Functional Meso Porous Nanostructures

The Sol-Gel Handbook Synthesis, Characterization, and Applications, First Edition. 

Starting from solution as in sol—gel processes, the key to design the desired dispersion and mixing of the different metal centers lies in an understanding of the chemical reactivity and structural chemistry of the precursors. Typical precursors for sol-gel processes are metal alkoxides, for example, Ti(OR)4 for titania or Si(OR)4 for silica. A challenge in hydrolytic sol-gel processing of these different alkoxides is the different reaction rate for the hydrolysis reaction. This reaction typically proceeds via a nucleophilic substitution mechanism and therefore depends on the partial charge, 5, on the metal atom (Si or Ti). The 

Depending on the desired materials properties and structure, these approaches have been explored for sol-gel processing of alkoxide-based precursors and further developed in combination with many experimental parameters that can also be adjusted to control the sol composition and particle size, including alkoxide/ water ratio, concentration of the precursors, reaction temperature, reaction time, drying methods, choice and concentration of the catalyst, and the solvent used [42]. Besides the well-known hydrolytic sol-gel processes, even nonhydro-lytic approaches have been reported as a powerful method in the synthesis of mixed oxide materials and will be discussed in more detail subsequently [43]. Another processing option to tailor the material properties is the application of soft or hard templates [44]. In addition to the chemical parameters, drying and thermal treatments or firing processes determine the final architecture, chemical 

Postsynthetic grafting or mixing of preformed nanoparticles as well as the thermal decomposition involving single-molecule precursors or occurring at high temperatures is not covered in this chapter. 

In principle, the synthetic approaches presented in this chapter are not only valid for Si/Ti binary oxides but can also be transferred (with the necessary adaptations) to other mixed metal oxide systems. 

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