ZnO nanoflakes self-assembled from the water splitting process using a hydroelectric cell

Abstract

Self-assembled ZnO nanoflakes grown at the zinc electrode of a hydroelectric cell by water splitting have been analyzed. ZnO nanoflake growth in the hydroelectric cell is a facile and cost effective green technique compared to other chemical synthesis processes. The formation of ZnO nanoflakes in the hematite based hydroelectric cell does not involve any template/capping agent/acid or base but only a few ml of water. The hydroelectric cell consists of a nonporous and oxygen deficient hematite pellet attached with the zinc anode and the silver inert cathode on opposite faces of the hematite square shaped pellet. The dissociation of water molecules into H₃O⁺ and OH⁻ ions occurs spontaneously on surface Fe cations and surface oxygen vacancies on hematite. The electrochemical reaction of water dissociated OH⁻ ions with the highly active Zn anode produces Zn(OH)₂ nanoparticles. It forms self-assembled zinc hydroxide nanoflakes from the green electricity source hydroelectric cell. Dehydrated self-assembled zinc hydroxide provided highly porous ZnO nanoflakes grown along the preferred (002) polar plane, verified by X-ray diffraction and TEM images. The surface area of ZnO nanoflakes has been obtained (144.28 m² g⁻¹) using a BET surface area analyser. Surface vacancies and interstitial defects in ZnO have been identified by Raman, photoluminescence, and XPS spectroscopy. The non-stoichiometric surface defects – Zn (Zn_i), O (O_i) interstitial defects and Zn (V_Zn), O (V_O) vacancies in the ZnO lattice have been reflected in time resolved photoluminescence measurements. Time-resolved photoluminescence spectra analysis showed that V_zn and V_o states are responsible for the blue and green emission. Such defective ZnO nanoflakes may be applicable as photo catalysts under sunlight/visible light due to surface oxygen defects observed by green emission at 540 nm. The self-growth efficient process of ZnO nanoflakes innovated in this work is an easily scalable technique for industrial production possessing huge potential applications in photocatalysis and paper industries.