Molecular Origin and Electrochemical Influence of Capacitive Surface States on Iron Oxide Photoanodes

Abstract

The origin, the nature, and the electronic structure of surface defects causing surface states on metal oxides and their role in solar water splitting have been under scrutiny for several decades. In the present study, the surface of hematite films is treated with an oxygen plasma and then subject to a detailed investigation with electroanalytical methods and element orbital specific X-ray spectroscopy. We observe a systemic variation of photoelectrochemical properties with oxygen treatment time. Fe 2p and O 1s core level X-ray photoelectron spectra and resonant valence band photoemission at the Fe 3p edge reveal the filling of prevalent oxygen vacancies with concomitant oxidation of Fe2+ to Fe3+ upon the oxygen treatment. The dc bias dependent impedance spectra confirm how a prevalent capacitive surface state, which evolves parallel with the photocurrent onset potential, becomes diminished upon oxygen treatment. Surface states of iron induce higher reactivity toward water oxidation than oxygen surface states. The correlation between oxygen vacancy filling, concentration of surface states, and photocurrent density in the course of treatment confirms that the surface defects are of a capacitive nature and that the onset of water splitting can be considered as a result of dielectric breakdown in an interfacial hydroxide layer between photoanode and water.

Systematic Investigation of the Electronic Structure of Hematite Thin Films

“…An introduction of other oxidation states of iron which would also alter the DOS can be ruled out as the satellite structure of the Fe2p‐peaks clearly shows Fe 3+ . In addition, Fe 2+ states would be expected to give rise to a photoemission signal at the Fermi energy, which is not observed in the present work …”

Identifying protons trapped in hematite photoanodes through structure–property analysis

“…Annealing under N 2 environments has been widely reported as a means to generate oxygen vacancies in a-Fe 2 O 3 photoanodes, which increases the charge carrier density and improves photocatalytic performance. [20][21][22][23][24] Here, we observe consistently improved performance for samples annealed under oxygen environments and no clear correlations between photocurrents and measured charge carrier densities ( Fig. 5D; Table S2 †).…”

“…The introduction of oxygen vacancies into a-Fe 2 O 3 has been shown to increase the relative intensity of the high energy component by a factor of 3. 21 The relative intensity of the high energy O 1s component here does not exhibit signicant changes for the three samples that lie on structure-property trend lines in 5; the relative intensity of the feature does decrease for the sample that was annealed for 10 min at 800 C under humidi-ed N 2 , which is an outlier in structure-property trends. The existence of a low energy shoulder on the Fe 2p 3/2 peak has been previously used to support the presence of oxygen vacancies by observation of associated Fe(II) within a-Fe 2 O 3 , 21 but detailed XPS studies show that electronic structure of Fe(III) based materials typically yield such features.…”

“…1 It has been reported that the concentration of oxygen vacancies (and the associated Fe II sites) can be increased in pure or doped forms of a-Fe 2 O 3 by annealing under O 2 -decient environments or decreased by annealing under O 2 -rich environments or in an oxygen plasma. [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.…”

Surface state assisted charge transfer and recombination processes at the semiconductor/electrolyte interface

“…These mechanisms are summarized in Figure 1.7. The first one (1), is a band to band recombination process. This process occurs when excitons, polarons or non interacting free electrons in the excited state recombine with holes in the ground state.…”