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%.
Hybrid photocatalysts consisting of a ruthenium complex and p-type photoactive N-doped Ta(2)O(5) anchored with an organic group were successfully synthesized by a direct assembly method. The photocatalyst anchored by phosphonate exhibited excellent photoconversion activity of CO(2) to formic acid under visible-light irradiation with respect to the reaction rate and stability.
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).
We demonstrated photocatalytic
CO2 reduction using water
as an electron donor under visible light irradiation by a Z-scheme
photocatalyst and a photoelectrochemical cell using bare (CuGa)0.5ZnS2 prepared by a flux method as a CO2-reducing photocatalyst. The Z-scheme system employing the bare (CuGa)0.5ZnS2 photocatalyst and RGO-(CoO
x
/BiVO4) as an O2-evolving photocatalyst
produced CO of a CO2 reduction product accompanied by H2 and O2 in a simple suspension system without any
additives under visible light irradiation and 1 atm of CO2. When a basic salt (i.e., NaHCO3, NaOH, etc.) was added
into the reactant solution (H2O + CO2), the
CO formation rate and the CO selectivity increased. The same effect
of the basic salt was observed for sacrificial CO2 reduction
using SO3
2– as an electron donor over
the bare (CuGa)0.5ZnS2 photocatalyst. The selectivity
for the CO formation of the Z-schematic CO2 reduction reached
10–20% in the presence of the basic salt even in an aqueous
solution and without loading any cocatalysts on the (CuGa)0.5ZnS2 metal sulfide photocatalyst. It is notable that CO
was obtained accompanied by reasonable O2 evolution, indicating
that water was an electron donor for the CO2 reduction.
Moreover, the present Z-scheme system also showed activity for solar
CO2 reduction using water as an electron donor. The bare
(CuGa)0.5ZnS2 powder loaded on an FTO glass
was also used as a photocathode for CO2 reduction under
visible light irradiation. CO and H2 were obtained on the
photocathode with 20% and 80% Faradaic efficiencies at 0.1 V vs RHE,
respectively.
Visible-light-driven Z-schematic CO2 reduction using H2O as an electron donor was achieved using a simple mixture of a metal-sulfide/molecular hybrid photocatalyst for CO2 reduction, a water oxidation photocatalyst and a redox-shuttle electron mediator. This is the first demonstration of a highly selective particulate CO2 reduction system accompanying O2 generation utilizing a semiconductor/molecular hybrid photocatalyst.
Stoichiometric water splitting under AM 1.5 irradiation without an external bias is demonstrated using a Pt/TiO2/N,Zn–Fe2O3/Cr2O3 photocathode connected with an n-SrTiO3−x photoanode.
Conspectus
The synthesis of organic chemicals from H2O and CO2 using solar energy is important for recycling
CO2 through cyclical use of chemical ingredients produced
from CO2 or molecular energy carriers based on CO2. Similar
to photosynthesis in plants, the CO2 molecules are reduced
by electrons and protons, which are extracted from H2O
molecules, to produce O2. This reaction is uphill; therefore,
the solar energy is stored as the chemical bonding energy in the organic
molecules. This artificial photosynthetic technology mimicking green
vegetation should be implemented as a self-standing system for on-site
direct solar energy storage that supports CO2 recycling
in a circular economy. Herein, we explain our interdisciplinary fusion
methodology to develop hybrid photocatalysts and photoelectrodes for
an artificial photosynthetic system for the CO2 reduction
reaction (CO2RR) in aqueous solutions. The key factor for
the system is the integration of uniquely different functions of molecular
transition-metal complexes and solid semiconductors. A metal complex
catalyst and a semiconductor appropriate for a CO2RR and
visible-light absorption, respectively, are linked, and they function
complementary way to catalyze CO2RR under visible-light
irradiation as a particulate photocatalyst dispersion in solution.
It has also been proven that Ru complexes with bipyridine ligands
can catalyze a CO2RR as photocathodes when they are linked
with various semiconductor surfaces, such as those of doped tantalum
oxides, doped iron oxides, indium phosphides, copper-based sulfides,
selenides, silicon, and others. These photocathodes can produce formate
and carbon monoxide using electrons and protons extracted from water
through potential-matched connections with photoanodes such as TiO2 or SrTiO3 for oxygen evolution reactions (OERs).
Benefiting from the very low overpotential of an aqueous CO2RR at metal complexes approaching the theoretical lower limit, the
semiconductor/molecule hybrid system demonstrates a single tablet-formed
monolithic electrode called “artificial leaf.” This
single electrode device can generate formate (HCOO–) from H2O and CO2 in a water-filled single-compartment
reactor without requiring a separation membrane under unassisted or
bias-free conditions, either electrically or chemically. The reaction
proceeds with a stoichiometric electron/hole ratio and stores solar
energy with a solar-to-chemical energy conversion efficiency of 4.6%,
which exceeds that of plants. In this Account, the key results that
marked our milestones in technological progress of the semiconductor/molecule
hybrid photosystem are concisely explained. These results include
design, proof of the principle, and understanding of the phenomena
by time-resolved spectroscopies, synchrotron radiation analyses, and
DFT calculations. These results enable us to address challenges toward
further scientific progress and the social implementation, including
the use of earth-abundant elements and the scale-up of the solar-driven
CO2RR system.
Highly crystalline Ni-doped β-FeOOH(Cl) nanorod catalysts for efficient electrochemical water oxidation were successfully synthesized by a one-pot ambient temperature synthesis.
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