| With the rapid depletion of fossil fuels and serious environmental pollution problems,how to achieve effective conversion of renewable energy has become an important and urgent problem for the scientists to solve.In a variety of energy conversion pathways,electrocatalytic and photocatalytic water splitting,and photoluminescence are commonly used in the effective energy conversion.The first two energy conversion ways are to convert large amounts of renewable electrical energy and solar energy into chemical energy,and photoluminescence is the conversion of sunlight and ultraviolet light into the light that is available in daily life.Therefore,these energy conversion methods that mentioned above play an important role in solving the current energy crisis.Of note,electrocatalysts,photocatalysts and fluorescent materials are definitely crucial for these energy conversion processes.Two-dimensional(2D)crystals,including quasi-graphene and inorganic graphene analogues,in respect to their ultrathin thickness and planar 2D structure,exhibit promising physical and unique chemical properties,resulting in enormous opportunities for applications in electrocatalysis,photocatalysis and photoluminescence.The purpose of this paper is to design the functional 2D crystal based on the analyses of the key points that influence and restrict the performances of the three energy conversion processes,in order to achieve high efficiency energy conversion.The author highlight the surface structural modulation of MXene nanosheets by introducing the surface rich fluorine termination groups in the structure can significantly improve the electrocatalytic activity for hydrogen evolution.By preparing ultrathin siloxene nano sheets to realize the application of silicon-based materials in the field of photocatalytic water splitting.By designing grain boundary engineering in the ternary ultrathin nanosheets with tunable bandgap,leading to the achievement of white light emission.The details of this dissertation are summarized briefly as follows:1.The electric conductivity and active sites density are two key points that influence the HER performances.Bear this in mind,we developed a strategy to simultaneous optimization of both active sites and electric conductivity for MXenes by surface structural modulation at atomic scale,achieving the superior HER activity.By taking the rich surface fluorine-terminated Ti2CTx nanosheets as an example,we systematically studied the role of rich surface fluorine termination groups play in their HER activities,disclosing that they can reduce both the hydrogen binding energy and charge-transfer resistance,which endow the Ti2CTx nanosheets with good electric conductivity and facile HER process.Synergistically,first-principles calculation and temperature-dependent resistance measurement reveal that the rich F-terminated Ti2CTx nanosheets are intrinsically metallic,which further enhance the fast electron transport during the HER process.In addition,the ultrathin thickness of Ti2CTx nanosheets provides larger electrochemical surface area and more catalytically active sites than their bulk counterparts,which are greatly beneficial for the HER catalysis.As a result,benefiting from the good electric conductivity and high exposure of effective active sites,the rich F-terminated Ti2CTx nanosheets exhibit outstanding HER performance with a small onset overpotential of 75 mV and large exchange current density of 0.41 mA cm-2.Our study will provide a valuable guideline for the design of MXenes-based materials with optimized electrocatalytic activity.2.Exploring new metal-free semiconductors for water splitting it is of great scientific significance.Therefore,we prepare ultrathin siloxene nanosheet by a topotactical reaction from the Zintl-CaSi2 and report for the first time it can serve as a photocatalyst for efficient water splitting to produce hydrogen without the addition of any cocatalyst or sacrificial agent.The optical absorption spectrum of the as-prepared siloxene nanosheets shows an absorption onset of ca.496 nm,from which the band gap was determined to be 2.50 eV.The conduction band of the as-obtained nanosheets was studied by Mott-Schottky analysis to be-0.92 V(vs.Ag/AgCl).Based on these,the siloxene nanosheet with the edge position of the conduction band located above the hydrogen-evolution potential and the valence band below the oxidation level for H2O to H2O2 or O2,indicating that siloxene nanosheets satisfy the necessary requirements for water splitting.Although siloxene nanosheets are unstable in water,siloxene exhibits excellent photocatalytic activity for hydrogen generation.After irradiation for 2 h,the rate of hydrogen generation can reach up to 11.4 mol g-1 h-1,far exceed than that of other reported metal-based photocatalysts.This study can provide a new insight into the study of low-dimensional silicon-based functional materials.3.Increasing surface states is the most efficient way to achieve white light emission from single-phased materials.We for the first time report a novel approach to realize white light emission by engineering abundant grain boundaries into the bandgap-tunable single-phased atomically-thin nanosheets.Taking the Zn-Cd-S system as an example,the existence of abundant grain boundaries on the nanosheets can significantly increase the number of the electron-hole recombination center,enrich the surface states for electron trapping and shorten the distance of charge transport from the photogeneration zone to the recombination center,thus leading to the remarkable increment of the charge density for radiative recombination and further enhancing the surface-state emission in a wide wavelength range.Benefiting from the tunable bandgap,the band-edge emission can be readily modulated,thus finally realizing the white light emission with the synergy of the surface-state emission enhanced by grain boundary engineering.As a result,excellent white light with CIE chromaticity coordinates of(0.35,0.32)can be achieved for the product with composition of Zn0.44Cd0.56S,which is much closer to the perfect white light(0.33,0.33).This work paves a new pathway for the achievement of white light emission for single-phased photoluminescent materials. |
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