Photoreduction of N₂ is among the most interesting and challenging methods for N₂ fixation under mild conditions. In this study, we determined that nitrogen vacancies (NVs) could endow graphitic carbon nitride (g-C₃N₄) with the photocatalytic N₂ fixation ability. Photocatalytic N₂ fixation induced by NVs is free from the interference of other gases. The reason is that NVs could selectively adsorb and activate N₂ because NVs have the same shape and size as the nitrogen atom in N₂. In addition to this, NVs could also improve many other properties of g-C₃N₄ to support photocatalytic N₂ fixation. These new findings could elucidate the design of efficient photocatalysts for photocatalytic N₂ fixation.

Selective photocatalytic N₂ fixation dependent on g-C₃N₄ induced by nitrogen vacancies

As an indispensable energy source, ammonia plays an essential role in agriculture and various industries. Given that the current ammonia production is still dominated by the energy-intensive and high carbon footprint Haber-Bosch process, photocatalytic nitrogen fixation represents a low-energy consuming and sustainable approach to generate ammonia. Heterostructured photocatalysts are hybrid materials composed of semiconductor materials containing interfaces that make full use of the unique superiorities of the constituents and synergistic effects between them. These promising photocatalysts have superior performances and substantial potential in photocatalytic reduction of nitrogen. In this review, a wide spectrum of recently developed heterostructured photocatalysts for nitrogen fixation to ammonia are evaluated. The fundamentals of solar-to-ammonia conversion, basic principles of various heterojunction photocatalysts and modification strategies are systematically reviewed. Finally, a brief summary and perspectives on the ongoing challenges and directions for future development of nitrogen photofixation catalysts are also provided.

Heterojunction photocatalysts for artificial nitrogen fixation: fundamentals, latest advances and future perspectives.

Photocatalytic Nitrogen Reduction by Ti3C2 MXene Derived Oxygen Vacancy-Rich C/TiO2

The unique InVO4 mesocrystal superstructure, particularly with cubical skeleton and hollow interior, which consists of numerous nanocube building blocks, closely stacking by stacking, aligning by aligning, and sharing the same crystallographic orientations, is successfully fabricated. The synergy of a reaction-limited aggregation and an Ostwald ripening process is reasonably proposed for the growth of this unique superstructure. Both single-particle surface photovoltage and confocal fluorescence spectroscopy measurements demonstrate that the long-range ordered mesocrystal superstructures can significantly retard the recombination of electron-hole pairs through the creation of a new pathway for anisotropic electron flow along the inter-nanocubes. This promising charge mobility feature of the superstructure greatly contributes to the pronounced photocatalytic performance of the InVO4 mesocrystal toward fixation of N2 into NH3 with the quantum yield of 0.50% at wavelength of 385 nm.

Hollow InVO4 Nanocuboid Assemblies toward Promoting Photocatalytic N2 Conversion Performance

Photocatalytic N2 fixation has emerged as a potential alternative pathway to synthesize NH3. However, poor light absorption, restricted charge separation, the extreme stability of N2 molecules caused by the N≡N bond, and the 6e--involved reactions are considered to be bottlenecks limiting the overall photocatalytic performance. In this context, the nature of the active sites plays a crucial role, which is associated with N2 adsorption, activation, and reduction. In this review, we provide forefront research on the development of highly active sites for photon-induced N2 fixation. Thus, three essential sections, namely (i) defect-rich photocatalysts, (ii) metal and nonmetal doping photocatalysts, and (iii) emerged co-catalysts, are highlighted to create a panorama of the materials approach to solar-driven N2 fixation. Finally, a summary and future outlook are given. It is hoped that this review will provide a broad picture and inspire the exploration of novel photocatalysts for efficient NH3 production.

Recent development of high-performance photocatalysts for N2 fixation: A review

Photocatalytic ammonia (NH3) synthesis from N2 and water driven by solar energy is a sustainable and environmentally friendly technology, which has gained considerable attention in recent years. In this review, the recent development in the fundamental understanding of photocatalytic NH3 synthesis and the methods of precise NH3 detection are summarized. More importantly the strategy for surface engineering and interface engineering of photocatalysts toward photocatalytic NH3 production has been thoroughly analyzed with the aim to stimulate critical thinking about the effective methodology for catalyst modification instead of exploring new materials. At the end the challenges and a few concerns are raised from the current reports and future perspectives in this research field are discussed targeting to clarify the reliability and reproducibility of the photochemical process and to direct the future research direction, such as flow reactor design and in-depth understanding of the underlying reaction pathway.

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Lunqiao Xiong

Papers

Progress and challenges in photocatalytic ammonia synthesis