Sensors
and
Actuators
B
190 (2014) 227–
233
Contents
lists
available
at
ScienceDirect
Sensors
and
Actuators
B:
Chemical
journal
h
om
epage:
www.elsevier.com/locat
e/snb
Room
temperature
gas
sensor
based
on
tin
dioxide-carbon
nanotubes
composite
films
Frank
Mendoza
a,b,∗
,
Dionne
M.
Hernández
a,c
,
Vladimir
Makarov
a,b
,
Emmanuel
Febus
a,b
,
Brad
R.
Weiner
a,c
,
Gerardo
Morell
a,b
a
Institute
for
Functional
Nanomaterials,
University
of
Puerto
Rico,
PR
00931,
USA
b
Department
of
Physics,
University
of
Puerto
Rico,
San
Juan,
PR
00936,
USA
c
Department
of
Chemistry,
University
of
Puerto
Rico,
San
Juan,
PR
00936,
USA
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
29
December
2012
Received
in
revised
form
10
August
2013
Accepted
14
August
2013
Available online 4 September 2013
Keywords:
HF-CVD
process
Gas
sensor
Carbon
nano-materials
a
b
s
t
r
a
c
t
Chemical
sensors
based
on
tin
dioxide-carbon
nanotubes
(SnO
2
-CNT)
composite
films
were
fabricated
by
hot
filament
chemical
vapor
deposition
(HF-CVD)
technique.
The
composite
films
consist
of
SnO
2
nanoparticles
highly
dispersed
on
the
CNTs
surface.
Their
resistivity
is
highly
sensitive
to
the
presence
of
adsorbates,
which
become
easily
attached
or
detached
at
room
temperature
and
ambient
pressure
depending
on
their
gas
phase
concentration.
We
systematically
studied
the
sensitivity
of
the
SnO
2
-CNT
composite
films
for
ethanol,
methanol
and
H
2
S.
The
results
were
also
compared
to
those
for
SnO
2
and
CNTs
separately.
It
is
shown
that
the
SnO
2
-CNT
composite
films
can
detect
ethanol,
methanol
and
H
2
S
down
to
ppm
levels
below
OSHA’s
permissible
exposure
limits
at
room
temperature
and
ambient
pressure.
Moreover,
they
self-recover
within
1
min
without
requiring
any
heating
or
energy
source.
© 2013 Elsevier B.V. All rights reserved.
1.
Introduction
Tin
dioxide
(SnO
2
)
has
been
studied
by
different
research
groups
due
to
its
capacity
to
adsorb
molecules
in
the
gas
phase
[1–3]
and
Li-
ions
in
rechargeable
batteries
[4–7],
improving
the
performance
of
bare
CNTs.
SnO
2
is
a
n-type
semiconductor
metal
oxide
with
a
band
gap
of
3.6
eV
[8],
making
it
a
favorable
material
for
sensing
devices
[9–11].
It
has
applications
in
environmental
problems
caused
by
the
emission
of
pollutants
from
assorted
sources,
such
as
CO,
NO
x
,
SO
2
[12]
and
H
2
S
[13],
which
represent
a
great
concern
for
public
safety.
Hydrogen
sulfide
is
well
known
in
the
oil
and
food
industry,
and
in
organic
waste
in
areas
such
as
sewers.
In
the
case
of
alcohols,
they
have
important
uses
in
the
pharmaceutical
and
food
industry.
SnO
2
is
widely
used
for
alcohol
detection,
especially
ethanol
[14].
Although
undoped
SnO
2
gas
sensors
have
low
sensitivity
and
selec-
tivity
[15],
they
can
be
improved
by
combining
with
other
materials
[16].
Studies
demonstrate
that
carbon
nano-materials
(CNMs)
[17,18]
(i.e.,
carbon
nanotubes,
nanobelts,
nanowires)
are
good
candi-
dates
as
potential
“dopants”
of
SnO
2
.
In
general,
CNMs
influence
the
materials’
charge
transfer,
electrostatic
environment,
surface
area,
and
molecular
adsorption
[19],
thus
improving
the
proper-
ties
required
for
gas
sensing
[20].
Gas
sensors
based
on
CNMs
∗
Corresponding
author.
E-mail
addresses:
frank.w.mendoza@uprrp.edu,
frankmendoza1@gmail.com
(F.
Mendoza).
in
their
bare
form
(without
using
dopants
or
hybrid
materials)
have
been
shown
to
detect
nitrogen
[21],
hydrogen
[22],
ammo-
nia
[23],
and
other
gas
and
vapors
[24].
Moreover,
their
sensitivity
can
be
enhanced
by
blending
with
activated
materials
[18,25,26].
However,
these
sensors
conventionally
operate
at
temperatures
above
∼200
◦
C
to
obtain
reasonable
responses
[18,27,28].
Further-
more,
the
baseline
gases
used
and
analyte
concentration
affect
the
response
and
recovery
times
[29]
of
the
sensors.
Wei
et
al.
[19]
combined
SnO
2
with
single
wall
carbon
nanotubes
to
develop
a
gas
sensor
for
NO
2
at
room
temperature.
They
reported
enhanced
sensitivities
of
the
hybrid
sensor
compared
to
pure
SnO
2
,
but
the
response
time
is
long,
in
the
order
of
minutes.
Chen
et
al.
[30]
pre-
pared
CNT/SnO
2
core/shell
structures
to
measure
ethanol
gas
at
various
concentrations
with
a
working
temperature
of
300
◦
C.
The
core/shell
configuration
and
working
temperature
enhanced
the
sensitivity
up
to
24.5
at
low
gas
concentrations.
We
hereby
report
the
room-temperature
sensor
characteristics
of
tin
dioxide-carbon
nanotubes
composite
films.
They
can
detect
ethanol,
methanol
and
H
2
S
down
to
ppm
levels,
with
excellent
response
and
recovery
times
at
room
temperature
and
ambient
pressure.
2.
Experimental
2.1.
Materials
and
methods
Fig.
1
depicts
a
flowchart
of
the
method
used
for
the
synthesis
of
tin
dioxide-carbon
nanotubes
composite
films
and
a
representative
0925-4005/$
–
see
front
matter ©
2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.snb.2013.08.050