Investigation of the Early Stages of Growth of Monazite-Type

J. Phys. Chem. C 2009, 113, 4763–4767

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ARTICLES Investigation of the Early Stages of Growth of Monazite-Type Lanthanide Phosphate Nanoparticles Katharina Hickmann,* Vanessa John, Anke Oertel, Karsten Koempe, and Markus Haase Department of Inorganic Chemistry I-Materials Research, Institute of Chemistry, UniVersity of Osnabrueck, Barbarastraβe 7, 49069 Osnabrueck, Germany ReceiVed: October 9, 2008; ReVised Manuscript ReceiVed: January 27, 2009

The growth of NdPO4 nanocrystals in diphenyl ether based reaction medium has been monitored by optical spectroscopy. Since the crystal field splitting and the intensity of the Nd3+ f-f transitions are affected by the coordination of the Nd3+ ions, different stages of the growth of NdPO4 nanocrystals can be distinguished by UV-visible absorption spectroscopy. It is found that a first reaction in homogeneous solution occurs already at room temperature when the phosphate-containing precursor solution is combined with the solution of the molecular Nd3+ precursor. Our results clearly indicate that a sol of NdPO4 nanoparticles with a mean particle diameter below 3 nm is thereby formed as the first product. Upon heating to 200 °C, this first product undergoes a complex sol-gel-sol transition, leading to a colloidal solution of 4-5 nm diameter NdPO4 nanocrystals. Further heating at 200 °C leads to annealing of the nanocrystalline lattice, but not to further particle growth if the amine concentration is sufficiently high. Introduction Because of the wide range of applications of nanomaterials, the controlling and monitoring of the growth of nanometerscale particles attracts large interest.1 Basic mechanisms controlling the nucleation, the growth, and the shape of nanocrystals2-4 as well as the in situ observation of the growth of nanoparticles on substrates have been discussed in several articles.5-8 For the rational development of synthesis strategies for nanocrystals with predetermined crystallinity, size and shape distribution methods are highly desired, which allow one to directly follow the nucleation and the growth of nanocrystals in solution.5,6,9-14 In this report, utilization of optical spectroscopy allows us to approach the growth of lanthanide phosphate nanoparticles and propose a growth mechanism. Owing to their variety of potential applications ranging from biolabeling15,16 and magnetic resonance imaging (MRI)17 to lighting,18,19 the synthesis of lanthanide-doped large bandgap materials has been a research issue for years. Phosphates of the lighter lanthanides, all of which crystallize in the monazite phase, form an important class of host lattices suitable for doping with lanthanide (III) ions. Nanocrystals of the lanthanide phosphates have been prepared by a variety of methods, including hydrothermal methods,20,21 solvothermal methods,22 and synthesis in high-boiling organic solvents20,23,24 and in ionic liquids.25 Protocols for the synthesis of nanoparticles as well as nanorods26,27 have been published by several groups. To investigate the early stages of particle growth, we determine NdPO4 nanoparticles because Nd3+ displays a large number of f-f transitions in the visible range, which can be * Corresponding author. E-mail: [email protected]. Phone: (+49)541-969-2382. Fax: (+49)541-969-3323.

easily investigated by UV-visible (UV-vis) absorption spectroscopy. The synthesis method applies to all phosphates of the lanthanides and leads to narrow particle size distributions for all lanthanide ions.23,28,29 Here we show that these particles grow by a complex sol-gel-sol mechanism, which, to the best of our knowledge, has not yet been described for lanthanide systems. Experimental Section Synthesis. NdPO4 and CePO4:Tb nanoparticles were prepared in a coordinating solvent mixture with an excess of phosphoric acid as described previously.23,28 To a clear solution of lanthanide chlorides (NdCl3 · 7H2O (3.59 g, 10 mmol, 99.9%, Treibacher) or CeCl3 · 7H2O (2.80 g, 7.5 mmol, 99.9%, Treibacher) and TbCl3 · 6H2O (0.94 g, 2.5 mmol, 99.9%, Treibacher) in methanol p.a. (approximately 10 mL), tributyl phosphate (TBP, 10.9 mL, 40 mmol, 99%, Riedel de Ha¨en) was added. The methanol was subsequently removed with a rotary evaporator. Diphenyl ether (30 mL, 98%, Merck) was added, and the water released by the hydrated metal chloride was distilled off under vacuum at 30-105 °C. At a temperature of less than 50 °C trihexyl amine (THA, 13.6 mL, 40 mmol, 99%; Fluka) was added, followed by a 2 M solution of phosphoric acid (7.0 mL, 14 mmol, 99%, Fluka) in dihexyl ether (97%, Fluka). Subsequently, the reaction mixture was heated to 200 °C under dry nitrogen. The heating was stopped after 0.5 h at 200 °C or after 16 h at 200 °C, the solution was allowed to cool down to room temperature, and 250 mL of methanol was added to the reaction mixture. The resulting clear solution was loaded into a diafiltration cell (Millipore) equipped with a 5000 Da filter. The volume of the colloidal solution was reduced to 50 mL, 250 mL of methanol was added, and diafiltration was resumed. This purification procedure was repeated four times. Nanocrystal

10.1021/jp808933f CCC: $40.75  2009 American Chemical Society Published on Web 03/05/2009

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Figure 1. Room temperature Tb3+ luminescence excitation spectra of the CePO4:Tb reaction mixture before heating to 200 °C (λobs ) 543 nm). The spectra are recorded before (dotted line) and after the addition of phosphate (black line), respectively. Directly after the addition of phosphate the Ce3+ f-d excitation band is observed (black line), indicating that the precursors react already at room temperature.

powders were obtained by removing the methanol from the purified colloid with a rotary evaporator. Characterization. UV-vis absorption spectra were recorded in 1 cm cuvettes using a Cary 6000i dual-beam spectrometer. Luminescence excitation spectra were measured with a Fluorolog 3 Spectrometer (SPEX). X-ray diffraction (XRD) patterns of powder samples were recorded with a PANalytical: X’pert Pro system using Cu KR radiation. Transmission electron microscopy (TEM) images were taken with a JEOL JEM 2100 transmission electron microscope using a LaB6-Cathode and an acceleration voltage of 200 kV. The viscosity of the reaction mixture was measured at 25 °C with an AMVn Microviscosimeter (Anton Paar). Results and Discussion The synthesis of lanthanide phosphate nanocrystals in diphenyl ether at 200 °C is based on the reaction between trihexyl ammonium phosphate and the coordination compound formed by the lanthanide chloride and TBP. If cerium chloride and a smaller amount of terbium chloride are employed, CePO4:Tb nanocrystals are formed, which show strong Tb3+ emission under UV-excitation. Because of the close proximity of cerium and terbium ions in the monazite lattice, efficient energy transfer between cerium and terbium takes place in that compound. The terbium excitation spectrum of CePO4:Tb nanocrystals therefore displays a strong band below 300 nm caused by the cerium 4f-5d transition. Figure 1 shows that this band is absent in the Tb3+ excitation spectrum of a diphenyl ether solution of CeCl3 · nTBP and TbCl3 · nTBP, indicating that the two compounds have a large mean distance in the solution. Weak Tb3+ emission is observed only if the Tb3+ ions are excited directly via their 7 F6f5D3 transition at 377 nm. However, already after the addition of THA and phosphoric acid at room temperature, which does not lead to changes apparent by eye, the broadband of the cerium 4f-5d transition is observed (Figure 1, black line). This shows that already at 298 K a reaction takes place leading to a product where cerium ions and terbium ions are already in close proximity to each other. Obviously, the high temperatures (200 °C) employed to obtain nanocrystals of high quality are not required for the molecular reaction between LnCl3 · nTBP

Figure 2. UV-vis absorption spectra of the reaction mixture at different stages of the synthesis displaying different crystal field splittings of the Nd3+ ions. Displayed are the solution spectra of hydrated NdCl3 in methanol (a), of the NdCl3 · nTBP complex in diphenylether (b), of the NdCl3 · nTBP complex in diphenylether and amine before (c) and after (d) the addition of phosphate, and of NdPO4 nanocrystals after heating at 200 °C for 16 h (e).

and trihexyl ammonium phosphate. Instead, the reaction seems to proceed via several intermediates some of which are formed already at room temperature. To investigate the early stages of particle growth we replaced Ce3+ and Tb3+ by Nd3+ because Nd3+ displays a large number of f-f transitions in the visible range which can be easily investigated by UV-vis absorption spectroscopy. The results obtained with Nd3+ should nevertheless apply to all larger ions of the lanthanide series since it has been shown earlier that Ce3+, Nd3+, and Pr3+ behave very similar in the diphenyl ether synthesis of monazite type lanthanide phosphate nanocrystals.23 Figure 2 shows UV-vis absorption spectra of the transparent reaction mixture at different stages of the NdPO4 nanoparticle synthesis. All spectra show the same Nd3+ f-f transitions from the 4I9/2 ground-state to higher excited states of the Nd3+ ion. The transition probabilities as well as the crystal field splitting of the transitions vary from one spectrum to the other indicating that the coordination of the Nd3+ ions changes during the synthesis. Spectrum a in Figure 2 shows the solution spectrum of hydrated NdCl3 in methanol. Spectrum b displays the absorption of the neodymium complex formed by TBP and NdCl3 in diphenyl ether after the removal of water. The spectrum is well structured, indicating that the crystal field and, hence, the ligand sphere is very similar for all Nd3+ ions in the sample. The addition of THA to the solution of the complex does not affect the crystal field splitting of the transitions excluding a direct interaction of the amine with the Nd3+ center (Figure 2c). The spectrum changes significantly, however, when the solution of phosphoric acid is added in the next step of the synthesis (Figure 2d). After the addition, broad structureless peaks are observed for all transitions, indicating that a product is formed that contains several different Nd3+ sites. This shows clearly

Growth of Lanthanide Phosphate Nanoparticles

Figure 3. Effect of the addition of aliquots of phosphoric acid on the UV-vis absorption spectrum of the NdCl3 · nTBP complex dissolved in diphenyl ether and amine. (a) Spectra for molar ratios between phosphate and neodymium from 0 and 0.3. (b) Spectra for molar ratios between phosphate and neodymium from 0.3 and 1.0. (c) Spectra for molar ratios between phosphate and neodymium from 1.0 and 1.4. (d) Spectra at a molar ratio between phosphate and neodymium of 1.4 before and after gelation of the colloidal solution. Each set of numbers is given in the same order as the absorbance values in the spectra between 580 and 585 nm. The spectra in part d are offset for clarity.

Figure 4. NdPO4 gel formed at room temperature without heating at 200 °C. Shown is a sample after storage for several weeks.

J. Phys. Chem. C, Vol. 113, No. 12, 2009 4765 that a fast reaction between the neodymium complex and the phosphoric acid takes place at room temperature, in accord with the result discussed above for CePO4:Tb nanoparticles. If the reaction mixture is then heated to 200 °C for 16 h, NdPO4 nanocrystals with a mean diameter of 4-5 nm are formed as discussed earlier.23 The crystal field splitting of the transitions in the absorption spectrum of the resulting colloid (Figure 2e) is identical to those known from bulk NdPO4 but the absorption lines are less well resolved, presumably because of the presence of a significant number of surface Nd3+ ions with different coordination. To investigate in more detail the reactions occurring already at room temperature, the phosphoric acid was added in small aliquots to the diphenyl ether solution of the neodymium complex and the amine, and the gradual transition from the well structured absorption peaks in spectrum 2c to the structureless peaks in spectrum 2d was monitored. Figure 3 shows that the absorption spectrum evolves in two consecutive steps. Up to a molar ratio between Ln3+ and PO43- of approximately 3:1, the addition of phosphoric acid leads to a first systematic change of the absorption spectrum, which is characterized by several well-defined isosbestic points (Figure 3a). The first of these points is located at 573 nm. Attempts to isolate and characterize the resulting product failed since no solvent was found that could be added to the reaction mixture without changing the absorption spectrum. The product reacts further with additional phosphoric acid, and this reaction is again characterized by several isosbestic points, the first of which is located at 575 nm (Figure 3b). If the molar ratio between Ln3+ and PO43- exceeds a value of approximately 1:1, however, further addition of H3PO4 only weakly affects the absorption spectrum (Figure 3c), which finally corresponds to spectrum d in Figure 2. We therefore conclude that the reaction is basically completed at a 1:1 ratio of Nd3+ and PO43-, indicating that the room-temperature product contains approximately equimolar amounts of neodymium and phosphate. The strong inhomogeneous broadening of the f-f transitions in Figure 3c and Figure 2d indicates that the Ln3+ ions in this product occupy several different sites, and the energy transfer

Figure 5. (a) UV-vis absorption spectra of NdPO4 nanocrystals in the reaction mixture after the addition of THA and H3PO4 (1) and after 0.5 (2) and 16 h (3) of heating at 200 °C. The inhomogeneous broadening of the transition lines decreases with time, indicating annealing of defects in the nanocrystalline lattice. (b) XRD data of NdPO4 nanocrystal powder in the reaction mixture after the addition of THA and H3PO4 (1) and after 0.5 (2) and 16 h (3) of heating at 200 °C. The similar width of the reflection peaks after 0.5 and 16 h shows that prolonged heating does not affect the particle size; i.e., particle growth is negligible. (c) TEM images of NdPO4 nanoparticles in the reaction mixture after the addition of THA and H3PO4 (1) and after 0.5 (2) and 16 h (3) of heating at 200 °C. The similar particle diameters after 0.5 and 16 h shows that prolonged heating does not affect the particle size; i.e., particle growth is negligible.

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SCHEME 1: Schematic Representation of the Different Stages of Growth of NdPO4 Nanocrystals in the Diphenyl Ether/Amine/TBP Solvent System

displayed in Figure 1 shows that the distance between adjacent ions is small. These properties are in accord with those expected for a sol of very small LnPO4 nanoparticles. In fact, the product in the reaction mixture undergoes a slow sol-gel transition upon storage at room temperature, which is faster upon heating to 50 °C or after the addition of polar solvents (Figure 4). The absorption spectrum of the transparent gel is very similar to the spectrum of the sol, as given by Figure 3d. The small deviations are probably caused by the different crystal field of Nd3+ ions located at the interfaces forming between the particles during the gelation process. The fact that the optical properties of the wet-gel basically resemble those of the solution further substantiates our assumption that already at room temperature a colloidal solution of small primary NdPO4 nanoparticles is formed. Moreover, a sol-gel transition of CePO4 nanoparticles has also been observed in aqueous solution by Rajesh et al.30 If the organic solvents are removed from the wet-gel by washing with methanol (as given in the experimental section for the nanocrystals prepared at 200 °C), a powder is obtained, displaying the powder XRD pattern given in Figure 5b.1. The reflection peaks are strongly broadened, as expected for very small crystallites. The peak width is comparable to the broadening observed for GdPO4 nanocrystals for which a particle size of approximately 2.5 nm was deduced from small-angle X-ray scattering (SAXS) experiments.23 Although the crystal phase and the exact particle size cannot be deduced from the XRD data of the dried gel, the strong broadening is in accord with the conclusions discussed above. In the standard synthesis procedure, gelation of the reaction mixture is also observed shortly during the heating step to 200 °C. Under these conditions, the gel is not stable but forms a sol again already before the final temperature is reached. At 195 °C the reaction mixture displays a viscosity of 6.57 mPa · s, measured after cooling at room temperature. This value is only slightly larger than the value of 5.95 mPa · s observed before the addition of H3PO4. The growth mechanism therefore combines the classic sol-gel transition observed for many oxide materials and the gel-sol transitions described by Sugimoto and others for oxide, sulfide and sulfate nanoparticles, such as Fe2O3,31 TiO2,32 BaTiO3,33 CdS,34 ZnS,34 PbS,34 CuS,34 and Al3(SO4)2(OH)5 · 2H2O.35 When the viscosity has reached a low value again, nanocrystals can be isolated from the reaction mixture. Figure 5 shows that the nanocrystals after heating for 0.5 h at 200 °C are larger than the particles that formed the gel, but do not further grow in size if heating at 200 °C is extended from 0.5 to 16 h. In earlier papers, it was shown in fact that the XRD patterns after 2 and 16 h at 200 °C are very similar,29 indicating that particle growth, for instance, by Ostwald ripening, is very slow. In a recent paper, we have shown that this, however, is only the case at the high concentrations of amine used in the

standard synthesis procedure also employed here.36 Despite the fact that particle growth at 200 °C is negligible, the inhomogeneous broadening of the transitions in the absorption spectra decreases with increased reaction time, indicating that an annealing of defects takes place in the nanocrystals (Figure 5a). We assume that, at elevated temperatures, two adjacent primary particles in the gel can either separate or, with low probability, fuse together. In this case, we are able to explain both the disintegration of the gel to slightly larger sol particles and also the observed annealing effect, which would, in this model, take place primarily at the interface between two fusing particles. Since the final particles are colloidally stabilized by organic surface ligands, these molecules may also influence the probability for fusion or separation of adjacent particles. Therefore, further studies on the influence of the ligands on the sol-gelsol-transition are required. Conclusion In conclusion, we can summarize the early stages of the growth of NdPO4 nanocrystals as schematically given in Scheme 1. Very small nanoparticles of NdPO4, most likely less than 3 nm in diameter are formed already at room temperature, when phosphoric acid is combined with a diphenyl ether solution of NdCl3 · nTBP. Upon heating, these particles first form a gel network, which disintegrates again upon further heating to higher temperatures. The fragmentation of the gel leads to NdPO4 nanocrystals with a diameter of 4-5 nm, which are presumably composed of a small number of primary particles. During heating at 200 °C, the nanoparticles do not grow to larger particles under the standard synthesis conditions employed here, but the inhomogeneous broadening of the optical transitions decreases, indicating an annealing of defects in the nanocrystalline lattice. A similar growth mechanism applies to CePO4 nanocrystals and presumably to all monazite-type LnPO4 nanoparticles of the lighter lanthanides. Acknowledgment. We thank Henning Eickmeier for TEM investigations and Marianne Gather for performing X-ray powder diffraction measurements and viscosity measurements. References and Notes (1) Thornton, G. Science 2003, 300, 1378. (2) Sugimoto, T. Funtai Kogaku Kaishi 2005, 42, 478. (3) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664. (4) Coelfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (5) Yeadon, M.; Yang, J. C.; Averback, R. S.; Bullard, J. W.; Olynick, D. L.; Gibson, J. M. Appl. Phys. Lett. 1997, 71, 1631. (6) Koga, K.; Matsuoka, Y.; Tanaka, K.; Shiratani, M.; Watanabe, Y. Appl. Phys. Lett. 2000, 77, 196. (7) Renaud, G.; Lazzari, R.; Revenant, C.; Barbier, A.; Noblet, M.; Ulrich, O.; Leroy, F.; Jupille, J.; Borensztein, Y.; Henry, C. R.; Deville, J.-P.; Scheurer, F.; Mane-Mane, J.; Fruchart, O. Science 2003, 300, 1416.

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