On the structure of concentrated detonation nanodiamond hydrosols with a positive ζ potential: Analysis of small-angle neutron scattering

Accepted Manuscript Research paper On the structure of concentrated detonation nanodiamond hydrosols with a positive ζ potential: analysis of small-angle neutron scattering Mikhail V. Avdeev, Oleksandr V. Tomchuk, Oleksandr I. Ivankov, Alexander E. Alexenskii, Artur T. Dideikin, Alexander Ya. Vul PII: DOI: Reference:

S0009-2614(16)30405-5 http://dx.doi.org/10.1016/j.cplett.2016.06.010 CPLETT 33919

To appear in:

Chemical Physics Letters

Received Date: Accepted Date:

28 March 2016 2 June 2016

Please cite this article as: M.V. Avdeev, O.V. Tomchuk, O.I. Ivankov, A.E. Alexenskii, A.T. Dideikin, A.Y. Vul, On the structure of concentrated detonation nanodiamond hydrosols with a positive ζ potential: analysis of smallangle neutron scattering, Chemical Physics Letters (2016), doi: http://dx.doi.org/10.1016/j.cplett.2016.06.010

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On the structure of concentrated detonation nanodiamond hydrosols with a positive  potential: analysis of small-angle neutron scattering Mikhail V. Avdeev1,2, Oleksandr V. Tomchuk1,3, Oleksandr I. Ivankov1,3, Alexander E. Alexenskii4, Artur T. Dideikin4, Alexander Ya. Vul4 1

Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna 141980,

Russia 2

Physical Faculty, St. Petersburg State University, Saint Petersburg 199034, Russia

3

Faculty of Physics, Taras Shevchenko National University of Kyiv, Kiev 03022, Ukraine

4

Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021, St. Petersburg,

Russia

Abstract Small-angle neutron scattering was applied for the structure characterization of detonation nanodiamond (DND) aqueous suspensions with a positive  potential. The contrast variation technique (based on mixtures of light and heavy water) was used to study the structure of DND particles and their clusters in solutions. The results were compared with the data of the previous similar experiments for DND suspensions with a negative  potential. The experimental range of the neutron scattering contrast in the system was extended to maximally possible by starting with the initial concentrated suspensions separately prepared in light and heavy water.

1. Introduction Nanodiamonds are of current interest in view of the rapidly growing number of their technical and, especially, biomedical applications (corresponding reviews have recently been published1,2). These nanoparticles with the characteristic size below 10 nm can be produced on an industrial scale using the detonation technique and known as detonation nanodiamonds (DND). An important issue in the physical and chemical studies of DND concerns the purification and synthesis of their sols in various solvents, which are used for the storage and subsequent chemical modifications. Starting from the first successful synthesis of stable aqueous suspensions by ‘steer milling’ of a chemically purified DND powder3 there has been an ever-rising interest in water-

soluble DND particles4-16 regarding the factors determining and affecting the aggregation stability of DND hydrosols. The latter is usually related to electrostatic repulsion between charged particles. Thus, the negative  potential (-stabilization) is accomplished by ionized carboxylic groups on the particle surface. The corresponding chemical surface modification is achieved by annealing a DND powder in the air at temperatures above 400 oC

10

. An alternative way is to form a positive 

potential (+-stabilization), when the chemical surface modification is achieved by annealing nanodiamonds in a hydrogen atmosphere9. As a result, the CH groups bind to the DND surface, and, when interacting with water, produce a positively charged layer. This effect has been known for more than a decade17, yet its nature is not fully understood. The availability of stable suspensions over a wide range of DND concentrations (up to the level of 10 wt.% makes it possible to apply the method of small-angle scattering of thermal neutrons that is very efficient in structural research with its powerful option of the contrast variation based on isotopic hydrogen-deuterium substitution in solvents. The latter employs the changes in the scattered intensity with a different content of the deuterated component in the liquid carrier. In suspensions, the contrast variation is generated by dissolving initial concentrated solutions in mixtures of light and heavy solvents. The previous SANS experiments7,14,18,19 performed for the concentrated DND suspensions of --type have revealed the presence in the solutions of the polydisperse fractal clusters in the size range around 100 nm. It has been concluded that due to their developed structure (fractal dimension of about 2.4), the clusters can penetrate each other in strongly concentrated solutions and form gel-like nets. The SANS contrast variation experiments have also shown that there is a significant shift in the mean scattering length density (SLD) of the DND particles as compared to the crystalline diamond. In combination with the specific deviation of the observed asymptotic scattering from the Porod law, this fact has been interpreted as an indication of the existence of a non-diamond component in DND responsible for a diffusive type of the particle surface. This component has been logically related to the near-surface graphite-like bonds in the structure of DND reflected in numerous spectroscopic studies1 and predicted by quantum mechanical calculations20. However, the regulation of the fraction of this kind of bonds and the possibility to remove them completely during purification are still under discussion1, which motivates further detailed SANS analysis of the SLD distribution in DND suspensions using different stabilizing methods. The present paper reports the results of the structural characterization of DND particles and their clusters in a hydrosol with +-stabilization by SANS based on the contrast variation technique. In addition to the previous SANS experiments on DND suspensions, here we extended the range of

the covered contrasts to the maximum possible one by starting with the initial concentrated suspensions separately prepared in light and heavy water. The two types of stabilization (+ and  stabilization) of DND in aqueous suspensions are compared regarding both the particle and the cluster structural level organization. An additional motivation concerns possible formation of a hydration shell around DND particles in water, which is actively discussed in recent studies5,12,13,14,21. The paper aims at clarifying the sensitivity of the SANS contrast variation technique to this shell basing on both the extended contrast range and the comparison of the two types of stabilization for which one can expect different organization of water at the interface with DND particles.

2. Experimental Two types of initial concentrated solutions based on light and heavy water (respective DND concentrations are 5.05 and 2.35 wt.%) were prepared according to the procedure in Ref.10. The solutions were then diluted in different D2O/H2O mixtures so that the content of heavy water in the final solution varied at the same DND concentrations in the ranges of 0-70 vol.% and 100-50 vol.% in the first and second cases, respectively. The measurements of ζ-potential via DLS using a Zetasizer ZS 3600 apparatus (Malvern Instruments Ltd.) at 25°C, at a scattering angle of 173° gave the characteristic number of adsorbed protons per DND particle in solution of about 70. SANS experiments were performed on the YuMO time-of-flight small-angle diffractometer at the IBR-2 pulsed reactor of the Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, Russia22. The scattered intensity (differential cross-section per sample volume) isotropic over the radial angle φ on the large-area detector (size 90 cm) was obtained as a function of the modulus of momentum transfer, q = (4π/λ)sin(θ/2), where θ is the scattering angle and λ is the incident neutron wavelength. The neutron wavelengths within an interval of 0.05–0.5 nm and the sample-detector distances of 4 and 16 m were used to obtain SANS spectra in a q-range of 0.08– 2.5 nm-1. A vanadium standard was used for an absolute calibration of the intensity. The raw data treatment was performed by the SAS program with a smoothing mode23. The liquid samples were measured in 1 mm-thick flat quartz cuvettes (Hellma). For the background correction, the scattering from the corresponding D2O/H2O mixtures was obtained in separate measurements and subtracted.

3. Results and discussion

The scattering curves for the initial suspensions in H2O and D2O are compared in Fig.1. Their character fully repeats the one observed previously in the SANS experiments with other kinds and modifications of DND suspensions7,14,18,19. The interpretation of the specific features of the scattering, namely the clearly seen two power-law type scattering levels, points to the fact that there are nano-sized (several nanometers) particles in the solutions, which are mainly combined into developed clusters with the fractal dimension D of 2.32.4. The surface of the basic particles is of a diffusive type19 with a small diffusivity index β = 0.080 ± 0.015. The two structural levels (particles and clusters) are characterized by high polydispersity, which smears fringes in the scattering curves one can expect in small-angle diffraction patterns from monodisperse objects in solutions. The dissolution (in Fig.1 it is followed for the H2O-based system) reveals no principal changes in the scattering behavior except for the effect of the residual incoherent background typical for neutron scattering experiments (largest q-values). In the case when the DND concentrations are close for the two types of the solvent the total scattering is significantly less for the D2O-based sample (see Fig.1), which is in agreement with the fact that the scattering contrast in solutions with carbon-based particles is lower for deuterated solvents than for protonated solvents. Despite the stronger effect of the residual incoherent background in the D2O-based system, still two power-law type scattering levels are resolved. As it has been previously shown, the observed kind of scattering curves fits well the unified exponential/power-law approximation24 combining two scattering levels:

I (q)  Gcl exp  q 2 Rgcl2 / 3  Bcl exp  q 2 Rg2 / 3 erf (1.1qRg cl / 61/ 2 ) / q   Pcl

GS exp  q R / 3   B  erf (qRg / 6 ) / q   Bg , 2

2 gS

1/ 2

P

(1)

where each level is represented by two terms corresponding to the Guinier law (scattering from the shape of the object) and the power law (scattering from the spatial correlations within the object), respectively; parameters without index and with index ‘cl’ correspond to the particle and cluster levels, respectively; Rg and Rgcl are the radii of gyration; G and Gcl are the forward scattered intensities; exponent P > 4 is related to the diffusive surface structure of the particles; exponent 1 < Pcl < 3 is related to the cluster fractal organization (mass fractal dimension D = Pcl); Bg is the residual incoherent scattering after the subtraction of the solvent scattering. The parameters of this approximation can be directly used for obtaining the characteristics of the particle size distribution function19. The corresponding results of the approach are shown in Fig.1 as solid lines and are summarized in Table 1, where for the particle size distribution function of the DND particles the

log-normal approximation is used. The size characteristics of DND particles in heavy water are omitted, since the approach relating to this point at the contrast scale can be hardly applied for Dsolvents where the systematic errors (due to the residual incoherent background) are significant 19. The found parameters of both levels do not show principal differences with the data obtained in the same way for DND suspensions with negative charge stabilization19. The used approach gives rather small mean size of the particles, however, the fact that the log-normal distribution is quite asymmetric means that the size of a significant part of the particles (~40 %) is within an interval of 3 - 6 nm, which is in qualitative agreement with the data of other structural methods10. The exact comparison of the size obtained by various methods for highly polydisperse nanoparticles is an intricate matter. The reason is that different methods probe different combinations of the moments of the particle size distribution. Thus, as it was previously noted for DND19, the analysis of the peak width in X-ray diffraction (XRD) gives25 the mean particle radius in the form ~/. Here, using the parameters of the particle size distribution in Table 1 one obtains for this combination 2 nm, which is in full agreement with the size of 4 nm from the previous XRD data. The difference is more significant, if the mean radius is compared with the corresponding estimate from the radius of gyration, R = (5/3)1/2 Rg = (5/3)1/2 (/)1/2, which is followed from the general theory of scattering by polydisperse particles 26. The radius of gyration is used in practice14 as a first approximation to the particle radius from small-angle scattering, when particles are assumed to be spherical, homogeneous and, most importantly, monodisperse. Here, this estimate gives R = 4 nm, i.e. twice larger than XRD. The found cluster gyration radius Rgcl is about 27 nm for all solutions. Again, in the spherical homogeneous approximation the cluster size can be estimated as 2(5/3)1/2Rgcl, which gives ≈70 nm. This estimate is the lower limit corresponding to the dense packing of the nanoparticles in the cluster and can be significantly affected by the developed cluster structure, as well as some possible contribution of the cluster-cluster interaction (structure-factor effect). The contrast variation for the two kinds of the initial suspensions is followed in Fig.2. Again, no principal changes in the character of the scattering curves are observed; the total intensity monotonously decreases with the growth of the D-component in both cases. As it has been mentioned above, this behavior is the result of a decrease in the contrast,  , of the DND particles against the solvent:

    s ,

(2)

where  is the mean scattering length density of DND, and  s is the solvent SLD determined by the volume fraction, , of the deuterated component in the mixture as

s  D  (1  ) H .

(3)

Here,  H and  D are the scattering length densities of light (H 2O) and heavy (D2O) water, respectively. In the case when the residual background effect can be neglected, the scattered intensity is proportional to the squared contrast

I ~ ( )2 .

(4)

From the q-behavior of the experimental curves in Fig.2, one can see that the influence of the residual background is minimal (less than two orders of magnitude) for the scattered intensity at the smallest q-values covered in the experiment. As a result, in accordance with (4) parabolic dependences are observed for the scattered intensity, I (qmin ) , in the minimum q-value, which are followed in the insets in Fig.2. The corresponding dependence of

I (qmin ) on  s should take a

linear form. Its extrapolation towards zero contrast gives the so-called match point, in which the solvent SLD formally meets the condition  s   . The corresponding dependences for the two kinds of samples are plotted in Fig.3. They show a perfect linear behaviour with the excellent overlap, which makes it possible to treat them simultaneously and, thus, as compared to the previous experiments on the contrast variation, expand the covered contrast range to the maximum possible for the H2O/D2O mixtures. The intersection of the fitting line in Fig.3 with the  s -axis gives the match point  = (10.4  0.6) × 1010 cm-2, which is significantly less than SLD of diamond,  = 11.8 × 1010 cm-2, calculated from its well-known crystalline structure. To emphasize the difference, the linear dependence of the scattered intensity (scaled to the unit concentration) from the equivalent diamond particles is additionally plotted in Fig.3. This difference was also observed for other types of DND suspensions studied previously by SANS and was considered as an evidence of the presence of the non-diamond component in the structure of dispersed DND particles. In the recent detailed analysis of the SANS data combining the results of the contrast variation and the deviation from the Porod law towards the diffusive character of the particle surface18,19, this component was associated with a rather wide transitional spatial layer (in terms of radially averaged SLD profile in the particle) between the diamond core and graphitelike shell on the surface. This layer has a diffusive structure and is responsible for the observed deviation from the Porod exponent (4) in the power-law type scattering at the particle level. For

comparison, the scattering length densities of diamond and graphite are indicated in Fig.3. The mean SLD of the DND particles in the continuous approximation18,19 according to the experimentally observed -parameter,  = (10.2  0.3) × 1010 cm-2, is well consistent with the discussed  . The difference observed here between the scattering length densities of DND and crystalline diamond is at the same level as for the DND suspensions with the negative charge stabilization studied previously by SANS in the similar way but starting only from H2O-based concentrated suspensions. This means that the match point is hardly affected by the difference in the stabilizing agents on the DND surface. The observable shift in the match point of DND because of the stabilizing shell should exceed the achieved experimental error which is (for the suspensions considered here) equivalent to the effective number of about 1000 atoms of hydrogen (provided the polydispersity of the particles is taken into account as well). Since the shift is not resolved, the number of hydrogen atoms in the shell is at least one order less, ~100, which agrees with the estimates based on the ζ-potential. The similar upper estimates can also be done for a hydration shell around DND particles which is discussed in recent studies5,12,13,14,21. Again, if one takes into account the polydispersity of DND particles, the estimates based on the SLD shift corresponding to the experimental error in the match point give the upper limit for the characteristic thickness of the hydration shell at the level of less than 0.1 nm at the 1-2% excess of the shell density over the density of the solvent. So, according to the SANS data, the difference in the densities of two kinds of water (in the shell and in the bulk) is extremely small. There are cases, when a more pronounced effect of the hydration shell on the SANS contrast variation for suspensions of nanoparticles is observed. The example is silica nanoparticles in water, where a specific H/D exchange in the interface layer with increasing thickness of the shell for smaller particles has been concluded27. An additional argument for the absence of the effect of the solvent ordering at the DND surface on the SANS data is the fact that the power-law part of the scattering corresponding to the particle surface repeats the one of DND particles in powders7,28, i.e. without any solvent. Again, the same power-law type scattering is observed for different solvents (comparison of SANS for DND in water and dimethyl sulfoxide has been made7,19).

4. Conclusions To summarize, no principal differences from the structural viewpoint between + and stabilization of the DND aqueous suspensions have been observed in the SANS experiments. The identity of the structure at a length scale of up to 100 nm has been proved with respect to the

developed clusters characterized by close fractal dimensions, as well as to the DND particles characterized by wide size distributions (polydispersity more than 30%) and diffusive surface. This testifies the existence of a unique mechanism of the cluster formation and growth in DND suspensions irrespective of the stabilization method. Our data also indicate that the most reasonable scenario of the cluster stabilization is the formation of a charged shell around the whole cluster rather than around individual particles in it. This explains the stop of the cluster growth in the suspensions and equilibrium of the clusters with some fraction of non-aggregated DND particles11, as well as the previously observed14 reproducibility of the cluster scattering level when redispersing dried DND powders in water. Despite the extension of the range of the neutron scattering contrast using two kinds of solutions based on light and heavy water, both the stabilizing shell and hydration shell had no effects on the results of the contrast variation experiments, which means that the corresponding density fluctuations of the solvent at the interface with DND particles are beyond the limits of the SANS experiment. For the stabilizing shell of adsorbed protons, this correlates with the data on the ζ-potential. For the hydration shell, this means that the theoretically predicted and experimentally found rearrangement of the bonds in the interface water on the surface of the DND particles has an extremely small influence on the interface water density as compared to that of the bulk water. It should be pointed out that the fact that the SANS contrast variation gives the same results for the two kinds of DND suspensions (i.e. based on H2O and D2O) with different incoherent background in neutron scattering experiments strongly supports the reliability of the previous SANS experiments with respect to the possible effects of the mentioned background. Finally, the aggregate structure of DND suspensions is a factor, which complicates the direct analysis of the scattering contribution from solvent-particle interface. In this regard, the synthesis and study of the concentrated and non-aggregated DND suspensions is a challenging task, which is likely to be solved by subsequent fractioning of initial solutions.

Acknowledgments The work has been carried out with the financial support of RFBR (grant 14-22-01054_ofi-m). A.E.Alexenskii and A.Y.Vul are grateful for the support of the Russian Scientific Foundation (project N 14-13-00795).

References

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Table 1. Parameters (gyration radii and fractal dimensions of clusters) of the best fits of unified exponential/power-law approximation (1) to the SANS experimental curves in Figure 1 and corresponding parameters of DND particle polydispersity (mean radii and standard deviations) obtained in the log-normal approximation19.

Sample

Concentration, wt. %

Rgcl, nm

D

‹R›, nm

σ, nm

ini DND/H2O

5.05

27(1)

2.33(1)

1.38(5)

0.49(2)

dis DND/H2O

2.25

28(1)

2.31(1)

1.38(7)

0.52(3)

ini DND/D2O

2.35

25(1)

2.32(2)

-

-

Figure captions

Fig.1. Experimental scattered intensity curves from the initial concentrated DND suspensions in light water (ini DND/H2O), 5.05 wt.%, and heavy water (ini DND/D2O), 2.25 wt.%, and from the dissolved suspension in light water (dis DND/H2O), 2.35 wt.%. The vertical dashed line conventionally separates two (particle and cluster) scattering levels for which specific power-law type scattering regimes are denoted. The solid lines correspond to the best models of the two-level approximation (1), which takes residual background into account. The corresponding parameters are collected in Table 1.

Fig.2. Experimental data on the SANS contrast variation for two series of samples prepared from the initial DND suspensions in light (a) and heavy (b) water. The weight fractions of DND in the samples are 1.26 and 1.12 wt.%, respectively. The volume fractions, , of D2O are indicated in the legends. The insets show the dependence of the scattered intensity at q = 0.1 nm-1 on the volume fraction of D2O in solution. The dashed lines in the insets follow the parabolic dependences.

Fig.3. The dependence of the experimentally found values [I(qmin)/c]1/2 on the solvent SLD, S, for the two series of samples prepared from the initial DND suspensions in light (ini DND/H2O) and heavy (ini DND/D2O) water, respectively. The experimental errors do not exceed the size of points. The solid line shows the best linear approximation fitted simultaneously to the experimental points of the two series. The dashed line follows the behavior expected for the particles of purely crystalline diamond. The arrows denote match points of graphite, DND and crystalline diamond in units of 1010 cm-2.

Fig.1.

Fig.2.

Fig.3.

Research Highlights 

Structure of nanodiamond hydrosols with a positive  potential is determined



Unique mechanism of cluster formation for opposite potentials is concluded



Similar particle characteristics in clusters are obtained for opposite potentials



The same structure organization of hydrosols was found for light and heavy water

Graphical Abstract