Immobilising molecular Ru complexes on a protective ultrathin oxide layer of p-Si electrodes towards photoelectrochemical CO2 reduction

Abstract

Photoelectrochemical CO₂ reduction is a promising approach for renewable fuel generation and to reduce greenhouse gas emissions. Owing to their synthetic tunability, molecular catalysts for the CO₂ reduction reaction can give rise to high product selectivity. In this context, a Ru^II complex [Ru(HO-tpy)(6-mbpy)(NCCH₃)]²⁺ (HO-tpy = 4′-hydroxy-2,2′:6′,2′′-terpyridine; 6-mbpy = 6-methyl-2,2′-bipyridine) was immobilised on a thin SiO_x layer of a p-Si electrode that was decorated with a bromide-terminated molecular layer. Following the characterisation of the assembled photocathodes by X-ray photoelectron spectroscopy and ellipsometry, PEC experiments demonstrate electron transfer from the p-Si to the Ru complex through the native oxide layer under illumination and a cathodic bias. A state-of-the-art photovoltage of 570 mV was determined by comparison with an analogous n-type Si assembly. While the photovoltage of the modified photocathode is promising for future photoelectrochemical CO₂ reduction and the p-Si/SiO_x junction seems to be unchanged during the PEC experiments, a fast desorption of the molecular Ru complex was observed. An in-depth investigation of the cathode degradation by comparison with reference materials highlights the role of the hydroxyl functionality of the Ru complex to ensure its grafting on the substrate. In contrast, no essential role for the bromide function on the Si substrate designed to engage with the hydroxyl group of the Ru complex in an S_N2-type reaction could be established.