Impact of electron–phonon coupling, Stokes shift and quantum confinement in boron nitride quantum dots for hydrogen production via water splitting

Two-dimensional-materials-based quantum dots are emerging materials platforms for energy conversion applications, for example as electrocatalysts for hydrogen production. The active sites or the surface defects (optically and electrically addressable), such as vacancies and dangling bonds, lead to electron-phonon scattering and hence result in the appearance of a phonon sideband. The quantification and systematic control of the electron-phonon interaction are needed in energy conversion and quantum technological applications. In this work, the electron-phonon coupling in hydrothermally synthesized boron nitride quantum dots (BNQDs) is controlled, and the strength of the electron-phonon coupling is quantified in terms of Huang-Rhys factors, Stokes shift, and quantum confinement. The application of controllably synthesized active sites in the BNQDs in hydrogen production is investigated. To quantify the strength of electron-phonon interaction, the Huang-Rhys factor, S, is calculated. The BNQDs with weak electron-phonon coupling, large Stokes shift (1.36 eV), and smaller size (≈4-5 nm) show enhanced electrocatalytic activity for hydrogen evolution. The sample with a greater number of surface states shows a lower average exciton lifetime, lower S factor, and reduction in charge transfer resistance. This is attributed to a greater number of active sites and good charge transfer. The higher electrochemically active surface area (26.1 cm²) confirms that there are numerous active sites, resulting in higher hydrogen evolution. Hence, the role of quantum confinement, electron-phonon coupling, and surface states is established. This research work gives insights into the application of surface states for energy applications such as hydrogen production, a new sustainable energy source for the world.