Metal–organic framework coated titanium dioxide nanorod array p–n heterojunction photoanode for solar water-splitting

This paper presents a p–n heterojunction photoanode based on a p-type porphyrin metal–organic framework (MOF) thin film and an n-type rutile titanium dioxide nanorod array for photoelectrochemical water splitting. The TiO₂@MOF core–shell nanorod array is formed by coating an 8 nm thick MOF layer on a vertically aligned TiO₂ nanorod array scaffold via a layer-by-layer self-assembly method. This vertically aligned core–shell nanorod array enables a long optical path length but a short path length for extraction of photogenerated minority charge carriers (holes) from TiO₂ to the electrolyte. A p–n junction is formed between TiO₂ and MOF, which improves the extraction of photogenerated electrons and holes out of the TiO₂ nanorods. In addition, the MOF coating significantly improves the efficiency of charge injection at the photoanode/electrolyte interface. Introduction of Co(Ⅲ) into the MOF layer further enhances the charge extraction in the photoanode and improves the charge injection efficiency. As a result, the photoelectrochemical cell with the TiO₂@Co-MOF nanorod array photoanode exhibits a photocurrent density of 2.93 mA/cm² at 1.23 V (vs. RHE), which is ~ 2.7 times the photocurrent achieved with bare TiO₂ nanorod array under irradiation of an unfiltered 300 W Xe lamp with an output power density of 100 mW/cm².