AVS 59th Annual International Symposium and Exhibition
    Thin Film Wednesday Sessions
       Session TF+SE+NS-WeM

Paper TF+SE+NS-WeM3
Nanostructured Homogenous CdSe/TiO2 Composite Visible Light Photoanodes Fabricated by Oblique Angle Codeposition

Wednesday, October 31, 2012, 8:40 am, Room 11

Session: Glancing Angle Deposition (GLAD)
Presenter: G.K. Larsen, University of Georgia
Authors: G.K. Larsen, University of Georgia
B.C. Fitzmorris, University of California Santa Cruz
C. Longo, University of Campinas, Brazil
J.Z. Zhang, University of California Santa Cruz
Y.-P. Zhao, University of Georgia
Correspondent: Click to Email
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 structural, optical, and photoelectrochemical properties of the films are investigated using a variety of experimental techniques. Ultrafast exciton dynamics are studied using femtosecond transient absorption (TA) spectroscopy. The CdSe/TiO2 composites are compared with pure CdSe and TiO2 filmsin order to determine their utility for photoelectrochemical (PEC) applications and to understand the mechanisms underlying the observed behaviors. The morphology of the deposited nanorods changes with film composition due to atomic collisions occurring in the vapor column, which can be modeled using a simplified Keller-Simmons expression. Furthermore, the two phase 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. Interestingly, 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. The composite nanorod structure is a more efficient photoanode under visible light illumination than both the pure CdSe and TiO2 nanorod array films. The higher efficiency of the composite films is attributed to efficient charge transfer and separation in the homogenously mixed composite. This is confirmed by the extremely high electron injection rate from CdSe into TiO2 observed in the ultrafast TA studies.