Benign Synthesis of Black Microspheres of Anatase TiO₂ with Paramagnetic Oxygen Vacancies through NH₃ Treatment

Abstract

Coloured TiO₂ is coveted for its ability to extract energy from the visible region of electromagnetic spectrum. Here a facile synthesis of black anatase titania microspheres (B-TiO₂ ) through a two-step process is reported. In the first step, amorphous white TiO₂ microspheres (W-TiO₂ ) are obtained by hydrolysing titanium tetraisopropoxide by ammonia vapours in ethanol. In the second step, the W-TiO₂ is thermally annealed at 500 °C to obtain B-TiO₂ . The diffuse reflectance analysis showed that B-TiO₂ absorbs across visible spectrum with absorption extending well into NIR region. Raman scattering together with EPR analysis showed compelling evidence of the existence of oxygen deficiency within the crystal in B-TiO₂ that induces black colouration in the sample. The defects present in the black anatase sample were confirmed to be single-electron-trapped (or paramagnetic) oxygen vacancies (V_o ⋅) by XPS and EPR studies. The magnetic susceptibility studies showed existence of antiferromagnetic interactions between these unpaired electron spins.