Dynamic Coupling of Photoinduced Charges with Evaporation-Driven Unipolar Ion Flow in Semiconducting Nanochannels

Unipolar ion flow in nanochannels, enabled by the overlapping electrical double layers at the water-solid interfaces, is essential in various energy conversion processes, such as osmotic and hydrovoltaic energy harvesting. Photoinduced charges in semiconductors have been reported to modulate solid surface charges and thus the selective transport of ions. Here, it is revealed that photoinduced charges can dynamically couple with the unipolar ion flow, instead of surface modulation, contributing to photogated water-evaporation-induced electricity. Specifically, the voltage output of the hydrovoltaic device, constructed by semiconducting tungsten oxide (WO₃) nanoparticles, is tuned from 2.7 to 0.2 V with high reproducibility by varying light intensities. Mechanistic investigations implied that photogating is attributed to the directional transport of photoinduced charges in WO₃, opposite to the unipolar ion flow. The voltage recovery dynamics after illumination cessation are found to be closely related to the ion concentration, explained by a circuit model involving the resistive and capacitive characteristics that depend on the charge carriers in the solid and solution. A new approach for low-concentration ion detection is further developed based on light and ion-induced regulation of evaporation-induced electricity. The findings offer new insights into the coupling of charge carriers in solid and liquid and can advance the applications of hydrovoltaic technologies.