A team of researchers from DOE’s Lawrence Berkeley National Laboratory (LBL) led by Samuel Mao has produced a photocatalyst that can extract hydrogen from water in the presence of light (including infrared and ultraviolet).
They used titanium dioxide (TiO2) whose lattice structure had been modified by changing its ordered pattern. They did that by hydrogenating the TiO2 crystals and found out that the disordered titanium dioxide they obtained was black, hence the idea that it absorbs light much better than the white one.
“We are trying to find better ways to generate hydrogen from water using sunshine,” says Samuel Mao, a scientist in Berkeley Lab’s Environmental Energy Technologies Division who led the research. “In this work, we introduced disorder in titanium dioxide nanocrystals, which greatly improves its light absorption ability and efficiency for the solar assisted extraction of hydrogen from water.”
Peter Yu and Lei Liu from Berkeley were the ones who explored how hydrogenation changed the electronic properties of titanium dioxide. They found out that by creating flaws in the lattice structure, the incoming photons excite the electrons easier, allowing them to jump the TiO2’s bandgap (an energy range in a solid where no electron states exist) and participate to the splitting of water.
“By introducing a specific kind of disorder, mid-gap electronic states are created accompanied by a reduced band gap,” says Yu, who is also a professor in the University of California at Berkeley’s Physics Department. “This makes it possible for the infrared part of the solar spectrum to be absorbed and contribute to the photocatalysis.”
By using a “sacrificial agent,” (a catalyst that gets consumed in the process) the scientists observed a 24 percent efficiency in the conversion of sunlight to hydrogen, but for the future they’ll be trying to eliminate the sacrificial agent and generate the same amounts of hydrogen just by using the black titanium dioxide and sunlight.
The black TiO2 didn’t show any signs of degradation during the 22-day testing period, so the researchers concluded it may as well be commercially-viable.