Yttrium doped ZnO nanorod arrays for increased charge mobility and carrier density for enhanced solar water splitting

An innovative procedure is presented, when for the first time, yttrium doped ZnO vertically aligned nanorods have been synthesized using a unique rapid microwave assisted method. In comparison with pristine ZnO NRs, the Y-doped samples present more favourable morphology along with reduced crystallinity due to substitutional defects, YZn. The Y acted as a shallow donor type defect, leading to an 80% increase in dopant density, to 1.36×1018 cm-2 in the 0.15% Y sample. The transmission line model was used to analyse the transport properties. It was found that a 1000-fold increase in conductivity and electron mobility was achieved by doping 0.15% Y, resulting in a high density of donors which fill charge traps. Meanwhile, a significant improvement in conductivity was accompanied by greater electron hole recombination and band gap reduction. Analysis of photoluminescence spectra reveals the effect of Y doping on native point defects, initially reducing Zn2+ vacancies by filling with YZn, followed by the reduction of O2- vacancies with interstitial doping at higher Y concentration. With a fine balance of superior conductivity and charge recombination rate, the photocatalytic water splitting performance was optimised achieving photocurrent of 0.84 mA cm-2 at 1.23 VRHE with 0.1% Y doping. This corresponded to a 47% enhancement in photoconversion efficiency compared to the pristine sample.