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

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 9314−9323

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* 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

ACS Sustainable Chem. Eng. 2018.6:9314-9323. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/26/19. For personal use only.

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ABSTRACT: Epoxy resins are the main encapsulation materials for light-emitting diodes (LEDs) due to their high transparency, appropriate mechanical strength, and excellent thermal stability. However, environmentally 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 LEDs (W-LEDs). 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 the ultrafast removing of lignin from wood using deep eutectic solvent (DES, oxalic acid and choline chloride) under microwave-assisted treatment, and then, CDs and poly(acrylic acid) (PAA) were filled into the delignified wood through an in situ polymerization. The transparent wood film embedding multicolor 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), 5237 K, and 83, respectively. This provides a simple route to prepare metal-free wood-based encapsulating materials for W-LEDs. KEYWORDS: Transparent wood, Deep eutectic solvent, Carbon dots, Encapsulating materials, White LED

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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 through a low-cost and green process.12 Deep eutectic solvents (DESs), a series of transparent liquid eutectic mixtures that are obtained through strong hydrogen-bonding interactions between hydrogen-bond acceptors (HBAs) and hydrogen-bond donors (HBDs),13,14 have been considered to be a new class of promising green solvents 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 overcoming 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,19 carboxylic acids,14,20 and polyols.21 With these remarkable characteristics, DESs can be used as green solvents for

INTRODUCTION Environmentally friendly, electricity-saving, and excellently stable solid-state lighting is 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 an LED chip, trichromatic systems, and encapsulation material. Epoxy resins have been frequently used as the encapsulation materials due to their high transparency, appropriate mechanical strength, excellent thermal stability, and inexpensiveness.5 However, the rather brittle and hydrophilicity of epoxy resins, as well as the fact that they easily suffer from yellowing during thermal process, 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 a 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 © 2018 American Chemical Society

Received: April 10, 2018 Revised: June 4, 2018 Published: June 10, 2018 9314

DOI: 10.1021/acssuschemeng.8b01618 ACS Sustainable Chem. Eng. 2018, 6, 9314−9323

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Fabrication Strategy of a White LED Based on UV Light-Emitting Diode Chip with the Multiple-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.

dihydrate and choline chloride (ChCl) were selected to prepare the DES to remove lignin from balsa wood. At the same time, multiple-color-emission CDs were synthesized by controlling the molar ratio and reaction temperature of citric acid and urea. For the preparation of the transparent wood composite containing CDs, poly(acrylic 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) (CRI, color rendering index; CCT, correlated color temperature; CIE, Commission Internationale Eclairage).

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.22−24 In addition, the trichromatic systems are usually constructed by mixing different primary-color-emitting materials in the proper ratio. The active phosphors are essential components for color conversion in many LEDs. Usually, most commercial phosphors contain rare earth metals,25 which are harmful to the human body and environment. Therefore, it is attractive to find low-cost and environmentally friendly alternatives with nontoxicity and suitable optical properties. Carbon dots (CDs) with excellent luminescent property and facile color tunability can be recognized as potential applications in W-LED fabrications.26,27 These nanoscale carbon dots can be uniformly dispersed in polymer encapsulation film as the alternative active phosphors. It has 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 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 multiple-color-emission CDs as a new encapsulation film for W-LEDs.30,31 As shown in Scheme 1, at first, oxalic acid

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RESULTS AND DISCUSSION Similar to ionic liquids, DESs can be prepared by selfassociation 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 9315


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

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

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 that a high content of lignin in wood chip was extracted. Figure 2b shows the color comparison of the chip during the bleach process to remove the residual lignin, and as expected, the color of the wood chip became white as lignin was almost completely removed (Figure S3). According to the NREL method, the Klason method was selected for the lignin quantitation.36 The content change of lignin and cellulose is shown in Figure 2c,d, and the cellulose content in process I and process II decreased just slightly (from 50.05 to 48.86 wt %), while the content of lignin decreased significantly from about 27.75 to 2.3 wt %, indicating that more than 90 wt % of lignin was removed. After removing the remaining lignin, the 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

quaternary ammonium halide salt (ChCl) to form a transparent ionic liquid.34 After mixing ChCl and oxalic acid at 80 °C for 1 h, a 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 prepared easily, and importantly, it showed poor solubility to cellulose with 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 microwave assistance. 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 9316


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

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

the wood chip could be almost completely removed while the skeleton structure of the wood could be preserved (Figure S4). Scanning electron microscopy (SEM) was used to observe the microstructure 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− 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 (Figure 3e) DES delignification and bleaching treatment did not reveal obvious damage on a microscale, but the stiff cell wall became flexible; their collapse can be clearly found, indicating the efficient removal of lignin. In addition, the separation of the cell walls near the middle 9317


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Figure 6. 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. High-resolution C 1s XPS spectra of (d) B-CDs, (e) G-CDs, and (f) R-CDs.

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,d). Multiple-color-emission CDs were prepared by hydrothermal treatment of the mixture of CA and urea under different temperatures. Figure 4 shows the UV−vis, 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 show a single and strong absorption peak at 340 nm, which corresponds to the conjugated systems of CN and CO. The emission peak of B-CDs is located at 450 nm (Figure 4b) with the excitation wavelength of 340 nm. Two absorbance peaks were observed at 340 and 470 nm (Figure 4d) for GCDs. The PL excitation spectrum of 550 nm emission (Figure 4e) indicates the green light emission. For R-CDs, a wide absorption peak in the red light region especially at 570 nm appeared. These peaks are attributed to the aromatic rings containing CO and CN bonds. The PLE spectrum of emission at 590 nm (Figure 4h) covers the three transitions band from 400 to 550 nm (Figure 4g). The color coordinates (x, y) of three CDs were calculated from corresponding PL emission spectra (Figure S5). The relative PL quantum yield (QY) of CDs was 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 morphologies of B-CDs, G-CDs, and R-CDs. As shown in the Figure 5a−c, the three CDs showed average diameters of 3.97 ± 0.67, 4.02 ± 0.58, and 4.13 ± 0.38 nm for B-CDs, G-CDs, and R-CDs, respectively. High-resolution transmission electron microscopy (HR-TEM) images demonstrate the high degree of crystallinity of the three CDs. The lattice fringe distances are 0.23 and 0.34 nm, respectively, which correspond to the crystal phase of graphene (100) and graphite (002) planes. Fourier transform infrared (FT-IR) and PL spectra of the three CDs are shown in Figures S6 and S7.

For characterization of the graphitic structure of the three types of CDs, the X-ray diffraction (XRD) pattern was employed. As shown in Figure 6a, two prominent peaks at 20.6° and 26.4° correspond to the (002) crystal plane of graphitic structure. The peak at 26.4° is sharp, which is the (002) diffraction peak of the graphitic structure. The increased intensity of the peak at 26.4° suggests that the graphitization of as-prepared CDs from B- to G- and R-CDs is gradually enhanced. Raman spectra, as shown in Figure 6b, exhibit two broad peaks at 1358 and 1584 cm−1, which are characterized by the 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 of 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 the range 1.5−3.5 wt %. When the temperature reached 270 °C, the weight loss was 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 because of the thermal reduction of CDs. The surface composition and elemental valence 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 reveal that the content of carbon and oxygen gradually increases from B-CDs to G-CDs and R-CDs. The high-resolution C 1s spectrum (Figure 6 d,e) of these CDs reveals that the four peaks could correspond 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 the COOH group remarkably enhances from B- to Gand R-CDs. The N 1s and O 1s high-resolution XPS spectra (Figure S9) show three kinds 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). For the fabrication of 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), 9318


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Figure 7. Characterization of microstructures of the as-prepared CDs-TW. (a, d, g) Photos of the CDs-TW. (b, c) SEM images of the delignified CDs-TW longitudinal section; cellulose fiber 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 CDs-TW (inset is photograph of PEO and composite with thickness of 550 μm). (b) Optical haze of CDsTW (inset is the photograph of PEO and composite with a 5 mm gap between transparent wood and underlying paper).

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 400−800 nm. As shown in Figure 8, the pure epoxy resin (PEO) and as-prepared CDsTW 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 because of the high optical transmittance and haze. The fiber skeleton structure in CDs-TW composites endows the CDs-TW with different anisotropic mechanical properties in R and L directions. As shown in Figure 8, to carry out the

and poly(methyl methacrylate) (PMMA), have been selected to fill the open channels of wood to reduce light scattering. Here, poly(acrylic acid) (PAA) was selected as the filling polymer material for its good optically transparency, cheapness, relatively low viscosity in water, and especially its 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,c shows SEM images of the CDs-TW filling with 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, the cellulose fiber alignment was dense, and the aligned cellulose fibers can significantly improve the mechanical properties of the resulting transparent wood. Figure 7h,i shows that the cell walls and the apertures can be well-preserved in the fabricated transparent wood composites. The interface between wood cell walls and 9319


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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.

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

mechanical test, R-original wood, L-original wood, CDs/Rtransparent wood (CDs/R-TW) composites, CDs/L-transparent wood (CDs/L-TW) composites, and PEO were fabricated with the same dimensions, 100 mm in length, 15 mm in width, and 1 mm in thickness. An 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 those of 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. In addition, the L-TW also has a higher toughness than the RTW. 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 that of L- original wood. The as-prepared CDs can be well-dispersed in numerous polar organic solvents such as ethanol, DMF, and DMSO, and it can form transparent solutions which exhibit various color emissions under different excitation wavelengths. From the PL emission spectra of the CD solution in different solvents, it can be found that there was no aggregation in various solvents, which means that it is possible to fabricate CDs/polymer composites by using water-soluble polymers such as poly9320


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

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CONCLUSIONS In summary, deep eutectic solvent (DES, oxalic acid and choline chloride) was used as a green solvent for delignification of wood to a 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-LEDs.

(acrylic acid) (PAA) for LEDs. Once having been uniformly embedded into the transparent wood, the CDs will endow the composites with a photoluminescence function (Figure 10, and Figure S11 and Table S1). First, the CDs were dissolved in the ethanol to form a transparent solution. Then, the acrylic acid aqueous solution (40 wt %) was mixed with the CD solution with a 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 the corresponding excitation wavelength. These emission positions of composite chips are the same as the corresponding CD solutions, indicating 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 (365 nm). 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 values are (0.33, 0.32), 5237 K, and 83, respectively (Figure S12 and Table S2). The 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 days. As shown in the Figure 11c, we tested the luminous stability of the white LED device again after 7 days. There is an 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 material for LEDs.

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EXPERIMENTAL SECTION

Materials. The balsa wood chip was obtained from Shandong Province. Deionized water with resistivity of 18 MΩ cm was produced by a Milli-Q (Millipore). 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, upon mixing ChCl and oxalic acid at 80 °C for 1 h, a clear DES was generated. The molar ratio of 1:1 was proven to be a priority in obtaining the optimum DES. Lignin Removal from Wood. The wood used in this work is balsa wood with dimensions 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 through microwave-assisted heating (800 W) for 10 min, followed by rinsing thoroughly with distilled water three times to remove most of the chemicals.34 Bleaching solution was prepared by mixing chemicals of deionized water, 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 and preserved in ethanol until use. 9321



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Synthesis and Characterizations of CDs. Multiple molar ratios of CA to urea in DMF were used to synthesize different-coloremissive CDs at preconcerted temperatures. These CDs with different emissive colors were prepared with the following 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, separately. Typically, the blueemissive CDs were synthesized by using 1 mmol of CA and 10 mmol of urea in 10 mL of DMF. Subsequently, the solution was transferred into a 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 a 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 BCDs, 3:1 for G-CDs, and 4:1 for R-CDs. Polymer Infiltration. Acrylic acid was diluted with deionized water to a concentration of 40 wt %. 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. Recycle of DES. After removing lignin, the rest of the dark-brown liquid was collected and poured into acetone/water (100 mL, 1:1 v/ v) antisolvent and stirred for 1 h. The mixture was vacuum-filtered and washed with the antisolvent 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 the 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, 1 mg of BCDs, 3 mg of G-CDs, and 2 mg of R-CDs were mixed with 50 mL of acrylic acid solution (40 wt %); 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 an epoxy resin-free LED chip carefully. Material Characterizations. Ultraviolet−visible absorption spectra and fluorescence spectra were recorded by a Shimadzu UV2410PC 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 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 the microscopy observation. The chemical structures in CDs were examined by means of Fourier transform infrared (FT-IR) spectra over a Bruker vector-2 spectrophotometer using the potassium bromide (KBr) assay method with the spectral range 400−4000 cm−1. The XPS spectra were analyzed on a Thermo ESCALAB 250Xi -technique surface analysis instrument. Raman spectra were obtained on a Lab RAM HR Raman microscope. The thermal stability of the CDs was recorded by thermogravimetric analysis (TGA).

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-551-63603748. Phone: +86-551-63606853. E-mail: [email protected]. ORCID

Lifeng Yan: 0000-0002-6063-270X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research is supported by the National Natural Science Foundation of China (51673180 and 51373162).

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REFERENCES

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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.8b01618. 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 (PDF) 9322


Research Article

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

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