Using Cu2ZnSn(S,Se)4 (CZTSSe) as a model system, we demonstrate the kinetic control of solid-gas reactions at nanoscale by manipulating the surface chemistry of both sol-gel nanoparticles (NPs) and colloidal nanocrystals (NCs). Specifically, we first identify that thiourea (commonly used as sulfur source in sol-gel processes for metal sulfides) can transform into melamine upon film formation, which serves as surface ligands for as-formed Cu2ZnSnS4 (CZTS) NPs. We further reveal that the presence of these surface ligands can significantly affect the outcome of the solid-gas reactions, which enables us to effectively control the selenization process during the fabrication of CZTSSe solar cells and achieve optimal film morphologies (continuous large grains) by fine-tuning the amount of surface ligands used. Such enhancement leads to better light absorption and allows us to achieve 6.5% efficiency from CZTSSe solar cells processed via a sol-gel process using nontoxic, low boiling point mixed solvents. We believe our discovery that the ligand of particulate precursors can significantly affect solid-gas reactions is universal to solid-state chemistry and will boost further research in both understanding the fundamentals of solid-state reactions at nanoscale and taking advantage of these reactions to fabricate crystalline thin film semiconductors with better morphologies and performances.