Oxidation of a metal surface is a phenomenon familiar to everybody. We have all seen green copper roofs, tarnished silver spoons, grey aluminum, and rusty iron. But few have stopped to think what actually happens in this phenomenon, and why different metals look so different when they are oxidized. It is also slightly surprising that despite all the years of research, even science has not managed to figure out all the details in this everyday phenomenon.
The starting point is clear: there is a clean unoxidized metal surface, and air with oxygen molecules. When oxygen molecules meet the surface, they breakup, and eventually an oxidized layer, the oxide, will form. The layer can be thin or thick, depending on the original metal. In some instances, this layer will start repel oxygen, and the surface can no longer oxidize any deeper.
This is the basis for corrosion resistance of aluminum and stainless steel, for example. But between the clean surface and the completely oxidized surface there is room for interesting structures formed by metal and oxygen, which can resemble a butterfly, diamond, or flower petals. Details of the structure depend on the metal in question, and also on the amount of oxygen, the temperature, etc.
The international research group, which includes members from the University of Genoa in Italy, University of Central Florida, and Professor of theoretical physics Matti Alatalo from the University of Oulu, have studied how oxidization progresses on the surface of silver.
Tunneling microscope measurements and quantum mechanical calculations reveal that a breaking oxygen molecule replaces the silver atom in the surface layer. In other words, it digs itself a hole where oxidation will start. It was previously assumed that the formation of oxidized structures requires steps on the surface, where the atoms on the edge move to the vicinity of oxygen atoms. The new observation also explains why the reflectivity of the surface changes in a certain temperature: oxygen molecules that have disintegrated in random locations cause oxidized structures to exist here and there in disarray, which decreases reflectivity.
Oxidation of a silver surface has a wider significance than just for granny’s spoon collection. Silver is a catalyst commonly used in the chemical industry for manufacturing, among other things, ethylene oxide, which is used as raw material for several products as well as in the raw in the disinfection of hospital equipment. Oxidation is an important phenomenon for catalysis, and in many cases it is the oxidized surface which acts as catalyst. Precise understanding of oxidation will help in planning even better catalysts in the future.
The article on the oxidation of silver was published in the leading general publication series of physics, Physical Review Letters.
Last updated: 15.6.2017