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