| The successful exfoliation of graphene has brought about a major change in the field of material development,and has aroused people’s interest in the study of nanoscale two-dimensional(2D)materials such as transition metal sulfides(TMDs)and graphitic carbon nitride(g-C3N4).Due to its large specific surface area,high carrier mobility,and adjustable electronic structure,it has shown great application potential in the fields of novel electronics,optics,spintronics,and valleytronics.2D polar materials such as Janus metal chalcogenides,as important derivatives of 2D materials,have become a research hotspot in recent years.Such Janus materials exhibit many novel properties due to their mirror asymmetry,such as the Rashba effect,piezoelectric polarization,etc.,and have broad application prospects in nonvolatile memory devices,sensors,and other electromechanical devices.Due to the smooth surface and no dangling bonds of 2D materials,there has been a lot of interest in exploring van der Waals heterostructures(vdWHs)constructed from various 2D materials.2D vdWHs,bonded by weak vdW forces,are flexible,tunable,and easy to assemble.By combining different 2D materials and adopting specific stacking methods,their unique properties can be organically combined to exhibit more novel physical and chemical properties.Currently,the combination of ultrathin 2D monolayer materials with polar materials in vdWHs provides novel and attractive candidates for optoelectronic applications and multiferroic studies.Since the discovery of photocatalytic technology based on semiconductor materials in the last century,it has been used as a promising and sustainable solution to energy depletion and environmental pollution,as well as an ideal way to help achieve future carbon peak and carbon neutrality goals.However,the rapid recombination of photogenerated electron-hole pairs is the main obstacle to enhancing the photocatalytic activity.In order to meet the needs of future new energy development and environmental governance,it is necessary to design a new photocatalytic system to improve photocatalytic efficiency.In the past decades,researchers have employed various strategies to develop novel photocatalysts to enhance photocatalytic efficiency.For example,higher photocatalytic efficiency can be obtained through proper texture design,doping technology,etc.Among them,constructing semiconductor heterostructures is an effective strategy to improve photocatalytic efficiency.The high charge separation efficiency of 2D vdWHs makes them promising for applications in photovoltaic devices and photocatalysis.Furthermore,constructing multiferroic vdWHs with ferroelectric and ferromagnetic coupling properties is crucial for the development of 2D ultra-low-power spintronic devices for processing and storing information because of their ability to control magnetism through electric fields.However,until now,intrinsic 2D multiferroic materials,especially those with strong magnetoelectric coupling,are still rare.Therefore,it is meaningful research to design polar vdWHs that can be used in photocatalysis,photoelectric conversion,and electrically controlled magnetic switching.In this thesis,based on first-principles calculations of density functional theory,we systematically studied the applications of vdWHs based on polar materials SnSSe,PtSSe,and ferroelectric materials In2Se3 on photocatalytic hydrogen production,photoelectric conversion,and electromagnetic switches.The main study contents and conclusions of this thesis are as follows:(1)Theoretical study on noble-metal-free direct Z-scheme photocatalysts for overall water splitting based on SnC/SnSSe vdWHsMimicking the natural photosynthesis system,direct Z-scheme heterostructures are promising photocatalysts for solar-driven water splitting and have attracted ever-growing attention.Many achievements are performed for such a scheme.But active direct Z-scheme photocatalysts are still rare.Herein,based on density functional theory,we propose a vdWH consisting of SnC and Janus SeSnS monolayers as a noble-metal-free direct Z-scheme photocatalyst for overall water splitting.The intrinsic built-in electric field of Janus SeSnS and the charge transfer from the SnC to the SeSnS layer give rise to a type-Ⅱ band alignment.Such band alignment benefits the formation of spatially separated reductive and oxidative active sites and the reduction of the global bandgap of the Janus vdWH.The reduced overall bandgap of the Janus vdWH significantly enhances light absorption in ultraviolet,visible,and even infrared regions.The high solar-to-hydrogen(STH)conversion efficiency of up to 60.8%makes it a compelling photocatalyst for overall water splitting.In addition,the light absorption coefficient is strain-tunable,e.g.,tensile strain promotes photocatalytic efficiency.(2)Theoretical study on high-efficiency photocatalytic and photovoltaic performances based on bifunctional GeC/SnSSe vdWHsDeveloping bifunctional materials which can satisfy concurrently for designing innovative high-efficient photocatalytic materials and photovoltaic nanodevices is a good strategy for solving the current serious energy and environmental issues.Based on first-principles and quantum transport calculations,we designed one of these kinds of bifunctional materials:Janus GeC/SnSSe vdWHs.We find that both vdWHs with opposite stacking manners can be utilized as direct Z-scheme photocatalysts for water splitting and novel photovoltaic devices.Unlike traditional direct Z-scheme photocatalysts,an unusual small energy separation between the low conduction band in the SnSSe layer and the high valence band residing in the GeC layer in the GeC/SnSSe vdWHs significantly enhances the separation of photogenerated electron-hole pairs,fostering catalytic activities and power conversion efficiency.The STH conversion efficiency of GeC/SSnSe vdWH is as high as 68.37%,which is superior to that reported in the literature so far.Large strain tunability of photovoltaic performance:4%tensile strain promotes an increased photocurrent as high as 40%in the GeC/SeSnS vdWH at a tensile photovoltaic device.Giant STH conversion efficiency and high photoresponsivity render GeC/SnSSe vdWH a promising bifunctional candidate for solar energy harvesting.This facilitates the design of efficient solar conversion materials.(3)Theoretical study on high-efficiency solar energy conversion applications based on SnC/PtSSe vdWHsDeveloping bifunctional materials that can be exploited as innovative high-efficient photocatalytic materials and concurrently serve as the basis of photovoltaic nanodevices is an appealing strategy for solving the current serious energy and environmental issues.Based on first-principles and quantum transport calculations,we designed a noble-metal-free bifunctional material:Janus SnC/PtSSe vdWH and proposed it could be utilized as a direct Z-scheme photocatalyst for water splitting and novel photovoltaic device.The intrinsic electric field and type-Ⅱ band alignment can greatly promote the separation of photogenerated electron-hole pairs.Applying proper biaxial strain can further enhance the photocatalytic and photovoltaic performance of the SnC/PtSSe vdWHs.More importantly,the STH conversion efficiency increased by 41%at 4%tensile strain.At 2%tensile strain,the energy conversion efficiency of the SnC/SPtSe vdWH was enhanced by about 77%.These findings provide new application prospects for designing direct Z-scheme photocatalysts and vdWHs-based photovoltaic devices.(4)Theoretical study on realization of nonvolatile and switchable magnetism based on InSe/In2Se3 ferroelectric vdWHsElectrical control of magnetism in vdW semiconductors has potential applications in developing ultralow-power-consumption spintronic devices for processing and storing information.However,the nonvolatile control of ferromagnetism and ferroelectricity in multiferroic materials has rarely been reported.Here,we demonstrate that introducing a semiconductor-aided layer into the ferroelectric layer is an extraordinary approach to realizing the nonvolatile control of magnetism.When stacking the semiconductor InSe monolayer with a ferroelectric In2Se3 monolayer and the ferroelectric polarization pointing to the InSe monolayer,the system evokes considerable magnetic polarization energy,implying a room temperature magnetism with proper hole doping.Furthermore,the magnetic state disappears after reversing its ferroelectric polarization by an electric field.Therefore,with ferroelectric polarization reversal,the magnetic state bears an ON-OFF switch.This provides a practical approach for the application of nonvolatile data storage devices. |
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