Flexible Transparent Sliced Veneer for Alternating Current

Jun 4, 2019 - If transparent wood could be used as the substrate for ACEL ... a weight ratio of 2:1) was stirred by magnetic force at 20 °C for 10 mi...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11464−11473

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Flexible Transparent Sliced Veneer for Alternating Current Electroluminescent Devices Tao Zhang,†,‡ Pei Yang,†,‡ Yanzhen Li,§ Yizhong Cao,†,‡ Yunlei Zhou,§ Minzhi Chen,†,‡ Ziqi Zhu,†,‡ Weimin Chen,*,†,‡ and Xiaoyan Zhou*,†,‡ †

College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China Fast-growing Tree & Agro-fibre Materials Engineering Center, Nanjing 210037, China § College of Engineering and Applied Sciences, National Laboratory of Solid State Microstructure, and Collaborative Innovation Centre of Advanced Microstructures, Nanjing University, Nanjing 210093, China Downloaded via GUILFORD COLG on July 26, 2019 at 06:24:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Flexible alternating current electroluminescent (ACEL) devices based on nanocellulose-derived transparent films have received a great deal of attention due to their numerous merits. However, the fabrication of nanocellulose-based functional materials usually requires complex treatments. Herein, we used transparent sliced veneer (TV) derived from commercial birch wood to fabricate ACEL devices. In comparison to nanocellulosebased transparent films, the preparation of TV is faster and cheaper. The total transmittance achieved a considerable value (up to 86%) compared to that of cellulose-based transparent films. To obtain a conductive transparent wood composite film, silver nanowires (AgNWs) were deposited on the TV substrate using a spray gun. The transmittance was 79.5% at an AgNWs area density of 450 mg/m2, and it displayed a low sheet resistance of 3 Ω sq−1. Finally, we successfully fabricated an ACEL device based on the conductive TV. The flexible transparent veneer-based alternating current electroluminescent (WACEL) device possessed waterproofing properties without the need for additional sealing. The luminance of the WACEL device approached a maximum value of 18.36 cd/m2 at a voltage of 220 V (frequency at 1k Hz), demonstrating its great potential for applications in signal expression and ambient lighting. KEYWORDS: Transparent sliced veneer, Silver nanowire, Commercial birch veneer, Flexible electroluminescent devices, Conductive wood

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over, in comparison to glass, plastic films are advantageous in terms of a simple manufacturing process, light weight, high stability, and excellent tortuosity. However, plastic wastes have been increasingly generated with the rapid updating of the electronic products. Another concern is that plastics, especially polyethylene terephthalate, are made from petrochemical products with limited resources. Therefore, the relevant nanocellulose products, such as cellulose-based transparent films and transparent paper, are regarded as promising alternatives for plastic films by virtue of their natural degradation, abundant raw materials, and high transparency,13−15 thus demonstrating their potential application in the fabrication of ACEL devices.16,17 Another effective method is to develop a transient device that can alleviate the environmental problems caused by electronic waste. Transient electronic devices are based on soluble materials or soluble

INTRODUCTION Recently, advanced light-emitting devices, especially flexible electroluminescent devices, have been considered one of the hottest research fields, owing to their wide application in largescale decoration, signal expression, ambient lighting, etc.1−6 Stretchable and flexible ACEL devices, which were studied by the Lee group,7 hold promise for applications in terms of stretchable light sources and soft robotics.8,9 As the basic support of these devices, the conductive substrate endows flexible alternating current electroluminescent (ACEL) devices with excellent mechanical properties. Glass, as a traditional conductor substrate, has been widely used in various lightemitting devices and electronic displays due to its high optical transparency (>80%) and dimensional stability. However, the rigidity and brittleness of glass lead to bending fracture in practical applications. Plastic films,10 transparent cellulose-based films,11 and paper12 have been reported as potential replacements for glass as the substrate for ACEL devices. Such plastic films could be commercially produced on a large scale, and the corresponding manufacturing costs would be reduced. More© 2019 American Chemical Society

Received: March 5, 2019 Revised: May 27, 2019 Published: June 4, 2019 11464

DOI: 10.1021/acssuschemeng.9b01129 ACS Sustainable Chem. Eng. 2019, 7, 11464−11473

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic of the preparation of flexible TV and the sample images under natural light. The total fabrication duration of flexible TV requires 4 h, including 2.5 h to remove lignin, 0.5 h to impregnate the epoxy resin, and 1 h to further polymerization.

an ACEL device using the flexible conductive transparent sliced veneer (FCTV) as a key component and established two illumination modes to demonstrate its capacity in building ACEL devices.

substrates, which can dissolve naturally in a short time. Currently, electrode materials including AgNWs and carbon nanotubes have been proven useful as soluble electrodes.18,19 Typically, previous studies have demonstrated that nanocellulose-based functional materials, especially those derived from wood, are fabricated via complex treatments.20 Traditional cellulose production involves chemical and mechanical methods, which, however, require complicated purification, alkali, and bleach treatments.21,22 In short, the fabrication of cellulose-based functional materials has usually required 50 h and more than 6 types of chemicals (e.g., sodium sulfate, sodium sulfite, and sodium hydroxide). These methods are expensive, time-consuming, and environmentally unfriendly.23 Transparent wood has drawn increasing attention as an emergent material owing to its low cost, high transparency, and high tensile strength.24,25 Recent years have witnessed great progress in the preparation of transparent wood for applications including heat shielding windows,26 luminescent transparent wood,27,28 smart windows,29 and centimeter-thick transparent wood.30 According to a previous study, it has been difficult for transparent wood to replace glass because the haze makes transparent wood show a high transmittance only when it is close to the surface of a material. If transparent wood could be used as the substrate for ACEL devices, the high haze would have no effect on the luminescence properties of the ACEL devices because of the close combination of wood substrates. Indium tin oxide (ITO), as a traditional conductor substrate, has been widely used in various light-emitting devices and electronic displays due to its high optical transparency (>80%), dimensional stability, and low sheet resistance (15 sq−1).31,32 However, the brittleness of ITO would lead to bending fracture in practical applications. In addition, the lack of indium resources and the complex manufacturing processes result in high production costs.33−40 Compared with ITO, silver nanowires (AgNWs), as a typical one-dimensional metal nanomaterial, present excellent mechanical flexibility, good transparency, and a high conductivity.41−43 AgNWs with a high length-to-diameter aspect ratio can form a low-area-density network structure. Therefore, to obtain a conductive transparent wood composite film, AgNWs were deposited on the flexible TV using a spray gun. Motivated by this result, we set out to design

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EXPERIMENTAL SECTION

Materials. Sliced birch veneer with a size of 50 mm × 30 mm × 0.25 mm was purchased from Jinhu Hongda Wood Industry Co., Ltd. (Jinhu, China). Sodium chlorite (NaClO2, Aladdin Biochemical Co., Ltd. (Shanghai, China)) and acetic acid (CH3COOH, Nanjing Chemical Reagent Co., Ltd. (Nanjing, China)) were used to remove lignin from birch veneer. Ethanol (Nanjing Chemical Reagent Co., Ltd.) was used as a solvent. Epoxy resin (Huasheng Tong Chuang Technology Co., Ltd. (Shenzhen, China)) was used as an infiltration polymer. Electrode patterns were formed by spraying AgNW dispersions (diameters of 70−120 nm and lengths of 30−50 μm, purchased from Shanghai Bohan Chemical Technology Co., Ltd. (Shanghai, China)). The AgNW solution was diluted in ethanol to a concentration of 1.0 mg·mL−1 prior to use. The EL particles were ZnS:Cu (purchased from Shanghai KPT company). Preparation of Delignification Sliced Wood Veneer. Birch veneer samples were first soaked in anhydrous ethanol. The veneer samples were extracted using 2 wt % of NaClO2 with an acetate buffer solution to reach a pH value of 4.6. Then, the birch veneer samples were immersed in a lignin removal solution at 85 °C until a white color was achieved. Subsequently, the delignified birch veneer samples were rinsed with hot distilled water (60 °C) several times to remove residual chemicals. Finally, the removed lignin samples were preserved in anhydrous ethanol for further use. Flexible Transparent Sliced Veneer (TV) Preparation. The preparation process for flexible TV is shown in Figure 1. Briefly, a delignified birch veneer sample was sandwiched between two glass slides and placed on a heating plate for 10 min at 60 °C to evaporate the anhydrous ethanol. Afterward, the delignified veneer was immersed in the liquid epoxy resin (two liquid components (#3601 epoxy resin and #3601 tetraethylenepentamine hardener) at a mass ratio of 1:1) under vacuum at 20 °C for 10 min to remove air. After the vacuum release, epoxy resin filled the wood structure by atmosphere pressure. This process was repeated three times to ensure sufficient infiltration. The veneer samples were taken out, and a polyethylene film was placed on both sides. Finally, the birch veneer samples were sandwiched between two glass slides and then cured at 60 °C for further polymerization. Fabrication of the Flexible Transparent Sliced Veneer-Based Alternating Current Electroluminescent (WACEL) Device. The 11465

DOI: 10.1021/acssuschemeng.9b01129 ACS Sustainable Chem. Eng. 2019, 7, 11464−11473

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ACS Sustainable Chemistry & Engineering

Figure 2. (a) FTIR spectra of the original wood and delignified wood, respectively. (b) Partially enlarged FTIR spectra of the original wood and delignified wood, respectively.

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structure of the WACEL device is shown in Figure S1 (Supporting Information). The WACEL device was fabricated using TV composite films as both bottom and top electrodes. It is a standard three-layer structure: TV/AgNW composite filmZnS:Cu embedded in epoxy resinAgNW/TV composite film cap. The preparation of the FCTV composite film was as follows: First, AgNWs were sprayed on the flexible TV using a spray gun. Then, birch veneer sample was heated for 10 min on a heating plate at 60°. Copper belt was glued at the edges of the AgNW networks as an external electrical circuit. The liquid epoxy resin ((two liquid components (# 3601 epoxy resin and # 3601 tetraethylenepentamine hardener) at a ratio of 1:1)) was kept under magnetic stirring at 20 °C for 5 min. Then the mixture of ZnS:Cu particles and epoxy resin (in a weight ratio of 2:1) was stirred by magnetic force at 20 °C for 10 min. Luminescent layers were spincoated (spin velocity 3000 rpm, duration time 30 s, temperature 25 °C) onto the bottom electrode. An additional AgNW/TV film was bonded to the luminescent layer to form a sandwich WACEL device before epoxy resin was fully cured. The lamination was conducted for 3 min under 80 kPa pressure (carried out in an automatic vacuum laminator). This process was accomplished in a vacuum environment for the purpose of ensuring intimate contacts between different functional layers without leaving bubbles. Characterizations. The surface and cross section of the flexible TV samples were characterized by a field-emission scanning electron microscope (FE-SEM, JSM-7600F, JEOL, Japan) operating at an acceleration voltage of 15 kV. The component changes in the birch veneer samples were determined on a Bruker FTIR spectrometer (VERTEX 80 V, Bruker, USA) from 4000 to 500 cm−1. The transmittance of the FCTV was measured with a Lambda 950 UV− vis spectrometer (PE, USA) within a spectrum range of 350−800 nm. To study the mechanical properties, an Instron 5966 testing machine was used at a cross-head speed of 5 mm min−1. Wood specimens were cut into dimensions of 50 mm × 5 mm × 0.25 mm. To ensure the accuracy of experiment, at least five transparent sliced veneer specimens were tested. The transparency measurement was according to “ASTM D1003 Standard Method for Haze and Luminous Transmittance of Transparent Plastics” (eq S(1), Supporting Information). The sheet resistance was measured using a Keithley 2110 digital multimeter with a four-point probe. The luminescence spectrum of the WACEL device was measured using a spectrophotometer (Photo Research, PR-655, USA). A high-voltage amplifier (Trek 10/10BHS) and a function generator (GW Instek AFG-2500) were used to power the WACEL device. A self-constructed motorized linear translation stage (acrylic sheets) was employed for the bending test. To measure the luminance of the WACEL device, a Konica Minolta CS-150 Luminance and Color Meter was used. The relation between EL intensities and bias voltages was evaluated by eq S(2) (Supporting Information).

RESULTS AND DISCUSSION Delignification Process for Birch Veneer Samples. In general, original wood is nontransparent, which is related to the structure and composition of wood. Although the structure of wood varies with growth location and environment, regularly aligned channels along the wood growth direction are a common feature of all types of wood. Because of the difference in refractive indices between birch wood cell wall and air, light scattering occurs when a beam of light passes through wood, thus changing the path of light propagation.44 Second, lignin possesses color as well as a strong absorptive capacity for light. Therefore, removing lignin from wood is the basis for the preparation of TV. Herein, we use commercial sliced birch veneer as the starting material. The most direct criterion of the delignification effect can be judged by color change of the wood sample, as shown in Figure S2a (Supporting Information). The original wood is yellowish, while the color of birch veneer sample turns white after lignin is removed. This result is attributed to the inherently colorless cellulose, hemicellulose, and successful removal of lignin.25 Figures S2c and S2d (Supporting Information) illustrate the wood microstructure before and after sodium hypochlorite treatment. From the cross section of the birch wood, it can be observed that empty lumen is tightly surrounded by the cell wall (Figure S2c, Supporting Information). Middle lamella acts to adhere adjacent wood cells. Lignin, the main component of the middle lamella, is considered an adhesive for cellulose and hemicellulose.20 Clearly, removal of lignin destroys the middle lamella, which leads to the release of stress and strain in wood structures and is accompanied by the formation of large voids during the drying process.24 Figure 2a shows the FT-IR spectra of the original wood and delignified wood. The broad absorption band at 3431 cm−1 represents the stretching vibration of hydroxyl groups. The peak at 2934 cm−1 can be attributed to the methyl, methylene, or methine stretching vibration. The emerging peak at 1737 cm−1 is related to the acetyl and uranic ester groups of hemicellulose or to the ester linkage of carboxylic groups of the ferulic and pcoumaric acids of lignin or hemicellulose, and the absorption band at 1638 cm−1 is assigned to the CO stretching vibration. The characteristic bands located at 1505, 1463, and 1425 cm−1 indicate the presence of aromatic rings in original wood sample.45 A partially enlarged FTIR image is shown in Figure 11466

DOI: 10.1021/acssuschemeng.9b01129 ACS Sustainable Chem. Eng. 2019, 7, 11464−11473

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Figure 3. (a) and (d) Optical image of original wood and flexible TV. (b) and (e) Microstructure of original wood and TV under low magnification. (c) and (f) Magnification SEM images of the cell wall of original wood and TV, respectively.

Figure 4. (a) Optical transmittance of original wood, delignification wood, and flexible TV. (b) Stress−strain curve for evaluating the mechanical properties of original wood and flexible TV.

which is essential for the rapid infiltration, as marked in Figures 3c and f (before and after resin impregnation). Optical Transmittance and Mechanical Properties of the Flexible TV Film. The optical transmittance of substrate ensures the light emitted by WACEL device passes through TV substrate effectively. The total transmittance of original wood is only 1.3% at 800 nm, which is almost negligible, mainly because of the strong absorption of lignin. As previously discussed, the transmittance of delignified wood is up to approximately 3.4% at 800 nm, and the reason why delignified wood is still nontransparent has been explained. In contrast, TV exhibits a significantly high transmittance after impregnating with epoxy resin, as illustrated in Figure 4a. The total transmittance can reach a maximum value of 86% at 800 nm. Based on the above analyses, it can be concluded that TV obtained in this experiment exhibits a comparable transmittance with that of nanocellulose films via a simpler process (only requiring 4 h).21 An evaluation of the mechanical properties of flexible TV is crucial for its application in electroluminescent devices. For the original wood, the fracture strength is up to 21.3 MPa, while the fabricated TV can reach up to 59.7 MPa; such a high strength is almost triple that of the original wood template (Figure 4b). The increase in strength is ascribed to the flexible transparent epoxy resin filling the pore space between cellulose nanofibers and improving the stress transfer.46

2b, and it is found that the characteristic peaks located at 1505, 1463, and 1425 cm−1 have barely any diagnostic signals in the FTIR spectrum of the delignified veneer, suggesting the successful removal of lignin from the veneer sample. Fabrication of the Flexible TV. Although the lignin was removed, the interface between wood cell wall (refractive index of cellulose ≈1.53) and air (refractive index of air ≈1.0) also cause a large light-scattering event in the visible range. Therefore, we choose a flexible transparent epoxy resin (refractive index ≈1.5) as the filler to greatly reduce the light scattering. Figures 3a and 3d show the images of original wood and TV, respectively (a 0.25 mm thick veneer with a size of 5 mm × 3 mm). Clearly, the fabricated TV exhibits a high transmittance in comparison to that of original birch veneer sample. The SEM image in Figure 3b shows that original wood has identical empty channels before impregnating with epoxy resin. After the vacuum was released, delignified wood channels are entirely filled by transparent epoxy resin (Figure 3e). The magnified SEM image shows that epoxy resin and wood cell wall are clearly distinguished. The flexible transparent epoxy resin can react strongly with cellulose and hemicellulose via hydrogen bonds or van der Waals forces,25 ensuring a good bonding strength between the epoxy resin and wood cells (Figure 3e). It should be noted that infiltration and delignification process do not destroy the channels of the original wood microstructures, 11467

DOI: 10.1021/acssuschemeng.9b01129 ACS Sustainable Chem. Eng. 2019, 7, 11464−11473

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Figure 5. (a) Total transmittance of flexible conductive transparent sliced veneer. (b) Stress−strain curves of flexible conductive transparent sliced veneer.

Figure 6. (a) Actual performances of electrical conductivity. (b) Sheet resistance of AgNWs with 100, 150, 200, 250, 300, 350, 400, and 450 mg/m2, respectively. (c) Characterization of AgNW/TV electrodes. SEM images of the polymerized TV electrode (left) and unpolymerized TV electrode (right). (d) The bending test for 500 cycles.

Fabrication of the FCTV. Given the excellent light transmittance and ideal mechanical properties, flexible TV is a

fascinating and promising substrate that can be combined with conductive materials to prepare flexible conductive composite 11468

DOI: 10.1021/acssuschemeng.9b01129 ACS Sustainable Chem. Eng. 2019, 7, 11464−11473

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ACS Sustainable Chemistry & Engineering

Figure 7. Design of electroluminescent device based on transparent flexible conductive wood substrate. (a) Schematic diagram of the main preparation of ACEL devices based on flexible conductive transparent sliced veneer. (b) Graphics and digital two modes of light emitting devices. (Photographs of the WACEL device at a voltage of 110 V, frequency of 1 kHz.) (c) Statistics of ZnS:Cu particles diameter with the average of 17.78 μm. Inset: Representative SEM image of ZnS:Cu particles. (d) X-ray diffraction pattern of ZnS:Cu particles. (e) SEM images of the ZnS:Cu/epoxy resin luminescent layer. Inset: Pictures of the ZnS:Cu particles before and after dispersion in epoxy resin.

for bending test. The fabricated TV composite film was tested for 1000 cycles in the original length and showed high damage resistance even completely bent to 180°, thus revealing its good mechanical durability and flexibility (Figures S4a and S4b, Supporting Information). In addition, TV can be completely wrapped around a 20 mm diameter test tube, bent into a “S” shape, bent into a “triangle” shape, completely wrapped around the finger, or folded (Figure S4c, Supporting Information). Sheet Resistance and Transmittance of the Transparent AgNW/TV Film. For a flexible transparent conductive electrode, especially when the equipment contains nanometer

films. Figure S3a (Supporting Information) shows a schematic of the preparation of FCTV. Figures S3b and S3c (Supporting Information) show the ON and OFF conditions of LED bulbs with or without contacting the FCTV. In contrast to the nonconductive TV, the transmittance of FCTV is decreased to 79.5% at 800 nm (Figure 5a), and such a performance can be attributed to light scattering caused by the difference in refractive indices between the TV and AgNWs. As shown in Figure 5b, the tensile strength of FCTV is 60.5 MPa, which is basically equal to the TV substrate (59.7 MPa). A selfconstructed motorized linear translation stage was employed 11469

DOI: 10.1021/acssuschemeng.9b01129 ACS Sustainable Chem. Eng. 2019, 7, 11464−11473

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ACS Sustainable Chemistry & Engineering

Figure 8. (a) Electroluminescent spectrum of the WACEL device. (b) Dependence of voltage and luminance of the WACEL device on different excitation frequencies. (c) Number of bending tests were conducted 400 times. Illustration: the actual bending image of the WACEL device. (d) Lightemitting stability by hollowing out, cutting the substrate in half with, or cutting off a part.

Fabrication of the WACEL Device. Encouraged by the above results, we set out to design an ACEL device based on FCTV. The schematic diagram of the preparation is shown in Figure 7a. The ACEL devices are principally dependent on excitation by an electric field. Accelerated carriers cause the excitation or ionization of luminescent centers, resulting in electron−hole pairs. The exciton relaxation produced by radiative recombination results in luminescence.48 Owing to the transparency of the two electrodes, the middle emitting region can be observed at the top and bottom sides (Video S1, Supporting Information). In this paper, AgNWs were sprayed into different shapes to form two electroluminescent patterns, including a rectangle and the number “151” (Figure 7b). Figure 7c shows that the average diameter of ZnS:Cu particles is 17.78 μm. X-ray powder diffraction (XRD) in Figure 7d confirmed that ZnS:Cu particles have a wurtzite structure (JCPDF No. 361450). The dispersion uniformity of ZnS:Cu in the epoxy resin affects the luminescent quality of the WACEL device. From the SEM image (Figure 7e), we can observe that the as-prepared EL layer consists of uniformly dispersed ZnS:Cu microparticles. To further prove the uniformity of dispersion, we spin-coated the luminescent layer on the FCTV substrate and then placed the resulting ZnS:Cu/epoxy resin luminescent layer under a commercial UV lamp to evaluate the uniformity of ZnS:Cu particles; as a control, the pure epoxy resin film and TV composite film were also luminated by UV light (Figures S6a− S6c, Supporting Information). It can be seen that the pristine

metal wires used as conductive materials, the sheet resistance is very important to evaluate electrode conductivity. It is worth mentioning that the LED bulbs can still give off bright light no matter whether they are bent (0°∼180°) or twisted, as shown in Figure 6a. In addition, the effect of the area density of the AgNWs on the sheet resistance is also investigated systematically. As illustrated in Figure 6b, the sheet resistance decreases from 75 to 3 Ω sq−1 by increasing the area density in the range of 100−450 mg/m2. The large sheet resistance between AgNWs is the common bottleneck in conductivity. When AgNWs were directly sprayed on the surface of TV (epoxy resin was fully polymerized), the hydrophobic surface prevented AgNWs from completely adhering to TV substrate, as shown in Figure 6c and Figure S5 (Supporting Information). To ensure stable conductivity of TV, we sprayed AgNW on the substrate before the epoxy resin was fully polymerized. The sheet resistance changes slightly during the bending test likely due to the bonding of the epoxy resin improving the interconnection between the AgNWs (Figure 6c). Even after 500 bending tests, the sheet resistance presents no change, which indicates that the FCTV possesses an excellent conductivity stability (Figure 6d). In addition, ZnS:Cu/epoxy resin composite layer can be well embedded in conductive structure. The position of AgNWs will not change during repeated bending and releasing progress, which is beneficial to the stable electrical conductivity of whole device.47 11470

DOI: 10.1021/acssuschemeng.9b01129 ACS Sustainable Chem. Eng. 2019, 7, 11464−11473

Research Article

ACS Sustainable Chemistry & Engineering epoxy resin film has no light signal, whereas the TV composite film displayed weak luminescence because of the residual of lignin. In contrast, the ZnS:Cu/epoxy resin luminescent layer exhibited strong and uniform cyan luminescence (Figures S6d− S6f, Supporting Information). In addition to the uniform luminescence, the SEM image of the luminescent layer also confirmed the good dispersity the of ZnS:Cu particles (Figure S6i, Supporting Information). Figure S7a (Supporting Information) shows the WACEL device is a standard three-layer structure. The thicknesses of the luminescent layer and the whole device are approximately 60 and 560 μm, respectively. Generally, when an ACEL device is subjected to bending or twisting, the interface is more vulnerable than the other parts of the device. Consequently, under different mechanical deformation conditions, the bonding strength is the key to ensuring device operation. In our experiment, since luminescent particles are directly dispersed in epoxy resin, the bonding strength between the luminescent layer and substrate can be guaranteed (Figure S7c, Supporting Information). Normally, the ZnS:Cu/epoxy resin composite layer can be embedded in a conductive structure. The position of the AgNWs do not change during repeated bending and releasing operations, which ensures the stable electrical conductivity of the whole device.48 Meanwhile, the presence of the composite layer greatly avoids contact between the AgNWs and air, protecting the AgNW networks from oxidation (Figure S7d, Supporting Information). The morphology of the AgNWs is examined and shown in Figure S7e (Supporting Information). The AgNWs exhibit a diameter of 90 nm. The EL emission spectrum of the WACEL is presented in Figure 8a. The luminescence peak is centered at 487 nm with a fwhm (full width at half-maximum) of 89 nm. The position of Commission Interationale de L’Echairage (CIE) color coordinates (x = 0.1637, y = 0.2785) demonstrates that light emitted from the WACEL device is in the blue-light region. Alternating voltage from the commercial alternating current electroluminescent driver (purchase from Shanghai KPT company) shows a largely distorted square waveform with amplitude of ∼120 V and a frequency of 1k Hz (Figure S8a, Supporting Information). Square wave voltage (120 V, 1k Hz) was generated by the high-voltage amplifier coupled with a function generator, which exhibits a high-fidelity waveform (Figure S8b, Supporting Information). Figure 8b shows the correlation between the luminance and applied voltages (from 25 to 220 V) under different excitation frequencies for the WACEL device. The WACEL device initiates light emission at a bias voltage of approximately 70 V. The luminance can achieve 18.36 cd/m2 at an applied voltage of 220 V (frequency at 1k Hz). The luminance values reach 9.31 cd/m2 at 400 Hz, 27.37 cd/m2 at 2k Hz, 38.40 cd/m2 at 5k Hz, and 54.34 cd/m2 at 10k Hz, revealing the monotonic increase as a function of the excitation frequency. The flexible performance was further evaluated under repeated bending cycles (a bending angle of 55°). The light intensity decreases as the WACEL device is bent, mainly due to the device being far away from the Luminance Meter in the bent state. The cycle number indicates a gradual decrease in brightness with repetitive bending, as shown in Figure 8c. The emission intensity is maintained at 80.77% of the original value after 400 times of flexing cycles. The actual bending state of the WACEL device is shown in Video S2 (Supporting Information). Figure 8d further shows the light-emitting stability by presenting images of the hollowed out from WACEL device, by cutting the substrate in half and by removing the WACEL device a part. Therefore, the

WACEL device designed in this paper has excellent durability to withstand external impacts and unexpected damages. To test the waterproof performance, we seal the voltage input electrode with tape to protect it from water and only expose the lightemitting part of the device (Figure S9, Supporting Information). The WACEL device exhibits waterproofing properties without any treatment. Even when the WACEL device is completely immersed in water, the brightness remains unchanged (Video S3, Supporting Information). Hence, it can be concluded that the fabricated WACEL device shows promise for application in waterproof ACEL devices.

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CONCLUSION In conclusion, we used commercial birch veneers to fabricate TV, which served as the substrate for ACEL devices. Compared with nanocellulose-based transparent films and transparent paper, the flexible TV with a light transmittance of 86% can be obtained by a simple treatment within no more than 5 h. The flexible conductive wood exhibits a low sheet resistance (3 Ω sq−1 at a density of 450 mg/m2) and an excellent high transmittance (79.5% at 800 nm). By controlling the spraying range between the AgNWs and the top electrode, we successfully designed two different luminescent modes, including a rectangle and the number “151”. The luminance of the WACEL device can reach a maximum value of 18.36 cd/m2 at a voltage of 220 V (frequency at 1k Hz). The WACEL device exhibits waterproofing properties without additional sealing process. Our results also show that the WACEL device designed in this paper has excellent durability to withstand external impacts and unexpected damages.

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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/acssuschemeng.9b01129. Middle emitting region observed at the top and bottom sides (AVI) Bending state of the WACEL device (AVI) WACEL device completely immersed in water, with the brightness remaining unchanged (AVI) Formula for calculating transmittance and brightness; deformation and measurement of flexibility of the TV; the average surface roughness and water contact angle of the TV; uniformity characterization of ZnS:Cu dispersed in epoxy resin; the waveform of the drive voltage; waterproof testing of the WACEL devices (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Tao Zhang: 0000-0001-8330-5018 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (Grant No. 31870549), the Jiangsu Nature Science Foundation (BK20161524), the Program for 333 11471

DOI: 10.1021/acssuschemeng.9b01129 ACS Sustainable Chem. Eng. 2019, 7, 11464−11473

Research Article

ACS Sustainable Chemistry & Engineering

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Talents Project in Jiangsu Province (Grant No. BRA2016381), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_839), and the Advanced Analysis and Testing Center of Nanjing Forestry University.

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DOI: 10.1021/acssuschemeng.9b01129 ACS Sustainable Chem. Eng. 2019, 7, 11464−11473

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DOI: 10.1021/acssuschemeng.9b01129 ACS Sustainable Chem. Eng. 2019, 7, 11464−11473