The influence of gold nanoparticles on the conductivity response of SnO2-based thin film gas sensors

Accepted Manuscript Title: The influence of gold nanoparticles on the conductivity response of SnO2 -based thin film gas sensors Author: G. Korotcenkov V. Brinzari L.B. Gulina B.K. Cho PII: DOI: Reference:

S0169-4332(15)01540-8 http://dx.doi.org/doi:10.1016/j.apsusc.2015.06.192 APSUSC 30713

To appear in:

APSUSC

Received date: Revised date: Accepted date:

9-4-2015 26-6-2015 29-6-2015

Please cite this article as: G. Korotcenkov, V. Brinzari, L.B. Gulina, B.K. Cho, The influence of gold nanoparticles on the conductivity response of SnO2 -based thin film gas sensors, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.06.192 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Highlights (for review)

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SILD method is acceptable for surface modification by gold nanoclusters (AuNCs). Modification by AuNCs can be used for improvement of SnO2 gas sensor parameters. AuNCs deposited on the SnO2 surface are active to reducing and oxidizing gases. The most significant increase in sensor response is observed during ozone detection. Effect of surface decoration by AuNCs depends on the number of deposition cycles. The maximal sensor response is observed for AuNCs with the size ~1-3 nm. Proposed models explain observed effects.

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Graphical Abstract (for review)

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*Manuscript Click here to view linked References

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The influence of gold nanoparticles on the conductivity response of SnO2based thin film gas sensors G. Korotcenkov1*, V. Brinzari2, L.B. Gulina3, B.K. Cho1** 1

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School of Material Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, Rep. of Korea 2 Department of Theoretical Physics, State University of Moldova, Chisinau, Rep. of Moldova 3 Institute of Chemistry, St. Petersburg State University, St. Petersburg, Russia

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Abstract

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The results presented in this study demonstrate that the successive ionic layer deposition (SILD) method for gold nanoparticle formation can be used for surface functionalization of SnO2 films to improve their gas sensing properties. As a result of successive treatments in HAuCl4·nH2O and NaBH4 solutions, gold nanoparticles can be formed on the surface of SnO2 crystallites. The size of the gold particles varies over the range of 1-50 nm depending on the number of SILD cycles. Gas sensing characteristics of the Au-modified SnO2 films are discussed as well. Unlike most studies focused on the development of CO sensors, the present research focuses on the specifics of the response of the SnO2:Au-based sensors to other gases, such as hydrogen and ozone. It is established that gold nanoparticles deposited on the SnO2 surface are active toward both reducing and oxidizing gases, and the effect of the SnO2 surface decoration by the gold nanoparticles on the gas sensing characteristics depends on the number of deposition cycles (i.e., the size of the gold particles). The sensitization to ozone and hydrogen suggests that the application of the surface modification by gold in the field of gas sensor design should not be limited by optimization of the CO sensor's parameters. Models showing the promotional role of Au additives are discussed, and a mechanism of sensitization in the SnO2:Au-based gas sensor is proposed. Keywords: SnO2; films; gold clusters; surface modification; SILD; gas sensors; characterization; optimization

Corresponding authors:

Prof. G. Korotcenkov School of Material Science and Engineering, Gwangju Institute of Science and Technology, 261 Cheomdan-gwagiro, Buk-gu, 500-712, Gwangju, Rep. of Korea Tel.: +82-62-715-2354 Fax: +82-62-715-2304 E-mail: [email protected] Prof. B.K. Cho Page 3 of 37

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School of Material Science and Engineering, Gwangju Institute of Science and Technology, 261 Cheomdan-gwagiro, Buk-gu, 500-712, Gwangju, Rep. of Korea Tel.: +82-62-715-2318 Fax: +82-62-715-2304 E-mail: [email protected]

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1. Introduction

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For many years, it was believed that only Pd and Pt, showing pronounced catalytic properties, were the most suitable materials for surface functionalization of metal oxides and optimization of gas sensor parameters [1-3]. However, studies carried out during recent decades have shown that gold, which was previously considered an inert material, at the transition to nanosized particles supported by metal oxides has started to show enhanced catalytic activity [47]. For example, it was found that a reaction of CO oxidation, with the participation of an Aubased catalyst, could proceed at an increased rate at significantly lower temperatures than when using conventional catalysts [8]. Therefore, during the last decade among the research aimed at using gold nanoparticles (AuNPs) in heterogeneous catalysis [9], there have been reports related to using AuNPs for surface functionalization of metal oxides in solid state sensors, including in conductometric gas sensors [10-15]. It was assumed that surface modification with gold nanoparticles would allow for the design of selective CO gas sensors that could be operated at low temperature. However, the same studies showed that achieving these indicated objectives is not a trivial task, and that achieving enhanced low-temperature sensitivity of the metal oxide sensors to CO requires the execution of several specific conditions to AuNPs synthesis and deposition, related to both the size and gold state in the cluster as well as the properties of the used metal oxide support. These conditions have been reviewed in [16]. Research has shown that, depending on the purpose, various methods can be used for gold nanoparticle deposition on the surface of metal oxides to form a gas-sensitive matrix. For example, AuNPs can be prepared using various physical deposition methods, such as evaporation, sputtering, laser ablation, and discharge plasma synthesis [17]. Methods of deposition from the gas phase, such as chemical vapor deposition (CVD), can also be used for the formation of gold nanoparticles [18]. However, physical methods, as well as the CVD method, are rather expensive. In addition, they have significant limitations for the formation of clusters and nanoparticles on the surfaces developed. Wet chemical methods do not have such constraints and have therefore recently begun to attract significant attention. Wet chemical methods include the Turkevich–Frens method [19], the Brust–Schiffrin method [20], impregnation [6,21], sol-gel [22], coprecipitation [6] and some other that can be found in the literature [12]. However, one should note that till now there are problems associated with the formation and stabilization of AuNPs of optimal size on the required surface. Present study shows that the technology of successive ionic layer deposition (SILD) of gold nanoparticles can also be used for the surface functionalization of metal oxide-based gas sensors. The specificity of this technique is based on multiple, successive treatments of required samples in solutions, which provide adsorption on the substrate surface of anions and cations and their subsequent interactions, and are accompanied by the formation of poorly soluble compounds. Page 4 of 37

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This technique has been described in detail in [23-25]. The advantages of this method, which is one version of the “layer by layer” (LbL) deposition technique and in many articles has the name successive ionic layer adsorption and reaction (SILAR) [26-29], include precise control of the thickness of the growing layer and the size of the surface clusters formed. The primary objective of this work was to evaluate the possibilities of this method for the formation of gold nanoparticles of a required size and to determine the optimal synthesis conditions for their use in the manufacturing of SnO2 conductometric gas sensors. Earlier studies in this area have not been conducted. Only a couple of papers are devoted to the formation of gold nanoparticles on Al2O3 and TiO2 substrates for plasmon excitation studies using this method [28]. Currently, the SILAR method is being used usually for the synthesis of metal chalcogenide semiconductor quantum dots [27,29]. Unlike most studies aimed at developing SnO2:Au-based CO sensors, the present research focused on studying the specifics of the SnO2:Au-based sensor response to other gases, such as hydrogen and ozone. It is well known that hydrogen and ozone are the basis for promising, environmentally clean technologies [30,31]. Therefore, it is important to determine whether there are any restrictions for using SnO2 films with surface modified with gold nanoparticles for improving parameters of sensors designed for the detection of the aforementioned gases. This study also aimed to understand the role of the gold nanoparticles in gas sensing effect and to determine the factors that are responsible for controlling the parameters of SnO2:Au gas sensors. SILD technology has previously been used for the surface modification of SnO2 films with gold nanoparticles [32,33]. However, in those studies, SILD technology was used for deposition of the SnO2-Au nanocomposites. Simultaneously with the deposition of the gold nanoparticles a fine dispersed SnO2 phase was formed on the surface of the SnO2 crystallites. It was established that surface modification by SnO2-Au nanocomposites could be used for improving the operating characteristics of conductometric SnO2-based gas sensors. In particular, an increase in sensor response and a decrease in response and recovery times have been observed. However, it should be noted that this situation, when the gold nanoparticles and the fine dispersed metal oxide phase are deposited simultaneously on the surface of the SnO2 crystallites, creates difficulties in interpreting the obtained results and understanding a real role for AuNPs in gas-sensitive effects. Therefore, we decided to conduct new research using only gold nanoparticles for the surface modification of SnO2 films. 2. Experimental details

For surface modification by gold nanoparticles, we have been using SnO2 films obtained by a spray pyrolysis deposition from SnCl4 water solutions at temperatures (Tpyr) of ~450°C. Some features of this process used for the deposition of gas sensing films have been described in [34,35]. The main advantages of this method for metal oxide film deposition include simplicity of design, ease of operation, the possibility of depositions of complex composition, low cost, and the ability to use cheap non-toxic precursors with low vapor pressure. The thickness of the SnO2 films, equal to ~40-50 nm, was chosen because of the need to achieve optimal parameters of gas sensors. According to [36], these thicknesses provide maximum sensor response to ozone at a minimum response time. At such thicknesses, diffusion processes do not limit the kinetics of the sensor response. The SnO2 films were deposited on alumina ceramic substrates. 2.1. Surface modification

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Deposition of the gold nanoparticles by SILD technology was conducted using the following route: in the first SILD cycle, a SnO2 film on the substrate was treated in a metal salt solution, and then the samples were washed with distilled water to remove the non-reacted reagents, treated with a water solution of NaBH4, and again washed with distilled water. Such a treatment represents one deposition cycle. As a precursor for surface modification by the gold nanoparticles, a water solution of HAuCl4 was used. Gold nanoparticles were formed according to reaction: (1)

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8HAuCl4 + 3NaBH4 + 6H2O = 8Au + 3NaBO2 + 32HCl

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One of the main advantages of the indicated process is the irreversibility of the reaction and high reaction rate at room temperature. This means that the reduction of HAuCl4 with NaBH4 can take place at room temperature. The size of the gold nanoparticles was controlled by changing the number of deposition cycles. During the present experiments, this varied from 1 to 16 SILD cycles. HAuCl4·nH2O with a gold mass content of 49.47% obtained from Aurat Ltd. (Russia) and NaBH4 obtained from Vekton (Russia) were used as the reactants for the gold nanoparticle synthesis. Working water solutions with required concentrations of HAuCl4 and NaBH4 were prepared directly before synthesis. For preparation of solutions as well as for washing procedures, Milli-Q high pure water with a resistivity higher than 18 MΩ was used. The choice of precursor concentration in solutions prepared for AuNPs synthesis was based on preliminary experiments, which showed that, in our case, 0.001 M HAuCl4 solution and 0.02 M NaBH4 solution are optimal concentrations of reactants. Significantly higher concentrations of the reducing agent are required for complete reduction of adsorbed gold ions. Further increase in concentration is undesirable because at higher concentrations of reagent the solution‟s pH is too high. It has been found that the indicated concentrations provide an acceptable process duration, which is required for the formation of gold nanoparticles with the desired size. The treatment time in each solution, including washing, varied in the range of 0.5-2 minutes. A more detailed description of this process can be found in [37]. 2.2. Film characterization

For structural characterization of the films, X-ray diffraction, scanning electron microscopy (SEM) and laser ellipsometry were used. These methods provide control of the thickness, microstructure and morphology of the deposited films. For these purposes, Philips XL30 ESEM and FEI Nova Nanolab SEM/FIB scanning electron microscopes were used. All SEM images were obtained in secondary electron detection mode using an acceleration voltage of 10-30 kV. X-ray photoelectron spectroscopy (XPS) was used for surface characterization. X-ray photoelectron spectra have been collected from an ultrahigh vacuum system equipped with the hemispherical electrostatic analyzer Omicron EA125 with the Al Kα line (hν = 1486.7 eV) for excitation. The O, Sn, In, Au, Cl, and C lines were recorded in detail with a band pass energy of 20 eV. A Zeiss Merlin scanning electron microscope equipped with an Oxford instruments Inca X-act EDX detector for X-ray microanalysis was also used for characterization of the surface composition. 2.3. Sensor fabrication and testing

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Gas sensor prototypes were fabricated on alumina ceramic substrates using thin-film technology. The tested samples had Au contacts. They were deposited by thermal gold evaporation on the SnO2:Au films already formed on alumina substrate using the technology described above. The distance between the measuring contacts was ~2-3 mm. Such a long distance between the contacts allows to reduce their influence on the analyzed gas sensing characteristics. The width of the samples was 2 mm. The gas-sensing properties of as-deposited and modified SnO2 films were investigated using a specially designed automated setup that can conduct measurements in either stationary conditions or in the conditions of controlling transition processes. For these measurements, the samples were placed in a special flow-type reactor with a controlled gas atmosphere and ambient humidity. For the samples‟ heating an external heater was used. The measured chamber volume was ~0.5 cm-3. The small volume of the chamber allowed to control transient processes with a time constant of more than 2-3 seconds. Measurements of sensor response were conducted in the temperature range of 25-450°C. However, the primary measurements were conducted at operating temperatures corresponding to the maxima of the sensor response, which were observed to be in the temperature range of 300-500°C. It is known that the most successful operation of conductometric sensors is observed in this precise temperature range. In this range, the influence of water vapor and surface poisoning on sensor parameters is substantially reduced. To exclude instability [38], we exposed the tested samples to stabilizing annealing at a temperature of 450оС, and all experiments were carried out at a constant humidity, which was maintained at 35-45% relative humidity (RH). While studying gas-sensitive properties, the sensor response (Rozone/Rair for ozone detection and Rair/Rgas for the detection of reducing gases) and the kinetics of the sensor response were controlled. Sensor testing was performed using the measurement cycle air → (air + gas) → air. Carbon monoxide, hydrogen and ozone were used as the test gases. For the ozone source, a special ozone generator based on an ultraviolet radiation lamp was used. The ozone concentration in the tested gas was ~1 ppm. The concentration of H2 and CO during these experiments was equal to 1000 ppm. The response and recovery times were determined during the measurement cycles air → (air + gas) and (air + gas) → air, respectively. Time constants of transient processes were determined on a 0.9 level from the steady state value of the film conductivity. Measurements of transient characteristics at each temperature point were conducted after temperature stabilization and establishment of the steady-state value of the film conductivity. To control the reproducibility of the results, we repeated every measurement cycle three to four times for a series of three samples manufactured with the same process. Deviations in the values obtained during the measurements are reflected in the figures. 3. Results and discussion

3.1. SEM of the SnO2:Au films Detailed analysis of gold nanoparticle formation on the surface of SnO2 films by the SILD method and further material characterization have been published previously [37]. This allows us to avoid repeating the previously published results and simply to state that SILD technology is capable to form gold nanopartiytsles with controlled size. As established in [37], the size of the AuNPs depends on the conditions of the synthesis and increases with the number of deposition cycles used. Representative SEM images of SnO2 films modified with Au by varying the

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numbers of SILD cycles are shown in Fig. 1. As can be seen in Table 1, the particle size, depending on the number of SILD cycles, can vary from 1-5 nm to 10-15 nm or higher. ***Fig. 1*** near here ***Table 1*** near here

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Analysis of the SEM images presented in Fig. 1 shows that the particle growth occurs mainly near the tops of the crystallites as well as within the cavities formed by several crystallites of the base material. SEM images also show that, after 4 deposition cycles, each crystallite contains ~1-2 gold particles. According to [31], these formations of 3D nanoparticles occur due to the aggregation of 1D and 2D clusters from the whole surface of crystallographic planes faceting the crystallite. This regularity generally coincides with regularities established for nanoparticles formed on smooth surfaces, where the growth and accumulation of the gold clusters first takes place at the step edges [39]. It is known that, in these places, the substrate atoms have the largest number of uncoordinated bonds. In particular, according to Wahlstrom et al. [40], the step edges of oxide surfaces can be considered a collection of oxygen vacancies. However, the synthesized particles unfortunately have a large variation in size [31], and along with small clusters, particles with a substantially larger size are present on the surface (Fig. 1c). This is a major disadvantage of the SILD method and of most methods of AuNPs synthesis [18], including wet chemical methods, because nanoparticles with different sizes have different functional properties [4-7]. However, the number of gold particles with an abnormally large size is significantly lower than the number of grains (see Fig. 1с). Therefore, they should not have a significant effect on the functional properties of modified films. Moreover, their appearance and growth was particularly intense when a large number of deposition cycles were used. Further research related to the study of gas-sensitive properties of SnO2 films (presented in section 3.3) show that SnO2 surface modification by gold using a large number of deposition cycles was not optimal for achieving high gas sensitivity.

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3.2. XPS of the SnO2:Au films

Typical full-range XPS spectra of the SnO2:Au films are shown in Fig. 2. The data indicate that the surface of the samples contains only gold, tin, oxygen and ordinary surface species, such as carbon elements. Quantitative analysis of the surface chemistry of SnO2 films was based on the area (intensity) of the main core level XPS O1s, Sn3d, and C1s, weighted by the corresponding atomic sensitivity factor (ASF) and showed that SnO2 is slightly non-stoichiometric with an [O]/[Sn] ratio of 1.7-1.8. This value is typical for nanostructured metal oxides [41], which have oxygen vacancy defects in the surface region of the SnO2 grains. XPS measurements also revealed that carbon is the main surface contaminant of the SnO2 grains. From our estimations, the [C]/[Sn] ratio is equaled to ~1. This means that the surface concentration of carbon is sufficient. However, this situation is typical for metal oxides stored under ambient conditions [41]. Energy-dispersive X-ray (EDX) spectroscopy data (see Fig. 3) also prove that the gold nanoparticles without impurities, such as chlorides and sodium, are really present on the surface. This means that the gold nanoparticles either do not contain contaminants that are present in the solutions used or their concentration is lower than the threshold sensitivity of the EDX and XPS methods. The absence or small concentration of chlorine is a good indication of the effectiveness of the washing procedure used in SILD. This feature is a significant advantage of the SILD method because the removal of excess reactants and reaction products already occurs during the Page 8 of 37

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gold nanoparticles deposition, in contrast to many other methods of chemical deposition used for AuNP synthesis. ***Fig. 2*** near here ***Fig. 3*** near here

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Typical XPS Au 4f (4f5/2 and 4f7/2) core level spectra are shown in Fig. 4. XPS spectra show that repetition of deposition cycles is accompanied by an increase in gold content at the metal oxide surface. Moreover, the change in the intensity of the Au 4f peaks corresponds to variations in the number of SILD cycles. For example, the XPS data revealed that the surface concentration of gold atoms [Au]/[Sn] increased from ~0.01 after 0.5 deposition cycles to ~0.03 after 2 deposition cycles and to ~0.06 after 4 SILD cycles. This means that SILD technology truly provides the opportunity to control the gold content on the surface of metal oxide films. It is important to note that XPS Au 4f peaks corresponding to binding energies of 84 and 88 eV are already clearly visible after surface modification when using a 0.5 deposition cycle. This means that an appreciable amount of gold already appears on the surface of the films after such a treatment. However, no AuNPs were found on the SEM images of these samples. This fact indicates that the size of the gold particles formed on the surface of the SnO2 grains after a 0.5 SILD cycle is less than the threshold resolution of the microscope used, which was in the range of 1-3 nm. It looks realistic. For example, the gold nanoparticles at initial stages of growth on a flat TiO2 surface during thermal evaporation have sizes in the range of 1-5 nm [42]. ***Fig. 4*** near here

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Analysis of the energy position of Au 4f peaks (binding energy (BE) = 84.2 and 87.8 eV) suggests that gold on the SnO2 surface is in the metallic state. For ionic Au3+, the BEs are equal to 85.3 and 88.9 eV [43]. In principle, this outcome is not surprising because the gold reduction to metal during thermal treatment is a typical process for all systems containing gold deposited using wet chemical methods. Typically, full gold reduction occurs at temperatures above 300°C [44,45]. In the present experiments, the samples were annealed at Tan = 450°C, and other chemical states of gold were therefore not expected. The XPS core level spectra, corresponding to the metal of the dominant oxide (i.e., Sn 3d, see Fig. 5a), showed that the appearance of the component corresponding to lower oxide states, such as Sn2+ and Sno, did not take place. Sn 3d spectra are identical for all samples regardless of the number of gold deposition cycles. Furthermore, there is no change in the energy position of the peaks. This means that there are no changes in the band bending. The presence of gold also does not affect the shape of the O1s peak (see Fig. 5b). ***Fig. 5*** near here All of this indicates that a significant chemical interaction with the oxide material does not take place during the gold deposition, and the appearance of gold on the SnO 2 surface is not accompanied by a change in the initial ratio of adsorbed species present on the SnO 2 surface. It is also important to note that the intensities of the primary XPS peaks of tin and oxygen, in contrast to Au 4f intensity, are changed very slightly, which is evidence of the fact that gold occupies a small part of the surface. 3.3. Gas sensing characteristics

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The results of the SnO2-based gas sensor testing after surface modification by AuNPs are shown in Figs. 6-9. In particular, Figs. 6 and 7 show the influence of the number of SILD cycles during gold deposition on sheet resistance of the SnO2:Au films measured in the air and in atmospheres containing hydrogen (Fig. 6) and ozone (Fig. 7). Figs. 8a and 9a present results related to the influence of the number of SILD cycles during gold deposition on sensor responses to reducing gases and ozone, respectively. The effect of the operating temperature on the sensor responses is shown in Figs. 8b and 9b. The results presented in Figs. 6–9 correspond to operating temperatures at which there is a maximum of sensor response. It is observed that the maximum response to reducing gases takes place at Toper of ~450°C, whereas the maximum response to ozone is observed at Toper of ~280°C. Such a major difference in the behavior of the sensor responses to reducing and oxidizing gases is typical for metal oxide sensors and can be explained by a difference in the reactivity of the tested gases [10,15,36,46]. ***Fig. 6*** near here ***Fig. 7*** near here ***Fig. 8*** near here ***Fig. 9*** near here

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To eliminate the side effects of treatments used during SILD of AuNPs on the sensor parameters, we performed additional experiments with the studied samples; tested sensors were subjected to the same treatments as they were during gold deposition, except that the treatment in an aqueous solution of HAuCl4 was replaced by washing with water. In Figs. 6–9, “0” deposition cycles corresponds to this treatment. This exact state was considered a reference point for the analysis of the influence of gold deposition on the properties of the SnO2 films. “0.5” deposition cycles corresponds to the treatment when the processing time in the HAuCl4 and NaBH4 water solutions was reduced two times in comparison with the time of treatments used in all of the other experiments. “Initial” in Figs. 6-9 corresponds to the parameters of the SnO2 gas sensors before any surface treatment. Detailed analysis of SnO2 gas sensor parameters with an unmodified surface can be found in previously published articles [34-36,46-49]. Analysis of the results presented in Figs. 6-8 establishes that the treatment with “0 deposition cycles”, unlike other treatments accompanied by deposition of gold, increases the film resistance (Figs. 6 and 7) and reduces the sensor response to reducing gases (Fig. 8) relative to the initial unmodified samples. Unfortunately, we do not know the real cause of the increase in the resistance after the “0 deposition cycle” treatment because this treatment included washing with distilled water, treatment with a water solution of NaBH4, washing again with distilled water and annealing at 450oC. Perhaps changes in the surface state of SnO2 films occurred during partial surface reduction after treatment with a water solution of the treatment NaBH4 and subsequent thermal reoxidation during annealing. However, such changes in SnO2 sample properties after the “0 deposition cycle” treatment, suggests that the increase in sensor response and the decrease in the film resistance observed for tested SnO2:Au samples after surface modification by gold clusters associated with the appearance of AuNPs on the SnO2 surface. There is an assumption that the active (i.e., the smallest) Au nanoparticles are negatively charged through electron donation from the conduction band of metal oxides. This assumption is based on the experimentally established fact that gold atoms prefer to bind to the site of oxygen vacancies (VO) [50]. It is also based on the results of theoretical studies [51] showing that gold atoms can take electrons from VO. This assumption is important because the presence of a negative charge on the gold nanoparticles promotes either the activation or dissociation of molecular oxygen and thus stimulates the oxidation of СО, Н2, alcohol, and so forth. The same Page 10 of 37

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conclusion was made by Tsunoyama et al. [52] regarding the Au:(poly(N-vinyl-2-pyrrolidone)based catalyst. Moreover, they found that the catalytic activity was enhanced with increasing electron density on the Au core. In addition, the same studies [51] conducted on TiO2 samples have shown that charge transfer is only possible for small clusters, such as Au1, Au3, Au5 and Au7 and for certain Au strips on the surface. This behavior provides a basis for the conclusion that the interaction of the large gold particles with the substrate is not as strong as that of the small clusters. As is known, electron transfer on gold nanoparticles should be accompanied by the formation of depletion zones under the particles and an increase in metal oxide resistance. However, this effect is not observed. Moreover, the resistance of the film modified by gold is slightly decreased (see Figs. 6 and 7). Such behavior can have the following explanations: (1) because of their large size, most of the gold particles on the SnO2 surface are neutral due to the lack of electron exchange with support; (2) the negative charge transferred from the metal oxide to gold is less than the negative charge captured by the species on the free SnO 2 surface at the area equivalent to the area of the gold particles; and (3) the appearance of gold nanoparticles on the surface of the SnO2 crystallites is accompanied by a decrease in the chemisorbed oxygen concentration. Figs. 6-9 also demonstrate that the gold nanoparticles deposited on the SnO2 surface are active to both reducing and oxidizing gases (Figs. 8a and 9a). However, the increase in sensor response to reducing gases is not significant (see Fig. 8a). In principle, a strong increase in sensitivity to reducing gases was not expected because thin films, especially those formed by thin-film technology due to the peculiarities of the morphology, are not optimal for detecting such gases [49]. Films formed using thin-film technology are usually characterized by lower porosity, a lack of necks, a larger area of intergrain contacts, and an increased size of the crystallites relative to gas sensing material prepared using thick-film technology [53]. In addition, due to the factors indicated above, the maximum sensitivity of sensors based on such films as a rule is shifted to higher temperatures in comparison to sensors fabricated by thick-film technology. An essential point in the obtained results was that the increase in sensitivity was observed relative to both CO and Н2 (see Fig. 8a). At the same time, a significant increase in the response of SnO2:Au-based sensors to ozone was unexpected. As shown in Fig. 9a, the growth of sensor response to ozone may be as large as two orders of magnitude. The sensitization to ozone leads to the conclusion that the field of using surface modifications by gold should not be limited to optimization of CO sensor parameters [14]. These results along with data given by Krivetskiy et al. [15] indicate that modification by gold can also be effective in the development of SnO2based sensors of ozone, acetone vapors, hydrogen, and so forth. Regarding NO2 sensors, more research is needed because studies presented in [15,53,54] gave inconsistent results. Furthermore, the results presented in Figs. 8 and 9 have shown that the effect of SnO2 surface decoration by AuNPs on gas sensing characteristics depends on the number of deposition cycles (i.e., the size of the gold particles; see Figs. 8a and 9a). Thus, as in the case of surface modification by noble metals such as Pd and Pt [1-3], there is an optimal level of surface doping, and this level of doping corresponds to a very low degree of surface coverage. As seen in Figs. 8 and 9, the maximum sensor response is observed for the samples corresponding to the “0.5” deposition cycle, for which the presence of gold nanoparticles on SEM images has not been observed. However, XPS studies indicated the presence of gold on the surface of these samples (see Fig. 4). This signifies that gold is responsible for the observed effect and that atomically dispersed gold or gold nanoparticles with a size not exceeding the resolution of microscopes are the most active in gas sensing effects. The resolution of the microscope used in the present Page 11 of 37

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studies was ~2-3 nm. This is consistent with data obtained for heterogeneous catalysis [7,8], where the most catalytically active clusters had a size of 2.5-3.0 nm. However, one should note that a further increase of deposition cycles, in which the gold nanoparticles appear on the SEM images and their size reaches 6-10 nm or more [37], is not accompanied by a sharp drop in sensor response, which should occur if the effect of the gold particles size on the catalytic activity is taken into account [7,8]. This result suggests that small particles can form on the surface of SnO2 rather than exclusively after the first deposition cycles and that the gas-sensitive effect involves not only small but also large particles. This conclusion is consistent with findings made by Carabineiro and Thompson [55], who found that gold-based catalysts are the most active in the 25 nm gold particle range, but that smaller and larger gold particles also have activity and may play significant roles. It is important to note that surface modification by the gold nanoparticles influences the SnO2 film conductivity measured in the air without the addition of any target gas (Figs. 6 and 7). Moreover, for reducing gases, the film resistance and sensor response have identical dependences on the number of deposition cycles (see Figs. 6 and 8). This suggests that most likely the same processes are responsible for the increase in the conductivity of the SnO2:Au films measured in the air without the addition of target gas and for the growth of sensor response to reducing gases. As indicated above, the gold particles formed on the surface of already deposited SnO2 films, and therefore AuNCs could not affect the bulk properties of the films or their structure. This means that the gold particles could only affect the processes having a surface nature. Analysis of such processes suggests that the back spillover of chemisorbed oxygen, which is intensified in the presence of reducing gases, is the most likely process, which may explain the observed behavior of both conductivity and sensor response in the presence of gold nanoparticles (see section 3.4). The absence of a shift in the response maximum in the lower temperature range for the SnO2 sensors with a modified surface is another important result of the present study. The maximum sensor response for the modified and unmodified SnO2:Au samples was observed at the same temperatures (Figs. 8b and 9b). From our point of view, the absence of a shift of the sensor response maximum in the range of lower temperatures is quite natural for the studied samples, because after the heat treatments used during gas sensor fabrication, (i) the coalescence of most catalytically active small gold particles with the formation of larger but less active particles takes place [4-7], and (ii) gold transforms to a metallic state (see section 3.2) in which gold does not show increased catalytic activity in low temperature range [44,56]. The increased size of the SnO2 crystallites and the large distance between gold particles can also contribute to the observed behavior of S = f(Toper) dependence. As seen in the SEM images (Fig. 1), the 20-50 nm SnO2 crystallites only have 1-2 gold nanoparticles on their surface. However, when the distance between the gold particles is too big, a significant gas sensing effect requires high temperature to provide high mobility of the surface species participating in the detection reactions (see section 3.4). Otherwise, the area of spillover zones around the gold particles is not sufficient for effective influence of the gold particles on the gas sensing properties of the SnO2 films. Regarding crystallite size, it is necessary to note that, according to [16], the achievement of high AuNPs reactivity at low temperature requires that metal oxide crystallites not exceed 5-6 nm. This statement is justified because oxides with such small crystallite size perform chemistry that is different than the chemistry of the same materials with larger crystallites. For example, the smaller metal oxide nanoparticles with a spherulite shape may supply more active sites and contribute to the strong interaction between gold and the nanocrystalline metal oxide. Unfortunately, forming porous metal oxides with such small crystallite size using thin-film

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technology is quite a difficult task [48,49]. As shown in [35], crystallites in films deposited on a heated substrate using thin-film technologies have a size that is proportional to the film thickness. It should be noted that the absence of a significant shift in the sensor response maximum of the SnO2:Au-based sensors in the low-temperature range that is characteristic of the current experiments is also typical of almost all such studies [10,11,54,57-60]. Almost all these studies were aimed at the development of sensors suitable for the sensor market, and therefore heat treatment at Тan ≥ 400°С was used in the fabrication technology. Such treatment is necessary for the sensor parameter stabilization. It is important that the absence of a temperature shift of the sensor response maximum was observed regardless of the method used for the surface modification by gold. Low temperature sensitivity to CO was observed only for sensors that were not subjected to stabilization annealing [61] or stabilized at temperatures below 200°С [12]. Low temperature sensitivity to CO was also observed by Ramgir et al. [62], who reported that the SnO2:Au films used in gas sensors were deposited by spray pyrolysis at Тspray = 450°С. However, in this case, we doubt that the temperature was correct because spray pyrolysis is accompanied by strong cooling of the substrate [63]. For a prolonged spray pyrolysis, the temperature can be decreased by several hundred degrees. This situation appears to have occurred in the experiments of Ramgir et al. [62], evidenced by the unusual morphology of the SnO2:Au layers studied [62]. Fig. 10 presents results related to the effect of surface modification on the recovery time of the sensor resistance after interacting with ozone. It is observed that surface modification by gold is accompanied by a decrease in the recovery time (τrec) to the initial state of the SnO2:Au sensors after interacting with ozone. It should be noted that this is an important effect because a long recovery time is one of the major disadvantages of SnO2-based ozone sensors [63]. As shown in Fig. 10, the maximum effect of τrec reduction was observed for the samples prepared using the maximum number of gold deposition cycles. Regarding the nature of this effect, one can then suggest the following mechanism: Because the recovery time in the case of ozone detection is determined by the lifetime of chemisorbed atomic oxygen remaining on the metal oxide surface after interacting with ozone [63], we presume that gold stimulates reactions that reduce this time. Reverse oxygen spillover and dissociative adsorption of water, carbon dioxide or hydrogen presenting in the atmosphere and interacting with chemisorbed oxygen presented on the SnO2 surface are examples of such reactions. The decreased recovery time with an increasing number of deposition cycles agrees with this hypothesis because the increase in the size of the clusters and their number should decrease the distance between the clusters and should thus increase the role of the spillover effect in the gas sensing phenomenon. It is clear that more specific statements will require more research. ***Fig. 10*** near here While analyzing the kinetics of the detection of reducing gases, no positive effects from the surface modification by gold were observed. In the range of operating temperatures between 300 and 400°C, surface modification by gold nanoparticles did not practically affect the kinetics of sensor response. We presume that such a situation exists because the kinetics of the SnO2:Au gas sensor response is controlled by adsorption/desorption processes taking place at the surface of SnO2 without participation of the gold nanoparticles, which is quite probable assuming that the kinetics of the sensor response are controlled by adsorption/desorption of oxygen and water on the SnO2 surface. Earlier in [64,65], this conclusion was confirmed for SnO2-based sensors with unmodified surfaces. 3.4. Sensitization mechanism in the SnO2:Au-based gas sensor Page 13 of 37

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Taking into account the close relationship of the gas sensitive effect observed in conductometric sensors with heterogeneous catalysis [66], one can state that the increased sensor response is conditioned by the appearance of the catalytically active gold nanoparticles on the SnO2 surface, which participate in a variety of reactions on the surface involved in sensor response, for example such as the oxidation of CO and H2. In Section 3.2, it was shown that SnO2 surface at room temperature is contaminated with carbon. However, we did not consider the influence of this contamination on gas sensing properties of SnO2:Au. During the surface modification experiments of SnO2 with gold, we did not observe any correlation between concentration of carbon on the surface measured at room temperature, and gas sensing properties. The concentration of carbon remained almost unchanged. However, a clear correlation between the concentration of gold on the surface and gas sensing effects was detected. In addition, it was established that at increased temperature and in an ozone-rich atmosphere, this concentration strongly decreases [67]. It has been suggested that the decrease in the concentration of carbon is due to the oxidation reaction that occurs with participation of chemisorbed oxygen and are accompanied by desorption of CO or CO2. For example Cataldo [68] showed that graphite nanoparticles react with O3 already at room temperature with evolution of CO2. Such behavior of surface carbon explains why carbon has a low influence on gas sensing effects measured at temperatures higher than 180-200oC. At these temperatures, highly reactive chemisorbed oxygen appears at the surface of metal oxides. Because of this, our attention was focused on gold nanoparticles. In addition, the inclusion of carbon in our model may further complicate the consideration of gas sensing properties of SnO2:Au, that even without carbon is rather complicated. It is known that the increased sensor response should only be observed in the case when electron transfer between surface catalysts and metal oxide accompanies this catalytic reaction [66,69]. In [6], based on the obtained data, H2 oxidation with the participation of Au/MeOx-based catalysts was presumed to occur without electron transfer, whereas the oxidation of CO with electron transfer is possible according to [39]. It appears from these studies that during CO and H2 detection, a significant difference in sensor parameters should be observed. However, in the present study, any essential difference in the behavior of the gas sensing characteristics of the SnO2-Au sensors during CO and H2 detection were not observed (see Fig. 6). On the basis of this fact, one can assume that for the studied SnO2-Au system, the mechanisms of the sensor response increase, observed during the detection of reducing gases CO and H2, are the same, at least at elevated temperatures. It should be noted that at present there is no consensus regarding the mechanism of gold nanoparticles participation in gas sensing effects. Therefore, both “electronic” and “chemical” [1,2,66,69] mechanisms of sensitization can be found in explanations proposed by various authors who studied gas-sensing effects in Au-MeOx-based sensors. For example, Rogers et al. [70], analyzing optical properties of Au-YSZ films, came to the conclusion that between AuNPs and YSZ at T = 500°C, charge transfer takes place, and the gas surrounding (CO, H2, and O2) influences this process. However, it is not clear what the reasons for charge transfer processes taking place at the Au surface are or the volume of YSZ (“redox”). In [57,71,72], it was also noted that, due to a charge transfer between the gold nanoparticles and SnO2, the depletion area is formed around gold nanoparticles on the surface of SnO2 nanobelts. In addition, it was concluded that this effect plays the primary role in the sensitization of SnO2:Au-based gas sensors. At the same time, Hubner et al. [11], studying SnO2:Au-based gas sensors fabricated using the gel impregnation method, believed that there is no electronic interaction between the gold Page 14 of 37

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nanoparticles and host oxide and that the sensitization effect of Au during CO and H2 detection can be attributed to the „„spillover effect‟‟. According to this model, the Au particles enrich the surface of the active metal oxide with oxygen species, which subsequently react with reducing gases such as CO and H2. This statement is based on the fact that the addition of gold during SnO2 synthesis was accompanied by an increase in the film resistance. However, it is known that molecular oxygen does not chemisorb dissociatively onto a gold surface within the temperature range of 25-500°C [73,74]. This eliminates the increase in the concentration of chemisorbed oxygen on the surface of metal oxide due to oxygen spillover effects. In addition, the experiment showed that the gold additive significantly modifies the conditions of SnO2 synthesis and thus has a significant effect on crystallite properties, such as the size, shape, and film morphology [10,15,62]. From our point of view, these precise changes may be the reason for the increased resistance observed in [13]. In the present study, the effect of gold on the structural properties of the SnO2 film is excluded because the gold deposition was conducted on an already formed film. This means that gold could not influence the structure of the film, and, therefore, changes observed in the gas sensing properties had a purely surface nature. Moreover, the data obtained (see Figs. 6 and 7) indicate that the resistance of the SnO2 film after modification by gold decreases. This suggests that in the temperature range used there is a reduction in the chemisorbed oxygen concentration on the surface of SnO2:Au crystallites but not its growth. This means that, in such a situation, oxygen back spillover is more likely than the direct spillover effect considered in [11]. However, at the same time, one can agree with Hubner et al. [11] that, during the detection of reducing gases, there is no change in the chemical state of Au. As shown in section 3.2, gold particles are already metallic in the initial state and therefore this state cannot change in a reducing atmosphere. This means that the “chemical” mechanism seems preferable for the detection of reducing gases. Certainly, one can agree with statements made in numerous papers [11,61,75] that the processes responsible for improvement in the sensor response of SnO2:Au-based devices to reducing gases take place at the periphery around the gold particles (i.e., at the perimeter interface between the gold particle and the metal oxide support, whereas oxygen species participating in oxidation reactions responsible for gas sensing effects are provided by the support (see Fig. 11a). It should be noted that the statements made above correlate with results discussed in the previous section, where it was determined that the sensor response is largely controlled by processes occurring at the free surface of the SnO2 crystallites. ***Fig. 11*** near here With regard to ozone detection, from our point of view, the “electronic” mechanism is thus the most preferred for explaining the observed effects. This mechanism is associated with oxidation of gold in the ozone atmosphere and a corresponding increase in the work function (see Fig. 11b). This should encourage the transition of electrons from the metal oxide on the cluster and thus promote an increase in the resistance of the metal oxide film. As shown by Miller et al. [76], small gold particles can be oxidized quite easily, whereas big clusters do not have such capability. Experiments have shown that a metallic gold film on TiO2 is stable toward heating in an oxygen atmosphere. However, Venkov et al. [77] have established that, in a NO + O2 gas mixture, metallic gold on the TiO2 surface is oxidized even at 573 K. The formation of a stable gold oxide, Au2O3, after treatment in oxygen d.c. plasma was also reported by Pireaux et al. [78] and Tsai et al. [79]. This oxide state of Au corresponds to a formal state 3+. This means that Au oxidation or at least significant oxygen chemisorption on the gold surface in the atmosphere of a strong oxidizing gas such as ozone (O3 + O2) is possible. In particular, it is known that gold is reactive enough to catalyze surface reactions, such as ozone dissociation [80]. The results Page 15 of 37

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obtained by Saliba et al. [81], Puckett et al. [82], Biener et al. [83], and King [84] also indicate an interaction of gold nanoparticles with ozone. For example, King [84] found that gold surfaces were oxidized by a combination of UV light and ozone generated from a mercury lamp, and after such treatment, the gold surfaces were enriched in oxygen. According to King [84], the oxide layer was found to be ~1.7 nm thick. Saliba et al. [81] also established that, after interaction with ozone, the Au(111) surface was covered by chemisorbed oxygen, and an electron transfer took place from the Au substrate to the oxygen adlayer, induced by the change of the work function up to 0.80 eV. The change in the work function of the Au surface after UV/ozone treatment was also observed by Suzue et al. [85]. They found that the work function changed at 0.55 eV. Here, it should be noted that this state is stable because the chemisorbed oxygen species could be removed from the gold surface only by heating the surface to temperatures of 600 K (375°C). For comparison, as mentioned above, dissociative O2 adsorption on the Au surface was observed only at temperatures exceeding 500°C [73,74]. However, taking into account the influence of gold particles on the magnitude of sensor response to ozone, it must be recognized that due to the small area of the gold nanoparticles on the surface of the SnO2 crystallites, the “electronic” mechanism of sensitization can hardly explain such a strong increase in sensor response. It appears in this case that a combined mechanism works when the oxidation/reduction of the gold nanoparticles is accompanied by a spillover of chemisorbed oxygen, which is formed at the surface of the gold clusters as a result of the dissociative adsorption of ozone [80,81]. The spillover effect in this case stimulates electronic exchange between SnO2 and the gold particles and also provides growth of the concentration of chemisorbed oxygen on the SnO2 surface. In other words, the spillover effect, which occurs during the interaction with ozone, promotes the growth of a negative charge captured by a surface species and thus contributes to the increase in band bending and the increased resistance of polycrystalline SnO2 films.

The results presented in this study demonstrate that successive ionic layer deposition (SILD) technology for gold nanoparticle formation can be used for surface functionalization of SnO2 films to improve their gas sensing properties. It was found that the size of the gold particles varies in the range of 1-50 nm depending on the number of SILD cycles. It is established that the gold nanoparticles deposited on the SnO2 surface are active to both reducing (CO and H2) and oxidizing (O3) gases and that the effect of the SnO2 surface decoration by AuNPs on the gas sensing characteristics also depends on the number of deposition cycles (i.e., the size of the gold particles). The maximum effect is observed with the smallest size gold nanoparticles. Sensitization to O3 and H2 suggests that applications of the gold surface modification in the field of gas sensor design should not be limited to the optimization of the CO sensor parameters. Models showing the promotional role of Au additives were also discussed, and a mechanism of sensitization in the SnO2:Au-based gas sensor was proposed. Based on the results obtained, it was assumed that (1) during interaction with reducing gases, gold does not change its chemical state (i.e., AuNCs remain in a metallic state); (2) processes that are responsible for improving the sensor response occur at the periphery around the Au nanoparticles; and (3) oxygen back spillover is more likely than is a direct spillover effect. With regard to the detection of ozone, it was concluded that an “electronic” mechanism, which is associated with the oxidation of gold nanoparticles in ozone atmosphere, and the increase of the work function seem

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to be preferred. The increase of the work function should stimulate the transition of electrons from the metal oxide to the gold particles and thus increase the resistance of the metal oxide film. However, the same analysis also confirms that our knowledge of the processes taking place in the Au-SnO2 system is rather limited and that to extend this fundamental understanding, additional research is required. The SILD technology analyzed in this paper also requires further development and optimization. In particular, from our point of view, it is necessary to find criteria that allow an increase in the number of gold nanoparticles of the required size formed on the surface of metal oxide crystallites (i.e., increasing the density of their distribution and decreasing the inter-particles distance). It is also important to elucidate the role of the size of the SnO2 crystallites in gas-sensitive effects involving gold nanoparticles.

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5. Acknowledgments

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This work was supported by National Research Foundation (NRF) grants funded by the Korean government (Nos. 2011-0028736 and 2013-K000315), by the Basic Science Research Program through the NRF funded by the Ministry of Science, ICT and Future Planning of Korea (2012-R1A1A2041564), by the Russian Foundation for Basic Research (Grant No. 15-0308045А), and by the Moldova Government under grant 15.817.02.29F and ASM-STCU project #5937. The authors are also thankful to Dr. X.Y. Chen and Prof. J. Schwank from the University of Michigan, USA for help in SnO2:Au film characterization and Prof. V.P. Tolstoy from St. Petersburg State University, Russia, for participation in the design of the SILD technology used for SnO2 surface modification by gold nanoparticles.

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Fig. 1. SEM image of SnO2 films deposited by spray pyrolysis and SnO2 films Au-modified by the SILD method: (a) 1, (b) 4, and (c) 8 deposition cycles (Tan = 400°C). Fig. 2. Full-range XPS spectra of the SnO2 films modified by Au nanoparticles deposited by the SILD method (8 deposition cycles).

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Fig. 3. EDX spectra of SnO2 films deposited on a Si substrate and modified by AuNPs using 3 SILD cycles.

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Fig. 4. XPS Au 4f core level spectra of SnO2 films with their surface modified by Au nanoparticles deposited by SI D methods: (1) 0.5, (2) 2, and (3) 4 deposition cycles. The “0.5 cycle” means that the time of the treatment in HAuCl4 and NaBH4 solutions was reduced 2-fold compared to the usual route.

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Fig. 5. XPS (a) Sn 3d and (b) O1s core level spectra of SnO2 films with their surface modified by Au clusters deposited by the SILD method: (1) initial SnO2 films and (2) SnO2:Au samples after 4 deposition cycles. Fig. 6. Conductivity of the SnO2 films modified by Au vs. the number of SILD deposition cycles in different atmospheres at the operation temperature Toper = 450°C: (1) air and (2) air + hydrogen (1000 ppm). Fig. 7. Conductivity of the SnO2 films modified by Au vs. the number of SILD deposition cycles in different atmospheres at the operation temperature Toper = 280°C: (1) air and (2) air + ozone (~1 ppm). Fig. 8. (a) Conductivity responses to H2 and CO (1000 ppm) of the SnO2 films modified by Au vs. the number of SILD deposition cycles. (b) Typical temperature dependencies of SnO2-based sensor responses to H2: (1) unmodified samples and (2) SnO2:Au sensors modified by AuNCs (1 deposition cycle). Fig. 9. (a) Influence of the number of Au deposition cycles on the SnO2:Au sensor response to ozone. (b) Typical temperature dependencies of SnO2-based sensors in response to ozone. Fig. 10. Influence of the number of deposition cycles on the recovery time of SnO2:Au sensors after interacting with ozone.

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Fig. 11. Schematic diagram illustrating mechanisms of AuNP influence on the gas sensing characteristics of SnO2:Au films in the case of (a) reducing gases (CO, H2) and (b) oxidizing gases (O3): (a) adsorption of reducing gases, back spillover of chemisorbed oxygen and reaction of these species at the periphery of gold nanoparticles as the main processes controlling reducing gas detection. (b) In the case of ozone, the mechanism of detection includes two processes: (1) oxidation/reduction of gold nanoparticles, which is accompanied by electron transfer between SnO2 and gold nanoparticles, and (2) dissociative adsorption of ozone with following spillover of chemisorbed oxygen.

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Figure-1 Click here to download high resolution image

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Figure-2 Click here to download high resolution image

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Figure-3 Click here to download high resolution image

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Figure-4 Click here to download high resolution image

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Figure-5a Click here to download high resolution image

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Figure-5b Click here to download high resolution image

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Figure-6 Click here to download high resolution image

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Figure-7 Click here to download high resolution image

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Figure-8 Click here to download high resolution image

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Figure-9 Click here to download high resolution image

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Figure-10 Click here to download high resolution image

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Figure-11 Click here to download high resolution image

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Table-1

Table 1. The average size of the gold clusters formed by the SILD technology on the surface of SnO2 films with the size of crystallites varied in the range of 25 ± 15 nm. 1 2 3–6 4-7

4 5 - 10

8 10 - 30

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Number of SILD cycles 0.5 Size of AuNPs, nm <3

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