Crystal Engineering of Nanomorphology for Complex Oxide Materials

ARTICLE pubs.acs.org/crystal

Crystal Engineering of Nanomorphology for Complex Oxide Materials via Thermal Decomposition of Metal-Organic Frameworks. Case Study of Sodium Tantalate Giovana G. Nunes,†,‡ Gulaim A. Seisenbaeva,† and Vadim G. Kessler*,† † ‡

Department of Chemistry, SLU, Box 7015, 75007 Uppsala, Sweden , Departamento de Química, Centro Politecnico - Jd. das Americas, Universidade Federal do Parana Curitiba - PR. CEP 81531-990, Brazil

bS Supporting Information ABSTRACT: Application of different solvating ligands drastically changes the composition and geometry of bimetallic sodium-tantalum pinacolates, forming insoluble and stable organic-inorganic hybrid materials through Van der Waals interactions. The loss of these neutral ligands on heating leads to contraction and densification of the structures, with formation of covalent metal-organic frameworks (MOFs) as intermediates, and resulting in direction-specific cracking and formation, on further thermal decomposition, of low-dimensional nanomorphologies—wires or plates of the same oxide material, containing well-crystalline monoclinic NaTaO3 as its major component. The crystallographic features of the precursor structures permitting us to control further morphological differentiation on the transformation to oxide material are identified.

’ INTRODUCTION Metal-Organic Frameworks (MOFs)1 have attracted major attention of researchers from different fields of chemistry during recent years because of their aesthetic beauty and tremendously broad spectrum of applications, ranging from hydrogen storage2 and gas separation3 to heterogeneous catalysis4 and luminescence5 and supramolecular magnetism.6 An emerging and barely explored opportunity of their use lies in the synthesis of nanomaterials with high surface and specific morphology via thermal decomposition of MOFs as (supra)molecular precursors. The few reported examples of material obtained through thermolysis of metal carboxylates feature “anemone-like” porous random 3D structures of interconnected spherical nanoparticles of copper metal7 or zinc oxide,8 respectively. In our work, we sought a possibility to exploit thermal decomposition of MOF precursors, insoluble metal alkoxide complexes, which are stable in ambient atmosphere. This approach is promising for creation of nanostructures because nucleation of the new phase can be driven as a solid state reaction, leading to creation of (nano)porous structure through combustion and removal of organic ligands.9 As the target oxide system we chose sodium tantalate, NaTaO3, a material highly attractive as visible light water splitting catalyst10 and a catalyst of methane conversion to hydrogen and carbon dioxide.11 New approaches to the chemically and phase-pure nanostructured NaTaO3 have recently become an object of intensive studies. The application of such techniques as solid state synthesis,12 coprecipitation,13 hydrothermal synthesis,14 and sol-gel technology15 has been reported so far. The nanofiber morphology with polycrystal r 2011 American Chemical Society

Table 1. Details of Data Collection and Structure Refinement for Compounds 1 and 2 1

2

chemical composition

C60H144O32Na4Ta4

formula weight

2193.51

816.72

crystal system space group

triclinic P1

monoclinic C2/m

μ (mm-1)

5.009

3.824

a (Å)

11.579(2)

17.394(12)

b (Å)

12.429(3)

10.049(7)

c (Å)

16.887(4)

8.404(6)

R (deg)

98.589(4)

90

β (deg)

92.109(4)

90.180(8)

γ (deg) V (Å3)

111.937(3) 2217.7(8)

90 1468.9(17)

T (K)

295(2)

295(2)

Z

1

2

C30H60O12NaTa

no. of independent reflns 3643 [R(int) = 0.007] 1489 [R(int) = 0.0210] no. of obsd reflns

3468 [I > 2σ(I)]

R1

0.0537

1475 [I > 2σ(I)] 0.0306

wR2

0.1626

0.0767

aggregates 50-100 nm in diameter and the length in micrometer range appears to be the most promising.15b,16 Received: November 2, 2010 Revised: January 5, 2011 Published: February 10, 2011 1238

dx.doi.org/10.1021/cg101456w | Cryst. Growth Des. 2011, 11, 1238–1243

Crystal Growth & Design

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Figure 1. The molecular and crystal structure of Na4Ta4O4(Pin)8(MeOH)12 (1).

No applicable Na-Ta precursor has been reported in literature. The bimetallic alifatic alkoxide complexes NaTa(OR)6, R = Me, Et, iPr, tBu, are highly soluble in organic solvents, easily melt and are strongly moisture sensitive.17 An attractive approach to insoluble and moisture insensitive materials is application of bidentate aryloxide ligands as it has been demonstrated earlier for Ti-derivatives by Wolczansky et al.18 The recently characterized homometallic tantalum pinacolates revealed, however, extremely high solubility and moisture sensitivity, resulting apparently from chelating mode in attachment of the [OCMe2CMe2O]2ligands.19 Introduction of bulky sodium cations had then a prospect to interconnect the alkoxo-tantalate units and lead to denser packing and stronger interactions, having a chance to produce insoluble and stable material. Application of a bulky and hydrophobic pinacolate ligand remained preferable as the hydrolysis of metal alkoxides occurs as a proton-assisted SN1 mechanism and hydrophobicity had to hinder the diffusion of protons or water.20

’ EXPERIMENTAL SECTION Materials and Methods. Synthesis of the precursor materials has been carried out in the dry nitrogen atmosphere in a drybox, while all subsequent separation and treatment procedures were implemented under ambient conditions. All the chemicals, tantalum methoxide, Ta(OCH3)5, tantalum ethoxide, Ta(OC2H5)5, sodium metal, pinacol, 18-crown-6, methanol, and toluene were received from Aldrich and used without further purification. FTIR spectra were recorded for pellets with KBr using Perkin-Elmer Spectrum 100 instrument. TGA-FTIR studies were made with Perkin-Elmer Pyris 1 instrument coupled to Spectrum 100 FTIR. X-ray studies for both single crystals and powders were carried out with multipurpose Bruker SMART Apex-II diffractometer at 25 °C using MoKR radiation, λ = 0.7093 Å. SEM-EDS characterization was carried out using Hitachi TM-1000-μDeX environmental tabletop

Figure 2. TGA curve for thermal decomposition of (a) 1 and (b) the resulting nanorods. 1239

dx.doi.org/10.1021/cg101456w |Cryst. Growth Des. 2011, 11, 1238–1243


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Figure 3. (a) Molecular structure of Na(18-crown-6)Ta(Pin)3 (2), displaying the disordered anion, (b) one of the possible configurations of the [Ta(Pin)3]- anion, (c) packing motif, resulting in layered structure arranged in 110 direction displayed along c-axis, (d) packing motif in the 110 plane. electron microscope, without coating of the samples in a charge-up reduction mode. The microanalysis data were obtained by MicroKemi AB, Uppsala, Sweden. Na4Ta4O4(Pin)8(MeOH)12 (1). In a typical procedure 0.75 g (2.2 mmol) of Ta(OCH3)5 and 0.05 g (2.2 mmol) of Na were dissolved in a mixture of 5 mL of toluene and 3 mL o CH3OH. A solution of 0.78 g (6.6 mmol) of pinacol, H2O2C6H12, in a mixture of the same composition was added to the initial solution. The produced clear solution was subjected to a short reflux (15 min) that resulted in massive precipitation of a white powder that was then isolated by filtration and dried in a vacuum. The yield was 0.65 g (54%). Found, %: C, 32.6; H, 6.4. Calcd for C60H144Na4O32Ta4, %: C, 32.9; H, 6.6. IR, cm-1: 3420 s br, 2980 s, 2932 s, 2876 sh, 1653 m br, 1461 m, 1444 m, 1383 s, 1370 m, 1360 s, 1251 w, 1231 vw, 1200 m, 1147 vs, 1081 w, 1061 vw, 1001 m, 974 s, 962 s, 891 s, 730 w, 699 s, 655 s, 607 s, 587 s, 531 s, 514 w, 491 m, 442 m. Single crystals of 1 were obtained by freezing the initial alkoxide solution in liquid nitrogen and adding the pinacol solution on top of the frozen material with subsequent melting and crystallization inside a Dewar vessel overnight. Na(18-crown-6)Ta(Pin)3 (2). 18-crown-6 0.5 g (1.9 mmol) and sodium metal (0.044 g, 1.9 mmol) were dissolved in a mixture of 0.7 mL of methanol and 2 mL of toluene. A solution of tantalum methoxide (0.64 g, 1.9 mmol) and pinacol (0.67 g, 5.7 mmol) in 2 mL of toluene and 0.5 mL of methanol was added quickly on vigorous stirring and the solution was frozen in liquid nitrogen with subsequent melting and crystallization in a Dewar vessel overnight. The resulting colorless transparent crystals were isolated by filtration and dried in a vacuum. The yield was 0.962 g (60%). Found, %: C, 43.9; H, 7.3. Calcd for

C30H60NaO12Ta, %: C, 44.1; H, 7.4. IR, cm-1: 2981 s, 2934 s, 1472 w, 1456 vw, 1444 w, 1384 m, 1370 m, 1361 m, 1285 w, 1252 w, 1231 vw, 1203 w, 1149 s, 1108 m, 1001 m, 962 s, 923 w, 885 s, 697 s, 609 m, 588 s, 515 m, 496 m, 439 m. X-ray Crystal Structure Determination. For details of data collection and refinement, please, see table 1. The data collection for 1 could be run only for one series in Omega scan in the view of the quick deterioration of the crystal due, apparently, to the loss of solvating methanol (even when sealed in a glass capillary), which explains the incompleteness of the data. Compound 2 was stable under the beam, when sealed in a glass capillary and a complete hemisphere data set could be collected for θ e 30°. The structures were solved by direct methods and refined by least-squares techniques in anisotropic approximation for all non-hydrogen atoms. The H atoms could be included into refinement of the structure of 1 in calculated position and refined in isotropic approximation, whereas severe disorder in the structure of 2 eliminated the possibility of introduction of the hydrogen atoms.

’ RESULTS AND DISCUSSION The addition of 3 equiv. of H2Pin to a solution of sodium and tantalum methoxides in either hydrocarbon solvent of methanol resulted practically immediately in precipitation with rather high yield of an oxocomplex Na4Ta4O4(Pin)8(MeOH)12 (1), insoluble in organic solvents. The molecules of 1 demonstrate an unprecedented octanuclear metal-oxygen core, which on closer consideration can be regarded as resulting from “fusion” of two dense packing related M4O16-type (tetramolybdate) blocks 1240


Crystal Growth & Design connected to each other via a double Na(μ-MeOH)2Na bridge. The predominantly electrostatic nature of bonding in alkoxide complexes leads generally to close-packed structures distinctly resembling those of oxometallates.21 It is worth noting that the structure of 1 should definitely not be considered as derived from an oxoalkoxide dimer of tantalum pinacolate, having as an individual homometallic compound Ta2O(OCMe2CMe2O)2(HOCMe2CMe2O)4 completely different structure and composition.19 Formation of an oxo-complex with high yield even on rigorous protection against moisture is not surprising for this system (see19 and refs therein) and could even be promoted by Bradley reaction, the ether elimination from metal alkoxides in solution, that even was first described for Nb(V) and Ta(V) complexes with bulky ligands22 and is known to be facilitated in the basic medium.23 The crystal structure of 1 is composed of these distinct bimetallic molecules (Figure 1) with elongated shape, forming linear chain-like packing along the volume diagonal of the triclinic cell. The chains are shifted in relation to each other by a half of the length of the diagonal. The space between molecules in the packing is due to the presence of solvating alcohol molecules. The latter are partially removed on drying or on heating, which is associated with the loss of rather exactly 10% of the mass of the sample. This is corresponding rather exactly to release of 7 of the total of 12 solvating molecules (Figure 2a). Such loss of solvating alcohol molecules should result in contraction of the structure in all 3 directions, but most strongly in the plane perpendicular to the direction of the packing (along this direction the contraction will apparently be stopped by formation of Na(μ-MeOH)2Na bridges, analogous to those observed in the central part of each molecule and leading apparently to a covalent metal-organic framework as an intermediate). This process is possible to follow on electron microscopic observation by SEM, as the block-shaped crystals distinctly crack in real time into thin nanorods with diameters of about 150 nm and the length of 1 μm and higher (aspect ratio >5) when the crystal is warmed up by the electron beam in vacuum. The product of thermal treatment of 1 (heating with a speed 10 °C/ min to 800 °C) consists of such rather uniform nanorods (Figure 2b). It appeared attractive to design an alternative solid-state structure with contraction possibility in only one dimension, providing on ligand removal and thermal treatment not rods but plates. This required creation of a layered structure with ligands to be situated between the layers. The challenge was thus in protecting of relatively symmetric [Ta(Pin)3]- anions and arranging them together with Naþ cations into a highly symmetric packing. A logical choice of a highly symmetric and planar ligand, strongly coordinating to sodium cations was apparently the 18-crown-6. Introduction of 1 equiv. of the crown-ether into the reaction medium with Na:Ta:Pin = 1:1:3 ratio resulted in clear solution, from which the complex Na(18-crown-6) Ta(Pin)3 (2) crystallized overnight at low temperature with high yields as hexagonally shaped rods. The molecular structure of 2 is a close ion pair with a cation [Na(18-crown-6)]þ, placed with the same probability either below or above the [Ta(Pin)3]- anion. The latter is disordered uniformly between 4 different orientations. The disordered ion pairs with a practically spheroidal shape are forming dense cubic packing (hexagonal layers situated on top of each other) with a pseudo 6-fold axis perpendicular to the layers (Figure 3).

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Figure 4. (a) TGA curve for thermal decomposition of 2, and (b) the resulting nanoplates.

In reality, this high symmetry cannot be ideal as it cannot be followed by alkoxide ligands and the true space group is monoclinic C2/m, which is together with P2/n or P2/c the most typical for pseudo cubically packed low symmetry alkoxide molecules.24,25 Underestimation of this insight has on several occasions hindered refinement of many important disordered structures.26 The layers are situated in the 110 plane of the real crystal, which is its predominant growth direction. On the thermal treatment the crown-ether ligands are removed and the structure is contracting in this single direction with perpendicular cracking into hexagonal nanoplates as a result (Figure 4). Even in this case, a true covalent 1D metal-organic framework has to be an intermediate. The release of neutral ligand occurs via two quite broad steps: first 12% of the initial weight are lost in the interval 64-125 °C and then about 5% more, in the interval 125-205 °C, which means that about half of the neutral ligands are lost until the complete thermal decomposition occurring as one-step process at 400 °C. The X-ray powder diffraction reveals quite unexpectedly not a single phase material, but a mixture composed to 80-85% of monoclinic NaTaO3 (PDF 01-074-2477) and 15-20% of equimolar mixture of Na3TaO4 and Ta2O5 (PDF 00-0270804 and 01-079-1375 respectively). Such material with combination of acidic and basic sites can be of special interest for catalytic applications. The product is a nanocomposite as the determination of the domain size using the Debye-Scherrer formula (see below) gives for the decomposition products of 1241



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Figure 5. Powder diffraction patterns of (a, d) thermal decomposition products of 1 and 2, respectively, heated up at a speed of 10 °C/min to 800 °C; (b, e) thermal decomposition products of 1 and 2 treated additionally at 900 °C for 3 h, (c, f) products of additional heat-treatment at 900 °C for 8 h. • NaTaO3 (PDF 01-074-2477), 9 Na3TaO4 (PDF 00-027-0804), 2 Ta2O5 (PDF 01-079-1375). The insets provide EDS spectra of the materials produced from compound 1 (to the left) and compound 2 (to the right).

both 1 and 2 average crystallite size of about 4 nm determined using the 1 0 0 line of the perovskite phase. Kλ τ¼ βτ cos θ where τ is the mean crystallite dimension, K is the shape factor (standard value 0.9 was applied), λ is the wavelength, and βτ is the line broadening (equal to B - b, B being the breadth of the observed diffraction line at its half-intensity maximum, and b the instrumental broadening). The size of the crystallites undergoes practically no change on the thermal treatment (see Figure 5), but the phase purity of the samples slowly improves, reaching ca. 97% for the NaTaO3 perovskite phase after 8 h at 900 °C. The morphology of the materials remains unchanged according to the SEM observations. This demonstrates possibility to obtain a phase-pure nanostructured sodium tantalate material, applying the proposed MOD approach. In conclusion, we can say that application of diolate structures supported by different solvating ligands appears to be a facile and promising approach to nanostructured oxide materials with controlled dimensionality.

’ ASSOCIATED CONTENT

bS

Supporting Information. X-ray crystallographic information files (CIF); detailed tables of bond distances and angles for compounds 1 and 2 (PDF). This information is available free of charge via the Internet at http://pubs.acs.org/

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ46-(0)18-671541. Fax: þ46-(0)18-673476. E-mail: vadim. [email protected].

’ ACKNOWLEDGMENT The authors express their gratitude to the Swedish Research for support to the project “Molecular Council (Vetenskapsradet) precursors and molecular models of nanoporous materials”. G.G.N. thanks CAPES (Coordenac-~ao de Pessoal de Nível Superior-Brazil) for the postdoc grant (Process 0481050). ’ REFERENCES (1) (a) Tranchemontagne, D. J. L.; Ni, Z.; O’Keeffe, M.; Yaghi, O. M. Angew. Chem. 2008, 47, 5136–5147. (b) Ferey, G. Chem. Soc. Rev. 2008, 37, 191–214. (c) Horike, S.; Shimomura, S.; Kitagawa, S. Nature Chem. 2009, 1, 695–704. (2) (a) Hu, Y. H.; Zhang, L. Adv. Mater. 2010, 22, E117–E130. (b) Yang, J.; Sudik, A.; Wolverton, C.; Siegel, D. J. Chem. Soc. Rev. 2010, 39, 656–675. (3) Phan, A.; Doonan, C. J.; Uribe-Romo, F. J.; Knobler, C. B.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2010, 43, 58–67. (4) Corma, A.; Garcia, H.; Xamena, F. X. L. Chem. Rev. 2010, 110, 4606–4655. (5) (a) Wang, P.; Ma, J. P.; Dong, Y. B. Chem.—Eur. J. 2009, 15, 10432–10445. (b) Allendorf, M. D.; Bauer, C. A.; Bhakta, R. K.; Houk, R. J. T. Chem. Soc. Rev. 2009, 38, 1330–1352. 1242



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Crystal Engineering of Nanomorphology for Complex Oxide Materials

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