7330
J. Phys. Chem. C 2007, 111, 7330-7334
Nanophase of Water in Nano-Diamond Gel Michail V. Korobov,*,† Natalia V. Avramenko,† Alexander G. Bogachev,† Natalia N. Rozhkova,‡ and Eiji O h sawa£ Department of Chemistry, Moscow State UniVersity, Moscow 119899, Russia, Institute of Geology, Karelian Research Centre RAS, PetrozaVodsk 185610, Russia, and NanoCarbon Research Institute, Asama Research Extension Centre, Shinshu UniVersity, 3-15-1 Tokita, Ueda, Nagano 386-8567, Japan ReceiVed: December 5, 2006; In Final Form: March 12, 2007
The nanosized water phase has been discovered while studying differential scanning calorimetry (DSC) of the aqueous gel of nano-diamond particles (diameter ca. 5 nm). Two endothermic peaks were observed in the DSC traces of frozen gel upon warming; a broad peak appeared at 265 K before the melting of bulk water, which is attributed to the melting of nanophase water adsorbed onto the surface of nano-diamond particles. The events can be reproduced in exothermic fashion upon cooling the same sample. The mass of nanophase water per one nano-diamond particle and the melting enthalpy of nanophase water were derived from the DSC data. Similar nanophase water was observed earlier in gels prepared from an aqueous dispersion of C60 clusters with an average diameter of 68 nm, but the effect was not as distinct as with nano-diamond particles. These results demonstrate that DSC could be a versatile tool to study the stability of carbon nanoparticles in liquid media.
Introduction Since 1963, novel nanocrystals of diamond, as small as 4-5 nm in diameter1c, have been known to grow upon detonation of certain explosive compositions in an inert atmosphere without addition of any extra carbon source.1a,b However, the crude product isolated from detonation soot consisted of tight agglutinates of nano-diamond (ND), 60-200 nm in diameter. Although the agglutinates of detonation ND have found limited applications for casting, coating, and reinforcing purposes, the performance of agglutinates was unremarkable.2-5 We have recently succeeded in disintegrating the agglutinates into primary particles by means of stirred-media milling in water to give a surprisingly stable aqueous colloid, which, after purification steps involving centrifugation, filtration, and sonication, did not produce any visible turbidity or precipitates upon storage for months.6,7 Disintegration of agglutinates by milling can also be accomplished in a few polar organic solvents like ethylene glycol monomethyl ether and dimethylsulfoxide.7 This report focuses on the reason of the unprecedented stability of aqueous colloids of ND. As the dried residue of colloidal solution showed strong IR absorption bands of the hydroxyl group, we initially attributed the stability to a large number of C-OH groups that could have been formed on the particle surface during oxidative removal of soot.6 Then, we noted an early work by Ji and his colleagues,8 who had assigned the origin of hydroxyl groups to water molecules absorbed on the particle surface by observing the disappearance of IR absorptions upon heating at 150 °C in high vacuum for 15 h in the infrared sample chamber. While they used agglutinates, we confirmed the disappearance of OH absorption bands in the IR spectra of disintegrated and similarly * To whom correspondence should be addressed. E-mail: korobov@ phys.chem.msu.ru. † Moscow State University. ‡ Karelian Research Centre RAS. £ Asama Research Extension Centre.
dried ND.7 Thus, the high stability of aqueous colloids of ND is not due to the surface C-OH groups, which never existed, but to water molecules adsorbed onto the particle surface. The OH absorption bands immediately came back when air was reintroduced into the IR sample chamber. The surprisingly fast adsorption of water onto the surface of ND particles, a phenomenon first observed by Ji et al.,8 means abnormally strong hydration. Similarly, stable aqueous dispersions of small C60 clusters (D ∼ 68 nm, ∼105 fullerene spheres in one cluster) have long been known and studied by a variety of experimental methods, including dynamic light scattering (DLS), small angle neutronscattering(SANS),andtransmissionelectronmicroscopy.9-11 In the case of a small C60 cluster, the effects were not clear enough for detailed analysis.12 The differential hydration suggests a significant effect of the size of the nanoparticles on the adsorption of water. In this paper, we report the results of DSC measurements on the gels (wet powders) of ND primary particles and present a realistic hydration model. Experimental Section The preparation of DSC samples is illustrated in Scheme 1. Commercial “diamond nanoparticles” (I) were manufactured by Gansu Lingyun Nano-Material Co., Ltd., Lanzhou, China, using the detonation method.1a,b The powder product was mixed conglomerates showing a discrete size distribution between 200 nm and 20 µm.6 A portion of sample I was mixed with distilled water in a 1:1 proportion to give an unstable slurry for DSC measurements (Ia). A larger portion of sample I was suspended in excess water in a 1:10 ratio and subjected to stirred-media milling with zirconia beads having an average diameter of 30 µm to give a clear colloidal solution of disintegrated primary ND particles (II). The milling procedure is described in detail elsewhere.7,13,14 A portion of colloid II was diluted to 0.2% with water and sonicated for DLS measurements, which gave a major peak at ca. 5 nm. Still, another portion of II was concentrated to a 1:1 ND/water suspension for DSC measurements (IIa). The
10.1021/jp0683420 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/03/2007
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J. Phys. Chem. C, Vol. 111, No. 20, 2007 7331
SCHEME 1: Preparation of DSC Samplesa
a
ND ) nano-diamond. See text for more details.
rest of II was heated at 450-630 K under nitrogen until no further loss of weight was detected. To the resulting powder was added Milli-Q water (18 MΩ‚cm) to obtain sample IIb. No sonication was used in preparation of sample IIb. Samples IIc and IId were prepared similary to sample IIb after removing water in an early stage, and the dried diamond was added with water at a pH value of ∼1 and cyclohexane. Samples Ia and IIa-d were studied by means of DSC. The mass ratio of liquid media to diamond powder in all of the samples was about 1:1. A DCS-30 TA Mettler instrument was used to capture the heating traces from 173 up to 323 K and the cooling traces from 293 to 173 K. The scanning rates were 10, 5, and 2 K/min. Each DSC sample weighed 15-25 mg. Results Endothermic Peaks in the DSC Traces. One or two endothermic peaks appeared near 273 K in the DSC heating traces of all of the samples studied. The feature at 273.2 ( 0.5 K corresponds obviously to the melting of bulk water. The area under this peak led to an average molar enthalpy of melting (QH2O,b; see below) of ∼6.0 kJ/mol, in good agreement with the standard value of 6.01 kJ/mol.15 This was the only peak observed for samples Ia and IIc. In samples IIa and IIb, an additional broad endothermic feature was observed at temperatures below T ) 273 K. Thirteen such samples were studied to obtain an average maximum temperature of 265 ( 0.5 K for the broad peak (Figure 1). We assign this peak to the melting of the nanophase of water (NPhW), as discussed below. Samples IIa and IIb were subjected to a heating-freezing cycle several times to confirm the reproducibility of DSC features. In between the runs, the sample was heated at T ) 343-373 K for several minutes to reduce the amount of water incorporated into the ND. After the DSC scan or the evaporation of water, the total weight of the sample was determined. In this way, the change in the DCS traces with the increase in the ND to water mass ratio in the sample could be followed. A typical evolution of the DCS curve is presented in Figure 2, which shows that the area of the peak at T ) 273.2 ( 0.5 K decreased with the evaporation of water down to zero, whereas the intensity of endothermic peak at 265 ( 0.5 K remained practically unchanged. Unless the bulk water was completely
removed, samples was saturated with NPhW. After the complete evaporation of the bulk water, the peak at 265 ( 0.5 K began to decrease until both endothermic features finally disappeared from the DSC trace. After that, no further loss of weight was detected. Two exothermic effects of freezing were observed upon cooling of the samples IIa and IIb (Figure 3). Sample IId gave only one sharp peak at the melting temperature of bulk cyclohexane (T ) 279.6 K) in the heating trace. The DSC traces (Figures 1-3) are similar in appearance to the solid-liquid phase transition in the mixture of bulk and finely divided liquids described in the literature.16-18 The solidliquid phase transition of finely divided liquid or the same liquid confined in small pores shifts to lower temperatures compared to the same transition in bulk liquid. In these cases, the depression of the melting temperature is attributed to the effect of surface curvature and tension of the small crystals formed inside the pores. The effect is clearly observed in DSC. It is safe to say that DSC is a method of choice in detecting the nanophases.16-19 Two peaks corresponding to the melting transitions in finely divided and in bulk liquid, respectively, appear one after the other in the DCS trace, as Plooster and Gitlin19 observed in the heating of water adsorbed on the nanoparticles of nonporous silica. In the present study, the endothermic peak at 265 ( 0.5 K, observed in the DSC traces of samples IIa and IIb, is attributed to the melting of NPhW occurring in the nano-diamond gels. It is worth noting that the DSC traces of these samples are well reproduced. Analysis of DSC data gave the following three important parameters of NPhW, namely, (1) the mass ratio of NPhW to ND, (2) the molar enthalpy of melting of NPhW, and (3) the characteristic diameter of NPhW. NPhW to ND “Saturated” Mass Ratio. The mass of ND, MC, in the sample was determined by weighing the sample after water was completely removed. The total amount of water in the sample, MH2O, was determined as the difference between the initial weight of the sample and that of MC. The mass of bulk water, MH2O,b, was calculated using the heat of melting of water
MH2O,b ) Q273K/QH2O,b
(1)
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Korobov et al.
Figure 1. Typical DSC scans: (1) aqueous gel of nano-diamond, sample IIa; total amount of water, MH2O ) 10.1 mg; mass of nano-diamond, MC ) 10.2 mg; (2) aqueous gel of C60 cluster; total amount of water, MH2O ) 2.7 mg; mass of fullerene C60, MC ) 2.0 mg.
Figure 3. Typical DSC cooling scan of nano-diamond gel, sample IIb. The sharp peak from the right corresponds to the freezing of the bulk water. The peak from the left corresponds to the freezing of NPhW.
TABLE 1: Pertinent Parameters of the Nanophase of Water (NPhW) in Gels of Nano-Diamond (ND) and Small C60 Clusters NDa
sample c
MH2O,nm/ MC ∆Hnm, kJ/mole ∆Tm, Kf
small C60 clusterb
(0.05)d
0.47 3.1 (0.5)d 8.32 (0.49)d
0.48 4.2 2
a Obtained by using ND samples IIa and IIb.; see text. b Taken from ref 12. c NPhW to ND “saturated” mass ratio. d Standard deviations. e Molar enthalpy of melting of NPhW. f Depression of melting point.
Molar Enthalpy of Melting of NPhW. This value, ∆Hnm, was calculated from the endothermic heat, Q265K, determined by DSC at 265 ( 0.5 K
∆Hnm ) Q265K/ MH2O,nm
Figure 2. A typical evolution of the DSC curve with the successive evaporation of water from the nano-diamond gel, sample IIb. The total amount of water in the sample was 12 mg (trace 1), 6 mg (trace 2), 3 mg (trace 3), and 0.5 mg (trace 4). See text for more discussion.
where Q273K denotes an enthalpy that corresponds to the endothermic feature at 273 K and QH2O,b the specific heat of melting of bulk water.15 The mass of NPhW, MH2O,nm, was calculated as the difference
MH2O,nm ) MH2O - MH2O,b
(2)
MH2O,nm remained almost constant in a sample until the bulk water was removed completely (Figure 2). We believe that this number corresponds to the saturation of the surface of nanodiamond with NPhW and is determined by the certain equilibrium with the bulk water. Finally, the ratio
MH2O,nm/MC
(3)
was calculated. The number was constant among samples IIa and IIb.
(4)
Characteristic Diameter of NPhW. The Gibbs-Kelvin equation20 (5) was used to calculate the diameter D
D ) 4TmσsflVS /∆TmQH2O,b
(5)
where σsfl is the surface tension of the solid-liquid interface,18 VS is the specific volume of solid water (ice), Tm is the melting point of bulk water, and ∆Tm is the depression of the melting point of bulk water and determined directly from the DSC curves. Pertinent values thus obtained are presented in Table 1 along with the corresponding values for small C60 clusters taken from our previous report.12 The samples of C60 clusters studied in our previous work are gels precipitated from the water dispersions of C60, prepared according to the method of Andrievsky.21 In Figure 1, a typical DSC heating trace for the fullerene gel is included for comparison. Discussion In this study, we found a nanophase of water, NPhW, in ND/ water systems IIa and IIb. On the other hand, water at pH ∼ 1 (IIc) and cyclohexane (IId) added to the dry ND from the same source as IIa and IIb did not form the nanophase; hence, not every liquid forms it. It could be assumed that water specifically interacts with ND by adhering to the particle surface. NPhW was easily removed from the system at T ) 343-373 K, that
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J. Phys. Chem. C, Vol. 111, No. 20, 2007 7333
Figure 4. Model of nanophase water (NPhW) surrounding a carbon nanoparticle.
TABLE 2: Model Parameters of a Primary Particle of ND and a Small C60 Cluster Surrounded by a Spherical Shell of NPhW parameters nma
d, D, nmb D′, nmd k, nme (D′ - d)/2, nmf (k - d)/2, nmg
ND
small C60 cluster
5.2 8.6 (0.6)c 7.2 6.5 1.0 0.6
68 36 83.4 73.4 7.7 2.7
a
Diameters of naked particles measured by DLS (ref 7) and the small-angle neutron scattering method (ref 22) for ND and by DLS (ref 12) and SANS (ref 9) for the C60 cluster. b Effective diameters of the naked particle or aggregates with the surrounding NPhW calculated by using eq 5. c Standard deviation. d Effective diameter of a naked particle or aggregates with the surrounding NPhW calculated by using eq 3. e Effective diameter of a primary particle (or cluster) consisting of a naked primary particle (or cluster) and the nonfreezing fraction of NPhW. f Thickness of the NPhW layer surrounding the naked particle (or aggregate). g Thickness of the layer of nonfreezing water.
is, at the same temperatures as the bulk water (see Results). It means that the interaction energy of NPhW with the surface is relatively low. Another important requirement for the appearance/detection of NPhW is the size of the diamond particle. In those systems where the diameter of the dominant species is larger than 200 nm, like Ia,7 the NPhW was not seen in DSC. According to eq 5, the depression of the melting point for nanospheres of water with the characteristic diameter D ) 200 nm should be less than 0.4 K, hardly detectable by the DSC instrument used. On the basis of the experimental results obtained above, one may propose a preliminary model of a ND particle present in aqueous gel and dispersion (Figure 4). The model consists of a nonporous nucleus surrounded by a spherical shell of adsorbed water, NPhW. Strictly speaking, Figure 4 describes the ND particle at temperatures below Tm. However, it is reasonable to assume that at least part of the water shell is preserved at temperatures above the melting point too. The simple model explains the ready transformation between the gel and sol, the high stability of both forms, and an observation that, even after the milled samples (II) were heated at 630K, the NPhW resumed in the samples (IIb) after addition of a new portion of water. It is also worth noting that pH ∼ 1, at which NPhW disappears (sample IIc), is just about the pH value at which ND particles (d ∼ 5 nm) start to lose their stability toward aggregation and form micron-sized clusters.7 On the basis of the core-shell model presented above, we performed additional calculations (see Table 2). Since the ratio MH2O,nm/MC is determined, one may calculate the outer diameter D′ and the thickness (D′ - d)/2 of the water shell surrounding
the diamond nanoparticle, assuming a uniform concentric layer of water around a spherical particle. The calculations are based on the diameter d of the diamond nanoparticle measured by DLS/SANS. To calculate D′, the following equation was used
4 D′ 3 4 d 3 M′H20,nm ) π + , π 3 2 3 2 F(H2O)
( )
()
(6)
which can be rewritten as
(
D′ ) d + 3
6M′H20,nm πF(H2O)
)
1/3
(7)
where M′H20,nm is the mass of NPhW per carbon nanoparticle with diametr d, and F(H2O) is the density of ice. The outer diameter D′ of a shell calculated from the ratio MH2O,nm/MC is equal to 7.2 nm, while the corresponding number obtained from the Gibbs-Kelvin equation is 8.6 ( 0.6 nm. Considering the initial assumptions made and the accuracy of the experimental methods used, we suggest that characteristic sizes of NPhW obtained from the two independent methods (without and with the use of eq 5) are in good agreement. The shell thickness D′ - d/2 is equal to 1 nm (ca. 3-4 monolayers of water, Table 2). The specific surface area of samples IIa and IIb was estimated by the BET method from N2 adsorption measurements as ∼500 m2/g. Assuming a crosssectional area of 10.5 × 10-20 m2 for the water molecule, the weight of the three statistical monolayers of H2O is 0.44 g per gram of nano-diamond. This result is in good agreement with the mass ratio of NPhW to ND (0.47; see Table 1). A large reduction in the enthalpy of melting of NPhW (Table 1) compared to that of bulk water (6.0 kJ/mol) indicates highly ordered structure of the adsorbed water on the surface. However, the inner part of the adsorbed layer itself may not participate in the melting/freezing process.18 The thickness of such an “inert” layer can be estimated from the experimental data obtained. The experimentally measured ratio
f)
∆HH2O,nm ∆HH2O,b
(8)
gives the fraction of melting/freezing NPhW to the total NPhW. The nonfreezing fraction of NPhW is then equal to 1 - f. The effective diameter k of a particle consisting of a naked primary ND particle (or cluster) surrounded by a shell of the nonfreezing water was calculated from the equations
7334 J. Phys. Chem. C, Vol. 111, No. 20, 2007
4 k 3 4 d 3 M′H20,nm π ) π + × (1 - f) 3 2 3 2 F(H2O)
()
()
Korobov et al.
(9)
Acknowledgment. M.K., N.A., and A.B. are supported by the RFBR Grant 06-03-32446. N.R. is supported by Ministry of Economic Development of KR Grant N7-06. E.O. is supported by NEDO International Cooperative Grant 2004IT081.
and
(
k ) d3 +
6M′H20,nm πF(H2O)
)
1/3
× (1 - f)
be a first step in developing a method to prepare stable liquid dispersions of carbon nanoparticles.
(10)
The thickness (k - d)/2 of an inert layer is equal to 0.6 nm (ca. 2 monolayers, Table 2). As can be seen from Table 2, for the C60 cluster, the diameter of the outer water shell has to be equal to 83 nm. The GibbsKelvin eq 3 gave a much smaller diameter (D ) 36 nm). This latter is even smaller than the diameter of the naked C60 cluster. One has to admit that the simple model of a uniform water shell, in this case, leads to conflicting conclusions. However, if we assume that NPhW forms a spherical cluster, one naked C60 cluster with d ) 68 nm has to be symmetrically surrounded by approximately six such spheres of water clusters with D ) 36 nm. The mass ratio of NPhW to a small C60 cluster (0.48, Table 1) gives the corresponding molar ratio, which is equal to 19.2 ( 1.0. On the basis of this number, it was proposed that an individual C60 molecule may be comfortably incorporated into the structure of liquid water as a “1:20 cluster”.23 This idea is, however, inconsistent with the SANS and DLS data, which gave the convincing evidence for the presence of small C60 clusters rather than individual fullerene molecules in the water dispersion. The simple model presented above comfortably incorporates all of the experimental data obtained so far using DSC, SANS, DLS, and adsorption measurements. On the other hand, we reserve discussion on the mechanism of the interaction of water with the corresponding carbon surfaces. In conclusion, we used DSC to detect the nanophase of water (NPhW) adsorbed on primary particles of ND. Conceivably, the same phase exists as well in the other carbon nanoparticles or nanoclusters smaller than ∼200 nm. The role of factors such as pH and ionic strength of water on the stability of NPhW can be easily studied by DSC. Formation of the protective nanophase shell of water may play a key role in preventing aggregation of carbon nanoparticles. If one knows the factors governing the stability of the adsorbed layer, it may be possible to form or destroy the layer deliberately. That will
References and Notes (1) (a) Shenderova, O. A.; Zhirnov, V. V.; Brenner, D. W. CRC Crit. ReV. Solid State Mater. Sci. 2002, 27, 227. (b) Sharda, T.; Bhattacharyya, S. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: Valencia, CA, 2004; Vol. 2, pp 337-370. (c) The latest value is 4.3 ( 0.3 nm; see Baidakova, M. V.; Vul’, A. Ya.; Siklitskii, V. I.; Faleev, N. N. Phys. Solid State 1998, 40, 715. (2) Park, S.; Srivastava, D.; Cho, K. J. Nanosci. Nanotechnol. 2001, 1, 75. (3) Chen, P. W.; Ding, Y. S.; Chen, Q.; Huang, F. L.; Yun, S. R. Diamond Relat. Mater. 2000, 9, 1722. (4) Burkat, G. K.; Fujimura, T.; Dolmatov, V. Yu.; Orlova, E. A.; Veretennikova, M. V. Diamond Relat. Mater. 2005, 14, 1761. (5) Spitsyn, B. V.; Davidson, J. L.; Gradoboev, M. N.; Galushko, T. B.; Serebryakova, N. V.; Karpukhina, T. A.; Kulakova, I. I.; Melnik, N. N. Diamond Relat. Mater. 2006, 15, 296. (6) Kru¨ger, A.; Kataoka, F.; Ozawa, M.; Fujino, T.; Suzuki, Y.; Aleksenskii, A. E.; Vul, A. Ya.; O h sawa, E. Carbon 2005, 43, 1722. (7) Ozawa, M.; Inakuma, M.; Takahashi, M.; Kataoka, F.; Kru¨ger, A.; O h sawa, E. AdV. Mater. In press. (8) Ji, S.; Jiang, T.; Xu, K.; Li, S. Appl. Surf. Sci. 1999, 133, 231. (9) Avdeev, M. V.; Khokhryakov, A. A.; Tropin, T. V.; Andrievsky, G. V.; Klochkov, V. K.; Derevyanchenko, L. I.; Rosta, L.; Garamus, V. M.; Priezzhev, V. B.; Korobov, M. V.; Aksenov, V. L. Langmuir 2004, 20, 4363. (10) Deguchi, S.; Alagrova, R.; Tsujii, K. Langmuir 2001, 17, 6013. (11) Andrievsky, G. V.; Klochkov, V. K.; Karyakina, E. L.; MchedlovPetrossyan, N. O. Chem. Phys. Lett. 1999, 300, 392. (12) Korobov, M. V.; Stukalin, E. B.; Ivanova, N. I.; Avramenko, N. V.; Andrievsky, G. V. In The Exciting World of Nanocages and Nanotubes; Kamat, P., Guldi, D., Kadish, K., Eds.; Electrochemical Society: Pennington, NJ, 2002; Vol. 12, p 799. (13) Kavan, L.; Zukalova, M.; Kalbac, M.; O h sawa, E.; Dunsch, L. Carbon 2006, 44, 3113. (14) Panich, A. M.; Shames, A. I.; Vieth, H.-M.; Takahashi, M.; O h sawa, E.; Vul’, A. Ya. Eur. J. Phys. B 2006, 52, 397. (15) CRC Handbook of Chemistry and Physics, 77th ed.; CRC Press: Boca Raton, FL, 1996. (16) Takei, T.; Onoda, Y.; Fuji, M.; Watanabe, T.; Chikazawa, M. Thermochim. Acta 2000, 199, 352. (17) Jackon, C.; McKenna, G. J. Chem. Phys. 1990, 93, 9002. (18) Ishikiriyama, G.; Todoki, G. M.; Motomura, K. J. Colloid Interface Sci. 1995, 171, 92. (19) Plooster, M.; Gitlin, S. J. Phys. Chem. 1971, 75, 3322. (20) Defay, R.; Prigogine, I.; Bellemans, A.; Everett, D. Surface Tension and Adsorption; Wiley: New York, 1966. (21) Andrievsky, G. V.; Kosevich, M. V.; Vovk, O. M.; Shelkovsky, V. S.; Vashchenko, L. A. J. Chem. Soc., Chem. Commun. 1995, 1281. (22) O h sawa, E.; Rozhkova, N. N.; Avdeev, M. V.; Aksenov, V. L. FLNP Annual Report 2005; FLNP, JINR, Dubna: Moscow, Russia, 2006. (23) Andrievsky, G. V.; Klochkov, V. K.; Bordyuh, A.; Dovbeshko, G. I. Chem. Phys. Lett. 2002, 364, 8.
