Zeta potential measurements of the suspensions were per-formed using a MALVERN 3000 HSA Zetasizer (Malvern In-struments Ltd.). In water suspensions with a concentration of
2.5 mg/ml, the pH values were adjusted from 2 to 11 using either 0.1 M nitric acid or 0.1 M sodium hydroxide. Then the
samples were allowed to stay undisturbed for 24 h prior to the
zeta potential measurements. Two runs were performed for all
samples. The particle size distribution (PSD) of the suspensions
was measured on a MALVERN 3000 HSA Zetasizer (Malvern
Instruments Ltd.) by a light-scattering spectrophotometer. Each
sample was measured 10 times, in which the refractive index of
TiO2was set to be 2.7.