Luminescent and Transparent Wood Composites Fabricated by Poly

Mar 8, 2017 - Research Center of Wood Bionic Intelligent Science, Northeast Forestry University, Harbin 150040, China. ‡ Dehua TB New Decoration Mat...
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Research Article pubs.acs.org/journal/ascecg

Luminescent and Transparent Wood Composites Fabricated by Poly(methyl methacrylate) and γ‑Fe2O3@YVO4:Eu3+ Nanoparticle Impregnation Wentao Gan,† Shaoliang Xiao,† Likun Gao,† Runan Gao,† Jian Li,*,† and Xianxu Zhan‡ †

Research Center of Wood Bionic Intelligent Science, Northeast Forestry University, Harbin 150040, China Dehua TB New Decoration Material Co., Ltd, Huzhou, 313200, China

‡

S Supporting Information *

ABSTRACT: Natural wood is functionalized using the index matching poly(methyl methacrylate) (PMMA) and luminescent γ-Fe2O3@ YVO4:Eu3+ nanoparticles to form a novel type of luminescent and transparent wood composite. First, the delignified wood template was obtained from natural wood through a lignin removal process, which can be used as a support for transparent polymer and phosphor nanoparticles. Then, the functionalization occurs in the lumen of wood, which benefits from PMMA that fills the cell lumen and enhances cellulose nanofiber interaction, leading to wood composites with excellent thermal properties, dimensional stability, and mechanical properties. More importantly, this wood composite displays a high optical transmittance in a broad wavelength range between 350 and 800 nm, magnetic responsiveness, and brightly colored photoluminescence under UV excitation at 254 nm. The unique properties and green nature of the luminescent wood composite have great potential in applications including green LED lighting equipment, luminescent magnetic switches, and anti-counterfeiting facilities. KEYWORDS: Luminescent wood, Transparent, Magnetic, γ-Fe2O3@YVO4:Eu3+

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INTRODUCTION Wood is a sustainable and renewable material with remarkable mechanical properties and sophisticated hierarchical structure, which has been widely used in daily life in various applications, including homes, furniture, artwork, heating, and decoration.1−3 However, the biocomposite nature of wood results in numerous harmful effects, including easy absorption of water and moisture, cracking, mildew, and biological and light degradation, which strongly reduce the durability of wood in service.4−6 Therefore, many modified approaches at the molecular or nanoscale level or surface of the wood have been proposed, in order to improve the shortcomings of the wood.7−10 With the development of materials science, wood materials have been increasingly considered as a biobased template to endow some novel properties.11 For example, Merk et al. reported on magnetic wood composites based on in-depth penetration of ferric and ferrous ions in wood matrices,12 which have some potential in the field of electromagnetic wave absorption, heating, and magnetic actuation.13−15 Wu et al. fabricated a superhydrophobic inorganic nanoparticle film on wood surface, yielding a durable superhydrophobic wood material with excellent dimensional stability and self-cleaning.16 Moreover, Hu et al. fabricated a unique transparent and haze wood materials by epoxy resin impregnation.17 First, colored lignin © 2017 American Chemical Society

were removed from the wood. The porous and hierarchical structure was well preserved after epoxy resin impregnation and curing.18 Further research conclusively showed that the transparent wood composites can be utilized for a range of optoelectronics and energy efficient building materials.19,20 Nowadays, considerable interest in the field of luminescent materials has been the focus of nanomaterials doped with lanthanide ions, these nanomaterials exhibit attractive chemical features such as blinking, low toxicity, resistance to photochemical degradation, and photobleaching.21−23 To the best of our knowledge, most of the available preparation involving the sol−gel method, hydrothermal process, the Pechini method, and solvothermal synthesis give excellent opportunities to obtain lanthanide-doped nanomaterials with various particles sizes, shapes, and doping levels,24−26 and these parameters greatly influence the chemical activity and luminescent properties of such materials. To expand the application of wood materials in the optical field, it is advantageous to impregnate some luminescent nanomaterials to the wood template, leading to the excellently Received: December 8, 2016 Revised: March 6, 2017 Published: March 8, 2017 3855

DOI: 10.1021/acssuschemeng.6b02985 ACS Sustainable Chem. Eng. 2017, 5, 3855−3862

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natural wood was treated with a NaClO2 (2 wt %) and glacial acetic acid solution (pH = 4.6) at 80 °C for 12 h. The treated wood templates were rinsed with distilled water, followed by placement in an aqueous solution of H2O2 (5 mol/L) and boiling for 4 h; then they were dried at 45 °C for 24 h. Then, the polymer solution was prepared by mixing the methyl methacrylate (MMA) and 2,2′-azobis(2methylpropionitrile) with a weight ratio of 1000 to 3, which was prepolymerized at 75 °C for 20 min. The prepolymerized methyl methacrylate solution was cooled down to room temperature in ice− water bath. After that, the prepolymerized polymer solution and γFe2O3@YVO4:Eu3+ nanoparticles were uniformly dispersed and mixed together with the weight ratios of 1000:1, 1000:2, 1000:5, and 1000:10 for luminescent wood containing different particle concentrations. The delignified wood template was infiltrated in the mixture under vacuum for 30 min, and this process was repeated three times to ensure the full infiltration. Finally, the infiltrated wood was sandwiched between two glass slides and kept static at 50 °C for 6 h. The luminescent and transparent wood composites was peeled off from the glass slides after the methyl methacrylate was completely solidified. In order to determine the concentration of Eu3+ in LW, corresponding ICP-MS analysis results of as-prepared wood samples were added in Table S1. Characterization. The surface morphology of the as-prepared samples was analyzed using scanning electron microscopy (SEM, HITACHI TM3030) and transmission electron microscopy (TEM, Tecnai G20). The surface components of the samples were determined via energy dispersive X-ray analysis (EDXA) and Fourier transformation infrared spectroscopy (Magna-IR 560). The phase structure of the as-prepared products was checked by the X-ray diffraction measurements (XRD, Rigaku, and D/MAX 2200) operated with Cu target radiation (λ = 1.54 Å). Inductively coupled plasma mass spectrometry (ICP-MS) was carried out on an Agilent 725. The thermal performances of the wood composites were measured by a thermal analyzer (TGA, SDT Q600) at a heating rate of 10 °C/min under nitrogen atmosphere. The transmittance of wood composites was analyzed with a U-4100 UV−vis spectrometer. The excitation and emission spectra of LW were recorded at room temperature using a FLsp920 spectrofluorometer. When the excitation wavelength (300 nm) was selected, the quantum yield values were calculated automatically. Magnetic properties of wood samples with the size of 4 mm × 4 mm × 0.5 mm were determined using a superconducting quantum interference device (MPMS XL-7, Quantum Design Corp.) at room temperature. The mechanical properties were performed using an Instron 1185 testing machine. The wood was selected without joints or fasteners with a dimension of about 50 mm × 10 mm × 3 mm. The dimensional stability was evaluated by placed the wood samples (20 mm × 20 mm × 0.5 mm) in chambers containing distilled water at the room temperature (25 °C). Volume change of the wood samples can be calculated according to eq 1:

luminescent wood composites. In this study, a novel approach, based on the removal of light absorbed lignin from the wood template and then impregnation of PMMA and γ-Fe2O3@ YVO4:Eu3+ nanoparticles, was proposed to synthesize highly transparent, luminescent, and magnetic wood composite. The resulting nanoscale structure of the wood template is analyzed, and effects on optical, magnetic, thermal, and mechanical properties are investigated. The luminescent wood with high transparency will offer diversified application toward the preparation of new types of green LED lighting equipment, luminescent magnetic switches, and anticounterfeiting facilities.

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EXPERIMENTAL SECTION

Materials. Wood slices (20 mm × 10 mm × 0.5 mm) of Cathay poplar (Populus cathayana Rehd) were used in this experiment. Ferric trichloride hexahydrate (FeCl3·6H2O), ferrous chloride tetrahydrate (FeCl2·4H2O), ammonia solution (NH3·H2O, 25%), sodium chlorite (NaClO2), glacial acetic acid, hydrogen peroxide (30% solution), sodium acetate (NaAc), ammonium metavanadate (NH4VO3), citric acid monohydrate, and ethanol in this study were supplied by Shanghai Boyle Chemical Company Limited.. Methyl methacrylate (MMA), 2,2′-azobis-(2-methylpropionitrile) (AIBN, Sigma-Aldrich), yttrium oxide (Y2O3, 99.9%), and europium oxide (Eu2O3, 99.9%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (China). All chemicals were of analytic-reagent grade. Synthesis of γ-Fe2O3@YVO4:Eu3+ Nanoparticles. Figure 1 shows the reactive scheme for synthesizing the luminescent and

VC(%) =

V1 − V0 × 100% V0

(1)

Where V0 and V1 are the volumes of wood before and after impregnation, respectively.

Figure 1. Experimental strategy for the fabrication of luminescent wood.

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RESULTS AND DISCUSSION For confirmation of particles size and distribution of YVO4:Eu3+ and γ-Fe2O3@YVO4:Eu3+, the TEM measurement was applied. Figure 2 shows the TEM images of YVO4:Eu3+ and γ-Fe2O3@YVO4:Eu3+ NPs. Figure S1 demonstrates the histograms of diameter of YVO4:Eu3+ NPs and γ-Fe2O3@YVO4:Eu3+ NPs. As revealed in Figure 2a, the YVO4:Eu3+ NPs are uniform and spherical in shape, with an average diameter of ∼15 nm (Figure S1a). Figure 2b and c shows the TEM images of γFe2O3@YVO4:Eu3+ NPs under different magnifications. The TEM images indicate that the YVO4:Eu3+ NPs with uniform size are assembled around the γ-Fe2O3 NPs. The diameter of the γ-Fe2O3@YVO4:Eu3+ NPs is mainly centered at 95 nm (Figure S1b). The energy dispersive spectrum elemental

transparent wood. The γ-Fe2O3@YVO4:Eu3+ nanoparticles were synthesized according to solvothermal method. First, 2 mmol of Y2O3, 0.1 mmol of Eu2O3, 24 mmol of citric acid, and 4 mmol of NH4VO3 were dissolved in dilute nitric acid with stirring at 80 °C for several minutes, and then dried at 120 °C for 24 h. After that, the samples was calcined at 1000 °C for 4 h to obtain the phosphor samples. Then, 0.1 g of phosphor particles, 0.25 g of CTAB, 0.92 g of FeCl3 and 0.34 g of FeCl2 were introduced into 50 mL distilled water with continuous stirring. The pH value of the mixture was adjusted to ca.10 through the addition of 25% ammonia solution. After stirring for 30 min, the system was transferred into a Teflon-lined stainless-steel autoclave and heated at 110 °C for 8 h. Finally, the powder samples was dried at 60 °C for 12 h. Synthesis of Luminescent Wood (LW). The delignified wood template was obtained thought a lignin removal process that the 3856

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Figure 2. TEM micrographs of (a) YVO4:Eu3+ and (b, c) γ-Fe2O3@ YVO4:Eu3+ NPs.

analysis data of γ-Fe2O3@YVO4:Eu3+ NPs shown in Table S2 confirms the presence of iron, oxygen, yttrium, vanadium, and europium in as-prepared nanocomposites. To determine the spatial distribution of PMMA and γFe2O3@Y2O3:Eu3+ NPs after polymer impregnation at the cell and tissue levels, a series of SEM pictures of top-view and crosssectional SEM images of the wood samples was taken. In Figure 3a and b, after PMMA impregnation, the original tracheid with

Figure 4. XRD patterns of (a) YVO4:Eu3+, (b) γ-Fe2O3@YVO4:Eu3+, (c) natural wood, (d) transparent wood, and (e) luminescent wood with a γ-Fe2O3@YVO4:Eu3+ NP concentration of 0.2 wt %.

successful crystallization of YVO4:Eu3+.28 Figure 4b shows the XRD patterns of γ-Fe2O3@YVO4:Eu3+. The XRD diffraction peaks at 2θ = 18°, 30°, 35°, 43°, 53°, 57°, and 62° of γ-Fe2O3 can be indexed according to the standard data of maghemite (JCPDS 39-1346),29 all diffraction peaks belonging to YVO4:Eu3+ are present, revealing the γ-Fe2O3@YVO4:Eu3+ NPs was successful synthesized in the experiments. In Figure 4c, two major diffraction peaks are observed for natural wood at 2θ = 16° and 22°, which are characteristic of cellulose.3,30 After polymer impregnation (Figure 4d), two broad peaks are observed for the transparent wood at 2θ = 15° and 30°, which are ascribed to PMMA.31 Upon further addition of γ-Fe2O3@ YVO4:Eu3+ NPs (Figure 4e), the main diffraction peaks at 2θ = 25°, 33°, 35°, 40°, 50°, 53°, 62°, and 63° of γ-Fe2O3@ YVO4:Eu3+ exist in LW; all diffraction peaks belonging to cellulose and PMMA are present, indicating that the γ-Fe2O3@ YVO4:Eu3+ and PMMA are immobilized into the wood template successfully. Figure 5 shows the FTIR spectra of natural wood, transparent wood, and LW with 0.2 wt % γ-Fe2O3@ YVO4:Eu3+ NP loading. The natural wood (Figure 5a) presents characteristic absorption peaks such as O−H stretching vibrations at 3435 cm−1, C−H stretching vibration at 2925 cm−1, acetyl groups of hemicelluloses at 1742 cm−1, CO stretching vibrations at 1632 cm−1, aromatic stretching vibrations of lignin at 1584 cm−1, aliphatic acid group vibrations at 1250 cm−1, and aromatic C−H in-plane deformation assigned to lignin at 1060 cm−1, which are in agreement with earlier research.32 After lignin removal and polymer infiltration, the absorption peaks at 1584 and 1060 cm−1 disappeared, indicating light-absorbing lignin was degraded during the preparation period. Moreover, the FTIR spectrum of the transparent wood also possesses the characteristic peaks of PMMA (2996 and 2950 cm−1 for C−H, 1733 cm−1 for CO, and 1210 and 1169 cm−1 for C−O, as presented in Figure 5b).33 However, when the γ-Fe2O3@YVO4:Eu3+ NPs were added to the polymer (Figure 5c), two absorption peaks at 835 and 450 cm−1 appeared, which corresponded to the characteristic absorption of V−O and Y(Eu)−O bonds, respectively.34 This further proves that YVO4:Eu3+ has impregnated into the wood template. Furthermore, the stretching vibrations of the

Figure 3. Top-view SEM image of (a) natural wood, (b) transparent wood, and (c) luminescent wood with a γ-Fe2O3@YVO4:Eu3+ NP concentration of 0.1 wt %, respectively. Cross-sectional SEM image of (d) natural wood, (e) transparent wood, and (f) luminescent wood with a γ-Fe2O3@YVO4:Eu3+ NP concentration of 0.1 wt %, respectively.

some pits of wood surface were observed to be covered by a continuous smooth polymer film. Upon addition of γ-Fe2O3@ YVO4:Eu3+ NPs, some particles are widely distributed on the wood surface (Figure 3c). The energy dispersive spectrum present in Figure S2 shows the chemical elements of LW: carbon, oxygen, iron, europium, yttrium, and gold (from the coating for SEM imaging). Moreover, the corresponding elemental maps of C, Fe, Y, and Eu (Figure S2) also confirm that the γ-Fe2O3@YVO4:Eu3+ NPs are deposited on the wood template. A cross-sectional SEM photograph of the untreated wood is shown in Figure 3d, in which the porous structure is well presented. During the experiment process, it is highly expected that polymerization takes place in the cell lumen, because the porous structure in wood will result in light scattering in the visible range and leads to the opacity of wood.27 Figure 3e and f shows that almost all of the cell lumen are filled with the polymer, while there are some nanoparticles located on cell lumen after the addition of γ-Fe2O3@YVO4:Eu3+ NPs. To determine the crystal structure of the samples obtained from different stages of the preparation, XRD was performed (Figure 4). The XRD pattern, Figure 4a of YVO4:Eu3+ shows the main XRD diffraction peaks at angles 2θ = 25°, 33.5°, and 50°, which are assigned to the diffraction of the (200), (112), and (312) planes of YVO4 (JCPDS 17-0341), suggesting the 3857

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Compared with the TG curve of PMMA, more residue of LW remained, which can be explained by the inorganic γFe2O3@YVO4:Eu3+ NPs, and the remaining carbon from the delignified wood template. And compared with the TG curve of natural wood, less residue was observed in LW, which may be attributed to the impregnation of complete combustion of PMMA in wood template. In Figure 6b, PMMA shows three main exothermic peaks at around 228, 294, and 377 °C in the DTA curve, corresponding to the three weight looses stages of PMMA in Figure 6a. As for the natural wood, two prominent exothermic peaks for flaming and glowing are observed at 305 and 367 °C, respectively.38 Moreover, compared with the DTA results of PMMA and natural wood, the exothermic peaks of LW are weakened and shifted to the high temperature, revealing the good thermostability of LW. The other noticeable characteristic of LW is optical properties. The transmittance of the wood samples obtained from different stages of processing is presented in Figure 7. Figure 5. FTIR spectra of (a) natural wood, (b) transparent wood, and (c) luminescent wood with a γ-Fe2O3@YVO4:Eu3+ NP concentration of 0.2 wt %.

Fe−O can be clearly seen around at 558 cm−1,35 which confirms that the γ-Fe2O3 NPs are deposited on the wood composites successfully. The results of the TG and DTA measurements of the samples are shown in Figure 6. In Figure 6a, PMMA undergoes three weight loss stages. The first step, started at around 160 °C, would be caused by the scissions of head-to-head linkages (H−H). The second step, started at around 275 °C, was attributed to the scissions at unsaturated ends including a hemolytic scission β to the vinyl group. The last step, started at around 340 °C, could be explained by the random scission within the polymer chain.36 However, the natural wood samples undergo two degradation steps. The first step started at around 100 °C could be caused by the evaporation of water, and the second step started at around 260 °C would be ascribed to the degradation of wood component such as cellulose, hemicellulose, and lignin.37 Similar degradation behaviors could be observed in LW. The first degradation step in the range of 100−260 °C due to the evaporation of water and the scissions of head-to-head linkages (H−H) in PMMA. The second degradation steps in the range of 260−420 °C could be caused by the degradation of wood component and PMMA.

Figure 7. Transmittance of the wood samples obtained from different stages of processing.

Natural wood shows negligible transmittance due to the light absorption of lignin and light scattering in porous wood structure. After lignin removal, light scattering still in porous wood structure, thus the transmittance of delignified wood is only 4.9%. Until polymer impregnation (without γ-Fe2O3@

Figure 6. (a) TG and (b) DTA curves of the PMMA, natural wood, and luminescent wood with a γ-Fe2O3@YVO4:Eu3+ NP concentration of 0.5 wt %. 3858

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Figure 8. (a) Excitation spectra and (b) emission spectra of luminescent wood.

YVO4:Eu3+ NPs), a highly transparent wood composite with the transmittance of 86.1% was achieved. As the concentration of γ-Fe2O3@YVO4:Eu3+ NPs increased, the transmittance of LW was decreased in 350−800 nm wavelength region, while it is generally higher than that of the natural wood (Figure 7). This is mainly caused by the optical absorption of as-prepared nanoparticles.39 Additionally, the larger amount of γ-Fe2O3@ YVO4:Eu3+ NPs that fills into the wood template can lead to the agglomeration of particles, which also will increase the light reflection and further decrease the transmittance of wood composites. However, the transmittance of LW that possesses γ-Fe2O3@YVO4:Eu3+ NPs of 0.1, 0.2, and 0.5 wt % is 80.6%, 73.2%, and 40.8%, respectively. The highest transmittance of the LW is 80.6%, which is good enough to be applied in transparent devices. The corresponding photoluminescence spectra of LW with different concentrations of γ-Fe2O3@YVO4:Eu3+ NPs are shown in Figure 8. Figure S3 demonstrates the photoluminescence spectra of YVO4:Eu3+ NPs and γ-Fe2O3@ YVO4:Eu3+ NPs. For the excitation spectrum of LW, there is a broad band extending from 250 to 350 nm with a maximum value at 300 nm in Figure 8a. The band corresponds to charge transfer bands of Eu−O and V−O and VO43− absorption bands.40,41 Both the LW, YVO4:Eu3+, and γ-Fe2O3@YVO4:Eu3+ NPs show similar excitation spectra in the range of 250−350 nm, but two main peaks at 397 and 467 nm disappear in the excitation spectra of LW and γ-Fe2O3@YVO4:Eu3+ NPs (Figure S3), which correspond to the electron transitions of VO43− groups to Eu3+ from 7F0−5L6 and 7F0−5L2, respectively. This phenomenon may be caused by the light absorption of γ-Fe2O3 NPs impregnated into the composites.42 Moreover, the excitation intensity was increased at first and then decreased with adding more γ-Fe2O3@YVO4:Eu3+ NPs. The highest intensity took place when the weight ratio of γ-Fe2O3@ YVO4:Eu3+ NPs to PMMA was 0.5 wt %. For the emission spectra of LW, as shown in Figure 8b, three peaks at 594, 650, and 698 nm are caused by 5D0−7F1, 5D0−7F3, and 5D0−7F4 transitions of Eu3+ ions in YVO4, respectively. However, the main emission peak at 619 nm corresponds to the 5D0−7F2 transition of Eu3+ ions, revealing that the 3p electrons of Eu3+ ions are occupied as a site without an inversion symmetry.43 As reported by Reisfeld et al.,44 when the ratio I(D0−F2)/I(D0−F1) is inferior to 1, indicating the Eu3+ site is totally symmetry. In our case, the ratio I(D0−F2)/I(D0− F1) is around 6, suggesting the Eu3+ site is low symmetry. The

inset of Figure 8b is plots of emission intensity at 619 nm variation with various (Eu3+ ions) γ-Fe2O3@YVO4:Eu3+ NPs doping concentration. A comparison among LW with different NPs doping concentration suggests that LW with the γ-Fe2O3@ YVO4:Eu3+ NPs concentration of 0.5 wt % has the strongest emission intensity. Higher Eu3+ concentrations in LW result in quenching the photoluminescence intensity. Magnetic γ-Fe2O3 NPs also make some side effects on emission intensity. Therefore, the optimum doping concentration of γ-Fe2O3@ YVO4:Eu3+ NPs is 0.5 wt %. Meanwhile, the effect on quantum yields via impregnation of different amounts of γ-Fe2O3@YVO4:Eu3+ NPs into the wood composites was studies, and the results are shown in Table 1. Table 1. Quantum Yields of As-Prepared Samples sample

quantum yield (%)

YVO4:Eu3+ γ-Fe2O3@YVO4:Eu3+ LW-0.1 wt % LW-0.2 wt % LW-0.5 wt % LW-1 wt %

74.44 1.15 0.10 0.11 0.64 0.44

The quantum yield of γ-Fe2O3@YVO4:Eu3+ NPs is found much smaller than YVO4:Eu3+ NPs, which may be caused by the quenching of the luminescence by magnetic γ-Fe2O3 NPs. Furthermore, compared with the bulk γ-Fe2O3@YVO4:Eu3+ NPs, there is a decline on the quantum yields of LW. A explanation may be that the wood template is nonluminescent material, so the PL efficiency of the LW will decrease with its percentage of increase. Thus, a low quantum yield of LW is obtained. For LW with different concentrations of γ-Fe2O3@ YVO4:Eu3+ NPs, with the increasing amount of Eu3+, more energy transfer takes place between vanadates groups to europium ions, contributing to a improvement of the quantum yield. More significantly, however, when the concentration of γFe2O3@YVO4:Eu3+ NPs reached 1 wt %, an additional quenching path occurs from energy transfer between neighboring Eu3+ which results in a drop of the quantum yield.45 All in all, the highest quantum yield of LW is obtained for the γ-Fe2O3@YVO4:Eu3+ concentration of 0.5 wt %. Not only does LW possess luminescent properties, but it is also a kind of superparamagnetic material, which provides potential for magneto-optical applications. Figure 9 is magnet3859

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the magnetic field. α is a function of the electron spin magnetic moment m (μB, μB is the Bohr magneton) of the individual molecules, which can be expressed as in eq 3: α=

(3)

Where, kB is Boltzmann’s constant and T is the absolute temperature. According to Figure 9, the magnetization (M) of LW increases with the increasing magnetic field (H) until it reaches the saturation value. The Ms values of LW which possesses γ-Fe2O3@YVO4:Eu3+ NPs of 0.1, 0.2, and 0.5 wt % are 0.26, 1.19, and 2.10 emu/g, respectively. As expected, the M s values of LW with the γ-Fe 2 O 3 @YVO 4 :Eu 3+ NP concentration of 1 wt % is 3.35 emu/g, which is greater than that of the LW with lower γ-Fe2 O3 @YVO4:Eu 3+ NP concentration (Figure S4). The results suggest a higher concentration of γ-Fe2O3@YVO4:Eu3+ NPs in wood template will better maintain the magnetic properties. The best fit to eq 2 is obtained using a nonlinear fitting of magnetization (M) and magnetic field (H) by Polymer software. The α parameter of for the γ-Fe2O3 NPs, γ-Fe2O3@ YVO4:Eu3+ NPs, and LW with the γ-Fe2O3@YVO4:Eu3+ NP concentrations of 0.1, 0.2, and 0.5 wt %, determined from the fit, are 4.75 × 10−3, 4.57 × 10−3, 4.50 × 10−3, 4.49 × 10−3, and 4.69 × 10−3 T−1, respectively. The fitting correlation coefficient R2 of all samples are 0.9967, 0.9970, 0.9988, 0.9986, and 0.9981, respectively. According to eq 3, the magnetic moment (m) was calculated from α, and the values of the γ-Fe2O3 NPs, γ-Fe2O3@YVO4:Eu3+ NPs, and LW with the γ-Fe2O3@ YVO4:Eu3+ NP concentrations of 0.1, 0.2, and 0.5 wt % are 2.04, 1.96, 1.94, 1.93, and 2.02 μB, respectively. The calculated magnetic moment (m) does not change much between the five samples, suggesting the fabrication process has little effect on the magnetic moment of γ-Fe2O3 NPs. For practical applications, it is important to evaluate the mechanical properties and dimensional stability of LW. The asprepared wood samples under the stretching process in the longitudinal direction are shown in Figure 10a. Natural wood possesses the fracture strength and modulus of 38.32 MPa and 5.04 GPa, respectively, because of the hierarchical structure of wood and the strong interactions among wood components such as cellulose and lignin.47 Compared with the natural wood, the delignified wood shows reduced mechanical properties with

Figure 9. Magnetization curves of luminescent wood at room temperature. (inset) Magnetization curves of as-prepared γ-Fe2O3 NPs and γ-Fe2O3@YVO4:Eu3+ NPs. The digital photograph shows that the luminescent wood with a γ-Fe2O3@YVO4:Eu 3+ NP concentration of 0.2 wt % is easily lifted with a magnet.

ization curves of LW with different concentrations of γ-Fe2O3@ YVO4:Eu3+ NPs at room temperature. The inset shows the magnetization curves of γ-Fe2O3 NPs and γ-Fe2O3@YVO4:Eu3+ NPs. The saturation magnetizations Ms of as-prepared γ-Fe2O3 NPs and γ-Fe2O3@YVO4:Eu3+ NPs are 68.02 and 27.77 emu/g, respectively. The large drop in Ms of the γ-Fe2O3@YVO4:Eu3+ NPs is caused by the presence of the nonmagnetic YVO4:Eu3+ NPs. Similar results are observed in LW. The natural wood is observed to be a nonmagnetic material as expected. Interestingly, all of the LW samples show polar properties and exhibit a typical superparamagnetic behavior at room temperature due to the immeasurable coercivity and remanence. Generally, the Langevin equation can use to define the magnetic properties of composites in a superparamagnetic system:46 M 1 = coth x − Ms x

m kBT

(2)

Where, M (emu/g) is the magnetization and Ms (emu/g) is the saturation magnetization. Usually, x = αH. Where, H (Oe) is

Figure 10. (a) Experimental stress−strain curves of natural wood, delignified wood, and luminescent wood with a γ-Fe2O3@YVO4:Eu3+ NP concentration of 0.5 wt %. (b) Volume changes of natural wood and luminescent wood with a γ-Fe2O3@YVO4:Eu3+ NP concentration of 0.5 wt % during the water immersion test. 3860

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Figure 11. Photographs of (a) natural wood, (b) delignified wood, (c) transparent wood, (d) luminescent wood with a γ-Fe2O3@YVO4:Eu3+ NP concentration of 0.1 wt %, and (e) luminescent wood with the γ-Fe2O3@YVO4:Eu3+ NP concentration of 0.1 wt % under UV light excitation at 254 nm.

a fracture strength and modulus down to 17.78 and 0.66 GPa, respectively. The low strength and modulus of the delignified wood are caused by the lack of strong load transfer mechanisms between the cellulose nanofibers after lignin removal.18 However, LW shows improved mechanical properties with a fracture strength and modulus up to 45.92 MPa and 2.66 GPa, respectively, which can be explained enhanced cellulose nanofibers interaction after PMMA infiltration. The results of the dimensional stability studies of LW are presented in Figure 10b. The gain in volume during storage in water of LW is reduced significantly compared with the natural wood. The maximum volume gain of natural wood reached 40.4% after 60 days of soaking, whereas LW displays a 15.8% volume gain, indicating that better dimensional stability of LW was obtained compared with the natural wood. We attribute this to the blocking effect of the filling PMMA, which are impregnating the cell lumen as presented in Figure 3f, thus resulting in reduced water absorption of LW. Figure 11 shows photographs of the wood samples. In Figure 11a, the wood slice with yellow color is optically opaque. After lignin removal, the wood slice is still opaque, but the color of wood is white (Figure 11b). Until the polymer infiltration, the wood slice is optically clear (Figure 11c). With the addition of γ-Fe2O3@YVO4:Eu3+ NPs, a highly transparent and brown wood slice was obtained (Figure 11d). In Figure 11e, it can be obviously seen that LW emits red luminescence, under UV excitation at 254 nm.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.L.). ORCID

Jian Li: 0000-0001-5795-5973 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by The National Natural Science Foundation of China (grant no. 31470584).

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REFERENCES

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CONCLUSIONS In summary, we have demonstrated a facile method for incorporating luminescent γ-Fe2O3@YVO4:Eu3+ NPs and index matching poly(methyl methacrylate) (PMMA) into a delignified wood template. The resultant luminescent wood glows a bright red color under a single wavelength excitation. Moreover, high optical transmittance of 80.6% and saturation magnetization of 0.26 emu/g were achieved at a luminescent wood with 0.1 wt % γ-Fe2O3@YVO4:Eu3+ NP loading. Optical transmittance decreased as the γ-Fe2O3@YVO4:Eu3+ NP mass fraction increased, whereas saturation magnetization increased for the same changes. Furthermore, the luminescent wood possessed higher thermal properties, dimensional stability, and mechanical properties than the natural wood. With the advantages of high transparency, unique luminescence, moderate magnetism, good thermal properties, dimensional stability, and excellent mechanical properties, these luminescent wood samples exhibit great promise as green LED lighting equipment, luminescent magnetic switches, and anticounterfeiting facilities.

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Histogram of diameter of nanocomposites, EDS elemental analysis, ICP-MS measurements, photoluminescence spectra, and magnetization curve (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02985. 3861

DOI: 10.1021/acssuschemeng.6b02985 ACS Sustainable Chem. Eng. 2017, 5, 3855−3862

Research Article

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