Attempt To Determine Pristine Points of Zero Charge of Nb2O5, Ta2O5, and HfO2

Many empirical formulas have been proposed in the literature to estimate the pristine points of zero charge of oxides from ionic radii and coordination numbers or from electronegativities, and good correlations between experimental and calculated values were obtained. The significance of such studies is limited unless accurate and reliable experimental pristine point of zero charge (PPZC) data are used. Comparison of various compilations of PPZC data shows that more attention should be paid to the proper choice of an unique PPZC among the many various values reported for the same oxide in the literature. Only for a few oxides (RuO2, TiO2, ZrO2, CeO2, Fe2O3, and Al2O3) can very reliable PPZC values be accurately selected. For the other oxides the existing literature data still need verification, and for some oxides PPZC data are not available at all. In order to add more oxides to this list, an attempt to determine the pristine points of zero charge of Nb2O5, Ta2O5, and HfO2 was made. For Nb2O5 the point of zero charge and isoelectric point exactly match at pH 4.1, and this value coincides with literature data. This result fills a critical gap in the listing of PPZC values; namely, this is one of very few reliable PPZC values in the acidic range. For Ta2O5 the point of zero charge at pH 5.2 and the isoelectric point at pH 5.3 in the presence of NaClO4 obtained in the present study are also consistent, but the literature data are very scattered and most of them fall at considerably lower pH values. For HfO2 the point of zero charge at pH 7.4 and the isoelectric point at pH 7.1 in the presence of NaClO4 were obtained in the present study. The difference between these values is too significant to claim a reliable PPZC.