LAUSR

Abstract Silver hollandite or argentina cryptomelane constituted by edge-sharing double [MnO6] octahedral chains forming (2 × 2) tunnel structure has attracted an increased interest in the field of mining, electric and catalytic material. In this study, we report the formation of silver hollandite by transformation from birnessite under hydrothermal conditions. We compare the effects of the type of birnessite precursor, the temperature, the concentration of Ag+ cation, the filling volume and the reaction time on the product phases, morphology and compositions of silver hollandite. Silver hollandite phase forms when 1 g birnessite reacts with 20 ml 0.08–0.1 M AgNO3 solutions for more than 0.5 h at above 300 °C. Otherwise in the lower temperature, silver concentration or filling volume conditions, bixbyite, hausmannite or manganite would coexist as major by-products. Ag-hollandite nanofibers are observed by TEM with diameters of 15–30 nm and lengths of 40–800 nm in Ag-K-OMS-400 °C sample and 40–100 nm width and 100–800 nm lengths in Ag-Na-OMS-400 °C sample. They show the characters of Type IV N2 adsorption isotherms that indicate the presence of the mesoporous structure nature. The average pore size and BET surface of Ag-hollandites are 8.3 nm, 57.4 m2/g in Ag-K-OMS-400 °C and 16.5 nm, 63.7 m2/g in Ag-Na-OMS-400 °C respectively. The silver species in Ag-hollandites are assumed to be a combination of both Ag+ and Ag° by XPS examination. Silver cations can be exchanged into the layers of birnessite, and in the high temperature and pressure hydrothermal conditions silver cations can prevent the phase from decomposing, but help the birnessite pillars to form the vertical set of octahedral sheets of hollandite structure, which yields the 2 × 2 tunnel structure.