Oxygen Vacancies Engineering of Iron Oxides Films for Solar Water Splitting

Abstract

International audienceHematite (α-Fe 2 O 3) can be considered as one of the top candidates to act as photoanode in the framework of clean hydrogen production through solar water splitting. The O:Fe ratio, that in this material plays a crucial role in the definition of its photoelectrochemical properties, has been investigated in detail. For this purpose, we examined thermal magnetite oxidation and hematite reduction as two possible routes to produce semiconducting iron oxides layers with controlled stoichiometry. We report on properties of single crystalline nanometric films elaborated by atomic oxygen plasma assisted molecular beam epitaxy as model systems to disentangle structural phase transition effects from pure stoichiometry ones. We provide new insights into the mechanisms related to hematite properties modifications and their correlation with photocurrent changes upon the presence of oxygen vacancies and phase mixing with magnetite, with respect to the vacancies concentration regimes. We show on one hand that crystallographic structure mixing appears as strongly detrimental for photoanodes synthesis whatever the oxygen vacancies concentration. On the other hand, oxygen vacancies in the optimal concentration range, while preserving the α-Fe 2 O 3 corundum phase, is highly favorable for solar water splitting, inducing a substantial reduction of 0.2 V for the onset potential and an overall photocurrent increase of 50% with respect to stoichiometric hematite. The present study demonstrates more generally the possibility of using oxygen vacancies as a degree of freedom for the optimization of hematite photoanodes

Activation of α‐Fe2O3 for Photoelectrochemical Water Splitting Strongly Enhanced by Low Temperature Annealing in Low Oxygen Containing Ambient

“…It was demonstrated that the introduction of oxygen vacancies considerably increased the electrical conductivity. However, under these reducing condition precursor oxides can easily form magnetite (Fe 3 O 4 ) which is highly detrimental for photolysis as it is either metal‐like or behaves like a narrow band gap (<1 eV) semiconductor . In other words, it is desired to introduce oxygen vacancies into α‐Fe 2 O 3 while suppressing the formation of Fe 3 O 4 .…”

“…Furthermore, literature generally describes that a thermal oxidative annealing leads to oxide layers that consist of a gradient of wustite (FeO), magnetite (Fe 3 O 4 ), and α‐Fe 2 O 3 . Since FeO and Fe 3 O 4 are either metal‐like or behave like a narrow band gap (<1 eV) semiconductor, they are not desired for photolysis . A similar double layer structure was also observed for the LO sample, but the thickness of the compact inner layer was slightly thinner due to the suppression of the thermal oxidation reaction in the low oxygen ambient.…”

“…[30] Since FeO and Fe 3 O 4 are either metal-like or behave like an arrow band gap (< 1eV) semiconductor,t hey are not desired for photolysis. [27,33] As imilar double layer structure was also observed for the LO sample, but the thickness of the compact inner layer was slightly thinner due to the suppression of the thermal oxidation reaction in the low oxygen ambient. For the layer formed after the Ar treatment (AR), the double layer structure was not observed because the thermal oxidation reaction is essentially suppressed in the oxygen deficient atmosphere.…”

“…[15,[28][29][30][31][32] Ling et al [29] reported that photoresponse of a-Fe 2 O 3 nanowires fabricated on an FTO (fluorine doped tin oxide) glass by hydrothermal synthesis was activated by annealing in an oxygen deficient atmosphere achieved in an evacuated furnacer efilled by pure N 2 .I tw as demonstrated that the introductiono fo xygen vacancies considerably increasedt he electricalc onductivity.H owever,u nder these reducing condition precursor oxidesc an easily form magnetite (Fe 3 O 4 )w hich is highly detrimental for photolysis as it is either metal-like or behaves like anarrow band gap (< 1eV) semiconductor. [27,33] In other words, it is desired to introduce oxygen vacancies into a-Fe 2 O 3 while suppressing the formation of Fe 3 O 4 .I nt he presentw ork, we investigate the effect of annealing in ad efined gas environment containing av ery low concentration of oxygen (Ar + 0.03 %O 2 atmosphere)o nt he PEC performance of nanostructured a-Fe 2 O 3 prepared by anodization of iron foils. We demonstrate that controlled annealing in low oxygen ambient can drastically improve the PEC performance of a-Fe 2 O 3 photoanodes even at al ow temperature of 400 8C.…”

Highly Selective Electrochemical Reduction of Dinitrogen to Ammonia at Ambient Temperature and Pressure over Iron Oxide Catalysts

“…The Cs ignals originated from the adsorbed carbon species onto the surface of the nanostructured materials and are commonly used as ac harge reference for XPS spectra. [31] To confirm the phase structure of the hematite samples before and after annealing, we obtainedX -ray diffraction (XRD) patterns for all samples (Figure 2c), in which all diffraction peaks matched well with those of the rhombohedral a-Fe 2 O 3 (JCPDS No. To further elucidatet his difference, the O1 ss pectra weref itted to three individual peaks to represent different oxygen environments (see the Supporting Information, Figure S1 b-d, Ta ble S1).…”