Low temperature TiO₂ based gas sensors for CO₂

Abstract

Monitoring the level of CO₂, especially in closed spaces, is more and more required in technological applications, or in human activities. Since most of the literature data reveal CO₂ detection materials with high sensitivities over 300 °C, here we have concentrated on the gas sensing abilities of Cr doped TiO₂ thin films in front of CO₂, close to the room temperature and at atmospheric pressure. The films were obtained by RF reactive sputtering. The undoped films contain a mixture of anatase and rutile phases. With the increase of Cr content, the crystallites size decreases, and the films become pure rutile for a 4 at% Cr concentration. We found out that these material based sensors are more sensitive to CO₂ for higher Cr concentration, the optimum operating temperature approaching to the room temperature, determining in fact low energy consumption. The explanation is related to the observed increase of oxygen vacancies number (which we have evidenced and clarified), and also to the presence of the rutile phase, whose higher dielectric constant (compared to anatase), and its finer crystallites, determine a better gas sensing. More, the surface active area in front of CO₂ increases, as the films become rougher for higher Cr contents. The increase of Cr³⁺ percentage enhance the power of interaction with the adsorbed species (O₂ and/or CO₂). A grain boundary model was proposed for the thermal activation of the electrical conductivity. The energy barrier height at the grain boundary, the impurities concentration (characteristic parameters of this model) were calculated and found to agree well with the data in the literature.

Introduction

Gas sensors are often needed in the technological applications or in human activities. Monitoring the air quality in the surrounding environment, especially in closed spaces, by detecting gases like formaldehyde, acetone, methane, carbon monoxide, carbon dioxide, etc. is more and more required [1], [2]. Semiconducting metal oxide in thin films are the most studied materials for gas sensors in atmospheric conditions, due to their low costs, rapid answer, simple implementation in small spaces (car motors, airplane cabin, etc.), ability to detect large number of gases [2]. Different metal oxide based materials have different reaction activation in front of different gases. TiO₂ is preferred because it works very well in high temperature environment, and it is chemical and mechanical stable [3]. Based on the largely observed fact that impurities doping is an effective way to enhance the sensor response magnitude, or its selectivity [4], [5], because of the possible decrease of the activation energy of the reaction occurring on the surface, we have investigated in this paper the gas sensing properties of TiO₂ films doped with different chromium percentages. The choice was based on the observation that Cr induces the transformation phase from anatase to rutile [5], [6], thus obtaining the most stable phase of TiO₂, which might confer the desired stability to the sensor. Cr-doped TiO₂ is already studied as sensing gas material, and it was found to be very sensitive especially in front of ethanol [7]. Here we proposed to get information on the detection of CO₂ which is less studied, especially for this material. CO₂ is considered the most important contributor to global warming, accounting for 64% of the increasing greenhouse effect [8]. There are industrial applications where the level of CO₂ needs to be permanently monitorized. The global airlines consume over 5 million barrels of oil per day, being a significant contributor to anthropogenic CO₂ emissions [9]. The review made by Wetchakun et al. [2] on the recent development of metal oxide gas sensors for environmentally hazardous gases including CO₂, revealed that there are few candidates for the CO₂ detection, but their high sensitivity is obtained at temperatures over 300 °C.

For fire sensor applications, Kim et al. [10] have proposed a CO₂ gas sensor workable at room temperature, by using a MIS structure having Pd/TiO₂/SiO₂/Si-wafer/Al layers. We proposed here a simpler way, by using a less expensive material, namely titania doped with chromium atoms in different percentages, which works under atmospheric pressure, and can detect comparable CO₂ amounts, at temperatures close to the room temperature, but with higher sensitivities.