Double-layerTiO2 nanotube arrayswerefabricatedbyatwo-stepanodizationprocesson
Ti foils.ThefirstTiO2 nanotube layerwasannealedafteranodizationandthenexposedto
the secondanodizationtogrowthesecondTiO2 nanotube layerbeneathit.Thecrystal-
lized upperlayeractsasaprotectivelayeragainstchemicaletchingofthelowerlayer
tubes topbyelectrolytethatleadstogrowingofthicklayerswithopen-top-tubesbeneath
the upperone.Theeffectofdifferentanodizationparametersonthefinalgeometryofthe
nanotubes, grownbeneaththeprotectivelayerwasinvestigated.TheupperTiO2 layerwas
detachedasanintactmembraneattheendofthetwo-stepanodizationprocessandused
in front-sideilluminateddye-sensitizedsolarcells(DSSCs)withhighconversionefficiency
up to8.66%.Also,theremainedTiO2 nanotubes onthesubstratewithdifferentdiameters
wereusedintheback-sideillumintedDSSCs