According to the concept of active sites, the activity of heterogeneous catalysts correlates with the number of available catalytic sites and the binding affinity of the substrates. Herein, we report a single-molecule, single-particle fluorescence approach to elucidate the inherent photocatalytic activity of exposed surfaces of anatase TiO(2), a promising photocatalyst, using redox-responsive fluorogenic dyes. A single-molecule imaging and kinetic analysis of the fluorescence from the products shows that reaction sites for the effective reduction of the probe molecules are preferentially located on the {101} facets of the crystal rather than the {001} facets with a higher surface energy. This surprising discrepancy can be explained in terms of face-specific electron-trapping probability. In situ observation of the catalytic events occurring at the solid/solution interfaces reveals the hidden role of the crystal facets in chemical reactions and their impact on the efficiency and selectivity of heterogeneous (photo)catalysts.

Pt-modified Au nanorods (NRs) synthesized by anisotropic overgrowth were used for producing H2 under visible and near-infrared light irradiation. The Pt-tipped sample exhibited much higher activity compared with fully covered samples. Using single-particle spectroscopies combined with transmission electron microscopy, we observed obvious quenching phenomena for photoluminescence and light scattering from individual Pt-tipped NRs. The analysis of energy relaxation of plasmon-generated hot electrons indicates the electron transfer from the excited Au to Pt.

Plasmonic photocatalysts were successfully synthesized by the modification of TiO2 mesocrystals with Au nanoparticles (NPs) by a simple impregnation method. The Au NP sensitizers show a strong photoelectrochemical response in the visible-light region (400-800 nm) due to their surface plasmon resonance (SPR). The diffuse reflectance spectroscopy measurements on a wide range of time scales (from picoseconds to minutes) demonstrate that a substantial part of electrons, injected from the Au NPs to the TiO2 mesocrystals through the SPR excitation, directionally migrate from the basal surfaces to the edges of the plate-like mesocrystals through the TiO2 nanocrystal networks and are temporally stored there for further reactions. This anisotropic electron flow significantly retarded the charge recombination of these electrons with the holes in the Au NPs, thereby improving the visible-light-photocatalytic activity (for organic-pollutant degradation) by more than an order of magnitude, as compared to that of conventional Au/TiO2 NP systems.

Titanium dioxide (TiO 2 ) has been extensively investigated for the photocatalytic purification of air and water. In this article, we have focused on the mechanisms of the one-electron redox reactions of organic compounds during the TiO 2 photocatalytic reactions and on the development of TiO 2 -based materials. It was clearly demonstrated that the adsorption dynamics of substrates and intermediates, the electronic interaction between TiO 2 and adsorbates, and the band structure and morphology of TiO 2 nanomaterials are crucial factors for establishing efficient photocatalytic reaction systems. The TiO 2 -based hybrid nanoparticles with various functional materials, such as polyoxometalates (POMs) and cyclodextrins (CDs), have been fabricated on the basis of the mechanistic aspects. New experimental methods, such as two-color two-laser flash photolysis and single-molecule fluorescence techniques, for the investigation of the TiO 2 photocatalytic reactions have been demonstrated.

Plasmonic bimetal nanostructures can be used to drive the conventional catalytic reactions efficiently at low temperature with the utilization of solar energy. This work developed Pd-modified Au nanorods, which work as the light absorber and the catalytically active site simultaneously, and exhibit efficient plasmon-enhanced catalytic formic acid dehydrogenation even when below room temperature (5 °C). Plasmon-induced interface interaction and photoreaction dynamics of individual nanorods were investigated by single-particle photoluminescence measurement, and a complete quenching phenomenon at the LSPR region was observed for the first time. More importantly, the spatial distribution of the SPR-induced enhancement, analyzed by the finite difference time domain (FDTD) simulation, shows that only tip-coated Pd can be affected for the occurrence of plasmon resonance energy transfer. This finding provides a route to decrease the amount of Pd species by the selective deposition only at the field-enhanced sites.

The photocatalytic oxidation reactivities of the photogenerated holes (h+) during ultraviolet or visible laser flash photolysis of pure anatase and sulfur- and carbon-doped TiO2 powders were investigated using time-resolved diffuse reflectance (TDR) spectroscopy. The one-electron oxidation processes of substrates such as methanol and 4-(methylthio)phenyl methanol (MTPM) by h+ at the TiO2 surface were examined. The TDR spectra and time traces observed for charge carriers and the MTPM radical cation (MTPM•+) revealed that the oxidation reactions of substrates by h+ generated during the 355-nm laser photolysis of TiO2 powders increased in the order of pure TiO2 > S-doped TiO2 > C-doped TiO2. On the other hand, no one-electron oxidation reactions of the substrates were observed during the 430-nm laser photolysis of the S- and C-doped TiO2 powders, although the charge carriers were sufficiently generated upon excitation. The effects of the trapping and detrapping processes of h+ at the doping sites on the oxidation reactions during the laser flash photolyses of the TiO2 powders are discussed.

Organometal halide perovskites have attracted considerable attention because of their striking electrical and optical properties that are desirable for application in solar cells and optoelectronic devices; however, the structure-related dynamics of photogenerated charges are almost always masked by ensemble averaging in conventional spectroscopic methods, making it difficult to clarify the underlying mechanism. Here we investigate the photoluminescence characteristics of CH 3 NH 3 PbBr 3 perovskite nanoparticles using single-particle spectroscopy combined with electron microscopy. The in situ analysis of light and Lewis-base-induced surface passivation revealed that the photoluminescence quenching and blinking phenomena of single CH 3 NH 3 PbBr 3 nanoparticles are most probably caused by charge trapping at surface states, where the number of effective trapping sites was estimated to be 1−4 per particle.O rganometal halide perovskites, mostly methylammonium lead halides (CH 3 NH 3 PbX 3 ; hereafter denoted as MAPbX 3 , where MA = CH 3 NH 3 and X = Cl, Br, or I), have attracted great attention for photovoltaic applications because of their low cost and ability to rapidly increase the power conversion efficiency by up to 20%. 1−4 The high efficiency is explained as the combined advantages of the broad lightabsorption range, high optical-absorption coefficient (∼10 5 cm −1 ), fast charge generation combined with slow recombination, and long-range diffusion length of charge carriers. 3−5 The tunability of the electronic levels of the perovskites by changing metal or halogen ions is a particularly important advantage because it provides an elegant means to adjust their optical absorption properties so that a good overlap with sunlight can be obtained. The same hybrid perovskite structures also offer prospects of promising applications in lasers and light-emitting diodes (LEDs). 6,7 In general, the active MAPbX 3 layer of the high-performance perovskite solar cells has been prepared by sequentially depositing a solution of PbX 2 and a solution of CH 3 NH 3 X (MAX) onto a mesoporous TiO 2 film, followed by removing the solvent to crystallize the perovskite structures with dimensions from tens to hundreds of nanometers. 8 The size and morphology of perovskite grains are definitely important for charge-generation, -transport, and -trapping properties that are essential to photovoltaic and optoelectronic devices; however, the fate of the photogenerated charge carriers on specific structures has remained unclear because almost all spectroscopic measurements have been conducted on bulk samples where the primary particles usually aggregate. Therefore, physicochemical examinations at the single-particle level are urgently required for elucidating the underlying mechanisms of charge dynamics.In this work, we synthesized colloidal MAPbBr 3 nanoparticles (NPs) by employing a ligand-f ree reprecipitation method. Organic ligands are usually essential for synthesis, but they may interact with the surface instead and capture the char...

Using the combination of fluorescence resonance energy transfer (FRET) and fluorescence correlation spectroscopy (FCS) technique, we investigate the mechanism and dynamics of the pH-induced conformational change of i-motif DNA in the bulk phases and at the single-molecule level. Despite numerous studies on i-motif that is formed from cytosine (C)-rich strand at slightly acidic pH, its detailed conformational dynamics have been rarely reported. Using the FRET technique to provide valuable information on the structure of biomolecules such as a protein and DNA, we clearly show that the partially folded species as well as the single-stranded structure coexist at neutral pH, supporting that the partially folded species may exist substantially in vivo and play an important role in a process of gene expression. By measuring the FCS curves of i-motif, we observed the gradual decrease of the diffusion coefficient of i-motif with increasing pH. The quantitative analysis of FCS curves supports that the gradual decrease of diffusion coefficient (D) associated with the conformational change of i-motif is not only due to the change in the intermolecular interaction between i-motif and solvent accompanied by the increase of pH but also due to the change of the shape of DNA. Furthermore, FCS analysis showed that the intrachain contact formation and dissociation for i-motif are 5-10 times faster than that for the open form. The fast dynamics of i-motif with a compact tetraplex is due to the intrinsic conformational changes at the fluorescent site including the motion of alkyl chain connecting the dye to DNA, whereas the slow intrachain contact formation observed from the open form is due to the DNA motion corresponding to an early stage interaction in the folding process of the unstructured open form.