The inclusion of metal nanoparticle electrocatalysts on semiconductor photoelectrodes is often used to increase the efficiency of photocatalytic reactions on semiconductors. It is well established that metal electrocatalysts can limit overpotential losses associated with many important photocatalytic reactions, significantly increasing the efficiency of the conversion of solar into chemical energy. On the other hand, the introduction of metal nanoparticles can also impact the light-absorbing properties of semiconductors. We have studied the impact of the introduction of Pt nanoparticles on the optical and catalytic properties of flat Si photoelectrodes for the photoelectrocatalytic evolution of hydrogen. We demonstrate that the deposition of platinum nanoparticles onto planar Si photocathodes results in improved catalytic rates for the hydrogen evolution reaction, but also a diminished light-limited rate of photon to hydrogen conversion. We also show that by embedding Pt nanoparticles into Si, the light-l...