Physicochemical characterization of sputtered iridium oxide
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. INTRODUCTION There is great interest in iridium oxide for use as oxygen and chlorine evolution anodes,1"4 electrochromic displays,5'6 pH sensors,7"12 electrodes for neural stimulation,13"15 and high rate, high charge capacitors.16 The properties of the iridium oxide films employed in any of these applications will be sensitive to the structure of the oxide film which, in turn, will be influenced by the method of film preparation. In the present work, the properties of sputtered iridium oxide films prepared on glass and titanium substrates under the same sputtering conditions are characterized by x-ray diffraction, differential scanning calorimetry (DSC), and impedance spectroscopy. A combined thermal, x-ray, and electrochemical characterization reported an exothermic transition at ca. 300 °C for iridium oxide deposited by reactive rf sputtering.1718 This transition was interpreted to be a crystallization of the sputtered film to IrO 2 . After the transition occurred, the sputtered films lost their electrochromic and electrocatalytic properties. In their study, the authors assumed that the properties of the sputtered films are independent of substrate, so they based their conclusions on pooled results obtained from films deposited on quartz, gold, and sapphire. The properties of the sputtered iridium oxide films, as of many thin films, depend both on the nature of the substrate and on the deposition conditions.19 For example, on alumina substrates, higher substrate temperature results in more crystalline films.10 II. EXPERIMENTAL Iridium oxide coatings were deposited by rf sputtering from an iridium target (99.9% Ir, 5 cm diameter, from Metron, in Allamunchy, New Jersey) in an oxygen plasma using a commercial sputtering system (IBEX 2000 from Microscience, Braintree, Massachusetts). The oxygen pressure in the chamber was 100 /xm Hg, and the distance from target to substrate was 4 cm. Deposition times were 96 min for samples intended for DSC and x-ray diffraction 440
http://journals.cambridge.org
J. Mater. Res., Vol. 4, No. 2, Mar/Apr 1989
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measurements, and 16 min for the samples intended for electrochemical measurements. The substrates coated were glass microscope slides (Corning 7059 quality, from Fisher Scientific, in Medford, Massachusetts) and titanium foil (m2N purity, 0.127 mm thickness, from Alfa Products, Danvers, Massachusetts) for x-ray and DSC measurements, and 1.1 mm diameter titanium disks, punched out from the same foil, for electrochemistry. Substrates were cleaned by washing in soapy water, distilled water, and methanol, prior to placement in the sputter chamber. Solutions were prepared with water distilled in an all-glass still. Sulfuric acid was analytical grade (from Fisher Scientific in Fair Lawn, New Jersey), and was used without further purification. The x-ray spectra were recorded in both diffractometer and Debye-Scherrer modes on a Rigaku Geigerflex 2013 (from Rigaku U.S.A., Danvers, Massachusetts) instrument using the Cu K a radiation. Diffraction spectra were rec
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