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By using nanoparticles of gold absorbing sunlight over the entire visible region and even near infrared light, researchers at the Tata Institute of Basic Research (TIFR) in Mumbai were able to desalt the water. seawater to produce drinking water. Unlike conventional reverse osmosis which consumes a lot of energy, gold nanoparticles do not require any external energy to produce potable water from seawater.
Using 2.5 mg of gold nanoparticles, the team led by Vivek Polshettiwar of the TIFR Chemistry Department was able to use sunlight to heat the water to 85 ° C and generate steam to produce water. drinking water from seawater. As the temperature reached is high, about 10% of the seawater becomes steam (and therefore drinkable) in about 30 minutes.
Alternatively, gold nanoparticles can be used to convert carbon dioxide to methane. This occurs when the light absorbed by the gold nanoparticles excites the electrons and as excited electrons, once transferred into carbon dioxide, convert it to methane in the presence of hydrogen. Hydrogen comes from the water used as a reaction solvent.
"Currently, the conversion of carbon dioxide to methane is low – about 1.5 micromoles per gram. It is desirable to increase the conversion from one millimeter to another. We find ways to improve the conversion rate, "says Professor Polshettiwar. The results of the study were published in the journal Chemical Science.
The gold nanoparticles decorate the surface of the 3D structure of the fibrous silica nanosphere. Silicon nanospheres of 400 to 500 nanometers are first functionalized with amines. In the presence of a reducing agent, the gold chloride is deposited on the nanospheres of silica. The gold nanoparticles were magnified during the deposition cycles.
"We used a different reducing agent that allows gold to settle on nanospheres already formed and not to form new nanoparticles," says Professor Polshettiwar. "A weak reducing agent does not allow gold to reach a critical concentration so that it can form new nanoparticles. But in some channels of fibrous material, the concentration of gold precursors was sufficient to form new nuclei leading to the formation of new nanoparticles. "
The formation of smaller gold nanoparticles allows for essential size variations for light recovery. Each gold nanoparticle has a cloud of electrons on the surface that resonates with the light. As gold nanoparticles get closer as they get bigger, the resonant electron cloud starts to couple. This allows gold nanoparticles to absorb light of different wavelengths – visible and near-infrared.
Although gold takes on different colors, including red to the size of the nanometer, it is not possible to blacken it by simply changing the size of the nanoparticle. "By changing the size and shape of the gold nanoparticles, we can adjust the characteristics of light absorption in the visible. When we have a lot of gold nanoparticles close to each other, we can achieve complete absorption of visible light, which leads to a black color, "says Mahak Dhiman of TIFR and one of the first authors of the article.
"There is a huge electromagnetic field and thermal heat produced at about 1 nanometer around the gold nanoparticle. This is called a hotspot. These hot spots are only present when there is a gap between the gold nanoparticles. The gaps provide a larger surface, "says Ayan Maity of TIFR and the other first author. More nanoparticles with empty spaces are needed to generate more thermal hot spots.
"This is just a preliminary study. The next step should be to replace gold with cheap metal to make it sustainable, "says Dhiman.
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