Ion Activity and Metal Ion Speciation Rationalize Outcomes in WO3 Nanoparticle Synthesis

Acid-mediated precipitation of aqueous tungstate solutions is widely used to synthesize WO3 nanoparticles, yet the coupling of reagent identity, ionic strength, and speciation equilibria across different synthetic protocols cloud which variables govern resulting polymorph type and nanoparticle morphology, as well as whether polymorph and nanoparticle morphology can be tuned independently. Here, we systematically vary acid and alkali salt identity, alkali cation concentration, and acid addition rate to isolate their individual contributions. By accounting for differences in ion activities and speciation equilibria, we show that apparent structure-directing effects previously attributed to anion identity instead arise primarily from solution-level differences rather than direct anion−WO3 interactions. In contrast, alkali cation concentration exerts an independent influence on the favorability of the metastable hexagonal phase that persists even after accounting for differences in ionic strength and speciation. We further demonstrate that acid addition rate modulates nanoparticle morphology without affecting polymorph outcome, identifying acidification kinetics as a synthetic handle acting independently of the factors governing polymorph selection. Using these mechanistic insights, we then show the systematic tunability of polymorph-dependent photochromic behavior in the resulting particles. Together, these findings provide independent synthetic handles for controlling WO3 nanoparticle phase and morphology which directly impact their functional properties, and offer a quantitative approach for reconciling and comparing outcomes across diverse reagent combinations and protocols in WO3 NP synthesis.