Crystalline TiO2 nanotube arrays are synthesized using a two-step process. The Ti foil sample is first treated with an oxidizing agent-containing electrolyte, either H2O2 or (NH4)(2)S2O8 then anodized at constant voltage in a fluoride-containing electrolyte. The resulting films are directly used for water photoelectrolysis and in dye-sensitized solar cells, see figure.