Improving Upconversion Photoluminescence of GdAlO3:Er3+, Yb3+ Phosphors via Ga3+, Lu3+ Doping
GdAlO3:Er3+, Yb3+ phosphors doped with Lu3+, Ga3+ ions were prepared via coprecipitation. The influences of substituting Ga3+ for Al3+ and substituting Lu3+ for Gd3+ on the structure and upconversion photoluminescence (UCPL) of the phosphors were studied. The experimental results show that the crystal textures did not change but the lattice parameters changed slightly via Lu3+, Ga3+ doping. This results in a decrease in the host phonon energy and a marked improvement in the green emission spectrum at 546nm. Moreover, the amounts of surface CO2, , and OH- species gradually decreased with Lu3+, Ga3+ doping. The combined effects led to an improvement in the UCPL efficiency with Ga3+, Lu3+ doping.
Upconversion (UC) materials can absorb two or three energy photons to convert infrared light to visible light [1, 2]. In recent years, Ln3+-doped UC materials have attracted much attention because of their potential applicability in many attractive research areas, such as solid-state lasers, biological imaging, solar cells, and the quantum dots [3–9]. It is well known that rare earth metals have a particular electronic structure with different 4f electronic numbers, and these are suitable for the UC process. Among the rare earth metals, Er3+ has a rich 4f level structure, and its long intermediate state energy level lifetime can absorb two or more photons to achieve UC luminescence under excitation of 650, 808, 980, and 1500 nm wavelengths. However, Yb3+ which has a large absorption cross-section and an energy level that matches with that of Er3+ is often codoped as the sensitizer. Yb3+ can absorb energy at 980 nm and can transfer the energy to Er3+ to improve the UC photoluminescence (UCPL) efficiency of phosphors [10–12].
Currently, phosphors with oxides as the host material exhibit high UC luminescence efficiency because of their low phonon energy, good chemical stability, mild synthesis conditions, and ecofriendliness. A lot of oxides, such as Sr2CeO4, Bi2WO6, NaGdTiO4, and KBaLu(MoO4)3, exhibit excellent UC luminescence properties as have been successively reported [13–16]. To the best of our knowledge, GdAlO3 belongs to the perovskite structure, orthogonal crystal system, and Pbnm space group. The density of GdAlO3 is 7.437g.cm−3, and the phonon energy is 670 cm−1, which is good for UCPL . In particular, the phonon energy and symmetry of the matrix have a great influence on UCPL efficiency. The phonon energy of the host material is mainly related to the relative atomic mass and bond length of the matrix elements. In this work, the lattice parameters of perovskite GdAlO3 are substituted with other ions, such as Ga3+, Lu3+, to adjust the phonon energy of the host material and then to improve the GdAlO3:Er3+, Yb3+ phosphor UCPL.
2. Experimental Section
2.1. Phosphors Preparation
The precursors of GdAlO3:Er3+,Yb3+ phosphors doped with Ga3+/Lu3+ were prepared using the coprecipitation method . Gd2O3 (99.99%), Yb2O3 (99.99%), Er2O3 (99.99%), Lu2O3 (99.99%), and Ga2O3 (99.99%) were prepared as the starting materials without further purification. These were dissolved in dilute nitric acid under heating, and then a required amount of Al(NO3)3·9H2O and ethanol aqueous solution were added sequentially under vigorous stirring until a homogenous solution formed in a 45°C water bath. Then 1 mol·L−1 NH4HCO3 aqueous solution was slowly added to obtain a white precipitate with magnetic stirring at a rate of 2 mL·min−1. After complete precipitation, the final pH of the slurry was around 7.5. The agitator was then turned off, and the precipitate was ripened at room temperature for 12 h. The precipitate was filtered and washed several times with deionized water. The precipitate was dried in an oven at 120°C for 12 h. In the end, the precipitate was ground and annealed in air at 1200°C for 6 h to obtain the white phosphor sample.
The samples were characterized using powder X-ray diffraction (XRD) performed on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ=0.154056 nm) operated at 30 mA and 40 kV. The UCPL spectra of the phosphors were recorded using an Ocean Optics PlasCalc-2000-UV-VIS-NIR plasma monitor and control system. The phosphors were excited with an MDL-H-980 model 980 nm infrared laser made by Changchun New Industries Optoelectronics Tech. Co., Ltd. Fourier-transform infrared (FT-IR) absorption spectra were recorded using a Nicolet 360 FTIR spectrometer. All of the measurements were carried out at room temperature. The energy level diagram was reproduced from Taoli Deng et al. (2018) [under the Creative Commons Attribution License/public domain] .
3. Results and Discussion
3.1. GdAl:, Phosphors Doped with
Figure 1 presents the XRD patterns of the Gd()O3: , (x=0.00,0.01,0.03,0.05) phosphors. All of the phosphor spectra match the standard data for perovskite GdAlO3 (PDF#46-0395) , and no other impurities were detected. Compared to the pattern of the AlO3:, phosphor, the spectral peaks of the phosphors are shifted to smaller diffraction following Ga3+ doping. Because the ionic radii of Ga3+ (0.0620nm, CN=8) are smaller than that of Al3+ (0.0535 nm, CN=8) , the host lattice expands, and this indicates that Lu3+ ions were successfully incorporated into the GdAlO3 host lattices. The average sizes of the crystallites are estimated using Scherrer’s equation : where D is the average crystallite size, λ is the wavelength of the Cu Kα line, β is the full width at half maximum in radians, and θ is the Bragg angle.
The strongest peak of the phosphor is at 28.6° (β=0.064) and was used to estimate the crystallite size using Scherrer’s equation. Using this procedure, the prepared phosphor particles had an average crystallite size of 2.44 nm by this procedure.
To optimize the UCPL of the phosphor, a series of samples with different doping concentrations of Ga3+ were synthesized, and their UCPL spectra were investigated. Figure 2 shows that all of the Gd()O3:, (x=0.00,0.01,0.03,0.05) phosphors have three main emission bands with maxima at 524 nm (green), 546 nm (green), and 659nm (red) excited by 980 nm radiation. The green emission peaks are assigned to 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+, and the red emission peak is ascribed to the 4F9/2 →4I15/2 transition of Er3+ . Moreover, the position of the emission peak did not shift significantly with the substitution of Ga3+. Clearly, a marked improvement in the emission spectra at 546nm is observed with Ga3+ doping. As seen in Figure 3, when the substitution concentration of Ga3+ was increased to 5%, the intensity ratio of the phosphor was 2.36 times greater than that of the phosphor without any ion doping, and when the Ga3+ doping concentration was increased, the intensity ratio of red/green emission decreased significantly.
To determine the reasons for changes in the UCPL properties with the substitution of different concentrations Ga3+ for Al3+, FT-IR spectra of Gd()O3: , phosphors were tested in Figure 4. All of the phosphors appear to have absorption peaks at 3460 cm−1 and 1635cm−1 and in the region less than 1000 cm−1. On one hand, absorption below 1000 cm−1 is ascribed to the vibration of the Gd()O3 host material, and the maximum vibration absorption peak of the phosphor matrix material shifts slightly in the low wavenumber direction with the substitution of Ga3+ for Al3+. Specifically, the matrix vibration is 670 cm−1 without substitution, and the matrix vibration is 665 cm−1 with 5% substitution. The results show that the phonon energy of the matrix decreases with Ga3+ doping, and this is helpful for improving the UCPL of the phosphors. On the other hand, the absorption at 3460 cm−1 and 1635 cm−1 is caused by the stretching vibration of OH− groups and the deformation vibration of HOH . Figure 4 shows that the concentration of the surface OH− groups decreased with Ga3+ doping, enhances the UC emission intensity, and decreases the red/green relative intensity.
3.2. GdAl:, Phosphors Doped with
XRD patterns for all of the samples ()AlO3:, (y=0.00, 0.01, 0.03, 0.05) with various doping concentrations of Lu3+ were measured as showed in Figure 5. All of the diffraction peaks can be assigned to GdAlO3 (PDF#46-0395), and no diffraction peak is detected. However, when the substitution concentration of Lu3+ was increased to 10% or even 25%, the XRD spectra of the phosphors no longer match the standard spectra of GdAlO3 because the radii of the ions (=0.0861 nm and =0.1053 nm) are quite different. It is difficult to replace Gd3+ with a large amount of Lu3+ in the same crystal because of the 15% difference in radius according to Vegard’s law .
Figure 6 shows the UC emission spectra of the samples () AlO3:, (y=0.00, 0.01, 0.03, 0.05) excited at 980 nm. In the spectra, there are three emission bands that peak at around 524, 546, and 656 nm, and the position of the emission peak did not shift significantly with the substitution of Lu3+, and this same observation is made with Ga3+ doping. From Figure 7 it can be seen that the strength of the green emission at 546 nm of the phosphor doped with 5% Lu3+ is 2.73 times greater than that of the phosphor Gd0.91AlO3:,. Also, the intensity ratio of the red/green emission decreased with Lu3+ doping.
Figure 8 shows the FT-IR spectra of the ()AlO3:, phosphors. All of the phosphors have absorption peaks at 3460 cm−1 (corresponding to the stretching vibration of OH− groups) and 1635 cm−1 (corresponding to the deformation vibration of HOH) and in the region less than 1000 cm−1 (corresponding to the vibration of the host materials). First, the vibration of the host material gradually shifts from 670 cm−1 to 663 cm−1 with substitution of Lu3+ for Al3+. At the same time, the concentration of the surface OH− groups also decreased with Lu3+ doping; this is because the rare earth metals Lu3+, Gd3+ have an abundant electronic structure with their 4f electronic number being different, so that they have relatively high coordination number which can possibly obtain the coordination bonds with OH-. This can enhance the green emission with a decrease in the red/green relative intensity.
3.3. UCPL Mechanism
The UC luminescence mechanisms for Er3+/Yb3+-codoped systems are well understood [25, 26] and can be described using the energy level diagram illustrated in Figure 9 reproduced from Taoli Deng et al. (2018) [under the Creative Commons Attribution License/public domain]. Under excitation at 980 nm, Yb3+ is excited to the 2F5/2 excited state. Meanwhile, Er3+ is excited from the 4I15/2 ground state to the excited state 4I11/2 via ET from Yb3+ in the 2F5/2 state. Because the lifetime of the 4I11/2 state of Er3+ is very long, it is expected that the population of this excited state can be further excited to the upper state of 4F7/2 through a second ET from another Yb3+ ion that is also in its excited state. After the fast 4F7/2→2H11/2 or 2H11/2→4S3/2 nonradiative relaxations, the excited Er3+ returns to the 4I15/2 ground state, producing the green emissions at 524 and 546 nm that radiate from the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions, respectively. At the same time, some of the populations of the 4F7/2 state relax nonradiatively to the 4F9/2 state, from which the red emission at 659 nm is observed for the 4F9/2 →4I15/2 transition. Alternatively, Er3+ in the 4I11/2 excited state is pumped by the first ET from Yb3+ and may relax to the 4I13/2 level via nonradiative relaxations. It may then be promoted to the red-emitting 4F9/2 level after a second ET from another Yb3+, which also produces red emission at 659 nm. First, when the phonon energy of a phosphor is high, the excited states of the activated ion can be coupled with the host lattice vibrational phonon, and this leads to an increased probability of nonradiative transitions for the activated ion in excited states to return to the ground state. Thus, the UC efficiency decreases significantly. Otherwise, from the schematic diagram, it is seen that the 4I11/2→4I13/2 and 4S3/2→4F9/2 nonradiative transitions of Er3+ play a key role in determining the green emission intensity and the ratio of red-to-green emission of UCPL spectra. In our previous work [17, 19], we showed that the energy gaps of 4I11/2 → 4I13/2 and 4S3/2→4F9/2 of Er3+ are about 3600 cm−1 and 3140 cm−1; these values are close to the vibrational frequency of OH− (3460 cm−1). The decrease in the OH− concentration in the phosphor had a marked influence on reducing the 4I11/2 → 4I13/2 and 4S3/2→4F9/2 nonradiative transitions. Consequently, the red emission at 659 nm from the 4F9/2 →4I15/2 transition is prevented, and the main green emission is enhanced. Therefore, the red/green emission ratio decreases with the decrease in the concentration of OH− in the phosphors.
The GdAlO3:Er3+,Yb3+ phosphors that have Al3+ and Gd3+ substituted by Ga3+ and Lu3+, respectively, were prepared via coprecipitation. All of the samples had three emission band peaks at around 524, 545, and 656 nm, and the positions of the emission peaks do not shift significantly with the substitution of Ga3+ and Lu3+. A marked improvement in the emission spectra at 546nm is observed with Ga3+ and Lu3+ doping. For the phosphors doped with 5% Ga3+ and Lu3+, the strengths are, respectively, 2.36 and 2.73 times greater than the strength of the Gd0.91AlO3:, phosphor, and the intensity ratio of the red/green emission decreased with an increase in the Ga3+ and Lu3+ doping concentration. The factors that influence the improvement of the UCPL phosphors are discussed. On one hand, the crystal textures did not change but the lattice parameters changed slightly via Lu3+ and Ga3+ doping, and this resulted in a decrease in the phonon energy of the host. On the other hand, the concentration of the surface OH− groups decreased with Lu3+ and Ga3+ doping, and this enhanced the main emission peak intensity and decreased the red/green relative intensity.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This work was financially supported by the Youth Growth S&T Personnel Foundation of Guizhou Education Department (KY 273 and KY 283), the Joint Science and Technology Funds of Guizhou S&T Department, Anshun City People’s Government, and Anshun University (LH 7271).
All of the GdAlO3:Er3+,Yb3+ phosphors that have Al3+ substituted by Ga3+ have three main emission bands with maxima at 524 nm (green), 546 nm (green), 659nm (red) excited by 980 nm radiation. The green emission peaks are assigned to 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+, and the red emission peak is ascribed to the 4F9/2 →4I15/2 transition of Er3. Moreover, the position of the emission peak did not shift significantly with the substitution of Ga3+. Clearly, a marked improvement in the emission spectra at 546nm is observed with Ga3+ doping. (Supplementary Materials)
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