  Nanoporosity in silicon results in an interface-dominated mechanics, fluidics
and photonics that are often superior to the ones of the bulk material.
However, their active control, e.g. as a response to electronic stimuli, is
challenging due to the absence of intrinsic piezoelectricity in the base
material. Here, for large-scale nanoporous silicon cantilevers wetted by
aqueous electrolytes, we show electrosorption-induced mechanical stress
generation of up to 600 kPa that is reversible and adjustable at will by
electrical potential variations of approximately 1 V. Laser cantilever bending
experiments in combination with in-operando cyclic voltammetry and
step-coulombmetry allow us to quantitatively trace this large electro-actuation
to the concerted action of 100 billions of parallel nanopores per square
centimeter cross section and to determine the capacitive charge-stress coupling
parameter upon ion ad- and desorption as well as the intimately related stress
actuation dynamics for perchloric and isotonic saline solutions. A comparison
with planar silicon surfaces reveals mechanistic insights on the observed
electrocapillarity (electrostatic Hellmann-Feynman interactions) with respect
to the importance of oxide formation and pore-wall roughness on the
single-nanopore scale. The observation of robust electrochemo-mechanical
actuation in a mainstream semiconductor with wafer-scale, self-organized
nanoporosity opens up entirely novel opportunities for on-chip integrated
stress generation and actuorics at exceptionally low operation voltages.
