Cetyltrimethylammonium bromide – Oleic acid (CTAB-OA) bilayer coated iron oxide nanocrystals for enhanced chromium (VI) photoreduction via ligand-to-metal charge transfer mechanism

Abstract

Photoactive cetyltrimethylammonium bromide-oleic acid (CTAB-OA) bilayer coated iron oxide nanocrystals (IONCs) were precisely synthesized and demonstrated for photo-enhanced hexavalent chromium (Cr(VI)) treatment. For these materials, an outward facing, CTAB layer offers specific affinity for Cr(VI), while the core IONC (8–25 nm, as Fe3O4 single domain) surface participates in Fe(II)/Fe(III) redox cycling. Further, at the coating-particle interface, the OA layer alters material photoactive dynamics (near UV and visible light) via a ligand-to-metal charge transfer (LMCT) mechanism, significantly enhancing Cr(VI) reduction. To delineate process variables, we first quantified photo-catalytic Cr(VI) reduction rates under UVA irradiation (sunlight mimic intensities) as a function of IONC core size, which correlates with the amount of surface available ferrous ion (Fe(II)). It was observed that 25 nm CTAB functionalized IONC showed outstanding Cr(VI) sorption and photoreduction performance compared with other IONC explored and controls ( e.g. TiO2 nanocrystals). We systematically explored the photoreduction mechanism(s) through a matrix of control experiments, considering solution pH, dissolved gas (air, O2, and CO2), along with hydroxyl and superoxide radicals. Results indicate that Fe(II) (at the IONC interface) acts as the primary Cr(VI) reductant and that Fe(III) (oxidized from Fe(II)) (re)cycles via a photocatalyzed pathway. Taken together, this work presents a clear physical and mechanistic description of CTAB-OA bilayer IONCs which, when optimized, are highly effective for Cr(VI) treatment under UVA irradiation. In addition to potential broad application for other contaminant targets that are reactive with Fe(II), which are many, this work also highlights the need for precise interfacial understanding of nano composite materials towards process optimization.