Decomposition Pathways of Thioureas in Oleylamine Control Reactive Sulfur Species in SrS Nanocrystal Synthesis

Understanding the decomposition and reaction mechanisms of sulfur precursors used in organometallic colloidal synthesis is critical for controlling the nucleation and growth of nanocrystals. In this work, we investigate the thermal decomposition of thiourea and various N,N′-substituted thioureas in oleylamine to elucidate their distinct decomposition mechanisms, sulfur release pathways, and the implications for strontium sulfide (SrS) nanoparticle syntheses due to these decompositions. Using a combination of in situ quadrupole mass spectrometry (QMS), NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), thermogravimetry (TGA), and DFT-calculated bond dissociation energies (BDEs), our findings reveal that substituent identity and symmetry significantly affect the decomposition onset temperatures and reaction intermediates. Unsubstituted thiourea undergoes initial isomerization with two competing subsequent fragmentation routes, releasing the gaseous products H2S and NH3, respectively. The other fragments from both routes, carbodiimide and isothiocyanic acid, react with oleylamine to form N,N′-dioleylthiourea. The N,N′-dioleylthiourea undergoes a second decomposition process, which releases H2S and the trisubstituted guanidine. Aliphatic N,N′-substituted thioureas exhibit the first fragmentation process, which releases the respective alkylamine, whereby the various chain-length substituents influence the decomposition onset temperature and the decomposition process, which releases H2S from N,N′-dioleylthiourea. Aromatic N,N′-substituted thioureas follow the same sequential decomposition mechanism, but resonance effects in N,N′-diphenylthiourea lower the decomposition temperature. These mechanistic differences are then directly correlated with the formation of strontium sulfide (SrS) nanocrystals. By tracking sulfur and amine release during synthesis, we show that the timing and concentration of reactive sulfur species, such as H2S, determine the nucleation rate, particle size, and morphology of the resulting nanocrystals. This study provides mechanistic insights into thiourea decomposition in oleylamine and establishes a direct link between the precursor structure and nanoparticle outcome. The results lay the foundation for more predictive and tunable synthesis strategies in the design of colloidal metal sulfide nanomaterials.