Ethanol Gas Sensing by Indium Oxide: An Operando Spectroscopic

Article pubs.acs.org/JPCC

Ethanol Gas Sensing by Indium Oxide: An Operando Spectroscopic Raman-FTIR Study Sandra San̈ ze and Christian Hess* Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Alarich-Weiss-Str. 8, 64287 Darmstadt, Germany 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.

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crystalline SnO27 and CuO/SnO28 at 100 °C, upon exposure to 300 ppm of H2S, the reversible formation of sulfide species (SnSx, Cu2S) was reported, which was correlated with the simultaneously measured decrease in resistance. In studies on nanostructured WO3 sensors, exposure to 10% CH4/H2 and 1.8% CO/N2 resulted at 150 °C in the formation of carbon species and a conductivity increase, whereas under oxidative conditions (1000 ppm of NO2) the carbon species disappeared and the conductivity strongly decreased.9 In the context of gas sensing, the adsorption of 1000 ppm of NO2 on nanocrystalline SnO2 was investigated within a temperature range of 25−300 °C using in situ Raman spectroscopy: based on analysis of differences in the temperature-dependent adsorption behavior of the observed NOx species (NO2 dimers, nitrite, nitrate) the conductivity decrease of SnO2 in the presence of NO2 was attributed to the formation of nitrite species.10 For the catalytic reaction of ethanol with metal oxides there exist two dominant mechanisms: the dehydration to ethene or the dehydrogenation to acetaldehyde, of which the latter route is more effective for the sensor response. Both routes can also lead to the total oxidation of the ethanol to carbon dioxide.11,12 The interaction of ethanol with In2O313−15 and SnO216−18 sensor materials has been studied in the past. In the case of In2O3-based gas sensors, oxidative dehydrogenation of ethanol to acetaldehyde is proposed to be the dominant process

INTRODUCTION Semiconducting 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 nano© 2014 American Chemical Society

Received: September 8, 2014 Revised: October 9, 2014 Published: October 14, 2014 25603

dx.doi.org/10.1021/jp509068s | J. Phys. Chem. C 2014, 118, 25603−25613

The Journal of Physical Chemistry C

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Figure 1. Raman spectra of the indium oxide gas sensor at 23 °C (red), 190 °C (black), and 325 °C (blue) in the presence of N2, 250 ppm of EtOH/N2, synthetic air, and 250 ppm of EtOH/syn.air as indicated. Spectra are offset for clarity.

responsible for the sensor response.13 A mechanism of ethanol detection by SnO2 proposed in the literature is based on the formation of surface ethoxy groups and acetaldehyde by means of abstraction of hydrogen atoms by the oxide surface. Alternatively, the acetaldehyde can undergo a further reaction with the surface to acetate. The ethanol gas-sensing mechanism of SnO2 is explained by the adsorbed molecular ethanol and the abstracted adsorbed hydrogen atoms acting as electron donors or by the adsorbate decomposition leaving oxygen vacancies.16 However, neither of the proposed mechanisms have been confirmed experimentally under the working conditions of the gas sensor. In this contribution we present a detailed operando study on the ethanol gas-sensing mechanism by indium oxide by simultaneous measurement of the sensor response (dc electrical conductivity), Raman spectra of the sensor material, and Fourier transform infrared (FTIR) spectra of the gas-phase composition. It will be shown that using such an approach, and depending on the gas environment and temperature, a correlation has been found between the sensor signal, the presence of adsorbates, the indium oxide oxidation state, and the intensity of surface hydroxyl groups. For comparison, the operando measurement was also repeated with ethene and acetaldehyde gas.

dropped onto the surface of an Al2O3 transducer substrate with interdigitated Pt electrodes to measure the sheet resistance. On the other side of the substrate was a meander Pt heater for heating the In2O3. The sensor was annealed at 100 °C for 24 h in a muffle furnace. Operando Experiments. Simultaneous electrical and Raman spectroscopic measurements were performed in a specifically designed in situ cell.5 To control the gas atmosphere and flow, the feed was supplied by mass flow controllers (Bronkhorst) using stainless steel tubes. At the outlet of the in situ cell, the gas composition was quantified by FTIR spectroscopy. To minimize condensation the tube between the cell and the FTIR spectrometer was heated at 100 °C. The body of the in situ cell is made of poly(tetrafluoroethylene) (PTFE, Teflon); the cell volume is 5 mL. The in situ cell is closed by an optical-quartz window (Heraeus). The sample resistance was measured using a Keithley 175A autoranging multimeter. The temperature of the Pt heater was calibrated prior to operation of the sensor. The sensitivity (S) of the sensor was defined as S = Ra/Rg, where Ra is the sensor resistance in the carrier gas and Rg the resistance in the target gas (ethanol, acetaldehyde, ethene). For gas-sensing experiments, the following gases (Westfalen AG) were employed: oxygen 5.0 (≤0.2 ppm of CO2, ≤0.2 ppm of CnHm, ≤3 ppm of H2O, ≤10 ppm of N2 + Ar), nitrogen 5.0 (≤3 ppm of O2, ≤1 ppm of CnHm, ≤5 ppm of H2O), 1000 ppm of ethanol (acetaldehyde, ethene) in nitrogen 4.0. The carrier gas was synthetic air (80% N2, 20% O2) or nitrogen cycled with 250 ppm of ethanol (acetaldehyde, ethene) in synthetic air or nitrogen; all gas mixtures were fed at 40 mL/min. Raman spectra were collected in a 180° backscattering geometry using a 20× objective (Olympus SLMPLN20x, WD: 25 mm, NA: 0.25). For excitation, the 514.5 nm output of an Ar+ laser (Melles Griot) was employed. The applied laser power was 7 mW as measured with a power meter at the location of the sample (Ophir). At such a power level, there was no damage on the sample even after several hours of laser irradiation. On the other hand, there was a small influence of the laser irradiation on the sheet resistance of the indium oxide,

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EXPERIMENTAL SECTION In2O3 Gas Sensor Preparation. For the preparation of indium oxide (In2O3), 2.5 g of indium(III) nitrate hydrate (In(NO3)3·5H2O, Sigma-Aldrich, 99.99%) was dissolved in 50 mL of deionized water. When 10 mL of ammonia solution (25 wt % in water) was added, white indium hydroxide was precipitated at pH 10. The precipitate was collected by decanting and centrifugation at 4500 rpm for 10 min. After being washed with deionized water, it was finally dried in air at 100 °C for 24 h. The product was ground and heated at 800 °C (10 °C/min) for 2 h in a muffle furnace. After cooling, the powder was ground again and sieved with a 56 μm sieve. For sensor preparation, approximately 20 mg of In2O3 was ultrasonically dispersed in deionized water for 10 min and then 25604



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which, however, was marginal compared to the resistance change by the gas-sensing process. The backscattered light was dispersed using a HoloSpec f/1.8i Raman spectrometer (Kaiser Optical Systems) with an axial transmission grating equipped with a Peltier-cooled charge-coupled device (CCD) detector. The resolution of the spectrometer is 5 cm−1; however, the wavelength stability was better than 0.5 cm−1. In the case of ethanol gas sensing, the Raman spectra were collected continuously with an accumulation time of 1000 s at 190 °C and 1400 s at 325 °C. For the acetaldehyde or ethene experiments the accumulation time was 1000 s. Spectra were independent of the measuring position and of the distance to the electrodes. The PTFE cell made a small contribution to the Raman signal (733 cm−1) due to backscattered light from the cell walls. In contrast, gas-phase signals or signals of adsorbates on the Al2O3 transducer substrate or the PTFE walls were not observed. For correlation of the Raman spectra with the gassensing measurements, first a baseline correction was made; then the intensity was normalized to the measurement time; and finally the Raman bands were integrated. Gas products were analyzed by FTIR spectroscopy (Bruker Vertex 70 FTIR spectrometer, pyroelectric DLaTGS detector) in a small-volume (25 mL) gas cell (Axiom, LFT) heated to 125 °C to avoid condensation. IR spectra were measured continuously with a resolution of 4 cm−1 and a measurement period of 600 scans (∼505 s) using the carrier gas as reference. To correlate the IR and gas-sensing measurements, the IR bands were baseline corrected, and their intensities were plotted against time. Any ethanol contribution was subtracted.

Table 1. Assignment of the Observed Raman Bands wavenumber [cm−1]

vibration

assignment

307 360 406 496 629 938 1330, 1563 2855, 2860, 2868 2939 2966 3643 3658

δ(InO6) υ(In−O−In)

c-In2O319 c-In2O319 reduced In2O322 c-In2O320 c-In2O320 acetate carbon23 formate,26 adsorbed ethanol24 acetate,25 adsorbed ethanol24 adsorbed ethanol24 bridging hydroxyl groups21 bridging hydroxyl groups21

υ(InO6) υ(InO6) υ(C−C) D-, G-band υ(CH) υ(CH) υ(CH) υ(OH) υ(OH)

Table 2. Assignment of the FTIR Gas-Phase Bands Used for the Correlation in Figures 6, 7, and 8 wavenumber [cm−1]

gas

1229 2178 2361 2733 2903 3140 3902

H3C−CO−CH3 CO CO2 H3C−CHO H3C−CH2−OH H2CCH2 H2O

shows an intensity decrease, and the band at 3658 cm−1 completely disappears. These changes are attributed to the reaction of ethanol with surface hydroxyl groups, leading to acetate formation. The formation of acetate groups is confirmed by the observation of the C−C and CH3 symmetric stretch vibrations at 938 and 2939 cm−1, respectively, although a small contribution of adsorbed ethanol to the latter band cannot be ruled out.24,25 Upon switching to nitrogen, initially no spectroscopic changes are observed. However, with time the ethanol-induced changes become weaker. In contrast, exposure to synthetic air immediately reestablishes the state of the sample prior to ethanol exposure, as indicated by the disappearance of the signals of the reduced indium oxide and acetate species as well as the intensity increase of the hydroxyl bands. In the presence of 250 ppm EtOH/syn.air a smaller degree of indium oxide reduction is observed as compared to EtOH/N2, based on the intensity of the 307 cm−1 band. Besides, an additional strong Raman feature at 2868 cm−1 is present, which is attributed to the C−H symmetric stretch vibration of a formate-like species, in agreement with the literature.26 Exposing the gas sensor to synthetic air results in removal of the ethanol-induced spectroscopic changes, indicating the reversibility of the spectroscopic response in synthetic air. Increasing the temperature to 325 °C in synthetic air does not induce any significant changes in the Raman spectrum (see Figure 1). Similarly, the spectrum recorded in the presence of ethanol strongly resembles the previous spectra at 325 °C. However, the situation in ethanol changes dramatically upon switching from synthetic air to nitrogen: the intensity increase of the bands at 325 and 406 cm−1 indicates reduction of indium oxide (see above). In addition, the appearance of new broad bands within 850−1650 cm−1 and 2700−3000 cm−1 as well as an increase in the background at higher wavenumbers are observed. As discussed in the following, these features are

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RESULTS AND DISCUSSION Raman Spectroscopy. Figure 1 shows the Raman spectrum of the prepared indium oxide sensor material in synthetic air at room temperature (23 °C). The spectrum is characterized by bands at 307, 360, 496, and 629 cm−1, confirming the presence of bixbyite-type indium oxide (cIn2O3).19 In detail, the Raman bands at 307 and 360 cm−1 are assigned to the bending vibration of the octahedron (δ(InO6)) and the In−O−In stretch vibration (υ(In−O−In)), respectively. The two bands at 496 and 629 cm−1 result from stretch vibrations of the octahedron (υ(InO6)).20 In the highfrequency part of the spectrum, a broad Raman band at around 3650 cm−1 is observed. Deconvolution yields two features located at 3643 and 3658 cm−1, which are assigned to bridged surface hydroxyl (type II) species, in agreement with the literature.21 The observed bands and their assignments are summarized in Tables 1 and 2. Figure 1 depicts also Raman spectra of indium oxide at 190 °C (black) and 325 °C (blue) in the presence of N2, EtOH/N2, syn.air, and 250 ppm EtOH/syn.air, as indicated. The spectrum at 190 °C in nitrogen is characterized by Raman bands at 307, 360, 496, 629, 3643, and 3558 cm−1, which can be assigned as discussed above (see Table 1). The red-shift of the In2O3related bands originates from thermally induced lattice expansion leading to mode softening. Upon exposure to 250 ppm of EtOH/N2, a strong intensity increase of the 360 cm−1 band is observed. Besides, a new band and a shoulder appear at 406 and 325 cm−1, respectively, which are attributed to nearsurface species of reduced indium oxide, in agreement with the literature.22 This assignment is consistent with their disappearance when the sample is exposed to synthetic air (see Figure 1). In addition, in the presence of ethanol new bands appear at 938 and 2939 cm−1; however, the hydroxyl band at 3643 cm−1 25605



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Figure 2. Temperature-dependent Raman spectra of the indium oxide gas sensor in the presence of 250 ppm EtOH/N2. Spectra are offset for clarity.

Figure 3. Temperature-dependent Raman spectra of the indium oxide gas sensor in the presence of 250 ppm EtOH/N2/40% O2. Spectra are offset for clarity.

associated with the formation of carbon species and CHx bands resulting from adsorbate decomposition, respectively. In contrast to the reaction at 190 °C, the changes caused by the presence of ethanol are reversible in pure nitrogen. Additionally, temperature-dependent Raman spectra of the indium oxide gas sensor exposed to 250 ppm EtOH/N2 were measured (see Figure 2). The spectrum of 250 ppm EtOH/N2 at 23 °C is characterized by Raman bands at 2872, 2930, 2966, and 3643 cm−1. The latter feature arises from bridged surface hydroxyl as discussed above. The other bands are attributed to adsorbed ethanol. On the basis of the literature24 the features at 2872, 2930, and 2966 cm−1 can be assigned to υs(CH2), υs(CH3), and υas(CH3), respectively. A temperature increase results in spectroscopic changes in both the low- and highwavenumber regions. At 100 °C, a new band appears at 942 cm−1, showing a red-shift with increasing temperature up to 938 cm−1 at 250 °C. Besides, between 23 and 250 °C, a change in the vibrational signature in the C−H stretching range is observed: the Raman features at 2872 and 2966 cm−1 decrease in intensity, whereas the band at 2930 cm−1 shows a 7 cm−1

blue-shift. These changes are attributed to the formation of adsorbed acetate due to the presence of the C−C (υ(C−C)) and CH3 (υ(CH3)) symmetric stretch vibrations at 938 and 2937 cm−1, respectively (see Table 1). This behavior is also consistent with the reference spectrum of acetaldehyde in the presence of indium oxide (see Figure 4). At 200 °C, the ethanol-related features have largely disappeared. Above 250 °C, the Raman spectrum completely changes: New broad bands appear within 850−1650 cm−1, which are characteristic for carbon species (see Table 1), whereas the acetaldehyderelated bands at 938 and 2937 cm−1 are no longer observed. These simultaneous spectroscopic changes are strong indications for a thermally induced decomposition of adsorbed species at temperatures >250 °C. A further increase in temperature results in the removal of the carbon features, which is attributed to oxidation processes induced by oxygen diffusion from the bulk. The temperature-dependent Raman spectra of the indium oxide gas sensor exposed to 250 ppm EtOH/N2/40% O2 are depicted in Figure 3, showing the oxygen influence on the 25606



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Figure 4. Raman spectra of the indium oxide gas sensor at 190 °C (black) and 325 °C (blue) in the presence of N2, 250 ppm CH3CHO/N2, synthetic air, and 250 ppm CH3CHO/syn.air, as indicated. Spectra are offset for clarity.

Figure 5. Raman spectra of the indium oxide gas sensor at 190 °C (black) and 325 °C (blue) in the presence of N2, 250 ppm C2H4/N2, synthetic air, and 250 ppm C2H4/syn.air, as indicated. Spectra are offset for clarity.

When oxygen is added (see Figure 3) the reduced In2O3 state is lower and could only be detected up to 200 °C. Figure 4 shows Raman spectra of indium oxide at 190 °C (red) and 325 °C (blue) in the presence of N2, CH3CHO/N2, syn.air, and 250 ppm CH3CHO/syn.air as indicated. The spectrum of indium oxide at 190 °C in nitrogen was discussed above (see Figure 1). Exposing the gas sensor to 250 ppm acetaldehyde in nitrogen (CH3CHO/N2) results in a strong intensity increase of the 361 cm−1 band as well as the appearance of a new band and a shoulder at 407 and 325 cm−1, respectively, which originate from indium oxide reduction (see above). Besides, additional new bands appear at 938 and 2939 cm−1, whereas the hydroxyl bands at 3643 and 3658 cm−1 completely disappear. These changes are consistent with those observed for ethanol exposure (see Figure 1) and are attributed to the reaction of acetaldehyde and surface hydroxyl groups to form acetate. The fact that the former bands are observed not only in the case of ethanol exposure but also in the presence of

ethanol reaction. At room temperature, the high-wavenumber spectrum resembles that in 250 ppm EtOH/N2 which is characteristic for adsorbed ethanol. Again, adsorbed acetate appears with increasing temperature until 250 °C, indicated by the Raman bands at 938 and 2939 cm−1 (see above). Unlike in Figure 2, the reaction in 250 ppm EtOH/N2/40% O2 must lead to additional adsorbates because of the different CH-vibration signature: Apart from the acetate signal the Raman band at 2872 cm−1 remains very intensive and shows a 9 cm−1 red-shift with increasing temperature. Furthermore, a small band at 1374 cm−1 can be seen. These new features were attributed to formate-like species.26 Compared to Figure 2, the hydroxyl bands are much more intensive between 200 and 350 °C in 250 ppm EtOH/N2/40% O2, which could be related to the detected increase in water concentration in the gas phase (not shown). In 250 ppm EtOH/N2 (see Figure 2), the reduced state of the indium oxide near the surface indicated by the band at 407 cm−1 could be proved over the complete temperature range. 25607



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Figure 6. Temporal correlation of spectroscopic data and sensor resistance of the operando experiments of the ethanol gas sensing of indium oxide (*10% O2 + 90% N2).

band at 2860 cm−1 appears, and at 325 °C in 250 ppm ethene in nitrogen (C2H4/N2), an increase of the 361 cm−1 feature is observed (see Discussion below). Besides, there are changes in the Raman background, which may originate from lowconcentration surface species. Temporal Correlation. Figure 6 depicts the temporal correlation of the spectroscopic Raman and IR gas-phase data with the sensor resistance during ethanol gas sensing of the indium oxide gas sensor. Switching the gas atmosphere from nitrogen to 250 ppm ethanol in nitrogen (EtOH/N2) at 190 °C results in indium oxide reduction, and bridged surface hydroxyl groups react with ethanol to adsorb acetate species. Under these conditions acetaldehyde and water were detected as gasphase products.27 Under ethanol exposure the resistance drops strongly as ethanol is oxidized to adsorbed acetate by indium oxide, as indicated by the intensity increase of the Raman bands at 360 and 407 cm−1. Another factor contributing to the strong change in sensor resistance is the acetate stability, which leads to only a slow recovery of the sensor and spectroscopic signals even after switching back to nitrogen. Addition of oxygen results in an immediate drop of the bands at 360 and 407 cm−1. The hydroxyl bands reappear and increase in intensity up to their original level prior to reaction with ethanol, whereas the acetate-related features vanish as a result of direct decomposition and/or further oxidation processes. The immediate increase in sensor resistance upon oxygen exposure is explained by rapid indium oxide reoxidation. However, the resistance does not return to its original value, most probably due to residual adsorbates that are below the Raman detection limit. Upon exposure to 250 ppm ethanol in synthetic air, the indium oxide reduction is less pronounced than in EtOH/N2 owing to immediate reoxidation by oxygen. Again, the hydroxyl bands largely disappear, but now less acetate is observed because of its partial decomposition to formate-like species. In the corresponding FTIR spectra, acetaldehyde, carbon dioxide, and water were detected as reaction products. As compared to the reaction in EtOH/N2, the main product acetaldehyde is now formed at a higher rate, which is due to a larger number of free

acetaldehyde strongly supports their assignment as acetate features. Switching back to pure nitrogen leads to a small intensity decrease of the acetaldehyde-induced features. However, as shown in Figure 4, at 190 °C in nitrogen the changes are slow. In contrast, the presence of synthetic air immediately restores the spectrum prior to acetaldehyde exposure. When adding acetaldehyde to the feed (CH3CHO/ syn.air), the overall spectrum shows strong similarity to that observed in CH3CHO/N2 except for the small feature at 2855 cm−1. Thus, in contrast to the behavior in ethanol, the presence of oxygen has only a minor influence on the Raman spectrum in the case of acetaldehyde, and the formation of formate-like species is largely suppressed. Switching back to synthetic air results in a spectrum resembling those prior to acetaldehyde exposure. A temperature increase to 325 °C in synthetic air does not induce any significant changes in the Raman spectrum (see Figure 4). Likewise, the spectrum obtained during exposure to acetaldehyde (CH3CHO/syn.air) strongly resembles the previous spectra. In contrast, in the presence of acetaldehyde in nitrogen major spectroscopic changes are observed, i.e., the appearance of broad carbon-related bands within 850−1650 cm−1, an intensity decrease of the indium oxide-related bands, as well as an increase in the background at higher wavenumbers (2700−3100 cm−1). On the other hand, there is no indication of adsorbate features besides carbon. Thus, similar to the behavior in ethanol (see Figure 1) these observations suggest the decomposition of adsorbates, leading to carbon formation. As a result of the carbon deposit, part of the visible radiation is absorbed at the sample surface, thereby significantly reducing the scattering intensity of the indium oxide features. In contrast to the reactions at 190 °C, the changes caused by the presence of acetaldehyde were reversible in pure nitrogen. In Figure 5 Raman spectra of indium oxide at 190 °C (red) and 325 °C (blue) in the presence of N2, C2H4/N2, syn.air, and 250 ppm C2H4/syn.air are shown. Overall, the presence of ethene results only in minor spectroscopic changes. Most notably, at 190 °C in 250 ppm ethene a small formate-related 25608



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Figure 7. Temporal correlation of spectroscopic data and sensor resistance of the operando experiments during exposure of acetaldehyde to indium oxide.

result from the deposition and thermally induced removal of carbon by oxygen and hydrogen.16 In comparison to 190 °C a higher ethanol conversion was observed. As gas products, CO2, H2O, acetaldehyde, ethene, and CO were detected. Switching from EtOH/N2 to pure nitrogen results in an increase in resistance that is not as fast as in the presence of oxygen at 325 °C but faster than at 190 °C. The resistance nearly returns to its initial value. The oscillation of the resistance is smaller than in ethanol/nitrogen (not shown). The ethanol-induced changes in the Raman spectra are reversible; i.e., both the bands related to reduced indium oxide and deposited surface carbon disappear, while those related to hydroxyl groups reappear. This behavior indicates that at 325 °C, in contrast to the behavior at 190 °C, in indium oxide diffusion processes from the bulk to the surface are fast enough to provide oxygen for indium oxide reoxidation and oxidation of residual carbon. Finally, when oxygen was switched on, the resistance increased at once to the initial state in synthetic air due to refilling of oxygen vacancies and oxidation of carbon residues. In Figure 7 the temporal correlation of the spectroscopic Raman and IR gas-phase data with the sensor resistance during reaction of 250 ppm acetaldehyde with the indium oxide gas sensor is shown. With a few exceptions the behavior is qualitatively consistent with the operando results obtained during ethanol conversion (see Figure 6). At 190 °C in acetaldehyde in nitrogen (CH3CHO/N2), indium oxide is reduced, and surface acetate is formed by reaction of hydroxyl groups with adsorbed acetaldehyde. Similarly to the behavior in ethanol, exposure to synthetic air leads to an abrupt increase in resistance; whereas indium oxide is reoxidized, the hydroxyl groups are restored, and the acetate-related bands disappear. Switching to 250 ppm CH3CHO/syn.air flow, again, results in acetate formation and consumption of hydroxyl groups. However, as compared to the behavior in ethanol, indium oxide is reduced to a smaller degree (see Figure 9), and in the Raman spectra a slightly red-shifted formate band at 2855 cm−1 is observed. Small amounts of water and CO2 are observed as gas-phase products. The latter may result from acetate

sites resulting from lower acetate stability in the presence of oxygen. As indium oxide is permanently oxidized the resistance does not decrease as much as in EtOH/N2, resulting in an overall smaller degree of indium oxide reduction. In synthetic air, the observed changes in the Raman spectra caused by the ethanol reaction were largely reversible. This is supported by the indium oxide reoxidation, the restoration of the hydroxyl groups, and the disappearance of the adsorbate-related bands. Subsequently, the sensor was heated to 400 °C to remove all adsorbates from the surface. Owing to their thermal decomposition, the formation of CO, CO2, and H2O is observed in the FTIR spectra. When the temperature was reduced to 190 °C again, the resistance had a significantly higher value compared to the value prior to heating, demonstrating the high sensitivity of the sensor signal for the presence of adsorbates even at spectroscopically not detectable quantities. Exposure to ethanol in synthetic air (EtOH/syn.air) at 325 °C results in significantly higher gas product concentrations as compared to 190 °C. As reaction products, CO2, H2O, acetaldehyde, ethene, and CO were detected. The increased rate of product formation is attributed to much faster surface reactions. As a result, no adsorbate-related bands can be observed. Besides, the Raman spectra do not show reduced indium oxide, which, however, does not exclude a permanent reduction and reoxidation of the indium oxide. Upon switching to synthetic air at 325 °C the resistance approaches its original value. The Raman spectra do not show any changes, and the gas product concentrations subside to zero. As shown in Figure 5, exposure to 250 ppm ethanol in nitrogen (EtOH/N2) at 325 °C leads to a stronger decrease in resistance as in EtOH/syn.air, which is a result of the missing reoxidation by oxygen. In contrast to the behavior at 190 °C, no hydroxyl bands can be observed due to the presence of carbon deposits caused by adsorbate decomposition. A detailed analysis of the sensor signal shows that carbon deposition is accompanied by periodic oscillations of the resistance (not shown). According to the literature, such periodic oscillations 25609



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Figure 8. Temporal correlation of spectroscopic data and sensor resistance of the operando experiments during exposure of ethene to indium oxide.

Figure 9. Influence of gas environment and temperature on the degree of reduction (left panel; intensity of Raman band at 361 cm−1 was normalized to the intensity of the band at 307 cm−1 and corrected for the intensity in the carrier gas) and conversion (right panel) of indium oxide gas sensors.

like species and adsorbed ethanol (see Table 1). However, no adsorbed ethene was observed, as the expected bands at 942− 951 cm−1, 1338−1342 cm−1, 1599−1618 cm−1, 2984−3010 cm−1, and 3060−3079 cm−1 are not observed.29 The formation of adsorbed ethanol is consistent with the proposed reaction mechanism of ethene with metal oxides by means of electrophilic addition of hydroxyl groups across the ethene double bond.30 Water and CO2 are the only reaction products detected, in agreement with the literature. In contrast to the behavior in ethanol and acetaldehyde, at 190 °C in 250 ppm of C2H4/N2 the resistance signal shows oscillations, indicating the presence of carbon deposits (not shown). Also in contrast to ethanol and acetaldehyde, heating to 400 °C leads to a decrease in resistance, which points to the presence of only small amounts of residues or no residues at all after ethene reaction in synthetic air at 190 °C. Thus, in comparison with ethanol and acetaldehyde, ethene shows a distinctly different adsorption/reaction behavior, as discussed in more detail below. Gas-Sensing Mechanism. The results from Figures 6−8 were compared regarding the degree of reduction of indium oxide as well as the gas conversion. As shown in the left panel of Figure 9, at 190 °C the degree of reduction under reaction conditions, based on the Raman band at 361 cm−1, decreases in

decomposition via formate-like species, as proposed in the literature.16 The spectroscopic and sensor behavior upon exposure to synthetic air and during heating to 400 °C resembles that in ethanol (see Figure 6). For exposure to 250 ppm acetaldehyde in synthetic air at 325 °C no changes in the Raman spectra are observed, while CO2, CO, and water are detected as gas products. In contrast, in acetaldehyde in nitrogen at 325 °C the Raman spectra show the characteristic bands due to reduced indium oxide and carbon deposits. Under these conditions CO2, CO, water, and acetone are detected as gas products.28 Figure 8 shows the temporal correlation of the operando spectroscopic data with the sensor resistance during reaction of 250 ppm ethene with the indium oxide gas sensor. In comparison with the behavior in ethanol and acetaldehyde, the Raman spectra show only minor changes as a function of the gas environment. In ethene no Raman band appeared at 407 cm−1. Thus, the phonon band at 361 cm−1 was used to determine the degree of indium oxide reduction (see Figure 9). On the basis of this feature, only in 250 ppm ethene in nitrogen at 325 °C was a weakly reduced state of indium oxide observed. Also, in ethene at 190 °C the hydroxyl groups showed only a small decrease in intensity. The spectra show a weak C−H stretching band at 2860 cm−1, which is attributed to formate25610



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Figure 10. Influence of gas environment and temperature on resistance (left panel) and sensitivity (right panel) of indium oxide gas sensors.

only minor (in contrast to ethanol), and we conclude that the oxygen-induced changes in the sensing behavior are a result of the varying degree of indium oxide reduction. In comparison to acetaldehyde, in ethanol at 190 °C there is a much smaller difference in the resistance for EtOH/N2 and EtOH/syn.air flow (see Figure 10). This may indicate that the formation of formate-like species influences the response of the gas sensor by reducing the resistance. A comparison of the ethanol and acetaldehyde reaction with indium oxide at 325 °C reveals that in the case of acetaldehyde the addition of oxygen results in larger changes of the resistance and sensitivity as well as a larger increase in gas conversion. On the other hand, for the CH3CHO/N2 feed the largest degree of reduction and the largest concentration of carbon deposits were observed (see Figure 9). Thus, the influence of oxygen on the sensing of ethanol and acetaldehyde at 325 °C may be explained by the different amounts of carbon residues and the degree of indium oxide reduction. From the above discussion it is apparent that the acetaldehyde reaction reflects a major part of the processes underlying ethanol gas sensing. Regarding the comparison of the ethene and ethanol sensing behavior, Figure 10 shows significant differences in the trends of the resistance and sensitivity. In general, the spectroscopic changes observed for the ethene reaction were minor; i.e., compared to the ethanol reaction there is a smaller amount of adsorbates, and indium oxide reduction was not detectable at all, except for C2H4/N2 feed at 325 °C. Oscillations of the resistance indicate the presence of carbon deposits, which are oxidized to CO2 when oxygen is added (not shown). The reaction mechanism is based on the oxidation of ethene to CO2 and H2O. In the course of the reaction either formate-like or carbon species may serve as intermediates. In contrast to ethanol, the ethene sensitivity in synthetic air increases for a temperature rise from 190 to 325 °C. As the major differences between the ethene and ethanol reactions concern the missing acetate, the smaller degree of indium oxide reduction, and the high selectivity for total oxidation, it appears conclusive that the ethene sensitivity in synthetic air is based on the total oxidation. The increasing ethene sensitivity with rising temperature in the absence of oxygen can be explained by the increasing amount of carbon deposits and the resulting degree of reduction. Like the acetaldehyde reaction, the ethene reaction represents part of the processes underlying ethanol sensing, namely, the contribution of the total oxidation. The results from ethene sensing show that at low temperatures (190 °C) the sensitivity is determined by the adsorbate concentration and their residence time on the sensor surface rather than the total amount of gas products. On the other hand, at higher

the order ethanol > acetaldehyde > ethene; in all cases the degree of reduction was smaller in the presence of oxygen. At 325 °C in oxygen no reduction was detectable. In the absence of oxygen at 325 °C the degree of reduction decreases as acetaldehyde > ethanol > ethene. Interestingly, the same trend was observed with respect to the amount of carbon deposits, indicating a correlation of the degree of indium oxide reduction and the amount of carbon residues. Similarly, at 190 °C the degree of reduction was higher the larger the amount of adsorbates. Despite the similarity of the detected adsorbates, the different degree of reduction observed for ethanol and acetaldehyde at 190 °C can be related to the different reaction pathways. In the case of the reaction of ethanol to acetate one more hydrogen atom needs to be separated compared to the acetaldehyde reaction. Thus, the oxidation number of carbon needs to undergo a larger change for the ethanol reaction compared to the acetaldehyde reaction. The right panel of Figure 9 shows a comparison of the gas conversion based on the FTIR gas-phase spectra. At 190 °C, only small conversion is observed, probably due to insufficient activation of adsorbed species at this temperature. In the presence of oxygen, the conversion increases, as the surface reactions are no longer limited by the availability of oxygen. As the amount of indium oxide was different for the experiments in ethanol, acetaldehyde, and ethene, a comparison of the absolute conversion values needs to be handled with care. Therefore, in the following only trends will be discussed. Thereby, the ratio of the ethene conversion in synthetic air as compared to nitrogen at 325 °C is significantly higher than in the case of ethanol and acetaldehyde. This behavior points to total oxidation as the dominant reaction pathway for ethene conversion and is consistent with the observed gas-phase products CO2 and H2O. In contrast, in acetaldehyde also CO and acetone were detected, whereas in ethanol FTIR spectra show the largest amounts of gas-phase products, consisting of CO2, H2O, CO, acetaldehyde, acetone, and ethene. In the following, the influence of the gas environment and temperature on the resistance and sensitivity of the indium oxide gas sensor will be discussed. For ethanol and acetaldehyde, Figure 10 depicts the same trends for the resistance and sensitivity. In both cases the operando results at 190 °C in nitrogen show strongly reduced indium oxide and formation of surface acetate. In contrast, in the presence of oxygen, differences in the nature of the adsorbates are observed in addition to the smaller degree of reduction. Apart from acetate, exposure to ethanol and acetaldehyde leads to different types and concentrations of formate-like species. As for acetaldehyde, the influence of oxygen on the adsorbates is 25611



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temperatures (325 °C) the total oxidation becomes more important, owing to increasing conversion to CO2.

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CONCLUSIONS We have used operando Raman/FTIR spectroscopy to elucidate the ethanol gas-sensing mechanism of indium oxide. Depending on the detailed working conditions (gas environment, temperature), different adsorbates, i.e., acetate, formate-like, and carbon species, are present on the indium oxide surface, and there are changes in the surface hydroxyl concentration and the degree of indium oxide reduction. A subtle dependence of the sensor response on the detailed state of the indium oxide surface is observed. In detail, during ethanol gas sensing at 190 °C surface acetate and reduced indium oxide are detected, which are correlated with a strong reduction in sensor resistance. Addition of oxygen results in the formation of surface formate-like species besides acetate, as well as in partial reoxidation of indium oxide. At the elevated temperature of 325 °C there is a significant increase in the amount of the gas-phase products acetaldehyde, ethene, carbon oxide, and water. Under these conditions the sensor surface is characterized by carbon species if oxygen is absent. It was shown that the reactions of acetaldehyde and ethene with indium oxide represent part of the processes underlying ethanol gas sensing. For ethanol gas sensing by indium oxide the following reaction mechanism is proposed: (i) Ethanol is adsorbed on the indium oxide surface; (ii) at 190 °C adsorbed ethanol may react further to acetate (iii) or decompose partially to formate-like species in the presence of oxygen; (iv) at 325 °C adsorbate decomposition results in the formation of carbon deposits, (v) which are oxidized to CO2 and H2O in the presence of oxygen. In conclusion, based on the operando results the sensor signal can be correlated with the nature of the adsorbates, the presence of surface hydroxyl groups, and the indium oxide oxidation state. Our findings underline that detailed spectroscopic studies under working conditions are essential for elucidating the mode of operation of gas sensors.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Aleksander Gurlo for his contribution in designing the experimental setup and Karl Kopp for technical support.

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REFERENCES

(1) Williams, D. E. Semiconducting Oxides as Gas-Sensitive Resistors. Sens. Actuators, B 1999, 57, 1−16. (2) Barsan, N.; Koziej, D.; Weimar, U. Metal Oxide-Based Gas Sensor Research: How to? Sens. Actuators, B 2007, 121, 18−35. (3) Korotcenkov, G. Metal Oxides for Solid-State Gas Sensors: What Determines our Choice? Mater. Sci. Eng., B 2007, 139, 1−23. (4) Gurlo, A.; Riedel, R. In Situ and Operando Spectroscopy for Assessing Mechanisms of Gas Sensing. Angew. Chem., Int. Ed. 2007, 46, 3826−3848. (5) Sänze, S.; Gurlo, A.; Hess, C. Monitoring Gas Sensors at Work: Operando Raman-FTIR Study of Ethanol Detection by Indium Oxide. Angew. Chem., Int. Ed. 2013, 52, 3607−3610. 25612



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(27) Traces of humidity in the tubing may have given contributions to the water signal. (28) In separate experiments using mass spectrometry, acetone was also detected during exposure of indium oxide to ethanol. (29) Busca, G.; Lorenzelli, V.; Ramis, G.; Saussey, J.; Lavalley, J. C. FT-IR Spectra of Ethylene Molecularly Adsorbed on Metal Oxides. J. Mol. Struct. 1992, 267, 315−329. (30) Harrison, P. G.; Maunders, B. Tin Oxide Surfaces. Part 14. Infrared Study of the Adsorption of Ethane and Ethene on Tin(IV) Oxide, Tin(IV) Oxide-Silica and Tin(IV) Oxide-Palladium Oxide. J. Chem. Soc., Faraday Trans. I 1985, 81, 1311−1327.

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Ethanol Gas Sensing by Indium Oxide: An Operando Spectroscopic

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