Ethanol Gas Sensing by Indium Oxide: An Operando Spectroscopic Raman-FTIR Study

Abstract

The ethanol gas-sensing mechanism of indium oxide has been investigated in detail by Raman spectroscopy in combination with resistance measurements of the indium oxide sensor material and Fourier transform infrared (FTIR) gas-phase analysis. The observed surface species depend on the gas environment and sensor temperature. Raman spectra taken at lower operating temperatures of the sensor (190°C) during ethanol gas sensing reveal the presence of surface acetate and reduced indium oxide. Addition of oxygen to the feed leads to the formation of surface formate-like species besides acetate. At elevated temperatures of 325°C an increase in the amount of the gas-phase products acetaldehyde, ethene, carbon oxide, and water is observed. Under these conditions the sensor surface is characterized by carbon species if oxygen is absent. The sensor signal is correlated with the nature of the adsorbates, the presence of surface hydroxyl groups, and the indium oxide oxidation state. The proposed gas-sensing mechanism is corroborated by detailed analysis of the spectroscopic and resistance response of indium oxide during exposure to acetaldehyde and ethene. Our results demonstrate the importance of detailed spectroscopic studies under working conditions to unravel the mode of operation of gas sensors. ■ INTRODUCTIONSemiconducting metal oxides have been used widely as gas sensor materials because of their high sensitivity to a large variety of target gases and their simple fabrication. 1−3 A metal oxide gas sensor reversibly changes its resistance in the presence of a target gas, which is caused by the adsorption of gas molecules on the semiconductor's surface. As a mechanistic explanation the ionosorption of the adsorbates is assumed, transferring electrons from or to the sensor's conduction band. Alternatively, the sensor behavior can be explained by reduction and reoxidation of the (sub)surface, producing and eliminating oxygen vacancies. The vacancies can be ionized, thereby releasing electrons to the conduction band. In both proposed mechanisms the oxygen from the air plays an important role, either as an ionosorbed species or as an oxidation source. Moreover, the sensor signal may be strongly influenced by the presence of the preadsorbed species (e.g., ionosorbed oxygen or hydroxyl groups). While there has been considerable progress in the field, a detailed understanding of the mode of operation of metal oxide gas sensors is still lacking. To this end, the development of experimental approaches will be essential, which (i) are applicable under realistic operating conditions of the gas sensor and (ii) allow for a correlation of the sensor response with adsorbed species, changes of the metal oxide material, and gas-phase composition (operando approach). 4,5 Despite the potential of Raman spectroscopy for studying gas sensors at work, only a few in situ Raman studies on metal oxide gas sensors have been published, 6−10 which in part were done in combination with resistance measurements. 7−9 For nanocrystalline SnO 2 7 and CuO/...

Shining Light on Indium Oxide Gas Sensors at Work: A Combined Operando Raman/UV-Vis/FT-IR Spectroscopic Study

“…Previous operando Raman studies on undoped In2O3 gas sensors have shown that during ethanol (EtOH) gas sensing the sensor signal can be directly correlated with the nature of the adsorbates, the presence of surface hydroxyl groups and the indium oxide oxidation state [2,3] Turning now to recent results on Ag doped In2O3, Figure 1 shows operando Raman spectra recorded after switching from air to 250 ppm EtOH/air at 190 °C. At higher wavenumbers, the disappearance of hydroxyl groups at 3639 and 3656 cm −1 (O-H stretch) is accompanied by the formation of acetate based on the bands at 871 cm −1 (C-C stretch) and 2935 cm −1 (C-H stretch) [3]. Interestingly, upon exposure to 250 ppm EtOH/air, the low wavenumber region shows dramatic changes, which are reversible and which can be related to the change in the Ag state during EtOH sensing.…”

“…Understanding the mode of operation of metal-oxide gas sensors (e.g., SnO2, In2O3) is of great scientific and economic interest. Such a knowledge based approach requires the development and application of spectroscopic tools to monitor the relevant surface and bulk processes under working conditions (operando approach) [1,2] In previous studies on In2O3 sensors, we applied combined operando Raman/gas-phase FT-IR spectroscopy to ethanol gas sensing [2,3] In this contribution, we will present recent results on ethanol and CO detection using undoped and Ag doped In2O3 gas sensors, demonstrating the advantages of (i) operando Surface Enhanced Raman Spectroscopy (SERS) to monitor the metal oxidation state, and (ii) extending the operando Raman/FT-IR setup by UV-Vis spectroscopy to reveal the degree of In2O3 reduction.…”

Urchin-like SnO2 nanoflowers via hydrothermal synthesis and their gas sensing properties

“…These 3D hierarchical nanostructures are assumed to be the most effective because of their porous nanostructures formed by the adjacent building blocks. On the other hand, gas sensing performances of SnO 2 can remarkably be improved once the size shrinks to a critical value of 14 nm (2 λD) [9,10]. This is confirmed by Yamazoe et al [11], claiming that the critical size of SnO 2 nanomaterials is 5-15 nm for the effective detection of H 2 and CO.…”

Trends and Advances in the Characterization of Gas Sensing Materials Based on Semiconducting Oxides

“…The temperature gap has a strong impact on the semiconducting properties and the surface chemistry of the gas sensing material, as the ionization of defects [ 23 , 24 ], adsorption of molecules [ 25 , 26 ] or chemical reactions [ 27 , 28 ] strongly depend on temperature. Heating the sample to typical operation temperatures between 200 and 400 °C is possibly restricted by the spectroscopic technique as such, e.g., requiring ultralow temperatures or by low pressures, which my trigger unwanted changes in the sample due to heating in the absence of oxygen.…”

“…The electronic structures can be probed by UV/vis Diffuse Reflectance Spectroscopy (DRS) [ 35 , 36 ], XAS and X-ray emission spectroscopies (XES) [ 34 , 37 , 38 ]. The surface chemistry is mainly investigated by Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) [ 28 , 39 , 40 , 41 ] and Raman spectroscopy [ 27 , 42 ]. An overview of operando methods for SMOX gas sensors is given in Figure 1 [ 14 ].…”