DOI: 10.1021/cg100100a
Synthesis and Characterization of Luminescent Nanoclays
2010, Vol. 10 2847–2850
Tamara Posati, Francesca Bellezza, Antonio Cipiciani, Ferdinando Costantino, Morena Nocchetti, Luigi Tarpani, and Loredana Latterini* Dipartimento di Chimica and Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Universit a di Perugia Via Elce di Sotto, 8, 06123 Perugia, Italy Received January 22, 2010; Revised Manuscript Received May 10, 2010
ABSTRACT: Hydrotalcite nanocrystals doped with Eu(III) ions were prepared by the microemulsion method. XRPD measurements revealed that europium ions are very likely incorporated in a low-symmetry hydrotalcite-type layer. TEM and AFM imaging indicated that nanocrystals have a 150 nm width and a plate shape. Luminescence measurements showed that the doped powder is luminescent and the Eu(III) ions are located in a low symmetry site. Introduction Layered solids have interesting physical and chemical properties, because of their structural anisotropy and because they can be easily functionalized by intercalation of atoms and molecules having specific properties.1,2 Among the layered solids, hydrotalcite-like compounds (HTlc), also known as layered double hydroxides (LDHs) or anionic clays, occupy a unique place because they are practically the only example of layered solids with positively charged layers and intercalated exchangeable interlayer anions to maintain charge neutrality. So far, layered double hydroxide matrices could gain designed optical properties by intercalation in their interlayer region of guest species possessing chromophoric groups, producing hybrid nanostructured materials.3 Actually, upon controlling the arrangement of the dye species in the lamellae, a full control of the optical and photochemical behavior of the intercalation compound can be achieved.4,5 The properties of the hybrid materials generally differ from those of the pure guest species, since the host-guest interactions that affect the distribution and orientation of guests in the host, and the guest-guest interactions6 allow modulation of the photophysical and photochemical properties of the guest, whose thermal and photochemical stability can also be improved.5,7,8 This approach to achieve inorganic matrices responsive to UV-vis irradiation can represent a limit for those applications in which the intercalation interaction is weakened (such as in electrolytic solutions) or when sharp and sensitive spectra are necessary (aslike in sensing or labeling materials). Furthermore, the intercalation of organic dyes in the interlayer region of HTlc to achieve detectable optical properties through ion exchange processes limits the availability of ionic sites for the further functionalization of the inorganic matrix. Particularly interesting, from this point of view, is the preparation of layered double hydroxides doped with luminescent metal ions such as rare earth metals. Rare earth metals are characterized by f-f transitions which are parity forbidden. As a result, rare earth luminescence arising from intershell transitions is sharp, photostable, and longlived.9 Eu(III) is luminescent in aqueous media and retains its luminescence when bound to complex systems.10 Furthermore, Eu(III) exhibits multiple emissions whose relative intensity and line splitting are sensitive to the environment of the metal.11 Despite the high interest in doping inorganic matrices with Eu(III), only a few examples can be found for the incorporation of the rare earth ions into the hydrotalcite lattice.12 Recently it has been reported that the size distribution of HTlc crystals can be controlled and reduced at nanometer level through
the double microemulsion method,13 and the obtained materials were then used for protein adsorption.14 In the present work, the constrained environment of the microemulsion is used to prepare HTlc nanocrystals with Eu(III) ions incorporated in the lamellae structures (EuHTlc). The material has been characterized by XRPD measurements, TEM and AFM imaging, while its luminescence properties were investigated by steady-state and time-resolved techniques.
Experimental Part
*Corresponding author. Telephone: þ39-75-5855636. Fax: þ39-75-5855598. E-mail:
[email protected].
Chemicals. Cetyltrimethylammonium bromide (CTABr) and Eu(NO3)3 3 5H2O were supplied by Aldrich. All other reagents were C. Erba RP-ACS products. Synthesis of EuHTlc Nanoparticles in Microemulsion. Cetyltrimethylammonium bromide as surfactant, n-butanol as cosurfactant, and isooctane as the oil phase were used to prepare the microemulsions. Two microemulsions, designated A and B, with identical composition (in terms of CTABr, n-butanol, isooctane, and water), were prepared by dissolving different reagents in their aqueous phase. The aqueous phase of A was a solution of Zn(NO3)2 3 6H2O (0.4 M), Al(NO3)3 3 9H2O (0.115 M), and Eu(NO3)3 3 5H2O (0.0104 M), while the aqueous phase of B was a 1.25 M NH3 solution. Microemulsions A and B were prepared by dispersing 6.25 g of CTABr and 7.75 mL of n-butanol in 18 mL of isooctane and then adding to each of these mixtures 6.75 mL of aqueous phase. Both the systems became clearly transparent after 15 min of mixing. The double-microemulsion processing route was then carried out by mixing equal volumes of the two initial microemulsions A and B to obtain the precipitation of ZnAlEu hydrotalcite in the reverse micelles. The resulting system was stirred at room temperature for 15 min. After this time, it became cloudy and was aged at 75 °C for 15 h. After aging, the particles were recovered by centrifuging (12000 rpm for 10 min), and a semitransparent gel was obtained. The gel was washed with isooctane (1 30 mL), with water (2 30 mL), and with a methanol-chloroform mixture (1:1) (3 30 mL) and then dispersed in water or dried at 60 °C under oil pump vacuum to give a fine powder. Chemical Analysis. Metal analyses were performed with Varian 700-ES series inductively coupled plasma-optical emission spectrometers (ICP-OES) using solutions prepared by dissolving the samples in concentrated HNO3 and properly diluted. Characterization of EuHTlc. The morphology of the sample was investigated with a Philips 208 transmission electron microscope (TEM). A small drop of the dispersion was deposited on a copper grid precoated with a Formvar film and then evaporated in air at room temperature. Atomic force microscopy (Solver-Pro, NT-MDT) was also used to provide information on particle size and shape. The measurements were carried out under semicontact conditions by using a 150-190 kHz cantilever having a 1-3 nm radius. The aqueous
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dispersions of the hydrotalcites were sonicated for 30 min and then deposited on mica support by spin coating (4000 rpm, 20 s). The samples were also characterized by chemical analysis, X-ray powder diffraction, and thermal analysis. The X-ray powder diffraction (XRPD) patterns were taken with a Philips X0 PERT PRO MPD diffractometer operating at 40 kV and 40 mA, with a step size 0.0170 2θ degree, and step scan 20 s, using Cu KR radiation and an X0 Celerator detector. Coupled thermogravimetric (TGA) and differential thermal (DTA) analyses were performed with a Netzsch STA 449C apparatus, in air flow and with a heating rate of 10 °C/min. A fluorimeter (Spex Fluorolog) equipped with a phosphorimeter (1934D) was used to record corrected luminescence spectra and luminescence decays of the samples using the right angle or front face configuration between the excitation and the emission light for the solutions and powder samples, respectively.
Results and Discussion The metal atomic percentage of the sample compared with those of the solution (reported in the parentheses) was the following: Zn, 73.1% (76.1%); Al, 26.0% (21.9%); Eu, 0.9% (2%). The solid contained a relatively higher amount of aluminum compared to the solution, very likely due to the lower aluminum hydroxide solubility. TGA and DTA measurements on the sample were carried out up to 1200 °C. Two stages of endothermic weight loss are evident; the first step, ranging from 80 to 350 °C, is due to the dehydration of the sample, the decomposition of cointercalated carbonate anions, and the dehydroxylation of the brucite layers. The second step is related to the loss of bromide anions. The exothermic peaks related to the combustion of the organic moieties are absent, which indicates the absence of adsorbed surfactant species. Given the metal atomic percentage and the fact that the metal oxides are formed at 1000 °C, it was possible to assign the following composition to the sample: [Zn0.731Al0.260Eu0.009(OH)2]Br0.156(CO3)0.0565 3 0.6H2O (metal weight percentage: Zn, 85.07%; Al, 12.50%; Eu 2.43%), which is slightly different compared to the sample prepared with the same method ([Zn0.72Al0.28(OH)2]Br0.28 3 0.69H2O)13 but without Eu(III). The X-ray diffraction patterns of EuHTlc are reported in Figure 1 (pattern b) and compared with the pattern recorded for the undoped HTlc prepared under the same experimental conditions (pattern a). First of all, it has to be noted that the doping procedure did not reduce the crystallinity of the sample, as observed for other systems.15 Furthermore, the lack of observing changes in the position of the interlayer distance reflections indicated that the doped material did not consist of an intercalation compound with Eu ions arranged in the interlayer region. The pattern of the EuHTlc (b) is similar to that of a typical Zn-Al hydrotalcite structure even if some new weak reflections can be observed, especially in the 35-60° 2θ region. These peaks are not ascribable to the classical hydrotalcite rhombohedral 2H type structure and do not correspond to the Eu(OH)3. The formation of similar weak reflections in the 2θ region has also been observed for the incorporation of little cations (like Li) in the HTlc layer.16 The authors have explained this feature with the formation of a low symmetry hydrotalcite with monoclinic structure. It is possible that also in the present case the Eu ions tend to reduce the symmetry and the unindexed peaks can be attributed to an ordering of the Eu ions into the sheet. For that reason, following the same strategy used in the LiAlHTlc,16 we have found the monoclinic C-centered unit cell derived from the hexagonal one with parameters a = 3.0794(5), b = 5.419(1), c = 15.518(5) A˚, and β = 95.16(5)°. The a parameter corresponds to that of the brucitic sheet, whereas the c axis is referred to a stacking of two layers. This unit cell is very similar to the HTlc Manasseite polytype.17 The space group C2/m was assigned on the basis of symmetric correlation between the rhombohedral and the monoclinic systems. A whole profile fit of the diffraction pattern with
Figure 1. XRPD patterns of HTlc (a) and EuHTlc (b): / indicates the reflections that cannot be indexed with the hexagonal cell; the inset shows the fitting profile of the diffraction pattern based on the LeBail method.
the LeBail method has then been performed, and the result is shown as the inset in Figure 1. The positions of the calculated reflections are shown as vertical bars under the pattern. The agreement factors of the LeBail fit are Rwp = 0.075 and Rp = 0.055, respectively. In order to carry out a deeper investigation on the formation of this monoclinic structure obtained after the Eu doping, the nanocrystals of EuHTlc have been grown in a hydrothermal bomb at 120 °C for about 12 h. This treatment induced a recrystallization of the product (see the Supporting Information, Figure SI3): the monoclinic phase weak reflections disappeared, and the formation of ZnO was also observed. A quantitative phase analysis of this product was carried out by using the Rietveld method, giving as results the segregation of about 17% of ZnO and a refined hydrotalcite Zn-Al structure with the following parameters: a = 3.0732(8) A˚; c = 22.708(1) A˚. These values correspond to those of Zn-Al hydrotalcites in carbonate form.18 The segregation of ZnO, during the recrystallization, shows that the nanometric hydrotalcite undergoes a dissolution/reprecipitation process in which about two-thirds of the europium ions remain in solution as bromide salt, as confirmed by the elemental analysis in the solid.19 Very likely, the Eu cations end up in the liquid phase during the recrystallization process because the high symmetry hydrotalcite is preferred in the crystal growth and this structure is not compatible with the europium high ionic radius. The presence of the low-symmetry structure containing the Eu ions can be observed only in the nanocrystalline product, thus precluding, at this stage, any further structural investigation. The comparison of the spectra with literature data allowed to exclude the formation of europium oxides on the HTlc surface.12 In order to better understand the arrangement of Eu cation in the HTlc structure, morphological analysis of the nanocrystals and a luminescence study have been carried out. AFM and TEM imaging gave an insight on the morphology and the dimensions of nanosized Eu-hydrotalcite crystals. TEM images showed quasi-hexagonal crystals with very sharp edges (Figure 2). These data suggest that the presence of Eu cations during the nanocrystal preparation affects their morphology. Their dimension was statistically analyzed and can be reproduced by a Gaussian function centered at 150 nm. A careful observation of TEM images revealed that on the crystals the contrast distribution is not constant, but darker regions can be observed. It is likely that this contrast enhancement is due to the accumulation of heavy atoms. This hypothesis is supported by the absence of any roughness on the crystal surface observed by AFM. The topography analysis indicated that the height profile of the nanocrystals is a few nanometers, which is well below the width measured by TEM imaging, indicating that the crystals have a
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Figure 2. TEM images (the scale bar for parts a and b is 500 and 100 nm, respectively); AFM image (c) and line scans (d) from the image in part c (solid line) of EuHTlc nanocrystals and from undoped HTlc nanocrystals (dotted line).
plate shape, different from what was previously observed for HTlc nanocrystals prepared without europium.13 The optical properties of europium were used to acquire information on the environment of the atom in the lamellar solid. The absorption spectrum of EuHTlc powder shows beyond the broad band due to the HTlc matrix (λ e 300 nm) a narrow peak at 393 nm (inset of Figure 3a) which can be assigned to the 7F0-5L6 transitions of Eu, thus suggesting that despite the small amount the lanthanide atoms are optically detectable. In particular, the luminescence properties of EuHTlc were investigated by steady-state and time-resolved techniques. The steady state luminescence spectrum of EuHTlc appeared broadened in comparison with the solution spectrum (Figure 3), which is in agreement with literature data; furthermore, the main peak is red-shifted when compared to the solution maxima. The band broadening can be due to a solid matrix effect, as previously observed with organic fluorophores,3 but the spectral shift can be ascribed to the arrangement of the emitters on the lamellae. In order to have more detailed information on the location of Eu atoms on the matrix, time-resolved luminescence measurements were carried out through a phosphorimeter. The room temperature photoluminescence (PL) excitation and emission spectra of the as-obtained EuHTlc were better resolved using a flash lamp as excitation source and monitoring the signal 5 μs after the flash. The PL emission spectrum upon excitation into the 7F0-5L6 transition of Eu(III) at 393 nm is shown in Figure 3. The emission peaks could be ascribed to the typical 5 D0-7FJ (J = 1, 2, 3, 4) transitions of Eu(III) ion. It is wellestablished that the Eu3þ emission is hypersensitive to the local chemical environment in terms of Judd-Ofelt theory.20 If Eu(III) occupies a site with an inversion center, the 5D0-7F1 magneticdipole transition is dominant, while in a site without an inversion center, the 5D0-7F2 electric-dipole transition is the strongest. In the present work, the dominating peaks are centered at 635 nm (corresponding to the 5D0-7F2 electric dipole transition) and they are much stronger than those located at 594 nm (correspond-
Figure 3. (a) Absorption and luminescence spectra of EuHTlc (black line, λexc= 393 nm) and luminescence spectrum of EuNO3 in H2O (red line, λexc= 393 nm). Inset: zoom in of the absorption spectrum of EuHTlc. (b) Luminescence excitation (recorded monitoring the luminescence at the main maxima, respectively) and emission spectra of EuHTlc (solid line, λexc= 393 nm; dashed line, λexc= 290 nm). Inset: luminescence decay traces (4, λexc = 393 nm and λem= 635 nm; 0, λexc= 290 nm and λem= 616 nm) in logarithmical scale together with the best fitting curves.
ing to the 5D0-7F1 magnetic dipole transition), suggesting that Eu is located in a low symmetry site of the clay.
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However, upon excitation at 290 nm, the luminescence redpeaks of Eu were observed at 614 nm; spectral shifts of the 5 D0-7F2 transitions were previously observed20-22 and were assigned to a J-mixing mechanism induced by the matrices. The spectral features appeared to be dependent on the excitation wavelength, since a shift of about 20 nm is observed on the 5 D0 - 7F2 peak, indicating a strong interaction (likely with electric nature) between the lamellae lattice and the rare earth ions. Time resolved luminescence measurements revealed a bimodal distribution of the emitting species (inset of Figure 3b) with decay times centered at 10 and 520 μs, independently from the excitation wavelength, within the experimental errors. This type of nonexponential decay is commonly observed for luminophores interacting with solid matrices,23 and the most correct and accurate method to analyze the data would be in terms of decay time distributions. Bimodal decay time distribution models were already proposed for organic fluorophore intercalated in HTlc layers;4 in the present systems, this behavior can be assigned to a distribution of decay times reflecting the Eu(III) ions localization on the clay particles or to the interactions that europium cations establish in different HTlc sites, although the occurrence of energy transfer processes cannot be excluded at this stage. In conclusion, the preparation procedure based on the double microemulsion method not only allowed control of the size distribution of the HTlc crystal but also made possible imposition of spatial constraints to the crystal growth, thus leading to the inclusion of cations with high ionic radius, such as europium. The inclusion of Eu(III) in the HTlc lamellae, although in low symmetry sites, allowed obtainment of nanoclays with sharp and easy detectable luminescence. The optical properties and the intercalation sites available in the interlayer region make these materials particularly interesting for applications in different fields, such as bioimaging, cellular delivery, or photonic materials. Acknowledgment. This work was supported by the Universit a di Perugia and the Ministero per l0 Universita e la Ricerca Scientifica e Tecnologica (Rome). Professor U. Costantino is acknowledged for helpful discussions and suggestions.
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Supporting Information Available: TG curves, AFM images and XRPD data. This material is available free of charge via the Internet at http://pubs.acs.org.
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