Sensors and Actuators B 96 (2003) 589–595 Optical CO sensitivity of Au–CuO composite film by use of the plasmon absorption change Masanori Ando a,∗ , Tetsuhiko Kobayashi b , Sumio Iijima c , Masatake Haruta d a Photonics Research Institute, AIST, Kansai Center, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan b Special Division for Green Life Technology, AIST, Kansai Center, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan c Research Center for Advanced Carbon Materials, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan d Research Institute for Green Technology, AIST, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Received 31 March 2003; accepted 15 July 2003 Abstract Gold–copper oxide (Au–CuO) composite films were prepared by the sputter-deposition of gold onto a glass plate substrate followed by pyrolysis of spin-coated copper naphthenate. The films comprise small Au particles and small CuO crystals both with diameters around 20nm, and exhibit optical response to CO in air. Carbon monoxide caused large increase in absorbance in the visible-near IR wavelength range. The absorbance change due to CO occurred near the plasmon absorption band of small Au particles with a maximum in the range of 700–800 nm, suggesting that the absorbance by small Au particles in the plasmon band was influenced by the changes in the dielectric constant and refractive index of copper oxide which surrounded small Au particles. The reversible absorbance change and the relatively fast response to CO over a concentration range from 50 to 10,000 ppm (1 vol.%) make the Au–CuO composite films to be useful for optical CO detection. © 2003 Elsevier B.V. All rights reserved. Keywords: Optochemical sensors; Carbon monoxide; Copper oxide; Small gold particles; Composite film; Plasmon absorption 1. Introduction Optical gas sensors have been attracting growing needs owing to several advantages over the conventional elec- tricity-based gas sensors [1,2]. These are high resistivity to electromagnetic noise, fire resistance and the capability of remote control and information transfer through an optical fiber network. Materials whose optical absorption properties are reversibly changed by atmospheric gases and moisture have considerable potential for use as optochemical sensors [1,2]. We have already reported that in the presence of CO the thin films of transition metal oxides, such as Co 3 O 4 , NiO and Mn 3 O 4 with p-type semiconducting property, show re- versible changes in the visible-near IR absorption band at 250–350 ◦ C [3]. The absorbance change was assumed to be caused by a change in positive hole density during catalytic CO oxidation. An appreciable enhancement in the optical gas sensitivity is observed when small Au particles are com- bined with those transition metal oxides. We have found that ∗ Corresponding author. Tel.: +81-72-751-9647; fax: +81-72-751-9637. E-mail address: m-ando@aist.go.jp (M. Ando). there are two types of enhancing effect (types 1 and 2) as follows. 1.1. Type 1: Absorption change of transition metal oxides In the type 1, the gas-sensitive optical absorption change comes simply from the transition metal oxide. The deposi- tion of Au nanoparticles on some kinds of transition metal oxides remarkably enhances the catalytic activity for oxi- dation of flammable gases [4]. Therefore, the gas-sensitive optical absorption change is enhanced by the enhancement of activity in catalytic oxidation of flammable gases. The optical gas sensing performance of type 1 is assumed to be closely related with the catalytic activity of the Au–transition metal oxide composite. 1.2. Type 2: Plasmon absorption change of Au particles In the type 2, the gas-sensitive optical absorption change arises from the plasmon absorption change of small Au par- ticles, and is not related directly with the activity in catalytic oxidation of flammable gases. The state of plasmon reso- nance near the surface of small Au particles are sensitively influenced by the physical properties such as dielectric 0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00645-2