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Controlling the Spatial Direction of Hydrothermally Grown Rutile TiO2 Nanocrystals by the Orientation of Seed Crystals

Hydrothermally grown TiO2 nanorods are a key material for several electronic applications. Due to its anisotropic crystal structure, the electronic properties of this semiconductor depend on the crystallographic direction. Consequently, it is important to control the crystal orientation to optimize charge carrier pathways. So far, the growth on common polycrystalline films such as fluorine tin oxide (FTO) results in randomly distributed growth directions. In this paper, we demonstrate the ability to control the growth direction of rutile TiO2 nanocrystals via the orientation of the seed crystals. The control of the orientation of such nanocrystals is an important tool to adjust the electronic, mechanical, and chemical properties of nanocrystalline films. We show that each employed macroscopic seed crystal provides the growth of parallel nanofingers along the [001] direction under specific angles. The parallel growth of these nanofingers leads to mesocrystalline films whose thickness and surface structure depends on the crystal orientation of the seed crystal. In particular, the structure of the films is closely linked with the known inner structure of hydrothermally grown rutile TiO2 nanorods on FTO. Additionally, comprehensive 1D structures on macroscopic single-crystals are generated by branching processes. These branched nanocrystals form expanded 2D defect planes, which provide the opportunity of defect doping-induced two-dimensional electronic systems (2DES).