Dehydrated halloysite intercalated mechanochemically with urea: Thermal behavior and structural aspects

Urea has been intercalated mechanochemically into dehydrated halloysite and analyzed by X-ray powder diffraction (XRPD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance ultraviolet/visible spectroscopy (DRUV–VIS), thermal analysis (TGA/DTA), transmission electron microscopy (TEM), and electron paramagnetic resonance (EPR). The basal distance expands from 7.4 to 10.7 Å and the interaction of urea to adjacent layers of halloysite through hydrogen bonds increases the structural order of the matrix. After heat treatment in air at different temperatures, decomposition products begin to appear starting from 100 °C. Although the basal distance remains constant up to 160 °C and collapses to the original value at 200 °C, urea and the decomposition products are still present in the sample. Starting from 125 °C, urea decomposition products reduce halloysite structural Fe³⁺ centers to Fe²⁺, as indicated by DRUV–VIS and EPR spectroscopy.