Controlled Synthesis and Optical Properties of Lanthanide-doped Na3ZrF7 Nanocrystals①

FU Hu-Hui LIU Yong-Sheng JIANG Fei-Long HONG Mo-Chun, c

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Controlled Synthesis and Optical Properties of Lanthanide-doped Na3ZrF7Nanocrystals①

FU Hu-Huia, b, c, dLIU Yong-Shenga, b②JIANG Fei-Longa②HONG Mao-Chuna, b, c②

a(350002)b(100049)c(201210)d(200050)

In this paper, we report for the first time the controlled synthesis of lanthanide ion (Ln3+)-doped tetragonal-phase Na3ZrF7nanocrystals (NCs)a high-temperature co-precipitation approach. The as-synthesized Na3ZrF7NCs are systematically studied by utilizing the XRD, TEM as well as high-resolution photoluminescence (PL) spectroscopy. The morphology and size for the as-synthesized Na3ZrF7NCs can be finely controlled by changing the experimental parameters such as the amount of precursor, solvent ratio, reaction temperature and time. By utilizing the red-emitting Eu3+ion as an efficient optical/structural probe, the successful hetero-valence doping of Ln3+activators in the lattices of Na3ZrF7NCs is well-established regardless of their different valences and radii between host Zr4+ion and Ln3+dopant. As a result, intense upconversion (UC) luminescence (UCL) ranging from UV to visible and to NIR spectral regions can be readily achieved after the doping of typical UCL couples of Yb3+/Er3+, Yb3+/Tm3+and Yb3+/Ho3+into the lattices of Na3ZrF7NCswhen excited by using a 980-nm NIR diode laser.

lanthanide ion, Na3ZrF7, nanocrystals, upconversion luminescence;

1 INTRODUCTION

Trivalent lanthanide (Ln3+)-doped upconversion (UC) nanocrystals (UCNCs) that convert low-energy irradiation into high-energy emission have shown promising applications in areas as diverse as bio- logical imaging, detection, therapeutics, photonics and full-color displays, owing to their exceptional optical properties such as large anti-Stokes shifts, sharp emission bandwidths, long excited-state lifetimes and tunable emission colors[1-28]. In parti- cular, with the rapid development of nanocrystal synthesis technology over the past decade, Ln3+- doped UCNCs can now be made with precisely controlled composition, morphology, phase, dimen- sion, emission color and lifetime[29-35]. Despite these significant advances, it should be noted that, in most reported cases, high-quality Ln3+-doped UCNCs with tunable crystal phase, nanocrystal size, mor- phology as well as upconversion luminescence (UCL) colors were mainly restricted to the Ln3+- doped hexagonal-phase NaYF4and NaGdF4NCs[36-46]. However, Na3ZrF7, as another suitable host material, has been rarely reported on its crystal structure, controlled synthesis and optical properties. Very recently, Wang and co-workers have synthe- sized Na3ZrF7NCs by using a solvothermal method, and then demonstrated that Na3ZrF7was an excellent host material which can exhibit intense single red UCL when co-doped with the typical Yb3+/Er3+UCL couple[47]. To date, the controlled synthesis of Ln3+-doped Na3ZrF7UCNCs by using other che- mical synthetic approaches such as thermal decom- position and high-temperature co-precipitation remains nearly untouched. Especially, due to the large discrepancy between ionic radius (e.g., 0.95 ? for Eu3+and 0.80 ? for Zr4+) and valences (+3 for Ln3+and +4 for Zr4+) for host Zr4+ion and Ln3+dopant, the successful hetero-valence doping of Ln3+ions in the lattices of Na3ZrF7crystal has been a great challenge for the broad community working in this field.

In this paper, the controlled synthesis of tetra- gonal-phase Na3ZrF7NCs with different mor- phology and size is achieved simply by varying the amount of NaOH, OA/ODE ratio, reaction tem- perature and time. In addition, the successful doping of Ln3+in Na3ZrF7crystal lattice is demonstrated for the first time by high-resolution photoluminescence spectroscopy of Eu3+at low temperature (10 K).Size-dependent UCL spectra of Ln3+-doped Na3ZrF7NCs with different amount of NaOH are deeply investigated. Intense UCL emissions of Ln3+-doped Na3ZrF7NCs ranging from UV to visible and to NIR spectral regions can be readily achieved by doping with typical UCL couples of Yb3+/Er3+, Yb3+/Tm3+and Yb3+/Ho3+.

2 EXPERIMENTAL

2.1 Chemicals and materials

NaOH and NH4F were purchased from Aladdin (China). Yb(CH3CO2)3·4H2O (99.999%), Er(CH3CO2)3·4H2O (99.99%), Tm(CH3CO2)3·4H2O (99.99%), Ho(CH3CO2)3·4H2O (99.99%), Eu(CH3CO2)3·H2O (99.99%), Ce(CH3CO2)3·H2O (99.99%), Tb(CH3CO2)3·H2O (99.99%), zirconium(IV) ace- tylacetonate, oleic acid (OA) and 1-octadecene (ODE) were purchased from Sigma- Aldrich (China). Cyclohexane, methanol, and ethanol were purchased from Sinopharm Chemical Reagent Co., China. All chemicals were used as received without further purification.

2.2 General procedure for the preparation of tetragonal-phase Na3ZrF7:Ln3+ NCs

The tetragonal-phase Na3ZrF7:Ln3+(Ln = Yb, Er, Tm, Ho) NCs were synthesizeda modified high-temperature co-precipitation method. In a typical procedure for the synthesis of Na3ZrF7:20%Yb3+/2%Er3+NCs, 0.78 mmol of zirconium (IV) acetylacetonate, 0.2 mmol of Yb(CH3CO2)3·4H2O and 0.02 mmol of Er(CH3CO2)3·4H2O were mixed with 10 mL of OA and 30 mL of ODE in a 100 mL three-neck round-bottom flask. The solution was heated to 150 ℃ under N2flow with constant stirring for 60 min to form a clear solution, and then cooled down to room temperature. Thereafter, 10 mL of methanol solution containing 3 mmol of NaOH and 7 mmol of NH4F was added and the resulting mixture was stirred for 30 min. After the removal of methanol by evaporation, the solution was heated to 290 ℃under N2flow with vigorous stirring for 60 min, and then cooled down to room temperature. The resul- ting NCs were precipitated by addition of ethanol, collected by centrifugation, then washed with ethanol for several times, and finally re-dispersed in cyclohexane.

2.3 Characterization

Powder XRD measurements were performed on a powder diffractometer (MiniFlex2, Rigaku) with Curadiation (= 0.154187 nm) from 10° to 70° at a scan rate of 5° min-1. Both the TEM and high-resolution TEM measurements were conducted on a transmission electron microscope (TEM, TECNAIG2F20) equipped with an energy dispersive X-ray spectroscope (EDS). UCL spectra were measured upon 980-nm NIR excitation from a continuous-wave diode laser. All the UCL decay curves for Yb3+/Er3+co-doped NCs were measured with a customized UV to mid-infrared steady-state and phosphorescence lifetime spectrometer (FSP920-C, Edinburgh Instrument) equipped with a digital oscilloscope (TDS3052B, Tektronix) and a tunable mid-band Optical Parametric Oscillator (OPO) pulse laser as the excitation source (410～2400 nm, 10 Hz, pulse width ≤5 ns, Vibrant 355II, OPOTEK), and the effective lifetime (eff) was determined by:

where0and(t) represent the maximum lumine- scence intensity and luminescence intensity at time t after cutoff of the excitation light, respectively.

3 RESULTS AND DISCUSSION

3.1 Structure and morphology

As shown in Fig. 1a, the Na3ZrF7crystal has a tetragonal structure (space group4/mmm) with the central Zr4+ions surrounded by eight F-ions formingthe edges of a cube. All Zr4+ions occupy the sitesymmetry of4h. High-quality tetragonal-phaseNa3ZrF7:Ln3+(Ln = Yb, Er, Tm, Ho, Eu, etc)NCs were synthesizeda modified high-temperature co-precipitation method with the assistance of OA and ODE as surfactant and solvent, respectively. Fig. 1b shows a typical low-resolution TEM image of the as-synthesized Na3ZrF7:20%Yb3+/2%Er3+NCs with uniform hexagonal shape. Corresponding high- resolution TEM (HRTEM) image of a single Na3ZrF7:Yb3+/Er3+nanocrystal shown in Fig. 1c displays clear lattice fringes with a d-spacing of 0.31 nm, which is in good accordance with the lattice spacing of the (112) plane for tetragonal Na3ZrF7crystal, indicative of the single crystalline nature of the obtained NCs. Selected-area electron diffraction (SAED) pattern obtained from the Fourier transform of the HRTEM image further confirmed the single- crystalline tetragonal structure of the Na3ZrF7:Yb3+/Er3+NCs (Fig. 1d). Componential analysis by energy-dispersive X-ray spectroscopy (EDX) revealed the presence of Na, Zr, F, Yb and Er elements, indicating the successful doping of Yb3+and Er3+ions in the as-synthesized Na3ZrF7: Yb3+/Er3+NCs (Fig. 1e).

Fig. 1. (a) Crystal structure of tetragonal-phase Na3ZrF7crystal. (b) Low-resolution TEM image of the as-synthesized Na3ZrF7:20%Yb3+/2%Er3+NCs. (c) High-resolution TEM image of a nanoparticle and (d) Corresponding Fourier-transform diffractogram. (e) EDX analysis of the as-synthesized NCs

TEM images of the Na3ZrF7:20%Yb3+/2%Er3+NCs synthesized with varied amount of NaOH in the raw materials ranging from 3 to 6 mmol are shown in Fig. 2. It can be seen that the morphology of Na3ZrF7:Yb3+/Er3+NCs gradually changed from larger nanohexagons to smaller ones with relatively low amount of NaOH of 3 and 4 mmol, to the mixture of nanohexagons and nanocubes with the NaOH amount of 4.5 mmol, then to small nanorods, big nanocubes, and eventually small irregular shaped nanospheres with the NaOH amount of 5, 5.5, and 6 mmol, respectively (Fig. 2a～2f). Corresponding nanocrystal sizes were measured to be 27.6 ± 2.1, 21.3 ± 1.6, 17.1 ± 3.9, 19.7 ± 1.5, 36.7 ± 4.5, and 19.5 ± 4.8 nm, respectively.

Fig. 2. TEM images of the Na3ZrF7:20%Yb3+/2%Er3+NCs synthesized with different amount of NaOH: (a) 3, (b) 4, (c) 4.5, (d) 5, (e) 5.5, and (f) 6 mmol, respectively. Insets in (a～f) are the corresponding size distributions

Notably, despite the morphology and size of Na3ZrF7:Yb3+/Er3+NCs changed significantly with the amount of NaOH increased from 3 to 6 mmol, the tetragonal crystal phase maintained, which was clearly revealed by the XRD patterns shown in Fig. 3. All the XRD patterns can be well indexed to the standard pattern for tetragonal Na3ZrF7crystal (JCPDS No. 12-0562). These results unambiguously demonstrated that the morphology and size of Na3ZrF7:Yb3+/Er3+NCs can be facilely tuned by varying the amount of NaOH, and a relatively less amount of NaOH is conducive to yield uniform large hexagonal-shaped Na3ZrF7:Yb3+/Er3+NCs.

Fig. 3. XRD patterns of Na3ZrF7:20%Yb3+/2%Er3+NCs synthesized with different amount of NaOH: (a) 3, (b) 4, (c) 4.5, (d) 5, (e) 5.5, and (f) 6 mmol, respectively

Moreover, the effect of solvent ratio of OA/ODE on Na3ZrF7:Yb3+/Er3+NCs was studied. We found that the morphology, size, and uniformity of Na3ZrF7:Yb3+/Er3+NCs were highly dependent on OA/ODE ratio. As shown in Fig. 4, relative to the total volume, a smaller OA/ODE ratio would result in larger and more uniform hexagonal NCs. Despite the size of Na3ZrF7:Yb3+/Er3+NCs decreased with the increase of OA/ODE ratio, hexagonal shape maintained. Corresponding nanocrystal sizes were measured to be 16.2 ± 1.3, 19.2 ± 6.7, 27.6 ± 2.1, and 41.2 ± 4.5 nm when the OA/ODE ratio was 10:15, 10:20, 10:30, and 10:40, respectively.

Fig. 4. TEM images of the Na3ZrF7:20%Yb3+/2%Er3+NCs synthesized with different OA/ODE ratios: (a) 10:15, (b) 10:20, (c) 10:30, (d) 10:40, respectively. Insets in (a～d) are the corresponding size distributions

Continually, the effects of reaction temperature and time on Na3ZrF7:Yb3+/Er3+NCs were investigated. As illustrated in Fig. 5, along with the increase of temperature from 270 to 290 ℃, the morphology of Na3ZrF7:Yb3+/Er3+NCs gradually changed from irregular shaped nanospheres to uniform nanohexagons (Fig. 5a-c), and keeping increasing reaction temperature to 300 ℃ would yield larger nanohexagons with a boarder size distribution (Fig. 5d).Corresponding nanocrystal sizes were measured to be 30.0 ± 10.8, 31.2 ± 4.5, 27.6 ± 2.1, and 41.2 ± 3.2 nm, respectively. This observation suggests that high reaction temperature was thermodynamically in favour of the generation of NCs with larger size and uniform morphology. The important role of tuning the size and morpho- logy was also verified by the resulting Na3ZrF7:Yb3+/Er3+NCs synthesized for different reaction time with an OA/ODE ratio of 10:30 at 290 ℃. The obtained Na3ZrF7:Yb3+/Er3+NCs owned irregular morphology with evident discrepancy in size and shape for relatively short reaction timeof 20 min (Fig. 6a), and regular hexagonal shape appeared when prolonging the reaction time to 40 min (Fig. 6b). Moreover, the morphology and size homogeneity were up to the optimum when the reaction time was increased to 60 min (Fig. 6c). Keeping increasing the reaction time to 90 min yielded larger hexagonal NCs with a boarder size distribution (Fig. 6d). Corresponding nanocrystal sizes were measured to be 12.5 ± 1.4 (small) and 49.6 ± 9.1 (large), 23.5 ± 7.9, 27.6 ± 2.1, and 35.6 ± 3.2 nm when the reaction time was set to 20, 40, 60, and 90 min, respectively.

Fig. 5. TEM images of the Na3ZrF7:20%Yb3+/2%Er3+NCs synthesized at different temperature: (a) 270 ℃, (b) 280 ℃, (c) 290 ℃, and (d) 300 ℃, respectively. Insets in (a～d) are the corresponding size distributions

Fig. 6. TEM images of the Na3ZrF7:20%Yb3+/2%Er3+NCs synthesized at 290 ℃ for different time: (a) 20 min, (b) 40 min, (c) 60 min and (d) 90 min, respectively. Insets in (a～d) are the corresponding size distributions

3.2 Optical properties of Na3ZrF7:Ln3+ NCs

It is well known that Eu3+ions are very sensitive to their coordination and local environment, thus usually employed as a sensitive spectral probe to study the crystal structure of host material through substituting the central cations in the crystal lattice[48-50]. To demonstrate the successful hetero- valence doping of trivalent lanthanide ions sub- stituted to Zr4+ions in the crystal lattice of Na3ZrF7NCs, we measured the high-resolution site-selectivespectra of Na3ZrF7:Eu3+NCs. Emission and excitation spectra and photoluminescent (PL) decays were collected at low temperature of 10 K to avoid the thermal broadening of spectral bands at room temperature[51]. As compared in Fig. 7a, the emission spectra of Na3ZrF7:Eu3+NCs upon excitation at 393.5 and 392.5 nm were highly different, where a distinct dissimilarity can be easily distinguished from the integrated intensity ratio of50→71to50→72transition. The50→71transition is magnetic dipole transition, which is unaffected by nearby structural changes. On the contrary, the50→72transition is electric dipole transition and it is hypersensitive to the local crystalline ?eld[52, 53]. In other words, the integratedintensity ratio () of50→72to50→71transition differingsignificantly in various sites is very sensitive to the local crystal field surroundings, and a smallerindicates a higher site symmetry of Eu3+ions doped in host crystal lattice. From the emission spectra in Fig. 7a,values were calculated to be 1.21 and 0.82 upon excitation at 393.5 and 392.5 nm, respectively. The decrease ofvalue indicated that the emissions of Eu3+ions upon excitation at 393.5 and 392.5 nm correspond to external low and interiorhigh symmetric sites in Na3ZrF7:Eu3+NCs, respectively, thus demonstrating the successful doping of Eu3+ions in Na3ZrF7crystal lattice. The big discrepancy in PL lifetimes displayed in Fig. 7b further confirmed the Eu3+ions of two different sites in Na3ZrF7:Eu3+NCs owing to the surface fluorescence quenching.

Fig. 7. (a) 10 K PL emission spectra of Na3ZrF7:Eu3+NCs upon excitation at 393.5 (red line) and 392.5 (black line) nm, respectively. (b) 10 K luminescence decays from5D0of Eu3+in Na3ZrF7:Eu3+NCs under site-selective excitation at 393.5 and 392.5 nm by monitoring the emission at 613.5 and 588.5 nm, respectively

After demonstrating the successful doping of Ln3+ions in Na3ZrF7crystal lattice, we have conducted a set of measurements for the optical properties of Na3ZrF7:Yb3+/Ln3+(Ln = Er, Tm, or Ho). Fig. 8 displays the typical UCL spectra of Na3ZrF7NCs doped with Yb3+/Er3+, Yb3+/Tm3+, and Yb3+/Ho3+upon near infrared (NIR) excitation at 980 nm, and intense visible and near-infrared (NIR) emissions were observed.As shown in Fig. 8a, the Na3ZrF7:Yb3+/Er3+NCsyielded intense red and extremely weak green emissions centred at 651 and 540 nm, respectively, which are assigned to the49/2→415/2and211/2/43/2→415/2transitions of Er3+, respectively. Next, as shown in Fig. 8b,intense single band NIR emissioncentred at 800 nm was detected in the Na3ZrF7:Yb3+/Tm3+NCs, which is assigned to the34→36transition of Tm3+. Furthermore, as shown in Fig. 8c, the Na3ZrF7:Yb3+/Ho3+NCsyieldedrelatively stronger green, weaker red and NIR emissions centred at 542, 655, and 750 nm, respectively, which are assigned to the54→58,55→58, and54→58transitions of Ho3+, respectively.

Fig. 8. UCL spectra of Na3ZrF7NCs doped with: (a) Yb3+/Er3+, (b) Yb3+/Tm3+and (c) Yb3+/Ho3+upon excitation at 980 nm with a power density of 30 W cm-2. (d) Log-log plots of UCL emission intensity against the excitation power density for Na3ZrF7NCs doped with (a) Yb3+/Er3+, (b) Yb3+/Tm3+and (c) Yb3+/Ho3+, respectively. (e) Schematic energy level diagrams of UC processes for Er3+, Tm3+, and Ho3+the sensitization of Yb3+in Na3ZrF7NCs. Inserts in (a) and (c) are the corresponding luminescent photos for Yb3+/Er3+and Yb3+/Ho3+co-doped Na3ZrF7NCs

To shed more light on the UC process in Ln3+- doped Na3ZrF7NCs, the dependence of the UCL emission intensity () of Er3+,Tm3+, and Ho3+ions on the excitation power density ()was analyzed (Fig. 8d). It is known that in the UC process,is proportional to the nthpower of, namely,μn, where n is the number of absorbed photon for one emitted UC photon. In other words, plots of logversus logcan be fitted into linear lines with different slope n values. Upon NIR excitation at 980 nm, plots of logversus logfor the emissions centred at 540 (211/2/43/2→415/2) and 660 (49/2→415/2) nm of Er3+, 800 (34→36) nm of Tm3+, and 542 (54→58), 655 (55→58) and 750 (54→58) nm of Ho3+ions can be fitted into linear lines with slopes of 1.91 and 1.95 for Er3+, 1.93 for Tm3+, and 1.87, 1.57, and 1.74 for Ho3+, respectively (Fig. 8d). These results suggest that the UCL emissions of Er3+, Tm3+, and Ho3+in Na3ZrF7NCs all occurredtwo-photon processes, and the proposed energy level diagrams are illustrated in Fig. 8e.

Moreover,the UCL spectra of Na3ZrF7:20%Yb3+/2%Er3+NCs synthesized with different amount of NaOH were collected upon NIR excitation at 980 nm. As depicted in Fig. 9a, as the amount of NaOH gradually increased, the UCL emission intensity of Na3ZrF7:20%Yb3+/2%Er3+NCs first decreased and reached to the minimum when the amount of NaOH was 4.5 mmol, then began to increase and achieved the maximum when the amount of NaOH was 5.5 mmol, and finally decreesed when keeping increasing the amount of NaOH.Corresponding UCL lifetimes were measured to be 0.25, 0.18, 0.11, 0.13, 0.28, and 0.21 ms, respectively (Fig. 9b), which agreed well with the UCL emission intensity of Na3ZrF7:20%Yb3+/2%Er3+NCs synthesized with different amount of NaOH.

Fig. 9. (a) UCL spectra of Na3ZrF7:20%Yb3+/2%Er3+NCs synthesized with different amount of NaOH upon excitation at 980 nm with a power density of 30 W cm-2. Inset shows the integrated intensity as a function of the amount of NaOH. (b) Corresponding UCL lifetimes of the as-synthesized Na3ZrF7:20%Yb3+/2%Er3+NCs

Furthermore, to validate the effect of Ln3+(Ln = Yb, Er, Tm, or Ho) concentration on the UCL emissions, we synthesized a series of Na3ZrF7NCs doped with different content of Yb3+, Er3+, Tm3+, and Ho3+ions. Fig. 10 shows the UCL spectra of Na3ZrF7NCs doped with different Yb3+concentration ranging from 0 to 30 mol%, Er3+con- centration from 0.5 to 5 mol%, Tm3+concentration from 0.5 to 2 mol%, and Ho3+concentration from 0.5 to 2 mol%, respectively. It can be seen clearly that the overall UCL emission intensity of Er3+in Na3ZrF7:Yb3+/Er3+NCs both increased at first then decreased with the increase of Yb3+and Er3+concentration, and the optimal Yb3+/Er3+co-doping concentration was 20%/2% mol (Fig. 10a-b). Besides, as shown in Fig. 10c and d, 1% mol was the optimal doping concentration for both Tm3+and Ho3+in Na3ZrF7NCs to achieve efficient UCL emissions.

Fig. 10. UCL spectra of Na3ZrF7NCs doped with different concentration of (a) Yb3+, (b) Er3+, (c) Tm3+, and (d) Ho3+, respectively. All the UCL spectra were measured under identical experimental conditions upon excitation at 980 nm with a power density of 30 W. cm-2

4 CONCLUSION

In summary, lanthanide-doped tetragonal-phase Na3ZrF7NCs were synthesized by a high-tempera- ture co-precipitation method. Monodisperse nano- crystals with tunable morphology and size can be readily obtained by varyingthe amount of NaOH, OA/ODE ratio, reaction temperature and time. The successful hetero-valence doping of Ln3+ionsin Na3ZrF7crystal lattice was demonstrated for the first time by taking advantage of high-resolution photo- luminescence spectroscopy of Eu3+at low tempera- ture (10 K). Size-dependent UC luminescence of Na3ZrF7:Yb/Er with differentamount of NaOH were deeply investigated. Efficient UC emissions of Yb3+/Ln3+(Ln = Er, Tm, and Ho) co-doped Na3ZrF7NCs were achieved upon NIR excitation at 980 nm. Tetragonal-phase Na3ZrF7NCs are highly expected to be a promising host lattice to fabricate lanthanide- doped luminescent nanomaterial, providing potential applications such as bioimaging and biodetection.

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2 April 2018;

22 May 2018

① This work was supported by the Strategic Priority Research Program of CAS (XDB20000000), the NSFC (Nos. 21390392, 21473205, and 21731006), Youth Innovation Promotion Association of CAS, and the Natural Science Foundation of Fujian Province (No. 2017J01038)

Liu Yong-Sheng. E-mail: liuysh@fjirsm.ac.cn; Jiang Fei-Long. E-mail: fjiang@fjirsm.ac.cn; Hong Mao-Chun. E-mail: hmc@fjirsm.ac.cn

10.14102/j.cnki.0254-5861.2011-2028