Photo-electrochemical and physical characterizations of a new single crystal POM-based material. Application in photocatalysis

Abstract A new inorganic-organic hybrid material [(H 2 pip) 3 ][α-PW 12 O 40 ] 2 ·4H 2 O, prepared by hydrothermal method, was structurally characterized by single-crystal X-ray diffraction. The compound based on a Keggin-type polyoxotungstate and piperazine (pip) displays a hybrid framework built from two (α-Keggin) 3− polyoxoanions and three (H 2 pip) 2+ hydrogen-bonded fragments, forming 3-D supramolecular architecture. The diffuse reflectance spectrum shows two optical transitions directly (3.27 eV) and indirectly (3.12 eV) allowed. The electrical conductivity follows an exponential law, indicating a semiconducting comportment with activation energy of 14 meV. The Mott-Schottky characteristic, plotted in Na 2 SO 4 (0.5 M) solution indicates n -type conduction with a flat band potential of −0.084 V SCE and electrons density of 4.24 × 10 18  cm −3 . As application, the photo-degradation of methylene blue (MB) upon UV irradiation was successfully achieved by OH • radicals. The improved activity is attributed to the potentials closeness of the valence and conduction bands with the radical levels.