In the field of precision polishing, nanodiamonds possess an extreme Mohs hardness of 10, along with high specific surface area and high surface activity at the nanoscale. When processing hard and brittle materials such as silicon carbide and sapphire, they are capable of atomic-level material removal. However, the native nanodiamond particles produced by the currently mainstream detonation and high-temperature high-pressure (HTHP) synthesis methods tend to spontaneously agglomerate. When used in workpiece polishing, this easily causes surface scratches, pits, poor flatness, and other issues, directly reducing product yield. It also limits their application in lubricating coatings, biosensing, high-end composite materials, and other fields. Therefore, effective deagglomeration and stable dispersion of nanodiamonds are key to fully leveraging their outstanding properties and unlocking high-end application markets. This article starts with the causes of nanodiamond agglomeration and reviews the four major deagglomeration technology routes prevalent in the industry.
Why do nanodiamonds cluster together?
Nanodiamond particles are extremely small, with diameters ranging from only 2 to 50 nanometers, and have a very large specific surface area. Physically, they are inherently in an unstable state. To achieve a more stable configuration, these ultrafine particles spontaneously adsorb to one another and agglomerate-this is the root cause of their tendency to cluster. At the same time, the particle surfaces have many unsaturated active sites that readily adsorb moisture and impurities from the air, further causing particles to adhere tightly together and exacerbating the clumping problem, forming conventional soft agglomerates. Typically, such soft agglomerates are mainly held together by physical forces such as van der Waals forces, electrostatic attraction, and capillary forces, with particles primarily in point or angular contact, resulting in relatively weak bonding forces.
However, nanodiamonds produced by the detonation method have residual impurities on their surfaces, such as graphitic carbon and amorphous carbon. During high-temperature synthesis, adjacent particles can directly form strong carbon–carbon bonds. Coupled with the dehydration and curing of surface groups during subsequent drying and storage, this ultimately results in dense, strongly bonded hard agglomerates. These hard agglomerates cannot be broken apart by ordinary external forces alone, posing a serious challenge to the application of nanodiamonds.
How to deagglomerate nanodiamonds?
In response to the above agglomeration mechanisms, the industry has developed multiple types of deagglomeration technologies for nanodiamonds, which can be classified into four major categories: physical dispersion methods, inorganic chemical methods, high-energy field methods, and surface chemical modification methods.
01 Physical Dispersion Methods
Physical dispersion methods rely on mechanical forces to overcome interparticle attraction and forcibly break up soft agglomerates. However, such physical dispersion means cannot guarantee long-term stability of the suspension and are typically used as auxiliary dispersion techniques. Common methods include:
(1) High-energy ball milling: This is the most common mechanical method. It uses the mechanical energy generated by high-speed collisions and shearing between grinding balls to crush micron‑sized agglomerates down to the nanometer scale. Typically, a dispersion medium (e.g., water or alcohols) provides lubrication and preliminary stabilization, so wet ball milling is more efficient than dry ball milling.
(2) Ultrasound‑assisted dispersion: This method utilizes the cavitation effect in liquids to deagglomerate clusters. During ultrasonic propagation, microbubbles form in the liquid. Under the periodic compression and rarefaction of the ultrasonic field, these bubbles grow to resonance size and then collapse violently. The instantaneous collapse generates localized high temperature, high pressure, and powerful shock waves as well as high‑speed microjets in a very small space, thereby disrupting van der Waals forces, electrostatic adsorption, and other binding forces between particles, and breaking large agglomerates into micron‑ to nanoscale individually dispersed particles.
02 Inorganic Chemical Methods
Inorganic chemical methods involve processes such as acid washing, hydrogenation annealing, and oxidation annealing to remove metallic compounds and non‑diamond carbon (including graphite and amorphous carbon) from the surface of nanodiamonds, thereby effectively reducing hard agglomeration.
(1) Acid washing: Treating with strong acids such as nitric acid or hydrochloric acid can dissolve and remove metallic impurities, while also introducing a large number of oxygen‑containing functional groups-such as carboxyl (-COOH), hydroxyl (-OH), and carbonyl (C=O) groups-onto the nanodiamond surface. In aqueous solutions, these polar groups ionize, imparting a negative surface charge to the particles, generating electrostatic repulsion between particles and thus hindering agglomeration.
(2) Oxidation annealing: Heating nanodiamonds in air or oxygen atmospheres similarly introduces oxygen‑containing functional groups on the surface. Compared with liquid‑phase acid oxidation, its advantage lies in the absence of liquid waste generation during processing, making it suitable for large‑scale continuous production.
(3) Hydrogenation annealing: In contrast to oxidation, hydrogenation annealing involves heating nanodiamonds at high temperature in a hydrogen atmosphere to remove oxygen‑containing surface groups, forming a hydrogen‑terminated surface (e.g., -CH₂ groups). After hydrogenation treatment, the nanodiamond surface becomes hydrophobic, significantly improving its dispersibility in non‑polar solvents.
03 High‑Energy Field Methods
High‑energy field methods use extreme physical fields such as lasers or plasma to act directly on the agglomerated structures, efficiently and cleanly destroying hard agglomerates of nanodiamonds and promoting stable dispersion in media.
(1) Plasma‑assisted dispersion: High‑energy particles (e.g., ions, electrons) generated by plasma bombard the nanodiamond agglomerates, not only producing strong physical impacts and microjets, but also directly breaking covalent bonds between nanodiamond particles and altering their surface chemical states, ultimately disrupting hard agglomeration.
(2) Laser‑assisted dispersion: The ultra‑high energy density of lasers can, on one hand, directly break covalent bonds between nanodiamond particles (especially the interfacial bonds between the surface sp²‑hybridized carbon layer and the sp³ diamond core) as well as intermolecular van der Waals forces. On the other hand, the laser energy can be absorbed by liquid media such as water, triggering avalanche ionization and generating plasma plumes. The instantaneous violent expansion of the plasma creates strong shock waves locally, impacting and breaking the physically bonded structure of nanodiamond agglomerates, thereby achieving efficient deagglomeration.
04 Surface Organic Chemical Modification
Agglomeration is a spontaneous process with low activation energy. Even if agglomerates are temporarily broken apart by external forces, as long as the particle surface state has not been fundamentally changed, the newly exposed surfaces will quickly re‑aggregate through the aforementioned forces. Surface organic chemical modification is therefore an essential means to maintain dispersion stability after deagglomeration by altering the particle surface state. Currently, the main approaches include arylation, sulfonation‑diamond functionalization, and photografting.
(1) Arylation: Introducing aromatic ring groups such as phenyl or naphthyl onto the nanodiamond surface can occupy surface dangling bond sites, while the steric hindrance provided by their rigid structures effectively prevents particle approach. Additionally, the introduced aromatic rings can serve as anchoring points for further functionalization.
(2) Sulfonation: Introducing sulfonic acid groups (-SO₃H) onto the nanodiamond surface via sulfonation reactions. Sulfonic acid groups are strongly acidic and fully ionize in aqueous solution, significantly altering the surface zeta potential of the particles. This generates strong electrostatic repulsion between nanodiamond particles, effectively counteracting van der Waals attraction and preventing re‑agglomeration, thus achieving deagglomeration and maintaining stable dispersion. At the same time, sulfonation changes the surface chemistry of nanodiamonds, enhancing surface polarity and markedly improving particle wettability in polar solvents (e.g., water).
(3) Photografting: Using ultraviolet or visible light irradiation to excite the nanodiamond surface or photosensitizers to generate free radicals, which initiate polymerization of olefinic monomers on the surface, grafting long polymer chains. Common grafting monomers include acrylic acid, acrylamide, methyl methacrylate, etc. The polymer chains assume extended conformations in solvents, forming a steric hindrance layer with thickness ranging from several nanometers to tens of nanometers. The repulsive force increases sharply as the interparticle distance decreases, making this one of the most effective means to prevent agglomeration. The advantages of photografting include mild reaction conditions and precise control of grafting density and chain length by adjusting irradiation time and monomer concentration.