Mitigation of Edge and Surface States Effects in Two-Dimensional WS₂ for Photocatalytic H₂ Generation

Large scale development of the 2D transition metal di-chalcogenides (TMDC) relies on landmark improvement in performance, which could emerge from nanostructuration. Using p-WS₂ nanoflakes with different degrees of exfoliation and fracturing, perspectives were provided to develop high-surface-area 2D p-WS₂ films for the photocatalytic hydrogen generation. The critical role of inter-nanoflakes contacts within high-surface-area 2D films was demonstrated, highlighting the benefit of plane/plane versus edge/plane contacts. Evidence of the high density of surface states displayed by these 2D films was provided through electrochemical measurements. In addition to operating as recombination centers, the surface states were shown to give rise to deleterious Fermi-level pinning (FLP), which dramatically decreased the efficiency of charge carrier separation. Lastly, promising strategies yielding FLP suppression via surface states modification were proposed. In particular, use of a multifunctional ultrathin film displaying healing, catalytic, and n-type semiconduction properties was shown to greatly enhance charge carrier separation and transport to the photo-electrode/electrolyte interface. When the 2D photoelectrodes were fabricated with the above prerequisites (i. e., a high proportion of plane/plane contacts and a successful surface states chemical modification), a photocurrent up to 4.5 mA cm^-2 was achieved for the first time on 2D p-WS₂ photocathodes for hydrogen generation.