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Nonstoichiometric Co-rich ZnCoO Hollow Nanospheres for High Performance Formaldehyde Detection at ppb Levels Hyung Ju Park, Jinmo Kim, Nak-Jin Choi, Hyunjoon Song, and Dae-Sik Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10862 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 22, 2016

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ACS Applied Materials & Interfaces

Nonstoichiometric Co-rich ZnCo2O4 Hollow Nanospheres for High Performance Formaldehyde Detection at ppb Levels Hyung Ju Park,†,‡ Jinmo Kim,§,‡ Nak-Jin Choi,†,⊥ Hyunjoon Song,*,§ and Dae-Sik Lee*,†

IT Convergence Technology Research Laboratory and Convergence Components & Materials Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 34129, Republic of Korea,† and Department of Chemistry, Korea Advanced Institute of Science and Technology, and Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (ibs), Daejeon 34141, Republic of Korea§ ⊥

Current address: R&D Team, Milaebo Co., Pyeongtaek 17709, Republic of Korea E-mail: [email protected],§ [email protected]†

KEYWORDS ZnCo2O4, hollow nanosphere, formaldehyde sensing, ppb level, selectivity

ACS Paragon Plus Environment

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ABSTRACT

Since metal oxide semiconductors were investigated as chemiresistors, rapid advances have been reported in this field. However, better performance metrics are still required, such as higher sensitivity and selectivity levels for practical applications. To improve the sensing performance, we discuss an optimal composition of the active sensing material, nonstoichiometric Co-rich ZnCo2O4, prepared by the partial substitution of Co2+ into Zn2+ in Co3O4 without altering a hollow sphere morphology. Remarkably, this Co-rich ZnCo2O4 phase achieved detection limits for formaldehyde as low as 13 ppb in experimental measurements and 2 ppb in theory, which were the lowest values ever reported from actual measurements at a working temperature of 225 °C. It was also unprecedented that the selectivity for formaldehyde was greatly enhanced with respect to the selectivity levels against other volatile organic compounds (VOCs). These excellent sensing performances are due to the optimal composition of the Co-rich ZnCo2O4 material with a proper hole concentration and well-organized conductive network.


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■ INTRODUCTION

Since metal oxides were proposed as sensitive chemiresistors, numerous gas sensors based on oxide semiconductors have been explored to detect toxic air pollutants.1-4 Although rapid advances have been reported in gas sensors, better performance metrics are still required, such as higher sensitivity and selectivity levels for practical applications. For instance, formaldehyde is a primary cause of sick building syndrome (SBS), and this substance is also known as an indoor and outdoor air pollutant. Due to its high toxicity, the World Health Organization (WHO) has established a limit for the long-term exposure to formaldehyde of 0.08 ppm in 30 min on average. Thus, a gas detection limit which is lower than this standard is required for sensing materials. Most metal oxides cannot meet this detection limit of sub-ppm levels,5,6 except in a few cases,7-11 such as SnO2-based films12,13 and ZnO tetrapods.14 These sensors are based on n-type semiconductors, which exhibit high sensitivity, but also have weak points of instability and slow response times. In p-type semiconductors, it is even more challenging to achieve such high sensitivity, although ptype materials have wide dynamic ranges and rapid recovery kinetics.15-17 In previous work, we selected p-type Co3O4 as an active sensing material.18 With the optimization of a hollow sphere morphology, the resulting device exhibited high activity with regard to formaldehyde sensing, with detection limits of 50 ppb, approaching the lowest detection limit of n-type semiconductors. To improve the sensing performance further, here we discuss an optimal composition of the active material, i.e. nonstoichiometric Co-rich ZnCo2O4, which was prepared by the partial substitution of Co2+ atoms in Co3O4 into Zn2+ without altering the original morphology. Remarkably, this Co-rich ZnCo2O4 phase achieved detection limits for formaldehyde as low as 13 ppb at a working temperature of 225 °C with a 12-fold higher response than that of Co3O4, which was the lowest value ever reported from actual measurements.5-14 It was also


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unprecedented that the selectivity for formaldehyde was greatly enhanced with respect to the selectivity levels against other volatile organic compounds (VOCs), especially against ethanol, with five-fold greater sensitivity. The ultra-high sensitivity levels, enhanced selectivity, and excellent stability of the Co-rich ZnCo2O4 material mostly meet the high standards of various sensing applications.

■ EXPERIMENTAL SECTION

Chemicals.

Cobalt(II)

nitrate

(Co(NO3)2,

98%),

1,5-pentanediol

(1,5-PD,

96%),

poly(vinylpyrrolidone) (PVP, Mw = 55,000), and zinc(II) acetylacetonate hexahydrate (Zn(acac)2·6H2O, 99.995%) were purchased from Sigma-Aldrich and used without further purification. Synthesis of ZnO@Co(OH)2 Core-Shell Spheres. Zn(acac)2·6H2O (0.10 g, 0.40 mmol) and PVP (1.0 g, 9.0 mmol) were dissolved in 1,5-PD (50 mL). Zn-PVP mixture was slowly heated to 250 °C for 10 min under an inert condition. Co(NO3)2 (0.12 g, 0.40 mmol) dissolved in 1,5-PD (5.0 mL) under an inert condition. The cobalt solution was injected into the hot Zn-PVP mixture solution after the Zn-PVP mixture maintained for 3 min at 250 °C. The mixture was stirred for 30 min at 250 °C. The product was cooled to room temperature using an ice bath and precipitated by the addition of ethanol (40 mL) and acetone (15 mL) with centrifugation. The precipitates were thoroughly washed with ethanol and dried in a vacuum oven at 80 °C for 12 h. Synthesis of ZnO@Co3O4 Core-Shell Spheres. The ZnO@Co(OH)2 powders were placed in an alumina boat, heated to 350 °C for 80 min, and calcined at 350 °C for 3 h in a tube furnace in air.


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Synthesis of Co-rich ZnCo2O4 Hollow Nanospheres. The ZnO@Co3O4 powders (0.20 g) were dispersed in ethanol (20 mL) by sonication. The dispersion was placed in a 50 mL polypropylene bottle. Hydrofluoric acid (4.0 mL, 50 wt %, aqueous solution) was added to the dispersion, and the mixture was stirred for 10 min at room temperature. The precipitates were collected by centrifugation and thoroughly washed with ethanol. The particles were dried in a vacuum oven at 80 °C for 12 h. Synthesis of Co3O4 Hollow Nanospheres. The ZnO@Co3O4 powders (0.20 g) were dispersed in ethanol (20 mL) by sonication. Hydrochloric acid (0.20 mL, 35 wt %, aqueous solution) was added to the dispersion, and the mixture was stirred for 10 min at room temperature. The precipitates were collected by centrifugation and thoroughly washed with ethanol. The particles were dried in a vacuum oven at 80 °C for 12 h. Characterization. The nanostructures were characterized by FEI Tecnai TF30 ST (300 kV, KAIST) and FEI Titan cubed G2 60-300 (Titan Double Cs corrected, KAIST) transmission electron microscopes. Samples were prepared by dropping a few samples of the corresponding solutions on carbon-coated 300 mesh copper grids (TED PELLA Inc.). X-ray diffraction patterns of the nanostructures were recorded on a Rigaku D/MAX-2500 diffractometer. Nitrogen sorption isotherms were measured at 77 K with a TriStar II 3020 (Micromeritics Instrument Corporation). Sensor Fabrication. The interdigitated electrodes (IDEs) were fabricated by the conventional photolithography, on conventional polymer resist and metal lift-off. The photoresist, AZ-5214, was spin-coated on a silicon oxide substrate at 5000 rpm for 30 s and baked at 90 °C for 3 min on a hot plate. Photo exposure was carried out for 45 s and then developing was for 30-60 s in the developer solution. After the metal layers (Cr/Au) were deposited by thermal evaporation, microand millimeter-sized structures were successfully generated on the substrate. The sensing materials


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were screen-printed with a 640 mesh on the IDEs and dried at 120 °C for 10 min in an electric oven. Finally, the devices were sintered at 350 °C for 2 h in an electric furnace. Gas Sensing Experiment. Gas sensing properties were measured using a computer-controlled characterization system.19 Gas sensors prepared on the silicon substrate were investigated on a hot chuck in a water-cooled steel chamber with a precise temperature control. The parent concentrations of gases and their balances were 10.3 ppm and N2 balance for formaldehyde, 20.3 ppm and dry air balance for ethanol, 10.5 ppm and dry air balance for acetone, 52.3 ppm and N2 balance for benzene, 51 ppm and dry air balance for NO2, 302 ppm and N2 balance for NH3, and 49.6 ppm and dry air balance for CO. The dry air was utilized as a carrier gas, which was supplied by a compressed air (N2:O2 = 8:2) control system including two oil free air compressors, two receiver tanks, air dryer (dew point: -70 ~ -80 oC) and three high efficiency particulate air (HEPA) filters (5, 1, and 0.45 µm). Gas analytes (RIGAS Co., Ltd.) and dry air were injected into the device chamber in turns with the flow rates of 1.3-100 and 1000 cc/min, respectively, using mass flow controllers (Brooks Instrument, Tylan FC-280SAV and Tylan FC-2900). The resistances of the sensor materials were measured by using a data acquisition board (DAQ) that was simultaneously able to acquire 24 channels of analogue inputs.

■ RESULTS AND DISCUSSION

The preparation of metal oxide hollow spheres was done in three steps (Figure 1). First, ZnO spheres were synthesized by the hydrolysis and condensation of Zn(acac)2 in 1,5-pentanediol at 250 °C. Co(NO3)2 was added to the hot ZnO dispersion in situ, and the mixture was stirred at 250 °C to yield ZnO@Co(OH)2 core-shell spheres (Figure 1a). Second, the powder form of the


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spheres was thermally treated at 350 °C in air for 3 h to provide ZnO@Co3O4 core-shell spheres (Figure 1b). The final step was the selective dissolution of the ZnO cores to yield metal oxide hollow spheres. The Co3O4 hollow shells were maintained by etching with a strong acid, HCl (Figure 1c);18 however, when the core-shell spheres were treated with a weak acid, HF, the Zn2+ ions dissolved in the original ZnO cores were incorporated into the shells and partially replaced the Co2+ ions in the Co3O4 lattice, generating ZnCo2O4 hollow shells.

Figure 1. Schematic representation of the three-step synthesis, and TEM images of a) ZnO@Co(OH)2 core-shell, b) ZnO@Co3O4 core-shell, and c) Co3O4 hollow nanospheres. The bars represent 50 nm.


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Transmission electron microscopy (TEM) images show that the morphology of the hollow spheres is maintained after the etching process (Figure 2). The average diameter of the hollow spheres is measured to be 72 ± 10 nm. The walls are composed of small granules with spatial contacts between neighbors, forming the overall hollow structure. High resolution TEM (HRTEM) image indicates that each granule is single-crystalline with distances between adjacent lattice fringe images of 0.244 and 0.286 nm (Figure 2c), in good agreement with those of the {311} and {220} crystallographic planes in spinel Co3O4 or ZnCo2O4, respectively. Importantly, elemental mapping by high angle annular dark field-scanning transmission electron microscopy (HAADFSTEM) shows that the Zn composition (dark-grey) is evenly distributed all over the region of a single hollow sphere identified by its Co composition (grey) (Figures 2d-f). The incorporation of

Figure 2. a,b) TEM and c) HRTEM images of Co-rich ZnCo2O4 hollow nanospheres. d) HAADFSTEM and EDS elemental mapping images of e) Zn and f) Zn/Co for a single hollow nanosphere. g) Ideal crystal structure of nonstoichiometric Co-rich ZnCo2O4 spinel. The bars represent a) 100 nm, b) 20 nm, c) 5 nm, and d-f) 20 nm.


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the Zn atoms was additionally confirmed by an elemental analysis using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) of the bulk sample, which estimated a mole fraction between Zn and Co of 21.77:78.23, nearly matching a 1:4 ratio. Energy-dispersive X-ray spectroscopy (EDS) in the microscopic region also found the mole fraction of Zn and Co to be 20.59:79.41. The HAADF-STEM image and the EDS line profile of an individual structure indicate that the morphology is a hollow sphere, but not a spherical agglomerates of small granules (Supporting Information, Figure S1). The X-ray diffraction (XRD) pattern of the hollow spheres matches both that of spinel Co3O4 (JCPDS No. 43-1003) and ZnCo2O4 (JCPDS No. 23-1390) (Figure 3a). The average singlecrystalline domain size of the hollow spheres was calculated to be 9.7 nm from the XRD peak using the Scherrer equation, which was close to the average size of individual granules and the shell thickness of the hollow spheres. The X-ray photoelectron spectroscopy (XPS) peaks indicated that the composition ratio between Zn and Co, as obtained by the integration of the peaks appearing in the Zn 2p and Co 2p regions multiplied by the sensitivity factors, is 1:4. In the Co 2p region, Co2+ and Co3+ peaks could be separately resolved by the deconvolution of the original pattern (Figure 3b). The large peaks at 780.3 and 795.3 eV and the small peaks at 783.3 and 797.6 eV were assigned to the peaks from the Co3+ and Co2+ species, respectively, while additional satellite splitting peaks also appeared. The area ratio between the Co2+ and Co3+ peaks was 1:5, in good agreement with the composition of Co-rich ZnCo2O4 depicted in Figure 2g. ZnCo2O4 is a well-known p-type zinc oxide spinel material which has been used in various applications.20-22 In normal spinel materials (A3+)2(B2+)O4, the low-valent B2+ cation occupies the tetrahedral site (B2+Td), and the high-valent A3+ cation occupies the octahedral site (A3+Oh). In ZnCo2O4, the crystal structure is isomorphic with Co3O4 but not with ZnO, which is correlated


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Figure 3. a) XRD and b) XPS spectra, c) temperature-dependent resistance, c inset) Arrhenius plot of the conductance, and d) N2 gas adsorption isotherms of Co-rich ZnCo2O4 hollow nanospheres.

with the fact that ∆H(CoTd) is smaller than ∆H(ZnOh).23 This indicates that the region of phase stability in ZnCo2O4 deviates toward the respective isomorphic phase, Co3O4; thus, ZnCo2O4 inherently exhibits a nonstoichiometric Co-rich characteristic. Consequently, the Co-rich ZnCo2O4 forms a stable spinel phase as a solid solution of Co3O4 and ZnCo2O4, whereas the Zn-rich ZnCo2O4 has a boundary against phase segregation in the Co3O4-ZnO phase diagram.23 The present hollow spheres can be assigned as a nonstoichiometric Co-rich ZnCo2O4 phase in which the ratio of Co3O4 and ZnCo2O4 in the solid solution is 2:3, meaning that the Td sites of the spinel


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are occupied statistically at a ratio of 2:3 by Co2+ and Zn2+, whereas the Oh sites are solely occupied by Co3+, as depicted in Figure 2g. At low temperatures, the ground-state structure of the spinel creates fully occupied bands and renders the material insulating. However, at high temperatures, cation cross-substitution gives rise to the formation of free carriers. In ZnCo2O4, the Co3+ ion is easy to substitute into the low valent Td site owing to the low ∆H(CoTd), but it is electrically inactive. Thus, the material is intrinsically p-type due to the small but significant presence of an acceptor, Zn2+ substituting onto a high-valent Oh site. An increase of the Zn content in ZnCo2O4 further increases the hole concentration and conductivity.24-26 The resistance of Co-rich ZnCo2O4 hollow spheres using an electrode with an 8 µm gap was measured to be 6.02×105 Ω at 225 °C, which converts into conductivity of 3.8 Scm-1. Conductivity measurements at variable temperatures estimated an activation energy level of 0.77 eV (Figure 3c). The conductivity and hole concentration of spinel ZnCo2O4 films at room temperature are known to be ~ 2 S cm-1 and ~ 2 × 1020 cm-3,27,28 respectively, and the latter is one order of magnitude higher than that of Co3O4 materials.29 When considering some increase of conductivity according to the elevation of working temperature, we believe that our conductivity (3.8 S cm-1) is reasonable. The Brunauer-Emmett-Teller (BET) surface area of the hollow spheres was measured to be 95.4 m2g-1, which was nearly three times larger than that (34.5 m2 g-1) of the Co3O4 nanoparticles (Figure 3d). The gas sensing properties were measured using a custom-made computer-controlled characterization system. Hollow nanospheres were mixed with a paste, after which the materials were deposited onto electrodes with a layer thickness of 10 µm (Figure 4). The sensing materials were stable under the calcination condition at 350 °C, which were characterized by TEM and XRD measurements (Supporting Information, Figure S2). After a thermal treatment, the resulting gas


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sensors were exposed to formaldehyde at the desired analyte concentration in a test chamber. For p-type semiconductors, the gas response is defined as R = Rg/Ra for reducing gases, where Ra and Rg are the resistance in air and in a gas atmosphere, respectively. A linear relationship between the response and the formaldehyde concentration was observed at all operating temperatures. The maximum responses were detected at 225 °C over a wide range of formaldehyde concentrations (Figure 4b), as was similarly observed with the Co3O4 material.18

Figure 4. a) SEM images of Co-rich ZnCo2O4 hollow nanospheres deposited onto gas sensors with 8 µm gaps. b) Response of Co-rich ZnCo2O4 hollow nanospheres as a function of the formaldehyde concentration at various operating temperatures.

The real-time response and recovery behaviors of both Co-rich ZnCo2O4 and Co3O4 hollow nanosphere devices were monitored at 225 °C with the formaldehyde concentration in the range of 13 ppb to 1000 ppb (Figure 5a). Throughout the measuring concentration range, the Co-rich ZnCo2O4 sensor showed much higher response values, from two times to twelve times as compared to that of the Co3O4 device. Under 13 ppb of formaldehyde, the response was estimated to be 1.17 with the root mean square noise of 0.0035 from the multiple measurements, in which the response change (0.17) was twice as large as that (0.08) of Co3O4. To the best of our knowledge, 13 ppb of


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formaldehyde is the lowest detection limit in actual measurements ever reported among n-type and p-type oxide semiconductors.5-14 For comparison with other sub-ppm level detections, Lv et al. reported that a sensitive SnO2-NiO film could detect 0.06 ppm of formaldehyde by increasing adsorbed oxygen sites on NiO,12 Iizuka et al. reported SnO2 porous film via plasma spray physical vapor deposition detect 0.04 ppm of formaldehyde by complex and expensive synthesis,13 and Calestani et al. used ZnO tetrapods for the detection of 0.05 ppm acetaldehyde.14 Figure 5b shows a linear increase of the response versus the formaldehyde concentration; the responses throughout the measured range are superior to those of other p-type semiconductor devices.15 By considering the true signal as defined by IUPAC at a signal-to-noise ratio exceeding 3,30 extrapolation from this linear dependence with the slope of 0.0063 and the root-mean-square noise of 0.0035 at 13 ppb provided a theoretical detection limit of 2 ppb.31 At the high concentrations, two repeated measurements of 1 ppm formaldehyde exhibited a reproducible response of 7.31 with a coefficient

Figure 5. a) Time-transient response curves of Co-rich ZnCo2O4 (grey) and Co3O4 (black) hollow nanospheres with respect to the formaldehyde concentration at 225 °C. b) Responses of Co-rich ZnCo2O4 and Co3O4 hollow nanospheres as a function of the formaldehyde concentration.


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of variation of 1.5%. The response and recovery times at this concentration were estimated to be 149 and 497 s at 225 °C, which decreased to 35 and 118 s at 300 °C, respectively. In order to detect extremely low concentrations of gas analytes, both the composition and the morphology of the active materials are critical. The sensing of formaldehyde mainly results from the modulation of the surface conductivity by the adsorption and desorption of gas molecules.32,33 In p-type semiconductors, the surface is readily covered with negatively charged chemisorbed oxygen, which generates a charge accumulation layer according to the reaction (1): 1/2 O2(g) → O-(ads) + h*

(1)

Then formaldehyde molecules are oxidized by O-(ads), leading to the reduction of the charge accumulation layer thickness via the reaction (2): HCHO(g) + 2 O-(ads) + 2 h* → CO2(g) + H2O(g)

(2)18

As a result, the sensor resistance increases (the conductivity decreases) by the exposure of formaldehyde. From the viewpoint of the particle structure, the charges mainly conduct through the particle surface, and the conductivity change is not much dependent upon the particle size. However, when the single-crystalline domain size is less than the layer thickness, the entire domain would accumulate holes; thus, the surface adsorption of gas molecules entirely dominates conductivity changes, particularly when the single-crystalline size is under 10 nm.2 It should be noted that the Co-rich ZnCo2O4 hollow spheres have multiple single-crystalline domains with the average diameter of ~ 10 nm, which is far less than the critical size of the grains (20-100 nm). Consequently, the surface adsorption of gas molecules and the resulting hole consumption largely influence the resistance (and the conductivity) of the sensing materials. The morphology of the hollow spheres is also suitable for the ideal sensing structure.3 The hollow structure of Co-rich ZnCo2O4 facilitates the easy penetration of gas molecules into the inner void space, allowing the


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full use of its granular surface. Moreover, the three-dimensional network generated in the densely packed solid sample directly affects the conductivity.18 The mechanical contacts between the hollow spheres simultaneously form three-dimensional connections with narrow thicknesses. In this configuration, the electrical conduction can readily be disrupted by a small number of particles of which the gas molecules alters the conductivity. In our comparison of Co-rich ZnCo2O4 to Co3O4 hollow spheres, however, these morphology effects with regard to the sensitivity are similar to each other; therefore, the composition effect may dominate the sensing performance. The crystal structure and morphology of Co-rich ZnCo2O4 are unchanged by cation substitution, resulting in high structural and chemical stabilities similar to those of pure Co3O4 crystals. However, the hole concentration is largely increased by the substitution,34 which greatly affects the conductivity change by a small amount of gas adsorption on the surface. The ZnCo2O4 phase is known as effective catalysts for CO oxidation35 and LPG combustion,36 as well as for electrocatalytic oxygen reduction reactions.37 This indicates that ZnCo2O4 strongly interacts oxygen gas to form reduced oxygen species, and readily generates the hole accumulation layer via the reaction (1), providing high sensitivity of the gas analytes. A further increase of the Zn content leads to the phase segregation of ZnO from the ZnCo2O4 lattices;8 thus, the present Co-rich ZnCo2O4 phase can be regarded as an optimal composition for maximum sensing performance. Humidity in atmosphere is one of the important obstacles to approach accurate measurements of VOCs, because water molecules are also chemisorbed on the surface and abruptly change electrical properties of the sensing devices. Figure 6 shows the effect of humidity with respect to the response of HCHO using the sensor based on the Co-rich ZnCo2O4 hollow spheres. The baseline resistance increases as the humidity increases. Though the HCHO response decreases by


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28% when the humidity increases from a dried gas to 80%, the gas sensors can easily and reliably detect the level of 100 ppb HCHO even under very humid conditions. A few mechanisms have been proposed for the explanation of surface conductivity change in the presence of water vapor.38,39 In general, water molecules can be adsorbed by physisorption or hydrogen bonding. At higher temperatures (100-500 °C), water molecules can react with Lewis acid sites (Metal) and Lewis base sites (oxygen) on the metal oxide surface forming (Metal.Metal+-OH-), and then release electrons (e-).37 As a result, the accumulation layer becomes thin and hinder the surface conductivity of the metal oxide sensors. In our device, the working temperature of the ZnCo2O4 sensor is 225 °C so that dissociative adsorption should take place and the pre-adsorbed oxygen on the surface of the ZnCo2O4 has been competitively displaced by physisorbed water.39 This device also shows reasonable stability, lasting for 1 d at a working temperature of 225 °C in ambient air with an average response change of 0.979 and a standard deviation of 0.008 (Figure 6c).

Figure 6. a) Time-transient response curves and b) response of 100 ppb HCHO as a function of the relative humidity at 225 °C, and c) stability curve of the sensor using Co-rich ZnCo2O4 hollow nanospheres upon exposure to air at 225 °C.


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Feasible selectivity for a target gas in a gas mixture is also one of the unsolved drawbacks related to solid-state sensors.40 In particular, formaldehyde, ethanol, benzene, and other aromatic hydrocarbons are VOCs having similar physical and chemical properties. The responses were measured under the standard conditions with the exposure of 400 ppb gases, including formaldehyde, carbon monoxide, nitrogen dioxide, ammonia, ethanol, acetic acid and benzene, all commonly found in indoor environments. Unexpectedly, the Co-rich ZnCo2O4 device showed a significantly large response change (ΔR) for formaldehyde (275%), which was more than five times greater than those of other VOCs, i.e., ethanol (53%), benzene (27%), and acetone (37%) (Figure 7). In contrast, the Co3O4 device did not exhibit distinct selectivity in VOCs. Such unprecedented selectivity for formaldehyde is presumably due to the hole concentration of the ZnCo2O4 materials, which is suitable for the effective discrimination of dipole moments between distinct gas molecules. Generally, a highly polarized surface can strongly adsorb gas molecules with a high dipole moment.41 In comparison with Co3O4, an increase of the Zn content in ZnCo2O4

Figure 7. Responses of the sensors using Co-rich ZnCo2O4 and Co3O4 hollow nanospheres to 400 ppb gas analytes.


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largely increases the hole concentration, which may strongly adsorb gas molecules with high dipole moments. It is reported that the better the adsorptive properties, the higher the catalytic activity in spinel-type oxide complexes in oxidation reactions.42 Among VOCs, formaldehyde has a relatively high dipole moment compared to ethanol, acetone, and benzene. Therefore, ZnCo2O4 can strongly chemisorb formaldehyde, resulting in high selectivity for formaldehyde against other VOCs. Further study needs to be explored for detailed mechanism.

■ CONCLUSIONS

In this study, nonstoichiometric Co-rich ZnCo2O4 hollow spheres were synthesized by the treatment of ZnO@Co3O4 core-shell particles with HF. The resulting sensing device exhibited ultra-high sensitivity for formaldehyde with detection limits as high as 13 ppb in experimental measurements at a working temperature of 225 °C and 2 ppb in theory, as well as good stability and unprecedented selectivity for formaldehyde against other VOCs. These excellent sensing performances are attributed to the optimal composition of the Co-rich ZnCo2O4 materials, which generates a proper hole concentration, as well as the optimal morphology, which forms a threedimensional conductive network consisting of a hollow structure with a large surface area. This rational optimization of the sensing materials has great potential as a platform for monitoring indoor and outdoor environments in real time. Success in this area can expand sensor applications to meet various needs, such as those associated with vehicles and mobile devices.

■ ASSOCIATED CONTENT Supporting Information. The STEM-HAADF image and EDS line spectrum of the Co-rich ZnCo2O4 hollows and the TEM image and XRD data of the Co-rich ZnCo2O4 hollows after paste


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calcination at 350 oC. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected] Author Contributions ‡ H.J.P. and J.K. contributed equally to this work. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was supported by IBS-R004-D1 and the National Research Foundation of Korea (NRF) funded by the Korea Government (MSIP) (NRF-2015R1A2A2A01004196). This work was also supported by the R&D Program of the Ministry of Science, ICT and Future Planning (12RC1510, Development of a breath gas analysis system for lung cancer screening).

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