Fe/Ru Oxide as a Versatile and Effective Cocatalyst for Boosting Z-Scheme Water-Splitting: Suppressing Undesirable Backward Electron Transfer

Abstract

The atificial Z-scheme is a promising and rational strategy for solar-to-chemical energy conversion such as water-splitting. In the Z-scheme, backward redox processes are an essential drawback that should be overcome to increase its efficiency. Here, we demonstrate that the simple co-loading of Fe/Ru oxide, (Fe,Ru)­O x , onto various photocatalysts effectively improves the efficiency of water oxidation by suppressing the undesirable backward oxidation of the redox reagent Fe2+. The (Fe,Ru)­O x co-loading on Bi4TaO8Cl afforded the highest water-splitting activity (apparent quantum efficiency of 1.6% at 420 nm) among the Z-scheme systems employing mixed-anion compounds as O2-evolving photocatalysts. The results of photoelectrochemical and electrochemical measurements along with time-resolved spectroscopy clarified the key roles of Fe/Ru oxide; the Ru oxide component functions as a “collector” of photogenerated carriers and active sites for surface redox reactions, while the Fe oxide component acts as a “blocker” against unfavorable Fe2+ oxidation. The versatile availability of Fe/Ru oxide has been demonstrated for other visible-light-responsive photocatalysts.

Conduction Band Control of Oxyhalides with a Triple-Fluorite Layer for Visible Light Photocatalysis

“…Given that all three compounds have suitable band potentials for both reduction and oxidation of water (Figure b), Bi 2 MO 4 Cl has a possibility of working as a photocatalyst for water splitting. Although no obvious differences in morphology and specific surface for all the particles were observed (Figure S11), there was a marked difference in photocatalytic activity; while the Bi 3 O 4 Cl and Bi 2 LaO 4 Cl showed negligible photocatalytic activities under visible light (λ > 400 nm) illumination, clear O 2 generation from water was demonstrated by Bi 2 YO 4 Cl, with the aid of a (Fe,Ru)­O x cocatalyst in the presence of an Fe 3+ electron acceptor (Figure a). Photocatalyzed H 2 production from water under the same visible light was also observed for Bi 2 YO 4 Cl, with the help of Pt cocatalyst in the presence of methanol as an electron donor (Figure b).…”

“…Thus, wider zigzag chain compounds ( n > 2) would be needed to improve photoconductivity. Notably, the photoconductivity of Bi 2 YO 4 Cl (2.9 × 10 –6 m 2 (V s) −1 ) is about 10 times larger than that of Bi 4 TaO 8 Cl (0.3 × 10 –6 m 2 (V s) −1 ), which is the most efficient mixed-anion compound as an O 2 -evolving photocatalyst in the Z-scheme water splitting . The smaller photoconductivity of Bi 4 TaO 8 Cl than Bi 2 YO 4 Cl may also be understood by comparing its crystal structure.…”

“…The external quantum efficiency (EQE) of water splitting was estimated to be 0.2% at 420 nm. At this stage, the EQE is lower than that of Bi 4 TaO 8 Cl (after several optimizations such as two-step synthesis), which is the most efficient mixed-anion compound as an O 2 -evolving photocatalyst in the Z-scheme water splitting . However, as mentioned above, the photoconductivity of Bi 2 YO 4 Cl is about 10 times larger than that of Bi 4 TaO 8 Cl, indicating that there is much room for improvement by modifying synthesis conditions and cocatalyst.…”

“…However, as mentioned above, the photoconductivity of Bi 2 YO 4 Cl is about 10 times larger than that of Bi 4 TaO 8 Cl, indicating that there is much room for improvement by modifying synthesis conditions and cocatalyst. Our initial attempts revealed that the photocatalytic activity of Bi 2 YO 4 Cl strongly depends on surface modification; for example, loading (Fe,Ru)­O x cocatalyst on Bi 2 YO 4 Cl increased the O 2 evolution rate by about 7 times (18 and 2.6 μmol h –1 , respectively, with and without cocatalyst), while nearly no difference was observed for Bi 4 TaO 8 Cl (24 and 23 μmol h –1 , respectively) …”

“…The total mass was set to 25 g. In the case of M = Y and La, the mixture was placed in an alumina crucible with a capacity of 30 cm 3 and heated at 1073 K for 20 h, while in the case of M = Bi the mixture was heated at 973 K for 1 h. After natural cooling, the product was thoroughly washed with deionized water, collected by filtration, and dried at room temperature. A (Fe,Ru)­O x cocatalyst (0.5 wt %) was loaded by the impregnation method according to the literature . Rh-doped SrTiO 3 powder with the atomic ratio of Sr/Ti/Rh = 1.07/1.0/0.01 was prepared according to the literature .…”