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Tailoring the Synthesis of LnF3 (Ln= La-Lu and Y) Nanocrystals via Mechanistic Study of the Co-precipitation Method Jordi Martinez-Esain, Josep Ros, Jordi Faraudo, Susagna Ricart, and Ramon Yáñez Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03454 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018
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Langmuir
Tailoring the Synthesis of LnF3 (Ln= La-Lu and Y) Nanocrystals via Mechanistic Study of the Co-precipitation Method Jordi Martínez-Esaín,†,‡ Josep Ros,† Jordi Faraudo,‡ Susagna Ricart‡ and Ramón Yáñez†,* †
Departament de Química, Universitat Autònoma de Barcelona, 08193, Bellaterra, Catalonia, Spain.
‡
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), 08193, Bellaterra, Catalonia, Spain.
ABSTRACT: Here, fifteen LnF3 nanocrystals are synthesized using co-precipitation method with citrate stabilization, to allow the fast, easy and reproducible synthesis of several nanoscaled structures in water. General trends related to the behavior of LnF3 nanocrystals are highlighted due to their broad range of application in several fields (e.g. medical applications). The same nature for all Ln3+ cations is expected due to the internal role of f-orbitals. However, we found that the use of different lanthanide element is crucial in the final size, shape, assembly and crystalline structure. In addition, the decrease of the cation size of the lanthanide series changes the behavior of these compounds, obtaining them in hexagonal, orthorhombic and cubic crystalline structures. In addition, we are able to tune the cubic crystalline phase to pure orthorhombic by modifying the pH of the system using HBF4 instead of tetramethylammonium citrate. Via 11B NMR, we demonstrated the mechanism of HBF4 as fluorinating agent if an additional source of F- is not added during the synthesis. 1H NMR and IR techniques were performed to unravel the picture of the surface chemistry of the two representative metal cations (Y and La). Finally, HRTEM and SAED were performed to uncover the shape of the obtained nanocrystals and the preferential orientation of the assembled particles, giving crucial information of the involved mechanisms. This study not only reveals the dependence of the crystalline structure with the used metal and pH, but also the achievement of LnF3 assembled particles or not depending on the final shape and temperature.
INTRODUCTION Currently, the interest to synthesize and directly apply different kind of nanocrystals (NCs) in several fields motivate the achievement of new frontiers and the enhancement of our living standards.1–3 During recent years, metal fluoride particles have been under deep investigation in several papers,4–7 explaining how one can synthesize them and a new application under exhaustive study.8–13 The highlighted interest in these type of particles lies in their potential application in several medical fields, such as Positron Emission Tomography (PET) imaging using 18F as radionuclei,14 Photodynamic Therapy,15–17 in Magnetic Resonance Imaging (MRI)18 and drug delivery systems.19 Their low solubility in water and their low toxicity make them suitable for fighting against cancer.20,21 The behavior of all non-radioactive lanthanide cations is summarized as Ln3+ cations with a remarkable uniformity of their chemical properties.22 Recent investigations demonstrate the non-bonding role of 4f-orbitals (lanthanoids) compared with 5f-orbitals (actinoids), which seem to play an important role in chemical coordination bond.23 Recently, Owen24 considered the covalent bond classification (CBC),25 used in coordination chemistry, as an useful analogy to study ligand-surface interaction. In
consequence with this new approximation, it is necessary to study deeply the different lanthanide cations to ensure the best approximation applied to nanocolloids. Due to the low effectivity of f-electrons to shield the outer shell when increasing nuclear charge, the stabilization of the metal cations nuclei produces a decrease in their sizes (from 103.2 pm for La3+ to 86.1 pm for Lu3+).26 This effect is observed directly in their chemical coordination (11-coordinated for big ones and 9-coordinated for the rest).22,26 Consequently, LnF3 present different stable crystalline structures, in which the most commonly reported are hexagonal (h), orthorhombic (o) and cubic (c).27,28 Big metal cations, from La3+ to Sm3+, crystallize preferentially in their hexagonal structure, while the small ones prefer the orthorhombic structure. However, YF3 presents cubic and orthorhombic stable crystalline phases. Considering this changeable role depending on their size, an accurate study of the behavior of each lanthanide cation is mandatory to extract periodical relation being applicable to LnF3 NCs design. Among the several reported synthetic approximations to obtain LnF3 NCs, co-precipitation method is claimed as the most convenient because it uses mild conditions, simple protocols and low-cost equipments.13 In addition, due to the water insolubility of all-LnF3 compounds compared with their analogous metal halide, the co-precipitation
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method is highlighted to be one of the most used to obtain these compounds.14,29 Polydentate carboxylate ligands allow the stabilization of NCs by direct carboxylic-to-metal coordination, being citrate the most reported stabilizer for several kinds of NCs.30–32 In addition, its biocompatibility and its high stability in water make, the obtained NCs, suitable for the direct application in medical fields.33,34 If we only centered our attention in aqueous co-precipitation routes, there are several reported studies in which someone can tune several synthetic parameters to enhance the obtained results. Different fluorinated precursors are claimed to tune the crystal phase,29 the selected stabilizing agent seems to produce a preferential coordination modifying the final shape,35 also in non-aqueous routes.36 The molar ratios of precursors also seem to play an important role in the final size and shape of the obtained NCs.14,18,36 Moreover, in several reported papers, some lanthanide metals tend to form aggregated LnF3 systems in a variety of synthetic conditions.37–39 Considering all these parameters, a complete study of the influence of each precursor and their different combination become impossible. However, a general trend of their crystalline structures and their general behavior could be extracted with little modifications applied to LnF3 NC synthesis. Here, fifteen LnF3 NCs have been obtained using the coprecipitation method with citrate as stabilizer; an easy, fast and reproducible method. ESI-MS was used to uncover the precursor solution (metal-stabilizer) before the formation of the NCs. (HR)TEM, XRD, NMR and IR were used to study the general trends and behavior of these compounds. The mechanisms related to the use of a covalent or ionic fluorinating source were unraveled, as well as the role played by an acidic media in the final obtained crystal phases. We found that what is responsible for the crystal phase transformation is not directly the fluorinating agent, as it was reported.29 We demonstrate that the pH, induced by the fluoride source, is the pivotal parameter to decide the final crystalline structure. Using SAED images of single supraparticles, we rationalized the formation of randomly oriented spherical supraparticles and highly oriented rodlike mesocrystals depending on the crystalline structure. The shape of the obtained NCs is postulated to affect directly the final colloidal solution, observing single NCs dispersions or big assembled particles. In addition, these assembled particles could be formed or avoided with a tunable temperature control. In contrast with a recently reported study,40 we demonstrated that the general trends related with metal cation radius (e.g. expected crystalline structure) are applicable in both methodologies, organic and aqueous synthesis. On the other hand, we stated that the difference in surface chemistry (e.g. ions used during the synthesis) and pH play a crucial role in the final behavior and crystalline structure of the NCs synthesized in aqueous media. In this work is presented the study of the general periodic properties expected by each lanthanide to predict the final obtained system in a well-known synthetic route performed in aqueous media.
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EXPERIMENTAL SECTION Materials. Yttrium (III) acetate hydrate 99.9%, Lanthanum (III) acetate hydrate 99.9%, Cerium (III) acetate hydrate 99.9%, Praseodymium (III) acetate hydrate 99.9%, Neodymium (III) acetate hydrate 99.9%, Samarium (III) acetate hydrate 99.9%, Europium (III) acetate hydrate 99.9%, Gadolinium (III) acetate hydrate 99.9%, Terbium (III) acetate hydrate 99.9%, Dysprosium (III) acetate hydrate 99.9%, Holmium (III) acetate hydrate 99.9%, Erbium (III) acetate hydrate 99.9%, Thulium (III) acetate hydrate 99.9%, Ytterbium (III) acetate hydrate 99.9%, Lutetium (III) acetate hydrate 99.9%, Citric acid 99%, Tetramethylammonium hydroxide 25% v/v in water, Ammonium fluoride >99.99%, Tetrafluoroboric acid 25% in water solution and Perchloric acid 70% in water were purchased from Sigma-Aldrich. Ethanol 96% from Panreac and Acetone 99.5% from Scharlau. Deuterium oxide 99.90% D was purchased from euriso-top All reagents were used as received without further purification. Particles Synthesis. Standard Procedure: Citric acid (2.25 mmol) in 16 ml of Milli-Q water was neutralized with tetramethylammonium hydroxide (6.75 mmol), followed by the addition of Ln(CH3COO)3 · H2O (1.5 mmol), in a 50 ml round-bottomed flask equipped with a condenser, magnetic stirrer and heater. The initial solution was heated until 100 °C, then NH4F (4.5 mmol) in 4 ml of Milli-Q water was injected dropwise. After 2 h of reaction, the final mixture was cooled down to room temperature. LnF3 NCs were separated from the reaction medium by the addition of 10 ml of ethanol or acetone as non-solvent (depending on the used metal), followed by centrifugation at 10,000 rpm for 20 minutes. Separated NCs were re-dispersed in 20 ml Milli-Q water as solvent forming a room temperature stable dispersion. Acidic method with fluoride injection: Ln(CH3COO)3 · H2O (1.5 mmol) in 16 ml of Milli-Q water was introduced into a 50 ml round-bottomed flask equipped with a condenser, magnetic stirrer and heater. The initial solution was heated until 100 °C, then HBF4 (2.52 mmol) or HClO4 (2.52 mmol) was injected followed by the dropwise addition of NH4F (4.5 mmol) in 4 ml of Milli-Q water. After 2 h of reaction, the final mixture was cooled down to room temperature. LnF3 NCs were separated from the reaction medium by the addition of 10 ml of ethanol as non-solvent, followed by centrifugation at 10,000 rpm for 20 minutes. Separated NCs were redispersed in 20 ml Milli-Q water as solvent. Acidic method without fluoride injection: Ln(CH3COO)3 · H2O (1.5 mmol) in 20 ml of Milli-Q water was introduced into a 50 ml round-bottomed flask equipped with a condenser, magnetic stirrer and heater. The initial solution was heated until 100 °C, then HBF 4 (2.52 mmol) was injected. After 2 h of reaction, the final mixture was cooled down to room temperature. LnF3 NCs were separated from the reaction medium by the addition of 10 ml of ethanol as non-solvent, followed by centrifugation at 10,000 rpm for 20 minutes. Separated NCs were redispersed in 20 ml Milli-Q water as solvent.
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Characterization. High-resolution Electrospray Ionization Mass Spectrometry (ESI-MS) measurements were recorded from water solution in a BRUKER Apollo microTOF system (Bruker Daltonics GmbH, Bremen, Alemanya) with electrospray ionization, autosampling and equipped with an HPLC pump (Series 1100, Agilent Technologies) at the Servei d’Anàlisi Química at the UAB. Transmission electron microscopy (TEM) micrographs were obtained on a 120 kV JEOL 1210 TEM, which has a resolution point of 3.2 Å. High Resolution Transmission Electron Microscopy (HRTEM) micrographs were obtained on a 200 kV JEOL 2011 TEM, which has a resolution point of 1.8 Å at 200 kV. Samples for TEM analysis were prepared by spreading a drop of as-prepared NCs diluted dispersion on amorphous carbon-coated grids and then dried in air. Powder X-ray diffraction (XRD) patterns of the samples were recorded with were performed using a Phillips XPert diffractometer equipped with a two circle diffractometers and Cu tube. Infrared spectroscopy (IR) analyses have been carried out in the Servei d’Anàlisi Química with the Bruker spectrophotometer IR Tensor 27. Nuclear Magnetic Resonance (NMR) analyses were recorded with a Bruker Advance II 400 spectrometer in D2O at 298 K. Samples for 1H NMR were prepared by washing the colloidal solution five times before drying NCs. Finally, the powder was dispersed in D2O. In the case of 11B NMR, samples were lyophilized and then dispersed in D2O. Freeze-drying was performed on a Telstar Lyoquest 80 freeze-dryer. Samples were first frozen at -80 °C in the same freeze-dryer, then dried at 0.05 mbar and room temperature for 24 hours.
RESULTS AND DISCUSSION Lanthanide (III) fluoride particles. We studied the synthesis of fifteen LnF3 NCs performed using a modified co-precipitation method in water. This was done in order to explore if all metals show the same behavior as is postulated in coordination chemistry by the non-bonding role of their f-orbitals. Although the participation of 4𝑓𝑦(3𝑥2 −𝑦2 ) orbital was recently demonstrated as important in chemical bond, in general, 4f-orbitals are considered as nonbonding compared with their 5f analogs.41 In addition, the general trends of this group should be unraveled to enhance their aqueous synthesis to be applied in medical fields. A single precipitation method was performed using acetate precursors with tetramethylammonium citrate as stabilizer following a reported method.42 To achieve a homogeneous nucleation, the complete dissolution of all precursors is needed. This solubility was achieved with a molar ratio Metal:Citrate up to 1:1.5.43 To assess the homogeneity of the system and evaluate our precursor solution, we performed Electrospray Ionization Mass Spectrometry (ESIMS) technique before the addition of ammonium fluoride. At these conditions, a dynamic equilibrium between different coordination complexes has been established, using Y and Gd, as it is presented in Supporting Information (SI). This is mainly composed of the mixture of different ionic complexes (Scheme 1).
We found the presence of (a), (b) and (c) in the mass spectrum of both metal cations. To ensure the presence of these complexes, positive and negative ESI were performed in the precursor solution. Although, species (c) was not encountered in yttrium precursor solution HR-MS ESI- spectrum, it was present in the HR-MS ESI+. Considering the intensity of the peaks in the spectrum and the match between both metal cation species, we postulate the same behavior for all lanthanide precursor solution. However, in large cations, more species can be present in the equilibrium due to their high coordinative ratio. Due to the lability of lanthanide complexes, a fast exchange between the different species is expected.44,45 Then, after the injection of ammonium fluoride, when the system is completely homogeneous at 100 °C, the nucleation process starts, followed by the growth during 2 h of reaction. Scheme 1. Dynamic equilibrium between the different precursor species observed by ESI-MS.
Ln is the lanthanide (Ln3+), Cit is citrate anion (C6H5O73-), Tma is tetramethylammonium cation (C4H12N+) and Ac is acetate anion (C2H3O2-).
Obtained particles show a variety of shapes and sizes (Table 1 and Figure 1) depending on the location of the metal in the lanthanide series, which determine the size of lanthanide cations. Considering all LnF3 NCs, three different behaviors could be encountered: (i) hexagonal nanoplatelets, (ii) spherical supraparticles and (iii) rod-like supraparticles. The first group (i) is formed by the three lanthanides with bigger metal cation size (La, Ce and Pr), which form a homogeneous dispersion of hexagonal platelets of ~7 nm with a thickness of ~3 nm (see Table 1 for size histograms). Then, the group (ii) is composed by Nd, Sm and from Tb to Lu. This group shows spherical fluoride supraparticles, made by self-assembly of smaller spherical nanoparticles, of different size, where the difference between their size could be rationalized considering the different cation radius. Neodymium, the first lanthanide that shows this behavior, forms small and non-defined spherical supraparticles. After that, Sm forms bigger spherical supraparticles, as in the case of Tb. Then, the size of the supraparticles start to decrease until the three last lanthanides (Tm, Yb and Lu), which are the smallest one, showing a different shape in the surface. They seem to be formed between spherical and worm-like NCs and the last one seems to be located perpendicular to the surface, showing these star-like shapes. The last group (iii) is formed by Eu and Gd, which form rod-like supraparticles.
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Table 1. Information of the NCs by the standard methodology. Size by histograms in TEM and Scherrer equation using XRD obtained patterns. Observed crystalline structure, metal cation radii and behavior of the final colloidal suspensions by TEM. NCs are ordered by their metal cation radii from bigger to smaller one. TEM size (nm)
Cation Radii26
XRD
NCs
Length
Width
LaF3
8.2 ± 1.8
2.2 ± 0.5
Scherrer(nm) 5.4
CeF3
7.6 ± 2.3
2.4 ± 0.5
6.6
102.0
PrF3
8.8 ± 1.6
3.4 ± 0.8
4.7
99.0
NCs
Crystalline Structure
Size (pm)
Behavior
Hexagonal
103.2
Nanoplatelets
NdF3
43 ± 5
-
5.2
98.3
Spherical
SmF3
151 ± 13
-
11.1
95.8
supraparticles
EuF3
346 ± 96
103 ± 22
8.0
94.7
Rod-like
GdF3
469 ± 51
128 ± 15
10.1
93.8
supraparticles
TbF3
137 ± 11
-
17.8
Orthorhombic and Hexagonal Cubic
92.3
Spherical supraparticles
DyF3
130 ± 10
-
12.8
91.2
HoF3
105 ± 12
-
9.4
90.1
YF3
83 ± 13
-
7.4
90.0
ErF3
107 ± 12
-
7.6
89.0
TmF3
58 ± 8
-
6.2
88.0
Star-like spherical
YbF3
90 ± 12
-
6.1
86.9
supraparticles
LuF3
72 ± 12
-
5.7
86.1
Figure 1. Brightness mode Transmission Electron Microscopy (TEM) of synthesized LnF3 particles at 100 °C in a co-precipitation method. All LnF3 images are ordered from left to right considering their cation size. From La to Nd, scale bar of 20 nm while the rest have a scale bar of 100 nm
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Different observed groups could be rationalized considering two key factors: (i) the size of the metal cation and (ii) the final shape of the NCs. (i) Metal cation size is claimed to affect directly the crystalline structure, metal coordination and hence, the final behavior of NCs. On the other hand, (ii) the shape of the NCs could affect directly the surface chemistry (organic ions distributed onto the surface). This effect could promote or avoid chemical interactions between different NCs, creating big self-assembled systems or stabilizing them electrostatically avoiding the aggregation. In our case, we found that big metal cations form single hexagonal nanoplatelets without aggregation. Their final shape is consequence of their hexagonal crystalline structure and their faceting seems to be able to avoid self-assembly to big supraparticles. When metal cation decreases in size, also decrease the faceted shape of NCs forming spherical NCs. These NC shapes promote the aggregation forming big assembled systems.
the literature we can find these two crystallographic phases as the most stable in LnF3,27 the mixture of both is expected for intermediate metal cations. Nevertheless, after Gd, the cubic phase was found as the dominant crystalline structure in our synthetic conditions.
Molar ratio metal:citrate was also studied for LaF3 and YF3 reveling the same behavior of those synthesized following the standard synthesis; aggregated particles for YF3 and single NC dispersions for LaF3 (Figure S5). In addition, we found that increasing the amount of citrate and, in consequence, the ionic strength of the medium, YF3 aggregates were bigger at high molar ratios (Table S2). Considering this effect, we chose a molar ratio metal:citrate of 1:1.5 to use the minimum amount of citrate in which lanthanide (III) citrate precursor was soluble.43 In addition, our recent reported study42 demonstrates that formed supraparticles, those with smaller metal cations, are produced by the controlled and temperature-tunable self-assembly of small NCs via the formation of citrate mediated ionic bridges. In our previous work, we demonstrated the applicability of the self-assembly observed in YF3 in other systems as: SmF3, EuF3 and LuF3. Here, we observe that this self-assembly (from nanoparticles to supraparticles) could be postulated from NdF3 to LuF3 (the smaller metal cations) due to the formation of supraparticles at 100 °C. First obtained results are enough to prove that the lanthanide group should not be summarized as metals cations with an equal role, they present different properties depending on the size of used lanthanide cation. Crystalline structure in standard synthesis. All samples were dried to perform the powder X-Ray Diffraction (XRD) to explore if the crystalline structure is directly related to the NC shapes and organization. Three different crystalline structures were detected depending on the lanthanide cation size; La3+ to Sm3+ (103.2 to 95.8 pm) show a pure h-LnF3, Eu3+ and Gd3+ (94.7 and 93.8 pm respectively) exhibit a mixture of h-LnF3 and o-LnF3 structures and Tb3+ to Lu3+ (92.3 to 86.1 pm) show a c-LnF3 structures, isostructural with YF3 (Figure 2). Results are compatible with a clear dependence between the cation size and the obtained crystalline structure. Large metal cations crystallize in the h-LnF3 form and the small ones in the cubic crystalline structure. Europium and gadolinium are in the middle of this transition and show a mixture of h-LnF3 and o-LnF3. Considering that in
Figure 2. X-Ray Diffraction patterns obtained for each LnF3 NC in their powder form. To summarize, diffractograms show the assignation of principal peaks considering the structure that the NCs present. The star in EuF3 and GdF3 shows the main peak of their hexagonal structure, the other peaks are overlapped with the orthorhombic structure. Reference patterns: (04-007-2483), (00-032-0483), (00-0080045), (00-046-1167), (00-009-0416), (00-032-0373), (00-0330542), (04-006-9968) and (00-012-0788) from the International Centre for Diffraction Data. All cubic structures were identified as isostructural with YF3 cubic pattern and assigned with this card.
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From these results, the rod-like shape of Eu and Gd could be explained by the presence of orthorhombic structure, which is only achieved by these two elements at the synthetic conditions. In addition, these elements show a mixture of crystalline structures, hexagonal and orthorhombic. They can adopt different crystalline structures depending on the synthetic conditions (e.g. ions present in the medium).35 This effect is explained by the close lattice energies between their different phases.46 The size of Eu and Gd cations could be stabilized in hexagonal, orthorhombic and cubic structures, giving to these elements a cross-border character. Considering that Nd and Sm show spherical supraparticles with a hexagonal crystalline structure, the hypothesis that groups (i), (ii) and (iii) are formed by different crystalline structures is rejected as an explanation of their shape and organization. Effect of pH. Once the crystalline structures were unraveled, the following step is to know if the NCs modify their crystalline structure with the pH of the precursor solution. To observe if these NCs show any structural modification under acidic conditions, we performed a modified acidic method using tetrafluoroboric acid instead of tetramethylammonium citrate as stabilizer. The first approximation carried out implies the injection of ammonium fluoride as fluorinating agent to prevent the hydrolysis of tetrafluoroboric acid, which only acts as acidic species. The formation of hexagonal NCs of ~50 nm was observed in the Ce and Nd cases (Figure 3A and 3B).
Figure 3. Transmission Electron Microscopy and X-ray Diffraction of obtained NCs using the acidic method with ammonium fluoride injection. CeF3 (A) and NdF3 (B) hexagonal nanoplatelets in their hexagonal crystalline structure. All images have the same scale bar.
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NCs show their classical hexagonal crystalline structure, corroborating that under acidic conditions, this structure remains unaltered. However, if we compare the NCs of same metal obtained with acidic condition (Figure 3A) with those obtained with the standard methodology (Figure 1) we find interesting difference to understand both methodologies. First, CeF3 increases their size if it is synthesized under acidic conditions. This means that citrate plays a pivotal role in the stabilization of NCs in the standard synthesis, hampering their growth. Likewise, while CeF3 NCs, stabilized with citrate show large colloidal stability in water, those obtained with the acidic method precipitate in hours. Although the ions present in the system could stabilize partially these NCs, they need the presence of a strong stabilizer onto the surface to avoid the growth and allow large stability. Focusing in NdF3 (Figure 3B), the same hexagonal phase was found with a NC size of ~50 nm. We have also found that, in the standard synthesis, they present non-defined assemblies formed by less-faceting NCs. On the other hand, under strongly acidic conditions, we achieve hexagonal nanoplatelets without aggregation. This evidence means that the pH modifies the crystalline habit of these particles to achieve a hexagonal final shape instead of the spherical obtained with neutral conditions. The stability of these particles is equal to CeF3 obtained with the same methodology Concerning to NCs obtained in their cubic crystalline structure using the standard synthesis, we have performed the same acidic methodology to know if the pH influence is able to modify the crystalline structure. In this case, we selected Y, Dy and Yb as representative elements of this group. TEM images show that under acidic conditions these particles are organized in a rod-like shape (Figure 4) and exhibit the typical orthorhombic structure of LnF3 compounds. This effect confirms our preliminary hypothesis that the orthorhombic phase shows a preferential growth forming finally rod-like supraparticles, as in the case of group (iii) under neutral conditions. The formed rods have different size and shape depending on the used metal in the synthesis and they are formed by the aggregation of smaller NCs. Here, we demonstrated the influence of the pH in the cubic crystalline structure. At acidic conditions, the orthorhombic structure is achieved from Tb to Lu. This effect proves how the formation of cubic structure in several nanomaterials is not allowed in acidic conditions.47 However, rod-like particles obtained with the standard methodology (EuF3 and GdF3) present larger stabilities than those obtained by the acidic method, which implies again the necessity of strong capping ligand to stabilize these particles in solution. To carry on with the pH study, we performed the last synthetic approximation applied to Ce and Y, the use of metal acetate precursor with HBF4 as acid and fluorinating precursor, without the addition of NH4F. Obtained particles (Figure S6) have the same size and shape as the ones using ammonium fluoride as precursor, and due to no other fluorine source being added, HBF4 acts as acid and
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Figure 5. 11B NMR of YF3 lyophilized samples after their synthesis with the acidic methodology using (A) HBF4 and NH4F as fluorinating precursors and (B) only using HBF4.
Figure 4. Transmission Electron Microscopy and X-ray Diffraction of obtained NCs using the acidic method with ammonium fluoride injection. YF3 (A), DyF3 (B) and YbF3 (C) rod-like NCs in their orthorhombic crystal phase. All images have the same scale bar.
fluorinating agent. Crystalline structure in both cases is the same as that obtained by the other methodology: pure hexagonal for CeF3 and pure orthorhombic for the YF3. To ensure that fluoride source is different in each synthesis, 11B Nuclear Magnetic Resonance (NMR) of each lyophilized sample (acidic method with and without ammonium fluoride injection) was carried out in YF3 samples directly after their synthesis (Figure 5). When ammonium fluoride is injected in the precursor solution, this act as fluorinating agent avoiding the hydrolysis of HBF4 reaction (1) and (2),48 showing only 2% of B(OH)3 (Figure 5A). However, the partial hydrolysis reaction (1) from BF4- into BF3OH- is done by their fast equilibrium. In this case, fluorides from NH4F precursor take place in the formation of LnF3 NCs. HBF4 + H2O HBF3OH + 2 H2O
HBF3OH + HF B(OH)3 + 3 HF
(1) (2)
Otherwise, if the synthesis is carried out with metal acetate and tetrafluoroboric acid as unique reactants, 11B NMR confirms the presence of a 54% of B(OH)3 (Figure 5B) as co-product of the HF formation following the reaction (2). In normal conditions, considering the high stability of HBF3OH, the formation of B(OH)3 and HF are not promoted by the low displaced equilibrium.48 However, the stability and insolubility of formed LnF3 displace this equilibrium to obtain all the HF necessary to achieve the NCs.
As HBF4 is used in all pH modified methodologies, we tested also the use of HClO4 with the addition of NH4F to ensure that the effect of BF4- anion in the medium does not affect the final crystalline structure (Figure S7). The formation of rod-like NCs with an orthorhombic structure confirms that the pH is the factor governing the final crystalline structure of these particles, while the fluorinating agent or the acidic source are not crucial factors to determine the final behavior at these synthetic conditions. Surface composition and NC shapes. These NCs show different crystalline structures depending on the used metal as well as the pH during the synthesis. However, the aggregation process, observed in some metals, is not yet clear with the performed studies. To get more information about these systems, 1H NMR and IR spectroscopy analysis were performed in La and Y as representative elements of non-aggregated and aggregated NCs respectively. Particles were washed five times and dried to perform IR and re-dispersion in D2O to NMR, ensuring that free ions in the system were removed. The presence of tetramethylammonium, citrate and acetate were observed in both cases (Figure 6A). Different relative amounts of each ion are observed comparing both spectra due to the different behavior of these particles (NCs and supraparticles). IR spectroscopy was also performed in both samples to ensure the same composition of the surface. The IR spectra (Figure 6B) present the same bands, suggesting the presence of carboxylate moieties onto particle surface (acetate and citrate). Bands at ~3200 cm-1 could be promoted by the OH group of citrates and the presence of ammonium or water adsorbed onto NC surfaces. Comparing both surface characterization techniques applied on both metals, we postulate a similar surface chemistry with the presence of the same attached molecules.
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HRTEM images of these particles, which show that supraparticles are not formed by the assembly of hexagonal nanoplatelets as in the case of LaF3 (Figure 7A). We did not find evidence of platelets in the performed HRTEM as those showed in the enlarged zone of Figure 7A. In consequence, we postulate that NdF3 and SmF3 form spherical NCs that spontaneously tent to aggregate forming big assembled spherical particles. This spherical behavior could indicate that, at this atomic radius size, formed NCs show a less-faceted shape and they seem spherical by HRTEM.
Figure 6. 1H NMR (A) and ATR-IR (B) of LaF3 (black, upper) and YF3 (blue, bottom) NCs washed five times.
High Resolution Transmission Electron Microscopy (HRTEM) was used to analyze the shape of NCs in the three observed groups: (i) hexagonal NCs (Figure 7A), (ii) spherical supraparticles (Figure 7B) and (iii) rod-like supraparticles (Figure 7C). In the first group (i), LaF3 shows single hexagonal nanoplatelets, in which two different exposed faces are observed. YF3 shows a spherical assembly of spherical NCs, as well as, EuF3 is formed also by spherical small NCs, although their final organization is preferential for obtaining rod-like supraparticles. In addition, we performed the HRTEM of NdF3 and SmF3 supraparticles to see if the NCs have the typical hexagonal shape as it is expected by particles with hexagonal crystalline structure. In Figure S8 it can be observed the
It is important to recall that in our synthetic procedure all hexagonal shape NCs avoid supraparticles formation while spherical NCs tend to organize to form highly stable supraparticle systems. In order to understand this different behavior, it is important to recall the mechanism inducing supraparticle formation in our capped NCs. The mechanism has been studied in detail42 (experimentally and by simulations) for the case of YF3 obtained with the same methodology as employed here. In that case, NCs are covered by adsorbed ions (acetate, citrate and ammonium) and supraparticle formation was due to ionic citrate bridges between NCs. In the case of hexagonal NCs, our results suggest that these ion mediated bridges are not present so the adsorption of ions in hexagonal NCs not only stabilizes the NCs but also prevents supraparticle formation. In this respect, it is important to note that in previous results without molecular ions as capping agents,49 the synthesis of LaF3 produces the self-assembly of big hexagonal NCs into bid rod-like crystals. Therefore, the key aspect to be considered here is why adsorbed ions play a different role in hexagonal or spherical NCs. We think that the difference is due to the different behavior of the exposed planes (hexagonal (001) Miller planes and rectangular (100) Miller planes), which allow the preferential attachment of the ions in one plane instead of the other one. This possibility will be investigated in more detail in a future work. Our present results therefore suggest that the selective binding of acetate, citrate and ammonium in a faceted system avoids NCs self-assembly in LaF3, CeF3 and PrF3. These results also emphasize the importance of the surface chemistry in the final size, shape and behavior of the obtained NCs.
Figure 7. HRTEM of LaF3 NCs (A) showing they two exposed faces, YF3 supraparticles (B) with an enlarged spherical NCs and EuF3 rod-like supraparticles (C) with one single spherical NC to demonstrate the spherical shape of these NC.
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On the other hand, the spherical behavior of the rest of metals (with the random coordination of the ions in their non-faceted surface) could allow ionic interactions easily between NCs surface, producing these temperature-tunable assembled systems by citrate mediated ionic bridges, as we commented before. Dependence of shape and crystalline structure. We found that in acidic conditions the cubic crystalline structure is replaced by the formation of pure orthorhombic structure, the common phase in LnF3 NCs. The reasons involved in the preference of one or other crystalline structure is a controversy, there are several factors and the main influences are not yet clear. Here, we postulate an approximation to understand this interesting effect concerning the nature of the first produced seeds in the colloidal solution. When the synthesis is carried out under acidic conditions, lanthanide cations are in their [Ln(H2O)x]3+ form before the formation of LnF3. In consequence, the acidic medium allows the formation of o-LnF3 and h-LnF3 NCs, the most stable thermodynamic structures, depending on the metal cation size as we explained before. When we produced the NCs following the standard method, the pH of the medium is few greater than 7. At this pH range, the formation of the cubic form of Ln2O3 is allowed forming the first nucleation seeds (kinetic product). After the addition of the fluoride source, the high excess of F- in the medium, compared with the amount of OH-, promotes the formation of LnF3 (thermodynamic product).50 Seeds of Ln2O3 could predetermine the cubic nature of LnF3, only observed in neutral-basic conditions. The high excess of fluorides in the media, as well as, the insolubility of LnF3 compounds could promote ionic exchange (substituting O2- by F-) in the first obtained seeds maintaining the crystalline structure adopted by the Ln2O3.51,52 Once crystalline structure is achieved, NCs show different assembly shapes depending on their structure. Cubic NCs form spherical assembled supraparticles while orthorhombic ones prefer a rod-like shape. If we perform a Selected-Area Electron Diffraction (SAED) of a single YF3 spherical supraparticle (Figure S9A), this shows a polycrystalline pattern with several spots distributed in circles depending on their d-spacing. This result is compatible with the symmetry of the cubic crystalline structure, which allows an assembly in all directions forming a spherical supraparticle randomly oriented. The analysis of a single rodlike EuF3 supraparticle by SAED (Figure 8A) show a high ordered crystalline core of the rod. In this case, we have a so called mesocrystal, an ordered superstructure formed by crystalline nanoparticles with external crystal faces in a scale between hundred nanometers to micrometers.53 We also examined the NCs obtained by acidic method (Figure S9B) showing also an ordered SAED in single spots. In consequence, we can conclude that orthorhombic structure allows a selective arrangement of obtained NCs forming a big mesocrystal. To uncover the growth direction, we used SAED images of different single rod-like supraparticles to simulate our
Figure 8. (A) Example of one EuF3 rod-like supraparticle with its respective SAED and the observed planes. (B) Representation of orthorhombic crystalline structure, made with VESTA software, with the planes found in SAED pattern.
particular case (Figure 8B). We can conclude that these supraparticles are interacting through [010] direction to form big assembled particles. This conclusion is uncovered considering that the cross product of two planes founded in SAED allows us to know the electron beam direction [101] direction. In consequence rods are assembled in the preferential [010] direction (see Figure S10 to other cases and Table S1). Among the different proposed formation mechanisms of mesocrystals,53 we postulate a kind of NC alignment by face selective molecules. This mechanism could summarize the behavior of all obtained LnF3 NCs and their assembled particles depending on the shape of the NC. Particles are interacting between them by ionic interactions due to their high fluoride-cation affinity onto the surface.42 In cubic NCs, high symmetry onto the surface, this interaction is promoted in all directions allowing spherical assembled systems. However, in orthorhombic NCs the interaction by [010] direction is highlighted due to the high fluoride-cation interaction, making a selective organization allowing rod-like supraparticles in the standard synthesis (+NH4 promoted interaction) and in acidic method (H3O+ mediated interaction), following a citrate mediated ionic bridges (standard method) or an ionic condensation (acidic methods).
CONCLUSIONS In this work, we studied the mechanistic insights in the synthesis of all non-radioactive LnF3 NCs to postulate general rules concerning these kinds of compounds. The cation size was stated as the pivotal factor to englobe each lanthanide in a defined crystalline structure. We were able to tune the crystalline structure and crystal habit in a set of particles by an easy pH modification. The effect of a strong acid during the synthesis was stated to play a crucial role in the reaction mechanism to form different crystalline phases. The pH is postulated to affect
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directly the precursor solution allowing different crystalline structures depending on the nature of metal in the medium. [Ln(H2O)x]3+ allows the formation of orthorhombic and hexagonal structures while the formation of Ln2O3 seeds promote the formation of c-LnF3 NCs for small-sized metal cations. The cross-border elements, Eu and Gd, show a mixture of hexagonal and orthorhombic structures due to their polymorphic behavior produced by their frontier cation sizes. The use of ionic or covalent (NH4F or HBF4) fluorinating precursors do not show differences in the final LnF3 NCs. Citrate is stated as high-effective stabilizer because all-LnF3 synthesized in the presence of this polydentate carboxylic ion were more stable than their analogs without it. Working at high temperatures (100 °C) promotes the self-assembly mediated ionic bridges from nanoparticles to supraparticles in those NCs with spherical shape. We demonstrated that the self-assembled mechanism mediated by citrate bridge is extensible to all smallcation sized LnF3 NCs. The final surface study reveals the presence of same organic ions attached to NC surface in single NCs and assembled particles. Due to the different shape of LnF3 NCs, we postulate that the hexagonal-shape NCs avoid the aggregation by their two different exposed faces. SAED images of single rods have been obtained demonstrating the formation of mesocrystals in the orthorhombic cases. An ordered [010] directional organization of NCs following a face selective molecules mechanism, while spherical assemblies show a random organization of NCs into supraparticles. Furthermore, this knowledge could be applied to prepare specific NCs on the design of nanoscaled system to be used in a broad range of applications.
ASSOCIATED CONTENT Supporting Information. High Resolution Electrospray Ionization Mass Spectrometry of Y and Gd precursor solution with their main peaks assignations, TEM images of LaF3 and YF3 NCs obtained at different molar ratios, TEM images and XRD patterns of YF3 and CeF3 nanocrystals obtained with acidic method without fluoride injection, TEM and XRD of YF3 nanocrystals obtained using HClO4 with ammonium fluoride injection, HRTEM of NdF3 and SmF3, TEM images and XRD of SmF3, GdF3 and YF3 NCs obtained at 5 °C, SAED patterns of YF3 obtained by standard method and by acidic method with ammonium fluoride injection, SAED patterns and orthorhombic structure representation of EuF3 supraparticles and Histograms and extra TEM image of all nanocrystals obtained in this work with the Scherrer size. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496), CONSOLIDER Excellence Network (MAT2015-68994-REDC), COACHSUPENERGY project (MAT2014-51778-C2-1-R, co-financed by the European Regional Development Fund) and SUPERINKS project (RTC-2015-3640-3, co-financed by the European Regional Development Fund). We also thank the support from the European Union for EUROTAPES project (FP7-NMP-Large-2011-280432) and from the Catalan Government with 2014-SGR-753 and Xarmae. The authors acknowledge the technical support of Servei de Microscòpia, NMR Service and Servei de Difracció de Raigs-X, all at the UAB. Also, the support of Dr. Judit Oró from Microscopy Service and Dr. Anna Crespi from X-ray Diffraction Service at the ICMAB-CSIC is acknowledged. Finally, J.M.E acknowledge the PIF pre-doctoral fellowship from the Universitat Autònoma de Barcelona. We thank Justin J. Charette for help with English usage.
ABBREVIATIONS Ln, lanthanide; Cit, citrate anion; Ac, acetate anion; Tma, tetramethylammonium cation; h, hexagonal; c, cubic; o, orthorhombic; PET, Positron Emission Tomography; NC, Nanocrystal; MRI, Magnetic Resonance Imaging; ESI-MS, Electrospray Ionization Mass Spectrometry; TEM, Transmission Electron Microscopy; HRTEM, High Resolution Transmission Electron Microscopy; XRD, X-ray Diffraction; NMR, Nuclear Magnetic Resonance; SAED, Selected-Area Electron Diffraction.
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