Highly Efficient and Stable Molecular-Based TiO₂ Photoanodes for Photoelectrochemical Water Splitting Achieved by Pyridyl Anchoring Technique

Abstract

Photoelectrochemical overall water splitting by semiconductor electrodes modified with functional molecules has attracted considerable attention in recent years. Various kinds of molecular-based photoanodes consisting of a semiconductor thin film modified with both a photosensitizer (PS) and a water oxidation catalyst (WOC) have been developed thus far, and overall water splitting is achieved by using such a molecular-based photoanode and a Pt cathode. Nevertheless, due to the desorption of a PS and/or a WOC from the semiconductor surfaces, almost all the reported molecular-based photoanodes lose their photoelectrocatalytic activity within an hour. Thus, there is a strong demand to greatly improve the long-term stability of the molecular-based photoanodes toward practical applications. Here, we demonstrate the effectiveness of the "pyridyl anchoring technique" developed by us, leading to the long-term stability of our molecular-based photoanodes owing to the high strength of the Ti–N (pyridyl) bonding. A molecular-based TiO2 photoanode modified with both a polypyridyl ruthenium PS, [Ru(dpbpy)2(qpy)]2+ (dpbpy = 4,4′-diphenyl-2,2′-bipyridine, qpy = 4,4′:2′,2″:4″,4‴-quaterpyridine) (Ru-qpy), and a Ru(bda)-type WOC, Ru(bda)(4,4′-bpy)2 (bda = 2,2′-bipyridine-6,6′-dicarboxylic acid, 4,4′-bpy = 4,4′-bipyridine) (Ru(bda)-py) by our technique promotes water oxidation with an almost quantitative Faradaic efficiency (94 ± 6%) at the applied potential of 0.05 V versus SCE over 3 h under solar light irradiation (λ > 410 nm). Moreover, a photoelectrochemical cell (PEC) consisting of this molecular-based photoanode and a Pt cathode promotes overall water splitting only by giving an extra bias of 0.4 V. Our PEC achieves the second highest solar-to-hydrogen (STH) conversion efficiency (0.07%) among such applied bias-compensating PECs, successfully demonstrating the usefulness of the stable anchoring of molecular components in order to fabricate highly efficient PECs for solar water splitting.