Morphological rank of nano-scale tin dioxide films deposited by spray pyrolysis from SnCl4·5H2O water solution

The morphology and some principal details of the crystallographic grain structure of tin dioxide thin (20–300 nm) films obtained from SnCl4·5H2O water solution by spray pyrolysis deposition were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) methods. Direct correlation between the pyrolysis temperature and several fundamental nanoscale grain shapes and crystallographic features successively replacing each other with Tpyr was shown. These were: (I) separate crystallites with a sphere-like shape (spherulites), existing at 300–350 °C; (II) agglomerated spherulites at T=350–410 °C; and (III) nanocrystals with regular crystal faceting at T=410–530 °C. In turn, the nanocrystals can be subdivided into three types of crystal habit: needle or long prismatic, prismatic and pyramidal habits, each of which corresponds to its own temperature range in increasing order. XRD study and crystallographic form analysis of the cassiterite phase allowed us to suggest identification of the grain facets observed in our experiments. An explanation is also suggested for the gas-sensing properties of such films, which are strongly dependent on the crystallographic grain habit {hkl} and may be attributed to surface orientation effects.