Self-Assembled Gold Nanoparticle-Mixed Metal Oxide Nanocomposites for Self-Sensitized Dye Degradation under Visible Light Irradiation

Gold nanoparticle (Au NP)-mixed metal oxide (MMO) nanocomposite photocatalysts for efficient self-sensitized dye degradations under visible light were prepared by an electrostatically driven self-assembly. Dihydrolipoic acid (DHLA)-capped Au NPs (building block I) were synthesized through a room temperature reaction. Their hydrodynamic size was determined as being around 4.9 nm by dynamic light scattering measurements. MMO nanoplates with lateral dimensions of 100-250 nm (building block II) were prepared by a calcination of zinc aluminum layered double hydroxides at 750 degrees C for 2 h in air. In a pH 7.0 aqueous solution, the DHLA-capped Au NPs had a negative zeta potential (-22 +/- 3 mV); on the other hand, the MMO nanoplates had a positive zeta potential (15 +/- 2 mV). Electrostatic self-assembly was achieved by stirring an aqueous solution (pH 7.0) containing DHLA-capped Au NPs and MMO nanoplates at room temperature for 1 h. The self-assembled and sequentially calcined nanocomposites exhibited the superior self-sensitized dye degradation efficiency under visible light to that of ZnO, TiO2 (P25), or pure MMO nanoplates. The enhanced degradation efficiency could be attributed to strong coupling interactions of ZnO and ZnAl2O4 phases of the MMO and the role of Au as an electron sink and mediator for formations of reactive oxidation species and as a light concentrator.