Theoretical Design And Exploration Of Novel 2D Energy Materials

Two-dimensional(2D)materials have drawn extensive attention since the successful exfoliation of graphene.Due to the unique structural characters,they always exhibit intriguing physical and chemical properties compared with their bulk counterparts.Up to now,a series of 2D materials have been proposed from both theory and experiments,such as transition metal dichalcogenides(TMDs),black phosphorus,silicene,germylene,MXenes and so on.They have been widely used in optics,magnetics,spintronics and nano-devices,and lead significant values in both scientific and practical areas.Especially for energy conversion,two-dimensional materials show unique superiority in the field of green energy production and transformation.For instance,in the studies of photocatalytic field,the carriers will migrate to the surface more quickly due to the decrease of dimensions,leading to a faster migration rate.As a consequence,the carrier recombination rate will be reduced while the photocatalytic efficiency will be enhanced.In addition,the large specific surface area always plays a positive role for improving the photocatalytic and electrocatalytic performance.In the field of photovoltaic systems,the fast carrier migration can improve the photoelectric conversion efficiency.All of these excellent properties provide a new route for solving the situations of energy shortage and environmental pollution.In this dissertation,we systematically investigate the mechanical,electronic and optical properties of novel 2D energy materials by using first-principles calculations.Then exploring the potential applications in photocatalytic,electrocatalytic and photovoltaic areas through adsorbing atoms and constructing heterostructures.This dissertation is divided into six chapters.In the first chapter,we introduce the background and current process of 2D materials in the field of photocatalysts,electrocatalysts and photovoltaic.In the second chapter,we introduce the basic theory,methods and relative softwares of the first-principles calculations.In the third chapter,we discuss how to improve the photocatalytic performance of 2D materials.In the fourth chapter,we investigate the properties of Ti4X3(X=C,N)for intrinsic electrocatalytic hydrogen evolution.In the fifth chapter,the potential application in solar cell of HfTeSe4 is explored by constructing heterostructures.In the sixth chapter,we summarize the main research contents and further give an outlook for photocatalysts,electrocatalysts and solar cells of 2D materials.The main research contents are listed as follows:(1)We investigate the photocatalytic properties of MoS2 by adsorbing Ag atom and clusters.It is confirmed that the composite systems are energetically favorable.The Fukui function results show that Ag nanoparticles adsorbing on monolayer MoS2 are presented as active reduction and oxidation sites,playing a role in mediating charge transfer from electron-rich MoS2 to target species.The induced impurity states are located at the middle of VBM and CBM.As a result,the intensity of light absorption can be enhanced.In addition,it is found that the work function of the composite system is decreased.Therefore,the enhanced surface activity of MoS2 leads to the high efficiency redox reaction.(2)The photocatalytic performance with clear atomic interface is investigated by constructing horizontal heterostructure CdS/ZnSe.Interestingly,the armchair configuration possesses a direct bandgap with type-? band alignment.Hence,the interfacial charge transfers from ZnSe to CdS results in a built-in field which can efficiently separate the electrons and holes.The higher carrier mobility ensures the high efficiency of redox reaction.More remarkably,both the bandgap and band edges of the lateral heterostructures can meet the requirements of the reduction and oxidation levels in both acidic and neutral solution.Such study provides a reference for the experimental design of photocatalytic systems(3)We show a promising way to achieve photocatalysts for overall water splitting based on monolayer SnS and GeSe.We demonstrate that the direction of charge transfer is dependent on the direction of stacking electric field.Most importantly,the decomposition of water reaction in the stacked Se-Ge-S-Sn can proceed spontaneously under light irradiation.This part of research provides theoretical reference for the design of similar systems.(4)We propose a novel family of Janus single-layer group-III monochalcogenides X-Ga-In-Y(X,Y=S and Se)using first-principles calculations.We find that these structures exhibit excellent stability by preforming AIMD and spectrum calculation.Their band gaps can be tuned effectively by external electric field,while the morphologies of band structures changed little.Importantly,their suitable band gaps(2.07 eV-2.60 eV)and band edge positions enable them promising candidates in photocatalytics.Moreover,the compressive strain can further make the band edge positions match with the redox potentials of water splitting better,and hence can increase the efficiency of solar energy conversion.Taking SeInGaS as an example,we study the two half reactions of the system by analyzing their Gibbs energy difference.The results reveal that only little external energy is needed to meet the thermodynamic requirements of the water decomposition.Our works provide a novel family of promising photocatalytic materials.(5)As a new kind of star materials,MXenes have been widely used in catalysis,gas sensing devices,energy conversion and basic components of storage devices.Herein,using first-principles calculations,we identify a family of promising HER catalytic materials with high intrinsic activity in MXene Ti4X3O2(X=C,N).We find that Ti4X3 are terminated by a mixture of OH*and O*under standard condition by plotting Pourbaix diagram.It is known that the Gibbs energy difference is a key parameter to evaluate the performance of the hydrogen evolution.The adsorption of atomic hydrogen on Ti4X3O2 exhibits a low Gibbs free energy difference,and the obtained activity barrier of Volmer-Heyrovsky step for Ti4N3O2 is only 30 meV,which indicates the excellent intrinsic HER activity.This may attribute to the change of p band center.Furthermore,we explore the HER activity of Ti4X3O2 in aqueous solution through applying biaxial strain,and find that the low Gibbs free energy is well preserved,demonstrating the high stability of the intrinsic HER activity of Ti4X3O2 in solution environment.(6)Herein,by using first-principles calculations,we propose an excellent two-dimensional photovoltaic material,monolayer HfTeSe4,which can be exfoliated feasibly from its layered bulk.The dynamical and thermal stability of the monolayer are proved by performing the phonon spectrum calculations and ab initio molecular dynamics(AIMD)simulations.It behaves in the semiconductor character with a moderate direct gap of 1.48 eV and exhibits remarkable absorbance coefficient of?105 cm-1 in the visible light region.Moreover,the heterostructure between HfTeSe4 and Bi2WO6 is proposed as potential solar cells with the solar conversion efficiency up to-20.8%.This work provides theoretical support for experimental feasibility of low-dimensional solar cells.