Synthesis of photoredox catalysts based on composite of graphene oxide and cobalt complexes

Abstract

Recently, graphene oxide (GO) has emerged to be a promising material for photocatalytic applications. Typically, its low photocatalytic activity can be improved by modification with visible light-absorbing molecules or dyes. In this work, we aim to develop an efficient photocatalyst based on GO by noncovalent fabrication with cobalt complexes containing porphyrin-derivative ligands. Meso-tetraphenylporphyrin cobalt(II) complex (CoTPP), cobalamin or vitamin B12 (B12), and TPP (for comparison) were used as our dyes in this study. First, GO was prepared by the modified Hummers’ method. Then, the dye-functionalized GO composites (GO-dye) were prepared by a simple self-assembly process using sonication method. It was found that only hydrophobic dyes (i.e., CoTPP and TPP) can form composites with GO. In addition, the interaction between GO and dyes was investigated by UV-vis and fluorescence. Absorption bands of dyes in GO-dye were considerably broadened and red-shifted as compared to those of dyes. The spectral changes are likely resulted from the dye aggregation on the GO solid substrate through π-π stacking and hydrophobic property. Fluorescence quenching of dyes by GO was also observed, presumably via photoinduced electron transfer (PET) process. Under light irradiation, dyes in GO-dye composites were found to exhibit better photostability than that of the dyes. Preliminary studies on reactivity of GO-dye composites as photocatalysts for substrate oxidation were carried out using benzyl alcohol as a representative substrate. It was found that GO-dyes exhibited higher reactivity than GO and the dyes, possibly due to cooperative photocatalysis.