Sol Gel Process

The sol-gel technique refers to a room-temperature chemical route that is used for preparing oxide materials by the hydrolysis of metal-organic compounds (particularly metal alkoxides) carried out under controlled pH, amount of water, and alkoxide concentration [333]. The process involves initially a homogeneous solution of metal alkoxide derivatives such as M(OR)X or MM' (OR)x+y, which is "activated" by a careful hydrolysis to initiate inorganic polymerization reactions that produce species containing M-O-M or M-O-M' linkages [194]. The fundamental reaction in the hydrolysis of metal alkoxides is a nucleophilic substitution (SN2) that involves a nucleophilic addition of an OH-8 group to the electrophilic metal center (M+8). As a result, the coordination number of the metal atom is increased in the transition state, which leads to the migration of a proton toward an alkoxy group, consequently knocking off the protonated ROH ligand (Scheme 8).

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