Green Synthesis and Catalytic Activity of Gold Nanoparticles/Graphene Oxide Nanocomposites Prepared By Tannic Acid.

Tannic acid is a phenolic compound that is abundant in plants. Five different concentrations of tannic acid were used as a reducing agent to synthesize gold nanoparticles (AuNPs). Three kinds of AuNPs were prepared to evaluate their catalytic activity for the 4-nitrophenol reduction reaction in the presence of sodium borohydride: (i) Colloidal solutions of AuNPs synthesized using tannic acid as a reducing agent (TA-AuNPs), (ii) Nanoparticles made by centrifuging the colloidal solution of TA-AuNPs followed by re-dispersion with deionized water (cf-TA-AuNPs), and (iii) Nanoparticles made by the in situ crystallization of TA-AuNPs on graphene oxide (TA-AuNPs-GO). TA-AuNPs, cf-TA-AuNPs and TA-AuNPs-GO exhibited a characteristic surface plasmon resonance band at 527~564 nm. High resolution transmission electron microscopy images revealed spherical-shaped nanoparticles. The rate constants of 4-nitrophenol reduction reaction increased for all three types of AuNPs with decreasing tannic acid concentration used during the synthetic process. The rate constants of cf-TA-AuNPs and TA-AuNPs-GO increased 1.45~1.64-fold and 2.25~4.49-fold, respectively, compared with the rate constants of TA-AuNPs. The conversion yields from 4-nitrophenol to 4-aminophenol in the presence of the cf-TA-AuNPs catalysts were measured using reverse phase high performance liquid chromatography. The conversion yield was excellent in a range of 97.57~99.43%.