Design And Simulation Of A Two-dimensional Photocatalyst For Water Splitting Based On First Principles

The development of clean renewable energy is of great significance in solving current energy shortages and environmental pollution problems.As a secondary energy source,hydrogen is considered to be an ideal alternative energy source because of its high calorific value,environmental friendliness and widely available.The use of solar light to generate hydrogen provides an effective solution to alleviate energy crises and solve environmental problems.However,conventional three-dimensional photocatalyst such as oxides and sulfides still face problems that the photogenerated electron holes are easily recombined,the visible light utilization rate is low,and the conduction band and the valence band potential cannot simultaneously satisfy the redox potential of the water splitting.Because of its unique geometric structure and electronic properties,two-dimensional nanomaterials have shown unique advantages in photocatalytic water splitting applications and are considered to be ideal materials for constructing high-efficiency photocatalyst.With the continuous improvement of theoretical calculation methods and computational level,structural search has become more practical and efficient.People can discover new two-dimensional nanomaterials in a short time and realize two-dimensional nanometers through first-principles calculation.Accurate descriptions of the photocatalytic properties of 2D materials can also be achieved by ab initio based modeling.Therefore,finding a highly efficient two-dimensional photocatalyst through theoretical calculation can not only enrich the types of existing photocatalysts,but also indicate the direction for experimental preparation.Oxides and sulfides containing elements of the group-VIA are the most commonly used photocatalysts.In this paper,we have used density functional theory(DFT)design four kinds of two-dimensional layered materials,and systematically studied their electrical,optical properties and photocatalytic performance.The main research results are as follows:(1)We calculated a two-dimensional GeTe single-layer material with α-As structure.The GeTe monolayer has good stability and can be obtained by exfoliation from corresponding bulk materials.The GeTe monolayer exhibits a suitable band edge position,remarkable visible light absorption efficiency,high carrier mobility,and can be used as an effective photocatalyst for water splitting.Strain engineering can effectively adjust the band gap and band edge position of GeTe monolayer and further enhance its photocatalytic performance.In addition,the GeTe monolayer has a strong adsorption effect on water molecules,which is favorable for surface hydrolysis reaction.Therefore,this study not only provides a forward-looking development of a two-dimensional photocatalyst for water splitting,but also proposes a general method of introducing strain to improve the performance of a photocatalyst in a two-dimensional material.(2)The bulk NbS2Cl2 material has an ideal optical band gap and water stability,but the ease of recombination of photo-generated carriers hinder its photocatalytic water splitting application.To this end,we have constructed a few layers of two-dimensional NbS2Cl2 materials to investigate it’s photocatalytic performance.The cleavage energy is small enough to allow us to obtain NbS2Cl2 monolayer via mechanical exfoliation techniques.The ultra-thin mono-layer,bi-layer and tri-layer NbS2Cl2 not only have suitable band gaps for capturing the visible portion of the solar spectrum,but also their band edge positions satisfy the potential requirements of the photocatalytic water splitting.In addition,the NbS2Cl2 monolayer has a high anisotropic carrier mobility,which facilitates the separation of electrons and holes.Therefore,two-dimensional NbS2Cl2 monolayer can be used as potential candidates for photocatalytic water splitting,and provide theoretical guidance for the development of two-dimensional photocatalyst for water splitting.(3)We systematically studied a layered material a-PdSeO3 by means of DFT computations.We found that the bulk α-PdSeO3 has a small cleavage energy and can be easilly obtain α-PdSeO3 monolayer via mechanical or liquid exfoliation.The calculations demonstrate that the α-PdSeO3 monolayer has a suitable band gap and has a relatively strong absorption capacity for visible light and ultraviolet light.In particular,the photogenerated electrons and holes have adequate driving to render that both water oxidation and hydrogen reduction half reactions proceed spontaneously on the different active sites of the α-PdSeO3 monolayer.Our results vividly revealed that α-PdSeO3 monolayer can act as a highly efficient photocatalyst for direct overall water splitting into H2 and O2 in a stoichiometric amount of 2:1 without using any sacrificial reagents or cocatalysts.Our findings will help facilitate the exploration and applications of α-PdSeO3 monolayer and related2D materials for overall water splitting.(4)The PdSeO3 material has another phase,referred to herein as the P-PdSeO3.This material has a cleavage energy(0.41 J/m2)similar to that of the α-PdSeO3,suggesting that it can be prepared by exfoliation from its bulk material.The β-PdSeO3 monolayer is kinetically and thermodynamically stable.Compared to α-PdSeO3 monolayer,Theβ-PdSeO3 is a semiconductor with a direct band gap of 2.25 eV and has significant light harvesting capability in the visible and ultraviolet regions.In addition,the band-edge inβ-PdSeO3 monolayer fit perfectly the water oxidation and reduction potentials,and the photoexcited electrons can provide sufficient driving force to promote the solar-driven H2 evolution reaction..This work possess a novel 2D visible photocatalyest,which is of great significance to fully utilize solar energy for water-splitting.