Facile Synthesis of Yb3+- and Er3+-Codoped LiGdF4 Colloidal Nanocrystals with High-Quality Upconversion Luminescence
Herein, we synthesized high-quality colloidal nanocrystals of Yb3+/Er3+-codoped LiGdF4 with intense green emission by using a facile route and turning the associated reaction parameters. Moreover, we probed the effects of reaction conditions on nanocrystal properties (crystal structure, morphology, and luminescence) and gained valuable mechanistic insights into nucleation and growth processes. Sample purity was found to depend on LiOH·H2O concentration, reaction temperature, and time, which allowed us to manipulate sample purity and thus obtain species ranging from mixtures of LiGdF4:Yb3+/Er3+ with GdF3 to pure tetragonal-phase LiGdF4:Yb3+/Er3+. Investigation of upconversion luminescence properties and the luminescence lifetime of as-prepared samples revealed that LiGdF4 is a promising host material for realizing the desired upconversion luminescence.
Trivalent lanthanide ion- (Ln3+-) doped nanocrystals, which can convert infrared radiation to visible luminescence, have attracted much attention in view of their excellent luminescence properties and unique application value [1–9]. The application fields are very wide including solid-state lasers, 3D displays, solar cells, photovoltaics, biological probe and label markers, and multimodal bioimaging [10–16]. Most of these nanocrystals correspond to fluorides, which exhibit several advantages over other halides, such as thermal and environmental stability, high refractive index, high transparency, low-frequency phonons, and lower emission threshold [17–20]. Among fluorides, those based on Gd3+ have been intensively researched because of their excellent chemical and optical properties [21–24]. Moreover, the energy gap between the 6P7/2 and the 8S7/2 levels of Gd3+ equals 32000 cm−1, which allows Gd3+ to be used as an intermediary to promote fluoride energy transfer and thus greatly improve the efficiency of upconversion luminescence [25, 26].
Much research has been performed on the monodisperse, well-shaped, uniform-size, and phase-pure NaGdF4 nanoparticles in recent decades [27–35]. For instance, Liu and coworkers prepared size-controllable, highly monodisperse, oleate-capped NaGdF4:Yb,Er nanocrystals that can be used as biological probes for in vivo testing of tiny tumors , while Johnson’s group showed that the regulation of reaction time and temperature allows the synthesis of size-tunable and ultrasmall NaGdF4 nanoparticles (2.5-8.0 nm) . There is no doubt that NaGdF4 is considered to be an ideal rigid host matrix for upconversion, and its synthesis has therefore become a research hotspot for the majority of scholars. However, LiGdF4 has been underexplored among the AGdF4 () hosts because of the difficulty of synthesizing pure tetragonal-phase LiGdF4 nanocrystals. To the best of our knowledge, LiGdF4 nanocrystals are mainly prepared using Czochralski, sol-gel, or thermal decomposition methods. For instance, Cornacchia and coworkers prepared LiGdF4:Tm3+ single crystals utilizing the Czochralski technique , while Lepoutre and coworkers prepared 90SiO210LiGd1-xEuxF4 ( or 0.05) composites using a sol-gel method . Moreover, Xiong’s group successfully synthesized LiGdF4 nanoparticles with different Ca2+ contents using a thermal decomposition method, revealing that Ca2+ ions are vital to the successful synthesis of these nanoparticles . Na and coworkers prepared pure tetragonal-phase LiGdF4 upconversion nanophosphors doped with Y3+ by thermal decomposition in a methanol-LiOH·H2O-NH4F mixture and showed that the orthorhombic GdF3 phase was produced at Y3+ doping degrees of 0-20 mol% . Initially, we tried to prepare LiGdF4:Yb,Er nanocrystals by similar methods of LiYF4:Yb,Er nanocrystals used, while we did not get the result we wanted. After careful analysis, we speculate that it may be caused by various facts, such as chemical factor itself, equipment factor, and experimental environmental factor. Furthermore, we noted that the vast majority of LiGdF4:Yb,Er nanocrystal syntheses employ solutions of oleic acid and 1-octadecenein methanol, which are highly toxic.
To address this challenge, we have developed a suitable synthetic route to prepare high-quality LiGdF4:Yb/Er nanocrystals avoiding extra ion doping and the use of methanol-LiOH·H2O-NH4F mixtures. An improved thermal decomposition method is introduced in this paper. Moreover, we determined optimal reaction conditions and investigated the influence of reaction temperature, reaction time, and LiOH·H2O content on the upconversion luminescence properties and luminescence lifetimes.
The synthesis was carried out using standard oxygen-free procedures and commercially available reagents. RE2O3 (, Yb3+ 99.99%, and Er3+ 99.99%), CF3COOH (analytical grade 99.0%), oleic acid (OA, analytical grade), and LiOH·H2O (>95.0%) were purchased from Sinopharm Chemical Reagent Beijing Co. Ltd. Absolute ethanol (), cyclohexane (analytical grade 99.5%), and 1-octadecene (ODE, technical grade 90%) were purchased from Tianjin Chemical Co., Kemeng, and Sigma-Aldrich, respectively. All chemicals were used without further purification.
2.2. Synthesis of Precursor Mixture
We improved and modified previously reported methods to synthesize trifluoroacetate precursors [42–44]. Compared with the traditional preparation process of single trifluoroacetates of the lanthanides [Ln(CF3COO)3, , Yb, and Er] and Li(CF3COO)3 samples, we are mixing the two to prepare precursor mixture. As for reaction vessel, we choose a 100 mL Teflon-lined autoclave to replace the traditional 100 mL three-neck flask. The process was carried out via adding Gd2O3 (0.78 mmol), Yb2O3 (0.2 mmol), Er2O3 (0.02 mmol), and a certain amount of LiOH·H2O to the mixture solution of trifluoroacetic acid/water (6 mL : 6 mL). This turbid solution was vigorously stirred and transferred to a 100 mL Teflon-lined autoclave. Subsequently, the emulsion was heated and maintained at 80°C. After confinement for 12 h, the solution was cooled to room temperature and transferred to a 100 mL three-neck flask and dryly heated up at 60°C for evaporating excess CF3COOH/H2O. Then, the precursor mixture was obtained. The profit about such doing is that the preparation process will be safer and more convenient. It is mainly because a mixed solution of methanol, LiOH·H2O, and NH4F for providing Li+ or other doped ions is required in the traditional preparation process, while none of these is required in our preparation.
2.3. Synthesis of High-Quality Yb3+/Er3+-Codoped LiGdF4 Colloidal Nanocrystals
The high-quality Yb3+/Er3+-codoped LiGdF4 colloidal nanocrystals were prepared by a thermal decomposition route. The process was carried out via adding 15 mL OA and 15 mL ODE to precursor mixture. The cloudy solution was vigorously stirred to yield a transparent solution. Subsequently, the transparent solution was heated to 110°C under vacuum condition for removing needless water/oxygen. An hour later, the solution was heated to 310°C (or 280°C, 290°C, and 300°C) under an argon gas atmosphere and maintained for a period of time (1 h, 2 h, 3 h, 4 h, 5 h, or 6 h). The reaction system was cooled to room temperature and added excess absolute ethanol to precipitate products. The as-synthesized products were washed several times with cyclohexane/ethanol (1 : 4) mixed solution to remove the residue of organic ligands and other mixtures on the products and isolated by centrifugation at 8000 rpm for 3 min. Finally, the products were dried under vacuum to 60°C for 12 h to obtain a white powder sample for reserving.
3. Results and Discussions
3.1. Synthetic Procedure and Reaction Mechanism
Figure 1 illustrates the synthesis of high-quality Yb3+/Er3+-codoped LiGdF4 colloidal nanocrystals, showing that it comprised two steps, namely, hydrothermal preparation of the precursor mixture, followed by thermal decomposition to afford LiGdF4 colloidal nanocrystals.
To get better understanding, the nucleation and growth mechanisms of LiGdF4:Yb0.2/Er0.02 colloidal nanocrystals are speculated in Scheme 1. Briefly, the process starts with the cothermolysis of Gd(CF3COO)3 and CF3COOLi in oleic acid and 1-octadecene systems. When the reaction system is heated to 100~120°C, trifluoroacetate ligands are partially exchanged for oleic acid residues, and the C-F bond of the former is broken to release F- when the reaction temperature further increases to 250~330°C. Subsequently, fluoride anions engage in fluorination and cleave Gd-OOCCF3 bonds to promote nucleation, and the thus obtained crystal nuclei agglomerate to form larger particles as the reaction progresses. With the increase of temperature, the growth rate of these nuclei crystal nucleus increases and eventually forms nanocrystals [45–48].
3.2. Structure and Morphology
Sample crystal structures were characterized by using a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation () at 200 mA and 40 kV. XRD patterns were recorded for dried powders in the range of at a step size of 8°/min. Figure 2 clearly demonstrates the growth kinetics of LiGdF4:Yb3+/Er3+ nanocrystals synthesized at different reaction times. It is not hard to find that the diffraction peaks of samples obtained at and , 2 h, and 3 h corresponded to a mixture of LiGdF4 (JCPDS No. 27-1236) and GdF3 (JCPDS No. 49-1804), whereas those obtained at and and 5 h contain LiGdF4 (JCPDS No. 27-1236). Hence, 4 or 5 h was concluded to be the optimal reaction times for synthesizing LiGdF4:Yb3+/Er3+ nanocrystals.
Next, we studied the optimum reaction temperature for preparing LiGdF4:Yb3+/Er3+ colloidal nanocrystals. Figure 3 clearly demonstrates the formation of LiGdF4 with GdF3 at and temperatures () of 280, 290, and 300°C, revealing that LiGdF4 nuclei were instantly formed even at 280°C and that the growth rate of these nuclei increased with the increasing temperature. A balance between nucleation and growth was established at 310°C, which was concluded to be the optimum reaction temperature for the synthesized tetragonal LiGdF4:Yb3+/Er3+ nanocrystals. Finally, the optimum Li/RE ratio was determined as three, although the optimization of Li+ concentration proved to be very tortuous. Details of the synthetic process and parameters of tetragonal-phase LiGdF4:Yb3+/Er3+ colloidal nanocrystals are given in the supporting information (Figure S1, S2, and Table S1). Thus, the above experimental results suggest that the optimum conditions for tetragonal LiGdF4:Yb3+/Er3+ nanocrystal synthesis were determined as , , and (Table S1).
Sample morphology was assessed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) with a FEI Tecnai G2 F20 S-Twin transmission electron microscope operating at 200 kV. Samples for TEM imaging were prepared by drying nanocrystal dispersions in cyclohexane on amorphous carbon-coated Cu grids. Representative TEM micrographs of as-synthesized LiGdF4 nanocrystals (relatively pure tetragonal phase) are displayed in Figure 4. Figures 4(a)–4(d) show low-magnification TEM images, while Figures 5(a) and 5(b) show high-magnification TEM images with related selected-area electron diffraction patterns, revealing that the as-synthesized LiGdF4:Yb3+/Er3+ nanocrystals can withstand irradiation with high-energy electrons. From the TEM images, we can also see that most of the samples are octahedron and sphere. And the average sizes of these crystals focus on 137.7 nm nearby in Figure 4(e), while the octahedral crystals focus on on average. HRTEM imaging revealed the presence of obvious lattice fringes, indicating the high crystallinity of individual particles. The adjacent lattice spacing calculated by FFT analysis (~0.47 nm) was assigned to the (101) crystal plane of tetragonal-phase LiGdF4, which confirmed that as-synthesized LiGdF4 nanocrystals exhibited high crystallinity and few defects.
3.3. Upconversion Luminescence
Upconversion emission spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer (, , and ) under excitation with an adjustable 980 nm NIR laser diode. Room temperature upconversion emission spectra are obtained by drying the nanocrystal dispersion in cyclohexane at a concentration of 2 mg/mL. It is formed into a colloidal solution by dispersing dried powder in cyclohexane for several hours’ ultrasound.
It is well known that Yb3+- and Er3+-codoped rare earth fluorides can exhibit strong upconversion luminescence upon 980 nm near-infrared excitation, as observed herein for the colloidal suspension of LiGdF4:Yb3+/Er3+. Two visible-light-region emission bands, positioned at 523 and 543 nm (green upconversion luminescence) and 672 nm (red upconversion luminescence), were observed for all samples in Figures 6(a) and 6(b). The above bands were ascribed to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions, respectively. Meanwhile, we also find that the intensity of upconversion luminescence increased with increasing reaction time, which was mainly attributed to the concomitant increase of tetragonal phase content. This phase could be obtained in relatively pure form at , , and , while mixtures of LiGdF4 with GdF3, exhibiting lower upconversion luminescence intensities because of the presence of the latter component and other impurities, were obtained otherwise. It is surprising that there is a sudden drop in the upconversion luminescence intensity of the sample, which is synthesized at , , and . This finding might be attributed to the fact that the content of GdF3 and other impurities in the above sample exceeded that in the optimal condition sample.
In addition, it might be that the growth rate of LiGdF4 crystal nucleus reached the maximum at 4 h, while the reaction time that further increased to 5 h might lead to excessive surfactant and activator ions accumulate on the crystal surface. All of these cause fluorescence quenching of the sample, which is synthesized at , , and . The clear contrast figures of emission intensity and reaction time are provided in the supporting information (Figure S3 and Table S2). To gain further insights into the upconversion emission process, we investigated the dependence of upconversion emission intensity on excitation power adopting the relation for data analysis in Figure 6(c). We regard the tetragonal LiGdF4 nanocrystals as example to illuminate the upconversion emission process. The slopes of Gaussian function-based log-log fits were determined as 2.11 and 1.95 for the dominant green emissions at 523 and 543 nm, respectively, which illustrated that these emissions involve a two-photon process. For red emission, the corresponding slope was obtained as 2.37, and the same conclusion was drawn. It corresponds with the analysis of upconversion emissions mechanism.
Figure 7 schematically illustrates energy transfer and upconversion emission processes occurring at an excitation wavelength of 980 nm and a pump power density of 55 mW/cm2. Under continuous excitation at 980 nm photon, sensitizer Yb3+ ions can be excited from the 2F7/2 ground state to the 2F5/2 state. Subsequently, the latter states decay back to the former, and the released energy is captured by nearby Er3+ ions, which are excited from the 4I15/2 ground state into the 4I11/2 state. Further energy capture by Er3+ ions in the 4I11/2 excited state results in the population of a higher-lying 4F7/2 state that can relax to the 2H11/2 and 4S3/2 levels (nonradiatively) and to the 4I15/2 level (radiatively) with dominant 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions resulting in green emission. Alternatively, Er3+ ions in the 4I11/2 excited state can nonradiatively relax to the 4I13/2 state and capture further energy to populate a higher-lying 4F9/2 state that subsequently radiatively relaxes to the 4I15/2 level with dominant 4F9/2 → 4I15/2 transitions resulting in red emissions. Notably, as-synthesized LiGdF4 nanocrystals not only showed higher upconversion emission intensity but also longer luminescence lifetime (Figure 8 and Figure S4).
The decay of upconversion luminescence was recorded at room temperature using a lifetime fluorescence spectrometer (Delta Flex TCSPC system, Horiba Scientific, Scotland) equipped with an adjustable pulse laser as excitation source (, , , and ). The obtained decay curves were fitted by a single-exponential function, and the effective UCL lifetime of nonexponential decay was calculated as where is the maximum upconversion luminescence intensity and is the upconversion luminescence intensity as a function of time [49, 50]. The UCL lifetimes of the 2H11/2 state, determined by monitoring Er3+ emission at 523 nm under 980 nm, were found to increase with a reaction time of up to 4 h, equalling 369.30 (2 h), 663.23 (3 h), 745.74 μs (4 h), and 540.45 μs (5 h), respectively (Figure 8). This result confirmed that the sample synthesized at , , and exhibited good photostability because of the relatively pure tetragonal phase. In other samples, the presence of impurity ingredients led to the rapid migration of energy to lattice defects or surface quenchers, inducing luminescence quenching. This behaviour was consistent with the trend of upconversion emission intensity in Figure 6(a). Finally, we measured the lifetimes of 2H11/2, 4S3/2, and 4F9/2 states of Er3+ under 980 nm excitation in LiGdF4:Yb3+/Er3+ nanocrystals, synthesized at , , and (Figure S4), revealing increases from 663.23 μs (523 nm) and 665.54 μs (543 nm) to 678.41 μs (672 nm), respectively.
We have successfully improved and modified previously reported methods to synthesize high-quality Yb3+/Er3+-codoped LiGdF4 colloidal nanocrystals with intense green emission. Specifically, the above synthesis involved the hydrothermal preparation of trifluoroacetate precursors Gd(CF3COO)3 and CF3COOLi that were subsequently thermolyzed to afford the desired nanocrystals. Importantly, the adopted approach obviated the need for additional ion doping and the use of toxic methanol-LiOH·H2O-NH4F mixture. Studies on the impact of LiOH·H2O concentration, reaction temperature, and time on the upconversion luminescence of nanocrystal samples showed that relatively phase-pure tetragonal LiGdF4 nanocrystals could be obtained under optimal conditions (, , and ). Moreover, as-synthesized LiGdF4:Yb3+/Er3+ nanocrystals not only showed a stronger upconversion emission intensity but also featured a longer luminescence lifetime. This work paves the way to the broad utilization of LiGdF4, which is viewed as an ideal alternative matrix material, since the ionic radius of Li+ is much smaller than that of Na+. Herein, this research work will be indispensable for further follow-up study.
All data are obtained through our own experiments. The public database is not used in this article. If the reader needs the data in this article, he can contact the corresponding author.
Conflicts of Interest
The authors declare that they have no conflict of interest.
We gratefully acknowledge professor Gejihu De for guiding and the testing Center of Inner Mongolia University. This work is supported by the Postgraduate Scientific Research Innovation Foundation of Inner Mongolia University (Grant No. CXJJS15082), Open Fund of the State Key Laboratory on Integrated Optoelectronics (Grant No. IOSKL2013KF08), National Science Foundation of China (Grant No. 21261016), and Talents Project Inner Mongolia of China (Grant No. CYYC5026).
Supplementary material including X-ray diffraction patterns of as-prepared samples, decay curves of LiGdF4:Yb3+/Er3+ nanocrystals, and discussion of influencing factors (such as temperature, time, and emission intensity). (Supplementary Materials)
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