Efficient Water Splitting Cascade Photoanodes with Ligand-Engineered MnO Cocatalysts

Abstract

The band edge positions of semiconductors determine functionality in solar water splitting. While ligand exchange is known to enable modification of the band structure, its crucial role in water splitting efficiency is not yet fully understood. Here, ligand-engineered manganese oxide cocatalyst nanoparticles (MnO NPs) on bismuth vanadate (BiVO₄) anodes are first demonstrated, and a remarkably enhanced photocurrent density of 6.25 mA cm^-2 is achieved. It is close to 85% of the theoretical photocurrent density (≈7.5 mA cm^-2) of BiVO₄. Improved photoactivity is closely related to the substantial shifts in band edge energies that originate from both the induced dipole at the ligand/MnO interface and the intrinsic dipole of the ligand. Combined spectroscopic analysis and electrochemical study reveal the clear relationship between the surface modification and the band edge positions for water oxidation. The proposed concept has considerable potential to explore new, efficient solar water splitting systems.

Keywords: MnO; band structure; ligand engineering; oxygen evolution catalysts; water splitting.

Figure 1

a,b) TEM image of 10 nm sized as‐prepared and BF₄‐treated MnO crystals. c) Photographic images of the as‐prepared MnO, BF₄‐treated, EDTA‐treated, and Ca–EDTA‐treated MnO NP solution. d) FT‐IR spectra of the as‐prepared, BF₄‐treated, EDTA‐treated, and Ca–EDTA‐treated MnO.

Figure 2

a) The corresponding TEM images of BF₄‐treated MnO/BiVO₄/WO₃ nanorods. b) Expanded image of BF₄‐treated MnO/BiVO₄/WO₃ nanorods. c–g) EDS images of W, Bi, V, Mn, and O, respectively. h) (112) and (102) of BiVO₄. i) Small square by HR‐TEM shows crystalline planes of (110) and (1−10) of BF₄‐treated MnO. j,k) Fast Fourier transform (FFT) pattern of BiVO₄ and BF₄‐treated MnO (* (c)–(g) scale bar is 100 nm).

Figure 3

a) Schematic illustration of MnO NP/BiVO₄/WO₃ photoanodes under back‐side illumination and the operation of water splitting cell. b) Photocurrent density of MnO/BiVO₄/WO₃ anodes with diverse ligands of MnO. c) Comparison of photocurrent density at 1.23 V versus RHE with different ligands of MnO. d) Electrochemical impedance spectra for three types of BiVO₄‐based anodes. e) Mott–Schottky plot of with BiVO₄/WO₃, BF₄‐treated MnO/BIVO₄/WO₃, and Ca–EDTA‐treated MnO/BiVO₄/WO₃ measured under light off. The inset is enlarged Mott–Schottky plot (* frequency: 1 kHz, amplitude: 10 mV). f) IPCE spectra of WO₃, BiVO₄, BiVO₄/WO₃, and BF₄‐treated MnO/BiVO₄/WO₃ at 1.23 V versus RHE. g) Stability test of BF₄‐treated MnO/BiVO₄/WO₃ anode. The photoactivities are measured in presence of Na₂SO₃.

Figure 4

a,b) UV–vis absorption spectra of WO₃, BiVO₄, BF₄‐treated MnO, Ca–EDTA‐treated MnO. c) The secondary electron emission spectra of the WO₃, BiVO₄/WO₃, BF₄‐treated MnO/BiVO₄/WO₃, Ca‐treated MnO/BiVO₄/WO₃, and reference Au foil electrodes. d) Valence band spectra, the energy difference between the Fermi level and the valence band maximum (E _F − E _V) of WO₃, BiVO₄/WO₃, BF₄‐treated MnO/BiVO₄/WO₃, Ca‐treated MnO/BiVO₄/WO₃. e,f) Flat band structure of BF₄‐treated MnO/BiVO₄/WO₃, Ca‐treated MnO/BiVO₄/WO₃, respectively.

Figure 5

The structural formulas and band structure of a) Ca–EDTA‐treated MnO/BiVO₄ and b) BF₄‐treated MnO/BiVO₄.