Synthesis Of G-C₃N₄-based Nanophotocatalysts Based On Charge Separation And Light Absorption Control

The growing energy crises and the unrestricted release of objectionable pollutants to the environment have caused comprehensive concerns in scientific society.Photocatalysis presents its environmental friendly service in this area.As a promising green technology,photocatalysis has received enormous attention owing to its mild nature,high efficiency and low cost operational techniques.Under solar irradiation,photocatalysts can make H₂O directly convert into H₂ and O₂,reduce CO₂ into solar fuels and degrade pollutants into H₂O and CO₂ without imposing any threat to the environment.Since the solar spectrum contains nearly 46%of visible light,,it is highly desired to develop narrow band gap photocatalysts for effective solar energy utilization.Polymeric graphitic carbon nitride,g-C₃N₄（CN）has been extensively utilized for water splitting,CO₂ reduction and pollutants degradation.However,high recombination rate of photogenerated charges,marginal visible-light absorption and weak surface catalysis still limit its practical applications in the photocatalytic conversion processes.Therefore,it is much meaningful to further improve its photocatalytic activities to obtain an efficient photocatalyst.Herein,we have carried out a series of modification strategies to improve charge separation,extend the optical absorption to visible light and improve surface catalysis of CN for enhanced photocatalytic activities to obtain energy rich compounds and safe environment for future generations.The main contents are as follow:1.Synthesis of plasmonic Au-（TiO₂/g-C₃N₄）nanocomposites as efficient visible-light photocatalysts to degrade bisphenol A（BPA）and 2,4-dichlorophenol（2,4-DCP）in aqueous solution under ambient conditions.Plasmonic Au-TiO₂/g-C₃N₄,（Au-T/CN）nanophotocatalysts were fabricated by coupling TiO₂ with CN by wet chemical method and then loading Au nanoparticles by photodeposition method.The photocatalytic activities were studied for the degradation of BPA and 2,4-DCP.The amount optimized2Au-6T/CN nanocomposite showed degradation activities of 37 and 46%for BPA and2,4-DCP respectively in 1 h under visible-light irradiation with 5.11 and 3.1 times improvement compared to that of pure CN.Interestingly,the photocatalyst extended the visible-light response of 2Au-6T/CN to 590 nm as proven by the degradation activity of2 and 4%for BPA and 2,4-DCP respectively at the given wavelength.These enhanced degradation activities are mainly attributed to highly promoted photogenerated charge separation via the excited electron transfer from plasmonic Au（590 nm>λ>470 nm）and CN（470 nm>λ>400 nm）to TiO₂ and extended visible-light absorption as indicated by the surface photovoltage spectra,photoelectrochemical I-V curves and transient-state surface photovoltage（TS-SPV）measurement.The highly promoted charge separation resulted in the availability of a large number of hydroxyl radicals（·OH）participating in the photocatalytic oxidation process as confirmed from the degradation of 2,4-DCP in the presence of iso-propyl alcohol as·OH scavenger.Moreover,the degradation rate constants of BPA and 2,4-DCP with 2Au-6T/CN were1.40 and 1.53 times higher than their respective degradation rate constants with P-25under UV-vis light irradiation showing the excellency of the fabricated photocatalyst.Finally,a possible reaction mechanism for the degradation of 2,4-DCP was proposed according to the analysis of the degradation intermediates.2.Synthesis of surface plasmon assisted Au/SnO₂/g-C₃N₄ nanophotocatalysts for hydrogen evolution.Firstly,g-C₃N₄（CN）nanosheets have been synthesized using dicyandiamide as precursor material and then coupled with different mass percent ratios of SnO₂（SO）nanoparticles.Au nanoparticles were photodeposited on the surface of the nanocomposites to form plasmonic Au/SnO₂/g-C₃N₄ photocatalysts and applied in the production of H₂ from water splitting.All the photocatalysts exhibited superior photocatalytic activities and favorable stabilities without any co-catalyst under visible-light irradiation.The 2Au/6SO/CN nanocomposite produced approximately 770μmol g^-1.h^-1 H₂ gas underλ>400 nm light illumination compared to the H₂ gas output of SO/CN（130μmol g-1）,Au/CN(112μmol g^-1 h^-1)and CN(11μmol g^-1 h^-1)under the same irradiation conditions.In addition,the photocatalytic activity of 2Au/6SO/CN continued unchanged for 6 runs in 30 h.It is confirmed by means of surface photovoltage spectra,photoelectrochemical I–V curves,electrochemical impedance spectra,examination of formed hydroxyl radicals and photocurrent action spectra as a function of different excitation wavelengths that the enhanced photoactivity for H₂evolution is attributed to the obviously promoted photogenerated charge separation via the excited electron transfer from plasmonic Au（≈520 nm）and CN（470 nm>λ>400nm）to SO.Moreover,the Kelvin probe test indicates that the conduction band of SO in the fabricated 2Au/6SO/CN nanocomposite is slightly shifted upward to attain a new and proper energy platform for the reduction process to evolve H₂ gas.3.Synthesis of cobalt phthalocyanine modified g-C₃N₄ nanophotocatalysts for degrading toxic and carcinogenic pollutants like 2,4,6-trichlorophenols（2,4,6-TCP）.In this section,porous CN nanosheets were synthesized by using melamine-cyanuric acid thermopolymerization method.Cobalt phthalocyanine（C）was then modified on porous CN.The photocatalytic activities of the fabricated photocatalysts were evaluated by degrading 2,4,6-TCP under visible-light irradiation and at different wavelengths.The amount optimized 0.5C-CN sample showed 27%degradation activity under visible light in 1 h.Interestingly,the photocatalytic activities were significantly improved up to 42%when cobalt phthalocyanine was loaded on phosphate bridged CN.These enhanced activities are attributed to the close contact between CN and cobalt phthalocyanine in the presence of phosphate bridges to extend the optical absorption of CN.From trapping experiments,it was confirmed that both holes and super oxide anions were involved in the photocatalytic degradation of 2,4,6-TCP.This research work would provide deep understanding of the photophysical and photochemical processes and feasible routes to improve the photocatalytic performance of visible-light responsive semiconductor photocatalysts for solar fuel production and pollutants degradation with clear mechanisms.