Metal-oxide semiconductors such as In 2 O 3 have been used widely as gas sensing materials because of their high sensitivity to target gases and their simple fabrication. [1][2][3] Their operation is based on changes in the electrical conductivity of the material through adsorption of gas molecules on the surface of the semiconductor. Despite progress in the field, a detailed mechanistic understanding of the gas sensing process is still lacking. The knowledge-based development of better gas sensors with increased selectivity and sensitivity will crucially depend on the development of experimental approaches which allow for simultaneous measurement of the sensor response, adsorbed species, changes in the metal-oxide material, and gas-phase composition (operando approach). [4] Ultimately, by correlating these results, the mode of operation of gas sensors can be elucidated.To demonstrate the potential of this approach, we present an operando Raman-FTIR study of the sensing of ethanol gas by bixbyite-type indium oxide (c-In 2 O 3 ). Adsorbed species and changes to the indium oxide during the gas sensing process can be observed using visible Raman spectroscopy Raman spectroscopy was chosen because it has the potential to detect both adsorbed species, including anionic oxygen species, and changes in the metal-oxide structure. Simultaneous recording of gas-phase FTIR spectra allows quantification of the gas-phase composition. Such operando Raman studies on gas sensors have not been reported before. The developed experimental setup is shown schematically in Figure 1. Simultaneous resistance and Raman spectroscopic measurements on the In 2 O 3 gas sensor were performed in a teflon cell equipped with a flat optical quartz window. The gas outlet was analyzed by FTIR spectroscopy. Figure 2 depicts representative Raman spectra of the operando experiments at 190 8C and 325 8C, with the spectra shown bottom-up in the measured sequences. The indium oxide bands at 304, 361, 493, and 624 cm À1 in the spectrum recorded at 190 8C in nitrogen confirm the bixbyite-type c-In 2 O 3 . [5] The Raman bands at 304 and 361 cm À1 are attributed to the bending vibration (d(InO 6 )) of the octahedron and the In-O-In stretch vibration (u(In-O-In)), respectively. The bands at 493 and 624 cm À1 are due to stretching vibrations of the octahedron (u(InO 6 )). [6,7] The high-frequency region of the Raman spectrum is characterized by two bands at 3643 and 3659 cm À1 , which are attributed to bridged surface hydroxy species. [8] No anionic oxygen species such as O 2 À ads were detected on the surface. Upon exposure of the sample to 250 ppm ethanol in nitrogen (EtOH/N 2 ) at 190 8C (flow rate: 40 mL min À1 ), the intensity of the band at 361 cm À1 increases and a new band and a shoulder appear at 407 and 325 cm À1 , respectively. [9,10] According to the literature, [12] the latter two features result from reduced indium oxide species near the surface, as these bands disappear again when the sample is exposed to oxygen Figure 1. The operando Raman-FTIR s...

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/...

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