Solvent-controlled synthesis routes in a premixed stagnation flame of high-entropy oxide nanoparticles

High-entropy oxides (HEOs) hold potential for energy conversion and storage applications, yet precise nanoscale morphological control remains a critical challenge. This study demonstrates the route-selective synthesis of single-phase (Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)₃O₄ spinel nanoparticles via a liquid-fed aerosol stagnation flame, achieved through the systematic formulation of precursor solvents (water, ethanol, and an ethanol/2-ethylhexanoic acid [EHA] mixture). We find that the particle formation mechanism is governed by the interplay between residence time and characteristic droplet evaporation and precursor decomposition and vaporization timescales. Water-based precursors follow a droplet-to-particle (DTP) route, producing uniform submicron solid spheres (∼500 nm). In this case, early solute precipitation and hydrolysis create a surface shell that traps low-volatility nitrates within the condensed phase. Replacing water with ethanol triggers localized micro-explosions, yet the low volatility of the nitrates prevents a complete mechanistic shift, leading to a bimodal size distribution. Conversely, adding EHA to ethanol triggers a transition to the gas-to-particle (GTP) route. EHA-mediated ligand exchange produces highly volatile metal carboxylates, reducing the vaporization timescale. Coupled with droplet micro-explosions driven by the preferential evaporation of solvents with contrasting volatilities, this synergy favors exclusive particle formation via GTP. This GTP route yields ultrafine, well-crystallized nanoparticles with high compositional uniformity. These results establish a framework based on reaction timescales for solvent-controlled flame synthesis, allowing for the morphological customization of complex multicomponent nanomaterials. Novelty and significance statement This work establishes a solvent-based route-switching strategy for liquid-fed flame synthesis of five-component high-entropy spinel oxides, enabling controlled selection between gas-to-particle (GTP) and droplet-to-particle (DTP) formation. This rarely reported approach achieves near-exclusive DTP synthesis, expanding solvent engineering beyond prior single- or two-metal systems to complex high-entropy oxides. Mechanistic investigations using a quasi-one-dimensional stagnation flame reactor accommodate pure aqueous precursors and support spatially resolved in-situ diagnostics. Across all solvent formulations, synthesized oxides maintain a single-phase solid-solution structure with tunable particle morphology and crystallite size. This provides a generalizable design principle for producing multi-component oxide nanoparticles suited to energy storage and catalysis without modifying the initial metal salt precursors or reactor configuration, achieving controlled route selection solely through adjusting the solvent formulation.