Solid-State NMR Study of Nanodiamonds Produced by the Detonation

May 26, 2009 - (F.M.) Phone: +32 2 650 5755. ... (M.D.) Phone: +33 4 73 40 71 05. ... 13C and 1H magic angle spinning (MAS) NMR and 13C MAS NMR with ...
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J. Phys. Chem. C 2009, 113, 10371–10378

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ARTICLES Solid-State NMR Study of Nanodiamonds Produced by the Detonation Technique Marc Dubois,*,† Katia Gue´rin,† Elodie Petit,† Nicolas Batisse,† Andre´ Hamwi,† Naoki Komatsu,‡ Je´roˆme Giraudet,§ Pascal Pirotte,§ and Francis Masin*,§ Laboratoire des Mate´riaux Inorganiques (UMR CNRS 6002), Clermont UniVersite´, 24 AVenue des Landais, 63177 Aubie`re, France, Department of Chemistry, Shiga UniVersity of Medical Science (SUMS), Seta, Otsu, Shiga 520-2192, Japan, and Matie`re Condense´e et Re´sonance Magne´tique, UniVersite´ Libre de Bruxelles (ULB), CP 232, BouleVard du Triomphe, B-1050 Bruxelles, Belgium ReceiVed: February 11, 2009; ReVised Manuscript ReceiVed: April 29, 2009

Nanodiamonds obtained by the detonation method have been investigated by means of solid-state magnetic nuclear resonance (NMR) and electron paramagnetic resonance. 13C and 1H magic angle spinning (MAS) NMR and 13C MAS NMR with 1H to 13C cross-polarization allow the determination of surface-hydrogenated groups (CH, CH2, and COH) and the quasi-absence of an sp2 carbon fullerene-like shell on the diamond surface to be underlined. The 1H and 13C spin-lattice relaxation time (T1) and second moment measurements are presented as a function of the temperature. Relaxation is shown to be mainly caused by paramagnetic centers in the case of 13C nuclei, whereas the presence of a molecular motion with an activation energy of 11.15 kJ · mol-1 is involved for 1H nuclei. 1. Introduction World-wide interest has developed over the past few years in nanometer-size diamond, called nanodiamond (ND), because of the intrinsic properties of diamond such as high thermal conductivity, wide band gap, extreme hardness, and high refraction index together with the high specific surface area as a characteristic of nanoparticles.1-3 As prepared, chemically purified or functionalized, their applications concern various fields: (i) biology and medicine such as in drug delivery and as fluorescent probes and (ii) mechanics as additives (lubricants, cooling fluids, electroplating baths,...).2,3 Nanodiamonds are prepared by the following two synthesis groups: the first one includes the methods involving the phase transition graphite f diamond. Graphite turns to diamond at high temperature and pressure. The second group consists of the method of chemical formation of diamond films. In the nanocrystalline form, diamond can be produced either as thin films using chemical vapor deposition (CVD) techniques or as a powder by the detonation of carbon-containing explosives such as trinitrotoluene (TNT) and hexogen in a steel chamber.4 This mixture of strong explosives results in a pressure increase during explosion and, consequently, in a higher diamond content in the detonation soot. The method of cooling is also a key point to obtain a high diamond content in the resulting soot.5-7 Using this method, metallic impurities and carbon phases distinct from diamond, such as amorphous carbons, graphite, and * To whom correspondence should be addressed. (F.M.) Phone: +32 2 650 5755. Fax: + 32 2 650 5023. E-mail: [email protected]. (M.D.) Phone: +33 4 73 40 71 05. Fax: + 33 4 73 40 71 08. E-mail: marc.dubois@ univ-bpclermont.fr. † Clermont Universite´. ‡ SUMS. § ULB.

fullerene-like carbons, can be synthesized in addition to nanodiamonds, and the resulting samples are then usually described as a diamond core (sp3 carbons) which is covered by an sp2 carbon fullerene-like shell. The shell structure and its thickness depend on the detonation condition. It is then of primary importance to remove these species to obtain the purest sample for the desired applications. Various acid treatments and thermal oxidation in air have been proposed for such an aim. The detonation method, which was rather referred to the first group mentioned before, can now be performed in ton quantities. Moreover, it allows the particles to be separated into a more narrow range of particle sizes (fractionalization or disintegration). As a matter of fact, the tendency to form sticky aggregates can be overcome either by oxidation in air to burn off the sp2 carbon shell8-10 or by mechanical treatment.10 Ultradisperse nanodiamonds can then be obtained with a narrow distribution of particle size centered around 5 nm. Nuclear magnetic resonance (NMR) is a very useful tool in studying the structural features and determination of different allotropic forms in nanocarbons since the position of the NMR signal of each nucleus depends on both the nature of chemical bonding and the carbon hybridization.11-15 The sp2 carbon shell as well as various surface groups can be underlined by NMR when these carbon atoms are present in sufficient amounts. Measurements of the spin-lattice and spin-spin relaxation times (T1 and T2, respectively) yield information about the molecular motion and interactions with paramagnetic centers (localized unpaired electrons). The NMR information must be correlated with electron paramagnetic resonance (EPR) to study these interactions. Although NMR studies about nanodiamonds are numerous, up to now, there has been no report on the temperature dependence of the nuclear relaxation times concerning both 1H and 13C nuclei. It is well established that, for the 13 C nucleus taking into account the stretched exponential

10.1021/jp901274f CCC: $40.75  2009 American Chemical Society Published on Web 05/26/2009

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character of the nuclear magnetization recovery, paramagnetic centers, i.e., dangling bonds, act in the nuclear relaxation. The surface properties of the nanodiamonds are of primary importance since, in addition to the surface chemistry (the presence of COH, CH, CH2), they determine the amount of physisorbed atmospheric oxygen molecules, which can act on nuclear relaxation as they do for electronic relaxation. In the present paper, NMR studies as a function of the temperature are shown for both 1H and 13C nuclei to investigate the nuclear relaxation processes, i.e., paramagnetic centers and/or molecular motion. The EPR spin density of the dangling bonds, sp2 content obtained by Raman spectroscopy, porosity, and specific surface area (measured by nitrogen adsorption at 77 K) are investigated to correlate the NMR data. First the physicochemical characterization using EPR, N2 adsorption, Raman spectroscopy, and magic angle spinning (MAS) NMR will be discussed to underline the nature of the detonation nanodiamonds, and 13C and 1H nuclear relaxation experiments will then be presented. 2. Experimental Section 2.1. Synthesis. The raw nanodiamonds (denoted r-NDs) were produced by Gansu Lingyun Nano-Material Co., Ltd., Lanzhou, China. The industrial synthesis consists of detonation of a 60/ 40 mixture of TNT/hexogen in a gaseous CO2 medium (socalled “dry synthesis”). The size distribution of the as-produced powder is dominated by a large particle size (100 nm) due to very tight aggregates that are formed during the cooling cycle of the detonation shock wave. The resulting slurry was then filtered and cleaned in aggressive acid treatments to produce a stable suspension of ND particles. Thus, the powder was vigorously mixed with a large excess of ZrO2 microbeads to reduce the particle size distribution, and the thus-obtained sample, called d-ND, was purchased as NanoAmando from NanoCarbon Research Institute. The final particle size is around 5 nm, and this sample was used as the smallest available core particle size diamond powder. More details of the d-ND including the procedure and the particle size were reported in ref 9. Although aggregates were successfully disintegrated by milling with ZrO2 microbeads, the primary particles thus obtained were black. 2.2. Physicochemical Characterizations. Raman spectra were measured at room temperature using a Jobin Yvon T64000 with a charge-coupled device multichannel detector. The excitation source was a 514.5 nm argon laser line. The laser power was adjusted to 200 mW. Nitrogen adsorption isotherms were measured at 77 K by a Micromeritics ASAP 2020 automatic apparatus. Before measurements, the samples were pretreated under a secondary vacuum at 300 °C for 2 h for sufficient removal of adsorbed impurities. EPR spectra were performed with a Bruker EMX digital X band (ν ) 9.653 GHz) spectrometer. Diphenylpicrylhydrazil (DPPH) was used as the calibration reference to determine both the resonance frequency and the densities of the spin carriers. 2.3. Solid-State NMR. NMR experiments were carried out on a Tecmag spectrometer (two working frequencies for each nucleus, 1H and 13C, 500.33 or 300.13 MHz and 125.81 or 75.47 MHz, respectively). A simple sequence (τ acquisition) was used with a single π/2 pulse length of 3.5 µs for 1H and 13C nuclei. The spin-lattice relaxation time T1 was measured using a saturation recovery sequence. The two-dimensional 1H-13C cross-polarization wide-line separation (CP-WISE) experiment was performed at 2 kHz with a cross-polarization contact time of 500 µs. The data matrix had a size of 32 points in the F1 (1H) dimension and 1024 data

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Figure 1. Visible (514.5 nm) Raman spectra of raw and disintegrated nanodiamonds.

points in the F2 (13C) dimension. The F1 and F2 dimensions were zero-filling to 256 and 2048 points, respectively, before two-dimensional Fourier transformation was carried out. 1H and 13 C chemical shifts were externally referenced to tetramethylsilane (TMS). 3. Results and Discussion 3.1. Physicochemical Characterization. We start this section with consideration about the color of the samples. Nanodiamonds exhibit a dark color after disintegration, whereas the raw product is gray. This suggests, in a first approach, a high content of graphite-like carbon in the former material. Nevertheless, neither X-ray diffraction nor the following characterization (Raman, 13C NMR) underlines such a surface shell. Amorphous carbons can be present, the band gap of these carbons strongly depends on their microstructure,16 and the difference could explain that the colors differ in the studied NDs. The analogy with the work of Iakoubovskii et al.6 is interesting; they compared detonation nanodiamonds obtained using a dry (CO2 as coolant) and wet (H2O) method. The colors of the resulting samples are gray and black, respectively. The content of the graphite-like carbon cannot explain such an observation since no presence of graphite in both samples was found using XRD. The authors reported the effect of the microstructure of the amorphous carbon shell which covers the nanodiamonds. 3.1.1. Raman Spectroscopy. The Raman spectra of raw and disintegrated NDs are nearly similar and exhibit three bands (Figure 1): (i) a narrow symmetrical diamond peak at 1325 cm-1 with a full width at half-maximum (fwhm) of 42 cm-1, which is downshifted and broadened in comparison with single-crystal bulk diamond at 1332 cm-1 with an fwhm in the range of 2.2-4.6 cm-1 (it corresponds to a zone center mode of T2g symmetry);17-19 (ii) the upshifted graphite G-band or pseudoG-band at 1624 cm-1; (iii) the disorder-induced doubleresonance D-band in the range 1250-1400 cm-1 which is not clearly observed (it is certainly superimposed with the base of the diamond line).20,21 In detail, the band at 1624 cm-1 can be decomposed into two lines, one centered at 1580 cm-1 assigned to residual sp2 carbon atoms of the fullerene-like shell and a second line at 1624 cm-1 which could be related to the presence of carbonyl oxygen containing functional groups on sp2- or sp3-hybridizated carbon atoms (pseudo-G-band).20 The Raman spectra of the nanodiamonds of the present study exhibit strong similarities with detonation NDs oxidized in an air atmosphere.8 The Raman bands corresponding to sp2-bonded carbons (Dand G-bands) are of low intensities as compared to the diamond peak, indicating that the acid washing, which is performed for

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Figure 2. EPR spectra and simulations of detonation NDs after (a, b) and before (c, d) disintegration.

the two samples, is efficient for reducing the amount of sp2 carbon atoms. 3.1.2. Electron Paramagnetic Resonance. Depending on both the synthesis and purification method, the possible spin carriers responsible for the EPR signal are identified as (i) uncoupled spins localized at broken carbon-carbon bonds, i.e., dangling bonds, which exhibit a g factor close to 2.0027 and a narrow line (the spin density of these paramagnetic centers was found to be around 1019 spin/g), (ii) metallic impurities, mainly iron(III) ion containing complex but also Cr and Ti (the g factor differs then from 2 with a value of 2.5, and the corresponding signal is broad (a few hundred gauss)), and (iii) spin localized on doped nitrogen atoms, which is underlined by a multiline hyperfine pattern (MHP).12,22,23 The EPR spectra of r-NDs and d-NDs and the simulations using WinSimfonia are given in Figure 2. First, it must be noted, within the detection limits of the spectrometer, the presence of neither the broad line related to metallic impurities nor the MHP of doped nitrogen atoms, indicating, for the first kind of spin carriers, that the acid washing is efficient to remove these impurities in both samples. After the purification, the atomic percentage of nitrogen is still around 2%, the absence of the MHP of doped nitrogen seems to indicate that this MHP is not resolved due to different neighboring of the spin carrier. Because of its g factor close to 2.003 ( 0.002, which is typical of free radicals and localized structural defects, the observed EPR signal is assigned to carbon dangling bonds having a localized spin. Simulation of the spectrum reveals a pure Lorentzian profile with ∆HPP (peak-to-peak line width) equal to 9.8 G for d-NDs (Figure 2a). The spin density (i.e., the number of electronic spins per mass unit of sample) is 5.0 × 1018 spins · g-1. On the contrary, two contributions are necessary to fit the spectrum of r-NDs (Figure 2c), and the parameters are ∆HPP ) 9.8 and 20 G and g ) 2.003 ( 0.002. The spin density of this sample is 3.7 × 1018 spins · g-1. Therefore, two types of dangling bonds with different neighborings coexist in the sample before disintegration. It is to be noted that the spin densities in our work are significantly lower than the ones reported in refs 12, 22, and 23 obtained by both EPR and SQUID measurements

on the samples from the same source. This difference could be explained by the extent of the sp2 carbon fullerene-like shell on the diamond surface. As a matter of fact, the amount of dangling bonds with unpaired electrons is high at the interface between the diamond core and this shell.27 The large extent of this shell, contrary to our case, could result in an increase of the spin density. Taking into account the high specific surface area of both raw and disintegrated NDs, which allows interaction of the dangling bonds with physisorbed paramagnetic oxygen molecules, we have investigate the change in the EPR signals after outgassing for 1 h at 120 °C and 2 h at 300 °C under a secondary vacuum (Figure 2b,d). After the outgassing, the samples are transferred under N2 and into a quartz EPR tube in a glovebox (filled with argon). The EPR tubes are then sealed off. Outgassing results in the removal of physisorbed molecules from the sample surface, in particular atmospheric paramagnetic O2. Different behaviors were obtained for raw and disintegrated nanodiamonds. For this latter sample, the spin density slightly increased for the outgassed sample, about 10% for d-NDs, in accordance with the results of Panich and Shames12 with no detectable changes in the signal shape. These authors explain such a phenomenon as follows: in air paramagnetic O2 molecules are physisorbed close enough to some of the paramagnetic centers (PCs), resulting in a strong dipolar and/or exchange interaction. Such an interaction leads to a significant broadening of the contribution of these closed PCs and a decrease of their contribution to the EPR signal. As this process is reversible, removal of the physisorbed O2 suppresses this interaction and the involved PCs contribute again to the signal intensity. The relative ratio Si/∑Si, where Si is the area of the line integral for the ith signal and ∑Si the sum of the area, in an air atmosphere for r-NDs, is 61.5% and 38.5% for the narrow and broad lines, respectively. The outgassing leads to values of 55.0% and 45.0%, indicating that the removal of oxygen molecules affects more the dangling bonds responsible for the broad line. A hypothesis of their location close to the surface could be made; these dangling bonds could be located in or close to the residual sp2 carbon shell and can be detected only

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SCHEME 1: Acid Washing and Disintegration of Detonation Nanodiamonds

for the r-NDs, where this shell is not completely removed after the acid washing (see Scheme 1, which is proposed taking into account all the characterization). Whereas the specific surface area available for oxygen adsorption is high, about 255 and 272 m2 · g-1, the EPR intensity increases only by 10% after removal of oxygen molecules for d-NDs and by 5% for r-NDs. Such a phenomenon has been underlined by Panich and Shames and tentatively explained by a shielding effect of the graphene layer on the short-range interaction of oxygen with dangling bonds (exchange interaction rather than dipole-dipole interaction). In our case, taking into account Raman and NMR data (shown hereafter), the extension of this sp2 carbon layer is limited for disintegrated and washed NDs. Moreover, the change upon outgassing is low. Therefore, the question of the location of the dangling bonds can be asked: Do dangling bonds sit in an sp2-enriched region and not in the crystalline core or in a homogeneous distribution in the overall volume? Nuclear relaxation measurements and, in particular, the analysis of the contribution of paramagnetic centers on the relaxation could give elements for this discussion. 3.1.3. N2 Adsorption at 77 K. The isotherm and pore size distribution curves are compared for the starting NDs and the resulting material after disintegration (Figure 3). No significant change of the BET surface occurred; the values are close, 255 and 272 m2 · g-1 for r-NDs and d-NDs, respectively. According to the classification of Brunauer, Deming, Deming, and Teller (BDDT),24 the shapes of the N2 isotherms at 77 K indicate a type IV isotherm typical of mesoporous materials. The porosity is essentially interparticular because, whatever the sample, the adsorption data deviate significantly from a flat to a curved feature above P/P0 ) 0.1 (above the monolayer formation), indicating uptake on the external surface. In spite of similar specific surface areas, the porosity significantly differs for the two samples. Whereas the pore size distribution is large and spread over the mesopore domain for r-NDs (Figure 3b), a strong peak appears on the dV/dφ distribution curve obtained on the desorption branch for the disintegrated sample corresponding to a narrow dispersion with pores of diameter close to 7.2 nm for d-NDs (V is the pore volume, φ the pore diameter). In both cases, the porosity is mainly interparticular, but due to the large dispersion of pores for the r-NDs, the desorption

Figure 3. (a) N2 adsorption-desorption isotherms at 77 K of raw and disintegrated NDs. The symbols O, ∆ and b, 2 represent the adsorption and desorption curves, respectively. The data of r-NDs are shifted by 300 cm3 · g-1 for clarity purposes. (b) Pore volume distributions calculated from the nitrogen desorption isotherms using the BJH model.

from the mesopores progressively occurs in a wide P/P0 range (0.6 < P/P0 < 1.0). On the contrary, the desorption in disintegrated NDs takes place abruptly at high relative pressure from pores with similar diameters (7.2 nm). This corresponds to the well-defined plateau on the desorption part for a relative pressure between 0.7 and 1. This comparison of the porosity for the two samples underlines that the disintegration results in ordered mesoporosity. The agglutinatites seem to be successfully disintegrated using milling with ZrO2 microbeads, and probably the sp2 carbon shell is also cracked, resulting in the opening of the interparticular mesoporosity (Scheme 1). 3.1.4. Nuclear Magnetic Resonance. 13C NMR spectra of detonation NDs after disintegration were recorded with MAS with a spinning rate of 8 kHz. In this experiment 1H decoupling was applied (Figure 4a). It is to be noted that (i) no significant difference is observed when the experiment is performed without 1 H decoupling, indicating the weakness of the 1H-13C interaction due to the low amount of hydrogenated groups and (ii) the spectra of r-NDs and d-NDs are strictly similar (not shown). Contrary to the nanodiamonds investigated by Panich13 (also produced by the detonation technique, i.e., a mixture of TNT and hexogene, 60/40), only one resonance line is observed, with a chemical shift of 34.9 ppm (TMS), which is assigned to sp3hybridized carbons. Panich assigned the additional line at 111 ppm (then close to aromatic carbons), not experimentally observed in our case, to the fullerene-like shell that covers the diamond core. Such a shell is predicted by Raty et al.25,26 This shell seems to be limited for the detonation nanodiamonds studied in the present investigation thanks to the successful washing and disintegration, which allow the sp2 layer to be washed away.

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Figure 6. Two-dimensional 1H f 13C CP-WISE of disintegrated detonation nanodiamonds. The spinning rate was 2 kHz.

Figure 4. Room temperature 13C NMR spectra (75.47 MHz) of disintegrated NDs: (a) MAS with proton decoupling and (b) 1H f 13C CP-MAS (spinning rate of 8 kHz). The asterisk marks the spinning sideband.

Figure 5. Room temperature 1H NMR spectra (300.13 MHz) of raw and disintegrated NDs (static and MAS with a spinning rate of 8 kHz).

As hydrogenated carbons (CH2, CH, or COH) could exist in nanodiamonds,6,12,13,15,27 1H f 13C CP with a spinning rate of 8 kHz (Figure 4b) and 1H MAS spectra (Figure 5) were also recorded to check this point. CP-MAS spectra reveal a line at 45 ppm which is typical of aliphatic carbons and is then assigned to CH and CH2 groups in accordance with the works of Panich et al.12,27 The sample also contains COH groups, which exhibit a 13C chemical shift of 75 ppm. Two lines with different line widths are present on the static 1H spectrum and assigned to COH, CH, and CH2 groups (Figure 5). Using MAS experiments, only the broad line displays spinning sidebands, and the isotropic chemical shifts of 4.5 and 1.3 ppm (TMS) are underlined. Due to the broadness, and as suggested by Iakoubovskii et al.,6 the

Figure 7. 13C magnetization semilogarithmic curve of NDs, 1 - Mz(t)/ Mz(0) vs t1/2, as a function of the temperature in air (at 293 K) and a N2 atmosphere (at 72, 100, 150, and 200 K).

line centered at 1.3 ppm can be an overlap of several peaks, which could be related to CH, CH2, and CH3 but also CdCH2, CsCH2, OsCH2, and NsCH2. The proportion of the two lines changes in the two samples; the relative intensity of the broad line is higher for disintegrated nanodiamonds. More precision about the assignments will be given taking into account 1H spin-lattice relaxation times and WISE measurements since the line widths suggest different molecular motions and T1 values. 3.1.5. Two-Dimensional (2D) 1H-13C CP-WISE. In addition to the analytical point of view, molecular dynamics in the nanodiamonds could be investigated using the two-dimensional 1 H f 13C CP-WISE experiment, which is used to compare the mobility of the two hydrogenated groups. Two important pieces of information could be extracted from 2D CP-WISE: 1H mobility and correlation between the 1H and 13C lines, allowing the unambiguous assignment of the 1H lines. The WISE spectrum is shown in Figure 6. The 1H fwhm’s for the two 13C chemical shifts are different, indicating different mobilities for the 1H atoms. The line for a δ13C of 75 ppm, i.e., related to the COH group, exhibits a fwhm of 12 kHz, significantly lower than that for a δ13C of 35 ppm (CH and CH2), fwhm ) 24 kHz. Molecular motion reduces the fwhm of the COH line. The narrow line at 4.5 ppm of the 1H MAS spectrum (Figure 5) can then be assigned to the COH group, whereas the broad one is related to a proton bonded to aliphatic carbons. By comparing the two samples, the relative amount of COH is higher in the nondisintegrated NDs. 3.2. Nuclear Relaxation. 3.2.1. 13C Measurements. To investigate the effect of PCs on the nuclear relaxation, we have compared the growth of the nuclear magnetization following an excitation pulse sequence (saturation recovery) with magic angle spinning (8 kHz). The outgassed samples are under a N2

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Figure 8. Evolution with temperature of the 13C NMR parameters: relaxation time A in air (O) and a N2 atmosphere (b), chemical shift (0), and spin-spin relaxation time (9).

Figure 10. 1H magnetization curve of NDs, Mz(t)/Mz(0) vs time, at room temperature for the two contributions (b) and for the first spinning sideband of the broad line (9).

TABLE 1: NMR Parameters at 72 and 380 K A (ms) T2 (ms) δ13C (ppm) (TMS)

72 K

380 K

1010 1.6 33.7

568 0.6 32

atmosphere during the experiments to avoid the possible effect of paramagnetic oxygen molecules, which can be physisorbed on the nanodiamond surface. To emphasize the nature of the magnetization growth, the magnetization as a function of t1/2 is plotted for some temperatures in the studied range (150 K < T < 293 K). The ordinate is 1 - Mz(t)/Mz(0) plotted on a logarithmic scale (Figure 7). The fittings as exp[-(t/A)β] (continuous lines in Figure 7), where A is the relaxation time, are in good agreement with the experimental data with β ) 0.5, indicating that the total magnetization seems to recover as exp[-(t/A)1/2]. This is typical of relaxation in the presence of PCs and without spin diffusion or in vanishing spin diffusion.28,29 This case is expected since 13C-13C dipolar interaction is limited by natural abundance. On the contrary, for rapid nuclear spin diffusion, β is equal to 1. In the case of solids with arbitrary space dimension D, in the vanishing spin diffusion regime, Furman et al.30 have shown that the magnetization growth is proportional to exp[-Atβ]. In a homogeneous distribution of the paramagnetic centers and nuclei, β is equal to D/6. On the other hand, for an inhomogeneous distribution, the sample must be divided into subsystems, each of them including only one PC surrounded by nuclei. These subsystems are packed in a d-dimensional space. In this case, β is equal to (D + d)/6. Moreover, using a saturation recovery sequence, the magnetic moments of the subsystems are aligned along the magnetic field, and then d ) 1, so for 3D

Figure 9. 13C (75.47 MHz) magnetization curve of NDs, Mz(t)/Mz(0) vs time, at room temperature with a working frequency of 125.81 (0) and 75.47 (b) MHz at room temperature.

Figure 11. Fwhm as a function of temperature for the 1H narrow line. The full line is a guideline for eyes.

Figure 12. 1H spin-lattice relaxation rate (s-1) as a function of temperature (K). The full line is a fit using the BPP model.

samples such as nanodiamonds, β ) 4/6 ) 0.66. This latter case is reported by Panich for detonation nanodiamonds.12 From our data, β is close to 0.5, as for the work of Alam et al.,13 corresponding to a space dimension of 3 for a homogeneous distribution, revealing then that dangling bonds, as internal paramagnetic centers apparently homogeneously distributed in the nanodiamond core, act on the 13C nuclear relaxation, resulting in an unusual 13C spin-lattice relaxation time. Values are reported in Figure 8 and in Table 1. The PCs seem to be uniformly distributed in the whole volume rather than close to the surface in the sp2 defect layer because of the limited expansion of this shell, highlighted by Raman and NMR data. As shown in Figure 9 the magnetization recovery (shown as Mz(t)/Mz(0) as a function of time) and then the experimental A equal to 439 ms) are independent of the working frequency (125.81 or 75.47 MHz) at room temperature. This indicates that molecular motion is not involved in the nuclear relaxation for NDs. As a matter of fact, in the case of molecular motions, the theory of Bloembergen Purcell Pound (BPP)31 gives the fol-

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TABLE 2: Parameters Obtained by Fitting the 1H 1/T1 Data parameter

value

2

error

3.9 × 10 5.3 × 10-12 1340 11.15 115.75 12

A (Hz ) τ0 (s) E (T) E (kJ mol-1) E (meV)

parameter

3.3 × 1010 4.9 × 10-13 27.2 0.23 2.39

[

]

(1)

where B is proportional to γ2h02, γ is the magnetogyric constant of carbon (6.73 107 s-1 T-1), h0 is the amplitude of the fluctuating magnetic field, ω is the nuclear spin resonance frequency (ω ) 2πν and ν ) 125.81 or 75.47 MHz for a carbon nucleus), and τ is the correlation time. The temperature dependence of the NMR parameters was investigated, and the values of A (related to the 13C NMR spin-lattice relaxation time T1) and the spin-spin relaxation time T2 of sealed NDs in a N2 atmosphere and in an air atmosphere were measured in the temperature ranges 70-320 and 290-390 K, respectively (Figure 8), at a frequency of 75.47 MHz. The dependence of the 13C chemical shift on temperature is also reported. The slight dependence of all the parameters confirms that molecular motion does not act in the nuclear relaxation. Due to the relaxation mode, where paramagnetic centers are dominant, the changes can be explained by the variations of the PC concentration. The relaxation time A exhibits a linear dependence on temperature whatever the atmosphere where the samples were investigated, i.e., N2 or air (Figure 8); this underlines that paramagnetic oxygen molecules, which are physisorbed on the high specific surface area of the nanodiamonds, are not involved in the nuclear relaxation. 3.2.2.1H NMR. In accordance with the 1H MAS spectra of NDs, the magnetization recovery curve exhibits two contributions (Figure 10) and the extracted 1H T1 values are 1.8 ( 0.3 and 61 ( 10 ms. The value of the longer T1 was confirmed by a saturation recovery sequence performed on the first sideband to avoid the contribution of the short relaxation time. Taking into account the WISE data and the line shape, the longer T1 is assigned to CH2 or CH groups, whereas the shorter value is related to COH. Only one T1, very short, around 1 ms, is reported by Cunningham et al.,15 who report that the reduction of T1 may be caused by molecular motion and/or strong interaction with paramagnetic centers. The temperature dependence of the 1H line width (fwhm) (Figure 11) is characteristic of dipole-dipole interactions. The reduction of the fwhm (or the second moment) at high temperature is a result of molecular motions which average the dipole-dipole interactions. At a temperature lower than 150 K, the fwhm is that of “frozen” molecules with no motion. The theory of BPP concerns the case of molecular motions;30 see eq 1 where γ is the gyromagnetogyric constant of the 1H nucleus (26.75 × 107 s-1 T-1). As shown by the temperature dependence, the process is thermally activated and described by an Arrhenius law:

τ ) τ0eE/T where E is the activation energy.

value

(2)

error

2.2 × 10 1736 14.43 149.96

τ0 (s) E (T) E (kJ mol-1) E (meV)

lowing expression for the T1 temperature dependence (A in the present study), which depends on the frequency:

1 τ 4τ )B + 2 2 T1 1+ωτ 1 + 4ω2τ2

TABLE 3: Parameters Obtained by Fitting the 1H Correlation Time Data 10

1.3 × 1010 89 0.7 7.69

Using eqs 1 and 2, the temperature dependence T1 curve was fitted (full line in Figure 12), and the corresponding parameters are given in Table 2. The found activation energy is E ) 11.15 kJ · mol-1 (1340 K). An experimental correlation time τ(T) for each temperature can be extracted from the second moment value M2(T) by the relation

τ(T) )

1 R4π√M2(T)

[

tan

2 2 π M2 (T) - l 2 L2 - l2

]

(3)

where R is a form factor near 1, L is the second moment of the rigid structure, and l is its reduced high-temperature value. The experimental τ values obtained from eq 3 (with the values of L and l obtained from Figure 11) are given in Figure 13 together with the fit realized with a temperature dependence; the process is thermally activated and described by an Arrhenius law (eq 2). The fit data are given in Table 3. In this way the extracted activation energy is slightly superior to that obtained by the 1H T1 data fit. 4. Conclusion First, Raman and NMR data underline that the detonation nanodiamond core, which consists of sp3 carbons, is very slightly covered by an sp2 carbon fullerene-like shell thanks to both acid washing and disintegration by milling with ZrO2 microbeads. The studied nanodiamonds can be described as the sp3 core covered with an amorphous carbon sheet containing surfacehydrogenated groups, such as CH2, CH, and COH as the main species (detectable by 1H f 13C CP-MAS NMR). The quasiabsence of the sp2 carbon fullerene-like shell changes the nuclear relaxation of both 13C and 1H. Taking into account the dependence of the NMR parameters on temperature, we underline that the relaxation processes differ for 1H and 13C nuclei. PCs, which consist of dangling bonds, act on the 13C relaxation contrary to molecular motion and to physisorbed paramagnetic oxygen, although a high specific surface area (272 m2 · g-1) area is available for oxygen molecule adsorption. This

Figure 13. Correlation time as a function of temperature. Experimental data are represented by circles (R ) 1, L ) 5.53 × 107 Hz2, and l ) 1.96 × 106 Hz2).

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PC effect results in a drastic shortening of the 13C spin-lattice relaxation times. Taking into account the effect of PCs on carbon nuclear relaxation, the distribution of PCs seems to be homogeneous in the NDs. The presence of a molecular motion with an activation energy of 11.15 kJ.mol-1 has been evidenced for 1 H nuclei using the temperature dependence of the relaxation times. This motion results in short spin-lattice relaxation times (1.8 ( 0.3 and 61 ( 10 ms). For the first time, the twodimensional 1H f 13C CP-WISE experiment was applied to nanodiamonds, allowing the unambiguous assignment of the two 1 H lines. References and Notes (1) Dolmatov, V. Y. Russ. Chem. ReV. 2001, 70, 607–626. (2) Williams, O. A.; Nesladek, M.; Daenen, M.; Michaelson, S.; Hoffman, A.; Osawa, E.; Haenen, K.; Jackman, R. B. Diamond Relat. Mater. 2008, 17 (7-10), 1080–1088. (3) Specht, C. G.; Williams, O. A.; Jackman, R. B.; Schoepfer, R. Biomaterials 2004, 25, 4073–4078. (4) Gruen, D. M. Annu. ReV. Mater. Sci. 1999, 29, 211–259. (5) Alexensky, A. E.; Baidakova, M. V.; Ya. Vul′, A.; Yu. Davydov Yu, V.; Pevtsova, A. Phys. Solid State 1997, 39, 1007–1015. (6) Iakoubovskii, K.; Baidakova, M. V.; Wouters, B. H.; Stesmans, A.; Adriaenssens, G. J.; Ya, A.; Grobet, P. J. Diamond Relat. Mater. 2000, 9, 861–865. (7) Baidakova, M. V.; Ya, A.; Siklitskii, V. I.; Faleev, N. N. Phys. Solid State 1998, 40, 715–718. (8) Osswald, S.; Yushin, G.; Mochalin, V.; Kucheyev, S. O.; Gogotsi, Y. J. Am. Chem. Soc. 2006, 128, 11635–11642. (9) Kruger, A.; Kataoka, F.; Ozawa, M.; Fujino, T.; Suzuki, Y.; Alesenskii, A. E.; Vul, A. Y.; Osawa, E. Carbon 2005, 43, 1722–1730. (10) Pichot, V.; Comet, M.; Fousson, E.; Baras, C.; Senger, A.; Le Normand, F.; Spitzer, D. Diamond Related Mater. 2008, 17, 13–22. (11) Shames, A. I.; Panich, A. M.; Kempiski, W.; Alexenskii, A. E.; Baidakova, M. V.; Dideikin, A. T.; Osipov, V. Yu.; Siklitski, V. I.; Osawa, E.; Ozawa, M.; Vul’, A. Ya. J. Phys. Chem. Solids 2002, 63, 1993–2001.

Dubois et al. (12) Panich, A. M. Diamond Relat. Mater. 2007, 16, 2044–2049. (13) Alam, T. M. Mater. Chem. Phys. 2004, 85, 310–315. (14) Donnet, J. B.; Fousson, E.; Delmott, L.; Samirant, M.; Baras, C.; Wang, T. K.; Eckhardt, A. C. R. Acad. Sci., Ser. IIc: Chim. 2000, 3, 831– 838. (15) Cunningham, G.; Panich, A. M.; Shames, A. I.; Petrov, I.; Shenderova, O. Diamond Relat. Mater. 2008, 17, 650–654. (16) Robertson, J. AdV. Phys. 1986, 35, 317–374. (17) Yushin, G. N.; Osswald, S.; Padalko, V. I.; Bogatyreva, G. P.; Gogotsi, Y. Diamond Relat. Mater. 2005, 14, 1721–1729. (18) Prawer, S.; W Nugent, K.; N Jamieson, D.; Orwa, J. O.; Bursill, L. A.; Peng, J. L. Chem. Phys. Lett. 2000, 332, 93–97. (19) Yanchuk, I. B.; Ya. Valakh, M.; Ya. Vul′, A.; Golubev, V. G.; Grudinkin, S. A.; Feoktistov, N. A. Diamond Relat. Mater. 2004, 13, 266– 269. (20) C Ferrari, A.; Robertson, J. Philos. Trans. R. Soc. London, Ser. A 2004, 362, 2477–2512. (21) Reich, S.; Thomsen, C. Philos. Trans. R. Soc. London, Ser. A 2004, 362, 2271–2288. (22) Yu, V.; Shames, A. I.; Enoki, T.; Takai, K.; Baidakova, M. V.; Ya, A. Diamond Relat. Mater. 2007, 16, 2035–2038. (23) Shames, A. I.; Panich, A. M.; Porro, S.; Rovere, M.; Musso, S.; Baidakova, M. V.; Yu, V.; Ya, A.; Enoki, T.; Takahashi, M.; Osawa, E.; Williams, O. A.; Bruno, P.; Gruen, D. M. Diamond Relat. Mater. 2007, 16, 1806–1812. (24) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1968, 60, 809. (25) Raty, J. Y.; Galli, G. Nat. Mater. 2003, 2, 792–795. (26) Raty, J. Y.; Galli, G.; Bosted, C.; van Buuren, T. W.; Terminello, L. J. Phys. ReV. Lett. 2003, 90, 037401. (27) Panich, A. M.; Shames, A. I.; Vieth, H.-M.; Osawa, E.; Takahashi, M.; Ya, A. Eur. Phys. J. B 2006, 52, 397–402. (28) Blumberg, W. E. Phys. ReV. 1960, 119 (1), >79–84. (29) Tse, D.; Hartmann, S. R. Phys. ReV. Lett. 1968, 21, 511–514. (30) Furman, G. B.; Kunoff, E. M.; Goren, D.; Pasquier, V.; Tinet, D. Phys. ReV. B 1995, 52 (14), 10182–10187. (31) Bloembergen, N.; Purcell, E. M.; Pound, R. V. Phys. ReV. 1948, 73, 679–712.

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