Topotactic Transformation Route to Monodisperse β-NaYF4:Ln3+

Feb 3, 2016 - Synopsis. β-NaYF4:Ln3+ microcrystals (Ln = Eu, Tb, Ce/Tb, Yb/Er, and Yb/Tm) with various morphologies have been directly synthesized by...
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Topotactic Transformation Route to Monodisperse β‑NaYF4:Ln3+ Microcrystals with Luminescence Properties Baiqi Shao,†,‡ Yang Feng,†,‡ Yan Song,† Mengmeng Jiao,†,‡ Wei Lü,† and Hongpeng You*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: A novel nonorganic wet route for direct synthesis of uniform hexagonal β-NaYF4:Ln3+ (Ln = Eu, Tb, Ce/Tb, Yb/Er, and Yb/Tm) microcrystals with various morphologies has been developed wherein the intermediate routine cubic−hexagonal (α → β) phase transfer process was avoided. The morphology can be effectively tuned into hexagonal disc, prism, and novel hierarchical architectures by systematically fine manipulating the Na2CO3/F− feeding ratio. It has been found that the routine α → β phase transfer for NaYF4 was not detected during the growth, while NaY(CO3)F2 emerged in the initial reaction stage and fast transformed into β-NaYF4 via a novel topotactic transformation behavior. Detailed structural analysis showed that β-NaYF4 preferred the [001] epitaxial growth direction of NaY(CO3)F2 due to the structural matching of [001]NaY(CO3)F2//[0001]β-NaYF4. Besides, the potential application of the as-prepared products as phosphors is emphasized by demonstrating multicolor emissions including downconversion, upconversion, and energy transfer (Ce−Tb) process by lanthanides doping.

1. INTRODUCTION Over the past few decades, the enthusiasm for research on lanthanide-doped upconversion nanoparticles (UCNPs) has been uplifting due to the potential applications in photonics, photovoltaics, and especially biological science since the integration of nanotechnology with biology.1−6 To date, great progress has been made in the fabrication and characterization on UCNPs with rapid development of nanoscience and nanotechnology.7,8 It is well established that the host matrix with low phonon energy is essential for the upconversion process for it can greatly decrease the multiphonon relaxation between closely spaced energy levels.9,10 Among various lowphonon candidates, β-NaYF4 is regarded as the most efficient upconversion host, and thus, great attention has been devoted to optimizing the size, morphology, and upconversion efficiency of β-NaYF4 to fulfill the stringent demands in targeting applications in modern science and technology.11,12 It is reported that NaYF4 exists in two phases: cubic (α) and hexagonal (β) phase. The upconversion efficiency of β-NaYF4 is much higher than that of the α-NaYF4 counterpart.13,14 However, α-NaYF4 is the kinetic phase formed first in the wet synthetic routes and gradually transforms into β-NaYF4 through a phase transformation process. Usually high temperature and long aging time are required for α → β phase transformation, and such forcing condition inevitably induces significant particle aggregation and size increase, which has always been a major concern in the synthesis of β-NaYF4 UCNPs.15,16 Despite the recent progress in β-NaYF4 UCNPs wet synthesis, the conventional techniques still could not avoid the α → β © 2016 American Chemical Society

phase transformation process, and they often suffer from drawbacks including toxic organometallic precursors, hazardous coordinating solvents, and stringent control conditions, which has been a bottleneck for further commercialization.17−19 Therefore, it is a pivotal point to directly synthesize β-NaYF4 phase by a facile synthetic strategy via avoiding the kinetic αNaYF4 intermediate phase. Recent advances have proved that a topotactic transformation strategy based on the structural matching was a practical feasible route to directly synthesize β-NaYF4 phase. One part of the notable cases was based on ion-exchange reactions. Zhao et al. synthesized a series of β-NaLnF4 nanotubes taking Ln(OH)3 nanotubes as precursors,20 and Wu et al. fabricated β-NaYF4 spindle-like mesocrystals from Y(OH)xF3−x.21 However, for such strategies an isostructure precursor and at least a two-step hydrothermal procedure are indispensable; as a result, they are strictly limited in generalization, while in the other part notable cases are based on the topotactic structural matching between a certain precursor and β-NaYF4. Wu et al. developed a solvothermal method to prepare β-NaYF4 microcrystals using Y2O3 as precursor on the basis of the structural matching [01̅1]Y2O3// [12̅2]β-NaYF4.22 Our group has developed a one-pot synthetic route to synthesize β-NaYF4 micro/nanocrystals using layered yttrium hydroxide Y2(OH)5NO3·nH2O (LYH) as precursor on the basis of the structural matching between LYH unit layer and Received: December 4, 2015 Published: February 3, 2016 1912

DOI: 10.1021/acs.inorgchem.5b02817 Inorg. Chem. 2016, 55, 1912−1919

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Inorganic Chemistry β-NaYF4 {0001} lattice plane.23 The rule for selection of precursor is more tolerant for the latter strategy based on topotactic structural matching, and thus, it is of practical significance and may be extended to other series of inorganic micro/nanocrystals. Herein, we developed a novel strategy for direct synthesis of monodisperse β-NaYF4 microcrystals using Na2CO3 as additive by a hydrothermal method. The as-prepared products were monodisperse microsingle crystals, and the morphology could be effectively tuned into hexagonal disc, prism, and hierarchical architecture by fine manipulating the fluorine source and Na2CO3/F− feeding ratio. (Note that, in our case, the Na2CO3/ F feeding ratio means the actual feeding amount of Na2CO3 and fluorine source not an arithmetical ratio. For example, 1/1 and 2/2 do not mean the same thing.) To the best of our knowledge, it is rare for systematic control on morphology of βNaYF4 microcrystals, because it is still a challenge to manipulate the morphology in the micrometer scale due to their low surface free energy. It is found that α-NaYF4 intermediate phase was not detected through the whole growth process, while NaY(CO3)F2 emerged in the initial stage and fast transformed into β-NaYF4 via a novel topotactic transformation behavior. Detailed structural analysis showed that β-NaYF4 preferred the [001] epitaxial growth direction of NaY(CO3)F2 due to the structural matching of [001]NaY(CO3)F2//[0001]β-NaYF4. Besides, the as-prepared samples exhibited strong multicolor emissions including downconversion, upconversion, and energy transfer (Ce−Tb) process by lanthanides doping. Our novel synthetic route may provide a new theoretical and experimental case for direct synthesis of β-NaYF4 by a green synthetic methodology.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Morphology. Figure 1 shows the XRD profile of the as-prepared β-NaYF4 sample with a

Figure 1. XRD pattern of the as-prepared β-NaYF4 sample with a Na2CO3/NaHF2 feeding ratio of 5/4.

Na2CO3/NaHF2 feeding ratio of 5/4. It can be seen that all peaks can be indexed to the standard data of hexagonal βNaYF4 (space group P63/m) with JCPDS no. 16-0334 without any other impurity peaks. The relative intensity of the peaks is changed compared with that of the standard pattern, suggesting the tropism of the particle and the probable anisotropy growth behavior. Figure 2 exhibits the SEM images of the

2. EXPERIMENTAL SECTION Chemicals. Ln(NO3)3 (Ln = Y, Ce, Eu, Tb, Yb, Er, Tm) stock solution was obtained by dissolving the corresponding rare-earth oxides (99.99%) in dilute HNO3 under heating. Commercially available NaHF2, NaF, and Na2CO3 are of analytical grade and were used without further purification. 2.1. Preparation of β-NaYF4 Microcrystals. In a typical synthesis, 10 mL of an aqueous solution containing certain amounts of Na2CO3 was added into 15 mL of an aqueous containing 1 mmol of Y(NO3)3 under magnetic stirring. The white precipitation occurred immediately. After stirring for 5 min, 15 mL of an aqueous solution containing certain amounts of NaHF2 or NaF was introduced into the above mixture. After stirring another 5 min, the mixture was sealed in a 50 mL Teflon-lined autoclave and maintained at 180 °C for 12 h. After hydrothermal treatment, the products were collected and washed with deionized water and absolute ethanol twice and dried at 60 °C for 12 h. The lanthanides-doped samples β-NaYF4:Ln3+ (Ln = Eu, Tb, Ce, Ce/Tb, Yb/Er, Yb/Tm) were synthesized followed the same procedure. 2.2. Characterization. The phase structure of the as-prepared products was characterized by powder X-ray diffraction with a D8 Focus diffractometer (Bruker, with Cu Kα radiation, λ = 0.15406 nm) at a scanning rate of 10° min−1. A field emission scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) (FE-SEM, S-4800, Hitachi, Japan) was employed to inspect the morphology and size of the as-prepared samples. Transmission electron microscopy images were obtained using a JEOL-2010 transmission electron microscope operating at 200 kV. Downconversion and upconversion (UC) photoluminescence spectra were recorded with a Hitachi F-7500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source, an external 980 nm diode laser, and a R928P photomultiplier tube.

Figure 2. SEM images of the representative as-prepared β-NaYF4 samples with Na2CO3/NaHF2 feeding ratios of (A) 4/2 and (B) 5/4 Na2CO3/NaF feeding ratios of (C) 3/12 and (D) 4/16. Scale bars are 10 μm.

representative as-prepared β-NaYF4 samples with different Na2CO3/NaHF2 (Figure 2A and 2B) and Na2CO3/NaF (Figure 2C and 2D) feeding ratios, respectively. All samples feature a hexagonal contour in morphology with perfect uniformity and well-defined crystallographic facets, agreeing well with the self-limitation behavior for hexagonal-phased crystal. The sizes of the particles are all in micrometer scale (for conciseness, the samples are denoted as “fluorine sourceNa2CO3/fluorine feeding ratio”, and the crystal size is denoted as “length × radius”: NaHF2-4/2:0.7 × 3 μm, NaHF2-5/4:1.6 × 1.7 μm, NaF-3/12:0.9 × 1.9 μm, and NaF-4/16:0.9 × 1.6 μm). The close-up views show that there are mulriple hexagonal contours in some particles but exhibit hexagonal disc or prism as a whole, suggesting the mesocrystal nature. The effects of the 1913

DOI: 10.1021/acs.inorgchem.5b02817 Inorg. Chem. 2016, 55, 1912−1919

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Inorganic Chemistry fluorine source and Na2CO3/F− on the morphology and size of the final products will be discussed in the later section. TEM and HRTEM characterization gives a more detailed view on the morphology and structure of the as-prepared βNaYF4 samples. Figure 3A shows the hexagonal contour feature

Figure 4. SEM images of the as-prepared samples at different reaction times of (A) 1, (B) 3, and (C) 6 h; (D) corresponding XRD patterns. Scale bars are 10 μm.

The macroscopic similarity in the shape framework between the two reflected the microcosmic topotactic nucleation and growth of β-NaYF4 along the definite crystalline direction of NaY(CO3)F2 flower structure due to the possible structural resemblance. As the reaction further prolonged to 6 h, more hexagonal microdiscs formed, and finally, phase-pure β-NaYF4 hexagonal microdiscs were obtained after 12 h hydrothermal treatment. During the crystallization process, the initially formed hexagonal disc hierarchical structure gradually merged intoan individual hexagonal disc architecture through the Ostwald ripening process, which gives a reasonable explanation for the multiple hexagonal contours of β-NaYF4 hexagonal microdisc. The topotactic structural resemblance between the two targets is the precondition for topotactic transformation. In βNaYF4 crystal structure, the Y3+ ion was coordinated by nine F− ions, which formed an YF9 trigonal prism with each vertical face bearing a pyramid. The YF9 polyhedra edge linked to other three adjacent ones along the a and b axes and face sharing to another two along the c axis, forming a hexagonal tunnel structure along the [0001] direction wherein the Na+ ions were inserted, as illustrated in Figure 5. In the tunnels, the crystal sites were occupied by one-half Na+ ions and one-half vacancies, and the Na+ ion was six coordinated, forming an irregular NaF6 octahedron,24,25 while in the NaY(CO3)F2

Figure 3. (A) TEM, (B) HRTEM iamges, (C) EDX spectrum, and (D) morphology schematic of an individual as-prepared β-NaYF4 hexagonal disc; (inset in B) corresponding FFT pattern.

of the β-NaYF4 sample with hexagonal disc morphology when viewed along the c axis. The HRTEM image (Figure 3B) projected along the [0001] direction recorded at the red rectangle area exhibits a well-resolved 2D lattice fringe, indicating the high crystallinity. The intersectant lattice fringes with a d spacing of 0.51 nm and an intersection angle of 120° can be indexed to the equivalent {101̅0} lattice plane, corresponding to the FFT pattern. On the basis of the above analysis, the structural geometry of the as-prepared β-NaYF4 sample with different aspect ratios can be dissected to be enclosed by six rectangular {1010̅ } and two hexagonal {0001} facets, as illustrated in Figure 3D. The EDX spectrum (Figure 3C) reveals the composition elements of Na, Y, and F, which further confirms the phase purity. 3.2. Topotactic Transformation Mechanism. To investigate the phase and morphology evolution of the samples during the growth process, a series of intermediate products at different reaction intervals was monitored by XRD and SEM measurements. Here, we took hexagonal disc NaHF2-4/2 as an example. One can see that after 1 h hydrothermal reaction the obtained products were monodisperse microflower structures with a size of about 2 μm (Figure 4A). The corresponding XRD pattern showed that the flower-like structure was phase-pure NaY(CO3)F2 with JCPDS no. 50-1633, not the kinetic αNaYF4 phase for most wet synthetic routes at the initial growth stage. As it was prolonged to 3 h, hexagonal microdiscs appeared beside the NaY(CO3)F2 microflower (Figure 4B), and weak peaks of β-NaYF4 could be detected in the corresponding XRD pattern, indicating the formation of βNaYF4 hexagonal microdiscs. A close-up view showed that the β-NaYF4 hexagonal disc sprouted from the body of the NaY(CO3)F2 flower, leading to an intermediate hybrid structure. A more careful observation revealed that the newly formed β-NaYF4 hexagonal disc hierarchical structure inherited the similar assembly mode of NaY(CO3)F2 flower, which implied the topotactic transformation mechanism from NaY(CO3)F2 to β-NaYF4, not via decomposition of the NaY(CO3)F2 followed by crystallization of the β-NaYF4 process.

Figure 5. Schematic illustration of the structural relationship between NaY(CO3)F2 and β-NaYF4. 1914

DOI: 10.1021/acs.inorgchem.5b02817 Inorg. Chem. 2016, 55, 1912−1919

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Figure 6. SEM images of as-prepared β-NaYF4 sample with different Na2CO3/NaHF2 feeding ratio. Na2CO3 and NaHF2 feeding amounts are fixed as constant in column and row, respectively. Row A: (A1) 3/2, (A2) 4/2, and (A3) 5/2. Row B: (B1) 3/3, (B2) 4/3, and (B3) 5/3. Row C: (C1) 3/ 4, (C2) 4/4, and (C3) 5/4. Scale bars are 10 μm.

crystal structure the Y3+ ion was coordinated by four F− and four O2− ions, forming a Y(F/O)8 bifurcated octahedron; the Na+ ion was coordinated by four F− and three O2− ions, forming a Na(F/O)7 bifurcated octahedron. The Y(F/O)8 polyhedra linked to three adjacent ones with edge sharing for two and vertex sharing for the other one along the a and b axes and vertex sharing to the adjacent two ones along the c axis, which also forms a similar hexagonal tunnel structure along the [001] direction with Na+ ions inserted.26 Such similarity in the atomic arrangement between NaY(CO3)F2 and β-NaYF4 is promising to greatly reduce the barrier energy for the topotactic nucleation of β-NaYF4 with overstepping the intermediate formation of α-NaYF4. There are also some other noteworthy structural similarities facilitated by topotactic transformation. One can see that the Y(F/O)8 polyhedron is analogous to the YF9 counterpart when viewed from certain directions as illustrated in Figure 5, and the Y(F/O)8 bifurcated octahedron can be intuitively seen as a quasi-YF9 trigonal prism. In addition, the Na(F/O)7 bifurcated octahedron is also similar to the NaF6 irregular octahedron (Supporting Information, Figure S1). On the basis of the above analysis, it is suggested that the topotactic structural similarity can epitaxially promote the nucleation and growth of β-NaYF4 along the [0001] direction, inheriting the [001] direction of NaY(CO3)F2; the corresponding structural relationship can be described as [001]NaY(CO3)F2//[0001]β-NaYF4. 3.3. Effects of the Experimental Variables. The experimental variables have a profound influence on the morphology, size, and composition of the final products. Figure 6 shows the morphology and size evolution versus the experimental variables of Na2CO3/NaHF2 feeding ratio. A whole scan of the series of products indicated that the Na2CO3/NaHF2 feeding ratio had a profound influence on the morphology and size evolutions. The experimental variables

and the corresponding crystalline sizes are listed in Table 1. By tuning the Na2CO3/NaHF2 feeding ratio, β-NaYF4 hexagonal Table 1. Na2CO3/NaHF2 Feeding Molar Ratios and the Corresponding Crystalline Sizes (μm) for the As-Prepared βNaYF4 Samplesa row A row B row C a

column 1

column 2

column 3

3/2-0.25 × 2.1 3/3-0.32 × 2 3/4-1.8 × 0.6

4/2-0.7 × 3 4/3-1.8 × 3.1 4/4-3.3 × 2

5/2-0.3 × 2.4 5/3-9 × 1.5 5/4-1.7 × 1.6

The abbreviation was described as Na2CO3/NaHF2-length × radius.

disc (NaHF2-3/2, NaHF2-4/2, and NaHF2-4/3), hexagonal disc hierarchical structure (NaHF2-3/3), and hexagonal prism microcrystals (NaHF2-3/4, NaHF2-4/4, and NaHF2-5/4) can be obtained. The hexagonal disc hierarchical structure still inherited the geometric imprint of the intermediate NaY(CO3)F2 flower in the early reaction stage, which further revealed the topotactic transformation mechanism. When the Na2CO3/NaHF2 feeding ratio increased over an upper limit, phase-pure NaY(CO3)F2 microdisc (NaHF2-5/2) and prism (NaHF2-5/3) can be obtained (the XRD pattern is shown in the Supporting Information, Figure S2), which may be due to the fact that deficient NaHF2 cannot initiate NaY(CO3)F2 → βNaYF4 TT. It can be seen that when the Na2CO3 feeding amount was increased, the rough tendency showed that the aspect ratio of the products gradually decreased, while it showed an inverse tendency when NaHF2 feeding amount was increased. Both tendencies projected more evidently with high feeding amounts for Na2CO3 and NaHF2. This may be due to the 2-fold role of NaHF2 in this reaction system. One is that introduction of increasing H+ ions can accelerate the NaY(CO3)F2 → β-NaYF4 topotactic transformation (TT) by protonating the CO32− ions (CO32− + 2H+ → H2O + CO2↑) 1915

DOI: 10.1021/acs.inorgchem.5b02817 Inorg. Chem. 2016, 55, 1912−1919

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Figure 7. SEM images of as-prepared β-NaYF4 sample with different Na2CO3/NaF feeding ratios. Na2CO3 and NaF feeding amounts are fixed as constant in column and row, respectively. Row A: (A1) 1.5/4, (A2) 2/4, and (A3) 3/4. Row B: (B1) 1.5/12, (B2) 2/12, and (B3) 3/12. Row C: (C1) 1.5/16, (C2) 2/16, and (C3) 4/16. Scale bars are 10 μm.

(Supporting Information); the distinct contrast between the central and fringe parts further confirmed the hollow structure. Such morphology is novel for β-NaYF4, which has never reported before, and the hierarchical mode may provide a prelude for designing of β-NaYF4 hollow nanoparticles. When the Na2CO3/NaF feeding ratio was increased to a high extent, phase-pure NaY(CO3)F2 microdiscs (NaF-3/4) were obtained, similar to that with using NaHF2 as the fluorine source. With increasing NaF feeding amounts, the aspect ratio of the hexagonal structures (including the hexagonal disc building blocks in the sphere structure) increased, which was consistent with the NaHF2 case. The sphere structure evolved into a doughnut-like, quasihexagonal disc and finally to hexagonal prisms. The doughnut-like structure was similar to the earthlike sphere except that the building block evolved from hexagonal disc into prism. However, with relatively higher feeding amounts of Na2CO3 and NaF, the aspect ratio of the products exhibited an increased tendency when the Na2CO3 feeding amounts gradually increased, which was contrary to the NaHF2 case. This may be due to the fact that without H+ ions the increasing CO32− ions greatly suppressed the growth speed of {101̅0} crystal facets by preferential adsorption. Therefore, Na2CO3 as the additive in our system has played two crucial roles: (1) inducing the direct formation of β-NaYF4 and (2) tuning the morphology of the final products. The morphology evolutions versus Na2CO3/F− feeding ratio for NaHF2 and NaF cases are illustrated in Figure 8. In conclusion, compared with the conventional wet synthetic methodology for β-NaYF4 nanocrystal, our green synthetic route is free of organic solvents and tedious experimental processes, and the topotactic transformation can effectively avoid the routine α → β phase transfer process for NaYF4, which may provide a practical and theoretical case for direct synthesis of β-NaYF4 nanocrystal. 3.4. Multicolor Luminescence Properties. Here, the potential application of the as-prepared β-NaYF4:Ln3+ (Ln =

extracted from NaY(CO 3 )F 2 . The other one is that introduction of increasing F − ions can facilitate the crystallization and Ostwald ripening processes of β-NaYF4. In this way, β-NaYF4 grew kinetic preferentially along the [0001] direction due to the [0001] tunnel structure properties, resulting in the formation of anisotropic hexagonal prisms with high aspect ratios and uniform crystal facets. All corresponding XRD patterns of the products are listed in Figure S3 (Supporting Information). When the fluorine source was changed to NaF, a new series of amazing morphology was observed, as shown in Figure 7. The experimental variables and the corresponding crystalline sizes are listed in Table 2, and the corresponding XRD patterns Table 2. Na2CO3/NaF Feeding Molar Ratios and the Corresponding Crystalline Sizes (μm) for the As-Prepared βNaYF4 Samplesa row A row B row C

column 1

column 2

column 3

1.5/4-3.3(0.2 × 0.5) 1.5/12-1.2 × 2.2 1.5/20-1.2 × 1

2/4-3(0.2 × 0.4) 2/12-1.8 × 3.1 2/16-2 × 1.7

3/4-0.2 × 1.5 3/12-0.9 × 1.9 3/16-1.6 × 0.9

The abbreviation was described as Na2CO3/NaF-length × radius. For the sphere particle, the size was expressed as radius combined with the length × radius of the hexagonal disc building block.

a

of the products are listed in the Figure S4 (Supporting Information). It can be seen that with low feeding amounts of Na2CO3 and NaF, the products (NaF-1.5/4, NaF-2/4) turned out to be monodisperse quasispheres, which were similar to the earth with two concaves at the two poles. A close-up view showed that the earth body was oriented densely packed with hexagonal discs along the latitude lines and the {101̅0} crystal facets pointed to the core, resulting in a hollow structure. The corresponding TEM images are shown in Figure S5 1916

DOI: 10.1021/acs.inorgchem.5b02817 Inorg. Chem. 2016, 55, 1912−1919

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Figure 8. Schematic illustrations of the morphology evolution versus experimental variable using (A) NaHF2 and (B) NaF as fluorine sources. Note that the when one experimental variable is varying, the other is fixed. For example, when NaHF2 is increased, the feeding amount of Na2CO3 is kept constant.

Figure 10. Excitation and emission spectra of the (A) βNaYF4:0.1Ce3+ and (B) β-NaYF4:0.1Ce3+,0.03Tb3+ samples.

emission spectra of the as-prepared β-NaYF4:0.1Ce3+ and βNaYF4:0.1Ce3+,0.03Tb3+ samples. It can be seen that the excitation spectrum of β-NaYF4:0.1Ce3+ consists of an intense broad band centering at 292 nm with a left shoulder at 262 nm when monitored at 336 nm, which can be indexed to the Ce3+ 4f−5d transition.28 Upon excitation into the Ce3+ ions at 292 nm, a broad emission band at around 336 nm can be found, owing to the characteristic Ce3+ 5d−4f emission. When codoped with Tb3+ ions, a strong green emission of Tb3+ with a relatively weak Ce3+ emission can be achieved under excitation into Ce3+ 4f−5d transition at 292 nm, indicating an efficient energy transfer process from Ce3+ to Tb3+ in the βNaYF4 host matrix. The doublet character of the excitation band reflected the splitting degree of the Ce 2F state, which is discussed in the Supporting Information (Figure S6). Thus, it makes a clear indication that sensitization of Ce3+ ions can greatly enhance the emission efficiency of the activators in the host matrix with low excitation efficiency. Figure 11 shows the UC emission spectra of the as-prepared β-NaYF4:0.18Yb 3+,0.02Er3+ and β-NaYF4:0.19Yb 3+,0.01Tm3+

Eu, Tb, Ce/Tb, Yb/Er, and Yb/Tm) products as phosphor was demonstrated by downconversion, upconversion, and energy transfer (Ce−Tb) emissions process. Figure 9 shows the

Figure 9. Excitation and emission spectra of the (A) βNaYF4:0.05Eu3+ and (B) β-NaYF4:0.05Tb3+ samples.

excitation and emission spectra of the as-prepared βNaYF4:0.05Eu3+ and β-NaYF4:0.05Tb3+ samples. Both excitation spectra consisted of series of characteristic f−f transitions from the ground state (Eu:7D0, Tb:7F6) to the excited state levels when monitored at the corresponding characteristic emissions. It is noteworthy that the F−-Eu3+/Tb3+ charge transfer bands cannot be detected compared with the oxide system, which is due to the higher electronegativity of the pure fluoride system that results in a blue shift toward the vacuum ultraviolet region.27 Upon excitation at 397 and 380 nm (Eu:7D0−5L6, Tb:5D3) the emission spectra exhibited the characteristic transitions of Eu3+ (5D0−7F1,2, 5D1−7F1,2) and Tb3+ (5D4−7F1,2,3,4), yielding orange-red and green emissions, respectively. We also investigated the energy transfer process by codoping of Ce3+ and Tb3+ ions. Figure 10 shows the excitation and

Figure 11. UC emission spectra of (A) β-NaYF4:0.18Yb3+,0.02Er3+ and (B) β-NaYF4:0.19Yb3+,0.01Tm3+. (Inset in A) Dependence of UC emission intensity on the pump power in β-NaYF4:0.18Yb3+,0.02Er3+. 1917

DOI: 10.1021/acs.inorgchem.5b02817 Inorg. Chem. 2016, 55, 1912−1919

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samples. It can be seen that the spectra of β-NaYF4:Yb 3+,Er3+ contained an intense red emission band (625−700 nm) corresponding to the Er3+4F9/2 → 4I15/2 transition and comparative green emission bands (500−535 and 535−575 nm), which can be assigned to the Er3+2H11/2−4I15/3 and 4 S3/2−4 I15/3 transitions, respectively. For β-NaYF4:Yb3+,Tm3+ samples, a strong red emission band (650−735 nm) and nearinfrared emission band (780−850 nm) dominate the spectra with a relatively weak blue emission band (445−510 nm) owing to the Tm3+1G4 → 3H6 transition. The red and nearinfrared emissions can be assigned to the Tm3+3F3 → 3H6 and 3 H4 → 3H6 transitions, respectively. To investigate the UC process, the power dependence of the UC emission intensity for β-NaYF4:Yb 3+,Er3+ sample was measured. It can be seen in the log(Iuc) vs log(IIR) curves (Iuc, output UC emission intensity; IIR, IR pump power) that the linear fitted slopes for the green (H11/2−4I15/3 and 4S3/2−4 I15/3 transitions) and red (4F9/2 → 4I15/2 transition) emissions are 2.39, 1.97, and 2.21, respectively, which indicate a two-photon UC process for the H11/2, 4S3/2, and 4F9/2 population of Er3+. The experimental observations agree with that of the previous reports;29,30 in such mechanism, 2H11/2(Er3+) and 4S3/2(Er3+) are populated through two-photon absorption with two-step nonradiative energy transfer from the adjacent Yb 3+ ions ( 4 I 11/5 (Er3+) → 4 I11/2(Er3+) → 4I7/2(Er3+)) and subsequent nonradiative relaxation (4I7/2(Er3+) → 2H11/2(Er3+), 4S3/2(Er3+)), yielding green emissions via 2H11/2(Er3+) → 4I15/2(Er3+) and 4S3/2(Er3+) → 4I15/2(Er3+) transitions. While the 4F9/2(Er3+) excited state is populated through two-photon absorption by two possible ways, the red emission can be yielded through the 4F9/2(Er3+) → 4I15/2(Er3+) transition, which are illustrated in Figure S7 (Supporting Information). For β-NaYF4:Yb3+,Tm3+ samples it is well established that the blue emission (1G4 → 3H6) was populated by a three-photon process, while the red (3F3 → 3 H6) and the near-infrared emission (3H4 → 3H6) undergo a two-photon process.31,32

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02817. Schematic illustration of Na(F/O)7 and the NaF6 polyhedra, XRD patterns of the samples with different experimental variables, FT-IR spectra of theβNaYF4:0.1Ce3+ and β-NaYF4:0.1Ce3+,0.03Tb3+ samples, and schematic illustration of the two-photon mechanism UC process of NaYF4: Yb3+,Er3+ (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (973 Program, grant no 2014CB643803), the National Natural Foundation of China (Grant No. 21271167), and the Fund for Creative Research Groups (Grant No. 21221061).

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4. CONCLUSION In summary, we described a novel nonorganic wet route that bypassed the intermediate routine cubic−hexagonal (α → β) phase transfer process to directly synthesize hexagonal β-NaYF4 microcrystals using Na2CO3 as additive by the hydrothermal method. Interestingly, the cubic−hexagonal phase transfer for NaYF4 was not detected during growth, while NaY(CO3)F2 emerged in the initial reaction stage and fast transformed into β-NaYF4 via a novel topotactic transformation behavior due to the structural matching of [001]NaY(CO3)F2//[0001]βNaYF4. We systematically investigated the influence of Na2CO3/F− feeding ratio on the evolutions of morphology and size of the final products, and a variety of morphologies including hexagonal prism, disc, and novel hierarchical architecture were obtained. Besides, the potential application of the as-prepared products as phosphors is emphasized by demonstrating multicolor emissions including downconversion, upconversion, and energy transfer (Ce−Tb) process by lanthanides doping. Our nonorganic green synthetic strategy may provide a new theoretical and experimental basis for direct synthesis of β-NaYF4 nano/microcrystals. 1918

DOI: 10.1021/acs.inorgchem.5b02817 Inorg. Chem. 2016, 55, 1912−1919

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DOI: 10.1021/acs.inorgchem.5b02817 Inorg. Chem. 2016, 55, 1912−1919