A Facile Synthesis and Photoluminescence Properties of SiO₂:Tb³⁺ Spherical Nanoparticles

Controlled synthesis of functional photoluminescent materials are of particular interest due to their fascinating optical properties. Herein, highly uniform SiO ₂:xTb ³⁺spherical nanoparticles are fabricated by a facile sol-gel method. The structure, morphology, compositions, and luminescence properties of As-prepared samples were well investigated using TEM, SEM, EDX, XRD, XPS and luminescence spectroscopy. The PL intensity of SiO ₂:xTb ³⁺spherical nanoparticles is Tb³⁺ ions concentration dependent achieved a maximum at 3 mol % of Tb³⁺. Particularly, SiO₂:xTb³⁺spherical nanoparticles exhibit a green emission corresponding to ⁵D₄ → ⁷ F_j transition (541 nm) of Tb³⁺. These results show that As-prepared phosphors may find potential application in solid-state lighting fields.

• Silica
• Sol-Gel
• Photoluminescence
• Terbium Doped-Silica

1. Introduction

Lanthanides have unique photoluminescence properties, which enables their utilization in diverse applications, such as optical devices [1] , biological fluorescence imaging and detections [2] [3] [4] [5] [6] , lighting and displays [7] [8] [9] depending on their morphology, size, and composition [10] . Particularly, Tb³⁺ is an intriguing trivalent lanthanides ion due to its green light emission under near-ultraviolet excitation. Silica is a fantastic oxide bolster material for an extensive variety of applications, such as electronics, displays, sensors, catalysts, and biomedical imaging, due to its thermal stability and endurable biocompatibility [11] - [17] . In addition, the silica spine cannot only serve to bulwark and firm the functional structure inside but also allow its optical properties to be inquired from outside due to the chemical activity and optical transparency of silica [18] . Furthermore, numerous researchers have been dedicated to the synthesis of rare earth ions doped SiO₂ materials with different morphologies via various synthesis methods. For example, Chen Jie et al. have successfully prepared SiO₂:Eu³⁺ fibers by both electrospinning technique and sol-gel process [19] . Gao Fei et al. prepared europium (III)-doped silica nanotubes by sol-gel method and discussed many influencing factors such as NH₄OH addition, stirring time and the formation mechanism [20] . Lidia Armelao et al. have reported the formation of strong O-Tb complex resulting in reducing the high frequency oscillators and enhancing the PL intensity [10] . The oxidation state of Tb(III) ions is very stable in a silica matrix, hence strong green luminescence can be obtained by a high temperature thermal-annealing treatment having reported by Choi et al. [21] . Inspired by this, herein we have synthesized SiO₂:xTb³⁺ nanospheres by sol-gel method in this work, which is facilitating homogeneous doping of Tb³⁺ into SiO₂ matrix without segregation that is highly required to increase the PL activity. The structure, morphology, and photoluminescence properties of SiO₂:xTb³⁺ nanospheres were investigated as a function of Tb³⁺ concentration.

2. Experimental Section

2.1. Materials

Cetyltrimethylammonium bromide (CTAB), Ammonia, ethanol, were purchased from Beijing Chemical. Tetraethyl orthosilicate (TEOS) was bought from Aladdin Reagent Database Inc. All chemicals were of analytical grade and were used directly without further purification. Ln(NO₃)₃ was prepared by dissolving the corresponding Sm₂O₃ (99.99%) and Tb₄O₇ (99.99%) powder in dilute HNO₃ solution at elevated temperature with ceaseless agitation.

2.2. Preparation

A series of rare earth-doped SiO₂ spherical nanoparticles were prepared via a simple CTAB-based sol-gel process. In a typical process, 0.35 g CTAB was dissolved in 5 mL ethanol and 20 ml deionized water. After stirring several minutes, 0.3 ml Ammonia, 2.2 ml TEOS and different amount of Ln(NO₃)₃ were added into the above solution, respectively. After additional agitation for overnight, the resulting precipitates were collected by centrifugation, washed three times with ethanol, deionized water, and then dried at 60˚C in air for 12 h. The final product was obtained through a heat treatment of the precursor at 600˚C in air for 2 h.

2.3. Characterizations.

X-ray powder diffraction (XRD) was measured by a Rigaku D/max-B II X-ray diffractometer with Cu Ka radiation. Transmission electron microscopy (TEM) images were obtained with a JEM-2000EX TEM (acceleration voltage of 200 kV). The scanning electron microscope (SEM) images were observed by S-4800, Hitachi. Energy-dispersive spectroscopy (EDS) analysis was performed with an H JEOL JXA 840 EDX system attached to the SEM microscope. The X-ray photoelectron spectra (XPS) were taken using a VG ESCALAB 250 electron energy spectrometer with Mg Ka (1253.6 eV) as the X-ray excitation source. The PL measurements were determined using Jobin Yvon FluoroMax-4 luminescence spectrophotometer (PL) equipped with a 150 W xenon lamp as the excitation source. All the measurements were performed at room temperature.

3. Results and Discussion

3.1. Morphologies and Structures

The morphologies and structures of samples were investigated by the SEM and TEM observations. (Figure 1(a)) shows the SEM image of the as-prepared SiO₂:Tb³⁺ spherical nanoparticles as we can see, the morphologies of these SNPs are uniform with a high purity approaching 100%. The decomposition of the organic components (such as CTAB) was further verified by the typical high- magnification transmission electron microscopy (TEM) images as shown in (Figure 1(b)). SiO₂:Tb³⁺nanoparticles are highly dispersed with a narrow size distribution (Figure 1(c)). The average diameter of products was 102.5 ± 4.2 nm and several tens to hundreds of nanometers in length, which were determined by manually measuring 50 randomly selected sphere (Figure 1(d)) by ImageJ. Interestingly, HR-TEM images reveal that these nanoparticles are spherical-like without any undesired impurities observed (Figure 1(e)). Moreover, the selected area electron diffraction (SAED) pattern (Figure 1(f)) shows that the silica doped Tb³⁺ ion is amorphous and no facilities which in agreement with the XRD pattern.

The structure and composition of the as-prepared SiO₂:Tb³⁺ spherical nanoparticles were examined by XRD. As shown in Figure 2, it was obvious that only

Figure 1

Morphology and structural analysis for the SiO 2 :Tb 3+ spherical nanoparticles: representative low and high-magnification SEM image (a), high-magnification TEM image (b), low-magnification TEM image (c), The diagram histogram of the spherical nanoparticles (d), HR-TEM image (e), SEAD pattern (f)

Figure 2

XRD patterns of The SiO 2 :xTb 3+ spherical nanoparticles doping different concentration after calcined at 600˚C (a) x = 0.00; (b) x = 0.02; (c) x = 0.03; (d) x = 0.04

Figure 3

The EDX spectra of SiO 2 : 3% Tb 3+ precursor spheres (a); SiO 2 :Tb 3+ spherical nanoparticles (b) and elemental mapping of SiO 2 :Tb 3+ spherical nanoparticles

broad peaks could be observed at 2θ = 24˚ - 25˚ in all samples, which corresponding to the characteristic diffraction peak of pure amorphous SiO₂ [22] , indicating no other phases or impurities were formed. Meanwhile, it also could be found that the doping could reduce the intensity of samples by increasing concentration. Here, Tb³⁺ ions were hardly substituted with Si⁴⁺ due to the large difference on the ionic radius between Tb³⁺ and Si⁴⁺ (The ion radius of Tb³⁺ and Si⁴⁺ are 1.04 Å and 0.26 Å, respectively) [23] . Thus we speculated that Tb³⁺ ions was incorporated into the network structure of SiO₂ by some weak interaction with O atoms, which reduced the symmetry of SiO₂ framework by deforming the distance of Si-O bond and/or the angle of Si-O-Si bond [24] .

3.2. Component Analysis

The EDX spectra for SiO₂:Tb³⁺ precursor spherical nanoparticles and SiO₂:Tb³⁺ spherical nanoparticles were demonstrated to Figure 3. That EDX characterization demonstrated that four components carbon (C), silicon (Si), oxygen (O) and terbium (Tb) were existed in the forerunner spherical nanoparticles, to which those atomic percent of C might have been high. Following calcination during 600˚C, it Might a chance to be seen main three elements, Si, O and Tb were existed in the SiO₂:Tb³⁺ spherical nanoparticles, which intended that C element resulting from organic component (such as CTAB) was completely removed after heat treatment and immaculate SiO₂:Tb³⁺ spherical nanoparticles were acquired. Those element-mapping images depicted those appropriation about Si, O and Tb components for SiO₂:Tb³⁺ spherical nanoparticles by mapping those same district Similarly as the SEM image, which plainly shown that Si, O and Tb atoms were homogeneously disseminated in the SiO₂:Tb³⁺ spherical nanoparticles. All the results above showed that the luminescent Tb³⁺ doped SiO₂ spherical nanoparticles were arranged effectively.

XPS analysis is conducted to get more insight into the chemical composition and electronic structure of the as-prepared SiO₂:Tb³⁺ spheres. Figure 4 reveals the presence of Si 2p, O 1s, Tb3d, and Tb 4d peaks respectively indicating the formation of SiO₂:Tb³⁺ sphere. The atomic ratio of Si 2P/O 1s/Tb 3d is estimated to be 38.6/61.19/0.77 and 37.6/61.43/0.88 via using 1 and 3 mol % of Tb³⁺. Intriguingly, the binding energy of Si 2P in SiO₂:Tb³⁺ is blue shifted by 0.12 eV and 0.18 eV upon increasing Tb concentration, similarly the biding energy of O1S is blue shifted by 0.06 eV and 0.12 eV (Figure 4). This is ascribed to the doping effect, which alter the electronic structure of Si [25] . Meanwhile, the binding energy of Tb 4d_3/2 is 154.35 and 154.6 eV via using 1 and 3 mol % of Tb³⁺. The same results were shown in Tb 3d spectra, two new intense peaks around 1277 eV and 1242.9 eV were assigned to the Tb 3d_5/2 and Tb 3d_3/2, respectively (Figure 4). These slightly difference arising from change the chemical environment of Tb³⁺ element by doping into the SiO₂ matrix [26] .

3.3. Photoluminescence Properties

In Figure 5 the luminescent properties of the Tb³⁺ doped silica spherical nanoparticles were investigated. The excitation spectra of the Tb³⁺ doped silica spherical nanoparticles after calcination at 600˚C was shown in Figure 5, the PL excitation spectra obtained by monitoring a green emission with various concentration of Tb³⁺ at 541 nm revealed a strong absorption (4f⁸-4f⁷5d¹) and several narrow peaks at 239 nm (⁷F₆-⁵D₀), 316 nm (⁷F6-⁵L₈), 328 nm (⁷F₆-⁵G4), 346 nm (⁷F₆-⁵G₅), 358 nm (⁷F₆-⁵D₃) and 468 nm (⁷F₆-⁵D₄), which were ascribed to the transitions from the 4f to 5d of the Tb³⁺ ions [27] . Under 377 nm UV radiation excitation, the emission spectrum of SiO₂:0.03Tb³⁺ spherical nanoparticles was composed of a group of sharp lines centered at about 487 nm, 541 nm, 583 nm and 618 nm, which corresponding to the ⁵D₄-⁷F_J (J = 6, 5, 4 and 3) transitions of the Tb³⁺ ions, respectively, indicating the as-prepared SiO₂:0.03Tb³⁺ spherical nanoparticles exhibit characteristic green emission.

Figure 6 presents the PL emission spectra of the SiO₂:xTb³⁺ samples with different Tb³⁺ concentrations at 377 nm irradiation. We could find that the spectra were almost same irrespective of the Tb³⁺ concentration, but with the increasing

Figure 4

Wide-scan XPS spectra and high-resolution Si (2p), O (1s), Tb (3d) and Tb (4d) XPS spectra of SiO 2 :Tb 3+ spheres

Figure 5

PL excitation and emission spectra of the SiO 2 :0.03Tb 3+ spherical nanoparticles

Figure 6

PL emission spectra of SiO2:x%Tb 3+ spherical nanoparticles with various Tb 3+ concentrations, (a) T he emission intensity of Tb 3+ as a function of Tb 3+ concentration, (b)

of the Tb³⁺ concentration from 1 mol% to 5 mol%, the PL intensity of the ⁵D₄ " ⁷F_J (J = 6, 5, 4, 3) transition increased at first, reaching a maximum value at the concentration of 3 mol%, and then decreased with the increasing of Tb³⁺ content due to the concentration quenching effect [28] . This might be due to the cluster of activators at high concentration would lead to the energy transfer by cross- relaxation between Tb³⁺ ions in the SiO₂:Tb³⁺ sphere. For most of rare-earth activators, the concentration quenching effect was ascribed to the non-radiative energy transfer from rare-earth ions to nearby quenching centers, which usually through the exchange interaction and multipole-multipole interaction [29] . At the same time, other non-radioactive processes such as energy transfer to hydroxyl ions and the defects in silica also could contribute to the luminescence quenching effects. It could be indicated that the optimal doping concentration of Tb³⁺ ions was 3 mol % of spherical nanoparticles. The luminescence property of the Tb³⁺ doped SiO₂ spherical nanoparticles was predominantly attributed to ⁵D₄ " ⁷F₆ and ⁵D₄ " ⁷F₅, and the ⁵D₄ " ⁷F₅ peak was dominant in comparison with other peaks, which was a hypersensitive forced electric dipole transition. It was known that the f-f transition arising from a forced electric dipole was forbidden and became partially allowed when the rare-earth ion was situated at a low symmetry site [23] . Therefore, the Tb³⁺ concentration as well as the silica framework structure affected the efficient luminescence of Tb³⁺ ions [13] . From the results discussed above, it can be deduced that the optimal efficient luminescence was observed at the 3 mol %Tb³⁺, which means that the concentration quenching was occurred above 3 mol % Tb³⁺.

The decay kinetics behaviors of Tb³⁺ in SiO₂:xTb³⁺ sphere were investigated. The lifetime decay curves for the ⁵D₄-⁷F₅ transition of Tb³⁺ (541 nm) at different concentration were measured at room temperature under excitation of 377 nm. As illustrated in Figure 7, the decay curves for the ⁵D₄-⁷F₅ transition of Tb³⁺ in all samples could be fitted well by a double-exponential decay [30] :

Figure 7

The decay kinetics behaviours of SiO 2 :xTb 3+ spheres for x = 0.01 (a); x = 0.02 (b); x = 0.03 (c); x = 0.04 (d); x = 0.05 (e).

where I and are the luminescence intensities at time t and 0, and are constants, t is the time, and and are the decay times for the exponential components. Furthermore, the average decay lifetimes (τ) can be calculated as

All the curves can be fitted by a double-exponential procedure, and the lifetime values of Tb³⁺ in SiO₂:Tb³⁺ can be determined to be 0.37, 0.74, 1.12, 0.69 and 0.32 ms corresponding to the Tb³⁺ concentration of 1%, 2%, 3%, 4% and 5% respectively. As seen in Figure 7, with the increase of the Tb³⁺ content, the lifetime values of SiO₂:xTb³⁺ spheres gradually extended until up to x = 0.03, then tended to decrease. The variation tendency of decay lifetime sequence was consisted with the luminescence intensity of samples. That means both the strongest luminescence intensity and longest lifetime value of Tb³⁺ in SiO₂:xTb³⁺ sphere ware at x = 0.03.

4. Conclusion

In summary, a series of SiO₂:xTb³⁺ spherical nanoparticles with various Tb³⁺ concentrations were successfully prepared via a simple CTAB-based sol-gel process. The as-synthesized samples present a well-defined spherical morphology. Under ultraviolet excitation, the Tb³⁺ singly doped SiO₂ samples show strong green emission. Furthermore, the luminescence intensity of the As-prepared Tb³⁺ doped SiO₂ spherical nanoparticles exhibited a high PL intensity determined by concentration of Tb³⁺ and achieved a maximum PL intensity (6.03027 × 10⁶) at 3 mol% Tb³⁺. These results indicate that the As-prepared Tb³⁺ doped SiO₂ spherical nanoparticles with variable concentration could be a potential phosphor to be used in the display and solid state lighting fields.

Acknowledgements

This present work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51272085 and 21671078), the Opening Research Funds Projects of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University (2016-06), and Project Supported by Graduate Innovation Fund of Jilin University (2016145). Murad M.A. Abualrejal greatly appreciates Chinese government scholarship 2013.

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