and energy crisis have become two major problems restricting the development of human society and their own development.Of all these,water pollution,especially,has seriously endangered human health.It is urgent to develop efficient sewage purification technology to alleviate the increasingly severe environmental problems.Semiconductor photocatalysis technology can degrade toxic and harmful pollutants into non-toxic CO₂,H₂O and minerals using the energy of sunlight.It has advantages of low energy consumption,no secondary pollution and low cost,which can purify sewage deeply under natural conditions and has attracted more and more attention in recent years.As a two-dimensional semiconductor material,MoS₂ has been widely studied in photodegradation of organic pollutions due to the appropriate bandgap,abundant active sites,and stable physical and chemical properties.However,photogenerated electrons and holes are easy to recombine in pure MoS₂ due to the high binding energy of excitons,which has hindered the application of MoS₂ in photocatalytic degradation of organic pollutants.In the present paper,a series of MoS₂-based photocatalytic materials were prepared by ion doping,semiconductor composite and other techniques to furtherly improve the photocatalytic activity of MoS₂.Testing as well as characterization of the as-prepared samples were carried out on X-ray diffractometer?XRD?,scanning electron microscope?SEM?,X-ray photoelectron spectroscopy?XPS?and UV-Vis spectrophotometer?UV-Vis?and electrochemical impedance spectroscopy?EIS?,the transient photocurrent density and photoemission spectrum?PL?.Finally,the photocatalytic activity of the synthetic samples were evaluated by degrading rhodamine B and methylene blue dyes under visible light irradiation.The main research findings as follows:?1?Research on controllable hydrothermal synthesis of nanoscale MoS₂.nanoscale MoS₂ have many different structures and morphologies.There are differences for MoS₂with different structures and morphologies in crystallinity,specific surface area and active site distribution,which affects the photocatalytic activity of MoS₂.MoS₂ microspheres,MoS₂ nanosheets and MoS₂ nanorods were successfully prepared by controlling the ratio of precursor liquid,reaction temperature,time,pH value and surfactant of hydrothermal reaction.The testing results indicated that:when the molybdenum-sulfur ratio of the precursor solution is 1:3,together with the hydrothermal reaction temperature is 200?,and the reaction time is 24 h,at the same time the precursor solution pH is 3,so that MoS₂nanosheests can be prepared,bearing a relatively largest specific surface area?124.52 m²/g?.The degradation efficiency of 15 mg MoS₂ nanosheets for 50 ml rhodamine B and methylene blue dye solutions with a concentration of 20 mg/L in 140 min can reach 96.11%and 97.97%,respectively.Compared with MoS₂ microspheres and MoS₂ nanorods,MoS₂nanosheets shows the highest degradation efficiency,which lay a foundation for ion doping and composite of MoS₂.?2?The energy band structure of MoS₂ was regulated by ion doping to improve its photocatalytic activity.P-doped MoS₂ nanosheests and Fe-doped MoS₂ nanosheests with different impurities concentrations were prepared by hydrothermal method.The testing results showed that:the degradation efficiency of 15 mg 5.1 at.%P-doped MoS₂ for 50 ml rhodamine B and methylene blue with concentration of 20 mg/L can reach 92.35%and98.12%,respectively,within 60 min.The degradation efficiency of 15 mg of 2.9 at.%Fe-doped MoS₂ for 50 ml rhodamine B and methylene blue with concentration of 20 mg/L can reach 98.10%and 98.54%,respectively,within 60 min.Compared with pure MoS₂nanosheets,the photocatalytic activity of doped MoS₂ nanocrystals was increased by 2times.The enhanced photocatalytic activity is attributed to three reasons:firstly,P ion and Fe ion mixed into MoS₂ can modulate the energy band structure of MoS₂ and enhance the absorption for sunlight.Secondly,additional active sites were introduced in MoS₂nanosheets due to the incorporation of P and Fe ions.Meanwhile,the addition of P and Fe ions also results in the increase of MoS₂ layer spacing and the improvement of dispersion.P ions can capture photogenic holes and Fe ions can capture photogenic electrons,which inhibits the combination of hotogenic electrons and holes and improves the activity of photocatalytic reaction.Thirdly,Fe²⁺ions reduced by hotogenic electrons can also trigger a series of Fenton reactions,which are coordinated by photocatalysis and Fenton reactions.?3?Synthesis of MoS₂/Carbon materials composite and enhanced photocatalytic properties.To solve the problem of hotogenic electrons and holes easy recombination exsiting in pure MoS₂,MoS₂/RGO and MoS₂/CC composite were prepared by using hydrothermal method.MoS₂ nanosheets were vertically grown on RGO and CC in situ.The testing results showed that:the degradation efficiency of 15 mg 7.5 wt.%MoS₂/RGO composite for 50 ml rhodamine B and methylene blue with concentration of 20 mg/L can reach to 96.61%and 98.82%,respectively,in 60 min.The degradation efficiency of MoS₂/CC composite with 8.4 mg MoS₂ for 50 ml rhodamine B and methylene blue with concentration of 20 mg/L can reach to 96.12%and 98.14%,respectively,in 60 min.Compared with pure MoS₂ nanosheets,the two composite materials have significantly improved the degradation performance for organic dyes.This enhanced photocatalytic activity due to the schottky junction formed between MoS₂ and carbon materials.Under the light irradation,hotogenic electrons in MoS₂ will transfer to carbon materials,and the hotogenic holes will stay in MoS₂.Therefore,hotogenic electrons and holes is separated in space.At the same time,carbon material not only provides nucleation sites for the growth of MoS₂ nanosheets,but also prevents MoS₂ nanosheets self-assembly into microspheres by forming the space steric effect in solution.The structure that MoS₂ nanosheets vertically grow on the RGO can produce more catalytic active sites,which is benefit to improve the photocatalytic efficiency of MoS₂.The MoS₂ nanosheets were vertically grown on carbon cloth,which not only improve the separation efficiency of electrons and holes by the schottky junction between MoS₂ and carbon materials,and produce more catalytic active sites,but also facilitate the recycling of catalyst and avoid secondary pollution.?4?The preparation of MoS₂/RGO/Bi₂WO₆ composite and their photocatalytic performance.In order to preparate a high active photocatalyst driven by visible light,MoS₂nanosheets has been vertically grown on RGO in situ by hydrothermal method.Then Bi₂WO₆ was grown on MoS₂/RGO composite in situ by hydrothermal method.By controlling hydrothermal process parameters and the proportion of each component in the precursor solution,MoS₂/RGO/Bi₂WO₆ composite with different composite proportions was prepared.The testing results showed that the degradation rate of the 15 mg MoS₂/RGO/Bi₂WO₆-2 sample for 50 ml methyl orange dye solution with the concentration of 20 mg/L can reached 93.54%within 60 min.Compared with MoS₂/RGO composite,the photocatalytic activity of MoS₂/RGO/Bi₂WO₆ composite increased 7 times.Compared with the pure Bi₂WO₆,the photocatalytic activity of MoS₂/RGO/Bi₂WO₆-2 composite increased 5 times.This enhanced photocatalytic activity benefited from the schottky junction formed by MoS₂,RGO and Bi₂WO₆.In the process of photocatalytic reaction,the hotogenic electrons of Bi₂WO₆ and MoS₂ will transfer to RGO,leaving hotogenic holes in the MoS₂ and Bi₂WO₆,which not only effectively separated the hotogenic electrons and holes.The hotogenic electrons on the RGO participate in reduction reaction,and the hotogenic holes on MoS₂ and Bi₂WO₆ participate in oxidation reaction.MoS₂ can make full use of the energy of sunlight,and the hotogenic holes of Bi₂WO₆ owns strong oxidation ability.The hotogenic holes of Bi₂WO₆ can degrade the large organic macromolecular into small intermediates.MoS₂ can furtherly degrade these small intermediates into CO₂,H₂O and inorganic matter.The photodegradation activity was greatly improved by three kinds of materials system.At the same time,the existence of RGO has not only provided a template for the growth of MoS₂ and Bi₂WO₆,but also form the space steric effect in aqueous solution,which make MoS₂/RGO/Bi₂WO₆ composite with larger specific surface area.