Yi-Jun Xu

TiO2−Graphene Nanocomposites for Gas-Phase Photocatalytic Degradation of Volatile Aromatic Pollutant: Is TiO2−Graphene Truly Different from Other TiO2−Carbon Composite Materials?

The nanocomposites of TiO(2)-graphene (TiO(2)-GR) have been prepared via a facile hydrothermal reaction of graphene oxide and TiO(2) in an ethanol-water solvent. We show that such a TiO(2)-GR nanocomposite exhibits much higher photocatalytic activity and stability than bare TiO(2) toward the gas-phase degradation of benzene, a volatile aromatic pollutant in air. By investigating the effect of different addition ratios of graphene on the photocatalytic activity of TiO(2)-GR systematically, we find that the higher weight ratio in TiO(2)-GR will decrease the photocatalytic activity. Analogous phenomenon is also observed for the liquid-phase degradation of dyes over TiO(2)-GR. In addition, the key features for TiO(2)-GR including enhancement of adsorptivity of pollutants, light absorption intensity, electron-hole pairs lifetime, and extended light absorption range have also been found in the composite of TiO(2) and carbon nanotubes (TiO(2)-CNT). These strongly manifest that TiO(2)-GR is in essence the same as other TiO(2)-carbon (carbon nanotubes, fullerenes, and activated carbon) composite materials on enhancement of photocatalytic activity of TiO(2), although graphene by itself has unique structural and electronic properties. Notably, this key fundamental question remains completely unaddressed in a recent report ( ACS Nano 2010 , 4 , 380 ) regarding liquid-phase degradation of dyes over the TiO(2)-GR photocatalyst. Thus, we propose that TiO(2)-GR cannot provide truly new insights into the fabrication of TiO(2)-carbon composite as high-performance photocatalysts. It is hoped that our work could avert the misleading message to the readership, hence offering a valuable source of reference on fabricating TiO(2)-carbon composites for their application as a photocatalyst in the environment cleanup.

Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions

Oxidation is an important method for the synthesis of chemical intermediates in the manufacture of high-tonnage commodities, high-value fine chemicals, agrochemicals and pharmaceuticals: but oxidations are often inefficient. The introduction of catalytic systems using oxygen from air is preferred for ‘green’ processing. Gold catalysis is now showing potential in selective redox processes, particularly for alcohol oxidation and the direct synthesis of hydrogen peroxide. However, a major challenge that persists is the synthesis of an epoxide by the direct electrophilic addition of oxygen to an alkene. Although ethene is epoxidized efficiently using molecular oxygen with silver catalysts in a large-scale industrial process, this is unique because higher alkenes can only be effectively epoxidized using hydrogen peroxide, hydroperoxides or stoichiometric oxygen donors. Here we show that nanocrystalline gold catalysts can provide tunable active catalysts for the oxidation of alkenes using air, with exceptionally high selectivity to partial oxidation products (∼98%) and significant conversions. Our finding significantly extends the discovery by Haruta that nanocrystalline gold can epoxidize alkenes when hydrogen is used to activate the molecular oxygen; in our case, no sacrificial reductant is needed. We anticipate that our finding will initiate attempts to understand more fully the mechanism of oxygen activation at gold surfaces, which might lead to commercial exploitation of the high redox activity of gold nanocrystals.

Engineering the Unique 2D Mat of Graphene to Achieve Graphene-TiO2 Nanocomposite for Photocatalytic Selective Transformation: What Advantage does Graphene Have over Its Forebear Carbon Nanotube?

Increasing interest has been devoted to synthesizing graphene (GR)-semiconductor nanocomposites as photocatalysts for potential applications, which is very similar to its forebear carbon nanotube (CNT)-semiconductor photocatalysts. Unfortunately, a thoughtful and inevitable comparison between GR- and CNT-semiconductors as photocatalysts is often neglected in literature. This situation may give incomplete or exaggerated information on the contribution role of GR to enhance the semiconductor photocatalytic activity, as compared to CNT. Thus, our knowledge regarding the specific advantage of GR over CNT on how to design more efficient GR-semiconductor nanocomposites and understanding the origin of their enhanced photocatalytic performance is far from satisfactory. By taking the TiO(2) semiconductor as an example, we conceptually demonstrate how to synthesize a more efficient GR-TiO(2) nanocomposite as a visible light photocatalyst toward selective oxidation of alcohols under mild conditions. Comparison between GR-TiO(2) and CNT-TiO(2) discloses the prominent advantage of GR over CNT on both controlling the morphology of GR-TiO(2) nanocomposite and enhancing the photocatalytic activity of TiO(2). This work clearly highlights the importance and necessity for a comparison investigation between GR- and CNT-semiconductors as photocatalysts, which will promote our in-depth fundamental understanding on the analogy and difference between GR and CNT on controlling the morphology of GR (or CNT)-semiconductor nanocomposites and enhancing the photocatalytic performance. Therefore, we appeal the photocatalysis community to pay attention to this respect rather than separately imposing hype on the miracle of GR in much the same way as its carbon forebears, which could significantly advance our rational fabrication of smart GR-semiconductor nanocomposites for artificial photosynthesis.

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 graphene–ZnO nanorod nanocomposites with improved photoactivity and anti-photocorrosion

A series of graphene–ZnO (GR–ZnO) nanorod nanocomposites with different weight addition ratios of graphene (GR) have been prepared via a facile hydrothermal reaction of graphene oxide (GO) and ZnO nanorods. X-ray diffraction (XRD), UV-vis diffuse reflectance spectra (DRS), field-emission scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS), photoluminescence (PL) spectra, and electron spin resonance (ESR) spectra are employed to determine the properties of the samples. It is found that GR–ZnO nanorod nanocomposites with a proper addition amount of GR exhibit higher photocatalytic activity and improved anti-photocorrosion than ZnO nanorods toward liquid-phase degradation of dye under ultraviolet (UV) light irradiation. The improved photoactivity and anti-photocorrosion of GR–ZnO nanorods can be ascribed to the integrative synergistic effect of enhanced adsorption capacity, the prolonged lifetime of photogenerated electron–hole pairs and effective interfacial hybridization between GR and ZnO nanorods. This study also shows that graphene sheets act as electronic conductive channels to efficiently separate the photogenerated charge carriers from ZnO nanorods. It is hoped that our current work could promote increasing interest in designing the nanocomposites of one-dimensional (1D) semiconductor and two-dimensional (2D) graphene for different photocatalytic applications.

Synthesis of One-Dimensional CdS@TiO2 Core–Shell Nanocomposites Photocatalyst for Selective Redox: The Dual Role of TiO2 Shell

One-dimensional (1D) CdS@TiO₂ core-shell nanocomposites (CSNs) have been successfully synthesized via a two-step solvothermal method. The structure and properties of 1D CdS@TiO₂ core-shell nanocomposites (CdS@TiO₂ CSNs) have been characterized by a series of techniques, including X-ray diffraction (XRD), ultraviolet-visible-light (UV-vis) diffuse reflectance spectra (DRS), field-emission scanning electron microscopy (FESEM), photoluminescence spectra (PL), and electron spin resonance (ESR) spectroscopy. The results demonstrate that 1D core-shell structure is formed by coating TiO₂ onto the substrate of CdS nanowires (NWs). The visible-light-driven photocatalytic activities of the as-prepared 1D CdS@TiO₂ CSNs are evaluated by selective oxidation of alcohols to aldehydes under mild conditions. Compared to bare CdS NWs, an obvious enhancement of both conversion and yield is achieved over 1D CdS@TiO₂ CSNs, which is ascribed to the prolonged lifetime of photogenerated charge carriers over 1D CdS@TiO₂ CSNs under visible-light irradiation. Furthermore, it is disclosed that the photogenerated holes from CdS core can be stuck by the TiO₂ shell, as evidenced by controlled radical scavenger experiments and efficiently selective reduction of heavy-metal ions, Cr(VI), over 1D CdS@TiO₂ CSNs, which consequently leads to the fact that the reaction mechanism of photocatalytic oxidation of alcohols over 1D CdS@TiO₂ CSNs is apparently different from that over 1D CdS NWs under visible-light irradiation. It is hoped that our work could not only offer useful information on the fabrication of various specific 1D core-shell nanostructures, but also open a new doorway of such 1D core-shell semiconductors as visible-light photocatalysts in the promising field of selective transformations.

A facile and green approach to synthesize Pt@CeO2 nanocomposite with tunable core-shell and yolk-shell structure and its application as a visible light photocatalyst

We report a very facile and green template-free hydrothermal approach to synthesize Pt@CeO2 nanocomposite in an aqueous phase with tunable core-shell and yolk-shell structure. The formation of such core-shell and yolk-shell Pt@CeO2 nanocomposites can be reasonably explained by a novel mechanism of combined synergy interaction of heterogeneous seeded growth process and Ostwald ripening process, which is distinctly different from the well-known physical phenomena, such as nanoscale Kirkendall effect, Ostwald ripening and oriented attachment, employed in wet-chemistry fabrications of core/yolk-shell inorganic nanostructures. Interestingly, we show that, using selective oxidation of benzyl alcohol as a probe reaction at room temperature and ambient pressure, the incorporation of Pt into the shell of semiconductor CeO2 can remarkably enhance the photocatalytic performance of CeO2 for selective oxidation of alcohol. This represents a first example on the application of metal core@semiconductor-oxide shell nanostructured composite materials as visible-light-driven photocatalyst to selective oxidation reactions. Therefore, our findings could not only offer a useful direction on scale-up fabrication of other metal-oxide-coated noble metal nanocomposites with tunable core/yolk-shell structure, but also point to promising vistas of such metal core@oxide shell semiconductor nanocomposites as a novel class of materials platform as visible-light-driven photocatalyst in selective organic transformations.

Identification of Bi2WO6 as a highly selective visible-light photocatalyst toward oxidation of glycerol to dihydroxyacetone in water

Glycerol, being either a primary by-product of biodiesel manufacture or a platform molecule from sugars, is of significant interest as a renewable biomass because it is a highly functionalized and versatile organic building block for the synthesis of value-added fine chemicals. In particular, selective oxidation of glycerol to various industrially valuable products by heterogeneous photocatalysis using solar light as free energy and molecular oxygen as benign oxidant under ambient conditions is extremely attractive. However, a highly selective, heterogeneous visible-light photocatalyst utilized for aerobic oxidation of glycerol has been unavailable. To date, the discovery or design of a visible-light-driven, highly selective photocatalyst for selective oxidation of glycerol to a specific product is particularly challenging in heterogeneous photocatalytic selective transformation. Herein, we for the first time, report the identification of flower-like Bi2WO6 as a highly selective visible-light photocatalyst toward aerobic selective oxidation of glycerol to dihydroxyacetone using oxygen as oxidant in water at room temperature and atmospheric pressure. A rationale for the observed high selectivity over photocatalyst flower-like Bi2WO6 is provided.

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.