Crystals size and surface chemistry dependent phase diagram for nanocrystals of rutile and anatase: Experimental studies and computer modeling

It is well known that rutile is the thermodynamically stable phase of TiO2 under ambient conditions at the macroscale, and that anatase is the thermodynamically stable phase at the nanoscale. Both anatase and rutile have superior performance in a range of advanced photochemical applications. It is important for our understanding of the stability of nanostructures in different chemical and physical environments, because both rutile and anatase nanocrystals are used in different chemical and engineering environments. Using a size-, shape- and temperature-dependent thermodynamic model we have generated the first phase diagram for anatase and rutile nanocrystals by incorporating more experimentally relevant parameters (both the equilibrium shape and surface chemistry). Results from hydrothermal synthesis and DFT-based computer modeling show acidic environment favors rutile formation. The acidic solution also favors OH2-terminated surfaces of both anatase and rutile. The boundary between rutile an danatase ranges from ~ 10 nm to ~ 50 nm, depending on temperature and surface composition. The calculated phase map indicates that the equilibrium boundary between anatase and rutile nano-crystals is surface charge chemistry dependent, which relates to both their formation and post-synthesis environments.