Quanjun Xiang

Synergetic Effect of MoS2 and Graphene as Cocatalysts for Enhanced Photocatalytic H2 Production Activity of TiO2 Nanoparticles

The production of H(2) by photocatalytic water splitting has attracted a lot attention as a clean and renewable solar H(2) generation system. Despite tremendous efforts, the present great challenge in materials science is to develop highly active photocatalysts for splitting of water at low cost. Here we report a new composite material consisting of TiO(2) nanocrystals grown in the presence of a layered MoS(2)/graphene hybrid as a high-performance photocatalyst for H(2) evolution. This composite material was prepared by a two-step simple hydrothermal process using sodium molybdate, thiourea, and graphene oxide as precursors of the MoS(2)/graphene hybrid and tetrabutylorthotitanate as the titanium precursor. Even without a noble-metal cocatalyst, the TiO(2)/MoS(2)/graphene composite reaches a high H(2) production rate of 165.3 μmol h(-1) when the content of the MoS(2)/graphene cocatalyst is 0.5 wt % and the content of graphene in this cocatalyst is 5.0 wt %, and the apparent quantum efficiency reaches 9.7% at 365 nm. This unusual photocatalytic activity arises from the positive synergetic effect between the MoS(2) and graphene components in this hybrid cocatalyst, which serve as an electron collector and a source of active adsorption sites, respectively. This study presents an inexpensive photocatalyst for energy conversion to achieve highly efficient H(2) evolution without noble metals.

Graphene-Based Photocatalysts for Hydrogen Generation

Graphene-based photocatalysts have gained increasing interest as a viable alternate to increase photocatalytic H2 production performance in converting solar energy into chemical energy. The use of graphene to enhance the efficiency of photocatalysts has been proved due to its unique two-dimensional conjugated structure and electronic properties. In this Perspective, we have summarized the recent significant advances on the design and applications of graphene-based photocatalytic composites. The rational designs for high-performance photocatalysts using graphene-based materials are described. The applications of the new materials in photocatalytic hydrogen evolution are presented. Finally, the ongoing challenges and opportunities for the future development of graphene-based photocatalysts are also proposed.

Graphene‐Based Photocatalysts for Solar‐Fuel Generation

The production of solar fuel through photocatalytic water splitting and CO2 reduction using photocatalysts has attracted considerable attention owing to the global energy shortage and growing environmental problems. During the past few years, many studies have demonstrated that graphene can markedly enhance the efficiency of photocatalysts for solar-fuel generation because of its unique 2D conjugated structure and electronic properties. Herein we summarize the recent advances in the application of graphene-based photocatalysts for solar-fuel production, including CO2 reduction to hydrocarbon fuel and water splitting to H2. A brief overview of the fundamental principles for splitting of water and reduction of CO2 is given. The different roles of graphene in these graphene-based photocatalysts for improving photocatalytic performance are discussed. Finally, the perspectives on the challenges and opportunities for future research in this promising area are also presented.

Fabrication and enhanced visible-light photocatalytic activity of carbon self-doped TiO2 sheets with exposed {001} facets

Novel carbon self-doped TiO2 sheets (CTS) with exposed {001} facets were synthesized by hydrothermal treatment of titanium carbide (TiC) in a HNO3-HF mixed aqueous solution. In this synthesis TiC was used as a precursor of TiO2 and a source of C, which was self-doped into the lattice of anatase sheets. The resulting CTS materials were examined as photocatalysts for degradation of methylene blue (MB) in aqueous solutions under visible light irradiation (λ > 420 nm). These materials exhibited an enhanced absorption in the whole visible-light region and an obvious red shift at the absorption edges. The first-principle density functional theory (DFT) calculations provided a further confirmation for the aforementioned red shift and for noticeable reduction of the band gap of C-doped TiO2 sheets with exposed {001} facets. The photocatalytic studies of CTS showed that these sheets exhibited much higher photocatalytic activity than that of the C-doped TiO2 nanoparticles due to the presence of exposed {001} facets. In addition, separation of CTS after photocatalytic reaction from slurry by filtration or sedimentation and their reuse is easier in comparison to conventional nanosized powder photocatalysts.

Nitrogen self-doped nanosized TiO2 sheets with exposed {001} facets for enhanced visible-light photocatalytic activity

Nitrogen self-doped TiO(2) nanosheets with exposed {001} facets (ca. 67%) were synthesized by solvothermal treatment of TiN in a HNO(3)-HF ethanol solution and exhibited much higher visible-light photocatalytic H(2)-production activity than nitrogen doped TiO(2) microcrystallites with exposed {001} facets (ca. 60%) by a factor of 4.1.

Microwave-hydrothermal preparation and visible-light photoactivity of plasmonic photocatalyst Ag-TiO2 nanocomposite hollow spheres.

Visible-light-driven plasmonic photocatalyst Ag-TiO(2) nanocomposite hollow spheres are prepared by a template-free chemically-induced self-transformation strategy under microwave-hydrothermal conditions, followed by a photochemical reduction process under xenon lamp irradiation. The prepared samples are characterized by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, N(2) adsorption-desorption isotherms, X-ray photoelectron spectroscopy, UV/Vis and Raman spectroscopy. Production of .OH radicals on the surface of visible-light illuminated TiO(2) was detected by using a photoluminescence method with terephthalic acid as the probe molecule. The photocatalytic activity of as-prepared samples was evaluated by photocatalytic decolorization of Rhodamine B (RhB) aqueous solution at ambient temperature under visible-light irradiation. The results show that the surface plasmon absorption band of the silver nanoparticles supported on the TiO(2) hollow spheres was red shifted, and a strong surface enhanced Raman scattering effect for the Ag-TiO(2) nanocomposite sample was observed. The prepared nanocomposite hollow spheres exhibits a highly visible-light photocatalytic activity for photocatalytic degradation of RhB in water, and their photocatalytic activity is higher than that of pure TiO(2) and commercial Degussa P25 (P25) powders. Especially, the as-prepared Ag-TiO(2) nanocomposite hollow spheres at the nominal atomic ratio of silver to titanium (R) of 2 showed the highest photocatalytic activity, which exceeds that of P25 by a factor of more than 2.

One-step hydrothermal fabrication and photocatalytic activity of surface-fluorinated TiO2 hollow microspheres and tabular anatase single micro-crystals with high-energy facets

Surface-fluorinated TiO2 hollow microspheres and tabular-shaped anatase single micro-crystals with highly energetic (001) facets exposed were prepared by a one-step hydrothermal strategy using ammonium bifluoride (NH4HF2) as a morphology controlling agent. The prepared samples were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and N2 adsorption–desorption isotherms. Production of ˙OH radicals on the TiO2 surface was detected by a photoluminescence (PL) technique using terephthalic acid as a probe molecule. The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic decolorization of Rhodamine B (RhB) aqueous solution at ambient temperature. The results indicate that the particle morphology, average crystallite size, specific surface areas, pore structures, and photocatalytic activity of the TiO2 samples can be readily controlled by changing the concentration of NH4HF2. With increasing NH4HF2 concentration, the average crystallite size and average pore size increase, whilst the specific surface area, pore volume and porosity steadily decrease. The photocatalytic activity of the as-prepared samples exceeds that of Degussa P25 (P25) when the molar ratios of fluorine to titanium (RF) are kept in the range of 0 to 1.

Improved visible-light photocatalytic activity of porous carbon self-doped ZnO nanosheet-assembled flowers

Hierarchical flower-like C-doped ZnO superstructures (ZnO flowers) assembled from porous nanosheets are obtained by pyrolysis of morphology-analogous Zn5(CO3)2(OH)6 precursors. The prepared ZnO flowers are characterized by X-ray diffraction, thermogravimetic and differential scanning calorimeter analysis, scanning electron microscopy, transmission electron microscopy, N2 sorption measurements, UV-vis diffuse reflectance spectra and X-ray photoelectron spectroscopy. The production of OH radicals on the ZnO surface under visible-light irradiation is detected by a photoluminescence technique using terephthalic acid as a probe molecule. The visible-light photocatalytic activity is evaluated by photocatalytic decomposition of the dye RhB in aqueous solution. The hierarchical organization of nanosheets, the multimode voids between and within porous nanosheets, together with annealing-induced in situ carbon self-doping within the ZnO lattice, account for the enhanced light-absorption capacity, extended light-response range and thus better photocatalytic activity of the ZnO flowers. Furthermore, first-principle density functional theory (DFT) calculation further confirms the C-doping induced red shift in the absorption edges of C-doped ZnO flowers.

Roles of MoS2 and Graphene as Cocatalysts in the Enhanced Visible-Light Photocatalytic H2 Production Activity of Multiarmed CdS Nanorods

The incorporation of cocatalysts into semiconductors is proved to be an effective approach to improving the efficiency of the photocatalytic H2 production. Noble metals such as Pt have been widely used as cocatalysts and can significantly improve the performance of photocatalytic H2 production. However, owing to the high cost and low abundance, the use of Pt in practical applications is restricted. Herein, we report two well‐known 2 D layered materials, MoS2 and graphene, as highly active cocatalysts for H2 production in CdS‐based photocatalytic systems. The CdS–MoS2 and CdS‐MoS2–graphene nanocomposites were prepared by using a facile two‐step solvothermal method, and the morphologies of CdS and MoS2 can be well controlled. The as‐prepared binary CdS–MoS2 nanocomposite exhibits the enhanced visible‐light photocatalytic activity for H2 production in lactic acid aqueous solution compared with a CdS–graphene nanocomposite and a conventional platinized CdS photocatalyst. Moreover, the ternary CdS–MoS2–graphene nanocomposite achieves the highest visible‐light photocatalytic H2 production activity of 621.3 μmol h−1 and the apparent quantum efficiency of 54.4 % at λ=420 nm. The enhanced photocatalytic activity of the CdS–MoS2–graphene nanocomposite can be primarily attributed to the positive synergistic effect between graphene sheets and thin MoS2 nanoplates. The graphene sheets can accelerate the efficient electron transfer from CdS nanorods to the active edge sites of MoS2 nanoplates, and the nanosized MoS2 can facilitate the photogenerated electrons participating in the photocatalytic H2 production. The mechanisms for improving the photocatalytic performance of the MoS2‐ and/or graphene‐modified CdS nanocomposites were proposed by using the electrochemical analysis and photoluminescence measurement.

Hierarchical Layered WS2 /Graphene-Modified CdS Nanorods for Efficient Photocatalytic Hydrogen Evolution.

Graphene-based ternary composite photocatalysts with genuine heterostructure constituents have attracted extensive attention in photocatalytic hydrogen evolution. Here we report a new graphene-based ternary composite consisting of CdS nanorods grown on hierarchical layered WS2 /graphene hybrid (WG) as a high-performance photocatalyst for hydrogen evolution under visible light irradiation. The optimal content of layered WG as a co-catalyst in the ternary CdS/WS2 /graphene composites was found to be 4.2 wt %, giving a visible light photocatalytic H2 -production rate of 1842 μmol h(-1)  g(-1) with an apparent quantum efficiency of 21.2 % at 420 nm. This high photocatalytic H2 -production activity is due to the deposition of CdS nanorods on layered WS2 /graphene sheets, which can efficiently suppress charge recombination, improve interfacial charge transfer, and provide reduction active sites. The proposed mechanism for the enhanced photocatalytic activity of CdS nanorods modified with hierarchical layered WG was further confirmed by transient photocurrent response. This work shows that a noble-metal-free hierarchical layered WS2 /graphene nanosheets hybrid can be used as an effective co-catalyst for photocatalytic water splitting.