Synthesis And Characterize Of CdS Composites With Enhanced Photocatalytic Activity And Mechanism Insight

Water pollution and energy crisis have become one of the major challenges worldwide,especially in developing countries.Consequently,it is imperative to develop a non-secondary-pollution water treatment as well as low energy consumption and high efficient.Photocatalysis technique,utilizing the solar energy,could non-selected oxidize the inorganic or organic pollution into innocuous and harmless materials in wastewater.There are many photocatalysts have been developed,such as CdS,ZnS,g-C₃N₄,TiO₂ and MoS₂.Amongst,CdS gained a great popular as a visible-light driven photocatalyst with a narrow bandgap.But the photocatalytic efficiency was refined by the poor adsorption,the fast recombination rate of photo-indued holes?h⁺?and electrons?e^-?and the phenomenon of photocorrosion.There are mainly four methods to modify CdS to improve its photocatalytic efficiency:increase the reaction area and active site;change the bandgap to burden the absorption spectra;inhibit the recombination rate of h⁺and e^-;suppress the occurrence of photocorrosion.The microcrystalline cellulose?MCC?as a high molecular weight polymer and graphitic carbon nitride?g-C₃N₄?as a semiconductor photocatalytic were selected to recombine with CdS to synthesis MCC/CdS composites and g-C₃N₄/CdS heterojunctions.The obtained MCC/CdS nanocomposites and g-C₃N₄/CdS heterojunctions were characterized by XRD,SEM,TEM,BET,EDS,HRTEM and UV-Vis to observe the microstructure and optical feature.The photocatalytic performance of samples was evaluated by the degrade of MB as simulation wastewater under visible light.The characteristic results of obtained MCC/CdS composites showed that,10%MCC/CdS showed narrow and sharp diffraction peak,indicating the good crystallinity;Compared with pure CdS,the bandgap of 10%MCC/CdS decreased,resulting the reduced energy to active the photocatalyst as well as expansive absorption spectra;The surface area of MCC/CdS also enlarged with the increased amount of micropores;The result of photocatalytic evaluation experiment showed that the photocatalytic and cycle performance of MCC/CdS nanocomposites improved greatly.Especially,the photocatalytic degradation rate of MB reached 80.90%when catalyzed by 10%MCC/CdS.In the photocatalytic degradation process,h⁺showed essential oxidation and·O₂^-in the second place and·OH was weakest among three active radicals.The CdS nanoparticles dispersed on the surface of g-C₃N₄.Among all the samples,15%g-C₃N₄/CdS heterojunction showed the best crystallinity and larger crystal size.The bandgap of 15%g-C₃N₄/CdS also decreased compared with CdS.The pseudo-first order model was fitter for this adsorption process.The photocatalytic degradation rate of MB was almost 100%when catalyzed by 15%g-C₃N₄/CdS heterojunction.Meanwhile,under the same condition,the photocatalytic degradation rate of methyl orange,methyl red and rhodamine B were also over90%.In this photocatalytic system,h⁺is the main active species,·OH played a secondary role and·O₂^-was the third active species.The 15%g-C₃N₄/CdS displays the highest photocurrent intensity,which is about 5.47 and 4.1 times higher than that of g-C₃N₄ and CdS indicating the existence of heterojunction.In consideration of the band gap of single g-C₃N₄?2.63 eV?and CdS?2.16 eV?,both can be easily activated by visible light and produce e^-and the h⁺.The E_CB of g-C₃N₄ and CdS are-1.13 eV and-0.34 eV and the E_VB of them are 1.75 eV and 1.70 eV,respectively.Thus,in the interface of heterojunction,the e^-in the CB of g-C₃N₄ can directly transfer to the CB of CdS while the h⁺transfer in opposite direction.Due to the specific ultrathin structure of g-C₃N₄,the distance of the movement of photogenerated carriers to interface can be greatly shortened.Given this,the recombination of photoelectron and holes can be inhibited effectively.Meanwhile,the photocorrosion of CdS caused by the oxidation of S^2-by h⁺also be avoided.Thus,the photocatalytic degradation performance and the stability improved significantly.