Hazardous Doping for Photo-Electrochemical Conversion: The Case of Nb-Doped Fe2O3 from First Principles

Abstract

Abstract:The challenge of improving the efficiency of photo-electrochemical devices is often addressed through doping. However, this strategy could harm performance. Specifically, as demonstrated in a recent experiment, doping one of the most widely used materials for water splitting, iron(III) oxide (Fe 2 O 3 ), with niobium (Nb) can still result in limited efficiency. In order to better understand the hazardous effect of doping, we use Density Functional Theory (DFT)+U for the case of Nb-doped Fe 2 O 3 . We find a direct correlation between the charge of the dopant, the charge on the surface of the Fe 2 O 3 material, and the overpotential required for water oxidation reaction. We believe that this work contributes to advancing our understanding of how to select effective dopants for materials.

Iron oxide surfaces

“…Consequently there have been many attempts to dope the material with metals to both engineer the band gap and reduce the required over-potential [167; 177-179]. In many cases this improves performance, but in the case of Nb it does not [180]. The valence band edge is more than 1 V lower than the water oxidation potential, but the kinetics of the oxygen evolution reaction (4OH -O 2 + 4e -+ 2H 2 O) at the surface are slow, possibly due to the presence of surface states at the hematite/electrolyte interface [175; 181].…”

The “Rust” Challenge: On the Correlations between Electronic Structure, Excited State Dynamics, and Photoelectrochemical Performance of Hematite Photoanodes for Solar Water Splitting

“…[73] Doping is commonly used in order to improve both the photocurrent and onset potential for water photo-oxidation. [11,18,32,55,60,[74][75][76][77][78][79][80][81][82][83][84] However, the role of doping on the photoelectrochemical performance of hematite photoanodes is not fully understood and has been attributed to a variety of factors including improvement in conductivity, [85] passivation of surface states and grain boundaries [86] , shifting of band edge positions, [32,55,80] reduction in effective mass, [55] and distortion of the crystal structure [82][83][84] which facilitates hopping for both electrons and holes. [59]…”

“…Understanding of the surface water oxidation reaction has advanced significantly through DFT+U calculations. [60,62,81,82,143,144] In particular, a successful comparison has correlated between the electronic structure of surface reaction intermediates and the experimentally measured absorption spectra. [31,41] Furthermore, study of overlayers on hematite has also yielded valuable insight into modifying the interfacial electronic structure for enhanced water photo-oxidation.…”

Water Oxidation Catalysis with Fe2O3 Constrained at the Nanoscale

“…Spin-polarized density functional theory calculations were performed with the VASP program. , The DFT+U formalism of Duradev et al was used with the Perdew–Burke–Ernzerhof (PBE) functional and an on-site Coulomb repulsion with an effective U–J term of 4.3 eV for Fe because we wish to compare to previous calculations with this methodology that was shown successful in predicting electronic structure , and catalytic ability of Fe 2 O 3 . − Projected-augmented wave (PAW) potentials replaced the core electrons of Fe 1s2s2p3s and O 1s. , …”