Giant conductivity enhancement of ferrite insulators induced by atomic hydrogen.

Hydrogen behavior in oxides has triggered much interest for its scientific and technological importance in a wide range of research fields from novel ion conductors to astrochemistry. Here, we report a giant conductivity enhancement in ZnFe2O4 ferrite insulators to the metallic state by over eleven orders of magnitude induced by electrochemically generated atomic hydrogen at room temperature. The conductivity and the amount of incorporated hydrogen increased in an exponential function with time. An activation energy for the atomic hydrogen chemisorption was measured to be 8.23 kJ mol(-1). Quantitative kinetics correlations among the adsorption of atomic hydrogen, hydrogen incorporation and conductivity enhancement are established, based on which hydrogen incorporation process is clarified herein. We demonstrate that the hydrogen incorporation in oxides can be adjusted by manipulating the kinetic factors. These findings have implications for research into hydrogen behavior in oxides in environments containing hydrogen atoms and offer possibilities for utilizing and controlling the modifications of oxide materials induced by atomic hydrogen.

Hydrogen Impurity Defects in Rutile TiO2.

Hydrogen-related defects play crucial roles in determining physical properties of their host oxides. In this work, we report our systematic experimental and theoretical (based on density functional theory) studies of the defect states formed in hydrogenated-rutile TiO2 in gaseous H2 and atomic H. In gas-hydrogenated TiO2, the incorporated hydrogen tends to occupy the oxygen vacancy site and negatively charged. The incorporated hydrogen takes the interstitial position in atom-hydrogenated TiO2, forming a weak O-H bond with the closest oxygen ion, and becomes positive. Both states of hydrogen affect the electronic structure of TiO2 mainly through changes of Ti 3d and O 2p states instead of the direct contributions of hydrogen. The resulted electronic structures of the hydrogenated TiO2 are manifested in modifications of the electrical and optical properties that will be useful for the design of new materials capable for green energy economy.