Electrochimica
Acta
108 (2013) 167–
174
Contents
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at
ScienceDirect
Electrochimica
Acta
jo
u
r
n
al
hom
ep
age:
www.elsevier.com/locate/electacta
Effect
of
etching
current
density
on
microstructure
and
NH
3
-sensing
properties
of
porous
silicon
with
intermediate-sized
pores
Mingda
Li,
Ming
Hu
∗
,
Peng
Zeng,
Shuangyun
Ma,
Wenjun
Yan,
Yuxiang
Qin
School
of
Electronics
and
Information
Engineering,
Tianjin
University,
Tianjin
300072,
PR
China
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
20
April
2013
Received
in
revised
form
16
June
2013
Accepted
18
June
2013
Available online 2 July 2013
Keywords:
Porous
silicon
Intermediate-sized
pores
Electrochemical
etching
NH
3
monitoring
a
b
s
t
r
a
c
t
In
this
work,
porous
silicon
with
intermediate-sized
pores
(intermediate–PS)
was
prepared
by
using
galvanostatic
electrochemical
etching
method
and
the
effect
toward
sensing
response
characteristics
of
NH
3
gas
was
also
studied.
The
morphology
and
surface
chemical
bonds
of
intermediate–PS
were
char-
acterized
by
using
field
emission
scanning
electron
microscope
(FESEM)
and
Fourier
transform
infrared
spectroscopy
(FTIR),
respectively.
The
results
showed
the
intermediate–PS
microstructure
can
be
signifi-
cantly
modulated
by
the
etching
current
density.
Moreover,
the
freshly
prepared
intermediate–PS
surface
could
achieve
reliable
passivation
after
storage
in
ethanol.
Furthermore,
the
gas-sensing
measurements
of
the
intermediate–PS
sensors
were
carried
out
versus
different
concentrations
of
NH
3
.
The
PS
sensor
exhibited
good
NH
3
-sensing
performances
at
room
temperature
owing
to
its
unique
microstructure
fea-
tures,
including
large
specific
surface
area
and
highly
ordered
pore
channels.
In
addition,
the
conceivable
pore
formation
mechanism
as
well
as
gas
sensing
mechanism
was
also
discussed.
© 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
As
a
kind
of
poisonous,
colorless
with
a
characteristic
pungent
odor
gas,
NH
3
gas
is
mainly
generated
by
chemical
plants,
fertil-
izer
and
motor
vehicles
[1].
In
particular,
the
high
levels
of
NH
3
in
the
atmosphere
are
extremely
harmful
to
human
health
and
environmental
protection
[2].
Based
on
the
US
National
Ambient
Air
Quality
Standards
for
NH
3
concentration,
the
threshold
limit
value
is
25
ppm
and
the
short-term
exposure
limit
value
is
35
ppm
[3].
As
a
matter
of
fact,
NH
3
-sensing
materials
have
been
attract-
ing
much
attention
recently
due
to
their
extensive
application
prospects,
including
food
freshness
monitoring,
chemical
indus-
tries,
medical
diagnosis,
environmental
protection
and
more
[4].
In
this
respect,
various
metal
oxide
semiconductors
(e.g.
SnO
2
[5],
ZnO
[6])
have
been
widely
used
and
exhibited
a
highly
sensitive
response
toward
NH
3
.
However,
most
of
them
need
to
be
operated
at
high
temperatures
due
to
their
poor
electrical
conductivity
at
near
room
temperature
(RT),
which
is
unfavorable
for
the
power
saving,
device
integration
and
thermally
stability
[7].
Therefore,
it
is
necessary
to
develop
a
cheap,
sensitive,
rapid
and
reliable
NH
3
gas
sensor,
which
is
still
a
challenge
for
conventional
sensing
mate-
rials.
According
to
the
IUPAC
standard,
porous
silicon
(PS)
can
be
classified
into
three
categories
depending
on
its
pore
diameter
∗
Corresponding
author.
Tel.:
+86
22
87890662;
fax:
+86
22
27401233.
E-mail
address:
huming@tju.edu.cn
(M.
Hu).
d:
micro-PS
(d
≤
10
nm),
meso-PS
(10
<
d
≤
50
nm)
and
macro-PS
(d
>
50
nm)
[8].
In
the
last
years,
the
PS
semiconductor
mate-
rial
has
attached
considerable
interest
owing
to
its
low
cost,
low
power
consumption,
and
potential
compatibility
with
sili-
con
integrated
circuit
technologies
[9,10].
Therefore,
PS
has
been
widely
investigated
for
a
variety
of
applications,
such
as
micro-
machining,
fuel
cells,
solar
cells,
photonic
crystals,
photovoltaic
device,
and
biosensors
[11,12].
What
is
more,
PS
is
considered
as
a
promising
candidate
for
high-performance
gas
sensor
fields
due
to
its
large
specific
surface
area
and
high
surface
chemi-
cal
activity
at
RT
[13,14].
Nevertheless,
the
widely
used
meso-PS
is
faced
with
two
main
obstacles
limiting
its
further
practical
applications.
The
first
is
the
instability
of
the
freshly
prepared
surface,
containing
a
high
density
of
metastable
silicon-hydride
termination
and
requires
prolonged
natural
oxidation
(lasts
sev-
eral
weeks,
even
several
months)
for
achieving
a
stabilized
surface
as
required
[15].
The
second
is
the
slow
recovery
speed
of
the
electrical
response
from
the
PS
and
analyte
interaction,
which
is
attributed
to
the
limited
mass
transport
inside
the
branching
pore
channels
[16].
So
far,
several
colleagues
have
focused
on
study-
ing
and
improving
the
performances
of
PS–based
gas
sensor.
For
instance,
Sun
et
al.
pointed
out
the
PS
gas-sensing
properties
can
be
improved
dramatically
by
modulating
desired
microstructure
[17]
.
Badilla
et
al.
demonstrated
the
functionalized
PS
surface
could
exhibit
a
better
sensing
stability
in
detecting
target
gas
[18].
Actually,
the
gas-sensing
properties
of
PS
are
known
to
be
highly
dependent
upon
its
porosity,
morphology,
structure,
resistivity,
and
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matter ©
2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.06.106