Micropatterning Silver Nanowire Networks on Cellulose Nanopaper

Oct 15, 2018 - Here, we report a reliable fabrication method for transparent and highly bendable microelectrodes based on conductive silver nanowires ...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 38517−38525

Micropatterning Silver Nanowire Networks on Cellulose Nanopaper for Transparent Paper Electronics Dabum Kim,† Youngsang Ko,† Goomin Kwon, Ung-Jin Kim, and Jungmok You* Department of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea

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ABSTRACT: Transparent microelectrodes with high bendability are necessary to develop lightweight, small electronic devices that are highly portable. Here, we report a reliable fabrication method for transparent and highly bendable microelectrodes based on conductive silver nanowires (AgNWs) and 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO)-oxidized cellulose nanofibers (CNFs). The AgNW-based micropatterns were simply fabricated on glass via poly(ethylene glycol) photolithography and then completely transferred to transparent TEMPO-CNF nanopaper with high bendability via vacuum-assisted microcontact printing (μCP). The AgNW micropatterns were embedded in the surface layer of TEMPO-CNF nanopaper, enabling strong adhesion to the nanopaper substrate. The resulting AgNW micropatterns on the TEMPO-CNF nanopaper showed an optical transparency of 82% at 550 nm and a sheet resistance of 54 Ω/sq when the surface density of AgNWs was as low as 12.9 μg/cm2. They exhibited good adhesion stability and excellent bending durability. After 12 peeling test cycles and 60 s sonication time, the sheet resistance of the AgNW networks embedded on TEMPO-CNF nanopaper increased by only ∼0.12 and ∼0.07 times, respectively. Furthermore, no significant change in electrical resistance was observed even after 3 bending cycles to nearly 90° and 500 cycles of 80% bending strain. Moreover, the AgNW patterns on TEMPO-CNF paper were successfully applied for constructing a transparent electric circuit as well as a solid-state electrochromic device. Overall, we proposed an effective way to fabricate AgNW micropatterns on transparent nanopaper, which can be expanded to various conductive materials for highperformance paper-based electronics. KEYWORDS: silver nanowire, TEMPO-oxidized cellulose nanofiber, transparent cellulose nanopaper, microelectrode, paper electronics

1. INTRODUCTION Transparent, bendable, conductive electrodes are important components for next-generation electronic devices such as smart tablets, rollable displays, foldable phones, and wearable sensors.1−6 Transparent conductive micropatterns on lightweight, bendable, and transparent substrates are particularly valuable for the development of future small, light, foldable, and portable electronic devices, as these foldable microelectrode arrays enable high-density integrated systems.7−9 Plastics have been extensively studied as lightweight alternatives to rigid glass in flexible optoelectronic devices over the past few decades, but they do not have the high bendability that is an essential requirement for future portable electronic devices.10,11 Indium tin oxide (ITO) has long been used as a traditional transparent electrode material due to its optical transparency and electrical conductivity. However, this material is brittle and requires an expensive deposition process, which makes it difficult to apply in next-generation microelectrode technology.12,13 There are a number of intrinsic requirements for fabricating an ideal transparent microelectrode for bendable and portable electronics: (1) the materials used for the transparent substrate must be low-cost, © 2018 American Chemical Society

lightweight, and highly bendable as well as easily integrated with conductive materials; (2) the transparent conductive materials must also be bendable or at least highly flexible and stable; (3) micropatterning of conductive materials on substrates must be a simple, fast, and high-throughput process. Optically transparent nanopapers, where cellulose nanofibers (CNFs) are densely packed and strongly interconnected with a number of hydrogen bonds, are of great interest in electronics and optoelectronics, because they can serve as low-cost, bendable, and transparent substrates that have a number of advantages over conventional glass, plastic, and general paper.14−18 For example, they are easy to prepare, highly transparent, lightweight, and possess excellent foldability, high tensile strength (2−6 GPa), and a low thermal expansion coefficient (6 ppm/K). Cellulose nanofibers, which are renewable and are the most common biopolymers in nature, are obtained by mechanically or chemically fibrillated wood pulp. Therefore, these nanopapers are perfect candidates for Received: September 3, 2018 Accepted: October 15, 2018 Published: October 15, 2018 38517

DOI: 10.1021/acsami.8b15230 ACS Appl. Mater. Interfaces 2018, 10, 38517−38525

Research Article

ACS Applied Materials & Interfaces

propyl) ketone (Irgacure 2959) was obtained from BASF. Silver nanowires (AgNWs, 1 wt %) with 20−40 nm diameters and 20−30 μm lengths were purchased from NANOPYXIS as a dispersion solution in isopropyl alcohol. These silver nanowires (AgNWs, 1 wt %) have been coated with a few nanometer polyvinylpyrrolidone capping layer as a stabilizer. 3-Acryloxypropyltrichlorosilane was obtained from Gelest, Inc. (Morrisville, PA). Phosphate-buffered saline (PBS) was purchased from Life Technologies. A mixed cellulose ester membrane filter was purchased from Advantec Co., Ltd. (Japan). 2.2. Characterization. Transmittance was measured with a spectrophotometer (UV-3600 plus, Shimadzu, Japan). The carboxylate content of TEMPO-CNF in water was determined by the electric conductivity titration method.39 A Zetasizer Nano ZS (Malvern Instruments) instrument was used to measure the surface charge on the TMEPO-CNF. Tensile tests of TEMPO-oxidized nanopaper were conducted with a CT3 Texture Analyzer (Brookfield Engineering). The morphology of the patterned AgNW electrodes on TEMPO-CNF paper was measured with a field emission scanning electron microscope (FE-SEM, Carl Zeiss, model SIGMA) and an optical microscope. The sheet resistance measurements were performed with a sheet resistance tester (CMT-100S, Advanced Instrument Technology). Thermogravimetric (TGA) analysis was conducted using a thermogravimetric analyzer (TGA N-1000, Scinco Co. Ltd.) with cellulose powder, pure CNF paper, and TEMPO-CNF paper from 25 to 620 °C at a heating rate of 10 °C /min in a nitrogen flow of 30 mL/min. FTIR spectroscopy data were collected using a Nicolet iS5 FTIR spectrometer (Thermo Fisher Scientific) in an attenuated total reflectance mode. 2.3. Preparation of the TEMPO-Oxidized Cellulose Nanofiber (TEMPO-CNF) Nanopaper. For the TEMPO oxidation of cellulose,40 0.016 g of TEMPO and 0.1 g of NaBr was dissolved in 100 mL of distilled water. Then, 1 g of cellulose fibers were added to the solution while stirring. 12% NaClO solution was adjusted to pH 10 by the addition of hydrochloric acid (0.1 M). Next, the TEMPO oxidation was started by adding the pH adjusted NaClO (5 mmol) and was continued at room temperature by stirring. The pH was retained at 10 by adding 0.5 M NaOH using a pH meter until no NaOH consumption was observed. The TEMPO-oxidized cellulose was washed with distilled water until the dispersion became neutral. After washing, the TEMPO-oxidized cellulose dispersion was homogenized at high pressure (Nano Disperser-NLM 100, Ilshin Autoclave Co. Ltd., 10 passes at 1000 bar) to produce the TEMPOCNF dispersion. Before filtration of the TEMPO-CNF dispersion, large cellulose fibers were collected by centrifugation (4000 rpm, 10 min) to make more transparent paper. Then, we used the dispersion, except for the sunken cellulose fibers. Then, 60 mL of the centrifuged TEMPO-CNF dispersion was carefully poured into a cellulose ester membrane (0.2 μm pore size, 47 mm diameter) and vacuum filtrated to fabricate the CNF matrix. After drying at 75 °C for 3 h, the thickness of the resulting nanopaper was found to be 18 μm by the thickness measurement device (2109S-10, Mitutoyo, Japan). 2.4. Fabrication of AgNW Patterns on Glass Substrate. We demonstrated the fabrication of AgNW patterns on a glass substrate in our previous papers.37,38,41 Briefly, the AgNW dispersions (0.05, 0.075, 0.1, 0.2, and 0.3 wt %) were spin-coated onto glass substrates at 5000 rpm and then annealed at 150 °C in air for 5 min. This process was repeated five times to achieve uniform AgNW networks on the glass substrate.6 After the fabrication of AgNW films on glass, we used a PEG photolithography method to pattern the AgNW films. PEGDA (MW 575) was mixed in PBS containing 1% w/v of a photoinitiator (Irgacure 2959) to achieve a 60% w/v gel precursor solution. The PEG precursor solution was dropped onto the AgNWcoated glass and then covered with silane-treated glass. The silane modification was employed to anchor the gel layer to the covered glass surface. The AgNW-coated glass was exposed to a UV light source (INNO Cure 2000, 2.32 mW/cm2) through a photomask and silane-treated glass for 1 s. After peeling off the silane-treated glass from pristine AgNW-coated glass, the UV-exposed AgNW film was observed to be transferred to the silane-treated glass along with the

substrates in the future production of electronic and optoelectronic devices.19−22 Conductive patterns have been prepared on nanopaper substrates by metal sputtering through a metal mask or by printing with metal pastes or inks.8,9,23 However, this metal sputtering approach requires expensive equipment, and is a very complex procedure and provides limited control over the pattern size. The inkjet printing technique requires specific ink properties including viscosity, surface tension range, specific metal particle size ranges to avoid clogging nozzles, and colloidal stability to allow droplet ejections.24,25 Moreover, poor adhesion of the printed metal track on the hydrophilic cellulose surface might limit the performance of bendable electronic devices.26 Thus, a simple process to fabricate transparent, stable, conductive micropatterns to function as various electrodes on bendable and transparent nanopaper is required. To the best of our knowledge, we are not aware of studies reporting the fabrication of transparent conductive micropatterns on optically transparent cellulose nanopaper (see the Supporting Information, Table S1).8,9,23,24,27−31 We now describe, for the first time, the successful fabrication of highly transparent, bendable, strongly adhesive, conductive microelectrodes on highly transparent nanopaper. Silver nanowires (AgNWs) were selected as a conductive material to achieve high-performance transparent microelectrodes because: (1) AgNW-based electrodes exhibit excellent flexibility, conductivity, and transparency, and (2) they can be easily fabricated by solution-process methods.32−36 AgNWbased microelectrodes were simply fabricated on a glass substrate using UV-induced poly(ethylene glycol) (PEG) photolithography. The PEG photolithographic technique based on PEG diacrylate (PEG-DA) as a UV-curable material has proven to be an attractive approach for both surface engineering and microstructure fabrication because of rapid photopolymerization under ambient conditions, low curing temperature, low energy requirement, and easy control over pattern shape and dimension.37,38 The AgNW-based microelectrodes on glass were clearly transferred to the transparent nanopaper by simple vacuum-assisted microcontact printing (μCP), resulting in AgNW-based microelectrodes on the transparent nanopaper. This study contains the first example of an efficient combination of PEG photolithography and vacuum-assisted μCP for pattern generation and pattern transfer, respectively. Our experiments demonstrated the high-performance of AgNW-based microelectrodes on nanopaper with high optical transparency (>85% at 550 nm), remarkable electrical conductivity (11−54 Ω/sq), good adhesion (increase in resistance of only ∼7% after 60 s of sonication), and high bendability (increase in resistance of ∼4 and ∼21% after 80 and 500 bending cycles, respectively). We envision that the AgNW-based microelectrodes on nanopaper may find broad utility in the development of a variety of nextgeneration bendable electronic devices.

2. EXPERIMENTAL SECTION 2.1. Materials. Cellulose powder (cotton linters), 2,2,6,6tetramethyl-1-piperidinyloxy (TEMPO), poly(ethylene glycol) diacrylate (PEG-DA, MW 575), poly(vinyl alcohol) (PVA, MW 130 000), ethyl viologen diperchlorate, sodium anthraquinone-2-sulfonate, potassium chloride, and borax anhydrous were purchased from SigmaAldrich. Sodium bromide (NaBr), a 12% sodium hypochlorite (NaClO) solution, sodium hydroxide (NaOH), ethyl alcohol, and hydrochloric acid (HCl) were purchased from Duksan Pure Chemicals Company. 4-(2-Hydroxyethoxy)phenyl-(2-hydroxy-238518

DOI: 10.1021/acsami.8b15230 ACS Appl. Mater. Interfaces 2018, 10, 38517−38525

Research Article

ACS Applied Materials & Interfaces

Figure 1. Fabrication of highly bendable conductive microelectrodes composed of AgNWs and cellulose nanopaper. (A) Schematic illustration of the fabrication of AgNW patterns on TEMPO-oxidized CNF paper: (i) preparation of TEMPO-oxidized CNF dispersion and transparent nanopaper; (ii) preparation of AgNW patterns on glass via PEG photolithographic process. Peeling off UV-exposed hydrogel layer left the unexposed AgNW region intact on the pristine AgNW-coated glass, leading to AgNW patterns; (iii, iv) transfer of the AgNW patterns from glass to TEMPO-oxidized CNF nanopaper through vacuum-assisted μCP and drying; (v) AgNW patterns on a transparent nanopaper. (B) (i) Photographs of transparent TEMPO-CNF paper, (ii) AgNW patterns on glass, and (iii) the resulting AgNW patterns on TEMPO-CNF paper. (C) (i) Photographic and (ii) microscopic images of AgNW micropatterns that were transferred to TEMPO-CNF paper via μCP without vacuumassistance. PEG hydrogel. The region of the AgNW film that was not exposed to UV light remained intact on pristine AgNW spin-coated glass to afford the AgNW patterns. The AgNW-patterned glass was washed with ethanol to remove the unexposed PEG precursor solution. 2.5. Fabrication of the AgNW Patterns on TEMPO-CNF Nanopaper. After the filtration of the TEMPO-CNF dispersion, the AgNW-patterned glass was carefully attached to the top of the wet TEMPO-CNF matrix for microcontact printing. Then, the TEMPOCNF matrix was vacuum filtrated for 10 min to remove air and water between the AgNW-patterned glass and the CNF matrix. A commercial vacuum pump (LAB 300, Lab Touch, vacuum rate: 675 mmHg) was used to provide uniform printing pressure between AgNW patterns and paper substrates. After filtration, the glass attached CNF matrix with its filter membrane was dried in an oven (75 °C) for 3 h under pressure with a low weight object (250 g). During drying, the AgNW patterns were uniformly transferred to a TEMPO-CNF matrix from glass. Then, the AgNW-embedded nanopaper was carefully peeled off the filter membrane. 2.6. Adhesion Tests. The tape test was manually carried out by attaching common 3 M tape to the AgNW-coated TEMPO-CNF paper (2.5 cm × 2.5 cm) and then removing it. This was repeated 12 times. AgNW-coated glass was also tested for comparison. A sonication test was carried out by soaking the AgNW-coated TEMPO-CNF paper (2.5 cm × 2.5 cm) in distilled water and sonicating for 2 min (SH-2140, Saehan-Sonic). AgNW-coated glass was also tested for comparison. 2.7. Flexibility Test. In the folding test, current−voltage (I−V) characteristics of AgNW-coated TEMPO-CNF paper (2.5 cm × 4 cm) were determined using a two-point probe method from −1 to 1 V with an electrochemical analysis device (PGSTAT204, Metrohm Autolab). In the bending cycle test, AgNW-coated TEMPO-CNF paper (2.5 cm × 1 cm) was bent at 80% strain and subsequently unbent, and this was repeated for 500 cycles. The sheet resistance of the sample was measured using a four-electrode system every 20 cycles and was compared to the initial value.

2.8. Light-emitting diode (LED) Bulb Test. Micro-patterned AgNW electrodes on TEMPO-CNF paper (2.5 cm × 2.5 cm) were prepared for the LED bulb test, and electrodes were connected to the AgNW patterns on TEMPO-CNF paper in a folded or unfolded state. A potential was applied via an electrochemical analysis device using chronoamperometry (potential: 2−3 V). 2.9. Fabrication of Solid-State Electrochromic Device (EC) Based on AgNW-Patterned TEMPO-CNF Nanopaper. A sandwich-type electrochromic (EC) device was fabricated with the AgNW pattern on TEMPO-CNF nanopaper as the working electrode and a bare ITO film as the counter electrode. To prepare a gel polymer electrolyte,42 electrochromic small molecule redox pairs of ethyl viologen diperchlorate (EV2+, 15 mmol/L) and sodium anthraquinone-2-sulfonate (AQS, 5 mmol/L) in combination with KCl (20 mmol/L) were dissolved in distilled water with 4 wt % PVA. This was mixed with 4 wt % borax in distilled water in a volume ratio of 4:1 to form the gel electrolyte. The resulting gel polymer electrolyte was spread on AgNW-patterned TEMPO-CNF nanopaper and then covered with a bare ITO film to fabricate the sandwich-type EC device (Figure 5C). An external potential of −2 and 0 V were applied to the EC device by the electrochemical analysis device (PGSTAT204, Metrohm Autolab) to achieve switchable color changes.

3. RESULTS AND DISCUSSION We developed a facile approach to construct highly bendable conductive microelectrodes where AgNWs were micropatterned on transparent cellulose nanopaper. Our approach is based on both UV-induced PEG photolithography and vacuum-assisted microcontact printing (μCP), which have been practically used for microfabrication. Figure 1A,B shows a schematic illustration and step-by-step photographic images of the fabrication of AgNW micropatterns on TEMPO-CNF paper. First, a highly transparent nanopaper (transmittance >85% at 550 nm) was fabricated through chemical 38519

DOI: 10.1021/acsami.8b15230 ACS Appl. Mater. Interfaces 2018, 10, 38517−38525

Research Article

ACS Applied Materials & Interfaces

Figure 2. Photographs and FE-SEM images of AgNW patterns with a line width of 500 μm on transparent TEMPO-CNF paper fabricated with (A−C) 0.3 wt % and (D−F) 0.075 wt % AgNW dispersed solutions. The surface density of (A−C) and (D−F) AgNW patterns is 51.6 and 12.9 μg/cm2, respectively. (D) White dotted line indicates optically transparent AgNW patterns. (G, H) Optical microscope images of AgNWs patterns with various sizes and shapes on TEMPO-CNF paper.

12.9 μg/cm2. The highly magnified SEM images of AgNW patterns (Figure 2C,F) indicate that AgNW networks were embedded in the surface layer of the nanopaper, which was strongly related to stability and flexibility of AgNW micropatterns on the nanopaper.46 Furthermore, the thickness of AgNW patterns was determined to be around 23 nm on glass and 13 nm on nanopaper by using atomic force microscopy (AFM) analysis, respectively (Figure S1), clearly demonstrating the AgNW-embedded nanopaper. This embedding phenomenon might occur when vacuum-assisted contact printing was conducted on wet nanopaper during the filtration process to draw in the wet state. In this study, 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO)-oxidized cellulose nanofibers (CNFs) were employed to fabricate the transparent nanopaper.47 We analyzed FT-IR spectra of three different types of cellulose materials to demonstrate TEMPO-mediated oxidation of primary alcohol to carboxylate in cellulose nanofibers. As observed in Figure S2A, the CO stretching at 1605 cm−1 (carboxylate) and 1740 cm−1 (acid) clearly appeared in TEMPO-oxidized CNFs, while these bands were absent in both mechanically treated CNFs and pure cellulose pulps.48 On the basis of more quantitative assessment of carboxylate groups via electric conductivity titration method, the carboxylate content on the TEMPO-CNF was determined to be 0.3 mmol/g. Furthermore, the zeta-potential at the TEMPO-CNF surface was calculated to be −33 mV, verifying the presence of carboxylate groups on the surfaces of TEMPO-CNF. This electrostatic repulsion between the negatively charged TEMPO-CNF leads to the formation of completely individualized TEMPO-CNF dispersed in water solution.49 The thermal stability of TEMPO-treated nanopaper was characterized by TGA analysis, and it was compared with mechanically treated nanopaper and pure cellulose pulps (Figure S2B). TEMPOtreated nanopaper started to degrade at around 220 °C, while mechanically treated nanopaper and cellulose pulp began a somewhat slower weight loss process around 250 and 300 °C, respectively.50 This result suggests that the decrease in thermal

defibrillation of cellulose pulp via TEMPO-mediated oxidation (Figure 1A-i and 1B-i). The TEMPO-oxidized CNFs were stably dispersed in aqueous solution because the introduction of a carboxylate in the C6 position decreased the inter-fiber hydrogen bonds and reduced the ability of the fibers to come into close contact (Figure 1A-i).43,44 In a separate step, AgNW micropatterns with various shapes and sizes were simply constructed on a glass substrate via PEG photolithography, as described in our previous studies.37,38,41 After detaching AgNWs along with the PEG hydrogel anchored on silanetreated glass from the region exposed to UV through a photomask, the UV non-exposed AgNW region remained on the glass, resulting in AgNW patterns on the glass (Figure 1A-ii and B-ii). In the final step, the AgNW-patterned glass substrate was carefully pressed against the wet TEMPO-oxidized CNF layer while performing vacuum filtration.45 After vacuumassisted μCP and complete drying for a couple of hours, we confirmed that the AgNW micropatterns were successfully transferred from the glass to the TEMPO-CNF paper (Figure 1A-iii,iv and B-iii). This step was specifically developed to create AgNW micropatterns which were embedded in the surface layer of the nanopaper, leading to strong AgNW adhesion to the nanopaper substrate, as discussed later. AgNW micropatterns were transferred from glass to the nanopaper substrate via μCP without vacuum-assistance to confirm the role of vacuumassistance for μCP. The photographs and FE-SEM images in Figure 2 show that the AgNW patterns with a line width of 500 μm were successfully constructed on transparent nanopaper via PEG photolithography and vacuum-assisted μCP. Additionally, we confirmed that the density of AgNW patterns, which is strongly associated with their optical and electrical properties, could be easily tuned by changing the concentration of AgNWs (0.3 and 0.075 wt %) dispersed in solution. As shown in the photographic image presented in Figure 2D, highly transparent AgNW micropatterns were constructed on transparent nanopaper by decreasing the surface density of AgNWs from 51.6 to 38520

DOI: 10.1021/acsami.8b15230 ACS Appl. Mater. Interfaces 2018, 10, 38517−38525

Research Article

ACS Applied Materials & Interfaces

Figure 3. (A) Optical transmittance of TEMPO-oxidized cellulose nanopaper and mechanically treated cellulose nanopaper. (B) Optical transmittance of AgNW-embedded TEMPO nanopaper depending on the surface density of AgNWs. (C) Transmittance at 550 nm and the sheet resistance of AgNW-embedded TEMPO nanopaper depending on the surface density of AgNWs. (D) Sheet resistance of AgNW-based micropatterns on glass before the direct transfer and TEMPO nanopaper after direct transfer via μCP.

Figure 4. Adhesion stability and mechanical flexibility of AgNW-embedded TEMPO nanopaper. (A) Changes in the sheet resistance of both AgNW-embedded TEMPO nanopaper and AgNW-coated glass as a function of (A) tape peeling cycles and (B) sonication time. (C) Current− voltage (I−V) characteristics of AgNW-embedded TEMPO nanopaper with different folding times (a folding angle is 90°). (D) Changes in the sheet resistance of AgNW-embedded TEMPO nanopaper as a function of bending cycles. The AgNW-embedded TEMPO nanopaper underwent bending around a diameter of 2 mm (around strain 80%).

stability was due to the introduction of carboxylation. However, it was noteworthy that TEMPO-treated nanopaper still showed better thermal stability compared to the common plastics widely used for transparent and flexible substrates, such

as polyethylene terephthalate, polyethylene naphthalate, and polycarbonate.51−54 It is worth noting that our transparent AgNW-patterned TEMPO nanopapers exhibited biodegradation behavior due to their natural cellulose fibers. In the soil 38521

DOI: 10.1021/acsami.8b15230 ACS Appl. Mater. Interfaces 2018, 10, 38517−38525

Research Article

ACS Applied Materials & Interfaces

Figure 5. Yellow and red LED light bulbs placed on highly transparent AgNW-patterned TEMPO nanopaper for (A) plate and (B) curved states (bending radius: 1.2 cm). AgNWs at a surface density of 12.9 μg/cm2 were patterned on TEMPO nanopaper via vacuum-assisted μCP for the LED test. The applied potential was 3 V. (C) Schematic illustration of the solid-state electrochromic (EC) device based on a sandwich structure of transparent AgNW patterns (“K” letter) on TEMPO nanopaper and the ITO film. Ethyl viologen diperchlorate was used as an active electrochromic small molecule. (D) The EC device was electrically switched from a dark purple state at −2 V to a transparent state at 0 V.

to 51.6 μg/cm2. The AgNW-embedded TEMPO-nanopapers prepared with 12.9 and 51.6 μg/cm2 surface density of AgNWs showed a sheet resistance of 54 and 11 Ω/sq with T550 nm values of around 82 and 69%, respectively. It is important to note that the AgNW-embedded TEMPO nanopaper with the smallest surface density of AgNWs (8.6 μg/cm2) represented the highest standard deviation value of 118 in the sheet resistance measurement, which means nonuniform AgNW distribution that was strongly associated with poor performance in electrode applications. Thus, the AgNW-embedded TEMPO nanopaper with 12.9 μg/cm2 surface density of AgNWs was the best substrate for transparent conductive microelectrodes for high-performance paper electronic devices. Figure 3D reveals almost the same sheet resistance for AgNWs coated on glass and embedded on nanopaper. This result demonstrates that AgNW-based micropatterns were completely transferred from the glass to the nanopaper without any damage to AgNW networks by the direct transfer of vacuum-assisted μCP. An investigation of the adhesion stability presented in Figure 4A,B demonstrated strong adhesion of AgNW networks on TEMPO nanopaper. In contrast to the AgNW spin-coated glass substrates which exhibit ∼8.9 and 3.7 times resistance increase after 1 peeling test and 1 s sonication, the sheet resistance of the AgNW-embedded TEMPO nanopaper remained almost unchanged even after 12 peeling test cycles and 120 s sonication (Figure 4A,B). To further examine the mechanical flexibility of AgNW-embedded TEMPO nanopaper, we analyzed the changes in electrical resistance during folding and bending tests. There were negligible changes in I− V curves and sheet resistance even though AgNW-embedded TEMPO nanopaper was bent three times to nearly 90 and

burial degradation tests, the AgNW-patterned TEMPO nanopaper was found to degrade slowly with a weight loss of 10% after 14 days (data not shown), which is similar to the reported degradation values for cellulose nanofibers.48,55 We next examined the mechanical properties of TEMPOCNF nanopaper of varying thickness by using stress−strain testing apparatus (see Supporting Information, Figure S3). The tensile strength was found to gradually increase from 15 to 79 MPa as the thickness of nanopaper increased from 18 to 65 μm, probably due to more fibers and more bonding surface in thicker nanopaper. These values of tensile strength of TEMPO-CNF paper were comparable to results reported for cellulose nanopaper.56,57 Figure 3 shows the transparent conductive performance of AgNW-embedded nanopaper substrates prepared by vacuumassisted μCP. TEMPO-oxidized nanopaper exhibited a much higher transmittance with a T550 nm of around 86% compared to only mechanically treated nanopaper with a T550 nm of around 39% (Figure 3A). Even though the transmittance of our TEMPO-oxidized nanopaper is slightly lower than 90% at 550 nm, this TEMPO-oxidized nanopaper possesses sufficiently high transmittance enough for practical applications (Figure 1B-i and iii). Such a large difference in transmittance is because TEMPO-nanopaper has denser nanostructures, causing reduced light scattering compared to mechanically treated nanopaper with nanopore structures. Improvement of both the optical transmittance and the electrical conductivity is very important to realize high-performance transparent conductive microelectrodes. As observed in Figure 3B,C, both the transmittance and the sheet resistance of AgNW-embedded nanopapers gradually decreased as the surface density of AgNWs increased from 8.6 38522

DOI: 10.1021/acsami.8b15230 ACS Appl. Mater. Interfaces 2018, 10, 38517−38525

ACS Applied Materials & Interfaces underwent a bending strain of 80% for 500 cycles (Figure 4C,D). These results strongly suggest that AgNW networks embedded on TEMPO nanopaper have good adhesion stability and excellent bending durability. As shown in Figure 5A,B, we have demonstrated that flexible AgNW micropatterns on TEMPO nanopaper prepared with 0.75% AgNW can be used as a transparent electric circuit to turn on LED bulbs. Furthermore, we have fabricated the solidstate electrochromic (EC) device to demonstrate the usability of our transparent AgNW-patterned TEMPO-CNF paper for electronics applications.42 As shown in Figure 5C, the AgNW pattern of “K” letter is used as a working electrode for EC devices with a sandwich structure. The absorption spectra and representative photographs presented in Figures S4 and 5D revealed that the “K” letter at −2 V was a deep purple color state while the “K” letter at 0 V was bleached in the visible region. This reversible electrochromism resulted from the redox reaction of viologen on the AgNW electrode of the “K” letter.58 This result indicates the broad utility of our AgNWpatterned TEMPO nanopapers as transparent flexible electrodes.

ACKNOWLEDGMENTS

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b15230.

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This study was supported by a grant from Kyung Hee University in 2017 (KHU- 20171194).

4. CONCLUSIONS In summary, we developed an efficient approach based on PEG photolithography and vacuum-assisted μCP to construct versatile transparent conductive microelectrodes. The TEMPO-treated cellulose nanopapers served as a transparent and flexible substrate for the conductive AgNW micropatterns. The AgNW micropatterns on glass fabricated by PEG photolithography were transferred completely to TEMPOnanopaper via a vacuum-assisted μCP method. The resulting AgNW micropatterns on TEMPO-nanopaper exhibited good adhesion stability and excellent mechanical bendability along with the control of transmittance and conductivity. We anticipate that our approach will be widely applicable for highly efficient micropatterning processes and will be valuable in constructing highly flexible conductive microelectrodes for transparent paper electronics.

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Research Article

AFM analysis of AgNWs on glass and TEMPO-CNF nanopaper, FT-IR spectra and TGA plots of cellulosebased materials, strain−stress curves of TEMPO-treated paper, absorption spectra of the EC device, a comparison of conductive patterns on cellulose-based papers (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-31-201-2626. Fax: +8231-204-8117. ORCID

Jungmok You: 0000-0002-9583-2242 Author Contributions †

D.K. and Y.K. contributed equally to this work.

Notes

The authors declare no competing financial interest. 38523

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ACS Applied Materials & Interfaces

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