We propose that embedding silicon nanoparticles (NP) into amorphous, nonstoichiometric ZnS leads to promising nanocomposites for solar energy conversion. Using ab initio molecular dynamics simulations we show that, upon high temperature amorphization of the host chalcogenide, sulfur atoms are drawn to the NP surface. We find that the sulfur content may be engineered to form a type II heterojunction, with complementary charge transport channels for electrons and holes, and that sulfur capping is beneficial to lower the nanoparticle gap, with respect to that of NPs embedded in oxide matrices. Our analysis is conducted using density functional theory with local and hybrid functionals and many body perturbation theory at the GW level.