Growth of β-NaYF4:Eu3+ Crystals by the Solvothermal Method with the Aid of Oleic Acid and Their Photoluminescence Properties

Lanthanide-doped hexagonal β-NaYF4 crystals have received much attention in recent years due to their excellent photoluminescence properties. However, lanthanide-doped β-NaYF4 crystals with micron and submicron scales as well as uniform morphology have received less attention. In this study, Eu3+-doped β-NaYF4 (β-NaYF4:Eu3+) crystals of micron and submicron size scales were synthesized using the solvothermal method with ethylene glycol as the solvent. The β-NaYF4:Eu3+ crystals were highly crystallized. A comparison of the characteristics of the β-NaYF4:Eu3+ crystals synthesized with and without the use of oleic acid as a surfactant was conducted. It was found that the utilization of oleic acid as a surfactant during their synthesis greatly decreased their particle size from micron to submicron scale, while adding a small amount of ethanol further reduced their particle size. In addition, they exhibited much smoother surfaces and more uniform morphologies, which were hexagonal prism bipyramids. The microstructural characteristics and photoluminescence properties of the β-NaYF4:Eu3+ crystals were studied in detail. Results showed that β-NaYF4:Eu3+ crystals prepared with the aid of oleic acid as a surfactant during their synthesis exhibited stronger photoluminescence.

The space groups of αand β-NaYF 4 are Fm3m (Z = 4) and P6 (Z = 1), respectively. The α-NaYF 4 is isomorphic with CaF 2 in which the sites of Ca 2+ ions are substituted randomly by 1/2 Na + ions and 1/2 Y 3+ ions and the O h symmetry of the cationic site has been retained [18]. Three types of cation sites exist in β-NaYF 4 : 1a sites are occupied by Y 3+ , 1f sites are occupied by 1/2Y 3+ and 1/2Na + ions randomly, while 2h sites are usually occupied by Na + ions only [19,20]. The symmetry of the two Y 3+ sites is C 3h . Thus, the phase transition from α-NaYF 4 to β-NaYF 4 must overcome energy barriers of cation rearrangement, and it, is therefore a disorder-to-order arrangement process. It has been found that β-NaYF 4 has a more stable crystal structure and better photoluminescence properties than α-NaYF 4 [1][2][3], which has resulted in β-NaYF 4 receiving more attention from researchers. However, Ln 3+ ions (e.g., Eu 3+ , Tb 3+ , Dy 3+ , etc.) have a relatively small optical absorption cross section and a narrow absorption spectrum because of the forbidden nature of the f-f transition. Thus the intrinsic photoluminescence of Ln 3+ ions in Ln 3+ -doped β-NaYF 4 phosphors is limited [21]. So far, numerous methods have been studied to enhance the absorption and photoluminescence intensity of Ln 3+ -doped β-NaYF 4 phosphors [22][23][24], including multiple types of Ln 3+ -doping and sensitization with organic ligands [8,25,26]. In this study, a surfactant was employed to control the morphology of the Ln 3+ -doped β-NaYF 4 phosphors, and their photoluminescence properties were enhanced.
Ln 3+ -doped β-NaYF 4 phosphors, in both micro-sized and nano-sized scales, have been previously synthesized using different methods, such as the molten salt method [27], thermal decomposition of organic precursors [28], sol-gel [17,29], and the hydrothermal/solvothermal method [30]. Some researchers [2,12,30,31] prepared NaYF 4 crystals using the hydrothermal/solvothermal method in a water/alcohol/oleic acid system, while others [32] prepared two kinds of NaYF 4 particles with different phase composition using an adjustable solvothermal method and by changing reaction temperature and molar ratio (Y 3+ /F − ) with a different phase composition. NaYF 4 nanorods, nanotubes, and nanodisks were prepared using oleic acid as hydrothermal synthesis-mediated agent [33], and UC β-NaYF 4 :Yb 3+ /Er 3+ microcrystals were synthesized using the hydrothermal method at 220 • C [34]. However, it is still difficult to obtain Ln 3+ -doped β-NaYF 4 particles with uniform and regular shape, high crystallinity, and strong photoluminescence performance. Moreover, these reported hydrothermal/solvothermal processes need either a high temperature or a long reaction time, which is neither economical nor environmentally friendly.
In this study, a simple and convenient solvothermal method was employed to synthesize highly dispersed Eu 3+ -doped β-NaYF 4 crystals of micron and submicron size, with controlled morphology and strong photoluminescence. The preparation method was based on our previous work of α-NaYF 4 :Eu 3+ , which had used ethylene glycol/ethanol as a solvent [8]. Oleic acid (OA) was used as a surfactant to control the shape and morphology of the β-NaYF 4 :Eu 3+ microcrystals with enhanced photoluminescence properties. Hexagonal prism bipyramidal β-NaYF 4 :Eu 3+ crystals with uniform particle size of micron and submicron scale were successfully synthesized. The different characteristics of the phase structures, microstructural characteristics, and photoluminescence properties between the as-synthesized β-NaYF 4 :Eu 3+ particles and those modified by OA were compared in detail. The mechanisms for their shape control, morphology optimization, and photoluminescence enhancement are discussed.

Synthesis of β-NaYF 4 :Eu 3+ Crystals without Surfactant
The β-NaYF 4 :Eu 3+ crystals were synthesized using a solvothermal method as previously described [8]. The doping concentration was 5 atm% with the formula β-NaY 0.95 Eu 0.05 F 4 . In a typical procedure, Y(NO 3 ) 3 ·6H 2 O (0.7277 g, or 1.9 mmol), Eu(NO 3 ) 3 ·6H 2 O (0.0446 g, or 0.1 mmol), and NaNO 3 (0.1700 g, or 2 mmol) were mixed and thoroughly dissolved in 15 mL EG to form a clear solution A at room temperature. NH 4 F (1.7779 g, or 48 mmol) was added to a 15 mL EG solution to form solution B at room temperature. It was found that the phase structure of the synthesized NaYF 4 strongly depended on the Y 3+ :F − molar ratio in the reaction system. To obtain β-NaYF 4 :Eu 3+ crystals, the molar ratio of Ln 3+ ions (Y 3+ + Eu 3+ ) in solution A to F − ions in solution B was set as Ln 3+ :F − = 1:24. Solutions A and B were mixed under vigorous stirring for 2 h to form a mixture solution, which was poured into a 50 mL Teflon-lined stainless steel autoclave and heated at 180 • C for 6 h to synthesize β-NaYF 4 :Eu 3+ crystals. After they were cooled down, the resultant β-NaYF 4 :Eu 3+ crystals were collected in a similar manner as described previously [8]. The undoped β-NaYF 4 crystals were synthesized as the standard sample, following the above procedure, except that no Eu 3+ was used.

Synthesis of β-NaYF 4 :Eu 3+ Crystals with the Aid of an OA Surfactant
OA was introduced into the synthesis as a surfactant to control the morphology and improve the photoluminescence of the β-NaYF 4 :Eu 3+ crystals. The synthesis procedure was the same as the above procedure for non-surfactant synthesis (Section 2.2), with the following two modifications. First, 5 mL of OA was added to solution A. Second, the 15 mL of EG of solution A had two sets of OA additions: (1) 15 mL of EG, or (2) 2.5 mL of absolute ethanol and 12.5 mL of EG. Thus, two β-NaYF 4 :Eu 3+ samples modified by an OA surfactant were synthesized with the two sets of OA additions.

Characterization
X-ray diffraction (XRD, PANalytical, Empyrean) and field emission scanning electron microscopy (FESEM, FEI-Quanta200F, Thermo Fisher, Waltham, MA, USA) were used to study the phase structures and observe the microstructural characteristics of the β-NaYF 4 :Eu 3+ crystals. The X-ray diffractometer, equipped with graphite-monochromatized Cu Kα radiation (λ = 1.5406 Å) was operated at the scanning step of 0.0131303 • in the 2θ range from 10 • to 90 • . The photoluminescence excitation and emission spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) equipped with a xenon lamp source. All of the measurements in this work were conducted at room temperature. Figure 1 shows the XRD patterns of the as-synthesized NaYF 4 crystals prepared using different Y 3+ :F − molar ratios. The Joint Committee on Powder Diffraction Standards (JCPDS) data for the cubic (α-NaYF 4 , PDF#77-2042) and hexagonal (β-NaYF 4 , PDF#16-0334) crystal structure of NaYF 4 are included in Figure 1 for reference. As shown in Figure 1, only the α-NaYF 4 phase could be obtained when Y 3+ :F − = 1:12, while mixed phases of α-NaYF 4 and β-NaYF 4 were present when Y 3+ :F − = 1:16 and 1:20. When Y 3+ :F − ≥ 1:24, only the β-NaYF 4 phase was obtained. This suggests that the crystal structure of NaYF 4 strongly depends on the Y 3+ :F − molar ratios in the reaction solution. For this reason, the Y 3+ :F − ratio was set at 1:24 to synthesize β-NaYF 4 :Eu 3+ in this study. Figure 2 shows the XRD patterns of the as-synthesized β-NaYF 4 :Eu 3+ crystals prepared without OA, with 5 mL OA and with 5 mL OA + 2.5 mL C 2 H 5 OH. All of the diffraction peaks ( Figure 2) can be readily indexed to the pure hexagonal NaYF 4 phase, and no secondary phase was observed in all samples. The sharp diffraction peaks in Figure 2 imply that all of the β-NaYF 4 :Eu 3+ particles were well crystallized. As seen in Figure 2, all of the XRD patterns were relatively similar, which suggests that the utilization of an OA surfactant did not have a noticeable impact on the crystal structures of the β-NaYF 4 :Eu 3+ crystals. Furthermore, the use of C 2 H 5 OH also had little influence on their crystal structures.  Figure 2 shows the XRD patterns of the as-synthesized β-NaYF4:Eu 3+ crystals prepared without OA, with 5 mL OA and with 5 mL OA + 2.5 mL C2H5OH. All of the diffraction peaks ( Figure 2) can be readily indexed to the pure hexagonal NaYF4 phase, and no secondary phase was observed in all samples. The sharp diffraction peaks in Figure 2 imply that all of the β-NaYF4:Eu 3+ particles were well crystallized. As seen in Figure 2, all of the XRD patterns were relatively similar, which suggests that the utilization of an OA surfactant did not have a noticeable impact on the crystal structures of the β-NaYF4:Eu 3+ crystals. Furthermore, the use of C2H5OH also had little influence on their crystal structures. The SEM images in Figure 3 show the microstructural characteristics of the as-prepared β-NaYF4:Eu 3+ particles. All of the β-NaYF4:Eu 3+ particles were highly dispersible. The β-NaYF4:Eu 3+ particles prepared without an OA surfactant were the largest (Figure 3a), which was about 1.5 μm. As shown in Figure 3b,c, the utilization of an OA surfactant in the reaction solution greatly reduced   Figure 2 shows the XRD patterns of the as-synthesized β-NaYF4:Eu 3+ crystals prepared without OA, with 5 mL OA and with 5 mL OA + 2.5 mL C2H5OH. All of the diffraction peaks ( Figure 2) can be readily indexed to the pure hexagonal NaYF4 phase, and no secondary phase was observed in all samples. The sharp diffraction peaks in Figure 2 imply that all of the β-NaYF4:Eu 3+ particles were well crystallized. As seen in Figure 2, all of the XRD patterns were relatively similar, which suggests that the utilization of an OA surfactant did not have a noticeable impact on the crystal structures of the β-NaYF4:Eu 3+ crystals. Furthermore, the use of C2H5OH also had little influence on their crystal structures. The SEM images in Figure 3 show the microstructural characteristics of the as-prepared β-NaYF4:Eu 3+ particles. All of the β-NaYF4:Eu 3+ particles were highly dispersible. The β-NaYF4:Eu 3+ particles prepared without an OA surfactant were the largest (Figure 3a), which was about 1.5 μm. As shown in Figure 3b,c, the utilization of an OA surfactant in the reaction solution greatly reduced The SEM images in Figure 3 show the microstructural characteristics of the as-prepared β-NaYF 4 :Eu 3+ particles. All of the β-NaYF 4 :Eu 3+ particles were highly dispersible. The β-NaYF 4 :Eu 3+ particles prepared without an OA surfactant were the largest (Figure 3a), which was about 1.5 µm. As shown in Figure 3b,c, the utilization of an OA surfactant in the reaction solution greatly reduced the size of the β-NaYF 4 :Eu 3+ particles. The average NaYF 4 :Eu 3+ particle size, prepared with 5 mL of OA and 5 mL of OA + 2.5 mL of C 2 H 5 OH was about 650 nm and 500 nm (Figure 3b,c), respectively. Thus, adding C 2 H 5 OH further reduced the size of the β-NaYF 4 :Eu 3+ particles. the size of the β-NaYF4:Eu 3+ particles. The average NaYF4:Eu 3+ particle size, prepared with 5 mL of OA and 5 mL of OA + 2.5 mL of C2H5OH was about 650 nm and 500 nm (Figure 3b,c), respectively. Thus, adding C2H5OH further reduced the size of the β-NaYF4:Eu 3+ particles. As seen in Figure 3a, the β-NaYF4:Eu 3+ particles, prepared without using an OA surfactant had irregular shapes with rough surfaces. However, the β-NaYF4:Eu 3+ particles prepared with an OA surfactant were regular and uniform hexagonal prism bipyramids [34] with smooth surfaces (insets of Figure 3b,c). Thus, the OA surfactant not only reduced the particle size of β-NaYF4:Eu 3+ from the As seen in Figure 3a, the β-NaYF 4 :Eu 3+ particles, prepared without using an OA surfactant had irregular shapes with rough surfaces. However, the β-NaYF 4 :Eu 3+ particles prepared with an OA surfactant were regular and uniform hexagonal prism bipyramids [34] with smooth surfaces (insets of Figure 3b,c). Thus, the OA surfactant not only reduced the particle size of β-NaYF 4 :Eu 3+ from the micrometer range to submicron scale, it also greatly improved their morphologies. Note that the hexagonal prism bipyramid shape is derived from top and bottom halves, where each half is a six-sided prism. The hexagonal prism bipyramid structure of β-NaYF 4 :Eu 3+ found in this experiment is similar to the β-NaYF 4 :Yb,Er micro-bipyramids shown by Ding et al. [34]. Their research [34] also proposed a possible morphology evolution mechanism for the β-NaYF 4 :Yb,Er micro-bipyramids, as illustrated in Figure 4. As pointed out by Liang et al. [2], the OA molecules are coated onto the outer face of the generated NaYF 4 particles during the early reaction process through the interaction between Ln 3+ (Y 3+ , Eu 3+ ) and the carboxyl of the OA, with the hydrophobic alkyl chains left outside. This interaction can be weakened under high temperature and pressure during the solvothermal process to allow for the Ln 3+ ions to be released gradually. The cubic α-NaYF 4 is grown first, and these particles usually have an isotropic shape. Because α-NaYF 4 is unstable, a more stable β-NaYF 4 is formed through an α→β phase transformation during the ongoing reaction process [34]. Eventually, hexagonal prisms are formed [34]. micrometer range to submicron scale, it also greatly improved their morphologies. Note that the hexagonal prism bipyramid shape is derived from top and bottom halves, where each half is a six-sided prism. The hexagonal prism bipyramid structure of β-NaYF4:Eu 3+ found in this experiment is similar to the β-NaYF4:Yb,Er micro-bipyramids shown by Ding et al. [34]. Their research [34] also proposed a possible morphology evolution mechanism for the β-NaYF4:Yb,Er micro-bipyramids, as illustrated in Figure 4. As pointed out by Liang et al. [2], the OA molecules are coated onto the outer face of the generated NaYF4 particles during the early reaction process through the interaction between Ln 3+ (Y 3+ , Eu 3+ ) and the carboxyl of the OA, with the hydrophobic alkyl chains left outside. This interaction can be weakened under high temperature and pressure during the solvothermal process to allow for the Ln 3+ ions to be released gradually. The cubic α-NaYF4 is grown first, and these particles usually have an isotropic shape. Because α-NaYF4 is unstable, a more stable β-NaYF4 is formed through an α→β phase transformation during the ongoing reaction process [34]. Eventually, hexagonal prisms are formed [34]. The main reason for the reduced particle size of β-NaYF4:Eu 3+ when using a surfactant is that OA covers the surface of the particles through the interaction between carboxyl and Eu 3+ ions ( Figure  3). Adding a small amount of C2H5OH into the reaction solution can further reduce the particle size because C2H5OH has a low boiling point, which boosts the internal pressure and consequentially improves the activity of ions. As a result, a large number of crystal nuclei form rapidly in a short time. Furthermore, the crystal nuclei will collide with each other violently, resulting in aggregation and uneven particle growth, which can be gradually uniformed to become microcrystals with increasing reaction time. Figure 5 shows the photoluminescence excitation and emission spectra of the three β-NaYF4:Eu 3+ samples prepared without OA, with 5 mL OA, and with 5 mL OA + 2.5 mL C2H5OH. All of the excitation peaks ranging from 200 to 500 nm are attributed to the characteristic excitation of Eu 3+ ions at 320 nm ( 7 F0→ 5 H0), 364 nm ( 7 F0→ 5 D4), 379 nm ( 7 F0→ 5 G2), 397 nm ( 7 F0→ 5 L6), 418 nm ( 7 F0→ 5 D3), and 468 nm ( 7 F0→ 5 D2) [8]. The strongest excitation peak originates from the 7 F0→ 5 L6 transition. Similarly, the emission peaks excited by an exciting light at 397 nm are the characteristic emissions of Eu 3+ ions, and they are assigned to the high-energy transition 5 D2→ 7 F3 at 515 nm and 5 D1→ 7 FJ (J = 0-2) at 530 nm, 541 nm, and 561 nm; low-energy transition 5 D0→ 7 FJ (J = 1-4) at 598 nm, 622 nm, 657 nm, and 702 nm, respectively [8,35]. The weaker emission peak at 598 nm corresponding to 5 D0→ 7 F1 is allowed by the magnetic dipole transitions due to Eu 3+ ions, located at a site with inversion symmetry, while the stronger emission peak at 622 nm, corresponding to 5 D0→ 7 F2, is allowed by the electric dipole transitions that are a result of absence of inversion symmetry at the Eu 3+ lattice site [36][37][38][39]. The main reason for the reduced particle size of β-NaYF 4 :Eu 3+ when using a surfactant is that OA covers the surface of the particles through the interaction between carboxyl and Eu 3+ ions (Figure 3). Adding a small amount of C 2 H 5 OH into the reaction solution can further reduce the particle size because C 2 H 5 OH has a low boiling point, which boosts the internal pressure and consequentially improves the activity of ions. As a result, a large number of crystal nuclei form rapidly in a short time. Furthermore, the crystal nuclei will collide with each other violently, resulting in aggregation and uneven particle growth, which can be gradually uniformed to become microcrystals with increasing reaction time. Figure 5 shows the photoluminescence excitation and emission spectra of the three β-NaYF 4 :Eu 3+ samples prepared without OA, with 5 mL OA, and with 5 mL OA + 2.5 mL C 2 H 5 OH. All of the excitation peaks ranging from 200 to 500 nm are attributed to the characteristic excitation of Eu 3+ ions at 320 nm ( 7 F 0 → 5 H 0 ), 364 nm ( 7 F 0 → 5 D 4 ), 379 nm ( 7 F 0 → 5 G 2 ), 397 nm ( 7 F 0 → 5 L 6 ), 418 nm ( 7 F 0 → 5 D 3 ), and 468 nm ( 7 F 0 → 5 D 2 ) [8]. The strongest excitation peak originates from the 7 F 0 → 5 L 6 transition. Similarly, the emission peaks excited by an exciting light at 397 nm are the characteristic emissions of Eu 3+ ions, and they are assigned to the high-energy transition 5 D 2 → 7 F 3 at 515 nm and 5 D 1 → 7 F J (J = 0-2) at 530 nm, 541 nm, and 561 nm; low-energy transition 5 D 0 → 7 F J (J = 1-4) at 598 nm, 622 nm, 657 nm, and 702 nm, respectively [8,35]. The weaker emission peak at 598 nm corresponding to 5 D 0 → 7 F 1 is allowed by the magnetic dipole transitions due to Eu 3+ ions, located at a site with inversion symmetry, while the stronger emission peak at 622 nm, corresponding to 5 D 0 → 7 F 2 , is allowed by the electric dipole transitions that are a result of absence of inversion symmetry at the Eu 3+ lattice site [36][37][38][39].

Photoluminescence Properties
As seen in Figure 5, the utilization of OA during the synthesis enhanced the photoluminescence intensity of the β-NaYF 4 :Eu 3+ crystals by about 20%. The β-NaYF 4 :Eu 3+ crystals prepared with either 5 mL OA or with 5 mL OA + 2.5 mL C 2 H 5 OH had comparable photoluminescence intensity ( Figure 5), with the latter being slightly higher than the former. The reason why OA can improve the photoluminescence of the β-NaYF 4 :Eu 3+ crystals could be explained as follows. The β-NaYF 4 :Eu 3+ crystals prepared with OA had a smoother surface and more integrated uniform shape than the ones prepared without using OA (Figure 3). Thus, the former should have a lower density of non-radiative traps on the surface than the latter, and therefore the former is expected to have stronger photoluminescence. As seen in Figure 5, the utilization of OA during the synthesis enhanced the photoluminescence intensity of the β-NaYF4:Eu 3+ crystals by about 20%. The β-NaYF4:Eu 3+ crystals prepared with either 5 mL OA or with 5 mL OA + 2.5 mL C2H5OH had comparable photoluminescence intensity ( Figure 5), with the latter being slightly higher than the former. The reason why OA can improve the photoluminescence of the β-NaYF4:Eu 3+ crystals could be explained as follows. The β-NaYF4:Eu 3+ crystals prepared with OA had a smoother surface and more integrated uniform shape than the ones prepared without using OA ( Figure 3). Thus, the former should have a lower density of non-radiative traps on the surface than the latter, and therefore the former is expected to have stronger photoluminescence.

Conclusions
In summary, highly crystallized β-NaYF4:Eu 3+ crystals with micron and submicron scale were successfully synthesized using the solvothermal method, with ethylene glycol as the solvent. The characteristics of the β-NaYF4:Eu 3+ crystals synthesized with and without the use of oleic acid as a surfactant during their synthesis were compared. The utilization of OA as a surfactant during the synthesis of the β-NaYF4:Eu 3+ crystals was found to greatly decrease their particle size from about 1.5 μm to about 650 nm. Adding small amount of ethanol could further decrease the particle to about 500 nm. In addition, the β-NaYF4:Eu 3+ crystals synthesized with the aid of OA as a surfactant also exhibited much smoother surfaces and more uniform morphologies, which were hexagonal prism bipyramids. The present study also found that the β-NaYF4:Eu 3+ crystals prepared with the aid of OA as a surfactant during their synthesis exhibited about 20% stronger photoluminescence than those synthesized without using an OA surfactant.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
In summary, highly crystallized β-NaYF 4 :Eu 3+ crystals with micron and submicron scale were successfully synthesized using the solvothermal method, with ethylene glycol as the solvent. The characteristics of the β-NaYF 4 :Eu 3+ crystals synthesized with and without the use of oleic acid as a surfactant during their synthesis were compared. The utilization of OA as a surfactant during the synthesis of the β-NaYF 4 :Eu 3+ crystals was found to greatly decrease their particle size from about 1.5 µm to about 650 nm. Adding small amount of ethanol could further decrease the particle to about 500 nm. In addition, the β-NaYF 4 :Eu 3+ crystals synthesized with the aid of OA as a surfactant also exhibited much smoother surfaces and more uniform morphologies, which were hexagonal prism bipyramids. The present study also found that the β-NaYF 4 :Eu 3+ crystals prepared with the aid of OA as a surfactant during their synthesis exhibited about 20% stronger photoluminescence than those synthesized without using an OA surfactant.
Author Contributions: J.H., X.W. and A.S. conducted the investigation and analysis work, and they wrote the manuscript draft. G.D. and N.C. designed the method and ssupervised the work. G.D. provided the funding for this work.