MetalâOrganic Framework-Templated PdO-Co3O4 Nanocubes

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Metal-organic framework templated PdO-Co3O4 nanocubes functionalized by SWCNTs: Improved NO2 reaction kinetics on flexible heating film Seon-Jin Choi, Hak-Jong Choi, Won-Tae Koo, Daihong Huh, Heon Lee, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11317 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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ACS Applied Materials & Interfaces

Metal-organic framework templated PdO-Co3O4 nanocubes functionalized by SWCNTs: Improved NO2 reaction kinetics on flexible heating film Seon-Jin Choi,†,⊥ Hak-Jong Choi,∥,⊥ Won-Tae Koo,‡ Daihong Huh,§ Heon Lee§,* and Il-Doo Kim‡,*

†

Biomaterials Innovation Research Center, Division of Engineering in Medicine, Department of

Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA 02139, USA ∥

Electrical & Systems Engineering, University of Pennsylvania, Moore Building, Levine

375200 South 33rd Street, Philadelphia, PA 19104, USA ‡

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

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

Department of Materials Science and Engineering, Korea University, Anam-ro 145, Seongbuk-

gu, Seoul 136-713, Republic of Korea ⊥

S-J Choi and H-J Choi contributed equally to this work.

*Address correspondence to [email protected], [email protected]

KEYWORDS: metal organic framework, Co3O4 nanocubes, carbon nanotube, metal mesh, flexible gas sensor

ACS Paragon Plus Environment

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ABSTRACT

Detection and control of air quality are major concerns in recent years for environmental monitoring and healthcare. In this work, we developed an integrated sensor architecture comprised of nanostructured composite sensing layers and a flexible heating substrate for portable and real-time detection of nitrogen dioxide (NO2). As sensing layers, PdO infiltrated Co3O4 hollow nanocubes (PdO-Co3O4 HNCs) were prepared by calcination of Pd-embedded Co based metal-organic framework (MOF) polyhedron particles. Single walled carbon nanotubes (SWCNTs) were functionalized with PdO-Co3O4 HNCs to control conductivity of sensing layers. As a flexible heating substrate, Ni mesh electrode covered with 40-nm thick Au layer (i.e., Ni(core)/Au(shell) mesh) was embedded in a colorless polyimide (cPI) film. As a result, SWCNTs functionalized PdO-Co3O4 HNCs sensor exhibited improved NO2 detection property at 100 °C with high sensitivity (S) of 44.11% at 20 ppm and low detection limit of 1 ppm. The accelerated reaction and recovery kinetics toward NO2 of SWCNTs functionalized PdO-Co3O4 HNCs were achieved by generating heat on the Ni(core)/Au(shell) mesh embedded cPI substrate. The SWCNTs-functionalized porous metal oxide sensing layers integrated on mechanically stable Ni(core)/Au(shell) mesh heating substrate can be envisioned as an essential sensing platform for realization of low temperature operation wearable chemical sensor.


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1. Introduction Nitrogen dioxide (NO2) is one of the most harmful gas species to human health. Excessive inhalation of NO2 at low concentrations (~1 ppm) can cause pulmonary disease and severe damage to respiratory system.1-2 The major sources of NO2 emission come from automobiles, power plants, and industrial combustion in air. Recently, National Aeronautics and Space Administration (NASA) revealed air pollution trends over most major cities in the world using high-resolution global satellite maps.3 In that study, a significant increase in NO2 concentration was observed in 2014, particularly in East Asia due to growing economics and demanding usage of fossil fuels.3 For this reason, new types of sensing platforms with flexible and wearable functions, including not only sensitive sensing layers but also mechanically stable heating substrate, should be developed for realization of real-time and fast NO2 detection, thereby warning harmful environment to the users. Metal-organic frameworks (MOFs) with incredibly high surface area, ultra-high porosity, and facile compositional tunability of versatile materials4-6 have been studied for applications in catalysts,7-8 gas separation,9 energy storage/conversion,10 and chemical sensors.11 In particular, diverse MOF-based nanomaterials have been intensively utilized for applications in energy storage and conversion such as supercapacitors,12-13 fuel cells,14 and electrocatalysts.15 For gas sensing application, MOFs have been utilized as effective templates to synthesize semiconductor metal oxide (SMO) nanostructures, which possess similar morphologies in comparison with pristine MOFs composed of metal nodes and organic linkers.16-17 In addition, sub-5 nm scale particles can be infiltrated into the cavity of MOF templates, which can serve as catalytic nanoparticles for ultra-sensitive gas sensors. For example, PdO functionalized Co3O4 hollow nanocages (PdO-Co3O4 HNC) were synthesized through high temperature calcination of Pd-


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embedded Co based zeolite imidazole framework (ZIF-67) and used as chemical sensing layers operated at 350 °C.18 However, the application of the MOF-templated nanostructures in flexible chemical sensors has been limited due to their poor reaction kinetics mainly attributed to the low conductivity of MOF-templated SMO nanostructures particularly at room temperature. To improve reaction kinetics toward chemical molecules, the operation of SMO sensors at elevated temperatures is imperative. In this sense, the use of robust and highly reliable heating film with a mechanical flexibility is essential for the realization of flexible chemical sensors with improved sensing performance. Conventionally, diverse conductive elements such as silver nanowires (Ag NWs),19-21 carbon nanotubes,22-23 and graphene24-25 were coated on plastic substrates for application in flexible heater. In particular, flexible heating film coated with Ag NWs and graphene was utilized as substrate for flexible chemical sensors. For example, Choi et al. investigated temperature dependent NO2 sensing characteristics of reduced graphene oxide and ruthenium oxide (RuO2) nanosheets using Ag NW-embedded colorless polyimide (cPI) heating film.26-27 In addition, Choi et al. demonstrated fast recovery rate of single layer graphene after NO2 reaction using double layer graphene heating film.28 More recently, a new type of transparent and flexible heating films has been proposed via conductive metal mesh.29-31 The mesh-type conductive microstructures are advantageous since the junctionless grid structures offer low power consumption, low operating voltage, and rapid/uniform heating capability for large area. For example, Khan et al. developed a flexible heater using Cu mesh-embedded plastic film prepared by Cu electroplating and subsequent embedding process in a polymer film.31 Moreover, the metal mesh patterned on a complex three-dimensional morphology was also prepared by using electrohydrodynamic jet printing,32 which can be potentially useful for wearable devices. As evidenced in previous studies, a majority of metal mesh electrodes as


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flexible heaters are based on Ni and Cu due to their high electrical conductivity and low price. However, the oxidation of metal mesh electrodes is still a major challenge, which results in severe degradation of heating property. Moreover, most mesh-type flexible heaters have been mainly applied to the flexible defroster so far and their application in flexible chemical sensors has yet been demonstrated. In this work, we firstly demonstrated flexible NO2 sensor using MOF-templated PdO-Co3O4 hollow nanocubes (HNCs) on a flexible heating film. Stable evaluation of chemical sensing characteristics using MOF-derived PdO-Co3O4 HNCs particularly at low temperature is very challenging due to poor electrical conductivity. The poor electrical conductivity of PdO-Co3O4 HNCs at low temperature (< 100 °C) was effectively manipulated by mixing single walled carbon nanotubes (SWCNTs) as fast charge carrier pathway with PdO-Co3O4 HNCs. For the rational design of highly robust flexible sensing substrate with a heater function, we fabricated Ni mesh electrode surrounded by ultra-thin Au film (i.e., Ni(core)/Au(shell) mesh electrode), which is tightly embedded in high temperature resistant cPI film, via template-assisted selective electrodeposition of Ni mesh followed by electroless deposition of Au. The substantial influence of Ni(core)/Au(shell) mesh electrode embedded cPI substrate on enhanced and real-time NO2 detection at moderate operating temperature is discussed. The proposed platform of SWCNTsloaded PdO-Co3O4 HNCs on the flexible heating film comprising of Ni(core)/Au(shell) mesh electrode will be an essential component for wearable NO2 sensors. 2. Experimental Section Fabrication of Ni(core)/Au(shell) mesh embedded cPI heater: The Ni(core)/Au(shell) mesh was fabricated using template-assisted electrodeposition and Au electroless deposition. First, polymer resist (LOR10B, MicroChem) was spin-coated on ITO/glass substrate with thickness of


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~ 1 µm and baked at 170 °C for 4 min. Si master mold with a pitch of 40 µm, a line width of 0.5 µm, a height of 1 µm, and square arrays was replicated using polydimethylsiloxane (PDMS) used as a polymer replica mold.33 Hydrogen silsesquioxane (HSQ) solution (Fox®-16, Dow Corning) was used as the resist for direct printing. Then, HSQ solution was spin-casted on the PDMS mold at 3000 rpm for 30 sec. Then, HSQ coated PDMS mold was directly printed on LOR10B coated ITO/glass with a pressure of 5 bar for 5 min. After detaching the PDMS mold, square arrayed HSQ pattern was formed on LOR10B coated ITO/glass. In order to keep the shape of the pattern, HSQ pattern was oxidized using UV/O3 treatment for 20 min. Then, HSQ pattern was transferred into LOR10B layer to selectively expose the ITO surface using reactive ion etching process. Residual layer of HSQ pattern and LOR10B layer were etched using CF4/CHF3 and O2 gases with RF power of 300 W and 100 W and pressure of 25 mTorr and 40 mTorr, respectively. Subsequently, Ni mesh was formed on selectively exposed ITO surface using Ni electrodeposition.34 After Ni electrodeposition, polycrystalline Ni mesh was formed (Figure S1 in the Supporting Information). Then, Ni surface was substituted by Au using electroless deposition. A solution of Au electroless deposition was prepared with 0.008 M of potassium dicyanoaurate (KAu(CN)2) and 0.18 M of potassium cyanide (KCN) in deionized (DI) water. Small amount of hydrogen chloride (HCl) solution was added to adjust the pH between 4 and 6. Ni mesh formed ITO/glass was immersed into the solution and heated up to 90 °C for several minutes to form thin Au shell on Ni surface. After that, HSQ/LOR10B square pattern was removed using N, N-dimethylformamide (DMF). Ni(core)/Au(shell) mesh on ITO/glass was cleaned using ethanol and DI water and dried using N2 blow. The Ni(core)/Au(shell) mesh on ITO/glass was utilized for fabrication of Ni(core)/Au(shell) mesh embedded colorless polyimide (cPI) film as the flexible heater (i.e., Ni/Au-cPI heater). As


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a precursor of cPI, polyamic acid (PAA) solution was prepared by dissolving 1.023 g of 4,4'(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 0.567 g of 3,3’-diaminodiphenyl sulfone (APS) in 3.5 mL of dimethylacetamide (DMAc) and stirred at 400 rpm for 6 h. The PAA solution was uniformly coated on the Ni(core)/Au(shell) mesh on ITO/glass using a doctor blade. Subsequently, imidization process was performed to form cPI film by heat-treatment at 100 °C, 200 °C, and 230 °C for 1 h at each temperature with a heating rate of 1 °C/min. The Ni/Au-cPI heater was obtained after lift-off from the ITO/glass substrate by immersing in DI water. The average thickness of the Ni(core)/Au(shell) mesh embedded cPI film was 230 µm after lift-off from the ITO/glass substrate. Synthesis of PdO-Co3O4 HNCs functionalized by SWCNTs: MOF-templated PdO-Co3O4 HNCs were synthesized by infiltration of Pd nanoparticles (NPs) into the cavity of ZIF-67 and subsequent reduction process.18 First, Co based ZIF-67 was prepared by precipitation reaction at room temperature. Two different solutions comprising of 2.933 g of Co(NO3)2·6H2O and 6.489 g of 2MeIm in methanol, respectively, were rapidly mixed together and stirred for 5 h. The Co based ZIF-67 was obtained after purification at 3000 rpm for 1 min using centrifugation and subsequent washing with ethanol. To infiltrate Pd NPs in the ZIF-67, 2 mg of Pd precursor (K2PdCl4) was introduced into a solution comprising of 40 mg of ZIF-67 dispersed in 1 mL of DI water. Subsequently, Pd ions were reduced by a reduction agent of NaBH4 solution (1.5 mg/mL). After infiltration and reduction processes, Pd infiltrated ZIF-67 was purified using centrifugation and washed with DI water. Finally, calcination process at 400 °C for 1 h resulted in the formation of PdO-Co3O4 HNCs. To increase conductivity of PdO-Co3O4 HNCs, 0.6 mg of SWCNTs (Nanostructured & Amorphous Materials Inc., 3 wt % in water) was mixed with 1.5 g PdO-Co3O4 HNCs dispersed in 100 mL ethanol.


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Interdigitated electrode (IDE) patterning: Interdigitated electrodes (IDEs) were patterned on cPI film to evaluate resistance transitions of SWCNTs-loaded PdO-Co3O4 HNCs during the exposure to NO2. The finger width and length were 200 µm and 2750 µm, respectively, with space of 200 µm between the electrodes. The IDE pattern was formed using a shadow mask and a 5 nm/50 nm-thick Cr/Au layer was deposited using e-beam evaporator. Investigation of NO2 sensing property: NO2 sensing characteristics were evaluated using a homemade measurement setup under different operating temperature.35 Temperature of metal mesh heater was controlled by applying different voltages to the metal mesh using a DC power supply (E3644A, Agilent). The operating temperatures were maintained at 22 °C (room temperature), 36 °C, and 100 °C by applying voltages of 0 V, 0.7 V, and 2.1 V, respectively, to the metal mesh heater. The resistance changes of SWCNTs-loaded PdO-Co3O4 HNCs were measured under different operating temperature. Before injection of NO2, sensor was stabilized approximately for 4 hours to maintain consistent baseline resistance. Cyclic NO2 exposure was performed in the concentration ranges of 1–20 ppm using mass flow controller and resistance transitions were monitored using a data acquisition system (34972A, Agilent) with a 16-channel multiplexer (34902A, Agilent). To investigate reversible reaction kinetics, the gas injection time was systematically controlled. Even though the reaction was not saturated, the injection of NO2 was consistently proceeded for 3 min followed by 3 min recovery using baseline air. The sensitivity (S=[Rair–Rgas]/Rair×100%) was calculated, where Rair and Rgas are the sensor’s resistances upon exposure to air and NO2, respectively.

3. Result and Discussion


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The fabrication process for Ni(core)/Au(shell) mesh embedded cPI film as a flexible sensing substrate with heating function is illustrated in Figure 1. In the first step, polymer resist (LOR10B) was spin-coated on ITO/glass substrate (Figure 1a). Then, hydrogen silsesquioxane (HSQ) solution was used to form square pattern arrays on LOR10B coated ITO/glass substrate using direct printing of replicable materials such as polydimethylsiloxane (PDMS),36 which is a simple and scalable method for forming the regular or random pattern arrays controlled by the geometry of the master stamp (Figures 1b and 1c). Square pattern arrays were then transferred into LOR10B layer for exposure of ITO surface by reactive ion etching process (Figure 1d). After then, Ni was electrodeposited on square pattern arrayed ITO/glass substrate to form Ni mesh (Figure 1e). Subsequently, HSQ and LOR10B were removed by dipping of the substrate in N,N-dimethylformamide (DMF) solution. After then, thin Au film as surface protection layer was uniformly coated on the outer surface of rectangular-shaped Ni electrode via electroless Au deposition for the creation of Ni(core)/Au(shell) mesh electrodes (Figure 1f). Lastly, polyamic acid (PAA) as a precursor solution for cPI film was screen-printed onto Ni(core)/Au(shell) mesh patterned on ITO/glass (Figure 1g). After imidization process, Ni(core)/Au(shell) mesh electrodes were embedded in cPI film and subsequently detached from ITO/glass, resulting in the free-standing and flexible Ni(core)/Au(shell) mesh based heater film for wearable sensor (Figure 1h). Figures 2a and 2b showed low- and high-magnification field-emission scanning electron microscopy (FE-SEM) images of Ni(core)/Au(shell) mesh electrodes on ITO/glass. The individual line comprising of rectangular-shaped rod of Ni covered with Au mesh exhibited a width of 1.4 µm and a pitch of 40 µm. After embedding process into cPI film and subsequent detaching process of the mesh electrodes from underlying ITO/glass, the Ni(core)/Au(shell)


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mesh electrodes tightly anchored on a cPI film was successfully fabricated (Figure 2c). In order to investigate composition and thickness of Ni(core)/Au(shell) mesh electrodes, cross-sectional structure was characterized by high-resolution transmission electron microscopy (HR-TEM) (Figure 2d). The cross-sectional area of single Ni(core)/Au(shell) line was observed as quasirectangle shape with approximately 1.4 µm (width) × 950 nm (height). As shown in Figure 2e, quasi-rectangular Ni electrodes were uniformly covered by 40 nm-thick Au layer, which was confirmed by an elemental mapping image of Ni(core)/Au(shell) mesh electrodes using energy dispersive X-ray spectroscopy (EDS). Optical transmittance of Ni(core)/Au(shell) mesh electrodes formed on a glass substrate and Ni(core)/Au(shell) mesh electrodes embedded in cPI film was investigated (Figure 2f). The Ni(core)/Au(shell) mesh electrodes formed on a glass substrate exhibited high optical transmittance of 86% at the wavelength of 550 nm. On the other hand, optical transmittance of the Ni(core)/Au(shell) mesh electrodes embedded in cPI film was decreased to 76% at the same wavelength due to the relatively low optical transmittance of bare thick-cPI film (230 µm, 78% at 550 nm). We expect that the optical transmittance of Ni(core)/Au(shell) mesh electrodes embedded cPI film can be further improved by reducing the thickness of cPI film. Highly conductive Ni(core)/Au(shell) mesh electrodes embedded cPI film was utilized as a flexible heater. The Ni(core)/Au(shell) mesh electrodes embedded cPI film (hereafter, Ni/Au-cPI heater) showed low sheet resistance of 2 Ω/sq. Current transitions of Ni/Au-cPI heater with the size of 1 cm × 2 cm were investigated by varying applied voltages in the range of 0 V–2 V (Figure 3a). Step-like current transitions were observed under the continuous increase of voltage with 0.2 V steps, exhibiting maximum current of 637 mA at 2 V. Correspondingly, temperature transitions of the Ni/Au-cPI heater were also characterized (Figure 3b). Similarly, step-like


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temperature transitions were observed as the voltage was increased with 0.2 V steps. The Ni/AucPI heater exhibited a maximum temperature of 170 °C at 2 V. In addition, fast temperature modulation was demonstrated with the responding time of 110 sec to generate heat from room temperature to 150 °C. To investigate long-term stability of the Ni/Au-cPI heater, the heater temperature and current level were continuously monitored under constant applied voltage at 1.2 V. (Figure 3c). The result revealed that very stable heater temperature (100–103 °C) and current levels (475–479 mA) were maintained for over 10 h. In addition, high mechanical flexibility of the Ni/Au-cPI heater was investigated under repeated cyclic bending stress up to 1700 times (Figure 3d). As confirmed by real-time current and temperature observation, stable temperature (70–80 °C) and current (440–452 mA) behaviors were achieved even under continuous heating of the film during the repeated bending cycles. The outstanding long-term stability and mechanical flexibility of the Ni/Au-cPI heater were attributed to the embedded structure of Ni/Au mesh in the thermally stable cPI film. In addition, conformal coating of 40 nm-thick Au layers surrounding Ni mesh electrodes allows stable on/off heating by preventing thermal oxidation of Ni mesh. To investigate enhanced NO2 detection capability of flexible sensors using Ni/Au-cPI heater, we synthesized composite sensing layers, i.e., porous PdO-Co3O4 HNCs functionalized by SWCNTs. Co based ZIF-67, consisted of Co as metal node and 2-methylimidazole as organic ligand, was used as the sacrificial template for producing PdO-Co3O4 HNCs. (Figure 4a). To functionalize Pd nanoparticles, Pd ions were infiltrated into the cavities of ZIF-67 and subsequent reduction process resulted in the metallic Pd-loaded ZIF-67 (Figure 4b). After high temperature calcination in air at 400 °C for 1 h, PdO decorated Co3O4 HNCs were obtained after oxidation of Pd and Co as well as thermal decomposition of organic linkers (Figure 4c). To


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improve electrical conductivity of PdO-Co3O4 HNCs sensing layers, SWCNTs were homogeneously mixed with the PdO-Co3O4 HNCs (hereafter, SWCNTs-loaded PdO-Co3O4 HNCs). The microstructural evolution of PdO-Co3O4 HNCs was observed by SEM and TEM (Figure 4d-i). As shown in Figure 4d and Figure S2a in the Supporting Information, ZIF-67 exhibited polyhedron structures with an average diameter of 450 nm. After calcination at 400 °C for 1 h, highly porous PdO-Co3O4 HNCs with an average pore size of 9.4 nm were achieved by the decomposition of ZIF-67 template and oxidation of Pd and Co (Figure 4e and Figure S2b in the Supporting Information). High BET surface area of 20.96 m2/g and high porosity with the maximum pore density at 58.84 nm were achieved for the PdO-Co3O4 HNCs (Figure S3a Supporting Information). Pd concentration was confirmed by inductive coupled plasma (ICP) analysis (Table S1 in the Supporting Information). The result revealed that very small amount of Pd content (2.45 mg/kg) was observed as compared to the content of Co (161 mg/kg) with the mass ratio of Co and Pd of 1:0.015. Minor differences in microstructure were observed after functionalization of SWCNTs with PdO-Co3O4 HNCs (Figure 4f). TEM analysis revealed that the PdO-Co3O4 HNCs exhibited hollow structure composed of small Co3O4 crystallites (20 nm) (Figure 4g). Magnified TEM analysis confirmed the functionalization of SWCNTs on PdOCo3O4 HNCs (yellow arrows in Figure 4h and Figure S2c-d in the Supporting Information). Substantially increased BET surface area (92.79 m2/g) was achieved by functionalization of SWCNTs on PdO-Co3O4 HNCs, which was mainly attributed to the increased pore density at mesoscale region (3.82 nm) by incorporation of SWCNTs (Figure S3b Supporting Information). The SWCNTs were employed for electrically bridging multiple PdO-Co3O4 HNCs, which can facilitate electrical conduction in between the PdO-Co3O4 HNCs and lead to improved charge


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collection onto sensing substrate. The functionalization of SWCNTs is essential for optimized control of the base resistance level of porous PdO-Co3O4 HNCs sensing layers. Without functionalization of SWCNTs, baseline resistance of PdO-Co3O4 HNCs was excessively high to measure due to their poor electrical conductivity. High-resolution TEM (HRTEM) analysis revealed the crystal structure of Co3O4 HNCs with the interplanar distances of 2.42 Å and 2.85 Å, which correspond to the crystal planes of (311) and (220), respectively (Figure 4i). Based on the HRTEM analysis, the crystal phase of Co3O4 HNCs was identified as cubic spinel structure. Selected area electron diffraction (SAED) pattern revealed crystalline structure of Co3O4 HNCs with the crystal planes such as (111), (220), (311), (400), (511), and (440), which were also partially observed in HRTEM (in the inset of Figure 4i). X-ray diffraction (XRD) pattern revealed that cubic spinel structure of Co3O4 was consistently observed (Figure S4 in the Supporting Information). An average grain size was 30.2 nm calculated by Scherrer equation with the peak position at 36.78°. However, the phase of PdO nanoparticles was not clearly observed in TEM image and XRD pattern due to their small particle size and low content. Alternatively, energy dispersive X-ray spectroscopy (EDS) analysis revealed the uniform functionalization of PdO nanoparticles on Co3O4 HNCs (Figure S2e-h in the Supporting Information). The SWCNTs-loaded PdO-Co3O4 HNCs were integrated on Ni/Au-cPI heater film to fabricate flexible chemical sensors (Figures 5a-c and Figure S5a in the Supporting Information). The conductive Ni/Au mesh electrodes were placed at the bottom of the cPI film to generate heat at applied voltages (CH). In addition, SWCNTs-loaded PdO-Co3O4 HNCs were printed at the top of cPI film by simple drop-coating of mixed inks. Characteristic resistance transitions (∆R) of SWCNTs-loaded PdO-Co3O4 HNCs were investigated under various heating voltages of 0 V–1.2


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V. The temperature of Ni/Au-cPI heater was steadily increased to 60 °C at 1.2 V (Figure 5d). Accordingly, the baseline resistance of SWCNTs-loaded PdO-Co3O4 HNCs was steadily decreased from 3.51 kΩ at 0 V (room temperature) to 3.06 kΩ at 1.2 V (60 °C), which is a typical semiconducting behavior (Figure 5e). Further reduction in baseline resistance (20.75 kΩ at 2.1 V) was achieved by elevating voltage to the Ni/Au-cPI heater (Figure S5b in the Supporting Information). Temperature dependent NO2 sensing characteristics were investigated using SWCNTs-loaded PdO-Co3O4 HNCs integrated on Ni/Au-cPI heater (Figure 5f and Figure S5 in the Supporting Information). The resistance transitions of SWCNTs-loaded PdO-Co3O4 HNCs were observed at the three different operating voltages, i.e., 0 V (room temperature), 0.7 V (36 °C), and 2.1 V (100 °C), with cyclic NO2 exposure in the concentration range of 1 ppm–20 ppm followed by refreshing with baseline air (Figure S6 in the Supporting Information). The NO2 exposure time was consistently maintained for 3 min to compare and investigate reversible reaction kinetics at different operating temperatures because the sensing signal was not saturated over prolonged NO2 exposure (Figure S7 in the Supporting Information). The resistance transitions upon NO2 exposures were converted into sensitivity (S) as defined by S=[(Rgas–Rair)/Rair]×100%, where Rair is the initial baseline resistance in air ambient and Rgas is the resistance of the sensor in NO2 ambient (Figure 5f). At 0 V (22 °C), relatively low sensitivity (S=27.33%) was observed at the first NO2 exposure at 20 ppm. Very sluggish recovery kinetics resulted in severe drift of baseline resistance, particularly at high NO2 concentration. Moreover, negligible responses were observed at low NO2 concentration ranges, i.e., 1–5 ppm. On the other hand, improved reaction kinetics were achieved at an elevated operation temperature. For instance, at 0.7 V (36 °C), similar sensitivity (S=26.25%) was observed at the first NO2 exposure at 20 ppm. Importantly, improved


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recovery kinetics were obtained at high NO2 concentration ranges (10–20 ppm) with appreciable responses at low concentration ranges (1–5 ppm). The improvement in reaction kinetics was evidently observed by further increasing operating voltage up to 2.1 V (100 °C). Much enhanced sensitivity (S=44.11%) was achieved at 20 ppm with detection capability at low NO2 concentration (S=3.09% at 1 ppm). Moreover, SWCNTs-loaded PdO-Co3O4 HNCs integrated on Ni/Au-cPI heater showed high mechanical stability, exhibiting negligible baseline resistance drift over cyclic bending test of 4000 cycles (Figure S8 in the Supporting Information). The improved reaction kinetics were quantitatively investigated by calculating adsorption and desorption rate constants. In previous studies, desorption rate constant (kdes), adsorption rate constant (kads), and equilibrium constant (K=kads/kdes) have been defined to evaluate the response and recovery kinetics as shown in the Equations (1) and (2) below.37-39 St = exp [− ∙ ]

St = ∙

(1)

1 − ! "−

∙ ∙ #$,

(2)

where S0 is the sensitivity when NO2 is removed after refreshing with baseline air, Smax is the maximum sensitivity toward NO2, and Ca is the NO2 concentration. To calculate the rate constants, Equations (1) and (2) were used by fitting to the sensitivity curves at the first reaction toward 20 ppm of NO2 at different operating voltages (Figure 6). It should be noted that the rate constants were separately obtained at the first reaction and second reaction states for accurate calculation of the rate constants while minimizing deviation from the original data. The adsorption and desorption rate constants as well as equilibrium constants of SWCNTs-loaded PdO-Co3O4 HNCs toward NO2 at 20 ppm were summarized in Table 1. The results revealed that the adsorption rate constants were increased in both first and second reactions when NO2 was


15


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exposed to the sensor as the operating voltages were increased. In particular, 2.15-fold enhancement in the adsorption rate constant was achieved in the first reaction at 2.1 V (100 °C) as compared to the sensing operation at 0 V (room temperature). In addition, when NO2 was removed by baseline air, the improved desorption rate constants were evidently observed in both first and second reactions as operating voltages were increased. However, the improvement in desorption rate constants at the second reaction was much larger than that at the first reaction. Specifically, 5.45-fold enhancement in desorption rate constant was obtained in the first reaction at 2.1 V (100 °C) from 0 V (room temperature), whereas 9.04-fold improvement was achieved in the second reaction. The result implies that the desorption process was more dominantly accelerated at an increased temperature. Indeed, higher desorption rate constants compared to adsorption constants were observed even after slightly increased temperature, i.e., 36 °C at 0.7 V. The sensing mechanism was investigated based on the resistance transition property and surface chemistry. In general, both Co3O4 and SWCNT exhibit p-type sensing properties, in which holes are majority carriers in the sensing signal transduction. Hole accumulation layer with certain thickness can be formed in air ambient during a stabilization process before NO2 injection. In particular, thick hole accumulation can be formed near the surface of Co3O4 HNCs due to the electron transfer from Co3O4 HNCs to SWCNTs (Figure S9 in the Supporting Information), which resulted in the increased electrical conductivity of SWCNTs-loaded PdOCo3O4 HNCs. In addition, catalytic PdO NPs can attract electrons from Co3O4 HNCs by further thickening the hole accumulation layer on the surface of Co3O4 HNCs and contribute surface reaction by electronic sensitization effect of PdO NPs.40 The surface reaction can be accelerated due to the large surface area (92.79 m2/g) and wide range of pore distribution in mesoscale and macroscale regions of SWCNTs-loaded PdO-Co3O4 HNCs facilitating gas penetration into the


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hollow structure and surface reaction. When an oxidizing gas such as NO2 is exposed to the ptype sensing layer, thickness of the hole accumulation layer is usually increased due to the electron trap on the surface, which results in the decreased resistance of sensor upon NO2 exposure. For this reason, the composite sensing layer of SWCNTs-loaded PdO-Co3O4 HNCs exhibited decreased resistance transition, similar to typical p-type sensing property (Figure S6 in the Supporting Information). The surface electron trap and charge transition were evidenced by investigating surface chemistry using X-ray photoelectron spectroscopy (XPS) (Figure 7). The SWCNTs-loaded PdO-Co3O4 HNCs composite sensors exhibited minor intensity of surface adsorbed nitrate (NO3–) at the binding energy of 406.6 eV (Figure 7a). However, significantly increased nitrate peak intensity was observed after the NO2 injection to the sensor (Figure 7b), which results in electron attraction by the formation of surface nitrate. In addition, XPS spectrum at O1s was investigated using the SWCNTs-loaded PdO-Co3O4 HNCs at room temperature (Figure S10 in the Supporting Information). The result revealed that diverse oxygen species including O2–, O–, and O2– with the high intensity of O2– peak were observed. Therefore, the formation of nitrate can occur by effective reaction of NO2 with surface oxygen species, as described in the following chemical reaction: O2 (gas) + e– O2– (adsorbed)

(3)

O2– (adsorbed)+ e– 2O– (adsorbed)

(4)

O– (adsorbed) + e– O2– (adsorbed)

(5)

NO2 (gas) + O– (adsorbed) NO3– (adsorbed)

(6)

On the other hand, additional peak at 406 eV was observed at an elevated temperature (35 °C) after NO2 exposure (Figure 7c). Previous studies revealed that the peak at around 406 eV was


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assigned as NO32–, which was mainly attributed to the coordination of NO2 with surface oxygen atoms related to a vacancy sites.41-42 In this regard, additional adsorption of NO32– occurred on the surface of sensing layer when NO2 is injected by the reaction as described below: NO2 (gas) + O2– (adsorbed) NO32– (adsorbed)

(7)

NO3– (adsorbed) + e– NO32– (adsorbed)

(8)

This result explains the formation of thick hole accumulation layer by accommodating additional surface adsorption sites and electron charge traps on the surface of sensing layer at an elevated temperature in the form of NO32–, which contributes to large resistance change toward NO2. 4. Conclusion In this work, we report an efficient processing strategy for a rational design of flexible gas sensor with improved sensitivity at low operation temperature (< 100 °C) and high mechanical stability. Ni(core)/Au(shell) mesh electrodes, which were prepared by embedding of imprinted Ni grid conformaly covered by thin Au layer into colorless polyimide (cPI) film, served as heating substrates for elevating temperature of sensing layers composed of SWCNTs-loaded PdO-Co3O4 HNCs. Inherent limitation of metal oxide based flexible sensors, i.e., excessively high base resistance particularly at room and/or low temperature (< 100 °C), was successfully overcome by incorporating SWCNTs as electrical conduction pathways onto porous PdOdecorated Co3O4 nanocubes as well as enhancing cPI substrate temperature. After integration of SWCNTs-loaded PdO-Co3O4 HNCs on the Ni/Au-cPI heater, the sensing characteristics were investigated at different operating temperatures controlled by applying voltages to the Ni/Au-cPI heater. More importantly, much improved reaction kinetics toward NO2 molecules were achieved with high sensitivity (S) of 44.11% at 2.1 V (100 °C) as compared to the sensitivity


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(S=27.33%) at 0 V (room temperature). By employing the unique sensing platform, a variety of sensing layers can be integrated on the Ni/Au-cPI heater substrate for real-time detections of diverse hazardous chemical species for wearable applications.


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ASSOCIATED CONTENT Supporting Information. SEM and TEM images of ZIF-67, PdO-loaded porous Co3O4 nanocubes (NCs), PdO-Co3O4 NCs functionalized by single walled carbon nanotubes (SWCNTs). Camera image of the fabricated sensor. Pore distribution analysis, ICP analysis, XRD analysis, and XPS analysis of PdO-Co3O4 NCs functionalized by SWCNTs. Dynamic resistance transition of PdO-Co3O4 NCs functionalized by SWCNTs toward NO2 gas. Resistance transition of flexible sensor film under cyclic bending stress. SEM image of Ni mesh. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Author Contributions ⊥

S-J Choi and H-J Choi contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by Wearable Platform Materials Technology Center (WMC) funded by the National Research Foundation of Korea (NRF) Grant of the Korean Government (MSIP; No. 2016R1A5A1009926). This research was supported by Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4905609) and by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (2016R1A2B3015400). National Research Foundation (NRF) of Korea grant funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1A2A1A16074901).


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(25) Kang, J.; Kim, H.; Kim, K. S.; Lee, S. K.; Bae, S.; Ahn, J. H.; Kim, Y. J.; Choi, J. B.; Hong, B. H. High-Performance Graphene-Based Transparent Flexible Heaters. Nano Lett. 2011, 11 (12), 5154-5158. (26) Choi, S. J.; Kim, S. J.; Jang, J. S.; Lee, J. H.; Kim, I. D. Silver Nanowire Embedded Colorless Polyimide Heater for Wearable Chemical Sensors: Improved Reversible Reaction Kinetics of Optically Reduced Graphene Oxide. Small 2016, 12 (42), 5826-5835. (27) Choi, S. J.; Jang, J. S.; Park, H. J.; Kim, I. D. Optically Sintered 2D RuO2 Nanosheets: Temperature-Controlled NO2 Reaction. Adv. Funct. Mater. 2017, 27 (13), 1606026. (28) Choi, H.; Choi, J. S.; Kim, J. S.; Choe, J. H.; Chung, K. H.; Shin, J. W.; Kim, J. T.; Youn, D. H.; Kim, K. C.; Lee, J. I.; Choi, S. Y.; Kim, P.; Choi, C. G.; Yu, Y. J. Flexible and Transparent Gas Molecule Sensor Integrated with Sensing and Heating Graphene Layers. Small 2014, 10 (18), 3685-3691. (29) Kim, H. J.; Kim, Y.; Jeong, J. H.; Choi, J. H.; Lee, J.; Choi, D. G. A Cupronickel-Based Micromesh Film for Use as a High-Performance and Low-Voltage Transparent Heater. J. Mater. Chem. A 2015, 3 (32), 16621-16626. (30) Lordan, D.; Burke, M.; Manning, M.; Martin, A.; Amann, A.; O'Connell, D.; Murphy, R.; Lyons, C.; Quinn, A. J. Asymmetric Pentagonal Metal Meshes for Flexible Transparent Electrodes and Heaters. ACS Appl. Mater. Interfaces 2017, 9 (5), 4932-4940. (31) Khan, A.; Lee, S.; Jang, T.; Xiong, Z.; Zhang, C. P.; Tang, J. Y.; Guo, L. J.; Li, W. D. HighPerformance Flexible Transparent Electrode with an Embedded Metal Mesh Fabricated by Cost-Effective Solution Process. Small 2016, 12 (22), 3021-3030. (32) Seong, B.; Yoo, H.; Nguyen, V. D.; Jang, Y.; Ryu, C.; Byun, D. Metal-Mesh Based Transparent Electrode on a 3-D Curved Surface by Electrohydrodynamic Jet Printing. J. Micromech. Microeng. 2014, 24 (9), 097002. (33) Choi, H. J.; Choo, S.; Jung, P. H.; Shin, J. H.; Kim, Y. D.; Lee, H. Uniformly Embedded Silver Nanomesh as Highly Bendable Transparent Conducting Electrode. Nanotechnology 2015, 26 (5), 055305. (34) Choi, H. J.; Ryu, S. W.; Jun, J.; Moon, S.; Huh, D.; Kim, Y. D.; Lee, H. Fabrication of a Transparent Conducting Ni-Nanomesh-Embedded Film Using Template-Assisted Ni Electrodeposition and Hot Transfer Process. RSC Adv. 2016, 6 (85), 81814-81817. (35) Choi, S. J.; Kim, S. J.; Cho, H. J.; Jang, J. S.; Lin, Y. M.; Tuller, H. L.; Rutledge, G. C.; Kim, I. D. WO3 Nanofiber-Based Biomarker Detectors Enabled by Protein-Encapsulated Catalyst Self-Assembled on Polystyrene Colloid Templates. Small 2016, 12 (7), 911-920. (36) Choi, H. J.; Choo, S.; Shin, J. H.; Kim, K. I.; Lee, H. Fabrication of Superhydrophobic and Oleophobic Surfaces with Overhang Structure by Reverse Nanoimprint Lithography. J. Phys. Chem. C 2013, 117 (46), 24354-24359.


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(37) Saetia, K.; Schnorr, J. M.; Mannarino, M. M.; Kim, S. Y.; Rutledge, G. C.; Swager, T. M.; Hammond, P. T. Spray-Layer-by-Layer Carbon Nanotube/Electrospun Fiber Electrodes for Flexible Chemiresistive Sensor Applications. Adv. Funct. Mater. 2014, 24 (4), 492-502. (38) Lee, C. Y.; Sharma, R.; Radadia, A. D.; Masel, R. I.; Strano, M. S. On-Chip Micro Gas Chromatograph Enabled by a Noncovalently Functionalized Single-Walled Carbon Nanotube Sensor Array. Angew. Chem., Int. Ed. 2008, 47 (27), 5018-5021. (39) Lee, C. Y.; Strano, M. S. Understanding the Dynamics of Signal Transduction for Adsorption of Gases and Vapors on Carbon Nanotube Sensors. Langmuir 2005, 21 (11), 5192-5196. (40) Choi, S. J.; Kim, S. J.; Koo, W. T.; Cho, H. J.; Kim, I. D. Catalyst-Loaded Porous WO3 Nanofibers Using Catalyst-Decorated Polystyrene Colloid Templates for Detection of Biomarker Molecules. Chem. Commun. 2015, 51 (13), 2609-2612. (41) Baltrusaitis, J.; Jayaweera, P. M.; Grassian, V. H. XPS Study of Nitrogen Dioxide Adsorption on Metal Oxide Particle Surfaces Under Different Environmental Conditions. Phys. Chem. Chem. Phys. 2009, 11 (37), 8295-8305. (42) Aduru, S.; Contarini, S.; Rabalais, J. W. Electron-Stimulated, X-Ray-Stimulated, and IonStimulated Decomposition of Nitrate Salts. J. Phys. Chem. 1986, 90 (8), 1683-1688.


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Figure Captions Figure 1. Schematic illustration of fabrication process for Ni(core)/Au(shell) mesh micro-heater embedded in colorless polyimide film (Ni/Au-cPI heater): (a) Sacrificial layer coating, (b) direct printing of hydrogen silsesquioxane (HSQ), (c) detaching of PDMS mold, (d) reactive ion etching, (e) template-assisted Ni electrodeposition, (f) Au electroless deposition, (g) polyamic acid (PAA) coating, and (h) colorless polyimide (cPI) lift-off after imidization. Figure 2. SEM images of Ni(core)/Au(shell) mesh: (a) Ni(core)/Au(shell) mesh ITO/glass, (b) magnified SEM image of (a), and (c) SEM image of Ni/Au-cPI heater. (d) TEM and (e) EDS elemental mapping images of Ni(core)/Au(shell) mesh on ITO/glass. (f) Optical transmittance of bare cPI film and Ni/Au-cPI heater. The insets in (f) show a photograph of Ni/Au-cPI heater. Figure 3. (a) Current transition and (b) heat generation properties of Ni/Au-cPI heater with respect to the applied voltage in the range of 0 V–2 V. (c) Long-term stability of the Ni/Au-cPI heater under the constant applied voltage of 1.2 V. (d) Bending stability of the Ni/Au-cPI heater under repeated bending cycles at 1 V. Figure 4. Schematic illustrations of (a) ZIF-67 synthesis, (b) catalytic Pd encapsulation in ZIF67, and (c) formation of PdO-decorated Co3O4 hollow nanocubes (PdO-Co3O4 HNCs) after calcination. SEM images of (d) ZIF-67, (e) PdO-Co3O4 HNCs, and (f) SWCNTs-loaded PdOCo3O4 HNCs. (g) TEM images of PdO-Co3O4 HNCs, (h) magnified TEM image, and (i) highresolution TEM (HRTEM) image SWCNTs-loaded PdO-Co3O4 HNCs. Figure 5. Schematic illustrations of (a) overall sensor platform, (b) cross sectional view , and (c) magnified view of SWCNTs-loaded PdO-Co3O4 HNCs on cPI film. (d) Current transition property (CH) of Ni/Au-cPI heater with respect to the applied voltages. (e) Resistance transition property (∆R) of SWCNTs-loaded PdO-Co3O4 HNCs with respect to the applied voltages to the Ni/Au-cPI heater. (f) Sensitivity [(Rair –Rgas)/Rair (%)] property of SWCNTs-loaded PdO-Co3O4 HNCs toward NO2 under the different applied voltages at 0 V (22 °C), 0.7 V (36 °C), and 2.1 V (100 °C). Figure 6. Response and recovery kinetics of SWCNTs-loaded PdO-Co3O4 HNCs on cPI film at the different applied voltages to the Ni/Au-cPI heater at (a) 0 V (22 °C), (b) 0.7 V (36 °C), and (c) 2.1 V (100 °C). Figure 7. High resolution X-ray photoelectron spectroscopy (XPS) analysis at N 1s: (a) Asprepared SWCNTs-loaded PdO-Co3O4 HNCs. (b) SWCNTs-loaded PdO-Co3O4 HNCs after NO2 exposure at room temperature. (c) SWCNTs-loaded PdO-Co3O4 HNCs after NO2 exposure at 35 °C.


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(Rair-Rgas)/Rair(%)

(a)

0 V (22 °C)

60 50 (i) K ads (first)

(ii) Kads (second)

(iii) Kdes (first)

(iv) Kdes (second)

40 30 20 10 0 0

1

2

3

4

5

6

Time (min) (Rair-Rgas)/Rair(%)

(b)

60

0.7 V (36 °C)

50 (i) K ads (first)

40

(ii) Kads (iii) Kdes (second) (first)

(iv) Kdes (second)

30 20 10 0 0

1

2

3

4

5

6

Time (min)

(c) 60 (Rair-Rgas)/Rair(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.1 V (100 °C) (i) Kads

(ii) Kads

50 (first) (second)

(iii) Kdes (first)

(iv) Kdes (second)

40 30 20 10 0 0

1

2

3

4

5

6

Time (min)


31


Intensity (a.u.)

(a)

410

400 eV (C-N)

406.6 eV − (NO3 )

408

406

404

402

400

398

396

Binding energy (eV)

(b)

406.43 eV − (NO3 )

Intensity (a.u.) 410

400 eV (C-N)

408

406

404

402

400

398

396

Binding energy (eV)

(c) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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410

406.6 eV − (NO3 )

406 eV 2− (NO3 ) 399.7 eV (C-N)

408

406

404

402

400

398

396

Binding energy (eV)


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Table 1. Adsorption rate, desorption rate, and equilibrium constants of SWCNTs-loaded PdOCo3O4 HNCs at the different film temperatures.

First reaction

Second reaction

Operating temperature

Kads [ppm–1·s–1]

Kdes [s–1]

K (kads/kdes) [ppm–1]

22 °C (0 V)

2.118 × 10–3

1.720 × 10–3

1.231

36 °C (0.7 V)

3.164 × 10–3

7.374× 10–3

0.429

100 °C (2.1 V)

4.571 × 10–3

9.378 × 10–3

0.487

22 °C (0 V)

0.961 × 10–3

0.270 × 10–3

3.559

36 °C (0.7 V)

1.306 × 10–3

1.505 × 10–3

0.868

100 °C (2.1 V)

1.439 × 10–3

2.442 × 10–3

0.589


33


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ToC figure


34

MetalâOrganic Framework-Templated PdO-Co3O4 Nanocubes

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