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A versatile single-source precursor for the synthesis of LaCoO3 films
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Materials Letters 62 (2008) 1179 – 1182 www.elsevier.com/locate/matlet
A versatile single-source precursor for the synthesis of LaCoO3 films Lidia Armelao a , Gregorio Bottaro a,⁎, Laura Crociani b , Roberta Seraglia a , Eugenio Tondello a , Pierino Zanella b a
Istituto di Scienze e Tecnologie Molecolari, CNR, e INSTM, Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131 Padova, Italy b Istituto di Chimica Inorganica e delle Superfici, CNR, C.so Stati Uniti 4, 35127 Padova, Italy Received 4 June 2007; accepted 2 August 2007 Available online 10 August 2007
Abstract The present communication is focused on the synthesis of the versatile heterobimetallic LaCo(ODiEt)5 (ODiEt = OC(CH2CH3)2CH2OCH3) alkoxide compound and on its innovative and unprecedented use as single-source precursor in the preparation of LaCoO3 films by both sol–gel and Chemical Vapour Deposition techniques. Nanostructured LaCoO3 films characterized by thermal stability of the perovskite structure up to 900 °C have been obtained thus evidencing the versatility of the adopted source compound. Morphological analysis revealed that high-quality smooth, well-adherent and crack-free layers have been prepared. © 2007 Elsevier B.V. All rights reserved. Keywords: Sol–gel preparation; Chemical Vapour Deposition; Single-source precursor; LaCoO3
1. Introduction Metal oxide films cover a wide range of applications including electronics, optics, magnetism and catalysis [1]. In the synthesis of oxide systems, the approach through mild chemical routes plays a pivotal role in the control of the final materials properties. Among others, sol–gel and CVD techniques are particularly suitable for yielding thin films with good control over chemical composition and microstructure. A common feature of these synthesis processes concerns the need of molecular precursors with controlled stoichiometry, suitable properties of solubility, volatility and clean decomposition pathways. In particular, in the synthesis of multimetallic oxide systems, the availability of source compounds containing in one molecule all the elements necessary for the formation of the final solid system in the proper atomic ratio, represents a further strategic advantage [1,2]. Such molecular compounds, defined as single-source precursors, can be synthesized with designed solubility and volatility being thus suitable as
⁎ Corresponding author. Tel.: +39 049 8275188; fax: +39 049 8275161. E-mail address: [email protected] (G. Bottaro). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.08.007
precursors for both liquid and vapour phase techniques such as sol–gel and CVD routes. Among metal oxide materials, an increasing interest is devoted to the development of systems with perovskite lattice such as lanthanum cobaltite (LaCoO3), due to their potential applications in energetics both as cathodes and solid electrolytes in solid oxide fuel cells (SOFCs), and in environmental catalysis for pollutant decomposition [3–5]. The present communication is focused on the synthesis of the versatile heterobimetallic LaCo(ODiEt)5 (ODiEt = OC (CH2CH3)2CH2OCH3) alkoxide compound and on its innovative and unprecedent use as single-source precursor in the preparation of LaCoO3 films by both sol–gel and CVD techniques. Due to the presence of the etheric oxygen atom in the organic moiety, the alkoxide precursor exhibits higher solubility in polar solvents and marked volatility due to the stabilization of lowmolecular weight species such as oligomers or monomers via intra-molecular interactions [6]. This compound is thus suitable as a molecular precursor in the synthesis of multimetallic oxides via liquid and vapour phase chemical approaches. As addressed in the following, both approaches have successfully led to the obtainment of smooth, homogeneous and crack-free layers. The high-quality nanostructured LaCoO3 films are characterized by stability of the perovskitic crystalline structure up to 900 °C.
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2. Experimental The LaCo(ODiEt)5 compound was first reported in 1995 by Herrmann et al. [7] as a heterobimetallic volatile complex of lanthanum and cobalt. Unfortunately, scarce experimental details were given to allow the reproducibility of the alkoxide synthesis. In this study, LaCo(ODiEt)5 was prepared starting form Co(II) and La(III) [N,N-bis(trimethylsilyl) amide], Co[N (SiMe3)2]2 and La[N(SiMe3)2]3 respectively. First, two different toluene solutions (25 mL each) containing Co[N(SiMe3)2]2 (379.8 mg, 1 mmol) and DiEtOH (2 mmol), La[N(SiMe3)2]3 (620.2 mg, 1 mmol) and DiEtOH (3 mmol) were prepared. After stirring for 24 h, the solutions were mixed and kept under vigorous stirring for further 72 h. All handling operations were performed at room temperature in a glove box. The mixed solution was then evaporated to dryness, the solid was dissolved into n-hexane (15 mL) and stored in a refrigerator at − 25 °C. After 24 h, the mixture was filtered and evaporated to dryness. The final product was obtained as a blue oil upon sublimation at 115 °C and 10− 3 Torr. Sol–gel solutions were prepared dissolving LaCo(ODiEt)5 (1.3 mmol) in ethanol (7.892 g, Fluka ≥ 99.8%) in the presence of HCl (2.0 mmol, 37%, Carlo Erba), and kept under stirring at room temperature for 2 h before film preparation. Sol–gel depositions of LaCoO3 layers were carried out by dip-coating on Herasil silica slides (Heraeus®, Quarzschmelze, Hanau, Germany) at room temperature with a controlled withdrawal speed of about 15 cm/min by means of a multidipping process, up to 3 depositions. All as-prepared layers appeared highly homogeneous, pale-blue coloured, and well adherent to the substrate. Handlings have been carried out under controlled atmosphere in a glove box. In the CVD experiment the precursor was vapourized at 120 °C (total pressure 10 − 3 Torr). The silica substrate, previously cleaned in an ultrasonic bath using trichloroethylene, was heated at 500 °C for a total deposition time of 2 h. Sol–gel and CVD films were subsequently annealed in air between 500 and 900 °C for 1 h. Compositional analysis was carried out by an analytical system working in energy dispersion (EDS-EDAX DX PRIME)
in a Philips XL-40 scanning electron microscope (SEM), equipped with a LaB6 source. EI mass spectra were recorded on a Finnigan Trace MS using a ThermoQuest direct injection probe. GIXRD (Glancing Incidence X-ray Ray Diffraction) patterns were recorded by a Bruker D8 Advance diffractometer equipped with a Göbel mirror and a CuKα source (40 kV, 40 mA), at a fixed incidence angle of 0.5°. The average crystallite dimensions were estimated by means of the Scherrer equation. Atomic Force Microscopy (AFM) analysis was carried out using a Park Autoprobe CP instrument operating in contact mode and in air. The background was subtracted from the images using the ProScan 1.3 software from Park Scientific. 3. Results and discussion The chemical composition of the alkoxide compound was determined by Energy Dispersive X-ray Analysis (EDX) and elemental analysis. As revealed by EDX, the Co:La atomic ratio is 1:1, which is the proper metal ratio in view of the final oxide material. Concerning the organic ligands, elemental analysis indicates a LaCo(ODiEt)5 formula for the synthesized compound (calculated for LaCoO10C35H75: C = 49.2%, H = 8.9%; found C = 49.7%, H = 8.8%). Further information on the molecular structure of the alkoxide compound was obtained by electron impact (EI) mass spectrometry. As a matter of fact, soft ionization mass spectrometric techniques such as Electrospray Ionisation (ESI) and Matrix Assisted Laser Desorption Ionisation (MALDI) are not suitable to this aim, due to the use of solvents (i.e. acetonitrile) or matrices (i.e. 2,5-dihydroxy-benzoic acid) which can coordinate the alkoxide compound [8]. Due to the high ionization energy employed, the molecular peak is not detected, but at least three fragments associated to heterobimetallic ionic species can be identified. The peak at m/z 476 (Rel.Ab.% = 42%) can be assigned to LaCoO(ODiEt)+2 ions which is likely to have an oxygen bridge between La and Co metal centres. Furthermore, the fragments at m/z 376 (Rel.Ab.% = 38%) and 276 (Rel.Ab.% = 24%) can be reasonably assigned to LaCoO(OMe)(ODiEt)+ and LaCoO(OMe)+2 ions, respectively. Besides the presence of heterobimetallic species, monometallic ions containing Co or La have been also detected. The signals at m/z 301 (Rel.Ab.% = 25%) and at m/z 201 (Rel.Ab.% = 100%) can be assigned to La(OMe)(ODiEt)+ and La(OMe)+2 species respectively. Furthermore, the peak at m/z 162 (Rel.Ab.% = 39%) can
Fig. 1. GIXRD pattern of sol–gel and CVD films. The mean crystallite size, as evaluated from diffraction peak broadening, was lower than 30 nm in both cases.
L. Armelao et al. / Materials Letters 62 (2008) 1179–1182
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Fig. 2. Representative 2D and 3D AFM micrographs (1 × 1 μm2) for LaCoO3 film prepared via sol–gel and annealed in air for 1 h at 900 °C.
be attributed to the monometallic species Co(OCHEtCH2OMe)+ generated from Co(ODiEt)2. These results clearly support the presence of a linkage between Co and La through oxygen bridges, as already observed in other bimetallic species containing the ODiEt moiety [7,9]. Moreover, the heterobimetallic species in the gas phase, with a 1:1 La: Co atomic ratio, exactly match the metal composition desired in the final material. LaCo(ODiEt)5 was thus adopted as a single-source precursor for the deposition of LaCoO3 films from both sol–gel and CVD routes. Irrespective of the preparation method, the as-prepared specimens resulted homogeneous, crack-free and with amorphous structure. No appreciable modifications of the layers' microstructure were evidenced upon thermal treatment up to 600 °C. For both sol–gel and CVD samples, the formation of the perovskitic LaCoO3 phase as the sole crystalline structure was detected after annealing at 700 °C, as evidenced by Glancing Incidence X-ray diffraction analysis (Fig. 1). The absence of intermediate or undesired phases with different composition such as La2CoO4, Co3O4, La2O2CO3 or La2O3 was ascribed to the peculiar nature of the adopted precursor. As a matter of fact, the La–O–Co chemical bonds present in the single-source precursor molecules are likely to be retained during the sol–gel and CVD processes which lead to the final oxide material. The formation of a ternary La–Co–O phase with a 1:1 La:Co ratio is thus favoured over the crystallization of the single oxides. The quality of the oxide layers and the stability of the LaCoO3 crystalline phase were observed after thermal treatments up to 900 °C. For CVD specimens, beside the reflections already observed after heating at 700 °C [2θ = 32.9° (110), 33.4° (104), 47.5° (024)], additional peaks associated to the rhombohedral LaCoO3 phase [2θ = 23.2° (012), 40.6° (202), 41.4° (006)] were detected after 900 °C treatment [10]. A more limited increase in the relative intensity of the diffraction peaks upon annealing was observed for sol–gel films, due to their lower thickness. The remarkable thermal stability at high temperature of LaCoO3 films is an important aspect. In fact, it is well known from literature as lanthanum cobaltite layers decompose when heated at temperature higher than 800 °C. This phenomenon is progressively promoted by increasing temperatures and duration of the thermal treatments [11], and seems to be related to diffusion processes and chemical reactions occurring at the LaCoO3/substrate interface. A similar behaviour has also been evidenced in our previous studies where LaCoO3 layers were synthesized starting from distinct molecular sources for lanthanum and cobalt [3–5]. It is our opinion that the enhanced thermal stability displayed by lanthanum cobaltite films prepared starting from LaCo(ODiEt)5 could be related to the intrinsic characteristics of the heterobimetallic compounds. As a matter of fact, phase separation or element segregation is less likely by using single-
source precursors [12] which represent, in a certain sense, the building block units of the final material. Besides the stability of the perovskitic LaCoO3 phase up to high temperatures, the layers exhibited also appreciable morphological features, i.e. substrate adhesion, smoothness (root-mean-square roughness lower than 1 nm) and integrity even under the more severe annealing conditions, as evidenced by AFM analysis (Fig. 2).
4. Conclusions In summary, a versatile La–Co heterobimetallic alkoxide has been synthesized and for the first time successfully employed as single-source precursor for the deposition of lanthanum cobaltite films both via liquid (sol–gel) and vapour (CVD) phase chemical ways to nanomaterials. High-quality smooth and nanostructured LaCoO3 films characterized by thermal stability of the perovskite structure up to high temperature have been obtained by both deposition routes, thus evidencing the versatility of the adopted LaCo(ODiEt)5 source compound. Acknowledgements This work has been supported by research programs CNRINSTM PROMO, FISR-MIUR “Inorganic hybrid nanosystems for the development and the innovation of fuel cells”, INSTMPRISMA “High-k oxide based thin films from liquid and vapour phase routes”, and FIRB-RBNE033KMA “Molecular compounds and hybrid nanostructured materials with resonant and non-resonant optical properties for photonic devices”. The authors are grateful to Dr. Laura Sperni (Department of Chemistry, Venezia University) for useful help in the mass spectrometry measurements. References [1] L.G. Hubert-Pfalzgraf, Inorg. Chem. Commun. 6 (2003) 102. [2] M. Veith, J. Chem. Soc., Dalton Trans. (2002) 2405. [3] L. Armelao, D. Barreca, G. Bottaro, A. Gasparotto, C. Maragno, E. Tondello, C. Sada, J. Nanosci. Nanotechnol. 5 (2005) 781. [4] L. Armelao, D. Barreca, G. Bottaro, A. Gasparotto, C. Maragno, E. Tondello, Chem. Mater. 17 (2005) 427. [5] L. Armelao, D. Barreca, L. Bertolo, E. Bontempi, G. Bottaro, L.E. Depero, E. Pierangelo, Cryst. Eng. 5 (2002) 291.
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[6] W.A. Herrmann, N.W. Huber, R. Anwander, T. Priermeier, Chem. Ber. 126 (1993) 1127. [7] W.A. Herrmann, N.W. Huber, O. Runte, Angew. Chem., Int. Ed. Engl. 34 (1995) 2187. [8] M. Karas, U. Bahr, U. Gießmann, Mass Spectrom. Rev. 10 (1991) 335. [9] R. Anwander, F.C. Munck, T. Priermeier, W. Scherer, O. Runte, W.A. Herrmann, Inorg. Chem. 36 (1997) 3545.
[10] JCPDS card no. 48-0123. [11] Y. Zhang, Y. Zhu, X. Ye, L. Cao, Surf. Interface Anal. 32 (2001) 310. [12] S. Mathur, H. Shen, R. Rapalaviciute, A. Kareiva, N. Donia, J. Mater. Chem. 14 (2004) 3259.
Materials Letters 62 (2008) 1179 – 1182 www.elsevier.com/locate/matlet
A versatile single-source precursor for the synthesis of LaCoO3 films Lidia Armelao a , Gregorio Bottaro a,⁎, Laura Crociani b , Roberta Seraglia a , Eugenio Tondello a , Pierino Zanella b a
Istituto di Scienze e Tecnologie Molecolari, CNR, e INSTM, Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131 Padova, Italy b Istituto di Chimica Inorganica e delle Superfici, CNR, C.so Stati Uniti 4, 35127 Padova, Italy Received 4 June 2007; accepted 2 August 2007 Available online 10 August 2007
Abstract The present communication is focused on the synthesis of the versatile heterobimetallic LaCo(ODiEt)5 (ODiEt = OC(CH2CH3)2CH2OCH3) alkoxide compound and on its innovative and unprecedented use as single-source precursor in the preparation of LaCoO3 films by both sol–gel and Chemical Vapour Deposition techniques. Nanostructured LaCoO3 films characterized by thermal stability of the perovskite structure up to 900 °C have been obtained thus evidencing the versatility of the adopted source compound. Morphological analysis revealed that high-quality smooth, well-adherent and crack-free layers have been prepared. © 2007 Elsevier B.V. All rights reserved. Keywords: Sol–gel preparation; Chemical Vapour Deposition; Single-source precursor; LaCoO3
1. Introduction Metal oxide films cover a wide range of applications including electronics, optics, magnetism and catalysis [1]. In the synthesis of oxide systems, the approach through mild chemical routes plays a pivotal role in the control of the final materials properties. Among others, sol–gel and CVD techniques are particularly suitable for yielding thin films with good control over chemical composition and microstructure. A common feature of these synthesis processes concerns the need of molecular precursors with controlled stoichiometry, suitable properties of solubility, volatility and clean decomposition pathways. In particular, in the synthesis of multimetallic oxide systems, the availability of source compounds containing in one molecule all the elements necessary for the formation of the final solid system in the proper atomic ratio, represents a further strategic advantage [1,2]. Such molecular compounds, defined as single-source precursors, can be synthesized with designed solubility and volatility being thus suitable as
⁎ Corresponding author. Tel.: +39 049 8275188; fax: +39 049 8275161. E-mail address: [email protected] (G. Bottaro). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.08.007
precursors for both liquid and vapour phase techniques such as sol–gel and CVD routes. Among metal oxide materials, an increasing interest is devoted to the development of systems with perovskite lattice such as lanthanum cobaltite (LaCoO3), due to their potential applications in energetics both as cathodes and solid electrolytes in solid oxide fuel cells (SOFCs), and in environmental catalysis for pollutant decomposition [3–5]. The present communication is focused on the synthesis of the versatile heterobimetallic LaCo(ODiEt)5 (ODiEt = OC (CH2CH3)2CH2OCH3) alkoxide compound and on its innovative and unprecedent use as single-source precursor in the preparation of LaCoO3 films by both sol–gel and CVD techniques. Due to the presence of the etheric oxygen atom in the organic moiety, the alkoxide precursor exhibits higher solubility in polar solvents and marked volatility due to the stabilization of lowmolecular weight species such as oligomers or monomers via intra-molecular interactions [6]. This compound is thus suitable as a molecular precursor in the synthesis of multimetallic oxides via liquid and vapour phase chemical approaches. As addressed in the following, both approaches have successfully led to the obtainment of smooth, homogeneous and crack-free layers. The high-quality nanostructured LaCoO3 films are characterized by stability of the perovskitic crystalline structure up to 900 °C.
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2. Experimental The LaCo(ODiEt)5 compound was first reported in 1995 by Herrmann et al. [7] as a heterobimetallic volatile complex of lanthanum and cobalt. Unfortunately, scarce experimental details were given to allow the reproducibility of the alkoxide synthesis. In this study, LaCo(ODiEt)5 was prepared starting form Co(II) and La(III) [N,N-bis(trimethylsilyl) amide], Co[N (SiMe3)2]2 and La[N(SiMe3)2]3 respectively. First, two different toluene solutions (25 mL each) containing Co[N(SiMe3)2]2 (379.8 mg, 1 mmol) and DiEtOH (2 mmol), La[N(SiMe3)2]3 (620.2 mg, 1 mmol) and DiEtOH (3 mmol) were prepared. After stirring for 24 h, the solutions were mixed and kept under vigorous stirring for further 72 h. All handling operations were performed at room temperature in a glove box. The mixed solution was then evaporated to dryness, the solid was dissolved into n-hexane (15 mL) and stored in a refrigerator at − 25 °C. After 24 h, the mixture was filtered and evaporated to dryness. The final product was obtained as a blue oil upon sublimation at 115 °C and 10− 3 Torr. Sol–gel solutions were prepared dissolving LaCo(ODiEt)5 (1.3 mmol) in ethanol (7.892 g, Fluka ≥ 99.8%) in the presence of HCl (2.0 mmol, 37%, Carlo Erba), and kept under stirring at room temperature for 2 h before film preparation. Sol–gel depositions of LaCoO3 layers were carried out by dip-coating on Herasil silica slides (Heraeus®, Quarzschmelze, Hanau, Germany) at room temperature with a controlled withdrawal speed of about 15 cm/min by means of a multidipping process, up to 3 depositions. All as-prepared layers appeared highly homogeneous, pale-blue coloured, and well adherent to the substrate. Handlings have been carried out under controlled atmosphere in a glove box. In the CVD experiment the precursor was vapourized at 120 °C (total pressure 10 − 3 Torr). The silica substrate, previously cleaned in an ultrasonic bath using trichloroethylene, was heated at 500 °C for a total deposition time of 2 h. Sol–gel and CVD films were subsequently annealed in air between 500 and 900 °C for 1 h. Compositional analysis was carried out by an analytical system working in energy dispersion (EDS-EDAX DX PRIME)
in a Philips XL-40 scanning electron microscope (SEM), equipped with a LaB6 source. EI mass spectra were recorded on a Finnigan Trace MS using a ThermoQuest direct injection probe. GIXRD (Glancing Incidence X-ray Ray Diffraction) patterns were recorded by a Bruker D8 Advance diffractometer equipped with a Göbel mirror and a CuKα source (40 kV, 40 mA), at a fixed incidence angle of 0.5°. The average crystallite dimensions were estimated by means of the Scherrer equation. Atomic Force Microscopy (AFM) analysis was carried out using a Park Autoprobe CP instrument operating in contact mode and in air. The background was subtracted from the images using the ProScan 1.3 software from Park Scientific. 3. Results and discussion The chemical composition of the alkoxide compound was determined by Energy Dispersive X-ray Analysis (EDX) and elemental analysis. As revealed by EDX, the Co:La atomic ratio is 1:1, which is the proper metal ratio in view of the final oxide material. Concerning the organic ligands, elemental analysis indicates a LaCo(ODiEt)5 formula for the synthesized compound (calculated for LaCoO10C35H75: C = 49.2%, H = 8.9%; found C = 49.7%, H = 8.8%). Further information on the molecular structure of the alkoxide compound was obtained by electron impact (EI) mass spectrometry. As a matter of fact, soft ionization mass spectrometric techniques such as Electrospray Ionisation (ESI) and Matrix Assisted Laser Desorption Ionisation (MALDI) are not suitable to this aim, due to the use of solvents (i.e. acetonitrile) or matrices (i.e. 2,5-dihydroxy-benzoic acid) which can coordinate the alkoxide compound [8]. Due to the high ionization energy employed, the molecular peak is not detected, but at least three fragments associated to heterobimetallic ionic species can be identified. The peak at m/z 476 (Rel.Ab.% = 42%) can be assigned to LaCoO(ODiEt)+2 ions which is likely to have an oxygen bridge between La and Co metal centres. Furthermore, the fragments at m/z 376 (Rel.Ab.% = 38%) and 276 (Rel.Ab.% = 24%) can be reasonably assigned to LaCoO(OMe)(ODiEt)+ and LaCoO(OMe)+2 ions, respectively. Besides the presence of heterobimetallic species, monometallic ions containing Co or La have been also detected. The signals at m/z 301 (Rel.Ab.% = 25%) and at m/z 201 (Rel.Ab.% = 100%) can be assigned to La(OMe)(ODiEt)+ and La(OMe)+2 species respectively. Furthermore, the peak at m/z 162 (Rel.Ab.% = 39%) can
Fig. 1. GIXRD pattern of sol–gel and CVD films. The mean crystallite size, as evaluated from diffraction peak broadening, was lower than 30 nm in both cases.
L. Armelao et al. / Materials Letters 62 (2008) 1179–1182
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Fig. 2. Representative 2D and 3D AFM micrographs (1 × 1 μm2) for LaCoO3 film prepared via sol–gel and annealed in air for 1 h at 900 °C.
be attributed to the monometallic species Co(OCHEtCH2OMe)+ generated from Co(ODiEt)2. These results clearly support the presence of a linkage between Co and La through oxygen bridges, as already observed in other bimetallic species containing the ODiEt moiety [7,9]. Moreover, the heterobimetallic species in the gas phase, with a 1:1 La: Co atomic ratio, exactly match the metal composition desired in the final material. LaCo(ODiEt)5 was thus adopted as a single-source precursor for the deposition of LaCoO3 films from both sol–gel and CVD routes. Irrespective of the preparation method, the as-prepared specimens resulted homogeneous, crack-free and with amorphous structure. No appreciable modifications of the layers' microstructure were evidenced upon thermal treatment up to 600 °C. For both sol–gel and CVD samples, the formation of the perovskitic LaCoO3 phase as the sole crystalline structure was detected after annealing at 700 °C, as evidenced by Glancing Incidence X-ray diffraction analysis (Fig. 1). The absence of intermediate or undesired phases with different composition such as La2CoO4, Co3O4, La2O2CO3 or La2O3 was ascribed to the peculiar nature of the adopted precursor. As a matter of fact, the La–O–Co chemical bonds present in the single-source precursor molecules are likely to be retained during the sol–gel and CVD processes which lead to the final oxide material. The formation of a ternary La–Co–O phase with a 1:1 La:Co ratio is thus favoured over the crystallization of the single oxides. The quality of the oxide layers and the stability of the LaCoO3 crystalline phase were observed after thermal treatments up to 900 °C. For CVD specimens, beside the reflections already observed after heating at 700 °C [2θ = 32.9° (110), 33.4° (104), 47.5° (024)], additional peaks associated to the rhombohedral LaCoO3 phase [2θ = 23.2° (012), 40.6° (202), 41.4° (006)] were detected after 900 °C treatment [10]. A more limited increase in the relative intensity of the diffraction peaks upon annealing was observed for sol–gel films, due to their lower thickness. The remarkable thermal stability at high temperature of LaCoO3 films is an important aspect. In fact, it is well known from literature as lanthanum cobaltite layers decompose when heated at temperature higher than 800 °C. This phenomenon is progressively promoted by increasing temperatures and duration of the thermal treatments [11], and seems to be related to diffusion processes and chemical reactions occurring at the LaCoO3/substrate interface. A similar behaviour has also been evidenced in our previous studies where LaCoO3 layers were synthesized starting from distinct molecular sources for lanthanum and cobalt [3–5]. It is our opinion that the enhanced thermal stability displayed by lanthanum cobaltite films prepared starting from LaCo(ODiEt)5 could be related to the intrinsic characteristics of the heterobimetallic compounds. As a matter of fact, phase separation or element segregation is less likely by using single-
source precursors [12] which represent, in a certain sense, the building block units of the final material. Besides the stability of the perovskitic LaCoO3 phase up to high temperatures, the layers exhibited also appreciable morphological features, i.e. substrate adhesion, smoothness (root-mean-square roughness lower than 1 nm) and integrity even under the more severe annealing conditions, as evidenced by AFM analysis (Fig. 2).
4. Conclusions In summary, a versatile La–Co heterobimetallic alkoxide has been synthesized and for the first time successfully employed as single-source precursor for the deposition of lanthanum cobaltite films both via liquid (sol–gel) and vapour (CVD) phase chemical ways to nanomaterials. High-quality smooth and nanostructured LaCoO3 films characterized by thermal stability of the perovskite structure up to high temperature have been obtained by both deposition routes, thus evidencing the versatility of the adopted LaCo(ODiEt)5 source compound. Acknowledgements This work has been supported by research programs CNRINSTM PROMO, FISR-MIUR “Inorganic hybrid nanosystems for the development and the innovation of fuel cells”, INSTMPRISMA “High-k oxide based thin films from liquid and vapour phase routes”, and FIRB-RBNE033KMA “Molecular compounds and hybrid nanostructured materials with resonant and non-resonant optical properties for photonic devices”. The authors are grateful to Dr. Laura Sperni (Department of Chemistry, Venezia University) for useful help in the mass spectrometry measurements. References [1] L.G. Hubert-Pfalzgraf, Inorg. Chem. Commun. 6 (2003) 102. [2] M. Veith, J. Chem. Soc., Dalton Trans. (2002) 2405. [3] L. Armelao, D. Barreca, G. Bottaro, A. Gasparotto, C. Maragno, E. Tondello, C. Sada, J. Nanosci. Nanotechnol. 5 (2005) 781. [4] L. Armelao, D. Barreca, G. Bottaro, A. Gasparotto, C. Maragno, E. Tondello, Chem. Mater. 17 (2005) 427. [5] L. Armelao, D. Barreca, L. Bertolo, E. Bontempi, G. Bottaro, L.E. Depero, E. Pierangelo, Cryst. Eng. 5 (2002) 291.
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[6] W.A. Herrmann, N.W. Huber, R. Anwander, T. Priermeier, Chem. Ber. 126 (1993) 1127. [7] W.A. Herrmann, N.W. Huber, O. Runte, Angew. Chem., Int. Ed. Engl. 34 (1995) 2187. [8] M. Karas, U. Bahr, U. Gießmann, Mass Spectrom. Rev. 10 (1991) 335. [9] R. Anwander, F.C. Munck, T. Priermeier, W. Scherer, O. Runte, W.A. Herrmann, Inorg. Chem. 36 (1997) 3545.
[10] JCPDS card no. 48-0123. [11] Y. Zhang, Y. Zhu, X. Ye, L. Cao, Surf. Interface Anal. 32 (2001) 310. [12] S. Mathur, H. Shen, R. Rapalaviciute, A. Kareiva, N. Donia, J. Mater. Chem. 14 (2004) 3259.