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Manipulating the gas–surface interaction between copper(II) oxide and mono-nitrogen oxides using temperature
Sensors and Actuators B 229 (2016) 57–62
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Manipulating the gas–surface interaction between copper(II) oxide and mono-nitrogen oxides using temperature Janosch Kneer, Jürgen Wöllenstein, Stefan Palzer ∗ Laboratory for Gas Sensors, Department of Microsystems Engineering, University of Freiburg, Germany
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Article history: Received 7 December 2015 Received in revised form 14 January 2016 Accepted 21 January 2016 Available online 25 January 2016 Keywords: Copper(II) oxide NO2 sensing Surface reactions NO sensing
a b s t r a c t In this contribution we investigate the effects of surface reactions of nitric oxide (NO) and nitrogen dioxide (NO2 ) on inkjet printed, p-type semiconducting copper(II) oxide (CuO) in different temperature regimes. For the first time we show that the gas-induced changes of the electrical conductivity of CuO upon NOx exposure may be tuned from oxidizing to reducing behavior and that the interaction may be effectively turned off. Using the experimental results of the transition from oxidizing to reducing characteristics we discuss the underlying surface reactions, which is of interest for metal-oxide based gas sensors for NOx in general. We demonstrate that the metal oxide reaction is solely governed by the temperature dependent chemical equilibrium of NO/NO2 . Based on these results temperature modulation schemes to enhance the selectivity of metal-oxide gas sensors may be devised. We furthermore show that the CuO material system is able to detect low levels of NO/NO2 , with a strongest oxidation response at 260 ◦ C and a strongest reduction response at 440 ◦ C for both oxides. Due to the high stability of the baseline resistivity of the layer, concentrations as low as 200 ppb may be detected. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nitric oxide (NO) and nitrogen dioxide (NO2 ) are often referred to as NOx and occur in many combustion processes. Both are high impact pollutants responsible for the formation of smog [1], a catalyst for the formation of ozone [2] and one cause for acid rain [3]. They may cause harmful effects on the human metabolism already at low ppm levels. Because the background levels of NOx in the atmosphere, especially in large urban agglomerations, are already close to hazardous levels, preventing a further increase as well as the control of NOx emissions is necessary. Accordingly, strict limits apply for their emission from various sources. The U.S. Occupational Safety and Health Administration gives a limit of 25 ppm and 5 ppm for exposure to NO and NO2 , respectively. Recent studies investigating the health implications have led to a recommendation of a limit of 1 ppm [4]. Even in this low ppm concentration range it may cause respiratory problems and 4 ppm may lead to anesthetize the human nose [5]. Consequently, in order to monitor and control NO2 levels selective, low-cost sensing solutions for large-scale deployment are in great demand. Low power consuming, MEMSbased metal oxide gas sensors are a promising candidate for this
∗ Corresponding author. E-mail address: [email protected] (S. Palzer). http://dx.doi.org/10.1016/j.snb.2016.01.104 0925-4005/© 2016 Elsevier B.V. All rights reserved.
task because they offer high sensitivity and robustness at low cost [6,7]. The most prominent functional materials investigated to date include n-type semiconducting tin dioxide (SnO2 ) [8–10], tungsten oxide (WO3 ) [11–13], and zinc oxide (ZnO) [14–17], p-type semiconducting metal-oxides have attracted less attention within the last two decades [18–20]. Among those, copper(II) oxide (CuO) shows interesting properties towards trace gases [21,22] but has not been investigated with respect to its temperature dependent NOx response over a large temperature range. While several different CuO nano-morphologies, including spheres [23], flakes [24], rods [25] and wires [26] have been investigated with respect to their reaction toward NO2 , none of the previous work has investigated the possibility to use pure CuO to increase the selectivity towards NO2 or investigated the impact and interactions of NO. In general, only few p-type materials have been studied with regard to their NO interaction and Bi2 O3 is the most well-known example [27]. Interestingly, CuO is one of the most prominent materials in heterogeneous catalysis [22] and subject to many recent studies on selective catalytic reduction (SCR) mechanisms for NOx using various other gases [28–31]. Moreover, the individual catalytic activity of CuO and the decomposition mechanisms of NO and NO2 on CuO surfaces have been studied for a large temperature interval [32,33] in the late 1960s and 70s already suggesting different impacts on the surface electronic structure.
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However, this has so far not been put to use in gas sensing using pure CuO surfaces. The n-type semiconducting metal oxides generally show high response both to NO and NO2 . Both have been characterized to show an oxidizing surface interaction, with NO2 having a stronger interaction and a maximum layer response around 250 ◦ C. For higher temperatures the sensor response rapidly decreases and past studies have shown only marginal response at 350 ◦ C [6,34–37]. So far, only Ruhland et al. [38] have reported on the reducing effect of NO on SnO2 surfaces but conducted their experiments at 400 ◦ C thus exceeding the temperature range of previous studies. Still, insufficient selectivity currently prevents the use of metal oxide based gas sensors for NOx detection which remains a scientific and technological challenge. Here we have employed an inkjet printing method to produce CuO sensing surfaces and show that surface reactions can be manipulated via the temperature. The experimental results indicate that the sensing layer reactions towards NO and NO2 are governed mainly by the chemical equilibrium of the ratio between NO and NO2 and not the metal oxide specific adsorption processes. The sensor response may be tuned all the way from oxidizing to reducing behavior which opens up the possibility to effectively turn off the sensor response using the layer’s temperature. This might pave the way towards CuO-based gas sensors for selective detection of NOx in case interfering gases are not present or may be compensated for. It furthermore should be possible to extend the scheme to other gas sensitive metal oxides.
2. Experimental The functional CuO particles were prepared in a dual step process described in detail in Ref. [39]. First, a base dispersion is generated using a wet grinding process employing a Retsch PM 100 planetary mill and zirconia balls to reduce the particle size. In a second step polyethylene glycol (PEG) 400 and deionized water (DI-H2 O) are added and mixed in order to match the viscosity specifications of the deployed Dimatix DMP2831 printer system [40]. The so-produced CuO ink has a viscosity of = 4.33 mPa s. A droplet pitch of 40 m and a piezo nozzle voltage of (30 ± 3) V was used to assemble the CuO layers by printing 6 stacked layers over an interdigitated Pt-electrode on a low power consuming MEMS chip described in Ref. [41]. The resulting layer morphology was examined using a scanning electron microscope (SEM) (Hitachi S-4700) and the grinded CuO purity and crystallinity was investigated in a capillary diffractometer featuring a Molybdenum X-ray source (Mo-K␣) with JCPDS card no. 45-0937 as reference. The gas dependent characterization of the CuO material system’s
sensor response was performed in a fully automated measurement apparatus [42]. Different NO and NO2 concentrations in the range between 0.2–5 ppm have been applied in dry and humid synthetic air (50% r.H., at 25 ◦ C). Four sensing elements were recorded simultaneously in order to evaluate the reproducibility of the sensor signal. The gas-dependent change in electrical resistivity has been recorded for different temperatures in the range between 120 ◦ C and 500 ◦ C.
3. Results and discussion The morphology of the gas-sensitive layers has been evaluated using scanning electron microscopy and the result is presented in Fig. 1. The grinding process causes a significant reduction of mean particle size and also alters the shape as a function of the grinding time considerably. Because size and shape influence the gas sensitive properties of functional materials [43,44], this techniques allows for a convenient way to increase the information obtained by the same material. Various CuO surface morphologies are accessible by adapting the grinding time as demonstrated in Fig. 1(a,b). It shows the sensing layer’s surface after grinding times of 16 h and 48 h resulting in flake-like and nano-sized spherical particles, respectively. The high purity of the functional CuO particles is confirmed by XRD analysis and is shown in Fig. 1(c). The peak intensities reveal no predominant direction in the monoclinic lattice. Analyzing the peak width using the Scherrer relation [45] reveals the particles to be polycrystalline with a crystallite size of (8.9 ± 1.3) nm. The porous, nano-sized particle layers shown in Fig. 1b are utilized in the NO/NO2 gas sensing experiments. Fig. 2a,b,d,e illustrates the NOx dependent resistance of a CuO sensing layer at two different temperatures, 250 ◦ C and 500 ◦ C and 50% r.H. A reference sensor (Figaro 2600) is operated in steady state conditions according to the producer’s recommendations in order to monitor the gas flow and exclude parasitic gas composition effects during all experiments. The reference sensor reveals the oxidizing nature of both gases with the typical higher sensitivity towards NO2 [35]. Due to the low noise and very stable baselines of the inkjet printed CuO sensors, a concentration of 500 ppb NO or NO2 can be clearly resolved and from the signal analysis we estimate a resolution limit of 200 ppb. For 250 ◦ C operating temperature the response times are on the order of 30 min which is much longer than the gas exchange time of our measurement chamber [42] indicating strongly chemisorbed surface states and slow saturation of the surface. The measured response R0 /RG towards NO2 exposure, in dry conditions is about 50% lower as compared to a background atmosphere containing 50% r.H. at 25 ◦ C, while for NO exposure the response is small for both conditions.
Fig. 1. SEM images of inkjet printed CuO layers prepared by (a) 16 h and (b) 48 h grinding time results in flake-like and nano-scale spherical particles, respectively. (c) XRD spectrum verifying pure monoclinic CuO.
J. Kneer et al. / Sensors and Actuators B 229 (2016) 57–62
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Fig. 2. NO and NO2 dependent resistance response of CuO layers recorded for various temperatures and humidity levels. The dashed graph is the response of a Figaro 2600 sensor for reference purpose. (a) Electrical resistivity of a CuO layer at 250 ◦ C for four different NO concentrations in synthetic air and 50% r.H. at 2 ◦ C. (b) The corresponding response for NO2 . (c) Analysis of the layer’s response at 250 ◦ C for various concentrations of NO and NO2 , respectively, in dry synthetic air and synthetic air at 50% r.H. at 25 ◦ C. (d) Electrical resistivity of a CuO layer at 500 ◦ C for four different NO concentrations in synthetic air and 50% r.H. at 25 ◦ C. (e) The corresponding response for NO2 . (f) Analysis of the layer’s response at 500 ◦ C for various concentrations of NO and NO2 , respectively, in dry synthetic air and synthetic air at 50% r.H. at 25 ◦ C. The layer’s response highlight the dependence of the sensor’s sensitivity towards NOx on the humidity levels.
NO2 is known to be a strongly oxidizing gas with a five times higher electron affinity as compared to oxygen [46] and depending on the temperature two net reaction routes are proposed. In the low temperature regime NO and NO2 react on CuO surfaces according to following net reactions [6,24,47]: NO(g) + e− → NO− (ads)
(1a)
− NO(g) + O− 2 → NO(ads) + O2(g)
(1b)
NO2(g) + e− →
NO− 2(ads)
− NO2(g) + O− 2 → NO2(ads) + O2(g)
(2a) (2b)
which leads to an increase in electrical conductivity in p-type semiconducting materials like CuO upon NO/NO2 exposure. Both, NO and NO2 are directly adsorbed to the surface or may substitute an oxygen adsorbate. In both cases the energy barriers at grain boundary surfaces are severely changed due to additional charges or pinning because of the electron affinity [46]. Ultimately, the process results in an increase of the CuO hole accumulation layer, i.e., increasing the electrical conductivity. When raising the temperature to 500 ◦ C the gas–surface processes lead to an increase in resistivity as shown in Fig. 2d,e both for exposure to NO and NO2 . At elevated temperatures several processes take place concurrently. The oxygen adsorbates dissociate and atomic oxygen emerges leading to an increase in + e− → 2O− [48]. This causes an addielectron uptake via O− 2(ads) (ads) tional increase in electrical conductivity and reduces the available adsorption sites for NOx . At about 400 ◦ C the chemical equilibrium between NO and NO2 shifts towards NO2 [27,28] and, simultaneously, the catalytic activity of the CuO surface increases leading to a decomposition of NO2 into nitrogen and oxygen in a dual step process: 1 O2(g) 2
(3)
1 1 NO(g) → k2 N2(g) + O2(g) 2 2
(4)
NO2(g) → k1NO(g) +
with nitrogen and atomic oxygen leaving the CuO surface [32]. These well-established catalytic processes directly impact the surface electronic characteristics and explain the observed net reducing interaction. The sensing signal is now mainly produced by the additionally produced oxygen and now react with the atomic surface adsorbates, which are in immediate vicinity and predominantly present in the temperature regime according to: 1 O2(g) + O− → O2(g) + e− ads 2
(5)
with an electron released and recombining in the hole accumulation layer, thereby increasing the resistance. In order to gain deeper insight into the transition from oxidizing to reducing surface interactions, measurements in the temperature range from 120 ◦ C to 500 ◦ C for equal exposure steps towards NO and NO2 have been performed. Subsequently, these measurement have been performed for different concentration levels. Fig. 3 shows the averaged sensor response RG /R0 of four individual sensing elements for a repeated exposure towards 2 ppm NO and NO2 .
Fig. 3. The CuO surface response upon 2 ppm NO and NO2 exposure in dry synthetic air for different temperatures reveals the transition from oxidizing to reducing behavior.
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Fig. 4. Scheme of the observed, temperature dependent surface reactions of NO and NO2 . (left) Oxidizing interaction after Eqs. (1) & (2) increasing the hole accumulation layer and leading to a decrease in sensing resistance and (right) catalytic decomposition after Eqs. (3) and (4) and subsequent reducing Reaction (5) with electrons released and recombining in the surface leading to the observed resistance increase. The observed transition temperatures are 320 ◦ C and 360 ◦ C for NO and NO2 respectively.
Based on this result the two suggested temperature and surface interaction regimes can be clearly identified and are graphically outlined in Fig. 4. The experimental results furthermore now allow for identification of the transition temperatures for NO and NO2 . The surface reactions are governed by the net reactions given in Eqs. (1) and (2) in the regime below 360 ◦ C shown on the left hand side in Fig. 4 and by Eqs. (3)–(5) above 360 ◦ C for NO2 shown on the right. The corresponding transition temperature for NO is 320 ◦ C. Within the low temperature regime the sensor reaction is strongest at 260 ◦ C for both gases, which is in accordance with previous reports [35] while in the high temperature regime the ideal operation temperature is 440 ◦ C again for both gases. These results also show that it is possible to tune the sensor response using the temperature and that furthermore the sensitivity of CuO towards NOx gases may be effectively turned off. In fact, given the general applicability of the reactions this should hold true for other thin metal oxide layers as well and an indication of this can be found in the results presented in [38], which shows changes in the type of behavior of a SnO2 layer at 400 ◦ C upon NO2 exposure. However, thick film layers might show a different behavior due to emerging diffusion processes in the bulk material [49]. With this in mind, temperature modulation schemes may be implemented for thin metal oxide layers that selectively turn off the reaction towards NOx which in turn should greatly enhance
the capabilities of pattern recognition techniques. The cross-over region around 360 ◦ C has been investigated in greater detail for different concentrations of NO2 in order to further investigate the surface processes. The results are shown in Fig. 5 and reveal that both reactions compete with each other on a time scale that is on the order of minutes and concentration dependent. Fig. 5a demonstrates that the evolution of the layer’s resistivity at 350 ◦ C is highly dependent on the applied NO2 concentration. For all values a short increase in resistivity indicates the dominance of Eq. (1) at first. However, after a few minutes the resistivity begins to drop until reaching a second, concentration dependent turning point. For concentrations below 1 ppm the onset of the second increase in resistivity appears to be fairly constant but for higher concentration the time decreases. This may indicate an accumulation process and after passing a coverage threshold the resistivity increases again. For concentrations below 1 ppm the surface reaction seems to follow Eqs. (3)–(5) on long time scales while for higher concentrations the competing oxidizing reaction (Eq. (2)) prevails. This behavior indicates that at least two different processes are involved in the depletion of surface adsorbed species. Operating CuO layers in this temperature regime may also be used in a dosimeter-like operation mode by evaluating the time evolution of the electrical resistivity. The concentration dependent behavior is evaluated for further temperatures and the results in Fig. 5(b) verify that by increasing
Fig. 5. (a) The sensor response at 350 ◦ C for different NO2 concentrations in dry synthetic air indicates several competing surface processes that affect the electrical resistivity. (b) Concentration dependent response slightly above the cross-over temperature. The values are calculated for long term exposure such that accumulation effects are neglected.
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the temperature the long-term response of CuO layers is steadily pushed towards Eqs. (3)–(5). Hence, because both surface reactions compete in the temperature regime between 350◦ –380 ◦ C, standard read-out techniques by simple comparison of the resistivity values is unsuitable. This is highlighted by the fact that the strongest reducing reaction is detected at 1 ppm for temperatures in the range between 370–390 ◦ C. For lower temperatures the strongest reducing effect is at even lower concentrations. 4. Conclusion Oxidizing and reducing interactions of NO and NO2 on the CuO surface have been investigated depending on the temperature. We identified a transition temperature regime of concurring surface reactions and investigated the additional role of concentration of NO2 . We have consequently shown that it is possible to control the influence of NOx on the electrical conductivity of CuO layers and turn it off via temperature. This may lead to CuO-based NOx sensors with a significant increase in selectivity and the determination of concentrations in the ppb range. The results also show a rich surface chemistry with several competing processes that influence the electrical resistivity. This may give rise to a new method to operate CuO-based sensors to detect NOx . The concentration dependent evolution of the layer’s electrical response should allow for the development of novel schemes to selectively determine the NOx content in air. Acknowledgement This work was supported by a grant from the German Federal ministry of Science and Education (BMBF) under grant number 16SV5943 (SensOdor). References [1] B. Dimitriades, Effects of hydrocarbon and nitrogen oxides on photochemical smog formation, Environ. Sci. Technol. 6 (1972) 253–260. [2] A.J. Haagen-Smit, C.E. Bradley, M.M. Fox, Ozone formation in photochemical oxidation of organic substances, Ind. Eng. Chem. 45 (1953) 2086–2089. [3] J.G. Irwin, M.L. Williams, Acid rain: chemistry and transport, Environ. Pollut. 50 (1988) 29–59. [4] Occupational Safety and Health Adminstration (OSHA). Limits for air contaminants Standard 1910.1000 Table Z-1. [5] H. Gong, Health effects of air pollution. A review of clinical studies, Clin. Chest. Med. 13 (1992) 201–214. [6] A. Afzal, N. Cioffi, L. Sabbatini, L. Torsi, NOx sensors based on semiconducting metal oxide nanostructures: progress and perspectives, Sens. Actuators B: Chem. 171 (2012) 25–42. [7] L. Wang, Z.-C. Hao, L. Dai, Y. Li, H. Zhou, A planar impedancemetric NO2 sensor based on NiO nanoparticles sensing electrode, Mater. Lett. 87 (2012) 24–27. [8] A. Sharma, M. Tomar, V. Gupta, SnO2 thin film sensor with enhanced response for NO2 gas at lower temperatures, Sens. Actuators B: Chem. 156 (2011) 743–752. [9] T. Hyodo, S. Abe, Y. Shimizu, M. Egashira, Gas-sensing properties of ordererd mesoporous SnO2 and effects of coatings thereof, Sens. Actuators B: Chem. 93 (2009) 590–600. [10] H.C. Wang, Y. Li, M.J. Yang, Fast response thin film SnO2 gas sensors operating at room temperature, Sens. Actuators B: Chem. 119 (2006) 380–383. [11] C. Zhang, M. Debliquy, A. Boudiba, H. Liao, C. Coddet, Sensing properties of atmospheric plasma-sprayed WO3 coating for sub-ppm NO2 detection, Sens. Actuators B: Chem. 144 (2010) 280–288. [12] Y. Qin, M. Hu, J. Zhang, Microstructure characterization and NO2 -sensing properties of tungsten oxide nanostructures, Sens. Actuators B: Chem. 150 (2010) 339–345. [13] E.K. Heidari, C. Zamani, E. Marzbanrad, B. Raissi, S. Nazaropour, WO3 -based NO2 sensors fabricated through low frequency AC electrophoretic deposition, Sens. Actuators B: Chem. 146 (2010) 165–170. [14] C. Zhang, M. Debliquy, H. Liao, Deposition and microstructure characterization of atmospheric plasma-sprayed ZnO coatings for NO2 detection, Appl. Surf. Sci. 256 (2010) 5905–5910. [15] E. Oh, H.-Y. Choi, S.-H. Jung, S. Cho, J.C. Kim, K.-H. Lee, S.-W. Kang, J. Kim, J.-Y. Yun, S.-H. Jeong, High-performance NO2 gas sensor based on ZnO nanorod grown by ultrasonic irradiation, Sens. Actuators B: Chem. 141 (2009) 239–243.
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Biographies Janosch Kneer, Dr. received his Diploma in Microsystems Engineering from the University of Freiburg in 2010. He joined the Chair for Gas Sensor Systems in 2011
working as a Ph.D. student in the field of metal oxide chemosensing. He received his Ph.D. in 2015. Jürgen Wöllenstein, Prof. Dr. received his degree in Electrical Engineering from the University of Kassel in 1994. In 1994 he joined the chemical sensors group at the Fraunhofer Institute for Physical Measurement Techniques in Freiburg. Since 2002 he is head of the gas sensor group at Fraunhofer-IPM. In 2009, he became a full professor at the Department of Microsystems Engineering of the University of Freiburg. He is author and co-author of more than 40 publications and holds several patents. Stefan Palzer, PhD received his Diploma in Physics from the University of Freiburg in 2006. He then joined the Cavendish Laboratory at Cambridge University where he graduated with a Ph.D. in Physics in 2010. In 2012 he joined the laboratory for gas sensor at the Department of Microsystems Engineering as a group leader.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Manipulating the gas–surface interaction between copper(II) oxide and mono-nitrogen oxides using temperature Janosch Kneer, Jürgen Wöllenstein, Stefan Palzer ∗ Laboratory for Gas Sensors, Department of Microsystems Engineering, University of Freiburg, Germany
a r t i c l e
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Article history: Received 7 December 2015 Received in revised form 14 January 2016 Accepted 21 January 2016 Available online 25 January 2016 Keywords: Copper(II) oxide NO2 sensing Surface reactions NO sensing
a b s t r a c t In this contribution we investigate the effects of surface reactions of nitric oxide (NO) and nitrogen dioxide (NO2 ) on inkjet printed, p-type semiconducting copper(II) oxide (CuO) in different temperature regimes. For the first time we show that the gas-induced changes of the electrical conductivity of CuO upon NOx exposure may be tuned from oxidizing to reducing behavior and that the interaction may be effectively turned off. Using the experimental results of the transition from oxidizing to reducing characteristics we discuss the underlying surface reactions, which is of interest for metal-oxide based gas sensors for NOx in general. We demonstrate that the metal oxide reaction is solely governed by the temperature dependent chemical equilibrium of NO/NO2 . Based on these results temperature modulation schemes to enhance the selectivity of metal-oxide gas sensors may be devised. We furthermore show that the CuO material system is able to detect low levels of NO/NO2 , with a strongest oxidation response at 260 ◦ C and a strongest reduction response at 440 ◦ C for both oxides. Due to the high stability of the baseline resistivity of the layer, concentrations as low as 200 ppb may be detected. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nitric oxide (NO) and nitrogen dioxide (NO2 ) are often referred to as NOx and occur in many combustion processes. Both are high impact pollutants responsible for the formation of smog [1], a catalyst for the formation of ozone [2] and one cause for acid rain [3]. They may cause harmful effects on the human metabolism already at low ppm levels. Because the background levels of NOx in the atmosphere, especially in large urban agglomerations, are already close to hazardous levels, preventing a further increase as well as the control of NOx emissions is necessary. Accordingly, strict limits apply for their emission from various sources. The U.S. Occupational Safety and Health Administration gives a limit of 25 ppm and 5 ppm for exposure to NO and NO2 , respectively. Recent studies investigating the health implications have led to a recommendation of a limit of 1 ppm [4]. Even in this low ppm concentration range it may cause respiratory problems and 4 ppm may lead to anesthetize the human nose [5]. Consequently, in order to monitor and control NO2 levels selective, low-cost sensing solutions for large-scale deployment are in great demand. Low power consuming, MEMSbased metal oxide gas sensors are a promising candidate for this
∗ Corresponding author. E-mail address: [email protected] (S. Palzer). http://dx.doi.org/10.1016/j.snb.2016.01.104 0925-4005/© 2016 Elsevier B.V. All rights reserved.
task because they offer high sensitivity and robustness at low cost [6,7]. The most prominent functional materials investigated to date include n-type semiconducting tin dioxide (SnO2 ) [8–10], tungsten oxide (WO3 ) [11–13], and zinc oxide (ZnO) [14–17], p-type semiconducting metal-oxides have attracted less attention within the last two decades [18–20]. Among those, copper(II) oxide (CuO) shows interesting properties towards trace gases [21,22] but has not been investigated with respect to its temperature dependent NOx response over a large temperature range. While several different CuO nano-morphologies, including spheres [23], flakes [24], rods [25] and wires [26] have been investigated with respect to their reaction toward NO2 , none of the previous work has investigated the possibility to use pure CuO to increase the selectivity towards NO2 or investigated the impact and interactions of NO. In general, only few p-type materials have been studied with regard to their NO interaction and Bi2 O3 is the most well-known example [27]. Interestingly, CuO is one of the most prominent materials in heterogeneous catalysis [22] and subject to many recent studies on selective catalytic reduction (SCR) mechanisms for NOx using various other gases [28–31]. Moreover, the individual catalytic activity of CuO and the decomposition mechanisms of NO and NO2 on CuO surfaces have been studied for a large temperature interval [32,33] in the late 1960s and 70s already suggesting different impacts on the surface electronic structure.
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However, this has so far not been put to use in gas sensing using pure CuO surfaces. The n-type semiconducting metal oxides generally show high response both to NO and NO2 . Both have been characterized to show an oxidizing surface interaction, with NO2 having a stronger interaction and a maximum layer response around 250 ◦ C. For higher temperatures the sensor response rapidly decreases and past studies have shown only marginal response at 350 ◦ C [6,34–37]. So far, only Ruhland et al. [38] have reported on the reducing effect of NO on SnO2 surfaces but conducted their experiments at 400 ◦ C thus exceeding the temperature range of previous studies. Still, insufficient selectivity currently prevents the use of metal oxide based gas sensors for NOx detection which remains a scientific and technological challenge. Here we have employed an inkjet printing method to produce CuO sensing surfaces and show that surface reactions can be manipulated via the temperature. The experimental results indicate that the sensing layer reactions towards NO and NO2 are governed mainly by the chemical equilibrium of the ratio between NO and NO2 and not the metal oxide specific adsorption processes. The sensor response may be tuned all the way from oxidizing to reducing behavior which opens up the possibility to effectively turn off the sensor response using the layer’s temperature. This might pave the way towards CuO-based gas sensors for selective detection of NOx in case interfering gases are not present or may be compensated for. It furthermore should be possible to extend the scheme to other gas sensitive metal oxides.
2. Experimental The functional CuO particles were prepared in a dual step process described in detail in Ref. [39]. First, a base dispersion is generated using a wet grinding process employing a Retsch PM 100 planetary mill and zirconia balls to reduce the particle size. In a second step polyethylene glycol (PEG) 400 and deionized water (DI-H2 O) are added and mixed in order to match the viscosity specifications of the deployed Dimatix DMP2831 printer system [40]. The so-produced CuO ink has a viscosity of = 4.33 mPa s. A droplet pitch of 40 m and a piezo nozzle voltage of (30 ± 3) V was used to assemble the CuO layers by printing 6 stacked layers over an interdigitated Pt-electrode on a low power consuming MEMS chip described in Ref. [41]. The resulting layer morphology was examined using a scanning electron microscope (SEM) (Hitachi S-4700) and the grinded CuO purity and crystallinity was investigated in a capillary diffractometer featuring a Molybdenum X-ray source (Mo-K␣) with JCPDS card no. 45-0937 as reference. The gas dependent characterization of the CuO material system’s
sensor response was performed in a fully automated measurement apparatus [42]. Different NO and NO2 concentrations in the range between 0.2–5 ppm have been applied in dry and humid synthetic air (50% r.H., at 25 ◦ C). Four sensing elements were recorded simultaneously in order to evaluate the reproducibility of the sensor signal. The gas-dependent change in electrical resistivity has been recorded for different temperatures in the range between 120 ◦ C and 500 ◦ C.
3. Results and discussion The morphology of the gas-sensitive layers has been evaluated using scanning electron microscopy and the result is presented in Fig. 1. The grinding process causes a significant reduction of mean particle size and also alters the shape as a function of the grinding time considerably. Because size and shape influence the gas sensitive properties of functional materials [43,44], this techniques allows for a convenient way to increase the information obtained by the same material. Various CuO surface morphologies are accessible by adapting the grinding time as demonstrated in Fig. 1(a,b). It shows the sensing layer’s surface after grinding times of 16 h and 48 h resulting in flake-like and nano-sized spherical particles, respectively. The high purity of the functional CuO particles is confirmed by XRD analysis and is shown in Fig. 1(c). The peak intensities reveal no predominant direction in the monoclinic lattice. Analyzing the peak width using the Scherrer relation [45] reveals the particles to be polycrystalline with a crystallite size of (8.9 ± 1.3) nm. The porous, nano-sized particle layers shown in Fig. 1b are utilized in the NO/NO2 gas sensing experiments. Fig. 2a,b,d,e illustrates the NOx dependent resistance of a CuO sensing layer at two different temperatures, 250 ◦ C and 500 ◦ C and 50% r.H. A reference sensor (Figaro 2600) is operated in steady state conditions according to the producer’s recommendations in order to monitor the gas flow and exclude parasitic gas composition effects during all experiments. The reference sensor reveals the oxidizing nature of both gases with the typical higher sensitivity towards NO2 [35]. Due to the low noise and very stable baselines of the inkjet printed CuO sensors, a concentration of 500 ppb NO or NO2 can be clearly resolved and from the signal analysis we estimate a resolution limit of 200 ppb. For 250 ◦ C operating temperature the response times are on the order of 30 min which is much longer than the gas exchange time of our measurement chamber [42] indicating strongly chemisorbed surface states and slow saturation of the surface. The measured response R0 /RG towards NO2 exposure, in dry conditions is about 50% lower as compared to a background atmosphere containing 50% r.H. at 25 ◦ C, while for NO exposure the response is small for both conditions.
Fig. 1. SEM images of inkjet printed CuO layers prepared by (a) 16 h and (b) 48 h grinding time results in flake-like and nano-scale spherical particles, respectively. (c) XRD spectrum verifying pure monoclinic CuO.
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Fig. 2. NO and NO2 dependent resistance response of CuO layers recorded for various temperatures and humidity levels. The dashed graph is the response of a Figaro 2600 sensor for reference purpose. (a) Electrical resistivity of a CuO layer at 250 ◦ C for four different NO concentrations in synthetic air and 50% r.H. at 2 ◦ C. (b) The corresponding response for NO2 . (c) Analysis of the layer’s response at 250 ◦ C for various concentrations of NO and NO2 , respectively, in dry synthetic air and synthetic air at 50% r.H. at 25 ◦ C. (d) Electrical resistivity of a CuO layer at 500 ◦ C for four different NO concentrations in synthetic air and 50% r.H. at 25 ◦ C. (e) The corresponding response for NO2 . (f) Analysis of the layer’s response at 500 ◦ C for various concentrations of NO and NO2 , respectively, in dry synthetic air and synthetic air at 50% r.H. at 25 ◦ C. The layer’s response highlight the dependence of the sensor’s sensitivity towards NOx on the humidity levels.
NO2 is known to be a strongly oxidizing gas with a five times higher electron affinity as compared to oxygen [46] and depending on the temperature two net reaction routes are proposed. In the low temperature regime NO and NO2 react on CuO surfaces according to following net reactions [6,24,47]: NO(g) + e− → NO− (ads)
(1a)
− NO(g) + O− 2 → NO(ads) + O2(g)
(1b)
NO2(g) + e− →
NO− 2(ads)
− NO2(g) + O− 2 → NO2(ads) + O2(g)
(2a) (2b)
which leads to an increase in electrical conductivity in p-type semiconducting materials like CuO upon NO/NO2 exposure. Both, NO and NO2 are directly adsorbed to the surface or may substitute an oxygen adsorbate. In both cases the energy barriers at grain boundary surfaces are severely changed due to additional charges or pinning because of the electron affinity [46]. Ultimately, the process results in an increase of the CuO hole accumulation layer, i.e., increasing the electrical conductivity. When raising the temperature to 500 ◦ C the gas–surface processes lead to an increase in resistivity as shown in Fig. 2d,e both for exposure to NO and NO2 . At elevated temperatures several processes take place concurrently. The oxygen adsorbates dissociate and atomic oxygen emerges leading to an increase in + e− → 2O− [48]. This causes an addielectron uptake via O− 2(ads) (ads) tional increase in electrical conductivity and reduces the available adsorption sites for NOx . At about 400 ◦ C the chemical equilibrium between NO and NO2 shifts towards NO2 [27,28] and, simultaneously, the catalytic activity of the CuO surface increases leading to a decomposition of NO2 into nitrogen and oxygen in a dual step process: 1 O2(g) 2
(3)
1 1 NO(g) → k2 N2(g) + O2(g) 2 2
(4)
NO2(g) → k1NO(g) +
with nitrogen and atomic oxygen leaving the CuO surface [32]. These well-established catalytic processes directly impact the surface electronic characteristics and explain the observed net reducing interaction. The sensing signal is now mainly produced by the additionally produced oxygen and now react with the atomic surface adsorbates, which are in immediate vicinity and predominantly present in the temperature regime according to: 1 O2(g) + O− → O2(g) + e− ads 2
(5)
with an electron released and recombining in the hole accumulation layer, thereby increasing the resistance. In order to gain deeper insight into the transition from oxidizing to reducing surface interactions, measurements in the temperature range from 120 ◦ C to 500 ◦ C for equal exposure steps towards NO and NO2 have been performed. Subsequently, these measurement have been performed for different concentration levels. Fig. 3 shows the averaged sensor response RG /R0 of four individual sensing elements for a repeated exposure towards 2 ppm NO and NO2 .
Fig. 3. The CuO surface response upon 2 ppm NO and NO2 exposure in dry synthetic air for different temperatures reveals the transition from oxidizing to reducing behavior.
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Fig. 4. Scheme of the observed, temperature dependent surface reactions of NO and NO2 . (left) Oxidizing interaction after Eqs. (1) & (2) increasing the hole accumulation layer and leading to a decrease in sensing resistance and (right) catalytic decomposition after Eqs. (3) and (4) and subsequent reducing Reaction (5) with electrons released and recombining in the surface leading to the observed resistance increase. The observed transition temperatures are 320 ◦ C and 360 ◦ C for NO and NO2 respectively.
Based on this result the two suggested temperature and surface interaction regimes can be clearly identified and are graphically outlined in Fig. 4. The experimental results furthermore now allow for identification of the transition temperatures for NO and NO2 . The surface reactions are governed by the net reactions given in Eqs. (1) and (2) in the regime below 360 ◦ C shown on the left hand side in Fig. 4 and by Eqs. (3)–(5) above 360 ◦ C for NO2 shown on the right. The corresponding transition temperature for NO is 320 ◦ C. Within the low temperature regime the sensor reaction is strongest at 260 ◦ C for both gases, which is in accordance with previous reports [35] while in the high temperature regime the ideal operation temperature is 440 ◦ C again for both gases. These results also show that it is possible to tune the sensor response using the temperature and that furthermore the sensitivity of CuO towards NOx gases may be effectively turned off. In fact, given the general applicability of the reactions this should hold true for other thin metal oxide layers as well and an indication of this can be found in the results presented in [38], which shows changes in the type of behavior of a SnO2 layer at 400 ◦ C upon NO2 exposure. However, thick film layers might show a different behavior due to emerging diffusion processes in the bulk material [49]. With this in mind, temperature modulation schemes may be implemented for thin metal oxide layers that selectively turn off the reaction towards NOx which in turn should greatly enhance
the capabilities of pattern recognition techniques. The cross-over region around 360 ◦ C has been investigated in greater detail for different concentrations of NO2 in order to further investigate the surface processes. The results are shown in Fig. 5 and reveal that both reactions compete with each other on a time scale that is on the order of minutes and concentration dependent. Fig. 5a demonstrates that the evolution of the layer’s resistivity at 350 ◦ C is highly dependent on the applied NO2 concentration. For all values a short increase in resistivity indicates the dominance of Eq. (1) at first. However, after a few minutes the resistivity begins to drop until reaching a second, concentration dependent turning point. For concentrations below 1 ppm the onset of the second increase in resistivity appears to be fairly constant but for higher concentration the time decreases. This may indicate an accumulation process and after passing a coverage threshold the resistivity increases again. For concentrations below 1 ppm the surface reaction seems to follow Eqs. (3)–(5) on long time scales while for higher concentrations the competing oxidizing reaction (Eq. (2)) prevails. This behavior indicates that at least two different processes are involved in the depletion of surface adsorbed species. Operating CuO layers in this temperature regime may also be used in a dosimeter-like operation mode by evaluating the time evolution of the electrical resistivity. The concentration dependent behavior is evaluated for further temperatures and the results in Fig. 5(b) verify that by increasing
Fig. 5. (a) The sensor response at 350 ◦ C for different NO2 concentrations in dry synthetic air indicates several competing surface processes that affect the electrical resistivity. (b) Concentration dependent response slightly above the cross-over temperature. The values are calculated for long term exposure such that accumulation effects are neglected.
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the temperature the long-term response of CuO layers is steadily pushed towards Eqs. (3)–(5). Hence, because both surface reactions compete in the temperature regime between 350◦ –380 ◦ C, standard read-out techniques by simple comparison of the resistivity values is unsuitable. This is highlighted by the fact that the strongest reducing reaction is detected at 1 ppm for temperatures in the range between 370–390 ◦ C. For lower temperatures the strongest reducing effect is at even lower concentrations. 4. Conclusion Oxidizing and reducing interactions of NO and NO2 on the CuO surface have been investigated depending on the temperature. We identified a transition temperature regime of concurring surface reactions and investigated the additional role of concentration of NO2 . We have consequently shown that it is possible to control the influence of NOx on the electrical conductivity of CuO layers and turn it off via temperature. This may lead to CuO-based NOx sensors with a significant increase in selectivity and the determination of concentrations in the ppb range. The results also show a rich surface chemistry with several competing processes that influence the electrical resistivity. This may give rise to a new method to operate CuO-based sensors to detect NOx . The concentration dependent evolution of the layer’s electrical response should allow for the development of novel schemes to selectively determine the NOx content in air. Acknowledgement This work was supported by a grant from the German Federal ministry of Science and Education (BMBF) under grant number 16SV5943 (SensOdor). References [1] B. Dimitriades, Effects of hydrocarbon and nitrogen oxides on photochemical smog formation, Environ. Sci. Technol. 6 (1972) 253–260. [2] A.J. Haagen-Smit, C.E. Bradley, M.M. Fox, Ozone formation in photochemical oxidation of organic substances, Ind. Eng. Chem. 45 (1953) 2086–2089. [3] J.G. Irwin, M.L. Williams, Acid rain: chemistry and transport, Environ. Pollut. 50 (1988) 29–59. [4] Occupational Safety and Health Adminstration (OSHA). Limits for air contaminants Standard 1910.1000 Table Z-1. [5] H. Gong, Health effects of air pollution. A review of clinical studies, Clin. Chest. Med. 13 (1992) 201–214. [6] A. Afzal, N. Cioffi, L. Sabbatini, L. Torsi, NOx sensors based on semiconducting metal oxide nanostructures: progress and perspectives, Sens. Actuators B: Chem. 171 (2012) 25–42. [7] L. Wang, Z.-C. Hao, L. Dai, Y. Li, H. Zhou, A planar impedancemetric NO2 sensor based on NiO nanoparticles sensing electrode, Mater. Lett. 87 (2012) 24–27. [8] A. Sharma, M. Tomar, V. Gupta, SnO2 thin film sensor with enhanced response for NO2 gas at lower temperatures, Sens. Actuators B: Chem. 156 (2011) 743–752. [9] T. Hyodo, S. Abe, Y. Shimizu, M. Egashira, Gas-sensing properties of ordererd mesoporous SnO2 and effects of coatings thereof, Sens. Actuators B: Chem. 93 (2009) 590–600. [10] H.C. Wang, Y. Li, M.J. Yang, Fast response thin film SnO2 gas sensors operating at room temperature, Sens. Actuators B: Chem. 119 (2006) 380–383. [11] C. Zhang, M. Debliquy, A. Boudiba, H. Liao, C. Coddet, Sensing properties of atmospheric plasma-sprayed WO3 coating for sub-ppm NO2 detection, Sens. Actuators B: Chem. 144 (2010) 280–288. [12] Y. Qin, M. Hu, J. Zhang, Microstructure characterization and NO2 -sensing properties of tungsten oxide nanostructures, Sens. Actuators B: Chem. 150 (2010) 339–345. [13] E.K. Heidari, C. Zamani, E. Marzbanrad, B. Raissi, S. Nazaropour, WO3 -based NO2 sensors fabricated through low frequency AC electrophoretic deposition, Sens. Actuators B: Chem. 146 (2010) 165–170. [14] C. Zhang, M. Debliquy, H. Liao, Deposition and microstructure characterization of atmospheric plasma-sprayed ZnO coatings for NO2 detection, Appl. Surf. Sci. 256 (2010) 5905–5910. [15] E. Oh, H.-Y. Choi, S.-H. Jung, S. Cho, J.C. Kim, K.-H. Lee, S.-W. Kang, J. Kim, J.-Y. Yun, S.-H. Jeong, High-performance NO2 gas sensor based on ZnO nanorod grown by ultrasonic irradiation, Sens. Actuators B: Chem. 141 (2009) 239–243.
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Biographies Janosch Kneer, Dr. received his Diploma in Microsystems Engineering from the University of Freiburg in 2010. He joined the Chair for Gas Sensor Systems in 2011
working as a Ph.D. student in the field of metal oxide chemosensing. He received his Ph.D. in 2015. Jürgen Wöllenstein, Prof. Dr. received his degree in Electrical Engineering from the University of Kassel in 1994. In 1994 he joined the chemical sensors group at the Fraunhofer Institute for Physical Measurement Techniques in Freiburg. Since 2002 he is head of the gas sensor group at Fraunhofer-IPM. In 2009, he became a full professor at the Department of Microsystems Engineering of the University of Freiburg. He is author and co-author of more than 40 publications and holds several patents. Stefan Palzer, PhD received his Diploma in Physics from the University of Freiburg in 2006. He then joined the Cavendish Laboratory at Cambridge University where he graduated with a Ph.D. in Physics in 2010. In 2012 he joined the laboratory for gas sensor at the Department of Microsystems Engineering as a group leader.