From a microwave flash-synthesized TiO2 colloidal suspension to TiO2 thin films
Introduction
Titanium dioxide has a wide range of applications in the form of thin films, such as optical films for photocatalytic or solar cell [1], [2], [3], taking advantage of the high refractive index of TiO2, and gas sensors [4], [5]. Titanium dioxide thin layers are mostly prepared by sol–gel processes or chemical vapor deposition, both methods involving the use of environmentally harmful chemicals or additives. Furthermore, sol–gel processes require time-consuming thermal treatment to react the deposited precursor into an oxide film.
Microwave one-step oxide synthesis gives new insights for enhancement in the field of thin oxide film deposition. Up to now, titanium dioxide microwave synthesis with simultaneous film deposition has been tried by Vigil et al. [6] from an aqueous solution, while Cirera et al. [7] have prepared thick films composed of SnO2 powders obtained by microwave heating of tin chloride in methanol solutions.
Before the experiments reported herein, the authors reported tin dioxide thin film deposition by dip-coating into tin dioxide colloids, previously synthesized with the RAMO system from aqueous tin tetrachloride solutions [8].
The context of the present work was a complete study of TiO2 rutile and anatase synthesis by microwave-induced thermohydrolysis. The microwave-prepared colloidal suspensions resulting from that study showed satisfying colloidal stability to be applied to thin film deposition by dip-coating. The benefits of such a deposition process, time and energy savings, arise from the microwave process but also from the properties of the microwave-prepared colloidal suspension, containing oxide nanoparticles and no surfactant molecules. As a consequence, we report on a “green” process for TiO2 thin film deposition, where the need for thermal treatment is lowered or even avoided by the existence of oxide nanocrystals in the suspension.
Section snippets
Colloidal suspension preparation
The TiO2 colloidal suspension was prepared by microwave heating of an aqueous solution containing 0.01 mol L−1 titanium tetrachloride and 0.1 mol L−1 hydrochloric acid. Titanium tetrachloride (99.995%, Aldrich, Ref. 25,431-2) and hydrochloric acid (Prolabo, RP Normapur) were used without further purification. Titanium(IV) chloride was added under vigorous stirring, in previously acidified water. The microwave device employed was specially designed by the authors. This microwave heating system,
Hydrodynamic diameter of particles within the microwave suspension
Prior to deposition, particle size in the suspension was measured by photon correlation spectroscopy. A narrow size distribution centered on 70 nm can be observed on Fig. 1. The whole set of characterizations performed on the sample proved that this size of 70 nm corresponds to the hydrodynamic diameter of the secondary particles, as shown by the inset into Fig. 1. Colloidal stability of this suspension, and thus its satisfying properties to be applied for any deposition technique, were also
Conclusion
Morphology, roughness, thickness, composition, and optical properties investigations proved the feasibility of preparation of TiO2 thin films by dip-coating from microwave-synthesized aqueous TiO2 nanoparticles colloidal suspensions, using the microwave autoclave reactor RAMO. The colloidal suspension was used as prepared, without any modification prior to deposition. The thin films were transparent, showed good resistance to delaminating and promising optical properties. The use of silicon
Acknowledgments
The authors thank C. Dumas and M. Sacilotti (LPUB, Université de Bourgogne) for access to the dip-coating and Micromap devices, S. Sen and S. Mahanty (Central Glass and Ceramic Research Institute, Kolkatta, India) for spectrophotometric measurements, and C. Santilli and A. Rizzato (IQ-UNESP, Araraquara, Brazil) for X-ray reflectometry.
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