Two-dimensional nanoparticles with great catalytic potential

Hydrogen is an environmentally friendly alternative to traditional fossil fuels. To date, expensive and rare materials such as platinum are required for its catalytic production, for example by water electrolysis. Easily available catalysts could make mass production possible in the future.

The research team from Helmut Cölfen (physical chemistry) and Peter Nielaba (statistical and computational physics) at the University of Konstanz have developed a general method for producing 2D nanoparticles from accessible materials, together with researchers from China Ocean University, Qingdao (China) and the Fritz Institute Haber of the Max Planck Society, Berlin (Germany).

2D nanoparticles have a high catalytic capacity, which is why this synthetic pathway is suitable for producing particularly active catalysts.

The corresponding synthesis process takes place in a simple aqueous solution. No toxic additives or particularly high temperatures, which are energy unfavorable, are required. The process is controlled simply by changing the concentration of ingredients and regulating the temperature. The research team succeeded in molding more than 30 different compounds into two-dimensional shapes using this method, which is now described for the first time in the journal. Synthesis of nature.

The advantage of two-dimensional nanoparticles

Two-dimensional (2D) nanoparticles have a large number of surface atoms, which have different properties than the atoms inside the particle. The bonds of the surface atoms are unsaturated because the surface lacks immediate neighboring atoms with which bonds are formed within the particle. This leads to surface or superficial tension. Since this unsaturated state takes up a lot of energy for the system as a whole, the nanoparticles try to aggregate together to saturate the bonds and reduce the surface area.

However, if the surface bonds remain unsaturated, this leads to an increase in the chemical reaction. The number of unsaturated bonds is particularly high in 2D nanoparticles because they contain unsaturated bonds not only on the top and bottom, but also on the sides and edges. This makes them particularly interesting for catalysis, which plays a major role in chemistry. However, the ordered nanocrystals are difficult to fabricate due to the unfavorable energy state at the surface.

Two-dimensional nanoparticles are anisotropic, and their properties depend on the orientation of their building blocks. The particle’s crystal lattice is crucial to its growth direction. If the nanoparticles contain lattice crystal layers as in clay, the particles grow two-dimensionally. However, materials of choice for catalysis rarely adopt the two-dimensional shape on their own.

If the crystal lattice dictates the crystal to grow rapidly along two crystallographic axes, 2D nanoparticles can be easily fabricated. Then, only a few molecular building units in solution are required to grow 2D nanoparticles. If the crystals grow in other directions at the same speed or slightly slower, the crystals take on a three-dimensional shape.

How do you grow two-dimensional nanoparticles?

The research team discovered how the concentration of molecular building units in solution could be used to manipulate this process: If the concentration of building blocks increases, the principle of “what grows fast also consumes more material” comes into play: the distance between the fast-growing and slower-growing crystal axes, resulting in two-dimensional particles.

The method of increasing the building block concentration does not work if the growth rate along the various relevant crystallographic axes is approximately the same. In this case, the researchers use another parameter. The growth rate of crystal surfaces is highly dependent on temperature. If the solution temperature is changed even by a few degrees, the difference in growth rate between the slow and fast growing crystal faces will increase. As a result, the nanoparticles grow in two dimensions.

The method works for more than 30 elements of the periodic table

This general procedure works for many materials. In the periodic table, the German-Chinese research team has been able to identify metals in many groups, more than 30 in all, that take the two-dimensional form as oxides or hydroxides, but also acids, sulfides, chloride oxides and phosphates. The advantage of this general approach, which was first described: In most cases, the materials are produced at room temperature in water — without toxic solvents or high temperatures.

Moreover, the yield of catalysts is highly scalable. In the lab, the researchers are working on a multi-gram scale. To mass-produce catalysts using easily accessible materials, an airtight vessel is all that is needed – rather than special devices such as pressure vessels.

Experiments confirm the theory

An empirical study also shows how theoretical knowledge can be put into practice. The experiments confirm theoretical simulations conducted by Peter Nielaba’s team in a joint project with Cölfen’s team at the collaborative research center 1214 “Anisotropic particles as building blocks: shape design, interactions and structures” at the University of Konstanz.

The physicist had already taken into account the differences in the concentration of the ingredients and the temperature. “The calculations and what we found empirically match exactly,” concludes Helmut Colvin.

The work has been published in the journal Synthesis of nature.