The periodic shell structure and surface reconstruction of metallic FePt nanoparticles with icosahedral structure has been quantitatively studied by high-resolution transmission electron microscopy with focal series reconstruction with sub-angstrom resolution. The icosahedral FePt nanoparticles fabricated by the gas phase condensation technique in vacuum have been found to be surprisingly oxidation resistant and stable under electron beam irradiation. We find the lattice spacing of (111) planes in the surface region to be size dependent and to expand by as much as 9% with respect to the bulk value of Fe52Pt48. Controlled removal of the (111) surface layers in situ results in a similar outward relaxation of the new surface layer. This unusually large layerwise outward relaxation is discussed in terms of preferential Pt segregation to the surface forming a Pt enriched shell around a Fe-rich Fe/Pt core.
Anomalous Hall effect at room temperature in perpendicular Hall balance with a core structure of [Pt/Co]4/NiO/[Co/Pt]4 has been tuned by functional CoO layers, where [Pt/Co]4 multilayers exhibit perpendicular magnetic anisotropy. A giant Hall resistance ratio up to 69 900% and saturation Hall resistance (RSP) up to 2590 mΩ were obtained in CoO/[Pt/Co]4/NiO/[Co/Pt]4/CoO system, which is 302% and 146% larger than that in the structure without CoO layers, respectively. Transmission electron microscopy shows highly textured [Co/Pt]4 multilayers and oxide layers with local epitaxial relations, indicating that the crystallographic structure has significant influence on spin dependent transport properties.
Supported precious metal catalysts play a significant role in many chemical transformations including energy generation and control of toxic emissions. Precious metals are, however, expensive and thus efficient usage of precious metals is of critical importance for practical applications. The development of single-atom catalysts (SACs), which maximize the efficiency of costly active components of supported metal catalysts, is important for both fundamental studies and industrial applications [1][2][3]. The issue with the current SACs is that they usually consist of extremely low loading levels of metal, resulting in low specific activity and conversion rate. High loading levels of isolated single metal atoms are not stable and sinter to larger particles during catalytic reactions. Stabilization of single metal atoms by anchoring sites, for example, surface/subsurface defects, hydroxyl groups, and structural stabilizers, becomes imperative for further development of practical SACs.The high loading Pt SACs were synthesized by a modified adsorption method. Briefly, H2PtCl6 solution was used as the Pt precursor and was mixed with the pre-formed NiO nanocrystallites. The nominal loading of the Pt metal was 2.0 wt%. The resultant precipitate was filtered, washed, dried and calcined at 400°C for 5 hours (denoted as 2Pt1/NiO). The catalytic performance of the synthesized catalysts for CO oxidation was evaluated in a fixed-bed reactor. The feed gas composition was 1vol% CO + 1vol% O2 and balance He with a flow rate of 33 ml/min, and 80 mg catalyst was directly used without reduction. The outlet gas compositions were on-line analyzed by a gas chromatograph and the CO conversion was calculated based on the inlet and outlet CO concentrations. Aberration-corrected high-angle annular dark-field (AC-HAADF) microscopy was used to characterize the synthesized and used catalysts.The low magnification image of Figure 1a clearly shows that there were no Pt particles in the assynthesized 2Pt1/NiO catalyst. The atomic resolution image of Figure 1b reveals only isolated Pt single atoms uniformly dispersed onto the surfaces of NiO nanocrystallites. By analyzing many such low and high magnification images of various regions of the as-synthesized catalyst, we unambiguously concluded that the as-synthesized 2Pt1/NiO catalyst contained only isolated single Pt atoms. Figure 2 displays CO conversion versus temperature and time-on-stream for the 2Pt1/NiO catalyst. Except the initial increase of the CO conversion (Figure 2b), the 2Pt1/NiO did not deactivate at all during the 1,000 minutes test for CO oxidation reaction at 500K. Figure 1c and 1d display the AC-HAADF images of the used catalyst, which contained only individually isolated Pt single atoms. Thus we have developed a stable, high number density Pt1/NiO SAC for CO oxidation. Similar synthesis strategies will be used for developing other types of SACs for various types of catalytic reactions.
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