Siloxane treatment of metal oxide semiconductor gas sensors in temperature-cycled operation – sensitivity and selectivity

Abstract

Abstract. The impact of a hexamethyldisiloxane (HMDSO) treatment on the response of doped SnO2 sensors is investigated for acetone, carbon monoxide and hydrogen. The sensor was operated in temperature cycles based on the DSR concept (differential surface reduction). According to this concept, the rate constants for the reduction and oxidation of the surface after fast temperature changes can be evaluated and used for quantification of reducing gases as well as quantification and compensation of sensor poisoning by siloxanes, which is shown in this work. Increasing HMDSO exposure reduces the rate constants and therefore the sensitivity of the sensor more and more for all processes. On the other hand, while the rate constants for acetone and carbon monoxide are reduced nearly to zero already for short treatments, the hydrogen sensitivity remains fairly stable, which greatly increases the selectivity. During repeated HMDSO treatment the quasistatic sensitivity, i.e. equilibrium sensitivity at one point during the temperature cycle, rises at first for all gases but then drops rapidly for acetone and carbon monoxide, which can also be explained by reduced rate constants for oxygen chemisorption on the sensor surface when considering the generation of surface charge.

Measuring Hydrogen in Indoor Air with a Selective Metal Oxide Semiconductor Sensor

“…SGP30 multilayer MOS sensors (four sensitive layers on one hotplate, Sensirion AG, Stäfa, Switzerland) were used for this study. One sensor device was installed as delivered and another one was pre-treated with Octamethylcyclotetrasiloxane at 2 ppm over 18 h yielding a H 2 selective sensor (compare [26,28]). The temperature cycle is specifically designed to achieve high sensitivity and selectivity by using the differential surface reduction (DSR) method, which is described in detail in previous publications [32][33][34].…”

“…In this manuscript we present an extended field-study over two months inside an office at the university with a focus on H 2 to get a better overall idea on the topic of H 2 in indoor air-as interferent and as target gas. Two approaches for achieving selective H 2 quantification were combined: TCO followed by pattern analysis with machine learning (ML) algorithms [10,11] and a pre-treatment of the sensitive layer with siloxane, which in other circumstances is called poisoning since it deteriorates all sensitivities except for H 2 [27][28][29][30][31]. The performance of the sensor signals is validated by release tests and a GC with reducing compound photometer (RCP) detector (Peak Performer 1, Peak Laboratories Inc., Mountain View, CA, USA), which to our knowledge is the only analytical device for online measurement of hydrogen with ppb-level resolution.…”

Silica: ubiquitous poison of metal oxide interfaces

“…Volatile organic silicon compounds (VOSCs) are widely used in consumer goods such as electronics, furniture, healthcare, pharmaceuticals, cosmetics, and cookware, 93 leading to significantly higher concentrations indoors than outdoors. [94][95][96] Over the last decades, a great deal of effort has been invested in identifying Si-impurity sources and developing means for avoiding contamination. For example, van Berkel et al prevented Si-poisoning in oxygen permeation membrane measurements by avoiding VOSC containing grease in the manual valves of the experimental setup.…”

“…Williams and Pratt, and Schultealbert et al, both found that exposure to VOSCs deactivate different surface sites on SnO2 and at varying rates (Figure 10a). 95,96 The transient response to all gases slows down as a result of Si poisoning, but the active sites for hydrogen appear to be deactivated to a lesser degree than those for the other gases, (Figure 10b). The authors argue that the high selectivity, and even increased steady state response to hydrogen, results from the fact that the readsorption of oxygen is more strongly hindered by the adsorption of VOSCs than the reduction by hydrogen.…”

“…10a). 95,96 The transient response to all gases slows down as a result of Si poisoning, but the active sites for hydrogen appear to be deactivated to a lesser degree than those for the other gases (Fig. 10b).…”