Preparation and Luminescence Properties of Europium(III) Ternary Complex-modified Poplar Wood-based Materials

Abstract

Inspired by the ultraviolet induced properties of biological materials, we developed fluorescent wood as a new type of functional material which can be applied to anti-counterfeit label technology and wood-emitting artwork. This study discusses the influence of temperature, time, ligand ratio, and reactant concentration on the structure and properties of fluorescent wood with europium(III) ternary complexes. The surface morphology and fluorescent properties of the modified wood were characterized by scanning electron microscopy (SEM) and fluorescence spectrometry. The results showed that a high reaction temperature of 75 ºC was beneficial in obtaining stronger fluorescent wood; a reaction time of 8 h exerted a significant influence on fluorescent wood when the reactant concentration of the sample was 0.03 mol/L. In addition, the results of infrared spectroscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray analysis showed that modification of wood takes place through the Si-OH groups of the silane coupling agent, which link to the surface of wood. After 120 hours of UV light irradiation, the surface fluorescence intensity of the modified wood can reach 500 arb. units. It can be considered that the modified wood has better anti-ultraviolet aging properties, thereby prolonging the life of poplar.

Keywords:
Fluorescent; europium ternary complexes; silane-modified; poplar wood-based materials

1. Introduction

Wood is a carbon-neutral material and hence does not result in a net increase in carbon dioxide due to cyclic carbon fixation during photosynthesis and degradation ¹. Apart from usage in houses, furniture, artwork, heating, and decoration, wood and its derivative materials are also widely used in green electronics ^2,3. With continuous development in the field of materials science, wood materials are increasingly being considered as biological templates with new properties ⁴. In recent years, many approaches for modifying wood surfaces have been proposed, such as magnetic biochar based on wood ⁵. There are many studies on the production of superhydrophobic wood materials ^6,7. Nowadays, only a few wood types are being used as templates for the preparation of luminescent materials ⁸. It is well known that europium-based fluorescent complexes, which are red-light emitting phosphor materials, exhibit long luminescence lifetimes and sharp emission bands ⁹.

The surface of the wood-based material has a large number of reactive functional groups, and grafting reaction can occur between the functional materials. The organic-inorganic hybrid wood-based fluorescent material with excellent performance, which not only realizes the light functionalization of wood, but also can be used for the construction of surface patterns of wood products, design of new wooden-based anti-counterfeit labels and exhibition of wooden art. The advantages of phosphor materials extracted from wood biotemplates may be of significant benefit in anisotropic structures, when combined with prospective lightguide properties ¹⁰.

In order to broaden the application of wood as a template material for optical functionality, this study describes a type of modified wood synthesized with europium ternary complexes; these modified woods, unlike untreated wood, exhibit fluorescence. To analyze the chemical nature of such fluorescent wood, several analytical techniques, were employed. It was found from our analysis that the developed fluorescent wood surfaces can prevent wood components from being destroyed by UV light and hence their life can be extended.

2. Methods

2.1 Materials and sample preparation

Ethanol (99.7%) and methylene dichloride (analytical grade) were provided by Tianjin Kaitong Chemical Reagent Co., Ltd. (China). Phthaloyl chloride (98%), aminopropyl trimethoxysilane (KH550), and Eu(NO₃)₃ 6H₂O (99.99%) were procured from Beijing Huaweiruike Co., Ltd. (China), while hydrochloric acid was supplied by Yantai Double Chemical Co., Ltd. (China).

Beams of poplar sapwood (10 mm × 5 mm × 2 mm) were obtained from Linbang Wood Industry Board Factory, ShanDong, China. The samples were impregnated by ultrasonic washing for 10 min with acetone, ethanol, and deionized water (in sequence) and then dried at 80 ºC in a vacuum oven for 2 h.

2.2 Synthesis of the modified complexes

Phthaloyl chloride (0.001 mol) in methylene chloride (10 mL) was added to a solution of aminopropyl triethoxysilane (0.002 mol) in methylene chloride (10 mL) at 0 ºC. Triethylamine was added to the reaction mixture and stirred at room temperature for 2 h. After washing and purification, a yellow viscous liquid of the modified ligand was obtained, as is showed in Fig.1. The modified ligand, 1,10-phenanthroline, and Eu(NO₃)₃·6H₂O were accurately weighed at pre-determined quantities and dissolved in 10 mL of anhydrous ethanol. The complexes were heated continuously under reflux at different temperatures (35, 45, 55, 65, and 75 ºC) for different time periods (4, 6, 8, 10, and 12 h)¹¹. Later, the mixtures were dried in an oven at 80 ºC for 12 h. (See the Fig.2)

Figure 1
Modified ligand structure

Figure 2
Modified complex structure

2.3 Synthesis of fluorescent wood

The infiltration solutions were prepared by dissolving the modified complexes in ethanol at different concentrations (0.005, 0.0075, 0.01, 0.0125, 0.015, 0.02, 0.03, and 0.04 mol/L). The wood to be treated was placed in the desired solution to which a few drops of dilute hydrochloric acid were added and then stirred at 70 ºC for 4 h. Finally, the wood samples were placed in a vacuum drying oven and dried at 120 ºC for 30 min.

3. Characterization

The elemental compositions of the wood samples were determined by IR spectroscopy (FT-IR, Magna-560, Nicolet) and XPS analysis (PHI Thermo Fisher Scientific Company Quantera). Thermogravimetric analysis (TGA) was performed on a thermal analyzer (Q500, American TA Company) in an inert (N₂) environment at a heating rate of 10 ºC/min between ambient temperature and 800 ºC. The infiltrated wood samples and native wood samples were transferred onto a sample holder for SEM analysis and sputtered with gold (Quanta 200, FEI, Hillsboro, Oregon). Luminescence spectra were recorded at room temperature using a LS55 fluorescence spectrometer. The excitation and emission monochromator slit widths were 10 nm and 5nm, respectively, and the speed was set at 600 nm/min. The wettability of the solid surfaces was measured using an optical contact angle meter with 5 µL deionized water droplets at room temperature.

4. Results and Discussion

4.1 The formation of chemical bonds on wood.

The phthalyl chloride reacts with KH550 to form a modified ligand. The C=O in the modified ligand and the N in 1,10-phenanthroline coordinate with the central ion Eu³⁺ to form a silane modification complex. The formation of chemical bonds on wood surfaces occurs due to modification with rare earth complexes (-Si-OC₂H₅ groups) in acid hydrolysis conditions, which generates a large number of Si-OH groups ¹². The Si-OH groups are absorbed by the wood surface and penetrate the fibers due to the porous nature of wood ¹³. Si-OH groups can efficiently undergo a self-condensation reaction to form Si-O-Si bonds. Therefore, after condensation polymerization under heating conditions, modified rare earth complexes are combined with wood surfaces by covalent bonds ¹⁴ (see the Fig. 3).

Figure 3
The formation of chemical bonds on wood

4.2 EDX analysis

EDX patterns of native wood indicate a composition consisting of 61.42% C and 38.58% O. In contrast, the modified fluorescent wood (By impregnated with 1:3:1 modified complex) also included 9.65% Si and 11.95% Eu. The mass ratio of silicon and europium is 0.81, while the material ratio of reactions is 1.1. The ligand modified by the silane coupling agents does not adequately react with europium ions. The modified fluorescent wood samples show an absorption peak corresponding to silicon at 1.84 keV and atomic europium at 5.9 keV. Meanwhile, the absorption peak of atomic chlorine at 2.63 keV could be ascribed to the presence of dilute hydrochloric acid that could accelerate hydrolysis (see the Fig. 4).

Figure 4
EDX analysis of native wood(a) and modified fluorescence wood(b)

4.3 IR analysis

Fig. 5 depicts the IR spectra (4000-400 cm^-1) of native wood and modified wood samples. The absorption peaks at 1031 cm^-1 and 2910 cm^-1 are attributed to cellulose, while the peak at 1736 cm^-1 is attributed to hemicellulose ^15-17. The -OH group vibration at 3350 cm^-1 and benzene carbon skeleton absorption peak near 1505 cm^-1 in Fig. 5(a) can be observed to be different from those in Fig. 5(b). The characteristic absorption peak at 3338 cm^-1 shifted to a high wavenumber, which corresponds to a change in the density of the electron cloud; this change is responsible for the weakening of σ covalent bonds and induces a red shift in the characteristic absorption peak. The absorption peak at 722 cm^-1 can be assigned to the stretching vibrations of the Si-C bonds of siloxane components. The typical absorption peaks of Si-O-Si bonds of siloxane components appearing at 1031 cm^-1 overlap with the characteristic peaks of cellulose ^18,19. Analysis of the spectra confirmed the presence of silicon atoms on the surface of the wood samples.

Figure 5
IR spectra of native and modified wood

4.4 XPS analysis

To further gather molecular-level information on Eu(III) adsorbed onto wood, XPS measurements were conducted for the surface-elemental analysis of 1,10-phenanthroline,modified ligand, native wood, modified complexes, and modified wood. The average binding energies of C_1s, O_1s, N_1s, Si_2p, and Eu_3d are listed in Table 1. As shown in Fig. 6(a), two major peaks of C and O are clearly distinguishable, indicating that the native wood was mainly composed of C (66.5 at.%) and O (33.5 at.%). The average binding energies of C_1s, O_1s, N_1s, Si_2p, and Eu_3d in the modified wood increased by 0.96, 0.85, 2.38, 0.46, and 1.14 eV, respectively, as compared to the modified complexes.

Table 1
The bonding energies of electrons (eV) in different samples

Samples	C1s	O1s	N1s	Si2p	Eu3d
Modified ligand	284.28	531.86	405.21	101.49	-
1,10-phenanthroline	283.74	-	397.59	-	-
Modified complexes	283.69	530.87	398.53	101.42	1133.47
Modified wood	284.65	531.72	400.91	101.88	1134.61
Native wood	284.99	532.02	-	-	-

Figure 6
XPS survey spectra (a) native wood, (b) modified w ood, (c) modified complexes,(d) modified ligand and (e) 1,10-phenanthroline

The bond energies of N_1s and O_1s in different samples are shown in Table 2. For 1,10-phenanthroline, the main peak at 397.6 eV is assigned to the N element (C=N-C) on the aromatic ring bonded to the C atom in the sp₂ hybrid form. The second peak at 400.25 eV is attributed to the N element (H-N-C₂) connected to the C atom in the sp₃ hybrid form. The weakest peak at the binding energy of 404.8 eV is due to the excitation of the π electron cloud in the N element ²⁰; The binding energy of N_1s of 1,10-phenanthroline and native wood was shifted toward the high binding energy direction. The formation of the modified complex involves the coordination of Eu-O and Eu-N, so the binding energy of both ligands N_1s is shifted toward the high binding energy compared to the modified complex. The O_1s binding energy of modified ligand, modified complexes and 1,10-phenanthroline was shifted toward the low binding energy direction compared with the modified wood. This is due to the efficient self-condensation reaction of the Si-OC₂H₅ groups in the complexes with the -OH groups on the surface of the wood to form Si-O-C groups, which led to a decrease in the electron density of the O atoms.

Table 2
The bonding energies of N_1s and O_1s in different samples

Samples	N1s(eV)
Modified ligand	405.20	-	-	531.50	532.50
Modified complex	406.60	-	-	531.30	532.30
Modified wood	-	400.96	399.80	531.20	532.00
1,10-phenanthroline	404.80	400.25	397.61	-	-
Native wood	-	-	399.45	531.98	-

4.5 Fluorescence spectroscopy

4.5.1 Ligand ratio of the sample

The effect of the ligand on the fluorescent wood was determined by changing the ratio of 1,10-phenanthroline and the modified silane ligand, as shown in Table 3. These samples were heated for 4 h at 70 ºC. When the molar ratio of 1,10-phenanthroline/Eu³⁺ is 1:1, the ratio of Eu³⁺/ligands peak intensity is equal to 1:1, the fluorescence emission of the 1:1:3 (Eu: 1,10-phenanthroline: silane ligand) complex is the strongest (see the Fig. 7).

Table 3
Preparation of different ratios of ligands/Eu ³⁺ions (0.01 mol)

Molar ratio of Eu3+:1,10-phenanthroline:modified ligand	Eu3+ peak intensity (I/arb.units)	Ligands peak intensity (I/arb.units)	Ratio of Eu3+/ligands
1:2:2	58.201	247.051	0.236
1:2:3	66.675	159.443	0.418
1:1:3	168.6086	168.8972	0.998
1:2:1	55.025	88.126	0.624
1:3:1	23.527	80.755	0.291

Figure 7
Emission spectra of different ratio

4.5.2 The reactant concentration of the sample

In order to investigate the fluorescent properties of the modified wood samples, their emission spectra were recorded in a homogeneous powder state at room temperature. Fig. 8 illustrates the spectra of wood samples containing 1:1:3 (Eu: phenanthroline: silane ligand) at different concentrations of 0.005, 0.0075, 0.01, 0.0125, 0.015, 0.02, 0.03, and 0.04 mol/L; small differences in the peak intensities could be observed. Under UV irradiation with 395-nm light, the emission spectra of modified wood exhibited characteristic ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃, and ⁵D₀→⁷F₄ electronic transitions at 590, 614, 650, and 686 nm, respectively. The most intense peak at 614 nm was an induced electric dipole for the ⁵D₀→⁷F₂ transition of Eu³⁺, which indicates that the complexes occupy a site of low symmetry surrounding the Europium ion ²¹. In this situation, according to the Judd-Ofelt theory, the ⁵D₀→⁷F₀ transition was forbidden and it was always absent or much weaker than ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ due to the same angular momentum ^22,23. And the ⁵D₀→⁷F₁ transition is responsible for only one peak in the spectra. These results suggest the presence of a uniform chemical environment around the Eu³⁺ ion. It can be seen in Table 4 that the 0.03mol/L wood sample has the strongest emission intensity among all the modified samples.

Figure 8
Emission spectra of different concentration

Table 4
Fluorescence intensity of modified fluorescent wood at different reaction concentration

Number	1	2	3	4	5	6	7	8
Samples concentration (10–1 mol/L)	0.05	0.075	0.1	0.125	0.15	0.2	0.3	0.4
Fluorescence intensity (I/arb.units)	78.9	114.7	168.7	200.6	333.0	487.3	557.2	422.4

4.5.3 Effect of reaction time

In order to investigate the effect of different reaction times on the modified fluorescent wood, the reaction temperature and impregnation ratio were maintained at constant values.

As shown in Table 5, the fluorescence emission intensity at 619 nm increased firstly and then decreased with an increase in the reaction time. Over time, the product on the surface of the wood gradually increased in quantity. After 10 h, the modified complex reacted completely with wood and the measured fluorescence emission intensity increased gradually. Therefore, the optimum reaction time for synthesizing modified fluorescent wood was determined to be 8 h (see the Fig. 9).

Table 5
Fluorescence intensity of modified fluorescent wood at different reaction times

Reaction time (h)	4	6	8	10	12
Fluorescence intensity (I/arb. units)	210.9	237.5	354.2	196.2	196.2

Figure 9
Emission spectra of different time

4.5.4 Effect of reaction temperature

With an increase in the reaction temperature, the fluorescence intensity of the modified wood samples increased. As shown in Table 6, an increase in temperature is beneficial for accelerating the reaction rate, resulting in a stronger fluorescence emission. The boiling point of absolute ethanol is 78.5 ºC, beyond which the reaction temperature cannot be increased. Under such conditions, the emission intensity of the wood sample at 75 ºC was found to be the strongest (see the Fig. 10).

Table 6
Fluorescence intensity of modified fluorescent wood at different reaction temperatures

Reaction temperature (ºC)	35	45	55	65	75
Fluorescence intensity (I/arb.units)	109.6	124.3	172.9	208.0	235.8

Figure 10
Emission spectra of different temperature

4.6 TG analysis

The TG and differential thermogravimetry (DTG) curves of the native wood and modified fluorescent wood samples are depicted in Fig. 11. In the DTG curves, the first step could be attributed to the evaporation of water present in the wood samples. The second step in the pattern of poplar wood revealed a peak decomposition rate at around 270 ºC, which was attributed to the degradation of wood components. However, modified fluorescent wood revealed a decomposition peak at 220 ºC. This stage is attributed to the dehydration of Eu³⁺ complexes and wood components ²⁴. On comparing the TG results of samples with different impregnation ratios at 800 ºC, the mass loss of modified fluorescent wood was found to be around 86% and poplar wood almost completely decomposed. Moreover, the exothermic peaks of the modified fluorescent wood samples were observed to be weakened in the differential thermal analysis (DTG) curves, which indicates their good thermal stability ²⁵.

Figure 11
(a)TG/(b)DTG curves of different impregnation ratio of the wood

In order to confirm the percentage of Si and Eu that was deposited in the modified wood (Eu: phenanthroline: silane ligand=1:3:1), the test of this reside in EDX analysis could quantify the actual percentage of Si and Eu deposited, as shown in Fig.12. Compared with Fig4(b), the chlorine in the residue disappears, mainly because the C-Cl bond is destroyed under high temperature conditions, and the Si-N of the silane coupling agent and 1,10-phenanthroline are also destroyed. The mass ratio of C, N, O and Eu elements in the modified wood residue decreased. The reside included 8.56% Si and 20.99% Eu. The actual percentage of silicon and europium is 0.41.

Figure 12
EDX analysis of residue modified wood (1:1:3)

4.7 Scanning electron microscopy

The images in Fig. 13 show the SEM images of native and modified fluorescent wood samples. Fig. 13(a) and (b) show the native wood surfaces, including vessel architecture and pit hole morphology. Fig. 13(c) shows that the microscopic details of the pit holes were covered. Fig. 13(d) indicates a relatively smooth surface for modified fluorescent wood. In contrast, a dense-packed layer and a smooth vessel inner wall were observed on the surfaces of modified fluorescent wood. The filling up of the crevices shows that the modified complex reacted with the -OH groups on the surface of wood and adhered to it.

Figure 13
SEM images of vertical section of different samples (a), (c) vessel of native wood and modified fluorescence wood(×500) (b), (d) pit hole of native wood and modified fluorescence wood(×1000)

4.8 Fluorescence stability

As shown in the Fig.14, the 1:3:1 modified wood was irradiated with 310nm UV lamps for 0 h, 24 h, 48 h, 72 h, 96 h, 120 h, respectively. When the aging time reached 24 h, the fluorescence intensity of the wood fluctuates within a certain range. This shows that fluorescent wood has a strong ultraviolet absorption capacity and can continuously emit light under the excitation of ultraviolet light. This property can reduce the loss of wood due to ultraviolet light and prolong its life. On the other hand, it realizes the application of fluorescent wood in anti-counterfeiting and art exhibits.

Figure 14
The fluorescence intensity of modified wood with different UV aging times

5. Conclusions

Fluorescent wood was synthesized from native wood by modification with rare earth Eu³⁺ ternary complexes. The optimized reaction conditions were determined by measuring the fluorescence emission intensity of Eu³⁺; they were as follows-reaction time of 8 h, reaction temperature of 75 ºC, Eu³⁺:1,10-phenanthroline:modified silane ligand ratio of 1:1:3, and reactant concentration of 0.03 mol/L. The strongest fluorescence emission intensity of 557 arb.units was assigned to the ⁵D₂-⁷F₀ transition of Eu(III) ion in the fluorescence spectrum. The results of IR, XPS, and EDX analyses indicate that modified complexes on the surface of the wood. It has been proposed that the Si-OH groups of the silane coupling agent link with the surface of wood. Compared to untreated wood, fluorescent wood exhibited better thermal properties and hydrophilicity.

The synthesized fluorescent wood can be used to construct new wood-based materials. Due to the presence of europium complexes, fluorescent wood samples can glow with a reddish photoluminescence. The application of such fluorescent wood will extend the usable life of poplar and prevent wood structural units from being destroyed by UV light. Further, such fluorescent woods not only enhance resource use and the efficiency of short wood, but also play an important role in anti-counterfeiting efforts.

6. Acknowledgements

This study was supported by the financial supports from the National Natural Science Foundation of China (GN:31770593) and the Fundamental Research Funds for the Central Universities (GN: 2572017CB18).

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Publication Dates

• Publication in this collection
  23 Sept 2019
• Date of issue
  2019

History

• Received
  05 Nov 2018
• Reviewed
  14 May 2019
• Accepted
  29 June 2019