Anodic Nanotubular/porous Hematite Photoanode for Solar Water Splitting: Substantial Effect of Iron Substrate Purity

Anodization of iron substrates is one of the most simple and effective ways to fabricate nanotubular (and porous) structures that could be directly used as a photoanode for solar water splitting. Up to now, all studies in this field focused on achieving a better geometry of the hematite nanostructures for a higher efficiency. The present study, however, highlights that the purity of the iron substrate used for any anodic‐hematite‐formation approach is extremely important in view of the water‐splitting performance. Herein, anodic self‐organized oxide morphologies (nanotubular and nanoporous) are grown on different iron substrates under a range of anodization conditions, including elevated temperatures and anodization supported by ultrasonication. Substrate purity has not only a significant effect on oxide‐layer growth rate and tube morphology, but also gives rise to a ninefold increase in the photoelectrochemical water‐splitting performance (0.250 vs. 0.028 mA cm−2 at 1.40 V vs. reversible hydrogen electrode under AM 1.5 100 mW cm−2 illumination) for 99.99 % versus 99.5 % purity iron substrates of similar oxide geometry. Elemental analysis and model alloys show that particularly manganese impurities have a strong detrimental effect on the water‐splitting performance. Purity matters: This study highlights that the iron substrate purity used for any anodic hematite formation or thermal oxide approach is extremely important in view of the performance as a photoanode for photoelectrochemical water splitting. A ninefold increase in the water‐splitting performance can be obtained by replacing the most common iron 99.5 % purity substrate with a 99.99 % purity.