Solid state nuclear magnetic resonance studies of functional materials

Abstract

The functional properties of materials are inherently linked to the structure, and therefore it is imperative that accurate and comprehensive characterisations are accomplished to improve performance. As improvements to materials are often related to a greater morphological complexity through disorder, analytical techniques are required to characterise these structural intricacies. Solid state NMR is excellent at obtaining short range information and has therefore been implemented for structural elucidation in the areas of heterogeneous catalysis, thermoelectric materials and optoelectronic materials. The large chemical shift range and high sensitivity of 17O solid state NMR allows for minor structural changes to be detected making it an attractive prospective tool in the structural characterisation of platinum group metal (PGM) catalysts. For the first time, 17O solid state NMR spectra for PtO2, PdO, Rh2O3 nanoparticles and bulk RuO2 are reported and correlated to their structures through the use of PXRD, Raman spectroscopy and TEM. Furthermore, the application of 17O solid state MAS NMR to Pt and Pd supported by the commonly used metal oxide supports; γ-Al2O3, SiO2 and TiO2 has been explored and complemented by 195Pt, 1H and 29Si solid state NMR. This has allowed for direct observation of catalyst-support bonding which provides new avenues for the structural characterisation of catalytic systems. A multiple technique approach has been adopted to probe the structure of Sr1−x/2Ti1−xMxO3 (M = Nb5+, Ta5+) systems for uses as high temperature thermoelectric materials. The presence of Sr (A) site vacancies were detected for doped SrTiO3 through the use of neutron diffraction, elemental analysis, Raman spectroscopy, and 93Nb and 87Sr solid state MAS NMR data. Comparing 93Nb MAS NMR with calculated NMR parameters generated from materials modelling structural realisations using the GIPAW DFT approach has allowed for the characterisation of three distinct Nb sites; Nb directly substituted into the SrTiO3 cubic lattice, a distorted Nb dimer about a Sr vacancy and disordered niobia nanodomains (> 5 mol% Nb) formed from significant Sr vacancy formation. 133Cs, 23Na and 39K solid state MAS NMR has been implemented to study the structure and mobility of direct band gap Pb-free double perovskite Cs2AgInxBi1−xCl6, Cs2NaxAg1−xInCl6:Bi and Cs2KxAg1−xInCl6:Bi nanocrystal systems. The alteration to the optical properties from Bi3+, Na+ and K+ incorporation is rationalised in terms of the 133Cs T1 data and the evolving structural defects comprising each system, whereby a passivation of the defects leads to an enhancement of the photoluminescence quantum yield (PLQY). Materials modelling using the Ab Initio Random Structure Search (AIRSS) method, and the calculation of the NMR parameters emanating from the generated structural realisations using the GIPAW DFT approach, showed that the introduction of K+ induces significant structural disorder and multi-phase formation as highlighted by the large 133Cs and 39K chemical shift dispersion. The 39K MAS NMR data demonstrates that the PLQY behaviour maps directly with the K+ incorporation into the cubic Cs2KyAg1−yInCl6 phase supporting B site occupancy.