Synthesis and characterization of lanthanum oxide nanoparticles from thermolysis of nanostructured supramolecular compound

Synthesis and characterization of lanthanum oxide nanoparticles from thermolysis of nanostructured supramolecular compound

Journal of Molecular Liquids 153 (2010) 129–132 Contents lists available at ScienceDirect Journal of Molecular Liquids j o u r n a l h o m e p a g e...

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Journal of Molecular Liquids 153 (2010) 129–132

Contents lists available at ScienceDirect

Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q

Synthesis and characterization of lanthanum oxide nanoparticles from thermolysis of nanostructured supramolecular compound Somaye Khanjani, Ali Morsali ⁎ Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14155-4838, Tehran, Islamic Republic of Iran

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Article history: Received 17 September 2009 Received in revised form 25 January 2010 Accepted 29 January 2010 Available online 6 February 2010 Keywords: Nanostructure Supramolecular Lanthanum(III) Crystal structure

a b s t r a c t A new nanostructured La(III) supramolecular compound, [La(4,4′-bipy)(H2O)2 (NO3)4]·(4,4′-Hbipy) (1), 4,4′-bipy = 4,4′-bipyridine} was synthesized by sonochemical method. The nanostructure of 1 was investigated using scanning electron microscopy, X-ray powder diffraction (XRD), IR spectroscopy and elemental analyses. The crystal structure of compound 1 was determined by single-crystal X-ray diffraction. The thermal stability of bulk compound and of nanosized particles was studied by thermal gravimetric (TG) and differential thermal analyses (DTA). Nanosized La2O3 was obtained by calcination of nanostructure of compound 1 at 700 °C. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Properties and applications of nanostructure materials have been extensively investigated in the past decades [1–10] and it has been shown that properties of inorganic materials depend deeply on the size and morphologies of the particles of which the compounds are made up from. The supramolecular architectures exhibit potential applications as molecular wires, electrical conductors, and molecular magnets, in host–guest chemistry and in catalysis. Nanosized coordination supramolecular materials are interesting candidates for applications in many fields, including catalysis, molecular adsorption, magnetism, nonlinear optics, luminescence, and molecular sensing, but nanoscale particles of metal–organic coordination supramolecules have rarely been reported. Lanthanum oxide (La2O3) ultrafine powders have a lot of attractive properties for industrial and technological applications. It can be used as an important component in automobile exhaust-gas convectors, as promising catalytic materials, as high κ gate dielectric materials, and as optical filters. Moreover, it is also used as a strengthening agent in structural materials [11]. The sensitivity of gas sensitive ceramics could be improved when La2O3 nanophase was added in the ceramics [12]. Various techniques for the preparation of rare earth oxides of ultrafine powders and nanopowders have been developed, including high energy ball milling, deposition method, hydrothermal process, sol–gel method and spray deposition method [13–22].

⁎ Corresponding author. Tel.: +98 2182884416; fax: +98 2188009730. E-mail address: [email protected] (A. Morsali). 0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2010.01.010

This article focuses on the simple synthetic preparation of nanopowders of a new La(III) supramolecular complex, [La(4,4′bipy)(H2O)2(NO3)4]·(4,4′-Hbipy) (1), 4,4′-bipy = 4,4′-bipyridine}, and on its conversion into nanostructured La2O3 by calcination at 700 °C. 2. Experimental 2.1. Materials and physical techniques All reagents for the synthesis and analysis were commercially available and used as received. Double distilled water was used to prepare aqueous solutions. A multiwave ultrasonic generator (Sonicator_3000; Misonix, Inc., Farmingdale, NY, USA), equipped with a converter/transducer and titanium oscillator (horn), 12.5 mm in diameter, operating at 20 kHz with a maximum power output of 600 W, was used for the ultrasonic irradiation. Microanalyses were carried out using a Heraeus CHN-O-Rapid analyzer. Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected. IR spectra were recorded using Perkin-Elmer 597 and Nicolet 510P spectrophotometers. The thermal behavior was measured with a PL-STA 1500 apparatus. Crystallographic measurements were made using a Bruker APEX area-detector diffractometer. The intensity data were collected using graphite monochromated Mo–Kα radiation. The structure was solved by direct methods and refined by full-matrix least-squares techniques on F2. Structure solution and refinement were accomplished using SHELXL-97 program packages [23]. The molecular structure plot and simulated XRD powder pattern based on single-crystal data were prepared using Mercury software [24]. X-ray powder diffraction (XRD) measurements were performed

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Scheme 1. Materials produced and synthetic methods.

using a Philips diffractometer of X'pert company with monochromated Cu-Kα radiation. The crystallite sizes of selected samples were estimated using the Scherrer method. The samples were characterized with a scanning electron microscope with gold coating. To prepare the nanoparticles of [La(4,4′-bipy)(H2O)2(NO3)4]·(4,4′Hbipy) (1), a proper amount of the solution of La(NO3)3 (10 ml, 0.1 M) in MeOH was positioned in a high-density ultrasonic probe, operating at 20 kHz with a maximum power output of 600 W then into this solution, a proper volume of ligand 4,4′-bipy solution (10 ml, 0.1 M) was added in drops. The obtained precipitates were filtered, subsequently washed with MeOH and then dried (Yield: 0.25 g, 34%), (Found C: 32.72, H: 2.73, N: 15.34: calculated for C20H21LaN8O14; C:32.60, H:2.85, N: 15.21%). IR (cm− 1) bands: 3520(br), 3085(w), 1626(w), 1593(m), 1435(vs), 1382 (vs), 1212(w), 1031(vw), 1022(m), 810(s), 722(w), 691(w), 614(w), 556(w). To isolate the single crystals of [La(4,4′-bipy)(H2O)2(NO3)4]·(4,4′Hbipy) (1), 4,4′-bipy (1 mmol, 0.156 g) La(NO3)3·3H2O (0.5 mmol, 0.159 g) were placed in the main arm of the branched tube. Methanol

was carefully added to fill the arms, the tube was sealed and the ligandcontaining arm immersed in an oil bath at 60 °C while the other arm was kept at ambient temperature. After 2–3 days, yellow crystals (d.pN 300 °C), had deposited in the cooler arm, were isolated, filtered off, washed with acetone and ether and air dried (0.37 g, yield 50%), (Found C: 32.70, H: 2.76, N: 15.46: calculated for C20H21LaN8O14; C:32.60, H:2.85, N: 15.21%). IR (cm− 1) bands: 3503(br), 1626(w), 1589(m), 1435(vs), 1382(vs), 1204(w), 1031(m), 1022(m), 808(s), 726(w), 691(w), 614(m), 560(w).

3. Results and discussion Scheme 1 shows the reaction between La(NO3)3 with 4,4′-bipy by two different methods. Compound 1 was obtained by reaction between 4,4′-bipy and lanthanum(III) nitrate in methanol. Single crystalline material was obtained using a heat gradient applied to a solution of the reagents (the “branched tube method”) and nanostructured particles of compound 1 were prepared by ultrasonication of the methanolic solution. Compound 1 is an air-stable and high-melting solid that is soluble in DMSO. The IR spectra are same for bulk and nanosized compound 1 and show characteristic absorption bands for the 4,4′-bipy ligands and

Table 1 crystal data and structure refinement for [La(4,4′-bipy)(H2O)2(NO3)4]·(4,4′-Hbipy) (1). Identification code Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit cell dimensions

Fig. 1. The XRD patterns of simulated from single-crystal X-ray data of compound [La (4,4′-bipy)(H2O)2(NO3)4]·(4,4′-Hbipy) (1) (bottom) and materials as synthesized of compound [La(4,4′-bipy)(H2O)2(NO3)4]·(4,4′-Hbipy) (1) (top).

Volume Z Density (calculated) Absorption coefficient Crystal size Theta range for data collection Index ranges

Reflections collected Independent reflections Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I N 2σ (I)] R Indices (all data) Fig. 2. SEM images of compound 1 nanopowders.

Largest diff. peak, hole

1 C20H21LaN8O14 736.36 100(2) 0.71073 Orthorhombic P212121 a = 7.2438(5) Å b = 18.4781(12) Å C = 19.8909(13) Å α = 90° β = 90° γ = 90° 2662.4(3)Å3 4 1.837 g cm− 3 1.691 mm− 1 0.33 × 0.28 × 0.14 mm3 2.20 to 31.41° − 10 ≤ h ≤ 10 − 26 ≤ k ≤ 26 −17 ≤ l ≤ 28 8219 7934 ω scan 0.672 and 0.789 Full-matrix Least-squares on F2 8219/4/400 1.035 R1 = 0.0215 wR2 = 0.0452 R1 = 0.0231 wR2 = 0.0464 0.866 and − 0.540 e Å− 3

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Table 2 Bond lengths/Å and angles/° [La(4,4′-bipy)(H2O)2(NO3)4]·(4,4′-Hbipy) (1). 1 La1–O1 La1–O4 La1–O5 La1–O7 La1–O8 La1–O10 La1–N5 O1–La1–O4 O1–La1–O8 O1–La1–O10 O1–La1–O13 O10–La1–O8 O13–La1–O8 O13–La1–O10 O14–La1–O1 O14–La1–O4 O14–La1–O8 O14–La1–O10 O14–La1–O13 O2–La1–N5 O4–La1–N5 O5–La1–N5 O7–La1–N5 O11–La1–N5

2.562(14) 2.653(15) 2.684(14) 2.719(15) 2.628(13) 2.607(14) 2.817(17) 71.64(5) 109.22(5) 79.44(5) 113.10(4) 117.31(5) 137.60(5) 73.87(5) 142.72(5) 73.70(5) 74.43(5) 133.33(5) 70.57(5) 126.70(5) 139.43(5) 128.36(4) 109.66(5) 69.02(5)

nitrate anions. The absorption bands with variable intensity in the frequency range of 1400–1580 cm− 1 correspond to ring vibrations of the py moiety of the 4,4′-bipy ligands. The IR spectrum of compound 1 shows ν(NO3) at ca 1382 cm− 1. Fig. 1 (top) shows the XRD patterns of typical sample of compound 1 prepared by the branched tube and sonochemical process. Fig. 1 (bottom) shows the XRD pattern simulated from the single-crystal X-ray data. Acceptable matches are observed for the compound indicating the presence of only one crystalline phase in the sample prepared using the branched tube and sonochemical process. Estimated from the Scherrer formula, calculation of particle sizes from the broadening of the XRD peaks (D=0.891λ /βcosθ, where D is the average grain size, λ is the X-ray wavelength (0.15405 nm), and θ and β are the diffraction angle and fullwidth at half maximum of an observed peak, respectively [25]) was found to be around 80 nm, which was in agreement with the value obtained from the SEM images (Fig. 2). For further investigation the other solvents such as water, acetonitrile, dichloromethane, benzene and chloroform were used, but particle size of the compound was not in nanoscales. Single-crystal X-ray analysis reveals that compound [La(4,4′-bipy) (H2O)2(NO3)4]·(4,4′-Hbipy) (1) crystallizes in an orthorhombic setting with space group of P212121 (Tables 1 and 2). Structural determination of compound 1 by X-ray crystallography showed the

Fig. 4. A fragment of the three-dimensional network formed by hydrogen bonding in [La(4,4′-bipy)(H2O)2(NO3)4]·(4,4′-Hbipy) (1).

complex in solid state (Fig. 3) consisting of monomeric units. The lanthanum atoms are linked by one nitrogen atom of 4,4′-bipy ligand and ten oxygen atoms of nitrate and water ligands. The coordination number of La atoms can be considered to be eleven with NO10 donor atoms array. One of the nitrogen atoms of the 4,4′-bipy ligand is not coordinated to La atoms (Fig. 3) and indeed this ligand acts as a less commonly unidentate ligand. There is another protonated 4,4′-bipy molecule that was not coordinated to the La atom. The nitrogen atoms from both the coordinated and uncoordinated 4,4′-bipy ligands and oxygen atom of water ligands are involved in hydrogen bonding and consequently, the two perpendicular anionic and cationic components form a three-dimensional network (Fig. 4). There are π–π stacking [26,27] interactions between aromatic rings of 4,4′-bipy ligands belonging to dimeric units in compound 1. The py groups of 4,4′-bipy are almost parallel and this parallel array of the planes of the aromatic moieties indicates that these interactions are of the face-toface π-stacking type [28]. The centroid–centroid distance of py groups is 3.94 Å and the angle between the ring normal and the centroid vectors is 100.56°. To examine the thermal stability of the nanostructure and single crystals of compound 1, thermal gravimetric (TG) and differential thermal analyses (DTA) were carried out between 30 and 610 °C in a static atmosphere of nitrogen (Fig. 5, Bulk). Single crystals of compound 1 are stable and do not decompose up to 147 °C, at which temperature endothermic removal of aqua ligands (observed: 4.12% and calculated: 4.88%) starts. The second step about 205 to 350 °C corresponds to removal of the two organic ligand, 4,4′-bipy and 4,4′-Hbipy+, (observed: 47.2% and calculated: 48.96%). The final

Fig. 3. X-ray crystal structure [La(4,4′-bipy)(H2O)2(NO3)4]·(4,4′-Hbipy) (1) (30% thermal displacement ellipsoids).

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The difference between the two maximum intensities in the DTA curves also shows the lower stability of the nanoparticles when compared with their single-crystal counterparts which is in good agreement with the TGA results. Nanostructures of La2O3 have been generated by thermal decomposition of nanostructured compound 1 at 700 °C (Fig. 6). Powder X-ray diffraction of the residue at 700 °C indicates the formation of La2O3 (Fig. 7c and JCPDS Card No. 05-0602). Fig. 7 shows the SEM images of nanostructured La2O3 obtained from calcination of nanostructured compound 1 under air at 700 °C. 4. Conclusion Fig. 5. TGA diagrams of [La(4,4′-bipy)(H2O)2(NO3)4]·(4,4′-Hbipy) (1) as single crystals (Bulk lines) and [La(4,4′-bipy)(H2O)2(NO3)4]·(4,4′-Hbipy) (1) as nano-size (Nano lines).

Nanoparticles of a new LaIII supramolecular compound, [La(4,4′bipy)(H2O)2(NO3)4]·(4,4′-Hbipy) (1) were synthesized by sonochemical method and characterized by IR, XRD and SEM. The structure of compound 1 was determined by X-ray crystallography. Reduction of the particle size of the supramolecular compound to a few dozen nanometers results in a lower thermal stability when compared to single crystalline samples. Calcinations of compound 1 under 700 °C produce La2O3. This study demonstrates that supramolecular compounds may be suitable precursors for the preparation of nanoscale materials with interesting morphologies and different products. Acknowledgement This work was supported by the Tarbiat Modares University.

Fig. 6. XRD patterns of calcinated compound 1 at 700 °C.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi: 10.1016/j.molliq.2010.01.010. References

Fig. 7. SEM photographs of La2O3 nanostructure (produced by calcination of compound 1 under at 700 °C).

residual weight is 34.68% (calculated: 38.4%) corresponding to La2O3. The DTA curve displays three endothermic peaks at 146, 222 and 245 °C and a distinct exothermic peak at 358 °C (Fig. 5, Bulk). Detectable decomposition of the nanoparticles of 1 thus starts about 70° earlier than that of its single-crystal counterparts, and probably due to the much higher surface to volume ratio of the nanosized particles, more heat is needed to annihilate the lattices of single crystal. This stabilization is annihilated approximately by the produced nanostructure of this compound by sonochemical method. The final residual weight is 37.13% (calculated: 38.4%) corresponding to La2O3. The DTA curve displays three endothermic peaks at 130, 213 and 277 °C and an exothermic peak at 375 °C (Fig. 5, Nano).

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