Zi-Rong Tang

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.

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.

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.

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.

Transforming CdS into an efficient visible light photocatalyst for selective oxidation of saturated primary C–H bonds under ambient conditions

Selective activation of saturated sp3 C–H bonds to high-value-added chemicals remains a significant but challenging task for the sustainable exploitation of available feedstocks. However, the selective oxidation of C–H bonds with environmentally benign oxygen is often very difficult to control. Research works available in thermal heterogeneous catalysis often involve the use of transition metal particles together with harsh reaction conditions, e.g., high temperature and high pressure, which results in the difficulty in controlling the selectivity. Here, we report a very simple room temperature method to prepare a cubic phase, sheet structured semiconductor CdS sample. The as-prepared CdS is able to be used as a visible-light-driven photocatalyst for the selective oxidation of saturated primary C–H bonds in alkyl aromatics with high activity and selectivity using molecular oxygen as a benign oxidant and benzotrifluoride as the solvent under ambient conditions, i.e., room temperature and atmospheric pressure. The superior photocatalytic performance of CdS can be attributed to its unique assembly of sheet structure with cubic phase, high surface area and efficient separation of photogenerated charge carriers. The possible reaction mechanism for the photocatalytic selective oxidation of such C–H bonds over the CdS semiconductor has also been proposed.

One-dimensional nanostructure based materials for versatile photocatalytic applications

One-dimensional (1D) nanostructures are believed to play a significant role on the horizon of material science, and are a promising class of ideal high performance candidates for energy storage and conversion owing to their unique optical, structural and electronic properties. In particular, 1D nanostructure-based photocatalysts have been attracting ever-growing research attention. In this review article, we mainly focus on systematically summarizing the applications of 1D-based nanocomposites in photocatalysis, including nonselective processes for the degradation of pollutants, direct solar energy conversion to storable fuels and selective transformations for organic synthesis. Particularly, we explore the new directions for boosting the photocatalytic performances of 1D nanostructures, including graphene-1D nanocomposites, surface modification, 1D core–shell nanostructures and different exposed facet effects. It is hoped that this article will promote the efficient harnessing and rational development of the outstanding structural and electronic properties of 1D nanostructures to design more efficient 1D-based photocatalysts towards numerous applications in the field of solar energy conversion.

Recent progress in carbon quantum dots: synthesis, properties and applications in photocatalysis

Carbon quantum dots (CQDs) as a rising star of carbon nanomaterials, by virtue of their unique physicochemical, optical and electronic properties, have displayed tremendous momentum in numerous fields such as biosensing, bioimaging, drug delivery, optoelectronics, photovoltaics and photocatalysis. In particular, the rich optical and electronic properties of CQDs including efficient light harvesting, tunable photoluminescence (PL), extraordinary up-converted photoluminescence (UCPL) and outstanding photoinduced electron transfer have attracted considerable interest in different photocatalytic applications for the sake of full utilization of the solar spectrum. This review aims to demonstrate the recent progress in the synthesis, properties and photocatalytic applications of CQDs, particularly highlighting the fundamental multifaceted roles of CQDs in photoredox processes. Furthermore, we discuss the challenges and future direction of CQD-based materials in this booming research field, with a perspective toward the ultimate achievement of highly efficient and long-term stable CQD-based photocatalysts.

Two-Dimensional MoS2 Nanosheet-Coated Bi2S3 Discoids: Synthesis, Formation Mechanism, and Photocatalytic Application

Myriad materials with desirable functional property resulting from their unique structures ignite enormous interest in synthesizing materials with controlled structural morphology toward achieving novel or enhanced properties for target applications. Herein, the novel and unique two-dimensional (2D) MoS2 nanosheet-coated Bi2S3 discoids composites, which feature a Bi2S3-core/MoS2-shell structure, have been elaborated via a facile anion-exchange strategy. Using the MoS2 nanosheets to coat the surface of Bi2S3 discoids boosts the light-harvesting efficiency and charge separation and promotes faster charge transport and collection, thus leading to the higher activity of the photocatalytic reduction of Cr(VI) under visible light irradiation (λ > 400 nm). In particular, the phase evolution and possible formation mechanism of the MoS2-Bi2S3 core-shell structure have been explored by virtue of temperature- and time-dependent experiments. It is anticipated that this work could promote further interest in adopting an anion-exchange strategy to fabricate semiconductor-based composite materials with controlled architectural morphology and enhanced photocatalytic performance toward diverse applications.

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.

A facile and high-yield approach to synthesize one-dimensional CeO2 nanotubes with well-shaped hollow interior as a photocatalyst for degradation of toxic pollutants

A facile, template-free and high-yield synthesis of single-crystalline cerium dioxide nanotubes (CeO2-NT) has been reported via a “casually-modified” approach based on the hydrothermal treatment of Ce(OH)CO3 precursors with alkali solution in an aqueous phase. This simple modification in synthesis procedures not only improves the yield of CeO2-NT remarkably, but also gives rise to the formation of CeO2-NT featuring excellent nanotubular open-ended structure with a well-shaped hollow interior. The collection techniques of BET, UV/visible diffuse reflectance spectra (DRS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis have been employed to characterize the morphology and optical properties of the as-prepared CeO2-NT. Significantly, we demonstrate that CeO2-NT exhibits a markedly enhanced photocatalytic activity and stability as compared with its counterpart of CeO2 nanoparticles and commercial TiO2 (P25) toward the degradation of aromatic benzene, a well-known toxic pollutant that commonly occurs in urban ambient air and is of significant concern regarding environmental health because of its toxic, mutagenic, or carcinogenic properties. This represents a first example to demonstrate the advantage of CeO2 nanotubes as photocatalyst as compared to its counterpart of CeO2 nanoparticles, clearly suggesting the morphology/shape-dependent photocatalytic behaviour of CeO2 materials. Therefore, our current work not only offers a simple approach for fabrication of open-ended CeO2-NT with well-shaped hollow interior, but also demonstrates the promising potential of the applications of CeO2-NT, CeO2-NT-based and other metal oxide nanotube-based materials in the area of photocatalysis, which will inevitably enrich the intriguing chemistry of morphology/shape-dependent heterogeneous photocatalysis and thermal catalysis.