Photo-electrochemical and physical characterizations of a new single crystal POM- based material. Application to Rhodamine B photodegradation

The physical and photoelectrochemical characterizations of a new inorganic-organic hybrid single crystal material, synthesized by hydrothermal route, based on α-Keggin-type polyoxotungstate and Cu(I)-(trz) coordination complex, namely [Cu6(trz)6][α-PW12O40]2·H2O, [trz=1H-1,2,4-triazole] are investigated for the first time. The crystal structure, determined by single-crystal X-ray diffraction, reveals a 3-D supramolecular host-guest structure with Keggin anion as template. The diffuse reflectance indicates an optical transition at 2.91 eV, directly allowed, attributed to the Keggin polyoxoanions. The physical characterization shows a semiconductor behavior and the conductivity follows an exponential type law associated to a small polaron hopping between neighboring ions. The thermo-power measurement indicates n-type conduction with an electron mobility (0.54×10−6 cm2 V−1 s−1), thermally activated. The photoelectrochemical study, performed in Na2SO4 medium, confirms n-type conduction with a flat band potential of −0.65 VSCE and electrons density of 6.59×1018 cm−3. The electrochemical impedance spectroscopy reveals the predominance of the bulk contribution with a constant phase element and Warburg diffusion. As application, the photo-degradation of Rhodamine B upon sunlight and UV irradiation is achieved by the radicals O2
- - and OH
- . The improved activity is attributed to the wide depletion width (460 nm) and the potentials of the valence and conduction bands close to the radicals levels.