Size- and Phase-Controlled Synthesis of Monodisperse NaYF4:Yb,Er

Aug 24, 2007 - Jing Zhou , Qian Liu , Wei Feng , Yun Sun , and Fuyou Li. Chemical Reviews 2015 115 (1), 395-465. Abstract | Full ... Chemistry of Mate...
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J. Phys. Chem. C 2007, 111, 13730-13739

Size- and Phase-Controlled Synthesis of Monodisperse NaYF4:Yb,Er Nanocrystals from a Unique Delayed Nucleation Pathway Monitored with Upconversion Spectroscopy Hao-Xin Mai, Ya-Wen Zhang,* Ling-Dong Sun, and Chun-Hua Yan* Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth Materials Chemistry and Applications & PKU-HKU Joint Lab in Rare Earth Materials and Bioinorganic Chemistry, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: May 21, 2007; In Final Form: July 6, 2007

Monodisperse R-NaYF4:Yb,Er and β-NaYF4:Yb,Er nanocrystals with controlled size, chemical composition, and surface state were synthesized from trifluoroacetate precursors in hot surfactant solutions via a unique delayed nucleation pathway. The growth mechanisms of both R- and β-phase nanocrystals and the underlying Rfβ phase transition process were uncovered by upconversion (UC) spectroscopy, transmission electron microscopy, and X-ray diffraction techniques. The UC emission was sensitive to the growth process of the NaYF4:Yb,Er nanocrystals; that is, the UC intensity is sensitive to the nucleation and phase transition process, and the ratio of green to red emission is also sensitive to the crystallite size and phase, which makes it easy to distinguish various nanocrystal growth stages and phase transition modes with UC spectroscopy. Differently sized monodisperse R-NaYF4:Yb,Er nanopolyhedra (5-14 nm) were readily obtained via only prolonging the reaction time. During the reaction, four consecutive nanocrystal growth stages including nucleation in a delayed time, particle growth by monomer supply, size shrinkage by dissolution, and aggregation were identified. The essential of the delayed nucleation was likely due to the requisite in accumulating enough NaF (from the decomposed Na(CF3COO)) for the nanocrystal growth. Monodisperse β-NaYF4:Yb,Er nanocrystals with tunable sizes in a broad range from 20 to 300 nm were obtained from R-NaYF4:Yb,Er monomers by restricting or enhancing the Ostwald-ripening process, in which the Rfβ transition happened in a delayed time.

Introduction Colloidal inorganic nanocrystals are of great fundamental and technical interest, due to their unique size-/shape-dependent properties and their promise in assembling advanced materials and devices from nanoscaled building blocks via bottom-up technology.1 Particularly, the rational design of synthetic protocol toward monodisperse colloidal nanocrystals and the revealment of the underlying chemical principles are crucially important not only for a reproducible large-scale fabrication of high-quality products applied in multidisciplinary areas, but also for the correct interpretation of the collective physical properties of an ensemble of particles in terms of the features of an individual particle.1,2 Upconversion (UC) luminescent nanomaterials have attracted much research interest due to their applications in various optical devices such as solid-state lasers, three-dimensional flat-panel displays, and low-intensity IR imaging.3-5,7,8 In particular, the UC nanomaterials with manipulated sizes have been widely studied not only in traditional optical devices and white light source powders, but also in sensitive bio-probes and bio-images,5 due to their features of low background light, high detection limits, and low toxicity as compared to the majority of current commercialized labels such as organic dyes and quantum dots.6 Among them, hexagonal phase (β phase) NaYF4:Yb,Er is known as one of the most efficient UC materials under near-infrared (NIR) excitation. More recently, much research effort has been * Corresponding authors. Fax: +86-10-6275-4179. E-mail: yan@ pku.edu.cn (C.-H.Y.).

devoted to the synthesis, characterization, and optical properties of dispersible NaYF4:Yb,Er/Tm nanocrystals. The solution methods including modified precipitation,7a hydrothermal method,7b and non-hydrolytic approaches8 have been developed to prepare these nanocrystals with controlled size/shape/phase. However, some challenges still remain in this field: how to fabricate the differently sized nanocrystals in a controllable and reproducible way to uncover the size-dependent UC behaviors and the detailed UC mechanisms, and how to obtain multicolor UC emissions without a filter for various applications.3-5,7,8 It is clear that the key point to face these challenges is to understand the detailed nanocrystal growth kinetics in the solution phase, based on the information collected by various robust modern characterization methods. The well penetrability excitation and the intensive emissions of UC materials are considered to facilitate UC spectroscopy to the monitoring of the different growth stages of UC nanocrystals. In this Article, we report the controlled synthesis of size tunable monodisperse R-NaYF4:Yb,Er (5-14 nm) and β-NaYF4: Yb,Er nanocrystals (20-300 nm) from trifluoroacetate precursors via a unique delayed nucleation pathway in solution phase. It is the first time that UC spectroscopy, combined with transmission electron microscopy and X-ray diffraction technique, were used to investigate the growth mechanisms of both R- and β-phase nanocrystals, and the underlying Rfβ phase transition process. The conceptual delayed nucleation is deeply discussed and demonstrated as a unique pathway for achieving size monodispersity in this work.

10.1021/jp073919e CCC: $37.00 © 2007 American Chemical Society Published on Web 08/24/2007

Synthesis of Monodisperse NaYF4:Yb,Er Nanocrystals Experimental Section The synthesis was carried out using standard oxygen-free procedures and commercially available reagents. Rare-earth oxides (RE: Y, Yb, Er), oleic acid (OA; 90%, Alpha), oleylamine (OM; >80%, Acros), 1-octadecene (ODE; >90%, Acros), trifluoroacetic acid (99%, Acros), CF3COONa (>97%, Acros), absolute ethanol, and cyclohexane were used as received. RE(CF3COO)3 was prepared by the literature method.8a,9 Synthesis of r-NaYF4:20%Yb,2%Er Nanocrystals. 1 mmol of CF3COONa and a suitable proportion of Y(CF3COO)3, Yb(CF3COO)3, and Er(CF3COO)3 were added to a mixture of OA (10 mmol), OM (10 mmol), and ODE (20 mmol) in a threenecked flask at room temperature. Next, the slurry was heated to 100 °C to remove water and oxygen, with vigorous magnetic stirring under vacuum for 30 min in a temperature-controlled electromantle, and thus to form an optically transparent solution. The solution was then heated to 250-320 °C at a heating rate of 20 °C min-1, and the solution became a bit turbid and was maintained at this temperature for 0.5-10 h under an Ar atmosphere. Next, an excess amount of ethanol was poured into the solution at room temperature. The resultant mixture was centrifugally separated, and the products were collected. The as-precipitated nanocrystals without any size-selection were washed several times with ethanol and then dried in air at 70 °C overnight, showing a yield between 50% and 70%. The weight of the capping ligands on the surfaces of the R-NaYF4: 20%Yb,2%Er nanocrystals was 22% of the whole products (Figure S1). The afforded nanocrystals could be easily redispersed in various nonpolar organic solvents (e.g., cyclohexane). Synthesis of β-NaYF4:20%Yb,2%Er Nanocrystals. The synthetic procedure was the same as that used to synthesize R-NaYF4:20%Yb,2%Er nanocrystals, except that quantitative CF3COONa and R-NaYF4:20%Yb,2%Er nanocrystals prepared were added to a mixture of OA (20 mmol) and ODE (20 mmol) in a three-necked flask at room temperature (Table S1). Instrumentation. The thermo-gravimetry (TG) runs were performed with a Universal V2.60 TA instrument at a heating rate of 5 °C min-1 from room temperature to 700 °C, using a-Al2O3 as a reference. Powder X-ray diffraction (XRD) patterns of the dried powders were recorded on a Rigaku D/MAX-2000 diffractometer (Japan) with a slit of 1/2° at a scanning rate of 2° min-1, using Cu KR radiation (λ ) 1.5418 Å). The lattice parameters were calculated with the least-squares method. Samples for transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analysis were prepared by drying a drop of nanocrystal dispersion in cyclohexane on amorphous carbon-coated copper grids. Particle sizes and shapes were examined by a TEM (200CX, JEOL, Japan) operated at 160 kV and a SEM (DB-235 focused ion beam (FIB) system). High-resolution TEM (HRTEM) was performed on a Philips Tecnai F30 FEG-TEM operated at 300 kV. Inductively coupled plasma (ICP-AES) (Plasma-Spec, Leeman Labs Inc.) was used to determine Y3+, Yb3+, and Er3+ contents in NaYF4:Yb,Er nanocrystals. FTIR spectra of the samples (prepared by directly depositing the product solutions in cyclohexane onto a KBr wafer) were obtained with a Nicolet Magna 750 spectrophotometer. Room-temperature UC fluorescence spectra of the asformed transparent reaction solutions (aspirated directly from the reaction system by a 1 mL plastic syringe without any posttreatment and then cooled to room temperature) and the NaYF4:Yb,Er/Tm nanocrystals redispersed in cyclohexane (1 wt %) were measured on a modified Hitachi F-4500 spectrophotometer with an external tunable 2 w 980 nm laser diode

J. Phys. Chem. C, Vol. 111, No. 37, 2007 13731 (excited power density: 1.22 W cm-2) as the excitation source in place of the xenon lamp in the spectrometer. Results and Discussion 1. Characteristics of Differently Sized r-NaYF4:Yb,Er and β-NaYF4:Yb,Er Nanocrystals. (13.7 ( 0.8), (8.0 ( 1.0), and (5.1 ( 0.6) nm R-NaYF4:Yb,Er nanopolyhedra in a pure cubic structure (space group: Fm-3m; Figure 1a and Table S2) were obtained via only prolonging the reaction time from 1, to 3, then to 5 h, respectively (Figure 1b-d), under the co-thermolysis of 1 mmol of CF3COONa and 1 mmol of RE(CF3COO)3 in 40 mmol of OA/OM/ODE at 250 °C. HRTEM inset in Figure 1b shows lattice fringes of the (111) planes for the 13.7 nm R-NaYF4:Yb,Er nanopolyhedra, while 5.1 nm R-NaYF4:Yb,Er nanopolyhedra are in the shape of truncated cubes enclosed by the {100} and {111} facets (Figure 1d). High-quality β-NaYF4:Yb,Er nanocrystals (nanospheres, hexagonal nanoprims, and hexagonal nanoplates) in a pure hexagonal structure (space group: P-6; Figure 2a and Table S2) with different sizes ranging from 20 to 300 nm were selectively synthesized under 330 °C for 15 min, via manipulating the precursor ratio of R-NaYF4:Yb,Er to CF3COONa and the synthetic conditions of R-NaYF4:Yb,Er precursor (Table S1 and Figure 2b-g). Panels b-g of Figure 2 show the TEM images of (20.2 ( 0.9) and (27.7 ( 1.3) nm nanospheres, (38.1 ( 1.9) nm × (30.8 ( 1.8) nm and (47.1 ( 2.0) nm × (45.8 ( 2.3) nm hexagonal nanoprisms, and (72.1 ( 4.8) nm × (45.5 ( 2.6) nm and (309 ( 18) nm hexagonal nanoplates of β-NaY0.78Yb0.2Er0.02F4 in turn, and the SEM image shows the hexagonal obverse and the quadrate side faces of the (185 ( 9) nm × (75 ( 3.4) nm β-NaY0.78Yb0.2Er0.02F4 nanoplates (Figure 2h). HRTEM measurements reveal that all of the as-prepared nanocrystals are single-crystalline, the nanospheres mainly show the lattice fringe of (11-20) planes (inset in Figure 2c), and the nanoprisms and nanoplates are enclosed by the {10-10} and {0001} facets (insets in Figures 2e,g,h and S2). 2. Monodisperse r-NaYF4:Yb,Er Nanocrystals Formed by A Delayed Nucleation Pathway. The results of UC spectroscopy and TEM measurements suggest that the growth kinetics of monodisperse R-NaYF4:Yb,Er nanocrystals includes four main stages (Scheme 1): (1) nucleation in a delayed time; (2) size growth by monomer supply; (3) size shrinkage by dissolution; and (4) aggregation. The UC spectral change of the reaction solution is typical in each stage. Stage I. Once the reaction temperature reached 250 °C, CO2 and fluorinated/oxyfluorinated carbon species were released from the reaction solution, detected by a gas chromatograph/ mass spectrometer, implying that the decomposition of CF3COOoccurred at the beginning of the reaction.8a However, no particles but some gels, which did not exhibit obvious XRD peaks and UC signals ascribable to R-NaYF4:Yb,Er, were isolated from the solution even prolonging the reaction time to 20 min (Figures 3a,b and S3a,b and Supporting Information). As seen from Figure 3a, faint UC signals appeared at 29 min, and the UC intensity sharply increased instantaneously from 29 to 30 min, indicating a sudden nucleation of R-NaYF4:Yb,Er in 1 min, as also verified by the TEM results. At 29 min, only some 1 nm R-NaYF4:Yb,Er nanoparticles coexisted with a great amount of gel (Figure 3c), which transformed to 10.8 nm R-NaYF4: Yb,Er nanopolyhedra in 1 min (at 30 min) (Figures 3d and S3a). At this moment, the solution became turbid suddenly. Beyond 30 min, the growth of R-NaYF4:Yb,Er nanopolyhedra seemed mild, and the size increased slightly from 10.8 nm at 30 min (Figure 3d), to 13.6 at 45 min,8a and to 13.7 nm at 60 min

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Figure 1. (a) XRD patterns of R-NaYF4:Yb,Er nanocrystals obtained at 1, 3, and 5 h under 250 °C in OA/OM/ODE. TEM and HRTEM (inset) images of R-NaYF4:Yb,Er nanocrystals obtained at (b) 1, (c) 3, and (d) 5 h at 250 °C in OA/OM/ODE.

(Figure 1b). These observations strongly suggested that the formation of R-NaYF4:Yb,Er nanopolyhedra took a delayed nucleation pathway (also see Figure S4 and Supporting Information), which is similar to the case reported by Alivisatos’s group for the growth of monodisperse Fe2O3 nanodisks from Fe(CO)5 precursor.2e As is well-known, the delayed nucleation is extremely useful for the synthesis of monodisperse nanocrystals because it does not need a rapid (and in many cases irreproducible) initial injection of precursor.2e Therefore, unfolding the intrinsic principles of this process is very important. In the Fe(CO)5Fe2O3 system, reaction temperature and precursor concentration are two key factors in the delayed nucleation.2e The higher are the concentration of the iron precursor and reaction temperature, the shorter is the retardation in nanocrystal nucleation, and the smaller is the size of the nanocrystals.2e In the present system, the induction time (ti) for the nucleation of R-NaYF4:Yb,Er nanocrystals and the crystallite size strongly rely on the reaction temperature, but are independent of the concentrations of CF3COONa and RE(CF3COO)3 (Figures 4a and S5). With increasing temperature, ti is gradually shortened (see Figure 3a; 230 °C, >30 min; 250 °C, 30 min; 270 °C, ∼12 min; 290 °C, ∼7 min; 310 °C, 5 min). In addition, the size of the R-NaYF4: Yb,Er nanocrystals collected at 1 min after the nucleation decreases with increasing temperature (Figure 3d, 250 °C, 10.8 nm; Figure 3e, 270 °C, 7.3 nm; Figure 3f, 290 °C, 6.8 nm). However, at 250 °C, ti is about 30 min and the particle size remains around 11 nm, whenever the amount of either CF3COONa or RE(CF3COO)3 varied from 1.5 to 0.5 mmol (Figure 4a). Furthermore, no particles were obtained at 250 °C for 1 h as the amount of CF3COONa was 0.25 mmol (Figure 4a). In the Fe(CO)5-Fe2O3 system, the delayed nucleation is caused by the gradual transformation of Fe(CO)5 into intermediate species (such as higher nuclear clusters of carbonyls or metal-surfactant complexes), which then serves as the active

“monomer” species during the nanocrystal growth.2e Consequently, both a high temperature and iron concentration speed up the reaction and induce the formation of small nuclei.2e However, in the present system, R-NaYF4:Yb,Er is produced by the coprecipitating reaction between NaF (decomposed from CF3COONa) and RE(CF3COO)3 (undecomposed below 310 °C1i). To further uncover the delayed nucleation process, 0.2, 0.5, and 1 mmol of NaF dissolved in OA was injected into the solution after reacting for 12 min at 250 °C. ti decreased with increasing amount of NaF injected (Figure 4b). Once too much NaF (>0.5 mmol) was injected, the nucleation took place immediately (Figure 4b). Particularly, the detectable UC signals appeared instantly as soon as 1 mmol of NaF instead of CF3COONa was injected at 250 °C, suggesting much less induction time for the nucleation of R-NaYF4:Yb,Er with NaF and RE(CF3COO)3 as the precursors (Figure 4b). Moreover, the nucleation threshold of R-NaYF4:Yb,Er was studied with the amount of RE(CF3COO)3 fixed at 1 mmol. Figure 4c shows that a higher temperature leads to a lower nucleation threshold (nNaF was 1 mmol at 180 °C, 0.5 mmol at 250 °C, and 0.25 mmol at 290 °C). A schematic process of delayed nucleation for R-NaYF4:Yb,Er is depicted in Scheme 1. CF3COONa decomposes into NaF gradually as the reaction temperature exceeds 250 °C. If the concentration of NaF formed at a short reaction time is lower than the nucleation threshold, no nanocrystals are formed through the coprecipitation of NaF and RE(CF3COO)3. The reaction time at this point is considered as ti. When NaF accumulates enough, a burst of nucleation occurs with a very fast growth rate of the R-NaYF4:Yb,Er nanocrystals soon after the nucleation, and the size of the nanocrystals increases abruptly. ti is short if the nucleation threshold is low. The decomposition rate of CF3COONa increases with the temperature, while it is independent of the concentration. Therefore, ti is invariable when the amount of CF3COONa exceeds the nucleation threshold. This process is

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Figure 2. (a) XRD patterns of the β-NaY0.78Yb0.2Er0.02F4 nanocrystals with different sizes. TEM and HRTEM (inset) images of (b) (20.2 ( 0.9) nm β-NaY0.78Yb0.2Er0.02F4 nanospheres, (c) (27.7 ( 1.3) nm β-NaY0.78Yb0.2Er0.02F4 nanospheres, (d) (38.1 ( 0.9) nm × (30.8 ( 1.8) nm β-NaY0.78Yb0.2Er0.02F4 hexagonal nanoprisms, (e) (47.1 ( 2.0) nm × (45.8 ( 2.3) nm β-NaY0.78Yb0.2Er0.02F4 hexagonal nanoprisms, (f) (72.1 ( 4.8) nm × (45.5 ( 2.6) nm β-NaY0.78Yb0.2Er0.02F4 hexagonal nanoplates, and (g) (309 ( 18) nm hexagonal nanoplates. (h) SEM image of (185 ( 9) nm × (75 ( 3.4) nm β-NaY0.78Yb0.2Er0.02F4 hexagonal nanoplates.

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SCHEME 1: Schematic Illustration of the Growth Stages of r-NaYF4:Yb,Er Nanocrystals via a Delayed Nucleation Pathway

further verified by the study of the nucleation threshold with changing the amount of RE(CF3COO)3. ti is unchanged with the amount of RE(CF3COO)3 decreasing from 1.5 to 0.2 mmol at 250 °C (Figure 4d), indicating that this reaction is mainly induced by NaF (also see Figure S6 and Supporting Information). At the nucleation threshold concentration (Figure 4c), the nucleation rate of the R-NaYF4:Yb,Er is higher (resulting in the decrease of the particle size) with increasing temperature (Figure S6c, 250 °C, ∼6 nm; Figure S6d, 290 °C, ∼4 nm). Hence, the higher is the temperature, the smaller are R-NaYF4: Yb,Er nanopolyhedra with CF3COONa and RE(CF3COO)3 as the precursors. Stages II and III. The controlled synthesis of monodisperse nanocrystals is extensively concerned nowadays. Generally, monodisperse nanocrystals are obtained via controlling the monomer concentration in the solution.1,2 The small crystal is unstable due to a large fraction of active surface atoms, and it tends to cut down the surface to reduce the surface energy. If the monomer concentration is high (at the beginning of the reaction or by injection), the growth rate of the small crystal is higher than the large one and results in a narrow size distribution. When the monomer concentration is low or even consumed completely, the small crystal is redissolved due to the high surface energy and the large one grows (Ostwald ripening), leading to a broad size distribution.2 In the present system, the size of R-NaYF4:Yb,Er nanopolyhedra increases uniformly in early time of stage II, indicating that the monomer concentration is high during this period (Figures 3d, S7d, 1b, and S7e), and then the size of the nanocrystals nearly remains invariant (13.7 ( 0.8 nm at 1 h; 13.5 ( 0.6 nm at 2 h), revealing that the monomers tend to be exhausted (Figures 1b, S7e, S7a, and S7f), whereas in stage III (from 2 to 5 h), the size of R-NaYF4:Yb,Er nanopolyhedra decreases (Figure 1c,d) and the size distribution of R-NaYF4:Yb,Er nanopolyhedra is broadened relatively (Figure S7f-h), indicating that the monomers are completely consumed and the R-NaYF4:Yb,Er nanopolyhedra are dissolved. Interestingly, it seems that in this stage the small nanocrystal is more stable than the large one (Table S3). To understand the reason accounting for the size variation of R-NaYF4:Yb,Er nanopolyhedra, FTIR spectra of 8.0 and 5.1 nm R-NaYF4:Yb,Er nanopolyhedra and the contrastive experiments are shown in Figure 5. The peak at about 1711 cm-1 was assigned as the carbonyl peak of free carboxylic acid groups (COOH), while the COO- asymmetric stretch was at about 1530 or 1540-1560 cm-1.1g,h No COO- asymmetric stretch was observed for pure OA (spectrum A). In the case that the bonding interactions between OA molecules and RE3+ ions were weak (1 mmol of Y(OH)3 reacted with 3 mmol of OA for 10 min), only the peaks of COOH and COO- were detected (spectrum B). However, when the bonding interactions became strong (1 mmol of Y(OH)3 reacted with 10 mmol of OA for 30 min), a peak at 1640 cm-1 ascribed to coordinated carbonyl groups was

detected (spectrum C). As Y(OA)3 could be obtained via heating the mixture of Y(OH)3 and OA under vacuum for a long time, it seemed that the intensity of the peak at 1640 cm-1 represented the strength of the coordinating interaction between Y3+ and COO-. To our knowledge, the interaction between nanocrystal surface and capping ligand of long alkyl chain organic compounds (such as OA) has two basic modes, that is, strong coordination interaction10a and weak van der Waals interaction.10b Therefore, for the 8.0 nm R-NaYF4:Yb,Er nanopolyhedra (spectrum D), the appearance of the peaks of free COOH and COO- (similar to the case of spectrum B) indicated a weak van der Waals interaction between OA molecules and the nanocrystal surfaces, whereas, for the 5.1 nm R-NaYF4:Yb,Er nanopolyhedra (spectrum E), besides the COO- asymmetric stretch, no peaks at 1711 cm-1 but that at 1640 cm-1 was detected, revealing that the 5.1 nm nanocrystal surfaces were strongly combined by coordinative OA molecules. HRTEM image inserted in Figure 1d reveals that the main exposed crystal facets of the 5.1 nm nanopolyhedra are the (100) and (111) planes, showing a shape of truncated cube, which is different from that of the typical R-NaREF4 nanopolyhedra and the 13.7 nm R-NaYF4:Yb,Er nanopolyhedra (Figure 1b).8a Thus, it seems that the exposed (100) facets of R-NaYF4:Yb,Er are stabilized by the strong binding of OA molecules on the R-NaYF4:Yb,Er nanopolyhedra surfaces (Figure S8 and Supporting Information). Therefore, the processes of stages II and III are supposed and illustrated in Scheme 1. The concentrations of the monomer are high in stages I and II. The nanocrystals grow rapidly and uniformly and expose the (111) and (110) planes.2,11 Upon further growing larger with time, they became more unstable for the reduced bonding interaction between nanocrystal surface and OA (presumably dominated by van der Waals interaction). When the monomers are consumed completely, the nanocrystals are redissolved and expose the (100) planes stabilized by the strongly coordinated OA ligands. Under this condition, the small truncated cubes are obtained. Stage IV. When the reaction develops further, the truncated nanocubes tend to form quite large nonuniform aggregates about 37 nm (Figure S7b,i) at 8 h and 45 nm at 10 h (Figure S7c,j), possibly due to the much high surface energy of the small nanocubes and the attractive forces from the condensation of the surface ligands at this stage. Noticeably, it is very easy to distinguish the above four stages from the temporal evolution of the UC spectrum (Figures 6 and S9, and Supporting Information). The UC intensity (IUC) and the intensity ratio of green to red emission (fg/r) are suddenly detected in stage I (0.5 h). In stage II (0.5-2 h), the IUC increases sharply while fg/r nearly keeps invariant. Contrarily, the variation of IUC is mild in stage III (2-5 h), but fg/r shows an abrupt decrease from 0.43 to 0.12. In stage IV (5-10 h), fg/r increases sharply but the IUC increases slightly. Therefore, UC spectros-

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Figure 3. (a) UC intensity (sum of integrated peak area: green plus red) variation of the solutions collected at different periods of times (0-30 min) at different temperatures (230-310 °C), using 1 mmol of CF3COONa and 1 mmol of RE(CF3COO)3 as the precursors in OA/OM/ODE. TEM and HRTEM images (inset) of the products obtained at (b) 20, (c) 29, and (d) 30 min at 250 °C in OA/OM/ODE. TEM images of the products collected at 1 min after the nucleation at different temperatures: (e) 270 °C at 12 min and (f) 290 °C at 6 min.

copy might serve as a powerful tool for the investigation of the crystal growth kinetics of UC materials. 3. Monodisperse β-NaYF4:Yb,Er Nanocrystals Formed via a Delayed rfβ Phase Transition. As is well-known, β-NaYF4: Yb,Er is one of the most efficient UC materials for performing infrared-to-visible photon conversion with green emission 10 times stronger and overall (green-plus-red) emissions 4.4 times higher than those for R-NaYF4:Yb,Er.3-5,7,8 We have recently demonstrated three modes for Rfβ transition of NaREF4 in the trifluoroacetate approach.8a In particular, the phase transition of NaREF4 in group II (Sm to Tb) is an Ostwald-ripening restricted process, wherein the size of the uniform nanocrystals gradually increases, while, for the NaREF4 in group III (Dy to

Lu, Y), the phase transition is an Ostwald-ripening promoted process, wherein the dissolution and recrystallization is dominant.8a In this study, the Rfβ transition behavior for the NaYF4:Yb,Er nanocrystals can be manipulated to either the mode of group III (Scheme 2, path A) or that of group II (Scheme 2, path B) by enhancing or restricting the Ostwald-ripening process, respectively. With R-NaYF4:Yb,Er (obtained at 310 °C for 30 min) as the precursor for a reaction in OA/ODE at 330 °C, due to the broad size distribution of the nanoparticles (Figure S9f), the Ostwaldripening process is promoted and the Rfβ transition adopts the mode of group III (Scheme 2, path A). When enough CF3COONa is added in this system, the Ostwald-ripening process

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Figure 4. (a) UC intensity variation of the solutions recorded at different periods of times (0-60 min) under 250 °C, using 0.25-1.5 mmol of Na(CF3COO) and 1 mmol of RE(CF3COO)3 as the precursors. (b) UC intensity variation of the solutions collected at different periods of times (0-30 min) under different conditions (A, 1 mmol of RE(CF3COO)3 and 1 mmol of CF3COONa as the precursors; B, 0.2 mmol of NaF was injected at 12 min; C, 0.5 mmol of NaF was injected at 12 min; D, 1 mmol of NaF was injected at 12 min; E, 1 mmol of RE(CF3COO)3 as the precursor, 1 mmol of NaF was injected once the temperature reached 250 °C). (c) Diagram of nucleation temperature versus the amount of NaF in OA/OM/ODE. (d) Diagrams of the induction time versus the amount of RE(CF3COO)3 in OA/OM/ODE, as the amount of Na(CF3COO) was fixed at 1 mmol.

Figure 5. FTIR spectra of various Y-contained samples: A, OA; B, 1 mmol of Y(OH)3 mixed with 3 mmol of OA upon heating at 140 °C for 10 min; C, 1 mmol of Y(OH)3 mixed with 10 mmol of OA upon heating at 140 °C for 30 min; D, 8.0 nm R-NaYF4:Yb,Er nanopolyhedra; E, 5.1 nm R-NaYF4:Yb,Er nanopolyhedra dissolved in 5 mL of cyclohexane.

is restricted due to the increase of the monomers, and then the Rfβ transition adopts the mode of group II (Scheme 2, path B). These two modes show remarkable differences in the UC spectra. For the mode of group III, the variation of IUC and fg/r is not co-incident (Figures 7a and S10a). The variation of IUC is mild before the phase transition (at about 8 min, Figure 8a). The IUC increases abruptly after 8 min, indicative of the beginning of the Rfβ transition (Figures 7a and S10a),3-5,7,8 while it increases tardily again (Figures 7a and S10a) at 10 min as the phase transition almost finishes (Figure 8a). In contrast,

Figure 6. Variation of UC intensity and the fg/r of the solutions recorded at different periods of times (0-10 h) at 250 °C in OA/OM/ODE (the different stages marked with dotted lines).

the variation tendency of the fg/r is different. fg/r increased nearly linearly at the initial stage of the whole process (up to 5 min) with a plateau at the stage of the phase transition (Figures 7a and S10a). fg/r increased sharply again after 10 min. As for the mode of group II, the variation of IUC and fg/r is similar except at the stage before the phase transition (Figures 7b and S10b). fg/r increased sharply in the whole process, while the IUC shows a sudden increase when the phase transition occurs (Figures 7b and S11a). It is suggested that these two different kinds of UC temporal evolution imply whether the Ostwald-ripening occurs. From the above discussion, IUC is not sensitive to the particle size at the

Synthesis of Monodisperse NaYF4:Yb,Er Nanocrystals

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Figure 7. UC intensity and fg/r variation of the solutions collected at different periods of times (0-15 min) at 330 °C in OA/ODE for the Rfβ transition of NaYF4:Yb,Er nanocrystals in the mode of (a) group III and (b) group II (dash lines marking the estimated timeline for the Rfβ transition).

SCHEME 2: Schematic Illustration of the Growth Process of β-NaYF4:Yb,Er Nanocrystals from r-NaYF4:Yb,Er Monomers via a Delayed rfβ Phase Transition

time before the phase transition (Figures 7 and S10). Therefore, IUC is unchanged in this stage. In contrast, fg/r is affected by the particle size. Figure 8b shows that in the mode of group III, two kinds of nanopolyhedra in the respective size of 9.4 and 5.2 nm coexist in the solution as the temperature reaches 330 °C. Because of the high reaction temperature and the strong surface coordination by OA, the small nanoparticles are redissolved and the size distribution of the nanocrystals decreases with prolonging the time (Figure 8c, 3 min, 9.7 ( 0.9 nm; Figure 8d, 5 min, 9.4 ( 0.6 nm). As the amount of small nanopolyhedra decreases, fg/r in the solution increases (Figures 7a and S10a). For the mode of group II, the behavior of the nanoparticles is similar in this stage. When the phase transition occurs, an abrupt increase of the IUC is observed in both group II and III processes (Figures 7 and S10). For the group III process (Scheme 2, path A), however, the dissolution of the R-NaYF4:Yb,Er nanocrystals is tempestuous as the great increase of the size of the β-NaYF4: Yb,Er nanocrystals (Figure 8e), leading to the rapid decrease of the size of the R-NaYF4:Yb,Er nanocrystals. Under the combined action of the increased fg/r caused by the formation of the β-NaYF4:Yb,Er nanocrystals and the decreased fg/r led by the increased amount of the small R-NaYF4:Yb,Er nanocrystals, the variation of fg/r of the solution is mild (Figures 7a and S10a). fg/r of the solution then increased sharply again (Figures 7a and S10a) when the phase transition finishes as the R-NaYF4:Yb,Er nanocrystals are nearly exhausted (10 min growth time). Finally, uniform 55.9 nm × 35.5 nm β-NaYF4: Yb,Er hexagonal nanoplates are formed (Figures 8f and S11b). For the group II process (Scheme 2, path B), as the concentration of the monomers is high, the Ostwald-ripening process is

restricted and the size of the NaYF4:Yb,Er increases uniformly during the phase transition stage (Figure 9a and b). As a result, there is no fg/r plateau (Figures 7b and S10b), implying that the small R-NaYF4:Yb,Er nanocrystals formed are few, which confirms our suggestion (Ostwald-ripening process is restricted). Finally, uniform 27.7 nm β-NaYF4:Yb,Er nanospheres are formed (Figure 2b). Similar to the case of R-NaYF4:Yb,Er nanopolyhedra, the surface states of the β-NaYF4:Yb,Er nanocrystals were different with different sizes. FTIR spectra of 20.2 nm β-NaYF4:Yb,Er nanospheres, 47.1 nm β-NaYF4:Yb,Er nanoprisms, and 185 nm β-NaYF4:Yb,Er nanoplates were shown in Figure 10. No intensive coordinating interactions existed between OA molecules and nanocrystal surfaces (no peak at 1640 cm-1). With the size increase, the relative intensity of the COO- asymmetric stretch weakened. Therefore, as the size of the β-NaYF4:Yb,Er nanocrystals increased, C17H33COO- tended to be transferred as C17H33COOH molecules in the coating layer, implying that van der Waals interaction between nanocrystal surface and OA might predominate for the large-sized nanocrystals. It should be pointed out that the Rfβ transition in these two processes does not occur instantaneously once the temperature reaches 330 °C, but undergoes a process of the redissolution of the small R-NaYF4:Yb,Er nanocrystals as the precursors. Therefore, the Rfβ transition observed in this work is also considered as a delayed process with the accumulation of the R-NaYF4:Yb,Er monomers (Scheme 2). To summarize, differently sized β-NaYF4:Yb,Er nanocrystals can be synthesized selectively via manipulating the Ostwald-ripening process into either group II mode or group III mode via the delayed Rfβ

13738 J. Phys. Chem. C, Vol. 111, No. 37, 2007

Mai et al.

Figure 8. (a) XRD patterns of NaY0.78Yb0.2Er0.02F4 nanocrystals separated from OA/ODE at 330 °C at 5, 8, and 10 min. TEM images of the products obtained at (b) 1, (c) 3, (d) 5, (e) 8, and (f) 15 min at 330 °C in OA/ODE (precursors: 1 mmol of R-NaY0.78Yb0.2Er0.02F4).

Figure 9. TEM images of the products obtained at (a) 5 and (b) 10 min at 330 °C in OA/ODE (precursors: 1 mmol of CF3COONa and 1 mmol of R-NaY0.78Yb0.2Er0.02F4).

transition (Scheme 2). When the phase transition adopts the group III mode (Scheme 2, path A), the size of the β-NaYF4: Yb,Er nanocrystals is large, while for the group II mode, the size of the β-NaYF4:Yb,Er nanocrystals is small (Scheme 2, path B). As the preparation temperature for the as-obtained R-NaYF4:Yb,Er precursors is high (with the reaction time long enough), the size distribution of R-NaYF4:Yb,Er nanopolyhedra becomes broad; therefore, Ostwald-ripening process is enhanced

Figure 10. FTIR spectra of 20.2 nm β-NaYF4:Yb,Er nanospheres, 47.1 nm β-NaYF4:Yb,Er nanoprisms, and 185 nm β-NaYF4:Yb,Er nanoplates dissolved in 5 mL of cyclohexane.

during the phase transition. Thus, the size of the β-NaYF4:Yb,Er nanocrystals is large (Table S1). Also, the decrease of the amount of the CF3COONa enhances the Ostwald-ripening process and results in the increased size of β-NaYF4:Yb,Er nanocrystals. When the R-NaYF4:Yb,Er precursors are obtained

Synthesis of Monodisperse NaYF4:Yb,Er Nanocrystals at relatively low temperature (250 °C), the size of the as-obtained β-NaYF4:Yb,Er nanocrystals is large (Table S1), possibly due to that the redissolution and recrystallization is easier for the relatively low crystallinity of the R-NaYF4:Yb,Er precursors. Conclusions Size-tunable high-quality R- and β-NaYF4:Yb,Er nanocrystals were synthesized from trifluoroacetate precursors in hot surfactant solutions of OA/OM/ODE via a special delayed nucleation pathway. Monodisperse R-NaYF4:Yb,Er nanopolyhedra in the size range of 5-14 nm were readily obtained by only prolonging the reaction time, in which four consecutive nanocrystal growth stages including nucleation in a delayed time, size growth by monomer supply, size shrinkage by dissolution, and aggregation were demonstrated. The essential of the delayed nucleation is verified to result from the requirement in the accumulation of enough NaF for the nanocrystal growth. Monodisperse β-NaYF4:Yb,Er nanocrystals in a broad range of 20-300 nm were obtained from the R-NaYF4:Yb,Er precursor via a restricted or enhanced Ostwald-ripening process, in which the Rfβ transition took place in a delayed time for the necessity of accumulating sufficient R-NaYF4:Yb,Er monomers. As the UC emission characters were confirmed to be sensitive to the growth process of the UC nanocrystals (i.e., IUC sensitive to the nucleation and phase transition, and fg/r sensitive to the crystallite size and phase), it is easy to distinguish various growth stages and different phase transition modes of the UC nanocrystals; therefore, UC spectroscopy can be applied to monitor the crystal growth kinetics of various UC nanocrystals. More importantly, we find that fg/r values of the present differently sized NaYF4:Yb,Er nanocrystals are decided by the content of R and β phases in the nanocrystals as well as the nature of their surface states (such as surface oxygen defects and binding modes of capping ligands). As a result, highly efficient multicolor UC emissions can be obtained only via tuning the size of the as-synthesized NaYF4:Yb,Er nanocrystals, which will be discussed in future work. We believe that the size- and phase-controlled high-quality NaYF4:Yb,Er nanocrystals are good candidate materials for many applications such as lasers, displays, imaging, and bio-sensing, and the unique delayed nucleation pathway can offer a promising synthetic protocol for obtaining many other monodisperse inorganic nanocrystals. Acknowledgment. We gratefully acknowledge the financial aids from the MOST of China (Grant No. 2006CB601104), the NSFC (Grant Nos. 20571003, 20221101, and 20423005), and the Research Fund for the Doctoral Program of Higher Education of the MOE of China (Grant No. 20060001027). Supporting Information Available: More results obtained by means of ICP-AES, TG, XRD, and TEM for NaYF4:Yb,Er

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