Solids containing Si-O-P bonds: is the hydrolytic sol-gel route a suitable synthesis strategy?

Materials based on silicon-phosphorus mixed oxides have traditionally attracted interest in electronics, optics, catalysis, and related fields. The preparation of a solid containing stable Si-O-P linkages is a huge challenge due to their intrinsic instability to hydrolysis in a wet atmosphere. On the other hand, most technological applications of these materials, such as protonic conductive membranes in fuel cells and water-tolerant solid acid catalysts, are related to their interaction with water; consequently, suitable synthesis procedures that positively face this tradeoff are mandatory. Besides the traditional high-temperature techniques, sol-gel synthetic methods represent a viable, low-cost alternative, allowing for the preparation of high-purity materials with a homogeneous distribution of the components at the atomic scale. Si-O-P linkages are easily obtained by nonhydrolytic sol-gel routes, but only in inert and dry atmosphere. Conversely, hydrolytic routes offer opportunities to control the structure of the products in a wide range of processing conditions. The present review aims at providing an overall picture of the research on the sol-gel synthesis of phosphosilicate and related materials and theisr different applications, emphasizing how the interest in these systems is still lively, considering both conventional and emerging applications, such as flame retardance. The incorporation of Si-O-P nanostructures in polymer composites, coatings, and textiles is indeed a promising strategy to improve properties like thermal stability and fire resistance; however, their in-situ synthesis brings about additional difficulties related to the reactivity of the precursors. The perspectives linked with the development of Si-P-based materials are finally outlined. Si-O-P linkages show an intrinsic low stability to hydrolysis.Hydrolytic sol-gel processing in suitable conditions can produce extended Si-P cross-condensation.The introduction of a metal in the phosphosilicate network can stabilize P and prevent its leaching.The formation of Si-P nanostructures can be favored in a polymer matrix, giving hybrid composites.Si-P oxides have interesting application prospects in fuel cells, acid catalysis, flame retardance.

Automated summary

Materials based on silicon-phosphorus mixed oxides have a wide range of applications, but their hydrolytic stability is low in wet environments. Sol-gel synthesis allows for the preparation of high-purity materials with a homogeneous distribution of components, but balancing hydrolytic stability with application requirements is necessary. The incorporation of metals can stabilize phosphorus and prevent leaching, while the formation of Si-P nanostructures is advantageous in polymer matrices. Si-P oxides have interesting application prospects in fuel cells, acid catalysis, and flame retardance.