Evolving surface properties of stirred wet milled aluminum-doped titanium dioxide: A discretely heterogeneous system

The stirred wet milling of aluminum-doped TiO2 was considered. At milling speeds of 2500–6000 rpm, the pHi.e.p. shifted from pH 5.7 to pH ~8, while at 8000 rpm the shift in pHi.e.p. was smaller. Milling at 8000 rpm, the reduced milling performance was attributed to a change in the predominant milling mechanism. XPS revealed an approximate linear correlation between the relative surface alumina content (at.%) and particle specific surface area, with the shifting pHi.e.p. corresponding to the surface alumina. The lower pHi.e.p. at 8000 rpm was rationalized by high resolution TEM image analysis. Samples milled at 8000 rpm (beyond mill energies used in pigment production) produced a significant quantity of ultra-fines (d50 ≪ 50 nm) which coated the larger particles. These ultra-fines were predominately titania-like and suppressed the shift in pHi.e.p.. The study confirmed the aluminum-doped TiO2 particles were initially titania surface-rich with bulk alumina increasingly exposed during milling.