The solar-powered home hydrogen fueling station just got a step closer to reality.
Scientists at Rutgers University–New Brunswick have discovered that star-shaped gold nanoparticles coated with a titanium semi-conductor can capture the energy in sunlight to produce hydrogen over four times more efficiently than existing methods. Even better, they have demonstrated a low temperature process for making the new material.
The trick lies in the points of the star. The star shape makes it possible for even low-energy wavelengths of light in the visible or infrared range to excite an electron in the nanoparticle. After a beam of light "excites" the particles in the material, the points efficiently inject that electron into the semi-conductor where it can react with the water molecules to free up gaseous hydrogen. This is known as photocatalysis.
There's a lot more physics in the details, including localized surface plasmon resonance (LSPR) which is a fancy way of describing how the photon of light affects the flow of electrons in the metal particle, a bit like tossing a stone into a pond produces ripples in the water. If you imagine the peaks of each ripple of water as having the energy to effect a change (such as lifting a rubber ducky), you can imagine how the peak in a wave of electron flow could have the energy to fling an electron at a water molecule where it can break the chemical bond holding the hydrogen and oxygen together.
There's some good luck here too. It turns out that the semi-conducting titanium oxide forms a defect-free interface with the gold in the nanostar when a thin layer of the crystalline titanium compounds is grown on the stars at low temperature. If this was not possible at low temperature, the production of the material would face more serious obstacles, because the gold nanostars get messed up by higher temperatures. It is important that the rays of the star remain long and narrow after the coating process, so that the ripple effect in the electron flow is optimized and the subsequent injection of an electron into the water reaction is promoted.
This hot electron injection technique has a lot of potential. In addition to generating hydrogen from water by photocatalysis, such materials could serve useful in converting carbon dioxide or for other applications in the solar or chemical industries.