Synthesis And Hydrogen Production Performance Research Of CdS-based Semiconductor Photocatalysts

With the continuous development of science and technology,some traditional non-renewable energy sources（such as coal,oil,and natural gas）have become less and less,and at the same time,it have caused some environmental pollution problems,such as haze,greenhouse gas emissions and other serious problems.Therefore,it is important to develop a new energy source that is low-carbon,green,and environmentally friendly.Among many renewable energy sources,solar energy possesses the advantages such as abundant reserves and low utilization costs,so converting solar energy into hydrogen energy is an important method.The key point of the converting solar energy into hydrogen energy is the design and development of semiconductor photocatalysts.CdS is a typical semiconductor with suitable conduction and valence band location.Moreover,its bandgap is 2.48 eV,which meets the condition of photocatalatic hydrogen production through water splitting.It is a good visible light response semiconductor photocatalytist.However,at the same time,CdS also has many shortcomings due to poor hydrogen production performance,which limits its application in the field of photocatalysis.In response to this problem,we can improve the photocatalytic activity by recombining semiconductors in two different conduction bands and valence band positions.We can improve the preparation of Zn_1-xCd_xS semiconductor photocatalysts mainly using ZIF-8 as precursor（ZIFs are zeolitic imidazole framework materials,and are porous crystalline materials with a zeolite structure connected by transition metal atoms and imidazole derivatives.They are one of the MOFs series.ZIF-8 and ZnCo-ZIF belong to the ZIFs series.）.On the other hand,due to the low efficiency of the separation and transmission of photogenerated electrons and holes on the surface of CdS catalysts,photocatalytic hydrogen production activity is greatly reduced,therefore,we combine CdS and MOF（metal-organic framework materials,a new type of crystalline material with a porous structure composed of organic ligands and metal ions or metal clusters）,heterojunctions are constructed to improve the photocatalytic hydrogen production efficiency.In addition,there are fewer active sites exposed on the surface of CdS,the photocatalytic activity is limited,the separation of photogenerated electrons and holes can be increased by supporting the promoter（MoS₂）on the surface.Specifically the following process:1.Preparation of Zn_1-xCd_xS semiconductor photocatalysts using ZIF-8 as precursor.ZIF-8 crystals were synthesized using zinc nitrate and dimethylimidazol as raw materials,Using ZIF-8 as a precursor,a cadmium source（Cd（NO₃）·4H₂O）and a sulfur source（thiourea）were added,and a porous Zn_1-xCd_xS（x=0.1,0.3,0.5,0.7,0.9,1）semiconductor photocatalyst with the similar appearance morphology of ZIF-8 crystal was synthesized by a hydrothermal method.Photocatalytic hydrogen production performance test found,under the visible light irradiation,the Zn_1-xCd_xS semiconductor photocatalyst（x=0.5）has the best photocatalytic performance of hydrogen production.Then ZnCo-ZIF（zinc nitrate,cobalt nitrate and dimethyl imidazole as raw materials）were used as precursors to synthesize Co-Zn_0.5Cd_0.5S semiconductor photocatalysts.The photocatalytic hydrogen production performance test shows that Co-Zn_0.5Cd_0.5S semiconductor photocatalyst（Co=0.5 at.%）has the greatest photocatalytic hydrogen production properties when the amounts of Co is 0.5 at.%,its H₂production rate are 45.2 and 422.2 times larger than those of Zn_0.5Cd_0.5S and the CdS semiconductor photocatalyst,respectively.The synthesis of Zn_1-xCd_xS and Co-Zn_0.5Cd_0.5S semiconductor photocatalysts using ZIFs as templates has demonstrated a highly porous microstructure and high photocatalytic hydrogen production performance,providing a favorable advantage way for the preparation of high performance nanoporous photocatalysts.2.Preparation and their hydrogen production properties.of CdS/MOF photocatalysts.The CdS nanorods were synthesized using ethylenediamine as a template,and based on CdS nanorods,CdS/ZIF-8 composite nanocrystals were synthesized by adding Zn（NO₃）·6H₂O and dimethylimidazole,the amount of ZIF-8 added was 5 at.%,20 at.%,50 at.%,100 at.%,200at.%,and 500 at.%of CdS nanorods.The photocatalytic hydrogen production performance test shows that the amount of ZIF-8 added was 100 at.%,it is 101.2 times that of pure CdS nanorods.In addition,some Zn elements in ZIF-8 were doped with Co,Ni,and Cu metal ions.and successfully synthesizing CdS/ZnM-ZIF（M=Co,Ni,Cu）heterogeneity structural photocatalysts,it was found that the amounts of Co,Ni,and Cu elements were 10 at.%,30at.%and 10 at.%,respectively,the photocatalytic performance of hydrogen production was best.The hydrogen production of CdS/ZnCo-ZIF（Co=10 at.%）composite photocatalyst was177.9 times and 1.8 times that of pure CdS and CdS/ZIF-8（ZIF-8=100 at.%）,respectively.The hydrogen production of CdS/ZnNi-ZIF（Ni=30 at.%）composite photocatalyst was 277.3and 2.74 times larger than those of CdS and CdS/ZIF-8（ZIF-8=100 at.%）photocatalyst.The hydrogen production of CdS/ZnCu-ZIF（Cu=10 at.%）composite photocatalyst was 131.9times and 1.3 times that of pure CdS and CdS/ZIF-8（ZIF-8=100 at.%）photocatalyst.The photocatalyst synthesized by semiconductor and MOF has inherited the highly porous morphology of MOF,and showed good separation efficiency of photogenerated carriers.further improving the hydrogen production efficiency of CdS.3.Preparation and their hydrogen production propertie of CdS/MoS₂ and CdS/P-MoS₂photocatalysts.CdS loaded with MoS₂ and P-MoS₂ as a cocatalyst,The load content of MoS₂was 2 at.%,3 at.%,4 at.%and different P additions of P-MoS₂（0.5 at.%,2 at.%,5 at.%）co-catalyst,.The photocatalytic hydrogen production performance test shows that the MoS₂content and P addition amount are 3 at.%and 2 at.%,respectively,the photocatalytic hydrogen production performance is best,the hydrogen production of CdS/MoS₂（3 at.%）photocatalyst was 28.8 times that of pure CdS,and the hydrogen production of CdS/P-MoS₂（2 at.%）photocatalyst was 44.9 and 1.6 times larger than those of CdS and CdS/MoS₂（3 at.%）.Semiconductors loaded with cocatalysts is an effective method and shows good photocatalytic properties for hydrogen production.This is because the S atoms exposed to the surface of the catalyst provide many active sites for the catalytic reaction and play a positive role in the separation of photogenerated electrons and holes,greatly improving the photocatalytic hydrogen production efficiency of the photocatalyst.