Sizing Nanoparticles - Determining the Particle Size of Nanomaterials by Micromeritics

Topics Covered

Background
Case Study
Summary

Background

Interest in nanoparticles has risen consistently and rapidly in recent years, but one problem has remained; what is the “best” way to determine the size of these materials? In other words, what is the particle size of my nano materials? In this study, we wanted to investigate what viable options were available to measure these types of materials. There are several techniques commercially available, but we choose to look only at dynamic light scattering (DLS), Microscopy (SEM or TEM), and finally a calculation of the average particle size from a BET surface area measurement.

Case Study

Samples of nanoparticles were chosen from a variety of industries and were simply identified as Nickel, Carbon, Titanium di-silicide (TiSi2), and Tantalum. Samples were analyzed using a Micromeritics TriStar 3000 gas adsorption instrument which calculated a BET surface area. From this, an average particle diameter was calculated assuming nonporous spherical particles and a theoretical density of the individual materials. Without going through the entire derivation, the equation for calculating the average particle diameter in nanometers is 6000/(BET surface area in m2 /g) x (density in g/cm3). The BET surface area and the average particle size results are found in the table below.

These samples were submitted to MVA Scientific Consultants for particle size analysis by TEM. Samples were also sent to various manufacturers of DLS instruments. Results varied with the energy applied in an attempt to disperse these samples. The results shown below were obtained after sonicating for at least two hours prior to analysis.


Tantalum


TiSi2


Nickel


Carbon

The images above are the results from the analysis using the transmission electron microscopy technique (TEM).

The table below summarizes the results obtained from each technique.

Sample Description BET S. A. (m2/g) (Micromeritics Analytical Services) Calculated Avg. Particle size from BET Mean Particle size Diameter by Volume Measured using TEM (MVA Scientific Consultants) Mean Particle size by DLS
Tantalum 89.73 8 nm 6.8 nm 316 nm
TiSi2 97.25 19 nm 12.6 nm 157 nm
Nickel 26.3 35 nm 23.6 nm 1.3 micrometers
Carbon 62.05 45 nm 31 nm Not Available

Summary

If the nanoparticles exist in a dry powder form and have formed aggregates such as the materials used in this study, then a reasonable primary particle size may be obtained by using either TEM or by calculating an average particle diameter using a measured BET surface area. It is important to point out that the difference observed between the calculated value from BET surface area and the TEM results is due to a small amount of surface being lost due to the primary particles forming aggregates.

If the aggregate particle size is preferred, a combination of microscopy or light scattering method, either dynamic or static light scattering, may be appropriate. So much energy is required to disperse the aggregate nanoparticle materials tested, that a primary particle size was not obtainable using DLS technique.

If your particles are suspended in a stable solution and have not formed aggregates, either TEM or dynamic light scattering method would provide good primary particle size information.

Micromeritics Instrument Corporation

Source: Micromeritics Instrument Corporation.

For more information on this source please visit Micromeritics Instrument Corporation.

Date Added: Jul 2, 2010 | Updated: Sep 11, 2013
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