Metal Oxide Gas Sensors with Au Nanocluster Catalytic Overlayer

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Functional Inorganic Materials and Devices

Metal Oxide Gas Sensors with Au Nanocluster Catalytic Overlayer: Toward Tuning Gas Selectivity and Response Using a Novel Bilayer Sensor Design Young Kook Moon, Seong-yong Jeong, Yun Chan Kang, and Jong-Heun Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11079 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

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ACS Applied Materials & Interfaces

Metal Oxide Gas Sensors with Au Nanocluster Catalytic Overlayer: Toward Tuning Gas Selectivity and Response Using a Novel Bilayer Sensor Design Young Kook Moon, † Seong-Yong Jeong, † Yun Chan Kang, † and Jong-Heun Lee*,† †Department

of Materials Science and Engineering, Korea University, Seoul 02841, Republic

of Korea *Author to whom correspondence should be addressed. E-mail: [email protected]; Fax: +82–2–928–3584; Tel: +82–2–3290–3282

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ABSTRACT

Noble metals or oxide catalysts have traditionally been loaded or doped to enhance the gas sensing properties of oxide semiconductor chemiresistors. However, the selective detection of various chemicals for a wide range of new applications remains a challenging problem. In this paper, we propose a novel bilayer design with an oxide chemiresistor sensing layer and nanoscale catalytic Au overlayer to provide high controllability for gas sensing characteristics. The Au nanocluster overlayer significantly enhances the methylbenzene response of a SnO2 thick film sensor by reforming gases into more reactive species and suppresses the responses to reactive interference gases through oxidative filtering, leading to excellent selectivity to methylbenzene. Gas sensing characteristics can be tuned by controlling the morphology, amount, and number density of Au nanoclusters through the variation in the Au coating thickness (0.5–3 nm) and thermal annealing conditions (0.5–4 h at 550 ºC). Furthermore, the general validity of the proposed Au-coated bilayer sensor design was confirmed through the enhancement of response and selectivity toward methylbenzenes by coating Au nanoclusters onto ZnO and Co3O4 sensors. The sensing mechanism, advantages, and potential applications of bilayer sensors are discussed from the perspective of the separation of sensing and catalytic reactions, as well as the reforming and oxidation of analyte gases in association with the configuration of the sensing layer and Au catalytic overlayer.

KEYWORDS: bilayer, metal oxide gas sensor, Au nanocluster, SnO2, ZnO, Co3O4, gas selectivity, gas response


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1. INTRODUCTION Metal oxide semiconductor gas sensors have been widely used for detecting various explosive and hazardous gases because of their distinctive advantages, including high sensitivity, rapid response, reversibility, stability, and cost effectiveness.1-6 Recently, gas sensors and sensor arrays using oxide chemiresistors have provided new opportunities in the applications of indoor air quality monitoring,7,

8

disease diagnosis,9 and food freshness

evaluation10 based on the rapid development of the internet of things, sensor networks, sensor integration technology, and artificial intelligence. Remarkable efforts have been devoted to enhancing gas sensing properties by loading/doping of noble metals or oxide catalysts.11-14 However, the numbers of detectable analyte gases and available gas sensing patterns for discriminating different chemicals are still limited and insufficient for use in new applications and high-performance electronic noses. Furthermore, the resistance of catalyst-loaded/doped sensors often becomes too high to measure using conventional electric circuits because of oxygen spillover,15 acceptor doping,16, 17 and charge transfer across hetero-interfaces with different work functions.18 Finally, the catalyst-assisted increase in gas response is frequently accompanied by an increase in responses to interference gases, which hampers selective gas detection. From this perspective, a novel sensing strategy that provides diverse sensing characteristics and high tunability for gas selectivity and gas response is highly required. Bilayer designs for sensing films coated with catalytic overlayers can separate sensing and catalytic reactions into independent processes, unlike films consisting of sensing materials that are uniformly doped/loaded with catalytic materials. Therefore, such designs can be considered as a possible approach to solving the catalyst-associated challenges described above. For 3 ACS Paragon Plus Environment


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example, when sensing electrodes are located on the lower layer of the film, sensor resistance is largely unaffected by the catalytic overlayer, which provides controllability for sensing characteristics without the variation of sensor resistance. In the early stages of bilayer design research, micrometer-thick catalyst-loaded or catalytic overlayers were explored for reducing the cross-responses to interference gases through oxidative filtering.19-22 Recently, we proposed a novel bilayer sensor design consisting of a micrometer-thick sensing film and nanoscale oxide catalytic overlayer that provides excellent control for gas selectivity and responses.23, 24 The excellent tunability of gas sensing behaviors is attributed to the two different roles of the nanoscale overlayer of catalytic oxide, namely the enhancement of response to less reactive gas through gas reforming and the suppression of cross-responses to interference gases by oxidative filtering. Accordingly, it is very important to confirm if these intriguing and versatile control characteristics for both gas selectivity and response are also achievable when using a nanoscale overlayer of noble metal catalysts because various combinations of sensing and catalytic materials in bilayer designs can offer new functionalities and opportunities. Nevertheless, the design of bilayer gas sensors is still in the nascent stage and to the best of our knowledge, the reforming-assisted enhancement of gas response using bilayer sensors with noble metal catalyst overlayers has not been reported. Among the noble metal catalysts, Au exhibits excellent catalytic performance for the oxidation of volatile organic compounds when nanoscale Au particles are loaded onto oxide supports, such as TiO2, Fe2O3, SnO2, ZnO, and CeO2.25-28 Therefore, in this study, various configurations (i.e., size, amount, and number density) of Au nanoclusters were formed on representative gas sensing SnO2 films by controlling the Au coating thickness (0.5–3 nm) and thermal annealing time (0.5–4 h) at 550 °C. The gas sensing characteristics of the resulting sensors were then measured. The configuration of Au nanoclusters in the overlayer 4 ACS Paragon Plus Environment

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significantly influenced the response and selectivity of the Au(c)/SnO2(s) (c: catalytic overlayer, s: sensing layer) bilayer sensors to methylbenzenes, indicating excellent potential for tuning gas sensing characteristics by simultaneously controlling gas reforming and oxidation. Furthermore, the general validity of the proposed bilayer sensor design was confirmed by demonstrating that Au nanocluster overlayers also increase the methylbenzene selectivity of ZnO and Co3O4 sensors. The main focus of this study was to analyze the Auconfiguration-dependent gas sensing behaviors of Au(c)/SnO2(s) sensors based on their underlying sensing mechanisms, as well as the role of Au nanoclusters in gas reforming and oxidation. 2. EXPERIMENTAL SECTION 2.1. Synthesis of SnO2 hollow spheres SnO2 hollow spheres were prepared using ultrasonic spray pyrolysis. The experimental setup for spray pyrolysis is shown in Figure. S1a. First, 0.1 mol of Tin (II) chloride dihydrate (SnCl2• 2H2O, ≥ 99.99%, Sigma-Aldrich, USA) and 0.025 mol of citric acid monohydrate (C6H8O7• H2O, ≥ 99.0%, Sigma-Aldrich, USA) were dissolved in 1 L of a diluted hydrochloric acid solution (35.0–37.0% HCl : H2O = 1 : 49 by volume). Droplets of this source solution were then generated by six ultrasonic transducers (resonance frequency: 1.7 MHz) and transferred into a quartz tube (inner diameter : 55 mm) in an electric furnace heated to 700 ºC in air (flow rate : 10 L/min) for pyrolysis. A Teflon bag filter was used to collect the precursor powders and the as-prepared precursor powders were converted into SnO2 hollow spheres via heat treatment at 600 ºC for 2 h in air. 5 ACS Paragon Plus Environment


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2.2. Preparation of gas sensing films and gold nanoclusters The SnO2 powders were screen printed onto Pt interdigitated electrode (IDE) (electrode gap : 100 m) patterns on SiO2/Si substrates (size = 8 mm x 8 mm) (Figure. S1b). The slurry for screen printing was prepared by mixing SnO2 hollow spheres with an organic binder (FCM, a terpineol-based ink vehicle, USA) (SnO2 : binder = 28 : 72 by weight). The thickness of the sensing films was manipulated by controlling the emulsion thickness of the screen. After screen printing, the sensing films were heat treated at 550 ºC for 2 h to remove the organic binder. Catalytic Au nanoparticles were deposited on the SnO2 thick films through e-beam evaporation of Au grains (99.998%, Kojundo Chemical Lab. Co., Ltd., Japan) in a vacuum chamber (below 2×10-6 Torr) at a rate of 0.1 Å sec−1 (nominal thicknesses : 0.5, 1, and 3 nm), as calibrated by a quartz crystal microbalance, which were converted into discrete configurations of Au nanoclusters through heat treatment at 550 ºC for 2 h. For simplicity, these sensors will be referred as 0.5Au/SnO2(2h), 1Au/SnO2(2h), and 3Au/SnO2(2h) for Au layer thicknesses of 0.5, 1, and 3 nm, respectively. Additionally, SnO2 thick films with 1-nm-thick Au overlayers were also heat treated at 550 ºC for 0.5 and 4 h (referred to as 1Au/SnO2(0.5h) and 1Au/SnO2(4h), respectively) to investigate the effects of Au configuration on gas sensing characteristics. The procedures to measure gas sensing characteristics and to analyze materials are shown in Supporting Information. 3. RESULTS AND DISCUSSION SnO2 spheres were prepared through a spray pyrolysis reaction and subsequent heat treatment at 600 ºC (Figure. S1a). The average diameter of ~100 SnO2 spheres was 1.27 ± 0.64 m (Figure. S2a). A TEM image reveals that the contour of the central region of the spheres is brighter than that of the edges, confirming a hollow morphology (Figure. S2b). The shells were 6 ACS Paragon Plus Environment

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24–32 nm thick (Figure. S2c). A crystalline SnO2 phase was confirmed by lattice fringes exhibiting (110), (011), and (101) planes with spacing values of 3.45, 2.69, and 2.66 Å, respectively (Figure. S2d). The N2 adsorption and desorption isotherms of the SnO2 hollow spheres exhibit type-IV characteristics with H3 hysteresis loops, indicating the presence of nanocavities with mesopores (Figure. S3). The SnO2 spheres consisted of abundant mesopores with diameters ranging from 2 to 50 nm and exhibited a surface area of 26.3 m2g−1, suggesting highly gas-accessible structures of sensing materials. The SnO2 thick films were formed by screen printing a slurry using a Pt IDE on a SiO2/Si substrate and sequent heat treatment at 550 ºC. A cross-sectional SEM image of a sensing film (Figure. 1a) reveals that the films consisting of SnO2 hollow spheres are approximately 32.0 m thick. The thicknesses of all the pure and Au-coated SnO2 sensing films in the experiment were similar (approximately 30 m) (Figure. 1a and b). Au was coated onto the SnO2 thick films through e-beam evaporation, which converted the Au particles into discrete Au nanoclusters through agglomeration and crystallization during thermal annealing at 550 ºC (Figure. S1b). The size, amount, number density, and configuration of the Au nanoclusters can be tuned by controlling the thickness of the Au coating (0.5, 1, and 3 nm) and the annealing time (0.5, 2, and 4 h). For example, a high-magnification SEM image (Figure. 1c) of the top of the 1Au/SnO2(2h) sensing film reveals that Au nanoclusters are uniformly coated onto the SnO2 spheres. The compositional profiles of the Sn and Au components were further analyzed by using an EPMA (field-emission electron probe microanalyzer) elemental mapping (Figure. 1df). The results clearly show that most of the Sn component is uniformly distributed over the entire sensing film, whereas the Au is located only on the upper part of the thick film. This is confirmed again by the compositional mapping in TEM analysis (Figure. 1g–i). 7 ACS Paragon Plus Environment


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Figure. S4 shows HR-TEM images of the 1Au/SnO2(2h) specimen. One can see that the Au nanoparticles are uniformly dispersed on the highly crystalline SnO2 (Figure. S4a and b). To verify the chemical state of the Au nanoparticles after annealing at 550 ºC, the SnO2(2h) and 1Au/SnO2(2h) specimens were analyzed using XPS (Figure. S4c-f). The C 1s (284.6 eV) line was used to calibrate the binding energies. The Sn 3d3/2 and 3d5/2 peaks of SnO2(2h) at 495.0 and 486.5 eV indicate the formation of SnO2 (Figure. S4c).29 The binding energies of the Au 4f5/2 and Au 4f7/2 peaks (4f5/2 = 87.3 eV and 4f7/2 = 83.6 eV) of 1Au/SnO2(2h) (Figure. S4f) are slightly lower than those of bulk metallic Au (4f5/2 = 87.7 eV and 4f7/2 = 84.0 eV),30 whereas those of the Sn 3d3/2 and Sn 3d5/2 peaks are shifted to higher levels based on the Au coating (Figure. S4c and d), indicating electron transfer from SnO2 to the Au nanoparticles. This suggests that the Au nanoparticles in the 1Au/SnO2(2h) specimen exist in a metallic state. The XRD results for the pure SnO2, 0.5Au/SnO2(2h), 1Au/SnO2(2h), and 3Au/SnO2(2h) specimens reveal a tetragonal SnO2 phase [ICDD# 041-1445] (Figure. S5). The crystallite size of this SnO2 phase was calculated to be 14.5 ± 1.7 nm using Scherrer’s equation. No peak shifts were observed in any of the Au-coated specimens and an Au phase was observed in the 3Au/SnO2(2h) specimen, suggesting that Au is not incorporated into the SnO2 lattice during heat treatment at 550 ºC. The TEM images of SnO2 hollow spheres with different configurations of Au nanoclusters are shown in Figure. 2, and the sizes and number densities of the Au nanoclusters are summarized in Figure. S6. The average sizes of the Au nanoclusters in the 0.5Au/SnO2(2h), 1Au/SnO2(2h), and 3Au/SnO2(2h) specimens were 5.8 ± 1.2, 8.0 ± 2.3, and 12.5 ± 4.0 nm, respectively (Figure. S6a–c). The number density of Au nanoclusters per unit area (ND-Au) increased from 2,436/m2 to 4,196/m2 when the Au coating thickness


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increased from 0.5 to 3 nm, which can be attributed to an increase of Au sources. When the 1Au/SnO2 specimen was annealed for a shorter time (0.5 h), the average size of the Au nanoclusters decreased to 5.4 ± 2.1 nm, but the ND-Au value increased to 5,550/m2 (Figure. S6b and d), indicating limited growth of Au nanoclusters. Nanocluster size and ND-Au tended to saturate when the heat treatment time increased further from 2 h to 4 h (Figure. S6b and e). The gas sensing characteristics of the pure and Au-coated SnO2 films were measured at temperatures in the range of 350–450 ºC (Figure. 3). For all of the sensors, sensor resistance decreased upon exposure to reducing gases and returned to its original value when the gas atmosphere was changed to air (Figure. S7). The gas responses (S = Ra/Rg) to 5 ppm of benzene, toluene, p-xylene, and ethanol were calculated from the sensing transients. The responses of the pure SnO2 sensor to 5 ppm of xylene and toluene (Sxylene = 13.7, Stoluene = 13.0) at 350 ºC are higher than its responses to benzene and ethanol (Sbenzene = 6.5, Sethanol = 3.7) (Figure. 3a), but all of the gas responses become similar and low as the sensing temperature increases to 450 ºC. For a fixed annealing time of 2 h (Figure. 3a–d), the overall gas responses tended to increase upon coating a 0.5-nm-thick Au layer (Figure. 3b), which became the highest with increasing a coating thickness to 1 nm (Figure. 3c). For example, the gas responses of the 1Au/SnO2(2h) sensor to 5 ppm of xylene and toluene are as high as 61.4 and 56.2 at 350 ºC, which are 4.5 and 4.3 times higher than those of the SnO2(2h) sensor, respectively. However, a further increase in Au coating thickness to 3 nm decreases the gas responses, although the gas responses are still higher than those of the pure SnO2(2h) sensor. Note that the Au nanoclusters are not only small (size: 2–25 nm), but are also only loaded onto the upper SnO2 hemispheres on the top of a sensing film (Figure. 1d–i), whereas the thicknesses of all of the SnO2 sensing films with and without Au overlayers are similar (~ 30 m). This suggests that the increase in gas response did not emanate from the morphological or microstructural variation of the sensing films, but 9 ACS Paragon Plus Environment


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from the catalytic role of Au nanoclusters, meaning gas response can be tuned by controlling the amount of Au nanoclusters. The gas responses of the 1Au/SnO2(0.5h) and 1Au/SnO2(4h) sensors were also investigated (Figure. 3e and f). It was found that the overall gas responses decreased when shortening or extending the annealing time, indicating that gas response is dependent not only on the absolute amount of Au nanoclusters, but also on the configuration of the Au nanoclusters. The optimal sensing temperature for sensitive and selective detection of methylbenzenes was determined to be 350 ºC. Therefore, the gas responses at 350 ºC and the probable sensing mechanisms are shown in Figure. 4. The methylbenzene responses of the SnO2(2h) sensor fabricated in this study are higher than the ethanol responses (Figure. 4a), whereas the responses of most pure SnO2 sensors to reactive ethanol in the literature are generally higher than those to less-reactive methylbenzenes.31-33 To understand this behavior, the gas responses of pure SnO2 sensors with different thicknesses (30 and 6 m) were measured at 300 and 350 ºC (Figure. S8). Note that the thin SnO2 sensor (thickness: 6 m) at a low sensing temperature (300 ºC) exhibited the highest response to ethanol. And the thicker sensing layer or higher sensing temperature leads to relatively lower responses to ethanol. This suggests that the transport of ethanol to the lower sensing region of the film that is close to the sensing electrodes is difficult because the reactive ethanol is oxidized into less-reactive or non-reactive species (e.g., H2O or CO2) in the upper region of a SnO2 sensing film when the sensing temperature is high or sensing film is thick. This is feasible considering that SnO2 is a representative catalytic material for oxidizing hydrocarbons.34 Therefore, the cross-responses to ubiquitous and highly reactive ethanol can be suppressed in part through the operation of a relatively thick SnO2(2h) sensor at a moderately elevated temperature (350 ºC). 10 ACS Paragon Plus Environment

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However, the oxidative filtering of interference gas through film thickening is often insufficient for achieving a significant response to analyte gases based on limited gas transport. The most intriguing result from this study is the significant enhancement of methylbenzene responses through the introduction of Au nanoclusters on top of sensing films (Figure. 4b–f). Because the lower part of the sensing film that is close to the electrodes is unaffected by Au decoration, this increase in methylbenzene responses can be explained by the Au-catalystassisted reforming of less reactive methylbenzene into more reactive and smaller species. This explanation matches the gas response enhancement through catalytic oxide overlayer coating in previous studies.23,

24

Most of volatile organic compounds (VOCs) such as alcohols,

aldehyde, amine, and aromatic compounds can be dissociated and/or partially oxidized into more reactive and smaller species.35-38 This suggests that the possibility of gas response enhancement can be accomplished through gas reforming reaction on various VOCs. However, when the thickness of the Au coating increases to 3 nm (3Au/SnO2(2h)), the responses to xylene and toluene decrease (Sxylene = 33.7, Stoluene = 30.7) (Figure. 4d), indicating that the oxidation of xylene and toluene to reduce gas responses is promoted more by the excessive amount of catalytic Au nanoparticles. These results demonstrate that the amount of Au nanoclusters is an important parameter for controlling gas sensing characteristics and should be optimized considering both the oxidation and reforming of analyte gas. At a fixed Au coating thickness (1 nm), the effects of annealing time (0.5 h, 2 h, and 4 h) on gas sensing characteristics were investigated (Figure. 4c,e, and f). The gas responses decreased with both long and short annealing times, resulting in the following trend: S(1Au/SnO2(2h)) > S(1Au/SnO2(4h)) > S(1Au/SnO2(0.5h). This indicates that the role of Au nanoclusters in the sensitive and selective detection of methylbenzenes is not simple. It should be noted that the size of Au nanoclusters and ND-Au of the 1Au/SnO2(0.5h) sensor are significantly smaller and 11 ACS Paragon Plus Environment


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greater than those of the 1Au/SnO2(2h) sensor, respectively (Figure. S5b and d). The catalytic activity of Au is known to be related to many different parameters, such as the size of Au nanoclusters,39, 40 portion of low-coordinated Au atoms,39, 41, 42 valence state of Au (Au0 or Au+),43,

44

support materials,39,

45, 46

and interface between Au and the oxide support.47 In

particular, Haruta et al.28, 48 reported that the catalytic activity per unit surface area of supported Au nanoparticles is approximately 100 times greater than that of unsupported Au nanoparticles. This means that irreversible oxygen adsorption at the perimeter interface determines the rate of catalytic oxidation. To examine the effects of Au catalysts on oxygen chemisorption, XPS analysis was carried out (Figure. S9). The asymmetric O 1s spectra of all specimens were separated into two peaks: a lattice oxygen peak (O2−lat = 530.37 eV) and chemisorbed oxygen peak (O−ads = 531.28 eV).29 The peak ratios of O−ads/O2−lat for pristine SnO2, 0.5Au/SnO2(2h), 1Au/SnO2(2h), and 3Au/SnO2(2h) are 0.48, 0.54, 0.93, and 1.2, respectively. This supports the hypothesis that the catalytic activity of Au nanoparticles increases as the amount of Au increases and agrees with the observation that ND-Au values and the perimeter interface length between Au and SnO2 surfaces increase with increasing Au amounts. Additionally, the peak ratios of O−ads/O2−lat for 1Au/SnO2(0.5h), 1Au/SnO2(2h), and 1Au/SnO2(4h) are 2.87, 0.93, and 0.65 respectively. This demonstrates that the longer the annealing time, the lower the catalytic activity of Au nanoparticles. For the same amount of Au, smaller Au nanoparticles exhibit higher catalytic activity because they provide increased surface area for reaction and a longer interface between Au and SnO2. Accordingly, the decrease in overall gas responses when shortening the annealing time from 2 h to 0.5 h for 1Au/SnO2 sensors can be attributed to the promotion of oxidative gas filtering (Figure. 4c and e). In comparison, the decrease of gas responses with


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extending the annealing time from 2 h to 4 h can be explained by the restrained gas reforming (Figure 4c and f). The results above clearly demonstrate a novel and versatile strategy for controlling gas response and selectivity using oxide semiconductor films coated with Au nanoclusters. To highlight the unique advantages of such bilayer sensor structures, an Au-loaded SnO2 sensor was also prepared by loading Au nanoparticles to the entire surfaces of SnO2 spheres and subsequent thick film (Figure. S10). The sensing characteristics of this film were compared to those of the films discussed above. Both the SnO2(2h) and 1Au/SnO2(2h) sensors exhibited similar Ra values (3.8 and 3.0 MΩ) (Figure. 5a and b) because the sensing materials near the electrodes are unaffected by the Au nanoclusters. In contrast, the Au-loaded SnO2 sensor exhibited a very high Ra value (22 GΩ) that is nearly immeasurable using a conventional electric circuit (Figure. 5c). This increase in the Ra values of SnO2 sensors based on Au loading has been reported frequently in the literature15,

49, 50

and has been explained either by Au-

assisted oxygen spillover effect15, 51 or by Schottky contact formation between SnO2 and Au nanoclusters with different work function values (𝛷𝑆𝑛𝑂2 = 4.9 eV and 𝛷𝐴𝑢 = 5.35–5.76 eV).49, 50, 52, 53 In any case, a significant increase in sensor resistance based on catalyst loading can be an obstacle to resistance measurement using conventional electric circuits. The control of gas selectivity and response without affecting sensor resistance in this study emanated from the separation of catalytic and sensing regions. This separation is very useful for designing real sensors. To investigate the general potential and validity of the proposed bilayer sensor design in the context of catalytic activation or filtering of analyte and interference gases, the effects of Au nanoclusters on the gas sensing characteristics of ZnO and Co3O4 thick films were investigated (Figure. 6a and b). The pure ZnO thick film sensor showed the highest response to ethanol (S 13 ACS Paragon Plus Environment


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= 88.7) at 350 ºC. However, the ethanol response became very low (S = 16.5) after applying a Au nanocluster overlayer, even lower than the response to methylbenzenes. Similarly, the ethanol response was the highest (S = 45.0) for the Co3O4 sensor at 200 ºC, but this response became lower than the xylene response (S = 33.6) after applying the Au nanocluster overlayer to the top of sensing film. Note that the response (S) of p-type Co3O4 sensor was defined as Rg/Ra. In both sensors, the responses to xylene were enhanced by Au nanoclusters, whereas those to ethanol were reduced. These results clearly confirm again that Au nanocluster overlayers are a general solution that can be used not only for suppressing ethanol responses through oxidative filtering, but also for enhancing xylene responses by reforming xylene molecules into more reactive species. Furthermore, a minimal amount of Au coating is sufficient for the proposed sensor design. From this perspective, the sensor coated with Au nanoclusters proposed in this study is a versatile, economic, and promising platform that can control gas response and selectivity without affecting sensor resistance by using a minimal amount of catalyst. Xylene has been widely used as solvent in various industries and is a representative indoor pollutant. Because the inhalation of xylene can cause various symptoms, such as headaches, dizziness, and nausea, the permissible exposure limit for xylene in the workplace is defined as 100 ppm (8-h time-weighted-average concentration).54 Additionally, chronic occupational exposure to 14 ppm of xylene is known to cause eye irritation, sore throat, and neurological effects.55 The responses of the 1Au/SnO2(2h) sensor to 0.25–5 ppm of xylene were measured at 350 ºC. The detection limit for p-xylene was calculated to be 28.3 ppb based on extrapolation when an Ra/Rg value > 1.2 was used as the criterion for gas sensing (Figure. 7a and b). The sensing transient of the 1Au/SnO2(2h) sensor remained stable upon repetitive exposure to 5 ppm of p-xylene at 350 ºC and the sensor exhibited good long-term stability for over 20 days 14 ACS Paragon Plus Environment

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(Figure. 7c and d). Accordingly, the sensor proposed in this study can be used to monitor ppmlevel and sub-ppm-level hazardous xylene gases in real applications. Oxide semiconductor chemiresistors have long been used to detect harmful gases. However, to expand gas sensor applications further, new sensing materials must be designed to achieve new functionality. Additionally, abundant gas sensing libraries for complex gases are essential for the discrimination of different odors. A bilayer sensor design with an oxide semiconductor sensing layer and catalytic overlayer can provide numerous combinations of different catalytic and sensing materials, which will facilitate the design of high-performance gas sensors and electronic noses. Furthermore, gas sensing characteristics can be tuned effectively by controlling the size, number density, and configuration of catalytic materials. Finally, the separation of gas sensing and catalytic reactions into independent processes can not only provide excellent controllability over sensing characteristics, but also avoid abrupt changes in sensor resistance in air after catalyst loading. Therefore, the proposed sensor design offers a novel and generalized solution for designing diverse gas sensing materials for a wide range of new applications. 4. CONCLUSION Bilayer sensors with oxide chemiresistor sensing layers and Au nanocluster overlayers were proposed as novel sensor structures with high controllability for gas sensing characteristics. The gas sensing mechanisms of the proposed sensors were investigated. A coating of Au nanoclusters on the top surface of a SnO2 sensing film can either enhance or reduce gas responses to volatile organic compounds. The former phenomenon is related to the reforming of less reactive gases into more reactive or smaller species, whereas the latter phenomenon is attributed to the oxidation of gases into less reactive or non-reactive species. Therefore, gas 15 ACS Paragon Plus Environment


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selectivity and response can be tuned by controlling the amount, size, number density, and configuration of Au nanoclusters in the overlayer. The general concept of a bilayer sensor design was validated by an increase in xylene responses and decrease ethanol cross-responses due to the coating of Au nanoclusters on ZnO and Co3O4 sensors. These results demonstrate the excellent potential of bilayer sensors with nanoscale Au catalytic overlayers for tuning gas sensing characteristics through the versatile control of gas reforming and gas oxidation. Such sensors can be used for new sensor applications that require diverse sensing materials with new functionality and abundant gas sensing libraries.


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ASSOCIATED CONTENT Supporting information. Supplementary Experimental Section; Schematic diagrams of the ultrasonic spray pyrolysis process for synthesizing the SnO2 hollow spheres and the fabrication of the Au/SnO2 sensor; Scanning electron microscopy (SEM) image of SnO2 hollow spheres, Transmission electron microscopy (TEM) images of SnO2 hollow spheres, and high resolution lattice fringe; N2 adsorption and desorption isotherms of SnO2 hollow spheres, Barrett-Joyner-Halenda pore size distribution and BET surface area of SnO2 hollow spheres; Schematic images of 1Au/SnO2(2h) and HRTEM images of the upper most region of the sensing film. XPS spectra for Sn 3d and Au 4f of SnO2 and 1Au/SnO2(2h) specimens; XRD patterns of pure SnO2, 0.5Au/SnO2(2h), 1Au/SnO2(2h), and 3Au/SnO2(2h) specimens; STEM images and Au number density (particles/m2) of xAu/SnO2(2h) specimen (x = 0.5 nm, 1 nm and 3 nm) and 1Au/SnO2(yh) specimen (y = 0.5 and 4); Gas sensing transients of the SnO2 sensor and

1Au/SnO2(2h) sensor toward 5 ppm analyte gases; SEM images and gas sensing

response of the thin film SnO2 sensor and thick film SnO2 sensor toward 5 ppm analyte gases at 300 ºC and 350 ºC; XPS spectra of the O1s peaks in pure SnO2; Experimental procedure to prepare SnO2 hollow spheres uniformly loaded with 3 wt% Au Notes The authors declare no competing financial interest. Acknowledgements This work was supported by a grant from the Samsung Research Funding & Incubation Center for Future Technology (SRFC), Grant No. SRFC-TA1803-04.



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Figure 1. Cross-sectional SEM images of the (a) pure SnO2 sensor and (b) 1Au/SnO2(2h) sensor, and (c) top-view SEM images of the 1Au/SnO2(2h) sensor. (d–i) Backscattered images and elemental (Au, Sn) mapping results for a sphere detached from the uppermost region of the sensing film.



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Figure 2. TEM images and elemental (Au) mapping results for a sphere detached from the uppermost region of the (a) 0.5Au/SnO2(2h) sensor, (b) 1Au/SnO2(2h) sensor, (c) 3Au/SnO2(2h) sensor, (d) 1Au/SnO2(0.5h) sensor, and (e) 1Au/SnO2(4h) sensor.


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Figure 3. Gas sensing characteristics and gas responses (Ra/Rg) of the (a) pure SnO2, xAu/SnO2(2h) sensor (x = (b) 0.5 nm, (c) 1 nm, and (d) 3 nm) and 1Au/SnO2(yh) sensor (y = (e) 0.5 and (f) 4) to 5 ppm of various gases (ethanol, benzene, toluene, p-xylene) at temperatures in the range of 350–450 °C.



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Figure 4. Gas sensing characteristics and sensing mechanisms of the (a) pure SnO2 sensor, xAu/SnO2(2h) sensor (x = (b) 0.5 nm, (c) 1 nm, and (d) 3 nm) and 1Au/SnO2(yh) sensor (y = (e) 0.5 and (f) 4) to 5 ppm of various gases (E: ethanol, B: benzene, T: toluene, X: p-xylene) at 350 °C.


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Figure 5. Schematic diagrams and dynamic sensing transients of the (a) pure SnO2 sensor, (b) 1Au/SnO2(2h) sensor, and (c) Au-loaded SnO2 sensor.



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Figure 6. (a) Gas sensing characteristics and gas responses (Ra/Rg or Rg/Ra) of the (a) pure ZnO and 1Au/ZnO(2h) sensors to 5 ppm of various gases at 350 °C, and (b) pure Co3O4(2h) and 1Au/Co3O4(2h) sensors to 5 ppm of various gases (E: ethanol, B: benzene, T: toluene, X: pxylene) at 200 °C.


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Figure 7. (a) Gas sensing transients of the 1Au/SnO2(2h) sensor to 0.25–5 ppm of p-xylene at 350 °C, (b) gas response as a function of p-xylene concentration, (c) repetitive sensing transients to 5 ppm of p-xylene at 350 °C, and (d) long-term stability of the sensor.



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Metal Oxide Gas Sensors with Au Nanocluster Catalytic Overlayer

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