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Identifying the active oxygen species in SnO2 based gas sensing materials: an operando IR spectroscopy study David Degler, Susanne Wicker, Udo Weimar, and Nicolae Barsan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04082 • Publication Date (Web): 08 May 2015 Downloaded from http://pubs.acs.org on May 14, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

INENTIFYING THE ACTIVE OXYGEN SPECIES IN SnO2 BASED GAS SENSING MATERIALS: AN OPERANDO IR SPECTRSOCOPY STUDY David Degler, Susanne Wicker, Udo Weimar and Nicolae Barsan* Institute of Physical and Theoretical Chemistry (iptc), University of Tuebingen, Auf der Morgenstelle 15, D-72076 Tuebingen, Germany

ABSTRACT: This work demonstrates that it is possible to follow the surface chemistry of oxygen on SnO2 based gas sensing materials using operando Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS). The inherent difficulties, due to the intrinsic properties of the studied oxide and the limitations of the method, were overcome by comparing the results obtained for two different materials and by using of isotopically labeled gases together with the simultaneous measurement of the sensor signals. In spite of the differences in the surface composition and reactivity between the different materials, the experimental results show that the reactive oxygen species are similar in nature and the gas recognition takes place by the interplay of surface reduction and (re-)oxidation.

1

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1. Introduction Gas sensing with Semiconducting Metal OXide (SMOX) sensors is based on the interplay of reception – adsorption/reaction of the target gas and the corresponding charge transfer processes between it and the SMOX – and transduction, which is the transformation of those charge transfer processes into measurable changes in electrical resistance of the sensing layer 1,2. For SnO2 gas sensors the generally accepted working principle is linked to the ionosorption of atmospheric oxygen onto the SnO2 surface that involves trapping electrons from the conduction band and, consequently, determines the appearance of an electron depleted region at the surface of the SnO2 grains and the increase of the sensor resistance 3. Reducing gases, like CO and H2, are considered to react with ionosorbed oxygen (reception) decreasing the band bending and consequently the sensor resistance (transduction). Until it was shown by our group that reducing gases dosed in the absence of oxygen strongly decrease the sensor resistance

4–6

, it was not considered that the sensing of reducing gases can take place in the

absence of the atmospheric oxygen; since in our experiments no reaction products of CO or H2, specifically CO2 or H2O, were found, it was assumed that donor species are formed, carbonyl-like (SnCO+) or protons (H+) respectively, which, by donating electrons to the SMOX are determining the appearance of a surface accumulation layer

4–6

. It was also assumed that the downwards bending of

the energy bands and the appearance of the corresponding accumulation layer is not taking place in realistic application conditions i.e. in air, which means that the clarification of the nature of the above mentioned donors is more of academic interest. Quite recently, though, we found that such a switch from a depletion layer controlled to an accumulation layer controlled conduction mechanism occurs under realistic conditions (20.5 %vol O2, r.h. > 6 %r.h., c(CO) < 300 ppm) 7. Besides the obvious consequences for the transduction – this switch has major implications on the effect of band bending changes onto the measured sensing layer resistance – there are significant implications also for the 2


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reception: additional to the reaction with ionosorbed oxygen species we have to take into consideration also the direct reaction between the reducing gases and the SnO2 surface. In the absence of experimental evidence, a good starting point is the theory: recent DFT calculations for SnO2 (101) and (110) surfaces, published by J.-M. Ducéré et al. 8 and X. Wang et al. 9 respectively, give a detailed description of the charge-transfer associated with CO oxidation. Consistently, it was found that CO reacts with 2-fold coordinated (bridging) oxygen forming CO2 and an oxygen vacancy (equation 1), which acts as an adsorption site for molecular and atomic oxygen species (equation 2). Adsorbed molecular oxygen may dissociate into two atomic oxygen species (equation 3a) or react with CO forming CO2 and bridging oxygen (equation 3b). The formation and healing of oxygen vacancies is coupled with a charge transfer of approximately one electron per site (α = 1) 8,9 between SnO2 and the adsorbed oxygen species. The chemical equations are simplified by disregarding the adsorption and desorption steps of the reaction educts/products. ܱைఈି + ‫ܸ → ܱܥ‬ை + ‫ܱܥ‬ଶ + ߙ ∙ ݁ ି

(1)

ܸை + ܱଶ + ߙ ∙ ݁ ି → ܱଶ ఈି ை

(2)

ܱଶ ఈି + ܸை + ߙ ∙ ݁ ି → 2ܱைఈି ை

(3a)

ܱଶ ఈି + ‫ܱ → ܱܥ‬ைఈି + ‫ܱܥ‬ଶ ை

(3b)

A Mars-van-Krevelen-like involvement of lattice oxygen for oxidation reactions is generally used to explain the catalytic properties of SnO2-based catalysts10–12. Noble metal doped SnO2 materials are reported to be suitable catalysts for the low-temperature oxidation of CO11,13 or methane14,15. While, in the case of CO oxidation, they present a high catalytic activity below 200 °C, undoped SnO2 is not reactive for low temperatures and shows a slowly increasing activity above 300 °C13. 3



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In addition to the oxidation of CO, a direct charge transfer from CO to SnO2 was also considered in both computational investigations with the finding that the charge transferred by a CO-donor species is much smaller than the charge transfer caused by a vacancy formation. Consequently, a large CO-donor concentration would be needed in order to determine sensor signals that are comparable to signals caused by vacancy formation. The formation of the latter is easy to understand; in case of the donor its chemical nature is unclear and species other than carbonyls have also to be taken into consideration. In various infrared spectroscopic studies surface carbonate species were found on SnO2 under CO exposure

11,16,17

. The calculations of X. Wang et al. indicate carbonates as an intermediate in the

reaction of molecular oxygen with CO (equation 4), while J.-M. Ducéré et al. propose that carbonates are formed by a subsequent reaction of CO2 with surface oxygen (equation 5). + ‫ܱܥ → ܱܥ‬ଷି → ܱைି + ‫ܱܥ‬ଶ ܱଶ ି ை

(4)

ܱைି + ‫ܱܥ‬ଶ ⇋ ‫ܱܥ‬ଷି

(5)

If the surface chemistry proposed by DFT calculations is a realistic model for CO oxidation/sensing, experimental evidence should be found using suitable operando techniques

18,19

; a good candidate is

Diffuse Reflectance Fourier-Transform Infrared Spectroscopy (DRIFTS), which was proven to be a suitable probe for the elucidation of the surface chemistry of gas sensing materials 17,20–22. In line with the discussed surface reaction mechanism (reception), one would expect to find the following: •

The process described by equation 1 should determine changes in the Sn-O vibration bands and its overtones.

•

A sufficiently high concentration of a surface donor, caused by CO exposure, is expected to determine changes in the range 2300 to 1300 cm-1 – corresponding to carbonyl and carbonate species. 4


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•

If surface carbonates are formed by a subsequent reaction – as proposed by J.-M. Ducéré – they should also be formed when SnO2 is exposed to the reaction product CO2.

Due to the fact that in the spectroscopic region of interest (4000 - 1000 cm-1) various bands of surface species (hydroxyls, oxygen, carbonyls, carboxylates and carbonates) appear, it is necessary to differentiate those in order to correctly interpret the measured spectra. This contribution presents such an approach, performed by utilizing isotopic labeled gases.

2. Experimental Section 2.1. Synthesis and Sample Preparation Over the past five decades various SnO2 materials were investigated using IR spectroscopy, but most of the materials differed in synthesis route and pretreatment; and hence in stoichiometry and crystal quality 11,16,17,21–25. In this study two materials were compared that: •

have different crystal qualities and particle sizes, but were still synthesized by the same route, and

•

are known to show good gas sensing performance.

The base material was synthesized starting with SnCl4 (Merck, purified by distillation) from which a cooled aqueous solution was made (23.5 % vol.) and carefully added to a cooled ammonia solution (3.8 % vol.). Afterwards 1 L bi-distilled water was added and the mixture was stirred for 2 h at room temperature. The precipitate was washed several times, until no more chloride was found in the solution, and dried at 80 °C; the as-obtained glassy aggregates where ground using a vibratory mill (3 h, 30 Hz) with ZrO2 balls (Ø 5 mm). In the next step, the resulting fine white powder was calcined for 8 h at 450 and 1000 °C, respectively and again ground for 3 h (same procedure as above). A detailed 5



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characterization of materials synthesized by the same route and calcination procedure is given elsewhere 26,27. Sensors were made from both powders by mixing them with 1,2-propanediol to obtain a printable paste, which was deposited on an alumina substrate equipped with interdigitated platinum electrodes and a backside platinum heater

28

. The printed sensors were dried at 70 °C and finally annealed by

subjecting them to a four temperature step procedure (300, 400, 500, 400 °C for 20 min each). The obtained sensors are referred to as UD450 and UD1000, corresponding to the respective calcination temperature of the powders. 2.2. Operando DRIFT spectroscopy For the operando characterization of gas sensors and calculation of absorbance spectra we followed the procedure described in

21

. For the experiments, a Bruker Vertex70v (mid-band MTC detector,

external high performance glowbar; 512 scans per spectrum, 1 cm-1 spectral resolution) and a Vertex80v (narrow-band MTC detector, internal glowbar; 1024 scans per spectrum, 4 cm-1 spectral resolution) FT-IR spectrometer were used for measurements in air and nitrogen, respectively. Hence the Vertex80v setup is dedicated to measurements in N2 the entire infrastructure consists of air tight stainless steel tubing (oxygen background < 15 ppm, measured with Zirox SGM400); additionally, a residual gas analyzer mass spectrometer (HIDEN HPR-20) is attached to the outlet of the sensing chamber, as well as a hygrometer (DMT152). The sensors were placed in a homemade sensing chamber with a KBr window, which was mounted in a diffuse reflectance spectroscopy cell (Harrick “Praying Mantis”), and were heated to 300 °C by using the backside heater. During all experiments the resistance of the sensing layer was recorded by a digital multimeter (Keithley 2000 and Agilent 34410A for the Vertex70v and Vertex80v setup, respectively). Gases were dosed by using homemade gas mixing stations. The background humidity in dry synthetic air was found to be below 10 ppm (Vertex 6


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70v) and 2 ppm in dry nitrogen (Vertex 80v), respectively. All experiments were performed at total flow rates of 150 sccm in air and 200 sccm in N2. Gases were supplied by Westfalen AG Münster; the carrier gases synthetic air (20.5 %vol O2) and N2 had a purity of 99.999 and 99.9999 %, respectively, and all target gases were purchased from the supplier as mixtures of H2 (99.999 %), CO (99.997 %) or CO2 (99.9 %) in synthetic air (99.999 %) and CO (99.997 %) in N2 (99.9999 %). Isotopic labeld gases had an isotopic purity of 99 % for 13CO and 99.8 % for D2. Humid conditions were achieved using evaporators filled with distilled water or heavy water (99.9 %).

3. Results and Discussion

Figure 1. Single Channel Spectra of SnO2 gas sensors produced from differently calcined SnO2 and measured during sensor operation. The spectra of UD1000 (1000 °C, 8 h) and UD450 (450 °C, 8 h) are plotted with a y-offset. Figure 1 shows the single channel spectra of the UD450 and UD1000 sensors operated in dry air (TSensor = 300 °C). Both samples exhibit typical features of a SnO2 surface: The presence of isolated hydroxyl 7



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groups at the surface is indicated by the appearance of sharp bands between 3750 and 3450 cm-1, while bridged (interacting) hydroxyls cause broad bands between 3600 and 2500 cm-1 17,21,22. The three bands between 3000 and 2800 cm-1 are artefacts due to a hydrocarbon contamination of the spectrometer and disappear when calculating absorbance spectra. A similar effect is observed for the bands between 1800 and 1500 cm-1, which are either artefacts or completely inert species. The bands between 1500 and 1000 cm-1 are assigned to hydroxyl deformation vibrations (1175 and 1250 cm-1 11) and Sn-O lattice vibrations/overtones (1380-1340 cm-1; 1120-960 cm-1 deformation vibrations appear between 1000 and 900 cm-1

11

29

). Several hydroxyl

. The strongly increasing absorption of

SnO2 around 900 to 800 cm-1 makes measurements below 900 cm-1 impossible 11. The overall shape of single channel and absorbance spectra measured on n-type SnO2 is - in addition to the absorption caused by functional groups of surface species and the SnO2 material itself - influenced by the absorption of free charge carriers which increases with decreasing wavenumber, showing a strong rise in absorbance below 2000 cm-1. We assume that the free charge carrier absorption and individual parameters of the spectrometer (light source, optics and detector) dictate the overall shape of the IR spectrum. Both materials show obvious differences in the nature and intensities of the hydroxyl groups: The UD450 material displays a series of vibrations assigned to isolated terminal (3723, 3662 and 3628 cm-1) and rooted (3527 and 3485 cm-1) hydroxyl groups

20,30,31

. Moreover, the hydroxyl region is dominated

by a broad band between 3680 and 2610 cm-1 indicating a high concentration of bridged hydroxyl groups. In contrast, the UD1000 material exhibits two bands of isolated terminal hydroxyl groups at 3736 and 3723 cm-1 and a broadened band centered around 3424 cm-1 assigned to bridged hydroxyl groups, which is less pronounced when compared to the UD450 material. For similarly synthesized SnO2 powders a decrease of the water and hydroxyls content was found as a function of the calcination 8


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temperature 26, hence it is most likely to assume that the UD450 surface has a higher degree of surface hydroxylation 26,27. With regard to UD450 the corresponding bands are less pronounced and, instead of a broad band, two separated bands at 1362 and 1334 cm-1 were found together with a stronger absorption in the hydroxyl regions 11. The differences found in the single channel spectra suggest that for UD450 the surface is dominated by hydroxyl groups, while UD1000 is mainly terminated by oxide species. The most interesting range in the quest of clarifying the interaction between CO and the surface of SnO2 is the one below 2400 cm-1. In this region the evidences for the different processes indicated by the experiments and the theory should be found: formation of donors such as carbonyls or oxygen related species. Because in that spectral region bands corresponding to our targets and the ones corresponding to hydroxyls and carbonates overlap, isotopic exchange experiments are chosen to discriminate between the different species. Moreover, in the case of the sensor’s electrical resistance recorded under exposure to N2 corresponds to the border between negative – dominant presence of oxygen ions – and positive charging of the SnO2 surface – dominant presence of CO related donors. From the transduction viewpoint, the former case indicates a conduction mechanism controlled by the presence of a surface depletion layer, while the latter points to conduction mechanism controlled by the presence of a surface accumulation layer. Consequently, the experimental conditions in this study were chosen in such a manner that the electrical resistance is either clearly above the nitrogen level or completely below it 7. They were: exposure to 100 ppm CO in dry air for the former and exposure to 100 ppm CO in humid air (10 %r.h.@25°C) for the latter case. Additionally, both materials were exposed to a situation in which almost no preadsorbed oxygen is playing any role by dosing 500 ppm CO in a N2 background.

9



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Figure 2. Absorbance spectra of UD1000 (a) and UD450 (b) during 100 ppm CO exposure in dry and humid (10 %r.h., 25 °C) air. For UD1000 the corresponding absorbance spectra (figure 2a) of the experiments performed in air show, in both dry and humid conditions, a decrease of O-H vibrations (3736, 3723 and 3424 cm-1) along with a decrease of various bands below 1400 cm-1. It is worth noticing that the overall shape of the absorbance spectra of UD1000 is different depending on the conditions (dry or humid). The possible cause can be the change in the electronic situation of SnO2, however a detailed analysis of this observation is out of the scope of this work. A similar change of the overall spectrum shape was not observed for UD450 under these conditions (figure 2b). Moreover, UD450 presents additional features when compared to UD1000: In dry conditions the intensity of terminal and rooted hydroxyl groups at 3723, 3628, 3527 and 3485 cm-1 and the broad absorption band of bridged hydroxyl groups (3680 to 2610 cm-1) is decreased, while the intensity of the band at 3662 cm-1 increases. Between 1550 and 1300 cm-1 four bands assigned to carbonate species arise

17,24

, but no decreasing bands below 1400

cm-1 are found. For both materials the formation of gas phase CO2 (2348 cm-1) is observed. All observed changes appear to be less pronounced in the presence of humidity. One should note that for 10


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both materials no carbonyl band was found during the exposure to CO under operation conditions, which casts a doubt over the presence of the corresponding type of donor. The explanation, for both materials, of the OH vibrations decrease (between 3800-2500 cm-1) is assumed to be caused by the competition between water vapor and CO for the same adsorption sites/reaction partners, namely preadsorbed oxygen ions, see also 21.

Figure 3. Absorbance spectra of UD1000 (a) and UD450 (b) recorded during the

12

CO-13CO exchange

experiment in dry air (500 ppm CO). Shifts are indicated by arrows. To confirm the absence of carbonate species on UD1000 and the correct assignment on UD450 we performed 12CO-13CO exchange experiments (figure 3). A high concentration (500 ppm) was chosen to amplify possible effects – maybe additional bands, e.g. carbonyls, are already there at much lower concentrations, but more difficult to observe. On UD1000 only shifts of gas phase CO and CO2 are observed, while in the spectrum of UD450 CO, CO2, and carbonate bands are shifted. Again, also in the exchange experiments no carbonyl species were found.

11



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Based on the harmonic oscillator approximation the isotopic shift can be estimated using the reduced masses of 12C=O and 13C=O: ෥భయ಴ೀ ఔ ෥భమ಴ೀ ఔ

ఓ

= ටఓభయ಴ೀ = 0.978 భమ಴ೀ

(6)

Table 1. Observed isotopic shifts during 12

C species

13

C species

12

CO-13CO exchange

shift

[cm-1]

[cm-1]

2348

2282

0.972

2142

2094

0.978

1568*

1536*

0.978

1525*

1483*

0.972

1440*

1402*

0.974

1375*

1331*

0.968

1317*

1277*

0.970

* UD450 only The observed shifts (table 1) are in agreement with the expected shifts, the deviation for all species except CO is due to the fact that the harmonic oscillator is only applicable to bi-atomic molecules

32

.

The isotopic labeling confirms that indeed carbonates are formed on UD450. The two intense bands (1440 and 1375 cm-1) are identified as unidentate carbonates

11,17,24

; the high and low frequency

shoulders (1568, 1523 and 1317 cm-1) are most probably carboxylates (1523 and 1317 cm-1) and bidentate carbonates (1568 cm-1) 11,17 because an identification as bicarbonates can be excluded by the absence of the corresponding C-OH vibration (~ 1220 cm-1 24).

12


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Figure 4. Absorbance spectra of UD450 exposed to 300 ppm CO2 and CO in dry air. The corresponding resistance data is shown in the supporting information (S1). It is noteworthy that the formation of carbonates under operation conditions occurs exclusively on UD450, while both materials are active in terms of CO conversion and CO sensing. This implies that carbonates are less relevant for CO oxidation and gas sensing in general on SnO2 than previously thought

3,17,23,25

. To determine whether the carbonates are an intermediate of the reaction of

molecular O2- and CO 9 or are created by the reaction of CO2 with the SnO2 surface 8, CO2 was dosed in dry air on UD450 (figure 4). It is evident that the exposure of CO2 causes the very same surface carbonates as observed during CO exposure, where CO2 is formed by CO oxidation. Based on the IR spectra we assume that carbonates are formed by a subsequent reaction of CO2 and hence the observed shifts (table 1) are caused by the reaction product

13

CO2 and not directly by

13

CO. With respect to the theoretical calculations for

carbonate formation our experimental findings support a subsequent reaction of CO2 with the SnO2 surface as proposed by J.-M. Ducéré et al. 8. However, a final determination of the origin of carbonates requires a time-dependent study of the concentrations of the gas-phase and surface species, as it, for example, should be possible by Steady-State Isotopic Transient Kinetics Analysis33.

13



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One should note that while the exposure to 300 ppm CO causes a large resistance change (S = Rair/Rair+CO = 210) the exposure of 300 ppm CO2 causes only a minor one (S = Rair/Rair+CO2 = 4), which can be traced back to a small variation in the background humidity (few ppm); the corresponding resistance measurements are shown in the supporting information (S1). This suggests that the presence of carbonates cannot be involved in the reception mechanisms linked to electrical resistance changes.

Figure 5. Absorbance spectra of UD1000 (a) and UD450 (b) exposed to 300 ppm H2 in dry and humid (10 % r.h., 25 °C) air. The sole formation of carbonates on UD450, could indicate that the differences between the two materials recorded below 1400 cm-1 are possibly caused by a masking effect, namely an overlap between decreasing bands as observed on UD1000 during CO exposure (figure 2a) combined with the increasing carbonates bands. To double check that, both materials were exposed to 100 ppm H2 in dry and humid air (figure 5). The spectra of UD1000 shows similar features like under exposure to 100 ppm CO (figure 2a). When comparing the H2 and CO exposure on UD450 (figure 5b and 2b respectively) no substantial differences are found in the hydroxyl region while, below 1400 cm-1, instead of increasing carbonate bands two decreasing bands at 1362 and 1334 cm-1 appear in addition to a decrease at 1058 14 ACS Paragon Plus Environment

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cm-1 and between 1037 and 970 cm-1. These findings suggest that for UD1000 and UD450 the exposure to reducing gases leads to a decrease of a series of bands that, between 1400 and 1300 cm-1, are masked by carbonates on UD450 in the case of CO exposure. The effect of both reducing gases on the SnO2 surface is weakened in the presence of humidity. However, the identification of the bands between 1400 and 1000 cm-1 is – besides the carbonates - still not clear-cut, since Sn-O overtones as well as hydroxyl deformation vibrations occur in this region. To be able to differentiate between those, two isotopic exchange experiments were performed by using 300 ppm H2/D2 (supporting information S2) and 10%r.h. H2O/D2O (figure 6); since no qualitative differences between the experiments were found – the exposure to 10% H2O/D2O shows a stronger effect due to higher concentrations – and the exchange during H2/D2 exposure is assumed to take place via the reaction products H2O and D2O, respectively 21, the following discussion will focus on the latter. When comparing the spectra of H2O and D2O exposure below 1500 cm-1 it is evident that the effect of the isotopic exchange occurs below 1000 or 1300 cm-1 on UD1000 or U450, respectively. Hence, the bands above these wavenumbers are not related to hydroxyl groups and can be safely identified as bands related to Sn-O vibrations and their overtones. The ones at 1362 and 1334 cm-1 (shoulder on UD1000) are common for both materials, the additional ones at 1271, 1206, 1159 and 1056 cm-1 on UD1000 are attributed to the oxide dominated surface termination and therefore are weak or absent on UD450.

15



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Figure 6. Exposure of H2O or D2O vapor (10 %r.h., 25 °C). The absorbance spectra of UD1000 (a) and UD450 (b) are referenced to dry air. The assignments are corroborated by previous works on highly crystalline SnO2 materials

25,29

and,

accordingly, most of these bands represent overtones of fundamental Sn-O vibrations found between 300 and 800 cm-1 11, a spectral region not accessible because of the experimental set up limitations and SnO2 characteristics. The differences in the nature and intensity of exchanged OH-bands during D2O exposure confirms a much lower hydroxyl concentration on UD1000 (a detailed description of the exchange of OH and OD vibrations is given elsewhere 21,22). The electrical effects of water vapor are quite different, as shown in the supporting information (S3). Under water vapor exposure the resistance of UD1000 drops strongly, , S = Rdry/R10%r.h. = 29, while the effect on UD450 is comparably small, S = Rdry/R10%r.h. = 5. Corresponding to the resistance measurement differences are also observed in the DRIFT spectra recorded during H2O exposure (figure 6). Considering first the effect of water vapor on UD450, which appears to be easier to understand, the following observations can be made: The overall intensity of isolated and bridged OH (3800 to 2000 cm-1) and hydroxyl deformation (1300-900 cm-1) vibrations is increased, while the bands 16


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at 1362 and 1334 cm-1 are decreased indicating the removal of oxygen, which is in line with a resistance drop. However, regarding the free terminal OH vibrations (3800-3500 cm-1) no uniform trend is observed; the band at 3723 cm-1 appears to be marginally effected by the water vapor, while the series of isolated OH vibrations between 3700 and 3500 cm-1 involve an overlapping between decreasing (3628 cm-1) and increasing (3662, 3588 and 3549 cm-1) bands. This can be explained by the reaction of water vapor with surface oxygen, which causes a decrease in the electrical resistance and, by creating additional hydroxyl groups that interact with the already existing ones, attenuate the corresponding intensities of the isolated hydroxyls. In contrast, the effect of water vapor on UD1000 cannot be explained as easily. In line with the significant resistance drop, a decrease of the concentration of the surface oxygen species is observed (1362, 1334, 1271, 1159, 1059 cm-1). Additionally, such a reaction is expected to increase the surface hydroxyl concentration, which is opposed to what is observable, namely that bands corresponding to hydroxyl vibrations (3736, 3723 and 3424 cm-1) are decreased. This effect seems contradictory and can possibly be explained by an attenuation of the hydroxyl vibration with increasing hydration, as reported for OH-(H2O)n clusters

34

and recently theoretically explained 35. The previous paragraph demonstrates the complexity of the interaction of water vapor with SnO2; because the effect of water is essential for the understanding of gas sensing under real conditions further research is needed. So far all experiments were done in conditions where the adsorption of atmospheric oxygen plays a significant role. However, it is not fully clear what the decrease of the metal-oxide/surface oxygen related bonds implies. They may indicate: the formation of a surface lattice oxygen vacancy or the elimination of an adsorbed molecular or atomic oxygen ion. Oxygen vacancies, generally surface defects, are considered to be possible adsorption sites for atmospheric oxygen

36,37

the adsorption of

which needs to involve the capture of an electron from the conduction band in order to justify the 17



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increase of the electrical resistance. So, it is possible that the result of oxygen adsorption is the relocalization of an electron that was delocalized in the conduction band and originated from donor states caused by the ionization of bulk or surface oxygen vacancies 36. The localization would mean the formation of a bond. So, removing the adsorbed oxygen could be seen as a decrease of the corresponding IR band. Under CO exposure, exactly the same result would be obtained if the surface lattice is reduced i.e. oxygen vacancies are formed; in this case we could look to the adsorption of oxygen as a process of oxygen vacancies cancellation and in the presence of both the equilibrium concentration of oxygen vacancies will determine the surface charge and the electrical resistance of the sensor. Therefore, it is essential to know if - starting from a condition in which there is no or very little adsorbed oxygen i.e. in a background of N2 – the exposure to atmospheric oxygen leads to an increase of the same bands that are decreased by the exposure to CO or H2. And it is as important to see if exposing, in the same conditions, the surface to reducing gases causes a decrease of the bands is observable. The absorbance spectra in figure 7 show the effect of 500 ppm CO as well as 1000 ppm O2 on both materials in dry nitrogen (< 15 ppm O2). The overall shape of the spectra is dominated by the absorption of free charge carriers, which are dramatically changed by the exposures. This leads for both materials, in the first case, to a strong increase of the absorption below 2000 cm-1 and, in the second case, to a decrease of the absorption in this region. Exposure to CO in nitrogen, for both materials, decreases the same Sn-O bands as for the CO, H2 and H2O exposure in air (1400 to 1000 cm-1 for UD1000 and 1400 to 1300 cm-1 for UD450). As observed already in synthetic air, no carbonyl-like donor species are formed.

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Figure 7. Exposure of 500 ppm CO or 1000 ppm O2 in nitrogen (oxygen background < 15 ppm). The absorbance spectra of UD1000 (a) and UD450 (b) are referenced to pure nitrogen. From these findings we conclude that in all conditions the CO reception takes place by one and the same surface reaction mechanism, which involves the formation of oxygen vacancies acting as donor species in SnO2. For both materials, an effect of oxygen exposure in nitrogen is the increase of the intensity of the Sn-O bands. This indicates the formation of additional Sn-O bonds and can be interpreted as healing of vacancies in the presence of atmospheric oxygen. All in all, our results confirm the theoretical results reported recently 8,9: the sensing of representative reducing gases, including humidity, seems to be governed by the interplay between surface reduction and reoxidation, which depends on the composition of the surrounding atmosphere, and will result in a certain concentration of oxygen vacancies i.e. surface charge and in the end sensor resistance.

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Figure 8. Exhaust gas analysis of the both materials operated in dry air and dry nitrogen. The graphs show the measured intensities for m/z = 44. A reduction of the surface appears to be in contradiction with the previous results obtained by monitoring the exhaust of the experiments performed by exposing the sensors to CO in N2, namely no detectable CO2 formation 6, which are in line with the results obtained during the current experiments (figure 8). It is worth noting that in similar conditions we observed considerable CO2 generation for Ptdoped SnO2 and WO3 for very similar or lower sensor effects 38,39. This suggests that the undoped SnO2 surface reduction is very limited, which is not unexpected: temperature programmed isotopic exchange experiments using mass spectrometry have shown that a high level of oxygen activation of undoped SnO2 surfaces occurs at temperatures above 450 to 550 °C

40

. Accordingly, at 300 °C the

oxidation/reduction sites should be not that many. The theoretical calculations already cited

8,9

,

indicate the two-fold coordinated surface oxygen ions as being the reactive ones. Looking at the sum of the experiments shown in the previous paragraphs we can make the following statements.

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•

We were able to identify a series of bands which decrease in the presence of reducing gases. In agreement with other authors and the isotopic exchange experiments presented in this work the bands were assigned to Sn-O bonds.

•

All reducing gases (CO, H2 and H2O) affect the same oxygen-related bands and hence suggest a competition between reducing gases for the same reactive oxygen species.

•

In no condition any carbonyl-like donor species was observed, while the decrease of Sn-O vibrations was found in all reducing conditions. Consequently the donor species is identified as oxygen vacancies.

•

Surface carbonates were only found on UD450, originating from a reaction with CO2 and having no direct effect on the CO reception mechanism.

Based on the theoretical calculations

8,9

and our experimental findings we have developed the

following general model for the SnO2 surface reaction using CO as an example for a simple reducing gas (figure 9). Starting in the situation in which oxygen is adsorbed at a surface site CO reacts with the oxygen forming CO2, an oxygen vacancy, and a free electron (equation 1). The reduction of the surface is followed by a reoxidation which can be assumed to take place in at least two steps: the formation of a molecular surface species from atmospheric oxygen (equation 2) and the dissociation of the molecular species into two atomic species resulting in the healing of the oxygen vacancy (equations 3a & 3b). So far reaction steps of the reoxidation (equations 2, 3a & 3b) are of a theoretical nature since no experimental information is available. The proposed reaction model brings fundamental changes in the understanding of the reception mechanism of SnO2 gas sensors. In general the sensors signal will still depend on the interplay of adsorption, desorption and reaction of surface oxygen and under common sensing conditions (T ≤ 300 °C) the reception will not affect the bulk. However, in the absence of atmospheric oxygen there will be still oxygen available at the surface, hence the solid will have some kind of bulk termination exposing loosely bound oxygen. In the absence of atmospheric oxygen and 21 ACS Paragon Plus Environment


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reducing gases the surface concentration of the terminating oxygen will be in equilibrium. Dosing reducing gases will further decrease the concentration of surface oxygen and increase the number of released electrons. Since in the absence of oxygen no reoxidation takes place – or only to a small extent with residual oxygen (supporting information S4) – a certain concentration of a reducing gas will have a higher electrical effect compared to the same amount dosed in the presence of oxygen. This is shown by the recovery of the sensor resistances; while in air 80 % of the baseline resistances are reached within one hour after CO exposure, in nitrogen the recovery after one hour is 22.5 % for UD1000 and 1.6 % for UD450 (supporting information S5). The massively decreased reoxidation explains the high sensor signals of CO and H2 in the absence of oxygen 5,6. The effect of humidity on the gas sensing mechanism of SnO2 can be explained by the competition of water vapor and reducing gases for the same reactive oxygen species and probably also by a site blocking effect of water vapor when interacting with the surface. The dosing of reducing gases in humid air can decrease the oxygen concentration of the surface below the concentration in nitrogen. Thus a switch in the transduction model 7 can be explained by this new reception model. However, the surface concentration of reactive oxygen in humid air is much lower and hence CO conversion and sensor signals are decreased in comparison with dry conditions. 4. Conclusion By using operando DRIFT spectroscopy and isotopic labeled gases we were able to propose a surface vacancy based reception model for the gas sensing with undoped SnO2. We also found that the difference in the calcination temperature determines different surface terminations for the SnO2 materials: In case of UD450 the surface is terminated mainly by hydroxyl groups, while in case of UD1000 hydroxyls are a less dominant surface species and major surface termination is of oxide nature. Still, we found clear analogies in the surface chemistry, which - in accordance with DFT calculations - suggests that at specific sites oxygen ions are removed from the surface by reducing 22


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gases (CO, H2, H2O), releasing trapped electrons to the conduction band. It was found that this mechanism occurs in dry and humid air as well as in N2 (c(O2) < 15 ppm) and in addition no donor species other than oxygen vacancies were found. The formation of carbonate species on UD450 was identified as a subsequent or parallel reaction, which has no impact on the gas sensing mechanism. The reaction mechanism summarized in figure 9 gives also an explanation for the high sensor signals and not measureable conversion in the absence of oxygen; on one hand the lack of reoxidation in the absence of atmospheric oxygen causes a high concentration of surface oxygen vacancies giving a strong rise in the conductance, while on the other hand the reaction rate approaches zero since no further oxygen is available for CO oxidation at 300 °C.

Figure 9 – Proposed model for the surface chemistry of SnO2 at temperatures below the activation temperature of the subsurface/bulk oxygen, describing the CO oxidation/gas recognition as the interplay of surface reduction (1) and reoxidation (2 & 3). The next task will be the further development of a revised transduction mechanism for SnO2 gas sensors based on the formation of oxygen vacancy. Although we were able to improve the understanding of the interaction of reducing gases with SnO2 materials under sensing conditions, other important questions, such like the effect of water vapor, the mechanism of the reoxidation of the

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surface or the relationship between the electronic situation of SnO2 and the overall shape of the IR spectrum, are still open and need further research. ASSOCIATED CONTENT Supporting Information. As mentioned in the text, additional spectroscopic data and resistance measurements are available as supporting information. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] +4970712978761 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. REFERENCES

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