Controlled NO2 Reaction

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metal oxide NSs is monolayer Ru oxide .... thin 2D metal oxide NSs has not been studied. .... then hydronium ions (H3O+) with higher molecular size were.

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Seon-Jin Choi, Ji-Soo Jang, Hee Jung Park,* and Il-Doo Kim* examples include transition metal dichalcogenides (MoS2, MoSe2, WS2, WSe2, etc.),[6] metal oxide (Ti oxides, Mn oxides, Ru oxides, etc.),[7–9] and layered double hydroxides.[9] In particular, 2D metal oxides NSs such as Ti oxides,[10,11] Mn oxide,[12,13] and Ru oxides[14] have been intensively studied for applications in dielectric nanodevices and electrochemical energy storage by facilitating metallic and semiconducting properties. One of the most promising metal oxide NSs is monolayer Ru oxide exhibiting a highly conductive property. In-depth studies have been performed to establish a synthesis method and investigate the material properties of Ru oxide NSs. Particularly, considerable progress has been made using Ru oxide NSs as an effective catalyst for electrochemical supercapacitors, oxygen reduction electrocatalysts, and photo­ catalysts.[15–18] Very interesting features of metal oxide NSs triggered extensive research; however, the applications of the layered metal oxides are mainly in energy conversion and storage systems. For this reason, exploration and discovery of new applications are imperative to expand the versatility of metal oxide NSs in many research fields. Recently, the detection of chemical species using wearable sensors has gained much interest due to the need of real-time and on-site monitoring of environmental and physical conditions.[19–21] Nanostructured metal oxides using 0D nanoparticles and 1D nanofibers have been widely studied for highly sensitive chemical sensors.[22–25] However, the inherently brittle property of metal oxides limits further application as sensing layers in wearable platform. As an emerging sensing structure, 2D-layered NSs are the most suitable candidate for wearable chemical sensors due to their mechanical flexibility as well as large surface reaction sites. Thus far, atomically thin 2D graphene and transition metal dichalcogenides (TMD) layers have been mainly investigated for chemical sensors.[26–32] Late et al. evaluated chemical sensing characteristics using a thin-layered MoS2 transistor, which revealed high NO2 sensitivity with five-layer MoS2.[33] In addition, the 2D hybrid structure of MoS2/graphene layers was proposed by Cho et al.[35] and Long et al.[34] for the detection of NO2 molecules. In particular, Cho et al. demonstrated flexible NO2 sensors by integration of the 2D hybrid MoS2/graphene layers on a yellow polyimide film.[35] Although numerous efforts have been made to investigate the chemical sensing characteristics of graphene and TMD layers, the chemical sensing property of atomically

2D Ru oxide nanosheets (NSs) with optically punched nanoholes are synthesized and integrated on a flexible heating substrate, i.e., silver nanowire (Ag NW)-embedded colorless polyimide (cPI) film, for application in wearable chemical sensors. Multiple discrete pores on the sub-5-nm scale are formed on the basal planes of Ru oxide NSs by irradiation of intense pulsed light. The chemical sensing characteristic of the porous Ru oxide NSs toward nitrogen dioxide (NO2) is investigated under controlled temperatures by applying DC voltage to the Ag NW-embedded cPI film. The improved NO2 responding and recovery kinetics are achieved using the porous Ru oxide NSs with sensitivity of 1.124% at 20 ppm at a film temperature of 80.3 °C. A wireless patch-type sensor module is developed to demonstrate wearable sensing of NO2 using the Ru oxide NSs on Ag NW-embedded cPI heating film. This work paved the new way for application of atomically thin and porous Ru oxide NSs in chemical sensors, which can detect hazardous species in real time.

1. Introduction Diverse 2D layered structures are gaining attention in recent years for manipulation of physical, electronic, chemical, and optical properties.[1–3] Such properties can be modulated by thinning the bulk materials to a thickness of a few atomic layers. Distinctive characteristics of such atomically thin layers include the electron confinement in 2D nanomaterials by preventing interlayer interaction.[4,5] In addition, the 2D-layered structure has high specific surface area, enabling enhanced performances for surface dominant reactions. Recently, unique 2D layers, i.e., nanosheets (NSs), have been synthesized to demonstrate their potential feasibility for specific applications such as nanoelectronics, energy storage, and optoelectronics. The Dr. S.-J. Choi Applied Science Research Institute Korea Advanced Institute of Science and Technology 291 Daehak-ro, Yuseong-gu Daejeon 34141, Republic of Korea Dr. S.-J. Choi, J.-S. Jang, Prof. I.-D. Kim Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea E-mail: [email protected] Prof. H. J. Park Department of Advanced Materials Engineering Daejeon University 62 Daehak-ro, Dong-gu, Daejeon 34520, Republic of Korea E-mail: [email protected]

DOI: 10.1002/adfm.201606026

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Optically Sintered 2D RuO2 Nanosheets: TemperatureControlled NO2 Reaction

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thin 2D metal oxide NSs has not been studied. A strong reason for unprecedented research of atomically thin metal oxide NSs for chemical sensors is the low sensitivity at room temperature because metal oxide sensing layers tend to exhibit a gasresponding property at elevated temperatures (>200 °C).[36,37] Therefore, the acceleration of adsorption and desorption processes toward analyte molecules is essential by controlling the operating temperatures of sensor substrate, which can be integrated with wearable sensor platforms. Based on the previous demonstrations of outstanding catalytic properties, Ru oxide NSs can provide characteristic gas adsorption and desorption capabilities. To further improve the sensitivity of Ru oxide NSs, porous structure is ideal for chemical sensing application by facilitating effective gas transport into the sensing layers and surface reaction in large areas. Conventionally, open pores on metal oxide NSs were formed by a thermal decomposition technique.[38–40] However, the synthesis of an atomically thin layer with nanometer-scale pores is extremely difficult to achieve. In this work, we propose a unique porous structure of 2D Ru oxide, i.e., RuO2, NSs to exploit a new application for chemical sensors on a wearable heating platform. The 2D Ru oxide NSs were prepared by protonation and an intercalation technique to exfoliate layered bulk crystals using organic ions. For the effective penetration of chemical molecules into the sensing layers, sub-5 nm-scale pores were generated on the Ru oxide NSs by an ultrafast optical sintering technique. For the wearable heating platform, silver nanowire (Ag NW)-embedded colorless polyimide (cPI) film was employed as a heating substrate to promote the sensing property of the porous Ru oxide NSs. The improved adsorption and desorption kinetics of the porous Ru oxide NSs were investigated by controlling the temperature of Ag NW-cPI film. In addition, a wearable patch-type sensor module was developed and combined with a sensing layer of atomically thin and porous Ru oxide NSs on Ag NW-cPI film for real-time monitoring of hazardous species via wireless transmission of chemical sensing information.

2. Result and Discussion The synthesis of 2D Ru oxide NSs and the fabrication process of Ag NW-cPI film were illustrated in Figure 1. The Ru oxide NSs were synthesized by the liquid phase exfoliation technique (Experimental Section). First, the layered structure of NaRuO2 was prepared by solid-state reaction (Figure 1a). Then, acid treatment using HCl was performed to exchange interlayer sodium (Na+) with proton (H+). Finally, ruthenate NSs were obtained by exfoliation of each layer using a tetrabutylammonium hydroxide aqueous solution (TBAOH) (Figures 1b,c). In parallel, Ag NW-cPI film was prepared as a heating substrate. The Ag NWs were synthesized by the polyol method (Experimental Section). These Ag NWs were randomly distributed on a glass substrate by a transfer technique from an Ag NWfiltrated membrane using a pressing machine (Figure 1d). For the preparation of a flexible and transparent heating substrate, polyamic acid (PAA) solution was prepared by dissolving 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,3′-diaminodiphenyl sulfone (APS) in N,N-dimethylformamide (DMF) solution. The Ag NW-coated glass was uniformly covered by the PAA solution using screen printing technique (Figure 1e). Finally, Ag NWs were embedded at the bottom of cPI film after the imidization process of PAA at an elevated temperature (Figure 1f). The Ag NW-cPI film was detached from glass substrate and utilized as an effective heater to control the operating temperature of Ru oxide NSs. The average thickness of the three different Ag NW-cPI films was 64 μm after the imidization process. The Ag NW-cPI film exhibited a conductive property owing to the interconnected Ag NW networks with sheet resistance in the range of 8–11 Ω per square. After patterning of interdigitated sensing electrodes (IDEs) on Ag NW-cPI film using an e-beam evaporator, Ru oxide NSs were integrated on the top surface of Ag NW-cPI film by the drop-coating method. The resistance changes (∆R) of Ru oxide NSs were measured during the cyclic NO2 exposure (Figure 1g, side view). In addition, DC voltage (VH) was applied to the Ag

Figure 1.  a) Layered bulk crystal of NaRuO2. b) Liquid phase exfoliation of layered NaRuO2 crystal using organic ions. c) Exfoliated 2D Ru oxide nanosheets (NSs). d) Silver nanowire (Ag NW) network on a glass substrate. e) Polyamic acid (PAA) screen printing on the Ag NW network. f) Imidization of PAA to form colorless polyimide (cPI) and lift-off of Ag NW-embedded cPI film from the glass substrate. g) 2D Ru oxide NSs on the Ag-NW-embedded cPI heating film.

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FULL PAPER Figure 2.  a) X-ray diffraction (XRD) patterns of bulk NaRuO2, acid-treated NaRuO2, and exfoliated Ru oxide nanosheets (NSs). b) Schematic illustration of NaRuO2. c) Atomic force microscopy (AFM) analysis of Ru oxide NSs. d) Transmission electron microscopy (TEM) and e) high-resolution TEM analysis of Ru oxide NSs with selected area electron diffraction (SAED) pattern in the inset of panel (e). f) Schematic illustration of Ru oxide NS.

NW networks at the bottom of cPI film to investigate characteristic sensing behaviors of Ru oxide NSs toward NO2 at different film temperatures (Figure 1g, cross-sectional view). The microstructure and crystallographic structure of Ru oxide NSs were investigated, as shown in Figure 2. X-ray diffraction (XRD) analysis revealed that all peaks of the pristine NaRuO2 were indexed based on a hexagonal unit cell (a primitive rhombohedral symmetry), showing that the target single phase (space group: R-3m) is well formed (Figure 2a). The refined unit cell constants, a ≈ 0.30 nm and c ≈ 1.63 nm, were very similar to the previously reported NaRuO2 with an α-NaFeO2-related structure.[15] The d-spacing of (003) plane was calculated to be ≈0.543 nm, as shown in the schematic (Figure 2b). The obtained NaRuO2 was a black powder with the average particle size of 22 µm (Figure S1, Supporting Information). After the aqueous acid treatment, the sodium ions within the pristine NaRuO2 were extracted from the interlayers, and then hydronium ions (H3O+) with higher molecular size were intercalated into the interlayers by replacing sodium, thereby increasing the d-spacing. Accordingly, some peaks relevant to (00l) shifted to a lower angle as seen in the acid-treated powder. The d-spacing was calculated to be ≈0.75 nm. The XRD study of the Ru oxide NSs was also carried out. The Ru oxide NSs were collected by filtering the colloidal suspension with an anodized aluminum oxide (Anodic47, Whatmanm) membrane; thus, the NSs were restacked in the out-of-plane direction. The distinctive peaks corresponding to (00l) planes are shown in Figure 2a. An atomic force microscopy (AFM) image clearly revealed the presence of Ru oxide NSs with an average thickness of ≈1.3 nm,

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which implies that Ru oxide NSs were exfoliated to monolayers (Figure 2c).[15] The lateral size was in the range from several hundred nanometers to a few micrometers. Transmission electron microscopy (TEM) analysis clearly confirmed atomically thin layered 2D structure (Figure 2d). In addition, highresolution TEM analysis revealed the hexagonal structure of Ru oxide with d-spacing of 2.50 Å (Figure 2e). The hexagonal structure was also confirmed by a selected area electron diffraction (SAED) pattern (in the inset of Figure 2e). The lattice constant was confirmed as 2.87 Å, corresponding to the (100) crystal plane of the Ru oxide NS (Figure 2f). The wearable heater was successfully fabricated by embedding Ag NWs at the bottom of cPI. The Ag NWs exhibited single crystalline 1D structure with an average diameter of 70 nm and a length distribution in the range of 20–50 µm (Figure S2a,b, Supporting Information). The Ag NWs were partially extruded on the surface of cPI film after PAA screen printing followed by imidization (Figure S2c, Supporting Information). The cross-sectional observation confirmed the interconnected Ag NWs, which can contribute to uniform electrical conduction over the entire film (Figure S2d, Supporting Information). The sensing layer of Ru oxide NSs was drop-coated on the IDEs at the top side of Ag NW-cPI film, to prevent electrical connection between Ru oxide NSs and Ag NWs. Highly porous Ru oxide NSs containing sub-5 nm-scale pores were achieved by irradiating intense pulsed light (IPL) using a Xenon flash lamp on the Ru oxide NS-coated Ag NW-cPI film (Figure 3a). The critical merit of IPL utilizing visible spectrum is the fast irradiation process with minimal damage to a

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Figure 3.  a) Schematic illustration of intense pulsed light (IPL) irradiation. b) Current–voltage characteristic of Ru oxide nanosheets (NSs) before and after IPL irradiation. c) In situ current transition of Ru oxide NSs during the IPL irradiation. d) TEM analysis of the porous Ru oxide NSs with fast Fourier transform (FFT) diffraction patterns in the inset. X-ray photoelectron spectroscopy (XPS) of Ru oxide NSs e) before and f) after IPL irradiation.

transparent plastic substrate.[41,42] The overall IPL process takes only a few seconds, and the modulation of the material properties occurs within a few milliseconds. Intense light was irradiated for 15 ms to the Ru oxide NS-coated Ag NW-cPI film under the fixed applied voltage of 150 V to the light source. After the IPL irradiation to the Ru oxide NSs, a 2.65-fold reduction in electrical conductivity was observed as compared to the pristine Ru oxide NSs (Figure 3b). The in-situ current transition during the IPL irradiation revealed that only 20 ms was required to modify the electrical property of the Ru oxide NSs (Figure 3c). The morphological observation using TEM revealed that plenty of pores smaller than 5 nm were created on the basal plane of Ru oxide NSs after IPL irradiation (Figure 3d). The average pore size was 2.58 ± 1.16 nm, which was confirmed by measuring 50 different pores (Figure S3, Supporting Information). Despite the formation of nanoscale pores by optical sintering, hexagonal crystal structure of Ru oxide NSs was maintained mainly due to very short IPL irradiation time, as confirmed by fast Fourier transform (FFT) diffraction patterns (in the inset of Figure 3d). Based on the TEM observation, the reduction of electrical conductivity was attributed to the formation of nanoscale pores on the Ru oxide NSs. The unique 2D structure with multiple pores on Ru oxide NSs is advantageous for chemical detection by effectively transporting the gas molecules into the sensing layers. The chemical binding states of Ru oxide NSs were investigated using X-ray photoelectron spectroscopy (XPS) before and after IPL irradiation. A survey scan confirmed the characteristic peaks related to Ru, N, O, and C (Figure S4a, Supporting Information). High-resolution XPS spectra of Ru 3d revealed a peak shift to the higher binding energy after IPL irradiation (Figure S4b, Supporting Information). A peak shift to high energy was previously observed in the case of 1606026  (4 of 9)

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oxidation from metallic Ru.[43] High-resolution XPS spectra of Ru 3p were investigated to understand the effect of IPL irradiation (Figure 3e,f). Characteristic peaks related to metallic Ru (Ru0), Ru oxide (RuO2), and hydrated Ru oxide (RuO2⋅xH2O) were observed (Figure 3e). For the as-prepared Ru oxide NSs before IPL irradiation, an oxygen deficient form of Ru oxide, i.e., RuO1.74, was confirmed by calculating the ratio of O/Ru. Higher binding energies were observed with the Ru oxide NSs after IPL irradiation compared with the as-prepared Ru oxide, which were mainly attributed to the oxidation of Ru oxide NSs (Figure 3f and Table 1). The O/Ru ratio confirmed the formation of fully oxidized RuO2 by supplying oxygen during the IPL irradiation. In addition, dehydration of Ru oxide NSs was observed, which may induce sub-5 nm-scale pores on the Ru oxide NSs by the evaporation of water molecules. The oxidation as well as dehydration of Ru oxide NSs was responsible for the intensive thermal energy during the IPL irradiation.[44] To investigate the heating characteristic of Ag NW-cPI film, DC voltage was applied to the film and current was monitored. The voltage-dependent current transition exhibited stable current variation from 0 to 1.4 V with 0.2 V step (Figure 4a). The maximum current of Ag NW-cPI film was 180.2 mA at 1.4 V in the flat state. To demonstrate the mechanical stability, the current transition of Ag NW-cPI film was investigated in the bent state with a bending angle of 30° (Figure S5, Supporting Information). Almost identical current values were obtained at the same applied voltages when the Ag NW-cPI film was in the bent state with a maximum current of 177.6 mA at 1.4 V. The stable current transition property was primarily attributed to the robust structure of Ag NW-cPI film, in which Ag NWs were mostly embedded in the cPI film. The temperature of Ag NW-cPI film at different applied voltages was confirmed by using an IR camera (FLIR, E8). The temperature transition of

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Orbital/ spin

Binding energy [eV]

Area under peak [%]

Assignment

Before IPL

After IPL

Before IPL

After IPL

3p3/2

461.18

462.19

77.20

71.88

Ru0

3p1/2

483.28

484.29

3p3/2

463.54

464.64

15.59

24.25

Ru (IV)

7.21

3.87

Ru (IV) (hydrate)

3p1/2

485.64

486.75

3p3/2

465.28

466.72

3p1/2

487.38

488.83

Ag NW-cPI film with respect to the current flow was investigated (Figure S6a, Supporting Information). The film temperature was increased with the applied voltage up to 80.3 °C at 1.4 V with a current flow of 178.7 mA (Figure 4b). In addition, only 14 s was required to increase the film temperature up to 80 °C, which demonstrated the fast temperature response of Ag NW-cPI film (Figure S6b, Supporting Information). The resistance transition of the porous Ru oxide NSs on Ag NW-cPI film was investigated with respect to the applied voltage to

the Ag NW-cPI film (Figure 4c). Applied-voltage-dependent resistance tuning of the porous Ru oxide NSs was achieved by effectively modulating the substrate temperature using Ag NW networks. The increased resistance of the porous Ru oxide NSs was observed with increased film temperature, which shows characteristic metallic behavior. Similar resistance transition was observed with the porous Ru oxide NSs in the bent state. Although slightly increased resistance was observed due to the bending stress, the resistance was recovered close to the initial

Figure 4.  a) Current transition property with respect to the applied voltage to the Ag NW-cPI film. b) IR camera images of the Ag NW-cPI film at different applied voltages. c) Resistance transition property of porous Ru oxide with respect to the applied voltage to the Ag NW-cPI film. Temperature-controlled NO2 adsorption and desorption properties in d) flat and e) bent states.

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Table 1.  Quantification of XPS spectra of Ru oxide NSs before and after IPL irradiation.

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resistance after releasing the bending stress, which demonstrated the mechanical stability of Ru oxide NSs (Figure S7, Supporting Information). To demonstrate temperature-controlled NO2 adsorption and desorption properties, resistance transitions were evaluated using the porous Ru oxide NSs at the different film temperatures in flat and bent states. The film temperature was controlled by applying voltages of 0 V (room temperature), 0.8 V (41.9 °C), and 1.4 V (80.3 °C) to the Ag NW networks. In the flat state without applied voltage to the Ag NW-cPI film, negligible NO2 sensing property was observed due to the low reaction kinetics of porous Ru oxide NSs at room temperature (Figure 4d). When a voltage of 0.8 V (41.9 °C) was applied to the Ag NW-cPI film, porous Ru oxide NSs exhibited noticeable response and recovery characteristics. Moreover, the dramatically improved response and recovery characteristics were observed at 1.4 V, which were achieved by the increased reaction kinetics of the porous Ru oxide NSs at the elevated film temperature (80.3 °C). In the bent state, a similar improvement in NO2 sensing property was observed at the elevated temperature at 1.4 V to the Ag NW-cPI film as compared with the sensing property at room temperature (Figure 4e). The selective sensing characteristic of Ru oxide NSs was investigated by exposing other interfering analytes such as hydrogen (H2), carbon mono­ xide (CO), toluene (C6H5CH3), acetone (CH3COCH3), and nitrogen monoxide (NO) (Figure S8, Supporting Information). Although other interfering analytes exhibited negligible sensing property, appreciable responding and recovery characteristics were observed toward NO. The NO2 sensing performance of the porous Ru oxide NSs was quantitatively investigated by calculating normalized resistance change as defined by sensitivity (S), i.e., 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. The porous Ru oxide NSs in flat state at 1.4 V (80.3 °C) exhibited the highest sensitivity of 1.124% at the first reaction to 20 ppm of NO2 (Figure S9a, Supporting Information). In addition, similar sensitivity of 1.116% at 20 ppm of NO2 was observed in the bent state at the same applied voltage of 1.4 V (Figure S9b, Supporting Information). On the other hand, almost negligible sensitivities were observed at 0 V (room temperature) and 0.8 V (41.9 °C) in both flat and bent states. Based on the sensitivity values of the porous Ru oxide NSs at the first reaction in flat and bent states, adsorption and desorption kinetics were evaluated to quantitatively understand the improved response and recovery processes. In previous studies, the desorption rate constant (kdes), adsorption rate constant (kads), and equilibrium constant (K = kads/kdes) have been widely used to evaluate the response and recovery kinetics as described in the Equations (1) and (2) below[45–47] S (t ) = S0 exp [−kdes ⋅ t ] S (t ) = Smax ⋅

(1)



 1 + C aK  C aK  ⋅ kads ⋅ t  1 − exp −   1 + C aK  K

(2)

where S0 is the sensitivity when NO2 is removed, Smax is the maximum sensitivity, and Ca is the NO2 concentration. The rate 1606026  (6 of 9)

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Table 2.  Adsorption rate constant (kads), desorption rate constant (kdes), and equilibrium constant (K = kads/kdes) of the porous Ru oxide NSs toward 20 ppm of NO2 at different applied voltages in flat and bending states. kads [ppm−1 s−1]

kdes [s−1]

0.8 V (flat)

3.55 × 10−2

4.386 × 10−3

1.4 V (flat)

10−2

1.4 V (bent)

6.89 × 5.277 ×

10−2

K (kads/kdes) [ppm−1] 8.094

−3

4.498

−3

4.479

15.330 × 10 11.782 × 10

constants as well as equilibrium constants were obtained by fitting the sensitivity curves to Equations (1) and (2) (Figure S10, Supporting Information), and the result was summarized in Table 2. It was revealed that 1.94-fold and 3.50-fold improvements were achieved in adsorption and desorption kinetics, respectively, by increasing the applied voltage to the Ag NW-cPI film from 0.8 V (41.9 °C) to 1.4 V (80.3 °C) due to the elevation of operating temperature. In addition, similar rate constants and equilibrium constants were observed in the flat and bent states at 1.4 V, which demonstrate the stable NO2 sensing characteristics of the porous Ru oxide NSs under mechanical stress. In terms of the sensing mechanism of the Ru oxide NSs toward NO2, the electrical resistivity of 2D sensing layers can be modulated by surface adsorption of charged molecules and charge transfer.[48,49] For example, charged chemical molecules can attract electrons or holes from the 2D sensing layers by adsorption on the surface, which results in either increased or decreased resistivity. To verify such a reaction directly, we performed ex-situ XPS analysis of the porous Ru oxide NSs at N1s spectra before and after NO2 exposure (Figure 5). The porous Ru oxide NSs after IPL irradiation exhibited two characteristic XPS spectra at 402.4 and 400.4 eV, which were attributed to the bonding of NO− and CN (Figure 5a). The characteristic CN bonding was ascribed to the binding energy from the cPI substrate. In addition, a low intensity spectrum at around 406.5 eV was observed due to the adsorption of NO3− . In this state, the porous Ru oxide NSs retain electrons as majority carriers (Figure 5a-1). Subsequently, an abrupt enhancement in intensity at 405.8 eV was observed after the injection of NO2 mole­ cules, which resulted from the adsorption of NO3− (Figure 5b). The formation of NO3− can contribute to the increased resistance of the porous Ru oxide NSs by attraction of electrons as described in the following chemical reaction (Figure 5b-1) 2NO2 + O2 + 2e − → 2NO3−

(3)

The increasing resistance trend is consistent with the observation in Figure 4d,e during NO2 injection to the porous Ru − oxide NSs. However, the noticeable intensity of NO3 at 406.6 eV was observed even after the recovery process using fresh air (Figure 5c). This result can be explained by the build-up of NO3− even though the recovery process was performed at an elevated temperature using Ag NW-cPI heating film (Figure 5c-1). The build-up phenomenon was also confirmed by the drift of baseline resistance after the recovery process during the cyclic NO2 exposure (Figure S11, Supporting Information). To fully recover the initial resistance, ≈500 s was required as confirmed

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FULL PAPER Figure 5.  Ex situ XPS analysis at N1s spectra with a) the pristine porous Ru oxide NSs, b) porous Ru oxide NSs after NO2 injection, and c) porous Ru oxide NSs after the recovery process using fresh air. Schematic illustration for NO2 adsorption and desorption mechanism: a-1) stabilization in air, b-1) NO2 injection and NO 3− formation, and c-1) recovery and NO 3− build-up.

by the prolonged recovery process (Figure S12, Supporting Information). To demonstrate the potential application of wearable chemical sensors for environment monitoring, we developed a patchtype sensor module that can transmit the measured data to a smartphone via Bluetooth communication (Figure 6a). The detailed specifications of the sensor module can be found in

Figure S13 in the Supporting Information. The sensor module can be attached to clothing for the detection of NO2, which is a hazardous chemical component generated from automobile exhaust.[50] The porous Ru oxide NS-coated Ag NW-cPI film was integrated on the patch-type sensor module under an applied voltage of 1.4 V to elevate the operating temperature to 70.3 °C (Figure 6b). Characteristic NO2 response and recovery properties were observed using the porous Ru oxide NSs during the cyclic exposure to 20 ppm NO2 at 1.4 V to the Ag NW-cPI film as compared with the operating voltages at 0 V (room temperature) and 0.8 V (34.4 °C) (Figure 6c). High-resolution resistance transition will be obtained by further development of reliable wearable chemical sensors to investigate response and recovery speeds.

3. Conclusion

Figure 6.  a) Camera image of a patch-type sensor module with the porous Ru oxide NS coated Ag NW-cPI film. b) IR camera image of the sensor module during the operation. c) Resistance transition property of Ru oxide NSs under the applied voltages to the Ag NW-cPI film.

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In summary, we first employed an optical sintering technique to obtain porous Ru oxide NSs for application in chemical sensing layers. Atomically thin Ru oxide NSs with sub-5 nm-scale pores were achieved by irradiation of IPL while maintaining the singlelayered hexagonal structure of as-prepared Ru oxide NSs. The unique 2D porous NSs were responsible for the transformation of oxygendeficient RuO1.74 to fully oxidized RuO2 as well as the dehydration of water molecules from Ru oxide NSs. To accelerate the surface

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reaction of the porous Ru oxide NSs toward NO2 molecules, Ag NW-embedded cPI film was used as a flexible heating substrate. Approximately 1.94-fold and 3.50-fold enhancement in adsorption and desorption kinetics were respectively achieved using the porous Ru oxide NSs by increasing the Ag NW-cPI film temperature from 41.9 °C (0.8 V) to 80.3 °C (1.4 V), leading to significantly enhanced sensitivity of 1.124%. This work demonstrates the potential suitability of atomically thin metal oxide NSs as gas sensing layers for application in wearable chemical sensors. Although the operating temperature was relatively high for wearable application, room-temperature operation can be achieved by compositional modification and catalyst functionalization. In addition, diverse 2D metal oxide can be further investigated to evaluate chemical sensing properties on well-established wearable sensing platforms such as Ag NW-cPI heating film.

4. Experimental Section Materials: 6FDA, APS, DMF, silver chloride (AgCl), ethylene glycol (EG), silver nitrate (AgNO3), potassium bromide (KBr), and polyvinylpirrolidone (PVP, Mw = 1300 k g mol−1) were purchased from Sigma-Aldrich (St. Louis, USA). Na2CO3 (Kanto Chem., 99.8%), Ru (R&D Korea, 99.99%), RuO2 (R&D Korea, 99.95%), and TBAOH (C16H37NO, Alfa Aesar) were used during the synthesis of 2D Ru oxide nanosheets. All chemicals were used without further purification. Silver Nanowire Synthesis: Polyol synthesis method was used to prepare silver nanowires (Ag NWs), which is described elsewhere.[21,51] A composite solution comprising 6.68 g of PVP, 0.1 g of KBr, and 0.5 g of AgCl dissolved in 200 mL of EG was prepared under stirring at 200 rpm for 1 h. Then, the composite solution was heated up to 170 °C. To grow Ag NWs, 2.2 g of AgNO3 dissolved in 5 mL of EG solution was slowly introduced to the composite solution using a syringe pump with a feeding rate of ≈0.08 mL min−1. The obtained Ag NWs were repeatedly washed using centrifugation to eliminate remaining residues such as solvent, PVP, and other impurities using deionized (DI) water and then stabilized in methanol. Preparation of Ag NW-Embedded Colorless Polyimide Film: Ag NW-embedded cPI (Ag NW-cPI) film was obtained by vacuum filtration and transfer technique. First, the Ag NWs were filtrated using a 0.2 µm pore Nylon membrane filter (Whatman, Germany). Then, the Ag NWs were transferred to a glass substrate (2.5 cm × 2.5 cm) under the pressure of 2 MPa for a few seconds using a pressure machine. PAA solution was prepared by dissolving 1.018 g of 6FDA and 0.569 g of APS in 3.5 g of DMF and stirred at 500 rpm for 8 h. To obtain embedded structure, the PAA solution was screen printed on the Ag NW dispersed glass substrate using a doctor blade. Imidization process was carried out to obtain cPI film after heat treatment at 100, 200, and 230 °C for 1 h at each temperature with a heating rate of 2 °C min−1. A flexible and transparent heating substrate was achieved after lift-off of the Ag NW-embedded cPI film from the glass substrate. The average thickness of the NW-embedded cPI films was 64 µm after the imidization process. Interdigitated Electrode Patterning: IDEs were patterned on cPI film to measure resistance changes of the porous Ru oxide NSs during the exposure to NO2 molecules. The spacing between the electrodes is 200 µm with finger width of 200 µm and length of 2750 µm. A 10 nm/100 nm thick Cr/Au layer was deposited using e-beam evaporator and shadow mask for IDEs. Synthesis of 2D Ru Oxide Nanosheets: Pristine NaRuO2 was first prepared by solid-state reaction at 900 °C for 6 d under nitrogen atmosphere with appropriate amounts of Na2CO3, Ru, and RuO2 powders (molar ratio of 2:1:3). The obtained pristine NaRuO2 was treated with 1 m HCl solution at room temperature for 3 d in order to exchange interlayer sodium (Na+) with protons (H+). The acid-treated powders were exfoliated to ruthenate NSs for 2 weeks by TBAOH

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(C16H37NO, Alfa Aesar). Dialysis process was followed using a polymer membrane for 3 d to remove the remaining organics from the colloidal suspension. The final NS colloid was obtained after the unexfoliated precipitations were removed by centrifuging at a speed of 10 000 rpm for 10 min. The absorbance of the colloidal suspension was measured using UV–vis spectroscopy (Agilent 8453) to investigate the colloidal concentration (0.7 g L−1). The remaining Na-ions within the NSs were evaluated by inductive coupled plasma measurement (Na/Ru ratio ≈0.03%). 2D Ru Oxide NSs Coating on the Ag-cPI Film: The Ru oxide NSs were coated by a simple drop-coating method on the IDE-patterned Ag-cPI film. To obtain very thin-layered Ru oxide sheets on the Ag-cPI film, 5 µL of Ru oxide NS dispersed in DI water was drop-coated on the Au IDE patterned Ag-cPI film using a micropipette and was dried in ambient air. The optical microscopy image of the Ru oxide sheets confirmed the multilayered structure (Figure S14, Supporting Information). Intensive Pulsed Light (IPL) Irradiation: As a light source, a Xenon flash lamp (ILC technology, L6755) was used to form sub-5 nm-scale pores on Ru oxide NSs directly on the Ag NW-cPI film. The spectrum of the light source was in the range of 400–1100 nm. The pulse light was irradiated to the as-synthesized Ru oxide NSs through quartz crystal. The Ru oxide NS-coated Ag NW-cPI film was located under the quartz with the pulse gap of 5 mm. Then, the pulse on/off time was controlled to be 15/30 ms. A 150 V was applied to the flash lamp to maintain the flash light energy as 1.15 J cm−2. Single light pulse was exposed to form porous Ru oxide NSs on the Ag NW-cPI film. NO2 Sensing Characterization: Temperature-controlled NO2 adsorption and desorption characteristics were evaluated using a homemade measurement setup.[52] The resistance changes of the porous Ru oxide NSs were measured during the cyclic exposure to NO2 using a data acquisition system (34972A, Agilent) with a 16-channel multiplexer (34902A, Agilent). The concentration of NO2 was ranging from 5 to 20 ppm with a constant injection time of 3 min followed by 3 min recovery time using baseline air. The operating temperature of the Ru oxide NSs was controlled by applying DC voltage to the Ag NW-cPI film using a DC power supply (E3644A, Agilent). The sensitivity (S = [Rgas − Rair]/Rair × 100%) was calculated, where Rair and Rgas are the sensor’s resistance upon exposure to air and NO2, respectively.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements 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 (Grant No. 2016M3A7B4905609). National Research Foundation (NRF) of Korea grant funded by the Ministry of Science, ICT and Future Planning (Grant No. NRF-2016R1E1A2A02945984). Received: November 16, 2016 Published online:

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