Metals absorb light using plasmon resonance (the collective motion of conduction electrons) and in metal nanoparticles this effect leads to very efficient light absorption. Earlier this year, a team led by Naomi Halas and Peter Nordlander at Rice University in Texas in the US discovered that gold nanoparticles immersed in liquids can absorb more than 80% of the power from sunlight and produce vapour. The phenomenon, which occurs even at light intensities as low as 106W/m2 – and without any separate source of applied heat – shows promise for a number of technology applications.

Although the researchers were previously unsure as to how exactly the nanoparticles produced steam, they have now found that light heats up the nanoparticles, evaporating the surrounding water. “A steam layer then forms around each particle, thermally insulating it from the liquid it is immersed in and drastically increasing the temperature of the particles,” explained Nordlander. “The thickness of this steam layer increases until a steady state is reached and we can detect this thickness indirectly by measuring the shift in plasmon resonance of the nanoparticle and by measuring its temperature.”

The finding leads on from another, now well known phenomenon, in which nanoparticles illuminated at a wavelength corresponding to their plasmon resonance can act as highly efficient localized heat sources at nanolength scales, he adds. The effect has already been exploited in a wide range of fields, including in energy generation, chemical catalysis, protein imaging and biomedicine.

Surface plasmon resonance shifts

The team, which includes researchers from the FOM Institute in Amsterdam, CSIC in Madrid, and the State Key Lab for Mesoscopic Physics at Peking University in Beijing, employed a technique called dark-field scattering to measure how the surface plasmon resonance of the nanoparticles shifts when the particles are exposed to focused sunlight and laser beams. The technique allowed the scientists to measure a range of parameters including: the radius of the nanobubbles that form around a nanoparticle surface when it is exposed to light; the internal nanobubble pressure; the surface temperature of the nanoparticle and how it behaves when exposed to varying intensities of light.

“We also found that micron-sized bubbles form thanks to two nanobubbles coalescing on adjacent particles,” said Nordlander. “Our experiments provide a detailed picture of how the particles produce steam when exposed to light – information that will be important for optimizing and developing this useful, sunlight-driven process,” he told nanotechweb.org.

The team says that it will now look at nanoparticles other than those made of gold to see if they produce steam just as efficiently. “We are also working on optimizing the shape and structure of the nanoparticles that can be used in this process,” revealed Nordlander.

The current work is detailed in Nano Letters.