Linear Sensing Response to Ethanol by Indium Oxide Nanoparticle Layers

Indium oxide nanoparticles having well-defined particle sizes were synthesized using a chemical capping method. These nanoparticles were used for making the nanoparticle layers without altering the size and morphology of these particles. These nanoparticles and nanoparticle layers were characterized using XRD, TEM, HRTEM and AFM. The ethanol sensing behavior of the nanoparticle layers were studied at different ethanol concentrations. It was observed that the sensor response was linear to the ethanol concentration in the range of 10–1000？ppm. The ethanol sensing behavior has been explained on the basis of the creation of a depletion region due to the adsorbed oxygen and release of the electron in the conduction band in the presence of ethanol (as it takes away the adsorbed oxygen). The explanation has been supported by EDAX results. 1. Introduction Indium oxide is a wide bandgap material (bandgap ~3.6？eV) [1]. It is nonstoichiometric due to the oxygen deficiency, and therefore it is an n-type highly degenerate semiconductor. It has a relatively low resistance (~10？4？Ωcm) and has high transmittance in visible region (~85%). Due to these properties, it has wide applications in gas sensing towards O3 and . It is also widely used as transparent conducting electrode and electroluminescent devices [2]. It has been shown in most of the reported works that the size of the nanoparticles in the layers/films used for gas sensing is not well defined because of the different methodologies used for making the particles as well as gas sensing layer, and therefore it is difficult to correlate the sensing properties with the size of the particles and other parameters [3]. In this study, the size of the nanoparticles has been preserved in the nanoparticle layers by using a two-step synthesis process. In the first step, nanoparticles having a well-defined particle size have been synthesized by a chemical capping method, and in the second step, the dispersion of the particles has been deposited on the substrates, without altering the size of the particles [4]. For any material to be useful for the gas sensing applications, it is important that its sensor response is linear for a wide range of the gas concentrations [5–8]. In this work, the effect of ethanol concentration on the gas sensing properties of indium oxide nanoparticle layers has been studied without altering the working temperature or the size of the nanoparticles of the nanoparticle layers. 2. Experimental In order to synthesize In2O3 nanoparticles, InCl3 3H2O has been dispersed in ethanol. This dispersion is