Materials Horizons Blog

Is it feasible to convert nitrogen to ammonia using water and light?

With international collaboration, scientists from China and Singapore have looked into the aspect of the state-of-the-art engineering of photocatalysts for the nitrogen (N₂) fixation toward understanding the ammonia (NH₃) synthesis. The work was recently reported by Dr. Wee-Jun Ong and co-workers in Materials Horizons, which is featured on the Inside Front Cover in Volume 5, Issue 1 in 2018.

(a) An overview of the N₂ cycle and circulation of N₂ in various forms. (b) Diagram of the state-of-the-art milestone in the development of photocatalysts for N₂ fixation.  Images adapted from Chen et al., Mater. Horiz., 2018, Advance Article with permission from The Royal Society of Chemistry. 

N₂ is one of the most abundant gases on the Earth, comprising 78% in our atmosphere. Nonetheless, N₂ in the gaseous state cannot be effectively utilized by organisms. Therefore, N₂ must be “fixed” to make it valuable by breaking the strong NN triple bonds to transform it into a form that can be consumed by plants, animals and human beings. Hitherto, two typical methods to realize the fixation of N₂ are: (1) a natural and bacterial process, and (2) the Haber-Bosch process in industry. For the last 100 years, the N₂ conversion has led to the commercial fertilizer production and sustained the food intake supply for the worldwide population. However, the Haber-Bosch process consumes high pressures and temperatures, hence demanding a huge quantity (~2%) of the fossil fuel source. Thus, it is envisaged that the alternative process, which utilizes nanomaterials to absorb photon to mimic the natural photosynthesis in green leaves, can act as a paradigm shift for fixing nitrogen.

In this Review, the photo(electro)catalysts are classified based on the chemical compositions ranging from metal oxide to metal sulfide, bismuth oxyhalides, carbonaceous nanomaterials and other potential materials. The significance and relationship between the modification (e.g. nanoarchitecture design, crystal facet engineering, doping, and heterostructuring) and influences on the photo(electro)chemical activity of the catalysts are highlighted. Last but not least, to divert from the present laboratory-scale level to industrial applications, additional thoughts must be devoted to translating from academic research to practicality. How to amplify the yield of developed catalysts while preserving the intrinsic structures for the commercialization of “ammonia photosynthesis” is of universal challenge.

Wee-Jun Ong is a member of the Community Board for Materials Horizons. Currently, he works as a Staff Scientist in the Institute of Materials Research and Engineering (IMRE) at Agency for Science, Technology and Research (A*STAR) in Singapore. His research interests focus on photocatalytic, photoelectrochemical and electrochemical H₂O splitting, CO₂ reduction, N₂ fixation and H₂O₂ production for energy conversion and storage via experimental and density functional theory (DFT) studies. At present, he also serves as the Associate Editor of Frontiers in Chemistry and Frontiers in Materials, and an Editorial Board Member of Scientific Reports, Nanotechnology and Nano Futures. Check out his personal research website here.

The synthesis of oriented metal oxide nanodots on graphene oxide (GO) sheets using a surfactant-directed assembly strategy was recently reported by Professor Liqiang Mai and co-workers in Materials Horizons. This technique presents a versatile and general method for the synthesis of carbon-confined metal oxide nanodots, as well as a way to significantly enhance the energy storage properties of metal oxide nanocomposites.

Tin dioxide (SnO₂) is a promising candidate electrode material for high performance lithium-ion batteries, due to its high theoretical capacity. However, the large volume expansion caused by lithium intercalation into SnO₂ (up to 300%) results in poor cycling stability. In this article, metal-ligand bonds were used to immobilise SnO₂ nanodot precursors onto a functionalised GO surface. The nanodots were complexed with organic ligands and subsequently carbonised to form nanocrystalline carbon-confined metal oxide nanodots (C@SnO₂@Gr). Nanocrystallinity was achieved through the mismatched coordination of the organic ligands, as the distortion prevented aggregation of the precursor and crystal growth across larger areas.

When tested in a lithium-ion battery, the C@SnO₂@Gr nanodots were found to have exceptional cycling stability and capacity over 1200 cycles in comparison to similar carbonised SnO₂ nanocomposites. The material also demonstrated excellent rate capabilities, facilitated by its high surface area.

This paper highlights a promising method for the general synthesis of metal oxide nanodots, including SnO₂, Cr₂O₃, Fe₃O₄, and Al₂O₃. Furthermore, this method could be used to enhance the lithium storage capabilities of metal oxide materials for future energy storage applications.

Markus Müllner is a member of the Community Board for Materials Horizons and an academic at The University of Sydney. Markus and Honours student Olivia McRae are interested in nanostructuring electrode materials to advance performance of lithium ion batteries. https://www.polymernanostructures.com/

Researchers have long been interested in peering into the brain. Added to the inherent challenge of imaging through biological medium, the skull presents a major barrier that highly attenuates light.

To overcome this barrier, in a recent communication in Materials Horizons, Guo et al. have synthesized an organic nanoparticle for photoacoustic imaging with absorbance in the second near-infrared window. At this wavelength, there is relatively low scattering from tissue allowing for deeper penetration of light.

Photoacoustic images of a brain tumor after nanoparticle injection. The grey ultrasound image shows the skin and the skull margin, and the green signal indicates the nanoparticle distribution. Image adapted from Guo et al., Mater. Horiz., 2017, Advance Article with permission from The Royal Society of Chemistry. 

Nanoparticles were made from benzodithiophene-benzobithiadiazole donor-acceptor pairs co-polymerized and nanoprecipitated using biocompatible materials. When these imaging nanoparticles were applied to mice with orthotopic brain tumors, tumors 3.4 nm below the skull were resolved with a nearly 100-fold increase in photoacoustic signal compared to before intravenous administration of nanoparticles. The stable, high contrast photoacoustic imaging nanoparticle presented in this work offers a versatile platform for simple chemical modifications such as ligand targeting or drug loading.

Future work remains on the horizon to advance these materials for imaging through the ~5 mm thickness of human skulls.

Ester Kwon is a member of the Community Board for Materials Horizons. Currently, she works as an Assistant Professor in the Department of Bioengineering at University of California San Diego, USA. Check out her personal website here.