Facile-Controlled Epitaxial Growth Direction of Heterogeneous Core/Shell Structured NaLnF4 Nanocrystals through Traditional Methods

Fabrication of nanoscale materials with desirable morphology and surface properties becomes more urgent when constructing hybrid nanocrystals with multiple functionalities. Here, we report a facile measure to control the outer layer growth direction combining with coprecipitation and thermal decomposition method for constructing a series of heterogeneous core/shell structured NaLnF4 nanocrystals, involving NaYF4 and NaNdF4 material. Our investigations suggest that it is feasible to control over the outer layer growth orientation by combining the two traditional methods.

There are three main approaches to produce high-quality NaLnF 4 and NaLnF 4 -based core/shell-structured nanocrystals including hydrothermal, coprecipitation, and thermal decomposition methods [23][24][25]. Liu's group have achieved the precise size and shape control with atomic scale in a family of rare-earth-doped nanomaterials in the presence of various at oleate anions (OA -)/(oleic acid molecules) OAH ratios by coprecipitation method [26]. Subsequently, they developed a hydrothermal-based epitaxial growth technique and found that EDTA or citric acid play an indispensable role in controlling epitaxial orientation [27]. Similar phenomena were observed by Zhang's group, they tuned the phase and morphology of the products by adjusting the composition of the oleic acid/NaOH ratio precisely in hydrothermal approach [25]. Zhao's group developed a thermal decomposition method to synthesize NaLnF 4 core-multishell-structured nanomaterials using the thermal decomposition method, and the nanoparticles have a disk shape, indicating the NaLnF 4 shell growing in the [10 10] direction [28]. But they did not explain the reason for this appearance further. High-quality homogeneous and heterogeneous core/shell structured nanoparticles could be constructed through the above methods and the morphology regulation of nanoparticles requires precise control of the proportion of reactants in the reaction solution.
Combined with the different methods of controlling the direction of shell growth in the above work, we used the β-NaYF 4 nanoparticles as core and coated the NaNdF 4 shell to construct monodisperse heterogeneous core/shell-structured NaYF 4 /NaNdF 4 nanocrystals by the coprecipitation and thermal decomposition methods, respectively. This method is based on a selective epitaxial core/shell growth process in the presence of oleic acid molecular (OAH) or oleate anions (OA -), which arises from the different sodium sources adopted in the two methods. OAH is often used as a coordination solvent during the synthesis of nanoparticles. Sodium hydroxide (NaOH), as sodium source in coprecipitation method, would cause OAH to dissociate into OA -. Sodium trifluoroacetate (NaTFA), as a commonly used sodium source in thermal decomposition method, has no such effect on OAH. This allows us to introduce an approach to controlling the epitaxial growth direction of NaLnF 4 shell to construct heterogeneous core/shell-structured NaLnF 4 nanocrystals combining these two methods.  3 ) and yttrium trifluoroacetate (Y(TFA) 3 ) was prepared following our previous work [29]. All other chemical reagents were of analytical grade and used directly without further purification. Deionized water was used in all experiments.

Experimental
2.2. Synthesis of Dumbbell-Like NaYF 4 /NaNdF 4 Nanoparticles through the Coprecipitation Method. The dumbbell-like nanoparticles were synthesized following the method reported by Liu et al. with slight modifications. We first prepared β-NaYF 4 nanoparticles with a size of 25 nm and then coated NaNdF 4 shell with coprecipitation method. In a typical experiment, for the β-NaYF 4 nanoparticle synthesis, YCl 3 ·6H 2 O (0.5 mmol) was added into a mixture of oleic acid (3 mL) and 1-octadecene (7.5 mL) in a 50 mL three-necked flask at room temperature. Under a nitrogen atmosphere and stirring, the mixture was heated at 150°C for 0.5 h to form lanthanideoleate complexes. Thereafter, the reaction solution was cooled down to room temperature followed by the addition of a 5 mL methanol solution containing NaOH (1.25 mmol) and NH 4 F (2 mmol). Subsequently, the mixture was heated at 70°C for 0.5 h to evaporate the methanol under vigorous stirring. The temperature was then increased to 120°C. After being maintained for 10 min, the reaction mixture was heated to 300°C at a heating rate of 15°C min -1 under a nitrogen atmosphere and stirring. Upon completion of the reaction at 300°C after 1 h, the solution was cooled down to room temperature. The resulting nanocrystals were collected by centrifugation, washed with a mixture of cyclohexane and ethanol several times, and dispersed in cyclohexane (2 mL) for NaNdF 4 shell growth.
The NaNdF 4 shell precursor was prepared by adding NdCl 3 ·6H 2 O (0.5 mmol) into an oleic acid (5 mL) and 1octadecene (5 mL) mixture in a 50 mL three-necked flask followed by heating at 150°C for 30 min before cooling down to 50°C. Subsequently, as-prepared NaYF 4 nanoparticles (0.5 mmol) dispersed in 2 mL of cyclohexane were added under vigorous stirring, and a 10 mL methanol solution of NaOH (1.25 mmol) and NH 4 F (2 mmol) was injected into the reaction flask after 5 min. The resulting mixture was stirred below 50°C for at least 60 min, heated to 300°C under a nitrogen atmosphere and kept for 1 h, and then cooled down to room temperature. The resulting nanoparticles were precipitated by adding a mixture of methanol and cyclohexane, collected by centrifugation, washed with ethanol several times, and redispersed in 5 mL of cyclohexane.
2.3. Synthesis of Pie-Shaped NaYF 4 /NaNdF 4 Nanoparticles through the Thermal Decomposition Method. NaYF 4 nanoparticles (0.5 mmol) dispersed in 2 mL of cyclohexane, and Nd(TFA) 3 (0.5 mmol) was added into an oleic acid (5 mL) and 1-octadecene (5 mL) mixture in a 50 mL three-necked flask followed by heating at 80°C for 20 min to remove cyclohexane. Subsequently, the reaction mixture was heated to 300°C under a nitrogen atmosphere and kept for 30 min, and then cooled down to room temperature. The treating process of the product is the same as above.
2.4. Synthesis of Nanoscale Hexagonal Prismatic through the Coprecipitation Method. YCl 3 ·6H 2 O (1 mmol) was added into an oleic acid (10 mL) and 1-octadecene (10 mL) mixture in a 100 mL three-necked flask followed by heating at 150°C for 30 min before cooling down to 50°C. Subsequently, asprepared pie-shaped NaYF 4 /NaNdF 4 nanoparticles (0.5 mmol) dispersed in 5 mL of cyclohexane were added under vigorous stirring, and a 20 mL methanol solution of NaOH (2.5 mmol) and NH 4 F (4 mmol) was injected into the reaction flask after 5 min. The resulting mixture was stirred below 50°C for 30 min, heated to 300°C under a nitrogen atmosphere and kept for 1 h, and then cooled down to room temperature. The resulting nanoparticles were precipitated by adding a mixture of methanol and cyclohexane, collected by centrifugation, washed with ethanol several times, and redispersed in 10 mL of cyclohexane.

Synthesis of Dumbbell-Like Nanoparticles with Three
Rings through the Thermal Decomposition Method. Dumbbell-like NaYF 4 /NaNdF 4 nanoparticles (0.5 mmol) dispersed in 5 mL of cyclohexane, and Y(TFA) 3 (1 mmol) was added into an oleic acid (10 mL) and 1-octadecene (10 mL) mixture in a 50 mL three-necked flask followed by heating at 80°C for 20 min to remove cyclohexane. Subsequently, the reaction mixture was heated to 300°C under a nitrogen atmosphere and kept for 30 min, and then cooled down to room 2 Journal of Nanomaterials temperature. The treating process of the product is the same as that of nanoscale hexagonal prismatic.
2.6. Characterization. Transmission electron microscopy (TEM) images were taken with a Tecnai G220 transmission electron microscope operating at 200 kV. High-resolution scanning transmission electron microscopy (STEM) and elemental mapping result were performed on a Tecnai G2 F30 transmission electron microscope operating at 300 kV. X-ray diffraction (XRD) patterns were recorded on a D/MAXRB X-ray diffractometer operated at 12 kW with Cu-Kα radiation (λ = 1:5418 Å).

Results and Discussion
We first prepared sub-25 nm β-NaYF 4 nanocrystals as seed particles (Figure 1(a)) and then coated NaNdF 4 shell on the β-NaYF 4 nanoparticles using the coprecipitation and thermal decomposition methods separately. Dumbbell-like NaYF 4 /NaNdF 4 nanoparticles were obtained with a high uniformity (58 ± 3 nm in length, 35 ± 2 nm, and 18 ± 1 nm in diameter for each end and the middle bar) when the NaNdF 4 shell was coated by coprecipitation method (Figure 1(b)). HRTEM image depicted clearly lattice fringes associated with (10 10) planes (d-spacing of 0.52 nm) and the (0002) plane (d-spacing of 0.18 nm) (Figure 1(c)), suggesting that the NaNdF 4 shell has grown in the [0001] direction (longitudinal growth). Elemental mapping of a single nanoparticle indicates that the Nd 3+ ions are embedded at the dumbbell ends, and Y 3+ ions are located in the middle of the dumbbell bars, respectively (Figure 1(d)). Furthermore, the middle diameter of the dumbbells is about 18 nm less than the diameter of NaYF 4 core, which showed that ligand etching had taken place on the side face ((10 10) planes) of the NaYF 4 core during epitaxial growth. We further demonstrated the formation of pie-shaped NaYF 4 /NaNdF 4 nanocrystals by the thermal decomposition method (Figure 2(a)). It shows that the as-prepared nanoparticles were monodisperse with a diameter of 42 ± 2 nm. In view of the (10 10) facets of β-NaYF 4 crystal with the same atomic arrangement, thus, the deposition rate of the NaNdF 4 shell on the (10 10) facets of the β-NaYF 4 core must be the same. We believe that the ring-shaped NaNdF 4 shell growth in the [10 10] direction (transverse growth). Considering the 3 Journal of Nanomaterials diameter of NaYF 4 core is about 25 nm, the NaNdF 4 shell thickness is about 8.5 nm. The HRTEM image confirms that the nanoparticles were single-crystalline with an interplane distance of the lattice fringes of~0.52 nm, which corresponds to that of the (0001) facets of β-NaYF 4 (Figure 2(b)). Lattice fringes associated with {10 10} planes (d-spacing of 0.52 nm) can be discerned in the HRTEM image, and there are no obvious defect at the core-shell interface, suggesting that the NaNdF 4 shell has grown preferentially along the [0001] direction. Energy dispersive X-ray (EDX) line-scan profiles of a single pie-shaped nanoparticle confirms that the Nd 3+ ions are embedded around the nanoparticle, and Y 3+ ions are located in the center of it, respectively (inset in Figure 2(b)).
The TEM image of the products obtained after 15 min of reaction (NaNdF 4 shell coating time is 30 min) shows two distinct particle morphologies that include small nano-spheres and large pie-like nanoparticles, well consistent with the presence of hexagonal and cubic phase NaLnF 4 observed by X-ray powder diffraction (Figure 2(d)). When the reaction time reached 30 minutes, the small cubic phase particles in the product disappeared. The result confirms that the shell precursor first formed cubic phase NaNdF 4 nanoparticles and then gradually coated on the β-NaYF 4 particles through the Oswald ripening process. The size along the [0001] direction, the part uncoated with outer layer, was almost unchanged (25 nm) (particle in dotted box in Figure 2(c)), indicating there is no obvious ligand etching phenomenon of NaYF 4 core when coating the NaNdF 4 shell through the thermal decomposition method.
To further confirm the influence of sodium sources on the shell growth direction, we coat the NaYF 4 shell on the as-prepared pie-shaped and dumbbell-like NaYF 4 /NaNdF 4 50 nm  Journal of Nanomaterials nanoparticles through the coprecipitation and thermal decomposition methods, respectively. Nanoscale hexagonal prismatic and dumbbells with three rings were obtained, respectively. As shown in Figures 3(a) and 3(b), the length of the hexagonal prismatic is about 38 nm, and the transverse size is about 43 nm, which indicated that the longitudinal growth is at least 13 nm and the transverse growth is only about 1 nm by contrast. A large number of hexagonal prisms have cracks in the side faces, which may be related to the lattice stress caused by the lattice mismatch between the NaYF 4 and NaNdF 4 crystals. When the NaYF 4 shell was coated on the dumbbells through thermal decomposition method, the transversal growth was observed, and the NaYF 4 rings were found around both ends and the middle of the dumbbell without a measurable change in the longitudinal direction (Figures 3(c) and 3(d)).
To reveal the underlying cause of these phenomena, we lay out the reaction parameters of the shell coating process: (i) For the coprecipitation and thermal decomposition method, the types and proportion of reaction solvents are identical (V/VðOA/1 − ODEÞ = 1/1). Types of rare-earth salts for shell precursors are different (rare earth chloride in coprecipitation method and rare earth trifluoroacetate salts in thermal decomposition method), but in the two-shell coating process, both shell precursors first form cubic phase NaLnF 4 nanoparticles and then wraps themselves on the inner layer when the shelling temperature exceed 270°C [22,24,30]. Therefore, we believe that the difference of precursor species is not the main reason for the different growth direction of shell; (ii) the sodium sources adopted in coprecipitation and thermal decomposition method are NaOH and NaTFA, respectively. As mentioned above, OHin the reaction solvent could dissociate OAH into OA -, and OApreferentially binds to Y 3+ ions exposed on the (10 10) facets of β-NaYF 4 nanocrystal, with a much higher binding energy (-35.4 eV) than that on the (0001) facet (-21.8 eV), which means that OApreferentially binds to Y 3+ ions exposed on the (10 10) facet of the β-NaYF 4 nanocrystal [26]. Due to a large concentration of passivating OAions on the (10 10) facets, a faster longitudinal deposition of NaNdF 4 shell on  Journal of Nanomaterials the both (0001) facets of β-NaYF 4 core would occur compared with transversal deposition on the (10 10) facets. Meanwhile, the dissolution (etching) phenomenon from the side face of β-NaYF 4 core takes place, which was attributed to the strong binding of OAto the exposed Y 3+ on the side surfaces in the presence of high concentration of OA - [31]. As a result, dumbbell-like nanoparticles formed. In contrast, when NaTFA was adopted as sodium source in thermal decomposition method, NaTFA could not dissociate OAH to OA -; thus, there are less OAin the reaction solution. Unlike OAions, OAH binds to the (0001) facets with a higher probability (-9.4 eV) than the (10 10) facet (-4.6 eV), which means that the shell precursor would preferentially deposit on the (10 10) facet (side face). As a consequence, epitaxial growth of shell in the transversal direction was observed, and NaNdF 4 rings around the NaYF 4 core were formed. In addition, there were no obvious dissolution phenomenon takes places on the noncoated surface due to the weak binding of OAH to the exposed Y 3+ . Based on these analyses, we suggest that sodium source in the preparation methods is playing a critical role on the epitaxial shell growth direction. (iii) The crystal lattice mismatch between inner layer and shell layer has influence on the epitaxial growth. Considering the morphology evolution of the coreshell-structured nanoparticles composed of β-NaYF 4 and β-NaNdF 4 involved in this work, the lattice mismatch between them may not be the main reason for the different direction of the epitaxial growth of the shell. Besides, this work does not explore the effect of temperature on the shell epitaxial growth. According to the previous work, the reaction temperature does not alter the direction of epitaxial growth but does affect the growth speed [26].

Conclusion
We have constructed heterogeneous core/shell-structured NaLnF 4 nanocrystals with various shapes using the traditional preparation methods. Microstructure results show that NaOH and NaTFA, as the sodium source, have different influence on the concentration ratio of OA-ions and OAH in the reaction solution and lead to longitudinal and transversal growth direction, respectively. We believe that the construction of heterostructure in nanoscale can be applied to precisely regulate the interparticle energy transfer. In addition, this work can be used to synthesize heterojunctions of other shell materials, such as NaLuF 4 and NaGdF 4 . The addition of NaLuF 4 enables X-ray computed tomography, whereas using NaGdF 4 enables magnetic resonance imaging. Furthermore, these studies could be facilitated by harnessing the anisotropic properties of different types of layer that permit diverse functionalizations for multimodal applications.

Data Availability
Findings of this research work will be provided by the corresponding author on reasonable demand.

Conflicts of Interest
The authors declare no conflicts of interest.