Keita Sekizawa

Artificial Z-Scheme Constructed with a Supramolecular Metal Complex and Semiconductor for the Photocatalytic Reduction of CO2

A hybrid for the visible-light-driven photocatalytic reduction of CO2 using methanol as a reducing agent was developed by combining two different types of photocatalysts: a Ru(II) dinuclear complex (RuBLRu′) used for CO2 reduction is adsorbed onto Ag-loaded TaON (Ag/TaON) for methanol oxidation. Isotope experiments clearly showed that this hybrid photocatalyst mainly produced HCOOH (TN = 41 for 9 h irradiation) from CO2 and HCHO from methanol. Therefore, it converted light energy into chemical energy (ΔG° = +83.0 kJ/mol). Photocatalytic reaction proceeds by the stepwise excitation of Ag/TaON and the Ru dinuclear complex on Ag/TaON, similar to the photosynthesis Z-scheme.

Visible-Light-Driven CO2 Reduction with Carbon Nitride: Enhancing the Activity of Ruthenium Catalysts†

A heterogeneous photocatalyst system that consists of a ruthenium complex and carbon nitride (C3N4), which act as the catalytic and light-harvesting units, respectively, was developed for the reduction of CO2 into formic acid. Promoting the injection of electrons from C3N4 into the ruthenium unit as well as strengthening the electronic interactions between the two units enhanced its activity. The use of a suitable solvent further improved the performance, resulting in a turnover number of greater than 1000 and an apparent quantum yield of 5.7% at 400 nm. These are the best values that have been reported for heterogeneous photocatalysts for CO2 reduction under visible-light irradiation to date.

Selective Formic Acid Production via CO2 Reduction with Visible Light Using a Hybrid of a Perovskite Tantalum Oxynitride and a Binuclear Ruthenium(II) Complex

A hybrid material consisting of CaTaO2N (a perovskite oxynitride semiconductor having a band gap of 2.5 eV) and a binuclear Ru(II) complex photocatalytically produced HCOOH via CO2 reduction with high selectivity (>99%) under visible light (λ>400 nm). Results of photocatalytic reactions, spectroscopic measurements, and electron microscopy observations indicated that the reaction was driven according to a two-step photoexcitation of CaTaO2N and the Ru photosensitizer unit, where Ag nanoparticles loaded on CaTaO2N with optimal distribution mediated interfacial electron transfer due to reductive quenching.

Structural Improvement of CaFe2O4 by Metal Doping toward Enhanced Cathodic Photocurrent

Various metal-doped p-type CaFe2O4 photocathodes were prepared in an attempt to improve the low quantum efficiency for photoreaction. CuO and Au doping enhanced the photocurrent by expansion of the absorption wavelength region and plasmon resonance, respectively. X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) analysis showed that doping with these metals further disturbed the originally distorted crystal structure of CaFe2O4. In contrast, doping with Ag relaxed the distorted crystal structure around the Fe center toward symmetry. Ag doping resulted in improvement of the carrier mobility together with a red-shift of photoabsorption with Ag-doped CaFe2O4 having a 23-fold higher photocurrent than undoped CaFe2O4.

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.

Metal-complex/semiconductor hybrids for carbon dioxide fixation

A hybrid photocatalyst consisting of a catalytic Ru complex and polymeric carbon nitride (band gap, 2.7 eV) was capable of reducing CO2 into HCOOH with ~80% selectivity under visible light (λ > 420 nm) in the presence of a suitable electron donor. Introduction of mesoporosity into the graphitic carbon nitride structure to increase the specific surface area was essential to enhancing the activity. However, higher surface area (in other words, lower crystallinity) that originated from excessively introduced mesopores had a negative impact on activity. Promoting electron injection from carbon nitride to the catalytic Ru unit as well as strengthening the electronic interactions between the two units improved the activity. Under the optimal condition, a turnover number (TON, with respect to the Ru complex used) greater than 1000 and an apparent quantum yield of 5.7% (at 400 nm) were obtained, which are the greatest among heterogeneous photocatalysts for visible-light CO2 reduction ever reported.