Surface Modifications of Detonation Nanodiamonds Probed by

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Surface Modifications of Detonation Nanodiamonds Probed by Multiwavelength Raman Spectroscopy Michel Mermoux,*,†,‡ Alexandre Crisci,§,∥ Tristan Petit,⊥,# Hugues A. Girard,⊥ and Jean-Charles Arnault⊥ †

Université Grenoble Alpes, LEPMI, F-38000 Grenoble, France CNRS, LEPMI, F-38000 Grenoble, France § Université Grenoble Alpes, SIMAP, F-38000 Grenoble, France ∥ CNRS, SIMAP, F-38000 Grenoble, France ⊥ CEA, LIST, Diamond Sensors Laboratory, F-91191 Gif-Sur-Yvette, France ‡

S Supporting Information *

ABSTRACT: Detonation nanodiamonds (DND) with different surface chemical and structural features were characterized using multiwavelength Raman analysis, from deep UV to more conventional visible wavelengths. In particular, effects of DND surface modifications induced by annealing (air or vacuum) or by hydrogen microwave plasma on the line shape of their corresponding Raman spectra were investigated. Indeed, the different surface treatments led to specific surface terminations and/or different shell structures or surface reconstructions. In addition, annealing treatments in air or hydrogen have also been performed along with in situ Raman spectroscopy measurements. This original combination allows a better understanding of the corresponding surface modifications. The effects of the different surface terminations on the line shape of the spectra as well as the choice of the Raman experimental conditions are discussed. The line shape of the Raman spectrum of DND is also discussed in detail.

1. INTRODUCTION Detonation nanodiamond (DND) has attracted a lot of attention over the past two decades, see for example refs 1−6, and references therein. It is produced in bulk quantities, tens of tons annually, by means of detonation of carboncontaining explosives followed by a thorough purification from the detonation soot using different chemical treatments.1,2 Different sources are now commercially available. In material sciences, those nanoparticles have great potential for a large variety of applications, including composite materials, lubricants, polishing compositions, electrode materials,1−3 seeds for further CVD diamond growth, etc.7,8 In addition, they have recently attracted wide interest in the field of biomedical applications9,10 because of some remarkable properties such as low toxicity and biocompatibility,11−13 the possibility of sophisticated surface functionalization,14,15 and their ability to fluoresce when excited with ultraviolet or visible wavelengths.16−18 These assets make DND a potential candidate for a wide range of applications in biotechnologies and medicine, including targeted drug delivery, bioimaging, biosensors, implant coating, or even artificial substrates for tissue engineering.19,20 For all these applications, a controlled and well-defined surface termination of DND is mandatory because their surface properties may alter significantly their chemical affinity.14,15 © XXXX American Chemical Society

Moreover, the surface properties, or more exactly the surface functional groups of DND particles, have a strong influence on the aggregation behavior of the low-concentrated DND aqueous suspensions.21,22 DND isolated particles are commonly described according to a core−shell model including a crystalline diamond core of about 4 nm in diameter including crystallographic defects like twins or dislocations,23 surrounded by a ∼0.7 nm thick more or less amorphous “shell”, and a chemically active “surface”. Atomistic modeling,24−27 Fourier transform infrared spectroscopy (FTIR),28−31 temperature-programmed desorption (TPD),32 nuclear magnetic resonance (NMR),33−39 electron paramagnetic resonance (EPR),33,38 high resolution transmission electron microscopy (HRTEM),23,40 and X-ray photoelectron spectroscopy (XPS)31,41 studies showed that the surface of diamond nanoparticles may have rather complex structures that may include mixed sp2/sp3 bonds, an sp2-like carbon surface, different reconstructed diamond surfaces (the so-called fullerene-like reconstructions24), radicals, and a large variety of oxygen-containing functional groups. DNDs containing a large amount of sp2 carbons are often labeled Received: July 23, 2014 Revised: September 12, 2014

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dx.doi.org/10.1021/jp507377z | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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“bucky diamond”24,40 or “onion-like carbon” when they are fully graphitized.42,43 Various protocols have been reported to control DND surface terminations. Thermal annealing in air at about 400− 450 °C leads to removal of the carbon soot and surface oxidation of DND,44 leading among other things to the creation of carboxylic groups. Vacuum annealing in the 700− 1200 °C temperature range promote surface sp2 reconstructions and progressive graphitization of the DND42,45,46 to create onion-like structures. Ozone treatment of DND mainly leads to the formation of carboxylic anhydride groups.47 Hydrogen treatment of DND in microwave plasma leads to the formation of a hydrogenated surface.48 In most of these studies, fullerene or onion-like reconstructions are highlighted with the help of HRTEM images while FTIR, XPS, and NMR are mostly used to probe the carbon/oxygen and carbon/ hydrogen bonds on the particles. At present, if the surface chemistry of the DND seems to be well controlled,14 there is no clear consensus on the surface structure of these nanoparticles and on the nature of the “shell”. In particular, it is suspected that the origin of the DND, thus the detonation synthesis conditions, may strongly affect their properties. Raman spectroscopy is known as a method of choice for the analysis of carbon materials and carbon nanostructures, allowing the identification of the type of bonding and estimations of the size of coherent domains.49,50 Furthermore, in some specific cases, this method can also be used to control surface modifications of carbon materials, in particular when strongly conjugated molecules are grafted.51,52 In such a case, monolayers can be detected and their chemical or electrochemical reactivity studied. Concerning DND, the use of Raman spectroscopy was mainly limited to simple identification of the diamond phase. However, over the past decade, there is an ongoing effort to understand the different features appearing in the Raman spectrum of DND.37,43,53−61 The Raman spectrum of purified DND usually consists of several characteristic features:53,56,61 (i) the first-order Raman mode of the cubic diamond lattice which is broadened and redshifted by about 3−8 cm−1 compared to bulk diamond, (ii) broad features with two apparent maxima in the 500−1250 cm−1 range, and (iii) a broad asymmetric line peaking in the 1600−1650 cm−1 (hereafter named “1650 cm−1 peak”) range, depending on the sample origin and its purification. The broadening and redshift of the diamond line is usually interpreted according to the phenomenological phononconfinement model,60,62,63 while various origins for the 1650 cm −1 peak are still proposed and discussed. Several interpretations, such as sp2 carbon,56 sp2 clusters,64 mixed sp2/sp3,61 surface hydroxyl groups,54 localized interstitial CC pairs or similar defects within the diamond lattice65−67 have been proposed. In addition, taking into account the complex surface chemistry of DND, this broad band may also result from overlapping contributions of different origins. DND are also characterized by a high surface/volume ratio. Crude calculations show that 5 nm diamond nanoparticles are made of about 11 000 carbon atoms. A 0.7 nm thick disordered shell should concern at least 30% of all carbon atoms. Finally, a recent NMR study39 convincingly shows that together, the observed C−H, C−OH, CO, and CC groups account for 12−14% of all carbon atoms, which matches the surface fraction expected for bulk-terminated 5 nm diameter diamond particles. According to the same paper, the sp2 carbon fraction

is only about 1% in pristine purified DND, while other studies report sp2 contents as high as 6%.36 Taking into account strong differences in scattering cross section of the different functional groups, these estimates tend to indicate that the DND surface or subsurface chemistry should affect the line shape of DND Raman spectra. In order to clarify the origin of the different features observed on the Raman spectra of DND, we performed a multiwavelength analysis on DND with well-defined surface chemistries. Thermal or plasma treatments were used to tune surface terminations and/or shell structure of DND from a same origin. Modifications in the Raman spectra line shapes were studied and correlated to surface terminations. In addition, in situ Raman analyses were also performed as an attempt to control annealing treatments in air or hydrogen.

2. EXPERIMENTAL SECTION Samples. Most of the experimental work was performed with detonation DND provided by NanoCarbon Research Institute Ltd. Their typical grain size was in the 3−6 nm range. High resolution transmission electron microscopy (HRTEM) pictures of the as-received DND are given in ref 68, in which the diamond core and the shell structure are clearly observed. The as-received powder was used without any further modification. Oxidation of the DND was performed by annealing in air, for a temperature of 400 °C during 4 h. For hydrogenation, samples were treated in an experimental MW plasma setup already described elsewhere.48 Briefly, DND were exposed for 20 min to a pure hydrogen (>N70) plasma using a Downstream source (Sairem). Typical process parameters were 300 W (2.45 GHz) and 15 mbars. Hydrogenation led to the complete removal the oxygen from the sample surface, as probed by XPS and FTIR (Supporting Information, Figure S1).48,68,69 One specific hydrogenated sample was analyzed. Finally, to promote some surface graphitization, the raw DND were progressively exposed to annealing treatments at 700, 900, and 1100 °C in ultrahigh vacuum conditions, maintaining the chamber pressure below 5 × 10−9 mbar.46 Two different samples were available, obtained at 700 and 1100 °C respectively. For Raman analysis, some of the DND powders were directly observed under the microscope. However, most of the samples were also dispersed in water and then deposited by drop casting on different substrates. Silicon substrates were preferred, giving a nearly perfect flat background signal in the 1050−2000 cm−1 wavenumber range. For comparison purposes, estimation of the sample temperature under the focused laser beam in particular, High Pressure High Temperature (HPHT) nanodiamonds (mean diameter: 20 nm) from Van Moppes and detonation nanodiamond from Sinta (mean size in the 4−8 nm range) were used. Preliminary measurements were also conducted on DND samples from PlasmaChem. Raman Measurements. Micro-Raman measurements were performed using three different instruments. The first was a Jobin-Yvon T64000 triple monochromator spectrometer that allowed measurements in both the visible and UV spectral ranges. For this purpose, it was equipped with a UV-enhanced liquid-nitrogen-cooled CCD detector, a microscope, two different confocal optics, and interchangeable gratings (2400, 1800, and 600 grooves/mm). The second and third instruments were Renishaw RM 1000 and InVia spectrometers that allowed measurements in the visible, near-UV, and near-IR spectral ranges. They were both equipped with an air-cooled B



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CCD detector and a microscope. For all instruments, a 50× (numerical aperture or NA = 0.85) objective and a UVdedicated 40× (NA = 0.5) objective were used to focus the laser on the sample surface and collect the scattered light. In the present work, samples were examined at different excitation wavelengths. The different excitation wavelengths used were the 785 nm line of a laser diode, the 514 nm line of a krypton−argon laser, the 363 nm line of an argon laser, the 325 nm line of a He−Cd laser, and the 244 nm line of a frequencydoubled argon laser. Using the 785 nm excitation wavelength, spectra were systematically dominated by a strong photoluminescence (PL) background and within some exceptions, it was nearly impossible to extract with certainty the Raman signal of the particles from the PL background. As a consequence, results obtained at this specific wavelength will not be discussed in the following. Using the 363 and 514 nm excitations, the Raman spectra were also observed superimposed on a strong PL background. This PL background could be fitted to polynomial (cubic) functions or cubic spline interpolations in the 100−2500 cm−1 wavenumber range and then subtracted to give a Raman spectrum of the nanoparticles. The different procedures used to correct for the PL background led to similar spectra line shapes. Spectra obtained with the 325 and 244 nm excitations were essentially PL-free.

Figure 1. Diamond peak position as a function of laser power at the sample surface for the 325 nm excitation wavelength. Blue trace: HTHP nanodiamonds from Van Moppes (mean size 20 nm). Black trace: detonation nanodiamond from Sinta (mean size 4−8 nm). Dotted line: observed trend from different sets of measurements for Nanocarbon Research Institute detonation nanodiamond (mean size 5 nm). Sample temperature was estimated form the line shift of the HTHP Raman diamond line, see ref 70. Unless specified, most of the experimental data discussed in this paper have been recorded for incident powers lower than 200 μW at the sample.

3. RESULTS Suitable Raman Experimental Conditions for DND Analysis. The main problem in Raman spectroscopy of DND was most probably related to the laser radiation commonly used as the excitation source. Local heating caused by the focused laser light must be taken in account, since it may affect the Raman spectrum even at relatively low laser power levels. In particular, it was observed that UV excitations used to get PLfree spectra induce considerable sample heating compared to visible ones and may lead to thermal damage and/or changes in sample composition. Even if no dramatic changes in the shape of the spectra could be observed with respect to the power at the sample, some damages at the sample surfaces could be visualized using simple optical microscopy. Thanks to neutral density filters, the power at the sample could be adjusted over a wide range. In order to monitor the impact of laser power on the DND Raman spectra, 20 nm HPHT nanodiamonds and 4− 8 nm SINTA DND were characterized with laser power at the sample ranging from about 30 μW to 1.5 mW. The sample temperature was evaluated from the measured line shifts.70 These nanodiamonds give relatively intense and narrow signals, and consequently line shifts can be quite easily detected. The main results are given in Figure 1. At 325 nm, strong heating is effectively observed for moderate laser powers for both kinds of particles, even if the effect is less intense for 20 nm particles. For about 1 mW at the sample, the DND effective temperature may be as high as 400 °C. This is clearly a problem for DND which can be strongly oxidized above 400 °C.44 Laser-induced heating was more difficult to estimate for the DND from Nanocarbon Institute studied here; the dotted line in Figure 1 shows the observed trend from different sets of measurement and is simply a guide for the eyes. Thus, depending on the excitation wavelength, we systematically tried to find a compromise between signal-to-noise ratio and acquisition times. In particular, using UV wavelengths, acquisition times could be as high as 1 h. To avoid some possible interferences or chemical modifications using deep UV wavelengths, no cooling

agents, water in particular, were used. Instead, the incident laser power was minimized. Characteristic Features of the DND Raman Spectrum. Figure 2 shows a typical Raman spectrum of the as-received

Figure 2. Raman spectrum of a Nanocarbon Research Institute detonation nanodiamond sample. For comparison purposes are also given the spectra of vitreous carbon (sample annealed at 1600 °C), graphite (HOPG sample) and the diamond second-order spectrum. The inset gives the diamond experimental and calculated phonon density of states, adapted from ref 75.

DND from Nanocarbon Research Institute. The excitation wavelength was the 325 nm line of the HeCd laser, allowing us to get a PL-free background. It is compared to spectra of three references carbon samples analyzed with similar acquisition conditions: a diamond single crystal (for clarity, only the second-order region is displayed), a graphite (HOPG) crystal, and a vitreous carbon sample heat-treated at 1600 °C. The main features in the DND first-order spectrum are seen to be a sharp asymmetric peak centered at about 1328 cm−1 and a broad asymmetric band peaking near 1650 cm−1. Broad features C



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are also observed in the 300−1000 cm−1 and the 1000−1350 cm−1 ranges. Finally, high frequency lines at about 2900 and 3300 cm−1 may be also distinguished. Spectra obtained for the two DND origins (NanoCarbon and PlasmaChem) overlap almost perfectly. The line shape of this spectrum, in the 900− 2000 cm−1 range in particular, is also found to be similar to those that can be found in the literature33,44,53−61 for different DND origins. This suggests that the characteristic features of the DND Raman spectrum do not strongly depend on their origins and are rather independent of the purification treatments, providing their mean sizes are similar. With the acquisition conditions used, the overall line shape of this spectrum was nearly independent of the surface chemistry, oxygen or hydrogen terminated surfaces. We shall return to this point later. Note that similar features are also found in spectra of ion beam amorphized diamond.65,66 The sharp asymmetric peak feature at about 1328 cm−1 obviously corresponds to the first-order Raman mode of the cubic diamond lattice. This line is broadened and red-shifted compared to bulk diamond (1332 cm−1), and the phonon confinement effect is mainly considered as the physical origin for these changes.60,62−65 Briefly, the q ≈ 0 selection rule for the optical phonon is strictly valid only for infinite crystals, i.e., when the translational symmetry is preserved over large distances. However, the bulk, plane wave-like phonon wave functions cannot exist within a crystallite with a diameter lower than ca. 10 nm. The confined phonon is therefore composed of a sum of bulk-like phonons with a range of wavevectors around the zone center. The spatial confinement, via a relationship of the uncertainty principle type, results in Iight scattering from a nominally “zone center” phonon whose wavevector has an “uncertainty” Δq and whose energy has an “uncertainty” Δω. Thus, the phonon’s spatial confinement results in a broadening of the Raman scattering features, reflecting the ”uncertainty” in its energy. For diamond, considering the diamond phonon dispersion functions, the additional lower-energy components within the confined phonon’s wave function results in a preferential broadening toward the lower-energy side of the Raman scattering peak, resulting in an asymmetric spectrum. In addition the phonon peak is expected to shift toward lower energies with increasing spatial confinement. The formalism derived by Campbell and Fauchet63 or Richter et al.62 was usually used to extract the particle size from the line shape of the diamond line along with more or less sophisticated descriptions of the phonon dispersion functions.60,71 However, examining the different results, some scatter exists between the different models, in particular when the line shape of the smallest particles has to be reproduced. Moreover, particle defects, microstrains, and even slight heating of the sample may contribute to affect significantly the position and line shape of the diamond first-order band, making it difficult to extract a true coherent domain size from the Raman measurements. This point will not be further discussed here. As already mentioned, broad signals are also observed in the 300−1000 and 1000−1350 cm−1 range. These features also nicely find their origins in the breakdown of selection rules because of coherent domains of very small sizes, which allow phonons from high-symmetry points at zone boundary to contribute to Raman spectra. Again, as the translational symmetry which characterizes the crystalline structure is lost, in particular by introducing bond angle disorder,72,73 the optical vibrations become localized instead of being extended in the whole lattice. If the coherence length is sufficiently small, the

Raman spectrum basically reflects the broadened vibrational density of states (VDOS) of the compound. The inset in Figure 2 gives the experimental and calculated VDOS of crystalline diamond according to the calculations of Pavone et al.74 and the measurements of Bosak et al.75 It is seen that there is a close match between the diamond VDOS and the broad signals observed in the DND Raman spectra. Again, note that similar broad signals were observed for ion beam amorphized diamond.66 Such lattice distortions in DND were already suggested from NMR35 and X-ray or neutron diffraction76 data. In agreement with most of the HRTEM studies, both methods have shown that a major part of DND volume constitutes a diamond lattice with few defects.23 However, both methods suggest that this diamond core is bordered by a distorted/ strained diamond lattice which forms the surface shell. The thickness of such a partially disordered shell has been estimated at about 0.5−0.7 nm. A 0.7 nm thick disordered shell should concern at least 30% of all carbon atoms of the particles. Thus, these broad bands may reflect some disorder in the outer region of the DND. The 1650 cm−1 broad and asymmetric band is also characteristic of the Raman spectra of purified DND. Most of the published data show this feature, provided the DND were purified from the sp2 carbon soot or was not vacuum-annealed at high temperatures. It should be noted again that the spectra obtained for the two DND origins investigated here (NanoCarbon and PlasmaChem) overlap almost perfectly. This feature cannot be assigned to any pure sp3 form of carbon, being far away from the cutoff frequency of the diamond VDOS (∼1336 cm−1). This feature is also different from the graphite “G” band, being upshifted by about 30−50 cm−1 from its expected position, see Figure 2. Such a shift cannot be realistically explained by high compressive stresses, it should in fact correspond to an isostatic pressure of at least 8−10 GPa.77,78 The absence of the corresponding defect-induced “D” line which is expected for samples or sp2 graphitic fragments with small coherence lengths is also a convincing evidence for this. From different NMR measurements, the sp2 content of DND particles has been recently estimated at about 1%,35,39 meaning that isolated sp2 bonds or small linear polyene chains or fragments should be an explanation for this band. Ab initio calculations show that such defects could be stable in the diamond lattice.65,67 Some surface reconstructions are also expected to produce sp2-like features.79 It was also suggested that this band should be due to the presence of surface hydroxyl groups.53 In such a case, this frequency should be characteristic of the water bending mode. However, the corresponding and much more intense O−H stretching mode of water is not observed in the 3200−3500 cm−1 wavenumber range. Moreover, both the bending and stretching modes of the O−H and C−O groups are expected at much lower wavenumbers for alcohols and phenols,80 below 1400 cm−1 in every case. To obtain additional information about the origin of this band, it seems necessary to find some means to modify the line shape of the characteristic DND spectrum. For such a purpose, different approaches may be considered: (i) modifying the surface chemistry or the surface structure of the particles, (ii) probing the modifications induced by a change of the excitation wavelength, as it was done earlier for the analysis of CVD nanocrystalline films,81 and (iii) performing analyzes at low and/or high temperatures under controlled atmospheres. These different alternatives will be discussed in the following. D



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Finally, other broad signals with weak intensities could be observed at higher frequencies, lying in the 2800 and 3500 cm−1 wavenumber range. They were mostly observed by strongly increasing the acquisition times for most of the excitation wavelengths used. However, they are hardly discriminated from the strong PL background at 514 nm. The first maximum is observed at about 2950 cm−1, i.e. in the frequency range for which the C−H bonds are clearly observed in the FTIR spectra. In this wavenumber range, the match between Raman and FTIR spectra is pretty good. However, It should still be interpreted with some caution because disordered sp2 carbon may give a second-order signal in this wavenumber range, see Figure 2. The second maximum is observed at about 3285 cm−1. The frequency of this line is almost twice of that of the 1650 cm−1 line. Thus, it may be tentatively interpreted as a second-order feature. However, more systematic studies will be necessary to go further in the interpretation of these signals. Multiwavelength Raman Analysis, As Received DND. Because Raman scattering from carbon materials is most often a resonant process, it is always useful to compare spectra obtained for different excitation wavelengths. Indeed, it is wellknown the Raman spectra of carbon-based materials may strongly depend on the excitation wavelength.49,50,81 Most often sp2 phases are probed with near-infrared and/or visible excitations, whereas the Raman signal of the diamond will be clearly visible for higher excitation energies. A striking example is that of nanocrystalline diamond layers obtained by microwave plasma CVD processes. Although such layers may contain more than 95% of sp3 carbon, the diamond Raman signal is usually obscured by that of sp2 phases when conventional visible excitation wavelengths are used, while a clear diamond signature is obtained for excitation wavelengths in the deep UV range.81 Note that the band gap of diamond is 5.4 eV. This corresponds to a wavelength of about 230 nm. This means that for the wavelengths used in this study resonant excitation of the diamond line is not really expected.82 The Raman spectra of the as received Nanocarbon DND excited at wavelengths ranging from 244 to 514 nm are shown in Figure 3. Here, the spectra were normalized to the diamond line intensity. One immediately sees that the shape of the spectra only slightly changes with the excitation wavelength. In contrast to nanocrystalline CVD layers,81 the intensity ratio of the diamond line to the 1650 cm−1 band is more or less constant. As expected, the profile of the diamond line and the broad line at about 1250 cm−1 are not affected by the wavelength change. However, even for low power levels, the slight frequency shifts of the diamond line are immediately ascribed to uncontrolled heating effects. The fact that the low frequency part (1000−1450 cm−1) of the spectra is unaffected by the wavelength change also precludes the presence of a strong graphitic “D” band in this frequency range. Regarding the 1650 cm−1 band, two small evolutions can be seen with the change of excitation wavelength. First, there is a noticeable apparent shift of the maximum of this line, from ∼1630 cm−1 at 514 nm to ∼1660 cm−1 at 244 nm. Second, a clear broadening of this line is also observed. Part of this broadening is explained by the development of a high frequency shoulder peaking at about 1750 cm−1. As reported earlier,53,61 this line is most probably tracks the presence of CO bonds located at the DND particle surface. The corresponding frequency is usually found in most of the FTIR spectra of DND particles (see the Supporting Information, Figure S1). Finally, note that a third

Figure 3. Raman spectra of the as-received Nanocarbon Research Institute detonation nanodiamond sample obtained with different excitation wavelength, from 514.5 nm (lower trace) to 244 nm (upper trace).

apparent maximum can be clearly identified at 244 nm, at about 1590 cm−1. Multiwavelength Raman Analysis, Surface-Modified DND. Annealing under different atmospheres may be used to modify the surface properties of DND. These treatments confer different surface properties, in particular in terms of surface terminations and zeta potentials. Different methods are used to analyze more or less quantitatively the different surface groups and the effects of different thermal and/or chemical treatments, but Raman spectroscopy has been used only scarcely for such a purpose. Here we examine the effects of air or vacuum annealing and hydrogen plasma treatment. Three excitation wavelengths were used. Some of the spectra obtained at 514 nm are shown in Figure 4. Again, the spectra were normalized in intensity. Differences between samples mainly concern the 1400 and 1650 cm−1 wavenumber range which is more or less broadened. A first visual inspection of the spectra allows isolating different maxima which are indicated by dotted lines in the figure. They are peaking at about 1530, 1600, and 1630 cm−1. Also seen is an apparent maximum at about 1350 cm−1 for both vacuum-annealed samples, as more or less expected for samples exhibiting graphitic surface reconstructions. The same maximum is also detectable for the hydrogenated sample. In addition, for the air-annealed DND, it is seen that the scattering in the 1400−1600 cm−1 range can be reduced significantly. The fact that the hydrogenated and air-annealed samples exhibit different surface terminations (carbon−oxygen or carbon− hydrogen bonds) but have rather similar Raman spectra suggests that the nature of the different surface functional group may have a limited effect on the line shape of the spectra for these acquisition conditions. A series of fits were performed as an attempt to extract characteristic frequencies in this set of spectra. Although one must remain very cautious with such fitting procedures, it is effectively possible to consistently reconstruct all the spectra using a set of characteristic frequencies near 1250, 1330, 1350, 1450, 1540, 1580, and 1630 cm−1 (see the Supporting Information, Figure S2). Note E



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Figure 4. Raman spectra of the modified Nanocarbon Research Institute detonation nanodiamond samples. From the bottom to the top traces: as-received, air-annealed at 400 °C, hydrogenated, vacuumannealed at 700 °C, and vacuum-annealed at 1100 °C. Excitation wavelength was the 514.5 nm line of an argon laser. The dashed lines indicate apparent maxima.

Figure 6. Deep UV Raman spectra of the modified Nanocarbon Research Institute detonation nanodiamond samples. From the bottom to the top traces: as-received, air-annealed at 400 °C, hydrogenated, and vacuum-annealed at 1100 °C. Excitation wavelength was the 244 nm line of a frequency-doubled argon laser. Concerning the vacuum annealed sample, note the presence of two additional lines at about 1000 and 1080 cm−1 that appeared after prolonged laser exposure.

that the diamond peak is only slightly downshifted, by about 2−3 cm−1. The same set of samples was subsequently examined with deep-UV excitations. Main trends are provided on Figures 5

in intensity and broadening of the 1650 cm−1 component for the hydrogenated sample. A simple subtraction of the airannealed and hydrogenated samples spectra immediately reveals two broad lines peaking at about 1600 and 1400 cm−1, the usual positions of the so-called D and G lines when excited at 325 nm (see the Supporting Information, Figure S3). The spectrum of the sample vacuum-annealed at 700 °C is rather similar in shape to the hydrogenated one. Again, the spectrum subtraction with the air-annealed sample clearly evidences a clear contribution of the D and G graphitic lines. For the vacuum-annealing at the highest temperature, the Raman spectrum now only exhibits the D and G lines while the signal characteristic of DND is nearly lost. Note that when observable, the diamond peak is now downshifted by about 4− 6 cm−1, in spite of the weak laser power at the sample. All the samples could not be examined at 244 nm. Some of the spectra are given in Figure 6. To the best of our knowledge, only ref 61 makes mention of DND Raman spectra with an excitation at 244 nm, and both sets of results are in agreement. Again, the spectra of the as-received, air annealed, hydrogenated samples are similar in shapes. Apart from the diamond signal, a broadband within a region of 1500−1800 cm−1 is still observed, which can clearly be interpreted as a superposition of three peaks at about 1600, 1660, and 1750 cm−1. Again, compared to the diamond signal, a slight decrease in intensity of this broad line can be discussed for the air-annealed sample. The 1750 cm−1 component is observed for all the three samples, and some variations in relative intensities can also be discussed. Finally, the vacuum-annealed sample (1100 °C) spectrum clearly exhibits a graphitic signal. However, the diamond band observed in the Raman spectra of this sample confirms that the diamond structure remains after annealing in vacuum at high temperature, which is consistent with the

Figure 5. UV Raman spectra of the modified Nanocarbon Research Institute detonation nanodiamond samples. From the bottom to the top traces: as-received, air-annealed at 400 °C, hydrogenated, vacuumannealed at 700 °C, and vacuum-annealed at 1100 °C. Excitation wavelength was the 325 nm line of an HeCd laser.

and 6. Using the 325 nm excitation wavelength, the spectra of the as received and air-annealed samples are very similar in shape. The main observable difference between both samples is perhaps a slight increase of the signal near 1750 cm−1 for the air-annealed sample, the position at which the CO bonds are detected. A slight increase of the 1650 cm−1 line for the as received sample may be also mentioned. The asymmetry of this broad component may be described with two contributions at about 1590 and 1650 cm−1. By contrast, there is a clear increase F



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HRTEM observations.46,69 Apart from this signal, the 1750 cm−1 line is still detected. Also observed is the presence of two additional lines at about 1000 and 1080 cm−1 that appeared after prolonged laser exposure. This clearly raises the question of the stability of the samples under UV irradiation. The diamond peak is always downshifted by about 6−8 cm−1. In Situ Measurements. Different in situ treatments under controlled atmospheres were performed on the as received sample from Nanocarbon Research Institute in order to track eventual modifications in the shape of the 1650 cm−1 feature. In all cases, to prevent interference from the PL background, the Raman measurements were conducted at 325 nm. The first measurements were carried out under vacuum at the liquid nitrogen temperature (77 K). For the lowest incident powers (up to about 300 μW at the sample), we did not detect strong changes in the overall signal line shape. In particular, no specific mode narrowing or clear C−H or O−H signatures were obtained. However, increasing the incident power (about 1−2 mW at the sample), graphitization of the irradiated spot was immediately evidenced. This nicely confirms the low stability of DND under focused UV irradiation. To further investigate the origin of the 1650 cm−1 peak, we monitored high temperature-induced changes in the Raman spectrum of DND. First in situ Raman spectra were recorded in flowing air to promote a controlled sample oxidation. For such a purpose, the sample was heated with 50 °C steps from room temperature to 500 °C. For each temperature, the exposure time was 15 min. Apart from small temperature induced shifts no clear signal evolutions were obtained in the 25−450 °C range. As expected, subsequent heating of the sample up to 500 °C led to its complete oxidation.44 Similar experiments were repeated in flowing hydrogen, as shown in Figure 7. The

control these experiments. In particular, the temperature is a simple thermocouple reading that does not take into account some additional heating induced by the focused laser beam. But the trend of a progressive surface graphitization for rather low temperatures was observed from different experiments.

4. DISCUSSION As already mentioned,53 the main problem in Raman spectroscopy of DND is related to the laser irradiation used as an excitation source. Conventional visible excitations effectively allow obtaining strong signals at the cost of extensive baseline subtractions while UV excitations allow getting PL-free spectra with a more or less controlled heating of the samples. Using excessive incident powers, sample degradation has been effectively clearly detected by simple optical microscopy. Moreover, because of the small Raman cross-section of diamond as compared to that of graphitic or amorphous carbon materials,50 UV excitations may be necessary to selectively probe the Raman signal of DND. Therefore, the issue of the stability of the different surface functional groups under UV irradiation arises. Using the 325 and 244 nm excitations, signals at about 1750 cm−1 have been almost systematically detected and have been previously assigned to CO bond stretching vibrations. If this assignment is correct, these chemical moieties are present on every DND surface, irrespective of the surface treatment. However, in the particular case the hydrogenated sample, carboxylic groups could not be detected by FTIR,68 although this method is highly sensitive to these functional groups. Thus, at present, we cannot ascertain that this method is not destructive, especially from the stability of surface chemical bonds viewpoint. Analyzing DND after different surface treatments, we have shown that Raman spectroscopy was poorly sensitive, or not at all, to the different surface terminations. For the moment, only graphitic reconstructions are clearly recognized from the spectra. For purified DND, even if 12−14% of the carbon atoms involve C−H or C−OH bonds,39 they do not seem to give specific Raman signals. If the analysis conditions, especially with visible excitations, are effectively nondestructive, this is perhaps not surprising. For example, Raman spectroscopy failed to detect such chemical moieties in graphite oxide, whose stoichiometry is close to C4OOH.77 Only the nonoxidized regions of graphitic planes are recognized with Raman spectroscopy. Indeed, the scattering cross section of such chemical bonds with low polarizability is known to be low.83 Finally, current literature mentions spectra obtained for fluorinated surfaces.37 It appears that the fluorination of the DND do not modify its Raman spectrum. Thus, with the present detection conditions, Raman spectroscopy is only poorly sensitive to the surface chemistry of DND. As previously mentioned, DND isolated particles are commonly described according to a core−shell model including a crystalline diamond core of about 4 nm in diameter, surrounded by a ∼0.7 nm thick disordered “shell”, and a chemically active “surface”, see for example the schematic models given in refs 35, 36, and 39. Again, a 0.7 nm thick disordered shell should concern at least 30% of all carbon atoms of the particles. This disordered part of the DND nicely explains the presence of the broad Raman signals that matches the diamond VDOS. Part of the 1250 cm−1 broad signal should partially reflect the presence of grains or subgrains with very low particles sizes.23,60 Nevertheless, we believe that most of this broad signal finds its origin in a disordered shell. In this

Figure 7. In situ UV (325 nm) Raman spectra recorded during annealing of DND powder in flowing hydrogen atmosphere. Spectra are roughly normalized with respect to the 1650 cm−1 band intensity. Note the progressive decrease of the diamond line intensity and the apparent downshift of the 1650 cm−1 line as the temperature is increased.

comparison of spectra suggests the progressive development of graphitic signals for temperatures as low as 175 °C, see the shoulder on the low frequency side of the 1650 cm−1 band. These signals clearly grow in intensity up to 275 °C. Again, an increase of the incident laser power contributes to modify the observed signals. At present, it is still difficult to completely G



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the low frequency shoulder of the 1650 cm−1 line may be the indication of the presence of some graphitic reconstructions in the purified as-received material. However, the surface coverage of such graphitic spots should be low. Other ab initio calculations also suggested that the surface of nonfully hydrogenated DND reconstructs in a fullerene-like manner.24 Such reconstructions implies the graphitization of the first atomic layer of the (111) facets which is followed by the formation of pentagons linking the graphene fragments with the underneath atoms. Such reconstructions are effectively suggested by HRTEM images for samples vacuum annealed at rather low temperatures (700 °C) that can be used for further chemical modifications of the DND surface14 starting from sp2 bonds. Even if these defects are quite common in graphitic compounds, little is known on the vibrational modes of 5- or even 7-membered rings in graphitic compounds. Doyle and Dennison,85 Tarrant et al.86 have estimated the mode frequencies using 5- or 7-membered rings coupled to a continuous random network85 or from selected structures containing 5- and 7-membered rings. In particular, Doyle and Dennison estimated the Raman vibrational modes of nonbenzene-ring structures and assigned peaks at about 1450 and 1530 cm−1 for a five member carbon ring structure. These modes can be seen as the equivalent of the breathing (D band with A1 symmetry) and stretching (G band with E2 symmetry) modes of graphite. Such frequencies match effectively those extracted from line fitting of the spectra obtained with the 514.5 nm excitation. These contributions are of low intensity for the air annealed sample while they grow in intensity during hydrogenation or vacuum annealing. However, we are aware that these frequencies have only been extracted from line fitting procedures, after subtraction of a strong PL background. To support this hypothesis, it would probably be interesting to analyze other samples annealed for different temperatures. The 1650 cm−1 broad signal of the Raman spectrum of DND can be partially modified or completely suppressed with plasma hydrogenation or graphitization processes. This is an indication that the sp2 defects are located at the DND surface, or close to it. Raman spectra of hydrogenated DND clearly indicate the formation of small graphitic clusters at the sample surface. If the assignment of this signal to small sp2 chain fragments is correct, this means that the sp2 chains will gradually form graphitic clusters at the DND surface as the temperature is increased or oxygen coverage decreases. This was also a quite unexpected result. However, ab initio calculations suggest that the difference in formation energy between nanodiamonds with hydrogenated surfaces and those with bare surfaces is rapidly decreasing as the size of the nanoparticle is increased. As written in ref 87, this perhaps suggests a threshold size in the nanometer range where a reversal of stability between hydrogenated and bare nanoparticles will occur and bucky diamonds (or parent structures) will become more stable than diamond nanoparticles with hydrogenated, reconstructed surfaces.

context, Raman spectroscopy confirms the DND core−shell model. Note that the Raman spectrum of purified DND does not strongly depend on its origin, at least for the origins investigated here. It remains to discuss the origin of the broad asymmetric signal peaking at about 1650 cm−1. Looking at the literature, this signal is clearly observed whatever the DND origin. It is hardy affected by simple chemical or high temperature treatments (air oxidation, or fluorination according to ref 37.). As already discussed, it does not strongly depend on the DND surface chemistry, even if about 10% of the carbon atoms are involved in surface functional groups. This feature cannot be assigned to any pure sp3 form of carbon, being far away from the cutoff frequency of the diamond VDOS. The overall signal behavior is also different from the graphite G band one, its maximum being upshifted by about 30−50 cm−1 from its expected position and exhibiting some dispersion with the excitation wavelength. However, it can reasonably be assigned to sp2 hybridized carbon atoms. Indeed, the line shape of this signal may be modified by different temperature treatments (vacuum annealing or H2 plasma treatment) leading to a partial or complete graphitization of the DND surface. Taking into account NMR results, this means that this broad signal accounts for only a few percent of all carbon atoms, about 1% according to ref 39. Lines at about 1600 cm−1 are usually due to the bond stretching of all pairs of sp2 atoms in both ring and chain configurations, and Raman spectroscopy is mainly sensitive to the configuration of sp2 sites because of their higher polarizability and thus higher scattering cross sections. Thus, Raman spectra of carbonaceous compounds are usually dominated by the sp2 sites. Signals well above 1600 cm−1 have already been reported for the so-called tetrahedral amorphous carbon (ta-C) films.50 Interestingly, there is a strong dispersion of this peak versus the excitation wavelength related to the sp2 content of the films. Depending on the sp2 content, peak frequencies can be downshifted by more than 50 cm−1 tuning the excitation wavelength from 244 to 514 nm. Similar frequencies and dispersions of the CC stretching modes also occur for polyenes with various chain lengths.84 Using visible excitations, a linear dependence of the Raman shifts of resonantly coupled modes with the inverse conjugation length was observed. Ref 84 shows for example that, when the number of alternated double bonds is varied from 2 to 5, the frequency of the CC stretching mode moves from about 1660 to 1580 cm−1. A distribution of conjugation lengths could explain both the asymmetry of the 1650 cm−1 line as well as its dispersion versus the excitation wavelength. Ab initio calculations have also suggested that isolated sp2 defects with more or less complex environments defects could be stable in the diamond lattice65,67 and could be a possible explanation for the 1650 cm−1 line. These two hypotheses are rather similar in their formulations. The 1650 cm−1 line profile can also result from overlapping contributions of different origins. Using UV excitations, the 325 nm one in particular, and neglecting the high frequency shoulder peaking at about 1750 cm−1, this line can be coherently described with two contributions peaking at about 1590 and 1650 cm−1 for all the samples examined. Moreover, as subtractions of the air-annealed with hydrogenated or some vacuum-annealed samples spectra clearly revealed two lines peaking at about 1600 and 1400 cm−1, i.e., the usual positions of the so-called G and D lines when excited at this frequency,

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SUMMARY The present paper investigates correlations between surface chemistry and surface structure of detonation nanodiamonds (DND) and their corresponding Raman spectra. DND from the same origin were treated either by annealing (under air or vacuum) or by plasma hydrogenation. A multiwavelength Raman approach, from deep UV to more conventional visible wavelengths, was chosen to characterize each treated DND. H



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ACKNOWLEDGMENTS The authors would like to thank Eiji Osawa (NanoCarbon Research Institute Ltd.) for providing samples of nanodiamond powders for this work.

Moreover, the effects of annealing treatments in air or hydrogen on DND were monitored in situ. Experimental conditions were adapted to minimize laserinduced effects on DND. Indeed, using excitation wavelengths in the deep UV range, the different surface functional groups may be modified under focused UV irradiation. This is clearly a problem when functionalized surfaces have to be analyzed. In our opinion, this is still an issue we have to work on. Apart from the diamond signal, the Raman spectrum of most of the DND samples exhibits broad signals centered at about 500 and 1250 cm−1. As this broad signal reflects the diamond VDOS, its origin is most probably related to a disordered shell. In this context, Raman spectroscopy confirms the DND core− shell model which was in part established from NMR and HRTEM measurements. The Raman spectrum of purified DND does not strongly depend on its origin, at least for the samples considered in this study. Analyzing DND of one origin with different surface preparations, we underlined that Raman spectroscopy is poorly sensitive to the different surface terminations, C−H and C− OH bonds in particular, even if these groups may account for 12−14% of all carbon atoms. Only a small contribution of carbonyl groups is detectable with the increase of the excitation energy. On the other hand, graphitic or other sp2-based reconstructions were clearly identified from the spectra line shapes. The 1650 cm−1 signal systematically observed along with the diamond line in the DND Raman spectra is thought to be due in part to small sp2 chain fragments that merge in small graphitic reconstructions upon vacuum heating and/or reduction of the oxygen coverage. From a chemical viewpoint, it is interesting to mention that hydrogenation, or annealing under flowing hydrogen, may promote a partial surface graphitization of the DND surface. It will be interesting to tune the hydrogenation conditions to understand their effects on the surface reconstructions. This issue is currently under examination. Similarly, it remains to be established whether or not Raman spectroscopy may allow monitoring the grafting of specific molecules on DND surfaces. At present, this is still a challenging task.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

FTIR data, analysis of the modified DND Raman spectra obtained with the 514.5 nm excitation, graphitic material incorporation in hydrogenated DND, Raman analysis at 325 nm and spectra subtractions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address #

Joint Ultrafast Dynamics Lab in Solutions and at Interfaces [JULiq] Institute of Methods for Materials Development at Helmholtz Zentrum Berlin, Albert-Einstein-Str. 15, 12489 Berlin, Germany Notes

The authors declare no competing financial interest. I



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