Ostwald Ripening, Particle Size Focusing, and Decomposition of Sub

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Ostwald Ripening, Particle Size Focusing, and Decomposition of Sub10 nm NaREF4 (RE = La, Ce, Pr, Nd) Nanocrystals A. Naduviledathu Raj, T. Rinkel, and M. Haase* Institute of Chemistry of New Materials, University of Osnabrück, Barbara Strasse 7, 49076 Osnabrueck, Germany S Supporting Information *

ABSTRACT: NaREF4 nanocrystals of the lighter rare earth elements RE = La, Ce, Pr, Nd are more difficult to prepare than those of the heavier lanthanides because the materials are sensitive to decomposition. Here, we present a procedure for the synthesis of sub-10 nm particles of β-NaLaF4, βNaCeF4, β-NaPrF4, and β-NaNdF4, as well as of α-NaCeF4, α-NaPrF4, and α-NaNdF4. These particles were used to study the decomposition reaction taking place in the standard oleic acid/octadecene solvent. Ostwald ripening of the β-phase particles in this solvent leads to the expected particle growth and broadening of the particle size distribution only in the case of RE = Ce, Pr, Nd, whereas particles of NaLaF4 decompose to LaF3. Ostwald ripening of the α-phase particles in oleic acid/octadecene yields two products: larger particles of the hexagonal β-phase with a very narrow size distribution and smaller particles of the rare earth trifluoride REF3. The yield of the nearly monodisperse β-NaREF4 particles decreases from Nd to Ce and becomes negligible for the latter. X-ray fluorescence analysis reveals that the decomposition into the REF3 phase is caused by a strong sodium deficiency of the cubic α-NaREF4 nanocrystals, which is observed even if these particles are prepared in the presence of a 6-fold excess of sodium ions. We show that larger particles of phase-pure β-NaCeF4, β-NaPrF4, and β-NaNdF4 can nevertheless be prepared by Ostwald ripening of the corresponding cubic α-NaREF4 particles, if the sodium deficiency of the latter is compensated for during their conversion to the β-phase.

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particles of the cubic α-phase, but if heating is continued for a sufficiently long time, small seeds of the hexagonal β-phase nucleate in solution. After nucleation of these seeds, the αphase particles rapidly dissolve, whereas the β-phase seeds rapidly increase in size, indicating a higher solubility of the thermodynamically less stable α-phase.26,51−54 The fast release of monomers during the rapid dissolution of the α-phase results in a monomer concentration so high that the particles of the less soluble β-phase grow under the condition of monomer supersaturation. This leads to focusing of their particle size distribution, as can be confirmed by numerical simulations based on the LSW theory of Ostwald ripening.8,55 If colloids of α-phase particles are heated, the final size of the β-phase particles depends on the number of seeds formed and, hence, on their nucleation kinetics, which is not wellunderstood. However, if synthesis procedures for small particles of both phases exist, as for instance in the case of NaSmF4, NaEuF4, NaGdF4, and NaTbF4, a well-defined number of βparticles can be added as seeds to the colloid of α-phase particles. In this case, the final size of the product particles solely depends on the molar ratio of the α-phase and β-phase particles and their initial sizes. Heating of such binary particle mixtures represents a convenient method to produce nearly monodiperse β-phase particles on a large scale, since the

INTRODUCTION Rare earth compounds such as NaREF4 and LiREF4 form a class of attractive host lattices for a variety of functional optical materials used as upconversion phosphors,1−9 quantum-cutting phosphors,10−14laser materials,15−17and in solar cell applications.18−22 In recent years, synthesis procedures for nanocrystals of these materials have been developed that yield particles of a well-defined crystal phase, high crystallinity, and narrow particle size distribution.1,2,6,9,23−33 In particular, nearly monodisperse nanocrystals of sodium rare earth fluorides (NaREF4) are intensively investigated, as particles doped with ytterbium and erbium display NIR-to-visible upconversion emission, while particles containing gadolinium enhance the contrast in magnetic resonance imaging (MRI).34−41 Most synthesis procedures for particles of small size are based on oleic acid as reaction medium, combined either with other highboiling apolar solvents, such as octadecene, oleylamine, squalene, or trioctylphosphine,2,24,42−45 or with polar solvents, such as water or alcohol, the latter combinations requiring that heating is performed in an autoclave.33,46−48 The growth of NaREF4 nanocrystals in oleic acid/octadecene solvent is interesting from a mechanistic point of view because it has been shown that the resulting narrow particle size distributions are connected to the polymorphism of the system.30,49,50 If particles of the hexagonal β-phase of NaSmF4, NaEuF4, NaGdF4, or NaTbF4 are heated in this solvent, Ostwald ripening of the nanocrystals leads to broad particle size distributions. Ostwald ripening is also observed for © 2014 American Chemical Society

Received: July 11, 2014 Revised: August 27, 2014 Published: September 1, 2014 5689

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to the excess of sparingly soluble sodium fluoride, which was removed by centrifugation after cooling to room temperature. The clear supernatant was merged with an equal volume of ethanol, leading to precipitation of the small particles, which were separated by centrifugation. The particles were purified by redispersing the precipitate in hexane (3 mL of hexane per 1 mmol of rare earth oleates), followed by precipitation with ethanol and separation by centrifugation. Synthesis of Small Hexagonal Phase Nanocrystals. Sub-10 nm particles of β-NaREF4 (RE = La, Ce, Pr, Nd) were prepared by using rare earth oleate and sodium oleate in a molar ratio of 1:2.5. The combined oleates were dissolved in a mixture of oleic acid, oleylamine, and octadecene (2.5 mL of oleic acid, 2.5 mL of oleylamine, and 5 mL of octadecene per 1 mmol of rare earth oleate), and the solution was degassed on a vacuum Schlenk line (1 mbar) for 1 h at 100 °C under vigorous stirring. Subsequently, 4 mmol of ammonium fluoride per 1 mmol of rare earth oleate was added under nitrogen counterflow. The setup was cycled three times between a nitrogen atmosphere and vacuum (vacuum was applied for only 5 s in each cycle) before the reaction mixture was heated at 290 °C for 60 min under nitrogen flow and vigorous stirring. After cooling to room temperature, the nanocrystals were precipitated by adding an equal volume of ethanol and separated by centrifugation. Purification was performed as given above for particles of the cubic phase. Synthesis of Large Hexagonal Phase Nanocrystals. Larger particles of β-NaREF4 (RE = Ce, Pr, Nd) were prepared by Ostwald ripening of smaller NaREF4 particles in oleic acid/octadecene, as follows. The reaction was performed in a 25 mL three-neck flask with an attached thermo sensor and reflux condenser. After filling 5 mL of oleic acid and 5 mL of octadecene into the flask, 2.5 mmol of small particles of the α-phase or the β-phase was added and the setup was connected to a Schlenk line. The particles were redispersed in the solvent by heating the mixture at 100 °C for 60 min under vacuum. Thereafter, the solution was heated to 320 °C under nitrogen flow with vigorous stirring. After the desired reaction time, the solution was rapidly cooled to room temperature. A 1:2 mixture of hexane and ethanol was added, and the nanocrystals were separated by centrifugation. The precipitate was purified as given above for the small particles. For TEM investigations, the nanocrystals were redispersed in hexane using ultrasonication. Instrumentation. TEM images were recorded with a JEM 2100 electron microscope (JEOL) operated at 200 kV. Size histograms of the samples were derived from TEM images using the software ImageJ.61 In the case of rod-shaped particles, the length and the width of each particle were determined from TEM images and the volume of each particle was calculated by assuming a cylindrical shape. The radius of a sphere with identical volume was then calculated from this value and used for the histogram. X-ray powder diffraction measurements were performed on an X’Pert Pro Diffractometer (PANalytical) with Bragg−Brentano geometry using Cu Kα radiation (40 kV, 40 mA) and a 2Θ step size of 0.0334°. The elemental composition of samples was determined with a 1 kW AXIOSmAX X-ray fluorescence spectrometer (PANalytical).

injection of large solvent volumes is not required.51 In the case of the lighter lanthanides RE = Ce, Pr and Nd, however, the synthesis of monodisperse nanocrystals of the β-phase proved to be significantly more difficult.56 Mai et al. mentioned βNaPrF4 and β-NaNdF4 in their pioneering work on the oleic acid based synthesis of the whole series of NaREF4 (RE = Pr− Lu, Y) particles but did not describe the synthesis of β-NaLaF4 and β-NaCeF4.54 Chow et al. obtained high-quality β-NaLaF4 particles by the decomposition of the metal trifluoroacetates in oleylamine but mentioned that the particles are formed only under drastic reaction conditions, i.e., at very high temperatures of 345 °C, and in the presence of excess trifluoroacetic acid.57 Y. Li et al. investigated the solvothermal synthesis of β-NaCeF4 in oleic acid/ethanol mixtures and reported the formation of rather large nanorods with a length of approximately 200 nm.58 Wu et al. prepared small β-NaCeF4 particles by using diethylene glycol as solvent and poly(acrylic acid) (PAA) as stabilizer but obtained comparatively broad size distributions.59 In order to identify the cause for the difficult formation of these NaREF4 nanomaterials, we have developed procedures to prepare sub-10 nm particles of both crystal phases. As in our previous work on NaSmF4, NaEuF4, NaGdF4, and NaTbF4 nanocrystals,51 we could then purify the sub-10 nm particles and use them as a single-source precursor for the synthesis of larger β-phase particles of the same material. The final particles are thereby formed solely by Ostwald ripening and in the absence of any molecular precursors apart from the “monomers” exchanged among the particles during Ostwald ripening. This “clean” reaction allows us to identify the problems as well as the conditions to obtain nearly monodisperse β-NaREF4 particles of the lighter lanthanides.

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EXPERIMENTAL SECTION

Materials. Sodium oleate (82%, Sigma-Aldrich), ammonium fluoride (98%, Sigma-Aldrich), oleic acid (90%, Alfa Aesar), octadecene (90%, Alfa Aesar), sodium hydroxide (>98%, Fluka), methanol (99.8%, Sigma-Aldrich), and hydrated rare earth chlorides of NdCl3, PrCl3, and CeCl3 (99.9%, Treibacher Industries AG) were used as received. Sodium fluoride was freshly prepared by slowly adding a solution (0.05 mol/L) of NaOH in methanol to a suspension (0.05 mol/L) of NH4F in methanol and stirring the mixture for 30 min at room temperature. The precipitate of NaF was isolated by centrifugation, washed two times with methanol, and dried at room temperature. Synthesis of Rare Earth Oleates. The rare earth oleates were prepared by reacting solutions of the corresponding rare earth chlorides with sodium oleate, as given in the literature.60 In a typical preparation of neodymium oleate, 60 mmol of neodymium chloride (NdCl3∙ 6H2O, 21.52 g) and 180 mmol of sodium oleate (54.8 g) were dispersed in 120 mL of ethanol, 90 mL of water, and 210 mL of hexane, and the resulting turbid mixture was heated under reflux (at about 60 °C). After 14 h of heating, the transparent pale purple colored organic phase containing the neodymium oleate was separated. The neodymium oleate was isolated as a waxy solid by removing the hexane with a rotavap. Praseodymium oleate (green), cerium oleate (pale brown) and lanthanum oleate (pale yellow) were prepared analogously. Synthesis of Small Cubic Phase Nanocrystals. Sub-10 nm particles of α-NaREF4 (RE = Ce, Pr, and Nd) were prepared by combining a 1:6 mixture (RE:Na) of rare earth oleates and freshly prepared sodium fluoride with oleic acid, oleylamine, and octadecene (2.5 mL of oleic acid, 2.5 mL of oleylamine, and 5 mL of octadecene per 1 mmol of rare earth oleate), and degassing the mixture on a vacuum Schlenk line (1 mbar) for 1 h at 100 °C under vigorous stirring. The reaction mixture was heated at 200 °C for 60 min under nitrogen flow and further stirring. A turbid solution was obtained due

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RESULTS AND DISCUSSION In all of our synthesis procedures, small NaREF4 particles of the cubic α-phase or the hexagonal β-phase were used as singlesource precursors. Nearly monodisperse sub-10 nm NaREF4 particles (RE = La, Ce, Pr and Nd) of the β-phase were prepared in gram amounts by the reaction of ammonium fluoride with sodium oleate and rare earth oleate dissolved in a solvent mixture of oleic acid, oleylamine, and octadecene. TEM images of the particles are shown in Figure 1. The use of oleylamine as part of the solvent mixture was found to be crucial to obtain particles of the β-phase. In the case of cerium and praseodymium, CeF3 and PrF3 particles are formed instead, if oleylamine was omitted. Size histograms corresponding to the TEM images in Figure 1 are given in the Supporting 5690

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Figure 2. TEM images and corresponding particle size histograms of polydisperse (a) β-NaCeF4, (b) β-NaPrF4, and (c) β-NaNdF4 nanocrystals obtained by heating the small β-phase particles shown in Figure 1 in oleic acid/octadecene for 1 h at 320 °C. Values for the mean particle diameter and the relative standard deviation (in %) are given in the histograms. The blue solid lines indicate Gaussian fits.

Figure 1. TEM images of nearly monodisperse sub-10 nm particles of (a) β-NaLaF4, (b) β-NaCeF4, (c) β-NaPrF4, and (d) β-NaNdF4 nanocrystals prepared by the reaction of the metal oleates with ammonium fluoride in oleylamine/oleic acid/octadecene. Size histograms are shown in Figure SI-1a (Supporting Information). Scale bars in insets correspond to 5 nm.

NaREF4 + 4 oleic acid

Information (Figure SI-1a). The figures show that the mean particle sizes ⟨d⟩ are in the range of 5−7 nm and that narrow size distributions with standard deviations of σ/⟨d⟩ = 5−10% are obtained in all cases. For all materials, the XRD data are in accord with the hexagonal β-phase of NaREF4 (Figure SI-1b, Supporting Information). Ostwald ripening of the particles is observed to be very slow in the reaction medium containing oleylamine. To investigate the growth behavior in the absence of oleylamine, the particles were precipitated with ethanol, washed, and subsequently redispersed in a solvent mixture containing only oleic acid and octadecene. Heating at 320 °C for 1 h increases both the mean particle size and the width of the size distribution. The TEM images and the corresponding size histograms as given in Figure 2. In the case of NaCeF4, NaPrF4, and NaNdF4 particles, the XRD data display no other phases after heating than the hexagonal β-phase, but a smaller peak width due to the increased particle size (Figure SI-2, Supporting Information). This growth behavior meets the expectations for Ostwald ripening and has already been observed for β-NaSmF4, β-NaEuF4, β-NaGdF4, and β-NaTbF4 nanocrystals.51,62 In the case of NaLaF4, however, the diffractogram shows the presence of some LaF3 already after 1 h of heating at 290 °C. After 1 h of heating at 320 °C, complete decomposition into LaF3 is observed (Figure SI-3, Supporting Information). Two conclusions can be drawn from these results. The first is that the β-phase of NaLaF4 is not stable in the more acidic solvent mixture containing no oleylamine. The second conclusion is that Ostwald ripening in the case of NaCeF4, NaPrF4, and NaNdF4 proceeds much faster than in the presence of oleylamine. The reason could be fast protonation of the fluoride ions, resulting in faster exchange of “monomers” among particles:

↔ Na‐oleate + RE‐(oleate)3 + 4 HF

Ostwald ripening under acidic conditions, therefore, rapidly increases the mean size of NaCeF4, NaPrF4, and NaNdF4 particles but leads also to broadening of the size distributions. To obtain narrow particle size distributions, a high concentration of monomers has to be somehow maintained during particle growth (supersaturation of monomers).63 In the case of the heavier lanthanides, the latter can be achieved by heating βphase particles in the presence of a large amount of α-phase particles, as mentioned in the Introduction.51,64 To use this method also in the case of the lighter lanthanides, we developed an improved procedure for preparing small particles of the cubic α-phase of the materials. Similar to literature procedures used for the synthesis of NaSmF4, NaEuF4, NaGdF4, and NaTbF4 particles,50,51 we replaced ammonium fluoride and sodium oleate by excess sodium fluoride to promote the formation of the cubic phase. Similar to Mai et al. in their trifluoroacetate based synthesis of α-phase NaPrF4 and NaNdF4 particles,54 we additionally replaced the oleic acid/octadecene solvent by a mixture of oleylamine, oleic acid, and octadecene with lower acidity. This modified procedure allows the preparation not only of α-NaPrF4 and α-NaNdF4 particles but also of α-NaCeF4, which, to our knowledge, has not been reported yet. Figure 3a−c displays TEM images of the resulting nanocrystals after precipitation and washing with ethanol. The XRD data are given in Figure SI-4 (Supporting Information). In all cases, the XRD peaks are well-indexed to the cubic α-phase, even for NaCeF4. We expected that heating of the α-phase particles in oleic acid/octadecene solvent would lead to Ostwald ripening until particles of the hexagonal β-phase nucleate in solution, as in the 5691

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observed, leading to a mixture of small NdF3 particles and a significant amount of large β-NaNdF4 particles: α ‐NaNdF4 particle + oleic acid → NdF3 particle + β ‐NaNdF4 particle + Na‐oleate + HF (2)

Note that the large particles of hexagonal phase NaNdF4 in fact display the expected narrow size distribution (Figure 3f). The low stability of the α-NaCeF4 and α-NaPrF4 particles is not unexpected as the α-phase is missing in the phase diagram of bulk NaCeF4, indicating that bulk α-NaCeF4 is thermodynamically instable.65 In the case of bulk NaPrF4, the α-phase exists but only within a narrow range of compositions. For all lanthanides for which the α-phase of NaREF4 exists, the phase is thermodynamically stable only at high temperatures and for sodium-deficient compositions Na1−xREF4−x. We, therefore, investigated the Na/RE ratio of our α-phase materials by X-ray fluorescence analysis (XRF). In the case of nanocrystals, the analysis of the Na/RE ratio is hampered by the fact that the ligand shell of the particles can contain some sodium oleate that cannot be washed off without negatively affecting the dispersibility of the particles. Nevertheless, the Na/RE value of all three cubic materials is significantly smaller than 1 (between 0.3 and 0.5) and 2−3 times lower compared to the corresponding β-phase particles. The high sodium deficiency of the α-phase particles is surprising as these particles are prepared in the presence of a 6-fold excess of sodium ions (see the Experimental Section). In the acidic oleic acid/octadecene solvent mixture, the sodium ions are obviously not easily incorporated into the Na1−xREF4−x lattice, at least not in case of the larger RE-ions, La, Ce, Pr, and Nd. Consequently, only partial conversion to the β-phase is possible according to Figure 3. TEM images of sub-10 nm particles of (a) α-NaCeF4, (b) αNaPrF4, and (c) α-NaNdF4 prepared by reacting the rare earth oleates with excess sodium fluoride in oleylamine/oleic acid/octadecene solvent. Products obtained by heating these particles at 320 °C in oleic acid/octadecene for 1 h are shown on the right-hand side (d−f). The XRD data given in Figure SI-3 (Supporting Information) show that the products consist of (d) CeF3 particles, (e) mainly PrF3 particles, and (f) a mixture of large NaNdF4 and small NdF3 particles, respectively.

α ‐Na1 − xREF4 − x → (1 − x) β ‐NaREF4 + x REF3

The absence of β-phase particles in Figure 3d,e indicates, however, that the sodium released by the sodium-deficient αphase particles of NaCeF4 and NaPrF4 is not even sufficient to form any significant number of β-phase particles. We, therefore, investigated the conversion of the α-phase particles in the presence of additional sodium fluoride in the solvent, since an excess of sodium and fluoride should favor the formation of the reactants on the left-hand side of the reaction 1. Thus, α-phase particles of NaCeF4, NaPrF4, or NaNdF4 were redispersed in oleic acid/octadecene, freshly prepared NaF was added, and the mixture was heated to 320 °C as above. As shown by the XRD data in Figure SI-6 (Supporting Information), complete conversion of the α-phase to the β-phase occurs. TEM images and particle size histograms of the resulting particles are shown in Figure 4. In all cases, phase-pure β-phase particles with a fairly narrow size distribution are obtained. This result shows how strongly the formation of β-NaREF4 particles is affected by the exchange of sodium ions between the crystal lattice of the particles and the acidic oleic acid/octadecene solvent. In addition, the procedure represents a convenient method to prepare NaCeF4, NaPrF4, or NaNdF4 particles with larger sizes than the sub-10 nm particles shown in Figure 1.

case of NaYF4: Yb, Er, NaSmF4, NaEuF4, NaGdF4, and NaTbF4.51 The resulting binary mixture of α- and β-phase particles should then show the aforementioned growth behavior, that is, complete dissolution of all α-phases particles along with fast particle growth and focusing of the size distribution of the β-phase particles.64 The particles of αNaCeF4, α-NaPrF4, and α-NaNdF4, however, behave differently. The TEM images of Figure 3d−f and the XRD data of Figure SI-5 (Supporting Information) show that heating of the purified α-phase particles to 320 °C in oleic acid/octadecene leads mainly to decomposition into lanthanide trifluoride particles. As precipitation of NaF is not observed, we may write α‐NaREF4 particle + oleic acid → REF3 particle + Na‐oleate + HF

(3)

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(1)

CONCLUSION In conclusion, we have investigated the synthesis and Ostwald ripening of NaLaF4, NaCeF4, NaPrF4, and NaNdF4 nanocrystals in oleic acid based solvent mixtures. Sub-10 nm particles of the hexagonal β-phase can be prepared for all four compounds

Complete decomposition is observed for cerium, whereas, in the case of praseodymium, a very small number of β-NaPrF4 particles can be detected among the vast majority of PrF3 particles. In the case of α-NaNdF4, partial decomposition is 5692

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Funding

This work was financially supported by a Georg Christoph Lichtenberg-Stipend. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

We thank Henning Eickmeier for providing the TEM images.

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Figure 4. TEM images and corresponding particle size histograms of (a) β-NaCeF4, (b) β-NaPrF4, and (c) β-NaNdF4 obtained by heating sub-10 nm particles of α-NaCeF4, α-NaPrF4, and α-NaNdF4, respectively, in the presence of sodium fluoride at 320 °C in oleic acid/octadecene for 1 h. Values for the mean particle diameter and the relative standard deviation (in %) are given in the histograms. The blue solid lines indicate Gaussian fits.

by reacting the corresponding metal oleates with ammonium fluoride in oleylamine/oleic acid/octadecene mixtures. In the presence of oleylamine, Ostwald ripening of the particles is very slow. In the absence of oleylamine, that is, when the particles are redispersed and heated in the more acidic oleic acid/ octadecene solvent, broadening of the size distribution due to Ostwald ripening takes place in the case of NaCeF4, NaPrF4, and NaNdF4, whereas decomposition to LaF3 is observed for NaLaF4. Decomposition to the trifluorides is also observed when sub-10 nm particles of the cubic α-phase of NaNdF4, NaPrF4, and NaCeF4 are heated in oleic acid/octadecene solvent. Only in the case of α-NaNdF4 is a significant amount of nearly monodisperse β-NaNdF4 particles formed in addition to small NdF3 particles. This decomposition of the α-phase particles can be suppressed by the addition of sodium fluoride to the acidic solvent mixture. In this case, Ostwald ripening in connection with the α/β-phase transition leads to the formation of larger β-phase particles with a narrow particle size distribution, similar to the heavier lanthanides.

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ASSOCIATED CONTENT

* Supporting Information S

Additional figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.H.). Author Contributions

The manuscript was written by means of contributions of all authors. 5693

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dx.doi.org/10.1021/cm502532r | Chem. Mater. 2014, 26, 5689−5694