Spectroscopic properties of electrochemically populated electronic states in nanostructured TiO2 films: anatase versus rutile

A thorough characterization of nanostructured materials under application-relevant conditions is a prerequisite for elucidating the interplay between their physicochemical nature and their functional properties in practical applications. Here, we use a spectroelectrochemical approach to study the population of electronic states in different types of nanostructured anatase and rutile TiO2 films in contact with an aqueous electrolyte. The spectroscopic properties of the two polymorphs were addressed under Fermi level control in the energy range between the fundamental absorption threshold and the onset of lattice absorption (3.3-0.1 eV). The results evidence the establishment of an equilibrium between localized Ti3+ centers absorbing in the vis/NIR and shallow (e(-))(H+) traps absorbing in the MIR upon electron accumulation in anatase electrodes. The absence of the MIR-active (e(-))(H+) traps on all rutile electrodes points to a crystal structure-dependent electron population in the films.