ReEtching produces nanostructured silicon when a catalytic agent, e.g. dissolved V2O5, is used to facilitate etching between Si and H2O2. H2O2 regenerates dissolved V in a 5+ oxidation state, which initiates etching by injecting holes into the Si valence band. Independent control over the extent of reaction (controlled by the amount of H2O2 added) and the rate of reaction (controlled by the rate at which H2O2 is pumped into the etchant solution) allows us to porosify Si substrates of arbitrary size, shape and doping, including wafers, single-crystal powders, polycrystalline powders, metallurgical grade powder, Si nanowires, Si pillars and Si powders that have been textured with metal-assisted catalytic etching (MACE). Similarly, improved control over the nucleation and etching in MACE is achieved by pumped delivery of reagents. Nanowires are not produced directly by MACE of powders, rather they form when a porosified layers is cleaved by capillary forces or sonication.