Electron Holographic Characterization of Nano-Hetero Interface Effect in Gold Catalysts
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Electron Holographic Characterization of Nano-Hetero Interface Effect in Gold Catalysts S. Ichikawa, T. Akita, K. Okazaki, K. Tanaka, and M. Kohyama Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST) 1-8-31 Midorigaoka, Ikeda, OSAKA, 563-8577, Japan ABSTRACT We investigated the atomic structure near the interface and the size dependence of the mean inner potential of gold in Au/TiO2 catalysts prepared by the deposition precipitation (DP) method and the vacuum evaporation (VE) method using high resolution electron microscopy (HREM) and electron holography. The TiO2 supports prepared by DP method and VE method are considered to have oxygen-rich surfaces and titanium-rich surfaces respectively. In case of the Au/TiO2 catalyst prepared by DP method, the mean inner potential of gold increased depending on the size of the particle. When the size of the gold particle is over 5nm, the mean inner potential of gold was the same as that of bulk Au. When the size is below 5nm, the mean inner potential became to increase. It increases suddenly over 40V, particularly in case of below 2nm. It indicates that the electronic state of gold on TiO2 changes from that of bulk state as the size decreases. On the other hand, the mean inner potential of gold in Au/TiO2 catalysts prepared by VE method also increased as the size decreased, but the behavior of the size dependence is different from that of DP method. The critical size of the mean inner potential change is around 3nm. The size of the gold particle was below 3nm, the mean inner potential of gold increased gradually. The mean inner potential of VE method is less than that of DP method with the same particle size in spite of the size below 3nm. The stoichiometry at the interface between Au and TiO2 should be one of the dominant reasons for the behavior difference of the size dependence of the mean inner potential between the preparations. INTRODUCTION Gold is well-known to show the catalytic activities when the gold particles is supported on a certain oxides with nanometer scale [1, 2]. The catalytic activities e.g. turn over frequency (TOF), vary depending on the size of the gold particle [3]. It is considered that the formation of the typical electronic structure at the nano-hetero interfaces between the gold particles and oxide supports causes the appearance of the catalytic activities. Using high resolution electron microscopy (HREM) and electron holography, we have observed Au/TiO2 catalysts prepared by the deposition precipitation (DP) method [4] and found that the morphology and the mean inner potential of the gold particle also vary depending on the size of particle similarly to the catalytic activities [5, 6]. When the size of the gold particle is over 5nm, the mean inner potential of gold was the same as that of bulk Au. When the size is below 5nm, the mean inner potential became to increase. It increases suddenly over 40V, particularly in case of below 2nm. It indicates that the electronic sta
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