Dual-Function Metal–Organic Framework-Based Wearable Fibers for

Dec 29, 2017 - Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center f...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 2837−2842

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Dual-Function Metal−Organic Framework-Based Wearable Fibers for Gas Probing and Energy Storage Kun Rui,† Xiaoshan Wang,† Min Du,† Yao Zhang,† Qingqing Wang,† Zhongyuan Ma,† Qiao Zhang,† Desheng Li,† Xiao Huang,† Gengzhi Sun,*,† Jixin Zhu,*,† and Wei Huang*,†,‡ †

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China ‡ Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an 710072, China S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) coupled with multiwalled carbon nanotubes (MWCNTs) have been developed with an ultrahigh sensitivity for hazardous gas monitoring. Both the MOF/MWCNT and as-derived metal oxides (MOs)/MWCNTs hybrid fibers deliver an ultralow detection limit for NO2 down to 0.1 ppm without external heating, and they can be further bent into different angles without loss of sensing performance. Also, a high specific capacitance of 110 F cm−3 can also be obtained for MO/MWCNT hybrid fibers, demonstrating promising application for integrated wearable devices. KEYWORDS: metal−organic frameworks, carbon nanotubes, flexible devices, gas sensors, supercapacitors

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INTRODUCTION Surging research interests have been attracted to the development of next-generation wearable technologies involving smart clothes, electronic skins, intelligent bracelets, and so on.1−3 In particular, wearable electronics have found significant applications for health monitoring such as for pressure detection,4 perspiration analysis,5 lactate monitoring,6 etc. and are no longer limited to basic analysis in laboratories or hospitals. Recently, gas sensors, by capturing and responding to human exhaled breath and environmental gases,7 have opened up new opportunities for early warning, personal protection, healthcare, and intelligent control. However, traditional gas-sensing devices reported so far need bulky systems (e.g., colorimetry and spectroscopy)8,9 or the incorporation of rigid electrodes (e.g., interdigital electrodes) and substrates (e.g., wafers, ceramics, glass),10,11 or they require high temperatures (>200 °C).12,13 These traits are mostly not compatible with the requirements of being flexible and lightweight or possessing weavability for wearable electronic devices. Conceivably, adaptable energy storage systems such as flexible supercapacitors and lithium batteries instead of conventional bulky and planar structures are in great demand to power wearable electronic devices.14−16 Therefore, a valid integration of advanced multifunctional © 2017 American Chemical Society

materials, delicate mechanical design, and specialized device architectures has been required urgently. In the past decades, metal−organic frameworks (MOFs) have attracted remarkable attention as prospective materials in a large variety of fields due to their exceptional tunability of structures and properties.17−19 Endowed with ultrahigh porosity, structural flexibility, and huge surface areas,20,21 MOFs as well as MOF-derived materials have been unsurprisingly exploited for sensory applications with improved performance.22,23 Nevertheless, developing wearable gas sensors based on MOFs still remains quite a challenge. Unlike randomly dispersed CNTs in macroscopic assembly, multiwalled carbon nanotube (MWCNT) fibers spun from vertically aligned arrays not only eliminate undesired disorders but also show excellent electrical conductivity with improved charge transport along the aligned direction.24 Coupled with high mechanical reliability, lightweight MWCNT fibers turn out to be a promising candidate as the conducting substrate in flexible electronics. Moreover, various guest materials can be Received: November 3, 2017 Accepted: December 29, 2017 Published: December 29, 2017 2837

DOI: 10.1021/acsami.7b16761 ACS Appl. Mater. Interfaces 2018, 10, 2837−2842

Research Article

ACS Applied Materials & Interfaces introduced for functional flexible electronic devices, such as microelectrodes,25 strain sensors,26 actuators,27 and supercapacitors,28 and could be further woven into future e-textiles. However, to the best of our knowledge, there is no report of MOF fiber-based wearable devices for gas probing integrated with energy storage. To this end, we present a flexible and wearable fibrous device with the dual function of both gas probing and energy storage. In this protocol, MOFs (e.g., ZIF-67-Co and MIL-88-Fe) coupled with highly conductive MWCNTs were twisted into organic−inorganic fibers followed by proper post-treatment. Endowed with high surface area, the presence of Co/Fe elements, and unique structure features (e.g., dodecahedral morphology), functional fibers loaded with ZIF-67-Co and MIL-88-Fe as well as their derivatives are expected to render not only high sensitivity for gas sensors but also good electrochemical activity for supercapacitors. In a typical procedure, spinnable MWCNT arrays were first grown on an Si/SiO2 substrate by chemical vapor deposition (CVD).29 The as-prepared MWCNTs are well aligned and free-standing on the substrate with a height of ∼500 μm (Figure S1). As illustrated in Figure 1, aligned MWCNT sheets were drawn

Figure 2. Characterizations of Co3O4/MWCNT hybrid fiber. (a−c) SEM images of Co3O4/MWCNT hybrid fiber at different magnifications. (d) Elemental mapping of Co (blue), O (red), and C (green) for Co3O4/MWCNT hybrid fiber, scale bar: 50 μm. (e) Raman spectra of Co3O4/MWCNT hybrid fiber and bare MWCNT fiber.

(Figure 2b), the uniform decoration of metal oxide on the surface of perfectly aligned MWCNTs can be observed. Figure 2c indicated that the metal oxide nanocrystals with sizes of ca. 300 nm inherited polygonal shapes from ZIF-67-Co precursors. Meanwhile, the alignment and structure of the MWCNTs were maintained without obvious aggregates during the heat treatment. More importantly, the hybrid fiber was strong and flexible enough to be easily tied into a knot (Figure 2d). Corresponding elemental mapping based on energy-dispersive X-ray (EDX) spectroscopy (Figure S4) was further conducted to reveal the uniform distribution of Co, O, and C in the knotted hybrid fiber. In addition, the formation of metal oxide was proven by the typical Raman spectra in Figure 2e. Apart from the characteristic peaks of the MWCNT at 1350, 1580, and 2699 cm−1 corresponding to the D, G, and 2D bands, respectively, well-defined peaks of the A1g (679 cm−1), Eg (473 cm−1), F2g (513 cm−1), and F2g (190 cm−1) modes were in good agreement with previous reports of Co3O4.30 X-ray diffraction (XRD) patterns (Figure S5) for pure ZIF-67-Co annealed under the same conditions also confirmed the cubic structure of Co3O4. As another sample, the Fe2O3/MWCNT hybrid fiber was also obtained by annealing MIL-88-Fe/MWCNT in air (see details in the Supporting Information). Aligned MWCNT sheets decorated by a spindle-like iron-based oxide were scrolled into a fiber (Figure S6). Uniform distribution of Fe, O, and C can be verified according to the elemental mapping results (Figure S7). Moreover, the presence of Fe2O3 was confirmed in light of the Raman spectrum and XRD pattern (Figure S8).31 In order to turn our proof-of-concept prototype into realization, the mechanical robustness (Figure S9) and flexibility of the hybrid fiber were confirmed in practical terms, by sewing them into a commercially available textile fabric without damage (Figure 3a). Endowed with high flexibility, the hybrid fiber can be deformed into various shapes in accordance with practical requirements (Figure S10). As for gas-sensing evaluation, the transparent polyethylene terephthalate (PET) flexible substrate was used to fix the MWCNTbased fiber with Ag paste and Cu wires (Figures 3a and S11), which can be connected to a signal output device. For comparison, the responsive resistances were normalized in the form of ΔR/R = (Rg − Rb)/Rb, where Rb and Rg stand for the stabilized baseline resistance and the real-time resistance, before and during the gas-sensing test, respectively. Herein, ZIF-67-

Figure 1. Schematic illustration of the stepwise design of the wearable fiber.

from the arrays with controlled dimensions and then stacked layer by layer. As proof of concept, the prefabricated MOFs, e.g., ZIF-67-Co and MIL-88-Fe, were introduced into the stacked MWCNT sheets in a controlled amount via a facile drop-casting method. By varying the volume of the MOFs/ ethanol suspension, the desired amount of the MOFs incorporated were tuned (30, 60, and 90 wt % of MOFs, respectively). The MOF-decorated MWCNT sheets were further twisted with a motor to form a desirably aligned MOF/MWCNT hybrid fiber. The as-deposited MOF particles were readily trapped and tightly encapsulated by interconnected MWCNTs. Furthermore, flexible MO/MWCNT hybrid fibers were fabricated with well-designed building blocks and a controllable composition after the MOF/MWCNT hybrid fiber was annealed in air.

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RESULTS AND DISCUSSION The morphology of the as-synthesized ZIF-67-Co/MWCNT hybrid fiber (designated as BC90, 90 wt % of ZIF-67-Co before annealing) can be observed by typical scanning electron microscopy (SEM) images (Figure S2). After subsequent heat treatment in air, ZIF-67-Co in the hybrid fiber was transformed into Co-based oxide. As seen in Figure 2a, the twisted aligned MO/MWCNT hybrid fiber after annealing at 300 °C for 2 h (designated as AC90) displayed an average diameter of approximately 30 μm, which increased in contrast with that of bare MWCNT fiber (Figure S3). At a higher magnification 2838

DOI: 10.1021/acsami.7b16761 ACS Appl. Mater. Interfaces 2018, 10, 2837−2842

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MWCNT fiber is also shown in Figures S13 and S14. Moreover, gas-probing behaviors of MWCNT-based hybrid fibers incorporated with MIL-88-Fe as well as corresponding Fe2O3 were characterized (Figures S15 and S16), exhibiting a sensitive response at 0.1 ppm in both cases. This can be likely attributed to the desirable sensing activity of the MWCNT fiber with nanoscale surface topography and porosity, which would facilitate rapid interactions with gas molecules and promote corresponding electron transfer reactions.34 Hence, this general approach incorporating well-aligned MWCNTs and gassensitive MOF-derived MOs holds a great potential to be extended for developing high-performance sensors with significant sensitivity at low concentrations in particular. To further demonstrate the potential of the fiber-based sensor as a wearable device, the MOF/MWCNT or MO/ MWCNT hybrid fiber sensor was bent into different angles without any damage in structure (Figure S17). Sensing behaviors depending on different shapes such as straight and bent forms (Figure 4a) were further monitored during exposure to NO2 gas of stepwise increasing concentrations at room temperature. Figure 4b summarized the response of hybrid fibers with different amounts of Co3O4, and no obvious performance decay was observed for each Co3O4 content (Figure S18), owing to the high flexibility of MWCNTs, which

Figure 3. Gas-probing properties of ZIF-67-Co-derived Co3O4/ MWCNT hybrid fibers. (a) A photograph of hybrid fiber woven into a lab coat and schematic illustration of the fabric gas sensor. Normalized real-time dependent response curves of Co3O4/MWCNT hybrid fibers with different Co3O4 loadings in a concentration range of (b) 0.1−20 ppm and (c) 20−1000 ppm. Calculated response for Co3O4/MWCNT hybrid fibers as a function of NO2 concentration in a range of (d) 0.1−20 ppm and (e) 0.1−1000 ppm.

Co/MWCNT hybrid fibers (e.g., BC30, BC60, and BC90) delivered desirable gas-sensing behaviors with a negative resistance response to NO2 (Figure S12). A distinct resistance decrease can be observed even at a low gas concentration of 0.1 ppm without external heating, indicating excellent sensitivity toward NO2. The sensor response was also calculated based on −R (%) = (Rg − Rn)/Rn × 100 (where Rg and Rn denote the resistance after and before exposure to NO2 gas, respectively). As seen, increasing sensitivities can be achieved unsurprisingly with increasing NO2 concentration. Furthermore, MOF-derived MO/MWCNT hybrid fibers were also obtained after a facile annealing process, whose dynamic response toward NO2 at room temperature was presented in Figure 3b,c. As indicated, synergistic enhanced gas-sensing behaviors can be expected due to the similar negative responsive nature of both Co3O4 and MWCNT, whose resistance would decrease after exposure to the oxidizing NO2 gas.32,33 With use of p-type Co3O4 as an example, it can be seen that the hole concentration would increase when adsorbed NO2 extracted electrons from its surface oxygen ion, leading to the increased conductivity. Upon exposure to NO2, the resistance of all three Co3O4/MWCNT hybrid fibers with different Co3O4 loadings decreased immediately as expected, displaying a sensitive response down to 0.1 ppm as well. The difference among the response of hybrid fibers with different Co3O4 loadings at low NO2 concentrations was rather small (Figure 3d), consistent with the nearly overlapped curves of Figure 3b. With the highest Co3O4 loading, the Co3O4/ MWCNT hybrid fiber (AC90) showed much enhanced sensitivity with increased NO2 concentrations higher than 20 ppm (Figure 3e). For contrast, the gas-sensing behavior of bare

Figure 4. Flexibility of the fiber-based gas probe. (a) Schematic illustration of deformed hybrid fibers and definition of bending angle. (b) Gas-probing response of the Co3O4/MWCNT hybrid fibers with and without the bending strain. (c) Comparison of normalized responsive resistance of Co3O4/MWCNT hybrid fiber (AC90) with and without a fixed bending angle of 60° in a concentration range of 0.1−20 ppm and (d) calculated response values. (e) Comparison of normalized responsive resistance of ZIF-67-Co/MWCNT hybrid fiber (BC90) with and without fixed bending angles (60 and 120°) in a concentration range of 0.1−50 ppm and (f) calculated response values. 2839

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CNT/RGO composite fiber,36 etc.), indicating promising application for integrated wearable devices with a self-power supply. The rate capability of Co3O4/MWCNT hybrid fiber (AC90, Figure 5c) was also examined from 5 to 100 mV s−1. Endowed with good electron conduction of the MWCNTbased fiber electrode, the shape of the CV curves was not significantly influenced by increasing scan rates. As seen from the corresponding Cv values shown in Figure 5d, the capacitance of 41.2 F cm−3 at 100 mV s−1 was still comparable to a series of metal oxide/sulfide-based fiber electrodes.29,37−39

was crucial for their wearable applications. To take a closer investigation, fiber-based gas sensors were further bent into fixed angles. In the case of the Co3O4/MWCNT hybrid fiber (AC90), a highly sensitive response of the bent sensor at 60° can be achieved down to 0.1 ppm with a response retention of 81% (Figure 4c). More importantly, a better response was obtained by bent sensors at plenty of NO2 concentrations ca. 0.4−10 ppm (Figure 4d). This can be likely ascribed to the improved electron transfer of MWCNT fibers along the radial direction, which was partially straightened under tension. Similar results can also be obtained for AC60 and AF90 (Figures S19 and S20). In addition, normalized responsive resistance curves remained almost unchanged under bending (60 and 120°) with increasing NO2 gas concentrations for ZIF67-Co/MWCNT hybrid fibers (Figure 4e,f). Benefiting from the excellent flexibility of MWCNTs and high sensitivity of MOF-derived Co3O4, the state-of-the-art fiber-based gas sensor is believed to offer great potential in the field of wearable electronics. Despite the fact that most current wearable devices are dependent on an external power supply, the as-prepared MO/ MWCNT hybrid fiber itself can serve as a supercapacitor for energy storage. Cyclic voltammograms (CV) of Co3O4/ MWCNT fiber-based supercapacitors at a scan rate of 5 mV s−1 (Figure 5a) displayed distinct redox peaks, indicating that

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CONCLUSIONS In summary, we have developed a flexible and wearable sensor based on MOFs coupled with MWCNT fibers for the sensitive monitoring of NO2 gas. We successfully introduced MOFs as the precursors of the metal oxides and well-aligned MWCNT fibers to support gas-sensing nanocrystals. Both the MOF/ MWCNT and as-derived MO/MWCNT hybrid fibers showed a remarkable detection sensitivity for NO2 down to 0.1 ppm without external heating. The highly flexible fiber device can be bent into different angles without loss of sensor performance. More importantly, this state-of-the-art fiber composite can be further woven into smart textiles endowed with NO2monitoring properties as well as energy storage functions. This study suggests that these fiber-based gas sensors with remarkable performance significantly enlarge their potential application for advanced wearable electronics for safety and healthcare purposes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16761. Experimental details; SEM images of MWCNT arrays and the ZIF-67-Co/MWCNT hybrid fiber; optical microscope images of bare MWCNT fiber andCo3O4/ MWCNT hybrid fibers; EDS spectrum of Co3O4/ MWCNT hybrid fiber; XRD pattern of pure ZIF-67Co annealed in air; SEM images, elemental mapping, and Raman spectrum of Fe2O3/MWCNT hybrid fiber; digital photographs of MO/MWCNT hybrid fiber bent into various shapes and fiber-based sensing unit on a PET substrate; gas-sensing behavior of ZIF-67-Co/MWCNT hybrid fibers; bare MWCNT fibers; Co3O4/MWCNT hybrid fibers compared with bare MWCNT fibers, MIL88-Fe/MWCNT hybrid fibers, and Fe2O3/MWCNT hybrid fibers; digital photographs of fiber sensor under different bending angles; gas-sensing response of the Co3O4/MWCNT and Fe2O3MWCNT hybrid fibers with and without the bending strain; cyclic voltammograms (CV) curves of bare MWCNT fiber-shaped supercapacitor (PDF)

Figure 5. Electrochemical properties of the fibrous Co3O4/MWCNT supercapacitors. (a) CV curves and (b) calculated capacitances (Cv) of Co3O4/MWCNT hybrid fiber-based supercapacitors with different Co3O4 loadings at 5 mV s−1. (c) CV curves and (d) calculated capacitances (Cv) of Co3O4/MWCNT hybrid fiber-based supercapacitors (AC90) at different scan rates.

the capacitance mainly resulted from pseudocapacitance rather than from the double-layer capacitance. By contrast, a typical rectangular shape was obtained for bare MWCNT fibers (Figure S21), whose contribution to the capacitance of hybrid fibers can be therefore neglected. As seen, with the increasing incorporation of Co3O4, a similar curve shape can be readily maintained. Specific volumetric capacitances (Cv) at 5 mV s−1 were therefore calculated, and corresponding results were displayed in Figure 5b. A desirable capacitance as high as 110 F cm−3 can be achieved for AC90, which was higher than that for previously reported carbon-based fiber electrodes (e.g., 80.8 F cm−3 for CW/PNC/PEDOT hybrid fiber,35 68.4 F cm−3 for

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.S.) *E-mail: [email protected] (J.Z.) *E-mail: [email protected] (W.H.) ORCID

Xiao Huang: 0000-0002-0106-7763 Gengzhi Sun: 0000-0002-8000-8912 2840

DOI: 10.1021/acsami.7b16761 ACS Appl. Mater. Interfaces 2018, 10, 2837−2842

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Jixin Zhu: 0000-0001-8749-8937 Wei Huang: 0000-0001-7004-6408 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of China (21501091), the NSF of Jiangsu Province (55135065), the Recruitment Program of Global Experts (1211019), the “Six Talent Peak” Project of Jiangsu Province (XCL-043), the National Key Basic Research Program of China (973) (2015CB932200), and the China Postdoctoral Science Foundation (2016M600404, 2017T100360).

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