Tuning Defects in Ni‐Doped Maghemite for Enhanced Solar Driven Oxygen Evolution: Insights From <i>Operando</i> X‐Ray Spectroscopies

Maghemite ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>γ</mml:mi> <mml:annotation>$\gamma$</mml:annotation></mml:semantics> </mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>Fe</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:annotation>${\rm Fe}_2$</mml:annotation></mml:semantics> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>O</mml:mi> <mml:mn>3</mml:mn></mml:msub> <mml:annotation>${\rm O}_{3}$</mml:annotation></mml:semantics> </mml:math> ) is a promising non-precious-metal-containing photocatalyst for water oxidation (OER). Despite being less studied than Hematite, it offers similar corrosion resistance and a favorable band structure, along with higher conductivity and the advantage of an adaptable spinel structure. Its catalytic performance can be optimized by tuning the concentration of a donor dopant. In this work, we synthesized via molecular beam epitaxy ultra-thin (6 nm) Ni-doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>γ</mml:mi> <mml:annotation>$\gamma$</mml:annotation></mml:semantics> </mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>Fe</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:annotation>${\rm Fe}_2$</mml:annotation></mml:semantics> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>O</mml:mi> <mml:mn>3</mml:mn></mml:msub> <mml:annotation>${\rm O}_3$</mml:annotation></mml:semantics> </mml:math> (111) films with different Ni concentrations. We present a comprehensive study that sheds light on the structural reconfigurations induced by tuning the Ni concentration and the resulting effects on the OER onset and photoconversion efficiency in alkaline media. Samples' electronic reconfiguration under operando conditions and the role of Ni dopant on the catalytic mechanism are studied with X-ray Absorption Spectroscopy. Light-driven surface chemistry modifications during OER are probed with a novel X-ray fluorescence pump-and-probe approach: Fixed energy X-ray absorption PhotoVoltammetry.