Heterogeneous Metal OxideâGraphene Thorn-Bush Single Fiber as a

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Functional Nanostructured Materials (including low-D carbon)

Heterogeneous Metal Oxide-Graphene ThornBush Single Fiber as Freestanding Chemiresistor Ji-Soo Jang, Hayoung Yu, Seon-Jin Choi, Won-Tae Koo, Jiyoung Lee, DongHa Kim, Joon-Young Kang, Yong Jin Jeong, Hyeon Su Jeong, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22015 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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ACS Applied Materials & Interfaces

Heterogeneous Metal Oxide-Graphene ThornBush Single Fiber as Freestanding Chemiresistor Ji–Soo Jang, ‡ Hayoung Yu,§ Seon–Jin Choi, δ Won-Tae Koo, ‡ Jiyoung Lee, ‡ Dong-Ha Kim, ‡ Joon-Young Kang, ‡ Yong Jin Jeong, ‡ Hyeonsu Jeong,§,* and Il–Doo Kim‡,* ‡Department

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

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

of Advanced Composite Materials, Korea Institute of Science and Technology

(KIST), Chudong-ro 92, Bongdong-eup, Wanju-gun, Jeonrabuk-do, 565-905, Republic of Korea δDepartment

of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue,

Cambridge, MA 02139. USA

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

KEYWORDS porous

graphene

fiber,

liquid

crystal,

tunicate,

nitrogen

ACS Paragon Plus Environment

dioxide,

gas

sensor

ACS Applied Materials & Interfaces 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

ABSTRACT The development of freestanding fiber-type chemiresistor, having high integration ability with various portable electronics including smart clothing system, is highly demanding for the nextgeneration wearable sensing platform. However, critical challenges stemming from irreversible chemical sensing kinetics and weak reliability of the freestanding fiber-type chemiresistor hinder their practical use. In this work, for the first time, we report on potential suitability of the freestanding and ultraporous reduced graphene oxide fiber functionalized with WO3 nanorods (porous WO3 NRs-RGO composite fiber) as a sensitive nitrogen dioxide (NO2) detector. By employing tunicate cellulose nanofiber (TCNF), which is a unique animal-type cellulose, the numerous mesopores are formed on wet-spun TCNF-GO composite fiber, unlike bare GO fiber with dense surface structure. More interestingly, due to the superior wettability of TCNF, the aqueous tungsten precursor is uniformly adsorbed on ultraporous TCNF-GO fiber and subsequent heat treatment results in the thermal reduction of TCNF-GO fiber and hierarchical growth of WO3 NRs perpendicular to the porous RGO fiber (porous WO3 NRsRGO fiber). The freestanding porous WO3 NRs-RGO fiber shows a notable response to 1 ppm of NO2. Furthermore, we successfully demonstrate reversible NO2 sensing characteristics of the porous WO3 NRs-RGO fiber, which is integrated on a wrist-type wearable sensing devices.


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INTRODUCTION Unlike pure graphene, graphene oxide (GO) shows intriguing colloidal liquid crystalline characteristics with thermodynamically stable long-range orientation order, which is defined as a GO liquid crystal (GOLC) behavior.1-3 Because of this behavior, the exceptional potential of GOLC has been explosively studied in numerous applications over the past few years, including spinning of graphene based fiber, energy storage/conversion, optoelectronics, and catalysis.4 Among them, wet-spun GO fibers, which have highly ordered graphene sheets in the fiber, are gaining much attention because it is an efficient way to translate high mechanical strength and electrical conductivity of graphene (or reduced graphene oxide) into the macroscopic structure.5 Taking these advantages, various research fields such as wearable supercapacitors, freestanding cable-type chemical sensors, and fiber-type hydrogen evolution reaction catalysts have been successfully opened up.6 However, although GOLC derived GO fiber actively provides great opportunities toward various scientific researches, it is often very difficult to develop mesopores (2-50 nm) on GO fibers, which are assembled from densely packed GO sheets owing to its LC characteristics. This is significantly unfavorable for applications to chemical sensors and catalysts, which are dependent on the surface chemical reactions. For chemical sensors, the ultrasmall pore size distribution of GO (0.289–0.33 nm)7 impedes spontaneous gas diffusion into the inner layer of the bare GO fiber. With the exception of hydrogen molecules, which has a small kinetic diameter of 0.289 nm,8 other toxic gas molecules such as NO2 (0.401–0.502 nm), H2S (0.360 nm), CH4 (0.380 nm), and CH3COCH3 (0.500 nm) can hardly penetrate into the densely assembled GO fiber.9 Accordingly, GO fibers inevitably exhibits poor sensing characteristics to many gas species. For example, Yun et al. developed a bare RGO single yarn as a NO2 gas sensing layer, which demonstrated poor



sensing characteristics such as low response and ultraslow recovery due to poor gas accessibility.10 This necessitates a rational design of a highly porous graphene based fiber. To further improve the chemical sensing characteristics of GO fiber, surface functionalization by catalysts or creation of heterogeneous interfaces has been employed. For instance, Yeo et al. synthesized Cu NPs decorated freestanding RGO fiber, and Niu et al. developed the MoS2 loaded graphene fiber.11, 12 Although a substantial improvement in sensing performance was observed compared to bare fibers, they still showed poor reversibility and low detection limit for low ppm level target gas molecules, owing to inhomogeneous catalyst loading. For these reasons, functionalization of GO fiber with catalysts or additives with high uniformity and superior stability is highly demanded to develop a sensitive GO fiber based sensor. As a new processing strategy, we firstly employed a tunicate cellulose nanofiber (TCNF), which is a unique animal-type cellulose nanofiber extracted from tunicate, to modify surface functionality of GO sheets for the development of meso- and macropores in wet-spun GO fiber and creation of carbon-metal oxide composite sensing layers (e.g. WO3-RGO). In fact, TCNF is very intriguing material owing to its high mechanical property, superior wettability, earthabundant and eco-friendly characteristics.13, 14 Due to the single nematic phase derived from LC behavior of TCNF-loaded GO (hereafter, TCNF-GO) solution, and random distribution property of TCNF, a porous, highly stable, and uniform TCNF-GO composite fiber was spontaneously formed when the TCNF-GO containing solution was wet-spun. Furthermore, the superior wettability of TCNF plays a crucial role in uniform immobilization of the aqueous tungsten precursor on TCNF-GO fiber (W_TCNF-GO fiber), enabling a very uniform decoration of catalytic WO3 nanorods (NRs) on porous RGO fiber after high temperature heat treatment. Because of the high porosity and heterogeneous junction effect, WO3 loaded porous


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RGO fiber exhibited highly reversible sensing kinetics even at 1 ppm level of NO2, and showed great compatibility with wrist-type wearable sensing devices.

RESULTS AND DISCUSSION To investigate the lyotropic transition of TCNF, graphene oxide (GO), and TCNF-loaded GO composite, we carried out polarized optical microscopy (POM) analysis of these samples with the same concentrations (0.5 wt%). The 0.5 wt% concentration of each target material was enough to observe the LC phase property of GO, TCNF, and TCNF-loaded GO composite. As shown in Figure 1a, the aqueous solution dope of TCNF, which is extracted from tunicate (see experimental section), showed high birefringence under crossed polarizers, a Schlieren texture which is typically observed for nematic phase. Similarly, a 0.5 wt% GO solution dispersed in deionized (DI) water showed brush patterns in texture, indicating the formation of lyotropic nematic phase in aqueous medium.15 Since both GO and TCNF possess the aqueous LC phase, high miscibility between the two materials in aqueous solution can be achieved, enabling the creation of single nematic LC phase (Figure 1c) without phase separation regardless of their proportions (Figure S1). Since highly concentrated dope solution (e.g. 2 wt%) for wet-spinning can be only achieved by using LC phase, the realization of LC phase in GO-TCNF composite solution is critical for the development of wet-spun TCNF-GO composite fiber (hereafter, TCNF-GO fiber).16-18 Taking advantage of composite LC nature, GO can be aligned in a uniform direction during wet-spinning, whereas the TCNF appears random distributed on the wet-spun GO fibers, thereby forming the mesopores (see the schematic of TCNF-GO fiber surface in Figure 1d). The TCNF has a nanofibrous morphology with a width of 11.48 nm and a few-micron size length (Figure S3), as measured by atomic force microscopy (AFM). In order to investigate the



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TCNF effect in wet-spun fibrous structures, we carried out the wet-spinning process with various dope solutions, namely, pristine GO solution, 5:5 w/w, and 2:8 w/w of GO/TCNF composite solution. The wet-spun GO fibers and TCNF-GO fibers stabilized and solidified in the form of freestanding continuous fibers, respectively. As a control sample, the as-spun GO fiber with a mean diameter of 65.5 μm showed one-dimensionally aligned wrinkled surface characteristics (Figure 1e). Due to the high concentration of GO, the GO fiber exhibited stable freestanding characteristics (inset of Figure 1e). However, the GO fiber did not show mesoand macroporosity in their microstructures as shown in the magnified scanning electron microscopy (SEM) image of GO fiber (inset of Figure 1e). Due to the dense packing behavior of GO sheets at high concentration, the GO exhibited regular alignment in wet-spun fiber. Interestingly, TCNF-GO composite spinning dope solution induced irregularly wrinkled surface morphology (Figure 1f). These unique morphological features of TCNF-GO fiber can be explained by outstanding hydrophilic nature of TCNF. During the wet-spinning of TCNFGO dope solution, numerous water molecules can be adsorbed on TCNF because of its high hydrophilicity, resulting in excessive volume expansion of TCNF-GO fiber.19 The subsequent coagulation of TCNF-GO gel after spinning induces extensive desorption of water molecules from TCNF; considerable shrinkage of TCNF-GO fiber occurs, leading to the formation of irregularly wrinkled surface morphology.20 In particular, TCNF-GO (8:2 w/w) fiber containing high concentration of TCNF clearly exhibited mesoporous morphology (inset of Figure 1f), while the 5:5 w/w TCNF-GO composite fiber did not show noticeable

mesoporosity but

showed highly wrinkled surface structure (Figure S2b, S2e, and S2h). Note that the optimized GO/TCNF ratio was determined based on the degree of mesoporosity of the wet-spun fiber; 2:8 w/w of GO/TCNF composite solution induced high porosity in the fiber structures (Figure S2). Although TCNF also exhibits LC behavior in the spinning dope solution, the random distribution characteristic of nanofibrous TCNF in TCNF-GO fiber enables facile creation of


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mesoporous fiber structure (inset of Figure 1f). The conventional LC materials (e.g. GO) usually align in the longitudinal direction of the wet-spun fiber; the random distribution behavior of TCNF in wet-spun TCNF-GO fiber, despite its LC behavior, is an exclusive nature of TCNF compared to other plant cellulose.20 Furthermore, the as-synthesized freestanding TCNF-GO fiber showed high mechanical strength of 124 MPa, which is similar value with pristine GO fiber (Figure S4).23 By using this intriguing phenomenon, we successfully introduced numerous mesopores in GO-based fiber. To quantitatively compare the surface area and pore distribution of GO fiber and TCNFGO

fiber,

we

carried

out

Brunauer-Emmett-Teller

(BET)

analysis.

The

N2

adsorption/desorption isotherms of GO fiber and TCNF-GO fiber were type IV (in accordance with IUPAC classification), indicating that GO fiber and TCNF-GO fiber were highly porous structures (Figure 2a).21 TCNF-GO fiber showed much higher BET surface area (324.1 m2/g) compared with that (198.55 m2/g) of GO fiber. The higher surface area of TCNF-GO fiber can be attributed to the wrinkled surface morphology and the high mesoporosity that were derived from anchoring of TCNF to the GO fiber. The corresponding pore width distribution curves were calculated using Barrett-Joyner-Halenda (BJH) method. As shown in Figure 2b, the TCNF-GO fiber exhibited main broad pore distribution peak in the range of 5–30 nm, which is significantly advantageous for high gas accessibility, while pristine GO fiber mainly showed tiny sized micropores (< 1 nm) that are disadvantageous for effective gas diffusion into the sensing layer.22 The fabrication of the WO3 nanorods (NRs) loaded porous reduced graphene oxide fiber (porous WO3 NRs-RGO fiber) is schematically depicted in Figure 3a and 3b. Considering a simple in situ growth of WO3 NRs on the target matrix23 (RGO fiber in this work) and high sensitivity of WO3-RGO composite especially toward NO2 gas,24, 25 we rationally designed the WO3 NRs loaded porous RGO fiber. The high density TCNFs, which are tightly immobilized



on GO fiber, serve as an effective adsorbing layer of aqueous tungsten precursor solution (ammonium metatungstate hydrate [(NH4)6H2W12O40 •xH2O], AMH). Owing to the outstanding wettability of TCNF, the aqueous solution of tungsten precursor was easily adsorbed on TCNF, resulting in the formation of AMH/DI-water molecules loaded TCNF-GO fiber (W_TCNF-GO fiber) (Figure 3a). After pyrolysis of W_TCNF-GO fiber in argon (Ar) atmosphere at 700 ℃, porous WO3 NRs-RGO fiber was obtained (Figure 3b). Interestingly, the uniformly distributed AMH vertically grew in the form of WO3 NRs on the W_TCNF-GO fiber during heat treatment.23, 26 Due to the sufficient oxygen species in AMH, the WO3 phase can be formed and the preferential growth of W atoms to [010] direction induced the WO3 NRs structures on TCNF-GO fiber.23 Thus, the chemical equation for the formation of WO3 phase can be described by (NH4)6H2W12O40‧xH2O → 12WO3 (s) + (4+x)H2O (g) + 6NH3 (g). The morphologies of the samples were analyzed using SEM (Figure 3c–f). Taking advantage of TCNF’s hydrophilicity, the aqueous solution of AMH was uniformly loaded on the TCNF-GO fiber without any obvious morphological transformation (Figure 3c and 3d). After high temperature heat treatment, we clearly observed numerous vertically grown WO3 NRs on the porous carbonized single fiber (porous WO3 NRs-RGO fiber) due to in-situ growth of WO3 NRs on TCNF (Figure 3e); the WO3 NRs showed a mean width of 197 nm and a mean length of 1.87 μm (Figure 3f). The wrinkled surface morphology, which is highly advantageous for effective gas sensing reaction, was well preserved even after the high temperature calcination. More quantitatively, the BET surface area (359.1 m2/g) of porous WO3 NRs-RGO fiber was well preserved even after the calcination as shown in Figure S5. Furthermore, the synthesized freestanding porous WO3 NRs-RGO fiber showed slightly degraded mechanical strength of 70 MPa, which is smaller value than TCNF-GO fiber (Figure S6). To investigate the critical role of TCNF as a receptor layer for the growth of WO3 NRs, we prepared a control sample by loading the AMH precursor on the conventional GO fiber


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without TCNF. After pyrolysis, the uniformity of as-grown WO3 NRs on RGO fiber was significantly different. As shown in Figure 4a–b, WO3 NRs-loaded RGO fiber derived from pristine GO fiber showed a poor uniformity, exhibiting severely aggregated WO3 regions (Figure 4a), while TCNF-GO fiber derived porous WO3 NRs-RGO fiber showed a uniform distribution of WO3 NRs on the entire surface of the porous RGO fiber (Figure 4b). The superimposed schematic images in Figure 4a and 4b show the corresponding structural images. Chemical binding states of porous WO3 NRs-RGO fiber were investigated by using Xray photoelectron spectroscopy (XPS) and Raman spectroscopy (Figure 4c–f). The peaks at 286.3 and 289.3 eV corresponding to -C-OH and -O=C-OH bonds, respectively, were observed in porous WO3 NRs-RGO fiber; the existence of -C-OH and -O=C-OH bonds in RGO fiber was reported in our previous study.27 In addition, the predominant peak at 284.6 eV (C-C bonds) was observed after carbonization, implying the formation of graphite-like aromatic structure.27 The predominant oxygen peaks in porous WO3 NRs-RGO fiber assigned to the C-OH bonds were observed at 533.15 eV (Figure 4d); the C-OH bonds mainly existed in RGO fiber. Furthermore, chemisorbed oxygen species (O-) and lattice oxygen (O2-) were also observed at 531 and 530.2 eV, respectively, due to the formation of WO3 NRs on porous RGO fiber28; chemisorbed oxygen species mainly serve as chemiresistive gas reaction sites especially at high temperature (e.g. 200 ℃).29 Two W 4f5/2 and 4f7/2 peaks were also observed at the binding energies of 38.1 and 35.9 eV, respectively, which corresponded to W6+ binding states (Figure 4e)30. The WO3 phases were well formed despite the high temperature heat treatment in reducing atmosphere. To further investigate the chemical binding state in porous WO3 NRsRGO fiber, we carried out the Raman spectroscopy analysis with 3 different samples, i.e., TCNF-GO derived porous RGO fiber, WO3-RGO fiber (without TCNF), and porous WO3 NRs-RGO fiber (with TCNF) (Figure 4f). The characteristic peaks of D-band and G-band were clearly observed at the wavelengths of around 1350 and 1600 cm-1, respectively, in all samples.



The D-band and G-band peaks indicated the breathing mode of aromatic rings and optical E2g phonons, which are attributed to defect and bond stretching of sp2 carbon pairs.31 The intensity ratio (ID/IG) of D-band and G-band peaks was calculated to investigate the in-plane crystallite sizes (Lc) of graphitic carbon structures. Based on the Tuinstra-Koenig relation in Equation 1, the Lc of WO3-RGO fiber and porous WO3 NRs-RGO fiber were smaller than porous RGO fiber due to higher ID/IG values of WO3-RGO fiber (0.994) and porous WO3 NRs-RGO fiber (0.991) compared with that (0.929) of porous RGO fiber. The decreased crystallite size of porous WO3 NRs-RGO fiber was mainly attributed to the introduction of defects into the graphene during the growth of WO3 NRs and pyrolysis (carbonization) of TCNF. However, due to the higher density of GO in GO fiber than in TCNF-GO fiber, the WO3-RGO (derived from GO) fiber showed higher ID/IG values than that of porous WO3 NRs-RGO fiber despite the growth of WO3. Lc [nm] = (2.4 × 10 ―10) ∙ λ4 ∙

() 𝐼𝐺

𝐼𝐷

(1)

Here, the λ is the Raman excitation wavelength, e.g. 514 nm. By comparing the Raman spectra of three samples, we also observed the three WO3 related Raman peaks at 272, 717, and 807 cm-1 in the WO3-RGO fiber and porous WO3 NRs-RGO fiber (Figure 4f). The Raman peaks at 717 and 807 cm-1 were mainly attributed to W-O-W stretching vibration mode, while the peaks centered at 272 cm-1 were derived from W-O-W bending mode vibration.32 Through the XPS and Raman analyses, we clearly confirmed that the phase of WO3 NRs were well-formed on the RGO fiber with crystallized carbon structures. To investigate gas sensing performance of TCNF-GO derived porous RGO fiber, WO3RGO fiber (without TCNF), and porous WO3 NRs-RGO fiber (with TCNF), we carried out the sensing tests toward 6 different toxic gases, namely, nitrogen dioxide (NO2), nitrogen monoxide (NO), hydrogen sulfide (H2S), toluene (C7H8), acetone (C3H6O), and ethanol


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(C2H5OH) using our sensing measurement system.33 The relative target gas concentration was controlled in the range of 1–5 ppm in dry condition (2.6% RH), and the normalized response (S, sensitivity) was defined as [|Rgas-Rair|/Rair] × 100 (%), where Rgas and Rair are resistances when target gas and baseline dry air (2.6% RH) are injected, respectively. Figure 5a exhibited the dynamic resistance kinetics of porous WO3 NRs-RGO fiber, WO3-RGO fiber, and porous RGO fiber toward NO2 (1–5 ppm) at 100 ℃. The porous WO3 NRs-RGO fiber showed reversible NO2 sensing properties with obvious resistance fluctuation, while WO3-RGO fiber and porous RGO fiber did not show notable sensing response to NO2 gases. The effect of WO3 NRs on improved NO2 gas reaction can be explained by comparing the sensing properties of porous WO3 NRs-RGO and porous RGO fibers. Although the RGO fiber was porous, it exhibited much lower NO2 detection ability compared to porous WO3 NRs-RGO fiber; this means that the WO3 NRs on porous RGO fiber played an important role in enhancing NO2 sensing characteristics. The functionalization effect of WO3 results in the difference of baseline resistance between porous WO3 NRs-RGO fiber (1800 Ω) and porous RGO fiber (1520 Ω); numerous heterogeneous junctions in porous WO3 NRs-RGO fiber are formed between p-type RGO fiber and n-type WO3 NRs, leading to increased resistance. Note that RGO and WO3 are well-known p-type and n-type sensing materials, respectively.34,

35

In addition, the non-

aggregation and uniform decoration of WO3 NRs on porous RGO fiber is crucial for improved NO2 sensing. For example, aggregated WO3 loaded RGO fiber did not show reversible gas sensing reaction. Considering the temperature-dependent sensing kinetics of porous WO3 NRs on RGO fiber (Figure 5b), we optimized the operation sensing temperature of porous WO3 NRs-RGO fiber at 100 ℃. The porous WO3 NRs-RGO fiber also showed reversible ppm level NO2 detection behavior at lower temperature (e.g. room temp., and 50 ℃). In general, the sensor operated at an elevated temperature induces faster response time. However, in our case, the sensing at 200 ℃ resulted in the transition of the sensing type of porous WO3 NRs-RGO



fiber from p-type to n-type, with a slower response speed (Figure S9a). To quantitatively calculate the response/recovery times of the porous WO3 NRs-RGO fiber, we plotted the normalized curves of response and recovery for porous WO3 NRs-RGO fiber at different sensing temperature (Figure 5c and 5d). Note that the response and recovery times were determined at ΔR/ΔRmax = 0.9 and ΔR/ΔRmax = 0.1, respectively. As shown in Figure 5c, the porous WO3 NRs-RGO fiber showed the fastest response time (180 sec) at 100 ℃ compared with response times at room temp. (480 sec), 50 ℃ (300 sec), and 200 ℃ (425 sec). On the other hand, the fastest recovery time of the porous WO3 NRs-RGO fiber was observed at 100 ℃ (432 sec) compared to the recovery times at room temp. (528 sec), 50 ℃ (436 sec), and 200 ℃ (448 sec) (Figure 5d). Based on the sensing mechanism reaction, the porous WO3 NRs-RGO fiber showed the physisorption/desorption (weak bond) NO2 sensing mechanism under 100 ℃, while the sensing mechanism at 200 ℃ was the chemisorption/desorption reaction (strong bond).36 Accordingly, the sensor operated at lower sensing temperature (e.g. 100 ℃) exhibited a faster sensing speed compared with the sensor operated at higher sensing temperature (e.g. 200℃). In addition to the sensing speed, we evaluated the response values of the porous WO3 NRs-RGO fiber at various operating temperatures (Figure 5e). The porous WO3 NRs-RGO fiber showed the highest response value (S = [Rair-Rgas]/Rgas × 100 (%), 9.67%) toward 5 ppm of NO2 gas at 100 ℃ compared to the sensors operated at room temp. (2.46%), 50 ℃ (6.87%), and 200 ℃ (9.48%). Since the porous WO3 NRs-RGO fiber showed a linear type response pattern, it can quantitatively detect the concentration-dependent NO2 gas molecules. Furthermore, the porous WO3 NRs-RGO fiber showed superior selectivity toward 5 ppm NO2 (9.67 %), and negligible responses to 6 interfering gas molecules, i.e., ethanol, acetone, toluene, H2S, and NO, (S < 2.45%) (Figure 5f). In terms of the sensing stability, the porous WO3 NRsRGO fiber showed a stable NO2 response under 10 repetitive cycling and humidity-stable NO2 sensing characteristics (Figure S7 and Figure S8).


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The distinctive NO2 sensing characteristics of porous WO3 NRs-RGO fiber are attributed to (i) highly porous structures with high surface area, and (ii) heterojunction effect between the n-type WO3 NRs and the p-type RGO fiber. Because the porous WO3 NRs-RGO fiber well maintained the wrinkled surface morphologies of TCNF-GO fiber, even after pyrolysis, the porous WO3 NRs-RGO fiber can deliver numerous gas reaction sites. The higher surface area of TCNF-GO fiber (324.1 m2/g) compared with that of GO fiber (198.55 m2/g) strongly supports that TCNF-GO fiber effectively induced a lot of gas adsorption sites. Secondly, the heterogeneous junction between the n-type WO3 NRs and the p-type RGO fiber enhanced the NO2 sensing characteristics. Because the uniformly loaded n-type WO3 NRs on p-type RGO fiber induced larger depletion of hole accumulation layer of p-type RGO fiber, the dramatic thickness modulation of hole accumulation layer occurred when the porous WO3 NRs-RGO fiber was exposed to NO2 gas, leading to huge resistance change at low temperature such as room temp., 50 ℃ and 100 ℃ (Figure 6a). Upon exposure to NO2 gas molecules, the electrons in the porous WO3 NRs-RGO fiber were consumed according to Equation 2.37 Due to the decrease in the number of electrons, the relative hole concentration increased, with a consequent decrease in the resistance of the porous WO3 NRs-RGO fiber. 𝑁𝑂2 (g) + 𝑒 ― →𝑁𝑂2― (𝑎𝑑𝑠)

(2) 1

𝑁𝑂2― (𝑎𝑑𝑠) +2𝑂 ― + 𝑒 ― →𝑁𝑂(g) + 2𝑂2 (𝑔) + 2𝑂2 ―

(3)

On the other hand, at higher temperature such as 200 ℃, the n-type WO3 NRs in the porous WO3 NRs-RGO fiber were mainly involved in the gas sensing reaction (red dotted box in Figure S9b). Since the electrons as main sensing carriers were mainly distributed on the porous WO3 NRs-RGO fiber at the 200 ℃, the recombination between holes in RGO fiber and electrons in WO3 NRs induced the electron-depleted region in WO3 NRs (Figure 6b). Afterward, the electrons in porous WO3 NRs-RGO fiber were consumed during the NO2



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sensing reaction (Equation 3),38 thereby resulting in higher resistance of the porous WO3 NRsRGO fiber. To quantitatively understand the effect of WO3 NRs on RGO fiber at the optimum temperature (~ 100 ℃), we calculated the NO2 adsorption/desorption constants and activation energy for the reaction using Equations 4–6, where S0 is the sensitivity when NO2 is desorbed, Smax is the maximum sensitivity, and Ca is the target gas concentration, Kdes is desorption rate constant, Kads is adsorption rate constant, and Ea is the activation energy for sensing reactions. The equilibrium constant K is defined as Kads/Kdes. Note that Equations 4 and 5 have been widely used for evaluating response/recovery kinetics.37, 39 𝐶𝑎𝐾

[

𝑆(𝑡) = 𝑆𝑚𝑎𝑥 ∙ 1 + 𝐶𝑎𝐾(1 ― exp ―

1 + 𝐶𝑎𝐾 𝐾

]

∙ 𝑘𝑎𝑑𝑠 ∙ 𝑡 )

(4)

𝑆(𝑡) = 𝑆0exp [ ― 𝑘𝑑𝑒𝑠 ∙ 𝑡]

(5)

𝐾𝑎𝑑𝑠𝑜𝑟 𝐾𝑑𝑒𝑠 = A0exp [ ― 𝐸𝑎/RT]

(6)

As shown in Table S1, the porous WO3 NRs-RGO fiber exhibited higher adsorption and desorption rate constants (0.183 ppm–1 s–1 for adsorption and 0.301 s–1 for desorption) than those (0.17 ppm–1 s–1 for adsorption and 0.22 s–1 for desorption) of the porous RGO fiber. This means that the uniformly decorated WO3 NRs played effective role in activating the NO2 sensing reaction (Equation 2). We further calculated activation energies (Ea) for NO2 sensing reaction of each sensor by employing the Arrhenius plots of ln (Kads or Kdes) versus 1000/T (Figure 6c and 6d). By calculating the slope of the corresponding Arrhenius plots, we successfully obtained the activation energies (Figure 6e). The activation energies for adsorption (6.38 kcal/mol) and desorption (1.23 kcal/mol) of porous WO3 NRs-RGO fiber are remarkably lower than those (7.63 kcal/mol and 11.86 kcal/mol) of porous RGO fiber. This means that NO2 adsorption/desorption reactions on porous WO3 NRs-RGO fiber occurred spontaneously


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compared to porous RGO fiber. Thus, we realized that the uniform decoration of WO3 NRs on freestanding RGO fiber effectively promoted the NO2 sensing reactions. To investigate the potential feasibility of the porous WO3 NRs-RGO fiber as a wearable sensing platform, we integrated the porous WO3 NRs-RGO fiber on various objects such as personal watches, glasses, and commercialized Kimtech paper (Figure 7a and 7b). Due to the high mechanical bendability and micrometer scale thickness (~ 45.4 μm), the porous WO3 NRs-RGO fiber was easily integrated on curved sides of each object. More importantly, the porous WO3 NRs-RGO single fiber was sewed on the Kimtech paper like a thread; this shows that the porous WO3 NRs-RGO single fiber can be potentially incorporated in textiles for a smart cloth-based gas-sensing platform (Figure 7c). In addition, due to the strong anchorage of WO3 NRs on porous RGO fiber, the porous WO3 NRs-RGO fiber stably maintained their microstructures even after rinsing with water (Figure S11). To further demonstrate their sensing properties as the wearable sensing platform, we loaded the porous WO3 NRs-RGO single fiber on a portable sensing device fabricated on a flexible printed circuit board that can deliver the sensing data to a smart device through a wireless Bluetooth communication (Figure 7d). 20 ppm of NO2 gas molecules was directly injected to the porous WO3 NRs-RGO single fiber sensor for approximately 10 s, and the injection was repeated at 10 cycles. Although the porous WO3 NRs-RGO fiber sensor was operated at room temp., the reliable and reversible NO2 sensing response (2.25–2.75 %) was observed with fast response and recovery times (3 and 6 s, respectively) during the repetitive sensing cycles (Figure 7e). However, in order to quantitatively investigate the response and the sensing speed, further development of a reliable high-resolution gas monitoring wrist-type wearable sensing system is needed.



CONCLUSIONS In this work, we fabricated a freestanding single fiber-type architecture as a toxic NO2 detector in which WO3 NRs were vertically loaded on highly porous RGO fiber. The TCNF (animal-type cellulose) was employed to syntheize the highly porous GO fiber through wetspinning. Afterwards, the as-spun TCNF-loaded GO fiber was simultaneously used as a superior receptor for aqueous tungsten precursor and sacrificial template for creation of highly porous WO3 NRs-RGO single fiber. Owing to superhydrophilicity of TCNF and its random distribution nature in the TCNF-GO fiber, WO3 NRs were uniformly functionalized on the entire surface of RGO fiber, and highly porous RGO fiber was simultaneously achieved by simple calcination. Very interestingly, WO3 NRs loaded porous RGO fiber (porous WO3 NRsRGO fiber) showed reversible NO2 sensing kinetics even at 1 ppm of NO2, and excellent compatibility with various objects. The porous WO3 NRs-RGO fiber integrated watch-type wearable sensing devices successfully detected NO2 gas molecules with fast and reliable sensing response even at room temp. This work demonstrated that the metal oxide decorated single fiber-type porous RGO fiber templated by superhydrophilic TCNF acts as a reliable sensing platform, which overcomes the poor sensing characteristics of the current flexible fiber-type sensing platforms.

EXPERIMENTAL SECTION Materials. Ammonium metatungstate hydrate (AMH, (NH4)6H2W12O40) was purchased from SigmaAldrich (St. Louis, USA). For graphene oxide (GO) synthesis, graphite (Asbury Carbons, initial size ≤ 300µm), nitric acid (HNO3, 65-66%, DAEJUNG), sulfuric acid (H2SO4, 98%, DAEJUNG), potassium permanganate (KMnO4, Sigma Aldrich), hydrogen peroxide (H2O2,


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DAEJUNG) and hydrochloric acid (HCl, DAEJUNG) were purchased. For TCNF preparation, 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO, 98%), sodium bromide (NaBr), sodium hypochlorite (NaClO, 12%) and sodium hydroxide (NaOH, 97%) were all purchased from Sigma Aldrich. All chemicals were used without further purification. Synthesis of GO fiber. The GO liquid crystal (LC) was prepared by a modified Hummer’s method as described in detail elsewhere.40 The prepared GO was concentrated by centrifugation at 20,000 rpm for 2 h. (Supra 30 K, Hanil Science, Korea), after which the precipitate was collected. For GO fiber, a 2 wt% of concentrated GO dope was spun into a coagulation bath (5 wt% CaCl2, ethanol/water mixture (50/50/v/v) using a spinning needle (0.25 mm inner diameter) at a rate of 0.1 mL/min, followed by continuous winding of the fiber onto a bobbin at a winding speed of 1.5 m/min. The GO fiber was washed with ethanol and water, and subsequently dried at 80 °C in vacuum overnight. Synthesis of TCNF-GO fiber. TCNF was prepared by a well-known TEMPO-mediated oxidation.20 Briefly, tunicate cellulose was isolated from Halocynthia roretzi (HR) in a series of alkaline treatment and bleaching process to remove protein, lipids, and other polysaccharides in HR as described in previous report.41 After isolation, the tunicate cellulose suspension (10 g, 100 mL) in water was mixed with TEMPO (0.016g, 0.1 mmol) and sodium bromide (0.1g, 1 mmol), followed by adding sodium hypochlorite (5 mmol) at room temperature. The pH of the reaction was adjusted to 10 via addition of 0.5 M sodium hydroxide for 1 h. Subsequently, the oxidized cellulose was obtained by filtration, and then washed with deionized water (DI-water). The tunicate nanocellulose nanofiber (TCNF) aqueous suspension (0.5 wt%) was obtained by ultrasonication at 750 W with 50 amplitude for 1 h (VCX-740, Sonics & Materials Inc. USA). The TCNF and GO suspension (both 5 mg/mL) was simply mixed through stirring by



controlling weight ratio (w/w, 3/7, 5/5 and 7/3), and then the mixed solution was concentrated to 2 wt% for spinning. As in the synthesis of GO fiber, wet-spinning process was applied to fabricate TCNF-GO fiber. Synthesis of porous WO3 NRs-RGO fiber and WO3-RGO fiber. As-synthesized TCNF-GO fiber was immersed in 1.8 g of ammonium metatungstate (AMH) dissolved in 25 g of DI-water for 3 h. Afterwards, the AMH loaded TCNF-GO fiber (W_TCNFGO fiber) was dried at 50 ℃ in air, followed by calcination at 700 ℃ in Ar atmosphere for 3 h. The final product exhibited black colored freestanding single fiber structure. To synthesize the WO3-RGO fiber as control sample, as-synthesized GO fiber was also immersed in 1.8 g of AMH dissolved in 25 g of DI-water for 3 h. And then, the dry and calcination processes of AMH loaded GO fiber were proceeded at 50 ℃ in air and 700 ℃ in Ar atmosphere for 3 h, respectively.

Characterization. Polarized optical microscopy (POM, LV-100POL, Nikon) was employed to investigate birefringence of GO, TCNF and TCNF-GO liquid crystals (LCs). The field-emission scanning electron microscopy (FE-SEM, Nova 230) analysis was conducted to analyze the microstructures and morphologies of the samples. Atomic force microscopy (AFM, MFP-3D, Asylum Research) analysis was carried out to investigate the surface morphology of TCNF and GO sheets. X-ray photoelectron spectroscopy (XPS, Sigma Probe, Thermo VG Scientific) with Al Kα radiation (1486.6 eV) was carried out to characterize the chemical bonding states in porous WO3 NRs-RGO fiber. Sensing properties of porous RGO fiber, porous WO3 NRs-RGO fiber, and WO3-RGO fiber were evaluated by a homemade testing equipment described elsewhere .33 The resistance variation of each sample was measured by using a data acquisition system (34972A, Agilent).


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Sensor fabrication and gas sensing measurement. Responses of sensors were evaluated by resistance changes (S=[Rgas-Rair]/Rair × 100 ) of sensing layers (porous RGO fiber, porous WO3 NRs-RGO fiber, and WO3-RGO fiber). The resistance of sensing layers was measured by the data acquisition system (34972A, Agilent). Note that Rair is the sensor baseline resistance upon exposure to air and Rgas is the resistance upon exposure to target gas. All of the sensors were stabilized in dry air (30% RH). The sensors were exposed to various toxic gas molecules (NO2, NO, H2S, toluene, acetone, and ethanol) with concentrations ranging from 1 to 5 ppm. The on/off interval of exposure to gases was 10 min. The operating temperature of the sensors was adjusted by applying a DC voltage to the microheater on the back of the sensor substrate using a DC power supply (E364A, Agilent). The interval between gold sensing electrodes was 70 μm and real-time NO2 gas monitoring was performed by wrist type sensing devices with 10 s gas/air injection interval.

ASSOCIATED CONTENT Supporting Information. These materials are available free of charge via the Internet at “http://pubs.acs.org.” Additional POM analysis of GO-TCNF composite, SEM, AFM, elongation property, additional sensing analysis results, microstructure after rinse process can be available. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORICD



Il-Doo Kim: 0000-0002-9970-2218 Hyeonsu Jeong: 0000-0003-0958-8173 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation (NRF) of Korea grant funded by the Ministry of Science and ICT. (NRF-2015R1A2A1A16074901, 2016M3A7B4905619). This work was also supported by Wearable Platform Materials Technology Center (WMC) funded by National Research Foundation of Korea (NRF) Grant of the Korean Government (MSIP) (No. 2016R1A5A1009926). This research was supported by the Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4905609). This work was supported by the National Research Foundation of Korea (NRF), grant no. 2014R1A4A1003712 (BRL Program).

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Figure 1. Polarized optical microscopy (POM) image of (a) TCNF, (b) graphene oxide (GO), and (c) TCNF-GO composite, (d) the schematic illustration of wet-spinning of TCNF-GO fiber, SEM images of (e) GO fiber, and (f) TCNF-GO fiber with magnified SEM images (inset).


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Figure 2. (a) Isothermal adsorption/desorption plot and BET surface area of TCNF-GO fiber and GO fiber, (b) pore size distribution of TCNF-GO fiber and GO fiber.



Figure 3. (a, b) Schematic illustration for the synthesis of porous WO3 NRs-RGO fiber, SEM images of (c, d) AMH loaded TCNF-GO fiber, and (e, f) porous WO3 NRs-RGO fiber.


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Figure 4. SEM images of (a) WO3-RGO fiber and (b) porous WO3 NRs-RGO fiber, XPS peaks of (c) C 1s, (d) O 1s, (e) W 4f for porous WO3 NRs-RGO fiber, (f) Raman spectroscopy analysis for porous RGO fiber, WO3-RGO fiber, and porous WO3 NRs-RGO fiber.



Figure 5. Dynamic resistance transition of (a) porous WO3 NRs-RGO fiber, WO3-RGO fiber, and RGO fiber, (b) porous WO3 NRs-RGO fiber depending on the sensing temperature during the NO2 sensing cycles, (c, d) normalized curve for NO2 response/recovery time of porous WO3 NRs-RGO fiber at each sensing temperature, (e) NO2 response value of porous WO3 NRs-RGO fiber at each NO2 concentration, and (f) selectivity characteristics of porous WO3 NRs-RGO fiber.


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Figure 6. Suggested sensing mechanism of porous WO3 NRs-RGO fiber (a) at low temperature, and (b) at high temperature. Arrhenius plots for (c) the response and (d) the recovery, and (e) calculated activation energy graph for the adsorption and desorption of NO2 on the porous WO3 NRs-RGO fiber and porous RGO fiber.



Figure 7. Digital images of (a–c) porous WO3 NRs-RGO fiber loaded curved surfaces on personal watch, glasses, and sewed porous WO3 NRs-RGO fiber on Kimtech paper, and (d) portable sensing module loaded with porous WO3 NRs-RGO fiber, (e) real-time NO2 monitoring using portable sensing module.


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