Available online at www.sciencedirect.com Sensors and Actuators B 130 (2008) 531–537 Surface plasmon resonance optical gas sensing of nanostructured ZnO films C. de Juli´ an Fern´ andez a,∗ , M.G. Manera b , G. Pellegrini a , M. Bersani a , G. Mattei a , R. Rella b , L. Vasanelli c , P. Mazzoldi a a Department of Physics, University of Padova, via Marzolo 8, 35131 Padova, Italy b IMM-CNR, Lecce, Campus Universitario, via Monteroni, 73100 Lecce, Italy c Innovation Engineering Department, University of Lecce, via per Arnesano, 73100 Lecce, Italy Available online 29 September 2007 Abstract The structural and optical gas sensing properties of thin nanostructured ZnO films are investigated by total attenuation surface plasmon resonance technique. Ten nanometers thick ZnO films have been prepared by sol–gel route and thermally treated at 150 ◦ C and 400 ◦ C. Grazing incidence X-ray diffraction studies showed that the thermal treatment promotes the increase of grain size as the annealing temperature increases. The optical properties of both films are similar being the energy gap of the films annealed at 150 ◦ C and 400 ◦ C equal to 3.33 ± 0.05 eV and 3.25 ± 0.05 eV, respectively, both smaller than the energy gap of the bulk ZnO (3.37eV). The surface plasmon resonance investigations show remarkable and reversible responses to different concentrations of methanol, ethanol, isopropanol and hexane vapours in dry-air. In both films the responses to ethanol and isopropanol are similar and larger than the responses to hexane and methanol. The responses to all vapours are larger in the film annealed at 400 ◦ C except for the methanol for which both films exhibit similar responses. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnO; Nanostructured films; Surface plasmon resonance; Optical gas sensors 1. Introduction The research on nanostructured semiconductors oxides is of emerging interest for gas sensing devices. These well-standing gas sensing materials prepared with grain sizes from some few to 100 nm and in thin films form exhibit new promising features in this applicative sector. Both chemical and physical properties of these materials are different from those found in the bulk ones because the grain sizes are smaller than the characteristic lengths associated with the physical properties and because their features are dominated by the properties of the interfaces distinctive of the nanomaterials. However a typical problem of these materials is that, due to the small amount of matter that are formed, the mea- sure of their sensing properties requires much larger amounts (e.g., film thickness). This produces that the nanometric-scaled ∗ Corresponding author. Present address: INSTM-Department of Chemistry, University of Florence, Sesto Fiorentino, Italy. Tel.: +39 0554573225; fax: +39 0554573372. E-mail address: cesar.dejulian@unifi.it (C. de Juli´ an Fern´ andez). properties are partially lost or that the diffusion of analyte in the material is limited. One possible solution is to increase the porosity of the films. In this work we have investigated the optical sensing prop- erties of ZnO films with only few nanometers thick. Different studies in the literatures [1,2] demonstrate that nanostructured ZnO films shows VIS photoemission, optical and transport prop- erties that are interesting for sensing applications but also for photovoltaic cells and optoelectronic devices. Several studies have shown that ZnO nanostructured thick films, nanoparticles and nanowires exhibit sensing response to volatile organic com- pounds (VOCs), CO, NO 2 ,H 2 , NH 3 ,O 3 gases and can be used as pH sensor [3–12] mainly monitored by electrical measure- ments. The presence of vacancies and defects, typical of ZnO nanostructured materials [1,2,13–15] and on the basis of their catalytical properties, has been demonstrated to make this oxide chemically and electrically sensible to the adsorption of different analytes. However, the optical sensing properties of ZnO nano- metric films have not been widely investigated. Mazingue et al. [5] have used m-lines technique to detect butane using nanos- tructured ZnO films. Tang et al. [12] proposed an ethanol optical 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.09.065