Abstract
TiO2-based materials are of great practical interest in several technological areas. Both the size and the morphology of the TiO2 particles are of critical importance for their applications. The current study explores the effect of several factors on the outcome of the TiO2 particle synthesis via the so-called ‘two-emulsion method’. In this technique, two water-in-oil emulsions—each of them containing different reactant in the dispersed water drops—are mixed under well controlled conditions. Upon such mixing, partial coalescence of the water drops from the two emulsions leads to mixing of the drop content, with chemical reaction occurring within the drops, and to synthesis of Ti(OH)4 particles. Afterwards, the latter are transformed by emulsion heating into TiO2 particles and aggregates of predominantly anatase structure. Our results show that—depending on the precursor and surfactant concentrations, oil viscosity, emulsification time, and mixing speed—the obtained nanoparticles could aggregate either on the drop surface, forming capsules with a very smooth surface, or inside the water droplets, thus leading to hierarchically structured aggregates of micrometer size. The spherical smooth capsules are constructed of very small monodisperse TiO2 nanoparticles with size below 5 nm. The hierarchical bulk aggregates, on the other hand, are formed from bigger primary particles of sub-micrometer size. The obtained results show that one can obtain various TiO2 structures by controlling the conditions during the emulsion preparation and mixing
Highlights
TiO2-based materials attract attention of the scientists for their potential and successful use in wide range of areas including photo-catalysis for self-cleaning of solid surfaces, water cleaning, in solar cells, sensors, memory devices, fillers, etc. [1,2,3,4,5,6,7,8,9,10]
We focused on the formation of TiO2 nanoparticle materials of high concentration which aggregate in different structures, including spherical capsules with smooth or rough surface, or hierarchical aggregates using the two-emulsion method with inverse W/O emulsions, see Figure 1
The results showed that only Brij 72 from the group of Lutensol and Brij series was able to support the formation of W/O emulsions
Summary
TiO2-based materials attract attention of the scientists for their potential and successful use in wide range of areas including photo-catalysis for self-cleaning of solid surfaces, water cleaning, in solar cells, sensors, memory devices, fillers, etc. [1,2,3,4,5,6,7,8,9,10]. The rutile and anatase are both tetragonal with distortion of the TiO6 octahedron being larger for the anatase phase [21,22]. Hu et al [27] have reported that TiO2 normally undergoes an anatase-to-rutile phase transformation in the range from 600–700 ◦C. This transformation was affected by factors such as preparation conditions, precursors, impurities, and oxygen vacancies. The transformation sequence among the three main titania polymorphs (anatase, brookite, and rutile) is reported to be size dependent, because the energies of the three polymorphs are rather close and can be inversed by small differences in the surface energy [30]. If the particle size of the three nanocrystalline phases is equal, anatase is most thermodynamically stable at sizes < 11 nm, brookite is most stable for crystal sizes between 11 and 35 nm, and rutile is most stable at sizes > 35 nm [30]
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