New Strategies For The Improvement Of The Photocatalytic Redox Performance Of Graphitic Carbon Nitride

Semiconductor photocatalysis technology has drawn broad interdisciplinary attention in the areas of environmental remediation because of its significant advantages such as green,high efficiency,energy saving and no secondary contamination.The key issue of this technology is to develop efficient,stable,and inexpensive visible light-driven photocatalysts.Among the reported semiconductor photocatalytic materials,graphitic carbon nitride（g-C3N4）polymer semiconductor,has exhibited the appealing features includingπ-conjugated electronic structure,high light and chemical stability,earth-abundant nature,and easy modification,which endow it a promising candidate of the next generation visible light-responsive photocatalyst.These unique properties ensure g-C3N4 great application potential in the field of photocatalytic removal of pollutants in wastewater.However,conventionally prepared bulk g-C3N4 suffers from the intrinsic obstacles and shortcomings including low surface area,moderate oxidation capacity,limited visible-light utilization and high recombination probability of the photogenerated charge carriers,which greatly limits its practical applications.To overcome these problems,this doctoral dissertation devotes to designing the modified g-C3N4-based photocatalysts with improved photocatalytic redox performance via novel strategies,which include surface functionalization,covalent modification and defect engineering.The composition and structure,morphology,surface physicochemical properties and optical properties of as-prepared photocatalysts are well-characterized.Finally,the photocatalytic activity of the catalysts for the removal of aqueous refractory organic pollutants and high-valent toxic heavy metals under simulated sunlight or visible-light irradiation are systematically investigated.On the basis of the experimental results,the relationship between the surface and interface properties,and intrinsic structure of the modified g-C3N4-based catalyst with the photocatalytic activity is revealed.Furthermore,combination of the experimental results and density functional theory（DFT）calculations,the mechanisms of the target photocatalytic reactions are deeply studied.In addition,the catalytic reusability of as-prepared catalysts are evaluated in detail.The work therefore provides an important guidance for the design of efficient visible-light response photocatalysts;additionally,it provides a new strategy to efficiently remove organic and inorganic pollutants in water.The specific research contents of this work are as follows.1.By using urea-derived g-C3N4（UCN）as the precursor,a series of hydroxyl-modified carbon nitride nanosheets（HUCNs）with interconnected open-framework is fabricated via hydrolysis of UCN under different alkaline concentrations followed by a self-assembly in dialysis process.As-prepared HUCNs exhibit enhanced simulated sunlight photocatalytic reduction activity toward Cr（VI）as compared with the UCN.Among them,3.0-HUCN,obtained by treatment of UCN with 3 mol L^-1 of Na OH aqueous solution shows the highest photocatalytic reduction activity to Cr（VI）.After simulated sunlight（320 nm（27）?（27）680 nm）irradiating the 3.0-HUCN for 45 min,the removal efficiency of Cr（VI）is 99.8%,and its apparent first-order rate constant（k）value is almost 13.4 times higher than that of the UCN.The excellent photocatalytic reduction activity of HUCNs is attributed to the surface modification by the hydroxyl groups,which not only promotes the adsorption of Cr（VI）on the surface of HUCNs,but also accelerates the separation of photogenerated electrons and holes;in addition,the interconnected open-framework structure of HUCNs is benefit for the electrons transfer to the surface and the improvement of the accessibility of the active sites.Moreover,the upward CB edge potential of HUCNs endows the photogenerated electrons stronger reduction ability.And the HUCNs possess considerably high photocatalytic reusability and stability,and the loss of the photocatalytic activity is negligible after five consecutive cycles.2.A series of perylene tetracarboxylic diimide（PTCDA）-g-C3N4（PDI/GCN）heterojunctions with various weight percentages of PTCDA-to-g-C₃N₄（0.5-30%）are prepared by one-step imidization reaction between PTCDA and g-C3N4 in aqueous solutions.The surface hybridization at the interface between PTCDA and g-C3N4 in the PDI/GCN heterojunctions occurs via O=C-N-C=O covalent linkage.Various PDI/GCN heterojunctions exhibit different visible light photocatalytic activities for the degradation of p-nitrophenol（PNP）and levofloxacin（LEV）in water.Among them,1%PDI/GCN heterojunction,prepared at a PTCDA-to-g-C3N4 weight percentage of 1%,exhibits remarkably higher visible light photocatalytic degradation and mineralization ability toward both target pollutants as compared with bulk g-C3N4,commercially available P25 Ti O2 and other PDI/GCN heterojunctions.The degradation of PNP and LEV over the 1%PDI/GCN is completed after visible light（400 nm<?<680 nm）irradiation for 45 and 15 min,respectively;additionally,after visible-light irradiation for 360 and 480 min,the TOC removal efficiency reaches 100%and 55.6%in the1%PDI/GCN-photocatalyzed PNP and LEV degradation system,respectively;moreover,the inhibition ratio of the 1%PDI/GCN to the toxicity is 0.9%for PNP（after visible-light irradiation for 360 min）and 10%for LEV systems（after visible-light irradiation for 480 min）,respectively.The surface hybridization in the PDI/GCN heterojunctions plays the key role for this enhanced photocatalytic activity by accelerating the migration and separation of the photogenerated charge carriers,producing more active species like superoxide anion radicals（·O₂^-）,holes(h⁺_VB),and hydroxyl radicals（·OH）for deep oxidation of PNP and LEV to CO₂,H2O and inorganic anions.Moreover,PDI/GCN heterojunctions exhibit excellent photocatalytic stability and reusability,no significant activity loss is observed after five catalytic cycles.3.A novel in-situ route is designed via pre-assembly followed by thermally induced copolymerization between urea and a small amount of malonamide to prepare bridging carbon and structural edge defect co-modified g-C3N4（CBCN）successfully.The quantity of bridging carbon and structural edge defects can be well-controlled by adjusting the dosage of malonamide（5,20 and 60 mg）.Under visible light（400 nm（27）?（27）680 nm）irradiation,various CBCN catalysts exhibit higher visible light photocatalytic degradation activity towards acetaminophen（APAP）and methylparaben（MPB）,as compared with bulk g-C3N4 and P25Ti O2.Among them,CBCN-20,obtained by adding 20 mg of malonamide,exhibits the highest visible-light photocatalytic oxidation activity.After visible-light irradiating for 30 and 90 min,APAP and MPB can be degraded completely over the CBCN-20;and the apparent first-order rate constant（k）for the photocatalytic degradation of APAP and MPB over the CBCN-20 is8.2 and 7.0 times higher than that of bulk g-C3N4.This excellent visible light photocatalytic activity is mainly due to the synergetic effect of the introduced bridging carbon and structural edge defects.The modification by bridging carbon not only optimizes the band structure of g-C3N4 by the improvement of the conjugation degree,but also causes the charge redistribution.Both factors improve the separation efficiency of the photogenerated charge carriers and visible-light utilization.In addition,structural edge defects originated from the introduction of bridging carbon can serve as active center to enhance the surface activity of the catalyst.The synergetic effect of bridging carbon and structural edge defects is beneficial for the generation of more plentiful photogenerated electrons,and thereby more abundant·O₂^-and a certain amount of·OH are generated in the CBCN system.Under the attack of h VB+,·O2^-and·OH,PNP and ATN can be degraded completely and gradually mineralized to CO2 and H2O.CBCN shows excellent catalytic reusability,and the photocatalytic oxidation activities of CBCN to APAP and MPB maintain unchangeable after five consecutive cycles basically.4.A unique and novel strategy,one-step formaldehyde-assisted thermal polycondensation of molten urea,is developed to fabricate carbon-deficient and oxygen-doped g-C3N4 porous nanosheets（VC-OCN）.Combination of many characterization techniques and DFT theoretical calculations,the electronic and band structure of VC-OCN have been studied.It is found that a small amount of oxygen atoms in VC-OCN come from the self-doping of molten urea,while carbon vacancies are simultaneously introduced owing to the addition of formaldehyde.Furthermore,by adjusting the dosage of formaldehyde（5,15 and 50?L）,the concentration of carbon vacancies in VC-OCN can be well-controlled and thereby the electronic structure as well as optical absorption properties.The VC-OCN catalysts exhibit excellent and interesting carbon vacancy concentration-dependent photocatalytic degradation activity to p-nitrophenol（PNP）and atrazine（ATN）,in which the VC-OCN15（prepared at formaldehyde dosage of 15?L）with the optimized carbon vacancy concentration displays the highest visible light photocatalytic activity,and the apparent first-order rate constant of VC-OCN15 for the PNP and ATN removal is 4.4 and 5.2 times higher than that of bulk g-C3N4.This excellent photocatalytic oxidation activity of VC-OCN is mainly due to the synergetic effect of carbon vacancies and oxygen doping sites,which can not only boost the generation,separation,migration and transportation of the photogenerated charge carriers but also enhances the adsorption and activation of the catalyst to oxygen molecules.As a consequence,multiple active oxygen species including·O₂^-anion radicals,·OH radicals,and singlet oxygen（¹O₂）are generated in the photocatalytic system,and they are responsible for not only degradation but also mineralization of the target pollutants.Additionally,the porous sheet-like nanostructure of VC-OCN with improved textual properties also contributes to the enhanced photocatalytic activity of VC-OCN via the improvement of the migration and diffusion of the photogenerated charge carriers.In addition,VC-OCN shows excellent reusability and stability,and the photocatalytic activity maintains a constant over five photocatalytic cycles;meanwhile,the phase structure,carbon vacancies,and morphology of the used VC-OCN15 are retained.