Preparation Of High-efficiency G-C₃N₄ Based Nanocomposite Heterojunction Photocatalyst And Its Photocatalytic Activity

As one of the most cost-effective potential solutions,photocatalysis technique has been widely investigated to alleviate environment pollution and energy crisis since 1990s.Among the multitudinous inorganic semicounductor photocatalysts,TiO2 has been turned out to be an outstanding delegate in environment and energy-related applications.However,TiO2 photocatalysts are only UV-light-response semiconductors,because of its relatively wide bandgap?Eg>3.0 eV?.Such intrinsic property greatly hinders its further practical applications.Because the UV light only accounts for 4%in the whole solar spectra,while the visible light occupies 45%.Therefore,the fabrication of a novel polymeric semiconductor with excellent visible-light-driven ability is of necessity and urgency,which can also dramatically complement the drawbacks of conventional inorganic metal oxides.Graphitic carbon nitride?abbreviated to g-C₃N₄?,has been attracted immensely global attention since it was first adopted as a metal-free polymeric semiconductor for photocatalytic water-splitting to produce H2 and O2 in 2009.With intriguing features such as good chemical and thermal stability,low cost,ease of preparation and visible light response,various applications especially in photocatalysis areas.Unfortunately,the low specific surface area,small absorption edge?460 nm?and fast photoinduced charges/holes recombination greatly obstruct it to achieve better performance.Therefore,structural and morphological modification approaches should be employed to enhance the photocatalytic performance of g-C₃N₄.In this study,firstly,g-C₃N₄ bulks?CN-Bare?were prepared by conventional thermal polycondensation approach.The effect of various N-rich precursors and polymerization temperature were sysmatically investigated.The above-mentioned two factors both played vital role in tuning the textrual structure,optical absorption and electric properties of g-C₃N₄.The CN-Bare sample prepared from melamine?named CN-M-550?really presented the weakest photooxidation ability than urea/dicyandiamide-derived g-C₃N₄ samples.Urea-derived g-C₃N₄ nanosheets?CN-U-550?showed the best photocatalytic activity for CH3CHO degradation among the three N-riched precursors,giving about 3.3 times higher than CN-M-550 counterpart.Thus g-C₃N₄ bulks?especially to CN-M-550?are still low,which needs further promotion.Then,due to the restrictions of low specific surface areas and low concentrations of active-sites,three physicochemical modification approaches were adopted to CN-Bare:planetary ball milling,ethanol/water solvothermal treatment and selenium assisted exfoliation.The as-prepared g-C₃N₄ samples were denoted as CN-PM,CN-HT and CN-Rods,respectively.And their photooxidation activities to convert CH3CHO to CO,showed 1.8,2.5 and 6.3 times higher than CN-Bare.Planetary ball milling is an efficient way to crush g-C₃N₄ bulks into micro/nano-sized fragements,thus much more active-sites could be introducted to surface.And ethanol/water solvothermal treatment has turned out to be a powerful route to remove numerous'unstable-zone' on g-C₃N₄ structure,resulting in big specific surface area and vast of mesoporous on generated nanosheets.Particularly,the morphology of g-C₃N₄ can vastly altered from bulk-like to nanorod arrays decorated with mesoporous by selenium assisted exfoliation?CN-Rods?for the first time.The formation mechanism was expatiated below:selenium molecular and melamine molecular both suffer from sublimation as temperature elevation.Then extremely strong interaction would take place between selenium and amine grous in melamine.Thus as an effective structrure-directing agent,selenium assisted the polymerization to fabricate g-C₃N₄ nanorod arrays;With further increasing the polymerization temperature,selenium molecular would be fully removed,leaving numerous mesoporous on g-C₃N₄ nanorod arrays.Such structural modified CN-Rods displayed superior photocatalytic performance,attributing to the synergistic effect of higher specific surface areas,higher concentrations of active-sites,enhanced optical absorption and better separation rate of charge carriers.In the third part,three different modification approaches were developed to extend the visible light absorption properties of g-C₃N₄ photocatalyst,so as to further enhance its photoredox performance.?1?Local Surface Plasmonic Resonance?LSPR?Au nanoparticles were loaded on g-C₃N₄ surface by deposition-thermal-reduction method.The results indicated that 1wt%Au/g-C₃N₄ treated at 450? presented the strongest optical absorption within the whole visible light spectrum?400-700 nm?,endowed with the best photoredox performance.?2?A new precursor thiourea oxide was utilized for the first time to produce a highly efficiency visible-light-driven g-C₃N₄ photocatalyst by one-pot calcination method.The technical issues of O-S co-doping for g-C₃N₄ was also achieved by this method,leading to its absorbance edge up to 600 nm,which was well consisted with our DFT?density functional theory?calculation results.?3?A facile two-steps thermal etching method was also adopted in this section.Fascinatingly,the optical absorption edge of g-C₃N₄ fabricated under Argon atmosphere condition by this method was extended from 460 nm to 650 nm,meaning that the utilization efficiency of optical spectra and photocatalytic performance by this novel g-C₃N₄ can be greatly enhanced.In the fourth section,in order to promoting the separation efficiency of photoinduced charge carriers,the fully removl of the gasous CH3CHO VOCs?volatile organic chemicals?as well as the CO₂ conversion by photoelectrochemical reduction reaction,the author adopted morphology-controlled CeO2 as co-catalyst to fabricate a series of CeO2/g-C₃N₄ nanohybrid heterojunction photocatalysts in this part.It is noteworthy that the intimate degree of established heterojuctions between CeO2 and g-C₃N₄ affects the photoredox activities tremendously.The in-situ grown Ce02 nanocrystals on CN-HT and CN-Rods achieved much better activity than CN-Bare both in CH3CHO photooxidation and CO₂ photoelectrochemical reduction reactions,which is attributed to the established intimate interfacial between CeO2 and g-C₃N₄,fast Ce3+/Ce4+ redox cycles in Cev2 and high separation efficiency of charge carriers.Besides,by above-established methods,the pretty good versatility and expansibility of other metal oxides?NiO and Bi₂O₂CO₃?on g-C₃N₄ to obtain various novel g-C₃N₄-based nanohybrid heterojunction photocatalysts with excellent photocatalytic performance were well-founded confirmed.A low-molecular-weight carbon nitride photocatalyst?denoted Melem-Bare?was also fabricated at relatively low polycondensation temperature and the as-prepared melem oligomers were further purified by ethanol/water solvothermal treatment?named Melem-ST?.The photocatalytic activity for CH3CHO oxidation by the melem-ST were found to be the most active photocatalysts,having up to at least 2.5 times higher activity of the Melem-Bare,which in turn has 4 times the activity of the CN-Bare,the literature benchmark.Astonishingly,the photoactivity of melem-ST for CH3CHO oxidation can be further promoted by in-situ grown CeO2,achieving 8 times up with respect to CN-Bare.Furthermore,the feasibility of the series established novel nanohybrid heterojunction photocatalysts were also confirmed in 2-propanol oxidation,manifesting similar enhanced results.In this paper,the author proposed and exemplified some cost-effective solutions to solve the following main issues which greatly obstruct the development and applications of g-C₃N₄ photocatalysts:low specific surface area,weak visible-light absorption and fast recombination rate of photogenerated electrons and holes.In summary,the textural morphology of g-C₃N₄ bulks were successfully regulated by planetary ball milling,ethanol/water solvothermal treatment and selenium assisted exfoliation.And its optical absorption edge were greatly extended from 460 nm to the whole visible light spectra by LSPR Au nanoparticles,O-S co-doping and two-steps thermal etching method.Meaningfully,a series of novel g-C₃N₄-based nanohybrid heterojunction photocatalysts with enhanced photocatalytic performance for CH3CHO degradation and CO₂ PEC conversion under visible light irradiation were successfully fabricated,which is bound to unfold a vast and applicable foreground in energy and environment-related areas.