Formation of Eu (III) Nanoparticles on Borosilicate Sol− Gel Studied

Oct 11, 2010 - Andreia Ruivo , Suzana M. Andrade , João Rocha , César A. T. Laia , and ... Isabel Coutinho , J. P. Leal , Luís C. Branco , and César A...
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J. Phys. Chem. C 2010, 114, 18414–18422

Formation of Eu(III) Nanoparticles on Borosilicate Sol-Gel Studied with Time-Resolved Luminescence Techniques Ma´rcia G. Ventura, Ce´sar A. T. Laia,* and A. Jorge Parola REQUIMTE, Departamento de Quı´mica, Faculdade de Cieˆncias e Tecnologia, UniVersidade NoVa de Lisboa, 2829-516 Monte da Caparica, Portugal ReceiVed: July 22, 2010; ReVised Manuscript ReceiVed: September 3, 2010

The formation of Eu(III) nanoparticles in borosilicate sol-gels and the glass formation heat treatment effect on those particles were studied using luminescence techniques. The presence of the particles was observed using transmission electron microscopy (TEM) images followed by analysis with energy dispersive X-ray spectroscopy (EDS). These experiments showed the presence of particles with a large quantity of europium and chlorine and only small amounts of oxygen with sizes ranging from 30 to 100 nm. Heat treatment at 400, 600, and 800 °C lead to glass samples in which those particles were no longer observed. Steady-state and time-resolved luminescence techniques allowed a detailed study of Eu(III) photophysics in sol-gel and glass samples. In sol-gel matrices, the 5D0 f 7F0 transition is very weak, hinting at Eu(III) species experiencing a rather symmetric crystal field. The 5D0 f 7F2 transition intensity is not very strong, which according to a Judd-Ofelt analysis indicates low interaction with the anions present in the sol-gel matrices. This picture reverses after heat treatment, indicating a replacement of chloride anions with oxygen as preferential ligands of Eu(III). Time-resolved luminescence shows in a more detailed way these aspects. Sol-gel samples display nonexponential kinetics, which are attributed to Eu(III) species present in the nanoparticles surface (bound to oxygen) and Eu(III) in the core of the nanoparticles (bound to chloride). Glass samples display singleexponential luminescence decays, in which the decay constant approaches the values calculated for the radiative rate constant with Judd-Ofelt analysis. It is concluded that, in sol-gel, mechanisms like electron-phonon coupling suppress the Eu(III) luminescence, which disappear as soon as the nanoparticles are disrupted after heat treatment. 1. Introduction Despite their name, rare earths are becoming increasingly common in a vast number of applications, such as electroluminescent devices, biomedical applications, photoluminescent materials (including lasers), chemical sensors, and many more.1 Recently, rare earths are becoming a truly interdisciplinary research topic with their use in fine arts through luminescent glass sculptures and other artistic creations.2 In the core of their applicability are the interesting (and tunable) luminescent colors that are obtained via photo- or electroinputs. Into this respect, rare earths present interesting basic science and technological challenges. Although their use as phosphors is relatively easy, this phenomenon arises from forbidden f-f transitions which display low extinction coefficients for the absorption of visible light.3-7 Much effort as been directed toward the design of photosensisitizers capable of transferring energy to rare earths and hence increasing their luminescent intensity, allowing the use of these elements in lower concentrations.8 On the other hand, the quantum luminescence efficiency also plays an important role. Namely, it is well known that through vibronic coupling the luminescence of rare earth ions such as Eu(III) is highly quenched.9 Finally, aggregation of rare earths in certain matrices can yield unexpected results, also resulting in luminescence quenching.10-13 The matrices in which rare earths can be dissolved are widespread, from glasses to ionic liquids, from LDH particles to sol-gel.10,11,14-16 Concerning this variety of hosts, the sol-gel * To whom correspondence should be addressed. E-mail cesar.laia@ dq.fct.unl.pt.

process has been seen as an alternative method for glass production with some advantages over the traditional method. Among these advantages are the lower temperatures used in the glass production, the high purity and homogeneity of the resulting materials, and the variety of compositions that can be achieved.17 Using this method, rare earth ions can be incorporated in silica gel to produce glasses. The luminescence properties of lanthanide ions make them useful as optical probes for the sol-gel process18-20 and in optical devices such solidstate lasers.18 In the case of optical devices, high concentrations of the rare earth ions as dopants are often necessary in order to achieve a good efficiency on the emission process due to their low light absorption. However, concentration quenching attributed to the dopant clustering, which can occur even at low amounts of the ion, limits the luminescence efficiency. The clustering has origin on the agglomeration of the rare earth ions through oxygen linkages, and quenching occurs via several mechanisms, including cross relaxation and an energy-transfer process.10-13,21 The clustering of lanthanide ions in silica glasses derived from sol-gel has been the subject of several studies in order to determine the influence of codopants. It was found that addition of aluminum as a codopant to a sol-gel system containing europium was effective on dispersing and isolating the europium ions, avoiding in this way the clustering, whereas Monteil et al.21 observed that the antiquenching effect was attributed to a modification in the europium local structure and not to the ion effective dispersion. As with aluminum, boron can be also introduced in the solgel system as a network modifier ion. When doped with rare

10.1021/jp106844t  2010 American Chemical Society Published on Web 10/11/2010

Formation of Eu(III) Nanoparticles on Borosilicate Sol-Gel

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earths, an advantage associated with borosilicate glasses is the lower sintering temperature. Boron is a small ion with a high ionic charge and thus can produce strong effects on the physical and chemical properties of the glass. The phase-separation phenomenon often takes place in the oxide glasses including the borosilicate glasses. During this process the composition as well as the structure of glass regions are known to change and thus may be responsible for changes in the local structure of the rare earth ions used as dopants. Ding et al.22 performed a study on sodium borosilicate glasses doped with europium produced according to the traditional method. They determined changes in the physical properties of glasses with increasing europium concentrations, which were associated to the predominance of europium ions on the boron-rich phase. The emission studies were not extensive and the authors advert for the need of further investigation on emission mechanisms. Our purpose in this study is to understand how Eu(III) is dispersed on sol-gel matrices containing boron and determine its photoluminescent properties, namely, how effects like clustering/aggregation affect photoluminescence in these hosts and what is the role of hydroxyl groups on the Eu(III) luminescence quenching, prior to synterization. TEM and EDS experiments allowed the direct detection of nanoparticles containing Eu(III) within the sol-gel matrix. With this knowledge, standard luminescence spectroscopy was used in order to probe local symmetry experienced by Eu(III) and, through the Judd-Ofelt theory,3,6,7,23 explore how the lanthanide interacts with the matrix. Time-resolved luminescence enabled direct measurements of excited-state dynamics and analysis of effects such as internal conversion and resonance energy transfer within the aggregates.

operating at 200 kV. Gel and glass samples for TEM were prepared by grinding the sample to powder, suspending in ethanol, and dropping and evaporating the suspension on a Formvar-coated copper grid. The images were obtained with a point-to-point resolution of 2.7 Å. Analysis by energy dispersive X-ray spectroscopy (EDS) was carried out in a semiquantitative way with a resolution of 138 eV. Infrared analyses were performed on a Nicolet Nexus spectrophotometer coupled to a Continuµm microscope (15× objective) with a MCT-A detector cooled by liquid nitrogen. The spectra were collected in transmission mode, in 50-100 µm areas resolution setting 4 cm-1 and 128 scans, using a Thermo diamond anvil compression cell. The CO2 absorption at ca. 2300-2400 cm-1 was removed from the acquired spectra. Spectroscopic Measurements. Luminescence spectra were measured using a SPEX Fluorolog-3 model FL3-22 spectrofluorimeter with 0.5 nm slits. The excitation wavelength was 393 nm, and experiments were performed at room temperature (22 °C). Lifetime measurements were run on a LKS.60 ns laser flash photolysis spectrometer from Applied Photophysics, with a Brilliant Q-Switch Nd:YAG laser from Quantel, using the second harmonic (λexc ) 532 nm, laser pulse half-width equal to 6 ns). Emission decays were obtained with a spectral resolution of 2 nm, with a perpendicular geometry in relation to the laser excitation, by averaging between 2 and 10 measurements at each emission wavelength depending on the emission intensity of the sample. An optical cutoff filter (570 nm) for the emitted light was used in order to avoid scattering light contamination. Luminescence decay traces at each wavelength were analyzed using least-squares fittings of the experimental data, using Solver from MS Excel.

2. Experimental Section

3. Results and Discussion

Reagents. The following reagents and solvents were used without further purification: europium chloride hexahydrate (Alfa Aesar, 99.9%), tetraethylorthosilicate (TEOS, Aldrich, 98%), trimethylborate (TMB, Merck, >99%), absolute ethanol (Aldrich, p.a.), concentrated hydrochloric acid (Aldrich, 37%), and ammonia (Merck, 25%). Gels and Glasses Samples Preparation. Transparent borosilicate gels and glasses doped with europium were produced through the sol-gel method. The gels were obtained by hydrolysis and further condensation of TEOS and TMB alkoxides using a two-step acid-base catalysis. The gels were prepared with molar ratios TEOS:H2O:EtOH:HCl of 1:4:6:0.003 and HCl:NH4OH of 1.5. The mass percent of TMB related to TEOS was 5% (w/w), and the Eu(III) content was varied from 1% to 20% (w/w) relative to the SiO2 content. The procedure was as follows (see Supporting Information): (i) EuCl3 · 6H2O was dissolved in the required amount of H2O containing 1/4 of the ethanol and the resulting solution added dropwise under stirring to the TEOS previously diluted with another 1/4 of the ethanol; (ii) HCl was then added and the reaction mixture stirred for 1 h at 60 °C in sealed containers; (iii) TMB dissolved with the second half of ethanol was then added, and after stirring for 15 min at 60 °C, NH4OH was added. The resulting homogeneous sol was left to gel with no further stirring, keeping T ) 60 °C; the gelling times were 2-3 days. After gelation, the samples were aged for 1 week at room temperature and then heat treated in air at three different temperatures, 400, 600, and 800 °C with an increment of 100 °C/h during the heating process. Sample Characterization. Transmission electron microscopy (TEM) images were obtained with a Hitachi H-8100 microscope

TEM, EDS, and FTIR Measurements. In order to check out heterogeneities at the micrometer scale, the sol-gel materials doped with EuCl3 · 6H2O were investigated with TEM. The images show relatively polydisperse particles with diameters ranging from 30 to 100 nm (see Figure 1A and 1B). The temperature treatment leads to formation of glass pieces, which do not show the same heterogeneities at this resolution (see Supporting Information); therefore, heat treatment disrupts the large particles existing in the sol-gel matrix. In order to check out the composition of the particles, EDS measurements were performed (see Figure 1C) showing peaks consistent with the presence of europium and chlorine, a small oxygen content, and no silicon, whereas in areas without particles only oxygen and silicon are detected. These results suggest that europium cation and chloride anions are segregated within the sol-gel matrix as nanoparticle aggregates which are disrupted after heat treatment, which leads to glass formation. Mixed nanoparticles containing oxygen and ligands such as sulfide and chloride were found using other synthetic methods.24,25 Since EDS measurements are less sensitive to oxygen than other heavier elements, mixed nanoparticles with a general formula EuOx(OH)yClz could explain the observed results, but the presence of chloride outside the nanoparticles area is not observed. Therefore, chloride is probably the predominant counterion in the nanoparticles. While TEM/EDS measurements reveal details about the nanoparticles, FTIR spectra allow mainly detection of water and/ or hydroxyl groups on the samples as a function of the heat treatment. Several bands typical of Si-O and B-O vibrational modes are detected (see Figure 2), which are relatively independent of the heat treatment. Water peaks are clearly found around 3300and 1630 cm-1, with intensities decreasing progres-

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Figure 1. TEM images of the sol-gel containing Eu(III) 10% (w/w) where nanoparticles with diameters of 94, 44, 34, and 30 (A) and 74 nm (B) can be identified. (C) EDS analysis of the particle in B (bottom) and in area without nanoparticles present (up).

Figure 2. FTIR measurements of samples with 5% Eu(III) before (sol-gel) and after heat treatment. B-O stretching mode is found at 1350 cm-1, Si-O at 1060 cm-1, and Si-OH at 930 cm-1. The band at 773 cm-1 is probably due to a B-O-B bending mode.26

sively with heat treatment. Also revelant is the band at 930 cm-1, characteristic of a Si-OH stretching mode, which disappears after treatment at 800 °C. The heat treatment has therefore a strong impact in the water/hydroxyl content of the samples, which is removed after heating at 800 °C (see Figure 2). Steady-State Luminescence Spectroscopy. The impact of the disruption of the nanoparticles on the Eu(III) luminescence spectra is shown on Figure 3. The luminescence spectra are characterized by several peaks corresponding to transitions from the Eu(III) 5D0 state to 7Fn states (n ) 0, 1, ..., 0.6). 5D0 and 7F0 are nondegenerate states, i.e., the presence of a crystal field does not lead to a splitting of the energy levels (the so-called Stark levels). However, depending on the crystal field point group symmetry, the remaining 7Fn states (n ) 1, ..., 0.6) can lead to

Figure 3. Emission spectra (λexc ) 392 nm) of sol-gel materials doped with EuCl3 (samples with different Eu(III) % (w/w) concentrations: green, 1%; red, 5%; blue, 10%; black, 20%) prior to heat treatment (sol-gel) and after treatment at T ) 400, 600, and 800 °C.

the presence of several Stark levels.4,5,27,28 The most important case is the 5D0 f 7F1 transition. This transition is forbidden in terms of the electric dipole (ED) moment but is allowed in terms of the magnetic dipole (MD) moment and therefore appears clearly in the Eu(III) luminescence spectra. The intensity of this band is almost insensitive to the environment, being affected by the refraction index solely.3,29 This allows the use of this transition as an internal calibration standard for other emission bands and enables comparison with experimental data published previously. This transition also gives important information about the crystal field point group symmetry, depending on the number of peaks present on its emission band. Since the 5D0 state is always nondegenerate, the number of peaks gives direct

Formation of Eu(III) Nanoparticles on Borosilicate Sol-Gel information about the number of Stark levels of the 7F1 state and therefore the possible crystal field point group symmetry.4,27 Another important transition is 5D0 f 7F0. Both states are always nondegenerate; therefore, only a single peak is expected. However this transition is forbidden under both mechanisms discussed above. The perturbation caused by the crystal field leads to J-J mixing with the electric dipole allowed 5D0 f 7F2 transition; therefore, the 5D0 f 7F0 transition appears in the luminescence spectra with relatively low intensity as a single peak.28,30 The same mechanism is also responsible for 5D0 f 7 F3 and 5D0 f 7F5 transitions, although these are always weak. 5 D0 f 7F2, 5D0 f 7F4, and 5D0 f 7F6 transitions are all allowed through the ED mechanism and highly sensitive to the environment, especially the 5D0 f 7F2 emission peak, sometimes called hypersensitive transition.5 On the sol-gel materials, before heat treatment, the 5D0 f 7 F1 transition shows a single peak within the present wavelength resolution for the compositions explored, although a small shoulder to the blue is also detected. As seen above, the origin of this shoulder can be connected with the 5D1 f 7F3 transition: since the excitation is at 393 nm, 5D1 states are also generated. Nevertheless, the steady-state luminescence intensity of other bands originated from 5D1 states is rather low. The 5D0 f 7F0 peak appears with relatively low intensity, indicating a Cs, Cn, or CnV symmetry group. These results are compatible with hexagonal, pentagonal, or tetragonal point group symmetry (C6V, C6, C5V, C5, C4V, C4, C3V, or C3). This implies that two peaks are overlapped in the 5D0 f 7F1 band (therefore two 7F1 Stark levels), which is hinted from the shoulder visible on those bands. After heat treatment, however, this band splits into three peaks, indicating a lower point group symmetry of the type CS, C2, or C2V, taking into account the presence of the 5D0 f 7F0 peak. A more evident result is the increase of the 5D0 f 7F2 band intensity when compared with the 5D0 f 7F1 one upon heat treatment. While in sol-gel the two bands have almost the same intensity, the 5D0 f 7F2 band intensity increases about 4-fold in glassy materials. The ratio of intensities can be analyzed according to the Judd-Ofelt theory.3,6,7,23 This theory is based in the model where the intensity of the forbidden f-f ED transitions appears from the admixture into the 4f N configuration of configurations of opposite parity. The crystal field potential generated by point charges located at atom positions of the ligands appears in the theory as the perturbation for mixing states of different parity into the 4f N configuration. In the Judd-Ofelt theory, it is also considered that the Stark levels at the ground state are equally populated, an assumption reasonable for experiments at room temperature, and with an optically isotropic host matrix. Thus, only ED transitions between different manifolds are considered, which simplifies the problem. Three phenomenological intensity parameters appear in this theory (Ωλ with λ ) 2, 4, and 6), which are related with the dipole strength D of the electronic transition through

D)

〈 | | 〉

(n2 + 2)2 2 1 Ωλ fNΨJ U(λ) fNΨ′J′ e (2J + 1) 9n λ)2,4,6

∑

J. Phys. Chem. C, Vol. 114, No. 43, 2010 18417 in which Ωλ can be calculated. On the emission spectra, one can analyze results through the probability of an ED transition in lanthanides (AJ) given by

AJED )

∑

(1)

〈fNΨJ|U(λ)|fNΨ′J′〉 are reduced matrix elements and were previously calculated by Carnall et al.,31 e is the elementary charge, and n is the medium refraction index which should be taken into account since the lanthanide ion is a dielectric medium and, therefore, affected by polarizability interactions with the host matrix. This is the framework to analyze Eu(III) spectra,

2

(2)

Now AJ can be calculated through eq 2, provided n and Ωλ are known. In the case where these values are unavailable, one can analyze individually each emission band. For Eu(III), it is known that the 5D0 f 7F1 transition depends weakly on the environment and has a negligible ED mechanism; therefore, it is fairly constant and its AJ)1 can be taken from the literature assuming some reasonable value for the refraction index.29 The dependence of a magnetic dipole transition with the refraction index is

AMD ) J

64π4e2ν¯ 3 3 n DMD 3h(2J + 1)

(3)

with DMDJ)1 ) 9.4 × 10-6 Debye2. One can therefore define the following relation

γJ )

AED J

(4)

MD AJ)1

which is the intensity ratio between each transition and the D0 f 7F1 band, a value that can be obtained from experimental data. Assuming DMDJ)1 taken from the literature, the remaining AJ are obtained in a straightforward way and Ωλ might be calculated (considering the MD mechanism negligible for the other transitions) 5

Ωλ )

( )〈 | | 〉

AJ)λ 9n2 ν¯ λ 2 2 AJ)1 (n + 2) ν¯ j)1

MD DJ)1

3

N

(λ)

f ΨJ U

(5)

N

f Ψ′J′

It is important to point out that AJ)λ/AJ)1 is an experimental value that can be calculated by the integral intensity ratio of each transition over the MD transition. Therefore, the only assumption is concerned with the refraction index. Another outcome of this procedure is that the radiative lifetime of Eu(III) can be calculated with the following equation

τr ) 2

〈 | | 〉

64π4e2ν¯ 3 n(n2 + 2)2 Ωλ fNΨJ U(λ) fNΨ′J′ 3h(2J + 1) 9 λ)2,4,6

1 6

(6)

∑ AJ

J)0

using the available experimental data. This is the expected luminescence lifetime if the luminescence quantum yield was equal to unity, so lower luminescence lifetimes indicate luminescence quenching of Eu(III). From the emission spectra on Figure 2, these values were calculated and are listed in Table 1, assuming a refraction index equal to 1.5 (a common value for glassy materials). Estimated values of Ω2 and Ω4 are also given. The Ω2 parameter gives insight into the covalent character

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TABLE 1: Data Analysis of the Emission Spectra Shown in Figure 2 According to Eqs 1-7 R

AJ)0/ms-1 a

AJ)2/ms-1 a

1 5 10 20

1.6 0.9 1.0 1.3

1.6 1.0 1.2 1.2

98.6 64.3 61.9 63.5

1 5 10 20

2.4 3.0 3.6 3.3

5.0 4.0 3.6 3.5

1 5 10 20

3.5 3.8 3.8 3.7

1 5 10 20

4.6 4.2 3.8 3.7

Material Eu(III) % (w/w)

AJ)3/ms-1 a

AJ)4/ms-1 a

τr/msa

Ω2a,b/10-20 cm2

Ω4a,b/10-20 cm2

sol-gel 6.1 5.2 4.5 3.3

52.2 51.3 53.6 51.4

3.9 4.5 4.5 4.6

3.0 1.9 1.9 1.9

3.3 3.2 3.4 3.3

129.8 158.9 173.6 170.3

400°C 12.5 8.6 9.2 8.5

55.0 58.5 72.0 70.0

3.3 3.0 2.6 2.7

3.9 4.8 5.2 5.1

3.5 3.7 4.6 4.4

5.7 5.8 5.8 5.8

173.8 179.7 179.6 178.1

600°C 15.5 12.8 12.7 12.5

48.1 48.2 49.1 50.2

2.9 2.9 2.9 2.9

5.3 5.4 5.4 5.4

3.0 3.1 3.1 3.2

8.5 8.5 6.6 6.3

204.6 190.5 173.3 168.8

800°C 15.8 13.9 12.9 12.9

47.9 46.7 50.9 54.0

2.7 2.8 2.9 2.9

6.2 5.8 5.2 5.1

3.0 3.0 3.2 3.4

Calculation performed assuming n ≈ 1.5. b The AJ)1 value was calculated from DMDJ)1 ) 9.4 × 10-6 Debye2 (taken from ref 29) using eq 3 and used to calculate all other parameters. a

Figure 4. Luminescence decays of (A) sol-gel samples with increasing EuCl3 concentration (Eu(III) ) 1%, 5%, 10%, and 20% (w/w), according to the arrow) and (B) samples with Eu(III) 10% (w/w) concentration before and after heat treatment (no heat treatment, 400, 600, and 800 °C according to the arrow). The decays are the sum of all traces in a wavelength range from 580 to 640 nm (5D0 f 7F1 and 5D0 f 7F2 transitions), with λexc ) 532 nm.

of the Eu(III)-O bond, being higher when the convalency character increases.32 The ratio

R ) IJ)2 /IJ)1

(7)

is also given in Table 1. This ratio is the maximum intensity of the 5D0 f 7F2 band divided by the maximum intensity of the 5 D0 f 7F1 band, which is an experimental value reported in many publications; therefore, it is also given for comparison purposes. The trends show that the covalency character increases with heat treatment, which means that Eu(III) is more tightly bound to the matrix. The Ω4 parameter significance is not so well established, but it was proposed to depend on the overall “viscosity” of the matrix, i.e., it is a measure of long-range interactions between Eu(III) and the host.32 Nevertheless, this parameter does not change appreciably within the experimental error. The Ω6 parameter could not be calculated since its emission (the 5D0 f 7F6 transition which appears at ca. 800 nm) is overlapped with the more intense harmonic scattering of the experimental setup (λexc ) 393 nm). At other excitation

wavelengths, the emission was too weak to give reliable results; thus, this band was not explored. However, it is known that a correlation between Ω4 and Ω6 does exist, with Ω4 ≈ Ω6.33 The correlation is quite scattered, but within the objective of this work it is used. The bigger source of error is the refraction index, which might explain some variations occurring, e.g., in samples after heat treatment of 800 °C, and can lead to error bars of about 10% but only if n values spread to large differences between them (e.g., from 1.5 to 2). It is interesting to see that the intensity of the 5D0 f 7F0 transition increases in the glasses. This shows that after heat treatment, Eu(III) is hosted in a crystal field with lower symmetry, enhancing the J-J mixing mechanism. Luminescence data show that Eu(III) in the nanoparticles in the sol-gel interacts less strongly with the crystal field matrix than in glass where those particles are absent. An explanation for this effect is, besides the disruption of the nanoparticles, the interaction of Eu(III) with oxygen, substituting the chloride ion. This would lead to the observed increase of Ω2 values and to a less symmetric crystal field. The radiative time constants are on the order of 3-4 ms, a common value for Eu(III).

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TABLE 2: Analysis of the Decays in Figure 3 with Double-Exponential Kinetics A1

τ1/µs

A2

τ2/µs

0.78 0.86 0.81 1.00

279 148 148 128

0.22 0.14 0.19

91 20 59

glass samples Eu(III) 10% (w/w) 400 °C 0.85 600 °C 1.00 800 °C 1.00

202 620 2114

0.15

34

material Eu(III) % (w/w) 1 5 10 20

However, no obvious effect due to the presence of the hydroxylic group on the sol-gel is seen from the present results. Also, the possibility of energy transfer between Eu(III) neighbors in the matrix (or the particles) cannot be solved solely with the present results. In order to understand such effects, time-resolved luminescence spectroscopy measurements were performed. Time-Resolved Luminescence Spectroscopy. Figure 4A shows a time-resolved luminescence decay integrated for all measured wavelengths (580 to 640 nm) of sol-gel materials doped with increasing quantities of EuCl3.6H2O. The luminescence lifetime decreases when the Eu(III) concentration is increased, with values much smaller than intrinsic radiative lifetimes shown in Table 1. The decays in glasses after heat treatment are shown on Figure 4B. In this case, there is an increase in the luminescence lifetimes with increasing temperature used in the heat treatment, approaching the values of the intrinsic radiative lifetimes calculated previously. These decays could be analyzed with single and double exponential functions (see Table 2). What mechanism is causing the luminescence quenching in the sol-gel material? There are two contributions that could be listed: (a) the presence of hydroxyl groups in the matrix and (b) the aggregation of Eu(III) in large particles, shown in Figure 1. Figure 5A shows the time-resolved luminescence spectra of a sol-gel sample between 580 and 640 nm, and Figure 5B shows the same results, normalized with the area of the 5 D0 f 7F1 band. There is a change of the emission spectra with time in the microsecond time scale, indicating excitedstate dynamics in these samples. Figure 6 shows emission decays with either fast decaying processes or luminescence rise times depending on the wavelength explored. Such effects are indicative of conversion from an excited state to another species. Since excitation is done at 532 nm, the transition

Figure 6. Luminescence decays at single wavelengths for a sol-gel sample with Eu(III) 5% (w/w).

electronic excitation is due to 7F1 f 5D1. Thus, this effect is attributed to a 5D1 f 5D0 internal conversion occurring in about 4 µs (see below, Table 3, τ1). Similar results are also observed, e.g., in soda-lime glasses.34 An important conclusion from this result is that the 5D1 state is weakly quenched when compared with other materials; otherwise, it would lead to faster decays. A second process is observed with a decay time of about 20 µs (Table 3, τ2), which is also present in the integrated decays (although with slightly higher values, see Table 2, τ2). This component is not present in all samples. In fact, sol-gel samples with “diluted” Eu(III) concentration do not display this process. Figure 7 shows the pre-exponential factors obtained as a function of the wavelength for sol-gel samples, after global analysis of the luminescence decays with fixed decay times shown in Table 3. For the shortest component, positive pre-exponential factors appear where 5D1 f 7F3 and 5D1 f 7F4 were expected to appear,5 while negative pre-exponential factors are in accordance with 5D0 f 7F1 and 5D0 f 7F2 transitions. The 20 µs component does not display rise times (negative pre-exponential factors), and within experimental noise, it has the appearance of a Eu(III) luminescence spectra. Finally, the long decay has a spectrum similar to those obtained in steady-state experiments. Figure 8 shows how the intensity ratio between 5D0 f 7F2 and 5 D0 f 7F1 bands changes with time (which can be related with AJ)2/AJ)1). For short times, a rise is observed which is due to the Eu(III) internal conversion from the 5D1 to the 5D0 state. Afterward, for samples with intermediate Eu(III) content, the 20 µs component

Figure 5. Time-resolved luminescence spectra of a sol-gel material doped with Eu(III) 5% (w/w): (A) non-normalized data; (B) data normalized with the area of the emission peak between 580 and 600 nm. Same experimental setup conditions of Figure 3.

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TABLE 3: Analysis of Kinetic Traces of Luminescence Decays (triexponential functions with global analysis for the 580-640 nm wavelength range) and Time-Resolved Intensity Ratios (fits with eq 11) luminescence decays τ1/µs

material 1 5 10 20

2.4 4.0 4.2 4.5

400 °C 600 °C 800 °C

2.2

time-resolved intensity ratios

τ2/µs

τ3/µs

22.8 22.6 22.5

228 149 141 131

τ1/µs Eu(III) % (w/w) 3.5 6.1 5.0

τ2/µs

R1/∆1

R2/∆2

R3/∆3

17.8 17.3

-7.6 -0.7 -0.7

2.5 2.1

2.1 1.2 1.1 1.0

glass samples Eu(III) 10% (w/w) 204 4.4 640 2090

appears, meaning it indeed leads to a change of the lanthanide emission spectra. Finally, as observed already by steady-state emission spectroscopy (see Table 1), the ratio decreases with increasing Eu(III) concentration. The decays on Figures 6 and 8 have different physical descriptions. For the luminescence decays in Figure 6, the experimental results were analyzed with the well-known sum of exponentials model. For the case of Figure 8, the analysis is slightly more complex. In this case one has to consider a generic model for the luminescence decay

∑

Φ(t) ) ∆(t)

(8)

ai(λem)fi(t)

2.7 3.1 3.4

as stretched exponential, distribution of luminescence lifetimes, or kinetic transients, as long the pre-exponential factors are strictly the same (i.e., the shape of the time-resolved spectra is constant). For the simple case of a monoexponential kinetics it is easily derived that the ratio should be constant with time. Now, Figure 8 shows features that hint for at least three preexponential factors. In such case, the following equation is obtained

n

I(λem, t) )

-0.2

Since the data is normalized with the 5D0 f 7F1 area at each time, eq 8 should be normalized with such area

R3 + ∆3

2

R

(( ((

∑ ∆3i exp i)1 2

)) ))

1 1 t τi τ3

∆i 1 1 1+ exp t ∆ τ τ 3 i 3 i)1

∑

(11)

with n

∆(t) )

∑ ( ∫λ

λ2 1

n

ai(λ)dλ)fi(t) )

∑ ∆ifi(t)

(9)

Ri )

∫λλ ai(λ) dλ 4

3

(12)

The result of the division is

(∑ ) (∑ ) n-1

Φ(t) ) ∆(t)

an ai fi(t) + ∆n fn(t) ∆n

i)1 n-1 i)1

∆i fi(t) +1 ∆n fn(t)

in order to deal with integrated intensity ratios displayed in Figure 8, instead of pre-exponential factors. Two special limits are particularly interesting. When t f 0, one obtains

(10)

This is the most generic result of this normalization in order to show that the time function fi(t) can assume the usual exponential form. However, the equation also holds for other forms such

3

∑ Ri

I(λem, t ) 0) i)1 ) ∆(t ) 0) ∆(t ) 0)

(13)

While for tf∞, one obtains

Figure 7. Pre-exponential factors of time-resolved decays of sol-gel samples with different Eu(III) concentrations (Eu(III)) 1%, 5%, 10%, and 20% (w/w)) of the shortest component (left, decay time constant in Table 3) of the 5D1 f 5D0 component; short component of the 5D0 f 7Fn transition (center, not seen for Eu(III) ) 1% (w/w) sample), and long component (right) of 5D0 f 7Fn transition.

Formation of Eu(III) Nanoparticles on Borosilicate Sol-Gel

I(λem, t ) ∞) R3 ) ∆(t ) ∞) ∆3

(14)

which is a value that can be compared with steady-state data in Table 1. Figure 8 shows the experimental time-resolved data fitted with eq 11. The obtained luminescence decay times and time-resolved intensity ratios are presented in Table 3. The features that give rise to the component with a lifetime of ca. 2-4 µs are due to internal conversion from the 5D1 to the 5D0 state as previously discussed. The 20 µs now clearly appears with an intensity ratio of about 2.3 (average value), corresponding to a Ω2 of ca. 3.9 × 10-20 cm2. This value hints that Eu(III) is bound to oxygen instead of chloride, which could be attributed to Eu(III) species present in the surface (or interface) of the nanoparticles. We should point out that the spectra of this species (see Figure 7, middle) are different from the species with longer luminescence lifetimes (Figure 7, right). This is an indication that the nonexponential decay at these time scales are not due to transient effects on the relaxation of a single species but rather to distinct species. Nature of the Nonradiative Processes. Luminescence decay times are 1 or 2 orders of magnitude shorter than radiative lifetimes. This shows that nonradiative processes are the dominant fate of excited-state Eu(III) species. Such processes do not appear to take place from the 5D1 state since it displays a behavior similar to those observed on other systems.34 Also, heat treatment disrupts such processes, which is accompanied by the disruption of Eu(III) nanoparticles (as TEM/EDS experiments show on Figure 1, Eu(III) is preferentially bonded to chloride in the nanoparticles, while it is bound to oxygen when they are disrupted). Sinterization leads to materials in which the 5D0 lifetime approaches the intrinsic radiative process. Therefore, whatever the process that leads to luminescence quenching in sol-gel, it disappears in glasses. Two explanations could be put forward for this effect: (i) the presence of hydroxyl groups in the sol-gel that eventually would disappear upon sinterization and (ii) energy transfer within the nanoparticles that eventually would dissipate excited-state energy to the matrix through electron-phonon coupling.35 Hydroxyl groups, wellknown excited-state quenchers for Eu(III), are good candidates, but EDS measurements showed little oxygen content within the nanoparticles (Figure 1C). It seems that nanoparticles “protect” Eu(III) from the sol-gel environment; therefore, its involvement would be only effective on the particle surface, where indeed it

J. Phys. Chem. C, Vol. 114, No. 43, 2010 18421 seems to display a shorter lifetime. On the other hand, electron-phonon coupling is a known mechanism for excitedstate energy dissipation for europium particles in a number of hosts and also in pure Eu2O3.35 The results here presented are also strikingly similar to those published for MCM-41 hosts36 but with some differences: (i) the previous work in MCM-41 hosts was with Eu2O3 particles instead of EuCl3 particles; (ii) nanosecond excited-state dynamics appears in MCM-41 hosts, while our present results do not show any sign of such very fast processes. The rest, including results from heat treatment, is similar, which hints at a common physical phenomenon. The observation of an electron-phonon side band from the 5D2 f 7F1 excitation spectra could shed some light in this respect,35 but emission intensity was too low to be measured with our experimental setup. Energy transfer within the aggregate could also be largely responsible for the 20 µs decay, here attributed to surface Eu(III), either to hydroxyl groups of the matrix or to species inside the nanoparticles. Such a large rate constant, however, seems incompatible to resonance energy transfer within the nanoparticles, so again, an electron-phonon mechanism could take place, as seen previously. Final Comments The doping of sol-gel materials with rare earths probably lead to formation of lanthanide nanoparticles in most systems. Addition of boron in this work did not lead to a different picture, and formation of Eu(III) nanoparticles is observed, with a large chloride anion content. However heat treatment with temperatures around 800 °C disrupt these objects, which increases its luminescence efficiency as well as the intensity of the 5D0 f 7 F2 emission peak, responsible for the red color of Eu(III) luminescence. The luminescence kinetics is rather complex due to relaxation of higher Eu(III) excited states to the 5D0 state and also due to the different interactions between Eu(III) and chloride and oxygen anions. Electron-phonon mechanisms as well as energy transfer between Eu(III) in nanoparticles can both contribute to nonradiative processes within nanoparticles. Acknowledgment. FCT-MCTES (Portugal) is acknowledged for financial support through project PTDC/EAT/67354/2006 and a postdoc grant SFRH/BPD/40008/2007 (M.V.). Ms. Catarina Miguel is acknowledged for assistance in FTIR measurements. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 8. Intensity ratio between 5D0 f 7F2 and 5D0 f 7F1 for sol-gel samples. Fits with eq 11.

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