Every textbook will tell you that the crystal structure of bulk ice is hexagonal, but also that the first molecules formed by the adsorption of water onto a surface, a process called nucleation, will also arrange themselves into hexagons. Now an international group of researchers have discovered that pentagonal structures of ice can be formed on copper surfaces consisting of Cu (110) substrates.
Cu (110) substrates have surfaces that look like corrugated metal roofs where ridges of copper atoms are closer to the outer surface and alternate with rows of deeper embedded atoms. The electron density in these ridges is higher than in the troughs, making such surfaces polarised. Scientists at the Surface Science Research Centre at the University of Liverpool (UK) recorded scanning tunnelling microscopy (STM) images of water adsorbed on a Cu (110) substrate under high vacuum conditions. These STM images, created by the recording of the tunnelling current between a microscopic tip scanning right over the copper surface, showed perfect chain-like structures, one molecule thick, that followed the copper atom ridges. These STM images did not allow the direct resolution of the structures of the units making up these chains.
Using density functional theory, a computational technique allowing the calculation of the distribution of electrons in structures, Javier Carrasco and Angelos Michaelides, both at the Fritz-Haber Institute of the Max Planck Society in Berlin (Germany) and the Department of Chemistry at the University College London (UK), could show that these structures had to be pentagons. 'This discovery was unexpected,' says Carrasco. The result was also supported by infrared vibrational spectra obtained from water molecules, which matched best to those computed for a pentagonal structure of water molecules.
In bulk ice the hydrogen bonds between water molecules interplay in such a way that the molecules arrange themselves into hexagonal structures. The understanding of what happens when these molecules touch a surface is still 'one of the big remaining challenges,' says Michaelides. 'At such interfaces we have a new interaction, the water-metal bonds, and what is important is the relative balance between the water-water and water-metal bonds.' The team has investigated many surface types over the years, reports Michaelides, but this research was the first time that something as striking as the pentagonal structures was observed. 'We are saying that if we go to nickel we might also see pentagonal structures, but if we go to surfaces with larger atoms we might revert to the traditional hexagonal structures,' says Michaelides. The use of these materials might be of interest in the development of cloud seeding agents, he adds.
Lars Pettersson of the Physics Department of Stockholm University in Sweden, who belongs to an international group of scientists studying the adsorption of water on similar surfaces - including Cu(110) substrates - was equally surprised by the finding of Michaelides and his colleagues: 'The water molecules like to sit on the positive ridges, and what is interesting is that the pentagon is preferred over the hexagon in these linear chains,' says Pettersson. 'I think it is nice to have found that the pentagon behaves as a more stable hydrogen network than the hexagon,' he says.