Electrochemical Actuation in Porous Silicon
Authors: Brinker Manuel , Huber Patrick , Dittrich Guido , Richert Claudia , Huber Norbert , Lakner Pirmin Hubert , Keller Thomas Florian , Krekeler Tobias 
Published: December 2020

We show large and reversible electrochemical actuation in a material based on self-organised nanoporosity in silicon (pSi) in combination with the electrically conductive polymer polypyrrole (PPy), which is introduced inside the pore space. On a single-pore scale the electrochemical actuation can be traced to the high porosity of 50 % and to potential-dependent pressures of up to 15 MPa acting at the single-pore scale. These pressures originate in charging or discharging and the related counter-ion movements. The exceptionally small operation voltages (0.4-0.9V) and the simple synthesis enable a silicon based actor.

INTRODUCTION

Self-organized nano-porosity in silicon has been attracting interest from a fundamental point of view and for applications due to its special thermal, electrical and optical properties and its bio compatibility [1]. However, the absence of the piezo-electric effect in silicon makes applications in the field of electro-mechanics for this widely used semiconductor difficult. The aim of this work is to study the sensor and actuator characteristics of bare pSi and pSi filled with PPy. An electrochemical change in the oxidation state of PPy can increase or decrease the number of delocalised charges in the polymer. Immersed in an electrolyte this is also accompanied by a reversible counter-ion uptake or expulsion and thus with a marcroscopic contraction or swelling under electrical potential control, making PPy one of the most employed artificial muscle materials [2]. The material is immersed in an electrolyte solution and studied by different techniques as in-situ dilatometry and in-situ X-ray diffraction.

EXPERIMENTAL RESULTS AND DISCUSSIONS

Porous silicon is prepared by an electrochemical anodization process in a solution of hydrofluoric acid and ethanol by applying a constant current between a platinum cathode and a highly doped silicon wafer. The resulting pores are characteristically straight and orthogonally aligned to the silicon surface. The pore diameter amounts to 7.6 nm and is determined via nitrogen sorption isotherm as is the porosity of 50 %. In a second step PPy is incorporated inside the pores via an electro-polymerisation in galvanostatic mode.
Dilatometry measurements in an in-situ electrochemical setup are performed in order to characterize the actuating properties of the resulting pSi-PPy hybrid material. The sample is therefore immersed in perchloric acid (HClO4) and positioned in the setup, so that the pores are pointing in a horizontal direction, i.e. the sample is standing upright.
Measuring the sample length change while recording Cyclovoltammograms (CVs) allows a detailed characterisation of the electrochemical actuation. The potential E is reversibly changed from 0.4 V to 0.9 V versus the normal hydrogen electrode potential. An increasing potential leads to an increased insertion of chlorate-anions in the PP. Vice versa, decreasing the potential expells anions from the PPy. It is found, that the repetitive change in sample length coincides linearly with E. So, the incorporation of anions leads to an expansion of the sample and the release of the anions lets the sample contract.
For obtaining a further understanding of the electrochemical actuation mechanism of the PPy filled pSi membranes, the micromechanical properties are modelled based on the microstructure as recorded by electron transmission micrographs.
REFERENCES
[1] M. J. Sailor, Porous Silicon in Practice - Preparation, Characterization and Applications, Wiley-VCH, 2011.
[2] E. Smela, Adv. Mater., 2003, 15, 481–494.