1
Piezoelectric Nylon-11 Nanowire Arrays Grown by Template Wetting for Vibrational
Energy Harvesting Applications
Anuja Datta, Yeon Sik Choi, Evie Chalmers, Canlin Ou and Sohini Kar-Narayan*
[*] Dr. A. Datta, Y. Choi, E. Chalmers, C. Ou, Dr. S. Kar-Narayan
Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles
Babbage Road, Cambridge CB3 0FS, UK, E-mail: sk568@cam.ac.uk
Keywords: piezoelectric nanogenerator, odd-numbered Nylon, nanowires, energy harvesting,
template wetting.
Piezoelectric polymers, capable of converting mechanical vibrations into electrical
energy, are attractive for use in vibrational energy harvesting due to their flexibility,
robustness, ease and low cost of fabrication. In particular, piezoelectric polymers
nanostructures have been found to exhibit higher crystallinity, higher piezoelectric
coefficients
and ‘self-poling’, as compared to films or bulk. The research in this area has been
mainly dominated by polyvinylidene fluoride (PVDF) and its co-polymers, which while
promising, have a limited temperature range of operation due to their low Curie and/or
melting temperatures. Here, we report the fabrication and properties of vertically aligned, and
‘self-poled’ piezoelectric Nylon-11 nanowires with a melting temperature of ~200 °C, grown
by a facile and scalable capillary wetting technique. We show that a simple nanogenerator
comprising as-grown Nylon-11 nanowires, embedded in an anodized alumina (AAO)
template, can produce an open-circuit voltage of 1 V and short-circuit current of 100 nA,
when subjected to small-amplitude, low-frequency vibrations. Importantly, the resulting
nanogenerator is shown to exhibit excellent fatigue performance and high temperature
stability. Our work thus offers the possibility of exploiting a previously unexplored low-cost
piezoelectric polymer for nanowire-based energy harvesting, particularly at temperatures well
above room-temperature.
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1. Introduction
Nanogenerators (NGs) based on piezoelectric materials have attracted increasing
interest in light of the growing demand for wireless, portable, embedded, implantable and/or
wearable self-powered devices, and the ubiquitous availability of ambient vibrational energy
sources as a potential power solution.
[1-4]
Piezoelectric polymers, in particular, have been
widely studied and exploited in NG design,
[2,5-8]
as in spite of exhibiting weaker piezoelectric
properties than commonly used ceramics, such as barium titanate,
[9]
lead zirconium
titanate,
[10]
and zinc oxide,
[11-13]
they possess a range of advantages over ceramics that render
them mechanically stable, chemically robust and possibly biocompatible. Importantly,
nanostructures and nanowires of ferroelectric (and hence piezoelectric) polymers, such as
polyvinylidene (PVDF) and polyvinylidene fluoride trifluoroethylene (P(VDF-TrFE)), have
been found to exhibit superior piezoelectric performance,
[7,14-22]
by virtue of their nanoscale
confinement. There have been several reports on polymer-based NGs, including nanopressure
sensor, acoustic NG and mechanical energy harvester,
[23-27]
but the typically low Curie and/or
melting temperatures of these polymers limit their use in applications at higher temperatures,
for example in early-fault detection systems for heavy machinery that may require a wider
operating temperature range. There is thus a growing need to explore alternative piezoelectric
polymers with enhanced thermal stability, and in particular to develop low-cost scalable
processes to fabricate nanowires based on these materials, which can offer reliable energy
harvesting performance at higher temperatures.
In this regard, odd-numbered Nylons with relatively high melting temperatures, are
known to possess ferroelectric and piezoelectric characteristics by virtue of the high degree of
hydrogen bonding and dipole orientation resulting from the arrangement of polyamide
molecules within adjacent chains upon crystallization.
[28]
While, non-ferroelectric
even-numbered Nylons (Nylon-6,6 and Nylon-6) weaved as fabrics have already been
successfully used in textile industry,
[29]
and have found suitable applications as integrated
3
sensors and as components of energy devices,
[30]
ferroelectric odd-numbered Nylons
[31]
have
received considerably less attention. Among odd-numbered Nylons, Nylon-11 (polyamide-11
= [C
11
H
21
ON]
n
) exhibits piezoelectric and ferroelectric properties that are comparable to
PVDF at room temperature.
[32-35]
Typically, Nylon-11 exhibits five crystalline modifications,
including triclinic α and pseudo-hexagonal γ phases, which have different dipole densities and
orientation.
[36,37]
The high piezoelectricity in Nylon-11 is attributed to its polar crystal
structure (γ form), although other configurations may also lead to piezoelectric behavior (see
Supporting Information S1). A piezoelectric charge constant d
31
between 3-12 pC N
-1
has
been reported for a variety of melt drawn, strained, solution-cast and poled Nylon-11
films,
[36,38-42]
with a d
31
of up to ~15 pC N
-1
reported at relatively higher temperature (up to
200
o
C).
[43]
Nylon-11 has previously been prepared as nanofibers and nanoribbons via
electrospinning which requires high voltages,
[32,33]
but their performance as energy harvesters
has never been studied. Furthermore, scalable solution-based synthesis methods for Nylon-11
nanowires have not been attempted, and the understanding of their piezoelectric properties in
relation to the crystal structure in nanowires is yet to be reported.
Here we describe a facile and scalable synthesis technique for Nylon-11 nanowire
arrays of uniform size and high aspect ratio of ~ 200 by a solution-processed capillary
template infiltration method within AAO templates. Unlike our previous work on
P(VDF-TrFE) nanowires grown within a similar nanoporous template via gravity
infiltration,
[7]
herein, ultralong Nylon-11 nanowires were prepared by capillary wetting, which
was necesary in order to prevent fast drying of the infiltrated Nylon-11 solution and which
reproducibly yielded large density of highly crystalline nanowires of narrow size distribution,
with a significant presence of the piezoelectric γ phase imparted by the pore confinement of
the templates. In order to carry out further characterization, the nanowires could be
subsequently transferred to any substrate in the form of nanowire mats that resulted upon
preferential dissolution of the AAO template in phosphoric acid. More importantly, the
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Nylon-11 nanowires were ‘self-poled’,
[2,7,8,15,44,45]
and were incorporated as-grown into
template-based NGs for energy harvesting with a wide temperature range of operation and
excellent fatigue performance, without the requirement of poling via large external electric
fields that are otherwise typically required for piezoelectric performance. Enhanced
piezoelectric properties of Nylon-11 nanowires make this material a potentially low-cost
candidate for smart fabrics in the wearable technology industry, as has been demonstrated
recently with piezoelectric PVDF fibers,
[46,47]
with the added advantage of having a wider
temperature range of operation.
2. Results and Discussion
2.1. Growth Mechanism of Nylon-11 Nanowires
A schematic of the Nylon-11 nanowire-array growth process via capillary wetting of
the pores of an AAO template is shown in Figure 1. A three-step process was adopted to
control the growth of Nylon-11 nanowires inside the AAO template which comprised: (i)
preparation of the Nylon-11 solution in formic acid by dissolving the required concentration
(10 wt. % optimum) of Nylon-11 pellets; (ii) promoting the growth of Nylon-11 nanowires
via capillary action by placing the AAO templates on a Nylon-11 solution pool; and (iii)
cleaning the resulting loosely adhered Nylon-11 nanoparticles from the surfaces of the
templates following the capillary wetting process and subsequent evaporation of the formic
acid solvent. In order to study the density, morphology and the phase of the prepared
Nylon-11 nanowires, the AAO template was dissolved in phosphoric acid and a mat of
nanowires could be obtained and transferred to any desired substrate. The Nylon-11 polymer
concentration (wt. %) in solution was varied at the start of the growth process, and scanning
electron microscopy (SEM) imaging was carried out on the resulting nanowires to get an
understanding of the growth mechanism.
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The advantage of capillary wetting process in the preparation of Nylon-11 nanowires
was verified by carrying out complementary experiments using the more conventional
gravity-induced template infiltration method that was previously reported by our group for the
fabrication of P(VDF-TrFE) nanowires.
[7]
This method was found to be unsuitable for
Nylon-11 nanowire formation as can be seen in Figure 2(a) and (b). In the case of
conventional gravity-induced template wetting where the solution was drop-cast on to the top
surface of the template, incomplete pore-infiltration of Nylon-11 solution resulted (Figure
2(a)) due to fast drying out of the drop of formic acid solution. As a result, a thick film of
Nylon-11 was obtained on top of the template surface (Figure 2(b)), preventing further
soaking of the pores, and hence no nanowires were obtained using this method.
Capillary wetting (Figure 1) was therefore adopted as a viable alternative to ensure
complete infiltration of the template pores, for the template-assisted fabrication of Nylon-11
nanowires. It has been shown that the maximum height at which the polymer solution can
reach via “capillary wetting” is inversely proportional to the radius of the template pore, given
by Jurin’s law as h= 2γ (cos θ)/ρgr,
[48]
where γ is the surface tension of the polymer solution,
θ is the contact angle of the meniscus at the pore wall, ρ is the density of the polymer solution,
g is the acceleration due to gravity, and r is the radius of the nanopore. In this case, taking γ =
0.033-0.043 N m
-1
,
[31]
θ = 75-82°,
[49,50]
ρ = 1.01-1.03 gm cm
-3
,
[31]
and r = 125 nm (pore
diameter ~ 250 nm), respectively, the maximum height that the Nylon-11 solution can reach
via capillary force is calculated to be ~ 2.2 × 10
7
μm, which is much higher than the length of
the pore channels (thickness of the template ∼60 μm). However, the time required for filling
the pores is also controlled by the viscosity of the Nylon-11 pool solution,
[32]
which in turn
depends on the concentration of the pool solution (i.e. wt. % of the Nylon-11 pellets dissolved
in formic acid), and importantly, the rate of evaporation of the formic acid.
[51]
Thus in practice,
the actual height that the solution can successfully infiltrate is significantly lower. Also it has
been observed that when templates were wetted with polymers slightly above their
