Structure determination of molecular nanocomposites by combining pair distribution function analysis and solid-state NMR

Abstract

Transparent mesoporous silica monoliths of well-controlled porosity and a narrow pore size distribution around 6 nm have been used to prepare sodium nitroprusside (SNP) nanocomposites. The obtained nanomaterials could be characterised using X-ray total scattering coupled to atomic pair distribution function analysis (PDF) and solid-state NMR spectroscopy. The PDF analysis allows for a structural description of the confined species, as well as for the identification of the various coexisting phases: SNP isolated molecules and SNP crystalline nanoparticles. The model obtained suggests that the nanocrystals have the same molecular structure as the bulk crystalline material and measure about 6 nm in diameter. This result is quite exceptional because the space available inside the pores is only about ten times the size of the molecules. The multi-nuclei solid state NMR investigation confirms the structural model proposed by the PDF analysis and assigns the isolated molecules to the dynamic disorder of a solvated phase. The latter approach additionally provides quantitative information on the relative ratio between the dynamic molecules and the rigid nanocrystals. This result is exploited to study the evolution of the two confined SNP phases with respect to solvating water molecules. We show that the confined SNP nanocrystals can be easily dissolved when storing the nanocomposites at increasing atmospheric relative humidity.