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.

A new and highly controllable technique to manufacture functional gradient nanocomposites has been reported in a recent article, published in Materials Horizons. The technique enables smooth and programmable stiff-to-compliant (or compliant-to-stiff) transitions within micro-scale regions.

This technique, developed by Dr. Zhengzhi Wang and colleagues at Wuhan University, is based on a typical two-step process:

1. Use a magnetic field to generate a desired concentration gradient of magnetic-responsive nano-reinforcements inside a polymer matrix in liquid state.
2. Polymerize and solidify the redistributed polymer nanocomposites.

Using this technique, Wang et al. fabricated various biomimetic interfaces and surfaces and found that the functional gradient designs, with reduced stress concentrations, simultaneously improved the mechanical strength and durability over an order of magnitude compared with the traditional homogeneous counterparts.

The magnetically-actuated functional gradient nanocomposites can be further integrated into advanced additive manufacturing techniques to create a wide range of functional heterogeneous materials with unprecedented combinations of mechanical properties.

TEM image of functional gradient nanocomposites for compliant-stiff-compliant transitions

Mengye Wang is a member of the Community Board for Materials Horizons. Currently, she works as a postdoctoral fellow in the Department of Applied Physics at The Hong Kong Polytechnic University. She has a keen interest in advanced materials for environmental and energy applications, including photocatalysis and electrocatalysis.

Hybrid double perovskites have recently gained a vast amount of attention in the research area of photovoltaics as lead-free alternatives to the ground-breaking parent material [CH₃NH₃]PbI₃.¹ In double perovskites with the general formula A₂B’B’’X₆, two Pb²⁺ cations are effectively replaced with a monovalent B’ and a trivalent B’’ cation. Among the many fascinating properties of hybrid inorganic-organic perovskites, it is arguably the combination of strong light absorbance and long carrier lifetimes that make them so interesting for photovoltaic applications and light-emission devices. Recent experimental and theoretical studies on [CH₃NH₃]PbI₃ revealed a direct-indirect character of the bandgap, i.e. [CH₃NH₃]PbI₃ exhibits a direct band gap which is only approximately 47-60 meV higher in energy than the indirect band gap. Presumably, this is the origin of the paradox of strong absorption and long charge carrier lifetimes. When now turning our attention to lead free double perovskites, examples such as [CH₃NH₃]₂KBiCl₆ and [CH₃NH₃]₂AgBiBr₆ exhibit an indirect band gap,¹ hence unfavourable light absorption properties. The symmetry mismatch that leads to the indirect band gap in such materials was recently studied by D. O. Scanlon and A. Zunger theoretically.^2,3 Consequently, it is important to ask the question: is it possible to experimentally design a direct band gap in double perovskites?

In the recent article in Materials Horizons, ‘Designing Indirect-Direct Bandgap Transitions in Double Perovskites’,⁴ T. M. McQueen and co-workers have tackled this important question, studying the solid solution Cs₂AgIn_1-xSb_xCl₆ as a prototypical example. By wisely choosing B’ and B’’, a direct band gap in Cs₂AgInCl₆ has been achieved. The beauty lies in the simplicity of the concept – the understanding of band theory, i.e. symmetry and formation of bands with s-type and p-type character, see Figure 1. Going along the solid solution Cs₂AgIn_1-xSb_xCl₆, the valence band remains basically unchanged, whilst the character of the conduction band is continuously altered from s-type to p-type character. Clearly, the use of a chloride and in turn the ionic character of the solid with a band-gap larger than 3.5 eV limits the application of Cs₂AgInCl₆ in optoelectronics. However, the results depict a textbook example of how to manipulate properties in crystalline materials and open exciting opportunities for going forward in the field. For instance, one can easily envision a computational screening study of potential A₂B’B’’X₆ perovskites by using symmetry-based descriptors. Furthermore, it is important to note, that band engineering is a common concept in related areas of materials science, such as thermoelectrics and magnetic materials, and is a common tool for solid state chemists in general. Therefore, it is refreshing to see that band engineering now enters arguably one of the most fascinating developments of materials science within the last decade.

Figure 1. Schematic presentation of the orbital overlap (a) and the energy as a function of k for bands of s and p-σ orbitals (b) in a linear chain.

[1] F. Wei, Z. Deng, S. Sun, F. Xie, G. Kieslich, D. M. Evans, M. A. Carpenter, P. D. Bristowe, A. K. Cheetham ‘The synthesis, structure and electronic properties of a lead-free hybrid inorganic-organic double perovskites (MA)₂KBiCl₆ (MA = methylammonium)’ Mater. Horiz. 2016, 3, 328.

[2] C. N. Savory, A. Walsh and D. O. Scanlon ‘Can Pb-Free Halide Double Perovskites Support High-Efficiency Solar Cells?’ ACS Energy Lett. 2016, 1, 949.

[3] X.-G. Zhao, D. Yang, Y. Sun, T. Li, L. Zhang, L. Yu, A. Zunger ‘Cu-In Halide Perovskite Solar Absorbers’ J. Am. Chem. Soc. 2017, 139, 6718.

[4] T. Thao Tran, J. R. Panella, J. R. Chamorro, J. R. Morey, T. M. McQueen ‘Designing Indirect-Direct Bandgap Transitions in Double Perovskites’ Mater. Horiz. 2017, DOI: 10.1039/C7MH00239D.

Dr Gregor Kieslich is a Liebig-Fellow at Department of Chemistry, Technical University of Munich and is a member of the Community Board for Materials Horizons. He is an inorganic chemist focusing on crystal chemistry and structure–property relations in functional solids and hybrid frameworks: https://kieslichresearch.wordpress.com/

Photographs of different 3D shapes generated from the same stretched Nafion/PDA films treated with a NIR laser with different facular region shapes.

A new and highly adaptable way to make 3D structures in a wide range of different shapes has been reported in a new HOT article, published in Materials Horizons. The technique allows adjustment of both the shape transition process and the final shape at the same time.

The strategy, which Jian Ji’s group at Zhejiang University describe as a “writing” process, uses polymer nanosheets as blank “paper”. These are guided into making specific shape changes with a near infra-red laser beam “pen”. By controlling which shape changes happen at which time, several sheets can be woven together into a complex interlocking structure. Unlike previous techniques, the order of these changes can be easily altered to change the interlocking pattern.

Ji’s group used pre-stretched composite sheets of Nafion, a shape memory polymer, and polydopamine. When a NIR laser was applied to specific parts of the nanosheet, the polydopamine converted the light energy into heat. This caused internal stress between the heated and non-heated parts, triggering a shape transition of the sheet to relieve the stress. Changing the shape or intensity of the laser beam or where it was applied modulated the shape change, giving rise to a huge number of possible shapes.

Because the nanosheets don’t require special pretreatment before forming each particular shape, a variety of shapes can be made from the same starting material in quick succession. The technique could in future be used to make “personalised” components for the healthcare industry.

Read the full article here:
A ‘‘writing’’ strategy for shape transition with infinitely adjustable shaping sequences and in situ tunable 3D structures
Tingting Chen, Huan Li, Zuhong Li, Qiao Jin and Jian Ji
Mater. Horiz., 2016, DOI: 10.1039/C6MH00295A

Susannah May is a guest web writer for the RSC Journal blogs. She currently works in the Publishing Department of the Royal Society of Chemistry, and has a keen interest in biology and biomedicine, and the frontiers of their intersection with chemistry. She can be found on Twitter using @SusannahCIMay.