Hematite (α-Fe2O3) is a promising and Earth-abundant material for solar fuel production, and Si-doping has been employed as a general strategy to improve its performance. However, an atomistic description that reconciles the modifications that Si-doping induces on the morphology, crystalline lattice, and electronic and magnetic properties of α-Fe2O3 has remained elusive. Here we report on the role of electron small polarons in driving the morphological transition from nearly rounded-shaped to nanowire nanocrystals in Si-doped hematite α-Fe2O3. Electron small polaron formation is evidenced by the formation of Fe2+ and the increase of FeO6 distortion at increasing Si content. Local analysis via pair distribution function highlights an unreported crossover from small to large polarons, which affects the correlation length of the polaronic distortion from short to average scales. Ferromagnetic double exchange interactions between Fe2+/Fe3+ species are found to be the driving force of the crossover, constraining the chaining of chemical bonds along the  crystallographic direction. This promotes the increase in the reticular density of Fe atoms along the hematite basal plane only, which boosts the anisotropic growth of nanocrystals with more extended  facets. Our results show that magnetic and electronic interactions drive preferential crystallographic growth in Si-doped α-Fe2O3, thus providing new insights for the nanoscale structural design of efficient solar fuel devices.
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