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Giant electrochemical actuation in a nanoporous silicon-polypyrrole hybrid material

Giant electrochemical actuation in a nanoporous silicon-polypyrrole hybrid material
Synthesis of a nanoporous PPy-silicon material. (A) High-angle annular dark-field scanning TEM top view on a nanoporous silicon membrane filled by electropolymerization with pyrrole. The green and red color codes indicate the N and Si concentration resulting from EDX detection measurements, respectively. (B) Voltage-time recording during galvanostatic electropolymerization of pyrrole in nanoporous silicon, with mean pore diameter d and thickness t. Characteristic regimes are indicated and discussed in the main text. Credit: Science Advances, doi: 10.1126/sciadv.aba1483

The absence of piezoelectricity in silicon can lead to direct electromechanical applications of the mainstream semiconductor material. The integrated electrical control of silicon mechanics can open new perspectives for on-chip actuators. In a new report, Manuel Brinker and a research team in physics, materials, microscopy and hybrid nanostructures in Germany, combined wafer-scale nanoporosity in single-crystalline silicon to synthesize a composite demonstrating macroscopic electrostrain in aqueous electrolytes. The voltage-strain coupling was three-orders of magnitude larger than the best performing ceramics. Brinker et al. traced the electro-actuation to the concerted action of a 100 billion nanopores-per-square-centimeter cross-section and obtained exceptionally small operation voltages (0.4 to 0.9 volts) alongside sustainable and biocompatible base materials for biohybrid materials with promising bioactuator applications. The work is now published on Science Advances.

Developing polymers with embedded electrochemical actuation

Electrochemical changes that occur during the oxidation of the conductive polymer polypyrrole (PPy) can increase or decrease the number of delocalized changes in the polymer backbone. When immersed in an electrolyte, the material is accompanied with reversible counter-ion uptake or expulsion with macroscopic contraction as well as swelling under electrical potential control to make PPy one of the most common materials to develop artificial muscle materials. In this work, Brinker et al. combined the actuator polymer with a three-dimensional (3-D) scaffold structure of nanoporous silicon to design a material for embedded electrochemical actuation. The new construct contained a few light and abundant elemental constituents including hydrogen (H), carbon (C), nitrogen (N), oxygen (O), silicon (Si) and chlorine (Cl).

During the experiment, the team prepared the porous silicon (pSi) membrane using an electrochemical etching process of doped silicon in hydrofluoric acid. The resulting pores were straight and perpendicular on the silicon surface. Using scanning electron microscopy profiles, Brinker et al. observed a homogenous sample thickness. They then filled the porous silicon (pSi) membrane with polypyrrole (PPy) through electropolymerization of pyrrole monomers. Polymer nucleation and partial oxidation of pSi increased the open circuit potential leading to a constant deposition of PPy inside the pores. The highly asymmetrical pores formed a chain-like polymer growth inhibiting the branching of the polymer and leading to lower electrical resistance. The team observed the resulting composite using transmission electron micrographs (TEM) with energy-dispersive X-ray (EDX) spectroscopy signals to indicate homogeneous PPy filling of the random pSi honeycomb structure.

Giant electrochemical actuation in a nanoporous silicon-polypyrrole hybrid material
Structural characterization by sorption isotherm, illustration of the polymerization cell and illustration of the electroactuation setup. (A) Nitrogen sorption isotherm at T = 77 K recorded for nanoporous silicon. Plotted is the volume filling fraction f against the relative vapour