Near-infrared light-inducible Z-scheme overall water-splitting photocatalyst without an electron mediator.

We facilely prepared a solid-state heterojunction photocatalyst in which silver vanadium oxide (Ag2V4O11) and zinc rhodium oxide (ZnRh2O4) as oxygen and hydrogen evolution photocatalysts, respectively, were directly connected to generate Ag2V4O11/ZnRh2O4. Ag2V4O11/ZnRh2O4 photocatalyzed overall pure-water splitting without any electron mediator under irradiation with near-infrared light at wavelengths of up to 910 nm. The key points are that the conduction bottom potential of Ag2V4O11 is almost the same as the valence band top potential of ZnRh2O4, and that the bandgaps of Ag2V4O11 and ZnRh2O4 are 1.4 and 1.2 eV, respectively.

Red light-inducible overall water-splitting photocatalyst, gold-inserted zinc rhodium oxide and bismuth vanadium oxide heterojunction, connected using gold prepared by sputtering in ionic liquid.

We prepared a solid-state Z-scheme photocatalyst in which zinc rhodium oxide (ZnRh2O4) and bismuth vanadium oxide (Bi4V2O11) that served as hydrogen (H2) and oxygen (O2) evolution photocatalysts, respectively, were connected with gold (Au) nanoparticles. The Au nanoparticles were prepared by sputtering in an ionic liquid, N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide, to generate Au/ZnRh2O4/Au/Bi4V2O11 with various amounts of Au in the 12 mol. %-29 mol. % range (vs 1.0 mol ZnRh2O4 + 0.2 mol Bi4V2O11). Au/ZnRh2O4/Au/Bi4V2O11 photocatalyzed overall pure-water splitting under irradiation with red light at a wavelength of 700 nm, and the dependence of the amounts of Au on the apparent quantum efficiency tended to increase in the measurement range.

Selective loading of platinum or silver cocatalyst onto a hydrogen-evolution photocatalyst in a silver-mediated all solid-state Z-scheme system for enhanced overall water splitting.

We selectively loaded a hydrogen (H2)-evolution cocatalyst, either platinum (Pt) or silver (Ag), onto a H2-evolution photocatalyst, zinc rhodium oxide (ZnRh2O4), in a Ag-inserted ZnRh2O4 and bismuth vanadium oxide (Bi4V2O11) hetero-junction photocatalyst (ZnRh2O4/Ag/Bi4V2O11) by a photo-deposition method. The selective loading of Pt or Ag was achieved by taking advantage of the band-gap difference between ZnRh2O4 (1.2 eV) and Bi4V2O11 (1.7 eV) and increased the overall water-splitting activity of the photocatalyst.

Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts.

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O403-), transition metal ions (Ce3+ or Co2+), and organic polymers, with the aim of photo-activating oxygen-evolving manganese oxide catalysts, which are important components in artificial photosynthesis. The cross-linking technique was applied to obtain a self-standing membrane with a high PW12O403- content. Incorporation and structure retention of PW12O403- within the polymer matrix were confirmed by FT-IR and micro-Raman spectroscopy, and optical characteristics were investigated by UV-Vis spectroscopy, which revealed successful construction of the metal-to-metal charge transfer (MMCT) unit. After deposition of MnOx oxygen evolving catalysts, photocurrent measurements under visible light irradiation verified the sequential charge transfer, Mn → MMCT unit → electrode, and the photocurrent intensity was consistent with the redox potential of the donor metal (Ce or Co). This method provides a new strategy for preparing integrated systems involving catalysts and photon-absorption parts for use with photo-functional materials.

Facile synthesis of a red light-inducible overall water-splitting photocatalyst using gold as a solid-state electron mediator.

We facilely prepared a solid-state heterojunction photocatalyst in which zinc rhodium oxide (ZnRh2O4) and bismuth vanadium oxide (Bi4V2O11) as hydrogen (H2) and oxygen (O2) evolution photocatalysts, respectively, were connected with gold (Au) to generate ZnRh2O4/Au/Bi4V2O11. ZnRh2O4/Au/Bi4V2O11 photocatalyzed the overall pure-water splitting under irradiation with red light at wavelengths of up to 740 nm.

Element strategy of oxygen evolution electrocatalysis based on in situ spectroelectrochemistry.

Oxygen evolution electrocatalysis has received extensive attention due to its significance in biology, chemistry, and technology. However, it is still unclear how the abundant 3d-elements can be used to drive the four-electron oxidation of water as efficiently as in Nature. In this Feature Article, we will propose a design strategy concerning the optimization of the charge accumulation process based on our ongoing spectroelectrochemical study on Mn, Fe, and Ir oxygen evolution catalysts. Spectroscopic identification of the reaction intermediates showed that the activity of MnO2 and Fe2O3 was dictated by the generation of Mn3+ and Fe4+, whereas in the case of IrOx, the activity did not correlate with the valence change of Ir. The efficiency of charge accumulation through valence change is closely linked with the spin configuration of the metal center, because charge disproportionation, which was found to inhibit charge accumulation in the high-spin 3d metals, requires an electron in the eg orbital. In addition to directly increasing the overpotential through the generation of an unstable intermediate, charge disproportionation inhibits charge accumulation by dissipating the total oxidative energy of the system. A favorable charge accumulation process may also be beneficial for electrode kinetics due to the enhanced coupling between reaction rates and electrochemical driving force. The model proposed in this study may help explain why low-spin 4d/5d rare metals are often more active than the abundant high-spin 3d materials for multi-electron transfer reactions in general, and provides new insight into how active 3d-metal catalysts can be synthesized by optimizing the energetics of both bond formation and charge accumulation.

Efficient oxygen evolution on hematite at neutral pH enabled by proton-coupled electron transfer.

The rate-determining step of the oxygen evolution reaction on hematite electrodes was switched from the sequential electron/proton transfer process to the concerted proton-coupled electron transfer (CPET) process by adding pyridine derivatives to the electrolyte. By inducing the CPET process, the overpotential for oxygen evolution at neutral pH decreased by approximately 250 mV.

A heterojunction photocatalyst composed of zinc rhodium oxide, single crystal-derived bismuth vanadium oxide, and silver for overall pure-water splitting under visible light up to 740 nm.

We recently reported the synthesis of a solid-state heterojunction photocatalyst consisting of zinc rhodium oxide (ZnRh2O4) and bismuth vanadium oxide (Bi4V2O11), which functioned as hydrogen (H2) and oxygen (O2) evolution photocatalysts, respectively, connected with silver (Ag). Polycrystalline Bi4V2O11 (p-Bi4V2O11) powders were utilized to form ZnRh2O4/Ag/p-Bi4V2O11, which was able to photocatalyze overall pure-water splitting under red-light irradiation with a wavelength of 700 nm (R. Kobayashi et al., J. Mater. Chem. A, 2016, 4, 3061). In the present study, we replaced p-Bi4V2O11 with a powder obtained by pulverizing single crystals of Bi4V2O11 (s-Bi4V2O11) to form ZnRh2O4/Ag/s-Bi4V2O11, and demonstrated that this heterojunction photocatalyst had enhanced water-splitting activity. In addition, ZnRh2O4/Ag/s-Bi4V2O11 was able to utilize nearly the entire range of visible light up to a wavelength of 740 nm. These properties were attributable to the higher O2 evolution activity of s-Bi4V2O11.

Development of optically transparent water oxidation catalysts using manganese pyrophosphate compounds.

One challenge in artificial photosynthetic systems is the development of active oxygen evolution catalysts composed of abundant elements. The oxygen evolution activities of manganese pyrophosphate compounds were examined in electrochemical and photochemical experiments. Electrocatalysis using calcium-manganese pyrophosphate exhibited good catalytic ability under neutral pH and an oxygen evolution reaction was driven with a small overpotential (η<100 mV). UV-vis diffuse reflectance measurements revealed that manganese pyrophosphates exhibit weak absorption in the visible light region while commonly used oxygen evolution catalysts exhibit intense absorption. Therefore, the efficient light absorption of a photocatalyst was retained even after surface modification with a manganese pyrophosphate, and photochemical oxygen evolution was achieved by using magnesium ferrite modified with manganese pyrophosphate nanoparticles under the illumination of visible light at wavelength of over 420 nm.

Photocatalytic hydrogen evolution over ß-iron silicide under infrared-light irradiation.

We investigated the ability of ß-iron silicide (ß-FeSi2) to serve as a hydrogen (H2)-evolution photocatalyst due to the potential of its conduction band bottom, which may allow thermodynamically favorable H2 evolution in spite of its small band-gap of 0.80 eV. ß-FeSi2 had an apparent quantum efficiency for H2 evolution of ∼24% up to 950 nm (near infrared light), in the presence of the dithionic acid ion (S2O6(2-)) as a sacrificial agent. It was also sensitive to infrared light (>1300 nm) for H2 evolution.

Regulating proton-coupled electron transfer for efficient water splitting by manganese oxides at neutral pH.

Manganese oxides have been extensively investigated as model systems for the oxygen-evolving complex of photosystem II. However, most bioinspired catalysts are inefficient at neutral pH and functional similarity to the oxygen-evolving complex has been rarely achieved with manganese. Here we report the regulation of proton-coupled electron transfer involved in water oxidation by manganese oxides. Pyridine and its derivatives, which have pKa values intermediate to the water ligand bound to manganese(II) and manganese(III), are used as proton-coupled electron transfer induction reagents. The induction of concerted proton-coupled electron transfer is demonstrated by the detection of deuterium kinetic isotope effects and compliance of the reactions with the libido rule. Although proton-coupled electron transfer regulation is essential for the facial redox change of manganese in photosystem II, most manganese oxides impair these regulatory mechanisms. Thus, the present findings may provide a new design rationale for functional analogues of the oxygen-evolving complex for efficient water splitting at neutral pH.

Inhibition of charge disproportionation of MnO2 electrocatalysts for efficient water oxidation under neutral conditions.

The development of Mn-oxide electrocatalysts for the oxidation of H(2)O to O(2) has been the subject of intensive researches not only for their importance as components of artificial photosynthetic systems, but also as O(2)-evolving centers in photosystem II. However, limited knowledge of the mechanisms underlying this oxidation reaction hampers the ability to rationally design effective catalysts. Herein, using in situ spectroelectrochemical techniques, we demonstrate that the stabilization of surface-associated intermediate Mn(3+) species relative to charge disproportionation is an effective strategy to lower the overpotential for water oxidation by MnO(2). The formation of N-Mn bonds via the coordination of amine groups of poly(allylamine hydrochloride) to the surface Mn sites of MnO(2) electrodes effectively stabilized the Mn(3+) species, resulting in an ~500-mV negative shift of the onset potential for the O(2) evolution reaction at neutral pH.

Multielectron-transfer reactions at single Cu(II) centers embedded in polyoxotungstates driven by photo-induced metal-to-metal charge transfer from anchored Ce(III) to framework W(VI).

Using carbon monoxide as a probe molecule for the oxidation state of Cu ions, we demonstrated that anchored polynuclear charge-transfer complexes consisting of Ce(III) ions and Cu(II)-substituted Keggin-type polyoxotungstates function as efficient visible-light-driven multielectron-transfer catalysts.

Mechanisms of pH-dependent activity for water oxidation to molecular oxygen by MnO2 electrocatalysts.

Manganese oxides function as efficient electrocatalysts for water oxidation to molecular oxygen in strongly alkaline conditions, but are inefficient at neutral pH. To provide new insight into the mechanism underlying the pH-dependent activity of the electrooxidation reaction, we performed UV-vis spectroelectrochemical detection of the intermediate species for water oxidation by a manganese oxide electrode. Layered manganese oxide nanoparticles, δ-MnO(2) (K(0.17)[Mn(4+)(0.90)Mn(3+)(0.07)□(0.03)]O(2)·0.53H(2)O) deposited on fluorine-doped tin oxide electrodes were shown to catalyze water oxidation at pH from 4 to 13. At this pH range, a sharp rise in absorption at 510 nm was observed with a concomitant increase of anodic current for O(2) evolution. Using pyrophosphate as a probe molecule, the 510 nm absorption was attributable to Mn(3+) on the surface of δ-MnO(2). The onset potential of the water oxidation current was constant at approximately 1.5 V vs SHE from pH 4 to pH 8, but sharply shifted to negative at pH > 8. Strikingly, this behavior was well reproduced by the pH dependence of the onset of 510 nm absorption, indicating that Mn(3+) acts as the precursor of water oxidation. Mn(3+) is unstable at pH < 9 due to the disproportionation reaction resulting in the formation of Mn(2+) and Mn(4+), whereas it is effectively stabilized by the comproportionation of Mn(2+) and Mn(4+) in alkaline conditions. Thus, the low activity of manganese oxides for water oxidation under neutral conditions is most likely due to the inherent instability of Mn(3+), whose accumulation at the surface of catalysts requires the electrochemical oxidation of Mn(2+) at a potential of approximately 1.4 V. This new model suggests that the control of the disproportionation and comproportionation efficiencies of Mn(3+) is essential for the development of Mn catalysts that afford water oxidation with a small overpotential at neutral pH.