Using gold nanoparticles that absorb sunlight over the entire visible region and even the near infrared light, researchers at the Tata Institute of Fundamental Research (TIFR), Mumbai, have been able to desalinate seawater to produce drinking water. Unlike the conventional reverse osmosis that is energy intensive, the gold nanoparticles require no external energy to produce potable water from seawater.
Using 2.5 mg of gold nanoparticles, the team led by Vivek Polshettiwar from TIFR’s Department of Chemical Sciences was able to use sunlight to heat the water to 85 degree C and generate steam to produce drinking water from seawater. Since the temperature reached is high, about 10% of seawater becomes steam (and hence drinking water) in about 30 minutes.
Alternatively, the gold nanoparticles can be used to convert carbon dioxide into methane. This happens when the light absorbed by the gold nanoparticles excites the electrons, and the excited electrons when transferred into carbon dioxide converts it into methane in the presence of hydrogen. The hydrogen comes from the water that is used as a reaction solvent.
“At present, the conversion of carbon dioxide to methane is low — about 1.5 micromole per gram. It is desirable to increase the conversion one-fold to millimole range. We are finding ways to improve the conversion rate,” says Prof. Polshettiwar. The results of the study were published in the journal Chemical Science.
The gold nanoparticles decorate the surface of 3D fibrous silica nanosphere structure. The silica nanospheres measuring 400-500 nanometres are first functionalised with amines. In the presence of a reducing agent, the gold chloride gets deposited on the silica nanospheres. The gold nanoparticles were made bigger though cycles of deposition.
“We used a different reducing agent that allows the gold to get deposited only on already formed nanospheres and not form new nanoparticles,” says Prof. Polshettiwar. “A weak reducing agent does not allow gold to reach a critical concentration for it to form new nanoparticles. But in certain channels of the fibrous material, the concentration of the gold precursors was sufficient to form new nuclei leading to the formation of new nanoparticles.”
The formation of smaller gold nanoparticles allows variation in size, which is essential for harvesting light. Each gold nanoparticle has an electron cloud on the surface that resonates with light. As the gold nanoparticles come closer when they grow bigger, the resonating electron cloud starts coupling together. This allows the gold nanoparticles to absorb light of different wavelength — visible and near infrared light.
While gold takes on different co lours including red at nanometre size, it is not possible to make it black by simply changing the size of the nanoparticle. “By changing the size and shape of gold nanoparticles we can tune the light absorption characteristic in the visible region. When we have plenty of gold nanoparticles in the vicinity of each other we can achieve completely absorption of visible light leading to black colour,” says Mahak Dhiman from TIFR and one of the first authors of the paper.
“There is huge electromagnetic field and thermal heat produced about 1 nanometre around the gold nanoparticle. This is called a hotspot. Such hotspots are present only when there is a gap between the gold nanoparticles. The gaps provide higher surface area,” says Ayan Maity from TIFR and the other first author. So more number of nanoparticles with gaps in between them are needed to generate more thermal hotspots.
“This is only a preliminary study. The next step should be to replace gold with some inexpensive metal to make it sustainable,” says Dhiman.
