Nanostructured homogenous CdSe–TiO2 composite visible light photoanodes fabricated by oblique angle codeposition

Abstract

A unique fabrication method, oblique angle codeposition, is used to deposit well-aligned nanorod arrays and thick films of homogenously mixed CdSe–TiO₂ composites. The composite films are characterized structurally, optically, and photoelectrochemically using a variety of experimental techniques. The CdSe–TiO₂ composites are compared with pure CdSe and TiO₂ films in order to determine their utility for photoelectrochemical (PEC) applications and to understand the mechanisms underlying the observed behaviors. The evaporation process of CdSe creates three different cluster types within the TiO₂ film structures: isolated Se, Se-deficient CdSe, and Se-rich CdSe. The prevalence of each cluster type is dependent on predicted film composition, and each is affected differently by open-air annealing. Isolated Se can be incorporated into the TiO₂ lattice, resulting in low energy rutile phase. Se-deficient CdSe clusters crystallize preferentially into cubic CdSe and are easily oxidized into CdO, while Se-rich CdSe clusters crystallize into hexagonal CdSe and are more stable. Furthermore, each of these cluster types interacts differently with the surrounding TiO₂ matrix, resulting in diverse optical and PEC behaviors. Importantly, the composite nanorod structure is a more efficient photoanode under visible light illumination than both the pure CdSe and TiO₂ nanorod array films. The stoichiometry of the CdSe domains is more important than overall CdSe content within the film in determining the structural, optical, and PEC properties of the films.