Suppression of the Reversible Thermal Behavior of the Layered

The layered double hydroxide of Mg with Al decomposes below 600 °C with the loss of ... in the formation of an oxide residue having the rock salt str...
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Langmuir 2007, 23, 7700-7706

Suppression of the Reversible Thermal Behavior of the Layered Double Hydroxide (LDH) of Mg with Al: Stabilization of Nanoparticulate Oxides A. V. Radha,† P. Vishnu Kamath,*,† N. Ravishankar,‡ and C. Shivakumara§ Department of Chemistry, Central College, Bangalore UniVersity, Bangalore 560 001, India, and Materials Research Centre and Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India ReceiVed NoVember 9, 2006. In Final Form: January 20, 2007 The layered double hydroxide of Mg with Al decomposes below 600 °C with the loss of nearly 48% mass, resulting in the formation of an oxide residue having the rock salt structure and nanoparticulate morphology. However, this product reconstructs back into the parent LDH, owing to its compositional and morphological metastability. The oxide can be kinetically stabilized within an amorphous phosphate network built up through an ex situ reaction with a suitable phosphate source such as (NH4)H2PO4. This oxide transforms into a thermodynamically more stable phase with a spinel structure on soaking in an aqueous medium. The oxide residue has a nanoparticulate morphology as revealed by the Scherrer broadening of the Bragg reflections as well as by electron microscopy. This work shows that the hydroxide reconstruction reaction and spinel formation are competing reactions. Suppression of the former catalyzes spinel formation as the excess free energy of the metastable oxide residue is unlocked to promote the diffusion of Mg2+ ions from octahedral to tetrahedral sites, which is the essential precondition to the formation of a normal spinel. This reaction taking place as it does at ambient temperature and in solution helps in the retention of a nanostructured morphology for the spinel. Another way of stabilizing the oxide is by incorporating the thermally stable borate anion into the LDH. This paves the way for an in situ reaction between the cations of the host LDH and the borate guest. The in situ reaction directly leads to the formation of an oxide with a spinel structure.

Introduction Figlarz classified metastability into three types: (i) topological metastability of microporous materials, (ii) structural metastability as in polymorphism, and (iii) morphological metastability of nanoparticulate materials.1 Of these, the last has gained tremendous importance on account of the observation of particle size-dependent physical and chemical properties among nanoparticulate materials.2 There is therefore considerable interest in the synthesis of nanomaterials, especially oxides.3 These syntheses are typically carried out at low temperatures to prevent sintering and consequent particle growth4 or in the presence of additives, which behave as capping agents to prevent agglomeration and crystal growth beyond nanometer dimensions.5 Metal hydroxides are conventionally used as precursors for the low-temperature synthesis of metal oxides,6 and mixed metal hydroxides decompose to yield ternary oxides.7 Among the metal hydroxides, layered double hydroxides (LDHs) comprising divalent and trivalent cations, crystallizing in a layered structure derived from that of mineral brucite, are * To whom correspondence should be addressed. E-mail: [email protected]. Tel: 91-80-22961354. † Bangalore University. ‡ Materials Research Centre, Indian Institute of Science. § Solid State and Structural Chemistry Unit, Indian Institute of Science. (1) Figlarz, M. Prog. Solid State Chem. 1989, 19, 1. (2) Cushing, B. L.; Kolesnichenko, V. L.; O’ Connor, C. J. Chem. ReV. 2004, 104, 3893. (3) Rockenberger, J.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 1999, 121, 11595. (4) Shafi, K. V. P. M.; Ulman, A.; Yan, X.; Yang, N.; Estournes, C.; White, H.; Rafailovich, M. Langmuir 2001, 17, 5093. (5) Thimmaiah, S.; Rajamathi, M.; Singh, N.; Bera, P.; Meldrum, F.; Chandrasekhar, N.; Seshadri, R. J. Mater. Chem. 2001, 11, 3215. (6) Rao, C. N. R.; Gopalakrishnan, J. Acc. Chem. Res. 1987, 20, 20. (7) Vidyasagar, K.; Gopalakrishnan, J.; Rao, C. N. R. J. Solid State Chem. 1985, 58, 29.

versatile precursors. Brucite, Mg(OH)2, comprises a hexagonal close packing of hydroxyl ions in which alternative layers of octahedral sites are occupied by Mg2+ ions, resulting in a stacking of charge-neutral layers having the composition [Mg(OH)2].8 When a fraction, x (0.2 e x e 0.33), of the Mg2+ ions are isomorphously substituted by a trivalent ion such as Al3+, the hydroxide layers acquire the composition [Mg1 - xAlx(OH)2]x+ and develop a positive charge. The layers incorporate anions, An-, and water molecules for charge neutrality and stability to yield an LDH of the composition [Mg1 - xAlx(OH)2](An-)x/n‚yH2O. The mineral form of this LDH having carbonate ions and x ) 0.25 is called hydrotalcite (HT) and has the formula Mg6Al2(OH)16CO3‚4H2O.9 HT has been extensively used as a precursor for oxide catalysts, and its thermal behavior has been widely studied by a number of techniques and recently reviewed by Rives.10 HT undergoes a two-step mass loss during thermal decomposition. The low (250 °C) temperature mass loss is due to the loss of intercalated water whereas the high-temperature (450 °C) loss is due to the combined decarboxylationdehydroxylation of the layers.11 Furthermore, the total mass loss is nearly 48% of the original hydroxide. The loss of such a large mass as a volatile gas profoundly affects the microstructure of the oxide residue, rendering it nanoparticulate. In an earlier paper,12 we examined the residue by transmission electron microscopy (TEM) and found it to be composed of particles in (8) Oswald, H. R.; Asper, R. In Preparation and Crystal Growth of Materials with Layered Structures; Lieth, R. M. A., Ed.; D. Reidel Publishing Company: Dordrecht, The Netherlands, 1977; Vol. 1, p 71. (9) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173. (10) Rives, V. Mater. Chem. Phys. 2002, 75. 19. (11) Miyata, S. Clays Clay Miner. 1983, 31, 305. (12) Rajamathi, M.; Nataraja, G. D.; Ananthamurthy, S.; Kamath, P. V. J. Mater. Chem. 2000, 10, 2754.

10.1021/la063276e CCC: $37.00 © 2007 American Chemical Society Published on Web 06/09/2007

Stabilization of Nanoparticulate Oxides

the size range of 850 °C) Phase b formed at the end of step II is thermodynamically unstable on account of the large number of cation vacancies. Hence, this metastable phase tries to achieve stability either by forming a MgAl2O4 spinel or by reverting back to the parent LDH structure. MgAl2O4 is a normal spinel in which Mg2+ ions occupy tetrahedral sites. In metastable phase b having the rock salt structure, all of (28) Sato, T.; Kato, K.; Endo, T.; Shimada, M. React. Solids 1986, 2, 253.

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Figure 2. Thermogravimetric data of (a) Mg-Al-NO3- LDH and (b) its mixture with (NH4)H2PO4. The inset show the temperature program used in the TGA studies.

the Mg2+ ions are in octahedral sites. Hence, the formation of MgAl2O4 spinel from this phase requires the movement of the Mg2+ ions from octahedral sites to tetrahedral sites. But Mg2+ has a small positive tetrahedral energy preference in the spinel structure. Also, the enthalpy difference calculated for the diffusion of Mg2+ ions from six-coordinate MgO structure to fourcoordinate MgO with ZnO structure is found to be positive.29 Hence, the MgAl2O4 spinel formation from phase b requires activation energy and takes place only at high temperature above 850 °C. In the absence of this activation energy, metastable phase b reverts back to the parent LDH structure. In such a situation, there are two possible ways by which the reversible behavior can be suppressed. (1) Decomposed metastable phase b is reported to be highly reactive because of its nanoparticulate nature. Such highly reactive (29) Navrotsky, A.; Kleppa, O. J. J. Inorg. Nucl. Chem. 1967, 29, 2701.

Table 1. TG Results of Mg-Al-NO3- LDH and Its Mixture with (NH4)H2PO4 Mg-Al-NO3- LDH

Mg-Al-NO3- LDH + (NH4)H2PO4

mass loss

observed mass %

expected mass %

observed mass %

expected mass %

I step (30-100 °C) II step (100-300 °C) III step (300-800 °C) total

6.4 7.9 37.7 52.0

13.1a

18.7 9.8 26.1 54.6

19.9 8.9 20.8b 47.8

39.1 52.3

a Expected weight loss for the complete dehydration step of LDH (30 to 300 °C). b Including the loss of excess (NH4)H2PO4.

particles can be stabilized by using suitable capping agents during the decomposition of LDHs. (2) The formation of any other thermodynamically stable phase may be facilitated immediately after the decomposition of the LDH. This can be realized by introducing a nonvolatile anion,

Stabilization of Nanoparticulate Oxides

Figure 3. Powder X-ray diffractions patterns of the product of the solid-state reaction between (NH4)H2PO4 and the Mg-Al-NO3LDH obtained (a) immediately after isothermal heat treatment at 600 °C and (b) after soaking the residue in part a in water.

which is capable of forming a thermodynamically stable phase either with Mg2+ or with Al3+ during the decomposition of LDH. Of these methods, we have employed the latter. We used nonvolatile anions such as PO43- and borates for our studies because they are stable at high temperature and can form stable compounds with Mg2+ and Al3+. Depending on the mode of introduction of these anions with Mg-Al LDHs, the decomposition reaction can proceed in two ways. Ex Situ Reaction. The external introduction of PO43- by mixing (NH4)H2PO4 with Mg-Al-NO3- HT can lead to an ex situ reaction. On decomposition, the LDH present in the mixture loses NO3- to form a phase b oxide. The latter is expected to react with PO43- formed by the decomposition of (NH4)H2PO4 at high temperature, leading to the formation of a stable phase as in a typical solid-state reaction. In Situ Reaction. Nonvolatile anions such as borate can also be intercalated into the Mg-Al LDH lattice itself. The

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decomposition of LDH here is expected to involve a host-guest reaction between the lattice cations and the intercalated anion, leading to the formation of a stable phase. We examined both mechanisms. Figure 1 encapsulates the results of the control experiment. In Figure 1a is shown the PXRD pattern of the as-prepared MgAl-NO3- LDH. It is indexed to a hexagonal cell with a ) 3.034 Å and c ) 26.4 Å. The first reflection at 8.8 Å in the PXRD pattern and a sharp absorption band at 1370 cm-1 in the IR spectrum (data not shown) confirm the presence of intercalated nitrate ions.30 Figure 1b shows the PXRD pattern of the sample after decomposition at 750 °C. The absence of basal reflections in the PXRD and the disappearance of nitrate-related vibrations in the IR spectrum show that the layered structure of the hydroxide has broken down. Broad peaks corresponding to the reflections of MgO are seen in Figure 1b. Figure 1c shows the PXRD pattern of the sample obtained by soaking the oxide residue in water. The reappearance of the basal reflection at a low angle (7.7 Å) in the PXRD pattern and a characteristic absorption band at 1383 cm-1 in the IR spectrum of this sample indicate the reconstruction of the Mg-Al LDH. These results show that Mg-Al-NO3LDH exhibits reversible thermal behavior at 750 °C. These observations are in keeping with our own earlier results.12 To suppress this reversible behavior, an ex situ reaction was carried out by heating the Mg-Al-NO3- LDH with a slight stoichiometric excess of (NH4)H2PO4. In Figure 2 are shown TG data of Mg-Al-NO3- and its mixture with (NH4)H2PO4. The pure LDH exhibits a three-step mass loss; the first step (up to 100 °C) is due to the loss of adsorbed water, the second step corresponds to dehydration (100-300 °C), and the third step (300700 °C) corresponds to dehydroxylation of the layers as well as the loss of NO3- ions. The observed and expected mass losses are given in Table 1. From the TG data, it is evident that the mass loss is complete at 700 °C for both Mg-Al-NO3- LDH as well as its mixture with (NH4)H2PO4. In the presence of the phosphate salt, the TG shows 18.7% mass loss below 100 °C, in contrast

Figure 4. IR spectra of the product of the solid-state reaction between (NH4)H2PO4 and the Mg-Al-NO3- LDH obtained (a) immediately after isothermal treatment at 600 °C and (b) after soaking the residue in part a in water.

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Figure 5. Powder X-ray diffractions pattern of Mg3(PO4)2 obtained by the solid-state reaction between (NH4)H2PO4 and the Mg-AlNO3- LDH at 850 °C.

to 6.5% observed in the pure LDH. In the presence of the phosphate salt, the distinction between the steps above 100 °C is also lost, and a continuous mass loss is observed up to 700 °C. The TG data of pure (NH4)H2PO4 does not show any perceptible mass loss up to 250 °C. These results clearly indicate an aggressive metathesis reaction between the LDH and (NH4)H2PO4 below 100 °C. Further mass loss observed below 100 °C corresponds to the formation of the Mg-Al-H2PO4- LDH. The metathesis reaction between the LDH and (NH4)H2PO4 can be represented as (mass loss, expected: 19.9%, observed: 18.7%)

[Mg2Al(OH)6]NO3‚2H2O + (NH4)H2PO4 f [Mg2Al(OH)6]H2PO4‚2H2O + NH4NO3v The mass loss observed above 100 °C corresponds to the dehydration of the phosphate-containing LDH (expected, 8.9%; observed up to 275 °C, 9.8%) and decomposition. The calculated and observed mass loss in the 300-700 °C range do not match, even after accounting for the excess ammonium phosphate taken for the reaction. The observed excess mass loss of 5.3% remains unexplained but nevertheless indicates the complete decomposition of the LDH. Figures 3 and 4 show the PXRD patterns and the IR spectra of the product of the solid-state reaction between (NH4)H2PO4 and Mg-Al-NO3- LDH obtained at 600 °C. The absence of low-angle reflections in the PXRD pattern (Figure 3a) indicates the collapse of the LDH structure. The PXRD pattern of this sample shows broad reflections corresponding to the MgO phase along with a hump in the 15-40° 2θ region. Furthermore, the IR spectrum of this sample shows strong absorption at 1072 cm-1 due to P-O stretching, indicating the presence of PO43ions. The oxide residue was soaked in water to determine if the LDH was reconstructed. Figures 3b and 4b show the PXRD pattern and the IR spectrum of the sample obtained after soaking the oxide residue in water. Unlike in the case of the pure MgAl-NO3- LDH, in the present instance the LDH structure is not regenerated. The oxide residue retains its long-range structure as well as its short-range coordination as revealed by the IR spectrum. From these results, it is evident that the ex situ introduction of phosphate ions inhibits the reversible thermal behavior of Mg-Al LDH by stabilizing oxide phase b. One possible mechanism of stabilization is the formation of thermodynamically stable amorphous phosphates of any one or both (30) Meng, W.; Li, F.; Evans, D. G.; Duan, X. Mater. Res. Bull. 2004, 39, 1185.

Figure 6. Bright-field image and selected-area diffraction pattern (inset) of (a) the defect rock salt phase obtained by the ex situ thermal decomposition of HT and (b) after soaking the sample in part a in water. The inset (to the left) is a dark-field image showing fine spinel particles on the order of 10 nm.

of the cations. To confirm this, we carried out isothermal heating of the mixture at 850 °C to facilitate crystallization and recorded the PXRD pattern of the product (Figure 5). This shows sharp reflections that could be indexed to the monoclinic Mg3(PO4)2 phase (PDF-33-876). Because in Mg3(PO4)2 the Mg2+ ions are present in octahedral coordination, it needs less activation energy for formation because it requires little rearrangement of its coordination shell. Hence, the Mg3(PO4)2 phase forms prior to MgAl2O4 spinel formation at 850 °C. The kinetic stability of metastable phase b could be attributed to the presence of amorphous magnesium phosphate by the ex situ reaction. The PXRD patterns of the oxide residues (Figure 3) obtained from the ex situ reaction exhibit considerable broadening of the Bragg reflections related to crystallite size effects. By applying the Scherrer formula, we estimate the crystallite size of the MgO

Stabilization of Nanoparticulate Oxides

Figure 7. Powder X-ray diffractions patterns of (a) the Mg-Alborate LDH, (b) immediately after decomposition at 750 °C, and (c) after soaking the sample in part b in water.

product of the ex situ reaction to be approximately 3 nm. This does not change on soaking in water. Assuming that the product of the ex situ reaction (Figure 3a) is metastable, two possible kinetic pathways to achieve stability on soaking in water can be envisaged. (1) It can revert back to the LDH structure as in the control experiment (Figure 1) or (2) It can transform into the spinel. The high solubility of the oxides of Mg can actually facilitate this transformation by a dissolution-reprecipitation mechanism.31 Figure 3b clearly discounts the first possibility because the low-angle reflections that are typical of the basal distances of the layered structure are not restored. No conclusion regarding the formation of the spinel or otherwise can be drawn from Figure 3b on account of (i) the very large Scherrer broadening of the peaks, (ii) the overlapping of the lines due to the periclase and spinel phases, and (iii) the masking of the 111 reflection of the spinel (19° 2θ; 4.66 Å) by the broad feature in Figure 3b.

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To resolve these difficulties, TEM studies were carried out on freshly decomposed and soaked samples (Figure 6). Figure 6a shows a representative bright-field transmission electron micrograph of the oxide residue obtained by the ex situ reaction. This sample exhibits a uniform distribution of particles in the 3-5 nm size range with agglomerated spherical morphology. This particle size is found to be consistent with that obtained from the Scherrer broadening observed in Figure 3a. The selected-area diffraction pattern (inset of Figure 6a) of this sample shows two reflections that correspond to the 200 and 220 reflections of MgO, confirming the formation of a defect rock salt oxide residue on the decomposition of the LDH. Figure 6b shows a bright-field image of the same oxide residue after soaking in water. There are significant changes in the morphology as well as the structure. The sample exhibits porous morphology. The dark-field image of this sample (top left inset of Figure 6b) clearly indicates that the walls of the porous structure are embedded with nanoparticles having a size on the order of 10 nm. The selected-area diffraction pattern (bottom left inset of Figure 6b) of these particles shows many reflections indicating a change in the structure of the oxide residue. These reflections could be indexed to those of the MgAl2O4 spinel (220, 311, 400, 511, and 440). This work shows that the hydroxide reconstruction reaction and spinel formation are competing reactions. Suppression of the former catalyzes spinel formation as the excess free energy of the metastable oxide residue is unlocked to promote the diffusion of Mg2+ from octahedral to tetrahedral sites, which is the essential precondition to the formation of a normal spinel. This reaction taking place as it does at ambient temperature and in solution helps in the retention of the nanostructured morphology for the spinel. More elegant than the ex situ reaction is to intercalate a nonvolatile reactive anion into the LDH. Because phosphate intercalation into the LDH is not unequivocally established,31 we intercalated the borate anion to investigate its effect on the thermal decomposition of LDHs.

Figure 8. IR spectra of (a) the Mg-Al-borate LDH, (b) immediately after decomposition at 750 °C, and (c) after soaking the sample in part b in water.

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Figures 7a and 8a show the PXRD pattern and IR spectrum of the borate-intercalated LDH. The basal reflections have shifted to lower angles (10.7 Å) indicative of the incorporation of the tetraborate [B4O5(OH)4]2- anion.32 Furthermore, three basal reflections are observed in keeping with the literature.32 The IR spectrum of this sample (Figure 8a) confirms the presence of intercalated borate species. TG data (not shown) indicate that borate-LDH decomposes completely at 750 °C. The PXRD pattern of the resultant oxide residue is shown in Figure 7b. This oxide was also soaked in water, and the PXRD pattern of the product after soaking is shown in Figure 7c. The IR spectra recorded before and after soaking in water are given in Figure 8b,c. It is clear that the layered structure breaks down on thermal decomposition and the oxide residue is stable upon soaking in water and does not reconstruct the original LDH. The PXRD pattern of the residue corresponds to an oxide of the spinel structure. The intercalated borate suppresses the reconstruction by catalyzing the formation of the spinel. The spinel obtained by the in situ reaction is anisotropic with a crystallite size of 4 nm along the a crystallographic axis and (31) Radha, A. V.; Kamath P. V.; Shivakumara, C. Solid State Sci. 2005, 7, 1180. (32) del Arco, M.; Gutierrez, S.; Martin, C.; Rives, V.; Rocha, J. J. Solid State Chem. 2000, 151, 272.

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2.5 nm along the [311]. However, no independent verification of the crystallite size by electron microscopy could be carried out because the oxide residues suffer extensive beam damage in the microscope.

Conclusions Spinel formation and the reversible reconstruction of the LDH are two competing kinetic pathways available to the metastable oxide residues obtained from the thermal decomposition of LDHs. Whereas the Mg-Al LDHs generally exhibit reversible thermal behavior, this can be suppressed in the presence of suitable reactive interlayer species. The suppression of reversible behavior induces spinel formation even at ambient temperatures, whereas under other conditions spinel formation is observed only above 1100 °C. Acknowledgment. P.V.K. thanks the Department of Science and Technology, Government of India, (GOI) for financial support. A.V.R. thanks the University Grants Commission, GOI, for financial support as a Senior Research Fellow (NET). LA063276E