Decomposition of tetra-alkylammonium thiomolybdates characterised by thermoanalysis and mass spectrometry

The decomposition pattern of tetraalkyl-tetrathiomolybdates with general formula (R4N)2MoS4 (with R increasing from methyl to heptyl) was determined by means of differential thermal analysis (DTA), thermogravimetric analysis (TGA) and mass spectroscopy (MS) techniques. The complexity of thermal decomposition reactions increases with the size of the R4N group. Prior to decomposition at least one phase transition seems to occur in all complexes. The onset of thermal reactions was also a function of the tetra-alkylammonium precursor. All compounds decompose without forming sulfur rich MoS2+x intermediates. For R = methyl to pentyl precursors the MoS2 produced was nearly stoichiometric, however for R = hexyl and heptyl the S content was significantly reduced with a Mo:S ratio of about 1.5. The carbon and hydrogen residual contents in the product increased with the number of C atoms in R4N; for N contamination no clear trend was obvious. SEM images show that the formation of macro-pores was also a function of the alkyl group in R4N. The MoS2 materials obtained show a sponge-like morphology. Results of DSC experiments in combination with in situ X-ray diffraction also revealed the complex thermal behavior of (R4N)2MoS4 materials; reversible and irreversible phase transitions and glass-like transformations were identified in the low temperature range (35–140 °C), before the onset of decomposition.