Porosity Control and Transfer in Silicon Nanostructures through Electrochemical & Chemical Etching

Porous silicon particles and nanostructures are receiving growing interest from the scientific community. Two important parameters usually used to define a porous material are porosity and pore size. However, internal surface area and pore morphology are also of high importance, as they determine the material performances in many applicative domains such as Li-ion batteries ⁽¹⁾, catalysis, separation, drug delivery ⁽²⁾ etc.

High surface area porous silicon structures were prepared through silicon anodization in acidic HF electrolyte under galvanostatic control. The layers were then either taken off mechanically, forming powders, or subjected to a metal-assisted chemical etching (MaCE) post-treatment to produce high aspect ratio silicon nanowires through the depth of the porous film.

In the first scenario, careful attention was paid to the electrolyte composition in order to increase the internal surface area of the powders. In acidic solutions (i.e. HF mixed with strong acids such as HCl or H₂SO₄), powders with surface area exceeding 850 m²/g were achieved by lowering the average pore size to around 3 nm, whilst maintaining moderate porosity (~ 65%) ⁽³⁾.

In the latter scenario, MaCE ⁽⁴⁾ on porous silicon thin films facilitated the transfer of the parameters defining the mesoporous framework (porosity, pore diameter, surface area, pore orientation) from the thin film to high aspect ratio nanowires. This consisted of periodic modulation of the porosity with depth and applying MaCE post-treatment as illustrated by electron microscopy images, below, the nanowires clearly displayed the porosity gradient of the original porous silicon thin film.

The control of both anodization conditions, through an accurate choice of the silicon wafer doping type and level and electrolyte composition, to achieve ultra-high surface area porous powder, and porous layer post-treatment such as MaCE, to convert the porous framework from thin film to nanowires, provide new and promising opportunities in many application fields.

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