Experimental observation of two-layer TiO2 nanotube arrays prepared by stepping-voltage anodization

Two-layer TiO2 nanotube arrays were produced by stepping down the anodic voltage during which three nanotube interface structures between the top layer and the second layer were observed by SEM. We detected a polygonal ring structure on the top surface of the second layer and offer direct evidence of the growth of this second tube layer both at the cell boundary and right beneath the bottom of the first tube layer. For these processes, a possible growth mechanism is presented. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)