Electron Dynamics in Nanocrystalline ZnO and TiO₂ Films Probed by Potential Step Chronoamperometry and Transient Absorption Spectroscopy

Abstract

Potential step chronoamperometry is employed to compare the capacitances of nanocrystalline ZnO and TiO 2 electrodes. These capacitance data are complemented by transient optical absorption studies of charge recombination following adsorption of molecular sensitizer dyes to these metal oxide electrodes. Both measurements are conducted as a function of electrochemical bias applied to the metal oxide film in a threeelectrode photoelectrochemical cell. For both metal oxides, a power law dependence was observed between the half times for charge recombination (t 50% ) and the metal oxide electron density n determined from integration of the capacitance data, t 50% ∝ n -1/R , where R ) 0.27 and 0.30 ( 0.05 for ZnO and TiO 2 , respectively. A numerical model for the recombination dynamics based upon a random walk of electrons between localized sub-bandgap states is found to be in good agreement with experimental observations for both metal oxides. At negative applied potentials, the film capacitance, and therefore electron density, is observed to increase more rapidly with increasingly negative applied potential for the ZnO film compared to the TiO 2 film. This observation is quantitatively correlated with a more rapid acceleration of the recombination dynamics observed for dye sensitized ZnO films under negative biases. It is suggested that the faster recombination dynamics observed under negative bias may be the origin of the lower open circuit voltages reported previously for dye sensitized photoelectrochemical cells employing ZnO electrodes relative to comparable devices employing TiO 2 .

Physical electrochemistry of nanostructured devices

“…Examples are metal oxides used in DSC such as TiO 2 , ZnO and SnO 2 . [11][12][13][14][15] Colloidal nanocrystals are usually deposited over a conducting substrate and thermally treated to form a connected array of nanoparticles that can be used as electroactive electrodes. Structures can be more or less ordered, and interparticle connection can also be controlled with molecular ligands, which can be used to promote the self-assembly of special architectures.…”

pH-Dependent Electron Transfer from Re-bipyridyl Complexes to Metal Oxide Nanocrystalline Thin Films

“…The corresponding density of states decay roughly exponentially below the band edge with an decay constant of a ) 15 (eV) -1 , similar to those used to model defect state densities of TiO 2 nanocrystalline electrodes in other studies. 31,32,77,78 This equation predicts that injection rate depends sensitively on the electronic coupling strength, the density of states per unit volume in the solid, and the relative position of adsorbate potential and band edge. Shown in Figure 10 are the calculated injection rate, k ET as a function of E CB -E(S + /S*).…”

Modulation of the Rate of Electron Injection in Dye-Sensitized Nanocrystalline TiO2 Films by Externally Applied Bias

“…Such studies have been largely based by spectroelectrochemical studies of the density of conduction band/trapped electrons induced by electrical bias, 17 complimented more recently by a range of electrochemical techniques. [17][18][19][20][21] These studies have generally concluded that such films exhibit an exponential tail of states below the conduction band, attributed variously to defect states such as oxygen vacancies, surface states, and/or states induced by proton or lithium cation surface binding or intercalation into the TiO 2 nanoparticles. The energetics of this density of states has moreover been found to be strongly dependent upon the electrolyte composition, and in particular the concentration of small cations in this electrolyte.…”