Nanometer-Thick Hematite Films as Photoanodes for Solar Water Splitting

Abstract

Photoelectrochemical (PEC) water splitting is one of the most promising sustainable methods for feasible solar hydrogen production. However, this method is still impractical due to the lack of suitable photoanode materials that are efficient, stable, and cost-effective. Here, we present a surprisingly simple fabrication method for efficient, stable, and cost-effective nanometer-thick hematite films utilizing a rapid, ambient annealing approach. In the oxygen evolution reaction, the fabricated hematite films exhibit a Faradaic efficiency of 99.8% already at 1 V versus the reversible hydrogen electrode (RHE), a real photocurrent density of 2.35 mA cm–2 at 1.23 V versus RHE, and a superior photo-oxidation stability recorded for over 1000 h. Considering the active surface area, the measured photocurrent density is higher than any value achieved so far by hematite and other single-material thin-film photoanodes. Hence, we show for the first time that undoped hematite thin films can compete with doped hematite and other semiconductor materials.

Tellurium Nanosphere/Hematite Hybrid Thin Films with Enhanced Light Absorption for Efficient Photocatalysis

“…Photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes provides a promising route for storing solar energy. , Several semiconductors (e.g., TiO 2 , Fe 2 O 3 , and ZnIn 2 S 4 ) have exhibited promising prospects in photocatalysis; however, materials with large optical absorption coefficients tend to show high reflectivity due to the impedance mismatch at interfaces with air or other transparent dielectrics, limiting the total amount of absorbed solar power. , Therefore, enhancing the absorption of light is of great importance for efficient energy conversion. A well-known approach to suppress interface reflection involves an ultrathin film photoabsorber that is a nanoscale energy transducer converting solar energy to hydrogen energy. , The ultrathin film could provide a fast channel for the migration of photogenerated carriers and suppress the recombination of photogenerated carriers effectively .…”

Ultrathin Hematite‐Hercynite Films for Future Unassisted Solar Water Splitting

“…The PEC performance of hematite photoanodes is generally strongly dependent on the oxidation parameters. [ 9,18 ] A relatively high oxidation temperature and an extended oxidation time appear both beneficial for fabricating efficient hematite photoanodes. [ 9,18 ] For example, Sivula et al.…”

“…However, annealing at 800 °C must be considered undesirable because of the instability of many glass substrates, and the overall limitation to material systems that can withstand high temperatures. Relatively lower oxidation temperatures, such as ≈630 °C, are generally favorable but require, for instance, 6 h of oxidation as reported elsewhere [ 9 ] However, we aim to fabricate efficient photoanodes at lower oxidation temperatures or in a shorter time by using iron–aluminum alloys. For the electrode synthesis, we used thermal oxidation of ≈100 nm thick iron–aluminum alloy thin films in ambient air following the so‐called direct annealing method as described in ref.…”

“…For the electrode synthesis, we used thermal oxidation of ≈100 nm thick iron–aluminum alloy thin films in ambient air following the so‐called direct annealing method as described in ref. [9]. In our study, either relatively high annealing temperatures for very short annealing time or a low annealing temperature for a longer annealing time were examined and correlated with the electrode PEC performance.…”

Laser-Induced Hydrothermal Growth of Iron Oxide Nanoparticles on Diverse Substrates for Flexible Micro-Supercapacitors

“…Iron oxide has several crystalline polymorphs, including wüstite (FeO), hematite (α-Fe 2 O 3 ), and magnetite (Fe 3 O 4 ). Because of their distinct physical/chemical properties, crystalline iron oxides have been broadly studied in various applications, such as photocatalysis, solar water splitting, , batteries, and drug delivery . Besides the crystalline phases of iron oxide, amorphous iron oxide nanomaterials have been widely studied in lithium-ion batteries and oxygen evolution reaction catalysis .…”