SrFeOx perovskite based thin films on silicon substrates for sensor ...

<strong>SrFeOx</strong> <strong>perovskite</strong> <strong>based</strong> <strong>thin</strong> <strong>films</strong> on
silicon substrates for sensor
applications
Abdul Majid, , Jim Tunney, Steve Argue,
Melissa Kunkel and Mike Post

ABSTRACT
Nanostructured coatings have recently attracted increasing interest because of the
possibilities of synthesizing materials with unique physical-chemical properties.
Highly sophisticated surface related properties, such as optical, magnetic, electronic,
catalytic, mechanical, chemical and tribological properties can be obtained by
advanced nanostructured coatings, making them attractive for various industrial
applications.
The nanostructuring of <strong>thin</strong> <strong>films</strong> can greatly enhance the sensor properties of metal
oxide <strong>films</strong>. This includes dramatic enhancements in chemical sensitivity, faster
response times and increased chemical selectivity. Metal oxides of the <strong>perovskite</strong>
ABO3-x (A= Sr, B = Fe) class of ceramic materials have interesting mixed ionicelectronic
conductivity properties which make them leading candidates as sensor,
membrane and solid oxide fuel cell materials. In this presentation we describe our
efforts at developing methodology to grow nanostructured <strong>thin</strong> <strong>films</strong> of SrFeO~2.9,
using a solution <strong>based</strong> synthesis method. Solution-<strong>based</strong> synthesis offers molecular
level mixing of the starting materials, which leads to a high degree of homogeneity
with minimum particle size and high surface area. The <strong>perovskite</strong> coating sol was
prepared via a sol-gel method.
May 19, 2004 4

INTRODUCTION-1
NANOSTRUCTURED MATERIALS GROUP
1. Nanomaterials preparation
2. Nanomaterials chracterization
3. Device nanochemistry
May 19, 2004 4

PEROVSKITES – MIXED TRANSITION METAL OXIDES
For the past several years we have been involved with <strong>perovskite</strong>s
which are mixed transition metal oxides. These type of materials
show a variety of useful properties, covering a wide range of
scientific areas, from electrical (ferroelectricity, superconductivity,
ionic, and electronic conductivity) to magnetic, optical, and catalytic
properties. These properties lead to a number of applications such
as:
Sensors: humidity, O 2 , NO x , H 2 S, HC, alcohols
Catalysis: FC, destruction of chlorinated HC, oxidation
of light hydrocarbons and decomposition of peroxides
May 19, 2004 4

PEROVSKITES – Preparation
Conventional Method: Solid-state reaction at high temperature
Product: non-porous, coarse, non-uniformity of particle size and
shape, low specific surface area
Wet Chemical Methods: Examples – sol-gel
Advantages
☞ Better homogeneity
☞ Better compositional controls
☞ Lower processing temperatures
☞ Improved mechanical, electrical, chemical and catalytic properties
of metal oxides because of nanocrystalline particle size and high
surface area
☞ Porosity control
☞ Simpler and more economical <strong>thin</strong> <strong>films</strong> formation potential
May 19, 2004 4

<strong>SrFeOx</strong> – 2.5≤x≤3.0
☞ Recently, we have been working with SrFeO x
(2.5≤x≤3.0), which is a mixed ionic electronic
conductor, in which the reversible oxygen nonstoichiometry
can be exploited for gas sensor
applications.
☞ These type of materials are widely applied for gas
sensing by making use of the chemical equilibrium
between reducing and oxidizing species at higher
temperatures.
May 19, 2004 4

EXPERIMENTAL -1
Materials handling: GB under Ar
Calcinations: Box furnace with temperature programming
Reaction Progress: XRD
Preparation Methods:
1.Wet method: Pechini’s modified Sol-Gel method
2.Solid-state method at 1100 o C for comparison
May 19, 2004 4

<strong>SrFeOx</strong> – Preparation
Flow Chart of Sol-Gel Process for preparation of SrFeO x
Sr (NO 3 ) 2 Fe (NO 3 ) 3
Add citric acid
Citrates of Fe and Sr
Viscous Solution
Add dilute ammonia, pH 6-7
Heat at 80°C and stirred
Add ethylene glycol, continue heating
Precursor solution
(For Film Deposition)
Heat at 180-200°C
Grey porous solid
Heat at 400°C
May 19, 2004 4

Typical XRD patterns of SrFeO x
1200
Sol-gel preparation at 600oC
800
Solid-state preparation from oxides at 900oC
400
Solid-state preparation at 1100oC
0
20 30 40 50 60 70 80
2-Theta-Scale
May 19, 2004 4

Effect of calcination temperature
on surface area of SrFeO x
Specific surface area (m2/g)
24
20
16
12
8
4
Correlation coefficient for linear fit: 0.993
Specific surface area of the sample prepared
by solid-state method: 0.1m 2 /g
0
500 600 700 800 900
Calcination temperature oC
May 19, 2004 4

Solid-state preparation
Effect of ashing time on Sr 3 Fe 2 O 6.75 impurity
1000
800
Solid-State Preparation from Fe2O3 and SrCO3 at 1100oC
600
Calcination Time at 1100 o C: 50 Hrs
400
Calcination time at 1100oC: 70 Hrs
200
0
Calcination time at 1100oC: 170 Hrs
25 27.5 30 32.5 35
2-Theta-Scale
May 19, 2004 4

Effect of Calcination Temperature on
Crystallite size of SrFeO x
140
120
Correlation coefficient for 2nd order fit: 0.967
Crystallite size (nm)
100
80
60
40
Crystallite size of the sample prepared
by solid-state method: 512 nm
20
600 700 800 900
Calcination Temperature (oC)
May 19, 2004 4

Perovskite Thin Films – Objectives
1. Developing methodologies for the use of these materials
as crack free coatings on various substrates for selective
gas sensing applications. The viscosity of the sols is
expected to allow the easy preparation of <strong>films</strong> by dipcoating,
spraying, or spin coating.
2. To design and synthesize mesoporous and macroporous
<strong>thin</strong> <strong>films</strong>, to characterize their properties at the molecular
scale, and to understand the factors that influence material
response and selectivity.
May 19, 2004 4

Perovskite Thin Films – Methodology
☞ Substrate Preparation
Si wafers were sonicated in CCl 4 for 15 minutes and rinsed with 2-prapanol and
water. They were then sonicated in a hot piranha solution of conc. H 2 SO 4 and
30% H 2 O 2 (3:1) for 30 min to remove any organic matter, rinsed with copious
amount of water and dried at 150 o C.
☞ Film Deposition
The deposition of <strong>films</strong> on the Si-substrate was attempted by both spin-coating
and dip-coating techniques. The coated samples were then dried and annealed
at elevated temperature to form the desired phase in the coatings.
☞ Adhesion
Adhesion of the heated <strong>films</strong> to the substrates was poor. Unfired <strong>films</strong> were
easily scratched and could be removed from the substrates by scraping. Films
heated to a temperature around 110 o C were not removed or damaged by limited
physical abrasion. Hence it seems that many of the primary covalent bonds to
the surface are formed due to condensation on surface hydroxyl groups at
temperatures between 25 and 110 o C.
May 19, 2004 4

Perovskite Thin Films – Characterization
☞ The Phase Characterization
XRD
☞ Surface Characterization
Optical Microscope, SEM, TEM
☞ Composition
XPS
☞ Gas Sensing Potential for O 2 , N 2 , Propane
Electrical Resistance Measurements
May 19, 2004 4

Perovskite Thin Films – Sensor Applications
‣ nanostructuring of <strong>thin</strong> <strong>films</strong> can greatly enhance the sensor properties
of metal oxide <strong>films</strong>. This includes:
1. dramatic enhancements in chemical sensitivity,
2. faster response times and
3. increased chemical selectivity
‣ Metal oxides of the <strong>perovskite</strong> ABO3-x (A= Sr, La; B = Fe, Co) class
of ceramic materials have interesting mixed ionic-electronic
conductivity properties which make them leading candidates as sensors
‣ Based on a relationship between the electrical conductivity of the film
and the partial pressure of oxygen, oxide <strong>films</strong> can detect the
concentration of oxygen in the atmosphere.
‣ Metal oxide sensors can also show response to reducing and oxidizing
gases and can detect impurity gases in air at elevated temperatures.
May 19, 2004 4

Perovskite Thin Films – Sensor Applications
PRINCIPAL
☞ SrFeO x , a p-type semiconductor
☞ The type of gas in contact with the film will determine whether
there is a decrease or increase in the absorbed surface oxygen.
☞ p-type sensors show an increase in resistance on exposure to a
reducing gas and a decrease on exposure to an oxidizing gas.
☞ Gas detection works on the principle that the change in
resistance is proportional to the concentration of gas.
May 19, 2004 4

Perovskite Thin Films – Sensor Applications
Surface Reactions
☞ The chemisorption of oxygen occurring on the surface of the film is
represented as:
4e - + O 2 → 2 O 2- adsorbed (1)
☞ Reducing gases such as carbon monoxide or propane are adsorbed onto the
surface and react with the absorbed oxygen ions as follows:
2CO + O 2 - → 2CO 2 + e - (2)
☞ This reaction injects an electron into the semiconductor, which then fills a
hole and causes an increase in resistance, for p-type sensors. This reaction
may not result in complete combustion and other compounds may be
formed.
☞ A similar reaction occurs with an oxidizing gas; the gas is absorbed onto
the surface, and extracts an electron, for p-type sensors. An example of a
reaction with nitrogen dioxide gas and the surface is:
NO 2 + e - ↔ NO 2
-
(3)
May 19, 2004 4

Optical Micrographs
May 19, 2004 4

Optical Micrographs
May 19, 2004 4

RESISTANCE MEASUREMENTS
Electrical conductivity of the <strong>films</strong> was measured using a two-wire method
in controlled environment where the temperature could be varied between
25 and 500°C and changing the flowing gas composition as required.
The sample was placed in a 1-liter chamber with synthetic air (79% N 2 ,
21% O 2 ) flowing at 200cm 3 /min at 300°C for 20 minutes. It was then
exposed to 99.999% purity N 2 with 0.56 ppm of O 2 for 20 minutes before
switching back to synthetic air.
The resistance was measured and recorded every second during the
experiment.
Instrument: Keithley multimeter; Software: LabView; Temperature
measurement: wi<strong>thin</strong> ±0.2°C, recorded every second
Experiments were performed at 300 °C, 400°C and 500°C with N 2 and
propane (3000 ppm propane, 20.2% O 2 ) as the gas.
May 19, 2004 4

RESULTS
RESISTANCE AND GAS RESPONSE
Preliminary test results at 300-500 o C showed that the
film had gas response to nitrogen and propane gas.
The tests could only be carried out at higher
temperatures because the resistance readings are out
of range for the multimeter at lower temperatures.
May 19, 2004 4

RESULTS
Figure 1: The Response for N 2
Gas at 400°C
100
90
20 minutes
80
% Gas Response
70
60
50
40
30
55%
20
10
200 ccm of AIR
200 ccm of NITROGEN
0
500 1000 1500 2000 2500 3000
Time (s)
200 ccm of AIR
May 19, 2004 4

RESULTS
Figure 2: Temperature Dependence of Response for N 2 Gas
100
90
80
70
60
50
40
% Gas Response
30
20
10
200 ccm of Nitrogen Gas
0
250 300 350 400 450 500 550
Temperature (C)
May 19, 2004 4

RESULTS
Figure 3: The Response for Propane at 400 o C
2500000
3000 ppm PROPANE
250
2250000
200
2000000
150
Resistance (ohm)
1750000
1500000
1250000
1000000
750000
AIR (79% N2, 21% O2) AIR (79% N2,
21% O2)
100
50
0
-50
-100
Propane Gas Volume (ccm)
500000
-150
250000
-200
0
500 1000 1500 2000 2500 3000 3500
Time(s)
Resistance
Propane Gas Volume
-250
May 19, 2004 4

RESULTS
Figure 4: The Variation of Propane Gas Response at 400 o C with Time
100
90
20 minutes
80
80%
70
60
50
40
30
% Gas Response
20
10
200 ccm of AIR
200 ccm of AIR
200 ccm of PROPANE
0
0 500 1000 1500 2000 2500
Time (s)
May 19, 2004 4

RESULTS
Figure 5: Temperature dependence of Propane Gas Response at 400 o C
100
90
80
70
60
50
40
% Gas Response
30
20
10
200 ccm of Propane Gas
0
250 300 350 400 450 500 550
Temperature (C)
May 19, 2004 4

CONCLUSIONS -1
1. A sol-gel method <strong>based</strong> on the modified Pechini-type reaction
was successfully used for the preparation of SrFeO ~2.9 <strong>based</strong>
coatings on silicon substrates.
2. The sol-gel chelating precursor approach is a simple, cost
effective method for synthesizing mixed transition metal oxide
<strong>based</strong> coatings on silicon substrates.
3. The X-ray diffraction results showed that the film contained
the correct phase with no impurities.
4. The resistance values of the film were higher than expected due
to the surface morphology of the film, revealed through
microscope analysis and SEM.
May 19, 2004 4

CONCLUSIONS -2
5. The film prepared was a p-type semiconductor and showed gas
response to nitrogen and propane gas at temperatures of 300°C
to 500°C.
6. The film showed the highest percentage gas response to
nitrogen at 500°C of 97%. The gas response for nitrogen gas
was temperature dependant.
7. The highest percentage gas response for propane was 84% at
500°C. The gas response to propane did not appear to be
temperature dependant in a 20-minute gas exposure time.
8. The film showed greater percentage gas response to the
reducing gas than to an oxygen deficient gas.
May 19, 2004 4

ACKNOWLEDGEMENTS
Steve DeCliff, Digital photos
Jim Morgeson, SEM
Dashan Wang, TEM
David Kingston, XPS
Dr. Pamela Whitfield, XRD
Dr. Mahesh Matam, Resistance measurements