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,Nî‐bis‐[3‐trimethoxysilyl)propyl]‐4,4′‐bipyridinium}dibromide, I, to yield large coverages (0.5–5 × 10⁻⁸ mol/cm²) of redox centers [PQ^2+/+]_surf in a polysiloxane network. The surface redox polymer is a fast electron acceptor and can be reduced photoelectrochemically, [PQ²⁺]_surf → [PQ⁺]_surf, by ≥ E_g = 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.