How to control the shape of nanocrystals

Nanocrystals are crystalline particles with sizes between 1 and 1000 nm and possess many unique properties. These nanoparticles are applied in many areas of science and engineering, including drug delivery, chemical catalysis, biological sensors, and optical devices. This article deals with methods to control the shape of nanocrystals, mainly to regulate the complexity of nanoparticles.

Image Credit: Georgy Shafeev/

The increased application of nanocrystals has highlighted the need to synthesize nanocrystals of specific shapes and sizes. There are different shapes of nanocrystals including spherical, rod, cubic, oval, triangular, hexagonal helical, point and prism. The shapes of the nanocrystals determine their properties, which are used for various applications. For example, a nanoparticle with a lower surface edge and reduced coordination number exhibits higher chemical reactivity, a polarized surface, and greater catalytic activity.

Formation of nanocrystals of different shapes

Consider the development of zinc sulphide nanocrystals, which occurs via the diffusion of sulfur in a suspension of Zn nanocrystals. This reaction results in the formation of building blocks which undergo nucleation or self-assembly in specific nucleation centers to grow nanocrystals.

All building blocks contain a specific number of atoms and the varied combination and types of interactions (chemical bonding, electrostatic attraction or repulsion, and mechanical stress).

The formation of a nanocrystal of defined shape depends on the interaction between the building blocks during the self-assembly step. Therefore, by controlling the interactions between the building blocks, the shape of the nanocrystals can be regulated. Typically, scientists control the interactions between building blocks by using a mixture of surfactants and changing reaction temperatures.

Chemical methods to control the shape of nanocrystals

The chemical processes associated with the synthesis of nanocrystals involve two strategies, namely co-precipitation and chemical reduction. Both methods use a surfactant to control the growth process. Surfactants are molecules that contain a polar hydrophilic head and a hydrophobic hydrocarbon chain. A surfactant molecule with a small polar head and branched hydrocarbon chains forms spherical water-in-oil droplets or reverse micelles.

Co-precipitation involves the thermal decomposition of organometallic precursors. The size of the newly synthesized nanoparticle is determined by the ratio of surfactant to precursor, and the shape of the nanoparticles is controlled by the specific adsorption of the surfactant, which has been used to regulate the relative growth rates of the crystal faces.

In the co-precipitation method, the reaction temperature and time play a critical role in the synthesis of a particular nanocrystal. The chemical reduction method, which occurs in colloidal assemblies, is another effective process for the controlled size and shape synthesis of nanocrystals.

The shape of the nanoparticle depends on the shape of the template. For example, if the reaction is carried out in spherical reverse micelles, the newly synthesized nanoparticles are predominantly spherical. Although some exceptions to the above observation have been reported, in most cases, especially for metallic (silver, gold, copper, etc.) and semiconductor (silver sulphide, cadmium sulphide, manganese cadmium sulfide, etc.), the above observation has been verified.

Scientists have successfully grown elongated copper nanocrystals by conducting the reaction in cylindrical reverse micelles or an interconnected cylindrical phase. However, this process failed to perfectly deliver optimally shaped nanocrystals in every reaction.

To overcome this limitation, the researchers formulated a strategy by combining the matrix method based on surfactants and the capping method using specific salts or molecules. This method could control the shape of the newly synthesized nanoparticles. In some cases, this method could also be used to obtain non-spherical shaped nanoparticles in spherical reverse micelles.

One of the determining factors associated with the shape of nanoparticles is their crystallographic characteristics related to the core and closed surfaces. Thus, in a nutshell, the shape of a nanocrystal could be controlled via the structure of the precursor and the growth kinetics of the particles could be regulated by the specific adsorption capacity of the surfactant molecules or other molecules associated with the nanocrystal in growth.

Over the years, scientists have determined the factors influencing the formation of cubic nanocrystals. For example, ion selective adsorption is one of the main factors linked to the crystallographic nature of the surface and to the surface energies, which vary greatly depending on the precursors used.

Mathematical methods to control the shape of nanocrystals

Besides chemical methods, many software systems based on mathematical models have been designed to predict the specific shape of a nanocrystal. The researchers developed several mathematically distinct strategies based on Wulff to predict the shapes of nanocrystals. The modified Winterbottom model has been implemented in many commercial software packages to predict the shape of nanocrystals.

Hollow nanocrystals have gained popularity due to their applications in the development of lightweight materials. Moreover, these nanocrystals are also used in nanoelectronics, nano-optics and drug delivery systems. Scientists manipulate the morphology of these nanostructures by mechanisms similar to the Kirkendall effect. Nanopores can grow inside a nanocrystal by a mechanism of diffusion followed by nucleation.

A mathematical model was used to reach the thermodynamic equilibrium shape of a nanocrystal based on the minimization of surface energy. This model was first formulated by Wulff in 1901. According to Wulff’s construction, the equilibrium shape of single crystals could be determined based on a gamma plot (a plot of the surface free energy depending on orientation). The modified model was implemented for the development of software to visualize and quantify the shapes of nanocrystals.

Continue Reading: Analyzing Semiconductor Nanocrystals with Raman Spectroscopy.

References and further reading

Boukouvala, C., et al. (2021) Approaches to modeling the shape of nanocrystals. Nano-convergence, 8(26).

Liu, Z et al. (2018) Chapter 2 – Shape control in the synthesis of colloidal semiconductor nanocrystals. Assemblies of anisotropic particles, synthesis, assembly, modeling and applications, pp. 37-54.

Tao, RA (2008) Shape control of colloidal metallic nanocrystals, Nano Micro Small4(3).

Pal, S. et al. (2007) Shape control of nanocrystals, Journal of Physical Chemistry C111(44), p. 16071–16075.

Lisiecki, I. (2005) Size, shape and structural control of metallic nanocrystals, Journal of Physical Chemistry B109, 25, p. 12231–12244.

Pontes, FV et al. (2001) Control of the shape and size of colloidal nanocrystals: the case of cobalt, Science291(5511), p. 2115-2117. DOI: 10.1126/science.1058495

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