Surface Reconstruction of Cobalt Species on Amorphous Cobalt Silicate-Coated Fluorine-Doped Hematite for Efficient Photoelectrochemical Water Oxidation

Abstract

The slow kinetics of photoelectrochemical (PEC) water oxidation reaction is the bottleneck of PEC water splitting. Here, we report a comprehensive method to improve the PEC water oxidation performance of a hematite (α-Fe 2 O 3 ) photoanode, that is, fluorine doping and an ultrathin amorphous cobalt silicate (Co-Sil) oxygen evolution reaction (OER) cocatalyst by photo-assisted electrophoretic deposition (PEPD). Detailed investigations reveal that fluorine doping can reduce the interfacial transfer resistance of charge and increase the carrier density to improve the conductivity of hematite. Also, simultaneously, the Co-Sil is used as an excellent OER cocatalyst to accelerate OER kinetics. Specifically, surface reconstruction of cobalt species occurred, and its average oxidation state increased significantly, which was more conducive to water oxidation. In addition, the presence of silicate groups could reduce the OOH* adsorption free energy. The synergistic effect of these efforts significantly reduced the onset potential and overpotential and enhanced the charge separation of the α-Fe 2 O 3 photoanode, resulting in an excellent photocurrent density around 2.61 mA cm −2 at 1.23 V vs RHE (4.75 times higher than the primitive α-Fe 2 O 3 ). This work provides a feasible strategy for the construction and development of a potential hematite photoanode.

Revealing the Essential Role of Iron Phosphide and its Surface‐Evolved Species in the Photoelectrochemical Water Oxidation by Gd‐Doped Hematite Photoanode

“…Futhermore, the double‐layer capacitance ( C dl ) values are obtained by performing cyclic voltammetry (CV) measurements (Figure S8) over an extremely small potential range, thereby approximately evaluating the electrochemically active surface area (ECSA) of the photoanode [19] . As shown in Figure 4c, the Gd−Fe 2 O 3 photoanode possesses increased C dl value (38.0 μF cm −2 ), which is attributed to the modulation of the electronic structure of α‐Fe 2 O 3 by Gd doping, which in turn improves the surface quality of the photoanode [15a] . FeP/Gd−Fe 2 O 3 has a larger C dl value (56.6 μF cm −2 ) than Fe−Pi/Gd−Fe 2 O 3 (49.0 μF cm −2 ), which indicates that the species evolved on the surface of in‐situ generated FeP has more active sites than intentionally loaded Fe−Pi, which is more conducive to water oxidation.…”

“…Therefore, the further increased N D of FeP/Gd−Fe 2 O 3 photoanode can be attributed to the co‐doping of Gd and P. The Gd doping in the bulk phase significantly increases the N D , while the P diffusion on the surface effectively degrades the surface carrier recombination center and increases the N D synergistically. In addition, α‐Fe 2 O 3 , Gd−Fe 2 O 3 , and Fe−Pi/Gd−Fe 2 O 3 exhibit similar flat‐band potential values (≈0.54 V RHE ), while FeP/Gd−Fe 2 O 3 exhibits a negatively shifted flat‐band potential value (≈0.50 V RHE ), indicating that FeP makes the photoanode more band‐bending [15a] . Futhermore, the double‐layer capacitance ( C dl ) values are obtained by performing cyclic voltammetry (CV) measurements (Figure S8) over an extremely small potential range, thereby approximately evaluating the electrochemically active surface area (ECSA) of the photoanode [19] .…”

“…It is easy to see that both Gd doping and Fe−Pi can markedly increase the Δ V OC of α‐Fe 2 O 3 photoanode, indicating that both can significantly passivate the surface capture state of the photoanode [3a] . Focusing on FeP/Gd−Fe 2 O 3 photoanode, which exhibits greater Δ V OC than Fe−Pi/Gd−Fe 2 O 3 , this suggests that FeP and Fe−Pi synergistically passivate the surface state and significantly improve the surface quality of Gd−Fe 2 O 3 photoanode, resulting in a photoanode with a better photoelectric conversion capability [15a] . As shown in Figure 4b, the Mott–Schottky plots of positive slopes for all samples indicate that the Gd doping is n‐type, and Fe−Pi and FeP do not change the n‐type semiconductor properties of α‐Fe 2 O 3 either [3a] .…”

Recent Progress on Semiconductor Heterojunction‐Based Photoanodes for Photoelectrochemical Water Splitting

“…In view of the photoanode/electrolyte interface, the carriers may accumulate on the surface to cause serious interface recombination and affect performance due to electron back transfer and sluggish transfer dynamics. [162][163][164][165] Therefore, an intermediate layer is added to promote carrier transport for the water splitting redox reaction. Tang et al [166] found that TiO 2 forced electrons to be transported to the electrolyte interface, causing serious carrier loss.…”

Surface-oxidized titanium diboride as cocatalyst on hematite photoanode for solar water splitting

“…[5][6][7] However, the PEC property of the α-Fe 2 O 3 photoanode is unsatisfactory in practical application and is affected by the following defects, for instance, slow water oxidation kinetics, serious bulk and surface recombination of electron and hole, and short hole diffusion distance. [8][9][10] Considering the above, the targeted modification of α-Fe 2 O 3 is necessary to increase its PEC activity by: (i) nanoengineering α-Fe 2 O 3 to compensate for the short minority carrier diffusion length by maximizing the semiconductor-electrolyte interface; 11,12 (ii) increasing the electroconductivity by elemental doping (e.g., F, Ti, Ta); 7,13,14 (iii) passivating the surface states by the decoration of a metal oxide transition layer; 15,16 (iv) heterostructure engineering by utilizing the synergistic effects of different semiconductors; 17,18 and (v) loading cocatalyst to improve the water oxidation kinetics. 19 The state-of-the-art cocatalysts mainly include transition metal oxides (CoO X ), 20 hydroxides (Ni(OH) 2 ), 21 oxyhydroxides (FeOOH, NiOOH, InOOH), [21][22][23] and phosphates (CoPi, FeP).…”