Synthesis and luminescence properties of water

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1 Photonic Materials Laboratory, Institute of Chemistry, UNESP, CP 355, Araraquara-SP,. 14801-970, Brazil ... result of the nanometric sizes of crystallites, there was a large surface free for ...... Encyclopedia of Chemical Technology vol 2 (New York: Wiley) p 291. [2] Misra C 1994 Aluminium oxide, hydrated in Kirk–Othmer.
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NANOTECHNOLOGY

Nanotechnology 18 (2007) 455605 (8pp)

doi:10.1088/0957-4484/18/45/455605

Synthesis and luminescence properties of water dispersible Eu3+-doped boehmite nanoparticles J M A Caiut1,2 , S J L Ribeiro1 , Y Messaddeq1 , J Dexpert-Ghys2 , M Verelst2 and H Dexpert2 1 Photonic Materials Laboratory, Institute of Chemistry, UNESP, CP 355, Araraquara-SP, 14801-970, Brazil 2 Centre d’Elaboration de Mat´eriaux et d’Etudes Structurales/CNRS, BP 4347, F-31055 Toulouse cedex 4, France

E-mail: [email protected]

Received 2 July 2007, in final form 27 August 2007 Published 10 October 2007 Online at stacks.iop.org/Nano/18/455605 Abstract Nanocrystallized boehmite γ -AlOOH·n H2 O had been synthesized by spray-drying (SD) of a solution of aluminium tri-sec-butoxide peptized by nitric acid. The sub-micronic spherical particles obtained had an average diameter of 500 nm and were built of 100 nm or less platelet-like sub-particles. The average crystallite size calculated from XRD was 1.6 nm following the b axis (i.e. one unit cell) and 3–4 nm perpendicular to b. As a result of the nanometric sizes of crystallites, there was a large surface free for water adsorption and it was found to be n = 1.18 ± 0.24H2O per AlOOH. The SD spheres spontaneously dispersed in water at room temperature and formed stable—over months—suspensions with nanometre-size particles (25–85 nm). Luminescent europium-doped nanocrystallized boehmites AlOOH:Eu (Al0.98 Eu0.02 OOH·n H2 O) were synthesized the same way by SD and demonstrated the same crystallization properties and morphologies as the undoped powders. It is inferred from the Eu3+ luminescence spectroscopy that partly hydrated europium species are immobilized on the boehmite nanocrystals where they are directly bonded to α(OH) groups of the AlOOH surface. The europium coordination is schematically written [Eu3+ (OH)α (H2 O)7−α/2 ]. The europium-doped boehmite from SD spontaneously dispersed in water: the luminescence spectroscopy proves that most of the Eu3+ ions were detached from the NPs during water dispersion. The AlOOH:Eu nanoparticles were modified by the amine acid asparagine (ASN). The modification aimed to render the NPs compatible for further bio-functionalization. After surface modification, the NPs easily dispersed in water; the luminescence spectra after dispersion prove that the Eu3+ ions were held at the boehmite surface.

1. Introduction

Boehmite is traditionally prepared by solid state decomposition of gypsite, or by precipitation from acidic or basic aluminium aqueous solutions. A third method employs sol– gel procedures where controlled hydrolysis–condensation reactions can lead to different crystal phases starting from aluminium alcoholates. The boehmite phase obtained by the sol– gel route, first described by Yoldas [4, 5], is nanocrystalline.

Boehmite is the most important precursor of the transition aluminas, which are widely used in the industry of adsorbents and catalysts [1, 2]. More recently, it has also been considered as the inorganic counterpart in new organic–inorganic hybrid materials leading to modified alumina-based ceramics [3]. 0957-4484/07/455605+08$30.00

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© 2007 IOP Publishing Ltd Printed in the UK

Nanotechnology 18 (2007) 455605

J M A Caiut et al

(spray generation and drying) was accomplished in about 10 s. Powders elaborated by spray drying will be denoted SD-boehmite in the following. Conventional drying was also performed for comparison purposes in an oven at 70 ◦ C during 24 h. The oven dried powders will be denoted OD-boehmites in the following. The preparation can be represented by the following equation:

The names ‘pseudoboehmite’ or ‘nanocrystalline boehmite’ have been used in contrast to ‘microcrystalline’ or ‘wellcrystallized boehmite’. The nanocrystalline boehmites exhibit crystallite size less than 10 nm, defects and numerous adsorbed water molecules: these features strongly depend on the preparation conditions, more especially on the drying conditions. Following the usual procedure established in [4, 5], the sol– gel transformation is conducted in a very limited temperature range: between 75 and 95 ◦ C; then the gel is dried at 70 ◦ C for 24 h. In this paper an original method is applied, derived from the well-known spray pyrolysis technique. Spray pyrolysis is an aerosol process commonly used to form a wide variety of materials in powder form including metals, metal oxides, ceramics, superconductors and fullerenes. During the last few years, a pilot-scale set-up was built in our laboratory for the synthesis of sub-micrometric phosphor powders [6, 7] or of nanometric zinc oxide [8]. One of the major advantages of spray pyrolysis is its flexibility: pyrolysis may be conducted at high temperatures (1000 ◦ C or more) so that high temperature phases may be achieved in one step. Alternatively, the whole process may also be conducted at lower temperatures. In the case considered here, micrometric droplets of the peptized sol in an air flow were subjected to temperatures between 100 and 300 ◦ C, for a few seconds. During this unique step, the sol–gel reaction and the gel drying were achieved. Under these conditions, highly hydrated nanocrystalline boehmite particles were obtained, which could be easily dispersed in water, leading to stable nanoparticulate dispersions. Herein, we describe the structure of the nanocrystalline boehmites from the spray-drying and from the conventional drying routes. Thermal analysis, electron microscopy, xray diffraction, infrared spectroscopy and granulometry were the techniques used in the characterization. Eu3+ -containing particles were also prepared. From luminescence studies, a model depicting the bonding of the trivalent ions to the NP’s surface is proposed. Modification of the particle’s surface by asparagine was found to hinder water leaching. The amino acid strengthened the attachment of the luminescent ion at the surface of the boehmite NPs, which gives a reasonable chance for using them as luminescent bio-markers.

1Al(O–C4 H9 )3(l) + x H2 O(l) + 0.07HNO3(aq)

→ 1AlOOH·n H2 O(S) + 3C4 H9 OH(g) + 0.07H3 O+ (aq) + 0.07NO− 3(aq) + (x − n − 2.07)H2 O(l) . Asparagine (C4 H8 O3 N2 )-modified samples were obtained by dispersing SD particles in asparagine water solutions, under agitation for 12 h. Thermal analysis scans were recorded with a TG-DTA Setaram, Labsys instrument. The powders were put in platinum holders, under O2 atmosphere. Heating rates of 5 or 10 ◦ C min−1 were employed. X-ray diffraction (XRD) patterns were obtained with a Seifert XRD3000 diffractometer. Crystallite sizes were calculated using the Scherrer equation D = 0.9λ/(FWHM) cos θ with the full width at half-maximum measured in (2θ ) and expressed in radians. Microstructures were observed by scanning electron microscopy with a SEM-JEOL 6700F. The size distributions of as-prepared boehmite dispersed in water were determined by dynamic laser light scattering using a Brookhaven apparatus with the BI-9000 particle sizing software. The samples were analysed at 90◦ with a 35 mW Spectra Physics (127) He–Ne laser, as light source. Fourier transform infrared (FT-IR) spectra were recorded at RT using a Perkin-Elmer spectrometer, model 2000. The powders were mixed with dried KBr in known proportions and pressed into pellets. Room temperature luminescence excitation and emission spectra were obtained with a SPEX Fluorolog spectrofluorimeter, model F212I system equipped with a double grating SPEX 1680 monochromator (at spectral resolution 0.9 nm). The excitation source was a 450 W xenon lamp. The signal was detected using a water-cooled Hammamatsu R928 photomultiplier. The luminescence decays were analysed by coupling a phosphorimeter (SPEX, F212I) to the spectrofluorimeter. Lifetimes values were obtained by fitting the experimental decay curves with single exponential functions. The luminescence spectra were also measured by a Hitachi Fl100 spectrofluorimeter at 2.5 nm spectral resolution.

2. Experimental details The precursor used in this work was a boehmite sol prepared by the methodology established by Yoldas [4, 5]. Aluminiumtri-sec-butoxide (25.93 g/0.1 mol) was added to distilled water (300 ml) at 83 ◦ C. After 2 h of stirring, nitric acid was added as a peptizing agent, up to 0.07 HNO3 /Al3+ (molar). Eu-containing samples (2% [Eu]/[Al]) were also prepared. EuCl3 (6.6 × 10−3 M) was added to the distilled water before addition of the aluminium-tri-sec-butoxide. The sol (0.2 mol l−1 Al3+ ) was spray dried in an experimental setup containing a 2.4 MHz ultrasonic pellet as piezoelectric oscillator. It generates an aerosol made of fine droplets ( ≈ 5 μm) from the precursor solution in air. The aerosol is then driven into two heating zones, the first one around 100 ◦ C with the second one maintained at a chosen temperature between 200 and 500 ◦ C. The powder is separated from the gas phase by an electrostatic collector. The whole process

3. Results and discussion 3.1. Thermal analysis: water content in boehmite The thermal evolution of the SD and OD boehmite were analysed by thermogravimetry. An important weight loss, corresponding to the dehydration–dehydroxylation of the samples was observed from 30 to 450 ◦ C, which could be separated in two more or less distinct steps. Discussions on the amount, the nature, the stability and the localization of excess water in boehmite had been the subject of numerous works. We will refer here to the well-documented 2

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Figure 1. Powder x-ray diffraction of (a) SD-boehmite, (b) SD-boehmite annealed at 250 ◦ C, (c) same sample annealed at 500 ◦ C (γ -Al2 O3 ).

Figure 2. Drawing of the boehmite γ -AlOOH structure. The aluminium-centred octahedra are drawn. Small circles: oxygen bonding two octahedra. Large circles: oxygens from hydroxyls. (This figure is in colour only in the electronic version)

Table 1. Thermal evolution of OD- and SD-boehmite samples according to the formula (Al1−x Eux )OOH·n H2 O.

Table 2. Coherent lengths in three crystallographic directions for OD-boehmite, SD-boehmite and reference sample from [9]. ˚ Coherent length (A)

Sample synthesis (OD, SD)

% weight loss 30–1000 ◦ C

% weight loss 30–450 ◦ C

n

Sample

(020)

(120)

(031)

SD (x = 0) (SD) (x = 0.02) (SD) (x = 0.02) (OD) (x = 0.02) (OD) (x = 0.02)

34.2 38.0 39.4 32.8 35.8

31.0 34.7 36.3 29.3 31.9

0.97 1.34 1.44 0.96 1.18

Nguefack et al [9] (OD) (SD)

27 30 16

43 42 30

40 43 30

heating at 250 ◦ C, and the γ -Al2 O3 phase was observed after heating at 500 ◦ C. The same observations hold for spray- and for oven-dried samples. The crystal structure of γ -AlOOH first described by Milligan and McAtee [10] is orthorhombic. The unit cell (figure 2) consists of two double layers of aluminium-centred distorted octahedra AlO4 (OH)2 . OH groups locate at the outer surface of the double layers and interact to hold the layers together. If there is only one cell per crystal following b, half the hydroxyls are inter-layer OH and half are surface OH. The coherent lengths of the SD and OD samples were evaluated with the Scherrer formula for three (hkl) sets (table 2). The first one, extracted from the (020) peak width, is simply related to one of the cell axes (b). The other two depend in a more complicated way on the sizes along a and b (120) or c and b (031). It may be concluded safely that the crystallites in SD were consistently smaller than in OD-boehmite, following the three cell axes. In [9] it had been established that, when the FMWH of the (020) peak increases, its position shifts to lower values with respect to 14.3◦ observed in micro-sized boehmite. We observed the same trend but to a lesser extent. The experimental values for the OD-boehmite (2θ = 14◦ , FMWH = 2.5◦ ) agree well with those reported in figure 3 of [9], but for the SD-boehmite the (020) FMWH equals 5◦ for 2θ = 13.6◦ , much less shifted than it was reported in the reference. This proves that, although the average crystallite ˚ following the b size was weak in SD-boehmite (about 16 A direction, that is just one crystal cell), the inter-layer spacing was kept very near to the one in micro-sized boehmite. The ability of water adsorption is related to the particle’s size: smaller particles with higher surface atoms/bulk atoms

paper by Nguefack et al [9]. The first step (30–200 ◦ C) corresponds to the dehydration of physisorbed water molecules and of part of the chemisorbed molecules, whereas the second step (200–450 ◦ C) is due to the removal of chemisorbed molecules and to the decomposition of boehmite into alumina. From 450 to 1000 ◦ C, the weight loss corresponds to the dehydroxylation of the surface of alumina. The processes can be expressed following the formulae: 30–450 ◦ C (Al1−x Eux )OOH·n H2 O −−−−→ 1/2 (Al1−x Eux )O3 ·m H2 O 

450–1000 ◦ C

−−−−−→ 1/2 (Al1−x Eux )O3 . 

The numerical values extracted for different samples are gathered in table 1: the average formulation for our hydrated boehmites is: (Al1−x Eux )OOH, (1.18 ± 0.24)H2 O (where the error range traduces the spreading of the experimental values). It must be noticed that the water content may be over-estimated because part of the weight loss is due to the removal of residual nitrates (from the synthesis mixture) and of adsorbed carbonates: these species were detected by IR spectroscopy (see section 3.4), but were not quantified. 3.2. X-ray diffraction: crystallite size The diffractograms of SD and OD samples exhibited the γ -AlOOH boehmite phase (JCPD no 21-1307). Some observations were performed on samples dried by both methods, then subjected to annealing in air (figure 1). The boehmite structure remained the only one detected after 3

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Figure 4. Distribution of hydrodynamic sizes for SD-boehmite dispersed in water at pH = 7.0.

morphologies of the synthesized particles and their abilities to form stable water suspensions. The SD particles exhibited a spherical morphology as expected for a spray drying process. Some representative images are displayed in figure 3. The SD particles were submicronic, the majority of spheres show a medium diameter at 500 nm, but particles between 2 μm and 100 nm were observed. The spherical particles were observed to be built of 100 nm or less platelet-like sub-particles, clearly visible on the image taken at higher magnification in figure 3(b). On the other hand, the slow drying of OD-boehmite gave plateletlike particles (several microns in size) with, in some cases, very well defined shapes and particles surfaces, as is shown in figure 3(c). SD samples exhibited the very interesting property to be spontaneously dispersed in water at room temperature. Upon addition of 500 mg of SD-boehmite in 15 ml of water, the pH was lowered to 4.5 then stabilized. By addition of the OD-boehmite in water at 500 mg/15 ml, the same pH drop was observed and some particles were dispersed but they reaggregated in a short period (a few hours). In contrast, the SD-boehmite sol was stable over several months. The particle’s size distribution in the stable sol was analysed by dynamic laser scattering. A representative histogram is displayed in figure 4. The hydrodynamic diameters of 60% of the particles fall in the range 25 and 40 nm, while higher values, up to 85 nm, were measured for the remaining 40% of particles.

Figure 3. Scanning electron microscopy of the SD-boehmite powder ((a) and (b)) and of the OD-boehmite (c).

will have more free sites for water adsorption. Taking the coherent lengths as a measure of the crystallite sizes in the direction perpendicular to the considered (hkl), the number of sites free for water adsorption may be estimated in a picture where each crystallite is isolated from the others. A model for the water content of nanocrystallized boehmite had first been proposed by Baker et al [11]. These authors arrived at the formula AlOOH·0.785H2 O for the model chosen. In [9], the number of adsorption sites for water molecules had also been calculated, starting from a model crystallite. In the model, the faces of the crystallite are parallel to the unit cell faces, with 20, 2 and 30 unit cells in the a , b and c directions, respectively. These dimensions match the coherent lengths measured in that ˚ (020), 43 A ˚ (120) and 40 A ˚ (031). There are 2640 work: 27 A surface sites free for water adsorption versus 4800 bulk atoms, giving the formula AlOOH·0.55H2 O. Now if we consider crystallites 50% smaller in each direction (10, 1 and 15 unit cells in the a , b and c directions), then for the same volume of matter there will be twice the surface free for adsorption: 5280 adsorption sites for 4800 bulk sites and the formula becomes AlOOH·1.10H2 O. The number of cells following the b axis can obviously not be smaller than 1 but half sizes following a and c can still be considered: with 5, 1 and 7 unit cells it becomes AlOOH·1.7H2 O. Our samples could have as much as 1.7 H2 O per AlOOH formula in the first hydration shell, when we measured between 0.96 and 1.44 H2 O per AlOOH (table 1). From these considerations, it is concluded that the amount of adsorbed water measured by thermal analysis is compatible with the crystallite sizes of the SD-boehmite.

3.4. FTIR Most of the modes we observed in the IR spectrum of SDboehmite (figure 5) could be assigned by comparison with the IR analysis of well-crystallized γ -AlOOH reported in [12, 13]: vibration modes of AlO6 at 481, 632 and 734 cm−1 and δOH modes at 1072 and 1157 cm−1 . For the assignment of the other features, we refer to more recent investigations by IR of amorphous and nanocrystallized boehmite [14, 15]. The two νOH modes at 3083 and 3310 cm−1 were partly obscured by the broad band around 3400 cm−1 , this last one assigned to adsorbed

3.3. Morphology and water dispersion Besides the smaller crystallite sizes, the main difference we observed between the OD-and SD-boehmites concerns the 4

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Figure 5. FTIR spectra of (a) SD-boehmite, (b) SD-boehmite annealed at 250 ◦ C, (c) same sample annealed at 500 ◦ C (γ -Al2 O3 ).

hydrogen bonded hydroxyls. The vibration relative to isolated OH groups (in the 3600–3700 cm−1 range) could not be observed in these experimental conditions (without evacuation of adsorbed hydroxyls). The presence of adsorbed water molecules was shown by the δHOH at 1630 cm−1 . Residual nitrates (from the synthesis mixture) were shown by the narrow peak at 1385 cm−1 , superimposed on broader features that we attribute to adsorbed carbonates groups as it had been proven that the boehmite surface is likely to absorb some CO2 from air [16]. There was no trace of residual organics from the preparation mixture in the SD-boehmite. Upon heating at 500 ◦ C, the boehmite structure collapsed and the IR spectrum exhibited two partly resolved bands at 635 and 785 cm−1 , in the AlOn vibrational range and a broad feature at 3500 cm−1 from residual adsorbed hydroxyls. A group of three well-resolved peaks was observed at 1396, 1530 and 1635 cm−1 . We sustain for these bands the assignment given in [15]: the vibrations at 1396 and 1530 cm−1 came from adsorbed carbonates and at 1635 cm−1 is the δHOH .

Figure 6. Luminescence spectra recorded at room temperature. Top: excitation spectra (λem = 592 nm) and bottom: emission spectra (λex = 394 nm); (a) OD-boehmite, (b) SD-boehmite and (c) γ -Al2 O3 . Three samples with 98Al/2Eu.

figure 6(c). The 5 D0 → 7 F0−4 emissions are inhomogeneously broadened, because the Eu3+ ions were in numerous different environments in the highly porous γ -Al2 O3 . The luminescence spectra of Eu3+ in the SD, ODboehmites are displayed in figures 6(a) and (b). Contrasting with the observations made on γ -Al2 O3 , the 5 D0 → 7 F0 emission in boehmite was narrow, meaning that all the Eu ions had similar environments (at most one 5 D0 → 7 F0 line is allowed per Eu3+ crystallographic site). The three components of 5 D0 → 7 F1 were not resolved, indicating a small splitting because the crystal field is weak. The excitation spectra exhibited the intra-4f6 narrow excitation lines. At lower wavelength, the broad absorption to the Eu–O charge transfer state (CTS) was almost not visible in boehmite. In most of the oxide matrices, excitation into the CTS (250 nm) is, in contrast, more powerful than intra-4f6 excitation at 390 nm, as was also observed here for γ -Al2 O3 . It is not clear if the band shifted at lower wavenumbers for the boehmite samples or if absorption to the CTS was simply less intense, in any case this effect traduces that the oxygen to metal interaction is weak in doped boehmite. Other peculiarities must be pointed out: emissions at 535, 555 and 585 nm were observed, that can be indexed as 5 D1 → 7 F0,1 , 5 D1 → 7 F2 and 5 D1 → 7 F3 , respectively, in agreement with the europium electronic levels diagram. Actually the emission of Eu3+ in SD- (or OD-) boehmite is quite different from that observed in γ -alumina. Several features are very similar to what is observed in highly hydrated environments, for instance in faujasite-type zeolites [24]. In Y- zeolite, the fully hydrated (Eu3+ , 8H2 O) species are trapped in the so-called ‘supercages’ of the structure. The emission spectrum is very typical and similar to the one observed for aqueous solutions of europium. The ratio of intensities 5 D0 → 7 F1 /5 D0 → 7 F2 equals 4, about the same is observed for the aquo-ion, and the 5 D0 emitting level lifetime is τ = 0.14 ms (for 0.11 ms in aqueous solution). In the X- zeolite,

3.5. Eu 3+ luminescence The measurements described in the preceding gave the same results (within the range of precision of each technique) for boehmite samples with or without Eu3+ : this is easily understandable because the samples were lightly doped by europium: [Eu]/[Al] = 2%. The analysis of the luminescence properties is necessary to understand the local structure at the Eu3+ sites since the Eu3+ emission spectrum is very sensitive to its environment. In many cases, reported in the literature, the data could be analysed to achieve a precise picture of the local field via a set of so-called ‘crystal field parameters’. In many other occurrences the quantitative study is not achievable because the experimental data are too limited, but nevertheless a qualitative description of the local structure remains feasible [17], as will be done here below for europium in the boehmite host. Despite the difference in ionic radii it has been shown that europium can, to some extent, be incorporated at the aluminium site in the α -Al2 O3 matrix employing special elaboration conditions [18–23]. Introducing Ln3+ cations into the transition aluminas is easier; it results in better qualities of the phases for catalysis applications. The luminescence spectra of Eu3+ in the γ -Al2 O3 transition alumina are displayed in 5

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Table 3. Emission characteristics for Eu in water, in nanocrystalline boehmite (this work) and in zeolites (from [24]). Compounds EuCl3 :10−3 M in H2 O Eu3+ @ Y-zeolite Eu3+ @ X-zeolite Eu3+ @ AlOOH SD OD

5D

7 0 → F1 7 0 → F2

τ (5 D0 ) (ms)

Reference

4 4 0.5

0.11 0.14 0.25

This work [24] [24]

0.33 0.70

0.16 0.16

This work This work

5D

partly hydrated europium ions enter also the sodalite cages where they are more tightly bound to the aluminosilicate network. The emission in that case is characterized by 5 D0 → 7 F1 /5 D0 → 7 F2 = 0.5 and τ = 0.25 ms; these characteristics are gathered in table 3. In the boehmite samples, Eu3+ displayed intermediate features: 5 D0 → 7 F1 /5 D0 → 7 F2 = 0.33(SD) or 0.70 (OD) and τ = 0.16 ms. The high energy vibrational modes of the hydroxyl groups (at 3400–3600 cm−1 ) quench efficiently the 5 D0 emission by non-radiative paths. The evolution of the 5 D0 decay times values can be connected to the number of OH in the first coordination (CN) shell of the metal ion. The more recent review made by Supkowski and Horrocks [25] collects and analyses the data on 25 europium complexes in aqueous solution. The authors deduce that the non-radiative probability ( PNR ) increases by 0.44 ms−1 per OH added in the first CN shell. In a previous work, some of us had monitored online the 5 D0 lifetime during the Eu(III)-promoted O-alkylation of glycolate with maleate [26] and we found, in very good agreement with [25], that ( PNR ) decreases by 0.45 ms−1 per OH removed from the Eu coordination. The 5 D0 emission probabilities measured in boehmite and in Y-zeolite are 6.25 and 7.14 ms−1 , respectively. From the difference (0.89 ms−1 ), we estimate that there are, on average, two OH less in the europium CN shell for boehmite (14 OH) than for Y-zeolite (16 OH). In (Eu3+ , 8H2 O), the water molecules form a square antiprism around the central ion (D4d point symmetry) [27]. This symmetry seems well maintained when hydrated europium is trapped in Y-zeolite since 5 D0 → 7 F1 is about four times more intense than 5 D0 → 7 F2 . We have concluded here that the europium ions have about 14 (OH) linked in the boehmite samples; we also notice that the D4d point symmetry is destroyed in boehmite where we measured 5 D0 → 7 F1 /5 D0 → 7 F2 = 0.7 (OD-boehmite) and 0.33 (SD-boehmite). We then propose to describe the bonding between the europium ions and the AlOOH host in the following way. Partly hydrated europium species are immobilized on the boehmite nanocrystals where they are directly bonded to α(OH) groups. It is not possible to give a more precise description of the structure, and in particular to define We propose to use the formulation the value of α . [Eu3+ (OH)α (H2 O)7−α/2 ], which traduces that there are, on average, 14 OH bonded to one Eu3+ . As pointed out in section 3.2, if there is only one cell per crystal following b, as is the case for our SD-boehmite, half the hydroxyls are interlayer OH and half are surface OH. It was also mentioned that the inter-layer spacing in SD-boehmite was kept very near to the one in micro-sized boehmite and this was observed for

Figure 7. Luminescence spectra recorded at room temperature. Top: excitation spectra (λem = 592 nm) and bottom: emission spectra (λex = 394 nm); (a) EuCl3 in water; (b) water dispersion of SD-AlOOH:Eu; (c) water dispersion of 0.3ASN:1AlOOH:Eu (for (a)–(c) [Eu] = 4 × 10−3 M); and (d) solid SD-AlOOH:Eu.

the doped Al0.98 Eu0.02 OOH as well. This sustains the model of attachment of the partly hydrated europium species at the crystallite surfaces rather than between the layers. The Eu3+ environment is not exactly the same in both (SD and OD) samples, but it is not possible to extract a more precise account from the data. 3.6. Luminescence of the nanoparticles dispersed in water Figure 7(b) displays the excitation (top) and emission (bottom) spectra recorded for water dispersed boehmite ([Al3+ ] = 2 × 10−1 mol l−1 and [Eu3+ ] = 4 × 10−3 mol l−1 ). The excitation and emission narrow lines of Eu3+ were visible in the suspension (figure 7(b)), superimposed on broad bands centred at 420 nm (excitation) and 450 nm (emission). These bands were also observed on the suspensions of non-doped boehmite. We may associate these broad bands with defects on the nanoparticles’ surfaces as done in [28], but more investigations are necessary to give further interpretations. Compared to the solid AlOOH:Eu (figure 7(d)), the 5 D0 → 7 F1 , 7 F2 emissions of the dispersed NPs were slightly shifted (1 nm towards lower wavelength), whereas the 5 D0 → 7 F0 was not observed. The point worthy of note is that the 5 D0 → 7 F1 /5 D0 → 7 F2 was reversed with respect to the AlOOH:Eu solid and the emission spectrum looked more like that of a water solution (EuCl3 : 4 × 10−3 mol l−1 ), displayed in figure 7(a). The 5 D0 lifetime decreased (0.11 ms after dispersion in water versus 0.16 ms in solid state). These observations, gathered in table 4, would signify that most europium ions were hydrated in [Eu3+ ·8H2 O], and probably had detached from the boehmite NP’. Of course this effect would be detrimental to potential applications of these particles as markers in aqueous media. 6

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Table 4. Emission characteristics for Eu3+ in nanocrystalline boehmite and in water-dispersed NPs. 5D

Compounds

5D

Eu3+ @ AlOOH solid 0.3ASN:1AlOOH:Eu in water, [Eu] 4 × 10−3 M AlOOH:Eu in water, [Eu] 4 × 10−3 M EuCl3 in water, [Eu] 4 × 10−3 M

7 0 → F1 7 0 → F2

0.33 0.69 2.5 4

τ (5 D0 ) (ms) 0.16 0.18 0.11 0.11

in a very large number of surface hydroxyls free for water adsorption. Actually the chemical composition of SD nanocrystallized boehmite was AlOOH, (1.18 ± 0.24)H2 O. The very hydrophilic nature of the surface of SD-boehmite crystallites is at the origin of their ability to form stable suspensions in water. The SD spheres (500 nm in size) spontaneously dispersed in water at room temperature and formed stable—over months— suspensions with nanometre-size particles (20–80 nm). The same observations were made when the optically active Eu3+ ion was added to the boehmite ([Eu]/[Al] = 2%). The visible emission properties of Eu3+ -doped boehmite nanoparticles were investigated because these NPs could be used as luminescent bio-markers. In addition, the Eu3+ luminescence allowed us to propose a model to describe the bonding between the europium ions and the AlOOH host. Partly hydrated europium species are immobilized on the boehmite nanocrystals where they are directly bonded to α(OH) groups of the AlOOH surface. The europium coordination is schematically written [Eu3+ (OH)α (H2 O)7−α/2 ]. When the europium-doped boehmite NPs (AlOOH:Eu) were dispersed in water, the Eu3+ emission had changed with respect to the SD-powder, demonstrating that most of the europium ions were hydrated in [Eu3+ , 8H2 O], and probably had detached from the boehmite NPs. This leaching of the luminescent species from the NPs could efficiently be prevented by using the amino acid asparagine (ASN) as a surface modifier of the boehmite and in that case the characteristic emission of AlOOH:Eu was still observed after dispersion in water. We thus conclude that the amino acid strengthened the attachment of the luminescent ions at the surface of the boehmite NPs after suspension in water, which gives a reasonable chance for using them as luminescent biomarkers. Further works are in progress to investigate the hybrid ASN:AlOOH:Eu and related compounds.

Another problem to be addressed for that particular use is that the NP surface must be modified for eventual further bio-functionalization. Many works have been done by the group of Barron [3], proving that the carboxylate ligand may be covalently bound to aluminoxane nanoparticles, for example by reacting the mineral boehmite with carboxylic acid. We followed this idea and chose the amino acid asparagine ASN = H2 N–CO–CH(NH2 )–COOH that could be linked to the boehmite by reaction of the carboxylate end, and could bring reactive amine groups on the NPs surface. Figure 7(c) shows the spectra observed for aspargine (ASN)modified nanoparticles dispersed in water. A completely different picture was detected. The 5 D0 → 7 F1 /5 D0 → 7 F2 ratio was reversed with respect to the unmodified boehmite NPs in water (figure 7(b)), and was actually very similar to that of the boehmite:Eu powder (figure 7(d)). The 5 D0 lifetime for the water suspension of modified boehmite nanoparticles (0.3ASN:1AlOOH:Eu, [Eu] 4 × 10−3 mol l−1 ) was longer than for the unmodified NPs: 0.18 ms versus 0.11 ms, respectively. From the luminescence spectra (numerical data gathered in table 4), we may conclude that the amine acid strengthened the attachment of the luminescent ions at the surface of the boehmite NPs after suspension in water, which gives a reasonable chance for using them as luminescent biomarkers. Further works are in progress in order to investigate the chemical relationships between the boehmite surface, europium and the amine-acids. From the first results discussed here, it may be said that the amino acid acts as a shield between the Eu@AlOOH and the aqueous solvent.

4. Conclusions Nanocrystallized boehmite powders with the formula γ AlOOH·n H2 O were obtained by the controlled drying of a sol of aluminium alkoxide. Samples synthesized by conventional slow drying in an oven (OD-boehmite) had an average crystallite size of 3–4 nm and a platelet-like shape (several microns in size). More interestingly, the spray-drying (SD) process, that is the fast drying of micrometric droplets generated from the alkoxide precursor, produced sub-micronic boehmite particles exhibiting rough surface, spherical shape and dense texture. The spheres were built of 100 nm or less plateletlike sub-particles and the average crystallite size was smaller than in the OD-boehmite. It appears that the rapid drying (a few seconds) stops the development of the aluminium hydroxide network at the level of a few crystal cells. In the boehmite cell (γ -AlOOH), layers of aluminium-centred octahedra AlO4 (OH)2 pile up following b. One cell contains two layers held together by hydrogen bondings of the hydroxyl groups. The average crystallite size following b did not exceed 1 cell in SD-boehmite, so that half the hydroxyl groups were in fact at the external surface of the crystallites. This resulted

Acknowledgments The authors acknowledge the FAPESP, and the CAPESCOFECUB Brazil–France cooperation program for financial support. The authors thank C Brouca-Cabarrecq for her help in the structural drawings.

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