ABSTRACT
The existing synthesis technology allows the production of UDD on an industrial scale (tons per year). UDD powders are produced commercially in several centers in Russia, Ukraine, Byelorussia, Bulgaria, China, and Japan. The technology of UDD production consists of two steps: detonation synthesis of diamond-carbon soot and post-synthesis chemical purification. The main range of UDD particle sizes (some nanometers) depends only slightly on the technological details of production [1, 20, 21]. It is interesting to note that nanodiamond particles of similar sizes were found in meteorites [22] and are believed to belong to the family of “presolar dust grains” formed during star explosions [23]. This “invariance” of nanodiamond particle size was theoretically investigated using the model of carbon clustering and diamond/graphite transition in detonation regime [24]. Due to the small sizes of UDD particles the surface may be covered by a variety of O-, H-, N-containing functional groups (up to 20% by weight) with different chemical composition and structure [25]. In fact, an UDD nanoparticle should be considered as an inert diamond core covered by the chemically active shell of complex nature. In contrast to particle size, the surface chemistry of UDD strongly depends on the details of production technology [26]. The stage of chemical purification, which can vary from producer to producer, is the main factor governing the formation of specific chemical properties of UDD surface [27-30]. The nature and content of surface functional groups and hence the chemical activity of UDD are specifically dependent on the production method. From this point of view, the nanoproducts offered by different producers sunder the global name “detonation nanodiamonds” should be considered as different materials. Our experience indicates that the surface chemistry of UDD samples from even the same trademark can be different due to uncontrolled variation in the details of synthesis technology. This difference can lead to the lack of reproducibility and must be taken into consideration for the technological applications of as received UDD. To decrease the polyfunctional character of the surface of as-received UDD and to obtain predominantly monofunctional surface layers, the UDD is subjected to chemical treatment (fluorination [31, 32], chlorination [33, 34], hydrogenation [35, 36], and others [37]). The key feature of UDD is the possibility to functionalize the surface by introducing specific chemical groups, including the
grafting of different organic functionalities, in order to modify the chemical activity of UDD in a way suitable for their subsequent practical applications (see reviews [19, 38]). The great importance of surface chemistry of UDD requires a deep investigation of the properties of functional groups, both at the stage of production and after subsequent chemical modification. Characterization of surface groups on UDD is usually achieved by Fourier transform infrared (FTIR) spectroscopy [25, 27, 29, 30, 34, 35], because diamond is transparent in IR-region and the IR-spectra of UDD are mainly produced by surface species. X-ray photoelectron spectroscopy (XPS) was also used for this purpose [31, 39, 40]. The main disadvantages of these methods are the relatively low sensitivity and difficulties in data interpretation to obtain direct information on the chemical composition of surface groups. The mass spectrometry analysis of chemical composition of gases evolved from the solid sample during programmed heating in vacuum or in inert atmosphere, so-called thermal desorption mass spectrometry (TDMS), is a technique providing information on the chemical composition and the thermal stability of surface species. TDMS was widely used to study the surface chemistry of dispersed carbon materials [41, 42], diamond monocrystals [43] and diamond powders [44]. TDMS seems to be a promising technique for characterization of UDD surface as well [26]. 6.2 A Short Survey of Applications of Thermal
Desorption Mass Spectrometry to the Study of the Surface of Diamond MaterialsTDMS is a method of characterizing the surface species by pro-grammed heating of the sample under vacuum and simultaneously detecting of the evolved gas by means of a mass spectrometer. During the temperature increase some adsorbed species and decomposition products of surface functional groups escape from the surface at
different temperatures causing a rise in the pressure of a specific mass component. The results of measurements usually are represented in the form of temperature profiles (spectra of thermal desorption) of ionic fragments with different masses in the mass spectrum. This method is characterized by high sensitivity and high information content. Besides the information about the chemical composition of
surface compounds and their relative amounts, the method allows to determine the temperature dependence of desorption rates, the thermal stability of surface groups, and the activation energy of corresponding processes. Several types of C-O and C-H bonds were found by TDMS on the surface of the diamond monocrystals after interaction of oxygen [44], hydrogen, and water vapor [45] with the diamond powders at elevated temperatures. Later, this method was applied to the study of the surface of diamond powders modified by oxidation [46], hydrogenations [47], or other treatments [48-50]. The high sensitivity of the method provided the feasibility to study the adsorption of oxygen [43], hydrogen [51], and other molecules [52-54] even on the surface of a single crystal of diamond. The obtained results allowed to establish the presence, in particular, of a variety of oxygen-containing functional groups, such as carbonyl (C=O), lactone [(C=O)O], carboxylic acid [(C=O)OH], cyclic ether (COC), and carboxylic anhydride [(C=O)O(C=O)] on the diamond surfaces. These groups differ by thermal stability and are decomposed with the formation of CO and/or CO2 at different temperatures [46, 47]. Only limited information, mostly of fragmentary nature, is available on the application of the TDMS for characterization of UDD surfaces. A substantial variation of the composition of the desorption products (hydrocarbons, chlorine-and sulfur-containing compounds) in vacuum at temperatures up to 400°C has been found for UDD samples purified by different procedures [28]. Some transformations of oxygen-containing groups were detected by TDMS for UDD heated in air, nitrogen, and hydrogen (temperature profiles of CO and CO2 up to 850oC) [55]. A complex form of the temperature profile for water desorption from UDD surface has been evidenced in the temperature range up to 800°C [56]. The effect of chemical treatment of UDD on the spectra of thermal desorption of H2O, HCl, and oxygen at temperatures up to 600°C has also been noted [57]. It has been shown by means of TDMS in a wide range of temperatures (up to 1100°C) that H2O (100-600°C), CO2 (200-600°C), CO (500-900°C), and H2 (above 800°C) along with some traces of hydrocarbons were the main products of thermal desorption from the different types of UDD used as analogs of meteoritic diamonds [58]. Similar results have been described in relation to the problem of UDD graphitization [59].
