Porous silicon fabrication by anodisation: Progress towards the realisation of layers and powders with high surface area and micropore content

• Investigation of silicon electrochemical anodisation with various types of chemical additives in HF electrolyte.
• Porous silicon internal surface areas of up to 864 m2/g in HF–HCl mixture.
• Pore sizes in the range of 2.8–3.2 nm.

With a view to producing thick and very high surface area microporous silicon layers (and subsequently powders) by electrochemical anodisation, the incorporation of various types of chemical additives has been investigated, these in combination with hydrofluoric acid electrolyte and high-resistivity p-type parent substrates. Comparison under constant charge conditions shows that anodisation using 50 wt% hydrofluoric acid, or inclusion of the additives hydrochloric acid, sulphuric acid, or ammonium dodecylsulfate with lower concentration hydrofluoric acid, can facilitate powders with internal surface areas of up to 864 m2/g, average pore sizes in the region of 2.8–3.2 nm, and pore volumes in excess of 0.8 cm3/g – all as determined using nitrogen gas adsorption and associated isotherm analysis. Porous silicon powders with appreciable micropore content have thus been achieved, for the first time. Relevant application areas for such material are diverse, and potentially include energetics, impurity gettering, gas sensing microchips, orthopaedic implants, hydrogen storage, and Li-ion battery anodes.

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