Modified P‐Type Si Photocathodes for Photochemical Hydrogen Generation: Surface Texturing, Molecular Derivatizing Reagents, and Noble Metal Catalysts for Multi‐Electron Processes

Modification of single-crystal (100), p-type Si photocathode material to improve photoelectrochemical H2 generation from H2O is reported. The (100) surface has been first modified by chemical etching to reveal pyramids (1–5 μM in height) having (111) sides to give a “textured” surface having lower specular reflection losses than polished (100) or (111) single-crystal Si. Second, the textured surface has been derivatized with {N,Ni-bis-[3-trimethoxysilyl)propyl]-4,4′-bipyridinium}dibromide, I, to yield large coverages (0.5–5 × 10−8 mol/cm2) of redox centers [PQ2+/+]surf in a polysiloxane network. The surface redox polymer is a fast electron acceptor and can be reduced photoelectrochemically, [PQ2+]surf [PQ+]surf, by ≥ Eg = 1.1 eV light at ∼0.0 to - 0.1 V vs. SCE in aqueous electrolyte solution, or ∼ 500 mV more positive than on a reversible electrode such as Pt. Third, the surface polymer is then further modified by depositing Pd(O) or Pt(O) onto the outermost surface to catalyze the reaction 2[PQ+]surf + 2H+ 2[PQ2+]surf + H2. Energy conversion efficiency from the modified photocathodes for 632.8 nm (∼ 10 mW/cm2) light-driven H2 evolution is ∼ 5% at pH = 4, representing significant improvement compared to polished, (100) or (111) p-type Si-based photoelectrochemical devices where the surface of the p-type Si is not modified.