The availability of pure metallofullerenes in milligram quantities has allowed a number of metallofullerenes to be characterized by a variety of spectroscopic means. ['] Although there have been some studies on Sc,@C2, ,[21 the main research effort has been directed towards metallofullerenes of the type MGC,, with a single encaged metal atom.['] The first soluble dimetallofullerene, La,@C,,, was reported soon after the method of synthesizing fullerenes on the macroscopic scale was developed.r31 Most recently, Achiba et al. described the first isolation and electrochemical studies of La,@C,, .I4] This dimetallofullerene was found to be an even better electron acceptor than any metallofullerene of the type M@C,, . The first derivatization of La,@C,, has also been achieved by the same group.L5' Calculations have predicted that the Zh isomer of C,, should be able to accommodate two La atoms to form a thermodynamically and kinetically stable endohedral f~llerene.[~* The remarkable stability of La, @& comes from the fact that all six valence electrons of the two La atoms are transferred to the C,, cage; these six electrons are just enough to form a closed-shell electronic structure for the carbon cage with a large HOMO-LUMO gap. However, direct experimental confirmation for the oxidation states of the metal atoms inside the carbon cage has not yet been available.We have recently reported a technique for the efficient separation of Ce@C,, and also its spectroscopic characterization.r6. ' 1 Ce was shown to have an oxidation state of I I I .~'~ This new method also allowed separation and characterization of the new dimetallofullerene Ce,@C,,, which like Ce@C,, was detected in previous laser desorption mass spectrometry experiments. ['] We report here on the separation of Ce,@C,, and on its characterization by UV-Vis-NIR absorption spectroscopy and X-ray photoelectron spectroscopy (XPS).The high purity of Ce,@C,, isolated by HPLC can be seen from the negative-ion desorption chemical ionization (DCI) mass spectrum shown in Figure 1; a purity of >99% can be estimated. Figure 2 shows the UV-Vis-NIR absorption spectrum of Ce,@C,, in the wavelength range of 300 to 2100 nm. The spectrum exhibits a monotonically decreasing absorption coefficient with increasing wavelength without well-defined, sharp features. This relatively featureless spectrum is in contrast to those of the empty fullerenes['O1 as well as of the monometallofullerenes.[lcl Whetten et al. proposed that in La,@C,, the C,, cage has Z , symmetry, resulting in overall D,, symmetry for the dimetall~fullerene.[~~ 9 3 ' 'I This is supported by a recent theoretical calculation.[61 In this and earlier workc5] it was demonstrated that La,@C,, can be formally represented by Lai+@C&, forming a closed-shell electronic structure with a HOMO-LUMO gap larger than that of M@C,, (M = rare earth metal). As will be shown below, C,, in Ce,@C,, also acquires the six valence electrons from the two Ce atoms, presumably resulting in the same electronic configuration as that of La,@C,...
Pr@C82 and Pr2@C80 have been isolated with a purity of >99.0% by an efficient solvent extraction procedure, followed by HPLC separation. UV−VIS−NIR spectra of the metallofullerenes were measured in the wavelength range from 300 to 2100 nm. The X-ray photoelectron spectra of Pr@C82 and Pr2@C80 suggest that the Pr atoms inside the carbon cages all exist in a cationic form represented by Pr3+.
Nanoporous materials are of great interest for various technological applications including sensors based on surface-enhanced Raman scattering, catalysis, and biotechnology. Currently, tremendous efforts are dedicated to the development of porous one-dimensional materials to improve the properties of such class of materials. The main drawback of the synthesis approaches reported so far includes (i) the short length of the porous nanowires, which cannot reach the macroscopic scale, and (ii) the poor organization of the nanostructures obtained by the end of the synthesis process. In this work, we report for the first time on a two-step approach allowing creating highly ordered porous gold nanowire arrays with a length up to a few centimeters. This two-step approach consists of the growth of gold/copper alloy nanowires by magnetron cosputtering on a nanograted silicon substrate, serving as a physical template, followed by a selective dissolution of copper by an electrochemical anodic process in diluted sulfuric acid. We demonstrate that the pore size of the nanowires can be tailored between 6 and 21 nm by tuning the dealloying voltage between 0.2 and 0.4 V and the dealloying time within the range of 150-600 s. We further show that the initial gold content (11 to 26 atom %) and the diameter of the gold/copper alloy nanowires (135 to 250 nm) are two important parameters that must carefully be selected to precisely control the porosity of the material.
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