Fabrication and Characterization of Semiconductor Photoelectrodes with Orientation-Controlled α-Fe₂O₃ Thin Films

Abstract

Epitaxial α-Fe2O3 thin films on conducting Ta-doped SnO2 bottom layers are prepared by pulsed-laser deposition in order to elucidate variation of the photoelectrochemical (PEC) properties for the water oxidation reaction on the α-Fe2O3 photoanodes with different orientation and thickness. Cyclic voltammetry under illumination shows that current density (photocurrent onset potential) of the m-axis oriented samples is considerably larger (lower) than that of the c-axis oriented ones. Using electrochemical impedance spectroscopy under illumination, we ascribe these behaviors to difference in surface states between c- and m-planes besides anisotropic conductivity along the individual axes. Our study demonstrates the power of utilizing well-defined epitaxial thin films to investigate the anisotropic PEC properties of visible-light-driven photoelectrodes.

Effect of Orientation on Bulk and Surface Properties of Sn-doped Hematite (α-Fe2O3) Heteroepitaxial Thin Film Photoanodes

“…This is in contrast to what is reported for undoped hematite films deposited on Ta:SnO2 films where c-axis oriented hematite photoanodes showed improved onset potential as compared to m-axis oriented samples for thicknesses greater than ~50 nm. 17 For comparison, a thinner Sn-doped hematite film thickness of ~30 nm was also studied and the photoelectrochemical measurements are displayed in Figure S1. Similar trends to the thick films are observed in both water photo-oxidation and hole scavenger measurements for the c(001), a(110), and r(110) samples whereas the m(100) samples produce ~20% more photocurrent at high potentials.…”

“…Additionally, the difference in onset potential for different orientations with the Sndoped hematite films reported in this work does not appear to be significantly affected by thickness (seen by comparing Figures 5 and S1) as was reported previously for undoped films. 17 These effects can be explained by the better reproducibility of heavily doped films whose performance is not as susceptible to incorporation of unintentional impurities in the film as compared to undoped films and also to the smaller depletion width due to the high doping level which will result in less interaction with additional space charge arising from the back interface.…”

“…Previously, two distinct surface states for undoped c-plane hematite films and only one for m-plane samples were reported based on impedance spectroscopy results. 17 For comparison, we also prepared undoped hematite films and measured them by the previously mentioned CV method as shown in Figure 7d. Indeed, undoped hematite c-plane and m-plane samples exhibited two and one cathodic current peaks, respectively, in agreement with those measured by impedance spectroscopy previously.…”

“…Mashiko et al also reported that for water oxidation at high pH, undoped epitaxial hematite photoanodes grown in the c(001) orientation showed lower onset potential than films having m(100) orientation. 17 The possibility exists that increased positive surface band bending consistent with higher work function and lower flat-band potential can explain the improved performance of the m(100) hematite photoanodes. However, the calculated depletion widths from the Mott Schottky analysis (Fig S2 and discussion above) of the various photoanodes did not scale with photoelectrochemical performance.…”

“…16 Very recently, Mashiko et al reported significantly improved onset potential for epitaxial undoped c-axis (001) hematite films as compared to m-axis (100) films. 17 Despite these findings, it is still unclear how orientation affects the photoelectrochemical properties of hematite photoanodes. Since most work has focused on nanostructured photoelectrodes, it is difficult to determine whether the improvements observed in water splitting performance is a result of enhanced bulk properties due to higher conductivity within the basal plane or from surface effects such as improved kinetics or reduced surface recombination.…”