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

Abstract

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–TiO2 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.