Many photocatalyst materials for water splitting have been developed, particularly since the second half of the 1990s. Highly efficient water splitting on tantalate photocatalysts under UV ...irradiation has been achieved. Moreover, band engineering by doping, valence band formation, and synthesis of solid solutions, has led to the development of a large number of visible light-driven photocatalysts for
H
2
or
O
2
evolution from aqueous solutions containing electron donors or acceptors. Z-scheme photocatalyst systems for water splitting to
H
2
and
O
2
under visible light irradiation have been developed. This progress in the development of visible light-driven photocatalysts for water splitting is reviewed.
A Rh-doped SrTiO3 (SrTiO3:Rh) photocatalyst electrode that was readily prepared by pasting SrTiO3:Rh powder onto a transparent indium tin oxide electrode gave a cathodic photocurrent under ...visible-light irradiation (λ > 420 nm), indicating that the SrTiO3:Rh photocatalyst electrode possessed p-type semiconductor character. The cathodic photocurrent increased with an increase in the amount of doped Rh up to 7 atom %. The incident-photon-to-current efficiency at 420 nm was 0.18% under an applied potential of −0.7 V vs Ag/AgCl for the SrTiO3:Rh(7 atom %) photocatalyst electrode. The photocurrent was confirmed to be due to water splitting by analyzing the evolved H2 and O2. The water splitting proceeded with the application of an external bias smaller than 1.23 V versus a Pt counter electrode under visible-light irradiation and also using a solar simulator, suggesting that solar energy conversion should be possible with the present photoelectrochemical water splitting.
Photocatalytic water splitting is a challenging reaction because it is an ultimate solution to energy and environmental issues. Recently, many new powdered photocatalysts for water splitting have ...been developed. For example, a NiO (0.2 wt %)/NaTaO
:La (2 %) photocatalyst with a 4.1-eV band gap showed high activity for water splitting into H
and O
with an apparent quantum yield of 56 % at 270 nm. Overall water splitting under visible light irradiation has been achieved by construction of a Z-scheme photocatalysis system employing visible-light-driven photocatalysts, Ru/SrTiO
:Rh and BiVO
for H
and O
evolution, and an Fe
/Fe
redox couple as an electron relay. Moreover, highly efficient sulfide photocatalysts for solar hydrogen production in the presence of electron donors were developed by making solid solutions of ZnS with AgInS
and CuInS
of narrow band gap semiconductors. Thus, the database of powdered photocatalysts for water splitting has become plentiful.
Conspectus Photocatalytic and photoelectrochemical CO2 reduction of artificial photosynthesis is a promising chemical process to solve resource, energy, and environmental problems. An advantage of ...artificial photosynthesis is that solar energy is converted to chemical products using abundant water as electron and proton sources. It can be operated under ambient temperature and pressure. Especially, photocatalytic CO2 reduction employing a powdered material would be a low-cost and scalable system for practical use because of simplicity of the total system and simple mass-production of a photocatalyst material. In this Account, single particulate photocatalysts, Z-scheme photocatalysts, and photoelectrodes are introduced for artificial photosynthetic CO2 reduction. It is indispensable to use water as an electron donor (i.e., reasonable O2 evolution) but not to use a sacrificial reagent of a strong electron donor, for achievement of the artificial photosynthetic CO2 reduction accompanied by ΔG > 0. Confirmations of O2 evolution, a ratio of reacted e– to h+ estimated from obtained products, a turnover number, and a carbon source of a CO2 reduction product are discussed as the key points for evaluation of photocatalytic and photoelectrochemical CO2 reduction. Various metal oxide photocatalysts with wide band gaps have been developed for water splitting under UV light irradiation. However, these bare metal oxide photocatalysts without a cocatalyst do not show high photocatalytic CO2 reduction activity in an aqueous solution. The issue comes from lack of a reaction site for CO2 reduction and competitive reaction between water and CO2 reduction. This raises a key issue to find a cocatalyst and optimize reaction conditions defining this research field. Loading a Ag cocatalyst as a CO2 reduction site and NaHCO3 addition for a smooth supply of hydrated CO2 molecules as reactant are beneficial for efficient photocatalytic CO2 reduction. Ag/BaLa4Ti4O15 and Ag/NaTaO3:Ba reduce CO2 to CO as a main reduction reaction using water as an electron donor even in just water and an aqueous NaHCO3 solution. A Rh–Ru cocatalyst on NaTaO3:Sr gives CH4 with 10% selectivity (Faradaic efficiency) based on the number of reacted electrons in the photocatalytic CO2 reduction accompanied by O2 evolution by water oxidation. Visible-light-responsive photocatalyst systems are indispensable for efficient sunlight utilization. Z-scheme systems using CuGaS2, (CuGa)1–x Zn2x S2, CuGa1–x In x S2, and SrTiO3:Rh as CO2-reducing photocatalyst, BiVO4 as O2-evolving photocatalyst, and reduced graphene oxide (RGO) and Co-complex as electron mediator or without an electron mediator are active for CO2 reduction using water as an electron donor under visible light irradiation. These metal sulfide photocatalysts have the potential to take part in Z-scheme systems for artificial photosynthetic CO2 reduction, even though their ability to extract electrons from water is insufficient. A photoelectrochemical system using a photocathode is also attractive for CO2 reduction under visible light irradiation. For example, p-type CuGaS2, (CuGa)1–x Zn2x S2, Cu1–x Ag x GaS2, and SrTiO3:Rh function as photocathodes for CO2 reduction under visible light irradiation. Moreover, introducing a conducting polymer as a hole transporter and surface modification with Ag and ZnS improve photoelectrochemical performance.
Various photocatalyst materials developed by the group of the author are described. Alkali and alkaline earth tantalates have arisen as a new group of photocatalyst materials for water splitting into
...H
2
and
O
2
under ultraviolet light irradiation. They showed the activities even without co-catalysts such as Pt, being different from titanate photocatalysts. When NiO co-catalysts were loaded on the tantalate photocatalysts, the photocatalytic activities were drastically increased. Among the tantalates,
NiO
/
NaTaO
3
doped with La showed the highest activity. Some metal sulfide photocatalysts such as
AgGaS
2
and
AgInZn
7
S
9
showed high activities for
H
2
evolution from aqueous solutions containing
S
2
-
and
SO
3
2
-
as electron donors under visible light irradiation
(
wavelength
>
420
nm
)
. The
H
2
evolution was observed even under irradiation using a solar simulator.
Pt
/
SrTiO
3
doped with Rh has also arisen as a novel oxide photocatalyst for
H
2
evolution from aqueous solutions containing reducing reagents under visible light irradiation. On the other hand,
BiVO
4
and
AgNbO
3
showed high activities for
O
2
evolution in the presence of sacrificial reagent
(
Ag
+
)
under visible light irradiation.
An efficient BiVO ₄ thin film electrode for overall water splitting was prepared by dipping an F-doped SnO ₂ (FTO) substrate electrode in an aqueous nitric acid solution of Bi(NO ₃) ₃ and NH ₄VO ₃, ...and subsequently calcining it. X-ray diffraction of the BiVO ₄ thin film revealed that a photocatalytically active phase of scheelite-monoclinic BiVO ₄ was obtained. Scanning electron microscopy images showed that the surface of an FTO substrate was uniformly coated with the BiVO ₄ film with 300–400 nm of the thickness. The BiVO ₄ thin film electrode gave an excellent anodic photocurrent with 73% of an IPCE at 420 nm at 1.0 V vs. Ag/AgCl. Modification with CoO on the BiVO ₄ electrode improved the photoelectrochemical property. A photoelectrochemical cell consisting of the BiVO ₄ thin film electrode with and without CoO, and a Pt counter electrode was constructed for water splitting under visible light irradiation and simulated sunlight irradiation. Photocurrent due to water splitting to form H ₂ and O ₂ was confirmed with applying an external bias smaller than 1.23 V that is a theoretical voltage for electrolysis of water. Water splitting without applying external bias under visible light irradiation was demonstrated using a SrTiO ₃∶Rh photocathode and the BiVO ₄ photoanode.
Ru/SrTiO3:Rh photocatalyst powder for H2 evolution and varied photocatalyst powders for O2 evolution such as BiVO4 and WO3 were suspended in acidified aqueous solutions, resulting in showing ...activities for water splitting into H2 and O2 in a stoichiometric ratio without an electron mediator under visible light irradiation. The photocatalytic activities were dependent on pH. The highest activity was obtained at pH 3.5. An optical microscope observation of the aqueous suspension containing Ru/SrTiO3:Rh and BiVO4 powders at pH 3.5 revealed that these powders aggregated with suitable contact. The condition of Rh doped in SrTiO3 also affected strongly the photocatalytic activity and quenching of the photoluminescence of BiVO4. The high photocatalytic activity was obtained and the luminescence was remarkably quenched, when SrTiO3:Rh containing Rh species with reversible redox properties was used and mixed. These results indicated that the photocatalytic water splitting and quenching of the photoluminescence occurred through interparticle electron transfer from the conduction band of BiVO4 to impurity level consisting of the reversible Rh species doped in SrTiO3. Thus, we succeeded in constructing unique and simple Z-scheme photocatalysis systems driven by interparticle electron transfer under visible light irradiation. In addition, the (Ru/SrTiO3:Rh)−(BiVO4) system split water under simulated sunlight (AM-1.5).
Bismuth vanadate (BiVO4) is incorporated with reduced graphene oxide (RGO) using a facile single-step photocatalytic reaction to improve its photoresponse in visible light. Remarkable 10-fold ...enhancement in photoelectrochemical water splitting reaction is observed on BiVO4−RGO composite compared with pure BiVO4 under visible illumination. This improvement is attributed to the longer electron lifetime of excited BiVO4 as the electrons are injected to RGO instantly at the site of generation, leading to a minimized charge recombination. Improved contact between BiVO4 particles with transparent conducting electrode using RGO scaffold also contributes to this photoresponse enhancement.
The effectiveness of reduced graphene oxide as a solid electron mediator for water splitting in the Z-scheme photocatalysis system is demonstrated. We show that a tailor-made, photoreduced graphene ...oxide can shuttle photogenerated electrons from an O2-evolving photocatalyst (BiVO4) to a H2-evolving photocatalyst (Ru/SrTiO3:Rh), tripling the consumption of electron–hole pairs in the water splitting reaction under visible-light irradiation.
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