Preparation And Hydrogen Evolution From Water Splitting Over G-C₃N₄ Based Photocatalyst

In recent years,photocatalytic technology using solar energy to split water into hydrogen has attracted considerable attentions as one of the most promising approaches for solving the energy crises and environmental issues causing by the depletion of fossil fuels,as well as meeting the growing energy demand globally.For photocatalytic technology,the strategic point lies in appropriately selecting and developing effective semiconductor photocatalysts.Nonmetal graphitic carbon nitride?g-C₃N₄?has attracted widespread attention due to its visible light activity,low cost,good chemical stability and unique layered structure.However,the low visible light utilization and fast carrier recombination of pure g-C₃N₄ still hinder the improvement of its photocatalytic performance.To address these problems,this thesis has focused on the rational design and preparation of g-C₃N₄ based photocatalysts with superior photocatalytic activity for H₂ evolution by employing different modification strategies.The structure-performance relationship was investigated in detail for g-C₃N₄ based photocatalyst.The novel nonmetal interlayer incorporated into the g-C₃N₄ framework was fabricated by thermal polymerization of the?-cyclodextrin??-CD?and melamine as precursors.Firstly,characterization results demonstrated that the interlayer was composed of oxygen-contained graphitized carbon.The influence of the morphology,structure and photoelectric properties on the photocatalytic activity was explored after the introduction of nonmetal interlayer into the g-C₃N₄ framework.Compared with that of pure g-C₃N₄,the photocatalytic H₂ evolution of nonmetal interlayer modified g-C₃N₄ was about 5 times more active,which originated from the narrowed band gap,negative-shifted conduction band position and efficient charge transfer caused by this metal-free interlayer incorporation.The nonmetal interlayer bridged the interlayer and extended the?-conjugated system,which facilitated the charge-carrier migration and separation.In addition,the 2D/2D carbon-based nanocomposites were constructed via mutual supporting strategy by using poly-?furfural alcohol?and melamine as the precursors.The CPFA and g-C₃N₄supported each other in the process of high temperature polycondensation and finally the 2D/2D CPFA/g-C₃N₄ nanocomposites were obtained.The introduction of carbonized poly-?furfural alcohol??CPFA?in g-C₃N₄ system for 2D/2D structure construction not only increased the contact area of CPFA and g-C₃N₄ but also acted as electron-conducting channels,which facilitated the charge carriers transport and separation.The CPFA/g-C₃N₄ nanocomposite thus displayed the dramatically enhanced photocatalytic H₂ evolution from water splitting.The ultrathin g-C₃N₄ nanosheets and 2D high-crystalline g-C₃N₄?HC-CN?nanosheets were constructed through the hydrogen bonds-assisted and template induced strategies.The mechanism of formation this ultrathin g-C₃N₄ nanosheet was investigated in detail by analyzing the interim product.The AFM results?2-3 nm?indicated the ultrathin g-C₃N₄ nanosheets have only of few-layer thickness.The high-crystalline g-C₃N₄?HC-CN?nanosheets with reduced structural defects has been constructed through the Ni-foam induced thermal condensation,where Ni-foam not only served as the template for the deposition of the 2D g-C₃N₄nanosheets with high surface area to prevent the stacking of g-C₃N₄ nanosheets,and but also acted as the catalyst to promote the polymerization and crystallization of g-C₃N₄ via the effective dehydrogenation of-NH₂ group.The obtained ultrathin g-C₃N₄ nanosheets and HC-CN nanosheets exhibted superior photocatalytic performance for H₂ evolution under visible light irradiation.At last,the photocatalytic reaction mechanism was investigated in detail.This reveal why improved photocatalytic activity was obtained.The barbituric acid modified porous ultrathin g-C₃N₄ nanosheets?CNB NS?photocatalyst was prepared through a combined methodology of precursor reforming and thermal condensation.The mechanism of constructing porous ultrathin g-C₃N₄ nanosheets architecture was investigated in detail through the controlled group experiment.SEM,TEM and AFM results indicated that the obtained sample has the porous ultrathin g-C₃N₄ nanosheets architecture.The UV-vis DRS analysis results showed improved light obsorption.The PL and the photoelectrochemical measurements suggested the CNB NS facilitates the electron-hole pair diffusion,transmission and separation.The synergistic effect of barbituric acid modification and porous ultrathin nanosheets architecture made the material not only improved light harvesting,regulated band structure,facilitated the electron-hole pair separation,and supplied numerous active reactive sites and electron diffusion channels.As a result,the average hydrogen evolution rate of the CNB NS is 1323.25?mol h^-1g^-1,which is about 13 times higher than that of pure g-C₃N₄.A new Pt²⁺/Pt⁰ hybrid nanodots modified g-C₃N₄ photocatalyst?CNV-P?was fabricated by a chemical reduction method,in which nitrogen vacancies in g-C₃N₄assist to stabilize Pt²⁺species.It was elucidated that the coexistence of metallic Pt⁰and Pt²⁺species in the Pt nanodots loaded on g-C₃N₄ resulted in superior photocatalytic H₂ evolution performance with very low Pt loadings.The turnover frequencies?TOFs?are 265.91 h^-1 and 116.38 h^-1 for CNV-P0.1?0.1 wt%Pt?and CNV-P0.5?0.5 wt%Pt?,respectively,which are much higher than most of those of other g-C₃N₄-based photocatalysts with Pt co-catalyst reported previously.The excellent photocatalytic H₂ evolution performance results from:?i?metallic Pt⁰facilitates the electron transport and separation,and Pt²⁺species prevented the undesirable H₂ backward reaction;?ii?the strong interfacial contact between Pt²⁺/Pt⁰hybrid nanodots and nitrogen vacancies of CNV facilitated the interfacial electron transfer;and?iii?the highly dispersed Pt²⁺/Pt⁰ hybrid nanodots exposed more active sites for photocatalytic H₂ evolution.Our findings are useful for the design of highly active semiconductor-based photocatalysts with extremely low contents of precious metals to reduce the catalyst cost while achieving good activities.