Study on Integration of Non-platinum Hydrogen Releasing Sites on Graphitic Carbon Nitride for Photocatalytic Hydrogen Evolution

Author:Zhang Yun Xiao

Supervisor:zhang wei de


Degree Year:2019





Replacing non-renewable fossil energy with clean energy is an effective way to solve environmental pollution and energy crisis.Compared with other solar fuels,hydrogen energy holds the advantages of easy storage,environmental friendliness,and high energy content.Nowadays,efforts are mainly devoted on looking for efficient,stable and inexpensive photocatalysts to enable industrial applications of photocatalytic hydrogen generation.Graphitic carbon nitride(g-C3N4)is a two-dimensional non-metallic photocatalyst with suitable band gap,excellent chemical stability and can be obtained by a cost-effective way.However,the photocatalytic activity of g-C3N4 is still low.Various strategies,such as doping metal or non-metal elements,constructing heterojunctions with other semiconductors,and regulating micro-nanostructures,were used to improve the utilization of visible light,the separation efficiency of photogenerated electrons and holes,and to enlarge specific surface area of g-C3N4,intending to enhance its photocatalytic activity.However,the above-mentioned modifications usually require the addition of a noble metal such as platinum as a cocatalyst to reduce the overpotential of photocatalytic reaction and promote hydrogen releasing.Therefore,in order to reduce the cost of photocatalytic reaction,this study focused on integration of metal sulfides and transitional metal complexes onto g-C3N4 as hydrogen releasing sites for efficient photocatalytic hydrogen production.The main contents are as follows:1.Carbon nanotubes(CNTs)-modified graphitic carbon nitride(CN)with synergistic effect of nickel(II)sulfide(NiS)and molybdenum(II)disulfide(MoS2)This work reports effective photocatalysts which are composed of carbon nitride,carbon nanotubes(CNTs),MoS2 and NiS,for hydrogen evolution aiming at energy crises and environmental pollutions.The morphologies and optical properties of the photocatalysts were carefully characterized and their photocatalytic performance towards water reduction was studied afterwards.MoS2 and NiS exhibit a significant synergistic effect working as co-catalysts.Compared to MoS2/CN nanohybrid,carbon nanotubes and NiS improved the absorption of visible light and the separation of charge carriers effectively.NiS-MoS2/CNTs/CN catalyst exhibits high performance for H2 evolution and the optimized rate is 309.9μmol·h-1·g-1 with no noble metals under visible light irradiation.2.Cobalt complex and amino-functionalized carbon quantum dots modified graphitic carbon nitrideTernary photocatalysts Co(dcbpy)2(NCS)2/CQDs/CN by amide bonds coupling Co(dcbpy)2(NCS)2[dcbpy=(4,4’-dicarboxy-2,2’-bipyridine)]with amino-functionalized carbon quantum dots(CQDs)or graphitic carbon nitride nanosheets(CN)were developed for efficient solar-to-hydrogen conversion.The amide bonds accelerate the combination of Co(dcbpy)2(NCS)2 and CQDs/CN and improve the dispersion of Co(dcbpy)2(NCS)2 on CN framework,thus leading to enhanced migration of charge carriers and absorption of visible light.In addition,the optimum photocatalytic hydrogen evolution activity of Co(dcbpy)2(NCS)2/CQDs/CN catalyst reaches 295.9μmol·h-1·g-1 with excellent stability after four cycling tests under visible light irradiation.The hydrogen evolution activity of3Co(dcbpy)2(NCS)2/CQDs/CN catalyst reaches 75.02μmol·h-1·g-1 under monochromatic light irradiation(λ=450 nm)and the apparent quantum efficiency reaches 0.71%,whereas pure CN is inactive under the same condition.3.Modification of graphitic carbon nitride by nickel-schiff based complex and nickel oxide nanoparticlesTwo tetracoordinated cobalt(CoLa)and nickel(NiLa)schiff base complexes were prepared by simple chemical reaction for electrocatalytic reduction of protons or water to produce hydrogen.The TOF values of CoLa and NiLa reached 738.23 and 1331.23 h-1,respectively,in a buffer solution with pH=7.0 at-1.45 V vs.Ag/AgCl.According to the preparation of NiLa,a novel nickel-schiff base complex(NiLb)was prepared.Moreover,a facile and simple synthetic route is developed to prepare earth-abundant and noble-metal-free hybrid catalysts,which are composed of graphitic carbon nitride(CN),nickel complex(NiLb),and NiOx nanoparticles.Bimolecular nucleophilic substitution reaction was employed to attach nickel complex onto graphitic carbon nitride framework through covalent bond to support its high loading and dispersion.NiOx nanoparticles were further incorporated into the catalysts to serve as hole transporting medium to enhance the separation of photogenerated electrons and holes for higher photocatalytic performance.The optimized photocatalytic H2 evolution rate of the binary photocatalysts yNiLb/CN reaches 303.3μmol·h-1·g-1,while that of the ternary photocatalysts yNiLb/NiOx/CN reaches 524.1μmol·h-1·g-1 with apparent quantum efficiency of 1.46%at 450 nm.4.Modification of in-plane benzene-ring doped graphitic carbon nitride nanosheets with nickel complexA new dinuclear cobalt(III)[Lc2Co2Cl2]complex was prepared using amine bis(phenol)as a ligand.The complex exhibits an excellent electrocatalytic activity at an overpotential of887.6 mV and a TOF of 844.63 h-1.In addition,a novel nickel-schiff base complex(NiLd)was prepared followed the similar way.Furthermore,an efficient visible-light-driven hydrogen evolution method was achieved through the synthesis of a novel and highly efficient photocatalyst based on nickel complex(NiLd)and in-plane benzene ring-modified graphitic carbon nitride nanosheets(CN-DAPS)by copolymerization of urea andπ-electron-rich 4,4’-diaminodiphenyl sulfone(DAPS)via a controlled thermal-etching step.The incorporation of in-plane benzene-ring into CN not only extends theπ-conjugated system,but also promotes the migration of photogenerated electrons and enhances absorption of visible light.In addition,NiLd is covalently integrated onto CN-DAPS framework,enabling the intramolecular electron transfer from CN-DAPS to the coordinately-bonded nickel complex.Moreover,covalent combination of NiLd with CN-DAPS framework further enables its dispersion in molecules.The novel photocatalysts NiLd/CN-DAPS exhibit superior H2 evolution activity,while the optimum hydrogen generation rate reaches 613.4μmol·h-1·g-1.The broadened absorption of visible light causes the NiLd/CN-DAPS to be active for hydrogen generation even under irradiation of light with wavelength of 550 and 600 nm.The apparent quantum efficiency(AQE)reaches 2.15%at 450 nm.