Nanostructured films of iron, tin and titanium oxides by chemical vapor deposition

doi:10.1016/j.tsf.2005.07.249

Thin Solid Films

Volume 502, Issues 1–2, 28 April 2006, Pages 88-93

https://doi.org/10.1016/j.tsf.2005.07.249 Get rights and content

Abstract

Nanostructured films of iron, tin and titanium oxides were grown on glass and quartz by gas phase decomposition of [Fe(O^tBu)₃]₂, [Sn(O^tBu)₄] and [Ti(OⁱPr)₄], respectively. Films obtained exhibit good adhesion and homogeneous morphology with structural features in the nanometer range. The influence of microstructure and phase on optical (α-Fe₂O₃ and Fe₃O₄), gas sensing (SnO₂) and photocatalytic (TiO₂) properties was systematically studied to explore the potential of these coatings for technological applications.

Introduction

Nanostructured coatings are finding increasing applications due to their useful optical, magnetic, sensing and mechanical properties [1], [2]. In conjunction to their functional attributes, oxide films provide additional advantages of chemical inertness, high temperature stability and superior mechanical behaviour, important for a broad range of applications. To be suitable for end-use performance, the oxide coatings need to possess good adherence, crack-free surface and a well-defined composition. Among the various physical and chemical thin film deposition technologies, chemical vapor deposition (CVD) is gaining more attention due to the following features: (i) excellent film uniformity, (ii) high deposition rates, (iii) control over material composition and phase, (iv) conformal coverage on complex geometries, and (v) good adhesion and dense microstructures. The chemical features of metal alkoxides (M(OR)_x) such as pre-existing metal–oxygen bonds in molecular units, high volatility and low decomposition temperature make them attractive precursors to oxide ceramics in CVD process [3], [4].

Given the various iron oxide phases (hematite, magnetite and maghemite) present in the Fe–O system and their different optical and magnetic properties, methods for phase selective deposition of iron oxide films are of significant interest [4]. For example, magnetite (Fe₃O₄) is a half-metal with optical absorption edge in infrared region (E_g ∼ 0.3 eV) whereas hematite (α-Fe₂O₃) is treated as a UV absorber (E_g ∼ 2.2 eV). Among gas sensitive materials, SnO₂ (pure and doped) is a promising candidate because of its semiconductor properties, high reactivity toward gases and extreme low cost. The processing routes determine the microstructure of SnO₂, which subject to the specific surface area, can modify the gas sensing properties [5]. In addition to film morphology, the functional properties strongly depend on the solid phase present in the coatings. For example, among the TiO₂ phases, anatase shows significant photocatalytic activity whereas rutile is not suitable for similar applications [6]. Moreover, in comparison to polycrystalline TiO₂ powder, the enhanced surface area of nanocrystalline TiO₂ speeds up the photodegradation rate of organic compounds. Herein, we report the application of CVD of three molecular precursors, namely [Fe(O^tBu)₃]₂, [Sn(O^tBu)₄] and [Ti(OⁱPr)₄] to deposit iron, tin and titanium oxide coatings, respectively. We also demonstrate the influence of phase and microstructure on their functional properties.

Section snippets

Molecular precursors and CVD

The precursors [Fe(O^tBu)₃]₂ and [Sn(O^tBu)₄] were synthesized as described in previous reports [4], whereas [Ti(OⁱPr)₄], a commercial product (97%, Aldrich), was purified by distillation under vacuum (10^− 2 Torr, 80 °C) before using in the CVD process. The films were deposited on glass and quartz substrates, the experimental parameters of CVD are listed in Table 1. The decomposition patterns of the alkoxide precursors were deduced by analyzing the gaseous fragments on-line in a mass spectrometer

Iron oxides

Iron oxides are well-known pigments [8] and due to different optical and magnetic properties of hematite and magnetite, development of transparent magnetic films is interesting for optical and magneto-optical applications. Phase selective deposition of hematite (α-Fe₂O₃) and magnetite (Fe₃O₄) on copper and quartz substrates was achieved by vapor phase decomposition of an iron (III) alkoxide, [Fe(O^tBu)₃]₂ [4]. A precise control over the composition of deposited iron oxides was monitored by

Conclusion

Thin films of iron, tin and titanium oxides have been deposited on glass and quartz substrates by chemical vapor deposition of molecular alkoxide precursors. A large body of data is available on the deposition of binary metal oxides by gas phase methods [20]. When compared to conventional CVD approaches, the application of single molecular species as precursors offers certain intrinsic advantages such as (i) low synthesis temperature because diffusion is not required due to pre-existent M–O

Acknowledgments

This study was partially supported by a NATO-CLG Grant (No. 979756). Authors are thankful to the Saarland state and central government for providing the financial assistance. SM acknowledges Prof. M. Veith and Prof. H. Schmidt for providing the necessary infrastructure facilities. Thanks are due to the German Science Foundation (DFG) for supporting this work in the frame of the priority programme on nanomaterials – Sonderforschungsbereich 277 – operating at the Saarland University,

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Nanostructured films of iron, tin and titanium oxides by chemical vapor deposition

Abstract

Introduction

Section snippets

Molecular precursors and CVD

Iron oxides

Conclusion

Acknowledgments

Solid State Commun.

Thin Solid Films

Sens. Actuators, B, Chem.

Sens. Actuators, B, Chem.

Appl. Surf. Sci.

Sens. Actuators, B, Chem.

Sens. Actuators, B, Chem.

J. Photochem. Photobiol., A Chem.

Langmuir

Mater. Sci. Forum

J. Mol. Catal., A Chem.

J. Mater. Chem.

J. Am. Ceram. Soc.

Chem. Mater.