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

Abstract

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.

1. Introduction

NaYF₄ has two polymorphic forms: α- (cubic) and β-phase (hexagonal) structures. In recent years, NaYF₄—especially the β phase—has been considered one of the most effective host materials for performing multicolor down-conversion (DC) and up-conversion (UC) photoluminescence of lanthanide ions (Ln³⁺) due to its unique properties [1], including low phonon energy (<400 cm⁻¹) [2], high thermal and optical stability, good transparency, low non-radiative decay rate, and high radiative emission efficiency [3]. Ln³⁺-doped NaYF₄ phosphors have been extensively studied [4,5,6,7,8,9,10] in both micro- and nano-sized scales because of their potential applications in light-emitting diodes (LEDs) [11], lasers [12], display devices [9], biological labels [13,14,15], solar cells [16,17], and many others.

The space groups of α- and β-NaYF₄ are Fm3m (Z = 4) and P6 (Z = 1), respectively. The α-NaYF₄ is isomorphic with CaF₂ in which the sites of Ca²⁺ ions are substituted randomly by 1/2 Na⁺ ions and 1/2 Y³⁺ ions and the O_h symmetry of the cationic site has been retained [18]. Three types of cation sites exist in β-NaYF₄: 1a sites are occupied by Y³⁺, 1f sites are occupied by 1/2Y³⁺ and 1/2Na⁺ ions randomly, while 2h sites are usually occupied by Na⁺ ions only [19,20]. The symmetry of the two Y³⁺ sites is C_3h. Thus, the phase transition from α-NaYF₄ to β-NaYF₄ must overcome energy barriers of cation rearrangement, and it, is therefore a disorder-to-order arrangement process. It has been found that β-NaYF₄ has a more stable crystal structure and better photoluminescence properties than α-NaYF₄ [1,2,3], which has resulted in β-NaYF₄ receiving more attention from researchers. However, Ln³⁺ ions (e.g., Eu³⁺, Tb³⁺, Dy³⁺, 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 Ln³⁺ ions in Ln³⁺-doped β-NaYF₄ phosphors is limited [21]. So far, numerous methods have been studied to enhance the absorption and photoluminescence intensity of Ln³⁺-doped β-NaYF₄ phosphors [22,23,24], including multiple types of Ln³⁺-doping and sensitization with organic ligands [8,25,26]. In this study, a surfactant was employed to control the morphology of the Ln³⁺-doped β-NaYF₄ phosphors, and their photoluminescence properties were enhanced.

Ln³⁺-doped β-NaYF₄ 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 NaYF₄ crystals using the hydrothermal/solvothermal method in a water/alcohol/oleic acid system, while others [32] prepared two kinds of NaYF₄ particles with different phase composition using an adjustable solvothermal method and by changing reaction temperature and molar ratio (Y³⁺/F⁻) with a different phase composition. NaYF₄ nanorods, nanotubes, and nanodisks were prepared using oleic acid as hydrothermal synthesis-mediated agent [33], and UC β-NaYF₄:Yb³⁺/Er³⁺ microcrystals were synthesized using the hydrothermal method at 220 °C [34]. However, it is still difficult to obtain Ln³⁺-doped β-NaYF₄ 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 Eu³⁺-doped β-NaYF₄ crystals of micron and submicron size, with controlled morphology and strong photoluminescence. The preparation method was based on our previous work of α-NaYF₄:Eu³⁺, 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-NaYF₄:Eu³⁺ microcrystals with enhanced photoluminescence properties. Hexagonal prism bipyramidal β-NaYF₄:Eu³⁺ 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-NaYF₄:Eu³⁺ particles and those modified by OA were compared in detail. The mechanisms for their shape control, morphology optimization, and photoluminescence enhancement are discussed.

2. Method

2.1. Materials

Yttrium nitrate (Y(NO₃)₃·6H₂O, 99.99%), europium nitrate (Eu(NO₃)₃·6H₂O, 99.99%), sodium nitrate (NaNO₃) (analytical grade, A.R.), and ammonium fluoride (NH₄F) (A.R.) were used as the raw materials. OA (A.R.) was used as a surfactant, and ethylene glycol (EG, C₂H₆O₂, A.R.) and absolute ethanol (C₂H₅OH, A.R.) were used as the solvents. All chemicals were used as received, without further purification. Deionized water was used throughout this work.

2.2. Synthesis of β-NaYF₄:Eu³⁺ Crystals without Surfactant

The β-NaYF₄:Eu³⁺ crystals were synthesized using a solvothermal method as previously described [8]. The doping concentration was 5 atm% with the formula β-NaY_0.95Eu_0.05F₄. In a typical procedure, Y(NO₃)₃·6H₂O (0.7277 g, or 1.9 mmol), Eu(NO₃)₃·6H₂O (0.0446 g, or 0.1 mmol), and NaNO₃ (0.1700 g, or 2 mmol) were mixed and thoroughly dissolved in 15 mL EG to form a clear solution A at room temperature. NH₄F (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 NaYF₄ strongly depended on the Y³⁺:F⁻ molar ratio in the reaction system. To obtain β-NaYF₄:Eu³⁺ crystals, the molar ratio of Ln³⁺ ions (Y³⁺ + Eu³⁺) in solution A to F⁻ ions in solution B was set as Ln³⁺: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-NaYF₄:Eu³⁺ crystals. After they were cooled down, the resultant β-NaYF₄:Eu³⁺ crystals were collected in a similar manner as described previously [8]. The undoped β-NaYF₄ crystals were synthesized as the standard sample, following the above procedure, except that no Eu³⁺ was used.

2.3. Synthesis of β-NaYF₄:Eu³⁺ 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-NaYF₄:Eu³⁺ 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-NaYF₄:Eu³⁺ samples modified by an OA surfactant were synthesized with the two sets of OA additions.

2.4. 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-NaYF₄:Eu³⁺ 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.

3. Results and Discussion

3.1. Phase Structures and Microstructural Characteristics

Figure 1 shows the XRD patterns of the as-synthesized NaYF₄ crystals prepared using different Y³⁺:F⁻ molar ratios. The Joint Committee on Powder Diffraction Standards (JCPDS) data for the cubic (α-NaYF₄, PDF#77-2042) and hexagonal (β-NaYF₄, PDF#16-0334) crystal structure of NaYF₄ are included in Figure 1 for reference. As shown in Figure 1, only the α-NaYF₄ phase could be obtained when Y³⁺:F⁻ = 1:12, while mixed phases of α-NaYF₄ and β-NaYF₄ were present when Y³⁺:F⁻ = 1:16 and 1:20. When Y³⁺:F⁻ ≥ 1:24, only the β-NaYF₄ phase was obtained. This suggests that the crystal structure of NaYF₄ strongly depends on the Y³⁺:F⁻ molar ratios in the reaction solution. For this reason, the Y³⁺:F⁻ ratio was set at 1:24 to synthesize β-NaYF₄:Eu³⁺ in this study.

Figure 2 shows the XRD patterns of the as-synthesized β-NaYF₄:Eu³⁺ 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:Eu³⁺ 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:Eu³⁺ 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:Eu³⁺ particles. All of the β-NaYF₄:Eu³⁺ particles were highly dispersible. The β-NaYF₄:Eu³⁺ 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-NaYF₄:Eu³⁺ particles. The average NaYF₄:Eu³⁺ 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:Eu³⁺ particles.

As seen in Figure 3a, the β-NaYF₄:Eu³⁺ particles, prepared without using an OA surfactant had irregular shapes with rough surfaces. However, the β-NaYF₄:Eu³⁺ 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:Eu³⁺ 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-NaYF₄:Eu³⁺ 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 Ln³⁺ (Y³⁺, Eu³⁺) 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 Ln³⁺ 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:Eu³⁺ when using a surfactant is that OA covers the surface of the particles through the interaction between carboxyl and Eu³⁺ 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.

3.2. Photoluminescence Properties

Figure 5 shows the photoluminescence excitation and emission spectra of the three β-NaYF₄:Eu³⁺ 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 Eu³⁺ ions at 320 nm (⁷F₀→⁵H₀), 364 nm (⁷F₀→⁵D₄), 379 nm (⁷F₀→⁵G₂), 397 nm (⁷F₀→⁵L₆), 418 nm (⁷F₀→⁵D₃), and 468 nm (⁷F₀→⁵D₂) [8]. The strongest excitation peak originates from the ⁷F₀→⁵L₆ transition. Similarly, the emission peaks excited by an exciting light at 397 nm are the characteristic emissions of Eu³⁺ ions, and they are assigned to the high-energy transition ⁵D₂→⁷F₃ at 515 nm and ⁵D₁→⁷F_J (J = 0–2) at 530 nm, 541 nm, and 561 nm; low-energy transition ⁵D₀→⁷F_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 ⁵D₀→⁷F₁ is allowed by the magnetic dipole transitions due to Eu³⁺ ions, located at a site with inversion symmetry, while the stronger emission peak at 622 nm, corresponding to ⁵D₀→⁷F₂, is allowed by the electric dipole transitions that are a result of absence of inversion symmetry at the Eu³⁺ lattice site [36,37,38,39].

As seen in Figure 5, the utilization of OA during the synthesis enhanced the photoluminescence intensity of the β-NaYF₄:Eu³⁺ crystals by about 20%. The β-NaYF₄:Eu³⁺ 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:Eu³⁺ crystals could be explained as follows. The β-NaYF₄:Eu³⁺ 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.

4. Conclusions

In summary, highly crystallized β-NaYF₄:Eu³⁺ 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:Eu³⁺ 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:Eu³⁺ 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:Eu³⁺ 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:Eu³⁺ 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.