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Metal-Organic Framework Templated Catalysts: Dual Sensitization of PdOZnO Composite on Hollow SnO2 Nanotubes for Selective Acetone Sensors Won-Tae Koo, Ji-Soo Jang, Seon-Jin Choi, Hee-Jin Cho, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 14, 2017

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ACS Applied Materials & Interfaces

Metal-Organic Framework Templated Catalysts: Dual Sensitization of PdO-ZnO Composite on Hollow SnO2 Nanotubes for Selective Acetone Sensors Won-Tae Koo, † Ji–Soo Jang, † Seon–Jin Choi,†,‡ Hee-Jin Cho, † and Il–Doo Kim †,* †

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡

Applied Science Research Institute, Korea Advanced Institute of Science and Technology,

291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

*Corresponding author e-mail: [email protected]

KEYWORDS Metal-organic frameworks, nanotubes, catalysts, gas sensors, exhaled breath analysis

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ABSTRACT

Metal-organic framework (MOF) derived synergistic catalysts were easily functionalized on hollow SnO2 nanotubes (NTs) via electrospinning and subsequent calcination. Nanoscale Pd NPs (~ 2 nm) loaded Zn based zeolite imidazole framework (Pd@ZIF-8, ~ 80 nm) was used as a new catalyst loading platform for effective functionalization of PdO@ZnO complex catalyst onto the thin wall of one-dimensional (1D) metal oxide NTs. The well-dispersed nanoscale PdO catalysts (3-4 nm) and multi-heterojunctions (PdO/ZnO and ZnO/SnO2) on hollow structures are essential for the development of high-performance gas sensors. As a result, the PdO@ZnO dual catalysts-loaded hollow SnO2 NTs (PdO@ZnO-SnO2 NTs) exhibited high acetone response (Rair/Rgas = 5.06 at 400 °C @ 1 ppm), superior acetone selectivity against other interfering gases, and fast response (20 s)/recovery (64s) time under highly humid atmosphere (95% RH). In this work, the advantages of hollow SnO2 NT structure with high surface area and open porosity were clearly demonstrated by the comparison with SnO2 nanofibers (NFs). Moreover, the sensor arrays composed of sensor array consisted of SnO2 NFs, SnO2 NTs, PdO@ZnO-SnO2 NFs, and PdO@ZnO-SnO2 NTs successfully identified the patterns of the exhaled breath of normal people and simulated diabetics by using a principal component analysis.


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1. INTRODUCTION Metal-organic frameworks (MOFs) or porous coordination polymers have received great attention due to their unique properties such as incredibly high surface area, ultrahigh porosity, and tunable structures.1,2 To utilize their excellent benefits, MOFs have been studied for catalysis,3 gas storage,4 drug delivery,5 and energy devices.6 One of striking features in MOFs is that noble metal nanoparticles (NPs) can be encapsulated in the cavities of the MOFs composed of metal ions and organic ligands. Not only is the size of noble metal NPs embedded in MOFs restricted, but also are nanoscale metals well-distributed in the cavities of MOFs without any aggregation. To utilize the nanoscale metal particles in MOFs, numerous researchers have studied the metal encapsulated MOF (M@MOF) structures.7 For instance, Lu and co-workers reported Au or Pt loaded Zn based zeolite imidazole framework (ZIF-8) for hydrogenation catalyst.8 In addition, Jiang et al. synthesized AuAg core-shell NPs in ZIF8 for heterogeneous catalysis.9 Furthermore, Yan and co-workers suggested composite catalysts composed of AuPd encapsulated ZIF-8, MnOx, and graphene for hydrogenation.10 Although such M@MOF composite materials have been reported as superior catalysts, there is limitation in that M@MOF can not be applicable in high-temperature operating devices due to the poor thermal stability of organic ligands in MOFs. Therefore, to overcome this obstacle, carbon composites and metal oxides obtained from the pyrolysis or calcination of M@MOF have been widely studied to apply them in various fields. Hollow one-dimensional (1D) metal oxide nanostructures have been widely studied in various applications such as gas sensors,11 energy storage system,12 and catalysis13 due to their high surface area and easy gas accessibility. So far, the hollow tubular structures have been achieved by various synthetic methods, e. g., sacrificial templating route, hydrothermal, and chemical/physical vapour deposition.14,15 However, most of these synthetic routes have some limitations, such as low reproducibility and poor uniformity due to their complex synthetic processes. On the other hand, electrospinning as a powerful synthetic method to produce 1D 3 ACS Paragon Plus Environment


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structures with easy fabrication and high productivity,16,17 has been utilized for producing hollow metal oxide nanostructures without using sacrifitial templates by Kirkendall effect and Ostwald ripening. For instance, Cho et al. synthesized hollow SnO2 hierarchical nanofibers using Kirkendall effect by adjusting the calcination conditions of the electrospun nanofibers.18 In addition, Yoon et al. synthesized tube-in-tube or tube-in-fiber structure via the co-solventsassistedelectrospinning followed by calcination, where the ramping rate can affect the oxidation rate of RuO2 and Mn2O3.19 Not only that, the multi-composite nanomaterials with hollow tubular structure, which additionally possess novel catalysts, can be easily produced via electrospinning. Jang et al. recently reported one-pot synthesis of Pt catalysts decorated porous SnO2 nanotubes (NTs) by Ostwald ripening effect during the calcination of electrospun nanofibers.20 The tiny nanoparticle catalysts were easily loaded on the wall of metal oxide nanotube by electrospinning, but to the best of our knowledge, the MOF derived metal-oxide composite catalysts have yet been employed in the hollow tubular structures. To achieve superior gas sensors, the increase of surface area and the sensitization of catalyst should be accompanied to accommodate surface reaction between target gas and sensing materials.21 In particular, the nanoscale catalysts decorated on hollow structures can effectively promote the surface reactions. However, they are easily aggregated to reduce their surface energy at high operating temperature, leading to the degradation of response and the delay of recovery.22 Therefore, for the development of high sensitivity, superior selectivity, and fast responding speed gas sensors, particularly in highly humid atmosphere, facile and effective catalyst loading route should be further developed.23,24 Herein, we synthesized SnO2 NTs sensitized with PdO NPs-ZnO (PdO@ZnO) composite catalyst (hereafter, PdO@ZnOSnO2 NTs) by using Pd loaded ZIF-8 (Pd@ZIF-8) templates. PdO@ZnO heterogeneous catalysts derived from Pd@ZIF-8 are tightly anchored to the wall of SnO2 NTs via the electrospinning followed by fast ramping rate-assisted calcination. Since nanoscale PdO 4 ACS Paragon Plus Environment

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particles are separately loaded on the ZnO complex, the PdO catalysts are fully and stably utilized for the surface reaction without agglomeration. In addition, the hollow tubular structure of SnO2 provides both inside and outside reaction sites with high gas accessibility. On the basis of previous studies, the nanoscale PdO, as a catalyst, can effectively improve the sensing properties toward reducing gas by donating or depriving electrons to sensing layers.25 In addition, ZnO and SnO2 are a typical acetone sensing materials,26–29 and the combination of ZnO and SnO2 can further improve sensing properties by modulating the depletion layers.30 Thus, the distinctive heterojunction structure of ZnO/SnO2 NTs functionalized by PdO catalyst can induce superior acetone sensing properties. In this work, the detailed morphological evolution for synthesis of PdO@ZnO-SnO2 NTs and underlying gas sensing mechanism of PdO@ZnO sensitized SnO2 NTs toward selective acetone sensing are disscussed. Moreover, the merits of MOF-templated hollow nanotube structures are highlighted by comparison with pure SnO2 and PdO@ZnO-SnO2 NFs with dense nanofiber topology.

2. MATERIALS AND METHODS 2.1. Materials. Polyvinylpyrrolidone (PVP, Mw ~ 1,300,000 g mol–1), 2-methylimidazole (Hmin, 99.0%), and sodium borohydride (NaBH4, 96%) were purchased from Aldrich. Zinc nitrate hexahydrate ([Zn(NO3)2·6H2O], 98%), Tin(II) chloride dihydrate (SnCl2·2H2O), ethanol (EtOH, 99.5%), methanol (MeOH, 99.9%), N,N-Dimethylformamide (DMF, 99.8%), and potassium tetrachloropalladate(II) (K2PdCl4) were purchased from Sigma-Aldrich. All chemicals were used without further purification.

2.2. Synthesis of SnO2 NTs and SnO2 NFs. SnO2 NTs as well as SnO2 NFs as control samples were prepared by electrospinning followed by calcination step. 0.35 g of PVP and 0.25 g of SnCl2·2H2O were dissolved in the mixed solution of EtOH (1.35 g) and DMF (1.35 5 ACS Paragon Plus Environment


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g). The solution was continuously stirred with a magnetic bar at room temperature for 5 h. The electrospinning was conducted using the prepared solution at a high voltage of 16 kV and a feeding rate of 0.1 ml min–1. The distance of the syringe needle (21 gauge) and collector was 15 cm. To synthesize SnO2 NFs, the collected as-spun NFs were calcined at 600 °C with ramping rate of 5 °C m–1 for 1 h in air atmosphere. On the other hand, SnO2 NTs were synthesized by the rapid calcination (ramping rate of 10 °C m–1) at 600 °C for 1 h in air atmosphere.

2.3. Synthesis of PdO@ZnO-SnO2 NTs and PdO@ZnO-SnO2 NFs. First of all, Pd@ZIF-8 was synthesized as a template for PdO@ZnO complex catalysts. The ZIF-8 was synthesized by room temperature precipitation of Zn(NO3)2·6H2O (0.293 g) and Hmin (0.649 g) dissolved MeOH (40 mL). The precipitated ZIF-8 was dried at 50 °C and purified using EtOH. After purification of ZIF-8, 40 mg of ZIF-8 was dispersed in 1 mL deionized water (DI-water). Then, 10 mg of K2PdCl4 was added into the suspension and stirred under 100 rpm for 1 h. To produce Pd@ZIF-8, NaBH4 solution (1.5 mg mL–1) was added in the suspension. The prepared Pd@ZIF-8 was purified by centrifugation and washing with EtOH, and dispersed in 3 mL of DI-water. The 60 mg of Pd@ZIF-8 suspension was added into the mixed solution which is consisted of EtOH (1.35 g) and DMF (1.35 g). Then, 0.35 g of PVP and 0.25 g of SnCl2·2H2O were dissolved in the mixed solution. The solution was stirred under 500 rpm for 1 h, and elctrospun at 25 °C with 30% RH. During the electrospinning, a high voltage of 16 kV between the tip of the syringe needle (25 gauge) and the stainless steel foil collector was maintained at a distance of 15 cm. A feeding rate of 0.1 mL min–1 was also maintained by using a syringe pump. The as-spun Pd@ZIF-8/PVP/Sn composite NFs were transformed to PdO@ZnO-SnO2 NTs after the calcination at 600 °C for 1 h in air atmosphere with the heating rate of 10 °C min–1. SnO2 NTs were synthesized by the calcination at 600 °C with moderate ramping rate (5 °C m–1) for 1 h. 6 ACS Paragon Plus Environment

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2.4. Sensor fabrication and gas sensing measurement. To evaluate sensing characteristics, we prepared a sensing substrate (area: 2.5 mm x 2.5 mm, thickness 0.2 mm) which was patterned with two parallel Au electrodes (width: 25 µm, gap size: 70 µm) on the front and Pt heater on the back of Al2O3 substrate. Then, SnO2 NFs, PdO@ZnO-SnO2 NFs, SnO2 NTs, and PdO@ZnO-SnO2 NTs dispersed solution (10 mg in 50 mL of EtOH) were drop-coated on the four sensor substrates, respectively. Before the sensing test at each operating temperature, the prepared sensors were stabilized by air injection for 5 h. In sensing measurement, air was firstly injected for 10 min and the analyte gas was injected for 10 min with a concentration ranging from 100 ppb to 5 ppm. In case of the simulated exhaled breath sensing test, we captured exhaled breath of 10 healthy people using a Tedlar bag, and the each exhaled breath was injected by using diaphragm pump with a 2 min on/off interval. Then, the mixed gas consisted of exhaled breath and 2 ppm of acetone was injected by using mass flow controller. The responses (Rair/Rgas) of the sensors consisted of SnO2 NFs, PdO@ZnO-SnO2 NFs, SnO2 NTs, and PdO@ZnO-SnO2 NTs were evaluated by monitoring the resistance changes using acquisition system (34972, Agilent). The operating temperature was modulated by applying a certain voltage to Pt microheater using DC power supply (E3647A, Agilent). The pattern recognition by using 4 sensor arrays was carried out using IBM SPSS software.

2.5. Characterization. The field emission scanning electron microscopy (Nova230, FEI) and field emission transmission electron microscopy (Tecnai G2 F30 S-Twin, FEI) were conducted to analyze the microstructures and morphologies of samples. The powder X-ray diffraction (D/MAX-2500, Rigaku) with Cu Kα radiation (λ = 1.5418 Å) was carried out to investigate the crystal structure. The porous structure and Brunauer-Emmett-Teller (BET) surface area were confirmed by N2 adsorption/desorption isotherms (Tristar 3020, Micromeritics) at 77 K. The thermal behavior of samples was evaluated by thermal 7 ACS Paragon Plus Environment


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gravimetric analysis (Labsys Evo, Setaram). The chemical binding states were investigated by X-ray photoelectron spectroscopy (K-alpha, Thermo VG Scientific) using Al Kα radiation. Ultraviolet photoelectron spectroscopy (UPS) (Sigma Probe, Thermo VG Scientific) using He I source was conducted to investigate the work function and valence band maximum of samples.

3. RESULTS AND DISCUSSION Firstly, Pd@ZIF-8, as a template for the formation of PdO@ZnO heterogeneous catalysts, was prepared by the infiltration and reduction of Pd2+ (Figure 1a).9 ZIF-8 was easily synthesized by precipitation method at room temperature. The synthesized ZIF-8 showed polyhedron morphology with an average diameter of 80 nm (Figure S1a). The micro-porous structure of ZIF-8 was confirmed by X-ray diffraction (XRD) analysis and N2 uptake isotherms at 77K (Figure S2). Pd2+ ions were diffused in the cavities of ZIF-8 by infiltration in deionized water. Then, the diffused Pd2+ ions were reduced to Pd NPs by sodium borohydride (NaBH4). As the Pd NPs were encapsulated in each cavity of ZIF-8, the size of Pd NPs was suppressed by the cavity diameter of ZIF-8 (11.6 Å) and the Pd NPs were embedded in the cavities of ZIF-8 matrix (Figure 1b). Although scanning electron microscopy (SEM) image of Pd@ZIF-8 was similar to that of ZIF-8 (Figure S1b), the high-resolution transmission electron microscopy (HRTEM) image of Pd@ZIF-8 clearly exhibited the ultrasmall Pd NPs (~ 2 nm) with the Pd (111) crystal plane (Figure 1c). In addition, scanning TEM (STEM) image showed the well-dispersed Pd NPs on the ZIF-8 (Figure S1c). Furthermore, the energy dispersive X-ray spectroscopy (EDS) analysis at orange sphere point in Figure 1b verified the Pd NPs in the ZIF-8 (Figure 1d). Although the size Pd NPs was slightly larger than the cavity dimensions of ZIF-8, the crystal structure and porous structure were not changed, which are 8 ACS Paragon Plus Environment

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consistent with previous reports.31,32 The XRD peaks of Pd@ZIF-8 were identical to those of ZIF-8 due to the small size of Pd NPs and the encapsulation by ZIF-8, and the N2 adsorption/desorption isotherms also exhibited micro-porous structure with a reduced surface area due to the mass contribution of non-porous Pd NPs (Figure S2). Figure 2a illustrates the synthetic process of PdO@ZnO-SnO2 NTs. To synthesize PdO@ZnO-SnO2 NTs, Pd@ZIF-8 was electrospun with polyvinylpyrrolidone (PVP) and tin(II) dichloride dehydrate (SnCl2·2H2O). Pd@ZIF-8 was uniformly distributed on 1D nanofibers comprising of PVP and Sn precursors by the electrospinning. Then, the followed calcination of as-spun NFs produced the hollow SnO2 NTs by controlling the ramping rate of calcination, and the PdO@ZnO heterogeneous catalysts derived from Pd@ZIF-8 were tightly immobilized on the wall of SnO2 NTs. To synthesize hollow SnO2 NTs, we firstly optimized the calcination condition using as-spun PVP/Sn precursor composite NFs (Figure S3a). During the calcination, the organic compounds in the composite NFs were decomposed and SnCl2·2H2O was oxidized to SnO2. Therefore, fibrous SnO2 was synthesized by the calcination at 600 °C for 1 h with moderate ramping rate of 5 °C min–1 (Figure S3b). On the other hand, tubular SnO2 was synthesized by the calcination at 600 °C for 1 h with high ramping rate of 10 °C min–1 (Figure S3c,d). Therefore, the rapid calcination of Pd@ZIF8/PVP/Sn precursor composite NFs at 600 °C for 1 h (10 °C min–1) also produced the PdO@ZnO catalysts loaded hollow SnO2 NTs. The SEM image of as-spun composite NFs exhibited a smooth 1D structure with diameter in the range of 200–400 nm (Figure 2b). Some of Pd@ZIF-8 was clearly decorated on the surface of as-spun NFs, and others were embedded in the composite NFs (yellow arrow in Figure 2b). TEM image evidently shows that Pd@ZIF8 was embedded and dispersed in the composite NFs (Figure 2c). After calcination, the composite NFs were finally transformed to PdO@ZnO-SnO2 NTs with reduced diameter of 150–300 nm (Figure 2d). Pd@ZIF-8 templates were transformed to ultrasmall PdO catalysts loaded ZnO complex catalysts during the calcination (Figure S4a,b), and they were decorated 9 ACS Paragon Plus Environment


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on the wall of NTs (Figure S4c). In addition, during the high temperature calcination, the organic ligands in ZIF-8 are thermally decomposed, leading to the creation of tiny mesopores on the wall of PdO@ZnO-SnO2 NTs. Thus, PdO@ZnO-SnO2 NTs with plenty of pore channels can facilitate fast gas diffusion inside the tubular structure (Figure S3d). Since the loading amounts of PdO@ZnO complex catalyst are relatively low (0.204 wt%), the calcination with high ramping rate of 10 °C min–1 produces the hollow tubular structure. On the other hand, in case of Pd@ZIF-8/PVP/Sn precursor composite NFs with the high loading amounts of PdO@ZnO (3.41 wt%), hollow tubular structure was not maintained after calcination (Figure S5a,b), because many PdO@ZnO particles derived from Pd@ZIF-8 impede the outer diffusion of Sn during the calcination. Since most of PdO@ZnO complex catalysts on the SnO2 NTs were not clearly observed in SEM images (Figure 2d and Figure S5c,d), we carried out TEM analysis to verify the formation of PdO@ZnO catalysts. The TEM image of PdO@ZnO-SnO2 NTs showed the hollow tubular structure (Figure 2e). In addition, HRTEM image exhibited the PdO@ZnO catalyst functionalized on SnO2 with the crystal plane of ZnO (101) and SnO2 (110), which corresponded to interplanar spacing of 2.48 Å and 3.35 Å, respectively (Figure 2f). The spherical dark spots with the size of 3–4 nm in the ZnO were presumably PdO NPs (red circle in the Figure 2f), the size of which was increased by the oxidation of Pd NPs (2 nm). Selective area electron diffraction (SAED) patterns exhibited the polycrystalline properties of SnO2 with the crystal plane of (110), (101), and (211) (Figure 2g). The SAED patterns of PdO (101) and ZnO (101) were relatively week due to the low concentration of PdO@ZnO in SnO2 NTs. The STEM images and EDS elemental mapping images clearly showed the well-distributed Pd and Zn in the SnO2 NTs (Figure S6a,b). In addition, the EDS line profiles of PdO@ZnO-SnO2 NTs exhibited the hollow tubular structure of SnO2 and low loading amounts of PdO@ZnO (Figure S6c). These results confirmed that the PdO@ZnO complex catalysts were uniformly distributed on the wall of SnO2 NTs. 10 ACS Paragon Plus Environment

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Thermal gravimetric (TG) and differential scanning calorimetric (DSC) analysis were conducted to understand the thermal decomposition behavior of Pd@ZIF-8/PVP/Sn precursor composite NFs at different ramping rate (Figure 3a). The organic compounds in the composite NFs were decomposed by the calcination, so the total weight of composite NFs was decreased. The weight loss at low temperature range (< 200 °C) was related to the evaporation of solvents in the composite NFs. Then, the sharp decrease in weight was observed at 300 °C due to the decomposition of ZIF-8, PVP side chain, and SnCl2·2H2O, where these reactions are related to the exothermic peak at 318 °C. In addition, Sn precursors in the outer surface start oxidation before the SnCl2 and Pd@ZIF-8 in the core part of composite NFs when the temperature reaches 300 °C (Figure 3b). After that, the PVP main chains in as-spun composite NFs are gradually decomposed in the temperature range of 350– 550 °C.33–35 Noticeably, the rapidly calcined NFs showed broader temperature range (342– 577 °C) and higher exothermic heat flow (< 6.5 mW mg–1) compared to the slowly calcined NFs (412–548 °C and lower than 2.8 mW mg–1). These wider and higher heat flow revealed that the combustion reaction of PVP to CO2 and H2O more quickly occurred at high ramping rate, which can facilitate the outer diffusion of SnCl2 in Pd@ZIF-8/PVP/Sn precursor composite NFs.36 Therefore, the inner voids are built in the composite NFs by Kirkendall effect,37 and hollow SnO2 NTs funtionalized by PdO@ZnO heterogeneous catalysts are finally achieved after the rapid calcination (Figure 3b). In addition, the crystal structure of PdO@ZnO-SnO2 NTs was investigated by XRD analysis. The XRD analysis revealed the tetragonal SnO2 (JCPDS no. 41-1445) and polycrystalline properties of PdO@ZnO-SnO2 NTs (Figure S7). However, the diffraction peaks of PdO and ZnO were not observed in the XRD analysis due to the low concentration of PdO@ZnO. To further investigate the chemical binding state of the PdO@ZnO-SnO2 NTs, Xray photoelectron spectroscopy (XPS) was carried out. The high resolution spectrum of Sn 3d in PdO@ZnO-SnO2 NTs showed the two noticeable peaks of Sn4+ at 486.5 eV for 3d5/2 and 11 ACS Paragon Plus Environment


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495.0 eV for 3d3/2 (Figure S8a).20 The oxygen peaks of O 1s exhibited three states of oxygen for O2– 1s at 530.2 eV, for O– 1s at 531.0 eV, and for O2– 1s at 532.2 eV (Figure S8b).38 The O2– state indicated oxygen in SnO2, ZnO, and PdO. The O– and O2– was related to the chemisorbed oxygen species on the PdO@ZnO-SnO2 NTs. In addition, the chemical state of Zn was confirmed as Zn2+, corresponding to peaks that were located at 1022.0 eV for 2p3/2 and 1045.0 eV for 2p1/2 (Figure S8c).39 Furthermore, the high resolution spectrum of Pd 3d revealed two characteristic peaks of 3d5/2 at 336.9 eV and 3d3/2 at 342.2 eV, which were identified as Pd2+ state (Figure S8d).40 The XPS results confirmed that Sn, Zn, and Pd in PdO@ZnO-SnO2 NFs were completely oxidized to SnO2, ZnO, and PdO, respectively. To verify the effect of heterogeneous catalyst on hollow tubular structure, the acetone sensing performance of PdO@ZnO-SnO2 NTs was evaluated and compared with dense SnO2 NFs, SnO2 NTs, and PdO@ZnO-SnO2 NFs. As a control sample, the same levels of PdO@ZnO catalyst (0.204 wt%) were sensitized on SnO2 NFs by the calcination with moderate ramping rate (5 °C min–1) (Figure S9). The sensing characteristics were evaluated toward low ppm of acetone at high humidity (95% RH), in consideration of the fact that the concentration of acetone, a biomarker for diabetes, is increased to more than 1.8 ppm in exhaled breath of patient compared with that (900 ppb) of normal people.24 The sensing materials were drop-coated onto Au electrode patterned on Al2O3 substrates, respectively (See more detailed experimental section). Then, we investigated the temperature dependent sensing properties toward 5 ppm of acetone to optimize the operating temperature (Figure S10a). The time dependent sensing properties of acetone were evaluated in the concentration range of 0.1–5.0 ppm at optimum operating temperature (400 °C) (Figure 4a). The PdO@ZnO-SnO2 NTs showed highest acetone sensing response (Rair/Rgas = 10.12) to 5 ppm of acetone compared with that of SnO2 NFs (Rair/Rgas = 3.47), SnO2 NTs (Rair/Rgas = 5.45), and PdO@ZnO-SnO2 NFs (Rair/Rgas = 6.55). In addition, the PdO@ZnO-SnO2 NTs exhibited noticeable response even in 1 ppm of acetone (Rair/Rgas = 5.06) and 0.1 ppm of acetone 12 ACS Paragon Plus Environment

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(Rair/Rgas = 1.82). The detection limit of acetone sensing was estimated to 10 ppb with the noticeable response (Rair/Rgas = 1.31) by linear approximation (Figure S10b). The selectivity of PdO@ZnO-SnO2 NTs was further investigated toward 1 ppm of various interfering biomarker gases such as hydrogen sulfide (H2S), toluene (C7H8), carbon monoxide (CO), pentane (C5H12), and ammonia (NH3) (Figure 4b). As a result, the PdO@ZnO-SnO2 NTs exhibited high response to acetone (Rair/Rgas = 5.06) compared with other gases (Rair/Rgas < 2.35). Noticeably, PdO@ZnO-SnO2 NTs and SnO2 NTs showed dramatically improved response time (< 20 s), compared with PdO@ZnO-SnO2 NFs (< 40 s) and SnO2 NFs (< 72 s) (Figure 4c). Besides, the recovery time of PdO@ZnO-SnO2 NTs was significantly decreased to less than 64 s while those (176 s for SnO2 NFs, 156 s for PdO@ZnO-SnO2 NFs, and 120 s for SnO2 NTs) of other control samples showed slow recovery process (Figure 4d). In particular, the PdO@ZnO complex catalysts loaded on SnO2 NTs more effectively enhance recovery and response time compared with PdO@ZnO-SnO2 NFs and SnO2 NTs. Furthermore, the PdO@ZnO-SnO2 NTs showed stable response even during repeated sensing test toward 1 ppm of acetone at 400 °C (Figure S10c). These sensing performances confirmed that the PdO@ZnO catalysts on hollow SnO2 NTs efficiently enhanced the acetone sensing properties, in terms of sensitivity, selectivity, speed, and stability. Comparing with other acetone sensing materials reported on previous literatures, the acetone sensing characteristics of the PdO@ZnO-SnO2 NTs exhibited superior response/recovery time and detection limit even at highly humid atmospheres (Table S1). In addition, other parameters of the sensors can be further improved by the optimization of catalyst loading amounts and the use of other catalyst materials. To further verify the potential feasibility of PdO@ZnO-SnO2 NTs in an exhaled breath analyzer, we carried out exhaled breath analysis using exhaled breath of normal people and simulated exhaled breath of diabetics containing 2 ppm of acetone molecules in normal exhaled breath (See detailed synthetic process of simulated exhaled breath in experimental 13 ACS Paragon Plus Environment


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section). The exhaled breath of ten healthy people was respectively collected by using a Tedlar bag (Figure S11), and the concentration of acetone was fixed to 2 ppm because the diabetes patients’ breath contains more than 1.8 ppm of acetone in exhaled breath.24 The sensor array consisted of SnO2 NFs, SnO2 NTs, PdO@ZnO-SnO2 NFs, and PdO@ZnO-SnO2 NTs were exposed to the exhaled breath of healthy people and simulated diabetes. Then, the sensing data from the sensor array were treated in a statistical analysis program: principal component analysis (PCA). PCA is a widely used pattern recognition system, which can assemble unexplored data based on the similarity of data. Figure 4e shows the PCA result of the sensor array. The healthy subjects and simulated diabetes subjects are separately assembled in the small area without any overlap. In other words, ten simulated breath samples are distinguished from normal breath by using pattern recognition analysis. These results demonstrate that the MOF derived heterogeneous catalyst functionalized on SnO2 NTs can be potentially applicable in exhaled breath analyzers. The improvement in acetone sensing properties of PdO@ZnO-SnO2 NTs is arising from two main reasons. Firstly, the hollow tubular structure can maximize the reaction site of PdO@ZnO-SnO2 NTs because gas molecules can be easily diffused into the inner site of hollow tubular structure. The sensing properties of metal oxide based chemi-resistive gas sensors are affected by the surface reaction between chemisorbed oxygen species (O2–, O– and O2–) and analytes.25 The adsorption of oxygen molecules deprives the electrons of SnO2 and the desorption of chemisorbed oxygen species donates the electrons to SnO2, resulting in the resistance change of SnO2. Since hollow tubular structure has smaller cross-sectional area compared with the solid the fibrous structure, it is reasonable that the baseline resistance of SnO2 NTs is higher than that of SnO2 NFs. Moreover, because more oxygen molecules were chemisorbed on the surface of SnO2 NTs, the baseline resistances of SnO2 NTs (55.4 kΩ) and PdO@ZnO-SnO2 NTs (59.3 kΩ) are higher than those of SnO2 NFs (22.9 kΩ) and PdO@ZnO-SnO2 NFs (28.6 kΩ) (Figure 4f). In addition, the acetone molecules easily 14 ACS Paragon Plus Environment

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penetrate to SnO2 NTs and react with chemisorbed oxygen species, causing the rapid resistance decrease of sensing layer. The respo

nse times of SnO2 NTs (< 20 s) and

PdO@ZnO-SnO2 NTs (< 20 s) are faster than those of SnO2 NFs (< 72 s) and PdO@ZnOSnO2 NFs (< 40 s). Particularly, the resistance change of PdO@ZnO-SnO2 NTs (Rair/Rgas = 10.12) is larger than that (Rair/Rgas = 6.55) of PdO@ZnO-SnO2 NFs because more chemisorbed oxygen molecules react with acetone molecules. The specific surface area determined by Brunauer-Emmett-Teller (BET) method supported these sensing results. PdO@ZnO-SnO2 NTs exhibited higher surface area (15.5 m2 g–1) than PdO@ZnO-SnO2 NFs (8.7 m² g–1) (Figure S12), demonstrating that the hollow tubular structure is more efficient backbone material for activation of PdO@ZnO complex catalyst than the fibrous structure. Secondly, the heterogeneous sensitization of PdO@ZnO catalyst on SnO2 NTs can dramatically improve the acetone sensing performances (Figure 5a). The synergistic enhancement of PdO@ZnO catalyst on SnO2 NTs are interpreted as follow: (i) n-n heterojunction of PdO@ZnO/SnO2 and (ii) catalytic effect of PdO NPs. Heterojunctions are induced on the interface of Pd loaded ZnO (band gap = 3.37 eV) and SnO2 (band gap 3.5 eV) by the difference in work functions.30 Figure 5b shows the Ultraviolet photoelectron spectroscopy (UPS) spectrum of SnO2 NTs and PdO@ZnO-SnO2 NTs using He I radiation (21.22 eV). The high binding energy cutoff is verified to be 16.82 eV for SnO2 NTs and 16.43 eV for PdO@ZnO-SnO2 NTs (Figure 5b-ii). Since the work function of samples is defined as the difference between incident photon energy and high binding energy cut-off, the work functions are therefore calculated to be 4.40 eV for SnO2 NTs and 4.79 eV for PdO@ZnOSnO2 NTs. Since the Pd loaded ZnO has higher work function (5.34 eV) than SnO2 NTs (4.40 eV) (Figure S13), the electrons in SnO2 conduction band transfer to ZnO. Therefore, the energy state shift (0.39 eV) toward the Fermi level are observed in UPS spectrum (Figure 5b), like p-type doping effect. In addition, the energy gaps between the conduction band and Fermi level are obtained to 0.87 eV for PdO@ZnO and 0.5 eV for SnO2 because the valence band 15 ACS Paragon Plus Environment


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maximum of PdO@ZnO and SnO2 NTs is 2.50 eV and 3.00 eV respectively (Figure 5b-iii, Figure S13c, and Figure S14). Then, both the conduction bands are bent upward due to the chemisorption of oxygen molecules, and the potential barrier is introduced on the interface of PdO@ZnO/SnO2 (Figure 5a). As a result, the baseline resistance of PdO@ZnO-SnO2 NTs is increased to 59.3 kΩ compared that (55.4 kΩ) of SnO2 NTs, leading to the large resistance changes during acetone sensing. Furthermore, the PdO NPs as a well-known electronic sensitizer can cause the electron migration from ZnO to PdO during acetone sensing.31 After cyclic sensing measurement of PdO@ZnO-SnO2 NTs toward 5 ppm of acetone, we carried out ex-situ XPS analysis to elucidate the catalytic effect of PdO NPs. When exposed to air, most of Pd atoms exist as the oxidized state (Pd2+) (Figure 5c). On the contrary, the PdO NPs are partially reduced to Pd0 state when exposed to 1 ppm of acetone (Figure 5d), leading to the electron donation from PdO to ZnO. Not only that, the reduced Pd NPs can lower the activation energy of the reaction between acetone and chemisorbed oxygen (CH3COCH3 + O– → CH3COC+H2 + OH– + e– or CH3COCH3 + 2O– → CH3O– + C+H3 + CO2 + 2e–).41 These additional electrons are easily transferred to SnO2 due to the lowering of potential energy barriers originated from the desorption of chemisorbed oxygen (Figure 5a). Therefore, the resistance of PdO@ZnO-SnO2NFs is rapidly decreased by the synergistic effect of PdO@ZnO catalysts (Figure 4f). The recovery property of PdO@ZnO-SnO2 NTs is also enhanced by PdO@ZnO heterogeneous catalysts. The Pd NPs are easily oxidized to PdO NPs in air, and the electrons in ZnO are transferred to PdO. Since the adsorbed oxygens can capture the electrons of ZnO and SnO2, the electrons in ZnO and SnO2 were also migrated to chemisorbed oxygen. Therefore, when PdO@ZnO-SnO2 NTs were exposed to air, the energy barrier between ZnO and SnO2 was rapidly increased (Figure 5a), causing the resistance increase of sensing materials (Figure 4f). As a result, recovery time of PdO@ZnO-SnO2 NTs (64 s) was faster than that of SnO2 NTs (120 s) toward 1 ppm of acetone at 400 °C. From all


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these reasons, MOF driven PdO@ZnO catalysts on hollow SnO2 NTs can efficiently enhance the acetone sensing performances.

4. CONCLUSIONS In summary, Pd@ZIF-8 derived PdO@ZnO heterogeneous catalysts were stably loaded on the SnO2 NTs by electrospinning followed by rapid calcination. The PdO@ZnO complex catalysts are tightly immobilized on the wall of SnO2 NTs, which lead to dramatically improved acetone sensing characteristics due to the creation of n-n (ZnO-SnO2) heterojunction and electronic sensitization effect of PdO. The PdO@ZnO-SnO2 NTs showed remarkable response toward 1 ppm of acetone (Rair/Rgas = 5.06 at 400 °C) with high selectivity and stability. The response and recovery time of PdO@ZnO-SnO2 NTs were also improved to less than 20 s and 64 s, respectively. In addition, the PdO@ZnO-SnO2 NTs obviously classified the exhaled breath of normal people and simulated diabetics with PCA patterns. These results confirm that M@MOF derived heterogeneous complex catalysts loaded 1D NTs are an efficient sensing platform for the superior gas sensors. Notably, this proposed design concept can pave a new way to the development of hollow architectures functionalized with the various synergistic catalysts derived from M@MOFs.

ASSOCIATED CONTENT Supporting Information. Additional SEM images, N2 adsorption/desorption isotherms, XRD data, STEM image, EDS analysis, XPS analysis, UPS data, and additional sensing characteristics. These materials are available free of charge via the Internet at “http://pubs.acs.org.” AUTHOR INFORMATION 17 ACS Paragon Plus Environment


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Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation (NRF) of Korea grant funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1A2A1A16074901). This work was also 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 funded by the Ministry of Science, ICT & Future

Planning

as

Biomedical

Treatment

Technology

Development

Project

(2015M3A9D7067418).

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Figure Captions Figure 1. (a) Schematic illustration of synthesis of Pd@ZIF-8, (b) TEM image of Pd@ZIF-8, (c) HRTEM image of Pd@ZIF-8, and (d) EDS spectrum of Pd@ZIF-8. Figure 2. (a) Schematic illustration of synthetic process of PdO@ZnO-SnO2 NTs, (b) SEM image of as-spun Pd@ZIF-8/PVP/Sn composite NFs, (c) TEM image of as-spun Pd@ZIF8/PVP/Sn composite NFs, (d) SEM image of PdO@ZnO-SnO2 NTs, (e) TEM image of PdO@ZnO-SnO2 NTs, (f) HRTEM image of PdO@ZnO-SnO2 NTs, and (g) SAED pattern of PdO@ZnO-SnO2 NTs. Figure 3. (a) TG and DSC analysis of Pd@ZIF-8/PVP/Sn composite NFs with the different heating rate and (b) Schematic illustration on the formation mechanism of PdO@ZnO-SnO2 NTs. Figure 4. (a) Dynamic acetone response transition in the concentration range of 0.1–5.0 ppm at 400 °C, (b) Selective sensing characteristics of PdO@ZnO-SnO2 NTs toward 1 ppm of various analytes at 400 °C, (c) Response time and (d) Recovery time toward acetone at 400 °C, (e) Pattern recognition based on PCA using SnO2 NFs, SnO2 NTs, PdO@ZnO-SnO2 NFs, and PdO@ZnO-SnO2 NTs, and (f) Dynamic resistance transition toward 1 ppm of acetone at 400 °C. Figure 5. (a) Schematic illustration of acetone sensing mechanism for PdO@ZnO-SnO2 NTs, (b) i) UPS spectrum of SnO2 NTs and PdO@ZnO-SnO2 NTs, ii) high binding energy region, and iii) low binding energy region, and ex-situ XPS analysis using high resolution spectra of PdO@ZnO-SnO2 NTs in the vicinity of Pd 3d (c) in air and (d) in acetone after 7 cycle sensing measurement to 5 ppm of acetone at 400 °C.


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