| Heterogeneous photocatalysis has been applied to oxidize various organic pollutants and reduce heavy metal ions by directly utilizing solar energy,and reveals good sterilization effect,showing a potential application prospect in the field of environmental purification.As for the application of photocatalysis technology,it is crucial to design and fabricate effective catalytic systems.Recently,graphitic carbon nitride(g-C3N4),as a kind of organic semiconductor,has aroused extensive attention due to its visible light absorption,easy preparation,low cost of raw materials,uniqueπ-conjugated layer structure,good stability,and so forth.However,bulk g-C3N4 suffers from the low solar energy utilization,poor quantum efficiency,fast recombination of photogenerated electron-hole(e--h+)pairs and low reaction activity,which limited its practical application in photocatalysis.Therefore,the development of highly efficient and stable visible-light-induced photocatalysts hasgradually become the hotspot in the field of environmental photocatalysis.Considering these issues,g-C3N4 is modified by the author to meet the requirements.Based on the regulation of energy level and construction of built-in electric field,the surface/interface functionalization of g-C3N4 is carried out to improve its photocatalytic activity and stability in this thesis,which is mainly performedby elements doping and defects engineering.Meanwhile,the charge separation effect induced by built-in electric field and mechanisms of enhanced photocatalytic performances are systematically studied.The detailed results are as follows:(1)To introduce more reactive sites and enhance the interaction between the layers of g-C3N4,a novel polyoxometalates-intercalated porous g-C3N4(Mo-pCN)catalyst was synthesized.Combined with experimental characterizations and DFT calculation,the formation of the built-in electric field in the modified g-C3N4 and the effects of[Mo7O24]6-on regulating energy level structure and interface charge transfer were revealed.The results showed that photodegradation rates of BPA and 4-CP for Mo-pCN were 20.2and 28.7 times faster than pure g-C3N4,respectively,upon being excited by visible light(λ>420 nm)irradiation.The main reasons were that,compared with pure g-C3N4,the specific surface area of Mo-pCN was remarkably enlarged.Additionally,the enhanced visible light absorption,lowered fluorescence intensity and prolonged carriers lifetime were achieved.The intercalated[Mo7O24]6-between the layers of g-C3N4 served as electron transfer channels,which extended the π-conjugated system and accelerated the separation and transfer of e--h+pairs.Meanwhile,the oxygen clusters with strong electronegativity synchronously acted adsorption and catalytic reaction sites,leading to the higher efficiency of mass transfer and photodegradation activity.This work provides a novel strategy for the design and construction of efficient g-C3N4-based visible light photocatalysts.(2)Based on the above findings,it is critical to solve problems such as high cost of transition metals,high consumption,and limitation in practical application,a metal-free catalytic system was developed with oxygen atoms as reaction centers.The dual oxygen-doped porous g-C3N4(OPCN)photocatalysts were successfully prepared via a facile thermal copolymerization method by using ammonium oxalate as oxygen source.It was discovered that oxygen atoms synchronously substituted for the two sp2-hybridized nitrogen atoms(N1’and N4’)at the para position intri-s-triazine rings,which resulted in the formation of big delocalized π bonds and built-in micro electric field.Accordingly,the energy level structure and charge distribution were also changed.During the synthesis process,NH3 and CO2 were released and worked as the bubble template to create a porous structure of g-C3N4.As a result,theincreased specific surface area of g-C3N4,theenhanced visible light absorption and thenarrowed band gapcollectively accelerated migration and separation of photo-induced carriers.From the result of photodegradation tests,OPCN exhibited outstanding catalytic performances under visible light irradiation in removing various organic pollutants,including phenols,chlorophenols,dyes,etc.,which displayed a great potential application fororganic wastewater remediation.(3)To further enlarge the reaction space and reduce mass transfer resistance in metal-free catalytic systems,a defect engineered mesoporous g-C3N4(DMCN)catalyst was successfully designed and synthesized by a facile hard template route.During thermal polymerization and etching processes,surface defects containing hydroxyl(-OH)and cyano(-C≡N)groups were created by self-induced introduction.The results suggested that mesoporous structure and surface defects were constructed on g-C3N4,which effectively adjusted the electronic energy levels,enhanced visible light absorption of and increased the mass transfer during reactions.Localized electron polarization was occurred on the surface of g-C3N4,derived from the generation of these electron withdrawing groups.Meanwhile,photogenerated electrons transferred from g-C3N4 to electron acceptor persulfate(PS)via the-OH and-C≡N groups,which acted as the unique bridge to induce a single and efficient charge transfer route.As a result,photogenerated charge carriers were effectively transferred and separated,and thus strong oxidized SO4·-and ·OH were yielded.Thus,the g-C3N4-based metal-free photocatalytic system revealed the significantly enhanced photodegradation activity and stability to remove BPA.Under visible light irradiation(420～780 nm),the BPA degradation rate for DMCN was～39.6 times than that for g-C3N4.After five cycling tests,the removal efficiency was still up to 97%.This study is of great significance to develop metal-free catalytic systems with surface defects.(4)Charge transfer in g-C3N4 is generally limited due to weak van der Waals force between layers.Considering this critical issue,from the construction of charge transfer channels,fluorine atom doped porous lamellar g-C3N4(F-pCN)were synthesized by the combined thermal polymerization and high temperature assisted exfoliation method.The results demonstrated that F-pCN shaped as porous-like sheets,and fluorine atoms were bonded to carbon atoms in the aromatic rings,which effectively controlled the energy level position and enhanced the visible light adsorption.Besides,it is confirmed by theoretical calculation that fluorine atom bridged between two-dimensional layers of g-C3N4 and formed much lone pair electrons.Benefiting from the strong electronegativity,electrons were enriched around fluorine atoms and also multi-electron transfer channels were created.Consequently,photogenerated electrons were fast transferred from the bulk to the surface of g-C3N4,and then directly reacted with PS and other electrons acceptors to generate strong active species(e.g.,SO4·-,·OH and h+),greatly inhibiting charge carriers recombination.With λ>420 nm visible light irradiation,the removal rate of sulfamethoxazole by F-pCN-0.3 could increase up to 0.102 min-1,which was 6.8 times higher than that of pure g-C3N4(0.015 min-1),and the excellent chemical stability was also achieved.This catalytic system exhibited outstanding behaviors in degrading organic antibiotics contaminants under visible light.In a word,the surface and interface properties are vital factors to affect the photocatalytic performances of catalysts.Based on the strategies of electronic structure regulation and built-in electric field,this thesis mainly focused on the surface functionalization of g-C3N4 via doping and engineering defects,which was applied to degrade various organic pollutants under visible light irradiation.The structure-activity relationships were also discussed in detail.Our research work provides some guidance on the design and development of high-performance g-C3N4-based catalytic systems,and also contributes to practical applications of photocatalysis technology in the environmental pollution remediation. |
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