<i>In situ</i> ambient pressure XPS observation of surface chemistry and electronic structure of α-Fe₂O₃ and γ-Fe₂O₃ nanoparticles

다국어 초록 (Multilingual Abstract)

<P><B>Abstract</B></P> <P>Fundamental understanding of charge transfer reaction is essential for the surface and interface engineering of transition metal oxides. In this study the chemical reactivity towards oxygen and hydrogen (13 Pa) under applied thermal conditions (423–673 K), of two polymorphic forms of Fe<SUB>2</SUB>O<SUB>3</SUB> nanoparticles (γ-Fe<SUB>2</SUB>O<SUB>3</SUB> and α-Fe<SUB>2</SUB>O<SUB>3</SUB>) are investigated with the combination of <I>in situ</I> ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and near edge X-ray absorption fine structure spectroscopy (AP-NEXAFS). Our data show that the reactivity of these two polymorphs has a similar character based on the contribution of oxygen vacancy defect states and related material non-stoichiometry. Their exposure to hydrogen at increased temperature results in both cases in the surface reduction. However, γ-Fe<SUB>2</SUB>O<SUB>3</SUB> exhibits more covalent character and undergoes the reduction preferentially with a contribution of metallic Fe<SUP>0</SUP> than Fe<SUP>2+</SUP>, in contrast to α-Fe<SUB>2</SUB>O<SUB>3</SUB>. Further, upon introduction of oxygen at low temperature of 423 K, rapid re-oxidation process takes place at the Fe<SUB>2</SUB>O<SUB>3</SUB> nanoparticles surface. Prepared γ-Fe<SUB>2</SUB>O<SUB>3</SUB> and α-Fe<SUB>2</SUB>O<SUB>3</SUB> nanostructures exhibit in general high n-type and p-type sensor response towards hydrogen, respectively, in a wide concentrations range.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Chemical reactivity investigation of Fe<SUB>2</SUB>O<SUB>3</SUB> polymorph NPs by <I>in situ</I> XPS and NEXAFS. </LI> <LI> Reactivity of Fe<SUB>2</SUB>O<SUB>3</SUB> polymorph NPs is based on the material non-stoichiometry. </LI> <LI> γ-Fe<SUB>2</SUB>O<SUB>3</SUB> more covalent than α-Fe<SUB>2</SUB>O<SUB>3</SUB> in particular upon interaction with H<SUB>2</SUB>. </LI> <LI> γ-Fe<SUB>2</SUB>O<SUB>3</SUB> undergoes reduction preferentially with contribution of Fe<SUP>0</SUP> unlike α-Fe<SUB>2</SUB>O<SUB>3</SUB>. </LI> <LI> Electronic structure changes of Fe<SUB>2</SUB>O<SUB>3</SUB> NPs upon O<SUB>2</SUB> and H<SUB>2</SUB> exposures are reversible. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>