Water-Dispersible, Ligand-Free, and Extra-Small (<10 nm) Titania Nanoparticles: Control Over Primary, Secondary, and Tertiary Agglomeration Through a Modified “Non-Aqueous” Route

Non-aqueous routes to inorganic nanoparticles are supposedly based on the absence of water; here, this view is partially challenged, showing that the presence of water (or moisture) is probably necessary, and is surely useful to achieve a precise control over the growth/aggregation phenomena leading to titanium dioxide nanoparticles. This study is focused on the preparation of size-controlled and ligand-free titania (anatase) nanoparticles in water dispersion. This is achieved through a three-step process: 1) production of primary (3–4 nm) nanoparticles from titanium alkoxides (Ti(O n Pr) 4 , Ti(O n Bu) 4 or Ti(O i Pr) 4 ) in benzyl alcohol through the controlled addition of water; 2) thermal growth phase, where the aggregation of primary nanoparticles at 80 °C leads to secondary nanoparticles with a typical fractal dimension of 2.2–2.4; the primary particles are still identifi able as the individual crystallites composing the secondary nanoparticles; 3) precipitation/re-dispersion in water, where secondary nanoparticles further agglomerate to yield tertiary nanoparticles. The size of the latter and their photocatalytic effi ciency is primarily controlled by the nature of residual alkoxide chains; in particular, isopropoxide groups allow to produce anatase nanoparticles with an average size of 7–8 nm in water dispersion and in the absence of any stabilizing ligand, which is an unprecedented result.