Cr-doped TiO₂ gas sensor for exhaust NO₂ monitoring

Abstract

A set of Cr-highly doped TiO₂ samples with Cr contents ranging from 5 to 30 at.% were prepared in a sol–gel route and calcined at a temperature between 600 and 900 °C. X-ray diffraction (XRD) analyses revealed the persistence of anatase phase up to the calcination temperature of 700 °C in all samples, above which rutile phase became dominant. The segregation of Cr₂O₃ remained modest, only detectable by surface-sensitive technique like X-ray photoelectron spectra (XPS), for the 5 and 10 at.% Cr-doped samples calcined at 600 or 700 °C, suggesting incorporation of major part of doped Cr in the lattice of TiO₂. Higher calcination temperatures or higher Cr contents lead to marked segregation of Cr₂O₃. XPS spectra in the valence band region of the samples calcined at 600 °C revealed a shift of the binding energy (BE) at the band edge to the lower energy side with increasing Cr contents, suggesting a tendency for the electronic conduction to alter from n- to p-type. As tested preliminarily, the thick and thin film devices prepared with these samples exhibited p-type conduction, and, particularly, a thin film device using 10 at.% Cr-doped sample calcined at 600 °C proved promising performances in the detection of dilute NO₂ in air at 500 °C.

Introduction

Atmospheric pollution in urban areas has become a critical problem in recent years. Processes involving combustion in automobile engines or industrial plants have been the main sources of contamination due to the consequent emission of harmful gases. In order to control such emissions, the study of chemical sensors with suitable sensing performances has focused increasing interest [1]. In this point, the in situ detection at high temperatures of the various oxides of nitrogen resulting from the exhaust gases is still an unsettled issue.

Semiconductor gas sensors have been widely investigated for gas sensing applications due to their low cost, good performances and easy implementation. The principle of operation consists of a variation of the resistivity of the sensing material when its surface is exposed to different kinds of reducing or oxidative gases. Usually, the behavior is based on an n-type electronic conduction, in which the resistance of the sensing layer decreases when exposed to reducing gases, such as CO or hydrocarbons. However, in the presence of oxidative gases such as NO₂, the resistance increases, sometimes rising beyond the detection limits of conventional circuitry [2]. This phenomenon is even more pronounced in the case of thin films, where the diminution in thickness naturally leads to increases in resistance in air and sample gas. It is therefore necessary to control accurately the electronic properties of the materials to fabricate films suitable for monitoring NO₂.

Transition metal oxides, such as SnO₂, TiO₂, Ga₂O₃, WO₃, Nb₂O₅ or MoO₃, have been proposed as candidates for gas detection [3], [4], [5], [6]. Among these, TiO₂ exhibits probably the best chemical stability at high temperatures and harsh atmospheres. However, full development of titania based devices requires an improvement of its characteristics by the introduction of catalysts or additives. Pt has been used as a catalyst to quicken oxygen response in lambda devices [7], [8]; Cu improves selectivity to CO [9], [10]; alcohol sensors can be made with the addition of Nb [11]. Titanium dioxide itself is a high resistive n-type semiconductor with rather poor conductivity to be adopted for sensing oxidative gases [12]. To overcome this disadvantage, the electronic structure should be altered into p-type by the addition of foreign atoms. A few works that show how the addition of Cr to TiO₂ alters the electronic conductivity from n to p-type have appeared, opening the development of novel gas sensors [13], [14], [15]. The p-type materials obtained under appropriate conditions responded with a sharp decrease in its resistance upon exposure to diluted NO₂ [16]. The nature of the electronic behavior, i.e. alteration from n- to p-type and vice versa, and the sensing characteristics of the oxide materials have been discussed or reviewed in several papers [17], [18], [19]. In this context, a systematic study of the roles that dopant concentration and thermal treatment play in the resulting materials could be useful for a better understanding of the alteration from n- to p-type. We modified therefore TiO₂ by the controlled addition of Cr from 5 to 30 at.%, and calcined the materials from 600 to 900 °C. The resulting powders were then chemically and structurally analyzed.

This paper aims at reporting the results of these analyses as well as of the electrical characterization carried out for thin or thick films using some of these modified materials.