Adsorption of Water on FeOOH as Studied by Electrical Conductivity Measurements

Abstract

Abstract Changes in the electrical conductivity of synthetic α-, β-, and γ-FeOOH crystals with the amount of adsorbed water were measured at 30 °C and various frequencies (dc, 100–10 MHz). The conductivity variations of wateradsorbed FeOOH with temperature were also examined over the range from −140 to 140 °C. For the adsorption coverages between 0 and 0.5, the electronic conduction increased with the coverage due to the transfer of electrons from chemisorbed water molecules to the FeOOH crystal. The decrease in activation energy for conduction with chemisorption was observed. The conductivity increase due to the chemisorption is related to the characteristic surface structure of each FeOOH polymorph. In the coverage range 0.7–2, protons in the adsorbed water layer transport electrical charges. The linear relationships between the logarithm of conductivity and the logarithm of the relative pressure of water vapor suggest the presence of clusters of physisorbed water molecules on FeOOH surfaces. A steep change in protonic conductivity by a factor of 102–104 was observed at a temperature close to −5 °C. The change seems to be caused by an order-disorder transition of protons in the adsorbed water layer.