Towards cheap and sustainable energy sources by exploiting self-organized catalyst micro and nanostructures

Reaction diffusion precipitation processes in hydrogels can result in highly periodic micro- and nano-structured patterns known as Liesegang bands. These structures offer a very simple and cheap way to increase the surface area of certain catalyst materials. However, investigating the chemical origin of the precipitation bands is crucial for understanding and controlling the precipitation process. For this reason, the classical Liesegang system, discovered more than 100 years ago, was investigated in the first place. In this system two types of precipitation band, namely thick primary, and very dense, fine structured secondary bands are observed. Based on the results of the investigations (using STXM, SEM, SEM-FIB and EDX), a model was proposed, which describes the chemical origin of the precipitation bands. In addition, it was found that the fine structured so-called secondary bands originate from a different chemical system than the larger primary precipitation bands. Similarly to the classical Liesegang system, the origin of these fine structured bands was further investigated, resulting in a new Liesegang system containing only fine structured bands. It was found form material analysis using SEM, SEM-FIB, EDX and XPS that these highly periodic and very dense (up to 1 band per µm) bands consist of silver (Ag0) nano particles. By optimizing the parameters of this system it was also possible to control the precipitation process. It was also found that different hydrogels can be used and the structural design of special shaped precipitation bands and other structures (grids) was achieved. Since silver is known to be the most conductive element we wanted to combine these structures with hematite for solar induced water splitting applications in photo electrochemical cells (PEC). For this purpose, it was necessary to remove the hydrogel without destroying the precipitation structures. A simple sintering protocol was developed, which allows full removal of the hydrogel and full transformation of the particles forming the bands into pure silver bands. Several systems were tested to combine the silver bands with PEC photoanodes containing hematite. Unfortunately it was not possible to obtain an increase in the water splitting efficiency of the PEC cells, further optimization is needed. In addition to these fine structured silver systems, a high throughput screening protocol was developed, which allowed fast screening for new precipitation bands forming systems. By using this technique seven new, Liesegang type precipitation systems have been discovered, of which two consist of catalyst materials, such as AgMoO4 an AgWO4, and the others of iron or ruthenium coordination compounds. To the best of our knowledge this is the first time that it was possible to show that Liesegang type precipitation structures are formed out of more complex materials than metal salts.