Defect-Mediated Crystallization of the Particulate TiO₂ Photocatalyst Grown by Atomic Layer Deposition

Nanopowders or films of pure and mixed oxides in nanoparticulate form have gained specific interest due to their applicability in functionalizing high-surface-area substrates. Among various other applications, our presented work primarily focuses on the behavior of TiO₂ as a photocatalyst deposited by atomic layer deposition (ALD) on a quartz particle. The photocatalytic activity of TiO₂ on quartz particles grown by ALD was studied in terms of ALD growth temperature and post-treatment heating rate. Amorphous TiO₂ thin films (30 nm) were grown from tetrakis(dimethylamido)titanium (TDMAT) at 100 and 200 °C on quartz particles (0.35-3.5 μm) and crystallized using oxidative heat treatment at 500 °C with variable heating rates. The growth temperature was found to affect the TiO₂ defect structure: TiO₂ grown at 200 °C is black due to Ti³⁺ defects, whereas the film grown at 100 °C is white but contains some traces of the TDMAT ALD precursor. During the oxidative heat treatment, precursor traces desorbed and Ti³⁺ defects were oxidized. ALD TiO₂ grown at 100 °C crystallized as anatase, whereas the rutile-to-anatase ratio of 200 °C grown TiO₂ increased with the heating rate. The hydrogen production rate of mixed-phase TiO₂ was found to outperform that of anatase TiO₂.