Anatase coarsening kinetics under hydrothermal conditions as a function of pH and temperature

Nanoparticles coarsen in solution via atom-by-atom dissolution and reattachment (Ostwald ripening) or via crystallographically controlled attachment of discrete particles (oriented attachment). We coarsened anatase nanoparticles in hydrothermal solutions over the temperature range 105 - 250 degrees C and the pH range 1 - 12 and documented three distinct coarsening mechanisms controlled by solution pH. Anatase growth at pH > 11 occurs primarily by Ostwald ripening, yielding a wide particle size distribution. Over the pH range of 2 through 11, anatase grows both by oriented attachment and Ostwald ripening. At pH values < 2, anatase simultaneously grows by oriented attachment and dissolves, precipitating as rutile. Oriented attachment is significant at values nearer to the pH of point of zero net proton charge (PZC), where anatase is very insoluble and has low surface charge. Rutile precipitation at low pH and anatase growth at high pH are promoted by enhanced Ti solubility at pH values far from the PZC. Understanding solution pH effects on assembly and coarsening kinetics may enable improved control of nanoparticle properties.