Zinc oxide superstructures: Recent synthesis approaches and application for hydrogen production via photoelectrochemical water splitting

“Superstructures” are basically three dimensional (3D) nano/microstructures which arise at the expense of one dimensional (1D) and two dimensional (2D) nanostructures with controlled morphology and orientation. As compared to 1D and 2D nanostructures ZnO superstructures are superior because they possess good crystallinity, integrated stacked arrays with periodic arrangement, high surface area, and good conductivity. Over the past few decades specific focus has been given on the synthesis of ZnO superstructures in view of their astonishing physicochemical properties and promising applications in numerous fields. ZnO superstructures show exceptional performance for hydrogen production via photoelectrochemical (PEC) water splitting because of their certain advantageous properties such as high internal surface area, enhanced scattering with improved light harvesting, reduced recombination rate, low charge transfer resistance, better crystallinity, channelled conducting pathways, etc. These properties are majorly dependent on the various morphologies of ZnO superstructures. Recently different synthesis strategies have been developed to obtain a variety of morphologies of ZnO superstructures. In view of the search for renewable energy sources in the form of hydrogen, produced through water splitting using PEC cell, it is crucial to study different recent approaches of synthesizing ZnO superstructures and understand their role in enhancing PEC performance. Herein, we recap the principles governing hierarchy, complexity, orientation and crystallinity of ZnO superstructures. In general, they are distinguished by considering their geometrical shapes and common names used in the literature and discussion is done on the strategies acquired for their synthesis. This review summarizes the reasons behind the exceptional performance of ZnO superstructures and discusses those in detail. In the field of ZnO superstructures synthesis and their applicability for hydrogen production through PEC this review will not only help in basic understanding, but also ignite innovative ideas.