Composition and Microstructure of Cobalt Oxide Thin Films Obtained from a Novel Cobalt(II) Precursor by Chemical Vapor Deposition Davide Barreca and Cristian Massignan Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Universita ` di Padova, Via Loredan 4, I-35131 Padova, Italy Sergio Daolio and Monica Fabrizio Istituto di Polarografia ed Elettrochimica Preparativa, CNR, Corso Stati Uniti 4, I-35020 Padova, Italy Clara Piccirillo High Vacuum Process, Via Azzoni 3b, I-43100 Parma, Italy Lidia Armelao and Eugenio Tondello* Centro di Studio sulla Stabilita ` e Reattivita ` dei Composti di Coordinazione del CNR, Via Marzolo 1, I-35131 Padova, Italy Received March 20, 2000. Revised Manuscript Received October 31, 2000 The present work reports the synthesis and the characterization of cobalt oxide thin films obtained by chemical vapor deposition (CVD) on indium tin oxide (ITO) substrates, using a cobalt(II) -diketonate as precursor. The complex is characterized by electron impact mass spectrometry (EI-MS) and thermal analysis in order to investigate its decomposition pattern. The depositions are carried out in a cold wall reactor in the temperature range 350-500 °C at different oxygen pressures, to tailor film composition from CoO to Co 3 O 4 . The crystalline nanostructure is evidenced by X-ray diffraction (XRD), while the surface and in-depth chemical composition is studied by X-ray photoelectron (XPS) and X-ray excited auger electron spectroscopy (XE-AES). Atomic force microscopy (AFM) is employed to analyze the surface morphology of the films and its dependence on the synthesis conditions. Relevant results concerning the control of composition and microstructure of Co-O thin films are presented and discussed. Introduction Materials based on cobalt oxides have attracted a great interest in view of their potential applications in scientific and technological fields. 1-3 Co 3 O 4 , the most stable phase in the Co-O system, is a mixed valence compound [Co II Co III 2 O 4 ] with a normal spinel structure. Cobalt oxide-based materials have received attention for the production of solid-state sensors 4,5 and heteroge- neous catalysts 6-8 and as intercalation compounds for energy storage. 9-11 One of their most important applications is the preparation of electrochromic devices (ECDs) 12 exploit- ing the reversible changes of their optical properties under an external electrical stimulus. 13 Co 3 O 4 has been used as an active material for its color change from brown to light yellow when Li + ions are inserted; 14 the corresponding electrochromic efficiency is relatively high (≈25 cm 2 ‚C -1 ). (The electrochromic efficiency is defined as ΔOD(λ)/Q, where Q is the injected charge (C·cm -2 ) producing the absorbance variation ΔOD for a given λ). Besides, cobalt monoxide is often used as a counter electrode because of its anodic coloration proper- ties. 12,15,16 * Corresponding author: E-mail: tondello@chin.unipd.it. Phone: +39-0498275220. Fax: +39-0498275161. (1) Jime ´nez, V. M.; Ferna ´ ndez, A.; Espino ´s, J. P.; Gonza ´ lez-Elipe, A. R. J. Electron Spectrosc. Relat. Phenom. 1995, 71, 65. (2) Hamada, H.; Haneda, M.; Kakuta, N.; Miura, H.; Inomi, K.; Nanba, T.; Qi Hua, W.; Veno, A.; Ohfune, H.; Udagawa, Y. Chem. Lett. 1997, 887. (3) Tanaka, M.; Mukaia, M.; Fujimuri, Y.; Kondoh, M.; Tasaka, Y.; Baba, H.; Usami, S. Thin Solid Films 1996, 281-282, 453. (4) Ando, M.; Kobayashi, T.; Iijima, S.; Harita, M. J. Mater. Chem. 1997, 7, 1779. (5) Yamaura, H.; Tamaki, J.; Moriya, K.; Miura, N.; Yamazoe, N. J. Electrochem. Soc. 1997, 144, L158. (6) Nkeng, P.; Koening, J.; Gautier, J.; Chartier, P.; Poillerat, G. J. Electroanal. Chem. 1996, 402, 81. (7) Weichel, S.; Møller, P. J. Surf. Sci. 1998, 399, 219. (8) Okamoto, Y.; Imanaka, T.; Teranishi, S. J. Catal. 1980, 65, 448. (9) Ramachandram, K.; Oriakhi, C. O.; Lerner, M. M.; Koch, V. R. Mater. Res. Bull. 1996, 31, 767. (10) Hutchins, M. G.; Wright, P. J.; Grebenik, P. D. Solar Energy Mater. 1987, 16, 113. (11) Barrera, E.; Gonzales, I.; Viveros, T. Sol. Energy Mater. Sol. Cells 1998, 51, 69. (12) Svegl, F.; Orel, B.; Hutchins, M. G.; Kalcher, K. J. Electrochem. Soc. 1996, 143, 1532. (13) Granquist, C. G. In Handbook of Inorganic Electrochromic Materials; Elsevier Science: Amsterdam, 1995. (14) Maruyama, T.; Arai, S. J. Electrochem. Soc. 1996, 143, 1383. 588 Chem. Mater. 2001, 13, 588-593 10.1021/cm001041x CCC: $20.00 © 2001 American Chemical Society Published on Web 01/17/2001