Carbon dioxide (CO2) is one of the major indicators of fire and therefore its measurement is very important for low-false-alarm fire detection and emissions monitoring. However, only a limited number of CO2 sensing materials exist due to the high chemical stability of CO2. In this work, a novel CO2 microsensor based on nanocrystalline tin oxide (SnO2) doped with copper oxide (CuO) has been successfully demonstrated. The CuO-SnO2 based CO2 microsensors are fabricated by means of microelectromechanical systems (MEMS) technology and sol-gel nanomaterial-synthesis processes. At a doping level of CuO: SnO2 = 1:8 (molar ratio), the resistance of the sensor has a linear response to CO2 concentrations for the range of 1 to 4 percent CO2 in air at 450 C. This approach has demonstrated the use of SnO2, typically used for the detection of reducing gases, in the detection of an oxidizing gas

Hunter, Gary W.

Liu, Chung-Chiun

Lukco, Dorothy

Ward, Benjamin J.

Xu, Jennifer C.

NASA Technical Reports Server

Jennifer C. Xu and Gary W. Hunter
Glenn Research Center, Cleveland, Ohio
Dorothy Lukco
ASRC Aerospace Corporation, Cleveland, Ohio
Chung-Chiun Liu
Case Western Reserve University, Cleveland, Ohio
Benjamin J. Ward
Makel Engineering Inc., Chico, California
Novel Carbon Dioxide Microsensor Based on Tin 
Oxide Nanomaterial Doped With Copper Oxide
NASA/TM—2008-215436
December 2008
https://ntrs.nasa.gov/search.jsp?R=20090007953 2019-08-30T06:13:10+00:00Z
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Jennifer C. Xu and Gary W. Hunter
Glenn Research Center, Cleveland, Ohio
Dorothy Lukco
ASRC Aerospace Corporation, Cleveland, Ohio
Chung-Chiun Liu
Case Western Reserve University, Cleveland, Ohio
Benjamin J. Ward
Makel Engineering Inc., Chico, California
Novel Carbon Dioxide Microsensor Based on Tin 
Oxide Nanomaterial Doped With Copper Oxide
NASA/TM—2008-215436
December 2008
National Aeronautics and
Space Administration
Glenn Research Center
Cleveland, Ohio 44135
Acknowledgments
The assistance of Drago Androjna (Retired from Sierra Labo, Inc.) in gas testing and the support of NASA Aviation Safety 
Program, and the Space Fire Detection and Integrated Vehicle Health Monitoring Projects are greatly appreciated.
Available from
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Level of Review: This material has been technically reviewed by technical management. 
 NASA/TM—2008-215436 1
Novel Carbon Dioxide Microsensor Based on Tin Oxide 
Nanomaterial Doped With Copper Oxide 
 
Jennifer C. Xu and Gary W. Hunter 
National Aeronautics and Space Administration 
Glenn Research Center 
Cleveland, Ohio 44135 
 
Dorothy Lukco 
ASRC Aerospace Corporation 
Glenn Research Center 
Cleveland, Ohio 44135 
 
Chung-Chiun Liu 
Case Western Reserve University 
Cleveland, Ohio 44106 
 
Benjamin J. Ward 
Makel Engineering, Inc. 
Chico, California 95973 
 
Abstract 
Carbon dioxide (CO2) is one of the major indicators of fire 
and therefore its measurement is very important for low-false-
alarm fire detection and emissions monitoring. However, only 
a limited number of CO2 sensing materials exist due to the 
high chemical stability of CO2. In this work, a novel CO2 
microsensor based on nanocrystalline tin oxide (SnO2) doped 
with copper oxide (CuO) has been successfully demonstrated. 
The CuO-SnO2 based CO2 microsensors are fabricated by 
means of microelectromechanical systems (MEMS) 
technology and sol-gel nanomaterial-synthesis processes. At a 
doping level of CuO: SnO2 =1: 8 (molar ratio), the resistance 
of the sensor has a linear response to CO2 concentrations for 
the range of 1 to 4% CO2 in air at 450 °C.  This approach has 
demonstrated the use of SnO2, typically used for the detection 
of reducing gases, in the detection of an oxidizing gas. 
Introduction 
Carbon dioxide (CO2) gas is one of the most challenging 
gas species to detect due to its high chemical stability. 
However, there is a significant need for CO2 sensors for 
aerospace and commercial applications, especially for low 
powered microsensors. These applications include low-false-
alarm fire detection which detect chemical species indicative 
of a fire (e.g., CO and CO2) (ref. 1), as well as for 
environmental and emissions monitoring (ref. 2). Due to the 
stable chemical properties of CO2 gas, only a limited number 
of CO2 sensing materials exist. Most existing CO2 sensors are 
bulky in size and involve complicated fabrication processes 
(ref. 3 and 4). The high power consumption for the bulky CO2 
sensor is also a significant issue which needs to be addressed. 
While actively working on miniaturizing CO2 sensors using 
existing solid electrolyte sensing material (refs. 2, 5, and 6), 
we have also been aggressively exploring new CO2 sensing 
materials. A novel CO2 sensing material, nanocrystalline tin 
oxide (SnO2) doped with copper oxide (CuO) has been 
successfully demonstrated for CO2 detection. Contrary to 
traditional electrochemical-based CO2 sensors, which involve 
a multiple-electrolyte structure (refs. 2 to 6) and are hard to 
miniaturize, the new sensing material is a solid-state resistor-
based CuO and SnO2 mixture allowing straightforward 
fabrication of the CO2 microsensor. 
Experimental 
Sensor Fabrication 
The CuO-SnO2 nanomaterial-based CO2 microsensors were 
fabricated utilizing the following process: First, platinum 
interdigitated electrodes (30 μm wide fingers and spacing) 
were microfabricated on a quartz substrate (250 μm in 
thickness) using photolithography and thin-film sputtering. 
Then, SnO2 sol gel was synthesized through a water-based sol-
gel process using tin chloride (SnCl4) as a precursor to react 
with ammonium hydroxide (NH4OH) (ref. 7). Freshly 
deposited CuO was produced by reacting copper chloride 
(CuCl2) with potassium hydroxide (KOH), followed by 
removal of excess potassium and chloride ions in the solution. 
The SnO2 sol gel was then homogeneously mixed with freshly 
deposited CuO in different ratios. The mixture was drop 
deposited on the interdigitated electrode area (1.10 by 0.99 
mm). Finally, the sensors were heated at 700 °C for 2 hr to 
convert the doped sol-gel mixture into a nanocrystalline 
sensing material, with particle diameters smaller than 20 nm.  
 
 NASA/TM—2008-215436 2
 
 
 
 
 
1.10 mm 
 
Figure 1.—AutoCAD drawing of the sensor structure, with an 
interdigitated electrode area of 1.10 by 0.99 mm, and two 
electrode contacts located at opposite sides. 
 
Figure 1 is a drawing of the interdigitated electrodes 
showing the size of the electrode area. A wafer of around 
50 mm diameter can be used to fabricate up to 100 sensors. 
Sensor Testing  
The CO2 microsensors fabricated with different CuO/SnO2 
ratios were tested in a chamber on a heating stage and 
connected via probes to resistance meters. They were operated 
by measuring the electrical resistances of the sensor in various 
gases at a flow rate of 4000 sccm at a temperature of 450 °C.  
Results and Discussion 
Table 1 lists the CuO doping levels in SnO2 as analyzed by 
X-Ray Photoelectron Spectroscopy (XPS), and the 
corresponding response of these materials to CO2 gases. 
Results showed that only at a molar ratio of CuO: SnO2 = 1: 8, 
does the microsensor respond to CO2 at 450 °C.  
 
TABLE 1.—XPS ANALYSIS OF CuO/SnO2 
NANOMATERIALS AND THEIR RESPONSES TO CO2 
Sample number 1 2 3 4 5 
CuO: SnO2  
(molar ratio) 1: 25.7 1: 15.4 1: 8.0 1: 3.4 1: 1.6 
Response to CO2  No No Yes No No 
 
Figure 2 shows the testing results of carbon dioxide 
microsensors (CuO: SnO2 = 1: 8 in molar ratio) in different 
gases. The sensor resistance was measured in air, nitrogen 
(N2), air (50%)/N2 (50%), CO2 (2%)/air (48%)/N2 (50%) and 
CO2 in air from 1 to 4% at 450 °C. Figure 3 is a linear fit of 
sensor resistance change (compared to the value measured in 
air) versus CO2 concentrations from 1 to 4% in air.  
Results from table 1, figures 2 and 3 show that linear 
responses to CO2 from 1 to 4% in air were achieved at a 
doping level of CuO: SnO2 = 1: 8 in molar ratio. No CO2 
response was seen at other doping levels. The baseline of the 
sensor drifted slightly in air. These observations are being 
further investigated.  
The CuO-SnO2 nanomaterial-based CO2 microsensor is a 
resistor-type sensor, which is fundamentally different both in 
structure and in measurement approach from the traditional 
solid electrolyte CO2 sensor. It can be integrated into a sensor 
 
Resistance of Cuo-SnO2 Sensor Versus CO2 Concentration
in Air (450°C)
air
N2
Air/N2
2% CO2/48% air/50% N2
CO2 in air
1%
2% 3%
4%
4%
N20.0E+00
1.0E+08
2.0E+08
3.0E+08
4.0E+08
5.0E+08
6.0E+08
7.0E+08
8.0E+08
0 20 40 60 80 100 120 140 160
Time (min)
R
es
is
ta
nc
e 
(O
hm
)
R
es
is
ta
nc
e 
(O
hm
)
R
es
is
ta
nc
e 
(O
hm
)
R
es
is
ta
nc
e 
(O
hm
)
R
es
is
ta
nc
e 
(O
hm
)
R
es
is
ta
nc
e 
(O
hm
)
 
 
Figure 2.—Resistances of CuO-SnO2 based 
microsensor (CuO: SnO2 = 1: 8 in molar ratio) 
tested in air, N2, air (50%)/N2 (50%), CO2 (2%)/air 
(48%)/N2 (50%) and CO2 in air from 1 to 4%, with 
a repeat measurement at 4% CO2.  
 
Resistance Change Versus CO2 Concentration
0.0E+00
1.0E+08
2.0E+08
3.0E+08
0% 1% 2% 3% 4% 5%
CO2 Concentration in Air
Re
si
st
an
ce
 C
ha
ng
es
 (O
hm
)
R a
ir
-R
C
O
2
 
 
Figure 3.—Linear fitting of sensor resistance change 
versus CO2 concentration tested from 1 to 4% CO2 
gases in air.  
 
array to provide signals for aerospace and commercial 
applications such as fire detection, emission and 
environmental monitoring. This innovation is also 
scientifically significant because SnO2 is an n-type sensing 
material that has been widely used for detecting reducing 
gases such as carbon monoxide, hydrogen, and hydrocarbons 
(ref. 8). This demonstration is the first time to our knowledge 
of a CuO-SnO2 sensing material responding to CO2 gas in a 
significant and consistent way. This development creates 
opportunities for the batch fabrication of simple and 
inexpensive CO2 microsensors with low power consumption 
due to their small sizes. It could also lead to research that 
could alter fundamental knowledge of SnO2 as a sensing 
material, leading to a wider range of detectable species. While 
there are some scientific speculations about the sensing 
mechanism, it is still not clear to us. Further exploration will 
include expanding the sensor detection range, improving the 
sensor baseline stability, and understanding the sensing 
mechanism. 
 
0.99 mm 
 NASA/TM—2008-215436 3
References 
1. W.L. Grosshandler, “A review of measurements and 
candidate signatures for early fire detection,” NISTIR 555, 
Nat. Inst. of Stand. and Tech., Gaithersburg, MD, January 
1995. 
2. G.W. Hunter, J.C. Xu, and D. Makel, “Case studies in 
chemical sensor development,” In BioNanoFluidic 
MEMS, Peter J. Hesketh ed. Springer Science+Business 
Media, LLC, 2008, pp. 197–231. 
3. N. Miura, S. Yao, Y. Shimizu, N. Yamazoe, “Carbon 
dioxide sensor using sodium ion conductor and binary 
carbonate auxiliary electrode,” J. Electrochem. Soc. 139, 
1992, pp. 2033–2036. 
4. C. Lee, S.A. Akbar, and C.O. Park, “Potentiometric CO2 
gas sensor with lithium phosphorous oxynitride 
electrolyte,” Sens. Actuators, B80, 2001, pp. 234–242. 
5. B.J. Ward, C.C. Liu, and G.W. Hunter, “Novel processing 
of NASICON and sodium carbonate/barium carbonate 
thin and thick films for CO2 Microsensor,” J. of Materials 
Science 38, 2003, pp. 4289–4292. 
6. G.W. Hunter, J.C. Xu, C.C. Liu, B. Ward, D. Lukco, M. 
Artale, P. Lampard, D. Androjna, J.W. Hammond, 
“Miniaturized amperometric solid electrolyte carbon 
dioxide sensors,” ECS Trans. 3, (10) 203 (2006), 210th 
Electrochemical Society Meeting, Cancun, Mexico, 
October 29 - November 3, 2006.  
7. Z.H. Jin, H.J. Zhou, Z.L. Jin, R.F. Savinell and C.C. Liu, 
“Application of nano-crystalline porous tin oxide thin 
film for CO sensing” Sens. Actuators, B52, 1998. 
pp. 188–194. 
8. T.G. Nenov and S.P. Yordanov. Ceramic Sensors—
Technology and Applications Technomic Publishing, 
Lancaster, PA (1996).  
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14. ABSTRACT 
Carbon dioxide (CO2) is one of the major indicators of fire and therefore its measurement is very important for low-false-alarm fire 
detection and emissions monitoring. However, only a limited number of CO2 sensing materials exist due to the high chemical stability of 
CO2. In this work, a novel CO2 microsensor based on nanocrystalline tin oxide (SnO2) doped with copper oxide (CuO) has been 
successfully demonstrated. The CuO-SnO2 based CO2 microsensors are fabricated by means of microelectromechanical systems (MEMS) 
technology and sol-gel nanomaterial-synthesis processes. At a doping level of CuO: SnO2 = 1: 8 (molar ratio), the resistance of the sensor 
has a linear response to CO2 concentrations for the range of 1 to 4 percent CO2 in air at 450 °C. This approach has demonstrated the use of 
SnO2, typically used for the detection of reducing gases, in the detection of an oxidizing gas. 
15. SUBJECT TERMS 
Carbon dioxide (CO2); Gas; Fire detection; Low-false-alarm; Copper oxide (CuO); Microsensors; Nanomaterial; Tin oxide (SnO2); 
Nanocrystalline; MEMS; Emission monitoring; Environmental monitoring  
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