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‘Golden sandwich’ has potential to make the world more sustainable

Scientists have developed a photoelectrode that has the potential to harvest 85 percent of visible light in a 30 nanometers-thin semiconductor layer between gold layers, which can convert light energy 11 times more efficiently as compared to most previous methods.

On the path to making a sustainable society, there exists an ever-increasing demand for revolutionary solar cells or artificial photosynthesis systems that can use visible light energy from the sun while only working with as few materials as possible.

The research team was led by Professor Hiroaki Misawa from the Research Institute for Electronic Science at Hokkaido University, and it has been trying to develop a photoelectrode that can harvest visible light across a wide spectral range by utilizing the potential of gold nanoparticles that are loaded on a semiconductor. But the mere application of a layer of gold nanoparticles did not provide a sufficient amount of light absorption, because these particles took in light with only a narrow spectral range.

In a study published in Nature Nanotechnology, the research team talks about how they sandwiched a semiconductor, a 30-nanometer titanium dioxide thin-film, between a 100-nanometer gold film and gold nanoparticles to improve light absorption. When the system is exposed to light from the gold nanoparticle side, the gold film functioned as a mirror. It trapped the light in a cavity between the two gold layers and led to the nanoparticles absorbing more light.

The researchers were surprised to see that more than 85 percent of all visible light was harvested by the photoelectrode- this is much more efficient than most previous methods that have tried to achieve the same. Gold nanoparticles are noted to exhibit a phenomenon called localized plasmon resonance which can absorb a certain wavelength of light.

“Our photoelectrode successfully created a new condition in which plasmon and visible light trapped in the titanium oxide layer strongly interact, allowing light with a broad range of wavelengths to be absorbed by gold nanoparticles,” says Hiroaki Misawa.

When these gold nanoparticles absorb light, it leads to additional energy that can trigger electron excitation in the gold, which then transfers these electrons to the semiconductor. “The light energy conversion efficiency is 11 times higher than those without light-trapping functions,” Misawa explained.

This boosted efficiency of the ‘gold sandwich’ also led to an enhanced water splitting: the electrons reduced hydrogen ions to hydrogen, while the remaining electron holes led to the oxidization of water that produced oxygen –which is a promising process to yield clean energy.

“Using very small amounts of material, this photoelectrode enables an efficient conversion of sunlight into renewable energy, further contributing to the realization of a sustainable society,” concluded the research team.

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