Surface-and Interface-controlled Preparation Of Graphitic Carbon Nitride And The Enhanced Photocatalytic Oxidation Performance Towards Organic Pollutants

Green photocatalysis offers a very promising technology that uses clean solar energy and semiconductors to reduce and mineralize organic pollutants in mild conditions with the extraordinary advantages and environmental benignity,and it has become an immense potential in environmental remediation.For various photocatalytic reactions,the photocatalytic efficiency strongly depends on the performance of photocatalysts.Titanium dioxide（Ti O₂）is the most prevalent photocatalyst with its superstrong photooxidizing ability,chemical stability and low cost,and it has been widely applied in the degradation of various organic pollutants.However,the practical applications of Ti O₂ are severely limited by its poor sunlight utilization as well as low quantum efficiency.Therefore,it has become a research hotspot in the field of environmental catalysis to search for novel and alternative photocatalysts to Ti O₂ and to study the photocatalytic performance for the degradation of organic pollutants.Graphitic carbon nitride（g-C₃N₄）with extraordinary advantages including appealingπ-conjugated electronic structures,chemical stability as well as visible-light response ability and facile synthesis from inexpensive nitrogen-rich precursors has become the most promising candidate of the new generation visible-light responsive photocatalyst with great potential in the removal of organic pollutants.However,bulk g-C₃N₄ suffers from a low efficiency in photooxidation applications because of its poor BET surface area,high recombination probability of the photogenerated charge carriers(e^-_CB and h⁺_VB)and low visible light harvesting ability.In order to solve the above problems,the doctoral dissertation devoted to rationally tailoring the surface and interface structures of g-C₃N₄ and thus the electronicstructureaswellasthepropertiesofthe generation–separation–diffusion–transportation of the photogenerated charge carriers in g-C₃N₄ can be well-regulated and optimized,which can improve their photocatalytic oxidation ability.Firstly,a novel in-situ routes have been designed by selecting proper semiconductors that possess strong oxidation ability and well-matched band structure(i.e.,H₃PW₁₂O₄₀ and WO₃),and then combining it with g-C₃N₄ through chemical interaction or hydrogen bonding to fabricate direct Z-scheme-dictated heterojunction(H₃PW₁₂O₄₀/g-C₃N₄and WO₃/g-C₃N₄);in addition,by rationally designed defect engineering strategies,potassium-doped and nitrogen-deficient g-C₃N₄（D_N-K-CN）were successfully prepared.The composition and structure information,morphology and textual properties as well as optical absorption properties of the prepared photocatalysts were well-characterized.Finally,to systematically evaluate the photocatalytic performance of the above newly prepared g-C₃N₄-based catalysts,a series of light insensitive organic pollutants including recalcitrant volatile organic compound（VOCs）such as benzene,toluene and m-xylene（BTEX）and emerging phenolic derivatives,acetaminophen（APAP）and methylparaben（MPB）,are chosen as the target pollutants.The present studies revealed the relationship between the surface and interface properties and the photocatalytic performance of as-prepared catalysts.Meanwhile,the reaction mechanism is put forward in detail,and the degradation pathway is reasonably speculated.The specific contents are as follows.1.H₃PW₁₂O₄₀/g-C₃N₄ film-coated optical fibers with different H₃PW₁₂O₄₀ loadings levels of 1.1 and 3.2%is successfully fabricated via sol-gel technology to fabricate H₃PW₁₂O₄₀/Si O₂ sol followed by dip-withdrawing to H₃PW₁₂O₄₀/g-C₃N₄ film.The photocatalytic and stability performance of H₃PW₁₂O₄₀/g-C₃N₄ film in the degradation of gaseous benzene,toluene and m-xylene under simulated sunlight irradiation（320 nm<λ<680 nm）were then systematically evaluated.The results show that at relative humidity of73%and air atmosphere,the H₃PW₁₂O₄₀/g-C₃N₄ film-coated optical fibers with H₃PW₁₂O₄₀doping level of 3.2%exhibit the highest photooxidation ability of BTEX,and the apparent first-order rate constant of H₃PW₁₂O₄₀/g-C₃N₄-3.2 for benzene,toluene and m-xylene removal is 2.42,1.75 and 3.67 times higher than that of g-C₃N₄ film.The enhanced photocatalytic activity of the H₃PW₁₂O₄₀/g-C₃N₄ film is ascribed to direct Z-scheme-dictated charge carrier migration mechanism induced by acid-base interaction and hydrogen bonding between H₃PW₁₂O₄₀ and g-C₃N₄ as well as well-matched band structures of two components,which impart not only superior photogenerated charge carrier separation ability but also undiminished redox ability of the photogenerated electrons and holes.Therefore,plentiful reactive oxygen species including superoxide anion radicals（·O₂^-）and hydroxyl radicals（·OH）are generated and involved in the reactions,which play a dominant role in the photodegradation of BTEX over the H₃PW₁₂O₄₀/g-C₃N₄ film;moreover,the increased contact area of the catalyst film with the substrates and the improved light utilization ability also give rise to the important contribution to enhance the photocatalytic performance of H₃PW₁₂O₄₀/g-C₃N₄ film.More importantly,the H₃PW₁₂O₄₀/g-C₃N₄ film exhibits excellent stability and recyclability,and it can be used thirty times without obvious catalytic activity loss.This excellent catalytic reusability is owing to the strong interaction between H₃PW₁₂O₄₀and g-C₃N₄ polymer as well as Si O₂ framework;meanwhile,as-designed H₃PW₁₂O₄₀/g-C₃N₄film coating technique is reasonable.2.A facile and environmental friendly short-chain carboxylic acid-induced self-assembly hydrothermal treatment followed by thermal polycondensation route to fabricate a series of porous sheet-like WO₃/g-C₃N₄ heterojunction with oxygen doping is designed;meanwhile,the electronic properties and morphology features of WO₃/g-C₃N₄ nanosheet can be further regulated by selecting different kinds of short-chain carboxylic acids.The photocatalytic degradation performance of emerging phenolic pollutants,APAP and MPB,over the WO₃/g-C₃N₄ heterojunctions under visible-light irradiation（400 nm<λ<680 nm）were then systematically evaluated.The results show that the heterojunctions exhibit enhanced visible-light photocatalytic activity than bulk g-C₃N₄,supramolecule-based g-C₃N₄and WO₃in the removal of APAP and MPB.The apparent first-order rate constant of the most active acetic acid induced heterojunction,WO₃/g-C₃N₄-AA,is 8.0 and 6.1 times higher than that of bulk g-C₃N₄,while the apparent first-order rate constant of WO₃/g-C₃N₄-AA is 5.5 and 5.4times higher than that of g-C₃N₄-AA induced by acetic acid,in photodegradation of APAP and MPB.A series of tests including active species trapping,photoelectrochemistry and steady-state photoluminescence were carried out to explore the origins of the enhanced photocatalytic removal efficiency over the WO₃/g-C₃N₄.The enhanced photocatalytic activity of the WO₃/g-C₃N₄ heterojunctions is attributed to the direct Z-scheme charge transfer mechanism induced by strong hydrogen bonding between WO₃ and g-C₃N₄ as well as well-matched band structures of both of components,which can accelerate charge carrier separation and retain powerful redox ability of electrons on conduction band of g-C₃N₄ and holes on valance band of WO₃ simultaneously,and therefore plentiful reactive oxygen species including·O₂^-and·OH radicals are generated in the reaction systems,which play a dominant role in the photocatalytic degradation of phenolic pollutants over the WO₃/g-C₃N₄;moreover,the electronic polarization effect induced by charge redistribution around the doped oxygen atoms in g-C₃N₄ lattice can lead to the formation of an internal electric field,which is beneficial to the separation of the photogenerated charge carriers;finally,the unique porous sheet-like nanostructures of the WO₃/g-C₃N₄ heterojunctions with the improved visible light harvesting ability,plentiful surface active sites and shortened charge carrier transfer distance are also beneficial to this excellent performance.In addition,the WO₃/g-C₃N₄ heterojunctions exhibit excellent stability and recyclability,and they can be used five times without obvious catalytic activity loss.3.An acid-induced in-situ self-assembly hydrothermal treatment followed by KOH-assisted thermal polycondensation route to prepare potassium-doped and nitrogen-deficient g-C₃N₄（D_N-K-CN）is developed.Meanwhile,the doping levels and defect concentration can be well controlled by changing the initial adding amount of KOH.The photocatalytic degradation performance of emerging phenolic pollutants,APAP and MPB,over the D_N-K-CN under visible-light irradiation（400 nm<λ<680 nm）were then systematically evaluated.The results show that the photocatalytic activity was positively correlated with potassium doping levels and defect concentration,in which D_N-K-CN-7.9with the highest potassium dopant and nitrogen defects concentration exhibits the highest visible-light photocatalytic degradation performance of APAP and MPB,and the apparent first-order rate constant of D_N-K-CN-7.9 for APAP and MPB removal is 6.0 and 4.6 times higher than that of supramolecule-based g-C₃N₄.The enhanced photocatalytic activity of the D_N-K-CN is attributed to the synergetic effects of potassium doped and nitrogen deficient,which not only boost the generation–separation–diffusion–transportation of photogenerated charge carriers in g-C₃N₄,but also promote the activation of molecule O₂.As a consequence,a large amount of active oxygen species including·O₂^-,·OH radicals and singlet oxygen（¹O₂）are generated in the photocatalytic systems,and they are responsible for not only degradation but also mineralization of the target pollutants.More importantly,the conduction band and valance edge potential can be continuously tailored by the simultaneous introduction of potassium ions and nitrogen defects,which can enhance the visible light harvesting ability and improve the photooxidation ability by increasing the valance band edge potential.Therefore,the electronic properties and band structure of D_N-K-CN can be well-optimized by the synergetic effects of potassium doped and nitrogen deficient in g-C₃N₄ framework,which can not only facilitate the separation and transportation efficiency of e^-_CB and h⁺_VB,but also enhance the photooxidation ability.Both factors are beneficial to improve the photocatalytic degradation performance of organic pollutants over the D_N-K-CN.Moreover,the D_N-K-CN exhibits excellent stability and recyclability,and it can be reused five times without obvious catalytic activity loss.