Relating the 3D Geometry and Photoelectrochemical Activity of WO₃-Loaded n-Si Nanowires: Design Rules for Photoelectrodes

Abstract

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 WO 3 /Si nanowire array photoanodes with various nanowire lengths (1.3, 2.7, 3.2, and 3.8 μm) and different WO 3 thicknesses (10, 30, and 50 nm) using wet chemical etching for nanostructuring of Si and atomic layer deposition for the deposition of WO 3 . 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 WO 3 located at the top part of the nanowires is more effective than that at the bottom part for the WO 3 /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.

Geometric Optimization of Bismuth Vanadate Core–Shell Nanowire Photoanodes using Atomic Layer Deposition

“…These geometric parameters act together to influence performance metrics including light absorption, charge separation, surface charge recombination, and mass transport within the electrolyte. − ,,, In a core–shell NW array, light is absorbed along the length of the NW, while charge carriers diffuse and are collected radially. − Thus, light absorption can be increased by lengthening the NWs without increasing the required carrier diffusion lengths.…”

“…Increased surface area provides a greater number of potential reaction sites, but it may also result in increased surface and interfacial recombination. High-aspect-ratio nanostructures can also introduce concentration gradients in the electrolyte phase within the electrode. , Furthermore, changing the nanostructure geometry (including shell thickness) can result in greater focusing of light absorption near the top of the photoelectrode, which reduces the usefulness of active surface area deeper within the electrode. ,,, In this study, to decouple these effects, independent control of multiple geometric parameters is used to provide fundamental insights into the factors that limit the performance of nanostructured photoelectrodes.…”

“…29,47 Furthermore, changing the nanostructure geometry (including shell thickness) can result in greater focusing of light absorption near the top of the photoelectrode, which reduces the usefulness of active surface area deeper within the electrode. 2,46,48,49 In this study, to decouple these effects, independent control of multiple geometric parameters is used to provide fundamental insights into the factors that limit the performance of nanostructured photoelectrodes.…”

Toward Excellence in Photocathode Engineering for Photoelectrochemical CO2 Reduction: Design Rationales and Current Progress

“…Nanostructuring p‐Si has been a promising approach in facilitating PEC CO 2 RR as opposed to its unadulterated versions. To date, many Si nanostructures have emerged which took diverse forms such as nanowires, [ 200 ] nanopyramids, [ 201 ] nanopillars, [ 202 ] arrays, etc., in the hopes of improving its light absorption capacity by regulating penetration depth for photons, reducing reflection losses, improving surface reaction area, and other benefits that stem from their high aspect ratios. In particular, p‐Si nanowires have been widely adopted for PEC CO 2 RR, as depicted in Figure 15 A.…”

Recent Advances in Self‐Supported Semiconductor Heterojunction Nanoarrays as Efficient Photoanodes for Photoelectrochemical Water Splitting

“…[37] Finally, engineering the morphology and microstructure of heterojunction nanoarrays can increase light scattering and decrease carrier diffusion paths, thus enhancing the solar energy utilization and water oxidation kinetics. [38,39] To date, most of the self-supported heterojunction photoanodes reported to date consist of ordered nanoarrays of one semiconductor as the core scaffold, onto which a second semiconductor is conformally coated in the form of nanostructured shell. In such systems, the core semiconductor is normally very efficient at generating electron-hole pairs under band gap excitation, whereas the shell component is chosen to assist with charge separation and to protect the core material from photo-corrosion under the harsh oxidizing conditions of the PEC OER.…”