Self-assembled photoactive heterojunction phase gradient

Abstract

The high photocatalytic activity of anatase is markedly enhanced by the presence of a rutile interface. Such phase heterojunctions have previously been formed by sintering admixtures, which results in biphasic particles or agglomerated powders. Current efforts to orientate these heterojunctions relative to the incident light and electrodes to optimize and further improve photoactivity have focussed on sputter coating, dip coating and doctor blading. These techniques offer two ways by which photoactivity may be increased; firstly by maximizing the area of the heterojunction relative to the volume of the two phases and secondly by optimizing spatial ordering of the phases, charge carrier flow direction and path length. Here we report a one step room temperature self-assembly technique which reproducibly creates microstructurally phase graduated photoanodes consisting of a base layer of rutile nanorods and increasing concentrations of anatase in subsequent layers. A large increase in current density and OCP was observed (0.4 to 0.75 mA cm⁻²) that coincided with the appearance of anatase in the coating i.e. the initial formation of a heterojunction, and longer deposition times were concomitant with increasing current density to a maximum of 1.1 mA cm⁻² The resulting nanostructured electrodes were evaluated in non-optimised N3-dye-sensitized solar cells (DSSCs) where they demonstrated 188% higher short current density than P25 controls.