Control of Carrier Recombination on ZnO Nanowires Photoelectrochemistry

Abstract

Surface recombination on ZnO nanowires (NWs) dominates over photogenerated carrier collection unless band bending and electrochemical reactions facilitate charge separation. We studied acetonitrile electrolyte with tetrabutylammonium perchlorate, with and without redox mediators (ferrocene/ferrocenium, iodide/triiodide, and p-benzoquinone, BQ0/–). Faradaic processes successfully compete with recombination under biases that cause band bending along the NW c axis. Redox processes control carrier recombination mainly by hole (h+) removal and this affects the NW stability. Without redox mediator, recombination controls the photoelectrochemical behavior unless h+ oxidizes oxide ions in the lattice. Iodide stabilizes the material because the rate of I– oxidation by h+ is faster than the decomposition of ZnO. The conduction band edge (EC) of NWs and of single crystals (SCs) terminated on the (1 0 1̅ 0) planes that constitute most of the NW surface was determined to be EC = −0.3 ± 0.1 V vs NHE; the decomposition potential is estimated to be 3.7 V. The NW photocurrent onset is positive of EC because surface recombination is S ≥ 53 cm/s; S increases with surface hydroxide content. The surface states responsible for this recombination lie under EC and are proposed to result from NW surface reconstruction.