Oxygen vacancy doping of hematite analyzed by electrical conductivity and thermoelectric power measurements

Abstract

Hematite (α-Fe 2 O 3) is known for poor electronic transport properties, which are the main drawback of this material for optoelectronic applications. In this study, we investigate the concept of enhancing electrical conductivity by the introduction of oxygen vacancies during temperature treatment under low oxygen partial pressure. We demonstrate the possibility of tuning the conductivity continuously by more than five orders of magnitude during stepwise annealing in a moderate temperature range between 300 and 620 K. With thermoelectric power measurements, we are able to attribute the improvement of the electrical conductivity to an enhanced charge-carrier density by more than three orders of magnitude. We compare the oxygen vacancy doping of hematite thin films with hematite nanoparticle layers. Thereby we show that the dominant potential barrier that limits charge transport is either due to grain boundaries in hematite thin films or due to potential barriers that occur at the contact area between the nanoparticles, rather than the potential barrier within the small polaron hopping model, which is usually applied for hematite. Furthermore, we discuss the transition from oxygen-deficient hematite α-Fe 2 O 3−x towards the magnetite Fe 3 O 4 phase of iron oxide at high density of vacancies.

Manipulation of Charge Transfer in FeP@Fe2O3 Core–Shell Photoanode by Directed Built-In Electric Field

“…Theoretically, the maximum photocurrent of Fe 2 O 3 photoanode under AM 1.5G illumination is 12.6 mA/cm 2 for water oxidation . However, the practical photocurrent is much lower than the theoretical value ascribed to limited electrical conductivity, slow charge separation, sluggish charge transfer, and short hole diffusion lengths (2–4 nm). − …”

Identifying protons trapped in hematite photoanodes through structure–property analysis

“…[20][21][22][23][24] X-ray photoelectron spectroscopy and X-ray absorption spectroscopy have been successfully employed to conrm and quantify these vacancies, 20,21 and to show that complete removal of oxygen vacancies from the photoanode surface can result in suppression of PEC performance. 20,21 Increased concentrations have been shown to increase both the density and mobility of charge carriers by orders of magnitude, 25 a feature which is frequently observed in PEC studies as an increased charge carrier density (N d ) measured through Mott-Schottky analysis; [22][23][24] increased N d has been widely shown to result in improved PEC performance but induce negligible changes in absorption proles. 24 Evidence has been provided that excessive removal of oxygen from the lattice leads to decomposition to Fe 3 O 4 , 25 suggesting a limitation in potential gains.…”

“… 20,21 Increased concentrations have been shown to increase both the density and mobility of charge carriers by orders of magnitude, 25 a feature which is frequently observed in PEC studies as an increased charge carrier density ( N d ) measured through Mott–Schottky analysis; 22–24 increased N d has been widely shown to result in improved PEC performance but induce negligible changes in absorption profiles. 24 Evidence has been provided that excessive removal of oxygen from the lattice leads to decomposition to Fe 3 O 4 , 25 suggesting a limitation in potential gains. The improved fundamental understanding of the relationship between PEC behavior and a specific structural defect marks a significant advance that brings the field closer to the rational development of photoanodes.…”

Laser-Induced Photoreduction for Selective Tuning of the Oxidation State and Crystal Structure of Hematite Nanorods

“…It is generally accepted that the OVs in hematite serve as electron donors, resulting in improved electrical conductivity. [6][7][8][9]56 The donor density (N d ) of the samples was estimated using Mott− Schottky analysis. Figure 5a In an electrochemical cell, the bulk capacitance (C bulk ) of an electrode is related to the applied potential (V) by the Mott− Schottky equation as follows…”