2.1 Bet surface area analysis of nanoparticles  (Page 7/4)

Introduction

In the past few years, nanotechnology research has expanded out of the chemistry department and into the fields of medicine, energy, aerospace and even computing and information technology. With bulk materials, the surface area to volume is insignificant in relation to the number of atoms in the bulk, however when the particles are only 1 to 100 nm across, different properties begin to arise. For example, commercial grade zinc oxide has a surface area range of 2.5 to 12 m ² /g while nanoparticle zinc oxide can have surface areas as high as 54 m ² /g . The nanoparticles have superior UV blocking properties when compared to the bulk material, making them useful in applications such as sunscreen. Many useful properties of nanoparticles rise from their small size, making it very important to be able to determine their surface area.

Overview of bet theory

The BET theory was developed by Stephen Brunauer ( [link] ), Paul Emmett ( [link] ), and Edward Teller ( [link] ) in 1938. The first letter of each publisher’s surname was taken to name this theory. The BET theory was an extension of the Langmuir theory, developed by Irving Langmuir ( [link] ) in 1916.

The Langmuir theory relates the monolayer adsorption of gas molecules ( [link] ), also called adsorbates, onto a solid surface to the gas pressure of a medium above the solid surface at a fixed temperature to [link] , where θ is the fractional cover of the surface, P is the gas pressure and α is a constant.

The Langmuir theory is based on the following assumptions:

• All surface sites have the same adsorption energy for the adsorbate, which is usually argon, krypton or nitrogen gas. The surface site is defined as the area on the sample where one molecule can adsorb onto.
• Adsorption of the solvent at one site occurs independently of adsorption at neighboring sites.
• Activity of adsorbate is directly proportional to its concentration.
• Adsorbates form a monolayer.
• Each active site can be occupied only by one particle.

The Langmuir theory has a few flaws that are addressed by the BET theory. The BET theory extends the Langmuir theory to multilayer adsorption ( [link] ) with three additional assumptions:

• Gas molecules will physically adsorb on a solid in layers infinitely.
• The different adsorption layers do not interact.
• The theory can be applied to each layer.