Background/aim

Background:

Particles on a nano-scale (10^-9) display different physical properties to those on a maco-scale. For metal nanoparticles, these differing properties are due to the fact that the delocalised electrons in bulk metal are now geometrically restricted in nano-particles.

These geometrically constrained free electrons interact differently with light than in bulk metal, in a phenomenon of plasmon resonance. As size of nano-particles increases, the free electrons of metals can oscillate and absorb EMR at a different range of energies. This results in nanoprisms displaying different colours in solution depending on their size. This size can then be estimated by measuring the maximum wavelength on the absorbance spectrum of solutions containing silver nanoprisms.

The formation of nanoprisms results from an equilibrium of reduction and oxidation caused by reducing agents such as sodium borohydride and oxidizing agents such as hydrogen peroxide. As the two reduce and oxidize Ag+(aq) and Ag(s) in solution only the most stable shape of nanoparticles survives, taking the form of prisms (platelets)

The addition of a charge-stabilizer such as sodium citrate forms silver-citrate complexes that associates with silver ions on the surface of a growing nanoparticle. This ensures that the surface of growing nanoparticles are electrostatically negative, repelling eachother, preventing aggregation on a bulk level.

Adding reagents such as potassium bromide acts to limit the size at which the particle stops growing. as Br- ions strongly bind to the Ag+ ions on the surface of the nanoprisms, growth of the prisms is halted. The higher the concentration of Potassium Bromide, the smaller the nanoprisms are when aggregation is halted.

Using spectrophotometry, which creates an absorbance spectrum, proportional to the amount of light is absorbed by solution over a range of wavelengths, the size of the nanoprisms can be estimated by comparing the results to published experimental data.

According to the Beer-Lambert law, the absorbance of a solution at Lambda_maximum is proportional to the concentration of solution as well as the path length of the light travelling through solution. These variables are related by a constant epsilon, which is the extinction coefficient. Thus absorption=epsilon*concentration*path_length

Aim:

The aim of this practical is to demonstrate the formation of silver nanoprisms of various size by altering the amount of Potassium bromide added as a reagent. We are then to observe the difference in colour of the solutions containing nanoprisms, and account for their colour differences based on the size of nanoprisms formed.

Using spectrophotometry, we are to measure the lambda_maximum of one sample of nanoprisms in solution and create a Beer-Lambert plot by diluting the solution and plotting the absorbance against the concentration.

METHOD:

refer to https://au-mynotebook.labarchives.com/share/Alice%2520Williamson%2520CHEM1%2520SSP%25202015/MjIuMXwyODUzLzE3LTUvVHJlZU5vZGUvMTM3ODY0MjI5Mnw1Ni4x