Nitrogen oxide gases �NOx�, exhausted from automobiles andother combustion sources, are harmful pollutants. Therefore, a suit-able sensing device is in demand for feedback control of combustionprocesses and emissions monitoring.1,2 For this purpose, the deviceshould be capable of measuring 10–1000 ppm NOx selectively inthe exhaust. For atmospheric monitoring, sensors able to detect lowNOx concentration �1–100 ppm� are required.1 Solid-state sensorsare advantageous due to their compact size, ease of use, and lowcost.3 In addition, their properties can be tailored by modifying elec-trode materials.
Many solid-state potentiometric sensors, based on a solid elec-trolyte and a semiconducting metal oxide electrode, have been de-veloped and tested.4-11 A wide screening of NOx sensing electrodematerials for a planar-type device was performed among a series ofsemiconducting oxides.5 As a result, several oxides such as CdO,NiO, SnO2, and CeO2, were found to be highly sensitive to NO2;however, only the values of the voltage response were tabulatedwithout describing electrode configuration and sensing property�temperature dependence, response time, selectivity, etc.�. Szabo etal. reported the NOx sensitivity of a tubular-type sensor based on aCr2O3 sensing electrode in 3–21% O2, and it produced �70 mV for600 ppm NO2 and �30 mV for 600 ppm NO.6,7
Several research groups have suggested strategies to enhance gasselectivity.7,12,13 For example, Szabo et al. introduced zeolite cata-lysts in the gas pathway to facilitate NO/NO2 equilibrium prior togas sensing, but the NO response was still interfered with by NO2concentration change, and the effect of other exhaust componentssuch as H2O and hydrocarbons was not investigated.7,12 West et al.reported enhanced NOx selectivity over limited O2 concentrationrange �7–20%� by a dc electrical biasing technique.13 However,these types of techniques for selectivity improvement are not satis-factory for practical application and do not provide a fundamentalunderstanding of the underlying mechanism for selectivity. Surpris-ingly, previous works including Ref. 5-13, rarely investigated thegas selectivity behavior, even though the importance of the gas se-lectivity has been frequently described, especially in combustionexhaust atmosphere. Therefore, limited selectivity of the solid-statepotentiometric sensor still remains to be overcome. This is due inpart to the limited understanding of the chemical and electrochemi-cal processes occurring at the gas-solid interface.
In order to address this, the NO2 selectivity of asymmetricalpotentiometric sensor that utilizes MOx �MOx = Cr2O3, SnO2, andCeO2� and Pt electrodes, was fabricated and tested in this work.Initially, an optimal NO2 sensing temperature was determined. Atthe determined temperature, the NO2 selectivity against CO, CO2,
* Electrochemical Society Student Member.** Electrochemical Society Active Member.
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O2, NO, and H2O was investigated. Recently, we reported aWO3-based sensor to have high-sensitivity to NO2 as low as 10 ppmand NO2 selectivity in the presence of CO, CO2, and H2O.11 Theselectivity results were discussed comparing them with one anotherand with the WO3 sensor.11
Experimental
Chromium oxide �Cr2O3� and tin oxide �SnO2� were prepared bythermal decomposition of SnCl2 �Fisher Scientific, anhydrous� andCrCl3·6H2O �Johnson Matthey Electronics�, respectively, at 600°Cfor 5 h. Cerium oxide �CeO2� precipitate was produced fromCe�NO3�3·6H2O �Alfa Aesar, 99.5%� in H2O by adding ammoniauntil the pH 7. The resulting precipitate was then calcined at 600°Cfor 5 h. The samples produced were confirmed to be impurity-freeby using X-ray diffraction �XRD, Philips APD 3720 automated dif-fractometer, Cu K� radiation, � = 1.54178 Å�.
Details of the procedure for preparation of a sensing electrodewere described earlier.11 A screen-printing slurry was prepared bymixing the metal-oxide powder with a dispersing agent �ElectronicMaterials V006, Heraeus�. An asymmetrical planar sensor was madeusing a tape-casted 8 mol % Y2O3-doped ZrO2 substrate �YSZ,20 � 12 � 0.15 mm� as an electrolyte, and the metal oxide and Ptas an electrode, as shown in Fig. 1. Micrographs of the sensingelectrodes were obtained using a scanning electron microscope�SEM, JEOL JSM 6400 SEM�.
The sensor ���� MOx/YSZ/Pt ���, where MOx = Cr2O3, SnO2,and CeO2� was installed in a homemade quartz tube where bothelectrodes were exposed to the same gas atmosphere and connectedto a Keithley 2000 multimeter for measurements of electromotiveforce �emf�. The total gas flow rate was fixed at 300 cm3/min.
The sensor response was measured with increasing and then de-creasing concentration steps of NO2 �0–300 ppm�. The retentiontime at every step was 200 s. First, temperature dependence of thesensor response was measured at 450–700°C, while varying the
Figure 1. Schematic diagram of a potentiometric sensor. �a� Top view, �b�side view.
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concentration of NO2. Then, the NO2 selectivity against NO, CO,CO2, and H2O was investigated in 21% O2 with N2 balance, andagainst O2 in N2 balance. Finally, the sensors were tested in a simu-lated exhaust gas �16% CO2, 100 ppm CO, 3% O2, and 3% H2O�.
Results and Discussion
Characterization of electrodes.— Figure 2a and Fig. 3a showscanning electron micrographs of the Cr2O3 electrode surface and across section between the Cr2O3 film and YSZ, respectively, pre-pared by sintering at 800°C for 10 h. The Cr2O3 layer exhibited aporous microstructure with 0.2–1 �m grains and 3–4 �m thickness.
Grains in the SnO2 and CeO2 film are generally agglomeratedand not well defined, showing dispersed grain size distribution�0.1–2 �m�. Both are very porous, as shown in Fig. 2b and c, andare 3–4 �m thick �Fig. 3b and c�. The porous Pt layer was depositeduniformly with �9 �m thickness.11
Figure 2. Scanning electron micrograph of metal-oxide electrode surface.�a� Cr2O3, �b� SnO2, and �c� CeO2.
Figure 3. Scanning electron micrograph of cross section between metaloxide and YSZ. �a� Cr O , �b� SnO , and �c� CeO .
2 3 2 2
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General sensing properties.— Cr2O3-based sensors.— The sen-sor emf, obtained with the Cr2O3-based sensor, is plotted againsttime with 9–230 ppm NO2 concentration steps in 21% O2 with N2balance at 450–700°C �Fig. 4a�. Values of the emf �mV� are positiveand escalate with increasing NO2 concentration. Such positive NO2response was also observed during the sensor experiments per-formed using a WO3-based potentiometric sensor with the exactsame cell configuration in 3–21% O2.11,14 The present Cr2O3 sensoris, however, less sensitive to NO2 than the WO3 sensor. For ex-ample, the emf value measured, using the Cr2O3 sensor, is �20 mVfor 100 ppm NO2 at 600°C �Fig. 4�. A NO2 response ��60 mV�,about three times higher in the same condition, was produced withthe WO3 sensor.14
The response time of the Cr2O3 sensor did not appear tempera-ture dependent at 450–700°C, and a new steady state was alwaysachieved within 150 s �Fig. 4a�. The sensor response is faster whenmeasured in the increasing NO2 concentration region than in thedecreasing region. This agrees with previous results obtained by thekinetic calculation assuming the first-order adsorption and desorp-tion reactions, claiming that the response �from a balance gas to asensed gas� is always faster than the recovery �from a sensed gas toa balance gas�.15 Moreover, measured emf values, while increasingthe NO2 concentration, are slightly larger than those in the regime ofdecreasing concentration, especially at lower temperatures. For ex-ample, at 450–500°C, the difference is 1.5–2 mV at NO2 �20 ppm,but the difference decreases with increasing temperature and be-comes negligible at T � 650°C. This is likely due to dissimilarkinetics between adsorption and desorption at lower temperatures.3
Considering both the NO2 sensitivity and the relative kinetics ofadsorption and desorption, the results at 600°C seem to be optimal.Therefore, NO2 selectivity experiments were performed at 600°C.An average of the emf values measured with increasing and decreas-ing concentration was used for the following selectivity analysis.
Plots of the sensor emf vs the logarithm of NO2 concentrationare presented in Fig. 4b. A linear relationship is shown at450–600°C and NO2 �30 ppm. The slopes obtained at 500–600°Care comparable, and at T � 600°C, the slope decreases with in-creasing temperature.
The measurements of the NO2 sensitivity of the Cr2O3 sensor at600°C show good reproducibility between different measurementsrepeated over 11 days under the same condition �21% O2�, as shownin Fig. 5. The slopes in the plots are similar to each other and thedifference of the emf values at the same concentration steps are assmall as 0.1–1.5 mV. This good reproducibility is also the case forthe SnO2 and CeO2 sensor over 10 days.
During the experiments, at least three different sensors of thesame design were built and tested for comparison. The emf valuesmeasured for NOx agreed within 10–20% difference on average un-der the same conditions. The response of each sensor was expectedto be slightly different because parameters affecting the sensitivity,such as porosity and thickness of electrode, might have varied with
Figure 4. NO2 sensitivity of Cr2O3-based sensor �CR� in 21% O2 and bal-ance N2 at 450–700°C. �a� Plots of sensor emf vs time. NO2 concentrationsare shown for each step in ppm and �b� plots of sensor emf vs NO2 concen-tration. The lines drawn in �b� are logarithmic fits.
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fabrication process.16 The differences between sensors are indicativeof the importance of accurate control of fabrication conditions.
SnO2-based sensors.— The NO2 sensing response of theSnO2-based sensor at 500–700°C is presented in Fig. 6. The direc-tion of the NO2 response of the SnO2 sensor is the same as theCr2O3 and the WO3 sensor, showing the positive slopes and emfvalues.11,14 The SnO2 sensor shows �17 mV response for the de-tection of 100 ppm NO2 at 600°C.
The time required to reach 90% of a steady-state value for theSnO2 sensor is less than 100 s at 500–700°C �Fig. 6a�. The responsetime is 10–30 s shorter when measured with increasing NO2 con-centration than with decreasing, as is observed in the Cr2O3 sensor.An average value of the sensor emf measured with increasing anddecreasing concentration is used for the sensitivity plot �Fig. 6b�. Inthe plot, a linear relationship is shown at 500 � T � 700°C andNO2 � 50 ppm, and the highest slope is found at 600°C.
CeO2-based sensors.— The NO2 sensing behavior of theCeO2-based sensor, such as the direction of response, temperatureeffect on the sensitivity, and the response time, is very similar to theCr2O3 and the SnO2 sensor �Fig. 7a�. The sensor voltage corre-sponding to 100 ppm NO2 is �19 mV at 600°C. Again, a decreaseof the slope appears with increasing temperature at T � 600°C �Fig.7b�. The response time of the CeO2 sensor is �15 s shorter whenmeasured in the regime of increasing the NO2 concentration thandecreasing. In the CeO2 sensor, the response begins to saturate be-low 550°C; therefore, the results are not provided in the plot.
Figure 5. Reproducibility of Cr2O3 sensor NO2 sensitivity repeated over11 days under the same condition �in 21% O2 at 600°C�.
Figure 6. NO2 sensitivity of SnO2-based sensor �SN� in 21% O2 and balanceN2 at 500–700°C. �a� Plots of sensor emf vs time. NO2 concentrations areshown for each step in ppm and �b� plots of sensor emf vs NO2 concentra-tion. The lines drawn in �b� are logarithmic fits.
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Selectivity.— NO2 selectivity against CO and CO2.— The effectof 0–400 ppm CO on the NO2 sensitivity of the sensors was inves-tigated in 21% O2 at 600°C �Fig. 8�. All the sensors are able todetect 10–200 ppm NO2 selectively in the presence of 200 ppm CO.Values of the sensor emf, with and without CO, are similar to eachother at the same concentration step, exhibiting a maximum differ-ence less than 1.0 mV.
The effect of 10–20% CO2 is also negligible �Fig. 9�. The sen-sors are all selective to NO2 without a significant interference byCO2. A small decrease in the emf values occurs by the addition ofCO2 in 21% O2 at 600°C, but without a change in the slopes. Thedecrease is less than 1.5 mV for the Cr2O3 and CeO2 sensor, andless than 2.5 mV for the SnO2 sensor, which is still within the gen-eral range of reproducibility.
Selective NO2 detection against CO and CO2 was also observedduring the measurements with the WO3 sensor in 3–21% O2 at650°C.11,14 This was explained based on temperature-programmedreaction �TPR� results, where a NO2 decomposition reaction �Eq. 1�at both Pt and WO3 electrode was not affected by the presence ofCO and CO2 �Ref. 14�
Figure 7. NO2 sensitivity of CeO2-based sensor �CE� in 21% O2 and balanceN2 at 600–700°C. �a� Plots of sensor emf vs time. NO2 concentrations areshown for each step in ppm and �b� Plots of sensor emf vs NO2 concentra-tion. The lines drawn in �b� are logarithmic fits.
Figure 8. Effect of CO on NO2 sensitivity in 21% O2 with N2 balance at600°C. �a� Cr2O3-based sensor �CR�, �b� SnO2-based sensor �SN�, and �c�CeO -based sensor �CE�.
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NO2 � NO + 12 O2 �1�
Therefore, no significant change in the NO2 sensitivity was made byCO and CO2. This was also the case for the present Cr2O3electrode.17
NO2 selectivity against O2, NO, and H2O.— Influence of O2 on theNO2 sensitivity at 600°C is shown in Fig. 10. When O2 concentra-tion increases from 1 �or 3%� to 21%, both values of the emf and theslopes decrease for all the sensors. The slopes in 1–3% O2 are con-siderably higher than those in �10% O2; however, the sign of theslopes does not change. It is noteworthy that the Cr2O3-, SnO2-, andCeO2-based sensors are very similar in the general trend of the NO2
Figure 9. Effect of CO2 on NO2 sensitivity in 21% O2 with N2 balance at600°C. �a� Cr2O3-based sensor �CR�, �b� SnO2-based sensor �SN�, and �c�CeO2-based sensor �CE�.
Figure 10. Effect of O2 on NO2 sensitivity in N2 balance at 600°C. �a�Cr2O3-based sensor �CR�, �b� SnO2-based sensor �SN�, and �c� CeO2-basedsensor �CE�. The lines are logarithmic fits.
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sensitivity change depending on the O2 concentration. Furthermore,the O2 effect on the NO2 sensitivity of the WO3 sensor at 650°Cwas similar to the present sensors.14 None of the sensors showed aresponse to a change in O2 concentration alone �in the absence ofNO2, data not shown�, indicating that catalytic and adsorptive be-havior associated with O2 response is symmetrical between themetal oxide and Pt electrode. Higher pO2
may increase the contribu-tion of the symmetrical �nonsensed� O2 electrode reactions to theoverall reactions and, in turn, may decrease the relative contributionof the NO2 electrode reactions. Therefore, this may reduce the over-all sensor response that is solely associated with NO2.14
Influence of NO on the NO2 sensitivity is also significant, aspresented in Fig. 11. Values of the emf generally decrease withincreasing NO concentration �0–300 ppm�, resulting in a downwardshift of the plots, which is likely due to added NO that shows theopposite direction of the sensor response.5,11 A small increase of theslope is commonly shown by the addition of 100 ppm NO. Yetagain, the sensors show very similar selectivity behavior with oneanother and with the WO3 sensor.11 This NO effect indicates that athermodynamic equilibrium for the NO2 decomposition �Eq. 1� isnot established over the MOx electrodes at 600°C.14 If thermody-namic equilibrium is attained during NO2 sensing, more than 80%NO2 would be converted to NO at 600°C. The results would be anegative emf due to the nonelectrochemically formed NO. However,a positive response is observed, indicating that this is not the case.
Plots showing the effect of 3% H2O are presented in Fig. 12. Anaddition of 3% H2O generally decreases both the slopes and valuesof the emf significantly. The NO2 selectivity behavior over H2O isagain very comparable among the sensors, including the WO3 one.11
Strong ionosorption of water in the form of OH− and H+ on thesurface of metal oxides is well known.3,18,19 However, a linear rela-tionship is still shown at 50 � NO2 � 200 ppm in the presence of3% H2O.
For the potentiometric sensors using the Cr2O3, SnO2, and CeO2as a sensing electrode coupled with the Pt on the other side, theirNO2 selectivity behaviors are very similar with one another, suchthat in general, the selective NO2 detection is achieved over CO andCO2, but the NO2 response is changed by the addition of O2, NO,and H2O. The selectivity behavior is also similar to that of the WO3sensor.11 This NO selectivity is likely due to similarity in the rela-
Figure 11. Effect of NO on NO2 sensitivity in 21% O2 with N2 balance at600°C. �a� Cr2O3-based sensor �CR�, �b� SnO2-based sensor �SN�, and �c�CeO2-based sensor �CE�. The lines are logarithmic fits.
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tive catalytic activity between the metal-oxide electrodes, with thePt being catalytically more active than the oxides.14,17
Sensing properties in simulated exhaust.— The NO2 responsebetween being measured in 3% O2 and in the simulated exhaustcondition is compared for the Cr2O3- and CeO2-based sensor at600°C �Fig. 13�. The simulated exhaust condition includes mostgases generally formed during the combustion process, except hy-drocarbons. Values of the emf and the slopes measured in the ex-
Figure 12. Effect of H2O on NO2 sensitivity in 21% O2 with N2 balance at600°C. �a� Cr2O3-based sensor �CR�, �b� SnO2-based sensor �SN�, and �c�CeO2-based sensor �CE�. The lines are logarithmic fits.
Figure 13. Comparison of NO2 sensitivity between in 3% O2 and in simu-lated exhaust �3% O2, 3% H2O, 16% CO2, and 100 ppm CO� at 600°C. �a�Cr2O3-based sensor �CR� and �b� CeO2-based sensor �CE�. The lines are
logarithmic fits.
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haust are generally lower than in 3% O2 for both sensors. Thechange made by the combustion gas resembles the one by the addi-tion of H2O alone �Fig. 12�, indicating that the water contained inthe simulated exhaust has a major effect on the NO2 sensing behav-ior over CO2 and CO, probably via adsorption on the surface of themetal oxides.3,20 During the NO2 selectivity measurements of theWO3 sensor in the same exhaust atmosphere, selective NO2 detec-tion against CO and CO2 was observed.11 This supports the ideadescribing the dominant role of water on the NO2 sensitivity.
Conclusion
NO2 selectivity of an MOx-based potentiometric sensor �MOx= Cr2O3, SnO2, and CeO2� was investigated. The sensors are allable to detect 10–200 ppm NO2 selectively in the presence of200 ppm CO or 20% CO2 in 21% O2 and at 600°C. However, theNO2 sensitivity is significantly modified by the addition of O2, NO,and H2O in the same condition. Interestingly, all the sensors are verysimilar in the general selectivity trend. This resemblance is mostlikely related to similarity in the catalytic activity of these metaloxides when coupled with the catalytically active Pt electrode on theopposite side.
Acknowledgment
This work was supported by the DOE under contract no. DE-FG26-02NT41533 and DE-FC26-03NT41614.
University of Florida assisted in meeting the publication costs of thisarticle.
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