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The authors synthesized planar Mg-silicate space dust analogues via ultra-high vacuum co-deposition of Si, Mg, and O atoms on graphite to evaluate how surface stoichiometry drives interstellar chemistry. In situ XPS, NEXAFS, and ex situ electron microscopy paired with DFT calculations show that adjusting the Mg:Si ratio from 3.5 to 1.8 and applying thermal annealing drives the composition toward Mg-silicate while persistently preserving non-stoichiometric, catalytically active surface motifs. This work establishes a reproducible 2D surface-science platform for directly measuring grain-surface reaction kinetics in star- and planet-forming regions.
Persistent non-stoichiometric surface defects survive thermal annealing on interstellar dust analogues, revealing why planetary building blocks maintain catalytic activity in harsh astrophysical environments.
The atomic structure at the surface of interstellar silicate dust particles likely plays a key role in a variety of chemical processes occurring in star- and planet-forming regions of the interstellar medium (ISM). Here, we use the photoelectric effect to characterize, in situ, the local chemical structure of Mg-silicate nanoparticulate films, and ex situ electron microscopy to trace changes to morphology resulting from different starting compositions. Nanoparticulate films are prepared via co-deposition of Si, Mg and O atoms on a graphitic substrate under ultra-high vacuum (UHV) and are characterised with X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure measurements. Analysis, supported by density functional theory calculations, shows that adjusting the Mg-to-Si ratio from 3.5 to 1.8 changes particle composition from a mixture of MgO and Mg-silicate towards predominantly Mg-silicate. Annealing in UHV also pushes the composition towards Mg-silicate, however, non-stoichiometric chemical motifs are always observed, presenting a distribution of potentially catalytically active sites. Electron microscopy images show composition-dependent film morphologies that restructure differently upon thermal annealing. The Mg-silicate nanoparticulate films presented here are characterised as a 2D version of ISM analogue dust particles prepared via laser ablation and are therefore a more readily suitable model for surface science investigations into the catalytic properties of interstellar Mg-silicate dust.