Titanium Oxide Nanoparticles Precipitated from Low‐Temperature Aqueous Solutions: II. Thin‐Film Formation and Microstructure Developments

Abstract

We explored effects of the degree of supersaturation, which depends on solution concentration, pH, and temperature, on the development of microstructures of the TiO 2 thin films deposited from the controlled hydrolysis of TiCl 4 aqueous solutions. It was shown that, with precursor (TiCl 4 ) solution of low degree of supersaturation, a porous flower-like dendritic structure was synthesized, while a densely packed particulate nanostructure was obtained with that of high degree of supersaturation. The former morphology was attributed to the directional growth of TiO 2 crystals from the already-deposited films, whereas the latter to the bulk precipitated TiO 2 nanoparticles nucleated in the solution, followed by their attachment and self organization. By establishing the processing-microstructure relations, this study provided a means of generating a wide spectrum of reproducible TiO 2 microstructures using the low-temperature aqueous solution processing.

Titanium Oxide Nanoparticles Precipitated from Low‐Temperature Aqueous Solutions: III. Thin Film Properties

“…Lower dye uptake and less exposure to electrolyte solution (because of denser morphology) also reduces I SC produced by such anodes. The films generated at s ~63.9, however, showed porous structure with highly crystalline rutile plates . This leads to higher charge conduction, better dye uptake, higher electrolyte contact area, and higher I SC .…”

“…Precursor solution environment not only determines the nanoparticle assembly and the film microstructure, but also influences the phase of titania (amorphous, anatase, or rutile). We previously described the contribution of nucleation, growth, and agglomeration of nanoparticles in generating the hierarchical structures in titania films . As homogeneous nucleation is the major formation mechanism, nature of substrate has a minimal effect on the crystallinity of the precipitate.…”

“…The high‐s solutions (s ~214.9 and 157.7) have produced spherical particulate morphology. Our previous publication indicated that these types of films on Si substrate had anatase nanocrystallites embedded in amorphous matrix . The topology of these films [Figs.…”

“…On the other hand, the films obtained from higher supersaturations (157.7 and 214.9; plot 2 and 1) did not show any titania peak (except FTO peaks). Due to very small crystallites (<10 nm) embedded in an amorphous matrix (see part II), low X‐ray interaction volume in thinner films (from slow deposition kinetics) and due to strong substrate peaks, titania peaks might not show up even though some nanocrystallites existed in the film.…”

Microstructure development of hydrothermally grown TiO2 thin films with vertically aligned nanorods

“…In our previous studies, TiO 2 nanostructures were systematically controlled by the calculated degree of supersaturation ( s ). Different phases of TiO 2 (anatase, rutile, amorphous) and shapes of the nanostructures (leaf, tree, and particle) were prepared by the CBD process via a wide range of degrees of supersaturation ( s = 70‐200) . At very low degree of supersaturation ( s ≤ 70), the crystallinity of TiO 2 nanostructures (nanoblades and nanorods) was dramatically improved .…”