Terahertz time domain spectroscopy of hydrothermally synthesized boehmite and ammonium dawsonite nanostructures

The frequency dependent optical and dielectric properties of boehmite (AlOOH) and ammonium aluminum carbonate hydroxide (AACH) nanostructures, prepared at different temperatures via hydrothermal synthesis, were studied by using terahertz time domain spectroscopy (THz-TDS). The complex refractive indices, absorption coefficients and complex dielectric constants were determined and compared for different synthesis temperatures. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were also performed to confirm the formation of boehmite and AACH. Scanning electron microscopy (SEM) was performed to study the morphology of nanostructures. The study reveals the formation of AACH at higher temperatures and decrease in refractive index with increasing synthesis temperature. The higher absorption coefficient and lower refractive index and dielectric constant were observed for AACH than for boehmite.     

Hydrothermal synthesis and thermodynamic analysis of dawsonite-type compounds

Dawsonite-type compounds of formula MAl(OH)₂CO₃ (M=Na,K,NH₄) were hydrothermally synthesized. The influences of the composition of starting materials, reaction temperature, acidity of the reaction medium on the formation, morphology and particle size of MAl(OH)₂CO₃ powders were systematically investigated. The experiment results show that increase in the amount of MHCO₃ favors the formation of MAl(OH)₂CO₃ phase with more slender particle morphology. The higher the reaction temperature, the grosser the particle is. It was also demonstrated that the particle size of the MAl(OH)₂CO₃ phase are most uniform when the pH value of reaction system is 10.3 for sodium dawsonite and potassium dawsonite, but it is 9.79 for that of ammonium dawsonite. The thermodynamics of the formation of dawsonite-type compounds have also been analyzed in details.