Morphological and phase dependence of nanotitania materials generated under extreme pH conditions for large scale production of TiO2 nanowires (basic) and nanosquares or nanrods (acidic)

Abstract

The effect that the phase of the starting nanoseed titania (TiO2), the pH of the solvent solution, and the processing methodology employed have on the properties of the resultant TiO2 nanomaterials were explored. This led to the development of a new process to produce large-scale, phase pure, thin nanowires of TiO2 at high pH and nanosquares at low pH. Anatase, rutile, and Degussa P25TM TiO2 nanoparticle starting materials (or nanoseeds) were processed in strongly basic (10 M KOH) and strongly acidic (conc. HX, where X = Cl, Br, I) solutions using solvothermal (SOLVO) and solution precipitation (SPPT) methodologies. Under basic SOLVO conditions, the nanoseeds were converted to H2Ti2O5·H2O nanowires. The SPPT basic conditions also produced the same phased nanowires for the rutile and anatase nanoseeds, while the Degussa nanomaterial yielded mixed phased [anatase:rutile (9:1)] nanowires. The SPPT method was found to produce substantially thinner nanowires in comparison to the SOLVO route, with comparable surface areas but the strong basic media led to etching of the glassware yielding HK3Ti4O4(SiO4)3·4H2O nanorods. Hybridization of these two processing routes led to the use of NalgeneTM bottle as the reaction flask termed the hybrid (HYBR) route, yielding even thinner H2Ti2O5·H2O nanowires on a large-scale. Switching to a concentrated halide acid (HX, where X = Cl, Br, I) system, SOLVO, SPPT, and HYBR routes were investigated. The resultant TEM images revealed that the rutile starting material yielded short rods, whereas the anatase seeds formed square or faceted materials.