Research Article | Open Access
Efficient Color Tuning of Upconversion Luminescence from Core-Shell Oxysulfide Nanoparticles
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.
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 . 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 . 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.
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].
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.
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 . 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%.
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 . 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.
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.
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.
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.
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.
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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