Monolithic GaAs Nanowires on Silicon with TiO2/MoS2 Surface Engineering for Efficient and Stable Photoelectrochemical Hydrogen Evolution

Photoelectrochemical (PEC) water splitting is a promising route to meet the escalating demand for clean energy and to mitigate its associated environmental impact. III–V semiconductors possess excellent optical properties and hold the record for solar-to-hydrogen conversion efficiency. However, their photocorrosion in aqueous electrolytes remains a major challenge. In this work, self-catalyzed GaAs nanowire (NW) photocathodes, grown monolithically on silicon (Si) substrates by molecular beam epitaxy, were protected with an amorphous titanium dioxide (TiO2) layer deposited by atomic layer deposition (ALD) and decorated with a conformal, ∼5.5 nm polycrystalline MoS2 co-catalyst grown by plasma-enhanced ALD. The TiO2 thickness was varied (15, 37, and 47 nm), where the 37 nm layer delivered the best performance: a saturated photocurrent density of 20 mA/cm2 at –0.54 V versus RHE and an onset potential of –0.30 V in 1.0 M H2SO4 under one-sun illumination, together with stable hydrogen evolution for more than 2 h of continuous operation. Growing the GaAs NWs directly on large-area Si substrates provided a scalable route toward high-performance PEC devices and low-cost multijunction solar-fuel systems.