Photocatalytic overall water splitting into H2 and O2 is expected to be a promising method for the efficient utilization of solar energy. The design of optimal photocatalyst structures is a key to efficient overall water splitting, and the development of photocatalysts which can efficiently convert large portion of visible light spectrum has been required. Recently, a series of complex perovskite type transition metal oxynitrides, LaMgxT 1-xO1+3xN2-3x, was developed as photocatalysts for direct water splitting operable at wide wavelength of visible light. In addition two-step excitation water splitting via a novel photocatalytic device termed as photocatalyst sheet was developed. This consists of two types of semiconductors (hydrogen evolution photocatalyst and oxygen evolution photocatalyst) particles embedded in a conductive layer, and showed high efficiency for overall water splitting. These recent advances in photocatalytic water splitting were introduced.
Transient absorption of visible light responsive powder photocatalysts, solid solution of GaN and ZnO (denoted as
GaN:ZnO), and other related materials were measured by using femtosecond diffuse reflectance spectroscopy in order to
evaluate the nature of photogenerated electrons and holes through spectral information in visible and near-infrared region
as well as kinetics from 100 fs to 500 ps. The GaN:ZnO is known to be one of successful photocatalysts which are able
to split water into oxygen and hydrogen molecules under visible-light irradiation. Since photoexcitation generates
electrons and holes in the conduction and valence bands, respectively, it is important to understand their trapping and
recombination processes in details. Generally efficient and quick trapping and slow recombination of them are required
to increase the chance of charge transfer of them to protons and water molecules. We have elucidated that the charge
trapping was within time resolution (< 1 ps) and recombination time was about 100 ps for 26% of carriers and much
longer for 74%, clearly indicating that most of photogenerated carriers have long lifetime. Other photocatalysts with
lower photocatalytic activity showed shorter lifetimes. These results indicated that the long carrier lifetime in GaN:ZnO
is one of the reasons for the efficient reactivity.
Photocatalytic overall water splitting promises to enable a sustainable large-scale hydrogen-based energy system using
solar light, and great attention has been paid to the development of photocatalysts. It is necessary to develop
photocatalysts that function under visible light to utilize sunlight efficiently. We have proposed non-oxide materials as
candidates for visible-light-driven photocatalysts for overall water splitting, and this manuscript presents our recent
research in photocatalyst development. Some oxynitride photocatalysts, modified with appropriate cocatalysts, showed
performance for overall water splitting under visible light irradiation. The modification with cocatalysts drastically
improved the efficiency of photocatalytic reactions, indicating the importance of controlling the surface active sites.
Two-step excitation systems, known as Z-schemes, can significantly expand the range of light available for water
splitting to longer wavelengths.
Overall water splitting to form hydrogen and oxygen on a heterogeneous photocatalyst using solar energy is an attractive
process for large-scale hydrogen production. In recent years, numerous attempts have been made for the development of
visible-light-responsive photocatalysts to efficiently utilize solar energy. In this article, recent research progress in the
development of visible-light-driven photocatalysts is described, specifically focusing on our efforts made on the
development of (oxy)nitride photocatalysts for overall water splitting.
Oxynitrides are presented as effective non-oxide photocatalysts for overall water splitting. RuO2-loaded germanium nitride (β-Ge3N4) is shown to achieve stoichiometric decomposition of H2O into H2 and O2 under ultraviolet irradiation (λ > 200 nm). A novel solid solution of GaN and ZnO, (Ga1-xZnx)(N1-xOx), with a band gap of 2.4-2.8 eV (depending on composition) achieves overall water splitting under visible light (λ > 400 nm) when loaded with an appropriate cocatalyst. The narrower band gap of the solid solution originates from the bonding between Zn and N atoms at the top of the valence band. The photocatalytic activity of (Ga1-xZnx)(N1-xOx) for overall water splitting is dependent strongly on both the cocatalyst and the crystallinity and composition of the material. The quantum efficiency of (Ga1-xZnx)(N1-xOx) with Rh and Cr mixed-oxide (Rh2-yCryO3) nanoparticles reaches 2-3 % at 420-440 nm, which is the highest reported efficiency for overall water splitting in the visible-light region.
Infrared-vision sum frequency generation (IR-vis SFG) spectroscopy and temperature programmed desorption (TPD) spectroscopy were used to investigate the systems prepared by the adsorption of formic acid onto MgO(001) and Pt(110) surfaces. Prominent vibrational bands observed on the SFG spectra were located at 2870 cm-1 on the MgO surface and a 2960 cm-1 on the platinum surface, which were assigned to the CH stretching band of formate ion (HCOO-) produced by the dissociative adsorption. On the MgO surface, three different sites, produced by the repetition of adsorption-thermal desorption cycles, were identified as responsible for the adsorption. The tilt angle of the CH group was estimated from the polarization characteristics of SFG signals and was derived as 0 degree(s) +/- 30 degree(s) for the group giving dominant SFG peak. On the Pt(110) surface, at least six vibrational bands were identified on the SFG spectra, and the temperature dependence indicated that the orientation of formate molecules changed right before their desorption. Expressions of SFG tensor components of a diatomic molecule were derived.