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Nanoscale PdO Catalyst Functionalized Co3O4 Hollow Nanocages using MOF Templates for Selective Detection of Acetone Molecules in Exhaled Breath Won-Tae Koo, Sunmoon Yu, Seon-Jin Choi, Ji-Soo Jang, Jun Young Cheong, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01284 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017

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ACS Applied Materials & Interfaces

Nanoscale PdO Catalyst Functionalized Co3O4 Hollow Nanocages using MOF Templates for Selective Detection of Acetone Molecules in Exhaled Breath Won-Tae Koo,† Sunmoon Yu,† Seon-Jin Choi,†,‡ Ji-Soo Jang,† Jun Young Cheong,† and IlDoo Kim*,† †

Department of Materials Science and Engineering and ‡Applied Science Research Institute,

Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea *Corresponding author e-mail: [email protected]

KEYWORDS Metal-Organic Framework, ZIF-67, Gas Sensors, Catalyst, Metal Oxide

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ABSTRACT

The increase of surface area and the functionalization of catalyst are crucial to develop highperformance semiconductor metal oxide (SMO) based chemi-resistive gas sensors. Herein, nanoscale catalyst loaded Co3O4 hollow nanocages (HNCs) by using metal-organic framework (MOF) templates have been developed as a new sensing platform. Nanoscale Pd nanoparticles (NPs) were easily loaded on the cavity of Co based zeolite imidazole framework (ZIF-67). The porous structure of ZIF-67 can restrict the size of Pd NPs (2–3 nm) and separate Pd NPs from each other. Subsequently, the calcination of Pd loaded ZIF-67 produced the catalytic PdO NPs functionalized Co3O4 HNCs (PdO-Co3O4 HNCs). The ultrasmall PdO NPs (3–4 nm) are well-distributed in the wall of Co3O4 HNCs, the unique structure of which can provide high surface area and high catalytic activity. As a result, the PdO-Co3O4 HNCs exhibited improved acetone sensing response (Rgas/Rair = 2.51 to 5 ppm) compared to PdO-Co3O4 powders (Rgas/Rair = 1.98), Co3O4 HNCs (Rgas/Rair = 1.96), and Co3O4 powders (Rgas/Rair = 1.45). In addition, the PdO-Co3O4 HNCs showed high acetone selectivity against other interfering gases. Moreover, the sensor array clearly distinguished simulated exhaled breath of diabetics from healthy people’s breath. These results confirmed the novel synthesis of MOF templated nanoscale catalyst loaded SMO HNCs for high performance gas sensors.


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1. Introduction Semiconductor metal oxide (SMO), including SnO2, WO3, ZnO, and Co3O4, based chemiresistive gas sensors have received great attention due to their fascinating features, such as fast response and recovery time, low cost, and portability.1–4 To utilize these advantages, SMO based gas sensors have been widely studied for applications in volatile organic compounds detection, such as exhaled breath analyzers, harmful environmental sensors, and indoor air quality sensors.5-7 As the environmental pollution and health care become a serious problem nowadays, there are numerous pollutants and volatile organic compounds near us, so that the SMO based gas sensors that can be possible for miniaturization have attracted a great attention as a future sensing system.8 In particular, the early diagnosis of human disease can be realized by detecting biomarker gases in human breath.9 For example, the exhaled breath of diabetics increases to more than 1.8 part per million (ppm) of acetone molecule, comparing with that (0.3–0.5 ppm) of normal people. However, the low response and selectivity of SMO based gas sensors are still present as big barriers to the actual commercialization. The enlargement of surface area and the promotion of surface reaction should be achieved to develop superior SMO based analyzers, since the gas sensors are operated by the surface reaction of target gas on the SMO.10 Particularly, the hollow nanostructures can maximize the surface area,11 and the uniformly dispersed nanoscale catalysts can effectively promote the surface reactions.12 For instance, Jung et al. synthesized Co3O4-SnO2 hollow nanostructures for methylbenzene gas sensors.13 They synthesized Co3O4 hollow spheres using spray pyrolysis, and thin layers of SnO2 were coated on the Co3O4 by galvanic replacement. In addition, Wang and co-workers reported nanoscale Co3O4 decorated SnO2 core-shell nanospheres for ammonia gas sensors through hydrothermal and following calcination.14 However, the nanoscale catalysts are easily aggregated at high operating temperature, which 3 ACS Paragon Plus Environment


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lead to significant degradation in the sensing properties. In addition, additional synthetic steps are required to synthesize the nanoscale catalyst and bind it onto the hollow SMO. Therefore, the stable functionalization of nanoscale catalyst onto sensing layers with facile synthetic methods is challenging issue. On the other hand, metal-organic frameworks (MOFs) are new emerging materials which have ultrahigh porosity and incredibly high surface area.15 To utilize such highly promising materials in practical devices, MOF has been studied as a potential application in various fields, such as gas storage,16,17 catalysis,18 energy storage system,19 and drug delivery.20 In addition, MOF derived hollow and porous metal oxides have emerged due to their innumerable reaction site, porosity, and easy gas accessibility, so that extensive efforts have been devoted for potential applications in various fields.21,22 For instance, Shao and coworkers synthesized MOF-derived Co3O4 hollow dodecahedrons through one-step or twostep calcination as anode materials for Li-ion battery.23 Wu et al. also reported high symmetric porous Co3O4 hollow dodecahedra using Co based zeolite imidazole framework (ZIF-67) for the enhanced Li storage capability.24 In addition, hollow Co3O4 derived from ZIF-67 was synthesized via two-step thermolysis process for CO oxidation catalysts.25 Furthermore, the chemical gas sensors using MOF templated metal oxide have been reported in few articles to utilize its high surface area. Li et al. synthesized hollow ZnO cubes using Zn based zeolite imidazole framework (ZIF-8) and investigated benzene gas sensing properties.26 Lu and co-workers reported porous Co3O4 concave nanocubes using pure MOF templates for ethanol sensing.27 Although these sensing properties are largely improved than the pure MOF based chemi-resistive gas sensors,28 it is still insufficient to realize the accurate detection of sub-ppm level of target gas in a highly humid atmosphere.


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One of fascinating properties of MOF is that nanoscale metal nanoparticles (NPs) can be easily encapsulated in its cavities.29 As the size of metal NPs can be restricted by the cavity structure of MOF, ultra-small metal NPs (sub-10nm) are uniformly loaded on the MOF with high dispersion state. Hermes and co-workers firstly reported the metal loaded MOF (metal@MOF) via metal organic chemical vapor deposition.30 Cu, Pd, and Au were infiltrated in the cavity of MOF-5 and these materials showed high catalytic activities in cyclooctene hydrogenation. After that, Au and Pt NPs were easily encapsulated in the ZIF-8 by using polymer surfactants.31 In addition, Au, Ag, and Au@Ag core-shell NPs (< 10 nm) were encapsulated in the ZIF-8 by the infiltration of metal ions followed by reduction.32 Furthermore, our group reported the WO3 nanofibers functionalized by Pd@ZIF-8 derived catalysts through the electrospinning and following calcination.7 The metal@MOF driven complex catalysts improved the sensing properties by the synergistic effect, however, it is difficult to synthesize the materials due to the complicated synthetic process. Therefore, it is still urgent to design and synthesize the metal@MOF based sensing materials with high performance and the easy of fabrication. Herein, we firstly propose a facile synthesis of nanoscale catalyst loaded metal oxide hollow nanocages (HNCs) using metal@MOF templates, as a new gas sensing platform. The hollow structure, nanoscale catalyst, and the functionalization of nanoscale catalyst onto the hollow structure can be simultaneously achieved by utilizing MOF as a template. The catalytic metal NPs were easily loaded on ZIF-67 by the infiltration of metal precursors and subsequent reduction process (metal@ZIF-67). Then, metal@ZIF-67 converted to catalyst loaded Co3O4 hollow HNCs through one-step heat-treatment. During the calcination, the ZIF67 was transformed to the highly porous Co3O4 HNCs by the Kirkendall effect, which have high surface area and high gas accessibility. In addition, the nanoscale catalysts were 5 ACS Paragon Plus Environment


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uniformly functionalized on the wall of Co3O4 HNCs. These two advantages, (i) plenty of reaction sites and (ii) well-dispersed nanoscale catalysts, are key points to develop high performance gas sensors. Thus, the novel sensing nanomaterials can effectively improve the sensing characteristics. As a proof of the concept, we prepared nanoscale PdO NPs (3–4 nm) functionalized Co3O4 HNCs (PdO-Co3O4 HNCs) using Pd@ZIF-67 for selective detection of acetone molecules. Co3O4 as a typical p-type sensing material can exhibit high selectivity when combined with catalysts.33 Since PdO NPs are well-known catalysts to improve the acetone sensing characteristics,34-36 well-distributed PdO NPs can dramatically improve the surface reaction of Co3O4 HNCs. As a result, we found that the PdO-Co3O4 HNCs showed the enhanced acetone sensing performances, in terms of response, selectivity, and stability.

2. Materials and Methods 2.1 Materials Methanol (CH3OH), ethanol (C2H5OH) cobalt nitrate hexahydrate (Co(NO3)2·6H2O), sodium borohydride (NaBH4), and potassium tetrachloropalladate(II) (K2PdCl4) were purchased from Sigma-Aldrich. 2-methylimidazole (2MeIm, C4H6N2, 99.0%) were purchased from Aldrich. All chemicals were used without further purification. 2.2 Synthesis of ZIF-67, Co3O4 powder, and Co3O4 HNCs ZIF-67 was synthesized by precipitation reaction at room temperature (RT). 2.933 g of Co(NO3)2·6H2O and 6.489 g of 2MeIm were dissolved in 200 mL of methanol, respectively. These solutions were rapidly mixed together and stirred for 5 h at RT. After precipitation, the ZIF-67 was purified by centrifugation with 3,000 rpm for 1 min, and washed by ethanol. 6 ACS Paragon Plus Environment

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Then, the obtained ZIF-67 was dried at 50 °C for 12 h. Co3O4 powder and Co3O4 HNCs can be synthesized by the calcination of ZIF-67. Co3O4 powders were produced by the calcination at 600 °C for 1 h with the heating rate of 10 °C min-1. On the other hand, Co3O4 HNCs were produced by the calcination at 400 °C for 1 h with the ramping rate of 10 °C min-1. 2.3 Synthesis of Pd@ZIF-67, PdO-Co3O4 powders, and PdO-Co3O4 HNCs. To synthesize the PdO-Co3O4 HNCs, Pd NPs were encapsulated in the cavity of ZIF-67 by the infiltration and reduction process. 40 mg of ZIF-67 was dispersed in 1 mL of deionized water (DI-water). Then, K2PdCl4 was dissolved in ZIF-67 dispersion, and Pd metal ions were reduced by NaBH4 solution (1.5 mg mL-1). After encapsulation process, Pd@ZIF-67 was also purified by centrifugation and washed by DI-water. Finally, PdO-Co3O4 powders were obtained after the calcination at 600 °C for 1 h (ramping rate of 10 °C min-1), and PdO-Co3O4 HNCs were prepared by calcination at 400 °C for 1 h (ramping rate of 10 °C min-1). 2.4 Sensor fabrication and gas sensing measurement Firstly, 4 mg of each samples (Co3O4 powders, Co3O4 HNCs, PdO-Co3O4 powders, and PdO-Co3O4 HNCs) was respectively dispersed in 200 mL of ethanol. Then, the suspensions were drop-coated on the individual substrates (Al2O3) which were patterned two parallel Au electrodes (width: 25 µm, distance: 70 µm) on the front side and a Pt micro heater on the back side. Gas sensing measurements were carried out toward various analytes (acetone, hydrogen sulfide, ethanol, pentane, toluene, ammonia, and carbon monoxide) at 250–400 °C. The operating temperature was controlled by changing the voltage of Pt-micro-heater using DC power supply (E3647A, Agilent). Before the sensing test, the sensors were stabilized by the injection of air for 5 h toward each operating temperature. After stabilization, the sensors were exposed to air for 10 min in the chamber. Then, analytes in the concentration range of 7 ACS Paragon Plus Environment


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400 ppb to 5 ppm was injected for 10 min. The relative humidity was fixed at 90% RH. For the exhaled breath analysis, the exhaled breath was captured in a Tedlar bag, and the exhaled breath was injected by using diaphragm pump with a 2 min on/off interval. For the simulated diabetics’ breath analysis, the exhaled breath with 2 ppm of acetone was injected by using mass flow controller. The response was defined as the ratio of resistance (Rgas/Rair to reducing gas and Rair/Rgas to oxidizing gas). The resistances of sensors were investigated by using acquisition system (34972, Agilent), and then the response values were calculated. 2.5 Characterization Field emission scanning electron microscopy (FE-SEM) (Nova230, FEI) was used to investigate the morphologies of samples. The micro structures of Pd@ZIF-67 and PdOCo3O4 HNCs were analyzed by field emission transmission electron microscopy (FE-TEM) (Tecnai G2 F30 S-Twin, FEI). Powder X-ray diffraction (XRD) analysis using X-ray diffractometer (D/MAX-2500, Rigaku) using Cu Kα radiation was conducted to verify the crystal structure of products. The chemical bonding states of PdO-Co3O4 HNCs were identified by X-ray photoelectron spectroscopy (XPS) (Sigma Probe, Thermo VG Scientific) using Al Kα radiation. The Brunauer–Emmett–Teller (BET) surface area was investigated by N2 adsorption/desorption isotherms (Tristar 3020, Micromeritics) at 77 K. The thermal behavior of ZIF-67 during calcination in air atmosphere was investigated by thermal gravimetric analysis (Labsys Evo, Setaram).

3. Results and Discussion The synthetic process of PdO-Co3O4 HNCs is illustrated in scheme 1. Firstly, Pd NPs were encapsulated in the cavity of ZIF-67 by the infiltration of Pd ions in deionized water (DI8 ACS Paragon Plus Environment

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water) followed by sodium borohydride (NaBH4) reduction. After the encapsulation of Pd, PdO-Co3O4 HNCs were finally achieved after the calcination under air atmospheres. First of all, highly porous ZIF-67 plays an important role in the formation of Co3O4 HNCs and the encapsulation of ultra-small Pd NPs. As a sacrificial template, ZIF-67 was easily prepared by room temperature (RT) precipitation. The synthesized ZIF-67 showed polyhedron structures with the average size of 500 nm (Figure 1a), and the crystal and porous structure of ZIF-67 were confirmed by X-ray diffraction (XRD) analysis and N2 adsorption/desorption isotherms at 77 K (Figure S1 and S2), respectively. The results clearly exhibited the microporous structure of synthesized ZIF-67. In addition, we investigate the calcination condition of ZIF67 to produce Co3O4 HNCs. Since the hollow Co3O4 structure can be obtained by controlling calcination condition,23 we scrutinized the thermal behavior of ZIF-67. Thermal gravimetric analysis of ZIF-67 under air atmosphere clearly showed that ZIF-67 was rapidly decomposed at 400 °C (Figure S3). Therefore, the calcination of ZIF-67 was performed at 400 °C, 500 °C, and 600 °C for 1 h in air atmospheres. After the high-temperature heat-treatment, XRD analysis of samples clearly showed the cubic spinel structure of Co3O4 (JCPDS 43-1003) (Figure S4). In addition, the crystallinity of products was increased at higher calcination temperature. The average grain size was further analyzed by using Scherrer equation based on XRD results. The Scherrer formula is defined as D=Kλ/βcos(θ), where K is the shape factor (typical value = 0.9), D is the average grain size at a specific peak, λ is the wavelength of the X-ray sources (0.154 nm for Cu Kα), and β is the line broadening at half-maximum. The average grain size of respective samples is 23.41 nm for Co3O4 calcined at 400 °C, 30.98 nm for Co3O4 calcined at 500 °C, and 35.00 nm for Co3O4 calcined at 600 °C. Therefore, in terms of grain sizes, the Co3O4 calcined at 400°C is more attractive as a sensing layer, because the small grain size can exhibit high sensing properties.10 The morphology of samples was observed by field emission scanning electron microscopy (FE-SEM). The 9 ACS Paragon Plus Environment


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hollow structures was obtained by the calcination at 400 °C (Figure 1b). On the other hand, the calcination at 500 °C produced the mixed structure of powder and HNC (Figure 1c). As the calcination temperature was increased to 600 °C, most of ZIF-67 templates were converted to Co3O4 powders (Figure 1d). During the calcination, the organic compounds of ZIF-67 were decomposed, and the Co ions in ZIF-67 were oxidized to Co3O4 from the outside of ZIF-67. The spaces occupied by the organic linkers become empty sites, and the Co3O4 oxidized from Co in ZIF-67 are condensed to reduce the surface energy during the calcination.23 Therefore, the particle size of PdO-Co3O4 HNCs is reduced than Pd@ZIF-67. In addition, at the beginning of the oxidation, the outer diffusion of Co was faster than the inner diffusion of O, which can create the inner void in ZIF-67 by Kirkendall effect that is a phenomenon that the pores are formed because of the diffusivity difference between two components.23,37,38 The inner voids were continuously created during the calcination, then ZIF-67 templates were finally transformed to Co3O4 HNCs after the calcination at 400 °C for 1 h. However, as the calcination temperature increased, the inner vacancies with high thermal energy diffused to the outer surface, so that the hollow structure was gradually collapsed.39 Therefore, the particle size of Co3O4 was shrunk to 200 nm for 500 °C and 100 nm for 600 °C, comparing with that (300 nm) of Co3O4 HNCs obtained by the calcination at 400 °C. In addition, catalytic metal NPs are be easily combined with Co3O4 HNCs using ZIF-67 templates. The cavity structure of ZIF-67 can limit the size of NPs to 2–3 nm and isolate the NPs from each other, because each metal NP is settled on the different cavity site of ZIF67.31,32 In this work, the nanoscale Pd particles were easily settled on the cavities of ZIF-67 by the infiltration of Pd ions followed by reduction. The synthesized Pd@ZIF-67 exhibited similar polyhedron structure of ZIF-67 with the average size of 500 nm (Figure 2a). To clearly investigate the existence of Pd NPs and microstructure of Pd@ZIF-67, transmission 10 ACS Paragon Plus Environment

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electron microscopy (TEM) analysis was carried out. Although the overall morphology of Pd@ZIF-67 was similar to that of pristine ZIF-67 (Figure 2b), the dark spherical points were noticeably observed in the surface of Pd@ZIF-67 (Figure 2c). The high resolution TEM (HRTEM) image clearly showed the Pd NPs (2–3 nm) with the lattice fringe of Pd (111) plane (Figure 2d). In addition, the Pd NPs were well dispersed in the ZIF-67 templates due to the cavity structure of ZIF-67. More importantly, the crystal structure of Pd@ZIF-67 was not changed by the encapsulation of Pd NPs, as shown in XRD analysis (Figure S1). The XRD patterns of Pd@ZIF-67 were similar to those of ZIF-67. The Pd related peaks were not observed in XRD data due to the small size of Pd NPs (2–3 nm) and low loading amounts of Pd in ZIF-67 (0.83–2.50 wt%).40 The results clearly confirmed that the well-dispersed and ultra-small Pd NPs were easily and stably loaded on the ZIF-67 templates. After calcination at 400 °C in air atmosphere, Pd@ZIF-67 templates were transformed to PdO-Co3O4 HNCs as shown in Figure 2e. During the high-temperature heat-treatment, Pd NPs and Co ions are oxidized to PdO and Co3O4, and organic components in Pd@ZIF-67 are thermally decomposed. In case of small loading amounts of Pd NPs (0.83–2.50 wt%), hightemperature heat treatment of Pd@ZIF-67 under the same conditions also produced hollow structure of PdO-Co3O4 HNCs. On the other hand, in case of higher loading amounts (8.33 wt%), the final products did not perfectly maintain hollow structure (Figure S5). The Pd NPs in the ZIF-67 can disturb the formation of hollow structure because of the interruption of outer diffusion of Co, leading to the partial collapse of HNCs. TEM image exhibited the highly porous structure of PdO-CO3O4 HNCs (Figure 2f) with wall thickness of 20 nm. The thin layer of wall can increase the surface-volume ratio, providing plenty of reaction sites for the surface reactions. In addition, the lattice planes of PdO (101) and Co3O4 (311) were observed in the HRTEM image (Figure 2g). The size (3–4 nm) of PdO NPs was slightly 11 ACS Paragon Plus Environment


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increased by the oxidation of Pd, compared to that (2–3 nm) of Pd NPs. Moreover, selective area electron diffraction (SAED) image revealed the polycrystalline properties of Co3O4 with the lattice planes of (111), (220), (311), (511), and (400) (Figure 2h). The weak diffraction ring pattern of PdO (101) was also observed in the SAED image due to the low loading amounts of PdO in Co3O4 HNCs. Furthermore, the scanning TEM (STEM) images also showed the hollow polyhedron structure of PdO-Co3O4 (Figure S6). The energy dispersive Xray spectroscopy (EDS) elemental mapping images clearly confirmed not only the existence of Pd element but also the high dispersibility of Pd in PdO-CO3O4 HNCs (Figure 2i), indicating that nanoscale PdO catalysts were uniformly distributed on the Co3O4 HNCs. The crystal structure of PdO-Co3O4 HNCs was investigated by XRD analysis (Figure 3a). The XRD patterns of PdO-Co3O4 HNCs exhibited the cubic spinel structure of Co3O4, which consistent with the lattice plane of Co3O4 (111), (220), (311), (222), (400), (422), (511), and (440). These results are consistent with the SAED pattern (Figure 2h). However, PdO related peaks were not observed in the XRD analysis due to the low loading amounts of PdO. To analyze the chemical binding state of PdO-Co3O4 HNCs, we carried out X-ray photoelectron spectroscopy (XPS) analysis. The high resolution spectrum of Co 2p exhibited two characteristics peaks and satellite peaks (Figure 3b). The two major peaks located at 780.0 and 781.1 eV were corresponding to Co3+ 2p3/2 and Co2+ 2p3/2 respectively, which confirmed the formation of spinel Co3O4.41 The Pd 3d spectrum presented a dominant peak of Pd2+ 3d5/2 at 336.9 eV and a minor peak of Pd4+ 3d5/2 at 337.5 eV, corresponding to PdO and PdO2 (Figure 3c).42 The encapsulated Pd NPs were oxidized to PdO or PdO2 during the calcination, which can play a role as electronic sensitizers during gas sensing. Furthermore, the O 1s spectrum revealed the three oxygen states of O2– at 530.2 eV, O– at 531.0 eV, and O2– at 532.2 eV (Figure 3d).43 The O2– peaks were related to the oxygen in the oxide (Co3O4, PdO, or 12 ACS Paragon Plus Environment

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PdO2), and the peaks of O– and O2– represented chemisorbed oxygen species. These XPS results showed that the Co and Pd in Pd@ZIF-67 were completely oxidized to the Co3O4, PdO, or PdO2. The gas sensing characteristics of Co3O4 powder, Co3O4 HNCs, and PdO-Co3O4 HNCs were evaluated toward acetone (CH3COCH3) molecules under highly humid atmospheres (90% RH), which is similar with the humidity condition of human breath.9 According to the previous studies, the temperature affects the surface reaction of sensing materials by changing the surface adsorption properties.44 In addition, the amount of catalyst has a significant influence on sensing properties.12 The insufficient catalyst loading cannot fully enhance the surface reaction, and immoderate catalyst loading can degrade the sensing performances by the agglomeration of catalysts. Therefore, we optimized the operating temperature (250–400 °C) and loading amounts of Pd catalyst (0.83–8.33 wt%) in ZIF-67 to find an optimized condition for highest acetone sensing. Based on the results, 1.67 wt% of Pd loaded ZIF-67 derived PdO-Co3O4 HNCs exhibited highest acetone sensing performances at 350 °C (Figure S7a and b). To clearly verify the effect of hollow structure and nanoscale catalyst loading, PdO-Co3O4 powders were prepared after the calcination of 1.67 wt% of Pd loaded ZIF-67 at 600 °C for 1 h. Thus, the dynamic sensing characteristics were investigated for Co3O4 powders, Co3O4 HNCs, PdO-Co3O4 powders, and PdO-Co3O4 HNCs (Figure 4a) in the acetone concentration range of 0.4–5.0 ppm at 350 °C. The PdO-Co3O4 HNCs showed the higher acetone sensing responses (Rgas/Rair = 2.51 to 5 ppm of acetone) than PdO-Co3O4 powders (Rgas/Rair = 1.98 to 5 ppm of acetone), Co3O4 powders (Rgas/Rair = 1.96 to 5 ppm of acetone) and Co3O4 HNCs (Rgas/Rair = 1.45 to 5 ppm of acetone). The results clearly confirm that the sensing characteristics were markedly improved by the effects of hollow structure and the nanoscale catalyst. In addition, the PdO-Co3O4 HNCs detected the 0.4 ppm of 13 ACS Paragon Plus Environment


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acetone molecules with the noticeable response value (Rgas/Rair = 1.16). We further investigated the selectivity of PdO-Co3O4 HNCs toward 1 ppm of acetone, in consideration of the fact that the concentration of acetone in diabetes’ exhaled breath is increased to more than 1.8 ppm.45 The selectivity of a sensor is defined by the cross sensitivity to other gases.46 Therefore, the sensing responses of PdO-Co3O4 HNCs were examined by various analytes, such as hydrogen sulfides (H2S), ethanol (C2H5OH), pentane (C5H12), toluene (C7H8), ammonia (NH3), and carbon monoxide (CO) (Figure 4b). The PdO-Co3O4 HNCs exhibited high acetone sensing response (Rgas/Rair = 1.55) compared to other analytes (Rgas/Rair < 1.24), which confirmed the high selectivity of the PdO-Co3O4 HNCs toward acetone. In addition, we further investigated the stability of the PdO-Co3O4 HNCs. The PdO-Co3O4 HNCs stably detected the 1 ppm of acetone with a low variation (Rgas/Rair = 1.52 ± 0.03) during the cyclic sensing measurement (Figure 4c), and showed relatively good stability over 14 days (Figure S7c). The limit of detection of PdO-Co3O4 HNCs was further calculated by using an exponential plotting (Figure S7d). The calculated results revealed that the PdO-Co3O4 HNCs can sense a 0.1 ppm of acetone with certain response (Rgas/Rair = 1.04). The humidity effect of the sensors was also examined. Since the adsorption of water vapor donates the electrons to the sensing materials, the baseline resistance of p-type SMO based sensors is increased due to the recombination of holes and electrons.47 Therefore, the baseline resistance of PdOCo3O4 HNCs is increased at a high humidity (Figure S7e), leading to the degradation of sensing properties. Although the acetone sensing response (Rgas/Rair = 2.51) in highly humid atmospheres (90% RH) is slightly lower than that (Rgas/Rair = 3.17) in dry condition (Figure S7f), the sensors showed high acetone sensing properties even in a high humidity. These sensing properties are superior to other recent studies on p-type Co3O4 based acetone sensors (Table 1),48–56 which demonstrate that the MOF-derived PdO-Co3O4 HNCs are highly feasible as a new sensing platform. However, the sensing properties are not outstanding 14 ACS Paragon Plus Environment

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compared to the n-type SMO based gas sensors (Table 1), because the n-type SMO gas sensors that are operated by electron transfer intrinsically show higher resistance modulation than p-type SMO that are operated by hole transfer.33 Nevertherless, we propose the possibility of nanoscale catalyst functionalized hollow nanocages in p–type SMO based gas sensors that have not been explored much, and it can be possible to develop superior chemical gas sensors using various SMOs through the proposed strategy in this work. The sensing characteristics of SMO based gas sensors are affected by the surface reaction on the sensing layers.57 The chemisorption of oxygen on Co3O4, a p-type SMO, create the hole accumulation layer on the surface of Co3O4. O2 (g) → 2O– (ads) + 2h+

(1)

Then, the acetone molecules react with the chemisorbed oxygen (O– and O2–) and donate the electrons to sensing layers,58 which lead to the decrease in hole accumulation layer by the recombination of electrons and holes. CH3COCH3 (g) + O– (ads) → CH3COC+H2 + OH– + e–

(2)

CH3COCH3 (g) + 2O– (ads) → CH3O– + C+H3 + CO2 (g) + 2e–

(3)

Therefore, the resistance of sensors is increased by the surface reaction when exposed to analytes, as shown in Figure 4d. The baseline resistance is increased by the formation of hollow structure and functionalization of catalysts. The Co3O4 HNCs exhibited higher baseline resistance (2020 Ω) than the Co3O4 powders (520 Ω) because hollow and porous structure rarely provide an electron pathway for the charge transport. In addition, when PdO catalysts are loaded on Co3O4, hetero-interfaces are created between PdO and Co3O4, leading



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to the formation of the potential barriers. Therefore, the base resistances of sensors are increased to 1320 Ω for PdO-Co3O4 powders, and 2670 Ω for PdO-Co3O4 HNCs. The enhancement in the acetone sensing characteristics of the PdO-Co3O4 HNCs is interpreted as follows; (i) the increase of active sites and (ii) the functionalization of catalysts (Figure 4e). Firstly, the porous and hollow structures offer large reaction sites compared to powder structure. The surface area of Co3O4 HNCs (20.96 m2 g-1) and PdO-Co3O4 HNCs (20.18 m2 g-1) is higher than that of Co3O4 powders (0.36 m2 g-1). In addition, the hollow structure allows gas molecules to diffuse both inside and outside of sensing layers. Therefore, more oxygen molecules are adsorbed onto the surface of the Co3O4 HNCs, and they are reacted with the acetone molecules, resulting in the large resistance change. (Figure 4d). Secondly, the nanoscale PdO catalysts in the Co3O4 HNCs enhance the acetone sensing properties. The catalytic effect of Pd in SMO based gas sensors has already been demonstrated in previous literatures.10 As an electronic sensitizer, Pd NPs can donate or deprive electrons of the sensing materials by reduction or oxidation during sensing. The PdO NPs loaded on Co3O4 HNCs are partially reduced by acetone molecules because the adsorption of acetone molecules can create the hydroxyl groups that facilitate the desorption of oxygen atoms in the PdO.59 Then, the reduction of PdO can donate the electrons to Co3O4, decreasing the hole accumulation layers of Co3O4. In addition, the reduced Pd NPs can activate the surface reactions (reaction 2 and 3) between chemisorbed oxygen and acetone molecules by lowering the activation energy.60 These additional electrons are donated to the Co3O4 due to the catalytic effect of Pd and recombined with the hole in the Co3O4, leading to the additional increase in resistance of PdO-Co3O4 HNCs (Figure 4d). From these reasons, Pd@ZIF-67 derived PdO-Co3O4 HNCs exhibited the enhanced acetone sensing performances.


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To verify the selective detection of acetone molecules, we carried out the principal component analysis (PCA), which is a widely used pattern algorithm system to assemble unexplored data, using a sensor array of Co3O4 powders, Co3O4 HNCs, and Pd-Co3O4 HNCs. As shown in Figure 5a, the PCA results indicate that the acetone molecules in the various concentrations (1–5 ppm) were clearly distinguished from the other interfering gases (hydrogen sulfide, ethanol, pentane, toluene, and ammonia). We further analyzed the exhaled breath of healthy subjects and simulated exhaled breath of diabetics using a sensing array system. The exhaled breath from 8 subject was separately collected in a Tedlar bag (Figure S8), then the sample was injected in the sensing measurement system using a diaphragm pump. Considering that the concentration of acetone molecules in the exhaled breath of diabetics was increased to more than 1.8 ppm, the mixed gas consisted of the exhaled breath and 2 ppm of acetone molecules was injected in the sensing chamber by a mass flow controller for the simulated diabetics’ breath. The patterns of exhaled breath of healthy people are randomly dispersed (Figure 5b), because the human breath contains a number of volatile organic compounds,7 such as hydrogen, ethanol, methyl mercaptan, acetone, ammonia, toluene, and others, and the results are consistent with pattern analysis of diverse analytes (Figure 5a). On the other hand, the patterns of simulated breaths are clearly assembled in a small area (Figure 5b). Since the PdO-Co3O4 HNCs in the sensor array are selectively reacted with acetone molecules, the eight subjects are gathered by introducing acetone molecules without any overlap. These results demonstrate the future feasibility of MOF-derived nanoscale PdO catalyst loaded Co3O4 HNCs based gas sensors in achieving accurate and selective detection of acetone molecules.

4. Conclusion 17 ACS Paragon Plus Environment


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In this work, we have demonstrated the novel synthesis of the nanoscale PdO catalyst decorated Co3O4 HNCs using ZIF-67 template. The Pd particles (2–3 nm) were easily loaded on the cavities of ZIF-67. The cavity structure of ZIF-67 can restrict the growth of Pd NPs and disperse Pd NPs effectively. Then, the controlled calcination process produced the PdO NPs (3–4 nm) loaded Co3O4 HNCs by the Kirkendall effect. The PdO catalysts were uniformly functionalized on the wall of Co3O4 HNCs which have high surface area and gas accessibility, and effectively improved the gas sensing characteristics of Co3O4 HNCs. Particularly, the PdO-Co3O4 HNCs exhibited high response toward acetone (Rgas/Rair = 2.51 to 5 ppm) and superior selectivity against interfering gases. In addition, the simulated exhaled breath of diabetics was clearly recognized by using PCA. The results demonstrated the novel synthetic route of ultra-small catalyst loaded hollow SMO structure, and its high feasibility as potential applications in exhaled breath analyzers.

ASSOCIATED CONTENT Supporting Information. Additional characterizations and sensing characteristics. These materials are available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT 18 ACS Paragon Plus Environment

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This work was supported by Wearable Platform Materials Technology Center (WMC) funded by National Research Foundation of Korea (NRF) Grant of the Korean Government (MSIP) (No. 2016R1A5A1009926). This work was also supported by the Ministry of Science, ICT & Future

Planning

as

Biomedical

Treatment

Technology

Development

Project

(2015M3A9D7067418).

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Scheme 1. Schematic illustration of synthetic process of PdO-Co3O4 HNCs derived from Pd@ZIF-67 by optimized thermal treatment. The actual SEM image and TEM images are shown below illustrations, respectively.


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Figure 1. SEM images of (a) ZIF-67, and Co3O4 obtained by the calcination of ZIF-67 at (b) 400 °C, (c) 500 °C, and (d) 600 °C.



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Figure 2. (a) SEM image of Pd@ZIF-67, (b, c) TEM image of Pd@ZIF-67, (d) HRTEM image of Pd@ZIF-67, (e) SEM image of PdO-Co3O4 HNCs, (f) TEM image of PdO-Co3O4 HNCs, (g) HRTEM of PdO-Co3O4 HNCs, (h) SAED patterns of PdO-Co3O4 HNCs, and (i) STEM image and EDS elemental mapping images of PdO-Co3O4 HNCs.


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Figure 3. (a) XRD analysis of Co3O4 HNCs and PdO-Co3O4 HNCs, and XPS high resolution spectrum of PdO-Co3O4 HNCs: (b) Co 2p, (c) Pd 3d, and (d) O 1s.



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Figure 4. Sensing characteristics under highly humid condition (90% RH) at 350 °C; (a) Dynamic acetone sensing transient properties of Co3O4 powders, Co3O4 HNCs, PdO-Co3O4 powders, and PdO-Co3O4 HNCs in the concentration range of 0.4–5.0 ppm, (b) Response values to 1 ppm of interfering analytes, (c) Cyclic sensing transient of PdO-Co3O4 HNCs toward 1 ppm of acetone, (d) Dynamic resistance transition toward 5 ppm of acetone molecules, and (e) Schematic illustration of acetone sensing mechanism for PdO-Co3O4 HNCs. 30 ACS Paragon Plus Environment

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Figure 5. Pattern recognition based on PCA using Co3O4 powders, Co3O4 HNCs and PdOCo3O4 HNCs for (a) various concentrations of gases and (b) exhaled breath.



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Table 1. Comparison with recent studies of SMO based acetone sensors. Gas species

Materials

Response definition

Sensitivity (Response)

Detectio n limit

Testing ambient

Operating temp.

Ref.

Rgas/Rair

2.51 at 5 ppm

100 ppb

90% RH

350 ˚C

In this work

Rgas/Rair

5.6 at 100 ppm

2 ppm

dry

240 ˚C

48

Rgas/Rair

6.1 at 50 ppm

5 ppm

40% RH

160 ˚C

49

Rgas/Rair

11.4 at 100 ppm

1.8 ppm

50% RH

150 ˚C

50

Rgas/Rair

11.7 at 50 ppm

1 ppm

40% RH

240 ˚C

51

Rgas/Rair

2.29 at 5 ppm

120 ppb

90% RH

300 ˚C

52

Rair/Rgas

6 at 50 ppm

–

dry

210 ˚C

53

Rair/Rgas

35.8 at 100 ppm

1 ppm

50% RH

240 ˚C

54

Rair/Rgas

4.11 at 2 ppm

120 ppb

85% RH

300 ˚C

5

Rair/Rgas

7.12 at 3 ppm

120 ppb

80% RH

400 ˚C

55

Rair/Rgas

34.8 at 1 ppm

100 ppb

90% RH

350 ˚C

56

PdO-Co3O4 HNCs Co3O4 nanorods Co3O4 nanosheets porous Co3O4 nanosheets Co3O4/α-Fe2O3 nanofibers Ir-GO-Co3O4 acetone

nanofibers Au@SnO2 nanospheres GO/ZnO nanosheets Pt-WO3 hemitubes Pt-SnO2 nanofibers Pt-PS-SnO2 nanotubes


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