Nanocrystalline structure and nanopore formation in modified thermal TiO2 films

The anodic TiO2 films on Ti0 substrates were synthesized using different electrochemical techniques including potential step, potentiodynamic, and galvanostatic technique, from a 1 M Na2SO4+10mM NaF solution. The AFM imaging of film surfaces revealed that thin films (h<20nm) are composed of small spherical-shaped nanocrystals, up to 20 nm in size. In thicker TiO2 films (h>20nm), cylindrical nanopores with diameter of ca. 30 nm, are formed. In contrast to the nanocrystalline nature of anodic TiO2 films, thermally grown TiO2 films at 850 °C are composed of oriented large pyramidal crystallites (200–500 nm in size), which are transformed into even larger moulds (2–3μm in size) at 1050 °C. Thermal films of this type have been known to show inferior surface properties leading to large reflectance losses and the presence of surface states, which promote the electron–hole recombination. In this paper, we propose a new way to remedy these disadvantages of thermal films by special low temperature electrochemical anodic treatment. We have found that by the anodic treatment of thermally grown TiO2 films, it is possible to control the surface properties, such as the nanocrystalline structure and nanopore formation. These features are desirable for solar energy conversion devices and solar hydrogen production, where the enhancement of electrocatalytic activity by increasing real surface area, reduction of reflectance losses, and removal of surface states created by thermal growth, are of primary importance. The tested electrochemical treatment included program waveforms inducing nanocrystallinity and nanopore formation. The pulse-voltammetric procedures are proposed to control surface non-stoichiometry of TiO2 films and surface-states density of the photoelectrodes.