| Driven by the increase in energy needs and the gradually decrease in fossil fuel resources, and also the environmental concerns of nuclear energy, clean and renewable alternative energy recourses are highly demanded. Hydrogen energy has been regarded as one of the potential energy alternatives to simultaneously address energy and environmental problems because of its environmental friendliness. Several challenges are encountered associated with the hydrogen production, storage, transportation and application. Conversion of fossil fuel resources is the most widely adopted method currently for hydrogen to production, which therefore can not fulfill the requirement of low carbon economy as a consequence of our fossil fuel-based energy system. Photocatalytic hydrogen production from water splitting via semiconductor nanomaterials has long been considered the ultimate solution as an alternative route for solar energy utilization.Titanium dioxide (T1O2) has been extensively studied and is highlighted as an important semiconductor photocatalyst due to the superior characteristics such as exceptional optoelectronic properties, strong oxidizing power, chemical stability, low cost, and so on. The catalytic efficiency in photocatalytic hydrogen generation is altered by the light absorption ability, the energy levels, the conversion processes of photogenerated charges including generation, recombination, separation, migration and trapping. The wide band gap of TiO2(3.2eV) leads to a low utilization efficiency of sunlight because only the ultraviolet light can meet the energy requirement of electron excitation from the valence band to the conduction band, which hinders the practical application of TiO2in photocatalysis. To extend the light absorption of TiO2, several strategies have been adopted such as the morphology modification, the doping of cation/anion and nonmetal elements (C, N, S, P, etc).On the other hand, graphene and its derivatives have been regarded as an important component for various functional composite materials owing to its intriguing properties such as superior mobility of charge carriers at room temperature (>200000cm2V-1s-1). Researches about graphene-based semiconductor photocatalysts, including TiO2/graphene nanocomposites, have recently made much rapid progress in the design and fabrication. It has been demonstrated that graphene retards the charge recombination, accelerates the electron transfer and improves the photocatalytic efficiency of hydrogen evolution from water-splitting. Currently, the development of visible-light-driven, stable and highly efficient photocatalysts is very crucial to large-scale hydrogen evolution utilizing solar energy.In order to obtain a desired performance, the present dissertation discusses the fabrication of a series of nanocomposites composed of titania with graphene&its derivatives. The nanocomposites were fabricated via different approaches, including the nitrogen doping of TiO2, the nitrogen doping of graphene, or an organic dye doping in enhancing the photocatalytic efficiency. These hybrids were characterized with Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and photoluminescent spectrometry, etc. The photocatalytic activity was evaluated by measuring the hydrogen evolution amount from the methanol aqueous solution by an on-lined gas chromatography (GC).Firstly, graphene oxide (GO) was prepared from graphite by Hummers’method. The obtained GO sheets contain hydroxyl and carboxyl functional groups on basal planes which allows GO to show a good dispersability in aqueous media, while the electric conductibility of GO is poor because of a mass of non-conjugated defects. Interestingly, the partial ion of conjugated structure in GO could be reconstructed by the reduction via sodium borohydride. TRG-COOH nanocomposites were prepared through simply blending TiO2with the partial reduced GO (RG-COOH), which has a red-shifted band gap of2.80eV as compared with that of P25(3.25eV). TRG-COOH nanocomposite showed a much improved photocatalytic activity under irradiation using500W Xenon lamp with a hydrogen evolution of73.1μmol·h-1·g-1as compared with that of P25(8.2μmol·h-1·g-1), which was increased about8.9times. Further, nanocomposites of N-doped TiO2with graphene oxide (NTG) were successfully prepared by hydrothermal process from GO and N-doped TiO2(NT) that was prepared via a solvothermal method from commercial P25with1,2-diaminoethane as a nitrogen resource. XPS spectra demonstrated the formation of Ti-C bond between TiO2and graphene. The as-prepared TNG has1.43%N atom, which showed a red-shifted band gap of2.69eV as measured from UV-vis diffuse reflectance spectroscopy. The extending of light absorption to the visible light region would enhance the photocatalytic activity for hydrogen evolution. The excited electrons would mostly transfer to graphene sheets as electron sinks, which was justified by the compared photoluminescence measurements of P25, NT, TG and NTG. UV light irradiation affords P25, NT, TG and NTG a photocatalytic efficiency of76.1,270.0,370.2and716.0μmol h-1g-1, respectively, which is to say that the photocatalytic efficiency of NTG was increased about9.2times as that of P25. The visible light irradiation under Xenon lamp produces a photocatalytic efficiency of8.2,41.0,57.8and112.0μmol·h-1·g-1for P25, NT, TG and NTG, respectively showing about a13.6times activity increase of NTG compared to P25.In order to further enhance the photocatalytic efficiency, nanocomposites of nitrogen-doped TiO2and nitrogen-doped graphene (NTNG) were prepared via solvothermal method from P25,1,2-diaminoethane and N-doped graphene. N-doped graphene was obtained via mixing lithium nitride (Li3N) with tetrachloromethane (CCl4) through solvothermal approach. The nitrogen content in NTNG was measured as8.6atom%which is much higher than that of NG (3.5atom%) from XPS analysis. The formation of C-Ti bond between TiO2and NG was demonstrated from the high resolution Cls XPS spectra of NTNG, which endowed a enhanced of light absorption. The observed photoluminescent quenching indicated that a strong interaction between the excited state of NT and NG, the fast electron-transportation process avoids the recombination of photo-generated charges, either the electron or the holes. The electronegative N atom with lone electron pair bonding with C atoms in the graphene sheet would change the spin density and induce a higher charge density for the active sites. The UV light irradiation endows P25, NT and NTNG a photocatalytic efficiency of76.1,270.0and996.8μmol·h-1·g-1, respectively showing that the photocatalytic efficiency of NTNG was increased about13.1times as that of P25.Finally, eosin Y (EY) dye sensitized nanocomposite of TiO2with graphene (T-G-EY) were prepared by hydrothermal process starting from EY, TiO2(P25) and graphene oxide (GO). T-G-EY showed a narrower band gap of2.75eV and a very strong absorption in the visible region of400-600nm from the UV-vis diffuse reflectance absorption spectra. Photoluminescent quenching was also observed, illustrating the interactions between each component in the nanocomposite. The photocatalytic efficiency of T-G-EY for hydrogen evolution from methanol aqueous solution under a500W Xenon lamp irradiation was measured as84.2μmolh-1g-1.The proposed strategy in this dissertation by the incorporation of graphene&its derivatives with titania enhanced effectively the photocatalytic activity in various degree. TiO2/graphene nanocomposites showed an enhanced photocatalytic activity, which is attributed to the narrowing of band gap and the enhancement of light absorption, the fast separation of photo-generated electron-hole pairs and the retardation of the bulk or surface recombination of photo-induced charges derived from the superior mobility of charge carriers. Nitrogen doping extended the light absorption region further and induced the active region with higher charge density for the reduction of proton in methanol aqueous solution. Improvement in the photocatalytic efficiency for hydrogen evolution is attributed to the synergistic effect of each components in the nanocomposites. |
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