Study On Synthesis And Photocatalytic Activity Of N Doped TiO₂ Nanowire Arrays And Graphene Quantum Dots Nanocomposite

In recent decades, the photocatalytic degradation of organic pollutants has received much attention. And because of its high catalytic activity, high oxidation resistance, light stability, chemical stability, low cost, nontoxicity, titanium dioxide has been extensively investigated and widely used as a semiconductor photocatalyst in photocatalytic applications. However, the photocatalytic efficiency of TiO₂ is limited by its wide band gap energy, which is only sensitive to the shorter wavelength UV light, contributes less than 5% of the solar spectrum, and its photocatalytic applications indoor were also limited.Since the graphene was discovered, this new 2D structure carbon nanomaterial is widely used due to its excellent properties. Graphene has excellent electrical conductivity and has a strong adsorption capacity, and so on. The graphene quantum dots can be viewed as fragments of graphene quantum level, in addition to its excellent performance with graphene, but also has good quantum domain and boundary effects limit. Based on the above, bringing the graphene quantum dots into photocatalysis maybe could make up the lack of titanium dioxide. So, in view of the above problem, the major contents and consequence of this thesis are summarized as following:In the second chapter, TiO₂ nanowires?TNW? array films with large aspect ratios were hydrothermally prepared in 1 mol·L-1 NaOH solution and titanium sheet as the base. We also systematically studied the effects of the hydrothermal parameters on the TiO₂ nanowires films and their growth mechanism. Then we use the different characterization techniques such as field emission scanning electron microscopy?FESEM?, and high resolution transmission electron microscopy?HRTEM?, X-ray diffraction?XRD? to carry out a detailed study and analysis. The results showed that nanowires film had strong adhesion to the substrate. The as-prepared TiO₂ nanowires were single crystalline anatase phase and were the result of transformation of Ti into Na2Ti2O4?OH?2 and H2Ti2O5·H2O.Then, TiO₂ nanowire arrays were thermal treatment under the NH3 atmosphere for 120 min, then through a spin coating method to produce N-GQDs-TNW samples. The samples were characterized by field emission scanning electron microscopy?FESEM?, X-ray diffraction?XRD?. The results showed that the samples' morphology and crystalline structure annealed under the NH3 atmosphere and modified using graphene quantum dot has not change. UV-Vis diffuse reflectance spectroscopy showed that the absorption band edge of the samples were almost the same, still absorbed ultraviolet light. Fluorescence spectra showed that the intensity fluorescence of the N-GQDs-TNW samples were very weak, indicating that the electron-hole separation effect good, which created favorable conditions for the photocatalytic process. At last, we passed degradation of methylene blue with all kinds of samples, results showed that photocatalytic properties of the composites have improved compared with pure TNW arrays.In the third chapter, we prepared hierarchical anatase TiO₂ nanowires?HNW? array films on Ti-foil substrate by a second-step hydrothermal synthesis method. Then, hierarchical anatase TiO₂ nanowire arrays were thermal treatment under the NH3 atmosphere for 120 min, and through a spin coating method to produce N-GQDs-HNW samples. SEM results showed that HNW arrays consisted of long single crystalline nanowire trunks and short single crystalline nanorod branches. Further, we analysed the crystal structure of the sample using XRD and Raman, and the results showed that the sample prepared after the second-step hydrothermal reaction of the TiO₂ nanowire could be still indexed as the anatase TiO₂ phase. And the crystalline structure and morphology of the samples did not change by nitrogen and graphene quantum dots modified. UV-Vis diffuse reflectance spectroscopy showed that the absorption band edge of the samples were almost the same, still absorbed ultraviolet light. Fluorescence spectra showed that the intensity fluorescence of the N-GQDs-HNW samples were very weak, indicating that the electron-hole separation effect good, which created favorable conditions for the photocatalytic process. At last, we passed degradation of methylene blue with all kinds of samples, results showed that photocatalytic properties of the composites have improved compared with pure TNW arrays.In the fourth chapter, we use a facile hydrothermal synthesis route to N and S, N co-doped graphene quantum dot was developed by using citric acid as the C source and urea or thiourea as N and S sources. then through a spin coating method to produce S,N:GQDs/TNW?N: GQDs/TNW composites. Then, we used UV-vis spectroscopy techniques to investigate N and S, N co-doped graphene quantum dot samples for characterization. The results showed that a broad absorption band in the visible region appeared in S, N co-doped graphene quantum dot. This is due to doping with sulfur, which alters the surface state of GQDs. Furthermore, the results of the PL spectra analysis showed that both N:GQDs and S,N:GQDs exhibited a strong emission and the emission of GQDs were almost completely quenched in both cases of the N:GQD/TNW and S,N:GQD/TNW composites. This means charge separation happened between GQDs and TiO₂ nanowires. The degradation of MB results indicated that the S, N:GQD/TNW composites showed excellent photocatalytic performance.