Highly Efficient\nand Stable Molecular-Based TiO₂ Photoanodes for Photoelectrochemical\nWater Splitting Achieved\nby Pyridyl Anchoring Technique

Abstract

Photoelectrochemical overall water splitting by semiconductor\nelectrodes\nmodified with functional molecules has attracted considerable attention\nin recent years. Various kinds of molecular-based photoanodes consisting\nof a semiconductor thin film modified with both a photosensitizer\n(PS) and a water oxidation catalyst (WOC) have been developed thus\nfar, and overall water splitting is achieved by using such a molecular-based\nphotoanode and a Pt cathode. Nevertheless, due to the desorption of\na PS and/or a WOC from the semiconductor surfaces, almost all the\nreported molecular-based photoanodes lose their photoelectrocatalytic\nactivity within an hour. Thus, there is a strong demand to greatly\nimprove the long-term stability of the molecular-based photoanodes\ntoward practical applications. Here, we demonstrate the effectiveness\nof the “pyridyl anchoring technique” developed by us,\nleading to the long-term stability of our molecular-based photoanodes\nowing to the high strength of the Ti–N (pyridyl) bonding. A\nmolecular-based TiO₂ photoanode modified with both a polypyridyl\nruthenium PS, [Ru(dpbpy)₂(qpy)]²⁺ (dpbpy = 4,4′-diphenyl-2,2′-bipyridine,\nqpy = 4,4′:2′,2″:4″,4‴-quaterpyridine)\n(<b>Ru-qpy</b>), and a Ru(bda)-type WOC, Ru(bda)(4,4′-bpy)₂ (bda = 2,2′-bipyridine-6,6′-dicarboxylic acid,\n4,4′-bpy = 4,4′-bipyridine) (<b>Ru(bda)-py</b>) by our technique promotes water oxidation with an almost quantitative\nFaradaic efficiency (94 ± 6%) at the applied potential of 0.05\nV versus SCE over 3 h under solar light irradiation (λ >\n410\nnm). Moreover, a photoelectrochemical cell (PEC) consisting of this\nmolecular-based photoanode and a Pt cathode promotes overall water\nsplitting only by giving an extra bias of 0.4 V. Our PEC achieves\nthe second highest solar-to-hydrogen (STH) conversion efficiency (0.07%)\namong such applied bias-compensating PECs, successfully demonstrating\nthe usefulness of the stable anchoring of molecular components in\norder to fabricate highly efficient PECs for solar water splitting.