CuInS2 nanocrystals (NCs) attract numerous attention as a “green” quantum
dot, because of its nontoxic and cost-effective nature, and ideal band
gap for a maximum conversion efficiency under the sun. Heterostructured
nanocrystals (HNCs), in which distinct phases are conjugated at heterointerfaces,
have been intensively studied because they can finely control the flow
of photo-generated carrier. The development of HNCs containing the CuInS2
NCs should enhance the optical functions.
In this study, we synthesized heterotetrapods composed of a CuInS2 core
and CdS arms by using the seeded growth method and elucidated light-stimulated
behaviour using fs-laser flash photolysis for the first time. The crystal
structure of the heterotetrapods and the heterointerfaces between the core
and arms were studied by using bright-field scanning transmission electron
microscopy (BF-STEM). Interestingly, we discovered that CuInS2/ CdS heterotetrapod
shows dual type I and II like optical phenomena owing to the quasi-type
II band alignment, in which the two semiconductor phases have the similar
conduction or valence band positions. HNC with quasitype II band alignment
is considered important because of its potential applications in emitters,
photocatalysts and photo-energy conversion systems using carrier multiplication.
Furthermore, it caused longer-lived charge separation in the tetrapods
than that in the isolated CuInS2 seeds. The present result provides a new
strategy to enhance the functionality of CuInS2 for the various application.
The reaction of free cobalt cluster anions Con − (n ≤ 10) with O2 was investigated experimentally and theoretically to obtain atomistic insights into the early stage of oxidation process. Two types of magic compositions were found by the population analysis of the oxidation products (ConOm −) as a function of m: the population decreases abruptly upon addition of a single O atom to and removal of a single O atom from the magic compositions. Magic compositions of the former type were further divided into oxygen-rich (n:m ~ 3:4) and oxygen-poor (n:m ~ 1:1) series. The oxygen-rich compositions most likely correspond to fully oxidized states since the compositions are comparable to those of Co3O4 in the bulk. Their appearance is ascribed to the significant reduction of binding energies of O atoms to fully oxidized clusters. In contrast, the oxygen-poor compositions correspond to the intermediates of the full oxidation states in which only the surface is oxidized based on theoretical prediction that oxidation proceeds by bonding O atoms sequentially on the surface of Con − while retaining its morphology. Their appearance is ascribed to the kinetic bottleneck against internal oxidation owing to significant structural change of the Con moiety. In contrast, magic compositions of the latter type are associated with the abrupt increase of the survival probability as anionic states during the relaxation of internally-hot Co oxide clusters based on the m-dependent behaviors of adiabatic electron affinities determined by photoelectron spectroscopy.
The metal nano-dimer structures show the unique optical properties such as polarization anisotropy, or light concentration at the gap due to the excitation of the collective oscillation of free electrons. It is well known that these optical characteristics strongly depend on metal species, its shape or the gap distance. With the gap distance of sub-nanometer, the strength of the generated near-field is extremely enhanced by the dipole coupling. On the contrary, for the case of the bridged dimer or the dimer separated with the gap distance of less than a few nanometers, the effect of the tunneling electron leads to the generation of the charge transfer plasmon (CTP). In this study, we have tried to control the shape of Au nano bridged dimer structures in an atomic scale via electrochemical method. By the electrochemical deposition/dissolution of Cu monolayer on the surface of the structures, we have successfully observed the reversible optical property changes through in-situ electrochemical dark-field microscope measurements. The optical measurements supported that the reversible fine-tuning of the optical property of Au nano-dimer structure had been achieved.
Development of photocatalysts has been incentivized by technological prospects for energy conversion, energy storage and environmental remediation. Herein, we successfully synthesized photocatalytic composite nanowires by using coordination polymers (CPs) as precursors. A series of CP nanowires, [Cd(L-cysteinate)]n and [ZnxCd1-x(Lcysteinate)] n (x = 0.03, 0.05), was prepared for the precursors of composite nanowires. Pyrolysis of CP nanowires gave CdS/C and ZnxCd1-xS/C composite nanowires that maintain their original morphologies. The resulting CdS/C and ZnxCd1-xS/C nanowires exhibited efficient photocatalytic hydrogen generation.
Water-splitting photocatalysts are generally composed of a semiconductor photocatalyst and metal nanoparticle cocatalyst that acts as the reaction site. Effective strategies to achieve highly active photocatalysts include improving the semiconductor photocatalyst and the cocatalyst. We are striving to accurately control the cocatalyst by utilizing precisely regulated metal clusters synthesized using a liquid-phase method to impart high activity to the watersplitting photocatalyst. Herein, we report a study on a gold (Au) cluster–supported BaLa4Ti4O15 photocatalyst. In this study, first, the influence of refining Au cluster cocatalyst particles on the water-splitting reaction was clarified at an individual reaction level. As a result, we found that suppressing the oxygen (O2) reduction reaction is an important factor to achieve high activity by refining the cocatalyst. Then, we formed a Cr2O3 shell on the Au cluster cocatalyst particles (Au25) to prevent the O2 reduction reaction. The resulting water-splitting photocatalyst was highly active and stable, exhibiting activity about 19 times higher than that of Au25-BaLa4Ti4O15 without a Cr2O3 shell.
Plasmonic photoelectric conversion system which consists of semiconductor substrate and plasmon-active metal nano-structures has been received much attention as a way to solve the world energy problem. In the system, the details on the charge transfer process are still addressing issues. In this study, single-layer graphene was introduced into plasmonic photo-electrode to investigate the absolute electrochemical potential of Au nano-structures under the light illumination via Raman measurements. It is known that Raman bands of graphene reflect its electronic state. Taking into account this property, the electrochemical potential of materials contacted with graphene would be easily estimated by Raman measurements. Through the present investigations, we have revealed that the plasmon-induced electron-hole pair generation plays crucial roles in this system. Our new findings would be a key information for the fabrication of the high-performance plasmonic photo-conversion devices.
Upconversion (UC) is an energy-conversion phenomenon from longer wavelength irradiation (absorption of
lower energy photons) to shorter wavelength luminescence (emission of higher
energy photons). In this study, we plan to enhance the UC efficiency due
to UC nanoparticles (NPs) combined plasmonic NPs. Yb/Er-doped NaYF4 (NaYF4:Yb/Er)
NPs and CuS nanodisks (NDs) were synthesized as the UC NPs and the plasmonic
NPs, respectively. We examined influence of UC NPs/CuS NDs concentration
ratios on the UC efficiency in two different states of their dispersion
solutions and their immobilized polymer films.