Pyrolysis mass-spectrometry study of detonation nanodiamonds surface chemistry

Abstract Surface thermochemistry of detonation nanodiamonds (DND) is investigated by time-of-flight electron ionization mass-spectrometry. FTIR spectroscopy analysis supports mass-spectrometry results. The impact of ambient air atmosphere as part of DND deagglomeration process is revealed. It is stated that the surface of DND agglomerates is covered with esters, lactones, nitro-groups and aliphatic hydrocarbons. DND deagglomerated via annealing under air are terminated by carboxyl groups and carboxylic anhydrides. Carboxyl groups are revealed to be transformed to lactones during pyrolysis.


Introduction
Nanodiamond is one of the most unique and fascinating member of nanocarbon structures. First of all, nanodiamonds are distinctive in physico-chemical and optical properties, meanwhile retain a diamond lattice and properties on nanometer scale. Due to small sizes, mechanical hardness and biocompatibility nanodiamonds attracts long-lasting attention of many researchers, particularly of medicine and biology scopes. [1][2][3][4] Another principal property includes a highly developed surface, allowing to bind any molecules, complexes, polymers, etc. [5] There are only two methods of diamond nanoparticles synthesis distinguished by developed technologies that provide obtaining hydrosols of sub-10 nm diamonds in industrial scale. One of them is mechanical grinding of submicron diamond obtained by static high pressure high temperature synthesis [6] ; and the other is a detonation of strong carbon-containing explosives. [7] The former is complicated by laborious milling and separation processes. [8] The last requires a tedious chemical purification and isolation of 4-5 nm diamond crystals by deagglomeration procedure. Generally, nanodiamonds obtained by the former method have a main application as biomarkers due to inherent luminescent NV-centers. [9,10] The last method allows to obtain nanodiamonds with a wider scope of applications. In particular, it has been recently demonstrated that these diamond nanoparticles with Gd-grafted surface are promising contrast agents in magnetic resonance imaging. [11] Deagglomeration implies a set of technological processes aiming at breaking up agglomerates and obtaining a stable colloidal solution containing isolated primary DND particles. Agglomerate break-up may be realized through different ways: mechanical grinding, [12] thermal annealing, [13] plasma treatment, [14] sonication in salt and sugar media, [15] advanced oxidation process. [16] Generally, thermal annealing is considered to be the most facile and feasible method in DND deagglomeration. For this reason, many commercial nanodiamonds are produced via thermal annealing. Annealing may be realized under various gas atmospheres: air, molecular hydrogen, or argon. The choice of the atmosphere depends on desired functional moieties on DND surface. [17] In this sense, examining the surface of nanoparticles acquires a primary concern.
In an investigation of surface, Fourier transformed infrared spectroscopy (FTIR) is the most powerful and common tool along with X-ray photoelectron spectroscopy and Raman spectroscopy. This is a routine analysis method which permits to identify a major of surface functionalities without costly and time-consuming sample preparation technique. The first FTIR measurements of DND surface dates back to 2000s, when DND agglomerates were intensively studied. [18,19] It is revealed that acid treatment of DND leads to formation of esters, hydroxyls, carbonyls, aldehydes as well as amides and lactams. It is also well established that annealing in air results in formation of carboxyl groups and carboxylic anhydrides that are reflected in FTIR spectra. [20] Recently Petit et al. have published a paper summarizing all FTIR data on nanodiamonds. [21] However FTIR spectroscopy as a method for surface identification has relevant constraints. The main limitation is that specific IR vibrational modes of various species are equal in frequency causing complication in the spectra interpretation. Therefore, FTIR data should be combined with another method. For this purpose, temperature programmed desorption mass spectrometry (TPDMS) and pyrolysis mass spectrometry (PyMS) are appropriate techniques. TPDMS is used to examine species desorbed from surface of solids upon heating. [22] PyMS provides information on gas products formed upon thermal destruction of surface thus deriving thermal decomposition mechanisms. [23] TPDMS is widely applied to studying catalysis surface and PyMS is mainly used in the field of polymer research. Both methods are also applied to carbon materials, and while TPDMS deals with products of decomposition of covalently bonded oxygen-containing functions, [24,25] PyMS implies a more comprehensive study encompassing a wide range of volatile organics. [26] Regarding diamond nanoparticles a role of TPDMS and PyMS in the surface investigation is significantly underestimated. In particular, PyMS is a especially relevant method in analysis of noble gas isotopes in meteorite nanodiamonds [27][28][29][30] while TDPMS analyses are highlighted only in a few publications. [31][32][33][34] To the best author's knowledge, among papers on application of TPDMS only one study [33] is devoted to deagglomerated DND while the rest covers DND agglomerates. Additionally, some conclusions on surface compounds are based only on two-three peaks mainly with m/z ¼ 18, 28, and 44 attributed to H 2 O þ , CO þ , and CO 2 þ , respectively. This paper is aimed to reveal surface chemistry of agglomerated and deagglomerated DND by isothermal PyMS method focusing on several groups of mass peaks. Isothermal step analysis has proved advantageous for separation of oxygen-containing groups as revealed by Heumann et al. [25] This approach can be also expanded to other type surface functionality as demonstrated in this paper.

Materials and methods
The pristine sample is obtained from nanodiamond powder purchased from Federal State Unitary Enterprise "Technolog" (St. Petersburg, Russia). The industrial nanodiamond powder has been additionally treated with strong acids in order to remove metallic and inert impurities. Such sample contains DND agglomerates and is named "agDND." Deagglomeration has been implemented by annealing agDND under air atmosphere at 450 C for 3 h followed centrifugation to separate primary crystals of DND. [35] The solvent of prepared hydrosols has been extracted in rotary evaporator thus obtaining powders. Prepared deagglomerated sample is designated as "DND-air". Size distributions of both aqueous colloid solutions are presented in Figure S1.
Infrared Fourier Transform Spectroscopy (DRIFTS) is carried out using SpectraLum FT-08 spectrophotometer equipped with Pike EasyDiff diffuse reflectance accessory. Spectra are collected as an average of 100 scans with a resolution of 4 cm À1 . For each measurement 2 mg of the sample is thoroughly ground with 200 mg of KBr powder. Asymmetric least squares approximated baseline correction and local maximum peak finding are used in the spectra analysis.
Mass spectrometry of nanodiamonds was carried out using a small-sized time-of-flight mass spectrometer FT-2, developed at the Ioffe Institute. [36] Mass analyzer length is only 0.5 meters, but despite this, it has a pretty high resolution of $250-300 at half-height of the mass line in the mass range of 20-50 Da. The device has a linear trajectory of ion movement, that is, the ion source, reflector and detector are located on the same straight line. The device uses an electron impact source, the energy of the emitted ions is $1500 eV, which is determined by the amplitude of the ejection pulses following at a frequency of 10 kHz. The test sample weighing 1-2 mg is placed in a quartz pot, which is a test tube with an inner diameter of 2 mm and a height of $10 mm, which is placed in a miniature heater under the ionization zone of the ion source. The vapors, released during sample heating, enter directly into the ionization zone, and then bombarded by an electron beam with an energy of 100 eV, and the resulting ions are analyzed in a mass analyzer. The design of the ion source with a pot for the sample makes it possible to heat the test substance up to a temperature of 800 C, which is measured using a chromel-alumel thermocouple. Each study cycle of test sample consists of the following measurements: a blank measurement, when an empty pot without a sample is heated from room temperature to 800 C with an interval of 100 C, and working measurements when the pot with the sample is sequentially heated to 800 C with the same temperature intervals. Thus, the measurements were carried out while holding the samples under isothermal conditions. After each increase in the heater current, a pressure jump occurs in the mass spectrometer chamber. After 2-3 min, the pressure reaches its maximum and at this moment the mass spectrum is recorded. Archiving of the obtained data in the form of the dependence of peak areas on the time has been carried out throughout the entire experiment. Based on the data obtained, linear correlation coefficients between individual peaks have been determined. Mass peaks with a correlation coefficient of at least 0.8 have been collected and analyzed.

Results and discussions
3.1. Mass spectrometry study of vapor phase evolved during pyrolysis

Pristine agglomerated DND (agDND)
Hydrocarbons. A series of mass peaks of hydrocarbons is observed on thermograms of agDND, as seen in Figure 1a.
The only mass peaks with the highest intensities are shown. The assignment to hydrocarbons is based on strong correlation between intensities of C n H 2n (C 3 H 6 þ , C 4 H 8 þ ), C n H 2nþ1 (CH 3 þ , C 3 H 7 þ ) and C n H 2n-1 (C 2 H 3 þ , C 3 H 5 þ , C 4 H 7 þ ) mass peaks. The largest fragment contains four carbon atoms, and hence relates to aliphatic compound.
However, it is difficult to deduce could it be a parent ion and to define a structure of compounds being desorbed.
The major amount of hydrocarbon species is removed from sample within 100-300 temperature interval. Moreover, whether such species are physically or chemically adsorbed is still an issue. Similar peaks are observed in a work of Cheng et al. [37] The origin of the hydrocarbons has been proposed to be an impurity from pump oil. Although in the present study such peaks are well reproduced on the agDND mass-spectra and its intensity is much higher than background one.
Mass peaks intensities at 100-300 C relate to decomposition of hydrocarbons as presented in Figure 1a. At 500 profiles of 28 and 44 masses slightly exceed the background and may be associated with commence of esters decomposition. At 600 C temperature degradation of the esters is proceeding (mass 44). The decomposition of lactones is occurred at 700 C. Interestingly, the release of CO 2 (mass 44) is proceeding at 800 C that has not been reported in literature on diamond to our best knowledge. Therefore, this peak is considered to be associated with pyrones as oxygencontaining groups with the highest temperature of decomposition. [41] Pyrones are stated by J. T. Paci et al. [42] to be typical for graphitized nanodiamonds surface, and therefore, this moiety is expected to be formed under pyrolysis at about 700-800 C. Noteworthy, pyrolysis formation is shown for carbon nanotubes when heated under UHV in high temperature range. [43] Overall, whether the release of CO 2 at 800 C is attributed to decomposition of the highly stable surface groups like pyrones or is caused by low gas diffusion, this issue requires further investigation.
Nitro-compounds. Peaks at masses 14 and 30 correspond to nitro-compounds as one can see in Figure 1c. Nitro-compounds are likely to form due to purification of detonation soot by nitric acid at high pressure for graphite oxidation.

DND obtained via annealing under atmospheric air
(DND-air) Carboxyl groups, carboxylic anhydrides, lactones, carbonyls. PyMS spectra of DND-air are distinguished by very intense peaks at m/z ¼ 12, 16, 28, 44 upon heating from 600 C as one can see in Figure 2. At 500 species with masses 28 and 44 are observed indicating the commence of carboxylic anhydrides decomposition with slow rate. Further heating results in accelerating decomposition of these species at 600 C. At 700 thermal destruction products with mass 28 has been in a such amount that it exceeds the maximum level of ion current causing the detector to switch off. When the amount of positive ions has been decreased, the mass measurements proceed. Fragment ion with mass 28 is apparently ascribed to inherent carbonyls. Additionally, the enormously high intense of mass 28 more likely to evidence the carbonyl formation due to anhydride decomposition as well. The mass 44 at 700 is due to lactones decomposition. [43]  Evolving CO 2 at 800 has been also observed for agDND and is presumably associated with pyrone formation as well.
Annealing nanodiamonds under air turns the surface to be covered predominantly by carboxyl groups as revealed by Boehm's titration method. [44,45] It should be noted that the surface of nanodiamonds in powder can be distinguished from one in hydrosol. The high sedimentation stability of nanodiamond particles is due to dissociation of surface ionogenic functional groups. [46] Therefore, ionized form of carboxylic acid (or conjugated base)are carboxylate ions comprise surface ionogenic groups. Evaporation of solvent leads to carboxyl groups being dehydrated, and hence carboxylic anhydrides are formed. [47] Whereas reverse reaction takes place upon interaction with water. Dispersing of DND powder results in hydrolysis of carboxylic anhydrides with subsequent carboxyl groups formation. [45,48,49] Herewith, it worth to specify that powders of deagglomerated nanodiamonds are known to contain a bulk water and nanophase of water. [50] The latter is apparently located within nanoscale porous system. [51] In this sense, formation of carboxyl groups is highly likely to be a result of interaction of anhydrides with rather water molecules within pores of powder.
In this connection, evolving of H 2 O is analyzed separately. Figure 2 demonstrates ion currents of the peak at mass 18 (H 2 O þ ). Notably, intensity of the mass peak, or rate of water desorption, remains constant during heating at 300 and 400 C. Such curve profile is different from that of background and indicates a slow rate of a process causing release of water. Evolving of water in these temperature ranges is associated with dihydroxylation of carboxyl groups or phenols as has been shown by Li et al. [39] and Muhler et al. [43] Lactones are formed as result of condensation of two adjacent carboxyl and phenol groups. Meanwhile acid anhydrides are products of dihydroxylation of two adjacent carboxylic groups. Assuming a relatively high amount of carboxylic anhydrides on air-treated DND surface the formation of lactones is believed to be more preferable. Consequently, the high peak of CO 2 is accounted for lactones formed as result of condensation of carboxyl groups and phenols. Furthermore, pyrolysis of acrylic acid polymer is accompanied by similar two-stage reaction: dehydration of carboxyl groups and decomposition of carboxylic anhydrides thus formed. [52] Therefore, observation of such peculiarities in water desorption profiles provides evidence that concentration of carboxyl groups is much higher in deagglomerated nanodiamonds.

Interpretation of FTIR spectra based on
PyMS analysis

Pristine agglomerated DND (agDND)
The fact that the surface of agDND is covered with hydrocarbons is supported by FTIR analysis. C-H stretching vibrations are observed in the region of 2900-3000 cm À1 in FTIR spectrum shown in Figure 3. FTIR band at 1740 cm À1 may be deconvoluted into two bands: one of them is centered at about 1728 cm À1 and another is about 1757 cm À1 . The peak at 1728 cm À1 is ascribed to lactones which decompose at 700 according to PyMS thermograms (Figure 1b). The peak at 1757 cm À1 is accounted for esters which decompose at 600 C. The region of 950-1450 cm À1 is associated with stretching vibrations of ethers. The FTIR band at 1560 cm À1 reveals presence of aliphatic nitro-compounds as reveled by PyMS (Figure 1c). The gap in ion current curves at 700 is caused by switching off electron source and ion detector due to abrupt increase in pressure (above 5 Â 10 À6 Torr). In such conditions the measurements do not guarantee correctness due to a huge amount of ions at ionization zone followed by ions collisions and loss.

DND obtained via annealing under atmospheric air
(DND-air) Figure 4 presents a typical FTIR spectrum of nanodiamonds annealed in air. One can observe the modes of ester groups within 900-1500 cm À1 . The wide band at 1790 cm À1 is ascribed to C ¼ O in carboxylic anhydrides. It is to be noted that this band is frequently associated with C ¼ O in carboxyl groups. Despite carboxylic acids have not been directly observed by PyMS method here, this moiety should not been exclude from consideration. Carboxylic groups are believed to be thermally too unstable, and their examination by mass-spectrometry is highly complicated. [53] Therefore, FTIR spectrum obtained at room temperature may evidence the presence of carboxylic groups. Deconvolution of the band at 1790 cm À1 provides at least three peaks: two peaks at 1831 and 1796 cm À1 are both typical for C-O stretching of carboxylic anhydrides; and a peak at 1759 cm À1 is associated with carboxyl groups.

Conclusion
The surfaces of agglomerated detonation nanodiamonds (DND) and deagglomerated DND obtained via annealing under air are studied by pyrolysis mass spectrometry at isothermal mode and FTIR spectroscopy. The present findings do not contradict inferences about nanodiamonds surface chemistry stated in prior publications. In particular, it is confirmed that the pristine DND are covered by esters, lactones and carbonyls, while annealing under air leads to surface terminations by carboxyl groups and carboxylic anhydrides. In addition, pristine DND is featured by the hydrocarbon moiety which mostly desorbs at 300 C.
For the first time it is demonstrated that, firstly, carboxyl groups via dehydration reaction are converted to lactones and carboxylic anhydrides. Secondly, nitro-compounds are observed in pristine DND due to purification by nitric acid.