Construction Of Graphite Carbon Nitride Based Composite Photocatalysts And Their Photocatalytic Performance And Mechanisms

Semiconductor photocatalytic technology is one of green technologies because it can convert low-density solar energy into chemical energy.It has been widely used in environmental pollutant control and solar energy conversion.Graphite carbon nitride（g-C₃N₄）,a nontoxic metal-free and visible-light-responsive semiconductor,has two-dimensional planar morphology which benefits to obtain more photocatalytic active sites and shortens the transfer path of photoinduced electron-hole pairs.Simultaneously,g-C₃N₄ has good thermal stability and chemical stability.However,g-C₃N₄ has the drawbacks of high recombination rate of photoinduced electron-hole pairs and weak oxidation ability of photogenerated hole,which restricts the photocatalytic efficiency of g-C₃N₄.This dissertation focused on the improvement of the photocatalytic efficiency of g-C₃N₄ via coupling with other suitable semiconductors to form heterojunctions.The internal electric field formed in the heterointerface of the heterojunction composites accelerates the separation and transport of photo-induced carriers,which enhanced the photocatalytic activity.Photocatalytic reduction of aqueous Cr（VI）is employed to assess the photocatalytic activity and stability of the constructed heterojunction photocatalysts.The relationship of structure and photocatalytic performance was investigated,and the involved photocatalytic mechanism was revealed.Details are listed as below:1.SnS₂/g-C₃N₄ composite with“face to face”heterogeneous structure was synthesized using the hydrothermal technique.XPS,TEM,HRTEM and PL characterizations revealed that stable heterointerface between the higher surface energy of SnS₂ nanosheets and g-C₃N₄ was formed by the mutual coupling forces,which enhanced the separation of photogenerated electron-hole pairs.UV-vis DRS displayed the SnS₂/g-C₃N₄ has higher visible-light-response than g-C₃N₄,which indicates SnS₂/g-C₃N₄ is a visible-light photocatalyst.The photocatalytic reduction of Cr（VI）by SnS₂/g-C₃N₄ fitted the first-order reaction kinetics characteristics.SnS₂/g-C₃N₄ with 40%contents of g-C₃N₄ can achieve the complete photocatalytic reduction of aqueous Cr（VI）after 60 min visible light irradiation.Active species capture experiments and ESR showed that·O₂^–is the active specie in the photocatalysis by SnS₂/g-C₃N₄.Based on the experimental results,Z-scheme mechanism of the migration of photogenerated carriers between the heterogeneous interface is proposed.2.SnO₂/g-C₃N₄ nanocomposites were fabricated via a hydrothermal route according to optimized reaction conditions of the exfoliated treatment of bulk-g-C₃N₄in Na₂CO₃ aqueous solution.The TEM and HRTEM results showed that the heterostructure was formed between SnO₂ nanoparticles with high surface energy and two-dimensional g-C₃N₄.TGA analysis showed that the gravity peak of g-C₃N₄ in SnO₂/g-C₃N₄ composites is moving to the lower temperature with increased SnO₂content,which indicated that the heterostructure between Sn O₂ and g-C₃N₄ changed the chemical environment of g-C₃N₄ that influences the thermal stability of g-C₃N₄.PL and EIS characterizations demonstrated that the heterogeneous interface of SnO₂/g-C₃N₄ decreased the charge-transfer resistance and accelerated the interfacial charge transfer,also reduced charge-carriers recombination.The experimental results showed that the photocatalytic activity of Sn O₂/g-C₃N₄ in photocatalytic reduction of aqueous Cr（VI）under UV light is superior to visible light.For investigating the photocatalytic mechanism,active species capture experiments and trapping the·OH from chemical fluorescence were employed to prove the active species in the photoreduction process under different light sources.The type-II heterojunction photocatalytic mechanism was proposed in the case of visible light irradiation,whereas the Z-scheme photocatalytic mechanism was proposed in the case of ultraviolet light irradiation.This study revealed that light source is the one of key conditions for constructing the Z-scheme photocatalytic system.3.Nitrogen-doped titanium dioxide（N-TiO₂/g-C₃N₄）was prepared by solvothermal technique,the influence of the amount of HNO₃ on the optical and electrical properties and photocatalytic performance were investigated.The role of nitric acid in the reaction was revealed based on the following characterization results:（i）HNO₃ played the role of oxidation etching and protonation depolymerization,also increased the specific surface area of g-C₃N₄ through etching or exfoliation.（ii）HNO₃acted as nitrogen source in the formation of visible-light-driven N-TiO₂ photocatalyst.The shift of Ti 2p_3/2 value of N-TiO₂/g-C₃N₄ heterojunction confirmed the interaction between N-TiO₂ and g-C₃N₄ in heterogeneous interface.The charge-transfer resistance of photogenerated charge carriers was decreased to promote the interfacial charge transfer in the N-TiO₂/g-C₃N₄ heterojunction according to the experimental results of UV-vis DRS,EIS and photocurrent.The photocatalytic reduction of Cr（VI）under visible light were fitted to the pseudo-first-order kinetics.According to the ESR experimental results,obvious signals of DMPO-·OH and DMPO-·O₂^–were detected under visible light.The Z-scheme mechanism of N-TiO₂/g-C₃N₄ was proposed combined with the matched band potentials.4.It is of paramount significance for promoting the spatial separation of photoexcited charge carriers in the photocatalytic reaction.After the binary Bi₂Ti₂O₇/TiO₂ photocatalyst was first obtained by solvothermal method,the ternary g-C₃N₄/Bi₂Ti₂O₇/TiO₂ photocatalyst was further prepared by calcining process.It displayed the interaction among Bi₂Ti₂O₇,TiO₂ and g-C₃N₄ in their heterogeneous interface changes the chemical bonding of g-C₃N₄ according to the characterization results of XRD,FESEM and XPS,which decreased the thermal stability of g-C₃N₄.The amount of raw materials determined the composition of the target photocatalyst which were revealed through surface element analysis,and also influenced the photocatalytic reduction efficiency of Cr（VI）.The PL spectra and photocurrent were employed to inspect the good separation of photo-induced electrons and holes for g-C₃N₄/Bi₂Ti₂O₇/TiO₂,the results of which exhibited that the recombination of the photo-induced carriers was restrained.The enhanced photocatalytic performance of g-C₃N₄/Bi₂Ti₂O₇/TiO₂ ascribed the synergistic effect among three components which can produce the three-level electron transfer mechamism to seperate photo-induced carriers efficiently.