Preparation, characterization and gas sensitivity ofpolypyrrole/g-Fe2O3hybrid materials

Preparation,

characterization

and

gas

sensitivity

of
polypyrrole/
g
-Fe
2
O
3
hybrid

materials
§
Lina

Geng
a,
*
,

Shihua

Wu

b
a
Department

of

Chemistry,

Hebei

Normal

University,

Shijiazhuang

050016,

People’s

Republic

of

China
b
Department

of

Chemistry,

Nankai

University,

Tianjin

300071,

People’s

Republic

of

China
1.

Introduction
Organic–inorganic

hybrid

materials

composed

of

oxides

and
conducting

polymers

that

can

synergize

or

complement

the
properties

of

pure

organic

and

inorganic

materials

have

been
used

in

many

applications,

such

as

in

electronics,

optics,

coating
and

catalysis

[1–5].

Polymer/ferric

oxide

hybrids

in

particular

have
superior

properties

to

those

of

pure

polymer

and

ferric

oxide,

with
both

magnetic

and

polymer

properties,

and

have

demonstrated
wide

uses

in

medicine,

biochemistry

and

industry

[6,7].
Gas

sensors

have

been

developed

to

measure

gas

concentration,
monitor

emissions

in

combustion

processes

and

provide

feedback
control

[8].

The

study

of

organic–inorganic

hybrid

materials

for
application

in

gas

sensors

is

a

current

research

hotspot,

as

these
hybrids

can

compensate

for

the

drawbacks

of

single

inorganic
sensors

with

high

operating

temperatures

and

low

selectivity

and
of

organic

sensors

with

poor

processability

and

long

response–
recovery

time

[9,10].

Itoh

et

al.

[11]

developed

a

(PNMA)
x
MoO
3
hybrid

thin

film

and

found

in

evaluating

its

VOC

(volatile

organic
compound)

–

sensing

properties

that

the

selectivity

of

organic/
MoO
3
hybrids

can

be

controlled

by

modifying

the

organic
components.

Nardis

et

al.

[12]

reported

that

cobalt

porphyrin/
tin

dioxide

has

superior

selectivity

to

methanol

vapor

and

lower
working

temperatures

than

pure

SnO
2
.

Meanwhile,

Hosono

and
Matsubara

[13,14]

synthesized

a

PPy/MoO
3
thin

film

and

PPy/

MoO
3
pressed

pellet

and

found

that

PPy/MoO
3
materials

have
better

selectivity

compared

with

polar

VOCs.

Suri

et

al.

[15]
reported

on

a

PPy/iron

oxide

material

that

is

sensitivity

to
humidity

and

to

N
2
,

O
2
,

CO
2
and

CH
4
gases

at

different

pressures.
Our

previous

experiments

confirmed

that

PPy/ZnO,

PPy/WO
3
and

PAni/SnO
2
hybrids

are

superior

to

single

polymer

and

oxide
sensing

material

in

terms

of

selectivity

and

working

temperature
[16–18].

In

this

work,

PPy/
g
-Fe
2
O
3
hybrids

were

prepared

by
simultaneous

gelation

and

polymerization

processes

and

then
characterized

by

FT-IR,

XRD,

TG–DTA

and

HRTEM.

The

gas
sensitivities

of

PPy/
g
-Fe
2
O
3
hybrids

compared

to

pure

PPy

and
g
-Fe
2
O
3
under

CO,

H
2
,

NH
3
,

ethanol

and

acetone

atmosphere

at

low
operating

temperatures

(<100

8C)

were

evaluated.

The

sensing
mechanism

of

polypyrrole/
g
-Fe
2
O
3
is

also

discussed.
2.

Experimental
2.1.

Preparation

and

characterization

of

PPy/
g
-Fe
2
O
3
Pyrrole

monomers

were

distilled

under

reduced

pressure,
placed

in

a

desiccator

and

stored

at

4

8C

until

use.

Methoxy

ethanol
was

added

to

Fe(NO
3
)
3
Á9H

2
O

in

a

100

ml

round

bottom

flask
Materials

Research

Bulletin

48

(2013)

4339–4343
A

R

T

I

C

L

E

I

N

F

O
Article

history:
Received

7

April

2013
Received

in

revised

form

1

July

2013
Accepted

7

July

2013
Available

online

15

July

2013
Keywords:
A.

Composites
B.

Sol–gel

chemistry
C.

Differential

scanning

calorimetry

(DSC)
C.

Thermogravimetric

analysis

(TGA)
A

B

S

T

R

A

C

T
Polypyrrole

(PPy)/
g
-Fe
2
O
3
hybrid

materials

were

prepared

by

sol–gel

polymerization

in

situ

and
characterized

by

Fourier

transform

infrared

(FT-IR),

X-ray

powder

diffraction

(XRD),

thermogravimetric
and

differential

thermal

analysis

(TG–DTA)

and

high-resolution

transmission

electron

microscope
(HRTEM).

The

gas

sensitivities

in

CO,

H
2
,

NH
3
,

ethanol

or

acetone

atmospheres

were

determined

at

30

8C,
60

8C

and

90

8C.

FT-IR

and

XRD

patterns

suggest

that

ferric

oxide

in

the

hybrids

was

g
-Fe
2
O
3

,

with

a
diameter

of

approximately

5

nm.

TG–DTA

and

HRTEM

analyses

showed

that

different

reactant

molar
ratios

of

pyrrole

monomer:

Fe(NO
3
)
3
Á9H
2
O

resulted

in

different

microstructures

of

g
-Fe

2
O
3
and
molecular

weights

of

PPy.

An

increased

amount

of

Fe(NO
3
)
3
Á9H
2
O

increased

the

degree

of

uniformity

of
the

molecular

weight

of

PPy

and

resulted

in

a

change

of

g
-Fe
2
O
3
microstructure

from

granular

to

stick
particles.

The

results

of

gas

sensitivities

showed

that

the

PPy/
g
-Fe
2
O
3
hybrids

exhibited

high

sensitivity
to

NH
3
at

mild

operating

temperature

(<100

8C).

Furthermore,

the

sensing

mechanism

was

also
discussed.
ß

2013

The

Authors.

Published

by

Elsevier

Ltd.

All

rights

reserved.
§
This

is

an

open-access

article

distributed

under

the

terms

of

the

Creative
Commons

Attribution-NonCommercial-No

Derivative

Works

License,

which
permits

non-commercial

use,

distribution,

and

reproduction

in

any

medium,
provided

the

original

author

and

source

are

credited.
*

Corresponding

author.

Tel.:

+86

311

80787400;

fax:

+86

311

87881815.
E-mail

address:



(L.

Geng).
Contents

lists

available

at

SciVerse

ScienceDirect
Materials

Research

Bulletin
jo

u

rn

al

h

om

ep

age:

ww

w.els

evier.c

o

m/lo

c

ate/mat

res

b

u
0025-5408/$

–

see

front

matter

ß

2013

The

Authors.

Published

by

Elsevier

Ltd.

All

rights

reserved.
/>containing

a

magnetic

stir

bar,

and

then

distilled

pyrrole

was
added

dropwise

to

the

stirred

solution

for

the

molar

ratio

of

pyrrole
monomer:

Fe(NO
3
)
3
Á9H
2
O

=

1:3

or

1:4

(the

products

are

denoted
as

S1

and

S2).

The

solution

was

continuously

stirred

and

heated

at

a
slow

rate

to

evaporate

the

solvent,

and

a

black

powder

precipitate
was

obtained.

The

chemical

reaction

equations

were

as

follows:
FeðNO
3
Þ
3
þ

3CH
3
OCH
2
CH
2
OH

¼

FeðOCH

2
CH
2
OCH
3
Þ
3
þ

3HNO
3
MðORÞ
n
þ

H
2
O

¼

MðORÞ
x
ðOHÞ
nÀx
þ

ðn

À

xÞHOR
½FeðOCH
2
CH
2
OCH
3
Þ
3
isabbreviatedasMðORÞ
n

ÀÀMÀÀOH

þ

HOÀÀMÀÀ

¼

ÀÀMÀÀOÀÀMÀÀ

þ

H
2
O
ÀÀMÀÀOR

þ

ROÀÀMÀÀ

¼

ÀÀMÀÀOÀÀMÀÀ

þ

ROH
After

washing

with

water

followed

by

ethanol,

the

products
were

dried

in

an

oven

and

then

annealed

at

different

temperatures
of

100

8C,

130

8C,

150

8C

or

180

8C.
The

properties

of

the

PPy/ferric

oxide

hybrids

were

analyzed
using

several

structural

methods:

FT-IR

(Avatar

360

FT-IR
spectrophotometer),

XRD

(DMAX-2500

diffractometer

with

Cu
K
a

radiation

at

40

kV

and

100

mA),

TG–DTA

(ZRY-2P

Simultaneous
Thermal

Analyzer)

and

HRTEM

(Philips

T20ST,

operated

at

200

kV).
2.2.

Determination

of

gas

sensing

characteristics
CO,

H
2
,

NH
3
,

ethanol

and

acetone

were

selected

for

testing

the
gas

sensitivity

of

the

materials.

The

detection

system

and

electric
circuit

have

been

described

in

our

previously

studies

[15–17].
Briefly,

the

materials

were

fabricated

on

an

aluminum

tube

with
Au

electrodes

and

platinum

wires.

A

Ni–Cr

alloy

through

the

tube
was

used

as

a

heating

filament.

The

voltage

of

the

sensor

was
measured

indirectly

by

an

external

resistor

in

the

testing

circuit.
Gas

sensitivity

is

defined

as

S

=

V
g
/V
a
,

where

V
a

and

V
g
are

the
voltages

of

the

sensor

in

clear

air

and

in

the

test

gas,

respectively
[19,20].

All

experiments

were

carried

out

at

a

fixed

humidity

of
60%.
3.

Results

and

discussion
The

FT-IR

spectra

of

S1

and

S2

annealed

at

150
8
C

were
compared

with

that

of

PPy

in

the

range

of

400–4000

cm
À1
(Fig.

1).
The

characteristic

bands

of

PPy

were

observed

at

1560,

1398,

1298,
1211,

1047,

930

and

790

cm
À1
,

which

were

close

to

those

reported
in

the

literature

[21]:

stretching

vibration

(1560

cm
À1
)

of

the

C55C
bond,

stretching

vibration

(1298

cm
À1
)

of

the

C55C

bond,

stretching
vibration

(1211

cm
À1
)

of

the

C–N

bond,

and

the

pyrrole

ring

bonds
(1407,

1398,

1047,

930,

790

cm
À1
).

In

the

S1
(150

8C)
and

S2
(150

8C)
spectra,

characteristic

peaks

of

PPy

were

also

found

at

1407,

1398,
1047,

930

and

790

cm
À1
,

and

the

g
-Fe
2
O
3
specific

bands

appeared
in

681,

578

and

468

cm
À1
.

In

the

spectra,

the

C55O

bond

in
pyrrolidone

at

about

1700

cm
À1
due

to

the

overoxidation

of

PPy
was

clearly

seen.

The

obvious

absorption

peak

at

$1390

cm
À1
corresponded

to

KBr.
The

XRD

patterns

revealed

that

the

diffraction

peaks

of

the

S1
and

S2

samples

annealed

at

different

temperatures

appeared

at

the
same

crystal

face

(Fig.

2).

These

peaks

were

consistent

with

those
from

the

Joint

Committee

on

Powder

Diffraction

Standards

(JCPDS)

data

file

(25-1402)

and,

along

with

the

FT-IR

spectra

analysis,
indicated

the

iron

oxide

in

S1

and

S2

was

g
-Fe
2
O
3
.

The

diffraction
4000

3500

3000

2500

2000

1500

1000

500
0
10
20
30
40
50
60
70
S2
S1
PPy
Transmittance (%)
Waveleng

th (nm
-1
)
Fig.

1.

FT-IR

spectra

of

PPy,

S1
(150

8C)
and

S2
(150

8C)
.
0 102030405060708090
0
500
1000
1500
2000
2500
3000
4
3
2
1
440
513
426
400
220
313

Intensity (a.u.)
2 The

ta (deg

.)
1.10

0
o
C
2.13

0
o
C
3.15

0
o
C
4.18

0
o
C
0

10

20

30

40

50

60

70

80

90
0
500
1000
1500
2000
2500
3000
4
3
2
1
440
513
426
400

220
313
Intensity (a.u.)
2 The

ta (deg

.)
1.10

0
o
C
2.13

0
o
C
3.15

0
o
C
4.18

0
o
C
(a)
(b)

Fig.

2.

XRD

patterns

of

sample

S1

(a)

and

S2

(b)

annealed

at

different

temperatures.
L.

Geng,

S.

Wu

/

Materials

Research

Bulletin

48

(2013)

4339–4343
4340
intensity

increased

with

increasing

annealing

temperature,
and

the

particle

sizes

of

S1

and

S2

samples

annealed

at

150

8C
were

5.3

nm

and

4.7

nm,

respectively,

according

to

the

Scherrer
formula.
The

S1

and

S2

samples

annealed

at

100

8C

were

heated

at

the
rate

of

10

8C/min.

The

TG–DTA

curve

for

S1

showed

three
exothermic

and

two

weight

loss

processes

in

the

range

of

20–
550

8C

(Fig.

3a).

The

two

points

of

weight

losses

in

the

TG

curve
corresponded

to

the

first

and

second

exothermic

processes

in

the
DTA

curve,

while

the

third

exothermic

process

had

no

quality
change.

These

two

exothermic

peaks

coupled

with

weight

losses
near

219

8C

and

308

8C

were

caused

by

the

degradation

of

PPy,

as
the

molecular

weight

of

PPy

in

S1

was

not

uniform,

the

small

forms
degraded

first,

while

the

larger

ones

degraded

later.

The

total
weight

loss

percentage

of

S1
(100

8C)
was

31.9%.

The

third

exother-
mic

peak

near

441

8C

was

the

crystal

phase

transition

of

g
-Fe
2
O
3
to
a
-Fe
2
O
3
and

therefore

caused

no

weight

loss.
The

two

exothermic

peaks

and

one

point

of

weight

loss

in

the
TG–DTA

curve

of

S2
(100

8C)
occurred

in

the

range

of

20–600

8C
(Fig.

3b).

The

base

of

the

first

exothermic

peak

near

261

8C

was
wide,

which

spanned

the

temperature

range

of

the

two

exothermic
processes

near

219

8C

and

308

8C

of

S1
(100

8C)
.

This

result

could

be
explained

by

the

molecular

weights

of

the

PPy

species

in

S2
(100

8C)
being

close,

and

therefore

degradation

of

PPy

appeared

continuous
during

heating.

The

total

weight

loss

percentage

of

S2
(100

8C)
was
29%,

similar

to

that

of

S1
(100

8C)
.

As

with

S2
(100

8C)
,

the

second
exothermic

peak

of

S2
(100

8C)
near

458

8C

was

the

crystal

phase
transition

of

g
-Fe
2
O
3
to

a
-Fe
2
O
3
,

which

did

not

result

in

weight
loss.
Although

the

percentages

of

weight

loss

of

S1
(100

8C)
and
S2
(100

8C)
were

similar,

the

number

of

exothermic

peaks

were
different

due

to

differences

in

molecular

weights

of

PPy

in

the

two
samples

(Fig.

3a

and

b).

In

addition,

the

phase-transition
temperature

of

S2
(100

8C)
was

17

8C

higher

than

that

of

S1
(100

8C)
.
These

results

could

be

explained

further

from

the

TEM

and

HRTEM
micrographs.

Fig.

4a

shows

the

polymer

characteristics

(i.e.,
amorphous

particles

and

blurry

boundaries)

of

S1
(150

8C)
,

even
though

it

was

the

hybrid

of

PPy

and

g
-Fe
2
O
3
.

The

crystal

lines

of

g
-
Fe
2
O
3
were

not

obvious

even

with

HRTEM

(Fig.

4b),

which

was

due
to

the

g
-Fe
2
O
3
being

enwrapped

by

PPy

(Fig.

5).
From

the

TEM

and

HRTEM

micrographs

of

S2
(150

8C)
,

the
amorphous

polymer,

granular

and

stick

g
-Fe
2
O

3
particles

could
be

seen,

and

the

length

and

width

of

the

stick

form

was

about
200

nm

and

15

nm,

respectively.

Only

g
-Fe
2
O
3
diffraction

peaks
appeared

in

the

XRD

patterns

of

S2,

indicating

that

the

granular
and

stick

particles

were

all

g
-type

ferric

oxide.

This

result

indicates
that

the

different

molar

ratios

of

py

to

Fe(NO
3
)
3
Á9H
2
O

can

affect

the
morphology

of

iron

oxide.

Brezoi

and

Ion

[22]

had

reported

that

the
amount

of

py

could

influence

the

crystal

phase

of

iron

oxide

in

PPy/
iron

oxide

hybrids.

Xia

and

Wang

[23]

and

He

[24]

all

reported

that
polymer

conformation

does

influence

the

crystal

shape

of
inorganic

oxide.

However,

these

authors

did

not

discuss

the
specific

effect

of

proportion

of

reactants

on

the

morphology

of
inorganic

oxide

in

the

crystal

phase.
Fig.

3.

TG–DTA

curves

of

sample

S1
(100

8C)
(a)

and

S2
(100

8C)
(b).
Fig.

4.

TEM

(a)

and

HRTEM

(b)

micrographs

of

sample

S1
(150

8C)
.
L.

Geng,

S.

Wu

/

Materials

Research

Bulletin

48

(2013)

4339–4343

4341
In

the

gas

sensing

study,

S1

and

S2

were

made

for

thick

film
sensors,

and

their

sensitivities

for

CO,

H
2
,

NH
3
,

ethanol

and
acetone

gases

were

tested

at

30

8C,

60

8C

and

90

8C.

PPy/
g
-Fe
2
O
3
hybrids

prepared

with

reactants

at

two

differ ent

ratios

and

four
annealing

temperatures

all

showed

no

gas

sensitivity

to
3000

ppm

CO,

H

2
,

ethanol

and

acetone

at

30

8C,

60

8C

or

90

8C,
but

showed

good

response

to

2000

ppm

NH
3
under

the

three
operating

temperatures.

In

addition,

S1

and

S2

showed

similar
sensitivity

characteristics.
The

response–recovery

curves

of

S1
(150

8C)
(Fig.

6a)

and

S2
(150

8C)
(Fig.

6b)

showed

that

they

had

good

reversible

and

quick

response–
recovery

times

to

2000

ppm

NH
3

at

the

operating

temperatures

of
30

8C,

60

8C

and

90

8C

(Fig.

6).

The

response

and

recovery

times

of
S1
(150

8C)
were

12–36

s

and

20–22

s,

respectively,

at

different
working

temperatures,

and

those

of

S2
(150

8C)
were

17–40

s

and
20–23

s.

The

results

shown

in

Fig.

6

also

suggest

that

the

testing
voltage

increased

when

the

NH
3
gas

was

inputted,

which

was

due
to

the

increase

in

resistances

and

decreased

conductivities

of
0

50 10

015

020

025

030

0
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24
0.26
gas ou

t
gas in
3
2
1
Voltage (V)
Time (s)
1.30
o
C
2.60

o
C
3.90
o
C
0

50 10

015

020

025

030

0
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24

0.26
0.28
0.30
0.32
gas ou

t
gas in
3
2
1
Voltage (V)
Time (s)
1.30
o
C
2.60
o
C
3.90
o
C
(b)
(a)
Fig.

6.

Response–recovery

curves

of

S1
(150

8C)
(a)

and

S2
(150

8C)
(b)

to

2000

ppm

NH
3
at

different

working

temperatures.
Fig.

7.

Varied

sensitivities

of

S1

(a)

and

S2

(b)

(annealed

at

different

temperatures)

with

different

concentrations

of

NH
3
at

90

8C.
Fig.

5.

TEM

(a)

and

HRTEM

(b)

micrographs

of

sample

S2
(100

8C)
.
L.

Geng,

S.

Wu

/

Materials

Research

Bulletin

48

(2013)

4339–4343
4342
S1
(150

8C)
and

S2
(150

8C)
in

the

NH
3
atmosphere.

Thus,

the

PPy/
g
-
Fe
2

O
3
hybrids

showed

characteristics

of

an

n-type

semiconductor,
although

they

contained

both

p-

and

n-type

semiconductors.

This
finding

may

be

attributed

to

the

relatively

high

content

of

g
-Fe
2
O
3
in

the

hybrids.

However,

the

pure

g
-Fe
2
O
3
prepared

as

described

in
reference

[25]

showed

no

gas

sensitivity

at

the

operating
temperatures

of

30

8C,

60

8C

or

90

8C,

which

was

due

to

g
-Fe
2
O
3
being

an

insulator

at

normal

temperatures.
The

sensitivities

of

S1

and

S2

annealed

at

100

8C,

130

8C,

150

8C
and

180

8C

under

different

concentrations

of

NH
3
gases

at

90

8C
increased

linearly

with

increasing

concentrations

of

NH
3
(Fig.

7).
The

sensitivity

curves

of

S1

and

S2

tested

at

30

8C

and

60

8C

were
similar

with

that

at

90

8C

(data

not

shown).

These

results

suggest
that

S1

and

S2

based

sensors

can

be

used

in

low

operating
temperatures

(<100

8C)

to

detect

a

wide

testing

range

NH
3
gas
concentrations.

Our

previous

studies

reported

that

PPy/ZnO

and
PPy/WO
3
had

good

selectivity

to

NO
x

and

H
2
S

respectively,

but

had
no

sensitivity

to

NH
3
at

the

high

concentration

of

2000

ppm
[16,26].

Therefore,

PPy/
g
-Fe
2
O
3
hybrids

can

be

developed

in
further

applications

as

NH
3
selectivity

sensors.
4.

Conclusions
The

reactant

ratio

of

pyrrole

monomer:

Fe(NO
3
)
3
Á9H
2
O

and
annealing

temperature

of

PPy/
g
-Fe
2
O
3
hybrids

prepared

by

sol–gel
polymerization

in

situ

were

shown

here

to

influence

their

micro-
structure

and

gas

sensitivity.

Increasing

amounts

of

Fe(NO
3
)
3
Á9H
2
O
increased

the

degree

of

PPy

uniformity

and

resulted

in

the
microstructure

change

of

g
-Fe
2
O
3
from

granular

to

stick

particle
form.

Furthermore,

the

PPy/
g
-Fe
2
O
3
hybrids

were

all

selectively
sensitive

to

NH
3
gas

at

low

temperatures

(<100

8C)

and

could
overcome

the

shortcomings

of

the

long

response

time

of

PPy

and

high
operating

temperature

of

g
-Fe
2
O
3
.

Therefore,

the

hybrids

presented
important

and

practical

features

for

manufacture

of

gas

sensors.
Acknowledgment
This

work

was

supported

by

grants

from

the

National

Natural
Science

Foundation

of

China

(31201305).
Appendix

A.

Supplementary

data
Supplementary

data

associated

with

this

article

can

be

found,

in
the

online

version,

at

/>07.020.
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4343