On the Theoretical and Experimental Control of Defect Chemistry and Electrical and Photoelectrochemical Properties of Hematite Nanostructures

Abstract

Hematite (α-Fe 2 O 3 ) is regarded as one of the most promising cost-effective and stable anode materials in photoelectrochemical applications, and its performance, like other transition-metal oxides, depends strongly on its electrical and defect properties. In this work, the electrical and thermomechanical properties of undoped and Sn-doped α-Fe 2 O 3 nanoscale powders were characterized in situ at controlled temperatures (T = 250 to 400 °C) and atmospheres (pO 2 = 10 −4 to 1 atm O 2 ) to investigate their transport and defect properties. Frequency-dependent complex impedance spectra show that interfacial resistance between particles is negligible in comparison with particle resistance. Detailed defect models predicting the dependence of electron, hole, and iron and oxygen vacancy concentrations on temperature and oxygen partial pressures for undoped and doped α-Fe 2 O 3 were derived. Using these defect equilibria models, the operative defect regimes were established, and the bandgap energy of undoped α-Fe 2 O 3 and oxidation enthalpy of Sn-doped α-Fe 2 O 3 were obtained from the analysis of the temperature and pO 2 dependence of the electrical conductivity. On the basis of these results, we are able to explain the surprisingly weak impact of donor doping on the electrical conductivity of α-Fe 2 O 3 . Furthermore, experimental means based on the results of this study are given for successfully tuning hematite to enhance its photocatalytic activity for the water oxidation reaction.

The role of carbon dots – derived underlayer in hematite photoanodes

“…4a), two dominant peaks corresponding to Fe 2p 1/2 (∼724.3 eV) and Fe 2p 3/2 (∼711.1 eV) are observed, accompanied by their shakeup satellite peaks centered at 718.7 eV and 732.9 eV (Fe 2p 1/2 , sat and Fe 2p 3/2 , sat), confirming the presence of Fe 3+ as Fe 2 O 3 . 55 Importantly, a careful inspection of Fe 2p spectrum reveals that a red shift (0.5 eV) of the Fe 2p 1/2 satellite towards lower energy (Fig. 4a) for the H/UCDs samples relative to that of the pristine hematite, suggesting the existence of Fe 2+ or Fe-C bonds, which is consistent with the EELS data.…”

“…4b) exhibited a main line at 530.0 eV with a shoulder peak at 531.5 eV for all samples, corresponding to the coordination of Fe-O bond and surface hydroxyl species, respectively. 55,59 Pristine hematite and H/UCDs show similar optical absorbance edge, from ∼600 to ∼400 nm, indicating the introduction of UCDs does not have a significant effect on the UV-vis absorption range of hematite photoanodes (Fig. S6a †).…”

Effect of Doping and Excitation Wavelength on Charge Carrier Dynamics in Hematite by Time‐Resolved Microwave and Terahertz Photoconductivity

“…After verifying that the TRMC response of the capped films probes bulk properties, epitaxial hematite films with different dopants were prepared. Hematite is often doped to change its magnetic, electronic, or photoelectrochemical properties . Undoped hematite behaves as a weak n‐type semiconductor due to the existence of oxygen vacancies which are compensated by electrons.…”

“…Hematite is often doped to change its magnetic, electronic, or photoelectrochemical properties . Undoped hematite behaves as a weak n‐type semiconductor due to the existence of oxygen vacancies which are compensated by electrons. Hematite is typically doped with donors such as Ti or Sn to make a stronger n‐type semiconductor and improve the photoelectrochemical performance .…”

Revealing the synergy of Sn insertion in hematite for next‐generation solar water splitting nanoceramics

“…As a consequence, bare hematite does not always refer to Sn‐free material and both strategies have been often reported as Sn doping methodologies. Based on these aspects, some controversial points have been disseminated, mainly by the indiscriminate use of the word “doping” referring to any Sn addition on hematite able to change the photoelectrochemical performance 42 . Moreover, tin incorporation into hematite has been reported as capable of occurring in crystalline structure, on the surface or at grain boundaries, not necessarily replacing Fe 3+ by Sn 4+ in a site.…”