Kei Mitsuhara

Origin of stabilization and destabilization in solid-state redox reaction of oxide ions for lithium-ion batteries

Further increase in energy density of lithium batteries is needed for zero emission vehicles. However, energy density is restricted by unavoidable theoretical limits for positive electrodes used in commercial applications. One possibility towards energy densities exceeding these limits is to utilize anion (oxide ion) redox, instead of classical transition metal redox. Nevertheless, origin of activation of the oxide ion and its stabilization mechanism are not fully understood. Here we demonstrate that the suppression of formation of superoxide-like species on lithium extraction results in reversible redox for oxide ions, which is stabilized by the presence of relatively less covalent character of Mn4+ with oxide ions without the sacrifice of electronic conductivity. On the basis of these findings, we report an electrode material, whose metallic constituents consist only of 3d transition metal elements. The material delivers a reversible capacity of 300 mAh g−1 based on solid-state redox reaction of oxide ions.

The source of the Ti 3d defect state in the band gap of rutile titania (110) surfaces

The origin of the Ti 3d defect state seen in the band gap for reduced rutile TiO(2)(110) surfaces has been excitingly debated. The probable candidates are bridging O vacancies (V(O)) and Ti interstitials (Ti-int) condensed near the surfaces. The aim of this study is to give insights into the source of the gap state via photoelectron spectroscopy combined with ion scattering and elastic recoil detection analyses. We have made three important findings: (i) The intensity of the gap state observed is well correlated with the sheet resistance measured with a 4-point probe, inversely proportional to the density of Ti-int. (ii) Sputter∕annealing cycles in ultrahigh vacuum (UHV) lead to efficient V(O) creation and condensation of Ti-int near the surface, while only annealing below 870 K in UHV condenses subsurface Ti-int but does not create V(O) significantly. (iii) The electronic charge to heal a V(O) is almost twice that to create an O adatom adsorbed on the 5-fold Ti row. The results obtained here indicate that both the V(O) and Ti-interstitials condensed near the surface region contribute to the gap state and the contribution to the gap state from the Ti-int becomes comparable to that from V(O) for the substrates with low sheet resistance less than ∼200 Ω∕square.

The mechanism of emerging catalytic activity of gold nano-clusters on rutile TiO2(110) in CO oxidation reaction.

This paper reveals the fact that the O adatoms (O(ad)) adsorbed on the 5-fold Ti rows of rutile TiO(2)(110) react with CO to form CO(2) at room temperature and the oxidation reaction is pronouncedly enhanced by Au nano-clusters deposited on the above O-rich TiO(2)(110) surfaces. The optimum activity is obtained for 2D clusters with a lateral size of ∼1.5 nm and two-atomic layer height corresponding to ∼50 Au atoms∕cluster. This strong activity emerging is attributed to an electronic charge transfer from Au clusters to O-rich TiO(2)(110) supports observed clearly by work function measurement, which results in an interface dipole. The interface dipoles lower the potential barrier for dissociative O(2) adsorption on the surface and also enhance the reaction of CO with the O(ad) atoms to form CO(2) owing to the electric field of the interface dipoles, which generate an attractive force upon polar CO molecules and thus prolong the duration time on the Au nano-clusters. This electric field is screened by the valence electrons of Au clusters except near the perimeter interfaces, thereby the activity is diminished for three-dimensional clusters with a larger size.

Role of gold nanoclusters supported on TiO2(110) model catalyst in CO oxidation reaction

It was reported previously that O adatoms adsorbed dissociatively on the five-fold Ti rows of rutile TiO2(110) made the surface O-rich and reacted with CO molecules to form CO2. An electronic charge transfer taking place from gold nanoclusters to the O-rich TiO2(110) support played a crucial role to enhance the catalytic activity [Mitsuhara et al., J. Chem. Phys. 136, 124303 (2012)]. In this study, the authors have further accumulated experimental data for the CO oxidation reaction enhanced by gold nanoclusters on the TiO2(110) surface. Based on the results obtained here and previously, the authors propose an “interface dipole model,” which explains the strong activity of Au nanoclusters supported on O-rich TiO2(110) in CO oxidation reaction. Simultaneously, the authors also discuss the cationic cluster model proposed by Wang and Hammer [Phys. Rev. Lett. 97, 136107 (2006)] and the d-band model predicted by Hammer and Norskov [Adv. Catal. 45, 71 (2000)]. The latter is, in particular, widely accepted to explain the activities of heterogeneous catalysts. Contrary to the d-band model, our ab initio calculations demonstrate that the d-band center for Au nanoclusters moves apart from the Fermi level with decreasing the cluster size and this is due to contraction of the Au-Au bond length.

The structure of SrTiO3(001)-2 × 1 surface analyzed by high-resolution medium energy ion scattering coupled with ab initio calculations

High-resolution medium energy ion scattering (MEIS) spectrometry coupled with photoelectron spectroscopy revealed unambiguously that the initial SrTiO3(001) surface chemically etched in a buffered NH4F-HF solution was perfectly terminated with a single-layer (SL) of TiO2(001) and annealing the surface at 600-800 [ordinal indicator, masculine]C in ultrahigh vacuum (UHV) led to a (2 × 1)-reconstructed surface terminated with a double-layer (DL) of TiO2(001). After annealing in UHV, rock-salt SrO(001) clusters with two atomic layer height grew epitaxially on the DL-TiO2(001)-2 × 1 surface with a coverage of 20%-30%. High-resolution MEIS in connection with ab initio calculations demonstrated the structure of the DL-TiO2(001)-2 × 1 surface close to that proposed by Erdman et al. [Nature (London) 419, 55 (2002)] rather than that predicted by Herger et al. [Phys. Rev. Lett. 98, 076102 (2007)]. Based on the MEIS analysis combined with the ab initio calculations, we propose the most probable (2 × 1) surface structure.

The atomic and electronic structures of NiO(001)/Au(001) interfaces

The atomic and electronic structures of NiO(001)∕Au(001) interfaces were analyzed by high-resolution medium energy ion scattering (MEIS) and photoelectron spectroscopy using synchrotron-radiation-light. The MEIS analysis clearly showed that O atoms were located above Au atoms at the interface and the inter-planar distance of NiO(001)∕Au(001) was derived to be 2.30 ± 0.05 Å, which was consistent with the calculations based on the density functional theory (DFT). We measured the valence band spectra and found metallic features for the NiO thickness up to 3 monolayer (ML). Relevant to the metallic features, electron energy loss analysis revealed that the bandgap for NiO(001)∕Au(001) reduced with decreasing the NiO thickness from 10 down to 5 ML. We also observed Au 4f lines consisting of surface, bulk, and interface components and found a significant electronic charge transfer from Au(001) to NiO(001). The present DFT calculations demonstrated the presence of an image charge beneath Ni atoms at the interface just like alkali-halide∕metal interface, which may be a key issue to explain the core level shift and band structure.