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Extremely cheap mesoporous silicon nanowires by metal-assisted chemical etching of inexpensive metallurgical silicon have a higher porosity and thus volume expansion possibility than simple silicon nanowires. Metallurgical Si nanowires (MG-SiNWs) show a reversible capacity of about 2111 mAh g−1 at a current rate of 0.2 C, an encouraging stability of over at least 50 cycles, and a good rate capability.

The origin of the photocurrent enhancement and the overpotential reduction in solar water splitting employing nanostructured silicon is still a matter of debate. A set of tapered Si nanowires (SiNWs) has been designed for clarifying the impact of nanostructured Si on the hydrogen evolution reaction (HER) while precisely tailoring several interference factors such as surface area, light absorption and surface defect density. We find that defect passivation by KOH achieved by tapering is much more beneficial than the optical gain. Surfactant-mediated modification of SiNWs is capable of engineering the band structure. As a result, we suggest a guideline for nanostructured Si photoelectrodes optimized for the HER.