Highly active Cu-freudenbergite/TiO2 heterojunction for solar-driven hydrogen evolution and 5-hydroxymethylfurfural oxidation

Abstract

The pursuit of efficient photocatalytic systems for solar light-driven hydrogen evolution (HER) drives the search for novel semiconductor materials capable of forming advanced heterojunctions. Herein, we report the first synthesis of a non-stoichiometric, Cu-substituted freudenbergite (Cu-FDT) via a facile co-precipitation method. Comprehensive characterization (PXRD, XPS, RAMAN, HR-TEM/STEM-EDS) confirms the formation of a phase-pure freudenbergite structure with nanoplatelet morphology and mixed-valent Ti4+/Ti3+. Electronic characterization (UV-DRS, Mott-Schottky) reveals a bandgap value of 2.95 eV enabling extended solar light harvesting and a band alignment perfectly suited for HER, coupled with a strong oxidation potential for the valence band (VB). A modified synthetic approach, involving the addition of water during Cu-FDT peptization, enabled the in situ fabrication of a Cu-FDT/TiO2 heterojunction. The TiO2 phase (anatase, mixed phase, rutile) was tuned by varying the calcination temperature. Photocatalytic performance toward HER was evaluated for all composites to elucidate the effect of excess surface Na+ on photocatalytic activity. The optimal catalyst, a 1 wt% Pt-loaded, desodiated Cu-FDT/anatase heterojunction (1 wt% Pt @ Cu-FDTA deNa), achieved a high hydrogen production rate of 7183 μmol ·g⁻¹·h⁻¹ under solar irradiation. Mott-Schottky analysis confirmed a direct Z-scheme charge transfer mechanism, enabling superior charge separation while preserving strong redox potentials. Furthermore, the high oxidative power of the heterojunction was further demonstrated by the near-complete mineralization of 5-hydroxymethyl furfural (5-HMF), with only minimal yields of partial oxidation products 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and 2,5-diformylfuran (DFF). These findings highlight the potential of this novel photocatalyst to simultaneously drive HER and challenging oxidation reactions, thus coupling renewable H2 evolution with the potent oxidative power of photogenerated holes (h+) in the VB of Cu-FDT.