Trends and Advances in the Characterization of Gas Sensing Materials Based on Semiconducting Oxides
Abstract
:1. Introduction
2. In-Situ and Operando Methodology
- Ex-situ studies may include all types of specimens and experimental conditions. However, in many cases gaps in material, pressure and temperature are encountered. An example is the study of sensing material as powders at room temperature by using typical vacuum techniques.
- In-situ studies are performed in more appropriate conditions, namely temperatures, pressures and atmospheric compositions which match real operation conditions. These measurements might be carried out on various types of specimens.
- Operando studies are performed under application relevant conditions and involve the real-time evaluation of the performance of the studied material. In case of a gas sensor the performance evaluation is the sensor response/signal, while for a catalyst the performance in evaluated by catalytic conversion. The real-time evaluation of the performance implies that the study is done on a real device, e.g., a gas sensor. Thus, operando studies close the three gaps between laboratory experiment and application as much as possible.
- The material characterization should be done as close as possible to real operation conditions, by minimizing gaps in pressure and temperature. Even if the characterization is still considered to be ex-situ, like in the shown example (Figure 2), minimizing these gaps provides more accurate information.
- Using several techniques to assess similar aspects provides further advantages: Independent results allow verifying the drawn conclusion, e.g., on the structure, and in many cases complementary information is obtained, e.g., STEM offers spatial information on the distribution, while XAS is feasible operando technique, which allows assessing the structure under various conditions.
3. Complementary Techniques and Multi-Probe-Approach
4. Identifying Active and Inactive Species
- Active species and processes change the concentration of surface charge as a result of changes in the atmosphere;
- Inactive species do not change the surface charge or are not involved in processes changing the surface charge.
5. Conclusions
- A proper material characterization should be done as close as possible to the real operation conditions, i.e., preferentially during sensor operation, and involve several complementary techniques.
- Mechanistic studies of the gas sensing process should be based on in-situ and operando methods, minimizing the effect of the material, pressure and temperature gaps. Special attention should be drawn to atmospheric compositions, which should match the ones in real application, especially in case of gas concentrations and interfering compounds, such as water vapor.
- Using complementary techniques in a multi-probe-approach allows to probe various properties, processes or species within the same experiment or a series of similar experiments. This involves techniques being complementary in information depth or in the nature of the probed properties.
- Determining structure-function-relationships requires to differentiate between active and inactive species. A proper design of the experiment, using selective techniques or time-resolved methods strongly enhances the ability to correctly identify active species.
Conflicts of Interest
References
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Degler, D. Trends and Advances in the Characterization of Gas Sensing Materials Based on Semiconducting Oxides. Sensors 2018, 18, 3544. https://doi.org/10.3390/s18103544
Degler D. Trends and Advances in the Characterization of Gas Sensing Materials Based on Semiconducting Oxides. Sensors. 2018; 18(10):3544. https://doi.org/10.3390/s18103544
Chicago/Turabian StyleDegler, David. 2018. "Trends and Advances in the Characterization of Gas Sensing Materials Based on Semiconducting Oxides" Sensors 18, no. 10: 3544. https://doi.org/10.3390/s18103544
APA StyleDegler, D. (2018). Trends and Advances in the Characterization of Gas Sensing Materials Based on Semiconducting Oxides. Sensors, 18(10), 3544. https://doi.org/10.3390/s18103544