Isotope Effect between H2O and D2O in Hydrothermal Synthesis

Synthetic chemists can use different solvents (for example, polar vs nonpolar) to affect solute–solvent interactions and therefore favor a particular reaction product. Using a different isotope of the same solvent to select the product, however, is uncommon. We find that water and heavy water (D2O) can select different products in the hydrothermal syntheses of iron-containing solids. Under hydrothermal and anaerobic conditions, water oxidizes iron to evolve hydrogen gas and a mixture of Fe2+- and Fe3+-containing solids. When we replace D2O for H2O under identical hydrothermal conditions, we can select thermodynamic products in our syntheses of iron-containing solids, whether it be oxides or chalcogenides. To explain this remarkable difference in product selectivity, we discover an unexpected difference in a fundamental property between H2O and D2O: their standard reduction potentials. Through electrochemical measurements and under strongly basic conditions (pH = 14), we find that the reduction potential of D2O is 109 mV lower than that of H2O. This difference in the reduction potential cannot be explained by the simple Nernst formulation. The unique product selectivity of D2O over H2O allows us to prepare iron chalcogenides with structures relevant to the iron-based superconductors.