Electronic Structure, Optical Properties, and Photoelectrochemical Activity of Sn-Doped Fe₂O₃ Thin Films

Abstract

Hematite (Fe 2 O 3 ) is a well-known oxide semiconductor suitable for photoelectrochemical (PEC) water splitting and industry gas sensing. It is widely known that Sn doping of Fe 2 O 3 can enhance the device performance, yet the underlying mechanism remains elusive. In this work, we determine the relationship between electronic structure, optical properties, and PEC activity of Sn-doped Fe 2 O 3 by studying highly crystalline, wellcontrolled thin films prepared by pulsed laser deposition (PLD). We show that Sn doping substantially increases the n-type conductivity of Fe 2 O 3 , and the conduction mechanism is better described by a small-polaron hopping (SPH) model. Only 0.2% Sn doping significantly reduces the activation energy barrier for SPH conduction from at least 0.5 eV for undoped Fe 2 O 3 to 0.14 eV for doped ones. A combination of X-ray photoemission, X-ray absorption spectroscopy, and DFT calculations reveals that the Fermi level gradually shifts toward the conduction band minimum with Sn doping. A localized Fe 2+ -like gap state is observed at the top of the valence band, accounting for the SPH conduction. Interestingly, different from the literature, only 0.2% Sn doping in Fe 2 O 3 significantly improves the PEC activity, while more Sn decreases it. The improved PEC activity is partially attributed to an increased band bending potential which facilitates the charge separation at the space charge region. The reduced activation energy barrier for SPH will facilitate the transport of photoexcited carriers for the enhanced PEC, which is of interest for further carrier dynamics study.

Defects in complex oxide thin films for electronics and energy applications: challenges and opportunities

“…233 On the other hand, it has been shown that Fe 2 O 3 can be p-type doped by divalent cations such as Mg 2+ , 234,235 and n-type doped with tetravalent cations such as Ti 4+ , Sn 4+ , etc. [236][237][238][239] Doped Fe 2 O 3 can remarkably enhance the PEC activity toward water oxidation compared with undoped counterparts. [239][240][241][242] The same trend of enhanced PEC activity was also achieved in Mo 6+ or W 6+ doped BiVO 4 .…”

“…[236][237][238][239] Doped Fe 2 O 3 can remarkably enhance the PEC activity toward water oxidation compared with undoped counterparts. [239][240][241][242] The same trend of enhanced PEC activity was also achieved in Mo 6+ or W 6+ doped BiVO 4 . [243][244][245][246] Defect engineering is another approach to tuning the band structure and therefore PEC performance of TMOs, especially V O , V C , and lattice distortion.…”

Solution-mediated nanometric growth of α-Fe2O3 with electrocatalytic activity for water oxidation

“…Importantly, Sn 4+ -doping has been found to be an effective approach for tailoring the electronic properties of a-Fe 2 O 3 . [41][42][43] In the present case, the catalytic activity was likely enhanced by an increased electron conductivity by Sn 4+ -doping. 43 Two binding energies of O 1s (529.5 eV and 531.0 eV) were assigned to the O 2À and OH À , respectively.…”

“…[41][42][43] In the present case, the catalytic activity was likely enhanced by an increased electron conductivity by Sn 4+ -doping. 43 Two binding energies of O 1s (529.5 eV and 531.0 eV) were assigned to the O 2À and OH À , respectively. 44,45 The ratio of OH À /O 2À decreased aer annealing at 300 C. Thus, we suggest that the observed lattice expansion in the as-deposited a-Fe 2 O 3 layer was caused by the OH species.…”

Identifying the Spectroelectrochemical Characteristics of Hematite Photoanodes for Water Oxidation

“…7 Introduction of n-type dopants such as Ti and Sn is known to elevate the Fermi level of hematite and remove oxygen vacancies, while introducing the Fe 2+ states too. 32,35 The gradient doping near the surface, and the concomitant upshifted local Fermi levels would therefore cause a downward band bending. As such, a more 36 anodic applied potential is needed to flatten the bands, causing shifts in Efb.…”