Self-Assembled Vertically Aligned Hetero-Epitaxial ZnO/CdS Core/Shell Array by all CBD Process: Platform for Enhanced Visible-Light-Driven PEC Performance

We report on an all chemical bath deposition (CBD) fabrication of self-assembled vertically aligned heteroepitaxy grown (002)_ZnO//(002)_CdS ZnO/CdS core/shell nanorod (NR) arrays. These single-technique processed NR arrays comprising of CdS QDs conformally and epitaxially grown on faceted single-crystalline ZnO NRs achieve effective charge separation and collection, resulting from visible light driven highest ever, reported photocurrent density without any postdeposition process step, unlike previous such studies. The low ion-flux controlled ion-by-ion deposition induces chemical-epitaxy growth mechanism, with a 38° tilt of CdS c-axis. Structural characterizations confirm the homogeneous growth of (002) CdS QDs (size ∼30 nm) on hexagonal prismaticc-axis oriented ZnO NRs (400 nm diameter). Compared with pristine ZnO NR array, a 20-fold enhancement in photocurrent density (∼8.5 mA/cm² at +1.0 V vs Ag/AgCl) under AM1.5 light illumination is observed without any additional passivation/sensitization layer, attributed to the enlarged defect-free sharp ZnO/CdS interface and improved optical properties involving high visible light absorption as well as suppressed recombination of photoinduced charge carriers due to staircase type-II band alignment and hence effective charge separation and transport. Low-temperature short-time anneal further enhances the photocurrent density to 9-10 mA/cm² and photoconversion efficiency to 4.4%. Our results demonstrate that formation of atomically aligned interface is key to realizing high performance junction without any postdeposition step and have significant bearing on photoelectrochemical as well as photovoltaic performance of ZnO/CdS radial junction devices.