The First-principles Study on the Photocatalytic Performance of Boron Nitride and Carbon Nitride-based Van Der Waals Heterostructures

Author:Wang Biao

Supervisor:chen hong


Degree Year:2019





Environmental pollution and energy shortage have become the most serious problems in 21st Century.In order to solve these problems,semiconductor photocatalytic technology has risen at the right moment and gradually formed two fields of environmental purification and photocatalytic water splitting for hydrogen production.Because the process of photocatalytic water decomposition is safe and pollution-free,and its product(hydrogen energy)is clean power,the technology of photocatalytic water splitting is highly appreciated by researchers.However,traditional semiconductor electrodes such as titanium dioxide can only take advantage of ultraviolet light,which only occupies 4%of the solar spectrum,to catalyze for the water decomposition and result in low photocatalytic efficiency.Because two-dimensional(2D)materials have different peculiar properties from bulk materials in electronic,optic,mechanical and catalytic aspects,the research on 2D materials has reached a high level since the discovery of graphene.Moreover,2D materials can maximize the surface area and thus improve the efficiency of photocatalytic water decomposition.Many photocatalytic research groups have turned their focus on 2D graphene-like materials,such as carbon nitride(g-C3N4和C2N),borophene,silicene,hexagonal boron nitride(h-BN),black phosphorus(BP),III-V groups 2D nanomaterials(e.g.,SnS2,GaS,GaSe and InSe,etc.),transition metal sulfides(TMDCS,e.g.,WS2,MoS2,ZrS2,HfS2,etc.),transition metal carbides,nitrides,and carbonitrides(MXenes;e.g.,Ti2C,Nb2C,etc.).However,there is a problem of rapid recombination of photo-generated electrons-hole pairs in the photocatalytic process of independent 2D materials,which may weaken the photocatalytic efficiency.Recent studies have found that ideal band gaps and band-edge positions can be obtained by constructing 2D van der Waals heterostructures to enhance photocatalytic performance.In addition,density functional theory(DFT)has become an important method to study the electronic structure of systems,and has been widely used in the field of computational condensed matter physics and chemistry.In recent years,the van der Waals force in heterostructures can be approximately corrected by using non-local hybrid exchange-correlation function.Moreover,the energy gap calculated by hybrid density functional theory is generally closer to the experimental value.Therefore,there are great scientific value and practical significance to study the photocatalytic performance of van der Waals heterojunction by using hybrid density functional theory.In this thesis,we aim at h-BN,g-C3N4C2N,GaS,GaSe,HfS2,and InSe single-layer materials.GaS/h-BN(g-C3N4),GaSe/g-C3N4,HfS2/h-BN(g-C3N4),and InSe/C2N heterostructures have been constructed,whose band gaps,band-edge positions,density of states,strain effects,and optical absorption spectroscop have been systematically studied by hybrid density functional theory.The mechanism of improving the photocatalytic performance of semiconductors has been investigated.The work has been carried out in the following aspects:1.We employ hybrid density functional calculations to explore the potential of the2D GaS-based heterojunctionsfor the design of efficient water redox photocatalysts.Two-dimensional(2D)gallium sulfide(GaS),hexagonal boron nitride(h-BN)and graphitic carbon nitride(g-C3N4)have been fabricated and expected to be promising photocatalysts under ultraviolet irradiation.Both GaS/h-BN(g-C3N4)heterostructures can be formed via van der Waals(vdW)interaction and are direct bandgap semiconductors,whose bandgaps are reduced comparing with isolated GaS,h-BN or g-C3N4 monolayers and whose bandedges straddle water redox potentials.Furthermore,the optical absorption of GaS/h-BN(g-C3N4)heterostructures is observably enhanced in the ultraviolet–visible(UV–VIS)light range.The electron–hole pairs in GaS/h-BN(g-C3N4)heterostructures are completely separated from different layers.In addition,the in-plane biaxial strain can effectively modulate the electronic properties of GaS/h-BN(g-C3N4)heterostructures.Thus the GaS/h-BN(g-C3N4)heterojunctions are anticipated to be promising candidates for photocatalytic water splitting to produce hydrogen.2.In this work,we employ hybrid density functional theory to investigate HfS2-based vdW heterojunctions for highly efficient photovoltaic and photocatalytic applications.2D hafnium disulfide(HfS2)has been synthesized and is expected to be a promising candidate for photovoltaic applications,and at the same time the h-BN and g-C3N4 sheet have also been fabricated and are expected to be applied in photocatalysis.HfS2/h-BN and HfS2/g-C3N4 heterostructures with direct bandgaps and efficient charge separation are both typical type-II semiconductors and have potential as photovoltaic structures for solar power.Moreover,compared with h-BN and g-C3N4 single-layers,HfS2/h-BN heterostructure with 6%tensile strain and HfS2/g-C3N4 heterostructure with 9%tensile strain have moderate bandgaps,whose optical absorption is obviously enhanced in the ultraviolet-visible(UV-VIS)light range and whose bandedges are suitable for photocatalytic water splitting.HfS2/h-BN heterostructure with 6%applied strain,being different from HfS2/g-C3N4 heterostructure with 9%strain,possess a direct bandgap and show complete separation of the photoinduced electron–hole pairs.Thus the HfS2/h-BN heterojunction with 6%strain has bright prospects for photocatalytic water splitting to produce hydrogen by using visible light.3.Employing hybrid density function theory,we have study the structural,optical,and electric properties of InSe/C2N vdW heterostructure.2D indium selenium(InSe)nanosheet and nitrogenated holey structure(C2N)have been successfully fabricated and show the potential in photocatalytic applications.Compared with InSe and C2N monolayers,InS/C2N heterojunction with desirable moderate bandgap and type-II band alignment can completely separate the electron-hole pairs.Their optical absorptions are definitely strengthened in the ultraviolet and visible(UV-VIS)light regions.Moreover,InS/C2N heterojunction applied with 3%compressive strain can be adjusted to direct bandgap semiconductor.Therefore,the InS/C2N heterojunction is a promising visible-light driven catalyst for efficient water splitting.4.Using hybrid density functional theory,the performance of photocatalytic water decomposition in GaSe/g-C3N4 heterojunction has been investigated.GaSe/g-C3N4heterojunction have been designed,whose band gaps,band edge positions,density of states,strain effects and optical absorption spectra have been studied.When GaSe/g-C3N4 heterojunction is applied with 3%pressure or 6%tension,the band-edge positions of the heterojunction is suitable for the generation of oxygen and hydrogen from water.When applied tension,the heterojunction can completely separate the photogenerated electron-hole pairs,and the optical absorption properties of the heterojunction in the visible region are significantly enhanced resulting in the improvement of performance of water splitting in the visible region.5.We have systematically investigated the electronic and optical properties of 2D Van der Waals SnS2/MoSe2 nanocomposite by employing the HSE06 hybrid density functional theory.Through combing 2D SnS2 and MoSe2 single-layers,the vdW SnS2/MoSe2 heterojunction has shown to be a direct Z-scheme composition without redox mediators.A strong built-in electric field,which can effectively improve the separation efficiency of photogenerated electrons and holes,and thus improve the photocatalytic performance,forms in SnS2/MoSe2 heterojunction.Moreover,SnS2/MoSe2 heterojunction can straddle the redox potential energy of water and has enhanced the redox ability of photocatalytic water splitting comparing with the SnS2and MoSe2 monolayers.The valence bands and conduction bands of the heterojunction have been separated in two different layers,which also can avoid the recombination of photogenerated electron-hole pairs and effectively separate hydrogen and oxygen.In addition,the band gap of SnS2/MoSe2 heterojunction has been reduced,and the heterostructure can improve the optical absorption intensity in the ultraviolet-visible light region.Therefore,van der Waals SnS2/MoSe2 heterojunction is a potential material for photocatalytic water splitting.In addition,these materials such as h-BN,g-C3N4,C2N and GaS studied in this dissertation widely exist on the earth,and the production cost of these materials is low.Moreover,these heterostructures,whose crystal lattice are well matched,have strong interlayer binding energy,and have high photocatalytic efficiency.With the industrial development of manufacturing thin film,a serie of heterostructures will been widely used.There will be more and more broad prospects in production and application.