Improvement of Photoelectrochemical Hydrogen Generation by Surface Modification of p-Type Silicon Semiconductor Photocathodes

The improvement of H₂ evolution from two different types of catalytic p-type photocathode surfaces has been examined. p-Type Si has been platinized by photoelectrochemically plating Pt(0) onto the Si surface. Such a photocathode shows significant improvement (compared to naked p-type Si) for photochemical H₂ evolution with respect to output photovoltage, fill factor, and overall efficiency. Such photocathodes having an optimum amount of Pt(0) give a pH-dependent output voltage with respect to the H₂O/H₂ couple, but the dependence is not a simple 59-mV/pH dependence. No pH dependence would be expected if Pt(0) formed a Schottky barrier when plated onto p-type Si. A second kind of H₂ evolution catalyst has been confined to the surface of p-type Si. Polymeric quantities of an electroactive N,N′-dialkyl-4,4′-bipyridinium reagent, (PQ^2+/+)n. have been confined to the surface. The Br^- counterions of the polymer are then exchanged by PtCl₆²⁻. Photoreduction then yields Pt(0) dispersed in the polymer. Such a surface is again significantly improved compared to naked p-type Si with respect to H₂ evolution. A comparison of the naked p-Si, the simply platinized, and the [(PQ^2+/+)_n·nPt(0)]_surf. system is made and contrasted to the expected behavior of an external Schottky barrier photocell driving an electrolysis cell with a Pt cathode. Experiments with n-type MOS₂, n-type Si, Pt, Au, and W cathodes functionalized with the [(PQ^2+/+)_n·nPt(0)]_surf. system compared to the same surfaces directly platinized confirm an important difference in the mechanism of H₂ evolution catalysis for the two surface catalyst systems. For the [(PQ^2+/+)_n·nPt(0)]_surf. system there is an optimum pH for the catalysis, consistent with the pH-independent formal potential of the (PQ^2+/+)_n system, −0.55 ± 0.05 V vs. SCE, relative to the formal potential of the (H₂O/H₂) couple that moves 59 mV per pH unit. Qualitative experiments with insulating glass surfaces derivatized with [(PQ^2+/+)_n]_surf. establish directly that the Pt(0) is necessary, and sufficient, to equilibrate (PQ^2+/+)n with (H₂O/H₂). p-Type Si modified with optimum amounts of Pt(0) by direct platinization appears to give improved H₂ evolution efficiency by a mechanism where the Pt(0) serves as a catalyst that does not alter the interface energetics of the semiconductor.