Band Gap Energy and Lattice Distortion in Anatase TiO Thin Films Prepared by Reactive Sputtering with Different Thicknesses.

TiO is an abundant material on Earth, essential for the sustainable and cost-effective development of various technologies, with anatase being the most effective polymorph for photocatalytic and photovoltaic applications. Bulk crystalline anatase TiO exhibits a band gap energy E = 3.2 eV, for tetragonal lattice parameters a = 0.3785 nm and c = 0.9514 nm, but these characteristics vary for amorphous or polycrystalline thin films. Reactive magnetron sputtering has proven suitable for the preparation of TiO coatings on glass fiber substrates, with structural and optical characteristics that change during growth. Below a minimum thickness (t < 0.2 μm), the films have an amorphous nature or extremely small crystallite sizes not observable by X-ray diffraction. Afterwards, compressed quasi-randomly orientated crystallites are detected (volume strain ΔV = -0.02 and stress σ = -3.5 GPa for t = 0.2 μm) that evolve into relaxed and preferentially (004) orientated crystallites, reaching the standard anatase values at t ~ 1.4 μm with σ = 0.0 GPa. The band gap energy increases with lattice distortion according to the relation ∆E (eV) = -6∆V, and a further increase is observed for the thinnest coatings (∆E = 0.24 eV for t = 0.05 μm).