Influence of Matrix on the Luminescent and Structural Properties of

Article pubs.acs.org/JPCC

Influence of Matrix on the Luminescent and Structural Properties of Glycerine-Capped, Tb3+-Doped Fluoride Nanocrystals Tomasz Grzyb, Marcin Runowski, Agata Szczeszak, and Stefan Lis* Faculty of Chemistry, Department of Rare Earths, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland ABSTRACT: Nanocrystalline REF3:Tb3+ fluorides (where RE = Y, La, Ce, Gd, and Yb) have been synthesized using a precipitation method in a water/glycerine system. Glycerine, which was added in 25% by volume to Ln(NO3)3 and NH4F reactant solutions, prevented an uncontrolled growth of the nanocrystals and enhanced the stability of the colloids formed in water. X-ray analysis and transmission electron microscopy confirmed the formation of the fluoride nanocrystals. Additionally, the presence of glycerine on the surface of the crystallites was established on the basis of IR measurements. The use of Tb3+ ions resulted in a green luminescence of the products, with a maximum of this emission at 545 nm. Spectroscopic analysis was performed to investigate the effects of the fluoride matrix on the luminescent properties of the Tb3+ ions. Quantum yields were also determined. In particular, the most efficient system, based on CeF3 doped with 15% Tb3+, was investigated in detail.

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INTRODUCTION Rare earth fluorides (REF3) have been extensively investigated during the past decade as hosts for luminescent lanthanide ions (Ln3+).1−5 Because of their low phonon energies, nonradiative relaxation of the electronic excited states of dopants is minimal, and high quantum efficiencies of luminescence can be achieved. Nanomaterials based on REF3 appear to be promising alternatives to the currently used phosphors. Selected lanthanide ions, used as dopants, can provide appropriately tuned properties relevant to potential applications. Today, REF3 are used, or are being tested for use, in optoelectronics, optical fibers and amplifiers, lasers, and medicine.5−8 In view of their chemical stability, materials based on REF3 could potentially be used in bioimaging as luminescent probes. Their neutral chemical properties and low dissociation in water are important factors for in vivo investigations and make them a good alternative for quantum dots.9,10 To prepare water-soluble REF3 nanoparticles, their surface must often be modified by adsorption of molecules, which can act as functional modifiers and receptors.9,11−13 In the results presented in this work, we have used glycerine/water solutions for the synthesis medium and, further, to modify the surface of the prepared nanocrystals. The presence of glycerine molecules on the surface of these nanocrystals resulted in a tendency to form stable colloids in water. The low phonon energy of the fluorides is one of the reasons for their high effectiveness as matrixes for displaying the luminescence of Ln3+ ions. In the case of LaF3, the energy of their phonons is close to 350 cm−1.14 These properties allow for intense, efficient luminescence of the fluorides doped with Eu3+ and Tb3+ ions.15,16 Fluorides are also important and frequently used as the matrixes for upconversion applications.5,17,18 © 2012 American Chemical Society

Most of the synthetic methods of REF3 are based on precipitation reactions. These wet chemistry routes were modified in many ways to make the synthesis more efficient and to prepare particles with nanodimensions. In most cases, the known synthetic routes consist of precipitation in microemulsions,19 reverse microemulsions,20 in the hydrothermal conditions,21 in thermal decomposition reactions of trifluoroacetate precursors in high boiling solvents,22 or in combinations of the above-mentioned methods. However, these methods usually require high temperature and pressure or toxic and expensive solvents for the synthesis of nanoparticles. A competitive way to prepare lanthanide fluoride nanocrystals is a coprecipitation method. Precipitation of the fluorides in the presence of organic compounds (such as oleic acid23 or polyethylene glycol,24 or reagents containing amino- groups such as 2-aminoethyl dihydrogenphosphate,24 polyethylenimine3) prevents agglomeration and excessive growth of nanocrystals. Our previous studies concerned the synthesis, using the coprecipitation method, and the photoluminescent properties of nanosized LaF3:Eu3+ and GdF3:Eu3+.25 However, fluorides are prone to oxidation under calcination in air resulting in oxyfluoride products.26 In this Article, we present the synthesis, spectroscopy, and morphological analysis of nanocrystals of the rare earth fluorides doped with Tb3+ ions prepared in the presence of glycerine. The advantage of using glycerine is its nontoxicity, its large solubility in water, and the possibility of further chemical modifications of the prepared materials. Received: February 1, 2012 Revised: July 19, 2012 Published: July 20, 2012 17188

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Figure 1. XRD characterization of the (a) LaF3:Tb3+, (b) CeF3:Tb3+, (c) GdF3:Tb3+, and (d) (H3O)(Y3F10)(H2O):Tb3+ and (H3O)(Yb3F10)(H2O):Tb3+ nanocrystals. The line patterns correspond to the ICDD standards database.

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EXPERIMENTAL SECTION

Photographs showing the luminescence and the transparency of the colloidal CeF3:Tb3+ 15% nanocrystals were taken after ultrasonification (45 min) of the 5 mg powder suspension in water and after 3 h of holding this colloid in a quartz cuvette at room temperature. Characterization. X-ray diffraction patterns (XRD) were measured with a Bruker AXS D8 Advance diffractometer using Cu Kα radiation (λ = 1.541874 Å) in the 2θ ranges from 20° to 60°. Transmission electron microscopy images (TEM) were measured on a JEM 1200 EXII, JEOL microscope, using an accelerating voltage of 80 kV. Average crystallites sizes were calculated from the Scherrer equation:

Synthesis. Glycerine CH 2 (OH)CH(OH)CH 2 (OH) (POCh S.A., pure, 99.5%) and ammonium fluoride NH4F (POCh S.A., ACS grade, 98+ %), lanthanide oxides Y2O3, La 2 O3 , Yb 2 O3 (Sigma Aldrich, 99.99%), Gd 2 O 3 , Tb 4 O7 (Stanford Materials, 99.99%), and CeCl3·6H2O (Carl Roth, 99.9%) were used as starting materials. Lanthanide oxides were dissolved in HNO3 (POCh S.A., ultrapure) and evaporated to dryness several times to remove the excess of HNO3. From the nitrates, 1 M solutions were prepared. In a typical procedure to synthesize the LnF3:Tb3+ 5% nanocrystals, 25 mL of glycerine, 4.79 mL of Ln(NO3)3 (or CeCl3), and 252 μL of Tb(NO3)3 solutions were mixed and diluted to 100 mL with distilled water. A second solution containing NH4F was prepared in a similar way: 25 mL of glycerine and 9.5 mL of 2 M NH4F solution (with 25% excess) were mixed and diluted to 100 mL with distilled water. The Ln3+ solution was intensely stirred and heated to 50 °C. Next, the second mixture was slowly added to the Ln3+ solution over a 30 min period while holding the temperature at 50 °C. After 30 additional minutes of stirring, a white precipitate was collected by centrifugation, washed with water several times, and dried in a vacuum at room temperature. Reactions were carried out with the pH maintained at around 6.

D=

kλ cos θ β 2 − β02

(1)

where D is the average grain size, the factor k = 0.9 is characteristic for spherical objects, λ is the X-ray wavelength, and θ and β are the diffraction angle and full-width at halfmaximum of an observed peak. The IR absorption spectrum was recorded between 500 and 4000 cm−1 on an FTIR spectrophotometer, Bruker FT-IR IFS 66/s. The material was mixed with KBr and then pressed into discs. 17189

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Figure 2. TEM images of prepared rare earth fluorides: (a) (H3O)(Y3F10)(H2O):Tb3+ 5%, (b) GdF3:Tb3+ 5%, (c) (H3O)(Yb3F10)(H2O):Tb3+ 5%, (d) CeF3:Tb3+ 5%, and (e) LaF3:Tb3+ 5%.

I = I0 + Ae−t / τ

The excitation and emission spectra as well as luminescence lifetime measurements were performed on a Hitachi F-7000 fluorescence spectrophotometer at room temperature with use of powder samples. Excitation and emission spectra were corrected for the instrumental response. The luminescence quantum yields of the synthesized nanocrystals were determined using the method described by Wrighton et al. and successfully applied by us earlier.27,28 This method for measuring absolute luminescence quantum yields is based on the determination of the diffuse reflectance of the sample relative to a nonabsorbing standard (KBr) at the excitation and emission wavelengths of the sample under the same conditions. The quantum yield, φ, has been calculated using the equation:

φ=

(R std

E − R smpl)

(3)

Luminescence lifetimes were calculated with help of OriginLab 8.5 software. The goodness of fit to the time traces was not lower than R2 = 0.998, and errors were not higher than τerr < 0.01 ms.

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RESULTS AND DISCUSSION Structure and Morphology. The XRD patterns of the prepared LnF3:Tb3+ samples were compared to the ICDD (International Centre for Diffraction Data) standards database. The XRD patterns of LaF3:Tb3+, CeF3:Tb3+, and GdF3:Tb3+ fluorides were comparable with standards for simple fluorides, but Y(NO3)3 and Yb(NO3)3 reacted with NH4F resulting in the precipitation of the complex fluorides having the formula (H3O)(Ln3F10)(H2O):Tb3+. Figure 1a shows the XRD pattern of the as-prepared LaF3:Tb3+. The positions of the peaks are in good agreement with the standard pattern for hexagonal LaF3 crystals with the P3c1̅ crystallography space group. Figure 1b shows the XRD pattern of CeF3:Tb3+, which demonstrates, similar to previous material, a purely hexagonal structure with the P3c1̅ space group. The XRD pattern of the GdF3:Tb3+ 5% sample, presented in Figure 1c, agrees well with orthorhombic GdF3 reported in the PDF (Powder Diffraction File) standard card for the Pnma space group. XRD pattern diffraction peaks

(2)

where E is the area under corrected emission curve of the sample, and Rstd and Rsmpl are corrected areas under the diffuse reflectance curves of the nonabsorbing standard and samples, respectively, at the excitation wavelength. The measured luminescence decays showed nonexponential character. The decrease of the lifetime may be approximated by an exponential function; therefore, to calculate lifetimes, decays were fitted to an exponential function: 17190

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of those three fluorides are remarkably broadened, which reveals the nanocrystalline structure of the samples. The greatest differences in the structural properties were observed for yttrium and ytterbium fluorides. The XRD patterns of the prepared (H3O)(Yb3F10)(H2O):Tb3+ 5% and isostructural (H3O)(Y3F10)(H2O):Tb3+ 5% are presented in Figure 1d. This fluorinated compound, according to the ICDD database, crystallizes in a cubic Fd3m̅ system. Previous studies reported that (H3O)(Yb3F10)(H2O) has a quite stable structure and decomposes to YbF3 above 275 °C. This explains why we obtained that form of the fluoride under the conditions of our synthesis. The morphologies of the synthesized samples are presented by the TEM images, shown in Figure 2. The structures and dimensions of the precipitated crystals are different for each lanthanide fluoride, although the syntheses were carried out under the same conditions. The smallest crystallite sizes were found for the LaF3:Tb3+ and CeF3:Tb3+ samples (Figure 2d and e), which have the same crystal structure. LaF3:Tb3+ was precipitated with irregular morphology, and its nanocrystals formed small agglomerations after drying at the room temperature. However, CeF3:Tb3+ was obtained as nanoplates with lengths around 25 nm and thicknesses of several nanometers. This type of nanocrystal shape can be found in the literature for CeF3.29,30 Figure 2b shows the dried GdF3:Tb3+ precipitate. It can be seen that the prepared sample consists of uniform flower-like oval crystals with the dimensions around 100 nm. Similar results have been reported previously by Ma et al.,31 where CeF3 nanoflowers were prepared by a microwave-assisted method. The authors investigated the details of the formation of flower-like structures. During the microwave irradiation of the reagents, CeF3 nanodisks were formed, and, after a few minutes, small amorphous nanoparticles covered the surface of the nanocrystals. These nanodisks began to assemble to form flower-like structures. A similar mechanism can explain the morphology of the prepared GdF3:Tb3+ structure where microwave irradiation was substituted by heating the reagents and extending the time of their addition to the reaction system. The (H3O)(Y3F10)(H2O):Tb3+ (Figure 2a) crystallized as regular cubic nanocrystals with dimensions below 100 nm, which confirmed the XRD analysis. The (H3O)(Yb3F10)(H2O):Tb3+ (Figure 2c), with a similar crystal structure, crystallized in the form of submicrometer crystals. Cubic and spherical crystals were mixed, and their sizes were definitely the largest of all of the samples studied. The relatively large crystallite size corresponds to the crystallographic structure of this material. The crystallographic cell volume in the case of (H3O)(Yb3F10)(H2O) was as high as 3599.97 Å3 (∼3.6 nm), which directly affected the size of the crystallites.32 During the synthesis of fluorides, amorphous particles probably precipitate at the surface of the growing crystals, changing the shape of regular crystals on the spherical particles. The average crystallites sizes calculated using the Scherrer equation and taken from the TEM images are compared in Table 1.33 The nanocrystals sizes, calculated from the broadening of the XRD reflections, are comparable with the data obtained from the analysis of the TEM images only in the cases of LaF3:Tb3+ and CeF3:Tb3+. The sizes obtained from the registered diffraction patterns for the remaining fluorides were much smaller than the real sizes of the nanocrystals because of the imperfections of the Scherrer equation.34

Table 1. Average Crystallite Sizes Calculated from the Scherrer Equation and Measured from the TEM Images crystallite sizes (nm) sample LaF3:Tb3+ 5% CeF3: Tb3+ 5% GdF3:Tb3+ 5% (H3O)(Y3F10)(H2O):Tb3+ 5% (H3O)(Yb3F10)(H2O):Tb3+ 5%

XRD 10.8 18.2 22.1 43.5 73.3

± ± ± ± ±

4.7 4.3 4.4 1.4 2.1

TEM 11.6 17.9 ∼100 × 50 108 280

± 2.4 ± 3.1 ± 11 ± 50

Because of the excellent properties of CeF3 as a matrix for Tb3+ ions (in particular, taking into account the spectroscopic properties shown below), a series of the samples was prepared with variable amounts of the dopant. This allowed us to investigate changes in the luminescent properties, with increasing Tb3+ concentration, and to determine the optimal composition of the fluoride. Increasing the concentration of the Tb3+ ions in the resulting materials did not affect significantly the crystallographic structure. The XRD patterns shown in Figure 3 are in good accord with a CeF3 hexagonal structure, presented in Figure 1. The visible shift of the reflections with an increasing dopant concentration is the result of changes in the crystal cell volume, which is reduced (Figure 3). The Tb3+ ions (1.235 Å, CN 9) have a smaller radius than those of the displaced Ce3+ matrix ions (1.336 Å, CN 9), which directly influences the unit cell parameters. An increasing dopant concentration had an impact on the size of the nanocrystals, which systematically grew. The reason for this unexpected phenomenon (size should decrease when the crystal cell becomes smaller) lies in the solubility of the precipitated fluorides. According to Wu et al., the value of the solubility constant has a large influence on the size of precipitated crystals.35 Comparison of the various fluorides, performed by the authors, leads to the conclusion that more soluble fluorides precipitate as larger nanocrystals. Introduction of the Tb3+ ions into a CeF3 matrix changes the solubility in water of the formed, mixed Ce−Tb fluoride: CeF3 pKs0 = 16.5; TbF3 pKs0 = 16.1.36 Because the differences are small, changes are also slight. The obtained nanofluorides were also analyzed using FT-IR spectroscopy to demonstrate the presence of glycerine molecules on their surfaces. In Figure 4, the FT-IR spectrum of the CeF3:Tb3+ 5% nanocrystals is presented. The most intense absorption band at 3406 cm−1 is connected with O−H stretching vibrations. However, the weak bands at 2932 and 2883 cm−1 are assigned to asymmetric and symmetric stretching of the −CH2 group. The two bands with maxima at 1428 and 742 cm−1 are related to the scissoring and rocking vibrations of the −CH2 groups. The small band at 1050 cm−1 can be attributed to C−O−H stretching.10,35−38 In the spectrum, there are also quite intense bands due to stretching (3046 cm−1) and bending motions (1641 cm−1) characteristic of water molecules, creating a crystal hydrate with the fluoride and adsorbed on its surface.10,37−40 On the basis of the registered FT-IR spectrum, we concluded that the synthesized nanoparticles have glycerine on their surfaces. Luminescent Properties. Spectroscopic properties of the synthesized materials are a consequence of their doping by Tb3+ ions. Their green emission results from transitions between excited electronic 5D4 level of the Tb3+ ions and their ground state 7FJ (J = 0−6). The characteristic narrow transition bands on the Tb3+ luminescence spectra are well split, in connection with large values of the J quantum number 17191

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Figure 3. Effects of the increasing amounts of Tb3+ ions, doped into CeF3 nanocrystals, on their XRD patterns, crystal cell volume, and nanocrystal sizes (calculated from the Scherrer equation).

forbidden transition 7FJ→9DJ.45 However, a different situation could also occur as the result of interactions between high-spin 5d and excited 4f8 levels.46 In the registered excitation spectra of LaF3:Tb3+, the 7FJ→9DJ transition (∼283 nm) has a larger intensity than the 7FJ→7DJ (∼247 nm) transition. There are also visible 4f8→4f8 transition bands displayed in Figure 5a. The luminescence spectra consist of transition bands from the excited 5D3 or 5D4 levels to one of the 7FJ ground-state components. The excitation mechanism of the emitting 5D4 level has been investigated previously.41 The population of the 5 D4 level involves energy transfer between two Tb3+ ions and could be treated as concentration quenching.43 It has been reported that in low concentration, Tb3+ ions interact effectively, and this is evident in the raising time of luminescence decays.41,42 In higher concentrations, energy transfer between the 5D3 excited level and the 5D4 of Tb3+ ions results in almost total quenching of the blue emission and a large increase in the green luminescence.41 The luminescent properties of the fluoride GdF3:Tb3+ are shown in Figure 5b. In the excitation spectra, besides numerous bands arising from f−f transitions of the Tb3+ ion, additional bands are visible as a result of energy transfer between Gd3+ and Tb3+ ions. These bands are associated with the transitions 8 S7/2→6IJ (∼272 nm) and 8S7/2→6PJ (∼310 nm). The energy of the lowest excited state of the Gd3+ ion is similar to the higher excited-state components of the Tb3+ ion, which permits energy transfer between these levels. As a result of GdF3:Tb3+ excitation with a wavelength corresponding to the absorption of the Gd3+ ions, an emission spectrum was obtained, characteristic of the Tb3+ ions, consisting of bands associated with electronic transitions between the 5D4 and the 7FJ levels. Luminescence properties of the (H3O)(Y3F10)(H2O):Tb3+ and (H3O)(Yb3F10)(H2O):Tb3+ fluorides are shown in Figure 5c and d. Characteristics of the recorded excitation spectra are similar to the profile obtained for LaF3:Tb3+. In these systems, energy transfer from the matrix to the dopant ions is impossible due to the electronic structure of the Y3+ and Yb3+ ions (in the registered range). Hence, additional bands in the excitation spectra were not observed. In the obtained emission spectra,

Figure 4. FT-IR spectrum of CeF3:Tb3+ 5% nanocrystals covered by glycerine molecules.

in the 7F manifold. The color of emission in Tb3+-doped materials strongly depends on the concentration of dopant ions. It has been reported previously that at sufficiently low concentrations, Tb3+ could emit mostly in the blue region of spectra, due to the transitions from the 5D3 level.41−43 Figure 5 shows the excitation (− − −) and the emission (−) spectra of the prepared fluorides. The luminescent properties of LaF3:Tb3+ fluoride are shown in Figure 5a. The excitation spectra can be divided into two parts. The first part, in the range of 225−300 nm, corresponds to the 4f 8 →4f7 5d1 transitions of the Tb3+ ions, while the second part, above 300 nm, is composed only from the 4f8→4f8 transitions.44 Not all transitions between the levels of 4f8 and 4f75d1 are allowed. As a result of the promotion of an electron from the 4f into the 5d shell, two excited levels were formed: high-spin 9DJ and lowspin 7DJ.44 According to the well-known Hund’s rule, the 9DJ state is energetically lower than the 7DJ state. The transition 7 FJ→7DJ is spin-allowed, and, therefore, the intensity of the associated transition band is usually larger than the spin17192

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Figure 5. Excitation (− − −) and emission (−) spectra of LnF3:Tb3+ fluorides: (a) LaF3:Tb3+ 5%, (b) GdF3:Tb3+ 5%, (c) (H3O)(Y3F10)(H2O):Tb3+ 5%, and (d) (H3O)(Yb3F10)(H2O):Tb3+ 5%.

Figure 6. Excitation and emission spectra of CeF3:Tb3+ fluoride (left), luminescence observed from water colloid under 245 nm UV lamp irradiation (inset), effect of Tb3+ concentration on the luminescence intensity, where λem = 545 nm, λex = 279 nm (upper right), and comparison between luminescence properties of pure CeF3 and excitation of Tb3+ ions in LaF3 matrix (lower right).

fluoride. This system was chosen for a more accurate analysis, to investigate the optimal concentration of the Tb3+ ion and its effects on the structure and crystallite size. Figure 6 shows the luminescence properties of CeF3:Tb3+. The excitation spectra are different from those presented above in the Figure 5. The most visible changes occurred in the short

characteristic bands are visible resulting from electronic transitions of the Tb3+ ions. Other than the intensity, the characteristics of the emission bands did not differ among the samples studied. Of the synthesized series of 5% Tb3+-doped fluorides, the most intense luminescence was observed for the CeF3:Tb3+ 17193

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wavelengths region, where a broad and intense band with a maximum at 279 nm could be observed as a result of the 4f1→ 4f05d1 transition of the Ce3+ ion.47 The presence of this broad band is evidence for energy transfer between the Ce3+ and Tb3+ ions. The narrow and less intense bands in the range of 320− 400 nm are connected with direct excitation of the Tb3+ ions. Because of the forbidden nature of these bands, they show a much smaller intensity than the spin-allowed 4f1→4f05d1 Ce3+ transition band. Energy transfer between Ce3+ and Tb3+ ions is possible due to the electron configuration of these ions. In pure CeF3, the Ce3+ ions show emission in the UV region, with a maximum around 330 nm, which is in the range where Tb3+ ions could absorb energy. This phenomenon is clearly seen in Figure 6 (lower-right spectra). This energy transfer strongly enhances the Tb3+ luminescence. The Ce3+ ions act as sensitizers that harvest the UV light and effectively transfer the absorbed energy to the Tb3+ ions. The best dopant concentration was established on the basis of the 5D4→7F5 emission band. The optimum amount was found to be 15%. Greater amounts of Tb3+ ions in the CeF3 matrix quenched the luminescence intensity. The Tb3+ ions, due to their electronic structure, are prone to concentration quenching (cross relaxation), which resulted in a decrease in the intensity of the luminescence, and also in the absence of emission from the 5D3 level.48 The inset to Figure 6 shows the luminescence of a colloidal solution of CeF3:Tb3+ 15% nanocrystals. Photographs of the colloid, taken 3 h after preparation, show its stability and intense green luminescence despite the presence of water. The solubility of the CeF3:Tb3+ nanocrystals was examined using several solvents such as water, ethanol, cyclohexane, and acetone. Because of the presence of glycerine, and therefore −OH groups, on the surface of the nanocrystals, the most stable solutions were obtained when water or ethanol was used. A comparison of the luminescence spectra of the prepared LnF3:Tb3+ 5% fluorides is presented in Figure 7. For the excitation of samples, the optimal wavelengths were chosen to give the most intense luminescence for each particular sample. Because of the energy transfer process (see above), CeF3:Tb3+

5% showed the most intense emission. Although there is a similar energy transfer phenomena in GdF3:Tb3+, the luminescence intensity of this material is much smaller than that of CeF3:Tb3+. This is because the f−f electronic transitions of Gd3+ are less efficient than the f−d transition of Ce3+ involved in the energy transfer between Gd3+ and Tb3+ ions. Table 2 summarizes the measured absolute quantum yields for Table 2. Luminescence Quantum Yields (φ) of Obtained Nanocrystals Doped with Tb3+ Ions sample

λex (nm)

LaF3:Tb3+ 5% GdF3:Tb3+ 5% (H3O)(Y3F10)(H2O):Tb3+ 5% (H3O)(Yb3F10)(H2O):Tb3+ 5% CeF3: Tb3+ 1% CeF3: Tb3+ 2.5% CeF3: Tb3+ 5% CeF3: Tb3+ 7.5% CeF3: Tb3+ 10% CeF3: Tb3+ 15% CeF3: Tb3+ 20%

365

279

φ 0.010 0.010 0.015 0.002 0.120 0.244 0.347 0.464 0.489 0.600 0.527

± ± ± ± ± ± ± ± ± ± ±

0.001 0.001 0.002 0.001 0.009 0.009 0.009 0.022 0.024 0.038 0.029

all of the synthesized samples. Except for CeF3:Tb3+, the obtained materials show relatively low quantum yields in the range of 0.2−1.5%. Unlike the others, CeF 3 :Tb 3+ is characterized by a high quantum yield, reaching 60% in case of 15%-doped nanocrystals. The general trend in the quantum yields is similar to the trend in the intensities of the measured luminescence presented in the Figure 7. (H3O)(Y3F10)(H2O):Tb3+ showed relatively good luminescent properties. Its emission intensity was the largest of the remaining fluorides: lanthanum, gadolinium, and ytterbium. The presence of water molecules in the crystal structure of this fluoride complex did not significantly affect the luminescence of the Tb3+ ions. The small luminescence intensity of (H3O)(Yb3F10)(H2O):Tb3+ is the result of energy transfer from the Tb3+ to the Yb3+ ions. In this case, the energy transfer (from dopant to host) is in the opposite direction to what occurred in CeF3:Tb3+ and GdF3:Tb3+ fluorides. In (H3O)(Yb3F10)(H2O):Tb3+, the Tb3+ ions sensitize the Yb3+ ions to the radiation wavelengths in the range of UV. This results in the luminescence of Yb3+ ions in the infrared range (950−1100 nm).49 From the point of view of the Tb3+ emission, the Yb3+ ions act as quenchers. Luminescence decay curves of the prepared fluorides are presented in Figure 8a. They show nonexponential character, as a result of the cross relaxation process between Tb3+ ions. The kinetics of cross relaxation are complex, and the model explaining this process has been previously reported.42 However, the decay profiles can be fitted with an exponential function as a first approximation to the reported kinetics model. The longest lifetimes were obtained for the LaF3:Tb3+ and CeF3:Tb3+ fluorides. They have similar crystal structure and Tb 3+ site symmetry, which are responsible for their corresponding lifetimes. The measured lifetimes do not differ from values reported in the literature, which are in the range of 1−6 ms.19,30,50 Definitely the shortest lifetimes were found in the (H3O)(Yb3F10)(H2O):Tb3+ fluoride, where Yb3+ strongly quenches the excited Tb3+ ions. Figure 8b shows the effect of increasing the Tb 3+ concentration in CeF3 matrixes on their luminescence decays.

Figure 7. Comparison of the luminescence spectra of the prepared fluorides, doped by 5% Tb3+ ions. For the excitation, the optimal wavelengths were used, giving the most intense emission for a particular sample (365 nm for LaF3:Tb3+, (H3O)(Y3F10)(H2O):Tb3+, and (H3O)(Yb3F10)(H2O):Tb3+, 272 nm for GdF3:Tb3+, and 279 nm for CeF3:Tb3+). 17194

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The presence of glycerine in the reaction system resulted in the capping of the precipitated nanocrystals and, hence, enhanced the stability of colloids formed. Spectroscopic properties of the synthesized fluorides were studied and compared. The most effective luminescence was measured in the case of CeF3:Tb3+ 15% nanocrystals as a result of energy transfer from the Ce3+ ions to the Tb3+ ions. The calculated quantum yields confirmed the high efficiency of emission in CeF3:Tb3+ for which φ reaches 60% in the case of the 15%-doped sample.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +48 61 8291345. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Polish Ministry of Science and Higher Education, grant no. N204 329736, is gratefully acknowledged.

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REFERENCES

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Figure 8. Luminescence decays of Tb3+-doped fluorides, λex = 365 nm, λem = 545 nm. In all of the cases, the R2 factor was not lower than 0.998.

From this figure, as well as the inserted graph, it can be seen that increasing amounts of Tb3+ ions resulted in shorter lifetimes. Because cross relaxation occurred in the higher doped samples, the lifetime value was systematically shortened, which confirmed that this process concerns the obtained nanocrystals. We assumed that the amounts of glycerine are small and do not affect the Tb3+ emission to a large extent. It was clearly evident in the case of the CeF3:Tb3+ nanocrystals, which became larger with increasing Tb3+ concentration. Therefore, also their surface was larger and could accommodate more adsorbed glycerine. However, the luminescence intensity increased with nanocrystals size up to Tb3+ concentration around 15%. This is evidence that the spectroscopic properties and the observed cross relaxation of the Tb3+ ions had a much more important effect on the luminescence of the nanocrystals than did any quenching by small amounts of glycerine. Moreover, the analyzed decay curves showed the longest lifetimes for the smallest nanocrystals (LaF3:Tb3+ and CeF3:Tb3+), which have the higher surface to volume ratio. If the glycerine would have been strongly quenching the emission of Tb3+ ions, then, in the case of these two groups, their luminescence lifetimes should have been the shortest in the series.

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CONCLUSIONS The fluorides, obtained by the precipitation method, have good crystallinity despite the low temperature of the syntheses and the relatively short reaction times. These studies show the influence of the crystallographic structure on the morphology of the obtained nanomaterials. Although identical synthetic conditions were used, the products differed from each other, especially when the crystallographic structures were changed. 17195

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