The present invention provides a method for treating silicon to form pillars, especially for use as the active anode material in Li-ion batteries. The process is simple to operate on a commercial scale since it uses a solution containing only a small number of ingredients whose concentration needs to be controlled and it can be cheaper to operate than previous processes. The solution comprises: 0.01 to 5M HF 0.002 to 0.2M of metal ions capable of nucleating on and foirning a porous layer comprising regions of elemental metal on the silicon surface; 0.001 to 0.7M of an oxidant selected from the group O2, 03, H2O2, the acid, ammonium or alkali metal salt of NO3 -, S2O8 2-, NO2 -, B4O7 2- and C1O4 - or a mixture thereof. The treated silicon is suitably removed from the solution.