Dual-mode gas sensor for ultrasensitive and highly selective detection of xylene and toluene using Nb-doped NiO hollow spheres

Sensors & Actuators: B. Chemical 301 (2019) 127140

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Dual-mode gas sensor for ultrasensitive and highly selective detection of xylene and toluene using Nb-doped NiO hollow spheres

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Tae-Hyung Kim, Seong-Yong Jeong, Young Kook Moon, Jong-Heun Lee

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

ARTICLE INFO

ABSTRACT

Keywords: Gas sensor Nb-doped NiO Xylene Toluene Dual function Gas reforming

A single gas sensor with dual functionality for ultrasensitive and highly selective detection of p-xylene and toluene was designed using NiO hollow spheres doped with Nb. The pure and Nb-doped NiO hollow spheres were prepared by one-pot ultrasonic spray pyrolysis and subsequent heat treatment at 500 °C for 2 h. The Nbdoped NiO hollow spheres ([Nb]/[Ni] = 0.1) showed an ultrahigh response to 5 ppm of p-xylene (resistance ratio = 1752) and toluene (resistance ratio = 607), with negligible cross-responses to 5 ppm ethanol, benzene, carbon monoxide, and formaldehyde. In contrast, pure NiO hollow spheres showed negligibly low responses to 5 ppm of all analyte gases. In addition, the Nb-doped NiO hollow spheres exhibited dual sensing characteristics for selectively detecting p-xylene and toluene at 350 °C and 400 °C, respectively. The significant improvement of the response and selectivity for p-xylene and toluene can be explained by the high gas accessibility of hollow spheres, the Nb-doping-induced decrease in the charge carrier concentration, and the catalytic promotion of gas reforming reaction of less reactive xylene and toluene into more active species. The dual function of selectively detecting p-xylene and toluene in Nb-doped NiO hollow spheres is explained by the competition between oxidative filtering and gas reforming reaction depending on the operation temperature and sensing film thickness. The Nb-doped NiO hollow spheres can be used to design a single gas sensor with dual selectivity of xylene and toluene for reliable monitoring of the indoor air quality.

1. Introduction Indoor air quality is one of the major environmental issues as humans spend 90% of their time indoors. The most representative indoor air pollutants are volatile organic compounds (VOCs). Among the VOCs, aromatic hydrocarbons such as benzene, toluene, and xylene (BTX) are ubiquitous pollutants emitted from paint, furniture, adhesives, coal combustion, and cigarette smoke. Because human exposure to BTX gases causes various adverse health effects on the central nervous system, respiratory, kidney, lung, and heart [1,2], the selective, sensitive, and cost-effective detection of gas using a miniaturized sensor is of crucial importance for reliable indoor air quality monitoring. Gas chromatography-mass spectroscopy [3] and fluorescence spectroscopy [4] can analyze harmful VOCs precisely; however, bulky and expensive equipment, complex sampling procedures, and prolonged analysis times limit their widespread application. Oxide semiconductor gas sensors with simple structure can be integrated into a smart phone or miniaturized device and exhibit various distinctive advantages for indoor air monitoring such as high sensitivity, fast response speed, and stability, all of which can be best used for wireless data collection

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through Internet of Things technology [5–10]. However, a simple sensing mechanism of oxide semiconductor gas sensors involving the charge transfer between the analyte gas and sensor surface [11–13], often leads to a lack of gas selectivity. Various approaches have been explored to enhance the gas selectivity of oxide semiconductor chemiresistors, which include the control of sensing temperature [14], morphological change of sensing materials [15,16], catalyst doping/ loading [17–22], and catalytic filtering of interference gases [23]. Nevertheless, highly selective detection of indoor VOCs still remains a challenging issue. The sources and health impacts of VOCs vary significantly. To find the exact source of VOCs and establish proper settlement, the selective detection of each VOC using an analyte-specific sensor is the best method; however, an increase in the number of sensors is inevitable. In this perspective, the use of a single sensor with dual or multiple functions can be a viable option to detect various indoor VOCs with a minimal number of sensors. To date, there have been studies on the detection of methylbenzenes without discrimination between xylene and toluene as well as the detection of specific VOCs such as ethanol and benzene using a single sensor [19,20,23]. However, to the best

Corresponding author at: Department of Materials Science and Engineering, Korea University, Anam-Dong, Sungbuk-Gu, Seoul 02841, Republic of Korea. E-mail address: [email protected] (J.-H. Lee).

https://doi.org/10.1016/j.snb.2019.127140 Received 2 July 2019; Received in revised form 10 September 2019; Accepted 11 September 2019 Available online 13 September 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.

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knowledge of the authors, it is still difficult to discriminate between xylene and toluene because they have similar molecular structures, sizes, weights, and chemical characteristics. In addition, there is no report of a single sensor with dual function to detect xylene and toluene. In the present study, we propose Nb-doped NiO hollow spheres as a single sensor with dual function to detect xylene and toluene in a highly selective and sensitive manner simply by modulating the sensing temperature. For this, the gas reforming into more active species and gas oxidation into non- or less-reactive species are rationally controlled through the change of catalytic activity via Nb doping. Moreover, the NiO sensor doped with Nb ([Nb]/[Ni] = 0.1) showed the unprecedentedly high response (resistance ratio = 1752) to 5 ppm xylene, which is the highest among those reported in the literature. Special focus is directed at the elucidation of the sensing mechanism underlying the intriguing dual-mode gas detection as well as the understanding of the origins for ultrahigh response and selectivity.

the curve-fitted diameter distribution histogram for pure and Nb-doped NiO spheres (in Fig. S3). The average diameters of pure, 1Nb-, 5Nb-, 10Nb-, and 20Nb-NiO spheres were 0.74 ± 0.21 μm, 0.72 ± 0.18 μm, 0.68 ± 0.15 μm, 0.67 ± 0.18 μm, and 0.75 ± 0.21 μm, respectively. The sensing film, coated using the screen-printing mask with a thickness of 85 μm, was ˜8 μm thick and exhibited uniform film thickness (Fig. 1c). The decrease of film thickness emanated from the evaporation of the solvent and the decomposition of organic materials during drying and heat treatment. All the spheres showed a hollow morphology, which was confirmed by the bright contours at the central regions of the spheres by TEM analysis (Figs. 1d, e and S4). In the HR-TEM image of the 10Nb-NiO hollow spheres (Fig. 1f), both the (111) lattice fringe of NiO (separated by 0.24 nm) and (200) lattice fringe of NiNb2O6 (separated by 0.28 nm) were observed, indicating the coexistence of the NiO and NiNb2O6 phases. The EDS elemental mapping showed the uniform distribution of the Nb component throughout all Nb-doped NiO hollow spheres (Figs. 1g and S4). The crystal structures and phases of pure and Nb-doped NiO hollow spheres were analyzed by X-ray diffraction (Fig. 2). All the specimens showed the cubic NiO, and the Nb-related second phase was not found (Figs. 2a–e). It should be noted that no substantial peak shift was observed in NiO (200) peak with increasing Nb doping concentration (Fig. 2f). The ionic radii of Nb5+ (0.64 Å) and Ni2+ (0.69 Å) at the coordination number of 6 are similar. Thus, the incorporation of Nb into the NiO lattice can be regarded as plausible. The absence of the NiNb2O6 peak in Fig. 2d, despite its appearance in the HR-TEM image of the 10Nb-NiO specimen (Fig. 1f), can be explained by the low detection limit of XRD. To investigate the presence of the NiNb2O6 second phase, the specimens were analyzed using Raman spectroscopy (Fig. 3). The strong peaks at approximately 500 cm−1, related to the non-stoichiometric NieO stretching vibrations, were observed in all the specimens, which were blue-shifted with Nb doping [24]. This indicates the incorporation of high-oxidation-state ions at the site of Ni2+ [25]. Two small peaks at 790 and 850 cm−1, found in the 10Nb- and 20Nb-NiO specimens, were assigned to the vibration of bridging NbeOeNi [24]. From these results, it can be concluded that the NiNb2O6 second phases below the detection limit of the XRD analysis exist in the 10Nb- and 20Nb-NiO hollow spheres, and their amounts increase with Nb concentration. The NiO crystallite sizes in the pure, 1Nb-, 5Nb-, 10Nb-, and 20Nb-NiO hollow spheres were calculated to be 34.9, 20.5, 16.7, 15.4, and 16.6 nm, respectively, using the Scherrer’s equation from XRD results. The decrease of NiO crystallite size with Nb doping can be attributed to the suppression of NiO grain growth due to the incorporation of Nb into NiO, which is consistent with the literature data [24,26]. The incorporation of Nb into NiO was further analyzed using XPS. In Nb-doped NiO specimens, the Nb 3d5/2 (206.5 eV) and Nb 3d3/2 (209.2 eV) peaks were found, indicating that Nb exists in the form of Nb5+ (Fig. S5) [27].

2. Experimental 2.1. Synthesis of sensing materials Pure and Nb-doped NiO hollow spheres were synthesized by ultrasonic spray pyrolysis and subsequent heat treatment (Fig. S1). Nickel (II) nitrate hexahydrate (2.91 g, Ni(NO3)2·6H2O, 99.999%, SigmaAldrich, USA), citric acid monohydrate (2.10 g, C6H8O7·H2O, 99%, Sigma-Aldrich, USA), and ammonium niobate (V) oxalate hydrate (C4H4NNbO9·xH2O, 99.99%, Sigma-Aldrich, USA) were dissolved in distilled water (100 mL) to prepare the spray solution, and a spray solution without ammonium niobate (V) oxalate hydrate was used to synthesize pure NiO hollow spheres. The molar ratios between Nb and Ni ions ([Nb]/[Ni]) were 0.01, 0.05, 0.1, and 0.2. The droplets of the spray solution were generated by six ultrasonic transducers (frequency: 1.7 MHz), which were carried into the high-temperature quartz tube (temperature: 700 °C, length: 1200 mm, diameter: 55 mm) by a carrier gas (air, 10 L·min−1). The as-prepared precursor powders prepared by spray pyrolysis were collected on a Teflon bag filter. The precursor powders were converted into pure and Nb-doped NiO hollow spheres by heat treatment at 500 °C for 2 h. For simplicity, four Nb-doped NiO hollow spheres will be referred to as 1Nb-NiO, 5Nb-NiO, 10Nb-NiO, and 20Nb-NiO, respectively. 2.2. Preparation of gas-sensing film The pure and Nb-doped NiO hollow spheres were mixed with an organic binder (FCM, a terpineol-based ink vehicle, USA) in the ratio of 35:65 wt% to form a slurry. The slurry was screen-printed on an alumina substrate (area: 1.5 × 1.5 mm; thickness: 0.25 mm) with two Au electrodes on the top surface and a micro-heater on the bottom surface. The thickness of the sensing film was controlled using different thicknesses (25 μm, 45 μm and 85 μm) of the screen-printing masks. The sensors were heat-treated at 450 °C for 2 h in an electric furnace and subsequently at 475 °C for 2 h using a micro-heater to remove organic components and stabilize the sensor. Detailed experimental methods for material characterization and gas sensing are explained in Supplementary data.

3.2. Gas-sensing characteristics The gas-sensing characteristics of pure and Nb-doped NiO sensors to 5 ppm ethanol, p-xylene, toluene, benzene, carbon monoxide, and formaldehyde were measured at 350–450 °C (Fig. 4). All the sensing transients showed the increased resistance upon exposure to reducing gases and decreased resistance upon exposure to air (Fig. S6). In n-type metal oxide semiconductors such as SnO2, ZnO, In2O3, WO3 and Fe2O3, the adsorption of oxygen and its ionization into O− or O2− establishes highly resistive electron depletion layer near the surface of sensing materials, which increases the sensor resistance. When the gas sensor is exposed to reducing gases, they are oxidized by negatively charged oxygen on the surface and the electrons released to sensing materials, which decreases the sensor resistance [6,12,28]. In p-type metal oxide semiconductors such as NiO, Cr2O3, Co3O4, and CuO, the adsorption of oxygen and its ionization into O− or O2− accumulates the counter charge of holes near the surface, which establishes relatively conductive hole accumulation layer (HAL). Under this configuration, the electrons

3. Results and discussion 3.1. Characterization of sensing materials Pure and Nb-doped NiO hollow spheres prepared by ultrasonic spray pyrolysis and subsequent heat treatment exhibited a spherical morphology (Figs. 1a, b and S2). Observing a change in the morphology and particle size with increasing Nb concentration was difficult (Fig. S2). We have measured the diameters of ∼ 200 spheres for each sample and attained 2

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Fig. 1. (a, b) SEM images, (c) cross-sectional SEM image, (d, e) TEM images, (f) lattice fringe, and (g) elemental mapping images of 10Nb-NiO.

Fig. 2. X-ray diffraction patterns of (a) pure NiO, (b) 1Nb-NiO, (c) 5Nb-NiO, (d) 10Nb-NiO, (e) 20Nb-NiO, and (f) comparison of NiO (200) peaks.

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(Rg: resistance in analyte gas, Ra: resistance in air). The sensing characteristics at < 350 °C were not investigated in the present study because of sluggish sensing and recovery kinetics. For all the sensors, the gas responses to analyte gases gradually decreased as the sensor temperature increased. The pure NiO sensor exhibited negligibly low responses to all the gases (S = 1.1–1.3) over the entire sensor temperature range (Fig. 4a). However, the gas responses were significantly enhanced with Nb doping. The maximum responses of 1Nb-, 5Nb-, 10Nb-, and 20Nb-NiO sensors to 5 ppm gas at 350 °C were Stoluene = 1.8, Stoluene = 31, Sxylene = 1752, and Sxylene = 1171, respectively (Figs. 4b–e). In particular, the response of 10Nb-NiO sensor to 5 ppm p-xylene (Sxylene = 1752, Fig. 4d) is 1460 times higher than that of NiO sensor (Sxylene = 1.2, Fig. 4a). In addition to the unprecedented increase of the gas response, Nb-doping induced a substantial change in gas selectivity as well. The response to toluene was the highest at 350 °C in the pure, 1Nb-, and 5Nb-NiO sensors, whereas the response to p-xylene was the highest at 350 °C in the 10Nb- and 20Nb-NiO sensors. In the case of the 10Nb-NiO sensor, the gas responses to p-xylene and toluene were reversed with increasing temperature, especially at 400 °C, the response to toluene (Stoluene = 103) was 5 times higher than that to p-xylene (Sxylene = 19) (inset in Fig. 4d). Accordingly, the 10Nb-NiO single sensor can be used to detect xylene and toluene sensitively and selectively as well as discriminate between xylene and toluene by temperature modulation (Figs. 4f and g). The Nb-doping-induced dramatic change of gas response can be discussed in relation to various key parameters to determine the gassensing characteristics as well as the gas-sensing mechanism. First, the change of specific surface area and pore size distribution by Nb doping was investigated (Fig. 5). The specific surface area of pure NiO was 19.0 m2/g [2], and the mesopores with a modal size of ∼10 nm were observed (Fig. 5a). The volume of mesopores (mode size: ∼10 nm) tends to increases with Nb doping. However, when excessive amount of Nb was added (20Nb-NiO specimen), the volume of mesopores with a modal size of ∼10 nm decreased, while that of ∼4 nm increased

Fig. 3. Raman spectra of (a) pure NiO, (b) 1Nb-NiO, (c) 5Nb-NiO, (d) 10NbNiO, and (e) 20Nb-NiO.

generated by the reaction between negatively charged oxygen and reducing gas are injected to HAL, which decreases the hole concentration in HAL by the electron-hole recombination [13]. This leads to the increase of sensor resistance. The sensing transients in the present study show the typical chemiresistive variation of p-type oxide semiconductor gas sensors. Accordingly, the gas response (S) was defined as ‘Rg/Ra’

Fig. 4. Gas-sensing characteristics of (a) pure NiO, (b) 1Nb-NiO, (c) 5Nb-NiO, (d) 10Nb-NiO, and (e) 20Nb-NiO sensors to 5 ppm various gases at 350–450 °C; polar plot of gas responses to 5 ppm analyte gases of 10Nb-NiO sensor at (f) 350 °C and (g) 400 °C (E: ethanol, X: p-xylene, T: toluene, B: benzene, C: carbon monoxide, F: formaldehyde). 4

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Fig. 5. BET specific surface areas and pore-size distributions of (a) pure NiO, (b) 1Nb-NiO, (c) 5Nb-NiO, (d) 10Nb-NiO, and (e) 20Nb-NiO spheres determined by nitrogen adsorption.

(Fig. 5e-2). It is considered that the formation of NiNb2O6 nanoparticles in 20Nb-NiO specimen partially blocked pores with a modal size of ∼10 nm and generated smaller pores (mode size: ∼4 nm). The specific surface areas of 1Nb-, 5Nb-, 10Nb-, and 20Nb-NiO were 23.6, 53.2, 53.3, and 52.3 m2/g [2], respectively, indicating the increase of surface area with Nb doping (Figs. 5b–e). This is in line with the decrease of crystallite size with Nb doping, according to the Scherrer’s analysis as well as general tendency to increase the volume of mesopores. The above suggests that the increase in gas accessibility due to Nb doping can be considered a possible origin for the enhanced gas response. However, it should be noted that the xylene response of the 10Nb-NiO sensor is ∼1460 times higher than that of the NiO sensor, whereas the corresponding change in the specific surface is only 2.8 times higher. This strongly suggests the presence of other key parameters to determine the gas response. The ‘electronic sensitization’ mechanism, the change of charge carrier concentration in sensors by Nb doping, can be considered as the other possible reason. NiO is a typical p-type oxide semiconductor, and the main charge carrier is a hole. The hole accumulation layers (HALs) are formed near the surface of p-type oxide semiconductors by the ionized adsorption of oxygen, which play the role of charge conduction path. When NiO is exposed to reducing gases, it reacts with the negatively charged adsorbed oxygen on the NiO surface, and the release of electrons by oxidation reaction decreases the hole concentration in the HALs through electron-hole recombination. If the amount of electrons released by the gas-sensing reaction is fixed, the lower hole concentration in the HALs will lead to a higher chemiresistive variation. Indeed, it has been reported that the gas response of the NiO nanostructures can be significantly enhanced by decreasing the background hole concentration through the doping of higher valance elements, such as Cr3+ and Fe3+ [19,20]. To examine the effect of aliovalent doping, the sensor resistances in air (Ra) were compared. Note that the Ra values significantly increase with Nb doping and the Ra value of 20Nb-NiO sensor at 350 °C (Ra = 5.4 MΩ) is 1800 times higher than that of the NiO sensor (Ra = 3 kΩ) at the same temperature (Fig. 6). Furthermore, the increase in Ra by Nb doping is consistent with the previous observation of the doping of Nb to NiO in XRD, Raman, and XPS analyses. This strongly indicates that the electronic sensitization mechanism is a key reason for the unprecedented increase of gas response. Although the electronic sensitization and the increase of gas

Fig. 6. Sensor resistance in air at 350–450 °C.

accessibility can explain the overall enhancement of gas response by Nb doping, it is still difficult to explain the improvement and change of pxylene and toluene selectivity. In our experiment, two electrodes are located under the sensing film. Under this electrode configuration, the gas response is determined by the transport, reforming, and oxidation of the analyte gas in the sensing film and following sensing reaction between the ionized oxygen and gas at the lower sensing region close to the electrodes. In all the sensors, the responses to ethanol, formaldehyde, and carbon monoxide are lower than those to xylene and toluene. This can be explained by the nearly complete oxidation of highly reactive ethanol, formaldehyde, and carbon monoxide into less or non-reactive species such as CO2 and H2O, during the diffusion through catalytic sensing films (NiO or Nb-doped NiO). In contrast, the low response to benzene, the most stable and, thus, least reactive species, can be attributed to the difficulties in the reaction with ionized oxygen at the sensing stage. Xylene and toluene have moderate reactivity and are known to be reformed into smaller and more active species such as benzaldehyde, benzyl alcohol, and benzoic acid, by partial oxidation [29–33]. In this perspective, the high response to pxylene and toluene can be understood in relation to the reforming of 5

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gases with moderate reactivity into more reactive species during the transport of gas through the sensing film (NiO or Nb-doped NiO) with catalytic activity. The pure, 1Nb-, and 5Nb-NiO sensors showed the highest response to toluene at 350 °C (Figs. 4a–c). In the 10Nb- and 20Nb-NiO sensors, in contrast, the p-xylene response was the highest at 350 °C but reversed by the toluene response at higher sensing temperatures (Figs. 4d and e). The dependence of gas selectivity on the composition of sensing materials and sensor temperature strongly indicate that catalytic promotion is a key parameter to determine the gas-sensing reaction. In the literature, the doping of Nb into NiO is known to decrease the nickel vacancy, [Ni3+]/[Ni2+] ratio, hole concentration, and amount of chemisorbed oxygen [26]. The amount of chemisorbed oxygen is directly related to the activity of catalysts. Accordingly, it is reported that NiO with abundant chemisorbed oxygen tends to induce the complete oxidation of ethane, while Nb-doped NiO with a moderate amount of chemisorbed oxygen facilitates the oxidative dehydrogenation of ethane into ethylene by suppressing the full oxidation [26,34]. The decrease of hole concentration with Nb doping in the present study was confirmed by the measurement of the sensor resistance (Fig. 6). Thus, the change of [Ni3+]/[Ni2+] and chemisorbed oxygen with Nb doping was analyzed using XPS (Fig. 7). In all the specimens, the Ni2+ and Ni3+ peaks of Ni 2p3/2 were observed at 853.8–854.1 and 855.7–860.0 eV, respectively. The [Ni3+]/[Ni2+] ratios of the pure, 1Nb-, 5Nb-, 10Nb-, and 20Nb-NiO specimens were determined to be 1.63, 1.53, 1.22, 1.11, and 1.07, respectively (Figs. 7a-1–e-1). That is, the [Ni3+]/[Ni2+] ratio decreases as the Nb concentration increases. Three different oxygen species were observed in O 1s peaks of all specimens: lattice oxygen (OI: 529.0–529.4 eV), oxygen deficient region (OII: 530.2–530.5 eV), and chemisorbed oxygen (OIII: 531.6–532 eV). It should be noted that the relative ratios of OIII significantly decrease with Nb doping (Figs. 7a-2–e-2). These XPS results are consistent with the literature [24,26] and support the decrease of catalytic activity to a moderate level by Nb doping. However, despite the decrease of [Ni3+]/ [Ni2+] ratio and OIII with the addition of Nb (Fig. 7), 20Nb-NiO sensor showed lower gas responses than 10Nb-NiO sensor (Figs. 4d and e). This might be attributed to the variation of catalytic activity [35] and/ or pore size distribution (Fig. 5e-2) due to the formation of NiNb2O6

nanoparticles. Xylene and toluene molecules have two methyl groups (eCH3) and one methyl group on the benzene ring, respectively, showing similar molecular structures, sizes, and weights. Thus, in a single sensor, the distinctive selectivity toward a gas (xylene or toluene) can be explained not by the difference in gas diffusion but by the change in reactivity. Because the reactivity of methylbenzenes is related to the number of methyl groups bonded to the benzene ring, it is generally acknowledged that xylene exhibits higher reactivity than toluene. Considering the decrease of catalytic activity with Nb doping, the highest toluene response in pure or lightly Nb-doped NiO (1Nb- and 5Nb-NiO) sensors at 350 °C are explained by the more oxidation of p-xylene than its reforming into active species, whereas the highest p-xylene response in 10Nb- and 20Nb-NiO sensors at 350 °C can be attributed to the more reforming of p-xylene into active species rather than to its full oxidation. Both the responses to toluene and p-xylene decrease with increasing sensing temperature, which indicates that the oxidation becomes more dominant over the reforming reaction by thermal activation. Note that the p-xylene response decreases more rapidly than the toluene response. This is also related to the facile oxidation of more reactive p-xylene with increasing temperature. To check the idea that the gas-sensing reaction in the present study is related to the competition between the oxidation and reforming of analyte gases in the sensing films, the gas-sensing characteristics of the sensors with different film thicknesses (3, 5.5 and 8 μm) were investigated (Fig. 8). The ethanol responses were similar in all sensors with different thicknesses, while the responses to p-xylene and toluene showed a significant dependence on film thickness (Figs. 8a-2–c-2). Again, this supports the fact that methylbenzene selectivity can be understood in the framework of competition between gas reforming and oxidation, while the low ethanol response can be described by the full oxidation of active gas species. There are two important features in Fig. 8. First, the p-xylene response of the sensor with a thickness of 8 μm is higher than those of the thinner two sensors (thicknesses 3 and 5.5 μm). If the sensor response is solely dependent on the diffusion of the analyte gas to the lower sensing region, the gas response of the thickest sensor should be the lowest. The opposite result in the present study supports again that the reforming of p-xylene into more active

Fig. 7. XPS spectra of (a) pure NiO, (b) 1Nb-NiO, (c) 5Nb-NiO, (d) 10Nb-NiO, and (e) 20Nb-NiO. 6

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Fig. 8. Gas responses of 10Nb-NiO sensors with different film thicknesses to 5 ppm ethanol, p-xylene, and toluene: (a) thick sensor (thickness: 8 μm), (b) mediumthickness sensor (thickness: 5.5 μm), and (c) thin sensor (thickness: 3 μm).

species is present. Second, in all three sensors, the p-xylene responses were higher than those of toluene at 350 °C. However, at 400 °C, the gas response to toluene was significantly higher than that to p-xylene only in the thick sensor (Fig. 8a-3), while similar or lower than those to pxylene in the medium-thickness and thin sensors (Figs. 8b-3 and c-3). This says that, even at the same elevated temperature (400 °C), p-xylene is more readily oxidized in the thicker sensor with longer retention of gas, suggesting that the selectivity toward p-xylene and toluene can be tuned by the parameters to determine the catalytic reaction such as sensing materials, sensing temperatures, and retention of analyte gas. Note that the 10Nb-NiO sensor showed highly selective detection of p-xylene and toluene at the sensing temperatures of 350 and 400 °C (Figs. 4f and g), respectively. This demonstrates the promising potential of a single sensor with two different functionalities. For example, if a tiny 10Nb-NiO sensor is made on a microheater platform and two different pulse heater voltages are applied, the gas-sensing characteristics at two sensing temperatures can be measured shortly using a sensor, which will facilitate the dual function of a sensor to detect p-xylene and toluene in a highly selective and sensitive manner. The cyclic performance of the 10Nb-NiO sensor for 5 ppm p-xylene and toluene was measured at 350 and 400 °C, respectively (Figs. 9a and d). The sensor showed very stable and reproducible gas-sensing characteristics, even after repetitive measurements. In addition, the sensing transients of the 10Nb-NiO sensor toward 0.25–5 ppm p-xylene at

350 °C and toluene at 400 °C were measured (Figs. 9b and e). Based on the sensing transients, the 90% response time (τres) and 90% recovery time (τrecov), the times to reach 90% variation of sensor resistance upon exposure to analyte gas and air, were calculated (Fig. S7). The average τres and τrecov values upon exposure to 5 ppm p-xylene and air at 350 °C were 202 and 57 s, respectively. And the average τres and τrecov values of upon exposure to 5 ppm toluene and air at 400 °C were 143 and 30 s, respectively. The p-xylene and toluene detection limits were calculated to be 140 ppb and 72 ppb, respectively, when Rg/Ra > 1.2 was used as the gas-sensing criterion (Figs. 9c and f). Moreover, the xylene response of the 10Nb-NiO sensor in the present study (Sxylene = 1752) is the highest value among those reported in the literature [17–19, 36–56]. The exposure to xylene and toluene may cause eye and skin irritation, dizziness, headaches, neurological effects, and respiratory-system impairment. Toluene is known to induce damage to the kidneys or liver [57], and repeated breathing in of toluene emitted from glue or paint thinners may cause brain damage. The Agency for Toxic Substances and Disease Registry suggested that the minimal risk level of inhalation of xylene and toluene in acute-duration (≤14 days) is 2 ppm and in chronic-duration (≥1 year) is 0.05 ppm and 1 ppm, respectively [1, 2]. Therefore, it is necessary to detect and distinguish the sub-ppm level of xylene and toluene. From this perspective, the present dual-mode 10Nb-NiO sensor, which can detect both xylene and toluene in a highly selective and sensitive manner, is an attractive platform for economic 7

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Fig. 9. Gas sensing characteristics of 10Nb-NiO sensor: (a) cyclic performance to 5 ppm p-xylene, (b) dynamic sensing transients to 0.25–5 ppm p-xylene, (c) p-xylene responses as a function of concentration at 350 °C and those reported in the literature, (d) cyclic performance for 5 ppm toluene, (e) Dynamic sensing transients to 0.25–5 ppm toluene, and (f) toluene responses as a function of concentration at 400 °C.

References

and reliable indoor air quality monitoring. Finally, the selective detection of both xylene and toluene can open various new applications for leak detection in petroleum industry [58, 59], air quality monitoring in gas station [60], and the detection of aromatic biomarker gases in exhaled breath [61, 62].

[1] https://www.atsdr.cdc.gov/toxguides/toxguide-71.pdf. (Accessed on May, 2019). [2] https://www.atsdr.cdc.gov/toxguides/toxguide-56.pdf. (Accessed on May, 2019). [3] J.J. Langenfeld, S.B. Hawthorne, D.J. Miller, Quantitative analysis of fuel-related hydrocarbons in surface water and wastewater samples by solid-phase microextraction, Anal. Chem. 68 (1996) 144–155. [4] P. Karlitschek, F. Lewitzka, U. Bünting, M. Niederkrüger, G. Marowsky, Detection of aromatic pollutants in the environment by using UV-laser-induced fluorescence, Appl. Phys. B 67 (1998) 497–504. [5] N. Yamazoe, Toward innovations of gas sensor technology, Sens. Actuators B 108 (108) (2005) 2–14. [6] A. Kolmakov, Y. Zhang, G. Cheng, M. Moskovits, Detection of CO and O2 using tin oxide nanowire sensors, Adv. Mater. 15 (2003) 997–1000. [7] F. Rӧck, N. Barsan, U. Weimar, Electronic nose: current status and future trends, Chem. Rev. 108 (2008) 705–725. [8] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles, Nano Lett. 5 (2005) 667–673. [9] M. Righettoni, A. Amann, S.E. Pratsinis, Breath analysis by nanostructured metal oxides as chemo-resistive gas sensors, Mater. Today 18 (2015) 163–171. [10] J.M. Baik, M. Zielke, M.H. Kim, K.L. Turner, A.M. Wodtke, M. Moskovits, Tin-oxideNanowire-Based electronic nose using heterogeneous catalysis as a functionalization strategy, ACS Nano 4 (2010) 3117–3122. [11] J.-H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B 140 (2009) 319–336. [12] P.G. Harrison, N.J. Willett, The mechanism of operation of Tin(IV) oxide carbon monoxide sensors, Nature 332 (1988) 337–339. [13] H.-J. Kim, J.-H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: overview, Sens. Actuators B 192 (2014) 607–627. [14] S. Chakraborty, A. Sen, H.S. Maiti, Selective detection of methane and butane by temperature modulation in iron doped tin oxide sensors, Sens. Actuators B 115 (2006) 610–613. [15] J.-W. Yoon, S.H. Choi, J.-S. Kim, H.W. Jang, Y.C. Kang, J.-H. Lee, Trimodally porous SnO2 nanospheres with three-dimensional interconnectivity and size tunability: a one-pot synthetic route and potential application as an extremely sensitive ethanol detector, NPG Asia Mater. 8 (2016) e244. [16] C. Su, L. Zhang, Y. Han, C. Ren, X. Chen, J. Hu, M. Zeng, N. Hu, Y. Su, Z. Zhou, Z. Yang, Controllable synthesis of crescent-shaped porous NiO nanoplates for conductometric ethanol gas sensors, Sens. Actuators B 296 (2019) 126642. [17] T.-H. Kim, C.-H. Kwak, J.-H. Lee, NiO/NiWO4 composite yolk-shell spheres with nanoscale NiO outer layer for ultrasensitive and selective detection of subppm-level p-xylene, ACS Appl. Mater. Interfaces 9 (2017) 32034–32043. [18] B.-Y. Kim, J.-W. Yoon, J.K. Kim, Y.C. Kang, J.-H. Lee, Dual role of multiroomstructured Sn-doped NiO microspheres for ultrasensitive and highly selective detection of xylene, ACS Appl. Mater. Interfaces 10 (2018) 16605–16612. [19] H.-J. Kim, J.-W. Yoon, K.-I. Choi, J.W. Jang, A. Umar, J.-H. Lee, Ultraselective and sensitive detection of xylene and toluene for monitoring indoor air pollution using Cr-doped NiO hierarchical nanostructures, Nanoscale 5 (2013) 7066–7073. [20] H.-J. Kim, K.-I. Choi, K.-M. Kim, C.W. Na, J.-H. Lee, Highly sensitive C2H5OH sensors using Fe-doped NiO hollow spheres, Sens. Actuators B 171–172 (2012)

4. Conclusion Pure and Nb-doped NiO hollow spheres were prepared by ultrasonic spray pyrolysis, and their gas-sensing characteristics were investigated. The pure NiO sensor showed negligibly low gas responses to all analyte gases. In contrast, the high-concentration Nb-doped NiO sensor ([Nb]/ [Ni] = 0.1) exhibited ultrahigh responses to p-xylene and toluene with negligibly low cross-responses to various interference gases such as ethanol, formaldehyde, carbon monoxide, and benzene. In addition, the sensor showed excellent dual sensing properties to selectively detect pxylene at 350 °C and toluene at 400 °C. The unprecedentedly high gas responses to p-xylene and toluene were attributed to the high gas accessible morphology, electronic sensitization by Nb doping, and improved gas reforming reaction due to the Nb-doping-induced change in the catalytic activity. Moreover, the dual sensing characteristics to pxylene and toluene could be achieved by controlling the oxidative filtering of interference gases and the reforming of analyte gases depending on the film thickness and operating temperature. The Nb-doped NiO hollow spheres can be used to design a single sensor with dual functionality to detect p-xylene and toluene in an ultrasensitive and highly selective manner, which can facilitate new cost-effective sensor applications and discriminate sensing of aromatic indoor air pollutants. Acknowledgement This work was supported by a grant from the Samsung Research Funding & Incubation Center for Future Technology (SRFC), Grant No. SRFC-TA1803-04. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.127140. 8

Sensors & Actuators: B. Chemical 301 (2019) 127140

T.-H. Kim, et al. 1029–1037. [21] J. Hu, T. Wang, Y. Wang, D. Huang, G. He, Y. Han, N. Hu, Y. Su, Z. Zhou, Y. Zhang, Z. Yang, Enhanced formaldehyde detection based on Ni doping of SnO2 nanoparticles by one-step synthesis, Sens. Actuators B 263 (2018) 120–128. [22] S. Wang, D. Huang, S. Xu, W. Jiang, T. Wang, J. Hu, N. Hu, Y. Su, Y. Zhang, Z. Yang, Two-dimensional NiO nanosheets with enhanced room temperature NO2 sensing performance via Al doping, Phys. Chem. Chem. Phys. 19 (2017) 19043–19049. [23] S.-Y. Jeong, J.-W. Yoon, T.-H. Kim, H.-M. Jeong, C.-S. Lee, Y.C. Kang, J.-H. Lee, Ultra-selective detection of sub-ppm-level benzene using Pd-SnO2 yolk-shell microreactors with a catalytic Co3O4 overlayer for monitoring air quality, J. Mater. Chem. A 5 (2017) 1446–1454. [24] E. Rojas, J.J. Delgado, M.O. Guerrero-Pérez, M.A. Bañares, Performance of NiO and Ni-Nb-O active phases during the ethane ammoxidation into acetonitrile, Catal. Sci. Technol. 3 (2013) 3173–3182. [25] Z. Wang, H. Zhou, D. Han, F. Gu, Electron compensation in p-type 3DOM NiO by Sn doping for enhanced formaldehyde sensing performance, J. Mater. Chem. C 5 (2017) 3254–3263. [26] E. Heracleous, A.A. Lemonidou, Ni-Nb-O mixed oxides as highly active and selective catalysts for ethene production via ethane oxidative dehydrogenation. Part I: characterization and catalytic performance, J. Catal. 237 (2006) 162–174. [27] H. Suzuki, O. Tomita, M. Higashi, A. Nakada, R. Abe, Improved visible-light activity of nitrogen-doped layered niobate photocatalysts by NH3-nitridation with KCL flux, Appl. Catal. B-Environ. 232 (2018) 49–54. [28] Y. Shi, M. Wang, C. Hong, Z. Yang, J. Deng, X. Song, L. Wang, J. Shao, H. Liu, Y. Ding, Multi-junction joints network self-assembled with converging ZnO nanowires as multi-barrier gas sensor, Sens. Actuators B 177 (2013) 1027–1034. [29] I.E. Wachs, R.Y. Saleh, S.S. Chan, C.C. Chersich, The interaction of vanadium pentoxide with titania (anatase): Part I. Effect ono-xylene oxidation to phthalic anhydride, Appl. Catal. 15 (1985) 339–352. [30] L. Cao, Z. Gao, S.L. Suib, T.N. Obee, S.O. Hay, J.D. Freidaut, Photocatalytic oxidation of toluene on nanoscale TiO2 catalysts: studies of deactivation and regeneration, J. Catal. 196 (2000) 253–261. [31] M.M. Ameen, G.B. Raupp, Reversible catalyst deactivation in the photocatalytic oxidation of dilute o-Xylene in air, J. Catal. 184 (1999) 253–261. [32] J. Blanco, P. Avila, A. Bahamonde, E. Alvarez, B. Sánchez, M. Romero, Photocatalytic destruction of toluene and xylene at gas phase on a titania based monolithic catalyst, Catal. Today 29 (1996) 437–442. [33] H.K. Matralis, C. Papadopoulou, C. Kordulis, A.A. Elguezabal, V.C. Corberan, Selective oxidation of toluene over V2O5/TiO2 catalysts. Effect of vanadium loading and of molybdenum addition on the catalytic properties, Appl. Catal. A-Gen. 126 (1995) 365–380. [34] I. Popescu, Z. Skoufa, E. Heracleous, A. Lemonidou, I.-C. Marcu, A study by electrical conductivity measurements of the semiconductive and redox properties of Nbdoped NiO catalysts in correlation with the oxidative dehydrogenation of ethane, Phys. Chem. Chem. Phys. 17 (2015) 8138–8147. [35] B. Savova, S. Loridant, D. Filkova, J.M.M. Millet, Ni-Nb-O catalysts for ethane oxidative dehydrogenation, Appl. Catal. A-Gen. 390 (2010) 148–157. [36] B.-Y. Kim, J.H. Ahn, J.-W. Yoon, C.-S. Lee, Y.C. Kang, F. Abdel-Hady, A.A. Wazzan, J.-H. Lee, Highly selective xylene sensor based on NiO/NiMoO4 nanocomposite hierarchical spheres for indoor air monitoring, ACS Appl. Mater. Interfaces 8 (2016) 34603–34611. [37] S.-J. Hwang, K.-I. Choi, J.-W. Yong, Y.C. Kang, J.-H. Lee, Pure and palladium-loaded Co3O4 hollow hierarchical nanostructures with giant and ultraselective chemiresistivity to xylene and toluene, Chem. Eur. J. 21 (2015) 5872–5878. [38] J.-M. Jeong, J.-H. Kim, S.-Y. Jeong, C.-H. Kwak, J.-H. Lee, Co3O4-SnO2 hollow heteronanostructures: facile control of gas selectivity by compositional tuning of sensing materials via galvanic replacement, ACS Appl. Mater. Interfaces 8 (2016) 7877–7883. [39] J.-W. Yoon, Y.J. Hong, G.D. Park, S.-J. Hwang, F. Abdel-Hady, A.A. Wazzan, Y.C. Kang, J.-H. Lee, Kilogram-scale synthesis of Pd-loaded quintuple-shelled Co3O4 microreactors and their application to ultrasensitive and ultraselective detection of methylbenzenes, ACS Appl. Mater. Interfaces 7 (2015) 7717–7723. [40] H.-S. Woo, C.-H. Kwak, J.-H. Chung, J.-H. Lee, Co-doped branched ZnO nanowires for ultraselective and sensitive detection of xylene, ACS Appl. Mater. Interfaces 6 (2014) 22553–22560. [41] J.-H. Kim, H.-M. Jeong, C.W. Na, J.-W. Yoon, F. Abdel-Hady, A.A. Wazzan, J.H. Lee, Highly selective and sensitive xylene sensors using Cr2O3-ZnCr2O4 heteronanostructures prepared by galvanic replacement, Sens. Actuators B 235 (2016) 498–506. [42] H.-S. Woo, C.-H. Kwak, J.-H. Chung, J.-H. Lee, Highly selective and sensitive xylene sensors using Ni-doped branched ZnO nanowire networks, Sens. Actuators B 216 (2015) 358–366. [43] Y.-M. Jo, T.-H. Kim, C.-S. Lee, K. Lim, C.W. Na, F. Abdel-Hady, A.A. Wazzan, J.H. Lee, Metal-organic framework-derived hollow hierarchical Co3O4 nanocages with tunable size and morphology: ultrasensitive and highly selective detection of methylbenzenes, ACS Appl. Mater. Interfaces 10 (2018) 8860–8868. [44] B.-Y. Kim, J.-W. Yoon, K. Lim, S.H. Park, J.-W. Yoon, J.-H. Lee, Hollow spheres of CoCr2O4-Cr2O3 mixed oxides with nanoscale heterojunctions for exclusive detection of indoor xylene, J. Mater. Chem. C 6 (2018) 10767–10774. [45] K.H. Lee, B.-Y. Kim, J.-W. Yoon, J.-H. Lee, Extremely selective detection of ppb

[46] [47] [48] [49]

[50] [51] [52] [53] [54] [55] [56] [57] [58] [59]

[60] [61] [62]

levels of indoor xylene using CoCr2O4 hollow spheres activated by Pt doping, Chem. Commun. 55 (2019) 751–754. Y. Zhu, X. Su, C. Yang, X. Gao, F. Xiao, J. Wang, Synthesis of TiO2-WO3 nanocomposites as highly sensitive benzene sensors and high efficiency adsorbents, J. Mater. Chem. 22 (2012) 13914–13917. N. Chen, D. Deng, Y. Li, X. Xing, X. Liu, X. Xiao, Y. Wang, The xylene sensing performance of WO3 decorated anatase TiO2 nanoparticles as a sensing material for a gas sensor at a low operating temperature, RSC Adv. 6 (2016) 49692–49701. J. Cao, Z. Wang, R. Wang, T. Zhang, Electrostatic sprayed Cr-loaded NiO core-inhollow-shell structured micro/nanospheres with ultra-selectivity and sensitivity for xylene, Cryst Eng Comm 16 (2014) 7731–7737. K.-Y. Choi, J.-S. Park, K.-B. Park, H.J. Kim, H.-D. Park, S.-D. Kim, Low power microgas sensors using mixed SnO2 nanoparticles and MWCNTs to detect NO2, NH3, and xylene gases for ubiquitous sensor network applications, Sens. Actuators B 150 (2010) 65–72. F. Qu, H. Jiang, M. Yang, Designed formation through a metal organic framework route of ZnO/ZnCo2O4 hollow core-shell nanocages with enhanced gas sensing properties, Nanoscale 8 (2016) 16349–16356. F. Li, S. Gua, J. Shen, L. Shen, D. Sun, B. Wang, Y. Chen, S. Ruan, Xylene gas sensor based on Au-loaded WO3∙H2O nanocubes with enhanced sensing performance, Sens. Actuators B 238 (2017) 364–373. F. Li, Q. Qin, N. Zhang, C. Chen, L. Sun, X. Liu, Y. Chen, C. Li, S. Ruan, Improved gas sensing performance with Pd-doped WO3∙H2O nanomaterials for the detection of xylene, Sens. Actuators, B 244 (2017) 837–848. P. Rai, S.M. Majhi, Y.-T. Yu, J.-H. Lee, Synthesis of plasmonic Ag@SnO2 core-shell nanoreactors for xylene detection, RSC Adv. 5 (2015) 17653–17659. K. Xu, J. Zou, S. Tian, Y. Yang, F. Zeng, T. Yu, Y. Zhang, X. Jie, C. Yuan, Singlecrystalline porous nanosheets assembled hierarchical Co3O4 microspheres for enhanced gas-sensing properties to trace xylene, Sens. Actuators B 246 (2017) 68–77. L. Deng, X. Ding, D. Zeng, S. Zhang, C. Xie, High sensitivity and selectivity of CDoped WO3 gas sensors toward toluene and xylene, IEEE Sens. J. 12 (2012) 2209–2214. H. Qin, Y. Cao, J. Xie, D. Jia, Solid-state chemical synthesis and xylene-sensing properties of α-MoO3 arrays assembled by nanoplates, Sens. Actuators B 242 (2017) 769–776. H. Pyta, BTX air pollution in Zabrze, Poland, Polish J. Environ. Stud. 15 (2006) 785–791. D.R. Blake, F.S. Rowland, Urban leakage of liquefied petroleum gas and its impact on Mexico city air quality, Science 269 (1995) 953–956. B. Heibati, K.J.G. Pollitt, J.Y. Charati, A. Ducatman, M. Shokrzadeh, A. Karimi, M. Mohammadyan, Biomonitoring-based exposure assessment of benzene, toluene, ethylbenzene and xylene among workers at petroleum distribution facilities, Ecotoxicol. Environ. Safe 149 (2018) 19–25. F.A. Esteve-Turrillas, A. Pastor, M. Guardia, Assessing air quality inside vehicles and at filling stations by monitoring benzene, toluene, ethylbenzene and xylenes with the use of semipermeable devices, Anal. Chim. Acta 593 (2007) 108–116. G. Peng, U. Tisch, O. Adams, M. Hakim, N. Shehada, Y.Y. Broza, S. Billan, R. AbdahBortnyak, A. Kuten, H. Haick, Diagnosing lung cancer in exhaled breath using gold nanoparticles, Nat. Nanotechnol. 4 (2009) 669–673. M. Phillips, K. Gleeson, J.M.B. Hughes, J. Greenberg, R.N. Cataneo, L. Baker, Volatile organic compounds in breath as markers of lung cancer: a cross-sectional study, Lancet 353 (1999) 1930–1933.

Tae-Hyung Kim studied Materials Science and Engineering and received his B.S. from Korea University, Korea, in 2014. He is currently studying for an M.S./Ph.D. integrated degree at Korea University. His research interest is p-type oxide semiconductor gas sensors. Seong-Yong Jeong studied Materials Science and Engineering and received his B.S. from Chonbuk National University, Korea, in 2015. He is currently studying for an M.S./Ph.D. integrated degree at Korea University. His research interest is oxide semiconductor gas sensors using multilayer structure. Young Kook Moon studied Materials Science and Engineering and received his B.S. from Korea University, Korea, in 2018. He is currently studying for an M.S./Ph.D. integrated degree at Korea University. His research interest is oxide semiconductor gas sensors using multilayer structure. Jong-Heun Lee joined the Department of Materials Science and Engineering at Korea University as an associate professor in 2003, where he is currently a professor. He received his B.S., M.S., and Ph.D. degrees from Seoul National University in 1987, 1989, and 1993, respectively. Between 1993–1999, he developed automotive air–fuel ratio sensors at the Samsung Advanced Institute of Technology. He was a Science and Technology Agency of Japan (STA) fellow at the National Institute for Research in Inorganic Materials (currently NIMS, Japan) from 1999 to 2000 and a research professor at Seoul National University from 2000 to 2003. His current research interests include chemical sensors, functional nanostructures, and photoelectrochemical water splitting.

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