NiMoO4

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Highly selective xylene sensor based on NiO/NiMoO nanocomposite hierarchical spheres for indoor air monitoring Bo-Young Kim, Jee-Hyun Ahn, Ji-Wook Yoon, Chul-Soon Lee, Yun Chan Kang, Faissal Abdel-Hady, Abdulaziz A. Wazzan, and Jong-Heun Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13930 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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ACS Applied Materials & Interfaces

Highly selective xylene sensor based on NiO/NiMoO4 nanocomposite hierarchical spheres for indoor air monitoring Bo-Young Kim,† Jee Hyun Ahn,† Ji-Wook Yoon,† Chul-Soon Lee,† Yun Chan Kang,† Faissal Abdel-Hady,‡ Abdulaziz A. Wazzan,‡ and Jong-Heun Lee*,†, ‡ †

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

Korea ‡

Department of Chemical and Materials Engineering, King Abdulaziz University, Jeddah 21589,

Saudi Arabia *Author to whom correspondence should be addressed Email: [email protected]; Fax: +82-2-928-3584; Tel: +82-2-3290-3282 Keywords: NiO, NiMoO4, hierarchical structures, selective gas sensor, xylene

ABSTRACT Xylene is a hazardous volatile organic compound, which should be measured precisely for monitoring of indoor air quality. The selective detection of ppm-level xylene using oxide semiconductor chemiresistors, however, remains a challenging issue. In this study, NiO/NiMoO4 nanocomposite hierarchical spheres assembled from nanosheets were prepared by hydrothermal reaction, and the potential of sensors composed of these nanocomposites to selectively detect xylene gas was investigated. The sensors based on the NiO/NiMoO4 nanocomposite hierarchical spheres exhibited high responses (maximum resistance ratio = 101.5) to 5 ppm p-xylene with low cross-responses (resistance ratios < 30) to 5 ppm toluene, benzene, C2H5OH, CH3COCH3,

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HCHO, CO, trimethylamine, and NH3. In contrast, a sensor based on pure NiO hierarchical spheres exhibited negligibly low responses to all 9 analyte gases. The gas-sensing mechanism underlying the high selectivity and response to xylene in the NiO/NiMoO4 nanocomposite hierarchical spheres is discussed in relation to the catalytic promotion of the xylene-sensing reaction by synergistic combination between NiO and NiMoO4, gas-accessible hierarchical morphology, and electronic sensitization by Mo addition. Highly selective detection of xylene can pave the road toward a new solution for precise monitoring of indoor air pollution.

1. INTRODUCTION Human exposure to residential volatile organic compounds (VOCs) such as benzene, toluene, xylene, and formaldehyde is known to induce sick building syndrome with symptoms of headache, throat irritation, itchy skin, dizziness, and fatigue. In particular, xylene is one of the most representative and ubiquitous indoor pollutants that may induce diseases of the central nervous system and attack various respiratory organs.1 Metal oxide semiconductor gas sensors have irreplaceable advantages such as high sensitivity, fast response speed, simple fabrication, facile integration, and cost effectiveness2-6 and have thus been considered a viable platform to detect VOCs. The greatest challenge facing metal oxide gas sensors in monitoring indoor xylene is their gas selectivity because most n-type metal oxide semiconductors exhibit high responses to C2H5OH and HCHO,7,8 and thus distinguishing among chemically similar benzene, xylene, and toluene is difficult.9 Moreover, the exposure limits of several indoor pollutants differ depending on the health effect of human exposure. Accordingly, simple detection of total VOCs without discrimination of the gases


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makes it difficult to draw attention to the danger of a specific VOC and establish an appropriate response to a gas leak. In this respect, the selective detection of each target gas is critical. In general, the responses of n-type oxide semiconductor gas sensors to less reactive benzene, xylene, and toluene (BTX) gases are lower than those to highly reactive C2H5OH and HCHO.9-13 In contrast, p-type oxide semiconductors such as NiO, Cr2O3, and Co3O4 with high catalytic activity to oxidize BTX gases have been studied as sensing and additive materials to promote the sensing of xylene and/or toluene.14-16 Transition metal oxides of both p-type and n-type oxide semiconductors are good oxidative catalysts. Accordingly, the synergistic combination of two different catalytic oxides either by doping or by forming ternary oxides would be more effective in promoting or tuning the sensing of xylene.17 Moreover, when a nanocomposite is formed, the gas response can be enhanced by the control of conduction across two oxides with different work functions18. In the present study, we report that highly crystalline and gas-accessible NiO/NiMoO4 composite hierarchical nanostructures exhibit excellent selectivity and high response to sub-ppmlevel xylene with negligible cross-responses to chemically similar benzene and toluene as well as other representative VOCs such as C2H5OH, HCHO, and CO, whereas pure NiO hierarchical nanostructures exhibit practically no responses to any of the VOCs. The main focus of the study was directed at understanding the mechanism of selective and sensitive xylene detection in relation to the hierarchical morphology, change of charge carrier concentration in NiO, catalytic activation of NiMoO4, and synergistic catalytic effect of the nanocomposite between NiO and NiMoO4.


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2. EXPERIMENTAL Preparation of NiO/NiMoO4 hierarchical spheres. The pure NiO hierarchical spheres were prepared by hydrothermal reaction and subsequent heat treatment. Nickel (II) nitrate hexahydrate (0.145 g, Ni(NO3)2·6H2O, 99.999%, Sigma–Aldrich, USA) and urea (0.300 g, (NH2)2CO, 99.0%, Junsei Chemical Co., Japan) were dissolved in 50 mL of distilled water and stirred until the solution became clear. The solution was transferred to a Teflon-lined stainless steel autoclave (volume: 100 cm3), which was then sealed and heated at 180 °C for 9 h. After cooling, the resulting product was washed five times with distilled water and once with C2H5OH and then dried at 70 °C for 24 h in an oven. The as-prepared nickel precursor was converted into NiO hierarchical spheres by heat treatment at 550 °C for 4 h. The hierarchical morphology of the precursors containing Ni and Mo was achieved via a hydrothermal reaction of the stock solution containing nickel (II) nitrate hexahydrate (0.145 g) and urea (0.3 g) and ammonium molybdatetetrahydrate (0.004, 0.008, 0.026, 0.052, and 0.08 g, (NH4)6Mo7O24·4H2O, 99.0%, Junsei Chemical Co., Japan) at 180 °C for 9 h. The [Mo]/[Ni] molar ratios of the precursor solutions were 0.05, 0.1, 0.3, 0.6, and 1.0. The measured [Mo]/[Ni] ratios of the hierarchical oxide powders prepared by hydrothermal reaction of the precursor solutions, filtering, and heat treatment at 550 °C for 4 h were 0.049, 0.086, 0.21, 0.26, and 0.28, respectively; these measurements were made using inductively coupled plasma atomic emission spectrometry (ICP-AES). Accordingly, these 5 specimens will be denoted as NM0.049, NM0.086, NM0.21, NM0.26, and NM0.28, respectively. Preparation of NiO/NiMoO4 composite powder by solid-state mixing. To investigate the effect of the hierarchical morphology on the gas-sensing characteristics, NiO/NiMoO4 composite powders ([Mo]/[Ni] = 0.21) were also prepared via a solid-state reaction for comparison.


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Commercial nickel oxide (NiO, 99.8 %, powder, Sigma–Aldrich, USA) powders were mixed with commercial molybdenum oxide (MoO3, ≥99.5 %, powder, Sigma–Aldrich, USA) powders ([Mo]/[Ni] molar ratio = 0.21) by ball milling in C2H5OH for 24 h. After drying for 24 h at 70 °C, this powder mixture was heat-treated at 550 °C for 4 h. For simplicity, the NiO/NiMoO4 composite powders prepared by ball milling of commercial NiO and MoO3 powders will be referred to as SS-NM0.21. Characterization. The morphologies of the hierarchical NiO and NiO/NiMoO4 spheres were characterized using field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi Co. Ltd., Japan) and transmission electron microscopy (TEM, Talos F200X, FEI Co., USA). The crystallinity and phase were analyzed using X-ray diffraction (XRD, SmartLab, Rigaku, Japan) with CuKα radiation and X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, UlvacPHI, Japan). The surface area was measured using the Brunauer–Emmett–Teller method (BET, Tristar 3000, Micrometrics Co. Ltd., USA) with N2 as the adsorbate gas. To determine the precise composition of the NiO/NiMoO4 hierarchical spheres, ICP-AES (OPTIMA 8300, PerkinElmer, USA) was used. Gas-sensing characteristics. The pure NiO and NiO/NiMoO4 hierarchical spheres were dispersed in deionized water, and the slurry was coated on an alumina substrate (area: 1.5 × 1.5 mm2, thickness: 0.25 mm) with two Au electrodes on its top surface (electrode width: 1 mm, separation: 0.2 mm) and a micro-heater on its bottom surface (purchased from Sentech Korea Corp., Korea). The sensor temperature was controlled using the micro-heater underneath the substrate and was measured using an infrared temperature sensor (Metis MP25, Sensortherm GmBH, Germany). The sensors were contained in a specially designed, low-volume (1.5 cm3) quartz tube to minimize any delay in changing their surrounding atmosphere. Before


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measurement, the sensors were heated to 475 °C for 24 h to remove residual water and to stabilize the sensors. The thicknesses of sensing films using pure NiO and NiO/NiMoO4 hierarchical spheres were similar (5.05-6.75 µm). The gas-sensing characteristics were measured at 325–425 °C by adjusting the heater power in the range of 293–438 mW. The concentration of gases was controlled by mixing between gases in dry synthetic air balance and dry synthetic air. The gas responses (S = Rg/Ra; Rg: resistance in gas, Ra: resistance in air) of the sensors to 5 ppm p-xylene, toluene, benzene, C2H5OH, acetone, HCHO, CO, trimethylamine (TMA), and NH3 were measured by switching gas atmospheres. The DC two-probe resistances were measured using an electrometer interfaced with a computer.


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3. RESULTS AND DISCUSSION Characterization of sensing materials. The hierarchical precursor spheres consisting of nanosheets were synthesized by the hydrothermal self-assembly reaction of the stock solution ([Mo]/[Ni] = 0, 0.05, 0.1, 0.3, 0.6, and 1.0) (Figure S1) and were converted into NiO and NiO/NiMoO4 nanocomposite (NM0.049, NM0.086, NM0.21, NM0.26, and NM0.28) hierarchical spheres by heat treatment at 550 °C for 4 h (Figure 1). This process ensures that the gas-accessible hierarchical morphology formed during the hydrothermal reaction is maintained after heat treatment. The average diameters of ~200 hierarchical spheres in the NiO, NM0.049, NM0.086, NM0.21, NM0.26, and NM0.28 specimens were 1.89 ± 0.43, 3.54 ± 0.53, 4.42 ± 0.54, 3.16 ± 0.57, 1.50 ± 0.22, and 1.90 ± 0.45 µm, respectively. The pure NiO specimen exhibited a cubic structure (JCPDS #47-1049) (Figure 2a). The XRD patterns of the NM0.049 and NM0.086 specimens were similar, and no second phase was detected (Figure 2b and 2c). The NM0.21, NM0.26, and NM0.28 specimens consisted of αNiMoO4 (JCPDS #33-0948), NiO, and a small amount of β-NiMoO4 (JCPDS #45-0142) (Figure 2d–f). The co-existence of α- and β-NiMoO4 in the powders heat-treated at 550 °C was in line with the reported transformation temperature of NiMoO4 from α- to β- phase (550–670 °C).19 The intensity of the α-NiMoO4 peak increased with increasing [Mo]/[Ni] ratio in the stock solution for the hydrothermal reaction, indicating an increase of the α-NiMoO4 content within the NiO/α-NiMoO4 nanocomposites. By applying the Scherrer equation to the (111) peak of NiO, the crystallite sizes of NiO in the NiO, NM0.049, NM0.086, NM0.21, NM0.26, and NM0.28 powders were calculated to be 22.7, 8.5, 7.3, 3.4, 4.5, and 4.9 nm, respectively. The crystallite size initially decreased with increasing Mo concentration, reaching a minimum for the NM0.21 specimen, and then increased with further increase of the Mo concentration. In the NM0.21,


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NM0.26, and NM0.28 specimens, the crystallite size of α-NiMoO4 was calculated to be 18.2 ± 0.3, 22.1 ± 2.8, and 22.6 ± 2.5 nm, respectively. To investigate the possibility of Mo doping in NiO, the (111) peaks of NiO of all the specimens were further examined (Figure 2g and 2h). No significant peak shift was observed for the (111) peak (at 2θ = 37.2°). The ionic radius of Mo6+ (0.6 Å) at a coordination number of 6 is significantly smaller than that of Ni2+ (0.69 Å) at the same CN,20 indicating no incorporation of Mo into NiO lattice. The hierarchical morphology of all the samples was confirmed by TEM examination (Figure 3). A bright contour at the center of the hierarchical spheres was observed in the NM0.049 and NM0.086 specimens, indicating that the interior was hollow (Figure 3b and 3c). In contrast, no hollow morphology was observed in the NiO, NM0.21, NM0.26, and NM0.28 specimens (Figure 2a and 2d–f). Note that the surfaces of the hierarchical spheres tended to become more blunt in the NM0.26 and NM0.28 specimens with high Mo content (Figure 3e and 3f). To investigate the reason for surface blunting, the microstructural evolution of the NM0.21, NM0.26, and NM0.28 hierarchical spheres was studied by increasing the heat treatment duration at 550 °C from 10 min to 4 h. The sharp morphology of the nanosheets in the precursor spheres was maintained in the NM0.21 specimen (Figure S2a) regardless of the increase in heat-treatment duration, whereas the sharp nanosheets on the surface became blunted after the 4-h heat-treatment for the NM0.26 and NM0.28 specimens (Figure S2b and S2c). Thus, the blunting of the surface at high [Mo]/[Ni] ratio can be explained by the sintering of nanosheets with high Mo concentration at elevated temperature, which is feasible considering that the melting points of MoO3 (795 °C) and NiMoO4 (1475 °C) are significantly lower than that of NiO (1,955 °C).21-23 Lattice fringes of the NiO phase were observed in the NiO, NM0.049, and NM0.086 specimens (Figure 3a–c), whereas lattice fringes of both the NiO and α-NiMoO4 phases were observed in the NM0.21,


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NM0.26, and NM0.28 specimens (Figure 3d–f, only lattice fringes of the NiMoO4 phase are shown in Figure 3e and 3f). These findings are consistent with the XRD results presented in Figure 2. The distribution of Ni and Mo in the hierarchical spheres was investigated by TEM elemental mapping (Figure S3). In the all specimens, the Mo component was uniformly distributed over the entire NiO hierarchical sphere (Figure S3). For further investigation, the specimens were analyzed using XPS (Figure 4). The binding energy was corrected by referencing the C 1s line to 284.6 eV. The Mo 3d5/2 and 3d3/2 peaks were observed at 232.0–232.3 and 235.1–235.4 eV, respectively, in the NM0.049 specimen, which correspond to the Mo 3d level of the NiMoO4 phase24 because the binding energy of the pure MoO3 phase is substantially higher (232.7 and 235.8 eV).25,26 This finding suggests that the Mo in the NM0.049 and NM0.086 specimens (Figure 4b1 and 4c1) is present in the form of the NiMoO4 phase even though it was difficult to detect by XRD analysis because of its low detection limit. The Ni 2p3/2 and Ni 2p1/2 peaks due to spin-orbit splitting and satellite peaks for each were observed in the Ni 2p core level spectra of every specimen (Figure 4a2–f2). The Ni 2p3/2 and Ni 2p1/2 peaks were separated by 17.6 eV.27 The two Ni 2p3/2 peaks at binding energies of 853.7 and 855.4 eV in the NiO specimen indicate the presence of Ni2+ and Ni3+, respectively (Figure 4a2).28 The two peaks at 856.4 and 874.0 eV in the NM0.049, NM0.086, NM0.21, NM0.26, and NM0.28 specimens can be considered the Ni 2p3/2 and Ni 2p1/2 levels of NiMoO4 (Figure 4b2–f2).24 Gas-sensing characteristics. In general, semiconductor oxide gas sensors exhibit a high response to C2H5OH because of its high reactivity.29,30 However, for selective xylene detection, the cross-response to C2H5OH should be low because C2H5OH is one of the most ubiquitous indoor gases, as it is emitted from culinary use, alcohol beverages, and cleaning products.31 Thus,


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the xylene, toluene, and C2H5OH sensing characteristics of the sensors were investigated (Figure 5 and 6) at 325–425 °C. The dynamic sensing transients of the sensors upon exposure to 5 ppm p-xylene at 375 °C are shown in Figure 5. The pure NiO sensor showed practically no response (Figure 5a), whereas the other 5 sensors based on the NiO/NiMoO4 nanocomposite hierarchical structures exhibited significantly high responses to 5 ppm p-xylene and stable recovery upon exposure to air (Figure 5b–f). The sensors exhibited the gas-sensing characteristics of p-type oxide semiconductors. Note that the sensor resistance in air tends to increase with increasing Mo concentration. The responses of the sensors to 5 ppm p-xylene, toluene, and C2H5OH at 325–425 °C are shown in Figure 6. The pure NiO sensor exhibited negligibly low responses to all three gases (Rg/Ra = 1.03–1.21) (Figure 6a). In contrast, the 5 NM series sensors exhibited high gas responses (Figure 6b-f). The response to p-xylene reached maximum values at 375 or 400 °C, with the maximum response to 5 ppm p-xylene increasing up to 101.5 (for the NM0.21 sensor at 400 °C) before decreasing with further increase of the Mo concentration (Figure 6d). Based on the sensing transients, the 90% response time (τres) and 90% recovery time (τrecov) (the times to reach 90% variation in resistance upon exposure to the analyte gas and air, respectively) were calculated. The τres values ranged from 10 to 50 s, and the τrecov values ranged from 20 to 200 s (Figure S4). All the NiO/NiMoO4 nanocomposite sensors exhibited the highest response to p-xylene and the lowest response to C2H5OH. To quantify the gas selectivity, the selectivity toward p-xylene was calculated as Sxylene/Sinterference, where Sxylene and Sinterference were the responses to 5 ppm pxylene and 5 ppm of the interference gas, respectively. The Sxylene/SC2H5OH and Sxylene/Stoluene values as a function of sensor temperature are plotted in Figure S5. If solely considering the


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xylene selectivity with interference from C2H5OH or toluene, the performance of the NM0.086 sensor at 325 °C and 350 °C, respectively, was optimal. However, considering both response and selectivity to p-xylene, the performance of the NM0.21 sensor at 375 °C and 400 °C, respectively, was more advantageous. The selectivity of the NM0.21 sensor against other interference gases was examined by measuring the responses to 5 ppm p-xylene, toluene, benzene, C2H5OH, CH3COCH3, HCHO, CO, TMA, and NH3 at 325–425 °C (Figure S6). The results clearly demonstrate that the NM0.21 sensor exhibited highly selective detection of p-xylene regardless of variation of the sensor temperature, indicating its promising potential for indoor air monitoring applications. The selectivity of the pure NiO and NM0.21 sensors were more thoroughly investigated by measuring the gas responses of 5 different sensors under each condition at 375 °C (Figure 7). The NiO sensor did not show any significant response to the specific gas (Figure 7a), whereas the NM0.21 sensor exhibited remarkably high responses to 5 ppm xylene compared with its responses to the other interference gases (Figure 7b). Note that the response of the NM0.21 sensor to 5 ppm p-xylene is ~100 times higher than that of the pure NiO sensor and more than 4 times higher than those to the interference gases (toluene, benzene, C2H5OH, CH3COCH3, HCHO, CO, TMA, and NH3). Accordingly, p-xylene can be selectively and sensitively detected using NiO/NiMoO4 nanocomposite hierarchical structures. To understand the key parameters that determine the gas-sensing characteristics, the pore size distribution and surface area of the specimens were investigated by nitrogen adsorption (Figure 8). The BET specific surface areas (SSAs) of the NiO, NM0.049, NM0.086, and NM0.21 specimens were 16.8, 92.7, 110.2, and 234.8 m2/g, respectively (Figure 8a–d). The surface area and volume of the pores with a mode size of 20–30 nm increased significantly with increasing


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Mo content. This finding can be attributed to the decrease of crystallite size due to the formation of the nanocomposite between NiO and NiMoO4 and is in line with the crystallite sizes of the NiO phase calculated from the XRD patterns. The surface area and volume of pores with mode sizes of ~20 nm in the NM0.26 and NM0.28 specimens (Figure 8e and 8f) are slightly lower than those in the NM0.21 specimen. This result can be explained by the formation of a more agglomerated configuration of the hierarchical spheres due to the sintering of the nanosheets, which is supported by the increase of the XRD crystallite size in both the NiO and NiMoO4 phases and the blunting of the surface morphology. Note that the variation of the gas response (Figure 6) with increasing Mo concentration is consistent with that of the surface area and pore volume (Figure 8). In this respect, the increase of the gas response with increasing Mo from the NiO to the NM0.21 specimen can be partially explained by the enhanced transfer of analyte gases toward the wider area of sensing surfaces via abundant mesopores (mode size: ~ 20 nm). To investigate the effect of particle size on the gas response and selectivity, NiO/NiMoO4 composite powders with [Mo]/[Ni] = 0.21 (referred to as ‘SS-NM0.21’) were prepared, and their gas-sensing characteristics were measured (Figure S7). Although the sensor using SS-NM0.21 powders exhibited selectivity to p-xylene above 375 °C, the maximum response to 5 ppm pxylene was relatively low (40.3) (Figure S7a). The BET surface area of the SS-NM0.21 powders was 59.1 m2/g. These findings confirm that a high surface area (or small crystallite size) with a highly gas-accessible mesoporous structure plays a key role in enhancing the gas response. However, the enhancement of the gas response cannot be solely explained by the increase of the surface-area-to-volume ratio because the Mo concentration and surface area increased together. This enhancement can be discussed in relation to the gas-sensing characteristics of pure NiO nanostructures with high SSA in the literature. For instance, the responses of NiO core–shell


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nanotubes (SSA = 161.6 m2/g),32 NiO nanoparticles (SSA = ~128 m2/g),33 and NiO nanosheets (SSA = 95.4 m2/g)34 to C2H5OH were determined to be 4.2 (to 50 ppm), 3.0 (to 100 ppm), and 2.64 (to 100 ppm), respectively. These values are considerably lower than the response of the NM0.21 sensor (Rg/Ra= 21.6 at 375 °C) and SS-NM0.21 sensor (Rg/Ra= 16.3 at 350 °C) to 5 ppm C2H5OH. The low gas response of pure NiO sensors32-37 is generally observed in the literature and is known to emanate from the distinctive conduction mechanism of p-type oxide semiconductors, which involves the parallel competition between conduction along the narrow near-surface hole accumulation layer and along the wide less-conducting core.38 The marked difference in the C2H5OH response between the pure NiO nanostructures in the literature and the NM0.21 sensor in the present study despite their similar or comparable surface areas strongly suggests that the formation of NiMoO4 or the composite between NiO and NiMoO4 also plays an important role in the enhancement of the gas response. NiMoO4 is a p-type oxide semiconductor,39,40 and the work function of NiO is 5.4 eV. Although the work function of NiMoO4 is not available in the literature, the formation of a heterointerface between p-NiO and p-NiMoO4 with different work functions may cause the substantial increase of the sensor resistance with increasing Mo concentration in Figure 5. If the work function of NiO is higher than that of NiMoO4, electrons will flow from NiMoO4 to NiO after formation of the interface, which will decrease the hole concentration in NiO because of electron–hole recombination and increase the sensor resistance. Moreover, the conductivity of NiMoO4 is known to be lower than that of NiO.40 Thus, increasing the Mo concentration can cause a decrease in the conductivity of the NiO/NiMoO4 composite both by establishing p–p heterocontacts and by increasing the portion of the insulating phase. A sensor with relatively lower background charge carrier concentration will exhibit higher chemiresistive variation when


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the same amount of electrons is injected into the sensing materials by the gas-sensing reaction. Indeed, one of authors of the present study has reported the significant enhancement of gas response with a decrease of the background hole concentration of NiO nanostructures achieved by doping aliovalent Fe41,42 or establishing a p–n junction by decorating n-type In2O3 nanoclusters on NiO hollow spheres.30 Thus, the increase of the sensor resistance with the formation of NiMoO4 due to the increase of the Mo concentration may contribute to the enhancement of the gas response. However, significant enhancement of the gas selectivity to xylene cannot be explained by the increase of the surface-area-to-volume ratio or the increase of the sensor resistance due to the formation of the NiMoO4 phase or NiO(p)-NiMoO4(p) heteronanostructures. Thus, the catalytic promotion of the gas-sensing reaction by the Mo component or NiMoO4 should be considered as a reason for the selective xylene detection. To date, the xylene-sensing characteristics of pure NiO nanostructures have rarely been investigated, most likely because of their low gas response.32-37 Recently, the possibility of selective xylene sensing in pure and W-doped NiO nanostructures has been reported;43 however, the response was relatively low for ppm-level detection in real applications. Note that Ni-doped branched ZnO nanowire networks showed selective and sensitive detection of ppm-level xylene.44 These findings indicate that NiO can be considered a good sensing and additive material to enhance the xylene-sensing reaction once the gas response is significantly enhanced using an appropriate method. This idea is feasible considering the reports that NiO is a good catalyst for oxidizing methanol,45 promoting the adsorption of methyl radicals,46 and oxidizing toluene/xylene.47 NiMoO4 is also known as a good catalyst for the promotion of the partial oxidation of hydrocarbons,48,49 oxidative


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dehydrogenation of alkanes,50-52 and oxidation of toluene. Thus, the NiMoO4 phase can promote the xylene-sensing reaction. To investigate the role of NiMoO4 in determining the gas-sensing characteristics, we attempted to prepare single-phase NiMoO4 by controlling the [Mo]/[Ni] ratio but failed. Thus, it was difficult to distinguish between the catalytic effects of NiO and NiMoO4. However, the selective xylene sensing in both the NM0.21 and SS-NM0.21 sensors clearly indicate that the coexistence of NiO and NiMoO4 is advantageous for the promotion of the xylene-sensing reaction. The presence of two different catalytic materials can further promote the gas-sensing reaction because a reaction intermediate produced by one catalyst can be further dissociated by or react with the adjacent other catalyst. When two catalytic materials coexist, the mixing configuration is also important. For example, one of the authors in the present study reported that uniform Cr2O3/ZnCr2O4 nanocomposite powders prepared by galvanic replacement reaction showed high selectivity to xylene, whereas Cr2O3/ZnCr2O4 composite powders consisting of coarse particles showed relatively low selectivity to xylene.17 To investigate the effect of the mixing configuration between NiO and NiMoO4, the xylene selectivity of the SS-NM0.21 sensor was determined (Figure S7b). The Sxylene/SC2H5OH and Sxylene/Stoluene values of the NM0.21 sensor (Figure S5d) were substantially higher than those of the SS-NM0.21 sensor (Figure S7b), suggesting that the synergistic catalytic effect is maximized in the nanoscale composites between catalysts. The NiO/NiMoO4 nanocomposite hierarchical spheres in the present study with the uniform and nanoscale mixing of Ni and Mo components, high surface area, and mesoporous structures are excellent candidates for catalysts to promote the xylene-sensing reaction. The sensing transients of the NM0.21 sensor to 5 ppm p-xylene were repetitively measured at 375 °C (Figure 9a). The sensor exhibited very stable sensing and recovery. In addition, the


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sensor exhibited relatively good stability over 14 days (Figure 9b). The sensing transients of the NM0.21 sensor to 0.25–5 ppm p-xylene at 375 °C indicated reliable sensing characteristics (Figure 9c). The xylene responses of the NM0.21 sensor are higher than those of other gas sensors using various pure oxide semiconductors53-56 as well doped, catalyst-loaded and composite oxide semiconductors14,17,43,44,57-62 in the literature (Figure 9d).14,17,43,44,53-62 Although, the sensor using Pd-loaded Co3O4 hollow hierarchical nanostructures exhibited slightly higher response to 5 ppm p-xylene at 275°C, the Sxylene/Stoluene value (~1.5) was significantly lower than that of NM0.21 sensor (Sxylene/Stoluene = 3.2).62 This clearly confirms the promising potential of NM0.21 sensor to detect sub-ppm-level xylene in a highly selective and sensitive manner. Xylene is frequently used as a solvent in the printing, rubber, paint, and leather industries and is also found in gasoline and cigarette smoke. This VOC is known to induce skin and eye irritation, impairment of the respiratory system, headaches, and dizziness. The Occupational Safety and Health Administration have suggested 100 ppm (8-h time-weighted-average concentration) as the permissible exposure limit of xylene in the workplace.1 According to the Centers for Disease Control and Prevention (CDC),63 acute-duration inhalation of as low as 50 ppm xylene is known to produce irritant effects to the eyes and skin as well as impairment of the respiratory system, and chronic-duration occupational exposure at 14 ppm xylene can increase eye irritation, sore throat, and neurological effects. The low detection limit of p-xylene was determined to be 0.02 ppm when Rg/Ra = 1.2 was used as the detection limit point. These results indicate that the NM0.21 sensor can detect ppb-level xylene and can be used to monitor indoor and outdoor air quality for real applications.


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4. CONCLUSIONS Pure NiO and NiO/NiMoO4 hierarchical nanostructures self-assembled from crystalline nanosheets were synthesized by a hydrothermal reaction, and their gas-sensing characteristics were investigated. The gas responses of the sensor using pure NiO hierarchical spheres were negligibly low. In stark contrast, the sensor using NiO/NiMoO4 ([Mo]/[Ni] = 0.21) nanocomposite hierarchical spheres exhibited a very high response to sub-ppm-level p-xylene with negligibly low cross-responses to various interference gases such as toluene, benzene, C2H5OH, CH3COCH3, HCHO, CO, TMA, and NH3. Moreover, the sensor exhibited excellent xylene selectivity over a wide range of sensing temperatures (325–425 °C) as well as good stability. The high selectivity and response of the NiO/NiMoO4 nanocomposite sensor to pxylene were attributed to the synergistic catalytic promotion of the xylene-sensing reaction assisted by NiO and NiMoO4, electronic sensitization due to the Mo addition, and highly gasaccessible hierarchical morphology with high surface area. The NiO/NiMoO4 nanocomposite hierarchical spheres can be used to monitor indoor/outdoor xylene with minimum interference from other gases.


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ASSOCIATED CONTENT Supporting Information. SEM images of NiO, NM0.049, NM0.086, NM0.21, NM0.26, and NM0.28 precursor spheres; SEM images of NM0.21, NM0.26, and NM0.28 specimens with variation of the heat-treatment duration; TEM images of NiO, NM0.049, NM0.086, NM0.21, NM0.26, and NM0.28; 90% gas response and 90% recovery times for all the gas sensors; The Sxylene/Sethanol and Sxylene/Stoluene values of the NiO, NM0.049, NM0.086, NM0.21, NM0.26, and NM0.28 sensor; The selectivity of NM0.21 sensor; Gas response and selectivity for SS-NM0.21.

Notes The authors declare no competing financial interest.

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

Figure 1. SEM images of (a) NiO, (b) NM0.049, (c) NM0.086, (d) NM0.21, (e) NM0.26, and (f) NM0.28 hierarchical nanostructures.


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Figure 2. XRD patterns of (a) NiO, (b) NM0.049, (c) NM0.086, (d) NM0.21, (e) NM0.26, and (f) NM0.28 hierarchical nanostructures.


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Figure 3. TEM images and high-resolution lattice images of (a) NiO, (b) NM0.049, (c) NM0.086, (d) NM0.21, (e) NM0.26, and (f) NM0.28 hierarchical nanostructures.


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Figure 4. XPS spectra of (a) NiO, (b) NM0.049, (c) NM0.086, (d) NM0.21, (e) NM0.26, and (f) NM0.28 hierarchical nanostructures.


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Figure 5. Dynamic sensing transients of sensors using (a) NiO, (b) NM0.049, (c) NM0.086, (d) NM0.21, (e) NM0.26, and (f) NM0.28 hierarchical nanostructures upon exposure to 5 ppm pxylene at 375 °C.


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Figure 6. Responses of sensors using (a) NiO, (b) NM0.049, (c) NM0.086, (d) NM0.21, (e) NM0.26, and (f) NM0.28 hierarchical nanostructures to 5 ppm p-xylene, toluene, and C2H5OH at 325–425 °C.


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Figure 7. Gas responses of sensors using (a) NiO and (b) NM0.21 hierarchical nanostructures at 375 °C (concentration of all analyte gases: 5 ppm; X: p-xylene, T: toluene, B: benzene, E: C2H5OH, A: acetone, F: HCHO, C: CO, Tm: TMA, N: NH3).


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Figure 8. Pore size distribution and SSA determined by nitrogen adsorption of (a) NiO, (b) NM0.049, (c) NM0.086, (d) NM0.21, (e) NM0.26, and (f) NM0.28 hierarchical nanostructures.


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Figure 9. Xylene-sensing characteristics of NM0.21 sensors at 375 °C: (a) repetitive sensing transients to 5 ppm p-xylene, (b) long-term stability of sensor over 14 days, (c) dynamic sensing transients to 0.25–5 ppm p-xylene, and (d) xylene response of NM0.21 sensors and those of other oxide semiconductor gas sensors in the literature.14,17,43,44,53-62 (blue circles: response of pure oxide semiconductors, grey circles: response of doped, catalyst-loaded and composite oxide semiconductors in the literature)


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