Structure and Bonding in Fluorinated Nanodiamond - American

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Structure and Bonding in Fluorinated Nanodiamond Alexander M. Panich,*,† Hans-Martin Vieth,‡ Alexander I. Shames,† Natalya Froumin,§ ˆ sawa,| and Akifumi Yao⊥ Eiji O Department of Physics, Ben-Gurion UniVersity of the NegeV, P.O. Box 653, Be’er SheVa 84105, Israel, Institute for Experimental Physics, Free UniVersity Berlin, Arnimallee 14, D-14195 Berlin, Germany, Department of Materials Engineering, Ben-Gurion UniVersity of the NegeV, P.O. Box 653, Be’er SheVa 84105, Israel, NanoCarbon Research Institute, Ltd., AREC, Shinshu UniVerstiy, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan, and Chemical Research Center, Central Glass Co., 5253, Okiube, Ube, Yamaguchi 755-0001, Japan ReceiVed: August 14, 2009; ReVised Manuscript ReceiVed: NoVember 18, 2009

We report on a combined investigation of the structure and chemical bonding in fluorinated detonation nanodiamond by means of nuclear magnetic resonance, electron paramagnetic resonance, X-ray photoelectron spectroscopy, and Raman measurements. The results of these methods are found to be consistent with each other and evidence formation of different fluorocarbon groups on the nanodiamond surface, which substitutes for hydrocarbon and hydroxyl groups. The data obtained provide detailed information about the structure and bonding in the fluorinated diamond nanoparticle. The fluorinated sample has a significant number of paramagnetic defects (∼1020 spin/g) located mainly near the surface of the diamond nanoparticle, resulting in fast 19F and 13C nuclear spin-lattice relaxation. 1. Introduction Diamond nanoparticles represent one of several classes of carbon nanostructures that have risen to the forefront of materials research over the last two decades due to great potential for a variety of materials applications.1–4 Among several means of fabrication, the detonation technique accompanied by subsequent purification is the most important from a technological point of view because it allows production of the diamond nanoparticles of ∼4-5 nm in size in bulk quantities. The resulting product is called detonation nanodiamond (DND). The DND particle consists of a mechanically stable and chemically inert diamond core and a chemically active surface with controllable properties. Calculations5–7 and nuclear magnetic resonance (NMR),8–14 electron paramagnetic resonance (EPR),8,12,13 high-resolution transmission electron microscopy (HRTEM),8,15,16 and X-ray photoelectron spectroscopy (XPS)13 studies show that the surface of diamond nanoparticles may have rather complicated structures that can include an sp2-like coating, a reconstructed surface (e.g., dimerization of an (001) surface), and hydrogen termination with creation of hydrocarbon groups. On-purpose functionalization of the DND surface with targeted species allows preparation of DNDs with specified chemical, physical, and electronic properties. For example, fluorinated DND (F-DND) is a valuable commercial product as its solubility in some polar organic solvents is much higher than that for pristine DND (P-DND).17 Subsequent derivatization of F-DNDs with different functional groups (alkyl-, amino-, and amino acid nanodiamond derivatives) expands engineering and biomedical applications of DNDs.17 F-DND has been shown to be a wet chemistry precursor for coating glass surfaces with * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +972-8-6472458. Fax: +972-8-6472903. † Department of Physics, Ben-Gurion University of the Negev. ‡ Free University Berlin. § Department of Materials Engineering, Ben-Gurion University of the Negev. | Shinshu Universtiy. ⊥ Chemical Research Center, Central Glass Co.

covalently attached diamond particles18 to be eventually used in optical and biosensor applications. Fluorinated diamond films showed an improvement in frictional properties and a reduction of the surface energy.19 The formation of C-F bonds reduces the dielectric constant (), implying that F-DNDs may be useful for applications in low- composites in microelectronics.20 A recent magnetic susceptibility study of fluorinated and thereafter defluorinated nanodiamonds21 revealed enlarged concentration of dangling bond spins on the DND surface. In the present paper, we report our NMR, EPR, XPS, and Raman investigations of fluorinated nanodiamond. 2. Experimental Section The fluorination procedure used in our work is analogous to that described previously.17 The fluorination was carried out by introducing fluorine and hydrogen gases in a flow rate of 3:1 over preheated commercial nanodiamond powder agglutinates at temperatures in the range 150 to 470 °C in a monel reactor for 48 h. This simple reaction is actually a very powerful one, leading to a fluorine content of up to 8.6%, a much reduced size of agglutinates to tens of nanometers, and nearly complete disappearance of hydrocarbon and OH surface groups. Static 13C NMR spectra, spin-lattice (T1), and spin-spin (T2) relaxation times have been measured in an applied magnetic field B0 ) 8.0196 T at a resonance frequency of 85.857 MHz. The π/2 pulse length was 3.9 µs. The 13C magic angle spinning (MAS) spectrum was measured at a frequency of 150.858 MHz (B0 ) 14.0954 T) with a spinning rate 12 kHz, a π/2 pulse of 1.9 µs, and a recycle time of 2 s. 13C chemical shifts δ are given relative to external tetramethylsilane (TMS). Specifically, the spectrum was externally referenced to the adamantane lines at chemical shifts of 39.0 and 29.9 ppm relative to TMS. The spin-lattice relaxation time T1 was measured using a saturationcomb sequence. The spin-spin relaxation time T2 was measured using the 2D set of nuclear spin echo signals obtained by Hahn echo sequence with a variable delay between pulses. T2 was determined using a plot of the decaying echo amplitude as the point at which this amplitude falls to 1/e of its initial value.

10.1021/jp9078629  2010 American Chemical Society Published on Web 12/28/2009

Structure and Bonding in Fluorinated Nanodiamond

Figure 1. Static 13C NMR spectrum of fluorinated nanodiamond in B0 ) 8.0196 T. Deconvolution into two signals is shown by dashed lines.

The static 19F NMR spectra, as well as the spin-lattice and spin-spin relaxation time measurements on this nucleus, were carried out at resonance frequencies 28.744 MHz (in an applied magnetic field of 0.717 T) and 321.189 MHz (in an applied magnetic field of 8.0196 T). At the low frequency, the 19F spectra were measured using Fourier transformation of the phase-cycled solid echo. At the high frequency, they were measured using Fourier transformation of the phase-cycled Hahn echo. The π/2 pulse lengths were 1.3 and 3.4 µs at 28.744 and 321.189 MHz, respectively. The spin-lattice relaxation time T1 was measured using a saturation-comb sequence. The spin-spin relaxation time T2 was measured using the solidecho sequence at 28.744 MHz and both the Hahn and solidecho sequences at 321.189 MHz. 19 F MAS NMR spectra were measured at 282.353 MHz (B0 ) 7.0492 T) using Fourier transformation of the free induction decays at spinning rates of 10, 16, and 27 kHz. 19F chemical shifts are given relative to CFCl3 as it is accepted in the literature. In our measurements, we used liquid CF3COOH (δ ) -77.9 ppm relative to CFCl3) and C6H5F (δ ) -110.6 ppm relative to CFCl3) as external secondary references. The static 1H NMR spectrum has been measured in an applied magnetic field of 8.0196 T at a resonance frequency of 341.406 MHz. XPS spectra were measured using ESCALAB 250 spectrometer with Al X-ray source and monochromator. Calibration was performed according to the position of the Au 4f7 line. General survey and high-resolution spectra of elements were recorded. EPR spectra of F-DND powder were recorded using a Bruker EMX-220 X-band (ν ≈ 9.4 GHz) spectrometer equipped with an Oxford Instruments ESR900 cryostat and an Agilent 53150A frequency counter. Processing of the EPR spectra was done using Bruker WIN-EPR and OriginLab software. The density of the paramagnetic centers observed in the present experiments was estimated by comparison of doubly integrated intensities (DIN) of the g ) 2.0 signals with the intensity of the radicallike signal in a well purified nanodiamond sample with a known amount of paramagnetic defects, Ns ) (6.3 ( 0.5) × 1019 spins/ g.22 Raman spectra were recorded with instruments containing lasers having primary emissions at λ ) 514 and 633 nanometers. Most of the measurements have been carried out at room temperature (RT), except for the 19F spin-lattice relaxation study, which was done in the temperature range 120-300 K, and the EPR study, which was carried out between 4 and 300 K.

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Figure 2. 13C MAS spectrum of F-DND in B0 ) 14.0954 T. Deconvolution is shown by dashed lines.

3. Experimental Results and Discussion 3.1. 13C NMR Spectra. The static 13C NMR spectrum of the fluorinated detonation nanodiamond (Figure 1) is well deconvoluted into an intensive narrow signal having a chemical shift δ(13C) ) 35 ( 1 ppm and a weak broader line at 53 ( 1 ppm. The chemical shift of the narrow signal is characteristic of bulk diamond23–25 and high-quality diamond film;26 thus this signal is definitely attributed to the sp3 carbons belonging to the nanodiamond core. The broader signal is assigned to the fluorinated DND surface. It is likely a superposition of several overlapping signals that are unresolved due to the 13C chemical shielding anisotropy and 13C-19F dipole-dipole coupling in the surface fluoro-carbon groups. The 13C MAS spectrum (Figure 2) reveals at least two wellseparated lines. Furthermore, the intense peak around δ ) 36 ppm is asymmetric and could, in principle, be deconvoluted into three components shown by dashed lines. The narrow line having a chemical shift of δ ) 36.1 ( 0.3 ppm and a line width of ∆ν ) 3.0 ( 0.5 ppm, which is characteristic of bulk diamond,23–25 is definitely attributed to the sp3 carbons of the diamond core. (Here and later on the line widths were determined from a Gaussian fit). The broader line with the similar chemical shift δ ) 37.2 ( 0.3 ppm and line width ∆ν ) 11 ppm ( 0.5 is attributed to somewhat nonequivalent carbon atoms of the diamond core; this inequivalency may result from some distortion of the tetrahedral sp3 coordination. The origin of the broad line showing δ ) 46.7 ( 0.3 ppm and line width ∆ν ) 23 ( 0.5 ppm is unclear. In accordance with the works of Panich et al.,27–31 Krawietz and Haw,32 Giraudet et al.,33,34 Dubois et al.,35 and Hagaman et al.36 who extensively studied the fluorine-graphite intercalation compounds and graphite fluorides; the line at δ(13C) ) 89.0 ( 0.3 ppm is assigned to carbon atoms involved into the C-F bonds. Such a line at 88-89 ppm is usually observed in purely covalent graphite monofluoride (CF)n and is assigned to covalently bound C-F groups.27,29–31,33,35 In partially fluorinated graphite such as (C2.5F)n, (C2F)n, and partially fluorinated charcoal this line is shifted to ∼82-86 ppm.34,36 3.2. 13C Spin-Spin and Spin-Lattice Relaxation. The 13C spin-spin relaxation time (T2) measurement of the F-DND sample shows that the semilogarithmic plot of the echo amplitude decay consists of three linear segments (Figure 3). Thus, the echo decay may be well fitted by a superposition of three exponentials (Figure 3, inset)

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J. Phys. Chem. C, Vol. 114, No. 2, 2010

( )

M(t) ) M1(0)exp -

Panich et al.

( ) ( )

t t + M2(0)exp + T21 T22 M3(0)exp -

t T23

(1)

This fact is an indication of existence of three types of carbons with the values of T21 ) 168 ( 18, T22 ) 921 ( 98, and T23 ) 2518 ( 306 µs. To determine the nuclear spin-lattice relaxation time for carbons corresponding to the diamond core, we measured T1(13C) at the peak of 35 ppm. This measurement shows that the magnetization recovery is well described by a stretched exponential

{

[ ( ) ]}

M(t) ) M∞ 1 - exp -

t T1

R

(2)

where M∞ is the equilibrium magnetization. This is clearly seen in Figure 4 that shows the variation of the magnetization as a function of recovery time to the power of 0.66 ( 0.01. The obtained value of T1 is 168 ( 5 ms. The anomalous reduction in the 13C spin-lattice relaxation time from several hours in natural diamond24,37–39 to 168 ms in F-DND, and the stretched exponential character of the magnetization recovery are attributed to the interaction of nuclear spins with paramagnetic centers observed by us in all nanodiamond samples8,9,12,13 as well as in the F-DND sample under study (section 3.5). We note that the stretched exponential relaxation is a widely used approach that fits many relaxation processes in systems with a number of different relaxation environments. Here the stretched exponential relaxation appears due to a distribution of the

Figure 3. Decay of 13C Hahn echo amplitude (T2 measurements) in a semilogarithmic scale in the fluorinated nanodiamond. Inset shows echo decay in a linear scale; here dashed and solid lines show single- and triexponential fits, respectively.

Figure 4. 13C magnetization recovery of the diamond core carbons (T1 measurements) in a semilogarithmic scale.

Figure 5. Static 19F NMR spectrum of the fluorinated nanodiamond in B0 ) 0.717 T. The Gaussian fit is shown by a dashed line.

nuclear spin-lattice relaxation times in a system of nuclear spins fixed at different distances from the paramagnetic centers. The theory of such relaxation40–42 shows that the parameter R ) 1 is typical for the case of rapid nuclear spin diffusion, while in the regime of vanishing spin-diffusion its value is between 0.5 and 1.41,42 Spin diffusion is driven by the dipole-dipole interaction causing mutual flips of adjacent nuclear spins. We have shown9 that it is unlikely in the case of 13C nuclear spins in nanodiamonds due to low natural abundance (1.1%) of the 13 C isotope (the only carbon isotope having nuclear spin) that makes the compound a dilute system with respect to nuclear spins. Theory yields Rtheor ) 0.66 for a nonuniform distribution of the paramagnetic centers. The value obtained from the experiment is practically in agreement with this theoretical value. Let us also compare the relaxation times of the diamond core carbons in nonfluorinated and fluorinated DND samples. In the former compounds, T1 varies from 377 to 455 ms,9 while the latter shows twice shorter T1 ) 168 ms. This finding correlates well with the twice larger amount of paramagnetic centers in fluorinated DND compared with that in nonfluorinated DND (see section 3.5), readily supporting the spin-lattice relaxation mechanism via paramagnetic centers. We note that the surface carbons also show a stretched exponential (R ≈ 0.66) but much faster (in comparison with the carbons of the diamond core) spin-lattice relaxation with T1 ≈ 17 ms. This finding supports our structural model of diamond nanoparticle9 in which paramagnetic centers (broken bonds) are mainly located close to the DND surface and therefore the surface carbons experience stronger electron-nuclear interaction that causes faster spin-lattice relaxation. This is also supported by 19F NMR data described in the next section. 3.3. 19F NMR Spectra. The static low field (B0 ) 0.717 T) 19 F NMR spectrum is shown in Figure 5. It is represented by a nearly Gaussian line with a second moment S2 ) 178 ( 9 kHz2. Since the 19F chemical shielding anisotropy of fluorine involved into C-F bonds is usually around 150 ppm,31 the contribution of the chemical shielding anisotropy to the second moment43 S2 ) 4(σ// - σ⊥)2/45 at the resonance frequency 28.744 MHz is estimated as ∼1.7 kHz2, which is much smaller than the experimental value. The room temperature contribution of the coupling of the fluorine spins with paramagnetic centers to the 19F second moment in our system in the low magnetic field is negligible. Thus the obtained broad spectrum reflects strong dipole-dipole interactions among 19F spins indicating clustering of the fluorine atoms. Therefore one can assume that the fluorinated spots of limited sizes alternate with nearly nonfluorinated zones on the surface of the sample under study. The existence of the strong dipole-dipole interactions among fluorine spins is also supported by the very short 19F spin-spin


Figure 6. Static19F NMR spectra of the fluorinated nanodiamond in B0 ) 8.0196 T received by Fourier transform of free induction decay (FID) and spin echo. Inset shows 1H and 19F signals for comparison.

Figure 7. 19F MAS NMR spectra of F-DND sample measured in B0 ) 7.0492 T at spinning rates 10, 16, and 27 kHz. Sidebands are indicated by stars. Deconvolution of the spectrum at 27 kHz MAS is shown by dashed lines.

relaxation time, T2 ) 32 ( 1 µs. We note that Scruggs and Gleason44 reported that some fluorine atoms in the fluorinated bulk powder diamond occur in aggregates of greater than 40 nuclei, which is similar to our finding. Such a clustering was also observed in surface-fluorinated graphite.45 Static high field (B0 ) 8.0196 T) 19F NMR spectra of the compound under study are shown in Figure 6. The spectra seem to consist of several overlapping broad resonances. We note that the 1H measurements of nonfluorinated nanodiamond usually show an intense proton signal originating from the surface hydrocarbon groups.9,13,14 One can find from the inset in Figure 6 that the fluorinated nanodiamond reveals a weak residual proton signal which however is dramatically reduced, as compared with the signal observed for nonfluorinated DND samples, and is much smaller than the 19F signal. The ratio of intensities of 19F and residual 1H signals is around 15. This finding leads to the conclusion that the hydrocarbon and hydroxyl groups are effectively removed from the DND surface in the process of fluorination and are substituted by the chemically active fluorine atoms, resulting in the fluorocarbon groups in the F-DND sample. 19 F MAS NMR spectra, measured at 7.04 T using the spinning rates of 10, 16, and 27 kHz, are shown in Figure 7. The first spectrum shows sidebands around -85, -125, -195, and -230 ppm, and some of them overlap with the 19F NMR lines, such as the line around -119 ppm is a superposition of center band and sideband. However, the sidebands of the spectrum measured with the spinning rate of 27 kHz are at -65 and -255 ppm,


Figure 8. Temperature dependence of the spin-lattice relaxation time T1(19F) in different magnetic fields.

making this spectrum to be more informative. Deconvolution of this spectrum exhibits an intensive line with chemical shift δ ) -162 ( 1 that overlaps with the weaker line at δ ) -181 ( 2 ppm and a weak signal at -120 ( 0.5 ppm. Following a well-established assignment of 19F spectral components in fluorinated graphite and fluorine-graphite intercalation compounds (GICs),27–36 the line at δ ) -120 ppm is attributed to CF2 groups, while the lines with the peaks around -162 and -181 ppm are assigned to C-F groups. The two latter 19F resonances reveal two nonequivalent fluorine atoms with different degree of covalence of the C-F bond and somewhat different hybridization.31 The line at δ ≈ -181 ppm is usually observed in completely fluorinated graphite, i.e., in carbon monofluoride (CF)n and is attributed to C-F bonds in which the carbon atom is also bound to three other carbon atoms, making a tetrahedral-like sp3 configuration. The low field shift δ ) -161 ppm indicates a more covalent chemical bond31 that is characteristic of (CxF)n compounds with 13 < x < 1.31,36 One can explain this result in the following manner: It was shown31,36 that increasing fluorination of graphite and charcoal reduces the covalent character of the C-F bond, and owing to reduced paramagnetic contribution to the shielding, the 19F resonance shifts upfield. Panich has shown31 that the C-F-groups adjacent to sp2 carbons in nonsaturated fluorinated graphite and fluorineGICs exhibit a downfield 19F chemical shift, while fully fluorinated and saturated compound (CF)n with sp3 hybridization exhibits a high-field chemical shift. Therefore one can speculate that the line at -181 ppm belongs to the C-F groups arising from fluorination of the uncovered spots of the diamond core that results in sp3 hybridization, while the signal at -161 ppm comes from fluorocarbon groups with a distorted of sp3 configuration. The above findings are in good agreement with the FTIR data of fluorinated nanodiamond prepared by cold plasma functionalization of the nanodiamond particles,20,46 which revealed the existence of CsF, CF2, and some CF3 surface groups. Recent FTIR and magnetic susceptibility studies of fluorinated nanodiamond samples by Baidakova et al.21,47 revealed that fluorine atoms can react with carbon atoms not only on the surface of the DND core but also in the surface regions partially coated by a sp2-hybridized shell and that the fluorination creates localized spins in the shell through the formation of incompletely fluorinated graphitic structures. These data support well our findings on formation of fluorocarbon groups and increase in the number of localized electron spins under fluorination. 3.4. 19F Spin-Lattice and Spin-Spin Relaxation. The 19F spin-lattice relaxation time in the compound under study

778


Panich et al.

(Figure 8) is very short; at that, the magnetization recovery is well described by a stretched exponential (eq 2) with R ≈ 0.86. These features are characteristic of a strong interaction of 19F nuclear spins with localized unpaired electrons detected by EPR (see section 3.5). The spin-lattice relaxation measurements (Figure 8) show a very weak temperature dependence of T1(19F), indicating no intensive motion of molecular groups. However, T1 values measured in magnetic fields B0 ) 8.0196 and 0.717 T are strikingly different, and one can find that T1 increases with increasing magnetic field (from 8.9 ( 0.1 ms to 39.6 ( 0.1 ms at room temperature). It is known that, in the absence of molecular motion, the only mechanism that causes increase in T1 with increasing magnetic field is the heat contact with the lattice via localized electronic states (e.g., paramagnetic centers).48,49 This mechanism yields either T1 ≈ B02 or T1 ≈ B01/2 in the cases of rapid spin diffusion and diffusion-limited relaxation, respectively.48,49 Thus the observed field dependence of the 19F spin-lattice relaxation rate indicates an occurrence of the spin-diffusion-assisted relaxation regime, which is realized through the mutual flips between neighboring nuclear spins due to dipole-dipole interaction terms of I+ i Ij type and results in 50 the magnetization transfer from the distant nuclear spins to the localized electron spins. This is characteristic of the 19F isotope having 100% natural abundance, which makes the compound to be a magnetically concentrated system with respect to 19F nuclear spins, and is also an indication that the fluorine nuclei are clustered as shown above in section 3.3. The aforementioned fact that the decay is a stretched exponential is also a hallmark of this sort of relaxation. Our data allow us to estimate the diffusion coefficient D, using the well-known formula48,49,51

a2 D≈ × S 30 √ 2

(3)

where a is the distance between nuclei and S2(19F) ) 178 kHz2 (in B0 ) 0.717 T) is the second moment of the 19F spectrum. One can roughly estimate the distance a as the sum of the van der Waals radii of fluorine atoms (rvdW ) 1.47 Å52), such as a ) 2.94 Å, and calculate D ≈ 4 × 10-13 cm2/s, that is a characteristic value for nuclear dipole-dipole interactions. The existence of the strong dipole-dipole interactions among fluorine spins causing spin diffusion is also supported by the very short 19F spin-spin relaxation time, T2 ) 31.6 ( 4 µs and 30 ( 1 µs measured by the solid-echo sequence53 in the applied magnetic fields B0 ) 0.717 and 8.0196 T, respectively. We note that high magnetic field allows also formation of the Hahn echo,54 which exhibits a decay with T2 ) 10.5 ( 0.1 µs. The obtained difference in the T2 values measured by Hahn and solid echo sequences is not surprising and is explained in the following way: In a low magnetic field (B0 ) 0.717 T), the dipole-dipole interaction considerably exceeds the chemical shielding anisotropy; thus only a solid echo is formed. However, in B0 ) 8.0196 T both a Hahn and solid echo are formed, reflecting the comparable magnitudes of the interactions. The Hahn echo is formed by interactions that are linear in the spin operators of the resonant nucleus, and its decay is caused by bilinear interactions, in the case in question by dipole-dipole coupling between the resonant spins. At the same time, the solid echo is formed by dipolar coupling among the spins, and its decay is caused by a modified (by rf pulse) Hamiltonian for the dipole-dipole interaction.53,55 Thus different mechanisms of the formation

Figure 9. (a) Normalized EPR spectra at fixed ν ) 9.467 GHz: black line, P-DND at T ) 300 K; green line, F-DND at T ) 300 K; red line, F-DND at T ) 4 K. Inset: zoom of the EPR lines. (b) Various deconvolutions of the EPR spectrum of F-DND at T ) 4 K: black open circles, experimental spectrum. Best least-squares fittings: red line, by sum of two Lorentzians; green line, by a single Lorentzian; magenta line, by sum of two Gaussians; blue line, by a single Gaussian. Inset: zoom of high field wing of this EPR signal. (c) Deconvolution of the EPR spectrum of F-DND at T ) 4 K: black open circles, experimental spectrum; red line, fitting by sum of two Lorentzian lines; green line, narrow Lorentzian line; blue line, broad Lorentzian line.

and decay of the Hahn and solid echoes result in a difference in the T2 values obtained in the experiment. 3.5. EPR Spectra. The RT EPR spectrum of F-DND (Figure 9a, green line) shows an intense singlet signal due to unpaired electrons located on dangling bonds in the carbon subsystem, usually observed in well-purified P-DND (black solid line).8,22 The sample demonstrates also weak signals within the halffield region (g ≈ 4, spectrum not shown) due to both residual Fe3+ paramagnetic and/or ferromagnetic impurities and exchangecoupled carbon inherited defects in the triplet state.22 The latter indicates quite high purity of these samples. Comparison of RT EPR spectra of the pristine and fluorinated DND samples shows that fluorination practically does not change the basic parameters of the singlet EPR line factor (g ) 2.0027 ( 0.0002) and line shape (Lorentzian-like) except for the line broadening: ∆Hpp ) 0.76 ( 0.01 mT for P-DND vs ∆Hpp ) 0.92 ( 0.01 mT for F-DND; see inset in Figure 9a. The F-DND sample also demonstrates some shortening of the electron spin-lattice relaxation time. The most evident difference in magnetic properties of the F-DND sample is the significant increase of the electron spin density Ns ) (1.23 (

Structure and Bonding in Fluorinated Nanodiamond 0.16) × 1020 spin/g, which is twice as large as than that observed for the P-DND sample, Ns ) (6.3 ( 0.5) × 1019 spin/g. These data correlate well with the shortening of the nuclear spin-lattice relaxation time in F-DND (T1 ) 168 ms) compared with that in P-DND sample (∼455 ms 9). Moreover, this finding is in a good agreement with data the magnetic susceptibility study reported by Baidakova et al.:21 the magnetic susceptibility study of the DND fluorinated at 500 °C and thereafter partially defluorinated nanodiamonds reveals the same double increase in concentration of unpaired spins on the surface of completely fluorinated DND (∼1.2 × 1020 spin/g) compared with the nontreated DND sample (∼0.6 × 1020 spin/g). Such a perfect agreement between results obtained by different techniques points out the specific role of fluorine atoms in breaking the C-C covalent bonds of diamond lattice and creation of new stable long-living dangling bonds. It was recently found that the singlet Lorentzian-like signal, observed in thin ultrananocrystalline diamond films at RT, may be successfully decomposed into two Lorentzian signals (narrow and broad) having very similar (within the experimental errors) g factor values.56 There the broad signal was attributed to dangling bonds inside the nanocrystalline grains of the diamond film and the narrow one to graphite inherited paramagnetic defects in the grain boundary regions. We applied a similar twocomponent model to simulating the singlet Lorentzian-like EPR signals observed in both P-DND and F-DND samples. Figure 9b demonstrates attempts of fitting the experimental line (both the line shape and all experimentally obtained EPR parameters) by various single and double Lorentzians or Gaussians. It is obvious that just the double Lorentzian model fits the experimental data with the R2 value nearly approaching unity. Especially the best fit result is clearly seen in the line wings (see inset in Figure 9b). Finally, all experimental EPR signals (first derivative of the EPR absorption signals) were fitted using the following equation


Y'(H) ∝

∑

i)narrow,broad

Ci

{[

(

)

H - Hri ∆Hppi + H - Hri 2 2 3+4 ∆Hppi H + Hri ∆Hppi H + Hri 2 3+4 ∆Hppi

(

(

[ (

)]

)

)]

2

}

(4)

where Hri is the resonance field of the corresponding (broad or narrow) signal and ∆Hppi is its line width. The use of the two terms in eq 4 accounting for the clockwise as well as the anticlockwise circularly polarized component of the microwave radiation is necessary for the precise simulation of the broad Lorentzian lines.57 Figure 9c demonstrates the results of the deconvolution of the singlet EPR line of the F-DND sample recorded at T ) 4 K (black solid line) into the sum of two Lorentzian lines (red open circles) with the same resonance field (g factor), narrow (green solid line) and broad (blue solid line) ones. It is worth mentioning that variations of resonance field values Hri within ( 0.06 mT (that corresponds to the experimental error of g factor determination) insignificantly affect the broad line parameters obtained by the fitting. The same fitting procedure was used for the deconvolution of EPR spectra of both P-DND and F-DND samples recorded within the wide range of temperatures (4 - 300 K). In all cases the two-line approach describes the experimental spectra quite satisfactorily. What could be possible sources of these two kinds of intrinsic paramagnetic centers? Simulation shows that both of them have the same g factor, and correspondingly, most probably are of the same origin.8 The differences are just in the line widths. It has been stated that in P-DND samples the width of the EPR signal of carbon-inherited defects somehow reflects electronnuclear hyperfine interactions with neighboring atoms and

Figure 10. Temperature dependences of EPR line parameters. (a) F-DND sample, doubly integrated intensities DIN normalized to the DIN value at T ) 300 K: green circles, DIN obtained by the numerical integration of the EPR lines; red circles, DIN for the narrow line, obtained by simulation; black open stars, DIN for the broad line, obtained by simulation. (b) Normalized values of DIN-1, obtained by the numerical integration: green circles, F-DND sample; blue open triangles, P-DND sample. Lines represent linear fits to the C-W law. (c) Line widths for P-DND sample. (d) Line widths for F-DND sample.

780


groups.8 Since the Lorentzian line shape indicates quite a strong exchange interaction between these paramagnetic centers, the observed width of the singlet lines depends on both hyperfine structure (which is unresolved here) and the exchange interaction that averages this structure, resulting in a singlet Lorentzian line. Thus it may be supposed that these defects, located closer to the DND surface, demonstrate broader singlet lines due to interplay between more developed hyperfine structure (since they are closer to fluorine, 19F, I ) 1/2, natural abundance 100%, other surface groups and interface layer imperfections) and weaker exchange interactions (having longer distances from each other than for more deeply located defects). Paramagnetic centers located deeper in the diamond core are responsible for the narrower Lorentzian components. Figure 10 represents the temperature dependences of parameters of the singlet EPR lines obtained by both direct experimental spectra processing and fitting. Figure 10a shows that the doubly integrated intensities (DIN, proportional to the EPR magnetic susceptibility) for both broad and narrow lines obey the Curie-Weiss (C-W) law. The contribution to DIN of the narrow line exceeds the contribution of the broad one by at least a factor of 3. There is a difference in C-W parameters for these lines. Comparison of the C-W plots (1/DIN vs temperature, Figure 10b) for P-DND and F-DND indicates some knee-like anomaly in the vicinity of 90 K. A discontinuity in the ∆Hpp(T) dependence for the F-DND sample is observed in the same temperature region whereas the P-DND sample shows no discontinuities in the line width behavior; see parts c and d of Figure 10. Subtle changes in structural, electronic, and/or magnetic state of F-DND nanoparticles and their aggregates could be responsible for the discontinuity observed. 3.6. XPS Spectra. The results of the XPS measurement of the surface composition of the F-DND sample are shown in Figure 11a and in Table 1. Figure 11b shows the C 1s spectrum that is deconvoluted into three lines assigned to three kinds of carbon atoms. Here the dominant C 1s peak with the binding energy 286.2 ( 0.2 eV was definitely assigned to the sp3 carbons of the diamond core17,58 that embodies most of the carbon atoms. The weaker peaks showing binding energies of 287.7 ( 0.2 and 290.0 ( 0.2 eV are attributed to carbons forming C-F bonds, which usually show larger binding energies compared to those of nonfluorinated carbons.59–62 The peak at 290.0 eV is assigned to the tertiary carbon atoms covalently bound to the fluorine atoms (C-F), while the peak at 287.7 eV is due to a carbon atom neighboring to that bound to fluorine (C-CF).59–61 The fluorine 1s spectrum (Figure 11c) reveals two peaks centered at 686.6 ( 0.2 and 689.2 ( 0.2 eV, corresponding to two different kinds of the C-F bonds,60 in accordance with the aforementioned C 1s XPS and NMR data. 3.7. Raman Spectra. RT Raman spectra of the fluorinated DND sample, measured with 514 and 633 nm lasers (Figure 12), show three bands around 1322 ( 4, 1626 ( 4, and 1817 ( 4 cm-1 for λ ) 633 nm and around 1343 ( 5, 1592 ( 5, and 1822 ( 10 cm-1 for λ ) 514 nm. The position of the first peak is close to that in a standard diamond crystal with sp3coordinated carbon atoms, which shows a narrow symmetric line at 1332.5 cm-1 corresponding to the transverse TO phonon of symmetry F2g with a line width at half height from 1.7 to 9.1 cm-1.63,64 A similar band is observed in high-quality diamond films.27 This band is definitely assigned to the nanodiamond sp3 band. We note, however, that it is much broader in comparison to that in bulk diamond. It was shown that as the crystallites decrease in size, this line undergoes a low-frequency shift accompanied by an asymmetric broadening, which can be

Panich et al.

Figure 11. (a) XPS measurement of the surface composition of F-DND sample. (b) Carbon 1s XPS spectrum, elemental ID and quantification. (c) Fluorine 1s XPS spectrum, elemental ID and quantification. Deconvolution into three and two bands is shown by dashed lines.

TABLE 1: Surface Composition of F-DND Sample from XPS Measurements state

peak BE, eV

at %

F1s O1s N1s C1s

686.6 531.8 399.8 286.2

15.70 0.91 1.21 82.18

explained by the phonon confinement model.63 We also note that the position of this band practically coincides with the socalled defect-induced or disorder-induced band (D band) observed around 1350 cm-1 in graphite, which, however, is expected to be very weak in the case in question, as shown below. The position of the second peak (around 1626 ( 4 cm-1 for λ ) 633 nm and 1592 ( 5 cm-1 for λ ) 514 nm) is close to that of the G band in the Raman spectrum of crystalline graphite that exhibits a single narrow band near 1580 cm-1, which is


J. Phys. Chem. C, Vol. 114, No. 2, 2010 781 the aforementioned paramagnetic centers are located very close to the surface fluorine atoms and yield a strong electron-nuclear interaction that causes fast nuclear spin-lattice relaxation. Acknowledgment. The authors thank L. Zeiri for the Raman measurements and T. Makarova for useful discussion on the Raman spectra interpretation. The work in Israel and Japan was supported by New Energy Development Organization of Japan Grant No. 04IT4; the work in Israel was also partially supported by joint grant of the Israeli Ministry of Science & Technology and Russian Foundation for Basic Research No. 3-5708.

Figure 12. Room temperature Raman spectra of fluorinated DND measured with 514 and 633 nm lasers.

assigned to the in-plane Eg mode at zone center. Therefore this band is assigned to sp2 carbons. In microcrystalline graphite, this band broadens and shifts toward higher frequencies, up to 1620 cm-1. We believe that this peak is due to some curved sp2 fullerenelike shell. This shell does not form graphitic stacking and thus does not contribute to the D peak. We note that the observed G peak is dispersive: the higher the laser energy, the lower the energy shift of the peak. Such an effect is usually observed in the Raman spectra of nanodiamonds.65 Though the intensities of the two aforementioned bands are similar, we note that in the visible range the scattering crosssection S of sp2 bonds is larger than that of sp3 bonds. In the literature the Raman cross-section is usually considered to be ∼50 times larger for graphite than for diamond.66,67 Therefore the relative amount of carbons contributed to the band at ∼1592-1626 cm-1 is expected to be much smaller in comparison with those contributed to the band at ∼1322-1343 cm-1. We note that Abello et al.68 revised the relative Raman scattering cross-section of graphite and diamond films and found a factor S(graphite)/S(diamond) between 2 and 4 instead of 50. A case in which the band at ∼1592-1626 cm-1 may come from sp2-like shell that partially covers the diamond nanoparticle or even from residual carbon soot, seems to be more like to that reported by Abello. The peak around 1820 cm-1, which is always observed in nanodiamonds, is presumably assigned to defects.65 Besides the aforementioned bands, some Raman spectra of F-DND measured with the 633-nm laser show an additional band at 1090 cm-1 of unknown nature (not presented), which is not observed with the 514-nm laser and therefore was attributed to an artifact. 4. Conclusion We have carried out a combined investigation of the structure and chemical bonding in fluorinated nanodiamond using NMR, EPR, XPS, and Raman techniques. This study led us to the conclusion that the hydrocarbon and hydroxyl groups are effectively removed from the DND surface in the process of fluorination and are substituted by chemically active fluorine atoms, resulting in the appearance of the fluorocarbon groups on the DND surface. Also, the fluorination presumably destroys a sp2-like carbon shell (if it exists). F is covalently bound to reactive C atoms of the surface shell of the particles. Fluorinated spots of limited size alternate with nearly nonfluorinated zones. Our model of the DND structure9 based on the EPR and NMR data reveals that paramagnetic centers (broken bonds) are mainly located near the DND surface. This model is well supported by the very fast 19F spin-lattice relaxation rate, which shows that

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