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Redox Active Mixed Metal Oxides

Goal: Electrochemical synthesis of metal oxides from aqueous precursors with unique chemical and physical properties. Recently we have prepared mixed-valent rhenium thin films using chronocoulometry and proposed a deposition mechanism where the aqueous precursor (ReO4-) adsorbs to the working electrode prior to reduction.

Studies:

  • Preparation of electrochemically deposited thin films using facile chronocoulometric methods, and tailoring the structural composition systematically by tuning the deposition potential, solution concentration and pH, sintering conditions, etc.  
  • Elucidation of the complex deposition mechanism, based on advanced characterization methods (electrochemical quartz crystal microbalance, variable angle spectroscopic ellipsometry, real-time spectroelectrochemical analyses)
  • Determine the structure and composition of resultant phases using X-ray methods (X-ray diffraction, X-ray photoelectron spectroscopy, X-ray reflection) and high spatial resolution imaging (conductive-probe atomic force microscopy with Raman microscope)


Figure 1. Real-time spectroelectrochemical experiment showing plot of differential absorbance and current versus potential (top) and absorbance versus wavelength (bottom) during a cathodic linear sweep of ITO immersed in ReO4- .



Figure 2. Cyclic voltammograms of ITO immersed in ReO4- in the absence of SO42- (top) and in the presence of SO42- (bottom).
Significance: Electrochemical deposition is a powerful method to develop new redox-active materials, because it is a relatively inexpensive procedure that can be used to tune the growth of uniform thin films. By adjusting the system parameters, such as the deposition potential or solution pH, we can deposit mixed-valent redox-active metal oxides and systematically tune the film composition.

The growth of rhenium thin films is particularly interesting, because rhenium has potential uses in many applications ranging from catalysts[1] to semiconductors[2]. Our work in this area has allowed us to propose a detailed electrodeposition mechanism for the reduction of rhenium at non-hydrogen adsorbing electrode materials (e.g. Au, C, indium-tin oxide)[3]. We are currently building upon this research by depositing rhenium simultaneously with other transition metals to prepare mixed metal oxides. This area is largely unexplored in the literature, but it offers promise as a facile route to “wire” metallic rhenium into other oxide systems (e.g., MoO3) to enhance electronic conductivity[4].     
Related Publications:

[1] Blom, R.H.; Kollonitsch, V.; Kline, C.H. Ind. Eng. Chem. 1962, 54, 16-22. 

[2] Giaddui, T.; Earwaker, L.G.; Forcey, K.S.; Aylett, B.J.; Harding, I.S.; Nucl. Instr. and Meth. in Phys. Res. B 1996, 113, 201-204.

[3] Hahn, B.P.; May, R.A.; Stevenson, K.J. Langmuir (in press).

[4] Hahn, B.P.; Stevenson, K.J. ECS Transactions (submitted).                                             
Students currently involved in this project: Ben Hahn