Carbon-based Heterojunction Design Based On Energy Band Engineering And Its Photocatalytic Performance

The semiconductor photocatalysis technology is one of the effective ways to solve energy crisis and environmental pollution.However,the low quantum efficiency and other factors of photocatalyst limits its large-scale application.Therefore,the development of high-efficiency photocatalysts is a challenge in the Physics of Condensed Matter and so on.Among the reported catalysts,carbon-based materials have received great attention due to the low cost and excellent performance,such as graphene oxide?GO?and graphitic carbon nitride?g-C₃N₄?which are capable of water splitting.However,the larger carrier recombination rate of the catalyst leads to lower photocatalytic performance,and both the small band gap and the strong redox potential are not compatible.The use of two?or more?different materials to construct heterojunction?such as type ? or Z-scheme?can not only solve this problem,but also effectively reduce the recombination rate of electrons and holes,and improve the photocatalytic efficiency.However,how to construct efficient heterojunctions and optimize the morphology,dimensions and contact interfaces of heterojunctions is still an urgent problem to be solved.In this paper,several high-efficiency carbon-based heterojunctions are designed based on energy band engineering,and the regulation of heterojunction types and the influence of band offset,dimensions,morphology and contact interface on catalytic performance are systematically studied.At the same time,a simple method to regulate the dimension and morphology of g-C₃N₄ has been developed,which provides a better component structure and general strategy for the construction of heterojunctions.The detailed research contents are as follows:?1?Aiming at the problem that the heterojunction type and the band size are difficult to adjust,we propose a new strategy to construct and adjust the heterojunction type and band offset by using Zn_x Cd_1-xS and reduced graphene oxide?RGO?with continuously adjustable band gap.The effects of Zn/Cd ratio and oxygen concentration in RGO on the electronic structure of Zn_x Cd_1-xS and RGO are studied by first principles.Based on the energy band arrangement of the Zn_x Cd_1-xS and RGO,it is theoretically possible to construct heterojunctions of different interface typ es such as two type I,two type II,and one type ?.When the oxygen content of RGO is moderate,the type ? heterojunction is constructed and promotes the photogenerated electron-hole pair separation,and it has a high light absorption capacity in the vi sible light range,which greatly improves the photocatalytic performance of the catalysts system.In order to verify the theoretical calculation results,we prepared Zn _xCd_1-xS-RGO heterojunctions with different oxygen concentrations.Excessively high and l ow oxygen concentrations are not conducive to interfacial carrier separation,which is evidenced by catalytic activity,photocurrent,and photoluminescence?PL?tests.When the C:O of RGO is 6:1,the heterojunction exhibits the highest carrier separation efficiency and catalytic activity,which has verifies the theoretical calculation results.This design principle based on energy band engineering is also applicable to the design of other heterojunction materials,which can be widely used in the fields of photocatalysis,solar cells and functional electronic devices.?2?We have developed a simple method to regulate the dimensions and morphology of g-C₃N₄.Deionized water is used as a solvent,and melamine and cyanuric acid are hydrogen-bonded to form a uniform M-CA hexagonal prism precursor.The three-dimensional hierarchical structure,two-dimensional nanosheets and one-dimensional nanotubes of g-C₃N₄ can be prepared by regulating the annealing atmosphere and the melamine adsorbed on the surface of M-CA precursor.We have found that residual melamine molecules that are not hydrogen bonded to the cyanuric acid,which adsorbed on the surface of the M-CA precursor to protect the structural integrity of g-C₃N₄,which greatly enriched the supramolecular strategy for regulating the dimensions and morphology of g-C₃N₄.The inert N₂ effectively have prevented the interlayer polycondensation of the M-CA precursor to form a g-C₃N₄ nanosheet with a thickness of only 1.6 nm.The BET specific surface area and yield is 20 8.8 m²g^-1 and¹0 wt%,respectively,which is higher than reported values in the literature.The photocatalytic hydrogen evolution rate of the g-C₃N₄ nanosheet is 478?mol h^-1g^-1 under visible light irradiation,which is 17.3 times than bulk g-C₃N₄.This work not only enriches the understanding of the relationship between the dimensional of the photocatalytic material and the morphology and catalytic p roperties,but also provides a general method that design and preparation of g-C₃N₄-based heterojunction composites.?3?We use the hexagonal prism precursor of M-CA as the substrate to construct the Z-type heterojunction of g-C₃N₄/ZnO in combination with in-situ growth.By change the mass ratio of the components,the interface type and morphology of the heterojunction are effectively control,and the sample with hollow prismatic structure is obtained,which greatly increase the reactive sites.Zn²⁺is anchored on the surface of M-CA precursor,and an amorphous ZnO thin film with a thickness of only 10²0 nm is formed during annealing,which significantly shortens the migration distance of photogenerated carriers from the inside to the surface.Since the ban d gap of amorphous ZnO is slightly smaller than that of g-C₃N₄,the absorption spectrum of the heterojunction is slightly red-shifted,which is beneficial to improve the utilization efficiency of sunlight.The highest hydrogen evolution rate of g-C₃N₄/ZnO heterojunction reached 432.6?mol h^-1g^-1,which is 6 times than g-C₃N₄ under visible light irradiation.This work not only expands the application of amorphous ZnO in photocatalysis,but also develops the regulation strategy of heterojunction contact interface,which provides a new idea for the design and preparation of heterojunction photocatalyst in the future.?4?The g-C₃N₄ quantum dot?QCN?is prepared by a top-down method,and a0D/0D heterojunction with excellent interface is constructed with commercial P25.We have developed a method that promotes the intimate contact between QCN and P25,which can effectively reduce the contact resistance of the heterojunction interface and promote charge transfer.Although QCN is in contact with anatase or rutile o f P25,the carrier transport paths are different,but their interfaces are type II heterojunctions.Transient photocurrent,PL,optical absorption spectrum and other tests have proved that QCN can effectively inhibit the recombination of electrons and holes in P25,and broaden the range of light absorption and improve its photocatalytic activity.The highest hydrogen evolution rate of the heterojunction under visible light irradiation is986?mol h^-1g^-1,which is 11.7 times that of P25.This work will help develop the P25optimization strategy and increase its commercial application value.