TiO2 doping by hydroxyurea at the nucleation stage: towards a new photocatalyst in the visible spectral range

We report an original method of preparation of OCN-doped TiO2 for photocatalysis in the visible spectral range. The preparation is achieved by a sol–gel route using titanium tetraisopropoxide precursor. Special attention was paid to fluid micromixing, which enables homogeneous reaction conditions in the reactor bulk and monodispersity of the produced clusters/nanoparticles. The dopant hydroxyurea (HyU, CH4N2O2) is injected into the reactive fluid at the nucleation stage, which lasts tens of milliseconds. The doping results in a strong yellow coloration of the nanocolloids due to the absorption band in the spectral range 380–550 nm and accelerates the aggregation kinetics of both nuclei at the induction stage and sub-nuclei units (clusters) at the nucleation stage. FTIR, Raman and UV-visible absorption analyses show the formation of a stable HyU–TiO2 complex. EXAFS spectra indicate no appreciable changes of the first-shell Ti atom environment. The doping agent takes available surface sites of TiO2 clusters/nanoparticles attaining ∼10% molar loading. The reaction kinetics then accelerates due to a longer collisional lifetime between nanoparticles induced by the formation of a weak O⋯Ti bond. The OCN-group bonding to titanium atoms produces a weakening of the CO double bond and a strengthening of the C–N and N–O bonds.