Dual Role of Multiroom-Structured Sn-Doped NiO Microspheres for

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Functional Inorganic Materials and Devices

Dual role of multiroom-structured Sn-doped NiO microspheres for ultrasensitive and highly selective detection of xylene Bo-Young Kim, Ji-Wook Yoon, Jin Koo Kim, Yun Chan Kang, and Jong-Heun Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02412 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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ACS Applied Materials & Interfaces

Dual role of multiroom-structured Sn-doped NiO microspheres for ultrasensitive and highly selective detection of xylene †,‡

†,‡

†

,†

,†

Bo-Young Kim , Ji-Wook Yoon , Jin Koo Kim , Yun Chan Kang* , and Jong-Heun Lee* †

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

of Korea; ‡

These authors contributed equally to this work

*Author to whom correspondence should be addressed. Email: [email protected]; Tel: +82-2-3290-3268 [email protected]; Fax: +82-2-928-3584; Tel: +82-2-3290-3282

ABSTRACT Sn-doped NiO multiroom spheres with a unique microreactor morphology were prepared by facile ultrasonic spray pyrolysis of solution containing tin oxalate, nickel nitrate, and dextrin, and subsequent heat treatment. The multiroom structure was formed by phase segregation between the molten metal source and liquid-like dextrin and sequent removal of dextrin during spray pyrolysis, which played the dual roles of enhancing gas response and selectivity. The response (resistance ratio) of the Sn-doped NiO multiroom spheres to 1 ppm p-xylene was as high as 65.4 at 300 °C, which was 50.3 and 9.0 times higher than those of pure NiO multiroom spheres and Sn-doped NiO dense spheres, respectively. In addition, the Sn-doped 1

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NiO multiroom sensors showed a high selectivity to xylene. The unprecedented high response that enables the sensing of sub-ppm xylene was explained by the high gas accessibility of the multiroom structures and the Sn-doping-induced change in oxygen adsorption as well as charge carrier concentration, whereas the high xylene selectivity was attributed to the decomposition/reforming of xylene into smaller or more active species within the unique multiroom structure of Sn-doped NiO microreactors characterized by high catalytic activities. The multiroom oxide spheres can be used as a new and generalized platform to design high performance gas sensors.

Keywords: gas sensor; selectivity; multiroom; microreactor; Sn-doped NiO; xylene

1. INTRODUCTION Oxide spheres with the microreactor morphology have been receiving a tremendous amount of attention for various applications such as photocatalysts,1 supercapacitors,2 Li-ion batteries,3–5 and gas sensors,6–8 which facilitate mass transfer to entire nanostructures and microscale control of chemical reactions.9–11 In particular, for gas sensor applications, excellent gas accessibility of microreactors promises high gas response, and the gas reforming within the microreactors provides a new strategy to achieve high gas selectivity. Note that most of the oxide semiconductor chemiresistors suffer from a lack of gas selectivity—although they exhibit high response, good reversibility, facile integration, and excellent stability—that hinders the employment of gas sensors in real applications. In this context, microreactors with enhanced gas accessibility and tunable catalytic activity should be investigated to explore new functions of oxide semiconductor chemiresistors. 2


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Previously, the authors12–15 suggested that pure or catalyst-loaded oxide yolk-shell spheres can play the role of microreactors to achieve high gas sensitivity and selectivity. For example, WO3 and Ag-loaded SnO2 yolk-shell spheres with high gas accessibility exhibited significantly enhanced gas responses,12,13 and the possibility of designing selective gas sensors through gas reforming was demonstrated by Pd-loaded SnO2 or Pd-loaded Co3O4 yolk-shell spheres.14,15 However, the design of a high-performance gas sensor using the microreactor concept is still in the nascent stage and, to the best of our understanding, gas sensors using microreactor spheres with multiroom structures have never been studied before. In this study, multiroom structured Sn-doped NiO spheres were prepared by liquid (metal salt)-liquid (carbon source: dextrin) phase segregation and sequent removal of dextrin during the spray pyrolysis reaction, and were suggested as high-performance gas sensors to detect trace concentrations of xylene. Note that the sub-ppm-level xylene needs to be measured in a selective manner for monitoring indoor air quality. We have chosen Sn-doped NiO as the sensing material because both SnO2 and NiO are known to show significant catalytic activity toward various reducing gases, including aromatic volatile organic compounds,16,17 and the doping of aliovalent elements into NiO is reported to enhance gas response.18–21 The multiroom structured Sn-doped NiO spheres exhibited an unusually high response and selectivity to sub-ppm-level p-xylene, with low interference from toluene, ethanol, formaldehyde, benzene, ammonia, hydrogen, and carbon monoxide, making them useful for indoor air quality monitoring. The gas-sensing mechanism for the high response and selectivity to sub-ppm-level xylene was discussed in relation to the dual role of the unique multiroom structures in enhancing gas accessibility and promoting the gas-reforming reaction.

2. EXPERIMENTAL 3



2.1 Preparation of gas-sensing materials. Undoped and Sn-doped NiO multiroom spheres (referred to as “m-NiO” and “m-Sn-NiO,” respectively) were synthesized by spray pyrolysis and sequent heat treatment. Ni(NO3)2·6H2O (98%, Junsei Chemical Co., Japan), SnC2O4 (98%, Sigma-Aldrich, USA), and dextrin (15 g, (C6H10O5)x⋅nH2O, Samchun, Korea) were dissolved in distilled water (1000 ml) to prepare spray solution, and a spray solution without tin oxalate was used to synthesize m-NiO spheres. The molar ratio between Ni ions and Sn ions ([Ni]/[Sn]) was fixed at 5.0, and the total concentration ([Sn] + [Ni]) was 1.0 M. The droplets of the solution were generated by five piezoelectric transducers (frequency = 1.7 MHz), which were carried to a tube (temperature = 400 °C, length = 1200 mm, diameter = 50 mm). Nitrogen (flow rate = 15 L/min) was used as the carrier gas. A Teflon bag filter was used to collect the as-prepared precursor powders. The m-NiO and m-Sn-NiO spheres with multiroom morphology were prepared by annealing the precursor powders at 500 °C for 3 h in air. To study the effects of particle morphology on gas-sensing characteristics, dense Sndoped NiO spheres (referred as “d-Sn-NiO”) were also synthesized by ultrasonic spray pyrolysis of the spray solution without dextrin at 400 °C and subsequent annealing at 500 °C for 3 h in air. 2.2 Characterization of gas-sensing materials. The scanning electron microscopy (SEM, S-4800, Hitachi, Tokyo, Japan) and transmission electron microscopy (HR-TEM, Talos F200X, FEI Co., USA) were used to observe the morphology of powders. The phase and crystal structure were studied with powder X-ray diffraction (XRD, SmartLab, Rigaku, Japan), using CuKα radiation (λ = 1.5418 Å) and X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, Ulvac-PHI, Japan). The specific surface area was measured by the N2 adsorption (TriStar 3000, Micromeritics, Norcross, MN, USA). Thermogravimetric analysis (TG, Q600, TA Instruments, USA) was carried out to determine the annealing temperature of 4


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the precursors. 2.3 Characterization of gas-sensing properties. The aqueous slurry containing powders was deposited on Al2O3 substrates (1.5× 1.5 mm2) with two Au electrodes by drop coating. Before the measurements, the sensor was heated at 450 °C for 2 h using a microheater at the bottom of the Al2O3 substrate in order to remove any residual organic component and stabilize the sensor. After placing the sensor within a small-volume (1.5 cm3) quartz tube, a constant flow rate = (200 cm3/min) of air and the analyte gas was switched. The variation in sensor resistance was recorded with a computer-interfaced electrometer. The responses (Rg/Ra, Rg: the resistance in the analyte gas, Ra: the resistance in air) of the sensors to 1 ppm p-xylene, toluene, ethanol, formaldehyde, benzene, ammonia, hydrogen, and carbon monoxide at 300– 400 °C were measured.

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3. RESULTS AND DISCUSSION 3.1 Material Characterization. Multiroom structured undoped and Sn-doped NiO spheres were prepared by modifying our previous method22, involving the synthesis of multiroom CoMoO4 spheres, using phase segregation between metal salts and dextrin. Aqueous droplets containing nickel nitrate and dextrin were generated with an ultrasonic transducer, and were transported to a high-temperature tubular quartz reactor (Scheme 1a, left). Water, as the solvent in the droplets, evaporated at the early stage of the reaction, and the nickel source and dextrin were then melted. Phase segregation occurred between the molten nickel source and the liquid-like dextrin, and “dextrin islands,” embedded in the multiroom-structured molten nickel source (Scheme 1a, middle), were formed. After spray pyrolysis, m-NiO spheres containing small amounts of residual carbon were formed by the decomposition of the metal salts and most of dextrin (Scheme 1a, right, Figure S1a-1, a-2). The pure m-NiO spheres were prepared by removing the residual carbon through annealing at 500 °C for 3 h in air. Similarly, the m-Sn-NiO spheres were synthesized by spray pyrolysis of the aqueous solution containing nickel nitrate, tin oxalate, and dextrin (Figure S1b-1, 2), and subsequent heat treatment (Scheme 1b). The d-Sn-NiO spheres were prepared from the precursor solution without dextrin (Scheme 1c). The TG analyses of the m-NiO and m-SnNiO spheres confirmed that the residual carbon components are completely removed during heat treatment in air at temperatures below 500 °C (Figure S2). The multiroom morphology was observed from the semitransparent shells of the m-NiO (Figure 1a) and m-Sn-NiO spheres (Figure 1b) after the heat treatment at 500 °C, which was further confirmed from the inner structures observed using TEM (Figure 1d, e). In contrast, a dense inner structure was found in the d-Sn-NiO spheres (Figure 1c, f) after annealing at 500 °C. The diameters of 200 spheres were measured, and the particle size distributions are given 6


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in Figure S3. The average diameters of the ~200 m-NiO, m-Sn-NiO, and d-Sn-NiO spheres were 1.74 ± 0.61, 2.09 ± 0.88, and 1.57 ± 0.53 µm, respectively. The macropores in the mNiO and m-Sn-NiO spheres were verified to have a diameter of approximately 200 nm. The elemental mapping image using EDS (Energy dispersive X-ray spectroscopy) analysis revealed that the Sn component was uniformly distributed over the entire m-Sn-NiO sphere (Figure 1g). The phases and crystallinities were studied by X-ray diffraction (Figure 2). All the specimens exhibited the face-centered cubic phase of the NiO phase (JCPDS# 47-1049), and no second phase (Figure 2a-1, b-1, c-1) was observed. Based on the Scherrer equation, the crystallite sizes of the m-NiO, m-Sn-NiO, and d-Sn-NiO spheres were calculated to be 41.9 ± 1.8, 15.1 ± 0.7, and 29.9 ± 1.5 nm, respectively, which were confirmed with the highresolution TEM images of the respective spheres (Figure S4). Obvious peak shifts in the diffraction angles, lowered by up to 0.15°, were observed in the Sn-doped specimens (m-SnNiO and d-Sn-NiO spheres), which indicate that the Sn ions are incorporated into the NiO lattice after annealing at 500 °C for 3 h (Figure 2a-2, b-2, c-2). This is feasible considering that the ionic radius of Sn4+ at a coordination number (CN) of 6 is 0.83 Å, which is larger than that of Ni2+ at CN = 6 (0.69 Å). In addition, the decrease in crystallinity with increasing Sn doping observed in the present study is in line with the knowledge that Sn incorporation into the NiO lattice prevents crystal growth.16,17 3.2 Gas-sensing characteristics. To investigate the effect of Sn doping and multiroom structures, the gas-sensing responses of all three sensors to 1 ppm p-xylene, toluene, ethanol, formaldehyde, benzene, ammonia, hydrogen, and carbon monoxide were measured. All the sensors showed the typical gas-sensing behaviors of p-type oxide semiconductors: an increase in sensor resistance towards reducing gases and a decrease in the resistance towards 7



air. Thus, the gas responses (S = Rg/Ra) were measured in the range 300–400 °C (Figure 3). The m-NiO sensor showed negligibly low responses (1.0–1.3) to all the analyte gases (Figure 3a-1). In stark contrast, the m-Sn-NiO sensor showed significantly high gas responses at 300– 400 °C (Figure 3c-1), suggesting that the gas response of the NiO sensor depends on the level of Sn doping. The gas-sensing characteristics of the m-Sn-NiO sensor at a lower temperature (275 °C) were also measured to determine the optimal sensing conditions, because the m-SnNiO sensor showed the highest gas response among the three. The xylene response at 275 °C was lower than that of ethanol. This indicates that the optimum temperature for the selective and sensitive detection of xylene using the m-Sn-NiO sensor is 300 °C. In order to further investigate the effect of Sn doping, Sn-doped NiO multiroom spheres, with low Sn concentrations, (“referred to as “m-Snlow-NiO,” [Ni]/[Sn]=20.0) were also prepared using the same synthetic process. The m-Snlow-NiO spheres also showed a well-developed multiroom morphology under SEM and TEM observations (Figure S5a and b). Note that the gas responses of the m-Snlow-NiO sensor are higher than those of the m-NiO sensor, but lower than those of the m-Sn-NiO sensor, confirming again that Sn doping plays a key role in sensitization. However, the ethanol responses of the m-Snlow-NiO sensor at 300–325 °C were higher than the xylene responses, and the responses to ethanol and xylene are similar to each other at 350–400 °C, suggesting that a higher concentration of Sn doping is required to achieve selective sensing of xylene. Note that the gas responses of the m-Sn-NiO sensor are also substantially higher than those of the d-Sn-NiO sensor (Figure 3c-1 and b-1), suggesting that the multiroom morphology is another key parameter determining the gas response. The unprecedented high gas responses in the case of the m-Sn-NiO spheres can be discussed with respect to the changes in gas accessibility, surface reactivity, and charge carrier concentration due to Sn doping and gas reforming in multiroom spheres with high catalytic activity. 8


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The pore size distribution and specific surface area of the specimens were measured from the nitrogen adsorption/desorption isotherm (Figure 4). It was difficult to analyze the macropores with BET analysis, although SEM and TEM analyses (Figure 1) revealed that the m-NiO and m-Sn-NiO spheres contained macropores of diameters ~200 nm. The N2 adsorption/desorption isotherms of the d-Sn-NiO and m-Sn-NiO spheres were similar to the type IV isotherm with H3 hysteresis loops related to mesoporous solids, whereas those of the m-NiO spheres exhibited the type II isotherm, indicating their microporous or nonporous structure (Figure 4a). Abundant mesopores, with a mode diameter of ~ 4 nm, were found in the m-Sn-NiO spheres (Figure 4b-1), whereas no substantial mesopores were found in the mNiO spheres (Figure 4d-1). Thus, the mesopores with the mode size of 4 nm were formed by the incorporation of Sn into NiO. In the d-Sn-NiO spheres, a relatively small volume of mesopores with a mode diameter of approximately 10 nm was observed (Figure 4c-1). When combining with SEM/TEM analysis, both macropores (diameter: ~200 nm) and mesopores (diameter: ~4 nm) are abundant in the m-Sn-NiO spheres, whereas only a small volume of mesopores (diameter: ~10 nm) are observed in the d-Sn-NiO spheres; only macropores are found in the m-NiO spheres. This clearly explains the high gas accessibility of the m-Sn-NiO spheres. Moreover, the crystallite size of the m-Sn-NiO spheres is the smallest among the three samples. Accordingly, the BET specific surface area (SSA) of the m-Sn-NiO spheres (87.9 m2/g) was significantly higher than those of the d-Sn-NiO (30.9 m2/g) and m-NiO spheres (23.2 m2/g) (Figure 4b-2, 4c-2, 4d-2). These results are in line with the gas responses in Figure 3, which are also in line with the literature reporting the enhanced gas response in highly

gas

accessible

nanoarchitectures

such

as

hollow,23

hierarchical,15

and

macro/mesoporous nanostructures.24 Accordingly, the co-existence of abundant macro and mesopores, as well as a high SSA, increases the gas accessibility and sensing area of the m9



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Sn-NiO spheres, which explains the enhanced gas response. For a further investigation of the incorporation of Sn in NiO, all the specimens were analyzed using XPS (Figure 5). The C 1s line (284.6 eV) was to calibrate the binding energies. The difference between binding energies for Ni 2p3/2 and Ni 2p1/2 peaks was 17.6 eV.25 The Ni2+ and Ni3+ peaks of Ni 2p3/2 were observed at 854.2 eV and 856.0 eV, respectively (Figure 5a).26 The Ni 2p3/2 peaks of the d-Sn-NiO and m-Sn-NiO spheres were shifted to a binding energy 0.5 eV higher (Figure 5b and 5c). The shifts suggest that interactions take place between Ni and Sn, because the electronegativity of Sn (χ = 1.96) is higher than that of Ni (χ = 1.91); this substantiates the doping of Sn into the NiO lattice. The Sn 3d5/2 and Sn 3d3/2 peaks were observed at 486.3 eV and 494.7 eV, respectively, in the dand m-Sn-NiO spheres (Figure 5e and 5f),27 which suggests that Sn is present in the form of Sn4+. The Ni3+/Ni2+ ratio of the m-NiO, d-Sn-NiO, and m-Sn-NiO specimens were calculated to be 0.78, 1.61, and 1.79, respectively. The Ni3+/Ni2+ ratios significantly increased with an increase in Sn doping. The adsorbed oxygen on NiO is known to ionize by the oxidation of Ni2+ into Ni3+,28 and an excess of negatively charged interstitial oxygen (Oi′′)

29

is also

reported as a reason for the formation of Ni3+. Accordingly, the high Ni3+/Ni2+ values in the dSn-NiO and m-Sn-NiO spheres imply an abundance of negatively charged oxygen at the surface or inside the lattice.

1 X '' '' • 2 NiO O2 ( g ) + 2 NiO  → Osurf (or Oi ) + 2 NiNi + 2OO 2

(1)

The O 1s peaks of all the specimens were separated into 3 different parts:17 from lattice oxygen (OⅠ: 529.4 eV), oxygen-deficient regions (OⅡ: ~530.5 eV), and chemisorbed oxygen (OⅢ: 532.0 eV) (Figure 4g–i). The lattice oxygen peak at 529.4 eV is related to Ni-O and Ni10


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O-Sn because of the presence of O2- ions in the metal oxides. The relative percentages of the three peaks (OⅠ, OⅡ, and OⅢ) were 69.8, 21.1, and 9.1%, respectively, in m-NiO, which changed to 37.2, 45.7, and 17.0% in d-Sn-NiO and 33.9, 35.9, and 30.2% in m-Sn-NiO, respectively. Obviously, the contents of the OⅡ and OⅢ components significantly increased with Sn doping. Generally speaking, the increase in OⅡ can provide more oxygen adsorption sites on the surface, and the increase in OⅢ, which is a surface-chemisorbed oxygen species, will promote the gas-sensing reaction. Accordingly, the change in oxygen adsorption due to the doping of Sn should also be considered as a reason for the high gas response of the m-SnNiO spheres. The change in charge carrier concentration due to Sn doping should also be considered. Note that the sensor resistances in air of m-Sn-NiO were significantly higher than those of mNiO at 300–400 °C (Figure 6a–c). The incorporation of Sn4+ in the sites of Ni2+ in the lattice can be compensated either by the generation of two electrons and oxygen gas (electronic compensation) or by the formation of Ni vacancies (ionic compensation).30

1 •• X NiO SnO2   → SnNi + 2e' + OO + O2 ( g ) 2

(2)

•• X NiO SnO2  2 → Sn Ni + 2OO + V '' Ni

(3)

The electrons generated by reaction (2) will lead to a decrease in the hole concentrations of p-type NiO s, which would in turn increase their sensor resistance in air (Ra). In contrast, the ionic compensation mechanism presented in reaction (3) does not change the Ra value. The considerable increase in sensor resistance by Sn doping (Figure 6a-c) suggests that the electronic compensation mechanism is more probable. If one assumes that the same number 11



of electrons are injected into the p-type oxide chemiresistors by the gas-sensing reaction, higher chemiresistive variations will be found in the sensor with the lower hole concentration in the hole-accumulation layer. This is consistent with the knowledge that the doping of Fe3+ or Cr3+ into a NiO-based sensor increases both Ra and gas response.18–20 Thus, the increase in gas response due to Sn doping in the present study can also be explained in part by the electronic sensitization mechanism. Taking all these considerations into account, the high gas response of the m-Sn-NiO sensor can be rationalized by the enhanced gas accessibility due to the increase in macro and mesoporosity, high surface area-to-volume ratio, improved gas reactivity assisted by abundant oxygen adsorption, and increased chemiresistive variation due to the decrease in hole concentration, all of which originate from the multiroom morphology and Sn doping of NiO. The sensing transients of the m-Sn-NiO sensor for 1 ppm p-xylene at 300 °C remained stable even after repetitive measurements (Figure 7a). Furthermore, the sensor exhibited stable response for 4 weeks (Figure 7b). Both suggest the short- and long-term stability of the sensors. The sensing transients of the m-Sn-NiO sensor to 0.05–1 ppm p-xylene at 300 °C were shown in Figure 7c. The xylene responses of the m-Sn-NiO sensor were observed to be higher than those of undoped oxide chemiresistors31–33, as well as those of doped, catalystloaded, and composite-based oxide chemiresistors reported in the literature (Figure 7d and Table S1).16,17,20,34–42 In particular, the response to xylene for the present m-Sn-NiO sensor was significantly higher than those of the Sn-doped NiO sensors with different morphologies, such as hierarchical nanostructures and inverse opal structures, reported in the literature.16,17 This suggests that the multiroom morphology is a promising nanoarchitecture for gas sensors. The xylene concentration in unpolluted indoor air is usually lower than 1 ppm. The shortterm exposure to > 50 ppm xylene or long-term exposure to > 14 ppm xylene may induce 12


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irritation to the eyes, skin, and respiratory organs, as well as neurological effects.43 The concentration of 0.05 ppm was suggested as the minimal risk level of xylene in order to curtail the damages caused to humans.44 These observations suggest that not only the ppmlevel but also the sub-ppm-level detection of xylene is imperative for air quality monitoring. The m-Sn-NiO sensor showed a high response (Rg/Ra = 7.2) to 0.05 ppm p-xylene, and the detection limit was calculated to be 3.4 ppb when S (S = Rg/Ra)>1.2 was used as the criterion for gas sensing. These results indicate the promising potential of the m-Sn-NiO sensor for indoor/outdoor air monitoring. Note that the overall gas responses of the d-Sn-NiO sensor are higher than those of the m-NiO sensor. This can be explained in part by the increase in oxygen adsorption and decrease in hole concentration because of Sn doping. In addition, the combination of two catalytic materials can enhance the gas response. SnO2 is known to be a good catalyst for oxidizing ethanol.45,46 In addition, NiO is known to be an excellent catalyst for methanol oxidation, thus can promote the oxidation of xylene through the adsorption of methyl radical.47–49 Accordingly, a relatively higher gas response of the d-Sn-NiO sensor compared to that of the m-NiO sensor can be explained in part by the synergistic combination of sensing and doping materials that exhibit high catalytic activity. Finally, it is worthwhile noting that the morphological variation changed the gas selectivity as well. For example, the m-Sn-NiO sensor showed a high selectivity to xylene at 300 °C, whereas the d-Sn-NiO sensor showed similar and comparable responses to xylene, ethanol, and toluene. Because both materials are of the same composition, the synergistic combination of the two catalytic materials can be excluded from the list of primary reasons that affect gas selectivity. Previously, we reported that Pd-doped SnO2 yolk-shell spheres14 and Co3O4-SnO2 core-shell spheres50 show selective detection of xylene. The high xylene 13



selectivity was attributed to the in-diffusion of the less-reactive xylene through the semipermeable shells and the gas-reforming of the less-reactive xylene into more active and smaller species within the microreactors with catalytic activity, which was supported by an increase in the gas response time.14,50 The 90% response time (τres, the time required for 90% resistance change upon exposure to 1 ppm p-xylene) of the m-Sn-NiO sensor at 300 °C was 280 s, which was significantly higher than that of the d-Sn-NiO sensor (179 s). This emphasizes the fact that the unique multiroom structures of the m-Sn-NiO sensor in the present study also play the role of a microreactor to promote the xylene reforming reaction. The enhancement and control of gas response and selectivity by adopting the multiroom morphology can be used as a powerful tool to design high-performance gas sensors.

4. CONCLUSION The Sn-doped NiO multiroom spheres were prepared by template-free one-pot spray pyrolysis of aqueous droplets, following which their gas-sensing characteristics were studied. The sensor using Sn-doped NiO multiroom spheres showed not only a higher response to various gases but also higher selectivity to xylene compared to those using pure NiO multiroom spheres and dense Sn-doped NiO spheres. Multiroom morphology with a high gas accessibility significantly increased the gas response, whereas the doping of Sn into NiO increased oxygen adsorption, decreased hole concentration, and increased catalytic activity, thereby stimulating the gas-sensing reaction. Moreover, the unique morphology of the multirooms in the spheres played the role of microreactors to promote the reforming of xylene into smaller and more reactive species, which led to the selective detection of xylene, a representative indoor pollutant. The Sn-doped NiO multiroom spheres can be used for gas sensors to monitor ppb-level indoor/outdoor xylene. Consequently, the enhancement in gas accessibility and reforming of a specific gas using the unique multiroom morphology 14


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provides a new and general strategy for the design of highly sensitive and selective gas sensors.

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ASSOCIATED CONTENT Supporting Information. Scanning electron microscopy images of the m-NiO and m-SnNiO microspheres obtained after spray pyrolysis and the HR-TEM images of the m-NiO, mSn-NiO, and d-Sn-NiO microspheres.

Acknowledgements This work was supported by a grant from the National Research Foundation of Korea (NRF), which was funded by the Korean government (Ministry of Education, Science, and Technology (MEST), Grant No. 2016R1A2A1A05005331).

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FIGURES

Scheme 1. Schemes illustrating the formation of (a) NiO multiroom spheres (m-NiO), (b) Sndoped NiO multiroom spheres (m-Sn-NiO), and (c) dense Sn-doped NiO spheres (d-Sn-NiO).

17



Figure 1. (a–c) SEM and (d–g) TEM images of (a, d) m-NiO, (b, e) m-Sn-NiO, and (c, f) dSn-NiO spheres after heat treatment at 500 °C; (g) EDS elemental mapping images of the mSn-NiO spheres.

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Figure 2. X-ray diffraction patterns of (a) m-NiO, (b) m-Sn-NiO, and (c) d-Sn-NiO spheres.

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Figure 3. Gas responses (S = Rg/Ra) of (a-1) m-NiO, (b-1) d-Sn-NiO, and (c-1) m-Sn-NiO spheres to 1 ppm p-xylene, toluene, ethanol, formaldehyde, benzene, ammonia, hydrogen, and carbon monoxide at 300–400 °C; gas responses of (a-2) m-NiO, (b-2) d-Sn-NiO, and (c-2)

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m-Sn-NiO spheres to 1 ppm p-xylene at 300–400 °C (the m-Sn-NiO sensor was measured at 275–400 °C).

Figure 4. (a) Nitrogen adsorption and desorption isotherms of m-Sn-NiO (blue), d-Sn-NiO (green), and m-NiO (black) spheres; pore-size distribution and the corresponding BET surface areas of the (b) m-Sn-NiO, (c) d-Sn-NiO, and (d) m-NiO spheres.

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Figure 5. XPS patterns of (a, d, g) m-NiO, (b, e, h) d-Sn-NiO, and (c, f, i) m-Sn-NiO spheres: (a–c) Ni 2p, (d–f) Sn 3d, and (g–i) O 1s.

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Figure 6. (a-c) Dynamic sensing transients and sensor resistances of m-NiO and m-Sn-NiO sensors at 300–400 °C in air; (d, e) schemes illustrating the effect of Sn doping on xylene sensing.

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Figure 7. (a) Xylene sensing characteristics of m-Sn-NiO sensors at 300 °C: (a) repetitive sensing transients to 1 ppm p-xylene, (b) long-term stability of the sensor over 4 weeks, (c) dynamic sensing transients to 0.05–1.0 ppm p-xylene, and (d) xylene responses of the m-Sn doped NiO sensor and those reported in the literature.16,17,20,31-42

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Sensing Materials Via Galvanic Replacement. ACS Appl. Mater. Interfaces 2016, 8, 7877−7883.

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Dual Role of Multiroom-Structured Sn-Doped NiO Microspheres for

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