Atomic layer deposited molybdenum disulfide on Si photocathodes for highly efficient photoelectrochemical water reduction reaction

Abstract

Molybdenum disulfide (MoS₂) is an earth-abundant and low-cost hydrogen evolving electrocatalyst that can substitute noble metal catalysts. Atomic layer deposition (ALD) is a reliable and scalable process where MoS₂ nanomaterials grow directly on Si with a precise film thickness and composition. Here, we demonstrate high-performance Si photocathodes with MoS₂ cocatalysts using ALD for the photoelectrochemical (PEC) water reduction reaction. While the morphology and thickness of MoS₂ is controlled by ALD reaction cycles, post-sulfurization at high temperatures is conducted to form stoichiometric MoS₂ and dramatically enhances the crystallinity of MoS₂ to maximize the catalytically active edge sites of basal planes. A systematic study was performed to investigate the role of ALD and post-sulfurization parameters on PEC performances of MoS₂ on Si photocathodes. By optimizing the crystallinity, edge site density, stoichiometry, and morphology of MoS₂ for maximum electrochemical HER performance and minimum optical and electrical losses, our Si photocathodes with ALD-MoS₂ cocatalysts showed reduction of an overpotential of about 630 mV compared to bare Si. The photocathodes also showed a saturating photocurrent density of 31 mA cm⁻² without noticeable degradation during a 24 hour stability test in 0.5 M H₂SO₄ under a simulated 1 sun illumination.