Stability and Controlled Composition of Hexagonal WO 3 Imre Miklo ´s Szila ´gyi,* ,† Ja ´nos Madara ´sz, ‡ Gyo ¨rgy Pokol, ‡ Pe ´ter Kira ´ly, § Ga ´bor Ta ´rka ´nyi, § Sami Saukko, | Ja ´nos Mizsei, ⊥ Attila L. To ´th, # Andra ´s Szabo ´, ∇ and Katalin Varga-Josepovits O Materials Structure and Modeling Research Group of the Hungarian Academy of Sciences and Department of Inorganic and Analytical Chemistry, Budapest UniVersity of Technology and Economics, H-1111 Budapest, Szt. Gelle ´rt te ´r 4, Hungary, Institute of Structural Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, H-1025 Budapest, Pusztaszeri u ´t 59-67, Hungary, Microelectronics and Materials Physics Laboratories, UniVersity of Oulu, FIN-90014 Oulu, Finland, Department of Electron DeVices, Budapest UniVersity of Technology and Economics, Goldmann Gy. te ´r 3, Budapest H-1521, Hungary, Research Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences, H-1121 Konkoly-Thege u ´t 29-33, Budapest, Hungary, and Department of Organic Chemistry and Technology and Department of Atomic Physics, Budapest UniVersity of Technology and Economics, H-1111 Budapest, Budafoki u ´t 8, Hungary ReceiVed NoVember 5, 2007. ReVised Manuscript ReceiVed April 11, 2008 This paper discusses the formation of nanosized hexagonal tungsten oxide (h-WO 3 ) during the annealing of hexagonal ammonium tungsten bronze (HATB), (NH 4 ) 0.33-x WO 3-y . This process was investigated by TG/DTA-MS, XRD, SEM, Raman, XPS, and 1 H-MAS NMR analyses. Through adjusting the temperature and atmosphere of annealing HATB, the composition (W oxidation state, residual NH 4 + and NH 3 content) of h-WO 3 could be controlled. The effect of composition on the conductivity and gas sensitivity of h-WO 3 was studied. New structural information was obtained about both HATB and h-WO 3 . It was found that NH 4 + and NH 3 could be situated at three different positions in HATB. Residual NH 4 + and NH 3 in the hexagonal channels seemed to be vital for stabilizing h-WO 3 : when they were completely released, the hexagonal framework collapsed. We propose that the structure of h-WO 3 cannot be maintained without some stabilizing ions or molecules in the hexagonal channels. Introduction Tungsten oxides have attracted much attention in the past few decades owing to their promising physical and chemical properties. As a wide bandgap n-type semiconductor, WO 3 , is a potential candidate to be used in photoelectrochemical cells. 1,2 In contrast to TiO 2 , which absorbs only UV light, WO 3 has absorption also in the visible spectrum, and this makes it a widespread material in photocatalysis. 3–6 WO 3 is also well-known as an effective catalyst in several acid- catalyzed reactions (isomerization and olefin polymerization, dehydration, and esterification of alcohols, etc.). 7–9 With its ability to change its color easily, WO 3 is the most re- searched material for chromogenic (electro-, 10–13 photo-, 14,15 gaso-, 16 and thermochromic 17,18 ) devices. Its performance as a gas sensor to various gases (NH 3 , NO 2 ,H 2 S, etc.) is remarkable. 19–22 It also was reported that sodium doped WO 3 proved to be a high temperature superconductor with T c ) 90 K. 23 Among tungsten oxides, hexagonal tungsten trioxide, h-WO 3 , is of particular interest due to its open-tunnel structure and intercalation chemistry. 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