Characterization of nanodiamonds and their aggregates in suspension
Creators: name: Eldemrdash, Samir
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 510405
subject: Soft condensed matter

description: Over the past decade, nanodiamonds (NDs) have become widely explored – they are attractive due to their outstanding optical and magnetic properties as well as excellent biocompatibility, in addition to their mechanical, and physico-chemical properties. These properties make NDs a promising material for many applications in areas including quantum sensing, biology, catalysis, fillers in nanocomposites, and electro-mechanical devices. For many of these applications the colloidal properties of the particles are vital, yet these remain poorly understood. This thesis uses different scattering techniques including dynamic light scattering (DLS), depolarised dynamic light scattering (DDLS) and small-angle X-ray scattering (SAXS) to investigate the complex colloidal behaviour of nanodiamond particles and their aggregates in suspension.

I specifically focus on two types of nanodiamonds, detonation nanodiamonds (DNDs) and high-pressure high-temperature nanodiamonds (HPHT NDs). First, using DLS and SAXS, I studied a wide concentration range of DNDs in water to understand the effects of particle concentration and salt concentration on the dynamic self-assembly of these particles.

I found that the DND self-assembly process changes as a function of the particle concentration. However I found it is independent on the salt concentration up to a concentration of 1 mM. Our results suggest that the complex surface chemistry of nanodiamond surfaces in combination with the small diameter (3-5 nm) of DND primary particles leads to a unique form of dynamic aggregation and self-assembly in suspension.

Furthermore, these results revealed that the aggregates become denser with increasing particle concentration, and the aggregate’s overall shape changes from disk-like to rod-like. This behaviour is unexpected for the dilute particle suspensions studied here but similar to the aggregate structures observed for concentrated colloidal particles in high salt media, described by classical colloid aggregation theories (DLCA-RLCA).

To better understand this complex colloidal behaviour, I investigated the size and shape of DND aggregates in both protic and aprotic solvents. I demonstrated that aprotic solvents inhibit aggregation and provide DND suspensions of primary particles. The results highlight the role of interactions between chemical groups on the DND surface and solvent molecules.

Attention is then turned to HPHT NDs, whose highly irregular and anisotropic shapes can limit their application and have not previously been characterised using ensemble measurements. As the particles shape is a vital parameter for optical and quantum sensing application of HPHT NDs, I systematically investigated the shape properties of HPHT NDs using scattering-based ensemble measurements of particles suspended in water.

I compared the structure and shape the HPHT NDs over a wide range of sizes and from various suppliers. Using single-particle atomic force microscopy in combination with ensemble X-ray and light scattering techniques, I demonstrated that a typical HPHT ND in the 50 – 150 nm size range has an aspect ratio of three i.e. is three times thinner (e.g. in z) than it is wide (e.g. in the x-y plane). This high aspect ratio of FNDs is important for many quantum sensing measurements as it will enable enhanced sensitivities compared to spherical or other isotropic particle geometries.