micromachines
Article
Fabrication of Graphene Aerogels with Heavily
Loaded Metallic Nanoparticles
Chen Shen 1,2 , Elizabeth Barrios 1,2 , Matthew McInnis 1,3 , Joseph Zuyus 1,3 and Lei Zhai 1,2,3, *
1
2
3
*
NanoScience Technology Center, University of Central Florida, Orlando, FL 32826, USA;
[email protected] (C.S.); [email protected] (E.B.); [email protected] (M.M.);
[email protected] (J.Z.)
Department of Materials Science and Engineering, University of Central Florida, Orlando, FL 32826, USA
Department of Chemistry, University of Central Florida, Orlando, FL 32826, USA
Correspondence: [email protected]; Tel.: +1-407-882-2847; Fax: +1-407-882-2819
Academic Editor: Hongrui Jiang
Received: 29 November 2016; Accepted: 3 February 2017; Published: 7 February 2017
Abstract: Natural biomaterials with hierarchical structures that enable extraordinary capability
of detecting chemicals have inspired the interest in producing materials that can mimic these
natural structures. This study reports the fabrication of hierarchically-structured, reduced graphene
oxide (rGO) aerogels with heavily loaded palladium (Pd), platinum (Pt), nickel (Ni), and tin (Sn)
metallic nanoparticles. Metal salts chelated with ethylenediaminetetraacetic acid (EDTA) were mixed
with graphene oxide (GO) and then freeze-dried. The subsequent reduction produces rGO/metal
nanoparticle aerogels. SEM and EDS results indicated that a loading of 59, 67, 39, and 46 wt %
of Pd, Pt, Ni, and Sn nanoparticles was achieved. Pd/rGO aerogels of different Pd nanoparticle
concentrations were exposed to H2 gas to monitor the resistance change of the composites. The results
suggest that rGO aerogels can achieve a higher nanoparticle loading by using chelation to minimize
electrostatic interactions between metal ions and GO. Higher loading of Pd nanoparticles in graphene
aerogels lead to improved hydrogen gas sensing performance.
Keywords: graphene; aerogels; metal nanoparticles; hydrogen sensors
1. Introduction
Nature has produced remarkable materials possessing unique characteristics and behaviors
that inspire scientists to replicate their structures synthetically [1]. Unlike synthetic materials, many
natural biomaterials possess a hierarchical structure on many different size scales that have a complex
interaction between each scale, leading to very unique behaviors. For example, the biomolecules
in pollens self-assemble to form a connective, highly porous network that enables efficient mass
transport [2]. Additionally, nanostructures on pollens possess a very large internal surface area,
endowing many possible sites for surface reactions to occur (Figure 1a,b) [3]. In pollen grains, these
two features enable a sensitive response of the pollen-stigma recognition, fertilization, and the defense
mechanism [4,5]. As such, the hierarchical porous network platform demonstrated in the pollen grain
provides the possibility to manufacture a highly sensitive and efficient gas sensor. These possibilities
have led to many researchers trying to fabricate biomimetic-sensing systems to advance today’s gas
sensing technologies [2,3,6–10]. For example, tin(IV) oxide (SnO2 ) porous particles with large surface
area have been produced using pollens as templates (Figure 1c,d) [3]. In this work, we demonstrate
a strategy to achieve a similar hierarchical porous network for hydrogen sensing capabilities through
the loading of metallic nanoparticles onto a graphene aerogel motif.
Micromachines 2017, 8, 47; doi:10.3390/mi8020047
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Figure 1. (a,b) are SEM images of Balsaminaceae pollen with a microscale porous network and
Figure 1. (a,b) are SEM images of Balsaminaceae pollen with a microscale porous network and
nanoscale structures on the surface [2]; (c,d) are SEM images of constructed SnO2 structures for gas
nanoscale structures on the surface [2]; (c,d) are SEM images of constructed SnO2 structures for gas
sensing
Peltophorum pterocarpum
pterocarpum pollen
pollen grains
grains as
as aa template
template [3].
[3].
sensing created
created using
using Peltophorum
Graphene is a planar monolayer of carbon atoms arranged in a honeycomb lattice and has
Graphene is a planar monolayer of carbon atoms arranged in a honeycomb lattice and has
demonstrated very intriguing electrical properties, high strength, and high in-plane electrical
demonstrated very intriguing electrical properties, high strength, and high in-plane electrical
conductivity. Additionally, graphene has a very large surface-to-weight ratio, making it highly
conductivity. Additionally, graphene has a very large surface-to-weight ratio, making it highly suitable
suitable for sensor applications. For example, various metal nanoparticles have been attached to
for sensor applications. For example, various metal nanoparticles have been attached to graphene
graphene to detect chemicals such as hydrogen peroxide [11–13] and hydrogen [14]. The hierarchical
to detect chemicals such as hydrogen peroxide [11–13] and hydrogen [14]. The hierarchical porous
porous structure demonstrated in the pollen grains can be achieved in graphene through the
structure demonstrated in the pollen grains can be achieved in graphene through the creation of
creation of a graphene aerogel (GA) decorated with nanostructures [15] where aerogels provide a
a graphene aerogel (GA) decorated with nanostructures [15] where aerogels provide a network for
network for charge transport and nanostructures grant large surface area to interact with hydrogen
charge transport and nanostructures grant large surface area to interact with hydrogen gas. GA is
gas. GA is a nanoscale graphene network with low density, large open pores, and high specific
a nanoscale graphene network with low density, large open pores, and high specific surface area.
surface area. With unique electrical properties and a large surface area of graphene, GAs are
With unique electrical properties and a large surface area of graphene, GAs are promising materials
promising materials for gas sensing and energy storage applications [16–31]. Historically, GA is
for gas sensing and energy storage applications [16–31]. Historically, GA is produced through
produced through freeze-drying a graphene oxide (GO) hydrogel, where the graphene oxide is
freeze-drying a graphene oxide (GO) hydrogel, where the graphene oxide is synthesized via a modified
synthesized via a modified Hummer’s method. The freeze-dried GO aerogel is then reduced to
Hummer’s method. The freeze-dried GO aerogel is then reduced to produce a reduced graphene
produce a reduced graphene oxide (rGO) aerogel. This method produces physical crosslinks
oxide (rGO) aerogel. This method produces physical crosslinks between the graphene sheets that are
between the graphene sheets that are oriented in the freezing direction, creating long, continuous
oriented in the freezing direction, creating long, continuous open pores throughout the GA [31–33].
open pores throughout the GA [31–33]. For the use in gas sensing applications, GA is often
For the use in gas sensing applications, GA is often functionalized with metallic nanoparticles like
functionalized with metallic nanoparticles like palladium, platinum, nickel, tin, and so on [34].
palladium, platinum, nickel, tin, and so on [34].
Loading metallic nanoparticles onto a graphene surface requires mixing GO dispersion with a
Loading metallic nanoparticles onto a graphene surface requires mixing GO dispersion with
metal salt solution. The mixture is freeze-dried and then reduced to produce graphene/metallic
a metal salt solution. The mixture is freeze-dried and then reduced to produce graphene/metallic
nanoparticles composites [35,36]. However, the loading capacity of metallic nanoparticles on
nanoparticles composites [35,36]. However, the loading capacity of metallic nanoparticles on graphene
graphene sheets is limited by the bridging effect between GO and metal cations, and this effect
sheets is limited by the bridging effect between GO and metal cations, and this effect diminishes the
diminishes the gas-sensing performance of the materials [35,37]. To overcome this limitation,
gas-sensing performance of the materials [35,37]. To overcome this limitation, chelating agents such as
chelating agents such as ethylenediaminetetraacetic acid (EDTA) can be used to screen the
ethylenediaminetetraacetic acid (EDTA) can be used to screen the electrostatic interaction between
electrostatic interaction between metal cations and GO, thus increasing the density of metallic
metal cations and GO, thus increasing the density of metallic nanoparticles exposed on the surface of
nanoparticles exposed on the surface of GO sheets [38].
GO sheets [38].
Herein, we report an effective approach for loading a large amount of metallic nanoparticles
onto the surface of graphene aerogels using EDTA. These functionalized GO aerogels were
fabricated by creating a metal salt solution containing EDTA prior to the introduction of the aqueous
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Herein, we report an effective approach for loading a large amount of metallic nanoparticles onto
the surface of graphene aerogels using EDTA. These functionalized GO aerogels were fabricated by
creating a metal salt solution containing EDTA prior to the introduction of the aqueous GO dispersion.
The resulting GO aerogels loaded with metal ions were reduced to obtain the rGO (i.e., graphene)
aerogels with nanoparticles. In this work, we demonstrate the successful preparation of palladium,
platinum, nickel, and tin nanoparticle-decorated graphene aerogels and the fabrication of a hydrogen
gas sensor using palladium nanoparticle-loaded graphene aerogels.
2. Materials and Methods
2.1. Materials and Instruments
Sulfuric acid, potassium permanganate, hydrogen peroxide, and 0.5 M ethylenediaminetetraacetic
acid (EDTA) solution were purchased from Fisher Scientific. Tin chloride pentahydrate (SnCl4 ·5H2 O),
nickel chloride hexahydrate (NiCl2 ·6H2 O), palladium chloride (PdCl2 ), and chloroplatinic acid
(H2 PtCl6 ) were purchased from Alfa Aesar. Graphite powders (microfyne grade) were purchased from
Dixon Inc. All chemicals were used as received without further purification.
A Labconco freeze drying chamber (FreeZone 1) was used for the lyophilization of the graphene
oxide hydrogels. The Labcono lyophilizer was operated at a temperature of −52 ◦ C under a pressure
of 0.02 mbar. VWR vacuum oven was used for the vacuum drying of GO powders. BRANSON digital
horn sonifier was used to disperse GO in water. A Thermo Scientific tube furnace (Lindberg Blue M)
was used for the hydrogen reduction of the freeze-dried graphene oxide aerogels.
2.2. Synthesis of Graphene Oxide (GO) Using Modified Hummers’ Method
Graphene Oxide (GO) was prepared by modified Hummers method using graphite powder.
Typically, 1 g of graphite powder was added to a 500 mL beaker that was immersed in an ice bath,
followed by a slow addition of 50 mL concentrated sulfuric acid and 6 g of potassium permanganate
while stirring. The mixture was stirred for an additional hour at 30 ◦ C. 80 mL of deionized water
(DI water) was added dropwise to the homogeneous mixture. After the addition of DI water, the
beaker was then heated to 60 ◦ C and maintained for 1 h. 6 mL of 30% H2 O2 was then added into
the beaker. The heat was turned off after 5 min. After the mixture was cooled to room temperature,
the crude products were washed with DI water followed by centrifugation to obtain the product.
The washing was finished when the supernatant had a pH above 5. The wet GO was then dried in a
vacuum oven at room temperature to avoid thermal reduction during the drying process [39,40].
2.3. Preparation of GO Aerogels Loaded with Metal Salts
In a typical preparation, 1 mM of a metal salt (i.e., Pd, Pt, Ni, or Sn salts) was added to a 10 mL
0.1 M EDTA solution. Thus, the molar ratio between the metal salt and EDTA was 1:1. Most solutions
were prepared at room temperature. However, the PdCl2 /EDTA solution was heated at 90 ◦ C for
15 min to fully dissolve the metal salt, and the solution was cooled to room temperature prior to the
next step. An aqueous GO dispersion was prepared by using a horn sonifier. A mixture of 200 mg of
GO and 10 mL of DI water was sonicated for 30 min to form an aqueous GO dispersion. As shown
is Figure 2a, the completely dissolved metal salt/EDTA solutions were mixed with 20 mg/mL GO
dispersion with a volume ratio of 1:1 to form a metal salt/GO dispersion. The dispersion was then
transferred to several 2 mL plastic cuvettes after vigorous mixing was applied. Dispersions in cuvettes
were frozen using dry ice for the following lyophilization. After an overnight lyophilization, bulk
metal salt loaded GO aerogels were formed as shown in Figure 2b.
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Figure
Schematic Illustration
Illustration of
of of
metal
nanoparticles
decorated
rGO aerogels.
(a)
Figure
2. 2.Schematic
ofthe
theformation
formation
metal
nanoparticles
decorated
rGO aerogels.
GO
+
Metal
Salt/EDTA
Solution.
(b)
Metal
Salt
loaded
GO
Aerogels.
(c)
Mental
Nanoparticles
(a) GO + Metal Salt/EDTA Solution. (b) Metal Salt loaded GO Aerogels. (c) Mental Nanoparticles
Loaded Rgo Aerogels.
Loaded Rgo Aerogels.
2.4. Preparation of Reduced Graphene Oxide (rGO) Aerogels Decorated with Metal Nanoparticles
2.4. Preparation of Reduced Graphene Oxide (rGO) Aerogels Decorated with Metal Nanoparticles
The metal salt loaded GO aerogels shown in Figure 2b went through vapor phase reduction,
The
salt loaded
GO aerogels
shown
in Figure
2b went
through
vapor phase
reduction,
and
and
anmetal
atmosphere
of hydrazine
vapor
mixed
in nitrogen
was
used during
this reaction.
The
anreduction
atmosphere
of
hydrazine
vapor
mixed
in
nitrogen
was
used
during
this
reaction.
The
reduction
was conducted at 90 °C for one hour. Epoxide functional groups on the basal plane of GO
◦ C for one hour. Epoxide functional groups on the basal plane of GO were
was
conducted
90hydrazine
were
removedatby
vapor to avoid potential structure damage of aerogels in the next
removed
by reduction
hydrazinestep
vapor
to avoid
structure
damage
of aerogels
in the next
hydrogen
hydrogen
[39,41].
Oncepotential
the aerogels
were cooled
down
from hydrazine
reduction,
reduction
step
[39,41].
Once
the
aerogels
were
cooled
down
from
hydrazine
reduction,
they
they were further reduced by pure hydrogen gas in a pure hydrogen atmosphere at 400 °C for 1were
h
◦ C for 1 h and at 900 ◦ C
further
by pure
hydrogen
gas in aflow
purerate
hydrogen
atmosphere
at 400
and atreduced
900 °C for
10 min.
The hydrogen
was kept
at 30 SCCM.
Such
two steps reduction
rGOhydrogen
aerogels decorated
nanoparticles
as shown
in Figure
2c, andproduced
the color of
forproduced
10 min. The
flow ratewith
was metal
kept at
30 SCCM. Such
two steps
reduction
rGO
aerogels
was converted
from brown
to black as
during
theinreduction
process.
Tocolor
form of
tinaerogels
decorated
aerogels
decorated
with metal
nanoparticles
shown
Figure 2c,
and the
was
rGO
aerogels,
hydrogen
reduction
only
needs
to
be
conducted
at
400
°C
for
one
hour
as
tin
converted from brown to black during the reduction process. To form tin decorated rGO aerogels,
nanoparticles
would
flow
away
temperatures
were
applied.
hydrogen
reduction
only
needs
toifbehigher
conducted
at 400 ◦ C
for one
hour as tin nanoparticles would flow
away if higher temperatures were applied.
2.5. Characterization
2.5. Characterization
The morphologies of the aerogels were characterized by using a scanning electron microscope
operating
at 10 KeV (SEM,
ZEISS ULTRA
55, Zeiss, Dublin,
CA, aUSA)
and was
coupled
with
The morphologies
of the aerogels
were characterized
by using
scanning
electron
microscope
energy-dispersive
X-ray
spectroscopy
(EDS) to
the surface
of was
the aerogels.
operating
at 10 KeV
(SEM,
ZEISS ULTRA
55,investigate
Zeiss, Dublin,
CA, chemistry
USA) and
coupledThe
with
chemical
states
of
elements
were
analyzed
by
X-ray
photoelectron
spectroscopy
(XPS,
PHIaerogels.
5400,
energy-dispersive X-ray spectroscopy (EDS) to investigate the surface chemistry of the
Ulvac-PHI, Inc., Kanagawa, Japan). The surface area of aerogels was measured by a Nova 4200e
The chemical states of elements were analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5400,
Brunauer, Emmett, and Teller (B.E.T.) surface area analyzer (Quantachrome Instruments, Boynton
Ulvac-PHI, Inc., Kanagawa, Japan). The surface area of aerogels was measured by a Nova
Beach, FL, USA).
4200e Brunauer, Emmett, and Teller (B.E.T.) surface area analyzer (Quantachrome Instruments,
Boynton
Beach,
FL, USA).
2.6. Sensing
Hydrogen
Gas Using rGO Aerogels Decorated with Pd Nanoparticles
In order
to determine
the rGO
influence
of processing
and experimental
conditions of the aerogels
2.6. Sensing
Hydrogen
Gas Using
Aerogels
Decorated with
Pd Nanoparticles
on hydrogen sensing, rGO aerogels attached to electrical leads was inserted into a homemade flow
In order to determine the influence of processing and experimental conditions of the aerogels
chamber. The flow chamber was placed in a water bath with stable temperature, and a thermocouple
on hydrogen sensing, rGO aerogels attached to electrical leads was inserted into a homemade flow
was added to the interior of the flow chamber to diminish the affect from the exterior temperature.
chamber.
The flowresponse
chamber(i.e.,
was resistance
placed in change
a waterof
bath
stableto
temperature,
and
a thermocouple
Conductometric
rGOwith
aerogels)
hydrogen gas
was
carried out
was
added
to
the
interior
of
the
flow
chamber
to
diminish
the
affect
from
the
exterior
by monitoring the current through each aerogel under the condition of a steady flow oftemperature.
argon gas
Conductometric
responseof(i.e.,
change
rGO
to hydrogen
gas was
carried out
with periodic inclusion
1000resistance
ppm hydrogen
gasofinto
theaerogels)
stream. The
relative change
in resistivity
by[(R
monitoring
the
current
through
each
aerogel
under
the
condition
of
a
steady
flow
of
argon gas
g − R0)/R0%] (R0 and Rg are the resistance of the device before and after the exposure to hydrogen
with
periodic
inclusion
of
1000
ppm
hydrogen
gas
into
the
stream.
The
relative
change
in
resistivity
gas, respectively.) was calculated from the current and the voltage applied. These values
were
[(Rplotted
%] (R0 and
Rg are
thebaseline
resistance
of the device before and after the exposure to hydrogen
as a0function
of time
after
correction.
g − R0 )/R
gas, respectively.) was calculated from the current and the voltage applied. These values were plotted
as a function of time after baseline correction.
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3.
Results and
and Discussion
Discussion
3. Results
The
with
a solution
containing
equal
moles
of a metal
salt and
a ligand,
The material
materialfabrication
fabricationstarted
started
with
a solution
containing
equal
moles
of a metal
salt
and a
specifically
EDTA,
in
this
study.
EDTA
was
crucial
in
order
to
successfully
form
GO
aerogels
during
ligand, specifically EDTA, in this study. EDTA was crucial in order to successfully form GO aerogels
lyophilization.
If a strong
ligand ligand
like EDTA
was not
used,
strong
electrostatic
interactions
were
during lyophilization.
If a strong
like EDTA
was
not the
used,
the strong
electrostatic
interactions
between
positively
charged
metal ions
theand
negatively
chargedcharged
functional
groups on
the edge
of
were between
positively
charged
metaland
ions
the negatively
functional
groups
on the
GO,
usually
the
carboxylic
acid
groups,
caused
the
gelation
of
GO
[42].
Such
a
GO
hydrogel
would
edge of GO, usually the carboxylic acid groups, caused the gelation of GO [42]. Such a GO hydrogel
shrink
during theduring
lyophilization.
As a result, As
GO aaerogels
microstructures
would significantly
shrink significantly
the lyophilization.
result, with
GO porous
aerogels
with porous
were
not
formed,
leading
to
the
loss
of
the
advantages
of
the
aerogels
originating
from
unique
microstructures were not formed, leading to the loss of the advantages of the aerogels this
originating
microstructure.
On
the
other
hand,
if
metal
salts
are
mixed
with
EDTA
before
being
added
to
the
GO
from this unique microstructure. On the other hand, if metal salts are mixed with EDTA before
being
suspension,
formation
of metal-EDTA
complexes
eliminated
the electrostatic
interactions
between
added to thethe
GO
suspension,
the formation
of metal-EDTA
complexes
eliminated
the electrostatic
metal
ions
and
GO
[43–45].
Therefore,
van
der
Waals
interactions
between
the
GO
sheets
became
interactions between metal ions and GO [43–45]. Therefore, van der Waals interactions between
the
the
the mixtures,
which
is the
same as
whenisonly
GO was
dispersed
water.
GO major
sheetsinteractions
became the in
major
interactions
in the
mixtures,
which
the same
as when
only in
GO
was
As
a result,in
nearly
noAs
shrinkage
was
observed
after the lyophilization,
containing metal
dispersed
water.
a result,
nearly
no shrinkage
was observed and
afteraerogels
the lyophilization,
and
salts
were
successfully
formed.
After
successfully
creating
the
aerogels
containing
metal
salts,
the
GO
aerogels containing metal salts were successfully formed. After successfully creating the aerogels
flakes
and metal
in the aerogels
were
to in
rGO
and were
metalconverted
nanoparticles
during
the
containing
metalions
salts,
GO flakes
andconverted
metal ions
thesheets
aerogels
to rGO
sheets
high
temperature
hydrogen
reduction.
and metal nanoparticles during the high temperature hydrogen reduction.
The microstructure
with
metal
nanoparticles
was
examined
by
microstructureof
ofaatypical
typicalrGO
rGOaerogel
aerogeldecorated
decorated
with
metal
nanoparticles
was
examined
SEM
as shown
in Figure
3. The stacking
of the rGO
can be
clearly
the seen
cross-sectional
by SEM
as shown
in Figure
3. The stacking
of sheets
the rGO
sheets
canseen
be from
clearly
from the
image
in
Figure
3a.
This
structure
indicates
that
the
van
der
Waals
interactions
among
the rGO
cross-sectional image in Figure 3a. This structure indicates that the van der Waals interactions
sheets
even after
the hydrogen
reduction
process.
Also, it is
shownAlso,
that the
sizes
in
amongremained
the rGO sheets
remained
even after
the hydrogen
reduction
process.
it ispore
shown
that
the aerogels
are
theaerogels
sub-millimeter
range.
The interconnected
grapheneporous
framework
with
pore sizes
ininthe
are in the
sub-millimeter
range. Theporous
interconnected
graphene
aframework
significant with
directionality
in the
longitudinal in
direction
was realized
(Figurewas
3b). realized
The well-defined
3D
a significant
directionality
the longitudinal
direction
(Figure 3b).
microstructure
is due
to the directional
freezing
from
placing
dry ice beneath
the cuvettes
The well-defined
3D microstructure
is due
to theresulting
directional
freezing
resulting
from placing
dry ice
filled
with
metal
salt/GO
dispersions.
beneath the cuvettes filled with metal salt/GO dispersions.
Figure
area; (b)
(b) SEM
SEM image
image of
of the
the longitudinal
longitudinal section.
section.
Figure 3.
3. (a)
(a) SEM
SEM image
image of
of the
the cross-sectional
cross-sectional area;
The loading of the metal nanoparticles was verified by the SEM under higher magnifications as
The loading of the metal nanoparticles was verified by the SEM under higher magnifications
shown in Figure 4. The morphologies of the aerogels loaded with palladium, platinum, nickel, and
as shown in Figure 4. The morphologies of the aerogels loaded with palladium, platinum, nickel,
tin nanoparticles are shown in Figure 4a,e; Figure 4b,f; Figure 4c,g; and Figure 4d,h, respectively.
and tin nanoparticles are shown in Figure 4a,e; Figure 4b,f; Figure 4c,g; and Figure 4d,h, respectively.
The presence of ripples and wrinkles on the sheets verified the existence of rGO sheets. These
The presence of ripples and wrinkles on the sheets verified the existence of rGO sheets. These features
features are typical in rGO sheets and mainly result from the stress produced at elevated
are typical in rGO sheets and mainly result from the stress produced at elevated temperatures during
temperatures during hydrogen reduction process. After hydrogen reduction, metal ions chelated
hydrogen reduction process. After hydrogen reduction, metal ions chelated with EDTA on the rGO
with EDTA on the rGO sheets were reduced into their metallic state. The sizes of palladium,
sheets were reduced into their metallic state. The sizes of palladium, platinum, and nickel metal
platinum, and nickel metal nanoparticles are comparable, which ranged from 10 to 120 nm in
nanoparticles are comparable, which ranged from 10 to 120 nm in diameter. The tin nanoparticles
diameter. The tin nanoparticles loaded on rGO sheets have larger sizes compared with the other
loaded on rGO sheets have larger sizes compared with the other three metal nanoparticles. Particles as
three metal nanoparticles. Particles as large as 400 nm in diameter were observed. However, small
large as 400 nm in diameter were observed. However, small tin nanoparticles with diameters around
tin nanoparticles with diameters around 50 nm can still be observed.
50 nm can still be observed.
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Figure
4. SEM
(c,g) Ni;
Ni; and
and (d,h)
(d,h) Sn
Sn nanoparticles
nanoparticles loaded
loaded aerogels
aerogels
Figure 4.
SEM images
images of
of the
the (a,e)
(a,e) Pd;
Pd; (b,f)
(b,f) Pt;
Pt; (c,g)
formed
after
hydrogen
reduction.
formed after hydrogen reduction.
To verify the distribution of metal nanoparticles on rGO sheets, the metal loaded aerogels were
To verify the distribution of metal nanoparticles on rGO sheets, the metal loaded aerogels were
characterized by using EDS elemental mapping. The SEM images of metal loaded aerogels and the
characterized by using EDS elemental mapping. The SEM images of metal loaded aerogels and the
mapping of the loaded metal in the same area are shown Figure 5. The EDS mapping results of rGO
mapping of the loaded metal in the same area are shown Figure 5. The EDS mapping results of
aerogels loaded with palladium, platinum, nickel, and tin are shown in Figure 5a,e; Figure 5b,f;
rGO aerogels loaded with palladium, platinum, nickel, and tin are shown in Figure 5a,e; Figure 5b,f;
Figure 5c,g; and Figure 5d,h, respectively. Figure 5a–d shows SEM images of the area characterized
Figure 5c,g; and Figure 5d,h, respectively. Figure 5a–d shows SEM images of the area characterized for
for EDS elemental mapping. Uniform elemental distribution of metals is clearly indicated by
EDS elemental mapping. Uniform elemental distribution of metals is clearly indicated by Figure 5e–h.
Figure 5e–h. Among them, Figure 5e–h presents the elemental mapping of palladium, platinum,
Among them, Figure 5e–h presents the elemental mapping of palladium, platinum, nickel, and tin on
nickel, and tin on their corresponding rGO sheets in aerogels, respectively. These results confirm the
their corresponding rGO sheets in aerogels, respectively. These results confirm the successful loading
successful loading of metal particles onto rGO aerogels with excellent uniformity.
of metal particles onto rGO aerogels with excellent uniformity.
Micromachines 2017, 8, 47
Micromachines 2017, 8, 47
7 of 14
7 of 14
Figure5.5.SEM
SEMimages
imagesof
of(a)
(a)Pd,
Pd,(b)
(b) Pt,
Pt, (c)
(c) Ni,
Ni, and
and (d)
(d) Sn
Sn loaded
loaded rGO
Figure
rGO aerogels
aerogels and
and the
the corresponding
corresponding
EDSspatial
spatialmapping
mappingimages
imagesof
ofloaded
loaded(e)
(e)Pd,
Pd, (f)
(f) Pt,
Pt, (g)
(g) Ni,
Ni, and
and (h)
(h) Sn
EDS
Sn on
on rGO
rGO sheets.
sheets. The
Thescale
scalebar
barisis
20
μm.
20 µm.
The amount of loaded metal nanoparticles in rGO aerogels after hydrogen reductions was also
The amount of loaded metal nanoparticles in rGO aerogels after hydrogen reductions was also
characterized by EDS. Based on the EDS spectra in Figure 6, carbon and the loaded metals are the
characterized by EDS. Based on the EDS spectra in Figure 6, carbon and the loaded metals are the
predominant components of the aerogels after hydrogen reduction. Oxygen in the palladium and
predominant components of the aerogels after hydrogen reduction. Oxygen in the palladium and
platinum loaded aerogels are not detectable to give a reliable composition with the oxygen;
platinum loaded aerogels are not detectable to give a reliable composition with the oxygen; however,
however, small peaks at 0.525 eV can be seen for these two trials. Oxygen in the nickel and tin loaded
small peaks at 0.525 eV can be seen for these two trials. Oxygen in the nickel and tin loaded aerogels
aerogels were detected in quantifiable quantities and is likely attributed to the oxides that formed on
were detected in quantifiable quantities and is likely attributed to the oxides that formed on the
the surfaces of the aerogels, which will be discussed further in the XPS studies. For the aerogels
surfaces
of the aerogels, which will be discussed further in the XPS studies. For the aerogels containing
containing tin, the excess oxygen content compared with the other samples is also due to the
tin,
the excessreduction
oxygen content
compared
with theduring
other samples
is conducted
also due toat
the
incomplete
reduction
incomplete
of the rGO
that occurred
reduction
the
significantly
lower
oftemperature
the rGO thatofoccurred
during
reduction
conducted
at
the
significantly
lower
temperature
of 400 ◦ C.
400 °C. The loading efficiency of the metal nanoparticles was evaluated by analyzing
The
efficiency
of the
metal
was of
evaluated
by analyzing
the EDS
spectra
in
the loading
EDS spectra
in Figure
6, and
thenanoparticles
mass percentage
metal nanoparticles
in rGO
aerogels
after
Figure
6, and
the mass was
percentage
of metal
nanoparticles
rGOand
aerogels
hydrogen
hydrogen
reduction
calculated
to be
59%, 67%, in
39%,
46% after
for Pd,
Pt, Ni,reduction
and Sn
was
calculated
to
be
59%,
67%,
39%,
and
46%
for
Pd,
Pt,
Ni,
and
Sn
nanoparticles,
respectively.
nanoparticles, respectively. Compared with existing approaches of loading metal nanoparticles
to
Compared
with existing
approaches
of loading
metal
nanoparticles
to graphene
(aroundefficiencies.
9 at. % and
graphene (around
9 at. %
and 26 wt %)
[46,47], our
method
grants much
higher loading
26 wt The
%) [46,47],
our states
method
muchnanoparticles
higher loading
efficiencies.
chemical
of grants
the metal
loaded
onto the aerogels, before and after
The
chemical
states
of
the
metal
nanoparticles
loaded
onto
the aerogels,
before 7.
and
after
hydrogen reduction, were analyzed via XPS, and the data are presented
in Figure
The
XPShydrogen
spectra
reduction,
were analyzed
XPS, and
are presented
Figure 7. The
XPS spectra
of palladium,
of palladium,
platinum,via
nickel,
and the
tindata
in aerogels
beforeinhydrogen
reduction
are presented
in
platinum,
nickel,respectively.
and tin in The
aerogels
before of
hydrogen
reduction
arenickel,
presented
7a,c,e,g,
Figure 7a,c,e,g,
XPS spectra
palladium,
platinum,
and tinininFigure
aerogels
after
respectively.
The XPS
of palladium,
platinum,
nickel, and
tindeconvoluted
in aerogels after
hydrogen
hydrogen reduction
arespectra
presented
in Figure 7b,d,f,h,
respectively.
The
XPS spectra
in
reduction
are show
presented
in Figure
7b,d,f,h, respectively.
The deconvoluted
XPS spectra
in Figure
Figure 7a,b
3d peaks
of palladium
before and after
hydrogen reduction.
In Figure
7a, 7a,b
the
2+ 3d5/2 peak
show
3d peaks XPS
of palladium
and after
hydrogen
reduction.
Figure
7a, the deconvoluted
deconvoluted
peaks arebefore
recognized
as Pd
(338.55 In
eV)
of palladium
(II) chloride
2+ 3d
2+
2+
XPS
peaks
are
recognized
as
Pd
peak
(338.55
eV)
of
palladium
(II)
chloride
(PdCl
Pd2+
(PdCl2), Pd 3d3/2 peak (344.71 eV) of 5/2
PdCl2, and Pd 3d5/2 peak (337.19 eV) of palladium (II)2 ),oxide
2+
3d
eV) of PdCl
Pd 3dof
peak
eV) of
palladium
(II) oxideof(PdO)
(PdO)
[48].(344.71
The presence
of a small
PdO
is (337.19
most likely
due
to the hydrolysis
PdCl2.[48].
In
2 , andportion
3/2 peak
5/2
The
presence
a small
portion of PdO
mostare
likely
due to the
hydrolysis
of (335.20
PdCl2 . eV)
In Figure
7b,
Figure
7b, theofmajor
deconvoluted
XPS is
peaks
recognized
as Pd°
3d5/2 peak
and Pd°
◦
◦
the
major
XPS
peaks are
recognized
as Pd
eV) and Pd
3d3/2 peak
3d3/2
peakdeconvoluted
(340.46 eV) of
metallic
palladium.
These
two 3d
major
XPS(335.20
peaks observed
in palladium
5/2 peak
loadedeV)
aerogels
afterpalladium.
hydrogen These
reduction
verifies
thepeaks
effective
formation
of metallic
palladium
(340.46
of metallic
two major
XPS
observed
in palladium
loaded
aerogels
during
reduction
[49]. Additionally,
minimalformation
deconvoluted
XPS peaks
corresponding
to PdO [49].
are
after
hydrogen
reduction
verifies the effective
of metallic
palladium
during reduction
2+ 3d5/2 peak (337.20 eV) and Pd2+ 3d3/2 peak (342.50 eV) of PdO [50].
observed
in
Figure
7b,
namely
Pd
Additionally, minimal deconvoluted XPS peaks corresponding to PdO are observed in Figure 7b,
The PdO
aerogels
after
likely eV)
from
the surface
namely
Pd2+in3dthe
(337.20
eV)hydrogen
and Pd2+ reduction
3d3/2 peakis(342.50
of PdO
[50]. Theoxides.
PdO inThe
the
5/2 peak
deconvoluted
XPS spectra
in Figure
7c,d from
show the
4f peaks
platinum
and afterXPS
hydrogen
aerogels
after hydrogen
reduction
is likely
surfaceofoxides.
Thebefore
deconvoluted
spectra
Theshow
deconvoluted
XPS
peaks inbefore
Figureand
7c are
recognized
Pt4+ 4f7/2 peak
eV), Pt4+
inreduction.
Figure 7c,d
4f peaks of
platinum
after
hydrogenasreduction.
The(74.56
deconvoluted
2+
2+
4+
4+
4f5/2peaks
peak (77.87
eV),7c
Pt are
4f7/2
peak (71.99
4f5/2(74.56
peak (75.50
[51–53]. The2+
XPS
in Figure
recognized
aseV),
Pt and
4f7/2Ptpeak
eV), PteV),4frespectively
5/2 peak (77.87 eV), Pt
2+
4+
of peaks for Pt is 2+
likely
to (75.50
the reduction
of Pt during
the step
mixing Pt
salts
4fexistence
4f5/2due
peak
eV), respectively
[51–53].
Theofexistence
of4+peaks
7/2 peak (71.99 eV), and Pt
2+
4+
4+
with
EDTA
[51].
Two
major
deconvoluted
XPS
peaks
in
Figure
7d
are
recognized
as
Pt°
4f
7/2
peak
for Pt is likely due to the reduction of Pt during the step of mixing Pt salts with EDTA [51].
◦ 4f verifies
(70.81
eV) deconvoluted
and Pt° 4f5/2 XPS
peak peaks
(74.22ineV),
respectively
[52–54]. This
the effective
Two
major
Figure
7d are recognized
as Ptresult
7/2 peak (70.81 eV) and
◦
reduction
to
form
metallic
platinum.
Besides,
minimal
deconvoluted
XPS
peaks
corresponding
to
Pt 4f5/2 peak (74.22 eV), respectively [52–54]. This result verifies the effective reduction to form
platinum (II) oxide (PtO) are observed in Figure 8d, namely Pt2+ 4f7/2 peak (71.98 eV) and Pt2+ 4f5/2
Micromachines 2017, 8, 47
8 of 14
metallic platinum. Besides, minimal deconvoluted XPS peaks corresponding to platinum (II) oxide
Micromachines
2017, 8,in
47 Figure 8d, namely Pt2+ 4f7/2 peak (71.98 eV) and Pt2+ 4f5/2 peak (75.45
8 of 14eV).
(PtO)
are observed
Similar to the case of palladium loaded aerogels, the PtO in the aerogels after hydrogen reduction
peak (75.45 eV). Similar to the case of palladium loaded aerogels, the PtO in the aerogels after
is also
likely from the surface oxides. The deconvoluted XPS spectra in Figure 7e,f show 2p peaks
hydrogen reduction is also likely from the surface oxides. The deconvoluted XPS spectra in Figure
of nickel before and after hydrogen reduction. In Figure 7e, the single major peak is recognized as
7e,f show 2p peaks of nickel before and after hydrogen reduction. In Figure 7e, the single major peak
Ni2+ 2p3/2 peak (860.202+ eV) [55,56]. In Figure 7f, the deconvoluted XPS peaks are recognized as Ni◦
is recognized as Ni 2p
3/2 peak (860.20 eV) [55,56]. In Figure 7f, the deconvoluted XPS peaks are
2p3/2recognized
peak (858.63
eV), Ni2+ 2p (858.63
peakeV),
(860.21
eV), and Ni3+ 2p eV),peak
(861.40 eV), respectively [57].
as Ni° 2p3/2 peak 3/2
Ni2+ 2p3/2 peak (860.213/2
and Ni3+ 2p3/2 peak (861.40 eV),
◦
The respectively
Ni 2p3/2 peak
eV)
the major deconvoluted XPS peak, and this result verifies the effective
[57].(858.63
The Ni°
2pis
3/2 peak (858.63 eV) is the major deconvoluted XPS peak, and this result
3+ after hydrogen reduction is
reduction
to
form
metallic
nickel.
Besides,
the existence
Ni2+ and
verifies the effective reduction to
form metallic
nickel. of
Besides,
the Ni
existence
of Ni2+ and Ni3+ after
probably
attributed
to
the
formation
of
nickel
(II)
oxide
(NiO)
and
nickel
(III)
oxide
(Ninickel
on the
2 O3 ) (III)
hydrogen reduction is probably attributed to the formation of nickel (II) oxide (NiO)
and
surface
of (Ni
aerogels
[51].
The deconvoluted
XPS The
spectra
in Figure XPS
7g,h spectra
show 3d
tin show
before3dand
oxide
2O3) on
the surface
of aerogels [51].
deconvoluted
in peaks
Figureof
7g,h
of tinreduction.
before andInafter
hydrogen
reduction. In Figure
7g, the
afterpeaks
hydrogen
Figure
7g, the deconvoluted
XPS peaks
aredeconvoluted
recognized asXPS
Sn4+peaks
3d5/2are
peak
◦ 4+
recognized
5/2 peak
peak
(487.09
eV)
and
Sn°
3d
5/2 peak
(484.95
eV),
respectively
[58–61].
The
(487.09
eV) andasSnSn
3d3d
(484.95
eV),
respectively
[58–61].
The
presence
of
a
small
amount
of
5/2
presence
of
a
small
amount
of
metallic
tin
is
likely
due
to
the
reduction
by
EDTA
while
mixing
tin
metallic tin is likely due to the reduction by EDTA while mixing tin salts with EDTA. In Figure 7h, the
◦ 3d major
salts
withpeak
EDTA.
In Figure 7h,asthe
peak is recognized as Sn° 3d5/2 peak (484.96 eV) [61],
single
major
is recognized
Snsingle
5/2 peak (484.96 eV) [61], and this result verifies the effective
and
this
result
verifies
the
effective
reduction
to
form metallic tin.
reduction to form metallic tin.
Figure
6. EDS
spectra
palladium,(b)
(b)platinum,
platinum, (c)
(c) nickel,
nickel, and
aerogels
after
Figure
6. EDS
spectra
of of
(a)(a)
palladium,
and(d)
(d)tin
tinloaded
loadedrGO
rGO
aerogels
after
hydrogen
reduction.
hydrogen reduction.
Overall, the effective reduction of metal salts during a hydrogen reduction process was
Overall, the effective reduction of metal salts during a hydrogen reduction process was confirmed
confirmed by the comparison of the deconvoluted XPS peaks before and after reduction. The
by the
comparison
of the deconvoluted
XPScorrespond
peaks before
and
afteratreduction.
Thestate,
appearance
appearance
of dominant
XPS peaks, which
to the
metals
their metallic
indicate of
dominant
XPS
peaks,
which
correspond
to
the
metals
at
their
metallic
state,
indicate
the
highly
effective
the highly effective reduction of the metal salts in this study. Additionally, only a small amount
of
reduction
of
the
metal
salts
in
this
study.
Additionally,
only
a
small
amount
of
metal
oxides
exist
in the
metal oxides exist in the aerogels loaded with palladium, platinum, and nickel after hydrogen
aerogels
loadedand
with
platinum,
afterattributed
hydrogentoreduction,
and the
metal
reduction,
thepalladium,
residual metal
oxidesand
arenickel
probably
oxides formed
on residual
the surface
after
Furthermore,
no XPSon
peaks
regarding
tin hydrogen
oxides were
seen whileFurthermore,
applying
oxides
arehydrogen
probablyreduction.
attributed
to oxides formed
the surface
after
reduction.
curve
fitting,
thus thetin
reduction
to form
metallic
nanoparticles
more thus
effective
than that oftothe
no XPS
peaks
regarding
oxides were
seen
whiletin
applying
curve is
fitting,
the reduction
form
other
three
kinds
of
metals.
metallic tin nanoparticles is more effective than that of the other three kinds of metals.
Micromachines 2017, 8, 47
Micromachines 2017, 8, 47
9 of 14
9 of 14
Deconvoluted XPS
XPSspectra
spectraofofthe
theloaded
loadedmetal
metalininthe
theaerogels.
aerogels.
loaded
aerogels
Figure 7. Deconvoluted
PdPd
inin
PdPd
loaded
aerogels
(a)(a)
as
as prepared
reduction,
Pt loaded
aerogels
as prepared
and
(d) reduction,
after reduction,
prepared
andand
(b) (b)
afterafter
reduction,
Pt inPtPtinloaded
aerogels
(c) as(c)
prepared
and (d)
after
Ni in
Ni loaded
in Ni loaded
(e) as prepared
andreduction,
(f) after Sn
reduction,
Sn inaerogels
Sn loaded
(g)and
as
aerogelsaerogels
(e) as prepared
and (f) after
in Sn loaded
(g) asaerogels
prepared
prepared
and (h) after
reduction are shown.
(h)
after reduction
are shown.
The reduction
reduction of
of carbon
carbon in
in aerogels
aerogels during
during hydrogen
hydrogen reduction
reduction was
was also
also confirmed
confirmed by
by the
the XPS
XPS
The
spectra
of
C
1
s.
The
XPS
images
of
carbon
in
aerogels
loaded
with
four
different
kinds
of
metals
are
spectra of C 1 s. The XPS images of carbon in aerogels loaded with four different kinds of metals are
shown in
in Figure
Figure S1.
S1. The
of
shown
The obvious
obvious shift
shift of
of XPS
XPS peaks
peaks to
to the
the lower
lower binding
binding area
area proves
proves the
the reduction
reduction of
GO
during
the
reduction.
For
aerogels
loaded
with
palladium,
platinum,
and
nickel,
the
presence
of
GO during the reduction. For aerogels loaded with palladium, platinum, and nickel, the presence of
major
deconvoluted
peaks
at
the
binding
energy
of
284.6
eV,
which
is
the
signature
peak
of
graphitic
major deconvoluted peaks at the binding energy of 284.6 eV, which is the signature peak of graphitic
C–C bonds
bonds [25,37,62,63],
[25,37,62,63], indicates
indicates the
the effective
effective reduction
reduction of
of carbon
carbon in
in hydrogen
hydrogen atmosphere
atmosphere at
at the
the
C–C
temperature
of
900
°C.
Besides,
the
dramatic
decrease
of
the
relative
intensity
for
the
XPS
peaks
◦
temperature of 900 C. Besides, the dramatic decrease of the relative intensity for the XPS peaks which
which correspond
to the oxidized
of hydroxyl
groups
with binding
energy
correspond
to the oxidized
carbons,carbons,
namelynamely
peaks ofpeaks
hydroxyl
groups with
binding
energy around
around
286.5
eV,
peaks
of
carbonyl
C
O
double
bond
with
binding
energy
of
around
288
eV,
and
286.5 eV, peaks of carbonyl C=O double bond with binding energy of around 288 eV, and peaks from
peaks
from
O–C
O
ester
functional
groups
with
binding
energy
around
289
eV
[25,37,62,63],
further
O–C=O ester functional groups with binding energy around 289 eV [25,37,62,63], further verifies
verifies
the effective
reduction
of carbonyl
and
ester groups.
tin loaded
aerogels,
the appearance
the
effective
reduction
of carbonyl
and ester
groups.
For tin For
loaded
aerogels,
the appearance
of the
of
the
deconvoluted
XPS
peak
at
the
binding
energy
of
284.6
eV—which
is
the
characteristic
peak of
deconvoluted XPS peak at the binding energy of 284.6 eV—which is the characteristic peak of graphitic
graphitic carbon—proves
the effective
carbon
in hydrogen
atmosphere.
much
carbon—proves
the effective
reductionreduction
of carbonofin
hydrogen
atmosphere.
BecauseBecause
a mucha lower
lower reduction
temperature
400 °C
was
for tinaerogels,
loaded aerogels,
a muchfraction
smalleroffraction
of
reduction
temperature
of 400 ◦of
C was
used
forused
tin loaded
a much smaller
graphitic
graphitic
carbon
was
formed.
Additionally,
the
decrease
of
the
relative
intensity
of
the
ester
groups
carbon was formed. Additionally, the decrease of the relative intensity of the ester groups verifies the
verifies the
of this
functional
group
aerogels.
At last,peaks
deconvoluted
peaks with
the
reduction
of reduction
this functional
group
in aerogels.
Atin
last,
deconvoluted
with the binding
energies
binding
energies
of
290.1
eV
to
290.8
eV,
which
exist
in
all
aerogel
samples
as
prepared
and
after
of 290.1 eV to 290.8 eV, which exist in all aerogel samples as prepared and after hydrogen reduction,
hydrogen
reduction,π–π
areshakeup
the characteristic
π – π inshakeup
peaks in [63,64].
aromatic carbon
are
the characteristic
transition peaks
aromatictransition
carbon compounds
compounds
[63,64]. have been deposited on GO surfaces for hydrogen sensing because they have
Pd nanoparticles
Pd
nanoparticles
haveSuch
beenaffinity
deposited
onthe
GOelectron
surfacesdensity
for hydrogen
sensing
because
have
high affinity to hydrogen.
causes
of the metal
particle
to bethey
reduced,
high
affinity
to
hydrogen.
Such
affinity
causes
the
electron
density
of
the
metal
particle
to
thereby increasing the work function of the graphene-metal-gas system, leading to a change be
of
reduced, thereby
increasing
the of
work
function
of the on
graphene-metal-gas
system,interfacial
leading area
to a
resistance
[47,65]. The
sensitivity
the sensors
depends
the Pd nanoparticle/GO
change
of resistance
[47,65]. The
of the
thegas
sensors
depends The
on the
Pd nanoparticle/GO
and
the response
and recovery
time sensitivity
is affected by
flow efficiency.
Pd nanoparticles
on GAs
interfacial
area
and
the
response
and
recovery
time
is
affected
by
the
gas
flow
efficiency.
Pd
offer a large contact area between Pd nanoparticles and graphene and efficient gas flow dueThe
to the
nanoparticles
on
GAs
offer
a
large
contact
area
between
Pd
nanoparticles
and
graphene
and
efficient
porous structures. In our study, different amount of Pd nanoparticles were loaded onto rGO aerogels
gas flow due to the porous structures. In our study, different amount of Pd nanoparticles were
(rGOAs) by adding different amount of PdCl2 /EDTA solutions to GO suspensions. In addition, PdCl2
loaded onto rGO aerogels (rGOAs) by adding different amount of PdCl2/EDTA solutions to GO
without EDTA was added to GO suspensions as a control. While the concentration of PdCl2 can be as
suspensions. In addition, PdCl2 without EDTA was added to GO suspensions as a control. While the
high as 70 mM with EDTA before GO is precipitated, more than 4 mM pure PdCl2 would precipitate
concentration of PdCl2 can be as high as 70 mM with EDTA before GO is precipitated, more than 4
GO. The produced rGOA composites were put between two electrodes and subjected to hydrogen
mM pure PdCl2 would precipitate GO. The produced rGOA composites were put between two
sensing testing as shown in the Figure 8a inset. The resistance change of each rGOA/Pd nanoparticle
electrodes and subjected to hydrogen sensing testing as shown in the Figure 8a inset. The resistance
change of each rGOA/Pd nanoparticle composite upon the exposure to 1000 ppm hydrogen gas is
shown inFigure 8a. First, heavy loading of Pd nanoparticles on rGOA using EDTA have a much
Micromachines 2017, 8, 47
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Micromachines 2017, 8, 47
10 of 14
composite upon the exposure to 1000 ppm hydrogen gas is shown inFigure 8a. First, heavy loading
higher
response to on
hydrogen
gas EDTA
than small
of Pdresponse
nanoparticles
usinggas
pure
2,
of Pd nanoparticles
rGOA using
have aloading
much higher
to hydrogen
thanPdCl
small
suggesting
thatnanoparticles
increased Pdusing
nanoparticles
rGOA can that
increase
sensitivity.
Second, the
loading of Pd
pure PdClon
increased
Pd nanoparticles
onsensor
rGOA
2 , suggesting
response
increases
with Pd
nanoparticle
loading
reach increases
a peak at 30
mM
precursor concentration,
can increase
sensitivity.
Second,
the sensor
response
with
PdPd
nanoparticle
loading reach
and
thenatdecreases.
With
increasing
Pd loading,and
the then
amount
of nanoparticles
increases,
resulting
a peak
30 mM Pd
precursor
concentration,
decreases.
With increasing
Pdthe
loading,
the
increased
nanoparticle
surface the
area
leads to
an increase
in sensitivity.
However,
thetoPd
amount ofPd
nanoparticles
increases,
resulting
increased
Pd nanoparticle
surface
area leads
an
nanoparticle
size will increase
with
increased size
Pd loading
(Figure
S2),
whichincreased
causes the
increase in sensitivity.
However,
thefurther
Pd nanoparticle
will increase
with
further
Pd
reduction
of Pd nanoparticle
surfacethe
area.
The surface
of compoiste
rGOA
fabricated
fromarea
30
loading (Figure
S2), which causes
reduction
of Pdarea
nanoparticle
surface
area.
The surface
2
2
2
2
mM
and 70 mM
was 328
m /g and
24930
m mM
/g, respectively.
8b m
shows
the change
of resistance
of
of compoiste
rGOA
fabricated
from
and 70 mMFigure
was 328
/g and
249 m /g,
respectively.
rGOA/Pd
composites
versusoftime
over four
on-off cycles
of 1000 versus
ppm hydrogen
Figure 8b nanoparticle
shows the change
of resistance
rGOA/Pd
nanoparticle
composites
time over gas.
four
The
performance
comparison
of ourgas.
hydrogen
sensor to the
reported of
hydrogen
sensors
using
on-off
cycles of 1000
ppm hydrogen
The performance
comparison
our hydrogen
sensor
to Pd
the
nanoparticles
is listed
in Table
1. ItPd
clearly
shows a fast
response
of hydrogen
gas
(25 s)aand
reported hydrogen
sensors
using
nanoparticles
is listed
in Table
1. It clearly
shows
fastrecovery
response
(175
s) (Figuregas
S3).(25
Wes)believe
that the(175
improved
response
and
recovery
our hydrogen
senors
of hydrogen
and recovery
s) (Figure
S3). We
believe
thatrate
the of
improved
response
and
isrecovery
attributed
increased
Pdsenors
loading
and enhanced
flow efficiency
in rGOAs.
Larger amount
ratetoofthe
our
hydrogen
is attributed
to thegas
increased
Pd loading
and enhanced
gas flow
of
Pd nanoparticles
more binding
sites for hydrogen
and
porous
structure
of rGOA
efficiency
in rGOAs. provide
Larger amount
of Pd nanoparticles
providegas
more
binding
sites
for hydrogen
gas
facilitates
the
accessibility
of
hydrogen
gas
to
Pd
nanoparticles.
and porous structure of rGOA facilitates the accessibility of hydrogen gas to Pd nanoparticles.
Figure8.8.(a)
(a)The
Thechange
change of
of resistance
resistance of
of GA/Pd
GA/Pdnanoparticle
nanoparticlecomposites
compositeswith
withdifferent
differentPd
Pdloadings
loadings
Figure
upon
exposure
to
1000
ppm
hydrogen
gas.
(b)
The
change
of
resistance
of
GA/Pd
nanoparticle
upon exposure to 1000 ppm hydrogen gas. (b) The change of resistance of GA/Pd nanoparticle
compositesupon
uponhydrogen
hydrogengas
gason/off
on/offcycles.
cycles.
composites
Table1.1.Comparison
Comparisonof
ofhydrogen
hydrogengas
gassensing
sensingperformance.
performance.
Table
Material
Material
Method
Method
Pd nanoparticles
Pd nanoparticles
on CNTs
on CNTs
chemical
chemical
reduction
reduction
Pd and C60
physical vapor
physical
vapor
deposition
Pd nanocubes on
chemical
deposition
CNTson
reduction
Pd nanocubes
chemical
galvanic
PdCNTs
nanoparticles on
reduction
displacement
graphene
galvanic
reaction
Pd nanoparticles
Pd nanoparticles on displacement
chemical
on graphene
graphene
reduction
reaction
nanoribbon films
Pd nanoparticles
Pd nanoparticles on
chemical
e-beam
graphene
on graphene
evaporation
reduction
nanoribbon
films
nanoribbon films
Pd nanoparticles on
chemical
Pd nanoparticles
graphene aerogels
reduction
e-beam
on graphene
evaporation
nanoribbon films
Pd nanoparticles
chemical
on graphene
reduction
aerogels
Pd and C60
Detection
Detection
Response
Recovery
Response
TimeTime Recovery
Time Time
ReferenceRef.
Range Range
min.
20 min
18 18
min.
(300(300
ppm)ppm)20 min (100
ppm)(100
and 77min
min and 55100–1000
ppm
and
min (3000 and 55
ppm)
100–1000
ppm [66]
(3000 ppm)
(1000 ppm)
ppm)
500 s
500 s
1%
500 s
15 min
500 s
5 min.
15 min
2 min
2 min
750 s
15 min
750 s
25 s
[66]
min (1000 ppm)
[67]
1%
[67]
10 ppm–1%
5 min.
5 min
[68]
10 ppm–1%
25 ppm–2%
5 min
tens of min
[68]
[69]
25 ppm–2%
100 ppm–1%
[70]
10 min
ppm
tens of min 40–8000100
ppm–1% [71]
175 s
100–1000 ppm
[69]
[70]
current work
15 min
10 min
40–8000 ppm
[71]
25 s
175 s
100–1000 ppm
current
work
Micromachines 2017, 8, 47
11 of 14
4. Conclusions
In this paper, rGO aerogels loaded with four kinds of metallic nanoparticles were successfully
prepared with excellent directionality and decent hieratical structures. The loading of 59, 67, 39, and
46 wt % of metallic palladium, platinum, nickel, and tin in rGO/metal aerogels was achieved after
hydrogen reduction at elevated temperatures. It is believed that other kinds of metallic nanoparticles
can also be loaded onto rGO aerogels with similar procedures. A hydrogel gas sensor was produced
using Pd nanoparticle loaded rGO aerogels. The large surface area and porous structure of rGO
aerogels offer high sensitivity and fast response/recovery time of the sensor. The metal nanoparticle
decorated aerogels which have open-pore structure and a large surface area are promising platforms for
numerous applications, including energy storage, gas sensing, and catalysts of vapor phase reactions.
Supplementary Materials: The following are available online at www.mdpi.com/2072-666X/8/2/47/s1.
Acknowledgments: The project is partially supported by Florida Space Institute, and E.B. thanks NASA for
financial support through NASA Space Technology Research Fellowships (NSTRF).
Author Contributions: L.Z., C.S. and M.M. conceived and designed the experiments; C.S., E.B., M.M. and J.Z.
performed the experiments; C.S., E.B., M.M. and J.Z. analyzed the data. All authors contribute to the writing of
the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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