Study On The Construction Of ZnIn₂S₄ Based Photocatalyst And The Improvement Of Hydrogen Production Performance

Because of increasing environmental awareness and excessive non-renewable power consumption,the conversion of solar energy into chemical energy via semiconductor photocatalysts has attracted extensive attention,particularly with regard to the utilization of hydrogen energy.H₂ is one of the cleanest fuels for energy production that comprises an easily accessible element.Two-dimensional?2D?materials,such as graphene,graphene derivatives,and transition metal dichalcogenides?TMDs?,have been extensively used owing to their extraordinary electronic and optical properties.Accompanied by the booming development of graphene and its derivatives,much attention was paid to ultrathin nanosheets of inorganic graphene analogues?IGA?with large specific surface area.Among the various inorganic graphene analogues,the layered transition-metal chalcogenides garnered great research interest.ZnIn₂S₄ is a ternary chalcogenide with a bandgap of ca.2.5 eV and has attracted increasing attention largely due to its easy availability,ability of absobing visible light,low-cost,and good stability.Cadmium sulfide?CdS?as a typical semiconductor,do not only provide most direct low-tech access to clean hydrogen,but also permit fundamentally new insight into the photophysics of hydrogen production,has attracted interests of researchers throughout the world due to its suitable gap of 2.42 eV and its potential applications in photocatalysts.Yet,to date,the overall photoconversion efficiency is still very low and requires further promotion for commercialization.Many methods were developed to enhance photocatalyst efficiency,primarily by modifying its morphology,crystallization,and electrical properties.Hydrogenation engineering has been considered as one of the promising methodologies to significantly enhance the visible light absorption and photocatalytic activities of semiconductor photocatalyst.However,hydrogenated catalyst with remarkably elevated photocatalytic activity has been commonly obtained so far under severe conditions such as high temperatures or pressures and huge doses of H₂ or plasma activation with complex treatment.We proposed in-situ hydrogenation of ZnIn₂S₄ obtained at room temperature under UV-visible light illumination for promoting hydrogen evolution reaction.In addition,fabricate the heterojunction structure is also a rational strategy to suppress charge recombination in CdS.In a heterojunction photocatalyst,internal electric field can be built between different components,which would promote the spatial separation of photogenerated electron hole pairs.However,a thorough understanding of the relationships between the properties and functions of photocatalytic materials plays a crucial role in the design and fabrication of efficient photochemical systems for water splitting.The construction of the heterojunction structure with intimate contact and good light adsorption ability is important for realizing the high effective photocatalytic H₂ evolution.The heterostructure of photocatalyst with different orientations has been proven to be a good candidate for photocatalytic water splitting.These two works combined with the current research hotspots to study the photocatalytic hydrogen production performance of ZnIn₂S₄,CdS and its composites.The main research contents and conclusions are as follows:?1?In this work,ZnIn₂S₄ was used as photocatalyst for photocatalytic hydrogen evolution reaction?PHER?with an on-line gas chromatography.ZnIn₂S₄ nanosheets were fabricated by a facile low-temperature refluxing method followed by a moderate exfoliation.After that,the colloidal ZnIn₂S₄ nanosheets were illuminated under UV-visible light in a 100 mL sealed quartz flask at ambient temperature and atmospheric pressure.Samples obtained with illumination time of 2h,4h,6h were denoted as HxZIS-2h,HxZIS-4h,HxZIS-6h,respectively.Sample BCHZIS was obtained via vigorous stirring?1000 r/min,12h?under dark after 9-hour illumination.It was found that the photocatalyst experienced a significant color change from yellow to black in several hours when illuminated by UV-visible light for administrating photocatalytic hydrogen evolution.The physicochemical properties of the catalysts were characterized by HNMR,XPS,XRD,SEM,TEM,PL,DRS,Raman and EIS.Studies have shown that the sample BCHZIS has the optimal bandgap of 2.08eV,low electron hole recombination rate and low electron transfer resistance,which is beneficial to optical absorption and electron hole separation and promote photocatalytic hydrogen production efficiency.The photocatalytic hydrogen evolution properties could be adjusted by optimize the amounts of adsorbed H on the surface of ZnIn₂S₄ that could be easily realized via vigorous stirring.The optimized ZnIn₂S₄ with appropriate adsorbed H exhibits the best photocatalytic performance during the test with a ten-fold H₂ production enhancement compared to pristine ZnIn₂S₄ nanosheets.Judging from the ¹H solid-state NMR spectroscopy,the two peaks at 3.35 and 4.0 ppm correspond to S-H bonds,giving direct evidence that ZnIn₂S₄ had been hydrogenated during the photocatalytic water reduction process,which is further demonstrated by the XPS results.Our DFT results suggest that the hydrogen incorporation on the ZnIn₂S₄surface facilitates easier electron excitation and allows for higher carrier density as well as efficient carrier transport along the two-dimensional?2D?conducting channels.This result represents significant advance in fundamental understanding for the HER catalysis,and we believe that such a facile and general methodology will open up a new path way for the creation of high performance catalyst.?2?Here,a novel and unique mushroom-like CdS nanowire-ZnIn₂S₄ tips?OFH-CZt?heterostructure was fabricated inspired by the progress in 1D nanomaterials for solar energy conversion,and the recent booming of 2D nanomaterials in this field.CdS and ZnIn₂S₄ as efficient photocatalysts were chosen as the building blocks because of their hexagonal crystal structure and similar lattice parameter.OFH-CZt was constructed by employing CdS NWs as nanobuilding blocks via a facile solvent refluxing method.The obtained mushroom-like CdS-ZnIn₂S₄ with oriented facet heterojunction?OFH-CZt?is beneficial for visible light water splitting.The heterostructure achieves a remarkable noble metal-free H₂ production rate 3.07 mmol h^-1g^-11 with excellent long-term durability.This activity is 10-fold higher than that of pure CdS NWs.This OFH-CZt catalyst maintained⁹5%of its initial activity after 30 h of continuous hydrogen evolution reaction operation.The physicochemical properties of the composites were characterized by XRD,SEM,TEM,XPS,PL,DRS,EIS,photocurrent and polarization curves.The oriented facet heterojunction?OFH?strategy to facilitate the separation of electron-hole pairs in CdS was introduced.Due to the internal electric field and unidirectional flow of charge carriers in CdS,ZnIn₂S₄ nanotips effectively extract holes from NWs for exciton separation and decrease the physical and electrical barriers for charge transfer.Density functional theory?DFT?calculations revealed that the OFH structure is beneficial for charge carrier separation,thus notably enhancing photocatalytic hydrogen evolution.In briefly,our present work deepens the understanding of the transition metal chalcogenides based epitaxial heterostructures and may boom research interests on new structures to design robust,cheap,and commercially available photocatalytic materials.