Article pubs.acs.org/JPCC
Influence of Lu3+ Doping on the Crystal Structure of Uniform Small (5 and 13 nm) NaLnF4 Upconverting Nanocrystals Elsa Lu, Jothirmayanantham Pichaandi, Loryn P. Arnett, Lemuel Tong, and Mitchell A. Winnik* Department of Chemistry, University of Toronto, 80 St George Street, Toronto, ON, Canada M5S 3H6 S Supporting Information *
ABSTRACT: While there have been many advances in techniques to synthesize uniform lanthanide-doped upconversion nanoparticles (UCNPs), it is still a challenge to synthesize small (ca. 5 nm) hexagonal phase UCNPs that are also bright. The most common method to obtain strongly emissive UCNPs is to synthesize core− shell structures with a passivating shell coating the luminescent core. This approach normally results in larger NPs (>20 nm) and requires two-step procedures. Here, we report a one-pot synthesis of 4 nm NaLuF4:Gd(37%),Yb(16%),Er(2%) UCNPs, whose colloidal solutions show upconversion luminescence (UCL) visible to the eye. We initially hypothesized that the origin of UCL from such small UCNPs was due to a Gd-rich hexagonal upconverting core containing Yb and Er with a Lu-rich passivating shell. This idea is based on the different nucleation rates of the NaLnF4 NPs. Interestingly, the 4 nm NaLuF4-based UCNPs are in the cubic phase, and subsequently undergo a phase transformation with prolonged heating to form larger (12−14 nm) uniform hexagonal phase UCNPs. We also found that if the molar ratio of Lu:Gd in the reaction mixture was decreased from 45:37 to 20:62, the resulting UCNPs still initially nucleated in the cubic phase. Additional studies in which we varied other reaction parameters (temperature, ratios of Na+/Ln3+ and F−/Ln3+, and solvent composition) also resulted in initial nucleation in the cubic phase. In contrast, both the NaGdF4:Yb,Er and NaYF4:Gd,Yb,Er UCNPs nucleated in the hexagonal phase. Our results suggest that the presence of Lu in the reaction mixture influences the nucleation of NaLnF4 NPs. Lanthanide compositions that would normally nucleate in the hexagonal phase appear to nucleate in the cubic phase when Lu is present.
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INTRODUCTION
from other types of NPs. This feature reduces the phototoxicity of these NPs. UCNPs also do not exhibit photobleaching. Lanthanide NPs also have important potential applications as high sensitivity reagents for mass cytometry (MC). Our group is particularly interested in developing NP reagents for this application.5−7 MC is a high throughput bioanalytical technique for characterizing biomarker expression on individual cells.8−10 In this technique, various antibodies are labeled with different metal isotopes. Cell suspensions are treated with a cocktail of antibodies and then injected individually but stochastically into the plasma torch of an inductively coupled plasma mass spectrometer with time-of-flight detection. Typical MC reagents are metal chelating polymers that allow each antibody to be labeled with 150−200 copies of a metal isotope, typically a lanthanide isotope. Lanthanide NPs, for example, NaLnF4 NPs, consist of 1000 or more Ln atoms. Since the signal in ICP-MS increases linearly with the number of metal isotopes per antibody, NP reagents can in principle enhance the signal intensities in MC analyses.
Luminescent nanoparticles (NPs) have exceptional promise as bioimaging probes. Examples include quantum dots (QDs), noble metal clusters and noble metal NPs.1 To have the least impact on biological function, these NPs should be nontoxic, and, especially for intracellular imaging, they should be sufficiently small. For example, antibodies have a characteristic dimension of 10 nm. The most effective NPs for intracellular imaging should have diameters (d) less than 10 nm and preferably on the order of 5 nm.2 More recently, certain lanthanide-doped NPs have garnered significant interest due to their unique upconversion (UC) properties. The primary advantage of upconversion NPs (UCNPs) over traditional luminescent probes is that their excitation wavelength lies in the NIR region of the electromagnetic spectrum, an optical window where there is the least absorption and scattering of light by biomolecules. These excitation wavelengths nearly eliminate autofluorescence from cells and tissues that would interfere with the measurement of NP emission. Furthermore, the emission wavelength can also be varied from the UV to NIR regions by changing the type of activator ion, or dopant concentration.3,4 UC can also be observed at lower excitation power densities than emission © XXXX American Chemical Society
Received: April 21, 2017 Revised: July 21, 2017
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DOI: 10.1021/acs.jpcc.7b03783 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
(2.3 nm) over an upconverting core (11 nm). van Veggel and co-workers19 have shown that a thin shell (2 nm) of NaGdF4 over 3.5 nm NaGdF4:Yb,Tm increases the 800 nm emission by a factor of 10. These core/shell NPs were prepared by first synthesizing and purifying the upconverting core and then adding them in a second step to a mixture containing Gd oleate. Liu et al.14 prepared 10 nm NaGdF4:Yb,Tm NPs and showed that surface defects in these NPs can be removed and the relative blue to NIR emission intensities improved to match bulk material when an inert shell (2.5 nm in thickness) of NaGdF4 was grown over the core NP. Cohen and co-workers20 reported that growing a shell of NaYF4 over a 5 nm NaYF4:Yb,Er UC core led to core−shell NPs (d ∼ 9 nm) with a 100-fold increase in quantum yield over that of the core alone. Current methods to introduce a shell onto an upconverting NaLnF4 core are time-consuming, require two steps, are challenging for the synthesis of core/shell NPs with overall diameters less than 10−15 nm.21 Here we explore a one-pot strategy to prepare small (ca. 5 nm) and somewhat larger (12−15 nm) core−shell UCNPs in solution. Our goal was to synthesize small UCNPs that exhibit upconversion luminescence visible to the eye at low excitation powers. The basic idea was to use a mixture of lanthanide precursors known to have different nucleation rates,22 assuming that, in this coprecipitation reaction,23 NaLuF4, the slowest to nucleate, would deposit as a passivating shell on core NPs formed earlier in the reaction. Accordingly, we chose a reaction composition consisting of Lu (45 mol %), Gd (37 mol %), Yb (16 mol %), and Er (2 mol %). By having Gd (a Group II lanthanide)22 in the same reaction flask with Yb, Er, and Lu (Group III lanthanides),22 we hypothesized that gadolinium would nucleate first to form a hexagonal core matrix onto which erbium and ytterbium, (the upconversion active ions) would be incorporated. Finally, lutetium would form an inert shell over the UC core. We refer to these nanoparticles in terms of their composition as NaLuF4:Gd,Yb,Er, or more concisely as Lu/Gd UCNPs. To optimize conditions to synthesize UCNPs in our target size range, we examined the OA to ODE ratio, the sodium and fluoride to lanthanide ratio, reaction time, and reaction temperature. We explored the upconversion luminescence properties of these NPs and compared them to similar sized NPs lacking Lu in the reaction mixture. We found conditions that yielded uniform 4 nm NPs whose colloidal solutions showed UC luminescence easily visible to the naked eye. To our surprise, these were cubic (α) phase NPs. We consistently found that reaction mixtures containing Lu yielded particles that nucleated in the α-phase and then evolved into larger hexagonal (β-phase) NPs. Omission of Lu from the reactants gave UCNPs that appeared to nucleate directly in the β-phase.
An irritating technical problem associated with the purification and surface modification of nonluminescent NaLnF4 NPs is that they have no characteristic color. They are virtually impossible to see. Upconversion is important to us because it enables us to monitor these small NPs through a series of surface modification procedures. We are interested in small (5−15 nm) NaLnF4 NPs for tagging intracellular biomarkers for MC analysis. To facilitate the purification and surface modification of these NPs, we sought to introduce dopants that would render their colloidal solutions luminescent and visible to the naked eye when excited with a hand-held 978 nm laser. Despite the advantages of UCNP NaLnF4 NPs, practical applications have been limited by poor luminescence intensities. The luminescence from UCNPs comes from partially forbidden intra4f transitions.11 As a result, the absorption cross-section of Ln3+ is low compared to organic dyes and to QDs. Of the lanthanides, Yb3+ ions have the largest absorption cross-section (∼10−20 cm−2),12 which is why it is commonly used as the sensitizer ion. In addition to the absorption cross section, luminescence can also be affected by other factors, such as the crystal structure of the host matrix, size of the NP, surface quenching from defects, and interactions of the NP with the solvent and ligand molecules at the NP surface. The choice of host matrix is very important as the quantum yield decreases with increasing phonon energy of the matrix. Hexagonal phase NaYF4 or NaGdF4 have been shown experimentally to be the most efficient matrices for UCNPs13 due to their minimal phonon energy. The luminescence quantum yield increases with size as the number of lanthanide dopant ions in each NP increases. Therefore, the most commonly used UCNPs (20−30 nm) are much larger than the 5−15 nm NPs optimum for intracellular bioimaging. Furthermore, since smaller UCNPs have larger surface area to volume ratios, they are more susceptible to luminescence quenching by solvent and from surface defects.14 Currently, only a handful of reports exist on sub-10 nm UCNPs. One report by Liu et al.15 has shown visible upconversion emission from ca. 8 nm UCNPs at low powers (ca. 200 mW). These NPs were synthesized by thermal decomposition of lanthanide trifluoroacetate precursors. This method tends to suffer from poor reproducibility and produces HF as a toxic byproduct. Haase and co-workers16 recently reported a two-pot synthesis of 5 nm β-NaYF4:Yb,Er core UCNPs, over which they grew a 2 nm shell. They first prepared and purified 5 nm β-phase cores by using a high Na/Y ratio in the synthesis. In a separate reaction, they prepared sacrificial αNaYF4 NP to form the shell. Then the two types of NPs were added to a mixture of oleic acid and octadecene and heated to 300 °C to produce the final core/shell UCNP. While this approach is attractive, photoluminescence spectra for these samples were only shown from the NP powders, but not from their colloidal solutions. Cohen and co-workers17 have also reported the synthesis of 5 nm NaYF4:Yb,Er UCNPs for single particle imaging using a coprecipitation method with lanthanide chloride precursors and the addition of oleylamine to the solvent mixture to promote the formation of smaller NPs. Improving the luminescence of small UCNPs is a subject of intensive research. Several strategies have been employed to improve the brightness of UCNPs, and the most common strategy is to grow a shell over the core UCNP. For instance, Capobianco and co-workers18 reported significant upconversion enhancements by growing an upconversion-active shell
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EXPERIMENTAL SECTION Materials. The following were purchased from SigmaAldrich: 1-octadecene (ODE, technical grade, 90%), oleic acid (OA, technical grade, 90%), LuCl3·6H2O (99.9%), YbCl3·6H2O (99.99%), ErCl3·6H2O (99.99%), GdCl3·6H2O (99.999%), Y(CH3CO2)3·xH2O (99.9%), Gd(CH3CO2)3·xH2O (99.9%), Yb(CH3CO2)3·4H2O (99.9%), Er(CH3CO2)3·xH2O (99.9%), CH3OH (≥99.8%), and NH4F (≥99.99%). Lu(CH3CO2)3· xH2O (99.9%) was purchased from Alfa Aesar. NaOH (97%) was purchased from Caledon Laboratory Chemicals. Multilanthanide standards were purchased from PerkinElmer. HCl (32−35%) and HNO3 (67−70%) were purchased from Seastar B
DOI: 10.1021/acs.jpcc.7b03783 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Table 1. Reaction conditions that led to uniform small NaLnF4 NPs sample
mmol Na
mmol F
temp (°C)
time (min)
formula
Lu/Gd-4a Lu/Gd-13a Lu/Gd-20a Lu/Gd-9e Y/Gd-4a Y/Gd-6d Gd-4a Gd-5d
3.75 3.75 3.75 5.0 3.75 5.0 3.75 5.0
6 6 6 8 6 8 6 8
300b 300b 300c 300c 300b 285c 300b 285c
15 45 45 60 15 80 15 80
NaLuF4:Gd,Yb,Er NaLuF4:Gd,Yb,Er NaLuF4:Gd,Yb,Er NaLuF4:Gd,Yb,Er NaYF4:Gd,Yb,Er NaYF4:Gd,Yb,Er NaGdF4:Yb,Er NaGdF4: Yb,Er
size (nm)
crystal phase
± ± ± ± ± ± ± ±
cubic hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal
4.4 13.2 20 8.9 3.8 6.1 3.7 5.0
0.2 0.2 1 0.8 0.4 0.4 0.4 0.2
a
1 mmol lanthanide salts, 6 mL OA, 17 mL ODE, OA/ODE = 0.35. bFabric heating mantle. cCeramic heating mantle. d2 mmol lanthanide salts, 12 mL OA, 34 mL ODE, OA/ODE = 0.35. e2 mmol lanthanide salts, 24 mL OA, 34 mL ODE, OA/ODE = 0.7.
mL) was transferred to a 1.5 mL centrifuge tube, precipitated using an equal volume of absolute ethanol, and centrifuged at 5000g for 5 min. The supernatant was discarded and the pellet was redispersed in 0.5 mL hexanes; then the centrifuge tube was topped up with absolute ethanol and centrifuged again at 5000g for 5 min. The redispersion−precipitation cycle was repeated once more, and the final pellet was redispersed in 0.75 mL of hexanes and stored in a 1.8 mL scintillation vial. Instrumentation. Transmission Electron Microscopy (TEM). Oleate-capped NPs were diluted in hexanes and dropcast on a carbon/Formvar TEM grid. Either a Hitachi H-7000 transmission electron microscope operated at 100 kV or a Hitachi H-7700 instrument operated at 80 kV were used to take the images. Particle measurements for size histograms were obtained using Image-J, and at least 200 NPs were examined for statistical analyses. TEM samples for EELS and EDS measurements were subjected to 10 rounds of precipitation in absolute ethanol and redispersion in hexanes, and drop-cast on carbon/Formvar grids. TEM grids were then cleaned using a Hitachi ZONE Cleaner for 15 min on each side of the grid at a pressure setting of 50. Images were taken using the Hitachi HF-3300 operated at 300 kV. Powder X-ray Diffraction (PXRD). Power X-ray diffraction measurements were performed on a Rigaku Miniflex 600 diffractometer using Cu Kα radiation and a 2θ step size of 0.0075° from 0° to 80°. Samples were either drop-cast onto the sample holder from hexanes, or placed onto the sample holder as a powder. Thermogravimetric Analysis (TGA). NP samples were dried in a vacuum oven to remove residual organic solvents. Approximately 5 mg of sample was measured into an alumina pan and heated in a SDT Q600 (TA Instruments) thermogravimetric analyzer. NPs were heated to 100 °C in air at a heating rate of 10 °C/min and held isothermally for 30 min to remove traces of organics and moisture, followed by heating at 10 °C/min to 800 °C. The mass fraction of ligands was calculated from the percent mass loss associated with the second heating step. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). NPs were dried in a vacuum oven to remove residual organic solvents and water. Dried NPs (∼1 mg) were digested in aqua regia (4 mL, HCl/HNO3 (3:1 v/v), Seastar Chemicals, Inc.) overnight in sealed polypropylene tubes (5 mL). Tenfold dilutions of the solution in aqua regia were made with 2% HNO3 until ppb concentrations were obtained. The dilutions were repeated in triplicate. All samples were measured with an ELAN 9000 ICP-MS instrument. The standard solution was prepared by a series of 10-fold dilutions of multilanthanide
Chemicals Inc. These chemicals were used without further purification. Synthesis of 4 and 12−14 nm NaLuF4-Based UCNPs. In a typical synthesis, GdCl3·6H2O (0.37 mmol, 0.138 g), LuCl3· 6H2O (0.45 mmol, 0.175 g), YbCl3·6H2O (0.16 mmol, 0.062 g), ErCl3·6H2O (0.02 mmol, 0.008 g), OA (6 mL), and ODE (17 mL) were added to a 100 mL three-neck round-bottom flask and heated at 125 °C under 2 mmHg vacuum for 2 h. The resulting colorless clear solution was then cooled to room temperature. After releasing the vacuum, a freshly prepared, presonicated solution containing NaOH (3.75 mmol, 0.150 g) and NH4F (6.0 mmol, 0.222 g) in methanol (10 mL) was added into the flask. The milky white mixture was stirred at room temperature in air for 1 h, before being heated at ca. 60 °C under a flow of argon to remove the methanol. Once the methanol had evaporated and the temperature reached 100 °C, the reaction mixture was heated rapidly (ca. 10 °C/min) to 300 °C. It was held at 300 ± 1 °C for 15 min to obtain the 4 nm UCNPs or 45 min to obtain the 12−14 nm UCNPs. Temperatures were monitored directly in the reaction medium. Initial experiments employed a fabric heating mantle for this step. Later in the project, some reaction mixtures were heated with a ceramic heating mantle. This seemingly small change in protocol led to measurable differences in final NP sizes. Replacing the heating source with a new fabric heating mantle allowed us to recover conditions for synthesizing the original sizes of NPs. Synthesis of 9 nm NaLuF4-Based UCNPs. The synthesis of 9 nm NaLuF4:Gd,Yb,Er UCNPs followed the same general procedure as that described above for the 4 and 12 nm UCNPs except that the reaction was run on a 2 mmol scale with 24 mL OA, 34 mL ODE, 5 mmol NaOH, and 8 mmol NH4F. The final reaction temperature was still 300 °C and the heating time at this temperature was 60 min. Synthesis of NaYF4:Gd,Yb,Er and NaGdF4:Yb,Er NPs. The synthesis of NaYF4:Gd,Yb,Er and NaGdF4:Yb,Er NPs followed the same general procedure outlined for the synthesis of NaLuF4-based UCNPs except that different amounts of lanthanide precursor salts were used. In the case of NaYF4:Gd,Yb,Er UCNPs, Y(CH3CO2)3·xH2O (0.45 mmol), Gd(CH3CO2)3·xH2O (0.37 mmol), Yb(CH3CO2)3·4H2O (0.16 mmol), and Er(CH3CO2)3·xH2O (0.02 mmol) were used. In the case of NaGdF4:Yb,Er UCNPs, Gd(CH3CO2)3· xH2O (0.82 mmol), Yb(CH3CO2)3·4H2O (0.16 mmol), and Er(CH3CO2)3·xH2O (0.02 mmol) were used. Optimized reaction conditions for all UCNP syntheses are listed in Table 1. Washing the Nanoparticles. To remove excess solvent, ligands, and salts, an aliquot of the reaction mixture (∼0.75 C
DOI: 10.1021/acs.jpcc.7b03783 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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synthesize NaYF4:Yb,Er UCNPs (Y UCNPs). For 1 mmol of Ln salts, the reaction mixture consisted of an OA/ODE ratio of 0.35 (6/17 mL OA/ODE), 2.5 mmol NaOH, 4 mmol NH4F, with a final reaction temperature of 300 °C. Most experiments employed a fabric heating mantle. The first parameters we varied were the Na+/Ln3+ and F−/Ln3+ ratios. Haase and coworkers25 found that increasing the ratio of sodium to rare earth ions led to the dissolution of cubic NaYF4 NPs and promoted the formation of a large number of β-phase NaYF4 nuclei. This ultimately resulted in small ∼5 nm β-phase NaYF4 NPs. Liu and co-workers14 also found that by increasing the amount of the fluoride source (NH4F), they were able to obtain smaller NPs. We varied both the Na+/Ln3+ and the F−/Ln3+ ratios for a series of reactions run on a 1 mmol scale of Ln salts with the OA/ODE ratio fixed at 0.35 and a final temperature of 300 °C. We initially increased the amount of NH4F to 6 mmol while keeping the amount of NaOH at 2.5 mmol. Under these conditions, we obtained only polydisperse NPs. However, when we also increased the amount of NaOH to 3.75 mmol, we were able to obtain NPs with d ≈ 4 nm and a narrow size distribution after 15 min of heating at 300 °C (Figure 1A). These NPs satisfy one of the key objectives of our synthesis design, small NPs of uniform size. We refer to this sample as Lu/Gd-4.
standard solution (10 mg/L, 2% HNO3, PerkinElmer). The blank solution was 2 vol % HNO3. Upconversion Luminescence Intensity Measurements. Dispersions of NaLuF4:Gd, Yb, Er (4 nm), NaLuF4:Gd, Yb, Er (14 nm), NaGdF4:Yb, Er, and NaYF4:Gd, Yb, Er in hexanes were prepared such that each had an equal concentration of Yb3+ ions. The solutions were excited using a 978 nm continuous wave laser attached to a fluorometer (SPEX Fluorolog 3), from which we obtained the upconversion luminescence intensities. Each scan was carried out from 400 to 700 nm in increments of 1 nm. The integration time was 2 s, and the slit widths were 12 nm. Intensities were corrected for wavelength-dependent instrument effects using the MCORRECT file included in the instrument’s software package.
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RESULTS AND DISCUSSION Our hypothesis in the design of the NP synthesis strategy was that the differing nucleation behaviors of the lanthanides could be used to our advantage to obtain hexagonal phase core/shell UCNPs in a one-pot procedure. As mentioned in the Introduction, the lanthanides can be grouped into three categories based on the nucleation behavior of their respective NaLnF4 NPs: Group I (Pr, Nd), Group II (Sm to Tb), and Group III (Dy to Lu, Y).22 We focus on Group II and Group III lanthanides, and try to take advantage of the preference of the Group II lanthanides to nucleate directly in the hexagonal phase and at lower temperatures (230−260 °C) in the hexagonal phase compared to the lanthanides in Group III. Group III lanthanides tend to nucleate initially in the cubic phase and subsequently undergo a phase transformation at higher temperatures to the hexagonal phase. Based on these ideas, we chose a lanthanide composition of Lu (45 mol %), Gd (37 mol %), Yb (16 mol %), and Er (2 mol %) with the expectation that gadolinium would nucleate first to form a hexagonal core matrix incorporating the upconversion active ions Yb3+ and Er3+ as dopants. Lastly, lutetium would nucleate to form an inert shell over the UCNP core, yielding NaLuF4:Gd,Yb,Er (Lu/Gd) UCNPs. Our target was uniform small NPs with diameters in the range of 5−15 nm. We also synthesized UCNPs with two other compositions as control experiments. One sample had a lanthanide composition of 82 mol % Gd, 16 mol % Yb, and 2 mol % Er (NaGdF4:Yb,Er, Gd UCNPs). The second sample had a lanthanide composition of 45 mol % Y, 37 mol % Gd, 16 mol % Yb, and 2 mol % Er (NaYF4:Gd,Yb,Er, Y/Gd UCNPs). With this sample, gadolinium is still expected to nucleate first, but yttrium is expected to nucleate earlier than ytterbium and erbium. There should be no “shell” material in either sample. According to the LaMer growth model for colloidal particles, the final size of the particles depends on both the nucleation and growth stages of particle formation, and size uniformity will depend on good temporal separation between the two stages.24 Increasing the rate of nucleation, which increases the number of nuclei, will lead to the formation of smaller particles. The rate of nucleation of Ln NPs depends on many factors, including the solvent ratio (OA to ODE), the ratio of the lanthanide precursor salts to the amount of the sodium and fluoride sources, as well as the reaction temperature. We investigated these parameters systematically in order to find windows in this parameter space that would yield obtain uniform NPs in our target size range. Synthesis of NaLuF4:Gd,Yb,Er UCNPs. As a starting point, we employed reaction conditions typically used to
Figure 1. TEM images of NaLuF4:Gd,Yb,Er UCNP samples taken from the reaction mixture at 300 °C after heating for (A) 15 min (Lu/ Gd-4), (B) 20 min, (C) 25 min, and (D) 45 min (Lu/Gd-13). Other reaction parameters: 6 mL/17 mL OA/ODE, 3.8 mmol NaOH, 6.0 mmol NH4F. Scale bars are 100 nm. Inset in (A) shows UCL from a colloidal solution of Lu/Gd-4 in hexanes (3 wt %) when excited with a 978 nm laser.
Colloidal solutions of Lu/Gd-4 in hexanes showed upconversion emission visible to the eye when excited using a 978 CW laser (Figure 1A inset). When we increased the reaction time from 15 to 20 min, this led to a broadening of the size distribution (Figure 1B), and a further increase in reaction time to 25 min led to a bimodal distribution of NPs (Figure 1C). However, the size distribution became narrow again by 45 min, leading to uniform NPs with diameters of 12−14 nm (Figure 1D). These 12−14 nm UCNPs will be referred to as D
DOI: 10.1021/acs.jpcc.7b03783 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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polydisperse NPs formed at early times that evolved into more uniform larger hexagonal particles upon longer heating at 300 °C (Figures S3−S7). Since a similar kind of growth behavior was observed with these NPs as with Lu/Gd-4, we believe the initial small polydisperse NPs are also in the cubic phase. In an attempt to understand the effect of the reaction temperature, we also carried out reactions at 295 and 305 °C while keeping all other reaction conditions the same (6/17 mL OA/ODE, 3.75 mmol NaOH, 6.0 mmol NH4F). At 295 °C, at short times, we obtained only small (ca. 5 to 10 nm) polydisperse NPs, while at 305 °C, we again saw that there was a transition from a bimodal distribution of NPs at earlier times to larger hexagonal phase NPs (d ∼ 30 nm) at later times. This appears to suggest a cubic to hexagonal phase transformation as well. Therefore, it seems that changing the reaction temperature does not affect the initial nucleation in the cubic phase. In the experiments described above, we investigated the effects of the reactant ratios Na+/Ln3+, F−/Ln3+, the solvent composition (amount of OA, OA/ODE ratio), and temperature. We found in all these cases that the UCNPs nucleated initially in the cubic phase. We then investigated the influence of the lanthanide dopants. If we omitted Lu from the reaction and used conditions similar to those used to prepare Lu/Gd-4, or if we replaced Lu with Y (another Group III rare earth), we obtained small NPs in the hexagonal phase (Figure S8). In the first instance (see Table 1), we obtained 4 or 5 nm NaGdF4:Yb,Er, Gd UCNPs (82% Gd, 16% Yb and 2% Er) with weak upconversion luminescence. In the second instance, we obtained 4 or 6 nm NaYF4:Gd,Yb,Er, UCNPs (45% Y, 37% Gd, 16% Yb and 2% Er). We also found that if we synthesized Gd and Y/Gd UCNPs using the typical amounts of NaOH and NH4F (2.5 and 4 mmol per mmol Ln salts) and a lower reaction temperature (285 °C) (Table 1), we still obtained small NPs that were in the hexagonal phase. Our results suggest that in the presence of Lu, regardless of the amount of Gd doping, or the reaction conditions, the NPs nucleate directly in the cubic phase and undergo a dissolutionrenucleation process to form hexagonal phase NPs. However, in the absence of Lu, we find that Gd aids the direct nucleation of the NPs in the hexagonal phase. This result is different from a suggestion in the literature that including Gd3+ in the synthesis of NaYF4 UCNPs lowers the cubic to hexagonal phase transformation temperature. This idea was based on the observation by the Liu group26 mentioned above, that hexagonal UCNPs were formed in the presence of Gd3+ under mild conditions (90 min at temperatures as low as 230 °C). They did not, however, report a kinetic study to see if the nanoparticles were initially formed in a cubic phase followed by a phase transformation or whether they nucleated directly in the hexagonal phase. Our results are more consistent with direct nucleation into the hexagonal phase. In summary, we were able to find only one set of reaction conditions to synthesize uniform small (sub-5 nm) Lu/Gd UCNPs. While we expected to obtain these NPs in the hexagonal phase, we were surprised to find that these uniform and emissive NPs were in the cubic phase Time evolution studies showed that all reactions involving Lu initially gave small cubic phase NPs that evolved by Ostwald ripening into larger and often uniform hexagonal NPs with diameters in the range of 10−15 nm. On the other hand, all reactions without Lu gave hexagonal phase NPs directly.
Lu/Gd-13. ICP-MS results show that the lanthanide compositions of both Lu/Gd-4 and Lu/Gd-13 UCNPs (and all other NPs reported here) match well with the feed ratio of the lanthanide precursors (Table S1). Cubic versus Hexagonal Phase NPs. Powder X-ray diffraction (XRD) peaks for samples Lu/Gd-4 and Lu/Gd-13 are presented in Figure 2A,B along with reference peaks
Figure 2. PXRD traces for NaLuF4:Gd,Yb,Er UCNPs. Lu/Gd-4 (d ≈ 4 nm) (A) and Lu/Gd-13 (d ≈ 13 nm) (B) with cubic and hexagonal line reference peaks (C, D). Hexane dispersions of the NPs were dropcast onto the sample holder for analysis.
expected for cubic and hexagonal NaGdF4 structures (Figure 2C,D). The PXRD peaks for the Lu/Gd-4 nm sample can be matched to those of cubic phase NaGdF4, belonging to the Fm3̅m space group. In contrast, the peaks in the XRD spectra of the Lu/Gd-13 nm NPs correspond to those of hexagonal phase NaLnF4. Diffraction peaks for the 4 nm Lu/Gd UCNPs are broader than those of the 13 nm Lu/Gd UCNPs, consistent with a smaller crystallite size. We were surprised to find that sample Lu/Gd-4 was in the cubic phase, since we hypothesized gadolinium would nucleate first, due its lower nucleation temperature, and form nuclei in the hexagonal phase. It is also unexpected to find NPs in the cubic phase for reaction mixtures containing large amounts of Gd (e.g., 37 mol %) and at temperatures as high as 300 °C. Even when we increased the amount of Gd to 62 mol % and reduced the amount of Lu to 20 mol % (Figures S1, S2), where NaGdF4 would be the major component, we found that the UCNPs still initially nucleated in the cubic phase. For example, Liu and co-workers26 reported that in the synthesis of NaYF4 UCNPs, with 30% gadolinium included in the reaction mixture, they observed hexagonal phase NPs after 90 min, even at temperatures as low as 230 °C. Our cubic phase NPs were stable for several months at room temperature, not only in the reaction medium (octadecene and oleic acid), but also as a hexane dispersion after washing the NPs, and as powders. They do not undergo Ostwald ripening to form hexagonal phase NPs even if left in the reaction mixture (with excess oleic acid) for several months. XRD measurements (not shown) on “old” samples aged several months were indistinguishable from that shown in Figure 2A below. To explore other features of the reaction conditions that could lead to the formation of NPs in the cubic phase at early times, we carried out a series of experiments at different OA/ ODE ratios, keeping other aspects of the reaction constant. In all of the reactions in which Lu was present, small, often E
DOI: 10.1021/acs.jpcc.7b03783 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
nm. TEM images of all batches of Lu/Gd-4 and Lu/Gd-13 are shown in Figure S9. Over the course of these studies, the fabric heating mantle used for these experiments stopped working. When we began working with a ceramic heating mantle, even though we measured temperatures inside the reaction flask, we could no longer obtain uniform 4 nm NPs. Only polydisperse NPs were obtained at short times and larger NPs (d ∼ 20 nm, Table 1) were obtained upon prolonged heating. When we replaced the heating source with another fabric heating mantle, we were able to repeat the results described above. These observations also demonstrate the sensitivity of the synthesis is to subtle changes in the reaction conditions. Selected samples were examined by TGA to determine the surface coverage by oleic acid after three cycles of precipitation with ethanol and redispersion in hexanes to remove excess OA. The TGA traces and a table indicating the wt % loss and the OA footprint (OA molecules/nm2) are presented in Figure S10. Generally, the larger nanoparticles had a lower weight fraction of ligands compared to the smaller nanoparticles. The surface coverage of ligands are also all within the theoretical limit for binding of carboxyl groups to a surface (5 carboxylates nm−2).27 Contrary to what one would expect, the smallest NPs, Lu/Gd-4 (d = 4.6 nm) had the lowest surface coverage of oleates (2.6 nm−2), while the largest NPs, NaYF4:Yb,Er (d = 26.4 nm) had the highest surface coverage (5.0 nm−2). While the washing procedure could have affected the surface coverage of certain samples,5 care was taken to make sure all the samples for TGA were washed and prepared in the same way. These values may reflect differences in the inorganic surface composition of the NPs. Upconversion Luminescence. The UCL properties of dispersions of three NaLuF4:Gd,Yb,Er samples and one NaYF4:Gd,Yb,Er sample were examined for dispersions in hexanes excited with a 978 nm CW laser. Samples of Lu/Gd-4 (cubic, d ≈ 4 nm) and Lu/Gd-13 (hexagonal, d ≈ 14 nm) UCNPs were examined at an overall Yb3+ concentration of 3 × 105 M, and the resulting spectra are shown in Figure 4A. We can see the typical emission peaks at 525 nm (2H11/2 → 4I15/2), 540 nm (4S3/2 → 4I15/2), and 655 nm (4F9/2 → 4I15/2) for both samples.15 The emission is much weaker for the 4 nm Lu/Gd UCNPs, and the relative emission intensities at 542−655 nm are reversed compared to those of the 14 nm Lu/Gd UCNPs. This phenomenon of reversal in the green to red ratio is typical of smaller UCNPs. As the size of the NP decreases, the surface area to volume ratio increases. The total amount of adsorbed oleate ligands increases, and the number of crystal defects increases. Since the vibrational energies of the −CH2 and −OH bonds from the oleate ligands match the energy difference between the 4S3/2 and 4F9/2 energy levels, quenching of the green emission can lead to an increase in the red emission.3,28,29 The TEM images in Figure 4B,C demonstrate the size uniformity of the samples. The most surprising observation is that UC luminescence of the cubic 4 nm Lu/Gd NPs as a colloidal solution in hexanes is visible to the eye when excited at 978 nm with a power density of 150 W/cm2 (Figure 1, insert). This emission intensity, however, is weak compared to the luminescence of the 13 nm hexagonal Lu/Gd UCNPs as seen in Figure 4A. Corresponding luminescence spectra (Yb3+ = 5 × 105 M) of samples of Lu/Gd-9 (hexagonal, d ≈ 9 nm) and Y/Gd-6 (hexagonal, d ≈ 6 nm) are shown in Figure 4D, with corresponding TEM images presented in Figure 4E,F. The
Through varying the OA/ODE ratio, we found several sets of conditions to obtain uniform hexagonal UCNPs with diameters in the range of 12 to 15 nm, and one set of conditions that led to smaller NPs. Here with OA/ODE = 0.7 (heated 60 min at 300 °C with a ceramic heating mantle), we obtained uniform hexagonal NPs with d = 8.9 ± 0.8 nm (Table 1, Figure S4). These very useful 9 nm NPs will be referred to as Lu/Gd-9. A summary of characteristic reaction conditions that led to uniform small NaLnF4 NPs is provided in Table 1. These reactions were all very sensitive to the final reaction temperature, as even a deviation of
