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Reply to a Comment by A. Panich and A. Shames on our paper "Quantification of C=C and C=O Surface Carbons in Detonation Nanodiamond by NMR" Klaus Schmidt-Rohr, and Jinfang Cui J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05507 • Publication Date (Web): 18 Aug 2015 Downloaded from http://pubs.acs.org on August 31, 2015

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The Journal of Physical Chemistry

Reply: This negative Comment on our two papers 1,2 is puzzling and unfortunate, given that Dr. Panich, in his recent review,3 has actually agreed with us on central aspects of NMR of detonation nanodiamond, and of nanodiamond structure. In the following, we will highlight the agreement on several important facts, but also refute some of the misleading specific claims made in this Comment. In order to properly support our statements, we will provide spectral evidence and specific quotes (several of them from Dr. Panich himself3) that confirm our conclusion that the fraction of C=C carbons in pristine, properly purified detonation nanodiamond is ≤ 2%.

NMR of nanodiamond. Most importantly, Dr. Panich has concurred that NMR has not detected aromatic carbons in pristine, properly purified detonation diamond, stating in his review’s Summary that there “is still a puzzle” concerning “the absence of the NMR signal coming from the surface sp2 carbons”.3 Therefore, it is surprising that this Comment suggests that our NMR experiments had some “methodological problems” that prevented detection of sp2-carbon signals. As a basis for a meaningful discussion of the NMR evidence, in Figure 1 we show 13C NMR spectra of four different detonation nanodiamond samples measured in different laboratories. None of these show signals of C=C carbons (between 100 and 150 ppm) on this vertical scale. Neither does Dr. Panich’s spectrum (bottom trace in Figure 3 of ref.4) of a nanodiamond material made in the Gogotsi group by the same procedure4,5 as the sample whose spectrum is shown in Figure 1b. We were, in fact, the first to detect and quantify the sp2-hybridized carbons in purified pristine detonation nanodiamond, by recording spectra of sufficient quality so that 30-fold vertical expansion could be used to make C=O and C=C peaks visible.1 By accounting for all surface carbons expected for a 4.8-nm nanodiamond particle, in terms of CH, COH, C=O, and C=C moieties,1 and pointing out that the literature shows no convincing evidence of a large sp2-carbon fraction,1 we have actually solved the ‘puzzle’ of ref.3. It should be noted that the only purported detection of a large aromatic fraction in nanodiamond by NMR quoted by Panich in ref.3 is actually a misattribution. Contrary to claims in ref.3 that are repeated in this Comment, Donnet et al.6 did not report a significant fraction of sp2carbon in properly purified detonation nanodiamond. The spectrum with an aromatic-carbon band

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reproduced in Figure 5 of Panich’s review3 is of insufficiently purified nanodiamond (the figure caption by Donnet et al. clearly refers to “spectra of soot” 6), which of course contains aromatic species. For fully purified nanodiamond, Donnet et al.6 reported that “Oxidized (a) sample (figure 4) displays for the MAS technique study only the larger sp3 peak, …, whereas sp2 part has disappeared.” (6 on p. 835, bold font added for emphasis).

Figure 1. Comparison of 13C NMR spectra of various samples of pristine purified detonation nanodiamond, consistently showing very little C=C signal (100-150 ppm) and similar peaks of sp3-hybridized C. (a, b): Direct polarization (DP) spectra, (c, d) spectra after cross-polarization (CP) from 1H, which highlights signals of carbons near the particle surface. (a) DP spectrum of detonation nanodiamond from Sigma-Aldrich. The small peak labeled “ssb” at 175 ppm, i.e. 14 kHz from the main signal, is a spinning sideband, due to paramagnetic shift anisotropy generated by the unpaired electrons and 14-kHz magic-angle spinning. (b) Corresponding DP spectrum of a detonation nanodiamond sample purified by Gogotsi et al. We thank Dr. Yury Gogotsi, Drexel University, for making this material available. A DP spectrum of a material made by the same procedure was recently reported by Panich et al.4 (Figure 3, bottom) and also does not show intensity between 100 and 150 ppm. The 12-kHz MAS spectra in Figure 3 of ref. 4 exhibit small peaks at 155 ppm, i.e. 12 kHz from the main signal, which must at least partially be spinning sidebands, which will occur at this frequency position under the given conditions. No other


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explanation for an isolated signal at 155 ppm is available4 and our corresponding 14-kHz nanodiamond spectrum does not show this peak. (c) CP spectrum of Sigma-Aldrich detonation nanodiamond. (d) CP spectra, reported by Dubois et al., 7 of (top trace) “raw detonation nanodiamonds from Gansu Lingyun Nano-Material Co.” and (bottom trace) acid treated and disintegrated “NanoAmando®” detonation nanodiamond. Reprinted from ref. 7 with permission from Elsevier. The prominent claim in the Comment that “(i) the 13C NMR tail signal coming from the shell varies from sample to sample” is clearly refuted by the spectra in Figure 1a,b, and c,d, which show similar line shapes for various different nanodiamond samples. Comparison of the spectrum in Figure 1b with the (relatively noisy) spectrum of the same material in Figure 3 of ref.4 , which appears to show less of a tail than our spectrum in Figure 1b, indicates that the variations in the “NMR tail signal” may have more to do with the experimental set-up than with differences in sample structure. Our spectra are of high quality, with excellent signal-to-noise ratios due to careful experiment set-up followed by sufficient signal averaging, and reliable baselines due to the use of a Hahn spin echo with EXORCYCLE before detection.1 Without the Hahn echo,4 the spectrum will be distorted due to loss of signal, in particular of the broad tail, during the excitation-pulse dead-time. In addition, the “tail” at < 100 ppm is quite irrelevant with regard to ref. 1, the primary target of the Comment. As clearly stated in the title of ref.1, the main focus of that paper was on C=O and C=C sites; these have resonance positions >110 ppm, which are distinct from the “tail” at < 105 ppm. To try to analyze these carbon species using unresolved 1H NMR spectra, as suggested in point (ii) of the Comment, seems rather imprudent given dipolar broadening and overlap from signal of adsorbed H2O. Nanodiamond structural models. There is also general agreement between Dr. Panich and us on most aspects of the overall structure of nanodiamond particles. In Figure 10 of his review in 2012,3 he presented as his model of nanodiamond an only slightly modified copy of our structural model2 from 2009 (Figure 11). The small fractions of “sp2 flakes” and unspecified “bare spots”3 added by Panich3 could now be identified with the 1/6 of the surface (2.5% out of 12% surface carbons) that we have shown to be covered by C=O and C=C carbons.1 (In this context, it must be


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pointed out that it is not accurate to claim that in our model, “surface carbons are bound to H and OH groups”: The title of ref.1, “Quantification of C=C and C=O Surface Carbons in Detonation Nanodiamond by NMR” and the graphic in the abstract1 clearly show that there are also C=C and C=O surface carbons in our model.) A reviewer has let us know about “the conclusion of Dr. Panich that nanodiamonds show some sp2 carbon patches which can be removed by additional purification”, which confirms that we have agreement that properly purified nanodiamond is essentially free of sp2 patches. The undetectably small sp2 signals in all the spectra in Figure 1 underline that this level of purification is typical in commercial nanodiamond. In their Comment, Panich and Shames claim that a model with “several percent to several tens percent” of sp2 carbons “has been supported by HRTEM, Raman, and XPS measurements” that are quoted in Dr. Panich’s review (ref. 3). However, this is not in agreement with the literature on purified detonation nanodiamond, even that referenced by Dr. Panich himself3, as we will show here with specific quotes. HRTEM has problems due to “partial graphitization of the DND surface caused by electron beam irradiation”,3 and most TEM studies are consistent with a “crystalline structure with negligible fractions of non-diamond carbons” 3 or report that after proper purification, “almost all non-diamond carbon has been effectively removed from the surface” 3 (bold font added for emphasis). In addition, a recent TEM study 8 states: “No organized graphitic structures could be clearly distinguished on the surface of pristine NDs.”; in addition, “No graphitic signature could be detected at this stage” by XPS (bold font added for emphasis). So there is no HRTEM evidence for aromatic patches in pristine, properly purified detonation nanodiamond. Furthermore, it has been reported that “only diffraction peaks corresponding to the diamond structure … were observed in XRD patterns of the acid-purified nanodiamond powder.”3 The strong claim in ref.3 that “sp2 carbons in DND samples are always seen in Raman measurements” is not supported by the literature: the author had to admit, just a few lines down,3 that the supposed sp2-carbon band at 1620 cm-1 has actually been re-assigned9 to “O-H bending vibrations at 1640 cm-1”,3 making detection of a small fraction of sp2-carbon unreliable. A beautiful series of Raman spectra as a function of pyrolysis temperature 10 shows nearly complete loss of the 1640 cm-1 peak at 800oC, corresponding to the well-documented loss of OH groups, 1 before a graphitic band comes up at a nearby but distinct spectral position after pyrolysis at higher temperatures. Panich goes on to claim3 that in ref. 33, a paper of his on fluorinated nanodiamonds,


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“this band was definitely assigned to the sp2 carbons.” while in fact the paper contains only a weak statement “We believe that this peak is due to some curved sp2 fullerenelike shell”.11 In the same work,11 the 13C NMR spectrum (Figure 2) and XPS spectra (Figure 11) showed no signals of sp2 carbon, so the tentative interpretation of the peak in the fairly noisy Raman spectrum cannot be taken as an indication of a significant sp2 C=C carbon fraction in this nanodiamond derivative. In summary, there is no convincing Raman evidence for an aromatic-rich shell in pristine purified detonation nanodiamond. In the many published XPS spectra of pristine purified detonation nanodiamond, no resolved peak or distinct shoulder from sp2 C is visible. 8,11-15 Therefore, the identification or quantification of small amounts of sp2 carbon by this method is unreliable. Specifically, ref. 62 in the review 3 did not study detonation nanodiamond, but CVD films; in addition, the sp2 peak is still completely unresolved. Furthermore, it is claimed in ref. 3 that ref. 49 “reported 5% of sp2 carbons”, but this short, 4-page paper does not contain or mention XPS. At least five recent XPS studies do not support a model with a significant sp2-fraction. 8,12-15 As we had pointed out already in ref.1, “Our model has been confirmed by XPS, which shows signals assigned to crystalline and disordered sp3-hybridized carbon, but none of sp2-hybridized C, in pristine detonation nanodiamond.” Specifically, Butenko et al. 15 reported the “absence of a pronounced sp2 component” (bold font added for emphasis) in their XPS spectra even after heating to ~830oC, and concluded that even “annealing of ND at 1170 K is insufficient to produce appreciable graphitization.” In summary, the vast majority of the NMR, HRTEM, Raman, and XPS literature shows that the aromatic-carbon fraction in purified pristine detonation nanodiamond is small, often below the detection limit. This review of the literature, in conjunction with the similar NMR spectra of different samples in Figure 1, also indicates that, contrary to claims in the Comment, the surface structure of pristine purified detonation nanodiamond from different sources is remarkably similar, at least in terms of the small sp2:sp3-carbon ratio.

Location of unpaired electrons. We also all agree on the location of unpaired electrons in nanodiamond. While Panich and Shames first claim that our1 “statement …’that most unpaired electrons are not at the nanoparticle surface’ seems not to be accurate”, in the next sentence they assert the same observation (that “unpaired electrons … are not on the DND surface”) as their


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own discovery. The claim that Panich and Shames established the correct location of the unpaired electrons in 2002 is on shaky ground. Schmidt-Rohr and coworkers, in 2009, were the first to show the unpaired electrons at the depth of > 0.4 nm from the nonaromatic surface as generally accepted today, having since been confirmed by EPR16. The 2002 model17 assumed a few graphene layers around the diamond core, which is incorrect, as we agree now (see Figure 10 of ref. 3). The unpaired electrons then ended up outside (above) the diamond surface, while we now all agree that they are inside (under) the surface. In a review in 2007,18 Dr. Panich still showed the unpaired electrons at the surface of the diamond nanoparticle (see the very clear Figure 3 in ref. 18). So until right before our publication in 2009,2 Panich and Shames insisted (incorrectly, as they agree now) that the unpaired electrons are at the diamond nanoparticle surface, while in reality they are > 0.4 nm inside (below) the surface. In any case, the poorly supported claim of priority for this finding by the authors of the Comment should not distract from the agreement on the location of the unpaired electrons that we have achieved now. At the end of the paragraph on relaxation and unpaired electrons, the authors of the Comment fault us for not showing any “values of relaxation times” in ref.1, while in fact we had exhaustively analyzed the relaxation behavior of this same nanodiamond material in ref.2. This analysis included quantitative fitting for core, shell, and surface carbons relaxed by unpaired electrons in the nanodiamond particles, something Dr. Panich has not presented in his publications. We also note that in his own recent paper,4 Dr. Panich seems to have shown neither “values of relaxation times” nor the recycle delays used for his direct-polarization 13C NMR spectra. Heat treatment of nanodiamond. The authors of the Comment call the fraction of sp2 carbons in our nanodiamond sample annealed at 800oC “unrealistically small”. They ignored that in our paper,1 we had already pointed out that our moderate (~3%) aromatic-carbon fraction after heating to 800oC is generally consistent with the literature. The reported data show significant scatter, possibly due to different heat-treatment protocols: “A Raman spectrum showed spectral changes but no graphitic peak10 while C1s XPS studies found no detectable,15 7%,14 or 12-25% 8 sp2-hybridized carbon.”1 The difference in the series of Raman spectra with temperature in refs. 10 and 19 is particularly striking; after heating to 800oC, the former showed very little intensity while the latter exhibited strong graphitic peaks (corresponding to the strong C=C NMR signals


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observed by Panich et al.4). This confirms that the structural change with heat treatment is sample- or process-dependent.

Conclusions and outlook. Given the overwhelming evidence that pristine purified detonation diamond contains less than 2% aromatic C, it would be most productive to take this welldocumented fact as a starting point for exploring other interesting structural questions, such as the structure of the disordered nonaromatic shell, the effect of oxidative treatment, the structure of the aromatic shell generated by heat treatment/annealing/pyrolysis processes as well as the structure underneath the shell, the location of nitrogen in nanodiamond particles, the structure of defects, unusual crystal modifications in the core of nanodiamond particles, or structural changes due to chemical surface functionalization. Klaus Schmidt-Rohr1*, Jinfang Cui2 1

: Department of Chemistry, Brandeis University, Waltham, MA 02453, USA.

2

: Department of Chemistry, Iowa State University, Ames, IA 50010, USA

*Author to whom correspondence should be addressed at [email protected]

References Cited (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) Fang, X.; Mao, J.; Levin, E. M.; Schmidt-Rohr, K. Nonaromatic Core-Shell Structure of Nanodiamond from Solid-State NMR Spectroscopy; J. Am. Chem. Soc. 2009, 131, 14261435. (3) Panich, A. M. Nuclear Magnetic Resonance Studies of Nanodiamonds; Crit. Rev. Solid State Mater. Sci. 2012, 37, 276-303. (4) 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. Cond. Matt. 2013, 25, 245303.


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(5) Osswald, S.; Yushin, G.; Mochalin, V.; Kucheyev, S. O.; Gogotsi, Y. Control of sp2/sp3 Carbon Ratio and Surface Chemistry of Nanodiamond Powders by Selective Oxidation in Air; J. Am. Chem. Soc. 2006, 128, 11635-11642. (6) Donnet, J.-B.; Fousson, E.; Delmotte, L.; Samirant, M.; Baras, C.; Wand, T. K.; Eckhardt, A. 13

C NMR Characterization of Nanodiamond; C. R. Acad. Sci. Paris, Serie IIc, Chimie 2000, 3,

831-838. (7) Dubois, M.; Guerin, K.; Batisse, N.; Petit, E.; Hamwi, A.; Komatsu, N.; Kharbache, H.; Pirotte, P.; Masin, F. Solid State NMR Study of Nanodiamond Surface Chemistry; Solid State Nucl. Magn. Reson. 2011, 40, 144-154. (8) Petit, T.; Arnault, J. C.; Girard, H. A.; Sennour, M.; Kang, T.-Y.; Cheng, C.-L.; Bergonzo, P. Oxygen Hole Doping of Nanodiamond; Nanoscale 2012, 4, 6792-6799. (9) Mochalin, V.; Osswald, S.; Gogotsi, Y. Contribution of Functional groups to the Raman Spectrum of Nanodiamond Powder; Chem. Mater. 2009, 21, 273-279. (10) Chen, J.; Deng, S. Z.; Chen, J.; Yu, Z. X.; Xu, N. S. Graphitization of Nanodiamond Powder Annealed in Argon Ambient; Appl. Phys. Lett. 1999, 74, 3651-3653. (11) Panich, A. M.; Vieth, H.-M.; Shames, A. I.; Froumin, N.; Osawa, E.; Yao, A. Structure and Bonding in Fluorinated Nanodiamond; J. Phys. Chem. C 2010, 114, 774-782. (12) Butenko, Y. V.; Kuznetsov, V. L.; Paukshtis, E. A.; Stadnichenko, A. I.; Mazov, I. N.; Moseenkov, S. I.; Boronin, A. I.; Kosheev, S. V. The Thermal Stability of Nanodiamond Surface Groups and Onset of Nanodiamond Graphitization; Fullerenes, Nanotubes, Carbon Nanostruct. 2006, 14, 557-564. (13) Kuznetsov, V. L.; Butenko, Y. V. In Synthesis, Properties and Applications of Ultrananocrystalline Diamond; Gruen, D., Shenderova, O., Vul’, A., Eds.; Springer Netherlands: 2005; Vol. 192, p 199-216. (14) Petit, T.; Arnault, J.-C.; Girard, H. A.; Sennour, M.; Bergonzo, P. Early Stages of Surface Graphitization on Nanodiamond Probed by X-ray Photoelectron Spectroscopy; Phys. Rev. B Condens. Matter Mater. Phys. 2011, 84, 233407/233401-233407/233405. (15) Butenko, Y. V.; Krishnamurthy, S.; Chakraborty, A. K.; Kuznetsov, V. L.; Dhanak, V. R.; Hunt, M. R. C.; Šiller, L. Photoemission Study of Onionlike Carbons Produced by Annealing Nanodiamonds; Phys. Rev. B 2005, 71, 075420.


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(16) Shames, A. I.; Osipov, V. Y.; Aleksenskiy, A. E.; Osawa, E.; Vul, A. Y. Locating Inherent Unpaired Orbital Spins in Detonation Nanodiamonds through the Targeted Surface Decoration by Paramagnetic Probes; Diamond Relat. Mater. 2011, 20, 318-321. (17) Shames, A. I.; Panich, A. M.; Kempinski, W.; Alexenskii, A. E.; Baidakova, M. V.; Dideikin, A. T.; Osipov, V. Y.; Siklitski, V. I.; Osawa, E.; Ozawa, M.; Vul, A. Y. Defects and Impurities in Nanodiamonds: EPR, NMR and TEM Study; J. Phys. Chem. Solids 2002, 63, 1993-2001. (18) Panich, A. M. Solid State Nuclear Magnetic Resonance Studies of Nanocarbons; Diamond Relat. Mater. 2007, 16, 2044-2049. (19) Etzold, B. J. M.; Neitzel, I.; Kett, M.; Strobl, F.; Mochalin, V. N.; Gogotsi, Y. Layer-byLayer Oxidation for Decreasing the Size of Detonation Nanodiamond, Chem. Mater. 2014, 26, 3479-3484.


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