Photoelectrochemical And Photocatalytic Performance Of Graphene-based Nanocomposites

Graphene as a two–dimensional graphitic material, has many excellent characteristics such as good conductivity, high specific surface area. Reduced graphene oxide（RGO） owning some oxygen-containing groups and sp2-hybrid domains can provide anchors for some semiconductors or precursors, as well as it can serve as a substrate fot synthesizing smaller or more dispersible nanocrystals. Moreover, graphene has suitable position of Fermi energy, which can be a potential material in the field of photoelectronic device and photocatalysis.Ti O₂ photocatalysts are one of hottest photocatalytic materials. Ti O₂/RGO nano-heterojunction has better dispersion of Ti O₂, larger separation efficiency of electron-hole pairs, higher conducticity in comparison with that of pure Ti O₂. Band gap and energy level of RGO can be tuned by containing-oxygen groups. Electrochemical properties and semiconducting nature of Ti O₂/RGO heterojuction were investigated in the second chapter of the thesis. GO was synthesized by Hummers’ method and activated by nitric acid and chloroacetic acid, respectively. They were photocatalytically converted to RGO, RGO-N, RGO-C via Ti O₂ nanosheets reduction, simultaneously forming heterojunctions of TG, TG-N, TG-C, respectively. Effect of nitric and chloroacetic acid on the RGO and heterojunctions were studied via photoelectrochemcial and photocatalytic measurements. The results revealed RGO was an n-type semiconductor and TG was an n/n heterojunction. GO-N was p-type doped after nitric acid treatment, and TG-N was an n/p heterojunction which had a less negative apparent Fermi energy（EF*）, higher conductivity and lower charge transfer resistance in comparison with that of TG. On the other hand, after chloroacetic acid treatment, an n-type semiconductor RGO-C had been modified with more-COOH. As a result TG-C n/n heterojunction had a more negative EF*, lower effective donor concentration comparing with that of TG. But as the simulated circuits have demonstrated a complex of variation for resistance and capacity of the acid-treated heterojunction. For example, heterojunction has smaller resistance of charge transfer, larger resistance of solid-interface, larger capacity of charge transfer and solid-interface via nitric acid treatment. Furthermore, the Ti-O-C bond enhaces the photocatalytic performance of TG-C. TG-N has lower photocatalytic ability than TG and TG-C bacuse of the decrease of sp2-hybrid domains.Nano-sulfur/RGO composite was studied in the third chapter. The compsite with about 5 nm sulfur dispersing on the graphene sheet was synthesized by one-pot hydrothermal method without addition of any toxic organic solvent. Nano-sulfur has larger band gap because of the quantum confinement effect comparing with that of bulk sulfur. Both RGO and acetic acid contribute to the decrease of sulfur size. The composite has higher conductivity, lower resistant of charge transfer, slightly smaller concentration of electron donor, larger flat potential and much better photocatalytic performance in comparison with bulk sulfur. Otherwise, composite also has excellent cyclic stability for photocatalytic those of degradation of methylene blue.Ti O₂/Cu₂O/RGO composites was investigated in the fourth chapter of the thesis. The results reveal that: Cu SO4 is a stable precursor for the synthesis of Cu₂ O via photo-reductive method; it is the photo-induced electron generated by the Ti O₂ and graphene oxide that determine the final size of Cu₂ O in comparison with the more reactive anchors provided by Ti O₂ and graphene oxide. Cu₂ O is nucleating, growing and transformed to Cu as the ultraviolet-irradiation goes on; Cl- suppresses the reduction of Cu₂+ and nucleation of Cu₂O; on the contrary F- facilitates the growing of Cu₂ O and has a complex impact on the final product because of the unclear phases. The polyethylene glycol（PEG） increases the quantity of crystalline Cu₂ O. Ti O₂, Ti O₂/Cu₂ O, Ti O₂/Cu₂O/RGO, Ti O₂/Cu₂O/RGO（with assistance of PEG） has a sequence of enhanced photocatalytic performance. Ti O₂ combining with Cu₂ O enhances the absorption of visible light and facilitates the separation of electron-hole pairs in comparison with pure Ti O₂. RGO can adsorb the methyl orange（MO） via π-π conjugation and receive the photo-induced electron. PEG as a linear template can provide anchors for nucleation of Cu₂ O, decrease the size of Cu₂ O, enhace stability of composite, and improve the photocatalytic performace of composite eventually.The Ti O₂/Ti F3 composites was synthesized by the ultraviolet irradiation with assistance of Na F in the fifth chapter. Ti F3 presents a rectangle shape with size of 100³00nm. Better light absorption, adsorption and photodegradation of MO are achieved by Ti O₂/Ti F3 composite comparing with that of pure Ti O₂.