Carrier Regulation And Performance Of Cadmium Sulfide-Based Photocatalytic Hydrogen Production Materials

One of the promising low-cost solutions to climate change and the energy issue is the water splitting for hydrogen evolution utilizing a suspended particle photocatalytic reaction system.Each nanoparticle photocatalyst is a complete photoelectric reaction system with the surrounding solution.Therefore,the band structures of semiconductor nanomaterials must be capable of absorbing photon energy to excite charges but also of having sufficient reduction and oxidation capacity for photogenerated electrons and holes.It is also crucial that high mobility and a long lifetime ensure their effective participation in surface reactions.These pose a considerable challenge to photocatalyst development,both thermodynamically and kinetically.Cadmium sulfide（CdS）is a promising candidate due to the appropriate bandgap and more negative conduction band minimum.However,the easy recombination of CdS photogenerated charges and the slow surface reactions limit its photocatalytic H₂-evolution rate.This thesis,therefore,takes full advantage of the thermodynamic properties of CdS to optimize its photogenerated carrier kinetics via morphological modulation and heterostructure design and make it an excellent catalyst for photoinduced H₂ evolution.First,we have synthesized CdS photonic crystal with hierarchically periodic macropores（HPM CdS）using a two-step impregnation method for ameliorating the problem of easy recombination of photogenerated charges induced by CdS nanoparticle aggregation.The 3D-ordered porous structure alleviates the problem of grain boundaries as charge recombination centers,shortens the carrier migration distance,and accelerates molecular fluid transport.Moreover,the long-range periodic structure of HPM CdS,as a photonic crystal,generates the slow photonic effect that facilitates the excitation and utilization of photogenerated charges.Combined with the above advantages,HPM CdS exhibits a substantially enhanced photocatalytic H₂-evolution rate that is 11 times higher than that of CdS nanoparticles.To control the spatial distribution of photogenerated carriers for effective separation,we have constructed a hierarchically periodic macroporous structure consisting of alternately bridged CdS–Zn O（HPM CdS–Zn O）.The alternate heterojunctions form a carrier transport mechanism with multiple quantum well-like（MQW-like）band alignments based on the double Z-scheme.The electron wave function of the system is mostly localized in the potential wells after illumination due to the exceptional band structure.As a result,high-energy photogenerated electrons and holes accumulate in the CdS and Zn O portions,respectively,resulting in spatial separation.Meanwhile,these charges can be rapidly utilized for chemical reactions by the efficient mass transfer of the HPM structure.These enable the HPM CdS–Zn O heterojunction to exhibit a photocatalytic H₂-evolution rate of up to 587.8μmol h^-1without a co-catalyst.Apart from prolonging the photogenerated carrier lifetimes,it is also crucial to accelerating the chemical reaction kinetics of carriers migrating to the catalyst surface.Here,we have designed a bottom-up strategy for the in-situ growth of ultra-small lateral-sized Mo S₂ with tunable layer number on CdS nanorods（CN）by controlling the decomposition temperature and concentration of substrate seed（NH₄）₂Mo S₄.The bilayer Mo S₂ and CN coupling（2L–Mo S₂/CN）exhibits the optimum photocatalytic activity.This is facilitated by the fact that the CBM of 2L–Mo S₂ has sufficient reduction capacity to drive photocatalytic H₂ evolution compared to thicker Mo S₂,and the ultra-small lateral size provides more active sites.Meanwhile,the indirect bandgap,in contrast to the direct bandgap of the monolayer,suppresses the carrier recombination transferred to 2L–Mo S₂.Under the synergistic effect of both,2L–Mo S₂/CN has fast surface chemical reactions,so the photocatalytic H₂-evolution rate is up to 837.1μmol h^-1.