Yttrium-Doped ZnO Nanorod Arrays for Increased Charge Mobility and Carrier Density for Enhanced Solar Water Splitting

Abstract

An innovative procedure is presented, when for the first time, yttrium-doped ZnO vertically aligned nanorods (NRs) have been synthesized using a unique rapid microwave-assisted method. In comparison with pristine ZnO NRs, the Y-doped samples present a more favorable morphology along with reduced crystallinity because of substitutional defects, YZn. 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 analyze 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. The 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 optimized, achieving a 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.