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Identification, quantification and modification of detonation nanodiamond functional groups
Diamond & Related Materials 22 (2012) 113–117
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Identification, quantification and modification of detonation nanodiamond functional groups L. Schmidlin a,⁎, V. Pichot a, M. Comet a, S. Josset a, P. Rabu b, D. Spitzer a a NS3E “Nanomatériaux pour Systèmes Sous Sollicitations Extrêmes” UMR 3208 ISL/CNRS, Institut franco-allemand de recherches de Saint-Louis (ISL), 5 rue du général Cassagnou, 68301 Saint-Louis, France b Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS- Université de Strasbourg, bât 69, 23 rue du Loess, Boîte Postale 43, 67034 Strasbourg Cedex 02, France
a r t i c l e
i n f o
Article history: Received 11 January 2011 Received in revised form 2 November 2011 Accepted 13 December 2011 Available online 20 December 2011 Keywords: Nanodiamond Purification Functionalization Surface characterization Oxidation
a b s t r a c t The chemical composition of the detonation nanodiamond (DND) surface has to be unambiguously known in order to use them for specific applications and to develop strategies of functionalization. To this end, the Boehm titration method was here applied to DNDs of different origins subjected to different purification procedures. This method allows the determination of the oxygenated functional groups present on the surface of carbonaceous materials according to their acidity. The DNDs synthesized at the Institut franco-allemand de recherches de Saint-Louis (ISL) and purified through a two-step method were compared to commercial nanodiamonds. This work proves that the nature of the DND functional groups is very dependent on the history of the post-treatment applied to the nanodiamonds during the purification process of the detonation soot. The nanodiamonds synthesized at ISL and commercial nanodiamonds possess 0.81 COOH sites/nm2 and 0.15 COOH sites/nm2, respectively. However, an oxidation treatment, similar to the one performed on the ISL nanodiamonds, applied to the commercial sample, increases the COOH ratio to 0.80 function/nm2. In addition, the existence of lactone sites was pointed out on oxidized samples, indicating the presence of hydroxyl groups on the surface of untreated nanodiamonds. From experimental values and modelling, it was suggested that these functions are located on the edges of the nanodiamond particles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Diamond nanoparticles are of great interest for surface grafting [1], uses in biology [2–4] or for drug delivery [5,6]. Applications and devices conception require the controlled functionalization of the diamond nanoparticle surface in order to get specific chemical behaviours or properties. Different interactions with functional molecules can be used such as covalent, ionic or hydrogen bonding, and are strongly dependent on the species present on the surface of the nanodiamonds. Therefore, it is necessary to know the chemical composition of the nanodiamond surface in order to adapt the molecules that should be grafted. Such chemical information is also crucial for comparing the results reported by using nanodiamonds of different origins. This is particularly the case of the detonation nanodiamonds (DND) produced by using a variety of explosive mixtures and commercialized after uneven purification treatments. Previous experiments on DNDs synthesized in our laboratory (ISL, Saint-Louis, France) lead to evaluating the nanodiamond size by means of different techniques [7]. Radiuses of 2.40 nm and 2.05 nm were found with the BET method and by powder X-ray diffraction (XRD),
⁎ Corresponding author. Tel.: + 33 389695158; fax: + 33 389695074. E-mail address: [email protected] (L. Schmidlin). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.12.009
respectively. The radius obtained by the XRD technique corresponds to the DND crystallite, i.e. the size of the coherent diffraction domains, whereas the one calculated with the BET technique corresponds to the DND core plus the surface functional groups with a possible noncrystalline shell. The thickness of 0.35 nm on the surface of the DND could correspond to small functional groups. In the literature, it is found that the nanodiamond surface is naturally covered with various functional groups such as alcohol, amine, amide, carboxylic acid and carbonyl [8–13]. The Boehm titration method allows us to confirm the presence of carboxylic acid groups on the surface of the DND, and to quantify the surface coverage by these functions [14]. This technique is commonly used to determine the presence of oxygenated groups (like carboxylic acids, lactones, phenols, hydroxyls) on solids such as activated charcoals [15], but it has never been applied to nanodiamonds yet. The principle of this technique consists in the reaction of the oxygenated groups present on the surface of the diamond with different bases, depending on their acidity. Sodium bicarbonate (NaHCO3), a weak base (pKa= 6.33), exclusively reacts with the carboxylic sites (pKa = 4.2) located on the surface of the nanodiamonds. Sodium carbonate (Na2CO3), a stronger base (pKa = 10.35), reacts with both carboxylic acids and lactones. The amount of base that did not react with the nanodiamond surface is determined by a back titration with hydrochloric acid. Nguyen et al. tried to quantify the carboxylic sites by using a strong base (NaOH) [16]. For
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L. Schmidlin et al. / Diamond & Related Materials 22 (2012) 113–117
Fig. 1. Titration of the DND surface by NaHCO3.
this purpose, they first dispersed DND into a sodium hydroxide solution which was subsequently titrated by an HCl solution. The use of NaOH is inappropriate for dosing only carboxylic sites, because sodium hydroxide reacts with several kinds of surface functional groups. In addition, the “simultaneous” titration of the excess of NaOH in the presence of DND carboxylate groups does not allow the accurate determination of the equivalence point by conductimetry [17]. In this study two different detonation nanodiamond samples were compared. The oxygen-containing functional groups of the two samples were titrated by the Boehm technique. Infrared spectroscopy and zeta potential measurements were used to corroborate the results obtained by the titration. The influence of the purification history of DND samples on their surface state was highlighted. 2. Experimental Two different detonation nanodiamond samples were compared. The first one, ND1, was synthesized in our laboratory. The second sample consists of commercial nanodiamonds purchased from JSC « Diamond centre» in Saint-Petersburg and will be identified as ND2. The two samples were synthesized by detonation. The ND1 sample was obtained from the detonation of an explosive mixture composed of 30 wt.% RDX (hexahydro-1,3,5-hexanitro-1,3,5-triazine) and 70 wt.% TNT (2,4,6-trinitrotoluene) in water. The detonation soot was recovered from the detonation tank with deionized water. Successive filtration steps and evaporation allowed the raw detonation soot to be recovered. To remove metallic impurities coming from the explosive charge environment, an acidic treatment using an aqua regia mixture of HF/HNO3 was performed [7,18]. Subsequently to the acidic treatment, an oxidation in the air atmosphere around 400 °C was achieved to remove residual sp2 carbon impurities. The commercial nanodiamonds ND2 were subjected to a one-step acidic and oxidative treatment with nitric acid (HNO3) at 240–260 °C under pressure (100 atm) [19]. The identification of the surface functional groups was achieved by means of a Bruker Tensor 27 FT-IR spectroscope in transmittance mode, using KBr pellets. The surface area was obtained by a Beckman Coulter SA 3100 Surface Area Analyzer; for this analysis, the samples were outgassed at 225 °C under helium atmosphere during 15 h. The surface charge of the nanodiamond particles in suspension was measured by a Malvern Zetasizer Nano Series Nano-ZS with 1 g/L nanodiamond suspensions in water. For the Boehm technique analysis, a 0.05 mol.L− 1 solution of NaHCO3 was prepared and titrated by conductimetry with a 0.1 M HCl solution. Detonation nanodiamonds (250 mg) were then dispersed in 25 mL of the as-prepared NaHCO3 solution. The suspension was stirred for 24 h under a nitrogen flux. At the end of the experiment, the medium was filtered through a 1 μm OMNIPORE membrane and the filtrate was titrated with a 0.1 M HCl solution. The same procedure was used to carry out the titration with Na2CO3.
the decomposition of carbonate ion into water and carbon dioxide (Fig. 1). NaHCO3 is added in excess in order to achieve the complete reaction with the carboxylic functions. The volume of the HCl solution at the equivalence point was determined from the titration curve of the filtrate (Fig. 2). The slight increase in the conductivity observed before the equivalence point is attributed to the progressive substitution of Cl− for the initial HCO3−. According to Vogel, the relative limiting ionic molar conductivity of these anions at 25 °C is 44.5 and 76.3 Ω− 1·cm 2·mol− 1 respectively [17]. The sudden slope change noticed beyond the equivalent volume is due to the presence of H3O+ (349.8 Ω− 1·cm2·mol− 1) in the solution. The mole number of the sodium bicarbonate that did not react with the carboxylic acid groups of the nanodiamonds is deduced from the HCl volume at the equivalence point. The initial mole number of sodium bicarbonate was determined by the titration of the stock solution. The subtraction of these two values gives the mole number of the sodium bicarbonate which has reacted with the nanodiamonds and consequently, the mole number of the carboxylic acid groups present on the surface of the nanodiamonds. The surface density of the carboxylic sites at the surface of the DND is then calculated from the following formula: COOH Sites ¼
nCOOH NA mDND SBET 1018
ð1Þ
where nCOOH is the mole number of the carboxylic functions on the surface, NA the Avogadro number (6.02 × 10 23 mol − 1), mDND the mass of titrated nanodiamonds (g) and SBET the specific area determined by nitrogen adsorption (m 2/g). 3.1.2. Titration of lactone sites with Na2CO3 A stronger base is necessary for the titration of the lactone sites. Na2CO3 (pKa = 10.35) is convenient to titrate not only the lactone, but also the carboxylic sites. Lactone cycles undergo saponification resulting in carboxylate and hydroxyl sites (Fig. 3). The species involved in the reactions with functional groups is Na2CO3 only, in large excess and leading to NaHCO3. The titration of sodium carbonate occurs in two steps (Fig. 4). The conductivity decrease is attributed to the transformation of carbonate
NaHCO3 H2O + CO2 (H2CO3) NaCl
H2O + CO2 (H2CO3) NaCl HCl Veq
3. Results and discussion 3.1. DND titration using Boehm's technique 3.1.1. Titration of the carboxylic sites with NaHCO3 The carboxyl groups of the DND surface reacted with NaHCO3. This reaction leads to the formation of sodium carboxylate groups and to
Fig. 2. Example of NaHCO3 titration with 0.1 M HCl for the determination of the ND1 surface chemistry.
L. Schmidlin et al. / Diamond & Related Materials 22 (2012) 113–117
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Fig. 3. Titration of the DND surface by Na2CO3.
Na2CO3 NaHCO3 NaCl
H2CO3 NaCl HCl
NaHCO3 H2CO3 NaCl
Veq1
Veq2
Fig. 4. Example of Na2CO3 titration with HCl 0.1 M for the determination of the ND1 surface chemistry.
(138.6 Ω − 1·cm 2·mol − 1) into bicarbonate and to the introduction of chloride anions coming from the HCl solution. The second step corresponds to the neutralization of bicarbonate by the titrating solution which results in a slight increase in conductivity, as previously discussed. The number of carboxylic and lactone moieties is given by the difference between the equivalent volume Veq2 determined for the stock solution and the filtrate. The number of lactone sites is then calculated by subtracting from this value the number of carboxylic sites previously determined by the NaHCO3 titration. The titration results are presented in Table 1, for samples ND1 and ND2. 3.2. Influence of the purification process on the surface functionalization As mentioned in the experimental section, ND1 and ND2 were obtained from detonation soots purified by different processes. The Boehm method applied to each nanodiamond sample gives 0.81 carboxylic site per nm 2 for ND1 and 0.15/nm 2 for ND2 (Table 1). There are over 5 times more carboxylic acid groups on the surface of the DND purified by the ISL method than on that of the commercial DND. Considering the titration of lactones, 0.13 lactone/nm 2 for ND1 were found and none for ND2 (Table 1). Fig. 5 shows the FT-IR spectra of the two different DNDs. The bands located at 3500 cm − 1 are characteristic of surface O-H bonds of water molecules. The sharp band at 1630 cm − 1 reveals the presence of adsorbed water molecules [20]. The broad band (1700– 1865 cm − 1) can be attributed to ν C =O bonds in carboxylic acids and lactones [21]. Absorption bands typical of nanodiamonds are observed in the 900–1500 cm − 1 range [22]. The spectra of the two samples essentially differ in the relative position and intensity of the band attributed to the C= O vibration. The
IR absorption assigned to the C=O bonds in carboxylic acids is observed at lower wavenumber values than the one of lactones. Indeed, the adjacent electron-attractive oxygen increases the force constant of the carbonyl bond (inductive effect) [23]. For ND2 the ν C= O band is located at 1770 cm − 1, whereas in ND1, this C= O band is shifted to 1795 cm − 1. This blueshift can be linked to the presence of lactones. This analysis is in good agreement with Boehm's titration results. The ND1 sample was also studied by infrared spectroscopy after its reaction with the Na2CO3 solution. As expected for sodium carboxylates, the ν C=O band is shifted to lower wavenumbers, compared to that of the carboxylic acid of the initial DND [23]. The band assigned to the deformation of the adsorbed water is slightly shifted to higher wavenumbers. According to Robert et al., the ability of oxygen atoms to form hydrogen bonds is reduced in bonded carboxylates [24]. Here, carboxylate sites interact with their Na + moiety, thus modifying the hydrogen bonding of water molecules on the surface. In aqueous medium, the carboxylic sites of the DND surface are ionized (COO −), leading to a negative value of the zeta potential. The zeta potential values are found to be − 52 mV and − 22 mV for ND1 and ND2, respectively. Therefore, the higher rate of carboxylic groups on ND1 is directly linked to the more negative surface charge of the DND. The different characterization techniques make it possible to conclude that the DND produced and purified with the ISL technique (ND1) contains a lot more carboxylic sites than the commercial nanodiamonds used in this study (ND2). 3.3. Influence of the thermal oxidation on the surface chemistry As described previously, the surface chemistry of DNDs is highly influenced by their purification process. The main difference between the ND1 and ND2 purification processes is the oxidative step applied to ND1. Therefore, ND2 was subjected to the same oxidation process as the one used for the purification of ND1. The as-prepared material (ND3) was studied by using the same techniques as for ND1 and ND2. Boehm's technique performed on ND3 highlighted a drastic modification in the surface chemistry after the oxidation process with a high increase in the carboxylic sites and the creation of lactone sites (see Table 2).
Table 1 Specific area and number of surface functional groups for ND1 and ND2.
ND1 ND2
Specific area (m2/g)
COOH functions (/nm2)
Lactone functions (/nm2)
386 331
0.81 0.15
0.13 0
Fig. 5. ND1 and ND2 FT-IR spectra.
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Table 2 Specific area and number of surface functional groups for ND1, ND2 and ND3.
ND1 ND2 ND3
Specific area (m2/g)
COOH functions (/nm2)
Lactone functions (/nm2)
386 331 292
0.81 0.15 0.80
0.13 0 0.27
The number of carboxylic sites increased from 0.15 site/nm 2 for ND2 to 0.80 site/nm 2 for ND3. The number of lactone sites -which are not present on ND2- is increased up to 0.27 site/nm 2 after the thermal oxidation. It can be assumed that the formation of lactone results from the esterification of a carboxyl group with an adjacent hydroxyl group. This hypothesis is strengthened by the fact that the thermal treatment at 400 °C favours the dehydration and thus the lactone formation. Furthermore, the determination of the number of lactones indicates that ND2 starting material possesses at least 0.27 hydroxyl site per nm 2. The ND1 and ND3 spectra are almost the same, which indicates that the surface state of the ND2 sample was drastically modified upon heating at 400 °C for 24 h (Fig. 6). The absorption band of the carboxylic groups ν C= O in the ND2 sample was shifted from 1770 cm − 1 to 1795 cm − 1 after the oxidation process. This observation is consistent with the results obtained with Boehm's technique, indicating the formation of lactones. The increase in the carboxylic sites after the oxidation treatment is also confirmed by the zeta potential measurement, with an increase of 90% of the negative charge, from -22 mV for ND2 to -42 mV for ND3. 3.4. Distribution of the oxygenated sites on the DND surface According to Barnard et al., a simple way of representing the DND surface is the assembly of the two distinct crystallographic planes (111) and (100) [25]. The geometrical structures built by these planes are a truncated octahedron or a cuboctahedron (Fig. 7). The truncated octahedron is composed of 76% of (111) planes and 24% of (100) planes, while the cuboctahedron is made of 36% of (111) planes and 64% of (100) planes. The surface of a single particle approximated to a 4.10 nm diameter sphere is 52.81 nm2. On the basis of the diamond lattice parameter, the area occupied by a single carbon atom on the surface is estimated to be 0.0546 and 0.0630 nm2 for the (111) and (100) planes, respectively. The (111) and (100) planes, respectively, form hexagons and squares in the truncated octahedron structure and triangles and squares in the case of the cuboctahedron. Generally, the edges of a particle are the most reactive part of the surface. The number of carbon atoms forming each geometrical structure (square, triangle and hexagon) is linked to the peripheral atoms.
Fig. 6. ND1, ND2 and ND3 FT-IR spectra.
Fig. 7. Representation of the truncated octahedron (A) and cuboctahedron (B) structures.
The following equations allow the calculation of the nanodiamond edge carbons from the surface atoms according to the geometrical structure: x¼
pffiffiffiffiffi SS −1
x¼3
x¼
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 1 2 1 − ð1−ST Þ − 4 9 2
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 1 − ð1−SH Þ − 4 3 2
ð2Þ
ð3Þ
ð4Þ
where x is the number of carbon atoms composing each edge. SS, ST and SH represent the number of carbon atoms forming the square, triangular and hexagonal faces, respectively. The number of edges is 36 for the truncated octahedron and 24 for the cuboctahedron. Considering that the steric repulsions caused by the carboxylic acid or lactone surface functions only allow the bonding of one edge carbon atom out of three, the number of edge carbons that could be bonded with a surface functional group is 60 for the truncated octahedron and 64 for the cuboctahedron. The results obtained with Boehm's technique show that there are 50 lactones and carboxylic acid sites on the ND1 surface and 57 on ND3. The calculated values are then consistent with the experimental values obtained with Boehm's technique. These results infer that the functional sites are located on the edges of the DNDs, with nearly complete coverage. 4. Conclusion The surface chemistry of different detonation nanodiamonds was investigated. Boehm's technique showed the presence of carboxylic acid and lactone functional groups on the surface of the DNDs, in agreement with FT-IR spectroscopy. The DND surface chemistry is highly influenced by the purification process and especially by the oxidation treatment. The nanodiamonds synthesized at ISL and commercial nanodiamonds possess 0.81 COOH sites/nm 2 and 0.15 COOH sites/nm 2, respectively. However, applying a post-oxidation treatment to commercial nanodiamonds modifies the surface chemistry of the DNDs and increases the quantity of carboxylic acid and lactone functions. The quantification of the oxygenated groups on the DND surface combined with a simple model approach revealed that the functional groups are located on the edges of the nanoparticles occupying all the available sites. This study shows the interest of Boehm's technique for the quantitative characterization of the chemical functions at the surface of nanodiamonds. This can help in developing specific strategies of functionalization adapted to DNDs of different origins.
L. Schmidlin et al. / Diamond & Related Materials 22 (2012) 113–117
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Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Identification, quantification and modification of detonation nanodiamond functional groups L. Schmidlin a,⁎, V. Pichot a, M. Comet a, S. Josset a, P. Rabu b, D. Spitzer a a NS3E “Nanomatériaux pour Systèmes Sous Sollicitations Extrêmes” UMR 3208 ISL/CNRS, Institut franco-allemand de recherches de Saint-Louis (ISL), 5 rue du général Cassagnou, 68301 Saint-Louis, France b Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS- Université de Strasbourg, bât 69, 23 rue du Loess, Boîte Postale 43, 67034 Strasbourg Cedex 02, France
a r t i c l e
i n f o
Article history: Received 11 January 2011 Received in revised form 2 November 2011 Accepted 13 December 2011 Available online 20 December 2011 Keywords: Nanodiamond Purification Functionalization Surface characterization Oxidation
a b s t r a c t The chemical composition of the detonation nanodiamond (DND) surface has to be unambiguously known in order to use them for specific applications and to develop strategies of functionalization. To this end, the Boehm titration method was here applied to DNDs of different origins subjected to different purification procedures. This method allows the determination of the oxygenated functional groups present on the surface of carbonaceous materials according to their acidity. The DNDs synthesized at the Institut franco-allemand de recherches de Saint-Louis (ISL) and purified through a two-step method were compared to commercial nanodiamonds. This work proves that the nature of the DND functional groups is very dependent on the history of the post-treatment applied to the nanodiamonds during the purification process of the detonation soot. The nanodiamonds synthesized at ISL and commercial nanodiamonds possess 0.81 COOH sites/nm2 and 0.15 COOH sites/nm2, respectively. However, an oxidation treatment, similar to the one performed on the ISL nanodiamonds, applied to the commercial sample, increases the COOH ratio to 0.80 function/nm2. In addition, the existence of lactone sites was pointed out on oxidized samples, indicating the presence of hydroxyl groups on the surface of untreated nanodiamonds. From experimental values and modelling, it was suggested that these functions are located on the edges of the nanodiamond particles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Diamond nanoparticles are of great interest for surface grafting [1], uses in biology [2–4] or for drug delivery [5,6]. Applications and devices conception require the controlled functionalization of the diamond nanoparticle surface in order to get specific chemical behaviours or properties. Different interactions with functional molecules can be used such as covalent, ionic or hydrogen bonding, and are strongly dependent on the species present on the surface of the nanodiamonds. Therefore, it is necessary to know the chemical composition of the nanodiamond surface in order to adapt the molecules that should be grafted. Such chemical information is also crucial for comparing the results reported by using nanodiamonds of different origins. This is particularly the case of the detonation nanodiamonds (DND) produced by using a variety of explosive mixtures and commercialized after uneven purification treatments. Previous experiments on DNDs synthesized in our laboratory (ISL, Saint-Louis, France) lead to evaluating the nanodiamond size by means of different techniques [7]. Radiuses of 2.40 nm and 2.05 nm were found with the BET method and by powder X-ray diffraction (XRD),
⁎ Corresponding author. Tel.: + 33 389695158; fax: + 33 389695074. E-mail address: [email protected] (L. Schmidlin). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.12.009
respectively. The radius obtained by the XRD technique corresponds to the DND crystallite, i.e. the size of the coherent diffraction domains, whereas the one calculated with the BET technique corresponds to the DND core plus the surface functional groups with a possible noncrystalline shell. The thickness of 0.35 nm on the surface of the DND could correspond to small functional groups. In the literature, it is found that the nanodiamond surface is naturally covered with various functional groups such as alcohol, amine, amide, carboxylic acid and carbonyl [8–13]. The Boehm titration method allows us to confirm the presence of carboxylic acid groups on the surface of the DND, and to quantify the surface coverage by these functions [14]. This technique is commonly used to determine the presence of oxygenated groups (like carboxylic acids, lactones, phenols, hydroxyls) on solids such as activated charcoals [15], but it has never been applied to nanodiamonds yet. The principle of this technique consists in the reaction of the oxygenated groups present on the surface of the diamond with different bases, depending on their acidity. Sodium bicarbonate (NaHCO3), a weak base (pKa= 6.33), exclusively reacts with the carboxylic sites (pKa = 4.2) located on the surface of the nanodiamonds. Sodium carbonate (Na2CO3), a stronger base (pKa = 10.35), reacts with both carboxylic acids and lactones. The amount of base that did not react with the nanodiamond surface is determined by a back titration with hydrochloric acid. Nguyen et al. tried to quantify the carboxylic sites by using a strong base (NaOH) [16]. For
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L. Schmidlin et al. / Diamond & Related Materials 22 (2012) 113–117
Fig. 1. Titration of the DND surface by NaHCO3.
this purpose, they first dispersed DND into a sodium hydroxide solution which was subsequently titrated by an HCl solution. The use of NaOH is inappropriate for dosing only carboxylic sites, because sodium hydroxide reacts with several kinds of surface functional groups. In addition, the “simultaneous” titration of the excess of NaOH in the presence of DND carboxylate groups does not allow the accurate determination of the equivalence point by conductimetry [17]. In this study two different detonation nanodiamond samples were compared. The oxygen-containing functional groups of the two samples were titrated by the Boehm technique. Infrared spectroscopy and zeta potential measurements were used to corroborate the results obtained by the titration. The influence of the purification history of DND samples on their surface state was highlighted. 2. Experimental Two different detonation nanodiamond samples were compared. The first one, ND1, was synthesized in our laboratory. The second sample consists of commercial nanodiamonds purchased from JSC « Diamond centre» in Saint-Petersburg and will be identified as ND2. The two samples were synthesized by detonation. The ND1 sample was obtained from the detonation of an explosive mixture composed of 30 wt.% RDX (hexahydro-1,3,5-hexanitro-1,3,5-triazine) and 70 wt.% TNT (2,4,6-trinitrotoluene) in water. The detonation soot was recovered from the detonation tank with deionized water. Successive filtration steps and evaporation allowed the raw detonation soot to be recovered. To remove metallic impurities coming from the explosive charge environment, an acidic treatment using an aqua regia mixture of HF/HNO3 was performed [7,18]. Subsequently to the acidic treatment, an oxidation in the air atmosphere around 400 °C was achieved to remove residual sp2 carbon impurities. The commercial nanodiamonds ND2 were subjected to a one-step acidic and oxidative treatment with nitric acid (HNO3) at 240–260 °C under pressure (100 atm) [19]. The identification of the surface functional groups was achieved by means of a Bruker Tensor 27 FT-IR spectroscope in transmittance mode, using KBr pellets. The surface area was obtained by a Beckman Coulter SA 3100 Surface Area Analyzer; for this analysis, the samples were outgassed at 225 °C under helium atmosphere during 15 h. The surface charge of the nanodiamond particles in suspension was measured by a Malvern Zetasizer Nano Series Nano-ZS with 1 g/L nanodiamond suspensions in water. For the Boehm technique analysis, a 0.05 mol.L− 1 solution of NaHCO3 was prepared and titrated by conductimetry with a 0.1 M HCl solution. Detonation nanodiamonds (250 mg) were then dispersed in 25 mL of the as-prepared NaHCO3 solution. The suspension was stirred for 24 h under a nitrogen flux. At the end of the experiment, the medium was filtered through a 1 μm OMNIPORE membrane and the filtrate was titrated with a 0.1 M HCl solution. The same procedure was used to carry out the titration with Na2CO3.
the decomposition of carbonate ion into water and carbon dioxide (Fig. 1). NaHCO3 is added in excess in order to achieve the complete reaction with the carboxylic functions. The volume of the HCl solution at the equivalence point was determined from the titration curve of the filtrate (Fig. 2). The slight increase in the conductivity observed before the equivalence point is attributed to the progressive substitution of Cl− for the initial HCO3−. According to Vogel, the relative limiting ionic molar conductivity of these anions at 25 °C is 44.5 and 76.3 Ω− 1·cm 2·mol− 1 respectively [17]. The sudden slope change noticed beyond the equivalent volume is due to the presence of H3O+ (349.8 Ω− 1·cm2·mol− 1) in the solution. The mole number of the sodium bicarbonate that did not react with the carboxylic acid groups of the nanodiamonds is deduced from the HCl volume at the equivalence point. The initial mole number of sodium bicarbonate was determined by the titration of the stock solution. The subtraction of these two values gives the mole number of the sodium bicarbonate which has reacted with the nanodiamonds and consequently, the mole number of the carboxylic acid groups present on the surface of the nanodiamonds. The surface density of the carboxylic sites at the surface of the DND is then calculated from the following formula: COOH Sites ¼
nCOOH NA mDND SBET 1018
ð1Þ
where nCOOH is the mole number of the carboxylic functions on the surface, NA the Avogadro number (6.02 × 10 23 mol − 1), mDND the mass of titrated nanodiamonds (g) and SBET the specific area determined by nitrogen adsorption (m 2/g). 3.1.2. Titration of lactone sites with Na2CO3 A stronger base is necessary for the titration of the lactone sites. Na2CO3 (pKa = 10.35) is convenient to titrate not only the lactone, but also the carboxylic sites. Lactone cycles undergo saponification resulting in carboxylate and hydroxyl sites (Fig. 3). The species involved in the reactions with functional groups is Na2CO3 only, in large excess and leading to NaHCO3. The titration of sodium carbonate occurs in two steps (Fig. 4). The conductivity decrease is attributed to the transformation of carbonate
NaHCO3 H2O + CO2 (H2CO3) NaCl
H2O + CO2 (H2CO3) NaCl HCl Veq
3. Results and discussion 3.1. DND titration using Boehm's technique 3.1.1. Titration of the carboxylic sites with NaHCO3 The carboxyl groups of the DND surface reacted with NaHCO3. This reaction leads to the formation of sodium carboxylate groups and to
Fig. 2. Example of NaHCO3 titration with 0.1 M HCl for the determination of the ND1 surface chemistry.
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Fig. 3. Titration of the DND surface by Na2CO3.
Na2CO3 NaHCO3 NaCl
H2CO3 NaCl HCl
NaHCO3 H2CO3 NaCl
Veq1
Veq2
Fig. 4. Example of Na2CO3 titration with HCl 0.1 M for the determination of the ND1 surface chemistry.
(138.6 Ω − 1·cm 2·mol − 1) into bicarbonate and to the introduction of chloride anions coming from the HCl solution. The second step corresponds to the neutralization of bicarbonate by the titrating solution which results in a slight increase in conductivity, as previously discussed. The number of carboxylic and lactone moieties is given by the difference between the equivalent volume Veq2 determined for the stock solution and the filtrate. The number of lactone sites is then calculated by subtracting from this value the number of carboxylic sites previously determined by the NaHCO3 titration. The titration results are presented in Table 1, for samples ND1 and ND2. 3.2. Influence of the purification process on the surface functionalization As mentioned in the experimental section, ND1 and ND2 were obtained from detonation soots purified by different processes. The Boehm method applied to each nanodiamond sample gives 0.81 carboxylic site per nm 2 for ND1 and 0.15/nm 2 for ND2 (Table 1). There are over 5 times more carboxylic acid groups on the surface of the DND purified by the ISL method than on that of the commercial DND. Considering the titration of lactones, 0.13 lactone/nm 2 for ND1 were found and none for ND2 (Table 1). Fig. 5 shows the FT-IR spectra of the two different DNDs. The bands located at 3500 cm − 1 are characteristic of surface O-H bonds of water molecules. The sharp band at 1630 cm − 1 reveals the presence of adsorbed water molecules [20]. The broad band (1700– 1865 cm − 1) can be attributed to ν C =O bonds in carboxylic acids and lactones [21]. Absorption bands typical of nanodiamonds are observed in the 900–1500 cm − 1 range [22]. The spectra of the two samples essentially differ in the relative position and intensity of the band attributed to the C= O vibration. The
IR absorption assigned to the C=O bonds in carboxylic acids is observed at lower wavenumber values than the one of lactones. Indeed, the adjacent electron-attractive oxygen increases the force constant of the carbonyl bond (inductive effect) [23]. For ND2 the ν C= O band is located at 1770 cm − 1, whereas in ND1, this C= O band is shifted to 1795 cm − 1. This blueshift can be linked to the presence of lactones. This analysis is in good agreement with Boehm's titration results. The ND1 sample was also studied by infrared spectroscopy after its reaction with the Na2CO3 solution. As expected for sodium carboxylates, the ν C=O band is shifted to lower wavenumbers, compared to that of the carboxylic acid of the initial DND [23]. The band assigned to the deformation of the adsorbed water is slightly shifted to higher wavenumbers. According to Robert et al., the ability of oxygen atoms to form hydrogen bonds is reduced in bonded carboxylates [24]. Here, carboxylate sites interact with their Na + moiety, thus modifying the hydrogen bonding of water molecules on the surface. In aqueous medium, the carboxylic sites of the DND surface are ionized (COO −), leading to a negative value of the zeta potential. The zeta potential values are found to be − 52 mV and − 22 mV for ND1 and ND2, respectively. Therefore, the higher rate of carboxylic groups on ND1 is directly linked to the more negative surface charge of the DND. The different characterization techniques make it possible to conclude that the DND produced and purified with the ISL technique (ND1) contains a lot more carboxylic sites than the commercial nanodiamonds used in this study (ND2). 3.3. Influence of the thermal oxidation on the surface chemistry As described previously, the surface chemistry of DNDs is highly influenced by their purification process. The main difference between the ND1 and ND2 purification processes is the oxidative step applied to ND1. Therefore, ND2 was subjected to the same oxidation process as the one used for the purification of ND1. The as-prepared material (ND3) was studied by using the same techniques as for ND1 and ND2. Boehm's technique performed on ND3 highlighted a drastic modification in the surface chemistry after the oxidation process with a high increase in the carboxylic sites and the creation of lactone sites (see Table 2).
Table 1 Specific area and number of surface functional groups for ND1 and ND2.
ND1 ND2
Specific area (m2/g)
COOH functions (/nm2)
Lactone functions (/nm2)
386 331
0.81 0.15
0.13 0
Fig. 5. ND1 and ND2 FT-IR spectra.
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Table 2 Specific area and number of surface functional groups for ND1, ND2 and ND3.
ND1 ND2 ND3
Specific area (m2/g)
COOH functions (/nm2)
Lactone functions (/nm2)
386 331 292
0.81 0.15 0.80
0.13 0 0.27
The number of carboxylic sites increased from 0.15 site/nm 2 for ND2 to 0.80 site/nm 2 for ND3. The number of lactone sites -which are not present on ND2- is increased up to 0.27 site/nm 2 after the thermal oxidation. It can be assumed that the formation of lactone results from the esterification of a carboxyl group with an adjacent hydroxyl group. This hypothesis is strengthened by the fact that the thermal treatment at 400 °C favours the dehydration and thus the lactone formation. Furthermore, the determination of the number of lactones indicates that ND2 starting material possesses at least 0.27 hydroxyl site per nm 2. The ND1 and ND3 spectra are almost the same, which indicates that the surface state of the ND2 sample was drastically modified upon heating at 400 °C for 24 h (Fig. 6). The absorption band of the carboxylic groups ν C= O in the ND2 sample was shifted from 1770 cm − 1 to 1795 cm − 1 after the oxidation process. This observation is consistent with the results obtained with Boehm's technique, indicating the formation of lactones. The increase in the carboxylic sites after the oxidation treatment is also confirmed by the zeta potential measurement, with an increase of 90% of the negative charge, from -22 mV for ND2 to -42 mV for ND3. 3.4. Distribution of the oxygenated sites on the DND surface According to Barnard et al., a simple way of representing the DND surface is the assembly of the two distinct crystallographic planes (111) and (100) [25]. The geometrical structures built by these planes are a truncated octahedron or a cuboctahedron (Fig. 7). The truncated octahedron is composed of 76% of (111) planes and 24% of (100) planes, while the cuboctahedron is made of 36% of (111) planes and 64% of (100) planes. The surface of a single particle approximated to a 4.10 nm diameter sphere is 52.81 nm2. On the basis of the diamond lattice parameter, the area occupied by a single carbon atom on the surface is estimated to be 0.0546 and 0.0630 nm2 for the (111) and (100) planes, respectively. The (111) and (100) planes, respectively, form hexagons and squares in the truncated octahedron structure and triangles and squares in the case of the cuboctahedron. Generally, the edges of a particle are the most reactive part of the surface. The number of carbon atoms forming each geometrical structure (square, triangle and hexagon) is linked to the peripheral atoms.
Fig. 6. ND1, ND2 and ND3 FT-IR spectra.
Fig. 7. Representation of the truncated octahedron (A) and cuboctahedron (B) structures.
The following equations allow the calculation of the nanodiamond edge carbons from the surface atoms according to the geometrical structure: x¼
pffiffiffiffiffi SS −1
x¼3
x¼
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 1 2 1 − ð1−ST Þ − 4 9 2
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 1 − ð1−SH Þ − 4 3 2
ð2Þ
ð3Þ
ð4Þ
where x is the number of carbon atoms composing each edge. SS, ST and SH represent the number of carbon atoms forming the square, triangular and hexagonal faces, respectively. The number of edges is 36 for the truncated octahedron and 24 for the cuboctahedron. Considering that the steric repulsions caused by the carboxylic acid or lactone surface functions only allow the bonding of one edge carbon atom out of three, the number of edge carbons that could be bonded with a surface functional group is 60 for the truncated octahedron and 64 for the cuboctahedron. The results obtained with Boehm's technique show that there are 50 lactones and carboxylic acid sites on the ND1 surface and 57 on ND3. The calculated values are then consistent with the experimental values obtained with Boehm's technique. These results infer that the functional sites are located on the edges of the DNDs, with nearly complete coverage. 4. Conclusion The surface chemistry of different detonation nanodiamonds was investigated. Boehm's technique showed the presence of carboxylic acid and lactone functional groups on the surface of the DNDs, in agreement with FT-IR spectroscopy. The DND surface chemistry is highly influenced by the purification process and especially by the oxidation treatment. The nanodiamonds synthesized at ISL and commercial nanodiamonds possess 0.81 COOH sites/nm 2 and 0.15 COOH sites/nm 2, respectively. However, applying a post-oxidation treatment to commercial nanodiamonds modifies the surface chemistry of the DNDs and increases the quantity of carboxylic acid and lactone functions. The quantification of the oxygenated groups on the DND surface combined with a simple model approach revealed that the functional groups are located on the edges of the nanoparticles occupying all the available sites. This study shows the interest of Boehm's technique for the quantitative characterization of the chemical functions at the surface of nanodiamonds. This can help in developing specific strategies of functionalization adapted to DNDs of different origins.
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