Preparation and Hydrogen Evolution from Water Splitting Over g-C3N4 Based Photocatalyst

Author:Xing Wei Nan

Supervisor:chen gang

Database:Doctor

Degree Year:2018

Download:59

Pages:146

Size:7953K

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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 theβ-cyclodextrin(β-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 theπ-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 1323.25μmol 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.