Photoactive silicon surfaces functionalized with high-quality and redox-active platinum diimine complex monolayers

Abstract

New platinum diimine dichloro (PtCl₂) and dithiolene (Ptdithiolene) complexes have been synthesized and covalently bound to hydrogen-terminated, oxide-free silicon(111) surfaces using a two-step procedure. This immobilization route resulted in the formation of ca. 2.0–2.5 nm-thick densely packed and perfectly clean Pt complex monolayers with the expected structure of the grafted molecular chains. The surface coverages of the attached PtCl₂ and Ptdithiolene were electrochemically estimated at 8.9 × 10⁻¹¹ and 7.5 × 10⁻¹¹ mol cm⁻², respectively, corresponding to 0.07 PtCl₂ and 0.06 Ptdithiolene per surface silicon atom. The Pt complex-functionalized photoactive silicon surfaces showed under simulated sunlight a redox activity similar to that observed for the complexes in solution at a non-photoactive electrode. The two one-electron reduction processes centered on the bipyridine ligand were however observed at much less negative potentials, owing to the photovoltage generated at the silicon/monolayer interface. These modified photocathodes showed great promise for solar photoelectrocatalysis, as exemplified with the photoassisted electrocatalytic reduction of CO₂. Based on the measured cathodic photocurrent densities, the Ptdithiolene-modified surface showed superior catalytic performance.