Peculiarities of nitrogen dioxide detection with sprayed undoped and indium-doped zinc oxide thin films R. Ferro a, ⁎ , J.A. Rodríguez a , P. Bertrand b a Fac. of Physics, University of Havana, San Lázaro y L, 10400 Vedado, Habana, Cuba b Département de Sciences des Matériaux et des Procédés, Université catholique de Louvain, Croix du Sud 1, B1348 Louvain-la-Neuve, Belgium Received 20 September 2006; received in revised form 3 July 2007; accepted 30 July 2007 Available online 8 August 2007 Abstract Nitrogen dioxide (NO 2 ) sensors based on sprayed zinc oxide (ZnO) thin films have been prepared. The effect of the film thickness and the In-doping on the sensor performance (sensor response and resistance) is analyzed. By adding 3 wt.% of indium nitrate to the spraying solution it is possible to enhance the film–gas response to 5 ppm of NO 2 at 275 °C. At the same time the film resistance is sensibly reduced. The film crystallographic structure, morphology and additive content are studied by means of X-ray Diffraction, Scanning Electron Microscopy, X-ray Photoelectron Spectroscopy and Atomic Force Microscopy. The possible sensitization mechanism is discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Nitrogen dioxide; Sensors; Semiconductors 1. Introduction The control and reduction of pollutant species in the atmosphere are of great importance for modern society. Dioxide nitrogen (NO 2 ) is a highly toxic gas and one of the main causes of air pollution. It is emitted from combustion facilities such as automobiles, industrial plants and domestic heaters. Low concentrations of NO 2 (a few ppm) are harmful for human beings and plants. For this reason, sensitive NO 2 sensors suitable for continuous air-quality monitor- ing are demanded. In the last decades, resistive gas sensors based on semiconductor oxides have been the subject of extensive investigations due to their high sensitivity to both oxidizing and reducing gases. Some reviews have been published about the subject [1–3]. These sen- sors fulfill the main market requirements such as low cost, small size and easy maintenance. Moreover, thin films seem to be more promising to improve the sensor performance as well as to open the possibility of reducing production cost and power consumption by producing small-integrated gas sensors [4]. For NO 2 detection, sensors based on semiconductor oxides such as tin oxide (SnO 2 ), tungsten oxide (WO 3 ), indium oxide (In 2 O 3 ), titanium oxide (TiO 2 ) and zinc oxide (ZnO) have been tested. However, these oxides have high resistivity, especially when they are obtained as thin films. Besides, NO 2 is an oxidizing gas and thus it induces a resistance increase on n-type semiconductors. If the sensor impedance becomes too high, a complicated associated electronic circuitry is required in order to avoid leakage currents [5]. It is one of the main drawbacks of sensors based on these materials for NO 2 detection. ZnO is a well known, stable, low cost, and non-pollutant material. However, as it has been mentioned above, thin films of this material are highly resistive. In a previous work [6] we pro- posed to improve the sensing performance of ZnO by mixing it with cadmium oxide (CdO), a low resistivity material, in adequate proportions. In the present paper, an alternative and simplest direction is considered through the introduction of indium as additive. The use of additives to improve the sensor performance has been widely reported in the literature [7,8]. From the other hand, spray pyrolysis is a simple, inexpensive and suitable for mass production Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 2225 – 2230 www.elsevier.com/locate/tsf ⁎ Corresponding author. E-mail address: rferro@fisica.uh.cu (R. Ferro). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.07.189