Photoinduced capacitive-to-conductive transition in Cu-doped tungsten oxide hydrates toward enhanced photoelectrochemical energy conversion

A photoinduced capacitive-to-conductive (C2C) transition is reported in Cu-doped WO 3 ·H 2 O during the electrochemical reactions, enabling light-controlled modulation of electronic transport and hydrogen evolution reaction (HER) activity. This transition occurs under cathodic potentials and is governed by Cu-doping level, illumination, and potential scan rate. To investigate this phenomenon, WO 3 ·H 2 O (tungstite) and Cu-doped WO 3 ·H 2 O nanoplates were synthesized using the submerged photosynthesis of crystallites (SPsC) method, an environmentally benign, light-driven aqueous approach. Electrochemical and photoelectrochemical analyses were performed using cyclic voltammetry, Mott–Schottky, and linear sweep voltammetry. These results reveal that under slow scan rate and cathodic potentials, proton intercalation leads to H x WO 3 ·H 2 O formation, triggering a C2C transition due to a semiconductor-to-metal conversion, while simultaneously promoting hydrogen evolution. Upon illumination, a rapid and reversible increase in cathodic current is observed, reflecting enhanced carrier density and electronic conductivity. Both the carrier density and photocurrent increase linearly with light intensity, while the HER overpotential decreases correspondingly. Cu doping narrows the bandgap and introduces oxygen-vacancy-related sub-band states, thereby improving light absorption and charge transport. These findings provide fundamental insights into light- and defect-mediated conductivity switching and highlight WO 3 ·H 2 O as a promising photoresponsive material for energy storage and photoelectrocatalysis.