To study the gas sensor mechanism of semi-conducting metal oxide sensors, a new operando Raman-setup was designed and its potential was demonstrated by means of the ethanol gas sensing of indium oxide. To this, the heatable metal oxide gas sensor was measured by in situ Raman-spectroscopy in a teflon gas cell equipped with a quartz window, while a defined gas atmosphere was piped continuously through the gas cell via a pipe system. Simultaneously, the sensor resistance was recorded with a multimeter and the gas composition was analysed with a FTIR-spectrometer behind the gas cell. During the reaction between the target gas and the gas sensor, the changes of the electrical sensor resistance, the oxide´s surface, the oxide´s bulk and the gas products could be studied simultaneously with the help of the operando setup and therefore compared temporally.
The measurement setup was investigated with regard to possible error sources. Thereby a local temperature increase on the In2O3 gas sensor was discovered due to the laser radiation. An influence on the in situ Raman measurement data by the laser wavelength/power was excluded in the case of the qualitative analysis. The quantitative evaluation of the gas phase was susceptible to errors because of the air contamination in the gas flow and because of the ethanol conversion at the sensor substrate at high temperatures. The gas sensing was falsified by the setup because the characteristic sensor values (sensor temperature, sensitivity, response time, recovery time) could be influenced by the gas flow. Moreover, the contamination in the gas stream could also bear on the gas sensing.
By varying the composition of the gas phase (ethanol, nitrogen, oxygen, humidity, carbon dioxide, air) and the sensor temperature (23-500°C), the ethanol gas sensing mechanism of the In2O3 gas sensor was studied. The used cubic bixbyite-type indium oxide sensor material was synthesised by basic precipitation of indium nitrate followed by calcination at 800°C. The BET-surface area was 15 m2/g and the mean crystal size was 34 nm.
In the presence of ethanol, the analysis of the operando measurement data showed a decrease of the electrical sensor resistance, a conversion to different gas products (as acetaldehyde, acetone, ethylene, carbon dioxide, carbon monoxide, methane, water, hydrogen), a varying In2O3 reduction degree near the oxide´s surface and a change of the oxide´s surface species (as the decrease of the hydroxy group concentration and the appearance of different adsorbates (as acetate, ethoxy, formate-like species, carbon)). Thereby the resistance change, the gas products, the In2O3 reduction degree as well as the adsorbate species were depending on the temperature and the presence of oxygen and humidity, respectively. The influence of the carrier gas on the ethanol gas sensing decreased as follows: O2 > H2O > CO2. A good comparability to the operando measurement results could be proven under real conditions (room air).
Due to the spectroscopic measurement data, the following chemical mechanism was proposed for the ethanol conversion by the indium oxide gas sensor: The ethanol adsorbs on the In2O3 surface and dissociates to ethoxy which can either be dehydrated to ethylene or dehydrogenated to acetaldehyde. By the attack of the hydroxy group to the adsorbed acetaldehyde, the acetate can arise. The formate can be formed by the decomposition of the ethoxy or the acetate. In the absence of oxygen, the adsorbates decompose to carbon above ~300°C. However, in the presence of oxygen, they will be oxidised to CO2. As a part of the redox reaction, the indium oxide is reduced near the surface, but can be reoxidised in the presence of oxygen or humidity.
The found mechanism proves the most common sensing mechanism: The adsorbates point to the ionosorption mechanism (charge transfer between the adsorbate and oxide), whereas the reduced indium oxide refers to the reduction-reoxidation mechanism (variation of oxide´s oxygen stoichiometry). | English |
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