Credit: © 2008 Wiley

Materials that expand when compressed are rare, and are the result of liquid molecules being incorporated into nanoscale pores. The phenomenon has been previously observed in materials such as zeolites, where small unit-cell increases of around 2% have been reported. Now, Alexandr Talyzin of Umeå University, Sweden and colleagues1 in France, Hungary and Germany have discovered that the lattice of graphite oxide, when pressurized in water, can expand by as much as 30%.

The researchers examined the graphite oxide under high pressure with in situ X-ray diffraction using synchrotron radiation. Samples of both 'dry' graphite oxide and graphite oxide immersed in water were examined. The dry samples gave a normal pressure response: the interlayer spacing (d) between oxidized graphene planes decreasing significantly with an increase in pressure. The samples that were immersed in water, however, exhibited a large increase in interlayer spacing as the pressure was raised, reaching a maximum at a pressure of around 1.5 GPa. This behaviour was attributed to the pressure-induced uptake of water into the interlayer space of the material. At pressures above 1.5 GPa, the material displayed the usual compressibility, a transition that was correlated with the freezing of the water into an ice structure known as ice VI.

Talyzin and co-workers also found that the effect was reversible, resulting in the structure 'breathing' with variations in pressure.