(122c) Advanced Characterization of Non-Spherical Nanoparticles

Excellent product properties of the nanoparticles (NPs) including optoelectronic properties are very often dependent on their disperse properties [1,2]. An accurate evaluation of the disperse properties including shape, size and density therefore becomes a necessity. However, simultaneous characterization of these disperse properties still remains a major challenge. Size evaluation is also quite complicated for non-spherical NPs, where equivalent diameters are used to represent the size.

Nanorods (NRs) and other two dimensional NPs like platelets, nanotubes, ellipsoids etc. require two parameters for complete size characterization. Gold NRs are very popular due to their enhanced optical properties while Zinc oxide NRs are relevant in a number of nanodevices and thin film applications. Ellipsoidal nanoparticles are also well researched for their enhanced surface plasmon resonance. Although electron microscopy based techniques are the obvious choice for their characterization, these methods require analysis of a large number of particles for statistical significance. Dynamic light scattering and similar techniques are more complicated for non-spherical NPs and the hydrodynamic diameters obtained need to be corrected for the effect of rotational diffusion. This cannot be accomplished with only one measurement technique.

There have been commendable advances in the recent past on the characterization of nanoparticles dispersed in colloidal and gas phase systems. Analytical ultracentrifugation equipped with an UV-Vis multiwavelength detector (MWL-AUC) is a powerful and highly accurate tool for the multidimensional analysis of NPs dispersed in colloids [2]. AUC measures the motion of concentration boundaries depending on the applied centrifugal field. The motion of the concentration boundaries can be related to the sedimentation coefficient distribution of the NPs which depends on their mass and hydrodynamic diameter.

Owing to the typical sizes of the NPs, their motion in gas phase systems cannot be represented by continuum equations and kinetic gas theory comes into effect [3]. Scanning mobility particle sizer (SMPS) is the most commonly used measurement technique for characterizing and classifying aerosol NPs. It measures the electrical mobility of the NPs which are often dependent on the average projected area of the NPs in addition to the hydrodynamic diameter. For NRs, platelets and ellipsoidal NPs, AUC and SMPS measurements can be simultaneously analyzed to provide information on the two length parameters, namely the long and the short axes [4]. Colloidal NPs can be transferred to be dispersed in gas phase and this opens up new possibilities in NP characterization. It has been shown that the combination of these methods gives excellent estimates of the average length and diameter for gold and zinc oxide NRs in samples [4].

The method is being extended for nanoellipsoids, nanocubes and nanoplatelets. The combination of the techniques is also used to predict the variation in length and diameter of the nanorods in a sample where the cumulative distributions obtained from AUC and SMPS measurements are combined to provide insights on the length and diameter distributions of the nanorods.

References

1. J. Walter, T. J. Nacken, C. Damm, et al., Small, 2015, 11, 814.
2. J. Walter, G. Gorbet, T. Akdas, et al., Analyst, 2017, 142 (1), 206.
3. C. Zhang, T. Thajudeen, C. Larriba, et al., Aerosol Sci. & Tech.,2012, 46(10),1065.
4. T.Thajudeen, J. Walter, R.Srikantahrajah, et al., Nanocale Horizons, 2017.