Core-substituted naphthalenediimides anchored on BiVO4 for visible light-driven water splitting

In this work, a novel catalytic system for the sunlight-driven water splitting reaction, which exploits the photocatalytic ability of BiVO4 coupled to a new kind of noble-metal-free organic dye molecules, is proposed. Hence, mono- and di-substituted naphthalenediimides (NDIs) were designed to have different functional groups that provide to them both tunable optical properties and adjustable HOMO/LUMO levels, and were selectively prepared (starting from 1,4,5,8-naphthalenetetracarboxylic acid) achieving yields >69%. Smart anchoring groups (i.e. carboxylates or aromatic amines) were added to the dyes in order to allow them to covalently bond to acidic –OH groups present on the BiVO4 surface. An easy and low-cost room temperature dip-coating technique was used to dye-sensitize both BiVO4 powders and thin films. NMR, MS, FT-IR, TG, FESEM, XRD, XPS and optical analyses confirmed the successful organic synthetic routes and good dyes/BiVO4 linkages. Photochemical and photoelectrochemical water oxidation reaction tests, together with DFT calculations, demonstrated that a proper alignment of the semiconductor/NDI-based dye energy levels is fundamental for enhancing the photocatalyst performance through a Z-scheme mechanism. The ability of the NDI organic molecules to delocalize the electronic charges was also a key factor for minimizing recombination processes and achieving more than a ten-fold increase in the photocurrent density of a 6 cm2 BiVO4 photo-electrode. The here reported results open new perspectives for the utilization of this new series of core-substituted NDIs, which are able to improve the activity of photocatalysts for different sunlight-driven applications, e.g. waste water treatment and organic contaminants’ degradation, other than the production of solar fuels by water splitting mechanisms.