Study On Preparation And Characteristic Of CD/CdS@MIL-101 Photocatalysts

The energy and environmental crisis has spurred researches on clean and environmentally friendly sustainable energy.It is commonly believed that hydrogen is one of the cleanest and sustainable energy carrier candidate to replace fossil fuels.Photocatalytic hydrogen evolution from water splitting is considered to be an ideal and promising method to obtain hydrogen energy,wherein improving the efficiency of photocatalyst is an important technical issue.In this work,a novel photocatalyst is proposed and demonstrated.Different band-gap matching semiconductors are loaded on the pore or surface of MOFs to form heterojunctions and restrain carrier reco mbination.Furthermore,doping in cocatalyst is analyzed on its improvement of the electron mobility and the photocatalytic efficiency.In this work,CD/CdS@MIL-101 composite photocatalyst materials were prepared by encapsulating CdS quantum dots and carbon quantum dots（CDs）in the cage of MIL-101 by one-step double solvents method and heat treatment method using MIL-101 as the carrier.The construct of using CdS@MIL-101 heterojunction and introducing CDs as co-catalyst aim to enhance the photocatalytic hydrogen evolution of the materials.The X-ray powder diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）experiments results of CD/CdS@MIL-101 indicate that the MIL-101 and CdS have been successfully mixed,and the intrinsic luminescence peaks of CDs are found in the infrared spectrum（IR）.From the results of N₂ absorption and desorption experiments,the specific area of the material surface decreases gradually after embedding CdS and CDs into the MIL-101 channel.The results of thermogravimetric analysis（TG）and energy-dispersive X-ray spectroscopy（EDS）show that the CdS and CDs content in the composite are close to the theoretical value.Scanning electron microscopy（SEM）indicates that the MIL-101 nanoparticles have a regular octahedral morphology.When CdS and CDs are introduced into the composite,the morphology is basically remained,which indicates that CdS and CDs were mainly loaded into the pores of MIL-101.The transmission electron microscopy（TEM）and high-resolution transmission electron microscopy（HRTEM）images show that there are（002）crystal plane of hexagonal phase CdS and amorphous distribution of CDs in the composite.And the particle sizes of CdS and CDs are about 3 nm.The optimum photocatalytic H₂-evolution rate of composite materials,with CDs and CdS content about of 5 and 10 wt%,respectively,exhibits a H₂ evolution rate of 4887μmol h^-1 g^-1_（CdS）of CD/CdS@MIL-101（50）,which is 8.5 and 18.6 times higher than that of CdS@MIL-101 and bare CdS under visible-light irradiation,respectively.UV-vis diffuse reflectance spectroscopy（UV-vis）and Mott-schottky measurements results show that the band gap of CD/CdS@MIL-101 composite is 2.36 eV and its conduction band minimum is-0.55 V Vs.NHE,which is more beneficial in photocatalytic hydrogen production from the prospective of thermodynamics.Electrochemical impedance spectra,transient photocurrent response,PL and time-resolved PL indicate that CD/CdS@MIL-101 composite has the lowest impedance,lowest fluorescence intensity and lifetime,and highest charge transfer kinetics.It is found that the construction of CdS and MIL-101 heterojunction and the co-catalysis of CDs can effectively seperate photo-generated electrons and holes,increase the lifetime of photo-generated electrons,and improve the transit of photo-generated electrons to H⁺.The photocatalytic hydrogen production mechanism of CD/CdS@MIL-101 ternary composite is that the CdS produces photo-generated electron and hole pairs under visible light irradiation,and then the photo-generated electrons on the conduction band transport to the conduction band of MIL-101 by CdS@MIL-101 heterojunction.With CDs cocatalyst,the photo-generated electrons on the conduction band of MIL-101 are transferred to H⁺through CDs,and finally,hydrogen gas is generated.