Tomiko M. Suzuki

Selective CO2 Conversion to Formate Conjugated with H2O Oxidation Utilizing Semiconductor/Complex Hybrid Photocatalysts

Photoelectrochemical reduction of CO(2) to HCOO(-) (formate) over p-type InP/Ru complex polymer hybrid photocatalyst was highly enhanced by introducing an anchoring complex into the polymer. By functionally combining the hybrid photocatalyst with TiO(2) for water oxidation, selective photoreduction of CO(2) to HCOO(-) was achieved in aqueous media, in which H(2)O was used as both an electron donor and a proton source. The so-called Z-scheme (or two-step photoexcitation) system operated with no external electrical bias. The selectivity for HCOO(-) production was >70%, and the conversion efficiency of solar energy to chemical energy was 0.03-0.04%.

Visible light-sensitive mesoporous N-doped Ta2O5 spheres: synthesis and photocatalytic activity for hydrogen evolution and CO2 reduction

Crystallized mesoporous tantalum pentoxide spheres (CMTS) with particle diameters of ca. 100–500 nm and composed of Ta2O5 nanocrystals were synthesized for the first time by a combination of the sol–gel process and heat-treatment with the aid of carbon reinforcement. The specific surface area of the CMTS was up to 105 m2 g−1 and the pore diameter was controllable in the range of 5.6–17 nm by changing the crystallization temperature. Visible light-sensitive p-type N-doped Ta2O5 (N-CMTS) containing 5 at% N was successfully obtained by treatment of CMTS with ammonia, while retaining the mesoporosity and morphology of CMTS. N-CMTS exhibited excellent photocatalytic activity for hydrogen evolution and CO2 reduction (with ruthenium-complex) under visible light irradiation (≥410 nm) due to their larger surface area and controlled morphology compared with previously reported N-doped Ta2O5 fine particles.

Dual functional modification by N doping of Ta2O5: p-type conduction in visible-light-activated N-doped Ta2O5

We report dual functional modulation, both p-type conduction and band gap narrowing, of Ta2O5 semiconductor induced by heavy doping of nitrogen in films sputtered in N2/Ar mixture and ammonia-treated powders. The N doping induced a redshift in the optical absorption edge from 320 to 500 nm, resulting in the absorption of visible light. Simultaneously, the N doping caused a change in the conduction from n-type to p-type. As a result, the N–Ta2O5 photoelectrode containing 7.6 or 16.1 at. % of N exhibited a distinct cathodic photocurrent (due to p-type conduction) in solutions under visible light irradiation (>410 nm).

Z-scheme water splitting under visible light irradiation over powdered metal-complex/semiconductor hybrid photocatalysts mediated by reduced graphene oxide

A powdered Z-scheme system for the photocatalytic water splitting reaction under visible light irradiation was successfully demonstrated utilizing a combination of a metal-complex catalyst, reduced graphene oxide (RGO), and semiconductor photocatalysts. A Ru-complex electrocatalyst, [Ru (2,2′-bipyridine)(4,4′-diphosphonate-2,2′-bipyridine)(CO)2]2+, linked with SrTiO3:Rh (Rh: 4 at%) as a H+ reduction photocatalyst was coupled with BiVO4 as a water oxidation photocatalyst and RGO as a solid-state electron mediator. H2 and O2 evolved stoichiometrically under visible light irradiation (λ > 420 nm) and the turnover number for the Ru-complex was calculated to be 450 after 8.5 h of irradiation time, which indicates that this Z-scheme system splits water photocatalytically and the Ru-complex functions as an efficient cocatalyst for hydrogen evolution by water splitting.

Nitrogen and transition-metal codoped titania nanotube arrays for visible-light-sensitive photoelectrochemical water oxidation.

Vertically aligned titanium dioxide nanotube (TNT) arrays codoped with nitrogen and 3d transition metals were successfully fabricated using anodization and nitridation processes. The codoping of N and Fe yielded the highest visible-light-induced photoelectrochemical water oxidation due to bandgap narrowing of impurity levels by N and Fe.

Highly crystalline β-FeOOH(Cl) nanorod catalysts doped with transition metals for efficient water oxidation

The application of the water oxidation reaction to extract electrons from water molecules is important for the future sustainable synthesis of useful chemicals such as hydrogen and organic compounds. Therefore, a cost-effective oxygen evolution reaction (OER) in alkaline, neutral or acidic solution is required, based on the use of catalysts incorporating earth abundant elements. This work demonstrates that β-FeOOH(Cl) nanorod catalysts with high crystallinity and small size (an average diameter of 3 nm and a length of 15 nm) provided the best performance in the OER activity among Fe-based oxide and (oxy)hydroxide catalysts. The pristine β-FeOOH(Cl) nanorods with the high crystallinity are realized by a new process enabling one-pot, fast, and room temperature synthesis, which is the key to forming colloidal suspensions of the highly crystallized pure-phase β-FeOOH(Cl) nanorods. The versatility of this process also enabled doping of a wide variety of transition metals. Doping of Ni2+ (at 1.2 at%) improved the OER activity and shifted the threshold potential in the negative direction by 100 mV in an alkaline electrolyte, which was comparable to that of conventional IrOx colloidal nanoparticles.

Stoichiometric water splitting using a p-type Fe2O3 based photocathode with the aid of a multi-heterojunction

Fe2O3-based photocathodes are one of the least expensive options for hydrogen generation by water splitting. Although p-type N,Zn-doped Fe2O3 (N,Zn–Fe2O3) has been reported to possess a negative conduction band minimum position sufficient for photocathodic hydrogen generation, the efficiency and stability of the resulting H2 production is low and the reaction is sacrificial. In the present work, analysis by hard X-ray photoelectron spectroscopy (HAXPES) showed that these negative characteristics result from the self-redox reaction of p-type Fe2O3. Based on this result, a TiO2 layer was introduced onto the surface of p-type N,Zn–Fe2O3 to passivate surface defects. In addition, to ensure efficient electron transfer, a thin Cr2O3 layer was also inserted between N,Zn–Fe2O3 and a bottom side conductive oxide layer to generate a favorable band alignment for hole transfer. The resulting Pt/TiO2/N,Zn–Fe2O3/Cr2O3 electrode exhibits a highly stable, significantly enhanced cathodic photocurrent during H2 production under AM 1.5 irradiation. The mechanism providing this improvement was investigated by combining electrochemical impedance spectroscopy, open-circuit voltage decay analysis and scanning tunneling electron microscopy-energy dispersive X-ray spectroscopy. Stoichiometric water splitting without an external electrical bias was also demonstrated by connecting the Fe2O3-based photocathode to an n-type SrTiO3−x photoanode, representing the first-ever example of stoichiometric overall water splitting using an Fe-based photocathode.

Efficient Catalytic Electrode for CO2 Reduction Realized by Physisorbing Ni(cyclam) Molecules with Hydrophobicity Based on Hansen's Theory.

An electrochemical electrode physisorbed with Ni(cyclam) complex molecules containing tetraphenylborate ions (BPh4(-)) as counteranions shows catalytic activity for the reduction reaction of CO2 to CO in an aqueous electrolyte, superior to that of an electrode physisorbed with conventional [Ni(cyclam)]Cl2 complex molecules. The BPh4(-)-containing Ni(cyclam) is inferred as having high hydrophobicity based on its Hansen solubility parameter (HSP), with an interaction sphere excluding HSPs of water in a three-dimensional vector space. The high hydrophobicity of BPh4(-)-containing Ni(cyclam) molecules inhibits their dissolution into aqueous electrolyte and retains their immobilization onto the electrode surface, which we believe to result in the improved catalytic activity of the electrode physisorbed with them. HSP analysis also provides an optimized mixing ratio of solvents dissolving BPh4(-)-containing Ni(cyclam) molecules.