Novel Green Biomimetic Approach for Preparation of Highly Stable Au-ZnO Heterojunctions with Enhanced Photocatalytic Activity

A simple, ecofriendly, and biomimetic approach using cumin seeds extract (CSE) was developed for the formation of Au-ZnO Schottky contact without employing any chemical capping agents or stabilizers. The various unique phytoconstituents available in cumin seeds extract synergistically convert Au3+ ions into Au-0 on the surface of ZnO, as each phytoconstituent is unique in context to its molecular structure and properties. The as-prepared biogenic Au-ZnO hybrid composites were examined using various spectroscopic and microscopic techniques. The TEM investigation and XRD patterns clearly depict the well dispersed AuNPs with the size range 10-15 nm and face centered cubic lattice on wurtzite ZnO nanostructures. The optical study of the nanocomposites showed two absorption bands: one intense band around 390 nm, which corresponds to ZnO, and a second broad band approximately around 540 nm, which corresponds to Au. The photocatalytic efficacy of Au-ZnO nanocatalysts was investigated by observing the mineralization of an aqueous solution of methylene blue (MB) dye under a 200 W tungsten filament lamp as visible light source. The apparent rate constants were also calculated for degradation processes, and it has been observed that 1 and 3 wt % Au-ZnO nanocomposites respectively have 2.27 and 3.2 times higher photoactivity, compared to pure ZnO. This enhanced photoactivity of biogenic Au-ZnO composite materials was resultant from formation of stable and effective Schottky contact between Au metal and ZnO surfaces.