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 Au0 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.