This work is the continuation of two previous works: in the former we developed a WO3 based gas sensor featuring high sensitivity to NO2 but with the drawbacks of low conductance values (below 1 nS) and long recovery times (over 20 min); in the latter we developed a low-cost control electronic circuit suited to measure a wide range of conductance values (10 pS–100 μS).
Here, we use this electronic system to control this WO3 sensor according to temperature profile protocols, with the aim to show the possibility to handle such low conductance devices by means of cheap instrumentation, featuring at the same time reduce response times and a degree of selectivity arising from the temperature profile protocol. In particular, we focus on two target applications: detection of NO2 and detection of reducing gases, namely ethanol and methane, in different humidity conditions, showing the usefulness of time constants extrapolated from the response dynamics for the purpose.
Metal oxide chemiresistors have attracted a large interest due to their high sensitivity to a broad range of chemicals as well as to the possibility to prepare devices featuring reduced size, weight and power consumption by means of cheap methods of preparation.
These devices have been the subject of several works dedicated both to the understanding of their basic mechanism and to the development and optimization of sensing layers as well as the exploitation in different applicative fields.
Material scientists dedicated to the development of sensing layers featuring a wide range of sensing properties through the optimization of the oxide structure, such as the grain size, morphology, porosity, chemical composition. For example, by properly adjusting synthesis parameters of WO3 layers, high sensitivity to oxidizing gases can be obtained at room temperature [1]. The n-type sensing properties of pure TiO2 can be modified in a p-type sensing material by the addition of Cr [2].
In order to fully exploit the new materials while keeping cheap the device use, the possibility to read the sensor electrical signal through low-cost readout electronic circuits is important as well. This often contrasts with some papers reported in the literature. It is the case, for example, of WO3, which is widely recognized as one of the most suited oxides to detect NO2, but its low conductance often falls below the Nanosiemens, especially during exposure to NO2[1], [3], [4] and [5]. These values are out of range for most of cheap electronic readout systems and expensive picoammeters are necessary to measure the sensor signal. Furthermore, the WO3 sensitivity to NO2 is optimized at low sensor temperature (usually below 250 °C), where interfering effects induced by humidity are enhanced [6] and the sensor response (and recovery) dynamics are slowed down [3].
To this aim, we have developed a low-cost readout electronic system suitable to perform fast measurements (every 10 ms) over a wide range of values (10 pS–100 μS) [7]. In a previous work, we used a first prototype of the system to control an array of four metal oxide (MOX) based chemiresistors. Working with isothermal protocols, this electronic nose was used to discriminate key-aromas developed during the bread baking process [8].
The electronic system has been further developed to control the sensor temperature, with the aim to develop an autonomous device (sensor and electronics) fulfilling the requirements of reduced cost, fast readings, wide range of measurable conductance [9] and [10]. In this way, it is possible to obtain a degree of selectivity using a single sensor working with temperature profile methods. These methods are based on the temperature dependence of the oxide–gas interaction and use temperature profiles to induce complex interaction dynamics. So far, the whole conductance profile along time can be exploited to extrapolate chemically sensitive information (features) [11], [12] and [13].
In this work, we use our recently developed control-system [9] to control a WO3 based gas sensors working with temperature profile mode to detect NO2 in air under different humidity conditions, showing the suitability of this sensing system (sensors and control electronics) to overcome drawbacks intrinsic in isothermal methods.
In this work we have presented the exploitation of a low-cost electronic system, developed in a previous work to measure a wide range of conductance values (10 pS–100 μS), to control a WO3 based gas sensors according to temperature profile protocol. In particular, the use of the developed control electronic allowed the exploitation of the high sensitivity of the WO3 layer to NO2 by means of a low-cost sensing system, avoiding the need of expensive picoammeters. At the same time, it has been possible to overcome the long recovery time intrinsic in isothermal desorption kinetics of the material through the exploitation of temperature profiles. Finally, the exploitation of dynamic (τC) and amplitude (ΔG) parameters allows obtaining a degree of selectivity by means of a single sensor device, according to results observed focusing on two target applications: detection of NO2 and detection of reducing gases (ethanol and methane) in different humidity conditions.
