Graphene Modified Tio < Sub > 2 < / Sub > Light Catalyst Preparation And Photocatalytic Hydrogen Production Performance Research

Hydrogen evolution from aqeous solution over semiconductors utilizing solar energy has attracted great attention as it is expected to be able to solve problems of energy shortage and environmental crisis. Among all kinds of reported semiconductor photocatalysts, TiO2is currently considered one of the most promising photocatalysts because of its low cost, non-toxicity and chemical stability. However, TiO2is a wide band gap semiconductor (～3.2eV) and can absorb only3～5%ultraviolet fraction of solar light; the photoinduced electron-holes are easily recombined together resulting in low efficiency.In this paper, the newly discovered graphene is used to modify TiO2aiming to improve its photocatalytic performance. Graphene oxide (GO) thin films and reduced graphene oxide (rGO) modified TiO2thin films have been fabricated by a dip-coating method, excellent photoelectrochemical performance was observed under ultraviolet-visible (UV-Vis) light irradiation. A series of TiO2based powder photocatalysts have been synthesized by sol-gel method, hydrothermal method and flame assisted method, respectively, showing higher photocatalytic activity for hydrogen evolution under UV-Vis light irradiation. The main results are as follows:Graphene oxide (GO) film was fabricated on freshly cleaned fluorine-doped tin oxide (FTO) substrates from GO solution by a dip-coating method, and its photoelectrochemical properties were investigated. It was found that the light absorbance of GO films increased with increasing film thickness, while the optical band gap changed little. Within a certain film thickness, the photocurrent density of GO film increased from0.1μA·cm-2to0.25μA·cm-2along with increasing the film thickness under UV-Vis light irradiation; while UV irradiation can decrease the photocurrent density of GO film from0.25μA·cm-2to0.05μA·cm-2after10h of UV irradiation. GO film surface exhibited photoinduced hydrophobic property. The mechanisim of the photoinduced decreased photocurrent density and hydrophobic property of GO film was investigated:the oxygen-containing functional groups on GO surfaces can be eliminated through reaction with excited electrons under UV irradiation, whereas these oxygen-containing functional groups are hydrophilic and can suppress the recombination of photoinduced electron-holes.A series of rGO-TiO2thin films with two different combination modes were fabricated using tetrabutyl orthotitanate (TBOT) and GO as precursors by a dip-coating method on FTO substrates. Effect of the combination mode on the photoelectrochemical performance of rGO-TiO2was investigated. The rGO-TiO2thin films were composed of anatase TiO2. Under UV-Vis light irradiation, rGO-TiO2thin films showed higher photocurrent density in comparison to pure T1O2film. The photocurrent density of rGO-TiO2thin film with rGO at bottom distribution was23μA·cm-2, which was2.5and3.8times higher than that of FTO/TiO2/rGO film and pure TiO2film, respectively. It was revealed that the higher photoelectrochemical performance was ascribed to the unique charge transfer property of rGO and energy band matching in FTO/rGO/TiO2film, which could afford faster electron transport and higher separation efficiency of electron-hole pairs, resulting in enhanced photocurrent density. The design idea of fabricating rGO-TiO2thin film with rGO at bottom distribution shows promising applications in solar water splitting for hydrogen evolution and dye sensitized solar cells.A series of TiO2/rGO powder photocatalysts were synthesized using TBOT and rGO as the starting materials by a sol-gel method. The introduction of rGO can increase the specific surface area of the nanocomposites, expand light absorption into visible light range and increase visible light absorbance. Under UV-Vis light irradiation, the synthesized TiO2/rGO nanocomposites showed higher activity than that of P25for photocatalytic hydrogen evolution. The highest hydrogen evolution rate was2.65μmol·h-1for TiO2/rGO nanocomposite with rGO content as5wt%. Moreover, effects of annealing atmosphere were investigated on the photocatalytic performance for the samples. It was found that the samples annealed in nitrogen atmosphere exhibited higher performance than those annealed in air atmosphere. The enhanced photocatalytic performance was due to the generation of oxygen vacancies on the surface of the samples when annealed in nitrogen atmosphere, which is beneficial to the separation of photoinduced electron-holes.TiCl4and GO were used as the starting materials to synthesize TiO2/rGO powders by a one-step hydrothermal method. Well dispersed TiO2nanoparticles were observed on the surface of rGO. High content of rGO (≥5wt%) can suppress the growth of TiO2nanoparticles and promote the generation of rutile TiO2. Under UV-Vis light irradiation, the synthesized TiO2/rGO with rGO content as2.0wt%exhibited higher photocatalytic performance with the hydrogen evolution rate as2.15μmol·h-1Microspherical carbon incorporated TiO2powders were prepared using TBOT and absolute ethanol as the precursors by a one-step flame assisted method. The obtained carbon incorporated TiO2powders are composed of anatse TiO2and show microspherical morphology with average diameter in the range of0.5～2.0μm. The microspheres are composed of TiO2nanoparticles. The volume ratio of TBOT/ethanol and the reaction solution volume can affect the morphology of the samples. Under UV-Vis light irradiation, excellent photocatalytic activity was observed for the prepared carbon incorporated TiO2for hydrogen evolution. The sample prepared with TBOT/ethanol ratio as1:7and solution volume as40mL showed the highest hydrogen evolution rate of3.99μmol·h-1. The enhanced photocatalytic performance is due to the carbon species incorporated in TiO2from the incomplete combustion of organics and the microspherical morphology. The incorporated carbon can act as sensitizers and suppress the recombination of photoinduced electron holes, and the microspherical morphology is beneficial to the absorption and utilizaition of photons.Platinum (Pt) modified carbon incorporated TiO2(Pt-C/TiO2) photocatalysts were prepared by a one-step flame assisted method using TBOT, Pt precursor and absolute ethanol as the starting materials. The platinum introduction had little effect on the phase composition of the samples, but can suppress the formation of microspherical morphology. The average diameter of the formed microspheres was reduced to0.2μm when using H2PtCl6as Pt precursor, while it was still around2.0μm when using Na2PtCl6as platinum precursor. Under UV-Vis light irradiation, the prepared Pt-C/TiO2showed higher photocatalytic performance for hydrogen evolution, suggesting Pt modification can further enhance the performance of TiO2. The Pt-C/TiO2prepared using H2PtCl6as Pt precursor showed higher performance than those prepared using Na2PtCl6. When Pt content was0.1wt%using H2PtCl6as Pt precursor, the synthesized0.1wt%Pt-C/TiO2showed higher photocatalytic performance with the hydrogen evolution rate as5.42μmol·h-1.