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Light-Based and Lightweight, Aerogel Photocatalyst Enables Efficient Hydrogen Production

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ZURICH, Dec. 22, 2021 — 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.

Tweezers hold a tablet-shaped aerogel composed of palladium and nitrogen-doped TiO<sub>2</sub> 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.
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.
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.

A SEM image of the sponge-like internal structure of the aerogel. Courtesy of the Laboratory for Multifunctional Materials/ETH Zurich.
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.
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).

Published: December 2021
Glossary
photochemistry
The study of chemical reactions stimulated by the properties of light.
plasmonics
Plasmonics is a field of science and technology that focuses on the interaction between electromagnetic radiation and free electrons in a metal or semiconductor at the nanoscale. Specifically, plasmonics deals with the collective oscillations of these free electrons, known as surface plasmons, which can confine and manipulate light on the nanometer scale. Surface plasmons are formed when incident photons couple with the conduction electrons at the interface between a metal or semiconductor...
nano
An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
photochemistrychemicalsphotocatalysisResearch & TechnologyeducationindustrialEuropeETH ZurichplasmonicsaerogelMaterialsAdvanced Materialsnanonanoparticlesultraviolet lightUV light

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