Single Crystalline α-Fe2O3 Nanosheets with Improved PEC Performance for Water Splitting

Abstract

We report the photoelectrochemical (PEC) performance of a densely grown single crystalline hematite (α-Fe₂O₃) nanosheet photoanode for water splitting. Unlike expensive ITO/FTO substrates, the sheets were grown on a piece of pure Fe through controlled thermal oxidation, which is a facile low cost and one-step synthesis route. The sheets grow with a widest surface parallel to basal plane (0001). Iron oxide formed on Fe consisting of layer structure α-Fe₂O₃-Fe₃O₄-Fe is elucidated from GIXRD and correlated to spectral features observed in Raman and UV-vis spectroscopy. The top α-Fe₂O₃ nanosheet layer serves as a photoanode, whereas the conducting Fe₃O₄ layer serves to transport photogenerated electrons to the counter electrode through its back contact. Time-resolved photoluminescence (TRPL) measurements revealed significantly prolonged carrier lifetime compared to that of bulk. Compared to the thin film of α-Fe₂O₃ grown on the FTO substrate, ∼3 times higher photocurrent density (0.33 mA cm^-2 at 1.23 V_RHE) was achieved in the nanosheet sample under solar simulated AM 1.5 G illumination. The sample shows a bandgap of 2.1 eV and n-type conductivity with carrier density 9.59 × 10¹⁷ cm^-3. Electrochemical impedance spectroscopy (EIS) measurements reveal enhanced charge transport properties. The results suggest that nanosheets synthesized by the simple method yield far better PEC performance than the thin film on the FTO substrate. The anodic shifts of flat band potential, delayed electron-hole recombination, and growth direction parallel to the highly conducting basal plane (0001) being some of the contributing factors to the higher photocurrent observed in the NS photoanode are discussed. Characterizations carried out before and after the PEC reaction show excellent stability of the nanosheets in an alkaline electrochemical environment.

Figure 1

XRD pattern recorded on the NS sample (a) before and (b) after the PEC experiment showing hematite (α-Fe₂O₃) and magnetite (Fe₃O₄) peaks denoted by H and M, respectively. The Fe(110) peak arising from the substrate is marked as “*”. The inset shows curve fitting of the peak at 35.5° having overlapping contribution from H and M layers.

Figure 2

(a) GIXRD pattern recorded on the NS sample at different grazing incidence angles. (b) Peak area obtained from fitting is plotted as ratio H/M against grazing angle. Magnetite and Hematite peaks are denoted as M and H, respectively, in (a,b).

Figure 3

(a) UV–vis diffuse reflectance spectra of the NS sample and bulk α-Fe₂O₃ powder converted to Kubelka–Munk (K–M) absorbance. The vertical dotted line shows the absorption edge at 2.1 eV in both the samples. (b) Absorption spectrum of the α-Fe₂O₃ thin film sample showing absorbance marked at two different wavelengths λ₁ and λ₂ at which the Raman spectra were recorded.

Figure 4

Raman spectra recorded on the NS sample using two different laser wavelengths: (a) 473 and (b) 633 nm. Raman modes of α-Fe₂O₃ (H) are visible in both (a,b). The inset shows the shaded region recorded for longer duration to obtain less intense modes of Fe₃O₄ (M).

Figure 5

FESEM images of the α-Fe₂O₃ NS sample; (a) low magnification image showing dense and uniform coverage, (b) image showing sheet-like morphology, (c) magnified image showing NSs of varying lateral dimensions from 100 nm marked as “a” and several hundred nm marked as “b”, and (d) cross-section image showing distinct morphology α-Fe₂O₃ NSs, layer of α-Fe₂O₃ grains, and the Fe₃O₄ layer in sequence.

Figure 6

TEM images of α-Fe₂O₃ NSs; (a) transmission image showing NSs having lateral dimensions of ∼100 nm. HRTEM image in the inset shows lattice fringes at spacing of 0.256 nm which correspond to (110) planes of the NS (b) SAED pattern with indexed diffraction spots. The zone axis is parallel to the c-axis of the NS. The spots forming vertices of hexagon resulting from hexagonal lattice of α-Fe₂O₃ are shown by a dotted line, except the (300) and (3®00) spots.

Figure 7

XPS spectra of the NS sample before and after PEC measurements. (a) Fe 2p spectra showing spin–orbit doublet and Fe³⁺ satellite characteristic to α-Fe₂O₃ (b) O 1s spectra recorded before (c) after PEC. Peak labels 1, 2, and 3 indicate Fe–O, Fe–OH, and chemisorbed Fe–OH, respectively.

Figure 8

Time-resolved photoluminescence spectra recorded on (a) bulk pallet of α-Fe₂O₃ and (b) α-Fe₂O₃ NS sample at an excitation wavelength (λ_ex) of 288 nm and emission wavelength (λ_em) measured at 465 nm.

Figure 9

LSV plot of the α-Fe₂O₃ NS and thin film samples shown for comparison. The photocurrent density at 1.23 V_RHE in the two samples is indicated by a dotted line.

Figure 10

(a) Mott–Schottky plot showing n-type conductivity for both NSs and bulk α-Fe₂O_3; the intercept on x-axis indicates the value of flat band potential V_FB. (b) Nyquist plot for the NS sample fitted with the equivalent circuit model.