Guangcheng Xi

Ultrathin W18O49 Nanowires with Diameters below 1 nm: Synthesis, Near‐Infrared Absorption, Photoluminescence, and Photochemical Reduction of Carbon Dioxide

Inorganic nanowires with ultrathin diameters below the magic size (i.e., less than 2 nm) and even one unit cell size, have attracted much research attention in the past few years owing to their unique chemical and physical properties. As an important semiconductor material, tungsten oxide (WO3 x) nanowires and nanorods have attracted considerable attention because of their wide applications in gas sensors, electrochromic windows, optical devices, and photocatalysts. In particular, monoclinic W18O49 is of great interest owing to its unusual defect structure and promising properties in the nanometer regime. Early on, Park and co-workers reported the synthesis of W18O49 nanorods with a diameter of 4 nm by decomposing [W(CO)6] in Me3NO2·2 H2O and oleylamine. [16] Subsequently, Niederberger and co-workers synthesized hybrid W18O49/ organic nanowires with a very thin diameter of 1.3 nm by a bioligand-assisted method. Recently, Tremel and coworkers prepared W18O49 nanorods with a diameter of 2 nm by decomposing tungsten ethoxide in a mixture of oleic acid and trioctyl amine. Although good control over nanocrystal dimensions can be realized in these methods, removal of the surfactants or organic residues from the nanowire surface requires multiple washing steps. For fundamental investigations on the ultrathin oxide nanowire itself, as well as for technological applications (such as sensing and catalysis), the presence of residues on the nanowire surface from the synthesis may be a significant drawback. Herein, we report the preparation of ultrathin W18O49 nanowires that are efficient in the photochemical reduction of carbon dioxide by visible light. The ultrathin W18O49 nanowires were prepared by a very simple one-pot solution-phase method (see the experimental section in the Supporting Information). In a typical procedure, WCl6 was dissolved in ethanol, and the clear yellow solution was transferred to a teflon-lined stainless-steel autoclave and heated at 180 8C for 24 h. A blue flocculent precipitate was collected, washed, dried in air, and obtained in a yield of approximately 100 %. The product is insoluble in water and in acid (HCl, pH 0), and has a high specific surface area. W18O49 is a monoclinic structure type (P2 m) with lattice parameters of a = 18.318, b = 3.782, and c = 14.028 . Monoclinic W18O49 has a distorted ReO3 structure in which cornersharing distorted and tilt WO6 octahedra are connected in the a-, b-, and c-direction, thereby forming a three-dimensional structure (inset in Figure 1a). The X-ray diffraction (XRD) pattern of our sample demonstrates that the sample consists of monoclinic-phase W18O49 (Figure 1 a). The narrow (010) and (020) peaks strongly suggest that the possible crystal growth direction of the sample is [010], since the close-packed planes of the monoclinic W18O49 crystal are {010}, which will be further demonstrated by the direct observation of the highresolution transmission electron microscopy (HRTEM) image (see below). Energy-dispersive X-ray spectroscopy (EDS) confirms that the sample only contains W and O elements (Figure 1b). Furthermore, the Fourier transform infrared (FTIR) spectrum exhibits the clear surface of our sample (Figure S1 in the Supporting Information). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the as-synthesized sample is composed of nanowires with large aspect ratio and lengths of up to several micrometers (Figure 1c, d). Interestingly, higher-magnification TEM images (Figure 1e and Figures S2 and S3 in the Supporting Information) clearly reveal that the nanowires shown in Figure 1d are composed of a lot of individual, thinner nanowires. The diameter of the thinner nanowires is only about 0.9 nm. The HRTEM image and corresponding fast Fourier transform (FFT) pattern demonstrate that the ultrathin nanowires are crystalline and grow along [010] direction (Figure 1 f and Figure S4 in the Supporting Information). A comparison of the unit cell of W18O49 projected along the [010] direction with the typical diameter of the nanowires of 0.9 nm (red circle) allows [*] Dr. G. C. Xi, Dr. S. X. Ouyang, P. Li, Prof. J. H. Ye International Center for Materials Nanoarchitectonics (WPIMANA), and Environmental Remediation Materials Unit National Institute for Materials Science (NIMS) 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan) E-mail: jinhua.ye@nims.go.jp

In Situ Growth of Metal Particles on 3D Urchin-like WO3 Nanostructures

Metal/semiconductor hybrid materials of various sizes and morphologies have many applications in areas such as catalysis and sensing. Various organic agents are necessary to stabilize metal nanoparticles during synthesis, which leads to a layer of organic compounds present at the interfaces between the metal particles and the semiconductor supports. Generally, high-temperature oxidative treatment is used to remove the organics, which can extensively change the size and morphology of the particles, in turn altering their activity. Here we report a facile method for direct growth of noble-metal particles on WO(3) through an in situ redox reaction between weakly reductive WO(2.72) and oxidative metal salts in aqueous solution. This synthetic strategy has the advantages that it takes place in one step and requires no foreign reducing agents, stabilizing agents, or pretreatment of the precursors, making it a practical method for the controlled synthesis of metal/semiconductor hybrid nanomaterials. This synthetic method may open up a new way to develop metal-nanoparticle-loaded semiconductor composites.

Fe3O4/WO3 Hierarchical Core–Shell Structure: High‐Performance and Recyclable Visible‐Light Photocatalysis

A facile solvothermal epitaxial growth combined with a mild oxidation route has been developed for the fabrication of a magnetically recyclable Fe(3)O(4)/WO(3) core-shell visible-light photocatalyst. In this core-shell structured photocatalyst, visible-light-active WO(3) nanoplates (the shells) with high surface area are used as a medium to harvest absorbed photons and convert them to photogenerated charges, while conductive Fe(3)O(4) microspheres (the cores) are used as charge collectors to transport the photogenerated charges. This is a new role for magnetite. The Fe(3)O(4)/WO(3) core-shell structured photocatalysts possess large surface-exposure area, high visible-light-absorption efficiency, stable recyclability, and efficient charge-separation properties, the combination of which has rarely been reported in other visible-light-active photocatalysts. Photoelectrochemical investigations verify that the core-shell structured Fe(3)O(4)/WO(3) has a more effective photoconversion capability than pure WO(3) or Fe(3)O(4). At the same time, the visible-light photocatalytic ability of the Fe(3)O(4)/WO(3) photocatalyst has significantly enhanced activity in the photodegradation of organic-dye materials. The results presented herein provide new insights into core-shell materials as high-performance visible-light photocatalysts and their potential use in environmental protection.

Ultrathin SnO2 Nanorods: Template- and Surfactant-Free Solution Phase Synthesis, Growth Mechanism, Optical, Gas-Sensing, and Surface Adsorption Properties

A novel template- and surfactant-free low temperature solution-phase method has been successfully developed for the controlled synthesis of ultrathin SnO(2) single-crystalline nanorods for the first time. The ultrathin SnO(2) single-crystalline nanorods are 2.0 +/- 0.5 nm in diameter, which is smaller than its exciton Bohr radius. The ultrathin SnO(2) nanorods show a high specific area (191.5 m(2) g(-1)). Such a thin SnO(2) single-crystalline nanorod is new in the family of SnO(2) nanostrucures and presents a strong quantum confinement effect. Its formation depends on the reaction temperature as well as on the concentration of the urea solution. A nonclassical crystallization process, Ostwald ripening process followed by an oriented attachment mechanism, is proposed based on the detailed observations from a time-dependent crystal evolution process. Importantly, such structured SnO(2) has shown a strong structure-induced enhancement of gas-sensing properties and has exhibited greatly enhanced gas-sensing property for the detection of ethanol than that of other structured SnO(2), such as the powders of nanobelts and microrods. Moreover, these ultrathin SnO(2) nanorods exhibit excellent ability to remove organic pollutant in wastewater by enormous surface adsorption. These properties are mainly attributed to its higher surface-to-volume ratio and ultrathin diameter. This work provides a novel low temperature, green, and inexpensive pathway to the synthesis of ultrathin nanorods, offering a new material form for sensors, solar cells, catalysts, water treatments, and other applications.

Synthesis of Multiple-Shell WO3 Hollow Spheres by a Binary Carbonaceous Template Route and Their Applications in Visible-Light Photocatalysis

Hollow go lightly: well-defined multiple-shell WO(3) hollow spheres were synthesized by a facile binary carbonaceous spheres template route. Compared with single-shell WO(3) hollow spheres, the unusual porous multiple-shell structure of the WO(3) hollow spheres proves to greatly enhance photocatalytic activity toward degradation of organic pollutants under visible-light irradiation.

General synthesis of hybrid TiO2 mesoporous "french fries" toward improved photocatalytic conversion of CO2 into hydrocarbon fuel: a case of TiO2/ZnO.

Ever since the first report by Fujishima and Honda in 1972, TiO2-based photocatalysts have attracted more and more attention owing to their potential applications in the fields of energy and environment, such as water splitting, pollutant degradation, self-cleanness, dye-sensitized solar cells (DSSCs), and CO2 reduction. [6] To achieve high solar energy conversion efficiency, many interesting TiO2 structures have been developed, such as nanotubes, heterojunctions, sheets with a highly reactive surface, hierarchical structures, and hollow spheres. Mesoporous photocatalysts are especially attractive as heterogeneous catalysts owing to interconnected pore systems and high specific surface area. 13] On the other hand, photocatalysts with hybrid composite have also attracted more and more interest because of their improved photogenerated charge separation and enhanced photocatalytic activity. Therefore, it can be expected that high-performance photocatalysts may be obtained when hybrid composition and mesoporous structure are combined together to create synergistic structural/electronic effects. However, there are three important and difficult requirements in the preparation of homogeneously hybrid mesoporous structures: 1) each component in the products should be homogenously distributed; 2) macroscopic phase separation should be avoided in the formation process of hybrid crystalline pore walls; and 3) the synthesized hybrid mesoporous structures must have both high surface area and high crystallinity, which is often incompatible in one synthesis. In addition, to obtain a highly crystalline mesoporous material, high-temperature heat treatment is usually required for crystallization of the product. However, this process probably induces collapse of mesostructures. Due to these synthetic difficulties, it is still a big challenge to explore alternative general and high-yielding routes for the fabrication of well defined homogeneously hybrid mesostructures. Herein, we report a general, high-yielding, and reproducible route to synthesize well-defined homogeneously hybrid TiO2 mesoporous “french fries” (MFFs) with high specific surface area, large pore volume, and nanojunction modified pore walls by a facile furfural alcohol-derived polymerization–oxidation route (FAPO). Six types of hybrid TiO2 MFFs (TiO2/ZnO, TiO2/Fe2O3, TiO2/CuO, TiO2/NiO, TiO2/ Cr2O3, and TiO2/CeO2) are obtained by this general method. The homogeneously hybrid TiO2/ZnO MFFs prepared by the FAPO method exhibited a much higher photocatalytic activity of CO2 reduction owing to their unusual hybrid mesoporous structure and high surface area compared to TiO2/ ZnO particles prepared by solid state reaction (SSR-TiO2/ ZnO) and traditional P25 nanopowders. The rate of CO2 to hydrocarbon fuel (CH4) production obtained in the hybrid TiO2/ZnO MFFs is at least fifty and six times higher than those obtained in the SSR-TiO2/ZnO and commercial P25, respectively. Scheme 1 outlines the typical synthesis of homogeneously hybrid TiO2/ZnO MFFs. In the case of synthesis of TiO2/ ZnO MFFs, ZnCl2, titanium tetraisopropoxide (TPT), poly(propylene glycol)-block-poly(ethylene glycol) (P123), concentrated HNO3, absolute ethanol, and furfural alcohol (FA) were mixed together to form a claret transparent solution at room temperature. The high transparency of the pre-

Large-scale, ultrathin and (001) facet exposed TiO2 nanosheet superstructures and their applications in photocatalysis

Large-scale, ultrathin, and (001) facet exposed anatase TiO2 nanosheet superstructures were synthesized by a facile one-step solvothermal method. The length and width of the nanosheets was as high as 1.5–2 μm, while the thickness of the nanosheets was only about 2–4 nm. The anatase TiO2 nanosheet superstructures have superior photocatalytic performance.

W18O49 nanowire networks for catalyzed dehydration of isopropyl alcohol to propylene under visible light

We report the facile one-pot synthesis of self-organized W18O49 networks. The W18O49 networks show strong localized surface plasmon resonance. Interestingly, the W18O49 networks show unexpected capability in photocatalytic dehydration of isopropyl alcohol to propylene depending on their defect structure caused by large quantities of oxygen vacancy.

A metallic molybdenum dioxide with high stability for surface enhanced Raman spectroscopy

Compared with noble metals, semiconductors with surface plasmon resonance effect are another type of SERS substrate materials. The main obstacles so far are that the semiconducting materials are often unstable and easy to be further oxidized or decomposed by laser irradiating or contacting with corrosive substances. Here, we report that metallic MoO2 can be used as a SERS substrate to detect trace amounts of highly risk chemicals including bisphenol A (BPA), dichloropheno (DCP), pentachlorophenol (PCP) and so on. The minimum detectable concentration was 10−7 M and the maximum enhancement factor is up to 3.75 × 106. To the best of our knowledge, it may be the best among the metal oxides and even reaches or approaches to Au/Ag. The MoO2 shows an unexpected high oxidation resistance, which can even withstand 300 °C in air without further oxidation. The MoO2 material also can resist long etching of strong acid and alkali.

Large-Scale, Three–Dimensional, Free–Standing, and Mesoporous Metal Oxide Networks for High–Performance Photocatalysis

Mesoporous nanostructures represent a unique class of photocatalysts with many applications, including splitting of water, degradation of organic contaminants, and reduction of carbon dioxide. In this work, we report a general Lewis acid catalytic template route for the high–yield producing single– and multi–component large–scale three–dimensional (3D) mesoporous metal oxide networks. The large-scale 3D mesoporous metal oxide networks possess large macroscopic scale (millimeter–sized) and mesoporous nanostructure with huge pore volume and large surface exposure area. This method also can be used for the synthesis of large–scale 3D macro/mesoporous hierarchical porous materials and noble metal nanoparticles loaded 3D mesoporous networks. Photocatalytic degradation of Azo dyes demonstrated that the large–scale 3D mesoporous metal oxide networks enable high photocatalytic activity. The present synthetic method can serve as the new design concept for functional 3D mesoporous nanomaterials.