Min-Quan Yang

Waltzing with the Versatile Platform of Graphene to Synthesize Composite Photocatalysts.

Composite Photocatalysts Nan Zhang,†,‡ Min-Quan Yang,†,‡ Siqi Liu,†,‡ Yugang Sun,* and Yi-Jun Xu*,†,‡ †State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, P.R. China ‡College of Chemistry, New Campus, Fuzhou University, Fuzhou 350108, P.R. China Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States

Synthesis of Fullerene–, Carbon Nanotube–, and Graphene–TiO2 Nanocomposite Photocatalysts for Selective Oxidation: A Comparative Study

A series of TiO(2)-graphene (GR), -carbon nanotube (CNT), and -fullerene (C(60)) nanocomposite photocatalysts with different weight addition ratios of carbon contents are synthesized via a combination of sol-gel and hydrothermal methods. Their structures and properties are determined by the X-ray diffraction (XRD), UV-vis diffuse reflectance spectra (DRS), transmission electron microscopy (TEM), nitrogen adsorption-desorption, and photoelectrochemical measurements. Photocatalytic selective oxidation of benzyl alcohol to benzaldehyde is employed as a model reaction to evaluate the photocatalytic activity of the TiO(2)-carbon (GR, CNT, and C(60)) nanocomposites under visible light irradiation. The results reveal that incorporating TiO(2) with carbon materials can extend the adsorption edge of all the TiO(2)-carbon nanocomposites to the visible light region. For TiO(2)-GR, TiO(2)-CNT, and TiO(2)-C(60) nanocomposites, the photocatalytic activities of the composites with optimum ratios, TiO(2)-0.1% GR, TiO(2)-0.5% CNT, and TiO(2)-1.0% C(60), are very close to each other along with the irradiation time. Furthermore, the underlying reaction mechanism for the photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over TiO(2)-carbon nanocomposites has been explored using different radical scavenger techniques, suggesting that TiO(2)-carbon photocatalysts follow the analogous oxidation mechanism toward selective oxidation of benzyl alcohol. The addition of different carbon materials has no significant influence on the crystal phase, particle size, and the morphology of TiO(2). Therefore, it can be concluded, at least for nanocomposites of TiO(2)-carbon (GR, CNT, and C(60)) obtained by the present approach, that there is no much difference in essence on affecting the photocatalytic performance of semiconductor TiO(2) among these three different carbon allotropes, GR, CNT, and C(60). Our findings point to the importance of a comparative study of semiconductor-carbon photocatalysts on drawing a relatively objective conclusion rather than separately emphasizing the unique role of GR and joining the graphene gold rush.

Synthesis of Uniform CdS Nanospheres/Graphene Hybrid Nanocomposites and Their Application as Visible Light Photocatalyst for Selective Reduction of Nitro Organics in Water

We report the self-assembly of uniform CdS nanospheres/graphene (CdS NSPs/GR) hybrid nanocomposites via electrostatic interaction of positively charged CdS nanospheres (CdS NSPs) with negatively charged graphene oxide (GO), followed by GO reduction via a hydrothermal treatment. During this facile two-step wet chemistry process, reduced graphene oxide (RGO, also called GR) and the intimate interfacial contact between CdS NSPs and the GR sheets are achieved. Importantly, the CdS NSPs/GR nanocomposites exhibit a much higher photocatalytic performance than bare CdS NSPs toward selective reduction of nitro organics to corresponding amino organics under visible light irradiation. The superior photocatalytic performance of the CdS NSPs/GR nanocomposites can be attributed to the intimate interfacial contact between CdS NSPs and the GR sheets, which would maximize the excellent electron conductivity and mobility of GR that in turn markedly contributes to improving the fate and transfer of photogenerated charge carriers from CdS NSPs under visible light irradiation. Moreover, the photocorrosion of CdS and the photodegradation of GR can be efficiently inhibited. The excellent reusability of the CdS NSPs/GR nanocomposites can be attributed to the synergetic effect of the introduction of GR into the matrix of CdS NSPs and the addition of ammonium formate as quencher for photogenerated holes. It is hoped that our current work could promote us to efficiently harness such a simple and efficient self-assembly strategy to synthesize GR-based semiconductor composites with controlled morphology and, more significantly, widen the application of CdS/GR nanocomposite photocatalysts and offer new inroads into exploration and utilization of GR-based semiconductor nanocomposites as visible light photocatalysts for selective organic transformations.

Toward the enhanced photoactivity and photostability of ZnO nanospheres via intimate surface coating with reduced graphene oxide

A series of uniform ZnO nanospheres–reduced graphene oxide nanocomposites (ZnO–RGO NCs) with different weight addition ratios of RGO are successfully synthesized via a facile yet efficient method by intimately coating ZnO nanospheres (NSs) with RGO, which is afforded by electrostatic attraction between positively charged ZnO NSs and negatively charged graphene oxide (GO) in an aqueous medium at room temperature. The photocatalytic test of degradation of Rhodamine B shows that the optimal ZnO–10% RGO NCs exhibit a 5-fold enhancement of photoactivity than bare ZnO NSs, which is ascribed to the integrative synergetic effect of enhanced adsorption capacity, the decreased recombination of the electron–hole pairs and the enhanced ultraviolet light absorption intensity. Significantly, the recycled photoactivity tests show that, for ZnO–RGO NCs, the anti-photocorrosion of ZnO NSs is improved remarkably which is attributed to the effective hybridization of ZnO NSs with the RGO sheet via intimate surface coating. Such a significant photoactivity enhancement and anti-photocorrosion phenomenon can not be obtained by simply integrating RGO with ZnO NSs that are not subject to surface charge modification, which thus indicates the importance of intimate surface coating of ZnO with RGO toward the efficiency of enhancement of photoactivity and particularly the anti-photocorrosion of ZnO.

Surface charge promotes the synthesis of large, flat structured graphene–(CdS nanowire)–TiO2 nanocomposites as versatile visible light photocatalysts

Ternary hybrids of (reduced graphene oxide)–(CdS nanowire)–TiO2 nanocomposites (CTG) featuring a large two-dimensional (2D) flat structure have been successfully synthesized via a simple surface charge promoted self-assembly method. Compared to the curly (reduced graphene oxide)–(CdS nanowire) nanocomposites (CG) synthesized by a similar approach, CTG possesses a large 2D flat structure, which not only provides high optical transparency and a large surface area but also facilitates the migration of photogenerated electrons. This large 2D flat structure of CTG leads to increased optical absorption of visible light and increased electrical conductivity as compared to the curly CG, which is attributed to the fact that the large 2D flat structure of reduced graphene oxide (RGO) in CTG provides more efficient contact between light and the RGO sheets and facilitates the transfer of charge carriers. Experimental evidence has proven that negatively charged TiO2 nanoparticles (NPs) both on the surfaces of the CdS nanowires (CdS NWs) and on the RGO sheets can prevent the RGO sheets from becoming curly or aggregated as a result of electrostatic repulsion, thereby forming the large 2D flat structure of CTG. In addition to using RGO as an electron “sink” to improve the transfer of photogenerated electron–hole pairs (EHPs) from CdS NWs, the TiO2 NPs on CdS NWs are able to further boost the transfer of charge carriers in the ternary CTG system due to the suitable energy band match between TiO2 and CdS. Such efficient, spatially separated charge carriers make CTG a versatile visible light photocatalyst for photo-redox processes. This work provides a new, simple strategy to construct these large 2D flat structured RGO-based multi-component composites by using the surface charge properties of materials to efficiently utilize their respective unique electronic properties toward diverse photo-redox processes in both energy conversion and environmental purification.

Morphology control, defect engineering and photoactivity tuning of ZnO crystals by graphene oxide--a unique 2D macromolecular surfactant.

Zinc oxide (ZnO) nanostructured materials have received significant attention because of their unique physicochemical and electronic properties. In particular, the functional properties of ZnO are strongly dependent on its morphology and defect structure, particularly for a semiconductor ZnO-based photocatalyst. Here, we demonstrate a simple strategy for simultaneous morphology control, defect engineering and photoactivity tuning of semiconductor ZnO by utilizing the unique surfactant properties of graphene oxide (GO) in a liquid phase. By varying the amount of GO added during the synthesis process, the morphology of ZnO gradually evolves from a one dimensional prismatic rod to a hexagonal tube-like architecture while GO is converted into reduced GO (RGO). In addition, the introduction of GO can create oxygen vacancies in the lattice of ZnO crystals. As a result, the absorption edge of the wide band gap semiconductor ZnO is effectively extended to the visible light region, which thus endows the RGO-ZnO nanocomposites with visible light photoactivity; in contrast, the bare ZnO nanorod is only UV light photoactive. The synergistic integration of the unique morphology and the presence of oxygen vacancies imparts the RGO-ZnO nanocomposite with remarkably enhanced visible light photoactivity as compared to bare ZnO and its counterpart featuring different structural morphologies and the absence of oxygen vacancies. Our promising results highlight the versatility of the 2D GO as a solution-processable macromolecular surfactant to fabricate RGO-semiconductor nanocomposites with tunable morphology, defect structure and photocatalytic performance in a system-materials-engineering way.

Visible-Light-Driven Oxidation of Primary C-H Bonds over CdS with Dual Co-catalysts Graphene and TiO2

Selective activation of primary C–H bonds for fine chemicals synthesis is of crucial importance for the sustainable exploitation of available feedstocks. Here, we report a viable strategy to synthesize ternary GR-CdS-TiO2 composites with an intimate spatial integration and sheet-like structure, which is afforded by assembling two co-catalysts, graphene and TiO2, into the semiconductor CdS matrix with specific morphology as a visible light harvester. The GR-CdS-TiO2 composites are able to serve as a highly selective visible-light-driven photocatalyst for oxidation of saturated primary C–H bonds using benign oxygen as oxidant under ambient conditions. This work demonstrates a wide, promising scope of adopting co-catalyst strategy to design more efficient semiconductor-based photocatalyst toward selective activation of C–H bonds using solar light and molecular oxygen.

Enhancing the visible light photocatalytic performance of ternary CdS–(graphene–Pd) nanocomposites via a facile interfacial mediator and co-catalyst strategy

By embedding noble metal palladium (Pd) into the interfacial layer matrix of graphene (GR) and semiconductor CdS, we have successfully constructed ternary CdS–(GR–Pd) nanocomposites with intimate interfacial contact. The CdS–(GR–Pd) nanocomposites show remarkably enhanced photocatalytic activity toward selective redox reactions under visible light irradiation as compared to blank-CdS and the optimum binary CdS–GR. It is revealed that the photocatalytic performance enhancement of CdS–(GR–Pd) is ascribed to the optimized spatial charge carrier transfer across the interface resulting from the introduction of Pd nanoparticles as mediators into the interfacial layer between GR and CdS. One role of Pd is to serve as electron reservoir to directly trap photogenerated electrons from CdS and the other role is as interfacial mediator to promote electron relay in the ternary CdS–(GR–Pd) photocatalysts along with conductive graphene as dual co-catalysts. Moreover, the negative light “shielding effect” of GR can be partially counterbalanced through such a facile strategy. This work substantiates the feasibility of adopting the “interfacial-mediator” strategy to optimize the interfacial charge carriers transfer pathway and efficiency for improved photoactivity of GR–semiconductor nanocomposites toward target photoredox reactions.

Photocatalytic water splitting for solar hydrogen generation: fundamentals and recent advancements

The expected depletion of fossil fuel reserves and its serious environmental impact have emphasised the issue of sustainable development of the human society. Solar hydrogen by photocatalytic water splitting is a promising alternative to conventional fossil fuels, which is of great potential to relieve the energy and environmental issues and bring an energy revolution in a clean and sustainable manner. This review is going to make a brief introduction of the basic principles of photocatalytic water splitting and the concept of different kinds of water splitting systems. Various engineering strategies for searching higher efficiency of water splitting based on the photocatalytic processes, including light harvesting, charge carriers separation and co-catalysts loading, have been outlined and discussed with selected typical examples on some elaborately designed semiconductor-based photocatalytic systems. Moreover, recent impressive progresses and advancements for photocatalytic water splitting with some promising materials are presented. Finally, this review is concluded with a summary and perspective in this hot area of research.

Photocatalytic conversion of CO2 over graphene-based composites: current status and future perspective

The continuous rise in the atmospheric CO2 level and the ever-increasing demand of energy have raised serious concerns about the ensuing effects on the global climate change and future energy supply. Photocatalytic conversion of CO2, which uses solar light energy to recycle CO2 into fuels and chemicals, provides a promising and straightforward strategy to simultaneously reduce the atmospheric CO2 level and fulfil the future energy demand. However, the lack of substantial development of state-of-the-art materials remains a major bottleneck of this technology. In recent years, graphene-based composite photocatalysts have gained increasing interest in CO2 conversion due to the introduction of graphene with a series of unique physicochemical properties, which has shown to play a significant role in promoting the photocatalytic solar energy conversion efficiency. In this review, we comprehensively summarize the typical literature reports on graphene-based composites for photocatalytic conversion of CO2 to produce solar fuels and chemicals. The main types of the reported graphene-based composites and the role of graphene in the composites in improving the photocatalytic performance have been elaborated. In particular, we have highlighted the possible role of graphene in tuning the product selectivity of photocatalytic reduction of CO2. Finally, perspectives on the existing problems and future research on graphene-based composites toward photocatalytic conversion of CO2 are critically discussed.