Chemisorption of Water on the Surface of Silicon Microparticles Measured by Dynamic Nuclear Polarization Enhanced NMR

Abstract

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 µm) 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% H 2 O and 20% D 2 O, 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% H 2 O and 80% D 2 O, the narrow bound-proton peak was observed to shrink due to exchange between * To whom correspondence should be addressed arXiv:1610.06544v1 [cond-mat.mtrl-sci] 20 Oct 2016 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.

DNP-NMR of surface hydrogen on silicon microparticles

“…The MW irradiation was produced via a previously described millimeter wave source. [30,62] Powdered silicon particles (nominally 1-5µm poly crystalline silicon) were bought from Alfa Aesar, and were used as is. The powder was stored for many years in the original packaging, under ambient conditions before use.…”

“…Note that in previous experiments on the same powder only a single resonance was observed, with a FWHM of 6.2 kHz. [30] In that work, however, after wetting the same with 20:80 H 2 O:D 2 O, the NMR line it had an additional shoulder indicating the appearance of a second upfield resonance. The separation between the two resonances was found to be 34 ppm, similar to the separation we see in this work.…”

“…Note also that these spectra are similar to the EPR lineshapes published by Cassidy et al and Guy et al, measured at X-Band and W-Band, respectively, for the same particles. [21,30] The observed width of the EPR line is approximately 600 MHz at 115 GHz, suggesting an inhomogeneous linewidth of 490 MHz at 94 GHz. Figure 5 show that most of the observed DNP enhancement falls within the width of the EPR line (between -170 MHz and 140 MHz), but that there is still enhancement outside (<-170 GHz and >140 GHz).…”

“…This may be because the frequency of the triangular modulation is effectively twice that of sawtooth modulation (see Supporting Information), because it has been shown but that when the modulation frequency is much faster than the electron spin-lattice relaxation time T 1e a decrease in enhancement can be expected. [30] However, it is a small effect, and not clear why the upfield peak is affected more than the downfield peak. Figure 6: DNP spectrum of (a) the upfield peak and (b) the downfield peak of sample B: integrated intensity of each peak divided by the integrated intensity of the thermal MW off signal, plotted as a function of the MW irradiation frequency, with 120 s of irradiation at each frequency.…”

“…Silicon particles are known to contain localized unpaired electrons at the interface between the silicon and the silicon-oxide layers. [44,45,46,30,47,48,49] At room temperature these Si−H and Si−OH species are able to move around the surface to adjacent unoccupied dangling bonds, if any. These systems also exhibit a significant degree of heterogeneity based on the conditions under which the host materials were grown, the methods by which the powders were prepared and sorted, and the storage conditions.…”