Sensors
and
Actuators
B
181 (2013) 802–
809
Contents
lists
available
at
SciVerse
ScienceDirect
Sensors
and
Actuators
B:
Chemical
journa
l
h
o
me
pa
ge:
www.elsevier.com/locate/snb
Highly
sensitive
humidity
sensor
based
on
high
surface
area
mesoporous
LaFeO
3
prepared
by
a
nanocasting
route
Jing
Zhao
a
,
Yinping
Liu
a
,
Xiaowei
Li
a
,
Geyu
Lu
a,∗
,
Lu
You
a
,
Xishuang
Liang
a
,
Fengmin
Liu
a
,
Tong
Zhang
a
,
Yu
Du
b,∗
a
State
Key
Laboratory
on
Integrated
Optoelectronics,
College
of
Electronic
Science
and
Engineering,
Jilin
University,
2699
Qianjin
Street,
Changchun
130012,
China
b
Department
of
Applied
Physics,
Shenzhen
University,
Shenzhen
518060,
China
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
5
July
2012
Received
in
revised
form
23
January
2013
Accepted
19
February
2013
Available online 28 February 2013
Keywords:
Mesoporous
LaFeO
3
High
surface
area
Humidity
sensor
a
b
s
t
r
a
c
t
Mesoporous
LaFeO
3
with
high
surface
area
and
pore
volume
was
prepared
through
a
nanocasting
route
by
using
mesoporous
silica
SBA-15
as
a
hard
template.
The
structure
and
chemical
composition
of
the
sample
were
characterized
by
X-ray
diffraction
(XRD),
nitrogen
adsorption–desorption,
transmission
electron
microscopy
(TEM)
and
inductively
coupled
plasma
mass
spectrometry
(ICP-MS).
The
humidity
sensing
properties
of
the
mesoporous
LaFeO
3
were
investigated.
Impedance
greatly
changes
by
more
than
five
orders
magnitude
(1.7
×
10
6
to
4.5
k)
when
the
relative
humidity
varies
from
11%
to
98%
at
10
Hz,
and
it
also
exhibits
the
satisfactory
response
time,
hysteresis
and
stability.
The
sensor
utilizing
mesoporous
LaFeO
3
via
nanocasting
method
displays
superior
humidity
performance
compared
to
that
using
bulk
LaFeO
3
via
sol–gel
technique.
Such
sensing
behavior
of
the
mesoporous
LaFeO
3
can
be
attributed
to
the
high
surface
area
(83.2
m
2
/g)
and
porosity,
which
lead
to
highly
effective
interaction
between
the
water
molecules
and
the
surface
active
sites.
A
possible
mechanism
is
discussed
to
explain
the
excellent
performance
of
the
humidity
sensing
device
using
the
mesoporous
LaFeO
3
.
© 2013 Elsevier B.V. All rights reserved.
1.
Introduction
Humidity
sensors
play
a
vital
role
in
numerous
fields
[1,2]
to
maintain
a
specific
conditions,
such
as
environmental
mon-
itoring,
food
processing
industries,
medicine,
meteorology
and
research
labs.
The
relative
humidity
(RH)
[3],
which
is
the
ratio
of
actual
water
vapor
pressure
to
the
saturated
vapor
pressure
at
a
given
temperature,
is
the
most
important
parameter
in
specify-
ing
humidity.
Many
types
of
humidity
sensing
devices,
including
ceramic
[4–6],
polymer
[7],
optical
type
sensors
[8,9]
and
mechan-
ical
hygrometer
[10]
have
been
exploited
to
detect
humidity
in
the
practical
environment.
Among
the
various
humidity
sensors,
the
ceramic
sensors
are
widely
applied
because
of
their
simple
struc-
ture,
low
cost
and
good
stability.
However,
further
investigation
is
requested
in
order
to
optimize
the
performance
of
ceramic
humid-
ity
sensors,
such
as
sensitivity,
reversibility,
long-term
stability
as
well
as
response
and
recovery
times
[11].
The
change
of
the
elec-
trical
signal
(impedance)
with
humidity
in
the
ceramic
humidity
sensors
is
originated
by
the
adsorption
of
water
molecules
exist-
ing
in
the
atmosphere
on
the
surface
of
the
sensing
materials
[12,13].
Therefore,
an
increased
specific
surface
area
[14]
and
the
porous
structure
[15]
of
the
sample
are
beneficial
to
the
sensing
∗
Corresponding
authors.
Tel.:
+86
431
85167808;
fax:
+86
431
85167808.
E-mail
addresses:
lugy@jlu.edu.cn (G.
Lu),
duyu@szu.edu.cn
(Y.
Du).
properties
of
the
humidity
sensor
owing
to
the
exposure
of
more
active
sites
to
the
adsorbed
water
molecules.
Mesoporous
materials
[16,17]
have
received
enormous
atten-
tion
due
to
their
high
internal
and
external
surface
area,
large
pore
volume,
uniform
pore
diameter
and
easy
surface
functional-
ization,
rendering
the
unique
materials
useful
in
wide
spectrum
of
fields.
Recently,
various
mesoporous
metal
oxides
have
been
largely
replicated
via
nanocasting
method
[18,19]
which
is
an
important
strategy
for
synthesizing
nonsiliceous
mesoporous
oxides.
In
this
route,
mesoporous
silicas
(SBA-15,
KIT-6
and
MCM-41)
or
car-
bons
(CMK-3)
are
usually
used
as
the
“hard
templates”
[20–24].
The
precursor
compound
(metal
salt)
for
the
desired
metal
oxide
is
introduced
into
the
void
mesoporous
channel
of
silica
or
car-
bon
templates,
and
subsequent
in
situ
thermal
processing
to
form
the
desired
crystalline
metal
oxide.
Mesoporous
silica
template
is
removed
using
an
aqueous
NaOH
or
HF
solution
and
carbon
template
is
removed
by
calcination.
A
negative
ordered
array
of
the
metal
oxide
is
obtained
by
replicating
the
ordered
mesostruc-
ture
of
the
hard
template.
In
the
conventional
sol–gel
processes
by
using
surfactants
and
block
copolymers
(“soft
templates”),
it
is
difficult
to
obtain
crystalline
walls
or
ordered
mesostructure
for
many
non-siliceous
mesoporous
metal
oxides
[25,26].
The
main
problem
is
that
temperatures
required
for
the
crystallization
of
metal
oxides
are
normally
high
and
the
liquid
crystal
surfactant
templates
would
decompose
before
the
crystallization
of
metal
oxides.
However,
when
mesoporous
silicas
are
used
as
the
hard
0925-4005/$
–
see
front
matter ©
2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.snb.2013.02.077