Influence of precursor composition and thermal annealing on structure, phase, purity, and functionality of spray-flame made FexOy

Despite spray-flame synthesis (SFS) enabling versatile production of functional iron oxide nanomaterials, systematic studies quantifying structure-purity-function relationships remain scarce. Moreover, γ-Fe 2 O 3 /Fe 3 O 4 differentiation is accompanied by methodological uncertainties, while methods to systematically compare material purities are lacking. Here, we present a systematic comparison of iron oxides synthesized from iron(III) nitrate nonahydrate (INN) in pure ethanol (EtOH) vs. ethanol/2-ethylhexanoic acid (EtOH + EHA) using the standardized SpraySyn1 burner, employing Mössbauer spectroscopy for unambiguous phase identification. We further introduce particle surface loading ( PSL ) as a size-independent metric for surface impurities, enabling systematic purity comparison across synthesis conditions. Evaluating structure, purity, and functionality measured by vibrational scanning magnetometry (VSM) and oxygen evolution reaction (OER) demonstrates that INN in EtOH + EHA predominantly yields small γ-Fe 2 O 3 particles (<10 nm, ~95 wt%) with ~60% higher organic PSL than pure EtOH, yet exhibiting superparamagnetism despite moderate polydispersities. In contrast, INN in EtOH produces bimodal distributions containing ~20 wt% α-Fe 2 O 3 , benefiting OER activity. Thermal annealing at 250 °C eliminates surface-bound carbonaceous species and induces diffusion-driven sintering. Consequently, PSL decreases while Fe 2 O 3 content increases and particles exhibit increased crystallinity and size. Thus, thermally annealed samples from INN in EtOH + EHA exhibit doubled magnetic response (~50 emu/g vs. ~20 emu/g as-synthesized) while maintaining superparamagnetism. However, annealing decreases catalytically active sites, diminishing OER activity across all samples. These structure-purity-function correlations establish application-specific design guidelines: INN in EtOH + EHA with thermal annealing yields γ-Fe 2 O 3 suitable for magnetic applications, while as-synthesized samples from pure EtOH exhibit higher OER activity. • Solvent choice alters structure, purity, and functionality of nanoscale iron oxide • PSL metric enables size-independent quantification of surface-bound by-products • Definitive phase identification via Mössbauer spectroscopy • Moderate size-polydispersity preserves superparamagnetism in γ-Fe 2 O 3 samples • Thermal annealing enhances M S but reduces OER via diffusion-driven sintering