  We use dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance
(NMR) at liquid helium temperatures to directly detect hydrogen attached to the
surface of silicon microparticles. The proton NMR spectrum from a dry sample of
polycrystalline silicon powder (1-5 $\mu$m) shows a distinctively narrow
Lorentzian-shaped resonance with a width of 6.2 kHz, indicative of a very
sparse distribution of protons attached to the silicon surface. These protons
are within a few atomic monolayers of the silicon surface. The high sensitivity
NMR detection of surface protons from low surface area ($0.26 - 1.3 \: m^2/g$)
particles is enabled by an overall signal enhancement of 4150 over the room
temperature NMR signal at the same field. When the particles were suspended in
a solvent with 80% H2O and 20% D2O, the narrow peak was observed to grow in
intensity over time, indicating growth of the sparse surface proton layer.
However, when the particles were suspended in a solvent with 20% H2O and 80%
D2O, the narrow bound-proton peak was observed to shrink due to exchange
between the surface protons and the deuterium in solution. This decrease was
accompanied by a concomitant growth in the intensity of the frozen solvent
peak, as the relative proton concentration of the solvent increased. When the
particles were suspended in the organic solvent hexane, the proton NMR spectra
remained unchanged over time. These results are consistent with the known
chemisorption of water on the silicon surface resulting in the formation of
hydride and hydroxyl species. Low-temperature DNP NMR can thus be used as a
non-destructive probe of surface corrosion for silicon in aqueous environments.
This is important in the context of using silicon MEMS and bioMEMS devices in
such environments, for silicon micro- and nano-particle MRI imaging agents, and
the use of nanosilicon for splitting water in fuel cells.
