Illumination through shading: boosting photoactivity of light-absorbing materials by optically inactive coatings

Abstract We report the first-ever direct growth of a few layered graphene on WS ₂ nanotubes (NTs). This pioneering synthesis induced unprecedented electronic, optical, and electro-optical properties. By encapsulating WS ₂ NTs in graphene, a hybrid structure is created, combining the semiconductive properties of WS ₂ with the exceptional electronic properties of graphene. Photo-absorption spectroscopy identified distinct excitonic transitions in WS ₂ NTs and an additional π → π* transition in the hybrid structure, indicating strong interactions between the WS ₂ and the graphene. Finite-difference time-domain (FDTD) simulations revealed enhanced light-matter interactions in the WS ₂ /graphene core-shell system, demonstrating an 18% enhancement in the electric field and cavity mode confinement compared to the uncoated WS ₂ . The designed photoresponse is achieved by modifying the dielectric environment and the overlap of Fermi levels within the hybrid NTs. Density functional theory (DFT) calculations revealed substantial bandgap modulation due to graphene encapsulation, with an 87.5% reduction in the bandgap of the tubular WS ₂ NTs, compared to a 38.5% reduction in the 2D planar structure. This effect is driven by electronic hybridization at the WS ₂ /graphene interface and the stress-induced properties unique to the nanotubular geometry. Although WS ₂ is the only photoactive component of the composite, the addition of the non-photoactive graphene significantly enhanced the photoresponse of the hybrid structure. This property was exploited for photo-electrocatalysis (PEC) of hydrogen evolution reaction (HER). The WS ₂ /graphene nanocomposites exhibited a 49.1% enhancement in reduction current when the electrodes were exposed to light, compared to only an 8.9% increase observed for the uncoated WS ₂ under the same conditions. These results underscore the potential of WS ₂ /graphene NTs as tuneable materials for improved light absorption and charge carrier dynamics, offering a promising avenue for efficient photocatalysis and energy conversion technologies.