Facet-selective adsorption of Fe() on hematite visualized by nanoscale secondary ion mass spectrometry

• Published in: Environmental science. Nano 2019-08, Vol.6 (8), p.2429-244
• Main Authors: Taylor, Sandra D, Kovarik, Libor, Cliff, John B, Rosso, Kevin M
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Crystal defects; Crystal structure; Crystallites; Crystals; Defects; Electron transfer; Fronts; Haematite; Hematite; Imaging techniques; Ions; Iron; Iron 57; Iron isotopes; Isotopic tracers; Mass spectrometry; Mass spectroscopy; Metal oxides; Metals; Oxides; Oxidoreductions; Scientific imaging; Secondary ion mass spectrometry; Selective adsorption; Spectroscopy; Surface structure; Thin films; Tracers; Uptake
• ISSN: 2051-8153; 2051-8161
• DOI: 10.1039/c9en00562e

Facet-specific reactivity of metal oxide particles is a well-known but at times difficult to probe phenomenon. Furthermore, in semiconductor metal oxides where crystal facets enclosing particles are electrically connected, separating them to enable detailed characterization defeats the purpose; the study of intact individual crystallites is necessary. Here we develop a mass-sensitive imaging approach to do so, and demonstrate its potential by unveiling the preferential binding of Fe( ii ) to various surfaces of the Fe( iii ) oxide hematite. Using isotopic tracers to follow iron provenance, 56 Fe-hematite microplatelets with various enclosing facets are reacted with aqueous 57 Fe( ii ) at circumneutral pH. The resulting distribution of 57 Fe across the hematite surfaces is directly visualized and quantified using nanoscale secondary ion mass spectrometry (NanoSIMS). The results unambiguously show Fe( ii ) sorption is highly selective for the basal (001) surface, while edge surfaces such as (012) and (110) are enriched to a lesser extent (up to 10× lower). Crystal intergrowth defects exposing poorly-ordered, nanoscale surface structures show the least enrichment. These results resolving Fe( ii )-Fe( iii ) reaction fronts across multi-facetted crystals provide a clear correlation between uptake and particle surface structure. The illustrated approach to understanding facet-specific ion uptake is also likely generalizable to other interfacial processes such as electron transfer and heterogeneous catalysis, across a broad range of particle and thin-film based systems. Novel isotopic labelling and imaging techniques are used to directly observe the autocatalytic reaction and facet-selective adsorption of Fe( ii ) onto individual hematite (Fe 2 O 3 ) crystallites.