Mechanical Engineering Publications Mechanical Engineering
2016
Platinum Nanoparticle Decorated SiO2
Micro&bers as Catalysts for Micro Unmanned
Underwater Vehicle Propulsion
Bolin Chen
Iowa State University, cbl0511@iastate.edu
Nathaniel T. Garland
Iowa State University, ngarland@iastate.edu
Jason Geder
United States Naval Research Laboratory
Marius Pruessner
United States Naval Research Laboratory
Eric Mootz
Iowa State University
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Authors
Bolin Chen, Nathaniel T. Garland, Jason Geder, Marius Pruessner, Eric Mootz, Allison Cargill, Anne Leners,
Granit Vokshi, Jacob Davis, Wya9 Burns, Michael A. Daniele, Josh Kogot, Igor L. Medintz, and Jonathan C.
Claussen
8is article is available at Iowa State University Digital Repository: h9p://lib.dr.iastate.edu/me_pubs/200
Platinum Nanoparticle Decorated SiO
2
Microfibers as Catalysts for
Micro Unmanned Underwater Vehicle Propulsion
Bolin Chen,
†
Nathaniel T. Garland,
†
Jason Geder,
§
Marius Pruessner,
∥
Eric Mootz,
†
Allison Cargill,
†
Anne Leners,
†
Granit Vokshi,
†
Jacob Davis,
†
Wyatt Burns,
†
Michael A. Daniele,
⊥,#
Josh Kogot,
∇
Igor L. Medintz,
∥
and Jonathan C. Claussen*
,†,‡
†
Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, United States
‡
Research Ames Laboratory, Ames, Iowa 50011, United States
§
Laboratories for Computational Physics and Fluid Dynamics, Code 6041, U.S. Naval Research Laboratory, 4555 Overlook Ave. SW,
Washington, DC 20375, United States
∥
Center for Bio/Molecular Science & Engineering, Code 6900, U.S. Naval Research Laboratory, 4555 Overlook Ave. SW,
Washington, DC 20375, United States
⊥
Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27606, United States
#
Joint Department of Biomedical Engineering, University of North Carolina-Chapel Hill/North Carolina State University, Raleigh,
North Carolina 27695, United States
∇
Naval Surface Warfare Center, Panama City, Florida 32407, United States
ABSTRACT: Micro unmanned underwater vehicles (UUVs)
need to house propulsion mechanisms that are small in size but
sufficiently powerful to deliver on-demand acceleration for
tight radius turns, burst-driven docking maneuvers, and low-
speed course corrections. Recently, small-scale hydrogen
peroxide (H
2
O
2
) propulsion mechanisms have shown great
promise in delivering pulsatile thrust for such acceleration
needs. However, the need for robust, high surface area
nanocatalysts that can be manufactured on a large scale for
integration into micro UUV reaction chambers is still needed.
In this report, a thermal/electrical insulator, silicon oxide
(SiO
2
)microfibers, is used as a support for platinum
nanoparticle (PtNP) catalysts. The mercapto-silanization of
the SiO
2
microfibers enables strong covalent attachment with PtNPs, and the resultant PtNP−SiO
2
fibers act as a robust, high
surface area catalyst for H
2
O
2
decomposition. The PtNP−SiO
2
catalysts are fitted inside a micro UUV reaction chamber for
vehicular propulsion; the catalysts can propel a micro UUV for 5.9 m at a velocity of 1.18 m/s with 50 mL of 50% (w/w) H
2
O
2
.
The concomitance of facile fabrication, economic and scalable processing, and high performanceincluding a reduction in H
2
O
2
decomposition activation energy of 40−50% over conventional material catalysts paves the way for using these nanostructured
microfibers in modern, small-scale underwater vehicle propulsion systems.
KEYWORDS: platinum nanoparticles, silicon microfibers, propulsion, hydrogen peroxide, micro unmanned underwater vehicles
■
INTRODUCTION
Micro UUVs, or vehicles measuring between half a meter and a
few centimeters in length, are advantageous for a variety of
applications including characterization of 3D flow dynamics,
1
remote oceanic environmental monitoring,
2
and ocean floor
surveying and reconnaissance;
3
all of which shed new light on
our understanding of marine life. Moreover, such remote
exploration is not limited to littoral waters but may soon
include a wide variety of under ice exploration such as
exploration of Saturn’s moon Europa,
4
which are becoming
more approachable with new technologies. These underwater
missions require vehicles with the ability to perform tight radius
turns, burst-driven docking maneuvers, and low-speed course
corrections.
5
In order to provide the micro UUVs with these
capabilities, jet propulsion technologies are beginning to replace
propeller-based systems which are more prevalent on larger
unmanned underwater vehicles. This jet propulsion can be
accomplished using energy dense fuel systems such as
bipropellant rockets,
6
air-breathing engines, supercapacitors,
thermal batteries, and Ni−Cd or Ag−Zn batteries.
7
However,
monopropellant and cold gas thrusters greatly outperform said
propulsion systems, showing up to 45 times the maximum
Received: August 15, 2016
Accepted: October 11, 2016
Published: October 11, 2016
Research Article
www.acsami.org
© 2016 American Chemical Society 30941 DOI: 10.1021/acsami.6b10047
ACS Appl. Mater. Interfaces 2016, 8, 30941−30947
power density of Ni−Cd batteries in small-scale applications.
7
In particular, the monopropellant H
2
O
2
shows promise for
propulsion and has already been used by NASA in reaction
control systems, primarily to generate thrust for 100 kg
satellites.
8
H
2
O
2
can be decomposed catalytically to produce steam and
oxygen at high temperatures, making it an environmentally
friendly fuel.
9
Furthermore, H
2
O
2
has a large power density
with a high specific thrust when compared to other green
monopropellants.
10
Many different catalysts have been utilized
to initiate H
2
O
2
decomposition for both macroscale and micro/
nanoscale applications including self-propelled microbots,
11−13
and polymer-based nanorockets.
14−16
H
2
O
2
decomposition
catalysts have been fabricated using a range of materials such as
metal oxides
17−19
(e.g., iron(III) oxide, manganese dioxide, and
potassium dichromate), metals such as silver (Ag)
14
and
platinum (Pt),
9
and bimetals such as Pt−palladium (Pt−Pd).
15
Metal oxides are consumed during H
2
O
2
decomposition and,
therefore, can negatively impact propulsion and mission
duration. Conversely, Pt metal has been shown to produce
catalysts with lower activation energy than all other group VIII
metals,
20
and nanostructured Pt nanoparticle (Pt-NP) nano-
wires have shown lower activation energies than Pt−Pd
catalysts.
9
This superior performance makes Pt catalysts well-
suited for H
2
O
2
decomposition as demonstrated in a wide
variety of small-scale applications including sensors/biosen-
sors,
21−23
Pt-loaded stomatocytes,
24
tubular bubble thrusters or
nanomotors/microengines,
25
and microelectromechanical sys-
tem (MEMS) based thrusters.
26
PtNPs have been grown on a wide variety of substrates
including highly conductive surfaces such as graphene,
27−29
carbon nanotubes (CNTs),
30,31
and graphene foam,
32
as well as
nonconductive surfaces such as oxides (e.g., SiO
2
, aluminum
oxide),
33
and paper/cellulose.
34
Templates such as mesoporous
silica networks, polycarbonate, and porous anodic alumina are
often employed to form Pt nanoparticles/nanowires onto
surfaces via electrodeposition.
35,36
Decoration of PtNPs onto
surfaces via nontemplated growth methods has also been
developed, along with PtNP surface immobilization via covalent
linkage, encapsulation, adsorption, biomolecule−nanoparticle
binding, and controlled (e.g., current-pulse) electrodeposi-
tion.
37,38
Deposition by the reduction of chloroplatinic acid is a
simple, one-step process offering several advantages. Most
notably, the morphology and density of PtNPs on carbon
structures can be controlled by varying the Pt salt bath
concentration, pH, and deposition time.
39
In our previous
work, we have also shown electroless deposition o f Pt
nanowires on carbon nanotube microchannel membranes
40
grown through chemical vapor deposition as well as on
microfiberous cellulose films.
9,40
However, this work uniquely
applies these techniques to a non-carbon, thermal insulator
substrate (i.e., SiO
2
one of the most abundant materials on
earth) to produce an inexpensive, robust, and efficient catalyst
for micro UUV propulsion.
Herein, we apply an electroless, template-free Pt deposition
technique to produce PtNPs on nonconductive SiO
2
micro-
fibers via the reduction of chloroplatinic acid (H
2
PtCl
6
)by
formic acid (HCOOH). Pt is nucleated on the SiO
2
microfibers
and subsequently forms an even coating of “spherical-like’’
nanoparticles on the microfiber surface. This spherical
nanoparticle morphology is distinct from the Pt nanowire
“urchin-like” structures that we previously developed using
similar techniques on carbon-based structures including
cellulose and carbon nanotubes (CNTs).
9,40
We demonstrate
that catalytic conversion of H
2
O
2
to oxygen and water provided
by the Pt−SiO
2
microfibers provides sufficient thrust to propel
micro UUVs while only adding a few grams of weight to the
vehicle. Thus, this novel nanostructured catalyst provides a
robust, low weight solution for the pulsatile propulsion of micro
UUVsan emerging field of research
9,40,41
nestled between, in
terms of length scale, recent advances in H
2
O
2
nano/microscale
motors
5,11−16,22,42−44
and large-scale satellites.
45,46
■
MATERIALS AND METHODS
Mercaptosilane Modification of SiO
2
Microfibers. Before
silanization, SiO
2
microfibers (Sigma-Aldrich) were washed by
ultrasonication in deionized water (18 MΩ·cm) for 30 min and
soaked in 1 M NaOH for 30 min. The mercaptosilane [-(mercapto-
propyl)trimethoxysilane, Sigma-Aldrich] was diluted with acetone to
2% (v/v), and the SiO
2
microfibers were incubated in the solution for
3 h at room temperature, and then triple rinsed with acetone and dried
under vacuum at 65 °C for 60 min (Figure 1a,b).
PtNPs Growth on SiO
2
Microfibers. A solution of chloroplatinic
acid hexahydrate, formic acid, and deionized water is used to initiate Pt
growth (Figure 1 c). Three different solutions were created with
varying amounts of chloroplatinic acid hexahydrate (37.5% Pt, Sigma-
Aldrich 206083). Each solution contained 2 mL of reagent grade
formic acid (88% HCOOH, Macron 2592-05) and 18 mL of deionized
water, to which chloroplatinic acid hexahydrate was added360, 180,
and 90 mg in each respective solution, creating three different strength
solutions: 34.7, 17.4, and 8.7 mM, respectively. Aliquots of
concentrated ammonium hydroxide solution (30.0% NH
3
basis)
were added to said metal salt solutions until each mixture had a pH of
1.75. After the solutions were prepared, mercaptosilane-coated glass
wool microfibers were placed in each mixture and left to soak for
Figure 1. Unmodified silicone oxide fibers (a) are silanizedcoated
with organofunctional molecules via self-assembly (b)so that bonds
can form which link nan oparticles to the fibers (c). H
2
O
2
is
decomposed via catalytic decomposition (d), which produces a
pressure increase that is monitored to evaluate power produced. (e)
Optical image showing size of glass wool microfiber mat (left) and
concentration of Pt coated on glass wool fiber mat without silane
treatment (middle) and with silane treatment (right).
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b10047
ACS Appl. Mater. Interfaces 2016, 8, 30941−30947
30942
approximately 16 h. The glass wool microfibers were finally rinsed
thrice with deionized water and air-dried at room temperature.
H
2
O
2
Decomposition. H
2
O
2
decomposition testing was per-
formed according to our previous protocols
9,40
for each catalyst sample
(34.7, 17.4, and 8.7 mM) at three different temperatures (0, 17.5, 35
°C). In brief, a concentration of 1% H
2
O
2
(diluted from 30% w/w in
H
2
O
2
from the manufacturer: Fisher Scientific BP2633-500) was used.
The mass of Pt−SiO
2
catalyst used in each test was held constant at
0.03 g to make a more accurate assessment of the effectiveness of each
developed catalyst with distinct molar concentrations of Pt salt. The
test apparatus included two 125 mL round-bottom glass flasksone
to test the catalyst and the other as a control flask without catalyst.
Magnetic stir bars were positioned in the bottom of the flasks to
simulate a “flowing” environment, thus maximizing the amount of
H
2
O
2
contacting the catalyst sample. The flasks are placed inside water
baths on top of magnetic stirrer hot plates to maintain quasi isothermal
conditions at 17.5 and 35 °C, respectively, while the flasks were placed
in ice water baths to maintain the flask temperatures at 0 °C. A
thermometer was dipped into the bath solutions to monitor and adjust
the bath temperatures accordingly. The amount of oxygen generated
during each test was represented as a pressure diff erential between the
testing (PtNP−SiO
2
and H
2
O
2
reaction) and reference environments.
Pressure differences versus time were graphically displayed on a
computer. In between each trial run, the nanowires were removed
from the flask with tweezers and dried to ensure a maximum reaction
for the following runs. Resultant diff erential pressure vs time data were
then used to determine catalyst efficiency, as shown in the Results and
Discussion section.
Micro UUV Fabrication and Testing. The micro UUV was
designed in SolidWorks, a computer aided design program, then
manufactured with an Objet500 Connex 3D Printer using a PMMA-
like resin for the printing material. Once fabricated, the micro UUV
was attached to a 30.5 in. (0.77 m) rigid arm via screw thread fastening
and submerged in a 350 gal tank filled with 250 gal of water. The
opposite end of the arm was attached to a torque transducer (Interface
model 5350-50:50 oz-in sensor) mounted above the water tank. The
transducer was capable of measuring torque moments about the
neutral axis of the micro UUV within a 0.001 N·m tolerance, and the
readings were outputted via a CPU connection. The thrust produced
by the micro UUV was calculated from the torque readings using
software on the CPU. High-purity H
2
O
2
was pumped into the reaction
chamber with a 50 mL syringe through a high strength silicone tube
(inner dia. 0.25 in./9.525 mm) fitted to the reaction chamber via a
plastic barbed fitting. Steady flow rates were produced by applying a
constant pressure via a syringe pump connected to the syringe.
Pulsating flow was produced by manually applying pressure to the
syringe. Note this micro UUV was designed to rigidly attach to the
strain gauge to measured underwater vehicle thrust measurements via
the decomposition of H
2
O
2
with the developed Pt−SiO
2
catalyst; the
developed micro UUV was not designed for free swimming.
■
RESULTS AND DISCUSSION
Design and Fabrication of Pt−SiO
2
. In previous studies,
mercaptosilane-activated surfaces have permitted direct cou-
pling of oligonucleotides to glass surfaces
37
and enzymes to
platinized surfaces.
9
Such glass-silanization and Pt-silanization
techniques offer strong lateral stabilization of biological agents
through covalent linkages. These techniques form the chemical
modification rationale for this work, in that we hypothesize that
mercaptosilane-activated SiO
2
fibers will form strong covalent
linkages with PtNPs. Figure 1a−c shows a schematic of these
silanization processes. A low pH (1.75) was selected to create
the needle-like Pt nanowires, as our previously published work
demonstrated that this low pH assists in the development of
dense PtNPs onto surfaces, namely, cellulose and CNTs.
9,40
Figure 1e qualitatively illustrates how mercaptosilane-activated
SiO
2
promotes the deposition of PtNPs onto the SiO
2
fibers,
where Pt nanoparticle density (black) is much denser on
mercaptosilane-activated SiO
2
fibers as compared to those that
are not. Scanning electron microscopy (SEM) micrographs of
these Pt dense SiO
2
fibers reveal a surface coverage of PtNPs
with large micro/macro cloudlike PtNPs at higher chloropla-
tinic acid hexahydrate concentrations, i.e., 34.7 mM (Figure 2).
The size and density of these Pt structures are subsequently
improved for enhanced catalytic decomposition of H
2
O
2
.
The size and density of the PtNPs can be tuned by adjusting
the concentration of the Pt salt bath used in the deposition
process. For example, a 34.7 mM chloroplatinic acid
hexahydrate concentration produces highly aggregated nano-
particles with larger micro/macroparticles on the mercaptosi-
lane-activated SiO
2
(Figure 3a). The density of the PtNPs
significantly decreases while the relative number of larger
micro/macroparticles stays relatively the sa me when the
chloroplatinic acid hexahydrate concentration is reduced by
half to 17.4 mM ( Figure 3b). Finally, a monodisperse layer of
PtNPs without larger micro/macroparticles is formed on the
mercaptosilane-activated SiO
2
by reducing the chloroplatinic
acid hexahydrate concentration by half again to 8.7 mM (Figure
3c). The effects of this dense, uniform surface coverage of
PtNPs on the SiO
2
microfiber surface significantly improves the
reaction kinetics of the decomposition of H
2
O
2
as compared to
the other Pt−SiO
2
microfibers (Figure 3d). These reaction
kinetics are discussed in detail in the next section of the paper.
Catalyst Characterization: H
2
O
2
Decomposition. It is
known that the effectiveness of Pt catalysts for H
2
O
2
decomposition is proportional to the contact area between Pt
and H
2
O
2
, as higher exposed surface area leads to increased
likelihood for Pt-(-OH) and Pt-(-H) binding.
19
The reaction of
H
2
O
2
decomposition at low temperatures produces liquid water
and oxygen (eq 1).
→+
2
HO 2HO(l) O(g)
22 2 2
(1)
Figure 2. Scanning electron microscopy (SEM) images obtained at 20
keV show the size, density, and morphology of Pt nanoparticles and
macro/microparticles decorated on mercaptosilane-ac tivated SiO
2
microfibers with 34.7 mM chloroplatinic acid hexahydrate. The SEM
magnification is (a) 1 k×, (b) 5 k×, (c) 10 k× and (d) 40 k×,
respectively.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b10047
ACS Appl. Mater. Interfaces 2016, 8, 30941−30947
30943
