Thermal Evolution and Luminescence Properties of Zn−Al-Layered

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Thermal Evolution and Luminescence Properties of Zn-Al-Layered Double Hydroxides Containing Europium(III) Complexes of Ethylenediaminetetraacetate and Nitrilotriacetate Cang Li, Lianying Wang,* David G. Evans, and Xue Duan State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing, 100029, P. R. China

Europium complexes of ethylenediaminetetraacetate (EDTA) and nitrilotriacetate (NTA), [Eu(EDTA)(H2O)3]-, and [Eu(NTA)2(H2O)]3-, which have similar geometries but different charge densities, have been incorporated in Zn-Al layered double hydroxides (LDHs). The orientation and loading of the guest anions as well as their thermal stability and reorientation behavior are significantly influenced by their charge density. The maximal dimension of the [Eu(EDTA)(H2O)3]- anion is nearly perpendicular to the LDH layers and a reversible reorientation occurs on mild heating. The structural changes that occurred on heating were followed by variable temperature photoluminescence (PL) measurements. The kinetics of the structural transformation of ZnAlEu(EDTA) LDH in the temperature range 30-150 °C have also been studied. In the case of [Eu(NTA)2(H2O)]3-, the anion adopts a flat orientation in the interlayer galleries with its maximum dimension nearly parallel to the metal hydroxyl layers and no reorientation is observed on heating. 1. Introduction Intercalation of guest species in two-dimensional layered host solids provides a means for developing new hybrids with tunable functionality.1 Layered double hydroxides (LDHs), also known as hydrotalcite-like materials or anionic clays, are almost unique in being able to accommodate anionic guests in their interlayer galleries. The structure of LDHs, which have the general formula [MII1-xMIIIx(OH)2](An-)x/n · mH2O,2-4 can be considered as being based on that of brucite, Mg(OH)2, with isomorphic substitution of some divalent cations by trivalent cations resulting in a net positive charge on the layers. Anions are intercalated between the layers in order to maintain electroneutrality. The very flexible intralayer composition of the host, coupled with a wide possible choice of anionic guests, affords a large variety of multifunctional LDH materials with potential applications in catalysis,5 optics,6 separation science,7 biology,8 photochemistry, and electrochemistry.9 Considerable attention has recently been focused on the intercalation of metal complex anions in LDHs in an effort to develop new functional materials. Many transition metal ions including Fe(III), Co(II), Ni(II), and Cu(II)10-13 have been introduced into LDHs in the form of anionic complexes with organic ligands, among which polycarboxylate chelate ligands such as ethylenediaminetetraacetate (EDTA, Figure 1a), or nitrilotriacetate (NTA, Figure 1b), have been extensively studied because of their strong chelating abilities with most metal cations. Rare earth complexes, especially those containing europium and terbium, have been the subject of extensive research because of their sharp and intense emission bands arising from f-f transitions.14-16 Application of rare earth complex salts in optical materials is limited by their poor thermal stability and mechanical properties, however, and incorporation of rare earth complexes in inorganic LDH hosts should alleviate these drawbacks to some extent. Furthermore, the complex network * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +8610-64425385. Tel.: +861064451027. Address: State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 98, 15 Beisanhuan Dong Lu, Beijing, 100029, China.

of hydrogen bonding and other electrostatic interactions within the interlayer galleries facilitates the homogeneous dispersion of the guest anions; this is a prerequisite for exploitation of their optical properties since aggregation leads to fluorescence quenching.17 There have been some attempts to intercalate rare earth ions into LDHs after chelation with organic ligands.18-23 For example: Kaneyoshi and Jones20 reported that an LDH preintercalated with NTA was able to take up La(III) ions, while Li et al.21 have shown that a preformed [Eu(EDTA)]- can be incorporated into an MgAl-LDH host by an ion-exchange method. In an alternative approach, Zhuravleva et al.23 have shown that [Ln(pic)4]- (Ln ) Tb(III), Eu(III); pic ) picolinate) complexes reacted with calcined LDHs to give LDHs containing anionic complexes. To prepare hybrid LDH materials with precisely tailored functional properties, it is desirable to have detailed knowledge of the arrangement of guest anions intercalated in the interlayer galleries,24 since the arrangement of interlayer guest anions can dramatically influence the physicochemical properties of LDH materials.25,26 It is well-known that the layer charge density is often an important factor in determining the supramolecular structure of LDHs by virtue of its influence on the guest-host interactions and the loading of the guest anions.27-29 Similar effects can be observed by varying the charge density of the interlayer anions,30,31 but no such study of rare earth complexes has been reported to date. Furthermore, calcination of LDHs results in the liberation of physisorbed and interlayer water and the decomposition of hydroxyl layers,4 which in turn may affect the structure of interlayer complexes. Therefore, it is important to study the thermal evolution of LDHs intercalated with

Figure 1. Molecular structures for (a) EDTA and (b) NTA. Polycarboxylate chelate ligands which have different carboxylate number and chelating abilities with Eu ion are present here.

10.1021/ie800342u CCC: $40.75  2009 American Chemical Society Published on Web 01/02/2009

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different rare earth complexes, in order to better correlate their luminescence performance with their supramolecular structure, and to study the thermal stability and the steps involved in their conversion to the luminescent mixed metal oxide materials obtained on calcination at high temperatures. In this work, preformed Eu-containing complexes, [Eu(EDTA)(H2O)3]- (denoted as [Eu(EDTA)]-) and [Eu(NTA)2(H2O)]3- (denoted as [Eu(NTA)2]3-), which have similar ninecoordinate distorted capped square antiprismatic geometries but very different charge densities, were intercalated into the same ZnAl-NO3 LDHs host. The changes with temperature in the supramolecular structures and luminescence properties of the resulting ZnAl-Eu(EDTA) LDHs and ZnAl-Eu(NTA)2 LDHs have been studied. The kinetic features of the structural transformation of ZnAl-Eu(EDTA) LDH in the temperature range 30-150 °C are also discussed. Our aim is to exploit the fluorescence properties of the rare-earth containing anion as a means of monitoring the evolution of the structure of the LDHs as a function of temperature. Such information should provide a better understanding of the structure of these materials and provide a framework for the synthesis of new luminescent LDHs and/or mixed metal oxide materials. 2. Experimental Details 2.1. Synthesis. Eu2O3 (purity 99.99%) was obtained from Beijing Founder Dongan Rare Earths New Materials Co. Ltd. and other reagents were all of AR grade and from Beijing Yili Fine Chemical Reagent Co. and used without further purification. Deionized water with a conductance below 10-6 S cm-1 was freshly decarbonated by boiling and bubbling N2 before use in any synthesis or purification steps. Complex salts of Eu(III) with EDTA4- and NTA3- were prepared following a method in the literature.32 A mixture of Eu2O3 (3.52 g, 0.01 mol) and H4EDTA (5.84 g, 0.02 mol) was refluxed in water (100 mL) until the solid had completely dissolved. Aqueous sodium hydroxide solution (1.0 M) was added until the pH reached about 6. After concentration and slow evaporation of the solution, a white powder was obtained and was washed with cold water. Anal. Found % (calcd for Na[Eu(EDTA)] · 3H2O): Eu, 30.15 (29.40); Na, 4.67 (4.45); C, 24.01 (23.21); H, 3.41 (3.48); N, 5.60 (5.42). An analogous procedure using Eu2O3 and H3NTA with a 1:4 ratio was used to prepare the NTA complex. Anal. Found % (calcd for Na3Eu(NTA)2 · 3H2O): Eu, 23.25 (23.35); Na, 10.93 (10.60); C, 21.92 (22.12); H, 2.85 (2.76); N, 4.39 (4.30). The ZnAl-NO3 LDHs precursor was prepared by a method involving separate nucleation and aging steps developed in our laboratory.33 The white precipitate obtained by centrifugation was washed several times and stored as a slurry under nitrogen for later use. A small portion was dried at 60 °C for 24 h for analysis. Anal. Found % (calcd for Zn0.66Al0.34(OH)2(NO3)0.32(CO3)0.01 · 0.42H2O): Zn, 37.95 (37.45); Al, 8.18 (8.09); C, 0.13 (0.11); H, 2.76 (2.49); N, 4.04 (3.95). ZnAl-Eu(EDTA) LDHs was obtained by anion exchange, performed in water containing a suspension of the as-prepared ZnAl-NO3 LDHs precursor (5.72 g, ca. 0.004 mol of dry solid) and the Na[Eu(EDTA)] · 3H2O complex (6.14 g, 0.012 mol). The reaction mixture was stirred for 6 h at 70 °C, under an N2 atmosphere to prevent contamination by carbonate arising from atmospheric CO2. The white solid was separated by centrifugation, thoroughly washed with hot water, and dried at 60 °C for 24 h. Anal. Found % (calcd for Zn0.66Al0.34(OH)2[Eu(EDTA)]0.14(CO3)0.05(NO3)0.1 · 0.63H2O): Zn, 24.83 (25.29); Al, 5.36 (5.46); Eu, 12.56 (12.76); C, 10.80 (10.43); H, 3.12 (2.94); N, 3.34

(3.18). The ZnAl-Eu(NTA)2 LDHs was prepared in an analogous manner using a 2-fold molar excess of the anionic complex over the anion exchange capacity and a reaction time of 2 h. Anal. Found % (calcd for Zn0.64Al0.36(OH)2[Eu(NTA)2]0.12 · 0.65H2O): Zn, 25.59 (26.14); Al, 6.06 (6.19); Eu, 12.11 (11.61); C, 10.72 (11.01); H, 2.99 (3.00); N, 2.09 (2.14). 2.2. Characterization. In situ powder X-ray diffraction (in situ XRD) data were recorded with a Shimadzu XRD-6000 powder diffractometer over the temperature range from 30 to 900 °C in air, using Cu KR radiation (λ ) 0.154 nm) at 40 kV and 30 mA. The samples, as unoriented powders, were stepscanned in steps of 5°/min in the 2θ range from 3 to 70° using a count time of 4 s per step. The rate of temperature increase was 10 °C/min with a holding time of 5 min before each measurement. Fourier transform infrared (FTIR) spectra were recorded with a Bruker Vector 22 spectrometer in the range 4000-400 cm-1 at 2 cm-1 resolution. The KBr pellet technique, with a typical pellet containing ca. 1 wt % sample in KBr, was employed. Elemental analyses for metal ions were performed by inductively coupled plasma emission spectroscopy using a Shimadzu ICPS-7500 instrument on solutions prepared by dissolving the samples in concentrated HNO3. Carbon, hydrogen, and nitrogen analyses were carried out using an Elementarvario elemental analysis instrument. Thermogravimetry-differential thermal analysis (TG/DTA) coupled to mass spectrometry (MS) was recorded using a PerkinElmer Diamond TG apparatus linked to a ThermoStar MS by a quartz capillary transfer line heated at 190 °C. The heating rate was 10 °C/min with an air flow rate of 200 cm3/min. The TG apparatus was at atmospheric pressure, and the MS at a working pressure of 6 × 10-6 Torr. Variable temperature photoluminescence (PL) measurements were performed on a Shimadzu 5301 spectrofluorophotometer equipped with a 150 W xenon lamp as the excitation source, using monochromator slit widths of 1.5 nm on both excitation and emission sides. Emission spectra of the thermally treated samples were obtained immediately after heating the solids at the designated temperature for 30 min. 3. Results and Discussion 3.1. Preparation and Characterization. Complex salts of Eu(III) with EDTA4- and NTA3- were prepared following a procedure described in the literature for Na[Eu(EDTA)] · 3H2O.32 The molecular structures of the [Eu(EDTA)]- and [Eu(NTA)2]3 complexes are illustrated in the Supporting Information (see S1). In this salt, the Eu(III) adopts a nine-coordinate distorted capped square antiprismatic geometry with one hexadentate EDTA ligand and three water molecules, as is observed for other rare earth ions.32,34 An Eu(III) complex with a 1:2 ratio of metal to NTA has been structurally characterized in the salt K3[Eu(NTA)2(H2O)] · 5.5H2O, where NTA3- acts as a tetradentate ligand with three five-membered chelated rings; the Eu(III) also has a nine-coordinate distorted capped square antiprismatic coordination shell.35 In this work, an Eu:NTA ratio of 1:2 was employed and analytical data for the material obtained were consistent with the presence of the same [Eu(NTA)2(H2O)]3complex anion. The XRD patterns for the ZnAl-NO3 LDHs, ZnAl-Eu(EDTA) LDHs, and ZnAl-Eu(NTA)2 LDHs recorded at 30 °C are shown in Figure 2. The positions recorded for the diffraction maxima coincide with those expected for a typical lamellar structure and can be indexed to a hydrotalcite-like structure with R3jm

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Figure 2. XRD patterns for (a) ZnAl-NO3 LDHs, (b) ZnAl-Eu(EDTA) LDHs, (c) ZnAl-Eu(NTA)2 LDHs. All display the characteristic X-ray powder diffraction patterns of hydrotalcite-like materials with a series of strong (00l) reflections at lower angle and weaker reflections at higher angle.

Figure 3. FTIR spectra for (a) ZnAl-NO3 LDHs, (b) sodium salt of [Eu(EDTA)]-, (c) ZnAl-Eu(EDTA) LDHs, (d) sodium salt of [Eu(NTA)2]3-, (e) ZnAl-Eu(NTA)2 LDHs. All the carboxyl groups of NTA and/ or EDTA are equivalently directly and ionically bonded to the Eu(III) ion as chelate after intercalation in the LDH host.

rhombohedral symmetry,3,4 in which case the lowest angle peak corresponds to the basal (003) reflection. The basal spacing is 8.9 Å for ZnAl-NO3 LDHs, which is similar to the values reported in the literature for LDHs with M2+/M3+ ratios of ∼2 and interlayer NO3- anions.21,28 Intercalation of [Eu(EDTA)]and [Eu(NTA)2]3- complexes results in the complete disappearance of the (00l) reflections of the ZnAl-NO3 LDHs and an increase in the basal spacing from 8.9 Å in the precursor to 14.5 and 12.5 Å, respectively, as shown by the shift of the (003) reflection to lower angle. The cell parameter a, which represents the average distance between metal cations in the brucite-like sheets and therefore depends on the average ionic radius of the cations,3 can be calculated from the position of the (110) peak close to 60°. The values of unit cell a parameter are comparable for ZnAl-NO3 LDHs (3.053 Å), ZnAl-Eu(EDTA) LDHs (3.050 Å), and ZnAl-Eu(NTA)2 LDHs (3.048 Å), respectively, suggesting that the Zn/Al ratios are similar in three lamellar materials, which is consistent with the analytical data. Given that the thickness of the brucite-like layers in an LDHs is 4.8 Å,3 the gallery heights of the lamellar materials are ca. 9.7 and 7.7 Å for ZnAl-Eu(EDTA) LDHs and ZnAl-Eu(NTA)2 LDHs, respectively. The maximum and minimum O · · · O distances between carboxylate groups in the [Eu(EDTA)]- anion as determined by single crystal XRD32 are ca. 9.6 and 5.8 Å, respectively. This suggests that the complex is oriented with its maximal dimension nearly perpendicular to the layers. Analytical data suggest that cointercalation of carbonate and nitrate anions are observed in the ZnAl-LDHs host. Similar results been reported in our previous work involving intercalation of [Eu(EDTA)]- into Mg-Al LDHs,21 where it was shown that the charge density on the layers is too high to be balanced by intercalation of [Eu(EDTA)]- alone, necessitating the cointercalation of small anions (NO3- or CO32-) which have a much higher charge density. In case of the [Eu(NTA)2]3- anion, the maximum and minimum O · · · O distance between carboxylate groups is ca. 10.2 and 7.4 Å, respectively.35 The observed gallery height (7.7 Å) of ZnAl-Eu(NTA)2 LDHs is therefore consistent with a flat orientation in the interlayer galleries with its maximum dimen-

sion nearly parallel to the hydroxyl layers. In contrast to the case of [Eu(EDTA)]-, analytical data suggest that no cointercalation of small anions such as nitrate or carbonate is observed when the [Eu(NTA)2]3- anion is intercalated into the ZnAl-LDHs host. This is presumably a reflection of the higher charge density of the anion which allows it to balance the charge on the layers without any steric crowding. Vibrational spectroscopy is a very useful tool for the study of LDHs, especially those containing interlayer organic anions, since it is a probe of the interactions between the guest anions, the host layers, and interlayer water.4 FTIR spectra in the region 4000-400 cm-1 for ZnAl-NO3 LDHs, the two pristine complex salts, ZnAl-Eu(EDTA) LDHs and ZnAl-Eu(NTA)2 LDHs are illustrated in Figure 3. The intense broad absorption band centered at 3447 cm-1 in ZnAl-NO3 LDHs (Figure 3a) corresponds to the stretching vibrations of the hydroxyl groups of both the layer hydroxide moieties and interlayer water molecules. The broadening of this band is due to the hydrogen bonding of interlayer water with the guest anions as well as with the hydroxide groups of the layers.36 The band close to 1629 cm-1 corresponds to the deformation mode (δH2O) of water molecules. The intense absorption peak at 1384 cm-1 and the weak peak at 830 cm-1 are assigned to the ν3 and ν2 vibration modes of NO3- with D3h symmetry, respectively.21,28 The bands observed in the low-frequency region from 800 to 400 cm-1 in the spectrum correspond to M-O lattice vibrations.4 The FTIR spectra of Na[Eu(EDTA)] · 3H2O and ZnAl-Eu(EDTA) LDHs are shown in Figure 3b,c, respectively. ETDA and its complexes are known to give a band in the region 2800-3000 cm-1 arising from C-H stretching of CH2 groups. Appearance of this peak around 2900-2950 cm-1 is strong evidence for the formation of a chelate, whereas the free acid and the mono- and disodium salts of EDTA show this peak at 3020-3030 cm-1, and the tri- and tetrasodium salts absorb at 2800-2810 cm-1.37-39 The presence of a band centered at ca. 2925 cm-1 in the spectra of Na[Eu(EDTA)] · 3H2O and ZnAlEu(EDTA) LDHs suggests that the COO- groups of EDTA are directly attached to the Eu(III) ion as a chelate. The strongest and most characteristic absorption band for the carboxylate group is due to the antisymmetric COO- vibration (Vas) in the

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region 1570-1660 cm . According to the literature for metal EDTA complexes,37-39 as the bonding of the COO- group to the metal becomes more covalent in character the absorption band shifts to higher frequencies since covalent bonding localizes the electrons around the free carbonyl group and increases the double bond character causing an increase in the absorption frequency; a free COO- stretching band below 1610 cm-1 is generally taken to indicate that the bonding is predominantly ionic. A single sharp peak at 1607 cm-1 for the free complex (Figure 3b) and 1602 cm-1 for the intercalated [Eu(EDTA)]- complex (Figure 3c) indicates that the carboxyl groups of EDTA are ionically bonded to the Eu(III) ion. The presence of two COO- peaks in this region has been reported for Co-EDTA41 and Pb-EDTA38 complexes, indicating that more than one kind of carboxylate group was present in the complexes. Only one peak is observed for [Eu(EDTA)]- which is strong evidence of the equivalence of the four carboxyl groups and the absence of free carboxyl groups. A band centered at 1400 cm-1 corresponds to the symmetric Vs(COO-) stretching mode of the COO- group and ∆ν ) Vs - Vs gives information about the coordination environment of the carboxyl group.37-40 The values of ∆ν for [Eu(EDTA)]- (204 cm-1, Figure 3b) and ZnAl-Eu(EDTA) LDHs (202 cm-1, Figure 3c) are essentially identical indicating that the metal-oxygen interaction between four COO- groups and Eu(III) ion in [Eu(EDTA)]- is not affected by intercalation in the LDH host. The FTIR spectra of the sodium salt of [Eu(NTA)2]3- and ZnAl-Eu(NTA)2 LDHs are shown in Figure 3d,e, respectively. The assignments of the important vibrational bands of NTA42-44 are similar to those for EDTA. Comparison between the spectra of the [Eu(NTA)2]3- salt and ZnAl-Eu(NTA)2 LDHs shows that incorporation of the complex anion in LDHs does not cause any significant changes in position or intensity of the major IR bands. The single sharp band at 1604 and 1601 cm-1 for the COO- group (Vas) in [Eu(NTA)2]3- and ZnAl-Eu(NTA)2 LDHs, respectively, indicates that all the carboxyl groups of NTA are equivalently chelated to the Eu(III) ion. This result is confirmed by the single C-H stretching band centered at about 2924 cm-1 for both samples. Intercalation of the [Eu(NTA)2]3- anion in the LDH host is accompanied by a slight decrease in the value of ∆ν, from 200 to 194 cm-1, and an increase in the C-N stretching frequency38 from 1100 to 1127 cm-1, which may be due to the different intermolecular hydrogen bonding environments experienced by the [Eu(NTA)2]3- in its sodium salt as compared with that in the interlayer gallery of the ZnAl-Eu(NTA)2 LDHs. 3.2. Analysis of Thermal Behavior. The thermal evolution of the LDHs were studied by means of TG/DTA coupled to MS, with peaks at m/e 18 (H2O+), 30 (NO+), and 44 (CO2+) being analyzed. Generally, four steps are observed in the thermal evolution of LDHs:4 desorption of physically adsorbed water, removal of interlayer structural water, dehydroxylation of the brucite-like sheets, and decomposition of the interlayer anions, although the first two steps may overlap in the low temperature range. For the thermal decomposition of the ZnAl-NO3 LDHs, three distinct weight loss stages were observed in the TG/DTA curve (Figure 4a) which is identical to the literature.31 The first two steps correspond to the removal of physically adsorbed and intergallery water (30-180 °C) and dehydroxylation of the brucite-like layers (180-290 °C). The coupled MS (Figure 4b) confirms this result since the (H2O+) ion current peaks in the corresponding temperature ranges, with Tm ) 151 and 253 °C, respectively, can be assigned to the evolution of water. The third weight loss (290-550 °C) is due to decomposition of the

Figure 4. TG/DTA (a) and coupled MS (b) data for ZnAl-NO3 LDHs. Three distinct weight loss stages were observed for the thermal decomposition of the ZnAl-NO3 LDHs: removal of physically adsorbed and intergallery water (30-180 °C), dehydroxylation of the brucite-like layers (180-290 °C) and decomposition of the interlayer nitrate anions (290-550 °C).

Figure 5. TG/DTA (a) and coupled MS (b) data for for ZnAl-Eu(EDTA) LDHs.

interlayer nitrate anions as evidenced by a broad NO+ ion current peak (Tm ) 353 °C). No signals for NO+ and H2O+ ion currents are observed on further calcination above 550 °C, suggesting that the ZnAl-NO3 LDHs had decomposed completely at this temperature. The TG/DTA-MS curves for ZnAl-Eu(EDTA) LDHs and ZnAl-Eu(NTA)2 LDHs, which are shown in Figure 5 and Figure 6, respectively, exhibit three distinct weight loss steps similar to those of ZnAl-NO3 LDHs. The removal of surface adsorbed water and interlayer water molecules, which occurs

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Figure 6. TG/DTA (a) and coupled MS (b) data for for ZnAl-Eu(NTA)2 LDHs. The removal of physically adsorbed and intergallery water molecules, which occurs with a maximum at around 151 °C for ZnAl-NO3 LDHs (Figure 4b), occurs at lower temperature for ZnAl-Eu(EDTA) LDHs (Figure 5b, Tm ) 129 °C) and ZnAl-Eu(NTA)2 LDHs (Figure 6b, Tm ) 121 °C). In contrast, the dehydroxylation process of the brucite-like sheets shifts from 253 °C for the ZnAl-NO3 LDHs to 308 and 317 °C for ZnAl-Eu(EDTA) LDHs (Figure 5b) and for ZnAl-Eu(NTA)2 LDHs (Figure 6b), respectively.

with a maximum at around 151 °C in the H2O+ ion current for ZnAl-NO3 LDHs (Figure 4b), occurs at lower temperature for ZnAl-Eu(EDTA) LDHs (Figure 5b, Tm ) 129 °C) and ZnAl-Eu(NTA)2 LDHs (Figure 6b, Tm ) 121 °C). Incorporation of [Eu(EDTA)]- and [Eu(NTA)2]3- complexes, which are much larger than the nitrate anion, leads to a significant expansion of the interlayer spacing which in turn creates greater voids between the sheets and may facilitate the elimination of the interlayer water.45 The liberation of interlayer water molecule is complete below 200 °C for both complex anion-intercalated LDHs, as the observed H2O+ ion current is not significantly above baseline at this temperature. In contrast, the dehydroxylation process of the brucite-like sheets, which shows a maximum at around 253 °C in the H2O+ ion current for the ZnAl-NO3 LDHs, shifts to higher temperature for the complex anion-intercalated LDHs, namely around 308 °C for ZnAl-Eu(EDTA) LDHs (Figure 5b) and 317 °C for ZnAl-Eu(NTA)2 LDHs (Figure 6b). Similar results have been reported in the literature: for example, Malherbe and Besse45 have shown that dehydroxylation of the brucite-like sheets can be retarded through incorporation of appropriate oxoanions which can hydrogen bond with the hydroxyl groups of the brucite-like layers. In our case, the carbonyl oxygen atoms of coordinated carboxylate groups of guest complex anions [Eu(EDTA)]- and/ or [Eu(NTA)2]3- are able to strongly hydrogen bond in a similar fashion, leading to the observed enhanced thermal stability of the hydroxyl layers. The higher thermal stability of the ZnAl-Eu(NTA)2 LDHs compared with that of the ZnAl-Eu(EDTA) LDHs can be attributed to the higher charge density and larger number of COO- groups in [Eu(NTA)2]3-, which lead to stronger hydrogen bonding and other electrostatic interactions between host and guest species.

Figure 7. In situ XRD patterns for ZnAl-Eu(EDTA) LDHs in the temperature range 30-900 °C.

The third weight loss step of the complex anion-intercalated LDHs results from the decomposition and combustion of the interlayer complex anions. The sharp weight loss in the range 340-450 °C for ZnAl-Eu(EDTA) LDHs (Figure 5a), with a corresponding large exothermic peak (Tm ) 393 °C), is due to the combustion of the organic ligand which is confirmed by the H2O+ (Tm ) 382 °C) and CO2+ (Tm ) 382 °C) ion currents in the MS traces. The decomposition process of guest complex anions [Eu(EDTA)]- overlaps with those of the NO3- and CO32anions in the temperature range 290-550 °C. It is difficult to distinguish between the complex anion and the NO3- and CO32anions. On the other hand, the introduction of [Eu(EDTA)]complex anions, which represent about 40% of the anionic charge, enhances the thermal stability of LDHs owing to the strong hydrogen bond between coordinated groups of guest complex anions [Eu(EDTA)]- and hydroxide layers. As a result no special attention was spent on the decomposition of NO3and/or CO32- anions in ZnAl-Eu(EDTA) LDHs and we merely focus on the changes brought about by the introduction of the complex anions. A similar weight loss process is observed in the case of ZnAl-Eu(NTA)2 LDHs (Figure 6a) in the range 350-450 °C with a large exothermic peak (Tm ) 382 °C). Decomposition of the complex anion-intercalated LDHs was apparently complete at 450 °C, as no signals for H2O+ and CO2+ ion currents were observed above this temperature. 3.3. Structural Transformation. The in situ variable temperature XRD patterns for the ZnAl-Eu(EDTA) LDHs and ZnAl-Eu(NTA)2 LDHs in the temperature range 30-900 °C in ambient atmosphere are shown in Figure 7 and Figure 8, respectively. The relationship between the d(003) basal spacing of the complex anion-intercalated LDHs and the temperature is shown in Figure 9. The evolution of the XRD patterns shows three distinct temperature regions in both cases. At low temperature (30-270 °C), the (00l) diffraction peaks move to a higher angle with increasing temperature, which may be related to the loss of interlamellar water molecules, indicated by a contraction in the interlayer spacing.3 The decrease in the diffraction intensity and the broadening of diffraction peaks is also related to the loss of interlamellar water, which results in some loss of cohesion between the hydroxyl layers.46 In both

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Figure 8. In situ XRD patterns for ZnAl-Eu(NTA)2 LDHs in the temperature range 30-900 °C. At low temperature (30-270 °C), the (00l) diffraction peaks move to higher angle with increasing temperature. A significant variation in the relative intensities of the (00l) reflections was observed in the case of ZnAl-Eu(EDTA) LDHs on heating from 120 to 180 °C. Further heating both samples results in complete dehydroxylation and decomposition of the anions and gives well-crystallized ZnO (JCPDS Card 05-0669) accompanied by the formation of AlEuO3 (JCPDS Card 300012).

Figure 9. Relationship between d(003) basal spacing and temperature for ZnAl-Eu(EDTA) LDHs (a) and ZnAl-Eu(NTA)2 LDHs (b). A significant contraction in basal spacing (∆d ) 2.6 Å) was observed in the case of ZnAl-E(EDTA) LDHs on heating from 30 to 150 °C. In contrast the basal spacing of ZnAl-Eu(NTA)2 LDHs showed a gradual contraction of 0.8 Å with increasing temperature from 30 to 210 °C.

cases, the layered structure is maintained up to about 270 °C and collapses46 rapidly at higher temperatures as a result of dehydroxylation of the brucite-like sheets, as shown by the disappearance of both the (00l) and nonbasal diffraction peaks.47 It is noteworthy, however, that the magnitude and evolution of the contraction observed on heating are very different for the two samples. A significant contraction (∆d ) 2.6 Å) was observed in the case of ZnAl-Eu(EDTA) LDHs on heating from 30 to 150 °C. No further obvious interlayer contraction occurred on heating at temperatures below the onset of dehy-

droxylation of the brucite-like layers. In contrast, the basal spacing of ZnAl-Eu(NTA)2 LDHs showed a gradual contraction of 0.8 Å with increasing temperature from 30 to 150 °C, and then remained unchanged even on heating at 210 °C. The relatively small contraction in basal spacing for ZnAl-Eu(NTA)2 LDHs can be explained by the loss of interlamellar water molecules, without any reorientation of the anions. This is consistent with the proposal on the basis of the XRD data (vide supra) that the [Eu(NTA)2]3- complex anion adopts a flat orientation in the interlayer galleries. In the case of ZnAl-Eu(EDTA) LDHs however, the significant decrease in basal spacing (2.6 Å) cannot be related to the removal of interlayer water alone. Similar behavior has been reported in the literature for other anions intercalated in LDHs, such as 1,1′-ferrocenedicarboxylate in ZnAl-LDH,31 terephthalate in MgAl-LDHs,27 and [Zn(EDTA)]2- in MgAl-LDHs.28 In each case, the considerable shrinkage in basal spacing observed was related to the reorientation of guest anions accompanied by the loss of the interlayer water; this can be reversed by rehydration. Similarly, when ZnAl-Eu(EDTA) LDHs was heated at 150 °C for 30 min and then cooled for 30 min in moist air, the resulting XRD pattern was similar to that of the freshly prepared sample, giving a basal spacing of 14.2 Å. The fact that the structural transformation of ZnAl-Eu(EDTA) LDHs is reversible suggests that a reorientation of the [Eu(EDTA)]- complex anion in the interlayer gallery, probably induced by the loss of the interlayer water, rather than covalent grafting of the complex anions to the hydroxyl layers is the origin of the observed significant contraction in basal spacing,28,31 since grafting processes are generally irreversible. The gallery height of the layered structure heated to 150 °C is 7.1 Å. The shortest O · · · O distance between carboxylate groups in the [Eu(EDTA)]- anion determined by single crystal XRD32 is ca. 5.9 Å while, as noted above, the maximum O · · · O distance between carboxylate groups is ca. 9.7 Å. This indicates that the shortest dimension of the complex anions is neither perpendicular nor parallel to the layers and we propose that the [Eu(EDTA)]- complex adopts a tilted orientation after calcination at 150 °C. The parallel orientation is the most stable orientation considering the case of ZnAl-Eu(NTA)2 LDHs. However, the charge density on ZnAl-Eu(EDTA) LDHs is too high to be balanced by intercalation of [Eu(EDTA)]- alone, necessitating the cointercalation of small anions (NO3- or CO32-) carbonate ions which have a much higher charge density. So it is too crowded in the interlayer gallery when the [Eu(EDTA)]- anions adopt a parallel orientation and we believe that the tilted orientation is the most stable orientation of Eu(EDTA) complex in gallery spaces among the suggested three phases upon heat treatment. The occurrence of guest reorientation is further confirmed by the significant variation in the relative intensities of the (00l) reflections in the in situ XRD patterns of ZnAl-Eu(EDTA) LDH as shown in Figure 7. When atoms with large scattering power are located at different positions along the c axis, the usual pattern of decreasing intensities of successive (00l) reflections with increasing l is often not observed because of the significant fluctuations in the one-dimensional electrondensity distribution along the c axis.19,21,28 When the intensity of the (006) reflection is greater than that of (003), it has been proposed that a heavy metal atom is located at the midpoint of the interlayer galleries.21,28 In the case of ZnAl-Eu(EDTA) LDHs, the (00l) reflections broaden significantly and become asymmetric in the range 100-120 °C, which possibly corre-

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Figure 10. Schematic illustration of the possible structures of ZnAl-Eu(EDTA) LDHs at 30 and 150 °C. When intercalated in a Zn-Al layered double hydroxide host, the complex anion [Eu(EDTA)(H2O)]- undergoes a reorientation within the interlayer galleries accompanied by the loss of interlayer water on heating to 150 °C.

Figure 11. Plot of extent of reaction R versus calcination temperature for ZnAl-Eu(EDTA) LDHs. The value of R increasing from 0 at the start of the reaction with increasing temperature up to 150 °C (423 K), where it essentially reaches an equilibrium plateau.

sponds to an unstable intermediate resulting from disorder in the arrangement of guests induced by loss of interlayer water from the hydrogen-bonded framework in the interlayer galleries,3,4 and an inversion in intensity between the (003) peak and (006) peaks appears on heating between 120 and 180 °C. Therefore it can be concluded that removal of interlayer water on heating results in loss of the hydrogen-bonding network within the interlayer galleries and a reorientation of [Eu(EDTA)]- complex anion, in which the metal atom moves toward the midpoint of the interlayer galleries, in order to maximize the electrostatic interaction of the anion with the layers and thus compensate for the loss of hydrogen bonding. A schematic illustration of the possible structures of ZnAl-Eu(EDTA) LDH at 30 and at 150 °C is proposed in Figure 10. In the case of ZnAl-Eu(NTA)2 LDHs, the relative intensities of the (00l) reflections remain almost unchanged with increasing temperature, suggesting that the orientation of the [Eu(NTA)2]3- anion within the interlayer galleries does not vary with temperature. This further confirms that the [Eu(NTA)2]3- complex adopts a flat orientation in the interlayer gallery with its maximum dimension parallel to the hydroxyl layers. The difference between the two materials can be traced to the very different charge densities of the guest anions. The [Eu(NTA)2]3- anion has a sufficiently high charge density to be able to balance the layer charge when it adopts a flat orientation in the interlayer galleries and no reorientation occurs on heating. The much lower charge density of the [Eu(EDTA)]- anion forces it to adopt an upright orientation in order to allow cointercalation of the requisite small anions (NO3or CO32-) and interlayer water molecules. Loss of interlayer

water molecules on heating leads to a reduction in crowding in the interlayer galleries, and as a result the interlayer complex anion become tilted and interacts more strongly with the layers. In the temperature region 270-500 °C, the appearance of a poorly crystalline ZnO phase (JCPDS Card 01-1136) with reflections at about 34.4 and 36.3° indicates that dehydroxylation of the hydroxide layers has occurred. This is consistent with TG/DTA and coupled MS and in situ FTIR data, which suggest that dehydroxylation of the brucite-like layers along with decomposition of interlayer complex anions occurs in a similar temperature range. Heating both samples at 900 °C results in complete dehydroxylation and decomposition of the anions and gives well-crystallized ZnO (JCPDS Card 05-0669) accompanied by the formation of AlEuO3 (JCPDS Card 30-0012). No reflections characteristic of Eu2O3 (JCPDS Card 43-1008) can be observed, which indicates that the Eu(III) ions are well dispersed in the Zn-Al oxide matrix, preventing segregation of a europium oxide phase. 3.4. Kinetics of Structural Transformation. To investigate the mechanism of the structural transformation of ZnAl-Eu(EDTA) LDH in the temperature range 30-150 °C, a kinetic study was performed (see S2 in the Supporting Information). A solid-state reaction involves diffusion processes, that is, the transportation of matter to and from the reaction zone, a chemical reaction involving breaking and forming of bonds, and nucleation of products at active sites with subsequent growth of nucleated particles. Accordingly, the known theoretical kinetic models are usually classified into three groups: the diffusion models, the chemical reaction models, and the nucleation models.48,49 The Avrami-Erofe‘ev expression has previously been found to be an appropriate model for a variety of solid state processes including phase transitions, decompositions, crystallization, and intercalation.48 The activation energy for the structural transformation of ZnAl-Eu(EDTA) LDH was calculated to be in the range 26-30 kJ mol-1 (see S2 in the Supporting Information). These values are larger than that reported for the reorientation of selenate, a much smaller and more symmetrical anion, intercalated in LDHs (12 kJ mol-1) but are still in the range expected for processes controlled by hydrogen bonding.50 3.5. Analysis of Luminescence Behavior. When Eu(III) is excited from its 7F0 ground-state to higher states such as 5L6 it undergoes radiationless decay to the 5D0 state, from which luminescence to the 7FJ (J ) 0, 1, 2, 3, 4) levels is observed at room temperature.51 The emission spectra for the ZnAl-Eu(EDTA) LDHs, ZnAl-Eu(NTA)2 LDHs, and the corresponding complex salts are shown in Figure 12. The bands in the emission spectra between 550 and 720 nm arising from transitions between the 5D0 state and the 7FJ manifold are of most interest, because their position, relative intensity, and fine structure vary with the coordination environment of the Eu(III) ion. The relative intensities of the transitions to different J levels depend on the symmetry of the Eu(III) ion environment and can be described in terms of the Judd-Ofelt theory.52,53 The 5D0f7F1 transition retains its magnetic dipole character even in low symmetry sites and its intensity is relatively insensitive to the coordination environment. In contrast, the 5D0f7F2 transition is predominantly electric dipole in nature and its intensity is hypersensitive to the local chemical environment of the Eu(III) ion. Thus the ratio of the intensity of the 5D0f7F2 transition to that of the 5D0f7F1 transition, denoted as Iratio, shows a strong correlation with the local environment of the Eu(III) ion.54 In particular, the lower this ratio, the closer the local symmetry around the Eu(III) is to centrosymmetric. Correspondingly as

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Figure 13. Variation in the 5D0f7F1 magnetic-dipolar transition intensity and the Iratio values for ZnAl-Eu(EDTA) LDHs with increasing temperature (λEx ) 395 nm). Figure 12. Emission spectra for (a) sodium salt of [Eu(EDTA)]-, (b) ZnAl-Eu(EDTA) LDHs, (c) sodium salt of [Eu(NTA)2]3-, (d) ZnAl-Eu(NTA)2 LDHs. The emission spectra of the [Eu(EDTA)]- complex before and after intercalation in the LDHs are essentially identical, with five bands associated with the 5D0f7FJ (J ) 0, 1, 2, 3, 4) transitions, respectively. There is a considerable lowering of the local symmetry of the Eu(III) center in [Eu(NTA)2]3- after intercalation in the interlayer galleries.

the local environment of the Eu(III) sites becomes less symmetric, the Iratio value increases. We therefore can use the luminescence of the Eu(III) complexes incorporated in the LDH hosts as a probe for understanding the structural and dynamic behavior of the guest complex anions with varying temperature. As shown in Figure 12a and 12b, the emission spectra of the [Eu(EDTA)]- complex before and after intercalation in the LDHs are essentially identical, with five bands centered at 579, 592, 617, 650, and 696 nm associated with the 5D0f7FJ (J ) 0, 1, 2, 3, 4) transitions, respectively. In particular, the Iratio value for Zn2Al-Eu(EDTA) LDH is 2.07, which is only slightly different from that (2.12) for the complex salt, indicating that the symmetry and local environment of the Eu(III) center remains unchanged after anion-exchange and that the interaction between the [Eu(EDTA)]- complex anions and the hydroxyl layers does not alter the local symmetry of the corresponding Eu(III) sites. This result is in agreement with the XRD and FTIR analysis. In the emission spectrum of the [Eu(NTA)2]3- complex salt (Figure 12c) the intensity of the 5D0f7F1 magnetic-dipole transition is higher than that of the 5D0f7F2 electric-dipole transition, indicating that the local symmetry involves an approximate inversion center at the Eu(III) site.55 No crystal field splitting of the 5D0f7F1 and 5D0f7F2 bands is observed. The emission spectrum of ZnAl-Eu(NTA)2 LDHs (Figure 12d) is significantly different however: the 5D0f7F1 and 5D0f7F2 bands are clearly split into at least two and three components, respectively, and the intensity of the latter increases relative to that of the former. The Iratio value increases from 0.89 in the pristine [Eu(NTA)2]3- complex salt to 2.85 in the ZnAl-Eu(NTA)2 LDHs. These changes suggest that there is a considerable lowering of the local symmetry of the Eu(III) center after intercalation in the interlayer galleries, where distortion of the complex anion presumably leads to optimization of the interaction between the anions and the host layers.

Figure 14. Variation in the 5D0f7F1 magnetic-dipolar transition intensity and the Iratio values for ZnAl-Eu(NTA)2 LDHs with increasing temperature (λEx ) 395 nm). On calcination of ZnAl-Eu(EDTA) LDH, the value of the ratio of the intensities of the 5D0f7F2 and 5D0f7F1 emission bands increases, indicating a reduction in the local symmetry around the Eu(III). In the case of ZnAl-Eu(NTA)2, there is no obvious change in the value of this ratio in the temperature range 60-180 °C, indicating that the local symmetry of the Eu(III) ion remains unchanged.

Emission spectra for the thermally treated samples of ZnAl-Eu(EDTA) LDHs and ZnAl-Eu(NTA)2 LDHs were obtained immediately after heating the samples at the designated temperature for 30 min. The variation in the intensities of 5 D0f7F1 electric dipole transition and Iratio values in the emission spectra for ZnAl-Eu(EDTA) LDHs and ZnAl-Eu(NTA)2 LDHs heated at different temperatures are illustrated in Figure 13 and Figure 14, respectively. Since the 5D0f7F1 transition does not depend on the chemical environment around the Eu3+ ion due to its magnetic dipole nature, it can be used as a reference to compare luminescence intensities of Eu3+-based materials heated at different temperatures. For both complex anion-intercalated materials, the intensities of the 5D0f7F1 transition emission bands remain almost constant on heating below 150 °C and then decrease dramatically on further heating, becoming very weak at 300 °C. The variation in the value of Iratio in the temperature range 30-180 °C is very different for the two LDHs however. In case of Zn2Al-Eu(EDTA) LDH, the marked increase of Iratio value with temperature indicates

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that there is a considerable reduction in symmetry around the Eu(III) center. This result suggests that the reorientation of the complex anions within the interlayer galleries, as shown by the in situ XRD results discussed above, is accompanied by a distortion of the local geometry around the Eu(III); this may arise in order to maximize the interaction of the anions with the host layers in the new orientation. In contrast, the Iratio value for ZnAl-Eu(NTA)2 LDHs (Figure 14) remains almost constant, suggesting that there is little change in the local symmetry of the Eu(III) cation. This is consistent with the in situ XRD results, which indicate that there is no reorientation of the anion within the interlayer galleries in this temperature range. The rapid decrease in the Iratio value for the both complex anion-intercalated LDHs above 180 °C may be rationalized in terms of liberation of complex anions from the interlayer domain resulting from the destruction of lamellar structure as shown by in situ XRD analysis. 4. Conclusion Organic-inorganic LDHs which have characteristic luminescence properties of Eu(III) have been prepared by an ionexchange method. The arrangement and loading of guest molecules, the thermal stability and dynamic behavior of the interlayer complex anions, and the luminescence properties of the hybrid LDHs are significantly influenced by the charge density of the interlayer anions. The monovalent [Eu(EDTA)]- complex anion is accommodated in the interlayer galleries with minimal deformation of its capped square antiprismatic geometry. The maximal dimension of the guest complex anion is nearly perpendicular to the hydroxyl layers, with the cointercalation of small anions (NO3- or CO32-) to balance the charge density on the layers. In contrast, the trivalent [Eu(NTA)2]3- anion adopts a flat orientation in the interlayer galleries with its maximum dimension nearly parallel to the hydroxyl layers. The structure of the guest complex anion [Eu(NTA)2]3- is significantly distorted in order to maximize the host-guest interactions with the hydroxyl layers. The thermal stabilities of the complex anion-intercalated LDHs are enhanced compared with that of the LDHs nitrate precursor, with the ZnAl-Eu(NTA)2 LDHs having higher thermal stability due to the stronger hydrogen bonding and other electrostatic interactions. A reorientation of the complex anion is observed in the case of ZnAl-Eu(EDTA) LDHs on heating from 30 to 150 °C, which leads to a significant contraction (2.6 Å) in basal spacing. A concomitant reduction in the local symmetry around the Eu(III) ion is observed. The reversible nature of the transformation is indicative of a reorientation taking place upon heating rather than a grafting process. The structural transformation mechanism can be described by a second-order nucleation and nuclei growth law with Ea ) 26-30 kJ mol-1. In the case of [Eu(NTA)2]3-, the interlayer complex anion preserves its flat orientation on heating, and there is no obvious change in the local symmetry of the Eu(III) ion in the temperature range 30-150 °C. Acknowledgment This work was supported by the National Nature Science Foundation of China, the Major International Joint Research Program, the 111 Project (No. B07004) and Changjiang Scholars and Innovative Research Team in Universities (IRT 0406). Supporting Information Available: The molecular structures of the [Eu(EDTA)]- and [Eu(NTA)2]3- complexes and the

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ReceiVed for reView March 1, 2008 ReVised manuscript receiVed November 29, 2008 Accepted November 30, 2008 IE800342U