Bias-Free Photoelectrochemical Water Splitting Cells Constructed by Calixarene Dyes and Molecular Ru Catalysts via Pyridyl Anchoring Groups

Two metal-free calix[4]­arene dyes with pyridyl anchoring groups (C4BTP and C4STP) and a comparative single-chain dye (M-BTP) for comparison have been synthesized and applied in dye-sensitized photoelectrochemical cells in corporation with a pyridyl anchor-based molecular water oxidation catalyst (RuPy). All the devices based on the three dyes can conduct overall water splitting reaction under visible light with low or zero bias. It is confirmed that the cone structure and multiple light-harvesting units of calixarene dyes bring superior durability and photoelectroconversion property compared with chain dyes M-BTP, and the introduction of alkalinous pyridyl anchoring groups improves the adsorption firmness on TiO₂ in aqueous solution through the Ti–N bond. The optimized TiO₂|C4BTP+RuPy photoanode gives the highest photocurrent density of over 800 μA cm^–2, while the TiO₂|C4STP+RuPy photoanode exhibits the best Faradaic efficiency of 64.9% for O₂ evolution. To meet the needs of practical applications, the TiO₂|C4BTP+RuPy photoanode is utilized in a two-electrode system with 0 V bias, giving a steady photocurrent density of 42 μA cm^–2 in 3600 s and a Faradaic efficiency of 45.9% for O₂ evolution. This study offers an inspiration for developing efficient and durable organic photosensitizers in photocatalytic water-splitting devices.