Growth of hexagonal phase sodium rare earth tetrafluorides induced by heterogeneous cubic phase core

Abstract

A heterogenous core/shell strategy has been introduced to induce the growth of hexagonal phase NaREF₄ shells. During the core-mediated hetero-shell growth process, the cores significantly affect the crystalline phase of shells. Using heterogeneous cubic nanocrystals as cores, the resulting sub-30 nm hybrid nanoparticles possessed both hexagonal phase shells and good water solubility.

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Upconversion (UC) nanophosphors have attracted considerable interest as zero background luminescent probes in biological labeling and imaging applications.^1–10 In terms of their actual performance, the ideal nanophosphors should have high UC luminescence efficiency and low excitation threshold in order to achieve high detection sensitivities. Among various UC nanophosphors, lanthanide ions doped sodium rare-earth (RE) fluoride (commonly tetrafluorides, NaREF₄) nanocrystals (NCs) are considered the most promising candidates due to their intensive UC luminescence (RE = Ln and Y).^4,11–13 However, NaREF₄ NCs generally exhibit different phase structures, such as hexagonal phase (β) and cubic phase (α), the UC efficiency in the former one is significantly higher than that in the latter one.^14,15 It has been reported that the green UC luminescence efficiency in hexagonal phase NaYF₄:Yb,Er NCs is approximately 10 times stronger than that in the cubic phase form.^16,17 The UC efficiency is also strongly dependent on the nanocrystal composition. As demonstrated very recently by our group and other researchers, the β-NaLuF₄:Yb,Er NCs exhibited enhanced UC efficiency (up to 10 times) as compared to the β-NaYF₄:Yb,Er NCs of the same sizes.^4,13 Therefore, extensive efforts are focused on synthesizing hexagonal phase NaREF₄ materials as bio-probes in order to achieve highly UC luminescence.

Besides the hexagonal phase, water solubility and suitable size are the prerequisites for these NaREF₄ materials used as bio-probes. There have been efforts in directly synthesizing water soluble hexagonal phase NaREF₄ crystals, but their sizes are generally larger than 100 nm.^3,9,18,19 In some biological applications, the particle sizes are required below 30 nm. With decreasing the particle size, phase transition occurs due to the high surface tension, resulting in small cubic phase NaREF₄ NCs.^15,20 Alternatively, solution-based co-thermolysis of metal complex precursors method has been developed to synthesize sub-30 nm β-NaREF₄ NCs at temperatures above 300 °C. However, the as-prepared NCs are hydrophobic, thus complicated surface encapsulation is required to render them hydrophilic for biological applications.^20–24 Despite a great deal of efforts in the synthesis of high quality β-NaREF₄ NCs, it is still a great challenge to directly synthesize sub-30 nm water dispersed hexagonal NaREF₄ NCs with high UC luminescence.

To directly synthesize water soluble NaREF₄ crystals, the most promising approach is based on the solvothermal reaction involving amphiphilic surfactants or water soluble ligands as chelating agents that render the particles hydrophilic.^3,9,18,19 However, such reaction conditions generally yield cubic nanocrystals when the particle sizes are controlled smaller than 30 nm. For NaREF₄ material system, although the growth mechanism may be different, the cubic phase counterpart is kinetically metastable and the precursors commonly preferentially nucleate as α-NaREF₄.^15,21,25 It means β-NaREF₄ crystals are obtained via the growth of metastable α-NaREF₄ nuclei followed by a cubic-to-hexagonal phase transition, or via direct nucleation as hexagonal phase nuclei through providing sufficiently high reaction temperatures to surpass the energy barrier of metastable stage.^15,25 These processes are generally associated with large crystal size or hydrophobic surface of resulting β-NaREF₄. To overcome these limitations, we develop a new strategy to obtain small hetero core/shell NCs that possess both hexagonal phase shells and good water solubility. Notably, while previous works were focused on improving the core properties by core/shell structures in UC NCs,^26–30 this study, in contrast, demonstrates the core structures significantly affect the epitaxial shell growth process, and carefully selected cores control the crystalline phase of shells, resulting in highly luminescent UC probes.

Our strategy is based on using small cubic phase NaREF₄ cores to induce the growth of heterogeneous hexagonal NaRE′F₄ shells (RE and RE′ are different elements). In terms of the NaREF₄ materials, the cubic-to-hexagonal phase transition involves a structure change from a more symmetric lattice to a less symmetric one.^15,16,25 For the core-mediated hetero-shell growth, the lattice mismatch originated from different fluorides can provide an extra driven force to grow the shell with a low symmetric structure. Base on this principle, we introduce sub-20 nm cubic phase NaREF₄ NCs into a precursor solution containing a different kind of RE ions to form a NaREF₄/NaRE′F₄ core/shell heterostructure. Because the chemical properties of different RE elements are similar, the NaRE′F₄ can deposit easily onto the cubic NaREF₄ core. During the shell growth process, the two kinds of RE ions can be replaced by each others due to the cation exchange and form a hetero interface.^29,31 However, the ionic radii difference can cause the lattice distortion of the hetero interface, forming a low symmetric structure. Therefore, the subsequently deposited NaRE′F₄ shell can epitaxially grow in hexagonal phase (Fig. 1).

Fig. 1 Core-mediated heterogeneous growth processes. Due to the cation exchange, different sodium RE fluorides forms a low symmetric interface. The subsequently deposited shell epitaxially grows in hexagonal phase.

First, α-NaLuF₄ NCs (∼16 nm) were used as cores for the subsequent growth of β-NaYF₄ shell. Both the core and the resulting core/shell hybrid NCs (HNCs) were synthesized by a facile solvothermal method using an amphiphilic surfactant, polyvinylpyrrolidone (PVP) as the chelating agent. All these samples were examined by X-ray powder diffraction (XRD) to determine their crystal structures. The XRD pattern of the cores confirms that they are pure cubic phase NaLuF₄ NCs (Fig. S1a, ESI†). After the core NCs subsequently reacted with NaYF₄ precursors, the XRD pattern of the final product shows extra diffraction peaks appeared besides the peaks of α-NaLuF₄ crystals (Fig. 2b, the cubic phase peaks of NaLuF₄ crystals are labeled with squares). All these new peaks can be well indexed to the standard β-NaYF₄ crystal (Fig. 2a, JCPDS file number 16-334), but not the β-NaLuF₄ crystal (Fig. 2d, JCPDS file number 27-726). It indicates the growth of β-NaYF₄. In one control experiment having the NaYF₄ reaction precursors but without the α-NaLuF₄ core NCs, only the cubic NaYF₄ nanocrystals were formed, indicating the growth of β-NaYF₄ phase is due to the presence of α-NaLuF₄ NCs.

Fig. 2 XRD patterns of two kinds of HNCs and two standard β-NaREF₄ data. (a) β-NaYF₄ (JCPDS 16-334); (b) α-NaLuF₄/β-NaYF₄:Yb,Er HNCs (the peaks labeled with square belong to the diffraction of α-NaLuF₄); (c) α-NaYF₄/β-NaLuF₄:Yb,Er HNCs (the peaks labeled with circle belong to the diffraction of α-NaYF₄); (d) β-NaYF₄ (JCPDS 27-726).

Transmission electron microscopy (TEM) and the energy-dispersive X-ray spectroscopy (EDX) analysis were also performed to characterize the core/shell heterostructure. The TEM images and particle size distributions confirm the successful deposition of the NaYF₄ shell on the α-NaLuF₄ NCs (Fig. 3a–d). The shape of core NCs is nearly spherical and the lattice fringes can be clearly observed (insert of Fig. 3a). The original core NCs have an average diameter about 16 nm (Fig. 3c) and after they were reacted with the NaYF₄ precursors, an obvious increment of the mean size is observed (Fig. 3d). The mean size of the HNCs is about 22 nm without obvious changes in the shape. In the HR-TEM image of the HNCs, the lattice fringes are found to be about 0.298 nm, which is consistent with the d-spacing of the (110) plane of β-NaYF₄ crystal (insert of Fig. 3b). Both the increment in size and the appearance of β-NaYF₄ crystal lattice fringes suggest the successful growth of β-NaYF₄ shell onto the α-NaLuF₄ core. To further confirm the core/shell geometry, the local elemental mapping by EDX line scans was carried out (Fig. 4a–d). The mapping images clearly show that the labeled area of element Lu is smaller than that of Y and Yb in a single HNC. That means Y and Yb cover across the outer layer and Lu is located in the center, which supports the formation of NaLuF₄/NaYF₄ core/shell heterostructure. All above analyses demonstrate the cubic phase NaLuF₄ NCs can induce the growth of hexagonal phase NaYF₄ shell through constructing the core/shell heterostructure.

Fig. 3 TEM images and size distribution of two kinds of cubic cores and corresponding α-NaREF₄/β-NaRE′F4 HNCs. (a and c) α-NaLuF₄ cores (insert: HRTEM of a single α-NaLuF₄ NC); (b and d) α-NaLuF₄/β-NaYF₄ HNCs (insert: HRTEM of a single α-NaLuF₄/β-NaYF₄ HNC); (e and g) α-NaYF₄ cores (insert: HRTEM of a single α-NaYF₄ NC); (f and h) α-NaYF₄/β-NaLuF₄ HNCs (insert: HRTEM of a single α-NaYF₄/β-NaLuF₄ HNC).

Fig. 4 Images of local elemental mapping by EDX line scans for two kinds of α-NaREF₄/β-NaRE′F₄ HNCs. For α-NaLuF₄/β-NaYF₄ HNCs: (a) image of EDX line scans area; (b) Lu in core; (c) Y in shell; (d) Yb in shell. For α-NaYF₄/β-NaLuF₄ HNCs: (e) image of EDX line scans area; (f) Y in core; (g) Lu in shell; (h) Yb in shell.

The successfully induced growth of β-NaYF₄ shell using α-NaLuF₄ NC as core indicates β-NaLuF₄ shell can also be grown using α-NaYF₄ NC as core due to the similar hetero interface. The α-NaYF₄ NCs (∼14 nm) were exposed to NaLuF₄ precursor solution under the otherwise identical conditions. Similar measurements were also performed to confirm the formation of α-NaYF₄/β-NaLuF₄ core/shell heterostructure. The β-NaLuF₄ shell on α-NaYF₄ core was confirmed by XRD and the new diffraction peaks appeared in the XRD pattern (Fig. 2c) well match with those of the standard β-NaLuF₄ crystal (Fig. 2d, JCPDS 27-726). The heterostructure were observed by TEM (Fig. 3e–h) and confirmed by the local elemental mapping (Fig. 4e–h). These results further confirmed the induced growth hexagonal phase NaRE′F₄ shell by the cubic phase hetero core.

To increase the thickness of hexagonal phase shells, the shell growth time was increased from 6 to 24 hours. Two sets of α-NaLuF₄/β-NaYF₄ and α-NaYF₄/β-NaLuF₄ core/shell HNCs were synthesized. The other growth conditions are identical except the shell growth time. All obtained HNCs are spherical and the mean size was gradually increased with the reaction time (Fig. S2 and S5, ESI†). The size increase is consistent with the growth of hexagonal phase shells around cores. Such gradual increases of hexagonal phase shells was also confirmed by the evolution of XRD data (Fig. S1 and S4, ESI†). The relative intensity of hexagonal phase diffraction peaks is gradually increased as the shell growth time is increased, indicating that the hexagonal phase counterparts are gradually increased. It should be noticed that the evolution of diffraction peak shifts also reflect the increase of β-NaRE′F₄ shells. Compared with the pure β-NaYF₄ and β-NaLuF₄ crystals, slight diffraction peak shifts of hexagonal phase are observed in both sets of α-NaLuF₄/β-NaYF₄ and α-NaYF₄/β-NaLuF₄ HNCs. The peak shifts are consistent with the formation of the hetero interface. The crystal structure of β-NaYF₄ and β-NaLuF₄ are similar, they all adopt the same space group (P6₃/m), but the lattice parameters are different (a = 5.96 Å, c = 3.53 Å for NaYF₄ and a = 5.901 Å, c = 3.453 Å for NaLuF₄). Whether Lu³⁺ precursor ions replace Y³⁺ ions in NaYF₄ cores or Y³⁺ precursor ions replace Lu³⁺ ions in NaLuF₄ cores, the lattice parameters of interface will be changed, resulting in a slight shift in diffraction peaks. In the XRD p0atterns of the two sets of samples, the diffraction peaks shift towards higher angles in α-NaLuF₄/β-NaYF₄ HNCs compared with the standard β-NaYF₄ crystal (Fig. S1,† JCPDS no. 16-334†), while the peak shifts towards lower angles in α-NaYF₄/β-NaLuF₄ HNCs according to β-NaLuF₄ crystal (Fig. S4,† JCPDS no. 27-726†). However, such peak shifts are gradually decreased by increasing the shell growth time. For the 24 hour α-NaLuF₄/β-NaYF₄ and α-NaYF₄/β-NaLuF₄ HNCs, the profile of the XRD patterns for the hexagonal phase counterparts nearly match with the standard β-NaYF₄ and β-NaLuF₄ crystals, respectively (Fig. 2b and c). This fact provides an additional evidence for the formation of Y–Lu hetero interface and core/shell heterostructure.

Two sets of control experiments were performed to examine whether the hexagonal shells could be induced to grow by homogenous cubic phase cores. The same batch of cubic NaYF₄ or NaLuF₄ cores were used in the above heterogeneous growth process. With the same reaction conditions as those for the hetero-shell process, the core NCs were introduced into the relative solutions containing the NaYF₄ or NaLuF₄ precursors to achieve homogenous core/shell structures. In the homo-shell growth process, there were no obvious hexagonal phase diffraction peaks observed in the XRD patterns even for 24 hour growth time (Fig. S7, ESI†). In contrast, obvious hexagonal phase diffraction peaks could be observed for the 6 hour samples in the heterogeneous growth process (Fig. S1 and S4, ESI†). It indicates that the heterogenous core is a key factor to inducing the growth of hexagonal phase shell. During the hetero-shell growth process, the cation exchange leads to a gradient of two sodium RE fluorides. The lattice distortion and the symmetry change of the hetero interface are most likely to be responsible for the cubic-to-hexagonal phase transition of the heterogeneous shell. However, during the homo-shell growth process, the same kind of RE ions can not cause enough lattice distortion and the subsequently deposited shell keeps cubic phase. We reason that the core/shell heterostructure greatly affects the growth process of the epitaxial shells and facilitates the cubic-to-hexagonal phase transition.

Both the successfully prepared α-NaLuF₄/β-NaYF₄ and α-NaYF₄/β-NaLuF₄ HNCs can be well dispersed in water and emit intensive UC luminescence when Yb³⁺ (20 mol%) and Er³⁺ (3 mol%) were codoped in the hexagonal phase shells (Fig. 5b and d). Under the same excitation conditions, both the two types of HNCs exhibited enhanced UC luminescence intensity as compared to the water dispersed α-NaYF₄:Yb,Er and α-NaLuF₄:Yb,Er NCs with similar size (Fig. 5a and c). UC emission measurements on the aqueous solutions indicated that the intensity of green emission of α-NaYF₄/β-NaLuF₄:Yb,Er HNCs is approximately 2 times stronger than that of α-NaLuF₄/β-NaYF₄:Yb,Er HNCs (Fig. 5e). It is worth noting that the α-NaYF₄/β-NaLuF₄:Yb,Er HNCs also have much lower pumping threshold (about 5 to 10 times for green and red UC emissions) compared to the α-NaLuF₄/β-NaYF₄:Yb,Er HNCs (Table S1, ESI†). It is also consistent with the previous works that β-NaLuF₄ is better host material for UC luminescence than β-NaYF₄.^4,13 These results indicate α-NaYF₄/β-NaLuF₄:Yb,Er HNCs are promising nanophosphors for biological probes because of their good water solubility, small size (sub-30 nm), low pumping threshold and high UC efficiency.

Fig. 5 Photographs for colloidal solutions (about 1 mg mL⁻¹) of (a) α-NaYF₄ NCs; (b) α-NaLuF₄/β-NaYF₄ HNCs; (c) α-NaLuF₄ NCs; (d) α-NaYF₄/β-NaLuF₄ HNCs. (e) UC luminescence spectra of α-NaLuF₄/β-NaYF₄ HNCs (red line) and α-NaYF₄/β-NaLuF₄ HNCs (blue line).

In summary, we have developed a new strategy based on the core/shell heterostructure to prepare sub-30 nm water dispersed NaREF₄ nanoparticles in hexagonal phase. Cubic NaREF₄ cores were used to induce heterogeneous growth of hexagonal phase shells. Codoping sensitizer and activator ions into the hexagonal shells yielded the nanophosphors with more intense upconversion luminescence compared with the corresponding cubic phase nanocrystals of identical sizes and morphologies. This strategy not only provides a convenient route for facile synthesis of small water soluble β-NaREF₄ UC nanophosphors without complex surface modification, but also helps to understand the formation mechanism of small core/shell heterostructure, which may finally promote the applications of lanthanide-based luminescent probes in biological studies.

This work was supported by the National Natural Science Foundation of China (61275189, 11274139, 61222508, 61111140393) and Scientific and Technological Developing Project of Jilin Province (201105004).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures, TEM images, XRD and pumping power threshold. See DOI: 10.1039/c3ra41373j

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