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Which correctly describes the principle of the BET method for measuring the specific surface area of a powder?

  1. ANitrogen adsorption is measured at about 77 K at various pressures, and the monolayer capacity from the BET equation is converted to a surface area
  2. BThe crystallite size is found from XRD peak widths with the Scherrer equation and converted to a surface area by assuming that the particles are spheres
  3. CThe powder is dispersed in a liquid, laser light scattering is measured, and the surface area is calculated from the size distribution assuming spheres
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Answer: A. Nitrogen adsorption is measured at about 77 K at various pressures, and the monolayer capacity from the BET equation is converted to a surface area

The BET method physically adsorbs a gas such as nitrogen onto the powder at liquid-nitrogen temperature (about 77 K) and measures the amount adsorbed while varying the relative pressure. The BET equation, which treats multilayer adsorption, is usually fitted to data in the relative pressure range of about 0.05–0.3 to obtain the monolayer capacity vm, and multiplying by the area occupied by one nitrogen molecule (about 0.162 nm²) gives the surface area. Its distinctive feature is that it gives the actual surface area, including the inner surfaces of open pores that gas molecules can enter. What XRD peak widths give is the crystallite size by the Scherrer equation, not the surface area of the particles themselves. Laser diffraction and scattering give the size distribution of particles (including agglomerates), but do not reflect surface roughness or pores.

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More Nanomaterials questions

  1. For an alumina nanopowder, the primary particle diameter observed by TEM was about 30 nm, and the particle size obtained from the BET specific surface area assuming spheres was also about 30 nm. However, the mean particle size measured by dynamic light scattering after dispersing it in water was about 400 nm. Which interpretation of these results is most appropriate?
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