Article pubs.acs.org/JPCC
Sensitization of Eu3+ Luminescence in Eu:YPO4 Nanocrystals Jiangchao Chen, Qingguo Meng, P. Stanley May, Mary T. Berry, and Cuikun Lin* Department of Chemistry, University of South Dakota, 414 E Clark Street, Vermillion, South Dakota 57069, United States S Supporting Information *
ABSTRACT: Eu:YPO4·xH2O (x = 0.5−1) nanocrystals were synthesized by a liquid−solid−solution (LSS) solvothermal method and dispersed in chloroform. In order to sensitize the emission from the Eu3+ ions, 2-thenoyltrifluoroacetone (HTTFA) was used to replace a significant fraction of the oleate capping ligand on the as-prepared Eu:YPO4·xH2O (x = 0.5−1) nanocrystals. During the ligand exchange, HTTFA reacts with oleate, forming oleic acid and 2-thenoyltrifluoroacetonate, TTFA. The negatively charged TTFA then displaces the neutral oleic acid ligand from the surface of the nanoparticles. The resulting surface-modified samples were less dispersible in chloroform than were the as-prepared, oleate-capped nanoparticles but were easily dispersed in pyridine, forming very clear mixtures. The resulting surface-modified nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), UV−vis absorption spectroscopy, photoluminescence (PL) spectroscopy, and time-resolved luminescence spectroscopy. XRD analysis indicates that the samples are crystalline with a hexagonal phase. The oleate-capped Eu:YPO4·xH2O (x = 0.5−1) nanocrystals have a zeolite structure with a porous surface. The morphology and quality of the nanoparticles remained unchanged upon ligand exchange. The FTIR spectrum of the surfacemodified (TTFA-sensitized) Eu:YPO4·xH2O (x = 0.5−1) nanocrystals shows signals for both 2-thenoyltrifluoroacetate and oleate. Using UV−vis absorbance and elemental analysis, it is estimated that approximately half of the native oleate capping ligands are replaced with TTFA. Colloidal dispersions in pyridine show characteristic emission of Eu3+ 5D0 → 7FJ (J = 0−4) when excited at the TTFA absorbance band at 350 nm. Ligand excitation at 350 nm results in an enhancement of external quantum efficiency of Eu3+ emission of up to 4700× relative to direct Eu3+ excitation at 464 nm. The ability to sensitize emission from these nanocrystals greatly increases their potential for application in display and lighting fields. The “antenna effect” is another common strategy used to enhance the luminescence of lanthanide complexes. In the antenna effect, a strongly absorbing ligand coordinated to the lanthanide sensitizes lanthanide luminescence.2,17−20 Strong absorbance by the ligand and efficient energy transfer from ligand to lanthanide overcome the drawback of low molar absorptivity associated with the 4f → 4f transitions of lanthanides, resulting in an increase in the excitation efficiency of the lanthanide complexes. This strategy also can be applied to lanthanide-doped nanoparticles, in which the capping ligand serves as a sensitizer. Recently, Zhang et al. used tropolonate capping ligands to sensitize the NIR emission from Yb3+- and Nd3+-doped NaYF4 nanoparticles.21 Kokuoz et al. and Janssens et al. demonstrated the sensitization of Eu3+ emission in Eu:LaF3.22,23 Previous work in our laboratory showed that dipicolinate (DPA) strongly sensitizes Eu3+ emission in citratestabilized 5% Eu:LaF3, increasing the 614 nm emission intensity by a factor of 100 with less than 1% citrate replacement.16 Strouse’s group reported 2,4-pentadione-capped Eu:Y2O3 nanocrystals as a phosphor for white light-emitting diodes.24 Vela et al. and Xiao et al. demonstrated sensitizing europium emission in europium-doped indium oxide nanocrystals.25,26
1. INTRODUCTION Luminescent materials play an important role through their broad use in information displays and lighting1,2 and more specialized use in labeling and sensor technologies. Lanthanidedoped inorganic nanophosphors have many potential applications in light-emitting devices, low-threshold lasers, optical amplifiers, biological fluorescence labels, and drug-release schemes.3−8 However, luminescent intensities from lanthanide-doped nanostructured materials are usually lower than those from the corresponding bulk materials due to the quenching effects of surface defects and capping ligands. This disadvantage greatly restricts their applications. As a result, much effort has been devoted to increasing the luminescence efficiency of nanostructured phosphors.2−6,9,10 One effective method of increasing nanoparticle luminescence is to form a suitable inorganic shell around the core nanocrystals, thereby suppressing energy-loss processes at the surface.3,10 This method has proven to be successful in enhancing the luminescence of quantum dots (QDs) such as II−VI CdSe/ZnS,11,12 Mn:ZnS/ZnS,13 and III−V InAs/InP.14 The core/shell method has also been applied recently in lanthanide-doped nanocrystals, such as Tb:CePO4/LaPO4, Tb:CeF3/LaF3, and Eu:LaF3/LaF3.3,15,16 For lanthanide-doped nanocrystals, the shell serves primarily to shield the emitting Ln3+ ions from the high-energy phonon environment of the capping ligands and solvent. © 2013 American Chemical Society
Received: November 4, 2012 Revised: February 27, 2013 Published: February 27, 2013 5953
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With regard to the sensitization of Eu3+, the β-diketonate ligand 2-thenoyltrifluoroacetone (HTTFA) has attracted considerable interest due to its absorbance in the soft UV (350 nm) and excellent energy transfer efficiency to several lanthanides. Assuming the commonly invoked Jablonski model, TTFA undergoes a singlet−singlet transition under 350 nm excitation, followed by intersystem crossing (ISC) to the ligand triplet state (at 21 670 cm−1), followed by energy transfer (ET) from the triplet state to produce excited states of Eu3+.2,27 Bünzli has suggested that energy transfer may also occur directly from the singlet state of the ligand.2 In either case, sensitization is a short-range effect, with ligand-to-metal energy transfer occurring via (i) an electron exchange mechanism or (ii) a dipole−dipole mechanism, in which energy transfer efficiency is proportional to the inverse sixth power of the distance between the ligand and the lanthanide ion.2,16 As a consequence, efficient sensitization only occurs for lanthanide ions at or near the surface of the nanoparticles, although Cross et al. have demonstrated that sensitization of Eu:LaF3 nanocrystals through a thin shell of LaF3 is still possible.16 As a result of the strong distance dependence of sensitization, the particle size and surface-to-volume ratio greatly affect the fraction of metal ions in the nanocrystal that are available for sensitization. To the best of our knowledge, there are no reports on the sensitization of rare-earth orthophosphates (REPO4), an important class of lanthanide inorganic compounds, which have been widely used in fluorescent lamps, cathode ray tubes (CRTs), plasma display panels (PDPs), laser materials, and moisture sensors.28−30 The crystallographic forms of the rareearth orthophosphates (REPO4) are numerous, with the form obtained depending on the synthesis temperature and method, and include (1) the hexagonal rhabdophane type, REPO4·nH20 (RE = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy, n = 0.5−l); (2) the monoclinic weinschenkite-type, REPO4·2H2O (RE = Y, Dy, Er, Yb, or Lu); (3) the orthorhombic type, DyPO4·1.5H2O, and the anhydrous polymorphs; (4) the tetragonal xenotimetype, REPO4 (RE = Y, Dy, Er, Yb, or Lu), obtained by heating the weinschenkite-type phosphates at or above 300 °C in air; and (5) the monoclinic monazite-type, REPO4, obtained by heating rhabdophane-type phosphates at or above 500 °C in air.31−33 Many methods have been developed to synthesize and control the structure, morphology, and size of lanthanide orthophosphate nanoparticles, including the polyol method,34 the high-boiling point solvent method,3 sonochemical synthesis,35 the hydrothermal/solvothermal method,36−38 and the liquid−solid−solution (LSS) method.39,40 The LSS method is a specific solvothermal approach developed by Li et al. to synthesize a variety of nanocrystals, including lanthanide-doped nanoparticles, and can be used to generate monodisperse nanocrystals whose size can be tuned over a wide range, from several nanometers to several hundred nanometers, by adjusting the concentration of precursors, reaction temperature, and reaction time.39,41,42 In this work, Eu:YPO4·xH2O (x = 0.5−1) nanocrystals were synthesized by the LSS method, and 2-thenoyltrifluoroacetone (HTTFA) was used to replace a significant fraction of the oleate capping ligand on the as-prepared Eu:YPO4·xH2O (x = 0.5−1) nanocrystals in order to sensitize the emission of Eu3+ ions. During the ligand exchange, HTTFA reacted with oleate forming oleic acid and 2-thenoyltrifluoroacetonate, TTFA. The negatively charged TTFA then displaced the neutral oleic acid ligand from the surface of the nanoparticles. The resulting
surface-modified samples were less dispersible in chloroform than were the as-prepared, oleate-capped nanoparticles but were easily dispersed in pyridine, forming very clear mixtures. The resulting surface-modified samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), UV− vis absorption spectroscopy, photoluminescence (PL) spectroscopy, and time-resolved luminescence spectroscopy. By comparing the measured internal quantum efficiency under UV excitation with the calculated intrinsic quantum efficiency of the Eu3+(5D0) emitting state, the sensitization efficiency of TTFAcapped Eu-doped YPO4·xH2O (x = 0.5−1) nanocrystals (TTFA-YPO) nanoparticles was calculated. Although sensitized excitation of dilute-doped TTFA-YPO nanoparticles results in a much lower internal quantum yield compared to direct Eu3+ excitation, a significant enhancement of external quantum yield is obtained, resulting in much brighter sample luminescence.
2. EXPERIMENTAL SECTION 2.1. Materials. Eu(NO 3 ) 3 ·6H 2 O (99.9%) and Y(NO3)3·6H2O (99.9%) were purchased from GFS Chemicals, ethyl alcohol (denatured, 95%) was from Fisher Scientific, Eu(TTFA)3·3H2O and oleic acid (OA) (90%) were from Alfa Aesar, NaH2PO4·H2O (98%) was from Macron Chemicals, methyl alcohol (MeOH) (99.8%) was from ACROS, and NaOH (99.8%), 2-thenoyltrifluoroacetone (HTTFA) (99%), and chloroform (CHCl3) (≥99.8%) were from Sigma-Aldrich. All chemicals were used as received. 2.2. Synthesis of Eu:YPO4·xH2O (x = 0.5−1) Nanocrystals (Oleate-YPO) and Ligand Exchange by HTTFA (TTFA-YPO). Eu:YPO4·xH2O nanocrystals were synthesized by an LSS method.40 Typically, 40 mL of 1.5 M NaOH/ethanol solution, 40 mL of oleic acid (OA), 10 mL of 0.2 M Ln(NO3)3(aq) solution (Ln = Y and Eu with Y:Eu = 19:1), and 10 mL of 0.2 M NaH2PO4(aq) were mixed together with vigorous stirring. After stirring for 20 min, the mixture was transferred into a 125 mL autoclave, then sealed, and heated at 120 °C for 10 h. The apparatus was then allowed to cool to room temperature. The white product spontaneously separated from the reaction mixture and was observed in the bottom of the autoclave. The resulting product was washed three times with a mixture of chloroform:ethanol (1:2) and then dried under vacuum at room temperature. To prepare TTFA-sensitized nanoparticles, 0.059 g of Eu:YPO4·xH2O (x = 0.5−1) was dissolved in 10.0 mL of chloroform under stirring. Then, 0.444 g of HTTFA (0.002 mol) was added into the solution followed by sonication for 5 h. The resulting nanoparticles were precipitated using CHCl3/ ethanol (v:v = 1:1), washed with CHCl3 three times, and dried under atmosphere. Samples were subsequently dispersed in pyridine, forming very clear mixtures. 2.3. Preparation of Eu-Oleate Chloroform Solution. The Eu-oleate chloroform solution was prepared according to a modified procedure in the literature. Typically, 2 mmol of EuCl3 and 4 mL of oleic acid were dissolved in a solvent mixture composed of 2 mL of methanol and 30 mL of chloroform. The resulting solution was heated to 70 °C and kept at that temperature for 2 h. When the reaction was completed, the reaction mixture was dried under vacuum. The resulting Eu-oleate complex was dissolved in 100 mL of chloroform in a volumetric flask. 2.4. Nanoparticle Characterization. Powder X-ray diffraction studies (XRD) were performed using a Rigaku 5954
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Ultima IV X-ray diffractometer. The instrument is fitted with a copper X-ray tube emitting Kα1 radiation at 1.54 Å. The powder XRD patterns were collected in continuous scan mode with a scan rate of 1.00 deg/min over a 2θ range of 10°−70°. Identification of the samples was made by comparing the sample XRD pattern to the International Centre for Diffraction Data (ICDD) powder diffraction file for YPO4·0.8H2O (PDF card 42-0082). TEM images were obtained using an FEI Tecnai Spirit with an acceleration voltage of 120 kV. For the TEM experiments, chloroform solutions of the as-prepared Oleate-YPO nanocrystals and pyridine solutions of TTFA-YPO nanocrystals were cast onto carbon-coated copper grids and allowed to dry naturally. Fourier transform infrared (FTIR) spectra were measured using an ALPHA FTIR spectrometer from Bruker, equipped with an attenuated total reflectance attachment (Platinum ATR). CHN elemental analysis was performed by M-H-W Laboratories, Phoenix, AZ, using a modified PerkinElmer 240 C−H−N analyzer. UV−vis absorbance spectra of chloroform solutions of the asprepared Oleate-YPO nanocrystals and pyridine solution of TTFA-YPO nanocrystals were measured using a Cary 5000 dual-beam spectrophotometer. In order to estimate the doping percentage of Eu3+ in the Eu:YPO4·xH2O (x = 0.5−1) nanocrystals, absorbance spectra were measured on an Olis Clarity absorbance spectrometer for a 118 mM Eu:YPO4·xH2O (x = 0.5−1) /chloroform solution and a 50 mM EuCl3 aqueous solution for the Eu3+ 7F0 → 5D1 transition region (518−530 nm). The Clarity sample chamber provides an effective optical path length of ∼10 cm, enabling accurate absorbance measurements on weakly absorbing samples. Emission and excitation spectra were acquired on a Fluoromax fluorometer (SPEX) using front-face collection from a 1 cm path length cuvette or, alternatively, on a Fluoromax-4 fluorometer (JY Horiba) using 90° collection from a 0.4 cm path length cuvette. All spectra were corrected for instrument response in terms of relative photon flux per constant wavelength interval. Emission spectra for quantum efficiency measurements were also corrected for the intensity of the excitation light. Luminescence lifetimes of visible emission were acquired using a 0.46 m flat-field monochromator (JobinYvon HR460) with a fast, red-sensitive, side-window photomultiplier (Hamamatsu, R928) and a time-resolved photoncounting detection system (Stanford Research Systems, SR430 multichannel scaler). The pulsed excitation source for lifetime measurements was provided by either a frequency-tripled Nd:YAG laser at 355 nm or an optical parametric oscillator (Opotek, Opolette) at 464 nm. 2.5. Comparison of a Eu-Oleate Coordination Complex with Added TTFA to the TTFA-Sensitized Eu-Doped YPO4·xH2O Nanocrystals. The emission spectrum of 20 mM Eu(oleate)3 with added NaTTFA was measured with [TTFA]/ [Ln] ratios ranging from ∼0 to 5, holding the Eu 3+ concentration constant. The Eu(oleate)3−TTFA chloroform solutions were prepared by dissolving the appropriate mass of NaTTFA powder in 5 mL of 20 mM Eu(oleate)3 chloroform solution. The emission spectra of the Eu(oleate)3−TTFA chloroform solutions were measured by front-face collection using a Fluoromax fluorometer, with excitation at 350 nm. The emission spectra of TTFA-YPO dispersions in pyridine with added NaTTFA were measured with [TTFA]/[Ln] ratios ranging from ∼0 to 5, holding the Ln3+ (Ln = Y, Eu)
concentration constant. (Here, [TTFA] represents the concentration of TTFA added as NaTTFA.) Each sample had a total volume of 4 mL with a constant TTFA-YPO nanoparticle concentration of 0.13 g/L. Samples were prepared by mixing 2 mL of 0.26 g/L TTFA-YPO with a 1.44 g/L NaTTFA solution and solvent (pyridine). The relative volumes of added solvent and added NaTTFA solution were adjusted to achieve the desired ratio of [TTFA]/[Ln3+] (Ln = Y, Eu). The emission spectra were measured via front-face collection using a Fluoromax fluorometer, with excitation at 350 nm.
3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Morphology of Oleate-YPO and TTFA-YPO Nanoparticles. Figure 1 shows the XRD
Figure 1. XRD patterns of (a) as-prepared Eu-doped YPO4·xH2O (x = 0.5−1) nanocrystals (Oleate-YPO) and (b) TTFA-capped Eu-doped YPO4·xH2O nanocrystals (TTFA-YPO) compared to ICDD PDF card 42-0082 for the rhabdophane-type hexagonal structure of YPO4·0.8H2O.
profiles of (a) as-prepared Eu-doped YPO4·xH2O nanocrystals (Oleate-YPO) and (b) TTFA capped Eu-doped YPO4·xH2O nanocrystals (TTFA-YPO). All of the peaks in panels a and b can be readily indexed to a rhabdophane-type hexagonal structure YPO4·0.8H2O according to International Centre for Diffraction Data (ICDD) PDF card 42-0082. No peaks of any other phases or impurities were detected. Figure 1b shows that ligand exchange has little effect on the XRD pattern beyond a slight sharpening of the peaks. This confirms that the OleateYPO nanocrystals are chemically stable in the TTFA− chloroform solution and that our ligand exchange method does not diminish the quality of the nanocrystals. The XRD of the Eu:YPO4·xH2O nanocrystals after ligand exchange was checked because we observed in previous studies that dipicolinate (DPA) in high concentrations can destroy LaF3 nanoparticles.16 The size of the nanoparticles can be estimated from the powder XRD spectrum using the Scherrer equation (d = kλ/β cos θ), where λ is the X-ray wavelength (0.154 05 nm), β is the full width at half-maximum (fwhm) of a diffraction peak, θ is the diffraction angle, k is a constant (0.89), and d gives the size of the nanoparticle along the ⟨hkl⟩ direction.43,44 Here, we take the highest peak at 2θ = 30.2° ((111) plane) to calculate the size of the nanoparticles. The estimated crystallite sizes of 5955
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Oleate-YPO and TTFA-YPO nanoparticles are 23.9 ± 0.9 and 29.1 ± 0.3 nm, respectively, which agree well with the TEM results discussed below. (The uncertainties given for the size, d, are propagated from the standard error of the fitted values of β and thus reflect the precision in the average size rather than the width of the size distribution.) After ligand exchange, the average size of the nanoparticles gets slightly bigger, probably due to a small degree of size selection that occurs in the washing process. During multiple washing steps, the larger particles are more-easily centrifuged out, resulting in the small difference observed in the XRD. TEM images of the Oleate-YPO nanoparticles are shown in Figure 2a,c. The nanoparticles are well separated and of quasi-
structure, with cylindrical, oxygen-lined tunnels in the direction of the c-axis, with a diameter of 5.3 Å, which can accommodate water.32,46 Those tunnels have a dimension of an order of magnitude smaller than the observed irregular pores shown in Figure 2a,c,d. The morphology and crystallinity of the nanoparticles remain unchanged upon ligand exchange, as shown in Figure 2d. The average diameter, calculated using 80 nanoparticles, was found to be 32.4 ± 8.4 nm, consistent with the Scherrer analysis, which gave 29 nm. After ligand exchange, the average size of the nanoparticles was found to be slightly larger, for reasons discussed above. 3.2. Eu3+ Doping Level in YPO Nanoparticles. The actual Eu3+ doping percentage in our YPO nanoparticles was estimated based on CHN analysis to determine total organic content and the absorbance spectrum of Eu:YPO4·xH2O nanocrystals in chloroform solution to determine the europium concentration. The method is described in detail in the Supporting Information. In brief, the absolute Eu3+ concentration in a nanoparticle solution was determined by comparing its absorbance due to the magnetic dipole transition at 526 nm to that of a standard 50 mM EuCl3(aq) solution. The Eu3+ doping level in YPO was then calculated based on the known concentration of YPO4·xH2O in solution. The experimentally determined level of Eu3+ doping is 5.9 ± 0.2%, which is in reasonable agreement with the nominal 5% doping level expected based on precursor concentrations. 3.3. Surface Composition Analysis of Oleate-YPO and TTFA-YPO Nanoparticles. The surface composition of the Oleate-YPO and TTFA-YPO sample was studied by FTIR spectroscopy. Figure 3 compares the FTIR spectra of (a) Oleate-YPO, (b) TTFA-YPO nanoparticles, (c) oleic acid, (d)
Figure 2. TEM images of (a) Oleate-YPO nanoparticles. (b) Simple model illustrating lattice orientations. (c) HRTEM image of OleateYPO nanoparticles. (d) HRTEM image of TTFA-YPO nanoparticles.
hexagonal shape. The nanoparticles have high crystallinity, as evidenced by the well-defined two-dimensional lattice fringes seen in Figure 2c. The interplanar distance between adjacent lattice fringes is determined to be 0.59 nm, which corresponds to the d-spacing value of the (101̅0) planes. During the growth stage, oleate preferentially binds to the (001) facets of YPO4·xH2O seeds, which lowers the surface energy of these facets, such that growth in the ⟨0001⟩ direction is inhibited, and the growth of seeds is, instead, driven along six symmetric directions: ±⟨101̅0⟩, ±⟨11̅00⟩, and ±⟨011̅0⟩, resulting in the formation of YPO4·xH2O nanoplates, as shown in Figure 2b.4,45 The average diameter, calculated using 100 nanoparticles, was found to be 23.2 ± 8.8 nm. The average size is in reasonable agreement with the crystalline size calculated by the Scherrer formula from XRD patterns (23.9 nm), but the TEM results reveal the broad distribution of sizes present in the sample. The Oleate-YPO nanoparticles show a very interesting morphology, with a combination of zeolite structure and a nanoporous surface, resulting in large surface areas. The TEM images clearly show that the nanoparticles have relatively large irregular pores on their surface, sometimes penetrating entirely through the crystal. Similar results were observed by Li’s group.40 As previously noted, YPO4·xH2O has a zeolite
Figure 3. FTIR spectra of (a) Oleate-YPO, (b) TTFA-YPO nanoparticles, (c) oleic acid, (d) sodium oleate, (e) sodium 2thenoyltrifluoroacetate (NaTTFA), and (f) 2-thenoyltrifluoroacetone (HTTFA). The features at 1540 and 1455 cm−1 are assigned to the carboxylate group of the oleate capping ligand, and features at 2924 and 2854 cm−1 can be assigned to the C−H stretch of the oleate capping ligand. After ligand exchange, the new bands 1611 and 1535 cm−1 band can be ascribed to the chelating carbonyl group and the carbon−carbon double bond of thenoyltrifluoroacetate (green line). 5956
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free TTFA ligand due to the chelation with the lanthanides on the surface of the nanoparticles. Similar red-shifting of ligand absorbance with metal binding has been observed in the literature.16,52−57 For comparison, the absorbance spectrum of Oleate-YPO nanoparticles is also provided in Figure 4c (green line). Using the absorbance of TTFA-YPO in pyridine (0.26 g/L) shown in Figure 4 and the extinction coefficient of TTFA at 350 nm (ε = 2.0 × 104 M−1 cm−1), the concentration of TTFA was determined via Beer’s law to be [TTFA] = 1.6 × 10−4 M. The concentration of carbon due to TTFA is, therefore, 0.015 g/L. The wt % C in TTFA-YPO due to TTFA is then (0.015 g/ L)/(0.26 g/L) × 100 = 5.8%. From CHN analysis of the dried sample, the total wt % C in TTFA-YPO is 18.4%. The wt % C in TTFA-YPO due to oleate is then 18.4% − 5.8% = 12.6%. Given that there are 96 g of C per mole of TTFA and 212 g of C per mole of OA, the molar ratio of TTFA to OA in the nanoparticle sample is 1.0:0.96. Therefore, TTFA-YPO nanocrystals have an approximately equal number of TTFA and OA capping ligands. We note that we had initially attempted the ligand exchange using NaTTFA instead of HTTFA. However, this resulted in a minimal (1%) replacement of the oleate capping ligands. With the current method, an excess of HTTFA relative to oleate (18:1 molar ratio) drives the following equilibrium to the right.
sodium oleate, (e) sodium 2-thenoyltrifluoroacetate (NaTTFA), and (f) 2-thenoyltrifluoroacetone (HTTFA). The vibrational spectra of the Oleate-YPO sample and sodium oleate are displayed in Figures 3a and 3d, respectively. The bands at 1455 and 1540 cm−1 in Figure 3a are assigned to the asymmetric and symmetric stretching of the carboxylate group (−COO−), respectively, which is consistent with the oleate anion as the capping agent for Oleate-YPO nanoparticles.47 The lack of a band at 1700 cm−1 indicates there is little oleic acid on the surface of Oleate-YPO nanoparticles. The new bands at 1611 and 1535 cm−1 for TTFA-YPO (relative to Oleate-YPO) can be ascribed to the chelating carbonyl group and the carbon− carbon double bond of thenoyltrifluoroacetate (green line).48−50 The bands at 2924 and 2854 cm−1, which can be assigned to the C−H stretch of oleate, were also observed. We measured the FTIR spectrum of the discarded wash solution after evaporating the CHCl3 and ethanol (Supporting Information), and the band at 1709 cm−1 of oleic acid was observed, supporting that, during the ligand exchange, HTTFA reacts with oleate forming oleic acid and TTFA (2thenoyltrifluoroacetate). TTFA has a strong absorbance band centered at 350 nm, which can be used to quantify the attachment of TTFA to the surface of the nanoparticles.51 To eliminate interference from unbound TTFA in the sample dispersions, TTFA-YPO nanoparticles were washed with CHCl3/ethanol (v:v = 1:1), so that NaTTFA and any free Eu-oleate or Eu-TTFA complexes, if present, could be removed. The red trace in Figure 4a shows the absorbance spectrum of the washed TTFAYPO nanoparticles in pyridine (0.26 g/L). The absorbance band at 350 nm can be ascribed to TTFA bound to the nanoparticles, consistent with the absorbance of NaTTFA in Figure 4d (black line). Note that the TTFA absorbance of TTFA-YPO nanoparticles is red-shifted relative to that of the
HTTFA + OA− ↔ TTFA− + OA
As result, negatively charged TTFA is competing with neutral oleic acid, instead of with negatively charged oleate, for binding to the nanoparticle surface. As discussed above, this strategy proved quite effective, resulting in a 50% replacement of oleate capping ligands. 3.4. Photoluminescence Properties of TTFA-YPO Nanoparticles. Figure 5 shows the excitation and emission
Figure 5. Excitation and emission spectra of the 0.036 g/L TTFAYPO nanoparticles in pyridine. The excitation spectrum of TTFAYPO nanoparticles was collected by monitoring the 613 nm emission of Eu3+. The emission spectra were collected by exciting the TTFA ligand at 350 nm (a, red line) and exciting Eu3+ directly at 464 nm (b, black lines) into the 7F0 → 5D2 transition. A scaling factor of 2000 was applied to the emission spectrum resulting from direct Eu3+ excitation.
Figure 4. UV−vis absorbance spectra of 0.26 g/L TTFA-YPO nanoparticles in pyridine (a, red), 6.6 g/L TTFA-YPO nanoparticles in pyridine (b, blue), 9.6 g/L Oleate-YPO nanoparticles in chloroform (c, green), and 0.024 g/L NaTTFA in pyridine (d, black). In the TTFAYPO spectrum, the absorbance at 350 nm is consistent with the absorbance peak of NaTTFA. The peak at 464 nm arises from the 7F0 → 5D2 transition of Eu3+. The solution for which this transition is observed was 25× more concentrated than the solution used to observe the TTFA transition in TTFA-YPO. An additional scaling factor of 2000× is applied to the spectrum in order to see this weak feature.
spectra of the 0.036 g/L TTFA-YPO nanoparticles in pyridine. The excitation spectrum was collected by monitoring the 613 nm emission of Eu3+ arising from the 5D0 → 7F2 transition. The two emission spectra were collected by exciting into the TTFA absorbance at 350 nm (a, red line) and, alternately, by exciting Eu3+ directly at 464 nm (b, black line). A scaling factor of 2000 5957
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was applied to the emission spectrum resulting from direct Eu3+ excitation. Ligand excitation at 350 nm results in Eu3+ emission that is ∼4700 times brighter than obtained by direct Eu3+ excitation at 464 nm. We note that the 4700× enhancement factor observed here is obtained in the low-concentration limit, as defined in section 3.5. As discussed in section 3.5, the enhancement factor is a function of sample concentration. Under 350 nm excitation, the emission spectrum of TTFAYPO nanoparticles is clearly different from that resulting from direct Eu3+ excitation at λex = 464 nm. This is particularly evident regarding the asymmetry ratio, R, where R is the ratio of the integrated intensity of hypersensitive 5D0 → 7F2 transition to that of the magnetic dipole 5D0 → 7F1 transition. Because the emission for direct Eu3+ excitation is so weak for the 0.036 g/L TTFA-YPO sample (see Figure 5), the asymmetry ratio, R, for 464 nm excitation was determined using a more concentrated TTFA-YPO sample (6.6 g/L). Figure 6 shows a comparison of the 5D0 → 7F1,2 emission
In demonstrating ligand sensitization of nanoparticle luminescence, it is important to eliminate the possibility of interference from luminescence arising from free Eu 3+ complexes, not bound in the particles. If the sensitizing ligand preferentially binds free Eu3+ ions in solution, even small numbers of such ions would lead to a significant contribution to sample luminescence. The most likely candidate for interference would be from a Eu(oleate)x(TTFA)y complex. To reduce this possibility, the TTFA-sensitized nanoparticles were repeatedly washed with a chloroform/ethanol solution in which the HTTFA, NaTTFA, oleic acid, and Eu(oleate)x(TTFA)y are all highly soluble. The absorbance and fluorescence of the wash solution were monitored for the eventual disappearance of TTFA absorbance and Eu 3+ luminescence. To further demonstrate that the luminescence from our TTFA-YPO samples is not from free Eu(oleate)x(TTFA)y complexes, we compared the effect of added TTFA on the emission properties of our nanoparticle samples and on the emission properties of solutions of Eu(oleate)x. In Figure 7, a
Figure 6. Comparison of the 5D0 → 7F1,2 emission of 0.036 g/L TTFA-YPO nanoparticles in pyridine using TTFA excitation at λex = 350 nm (a, red line) with that from 6.6 g/L TTFA-YPO nanoparticles in pyridine using direct Eu3+ excitation at λex = 464 nm (b, black line). Excitation into TTFA sensitizes surface and near surface sites, which exhibit a higher asymmetry ratio, R, than does the direct excitation.
Figure 7. Plots of the asymmetry value, R, as a function of [TTFA]/ [Ln3+] in a chloroform solution of Eu(oleate)x(TTFA)y (black line) and a dispersion of TTFA-YPO nanoparticle in pyridine (red line). The value of [Ln3+] was held constant in these experiments. The dramatically different trends observed for the two samples are a strong indication that there is no substantive contribution from free Eu(oleate)x(TTFA)y complexes to the sensitized luminescence observed from the TTFA-YPO nanoparticle samples and that sensitization is occurring at Eu3+ surface/near-surface sites on the nanoparticles. The errors are propagated assuming a 5% uncertainty in the determination of each intensity value.
region for 0.036 g/L TTFA-YPO nanoparticles in pyridine with λex = 350 nm (a, red line) with that for 6.6 g/L TTFA-YPO nanoparticles in pyridine with λex = 464 nm (b, black line). The asymmetry ratio of TTFA-YPO with λex = 350 nm (R = 4.2) is much larger than with λex = 464 nm (R = 2.8). The photoluminescence from Eu3+ in YPO4 nanoparticles contains at least two components: one from the (near) surface sites and the other from interior sites.16,58 A consistent interpretation of differences in the emission spectra seen in Figure 6 is that the emission spectrum resulting from TTFA excitation at 350 nm is due primarily to Eu3+ surface and near-surface sites, which are closely associated with a TTFA ligand, whereas the 4f → 4f excitation at 464 nm generates emission from all Eu3+ ions in the nanoparticles. This implies that Eu3+ surface sites have a higher asymmetry ratio, R, compared to Eu3+ ions within the core, which is consistent with the previous observations of Sudarsan59 and our previous results for DPA sensitized 5% Eu:LaF3 nanoparticles.16
comparison is made of the dependence of R on added NaTTFA for a solution of Eu(oleate)x(TTFA)y and a dispersion of TTFA-YPO under 350 nm excitation. Higher R values are consistent with increased coordination number by TTFA. In the case of Eu(oleate)x(TTFA)y, the asymmetry ratio increases from 6.2 to 14 as the coordination of TTFA increases from y = 1 to an expected maximum of y = 3. In the TTFA-YPO case, the R value remains stable, dropping only slightly from 4.2 to 4.0. This is consistent with the case wherein surface ions in the nanoparticles bind at most one TTFA ligand, and adding TTFA simply results in more sites with bound TTFA, rather than higher TTFA coordination at any one site. It should be noted that the Eu(oleate)x complexes, with no bound TTFA, are silent in this experiment because excitation is into the TTFA ligand. Therefore, the value of R = 6.2 can be interpreted at the 5958
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Table 1. Ratio Itot/IMD, of the total Eu3+(5D0) emission to that of the the 5D0→7F1 magnetic dipole transition; observed Eu3+(5D0) decay rate constant, k; calculated radiative decay rate constant, kR; internal quantum efficiency, Φ; intrinsic quantum efficiency, ΦEu, and sensitization efficiency, ηsens of Oleate-YPO and TTFA-YPO nanoparticles, and Eu(TTFA)3. The errors are propagated assuming a 5% uncertainty in the determination of each I and A value sample
λex (nm)
Itot/IMD
464 350 350
4.95 ± 0.35 7.21 ± 0.51 23.4 ± 1.6
a
Oleate-YPO TTFA-YPOb Eu(TTFA)3c a
refractive index n a
1.49 1.51b 1.36c
k (s−1)
kR (s−1)
1388 ± 69 1409 ± 70 2771 ± 139
240 ± 17 364 ± 26 859 ± 61
Φ
ΦEu
ηsens
7.7 × 10−3 ± 0.8 × 10−3 0.15d
0.17 ± 0.02 0.26 ± 0.02 0.31 ± 0.03
0.030 ± 0.002 0.48 ± 0.04
CHCl3. bPyridine. cAcetone. dReference 63.
⎛I ⎞ kR = AMD,0n3⎜ tot ⎟ ⎝ IMD ⎠
asymmetry ratio for the Eu(oleate)x(TTFA)1 complex. The asymptotic value of R = 14 is interpreted as the asymmetry ratio for Eu(oleate)x(TTFA)3, and the intermediate values result from equilibrium mixtures. Strikingly, there exist no Eu(oleate)x(TTFA)y complex or mixture of complexes for which the R value is as low as that observed for the TTFA-YPO nanoparticle sample. This is strong evidence that the sensitized luminescence we report in this work arises from surface sensitization of the YPO nanoparticles. 3.5. Quantum Efficiencies and Sensitization Efficiency of TTFA-YPO Nanoparticles. The internal quantum efficiency of Eu3+emission, Φ, in TTFA-YPO when exciting into the TTFA absorbance band, is given by60−62
Φ = ηsensΦEu
(2)
where AMD,0 is the Einstein spontaneous emission coefficient for the 5D0 → 7F1 magnetic-dipole transition (in vacuum), n is the refractive index of the medium, and Itot and IMD are the integrated intensities of the total Eu3+(5D0) emission spectrum and the 5 D 0 → 7 F 1 magnetic-dipole transition peak, respectively. The value for AMD,0 is taken from Werts et al. as 14.65 s−1.67 The values of k, kR, and ΦEu for Oleate-YPO, TTFA-YPO, and Eu(TTFA)3 are given in Table 1. For Oleate-YPO, the total rate constant, k, was determined using 464 nm excitation. Table 1 also compares the values of Φ and ηsens for TTFA-YPO and Eu(TTFA)3. The intrinsic quantum efficiencies, ΦEu, of TTFA-YPO and Eu(TTFA)3 are similar, so that the lower internal quantum efficiency, Φ, observed for TTFA-YPO (Φ = 7.7 × 10−3 ± 0.8 × 10−3 vs 0.15) is due almost entirely to the decreased sensitization efficiency in TTFA (ηsens = 0.030 ± 0.002 vs 0.48 ± 0.04). For the TTFA-YPO nanoparticles, however, the majority of the sensitizing ligands are presumably coordinated to Y3+ ions. If we reasonably assume that only 5− 6% of TTFA ligands are bound to surface Eu3+ ions in TTFAYPO, consistent with the Eu3+ doping level in the particles, and that only ligands directly bound to Eu3+ participate in sensitization, then the value of ηsens for a TTFA ligand bound to a Eu3+ on the surface of TTFA-YPO is approximately 0.030 ± 0.002/0.059 ± 0.002 = 0.51 ± 0.05, which is similar to the value of ηsens = 0.48 ± 0.042 observed for Eu(TTFA)3 in acetone. Therefore, the intrinsic sensitization efficiency by TTFA for Eu3+ sites on the surface of the nanoparticles is similar to that for free Eu3+ ions in solution. We note that the value of ηsens = 0.51 ± 0.05 for TTFA bound to Eu3+ on TTFAYPO will have been slightly overestimated, if there is measurable sensitization of Eu3+ from TTFA ligands bound to adjacent Y3+. Cross et al. have presented significant evidence that dipicolinate can sensitize Eu3+ emission in 5%Eu:LaF3 even when not directly coordinated to Eu3+.16 Still, it is clear that the sensitization efficiency for a ligand bound to Eu3+ on the surface of the nanoparticle is similar to that observed for the Eu(TTFA)3 complex in solution. Finally, it is interesting to discuss the enhancement factor (EF) of the sensitized TTFA-YPO nanoparticles in terms of the various measures of quantum efficiency introduced in our analysis. For a given excitation flux, the “brightness”, or intensity of luminescence from the sample, is determined by the external quantum yield, ΦEXT, which can be related to the internal quantum yield, Φ, as follows:
(1)
where ΦEu is the intrinsic quantum efficiency of emission from Eu3+(5D0) and ηsens is the sensitization efficiency, which represents the fraction of photons absorbed by TTFA that produce a Eu3+(5D0) excited state. As discussed below, it is possible to determine Φ using a secondary luminescence standard; ΦEu can be determined from the luminescence lifetime and a corrected Eu3+(5D0) emission spectrum, as described below, and ηsens can, therefore, be determined using eq 1 and the known values of Φ and ΦEu. In this way, the factors that contribute to the overall internal quantum efficiency can be separately quantified and the sensitization process in TTFA-YPO be more completely understood. For comparison purposes, a reference solution of 1.0 × 10−3 M Eu(TTFA)3 in acetone solution was prepared, which, according to the literature, has an internal quantum efficiency for sensitized emission of 0.15.63 Of particular interest is a comparison of the sensitization efficiency, ηsens, of TTFAYPO and Eu(TTFA)3. The value of Φ in eq 1 can be estimated by comparison to a secondary standard, quinine sulfate (QS) in 1 N H2SO4 (Φstd = 0.546) (Supporting Information).64 It is determined to be Φ = 7.7 × 10−3 ± 0.8 × 10−3 for TTFA-YPO nanoparticles. As discussed below in detail, the low value of Φ = 7.7 × 10−3 ± 0.8 × 10−3 observed for sensitized emission from TTFA-YPO nanoparticles is due primarily to the fact that only 5−6% of the absorbing TTFA ligands are bound to Eu3+ and the rest are bound to nonemitting Y3+ sites. The intrinsic quantum efficiency, ΦEu, equal to the fraction of Eu3+ ions that relax radiatively once the Eu3+(5D0) state has been populated, is calculated from the ratio of the radiative rate constant, kR, to the total decay rate constant, k, according to ΦEu = kR/k. The total rate constants were determined from lifetime measurements (Supporting Information). Corrected total emission spectra were used to estimate kR of Eu3+(5D0), using the 5D0 → 7F1 magnetic dipole transition, which is largely unaffected by the crystal field, as an internal reference.65−67 The radiative rate constant, kR, is given by67
ΦEXT = 5959
NAbs Φ N
(3)
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where NAbs is the number of photons absorbed by the sample and N is the number of photons incident on the sample. The enhancement factor (EF) for ligand excitation at 350 nm relative to direct Eu3+ excitation at 464 nm can be expressed as Φ EF = EXT − 350 = ΦEXT − 464
NAbs Φ N 350 350 NAbs Φ N 464 Eu
( ) ( )
=
Ligand exchange results in an increase in external quantum yield of up to 4700× when using ligand excitation compared to direct Eu3+ excitation at 464 nm. The sensitized emission from the nanoparticles (TTFA-YPO) shows features characteristic of surface and near-surface Eu3+ sites with a high asymmetry ratio, R, compared to interior sites. The sensitized emission from TTFA-YPO also shows a higher R compared to the oleatecapped nanoparticles but a lower R compared to Eu(oleate)x(TTFA)y complexes. Monitoring the change of the R values with added NaTTFA proved to be a useful method for distinguishing true nanoparticle sensitization from potential interference by any free Eu3+ complexes not bound in the nanoparticle. The internal quantum efficiency, Φ, of sensitized emission from TTFA-YPO was determined to be 7.7 × 10−3, suggesting a TTFA to Eu3+(5D0) sensitization efficiency of 3.0%. This sensitization efficiency is comparable to the nominal 5−6% Eu3+ doping concentration and, thus, to the fraction of TTFA ligands expected to be bound to Eu3+, as opposed to nonemitting Y3+ surface sites, suggesting that the transfer efficiency for ligands bound directly to Eu3+ sites is quite high. In fact, the sensitization efficiency for surface/near-surface Eu3+ sites on TTFA-YPO nanoparticles was shown to be very similar to that of free Eu3+ ions in solution. Thus, we have demonstrated that surface-bound-ligand sensitization of the Eu3+-doped YPO4 nanocrystals provides significant luminescence enhancement and propose that ligand sensitization of lanthanide-based nanophosphors holds great promise for applications in luminescent sensing and in display and lighting technologies.
(1 − 10−A350)Φ350 (1 − 10−A464)ΦEu (4)
where A350 and A464 are the optical densities of the sample at 350 and 464 nm, respectively, and Φ350 is the internal quantum efficiency, Φ, of the TTFA-YPO sample excited at 350 nm. (Equation 4 makes the reasonable approximation that the internal quantum efficiency for 464 nm excitation equals the intrinsic quantum efficiency for Eu3+(5D0) luminescence.) Since Φ350 ≪ ΦEu, it is obvious that the enhancement from sensitization is due completely to the larger sample absorbance values at 350 nm. The relative absorbance of TTFA at 350 nm and Eu3+ at 464 nm in TTFA-YPO can be roughly estimated from the absorbance spectra a and b shown in Figure 4. Because the absorbance at 464 nm for the Eu3+ transition is very weak relative to the 350 nm TTFA transition in TTFAYPO, a more concentrated solution (6.6 g/L vs 0.26 g/L) was used, and a scaling factor of 2000× was applied to obtain the spectrum labeled as b. Thus, the absorbance ratio at 350 nm vs 464 nm, for a given concentration of TTFA-YPO, is estimated to be A350/A464= 1.6 × 105. According to eq 4, the enhancement factor declines with increasing sample concentration, as A350 and A464 grow larger. Maximum enhancement is achieved in the low-optical-density range (A < 0.05), in which the external quantum efficiencies vary linearly with sample concentration. In the low-opticaldensity range, eq 4 simplifies to A Φ EF = 350 350 A464 ΦEu
(A350 < 0.05)
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Estimation of the Eu doping percentage in Eu:YPO4·xH2O nanocrystals; FTIR spectrum of the first wash solution of TTFA-YPO nanoparticles after evaporating the CHCl3 and ethanol; excitation spectrum of Oleate-YPO nanoparticles; estimation of the internal quantum efficiency, Φ, of TTFA-YPO nanoparticles; decay curves for the luminescence of Eu3+ in the TTFA-YPO and Oleate-YPO nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.
(5)
which represents the maximum value of EF. From the observed enhancement factor of 4700 for samples with A350 < 0.05 and the values of Φ and ΦEu in Table 1, eq 5 was used to calculate A350/A464 = 1.6 × 105. This absorbance ratio value is the same as was estimated from the absorbance spectra in Figure 4. We note that ΦEXT and EF can be further increased by increasing the number of TTFA ligands on the nanoparticle surface or by increasing the doping level of Eu3+.
Corresponding Author
*E-mail:
[email protected]. Notes
4. CONCLUSION In this work, oleate-stabilized Eu:YPO4·xH2O (x = 0.5−1) nanocrystals were synthesized by an LSS solvothermal method, and luminescence from the Eu3+ dopant sites was sensitized by subsequent addition of surface-coordinating 2-thenoyltrifluoroacetonate (TTFA) using HTTFA. Powder XRD results indicate that the samples crystallized in the rhabdophane-type hexagonal phase, likely stabilized by zeolitic water. Ligand exchange has essentially no effect on the XRD pattern. TEM images confirm the structure, showing quasi-hexagonal shaped crystallites of ∼23.2 nm with well-defined crystallographic planes and large irregular pores appearing as defects. After ligand exchange, the average size of the nanoparticles, as isolated, becomes slightly larger, probably due to incidental size selection in the washing procedure. The ligand exchange results in addition of TTFA to the surface of the nanocrystals, with an approximate 1:1 molar ratio of TTFA to oleate capping ligands.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support for this work was provided by the National Science Foundation (EPS-0903804, CHE-0722632 and CHE0840507), NASA under Cooperative Agreement NNX10AN34A, and by the State of South Dakota through the Governor’s Office of Economic Development.
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REFERENCES
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dx.doi.org/10.1021/jp3109072 | J. Phys. Chem. C 2013, 117, 5953−5962
