X-ray Emission Studies of the Valence Band of Nanodiamonds Annealed at Different Temperatures

Abstract

X-ray emission spectroscopy has been applied to examine the electronic structure of onion-like carbon (OLC) generated by the annealing treatment of nanodiamonds (ND). The C Kα spectra of OLC produced in the temperature range of 1600−1900 K were found to be markedly different from the spectrum of particles formed at 2140 K and to be characterized by better ordering of graphitic shells. The latter spectrum was shown to be very similar to the C Kα of polycrystalline graphite, while the former ones exhibited a significant increase of the high-energy maximum that might be caused by the defect structure of graphitic networks forming at the intermediate temperatures. The experimental spectra were compared with the theoretical spectra from quantum-chemical semiempirical AM1 calculation of several models: a fullerene molecule, C240, having icosahedral structure, a C240 molecule incorporating a greater number of nonhexagonal rings, and a holed structure formed by removing pentagons from the icosahedral molecule. The density of high-energy electronic states in the valence band of the graphitic cage was found to be practically invariant to a change in ring statistics but to significantly increase because of localization of electrons on the zigzag sites of a hole boundary.

Photoemission study of onionlike carbons produced by annealing nanodiamonds

“…The schematics summarize the results of our previous HRTEM investigations of OLC and intermediates of ND to OLC transformation. 25,26,32,43 Due to the strong aggregation of the initial particles of ND the resulting OLC ͑shown in the schematic for annealing to 1800-1900 K͒ mostly consist of aggregates of carbon onions. OLC particles are connected together by joint curved graphite layers.…”

“…This process is accompanied by an increasing intensity in the shoulder at 4.2± 0.1 eV in the valence band data, which indicates growth of the population of mixed -states. 32,38 The second peak at 7.5± 0.1 eV is related to bonding in graphite. 32,38 It should be noted that in our previous work ͑Ref.…”

“…The first peak, at 2.8± 0.1 eV, is related to bonding in graphite. 32,38,62 Progressive graphitization of the ND in samples annealed at 1420 and 1600 K, and the eventual vanishing of sp 3 carbon from the C1s photoemission spectra ͑Fig. 4͒ in the sample annealed at 1800 K results in increasing intensity and sharpness of the peak at 2.8± 0.1 eV in the valence band spectra.…”

“…30 OLC also demonstrates high selectivity and catalytic activity in the oxidative dehydrogenation of ethyl benzene to styrene. 31 The properties of OLC produced by annealing ND have been investigated by x-ray emission spectroscopy, 32 Raman spectroscopy, 33 electron energy-loss spectroscopy, 34 electron-spin resonance, 35 x-ray diffraction, 36 and ultraviolet-visible absorption spectroscopy. 37 A photoemission study of potassium intercalated polygonal hollow onions produced at 2140 K showed that these carbon onions behave as small graphite crystals and hence display bulklike rather than molecularlike behavior.…”

“…38 The x-ray emission study of OLC produced in the temperature range of 1600-1900 K revealed that their C K ␣ spectra exhibit a significant increase of the high-energy maximum and are markedly different from the spectrum of particles formed at 2140 K, which is very similar to the C K ␣ spectrum of polycrystalline graphite. 32 However, due to dipole selection rules, the x-ray emission spectra yield information on the partial density of states. Until now, the full electronic structure of OLC has not been studied.…”