Si-based heterojunctions have been intensively employed to reduce the large overpotential of Si single-absorber photoelectrodes in the field of photoelectrochemical (PEC) water splitting, but achievement of high-quality heterojunctions with an appropriate bandgap on the Si nanostructure host remains a great challenge. Herein, the p-Si/n-CdS core/shell nanowire heterojunction photocathode has been achieved by a combination of nanosphere lithography, metal-assisted chemical etching, atomic layer deposition , and chemical bath deposition. Compared to the bare Si nanowire photocathode , the overpotential maintaining a photocurrent density of 10 mA cm −2 for the optimized p-Si/n-CdS core/shell nanowire heterojunction photocathode under AM 1.5G illumination is reduced by 0.93 V, and simultaneously the saturated photocurrent density is increased by 4.4 mA cm −2 . Analyses indicate that the introduced CdS shell can greatly facilitate the separation and transfer of the photogenerated carriers , as well as partly enhancing the optical absorption .
The CdS shells are epitaxially grown onto monodisperse zinc-blende CdSe core nanocrystals in octadecene mixed with fatty amines, with the volume fraction of fatty amines in the epitaxy solution (VNH2) varying between 0 and 100%. The shape─except the nanoplatelets from the reactions with medium VNH2 (especially 20% VNH2)─and surface ligands of the resulting CdSe/CdS core/shell nanocrystals from all reactions can be unified through a two-step surface treatment. The internal lattice defects originated from zinc-blende/wurtzite stacking fault(s), i.e., wurtzite lattice domain(s) in the CdS shells and twin fault(s) in the zinc-blende region, can be systematically controlled by varying the VNH2 and remain unaltered during the surface treatments. Both twin faults and wurtzite CdS lattice domains neither affect the electronic structure of the core/shell nanocrystals nor create internal electronic defects for the nanocrystals. Photoluminescence decay dynamics of the dot-shaped core/shell nanocrystals with solely twin faults can be mono-exponential and near-unity radiative decay, but photoluminescence of those with wurtzite CdS domains would always decay with double-exponential even if the photoluminescence quantum yield is unity. The results here imply that the most common lattice defects related to the stacking faults for II–VI and III–V semiconductor nanocrystals can be managed for achieving high-quality optical properties.
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