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High Nitrogen Doping of Detonation Nanodiamonds Vincent Pichot,* Odile Stephan,† Marc Comet, Eric Fousson, Julien Mory, Katia March,† and Denis Spitzer NS3E “Nanomate´riaux pour Syste`mes Sous Sollicitations Extreˆmes” UMR 3208 ISL/CNRS Institut franco-allemand de recherches de Saint Louis (ISL), 5 rue du Ge´ne´ral Cassagnou, 68301 Saint Louis, France, and Laboratoire de Physique des Solides UMR CNRS 8502, baˆt. 510, UniVersite´ Paris Sud, 91405 Orsay, France ReceiVed: December 24, 2009; ReVised Manuscript ReceiVed: April 29, 2010
The main goal of doping detonation nanodiamonds with nitrogen is to obtain stable photoluminescence properties. Small quantities of nitrogen have already been evidenced in such nanodiamonds. This article deals with the possibility of increasing the nitrogen content inside nanodiamond samples. Electron energy loss spectroscopy is a very useful tool for determining the location and the quantity of nitrogen within individual nanodiamonds. The experimental results demonstrate that the nitrogen content strongly depends on the precursors used in the composition of the explosive charge: the incorporation of melamine leads to an increase by a factor 2 or 3 in the nitrogen content in the nanodiamond core. To the best of our knowledge, this is the first time that such high nitrogen contents have been obtained for detonation nanodiamonds. It is also shown that nitrogen is always present inside the nanodiamond core in an sp3 hybridization configuration. Introduction The size and intrinsic properties of detonation nanodiamonds make them of great interest for potential applications in various domains. Nanodiamonds can be used as seeding for chemical vapor deposition (CVD) growth, for the improvement of the mechanical properties of metallic electrolytic deposits, and as a polishing agent.1-3 Moreover, the interest in nanodiamonds for medical applications is linked to their size, biocompatibility, and photoluminescence properties. The small size of the nanodiamond (5 nm) allows them to penetrate living cells by passing through their membrane. Their easily accessible and chemically active surface enables their functionalization with a wide variety of molecules and, more particularly, with drugs. Nanodiamonds can be easily located because they exhibit photoluminescence properties, which are due to the presence of nitrogen/vacancies at their surfaces or more probably within their cores. For all of these reasons, nanodiamonds could be injected into living organisms and thus be used as drug vectors to heal diseases as well as markers to control the drug progression.4,5 Nitrogen is usually found in detonation nanodiamonds as a small fraction: 1-3 wt %. It can be located at two different places: either at the nanodiamonds’ surface (belonging to chemical functions) or inside the nanodiamond core. Experimental studies reporting both situations can be found in the literature. For instance, Iakoubovskii et al. found by electron spin resonance (ESR) that the absence of nitrogen as single substitutional centers (P1 centers) suggests that nitrogen does not incorporate into the nanodiamond grains,6 on the contrary to Baranov et al., who revealed by the same technique the presence of individual N0 centers in sintered detonation nanodiamonds.7 Maillard-Schaller et al. showed by X-ray photoelectron spectroscopy measurements that the nitrogen content in detonation nanodiamond films is around 1-2 wt %, but they could not determine whether it was at the surface or * To whom correspondence should be addressed. Phone: +33 3 89 69 50 71. Fax: +33 3 89 69 50 74. E-mail:
[email protected]. † Laboratoire de Physique des Solides.
within the nanodiamond core.8 On the other hand, Mikov et al., using two-photon-excited luminescence, revealed that various nitrogen defects could be found within the nanodiamond grains but could not provide any quantification of the amount of nitrogen.9 More recently, studies were performed using a local probe by electron energy loss spectroscopy (EELS) measurements. Kvit et al. concluded that nitrogen is embedded within the core of nanodiamonds. However, the nitrogen content was not quantified, and no information on the nitrogen environment was obtained.10 Turner et al. found that nitrogen is present within the core of the nanodiamonds at around 3.5-4 wt %.11 Theoretical studies examining the potential energy surface for nitrogen substitution in diamond nanocrystals along a variety of crystallographically inequivalent substitution paths and sites revealed that nitrogen is unlikely to be stable as a dopant within nanodiamonds with sizes from 1 to 2.3 nm.12 Complementary simulations and fluorescence measurements showed that larger particles (above 5 nm) may contain a larger N content within the nanodiamond core.13 To the best of our knowledge, no experimental results report a nitrogen content higher than a few percent inside detonation nanodiamonds. In the present article, it is demonstrated that it is possible to increase the nitrogen content within the detonation nanodiamonds by using a high-nitrogen-content precursor. The EELS technique is used to locate and quantify accurately the presence of nitrogen inside nanodiamond particles as a function of the nature of the precursors that were added to the explosive charges. Experimental Section Three nanodiamond samples were prepared at the ISL by detonation of carbon-rich high explosives. Sample ND1 was obtained by the detonation of a hexogen (RDX)/trinitrotoluene (TNT) (hexolite 30/70) explosive charge.14 The second sample, ND2, was achieved by firing an explosive charge containing a mixture of RDX/melamine (80/20). This composition was tested in an attempt to increase the nitrogen content in the nanodiamonds. Melamine was used as the nitrogen source as it is the organic molecule that contains the highest relative nitrogen
10.1021/jp9121485 2010 American Chemical Society Published on Web 05/13/2010
High Nitrogen Doping of Detonation Nanodiamonds weight proportion (66.67 wt %) without being an explosive substance. Moreover, due to its high thermal stability (280 °C), it does not react with the explosive molecule it is mixed with during the preparation process. Therefore, its decomposition during the detonation experiment should permit the trapping of nitrogen atoms inside the diamond crystals during the nanodiamond formation occurring in the detonation process. The third sample, ND3, was prepared from a concentric explosive charge in which the external cover is composed of a 65/35 hexolite and the internal part is melamine. The proportion of melamine was 19 wt %. In all cases, the detonation soot obtained after the detonation experiment was purified from mineral impurities by using the hydrofluoric/nitric acid treatment described elsewhere.14 No subsequent oxidation treatment was performed. Elemental analyses were carried out by the “Service Central d’Analyses” of CNRS (Solaize, France). HADF imaging and EELS analysis were carried out at the “Laboratoire de Physique des Solides” with a VG HB 501 scanning transmission electron microscope (STEM) equipped with a field emission gun operated at 100 kV and fitted with a Gatan 666 parallel-EELS spectrometer, optically coupled to a CCD camera. Such an STEM instrument delivers a 0.5 nm electron probe of high brightness and offers a 0.3-0.5 eV energy resolution. This provides appropriate conditions for recording spectroscopic information at the nanometer scale on individual nanostructures. The smallest probe (0.5 nm in diameter) was used with a convergence angle of 7.5 mrad and a collecting angle of 24 mrad. More specifically, EELS data were acquired using the spectrum imaging (spectrum line) mode.15 In such a mode, the subnanometer electron probe is scanned in 2D (1D) over the area of interest and a whole EELS spectrum is acquired at every pixel of the 2D (1D) scan in parallel. For these experiments, the soot was sonicated in ethanol and one droplet of the suspension was deposited on a holey carbon film for STEM measurements. High-resolution bright-field STEM images where acquired with an aberrationcorrected dedicated STEM microscope NION USTEM 100 operated at 100 kV.
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Figure 1. (a) High-angle annular dark-field (HAADF) STEM image of nanodiamond particles in sample ND1. The inset shows a single isolated particle; the arrow shows the scan performed by the electron probe for spatially resolved EELS analysis. (b) Aberration-corrected high-resolution bright-field STEM image of a 5 nm nanodiamond showing the truncated octahedral morphology.
Results Nitrogen in Detonation Nanodiamonds Synthesized from a TNT/RDX Mixture. EELS analyses were performed on isolated nanoparticles sticking out from aggregates (typically hundreds of nanometers in size) into vacuum. Figure 1a shows a high-angle annular dark-field (HAADF) STEM image of such a single particle. The HAADF signal is proportional to the local projected mass. Therefore, it provides straightforward information about the morphology of the sample. This particle exhibits a size of about 5 nm, which is the typical value for classical nanodiamonds produced by detonation of TNT/RDX mixtures and a truncated octahedral morphology (see Figure 1b). A spectrum line was performed horizontally on this particle, giving a whole EELS spectrum every 3 Å across the nanodiamond (see the inset of Figure 1 for a display of the line trace). Figure 2a shows the resulting set of 32 spectra acquired along this line scan (the acquisition time per spectrum is 200 ms). The main detected signal is the carbon signal (C-K). It increases in intensity as the probe is scanned through the core of the nanoparticle. More importantly, slight changes in the nearedge fine structures (ELNES) can be observed: the spectra from the surface of the particles have a significant contribution from the signature of sp2 carbon bonding. This is clearly observed in Figure 2b, where two spectra extracted from the spectrum line and associated with surface and core probe positions are displayed. A peak at 285 eV (see arrow), which is characteristic
of transitions to π* states, displays a higher intensity in the spectrum from the surface location. The ELNES at the core location are characteristic of sp3 carbon with only a very small contribution from the sp2 surface layer. The signature of nitrogen (encircled N-K signal) is detected as a small signal for the central pixels of the line scan corresponding to locations at the core of the nanoparticle. Such a spatially resolved experiment can furnish quantitative elemental maps obtained by plotting the total elemental signal as a function of the probe position. This can be combined with a more sophisticated type of cartography: the mapping of the chemical bond (for example, sp2 and sp3 bonding) when the ELNES variations are plotted. Here, the intensity profile of sp2 carbon has been obtained by plotting the π* signal occurring at 285 eV (integrated over a 1 eV energy window) as a function of the probe position. This profile has been normalized by the total integrated C-K signal and compared to it. The profile for the total N-K signal normalized by the total C-K signal has been also calculated (Figure 3). It can be clearly seen that sp2 carbon is mainly located at the surface of the nanodiamond particle. The particle itself is composed of sp3 carbon, and it clearly appears that nitrogen is located within its core. We mention that the apparent larger diameter, as deduced from the carbon profile compared to the one observed in the
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Figure 2. (a) Set of 32 EELS spectra acquired with 3 Å spatial increments along the line trace displayed in Figure 1. (b) Spectra extracted from this spectrum line, associated with locations at the surface and at the core of the nanodiamond particle.
Figure 3. Intensity profiles extracted from the set of spectra displayed in Figure 2a: total carbon signal, that is, mainly sp3 carbon (hatched bars), sp2 carbon (white bars), and nitrogen (black bars).
HAADF image of Figure 1 (7 nm instead of 4 nm), is the consequence of a shift of the particle during the acquisition of the line spectrum, probably due to charging effects under the electron beam illumination.
Pichot et al. The sp2 layer found at the nanodiamond surface is usually observed in nanodiamond samples; in this case (sample ND1), this is all the truer because the sp2 carbon was not removed by any oxidation treatment.16 It was observed that, under the electron beam, the transformation of the nanodiamond through sp2 carbon occurs.17-19 When the sample ND1 is first thermally treated under air at 400 °C, EELS characterizations show that sp2 carbon layers are no longer observed for short acquisition times. The carbon and nitrogen contents can be calculated by summing all the spectra obtained on this particle. Carbon and nitrogen concentrations were found to be around 97.3 atom % (96.9 wt %) and 2.7 atom % (3.1 wt %), respectively. Similar values ((0.5 atom %) were found in other particles that were analyzed the same way. These results are in good agreement with previous results obtained by Turner et al.11 Elemental analysis, which is performed on macroscopic samples, has much better statistics than EELS. The nitrogen content found by this technique, 3.6 wt %, is in good agreement with the local EELS analysis. As a comment, we mention that we do not intend to give any reliable information about the surface state of the nanodiamonds. The use of fast electrons does not allow the detection of adsorbed molecules on a surface. Due to knock-on processes, the molecules are either desorbed or destroyed. However, the presence of adsorbed nitrogen originating from nitric acid is weakly probable due to the multiple washings with water performed during the purification processsnitrate anions being extremely soluble in water. Nitrogen in Doped Detonation Nanodiamonds. In the case of sample ND2, the obtained material was quite different. STEM images revealed the presence of graphite-like structures (see Figure 4a), nanodiamonds with a diameter of 5 nm and bigger nanodiamonds with a diameter around 10-20 nm (Figure 4b). These later particles are the most interesting material found in this sample. They generally exhibit a multiply twinned structure (Figure 4a), as mentioned by Turner et al.11 EELS spectra were recorded on the different entities present in the sample, and the nitrogen content was evaluated for each of them. For the sp2 (graphite-like) nanostructures, it was found that the nitrogen content is around 6 atom % (7 wt %). Concerning the 5 nm nanodiamonds, the nitrogen content was found to be around 2.5 atom % (2.9 wt %) on different particles and the shapes of the signals corresponding to sp2, sp3 carbon and nitrogen were the same as in sample ND1. It can be concluded once more that the nitrogen is included inside the nanodiamond particle as in sample ND1. Spectra from five different bigger nanodiamonds were recorded. The carbon and nitrogen contents were calculated by summing all the spectra obtained on these particles. The results were found to be around 95-96 atom % (94.2-95.4 wt %) carbon and 4-5 atom % (4.6-5.8 wt %) nitrogen. Sample ND3 exhibits a different morphology from both previous samples. Although small nanodiamonds with diameters of 5 nm and sp2 structures were observed, faceted nanodiamonds with a size around 40-50 nm were also found (Figure 5). The 5 nm nanodiamonds were found to contain the same composition as in both previous samples. The average nitrogen content measured by examining several 40-50 nm nanodiamonds was found to be around 8 atom % (9.2 wt %) with a minimum value of 5.6 atom % (6.6 wt %) and a maximum of 15 atom % (17.1 wt %) (see Table 1). Once more, the nitrogen was found to be incorporated within the nanodiamond core.
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Figure 4. (a) Aberration-corrected high-resolution bright-field STEM image of a 10 nm nanodiamond and graphite-like structures. (b) HAADF STEM image of nanodiamond particles in sample ND2.
Figure 6 shows the EELS spectra (normalized in intensity with respect to the carbon signal) from the 5 nm nanodiamonds of ND1, the 10-20 nm nanodiamonds of ND2, and the 40-50 nm nanodiamonds of ND3. The N-K signal is clearly proved to be more intense in the case of the bigger nanodiamonds, indicating the higher nitrogen content in these particles. Discussion A closer look at the N-K ELNES gives some insight into the nature of the local chemical environment of nitrogen in the nanodiamonds. Figure 7 shows a comparison of the nitrogen signal acquired on one large nanodiamond from sample ND2 (spectrum 2), one small nanodiamond in sample ND1 (spectrum 1), and one 40-50 nm nanodiamond from sample ND3 (spectrum 3). These spectra are compared to the N-K signal detected in cubic BN (spectrum 4) and in partly graphitized carbon nanoareas (spectrum 5). The N-K ELNES from spectrum 5 shows many similarities with those observed in thin CNx films, with a prepeak around 400 eV and a triangular σ* band.20 The contrast with the N-K ELNES from nanodiamond cores is striking. A faint contribution from the prepeak at 400 eV is observed for the small nanodiamond (spectrum 3) similar to the signature of N in an sp2 environment at the surface of the nanoparticle. Concerning the σ* band, it shows a drastically different shape from that usually observed in CNx materials. In fact, it is shifted to lower energies and shows a much sharper profile that is comparable to that observed for N in a cubic environment, such as c-BN (spectrum 4); this signal is present in all three signals from the nanodiamonds.21 We suggest that
Figure 5. HAADF STEM images of nanodiamond particles in sample ND3.
TABLE 1: Nitrogen Content for Several 40-50 nm Nanodiamonds in Sample ND3 nanodiamonds
N content atom %
1 2 3 4 5 6 7 8
5.7 7.5 7.6 7.7 7.8 9.3 11.9 15.0
these features are the signature of N in an sp3 coordination bonding within the core of the nanodiamond particle. The different populations and sizes of the nanodiamonds found in the samples clues to the mechanism of nitrogen incorporation into the nanodiamonds. Indeed, in sample ND1, only one population of nanodiamonds is found, suggesting that these diamonds originate from the explosive molecules of the hexolite (30/70). For ND2, two populations of nanodiamonds are observed, 5 and 10-20 nm in diameter. The small nanodiamonds are very similar to those of sample ND1, whereas the bigger ones contain about twice as much nitrogen as the small ones. Therefore, two sources for the nanodiamonds’ formation can be envisaged: the small nanodiamonds come from the explosive molecules, whereas the bigger ones are probably formed by a compaction effect on the melamine grains. Indeed, in the explosive charge prepared
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Figure 6. EELS spectra of nanodiamonds in samples ND1, ND2, and ND3.
Pichot et al. energetic composition. Melamine grains were also compressed under huge pressures, giving nanodiamonds with a bigger diameter of 40-50 nm that contain twice the nitrogen content observed previously for the explosive charge where melamine was more homogenously dispersed. Typical small nanodiamonds were also found coming directly from the external explosive casing. This is in agreement with the results from Bradac et al.13 showing that the larger the particles, the larger the core, the more N was detected. This was explained by the existence of a large kinetic energy barrier at the core/shell (sp3/sp2) interface trapping N in the core while N in the shell will escape the particle. These results emphasize the effect of the homogeneity of the composition of the explosive charge on the morphology of the synthesized sample. They suggest that the size of the nanodiamonds obtained by detonation and the amount of N doping may be controlled by the distribution of both the precursor and the molecules composing the explosive charge. Conclusions The nitrogen content in nanodiamonds has been found to depend on the nature and the homogeneity of the explosive composition used in the detonation experiment. The use of a nitrogen-rich precursor inside the explosive composition in the detonation experiment yields nanodiamonds containing 2 (sample ND2) or 3 (sample ND3) times more nitrogen than in typical nanodiamonds synthesized by detonation of pure TNT and RDX mixtures. The evidence for the nitrogen location inside the diamond core was provided by EELS measurements. In all cases, the nature of the local chemical environment of nitrogen in the nanodiamonds is similar to a cubic environment, such as c-BN. These results pave the way toward a new means of controlling the size and chemistry of highly N-doped nanodiamonds. Acknowledgment. This work was partly supported by the NADIA project (ANR PNANO) and the EU I3-026019ESTEEM contract. Mike Walls is greatly acknowledged for his careful reading of the manuscript and valuable suggestions. Mathieu Kociak is acknowledged for his help in the acquisition of the high-resolution STEM bright-field images. References and Notes
Figure 7. Nitrogen signals extracted from EELS spectra obtained from sample ND1 nanodiamonds (spectrum 1), sample ND2 (spectrum 2, nanodiamond; spectrum 5, sp2 structure), and sample ND3 (spectrum 3). Spectrum 4: profile observed for N in a cubic environment, such as c-BN.21
for sample ND2, the mixing between hexogen and melamine induced micrometric or millimetric domains of melamine inside the explosive charge. During the explosion, these domains were compressed by the surrounding pressure provided by the detonation process, resulting in nitrogenrich nanodiamonds with a bigger diameter. Sample ND3 confirms this hypothesis. In this case, the melamine molecules were placed at the center of the charge, forming bigger melamine domains-centimeters in sizesand were surrounded by a high explosive mixture (hexolite 65/35). During the explosion, the melamine at the center of the charge was subjected to a severe shock wave generated by the external
(1) Williams, O. A.; Douhe´ret, O.; Daenen, M.; Haenen, K.; Osawa, E. Chem. Phys. Lett. 2007, 445, 255–258. (2) Burkat, G. K.; Dolmatov, V. Yu. Phys. Solid State 2004, 46, 703– 710. (3) Artemov, A. S. Phys. Solid State 2004, 46, 687–695. (4) Huang, H.; Pierstorff, E.; Osawa, E.; Ho, D. Nano Lett. 2007, 7, 3305–3314. (5) Schrand, A. M.; Ciftans Hens, S. A.; Shenderova, O. A. Crit. ReV. Solid State Mater. Sci. 2009, 34, 18–74. (6) Iakoubovskii, K.; Baidakova, M. V.; Wouters, B. H.; Stesmans, A.; Adriaenssens, G. J.; Vul’, A. Ya.; Grobet, P. J. Diamond Relat. Mater. 2000, 9, 861–865. (7) Baranov, P. G.; Il’in, I. V.; Soltamova, A. A.; Vul, A. Ya.; Kidalov, S. V.; Shakhov, F. M.; Mamin, G. V.; Orlinskii, S. B.; Salakhov, M. Kh. JETP Lett. 2009, 89, 409–413. (8) Maillard-Schaller, E.; Kuettel, O. M.; Diederich, L.; Schlapbach, L.; Zhirnov, V. V.; Belorov, P. I. Diamond Relat. Mater. 1999, 8, 805– 808. (9) Mikov, S. N.; Igo, A. V.; Gorelik, V. S. Phys. Solid State 1999, 41, 1012–1014. (10) Kvit, A. V.; Zhirnov, V. V.; Tyler, T.; Hren, J. J. Composites, Part B 2004, 35, 163–166. (11) Turner, S.; Lebedev, O. I.; Shenderova, O.; Vlasov, I. I.; Verbeeck, J.; Van Tendeloo, G. AdV. Funct. Mater. 2009, 19, 2116–2124. (12) Barnard, A. S.; Sternberg, M. Diamond Relat. Mater. 2007, 16, 2078–2082.
High Nitrogen Doping of Detonation Nanodiamonds (13) Bradac, C.; Gaebel, T.; Naidoo, N.; Rabeau, J. R.; Barnard, A. S. Nano Lett. 2009, 9, 3555–3564. (14) Pichot, V.; Comet, M.; Fousson, E.; Baras, C.; Senger, A.; Le Normand, F.; Spitzer, D. Diamond Relat. Mater. 2008, 17, 13–22. (15) Colliex, C.; Tence´, M.; Lefe`vre, M.; Mory, C.; Gu, H.; Bouchet, D.; Jeanguillaume, C. Microchim. Acta 1994, 114, 71–84. (16) Baidakova, M.; Vul’, A. J. Phys. D: Appl. Phys. 2007, 40, 6300–6311. (17) Banhart, F. Philos. Trans. R. Soc. London, Ser. A 2004, 362, 2205– 2222.
J. Phys. Chem. C, Vol. 114, No. 22, 2010 10087 (18) Qin, L. C.; Iijima, S. Chem. Phys. Lett. 1996, 262, 252–258. (19) Roddatis, V. V.; Kuznetsov, V. L.; Butenko, Yu. V.; Su, D. S.; Schlo¨gl, R. Phys. Chem. Chem. Phys. 2002, 4, 1964–1967. (20) Alonso, F.; Gago, R.; Jimenez, I.; Gomez-Aleixandre, C.; Kreissig, U.; Albella, J. M. Diamond Relat. Mater. 2002, 11, 1161–1165. (21) Jaouen, M.; Hug, G.; Gonnet, V.; Demazeau, G.; Tourillon, G. Microsc., Microanal., Microstruct. 1995, 6, 127–139.
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