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Aggregate Structure in Concentrated Liquid Dispersions of Ultrananocrystalline Diamond by Small-Angle Neutron Scattering M. V. Avdeev,*,† N. N. Rozhkova,‡ V. L. Aksenov,†,§ V. M. Garamus,| R. Willumeit,| and j sawa⊥ E. O Joint Institute for Nuclear Research, Dubna, Russia, Institute of Geology, Karelian Research Centre RAS, PetrozaVodsk, Russia, Russian Research Center “KurchatoV Institute”, Moscow, Russia, GKSS Research Centre, Geesthacht, Germany, and NanoCarbon Research Institute, Asama Research Extension Centre, Shinshu UniVersity, Nagano, Ueda, Japan ReceiVed: January 15, 2009; ReVised Manuscript ReceiVed: April 14, 2009
Aggregates of nanodiamond particles dispersed in polar liquids (H2O and DMSO) upon undergoing a special milling procedure are studied by small-angle neutron scattering at a scale of 1-100 nm. The size and fractal character of the aggregates as well as the structural features of nanodiamond particles (size, surface) are compared for both solutions. The structural difference between liquid dispersions and initial nanodiamond powder is analyzed. The concentration effect is followed within an interval of 0.2-10 wt % to conclude about the interaction of the aggregates. It is shown that the developed structure of the aggregates allows their interpenetration in concentrated solutions. The contrast variation procedure using mixtures of nondeuterated and deuterated solvents is performed to judge the homogeneity of the aggregates and to find out their mean scattering length density. The existence of a nondiamond component in the particles is discussed on the basis of the scattering data. 1. Introduction Detonation nanodiamond (DND) is formed during an explosion of oxygen-imbalanced explosives in the absence of any extra carbon source.1,2 DND particles are ultrafine single crystals of cubic diamond with diameters of 4-5 nm as estimated1-3 from the half widths of powder X-ray diffraction peaks, thus offering nanosized diamond, apparently a highly attractive material in nanotechnology,4-10 including biomedical applications.5,11,12 However, until very recently DND has never been successfully purified. The main reason was the lack of knowledge on the remarkable tendency of nanoparticles to form tight aggregates. The particles in DND powders available now are assumed13-18 to be of a complex structure of conglomerates of variously sized aggregates containing extremely tight formations in the core with a size ranging between 60 and 200 nm. For these formations, the term “agglutinates” was proposed.18 A way to decompose agglutinates in DND by the stirredmedia milling together with the powerful sonication in wet conditions was recently developed.18,19 For milling, ultrafine ceramic beads of 30-µm zirconia are used. Complete disintegration of agglutinates into single nanodiamond particles (size ≈ 4.5 nm) was reported18,20,21 on the basis of dynamic light scattering (DLS) data. Particles of DND dispersed in liquids after such treatment show a tendency toward aggregation, which, in contrast to agglutinates, can be destroyed by sonication. Despite the formation of aggregates, the formed liquid dispersions known as dispersed ultrananocrystalline diamond (DUNCD)18 are surprisingly stable. Especially favorable carriers * To whom correspondence should be addressed. Telephone: 007 496 21 62 674. Fax: 007 496 21 65 484. E-mail:
[email protected]. † Joint Institute for Nuclear Research. ‡ Karelian Research Centre RAS. § Russian Research Centre “Kurchatov Institute”. | GKSS Research Center. ⊥ Shinshu University.
for producing stable DUNCDs are water and polar aprotic solvents such as dimethyl sulfoxide (DMSO). The stabilization mechanism in these colloids is under study.20,21 In the present article, we focus our attention on DUNCDs in water and DMSO with concentration within 1-10 wt %. Such concentration allows us to apply small-angle neutron scattering (SANS) in the most informative way at a wide scale of 1-100 nm. First, we follow the interaction change by dilution of initial concentrated samples. Second, the contrast variation procedure is carried out by diluting the initial samples with the proper mixtures of nondeuterated and deuterated solvents, which makes it possible to conclude about the inner structure of aggregates. The scattering from the liquid dispersion is also compared with that from initial dry nanodiamond powder, from which the DUNCDs are prepared. General interest in liquid dispersions of carbon materials should be pointed out. In addition to nanodiamond dispersions, it concerns aqueous dispersions of fullerenes,22-27 carbon black,28-32 and shungite carbon.33 The reason for this is connected with wide possibilities in modifying and functionalizing carbon-containing interfaces for various applications. 2. Experimental Section The commercial nanodiamond powder was purchased from Gansu Lingyun Nano-Material Co., Ltd., Lanzhou, China. The milling procedures and dispersion into liquids18,21 were carried out in the NanoCarbon Research Institute, Nagano, Japan. Dispersions in water and DMSO with concentration of 10 wt % were prepared. Details of the milling and purification operations were described elsewhere,2 where the dispersed primary particles of detonation nanodiamond were called singlenano buckydiamond (SNBD). Amorphous carbon was completely removed in two steps. The purification was first applied to agglutinates and then at the stage of beads-milling. The
10.1021/jp900424p CCC: $40.75 2009 American Chemical Society Published on Web 05/08/2009
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absence of amorphous carbon in DUNCDs was supported by HRTEM and TGA. At the same time, the initial powder and DUNCDs showed a sharp sp2-C signal at δ ) 111 ppm in 13C MAS NMR spectra, which suggests the presence of some graphene-layered structures with “decent perfection”.34 SANS experiments were carried out on the YuMO smallangle time-of-flight diffractometer at the IBR-2 pulsed reactor, Joint Institute for Nuclear Research, Dubna, Russia and on the SANS-1 small-angle instrument at the FRG-1 steady-state reactor of the GKSS Research Center, Geesthacht, Germany. The differential cross section per sample volume (scattering intensity) isotropic over the radial angle φ on the large-area detector was obtained as a function of the module of momentum transfer, q ) (4π/λ) sin(θ/2), where λ is the incident neutron wavelength and θ is the scattering angle. On the YuMO diffractometer,35,36 the neutron wavelengths within an interval of 0.05-0.5 nm and the sample-detector distances (SD) of 4 and 16 m (detector size 90 cm) were used to obtain scattering curves in a q-range of 0.1-3 nm-1. The wavelength of the scattered neutrons registered by the detector was determined according to the time-of-flight method. The calibration procedure was made using vanadium.35 On the SANS-1 instrument,37 measurements were carried out at a neutron wavelength of 0.81 nm (monochromatization ∆λ/λ ) 10%) and a series of sampledetector distances within an interval of 0.7-9.7 m (detector size 55 cm) to cover a q-range of 0.04-2.3 nm-1. H2O was used to calibrate the curves.38 At large SD (>1.8 m), the calibrated curves were obtained by recalculating the curves for H2O obtained at SD of 1.8 m with the corresponding distance coefficient. To check out the results of different corrections and calibration procedures, we obtained the scattering curves for several samples at two instruments and the curves showed complete reproducibility in overlapping q-regions. The liquid samples were put into 1-mm-thick quartz plane cuvettes. The concentration dependence of the scattering was determined by diluting the concentrated samples by several concentration values down to 0.5 wt %. To perform the contrast variation procedure, the concentrated samples were diluted by half with different mixtures of H2O/D2O and DMSO/D-DMSO, respectively, so that the content of the deuterated solvent in the final solutions was varied as 0, 10, 20, 30, 40, and 50 vol % for DUNCD in H2O and 0, 20, 30, 40, and 50 vol % for DUNCD in DMSO. As a background in the case of liquid samples, the scattering from the corresponding pure solvents was measured with the same experimental parameters and subtracted. The measurements of scattering curves of the concentrated DUNCDs at the two instruments show complete agreement between the obtained data within an overlap in the q-interval. Additionally, the cuvettes of the same type were filled with an initial nanodiamond powder. The bulk mass density of the sample, Fm ) 0.178 g cm-3, was determined from the measurements of their mass and filled volume. The scattering from an empty cuvette was subtracted from the powder scattering as a background. We tried to repeat the procedure39 of D2O absorption by the powder to separate scattering from closed and open pores in this sample. No significant absorption was found within two weeks, which allows us to conclude that the pores are almost completely closed in the given powder. 3. Results The scattering curves from initial and diluted samples of DUNCD are given in Figure 1. One can see that they are quite close for the two media. Scattering from the low-concentrated samples (0.5 wt %) reflects two-level organization of particles
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Figure 1. Comparison of experimental scattering curves (points) from initial nanodiamond powder with DUNCD in H2O and DMSO. Two concentrations for DUNCDs are shown. Experimental errors do not exceed the point size. Lines denote best models of eq 1 with corresponding parameters given in Table 1. For convenient view, curves for DUNCD in H2O are divided by 10 and 100 for 10 and 0.5 wt %, respectively. Curves for DUNCD in DMSO are divided by 40 and 400 for 10 and 0.5 wt %, respectively. Regions corresponding to different exponents in the scattering law are denoted.
TABLE 1: Parameters of Eq 1 Fitted to the Experimental Data for Low-Concentrated Samples of DUNCD in H2O and DMSO and Initial Powder sample
P
PS
GS, cm-1
RS, nm
DUNCD in H2O DUNCD in DMSO powder
2.34 2.29 1.26
4.13 4.23 4.19
0.5 0.5 32
2.9 2.7 3.2
in DUNCDs within the range of 1-100 nm. It was treated in terms of the exponential/power-law approach:39-42
I(q) ) B exp(-q2RS2/3)(1/q)P + GS exp(-q2RS2/3) + BS(1/qS*)PS
(1)
Here, parameters with the S index correspond to elementary scattering subunits in the system, which are, in the given case, nanodiamond particles. With respect to these particles, RS is their radius of gyration, PS is the exponent reflecting the character of the surface of these particles, and GS is the factor in power and exponential terms; variable qS* denotes the renormalized q according to qS* ) q/[erf(qkRS/61/2)]3, where k ) 1.1 is an empirical constant. Parameter GS can be considered as the scattering intensity at zero angle for subunits, as if they were independent, GS ) nS〈VS2(FS - Fsolv)2〉, where nS is the number density of subunits, VS is the subunit volume, Fsolv and FS are the mean scattering length densities of the bulk medium and subunits, and brackets denote the averaging over the possible size distribution. Parameters P and B in eq 1 describe the next structural level, whose units are aggregates of nanodiamond particles. Since the P value is less than 3, this means that one deals with fractal clusters. The used approach (eq 1) is the generalization of the ideas developed earlier for surface43,44 and mass45,46 fractals. Experimental data were fitted to eq 1 in the q-range of 0.15-2.2 nm-1, where two scattering levels of the power-law type are the most pronounced. Direct use of eq 1 faces difficulties, because of quite a number of free parameters. As in refs 41 and 42, we used a two-step procedure. First, we estimated the parameters B and P fitting the equation B(1/q)P
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Figure 2. (a) SANS curves from liquid DUNCDs referred to one concentration. The curves are shown for small q-values, where the concentration effect is observed. For convenient view, curves for DUNCD in DMSO are additionally divided by 10. Experimental errors do not exceed the point size. (b) Zimm plots with corresponding linear fits according to eq 2.
to the curves over the interval of 0.1-0.5 nm-1 and BS and PS fitting the equation BS(1/q)PS over the interval of 1-2.2 nm-1. Then, these parameters were fixed and eq 1 was fitted to the data by varying GS and RS. It should be pointed out that the parameters BS, PS, GS, and RS are very sensitive to the remaining background in the scattering curves, so their precision is not well determined. Nevertheless, the obtained results qualitatively satisfy the concept of the aggregate structure that we shall discuss below. The results of the fitting are given in Table 1. Below q ) 0.15 nm-1, a small deviation from the power-law scattering is observed for both samples. The reason is that the aggregate size starts to affect the scattering curves. However, the effect is rather small and not so well resolved to be reliably treated as an additional term G exp(-q2R2/3) in eq 1, where G and R correspond to aggregates. Still, from the q-point where the deviation from the power law takes place, one can estimate the characteristic aggregate size D ≈ 2π/q, which gives D ≈ 40 nm. The effect of this size on the scattering is not so pronounced. One cannot see an explicit Guinier regime corresponding to this value, which is a reflection of the high polydisperse nature of the aggregate. The organization of subunits (nanodiamond particles) in aggregates is fractal, which follows from the values of exponent P < 3.47 The mass fractal dimension is equal to P, that is, one deals with branched clusters, which differ significantly from closed packed structures (formal fractal dimension 3). The subunit interface is not smooth. The values of PS > 4 in both samples indicate the so-called diffusive character of the particle surface,47 which is consistent with the previous SANS data on nanodiamond powders.42 For spherical particles with diffusive surface, the scattering length density behaves at the interface as:
F(r) ) F0
( R -a r ) , R - a < r < R β
(2)
where R is the maximal particle radius, a is the effective thickness of the interface, F0 is the scattering length density of the homogeneous core, and the exponent 0 e β < 1 determines how F(r) approaches the singular point r ) R, where its derivative becomes infinite. The β-exponent is related to the exponent observed in the scattering as PS ) 4 + 2β,47 so that β ) 0 corresponds to the Porod law with PS ) 4. For concentrated samples (10 wt %), a deviation at small q-values from the power law type is more significant. This
is a structure-factor effect because of the interaction between aggregates in solution. It can be followed in more detail in Figure 2a where the data referred to one concentration are shown. As one can see, an increase in the concentration results in a small wide peak at q < 0.12 nm-1. For 10% samples its position, q ≈ 0.1-0.12 nm-1, is close to that produced by the interaction of polydisperse hard spheres,48 q ≈ 0.1-0.15 nm-1, for such aggregate concentration and their estimated size. The correlation length in this case is estimated as ξ ≈ 2π/q > 50 nm. For q > 0.2 nm-1, the curves fully coincide, which means that the structure of aggregates is concentration-independent at the scale of 1-40 nm. The repulsive type of the interaction is followed from the concentration dependence of the forward scattering intensity, I(0), referred to one concentration. For concentrations above 2 wt % for the two kinds of DUNCDs, the parameter I(0) can be estimated well using the far left points of the scattering curves. The Zimm equation49,50 can be applied to the concentration dependence of I(0):
C ) K + LA2C I(0)
(3)
where C is the mass fraction of the clusters, K and L are some positive constants, and A2 is the second virial coefficient, whose sign reflects the character of interaction between clusters. The corresponding Zimm plots for C/I(0) vs C for the two solutions show (Figure 2b) positive slopes corresponding to A2 > 0, which means that we deal with the repulsive type of the cluster-cluster interaction. Detailed analysis of the parameters from the linear approximation is not possible because of their low precision. In Figure 1 and Table 1, one can also compare the scattering from DUNCDs and initial diamond powder composed, as mentioned in the Introduction, from agglutinates. Two scattering levels corresponding to agglutinate structure can be seen in the q-range of 0.15-2.2 nm-1. A new level, as compared to DUNCDs, is observed for q < 0.15 nm-1, which corresponds to the association of agglutinates and is treated as an additional term B1/qP1 in eq 1 (Figure 1). The influence of the agglutinate size (which can be also estimated as >40 nm) on the scattering from this level is negligible. The obtained value 3 < P1 ≈ 3.5 < 4 corresponds to the scattering from the developed agglutinate surface, which can be characterized by the fractal dimension DS ) 6
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Figure 3. Contrast variation in 5 wt % liquid DUNCDs. For convenient view, curves for three samples are given. Insets show details of the scattering curves at small q-values for all samples. Experimental errors do not exceed the point size.
- P1 ) 2.5. The scattering comes from gaps in the agglutinate packing; the situation is very similar to what was observed in shungites at the same level.39 For the level of agglutinate subunits (nanodiamond particles), q > 0.4 nm-1, the scattering from the powder is similar to that from DUNCDs regarding both the subunit radius and diffusive character of the surface. But there is a principal difference in the organization of subunits in agglutinates, 0.15 < q < 0.5 nm-1. The fractal dimension of this level in the powder, P ≈ 1.3, is significantly smaller than in DUNCDs. The reason is that the main scattering contribution in nanodiamond powders comes from the pores.42 There is contrast between the pores and the matrix consisting of nanodiamond particles. This matrix is quite dense in comparison with the system of pores, which is followed from a rather low fractal dimension of pores in the powder. For aggregates in liquid DUNCDs the situation is different; the scattering comes from the nanodiamond particles against the solvent, which penetrates the aggregates. The system of nanodiamond particles is denser, which results in a higher fractal dimension for them. The contrast variation procedure was performed for 5% samples (Figure 3) to find the match point of the aggregates (i.e., the content of the deuterated solvent in the solution when the particles become invisible against the scattering from the carrier (zero contrast)). One can see that the scattering decreases quite monotonously with an increase in the volume fraction of the D-solvent, η, which shows that the studied aggregates are homogeneous. While the parameter P remains the same with the contrast, PS is affected by small noncompensated background. This fact leads to poor precision of GS and RS when treating the curves as described above, which does not allow a standard analysis of the contrast dependence to be made for these parameters. Still, a decrease in the RS radius from the bend in the curves around q ) 1 nm-1 can be seen in Figure 3. Estimates by eq 1 give a systematic decrease in the RS radius from 2.9 in nondeuterated solvents toward about 2.5 nm in 50% deuterated solvents. As in Figure 2a, because of the structure-factor effect (see insets) the forward scattering can be determined well from the far left points in the scattering curves. We used this parameter to find the match point of aggregates. Dependences of the square root of the intensity at zero angle as a function of mean scattering length density of the solvent, Fsolv, for
Figure 4. Finding of the match points for two kinds of liquid DUNCDs using data in Figure 3. Dependences (I(0))1/2 vs Fsolv are fitted to linear functions (coefficient of linear correlation is -0.997 in both cases). Sizes of the experimental points coincide with the corresponding error bars. The determined mean scattering length densities of nanodiamonds in the solutions are shown as open ellipses reflecting experimental errors. The calculated scattering length density of the crystalline diamond is shown as a full ellipse.
the two DUNCDs are given in Figure 4. The Fsolv values were calculated according to:
Fsolv ) ηFD + (1 - η)FH
(4)
where FD and FH are the scattering length density of the deuterated and nondeuterated solvents, respectively. We used FD ) 6.34 × 1010 cm-2, FH ) -0.56 × 1010 cm-2 in the case of H2O, and FD ) 5.26 × 1010 cm-2, FH ) -0.046 × 1010 cm-2 in the case of DMSO. Experimental graphs in Figure 4 fit well the linear dependences, whose intersections with abscissa give us the mean scattering length density of subunits F0 ) 10.5(5) × 1010 cm-2 for H2O and F0 ) 10.2(5) × 1010 cm-2 for DMSO. The corresponding volume fraction of deuterated component (match point) from eq 4 for the two liquid carriers are η0 ) 1.61(7) for H2O and η0 ) 1.94(9) for DMSO. One can see that the obtained values of the mean scattering length density of subunits can be considered to be the same for the two solvents within the errors. 4. Discussion The obtained value F0 points to a high contrast of the studied particles in nondeuterated liquids. In this case, they are close
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Figure 5. Conventional view of supposed different colloidal states of diamond nanocrystallites in the studied systems. (I) Initial powder of agglutinates are shown. Agglutinate size is more than 40 nm, and agglutinates possess a developed surface with fractal dimension DS ≈ 2.5. In addition to the model in ref 18, closed pores in agglitinates are observed in the given work; they are organized in a fractal structure with dimension D ≈ 1.26. (II) Liquid DUNDC right after dispersing procedure.18,21 Single diamond nanocrystallites in a liquid carrier were reported.18,21 Amorphous carbon is completely removed from solution. (III) Liquid DUNDC of low concentration (C ≈ 1 wt %) after aggregate formation, which is observed in the given work. Fractal dimension of aggregates D ≈ 2.3. Aggregate size > 40 nm. Characteristic size of crystallites composing aggregates ≈ 7 nm. Thickness of the characteristic nondiamond shell around crystallites ≈ 0.5 nm. (IV) Liquid DUNDC of high concentration (C ≈ 10 wt %) studied in the given work. Aggregates interpenetrate each other. Appearing pores are accessible for liquid. (V) DUNDC after evaporation of bulk solvent, in contrast to that in ref 20 remaining liquid in nanosized pores is assumed.
to homogeneous particles, which justifies the use of eq 1 in the treatment of the corresponding scattering curves. When dividing the curves in Figure 1 for the two solvents by the corresponding contrast factors, (∆F)2 ) (F0 - Fsolv)2, which are 1.19 × 1022 cm-4 (H2O) and 1.08 × 1022 cm-4 (DMSO), the scattering coincides well at q > 0.5 nm-1 for both concentrations. This q-interval corresponds to the inner structure of aggregates at the scale of 1-10 nm. Again, this testifies that the aggregates at this level are quite homogeneous and identical from the structural viewpoint in both solutions. At low q-values there is some difference in the scattering curves for the two solvents. The reason can be explained by a difference in the aggregate size distribution. Also, different influence of dielectric properties of H2O and DMSO on the structure factor should be taken into account. This is reflected in Figure 2b, where the lines corresponding to second virial coefficients, A2, show different slopes, thus indicating that the interaction between the aggregates in DMSO is more repulsive (A2 is larger). The obtained structural parameters (overall size of aggregates, size of primary particles, and fractal dimension) of DUNCDs are very similar to the parameters obtained for amorphous carbon (i.e., carbon blacks)29 dispersed in water by ultrasonication and stabilized by nonionic surfactants. The main difference is that in the case of carbon blacks it was impossible to get the match point of solution, which can be connected with a higher volume fraction of closed pores and the presence of nonionic surfactants in the solution, which makes the system very inhomogeneous for neutrons. Here, we discuss the obtained results with respect to formation of a graphitic ribbonlike shell at the nanodiamond surface.18 The existence of such a shell is testified by our SANS data. First, the difference in the size obtained in the X-ray diffraction and SANS experiments can be considered as an argument for the presence of such a shell. In X-ray diffraction the shell does not contribute to diffraction patterns, but it can be seen in SANS. The nondiamond shell is concluded also from different spectroscopic data (UV-Raman, XANES, FTIR),51 as well as EPR and NMR.34 Assuming nanodiamond particles to be spherical, from the radius of gyration observed in SANS one can estimate
their size to be about 7.8 nm, which is higher than 4-5 nm obtained by X-ray diffraction. The difference corresponds to a rather thick shell of about 1.4-1.7 nm. However, the precision is not so well determined. X-ray patterns give qualitative estimations of this size. Also, the particle size distribution for nanodiamond particles, whose form, in fact, is unknown, can affect this difference. The SANS contrast variation gives other strong evidence for the shell. A more precise estimate for the thickness of the nondiamond shell follows from the comparison of the obtained scattering length density of nanodiamond particles with that for crystalline diamond, Fdiam ) 11.8(3) × 1010 cm-2. This density for graphitic carbon of the shell can be estimated as Fgraph ) 7 × 1010 cm-2. Then the volume fraction of such carbon, ε, in the nanodiamond particle is found from:
F0 ) εFgraph + (1 - ε)Fdiam
(5)
It is about 0.3 for the obtained F0 value. Taking into account the determined size of nanodiamond, one obtains the thickness of the discussed shell to be 0.5 nm, which, from the chemical viewpoint, is more reasonable than the previous estimation. Also, a decrease in the radius of gyration of the nanodiamond particles with the performed contrast variation points to the existence of the shell. When the scattering length density of the solvent approaches the density of the shell, the influence of the shell on the scattering decreases. Effectively, one starts to see mainly the core particles with smaller radius. This shell can be responsible for the specific diffusive character of the particle surface, which produces PS > 4 (Table 1). The corresponding slope in the double logarithmic scale in Figure 3 at q > 1 nm-1 decreases with the contrast variation toward PS ) 4 (the Porod law) for the smooth interface of crystallites. An additional contribution into the difference between the mean scattering length densities of aggregates and crystalline diamond can be the system of closed pores in the aggregates filled with the solvent. The mean scattering length density of such pores is significantly less than that of pure diamond. The upper estimate for the volume of such closed pores in aggregates follows from eq 5, if one substitutes Fgraph with density of the
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solvent. It gives reasonable ε ≈ 0.1. However, the argument against this is that for the model with closed pores the particle radius of gyration should grow with an increase in the absolute scattering length density of the outer solvent as a result of an increase in the contrast between the pores and the solvent (solvent in the pores is not mixed with the outer solvent). This is not the case in our experiments. To summarize, the schematic model for different colloidal states of nanodiamond crystallites is given in Figure 5 on the basis of the SANS data. The important feature is the developed structure of the aggregates in liquid DUNCD. Such structure determines interpenetration of aggregates in concentrated liquid dispersions. Also, the explicit evidence for the nondiamond shells by SANS follows the estimate of the mean shell thickness. A possible shell organization as graphitic ribbons with turbostratic structure or spherical graphitic layers was discussed in ref 18. Stabilization of DUNCD aggregates is under study now. It is clear that it has charged character, since nanoparticles coagulate on addition of salts.21 Oxygen-containing groups on the surface of carbon particles (achieved by surface modification of nanoparticles) are known to be effective for stabilizing micrometer-sized nanodiamond suspensions in water.52 However, for DUNCDs, whose particles are significantly smaller, it has been shown21 that the solute-solvent interaction plays an important role in the stabilizing mechanism of these systems. It follows from the comparison of stability properties of DUNCDs in various solvent types. As a result, it has been suggested that the solvation shell forms on the diamond surface because of the hydrogen bonding. Also, the stabilization mechanism in water-based DUNCDs was recently connected20 with specific water (nanophase of water, NPhW) at the particle interface, which was concluded from experiments on the differential scanning calorimetry (DSC) of gels prepared by drying DUNCDs. It supports the fact that there is a specific adsorption of water on the curved carbon surface.53,54 We, however, suppose that an important factor responsible for the nanosized water in nanodiamond gels is the pore formation. The obtained characteristic size of NPhW (between 7 and 8 nm) can be explained by formation of pores in the association of the aggregates studied in the current work. In contrast to the initial powder, where the pores between agglutinates are inaccessible to water, wet conditions of the secondary aggregation in dried DUNCDs determine the formation of nanoscale pores filled with water, which is reflected in DSC experiments. 5. Conclusions Our SANS data reveal large aggregates (size more than 40 nm) in concentrated liquid dispersions of nanodiamond. Despite this fact, the dispersions are quite stable (at least for one year). The observed aggregates differ radically from the tight formations in initial dry powder showing developed fractal organization (characteristic fractal dimension 2.3). The structures of aggregates are identical for two different liquid carriers (H2O and DMSO) in respect to both the fractal organization and the structure of primary particles composing the aggregates. The interaction between the aggregates is of repulsive character in the two solutions, while the slight quantitative difference is observed (it is more repulsive in DMSO). A nondiamond shell with the characteristic thickness of 0.5 nm around diamond nanocrystallites (size ≈ 7 nm) is concluded from the mean scattering length density of the aggregates found by the contrast variation. The shell is responsible for the diffusive character of the nanodiamond surface.
Avdeev et al. The developed structure of the aggregates in DUNCDs allows the interpenetration of the aggregates in concentrated solutions. Also, it explains the appearance of nanophases of water20 when drying DUNCDs. Acknowledgment. NEDO International Cooperative Grant j .). RFBR Grant No. 08No. 2004IT081 is acknowledged (E.O 04-98825 is acknowledged (N.N.R.). References and Notes (1) Shenderova, O. A.; Zhirnov, V. V.; Brenner, D. W. Crit. ReV. Solid State Mater. Sci. 2002, 27, 227. j sawa, E. Pure Appl. Chem. 2008, 80, 1365. (2) O (3) Aleksenskii, A. E.; Baidakova, M. V.; Vul’, A. Y.; Siklitskii, V. I. Phys. Solid State 1999, 41, 668. (4) Hanada, K.; Matsuzaki, K.; Sano, T. Surf. Sci. 2007, 601, 4502. (5) Holt, K. B. Philos. Trans. R. Soc. London, Ser. A 2007, 365, 2845. (6) Baidakova, M.; Vul’, A. J. Phys. D 2007, 40, 6300. (7) Kru¨ger, A.; Ozawa, M.; Jarre, G.; Liang, Y.; Stegk, J.; Lu, L. Phys. Status Solidi A 2007, 204, 2881. (8) Shenderova, O.; Tyler, T.; Cunningham, G.; Ray, M.; Walsh, J.; Casulli, M.; Hens, S.; McGuire, G.; Kuznetsov, V.; Lipa, S. Diamond Relat. Mater. 2007, 16, 1770. (9) Zhang, Q.; Naito, K.; Tanaka, Y.; Kagawa, Y. Macromol. Rapid Commun. 2007, 28, 2069. (10) Dolmatov, V. Y. Usp. Khim. 2007, 76, 375. (11) Chao, J. I.; Perevedentseva, E.; Chung, P. H.; Liu, K. K.; Cheng, C. Y.; Chang, C. C.; Cheng, C. L. Biophys. J. 2007, 93, 2199. (12) Liu, K. K.; Cheng, C. L.; Chang, C. C.; Chao, J. I. Nanotechnology 2007, 18, 325102. (13) Sakovich, G. V.; Gubarevich, V. D.; Badaev, F. Z.; Brylyakov, P. M.; Besedina, O. A. Dokl. Akad. Nauk SSSR 1990, 310, 402. (14) Chiganova, G. A. Colloid J. Russ. Acad. Sci. 1997, 59, 87. (15) Chiganova, G. A. Colloid J. Russ. Acad. Sci. 2000, 62, 238. (16) Lin, E. E. SoV. J. Chem. Phys. 1994, 12, 404. (17) Aleksenskii, A. E.; Osipov, V. Y.; Dideykin, A. T.; Vul’, A. Y.; Adreaenssens, G. J.; Afanasev, V. V. Tech. Phys. Lett. 2000, 26, 819. (18) Kru¨ger, A.; Kataoka, F.; Ozawa, M.; Fujino, T.; Suzuki, Y.; j sawa, E. Carbon 2005, 43, 1722. Aleksenskii, A. E.; Vul’, A. Y.; O (19) Eidelman, E. D.; Siklitsky, V. I.; Sharonova, L. V.; Yagovkina, j sawa, E. M. A.; Vul’, A. Y.; Takahashi, M.; Inakuma, M.; Ozawa, M.; O Diamond Relat. Mater. 2005, 14, 1765. (20) Korobov, M. V.; Avramenko, N. V.; Bogachev, A. G.; Rozhkova, j sawa, E. J. Phys. Chem. C 2007, 111, 7330. N. N.; O (21) Ozawa, M.; Inakuma, M.; Takahashi, M.; Kataoka, F.; Kru¨ger, A.; j sawa, E. AdV. Mater. 2007, 19, 1201. O (22) Scrivens, W. A.; Tour, J. M.; Creek, K. E.; Pirisi, L. J. Am. Chem. Soc. 1994, 116, 4517. (23) Andrievsky, G. V.; Kosevich, M. V.; Vovk, O. M.; Shelkovsky, V. S.; Vashchenko, L. A. J. Chem. Soc., Chem. Commun. 1995, 12, 1281. (24) Deguchi, S.; Alargova, R. G.; Tsujii, K. Langmuir 2001, 17, 6013. (25) 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. (26) Scharff, P.; Risch, K.; Carta-Abelmann, L.; Dmytruk, I. M.; Bilyi, M. M.; Golub, O. A.; Khavryuchenko, A. V.; Buzaneva, E. V.; Aksenov, V. L.; Avdeev, M. V.; Prylutskyy, Y. I.; Durov, S. S. Carbon 2004, 42, 1203. (27) Brant, J. A.; Labille, J.; Bottero, J. Y.; Wiesner, M. R. Langmuir 2006, 22, 3878. (28) Bele, M.; Kodre, A.; Arcon, I.; Grdadolnik, J.; Pejovnik, S.; Besenhard, J. O. Carbon 1998, l36, 1207. (29) Garamus, V. M.; Pedersen, J. S. Colloids Surf., A 1998, 132, 203. (30) Basch, A.; Horn, R.; Besenhard, J. O. Colloids Surf., A 2005, 253, 155. (31) Barrie, C. L.; Griffiths, P. C.; Abbott, R. J.; Grillo, I.; Kudryashov, E.; Smyth, C. J. Colloid Interface Sci. 2004, 272, 210. (32) Zhao, J.-Y.; Wang, C.-Y. Chem. Lett. 2005, 34, 580. (33) Rozhkova, N. N.; Gribanov, A. V.; Khodorkovskii, M. A. Diamond Relat. Mater. 2007, 16, 2104. j sawa, (34) Panich, A. M.; Shames, A. I.; Vieth, H.-M.; Takahashi, M.; O E.; Vul’, A. Ya. Eur. Phys. J. B 2006, 52, 397. (35) Ostanevich, Yu. M. Macromol. Chem., Macromol. Symp. 1988, 15, 91. (36) Serdyuk, I. N. Physica B 1997, 234-236, 892. (37) Zhao, J.; Meerwinck, W.; Niinikoski, T.; Rijllart, A.; Schmitt, M.; Willumeit, R.; Stuhrmann, H. B. Nucl. Instrum. Methods Phys. Res., Sect. A 1995, 356, 133. (38) Jacrot, B. Rep. Prog. Phys. 1976, 39, 911.
Structure of Ultrananocrystalline Diamond (39) Avdeev, M. V.; Tropin, T. V.; Aksenov, V. L.; Rosta, L.; Garamus, V. M.; Rozhkova, N. N. Carbon 2006, 44, 954. (40) Beaucage, G. J. Appl. Crystallogr. 1996, 29, 134. (41) Avdeev, M. V.; Aksenov, V. L.; Kohlbrecher, J.; Rosta, L. Physica B 2004, 350, E905. (42) Avdeev, M. V.; Aksenov, V. L.; Rosta, L. Diamond Relat. Mater. 2007, 16, 2050. (43) Freltoft, T.; Kjems, J.; Sinha, S. K. Phys. ReV. B 1986, 33, 269275. (44) Teixeira, J. J. Appl. Crystallogr. 1988, 21, 781. (45) Bale, H. D.; Schmidt, P. W. Phys. ReV. Lett. 1984, 53, 596. (46) Kjems, J. K.; Schofield, P. In Scaling Phenomena in Disordered Systems; Pynn, R., Skjeltorp, A., Eds.; NATO Advanced Study Institute Series B; Plenum Press: New York, 1985; Vol. 133, pp 141-158. (47) Schmidt, P. W. In Modern Aspects of Small-Angle Scattering; Brumberger, H., Ed.; NATO Advanced Study Institute Series C; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1995; Vol. 451; pp 156.
J. Phys. Chem. C, Vol. 113, No. 22, 2009 9479 (48) Frenkel, D.; Vos, R. J.; de Kruif, C. G.; Vrij, A. J. Chem. Phys. 1986, 84, 4625. (49) Zimm, H. J. Chem. Phys. 1948, 16, 1093. (50) Svergun, D. I.; Feigin, L. A. Structure Analysis by Small-Angle X-ray and Neutron Scattering; Plenum Press: New York, 1987. (51) Osswald, S.; Yushin, G.; Mochalin, V.; Kucheyev, S. O.; Gogotsi, Y. J. Am. Chem. Soc. 2006, 128, 11635. (52) Zhu, Y. W.; Shen, X. Q.; Wang, B. C.; Xu, X. Y.; Feng, Z. J. Phys. Solid State 2004, 46, 665. (53) Ellison, M. D.; Good, A. P.; Kinnaman, C. S.; Padgett, N. E. J. Phys. Chem. B 2005, 109, 10640. (54) Lin, T.; Zhang, W.-D.; Huang, J.; He, Ch. J. Phys. Chem. B 2005, 109, 13755.
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