Structural Regulation In Low-dimensional Multinary Semiconductors For Optimizing The Optical And Electrical Behaviors

Since the industrial revolution,fossil fuels(such as coal,oil and natural gas)have been exploited to promote rapid economic and social development.With increasing demand on energy resources,and growing environmental concerns,it has become urgent to develop clean and sustainable energy conversion and storage systems with high efficiency and low cost.As the forefront of clean and sustainable energy technologies,electrocatalytic water splitting,photocatalysis,carbon dioxide fixation and utilization have attracted tremendous attention,due to their great potential in solving the energy crisis and greenhouse effect.However,most of catalysts still suffer from low efficiency,high cost,instability,low quantum efficiency,which will undoubtedly limit their commercial utilization.Therefore,developing new advanced catalysts and energy conversion materials will be crucial for solving the energy crisis and greenhouse effect.Low-dimensional materials have attracted extensive attention from scientists for their specific properties different from those of bulk materials in terms of optical,thermal,mechanical,electrical and other properties.It is well known that the construction of crystallographic defects,such as elemental doping,vacancies and distortion,is the most effective approach to notably improve the activities of photocatalytic or electrocatalytic water splitting and C02 reduction via the regulation of electronic structures.Meanwhile,effective regulation of excitonic process will help to design advanced photocatalyst and achieve some specific photocatalytic reactions.Here,by taking a series of low-dimensional semiconductors(nanoparticles and nanosheets)as a platform,this paper mainly focuses on designing advanced functional materials via regulation of structure distortion,heterojunction and vacancies in clean and sustainable energy technologies,including oxygen evolution reaction,photodetec-tor and CO2 fixation.And based on advanced technologies,such as neutron powder diffractions,ultrafast transient absorption spectroscopy and density functional theory(DFT)calculations,the influence of structural aspects(such as structure distortion,heterojunction and vacancies)on electronic structure and excitonic processes was investigated for enhancing catalytic activities and photoresponse properties in low-dimensional multinary semiconductors.The details of this dissertation are summarized briefly as follows:1.The activity of electrocatalyst is dominant by the spin configuration of active sites,which is strongly related to its local structure.Herein,the intrinsic relationships among local structural distortion,spin configuration and oxygen evolution reaction(OER)activity of the sample are systematically studied by taking the solid solution of transition metal tungstates(CoxFe1-XWO4)with tunable degrees of local structural distortions as a platform,and proposed that the local structural distortion induced spin transition is account for its high catalytic activity.Benefiting from the co-presence of high spin and low spin states,the Co0.708Fe0.292WO4 exhibits the best OER activity among all samples with a small overpotential of 327 mV for 10 mA cm-2 and a large current density of 254 mA cm-2 at 1.825 V(vs RHE)in 1 M KOH solution.This work offers a pathway for enhancing activity of electrocatalyst based on the regulation of structure distortion.2.Excitonic effects,originating from the interactions between charge carriers,influence and even dominate the photoresponsive properties of low-dimensional materials.For efficient carrier-related photoresponse,it is imperative to develop ap-propriate strategies to promote exciton dissociation in these systems.Herein,by taking black phosphorus nanosheets/poly(3-hexylthiophene)(BP/P3HT)as a prototype,this work propose that the construction of heterojunction with certain band alignment and transport property can facilitate exciton dissociation into free carriers.Analyses on band structures and carrier kinetics confirmed the directional injection of holes from BP to P3HT and the excellent transport property associated with the injected holes in P3HT.Benefiting from these features,BP/P3HT heterojunction yielded a high photocurrent on-off ratio of 18.3,contrasting with the much lower values in pristine BP nanosheets and P3HT.This work not only provides a heterojunction engineering strategy on excitonic regulation but also offers a way for enhancing photoresponsive properties of low-dimensional materials3.Vacancies in two-dimensional materials would significantly influence the photocatalytic activity.Here,by taking layered ternary chalcogenide FePS3 nanosheets as a platform,this work illustrate that sulfur vacancies can lower the adsorption energy of C02 on the reactive sites,and hence enhance the catalytic activity.First,this research successfully constructed S-vacancy-FePS3 nanosheets and FePS3 nanosheets,which is clearly confirmed by HRTEM and HAADF-STEM.ESR and XPS clearly confirmed that the existence of sulfur vacancies in FePS3 nanosheets,leading to a regulated electronic structure.And first-principles calculations provide clear evidence that sulfur vacancies can serve as active sites to effectively adsorb C02 and propylene oxide.Benefiting from the synergistic effect of specific electronic structure and exposed reactive sites,S-vacancy-FePS3 nanosheets show a enhanced conversion yield of cycloaddition reactions of C02 with epoxides to cyclic carbonates.This work offers a pathway for fixation and utilization of carbon dioxide.