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

Abstract

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.

Annealing temperature-dependent crystallinity and photocurrent response of anodic nanoporous iron oxide film

“…For instance, Lee et al reported on photocurrent of 0.25 mA/cm 2 at 0.39 V versus Ag/AgCl in 1 M KOH (100 mW/cm 2 , AM 1.5) for air-annealed nanoporous film with a thickness of ;0.2 lm. 15 Similarly in our previous report, a photocurrent of 0.33 mA/cm 2 at 0.39 V versus Ag/AgCl was reported utilizing an anodic nanoporous photoanode with a thickness of ;1.8 lm annealed in air at 450°C for 3 h. 13 The relatively low photocurrent values may be attributed to the above mentioned problems: oxidation of iron during annealing, thickening of barrier layer and Fe 3 O 4 and/or FeO formation. In this present work, we investigated on the formation of the nanoporous iron oxide and on the effect of nitrogen annealing to the properties of the nanoporous oxide and on the photocurrent response when the oxide film was illuminated.…”

Enhanced Solar Water Splitting by Swift Charge Separation in Au/FeOOH Sandwiched Single‐Crystalline Fe2O3 Nanoflake Photoelectrodes

“…Moreover, Fe 2 O 3 photoanodes composed of nanoparticles usually suffer from charge migration and recombination problems at or in grain boundaries. Recently, photoanodes comprised of one‐dimensional (1D) nanoarrays (e.g., nanorods and nanotubes) grown perpendicularly to the conductive substrate [e.g., fluorine‐doped tin oxide (FTO)] have become of particular interest because of a direct electron‐transport pathway and short hole‐diffusion length to the interface, which result in improved PEC properties compared to traditional particles or porous films . More recently, vertically aligned nanosheet arrays with specifically designed different crystal facets were reported .…”

“…Recently,photoanodes comprised of one-dimensional (1D) nanoarrays (e.g.,n anorods and nanotubes) grown perpendicularly to the conductive substrate [e.g.,f luorine-doped tin oxide (FTO)] have become of particulari nterest because of ad irecte lectron-transport pathway and short hole-diffusion lengtht ot he interface, which result in improved PEC properties comparedt ot raditional particles or porousf ilms. [7][8][9][10][11] More recently,v ertically aligned nanosheet arrays with specifically designed different crystal facets were reported. [12] Owing to the different relative energies of different facets, photoexcited electrons and holes can be driven to different crystal facets, and thus certain facets of as emiconductor effect reduction whereas others facilitate oxidation.…”

Templating Sol–Gel Hematite Films with Sacrificial Copper Oxide: Enhancing Photoanode Performance with Nanostructure and Oxygen Vacancies

“…9,10 Therefore, nanorods, nanowire arrays, nanonets and porous colloidal based films are attractive morphologies for a high photon harvesting efficiency. [11][12][13] However, realizing a high surface area does not improve the intrinsic optical and electronic properties of the electrode material. Another strategy to improve the performance of hematite photoanodes is to increase the majority carrier concentration through the use of n-type subsitutional dopants (e.g.…”