Density functional theory (DFT) and time-dependent DFT calculations have been performed on a set of 34 titanium dioxide clusters ((TiO2)n with n ≤ 125) to investigate structural and electronic properties of nanostructured TiO2 (nano-TiO2) materials. The investigated clusters include models of the three low-energy polymorphic forms of TiO2 anatase, rutile, and brookite. A systematic comparison of clusters of increasing size show clear trends for emerging bulk properties in the investigated systems as the surface-to-bulk ratio changes from small clusters dominated by undercoordinated surface atoms to more realistic model nanocrystals with significant bulk components. Differences and similarities in terms of atomic coordination, structural stability, and electronic properties for the three different polymorphic forms of nano-TiO2 are discussed. The calculations provide evidence for emerging polymorphism with increasing cluster sizes so that the different TiO2 forms can be clearly distinguished based on structural characteristics associated with the local bonding environment of the constituent atoms. © 2013 Wiley Periodicals, Inc.