(Invited) Plasmon-Driven Nitrogen Photofixation
Authors: Wang Jianfang 
Published: August 2024

Ammonia is one of the most important chemicals as well as an efficient energy carrier. The nitrogen element is indispensable for nearly all living species on the earth. However, dinitrogen molecules that are abundant in the atmosphere cannot be used directly by living species. They need to be converted into ammonia or nitrate for use. Such conversion is known as nitrogen fixation. Because natural nitrogen fixation is insufficient for sustaining all population on the earth, the industrial Haber-Bosch process has been developed for producing ammonia from dinitrogen and dihydrogen. The process consumes a large amount of energy and generates a large amount of carbon dioxide. Photocatalytic nitrogen fixation can operate under mild conditions and uses solar energy and water. It offers an intriguing alternative approach for the conversion of atmospheric dinitrogen into ammonia.
Plasmonic metal nanocrystals can effectively absorb light and efficiently generate hot charge carriers. Compared to traditional interband excitation in semiconductors, plasmon excitation offers a new approach for generating hot charge carriers, which can thereafter be utilized for various physical and chemical processes. The use of plasmonic hot charge carriers for photocatalysis has become a hot research topic in the recent years. Plasmonic hot charge carriers can not only enhance the reaction yield and selectivity, but also introduce new reaction pathways. Moreover, the plasmon resonance wavelengths of various nanoparticles can be synthetically adjusted over a broad range that surpasses the solar spectrum.
We have demonstrated the powerfulness of localized plasmon resonance on driving photocatalytic reduction of atmospheric nitrogen, where plasmonic hot electrons play an essential role. First, Au nanoparticles are anchored on ultrathin titania nanosheets with oxygen vacancies. The oxygen vacancies chemisorb and activate nitrogen molecules, which are subsequently reduced to ammonia by plasmonic hot electrons generated from the Au nanoparticles. Our synthesized all-inorganic hybrid structures mimic the function of biological nitrogenase enzymes, where the plasmonic Au nanoparticles play the role of the Fe proteins and the titania nanosheets play the role of the MoFe proteins. Second, we have extended the biomimetic idea into another popular semiconductor. A plasmonic hybrid catalyst is produced by uniformly embedding Au nanoparticles in the mesopores of nitrogen-deficient hollow carbon nitride spheres. The carbon nitride spheres possess abundant nitrogen vacancies, which serve as the sites for nitrogen chemisorption and activation, and capture photoexcited electrons stemmed from the carbon nitride spheres and the plasmon resonance of the embedded Au nanoparticles for the efficient reduction of the adsorbed nitrogen molecules to ammonia. Third, when metal-semiconductor hybrid nanostructures are designed as photocatalysts for nitrogen fixation, the introduction of a Schottky barrier at the junction is unavoidable. This Schottky barrier will undoubtedly lower the utilization efficiency of plasmonic charge carriers because only plasmonic hot charge carriers with sufficient energies and appropriate momenta can overcome the barrier and inject into the semiconductor. In addition, noble metals are well-known to be expensive. We have synthesized different metal oxide nanoparticles that are degenerately doped and thus exhibit strong plasmon resonance and demonstrated Schottky-barrier-free plasmonic photocatalysts for nitrogen fixation. The excitation of the plasmon resonance in these metal oxide nanoparticles can efficiently generate hot charge carriers. Because of the abundance of defects in these degenerately doped semiconductor nanoparticles, the photogenerated hot charge carriers can rapidly migrate to the defect sites and get trapped there. In this way, the lifetime of the plasmonic hot charge carriers is Moreover, these metal oxide nanoparticles possess active sites for adsorbing and activating dinitrogen molecules. Therefore, they function in a “one-stone-two-birds” manner. As a result, a record-high solar-to-chemical energy conversion efficiency has been achieved.