Interfacial oxygen vacancies yielding long-lived holes in hematite mesocrystal-based photoanodes

Abstract

Hematite (α-Fe₂O₃) is one of the most promising candidates as a photoanode materials for solar water splitting. Owing to the difficulty in suppressing the significant charge recombination, however, the photoelectrochemical (PEC) conversion efficiency of hematite is still far below the theoretical limit. Here we report thick hematite films (∼1500 nm) constructed by highly ordered and intimately attached hematite mesocrystals (MCs) for highly efficient PEC water oxidation. Due to the formation of abundant interfacial oxygen vacancies yielding a high carrier density of ∼10²⁰ cm^-3 and the resulting extremely large proportion of depletion regions with short depletion widths (<10 nm) in hierarchical structures, charge separation and collection efficiencies could be markedly improved. Moreover, it was found that long-lived charges are generated via excitation by shorter wavelength light (below ∼500 nm), thus enabling long-range hole transfer through the MC network to drive high efficiency of light-to-energy conversion under back illumination.

Fig. 1

Morphological characterization of Ti–Fe₂O₃ MC. a TEM and corresponding SAED (inset), and b HRTEM image of the as-synthesized Ti–Fe₂O₃ MC. c TEM and corresponding SAED (inset), d HAADF-STEM image and e corresponding EELS chemical composition maps of a typical Ti–Fe₂O₃ MC particle collected after annealing at 700 °C. f EEL spectra of the selected regions in panel d. g Top-view, and h cross-sectional SEM images of the optimized Ti–Fe₂O₃ MC photoanode. i XRD patterns of the Ti–Fe₂O₃ MC, Fe₂O₃ MC, and Fe₂O₃ SC photoanodes

Fig. 2

Interfacial oxygen vacancies (V_O) in MCs. a, b HAADF-STEM images of Fe₂O₃ MC and c corresponding EELS chemical composition maps of panel b. d EEL spectra of panel b and e corresponding EELS Fe-L_2,3 spectra of the selected region in panel b. The rightmost one of panel c is merged image of components 1 (red), 2 (green), and 3 (blue). f EPR spectra of the samples. g Schematic illustration of the V_O formation in MC, where (100) facets are tentatively assumed to form the interface

Fig. 3

PEC performance. a Current density–voltage curves of Ti–Fe₂O₃ MC/Co–Pi, Ti–Fe₂O₃ MC, Fe₂O₃ MC, and Fe₂O₃ SC photoanodes prepared by 50 cycles of spin-coatings measured under back illumination with AM 1.5 G simulated sunlight in 1.0 M NaOH. b IPCE curves measured at 1.23 V vs. RHE for the Ti–Fe₂O₃ MC, Fe₂O₃ MC, and Fe₂O₃ SC photoanodes. c Gas evolved from Ti–Fe₂O₃ MC photoanode and Pt counter electrode with an applied potential of 1.23 V vs. RHE under back illumination during 3 h and the corresponding Faradaic efficiencies (FEs). d Current density–time curve of the Ti–Fe₂O₃ MC photoanode measured at 1.23 V vs RHE under back illumination. e Photograph showing gas evolution from Ti–Fe₂O₃ MC photoanode and Pt counter electrode under an applied potential of 1.23 V vs. RHE

Fig. 4

Charge transfer efficiencies. a Mott–Schottky plots of the samples. Capacitances (Cs) were determined from electrochemical impedance measurements at a frequency of 10 kHz in the dark. b Electrochemical impedance spectra measured for the Ti–Fe₂O MC, Fe₂O₃ MC, and Fe₂O₃ SC photoanodes. The inset shows the equivalent circuit. c The energetic distribution of N_SS as a function of the applied potentials. d The correlation between N_SS, N_d, and current density at 1.23 V vs. RHE

Fig. 5

Superior back illumination current generation. a Lambert–Beer law-based simulation of light intensity in hematite film under different illumination modes (dash lines) and the measured light intensity in hematite with different thicknesses (dots) and the corresponding fitted line (solid lines). b The method used to measure the light intensity (I_x/I₀). c The current density–voltage curves of Ti–Fe₂O₃ MC photoanode (with thickness of 1500 nm) measured under different illumination modes. d The current densities at 1.23 V vs. RHE of the Ti–Fe₂O₃ MC photoanodes with different film thicknesses measured under different illumination modes

Fig. 6

Time-resolved PL measurements and wavelength-dependent current generation. a Emission spectrum (Inset shows the corresponding optical transmission images) and b decay profiles collected from isolated small aggregates of Ti–Fe₂O₃ MCs, Fe₂O₃ MCs, and Fe₂O₃ SCs. c The proposed electronic transitions and charge recombination process in MC. d Current density at 1.23 V vs. RHE of the Ti–Fe₂O₃ MC photoanode under monochromatic light illumination (430 nm and 530 nm) from the back side. The normalized current density at 1.23 V vs. RHE of the Ti-Fe₂O₃ MC photoanodes with different film thicknesses under e 430-nm and f 530-nm monochromatic light illumination from the back side

Fig. 7

Proposed charge carrier dynamics in thick hematite MC film. a Illustration of the charge transfer at the SEI of Ti–Fe₂O₃ MC. b Illustration of the hole transport in the optimized Ti–Fe₂O₃ MC photoanode with film thickness of 1500 nm