Chemoresistive Gas Sensors Based on SnO2 and Sn3O4 Nanobelts to Volatile Organic Compounds Detection: A Comparative Investigation
Authors: Suman Pedro H. , Weimar Udo , Barsan Nicolae , Orlandi Marcelo O. 
Published: July 2021

Introduction

Volatile organic compounds (VOCs) are organic chemicals with a relatively high vapor pressure at low temperatures released to the environment from biogenic and anthropogenic sources. Hundreds of VOCs have been identified in the human breath, and many of them act as potential biomarkers for noninvasive diagnosis and monitoring of several diseases [1]. Contrarily, the increasing emission of these hazardous compounds in urban areas, mainly from industry and automotive exhaust systems, has been a critical matter of modern society in terms of safety, environmental pollution, and public health [2].
Chemoresistors based on semiconducting metal oxides (SMOx) offer unique opportunities for trace detection of VOCs, considering their compatibility for portable devices, simple design, low-cost, and reduced power consumption [3]. While SnO2 has been the most used material in the active layer of market sensors, n-type Sn3O4 is another possible stoichiometry of tin oxide with emergent interests in the gas sensing field over the last years [4,5]. However, only limited research has reported the response of Sn3O4 nanostructures to VOCs detection.
In this work, the sensing properties of single-crystalline Sn3O4 nanobelts towards some relevant volatile organic compounds (ethanol, acetone, and toluene) were investigated comparatively to SnO2 in multiple operating conditions. A comprehensive understanding of the conduction mechanism associated with the performance of the sensors was additionally discussed.

Materials and Methods

Sn3O4 and SnO2 nanobelts were synthesized in a tube furnace by the carbothermal reduction method following optimized synthesis parameters described in our previous work [4]. The materials were collected from different regions of the growth site and separately dispersed in 1,2-propanediol by sonication. Several drops of the solutions were deposited onto alumina substrates containing interdigitated Pt electrodes on the topside and a Pt heater on the backside. The sensors were dried at 80 °C overnight and then annealed at 350°C for 10 minutes.
The devices were inserted into a homemade measurement chamber coupled with a gas mixing system equipped with mass-flow controllers. Several concentrations of the target gases, including ethanol, acetone, and toluene, were introduced into the chamber, and the DC electrical resistances were continually monitored at 200, 250, and 300°C. Synthetic air in dry and humid conditions (5%, 30%, and 70% r.h. at 25°C) was used as the reference gas. A constant gas flow of 200 sccm was maintained during all the experiments.

Results and Conclusions

The gas sensing measurements performed from 200 to 300°C revealed that the baseline resistance of SnO2 and Sn3O4 sensors changes accordingly with the humidity level in the background. It occurs due to the reducing nature of the water vapor on the surface of tin oxide. After exposure to ethanol, acetone, and toluene, the resistance of both sensors decreases following the gas concentrations. These reducing gas species act as surface donors, which improves the number of free electrons in n-type semiconductors. From the calibration curves, the signals of the SnO2 sensor toward toluene, acetone, and ethanol drop as a result of the increment in the relative humidity over the monitored operating temperatures. The same profile was observed in the Sn3O4 sensors at 200°C and 250°C, but the opposite outcome takes place at 300°C, i.e., higher sensor signals were achieved in humid conditions for the tested VOCs. In addition, the sensors exhibit remarkable selectivity to potential interferents such as H2 and CO in the full range of concentrations and working temperature investigated. Changes in the surface band bending were correlated with electrical resistance data based on a depletion layer model. It allowed obtaining information about the electronic structure as a function of the charge carrier concentration induced by the analytes chemisorption on the sensing layers. To the best of our knowledge, this is the first report on 1D single-crystalline Sn3O4 materials towards VOCs detection, opening up new possibilities for further investigations.

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