Specialists from the Krasnoyarsk Scientific Center (KSC) SB RAS have developed new photonic crystal structures that increase the efficiency of photoinduced water splitting to produce hydrogen, local publication “City News” reported on July 8.
Researchers have created a new structure for photoelectrodes used to produce hydrogen from water using solar energy based on titanium oxide (TiO₂), which is highly stable, non-toxic and low-cost.
Junior researcher at the Institute of Chemistry and Chemical Technology SB RAS, graduate student Nikolai Zosko explained:
“Titanium oxide is chemically stable and corrosion-resistant, which ensures its durability during operation. This material is non-toxic and low-cost. Thus, titanium oxide has unique properties that make it one of the most promising photocatalysts for the photoelectrochemical decomposition of water.”.
The scientists presented the results of the study in a series of papers, including the latest article “Preparation and activation of TiO₂ photonic crystal structures for increasing the efficiency of the photoelectrochemical decomposition reaction of water,” published in the Journal of the Siberian Federal University of Chemistry.
Photoelectrochemical decomposition of water to produce hydrogen uses semiconductor photoelectrodes that, by absorbing solar energy, split water into hydrogen and oxygen. Using solar energy can reduce the cost of producing hydrogen as a green fuel.
Hydrogen fuel is pure hydrogen used in fuel cells and internal combustion engines. Its advantages are high efficiency and quiet engine operation.
Hydrogen-powered cars travel three times longer on less fuel than gasoline-powered cars. They are just as environmentally friendly as electric cars, but take less time to refuel.
However, currently, hydrogen fuel production requires high costs, largely to ensure the safety of this process. And this is where photoelectrochemical decomposition of water comes to the rescue.
This process occurs in special devices – photoelectrochemical cells, in which, when semiconductor photoanodes are irradiated with light, water molecules are broken down into hydrogen and oxygen. The latter is released into the environment, and the hydrogen is used to produce environmentally friendly fuel.
The efficiency of such a device depends on stable photoelectrodes, which must effectively absorb sunlight. But stable titanium dioxide is a wide-bandgap semiconductor, meaning it has a wide bandgap that allows it to operate only in the ultraviolet region of the spectrum.
Sunlight reaching the Earth’s surface contains only about 4% of ultraviolet light, so the Krasnoyarsk researchers were faced with the task of increasing the efficiency of titanium dioxide nanotubes in visible light. To solve this problem, scientists from the Kola Scientific Center of SB RAS and the Siberian Federal University proposed using a photonic crystal structure.
Almost all of us have encountered photonic crystal structures in our daily lives. Natural opal has such a structure. It consists of highly ordered rows of silicon oxide beads. Another example is the bright green backs of beetles that shine in the sun and are made of ordered particles of chitin.
The solid phase structure of photonic crystals leads to periodically changing dielectric constants of the medium, forming the so-called photonic band gap: in it light cannot propagate in the crystal due to its periodic structure.
The group velocity of photons near the photonic band gap decreases and the “slow photon” (“slow light”) effect occurs, as a result, the efficiency of capturing sunlight can be significantly improved.
“When working with photonic crystals, the problem is the production process itself. Basically, rather expensive and labor-intensive methods are used. In our work, we use a simple and inexpensive two-stage electrochemical anodization method.”– said Nikolai Zosko.
First, an embossed substrate was prepared that will serve as a template for future nanotubes. Next, the nanotubes themselves were grown using an alternating high- and low-voltage pulse. In this case, the morphology of the photonic crystal, which determines its optical properties, can be changed within significant limits by changing the voltage and pulse duration.
The nanotubes grown in this way resemble a bamboo stalk: they expand when pulsed with high voltage and contract when pulsed with low voltage. As a result, a photoelectrode with a new structure became more efficient at absorbing and converting light with a longer wavelength, including the infrared region of the spectrum.
Experiments showed that when bamboo-like nanotubes were used to create a photoelectrode, the efficiency of converting an incident photon into an electron compared to smooth nanotubes increased from 1.3 times in visible light to three times under ultraviolet irradiation, proportionally increasing the amount of hydrogen released.
Thus, scientists from Krasnoyarsk were among the first to demonstrate the possibility of increasing the photoelectrochemical activity of titanium nanofilms during water splitting by using photonic crystal nanostructures. At the same time, the proposed method of their manufacture is simple, environmentally friendly and quite effective, which makes the use of photoanodes from this material promising for hydrogen production.
Source: Rossa Primavera
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