报告一：Pico-Newton Force Sensing at Liquid-Solid Interfaces

报告摘要：

Frequency-modulation atomic force microscopy (FM-AFM) is a promising tool to observe solid topography and also liquid structure at liquid-solid interfaces. The cantilever with a tip is mechanically oscillated. The shift of the resonance frequency, Δf, represents the force pushing or pulling the tip. Microscopes with a force sensitivity of 10 pN or better in water and organic solvents have been developed [1] and commercialized to date [2]. Using the advanced microscopes, we have examined structured liquids at a number of interfaces including water-Al₂O₃ [3], water-hydrophilic molecular monolayers [4], water-H₂NC₆H₄NO₂ [5, 6], water-CaCO₃ [7] (see Fig. 1), etc.

This talk will be focused on two-dimensional Δf distribution observed on water-CaCO₃ interfaces. The observed Δf distributions are interpreted with water density distribution through Gibbs free energy perturbed by the solid surface. The force sensitivity of 10 pN is the key for probing force on single liquid molecules. Possible application of Δf mapping to tribology research is also mentioned. 

References:

[1]     [1]  T. Fukuma et al., Rev. Sci. Instrum. 76, 053704 (2005).
[2]       http://www.shimadzu.com/an/news-events/2014/spm-8000fm.html
[3]       T. Hiasa et al., J. Phys. Chem. C 114, 21423 (2010).
[4]       T. Hiasa et al., Phys. Chem. Chem. Phys. 14, 8419 (2012).
[5]       R. Nishioka et al., J. Phys. Chem. C 117, 2939 (2013).
[6]       P. Spijker et al., J. Phys. Chem. C 118, 2058 (2014).
[7]       H. Imada et al., Langmuir 29, 10744 (2013).

报告二：Sr-doped NaTaO₃ Photocatalysts for Water Split Reaction: Mechanisms for Restricted Electron-Hole Recombination

报告摘要：

Visible-light sensitivity and high quantum efficiency are simultaneously required to achieve artificial photosynthesis in a practical scale. Kudo and coworkers offered a promising route to avoid electron-hole recombination in metal-oxide photocatalysts by doping foreign metals. They developed NaTaO₃ photocatalysts doped with alkaline-earth metals [1] thru the solid-state reaction. Quantum efficiency for overall water splitting increased to be near 50% or better by doping these selected metal cations at 1-5 mol%. Onishi and coworkers[2] then showed recombination of bandgap excited electrons and holes restricted by doping alkaline-earth metals. Their recent study [3] was focused on Sr-doped NaTaO₃ to propose formation of a NaTaO₃–SrSr_1/3Ta_2/3O₃ solid solution as the key for restricted recombination. In the solid solution, A site and B site of the perovskite-structured lattice are simultaneously doped with Sr²⁺.

Possible mechanisms behind restricted recombination were further examined in the present study. By preparing in NaCl flux, Sr concentration gradient was controlled in sub-micrometer sized particles with the particle-averaged concentration intact at 2 mol%. Heating in the flux at 1423 K for 1 (60) h made NaTaO₃–SrSr_1/3Ta_2/3O₃ particles with a large (small) gradient along particle radius, as evidenced by high-angel annular dark field (HAADF) scanning transmission electron microscopy. Electron-hole recombination rate was deduced from infrared light absorption by bandgap-excited electrons not yet recombined with holes. The 1h-heated particles presented intense absorption and longer heating resulted in weakened absorption, as shown in Fig. 1. This suggested Sr concentration gradient determines recombination rate. The conduction band of NaTaO₃ is composed of unoccupied Ta5d orbitals. In the solid solution, SrO₆ octahedra are embedded in the corner-shared chains of TaO₆ octahedra. The embedded octahedra hinder overlap of Ta5d orbitals. Hindered overlap makes the conduction band narrow with the band minimum shifted upward. The energy of conduction band minimum is sensitive to local concentration of SrO₆ octahedra and hence concentration gradient makes energy gradient of conduction band minimum. Photoexcited electrons are driven on the gradient leaving holes in the valence band. Electron-hole recombination can be restricted in this picture.

     [1] A. Iwase, H. Kato and A. Kudo, ChemSusChem 2, 873 (2009).

     [2] M. Maruyama, A. Iwase, H. Kato, A. Kudo and H. Onishi, J. Phys. Chem. C 113, 13918 (2009).

     [3] L. An and H. Onishi, ACS Catal. 5, 3196 (2015).