Solvent-free synthesis of lanthanide oxide and mixed lanthanide oxide nanoparticles

Abstract

Lanthanide oxides and mixed lanthanide oxides can be produced using furnace or microwave assisted solid-state synthesis. The use of Ln-tri(methylsilyl)amide-based precursors yields spherical nanoparticles. The formation of spherical shaped nanoparticles is likely due to the preferential single-step decomposition of the Ln-TMS as well as the low activation energy to overcome decomposition. Reaction temperature, initial metal ion ratio, and reaction dwell time can be used to control the final nanoparticle size. The method enables solvent-free, high-yield synthesis of morphology-controlled lanthanide oxides.

Claims

1. A method to synthesize lanthanide (Ln) oxide nanoparticles, comprising heating of at least one Ln-tri(methylsilyl)amide precursor at a reaction temperature for a dwell time to produce Ln-oxide nanoparticles.

2. The method of claim 1, wherein more than one Ln-tri(methylsilyl)amide precursor are combined to produce mixed Ln-oxide nanoparticles.

3. The method of claim 2, wherein the mixed Ln-oxide nanoparticles comprise binary, ternary, quaternary mixed Ln-oxides.

4. The method of claim 1, wherein the at least one Ln-tri(methylsilyl)amide precursor comprises cerium (III) tri(methylsilyl)amide or samarium (III) tri(methylsilyl)amide.

5. The method of claim 1, wherein the reaction temperature is greater than 500° C.

6. The method of claim 1, wherein the reaction temperature is less than about 1000° C.

7. The method of claim 1, wherein the dwell time is greater than 5 minutes.

8. The method of claim 2, wherein the Ln-oxide or mixed Ln-oxide nanoparticles are spherical.

9. The method of claim 8, wherein the spherical Ln-oxide or mixed Ln-oxide nanoparticles have a diameter of less than 500 nm.

10. The method of claim 1, wherein the heating is performed in a microwave oven.

11. The method of claim 1, wherein the heating is performed in a thermal furnace.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The detailed description will refer to the following drawings, wherein like elements are referred to by like numbers.

[0010] FIG. 1A shows powder X-ray diffraction (PXRD) spectra of powders isolated from the Ce-precursors (acetate, nitrate, and TMS) reaction, confirming that all precursors lead to highly crystalline cerium oxide. FIG. 1B is a scanning electron microscope (SEM) image of nanoparticles synthesized from a Ce-acetate precursor at a reaction temp of 750° C. and a reaction time of 10 minutes.

[0011] FIG. 1C is an SEM image of nanoparticles synthesized from a Ce-nitrate precursor. FIG. 1D is an SEM image of nanoparticles synthesized from a Ce-TMS precursor.

[0012] FIG. 2A is a PXRD spectra of Sm.sub.xCe.sub.1−xO.sub.2−δ prepared with variable starting Ce:Sm ratios. The PXRD inset in FIG. 2B shows the downfield shift in the spectra of CeO.sub.2 upon the addition of samarium ions. FIG. 2C is an SEM image of Sm.sub.xCe.sub.1−xO.sub.2−δ nanoparticles shows spherical morphology.

[0013] FIG. 3 shows Raman spectra of CeO.sub.2 and Sm.sub.xCe.sub.1−xO.sub.2−δ.

[0014] FIG. 4A is an SEM image of Sm.sub.xCe.sub.1−xO.sub.2−δ nanoparticles synthesized with a starting Ce:Sm ratio of 4:1, a reaction time of 10 minutes, at a temperature of 500° C. FIG. 4B is an SEM image of Sm.sub.xCe.sub.1−xO.sub.2−δ nanoparticles synthesized at 750° C. FIG. 4C is an SEM image of Sm.sub.xCe.sub.1−xO.sub.2−δ nanoparticles synthesized at 1000° C. The corresponding particle size distributions are shown in FIGS. 4D-F.

[0015] FIG. 5A is an SEM image of Sm.sub.xCe.sub.1−xO.sub.2−δ δ nanoparticles synthesized with a starting Ce:Sm ratio of 4:1, a reaction temperature of 750° C., and a reaction time of 5 minutes. FIG. 5B is an SEM image of Sm.sub.xCe.sub.1−xO.sub.2−δ δ nanoparticles synthesized at a reaction time of 10 minutes. FIG. 5C is an SEM image of Sm.sub.xCe.sub.1−xO.sub.2−δ δ nanoparticles synthesized at a reaction time of 20 minutes. The corresponding particle size distributions are shown in FIGS. 5D-F.

[0016] FIG. 6A shows the thermogravimetric analysis (TGA) of cerium-based precursors. FIG. 6B shows the TGA of samarium-based precursors.

[0017] FIG. 7 is a potential energy surface (PES) scan of Ce-TMS molecule. Insets are snapshots of Ce-TMS molecule with Ce-N distances of 2.30 Å and 4.95 Å. Atom colors: Ce (pink), Si (yellow), C (grey), N (blue), and H (white).

[0018] FIG. 8 is a schematic illustration of the first step of decomposition for Ln-acetate and Ln-TMS precursors. Atom colors: Ln (pink), C (grey), H (white), Si (yellow), N (blue).

[0019] FIG. 9 is a graph showing Sm.sub.xCe.sub.1−xO.sub.2−δ spherical nanoparticle formation based on Lamer nucleation theory.

DETAILED DESCRIPTION OF THE INVENTION

[0020] According to the present invention, Ln oxide and mixed Ln oxide (binary, ternary, quaternary, etc.) nanoparticles can be made using a systematic synthesis approach that uses varying reaction conditions with Ln-tri(methylsilyl)amide (TMS) precursors. In general, the Ln-TMS precursor can comprise any lanthanide series element. Temperature, dwell time, and chemical composition play a role in the generation of nanomaterials with tailored physical properties (i.e., crystalline phase, size, and morphology). The invention can use a conventional furnace as well as microwave assisted solid-state method with Ln-TMS precursors. Thermal furnace and microwave solid-state synthesis removes the need for solvents of any kind and dramatically reduces the overall reaction time.

[0021] As examples of the invention, CeO.sub.2 and Sm.sub.xCe.sub.1−xO.sub.2−δ mixed oxides were synthesized by microwave assisted solid-state reaction of TMS-based precursors. The use of TMS-based cerium precursors leads to the formation of spherical CeO.sub.2 nanoparticles. Additionally, combining TMS-based cerium and samarium precursors leads to the formation of spherical Sm.sub.xCe.sub.1−xO.sub.2−δ nanoparticles. Optimization of TMS-based Sm.sub.xCe.sub.1−xO.sub.2−δ nanoparticles can be performed by controlling the processing temperature, initial Ce:Sm ratio, and the dwell time at the final reaction temperature.

Microwave Assisted Solid-State Synthesis of Sm.SUB.x.Ce.SUB.1−x.O.SUB.2−δ Powders

[0022] Synthesis of cerium oxide (CeO.sub.2), samarium oxide (Sm.sub.2O.sub.3), and samarium-doped cerium oxide (Sm.sub.xCe.sub.1−xO.sub.2−δ) nanoparticles was accomplished via solid-state microwave irradiation of the respective metal precursors (i.e., cerium (III) nitrate hexahydrate, samarium (III) nitrate hexahydrate, cerium (III) acetate hydrate, samarium (III) acetate hydrate, cerium (III) TMS, and samarium (III) TMS). Cerium (III) TMS and samarium (III) TMS were prepared using established literature routes. See B. Donald et al., J. Chem. Soc., Dalt. Trans. 1021 (1973). CeO.sub.2 and Sm.sub.xCe.sub.1−xO.sub.2−δ nanoparticles were prepared via solid-state microwave radiation of the respective metal precursors (nitrate, acetate, TMS) in the presence of air only. All reactions involving acetate and nitrate precursors were prepared and performed in ambient conditions, while the TMS precursors were handled in a glove box prior heating in ambient conditions. In a standard synthesis method, the precursor was grounded in a mortar to insure a homogenous mixture. After mixing the precursor, the reaction was heated in a Microwave Research and Application BP-210 under atmospheric conditions at a reaction temperature, only using air as the reaction media. For the formation of Sm.sub.xCe.sub.1−xO.sub.2−δ various ratios and reaction conditions of Ce-TMS and Sm-TMS, shown in Table 1, were attempted to increase the Sm dopants concentration and control the physical nature of the material (size, morphology, and crystallinity). After the reactions were conducted as described the isolated materials were collected and analyzed without further modifications.

TABLE-US-00001 TABLE 1 Sm.sub.xCe.sub.1−xO.sub.2−δ Reaction Conditions Reaction Condition A B C D E F G H Ce:Sm Ratio 1:0 4:1 4:1 4:1 3:1 2:1 2:1 2:1 Reaction 750 1000 750 500 750 750 750 750 Temperature (° C.) Dwell Time 10 10 10 10 10 10 5 20 (min)

[0023] The robust and diverse physical properties of CeO.sub.2 and Sm.sub.xCe.sub.1−xO.sub.2−δ have led to numerous investigations detailing the morphological impact on their optical and electronic properties; however, there are no reports on understanding the generation of the aforementioned materials using traditional vs non-traditional precursors in a microwave in the absence of liquid media and surfactant. See S. Gnanam and V. Rajendran, J. Nanoparticles 2013, 1 (2013); and R. Suresh et al., Appl. Surf. Sci. 273, 457 (2013). Due to the shape-dependent properties of nanomaterials and the void in literature on a microwave assisted solid-state synthesis of various precursors, the effect of traditional and non-traditional precursors on the properties of the resultant CeO.sub.2 and Sm.sub.xCe.sub.1−xO.sub.2−δ nanoparticles was investigated.

Formation of Spherical Cerium Oxide Nanoparticles

[0024] Powder X-Ray diffraction (PXRD) data were collected on the powders isolated from the Ce-precursors (acetate, nitrate, and TMS) reaction. PXRD patterns for each sample are shown in FIG. 1A. For each sample the major peaks of the XRD patterns were indexed to the cubic fluorite CeO.sub.2 phase with the space group Fm-3m and no other impurities could be observed in the patterns. SEM images of all products generated from the microwave reaction of the individual Ce-precursors are shown in FIGS. 1B-D. FIGS. 1B and 1C show SEM images of the products formed from Ce-acetate and Ce-nitrate, respectively. For these systems, highly irregular CeO.sub.2 nanoparticles are present as well as agglomerated particles. The irregular morphology of the acetate- and nitrate-based CeO.sub.2 indicates that there are adsorbed surface species during the conversion reaction, and that transport of ions on the surfaces is not rapid enough to form regular facets based on crystal symmetry. However, FIG. 1D shows that the decomposition of Ce-TMS precursor yields CeO.sub.2 nanoparticles with a highly defined spherical morphology. The round spherical morphology of the TMS-based CeO.sub.2 system agrees with a rapid decomposition process, wherein cation transport to the nuclei is diffusion limited.

Formation of Samarium-Doped Ceria Nanospheres

[0025] Due to the serendipitous generation of spherical CeO.sub.2 nanoparticle from microwave irradiation of the Ce-TMS precursor, samarium-doped ceria (SDC) nanoparticles were synthesized by combining Ce-TMS and Sm-TMS precursors. PXRD data were collected on the powders isolated from the different reaction conditions. The crystalline product, Sm.sub.xCe.sub.1−xO.sub.2−δ, was indexed to the cubic fluorite phase, with no secondary phase present (Sm.sub.2O.sub.3; cubic la-3 phase). PXRD spectra are shown in FIG. 2A for Sm.sub.xCe.sub.1−xO.sub.2−δ generated with initial Ce:Sm ratios of 4:1, 3:1, and 2:1 and a reaction time of 10 minutes at 1000° C. Upon addition of Sm precursors, the PXRD patterns exhibit a downfield shift relative to CeO.sub.2 for all the major reflections, as shown in FIG. 2B. The shift to lower angles also shows a dependence on the amount of initial Sm-TMS precursor. The shift to lower angles is due to lattice strain from size mismatch caused by the uptake of different size Sm.sup.3+ ions, suggesting the successful incorporation of Sm.sup.3+ in the cubie lattice of CeO.sub.2. See J. P. Hos and P. G. McCormick, Scr. Mater. 48, 85 (2003). The production of Sm.sub.xCe.sub.1−xO.sub.2−δ yielded spherical nanoparticles, as shown in FIG. 2C, indicating that Sm.sup.3+ ions did not affect the overall morphology.

[0026] In order to confirm the uptake of Sm.sup.3+ ions in the cubic fluorite CeO.sub.2 lattice as well as to ascertain any structural changes, Sm.sub.xCe.sub.1−xO.sub.2−δ powders were analyzed via Raman spectroscopy. Representative Raman spectrum for as synthesized CeO.sub.2 and Sm.sub.xCe.sub.1−xO.sub.2−δ are shown in FIG. 3. Metal dioxide crystals with the cubic fluorite structure, such as CeO.sub.2 only have a single Raman mode, corresponding to the breathing of o atoms around the central metal cation. See V. G. Keramidas and W. B. White, J. Chem. Phys. 59, 1561 (1973). The single Raman mode for Ce.sub.02 appears at −460 cm.sup.−1. Upon Sm.sup.3+ incorporation in Ce.sub.02, the intense singe peak at ˜460 cm.sup.−1 becomes asymmetric, broadens, and shifts to lower frequencies due to increased oxygen vacancy and changes in the lattice structure. The Raman spectra of Sm.sub.xCe.sub.1−xO.sub.2−δ also exhibits a second peak at ˜570 cm.sup.−1, which is a direct result of increased oxygen vacancy due to the introduction of Sm.sup.3+ ions into the CeO.sub.2 lattice. See J. R. McBride et al., J. Appl. Phys. 76, 2435 (1994). This result is consistent with other reports of Sm.sub.xCe.sub.1−xO.sub.2−δ. See T. Deng et al., Bull. Mater. Sci. 38, 1149 (2015); and S. F. Wang et al., J. Mater. Res. Technol. 2, 141 (2013).

Effect of Synthetic Parameters on Nanoparticles Size

[0027] To understand the effect of temperature of the formation of Sm.sub.xCe.sub.1−xO.sub.2−δ, the original Ce:Sm ratio (4:1) of TMS precursors were investigated at 500° C., 750° C., and 1000° C. for 10 minutes, respectively. SEM images of the product illustrating the temperature effect are shown in FIGS. 4A-C. Spherical nanoparticles were obtained at all temperatures, however, the size of the particles varies. Therefore, to quantify the effect of temperature on the dispersion of the product, the diameter of a statistically significant number of particles were analyzed and plotted in histograms for each reaction, shown in FIG. 4D-F. At a final reaction temperature of 500° C. the majority of the nanoparticles have diameters between 10-50 nm, at 750° C. the average diameter increases to 75-125 nm, and at 1000° C. most particles have diameters between 175-250 nm. It is worth noting that the particle distributions follow a log-normal distribution as illustrated by the overlays on the histograms.

[0028] Additionally, the effect of reaction temperature on the formation of Sm.sub.xCe.sub.1−xO.sub.2−δ nanoparticles, a Ce:Sm ratio of 2:1 at 750° C. was investigated for dwell times of 5, 10, and 20 minutes, respectively. SEM images of the final products are shown in FIGS. 5A-C. After analyzing a statistically significant number of nanoparticles, it is evident that increasing the reaction temperature increases the average final nanoparticle size, as shown by the histograms in FIGS. 5D-F. An increase in final dwell time leads to an overall increase in final particle size. With all other conditions kept constant, there should be identical nucleation events and the particle size distribution should be identical upon reaction at the final dwell temperature. However, the longer the reaction proceeds at the maximum temperature, more growth will occur, particularly in the form of Ostwald ripening. See A. Baronov et al., Phys. Chem. Chem. Phys. 17, 20846 (2015). According to the Gibbs-Thompson description of nucleation, dissolution can occur as the reaction proceeds due to the curvature radius of formed particles. These smaller particles can then be re-dissolved into solution and incorporated into larger particles. See L. C. Varanda et al., An. Acad. Bras. Cienc. 91, (2019).

Decomposition Mechanism of Sm.SUB.x.Ce.SUB.1−x.O.SUB.2−δ Precursors

[0029] In solid-state reactions, the mechanism by which precursors breakdown plays a major role in the final morphology and properties of the resulting product. To help understand the effect of Ln-precursors on final nanoparticle morphology, the thermolysis properties of the Ln-precursors were studied using TGA as shown in FIGS. 6A and 6B. The commercial Ln-acetate/nitrate precursors display multi-step decomposition as reported elsewhere in literature, whereas Ln-TMS precursors display a single-step decomposition. See P. Melnikov et al., J. Therm. Anal. Calorim. 118, 1537 (2014); T. Arii et al., Anal. Sci. 17, 875 (2001); and K. C. Patil et al., Can. J. Chem. 46, 257 (1968). Additionally, Ln-TMS exhibits a much lower decomposition temperature. For example, Sm-TMS decomposes at ˜200° C., but Sm-acetate and Sm-nitrate do not decompose fully until >400° C. The simpler decomposition mechanism and lower decomposition temperature of Ln-TMS are indicators for classical nucleation behavior that can ultimately lead to well defined spherical nanoparticles.

[0030] To identify the particle nucleation mechanisms, density functional theory (DFT) calculations can provide insight into the breakdown of Ln-precursors. The presence of a single-step decomposition in Ln-TMS precursors suggests that TMS is removed from the Ce/Sm center without the formation of intermediate states. To evaluate the effect of TMS ligand removal, a potential energy surface (PES) scan was performed on Ce-TMS by iteratively stretching the Ce-N bond in 0.05 Å steps and allowing the structure to completely relax. The PES, shown in FIG. 7, does not indicate the formation of significant intermediate states as indicated by the Lorentz potential like behavior. The small change in slope at ˜3.6 Å is a result of the methyl groups loosely interacting with the Ce center but is not a strong enough interaction to suggest the existence of an intermediate structure.

[0031] The effect of intermediate structures can be observed quantitatively by calculating the dissociation energy (E.sub.D) of the first decomposition step for Ln precursors. Shown in Table 2 are the pathways and E.sub.D for the first decomposition step of Sm/Ce TMS and acetate-based precursors calculated as the difference between the relaxed energies of the products and reactants shown schematically in FIG. 8. For the cerium precursors, Ce-TMS displays a lower binding energy of −4.348 eV as compared to −1.574 for Ce-acetate. Similarly, Sm-TMS displays a lower binding energy of −3.334 as compared to −1.598 for Sm-acetate. The intermediate structures formed during acetate decomposition result in higher binding energies that make it difficult for molecular species to become available for particle nucleation. Nitrate decomposition also displays a multi-step profile; nitrates are known in molten salt chemistry to break down into nitrite ions before complete decomposition, but provide an oxygen source before full loss. Due to this behavior, it is expected that the binding energies would be in a similar range as that of the Ln-acetate precursors.

TABLE-US-00002 TABLE 2 Calculated dissociation energies (E.sub.D) for the first decomposition step of Ln-acetate and Ln-TMS precursors. Dissociation Energy (eV) Precursor Decomposition Step Ce Sm Ln-Acetate 2Ln(CH.sub.3CO.sub.2).sub.3.fwdarw.Ln.sub.2O(CH.sub.3CO.sub.2).sub.4 + −1.574 −1.598 (CH.sub.3).sub.2CO + CO.sub.2 Ln-TMS Ln(TMS).sub.3.fwdarw.Ln(TMS).sub.2 + TMS −4.348 −3.334

[0032] According to the TGA data and relevant calculations, it is apparent that the single-step decomposition of TMS and subsequent formation of spherical nanoparticles falls in line with LaMer's classical nucleation theory, shown schematically in FIG. 9. In step I, the concentration of Ln monomers increases at a steady rate as TMS ligands begin to separate from the metal center. In step II, the saturation of monomers overcomes the energy barrier at some critical solute concentration (C.sub.min.sup.nu) to achieve a burst nucleation event. In step III, crystallization occurs as the concentration of monomers decreases due to continued growth of stable nuclei. In step IV, nuclei continue to grow due to continued monomer diffusion and Ostwald ripening. Spherical nanoparticles are formed due to rapid nucleation followed by the diffusion of readily available monomer species leaving no time for faceted surfaces to form around nuclei. In the case of nitrate- and acetate-based precursors, the multi-step decomposition profiles are indicative of a sol-gel polymerization nucleation mechanism that leads to irregular morphologies as facets form semi-randomly on nuclei surfaces.

[0033] The present invention has been described as solvent-free furnace and microwave-assisted synthesis of Ln-oxide and mixed Ln-oxide nanoparticles. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.