Photoelectrochemical water splitting with porous α-Fe2O3 thin films prepared from Fe/Fe-oxide nanoparticles

• Synthesis of Fe/Fe-oxide nanoparticles (NPs) was conducted by thermal decomposition of iron pentacarbonyl.
• Spin-coating of Fe/Fe-oxide NPs allows fabrication of porous α-Fe2O3 thin films.
• α-Fe2O3 thin films shows photoelectrochemical water splitting properties.
• Deposition of NiOOH catalyst onto α-Fe2O3 improoves onset potential for water splitting.

We report on the photoelectrochemical (PEC) performance related to water oxidation of porous hematite (α-Fe2O3) thin films, which were prepared by spin coating of colloidal core/shell Fe/Fe-oxide nanoparticles (NPs) on fluorine-doped SnO2 (FTO) substrates. Oxidation of the obtained Fe/Fe-oxide films at 600 °C, 700 °C, and 800 °C in air yielded porous α-Fe2O3 thin films. The advantage of using Fe/Fe-oxide NPs is that they form stable suspensions in organic solvents and are suitable for spin coating. The highest photocurrent density of 0.75 mA/cm2 at 1.23 V (vs. a reversible hydrogen electrode, RHE) was achieved with an α-Fe2O3 thin film calcined at 800 °C. Incident photon-to-current conversion efficiency (IPCE) data showed that the quantum efficiency of the thin films was about 15% at 350 nm at an applied bias of 1.4 V vs. RHE. To improve the oxygen evolution reaction, we electrodeposited a Ni(OH)2/NiOOH catalyst (given as NiOOH) onto the α-Fe2O3 film and achieved the reduction of onset potential from 0.85 to 0.69 V vs. RHE. Electrochemical impedance spectroscopy and open-circuit photovoltage (OCP) measurements were used to estimate the flat-band potential of the thin films.

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