Unraveling the charge transfer/electron transport in mesoporous semiconductive TiO₂ films by voltabsorptometry

Abstract

In this work, we demonstrate that chronoabsorptometry and more specifically cyclic voltabsorptometry are particularly well suited techniques for acquiring a comprehensive understanding of the dynamics of electron transfer/charge transport within a transparent mesoporous semiconductive metal oxide film loaded with a redox-active dye. This is illustrated with the quantitative analysis of the spectroelectrochemical responses of two distinct heme-based redox probes adsorbed in highly-ordered mesoporous TiO2 thin films (prepared from evaporation-induced self-assembly, EISA). On the basis of a finite linear diffusion-reaction model as well as the establishment of the analytical expressions governing the limiting cases, it was possible to quantitatively analyse, predict and interpret the unusual voltabsorptometric responses of the adsorbed redox species as a function of the potential applied to the semiconductive film (i.e., as a function of the transition from an insulating to a conductive state or vice versa). In particular, we were able to accurately determine the interfacial charge transfer rates between the adsorbed redox species and the porous semiconductor. Another important and unexpected finding, inferred from the voltabsorptograms, is an interfacial electron transfer process predominantly governed by the extended conduction band states of the EISA TiO2 film and not by the localized traps in the bandgap. This is a significant result that contrasts those previously observed for dye-sensitized solar cells formed of randomly sintered TiO2 nanoparticles, a behaviour that was ascribed to a particularly low density of localized surface states in EISA TiO2. The present methodology also provides a unique and straightforward access to an activation-driving force relationship according to the Marcus theory, thus opening new opportunities not only to investigate the driving-force effects on electron recombination dynamics in dye-sensitized solar cells but also to study the electron transfer/transport mechanisms in heterogeneous photoelectrocatalytic systems combining nanostructured semiconductor electrodes and heterogeneous redox-active catalysts.

Investigating Charge Transfer in Functionalized Mesoporous EISA–SnO2 Films

“…It is interesting to note that in contrast to the reversible wave we observed here for MP-11 in EISA-SnO 2 , an irreversible CV peak was previously reported for the same molecule in EISA-TiO 2 films. 19 This distinctive behavior is fully consistent with a conduction band potential of TiO 2 that is ~0.4 V more negative than SnO 2 . insulating, it can be anticipated that direct interfacial electron transfer between SnO 2 and OsP should proceed irreversibly once the potential applied to the SnO 2 film is sufficiently negative to convert the latter reasonably conductive (i.e.…”

“…19 The second equation is related to the irreversible interfacial electron transfer reaction between the adsorbed Os III species and the electrons at the SnO 2 film/solution interface, wherein k is an average value of the interfacial electron transfer rates characterizing the different electronic states involved in the electron transfer. In the limiting case for which electron diffusion throughout the semi-conductive network is not interfering in the kinetics, the irreversible reduction wave can be described by the following analytical expression (see SI for details):…”

“…The methodology used for extracting the key information is similar to the one we have recently developed for analyzing the interfacial electron transfer parameters in mesoporous films of TiO 2 functionalized by iron-porphyrin-based compounds. 19 The SnO 2 films are prepared by evaporation-induced self-assembly (EISA), a sol-gel surfactant-templated synthetic method which leads to the formation of highly periodically-organized nanoporous metal oxides thin films on a flat solid substrate (i.e. a transparent conductive ITO electrode in the present case).…”

“…From the cathodic peak intensity of this reversible process, an apparent diffusion coefficient of ∼10 This biphasic reduction process is reminiscent to that previously reported at mesoporous EISATiO 2 electrodes functionalized by an iron porphyrin. 19 It is indicative of the co-existence of two charge transfer mechanisms: a first diffusion-like mechanism involving most likely electron hopping transport between adjacent chemisorbed OsP complexes across the insulating SnO 2 network, and a second mechanism involving direct interfacial electron transfer from SnO 2 to the oxidized adsorbed molecule. This second mechanism is only efficient once a significant concentration of electrons is injected into the semiconductive material (i.e., at E < 0.2 V at pH…”

“…These two molecules were selected because of (i) their intense redox-dependent visible absorption features Consequently, CVAs and DCVAs allow for a more specific and accurate monitoring of electron transfer reactions with adsorbed redox chromophores than CVs, for which the huge capacitive contribution at negative potentials tends to mask the faradaic response. 19 During a CV scan, reduction/oxidation of the adsorbed redox chromophore can occur through one or a combination of the following three electron transport mechanisms:…”