Influence of surface modification adopting thermal treatments on dispersion of detonation nanodiamond

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Abstract

In order to improve the dispersion of detonation nanodiamonds (ND) in aqueous and non-aqueous media, a series of thermal treatments have been conducted in air ambient to modify ND surface. Small angle X-ray scattering (SAXS) technique and high resolution transmission electron microscopy (HRTEM) were introduced to observe the primary size of ND. Differential thermal analysis (DTA), X-ray diffraction (XRD) methodology, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy were adopted to analyze the structure, bonds at surfaces of the treated ND. Malvern instrument Zetasizer3000HS was used for measuring the surface electric potential and the size distribution of ND. As thermal treatments can cause graphitization and oxidization of functional groups at the surface, ND treated at high temperature is correspondingly more negatively charged in an aqueous medium, and the increased absolute value of zeta potential ensures the electrostatic stability of ND particles. Specially, after being treated at a temperature more than 850 K, ND can be well dispersed in various media.

Graphical abstract

Detonation synthesized diamond has a primary particle size of around 10 nm, but serious agglomeration both in air and liquid media hinders its application as a nanoscaled material. As thermal treatments can cause graphitization and oxidization of functional groups at nanodiamond surface, the nanodiamond treated at high temperature is correspondingly more negatively charged in an aqueous medium, and the increased absolute value of zeta potential ensures the electrostatic stability of particles. Well-dispersed nanodiamond suspension can be prepared in aqueous medium. When a polymer dispersant was introduced, excellent dispersion in a non-aqueous medium, white oil, can also be realized.

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Introduction

Detonation nanodiamond (ND) explosively synthesized from TNT and RDX possesses excellent characteristics such as nano-size with even and round shape, superhardness and chemical stability [1], [2], [3], [4]. Therefore, it has great application potentials in many fields of ultrafine polishing, composite plating, lubricant, anti-friction composite materials, low-field electron emission, etc. [5], [6], [7], [8], [9], [10], [11], [12], [13]. Physical and chemical properties such as purity, crystal structure, surface composition and dispersion behavior of ND and their corresponding influential factors such as synthesis process, purification methods and dispersion media have been widely investigated [14], [15], [16], [17], [18]. The thermal stability, as well as the surface graphitization of the ND in thermal treatment, has also been studied [19], [20], [21].

Although the primary sizes of ND particles stay at around 10 nm [14], [18], [22], [23], they can easily aggregate during synthesis and subsequent treatments, especially when added into a variety of media. Consequently, further deagglomeration and dispersion are necessary. Some techniques such as ultrasonic and surface modification using inorganic electrolytes or hydrophobic organosilyl [24], [25], [26], [27], were introduced for deagglomeration and dispersion. The thermal treatment was also used for the dispersion of ND. Xu and Xue [28] adopted a graphitization–oxidation method to actualize the deaggregation and dispersion of ND in aqueous medium. Graphitization was conducted at first in N2 at 1273 K, whereafter an oxidation treatment in air ambient at 723 K for about 2 h was conducted. After that, the powder was dispersed in water with ultrasonic treatment, and a suspension containing over 50% of the particles with a diameter smaller than 50 nm was prepared. But a portion of aggregates, with even coarser size than the original particles was formed during these treatments. They consider that this phenomenon may attribute to the formation of C–O–C bonding (bridged oxygen bonds) between ND crystallites during heat-treatment, and further chemical measures are needed to disperse these particles.

Till now, the effect of thermal treatment on ND properties and the mechanism of the surface modification are not yet very clear. In the work presented in this paper, the surface properties of ND amid thermal treatments and the consequent influence on its dispersion in aqueous and non-aqueous media were investigated. The surface modification of ND has been discussed.

Section snippets

Experimental and analysis

ND sample used for this investigation was provided by Lingyun Company, a professional detonation diamond producer. This gray powder is one of its purified products from detonation synthesized black powder, where sulfuric acid and perchloric acid were used for purification treatments. Some characteristics of this powder are given in Table 1. In order to investigate the dispersion behavior of sample after thermal treatment, ND samples were loaded in the crucible, put in the stove and heated in

Primary size of original sample

Primary particle size of ND was measured adopting SAXS (Table 2). The result shows that this ND sample has a dimension smaller than 60 nm with the mean size of 12.0 nm and the median size of 8.5 nm. 88.3% of the particles are less than 18 nm in diameter. HRTEM micrographs, as given in Fig. 1, show that the primary ND has a nanometer size and serious agglomeration. The size of aggregates is broadly distributed from around 10 nm to more than 1000 nm.

Crystal structure

DTA results, shown in Fig. 2, illustrate the effect

Conclusions

After heat-treatment in air ambient, the formation of more graphite ingredients and obvious oxidation on the ND surface can be observed, while the latter, may impact the surface properties of ND particle greatly. As the oxidation of hydrocarbon groups on ND surface, the density and intensity of carboxyl groups turns to be stronger stepwise correspondingly along with the increase of temperature, which causes ND surface to be more negatively charged in an aqueous system. And as a result of

Acknowledgments

Part of this work has been carried out under the financial support by Hunan Provincial Natural Science Foundation of China (Project number: 04JJ3074).

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