For semiconductor nanowire structures the ratio of the minority charge carrier diffusion length over the light absorption depth is significantly reduced compared to bulk materials. In addition, nanostructured semiconductors exhibit larger surface-to-volume ratio. The dimensions of the nanostructures and their geometrical arrangement influence relevant processes such as light absorption, as well as charge separation and transport. Ion-track nanotechnology combined with electrodeposition and atomic layer deposition, enables the controlled synthesis of 3D architectures of semiconductor nanostructures with tailored composition, size and density.

Among the various materials studied as photocathodes for solar hydrogen production, Cu₂O is a promising candidate with a predicted solar-to-hydrogen conversion efficiency of ~18%. Moreover, Cu₂O is cheap, earth-abundant, non-toxic, and is also scalable and compatible with low-cost fabrication processes. Currently, the main challenge for Cu₂O-based photocathodes is their chemical instability in aqueous solution, which can be improved by the use of suitable passivation coatings.

Here, we present the synthesis and characterization of two types of semiconductor nanowire-based photocathodes: (i) highly textured single-crystalline p-type Cu₂O nanowire arrays prepared by electrodeposition in etched ion-track polymer membranes, (ii) Au/Cu₂O core-shell nanowire arrays prepared by electrodeposition of Cu₂O on arrays of free-standing single-crystalline Au nanowires. Polymer membranes with controlled nanochannel density (typically 10⁸ - 10¹⁰cm^-2) and channel diameter (~20 - 250 nm) are fabricated by swift heavy ion irradiation and selective chemical etching. Subsequently, either Cu₂O or Au nanowires with lengths up to 10 µm are synthesized by electrodeposition in the etched nanochannels. Optimized deposition conditions yield single-crystalline nanowires of both materials. After electrodeposition, the polymer membranes are dissolved in an organic solvent. The from the template released Cu₂O nanowire arrays are directly coated with a thin and conformal TiO₂ passivation film by atomic layer deposition (ALD). The Au nanowire arrays, in turn, are first coated with a Cu₂O layer by electrodeposition and then with a TiO₂ film by ALD. For optimization, the thickness of both layers is systematically varied during the synthesis.

The photoelectrochemical performance of both types of nanowire-based photoelectrodes is studied as a function of parameters such as nanowire length, diameter, and density. In particular, the influence of the core-shell geometry, and the resulting minimization of the diffusion length for both minority and majority charge carriers is discussed.