Interplay of porosity in gamma-Al2O3-doped ZnO nanocomposites: A comparative study of sonochemical and microwave reaction routes

Porous gamma-Al2O3-doped porous ZnO nanocomposites were synthesized by sonochemistry and microwave techniques, and the changes in the pore characteristics as a function of the dopant concentration were investigated. The insertion of gamma-Al2O3 or partial and/or total blockage of the pores of porous ZnO by porous gamma-Al2O3 largely depends on the technique employed. The porous nanostructures were formed without the use of any template and consisted of a mixture of micropores (1.3-1.4 nm) and mesopores (3.8-4.5 nm). The electron microscopic studies show that the pores are formed surrounded by nanoparticles whose size ranges between 4.1 and 5.0 nm and 9.3 and 11.9 nm in the sonochemical and microwave synthetic techniques, respectively. Fourier transform infrared (FTIR) measurements reveal the retained organic framework in the final products, resulting from the incomplete decomposition of the organic precursors in argon. The excitonic absorption in the diffusion reflection optical spectroscopy (DRS) is blue-shifted, as compared to that of bulk ZnO (373 nm), due to the quantum confinement effect. However, it is also possible that the Al doping creates charged defects which are the reason for the shifts of the absorption bands of ZnO to higher energy values.