Enhancement And Mechanism Of G-C₃N₄ Photocatalytic Hydrogen Production

Nowadays,photocatalytic technology is considered to be one of the most potential ways to solve the energy shortage,which can convert solar energy into chemical energy.Graphitic carbon nitride（g-C₃N₄）has attracted most researchers’attention because of its unique structure,stable physical-chemical properties,simple synthesis pathway and low cost.However,the pure g-C₃N₄ photocatalyst has some disadvantages such as small specific surface area,limited solar light utilization and low separation rate of photogenerated carrier,leading to the poor photocatalytic activity.Therefore,in order to overcome these shortcomings,three different modification methods are carried out to promote the photocatalytic activity of g-C₃N₄in the thesis discusses.The main contents are as follows:1.The novel in situ g-C₃N₄ p-n homojunction photocatalyst with nitrogen vacancies（NV-g-C₃N₄）is successfully fabricated via hydrothermal synthesis followed by two-step calcination.The in situ NV-g-C₃N₄ homojunction can be employed as an effective photocatalyst for hydrogen generation through water splitting under visible light,and the optimum rate constant of 3259.1μmol·g^-1·h^-1 is achieved,which is 8.7times as high as that of pristine g-C₃N₄.Moreover,the markedly increased photocatalytic performance is ascribed to the enhanced light utilization,large specific surface area and unique nitrogen-vacated p-n homojunction structure,which provides more active sites and improves the separation of photo-excited electron-hole pairs.Besides,the underlying mechanism for efficient charge transportation and separation is also proposed.This work demonstrates that the remodeling of g-C₃N₄ p-n homojunction with nitrogen vacancies is a feasible way as highly efficient photocatalysts and might inspire some new strategies for energy and environmental applications.2.A novel hierarchical porous NiO is successfully fabricated by simple precipitation and calcination by using Ni（CH₃COO）₂.4H₂O as nickel source,butanedioxime as ligand.Subsequently,the precursor of NV-g-C₃N₄ is loaded on the hierarchical porous NiO and calcined in air to prepare NV-g-C₃N₄/NiO.The NV-g-C₃N₄/NiO photocatalysts exhibited outstanding H₂ evolution rate under visible light irradiation in absence of expensive and rare noble metal co-catalysts.The optimized NV-g-C₃N₄/NiO（the mass ratio of NiO is 1.7%）achieved a maximum H₂evolution with 170.60μmol·g^-1·h^-1,exhibiting⁸.3-fold enhancement compared to NV-g-C₃N₄.NiO as co-catalyst provided more active sites for photocatalytic H₂evolution.Moreover,an inner electric field is formed between the interface of NiO and host nitrogen-vacanted g-C₃N₄,resulting in the photogenerated electrons transfer from NV-g-C₃N₄ to NiO co-catalyst,and thus significantly promoting the migration and separation of photogenerated charge carriers.3.NV-g-C₃N₄/NiO/Co-Pi composite photocatalyst is prepared by simple photochemical deposition method for depositing Co-Pi onto NV-g-C₃N₄/NiO by using cobalt chloride as cobalt source.As an oxidized co-catalyst,Co-Pi can effectively capture the photogenerated holes in photocatalytic reaction,and hierarchical porous NiO can quickly transfer the photoinduced electrons,resulting in the fast separation of photoinduced e^--h⁺pairs.The rate of photocatalytic hydrogen production is evaluated under the visible light in absence of the noble metal co-catalyst.The experimental results show that the NV-g-C₃N₄/NiO/Co-Pi composite exhibits the best photocatalytic activity.That may be ascribed to the fact that the synergistic effect of NiO and Co-Pi effectively promote the separation rate of photogenerated e^--h⁺pairs and thus the corresponding photocatalytic activity is significantly enhanced.