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Influence of Different Ligand Isomers on the Growth of Lanthanide Phosphate Nanoparticles Rajesh Komban,* Karsten Koempe, and Markus Haase* Department of Inorganic Chemistry I- Materials Research, Institute of Chemistry, University of Osnabrueck, Barbarastrasse 7, 49076 Osnabrueck, Germany
bS Supporting Information ABSTRACT: The steric position of the methoxy group in different isomers of methoxycinnamic acid ligands is observed to have a strong impact on the particle growth of lanthanide phosphate nanoparticles. Very small nanoparticles with a mean size of less than 4 nm are obtained for all phosphates from lanthanum to europium, if 3-methoxycinnamic acid or trans-4-methoxycinnamic acid are used as ligands or if these methoxy isomers are replaced by cinnamic acid or phenylpropionic acid. However, if trans-2-methoxycinnamic acid is employed in the synthesis, anisotropic particle growth along the [200] axis is observed, yielding elongated nanoparticles with an average length of up to 25 nm. Our results indicate that in all cases small nanoparticles are formed first at early stages of the synthesis and that these particles grow to larger particles only if trans-2-methoxycinnamic acid is used as the ligand. We assume that the position of the methoxy group in trans-2-methoxycinnamic acid leads to an easier displacement or removal of these ligands from the surface of the small particles, resulting in a less effective shielding of the particles against particle growth.
’ INTRODUCTION The size and shape-dependent properties of inorganic nanoparticles (NPs) enable a wide range of promising applications in frontier areas such as electronics, photonics, magnets, catalysis, and biological and chemical sensors, etc.110 Among them, lanthanide phosphate (LnPO4) class of nanomaterials have attracted many researchers and the properties of these materials have been investigated with respect to solid state proton conductivity,11,12 immobilization of radioactive isotopes13 and magnetic resonance imaging (MRI).14 Lanthanide-doped LnPO4 luminescent materials have been proposed for biotechnological and biomedical applications such as biological labels and sensors.15,16 Advantages of these luminescent particles include high resistance to photobleaching, low toxicity, high working temperature, narrow emission lines, composition tunable emission from visible to infrared wavelengths, high levels of brightness, and long emission lifetime. Other projected applications propose as active components in optoelectronic devices, solid state lasers, displays, optical amplifiers, and fluorescent lamps. Controlling and monitoring the growth of NPs is therefore gaining considerable interest and the basic mechanisms determining the nucleation, the growth, and the shape of NPs have been discussed in several articles.1719 Concerning the synthesis of LnPO4 NPs, significant research efforts have been made and different methods were used such as hydrothermal,2023 solvothermal,24 microwave-assisted,2426 solgel27 and pechini r 2011 American Chemical Society
solgel,28 synthesis in high-boiling organic solvents2931 and ionic liquids.32 Moreover, the coreshell strategy has also been successfully applied to synthesis LnPO4 phosphor to improve their fluorescence efficiency.33,34 Various complexing agents such as tris-ethylhexyl phosphate and tributyl phosphate,2931,3538 sodium tripolyphosphate,39 EDTA,40 citric acid,28,41 and oleic acid42 have been used in the synthesis. For the present study we used phenylpropionic acid (PhyProA), trans-cinnamic acid (CinA) and three commercially available isomers of methoxycinnamic acid (i.e., trans-2-methoxycinnamic acid (2-OmeCinA), 3-methoxycinnamic acid (3-OmeCinA) and trans-4-methoxycinnamic acid (4OmeCinA) as complexing agents. Because of their interesting optical properties, such as narrow emission lines and long luminescence life times, compounds of the lanthanides with carboxylic acids have been intensively investigated. One example are europium compounds with cinnamic acid, which display very effective energy transfer from the triplet state of cinnamic acid to the f-electron levels of europium.43 In the synthesis procedures presented here, lanthanide cinnamates are employed as precursors and the energy transfer is used to show that europium ions are attached to cinnamate ligands. It is found that the steric position of the methoxy group in
Received: August 5, 2010 Revised: February 1, 2011 Published: March 22, 2011 1033
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’ EXPERIMENTAL SECTION Synthesis. In a typical reaction, 3 mmol of the hydrated lanthanide chloride (99.9% Treibacher) were dissolved in 10 mL of methanol, the clear solution was transferred into a 100 mL three-necked round-bottom flask and 30 mL of diphenyl ether (98%, Merck) were added. An amount of 12 mmol methoxy cinnamic acid (2.138 g, Alfa Aesar, 98%) or CinA (1.778 g, Fulka, 98%) was added under stirring. Then the methanol was removed at 30 °C under rotoevaporation and subsequently the water was released by the hydrated metal chloride which was distilled off under vacuum by slowly increasing the temperature to 105 °C. After cooling the suspension obtained to below 50 °C, 9 mL of a previously prepared solution of 0.5 M phosphoric acid (99%, Fluka) and 0.75 M tridodecyl amine (TDDA, 95%, Fluka) in diphenyl ether were added and the temperature subsequently raised to 200 °C under dry nitrogen. At approximately 140 °C the turbid solution became transparent. However, the solution became turbid again after the final temperature of 200 °C had been reached. After 17 h of heating at 200 °C, the solution was allowed to cool down to room temperature, and the nanocrystals were precipitated with methanol. Finally the particles were washed 4 times with methanol and vacuum-dried at 50 °C. Characterization. X-ray diffraction (XRD) patterns of powder samples were recorded with a PANalytical: X’pert Pro system using Cu KR radiation. The luminescence spectra of powder samples were measured with a fluorolog 3 spectrometer (Horiba). Each powder was filled in a 1 mm quartz cuvette and measured right angle, using a solidsample holder (model 1933, Horiba). 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. Infrared (IR) spectra of the samples were measured with a vertex 70 FTIR-spectrometer (Bruker) using a single reflection diamond ATR system (golden gate). Rietveld Refinements. The X-ray diffraction patterns of the samples were analyzed by the Rietveld method using ICSD data sets for the monoclinic lanthanide phosphates (LaPO4, 079747; CePO4, 079746; PrPO4, 062161; NdPO4, 062162; SmPO4, 079751; EuPO4, 079752) and the FullProf program.44,45 In all cases, the nanostructures were assumed to be strain-free and the ThompsonCoxHasting function was used as the profile function. The scale factor and the lattice parameters were refined first followed by background parameters and peak profiles. Subsequently, the parameters corresponding to texture, size and shape of the nanoparticles were refined. To account for the anisotropic width of the Bragg peaks, the structure model ISize Model-1 was used in the refinement procedure. In this model the particles are assumed to have rod shape with the main axis of the rod being given by a predefined [hkl] direction. The length and the width of the rod determine the average thickness of the rod along each [hkl] direction. From these thickness values the widths of the corresponding Bragg peaks are calculated by using a Scherrer type formula. Thus, the diameter and the length of the particles are directly obtained from the Rietveld data. Details can be found in the literature.44,45 In our case, the length of the rod shaped particles correspond to the dimension along the main axis in [200] direction and their diameter is given by the width in the [002] direction.
’ RESULTS AND DISCUSSION Figure 1A depicts molecular models (energy-minimized models, Argus Lab software) of the carboxylic acid ligands used in this work, that is, cinnamic acid (CinA), trans-2-methoxycinnamic acid (2-OmeCinA), 3-methoxycinnamic acid (3-OmeCinA), trans-4-methoxycinnamic acid (4-OmeCinA), and phenylpropionic
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acid (PhyProA). Many carboxylic acids form crystalline salts with lanthanide ions and some of these salts show efficient energy transfer between the organic acid ligand and the ion. One such example is the europium salt of cinnamic acid, where efficient Eu3þ ff emission is observed upon excitation of the strongly absorbing cinnamic acid moiety. The luminescence excitation spectra (λem = 613 nm) in Figure 1B and the emission spectra (λex = 340 nm) in Figure 1C show that this energy transfer is also observed for the europium salts of the methoxy isomers of CinA. All these compounds display broad bands in the UV-region of the excitation spectra caused by the absorption bands of the ligands. The energy transfer verifies that not only CinA but also the methoxycinnamic acid ligands bind to Eu3þ, independent of the position of the methoxy group. No energy transfer is observed between PhyProA and europium although the phenyl group of the acid does absorb UV light below 275 nm. This is expected since the phenyl group and the COOH-group do not form a conjugated system due to the absence of the double bond in this molecule. Consequently, the excitation spectrum of europiumPhyProA shows only the ff transitions of the Eu3þ-ions above 275 nm and the compound is poorly exited at 340 nm (Figure 1Ce). All europium compounds form crystalline solids different from EuCl3 as shown by their XRD data given in the Supporting Information (Figures S1 and S2). Very recently, the crystal structures of the lanthanide cinnamates have been solved by Wong et al.46 The XRD patterns of our lanthanide cinnamates including europium cinnamate are in accord with these data (Supporting Information Figure S1A). The XRD data of the lanthanide salts of the methoxy cinnamates and of PhyProA differ from those of the cinnamates and their crystal structures are not yet known (Supporting Information Figures S1 and S2). The IR spectra show for all compounds a shifting of the carbonyl stretching frequency to lower wavenumbers compared to the respective carboxylic acid (Supporting Information Figure S3AE), which indicates the formation of the carboxylate anions in the metal salts. All europium compounds react with phosphoric acid when they are heated in the high boiling solvent mixture. Even if the reaction mixtures are heated only to 120 °C and subsequently quenched, the addition of methanol leads to the precipitation of a solid product in all cases. The XRD patterns of these products are very similar and are thus independent of the solid state structure of the europium complex employed or the nature of its carboxylic acid ligand (Figure 2). In all cases the peaks are strongly broadened indicating a very small particle size of the crystallites. In fact, similar XRD patterns were reported earlier for a different synthesis of EuPO4 and GdPO4 nanoparticles for which a size of about 23 nm was determined by small-angle X-ray scattering.30 The results therefore indicate that already at low temperatures very small nanoparticles are formed as the first product, similar to other synthesis procedures for lanthanide phosphate nanoparticles.29 If we assume a metal-to-phosphate ratio of unity in the small EuPO4 nanoparticles, yields in the range of 90% are calculated from the amounts of obtained precipitate. If the reaction mixtures are heated to 200 °C and kept at this temperature for 17 h, particle growth is observed in the case of 2-OmeCinA. This is shown in Figure 3A displaying five XRD patterns of EuPO4 synthesized with PhyProA, CinA, 4-OmeCinA, 3-OmeCinA, or 2-OmeCinA, respectively. It is clearly seen that only in the presence of 2-OmeCinA the EuPO4 nanoparticles grow to a particle size, which is large enough to determine the crystal phase from the XRD pattern. The peak positions are well 1034
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Figure 1. (A) Molecular models of the ligands used in this study. From left to right: CinA, 2-OmeCinA, 3-OmeCinA, 4-OmeCinA, and PhyProA (different colors represent different atoms: gray, carbon; white, hydrogen; red, oxygen). (B) Excitation (λem 613 nm) and (C) emission (λex 340 nm) spectra of the europium complexes of (a) CinA, (b) 2-OmeCinA, (c) 3-OmeCinA, (d) 4-OmeCinA, and (e) PhyProA.
Figure 2. XRD pattern of EuPO4 nanocrystals synthesized at 120 °C in the presence of (a) PhyProA, (b) CinA, (c) 4-OmeCinA, (d) 3-OmeCinA, and (e) 2-OmeCinA.
matched with those of monoclinic EuPO4 (JCPDS no. 83-0656), as given in the figure (Figure.3A). A closer inspection of the pattern reveals a smaller peak width of the [200] peak compared to the width of, for instance, the [002]-peak. In the case of PhyProA, CinA, 4-OmeCinA, and 3-OmeCinA the XRD peaks remain strongly broadened as in Figure 2. The growth process in the presence of 2-OMeCinA was studied further by quenching the reaction at different temperatures. The diffraction patterns of the resulting set of samples (Figure 3B) show that the more
narrow peaks of the larger EuPO4 particles are observed only when the reaction temperature exceeds approximately 140 °C. XRD patterns analogous to those given in Figure 3 are displayed in the Supporting Information for LaPO4 (Figure S4) and NdPO4 (Figures S5A and S5B) nanoparticles. In fact, similar results are obtained for all the lighter lanthanides. Figure 4A and B display the XRD patterns of all lanthanide phosphate samples prepared in the presence of 4-OmeCinA and 2-OmeCinA, respectively. In the case of 4-OmeCinA the XRD patterns show very broad diffraction peaks and the series displays a similar variation as was observed earlier for LnPO4 nanoparticles prepared in the presence of trialkyl phosphate ligands.30 The corresponding bulk phosphates are known to crystallize in the monoclinic monazite phase from lanthanum to gadolinium whereas the tetragonal xenotime phase is observed from dysprosium to lutetium phosphate.30,47 Terbium phosphate at the border of the two phase regions is reported to be dimorphic.47 Similar to our earlier results, the peak width in the XRD data given in Figure 4A indicate that very small nanoparticles are obtained in the vicinity of the monoclinic/tetragonal phase transition regime. Slightly larger particle sizes are obtained for LaPO4 and CePO4, where the XRD patterns already indicate the formation of the monazite phase. If 4-OmeCinA is replaced by 2-OmeCinA, the peak broadening is much less pronounced and the peak positions are in accord with the monoclinic monazite phase. Formation of the monazite phase is now observed from lanthanum to europium and only in the case of gadolinium phosphate, the next neighbor to the dimorphic terbium phosphate, very small particles are obtained. 1035
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Figure 3. (A) XRD pattern of EuPO4 nanocrystals synthesized at 200 °C in the presence of (a) PhyProA, (b) CinA, (c) 4-OmeCinA, (d) 3-OmeCinA, and (e) 2-OmeCinA. (B) XRD pattern of EuPO4 synthesized in the presence of 2-OmeCinA obtained by quenching the reaction at different temperatures.
Figure 4. XRD patterns of lanthanide phosphates synthesized in the presence of (A) 4-OmeCinA and (B) 2-OmeCinA, where (a) LaPO4, (b) CePO4, (c) PrPO4, (d) NdPO4, (e) SmPO4, (f) EuPO4, and (g) GdPO4.
Rietveld fits of the XRD data (Figure 4B) from lanthanum to europium phosphate are depicted in Figure 5 (where red circles, black line, and the green vertical lines correspond to the data point, the calculated XRD pattern and the peak positions, respectively). The deviations between the measured and the calculated pattern are very small, as given by the residua (blue lines) in the figure and the low R-factors (Rp < 1.5 and Rwp < 2) of the refinement. Rietveld fits of high quality require to account for the strong anisotropic broadening of the peaks by using a rodmodel for the particle shape with a preferred growth direction along the [200] axes. Therefore, the mean length of the particles in the direction of the [200] planes, as well as the mean diameter in the [002] direction, was obtained from the refinements. These values are plotted in Figure 6A and show that the aspect ratio of the nanorods increases from Nd to Eu. They are in accord with the average particle size and shape determined from TEM images as
shown for CePO4 and EuPO4 nanoparticles in Figure 6B and C, respectively. Since the results given above indicate that small nanoparticles are formed as the first product in all cases and in high yield, the larger particles displayed in Figure 6 must grow from these small particles, either by Ostwald ripening or by an attachment and fusion process. After the supply of small particles is spent, only small changes of the particle size are observed. This is shown in Figure 7, displaying the XRD refined data of the dimensions of the elongated particles after prolonged heating at 200 °C (corresponding refined diffractograms are presented in Figure S6 of the Supporting Information). Even after 50 h of heating at 200 °C, the average dimensions of the crystalline domains increased only by less than 10%, indicating that particle growth is slow. In the case of europium phosphate particles, the presence of the ligands on the particle surface can be investigated by luminescence excitation spectroscopy. As in the case of the 1036
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europium cinnamates (Figure 1), energy transfer from the ligands to europium lead to a broad UV-band in the excitation spectrum of the particles. However, these excitation bands are
weaker than for the corresponding complexes, indicating that a smaller number of europium ions are bound to the ligands (Supporting Information Figure S7). This is expected since only the surface ions of the particles can bind to the ligands. The weakest excitation band is observed in the case of 2-OmeCinA which could be taken as an indication that only a small number of these ligands are attached to the particles surface. However, as
Figure 5. Rietveld fits of the XRD patterns of figure 4B, where (a) LaPO4, (b) CePO4, (c) PrPO4, (d) NdPO4, (e) SmPO4, and (f) EuPO4.
Figure 7. Length and diameter of EuPO4 nanorods synthesized in the presence of 2-OmeCinA at 200 °C with different dwell times.
Figure 6. (A) Length and diameter of LnPO4 particles (Ln = LaEu), TEM images of (B) CePO4 and (C) EuPO4 synthesized at 200 °C for 17 h in the presence of 2-OmeCinA. 1037
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’ CONCLUSIONS Our results show that the growth of nanoparticles can be strongly affected by using different isomers of the capping agent in the synthesis. In the case of methoxycinnamic acid, the position of the methoxy group in different isomers of the ligands determines the final size of LnPO4 NPs. At an early stage of the synthesis small nanoparticles with a mean size of less than 4 nm are formed in the presence of all ligands, that is, 2-OmeCinA, 3-OmeCinA, 4-OmeCinA, CinA, or PhyProA. However, only in the case of 2-OmeCinA these particles grow further to larger elongated particles. We assume that the position of the methoxy group in 2-OmeCinA leads to an easier displacement or removal of these ligands during particle growth, resulting in a less effective shielding of the small particles against growth. ’ ASSOCIATED CONTENT Figure 8. TGA curves of EuPO4 nanocrystals synthesized in the presence of (a) 2-OmeCinA, (b) 3-OmeCinA, and (c) 4-OmeCinA at 140 °C.
the 2-OmeCinA ligand leads to the largest particle size, the latter conclusion cannot be drawn with certainty due to the smaller surface-to-bulk ratio of the particles in this case. On the basis of these results, we propose a growth mechanism where primary nanoparticles with a size of less than 4 nm are formed in all cases at reaction temperatures below 140 °C. When the temperature is increased to 200 °C, the primary particles form larger elongated particles only if 2-OmeCinA is used. As the concentration of tridodecylamine, the second coordinating compound in the reaction mixture, is the same in all cases, the 2-OmeCinA ligand obviously protects the small particles less well against this growth or attachment process. We assume that the steric position of the methoxy group in 2-OmeCinA leads either to a less dense packing of the ligands on the surface of the small particles or to a weaker binding of the ligand. Since the particles predominantly grow along the “a” axis, this effect seems to be most pronounced for the [001] facets. Thermogravimetric measurements of the samples prepared at 140 °C show a similar loss of mass (Figure 8), independent of the ligand isomer used in the synthesis. IR spectra of the samples recorded before and after heating to 500 °C show that the sharp IR bands of the ligands have disappeared after heating whereas the bands corresponding to the phosphate stretching and bending modes are barely affected (Figure S8A to S8C in the Supporting Information. PO stretching mode at 1030 cm1 and OdPO bending mode at 616 cm1).27 These results thus indicate the presence of comparable amounts of organic ligands at the particle surface and the removal of the ligands upon heating to 500 °C. Therefore, the coverage of the particle surfaces with ligand molecules seems to be similar in all cases. We take this as an indication that the higher growth rate in the case of 2-OmeCinA is probably caused by an easier displacement or removal of these ligands during particle growth. A more detailed investigation would require the calculation of the binding energies of the ligands and their interaction energy on the different crystal facets. Owing to the heavy elements involved, such a theoretical description is far beyond the scope of this paper. Our results, however, indicate that it should be worthwhile investigating the properties of isomers of other ligands employed in the synthesis of nanocrystals, such as oleic acid, TOPO, etc.
bS
Supporting Information. Figures showing XRD patterns of the CinA-lanthanide complexes, XRD patterns of the complexes of PhyProA-Eu, CinA-Eu, and EuCl3, XRD patterns of the 2OMeCinA-lanthanide complexes, XRD patterns of the 3OMeCinA-lanthanide complexes, XRD patterns of the 4OMeCinAlanthanide complexes, IR spectra of the carboxylic acids used as ligands for the synthesis, IR spectra of the CinA-lanthanide complexes, IR spectra of the 2OmeCinA-lanthanide complexes, IR spectra of the 3OMeCinA-lanthanide complexes, IR spectra of the 4OmeCinA-lanthanide complexes, XRD patterns of LaPO4 synthesized in the presence of CinA, 4-OmeCinA, 3-OmeCinA, and 2-OmeCinA, XRD patterns of NdPO4 synthesized in the presence of CinA, 4-OmeCinA, 3-OmeCinA, and 2-OmeCinA, XRD pattern of NdPO4 synthesized in the presence of 2-OmeCinA obtained by quenching the reaction at different temperatures, refined XRD patterns of EuPO4 synthesized in the presence of 2-OmeCinA at different dwell times at 200 °C, excitation spectra of europium phosphate synthesized at 200 °C/ 17 h, IR spectra of europium phosphate synthesized at 140 °C in the presence of 2-OmeCinA, IR spectra of europium phosphate synthesized at 140 °C in the presence of 3-OmeCinA, and IR spectra of europium phosphate synthesized at 140 °C in the presence of 4-OmeCinA. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (R.K.); markus.haase@ uni-osnabrueck.de (M.H.). Tel: 0049 541 969 2386. Fax: 0049 541 969 3323.
’ ACKNOWLEDGMENT We thank Mr. Henning Eickmeier, Mrs. Marianne Gather and Mrs. Kerstin Ruecker, University of Osnabrueck, Germany, for TEM investigations, X-ray powder diffraction, and thermogravimetric analysis, respectively. ’ REFERENCES (1) Goesmann, H.; Feldmann, C. Angew. Chem., Int. Ed. 2010, 49, 1362–1395. (2) A special Issue on Nanoscale Materials. Acc. Chem. Res. 1999, 32, 387-454. (3) Alivisatos, A. P. Science 1996, 271, 933–937. 1038
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