Preparation Of ZnIn₂S₄ Based Heterojunction Photocatalyst For Photocatalytic Hydrogen Production And Degradation Of Lignin

Photocatalytic technology provides a low energy consumption,safety,and green strategy for hydrogen production and degradation of lignin.Zn In₂S₄（ZIS）has visible light response（λ>420 nm）,simple preparation,good chemical stability,suitable energy band,etc.However,the serious recombination of photogenerated charge carriers,weak oxidation ability,narrow light absorption range,and the need for noble metal cocatalyst modification of ZIS restrict its application in fields such as hydrogen production and degradation of lignin.This study designed and prepared ZIS based single photon excited heterojunctions,S-scheme heterojunctions,metal doped S-scheme heterojunctions,and metal doped/non-noble metal cocatalyst modified S-scheme heterojunctions for photocatalytic hydrogen production,lignin degradation,and papermaking wastewater.Aiming at the problem of poor photogenerated carrier separation of ZIS,the 0D Nb₂O₅loaded on 2D Zn In₂S₄（ZIS）heterojunction（Nb-ZIS）via single photon excited pathway was prepared via the solvothermal strategy assisted by thioacetamide and sodium carbonate for photocatalytic H₂ evolution and degradation of sodium lignosulfonate（LS）.Scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and other characterization techniques confirmed that the Nb₂O₅ nanoparticles with a size of 10 nm were successfully prepared and uniformly loaded on 2D ZIS.The optimized Nb-ZIS heterojunction photocatalyst has a photocatalytic hydrogen production rate of 5.4 mmol/h/g using Pt as a cocatalyst,which exceeds the hydrogen evolution activity of ZIS（Pt as a co catalyst,with a hydrogen evolution rate of 3.6 mmol/h/g）;The Nb-ZIS heterojunction photocatalyst degrades 78.5%LS in 120 min,and the removal rate of Total organic carbon（TOC）is 34.0%,while ZIS degrades only 30.0%LS in 120 min.The optimized Nb-ZIS photocatalytic activity is superior to the commercial Nb₂O₅ bulk loaded ZIS heterojunction（Nb-ZIS-C）and the 0.2Nb/ZIS prepared without the addition of Na₂CO₃.Density functional theory（DFT）calculations and electron paramagnetic resonance（EPR）spectroscopy and other characterization techniques confirmed that Nb-ZIS heterojunctions via single photon excited pathway,which improved the separation efficiency of photogenerated carriers;the small-sized Nb₂O₅ can reduce the obstruction of 2D ZIS and effectively promote the absorption of light by ZIS.The apparent quantum efficiency（AQY）at420 nm is 3.3%.At the same time,Nb-ZIS can promote the mass transfer process of photocatalytic reaction to enhance the photocatalytic reaction.Aiming at the problem of low oxidation potential of ZIS,Zn In₂S₄ semi-encapsulated TiO₂nanoparticles S-scheme heterojunction（Ti-ZIS）was successfully prepared though N,N-dimethylformamide（DMF）assisted isopropanol solvothermal reaction for photocatalytic H₂evolution,degradation of LS,and secondary treatment of wastewater.The research shows that DMF inhibits the growth of TiO₂ nanoparticles,and at the same time,by optimizing the concentration of tetrabutyl titanate to control the agglomeration morphology of TiO₂,Zn In₂S₄semi-encapsulated TiO₂ nanoparticles heterojunction was prepared.The photocatalytic hydrogen production rate of the optimized Ti-ZIS using Pt as a cocatalyst is 28.0 mmol/h/g,which is 1.96 times the hydrogen evolution rate of 0.8Ti/ZIS（without DMF）.The AQY value at 420 nm is 1.0%.Ti-ZIS degraded 76.7%of LS at 120 minutes,with a TOC removal rate of49.4%.Ti-ZIS degraded 85.6%of papermaking wastewater after industrial secondary treatment in 120 min.Research has shown that the Ti-ZIS semi encapsulated core-shell structure shortens the transmission distance of photogenerated carriers and improves the utilization of scattered light;DFT calculation and EPR and other characterization techniques confirmed that Ti-ZIS formed S-scheme heterojunction,promoted the separation of photogenerated carriers,changed the migration path of photogenerated carriers,and generated superoxide radical（O₂^-）and hydroxyl radical（·OH）,wherein·OH participated in the deep oxidation of LS,so the TOC removal rate（49.4%）of Ti-ZIS degraded LS was significantly higher than that of Nb-ZIS（34.0%）.In order to further improve the photocatalytic hydrogen production performance of ZIS,the S-scheme heterojunction of in-situ Mo⁵⁺doped Zn In₂S₄ wrapped MoO₃（MoO₃@Mo-ZIS）was prepared through the"thermal solubilization strategy"for photocatalytic H₂ evolution.As the temperature increasing,MoO₃ gradually dissolves in ethylene glycol,prompting Zn²⁺and In³⁺to adsorb on its surface and react with S^2-;At the same time,the dissolved Mo⁶⁺is reduced to Mo⁵⁺by ethylene glycol,replacing some of the Zn²⁺sites.Finally,a Mo⁵⁺doped Zn In₂S₄encapsulated MoO₃ heterojunctionphotocatalyst photocatalyst were prepared.The optimized MoO₃@Mo-ZIS showed the good hydrogen evolution rate of 13.6 mmol/g/h.UV-visible diffuse reflectance spectra（UV-Vis DRS）demonstrates that the doping of Mo⁵⁺broadens the light absorption range to improve the light utilization,with an AQY value of 4.82%at 420 nm.DFT calculations confirm that Mo-S bond form new hybrid states near the Fermi level,thereby reducing free energy of hydrogen intermediate state adsorption to enhance photocatalytic hydrogen evolution.The formation of S-scheme heterojunction of MoO₃@Mo-ZIS proved by EPR and Kelvin probe force microscopy（KPFM）inhibits the recombination of photogenerated carriers,changes the migration path of photogenerated carriers,and retains strong oxidation and reduction ability,thereby enhancing photocatalytic hydrogen production.Aiming at the problem that ZIS needs to be modified with a noble metal cocatalyst,in-situ Mo⁵⁺doped and metallic Ni⁰ supported Zn In₂S₄ wrapped NiMoO₄ S-scheme heterojunction photocatalyst（NMO@M-ZIS-N）were developed through the"thermal solubilization reduction strategy"for photocatalytic H₂ evolution and reforming sodium lignosulfonate into hydrogen.With the increasing of temperature,NMO dissolved in ethylene glycol,which promoted the adsorption of Zn²⁺and In³⁺on its surface,and then reacted with S^2-.At the same time,the dissolved Mo⁶⁺and Ni²⁺were reduced to Mo⁵⁺and Ni⁰,respectively,and the Mo⁵⁺doped and metallic Ni⁰ supported Zn In₂S₄ wrapped NiMoO₄ heterojunction photocatalyst was prepared.The optimized NMO@M-ZIS-N exhibits excellent photocatalytic hydrogen evolution activity（19.4 mmol/h/g）.NMO@M-ZIS-N exhibits excellent the reforming sodium lignosulfonate into hydrogen（2.3 mmol/h/g）.The UV-Vis DRS spectra confirms that Mo⁵⁺doping broadens the light absorption range,thereby broadening the light absorption range;XPS indicates the presence of Ni⁰ as a hydrogen evolution cocatalyst,promoting photocatalytic hydrogen production activity.The formation of S-scheme heterojunction of NMO@M-ZIS-N confirmed by EPR and KPFM promoted the separation of photogenerated carrier,retained strong oxidation capacity,and improved the reforming sodium lignosulfonate into hydrogen.Therefore,NMO@M-ZIS-N has the advantage of synergistic Mo⁵⁺doping,Ni⁰ loading,and S-scheme heterojunction,realizing efficient photocatalytic reforming LS for hydrogen production.It solves the problems of difficult separation of photogenerated carriers,low oxidation potential,narrow light absorption,and the need for noble metal cocatalysts,respectively,via developing single photon excited heterostructures of Nb-ZIS,S-scheme heterostructures of Ti-ZIS,metal doped S-scheme heterojunction of MoO₃@Mo-ZIS,and S-scheme heterojunction modified with metal doped/non-noble metal cocatalysts of NMO@M-ZIS-N,achieving the efficient and stable photocatalytic hydrogen evolution,deep degradation of lignin and papermaking wastewater.