ISSN 10637850, Technical Physics Letters, 2013, Vol. 39, No. 3, pp. 244–247. © Pleiades Publishing, Ltd., 2013. Original Russian Text © S.V. Konyakhin, L.V. Sharonova, E.D. Eidelman, 2013, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2013, Vol. 39, No. 5, pp. 33–40.
Labeling Detonation Nanodiamond Suspensions Using the Optical Methods S. V. Konyakhin, L. V. Sharonova, and E. D. Eidelman* Ioffe Physical Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia St. Petersburg State Chemical–Pharmaceutical Academy, St. Petersburg, 197376 Russia *email:
[email protected] Received June 6, 2012
Abstract—The percentage of large particles in the water suspension of diamond nanoparticles to a large extent determines the potential of using these suspensions in technology and medicine. It is demonstrated that there is an error in determining the percentage of large diamond nanoparticles in water suspensions using the dynamic light scattering (DLS) method to measure the size distribution of the particles. A method for labeling these suspensions based on supplementing the DLS measurements with recording of an optical spec trum in the visible region is proposed. The labeling allows one to perform more reliable determination of the percentage of large particles in a suspension. DOI: 10.1134/S1063785013030073
Hydrosols of diamond nanoparticles with the char acteristic size of less than 10 nm are required for a number of applications in biology and medicine [1]. Two methods have recently been used to prepare these hydrosols of detonation nanodiamonds (DNDs): by grinding the powder of commercially available DND in a ball mill with ZrO2 microballs [2] and via special ized chemical treatment [3, 4]. The optical studies [5, 6] have demonstrated that absorbance in the visible part of the spectrum of hydrosols of DND is observed when using either method to obtain them, although both diamond and water are transparent in the entire visible range. As was demonstrated previously [5, 6], the absor bance observed is associated with the regions of sp2 hybridized carbon atoms emerging due to the action of two different mechanisms. First, it is obvious that any mechanical treatment causes a certain degree of graphitization of the surface of diamond nanoparticles [5]. Second, the decrease in the surface energy in the crystals is known to be accompanied by surface recon struction [7, 8]. The presence of a small percentage of particles of considerably larger size (50–100 nm) hinders the practical use of DND hydrosols with the characteristic particle size below 10 nm. Therefore, there inevitably emerges a demand for estimating the percentage of large particles in a DND hydrosol using relatively sim ple methods, thus “labeling” the suspension of dia mond nanoparticles by specifying all of their major parameters. As already mentioned, the regions with sp2hybrid ized carbon atoms are formed on the surface of a par
ticle (Fig. 1). However, these regions can be consid ered to be small even for nanosized particles. Since optical absorption can be induced by the emergence of a closed graphitelike shell on the parti cle surface [9–11], a simulation was carried out in order to reveal the possibility that this absorption can emerge. The regions of sp2hybridized atoms that can be considered to be capable of conducting electricity and, therefore, capable of absorbing light, whereas the homogeneous diamond nucleus can be considered to scatter light. The simulation was carried out by arranging graph itelike regions on the surface of a spherical diamond particle in a random manner (spots in Fig. 1); the size of these regions was much smaller than that of the par ticle diameter [9]. As soon as 70% of the surface was covered by these regions, they merged to form a single shell. The meaning of this critical value (the percola tion threshold) is close to that of the percolation threshold in percolation theory [10]. This threshold is independent of the size of graphitelike defects arranged on the surface and is close to that of a hexag onal lattice: 0.65 and 0.7 for the problems of edge and anglebased percolation, respectively. Figure 1 shows the instant of crossing the percola tion threshold. A model in which the nucleus is formed by sp2hybridized carbon atoms and the surface region consists of sp3hybridized atoms was eventually used. Only the nanoparticles at the surface of which the per colation threshold was overcome contribute to the absorption.
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LABELING DETONATION NANODIAMOND SUSPENSIONS εGper (a) εD
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Fig. 1. Proceeding from the model with incomplete graph itization (a), where a number of unconnected graphitized regions are present, to the twolayer model with a closed graphitelike shell (b). A particle partially covered with the individual sp2regions is shown (c). After further impact, the percolation threshold is overcome (d).
Fig. 2. The typical histogram of size distribution of the fraction of scattering intensity (blank bars), volume frac tion (solid bars), and the values of numerical concentra tion of diamond nanoparticles (squares) in the suspension. The particles were obtained without grinding. The total mass fraction of diamond was 0.05%.
It is a known fact that the scattering cross section of a particle is proportional to the square of its length. Hence, it follows that larger particles contribute sig nificantly more to the scattering. Thus, 106 small par ticles will contribute to scattering, identically to how the only particle of a tenfold larger size does so. This means that scattering will drastically decrease after large particles are removed from the hydrosol. Mean while, the effect on light absorption by the hydrosol after large particles are removed from it will be less sig nificant, since, unlike scattering, absorption is deter mined by the surface of particles containing the sp2 phase rather than by their volume.
tion. On the contrary, the intensity maximum is shifted toward a larger particle size. Thus, it is clear from Fig. 2 that the volume fraction of the compound in large particles is smaller than that in small particles; the concentration of the large parti cles is lower by many orders of magnitude. Moreover, it should be borne in mind that not every large particle has an absorbing surface. The combination of these facts gives grounds to use a formal simplified bimodal model of particle distribution over hydrosol to describe the optical spectra. This model allows one not to take into account the errors of measuring the size using the DLS method and determine the percentage of the par ticles contributing to the absorption. Figure 3 shows the characteristic optical density spectrum of a hydrosol with size distribution presented in Fig. 2. The presence of particles belonging to the major size type with D1 = 15 nm in order to interpret the absorption and scattering of light is insufficient: the experimental values of the optical density in the shortrange region (where scattering plays the main role) is higher than the computed values by one to two orders of magnitude. The presence of large particles has to be taken into account, which can be done by using the second group of particles with effective size D2 much larger than that of main particles D1 in the computation. Within the selected model, diamond nanoparticles with effective size D2 substitute both the large particles (which are present in the suspension and are characterized by good scattering properties) and aggregates of small particles. Concentration n1 of small particles is considerably higher than concentra tion n2 of large ones. Particle structure has been dis
The typical distribution of DND in a suspension is determined by the dynamic laser scattering (DLS) method. A histogram of one of the hydrosols is given for illustration in Fig. 2; it shows the volume fractions of the compound comprising the particles of a certain size depending on particle size (the distribution over the size of the volume fraction of particles). This his togram is the main result of the measurements per formed using DLS; it can be recalculated into the size distribution of a certain number of particles (numeri cal concentration). The typical curve showing the dependence of the numerical concentration of parti cles in the hydrosol on their size is also presented in Fig. 2. It also shows the size distribution of the inten sity of the form of a histogram. Particle size Dmax cor responding to the maximum of the numeric concen tration is apparently shifted toward the region of small particles with respect to the size corresponding to the maximum at the histogram showing the volume frac TECHNICAL PHYSICS LETTERS
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KONYAKHIN et al. Optical density 10
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0.001 200 300 400 500 600 700 800 900 10001100 Wavelength, nm Fig. 3. The characteristic dependence of optical density of the suspension on wavelength λ. Computed: scattering from large particles, dashed lines; scattering from small particles, short dashed line; absorption by large particles, dotted line; and absorption by small particles, dashand dot line. The solid line shows the resulting extinction. The computation was done based on the bimodal model. Dia monds: experimental results.
cussed earlier (Fig. 1). In accordance with the accepted model, both types of particles (regardless of their size) are considered to consist of a diamond (dia mondlike) nucleus and the surface region made of graphitelike material. The size of the diamond nucleus is equal to fraction qi (i = 1, 2) of the size of a particle of the corresponding type. Therefore, the thickness of the graphitelike region will be δiDi = 0.5Di(1 – qi). We assume the thickness of the closed graphitelike shell to be a parameter in this problem. The numerical values of optical density y = ln(I0/I) at various wavelengths λ can be obtained with allow ance for the presence of two types of particles in the hydrosol according to the following proportion: y = n 1 σ ext1 ( λ ) + n 2 σ ext2 ( λ ). In turn, the extinction crosssections of small and large particles, σext1 and σext2, respectively, are the sum of the absorption cross sections and scattering cross sections; the exact formulas to calculate them were given in [11]. Figure 3 shows the results of comparing the exper imental and computed optical density spectra in the bimodal model for the sample for which the size distri butions presented in Fig. 2 are valid. The parameters used in the computation were as follows: diameter of small particles, 10–15 nm (corre sponds to the maximum of distribution of the numer
ical density in Fig. 1); their concentration is almost 100%. The diameter of large particles and their con centration are parameters. Thus, the bimodal model contains the computed spectrum describing the exper imental result. The longwavelength region is determined by the absorption; it can be used to determine the parameters characterizing large particles. The following proce dure for suspension labeling can be proposed. The DLS method is used to determine the size distribution of the particles in hydrosol and to record the optical density spectrum of the hydrosol. The characteristic size of small particles is determined from the maxi mum of distribution of the numerical concentration from the DLS measurements. It can be generally assumed to be equal to 10–15 nm. The effective size of large particles can be estimated to correspond to the maximum in the distribution of scattering intensity. The size of large particles can be generally assumed to be approximately 100–150 nm [1]. Based on matching the experimental blue part of the spectrum with the computed data, the fraction of large particles is selected. Then, based on matching the experimental red part of the spectrum with the computed data at λ ≈ 800 nm, we obtain the graphitization degree. It is assumed that the thicknesses of the graphitelike layer coincide for both types of particles: δ1D1 = δ2D2, and this value is typically equal to several tens of ang stroms, which corresponds to the fact that only a small part of the particle surface is graphitized. This finding is rather interesting; however, it is the percentage of large particles that is the major result obtained by labeling. The labeling enables standardization of sus pensions of diamond nanoparticles, which can open new possibilities for their application in biology, med icine, and pharmacy. It is important that it is impossible to match the computed and experimental data in any of the models under consideration without graphitization (the pres ence of the absorbing region (a layer) consisting of sp2 hybridized carbon atoms). This fact supports the assumptions (hypotheses) of the existence of graphiti zation, which have been previously postulated based on completely different reasons. The percentage of actually graphitized surface can be easily calculated using the δi values. It is quite sufficient to divide the δiDi value by the lattice constant. Thus, in order to bring the experimental and com puted results to agreement in the natural model with allowance for contribution of all the histogram bars, one needs to drop the bars corresponding to large par ticles. This fact indicates that there is an error in mea suring the particle size using the DLS method. This conclusion was supported by the fact that the mea surements for the particles of this very size using the X ray diffraction method provide considerably different results [11]. A conclusion can be drawn that the label ing for determining the percentage of large particles proposed in this study is close to reality.
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This method of labeling will allow one to predict the effect of using certain water suspensions of detona tion diamond nanoparticles with a higher accuracy. Acknowledgments. The authors are grateful to A.Ya. Vul’, who initiated this study, and to A.E. Ale ksenskii for conducting the measurements. This work was supported by the programs of the Presidium of the Russian Academy of Sciences “Fun damental Foundations of the Technologies of Nano structures and Nanomaterials” and “Quantum Meso scopic and Unordered Systems,” as well as by the Rus sian Foundation for Basic Research, project no. 12 0800174a. REFERENCES 1. A. M. Schrand, S. A. C. Hens, and O. A. Shenderova, Crit. Rev. Solid State Mater. Sci. 34 (1), 18 (2009). 2. E. Osawa, Synthesis, Properties and Applications of Ultrananocrystalline Diamond (Springer, Utrecht, 2005).
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3. A. E. Aleksenskiy and E. D. Eidelman, Nanosci. Nan otechnol. Lett. 3, 68 (2011). 4. O. A. Williams, J. Htts, C. Dieker, W. Jager, W. Kirste, and C. E. Nebel, ACS Nano 4, 4824 (2010). 5. A.Ya. Vul’, E. D. Eydelman, M. Inakuma, and E. Osawa, Diamond Relat. Mater. 16, 2023 (2007). 6. A. E. Aleksenskii, A. Ya. Vul’, S. V. Konyakhin, K. V. Reich, L. V. Sharonova, and E. D. Eidel’man, Phys. Solid State 54, 578 (2012). 7. K. C. Pandey, Phys. Rev. B 25, 4338 (1982). 8. K. V. Reich, JETP Lett. 94, 22 (2011). 9. S. V. Konyakhin and E. D. Eidel’man, Vestn. Polotsk. Gos. Univ., No. 9, 93 (2008). 10. A. L. Efros, Physics and Geometry of Disorder (URSS, Moscow, 1994) [in Russian]. 11. A. Ya. Vul’, E. D. Eidelman, L. V. Sharonova, A. E. Alek senskiy, and S. V. Konyakhin, Diamond Relat. Mater. 20 (3), 279 (2011).
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Translated by D. Terpilovskaya
