Article pubs.acs.org/JPCC
Comparative NEXAFS, NMR, and FTIR Study of Various-Sized Nanodiamonds: As-Prepared and Fluorinated Elena M. Zagrebina,† Alexander V. Generalov,†,‡ Alexander Yu. Klyushin,† Konstantin A. Simonov,†,‡,∥ Nikolay A. Vinogradov,†,‡,∥,⊥ Marc Dubois,§ Lawrence Frezet,§ Nils Mårtensson,∥ Alexei B. Preobrajenski,†,‡ and Alexander S. Vinogradov*,† †
V.A. Fock Institute of Physics, St. Petersburg State University, 198504 St. Petersburg, Russia MAX IV laboratory, Lund University, Box 118, 22100 Lund, Sweden ∥ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden § UBP, Institut de Chimie de Clermont-Ferrand (ICCF), UMR CNRS 6296, Clermont Université, 63171 Aubière Cedex, France ‡
ABSTRACT: Various 4−50 nm in size diamond nanoparticles prepared by different synthesis methods and their fluorinated derivatives were studied by NEXAFS, solid state NMR and FTIR spectroscopy. C 1s and F 1s NEXAFS spectra of as-prepared and fluorinated nanodiamonds (NDs and F-NDs) were analyzed based on a comparison with the known ones of reference compounds (graphitized carbon nanodiscs, phenol and amino acid molecules, graphite oxide and monofluoride). It has been found that all the studied diamond nanoparticles have crystalline diamond cores and their surfaces are covered with graphite-like carbon clusters. These clusters are partially amorphized and oxidized with the formation of functional groups C−OH, CO, or OC−OH and the properties of these surface shells depend on the synthesis method of nanodiamonds. The fluorination of diamond nanoparticles has a purely superficial character; it almost completely cleans the NDs particles from carbon clusters and saturates dangling bonds on the surface of the diamond nanoparticles with F atoms forming covalent σ(C−F) bonds. NEXAFS data are further supported by NMR and FTIR spectroscopy, leading to similar conclusions concerning the properties of various NDs and the chemical bonding between C and F atoms in F-NDs. A combination of NEXAFS, solid state NMR and FTIR spectroscopy is demonstrated to be very efficient in investigating various NDs and their functionalized derivatives.
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INTRODUCTION Much attention is currently given to crystalline diamond nanoparticles (nanodiamonds, NDs) owing to their unique physical and chemical properties that provide various potential applications in luminescence biomedical imaging, drug delivery, tribology, quantum engineering, surface coatings etc.1,2 Nanodiamonds are commonly synthesized as powders using the graphite-diamond phase transition at high temperature and pressure (by detonation of a mixture of strong organic explosives with a negative oxygen balance)3−12 or as films using chemical formation with plasma assisted chemical vapor deposition (CVD) techniques.13,14 Other techniques for the synthesis of NDs are laser ablation,15,16 high-energy ball milling of high-pressure high-temperature (HPHT) diamond microcrystals,17,18 shock compression (SC) of different carbon precursors in a metallic matrix,19,20 autoclave synthesis from supercritical fluids21 etc. The nanoparticles obtained have a stable crystalline diamond core and therefore their physical and chemical properties are mainly governed by the characteristics of the atomic and electronic structure of the surfaces. In turn, the latter depends on the synthesis method of the NDs and on the structure of various carbon groups, which are formed on the surface in the growth processes and which stabilize the nanoparticles by terminating the surface dangling bonds.1 © 2014 American Chemical Society
Further modification of the NDs surface can be accomplished by removing these original groups and attaching other functional groups. This grafting (functionalization) provides new properties of considerable promise for various applications. In the context of this surface chemistry, pre-halogenated nanoparticles, especially fluorinated NDs (F-NDs), are considered to be a suitable starting material for the grafting of more complex polyatomic species.2 Therefore, a detailed characterization of the surface properties by various experimental techniques is required at all stages of the ND functionalization process. Raman spectroscopy is commonly used for obtaining information on the structure, composition and homogeneity of materials as well as information on the structure of surface groups.22−24 Fourier transform infrared (FTIR) spectroscopy allows to detect functional groups and adsorbed molecules on the surface as well as changes in the surface chemistry of functionalized nanodiamonds.25,26 Surface functional groups are often studied using methods of nuclear magnetic resonance (NMR).27−29 Received: October 22, 2014 Revised: December 8, 2014 Published: December 8, 2014 835
DOI: 10.1021/jp510618s J. Phys. Chem. C 2015, 119, 835−844
Article
The Journal of Physical Chemistry C Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is an effective technique for obtaining data on electronic structure, symmetry and energy distribution of unoccupied electron states in carbon-containing substances as well as on the coordination and the chemical state of carbon atoms.30 Previously, C 1s NEXAFS spectra were measured in particular to characterize the growth processes of diamond nanocrystalline films13,31−34 and the different purification treatments of detonation NDs,35 in order to estimate the fraction of graphitized carbon shells of diamond nanoparticles,35−37 to study quantum confinement effects in NDs38,39 and to study the σ exciton in the C 1s absorption spectra of nanodiamonds.13,39 However, no NEXAFS studies of F-NDs are reported so far to the best of our knowledge. On the other hand, these studies are necessary to get deeper insight into the character of the chemical interaction between the surface carbon and fluorine atoms and the electronic structure of fluorinated diamond nanoparticles. The present paper reports findings of the comparative study of the as-prepared NDs and their fluorinated derivatives (FNDs) by a combination of modern experimental techniques NEXAFS, NMR, and FTIR spectroscopies. At first, C 1s NEXAFS spectra of diamond nanoparticles, synthesized by different methods and having various sizes, were analyzed by comparing with corresponding spectra from reference compounds with sp2 and sp3 hybridized carbon atoms. Such compounds are graphitized carbon nanodiscs (graph-CNDs), crystalline diamond as well as compounds containing carbon− oxygen functional groups such as phenol and amino acid molecules and graphite oxide. The changes in the C 1s absorption spectra in going from NDs to F-NDs and F 1s NEXAFS of F-NDs are discussed in comparison with C 1s and F 1s absorption spectra of graphite monofluoride (CF)n. The results of this NEXAFS study are further discussed along with data of NMR and FTIR measurements in order to characterize in detail the atomic and electronic structure of NDs and F-NDs, in order to reveal the NDs fluorination mechanism and for determining the type of chemical bonding between carbon and fluorine atoms in F-NDs.
For preparing fluorinated derivatives the as-prepared ND particles were fluorinated using a flux of pure F2 (1 atm) at 500 °C for 12 h. It should be noted that the weight change was very small during the treatment, less than 2% for all samples except for F-ND50. This fact is indicative of the stability of the NDs in the harsh conditions of the fluorination processes. However, it is expected that such treatment can be accompanied by chemical interaction between C atoms in amorphous shells at the surface of the ND nanoparticles and F atoms. In this study, graphitized carbon nanodiscs (graph-CNDs) and graphite monofluoride (CF)n were used as reference materials.41 The CNDs were produced by pyrolysis of heavy oil using the Kvaerner carbon black and hydrogen process (CBH) at 2000 °C plasma temperature.42 The CNDs in the present work were used after a consecutive graphitization in argon at the 2700 °C (graph-CNDs). The graphite fluoride of (CF)n structural type was obtained by fluorination of graphite using molecular fluorine at 600 °C for 3 h.41 All NEXAFS measurements were carried out using monochromatic synchrotron radiation at the D1011 beamline at the MAX II electron storage ring (the MAX IV laboratory, Lund University, Sweden).43 This beamline is based on a modified Zeiss SX-700 plane grating type monochromator with a 1200 mm−1 groove density grating which covers a range of photon energies from 30 to 1500 eV with high energy resolution. The samples for absorption measurements were prepared ex situ in air by rubbing powders of NDs, F-NDs and reference compounds into the scratched surface of copper plates 5 × 5 mm2 in size in order to ensure uniform surface coatings without noticeable gaps. The substrate surfaces were cleaned mechanically and by sonication in ethanol. The samples of NDs and F-NDs were annealed in UHV at 250 °C for 30 min before measurements to eliminate any possible influence of adsorbed carbon contaminations on the C 1s absorption structures. All measurements of absorption spectra were performed under ultrahigh vacuum conditions with a residual gas pressure in the experimental chamber ∼10−9 mbar. The samples were placed at an angle of ∼45° with respect to the incident beam of the monochromatic radiation and the size of the focal spot on the sample was around 1 × 1 mm2. The NEXAFS spectra at the C 1s and F 1s thresholds of the NDs and F-NDs were measured by recording the total and partial electron yield (TEY and PEY) as a function of incident photon energy. It is well-known that the electron yield near the X-ray absorption edge is proportional to the absorption cross section.44,45 The TEY spectra were acquired by measuring the drain current from the sample, while the PEY spectra were recorded by a multiple-channel plate detector with a retarding voltage of Vret = −150 V. In the case of ND and F-ND spectra, the TEY and PEY techniques for measuring X-ray absorption are characterized by probing depths of >10 nm and 0.5−0.8 nm, respectively.30 All high kinetic energy Auger electrons, slow secondary electrons and photoelectrons are detected when measuring TEY spectra. The large probing depth is due to the large mean free path of the slow secondary electrons. Using a retarding potential, the slowest photoelectrons and secondary electrons are cut off. The signal is mainly due to Auger electrons, giving a probing depth of
