With appropriate pretreatment, aerogels could serve as efficient, visible light-active photocatalysts for industrial use. The Laboratory for Multifunctional Materials at ETH Zurich, headed by professor Markus Niederberger, demonstrated that nanoparticle-based aerogels can be doped with nitrogen to make them visible-light active for photocatalytic hydrogen (H₂) production.

The group works with aerogels composed of crystalline semiconductor nanoparticles. In its work, it surmised that if photocatalysis is to be more efficient and useful to industry, the catalyst must be able to absorb light from a broad range of wavelengths.

The material of choice for photocatalysis, the semiconductor titanium dioxide (TiO₂), only absorbs lights in the ultraviolet (UV) wavelength, which comprises only about 5% of the spectrum. While searching for a way to optimize TiO₂ for photocatalysis, researcher Junggou Kwon discovered that doping the aerogel with nitrogen caused individual oxygen atoms in the aerogel to be replaced with nitrogen atoms, making it possible for the aerogel to absorb more visible portions of the spectrum.

Kwon produced an aerogel using TiO₂ nanoparticles and small amounts of the noble metal palladium. She then placed the aerogel in a reactor and infused it with ammonia (NH₃) gas. The NH₃ gas caused individual nitrogen atoms to embed themselves in the crystal structure of the TiO₂ nanoparticles.

Plasma-enhanced chemical vapor deposition at low temperature using NH₃ gas enabled the nitrogen to be efficiently integrated into the preformed TiO₂ aerogels, improving their optical properties while preserving their favorable characteristics — their large surface area, extensive porosity, and nanoscale properties.

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Tweezers hold a tablet-shaped aerogel composed of palladium and nitrogen-doped TiO₂ nanoparticles. ETH Zurich researchers developed a photocatalyst made from an aerogel that could enable more efficient hydrogen production. The fabrication method and aerogel require sophisticated pretreatment of the material. Courtesy of Markus Niederberger/ETH Zurich.

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To test whether the modified aerogel could increase the efficiency of a chemical reaction — specifically, the production of H₂ from methanol and water — Kwon placed the aerogel monolith in a specially built reactor. She added a vapor of water and methanol to the reactor and irradiated it with two LED lights. The gas mixture diffused through the aerogel’s pores, where it was converted into H₂ on the surface of the TiO₂ and palladium nanoparticles.

Aerogels with palladium produced up to 70× more H₂ than aerogels without the addition. The nitrogen-doped TiO₂ nanoparticle-based aerogels, when loaded with palladium nanoparticles, showed a significant enhancement in visible-light-driven photocatalytic H₂ production, demonstrating excellent stability continuously for a period of five days, at which point the experiment was concluded. “The process would probably have been stable for longer,” Niederberger said. “Especially with regard to industrial applications, it’s important for it to be stable for as long as possible.”

As a new class of photocatalysts with an exceptional 3D structure, aerogels offer the potential for additional gas-phase reactions beyond H₂ production. Compared to electrolysis, which uses electric current to drive chemical reactions, photocatalysis requires only light.

The aerogel developed by Niederberger’s group was done primarily as a feasibility study; further investigation is needed to determine whether the group’s technique could be used to produce H₂ on a large scale.

For example, the researchers still need to resolve how to accelerate the flow of gas through the extremely small pores of the aerogel.

“To operate such a system on an industrial scale, we first have to increase the gas flow and also improve the irradiation of the aerogels,” Niederberger said.

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A SEM image of the sponge-like internal structure of the aerogel produced by ETH Zurich researchers using a nitrogen-doping process. Courtesy of the Laboratory for Multifunctional Materials/ETH Zurich.

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The efficient photocatalytic performance demonstrated by the ETH Zurich group is a result of optimizing the doping conditions, which provided an appropriate trade-off between photoabsorption and charge separation efficiency, the researchers said. This gas-phase nitriding process for modifying preformed aerogel monoliths is a way to systematically improve the photocatalytic efficiency of nanoparticle-based aerogels under visible light, significantly expanding the application potential of aerogels as photocatalysts.

The research was published in ACS Applied Materials & Interfaces (www.doi.org/10.1021/acsami.1c12579).