Non-innocent Spectators: Decoupling the Role of Cations and Anions in Modulating Nanoparticle Synthesis

Solution-phase nanomaterials synthesis is sensitive to the interplay between the many species in solution. Additives that do not directly participate in the net chemical reaction often moderate nanoparticle formation and growth, affecting the polymorphic phase distribution, particle size, and morphology. While ionic salts are common reaction-modifying additives, they necessarily introduce two distinct species in tandem, a cation and an anion, each of which can influence reaction evolution in different ways. Here, we separate the influence of cationic and anionic additives during TiO₂ nanoparticle synthesis by comparing reactions with systematically varied additive compositions. We evaluate 4 cations (Li⁺, Na⁺, K⁺, and NH₄⁺) and 5 anion compositions across a NO₃^--Cl^- gradient, comprising 20 total salt compositions. We combine in situ time-lapse photography and image analysis, which reveal the kinetics of product formation and sedimentation, with post facto depth-resolved X-ray pair distribution function (PDF) analysis to map the polymorph distribution accumulated at different stages of the reaction. The results show that ion additives most strongly perturb the reaction during the first of two distinct TiO₂ formation events, where they bias hydrolysis, nucleation, colloidal destabilization, and sedimentation in a coupled way. These findings show that the outcome of TiO₂ nanoparticle synthesis is controlled by the evolving solution environment and that spectator ions exert systematic control over how the system first departs from the initial state.