Selectivity of Tungsten Oxide Synthesized by Sol-Gel Method Towards Some Volatile Organic Compounds and Gaseous Materials in a Broad Range of Temperatures

Abstract

In this research, the investigation of sensing properties of non-stoichiometric WO3 (WO3−x) film towards some volatile organic compounds (VOC) (namely: Methanol, ethanol, isopropanol, acetone) and ammonia gas are reported. Sensors were tested at several temperatures within the interval ranging from a relatively low temperature of 60 up to 270 °C. Significant variation of selectivity, which depended on the operational temperature of sensor, was observed. Here, the reported WO3/WO3–x-based sensing material opens an avenue for the design of sensors with temperature-dependent sensitivity, which can be applied in the design of new gas- and/or VOC-sensing systems that are dedicated for the determination of particular gas- and/or VOC-based analyte concentration in the mixture of different gases and/or VOCs, using multivariate analysis of variance (MANOVA).

Tuning the Photo-Luminescence Properties of WO3 Layers by the Adjustment of Layer Formation Conditions

“…Two different charge transfer mechanisms are observed for WO 3 layers: the first is based on n-type conductivity that is typical to stoichiometric WO 3 [58][59][60] and the other mechanism is based on p-type conductivity that is characteristic for non-stoichiometric tungsten oxide WO 3-x . The latter was reported and can be responsible for the low-temperature sensitivity of WO 3-x -based layers towards gaseous analytes [61,62]. Time-resolved photoluminescence decay measurements presented in Figure 5 reveal that the average photoluminescence decay time of PrOH-140, PrOH-180, EtOH-140, EtOH-180 samples is 11.2, 11.3, 4.5 and 10.9 ns, respectively (Table 2).…”

“…Two different charge transfer mechanisms are observed for WO 3 layers: the first is based on n-type conductivity that is typical to stoichiometric WO 3 [ 58 , 59 , 60 ] and the other mechanism is based on p-type conductivity that is characteristic for non-stoichiometric tungsten oxide WO 3-x . The latter was reported and can be responsible for the low-temperature sensitivity of WO 3-x -based layers towards gaseous analytes [ 61 , 62 ].…”

Insights in the Application of Stoichiometric and Non-Stoichiometric Titanium Oxides for the Design of Sensors for the Determination of Gases and VOCs (TiO2−x and TinO2n−1 vs. TiO2)

“…Among many different options to measure analytical signal by semiconductor-based gas and VOC sensors, one of the simplest methods is to measure electrical resistivity of the layer. Here, it should be noted that in order to increase reliability of gas and VOC sensors in the analysis of gaseous compounds mixtures, in addition to basic resistivity measurements, it is reasonable to add some additional physicochemical methods, such as an evaluation of photoluminescence signals [1,2,7], which can be applied due to remarkable optical properties of a TiO 2 semiconductor [43]. It is worth noting that various TiO 2 -based structures can be designed and enhanced by laser pulses, which improve the photocatalytic and photovoltaic performance of designed nano-structures [44].…”

“…The mechanism of conductance variation in the presence and absence of target gases (e.g., CO) has been discussed in detail by several research groups [78,79] (Figure 3.). The electrostatic interaction between the particular parts of the target analyte and TiO 2 surface, as well as with the surface of other similarly behaving semiconductors (e.g., WO 3 [2,7]), also play a significant role in the development of both (i) changes of resistivity [1,2] and (ii) variations of photoluminescence spectra [7,25], which can both be applied for the assessment of the analytical signal [1]. The 'Debay radius/length' and the size of grains and are the factors that are significantly affecting charge-transfer efficiency in TiO 2 -and TiO 2−x -based layers.…”

“…Gas and volatile organic compound (VOC) sensors are important for various safety and environmental control issues. Recently, rather simple gas and VOC sensors based on the measurement of the electrical resistance of the semiconducting layer are developed [1][2][3][4][5][6]. Mostly, semiconducting materials are applied in the design of sensing layer, which is responsible for the formation of analytical signal towards target gases and/or VOCs.…”