| As an organic semiconductor material,graphitic carbon nitride（g-C3N4）is only made up of C and N elements.g-C3N4 possesses unique physical and chemical characteristics unmatched by traditional inorganic semiconductor materials,including suitable band structure,tunable molecular structure,cheap and plentiful raw materials,structural regulation and high thermal/mechanical/chemical stability.So that,it has attracted wide attention in many applications including photocatalysis,solar cells,electrochemistry and gas sensor.Among them,photocatalytic hydrogen production from water is a green and sustainable technology that can directly convert solar energy with low energy density into hydrogen energy with high heat value.It is expected to solve the two major problems of environmental degradation and energy shortage faced by today’s society.Thus,photocatalytic hydrogen production from water is considered to be an emerging technology with great development prospects.However,the bulk g-C3N4 prepared by a traditional thermal polycondensation method shows obvious shortcomings,such as exciton recombination,low electrical conductivity,low specific surface area and narrowed light absorption range.As a result,its photocatalytic hydrogen production performance is unsatisfactory,and eventually bulk g-C3N4 cannot be used for large-scale commercial hydrogen production.In view of this,researchers have explored and developed a series of strategies to improve the photocatalytic performance of g-C3N4.The most common and effective strategies mainly include elemental/molecular doping,functional group modification,morphology control,defect engineering,cocatalyst loading and construction of heterojunctions.Considering several key factors restricting the photocatalytic hydrogen production perfomance of g-C3N4,this thesis focuses on two typical control strategies of nanostructure and molecular structure engineering to achieve the optimization of electronic structure,photoelectrochemical and photocatalytic performance about g-C3N4 under guaranteeing its inherent advantages.In this thesis,two-dimensional（2D）porous ultrathin and broadband g-C3N4 nanosheets were prepared by rapid thermal oxidation,and a series of one-dimensional（1D）g-C3N4 nanotubes were prepared by the supramolecular self-assembly method,which realize the design of g-C3N4 nanostructures.Moreover,the regulation of the molecular structure of g-C3N4was completed by introducing defects,functional group modification,element or molecular doping,etc.A series of as-prepared low-dimensional g-C3N4 materials showed good photocatalytic hydrogen production performance.Based on this,this thesis focuses on exploring the underlying mechanism of enhanced photocatalytic hydrogen production performance of these low-dimensional g-C3N4 materials,and systematically investigated the relationship between their morphological/electronic structure and the photocatalytic hydrogen production performance,and the photocatalytic overall water splitting is ultimately achieved.It provides a reference for further understanding of the nanostructure and molecular structure regulating the photocatalytic activity of g-C3N4.The specific research contents are as follows:1.Preparation of ultrathin and broadband graphitic carbon nitride nanosheets and their photocatalytic hydrogen evolutionAiming at the disadvantage of bulk g-C3N4’s limited visible light response capability,a novel short-time thermal oxidation method is used to prepare porous ultrathin and broadband-responsed g-C3N4 nanosheets.SEM and TEM tests show that g-C3N4 nanosheets have a porous ultrathin structure,which can increase the specific surface area,expose a large amount of surface adsorption and active sites,shorten the charge migration distance.DRS test shows that the g-C3N4 nanosheets possess a significantly wide visible light response range and can improve the utilization of light.The porous ultrathin and broadband-responsed g-C3N4 nanosheets have excellent photocatalytic hydrogen production performance with H2 evolution rate of 240.60μmol h-1 in the visible light region（λ≥400 nm）.Meaningfully,it also shows good photocatalytic activity with H2 evolution rate of 5.08μmol h-1 in a broad spectral region（λ≥500 nm）.Furthermore,the corresponding EQE is approximately 12.9%atλ=420 nm.2.Preparation of nitrogen deficient graphitic carbon nitride nanotubes and their photocatalytic hydrogen evolutionAiming at the disadvantages of the traditional hard-template synthetic method for the preparation of g-C3N4 nanotubes,including the complex steps,long preparation cycle and difficult template removal,a preliminary supramolecular self-assembly method is used to prepare g-C3N4 nanotubes.The study indicates that the tubular structure can increase the specific surface area,improve light absorption performance,promote fast and long-distance transport of electrons,expose more adsorption and reaction sites.In addition,there are abundant N defects on the surface of the obtained g-C3N4 nanotubes,which can capture electrons or holes,thereby inhibiting exciton recombination.Owing to the tubular structure and abundant N defects,g-C3N4nanotubes show enhanced photocatalytic hydrogen production performance,and its hydrogen evolution rate is up to 118.5μmol h-1（λ≥400 nm）and the EQE atλ=420nm is 6.8%.At the same time,the supramolecular self-assembly method also lays the foundation for subsequent research work.3.Preparation of amino groups-rich graphitic carbon nitride nanotubes and their photocatalytic hydrogen evolutionAiming at the problems of insufficient order and large size of the g-C3N4nanotubes prepared in the previous chapter,the electronic and tubular structure of g-C3N4 nanotubes are further optimized by adding hydroxylamine hydrochloride with both amino and hydroxyl groups in the molecular structure during the supramolecular self-assembly process.Supported by the synchrotron-based X-ray absorption near-edge structure（XANES）spectroscopy,it is found that there are abundant amino groups in the structure of g-C3N4 nanotubes,which is conducive to enhancing the Lewis basicity and hydrophilicity of g-C3N4 nanotubes,and strengthing the ability to adsorb H2O or CO2 molecules.The tubular structure endows g-C3N4 nanotubes a large specific surface area,fast and long-range electron transport capabilities,and large amounts of adsorption and reaction sites.Combining the advantages of amino functionalization and tubular structure,the amino-rich g-C3N4 nanotubes prepared in this chapter are a dual-function photocatalytic material with both photocatalytic hydrogen production and CO2 reduction ability.The photocatalytic hydrogen activity of amino-rich g-C3N4 nanotubes is about 24.7 times that of bulk g-C3N4,the hydrogen evolution rate is up to 22.98 mmol g-1 h-1.The amino-rich g-C3N4 nanotubes exhibit a superior visible-light-induced CO2-to-CO conversion rate of 103.6μmol g-1 h-1,which is 17 times higher than that of bulk g-C3N4.4.Preparation of B,S codoped graphitic carbon nitride nanotubes and their photocatalytic hydrogen evolutionAiming at the problem that bulk g-C3N4 has few surface active sites,B,S co-doped g-C3N4 nanotubes are prepared by using the supramolecular self-assembly method in the presence of B-containing boric acid,S-containing thiourea and cyanuric acid as precursors.It is found that B atom can coordinate with two N atoms in the cavity between adjacent mesazine ring units,and S atom can replace the N atom at the edge of the mesazine unit.The simultaneous introduction of the B and S elements improves the charge separation and transport behavior of the g-C3N4 nanotubes and obtains a more positive valence band position,which further enhances the oxidation ability of the g-C3N4 nanotubes.Therefore,B,S co-doped g-C3N4 nanotubes have achieved improved photocatalytic performance in the fields of photocatalytic hydrogen production and degradation of pollutants.B,S co-doped g-C3N4 nanotubes display an excellent H2 evolution rate of 93.21μmol h-1 with the EQE of 5.3%atλ=420 nm.5.Preparation of carbon rings-conjugated graphitic carbon nitride nanotubes with all-organic D-A system and their photocatalytic hydrogen evolutionAiming at the problem of easy exciton recombination in bulk g-C3N4,carbon rings-conjugated g-C3N4 nanotubes with donor-acceptor（D-A）structure is prepared by using supramolecular self-assembly method,which accords to the successful experience of D-A structure in the field of solar cells.The 13C solid-state NMR and the XANES spectroscopy reveal that the C-ring structure is successfully implanted into the g-C3N4 nanotube framework,indicating that a photocatalyst with a D-A structure is successfully constructed.The D-A structure realizes the intra-molecular charge transfer,thereby suppressing the recombination of excitons,accelerating the separation and transmission of charges,and finally achieving high-efficiency photocatalytic hydrogen production performance.More importantly,the D-A structure further realizes the full photocatalytic overall water splitting without using any sacrificial agents. |
