- Email: [email protected]
Deaggregation of diamond nanoparticles studied by NMR
Diamond & Related Materials 27–28 (2012) 45–48
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Deaggregation of diamond nanoparticles studied by NMR A.M. Panich a,⁎, A.E. Aleksenskii b a b
Department of Physics, Ben-Gurion University of the Negev, Be'er Sheva 84105, Israel Ioffe Physical-Technical Institute, St. Petersburg 194021, Russia
a r t i c l e
i n f o
Article history: Received 23 March 2012 Received in revised form 11 May 2012 Accepted 12 May 2012 Available online 24 May 2012 Keywords: Nanoparticle Nanodiamond Deaggregation Nuclear magnetic resonance (NMR)
a b s t r a c t Nanodiamond (ND) particles are usually bound in aggregates that are difficult to separate into a single nanoparticle form, which restrains their applications. Recently, a stable suspension of deaggregated NDs [1] has been prepared using annealed ND samples. We discuss the mechanism of deaggregation of these NDs based on our 1H NMR data. We show that in contrast with the spectra of the initial sample, those of the annealed sample evidence fast proton exchange between the surface carboxyl groups and adsorbed water molecules and the formation of charged ND–COO − ↔ H3O+ structures, which results in repulsion of ND particles and yields a stable aqueous suspension of isolated NDs. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Diamond nanoparticles with diameters of 4–5 nm and narrow size distribution show great potential for a variety of applications, such as fluorescence markers, biosensors, for drug delivery (since nanodiamond is a non-toxic and biocompatible material), in polymer nanocomposites, in cosmetic products, to name just a few [2–4]. For majority of these applications, the particles must be individually accessible and treatable. However, the ND particles are usually tightly bound in aggregates that are difficult to separate into a single nanoparticle form. The aggregate size may be up to 100–200 nm, far in excess of the 4–5 nm core particle size. This circumstance restrains ND applications. Many efforts have been made to reveal the origin of the ND aggregation and to achieve single deaggregated ND particles. Long-term speculation that the aggregation is caused by van der Waals interactions or by chemical bonds between the individual particles was rejected [5] since the (i) aggregates strongly resist dispersion and a large driving force is required to disintegrate them, and since (ii) XRD measurements show that the diamond nanoparticles remain discrete [5]. A speculative assumption that the sp 2 matrix can bind the core sp 3 particles together was also abandoned since in highly purified NDs the sp 2 shell is removed [6–11], while agglomeration is observed. The most reasonable explanation of the agglomerate formation was suggested by Barnard and Sternberg who predicted the selfassembly of the ND particles by electrostatic attraction of the oppositely charged ND surfaces [12]. This mechanism has recently been
⁎ Corresponding author. E-mail address: [email protected] (A.M. Panich). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2012.05.005
confirmed by aberration-corrected electron microscopy measurements [5] and is now being accepted by the nanodiamond community [5,13–16]. Single, deaggregated ND particles have been achieved by several groups using different approaches. Ōsawa et al. [17,18] reported on primary particles obtained by stirred-media milling with micronsized ceramic beads, which gives a stable colloidal solution with separate particles having diameters of 4–5 nm. Williams et al. [19] and Aleksenskiy et al. [1] have recently shown that after annealing of aggregated ND powder in hydrogen or in air and following intensive sonication, the large aggregates are broken into their core (~4 nm) particles and yield a stable suspension of deaggregated NDs. The latter work [1] suggested formation of the carboxyl (СООН) groups by oxidation of surface hydrocarbon groups upon annealing and appearance of a double electric layer COO −–H3O + around each ND particle. Here we present our 1H NMR data for the annealed ND sample described in [1] and discuss the mechanism of deaggregation of such NDs at the atomic level. 2. Experimental Initial powder ND sample was carefully purified, dried in nitrogen flow at 130 °C and then annealed in the ambient atmosphere at 400 °C [1]. IR measurements [1,20] show that the annealing results in formation of the carboxyl groups by oxidation of the surface hydrocarbon groups. The X-ray diffraction and EPR control show the absence of the patterns associated with impurities. 13C NMR spectra reveal intense peak at 35 ppm coming from sp 3 carbons; no visible signal at 120 ppm coming from sp 2 carbons was detected. All 1H NMR measurements have been carried out in the applied magnetic field of B0 =8.0196 T at a resonance frequency of 341.41 MHz using a
46
A.M. Panich, A.E. Aleksenskii / Diamond & Related Materials 27–28 (2012) 45–48 1.0
1.0
0.8
1
H NMR, RT
0.6
0.8
Initial DND sample, 1 H NMR, RT
0.4 0.2
0.6
0.0 -10 -8 -6 -4 -2
0
2
4
6
8
10
Normalized amplitude
Normalized amplitude, arb. units
1.0
Frequency, kHz
0.4
0.2
0.0 -60
-40
-20
0
20
40
60
0.8
0.6
0.4
0.2
0.0 -100
-75
-50
-25
0
25
50
75
100
Frequency, kHz
Frequency, kHz Fig. 1. H NMR spectrum of the initial ND sample. Deconvolution into two components is shown by dashed lines. Separately detected narrow component is shown in the inset.
Fig. 3. 1H NMR spectrum of annealed ND sample at room temperature. Red dash line is a Lorentzian fit.
Tecmag pulse spectrometer and an Oxford Instruments superconducting magnet. The 1H spectra were measured by the solid echo method. In addition to the total spectrum, a narrow spectral component of the initial sample was measured separately with a delay of 128 μs between pulses, which causes the entire decay of the echo signal coming from the broad component (dipolar dephasing). The measurements have been made in the temperature range of 120–295 K.
consists of two lines. A broad component with a line width Δν~ 25 kHz is attributed to closely set hydrocarbon and hydroxyl groups, while the narrow component showing Δν ~ 2.3 kHz is assigned to moisture adsorbed on the ND surface. The latter can be removed by pumping out the sample at ~10− 4 Torr [6,10,11,16,25]. The broad line reflects strong dipole–dipole interactions among the 1H spins that indicate a clustering of the surface hydrogen atoms, i.e. that the hydrogenated spots of limited sizes alternate with nearly non-hydrogenated zones. The latter zones allow aggregation of the ND particles owing to electrostatic attraction of the oppositely charged surface spots (Fig. 2a) predicted by Barnard and Sternberg [12]. We note that the authors calculated the structure of ND without a surface hydrogen layer. This represents a limitation of the above-mentioned model [16]. It is intuitively clear that the electrostatic mechanism is hardly valid if the diamond core is completely covered by hydrogen atoms leaving no room for charged surfaces (Fig. 2b), as well as for partially hydrogenterminated core with regular positioning of hydrogen atoms preventing the aggregation (Fig. 2c). More recently Lai and Barnard [26] have discussed the model of aggregation of functionalized nanodiamonds and found that functionalization greatly reduces the inter-particle Coulombic interactions characteristic of unsaturated particles, well supporting the aforementioned speculations. In contrast to the spectra discussed above, the RT 1H NMR spectrum of the annealed ND under study (Fig. 3) is strikingly different from that of the initial sample and exhibits the only narrow component with Δν~ 3.1 kHz. This behavior is evidently caused by a fast chemical exchange between the protons of the surface carboxyl groups and those of adsorbed water molecules, which results in the line narrowing.
1
3. Results and discussion ND nanoparticles are known to be partially terminated by hydrogen atoms. NMR and FTIR measurements [6,10,11,16,21–23] have also revealed hydrocarbon (CH, CH2 and CH3) and hydroxyl surface groups depending on the particle pretreatment. Similar to the spectra of ND samples described in the literature [6,10,11,16,21,22,24,25], static room temperature (RT) 1H NMR spectrum of our initial ND (Fig. 1)
a + -
+ -
b
40
1
c
Δν, kHz
30
H NMR
20
10
Fig. 2. Sketch of three models of hydrogen-terminated nanodiamond particles: (a) hydrogenated spots of limited size alternate with non-hydrogenated zones showing oppositely charged surfaces; (b) diamond core completely terminated by hydrogen atoms; (c) partially hydrogen-terminated diamond core with regular positioning of hydrogen atoms. Diamond cores are shown by hexagons and hydrogen atoms are shown by small open circles.
0 120
150
180
210
240
270
T, K Fig. 4. Temperature dependence of the 1H line width.
300
A.M. Panich, A.E. Aleksenskii / Diamond & Related Materials 27–28 (2012) 45–48
of the monodispersed primary nanoparticles. Our experiments with the hydrogen-annealed ND sample confirm this result. All these findings support that our aforementioned mechanism is operative. In addition, we have also measured 13C NMR of ND suspension described in [1]. 13C spectrum revealed an intense peak of sp 3 carbons of the diamond core at 35 ppm and no visible signal of sp 2 carbons at 120 ppm. Owing to a low concentration of the ND particles in the suspension, signal-to noise ratio was not high (s/n ≈ 11). 13C spin-lattice relaxation time T1 was found to be ~ 400 ms, which is characteristic of powder samples of purified nanodiamonds [6, 8–11, 21–25].
1.0
Normalized amplitude
1
0.8
47
H NMR 125 K
0.6
0.4
0.2
4. Conclusion 0.0 -100
-75
-50
-25
0
25
50
75
100
Frequency, kHz Fig. 5. 1H NMR spectra of annealed ND sample at T = 125 K. Deconvolution into two components is shown by dashed lines, the blue line is the spectrum of hydroxonium ion, and the red line belongs to the carboxyl groups.
Such an effect is characteristic of COOH groups due to their acidic dissociation (COO −–H+) and COOH–H2O ↔ COO−–H3O + process, which results in formation of hydroxonium (H3O +) ions. Temperature reduction causes a gradual broadening of the 1H resonance that nearly stops around 165 K (Fig. 4). This broadening is caused by freezing of the chemical exchange and water and hydroxonium mobility. The low temperature spectrum is deconvoluted into two components (Fig. 5): a triplet attributed to the H3O + ions and a singlet coming from the hydrogen atoms of the carboxyl groups. The line shape of the former with the separation of outer maxima ~ 55 kHz (determined from the spectrum derivative) is characteristic of H3O + ions [27]. The data obtained elucidate the mechanism of deaggregation of the annealed ND particles. In an aqueous medium, the positively charged H3O+ ions and negatively charged COO− groups form a double electric layer around each ND particle (Fig. 6). Such a structure causes effective electrostatic repulsion in the system and formation of a stable suspension of single deaggregated nanoparticles [1]. The aforementioned interaction provides high zeta potential of 40–45 mV over a wide range of pH from 3 to 12 [1,20], which is necessary for the stability of suspensions. Experiments show that lowering pH below pH=2 results in coagulation of the nanoparticles due to elimination of the electrostatic interactions. This fact is likely caused by a suppression of the dissociation of the carboxyl groups. Note that relatively long COOH groups could also somewhat prevent the aggregation. Thus our data provides good support for the hypothesis of Aleksenskiy et al. [1] and explains the mechanism of the deaggregation at the atomic level. We note that very recent calculation [26] shows that hydrogenating the ND surface essentially reduces the electrostatic forces. Next, as reported by Williams et al. [19], hydrogenation via annealing NDs in hydrogen gas results in breakdown of ND aggregates and the recovery
The mechanism of aggregation and de-aggregation of diamond nanoparticles is discussed at the atomic level based on the 1H NMR data. Occurrence of differently charged non-hydrogenated zones allows the aggregation of diamond nanoparticles owing to electrostatic attraction between them. Deaggregation of the ND particles becomes possible after the sample annealing, resulting in the formation of the surface carboxyl group by oxidation of hydrocarbon groups. Our data evidence fast proton exchange between carboxyl groups and water molecules and formation of H3O + ions, which causes appearance of the double electric layer around the ND particles and electrostatic repulsion between them. This process results in deaggregation of ND particles and formation of stable suspension. Acknowledgments We thank A.I. Shames for help in spectrum processing and EPR measurement, A. Vul' for discussion and A. Barnard for supplying us with her new paper [26]. Research of A. E. Aleksenskii was supported by the Program N24 of the Russian Academy of Sciences. References [1] A.E. Aleksenskiy, E.D. Eydelman, Nanosci. Nanotechnol. Lett. 3 (2011) 68–74. [2] A.M. Schrand, S.A. Ciftan Hens, O.A. Shenderova, Crit. Rev. Solid State Mater. Sci. 34 (2009) 18–74. [3] V.Yu. Dolmatov, Russ. Chem. Rev. 76 (2007) 339–360. [4] A.S. Barnard, Analyst 134 (2009) 1751–1764. [5] L.-Y. Chang, E. Ōsawa, A.S. Barnard, Nanoscale 3 (2011) 958–962. [6] O. Shenderova, A.M. Panich, S. Moseenkov, S.C. Hens, V.L. Kuznetsov, H.-M. Vieth, J. Phys. Chem. C 115 (2011) 19005–19011. [7] S. Turner, O.I. Lebedev, O. Shenderova, I.I. Vlasov, J. Verbeeck, G. Van Tendeloo, Adv. Funct. Mater. 19 (2009) 2116–2124. [8] A.M. Panich, H.-M. Vieth, A.I. Shames, N. Froumin, E. Ōsawa, A. Yao, J. Phys. Chem. C 114 (2010) 774–782. [9] L.B. Casabianca, A.I. Shames, A.M. Panich, O. Shenderova, L. Frydman, J. Phys. Chem. C 115 (2011) 19041–19048. [10] A.M. Panich, A.I. Shames, B. Zousman, O. Levinson, Diamond Relat. Mater. 23 (2012) 150–153. [11] A.M. Panich, H.-M. Vieth, O. Shenderova, Fullerenes Nanotubes Carbon Nanostruct. 20 (2012) 579–586. [12] A.S. Barnard, M. Sternberg, J. Mater. Chem. 17 (2007) 4811–4819. [13] J. Hees, A. Kriele, O.A. Williams, Chem. Phys. Lett. 509 (2011) 12–15. [14] E. Ōsawa, D. Ho, H. Huang, M.V. Korobov, N.N. Rozhkova, Diamond Relat. Mater. 18 (2009) 904–909.
Fig. 6. Sketch of formation of hydroxonium ions and double electric layer around nanodiamond particles due to dissociation of the carboxyl groups.
48
A.M. Panich, A.E. Aleksenskii / Diamond & Related Materials 27–28 (2012) 45–48
[15] N.M. Gibson, T.-J.M. Luo, O. Shenderova, A.P. Koscheev, D.W. Brenner, J. Nanopart. Res. 14 (2012) 700–712. [16] A.M. Panich, A. Altman, A.I. Shames, V.Yu. Osipov, A.E. Alexenskiy, A.Ya. Vul', J. Phys. D Appl. Phys. 44 (2011) 125303/1–5. [17] A. Krüger, F. Kataoka, M. Ozawa, T. Fujino, Y. Suzuki, A.E. Aleksenskii, A.Ya. Vul, E. Ōsawa, Carbon 43 (2005) 1722–1730. [18] M. Ozawa, M. Inaguma, M. Takahashi, F. Kataoka, A. Krüger, E. Ōsawa, Adv. Mater. 19 (2007) 1201–1206. [19] O.A. Williams, J. Hees, C. Dieker, W. Jager, L. Kirste, C.E. Nebel, ACS Nano 4 (2010) 4824–4830. [20] A.E. Aleksenskii, A.T. Dideykin, A.V. Shvidchenko, A.Ya. Vul', 22nd European Conference on Diamond, Diamond-Like Materials, Carbon Nanotubes and Nitrides (DIAMOND 2011) Garmish–Partenkirchen, September 04–08, 2011.
[21] A.M. Panich, A.I. Shames, H.-M. Vieth, E. Ōsawa, M. Takahashi, A.Ya. Vul', Eur. Phys. J. B 52 (2006) 397–402. [22] A.M. Panich, Diamond Relat. Mater. 16 (2007) 2044–2049. [23] M. Dubois, K. Guerin, E. Petit, N. Batisse, A. Hamwi, N. Komatsu, J. Giraudet, P. Pirotte, F. Masin, J. Phys. Chem. C 113 (2009) 10371–10378. [24] A.M. Panich, A.I. Shames, O. Medvedev, V.Yu. Osipov, A.E. Alexenskii, A.Ya. Vul', Appl. Magn. Reson. 36 (2009) 317–329. [25] A.M. Panich, Crit. Rev. Solid State Mater. Sci. 37 (2012). [26] L. Lai, A.S. Barnard, J. Phys. Chem. Lett. 3 (2012) 896–901. [27] R.E. Richards, J.A.S. Smith, Trans. Faraday Soc. 47 (1951) 1261–1274.
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Deaggregation of diamond nanoparticles studied by NMR A.M. Panich a,⁎, A.E. Aleksenskii b a b
Department of Physics, Ben-Gurion University of the Negev, Be'er Sheva 84105, Israel Ioffe Physical-Technical Institute, St. Petersburg 194021, Russia
a r t i c l e
i n f o
Article history: Received 23 March 2012 Received in revised form 11 May 2012 Accepted 12 May 2012 Available online 24 May 2012 Keywords: Nanoparticle Nanodiamond Deaggregation Nuclear magnetic resonance (NMR)
a b s t r a c t Nanodiamond (ND) particles are usually bound in aggregates that are difficult to separate into a single nanoparticle form, which restrains their applications. Recently, a stable suspension of deaggregated NDs [1] has been prepared using annealed ND samples. We discuss the mechanism of deaggregation of these NDs based on our 1H NMR data. We show that in contrast with the spectra of the initial sample, those of the annealed sample evidence fast proton exchange between the surface carboxyl groups and adsorbed water molecules and the formation of charged ND–COO − ↔ H3O+ structures, which results in repulsion of ND particles and yields a stable aqueous suspension of isolated NDs. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Diamond nanoparticles with diameters of 4–5 nm and narrow size distribution show great potential for a variety of applications, such as fluorescence markers, biosensors, for drug delivery (since nanodiamond is a non-toxic and biocompatible material), in polymer nanocomposites, in cosmetic products, to name just a few [2–4]. For majority of these applications, the particles must be individually accessible and treatable. However, the ND particles are usually tightly bound in aggregates that are difficult to separate into a single nanoparticle form. The aggregate size may be up to 100–200 nm, far in excess of the 4–5 nm core particle size. This circumstance restrains ND applications. Many efforts have been made to reveal the origin of the ND aggregation and to achieve single deaggregated ND particles. Long-term speculation that the aggregation is caused by van der Waals interactions or by chemical bonds between the individual particles was rejected [5] since the (i) aggregates strongly resist dispersion and a large driving force is required to disintegrate them, and since (ii) XRD measurements show that the diamond nanoparticles remain discrete [5]. A speculative assumption that the sp 2 matrix can bind the core sp 3 particles together was also abandoned since in highly purified NDs the sp 2 shell is removed [6–11], while agglomeration is observed. The most reasonable explanation of the agglomerate formation was suggested by Barnard and Sternberg who predicted the selfassembly of the ND particles by electrostatic attraction of the oppositely charged ND surfaces [12]. This mechanism has recently been
⁎ Corresponding author. E-mail address: [email protected] (A.M. Panich). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2012.05.005
confirmed by aberration-corrected electron microscopy measurements [5] and is now being accepted by the nanodiamond community [5,13–16]. Single, deaggregated ND particles have been achieved by several groups using different approaches. Ōsawa et al. [17,18] reported on primary particles obtained by stirred-media milling with micronsized ceramic beads, which gives a stable colloidal solution with separate particles having diameters of 4–5 nm. Williams et al. [19] and Aleksenskiy et al. [1] have recently shown that after annealing of aggregated ND powder in hydrogen or in air and following intensive sonication, the large aggregates are broken into their core (~4 nm) particles and yield a stable suspension of deaggregated NDs. The latter work [1] suggested formation of the carboxyl (СООН) groups by oxidation of surface hydrocarbon groups upon annealing and appearance of a double electric layer COO −–H3O + around each ND particle. Here we present our 1H NMR data for the annealed ND sample described in [1] and discuss the mechanism of deaggregation of such NDs at the atomic level. 2. Experimental Initial powder ND sample was carefully purified, dried in nitrogen flow at 130 °C and then annealed in the ambient atmosphere at 400 °C [1]. IR measurements [1,20] show that the annealing results in formation of the carboxyl groups by oxidation of the surface hydrocarbon groups. The X-ray diffraction and EPR control show the absence of the patterns associated with impurities. 13C NMR spectra reveal intense peak at 35 ppm coming from sp 3 carbons; no visible signal at 120 ppm coming from sp 2 carbons was detected. All 1H NMR measurements have been carried out in the applied magnetic field of B0 =8.0196 T at a resonance frequency of 341.41 MHz using a
46
A.M. Panich, A.E. Aleksenskii / Diamond & Related Materials 27–28 (2012) 45–48 1.0
1.0
0.8
1
H NMR, RT
0.6
0.8
Initial DND sample, 1 H NMR, RT
0.4 0.2
0.6
0.0 -10 -8 -6 -4 -2
0
2
4
6
8
10
Normalized amplitude
Normalized amplitude, arb. units
1.0
Frequency, kHz
0.4
0.2
0.0 -60
-40
-20
0
20
40
60
0.8
0.6
0.4
0.2
0.0 -100
-75
-50
-25
0
25
50
75
100
Frequency, kHz
Frequency, kHz Fig. 1. H NMR spectrum of the initial ND sample. Deconvolution into two components is shown by dashed lines. Separately detected narrow component is shown in the inset.
Fig. 3. 1H NMR spectrum of annealed ND sample at room temperature. Red dash line is a Lorentzian fit.
Tecmag pulse spectrometer and an Oxford Instruments superconducting magnet. The 1H spectra were measured by the solid echo method. In addition to the total spectrum, a narrow spectral component of the initial sample was measured separately with a delay of 128 μs between pulses, which causes the entire decay of the echo signal coming from the broad component (dipolar dephasing). The measurements have been made in the temperature range of 120–295 K.
consists of two lines. A broad component with a line width Δν~ 25 kHz is attributed to closely set hydrocarbon and hydroxyl groups, while the narrow component showing Δν ~ 2.3 kHz is assigned to moisture adsorbed on the ND surface. The latter can be removed by pumping out the sample at ~10− 4 Torr [6,10,11,16,25]. The broad line reflects strong dipole–dipole interactions among the 1H spins that indicate a clustering of the surface hydrogen atoms, i.e. that the hydrogenated spots of limited sizes alternate with nearly non-hydrogenated zones. The latter zones allow aggregation of the ND particles owing to electrostatic attraction of the oppositely charged surface spots (Fig. 2a) predicted by Barnard and Sternberg [12]. We note that the authors calculated the structure of ND without a surface hydrogen layer. This represents a limitation of the above-mentioned model [16]. It is intuitively clear that the electrostatic mechanism is hardly valid if the diamond core is completely covered by hydrogen atoms leaving no room for charged surfaces (Fig. 2b), as well as for partially hydrogenterminated core with regular positioning of hydrogen atoms preventing the aggregation (Fig. 2c). More recently Lai and Barnard [26] have discussed the model of aggregation of functionalized nanodiamonds and found that functionalization greatly reduces the inter-particle Coulombic interactions characteristic of unsaturated particles, well supporting the aforementioned speculations. In contrast to the spectra discussed above, the RT 1H NMR spectrum of the annealed ND under study (Fig. 3) is strikingly different from that of the initial sample and exhibits the only narrow component with Δν~ 3.1 kHz. This behavior is evidently caused by a fast chemical exchange between the protons of the surface carboxyl groups and those of adsorbed water molecules, which results in the line narrowing.
1
3. Results and discussion ND nanoparticles are known to be partially terminated by hydrogen atoms. NMR and FTIR measurements [6,10,11,16,21–23] have also revealed hydrocarbon (CH, CH2 and CH3) and hydroxyl surface groups depending on the particle pretreatment. Similar to the spectra of ND samples described in the literature [6,10,11,16,21,22,24,25], static room temperature (RT) 1H NMR spectrum of our initial ND (Fig. 1)
a + -
+ -
b
40
1
c
Δν, kHz
30
H NMR
20
10
Fig. 2. Sketch of three models of hydrogen-terminated nanodiamond particles: (a) hydrogenated spots of limited size alternate with non-hydrogenated zones showing oppositely charged surfaces; (b) diamond core completely terminated by hydrogen atoms; (c) partially hydrogen-terminated diamond core with regular positioning of hydrogen atoms. Diamond cores are shown by hexagons and hydrogen atoms are shown by small open circles.
0 120
150
180
210
240
270
T, K Fig. 4. Temperature dependence of the 1H line width.
300
A.M. Panich, A.E. Aleksenskii / Diamond & Related Materials 27–28 (2012) 45–48
of the monodispersed primary nanoparticles. Our experiments with the hydrogen-annealed ND sample confirm this result. All these findings support that our aforementioned mechanism is operative. In addition, we have also measured 13C NMR of ND suspension described in [1]. 13C spectrum revealed an intense peak of sp 3 carbons of the diamond core at 35 ppm and no visible signal of sp 2 carbons at 120 ppm. Owing to a low concentration of the ND particles in the suspension, signal-to noise ratio was not high (s/n ≈ 11). 13C spin-lattice relaxation time T1 was found to be ~ 400 ms, which is characteristic of powder samples of purified nanodiamonds [6, 8–11, 21–25].
1.0
Normalized amplitude
1
0.8
47
H NMR 125 K
0.6
0.4
0.2
4. Conclusion 0.0 -100
-75
-50
-25
0
25
50
75
100
Frequency, kHz Fig. 5. 1H NMR spectra of annealed ND sample at T = 125 K. Deconvolution into two components is shown by dashed lines, the blue line is the spectrum of hydroxonium ion, and the red line belongs to the carboxyl groups.
Such an effect is characteristic of COOH groups due to their acidic dissociation (COO −–H+) and COOH–H2O ↔ COO−–H3O + process, which results in formation of hydroxonium (H3O +) ions. Temperature reduction causes a gradual broadening of the 1H resonance that nearly stops around 165 K (Fig. 4). This broadening is caused by freezing of the chemical exchange and water and hydroxonium mobility. The low temperature spectrum is deconvoluted into two components (Fig. 5): a triplet attributed to the H3O + ions and a singlet coming from the hydrogen atoms of the carboxyl groups. The line shape of the former with the separation of outer maxima ~ 55 kHz (determined from the spectrum derivative) is characteristic of H3O + ions [27]. The data obtained elucidate the mechanism of deaggregation of the annealed ND particles. In an aqueous medium, the positively charged H3O+ ions and negatively charged COO− groups form a double electric layer around each ND particle (Fig. 6). Such a structure causes effective electrostatic repulsion in the system and formation of a stable suspension of single deaggregated nanoparticles [1]. The aforementioned interaction provides high zeta potential of 40–45 mV over a wide range of pH from 3 to 12 [1,20], which is necessary for the stability of suspensions. Experiments show that lowering pH below pH=2 results in coagulation of the nanoparticles due to elimination of the electrostatic interactions. This fact is likely caused by a suppression of the dissociation of the carboxyl groups. Note that relatively long COOH groups could also somewhat prevent the aggregation. Thus our data provides good support for the hypothesis of Aleksenskiy et al. [1] and explains the mechanism of the deaggregation at the atomic level. We note that very recent calculation [26] shows that hydrogenating the ND surface essentially reduces the electrostatic forces. Next, as reported by Williams et al. [19], hydrogenation via annealing NDs in hydrogen gas results in breakdown of ND aggregates and the recovery
The mechanism of aggregation and de-aggregation of diamond nanoparticles is discussed at the atomic level based on the 1H NMR data. Occurrence of differently charged non-hydrogenated zones allows the aggregation of diamond nanoparticles owing to electrostatic attraction between them. Deaggregation of the ND particles becomes possible after the sample annealing, resulting in the formation of the surface carboxyl group by oxidation of hydrocarbon groups. Our data evidence fast proton exchange between carboxyl groups and water molecules and formation of H3O + ions, which causes appearance of the double electric layer around the ND particles and electrostatic repulsion between them. This process results in deaggregation of ND particles and formation of stable suspension. Acknowledgments We thank A.I. Shames for help in spectrum processing and EPR measurement, A. Vul' for discussion and A. Barnard for supplying us with her new paper [26]. Research of A. E. Aleksenskii was supported by the Program N24 of the Russian Academy of Sciences. References [1] A.E. Aleksenskiy, E.D. Eydelman, Nanosci. Nanotechnol. Lett. 3 (2011) 68–74. [2] A.M. Schrand, S.A. Ciftan Hens, O.A. Shenderova, Crit. Rev. Solid State Mater. Sci. 34 (2009) 18–74. [3] V.Yu. Dolmatov, Russ. Chem. Rev. 76 (2007) 339–360. [4] A.S. Barnard, Analyst 134 (2009) 1751–1764. [5] L.-Y. Chang, E. Ōsawa, A.S. Barnard, Nanoscale 3 (2011) 958–962. [6] O. Shenderova, A.M. Panich, S. Moseenkov, S.C. Hens, V.L. Kuznetsov, H.-M. Vieth, J. Phys. Chem. C 115 (2011) 19005–19011. [7] S. Turner, O.I. Lebedev, O. Shenderova, I.I. Vlasov, J. Verbeeck, G. Van Tendeloo, Adv. Funct. Mater. 19 (2009) 2116–2124. [8] A.M. Panich, H.-M. Vieth, A.I. Shames, N. Froumin, E. Ōsawa, A. Yao, J. Phys. Chem. C 114 (2010) 774–782. [9] L.B. Casabianca, A.I. Shames, A.M. Panich, O. Shenderova, L. Frydman, J. Phys. Chem. C 115 (2011) 19041–19048. [10] A.M. Panich, A.I. Shames, B. Zousman, O. Levinson, Diamond Relat. Mater. 23 (2012) 150–153. [11] A.M. Panich, H.-M. Vieth, O. Shenderova, Fullerenes Nanotubes Carbon Nanostruct. 20 (2012) 579–586. [12] A.S. Barnard, M. Sternberg, J. Mater. Chem. 17 (2007) 4811–4819. [13] J. Hees, A. Kriele, O.A. Williams, Chem. Phys. Lett. 509 (2011) 12–15. [14] E. Ōsawa, D. Ho, H. Huang, M.V. Korobov, N.N. Rozhkova, Diamond Relat. Mater. 18 (2009) 904–909.
Fig. 6. Sketch of formation of hydroxonium ions and double electric layer around nanodiamond particles due to dissociation of the carboxyl groups.
48
A.M. Panich, A.E. Aleksenskii / Diamond & Related Materials 27–28 (2012) 45–48
[15] N.M. Gibson, T.-J.M. Luo, O. Shenderova, A.P. Koscheev, D.W. Brenner, J. Nanopart. Res. 14 (2012) 700–712. [16] A.M. Panich, A. Altman, A.I. Shames, V.Yu. Osipov, A.E. Alexenskiy, A.Ya. Vul', J. Phys. D Appl. Phys. 44 (2011) 125303/1–5. [17] A. Krüger, F. Kataoka, M. Ozawa, T. Fujino, Y. Suzuki, A.E. Aleksenskii, A.Ya. Vul, E. Ōsawa, Carbon 43 (2005) 1722–1730. [18] M. Ozawa, M. Inaguma, M. Takahashi, F. Kataoka, A. Krüger, E. Ōsawa, Adv. Mater. 19 (2007) 1201–1206. [19] O.A. Williams, J. Hees, C. Dieker, W. Jager, L. Kirste, C.E. Nebel, ACS Nano 4 (2010) 4824–4830. [20] A.E. Aleksenskii, A.T. Dideykin, A.V. Shvidchenko, A.Ya. Vul', 22nd European Conference on Diamond, Diamond-Like Materials, Carbon Nanotubes and Nitrides (DIAMOND 2011) Garmish–Partenkirchen, September 04–08, 2011.
[21] A.M. Panich, A.I. Shames, H.-M. Vieth, E. Ōsawa, M. Takahashi, A.Ya. Vul', Eur. Phys. J. B 52 (2006) 397–402. [22] A.M. Panich, Diamond Relat. Mater. 16 (2007) 2044–2049. [23] M. Dubois, K. Guerin, E. Petit, N. Batisse, A. Hamwi, N. Komatsu, J. Giraudet, P. Pirotte, F. Masin, J. Phys. Chem. C 113 (2009) 10371–10378. [24] A.M. Panich, A.I. Shames, O. Medvedev, V.Yu. Osipov, A.E. Alexenskii, A.Ya. Vul', Appl. Magn. Reson. 36 (2009) 317–329. [25] A.M. Panich, Crit. Rev. Solid State Mater. Sci. 37 (2012). [26] L. Lai, A.S. Barnard, J. Phys. Chem. Lett. 3 (2012) 896–901. [27] R.E. Richards, J.A.S. Smith, Trans. Faraday Soc. 47 (1951) 1261–1274.