Characterization and butanol/ethanol sensing properties of mixed tungsten oxide and copper tungstate films obtained by spray–sol–gel

Abstract

Mixed WO₃–CuWO₄ films have been prepared from an aqueous solution of copper sulfate and polytungsten gel with a molar ratio of Cu/W from 0 to 100%. These solutions were sprayed on to alumina substrates at 220 °C. The obtained films were amorphous and crystallized after annealing at 300 °C in air for 3 h. The annealed films were composed of a mixture of CuWO₄ and WO₃ phases. The film obtained from a solution with an equimolar ratio of Cu/W was pure CuWO₄. The pure WO₃ films obtained have high surface ‘irregularities’, such as porosity. These surface ‘irregularities’ in the films were maintained or even increased as Cu/W molar ratio augments in the starting solution up to 10%. The gas sensitivity to butanol and ethanol vapors is also enhanced when the CuWO₄ phase increases in the film by up to 7–10%; with further increments to this proportion the detection sensitivity decreases, so compromises are approximately 7–10%. The gas sensitivity of pure CuWO₄ was lower than that of pure WO₃.

Introduction

Mixed oxides have been investigated intensively to improve or modify their electrochromic, gas sensing and photocatalitic properties. For example, coloration efficiency decreases slightly, but the lifetime of WO₃–TiO₂ thin films can be five times longer than those made of pure WO₃ [1], [2], mixed oxides have recently emerged as a promising candidate for gas detection [3], and the degradation rate of 1,4-dichlorobenzene is enhanced by the addition of WO₃ to TiO₂ [4]. It has been found that most metal oxide mixtures exhibit increased surface activity.

It is well-known that the conductance of simple metal oxides such as SnO₂, WO₃, ZnO and TiO₂ changes when the composition of the surrounding atmosphere is altered [5]. Different metals and oxides are used as dopants or catalyst in order to improve gas-sensing properties [6]. It has been concluded that the nature of the surface sites and the electron donor/acceptor properties of the gas, the adsorption, the surface reactions and desorption of gases are key features for the performance of semiconductor gas sensors [5]. Surface properties are expected to be influenced by grain boundaries between the grains of different chemical compositions. These phenomena will contribute to the gas-sensing properties. Mixed oxides that form distinct chemical compounds as in the systems Zn–Sn–O [7], Cd–In–O [8] and Sn–W–O [9], [10], [11], [12] have been used successfully in gas detection.

Tamaki et al. [13] have studied different metal tungstates in order to detect nitrogen oxides. However, CuWO₄ was missing in that study. A mixture of tungsten oxide and copper oxide heated in vacuum produces CuWO₄ with a distorted wolframite type structure [14] and CuWO₃ with a cubic structure [15]. The sol–gel technique is well suited for making mixed oxides, and work with W–Ti oxide [1], W–V oxide [16], V–Ti oxide [17] and Fe–Ti oxide [18] has been reported. W_xOP_y films, obtained by spraying a polytungsten gel mixed with H₃PO₄ on to a glass substrate at 430 °C, have shown improved electrochromism [19]. Combining the spray pyrolysis and the sol–gel techniques has produced very rough films [19]. This technique is suitable for producing semiconductor metal oxides for gas-sensing applications; due to the fact that it yields a large interface between a solid and a gaseous medium.

We report in this work the characterization and gas sensing properties of mixed WO₃–CuWO₄ films obtained by spraying aqueous solutions of copper sulfate and polytungsten sol on to alumina substrates at 220 °C. The incorporation of the CuWO₄ phase into WO₃ improved the gas response to ethanol and butanol with respect to pure WO₃.