Kinetics of light-driven oxygen evolution at nanostructured hematite semiconductor electrodes

Hematite nanostructures have been electrochemically grown by ultrasound-assisted anodization of iron substrates in an ethylene glycol based medium. This hematite nano-architecture has been tuned from a 1-D nanoporous layer (grown onto a bare iron foil substrate) to a high aspect self-organized nanotube one (grown onto a pretreated iron foil). Well-developed hematite nanotube arrays perpendicular to the substrate with a 1 μm in length have been obtained. The nanoporous sample was characterized by pores of a mean diameter of 30 nm and an interpore distance of 150 nm, whereas the self-organized nanotube layer consisted of nanotube arrays with a single tube inner diameter of approximately 50 nm and average spacing of approximately 90 nm. The wall thickness of the hematite nanotubes was of approximately 30 nm. A comparative study of the photoelectrochemical properties of these two different hematite nanostructures under water-splitting conditions have been studied through EIS and PEIS methods. The strong correlation between the CSS increase with the R_SS,ct decrease and the photocurrent development as the potential is made more anodic, indicated that holes transfer for the water splitting reaction takes place through the surface states and not directly from valence band holes. From the PEIS spectra the rate constants of the elementary reactions responsible for the competing processes of interfacial charge transfer (k_tr) and electron-hole recombination (k_rec) have been determined. A better photoresponse kinetic was observed from the hematite nanotubular structure as compared to the nanoporous one. The last indicates that in the hematite nanotubular structure it exists a very well length scale matching between the nanotube wall thickness and the hole diffusion length (maximize light absorption while maintaining the bulk within hole collection length), diminishing then the recombination processes.