1. Introduction
Wide-band semiconductor metal oxides have proven to be excellent sensitive materials for gas sensors. Bulk doping of these oxides or modification of their surface by various catalytic clusters can increase their sensitivity and selectivity in detecting most toxic gases [
1]. In addition to high sensitivity and selectivity, the most important characteristic of a sensor material is the stability of its properties. When working with sensors based on semiconductor metal oxides, a drift in resistance values can be observed over time. The main reason for this effect is an increase in the size of the nanoparticles of a semiconductor material at operating temperatures [
2,
3]. As was briefly summarized in References [
4,
5] the introduction of SiO
2 into the SnO
2 semiconductor matrix is one of the promising approaches for creating sensor materials with a high stability of microstructure parameters under conditions of semiconductor gas sensors’ operation. It was found [
4,
5] that the modification of tin dioxide with amorphous SiO
2, on the one hand, prevents the growth of SnO
2 nanoparticles during high-temperature post-synthetic processing that ensures the long-term stability of sensor characteristics at high temperatures even in humid air. The temperature stability of SnO
2/SiO
2 nanocomposites allows it to be used for detecting gases that require a high operating temperature (300–400 °C or higher) [
6,
7,
8]: methane, hydrogen, volatile organic compounds (VOCs), which include ethanol, benzene, toluene, and others. VOCs have a different toxicity. For example, poisoning with ethanol vapor (the threshold limit value, TLV is 522 ppm [
9]) leads to difficulty breathing, nausea, headache and dizziness [
10]. Mixtures of air with ethanol in a concentration of 2.6–18.9 vol% are explosive [
9]. Benzene (TLV is 4.6 ppm in working zone and 1.6 ppb in residential area), which is a carcinogen, has a significantly higher toxicity. Benzene vapors at a concentration of 0.8–8.6 vol% are explosive. Chronic poisoning leads to damage to the liver and central nervous system [
9]. Thus, it is extremely important to accurately and quickly detect VOCs in the air of residential and work areas. Based on the data presented in the reviews [
6,
8,
11,
12,
13], the optimal detection temperatures for ethanol and benzene using metal oxide based sensors are 250–300 °C and 300–400 °C, respectively. This indicates that it is necessary to use materials that retain a high specific surface area at very high temperatures to detect these gases. This requirement is fully met by SnO
2/SiO
2 nanocomposites.
Gold clusters deposited on metal oxides exhibit high catalytic activity in the reactions of partial hydrogenation, synthesis of hydrogen peroxide, oxidation of CO, hydrocarbons, and also volatile organic compounds [
14,
15,
16,
17,
18,
19,
20]. The activity of Au-based catalysts depends on many factors, including deposition methods on the oxide surface [
14,
21], the chemical nature of the support [
22,
23], the concentration [
24,
25,
26,
27] and the size [
28,
29] of Au nanoparticles. A summary of the literature data about the sensor properties of Au-modified SnO
2 sensor materials towards VOCs ethanol and benzene is presented in
Table 1.
This work is devoted to the study of the sensor properties of the thermally stable nanocrystalline SnO
2/SiO
2 composite to ethanol and benzene. To increase the sensitivity, the surface of SnO
2/SiO
2 nanocomposite was modified with Au nanoparticles. Based on the data from [
24,
25,
26,
27] for a combination of high catalytic activity and optimal resistance values for sensor measurements, the selected Au concentration was 1 wt.%. The most important result is that Au-modified SnO
2/SiO
2 retains a high sensitivity to ethanol and benzene in a humid atmosphere. Based on the DRIFTS investigations, this fact is discussed in terms of the synergistic effect of two modifiers (Au and SiO
2) in the formation of sensor properties of the SnO
2/SiO
2-Au nanocomposite.
3. Results
The analysis of the obtained SnO
2-300, SnO
2-Au, SnO
2/SiO
2, SnO
2/SiO
2-Au samples by X-ray diffraction showed the presence of only crystalline phase SnO
2 with the cassiterite structure (ICDD 41–1445) (
Figure S1, Supplementary Information). No Au-containing phases were detected because of the low Au content. The results of the analysis of the elemental composition, microstructure parameters, and electrophysical properties of the obtained samples are shown in
Table 2. The addition of SiO
2 at the stage of hydrothermal treatment of β-tin acid allows the stabilizing of the growth of SnO
2 nanoparticles at high-temperature annealing and samples with a high specific surface area, similar to pure SnO
2 annealed at lower temperature 300 °C, can be obtained. The increase in the sample’s resistance in the raw R (SnO
2-300) < R (SnO
2-Au) < R (SnO
2/SiO
2) < R (SnO
2/SiO
2-Au) is associated with the formation of Schottky barriers at the metal/semiconductor interface and dielectric properties of silicon dioxide. When the SnO
2/SiO
2 surface is modified by gold, due to the difference in the electron work function of Au (5.1 eV) [
39] and SnO
2 (<4.8 eV) [
40], a Schottky barrier arises at the metal/semiconductor interface. This results in an increase in the thickness of the electron-depleted layer near the surface of the SnO
2 crystallites and, consequently, an increase in the sensor resistance.
At the HRTEM image of SnO
2/SiO
2 (
Figure 2a), the crystalline SnO
2 phase with interplanar distances d = 0.34 nm (110) and amorphous silica particles distributed evenly on the surface of the semiconductor oxide are clearly distinguished. The cluster size of the Au modifier is one of the key factors affecting catalytic activity. According to the literature data, the optimal diameter of gold nanoparticles, which are highly active in oxidation reactions, is in the range of 1–10 nm [
14,
26,
27,
29,
41]. A larger particle size leads to an insufficient degree of dispersion of the deposited modifier. According to HRTEM, the size of Au clusters on the surface of modified SnO
2/SiO
2 nanocomposites is 8 ± 1 nm (
Figure 2b).
Active sites on the surface of the samples with oxidizing properties were studied by the method of thermally programmed reduction with hydrogen (TPR-H
2).
Figure 3 shows the temperature dependences of hydrogen consumption rates for SnO
2-300, SnO
2-Au, SnO
2/SiO
2, and SnO
2/SiO
2-Au samples. The TPR-H
2 profiles of all samples contain two peaks. The appearance of a peak in the low-temperature region at 170–320 °C is associated with the reduction in oxygen-containing O
2-, O- and OH-groups on the surface of semiconductor materials. On TPR-H
2 profiles of the Au-modified samples, a shift of the maximum in this peak to higher temperatures is observed by 64 °C and 93 °C in the case of SnO
2-Au and SnO
2/SiO
2-Au, respectively. This effect of increasing the activation energy of the reaction of reduction of surface groups can be explained by the presence of additional reactive oxygen species and their localization in the region of the Au/SnO
2 interface [
14,
42,
43].
In the temperature range 450–700 °C, the consumption of hydrogen is due to a Sn
4+ → Sn
o reduction. In the case of the SnO
2/SiO
2 sample, in a high-temperature peak, two components are distinguished. The appearance of a signal with a maximum at 485 °C is due to the partial reduction of Sn
4+ → Sn
2+. The maximum of the component responsible for the reduction in metal tin in SnO
2/SiO
2 and SnO
2/SiO
2-Au samples is shifted by 15 and 26 degrees, respectively, to higher temperatures, compared to the unmodified SnO
2-300 sample, due to the presence of the SiO
2 phase, which is resistant to reduction with hydrogen under given conditions. At the TPR-H
2 profile of SnO
2/SiO
2 nanocomposite, one can distinguish an additional high-temperature shoulder at 640 °C. This may be due to the difficult reduction in tin cations linked with SiO
4 groups. The introduction of Au clusters leads to the disappearance of this shoulder, which can indicate the weakening of the Sn-O bond on the perimeter of the SnO
2-SiO
2 heterocontact under the influence of the modifier. The amount of hydrogen consumed during sample reduction that was determined is shown in
Table 3.
The total amount of consumed H
2 per 1 mol of tin dioxide is close to the theoretical value (
n = 2) determined by the reaction
For the reduction in oxygen-containing groups on the surface of SnO
2/SiO
2 and SnO
2/SiO
2-Au, a greater amount of hydrogen is needed than in the case of samples unmodified with silicon dioxide. This can be explained by the additional contribution of silanol Si-OH groups present on the surface of these samples. The broad absorption band with a maximum at 1100 cm
–1 in the IR spectra of SiO
2 containing samples is due to the superposition of antisymmetric vibrations of Si-O-Si bridge bonds and vibrations of silanol groups (1250–870 cm
–1) (
Figure 4).
The surface composition of the obtained sensor materials was studied by the XPS method. The Sn 3d component is described by a doublet with peak maxima at 495.6 (Sn 3d
5/2) and 487.2 (Sn3d
3/2), which corresponds to Sn (IV) in SnO
2 [
44]. The spectra of samples containing SiO
2 contain a signal from Si 2p with an energy of 103 eV [
4]. The O 1s XP spectrum has an asymmetric form and decomposes into two peaks with energies of 531 eV (O
lat) and 532 eV (O
surf). The first component has a much higher intensity and corresponds to the lattice oxygen of SnO
2. The presence of the second peak corresponds to the different forms of chemisorbed oxygen and hydroxyl groups on the surface of the obtained materials. The ratio of integrated intensities O
surf/O
lat increases in the series SnO
2-300 < SnO
2-Au < SnO
2/SiO
2 < SnO
2/SiO
2-Au (
Figure 5,
Table 4), which is consistent with the data of TPR-H
2 and IR-spectroscopy. Thus, the modification of tin dioxide by SiO
2 and Au makes it possible to increase the concentration of surface oxygen-containing groups compared to pure SnO
2 with the same specific surface area.
The signal from Au 4f
7/2 (83.4 eV) in the SnO
2-Au spectrum is somewhat shifted from the reference value of Au 4f
7/2 (84.0 eV [
44]). The main form of the modifier is metallic gold (
Figure 6a), which is consistent with the data on the study of the charge state of gold catalysts deposited on oxide supports [
45]. However, the corresponding spectral region of the SnO
2/SiO
2-Au sample has a more complex structure (
Figure 6b), in which the components of metallic gold (83.9 eV 4f
7/2), as well as Au
+ (85.3 eV 4f
7/2) and Au
3+ cations (86.8 eV 4f
7/2) [
46] are present. The relative fractions of Au
+ and Au
3+ states are 11.2% and 11.4%, respectively. In addition, the region of the Au f-lines overlaps with the broad peak of Sn 4p (92.5 eV) [
47]. The appearance of peaks corresponding to Au
3+ in the SnO
2/SiO
2-Au spectrum can be explained by the ability of Au
3+ (r(Au
3+) = 0.068 nm) to integrate at the positions of coordination-unsaturated tin cations Sn
4c (r(Sn
4+) = 0.069 nm) on the non-stoichiometric SnO
2 surface. In our previous work [
5], it was found that the addition of SiO
2 in an amount of 13 mol.% leads to an increase in the concentration of paramagnetic defects Sn
3+ by 6.8 times in comparison with pure tin dioxide. The appearance of Au
Sn sites leads to a change in the electron density distribution on the Sn-O bond, which is confirmed by a decrease in ΔE (Sn 3d
5/2-O1s) for the SnO
2/SiO
2-Au sample by 1.1 eV (
Table 4) compared to the standard value for SnO
2 (44 eV [
44]). Cabot et al. [
45] found a localization of Au
δ+ at the sites of distortion of the SnO
2 lattice. Yinga et al. [
46] described the effect of the degree of defectiveness of the carrier on the charge state of Au. An additional effect exerted by silicon dioxide is the stabilization of cationic forms of gold by hydroxyl groups, due to the formation of an Au
δ+-OH-bond [
43,
48,
49] whose concentration on the surface of the SnO
2/SiO
2-Au sample is increased compared to SnO
2-Au due to the contribution of silanol groups. It is worth noting that it is the presence of cationic forms Au
+ and Au
3+ that are associated with the high catalytic activity of gold catalysts [
18,
29,
50,
51], due to the preferred adsorption of oxygen on Au
δ+ compared to metallic gold, as well as free d-orbitals of Au
3+.
The sensor properties of the obtained semiconductor materials were studied in the detection of ethanol (
Figure 7) and benzene (
Figure 8) in dry air (RH = 1% at 25 °C) and at a relative humidity of RH = 20% (at 25 °C) in the temperature range of 150–400 °C with increments of 50 °C. The maximum measurement temperature for sensors based on SnO
2-300 and SnO
2-Au did not exceed the temperature of their post-synthetic treatment, i.e., 300 °C. In the presence of ethanol vapor (
Figure 7a,b), the resistance of all sensors is
reduced. After each cycle of interaction with the gas being determined, the resistance value in air reaches the initial value. The sensor signals are reproducible (
Figure 7c,d).
According to the literature data, the oxidation of ethanol on oxide surfaces proceeds in several stages. First, dissociative chemisorption of C
2H
5OH occurs with the formation of ethoxide by the reaction
C
2H
5OH adsorption occurs through the oxygen atom of the hydroxyl group of the alcohol molecule on coordinatively unsaturated metal cations [
52], while the hydrogen atom forms a weak bond with the surface oxygen anion. Further, depending on the conditions, ethoxide can be oxidized to acetaldehyde when reacted with chemisorbed oxygen species O- or O
2- [
17,
52,
53]
or form ethylene [
52,
54]:
The reactions responsible for the sensor response proceed via oxidation of organic species by O
– as the predominant form of chemisorbed oxygen on the SnO
2/SiO
2 surface at 300–400 °C [
5]. The resulting acetaldehyde enters into a decarbonylation reaction with the formation of methane and carbon monoxide
or further oxidizes to acetic acid
The incomplete oxidation reactions (3) and (6) mainly proceed up to 220–250 °C [
52,
53]. At higher temperatures, ethoxide and intermediates are oxidized to carbon dioxide
The sensitivity of pure tin dioxide decreases significantly in humid air (
Figure 7d), while the sensor signal of SnO
2/SiO
2 remains almost unchanged due to its resistance to changes in relative humidity over a wide range [
4]. Modification by the Au also made it possible to neutralize the negative effects of humidity. The Au-modified SnO
2/SiO
2 nanocomposite demonstrates the highest sensor response to ethanol among the obtained semiconductor materials. The temperature of the maximum signal to 100 ppm C
2H
5OH in both dry and wet air was 300 °C (
Figure 7c,d). The absolute values of the response also coincide within the error. Sensor properties of SnO
2-300, SnO
2-Au, SnO
2/SiO
2 and SnO
2/SiO
2-Au towards 100 ppm C
2H
5OH at 300 °C are summarized in
Table 5. Modification by silicon dioxide prevents the increasing of sensors’ response and recovery times in humid air.
To determine the reason for the stability of the sensor response of Au-modified SnO
2/SiO
2 nanocomposite with an increase in air humidity, the interaction of this material with ethanol vapor in dry and wet air was studied by DRIFT spectroscopy in operando mode. The negative change in the vibration intensities of hydroxyl groups in the range 3300–3700 cm
−1 (
Figure 9) during the interaction of the surface of the SnO
2/SiO
2-Au sample with C
2H
5OH vapor indicates their direct participation in the adsorption of ethanol molecules
Due to the high noise DRIFT IR spectra caused by an apparently more complex composition the surface of the test sample, we were not able to detect the oscillations of C-O bonds (1080–1115 cm
−1), ν
ass. CH
3-, CH
2- groups (2900–2990 cm
−1), corresponding to adsorbed ethanol, ν
s. CH
3-, CH
2- located in the region (1300–1500 cm
−1) [
53,
55,
56], as well as vibrations of mono- (1530–1470, 1370–1300 cm
−1) and bidentant carbonates (1630–1590, 1270–1260 cm
−1) [
53], Sn
4+-CO, SiO
2-CO (2100–2200 cm
−1), corresponding to intermediate oxidation products.
At temperatures below 250 °C, the stability of Au-modified sensor materials to increased relative humidity can be due to the involvement of water molecules adsorbed on the surface of the semiconductor oxide in the process of oxygen activation along the perimeter of the Au/SnO
2 contact
where (
p) indicates the particles adsorbed in the region of the perimeter of the Au/SnO
2 contact, (surf) indicates particles adsorbed on the surface of the semiconductor oxide, (*) denotes the active forms of chemisorbed oxygen [
23]. DFT calculations [
57,
58] showed that the energy of co-adsorption of H
2O and O
2 on the Au (111) surface is much lower than the sum of the individual adsorption energies. The adsorption of water molecules at an increased humidity (RH = 20%) also reduces the electron work function for Au [
58]. The resulting excess electrons on gold are transferred to electron-acceptor O
ads. In sensor measurements in humid air, the effect of stabilization of the cationic forms of gold by hydroxyl groups appears. The described effects are clearly manifested in an increase in the conversion of CO oxidation reactions on Au catalysts deposited on various oxide supports at high air humidity and low temperatures (below the onset of H
2O desorption) [
23,
43,
59]. Another factor affecting the increase in the sensor signal to 100 ppm C
2H
5OH, materials modified with gold, is the possibility of the adsorption of alcohol molecules directly onto Au nanoparticles [
53]. It can also be assumed that the reaction with H
ads [
60], which was formed during the deprotonation of ethanol (Equation (2)), contributes to the formation of such a high sensor signal of the obtained semiconductor materials
Sensors based on SnO
2-300 do not show sensor sensitivity to benzene in either dry or wet air (
Figure 8). A small sensor response is found for SnO
2/SiO
2 at high temperatures above 300 °C. Such high temperatures are caused by the stability of the benzene molecule, the complete oxidation of which is a complex multi-electron process [
7]
The modification of semiconductor materials with Au nanoparticles made it possible to detect benzene at a concentration of 2 ppm (
Figure 8c,d). The sensitivity of the SnO
2-Au sample in the atmosphere of humid air is somewhat reduced, apparently due to the competitive adsorption of C
6H
6 and H
2O molecules. As in the case of ethanol, the sensor based on SnO
2/SiO
2-Au has the highest sensitivity and fastest response and recovery times. The temperature of the maximum sensor signal at RH = 1% was 300 °C. With an increase in relative air humidity of up to 20%, the maximum sensor response shifts to the region of higher temperatures (400 °C). This can indicate the involvement of SnO
2 lattice oxygen in the benzene oxidation by Mars-van Krevelen mechanism [
61]. The difference in energy between the lattice and surface O
2– is about 20 eV, while the corresponding value for O
2(gas) and O
2-(ads.) is 1.5 eV [
62]. According to XPS, the modification of SnO
2/SiO
2 with Au leads to a decrease in the Sn-O binding energy (
Table 4). The weakening of this Sn-O bond, combined with an increase in the measurement temperature, makes it possible for lattice oxygen (in addition to oxygen chemisorbates) to participate in the process of benzene oxidation, leading to a change in the resistance of the sensitive material. This mechanism was proposed by Jiang et al. [
63]. The sensor properties of SnO
2-300, SnO
2-Au, SnO
2/SiO
2 and SnO
2/SiO
2-Au towards 2 ppm C
6H
6 at 300 °C are summarized in
Table 6.
Thus, the synergistic effect of two modifiers (Au and SiO2) in the formation of sensor properties of SnO2/SiO2-Au is that the amorphous SiO2 provides the formation of centers on the surface of SnO2 that stabilize the cationic forms of gold Au+ and Au3+. These forms on the one hand exhibit higher catalytic activity in the oxidation of organic compounds, and, on the other hand, reduce the energy of the Sn-O bond that, at a high temperature, provides the involvement of SnO2 lattice oxygen in the benzene oxidation by Mars-van Krevelen mechanism.