Advancing solution combustion synthesis for control and tunability of inorganic nanostructures

Abstract

Synthesis of pore and shape controlled nanostructures on a large scale with low costs and simplicity is a present challenge in nanotechnology. The solution combustion synthesis (SCS) is an established energy and time efficient method for mass-producing nanocrystalline metal oxides and metals. A main uniqueness of this method is that the temperature required for crystalline nanostructure formation is achieved through exothermic reaction between precursors. There is, however, little controllability in product structures. This thesis develops novel synthetic methods based on solution combustion processing for scalable synthesis of inorganic materials with controlled nanostructures. An innovated method denoted as colloidal solution combustion synthesis (CSCS) is developed to prepare uniform mesoporous metal oxides with tunable porosity by combustion of a colloidal solution. The tunability of CSCS synthesized oxides is well illustrated by the control and correlation of surface area, pore volume, and pore size to amount of colloid added as demonstrated in the results for CeO2 and NiO. Colloids play a significant role beyond that of a template. The addition of a colloid limits combustion to take place in a confined and uniformly distributed nanospace, leading to a highly uniform mesoporous structure after template removal. Crystalline mesoporous CeO2 with 22 and 12 nm uniform pores, high specific surface areas (82 and 197 m2/g) and large pore volumes (0.6 and 0.37 ml/g) are successfully synthesized by CSCS. Nickel oxide powder having 223 m2/g, 0.39 ml/g pore volume, and uniform 22 nm mesopores is also prepared. Both oxides demon-strated excellent catalytic activities for carbon monoxide and soot oxidation. Another method developed in this thesis uses metal-organic framework (MOF) as a sacrificial template in a combustion synthesis. It has been demonstrated that instead of using external heating source to decompose the organic framework, combustion reaction inside the framework can produce pure Cr2O3 octahedra with the octahedral shape of MOF retained. A new one-step method of producing dense metallic Pd films from aqueous solution on a large scale via SCS is proposed. By using palladium nitrate and glycine, mirror-bright dense Pd films with good adhesion are deposited on different substrates at relatively low temperature of 300 oC. Interestingly, a complex formed between palladium nitrate and glycine is observed and confirmed by characterization. The complex formation gives new insights about the mechanism of the process. The Pd film deposited directly onto glassy carbon electrode exhibited excellent electrocatalytic activity for both alcohol oxidation and oxygen reduction reactions in alkaline media.