School of Chemical and Physical Sciences

Research project - Catalysis / shape control

Much recent light has been shed on the interesting size and shape dependent properties of nanomaterials. These can include quantum confinement (see Quantum Dots research section), magnetic susceptibility (see MRI contrast agents), catalytic enhancement, and surface plasmons.

In order to study many of these properties it is important to control a variation of size, shapes and compositions of various materials. However, many metal nanoparticles crystallize in very symmetric shapes. Through control of certain synthesis conditions during growth of asymmetric shapes, we may gain insight into the factors which influence nanoparticle formation. We work specifically on looking at the crystallography of the shape that is formed and defects present in the structure, as well as the growth mechanism and kinetics of the grown shapes.The way in which shapes assemble is also interesting, as pseudo-crystals of certain shapes may differ from those of typical spherical nanoparticles.

To date, we have produced a range of shapes in the group; focusing on the late transition and noble metals. These include cubes, tripods, and multiply branched particles from materials such as iron, platinum, palladium, nickel and ruthenium.

Shape control of nanoparticles

(A) Is a low resolution TEM image of an array of 12 nm nickel nanocubes - (B) Is a TEM image of decahedral rods standing on end.

While various types of nanomaterials have shown potential use in a wide range of applications, a prime example is for use as industrial or commercial catalysts. Catalytic properties are exploited daily in almost any industry from refining natural gas, oils, and drugs; to the reduction of air pollution or exhaust emissions. Currently many of the catalysts used can include rare earth metals such as gold or platinum. As the commodity price of these materials increase, it is important to find a suitable replacement that is both economically and commercially viable.

Nanocatalysts are already used for this purpose, as they have shown increased efficiency with a lower quantity of metal used. These nanoparticles are being studied mainly for their catalytic properties due to their size and shape dependent properties. In catalysis a decrease in the size of the metal to the dimensions of nanoparticles creates a greater surface area material. The shape of nanoparticles determines the exposure of different crystal faces which give rise to different reactivity and selectivity of the products from the catalysis. Other applications for these materials derive from the interesting optical properties that plasmonic noble metal nanoparticles have. Such an example is in Surface Enhanced Raman Scattering (SERS), where plasmonic coupling and surface enhancement is shape dependent.

While activity increases at smaller nanoparticle size, selectivity towards different products may change depending on the crystallography of the surface. By controlling the growth of these particles, we can achieve not only uniformity, but also unusual shapes with differing enclosed surfaces. By further adapting the composition of a nanoparticle we may further tune the catalytic properties to our desired catalytic system. We therefore aim to gain further understanding on how size, shape and composition effect catalytic properties.

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