The formation of Z-scheme CdS/CdO nanorods on FTO substrates: The shell thickness effects on the flat band potentials

CdS/CdO core/shell nanorod photoelectrochemical electrodes were prepared and the working mechanism of n-/n-type Z-scheme band structures is proposed based upon the shell thickness variation. A mild solution chemistry enabled the vertical growth of CdS nanorods on the low-cost conductive glass substrates. The thermal decomposition of Cd(NO₃)₂·4H₂O coated on the CdS nanorod surface successfully introduced CdS/CdO core/shell nanorod electrodes. The formation of CdO shell on CdS nanorods led to the improved photoelectrochemical performance through constructing the n-/n-type Z-scheme band structure. From the Mott-Schottky and the open-circuit potential analyses, the variation in the flat band potentials were monitored to understand the change in the photovoltage of the n-/n-type Z-scheme nanorod electrodes regarding the CdO shell thickness. The core/shell nanorod electrodes having the shell thickness equal to or smaller than the space charge region width exhibited higher photovoltages than those of others. This could be the outcome of the effective charge separation coming from the absence of the bulk region in which the electron transport is disturbed by boundaries. Finally, the optimum CdO shell thickness of CdS/CdO electrodes was determined to be ~2 nm and their corresponding photocurrent density was measured to be ~4.35 mA/cm², which is a ~22% increased value compared to that of bare CdS nanorod electrodes.