Study on the Defect Structure of SnO2:F Nanoparticles by High-Resolution Solid-State NMR

In this contribution the preparation and structural characterization of nanoscale fluorine doped tin-oxide (SnO2:F, FTO) is described. By using a microwave assisted polyol approach, nanoparticles with different doping levels are prepared, which show narrow size distribution as measured by X-ray diffraction, electron microscopy and dynamic light scattering. They were converted into electrically conductive optically transparent films at 500 degrees C by a specific thermal treatment (500 degrees C in air followed by 250 degrees C in forming gas), exhibiting a specific resistivity of (1.9 x 10(-1) Omega cm). Solid-state MAS NMR and Sn-119 Mossbauer spectroscopy were used to study how F atoms are incorporated into the SnO2:F nanoparticles. Distance constraints were determined by Sn-119{F-19} REDOR, fluorine-doping homogeneity by homonuclear dipolar recoupling experiments (SR6(6)(2)). Cross-polarization was used to investigate the immediate environment of the dopant. The experiments were supplemented by first-principles quantum-chemical calculations for possible defect site models. The combined data strongly indicate that F doping is not directly related to an increase in charge-carrier concentration, even though F atoms do occupy O vacancy sites in SnO2:F. For this study we have implemented background compensated NMR. 2D pulse-sequences which reliably suppress the fluorine background originating from the NMR probe. Moreover we show that duster calculations on the basis of the extended embedded ion method (EEIM) can be used to study the structure of diluted defects in crystalline host structures and predict NMR properties.