(Nanowerk News) A German-Chinese research team with participants from the University of Konstanz has discovered how catalysts and many other nanoplatelets can be produced in an environmentally friendly way from readily available materials and in sufficient quantities.
Hydrogen is considered as an environmentally friendly alternative to conventional fossil fuels. Until now, expensive and rare substances such as platinum were required for their catalytic production, for example via electrolytic water splitting. A more readily available catalyst could enable the production of large quantities in the future. The research team of Helmut Cölfen (Physical Chemistry) and Peter Nielaba (Statistical and Computational Physics) at Konstanz University have developed a general method for generating two-dimensional nanoparticles from easily accessible materials, together with researchers from Ocean University of China, Qingdao (China) and Fritz Haber Institute of the Max Planck Society, Berlin (Germany).
Two-dimensional nanoparticles have high catalytic potential, therefore this synthetic route is suitable for producing highly active catalysts.
The corresponding synthesis process is carried out in a simple aqueous solution. No toxic additives or extremely high temperatures, which are energetically unfavorable, are required. The process is controlled simply by varying the concentration of the components and by temperature regulation.
The research team managed to mold more than 30 different compounds into two-dimensional shapes using this method, which has been described for the first time in a scientific journal. Natural Synthesis (“Growth strategy for solution phase growth of two-dimensional nanomaterials via an integrated model”).
Advantages of two-dimensional nanoparticles
Two-dimensional (2D) nanoparticles have a large number of surface atoms, which have different properties from the atoms in the particle. The bonding of surface atoms is unsaturated because the surface has no nearest neighbor atoms forming bonds within the particle. This causes surface or interfacial tension. Since this unsaturated state is quite energy consuming for the entire system, the nanoparticles try to cluster together to saturate the bonds and minimize surface area.
However, if the surface bonds remain unsaturated, this results in increased chemical reactivity. The number of unsaturated bonds is very high in two-dimensional nanoparticles because they have unsaturated bonds not only at the top and bottom, but also at the sides and edges. This makes them very attractive for catalysis, which plays a major role in chemistry. However, the required nanocrystals are difficult to prepare due to the unfavorable energy state on the surface.
Two-dimensional nanoparticles are anisotropic, and their properties depend on the orientation of their building blocks. The crystal lattice of the particles determines the direction of their growth. If the nanoparticles have a layered crystal lattice like that of clay, the particles grow two-dimensionally. However, materials that are advantageous for catalysis rarely adopt a two-dimensional shape of their own. If the crystal lattice determines that the crystals grow rapidly along the two crystal axes, two-dimensional nanoparticles can be easily synthesized. Then, only a few molecular building blocks are needed in solution to grow the nanoparticles in two dimensions. If the crystal grows in the other direction just as fast or only slightly slower, it will take on a three-dimensional shape.
How nanoparticles grow in two dimensions
The research team has discovered how the concentration of the building blocks of molecules in solution can be used to manipulate this process: If the concentration of the building blocks is increased, the principle “what grows fast also consumes more material” comes into play: The distance between the axes of the fast-growing and faster-growing crystals slowly increases, producing two-dimensional particles.
The method for increasing the concentration of the building blocks does not work if the growth rates along the different relevant crystal axes are roughly the same. In this case researchers use other parameters. The growth rate of the crystal surface depends exponentially on temperature. If the temperature of the solution is changed by even a few degrees, the difference in growth rates between the slow-growing and fast-growing crystal surfaces 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 was able to identify metals in many groups, more than 30 in total, that exist in two dimensions as oxides or hydroxides, but also acids, sulfides, oxychlorides and phosphates. The advantages of this general approach, which have been described for the first time: In most cases, the material is produced at room temperature in water – without toxic solvents or high temperatures.
In addition, the yield of catalytic materials is highly measurable. In the lab, researchers are working on a multigram scale. To produce catalysts in large quantities using readily available substances, all that is required is a sealed vessel – rather than special equipment such as pressure vessels.
Experiments confirm the theory
Experimental studies also show how theoretical knowledge can be put into practice. The experiments confirm the theoretical simulations carried out by Peter Nielaba’s team in a joint project with the Cölfen team at the Collaborative Research Center 1214 “Anisotropic Particles as Building Blocks: Sewing Shape, Interaction and Structure” at the University of Konstanz. Physicists have accounted for variations in component concentrations and temperature. “The calculations and what we found experimentally really agree,” concludes Helmut Cölfen.