Characterization of noncrystalline nanomaterials:: NMR of zinc phosphate as a case study

Zinc phosphate nanoparticles are prepared via a polyol-mediated synthesis. The nanomaterial turns out to be nonagglomerated and very uniform in size and shape, in particular 20 nm in diameter. X-ray powder diffraction analysis and high-resolution transmission electron microscopy indicate as-prepared nanoparticles to be noncrystalline. To investigate the chemical composition (stoichiometry, material homogeneity, amount of ortho-/metaphosphate, water content, type of surface-allocated adsorbents, differentiation of surface/inner core), X-ray diffraction, NMR-spectroscopy, energy-dispersive X-ray analysis, infrared spectroscopy, and thermal analysis are performed. To validate the local structure and composition, we performed H-1, C-13, and P-31 magic angle spinning nuclear magnetic resonance spectroscopy and multidimensional homo- and heteronuclear multiple-pulse solid-state NMR experiments. Moreover, P-31{H-1} rotational echo double-resonance experiments for various spin topologies are analyzed analytically and numerically, in order to differentiate between homogeneous nanoparticles and core-shell nanoparticles. The analysis gives a length scale to homogeneity and for bulk materials allows us to differentiate between mono- and dihydrogen phosphates, and phosphate hydrates.