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Hydroxylated Detonation Nanodiamond: FTIR, XPS, and NMR Studies O. Shenderova,*,† A. M. Panich,‡ S. Moseenkov,§ S. C. Hens,† V. Kuznetsov,§ and H.-M. Vieth|| †
International Technology Center, 8100 Brownleigh Dr. Suite 120, Raleigh, North Carolina 27617-7300, United States Department of Physics, Ben-Gurion University of the Negev, Be 0er Sheva 84105, Israel § Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russia Institute for Experimental Physics, Free University Berlin, Arnimallee 14, D-14195 Berlin, Germany
)
‡
ABSTRACT: Detailed and unambiguous characterization of the surface structure of detonation nanodiamond (DND) particles remains one of the most challenging tasks for the preparation of chemically functionalized nanodiamonds. In the present paper, a combination of FTIR, NMR, and XPS was used to characterize DND particles that were treated in a reduction reaction that results in the enrichment of hydroxyl and hydroxymethyl functional groups. FTIR spectra and quantum-chemistry modeling demonstrated that the vacuum treatment of the sample, with the purpose of the removing adsorbed water and other volatile contaminates, is mandatory to obtain the correct data on the nature and relative content of the OH surface groups on DND. 13C and 1H NMR spectra show signals from the diamond core, hydroxyl, hydrocarbon groups, and moisture on the diamond surface. NMR data were taken for as-prepared DNDs, as well as those that were dried under vacuum conditions of 104 Torr, in order to distinguish between the NMR signal contributions due to moisture and other hydrogen-containing groups.
1. INTRODUCTION Diamond nanoparticles represent one of several classes of carbon nanostructures that have great potential for a variety of materials applications.13 They are currently produced in bulk quantities by means of detonation of carbon-containing explosives followed by purification from the detonation soot by chemical treatment. The resulting material, detonation nanodiamond, contains the diamond nanoparticles with primary particle size ∼45 nm. DND nanoparticles consist of a mechanically stable and chemically inert diamond core and a chemically active surface. Atomistic modeling,46 Fourier transform infrared spectroscopy (FTIR),7,8 temperature programmed desorption (TPD),8,9 nuclear magnetic resonance (NMR),1013 electron paramagnetic resonance (EPR),10,14 high resolution transmission electron microscopy (HRTEM),10,15,16 and X-ray photoelectron spectroscopy (XPS)17 studies show that the surface of diamond nanoparticles may have rather complicated structures that can include an sp2-like carbon coating, a reconstructed diamond surface, radicals, and a wide variety of functional groups. Since oxygen, nitrogen, and hydrogen are components of the carboncontaining explosive materials used for DND production, functional groups including these elements are present in abundance on the DND surface. Inhomogeneity of pristine DND surface hampers control of the surface functionalization including modification with biologically active moieties.18 Moreover, as it was pointed out by Chiganova,19 the hydrophilichydrophobic mosaic structure of the surface of DND particles has a strong influence on the aggregation behavior of the low-concentrated DND aqueous suspensions. Since surface energies of major diamond r 2011 American Chemical Society
facets are quite distinct and various surface groups have noticeably different binding energies with different diamond facets,2022 there can be fundamental reasons why uniformity of the surface groups on DND can not be achieved completely. Thus the long-term goals for modern DNDs are purity (absence of incombustible impurities and nondiamond carbon) and a surface that is chemically homogeneous. As it was pointed out by Krueger,18 further oxidation of typical DND as-received from a vendor can lead only to partially oxidized surface groups. If so, the reduction strategy was exploited transforming ketones, esters, carboxylic acids, and aldehydes to hydroxyl and hydroxymethyl functions using lithium aluminum hydride or borane in THF.18 Hydroxylated DND can be used as a starting material for a wide variety of subsequent functionalizations of the DND;18,23 this allows for further control of DND’s chemical and physical properties, as well as the development of related applications. Owing to the importance of hydroxylated DND, DND particles with the surface enriched with the OH groups had been studied in detail in the present paper by a variety of complementary techniques: NMR, FTIR, and XPS. It is known24,25 that peaks attributed to OH stretching and bending vibrations of adsorbed water obscure signatures of other moieties. Thus, in addition to the typical spectra taken in the air environment, the DNDOH samples were analyzed using a vacuum IR cuvette to avoid strong absorbance of adsorbed water and ensure the surface coverage by Received: June 8, 2011 Revised: August 9, 2011 Published: August 12, 2011 19005
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chemically attached hydroxyl groups took place. Also, the samples evacuated down to 104 Torr have been studied by NMR. As was reported by Krueger,18 thermal gravimetric analysis revealed that hydroxylated ND can adsorb up to 89 wt % of water, thus identification of chemically attached hydroxyls is of high importance. In the current approach, hydroxylation was realized using reduction with lithium aluminum hydride. Complementary techniques confirmed enrichment of the sample with hydroxymethyl species and the disappearance of carbonyl species.
2. EXPERIMENTAL SECTION Raw DND material was supplied by New Technologies, Co., Chelyabinsk, Russian Federation. The raw polydispersed nanodiamond material was produced using trinitrotoluene and cyclotrimethylenetrinitramine explosives by detonation in an icecooled chamber and purified using a solution of chromic anhydride in sulfuric acid. The average primary particle size for the detonation nanodiamond is 4 nm based on Rietveld XRD analysis. Additional purification of DND included treatment in the mixture of NaOH/H2O2, treatment by ion-exchange and fractionation down to 120 nm average volumetric particle size. Incombustible impurity content in the additionally purified sample is 0.6 wt.%. Elemental composition defined using Carlo Ebra EA 1108 CHNS Analyzer includes: C, 86.4 wt %; H, 0.5 wt %; N, 2.7 wt % (O, the rest). Resulting sample is called Ch-F6. A fraction of Ch-F6 with average aggregate sizes 30 nm was obtained by centrifugation. Zeta potentials of Ch-F6 and its 30 nm fraction were 45 mV. Another DND sample with negative zeta potential 45 mV and particle size 30 nm was obtained by centrifugation of material purchased from Real Dzerzinsk Inc., Russia. Reduced DNDOH was obtained as follows.23 Ch-F6 powder (5 g) was added to a round-bottom flask, which was then purged several times using standard Schlenk techniques. To the round-bottom flask were added 20 mL of degassed anhydrous tetrahydrofuran (THF) and 50 mL of a 2.0 M solution of LiAlH4 in THF by cannula. The sample was stirred under a nitrogen atmosphere at room temperature overnight. The reaction was quenched by dropwise addition of 1 M HCl, which solubilized the lithium and aluminum, and then the solution was basified to neutral pH. The product was collected by centrifugation and rinsed several times with water followed by drying with acetone. The product was dried in vacuum at 100 °C for 12 h. X-ray photoelectron spectroscopy (XPS) analysis did not reveal residual lithium or aluminum. FTIR analysis of the composition of the surface groups for initial and hydroxylated DND samples was performed using a Shimadzu FTIR-8300 spectrometer. For FTIR measurements, 10 mg of the ND sample was mixed with 320 mg of KBr powder and pressed into plates 0.50.7 mm thick with pressures up to 150 kg/cm2. The tablets were placed in an IR vacuum cell and heated at 100200 °C under vacuum (1 102 Torr) for 1 h to remove traces of adsorbed water. Following this procedure, FTIR spectra were recorded without exposing the samples to air to avoid the influence of atmospheric water on the spectra. XPS analysis was performed using a Kratos Axis Ultra with a monochromatic Al Kα X-ray source and a silicon wafer as a supporting substrate. The survey and high resolution scans were performed at 160 eV pass energy with 1 eV step and 10 eV pass energy with 0.1 eV step, correspondingly. The peak position calibration was done against Si 2p3/2 at 99.3 eV.26 The collected data were processed with CasaXPS software.
Figure 1. Schematics for the transformations of different surface groups existing on a typical DND that are converted into hydroxymethyl functions using a reduction reaction.
Zeta-potential and particle sizes were measured with a Malvern Zetasizer Nano ZS instrument. All NMR data have been collected at room temperature. The 13 C MAS (magic angle spinning) and 1H13C cross-polarization MAS (CPMAS) NMR measurements have been carried out at a resonance frequency of 150.858 MHz (B0 = 14.0954 T) with a spinning rate of 10 kHz. 13C chemical shifts are referenced to tetramethylsilane (TMS). Static 1H spectra, 1H and 13C spinlattice (T1) and spinspin (T2) relaxation times have been measured in magnetic field of B0 = 8.0196 T at resonance frequencies of 341.41 MHz for 1H and 85.85 MHz for 13C (i) at ambient conditions and (ii) after pumping out the DNDOH sample down to 104 Torr at room temperature and sealing into a glass tube. IR spectroscopy and NMR allow obtaining some data on the structure of the water molecules bound with the ND surface, but they do not make it possible to separate the individual configurations and assess their stability. This can only be done using computer modeling methods, quantum-chemical calculations in particular. The calculations were carried out using a standard semiempirical method, namely, PM327,28 (HyperChem 8,0 software of Hypercube, Inc.), which allows for satisfactory calculation of the structural features of chemical systems with hydrogen bonds. The calculations are within restricted HartreeFock theory and include only the valence electrons. 19006
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Figure 2. FTIR spectra of DND starting material Ch-F6 (top) and hydroxylated DNDOH (bottom). Spectra are taken for the ND-KBr pellet exposed to air and the pellet treated under vacuum in an IR cuvette at 200 °C to remove adsorbed water.
3. RESULTS AND DISCUSSION The parameters of nanodiamond slurries that are important for applications are the average aggregate size and the zeta potential, which characterize the slurry’s colloidal stability. The zeta potential of the starting material, Ch-F6, is 45 mV and has an average aggregate size of 120 nm. After the reduction reaction, the zeta potential was decreased to 38 mV, but no significant change in the average aggregate size was observed. When a mixture of starting material of 3040 nm in size with a positive zeta potential 45 mV and a negative zeta potential 45 mV (2:1 by weight) underwent a reduction reaction, the zeta potential of the reduced ND became 38 mV and the average aggregate size 3040 nm was preserved. It is important for many applications that the colloidally stable NDOH with a 3040 nm average aggregate size can be produced. LiAlH4 is expected to convert esters, carboxylic acids, aldehydes, anhydrides, lactones and ketones into their corresponding alcohols (Figure.1). As such, carbonyl related signatures in spectral signatures should decrease in intensity while the alcohol group signature intensities should increase. Since the resulting products are not only pure hydroxyl groups attached to the diamond surface but also include hydroxymethyl functionalities (Figure.1), the appearance of more pronounced signatures of hydrocarbons is expected. In the following sections, we provide detailed FTIR, XPS, and NMR spectral analysis for the starting and reduced nanodiamonds surface groups. Shown in Figure 2 are FTIR spectra taken in air and in vacuum for the initial Ch-F6 and reduced DNDOH samples. Due to the oxidative treatment of soot with a mixture of Cr2O3/H2SO4, as well as the additional oxidation of nondiamond carbon using a mixture of NaOH/H2O2, the surface of Ch-F6 DND contains various oxygen-containing groups, such as the following: gCOCe (ν 11001370 cm1 for ethers, acid anhydrides, lactones, epoxy groups) and carbonyl >CdO peak at 1730 cm1 (17001865 cm1 region is assigned for carbonyls in ketones, carboxylic acids, acid anhydrides, esters and lactones). It is important to note that adsorbed water provides strong absorption bands in the 35003300 (max 3420 cm1 νOH) and 16201630 cm1
(bending mode) regions. As can be seen from Figure 2, the FTIR spectrum of the initial DND Ch-F6 taken in air contains peaks related to adsorbed water and possibly to hydroxyl groups of DND (signatures for νOH of gCOH are located in the 32003600 cm1 region and are related to hydroxyl groups of carboxylic acids and alcohols). However, the dehydrated Ch-F6 sample peaks in the 35003300 cm1 region diminish appreciably (Figure.2); this result suggests the removal of adsorbed water, as well as a small amount of hydroxyl-containing groups that are chemically bonded to ND Ch-I6. The peak at 1630 cm1 also significantly diminishes after heating at 200 °C in vacuum and a peak with a maxima at 1594 cm1 becomes well pronounced instead (possibly this peak was overshadowed by 1630 cm1 peak in the sample with adsorbed water). This peak can be attributed to other functionalities on the carbon surface such as amide related bands or stretching vibrations of aromatic CdC bonds, which are polarized by oxygen atoms bound near one of the carbon atoms.29 In the reduced sample, carbonyl related peaks should disappear and the alcohol related groups should become more pronounced. The appearance of the more pronounced signatures of hydrocarbons is also expected. As can be seen in Figure 2 from the spectra of DNDOH sample taken in both air and in vacuum, the carbonyl peak of 1730 cm1 is well pronounced in the Ch-F6 sample, is indeed greatly diminished in DNDOH, and disappears in DNDOH treated in vacuum. The FTIR spectrum of the reduced DNDOH sample shows several intensive peaks that are absent or insignificant in the Ch-F6 sample and that are attributed to the vibrations of the OH group. The strongest evidence that the hydroxylation reaction took place is the presence of a band around 34003500 cm1 (stretch vibration of the OH group), which exist in the spectrum taken in vacuum after desorption of water. In addition, the peak at 1630 cm1 assigned to OH bending vibrations is also observed in the spectra taken in air and in vacuum. An appearance of a strong band at 1572 cm1 (aromatic CdC bonds) took place. New peaks within the 12501400 cm1 region can be attributed to δOH vibrations. Peaks within the 10001200 cm1 region can be assigned to CO vibrations in alcohol groups. 19007
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Figure 3. Illustration of DND cluster with 3 (a) and 7 (b) adsorbed water molecules used in the calculation of the vibrational density of states (c).
Aliphatic CH stretching vibrations at 28003000 cm1 are present with a significant abundance on both Ch-F6 and DNDOH samples. Those can be attributed to symmetric and asymmetric stretching vibrations of CH3 (2957 and 2880 cm1) and CH2 (2924 and 2851 cm1), correspondingly.8 Peaks at ∼1462 (as) and 1375 cm1 (s) and ∼14401465 cm1 can be ascribed to δCH for CH3 and CH2, correspondingly. From Figure 2, it can be seen that adsorbed water has a significant influence on the spectra: the intensity of the vibrations of some surface groups is greatly diminished, relative intensity of bands are changed, and the position of the bands can be shifted and some bands can disappear. These changes can be explained by the interaction of the adsorbed water with surface groups on DND, mostly due to the formation of hydrogen bonds with oxygen-containing groups, resulting in shifting of the bands position and changes in the intensity of vibrations. First of all, the presence of water effects the position and intensity of the oxygen-containing groups in the vibrational spectra. Therefore, heat treatment of the sample in vacuum is a mandatory process for correct identification of the surface groups. In addition, as can be seen in Figure 2, during vacuum treatment of the initial Ch-F6 sample, bands attributed to CH3 groups disappear. The presence of chemically attached CH3 groups on a surface of DND particles purified from sp2 carbon by oxidation is unlikely, since during purification such groups should be oxidized into carboxylic acid or alcohol groups. Thus during vacuum treatment the contaminant from volatile hydrocarbons, containing CH3 groups, were removed. The presence of volatile hydrocarbons on DND surface was also confirmed in thermal programmed mass spectrometry studies.9 This fact reemphasizes importance of vacuum treatment of DND samples before spectroscopic studies with a purpose of removing hydrocarbon-type volatile impurities which can be adsorbed during purification or from air. To further understand the role of adsorbed water on the position and intensity of the vibrational bands of DND samples, molecular modeling of the vibrational spectra was performed using a semiempirical PM3 approach at temperature 0 K. Twolayer cluster C34H35(OH)3, shown in Figure 3a,b, were chosen to model the interaction of the fragment of hydroxylated (111)
diamond surface with water molecules. The top carbon atoms (surface layer) contain an equal number of COH and CH groups. The unsaturated bonds of the bottom layer and edges of the top layer were terminated with hydrogen atoms to get closed cluster structure and avoid dangling bonds. We have started using the positioning of water molecules near the surface of clusters. ND clusters terminated with H and OH groups containing 3 and 7 molecules of adsorbed water were used in the molecular dynamics simulations (Figure 3a,b). The size of a bare cluster does not influence significantly the band position of a free COH group, whereas the variation of the number of water molecules around a COH group leads to a variety of new bands because all of these molecules form a variety of adsorbed ensembles with a variable number of hydrogen bonds. Figure 3c illustrates the vibrational density of states of the original DND cluster, as well as the cluster containing 3 and 7 molecules of the adsorbed water. As can be seen from Figure 3c, the number of adsorbed water molecules is increased, and additional vibrational bands appear in the spectra, due to hydrogen bonding between water molecules and OH groups on the DND surface (Figure 3a,b, dotted lines). Simultaneously, the intensity of vibrations is increased, as can be seen for CH and CH2 bands. Changes in the relative intensity of the bands can be also clearly seen (especially in the low-frequency region of the spectra). In addition, vibrations of the adsorbed water molecules themselves also mask vibrations of the surface groups. These results qualitatively demonstrate that adsorbed water must be removed to get the information on the surface groups. Molecular dynamics simulation using PM3 method for the calculation of interaction of the water molecules with diamond cluster surface was also repeated for the temperature range 0300 K. The results of the simulations demonstrated the stability of adsorbed ensembles at low temperatures but their decomposition at elevated temperatures (starting at T higher than 130 K). Thus, as can be seen from the experimental spectra and from the results of the quantum-chemistry modeling, vacuum treatment of the sample with the purpose of removing adsorbed water and other volatile contaminates is a requirement for obtaining the correct data on the nature and relative content of the surface groups on DND. 19008
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Figure 4. High resolution C1s and O1s XPS spectra of DND starting material Ch-F6 (a and c) and hydroxylated ND (b and d).
Figure 4 illustrates the XPS spectra for the starting Ch-F6 and the hydroxylated DND samples. After deconvolution of the C 1s high-resolution spectra, several characteristic peaks are suggested in the initial Ch-F6 and hydroxylated DND samples attributed to carbon in sp2 (∼ 282.3 eV) and sp3 (∼283.2 eV) type environments and in the oxygen-containing hydroxyl, ether, carbonyl, and carboxylic acid groups.30,31 A fitted peak, indicative of hydroxyl groups (∼283.6 eV), increased relative to the sp3 diamond peak in the deconvoluted spectra as well as the same can be concluded from the broadening of the top of the experimental not-deconvoluted peak (Figure 4a,b). Since part of the oxygen groups in the starting material can be confined within aggregates, they are inaccessible to the reducing media and will thus still contribute to the corresponding signatures in the DNDOH sample. In addition, due to the complicated nature of the surface structure of detonation ND with multiple types of groups and its existence in the form of tight aggregates, spatial hindrance may affect the reactivity of different surface groups. Furthermore, it is known that different nanodiamond facets themselves have different binding energies that will result in a different reaction potential of these surfaces.20 Thus, one can expect that the intended
reactions will not proceed for all groups of a given type present on the surface of the DND aggregates. On the contrary, the O1s spectra in Figure 4c,d clearly demonstrates that the amount of nonhydroxyl functionalities was greatly reduced and disappeared, whereas a peak related to hydroxyl groups (∼529.3 eV) became very well pronounced. The 13C MAS NMR and 1H13C CPMAS spectra of the DNDOH sample are shown in Figure 5. The only peak at 35 ppm in the 13C MAS spectrum is definitively assigned to the sp3 carbon atoms of the diamond core. The 1H13C CPMAS spectrum exhibits three features. The peak at δ = 37 ppm arises from the carbon atoms of the diamond core located close to the surface. The shoulder at δ ≈ 45 ppm is assigned to the signals of the CH and CH2 groups.32 The peak at δ = 71 ppm is assigned to the COH groups.32 The ratio of the intensities of COH to CH and CH2 signals is larger than that in nonhydroxylated DND.11,33 13 C nuclear spinlattice relaxation time T1 of the diamond core carbons of DNDOH, T1 = 542 ( 29 ms, is several orders of magnitude shorter than that in natural diamond, in which T1 varies from several hours to several days.3438 At that, the magnetization recovery in DNDOH sample is described by a 19009
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Figure 5. (a) 13C NMR MAS and (b) 1H13C CPMAS NMR spectra of DNDOH sample.
Figure 6. (a) 1H NMR spectrum of the as-prepared DNDOH sample. Deconvolution into two components is shown by dashed lines. Separately detected narrow component is shown in inset. (b) 1H NMR spectrum of the DND-OH sample after pumping out down to 104 Torr. Deconvolution into two components is shown by dashed lines.
stretched exponential M(t) = M∞{1 exp[(t/T1)α]} with α = 0.55 ( 0.03. Similar effects have been obtained in a number of DND samples10,11,3941 and attributed to the interaction of nuclear spins with unpaired electrons of paramagnetic defects detected
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Figure 7. 1H spin echo decay (T2 measurements) of the as-prepared DNDOH sample on semilogarithmic and linear (inset) scales. Inset reveals an initial part of the decay at short times (up to 0.4 ms).
by EPR. We note that the values of T1 > 500 ms are characteristic of well-purified DNDs, which was supported by our EPR measurements. Hydrogen atoms at the DND surface become apparent in the 1 H NMR measurements. 1H NMR spectrum of the as-prepared DNDOH sample consists of two lines (Figure 6a). A broad component with a line width Δν ≈ 25 kHz is attributed to closely set rigid hydrocarbon and hydroxyl groups, whereas the narrow component showing Δν ≈ 2 kHz is mainly assigned to the moisture adsorbed on the DND surface. The obtained broad line reflects strong dipoledipole interactions among the 1H spins that arises from the clustering of the hydrogen atoms, such that hydrogenated spots of limited sizes alternate with nearly nonhydrogenated zones in the sample under study. Similar clustering of hydrogen and fluorine atoms has recently been obtained in the initial DND42 and in fluorinated DND,39 respectively. The assignment of the narrow component results from the hydrophilicity of the DND surface18 and from the significant reduction of the intensity of the narrow component after pumping out the sample down to 104 Torr (Figure 6b). The latter fact reveals that nearly all physisorbed water has been removed with the vacuum treatment. One can definitively distinguish between both the narrow and the broad lines of the NMR spectrum by measuring the 1H NMR spectrum with dipolar dephasing, using a long delay between the pulses of the spin echo sequence, such as this delay exceeds the spinspin relaxation time T2 of the broad line. This makes the broad component disappear and allows for the detection of the narrow component only (inset in Figure 6a). The 1H spinspin relaxation time (T2) measurements of the as-prepared OH-DND sample shows that the magnetization is well described by a superposition of two exponentials, corresponding to two resonance lines observed in the experiment and assigned to the two hydrogen species. It is clearly seen from both linear and semilogarithmic plots of the magnetization decay (Figure 7). Here, the longer T21 = 2.16 ( 0.02 ms is assigned to moisture (narrow line), while the shorter T22 = 0.025 ( 0.001 ms is assigned to the hydroxyl and hydrocarbon groups covalently bound to carbon atoms (broad line). 1 H spinlattice relaxation measurement of the as-prepared DNDOH sample yields a confusion pair of T1’s around 41 ms for the narrow and broad spectral components. However, the exhausted sample reveals something quite different: T11 = 124.8 ( 13.1 ms for the OH and CHx groups and T12 = 84.2 ( 1.6 ms for a small portion of a residuary moisture. Elongation of the spinlattice relaxation time after pumping out the sample is caused by two reasons: (i) by removal of the major portion of 19010
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4. SUMMARY Reduction with lithium aluminum hydride of a DND sample containing a broad range of different surface groups was demonstrated. It was possible to produce DNDOH with average particle size 3040 nm, which is appealing for numerous applications. Characterization with 1H13C CP MAS and 1H MAS NMR, XPS, and FTIR measurements shows the appearance of the intensive signal of the OH groups after the functionalization procedure. Similar to DND functionalized with other surface groups, short 13C spinlattice relaxation time and nonexponential behavior of the 13C magnetization recovery, found in the T1 measurements of DNDOH, are attributed to the interaction of nuclear spins with the paramagnetic centers. Experimental FTIR and NMR spectra and results of the quantum-chemistry modeling demonstrate that a vacuum treatment of DND samples with a purpose of removing adsorbed water and other volatile contaminates is a mandatory process for obtaining correct data on the nature and relative content of the surface groups on DND. This treatment is critical in studies of hydroxylated DND. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT O.S and S.H. are thankful for the support from the Space and Naval Warfare Systems Centers (SSC) under Grant N66001-041-8933. The work in Israel was partially supported by the New Energy Development Organization of Japan (NEDO) Grant No. 04IT4 and MOST-RFBR Grant No. 3-5708. We greatly acknowledge the help of Garry Cunningham in sample preparation, Mark Walters of SMIF, Duke University for taking XPS spectra, A. Shames for EPR measurements, and Mark Hens and Jason Kelly for discussions on the paper. ’ REFERENCES (1) Ultrananocrystalline Diamond: Synthesis, Properties and Applications; Shenderova, O., Gruen, D., Eds.; William-Andrew Publishing: Oxford, U.K., 2006. (2) Shenderova, O. A.; Zhirnov, V. V.; Brenner, D. W. Crit. Rev. Solid State Mater. Sci. 2002, 27, 227–356. (3) Krueger, A. Chemistry 2008, 14, 1382–1390. (4) Raty, J. Y.; Galli, G. Nat. Mater. 2003, 2, 792–796. (5) Raty, J. Y.; Galli, G.; Bostedt, C.; van Buuren, T. W.; Terminello, L. J. Phys. Rev. Lett. 2003, 90, 037401/1–4. (6) Barnard, A.; Sternberg, M. J.Mat.Chem. 2007, 17, 4811–4819.
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