DOI: 10.1595/147106705X45631
Iridium/Carbon Films Prepared by MOCVD
OBSERVATIONS AND ELECTROCHEMICAL PROPERTIES RELATING TO OXYGEN ADDITIONS
By Changyi Hu* and Jigao Wan
Kunming Institute of Precious Metals, Kunming, Yunnan 650221, China; *E-mail: hcy@ipm.com.cn
and Jiaoyan Dai
Institute of Materials and Engineering, Central South University, Changsha, Hunan 410083, China
Iridium/carbon (Ir/C) films were prepared by MOCVD using iridium acetylacetonate as the
precursor and some electrochemical properties were studied, in particular the effects of oxygen
on the carbon content of the Ir/C films. Small additions of oxygen (4 ml min–1) to the source
gas drastically decrease the carbon content of the films. Ir grains are formed, up to ~ 3 nm
in diameter, in the amorphous carbon. It was found that Ir/C films with higher carbon content
have better catalytic performance – for measuring the oxygen concentration – than Ir/C films
with lower carbon content. The Ir/C films were used as electrodes in an oxygen concentration
cell, and the sensitivity of the cell to oxygen was recorded. The Nernstian electromotive
force of the cell is almost the same as that of a similar type of commercial oxygen concentration
sensor from Bosch, but the response time is faster.
Noble metals are widely used as electrodes in
gas sensors because of their unique physical and
chemical properties, such as their inertness, good
oxidation resistance, electrical conductivity and
catalytic performance. However, due to sluggish
charge transfer reactions at the sensing electrode
interface at low temperature (less than 500ºC) (1),
a gas sensor constructed with traditional Pt electrodes and ZrO2 electrolyte needs to be heated to a
higher temperature to obtain sufficient voltage output and a shorter response time. In order to
improve the properties of these sensors, Ir cluster
films have been prepared by MOCVD (metalorganic chemical vapour deposition) and
investigated (2–8). This paper reports on the composition, structure and electrochemical properties
of some Ir/C films.
Experimental Procedure
A schematic diagram of a horizontal hot-wall
MOCVD apparatus is shown in Figure 1. The precursor for the Ir/C films was 500 mg of iridium
tris-acetylacetonate,
(CH3COCHCOCH3)3Ir,
Ir(acac)3. Oxygen and argon were used as the reactant and transmission gases, respectively. The
substrates were quartz (10 mm × 10 mm × 1 mm
thick) and YSZ (yttria stabilised zirconia): 6 mol %
Y2O3, (10 mm Φ × 2 mm thick). The temperature
of the precursor (Tsor) was kept at 190ºC. The total
gas pressure in the chamber was fixed at 500 Pa,
with argon flow maintained at 50 ml min–1. The
precursor was placed in a small quartz boat in the
MOCVD apparatus. The deposition temperature
(Tdep) was varied from 450 to 650ºC, for a deposition time of 60 minutes. The flow of oxygen (FO2)
Furnace
Furnace
Manometer
Argon
Oxygen
Precursor
Substrate
Vacuum pump
Fig. 1 Schematic diagram of chemical vapour deposition equipment used for the preparation of Ir/C films
Platinum Metals Rev., 2005, 49, (2), 70–76
70
Fig. 2 Schematic diagram
of apparatus to measure
e.m.f. values of an oxygen
concentration cell. The cell
has YSZ solid electrolyte
and an Ir/C electrode
Volt-ohm-milliammeter
Furnace
Thermocouple
Sample
Oxygen
flowmeter
Oxygen
gas
was varied from 0 to 10 ml min–1.
The composition of the deposits was analysed
by X-ray photoelectron spectroscopy (XPS). The
exciting source of the XPS is Al (Kα), the sensitivity factors are 4.4, 0.25 and 0.66 for Ir, C and O,
respectively. The film structures were also investigated by XRD and SEM.
Figure 2 shows a schematic diagram of the
measurement of the Nernstian electromotive force
(e.m.f.) of the oxygen concentration cell having an
Ir/C electrode attached to both sides of a YSZ
solid electrolyte. Values of the e.m.f. were measured by changing the partial pressure of the
oxygen at temperatures from 300 to 600ºC.
A dynamic test apparatus (9) was used to assess
the performance of the oxygen sensor. The air and
(a)
Ir4f
Ir4d
120000
INTENSITY, cps
O1s
90000
Argon gas
1#k
(b)
Ir4f
Ir4d
240000
O1s
200000
C1s
Oxygen gas
fuel (natural gas) were adjusted to obtain the
desired λ values (normalised air/fuel ratios). The
exhaust gas was usually maintained in a rich condition, at λ = 0.95. A solenoid valve allowed
additional air to the burner to switch the exhaust
composition quickly to lean, when λ = 1.05, then
cutting off the additional air and switching back to
rich.
The sensor voltage output was measured by a
voltmeter having an input impedance of 107 Ω.
The voltage switching response was determined
using an oscilloscope, also with an input impedance of 107 Ω connected in parallel to the
voltmeter. The response time was defined as the
time taken for the output voltage, recorded on the
oscilloscope, to sweep between 600 and 200 mV.
280000
Ir4p3
Oxygen
flowmeter
Argon flowmeter
Ir4p3
C1s
160000
60000
120000
1#
6#k
80000
30000
40000
0
6#
0
600
500
400
300
B.E., eV
200
100
0
600
500
400
300
200
100
0
B.E., eV
Fig. 3 XPS spectra before argon sputtering (lower curves) and after argon sputtering (upper curves) for Ir/C films
prepared: (a) without oxygen addition and (b) with oxygen addition. B.E. is the binding energy
Platinum Metals Rev., 2005, 49, (2)
71
Results
Composition of Ir/C Films
Figure 3 shows the XPS spectrum before and
after argon sputtering (5 kV, 2 min) for Ir/C films
prepared on quartz substrates with and without
oxygen addition. Before argon sputtering, carbon
was observed at the surface of the two samples.
After sputtering, no observable signals from carbon were detected for the Ir/C films prepared with
oxygen addition (trace 6#k), but signals of carbon
were observed from films prepared without oxygen addition after sputtering (trace 1#k).
The effects of oxygen and temperature on the
contents inside Ir/C films prepared on quartz are
shown in Table I. The addition of oxygen (from 0
to 4 ml min–1) is seen to decrease the carbon content and thus increase the iridium content. There is
an increasing trend of carbon content inside the
films prepared without oxygen addition with
increasing deposition temperature.
Films obtained with the addition of oxygen
were smooth with silver-coloured surfaces: due to
the oxygen reacting with carbon. The reaction
products (carbon dioxide or carbon monoxide) are
exhausted from the deposition chamber.
Structure of Ir/C Films
Figure 4 shows the surface appearances and elemental maps of Ir/C films prepared without and
with oxygen addition. Film prepared without oxy-
Fig. 4 Elemental maps of carbon for Ir/C films. Top map: Film prepared at 650ºC without oxygen addition.
Bottom map: Film prepared at 600ºC with 4 ml min–1 oxygen addition
Platinum Metals Rev., 2005, 49, (2)
72
Table I
Composition of Ir/C Cluster Films Prepared under Different Deposition Conditions
Tdep
500ºC
Ir, wt.%
C, wt.%
O, wt.%
550ºC
600ºC
650ºC
0
4
0
4
0
4
0
4
89.5
9.8
0.7
98.6
0
1.4
82.5
17.2
0.3
94.9
4.7
0.4
83.6
15.1
1.3
97.4
2.1
0.5
66.9
32.2
0.9
98.8
0.6
0.6
gen addition is seen to have a higher carbon content than film with 4 ml min–1 oxygen addition.
The carbon is dispersed in the grain boundaries of
the iridium.
Figure 5 shows characteristic X-ray patterns of
Ir wire and Ir/C film prepared under different
oxygen flows. The height of the peak increases
with increasing oxygen flow, indicating that the
carbon content of the films is decreasing. The Xray peaks of Ir/C films are displaced in the same
direction, comparable to the X-ray peak of the Ir
wire. This indicates that the states of carbon
deposited in these films are the same.
The peaks in Figure 5 starting at the highest
represent: standard Ir wire; Ir/C films prepared
with oxygen additions of 10, 8, 4, 0 ml min–1,
respectively.
Figure 6 shows characteristic XRD patterns of
the Ir/C films prepared under different deposition
conditions. The Ir/C film with higher carbon content has lower broader XRD peaks. Based on
calculations from the half-width of the X-ray
peaks, the Ir grains are ~ 3 nm in size, Fig. 6(a),
consistent with direct observations by TEM (6).
(a)
CPS
Flow O2, ml min
–1
8000
10.00
7000
50.00
2θ
100.00
6000
(b)
COUNTS
5000
4000
CPS
3000
2000
1000
10.00
6.0200
6.0400
WAVELENGTH, Å
6.0600
Fig. 5 Characteristic X-ray patterns of Ir wire and Ir/C
films prepared in different oxygen flows.
Peaks are: top: Ir wire, followed by Ir/C films prepared
with oxygen additions of 10, 8, 4, 0 ml min–1
Platinum Metals Rev., 2005, 49, (2)
50.00
2θ
100.00
Fig. 6 Characteristics of the XRD patterns of Ir/C films
prepared under different deposition conditions:
(a) Film prepared at Tdep = 650ºC, FO2 = 0 ml min–1
(carbon content 32.2 wt.%)
(b) Film prepared at Tdep = 550ºC, FO2 = 9 ml min–1
(carbon content 9.9 wt.%)
73
etched by argon sputtering, were identified by
wavelength dispersive X-ray spectroscopy (WDS),
see Figure 8 and 9. These figures show that the
black and white granules (in Figure 7) represent
carbon and iridium, respectively. The carbon exists
as an amorphous structure determined from the
WDS.
Properties of Ir/C Film Electrodes
7µm
SEI
Fig. 7 SEM of Ir/C film deposited without addition of
oxygen (prepared at Tdep = 550ºC, FO2 = 0 ml min–1).
The upper arrow indicates a black granule and the lower
a white granule formed in the Ir/C films
An SEM surface observation of Ir/C film
deposited without addition of oxygen is shown in
Figure 7. The granules on the surface of the film,
COUNTS
100
carbon
The relationship between e.m.f. values at different temperatures and the ratio of the oxygen
partial pressures (P1/P2) in the oxygen concentration cell is shown in Figure 10. The Ir/C film
electrodes were deposited under various conditions. P1 is fixed at 0.1 MPa. The theoretical values
are calculated from the Nernstian equation (10):
e.m.f. = RT/4F ln P1/P2
where R is the gas constant, F is the Faraday constant and T is the absolute temperature.
100
80
80
60
60
40
40
20
20
0
carbon
0
43.000
44.000
45.000
46.000
43.000
100
45.000
iridium
80
COUNTS
44.000
46.000
WAVELENGTH, Å
WAVELENGTH, Å
iridium
10000
60
5000
40
20
0
0
6.255
6.260
6.265
6.270
WAVELENGTH, Å
Fig. 8 WDS of the black granules on the surface of the
Ir/C film after argon sputtering
Platinum Metals Rev., 2005, 49, (2)
6.240
6.250
6.260
6.270
WAVELENGTH, Å
Fig. 9 WDS of the white granules on the surface of the
Ir/C film after argon sputtering
74
30
25
35
(a)
¡ñ
¡ñ
e.m.f., mV
20
Measuring
temperature
¡ø
30
¡ø
25
¡ö
(a)
Graph
(b)
!
¡ö
" 0 ml min
20
! 4 ml min
15
500ºC
(b)
600ºC
theoretical value
–1
550ºC
650ºC
–1
550ºC
650ºC
15
10
10
5
5
1
1
2
2
ln P1/P2
Fig. 10 Relationship between e.m.f. values and the oxygen partial pressure ratio of the oxygen concentration cell
The difference between the experimental and
the theoretical values may be caused by electrical
leakage (11). The e.m.f. values of Ir/C films prepared without oxygen addition were found to be
higher than those prepared with 4 ml min–1 oxygen
addition. This means the catalytic response, to
oxygen, of film with more carbon content (prepared without oxygen addition) is higher.
Lastly, the response curves of the commercial
sensor (BOSCH LSH6) and the oxygen concentration cell constructed with Ir/C film and YSZ
are shown in Figure 11. The voltage outputs are
almost identical, but the response time of the cell
is shorter than that of the sensor.
Conclusions
Ir/C films were prepared by MOCVD using
iridium acetylacetonate as the precursor. Small
additions of oxygen to the source gas greatly
decrease the carbon content of the films. Ir grains
are formed up to ~ 3 nm in diameter by the amorphous carbon. Ir/C films with higher carbon
content have better catalytic performance than
Ir/C film of lower carbon content. The electrochemical properties of the oxygen concentration
cell using Ir/C films as the electrodes is almost the
same as that for a commercial sensor, but the
response time is shorter.
A research programme is currently being
undertaken to use the Ir/C films as electrodes for
commercial sensors.
Acknowledgements
This project was supported by National Natural Science
Foundation of China, Grant No. 50171031, and Yunnan
Scientific Project (Program No. 2003 PY10). The authors would
like to thank Mr Y. Wang and Senior J. M. Yang for their help
with sample preparation and SEM observation, respectively.
e.m.f., mV
References
1000
800
600
400
200
0
1000
2000
3000
4000
5000
TIME, ms
CVD Ir/C
Bosch
Fig. 11 Voltage-time response curves operating at 300ºC
Platinum Metals Rev., 2005, 49, (2)
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The Authors
Professor Changyi Hu is Professor of
Materials Science at the Research and
Development Center, Kunming
Institute of Precious Metals, China. His
major work is the preparations of films
and coatings of precious metals and
work pieces of refractory metals by
MOCVD and CVD.
Dr Jiaoyan Dai is an engineer at the
Institute of Materials and Engineering,
Central South University, China. Her
interests include CVD of precious metal
films, catalysis of precious metals and
electronic materials.
Jigao Wan is a Senior Researcher in the
Functional Materials Division, Kunming
Institute of Precious Metals, China. His
current research is on oxygen gas sensors
and other sensors.
Platinum Metals Rev., 2005, 49, (2)
76