Synthesis of brookite TiO₂ nanorods with isolated Co(ii) surface sites and photocatalytic degradation of 5,8-dihydroxy-1,4-naphthoquinone dye

Decomposition of Co₂(CO)₈ in the presence of ca. 4 nm × 20 nm oleic acid stabilized brookite TiO₂ nanorods was performed according to a prior report in the literature in which Co-TiO ₂ hybrid nanocrystals had been observed. The hybrid nanocrystals could not be duplicated; however, we report a procedure that consistently led to a mixture of blue Co(ii)-TiO₂ nanorods and Co precipitate. The Co(ii)-TiO₂ nanorods have single-site Co(ii) ions selectively attached to the TiO₂ nanocrystal surface. Transmission electron microscopy and powder X-ray diffraction data show the crystal phase and morphology of the nanorod is unchanged on addition of Co(ii) and no new crystal phases or particulate domains are associated with the colloid. A combination of UV-visible, X-ray photoelectron, and X-ray absorption spectroscopic analysis indicates Co(ii) sites are present on the surface of the TiO₂ nanorods in octahedral and tetrahedral coordination in ∼1:1 ratio. A mechanism is proposed in which the Co-Co bond of the precursor undergoes heterolysis followed by disproportionation of Co(i) to yield Co(ii) and Co(0) precipitate. The Co(ii)-TiO₂ nanorods were shown to exhibit greater activity than TiO₂ nanorods in the degradation of 5,8-dihydroxy-1,4-naphthoquinone dye under visible light irradiation.