Direct Identification of Reactive Electron Species Driving Photocatalytic Hydrogen Evolution on Metal-Loaded Oxides.

Performing spectroscopy under practical reaction conditions and extracting spectral components correlating with reaction activity are crucial in elucidating the reactive species in photocatalysis. However, the observation of weak signals corresponding to reactive photogenerated species is frequently hampered under reaction conditions owing to intense background signals originating from thermally induced species unrelated to the photoinduced reactions. Herein, by synchronizing the millisecond periodic excitations of photocatalysts with a Michelson interferometer used for FT-IR spectroscopy, we succeeded in significantly suppressing the signals derived from thermally excited electrons and observing the reactive photogenerated electrons contributing to the photocatalytic hydrogen evolution.

Positive and Negative Impacts of Interfacial Hydrogen Bonds on Photocatalytic Hydrogen Evolution.

Understanding the behavior of water molecules at solid-liquid interfaces is crucial for various applications such as photocatalytic water splitting, a key technology for sustainable fuel production and chemical transformations. Despite extensive studies conducted in the past, the impact of the microscopic structure of interfacial water molecules on photocatalytic reactivity has not been directly examined. In this study, using real-time mass spectrometry and Fourier-transform infrared spectroscopy, we demonstrated the crucial role of hydrogen bond (H-bond) networks on the photocatalytic hydrogen evolution in thickness-controlled water adsorption layers on various TiO photocatalysts.

Broadband Tip-Enhanced Nonlinear Optical Response in a Plasmonic Nanocavity.

We report a significantly broad nonlinear optical response enhanced in a tip-substrate plasmonic nanocavity. Focusing on the near-field second harmonics of the wavelength-tunable femtosecond laser, we demonstrate that the tip-enhancement of nonlinear optical effects efficiently works over the broad wavelength range through the visible to infrared region. We also found that this broadband nonlinear optical property is directly affected not only by the nanometer-scale sharpness of the tip apexes but also by the micrometer-scale surface geometry of the tip shafts.

Beyond Reduction Cocatalysts: Critical Role of Metal Cocatalysts in Photocatalytic Oxidation of Methane with Water.

Environmentally sustainable and selective conversion of methane to valuable chemicals under ambient conditions is pivotal for the development of next-generation photocatalytic technology. However, due to the lack of microscopic knowledge about non-thermal methane conversion, controlling and modulating photocatalytic oxidation processes driven by photogenerated holes remain a challenge. Here, we report novel function of metal cocatalysts to accept photogenerated holes and dominate selectivity of methane oxidation, which is clearly beyond the conventional concept in photocatalysis that the metal cocatalysts loaded on the surfaces of semiconductor photocatalysts mostly capture photogenerated electrons and dominate reduction reactions exclusively.

Critical impacts of interfacial water on C-H activation in photocatalytic methane conversion.

On-site and on-demand photocatalytic methane conversion under ambient conditions is one of the urgent global challenges for the sustainable use of ubiquitous methane resources. However, the lack of microscopic knowledge on its reaction mechanism prevents the development of engineering strategies for methane photocatalysis. Combining real-time mass spectrometry and operando infrared absorption spectroscopy with ab initio molecular dynamics simulations, here we report key molecular-level insights into photocatalytic green utilization of methane.

Orientational ordering in heteroepitaxial water ice on metal surfaces.

Heteroepitaxial growth of crystalline ice thin films of water on metal substrates under ultrahigh vacuum provides an excellent opportunity to investigate the interior and surface structures of crystalline ice that are closely related to their physicochemical properties. Here we present the spectroscopic studies of the orientational ordering and the surface relaxation of crystalline ice films grown on two representative metal surfaces: Pt(111) and Rh(111). A versatile tool for exploring these structures is sum frequency generation (SFG) vibrational spectroscopy; homodyne detection of SFG signals serves as a good measure of orientational ordering in the interior of crystalline ice films while heterodyne detection enables us to determine the direction of water molecules at the interface with metal substrates, in the interior of crystalline ice films, and at their surfaces.

Direct Experimental Evidence for Markedly Enhanced Surface Proton Activity Inherent to Water Ice.

Autoionization and subsequent proton transfer processes determine the proton activity inherent to water molecular systems. In this study, we provide direct experimental evidence that the proton activity is markedly enhanced at the surface of crystalline ice, on the basis of the simultaneous observation of H/D exchange of water molecules at the surface and in the interior of well-defined double-layer ice films composed of HO and DO. Thermal desorption mass spectrometry showed clear signatures derived from the surface H/D exchange equilibrium, whereas infrared absorption spectroscopy indicated no appreciable H/D exchange progress in the interior.

Thickness dependent homogeneous crystallization of ultrathin amorphous solid water films.

The crystallization mechanism and kinetics of amorphous materials are of paramount importance not only in basic science but also in the application field because they are closely related to their thermal stability. In the case of amorphous nanomaterials, thermal stability distinctively different from that of bulk materials often emerges. Despite intensive studies in the past, a thorough understanding of the stability at the molecular level has not been reached particularly on how crystallization processes depend on size and are influenced by their surface and interface.

Enhanced structural disorder at a nanocrystalline ice surface.

Detailed knowledge of the structure and dynamics of the surface of ice particles is of considerable importance for understanding catalytic reactions in the upper atmosphere. Here we report the enhanced structural disorder specific at a nanoscale ice island studied by using heterodyne-detected vibrational sum-frequency generation spectroscopy under ultrahigh vacuum. Ultrathin films of isotopically diluted HOD crystalline ice are grown on Rh(111), whose average height (≥1.

Water-Assisted Hole Trapping at the Highly Curved Surface of Nano-TiO Photocatalyst.

Heterogeneous photocatalysis is vital in solving energy and environmental issues that this society is confronted with. Although photocatalysts are often operated in the presence of water, it has not been yet clarified how the interaction with water itself affects charge dynamics in photocatalysts. Using water-coverage-controlled steady and transient infrared absorption spectroscopy and large-model (∼800 atoms) ab initio calculations, we clarify that water enhances hole trapping at the surface of TiO nanospheres but not of well-faceted nanoparticles.

Coherent singlet fission activated by symmetry breaking.

Singlet fission, in which a singlet exciton is converted to two triplet excitons, is a process that could be beneficial in photovoltaic applications. A full understanding of the dynamics of singlet fission in molecular systems requires detailed knowledge of the relevant potential energy surfaces and their (conical) intersections. However, obtaining such information is a nontrivial task, particularly for molecular aggregates.

Fast crystalline ice formation at extremely low temperature through water/neon matrix sublimation.

Crystalline ice formation requires water molecules to be sufficiently mobile to find and settle on the thermodynamically most stable site. Upon cooling, however, diffusion and rearrangement become increasingly kinetically difficult. Water ice grown by the condensation of water vapor in laboratory is thus generally assumed to be in a metastable amorphous form below 100 K.

Disentangling Multidimensional Nonequilibrium Dynamics of Adsorbates: CO Desorption from Cu(100).

Hot carriers at metal surfaces can drive nonthermal reactions of adsorbates. Characterizing nonequilibrium statistics among various degrees of freedom in an ultrafast time scale is crucial to understand and develop hot carrier-driven chemistry. Here we demonstrate multidimensional vibrational dynamics of carbon monoxide (CO) on Cu(100) along hot-carrier induced desorption studied by using time-resolved vibrational sum-frequency generation with phase-sensitive detection.

Effect of Water Adsorption on Carrier Trapping Dynamics at the Surface of Anatase TiO2 Nanoparticles.

Charge carrier trapping plays a vital role in heterogeneous photocatalytic water splitting because it strongly affects the dynamics of photogenerated charges and hence the photoconversion efficiency. Although hole trapping by water at water/photocatalyst interface is the first step of oxygen evolution in water splitting, little has been known on how water adsorbate itself is involved in hole trapping dynamics. To clarify this point, we have performed infrared transient and steady-state absorption spectroscopy of anatase TiO2 nanoparticles as a function of the number of water adsorbate layers.

Effects of rotational-symmetry breaking on physisorption of ortho- and para-H2 on Ag(111).

Quantum-state-selective thermal desorption of H2 weakly physisorbed on Ag(111) demonstrates significantly different desorption features between the nuclear-spin modifications. An energy shift due to the rotational-symmetry breaking induced by an anisotropic interaction affects not only the enthalpy but also the entropy of adsorption. The preexponential factor for desorption of the ortho-H2 is about three times as large as that of the para-H2.

Ultrafast exciton dynamics in dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene thin films.

Ultrafast dynamics of excitons in organic semiconductors is essential for a deep understanding of the working mechanism of plastic opto-electronic devices. In this work, excited state dynamics in dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene thin films has been studied with femtosecond transient absorption and time-resolved photoluminescence spectroscopy. Upon the excitation with a femtosecond pulse at 400 nm, a broad positive absorption band at 1.