Journal of Nanomaterials
Volume 2019 (2019), Article ID 7152690, 6 pages
https://doi.org/10.1155/2019/7152690

Research Article

Efficient Color Tuning of Upconversion Luminescence from Core-Shell Oxysulfide Nanoparticles

Ying Tian, Jianxin Jiao, Xin Feng, Mingming Xing, Yong Peng, and Xixian Luo

School of Science, Dalian Maritime University, Dalian, Liaoning, 116026, PR., China

Correspondence should be addressed to Mingming Xing; mingmingxing@dlmu.edu.cn and Xixian Luo; luoxx@dlmu.edu.cn

Received 11 March 2019; Accepted 10 April 2019; Published 7 October 2019

Academic Editor: William Yu

Copyright © 2019 Ying Tian et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺,Ho³⁺ core-shell up-conversion (UC) nanoparticles were successfully synthesized by the homogeneous co-precipitation method. The Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺,Ho³⁺ core-shell nanoparticles exhibit bright green emissions under 980 nm excitation, while the triple-ion doped Y₂O₂S:Er³⁺,Yb³⁺,Ho³⁺ sample presents mainly red emissions. The intensity ratio of green-to-red emission of the core-shell and conventional triple-ion doped samples are 2.8 and 0.3, respectively. Investigations on the UC mechanisms show that emissions from Er³⁺ and Ho³⁺ ions are achieved simultaneously in the core-shell nanoparticles. This is due to the efficient energy transfers of Yb³⁺→Ho³⁺ within the shell layer and Yb³⁺→Er³⁺ between the shell and the core. While the triple-ion doped Y₂O₂S: Er³⁺,Yb³⁺,Ho³⁺ sample exhibits mainly the emissions of Er³⁺ along with weak luminescence of Ho³⁺ ion. Since the cross relaxation between Er³⁺ and Ho³⁺ ions in the Y₂O₂S:Er³⁺,Yb³⁺,Ho³⁺ nanoparticles can effectively suppress the emissions of Ho³⁺ ions. Yet, in the core-shell structure, this cross relaxation can be successfully restrained in the core-shell structure where Er³⁺ is in the core and Ho³⁺ is in the shell. Therefore, the construction of core-shell structure can improve the luminescence efficiency and provide a route for adjustment of emission color.

1. Introduction

Upconversion luminescence (UCL) materials with unique luminescent properties have become the research focus due to their promising applications in anti-counterfeiting, solar cells, three-dimensional display and solid-state lasers [1–3]. Rare earth oxysulfide (RE₂O₂S (RE = rare earth)) is an ideal optical functional material [4–11] due to their excellent thermal and chemical stability. In addition, RE₂O₂S materials possess low phonon energy, which is important for efficient UCL [12–16]. It has been reported that the UCL efficiency of RE₂O₂S is comparable to that of β-NaYF₄ at excitation of 980 nm [12–14]. Meijerink et al. have shown that the UC internal quantum efficiency of Gd₂O₂S:Er³⁺ is higher than that of β-NaYF₄:Er³⁺ under 1550 nm excitation [15]. Therefore, rare earth oxysulfide is an ideal UC host material holding various promising properties. Furthermore, it is known that the efficiency of UCL is lower than that of conventional luminescence due to its intrinsic luminescence processes, and the tuning of emission color is very limited [16]. Recently, it has been proven that the core-shell structure can effectively improve the properties of UCL and the intensive researches focus on the fluoride material [17, 18]. There are only a few reports about the core-shell structures constructed based on the host material of oxysulfide [19, 20].

In this work, we have successfully synthesized the core-shell nanoparticles (NPs) of Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺,Ho³⁺ by using homogeneous co-precipitation method combined with the solid-gas sulfidation route. The UCL properties of Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺,Ho³⁺ and triple-ion doped Y₂O₂S:Er³⁺,Yb³⁺,Ho³⁺ samples were investigated under excitation of 980 nm laser. We found that the core-shell Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺,Ho³⁺ exhibits bright green emission, while Y₂O₂S:Er³⁺,Yb³⁺,Ho³⁺ presents red emission due to the new channels of energy transfers in the core-shell structure. Further, the cross relaxation between Er³⁺ and Ho³⁺ ions can be successfully restrained in the core-shell structure, which lead to the emissions from both of the Er³⁺ and Ho³⁺ ions. Therefore, the core-shell structure provides a new route for adjustment of luminescence color and improvement of luminescence efficiency.

2. Material and Methods

2.1. Synthesis of the Core-Shell NPs

The precipitates of the core material, Y(OH)CO₃:12% Er³⁺, were obtained by mixing the solution of urea (99% purchased from Tianjin Bodi Chemical Co., Ltd) in 0.8 L solution (7.5 mol/L) and rare earth nitrates Re(NO₃)₃ (99.99%, Guangzhou Rare Earth Industry Group CO., Ltd) in 0.2 L solution. The mole ratio of Y, Er ions is 88 : 12. After the centrifuging and washing with water and isopropanol, the core material of Y₂O₃:Er³⁺ powder was achieved by annealing the precipitates at 600°C for 1 h. To coat the shell layer on the core material, we first added the Y₂O₃:Er³⁺ into the urea solution (6 mol, 0.8 L) at 60°C, and then mixed the solution (0.2 L) of Re(NO₃)₃ (Re = Y, Yb, Ho, with mole ratio of 91 : 8 : 1) by bath sonication at 80°C for 30 mins. After the similar processes of cooling down, centrifuging, washing and drying, we obtained the precursor of Y₂O₃:Er³⁺@Y(OH)CO₃:Yb³⁺, Ho³⁺. The precursor of the reference sample, Y(OH)CO₃:12% Er³⁺,8%Yb³⁺,1%Ho³⁺, were obtained in the similar processes by mixing the solution of urea with Re(NO₃)₃ (Re = Y, Er, Yb, Ho with mole ratio of 79 : 12 : 8 : 1). Then, the core-shell NPs of Y₂O₃: 12%Er³⁺@Y₂O₃:8%Yb³⁺ and Y₂O₃:Er³⁺,Yb³⁺,Ho³⁺ were finally achieved by annealing the precursor at 600°C. The last step is the sulfidation process. The oxides and sulfur powders were put into a quartz tube which was heated up to 800°C for 30 mins. In this process, Argon was used as protection atmosphere. The above RE-ions doping concentrations were optimized on basis of a series of experimental results as shown in the supporting information.

2.2. Characterization

X-ray diffraction (XRD) patterns were recorded at 40 kV and 40 mA by using a Rigaku D/MAX-Ultima X-ray diffractometer with Cu K ( = 0.15406 nm) radiation. The UC luminescence spectra were measured by using the Hitachi F-4500 spectrometer equipped with a 980 nm laser diode (with power density of 0.15w/mm²). The slit width is 2.5 nm and the scanning speed is 2400 nm/min. The morphology and size of NPs was characterized by using a JEM-2000EX transmission electron microscope (TEM).

3. Results and Discussion

Figure 1 (a) and (b) show the XRD patterns of Y₂O₂S:Er³⁺ and Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺,Ho³⁺ samples. It can be observed that all the diffraction peaks can be indexed to the hexagonal Y₂O₂S compared with the standard card (JCPDS: No.24-1424) in Figure 1 (c). According to the Scherrer formula,

where D is the mean crystalline size, is the X-ray wavelength, B is the full width at half maximum (FWHM), and is the diffraction angle, the calculated crystalline size of Y₂O₂S:Er³⁺ and Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺, Ho³⁺ are 27.5 and 38.8 nm, respectively. The increase of the nanocrystalline size after the shell coating indicates the formation of core-shell structure which have the same matrix compositions in the core and shell layers [21, 22].

Figure 1: XRD spectra of (a) Y₂O₂S: Er³⁺, (b) Y₂O₂S: Er³⁺@Y₂O₂S: Yb³⁺, Ho³⁺ nanocrystals and (c) standard card of hexagonal Y₂O₂S (JCPDS: No.24-1424).

The size and morphology of the samples are studied by TEM measurements, as shown in Figure 2. The precipitates of the core material (Y(OH)CO₃:Er³⁺) are dispersed spherical nanoparticles with a mean particle size about 41 nm (Figure 2(a)). After annealing the Y(OH)CO₃:Er³⁺ sample at 600°C and further coated with the shell of Y(OH)CO₃:Yb³⁺, Ho³⁺, the precipitates of Y₂O₃:Er³ + @Y(OH)CO₃:Yb³⁺, Ho³+ were obtained as shown in Figure 2(b). It can be observed that the NPs remain the spherical shape with an enlarged mean particle size of ≈55 nm. This increase of the particle size after the shell coating indicates the formation of the core-shell structure [21, 22]. The final products of Y₂O₂S: Er³⁺ @Y₂O₂S: Yb³+, Ho³⁺ were obtained after the sulfidation process at a high temperature of 800°C, as shown in Figure 2(c). It can be seen that the particles aggregate together after calcination. These agglomerating particles make it challenging to obtain an accurate statics particle size distribution.

(a)

(b)

(c)

Figure 2: TEM images of (a) the precursor of the core NPs (Y(OH)CO₃:12% Er³⁺), (b) the precursor of the core-shell NPs (Y₂O₃:Er³⁺@Y(OH)CO₃:Yb³⁺, Ho³⁺), and (c) the final products of Y₂O₂S: Er³⁺@Y₂O₂S:Yb³⁺,Ho³⁺ NPs.

The core-shell Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺,Ho³⁺ UC NPs exhibit bright UCL of green color, while the Yb³⁺, Er³⁺, Ho³⁺ triple-doped Y₂O₂S NPs show red emission color. As shown in Figure 3, both samples present the green UC emission in the range of 516-570 nm and red ones at 623-698 nm. Compared with the spectrum of Y₂O₂S:Yb³⁺,Ho³⁺ sample, the visible emissions of the core-shell sample centered at 545, 655 and 750 nm, correspond to the ⁵S₂, ⁵F₄ → ⁵I₈, ⁵F₅ → ⁵I₈ and ⁵S₂, ⁵F₄ → ⁵I₇ transitions of Ho³⁺ ions, respectively [1]. Similarly, compared with the Y₂O₂S:Yb³⁺,Er³⁺ NPs, the green emissions of the core-shell sample centered at 540 and 555 nm contribute from the ²H11/2 → ⁴I15/2 and ⁴S3/2 → ⁴I15/2 transitions of Er³⁺ ions, respectively. And the red emission at ≈ 675 nm is resulted from the ⁴F9/2 → ⁴I15/2 transition of Er³⁺ ions [23, 24]. The measured quantum yield of Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺,Ho³⁺ UC NPs is about 0.45%.

Figure 3: Up-conversion emission spectra of Y₂O₂S: Yb³⁺, Er³⁺, Y₂O₂S: Yb³⁺, Ho³⁺, Y₂O₂S: Er³⁺ @ Y₂O₂S: Yb³⁺, Ho³⁺ and Y₂O₂S: Yb³⁺, Er³⁺, Ho³⁺ samples under 980 nm excitation.

Although the UCL from the Ho³⁺ and Er³⁺ ions are realized in both samples of Y₂O₂S:Yb³⁺,Er³⁺,Ho³⁺ and core-shell Y₂O₂S:Er³⁺ @Y₂O₂S:Yb³⁺,Ho³⁺, the green emissions from Ho³⁺ ions are significantly enhanced in the core-shell sample. While the red UCL from Er³⁺ ions is dominant in the Y₂O₂S:Yb³⁺,Er³⁺,Ho³⁺ sample. The intensity ratios (I) of green (G) to red (R) emissions (IG/IR) of Y₂O₂S:Er³⁺ @ Y₂O₂S:Yb³⁺, Ho³⁺ and Y₂O₂S:Yb³⁺, Er³⁺, Ho³⁺ samples are are 2.8 and 0.3, respectively. This is due to the cross relaxation between Er³⁺ and Ho³⁺ ions in the Y₂O₂S:Yb³⁺, Er³⁺,Ho³⁺ sample, which suppresses the emission of Ho³⁺ and enhances the red emission simultaneously. However, in the core-shell sample of Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺Ho³⁺, the cross relaxation between Er³⁺ and Ho³⁺ ions is effectively suppressed by constructing the core-shell structure where Er³⁺ is in the core and Ho³⁺ is in the shell. Therefore, the core-shell sample exhibits dominant green emissions from Ho³⁺ ions, yet the conventional triple-ion doped sample present red UCL from Er³⁺ ions.

The possible UCL processes of Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺, Ho³⁺ sample under 980 nm excitation are shown in Figure 4. The UCL of Ho³⁺, Yb³⁺ ions present green, red and weak NIR emissions. Firstly, the sensitizer of Yb³⁺ ion is excited to the ²F5/2 energy level after absorbing the 980 nm photon via ground state absorption (GSA) process. Then the Ho³⁺ ion is excited to the high levels of ⁵S₂ and ⁵F₄ via continuous energy transfer (ET) processes of ET1 and ET2 from Yb³⁺ ion (process (1) and process (2) in Figure 4). The excited Ho³⁺ ion at levels of ⁵S₂ and ⁵F₄ generates green emission through ⁵S₂, ⁵F₄ → ⁵I₈ transition, and a small portion of them decays to the intermediate state of ⁵I₇ via ⁵S₂, ⁵F₄ → ⁵I₇ to exhibit weak NIR emission [1]. This process of NIR emission will populate the ⁵I₇ level. In addition, the cross relaxation (CR) processes of ⁵S₂, ⁵F₄ (Ho³⁺) + ⁵I₈ (Ho³⁺) → ⁵I₄ (Ho³⁺) + ⁵I₇ (Ho³⁺) (CR1) and ⁵I₄(Ho³⁺) + ⁵I₈ (Ho³⁺) → ⁵I₆ (Ho³⁺) + ⁵I₇ (Ho³⁺) (CR2) may occur and further populate the energy level of ⁵I₇. Then Ho³⁺ ions at the state of ⁵I₇ will be pumped to the high excited state of ⁵I₅ after absorbing the 980 nm photon, and then emit red UCL through the ⁵I₅ → ⁵I₈ transition.

Figure 4: The UCL processes of Y₂O₂S: Er³⁺ @ Y₂O₂S: Yb³⁺, Ho³⁺ and Y₂O₂S: Yb³⁺, Er³⁺, Ho³⁺ samples under 980 nm excitation.

In terms of the UCL processes of Yb³⁺, Er³⁺ ions, the Yb³⁺ is firstly pumped to the excited level of ²F5/2 by absorbing a 980 photon, and then return to the ground state by transferring the energy to an Er³⁺ ion in the core via ET3 and ET5, as shown in Figure 4. These ET processes will excite the Er³⁺ from ground state of ⁴I15/2 to the high level of ⁴F7/2 through ⁴I11/2 level. The Er³⁺ ion at ⁴F7/2 then decays to the ²H11/2, and ⁴S3/2 levels via non-radiative relaxation, producing green emission through ²H11/2 → ⁴I15/2, and ⁴S3/2 → ⁴I15/2 transitions [6, 7]. The excited Er³⁺ ion at ⁴I11/2 level can also decay to the ⁴I13/2 state, and then populate to the ⁴F9/2 by absorbing a 980 photon. Then the red emission from Er³+ ion occurs via transition of ⁴F9/2 → ⁴I15/2.

Notably, due to the small distance between the Er³⁺ and Ho³⁺ ions in the Y₂O₂S: Yb³⁺, Er³⁺, Ho³⁺ sample, the cross relaxations of ⁵S₂, ⁵F₄ (Ho³⁺) + ⁴I11/2 (Er³⁺) → ⁵I₄ (Ho³⁺) + ⁴F9/2 (Er³⁺) (CR3) can easily occur in the conventional triple-ion doped sample. This CR3 process significantly increases the population of Er³⁺ ion at the ⁴F9/2 level, which results in the much stronger red emission (due to ⁴F9/2 → ⁴I15/2) in the Y₂O₂S: Yb³⁺, Er³⁺, Ho³⁺ sample than that of the core-shell Y₂O₂S: Er³⁺ @ Y₂O₂S: Yb³⁺, Ho³⁺ sample. Meanwhile, this CR3 process also decreases the population of Ho³⁺ ion at the ⁵S₂, ⁵F₄ states, thereby, suppressing the green emission in the Y₂O₂S: Yb³⁺, Er³⁺, Ho³⁺ sample. The different UCL and ET processes in the Y₂O₂S: Yb³⁺, Er³⁺, Ho³⁺ and core-shell Y₂O₂S: Er³⁺ @ Y₂O₂S: Yb³⁺, Ho³⁺ samples indicate the formation of the core-shell structure and provide a possible route for adjustment of emission color.

Further, the fluorescence decay curves of Y₂O₂S: Er³⁺ @ Y₂O₂S: Yb³⁺, Ho³⁺ (Core-Shell) and Y₂O₂S: Yb³⁺, Er³⁺ samples were measured as shown in Figure 5. All the decay curves present non-exponential profile due to the relaxation and energy transfer processes between the Yb³⁺ ions and Er³⁺, Ho³⁺ ions. According to the lifetime equation of , where τ is the calculated luminescence lifetime and I(t) is the luminescence intensity at time after the cutoff of the excitation light, the calculated lifetimes of green and red emissions of Y₂O₂S: Yb³⁺, Er³⁺ and Y₂O₂S: Er³⁺ @ Y₂O₂S: Yb³⁺, Ho³⁺ samples are listed in Table 1. It shows that lifetimes of red and green emissions of the core-shell Y₂O₂S: Er³⁺ @ Y₂O₂S: Yb³⁺, Ho³⁺ sample are much longer than that of the Y₂O₂S: Yb³⁺, Er³⁺ sample. This is owing to the effective protection of the shell layer for the emissions of Er³⁺ ions in the core. In addition, due to the non-radiative relaxation of ⁴I11/2 → ⁴I13/2 in the red emission process, the rising time of the red emission at 670 nm is much longer than that of the green emission at 548 nm. Therefore, the fluorescence decay measurements support the luminescence mechanisms.

Figure 5: Fluorescence decay curves of red (670 nm) and green (548 nm) emissions of Y₂O₂S: Er³⁺ @ Y₂O₂S: Yb³⁺, Ho³⁺ (Core-Shell) and Y₂O₂S: Yb³⁺, Er³⁺ samples under 980 nm excitation.

Table 1: Calculated luminescence lifetimes of Y₂O₂S: Er³⁺ @ Y₂O₂S: Yb³⁺, Ho³⁺ and Y₂O₂S: Yb³⁺, Er³⁺ samples.

Transition
Lifetime (μs)

Y₂O₂S: Er³⁺ @ Y₂O₂S: Yb³⁺, Ho³⁺
Y₂O₂S: Yb³⁺, Er³⁺

⁴S3/2 → ⁴I15/2 (548 nm)
37.8
21.8

⁴F9/2 → ⁴I15/2 (670 nm)
139.1
46.1

4. Conclusion

The Y₂O₂S:Er³⁺@Y₂O₂S:Yb³⁺, Ho³⁺ core-shell NPs are synthesized by the homogeneous co-precipitation method combining with the solid-gas sulfidation route. Investigations on the UCL show that the emissions from Er³⁺ and Ho³⁺ ions are achieved simultaneously in the core-shell NPs. This is due to the efficient energy transfers of Yb³⁺ → Ho³⁺ within the shell layer and Yb³⁺ → Er³⁺ between the shell and the core. However, the core-shell Y₂O₂S: Er³⁺ @ Y₂O₂S: Yb³⁺, Ho³⁺ and the triple-ion doped Y₂O₂S: Yb³⁺, Er³⁺, Ho³⁺ samples present mainly green emission from Ho³⁺ ions (IG/IR = 2.8) and red luminescence from Er³⁺ ions (IG/IR = 0.3), respectively. The reason is that the cross relaxation between Er³⁺ and Ho³⁺ ions can easily occur due to the small distance between them in the Y₂O₂S: Yb³⁺, Er³⁺, Ho³⁺ sample. While on the other hand, this cross relaxation can be successfully suppressed by the core-shell structure where Ho³⁺ is in the shell and Er³⁺ is in the core. Therefore, the unique core-shell Y₂O₂S nanostructure could offer new channels for energy transfers and presents novel UC luminescence properties.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

There is no conflict of interest regarding the publication of this paper.

Acknowledgments

The authors thank the National Natural Science Foundation of China (11504039), Fundamental Research Funds for the Central Universities (Grant No. 017192610, 017192617) for their financial support.

Supplementary Materials

The supplementary material provides the experiments on the optimum rare earth doping concentrations and the particle size distribution of the precursors of the core and core-shell nanoparticles. (Supplementary Materials)

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