Size-dependence of silicon nanoparticle etching: Surprisingly rapid hydrogen generation and novel intermediate structures

Many phenomena involving silicon nanoparticles have demonstrated size-dependence. The efficiency and rate of water splitting to form hydrogen gas by oxidation of silicon are much greater for 10 nm silicon nanocrystals than for larger silicon particles. In the work presented here, silicon nanocrystals of ∼10 nm in diameter produced hydrogen 150 times faster than 100 nm silicon, greatly exceeding the expected 10-fold increase expected from increased surface area. Hydrogen surface termination results in particles generating hydrogen gas in excess of the theoretical stoichiometric yield of 2 mol H2 per mol of Si. Transmission electron microscopy (TEM) and powder x-ray diffraction (XRD) show that 10 nm silicon particles are etched isotropically, in contrast to larger particles that etch anisotropically. A change in etching mechanism results in surprisingly rapid rates of etching and hydrogen generation. Silicon nanoparticles <100 nm in diameter and <40 µm in diameter were etched to quantify the size-dependence of hydrogen generation. TEM imaging was used to observe partially etched samples and gain insights into the etching process. Images of partially etched <100 nm silicon particles show hollow nanocapsule structures, markedly different from the solid nanospheres observed before reaction. These nanocapsules have oriented silicon walls. Formation of hollow structures was observed with different etchant types and concentrations, but the morphology of the nanocapsules depended on the etchant type and concentration. Hydrazine, KOH and NaOH form hollow nanospheres, nanoballoons and ruptured nanoballoons respectively when reacted with <100 nm particles. Hollow core formation is size-dependent and was not observed using 10 nm particles or microsilicon particles with any of the three etchants. Both hydrogen generation and nanocapsule formation exhibit qualitative size dependence, with instantaneous hydrogen generation only from 10 nm particles, and hollow capsule formation only with <100 nm particles.