Co3O4âSnO2 Hollow Heteronanostructures: Facile Control of Gas

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Co3O4-SnO2 hollow hetero-nanostructures: Facile control of gas selectivity by compositional tuning of sensing materials via galvanic replacement Hyun-Mook Jeong, Jae-Hyeok Kim, Seong-Yong Jeong, Chang-Hoon Kwak, and Jong-Heun Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00216 • Publication Date (Web): 11 Mar 2016 Downloaded from http://pubs.acs.org on March 14, 2016

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ACS Applied Materials & Interfaces

Co3O4-SnO2 hollow hetero-nanostructures: Facile control of gas selectivity by compositional tuning of sensing materials via galvanic replacement Hyun-Mook Jeong,† Jae-Hyeok Kim,† Seong-Yong Jeong,† Chang-Hoon Kwak,† and Jong-Heun Lee*,† †

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

Korea *Author to whom correspondence should be addressed Email: [email protected]; Fax: +82-2-928-3584; Tel: +82-2-3290-3282 Keywords: galvanic replacement, gas sensor, methyl benzene, Co3O4, SnO2, heterostructure.

ABSTRACT Co3O4 hollow spheres prepared by ultrasonic spray pyrolysis were converted into Co3O4−SnO2 core-shell hollow spheres by galvanic replacement with subsequent calcination at 450 oC for 2 h for gas sensor applications. Gas selectivity of the obtained spheres can be controlled by varying the amount of SnO2 shells (between 14.6, 24.3, and 43.3 at.%) and sensor temperatures. Co3O4 sensors possess an ability to selectively detect ethanol at 275 oC. When the amount of SnO2 shells was increased to 14.6 and 24.3 at.%, highly selective detection of xylene and methylbenzenes (xylene + toluene) was achieved at 275 oC and 300 oC, respectively. Good selectivity of Co3O4 hollow spheres to ethanol can be explained by a catalytic activity of Co3O4; while high selectivity of Co3O4−SnO2 core-shell hollow spheres to methylbenzenes is attributed

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to a synergistic effect of catalytic SnO2 and Co3O4 and promotion of gas sensing reactions by a pore-size control of microreactors.

1. INTRODUCTION Oxide hetero-nanostructures provide unique physico-chemical properties for the applications of photocatalysts,1,2 photovoltaic devices,3-5 and sensors6 due to a synergistic combination of different materials. In oxide semiconductor gas sensors, oxide nanostructures such as nanowires, nanosheets, and nanospheres are coated with either a discrete ensemble of nanoclusters or continuous nano-scale overlayer to improve their sensing characteristics. As possible means to control conduction in chemiresistors and promote specific gas sensing reactions, which significantly enhance gas response and selectivity, p-n, p-p, and n-n hetero-nanojunctions with different morphological configurations were investigated.7-13 Various physicochemical techniques have been used to prepare hetero-nanostructures, which include metal-organic vapor phase epitaxy,14 sputtering,15 atomic layer deposition,16 solutionbased coating,17 and galvanic replacement.18 Among these, galvanic replacement is a facile chemical route to prepare hetero-nanostructures by replacing the host cations with other cations of stronger ionization tendency dissolved in solution.19 Oxide/metal/organic heteronanostructures prepared by galvanic replacement have been applied to electrode materials for Li ion batteries,18,19 electrochemical bio sensors,20 and electrocatalysts.21,22 Uniform and gradual substitution that minimizes changes in host nano-architectures is advantageous for manufacturing highly gas accessible oxide hetero-nanostructures and for controlling the composition. However,


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to the best knowledge of the authors, galvanic replacement has not been widely used to produce high-performance gas sensors. In this work, highly gas accessible Co3O4 hollow spheres were prepared by ultrasonic spray pyrolysis and then converted into Co3O4−SnO2 core-shell hollow spheres by a galvanic replacement with Sn4+ ions. High selectivity to ethanol was observed for single-phase Co3O4 hollow spheres, while Co3O4−SnO2 spheres were characterized by high selectivity to xylene and/or methylbenzenes. The reasons for changes in gas selectivity were investigated in relation to compositional, morphological, and structural variations of hollow spheres during galvanic replacement.


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2. EXPERIMENTAL Preparation of Co3O4 hollow spheres. Single-phase Co3O4 hollow spheres were prepared by ultrasonic spray pyrolysis. 9.70 g of cobalt (II) nitrate hexahydrate (Co(NO3)2 ⋅6H2O, 99.999% trace metal basis, Sigma-Aldrich) and 3.94 g of citric acid monohydrate (C6H8O7⋅H2O, ACS reagent ≥ 99.0%, Sigma-Aldrich) were dissolved in 800 mL of distilled water. The obtained mixture was stirred at 25 oC until the solution became homogenized, which was used for spray solution. Citric acid was added to develop hollow morphology during ultrasonic spray pyrolysis.23 An experimental system of ultrasonic spray pyrolysis consisted of an ultrasonic nebulizer, a quartz tube inside an electric furnace, and a particle-collecting chamber. Six ultrasonic transducers with resonance frequencies of 1.7 MHz generated a large number of droplets. The droplets were then transferred by a carrier gas (oxygen, 20 L min-1) into the quartz tube (with an inner diameter of 55 mm) of the electric furnace heated up to 800 oC, which were converted into Co3O4 hollow spheres by a spray pyrolysis reaction. The obtained powder was placed on a Teflon bag filter in the particle-collecting chamber heated to 250 °C to protect the powder from water condensation. Co3O4-SnO2 core-shell hollow spheres attained by galvanic replacement reaction: To investigate the effects of SnO2 coatings of Co3O4 hollow spheres on their gas-sensing characteristics, Co3O4−SnO2 core-shell hollow spheres were prepared by galvanic replacement according to the procedure suggested by Oh et al.18 0.06 g of Co3O4 hollow spheres were dispersed in 15 mL of xylenes (C6H4(CH3)2, ACS reagent, ≥ 98.5% xylenes + ethylbenzene basis, Sigma-Aldrich) and stirred for 2 h. and then 0.14 g of oleic acid (C17H33COOH, technical grade 90%, Sigma-Aldrich) and 2.00 g of oleylamine (C18H35NH2, technical grade 70%, SigmaAldrich) are added to the Co3O4 slurry. Three different solutions of Sn (II) chloride (SnCl2,


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99.99% trace metal basis, Sigma-Aldrich) were prepared by adding 0.2, 0.4, and 1.6 mL of 2 M Sn (II) chloride aqueous solutions to the mixtures containing 1 mL of distilled water and 0.4 mL of hydrochloric acid (HCl, ACS reagent, 35 – 37.0 %, Samchun). Co3O4 powders were added to the above SnCl2 solutions ([Sn]/([Co]+[Sn]) ratios: 6.7, 13.4, and 53.5 at.%). The slurries containing Co3O4 hollow particles and SnCl2 aqueous solutions were heated to 90 °C in ambient air and kept at that temperature for 2 h to convert the Co3O4 hollow spheres into Sn-precursorcoated Co3O4 hollow spheres by a galvanic replacement reaction. After washing 5 times with ethanol, the Sn-precursor-coated Co3O4 hollow spheres were converted into Co3O4−SnO2 coreshell hollow spheres by a heat treatment at 450 °C for 2 h in ambient air. The Sn fractions of the Co3O4−SnO2 core-shell hollow spheres prepared from the solutions with [Sn]/([Co]+[Sn]) ratios of 6.7, 13.4, and 53.5 at.% were measured by inductively coupled plasma mass spectroscopy (ICP-AES); the obtained values were 14.6, 24.3, and 43.3 at.%. For simplicity, Co3O4-SnO2 (14.6, 24.3, and 43.3 at%) core-shell hollow spheres after heat treatment are herein referred to as “Co3O4-0.15S”, “Co3O4-0.24S”, and “Co3O4-0.43S”, respectively. Characterization and gas-sensing measurements. The prepared specimens were used for gas sensor measurements. Crystallinity and different phases of the Co3O4 hollow spheres and Co3O4−SnO2 core-shell hollow spheres were analyzed by X-ray diffraction (XRD, CuKα, D/MAX-2500V/PC, Rigaku, Japan), whereas morphologies of the precursors and specimens were investigated by field-emission scanning electron microscopy (FE-SEM, S-4900, Hitachi Co. Ltd., Japan) and high-resolution transmission electron microscopy (HR-TEM, TALOS F200X, FEI Co. Ltd., USA). Specific surface areas of the prepared powders were determined by a Brunauer–Emmett–Teller (BET) method (Tristar 3000, Micromeritics Co. Ltd., USA), and chemical states of the single-phase Co3O4 hollow spheres and Co3O4-SnO2 core-shell hollow


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spheres were examined by X-ray photoelectron spectroscopy (XPS, Multilab 2000, Thermo Fisher Scientific Inc., USA). Sn concentrations in the Co3O4−SnO2 core-shell hollow spheres were determined by ICP-AES (720 ICP-AES, Agilent Technologies Co. Ltd., USA). Alumina substrates (area of 1.5 × 1.5 mm2; thickness of 0.25 mm) with two Au electrodes (with widths of 1 mm and separation of 0.2 mm) on their top surfaces and microheaters on their bottom surfaces were used as supports. Single-phase Co3O4 hollow spheres and Co3O4−SnO2 core-shell hollow spheres were dispersed in deionized water, and the resulting slurries were drop-coated onto the Au electrodes using a micropipette. Sensing temperatures were measured using an IR temperature sensor (Metis MP25, Sensortherm GmbH., Germany) and controlled by the microheaters located underneath the alumina substrates. The sensors were contained in specially designed, low-volume (1.5 cm3) quartz tubes to minimize delays in changing their surrounding atmospheres. Sensor elements were annealed at 400 °C for 24 h to obtain thermally stable sensors at required sensing temperatures (275 – 375 °C) and remove any traces of remaining solvents. Gas responses (S = Rg/Ra for reducing gas; Rg: resistance in target gas, Ra: resistance in air) to 5 ppm of acetone (C3H6O), benzene (C6H6), carbon monoxide (CO), ethanol (C2H5OH), formaldehyde (HCHO), toluene (C6H5CH3), and p-xylene (1,4-dimethylbenzene, C6H4(CH3)2) were measured at 275 − 375 °C. Gas concentrations were controlled by changing mixing ratios between parent gases (5 ppm of acetone, benzene, carbon monoxide, ethanol, formaldehyde, toluene, and p-xylene in dry air balance) and dry synthetic air. DC two-probe resistances were measured using an electrometer connected to a computer.


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3. RESULTS AND DISCUSSION Characterization of Co3O4 hollow spheres and Co3O4−SnO2 core-shell hollow spheres. Co3O4 hollow spheres are characterized by a cubic Co3O4 (JCPDS no. 42-1467) phase (Figure 1a). A tetragonal SnO2 (JCPDS no. 41-1445) phase was observed for the Co3O4−0.24S and Co3O4−0.43S powders (Figure 1c,d) owing to an increase in the SnO2 amount with increasing the SnCl2 concentration for galvanic replacement, which indicates that the composition of the Co3O4−SnO2 composite powders can be tuned by a galvanic replacement reaction. For all specimens, neither secondary phases nor peak shifts were observed except for the Co3O4 and SnO2 phases indicating that these phases exist in the mixed form. The crystalline sizes of the Co3O4 phases calculated from the (311), (511) and (440) peaks by using the Scherrer’s formula were found to be 13.4 ± 0.8, 13.7 ± 0.7, 13.2 ± 1.3, and 13.5 ± 0.9 nm for the Co3O4, Co3O4−0.15S, Co3O4−0.24S, and Co3O4−0.43S powders, respectively. All the Co3O4 and Co3O4−SnO2 specimens revealed a spherical morphology after spray pyrolysis and subsequent heat treatment at 450 °C for 2 h (Figure 2). The diameters of the spheres ranged from 0.4 µm to 2 µm, and no significant changes in diameters were found with variations in sample compositions. A hollow morphology was frequently observed from broken spheres in all specimens (see arrows in Figure 2). In TEM images, all the Co3O4, Co3O4−0.15S, Co3O4−0.24S and Co3O4−0.43S spheres showed bright contours around the inner parts of the spheres (Figure 3a-d). This confirms their hollow nature. The shell thicknesses for the Co3O4−0.15S, Co3O4−0.24S, and Co3O4−0.43S hollow spheres were almost the same (around 30 nm). The elemental mapping analysis shows a uniform distribution of the Co and Sn elements inside the Co3O4−0.24S and Co3O4−0.43S spheres (Figure 3e,f,i,j). The Sn concentrations of the


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spheres tend to increase with increasing the Sn concentration in solution without variations in shell thickness, indicating a gradual replacement of Co sites with Sn species through a galvanic reaction. To analyze local concentrations of the Sn and Co components, cross-sections for the Co3O4−0.24S and Co3O4−0.43S specimens were obtained by focused ion beam treatment (Figure 3g,k). In both specimens, the compositional profiles of the Sn and Co elements obtained by energy dispersive spectroscopy (EDS) line scanning show Sn-rich compositions for the outer parts of the shells (Figure 3h,l), confirming the core-shell configuration of the Co3O4−0.24S and Co3O4-0.43S hollow spheres and suggesting that a galvanic replacement reaction begins at the outer surfaces of the Co3O4 hollow spheres. Gas-sensing characteristics of the Co3O4 and Co3O4−SnO2 core-shell hollow spheres. The dynamic sensing transients of the single-phase Co3O4 and Co3O4−SnO2 core-shell hollow spheres to 5 ppm of ethanol, toluene, and xylene at 300 °C are shown in Figure 4. All the sensors exhibit gas sensing characteristics of p-type metal oxide semiconductors corresponding to an increase in resistance upon exposure to reducing gases, which indicates that conduction along continuous p-type Co3O4 phases dominates sensing characteristics. The gas sensing characteristics of the sensors to 5 ppm of ethanol, toluene, xylene, acetone, formaldehyde, carbon monoxide and benzene were measured at 275-375 °C (Figure 5). The ethanol response is the highest at 275 °C (Rg/Ra = 13.5) and 300 °C (Rg/Ra = 9.9) in Co3O4 sensors (Figure 5a), which becomes similar to the responses to xylene, toluene, formaldehyde, and acetone at ≥ 325 °C. In a stark contrast, the Co3O4−0.15S and Co3O4−0.24S sensors reveal the highest response to 5 ppm of xylene (Rg/Ra = 15.6 and 18.6) and second highest response to toluene (Rg/Ra = 6.2 and 7.0, respectively) at 275 °C (Figure 5b,c). At 300 °C, the responses to 5 ppm of xylene and toluene become similar (Figure 5b) or comparable (Figure 5c), while those to


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other interference gases remain low (Figure 5b,c). Finally, the responses to xylene and toluene become similar to those to other interference gases at ≥ 325 °C. Further replacements of Co3O4 spheres with SnO2 significantly deteriorate their response and selectivity to xylene, toluene, and ethanol (Figure 5d). The polar plots of the gas responses suggest that selective detection of xylene can be achieved by operating the Co3O4−0.15S and Co3O4−0.24S sensors at 275 °C, whereas selective detection of methylbenzenes (xylene and/or toluene) is possible by operating the same sensors at 300 °C. Operating the single-phase Co3O4 sensors at 275 °C can be used to detect ethanol in a selective manner, which indicates that tuning of sensing materials by galvanic replacement and varying sensor temperatures can be used to control gas selectivity to representative indoor air pollutants. In our previous works, we have suggested that gas responses of ZnO nanowire networks can be significantly enhanced by decorating n-type ZnO nanowires with p-type nanoclusters such as NiO,24 Mn3O4,25 and Cr2O3.26 We also reported enhanced gas responses by decorating p-type NiO hollow spheres with n-type In2O3 nanoclusters.27 The observed increases in gas responses were explained by radial extensions of electron depletion layers in ZnO nanowires beneath the ptype nanoclusters and by a decrease in hole concentrations in hole accumulation layers of NiO hollow spheres beneath the n-type In2O3 nanoclusters. In both cases, the Ra values were significantly increased by the decoration of host material with p-type/n-type nanoclusters. The Ra values of the sensors in the present study were also significantly increased by increasing SnO2 concentrations in the Co3O4−SnO2 hollow spheres (Figure 4), which can be explained in part by the formation of nano-scale p-n junctions between the Co3O4 and SnO2 phases. And this is consistent with the presence of SnO2 second phase in Co3O4−0.24S and Co3O4−0.43S specimens.


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The samples were analyzed using XPS in order to investigate the valence states of the specimens (Figure S1). The binding energies of Co 2p3/2 peaks in single-phase Co3O4 and Co3O4−0.15S, Co3O4−0.24S, and Co3O4−0.43S specimens were 780.2, 782.5, 783.0, and 783.1 eV, respectively. The binding energies of Co 2p1/2 peaks also tended to increase with the increase of SnO2 contents. The shift of peaks to higher binding energy is attributed to the decrease in electron screening effect in Co3O4 by the incorporation of Sn with higher electronegativity to the Co3O4.28 And this is feasible considering that the ionic radius of Sn4+ (0.55 Å at coordination number of 4) is similar to those of Co2+ (0.58 Å at coordination number of 4) and Co3+ (0.61 Å at coordination number of 6).29 Accordingly, the XPS analysis indicates that Sn component is not only coated on the surface of Co3O4 but also incorporated into Co3O4 lattice during galvanic replacement. This indicates the doping of higher valence of Sn into Co3O4 is another possible reason to increase the Ra values of the sensors. However, the gas responses of the Co3O4−SnO2 hollow spheres were not merely enhanced with increasing SnO2 concentrations, but showed a relatively complex behavior depending on the sensor compositions. Moreover, good selectivity to methylbenzenes was achieved only for the Co3O4−0.15S and Co3O4−0.24S sensors, while that to ethanol was obtained only for the Co3O4 sensor. Thus, it is difficult to explain the composition-dependent variations in gas response and gas selectivity observed in the present study only by the formation of p-n junctions or Sn doping into Co3O4 lattice. For the Co3O4−0.43S sensors, the responses to all the gases were relatively low (< 4.0) with no significant selectivity observed regardless of the sensing temperature. Although the sensors show an increase in resistance upon exposure to reducing gases (i.e. p-type gas sensing characteristics) (Figure 5), it is probable to create continuous conduction paths along the n-type


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SnO2 phases in Co3O4−SnO2 hollow spheres considering the high concentration of SnO2 phase. When both the p-type Co3O4 and n-type SnO2 form continuous conduction paths in parallel, opposite variations in sensor resistance upon exposure to analyte gases can be nullified by each other, leading to low gas responses. The high sensor resistance in spite of two different parallel conduction paths can be attributed to the narrowing of n- and p-conduction channels due to the formation of nano-scale p-n junction. Selective detection of ethanol and relatively high gas responses to xylene, toluene, and acetone for single-phase Co3O4 sensors are consistent with the gas sensing characteristics of other Co3O4 nanostructures reported in the literature,30-33 which can be attributed to a relatively high catalytic activity of Co3O4 toward the abovementioned gases. However, selective detection of xylene and/or toluene by the Co3O4−0.15S and Co3O4−0.24S sensors should be understood in connection with the role of SnO2 added during galvanic replacement. To investigate the effect of sensor composition on gas selectivity, the Co3O4-SnO2 composite powders whose composition is same to Co3O4-0.24S were prepared by solid-state mixing of Co3O4 (Cobalt(II, III) oxide, nanoparticle < 50 nm, Sigma-Aldrich) and SnO2 (Tin(IV) oxide, nanoparticle < 100 nm, Sigma-Aldrich) commercial powders and subsequent heat treatment at 450 °C for 2 h (Figure S2a,b) followed by measurements of their gas sensing characteristics. Relatively low gas responses (Ra/Rg < 5) can be attributed to a lower BET surface area of the powders (22.1 m2g-1) prepared by a solid state reaction (Figure S2c). However, selective detection of methylbenzenes can be also achieved at 275 °C, and the responses to the all three gases become similar at ≥ 325 °C indicating that a composite formation between the Co3O4 and SnO2 phases enhances the selectivity to methylbenzenes. Both Co3O434,35 and SnO236 are known as good catalysts for ethanol oxidization at relatively low temperatures. Thus, a


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majority of ethanol can be oxidized into non-reactive gases such as CO2(g) and H2O(g) at the upper part of sensing films, and only its limited amounts can reach the lower active part close to the sensing electrodes. The responses to ethanol for the Co3O4−0.15S and Co3O4−0.24S sensors at 275 °C were very low, while for single-phase Co3O4 sensors they were relatively high at the same temperature, which supports the abovementioned explanation of the promotion of ethanol oxidation due to a synergistic combination of two different catalytic materials. In addition, morphological variations caused by galvanic replacement should be also considered as possible factors affecting gas sensing reactions. The pore size distributions of the spheres were determined by N2 isotherm desorption (Figure 6). For single-phase Co3O4 spheres, the maximum amount of pores was observed near diameters of around 10 nm (Figure 6a). It is interesting to note that with the progress in galvanic replacement, the volume of pores larger than 10 nm gradually decreases, while new mesopores with diameters of approximately 4 nm become more abundant. The galvanic replacement reaction starts from the mesoporous region where a Sn aqueous phase and Co3O4 primary particles coexist. Accordingly, mesopores with diameters of around 4 nm are formed by gradual filling of larger 10 nm mesopores due to a galvanic replacement process. Porous or hollow spheres with different sizes and volume of mesopores can be prepared by selecting various polymeric precursors to form pores during thermal decomposition37, controlling the amount/size of organic/inorganic sacrificial templates in spray solution38-40, and subsequent tuning of pore sizes by galvanic replacement, which can serve as microreactors reforming target gases and change gas sensing characteristics.41 For example, we have previously reported that Pd-loaded SnO2 yolk-shell spheres42 and Pd-loaded Co3O4 quintuple-shells43 showed selectivity to methylbenzenes, which was attributed to dissociation of less reactive xylene and toluene into smaller more reactive species in the microreactors between


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catalyst-loaded yolks and inner shell parts. In the present study, an increase in the amount of smaller mesopores (~4 nm) at the expense of larger mesopores (~10 nm) results in the increase in the retention time of gases inside the spheres (Figure 6e,g), which can enhance gas reforming and gas response due to a catalytic oxidation effect of Co3O4.44-46 And this is feasible if one considered that the kinetic diameter of xylene gases is 0.585 nm at 380 K.47 To test this hypothesis, we measured 90% response times (τres, the time required to reach 90% of the final resistance upon exposure to 5 ppm of a gas mixture) to 5 ppm ethanol, toluene, and xylene for the single-phase Co3O4, Co3O4−0.15S, and Co3O4−0.24S sensors at 275 oC (Figure S3). The response time tends to increase with increasing the concentration of SnO2, which supports the previous explanation. Thus, selective detection of methylbenzenes observed for the Co3O4−0.15S and Co3O4−0.24S sensors can be explained by a synergistic combination between the two catalytic materials and unique microreactor-assisted gas reforming reaction.

4. CONCLUSIONS Single-phase Co3O4 hollow spheres were prepared by ultrasonic spray pyrolysis and converted into Co3O4−SnO2 core-shell hollow spheres by galvanic replacement. Selective detection of ethanol, xylene, and methylbenzenes (xylene + toluene) was achieved by operating single-phase Co3O4 sensors at 275 °C, Co3O4−SnO2 sensors at 275 °C, and Co3O4−SnO2 sensors at 300 °C.


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The ability to control gas selectivity by galvanic replacement was attributed to catalytic activity tuning due to compositional changes and microreactor-assisted gas reforming reactions promoted by variations in mesoporosity of the hollow spheres. Galvanic replacement provides a facile chemical route for creating nano-scale oxide semiconductor heterojunctions (p-n, n-n, and p-p types) and regulating mesoporosity of sensing materials, which can be used to control gas sensing characteristics of oxide semiconductor chemiresistors.

ASSOCIATED CONTENT Supporting Information. The result of XPS analysis for Co 2p1/2 and Co 2p3/2 to the Co3O4, Co3O4−0.15S, Co3O4−0.24S, and Co3O4−0.43S specimens; the SEM image, diffraction pattern, and gas sensing characteristics for the solid-state mixed 24.3 at.% SnO2−Co3O4; the 90% response times for the single-phase Co3O4 hollow spheres, Co3O4−0.15S, and Co3O4−0.24S.

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (No. 2013R1A2A1A01006545).


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FIGURES

Figure 1. Diffraction patterns of the (a) single-phase Co3O4 hollow spheres, (b) Co3O4−0.15S, (c) Co3O4−0.24S, and (d) Co3O4−0.43S specimens.


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Figure 2. FE-SEM images of the (a) single-phase Co3O4 hollow spheres, (b) Co3O4−0.15S, (c) Co3O4−0.24S, and (d) Co3O4−0.43S specimens.


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Figure 3. TEM images of (a) Co3O4, (b) Co3O4−0.15S, (c) Co3O4−0.24S, and (d) Co3O4−0.43S spheres; TEM elemental mapping images, cross-sectional images, and line EDS images of (e-h) Co3O4−0.24S and (i-l) Co3O4−0.43S spheres.


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Figure 4. Sensing transients of the Co3O4, Co3O4−0.15S, Co3O4−0.24S, and Co3O4−0.43S sensors to 5 ppm of (a) ethanol, (b) toluene, and (c) xylene at 300 oC.


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Figure 5. Polar graphs and gas responses (Rg/Ra) of the (a) Co3O4, (b) Co3O4−0.15S, (c) Co3O4−0.24S, and (d) Co3O4−0.43S sensors to various gases at 275, 300, 325, 350, and 375 oC, respectively. (E: ethanol, T: toluene, X: xylene, A: acetone, H: formaldehyde, C: carbon monoxide, B: benzene)


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Figure 6. Pore-size distributions and specific surface areas of the (a and b) Co3O4, (c and d) Co3O4−0.15S, (e and f) Co3O4−0.24S, and (g and h) Co3O4−0.43S specimens measured by N2 isotherm desorption.


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Co3O4âSnO2 Hollow Heteronanostructures: Facile Control of Gas

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