RAPID COMMUNICATION Facile synthesis of nanocrystalline hexagonal tungsten trioxide from metallic tungsten powder and hydrogen peroxide Isao Tsuyumoto Department of Applied Chemistry, College of Bioscience and Chemistry, Kanazawa Institute of Technology, Nonoichi, Ishikawa, Japan Correspondence Isao Tsuyumoto, Department of Applied Chemistry, College of Bioscience and Chemistry, Kanazawa Institute of Technology, Ishikawa, Japan. Email: tsuyu@neptune.kanazawa-it.ac.jp Funding information Grant-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science (JSPS), Grant/Award Number: No. 16K05946 Abstract A hexagonal form of tungsten trioxide (h-WO 3 , particle size: 15.9-57.1 nm) was found to be formed by a direct reaction between metallic tungsten powder (W, particle size: 0.45-0.59 lm) and 15%-30% hydrogen peroxide (H 2 O 2 ) aq solution. Oxide film on the powder surface having the similar crystal structure as h-WO 3 was essential for the formation, and the surface oxide film was formed by aging the powder in air at 45°C, a relative humidity of 100% (P H2O 96 hPa) for 3-28 days or in ambient atmosphere at room temperature for 12 years. The Riet- veld analysis performed in the space group P6 3 /mcm (Z = 6) indicated the crystal structures were the same as those of the reported h-WO 3 and that the crystallo- graphic characteristic was as follows: a = 0.74219 nm, c = 0.77198 nm for h-WO 3 from the 28-day aged powder, and a = 0.74538 nm, c = 0.77194 nm for h-WO 3 from the 12-year aged powder. KEYWORDS nanoparticles, oxides, synthesis, tungsten/tungsten compounds, X-ray methods 1 | INTRODUCTION Tungsten trioxide, WO 3 , has received much attention for a wide variety of applications such as photocatalyst in water splitting, 1-8 dye-sensitized solar cells, 9 electrode materials for lithium-ion batteries, 10 photochromic films, 11 and elec- trochromic displays. 12,13 Many methods have been reported for the preparation of WO 3 nanoparticles as well as their properties and applications. 14 Tungsten trioxide crystallizes in several polymorphic forms, which can be categorized into two general crystal structures. One is based on the ReO 3 -type structure, i.e., WO 6 octahedra link together by corner sharing and form a three-dimensional framework. In this case, distortions of the octahedra bring about lower symmetries than ReO 3 , and these polymorphs have been reported to be monoclinic phase (II) from 5 to 278 K, tri- clinic phase from 248 to 290-300 K, monoclinic phase (I) (c-phase) from 290-300 K to 600 K, orthorhombic phase from 600 to 1010 K, and tetragonal phase from 1010 K to the melting point, 1746 K. 15 The other general crystal structure is a form with hexagonal symmetry first reported by Gerand et al, 16 i.e., WO 6 octahedra share their corners to form a framework with hexagonal channels along c-axis. This hexagonal WO 3 (h-WO 3 ) is a metastable phase, which has not been described in phase diagrams, and its synthetic routes are limited to low-temperature processes because high temperature causes transformation to the stable mono- clinic phase (I). 17 The h-WO 3 was first prepared by dehy- dration of WO 3 Á1/3H 2 O, and this hydrate was obtained by hydrothermal treatment of an aqueous suspension of either tungstic acid gel or WO 3 Á2H 2 O at 120°C. 16 The h-WO 3 has been also prepared by other methods such as hydrogen peroxide oxidation of hexagonal ammonium tungsten bronze, 18 and thermal decomposition of ammonium per- oxo-polytungstate. 17 These preparation processes of h-WO 3 require precise control of experimental conditions and are not suitable for inexpensive large-scale production. As the h-WO 3 as well as the monoclinic phase (I) has attracted a lot of attention as an alternative material for photocatalyst, 3 a facile and inexpensive preparation method of h-WO 3 is highly desirable. Our research group has been investigating a soft-chemical synthesis of metal oxides starting from a Received: 30 May 2017 | Accepted: 20 September 2017 DOI: 10.1111/jace.15250 J Am Ceram Soc. 2018;101:509–514. wileyonlinelibrary.com/journal/jace © 2017 The American Ceramic Society | 509