Research On Composite Modified Cadmium Selenide Quantum Dots And Its Photocatalytic Hydrogen Production

The photocatalytic hydrogen production as an effective way via transforming solar energy into hydrogen energy to solve the energy crisis and environment problem is widely concerned.However,low photocatalytic efficiency limits the application prospect of photocatalysis hydrogen production,so we need to design high efficiency photocatalyst to expand its application field.CdSe QDs is considered as one of the photocatalytic materials with research potential because of its characteristics of visible light response,controllable bandgap and rich surface properties.At present,it is necessary to modify CdSe QDs to solve the problem of fast charge recombination rate and low photocatalytic activity.In this paper,a series of composite systems are constructed by means of interfacial control,surface modification and cocatalyst introduction,which gradually enhance the photocatalytic performance of CdSe QDs.The photocatalytic mechanism of the composite system and the role of TiO2,polyethylene glycol,g-C3N4 or oxidized graphene in photocatalytic hydrogen production are studied in detail by the analysis of morphology,structure,composition,the light capture ability,electron hole separation efficiency and other properties.In addition,the main conclusions are as follows:(1)A binary compound system of TiO2/CdSe QDs is constructed with sulfhydryl propionic acid(MPA)as a connecting agent,and the influence of the crystal surface of three-dimensional semiconductor TiO2 on the photocatalytic activity of CdSe QDs was explored.The amount of photocatalytic hydrogen evolution from{001}-TiO2/CdSe QDs under visible light irradiation respectively is 8.9 and 213.1 times that of the single CdSe QDs and TiO2.Notably,the photocatalytic hydrogen production rate of {001}-TiO2/CdSe QDs(795.3 ?mol g-1h-1)is 2.2 times that of{101}-TiO2/CdSe QDs.The mechanism of photocatalytic hydrogen production of TiO2/CdSe QDs is instituted by thermodynamics and kinetic analysis as follows:the photo-generated electrons of CdSe QDs transport to TiO2,and then transmit to Ni for proton reduction reaction to produce hydrogen.The main reason for the excellent photocatalytic hydrogen activity of {001}-TiO2/CdSe QDs is that the electron transfer rate between {001}-TiO2 and CdSe QDs is accelerated by the joint action of large aperture,high surface energy and large electron transfer driving force,so that more electrons are involved in the proton reduction reaction.This shows that crystal control of electron transfer rate can enhance photocatalytic efficiency of composites.(2)PEG-CdSe QDs is prepared by one-step aqueous solution method with the polyethylene glycol(PEG)as a hole-scavenger,and its photocatalytic performance is studied.Photocatalytic hydrogen evolution efficiency of PEG-CdSe QDs is 1.5 times that of CdSe QDs.Thus,photocatalytic hydrogen production process shows that the photoholes of PEG-CdSe QDs are generated under light irradiation,transported to PEG and trapped via surfacial hydroxyl groups to improve efficiency of charge separation.This promotes more electron involved in a reduction reaction,thereby increasing photocatalytic activity.(3)With a two-dimensional sheet semiconductor material(g-C3N4)as base material,CdSe QDs/g-C3N4 composites are prepared,and the effects of sacrificial reagents and pH value on the photocatalytic activity are studied.When the loaded amount of CdSe QDs is 13.6wt%,the photocatalytic hydrogen production rate of CdSe QDs/g-C3N4 can reach to be 615.0 ?mol g-1h-1,and is higher than that of CdSe QDs(2.0 times)and g-C3N4(75.9 times).The Z model of photocatalytic mechanism is as follows:after CdSe QDs and g-C3N4 both generate electron-hole pairs,the electrons of g-C3N4 combine with the holes of CdSe QDs.Thus,it inhibits CdSe QDs own photogenerated electron hole recombination to promote more electrons involved in the reduction reaction.(4)The GO/CdSe QDs complex is prepared with good conductive graphite oxide as a cocatalyst,and the influence of sacrificial reagent,pH and GO content on the phtocatalytic activity of CdSe QDs and the role of GO in photocatalysis process are studied.The experimental results show that GO/CdSe QDs(lwt%)possesses the best hydrogen production rate(33881.4 ?mol g-1h-1)in the ethanol solution with pH 11,which is 20.7 times that of CdSe QDs.The mechanism of the activity improvement is as follows:GO not enhances the light capture ability of CdSe QDs,but provides active sites for hydrogen reduction and transmits electrons,which facilitates more electrons to participate in proton reduction reaction.Therefore,the above work provides some theoretical guidance for solving the problem of low charge separation efficiency of quantum dots photocatalyst by constructing the complex,and expands the prospect of the application of photocatalytic technology.