Relating the 3D geometry and photoelectrochemical activity of WO₃-loaded N-Si nanowires: Design rules for photoelectrodes

Nanostructured electrodes for photoelectrochemical (PEC) applications, such as water splitting, have a rather low photocurrent density regarding their highly enlarged surface area compared to plain electrodes. This demands for further understanding of the relation between the three-dimensional (3D) geometry and the PEC activity. To this end, we fabricate WO₃/Si nanowire array photoanodes with various nanowire lengths (1.3, 2.7, 3.2, and 3.8 μm) and different WO₃ thicknesses (10, 30, and 50 nm) using wet chemical etching for nanostructuring of Si and atomic layer deposition for the deposition of WO₃. It is found that by increasing the etching time, the nanowires become longer and the top surface area decreases. The photocurrent density first increases and then decreases with increasing Si etching time. This behavior can be explained by different and opposite effects regarding absorption, geometry, and material-specific properties. Particularly, the decrease of the photocurrent density can be due to: (1) the longer the nanowires, the heavier the recombination of the photogenerated carriers and (2) the long-time Si etching results in a loss of top part of the nanowire arrays. Because of shadowing, the WO₃ located at the top part of the nanowires is more effective than that at the bottom part for the WO₃/Si nanowire arrays and therefore the photocurrent is decreased. It reveals a trade-off between the top part surface area and the length of the nanowires. This study contributes to a better understanding of the relation between the geometry of nanostructures and the performance of PEC electrodes.