Band-gap energy of heteropoly compounds containing Keggin polyanion-[PV_xMo_(12-x)O_(40)]~(-(3+x)) relates to counter-cations and temperature studied by UV-VIS diffuse reflectance spectroscopy

摘要

The band gap energy (absorption edge energies) of the pure H_3[PMo_(12)O_(40)]·13H_2O and H_4[PVMo_(11)O_(40)]·13H_2O, respectively, supported on SiO_2 and SiC and some of its NH_4~+ and Cs+ salts were determined by different methods. The influence of the counter-cations and the temperature on band gap energy was studied. In this purpose, the diffuse reflectance spectra of above mentioned compounds were registered at different temperatures, and it were transposed in the curves of the Kubelka-Munk function vs. wavelength. The band gap energies were determined by processing of low field energy of the ligand-metal charge transfer band (O~(2-)→Mo~(6+) and O~(2-)→V~(5+)) usually observed between 200 and 400 nm on these curves. In this aim, the Tauc's relation was adapted for Kubelka-Munk function use and it was plotted for n = 1/2 (direct transition) and 2 (indirect transition) vs. wave energy (photon energy). The intersection of the curves' tangent drawn to their point of inflection with horizontal axis gives the band gap energy. The other method for calculation of band gap energy was the differential calculus on the Kubelka-Munk function vs. wave energy curve where the x value corresponding to curves' maximum is the found value. The comparison between experimental band gap values and literature data showed their partial fit. The higher temperature produces the band gap energy diminution as a result of a stronger interaction between Keggin Units, which occurs especially by the crystallization water loss. The Keggin Units isolation one from another by voluminous counter-ion or their spreading on a support leads to a weaker interaction between them and as a consequence, the increasing of absorption edge energy. A linear correlation of the crystallites size with band gap energy was observed.