Realization of red shift of absorption spectra using optical near-field effect.

In many applications such as CO2 reduction and water splitting, high-energy photons in the ultraviolet region are required to complete the chemical reactions. However, to realize sustainable development, the photon energies utilized must be lower than the absorption edge of the materials including the metal complex for CO2 reduction, the electrodes for water splitting, because of the huge amount of lower energy than the visible region received from the sun. In the previous works, we had demonstrated that optical near-fields (ONFs) could realize chemical reactions, by utilizing photon energies much lower than the absorption edge because of the spatial non-uniformity of the electric field. In this paper, we demonstrate that an ONF can realize the red shift of the absorption spectra of the metal-complex material for photocatalytic reduction. By attaching the metal complex to ZnO nano-crystalline aggregates with nano-scale protrusions, the absorption spectra by using diffuse reflection of the metal complex can be shifted to a longer wavelength by 10.6 nm. The results of computational studies based on a first-principles computational program including the ONF effect provide proof of the increase in the absorption of the metal complex at lower photon energies. Since the near-field assisted field increase improves the carrier excitation in the metal-complex materials, this effect may be universal and it could applicable to CO2 reduction using the other metal-complex materials, as well as to the other photo excitation process including water splitting.

Hydrogen storage mechanism and diffusion in metal-organic frameworks.

Diffusion and storage of hydrogen molecules in metal-organic frameworks are crucial for the development of next-generation energy storage devices. By resorting to the first principles modeling, we compute the diffusion coefficient of molecular hydrogen in these systems in a range of temperatures where MOF-based devices are expected to operate. The explicit inclusion of the electronic structure shows that diffusivities are one order of magnitude smaller than those reported by classical simulations, evidencing the insufficiency of the empirical force fields used so far. We show that hydrogen is mainly rolled up around the metal oxide nodes both in MOF-5 and IRMOF-6, and partly around the carbon atoms in the case of IRMOF-6, where charged linkers are present. Metal ions embedded in the junction sites exert an electrostatic attraction toward hydrogen and the resulting distribution shows some ordering around these same sites at low temperature, whereas this tendency vanishes at room temperature. The induced polarization of hydrogen molecules generates an electrostatic interaction with charged atoms inside these nano-scaffolds and this is a key factor for the enhancement in hydrogen storage both in MOF-5 and IRMOF-6. The mechanism discussed hereby provides a novel understanding of metal-organic frameworks and acts as a guide to tune their efficiency for hydrogen storage. Moreover it paves the way to a computer-aided design of effective MOFs indicating that a fine control of the distribution of electrostatic charges inside the hydrogen hosting structure is crucial.

A detailed insight into the catalytic reduction of NO operated by Cr-Cu nanostructures embedded in a CeO2 surface.

Replacing rare and expensive elements, such as Pt, Pd, and Rh, commonly used in catalytic devices with more abundant and less expensive ones is mandatory to realize efficient, sustainable and economically appealing three-way catalysts. In this context, the surface of a Cr-Cu/CeO2 system represents a versatile catalyst for the conversion of toxic NO into harmless N2. Yet, a clear picture of the underlying mechanism is still missing. We provide here a detailed insight into such a reaction mechanism by means of a combined experimental and theoretical study. Fourier-transform infrared spectroscopy is used to detect all the products resulting from catalytic reactions of NO and CO on the surface of a Cr-Cu/CeO2 nanocatalyst. CO pulsing experiments unveil that reactions of CO with O atoms at the Cr-Cu/CeO2 surface are the major factors responsible for the formation of surface vacancies. On these grounds, a comprehensive picture of the NO reduction and the role of both Cu and Cr dopants and vacancies is rationalized by first-principles modeling. Our findings provide a general route for the realization of ceria-based cost-effective catalysts.

Simple but Efficient Method for Inhibiting Sintering and Aggregation of Catalytic Pt Nanoclusters on Metal-Oxide Supports.

A simple and efficient method to inhibit aggregation of Pt clusters supported on metal oxide was developed, preserving the accessible clusters surface where catalytically active sites are located even at relatively high temperatures up to 700 K. The key idea was the inclusion of transition metal atoms such as Ni into the Pt clusters, thus anchoring the clusters through formation of strong chemical bonds with oxygen atoms of the metal-oxide support. To elucidate the efficiency of the method, first-principles molecular dynamics enhanced with free-energy sampling methods were used. These virtual experiments showed how doped Ni atoms, having a stronger affinity to O than Pt, anchor the Pt clusters tightly to the metal-oxide supports and inhibit their tendency to aggregate on the support.

An atomic-level insight into the basic mechanism responsible for the enhancement of the catalytic oxidation of carbon monoxide on a Cu/CeO2 surface.

The reaction mechanisms of CO molecules interacting with a Cu/CeO2 surface and related morphological modifications occurring upon removal of O atoms to generate CO2 are investigated by first-principles dynamical simulations complemented by a free-energy sampling technique. We show that the reactivity of oxygen atoms located in the first layer in the vicinity of the Cu site is remarkably high because of a reduction of the O coordination number. Moreover, we evidence that the doped Cu atoms are responsible for an enhancement of the exposure of other surrounding O atoms, even below the first surface layer, which can then easily react with CO and are gradually removed from the system in the oxidation process. The underlying mechanism responsible for such a high catalytic reactivity of the Cu/CeO2 surface toward CO oxidation is rationalized in terms of the characteristics of the doped Cu. In fact, this copper site is responsible for providing an increasing number of O atoms participating in the catalysis by exposing subsequently all O atoms in the vicinity which are likely to react with an approaching CO. This peculiarity of the Cu atoms extends to O atoms which initially can be deeply buried up to the fourth layer underneath the surface. The mechanism unveiled here provides useful insights into the fundamental mechanism and suggests strategies for the engineering and design of more effective ceria-based catalysts via metal doping.

How seaweeds release the excess energy from sunlight to surrounding sea water.

We report an atomistic insight into the mechanism regulating the energy released by a porphyra-334 molecule, the ubiquitous photosensitive component of marine algae, in a liquid water environment upon an electron excitation. To quantify this rapidly occurring process, we resort to the Fourier analysis of the mass-weighted auto-correlation function, providing evidence for a remarkable dynamic change in the number of hydrogen bonds among water molecules and between the porphyra-334 and its surrounding hydrating water. Hydrogen bonds between the porphyra-334 and close by water molecules can act directly and rather easily to promote an efficient transfer of the excess kinetic energies of the porphyra-334 to the surrounding solvating water molecules via an activation of the collective modes identified as hydrogen-bond stretching modes in liquid water which eventually results in a disruption of the hydrogen bond network. Since porphyra-334 is present in seaweeds, aquatic cyanobacteria (blue-green algae) and red algae, our findings allow addressing the question how algae in oceans or lakes, upon sunlight absorption, can release large amounts of energy into surrounding water without destabilizing neither their own nor the H2O molecular structure.

Development of theoretical approach for describing electronic properties of hetero-interface systems under applied bias voltage.

We have developed a theoretical approach for describing the electronic properties of hetero-interface systems under an applied electrode bias. The finite-temperature density functional theory is employed for controlling the chemical potential in their interfacial region, and thereby the electronic charge of the system is obtained. The electric field generated by the electronic charging is described as a saw-tooth-like electrostatic potential. Because of the continuum approximation of dielectrics sandwiched between electrodes, we treat dielectrics with thicknesses in a wide range from a few nanometers to more than several meters. Furthermore, the approach is implemented in our original computational program named grid-based coupled electron and electromagnetic field dynamics (GCEED), facilitating its application to nanostructures. Thus, the approach is capable of comprehensively revealing electronic structure changes in hetero-interface systems with an applied bias that are practically useful for experimental studies. We calculate the electronic structure of a SiO2-graphene-boron nitride (BN) system in which an electrode bias is applied between the graphene layer and an electrode attached on the SiO2 film. The electronic energy barrier between graphene and BN is varied with an applied bias, and the energy variation depends on the thickness of the BN film. This is because the density of states of graphene is so low that the graphene layer cannot fully screen the electric field generated by the electrodes. We have demonstrated that the electronic properties of hetero-interface systems are well controlled by the combination of the electronic charging and the generated electric field.

Gold Quantum Boxes: On the Periodicities and the Quantum Confinement in the Au28, Au36, Au44 , and Au52 Magic Series.

Revealing the size-dependent periodicities (including formula, growth pattern, and property evolution) is an important task in metal nanocluster research. However, investigation on this major issue has been complicated, as the size change is often accompanied by a structural change. Herein, with the successful determination of the Au44(TBBT)28 structure, where TBBT = 4-tert-butylbenzenethiolate, the missing size in the family of Au28(TBBT)20, Au36(TBBT)24, and Au52(TBBT)32 nanoclusters is filled, and a neat "magic series" with a unified formula of Au8n+4(TBBT)4n+8 (n = 3-6) is identified. Such a periodicity in magic numbers is a reflection of the uniform anisotropic growth patterns in this magic series, and the n value is correlated with the number of (001) layers in the face-centered cubic lattice. The size-dependent quantum confinement nature of this magic series is further understood by empirical scaling law, classical "particle in a box" model, and the density functional theory calculations.

Reducing the Cost and Preserving the Reactivity in Noble-Metal-Based Catalysts: Oxidation of CO by Pt and Al-Pt Alloy Clusters Supported on Graphene.

The oxidation mechanisms of CO to CO2 on graphene-supported Pt and Pt-Al alloy clusters are elucidated by reactive dynamical simulations. The general mechanism evidenced is a Langmuir-Hinshelwood (LH) pathway in which O2 is adsorbed on the cluster prior to the CO oxidation. The adsorbed O2 dissociates into two atomic oxygen atoms thus promoting the CO oxidation. Auxiliary simulations on alloy clusters in which other metals (Al, Co, Cr, Cu, Fe, Ni) replace a Pt atom have pointed to the aluminum doped cluster as a special case. In the nanoalloy, the reaction mechanism for CO oxidation is still a LH pathway with an activation barrier sufficiently low to be overcome at room temperature, thus preserving the catalyst efficiency. This provides a generalizable strategy for the design of efficient, yet sustainable, Pt-based catalysts at reduced cost.

Electric field effects on the electronic properties of the silicene-amine interface.

We performed first-principles studies of electric field (EF) effects on the electronic properties of silicene-amine (NH3 and NH2CH3) hetero-interface systems focusing on the electronic interactions at the interface. The band gaps of the systems increase with a positive applied EF but decrease with a negative EF; that is, the band gaps monotonically vary on changing the applied EF from negative to positive. The phenomenon of band gap variation with the sign of the applied EF is a characteristic feature of hetero-interface systems. We revealed the mechanism of the electronic structure change in silicene-amine due to an applied EF by visualizing the electron density change. It is shown that the electronic polarizations in both the Si-N chemical bond region and the silicene-layer region determine the characteristic band gap variation. Furthermore, the tunable energy range of the band gap of the silicene-amine is considerably higher than the range of a silicene monolayer; thus, the idea of controlling the band gaps of hetero-interface systems in combination with application of an EF bias is suitable for designing various devices that are difficult to fabricate with homogeneous two-dimensional materials such as silicene and graphene.

The absence of a gap state and enhancement of the Mars-van Krevelen reaction on oxygen defective Cu/CeO2 surfaces.

We report a detailed first-principles analysis of the electronic structures of oxygen defective CeO2 and Cu/CeO2 surfaces aimed at elucidating the disappearance of the gap state of defective CeO2 when a Cu atom is added at the surface. The top of the valence band of Cu/CeO2 originates from the O 2p states around this Cu atom. We show that this redistribution of electronic states at the defective Cu/CeO2 surface enhances the reactivity of the surface O atoms. Indeed, dynamical simulations show an acceleration of catalytic NO oxidation occurring via the Mars-van Krevelen mechanism mediated by these highly reactive oxygens.

Control of optical response of a supported cluster on different dielectric substrates.

We develop a computational method for optical response of a supported cluster on a dielectric substrate. The substrate is approximated by a dielectric continuum with a frequency-dependent dielectric function. The computational approach is based on our recently developed first-principles simulation method for photoinduced electron dynamics in real-time and real-space. The approach allows us to treat optical response of an adsorbate explicitly taking account of interactions at an interface between an adsorbate and a substrate. We calculate optical absorption spectra of supported Agn (n = 2, 54) clusters, changing the dielectric function of a substrate. By analyzing electron dynamics in real-time and real-space, we clarify the mechanisms for variations in absorption spectra, such as peak shifts and intensity changes, relating to various experimental results for optical absorption of supported clusters. Attractive and repulsive interactions between an adsorbate and a substrate result in red and blue shifts, respectively, and the intensity decreases by energy dissipation into a substrate. We demonstrate that optical properties can be controlled by varying the dielectric function of a substrate.

Observation of Body-Centered Cubic Gold Nanocluster.

The structure of nanoparticles plays a critical role in dictating their material properties. Gold is well known to adopt face-centered cubic (fcc) structure. Herein we report the first observation of a body-centered cubic (bcc) gold nanocluster composed of 38 gold atoms protected by 20 adamantanethiolate ligands and two sulfido atoms ([Au38S2(SR)20], where R=C10H15) as revealed by single-crystal X-ray crystallography. This bcc structure is in striking contrast with the fcc structure of bulk gold and conventional Au nanoparticles, as well as the bi-icosahedral structure of [Au38(SCH2CH2Ph)24]. The bcc nanocluster has a distinct HOMO-LUMO gap of ca. 1.5 eV, much larger than the gap (0.9 eV) of the bi-icosahedral [Au38(SCH2CH2Ph)24]. The unique structure of the bcc gold nanocluster may be promising in catalytic applications.

Structure determination of [Au18(SR)14].

Unravelling the atomic structures of small gold clusters is the key to understanding the origin of metallic bonds and the nucleation of clusters from organometallic precursors. Herein we report the X-ray crystal structure of a charge-neutral [Au18(SC6H11)14] cluster. This structure exhibits an unprecedented bi-octahedral (or hexagonal close packing) Au9 kernel protected by staple-like motifs including one tetramer, one dimer, and three monomers. Until the present, the [Au18(SC6H11)14] cluster is the smallest crystallographically characterized gold cluster protected by thiolates and provides important insight into the structural evolution with size. Theoretical calculations indicate charge transfer from surface to kernel for the HOMO-LUMO transition.

First-principles computational visualization of localized surface plasmon resonance in gold nanoclusters.

Cluster-size dependence of localized surface plasmon resonance (LSPR) for Aun nanoclusters (n = 54, 146, 308, 560, 922, 1414) is investigated by using our recently developed computational program of first-principles calculations for photoinduced electron dynamics in nanostructures. The size of Au1414 (3.9 nm in diameter) is unprecedentedly large in comparison with those addressed in previous first-principles calculations of optical response in nanoclusters. These computations enable us to clearly see that LSPR gradually grows and the LSPR peaks red shift with increasing cluster size. The growth of LSPR is visualized in real space, demonstrating that electron charge distributions oscillate in a collective manner around the outermost surface region of the clusters. We further illustrate that the core d electrons screen the collective oscillation of the conduction-like s electrons.

Theoretical approach for optical response in electrochemical systems: application to electrode potential dependence of surface-enhanced Raman scattering.

We propose a theoretical approach for optical response in electrochemical systems. The fundamental equation to be solved is based on a time-dependent density functional theory in real-time and real-space in combination with its finite temperature formula treating an electrode potential. Solvation effects are evaluated by a dielectric continuum theory. The approach allows us to treat optical response in electrochemical systems at the atomistic level of theory. We have applied the method to surface-enhanced Raman scattering (SERS) of 4-mercaptopyridine on an Ag electrode surface. It is shown that the SERS intensity has a peak as a function of the electrode potential. Furthermore, the real-space computational approach facilitates visualization of variation of the SERS intensity depending on an electrode potential.

Intrinsic Visible Plasmonic Properties of Colloidal PtIn2 Intermetallic Nanoparticles.

Materials that intrinsically exhibit localized surface plasmon resonance (LSPR) in the visible region have been predominantly researched on nanoparticles (NPs) composed of coinage metals, namely Au, Ag, and Cu. Here, as a coinage metal-free intermetallic NPs, colloidal PtIn2 NPs with a C1 (CaF2 -type) crystal structure are synthesized by the liquid phase method, which evidently exhibit LSPR at wavelengths similar to face-centered cubic (fcc)-Au NPs. Computational simulations pointed out differences in the electronic structure and photo-excited electron dynamics between C1-PtIn2 and fcc-Au NPs; reduces interband transition and stronger screening with smaller number of bound d-electrons compare with fcc-Au are unique origins of the visible plasmonic nature of C1-PtIn2 NPs. These results strongly indicate that the intermetallic NPs are expected to address the development of alternative plasmonic materials by tuning their crystal structure and composition.

Nonsuperatomic [Au23(SC6H11)16]- nanocluster featuring bipyramidal Au15 kernel and trimeric Au3(SR)4 motif.

We report the X-ray structure of a cyclohexanethiolate-capped [Au23(SR)16](-) nanocluster (counterion: tetraoctylammonium, TOA(+)). The structure comprises a cuboctahedron-based bipyramidal Au15 kernel, which is protected by two staple-like trimeric Au3(SR)4 motifs, two monomeric Au(SR)2 and four plain bridging SR ligands. Electronic structure analysis reveals nonsuperatomic feature of [Au23(SR)16](-) and confirms the Au15 kernel and surface motifs. The Au15 kernel and trimeric staple motif are unprecedented and offer new insight in understanding the structure evolution of gold nanoclusters.

Raman enhancement by plasmonic excitation of structurally-characterized metal clusters: Au8, Ag8, and Cu8.

The optical responses of metal clusters, M8 (M = Au, Ag and Cu), are investigated by the linear response theory based on the density functional theory. Unlike sodium clusters [Yasuike et al., Phys. Rev. A, 2011, 83, 013201], the plasmonic excitations in the present metal clusters are strongly reduced by the background d-electron excitations, i.e., Landau damping. To avoid the reduction of plasmon intensity, the control of cluster structures is one of the promising strategies. We demonstrate that the plasmonic excitations of the linear clusters are partially decoupled with the background d-electron excitations and their intensities are much stronger than those of the three-dimensional bicapped octahedral isomers. The linear isomer gives a strong enhancement of the Raman vibrational spectrum of a pyrazine molecule.

Toward the creation of stable, functionalized metal clusters.

Nanomaterials which exhibit both stability and functionality are currently considered to hold the most promise as components of nanotechnology devices. Thiolate (RS)-protected gold nanoclusters (Aun(SR)m) have attracted significant attention in this regard and, among these, the magic clusters are believed to be the best candidates since they are the most stable. We have investigated the effects of heteroatom doping, protection by selenolate ligands and protection by photoresponsive thiolates on the stability and physical/chemical properties of these clusters. Through such studies, we have attempted to establish methods of modifying magic Aun(SR)m clusters as a means of creating metal clusters that are both robust and functional. This paper summarizes our studies towards this goal and the obtained results.