Highly Efficient Porous Materials Inspired by Leaf Veins and Insect Spiracles

Highly Efficient Porous Materials Inspired by Leaf Veins and Insect Spiracles

By Ezinne Achinivu

Nature serves as one of life’s most creative and innovative scientists, and can sometimes provide us with examples of highly optimized and efficient systems. An international team of researchers led by Professor Bao-Lian Su, a life member of Clare Hall, University of Cambridge and who is also based at the University of Namur, Belgium and the Wuhan University of Technology in China, have discovered a way to emulate the natural porous structure found within leaves and insects in synthetic materials. This could yield highly enhanced materials and effective systems that demonstrate up to 20 times improvement in their performance with respect to applications like rechargeable batteries, photocatalysis and gas sensing.

From Murray’s Law to Murray Material

Plants and animals have similar tissues that contain multi-leveled networks of pores, which have evolved to maximize transport and reactions. The underlying physical principles of this optimized multi-level design are embodied in Murray’s law. Murray’s law defines the basic geometric features for porous materials with optimum transfer properties. Until now, researchers have not benefited from copying nature’s Murray networks into synthetic materials due to the challenges in developing these porous structures.

The team of researchers used a generalized Murray’s law to design and optimize the structures of the multi-leveled porous materials via a bottom-up approach. Well-defined microporous zinc oxide (ZnO) nanoparticles were used as the primary building blocks. They were are assembled into the multi-level porous Murray networks with interconnected macropores, mesopores and micropores. The secondary and tertiary mesoporous and macroporous networks were constructed by a bottom-up layer-by-layer evaporation-driven self-assembly procedure.

This concept has led to the development of materials, termed as the Murray material, whose pore sizes are multiscale (macro–meso–micro) and designed with diameter ratios obeying the revisited Murray’s law. The resultant materials have a multi-leveled network of pores, similar to that of the leaf veins and insect spiracles, which will improve their transport properties and reactivity. According to Professor Su, this new material could be beneficial to a wide array of porous materials and enhance functional ceramics and nano–metals used in environmental and energy applications.

Extending the Life of Batteries

The Li-storage performances of the ZnO Murray materials (ZOMM) were presented for application in Lithium ion batteries. The multi-leveled ZOMM exhibited a long-life cycling ability of up to 5,000 cycles with a reversible capacity that was 40 times that of a ZnO macroporous material and 25 times higher than that of state-of-the-art graphite. Utilizing the bio-inspired ZnO Murray network as an anode material delivered ultrahigh capacities and rate capabilities, along with long-life cycling stability. According to the research published in Nature, “Murray materials can meet both the criteria of fully space-filling pores, enabling ultra-short solid-phase Li-diffusion, and an optimum electrolyte-filled porous network favoring full and rapid Li-ion transfer with the electrolyte.”

Environmental Applications: Photocatalysis and Gas-Sensing

Photocatalysis, the acceleration of a chemical reaction by light, has widely been used for degrading organic pollutants based on fully dispersed nano-semiconductors—amongst a variety of applications. Using the bio-inspired 3D Murray networks, the relative photocatalytic rate of the ZOMM is compared to that of fully dispersed ZnO nanoparticles. The results show that the ZOMM yields one of the best photocatalytic activities ever reported for ZnO nanomaterials—displaying up to 17 times higher rates than that of the ZnO bulk sample. Additionally, the ZOMM film is also easily recyclable for repeated reactions.

The gas-sensing performance of the ZOMM under ethanol vapor exposure is also demonstrated. The fully branching and space-filling porous network enables the material to have increased surface area for oxygen adsorption, as well as, full and fast diffusion of gas molecules. According to the results, the ZOMM achieves a sensitivity of 457, the highest value ever reported for ethanol detection, which exceeds by at least 20 times that of commercial SnO2 sensors. The Murray material has a structure similar to the breathing network of insects, thereby, allowing gas detection over a broad concentration range with a higher, faster and much more stable response compared to state-of-the art material.

Enabling the Future

Professor Su and his research team anticipate that this strategy could be used effectively to design porous materials for energy and environmental applications. Utilizing a bio-inspired Murray network can enable a vast range of porous materials with enhanced transport properties and reactivity for high-impact applications in several industries.

Image courtesy of pixabay.com

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