A Hollow Assembly and Its Three-Dimensional Network Formation of

Oct 20, 2014 - The detection of formaldehyde at a very low concentration is a significant research topic, due to its detrimental impact on human healt...
0 downloads 0 Views 6MB Size
Download PDF
Article pubs.acs.org/JPCC

A Hollow Assembly and Its Three-Dimensional Network Formation of Single-Crystalline Co3O4 Nanoparticles for Ultrasensitive Formaldehyde Gas Sensors Jae Young Kim,†,⊥,§ Nak-Jin Choi,‡,§ Hyung Ju Park,‡ Jinmo Kim,†,⊥ Dae-Sik Lee,*,‡ and Hyunjoon Song*,†,⊥ †

Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, Daejeon, 305-701, Korea ‡ Convergence Components & Materials Research Laboratory and IT Convergence Components Laboratory, Electronics and Telecommunications Research Institute, 131 Gajeong-dong, Yuseong-gu, Daejeon, 305-700, Korea ⊥

S Supporting Information *

ABSTRACT: The detection of formaldehyde at a very low concentration is a significant research topic, due to its detrimental impact on human health. In the present study, we fabricated a hierarchical structure by the rational assembly of single-crystalline Co3O4 nanoparticles. A hollow morphology using sacrificial ZnO spheres could form a three-dimensional conducting network in a solid state. The resulting structure was selectively active for formaldehyde sensing, and the detection limit was 50 ppb, which was nearly close to the record-high value among the other semiconducting materials. Such superior properties were attributed to the regular, hierarchically assembled structures with a small crystalline domain size, a thin hollow morphology with a large surface area, and a three-dimensional conductive network with a narrow diameter. We believe that this hierarchical assembly can show great potential as a platform for improving human health through the monitoring of indoor environments.

■

concentration.25 Therefore, the dynamic range of p-type gas sensors is wider than that of n-type counterparts when exposed to a reducing gas. Cobalt oxide (Co3O4) is a well-known p-type magnetic semiconductor with direct optical band gaps at 1.48 and 2.19 eV.26 It is used in many applications, such as heterogeneous catalysts, electrochromic devices, and magnetic materials.27−30 Recently, cobalt oxide is of special interest as an anode material for lithium-ion batteries, owing to its high theoretical charge capacity.31 It has also been used for solid-state gas sensors, where the sensing characteristics are greatly influenced by the morphology and porosity of the nanostructures. Special morphologies of Co3O4 nanosheets, nanorods, and mesocrystals exhibit high sensitivity and a rapid response speed for ethanol and formaldehyde detection.25,32−38 In general, multidimensional and hollow structures are known to be highly active for gas sensing due to their high surface area and wellaligned porous structures without agglomeration. If the hollow shells are sufficiently thin, the entire surface became active and the gas response speed increases by means of rapid gas diffusion.39,40 Although these structural parameters have been optimized, the sensitivity of cobalt oxides has not thus far

INTRODUCTION Formaldehyde is an industrial chemical that is widely used to manufacture building materials and household products.1 One problem with formaldehyde is, however, its detrimental impact on human health, owing to its potentially carcinogenicity and its capability of forming toxic intermediates.2−4 The World Health Organization (WHO) has set a standard for safe exposure of 0.08 ppm averaged over 30 min.5 Apparently, the detection of formaldehyde at a very low concentration is a significant research topic, but traditional techniques such as gas chromatography and mass spectrometry are often costly and time-consuming without real-time measurements.6,7 Among these techniques, gas sensors using metal oxide semiconductors are attractive due to their simple working principle, high sensitivity, good portability, and low cost.8−11 Various n-type semiconductors, which conduct with electrons as the major charge carriers, have been investigated, including SnO2,12,13 WO3,14 NiO,15−17 CdO-mixed In2O3 ,18 and ZnO.19−21 These materials exhibit good sensitivity up to ppm or sub-ppm levels of formaldehyde with excellent stability sufficient for real situations. On the other hand, p-type semiconducting materials, which conduct with positive holes, have rarely been reported in relation to sensing, despite the fact that p-type materials are the main components used in the fabrication of p−n junctions and multiplex sensing devices.22−24 In p-type semiconductors, the resistance is increased by a reducing gas that lowers the positive (hole) charge carrier © XXXX American Chemical Society

Received: June 11, 2014 Revised: October 17, 2014

A

dx.doi.org/10.1021/jp505791v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

exceeded a few ppm at best, whereas n-type SnO2 nanoparticles can detect sub-ppm levels of formaldehyde.32,41−45 The key parameters for enhancing the sensing characteristics are not only the particle morphology and the porosity of the structures but also the particle domain size and connectivity forming a three-dimensional network.25 If the feature sizes of the nanostructures approach the Debye length, the gas response of sensing materials to chemical analytes increases significantly.46−48 In the present study, we fabricated a hierarchical structure by the rational assembly of singlecrystalline Co3O 4 nanoparticles. A hollow morphology composed of Co3O4 nanoparticles was generated on the surface of sacrificial ZnO spheres. After the removal of the ZnO templates, the Co3O4 hollows were densely packed in a solid state, creating an active sensing material and eventually forming a three-dimensional conducting network with large cavities (Scheme 1). The resulting hierarchical structure was selectively Scheme 1. Formation of the Hierarchical Structure of Co3O4 Nanoparticles for Active Sensing Materials

active for formaldehyde sensing, and the detection limit was ∼5 ppb, which was nearly close to the record-high value of n-type SnO2 nanoparticles. The device showed long-term stability upon exposure to air. The superior sensing characteristics were attributed to the synergistic structural hierarchy that included the small crystalline domains, hollow morphology, and a threedimensional conductive network of Co3O4.

Figure 1. TEM and (inset) HRTEM images of (a, b) ZnO@Co(OH)2 core−shell, (c, d) ZnO@Co3O4 core−shell, and (e, f) Co3O4 hollow aggregates. The bars represent (a, c, e) 50 nm and (b, d, f) 10 nm.

demonstrate that the product was a ZnO@Co(OH)2 core− shell spherical aggregate. The XRD spectrum only showed the major peaks of hexagonal wurtzite ZnO, as the Co(OH)2 phase was not fully developed. The Co(OH)2 particles were not chemically nor thermally stable, and thus the core−shell aggregates were oxidized by thermal treatment in air at 350 °C for 3 h. The TEM image showed that particles with an average diameter of 15 ± 3 nm were attached onto spheres with an average size of 110 ± 7 nm (Figure 1c,d), as was similarly observed in the ZnO@Co(OH)2 core−shell aggregates. However, the average distance between adjacent lattice fringe images was changed to be 0.286 nm, which is assignable as the distance between the [110] crystallographic planes of the Co3O4 phase (Figure 1d, inset). The XRD peaks of a face-centered cubic Co3O4 phase also appeared with the ZnO peaks (Figure 2a, black line), which match well with JCPDS card Nos. 42-1467 and 79-0208, respectively. These results allow the conclusion that the thermal treatment effectively converted the Co(OH)2 phase into Co3O4 without structural deformation. The final step was the selective dissolution of the sacrificial ZnO cores by chemical etching. The Co3O4 shells were highly resistant against a strong acid, and the Co3O4 hollow nanostructure was successfully generated by hydrochloric acid treatment at room temperature. The TEM image of the final product clearly showed a hollow morphology with a large inner cavity. The average diameter of the hollow shells was 106 ± 8 nm, not distinct from the previous core−shell aggregates. The average shell thickness was measured to be 15 ± 3 nm, which precisely matched the diameter of the individual Co3O4 nanoparticles. The average distance between adjacent fringe

■

RESULTS AND DISCUSSION In order to generate Co3O4 nanoparticles with a hollow morphology, ZnO spheres were utilized as a sacrificial template. The ZnO spheres were prepared by the hydrolysis and condensation of a Zn precursor, zinc(II) acetylacetonate hydrate, in the presence of poly(vinylpyrrolidone) (PVP) in 1,5-pentanediol (PD), according to a method presented in the literature.49 When the reaction was quenched at this stage, the transmission electron microscopy (TEM) image of the precipitates showed uniform ZnO spheres with an average diameter of 101 ± 4 nm. The X-ray diffraction (XRD) data were assigned as a hexagonal wurtzite ZnO phase (Supporting Information, Figure S1). The Co precursor solution, cobalt(II) nitrate in PD, was injected in situ into the hot ZnO-PVP mixture, and the mixture was stirred for 30 min at 250 °C. The TEM image of the product indicated that small particles were attached onto the surface (Figure 1a,b). The particles had an average diameter of 15 ± 3 nm. The average distance between adjacent fringes in the high-resolution TEM (HRTEM) image is 0.46 nm, in accordance with the distance between the [0001] crystallographic planes of the β-Co(OH)2 phase (Figure 1b, inset). The line scanning profile of the energy-dispersive X-ray (EDX) spectrum for Zn and Co across a single nanoparticle showed the typical pattern of a core−shell morphology, locating the Zn at the core and the Co at the shell. The molar ratio of Zn to Co was 41:50 on average (Supporting Information, Figure S2). These spectra comprehensively B

dx.doi.org/10.1021/jp505791v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

indicating that the product was Co3O4 hollow aggregates. The average crystallite size of the Co3O4 particles was calculated to be 14 nm from the XRD peak using the Debye−Scherrer equation, which was close to the single-crystalline particle domain size (15 nm) estimated from the TEM image, meaning that the hollow shells were composed of single particle layers. The particles on the hollow shells seemed to be loosely connected to the neighboring particle domains in the TEM images (Figure 1e,f), but the hollow aggregates showed high thermal and mechanical stability, presumably owing to a partial fusion of the contacts between the single-crystalline domains. As shown in the TEM images, the Co3O4 hollow shells had numerous pores on the walls. Nitrogen sorption isotherms directly confirmed the porosity of the materials. The ZnO@ Co3O4 core−shell and Co3O4 hollow aggregates exhibited type IV adsorption−desorption hysteresis. In particular, the Co3O4 hollows showed delayed capillary evaporation at a relative pressure of 0.77 and sharp condensation at 0.92−1.0 (Figure 2b). This feature is typically observed in a cage-like mesoporous structure, which was another indication of the porous hollow morphology of Co3O4. The Brunauer−Emmett−Teller (BET) surface areas of the core−shell and hollow aggregates were 34.5 and 51.2 m2 g−1, respectively (Figure 2b). Wang et al. reported the formation of porous Co3O4 hollow spheres using carbon spheres as a template.49 The thickness of the hollow structure was successfully tailored by surface modification of the carbon template. In our synthesis, the Co precursor was added in situ during the synthesis of the ZnO template, and thus the Co precursor concentration was a key factor to change the hollow thickness. By the addition of twice the concentration, the thickness increased to 21 nm (Supporting Information, Figure S3). Another advantage of the ZnO template is a versatility to form a core−shell morphology with various metal oxides. ZnO@Cu2O and CuO core-particle structures were synthesized and used as

Figure 2. (a) XRD spectra and (b) nitrogen adsorption isotherms of ZnO@Co3O4 core−shell (black line) and Co3O4 hollow aggregates (blue line).

images was 0.286 nm (Figure 1f, inset), and all XRD peaks were assigned as reflections of face-centered cubic Co3O4,

Figure 3. Scanning electron microscopy (SEM) images of the gas sensors using active materials of Co3O4 nanoparticles (NP) and hollow aggregates (Hollow), respectively. C

dx.doi.org/10.1021/jp505791v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

catalysts for azide−alkyne cycloaddition reactions,50 and other metal oxides, such as Fe3O4 and NiO, were also successfully layered on the surface of the ZnO template (Supporting Information, Figure S4). Prompted by the hollow structure and porous appearance of the as-prepared Co3O4 hollow aggregates, the gas sensing properties were measured using a computer-controlled characterization system.46−48 As a control experiment, Co3O4 nanoparticles with an average particle size of 17 ± 3 nm were synthesized (Supporting Information, Figure S5). The active sensing materials were prepared in a paste form by mixing them with α-terpineol (C10H18O), after which they were deposited onto interdigitated electrodes on a Si substrate by a screen printing method with a layer thickness of 10 μm. After thermal treatment at 400 °C for 2 h, the surface morphology of the sensing materials on the sensor electrodes was uniform with very fine particles (Figure 3 and Supporting Information, Figure S6). The resulting gas sensors were investigated on a hot chuck in a water-cooled steel chamber. The formaldehyde gas samples were mixed with air to meet the desired analyte concentration, which ranged from 50 to 3000 ppb, using mass flow controllers. For p-type semiconductor sensors, the electrical resistance is changed by the exposure to the gas, with the response defined as R = Rg/Ra (for reducing gases), where Ra and Rg are resistance values exhibited by the sensors in air and in a gas atmosphere, respectively. For the Co3O4 hollow aggregates and nanoparticles, the gas responses were checked at different operating temperatures ranging from 150 to 350 °C (Figure 4a and Supporting Information, Figure S7). When 3 ppm of formaldehyde was exposed, the response of the Co3O4 hollow samples was 4.5 times higher than that of the nanoparticles tested at an operating temperature of 150 °C. Even the response of the hollows at a level of 800 ppb was 2.5 times higher. By increasing the operating temperature, the HCHO responses decreased owing to the increase in thermally activated desorption of the gas molecules from the sensing materials. On the contrary, the response and recovery speeds tend to increase as the operating temperature increases (vide infra). Recovery speed is an important factor directly related to the materials’ stability for practical applications. Apparently, there is an optimum temperature range to manage both high response and high recovery speed, and in our experiments, the range is between 200 and 250 °C (Figure 4a, inset). In this range, the response of each sample provides a consistent plateau region that is not sensitive to a small temperature variation. The operating temperature of 220 °C is selected as an optimal temperature for the formaldehyde sensing purpose, because the response reaches the maximum value. The reduced resistance of the Co3O4 hollow aggregates at higher temperatures reflected the intrinsic thermal behavior of semiconductors against the resistance (Supporting Information, Figure S8). The sensing mechanism of formaldehyde mainly stems from the modulation of the surface conductivity by the adsorption and desorption of gas molecules. The surface of ptype Co3O4 is known to be readily covered with monolayers of negatively charged chemisorbed oxygen (O−(ads)),51,52 generating a charge accumulation layer on the particle surface according to the following reaction:

Figure 4. (a) The responses of the samples under different operating temperatures; Co3O4 hollow aggregates with 3 ppm (blue line) and 800 ppb (red line) of formaldehyde, and Co3O4 nanoparticles with 3 ppm (black line) of formaldehyde. Inset is the responses of the samples at a working temperature range of 200−250 °C. (b) The respective responses of Co3O4 hollow aggregates to different analytes (the gas concentrations were 800 ppb for HCHO, CO, and NO2, and 350 ppm for CO2) at an operating temperature of 220 °C.

HCHO(g) + 2O−(ads) + 2h* → CO2 (g) + H 2O(g)

When formaldehyde molecules are exposed, the charge accumulation layer becomes thinner due to the consumption of holes, which increases the sensor resistance. Literally, electrical conduction occurs through the competition between the charge accumulation layer on the surface and the resistive core and the junction (or the contact) between the particles. In this mechanism, a high response to a reducing gas is not expected in p-type semiconductors, and the response is less dependent on the particle size than that of the n-type counterparts, as the charges mainly conduct through the surface. However, if the single-crystalline particle size is smaller than the critical size of the grain value (20−100 nm) and the normal thickness (up to 10 nm) of the charge accumulation layer, and if it is larger than the minimum grain size (∼5 nm),8−11,46−48,53−55 nearly the entire area of the particles can accumulate charges, and the interaction with gas molecules abruptly changes the resistance of the particles. A large surface-to-volume ratio of the small nanoparticles also influences the total resistance change through the surface reaction. The Co3O4 hollow aggregates were composed of single-crystalline particles with a diameter of 15 nm, which were distinctly smaller than sensing materials reported thus far. However, the large enhancement of the response in the Co3O4 hollow samples compared to that in the

1 O2 (g) → O−(ads) + h* 2

Formaldehyde molecules are fully oxidized by O−(ads), as follows: D

dx.doi.org/10.1021/jp505791v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

Co3O4 nanoparticles indicates that the junction property and network formation were more important parameters than the particle domain size in our experiments (vide infra). Under an optimal condition at 220 °C, the Co3O4 hollow aggregates were tested with different types of gases, i.e., types that commonly exist in indoor building environments (Supporting Information, Figure S9). The gas concentrations were 800 ppb for formaldehyde, carbon monoxide, and nitrogen dioxide, and 350 ppm for carbon dioxide (Figure 4b). The response to formaldehyde showed exclusively high selectivity. Carbon monoxide also gave a response due to its strong reducing power, but the absolute value was 3 times smaller than that of formaldehyde. Meanwhile, it is known that Co3O4 materials have some sensitivity on various VOCs, including formaldehyde and ethanol. We also see that our hollow sample shows high activity on ethanol with no distinct activity difference from formaldehyde (Supporting Information, Figure S9d). However, ethanol does not commonly exist in indoor environments and not a chemical material under indoor environment regulation. The real-time response and recovery behaviors of the Co3O4 hollow aggregates and nanoparticles were investigated under optimal conditions after being sequentially exposed to different formaldehyde concentrations ranging from 50 to 3000 ppb (Figure 5a). After a stabilization process lasting for 2 min, formaldehyde and fresh air were injected in turns into the device chamber at the flow rates of 2−200 and 2000 cc/min,

respectively. For the Co3O4 hollow samples, the resistance clearly increased by 5% (the signal-to-noise ratio (SNR) = 31.25) with respect to the resistance in air (Ra) after exposure to 50 ppb of formaldehyde. This is a remarkably low concentration and the lowest ever reported in previous results using p-type semiconductors, as far as we know. The Li-doped NiO thin film showed the detectable concentrations up to 25 ppm,56 and the p−n homojunction film of Cu2O showed detection at the 100 ppm level.57 Even for n-type semiconductors, the optimum detection limits were at the same levels noted for the present Co3O4 hollow samples. A SnO2− NiO film could detect 60 ppb of formaldehyde with a conductance change of 2.6% at 300 °C,15−17 and the SnO nanoparticles showed a detection limit of 25 ppb at 250 °C.41−45 The response increased with the formaldehyde concentration, and the exposure of 3 ppm led to a response of 1.9, which could be extrapolated to the response of 30 for a concentration of 100 ppm for comparison. The concave Co3O4 mesocrystals exhibited a response of 1.8 at 20 ppm,32 and the Co3O4 nanostrings show a response of 4.4 at 100 ppm.44 These indicate that our materials produced high response values due to the small crystallites with high surface area compared to the other morphologies. Three repetitions of the 3 ppm exposure showed identical responses with a standard deviation of 1%, illustrating excellent reversibility for sensing applications. The Co3O4 nanoparticles were also able to detect 50 ppb of formaldehyde, but the responses were 4.5 times lower than those for the Co3O4 hollow samples over all concentration ranges. The average noise level of gas responses is 0.0016 ± 0.0004 for all of the formaldehyde sensors in Figure 5a. By increasing the operating temperature, the response time decreased because HCHO oxidation is a thermally activated reaction. Figure 5b indicates a linear increase of the response versus the formaldehyde concentration over all ranges for each sample. The inset emphasizes that the response was still large at low concentrations. According to the IUPAC definition, when the SNR equals 3, the signal is considered to be a true signal.58,59 The present experimental detection limit of the Co3O4 hollow samples was 50 ppb, and by reasonable extrapolation, the plausible detection limit reached to 5 ppb based on the SNR value. Considering the environmental guideline value of formaldehyde of 100 ppb under a building environment by the WHO,5 our device using the Co3O4 hollow aggregates has a far lower detection limit, demonstrating its potential to be applied in indoor environmental monitoring systems. The small crystalline domain size is one of the key factors behind such an extremely low detection limit, as observed in ntype SnO nanoparticles with fine grain sizes. Another key parameter is the thin hollow morphology of the Co3O4 aggregates. In n-type semiconductor gas sensors with a hollow morphology, all of the entire primary particles became active and the gas response speed increases through the regular pore structure. In our experiment, the total surface area of the Co3O4 hollow samples was 1.5 times larger than that of the Co3O4 nanoparticles, despite the fact that the single crystalline particle sizes were nearly identical. However, the gas response speed of the hollow morphology was not distinct from that of the nanoparticles, indicating that such a large enhancement of the sensitivity was not solely an effect of the morphology. This phenomenon would be greatly improved by a thin film fabrication process using alternative deposition methods.60 More importantly, the Co3O4 hollow aggregates had very thin

Figure 5. (a) Plots of the response versus time for Co3O4 hollow aggregates (red line) and Co3O4 nanoparticles (black line) upon exposure to formaldehyde gas at 220 °C. (b) Response variation of Co3O4 hollow aggregates (red line) and Co3O4 nanoparticles (black line) as a function of the formaldehyde concentration. Inset is the response at 50−200 ppb of formaldehyde. E

dx.doi.org/10.1021/jp505791v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

shells composed of single particle layers. In order to create active sensing materials, the Co3O4 hollow samples were densely packed in a solid state. As shown in Scheme 1, the Co3O4 hollow samples packed in the active materials simultaneously formed a three-dimensional network due to the physical contacts between the neighboring hollow structures (as emphasized in blue), and conductance actually occurred through this network. Interestingly, each network was only two-particles wide due to the single particle layer on each hollow structure. Therefore, the network could be simply regarded as woven conductive wires with a thickness of twice the particle diameter (∼30 nm). In this configuration, the electrical conduction was readily disrupted by changing a small number of conductive particles into nonconductive ones. The diameter of the primary particles was less than the thickness of the charge accumulation layer. Therefore, exposure to gas could effectively alter the electrical properties of each particle. In contrast, the dense packing of the Co3O4 nanoparticles had many contacts with neighboring particles. For instance, 12 junctions could form in an ideal hexagonally close-packed structure. Consequently, even if some of the nanoparticles became nonconductive, the electrical conduction of the entire agglomerate would not be significantly influenced. We have also analyzed the formaldehyde sensing properties of the Co3O4 hollow aggregates in terms of response and recovery time at variable temperatures (Supporting Information, Figures S7 and S10). The response and recovery times for a formaldehyde concentration of 3000 ppb are measured to be 279 and 564 s at 150 °C, 88 and 108 s at 200 °C, 75 and 105 s at 220 °C, 28 and 60 s at 300 °C, and 30 and 11 s at 350 °C, respectively. For the concentration of 50 ppb, the response and recovery times are measured to be 183 and 135 s at 150 °C, 67 and 88 s at 200 °C, 88 and 110 s at 220 °C, 52 and 20 s at 300 °C, and 17 and 7 s at 350 °C, respectively. As the operating temperature increases, the recovery speed tends to be largely enhanced. These response and recovery times are superior to the values of other p- and n-type materials. A Li-doped NiO thin film shows the response and recovery times of 10 s and 1− 2 min for 2500 ppm formaldehyde at 600 °C, of which the concentrations are 3 orders of magnitude larger than our experimental conditions.56 Among the representative n-type materials, the response and recovery times of a SnO2−NiO film are 200 and 230 s for 0.12 ppm and 100 and 94 s for 1 ppm formaldehyde at 300 °C.15 ZnO tetrapods exhibit a response time of 35 s and a recovery time of 5 min for 50 ppm aldehyde at 400 °C.61 The sensor consisting of Co3O4 hollow aggregates as an active material showed a stable long-term response for 3 days at the operating temperature in ambient air (Figure 6). It shows very stable values in the initial baseline resistance with a coefficiency of variance (CV) of 0.0016, as defined by CV = σ/ μ, where σ is the standard deviation and μ is the mean.62 For the Co 3O 4 hollow aggregates, although the resistance distinctively increased by only 5%, the SNR is ∼30 with respect to the resistance in air (Ra) after the exposure to 50 ppb formaldehyde, which was a remarkably low concentration compared to the previous results using p-type semiconductors. To the best of our knowledge, there have been no reports on the ppb-level formaldehyde detection using the Co 3 O 4 materials thus far.

Figure 6. Long-term stability curve of the sensing device using Co3O4 hollow aggregates upon exposure to air at 220 °C.

■

CONCLUSION A hierarchical structure of the single-crystalline Co3 O 4 nanoparticles was synthesized using ZnO spheres as sacrificial templates. This material exhibited remarkable sensing performance, showing extremely high sensitivity with a detection limit of 50 ppb, good selectivity for formaldehyde among the various indoor environmental gases, and good long-term stability for up to 3 days in air. These superior properties stemmed from the regular, hierarchically assembled structures with a small crystalline domain size under a critical size of a grain (20 nm), a thin hollow morphology with a large surface area, and a three-dimensional conductive network with a two-particle-wide diameter. We believe that this hierarchical assembly can be widely applied for other semiconductors and hybrid materials to enhance the solid-state sensing properties of various gases, showing great potential as a platform for improving human health through the monitoring of indoor environments in the field.

■

METHODS Chemicals. Cobalt acetylacetonate (Co(acac)2, 97%), cobalt(II) nitrate (Co(NO3)2, 98%), pentanediol (PD, 96%), poly(vinylpyrrolidione) (PVP, Mw = 55 000), oleylamine (70%), and zinc(II) acetylacetonate hexahydrate (99.995%) were purchased from Sigma-Aldrich and used without further purification. Synthesis of ZnO@Co(OH)2 Core−Shell Spherical Aggregates. Zinc acetylacetonate hydrate (0.10 g, 0.40 mmol) and PVP (1.0 g, 9.0 mmol) were dissolved in PD (50 mL), followed by slow heating to 250 °C for 12 min under an inert condition. Cobalt(II) nitrate (0.12 g, 0.40 mmol) dissolved in PD (5.0 mL) was injected into this hot zinc− PVP mixture solution, and the mixture was stirred for 30 min at 250 °C. The colloidal dispersion was cooled to room temperature, and the product was separated by adding ethanol (120 mL) with centrifugation. The precipitates were thoroughly washed with ethanol and dried in a vacuum oven at 80 °C for 12 h. Synthesis of Co3O4 Hollow Aggregates. The ZnO@ Co(OH)2 aggregate powder was placed in a ceramic boat. The sample was heated at a ramping rate of 4 K/min to 350 °C, and calcined at 350 °C for 3 h in air in a glass tube oven. The resulting powder (0.20 g) was dispersed in deionized water (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 F

dx.doi.org/10.1021/jp505791v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

*E-mail: [email protected] (H.S.).

ethanol. The particles were dried in a vacuum oven at 80 °C for 12 h. Synthesis of Co3O4 Nanoparticles. A mixture of Co(acac)2 (1.5 g, 6.0 mmol) and oleylamine (80.3 g, 300 mmol) was slowly heated from room temperature to 220 °C for 25 min under inert conditions, and allowed to stir at 220 °C for 35 min. The product was collected by centrifugation, thoroughly washed with ethanol, and dried in a vacuum oven at 80 °C for 12 h. After drying, the CoO nanoparticle powder were placed in the ceramic boat in a glass tube oven, heated at a ramping rate of 4 K/min to 350 °C, and calcined at 350 °C for 3 h in air. Sensor Fabrication. A paste for screen printing was prepared using the roll-milling process with α-terpineol as a solvent. The paste had a 50% solid content with ∼100 000 cP viscosity. A Si wafer was used as a substrate that could adapt the MEMS (microelectromechanical systems) technique. All fabrication processes were compatible with the CMOS (complementary metal oxide semiconductor) process available in a mass production and in a high yield. A SiO2 thin film was deposited by the LPCVD (low pressure chemical vapor deposition) process on a 4 in. 100-plane p-type two-sided polished Si wafer with a 500 μm thickness. A Pt thin film as a sensing electrode was deposited by the e-beam evaporation process, and the electrode pattern was made by the lift-off process in acetone. The sensing electrode has an interdigit (IDT) structure with a 10 μm width and a 5 μm gap. The sensing materials were screen-printed with a 325 mesh on the IDT electrode and dried at 120 °C for 10 min in an electric oven. The area of the sensing film is 2 × 2 mm2 (Figure 4). Finally, the sample was treated at 400 °C for 2 h in an electric furnace. Characterization. The nanostructures were characterized by Omega EM912 (120 kV, Korea Basic Science Institute) and Philips F20 Tecnai (200 kV, KAIST) transmission electron microscopes. Samples were prepared by putting a few drops of the corresponding colloidal solutions on carbon-coated copper grids (Ted Pellar, Inc.). X-ray powder diffraction patterns of nanostructures were recorded on a Rigaku D/MAX-RB (12 kW) diffractometer with Cu Kα radiation. Nitrogen sorption isotherms were measured at 77 K with a BELSORP mini-II (BEL Japan Inc.). Gas sensing properties were measured using a computer-controlled characterization system. Gas sensors prepared on the Si substrate were investigated on a hot chuck in the water-cooled steel chamber which is possible to control temperature precisely. Formaldehyde and fresh air were injected in turns into the device chamber, with the flow rates of 2−200 and 2000 cc/min, respectively, using a mass flow controller (MFC). Also, we used carbon dioxide, carbon monoxide, and nitrogen oxide, existing in building, for crosssensitivity. The resistances of the sensor materials were measured by using a data acquisition board (DAQ) that was simultaneously able to acquire 24 channels of analog inputs.

■

Author Contributions §

J.Y.K. and N.-J.C. 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) (2012-005624, R11-2007-050-00000-0). 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), and by the R&D Program of MKE/KEIT 276 (10035570, Development of self-powered smart sensor node platform for smart and green buildings).

■

ASSOCIATED CONTENT

S Supporting Information *

Additional crystal information, TEM images, XRD spectra, and gas sensing results. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) He, Z. K.; Zhang, Y. P.; Wei, W. J. Formaldehyde and VOC Emissions at Different Manufacturing Stages of Wood-Based Panels. Build. Environ. 2012, 47, 197−204. (2) Cogliano, V. J.; Baan, R. A.; Straif, K.; Secretan, M. B.; Ghissassi, F. E. Summary of IARC Monographs on Formaldehyde, 2Butoxyethanol, and 1-tert-Butoxy-2-Propanol. Environ. Health Perspect. 2005, 113, 1205−1208. (3) Salthammer, T. Formaldehyde in the Ambient Atmosphere: From an Indoor Pollutant to An Outdoor Pollutant? Angew. Chem., Int. Ed. 2013, 52, 3320−3327. (4) Siew, S. S.; Kauppinen, T.; Kyyrönen, P.; Heikkilä, P.; Pukkala, E. Occupational Exposure to Wood Dust and Formaldehyde and Risk of Nasal, Nasopharyngeal, and Lung Cancer among Finnish Men. Cancer Manage. Res. 2012, 4, 223−232. (5) Kawamura, K.; Kerman, K.; Fujihara, M.; Nagatani, N.; Hashiba, T.; Tamiya, E. Development of a Novel Hand-Held Formaldehyde Gas Sensor for the Rapid Detection of Sick Building Syndrome. Sens. Actuators, B 2005, 105, 495−501. (6) Hunter, M. C.; Bartle, K. D.; Seakins, P. W.; Lewis, A. C. Direct Measurement of Atmospheric Formaldehyde Using Gas Chromatography-Pulsed Discharge Ionisation Detection. Anal. Commun. 1999, 36, 101−104. (7) Kim, S.; Kim, H.-J. Comparison of Standard Methods and Gas Chromatography Method in Determination of Formaldehyde Emission from MDF Bonded with Formaldehyde-Based Resins. Bioresour. Technol. 2005, 96, 1457−1464. (8) Barsan, N.; Koziej, D.; Weimar, U. Metal Oxide-Based Gas Sensor Research: How to? Sens. Actuators, B 2007, 121, 18−35. (9) Barsan, N.; Simion, C.; Heine, T.; Pokhrel, S.; Weimar, U. Modeling of Sensing and Transduction for p-Type Semiconducting Metal Oxide Based Gas Sensors. J. Electroceram. 2010, 25, 11−19. (10) Chung, P.-R.; Tzeng, C.-T.; Ke, M.-T.; Lee, C.-Y. Formaldehyde Gas Sensors: A Review. Sensors 2013, 13, 4468−4484. (11) Chen, P.-C.; Shen, G.; Zhou, C. Chemical Sensors and Electronic Noses Based on 1-D Metal Oxide Nanostructures. IEEE Trans. Nanotechnol. 2008, 7, 668−682. (12) Han, L.; Wang, D.; Cui, J.; Chen, L.; Jiang, T.; Lin, Y. Study on Formaldehyde Gas-Sensing of In2O3-Sensitized ZnO Nanoflowers under Visible Light Irradiation at Room Temperature. J. Mater. Chem. 2012, 22, 12915−12920. (13) Kida, T.; Doi, T.; Shimanoe, K. Synthesis of Monodispersed SnO2 Nanocrystals and Their Remarkably High Sensitivity to Volatile Organic Compounds. Chem. Mater. 2010, 22, 2662−2667. (14) Chen, T.; Zhou, Z.; Wang, Y. Effects of Calcining Temperature on the Phase Structure and the Formaldehyde Gas Sensing Properties of CdO-Mixed In2O3. Sens. Actuators, B 2008, 135, 219−223. (15) Lv, P.; Tang, Z. A.; Yu, J.; Zhang, F. T.; Wei, G. F.; Huang, Z. X.; Hu, Y. Study on a Micro-Gas Sensor with SnO2−NiO Sensitive

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.-S.L.). G

dx.doi.org/10.1021/jp505791v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

Film for Indoor Formaldehyde Detection. Sens. Actuators, B 2008, 132, 74−80. (16) Castro-Hurtado, I.; Herran, J.; Mandayo, G. G.; Castano, E. Studies of Influence of Structural Properties and Thickness of NiO Thin Films on Formaldehyde Detection. Thin Solid Films 2011, 520, 947−952. (17) Lee, C.-Y.; Chiang, C.-M.; Wang, Y.-H.; Ma, R.-H. A SelfHeating Gas Sensor with Integrated NiO Thin-Film for Formaldehyde Detection. Sens. Actuators, B 2007, 122, 503−510. (18) Maekawa, T.; Tamaki, J.; Miura, N.; Yamaoze, N. Gold-Loaded Tungsten Oxide Sensor for Detection of Ammonia in Air. Chem. Lett. 1992, 4, 639−642. (19) Kolmakov, A.; Zhang, Y.; Cheng, G.; Moskovits, M. Detection of CO and O2 Using Tin Oxide Nanowire Sensors. Adv. Mater. 2003, 15, 997−1000. (20) Zhang, S.-L.; Byun, H.-G.; Lim, J.-O.; Huh, J.-S.; Lee, W. Controlled Synthesis of ZnO Nanostructures for Sub-ppm-Level VOC Detection. IEEE Sensors J. 2012, 12, 3149−3155. (21) Tak, Y.; Yong, K. A Novel Heterostructure of Co3O4/ZnO Nanowire Array Fabricated by Photochemical Coating Method. J. Phys. Chem. C 2008, 112, 74−79. (22) Kim, H. G.; Borse, P. H.; Choi, W. Y.; Lee, J. S. Photocatalytic Nanodiodes for Visible-Light Photocatalysis. Angew. Chem., Int. Ed. 2005, 44, 4585−4589. (23) Na, C. W.; Woo, H. S.; Kim, I.-D.; Lee, J.-H. Selective Detection of NO2 and C2H5OH Using a Co3O4-Decorated ZnO Nanowire Network Sensor. Chem. Commun. 2011, 47, 5148−5150. (24) Bekermann, D.; Gasparotto, A.; Barreca, D.; Maccato, C.; Comini, E.; Sada, C.; Sberveglieri, G.; Devi, A.; Fischer, R. A. Co3O4/ ZnO Nanocomposites: From Plasma Synthesis to Gas Sensing Applications. ACS Appl. Mater. Interfaces 2012, 4, 928−934. (25) Yoon, J.-W.; Choi, J.-K.; Lee, J.-H. Design of a Highly Sensitive and Selective C2H5OH Sensor Using p-Type Co3O4 Nanofibers. Sens. Actuators, B 2012, 161, 570−577. (26) Gulino, A.; Fiorit, G.; Fragal, I. Deposition of Thin Films of Cobalt Oxides by MOCVD. J. Mater. Chem. 2003, 13, 861−865. (27) Weichel, S.; M?ller, P. Annealing-Induced Microfaceting of the CoO(100) Surface Investigated by LEED and STM. J. Surf. Sci. 1998, 399, 219−224. (28) Svegl, F.; Orel, B.; Hutchins, M. G.; Kalcher, K. Structural and Spectroelectrochemical Investigations of Sol-Gel Derived Electrochromic Spinel Co3O4 Films. J. Electrochem. Soc. 1996, 143, 1532− 1539. (29) He, T.; Chen, D.; Jiao, X.; Wang, Y. Co3O4 Nanoboxes: Surfactant-Templated Fabrication and Microstructure Characterization. Adv. Mater. 2006, 18, 1078−1082. (30) Ando, M.; Kobayashi, T.; Lijima, S.; Haruta, M. Optical Recognition of CO and H2 by Use of Gas-Sensitive Au−Co3O4 Composite Films. J. Mater. Chem. 1997, 7, 1779−1783. (31) Lou, X. W.; Deng, D.; Lee, J. Y.; Feng, J.; Archer, L. A. SelfSupported Formation of Needlelike Co3O4 Nanotubes and Their Application as Lithium-Ion Battery Electrodes. Adv. Mater. 2008, 20, 258−262. (32) Liu, Y.; Zhu, G.; Ge, B.; Zhou, H.; Yuan, A.; Shen, X. Concave Co3O4 Octahedral Mesocrystal: Polymer-Mediated Synthesis and Sensing Properties. CrystEngComm 2012, 14, 6264−6270. (33) Li, W.-Y.; Xu, L.-N.; Chen, J. Co3O4 Nanomaterials in LithiumIon Batteries and Gas Sensors. Adv. Funct. Mater. 2005, 15, 851−857. (34) Cao, A.-M.; Hu, J.-S.; Liang, H.-P.; Song, W.-G.; Wan, L.-J.; He, X.-L.; Gao, X.-G.; Xia, S.-H. Hierarchically Structured Cobalt Oxide (Co3O4): The Morphology Control and Its Potential in Sensors. J. Phys. Chem. B 2006, 110, 15858−15863. (35) Nguyen, H.; El-Safty, S. A. Meso- and Macroporous Co3O4 Nanorods for Effective VOC Gas Sensors. J. Phys. Chem. C 2011, 115, 8466−8474. (36) Choi, K. I.; Kim, H.-R.; Kim, K.-M.; Liu, D.; Cao, G.; Lee, J.-H. C2H5OH Sensing Characteristics of Various Co3O4 Nanostructures Prepared by Solvothermal Reaction. Sens. Actuators, B 2010, 146, 183− 189.

(37) Geng, B.; Zhan, F.; Fang, C.; Yu, N. A Facile Coordination Compound Precursor Route to Controlled Synthesis of Co3O4 Nanostructures and Their Room-Temperature Gas Sensing Properties. J. Mater. Chem. 2008, 18, 4977−4984. (38) Wen, Z.; Zhu, L.; Mei, W.; Li, Y.; Hu, L.; Sun, L.; Wan, W.; Ye, Z. A Facile Fluorine-Mediated Hydrothermal Route to Controlled Synthesis of Rhombus-Shaped Co3O4 Nanorod Arrays and Their Application in Gas Sensing. J. Mater. Chem. 2013, 1, 7511−7518. (39) Lee, J.-H. Gas Sensors Using Hierarchical and Hollow Oxide Nanostructures: Overview. Sens. Actuators, B 2009, 140, 319−336. (40) Lai, X.; Li, J.; Korgel, B. A.; Dong, Z.; Li, Z.; Su, F.; Du, J.; Wang, D. General Synthesis and Gas-Sensing Properties of MultipleShell Metal Oxide Hollow Microspheres. Angew. Chem., Int. Ed. 2011, 50, 2738−2741. (41) Choi, N. J.; Lee, H. K.; Moon, S. E.; Yang, W. S. Fast Response Formaldehyde Gas Sensor for USN Application. Sens. Actuators, B 2012, 175, 132−136. (42) Xing, R.-Q.; Xu, L.; Zhu, Y.-S.; Song, J.; Qin, W.-F.; Dai, Q.-L.; Liu, D.-L.; Song, H.-W. Three-Dimensional Ordered SnO2 Inverse Opals for Superior Formaldehyde Gas-Sensing Performance. Sens. Actuators, B 2013, 188, 235−241. (43) Jiao, Q.; Fu, M.; You, C.; Zhao, Y.; Li, H. Preparation of Hollow Co3O4 Microspheres and Their Ethanol Sensing Properties. Inorg. Chem. 2012, 51, 11513−11520. (44) Liu, Y.; Zhu, G.; Chen, J.; Xu, H.; Shen, X.; Yuan, A. Co3O4/ ZnO Nanocomposites for Gas-Sensing Applications. Appl. Surf. Sci. 2013, 265, 379−384. (45) Park, J.; Shen, X.; Wang, G. Solvothermal Synthesis and GasSensing Performance of Co3O4 Hollow Nanospheres. Sens. Actuators, B 2009, 136, 494−498. (46) Xu, C.; Tamaki, J.; Miura, N.; Yamazoe, N. Grain Size Effects on Gas Sensitivity of Porous SnO2-Based Elements. Sens. Actuators, B 1991, 3, 147−155. (47) Pinna, N.; Neri, G.; Antonietti, M.; Niederberger, M. Nonaqueous Synthesis of Nanocrystalline Semiconducting Metal Oxides for Gas Sensing. Angew. Chem., Int. Ed. 2004, 43, 4345−4349. (48) Franke, M. E.; Koplin, T. J.; Simon, U. Metal and Metal Oxide Nanoparticles in Chemiresistors: Does The Nanoscale Matter? Small 2006, 2, 36−50. (49) Wang, C.-A.; Li, S.; An, L. Hierarchically porous Co3O4 hollow spheres with tunable pore structure and enhanced catalytic activity. Chem. Commun. 2013, 49, 7427−7429. (50) Park, J. C.; Kim, A. Y.; Kim, J. Y.; Park, S.; Park, K. H.; Song, H. ZnO−CuO Core−Branch Nanocatalysts for Ultrasound-Assisted Azide−Alkyne Cycloaddition Reactions. Chem. Commun. 2012, 48, 8484−8486. (51) Hagen, J. Heterogeneous Catalysis: Fundamentals; Wiley-VCH: Weinheim, Germany, 1999. (52) Kohl, D. Function and Applications of Gas Sensors. J. Phys. D: Appl. Phys. 2000, 34, R125. (53) Lu, F.; Liu, Y.; Dong, M.; Wang, X. P. Nanosized Tin Oxide as the Novel Material with Simultaneous Detection Towards CO, H2 and CH4. Sens. Actuators, B 2000, 66, 225−227. (54) Liu, A.; Jones, R.; Liao, L.; Samara-Rubio, D.; Rubin, D.; Cohen, O.; Nicolaescu, R.; Paniccia, M. High Speed Silicon Mach-Zehnder Modulator. Nature 2004, 427, 615−618. (55) Zaretskiy, N. P.; Menshikov, L. I.; Vasiliev, A. A. Theory of Gas Sensitivity of Nanostructured MOX Layers: Charge Carrier SelfExhaustion Approach. Sens. Actuators, B 2012, 175, 234−245. (56) Dirksen, J. A.; Duval, K.; Ring, T. A. NiO Thin-Film Formaldehyde Gas Sensor. Sens. Actuators, B 2001, 80, 106−115. (57) Jiang, T.; Xie, T.; Yang, W.; Fan, H.; Wang, D. Photoinduced Charge Transfer Process in p-Cu2O/n-Cu2O Homojunction Film and Its Photoelectric Gas-Sensing Properties. J. Colloid Interface Sci. 2013, 405, 242−248. (58) Li, J.; Lu, Y.; Ye, Q.; Cinke, M.; Han, J.; Meyyappan, M. Carbon Nanotube Sensors for Gas and Organic Vapor Detection. Nano Lett. 2003, 3, 929−933. H

dx.doi.org/10.1021/jp505791v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

(59) Currie, L. A. Nomenclature in Evaluation of Analytical Methods Including Detection and Quantification Capabilities. Pure Appl. Chem. 1995, 67, 1699−1723. (60) Sakai, G.; Matsunaga, N.; Shimanoe, K.; Yamazoe, N. Theory of Gas-Diffusion Controlled Sensitivity for Thin Film Semiconductor Gas Sensor. Sens. Actuators, B 2001, 80, 125−131. (61) Calestani, D.; Mosca, R.; Zanichelli, M.; Villani, M.; Zappettini, A. Aldehyde detection by ZnO tetrapod-based gas sensors. J. Mater. Chem. 2011, 21, 15532−15536. (62) Li, J.; Lu, Y.; Ye, Q.; Cinke, M.; Han, J.; Meyyappan, M. Carbon nanotube sensors for gas and organic vapor detection. Nano Lett. 2003, 3, 929−933.

I

dx.doi.org/10.1021/jp505791v | J. Phys. Chem. C XXXX, XXX, XXX−XXX