Lattice small polarons and magnetic interactions drive preferential nanocrystal growth in silicon doped hematite

TL;DR

This study reveals how lattice small polarons and magnetic interactions influence the anisotropic growth of silicon-doped hematite nanocrystals, offering insights for designing functional nanomaterials.

Contribution

It uncovers the role of small polarons and magnetic interactions in controlling nanocrystal morphology in doped hematite, a novel insight into material design.

Findings

Lattice small polarons increase with Si doping, affecting crystal strain.

A crossover from small to large polarons influences polaronic correlation length.

Magnetic double exchange interactions promote anisotropic nanocrystal growth.

Abstract

Understanding the interplay between the structural, chemical and physical properties of nanomaterials is crucial for designing new devices with enhanced performance. In this regards, doping of metal oxides is a general strategy to tune size, morphology, charge, lattice, orbital and spin degrees of freedoms and has been shown to affect nanomaterials properties for photoelectrochemical water splitting, batteries, catalysis, magnetic applications and optics. Here we report the role of lattice small polaron in driving the morphological transition from nearly isotropic to nanowire crystals in Si doped hematite ($\alpha-Fe_2O_3$). Lattice small polaron formation is well evidenced by the increase of hexagonal strain and degree of distortion of $FeO_6$ showing a hyperbolic trend with 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 $Fe^{2+}/Fe^{3+}$ species is found to be the driving force of the crossover, constraining the chaining of chemical bonds along the [110] 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 [110] facets. Our results show that magnetic and electronic interactions drive preferential crystallographic growth in doped metal oxides, thus providing a new route to design their functional properties.