Noritake Isomura

Size-dependent catalytic activity and geometries of size-selected Pt clusters on TiO2(110) surfaces

Scanning tunneling microscope observations show a geometrical transition from a planar structure to a 3D structure at n = 8. This geometrical transition resulted in a significant decrease in the activation energy of the CO oxidation reaction. The upper-layer Pt atoms of the 3D cluster structure that starts to form at n = 8 are low-coordinated Pt atoms, and they may play an important role in the CO oxidation reaction.

A new experimental setup for high-pressure catalytic activity measurements on surface deposited mass-selected Pt clusters

A new experimental setup to study catalytic and electronic properties of size-selected clusters on metal oxide substrates from the viewpoint of cluster-support interaction and to formulate a method for the development of heterogeneous catalysts such as automotive exhaust catalysts has been developed. The apparatus consists of a size-selected cluster source, a photoemission spectrometer, a scanning tunneling microscope (STM), and a high-pressure reaction cell. The high-pressure reaction cell measurements provided information on catalytic properties in conditions close to practical use. The authors investigated size-selected platinum clusters deposited on a TiO2(110) surface using a reaction cell and STM. Catalytic activity measurements showed that the catalytic activities have a cluster-size dependency.

Atomic-resolution imaging of size-selected platinum clusters on TiO2(110) surfaces

Size-selected Pt(n) (n=4,7-10,15) clusters were deposited on TiO(2)(110)-(1x1) surfaces and imaged at atomic resolution using an ultrahigh-vacuum scanning tunneling microscope with a carbon nanotube tip. Clusters smaller than Pt(7) lay flat on the surface with a planar structure and a planar-to-three-dimensional transition occurred at n=8 for Pt(n) clusters on TiO(2). However, both Pt(8) and Pt(9) had two types of geometric structures. The geometric structures depend strongly on the number of atoms in the deposited cluster possibly because of the differences in binding energies in different-sized clusters and different degrees of interaction with the surface. We obtained atomic-resolution images of size-selected clusters on surfaces for the first time, enabling the identification of atomic alignments in the clusters on the surface.

Effect of the addition of Mo and Na to Pt catalysts on the selective reduction of NO

The selective reduction of NOx by reductants such as C3H6 and CO in oxidizing feedstreams simulated exhaust from automobile engine and three-way behavior around the stoichiometric point have been investigated on trimetallic PtMoNa/SiO2 catalysts, over a wide range of temperatures compared with bimetallic PtMo/SiO2 catalyst and monometallic Pt/SiO2 catalyst. The simultaneous addition of Mo and Na to Pt catalysts has been found to improve the following reaction characteristics on selective reduction of NOx and three-way activity of conventional Pt catalyst. The temperature window on selective reduction of NOx on trimetallic PtMoNa/SiO2 catalysts was found to be wider and to shift at higher temperature than that on bimetallic PtMo/SiO2 and monometallic Pt/SiO2 catalysts under lean static conditions. The redox ratio window, in which three-way activity occurred on trimetallic PtMoNa/SiO2 catalysts, was also found to be wider than that on bimetallic PtMo/SiO2 catalysts and monometallic Pt/SiO2 around stoichiometric point. On the other hand, XPS, IR and CO adsorption data indicated that the oxidation of Pt on PtMoNa/SiO2 catalysts was depressed by the added Mo and Na even under excess oxygen conditions, so that the reaction characteristics of trimetallic PtMoNa/SiO2 catalysts was improved.

Epitaxial growth of CeO2(111) film on Ru(0001): scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS) study.

We formed an epitaxial film of CeO2(111) by sublimating Ce atoms on Ru(0001) surface kept at elevated temperature in an oxygen ambient. X-ray photoemission spectroscopy measurement revealed a decrease of Ce(4+)/Ce(3+) ratio in a small temperature window of the growth temperature between 1070 and 1096 K, which corresponds to the reduction of the CeO2(111). Scanning tunneling microscope image showed that a film with a wide terrace and a sharp step edge was obtained when the film was grown at the temperatures close to the reduction temperature, and the terrace width observed on the sample grown at 1060 K was more than twice of that grown at 1040 K. On the surface grown above the reduction temperature, the surface with a wide terrace and a sharp step was confirmed, but small dots were also seen in the terrace part, which are considerably Ce atoms adsorbed at the oxygen vacancies on the reduced surface. This experiment demonstrated that it is required to use the substrate temperature close to the reduction temperature to obtain CeO2(111) with wide terrace width and sharp step edges.

Cluster size dependence of Pt core-level shifts for mass-selected Pt clusters on TiO2(110) surfaces

In order to examine cluster size dependence, mass-selected platinum clusters, Ptn (n=2–5, 7, 8, 10, 15), were deposited on TiO2(110) surfaces at room temperature under soft landing conditions, and their core levels were investigated by x-ray photoelectron spectroscopy. Pt core-level shifts with cluster size are observed. The binding energies of Pt 4f7/2 decrease steeply with increasing cluster size up to n=7 for Ptn and decrease gradually for n≥8. This inflection point (n=8) agrees well with the cluster size at a geometric transition (planar-to-three-dimensional) seen with scanning tunneling microscopy (STM) [N. Isomura, X. Wu, and Y. Watanabe, J. Chem. Phys. 131, 164707 (2009)]. It was found that the core-level shifts of mass-selected Pt clusters deposited on TiO2 are closely correlated with cluster geometries determined directly by atomic-resolution STM imaging.

Alumina-supported sub-nanometer Pt10 clusters: amorphization and role of the support material in a highly active CO oxidation catalyst

Catalytic CO oxidation is unveiled on size-selected Pt_(10) clusters deposited on two very different ultrathin (≈0.5–0.7 nm thick) alumina films: (i) a highly ordered alumina obtained under ultra-high vacuum (UHV) by oxidation of the NiAl(110) surface and (ii) amorphous alumina obtained by atomic layer deposition (ALD) on a silicon chip that is a close model of real-world supports. Notably, when exposed to realistic reaction conditions, the Pt_(10)/UHV-alumina system undergoes a morphological transition in both the clusters and the substrate, and becomes closely akin to Pt_(10)/ALD-alumina, thus reconciling UHV-type surface-science and real-world experiments. The Pt_(10) clusters, thoroughly characterized via combined experimental techniques and theoretical analysis, exhibit among the highest CO oxidation activity per Pt atom reported for CO oxidation catalysts, due to the interplay of ultra-small size and support effects. A coherent interdisciplinary picture then emerges for this catalytic system.

Photoemission Spectroscopy of the Interface between Indium-Tin-Oxide and Copper Phthalocyanine for Transparent Organic Light-Emitting Devices

We investigated chemical and electronic structures of copper phthalocyanine (CuPc) for the electron-injection layer between indium tin oxide (ITO) and emission layers in transparent organic light-emitting devices (TOLEDs). Metallic Cu or dicopper oxide (Cu2O) is formed at the ITO/CuPc interface as a damaged layer during the sputtering deposition of ITO and plays a significant role in efficient electron injection from ITO to the emission layer. Sophisticated photoemission experiments revealed that the cause of the formation of the damaged layer is due to the exposure of CuPc to the oxygen plasma, while no significant damage is introduced by argon plasma and reaction with indium and tin atoms. To achieve efficient TOLEDs, the control of the oxygen plasma is essential.

Chemical Structure of Aluminum/8-Hydroxyquinoline Aluminum Interface.

The chemical structure of interfaces between aluminum (Al) and 8-hydroxyquinoline aluminum (Alq3) has been studied by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). New subpeaks of N1s and O1s in XPS spectra were observed after the deposition of Al on Alq3. The secondary ion intensity of quinoline in TOF-SIMS spectra was found to increase with Al coverage on Alq3. We, therefore, conclude that Alq3 is decomposed by the deposition of Al on Alq3, and that quinoline is formed at the Al/Alq3 interface.

Band slope in CdS layer of ZnO:Ga/CdS/Cu2ZnSnS4 photovoltaic cells revealed by hard X-ray photoelectron spectroscopy

For compound semiconductor photovoltaic cells with a common structure of the window-layer (WL)/buffer-layer (BL)/absorbing-layer (AL), the band slope in BLs, affecting the conversion efficiency, was directly and non-destructively measured by hard X-ray photoelectron spectroscopy. We demonstrated that the band slope in CdS-BLs sandwiched between WLs and Cu2ZnSnS4 (CZTS)-ALs reflected the trend of the work functions of WLs ( ϕ WL). This result implies that the larger downward band slope to the WL can be achieved using a smaller ϕ WL. The relatively large downward band slope of ∼0.5 eV to the WL was estimated in our ZnO:Ga/CdS/CZTS sample with a higher conversion efficiency of 9.4%, which indicates that the conversion efficiency of CZTS cells can be improved by a larger downward band slope to the WL.