2, Eindhoven University of Technology (TU/e), Eindhoven, , Netherlands
Nanostructuring of visible light absorbing materials such as BiVO4, WO3 and Fe2O3 is an effective way to increase their photoelectrochemical (PEC) water splitting performance. The improvement in performance is related to an increase in the surface area available for water splitting as well as shortened charge carrier pathways between the bulk and interface of the material .
Herein, we utilize a novel technique to obtain nanostructured thin film photoanodes by exposure to high-ion flux helium plasma. We observed that the degree of nanostructuring could be controlled by varying the main plasma exposure parameters- duration, ion flux and temperature . Hematite thin films are chosen as a model system for this study due to the well-known limitations of this material such as poor light absorptivity and the short minority carrier diffusion length (2-4 nm), both of which are seen to be improved by nanostructuring .
We found in this work that the PEC performance of the films was related, in a non-intuitive manner, not only to the nanostructured morphology but also to the crystal structure and chemical composition obtained on plasma exposure . Plasma exposed films contained multiple iron oxide phases, including α-Fe2O3, γ-Fe2O3 and Fe3O4. Electrochemical impedance spectroscopy (EIS) was used to relate the PEC water splitting activity of the plasma-exposed films to their physical and chemical properties.
We established that the increase in PEC performance of the plasma-exposed films can be attributed to two key factors. Firstly, an increase of up to 40 times in the surface area available for water splitting on plasma exposure, as compared to unexposed films. Secondly, the presence of γ-Fe2O3, which provides a second OER pathway for water splitting at high potentials. This is made possible due to a rapid drop in the charge transfer resistance for OER through γ-Fe2O3 with increase in applied potential, which allows for a parallel OER pathway to that of α-Fe2O3. Combination of these two factors leads to a 5 times increase in the photocurrent density for plasma-exposed films over unexposed films.
Thus, this study shows that plasma nanostructuring of thin films is a viable method for obtaining significantly improved photoelectrodes for water splitting. Moreover, this technique is also viable to attain controlled nanostructuring of materials for other applications, such as solar cells, energy storage and solid-oxide fuel cells.
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 Sinha R., Tanyeli I., Lavrijsen R., van de Sanden M. C. M., Bieberle-Hütter A., Electrochim. Acta, 2017, Accepted