Surface Composition of Carbon Nanotubes-Fe-Alumina Nanocomposite Powders: An Integral Low-Energy Electron Mössbauer Spectroscopic Study

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• 作者：Valdirene G. de Resende ; Eddy De Grave ; Alain Peigney ; Christophe Laurent
• 刊名：Journal of Physical Chemistry C
• 出版年：2008
• 出版时间：April 17, 2008
• 年：2008
• 卷：112
• 期：15
• 页码：5756 - 5761
• 全文大小：265K
• 年卷期：v.112,no.15(April 17, 2008)
• ISSN：1932-7455

文摘

The surface state of carbon nanotubes-Fe-alumina nanocomposite powders was studied by transmission andintegral low-energy electron Mössbauer spectroscopy. Several samples, prepared under reduction of the ages/gifchars/alpha.gif" BORDER=0>-Al_1.8Fe_0.2O₃ precursor in a H₂-CH₄ atmosphere applying the same heating and cooling rate and changing only themaximum temperature (800-1070 ages/entities/deg.gif">C) were investigated, demonstrating that integral low-energy electronMössbauer spectroscopy is a promising tool complementing transmission Mössbauer spectroscopy for theinvestigation of the location of the metal Fe and iron-carbide particles in the different carbon nanotube-nanocomposite systems containing iron. The nature of the iron species (Fe³⁺, Fe₃C_, ages/gifchars/alpha.gif" BORDER=0>-Fe, ages/gifchars/gamma.gif" BORDER=0 >-Fe-C) is correlatedto their location in the material. In particular, much information was derived for the powders prepared byusing a moderate reduction temperature (800, 850, and 910 ages/entities/deg.gif">C), for which the transmission and integrallow-energy electron Mössbauer spectra are markedly different. Indeed, ages/gifchars/alpha.gif" BORDER=0>-Fe and Fe₃C were not observed assurface species, while ages/gifchars/gamma.gif" BORDER=0 >-Fe-C is present at the surface and in the bulk in the same proportion independentof the temperature of preparation. This could show that most of the nanoparticles (detected as Fe₃C and/orages/gifchars/gamma.gif" BORDER=0 >-Fe-C) that contribute to the formation of carbon nanotubes are located in the outer porosity of the material,as opposed to the topmost (ca. 5 nm) surface. For the higher reduction temperatures T_r of 990 ages/entities/deg.gif">C and1070 ages/entities/deg.gif">C, all Fe and Fe-carbide particles formed during the reduction are distributed evenly in the bulk andthe surface of the matrix grains. The integral low-energy electron Mössbauer spectroscopic study of a powderoxidized in air at 600 ages/entities/deg.gif">C suggests that all Fe₃C particles oxidize to ages/gifchars/alpha.gif" BORDER=0>-Fe₂O₃, while the ages/gifchars/alpha.gif" BORDER=0>-Fe and/or ages/gifchars/gamma.gif" BORDER=0 >-Fe-Care partly transformed to Fe_1-xO and ages/gifchars/alpha.gif" BORDER=0>-Fe₂O₃, the latter phase forming a protecting layer that prevents totaloxidation.