Gas Selectivity Control in Co3O4 Sensor via Concurrent Tuning of GasReforming and Gas Filtering using Nanoscale Hetero-Overlayer ofCatalytic OxidesHyun-Mook Jeong,† Seong-Yong Jeong,† Jae-Hyeok Kim,† Bo-Young Kim,† Jun-Sik Kim,†

Faissal Abdel-Hady,‡ Abdulaziz A. Wazzan,‡ Hamad Ali Al-Turaif,‡ Ho Won Jang,§

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§Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826,Korea

*S Supporting Information

ABSTRACT: Co3O4 sensors with a nanoscale TiO2 or SnO2 catalytic overlayer were prepared by screen-printing of Co3O4yolk−shell spheres and subsequent e-beam evaporation of TiO2 and SnO2. The Co3O4 sensors with 5 nm thick TiO2 and SnO2overlayers showed high responses (resistance ratios) to 5 ppm xylene (14.5 and 28.8) and toluene (11.7 and 16.2) at 250 °C withnegligible responses to interference gases such as ethanol, HCHO, CO, and benzene. In contrast, the pure Co3O4 sensor did notshow remarkable selectivity toward any specific gas. The response and selectivity to methylbenzenes and ethanol could besystematically controlled by selecting the catalytic overlayer material, varying the overlayer thickness, and tuning the sensingtemperature. The significant enhancement of the selectivity for xylene and toluene was attributed to the reforming of less reactivemethylbenzenes into more reactive and smaller species and oxidative filtering of other interference gases, including ubiquitousethanol. The concurrent control of the gas reforming and oxidative filtering processes using a nanoscale overlayer of catalyticoxides provides a new, general, and powerful tool for designing highly selective and sensitive oxide semiconductor gas sensors.

KEYWORDS: gas sensor, catalytic overlayer, gas selectivity, methylbenzene, gas reforming, gas filtering, Co3O4

1. INTRODUCTION

Oxide semiconductor chemiresistors provide many irreplace-able advantages such as high sensitivity, fast response, simplestructure, facile integration, and cost effectiveness.1−8 However,a simple sensing mechanism based on charge transfer between asemiconductor surface and analyte gas often leads to similar orcomparable responses to a range of different gases. Accordingly,the selective detection of a specific gas has been a majorchallenge for practical application of oxide semiconductor gassensors.To date, various approaches to achieve selective gas sensing

have been explored, including control of the sensor temper-ature,9 loading with noble metals10,11 or oxide catalysts,12,13

exploiting acid−base interaction between the gas and sensingsurface,14,15 and the use of a catalytically active overlayer.16−21

The roles of the catalytic overlayer can be divided into“oxidative filtering” and “gas reforming”. The oxidative filteringachieves gas selectivity by oxidizing interference gases intononreactive or less reactive CO2 and/or H2O. For instance,porous Ga2O3 and Pd-loaded Al2O3 overlayers have been usedto suppress the cross-responses to ethanol and/or CO.17,21 Incontrast, the gas reforming enhances gas selectivity by

Received: September 15, 2017Accepted: November 7, 2017Published: November 7, 2017

Research Article

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© 2017 American Chemical Society 41397 DOI: 10.1021/acsami.7b13998ACS Appl. Mater. Interfaces 2017, 9, 41397−41404

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reforming the less-reactive analyte gas into more active andsmaller species. The SnO2 nanosheets and Pt-loaded Al2O3overlayers have been coated on oxide semiconductor sensingfilms to enhance the responses to 1-nonanal and benzene,respectively.18,20

Strictly speaking, the broad concept of gas reforming includesoxidative filtering. However, in the present study, wedistinguish between gas reforming to enhance the response tothe analyte gas and oxidative filtering to suppress the responsesto interference gases. Early stage research has demonstrated thepossibility of enhancing the gas selectivity either by oxidativefiltering16,17,21 or gas reforming.18−20 Although one report22

demonstrated the combined effect of gas reforming and filteringusing a noble metal-loaded SnO2 overlayer, most prior studiesused only one strategy, and systematic control of the gasselectivity by concurrently tuning the gas reforming andfiltering using various catalytic oxide hetero-overlayers hasbeen barely investigated.In the present study, we prepare Co3O4 sensing films with

and without a SnO2 or TiO2 overlayer and investigate the effectof the catalytic overlayer on the gas sensing characteristics.Interestingly, coating with the nanoscale SnO2 or TiO2overlayer significantly enhanced the responses of the Co3O4sensors to methylbenzenes while decreasing the cross-responsesto other interference gases, including ubiquitous ethanol, to anegligible level. The role of the catalytic overlayer on the gasselectivity to methylbenzene, the representative indoorpollutant, is explained in relation to the concurrent control ofgas reforming and gas oxidation via tuning the catalytic activityof the overlayer materials, the thickness of the overlayer, andthe synergistic catalytic effect between the overlayer andsensing films.

2. EXPERIMENTAL SECTION2.1. Synthesis of Co3O4 Yolk−Shell Spheres. The Co3O4 yolk−

shell spheres were synthesized by ultrasonic spray pyrolysis. Cobalt(II)nitrate hexahydrate [Co(NO3)2·6H2O; 0.020 M, 99.999% trace metalbasis, Sigma-Aldrich], citric acid monohydrate [C6H8O7·H2O; 0.020M, ACS reagent ≥99.0%, Sigma-Aldrich], and sucrose [C12H22O11;0.010 M, ≥99.5%, Sigma-Aldrich] were dissolved in 600 mL ofdistilled water. The mixture was stirred at room temperature tohomogenize the solution and was used as a spray solution. The systemfor ultrasonic spray pyrolysis consisted of an ultrasonic nebulizer (sixultrasonic transducers, resonance frequency: 1.7 MHz), a quartz tubein an electric furnace, and a particle-collecting chamber. The droplets

(size: ∼3 μm) were continuously generated by the ultrasonic nebulizerand moved by nitrogen gas (flow rate: 5 L min−1) into the quartz tube(inner diameter: 55 mm) in an electric furnace heated to 900 °C. TheCo3O4 yolk−shell spheres generated after the spray pyrolysis reactionwere collected on a Teflon bag filter. The collecting chamber wasgently heated to ∼250 °C to prevent water condensation.

2.2. Preparation of Gas-Sensing Film and Catalytic Over-layer. The slurry for screen printing of sensing film was prepared bymixing Co3O4 yolk−shell spheres with an organic binder (FCM, aterpineol-based ink vehicle, USA). The slurry contained 20 wt % ofCo3O4 yolk−shell spheres. The gas-sensing film was screen-printedusing slurry on an alumina substrate (area: 1.5 mm2; thickness: 0.25mm) with two Au electrodes on the top surface and a Ru microheateron the bottom surface. The Au electrodes were 1.0 mm wide, and thegap between electrodes was 1.0 mm. A SnO2 or TiO2 catalyticoverlayer was deposited on top of the Co3O4 thick film by e-beamevaporation of polycrystalline SnO2 and TiO2 grains (99.7%, KojundoChemical Lab. Co., Ltd., Japan). The setting values of voltage andcurrent of the e-beam were fixed to 6.78 kV and 7 mA, respectively.This leads to a deposition rate of sensing film to 0.04 Å s−1. Afterdeposition of the catalytic overlayer, the sensing films were heat-treated at 210 °C for 3 h and at 450 °C for 4 h to remove the residualorganic components and to stabilize the sensor at the operatingtemperature. For simplicity, the Co3O4 sensors with the catalyticoverlayer of TiO2 or SnO2 (thickness: 2, 5, and 20 nm) are hereafterreferred to as xTiO2/Co3O4 (x = 2, 5, and 20) and ySnO2/Co3O4 (y =2, 5, and 20).

2.3. Characterization and Gas-Sensing Measurement. Thecrystallinity and phases of the sensing films were analyzed by X-raydiffraction (XRD, Cu Kα, D/MAX-2500 V/PC, Rigaku, Japan). Themicrostructure and morphology of the sensing materials were observedby field-emission scanning electron microscopy (FE-SEM, S-4700,Hitachi Co., Ltd., Japan) and high-resolution transmission electronmicroscopy (HR-TEM, TALOS F200X, FEI Co., Ltd., USA). Thepore distribution and specific surface area were determined by theBrunauer−Emmett−Teller (BET) method (Tristar 3000, Micro-meritics Co., Ltd., USA). Depth profiling and elemental mapping ofthe SnO2/Co3O4 and TiO2/Co3O4 sensing films were performed byusing a time-of-flight secondary ion mass spectrometer (TOF-SIMS,TOF.SIMS 5, ION-TOF GmbH, Germany). The cross-sectionalimages and composition of the prepared films were investigated byusing a field emission electron probe microanalyzer (FE-EPMA, JXA-8530F, JEOL Ltd., Japan).

The sensor was annealed at 400 °C for 12 h prior to measurementof the sensing characteristics. The sensor temperatures were controlledby Ru microheaters located underneath of the Au-electrode patternedalumina substrates and continuously measured by IR temperaturesensor (Metis MP25, Sensortherm GmbH., Germany). Then, thesensors were contained in low-volume (1.5 cm3) quartz tubes to

Figure 1. (a) FE-SEM image, (b−d) high-resolution TEM images, and (e) schematic diagram of the development of single-phase Co3O4 yolk−shellsvia spray pyrolysis.

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decrease the delay in atmosphere change. The gas response (S = Rg/Ra; Rg: resistance in gas, Ra: resistance in air) to 5 ppm of benzene(C6H6), carbon monoxide (CO), ethanol (C2H5OH), formaldehyde(HCHO), toluene (C6H5CH3), and p-xylene (1,4-dimethylbenzene,C6H4(CH3)2) was measured at 200−300 °C. For simplicity, the gasresponse (to a specific gas) is referred to as Sgas (gas: benzene (B),carbon monoxide (C), ethanol (E), formaldehyde (F), toluene (T),and xylene (X)). The dry synthetic air (ratio of N2/O2 = 0.79:0.21)and 5 ppm of the analyte gases (benzene, carbon monoxide, ethanol,formaldehyde, toluene, and p-xylene in dry synthetic air balance) wereused to control the gas concentrations by varying the mixing ratios.The electrometer connected to a computer was used to measure thedc two-probe resistance of gas sensors.

3. RESULTS AND DISCUSSION

The Co3O4 spheres were prepared by spray pyrolysis. Thediameters of the spheres ranged from 0.3 to 1.5 μm (Figure 1a).The TEM image (Figure 1b−d) showed that most of thespheres consisted of a thin shell and a smaller sphere within theshell. The development of the yolk−shell morphology with thesphere-in-sphere configuration can be explained by thefollowing reactions during spray pyrolysis: (1) solventevaporation from the droplets, (2) formation of carbon−Co(C−Co) composite spheres by the polymerization andcarbonization of sucrose, (3) formation of the Co3O4 shell byoxidation and decomposition of the C−Co precursors at theoutermost region of the sphere and subsequent contraction ofthe inner C−Co precursors, and (4) formation of Co3O4 yolk−shell spheres by the oxidation and decomposition of the inneryolk region of the C−Co precursors (Figure 1e). The nitrogenadsorption and desorption isotherm of the Co3O4 yolk−shellspheres consisted of type IV and type H3 hysteresis loops,indicating the presence of mesopores (10−100 nm, mode size:37.2 nm) (Figure S1). This suggests that the Co3O4 yolk−shellspheres had highly gas-accessible structures with abundantmeso- and macroporous channels. The surface area was 16.2m2/g.A schematic of the procedure for preparing the sensors is

shown in Figure 2a. A thick Co3O4 sensing film was fabricated

by screen printing of a slurry containing the Co3O4 yolk−shellspheres. Subsequently, TiO2 and SnO2 catalytic overlayers withdifferent thicknesses (2, 5, and 20 nm) were deposited on theCo3O4 sensor by e-beam evaporation. The thicknesses of all theCo3O4 sensing films without and with SnO2/TiO2 overlayerswere similar (∼5.0 μm) (Figures 2b−d and S2).Because the SnO2 or TiO2 overlayer was too thin to observe

by SEM analysis, the compositional variation at the upper partsof the sensing films was analyzed by EPMA (Figures 2e,f andS2). The concentrations of TiO2 and SnO2 were locally highernear the uppermost part of the sensing film, and this trendbecame more pronounced with thickening of the overlayerfrom 2 to 20 nm. The TOF-SIMS 3D and 2D compositionmapping images and depth profiles of the 5TiO2/Co3O4 and5SnO2/Co3O4 sensors showed that the Ti/Co and Sn/Coratios gradually decreased with increasing deposition time(Figure S3). This result again confirms the successfuldeposition of TiO2 and SnO2, which is consistent with theimages obtained by FE-EPMA, as shown in Figure 2.Note that the TiO2/SnO2 abundant region (Figures 2e,f and

S2) was typically 0.5−1 μm thick, which is significantly widerthan the film thickness (2−20 nm) expected from thedeposition rate. The side-view TEM image and compositionalmapping of a sphere located at the uppermost region of the5TiO2/Co3O4 sensor confirmed that a TiO2 layer with athickness of ∼20 nm was coated on the upper half of the Co3O4yolk−shell spheres (diameter: 0.4−1.0 μm) (Figure S4a−d).The top-view TEM image and compositional mapping of thespheres at the uppermost region of the 5SnO2/Co3O4 sensorindicates that the SnO2 layer was uniformly coated on the outerpart of the Co3O4 yolk−shell spheres (Figure S4e−h).Considering the EPMA analysis using SEM (Figure 2e,f) andelemental mapping using TEM (Figure S4), it can be confirmedthat nanoscale (thickness: ∼20 nm) SnO2 and TiO2 overlayerswere uniformly coated on the outer surface of the Co3O4sensing films with embossed morphology. The Co3O4, 5TiO2/Co3O4, and 5SnO2/Co3O4 films were indexed to cubic Co3O4(Powder Diffraction File no. 42−1467, Joint Committee onPowder Diffraction Standards, 1990) and no other peak relatedto SnO2 or TiO2 was observed (Figure S5). This can beattributed to the nanoscale thickness of the overlayers, which isbelow the detection limit of X-ray diffraction.The gas-sensing characteristics of the sensors were measured

at 200−300 °C (Figure 3). All the sensors with and withoutTiO2 or SnO2 overlayers showed p-type gas sensing character-istics, that is, the sensor resistance increased upon exposure toreducing gases (see the sensing transients of Co3O4, 5TiO2/Co3O4, and 5SnO2/Co3O4 sensors to ethanol, xylene, andtoluene in Figure S6). This confirms that conduction along thelower part of the p-type Co3O4 thick films near the sensingelectrode is hardly affected by the nanoscale n-type SnO2 andTiO2 overlayers, which is feasible considering the thickness ofCo3O4 sensing film (∼5 μm) and electrode configuration.The Co3O4 sensor showed high responses to ethanol,

toluene, and xylene (Figure 3a). At 200 °C, the response toethanol was the highest but decreased rapidly as thetemperature of the sensor increased, whereas the responses toxylene and toluene decreased gradually. Accordingly, the sensorshowed selectivity to ethanol below 250 °C and tomethylbenzenes (toluene and xylene) above 275 °C. This isconsistent with the gas sensing characteristics of pure Co3O4sensors reported in the literature.13,23−25 Note that theresponses of the Co3O4 sensor to 5 ppm xylene and toluene

Figure 2. (a) Schematic diagram of the fabrication of Co3O4, TiO2/Co3O4, and SnO2/Co3O4 sensors and cross-sectional FE-SEM imagesof the (b) Co3O4, (c) 5TiO2/Co3O4, and (d) 5SnO2/Co3O4 sensingfilms and FE-EPMA analysis of compositional variation at the upperparts of the (e) 5TiO2/Co3O4 and (f) 5SnO2/Co3O4 sensors.

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were relatively low (3.23−3.51) at 300 °C, although selectivedetection of methylbenzene was possible.The gas responses of the 2TiO2/Co3O4 (Figure 3b) and

2SnO2/Co3O4 (Figure 3e) sensors to ethanol, toluene, andxylene at 200 °C were significantly higher than those of theCo3O4 sensor at 200 °C (Figure 3a). This indicates that thethin TiO2 and SnO2 overlayers catalytically reformed theanalyte gases into more reactive species for the gas sensingreaction. Although the details of gas reforming reaction needfurther systematic investigation, toluene and xylene are knownto be reformed or partially oxidized into various species such as4-methylbenzaldehyde,26−28 benzyl alcohol,29 benzalde-hyde,28−30 and phthalic anhydride26 at elevated temperaturein the presence of catalysts. The oxide semiconductor sensorsusing SnO2 and WO3 are known to show high responses tobenzaldehyde and tolualdehyde,31,32 which suggest that gasresponses to xylene and toluene can be enhanced by catalyst-assisted gas reforming reaction. The ethanol is also known todissociated into CH3CHO + H2 or C2H2 + H2O at basic oracidic surfaces of oxides,14 demonstrating that the enhancementof ethanol response by reforming is probable.The SE value (101.7) of the 2TiO2/Co3O4 sensor at 200 °C

(Figure 3b) decreased to 68.4 and 28.4 by further thickening ofoxide overlayer to 5−20 nm (Figures 3c,d), whereas the SX andST values showed minor decreases. This leads to the downshiftof the minimum sensing temperature to show the selectivedetection of xylene and toluene (Tmin,TX). For example, theTmin,TX values of the Co3O4, 2TiO2/Co3O4, 5TiO2/Co3O4, and20TiO2/Co3O4 sensors were 275, 250, 250, and 200 °C,respectively. An abrupt decrease in the SE values and a relatively

gradual decrease in the SX and ST values were also observedwhen the thickness of the SnO2 overlayer was increased from 2to 20 nm (Figure 3e−g). The Tmin,TX values of the 2SnO2/Co3O4 (250 °C), 5SnO2/Co3O4 (225 °C), and 20SnO2/Co3O4(200 °C) sensors support this observation. The temperaturewindows for selective and sensitive detection of methylben-zenes are highlighted in gray dotted boxes in Figure 3.Ethanol is one of the most ubiquitous indoor gases33 and

oxide semiconductor gas sensors often show the highestresponse to ethanol.9,34−37 Thus, it is important to decrease thecross-response to ethanol to a negligible level. The selectivity toxylene and toluene over ethanol interference (SX/SE and ST/SE)were calculated (Figure 4). Note that SX/SE > 1 and SX/SE < 1indicate the selective detection of xylene over ethanol andselective detection of ethanol over xylene, respectively. Coatingof both TiO2 and SnO2 overlayers significantly increased theSX/SE and ST/SE values, confirming a significant effect of thecatalytic overlayer in enhancing the methylbenzene selectivity.The highest SX/SE and ST/SE values were observed at 250 °Cfor the 5TiO2/Co3O4 and 5SnO2/Co3O4 sensors.The in-diffusion of the analyte gases from the upper part of

the sensing film to the lower part close to the sensingelectrodes can be regarded as the same because the thicknessesof all the sensing films in the present study are similar. Thus,the variation in the gas response and selectivity with theintroduction of the nanoscale oxide overlayer can be under-stood in relation to the reforming of the gas into more activespecies and/or oxidation of the gas into nonreactive species.Figure 5 shows a schematic diagram illustrating the xylene andethanol sensing reaction at 250 °C.

Figure 3. Gas-sensing characteristics and gas responses (Rg/Ra) of the (a) Co3O4, (b) 2TiO2/Co3O4, (c) 5TiO2/Co3O4, (d) 20TiO2/Co3O4, (e)2SnO2/Co3O4, (f) 5SnO2/Co3O4, and (g) 20SnO2/Co3O4 sensors to 5 ppm of various gases at 200, 225, 250, 275, and 300 °C, respectively.

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The SX value at 250 °C increased significantly from 5.8 to22.2 with the deposition of a 2 nm thick TiO2 overlayer (Figure5a,b-1), indicating that less-reactive xylene was reformed intomore reactive and smaller species by the catalytic TiO2overlayer. The variation of the gas response can be eitherenhanced by reforming of the analyte gas into more activespecies during transport of the gas to the lower part of thesensing film or may decrease due to complete oxidation of theanalyte gas into nonreactive or less-reactive CO2 and H2O atthe upper part of the sensing films. The decrease in the SXvalues to 14.5 and 3.3 at 250 °C when the thickness of the TiO2overlayer was increased to 5 and 20 nm (Figure 5b-2,b-3),respectively, can be attributed to the promotion of xyleneoxidation to form non- or less-reactive species, assisted by thethicker catalytic overlayer. For the SnO2-coated sensors, the SXvalues increased when the thickness of the overlayer wasincreased from 2 to 5 nm and then decreased when thethickness was further increased to 20 nm (Figure 5a,c-1,c-2,c-3). The former and latter observations can be also attributed tothe promotion of gas reforming and gas oxidation, respectively.In contrast, at 250 °C, the SE values decreased monotonouslyfrom 5.1 to 1.5 (Figure 5d-1,d-2,d-3) and from 4.6 to 1.6(Figure 5e-1,e-2,e-3) with thickening TiO2 and SnO2 overlayersfrom 2 to 20 nm. This suggests that suppression of the gas

response via oxidation of the highly reactive ethanol into non-or less-reactive species by the catalytic overlayer was dominantover the enhancement of the gas response via the gas reformingeffect.38,39 Accordingly, coating with the catalytic oxideoverlayer and control of its thickness provide a powerful toolfor tuning the gas selectivity as well as the gas response.A Co3O4 sensor coated with 5 nm of WO3 was also

fabricated, and its sensing characteristics were measured inorder to understand the role of the catalytic overlayer in the gassensing reaction (Figure S7). Coating with the WO3 overlayerhardly influenced the gas response and selectivity. This suggeststhat the materials of the overlayer should be carefully chosenbased on the catalytic activity for gas reforming and oxidation.The 90% response time (τres) and 90% recovery time (τrecov),

i.e., the time to reach 90% variation in the resistance uponexposure of the Co3O4, 5TiO2/Co3O4, and 5SnO2/Co3O4sensors to the analyte gas and air were calculated from thesensing transients in Figure S6 at 250 °C. The τrecov values ofthe 5TiO2/Co3O4 and 5SnO2/Co3O4 sensors upon exposure to5 ppm ethanol (112 and 104 s) are similar to that of the Co3O4sensor (99 s). In stark contrast, the τres values of the 5TiO2/Co3O4 and 5SnO2/Co3O4 sensors upon exposure to 5 ppmtoluene and xylene (216−317 s) were significantly higher thanthose of the Co3O4 sensor (73−78 s). This strongly suggests

Figure 4. (a) Toluene selectivity (ST/SE) and (b) xylene selectivity(SX/SE) of the Co3O4, 2TiO2/Co3O4, 5TiO2/Co3O4, and 20TiO2/Co3O4 sensors and (c and d) those of 2SnO2/Co3O4, 5SnO2/Co3O4,and 20SnO2/Co3O4 sensors at 200, 225, 250, 275, and 300 °C,respectively.

Figure 5. Schematic diagram of mechanism of catalytic promotion of(a−c) xylene and (d−e) ethanol sensing at 250 °C via gas reformingand oxidative filtering induced by coating with (b-1 and d-1) 2 nmTiO2, (b-2 and d-2) 5 nm TiO2, (b-3 and d-3) 20 nm TiO2, (c-1 ande-1) 2 nm SnO2, (c-2 and e-2) 5 nm SnO2, and (c-3 and e-3) 20 nmSnO2 catalytic hetero-overlayer.

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that the catalytic TiO2 or SnO2 overlayer promotes thereforming of toluene and xylene.When sensing electrodes are located at the lower part of a

film and the catalytic oxide overlayer is coated on the sensingfilm, the response to a reducing gas is determined by (1)reforming/oxidation of the analyte gases on the catalyticoverlayer, (2) transport of the analyte gas or reformed gas tothe lower part of the sensing film close to the electrodes, and(3) the surface reaction between the gas and negatively chargedadsorbed oxygen. The TiO2 and SnO2 overlayers were not onlyvery thin (≤20 nm) compared to the sensing film (∼5 μm) butalso showed noncontinuous configurations, plausibly due toembossed top morphology of Co3O4 sensing film (Figure 2e,f).Moreover, all the sensors showed similar size of Co3O4 yolk−shell spheres, film thickness (∼5 μm), and packing config-uration. In this respect, the effect of gas transport to the lowersensing part can be excluded as a key parameter influencing thegas response.The oxide semiconductor gas sensors generally show bell-

shaped gas responses as a function of the sensor temperature.The lower gas response at low sensing temperature is attributedto the difficulties in the surface reaction between the reducinggas and negatively charged adsorbed oxygen and/or lowerdegree of oxygen adsorption. If the sensing temperaturebecomes too high, then the extent of oxygen adsorption alsodecreases and oxidative filtering of the analyte gas becomesmore dominant during gas diffusion through the catalyticallyactive sensing film although the surface reaction between thegas and adsorbed oxygen is thermally activated, leading to a lowgas response. Thus, the gas response shows the maximum valueat moderate sensing temperatures. In the present study, theresponses of the sensors to xylene, toluene, and ethanoldecreased monotonously with the sensing temperature (200−300 °C), indicating that the gas response is not governed by thesurface reaction between the gas and adsorbed oxygen.Accordingly, the variation of gas response in the present

study should be understood in the framework of reforming/oxidation of the analyte gases on the catalytic overlayer.Reforming or oxidation of analyte gases is dependent not onlyon the catalytic overlayer, but also on the sensing temperature.For example, the response of the Co3O4 sensor to 5 ppmethanol at 200 °C (18.5) increased dramatically to 101.7 and332.1 upon coating with 2 nm thick TiO2 and SnO2 overlayers,respectively. In contrast, the response of the Co3O4 sensor to 5ppm ethanol at 300 °C (1.7) decreased to 1.36 and 1.39 byintroducing the same overlayers. These suggest that thecatalytic overlayers enhance the gas response by promotingthe reforming of ethanol into more reactive species at lowsensing temperature (200 °C) but suppress the gas response bypromoting the oxidation of ethanol into nonreactive species athigh sensing temperature (300 °C). It should be noted that theresponse of the 2TiO2/Co3O4 and 2SnO2/Co3O4 sensors toethanol becomes negligibly low beyond 250 °C and that of the20TiO2/Co3O4 and 20SnO2/Co3O4 sensors declines beyond225 °C. This clearly shows that the oxidative filtering effectbecomes predominant when the catalytic overlayer is thicker orthe sensing temperature is higher.The SX and ST values of the 2TiO2/Co3O4 sensor (80.9 and

61.4) and 2SnO2/Co3O4 sensor at 200 °C (139.8 and 63.7)were also significantly higher than those of the Co3O4 sensor(9.0 and 8.1), indicating enhancement of the gas response dueto the gas reforming effect. This decrease in the gas responsewith increasing sensor temperature can be understood as

strengthening of the oxidative filtering effect. The temperatureat which the SX and ST values became negligible was 275 °C forthe 2TiO2/Co3O4 and 2SnO2/Co3O4 sensors, which is higherthan that at which the SE values became negligible, i.e., 250 °C.Thus, the highest selectivity to xylene and toluene can beobtained at 250 °C. Note that the temperature for achievingnegligible SX and ST values still remained high (275 °C for the20TiO2/Co3O4 and 20SnO2/Co3O4 sensors) and was signifi-cantly higher than that at which the SE value became negligible(225 °C) for the same sensors. The high gas responses toxylene and toluene even at high sensing temperature and with athicker overlayer, leading to high selectivity to methylbenzenes,can be attributed to the promotion of gas reforming with lessoxidative filtering. This is feasible considering that xylene ismore stable against oxidation than ethanol. Therefore, theoverlayer materials for promoting gas reforming reaction of theanalyte gas and oxidation reaction of the interference gasshould be chosen based on their catalytic activity, and thecatalytic promotion can be controlled either by varying theoverlayer thickness or the sensing temperature.In the literature, a two-layer configuration of sensors has

been explored for controlling the gas sensing characteristics viagas reforming or oxidative filtering. Masuda et al.18 reportedthat the coating of SnO2 nanosheets with (101) crystal faces ona Pt:Pd:Au-loaded SnO2 sensing film significantly enhanced theresponse to 10 ppm 1-nonanal. Considering the sensorconfiguration, this can be attributed to the gas reformingwithin catalytic SnO2 nanosheets overlayer with preferredorientation. Hubalek20 reported that the response of a WO3sensor to benzene was significantly increased by coating with aPt-loaded Al2O3 catalytic overlayer, whereas the responses toethanol, NH3, and NO2 remained similar. Finally, one of thepresent authors19 demonstrated that the responses of the Pd-loaded SnO2 sensors to benzene can be enhanced by coatingwith a Co3O4 overlayer. These observations are also in line withthe gas reforming effect.Suppression of the cross-responses to interference gases by

oxidative filtering has also been reported. Fleischer et al.17

dramatically suppressed the cross-responses of dense and thin(∼0.2 μm) Ga2O3 sensors to ethanol and acetone withoutsacrificing the response to methane by coating with a porousand thick (∼300 μm) Ga2O3 overlayer. This demonstrates thateven the same material can be employed as an oxidativefiltering layer if the reaction between the material surface andgas is maximized during transport of the gas through the thickand highly gas-accessible porous overlayer. Sahm et al.21

prepared a SnO2 sensing film without and with a Pd-loadedAl2O3 overlayer by flame spray pyrolysis and observedsuppression of the responses to CO and ethanol at 400 °Cwithout significant variation in the response to methane.Note that gas reforming is generally effective for increasing

the response to a specific gas but is limited in terms of thedecrease in the cross-responses to other interference gases. Incontrast, it is difficult to increase the response to the targetanalyte gas only by oxidative filtering. Thus, the combination ofgas reforming and oxidative filtering is advantageous. Forexample, Rebholz et al.22 reported that coating a 3 wt % Pd-SnO2 overlayer on a 1 wt % Sb/0.01 wt % Pt-SnO2 sensing filmincreased the ethanol response but decreased the CO response.Although this demonstrates the possibility for discriminationbetween ethanol and CO, the design of highly selective andsensitive gas sensors by simultaneous gas reforming andfiltering remains in the nascent stage. From this perspective,

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the increase in the response via gas reforming while suppressingthe cross-responses to interference gases via oxidative filteringachieved in the present study provides a promising direction forthe design of highly selective and sensitive gas sensors.As mentioned before, the overlayer material for gas

reforming and oxidative filtering can be the same as thesensing materials if their microstructure17 or crystallographicorientation18 are different. However, only limited control of thecatalytic activity of the overlayer can be achieved bymanipulating the crystallinity and diffusion path. If sensingand overlayer materials with different catalytic activity are used,then as demonstrated in the present study synergistic catalyticpromotion can be obtained and the selectivity can be moresystematically tuned by varying parameters such as theoverlayer materials, overlayer thickness, and sensing temper-ature. Moreover, the numerous combinations of sensing andoverlayer materials can provide a new, powerful, and versatilemethod for designing high-performance gas sensors with highlytunable selectivity. Note that we have only limited number ofsensing materials, but should detect a vast number of differentchemical species. From this perspective, various combinationsof sensing and overlayer materials are promising.The 5TiO2/Co3O4 and 5SnO2/Co3O4 sensors showed

reversible and reliable sensing properties upon repetitiveexposure to analyte gases (Figure S8a) and exhibited stablesensing characteristics over 30 days (Figure S8b). Deposition ofa SnO2 or TiO2 overlayer on Co3O4 sensing films provides anew strategy for designing highly selective and sensitivemethylbenzene sensors and an effective algorithm for tuningthe gas selectivity via simultaneous control of the gas reformingand gas oxidation reactions.

4. CONCLUSIONS

Highly selective methylbenzene sensors were designed bycoating a nanoscale TiO2 or SnO2 overlayer on a Co3O4 film.The selectivity of the Co3O4 sensor to methylbenzene could besystematically controlled by introducing a catalytic hetero-overlayer and by precise control of the overlayer thickness andsensing temperature. The highly selective and sensitivedetection of methylbenzenes was attributed to the reformingof less reactive methylbenzenes into more active and smallerspecies and the oxidation of more reactive interference gasesinto non- or less-reactive CO2 and H2O. Concurrent control ofthe gas reforming and oxidative filtering assisted by a synergisticcombination of sensing and overlayer materials with differentcatalytic activity can pave new way for the design of high-performance gas sensors with high selectivity, excellentsensitivity, and new functionality.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.7b13998.

N2 adsorption/desorption and pore-size distribution ofCo3O4 yolk−shell spheres; cross-sectional images of2TiO2/Co3O4, 20TiO2/Co3O4, 2SnO2/Co3O4, and20SnO2/Co3O4 sensing films; TOF-SIMS data, TEMimage, diffraction pattern, and dynamic sensing transientfor Co3O4, 5TiO2/Co3O4, and 5SnO2/Co3O4 sensors;comparison of gas-sensing characteristics of Co3O4,5TiO2/Co3O4, 5SnO2/Co3O4, and 5WO3/Co3O4 sen-

sors; long-term stability of 5TiO2/Co3O4 and 5SnO2/Co3O4 sensors to 5 ppm xylene (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: jongheun@korea.ac.kr. Fax.: +82-2-928-3584. Tel.:+82-2-3290-3282.

ORCIDHo Won Jang: 0000-0002-6952-7359Jong-Heun Lee: 0000-0002-3075-3623NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is supported by the Deanship of Scientific Research(DSR), King Abdulaziz University (KAU), under grant No. 2-135-36-HiCi, a National Research Foundation of Korea (NRF)grant funded by the Korea Government (MEST) (No.2016R1A2A1A05005331), and Development of intelligentceramic electrode/catalyst for Pt reducing in exhaust gassensor by nonstoichiometric oxide material (No. 10062222)funded by Korea Government (MOTIE). The authors,therefore, acknowledge the technical and financial support ofKAU, MEST, and MOTIE.

■ REFERENCES(1) Yamazoe, N. Toward Innovations of Gas Sensor Technology.Sens. Actuators, B 2005, 108, 2−14.(2) Franke, M. E.; Koplin, T. J.; Simon, U. Metal and Metal OxideNanoparticles in Chemiresistors: Does the Nanoscale Matter? Small2006, 2, 36−50.(3) Lee, J.−H. Gas Sensors using Hierarchical and Hollow OxideNanostructures: Overview. Sens. Actuators, B 2009, 140, 319−336.(4) Kolmakov, A.; Klenov, D. O.; Lilach, Y.; Stemmer, S.; Moskovits,M. Enhanced Gas Sensing by Individual SnO2 Nanowires andNanobelts Functionalized with Pd Catalyst Particles. Nano Lett.2005, 5, 667−673.(5) Li, K.−M.; Li, Y.−J.; Lu, M.−Y.; Kuo, C.−I.; Chen, L.−J. DirectConversion of Single-Layer SnO Nanoplates to Multi-Layer SnO2Nanoplates with Enhanced Ethanol Sensing Properties. Adv. Funct.Mater. 2009, 19, 2453−2456.(6) Hoa, N. D.; El-Safty, S. A. Highly Sensitive and Selective VolatileOrganic Compound Gas Sensors Based on Mesoporous Nano-composite Monoliths. Anal. Methods 2011, 3, 1948.(7) Hoa, N. D.; El-Safty, S. A. Gas Nanosensor Design PackagesBased on Tungsten Oxide: Mesocages, Hollow Spheres, andNanowires. Nanotechnology 2011, 22, 485503.(8) Liu, Y.; Zhu, G.; Ge, B.; Zhou, H.; Yuan, A.; Shen, X. ConcaveCo3O4 Octahedral Mesocrystal: Polymer-Mediated Synthesis andSensing Properties. CrystEngComm 2012, 14, 6264−6270.(9) Jing, Z.; Zhan, J. Fabrication and Gas-Sensing Properties ofPorous ZnO Nanoplates. Adv. Mater. 2008, 20, 4547−4551.(10) Rai, P.; Yoon, J.−W.; Kwak, C.−H.; Lee, J.−H. Role of PdNanoparticles in Gas Sensing Behaviour of Pd@In2O3 Yolk-ShellNanoreactors. J. Mater. Chem. A 2016, 4, 264−269.(11) Kim, S.−J.; Hwang, I.−S.; Na, C. W.; Kim, I.−D.; Kang, Y. C.;Lee, J.−H. Ultrasensitive and Selective C2H5OH Sensors using Rh-loaded In2O3 Hollow Spheres. J. Mater. Chem. 2011, 21, 18560−18567.(12) Woo, H.−S.; Kwak, C.−H.; Kim, I.−D.; Lee, J.−H. Selective,Sensitive, and Reversible Detection of H2S using Mo-doped ZnONanowire Network Sensors. J. Mater. Chem. A 2014, 2, 6412−6418.(13) Jeong, H.−M.; Kim, J.−H.; Jeong, S.−Y.; Kwak, C.−H.; Lee, J.−H. Co3O4−SnO2 Hollow Heteronanostructures: Facile Control of Gas

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.7b13998ACS Appl. Mater. Interfaces 2017, 9, 41397−41404

41403

Selectivity by Compositional Tuning of Sensing Materials via GalvanicReplacement. ACS Appl. Mater. Interfaces 2016, 8, 7877−7883.(14) Jinkawa, T.; Sakai, G.; Tamaki, J.; Miura, N.; Yamazoe, N.Relationship Between Ethanol Gas Sensitivity and Surface CatalyticProperty of Tin Oxide Sensors Modified with Acidic or Basic Oxides. J.Mol. Catal. A: Chem. 2000, 155, 193−200.(15) Cho, Y. H.; Kang, Y. C.; Lee, J.−H. Highly Selective andSensitive Detection of Trimethylamine using WO3 Hollow SpheresPrepared by Ultrasonic Spray Pyrolysis. Sens. Actuators, B 2013, 176,971−977.(16) Jansat, S.; Pelzer, K.; Garcia-Anton, J.; Raucoules, R.; Philippot,K.; Maisonnat, A.; Chaudret, B.; Guari, Y.; Mehdi, A.; Reye, C.; Corriu,R. J. P. Synthesis of New RuO2@SiO2 Composite Nanomaterials andTheir Application as Catalytic Filters for Selective Gas Detection. Adv.Funct. Mater. 2007, 17, 3339−3347.(17) Fleischer, M.; Kornely, S.; Weh, T.; Frank, J.; Meixner, H.Selective Gas Detection with High-Temperature Operated MetalOxides using Catalytic Filters. Sens. Actuators, B 2000, 69, 205−210.(18) Masuda, Y.; Itoh, T.; Shin, W.; Kato, K. SnO2 Nanosheet/Nanoparticle Detector for the Sensing of 1-Nonanal Gas Produced byLung Cancer. Sci. Rep. 2015, 5, 10122.(19) Jeong, S.−Y.; Yoon, J.−W.; Kim, T.−H.; Jeong, H.−M.; Lee,C.−S.; Kang, Y. C.; Lee, J.−H. Ultra-selective Detection of Sub-ppm-level Benzene using Pd-SnO2 Yolk-Shell Micro-Reactors with aCatalytic Co3O4 Overlayer for Monitoring Air Quality. J. Mater.Chem. A 2017, 5, 1446−1454.(20) Hubalek, J.; Malysz, K.; Prasek, J.; Vilanova, X.; Ivanov, P.;Llobet, E.; Brezmes, J.; Correig, X.; Sverak, Z. Pt-loaded Al2O3

Catalytic Filters for Screen-printed WO3 Sensors Highly Selective toBenzene. Sens. Actuators, B 2004, 101, 277−283.(21) Sahm, T.; Rong, W.; Barsan, N.; Madler, L.; Friedlander, S. K.;Weimar, U. Formation of Multilayer Films for Gas Sensing by in situThermophoretic Deposition of Nanoparticles from Aerosol Phase. J.Mater. Res. 2007, 22, 850−857.(22) Rebholz, J.; Grossmann, K.; Pham, D.; Pokhrel, S.; Madler, L.;Weimar, U.; Barsan, N. Selectivity Enhancement by Using Double-Layer MOX-Based Gas Sensors Prepared by Flame Spray Pyrolysis(FSP). Sensors 2016, 16, 1437.(23) Lu, Y.; Zhan, W.; He, Y.; Wang, Y.; Kong, X.; Kuang, Q.; Xie, Z.;Zheng, L. MOF-Templated Synthesis of Porous Co3O4 ConcaveNanocubes with High Specific Surface Area and Their Gas SensingProperties. ACS Appl. Mater. Interfaces 2014, 6, 4186−4195.(24) Yoon, J.−W.; Hong, Y. J.; Park, G. D.; Hwang, S.−J.; Abdel-Hady, F.; Wazzan, A. A.; Kang, Y. C.; Lee, J.−H. Kilogram-scaleSynthesis of Pd-Loaded Quintuple-shelled Co3O4 Microreactors andTheir Application to Ultrasensitive and Ultraselective Detection ofMethylbenzenes. ACS Appl. Mater. Interfaces 2015, 7, 7717−7723.(25) Wen, Z.; Zhu, L.; Li, Y.; Zhang, Z.; Ye, Z. Mesoporous Co3O4

Nanoneedle Arrays for High-performance Gas Sensor. Sens. Actuators,B 2014, 203, 873−879.(26) Wachs, I. E.; Saleh, R. Y.; Chan, S. S.; Chersich, C. C. TheInteraction of Vanadium Pentoxide with Titania (Anatase): Part I.Effect on o-Xylene Oxidation to Phthalic Anhydride. Appl. Catal. 1985,15, 339−352.(27) Ameen, M. M.; Raupp, G. B. Reversible Catalyst Deactivation inthe Photocatalytic Oxidation of Dilute o-Xylene in Air. J. Catal. 1999,184, 112−122.(28) Blanco, J.; Avila, P.; Bahamonde, A.; Alvarez, E.; Sanchez, B.;Romero, M. Photocatalytic Destruction of Toluene and Xylene at GasPhase on a Titania Based Monolithic Catalyst. Catal. Today 1996, 29,437−442.(29) Cao, L.; Gao, Z.; Suib, S. L.; Obee, T. N.; Hay, S. O.; Freihaut, J.D. Photocatalytic Oxidation of Toluene on Nanoscale TiO2 Catalysts:Studies of Deactivation and Regeneration. J. Catal. 2000, 196, 253−261.(30) Matralis, H. K.; Papadopoulou, C.; Kordulis, C.; Elguezabal, A.A.; Corberan, V. C. Selective Oxidation of Toluene over V2O5/TiO2

Catalysts. Effect of Vanadium Loading and of Molybdenum Additionon the Catalytic Properties. Appl. Catal., A 1995, 126, 365−380.

(31) Fang, C.; Wang, S.; Wang, Q.; Liu, J.; Geng, B. Coralloid SnO2with Hierarchical Structures and Their Application as Recoverable GasSensors for the Detection of Benzaldehyde/Acetone. Mater. Chem.Phys. 2010, 122, 30−34.(32) Fruhberger, B.; Stirling, N.; Grillo, F. G.; Ma, S.; Ruthven, D.;Lad, R. J.; Frederick, B. G. Detection and Quantification of NitricOxide in Human Breath Using a Semiconducting Oxide BasedChemiresistive Microsensor. Sens. Actuators, B 2001, 76, 226−234.(33) Wolkoff, P.; Nielsen, G. D. Organic Compounds in Indoor Air−Their Relevance for Perceived Indoor Air Quality? Atmos. Environ.2001, 35, 4407−4417.(34) Li, W.−Y.; Xu, L.−N.; Chen, J. Co3O4 Nanomaterials inLithium-ion Batteries and Gas Sensors. Adv. Funct. Mater. 2005, 15,851−857.(35) Liu, Y.; Koep, E.; Liu, M. A Highly Sensitive and Fast-responding SnO2 Sensor Fabricated by Combustion Chemical VaporDeposition. Chem. Mater. 2005, 17, 3997−4000.(36) Wang, L.; Fei, T.; Lou, Z.; Zhang, T. Three-dimensionalHierarchical Flowerlike α-Fe2O3 Nanostructures: Synthesis andEthanol-sensing Properties. ACS Appl. Mater. Interfaces 2011, 3,4689−4694.(37) Sun, X.; Hao, H.; Ji, H.; Li, X.; Cai, S.; Zheng, C. NanocastingSynthesis of In2O3 with Appropriate Mesostructured Ordering andEnhanced Gas-Sensing Property. ACS Appl. Mater. Interfaces 2014, 6,401−409.(38) Kim, W. J.; Lee, S. W.; Sohn, Y. Metallic Sn Spheres and SnO2@C Core-shells by Anaerobic and Aerobic Catalytic Ethanol and COOxidation Reactions over SnO2 Nanoparticles. Sci. Rep. 2015, 5,13448.(39) Kim, K. S.; Barteau, M. A. Reactions of Aliphatic Alcohols onthe {011}-Facetted TiO2 (001) Surface. J. Mol. Catal. 1990, 63, 103−117.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.7b13998ACS Appl. Mater. Interfaces 2017, 9, 41397−41404

41404