The liquid phase deposition (LPD) of metal oxide thin films at the liquid—liquid interface was investigated. Just after the start of the LPD reaction, depositions were observed continually at the liquid—liquid interface. The deposition grew two-dimensionally into a self-standing film with thickness of 1 μm and domain size of up to ca. 1 mm without any solid substrates. The self-standing film was formed with asymmetrical morphology, which consisted of flat surface on the side of the liquid—liquid interface and relatively rough surface on the side of the bulk solution. The structure was characterized by Raman spectroscopy, confirming that metastable ammonium titanium oxide fluoride (NH4TiOF3) was first deposited at the liquid—liquid interface, on which anatase-type titanium oxide (TiO2) was deposited second to forming the asymmetrical bilayer structure. It was suggested that the metal fluoride complex of the precursor was concentrated in the vicinity of the liquid—liquid interface compared with the solid—liquid interface due to the difference of the interfacial free energy, which could cause formation of the asymmetrical structure. The liquid—liquid interface could be confirmed as the specific reaction field for the LPD reaction. Also, the other metal oxide self-standing films such as tin oxide (SnO2) and iron hydroxide oxide (β-FeOOH) were obtained in this process. The process also has great potential not only for basic science but also in the engineering field such as a soft solution process for template-free synthesis.
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