Flame chemistry control of growth–etching dynamics: Antagonistic effects of iron doping in iron-oxide nanoparticle synthesis

We investigate the impact of iron doping on flame temperature and hydroxyl (OH) radical concentration in flames used for iron-oxide nanoparticle synthesis, with direct implications for growth–etching dynamics governing particle formation. Two flame configurations are studied: low-pressure (30 mbar) H₂/O₂/Ar flames with equivalence ratios ( ϕ ) ranging from 0.25 to 1.5 doped with iron pentacarbonyl (Fe(CO)₅), and atmospheric-pressure spray flames fed with combustible iron nitrate nonahydrate (INN) solutions. Flame temperature and OH radical concentrations are quantified by laser-induced fluorescence (LIF) imaging. In the low-pressure configuration, Fe(CO)₅ addition leads to a modest increase in OH concentration arising from two competing effects: an iron-induced temperature increase promoting OH concentrations, and OH consumption via reactions with iron-containing species. The balance between these antagonistic pathways depends strongly on the equivalence ratio, thereby modifying the local oxidative environment that controls nanoparticle growth and surface etching. Chemical kinetics simulations predict a stronger OH enhancement than observed experimentally, indicating that OH scavenging by iron-containing species is underestimated in the current reaction mechanisms. In contrast, in spray flame synthesis, INN addition has a negligible effect on both temperature and OH concentration, constituent with the low abundance of Fe–O–H species under these conditions. These results demonstrate that precursor-dependent flame chemistry can significantly alter the reactive environment governing particle formation with direct implications for particle-size distributions and phase composition, highlighting the need to account for metal–radical interactions when designing flame synthesis processes for oxide nanopowders.