Construction Of MXene-based Composite Photocatalysts And The Study On The Underlying Mechanism For Their Enhanced Performance

Photocatalysis is a desired technology to alleviate the increasingly serious environmental pollution and energy crisis with the usage of solor energy as the driving force.Since the performance of single semiconductor photocatalyst often fails to achieve ideal results,supported cocatalyst is one of the promising modification strategies to enhance the photocatalytic activity.Transition metal carbides/nitrides/carbonitrides,known as MXenes,possess rich electronic properties,which not only enable the regulation for the growth of semiconductors,but also facilitate the separation of photogenerated charge carriers and the absorption of reactants to collectively contribute to photoactivity enhancement.This study mainly focuses on the controllable preparation of MXene-based composites and their synergistic mechanism of the enhanced photocatalytic activities.（1）2D/2D In₂S₃/Ti₃C₂T_x composites were prepared by a one-step low-temperature refluxing method,during which Ti₃C₂T_x nanosheets acted as substrates to provide nucleation-growth sites.While bare In₂S₃ tends to evolve as nanoparticles,it is found that Ti₃C₂T_x acts as the platform for directing the growth of 2D In₂S₃ nanoflakes,thereby generating 2D/2D In₂S₃/Ti₃C₂T_x heterostructures.In this study,the evolution of In₂S₃morphology was investigated by controlling Ti₃C₂T_x content.According to the results of the control experiments,it is reasonable to propose that low content of Ti₃C₂T_x results in high density of In₂S₃ nuclei,which favors the vertical growth of nanosheets based on the theory than the steric hindrance from adjacent seeds can hinder the in-plane direction growth.For the probe reaction of nitroaromatic compounds hydrogenation,In₂S₃/Ti₃C₂T_xcomposite exhibited boosted photocatalytic activities compared to bare In₂S₃,which can be ascribed to the favorable active sites exposure in the 2D/2D heterostructures and the“electron sink”effect of Ti₃C₂T_x nanosheets.Consequently,a feasible mechanism of the reinforced photoactivity over In₂S₃/Ti₃C₂T_x for the reductive transformation of nitroaromatic compounds with reference to In₂S₃ counterpart has been proposed.（2）Insufficient active sites for photocatalytic hydrogen evolution over In₂S₃/MXene composites leads to unsatisfactory results.Herein,In₂S₃/MXene@NiS composites are constructed by a photo-deposition method with the aim to boost the efficacy of photocatalytic hydrogen evolution through enriching hydrogen-evolution sites and accelerating the transfer of photo-induced charges.As expected,the introduction of MXene and NiS dual cocatalysts leads to the improved photocatalytic water splitting activity(433μmol·g^-1·h^-1),which was 41.2-fold that of pristine In₂S₃(10.5μmol·g^-1·h^-1),and even better than that of In₂S₃@Pt-1%(206.5μmol·g^-1·h^-1)by 2.1 times.The improved photocatalytic hydrogen evolution performance results from abundant hydrogen evolution sites stemmed from NiS and the dual channel directed charge transfer.This study provides a simple,feasible and economical protocal for engineering noble metal-free cocatalyst for enhancing photocatalytic hydrogen evolution as well as a new insight into the design of composite photocatalysts with dual cocatalysts.