Topotactic Growth, Selective Adsorption, and Adsorption-Driven Photocatalysis of Protonated Layered Titanate Nanosheets

Layered titanates with selective adsorption ability and adsorption-driven photocatalytic property can be quite attractive due to their potential applications in water purification. In this work, lepidocrocite-like layered protonated titanate (H2Ti2O5·H2O, denoted as HTO) nanosheets were successfully synthesized by an ion-exchange process. It turns out that this layered structure displays an abundant and selective adsorption toward the fluoroquinolone pharmaceutical compared with some large dye molecules due to a size selectivity of the interlayer spacing of HTO and the molecular horizontal size, as well as their electrostatic interaction. The uptake ability of HTO could be readily controlled through adjusting the pH values of adsorbate solution, and the maximum uptake capacity was achieved at the pH value of about 5.5 for ciprofloxacin (CIP) and 6.5 for moxifloxacin (MOX). The adsorption amount of smaller nalidixic acid (NAL) showed an increasing tendency as the pH value decreased. Moreover, the two-dimensional layered crystal structure also permits such HTO nanosheets to have a large percentage of (010) faces exposed, which is considerably provided by the interlayer surfaces of these nanosheets. The (010) surface has a similar Ti and O atomic arrangement as to the highly reactive anatase TiO2(001) one. Due to these specific characteristics, these HTO nanosheets show excellent photocatalytic activity in degrading CIP under UV light irradiation as well as possess a superior adsorption ability to remove CIP from aqueous solution selectively and efficiently. The photocatalytic reaction is believed to be mainly conducted on the active anatase (001)-like interlayer (010) surfaces of the layered structures since the as-prepared HTO performs an adsorption-driven molecular recognitive photocatalytic reaction.