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Surface peculiarities of detonation nanodiamonds in dependence of fabrication and purification methods
Diamond & Related Materials 16 (2007) 776 – 780 www.elsevier.com/locate/diamond
Surface peculiarities of detonation nanodiamonds in dependence of fabrication and purification methods D. Mitev a,⁎, R. Dimitrova b , M. Spassova b , Ch. Minchev b , S. Stavrev a a
b
Space Research Institute-BAS, Bulgaria Institute of Organic Chemistry-BAS, Bulgaria Available online 24 January 2007
Abstract A variety of post-treatments, such as thermal annealing and liquid phase oxidation, were used to determine the surface properties of six blasting synthesized nanodiamond powders, produced by various purification conditions. Methods of FTIR-spectroscopy, oxidative titration and pHmeasurement were applied for comparing the surface functional groups. All spectra exhibited the characteristic IR-bands of detonation diamond in the regions 3400–2700 cm− 1 and 1000–400 cm− 1. The differences were revealed by the dissimilarity of the bands in the region 2000–1000 cm− 1. The nanodiamond powder annealing up to 600 °C in air most fully revealed the specificity of the attached groups on diamond surface. The chemical oxidation with H2O2 modified nanodiamond surface groups and prevented them from further oxidative attack during thermal treatment. © 2007 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Nanoparticles; Surface characterization
1. Introduction The detonation synthesis of ultra-dispersed diamonds (UDDs) is commonly used in the last decades, since it offers a simple method to reach the necessary pressure and temperatures. Various targets are used for the purpose of diamond synthesis, such as pure carbon materials or carbon-containing compounds, including explosives with negative oxygen balance [1]. A variety of methods for UDDs purification are stated in the literature and it is established that the properties of the UDDs are largely determined by the applied purification methods [2,3]. The UDDs surface is chemically multifunctional, but the chemical groups can be formed and changed also by means of post-manipulation of detonation diamond [4–6]. The aim of the present work is to identify the chemical groups on the diamond surface. We have investigated six diamond samples produced by two detonation synthesis, and were refined using various purification conditions. The methods of thermal annealing in static and flow air conditions, the liquid phase oxidation, FTIRspectroscopy, pH and titration are applied for comparing the surface functional groups. Attention has been devoted to ⁎ Corresponding author. E-mail address: [email protected] (D. Mitev). 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.01.005
distinguish functional groups, which are the result of purification and those which are formed from subsequent autoannealing, oxidation, etc. 2. Experimental 2.1. Samples The UDDs synthesis was carried out in the Department SMN of SRI-BAS by the use of carbon-containing explosives with negative oxygen balance [7,8]. In order to obtain nanodiamonds with fundamental particles about 4 nm, detonation of cylindrical charge (0.4 kg), consisting of explosive molecules such as trinitrotoluene and cyclotrimethylenetrinitramine (60/40 wt.%), was carried out in a spherical explosion camera with an inner volume of 3 m3. The blasting of the charge was done in the presence of two cooling media — water and salt-containing water solutions (condition of reduced heat capacity of the cooler). The purification of the diamond powder from the attending admixtures was done as follows: i/ oxidation with concentrated nitric acid at temperatures up to 200–250 °C, followed by additional removal of the metal impurities by HCl-treatment (80–100 °C), washing
D. Mitev et al. / Diamond & Related Materials 16 (2007) 776–780 Table 1 Samples' designation, functional carbon atoms amount and acidity Samples
N B O OBred Bred Ored a b c
1
2
3
4
Parent
After heating at 400 °C
After H2O2 and heating at 400 °C
After heating at 600 °C
%“oxidized C” a (pH)
%“released C” b (pH)
%“released C” b (pH)
%“remained C” c (pH)
48 37 15 20 35 15
2.05 (5.2) 1.95 (3.6) 2.15 (4.6) 1.15 (4.6) 2.65 (4.3) 3.10 (6.0)
4.9 3.8 2.5 2.0 5.5 3.5
30.9 (6.5) 35.9 (6.5) 35.0 (6.5) 42.3 (6.8) 30.2 (6.0) 35.0 (6.8)
(3.5) (4.8) (6.2) (4.0) (4.5) (5.0)
(3.5) (3.5) (3.5) (3.5) (3.5) (3.5)
As determined by titration method. As determined by Ascarite sorption. As determined by thermal annealing in shallow bed.
with water until pH 7 and drying. The so obtained gray in color sample was denoted here as N sample. ii/ boiling in a solution of potassium dichromate (K2Cr2O7) in sulfuric acid (H2SO4) at 100–115 °C, followed by additional removal of metal impurities as described above. The gray-colored samples were denoted B, when water is used as a cooling agent, and Bred when reduced heat capacity of the cooler was applied. iii/ catalytic burning in air conditions (temperatures up to 500 °C), followed by removing of metal impurities. The samples of dark-grey appearance were respectively denoted O and Ored. iiii/ catalytic burning in air, followed by boiling in a solution of K2Cr2O7 and H2SO4. The so obtained gray sample (synthesized in reduced heat capacity condition) was denoted OBred. 2.2. Surface pH-value The pH determination was carried out as follows. A 20-ml portion of boiled de-ionized water was added to 0.2 g of the corresponding sample in a flask equipped with a reflux condenser. The mixture was heated and boiled gently for
777
10 min and filtered after cooling below 50 ± 5 °C. The pHvalues were determined at room temperature and the results were presented in Table 1. 2.3. Determination of surface carbon–oxide groups through titration The information for the surface carbon–oxygen functional groups was obtained performing standard titration. The procedure was described in [6]. 0.1 g of the corresponding sample with 8.3 ml de-ionized water and 10 ml K2Cr2O7 (0.25 N) were stirred with a magnetic stirrer at room temperature in two flasks. Further on, 20 ml of H2SO4 and trace of Ag2SO4 were added. The flasks were heated for 2 h at 90 °C. After hot filtration and cooling of the filtrates, the titration was carried out with 0.25 N Moor salt in the presence of an indicator (0.2% N-phenylantranilic acid). Its equilibrium was violet to green transition. The obtained results (“oxidized C”) were given in Table 1. 2.4. Thermal annealing i/ 0.2 g of the sample was placed in a flow type reactor. The temperature was raised with 10 °C/min from ambient up to 400 °C in air atmosphere and kept for 2 h. The amount of the “released C” was calculated from the weight of the CO2 absorbed by Ascarite-II as: %carbon = A(0.2729)(100) / B; where A is the weight (g) of CO2 determined by weighting the Ascarite tube before and after annealing; 0.2729 is the ratio of the molecular weight of carbon to the molecular weight of CO2 and B is the weight (g) of the sample. ii/ 0.2 g of the sample was placed in a laboratory pottery vessel and heated till constant weight at 600 °C in a furnace (shallow bed). The weight of the initial and heated samples was determined with accuracy of ±0.0005 g. The amount of the “remained C” was estimated as: %remained C = (M2 − M) (100) / (M1 − M); where, M — was the weight of the pottery vessel after heating, g; M1 — weight of the pottery vessel with initial sample, g; and M2 — weight of the pottery vessel with the diamond residue, g. The results were presented in Table 1.
Fig. 1. FTIR-spectra of the parent diamond powders: (A): line 1 — N; line 2 — B; line 3 — O; (B): line 1 — OBred; line 2 — Bred; line 3 — Ored.
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3. Results and discussion It is well proved that the detonation diamonds, chemically extracted from a detonation soot have a mean size of the core ∼4 nm and a shell made up of sp2-coordinated carbon atoms as well as unusual tight aggregation [16]. The SEM images (presented in Ref. [15]) evidenced that the particle sizes are in the nanometer range with a well-developed facet on it. The FTIRspectra of the investigated samples bear the general resemblance to known spectra as presented in Fig. 1. The spectra of N, B and O samples are given in the whole spectral region (Fig. 1A), while the spectra of OBred, Bred and Ored samples are shown in the specific diamond framework spectral region (Fig. 1B), since all spectra at the higher frequencies are identical. A wide, complex band centered at 600 cm− 1 was observed in all spectra, with the exception of the spectrum of sample O, where identifiable bands were presented. The broad structureless band
Fig. 2. FTIR-spectra of sample N (A — line 1), B (B — line 1) and O (C — line 1) impacted by the heating temperature: lines 2 — at 400 °C; lines 4 — at 600 °C and lines 3 — oxidation with H2O2 followed by thermal treatment at 400 °C.
2.5. Combined chemical–thermal oxidation 0.2 g of the sample were put in a flask and stirred with a magnetic stirrer at room temperature with a 30 ml aqueous H2O2 (26 wt.%) for 2 h. After that the sample was filtered, washed with water and dried. The thermal annealing of the oxidized sample was carried out at 400 °C. The samples' surface functional groups, before and after the different treatments, were registered by FTIR-spectra obtained on a Brucker-Vector 22 FTIR-spectrometer in the region of framework vibrations by using KBr pellet technique. A portion of the 0.5–1 mg sample to 100–150 mg KBr, depending on the sample type, was used. The assignment of the absorption bands is based on experience with molecular organic compounds and literature data [9–14]. The samples' nanocrystalline morphology was checked by SEM microphotographs taken on a Philips XL 30S model scanning electron microscope at 5 kV accelerating voltage and a beam current of 10 μA [15].
Fig. 3. FTIR-spectra of samples OBred (A — line 1), Bred (B — line 1) and Ored (C — line 1) impacted by the heating temperature: lines 2 — at 400 °C; lines 4 — at 600 °C and lines 3 — oxidation with H2O2 followed by thermal treatment at 400 °C.
D. Mitev et al. / Diamond & Related Materials 16 (2007) 776–780
could be associated with recombination through a system of continuously distributed energy levels in a sample band gap. The band at 672 cm− 1 (Fig. 1A, line 3) could be assigned to C–O bond, as it was done in [14,17] for meteoritic diamonds. The comparison of the spectra was based on the assumption that all detonation diamonds should resemble their morphological and structural characteristics. A broad peak consisting of identifiable strong bands at 1118 cm− 1 (deformation OH), at 1128 cm− 1 (generally assigned to C–O–H and C_C vibrations) and around 1260 cm− 1 (for amorphous diamond and/or vibration (–C–O– C–) in ether group) was observed in different ratios in all spectra. The structure of the band in this region originated from the absorption of the disordered nanocluster surface containing threefold-coordinated carbon. It should be marked that depending on the cooling condition, some differences in this spectral region were also refined. Relatively strong bands at 1330 cm− 1 (diamond peak), 1535 cm− 1 (sp2 coordinated carbon) and 2100 cm− 1 were registered (Fig. 1B). Further on, carbonyl groups were ascertained for all samples, by bands in the region 1701–1768 cm− 1. The existence of a strong band of the bending mode δ(OH) of water at 1630 cm− 1 and a broad asymmetric band centered at 3430 with shoulder at 3250 cm− 1, ν(OH) were seen. As stated in [18], the band at 3440 cm− 1 was apparent for small water clusters, while the shoulder at lower frequency revealed a coordinated type of water. Thus, our spectra provided evidence for the existence mainly of intermolecular hydrogen bonding of adsorbed water molecules. The characteristic absorption bands at 2890 cm− 1 and at 2925 cm− 1, which corresponded to the asymmetric and symmetric stretching C–H vibration respectively, were seen only in the spectrum of sample B. From the performed analyses we concluded that different acid groups and admixtures presented on diamond powder surface in dependence of the purification method. Both acid purification procedures resulted in the formation of groups with high acidity, as the effect is more pronounced in the case of the nitric acid purification. As it was presented in Table 1 — column 1, the pH-values were in the range from 3.5 to 6.2 and the amount of the “oxidized C” evidenced for multifunctional carbon atoms. A higher amount of amorphous diamond carbon, especially in the powders synthesized under reduced heat capacity conditions, was registered (the broad peak at 1260 cm− 1). The method of the catalytic burning provided fairly good removal of the amorphous carbon from the diamond powder. The nanocrystalline morphology of the samples was not affected as proved by the SEM images [5]. In order to follow the stability and the type of the existing surface functional groups, we performed oxidative treatment in air at two temperatures (400 and 600 °C) and combined chemical–thermal oxidation. All samples heated at 400 °C demonstrated a weight loss of about 12 wt.% and carbon diminishing of 1–3% (Table 1, column 2). A transformation of the surface functional groups during the upper mentioned treatments was registered in the FTIR-spectra as shown in Fig. 2A, B and C for the N, B and O samples, respectively. Strong and defined bands, between 500 and 800 cm− 1, came into view with the temperature rise, as for sample “O” the bands were with high intensity at 400 °C. The appearance of bands
779
with high intensity was due to the loss of CO, CO2, H2O, or other volatiles during the thermal treatments, thus showing an existence of diamond carbon bonded to other atoms such as N, Cl or Fe [19]. A significant change in the intensity of the bands in the spectral region 800–1500 cm− 1 was observed in all spectra (Fig. 2 — lines 4), which evidenced for sp2 coordinated carbon (bands at 1030, 1128 and 1260 cm− 1), as well as at 1535 cm− 1. As the temperature rise resulted in a release not only of the C_O groups but of hydrogen as well, an auto-oxidation process could not be excluded, which led to graphitization of diamond through surface reconstruction. In the spectral region 1500–2000 cm− 1, thought to be characteristic for the diamond surface, the existence and the type of surface acid groups were compared (Fig. 2 — lines 2 and 3). Different carboxyl structures were formed as a result of the thermal decomposition of ether structures (disappearance of the band at 1260 cm− 1) and/or oxidation processes. It should be marked that an enhancement and a blue shift of the C_O stretching vibration of the carbonyl groups (from 1701 to 1768 and 1785 cm− 1) were observed, simultaneous with the distinct vanishing of the C–H vibration. The effect was most probably due to the process of thermo-oxidative destruction of the amorphous carbon. Comparing the intensity of the bands, it should be marked that high amounts of acid groups were obtained after the samples underwent chemical oxidation with hydrogen peroxide followed by thermal treatment at 400 °C (Fig. 2A, B and C — lines 3). All samples had a pH-value of 3.5 and the percent of the “released C” was almost twice higher in comparison with the values of the initial samples (Table 1, column 3). In the spectra of the combined chemical–thermal oxidized samples, an appearance of a shoulder at around 3695 cm− 1 was observed belonging to aliphatic C–H and/or “isolated” O–H groups. Sample annealing at 600 °C for a rather long period could not remove all hydroxyl (O–H) and carboxyl (C_O) groups. The pH-measurement of the samples' water suspensions proved the retained slight acidic reaction of about pH = 6.5 (Table 1, column 4). It should be marked that the diamond particles get more stable than their counterparts with temperature rise and some specific surface reconstructions were revealed through the FTIR measurements. The transformation of the surface functional groups with the different oxidative treatments for the OBred, Bred and Ored samples was presented in Fig. 3A, B and C, respectively. For this sample series the amount of the oxidized “C” was less in comparison with the one measured for the first series (Table 1, column 1). Besides, the carbon loss varied considerably with the purification procedure used (Table 1) [20]. The main features of the FTIR-spectra, such as the blue shift of the C_O stretching vibration of carbonyl groups, the vanishing of the C–H vibration, the decrease but not the removal of all hydroxyl (O–H) groups, were followed too. Some new aspects, such as the presence of bands with higher intensity at 1350 cm− 1 and 1534 cm− 1 in all spectra should be marked, which evidenced that the diamond samples, synthesized at the presence of cooling media with reduced heat capacity, were with more distinct properties after annealing.
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4. Conclusion The FTIR-spectra could provide information for diamond surface functional groups. The samples' individual spectra were dependent on the extent of the defects arising from the addition of non-carbon atoms such as O, N, H or Fe during detonation of explosive molecules and purification procedures. The surface was sensitive to post-synthesis oxidative treatment both with air or hydrogen peroxide and provided opportunity for modification of the diamond surface. The different acid purification methods modified the diamond surface in an approximately equal manner. The surface of the catalytic burned diamonds was stable and hardly modified by additional oxidative treatments. References [1] O. Shenderova, V. Zhirnov, D. Brenner, Critical Reviews in Solid State and Materials Sciences 3–4 (2002) 266. [2] A. Vereshchagin, I. Larionova, Polzunov's Almanac (3) (1999) 596. [3] H. Fenglei, T. Yi, Y. Shourong, Physics of Solid State 46 (2004) 601. [4] Kang Hu, Qunji Hue, Physics of Solid State 46 (2004) 633. [5] I. Kulakova, Physics of Solid State 46 (2004) 621.
[6] [7] [8] [9] [10] [11] [12]
[13] [14] [15] [16] [17] [18]
[19] [20]
H.P. Boehm, Advances in Catalysis 16 (1996) 179. U.S. Patent 5353708 (1994). BG. Patent 49267A (1991). Ti. Jiang, K. Hu, Carbon 33 (1995) 1663. E. Mironov, A. Koretz, E. Petrov, Diamond and Related Materials 11 (2002) 872. A. Dandekar, R. Baker, H. Vannice, Journal of Catalysis 183 (1999) 131. G. Bogatyreva, M. Voloshin, V. Malogolovetz, V. Gvyazdovskaya, D. Ilnitskaya, Journal of Optoelectronics and Advanced Materials 2, 5 (2000) 469. V. Dolmatov, Uspekhi Khimii 70 (2001) 687. B. Azambre, S. Collura, J. Weber, Diamond and Related Materials 14 (2005) 1312. S. Stavrev, D. Mitev, L. Markov, J. Karadjov, Nanoscience and Nanotechnology 5 (2005) 121. A. Krüger, M. Ozawa, F. Kataoka, T. Fujino, Y. Suzuki, A.E. Aleksenskii, A.Ya. Vul', E. Osawa, Carbon 43 (2005) 1722. A. Koscheev, A. Serzhantov, S. Merchel, U. Ott, O. Guillois, C. Reynaud, Lunar and Planetary Science 34 (2003) 1287. Ch. Koike, N.Ch. Wickramasinghe, N. Kano, K. Yamakoshi, T. Yamamoto, Ch. Kaito, S. Kimura, H. Okuda, Monthly Notices of the Royal Astronomical Society 277 (1995) 986. Sh. Ji, T. Jiang, K. Hu, Sh. Li, Applied Surface Science 133 (1998) 231. A. Koscheev, N. Zaripov, U. Ott, Lunar and Planetary Science 36 (2005) 1406.
Surface peculiarities of detonation nanodiamonds in dependence of fabrication and purification methods D. Mitev a,⁎, R. Dimitrova b , M. Spassova b , Ch. Minchev b , S. Stavrev a a
b
Space Research Institute-BAS, Bulgaria Institute of Organic Chemistry-BAS, Bulgaria Available online 24 January 2007
Abstract A variety of post-treatments, such as thermal annealing and liquid phase oxidation, were used to determine the surface properties of six blasting synthesized nanodiamond powders, produced by various purification conditions. Methods of FTIR-spectroscopy, oxidative titration and pHmeasurement were applied for comparing the surface functional groups. All spectra exhibited the characteristic IR-bands of detonation diamond in the regions 3400–2700 cm− 1 and 1000–400 cm− 1. The differences were revealed by the dissimilarity of the bands in the region 2000–1000 cm− 1. The nanodiamond powder annealing up to 600 °C in air most fully revealed the specificity of the attached groups on diamond surface. The chemical oxidation with H2O2 modified nanodiamond surface groups and prevented them from further oxidative attack during thermal treatment. © 2007 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Nanoparticles; Surface characterization
1. Introduction The detonation synthesis of ultra-dispersed diamonds (UDDs) is commonly used in the last decades, since it offers a simple method to reach the necessary pressure and temperatures. Various targets are used for the purpose of diamond synthesis, such as pure carbon materials or carbon-containing compounds, including explosives with negative oxygen balance [1]. A variety of methods for UDDs purification are stated in the literature and it is established that the properties of the UDDs are largely determined by the applied purification methods [2,3]. The UDDs surface is chemically multifunctional, but the chemical groups can be formed and changed also by means of post-manipulation of detonation diamond [4–6]. The aim of the present work is to identify the chemical groups on the diamond surface. We have investigated six diamond samples produced by two detonation synthesis, and were refined using various purification conditions. The methods of thermal annealing in static and flow air conditions, the liquid phase oxidation, FTIRspectroscopy, pH and titration are applied for comparing the surface functional groups. Attention has been devoted to ⁎ Corresponding author. E-mail address: [email protected] (D. Mitev). 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.01.005
distinguish functional groups, which are the result of purification and those which are formed from subsequent autoannealing, oxidation, etc. 2. Experimental 2.1. Samples The UDDs synthesis was carried out in the Department SMN of SRI-BAS by the use of carbon-containing explosives with negative oxygen balance [7,8]. In order to obtain nanodiamonds with fundamental particles about 4 nm, detonation of cylindrical charge (0.4 kg), consisting of explosive molecules such as trinitrotoluene and cyclotrimethylenetrinitramine (60/40 wt.%), was carried out in a spherical explosion camera with an inner volume of 3 m3. The blasting of the charge was done in the presence of two cooling media — water and salt-containing water solutions (condition of reduced heat capacity of the cooler). The purification of the diamond powder from the attending admixtures was done as follows: i/ oxidation with concentrated nitric acid at temperatures up to 200–250 °C, followed by additional removal of the metal impurities by HCl-treatment (80–100 °C), washing
D. Mitev et al. / Diamond & Related Materials 16 (2007) 776–780 Table 1 Samples' designation, functional carbon atoms amount and acidity Samples
N B O OBred Bred Ored a b c
1
2
3
4
Parent
After heating at 400 °C
After H2O2 and heating at 400 °C
After heating at 600 °C
%“oxidized C” a (pH)
%“released C” b (pH)
%“released C” b (pH)
%“remained C” c (pH)
48 37 15 20 35 15
2.05 (5.2) 1.95 (3.6) 2.15 (4.6) 1.15 (4.6) 2.65 (4.3) 3.10 (6.0)
4.9 3.8 2.5 2.0 5.5 3.5
30.9 (6.5) 35.9 (6.5) 35.0 (6.5) 42.3 (6.8) 30.2 (6.0) 35.0 (6.8)
(3.5) (4.8) (6.2) (4.0) (4.5) (5.0)
(3.5) (3.5) (3.5) (3.5) (3.5) (3.5)
As determined by titration method. As determined by Ascarite sorption. As determined by thermal annealing in shallow bed.
with water until pH 7 and drying. The so obtained gray in color sample was denoted here as N sample. ii/ boiling in a solution of potassium dichromate (K2Cr2O7) in sulfuric acid (H2SO4) at 100–115 °C, followed by additional removal of metal impurities as described above. The gray-colored samples were denoted B, when water is used as a cooling agent, and Bred when reduced heat capacity of the cooler was applied. iii/ catalytic burning in air conditions (temperatures up to 500 °C), followed by removing of metal impurities. The samples of dark-grey appearance were respectively denoted O and Ored. iiii/ catalytic burning in air, followed by boiling in a solution of K2Cr2O7 and H2SO4. The so obtained gray sample (synthesized in reduced heat capacity condition) was denoted OBred. 2.2. Surface pH-value The pH determination was carried out as follows. A 20-ml portion of boiled de-ionized water was added to 0.2 g of the corresponding sample in a flask equipped with a reflux condenser. The mixture was heated and boiled gently for
777
10 min and filtered after cooling below 50 ± 5 °C. The pHvalues were determined at room temperature and the results were presented in Table 1. 2.3. Determination of surface carbon–oxide groups through titration The information for the surface carbon–oxygen functional groups was obtained performing standard titration. The procedure was described in [6]. 0.1 g of the corresponding sample with 8.3 ml de-ionized water and 10 ml K2Cr2O7 (0.25 N) were stirred with a magnetic stirrer at room temperature in two flasks. Further on, 20 ml of H2SO4 and trace of Ag2SO4 were added. The flasks were heated for 2 h at 90 °C. After hot filtration and cooling of the filtrates, the titration was carried out with 0.25 N Moor salt in the presence of an indicator (0.2% N-phenylantranilic acid). Its equilibrium was violet to green transition. The obtained results (“oxidized C”) were given in Table 1. 2.4. Thermal annealing i/ 0.2 g of the sample was placed in a flow type reactor. The temperature was raised with 10 °C/min from ambient up to 400 °C in air atmosphere and kept for 2 h. The amount of the “released C” was calculated from the weight of the CO2 absorbed by Ascarite-II as: %carbon = A(0.2729)(100) / B; where A is the weight (g) of CO2 determined by weighting the Ascarite tube before and after annealing; 0.2729 is the ratio of the molecular weight of carbon to the molecular weight of CO2 and B is the weight (g) of the sample. ii/ 0.2 g of the sample was placed in a laboratory pottery vessel and heated till constant weight at 600 °C in a furnace (shallow bed). The weight of the initial and heated samples was determined with accuracy of ±0.0005 g. The amount of the “remained C” was estimated as: %remained C = (M2 − M) (100) / (M1 − M); where, M — was the weight of the pottery vessel after heating, g; M1 — weight of the pottery vessel with initial sample, g; and M2 — weight of the pottery vessel with the diamond residue, g. The results were presented in Table 1.
Fig. 1. FTIR-spectra of the parent diamond powders: (A): line 1 — N; line 2 — B; line 3 — O; (B): line 1 — OBred; line 2 — Bred; line 3 — Ored.
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3. Results and discussion It is well proved that the detonation diamonds, chemically extracted from a detonation soot have a mean size of the core ∼4 nm and a shell made up of sp2-coordinated carbon atoms as well as unusual tight aggregation [16]. The SEM images (presented in Ref. [15]) evidenced that the particle sizes are in the nanometer range with a well-developed facet on it. The FTIRspectra of the investigated samples bear the general resemblance to known spectra as presented in Fig. 1. The spectra of N, B and O samples are given in the whole spectral region (Fig. 1A), while the spectra of OBred, Bred and Ored samples are shown in the specific diamond framework spectral region (Fig. 1B), since all spectra at the higher frequencies are identical. A wide, complex band centered at 600 cm− 1 was observed in all spectra, with the exception of the spectrum of sample O, where identifiable bands were presented. The broad structureless band
Fig. 2. FTIR-spectra of sample N (A — line 1), B (B — line 1) and O (C — line 1) impacted by the heating temperature: lines 2 — at 400 °C; lines 4 — at 600 °C and lines 3 — oxidation with H2O2 followed by thermal treatment at 400 °C.
2.5. Combined chemical–thermal oxidation 0.2 g of the sample were put in a flask and stirred with a magnetic stirrer at room temperature with a 30 ml aqueous H2O2 (26 wt.%) for 2 h. After that the sample was filtered, washed with water and dried. The thermal annealing of the oxidized sample was carried out at 400 °C. The samples' surface functional groups, before and after the different treatments, were registered by FTIR-spectra obtained on a Brucker-Vector 22 FTIR-spectrometer in the region of framework vibrations by using KBr pellet technique. A portion of the 0.5–1 mg sample to 100–150 mg KBr, depending on the sample type, was used. The assignment of the absorption bands is based on experience with molecular organic compounds and literature data [9–14]. The samples' nanocrystalline morphology was checked by SEM microphotographs taken on a Philips XL 30S model scanning electron microscope at 5 kV accelerating voltage and a beam current of 10 μA [15].
Fig. 3. FTIR-spectra of samples OBred (A — line 1), Bred (B — line 1) and Ored (C — line 1) impacted by the heating temperature: lines 2 — at 400 °C; lines 4 — at 600 °C and lines 3 — oxidation with H2O2 followed by thermal treatment at 400 °C.
D. Mitev et al. / Diamond & Related Materials 16 (2007) 776–780
could be associated with recombination through a system of continuously distributed energy levels in a sample band gap. The band at 672 cm− 1 (Fig. 1A, line 3) could be assigned to C–O bond, as it was done in [14,17] for meteoritic diamonds. The comparison of the spectra was based on the assumption that all detonation diamonds should resemble their morphological and structural characteristics. A broad peak consisting of identifiable strong bands at 1118 cm− 1 (deformation OH), at 1128 cm− 1 (generally assigned to C–O–H and C_C vibrations) and around 1260 cm− 1 (for amorphous diamond and/or vibration (–C–O– C–) in ether group) was observed in different ratios in all spectra. The structure of the band in this region originated from the absorption of the disordered nanocluster surface containing threefold-coordinated carbon. It should be marked that depending on the cooling condition, some differences in this spectral region were also refined. Relatively strong bands at 1330 cm− 1 (diamond peak), 1535 cm− 1 (sp2 coordinated carbon) and 2100 cm− 1 were registered (Fig. 1B). Further on, carbonyl groups were ascertained for all samples, by bands in the region 1701–1768 cm− 1. The existence of a strong band of the bending mode δ(OH) of water at 1630 cm− 1 and a broad asymmetric band centered at 3430 with shoulder at 3250 cm− 1, ν(OH) were seen. As stated in [18], the band at 3440 cm− 1 was apparent for small water clusters, while the shoulder at lower frequency revealed a coordinated type of water. Thus, our spectra provided evidence for the existence mainly of intermolecular hydrogen bonding of adsorbed water molecules. The characteristic absorption bands at 2890 cm− 1 and at 2925 cm− 1, which corresponded to the asymmetric and symmetric stretching C–H vibration respectively, were seen only in the spectrum of sample B. From the performed analyses we concluded that different acid groups and admixtures presented on diamond powder surface in dependence of the purification method. Both acid purification procedures resulted in the formation of groups with high acidity, as the effect is more pronounced in the case of the nitric acid purification. As it was presented in Table 1 — column 1, the pH-values were in the range from 3.5 to 6.2 and the amount of the “oxidized C” evidenced for multifunctional carbon atoms. A higher amount of amorphous diamond carbon, especially in the powders synthesized under reduced heat capacity conditions, was registered (the broad peak at 1260 cm− 1). The method of the catalytic burning provided fairly good removal of the amorphous carbon from the diamond powder. The nanocrystalline morphology of the samples was not affected as proved by the SEM images [5]. In order to follow the stability and the type of the existing surface functional groups, we performed oxidative treatment in air at two temperatures (400 and 600 °C) and combined chemical–thermal oxidation. All samples heated at 400 °C demonstrated a weight loss of about 12 wt.% and carbon diminishing of 1–3% (Table 1, column 2). A transformation of the surface functional groups during the upper mentioned treatments was registered in the FTIR-spectra as shown in Fig. 2A, B and C for the N, B and O samples, respectively. Strong and defined bands, between 500 and 800 cm− 1, came into view with the temperature rise, as for sample “O” the bands were with high intensity at 400 °C. The appearance of bands
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with high intensity was due to the loss of CO, CO2, H2O, or other volatiles during the thermal treatments, thus showing an existence of diamond carbon bonded to other atoms such as N, Cl or Fe [19]. A significant change in the intensity of the bands in the spectral region 800–1500 cm− 1 was observed in all spectra (Fig. 2 — lines 4), which evidenced for sp2 coordinated carbon (bands at 1030, 1128 and 1260 cm− 1), as well as at 1535 cm− 1. As the temperature rise resulted in a release not only of the C_O groups but of hydrogen as well, an auto-oxidation process could not be excluded, which led to graphitization of diamond through surface reconstruction. In the spectral region 1500–2000 cm− 1, thought to be characteristic for the diamond surface, the existence and the type of surface acid groups were compared (Fig. 2 — lines 2 and 3). Different carboxyl structures were formed as a result of the thermal decomposition of ether structures (disappearance of the band at 1260 cm− 1) and/or oxidation processes. It should be marked that an enhancement and a blue shift of the C_O stretching vibration of the carbonyl groups (from 1701 to 1768 and 1785 cm− 1) were observed, simultaneous with the distinct vanishing of the C–H vibration. The effect was most probably due to the process of thermo-oxidative destruction of the amorphous carbon. Comparing the intensity of the bands, it should be marked that high amounts of acid groups were obtained after the samples underwent chemical oxidation with hydrogen peroxide followed by thermal treatment at 400 °C (Fig. 2A, B and C — lines 3). All samples had a pH-value of 3.5 and the percent of the “released C” was almost twice higher in comparison with the values of the initial samples (Table 1, column 3). In the spectra of the combined chemical–thermal oxidized samples, an appearance of a shoulder at around 3695 cm− 1 was observed belonging to aliphatic C–H and/or “isolated” O–H groups. Sample annealing at 600 °C for a rather long period could not remove all hydroxyl (O–H) and carboxyl (C_O) groups. The pH-measurement of the samples' water suspensions proved the retained slight acidic reaction of about pH = 6.5 (Table 1, column 4). It should be marked that the diamond particles get more stable than their counterparts with temperature rise and some specific surface reconstructions were revealed through the FTIR measurements. The transformation of the surface functional groups with the different oxidative treatments for the OBred, Bred and Ored samples was presented in Fig. 3A, B and C, respectively. For this sample series the amount of the oxidized “C” was less in comparison with the one measured for the first series (Table 1, column 1). Besides, the carbon loss varied considerably with the purification procedure used (Table 1) [20]. The main features of the FTIR-spectra, such as the blue shift of the C_O stretching vibration of carbonyl groups, the vanishing of the C–H vibration, the decrease but not the removal of all hydroxyl (O–H) groups, were followed too. Some new aspects, such as the presence of bands with higher intensity at 1350 cm− 1 and 1534 cm− 1 in all spectra should be marked, which evidenced that the diamond samples, synthesized at the presence of cooling media with reduced heat capacity, were with more distinct properties after annealing.
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4. Conclusion The FTIR-spectra could provide information for diamond surface functional groups. The samples' individual spectra were dependent on the extent of the defects arising from the addition of non-carbon atoms such as O, N, H or Fe during detonation of explosive molecules and purification procedures. The surface was sensitive to post-synthesis oxidative treatment both with air or hydrogen peroxide and provided opportunity for modification of the diamond surface. The different acid purification methods modified the diamond surface in an approximately equal manner. The surface of the catalytic burned diamonds was stable and hardly modified by additional oxidative treatments. References [1] O. Shenderova, V. Zhirnov, D. Brenner, Critical Reviews in Solid State and Materials Sciences 3–4 (2002) 266. [2] A. Vereshchagin, I. Larionova, Polzunov's Almanac (3) (1999) 596. [3] H. Fenglei, T. Yi, Y. Shourong, Physics of Solid State 46 (2004) 601. [4] Kang Hu, Qunji Hue, Physics of Solid State 46 (2004) 633. [5] I. Kulakova, Physics of Solid State 46 (2004) 621.
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