Luminescence of LaF3:Ln3+ Nanocrystal Dispersions in Ionic Liquids

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J. Phys. Chem. C 2009, 113, 13532–13538

Luminescence of LaF3:Ln3+ Nanocrystal Dispersions in Ionic Liquids Kyra Lunstroot,† Linny Baeten,† Peter Nockemann,† Johan Martens,‡ Pieter Verlooy,‡ Xingpu Ye,§ Christiane Go¨rller-Walrand,† Koen Binnemans,*,† and Kris Driesen† Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F bus 2404, 3001 HeVerlee, Belgium, Centre for Surface Chemistry and Catalysis, Katholieke UniVersiteit LeuVen, Kasteelpark Arenberg 23 - bus 2461, 3001 HeVerlee, Belgium, and Department of Metallurgy and Materials Engineering, Katholieke UniVersiteit LeuVen, Kasteelpark Arenberg 44 - bus 2450, 3001 HeVerlee, Belgium ReceiVed: February 19, 2009; ReVised Manuscript ReceiVed: June 11, 2009

Ionic liquids were used as solvents for dispersing luminescent lanthanide-doped LaF3:Ln3+ nanocrystals (Ln3+ ) Eu3+ and Nd3+). To increase the solubility of the inorganic nanoparticles in the ionic liquids, the nanocrystals were prepared with different stabilizing ligands, i.e., citrate, N,N,N-trimethylglycine (betaine), and lauryldimethylglycine (lauryl betaine). LaF3:5%Ln3+:betaine could successfully be dispersed in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C4mpyr][Tf2N], 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate [C4mpyr][TfO], and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4mim][Tf2N] but only in limited amounts. Red photoluminescence was observed for the europium(III)-containing nanoparticles and near-infrared luminescence for the neodymium(III)-containing systems. Introduction The light emitted by trivalent lanthanide ions can be distinguished from the luminescence of organic molecules or most transition-metal complexes by its highly monochromatic nature. The spectroscopically active 4f electrons of the lanthanide ions are not involved in any binding molecular orbital, and they are protected from the environment by the fully occupied 5s and 5p orbitals. This results in the typical narrow emission lines in lanthanide luminescence spectra.1,2 Examples of luminescent lanthanide ions are europium(III) (red color), terbium(III) (green color), samarium(III) (orange-red color), and the near-infrared emitting lanthanide ions ytterbium(III), neodymium(III), and erbium(III). Lanthanide compounds have been successfully used as phosphors in numerous applications such as OLEDs (organic light-emitting diodes),3,4 fluorescent lamps,5 lasers,6 and luminescent probes in biology.7 Ideally, a phosphor emits light with a high intensity and a long luminescence lifetime. Because the electronic transitions that give rise to the lanthanide luminescence are forbidden intraconfigurational f-f transitions (Laporte selection rule), excited lanthanide ions typically have long radiative lifetimes. Intense metal-centered luminescence is observed for molecular lanthanide complexes where excitation of the lanthanide ions can be enhanced by the antenna effect.8 However, one of the inherent limitations of complexes with organic ligands is that they possess high-energy vibrations that can deactivate the excited states of the lanthanide ions and result in shortened lifetimes. On the other hand, inorganic matrixes such as fluorides have lower phonon energies, resulting in higher luminescence quantum yields and longer lifetimes. This advantage applies to inorganic lanthanide-doped single crystals and nanocrystals. Typical fluoride host crystals for lanthanide ions are LiYF4,9 KY3F10,10 and LaF3.11 Unfortunately, large single crystals are * Corresponding author: E-mail: [email protected]. Fax: + 32 16 32 79 92. † Department of Chemistry. ‡ Centre for Surface Chemistry and Catalysis. § Department of Metallurgy and Materials Engineering.

often difficult to grow. The dispersion of nanoparticles in solutions provides a way to incorporate an inorganic phase in a liquid medium. Therefore, luminescent lanthanide-doped nanocrystals are popular research objects.12-20 These nanocrystals were also dispersed in conventional solvents (e.g., chloroform, dimethylsulfoxide), and these solutions showed good luminescent properties.18-20 Recently, several papers on the use of ionic liquids as optical solvents for lanthanide luminescence have appeared.21-29 Ionic liquids are organic liquids consisting entirely of ions. They have interesting properties, such as a high thermal, chemical, and electrochemical stability,30 and they can be transparent through a broad range of the electromagnetic spectrum.31-33 High quantum yields29 and an enhanced photostability of lanthanide β-diketonate complexes26 in ionic liquids were reported. Confinement of lanthanide-containing ionic liquids in a porous silica matrix resulted in new luminescent monolithic hybrid materials referred to as “ionogels”.34,35 Ionic liquids have also been used for the synthesis of lanthanide-doped nanoparticles.36,37 Dispersion of luminescent lanthanide-containing nanoparticles in an ionic liquid instead of a conventional solvent could optimize the luminescent system. Ionic liquids are interesting solvents for luminescent lanthanide complexes because they can be thoroughly dried, so quenching of the luminescence by water molecules can be minimized. Because of the large choice of constituent anions and cations, the ionic liquid can be designed so as to optimize the solubility properties and decrease the luminescence quenching by solvent molecules, e.g., by avoiding high-energy vibrations. However, ionic liquids generally show a low solubility for inorganic species. Therefore, dispersion of inorganic nanocrystals in an ionic liquid is not an easy task. Although most of the reports that can be found in the literature on nanocrystals and ionic liquids involve the synthesis of nanocrystals in ionic liquids,38-44 some authors describe the incorporation of nanocrystals in ionic liquids, for instance, for catalytic purposes.45,46 A well-established strategy to mask the inorganic nature of the nanocrystal is by adding a coordinating ligand such as

10.1021/jp9015118 CCC: $40.75  2009 American Chemical Society Published on Web 07/09/2009

Luminescence of LaF3:Ln3+ Nanocrystal Dispersions citrate12 or derivatized poly(ethylene glycol)47 to the reaction medium of nanocrystals. The interaction between the ligands and the lanthanide ions in the solution can restrict the crystal growth; i.e., nanosize crystals are obtained, as the ligands will be attached to the crystal surface. The resulting ligand-capped nanoparticles show an enhanced solubility in organic solvents or water. In this paper, LaF3:Ln3+ nanocrystals are presented, prepared with lauryldimethylglycine (referred to as lauryl betaine) and N,N,N-trimethylglycine (referred to as betaine) as stabilizing ligands. These ligands have the possibility to form zwitterions. Therefore, interaction of the ligands with the inorganic nanocrystals results in a positive charge on the crystal surface. This charge prevents aggregation of the crystals in solutions and as a result improves the solubility properties of the nanocrystal in an ionic liquid or in a conventional solvent. The carboxylate group is also known for its strong interactions with the oxophilic trivalent lanthanide ions. Similar, betaine derivative ligands have been reported by Liu et al. for the stabilization of ZnO nanocrystals.48 For comparison reasons, LaF3:Ln3+ nanocrystals without stabilizing ligands or with citrate molecules were also prepared. Experimental Section Synthesis of the Nanocrystals. LaF3:Ln3+ nanocrystals without stabilizing ligands were prepared following a method described by Wang et al.:14 LaCl3 · 7H2O (1.9 mmol), LnCl3 · xH2O (0.1 mmol), and NH4F (6 mmol) were dissolved in 50 mL of water. The mixture was stirred and heated at 75 °C for 2 h. The nanocrystals were precipitated and collected with a centrifuge (3000 r/min, 15 min). The precipitate was washed three times with water and centrifuged again. Finally, the product was dried in Vacuo at room temperature. The LaF3 nanocrystals were prepared doped with Eu3+ (5%) and Nd3+ (5%). Core and core-shell LaF3:Ln3+ nanocrystals stabilized by citrate molecules were synthesized as reported by Sudarsan et al.12 The syntheses of LaF3 nanocrystals obtained with lauryldimethylglycine (referred to as lauryl betaine) and N,N,Ntrimethylglycine (referred to as betaine) were also based on the method described by Sudarsan et al.:12 Core Nanocrystals. 10.4 mmol of the ligand (citric acid, N,N,N-trimethylglycine, or lauryldimethylglycine) was dissolved in water together with 3 mmol of NaF. In the case of the citrate, the solution was neutralized (to pH 6) by adding a diluted aqueous NH3 solution. The mixture was stirred and heated up to 75 °C. La(NO3)3 · 6H2O or LaCl3 · 7H2O (0.95 mmol) and Ln(NO3)3 · xH2O or LnCl3 · xH2O (0.05 mmol) were dissolved in 2 mL of water and added dropwise to the solution. The mixture was left at 75 °C for 2 h. The nanocrystals were precipitated by adding 75 mL of ethanol, which was followed by the centrifugation (2500 r/min, 10 min) and washing of the precipitate with ethanol. The crystals were dried in Vacuo at 50 °C. The LaF3 nanocrystals were prepared doped with Eu3+ (5%) and Nd3+ (5%). LaF3:10% Eu3+:betaine was prepared analogous to this procedure. Only the molar ratio of the lanthanide ions (La3+/Eu3+) was altered. Core-Shell Nanocrystals: LaF3:Ln3+-LaF3. 15.6 mmol of the ligand (citric acid or N,N,N-trimethylglycine) was dissolved in water (35 mL) and neutralized (to pH 6) with a diluted aqueous NH3 solution. The mixture was stirred and heated up to 75 °C. La(NO3)3 · 6H2O (1.26 mmol) and Ln(NO3)3 · xH2O (0.067 mmol) were dissolved in 3 mL of methanol and added dropwise to the solution. NaF (6.34 mmol) was dissolved in

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13533 water (3 mL) and also added dropwise to the solution. The reaction mixture was stirred for 10 min. La(NO3)3 · 6H2O (1.39 mmol) was dissolved in methanol (3 mL) and added to the solution for the formation of the shell. The mixture was left for 2 h at 75 °C. The nanoparticles were precipitated by adding 75 mL of ethanol followed by centrifugation (3000 r/min, 10 min) and washing of the precipitate with ethanol. The final product was dried in Vacuo at 50 °C. The LaF3 core-shell nanocrystals were prepared doped with Eu3+ (5%) and Nd3+ (5%). Synthesis of the Ionic Liquids. Different ionic liquids were used to test the solubility of the LaF3:Ln3+ nanocrystals. 1-Butyl1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C4mpyr][Tf2N] and 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate [C4mpyr][TfO] were obtained from IoLiTec (Denzlingen, Germany). 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4mim][Tf2N]31 and choline saccharinate49 and choline acesulfamate49 were synthesized according to methods described in the literature. General Techniques. Powder X-ray diffractions were measured on a Stoe Stadi P diffractometer using Cu KR1 radiation (λ ) 1.54056 Å) and an image plate detector covering an angular range of 80°. The powder diffractograms were analyzed with STOE WINXPOW (Stoe & Cie Darmstadt).50 High-resolution images from nanocrystals were obtained with transmission electron microscopy (TEM). They were taken from a Philips CM 200 FEG instrument equipped with a super twin-R lens, field emission gun (FEG) operating in the Schottky mode, superultrathin window (SUTW) energy dispersive X-ray spectrometer (EDS), and a GATAN energy selective image filter. CHN elemental analyses (carbon-hydrogen-nitrogen) were performed on a CE Instruments EA-1110 elemental analyzer. FTIR spectra were recorded on a Bruker IFS-66 spectrometer, using the KBr pellet method. Photoluminescence spectra and luminescence lifetime measurements in the visible region have been recorded on an Edinburgh Instruments FS900 steady state spectrofluorimeter. This instrument is equipped with a xenon arc lamp (450 W), a microsecond flash lamp, and a red-sensitive photomultiplier (200-850 nm). For the infrared spectra, an Edinburgh Instruments FS-920 spectrofluorimeter equipped with a xenon arc lamp (450 W), a continuum Minilite II YAG(Nd), a double excitation monochromator, a blue-sensitive photomultiplier (200-650 nm), and a liquid nitrogen cooled Hamamatsu R550972 NIR photomultiplier (600-1700 nm) was used. Results and Discussion Fluoride matrixes such as LaF3 typically have low phonon energies. However, the high-energy vibrations of the ligands attached at the surface of LaF3:Ln3+ nanoparticles could quench the excitation energy of the luminescent lanthanide ions located near the surface of the nanocrystal. Therefore, nanocrystals were prepared also with an additional protecting LaF3 shell around the original LaF3:Ln3+ “core”. The molecular structures of the protonated ligands, citric acid, protonated lauryl betaine, and protonated betaine are shown in Figure 1. The powder X-ray diffraction peaks of the synthesized LaF3:Ln3+ nanocrystals were compared with the position of the diffraction peaks, as predicted from a known LaF3 crystal structure (from Gregson et al.51). This crystal structure consists of a hexagonal phase with space group P63/mmc. The La3+ ion has a coordination number of 11 and occupies a site with C2 symmetry. The XRD is shown for LaF3:5%Eu3+:betaine together with the predicted lines in Figure 2. The diffractogram is in good agreement with the predicted peaks from the known LaF3

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Lunstroot et al. TABLE 1: Average Size (Diameter) of the Synthesized LaF3:Ln3+ Nanocrystals Calculated from the Powder X-ray Diffractograms Using the Debye-Scherrer Formula product

Figure 1. Stabilizing ligands used in the synthesis of the LaF3:Ln3+ nanocrystals:. (1) citric acid; (2) protonated betaine; (3) protonated lauryl betaine.

Figure 2. Powder X-ray diffractogram of LaF3:5%Eu3+:betaine (black line) compared with the calculated diffraction pattern predicted on the basis of the known crystal structure of LaF351 (blue line). The peak marked by an asterisk is due to the mylar film used for sample preparation.

crystal structure. The diffractograms of the other LaF3:Ln3+ nanocrystals were similar to Figure 2. The average size (diameter) of the nanocrystal can be calculated from the broadening of the diffraction peaks using the Debye-Scherrer formula:52

d)

kλ B(cos θ)

(1)

Here, B stands for the full width of the peak at half-maximum in radians, k is the Scherrer constant (k ) 0.89), λ is the radiation wavelength of the X-ray source (λCu ) 1.54056 Å), and θ is the position of the maximum of the diffraction peak. A good fit of the data was obtained by a Gaussian distribution of the diffraction peak at 2θ ) 28.3°. In the case of LaF3:5%Eu3+: betaine (Figure 2), an average value of 11 nm (diameter) was found for the size of the particles. The different sizes of the nanocrystals calculated from the powder X-ray diffractograms are summarized in Table 1. High-resolution images were taken from LaF3:5%Eu3+:betaine with a transmission electron microscope (TEM) and are shown in Figure 3. The particles show a large size distribution. The largest nanocrystals have a diameter of about 20 nm, while the smallest have a diameter of 5 nm. The diameter for LaF3:Ln3+: betaine, as calculated from the diffractograms with the Debye-Scherrer formula, was 11 nm. This is an average value and does not take into account a possible size distribution or

LaF3:5%Eu LaF3:5%Nd LaF3:5%Eu LaF3:5%Nd LaF3:5%Eu-LaF3 LaF3:5%Nd-LaF3 LaF3:5%Eu LaF3:5%Nd LaF3:5%Eu LaF3:5%Nd LaF3:10%Eu LaF3:5%Eu-LaF3 LaF3:5%Nd-LaF3

core/core shell core core core core core core core core core core core core core

shell shell

shell shell

stabilizing ligand

size crystal (nm)

none none citrate citrate citrate citrate lauryl betaine lauryl betaine betaine betaine betaine betaine betaine

13 16 6 3 3 3 6 6 11 11 10 11 10

shape anisotropy. Therefore, the crystal size calculated from the powder X-ray diffractograms is a good estimate for the average size of the nanocrystals. In the infrared absorption spectra, a peak around 1650 cm-1 was attributed to the vibration of the carboxylate group (COO-). Two components were found for this peak in the case of LaF3:Ln3+:citrate (maxima at 1700 and 1590 cm-1), which were assigned to the un-ionized and the ionized, coordinating COO stretching vibrations, respectively. Indeed, citrate molecules have two “free” carboxylic acid groups after coordination to a lanthanide(III) ion. For LaF3:Ln3+:betaine, only one peak could be observed around 1620 cm-1. This is consistent with the fact that for LaF3:Ln3+:betaine all of the carboxylate groups (one per molecule) are deprotonated and can coordinate to the lanthanide ions at the surface of the nanocrystal. As a reference, for betaine hydrochloride (protonated carboxylate group), the COO stretching band is located at 1730 cm-1 (obtained from the Acros database53). From each type of nanocrystalline sample (LaF3:Ln3+ with citrate, betaine, or lauryl betaine or without stabilizing ligand), the LaF3:5%Eu3+ nanocrystal was taken and used for the determination of the solubility properties in the ionic liquid. As the solvation properties of the ionic liquid can vary dependent upon the constituent ions, different ionic liquids were selected: 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C4mpyr][Tf2N], 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate [C4mpyr][TfO], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4mim][Tf2N], choline saccharinate, and choline acesulfamate. The molecular structures of the ionic liquids are shown in Figure 4. [C4mpyr][Tf2N] and [C4mim][Tf2N] are hydrophobic ionic liquids, while [C4mpyr][TfO], choline saccharinate, and choline acesulfamate have hydrophilic properties. [Tf2N]- and [TfO]- are weak coordinating ligands. Dissolution of the betaine nanocrystals could result in solvation by for instance [Tf2N]- anions around the positively charged particles. This involves the replacement of the counterions, probably Cl- or NO3- ions that are bound to the positively charged betaine nanocrystals by [Tf2N]- anions. A sample of nanocrystals (0.5 mg) was dissolved in 2 mL of ionic liquid. The ionic liquid was dried on a rotary evaporator before use (water content was checked by Karl Fischer titration and was on average 130 ppm). The mixtures of nanocrystals and ionic liquids were stirred and heated to 75 °C. After 1 h, only the LaF3:5%Eu3+ nanocrystals stabilized with betaine molecules were found to be soluble in some of the ionic liquids. They were not soluble in an organic solvent, i.e., dichlo-

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Figure 3. TEM images of LaF3:5%Eu3+ with betaine as a stabilizing ligand. The scale as shown in the pictures is 50 nm (left picture) and 20 nm (right picture).

Figure 4. Overview of the ionic liquids for which the solubility of the LaF3:Eu3+ nanocrystals was tested: (1) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C4mpyr][Tf2N]; (2) 1-butyl1-methylpyrrolidinium trifluoromethanesulfonate [C4mpyr][TfO]; (3) 1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide[C4mim][Tf2N]; (4) choline saccharinate; (5) choline acesulfamate.

romethane, toluene, methanol, ethanol, chloroform, dimethylsulfoxide, and acetonitrile. The maximum amount of LaF3:5%Eu3+:betaine that could be dispersed in the ionic liquids [C4mpyr][Tf2N] (density 1.41 g mL-1), [C4mim][Tf2N] (density 1.44 g mL-1), and [C4mpyr][TfO] (density 1.20 g mL-1) was 1 mg in 2 mL of ionic liquids, corresponding to 0.036, 0.035, and 0.042 wt %, respectively. LaF3:5%Eu3+:lauryl betaine nanocrystals were found to be soluble in chloroform (transparent solution), while LaF3:5%Eu3+:citrate and LaF3:5%Eu3+ prepared without stabilizing ligand could not be dispersed in any of the tested ionic liquids or organic solvents. All LaF3:5%Eu3+ nanocrystals could be partly dispersed in water. To check whether the nanocrystals remained intact after dispersion in the ionic liquid and were not decomposed, the nanocrystals were isolated from the ionic liquid phase by solvent extraction. An 8 mg sample of LaF3:5%Eu3+:betaine was dissolved in 16 mL of [C4mpyr][Tf2N] followed by solvent extraction with water. After the extraction step, water was

evaporated from the aqueous phase and the residue left after evaporation was investigated by powder XRD and FTIR spectroscopy. The resulting powder X-ray diffractogram and infrared spectrum were compared with the diffractogram and infrared spectrum of LaF3:5%Eu3+:betaine taken before dispersion in the ionic liquid. They were found to be very similar. The average diameter of the nanocrystals was 10 nm. This corresponds well to the size found for the particles before they were incorporated in the ionic liquid (11 nm). These results indicate that there is no degradation or agglomeration of the nanocrystals in the ionic liquid phase. Visible europium(III) luminescence was observed for LaF3:Eu3+:betaine nanocrystals dispersed in the ionic liquids [C4mpyr][Tf2N], [C4mim][Tf2N], and [C4mpyr][TfO]. The luminescence spectra of LaF3:5%Eu3+:betaine (“core”) and LaF3:5%Eu3+-LaF3:betaine (“core shell”) are shown in Figure 5 for the nanocrystals in the solid state. The observation of transitions starting from the 5D1 excited state of Eu3+ indicates that the inorganic fluoride matrix is not an efficient quencher for this energy level, which is located only at about 2000 cm-1 above the 5D0 level. In organic europium(III) complexes, many vibrations are present that can efficiently quench the 5D1 excited state, and as a result, no, or only very weak, transitions are observed from the 5D1 excited state. The splitting of the 5D0 f 7 F1 transition (located at 590 nm (17000 cm-1)) into three components reveals a low site symmetry. The peak at 613 nm (16300 cm-1) (5D0 f 7F2 transition) is not completely resolved, making a further analysis of the site symmetry from the spectroscopic data difficult. However, the symmetry and analysis of the luminescence spectra of LaF3:Eu3+ have been the topic of several papers.54-56 The site symmetry of La3+ in the hexagonal tysonite structure of LaF3 is C2.57 This can be regarded as a distortion of a tricapped trigonal prism (D3h symmetry) and can be approximated by C2V (also a distortion of D3h) for the analysis of the crystal-field splitting in the spectra of LaF3:Eu3+. The fact that there are not five distinct peaks present for the 5D0 f 7 F2 transition in the luminescence spectrum (Figure 5) of LaF3:Eu3+ is a strong indication for the resemblance of the coordination polyhedron of LaF3 to a higher symmetry, such as a (distorted) tricapped trigonal prism (D3h symmetry). LaF3:

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Figure 5. Emission spectra of (I) LaF3:5%Eu3+:betaine (“core”) and (II) LaF3:5%Eu3+-LaF3:betaine (“core shell”) in the solid state. The excitation wavelength was set at 393 nm, and the spectra were measured at room temperature.

Lunstroot et al.

Figure 7. Room temperature emission spectrum of LaF3:10%Eu3+: betaine in [C4mim][Tf2N]. The excitation wavelength was 393 nm.

TABLE 2: Summary of the Luminescence Decay Times (τ) for the Europium(III)-Doped Nanocrystals in the Solid State or Dispersed in Chloroform product 3+

LaF3:5%Eu (core) LaF3:5%Eu3+:citrate (core) LaF3:5%Eu3+-LaF3:citrate (core shell) LaF3:5%Eu3+:lauryl betaine (core) LaF3:5%Eu3+:lauryl betaine (core) in CHCl3 LaF3:5%Eu3+:betaine (core) LaF3:5%Eu3+-LaF3:betaine (core shell) LaF3:10%Eu3+:betaine (core)

Figure 6. Room temperature emission spectra of LaF3:5%Eu3+:betaine in (I) [C4mpyr][TfO], (II) [C4mpyr][Tf2N], and (III) [C4mim][Tf2N]. The excitation wavelength was 393 nm.

5%Eu3+:betaine was dispersed in the ionic liquids and excited at 393 nm (25445 cm-1). The resulting emission spectra are shown in Figure 6. Unfortunately, the intensity of the europium(III) luminescence was rather low and there is a strong background fluorescence from the ionic liquid solvent. Since it was not possible to simply increase the concentration of the nanocrystals in the ionic liquid, the problem was overcome by preparing nanocrystals with 10% europium(III) instead of 5%. The luminescence spectrum of LaF3:10%Eu3+:betaine in [C4mim][Tf2N] is shown in Figure 7. The europium(III) spectra are comparable to those recorded for LaF3:5%Eu3+:betaine in the solid state (Figure 5), indicating that higher europium(III) concentrations (10% instead of 5% in the nanocrystals) do not affect the crystalline matrix and that incorporation of the nanocrystal in the ionic liquid does not lead to destabilization of the nanocrystalline host (as was demonstrated before). An overview of the luminescent lifetimes found for the europium(III)-doped nanocrystals is given in Table 2. The data show a biexponential decay for all of the nanoparticles. The

τ1 (ms)

%

τ2 (ms)

%

3.9 ( 0.2 3.5 ( 0.1 3.7 ( 0.1

84 78 75

0.8 ( 0.5 0.9 ( 0.1 0.9 ( 0.1

16 22 25

5.0 ( 0.5

78

1.4 ( 0.2

22

4.2 ( 0.1

91

1.0 ( 0.1

9

4.4 ( 0.1 4.5 ( 0.1

94 96

1.1 ( 0.1 1.1 ( 0.1

6 4

4.7 ( 0.4

94

1.1 ( 0.1

6

average lifetime of the main component is 4 ms for the europium(III)-doped nanocrystals. For comparison, the lifetime of Eu3+ luminescence in organic europium(III) complexes is usually around 0.5 ms. It was not possible to determine the luminescent lifetime of the LaF3:Eu3+:betaine nanocrystals in the ionic liquids because of the low intensity. Since the background fluorescence of the ionic liquid is only located in the visible part of the spectrum and only arises when excited in the UV range, this will not interfere with the luminescence of the near-infrared emitting lanthanide ions. The luminescence spectra of LaF3:5%Nd3+:betaine in the solid state and dissolved in [C4mim][Tf2N] are shown in Figure 8. The emission peaks correspond to transitions from the 4F3/2 state toward the different J-levels of the 4I term (4IJ, J ) 13/2, 11/2, 9/2). The intensity of the neodymium(III) emission was lower in the ionic liquid than for the nanocrystal in the solid state, resulting in spectra with lower resolutions for LaF3:5%Nd3+: betaine dispersed in the ionic liquid. The emission of LaF3:5%Nd3+:betaine was also recorded in different ionic liquids. This is demonstrated in Figure 9. The luminescence lifetimes were measured for the LaF3:5%Nd3+:betaine (“core”) in the solid state and dissolved in the different ionic liquids as well as for LaF3:5%Nd3+-LaF3:betaine (“core shell”) in the solid state and dissolved in [C4mim][Tf2N]. This is tabulated in Table 3. From Table 3, it can be derived that there is a minor effect from the optical inactive shell in core shell nanocrystals

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Figure 8. Emission spectra of LaF3:5%Nd3+:betaine (I) in the solid state and (II) in [C4mim][Tf2N]. The excitation wavelength was 578 nm, and the spectra were measured at room temperature.

Figure 9. Emission spectra of LaF3:5%Nd3+:betaine in (I) [C4mpyr][TfO], (II) [C4mpyr][Tf2N], and (III) [C4mim][Tf2N]. The excitation wavelength was 578 nm, and the spectra were measured at room temperature.

TABLE 3: Summary of the Luminescence Decay Times (τ) for the Neodymium(III)-Doped Nanocrystals in the Solid State as Well as Dissolved in Ionic Liquids product

τ1 (µs)

%

τ2 (µs)

%

LaF3:5%Nd3+:betaine (core) LaF3:5%Nd3+-LaF3:betaine (core shell) LaF3:5%Nd3+:betaine (core) in [C4mpyr][Tf2N] LaF3:5%Nd3+:betaine (core) in [C4mpyr][TfO] LaF3:5%Nd3+:betaine (core) in [C4mim][Tf2N] LaF3:5%Nd3+-LaF3:betaine (core shell) in [C4mim][Tf2N]

54 ( 2 61 ( 1

58 54

16 ( 1 20 ( 1

42 46

40 ( 2

74

9(1

26

41 ( 2

78

9(1

22

41 ( 2

77

9(2

23

60 ( 2

62

18 ( 1

38

compared to the core particles. The lifetimes are slightly higher for the core shell nanocrystals in the solid state (61 µs (τ1) and 20 µs (τ2)) than for the core analogues (54 µs (τ1) and 16 µs

(τ2)). The same is true for the particles in [C4mim][Tf2N]: 60 µs (τ1) and 18 µs (τ2) for the core shell nanocrystals compared to 41 µs (τ1) and 9 µs (τ2) for core neodymium-doped nanocrystals in [C4mim][Tf2N]. The LaF3 shell protects the luminescent Nd3+ ions from quenchers on the surface of the nanocrystal. This effect was not observed for the europium(III)doped nanocrystalline analogues. Neodymium(III) ions are more susceptible to energy quenching from surrounding high-energy vibrations, since the energy gap between the emitting level and the next lower energy level is smaller for Nd3+ ((5500 cm-1) than for Eu3+ ions ((12500 cm-1). The luminescence lifetimes of the particles dissolved in the ionic liquids are very similar (41 µs (τ1) and 9 µs (τ2)), all being slightly lower than those for LaF3:5%Nd3+:betaine in the solid state (54 µs (τ1) and 16 µs (τ2)). For the core shell nanocrystals, there is no difference between the lifetime of the particles in the solid state (61 µs (τ1) and 20 µs (τ2)) and LaF3:5%Nd3+-LaF3:betaine dissolved in [C4mim][Tf2N] (60 µs (τ1) and 18 µs (τ2)), indicating again protection of the Nd3+ ions from the environment by the LaF3 shell. Conclusion Dispersing or dissolving LaF3 nanocrystals in an ionic liquid is not a straightforward task. The LaF3:Ln3+ nanocrystals without capping ligands or with citrate ions could not be dissolved at all in any ionic liquid tested, whereas LaF3:Ln3+ nanocrystals stabilized with betaine ligands could be dissolved in small amounts (about 1 mg of nanocrystals in 2 mL of ionic liquid) in the ionic liquids [C4mpyr][Tf2N], [C4mpyr][TfO], and [C4mim][Tf2N]. The particles remain intact upon dispersion in the ionic liquid, as was shown by XRD and FTIR analysis of the nanoparticles extracted from the ionic liquid. Europium(III) and neodymium(III) luminescence could be detected for the ionic liquid mixtures containing the inorganic nanoparticles. These results are promising and should be regarded as a first attempt to obtain lanthanide(III) luminescence from nanocrystals dispersed in an ionic liquid. In the future, research efforts will be focused on increasing the solubility of the nanocrystals in ionic liquids. This could be achieved either by using other stabilizing ligands or by adapting the ionic liquid composition so as to match the nanocrystalline system, e.g., by using citrate as an ionic liquid component for citrate-stabilized nanoparticles. Acknowledgment. K.L. (research assistant) and K.D. (postdoctoral fellow) thank the F.W.O.-Flanders (Belgium) for research fellowships. P.V. acknowledges the Flemish IWT for a fellowship. This project was financed by the F.W.O.-Flanders (project G.0508.07) and by the K.U.Leuven (project GOA 08/ 05 and project IDO/05/005). The authors thank IoLiTec (Denzlingen, Germany) for support. References and Notes (1) Bu¨nzli, J.-C. G. Acc. Chem. Res. 2006, 39, 53–61. (2) Bu¨nzli, J.-C. G.; Piguet, C. Chem. Soc. ReV. 2005, 34, 1048–1077. (3) Reyes, R.; Cremona, M.; Teotonio, E. E. S.; Brito, H. F.; Malta, O. L. Chem. Phys. Lett. 2004, 396, 54–58. (4) Kido, J.; Okamoto, Y. Chem. ReV. 2002, 102, 2357–2368. (5) Feldmann, C.; Justel, T.; Ronda, C. R.; Schmidt, P. J. AdV. Funct. Mater. 2003, 13, 511–516. (6) Weber, M. J. J. Non-Cryst. Solids 1990, 23, 208–222. (7) Hemmila, I.; Laitala, V. J. Fluoresc. 2005, 15, 529–542. (8) Sabbatini, N.; Guardigli, M.; Lehn, J. M. Coord. Chem. ReV. 1993, 123, 201–228. (9) Go¨rller-Walrand, C.; Binnemans, K.; Fluyt, L. J. Phys.: Condens. Matter 1993, 5, 8359–8374. (10) Porcher, P.; Caro, P. J. Chem. Phys. 1976, 65, 89–94.

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