Chemical gardening may be an option for chemists who are not good with plants but would like to have a nice little garden. With a sodium metasilicate solution and some metal salts you can create beautiful gardens in a beaker.

Dimitra Spanoudaki of the Free University of Brussels and Eleni Pavlidou and Dimitra Sazou of the Aristotle University of Thessaloniki wanted to make it a little more electrifying. Literally, because they wondered if they could also form such structures in an electrochemical way.

The researchers started with a zinc disc electrode in a sulfuric acid solution with a platinum counter electrode. If you put a voltage on the zinc electrode, the electronic decomposition of the metal follows. First, dissolved zinc sulphate forms, adsorbed to the electrode ([ZnHSO₄]_ads). Next, the acidic environment protonates the zinc sulphate to [ZnHSO₄H⁺]_ads. Depending on the potential, either free zinc and sulphate ions form in solution or the [ZnHSO₄H⁺]_ads precipitates to a solid, hydrated salt ZnSO₄∙nH₂O.

When ZnSO₄ precipitates, the precipitate eventually forms micro-rods about 5 µm long. This proceeds as follows: the precipitate acts as a bipolar diffusion membrane between the sulfuric acid solution and the disk electrode. Through convection and the formation of hydrogen bubbles on the disk, you get openings in the membrane, which then leads to the dissolution of Zn²⁺ ions. Along the edges of the diffusion membrane, the Zn²⁺ and SO₄^2– ions find each other and form rods of the hydrated salt. The researchers made the pictures with scanning electron microscopy.

Chemical gardening can also be done upside down: instead of putting the salt crystal – which serves as a starting point – at the bottom of a beaker, you attach it to the top. As a result, the salt crystal formations grow from top to bottom. The Greeks also successfully tested this principle, though the images were less spectacular.

You get more spectacular pictures if you apply a high frequency current oscillations. That way you get more complex, almost flower-like structures (see image). The researchers hope that this will enable you to manipulate electrodes in such a way that they will have interesting catalytic or optical applications. But first, some more ploughing and raking is needed.

Spanoudaki, D. et al. (2021) ChemSysChem 2