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Comment on "Quantification of C=C and C=O Surface Carbons in Detonation Nanodiamond by NMR" Alexander M. Panich, and Alexander I. Shames J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5125074 • Publication Date (Web): 11 May 2015 Downloaded from http://pubs.acs.org on June 10, 2015
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Comment on "Quantification of C=C and C=O Surface Carbons in Detonation Nanodiamond by NMR" A. M. Panich*, A. I. Shames Department of Physics, Ben-Gurion University of the Negev, Be'er Sheva 8410501, Israel
* Corresponding author. Email:
[email protected], tel. +972-8-6472458
Schmidt-Rohr et al. have recently published an article on NMR study of detonation nanodiamond (DND)1 containing criticism of some results and findings reported in our review on the subject.2 Based on their own results and citation of data consistent with, Schmidt-Rohr et al.
1, 3
suggested the model of DND structure comprising an ordered diamond core covered by
quite a thick partially disordered shell with a non-aromatic surface, which account 39 and 61% of carbons, respectively. In this model, surface carbons are bound to H and OH groups. The shell is responsible for a tail in the region of ~40-85 ppm in the 13C NMR spectrum. Our review 2
provides an analysis of practically all available at that time data received by a number of
research groups, which shows that: (i) 13C NMR tail signal coming from the shell varies from sample to sample and may be less intense than that reported in Ref. [3] (e.g., see Fig.6 of Ref. [2]); (ii) DND surface coverage by hydrogen-containing groups is usually incomplete and differs for various samples, which is reflected in noticeable variations of the width of broad component in the static 1H NMR spectra2 caused by dipole-dipole interaction between protons of the surface hydrogen-containing groups; (iii) the content of sp2 carbons is different in pristine DND samples obtained from different sources and may account from several percent to several tens percent of the entire DND carbons.
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Such a model of a DND particle2 has been supported by HRTEM, Raman, XANES and XPS measurements (Ref.[2] and refs therein) and is widely accepted by the nanodiamond community. The real fraction of the surface sp2 carbons in a certain DND sample depends on sample fabrication and purification procedures, as well as on specific treatment processes and, in some cases, may be effectively reduced (e.g., as demonstrated in Ref.[4]). All aforementioned facts are presented in the review2 but were ignored by the authors of the articles,1,3 whose DND model is, actually, just a specific case of the more general model proposed by us.2 The statement of Schmidt-Rohr et al. (p.9624, Ref.[1]) that "we have shown by NMR relaxation analysis and others have confirmed by EPR that most unpaired electrons are not at the nanoparticle surface" seems not to be accurate. The finding that the acceleration of 13C spinlattice relaxation in DND is caused by unpaired electrons of dangling bonds, most of which are not on the DND surface but between the core and shell, was first done by us in 2002,5 long before the publication of Ref. [3]. Then it was confirmed by numerous EPR and NMR studies done at our lab – see Ref. [2] and references therein. We also found out that nuclei located in the shell show faster relaxation compared with those in DND core and suggested a model of paramagnetic defect localization.6,7 We note that the analysis of spin-lattice relaxation done in Ref.[3] is questionable since those simulations were based on the number of unpaired electron spins per particle which contradicts well proven EPR and SQUID data showing at least three times smaller value.8 The matter is that as-received samples from most of vendors (such as studied in Refs. [1,3]) usually contain some para- and ferromagnetic metal and metal oxide impurities. Careful EPR study of these impurities should precede any magnetic measurements and subsequent conclusions. Ref. [3] shows no EPR evidences on absence of magnetic impurities in the sample under study whereas their contribution to magnetic SQUID measurement data may lead to overestimation of the reported density of paramagnetic defects.
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Moreover, in contrast to our studies, Ref.[3] does not show any values of relaxation times. It is worth mentioning that general characterization of ‘pristine’ DND samples studied in Refs.[1,3] leaves much to be desired. In many important aspects like sample purification procedures, actual impurities content etc. these sample remain to be a ‘black box’. The origin of sp2 carbons in nanodiamonds (NDs) of different origin is still under debates. Atomistic simulations9,10 showed that bare ND surfaces are unstable and subjected to reconstruction from sp3 to sp2 carbon bonding. Herewith sp2 graphene flakes may be formed only on the non-hydrogenated (111) facets of the “relaxed” bare diamond. Thus just a part of surface atoms, which account 15.7% for 4.2 nm ND particle,
11
is involved to the sp2
hybridization. Therefore one can suggest that graphitic-like structures being observed in the most of HRTEM images (e.g, see Fig.9 in Ref.[2]) may also originate from soot accompanying the DND synthesis. We note that many DNDs consist of ~100-200 nm tight agglomerates formed during synthesis. Theoretically, bare diamond particles are suggested to aggregate owing to electrostatic interaction.10 However, experimentally, during the detonation, sp3 carbon is energetically preferred only for around 0.5 µs. For the rest of the detonation duration, sp2 is the preferred carbon configuration. Such sp2 carbons may cover diamond core and fasten together primary DND particles (Fig.8 in Ref.[12]), perhaps by interparticle C–C bonds. Next, it is obviously that the surface state of the particles located closer to the periphery of the aggregate, where oxidation of the sp2 carbon is most rigorous, is quite different from that of particles of the interior part of the aggregate, where graphitic-like flakes may be preserved after purification. These flakes may be efficiently removed by cluster deaggregation and deep purification. However, high grade purification noticeably increases the production cost and therefore is quite moderate with vendors selling "pristine purified DNDs". As a rule such asreceived samples show some sp2 carbons detected in HRTEM, XPS and Raman measurements. NMR studies of purified DNDs of Donnet et al.13 (Fig.5 in Ref.[2]) show sp3/sp2 ratios varying
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from 0.83 to 2.01. Further, our recent NMR study of as-received purified NDs of the SSX series synthesized by a shock wave compression technique (Van Moppes & Sons, Switzerland)14 shows the presence of ~13.4% of sp2 carbons that were successfully removed by special deep purification in the laboratory. Unfortunately such facts, probably due to their inconsistence with the results and conclusions claimed in Ref. [1, 3], have been ignored there. In fact, Ref. [1] reports on the detection of unrealistically small content of sp2 carbons in the DND sample annealed at 800oC, whereas its significant surface graphitization has been well established by HRTEM which has revealed the conversion of the outer surface into discontinuous sp2 layers around the diamond core.15 The latter correlates well with our NMR and XPS study revealing ~23% sp2 carbons in the DND sample annealed at 800oC
16
as well as with XPS and HRTEM
data on the DND samples annealed at 750 oC,17 but is in a striking contrast with the data of Ref.[1] (Table 1, p. 9623) showing only 2.7% of C=C carbons in DND annealed at 800oC. This contradiction may indicate some methodological problems and disputes very low (less that 1%) content of sp2 carbons detected in as-received DND sample as per Refs. [1,3]. At last, the conclusion of Schmidt-Rohr et al. on pristine purified DNDs that "any significant disagreement between conclusions from NMR data cannot be attributed to differences in samples obtained from different sources but must be due to a difference in analysis, for instance assuming different surface carbon fractions as discussed above" (Ref.[1], p.9626) is groundless and may mislead the readers. Just the opposite: there is a variety of different DND samples synthesized at different conditions and subjected to different purification procedures. This diversity causes significant differences in the DNDs surface and core structures making results of any study to be a sample dependent. This fact is well known and widely accepted by the nanodiamond community. We are grateful to anonymous referees for valuable comments. Bibliography 4 ACS Paragon Plus Environment
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1. Cui, J.-F.; Fang, X.-W.; Schmidt-Rohr, K. Quantification of C=C and C=O Surface Carbons in Detonation Nanodiamond by NMR. J. Phys. Chem. C 2014, 118, 9621-9627. 2. Panich A. M. Nuclear Magnetic Resonance Studies of Nanodiamonds. Crit. Rev. Solid State Mater. Sci. 2012, 37, 276 – 303. 3. Fang, X. W.; Mao, J. D.; Levin, E. M; Schmidt-Rohr K. Nonaromatic Core#Shell Structure of Nanodiamond from Solid-State NMR Spectroscopy. J.Am. Chem. Soc. 2009, 131, 14261435. 4. Turner, S.; Lebedev, O. I.; Shenderova, O.; Vlasov, I. I.; Verbeeck, J.; Tendeloo G. Van. Determination of Size, Morphology, and Nitrogen Impurity Location in Treated Detonation Nanodiamond by Transmission Electron Microscopy. Adv. Funct. Mater. 2009, 19, 2116–2124. 5. Shames, A. I.; Panich, A. M.; Kempiński, W.; Alexenskii, A. E.; Baidakova, M.V.; Dideikin, A.T. et al. Defects and Impurities in Nanodiamonds: EPR, NMR and TEM study. J. Phys. Chem. Solids 2002, 63, 1993-2001. 6. Panich, A. M.; Shames, A. I.; Vieth, H.-M.; Ōsawa, E.; Takahashi, M.; Vul’ A. Ya. Nuclear Magnetic Resonance Study of Ultrananocrystalline Diamonds. Europ. Phys. J. B 2006, 52, 397– 402. 7. Shames, A.I.; Osipov, V. Yu.; Aleksenskiy, A. E.; Ōsawa, E.; Vul', A. Ya. Locating Inherent Unpaired Orbital Spins in Detonation Nanodiamonds Through the Targeted Surface Decoration by Paramagnetic Probes. Diamond & Related Materials 2011, 20, 318–321. 8. Osipov, V. Yu.; Shames, A. I.; Enoki, T; Takai, K.; Baidakova, M. V.; Vul’, A. Ya. Paramagnetic Defects and Exchange Coupled Spins in Pristine Ultrananocrystalline Diamonds. Diamond & Relat. Mater. 2007, 16, 2035–2038.
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9. Barnard, A. S.; Russo, S. P.; Snook I. K. Structural Relaxation and Relative Stability of Nanodiamond Morphologies. Diamond & Relat. Mater. 2003, 12, 1867–1872. 10. Barnard, A. S; Sternberg, M. Structural Relaxation and Relative Stability of Nanodiamond Morphologies. J. Mater. Chem. 2007, 17, 4811–4819. 11. Shenderova, O.; Koscheev, A.; Zaripov, N.; Petrov, I.; Skryabin, Y.; Detkov, P.; Turner, S.; Tendeloo, G. Van. Surface Chemistry and Properties of Ozone-Purified Detonation Nanodiamonds. J. Phys. Chem. C 2011, 115, 9827–9837. 12. Krüger, A.; Kataoka, F; Ozawa, M; Fujino, T.; Suzuki, Y; Aleksenskii et al. Unusually Tight Aggregation in Detonation Nanodiamond: Identification and Disintegration. Carbon 2005, 43, 1722–1730. 13. Donnet, J. B.; Fousson, E.; Delmott, L.; Samirant, M.; Baras, C.; Wang, T. K.; Eckhardt, A. 13
C NMR Characterization of Nanodiamonds. C. R. Acad. Sci. Paris, Serie IIc, Chimie 2000, 3,
831–838. 14. Shames, A. I.; Panich, A. M.; Mogilyansky, D.; Sergeev, N. A.; Olszewski, M.; Osipov, V. Yu.; Boudou, J.-P. XRD, EPR and NMR Study of Commercial Polycrystalline Micro- and Nanodiamonds. Presented at Hasselt Diamond workshop 2015 - SBDD XX, Hasselt, Belgium, February 25-27, 2015, Paper 5.9. 15. Cebik, Jonathan; McDonough, J. K.; Peerally, F.; Medrano, R.; Neitzel, I.; Gogotsi, Y.; Osswald, S. Raman Spectroscopy Study of the Nanodiamond-to-Carbon Onion Transformation. Nanotechnology 2013, 24, 205703. 16. Panich, A. M.; Shames, A. I.; Sergeev, N. A.; Olszewski, M.; McDonough, J. K.; Mochalin V. N.; Gogotsi Y. Nanodiamond Graphitization – a Magnetic Resonance Study. J. Phys.: Condens. Matter 2013, 25, 245303. 17. Petit, T.; Arnault, J-Ch.; Girard, H. A.; Sennour, M.; Kang, T.-Y.; Cheng, C. L.; Bergonzo, P. Oxygen Hole Doping of Nanodiamond. Nanoscale 2012, 4, 6792-6799. 6 ACS Paragon Plus Environment
