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Transparent Wood Film Incorporating Carbon Dots as Encapsulating Material for White Light-Emitting Diodes Zhihao Bi, Tuanwei Li, Hui Su, Yong Ni, and Lifeng Yan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01618 • Publication Date (Web): 10 Jun 2018 Downloaded from http://pubs.acs.org on June 10, 2018

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ACS Sustainable Chemistry & Engineering

Transparent Wood Film Incorporating Carbon Dots as Encapsulating Material for White Light-Emitting Diodes Zhihao Bi, Tuanwei Li, Hui Su, Yong Ni, Lifeng Yan * CAS Key Laboratory of Soft Matter Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, and Department of Chemical Physics, iCHEM, University of Science and Technology of China. Hefei, 230026, P.R.China. Fax: +86-551-63603748; Tel: +86-551-63606853; E-mail: [email protected]

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ABSTRACT. Epoxy resins are the main encapsulation materials for light-emitting devices (LEDs) owing to their high transparency, appropriate mechanical strength, and excellent thermal stability. However, environmental benign materials needed to be developed with improving performances. Transparent wood and its nanocomposites prepared from natural biomass are potential alternative materials to them. Green preparation of transparent wood incorporating with trichromatic systems is an attractive topic, especially for white LED (w-LED). Here, multiple-color-emission carbon dots (CDs), serving as trichromatic systems in w-LEDs, were synthesized by tuning the extent of graphitization and surface function of the nanoparticles using citric acid and urea as feedstocks. Then, a green, facile and energy-efficient method for the preparation of carbon dots/transparent wood (CDs-TW) composites was raised by ultrafast removing lignin from wood using deep eutectic solvent (DES, oxalic acid and choline chloride) under the microwave-assisted treatment, and then CDs and polyacrylic acid (PAA) were filled into the delignified wood through an in-situ polymerization. The transparent wood film embedding multi-color CDs was fabricated, which showed white light emission under ultraviolet light excitation and enhanced mechanical tensile strength (60.92 MPa). Simultaneously, the as-prepared film can be used as an encapsulation film for white LEDs, which exhibited excellent color characteristics with the commission internationale eclairage (CIE) color coordinates, a correlated color temperature (CCT), and a color rendering index(CRI) of (0.33,0.32), 5237K, and 83, respectively. It provides a simple route to prepare metal-free wood based encapsulating materials for w-LEDs. 2


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Keywords. Transparent wood, deep eutectic solvent, Carbon Dots, encapsulating materials, white LED.

Introduction. Environment-friendly, electricity-saving and excellently stable solid state lightings are in compliance with the green and sustainable development perspective.1, 2 Among numerous devices, white light-emitting diodes (w-LEDs), especially the blue LED chips with yellow phosphors,3 have been used widely in many fields.4 w-LEDs are usually composed of LED chip, trichromatic systems, and encapsulation material. Epoxy resins have been frequently used as the encapsulation materials owing to their high transparency, appropriate mechanical strength, excellent thermal stability and inexpensiveness.5 However, the rather brittle, easily suffer from yellowing during thermal process and hydrophilic of epoxy resins impel us continuously to exploit new green alternative encapsulation materials for LEDs.6 Transparent wood can be prepared directly by removing lignin from wood, followed by filling the vascular structures and pores inside the wood with suitable matrix, such as polymers with similar refractive index.7-9 For example, Hu et al. successfully prepared a series of transparent wood composites by removing lignin using sodium hydroxide, sodium sulfite and hydrogen peroxide, and then infiltrating suitable epoxy resin as the matrix.10, 11 In recent years, although numerous methods have been developed for removing lignin from wood while retaining the cellulose concurrently, it is still a challenge to break the bonds between lignin and carbohydrate 3



through a low-cost and green process.12 Deep eutectic solvents (DESs), a series of transparent liquid eutectic mixtures that obtained through strong hydrogen-bonding interactions between hydrogen-bond acceptors (HBA) and hydrogen-bond donors (HBD),13, 14 have been considered to be a new class of promising green solvent for their properties of both organic solvents and ionic liquids,15, 16 which can partially or entirely dissolve lignin of wood.17, 18 DESs share most of the outstanding chemical or physical properties while overcome many drawbacks of conventional ionic liquids. For instance, many DESs can be prepared by low-cost, widely available, and low-toxic/nontoxic compounds, such as ChCl,

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carboxylic acids14, 20 and polyols.21

With these remarkable characteristics, DESs can be used as green solvents for preparation of transparent wood. However, the application of microwave-assisted DESs in removing the lignin and fabricating outstanding woodiness material is still in its infancy so far. 22.23, 24 In addition, the trichromatic systems usually constructed by mixing different primary color emitting materials in proper ratio. The active phosphors are essential components for color conversion in many LEDs. Usually, most commercial phosphors contain of rare earth metals,25 which are harmful to human body and environment. Therefore, it is attractive to find low-cost and environment-friendly alternatives with non-toxic and suitable optical properties. Carbon dots (CDs) with excellent luminescent property and facile color tunability can be recognized as potential applications in white w-LED fabrications.26, 27 These nanoscale carbon dots can be uniformly dispersed in polymer encapsulation film as the alternative active phosphors. 4


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There have been reported that multiple color emissive CDs were successfully synthesized through tuning the carbonization and surface functionalization of nanoparticles when citric acid (CA) and urea were used as feedstocks.28,29 The emission position of the resulting CDs ranges from 450 nm to 610 nm with higher photoluminescence quantum yields. Here, we not only focus on the dissolution of lignin of wood in DESs, but also aim to fabricate transparent wood combining multi-color emission CDs as new encapsulation film for w-LEDs.30,

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As shown in Scheme 1, at first, oxalic acid

dihydrate and choline chloride (ChCl) were selected to prepare the DES to remove lignin from Balsa wood. At the same time, multi-color emission CDs were synthesized by controlling the molar ratio and reaction temperature of citric acid and urea. In order to prepare the transparent wood composite containing CDs, polyacrylic acid (PAA) was chosen as the filled polymer for its transparency over the entire visible spectrum and the match of refractive index between delignified wood (1.54) and PAA (1.51), through the in-situ polymerization of acrylic acid monomer inside delignified wood in the presence of CDs. Then a white LED was prepared with a commercial blue LED chip as the exciting light source and the as-prepared Carbon Dots/transparent wood (CDs-TW) film as the encapsulating film. The encapsulated white LED shows excellent optical characteristic and good luminous stability, a CRI value of 83 and a CCT value of 5237 K at CIE color coordinates of (0.33,0.32).

Results and discussion 5



Similar to ionic liquids, DESs can be prepared by self-association of HBAs and HBDs and have a series of unique physicochemical properties. The freezing point (Tf) or melting point (Tm) of a DES are generally lower than its individual components (Figure 1a).32, 33 Here, DES was prepared by mixing and heating an organic HBD (oxalic acid) with a quaternary ammonium halide salt (ChCl) to form a transparent ionic liquid.34 After mixing ChCl and oxalic acid at 80 oC for 1 h, transparent DES solution was formed and kept liquidity at room temperature (Figure 1b), which indicates that the optimum DES could be obtained at a molar ratio of 1:1 of HBA to HBD, and the atom economy was 100 % The ChCl/oxalic acid DES was prepare easily, and importantly it showed poor solubility to cellulose while good solubility to hemicellulose and lignin. Therefore, hemicellulose and lignin were dissolved or reacted selectively, while cellulose and the main structure of wood were retained during the delignification process. Here, a balsa wood chip with dimensions of 20 mm×20 mm×1 mm was cut at first (Figure S1). Then the as-prepared DES was used as solvent to remove the lignin under the assistance of the microwave. In brief, the wood chip was immersed in DES with a mass ratio of 1:20 and the reaction was mediated by microwave process (process I, Figure 2a and Figure S2) to dissolve part of the lignin. Next, the residual wood chip was transferred into alkaline H2O2 bleaching solution to remove the residual lignin by a typical bleach process (process II, Figure 2b).35 The color of the wood chip turned dark brown (Figure 2a) after the microwave-assisted heating in DES, indicating a high content of lignin in wood chip were extracted. Figure 2b shows the 6


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color comparison of the chip during bleach process to remove the residual lignin, and as expected, the color of wood chip became white as lignin was almost completely removed (Figure S3). According to the NREL method, Klason method was selected for the lignin quantitation.36 The content change of lignin and cellulose was shown in Figure 2c and 2d, and the cellulose content in process I and process II decreased just slightly (from 50.05 wt% to 48.86 wt%), while the content of lignin decreased significantly from about 27.75 wt% to 2.3 wt%, indicating that more than 90 wt% of lignin was removed. After removing of the remaining lignin, brown color of the wood chip vanished gradually and the color changed to white because of the significant light reflection at the interfaces. Through the delignification process, the lignin of wood chip could be almost completely removed while the skeleton structure of the wood could be preserved (Figure S4). Scanning electron microscope (SEM) was used to observe the micro-structure of the wood chip before and after DES delignification. As shown in Figure 3, there were many mesoporous channels in Balsa wood, with a diameter of 150 µm to 200 µm. The existence of the open and broad channels is convenient for the fast removal of lignin from the thin balsa wood chip. After the lignin removal processes, the microstructure of wood was preserved, but the large amount of lignin had been removed in the cell wall. SEM images of the cell wall before (Figure 3b) and after DES delignification and bleaching treatment (Figure 3e) did not reveal obvious damage on a micro-scale, but the stiff cell wall became flexible and collapse of them can be clearly found, indicating the efficient removal of lignin. In addition, the separation of the cell walls 7



near the middle lamella occurred after delignification, as seen in Figure 3f. The empty space between the cell walls was much larger than the original space occupied by lignin in the middle lamella. Simultaneously, the dominating cellulose skeleton was preserved between wood cells. The weight change of the composition of wood confirmed better preservation of the cellulose skeleton (Figure 2c and 2d). Multi-color emission CDs were prepared by hydrothermal treatment of the mixture of CA and urea under different temperatures. Figure 4 shows the UV–vis spectra, photoluminescence (PL) excitation and emission spectra of blue carbon dots (B-CDs), green carbon dots (G-CDs) and red carbon dots (R-CDs). As shown in Figure 4a, B-CDs shows a single and strong absorption peak at 340nm, which corresponds to the conjugated systems of C=N and C=O. The emission peak of B-CDs locates at 450nm (Figure 4b) with the excitation wavelength of 340nm. Two absorbance peaks were respectively observed at 340nm and 470nm (Figure 4d) for G-CDs. The PL excitation spectrum of 550nm emission (Figure 4e), indicating the green light emission. For R-CDs, a wide absorption peak in red light region especially at 570nm appeared. These peaks are attributed to the aromatic rings containing C=O and C=N bonds. The PLE spectrum of emission at 590nm (Figure 4h) covers the three transitions band from 400 to 550nm (Figure 4g). The color coordinates (x, y) of three CDs calculated from corresponding PL emission spectra (Figure S5). The relative PL quantum yield (QY) of CDs were calculated to be 53.82% for B-CDs, 36.18% for G-CDs, and 12.73% for R-CDs using quinine sulfate as the references. Transmission electron microscopy (TEM) was used to characterize the 8


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morphologies of B-CDs, G-CDs, and R-CDs. As shown in the Figure 5 (a-c), the three CDs showed average diameters are 3.97 ± 0.67, 4.02 ± 0.58, and 4.13 ± 0.38nm for B-CDs, G-CDs, and R-CDs, respectively. High-resolution transmission electron microscopy (HR TEM) images demonstrate that the high degree of crystallinity of the three CDs. The lattice fringe distances are 0.23 nm and 0.34nm respectively, which correspond to the crystal phase of graphene (100) and graphite (002) planes. Fourier transform infrared (FI-IR) and PL spectra of the three CDs were shown in Figure S6 and S7. To characterize the graphitic structure of the three types of CDs, X-ray diffraction (XRD) pattern was employed. As shown in Figure 6a, two prominent peaks at 20.6° and 26.4°, which corresponds to the (002) crystal plane of graphitic structure. The peak at 26.4° is sharp, which is the (002) diffraction peak of graphitic structure. The increased intensity of peak at 26.4° suggest that the graphitization of as-prepared CDs from B-, G- to R-CDs is gradually enhanced. Raman spectrums, as shown in Figure 6b, exhibits two broad peaks at 1358 and 1584 cm-1, which are characterized by D and G bands of these three CDs. The value of ID/IG is about 1.12, 0.94, and 0.89 for B-CDs, G-CDs, and R-CDs, respectively. The results reveal that the extent of graphitization of obtained CDs is gradually increased with the improvement of reaction temperature and mole ratios CA/urea. Thermal gravimetric analysis (TGA) was employed to investigate the thermal stability of these CDs under nitrogen gas atmosphere. As seen in Figure 6c, the water content on the surface of CDs is in range of 1.5 wt% to 3.5 wt%. When the temperature reached 270 °C, the weight loss was 9



caused by the decomposition of the surface carboxy group and other organic molecules of CDs. When the temperature increased to 800 °C, the carbonized residue formed due to the thermal reduction of CDs. The surface composition and elemental valance state of these CDs were characterized by X-ray photoelectron spectroscopy (XPS). Figure S8 shows the full survey XPS spectra of these three types of CDs, and the results reveals that the content of carbon and oxygen gradually increase from B-CDs, G-CDs, and R-CDs. High-resolution C1s spectrum (Figure 6 d-e) of these CDs reveals that the four peaks could be corresponded to C=C/C-C (284.3 eV), C-O/C-N (285.7 eV), -C=O (288 eV) and -COOH (289.9 eV), respectively. The content of -COOH group remarkably enhance from B-, G-, to R-CDs. The N1s and O1s high resolution XPS spectra (Figure S9) show three kind of N and two types of O, including pyridinic N (399.7 eV), pyrrolic N (400.5 eV), graphitic N (401.5 eV), C-O (534 eV) and C=O (531.9 eV). To fabricate the carbon dots/transparent wood (CDs-TW) composite with a high optical transmittance, many kinds of index-matching polymers, such as epoxy resin (PEO), polyvinylpyrrolidone (PVP), polydimethylsiloxane (PDMS), and polymethyl methacrylate (PMMA), have been selected to fill the open channels of wood to reduce light scattering. Here, polyacrylic acid (PAA) was selected as the filling polymer material for its good optically transparency, cheapness and relatively low viscosity in water, especially similar refractive index to cellulose. These characteristics of PAA enabled it to completely immerse through the microscale apertures and open channels in the wood chip. Figure 7b and 7c show SEM images of the CDs-TW filling with 10


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PAA, where all opened channels have been completely occupied by PAA. The cellulose fibers can be neatly glued together by the polymer infiltration into the channels of wood. In addition, cellulose fibers alignment was dense and the aligned cellulose fibers can significantly improve the mechanical properties of the resulting transparent wood. Figure 7h and 7i show the cell walls and the apertures can be well preserved in the fabricated transparent wood composites. The interface between wood cell walls and PAA can be clearly observed, and both contents contact closely with each other, which is the source of transparency. Optical transmittance and haze of the CDs-TW composite were also measured in the range of 400 to 800nm. As shown in Figure 8, the pure epoxy resin (PEO) and as-prepared CDs-TW composite all showed high optical performance, and the transmittance of PEO and CDs-TW were up to 92% and 85% for a thickness of 550 µm. Interestingly, CDs-TW also showed large haze (85%) covering the whole visible light region, indicating that the light scattering intensity has nothing to do with the wavelength because the roughness of the interface between the cellulose and the PAA is larger than the wavelength of incident light. The CDs-TW can be promisingly used for some light management applications due to the high optical transmittance and haze. The fiber skeleton structure in CDs-TW composites endow the CDs-TW with different anisotropic mechanical properties in R and L directions. As shown in Figure 8, to carry out the mechanical test, R-original wood, L-original wood, CDs/R-transparent

wood

(CDs/R-TW)

composites,

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CDs/L-transparent

wood


(CDs/L-TW) composites and PEO were fabricated with a same dimension, 100 mm in length, 15 mm in width and 1mm in thickness. Instron E3000 tester was used to conduct the stress-strain test for the chip. Compared with original wood, the PEO and CDs-TW composites exhibit promoted mechanical properties. Compared with the R-original wood, the CDs/R-TW composite exhibits an approximately equal ductility but a significant promotion in fracture strength. The fracture strength and modulus of CDs/R-TW composites were up to 27.38 MPa and 1.24 GPa, respectively (Figure 9c), and the R-original wood were only 4.56 MPa and 0.21 GPa, respectively. It is worth noting that the CDs/R-TW shows a similar strain with the R-original wood (about 2.2%). Compared to the L-original wood, the PEO and CDs/L-TW composite possesses a significant improvement in both fracture strength and toughness (Figure S10). The fracture strength and modulus of L-TW can reach 60.92 MPa and 1.43 GPa, respectively. Besides, the L-TW also has a higher toughness than the R-TW. Compared with the PEO and L-original wood, the fracture strength and toughness of CDs/L-TW have a dramatic increase due to the polymer filtration. It is highly desirable but difficult to achieve simultaneous increase of mechanical strength and ductility for most compositely structural materials. Strain of CDs/L-TW can reach up to 4.27%, which is much higher than L- original wood. The as-prepared CDs can be well dispersed in numerous polar organic solvents such as ethanol, DMF, DMSO and it can form transparent solutions which exhibit various color emissions under different excitation wavelength. From the PL emission spectra of CDs solution in different solvents, it can be found that there was no 12


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aggregation in various solvents, which means it is possible to fabricate CDs/polymer composites by using water-soluble polymers such as poly (acrylic acid) (PAA) for LEDs. Once been uniformly embedded into the transparent wood, the CDs will endow the composites with photoluminescence function (Figure 10, Figure S11 and Table S1). Firstly, the CDs were dissolved in the ethanol to form a transparent solution. Then the acrylic acid aqueous solution (40 wt%) was mixed with CDs solution with 5:1 volume ratio. Finally, the delignification wood was placed at the bottom of a beaker and immersed in the mixture solution to achieve the CDs-TW. After polymerization, these plates were transparent and exhibited different shades of color because of the insertion of different CDs. Figure 10 shows the optical micrographs of CDs-TWs. These composite chips are optical transparent and show different color emissions from blue to red under corresponding excitation wavelength. These emission positions of composite chips are same as the corresponding CDs solutions, indicates that the CDs can be well dispersed with no aggregation in these composites. These results confirm that the composite chips are a kind of promising encapsulation material for LEDs. To fabricate the white LEDs, the three as-prepared CDs were mixed together into the acrylic acid monomer water solution followed by embedding in the delignified wood for encapsulation of a blue UV-chip (365nm). The UV-chip encapsulated by CDs-TW can emit pure white light when the three CDs were mixed and uniformly dispersed in CDs-TW with 1:3:2 weight ratio. The CIE color coordinates, CCT and CRI are (0.33, 0.32), 5237 K and 83, respectively (Figure S12 and Table S2). The 13



emitting light spectrum emitted by white LED covers the visible light region from 400 to 800 nm. Luminous stability is another essential parameter to evaluate the performance of the white LED. To test the luminous stability of the white LED, the device was detected after 7 d. As shown in the Figure 11 c, we tested the luminous stability of the white LED device again after 7 d. There is almost negligible decrease, and the intensity maintains 85% after exposure of UV light, which indicates that the white LED encapsulated with CDs-TW has excellent stability (Figure S13 and Table S3). These stability results further confirm the composite as a kind of promising encapsulation materials for LEDs.

Conclusions In summary, deep eutectic solvent (DES, oxalic acid and choline chloride) was used as a green solvent for delignification of wood to porous cellulose-based frame, and then acrylic acid (AA) was polymerized therein to form a transparent wood film with high mechanical strength. At the same time, CDs with low-cost, nontoxicity and multiple color emission were synthesized by controlling thermal pyrolysis of CA and urea. Then multiple color emission transparent wood can be obtained by incorporating the as-prepared CDs into the transparent wood, and the as-formed composite showed both excellent optical and mechanical properties, which can be used as an alternative encapsulating material for w-LED.

Experimental section 14


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Materials. Balsa wood chip was obtained from Shandong Province. Deionized water with resistivity of 18 MΩcm was produced by a Milli-Q (Millipore, USA). Choline chloride (ChCl2, 98%) and oxalic acid dihydrate were purchased from Aladdin Chemical Reagent Co. Ltd. In addition, analytical grade hydrogen peroxide (H2O2, 98%), sodium silicate, magnesium sulfate and ethylene diamine tetraacetic acid (EDTA) were purchased from Shanghai Chemical Reagents Company and were used directly without further purification. Other reagents such as sulfur acid (98%), acrylic acid (98%) and ammonium persulfate (98%) were purchased from Sinopharm Chemical Reagent Co. Ltd.

Preparation of DES Typically, mixing ChCl and oxalic acid at 80 oC for 1 h, a clear DES was generated. The molar ratio of 1:1 was proved to be a priority in obtaining the optimum DES.

Lignin removal from wood The wood used in this work is Balsa wood with dimension of 200 mm× 200 mm ×1 mm. The wood chip was mixed with ChCl/oxalic acid DES in a weight ratio of 20 :1. The mixture was heated at microwave-assisted heating (800 W) for 10 min, followed by rinsing thoroughly with distilled water three times to remove most of the chemicals34. Bleaching solution was prepared by mixing chemicals of deionized water, 15



sodium silicate (3 wt%), sodium hydroxide solution (3 wt%), magnesium sulfate (0.1 wt%), EDTA (0.1 wt%), and H2O2 (4 wt%). The wood substrates were immersed in the bleaching solution and kept boiling until the yellow color of the wood slice disappeared. The wood substrates were then thoroughly rinsed with cold water preserved in ethanol until use.

Synthesis and Characterizations of CDs: Multiple molar ratios of CA to urea in DMF were used to synthesis different color emissive CDs at preconcerted temperatures. These CDs with different emissive colors were prepared at conditions: CA/urea = 0.1 at 140 °C for B-CDs, CA/urea = 0.3 at 160 °C for G-CDs and CA/urea = 0.75 at 200 °C for R-CDs, respectively. Typically, the blue emissive CDs was synthesized by using 1 mmol CA and 10 mmol urea in 10 mL DMF. Subsequently, the solution was transferred into 25 mL Teﬂon-lined stainless-steel autoclave and heated to 140 °C for 12 h. The reaction mixture was centrifuged at 12 000 rpm for 10 min by using high speed centrifuge. The supernatant was collected and then added into the mixed solvent of petroleum ether and ethyl acetate to remove unreacted chemicals. Then the solid precipitated out and was dried in vacuum oven at 70 °C overnight. Generally, the ratio of petroleum ether to ethyl acetate is 1:1 for B-CDs, 3:1 for G-CDs, and 4:1 for R-CDs, respectively.

Polymer infiltration Acrylic acid was diluted with deionized water to a concentration of 40wt%. 16


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Subsequently, ammonium persulfate (1 wt%) was added to the mixture. After full dissolution, the lignin-removed wood was placed at the bottom of a beaker and immersed in PAA solution. The solution was then degassed under 200 Pa for approximately 10 min to ensure full infiltration of solution into wood. Finally, the monomer-infiltrated wood sample was sandwiched between two glass slides, wrapped with aluminum foil, and heated in an oven at 75 °C for 4 h in air atmosphere for polymerization.

The recycle of DES After removing lignin, the rest of dark-brown liquid was collected and poured into acetone/water (100 mL, 1:1 v/v) anti-solvent and stirred for 1 h. The mixture was vacuum-filtered and washed with the anti-solvent several times. After filtration, the filter liquor was heated at 60 °C in a rotary evaporator to remove acetone, and then the concentrated solution was filtrated to obtain lignin fraction. Finally, the concentrated solution was heated at 70 °C in a rotary evaporator to obtain recycled DES (Figure S14).

Fabrication of the LED Devices For the white LED, 1mg B-CDs, 3mg G-CDs, and 2mg R-CDs were mixed with 50 mL acrylic acid solution (40wt%), then the lignin-removed wood was placed at the bottom of a beaker and immersed in the homogeneous mixture following polymerization to fabricate the CDs-TW. Finally, the white-CDs-TW was covered on 17



an epoxy resin-free LED chip carefully.

Material Characterizations Ultraviolet–visible absorption spectra and fluorescence spectra were recorded by a Shimadzu UV-2410PC spectrophotometer and a Shimadzu RF-5301PC ﬂuorescence Spectrophotometer. High-resolution TEM images were conducted with a JEOL JEM-2010 transmission electron microscope. The NMR spectra were carried out on a Bruker AVIII 400 MHz spectrometer equipped with a DCH cryoprobe using D2O as the solvent. The morphologies of the several samples were observed and detected by the field emission scanning electron microscopy (FESEM, Leo Co., Oberkochen, Germany). The wood and composite chips were sputter coated with a thin layer of platinum before microscopy observation. The chemical structures in CDs were examined by means of Fourier transform infrared (FT-IR) spectra over a Bruker vector-2 spectrophotometer (Germany) using potassium bromide (KBr) assay method with the spectral range of 400-4000 cm-1. The XPS were analyzed on a Thermo ESCALAB 250Xi -technique Surface Analysis. Raman spectra were obtained on Lab RAM HR Raman microscope. The thermal stability of the CDs was recorded by thermogravimetric analysis (TGA).

Author Information Corresponding Authors *Prof. Lifeng Yan: [email protected] 18


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ORCID http://orcid.org/0000-0002-6063-270X

Acknowledgement The research is supported by the National Natural Science Foundation of China (No. 51673180 and 51373162).

Supporting Information Photos of wood samples and treatment, XPS and PL spectra of CDs FT-IR and XRD results of samples, and CIE color coordinates of the samples can be found.

Notes The authors declare no competing financial interest.

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10. Zhu, M.; Li, T.; Davis, C. S.; Yao, Y.; Dai, J.; Wang, Y.; AlQatari, F.; Gilman, J. W.; Hu, L., Transparent and haze wood composites for highly efficient broadband 20


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light management in solar cells. Nano Energy 2016, 26, 332-339. 11. Jia, C.; Li, Y.; Yang, Z.; Chen, G.; Yao, Y.; Jiang, F.; Kuang, Y.; Pastel, G.; Xie, H.; Yang, B.; Das, S.; Hu, L., Rich Mesostructures Derived from Natural Woods for Solar Steam Generation. Joule 1, (3), 588-599. 12. Chang, K.-L.; Chen, X.-M.; Wang, X.-Q.; Han, Y.-J.; Potprommanee, L.; Liu, J.-y.; Liao, Y.-L.; Ning, X.-a.; Sun, S.-y.; Huang, Q., Impact of surfactant type for ionic liquid pretreatment on enhancing delignification of rice straw. Biorespurce Technol. 2017, 227, 388-392. 13. Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V., Novel solvent properties of choline chloride/urea mixtures. Chem. Commun. 2003, (1), 70-71. 14. Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K., Deep eutectic solvents formed between choline chloride and carboxylic acids: Versatile alternatives to ionic liquids. J. Am. Chem. Soc. 2004, 126, (29), 9142-9147. 15. Russ, C.; Konig, B., Low melting mixtures in organic synthesis - an alternative to ionic liquids? Green Chem. 2012, 14, (11), 2969-2982. 16. Zhang, Q. H.; Vigier, K. D.; Royer, S.; Jerome, F., Deep eutectic solvents: syntheses, properties and applications. Chem. Soc. Rev. 2012, 41, (21), 7108-7146. 17. Procentese, A.; Johnson, E.; Orr, V.; Campanile, A. G.; Wood, J. A.; Marzocchella, A.; Rehmann, L., Deep eutectic solvent pretreatment and subsequent saccharification of corncob. Bioresource Technol. 2015, 192, 31-36. 21



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18. Gunny, A. A. N.; Arbain, D.; Nashef, E. M.; Jamal, P., Applicability evaluation of Deep

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Bioresource Technol. 2015, 181, 297-302. 19. Larriba, M.; Ayuso, M.; Navarro, P.; Delgado-Mellado, N.; Gonzalez-Miquel, M.; Garcia, J.; Rodriguez, F., Choline Chloride-Based Deep Eutectic Solvents in the Dearomatization of Gasolines. Acs Sustain. Chem. Eng. 2018, 6, (1), 1039-1047. 20. Li, J. J.; Xiao, H.; Tang, X. D.; Zhou, M., Green Carboxylic Acid-Based Deep Eutectic Solvents as Solvents for Extractive Desulfurization. Energ Fuel 2016, 30, (7), 5411-5418. 21. Maugeri, Z.; de Maria, P. D., Novel choline-chloride-based deep-eutectic-solvents with renewable hydrogen bond donors: levulinic acid and sugar-based polyols. Rsc Adv. 2012, 2, (2), 421-425. 22. Li, T. F.; Lyu, G. J.; Liu, Y.; Lou, R.; Lucia, L. A.; Yang, G. H.; Chen, J. C.; Saeed, H. A. M., Deep Eutectic Solvents (DESs) for the Isolation of Willow Lignin (Salix matsudana cv. Zhuliu). Int. J. Mol. Sci. 2017, 18, (11). 23. Smith, E. L.; Abbott, A. P.; Ryder, K. S., Deep Eutectic Solvents (DESs) and Their Applications. Chem. Rev. 2014, 114, (21), 11060-11082. 24. Wagle, D. V.; Zhao, H.; Baker, G. A., Deep Eutectic Solvents: Sustainable Media for Nanoscale and Functional Materials. Accounts Chem. Res. 2014, 47, (8), 2299-2308. 25. Xu, J.; Cherepy, N. J.; Ueda, J.; Tanabe, S., Red persistent luminescence in rare earth-free AlN:Mn2+ phosphor. Mater. Lett. 2017, 206, 175-177. 22


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26. Li, J. C.; Yang, Z. Q.; Tang, D. M.; Zhang, L.; Hou, P. X.; Zhao, S. Y.; Liu, C.; Cheng, M.; Li, G. X.; Zhang, F.; Cheng, H. M., N-doped carbon nanotubes containing a high concentration of single iron atoms for efficient oxygen reduction. Npg Asia Mater. 2018, 10. 27. Zhu, S. J.; Zhao, X. H.; Song, Y. B.; Lu, S. Y.; Yang, B., Beyond bottom-up carbon nanodots: Citric-acid derived organic molecules. Nano Today 2016, 11, (2), 128-132. 28. Miao, X.; Qu, D.; Yang, D. X.; Nie, B.; Zhao, Y. K.; Fan, H. Y.; Sun, Z. C., Synthesis of Carbon Dots with Multiple Color Emission by Controlled Graphitization and Surface Functionalization. Adv. Mater. 2018, 30, (1), 1704740. 29. Ding, H.; Yu, S. B.; Wei, J. S.; Xiong, H. M., Full-Color Light-Emitting Carbon Dots with a Surface-State-Controlled Luminescence Mechanism. Acs Nano 2016, 10, (1), 484-491. 30. Selkala, T.; Sirvio, J. A.; Lorite, G. S.; Liimatainen, H., Anionically Stabilized Cellulose Nanofibrils through Succinylation Pretreatment in Urea-Lithium Chloride Deep Eutectic Solvent. Chemsuschem 2016, 9, (21), 3074-3083. 31. Francisco, M.; van den Bruinhorst, A.; Kroon, M. C., New natural and renewable low transition temperature mixtures (LTTMs): screening as solvents for lignocellulosic biomass processing. Green Chem. 2012, 14, (8), 2153-2157. 32. Zhekenov, T.; Toksanbayev, N.; Kazakbayeva, Z.; Shah, D.; Mjalli, F. S., Formation of type III Deep Eutectic Solvents and effect of water on their intermolecular interactions. Fluid Phase Equilibr 2017, 441, 43-48. 23



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33. Guo, W. J.; Hou, Y. C.; Ren, S. H.; Tian, S. D.; Wu, W. Z., Formation of Deep Eutectic Solvents by Phenols and Choline Chloride and Their Physical Properties. J. Chem. Eng. Data 2013, 58, (4), 866-872. 34. Liu, Y. Z.; Chen, W. S.; Xia, Q. Q.; Guo, B. T.; Wang, Q. W.; Liu, S. X.; Liu, Y. X.; Li, J.; Yu, H. P., Efficient Cleavage of Lignin-Carbohydrate Complexes and Ultrafast

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Captions of Figures Scheme 1. Fabrication strategy of white LED based on UV light-emitting diodes chip with the multi-color emission CDs-TW as encapsulating materials. Figure 1. a) Schematic representation of the eutectic point between HBA and HBD, b) DES solution and its contents of choline chloride and oxalic acid employed here. Figure 2. Color changes of wood during lignin removal process I (a) and II (b); Lignin and cellulose content of wood during lignin removal Process I (c) and II (d), respectively. Figure 3. Photo-images of original wood (a) and lignin-removal wood (d), and cross-section SEM images of cell wall structures of original wood (b) and the lignin-rich middle lamella (c), and SEM images of cell wall structures of the delignified wood chip (e) and the empty middle lamella (f). Figure 4. UV–vis spectra, photoluminescence spectra and fluorescent imaging of B-CDs (a, b, c), G-CDs (d, e, f), and R-CDs (g, h, i). Figure 5. a-c) TEM images of B-CDs (a), G-CDs (b), and R-CDs (c). Insets are the corresponding high resolution TEM. d-f) The particles size distributions of B-CDs (d), G-CDs (e), and R-CDs (f), respectively. Figure 6. The structural characterization, thermal stability and surface composition of the CDs. a) XRD patterns of the three types of CDs. b) Raman spectra of the three types of CDs. c) TGA images of the three types of CDs. d–e) The high-resolution C1s XPS spectra of B-CDs (d), G-CDs (e), and R-CDs (f), 25



respectively. Figure 7. Characterization of microstructures of the as-prepared CDs-TW wood: a, d, g) Photos of the CDs-TW; b, c) SEM images of delignified CDs-TW longitudinal section, cellulose fibers alignment is dense and can be clearly observed. e, f) SEM image of the surface of the transparent film, displaying the aligned microfibers. h, i) Cross-section SEM image of the CDs-TW, showing interface gaps between the wood cell walls and PAA. Figure 8. a) Optical transmittance of PEO and CDs-TW (inset is photograph of composite with thickness of 550 µm), b) Optical haze of PEO and CDs-TW (inset is the photograph of PEO and composite with a 5 mm gap between transparent wood and underlying paper). Figure 9. a, b) Schematic to illustrate mechanical tests for R- and L-transparent wood, respectively. c) Experimental stress–strain curves for the R-original wood and CDs/R-TW composite. d) Experimental stress–strain curves for L-original wood and CDs/L-TW composite. Figure 10. Optical photographs (a-c) and fluorescent images (d-f) of the CDs-TW composite chip under different excitation light. Figure 11. Characterization of white LED encapsulated using the CDs-TW. a) The CIE color coordinates of the white LEDs. Inset is the optical photograph of the encapsulated white LED at lighting state. b) The emitting light spectrum of white CDs. c) The emitting spectra of the encapsulated white LED at initial and after lighting over 7 d. 26


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Scheme 1. Fabrication strategy of white LED based on UV light-emitting diodes chip with the multi-color emission CDs-TW as encapsulating materials.

27



Figure 1. a) Schematic representation of the eutectic point between HBA and HBD, b) DES solution and its contents of choline chloride and oxalic acid employed here.

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Figure 2. Color changes of wood during lignin removal process I (a) and II (b); Lignin and cellulose content of wood during lignin removal Process I (c) and II (d), respectively.

29



Figure 3. Photo images of original wood (a) and lignin-removal wood (d), and cross-section SEM images of cell wall structures of original wood (b) and the lignin-rich middle lamella (c), and SEM images of cell wall structures of the delignified wood chip (e) and the empty middle lamella (f).

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Figure 4. UV–vis spectra, photoluminescence spectra and fluorescent imaging of B-CDs (a, b, c), G-CDs (d, e, f), and r-CDs (g, h, i).

31



Figure 5. a-c) TEM images of B-CDs (a), G-CDs (b), and R-CDs (c). Insets are the corresponding high resolution TEM. d-f) The particles size distributions of B-CDs (d), G-CDs (e), and R-CDs (f), respectively.

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Figure 6. The structural characterization, thermal stability and surface composition of the CDs. a) XRD patterns of the three types of CDs. b) Raman spectra of the three types of CDs. c) TGA images of the three types of CDs. d–e) The high-resolution C1s XPS spectra of B-CDs (d), G-CDs (e), and R-CDs (f), respectively.

33



Figure 7. Characterization of microstructures of the as-prepared CDs-TW: a, d, g) Photos of the CDs-TW; b, c) SEM images of delignified CDs-TW longitudinal section, cellulose fibers alignment is dense and can be clearly observed. e, f) SEM image of the surface of the transparent film, displaying the aligned microfibers. h, i) Cross-section SEM image of the CDs-TW, showing interface gaps between the wood cell walls and PAA.

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Figure 8. a) Optical transmittance of CDs-TW (inset is photograph of PEO and composite with thickness of 550 µm), b) Optical haze of CDs-TW (inset is the photograph of PEO and composite with a 5 mm gap between transparent wood and underlying paper).

35



Figure 9. a, b) Schematic to illustrate mechanical tests for R- and L-transparent wood, respectively. c) Experimental stress–strain curves for the R-original wood, PEO and CDs/R-TW composite. d) Experimental stress–strain curves for L-original wood, PEO and CDs/L-TW composite

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Figure 10. Optical photographs (a-c) and fluorescent images (d-f) of the CDs-TW composite chip under different excitation light.

37



Figure 11. Characterization of white LED encapsulated using the CDs-TW. a) The CIE color coordinates of the white LEDs. Inset is the optical photograph of the encapsulated white LED at lighting state. b) The emitting light spectrum of white CDs. c) The emitting spectra of the encapsulated white LED at initial and after lighting over 7 d.

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TOC

Green delignification process was used to prepare all-carbon transparent wood encapsulation material for white LEDs.

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Transparent Wood Film Incorporating Carbon Dots ... - ACS Publications

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