Preparation method of Ce_(1-x)Zr_xO_2/tourmaline nanocomposite with high far-infrared emissivity and its mechanism

摘要

Far-infrared functional nanocomposites were prepared by the coprecipitation method using natural tourmaline (XY_3Z_6Si_6O_(18)(BO_3)_3V_3W, where X is Na~+, Ca~(2+), K~+, or vacancy; Y is Mg~(2+), Fe~(2+), Mn~(2+), Al~(3+), Fe~(3+), Mn~(3+), Cr~(3+), Li~+, or Ti~(4+); Z is Al~(3+), Mg~(2+), Cr~(3+), or V~(3+); V is O~(2-), OH~-; and W is O~(2-), OH~-, or F~-) powders, ammonium cerium(Ⅳ) nitrate and zirconium(Ⅳ) nitrate pentahydrate as raw materials. The reference sample tourmaline modified with ammonium cerium(Ⅳ) nitrate alone was also prepared by a similar precipitation route. The results of Fourier transform infrared spectroscopy show that Ce-Zr can further enhance the far-infrared emission properties of tourmaline than Ce alone. Through characterization by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), the mechanism by which Ce(-Zr) acts on the far-infrared emission property of tourmaline was systematically studied. The XPS spectra show that the Fe~(3+) ratio inside tourmaline powders after heat treatment can be raised by doping Ce and further raised after adding Zr. Moreover, it is showed that Ce~(3+) is dominant inside the samples, but its dominance is replaced by Ce~(4+) outside. In addition, XRD results indicate the formation of CeO_2 and Ce_(1-x)Zr_xO_2 crystallites during the heat treatment, and further, TEM observations show they exist as nanoparticles on the surface of tourmaline powders. Based on these results, we attribute the improved far-infrared emission properties of Ce-Zr-doped tourmaline to the enhanced unit cell shrinkage of the tourmaline arisen from much more oxidation of Fe~(2+) (0.074 nm in radius) to Fe~(3+) (0.064 nm in radius) inside the tourmaline caused by Zr enhancing the redox shift between Ce~(4+) and Ce~(3+) via improving the oxygen mobility in the Ce-Zr crystal.