Black titania is a class of materials that are of interest due to their increased absorption in the visible spectrum, as well as their catalytic properties. They have potential applications as materials for photovoltaic (PV) cells, water splitting and waste treatment. Black titania has been synthesized via numerous reduction methods and with various dopants. While the resulting structure and performance of the material is dependant on synthesis, black titania is often described as a core-shell structure of anatase TiO2 nanoparticles surrounded by a layer of amorphized TiO2. This amorphized layer usually has an increased concentration of oxygen vacancies and Ti3+ ions relative to white TiO2. The possibility of ion irradiation-synthesized black titania is interesting because the passage of medium energy (< 1MeV) ions through a TiO2 particle has the potential to amorphize the entire volume rather than just a surface layer. Heavily Si-doped materials are also of interest to PV applications as Si quantum dots (QD) are strong absorbers in the infrared. The porosity of the TiO2 may decrease the Si-QD formation temperature to that which the porous TiO2 can survive.
In the present study, a porous layer of TiO2 nanoparticle film was spin coated on pieces of Pyrex and fused silica. These samples were implanted with 90 keV Si+ ions at the following fluences: 5.00x1014 cm-2, 1.88x1015 cm-2, 7.07x1015 cm-2, 2.66x1016 cm-2 and 1.00x1017 cm-2. The films were observed to be various shades of grey in colour and became increasingly darker at higher fluence. UV-vis absorption measurements confirmed near-uniform increased absorption across the visible spectrum. X-ray diffraction results showed the peaks of anatase-TiO2 superimposed on the broad amorphous background of the substrate. The peak intensity decreased with increasing fluence and almost disappeared completely at the highest fluence – suggesting complete destruction of TiO2 crystalline order. Depth-resolved positron annihilation spectroscopy (PAS) is a technique sensitive to point defects and open volume in thin films and layered structures. PAS results on these samples indicate increasing defect concentration with increasing fluence in a way that is uniform throughout the TiO2 layer.
These results indicate that the Si-ion synthesized black titania is more thoroughly amorphized than previously reported black titania materials – at the highest fluence the entire volume of the nanoparticles through the entire width of the film. Further studies will investigate the role of the implanted Si-ion and the impact of annealing.