**3.22. Using sacrificial template**

As depicted in **Figure 23**, firstly, an array of ZnO nanowires is grown on the substrate. The specimen is then placed in a CVD reactor in which silane is used to form a silicon coating around the ZnO nanowires. The thickness of the over-coating silicon shell, which can be determined by the duration of the silane exposure, is extremely crucial for the formation of porous silicon. Indeed, only silicon sidewall thicknesses of about 12 nm or less finally lead to porous nanotubes. The temperature of the CVD process determines the crystallinity of the silicon over-coating layer. Deposition at 500°C results in amorphous silicon, while formation of crystalline shells requires at least 600°C. In order to remove the ZnO template and form silicon nanotubes, the sample is heated up to 450°C at the presence of ammonium chloride NH4 Cl. Ammonium chloride sublimes and decomposes into ammonia and hydrogen chloride gas. The latter reacts with ZnO creating ZnCl2 liquid which is then converted into zinc amide species in the presence of ammonia. If the sidewall would be less than 12 nm, a further thermal annealing process porosifies the nanotubes. Porosification has been attributed to a strain-influenced mechanism [77]. The pore sizes of the obtained structure ranges from 5 to 10 nm. These porous nanotubes can be used for therapeutic applications due to their solubility in water at room temperature.
