The Research Of Photocatalytic Water Splitting Of ZnO/CdS Nanoarrays

Hydrogen is a regenerative and environmentally friendly fuel of the future since its combustion generates only water,and thus it has received a great deal of attention by scientists over the past three decades.Photocatalytic or photoelectrochemical（PEC）water splitting into H2 using solar energy and semiconductors is a promising way to solve both the solar energy storage and green production of H2 fuel.CdS is the well-known semiconductor material for PEC water splitting due to its good visible-light absorption properties relating to the proper band gap width（2.4 eV）and the sufficiently negative flat band potential.ZnO is a direct bandgap semiconductor with the highest reported electron mobility and the highest conduction band edge among various electron transporters.Especially,onedimensional ZnO nanorod arrays are superior owing to its outstanding features in the facile scale-up synthesis,direct electron transport pathway,and the increased light absorption due to light scattering effect.The formation of type-II heterojunction photocatalysts with ZnO and CdS compounded the advantages of both,but the further development and application were limited in the lower efficiency and stability of ZnO/CdS heterojunction photocatalytic water splitting.Chemical bath deposition（CBD）,anneal and mechanisms of ZnO/CdS nanoarrays as well as CdCl₂ treatment were investigated.In this study,ZnO/CdS nanoarrays were synthesized by hydrothermally grown ZnO nanoarray on FTO with CdS via a CBD process combined with a simple post-annealing treatment.The phase composition,elemental composition,crystalline morphology,transittance,energy band gap and emission wavelength of ZnO/CdS nanoarrays analyzed by XRD,SEM,TEM,EDS,UV-vis and PL,its PEC property measured in a threeelectrode system in an electrochemical workstation.The experimental results showed that Cd（CH3COOH）2,CH3COOHNH4,CH4N2 S and pH had considerable influence on the CBD and the structure of ZnO/CdS.The results show that these parameters have important influence on the grain size,deposition rate and density of CdS in chemical bath deposition.This study provides a simple and effective method for uniformly densifying the CdS layer on the surface of ZnO nanorod arrays and can be used in practical production.The effects of annealing temperature on the photocatalytic activity and stability of ZnO/CdS nanodubes were investigated.A higher calcining temperature was used to improve the crystallinity of CdS and form a Zn_xCd_1-xS junction between ZnO and CdS.The H2 evolution efficiency and the photocatalytic stability were significantly improved,which could be contributed to the reduction of interface defects,the improvement of the electrons inject and crystallinity of CdS.The maximum photocurrent and most stable performance were 5.1 mA/cm² at 0.2 V bias（Ag/AgCl）and 25% decay in 12 h respectively,which corresponding to the sample calcined at 550 oC and 600 oC.And the photocurrent decay is due to the ZnO corrosion by the electrolyte because of the inevitably exposed ZnO or the shell cracking during calcining.Finally,the effect of Cd Cl2 wet annealing on the photocatalytic activity and stability of ZnO/CdS nanoarrays was studied.The results show that the photocatalytic activity and stability of the nanostructures are significantly improved.The photocatalytic activity of ZnO/CdS nanocrystals treated with CdCl₂ did not appear on the surface after photocatalytic test.The photocatalytic current density was 6.5mA/cm²（0.4V vs Ag/AgCl）.The current density keep 93% after 20 hours photocatalytic test.