The substrate-free hydrolysis of Zn nanocrystals was investigated as the second step in a ZnO/Zn solar water-splitting thermochemical cycle. In this work, we combined two different ion-mobility schemes in series to study the hydrolysis kinetics of size-selected zinc nanocrystals (NCs). The first mobility characterization size selects particles with a differential mobility analyzer (DMA). The second mobility characterization employs an aerosol particle mass analyzer (APM) and measures changes in mass resulting from a controlled hydrolysis of the Zn NCs. A low temperature reaction mechanism is proposed to explain the mass change behavior of Zn NCs hydrolysis at 100-250 degrees C. An Arrhenius law was used to extract the reaction kinetic parameters. The hydrolysis activation energy and the order of the reaction with respect to water mole fraction were found to be 24 +/- 2 kJ/mol and 0.9 +/- 0.1, respectively. Complete conversion of 70 nm Zn NCs was achieved at 175 degrees C with a residence time of about 10 s and water vapor mole fraction of 19%. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.