Plasmon-enhanced sub-Debye-length nanogap photoelectrochemical cells for field-assisted electrolyte-free water splitting

Abstract

This work studies pure water splitting (i.e., no additional electrolyte) using plasmonic sub-Debye-length nanogap photoelectrochemical cells (NPECs). With 60 nm Au gratings covered by 4 nm amorphous TiO2 under 638 nm normal incident illumination, we demonstrate up to 66 × higher photocurrent at 1.5 V than that of Au-gratings-only NPECs. In addition, we are able to efficiently drive pure water splitting at voltages as low as 1.3 V, which is very close to the theoretical thermodynamic potential threshold of 1.23 V. Our electromagnetic finite difference time domain (FDTD) simulation results (Lumerical Inc.) show a 240 × increase in the electric field intensity near the TiO2 layer compared to the incident electric field. We attribute our observations to 1) Hot spots generated by surface plasmonic resonance (SPR) on Au gratings, which greatly increase the number of electron-hole pairs; 2) The 60 nm gap distance between electrodes, which is smaller than the Debye length of pure water (∼220 nm) resulting in a much higher electric field by eliminating the shield effect and by coupling two half-reactions of water splitting together; and 3) The 4 nm amorphous TiO2, which improves hot carriers separation. Our NPEC is a promising device for efficient hydrogen production enabling the utilization of visible light.