As-prepared highly (00.1)-textured WSe 2 films are only weakly photoactive, presumably at the few edge steps/grain boundaries of the WSe 2 crystallites. To achieve an efficient hydrogen evolution, the number of catalytically active sites must be increased. This can be done by Pt deposition, which activates the grain boundaries or by chemical/electronic activation of the (00.1) vdW surfaces. The Mo 3 S 13 clusters in ATM exhibit a higher number of S-active sites compared to MoS 2 nanoparticles. There are two possible charge transfer mechanisms at the ATM/WSe 2 interface: the charge transfer through the ATM/WSe 2 junction, i.e. parallel to the c-axis (normal to vdW planes) as well as across the WSe 2 edges/steps, i.e. perpendicular to the c-axis (along vdW planes), where the ATM chemically passivates the WSe 2 . The diffusion length along the c-axis is one to two orders of magnitude less than the diffusion length perpendicular to the c-axis caused by the strongly anisotropic structure of the transition metal chalcogenides. The bonding in these materials within the van der Waals (vdW) planes is ionic-covalent, i.e. strong, while it is weak between the vdW planes. Both of these charge transfer mechanisms contribute to the overall photocurrent density at the SEI.