Ž . Thin Solid Films 323 1998 18–22 Preparation of ceria thin films and microtubes by vapor-phase deposition using NiO as oxygen source Minoru Inaba a, ) , Atsushi Mineshige a , Shinji Nakanishi a , Isao Nishimura b , Akimasa Tasaka b , Kenji Kikuchi c , Zempachi Ogumi a a Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto UniÕersity, Sakyo-ku, Kyoto 606-01, Japan b Department of Molecular Science and Technology, Faculty of Engineering, Doshisha UniÕersity, Tanabe-cho, Tsuzuki-gun, Kyoto 610-03, Japan c Department of Materials Science, The UniÕersity of Shiga Prefecture, Hikone, Shiga 522, Japan Received 20 May 1997; accepted 6 November 1997 Abstract Ž . A modified version of electrochemical vapor deposition EVD using NiO as an oxygen source was applied to the fabrication of CeO 2 thin films and microtubes. Cerium trichloride was used as a metal source, and NiO pellets and surface-oxidized Ni wires of 125 mm diameter as substrates for deposition. Thin films of cubic CeO were formed on both substrates by the vapor-phase reaction at 900 and 2 9508C. Furthermore, ceria microtubes of 130 mm diameter were obtained by soaking the ceria-coated Ni wires in hydrochloric acid. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Electrochemical vapor deposition; Ceria; Thin film; Microtube; Nickel oxide 1. Introduction Oxygen semipermeable membranes separate oxygen se- lectively from air, and the resulting oxygen-enriched air is effectively used to enhance combustion efficiency. While Ž . polymeric membranes such as poly dimethylsiloxane have been mainly studied for this purpose, the use of oxide-ion Ž 2y . O conductors as oxygen semipermeable membranes at wx elevated temperatures will also be promising 1 . Oxide-ion conductors exhibit absolute selectivity for oxygen, whereas it is not the case for polymeric membranes. Furthermore, they offer more chemical, thermal and structural stability w x than polymeric ones 1,2 . Ž . Yttria-stabilized zirconia YSZ is a well-known pure oxide-ionic conductor and has been studied for use as wx oxygen semipermeable membranes 1 . YSZ does not have an appreciable electronic conductivity, and hence, addi- tional electrodes and an external electrical circuit are nec- essary to carry electrons produced and consumed by oxy- gen evolution and ionization, respectively. In addition, various polarization effects may critically control the over- wx all performance 1 . On the other hand, so-called mixed conductors, which have both oxide-ionic and electronic ) Corresponding author. conductivities, such as CeO and ABO -type perovskite 2 3 oxides are more suitable for use as oxygen semipermeable membranes because they have paths for both oxide-ions and electrons and thereby, do not need additional elec- w x trodes and an external electrical circuit 1,3–6 . Pure ceria is one of such promising mixed conductors. It is stable over a wide range of temperature or oxygen partial pres- sure, and its electronic conductivity and oxide-ionic trans- ference number of CeO are about 10 y3 Scm y1 and 0.4, 2 w x respectively, at 9008C in air 7–10 . Among several techniques available for preparing thin and dense films of oxide-ionic conducting materials such Ž . as YSZ, electrochemical vapor deposition EVD has been w x demonstrated to be a very promising technique 5,11–16 . We recently developed a technique to prepare thin films of oxide-ionic conductors by a modified EVD method, in which we used nickel oxide as an oxygen source instead of w x gaseous oxygen sources 17–19 . Our method has an ad- vantage that thin films can be prepared on NiO substrates of desired shapes, and we have applied this method to the fabrication of YSZ microtubes of ca. 100 m m diameter w x using surface-oxidized Ni wires as substrates 20,21 . The morphology gives a large surfacervolume ratio, and thereby, microtubes are very attractive as electrolytes in various electrochemical devices. As mentioned earlier, ce- 0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.