Lightsources em 04/04/2016
Applications depend on the properties of different shapes
Nanotechnology, the production and manipulation of structures in the nanometric scale, is one of the current frontiers in the development of new materials. The applications of nanoparticles, which are comprised of up to hundreds of atoms, are diverse and can cover anything from encapsulation of drugs to catalysis in fuel cells. Their use will depend on the particle’s physical properties, which in turn depend on their shape.
In this context, scientists from the Universidade de Santiago de Compostela (Spain) and from the Instituto de Investigaciones Fisicoquímicas Teóricas Y Aplicadas, in La Plata (Argentine), used the experimental facilities of the Brazilian Synchrotron Light Laboratory (LNLS) to study the stability of different forms of gold nanoparticles – spheres, rods and prims – when exposed to low power ultraviolet radiation. They observed that, while spherical nanoparticles were stable, rods and prisms quickly corroded and dissolved back into gold ions.
They also investigated the role played by small silver clusters in the formation of the gold nanoparticles and on their photostability, an important property for the application of nanoparticles. One possible application is as catalyst for the degradation of organic contaminants in water and consequent hydrogen production, which can be used as fuel.
Different shapes and properties
Nanoparticles can be obtained from a variety of physical, chemical and biological processes in different shapes: spheres, cubes, tubes, rods, prims, and others. Each shape has a different set of physical – electric, magnetic, catalytic, optical – properties, which can be adjusted by modifying, for example, the ratio between their length and diameter.
Different physical properties lead to different applications. For example, nanofibers – thin and long nanorods – can be used for the production of transparent thin conductive films for touchscreens. Other nanostructures, with magnetic properties, can be used for cancer treatment through the so-called magnetic hyperthermia, by which nanoparticles heat up and destroy cancerous cells when subject to magnetic fields. In addition, drugs encapsulated by nanoparticles can be better directed and allowed to act only where they are needed, reducing their toxicity to the rest of the organism.
Gold atoms usually form spherical and cubic shaped nanoparticles. The addition of silver in different concentrations allows the growth of other shapes such as nanorods and nanoprims. In the scientists’ hypothesis, the anisotropic growth is caused by the presence of silver clusters, smaller than one nanometer, in the surface of the nanoparticles.
Then, the researchers exposed nanospheres, rods and prims to low-power ultraviolet radiation to investigate their photostability. They observed that, while spherical nanoparticles were stable, rods and prisms quickly corroded and dissolved back into gold ions.
According to the corresponding scientist, M. Arturo López-Quintela, the experimental stations of LNLS’ SXS beamline were essential to prove their hypothesis that the instability of nanorods and nanoprisms would be due to the same silver clusters that promoted their formation. Those clusters would act as catalysts for the degradation of the nanostructures.
The XANES (X-ray Absorption Near Edge Structure) technique was used to analyze the electronic structure of the silver clusters and their interaction with the gold nanorods. The results indicated the transference of electrons from gold to silver atoms when under irradiation by ultraviolet light.
When irradiated, the silver clusters release electrons that are quickly absorbed by the oxygen dissolved in the solution. The resulting charge imbalance causes oxidation and degradation of the clusters into gold ions.
These photocatalytic properties of the silver clusters attached to the nanoparticles can be used to degrade substances that are easier to oxidize than gold. For instance, by adding an organic compound to the nanoparticle solution, deoxygenating and then irradiating the ensemble, the compound oxidize in place of the nanoparticles. The released electrons, instead of reacting with the free oxygen (which was removed from the solution), react with the hydrogen in the water subsequently producing hydrogen gas.
That way, gold nanorods and nanoprisms could be used as catalysts for producing hydrogen for fuel, from, for example, organic contaminants in water.
Photostability of gold nanoparticles with different shapes: role of Ag clusters – Y.A. Attia, D. Buceta, F. Requejo, L.J. Giovanetti and M.A. Lopez-Quintela, Nanoscale, 2015, 7, 11273-11279. doi: 10.1039/C5NR01887K
Structure-Directing and High-Efficiency Photocatalytic Hydrogen Production by Ag Clusters – Y.A. Attia, D. Buceta, C. Blanco-Varela, M.B. Mohamed, G. Barone e M.A. López-Quintela, J. Am. Chem. Soc., 2014, 136, 1182–1185. doi: 10.1021/ja410451m