# The Influence of SiO2 Shell on Fluorescent Properties of LaF3:Nd3+/SiO2 Core/Shell Nanoparticles

Authors: Cui Kai; Gao Chao; Peng Bo; Wei Wei
Journal: Journal of Nanomaterials
 (2010)

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## Abstract

Distinct effects of the SiO2 shell on fluorescence properties of LaF3:Nd3+/SiO2 core/shell nanoparticles were demonstrated by annealling the nanoparticles at different temperatures. Emission spectra, excitation spectra, and decay curves of the nanoparticles were measured. A significant improvement of fluorescence intensity was observed for LaF3:Nd3+/SiO2 core/shell nanoparticles annealed at 900∘C. The phenomenon is ascribed to the change of environment of LaF3:Nd3+ core which is imposed by SiO2 shell. And the change is confirmed by the excitation spectra. It provides a useful way to improve fluorescent intensity of the SiO2-coated LaF3:Nd3+ nanoparticles. The lifetime for nanoparticles annealed at 900∘C shows a slight decrease contrast with nanoparticles annealed at 400 and 600∘C. This is caused by the higher phonon energy of SiO2 than that of LaF3.


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## Body

## 1. Introduction

In the past decade, the synthesis of lanthanide-doped nanoparticles has attracted a great deal of attention, since the materials are considered as potentially useful phosphors in lamps, display devices [1], components in optical telecommunication [2], new optoelectronic devices [3],and probes in biomedical imaging and detection [4]. LaF3 possessing low phonon energy, adequate thermal and environmental stability, is regarded as excellent host matrixes for performing luminescence [5, 6]. Nanoparticles of LaF3 doped with lanthanide ions have been studied for years for their luminescence properties [7, 8]. However, the water and organic molecules absorbed on nanoparticles noticeably hampered their optical efficiency, when the nanoparticles are dispersed into aqueous and organic environment. The O-H and C-H groups have a high vibration frequency and can efficiently quench the luminescence of lanthanide ions [9, 10]. This is in particular true for the lanthanide ions emitted in the near-infrared region, like Nd3+, Yb3+, and Er3+ because the energy gap between excited state and ground state is small [11]. Fortunately, these problems can be overcome when an appropriate shell is grown around the lanthanide ions doped LaF3 core, and silica is usually used as a coating layer due to its high chemical stability, optical transparency, and biocompatibility [12]. LaF3 nanoparticles with different thickness of SiO2 shell were synthesized and the LaF3:Nd3+/SiO2 core/shell nanomaterials used for biological NIR probes has been reported [13, 14]. However, except the protection effect of SiO2 layer, the influence of SiO2 shell on fluorescent properties of the Lanthanide-doped LaF3 core has seldom been discussed.In this work, To investigate the interactions between SiO2 shell and lanthanide ions doped LaF3 core, a series of Nd3+-doped LaF3 nanoparticles capped with SiO2 shell were synthesized and annealed at different temperatures. When the anneal temperature is 900°C, spectroscopic evidence for the change of LaF3 environment created by SiO2 shell was observed. And the change of environment leads to a significant improvement of the fluorescent intensity of LaF3:Nd3+/SiO2 core/shell nanoparticles. This provides a simple and useful way to improve the fluorescent properties of lanthanide-doped LaF3/SiO2 core/shell nanomaterials.


## 2. Experimental

The SiO2-coated LaF3:Nd3+ nanoparticles were synthesized as follows. NH4F (0.44 g, 12 mmol) was dissolved in methanol (20 mL), and then the solution was heated to 65°C. Another solution of La(NO)3·6H2O (1.694 g, 3.89 mmol), Nd(NO)3·6H2O (0.052 g, 0.11 mmol) in  methanol (10 mL) was added dropwise to the NH4F solution. The resulting solution was stirred at 65°C for 2 h. And the LaF3:Nd3+ nanoparticles were collected by centrifugation. Stöber method was adopted for the SiO2 coating process. LaF3:Nd3+nanopartilces (0.5 g) were dispersed in ethanol (100 mL). Then tetraethyl orthosilicate (TEOS) (0.2 mL, 1 mmol) was added dropwise to the solution. After mixing for 1 min, NH4OH (25%) (3 mL) were added in the mixture under stirring. The mixture was stirred for 3 h to get the as-grown sample. For better crystallinity and enhanced luminescence, the as-grown sample was annealed in the air for 4 h to get the final product.The Inductively coupled plasma (ICP) analyses were carried out on a Hitachi P-4010 inductively coupled plasma emission spectrometer. X-ray diffraction (XRD) patterns were measured on a Rigaku D/max-2400 X-ray powder diffractometer. The size and morphology of nanoparticles were determined at 300 kV by a JEOL JEM-3010 transmission electron microscope (TEM) and XL30 field-emission scanning electron microscope (SEM). Photoluminescence emission spectra were recorded on a Zolix Omini-k 300 spectrophotometer pumped by a laser diode at 800 nm. The Fourier transform infrared (FTIR) spectra were made with a Shimadzu FT-IR 8900 spectrometer. The excitation spectra were recorded on an Edinburgh Instruments FLS920 spectrofluorimeter.


## 3. Results and Discussion

The concentrations of Nd, La, and Si in LaF3:Nd3+/SiO2 core/shell nanoparticles were determined to be 1.79, 57.1, and 7.93% by ICP. The XRD patterns of samples annealed at different temperatures are shown in Figure 1. When the as-prepared sample was heated at 900°C for 4 h, well-defined diffraction peaks were obtained. All the peaks can be well indexed to the hexagonal LaF3 crystal structure, and no trace of other characteristic peaks were observed. The TEM and SEM images provide direct information about the sizes and typical shapes of the nanoparticles. Figure 2 illustrates the representative TEM and SEM images of the LaF3:Nd3+ nanoparticles and those coated of SiO2 shells. The bare LaF3:Nd3+ sample contains nanoparticles with an average size of 8 nm. After coating the SiO2 shell, it is clearly observed that the particles have a core-shell structure and the silica shell thickness is about 5–7 nm. When the LaF3:Nd3+/SiO2 nanoparticles were annealed at 900°C for 4 h, the morphology of the sample aggregates with a size from 30–60 nm, and the SEM image (Figure 2(d)) showed that the annealed LaF3:Nd3+/SiO2 nanoparticles consists of spherical particles with a size between 50–150 nm.Figure 1
XRD patterns of LaF3:Nd3+/SiO2 core/shell nanoparticles annealed at different temperatures.TEM images of (a) LaF3:Nd3+, (b) LaF3:Nd3+/SiO2, (c) LaF3:Nd3+/SiO2 annealed at 900°C,SEM image of (d) LaF3:Nd3+/SiO2 annealed at 900°C.
(a)(b)(c)(d)The FTIR spectra of LaF3:Nd3+/SiO2 core/shell nanoparticles are presented in Figure 3. Strong vibrational absorption bands at 3400–3600 and 1350–1600 cm-1 were observed in as-prepared sample, which correspond to O-H mode. So the physically adsorbed solvent and O-H groups on the as-prepared nanoparticles are still not removed. Whereas for the nanoparticles annealed at 400°C, the former absorption peaks show a great decrease. When the anneal temperature raised to 900°C, the absorption peaks of O-H mode completely disappeared. Thus, the nonradiative vibrational excitation of Nd3+ in the nanoparticles created by O-H and C-H groups can be excluded.Figure 3
FTIR spectra of LaF3:Nd3+/SiO2 core/shell nanoparticles annealed at different temperatures.Figure4 shows the room temperature emission spectra of LaF3:Nd3+/SiO2 core/shell and LaF3:Nd3+nanoparticles under excitation at 808 nm. The emission lines centered at 880, 1060, and 1330 nm correspond to the transitions from 4F3/2to 4I9/2, 4I11/2, and 4I13/2, respectively [15]. For LaF3:Nd3+/SiO2 core/shell nanoparticles as-prepared and annealed at 400 and 600°C, the emission pattern is similar with that of LaF3:Nd3+ nanoparticles in both the peak positions and shapes, which means that the SiO2 shell have minimal effect on LaF3:Nd3+ core. However, when the annealed temperature is 900°C, emission spectra of the LaF3:Nd3+/SiO2 core/shell nanoparticles show a very unusual manner. (1) Their fluorescence intensity show a great increase compared with that of the LaF3:Nd3+/SiO2 core/shell nanoparticles annealed at 600°C. (2) The strongest emission line for 4F3/2→4I11/2 transition peaking at 1073 nm are redshifted by about 17 nm contrasted with that of nanoparticles annealed at 600°C (1056 nm). (3) The 4F3/2→4I9/2 and 4F3/2→4I13/2 emission of LaF3:Nd3+/SiO2 core/shell nanoparticles annealed at 900°C have remarkable Stark splittings which have not been found in other samples. In contrast, emission spectrum of 900°C annealed LaF3:Nd3+ nanoparticles which have no SiO2 shell has also been recorded, and is shown in Figure 4(b). The fluorescence intensity is only about 1/5 compared with that of LaF3:Nd3+/SiO2 core/shell nanoparticles annealed at 900°C, the 4F3/2→4I11/2 emission is peaking at about 1058 nm, and no emission lines have Stark splittings. The results mean, for LaF3:Nd3+/SiO2 core/shell nanoparticles, that the influence of SiO2 shell has become clearly when the anneal temperature is 900°C, and the remarkable improvement of fluorescence intensity is caused by the SiO2 shell.Emission spectra of nanoparticles annealed at different temperatures: (a) LaF3:Nd3+/SiO2, (b) LaF3:Nd3+.
(a)(b)To get more information on the functions of SiO2 shell, the excitation spectra by monitoring the 4F3/2→4I11/2 emission in LaF3:Nd3+/SiO2 core/shell and LaF3:Nd3+ nanoparticles annealed at 900°C are compared in Figure 5. The spectra displays well-resolved lines, centered at 328, 352, 430, 472, 519, 578.9, 737, 801, and 881 nm corresponding to the direct excitation of Nd3+ from 4I9/2 to the higher excited states:2D5/2, 2P1/2, 4G11/2, 2K15/2 + 2D3/2 + 2G9/2, 2K13/2 +4G7/2 + 4G9/2, 2G7/2 +2G5/2, 2H11/2, 4F9/2, 4S3/2 +4F7/2, 4F5/2 + 2H9/2, and4F3/2. Interestingly, remarkable differences are observed in excitation spectra of the two samples. As shown in the inset of Figure 5, the shape of peaks for 4I9/2→2P1/2 transition are different between the two nanoparticles, and the lines for 4I9/2→4S3/2 + 4F7/2, and 4F5/2 + 2H9/2 transitions of LaF3:Nd3+/SiO2 core/shell nanoparticles are much broader compared with that of LaF3:Nd3+ nanoparticles. The changes of excitation spectra mean that the environment of LaF3:Nd3+ is different for the two nanoparticles. The Stark splittings of 4F3/2→4I9/2 and 4F3/2→4I13/2 emission for LaF3:Nd3+/SiO2 core/shell nanoparticles annealed at 900°C also confirmed the change. In previous reports, the phenomena were often caused by the host matrixes or surfactants [15]. In this work, for SiO2 shell and LaF3 core have not the same lattice structures, the shell could bring a noncentrosymmetric environment of the LaF3:Nd3+ core [16]. And the new noncentrosymmetric environment causes a significant improvement of fluorescent intensity of LaF3:Nd3+/SiO2 core/shell nanoparticles [17].Excitation spectra of nanoparticles annealed at900°C: (a) LaF3:Nd3+/SiO2, (b) LaF3:Nd3+.
(a)(b)To have a further investigation of influence of the SiO2 shell on LaF3:Nd3+ core, fluorescent decay curves of LaF3:Nd3+/SiO2 core/shell nanoparticles were measured by monitoring the 4F3/2→4I11/2 emission at about 1060 nm. As shown in Table 1. For as-prepared nanoparticles, the lifetime is very difficult to be detected due to the presence of a large amount of O-H groups. When the nanoparticles were annealed at 400°C, the lifetime is 159 μs. Surprisingly, with a further rising of the anneal temperature, the lifetimes show a slight decrease. When annealed at 600 and 900°C, the lifetimes are 156 μs and 148 μs, respectively. However, for 900°C annealed LaF3:Nd3+ nanoparticles which have no SiO2 shell, the lifetime is 196 μs. It can be deduced that the lifetimes decrease in LaF3:Nd3+/SiO2 core/shell nanoparticles is not caused by the LaF3:Nd3+ core itself. In general, there are two factors that could affect  radiative lifetime of Nd3+ in nanoparticles. (1) The existence of O-H groups and impurity on nanoparticles surfaces which could effectively reduce the lifetime of Nd3+; (2) vibrational energies of host matrix. For our samples, the difference is just the anneal temperature. For LaF3:Nd3+/SiO2 core/shell nanoparticles annealed at 900°C, the effect of O-H groups can be obviated (Figure 3) and the LaF3 host has a good crystallinity (Figure 1).Table 1
Lifetimes of nanoparticles annealed at different temperatures.
NanoparticlesAnnealed temperatureLifetime(°C)(μs)LaF3:Nd3+/SiO2 core/shell400159600156900148LaF3:Nd3+900197The LaF3:Nd3+/SiO2 core/shell nanoparticles were synthesized in two steps. In as-prepared nanoparticles, SiO2 shell is just coated on LaF3:Nd3+ core and interaction between the two parts is very weak. When the anneal temperature is 400°C, most solvent remained on nanoparticles is removed. Thus its lifetime is of 159 μs. When the anneal temperature was raised to 900°C, the interaction between SiO2 shell and LaF3:Nd3+ core increases. Since the phonon energy of SiO2 (1100 cm-1) is higher than that of LaF3 (350 cm-1) and the nanoparticles have a larger surface-to-core ratio [16], lifetime of Nd3+ near or on the surfaces of nanoparticles is slightly shorter than that of Nd3+ within the core structures and the measured lifetime of LaF3:Nd3+/SiO2 core/shell nanoparticles is decreased.


## 4. Conclusions

To summarize, we have shown that, for the SiO2-coated LaF3:Nd3+ nanoparticles, the SiO2 shell has an interesting influence on their LaF3 core. When the anneal temperature is 900°C, the change of environment of LaF3:Nd3+ core caused by SiO2 shell has become obvious. As a result, Stark splittings of 4F3/2→4I9/2 and 4F3/2→4I13/2 transition peaks and the redshift of 4F3/2→4I11/2 transition peak were observed in their emission spectrum. The fluorescent intensity of LaF3:Nd3+/SiO2 core/shell nanoparticles also has a great improvement, which can be very beneficial for applications.



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*Source: 238792-2010-10-25.xml*