Cadmium Sulfide-Based Gel Beads For Photocatalytic Hydrogen Evolution From Water Splitting

The development of efficient and stable photocatalysts is an important issue for improving the efficiency of photocatalytic hydrogen evolution from water splitting.Cadmium sulfide（CdS）is a photocatalyst with the potential of photocatalytic hydrogen production from water splitting because the position of the bottom of its conduction band meets the thermodynamic requirements of the photocatalytic reduction of water reaction.However,due to the serious photocorrosion in the photocatalytic reaction process,the rapid recombination of photo-generated carriers,and the difficulty in solid-liquid separation,which limits the improvement of photocatalytic hydrogen production and obstructs its application.To overcome the above shortcomings,in this work,millimeter-scale chitosan gel beads were used as confinement template to construct different chitosan/CdS based composite photocatalysts by combining with morphology control,co-catalyst surface modification and band structure regulation for CdS,aimed at improving the CdS photocorrosion resistance,carriers separation efficiency,and the hydrogen production performance from water splitting.The interface structure,band structure,photostability,carriers separation behavior,etc.were revealed by means of hydrogen production performance testing,characterization analysis,and density functional theory（DFT）calculations.The main research contents and innovative results were as follows:（1）Construction of surface state and confined CdS-based chitosan xerogel beads for photocatalytic hydrogen evolution from water splitting.A macroscopic millimeter-scale CdS-based chitosan xerogel beads photocatalyst（CXB@CdS）with easy separation was prepared by a mild adsorption-sulfurization process.The concentration of Na₂S could regulate the size of the surface CdS on CXB@CdS,and the surface CdS with smaller size showed a higher photocatalytic hydrogen production performance.The–NH₂ and–OH on the chitosan framework could induce an electron-rich microenvironment to guide the directional separation of photogenerated holes,which effectively inhibited the photocorrosion of CdS.Compared with pure CdS,CXB@CdS showed better photocatalytic hydrogen production performance.Moreover,the photogenerated holes generated by CdS reacted with hole scavenger（Na₂S-Na₂SO₃）preferentially,thus avoiding the oxidative destruction of chitosan in the photocatalytic hydrogen production process.（2）Controllable synthesis of Cd modified CdS micron-scale supported thin layer for photocatalytic hydrogen evolution from water splitting.Firstly,Cd（II）was loaded on the surface of chitosan/Mg（OH）₂ xerogel beads（CMB）by an ion exchange process,then Cd（II）was reduced to metallic Cd by hydrated electrons(e_aq^·-)with strong reducing power produced by ultraviolet irradiation of Na₂SO₃ solution during in-situ sulfurization process.Thus,the construction of metallic Cd modified CdS micron-scale supported thin layer photocatalyst（CMB@CdS/Cd）was realized.The hydrogen production efficiency of CMB@CdS/Cd under visible light was higher than that of CMB@CdS,when the ultraviolet irradiation time was 3 h and CdS content was 22.0%,the hydrogen production efficiency of CMB@CdS/Cd was the highest（117.9μmol/h）,which was about 3 times than that of CMB@CdS,and delivered good photocatalytic stability.The excellent electrical conductivity of metallic Cd,the formation of built-in electric field,and the Ohmic contact between Cd and CdS promoted synergistically the separation of photogenerated carriers and improvement of the hydrogen production performance.（3）In-situ growth of amorphous NiS_x on CdS-based macroporous hydrogel beads for photocatalytic hydrogen evolution from water splitting.A CdS-Ni S_x photocatalyst-cocatalyst system confined inside chitosan hydrogel beads（CHB@CdS-Ni S_x）was built by two successive adsorption-sulfurization processes.Amorphous Ni S_x was uniformly dispersed on the surface of CdS,forming an intimate interface contact to accelerate the electron migration from CdS to Ni S_x.Owing to irregular loose structure,Ni S_x possessed abundant unsaturated edge active S as H₂-evolution sites to facilitate performance of hydrogen production.The hydrogen production performance of the optimal CHB@CdS-0.02Ni S_x（the molar ratio of Ni to Cd was 0.02）could reach 218.7μmol/h and still maintain the initial hydrogen production performance of 90.0%after 11 times recycling for a total of 60 h.As investigated by DFT calculation,Ni S_x could reduce the activation energy of water,promote the dissociation of water and optimize the adsorption and desorption of hydrogen,revealing that its hydrogen evolution path（H₂O→H*→H₂）was more energetically feasible.（4）Functional group-oriented synthesis of Zn_xCd_1-xS-based hydrogel beads for photocatalytic hydrogen evolution from water splitting.Considering the guided enrichment of the–NH₂ and–OH distributed on the chitosan framework,Zn_xCd_1-xS solid solution was confined nucleated and grown on macroporous chitosan hydrogel beads(CHB@Zn_xCd_1-xS,x=0,0.17,0.33,0.44 and 1).The water molecules inside hydrogel beads as the supporting medium maintained the interconnected macroporous structure in CHB@Zn_xCd_1-xS,which could promote the confined Zn_xCd_1–xS to be driven by light for effective use.The band gap and conduction band position of CHB@Zn_xCd_1-xS can be adjusted by changing the content of Zn.The optimal CHB@Zn_xCd_1-xS exhibited the hydrogen production performance of 298.2μmol/h,which was attributed to its strong photoinduced carrier separation ability,suitable band structure,and good hydrophilicity of confinement template of chitosan.