Synthesis, Mechanism, and Gas-Sensing Application of Surfactant Tailored Tungsten Oxide Nanostructures By Suman Pokhrel,* Cristian E. Simion, Valentin S. Teodorescu, Nicolae Barsan, and Udo Weimar 1. Introduction Nanostructured materials, especially transition- metal oxides, play a crucial role in future technological applications. Among various transition-metal oxides, WO 3 offers a particularly wide spectrum of useful properties, including high structural flexibility, [1] switchable optical properties, [2] catalytic behavior, [3] electrochromic, [4,5] and gas-sen- sing properties. [6–10] To harvest specific properties from a material, the synthetic strategies play a significant role, and different synthetic routes developed in the last few years are achieving control of crystallite size, shape, and assembly beha- vior. [11–14] In spite of these developments, the preparation of WO 3 nanostructures with reduced dimensionality and the associated mechanistic pathways are not yet widely available. It is known that sol–gel routes using aromatic alcohols and tungsten alkoxides allow some control over particle size and crystallanity, and the use of block copolymers added to an alcohol solution of inorganic metal salts induces mesoscopically ordered metal oxide nano- structures giving rise to a mesoporous assembly. [11,15–17] Though the use of surfac- tants are promising for the soft-chemistry fabrication, enabling synthesis of 1D WO 3 nanostructures, [18–31] not much has been explored with various diversified surfactants. Herein, we report a number of soft-chemistry routes to self-assembled crystalline WO 3 of different morphologies synthesized with structure-directing agents deferoxamine mesylate (DFOM), hexade- cyltrimethylammoniumbromide (CTAB), and poly(alkylene oxide) block copolymer (Pluronic P123) to obtain the various morphologies at low-to- room temperature. The use of structure-directing agents and a metal halide in our synthetic approach allows the preparation of particularly promising nanomaterials for gas-sensing applica- tions. We report exploitation of these nanomaterials as toxic gas sensors particularly for NO 2 and CO in application-relevant concentration ranges. 2. Materials Characterization 2.1. Powder X-ray Diffraction The powder X-ray diffraction (XRD) spectra of as-prepared powders and heated samples are presented in Figure 1. A FULL PAPER www.afm-journal.de [*] Dr. S. Pokhrel, N. Barsan, U. Weimar Institute of Physical Chemistry, Tu¨bingen University Auf der Morgenstelle 15, 72076 Tu¨bingen (Germany) C. E. Simion, Dr. V. S. Teodorescu National Institute of R & D for Material Physics 77125, Bucharest-Magurele (Romania) DOI: 10.1002/adfm.200801171 Widely applicable nonaqueous solution routes have been employed for the syntheses of crystalline nanostructured tungsten oxide particles from a tungsten hexachloride precursor. Here, a systematic study on the crystallization and assembly behavior of tungsten oxide products made by using the bioligand deferoxamine mesylate (DFOM) (product I), the two chelating ligands hexadecyltrimethylammoniumbromide (CTAB) (II) and poly(alkylene oxide) block copolymer (Pluronic P123) (III) is presented. The mechanistic pathways for the material synthesis are also discussed in detail. The tungsten oxide nanomaterials and reaction solutions are characterized by Fourier transform IR, 1 H, and 13 C NMR spectroscopies, powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, and selected-area electron diffraction. The indexing of the line pattern suggests WO 3 is in its monoclinic structure with a ¼ 0.7297 nm, b ¼ 0.7539 nm, c ¼ 0.7688 nm, and b ¼ 90.91 8. The nanoparticles formed have various architectures, such as chromosomal shapes (product I) and slates (II), which are quite different from the mesoporous one (III) that has internal pores or mesopores ranging from 5 to 15 nm. The nanoparticles obtained from all the synthetic procedures are in the range of 40–60nm. The investigation of the gas-sensing properties of these materials indicate that all the sensors have good baseline stability and the sensors fabricated from material III present very different response kinetics and different CO detection properties. The possibility of adjusting the morphology and by that tuning the gas-sensing properties makes the preparation strategies used interesting candidates for fabricating gas-sensing materials. Adv. Funct. Mater. 2009, 19, 1767–1774 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1767