- Email: [email protected]
ZnO laminated heterostructured configuration
Sensors and Actuators B 195 (2014) 500–508
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Enhanced response to NO2 with CuO/ZnO laminated heterostructured configuration Li Yang a , Changsheng Xie a,b , Guozhu Zhang a , Jianwei Zhao a , Xueli Yu a , Dawen Zeng a,b , Shunping Zhang a,b,∗ a
State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China Nanomaterials and Smart Sensors Research Laboratory, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b
a r t i c l e
i n f o
Article history: Received 22 September 2013 Received in revised form 9 November 2013 Accepted 8 January 2014 Available online 22 January 2014 Keywords: CuO/ZnO Laminated heterostructured configration Charge separation Gas sensor
a b s t r a c t The charge separation behavior is easily ignored in the field of gas sensor. In this work, a novel perspective focuses on the charge separation theory for the exploration of the gas sensing mechanism. For this target, the novel laminated CuO/ZnO p–n heterostructured systems are introduced to achieve this attempt. The desired system is composed of the parallel Au electrodes, CuO and ZnO functional layers. It is worth noting that the synergistic effect of the laminated heterostructure and external electric field can strengthen the unidirectional charge separation in both CuO and ZnO layers. Comparing with the plain CuO, the optimal response is enhanced 3.58 times by utilizing the [electrode/CuO/ZnO] heterostructured configuration at 350 ◦ C. Interestingly, we observe that the resistance changed remarkably once the NO2 gas is cut off, and its detailed formation mechanism is discussed. We believe that the transformation process from double electrons to single electron is responsible for the abnormal results. The analysis method for gas sensing mechanism has a certain guiding significant for the design of gas sensor with high performance by using the laminated heterostructured configuration. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Primarily known for its presence in atmospheric pollution as a toxic gas, nitric oxide (NOx ) is an important kind of target molecule for MOS gas sensors. Nitrogen dioxide (NO2 ) is a major component of so-called NOx gases [1], which is involved in the formation of ground level ozone, participates in global warming, and also forms toxic chemicals, nitrate particles and acid rain [2]. Many relevant efforts are tried to reduce the environmental pollution, for example, developing the sensors to detect NOx gas and adopting the novel catalysts to degrade NOx into the harmless gaseous species [3–6]. Metal oxide semiconductors (MOS) with chemo-resistance have shown good gas sensing properties, such as extensive response, short response-recovery time [7–10]. For this reason, MOS sensors are used as an effective tool to detect various gaseous volatile organic compounds (VOCs), nitric oxide and sulfide fumes [5,8,9]. Recently, CuO and ZnO semiconductors exhibit great potential for broad applications in detection of low concentration NO2 [10,11]. Nevertheless, to improve the performance of gas sensor, some
∗ Corresponding author at: Huazhong University of Science and Technology, Department of Materials Science and Engineering, 1037 Luoyu Road, Wuhan, China. Tel.: +86 27 87556544/+86 27 87543778; fax: +86 27 87543776. E-mail address: [email protected] (S. Zhang). 0925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2014.01.030
widely accepted strategies have been utilized via regulating the content and distribution of foreign chemical elements in sensing films. For example, the modification of nanostructures by the introduction of functional activators, such as noble metal elements (Au, Pt, Pd, etc.) or the rare earth elements onto the surface of MOS, has been confirmed as an effective surface modification technology for enhancing the performance of gas sensor [12–14]. Aside from noble metals modification, MOS also has been extensively designed as the laminated heterostructured configuration by resorting to the diversified deposition technologies. As a rule, the heterostructure can extend the space charge region and regulate the carrier transportation among the grains to improve functional properties [10]. Among these heterostructured configurations applied in a range of gas sensor, CuO/ZnO has become a focus of intense research because the p–n heterostructured interface can provide electric junction properties under an applied external electric field. Unfortunately, the basic surface reaction underlying the sensing mechanism of CuO/ZnO is not fully understood. Until now, many relative papers have revealed that CuO/ZnO heterostructured system is suitable for the application of detecting various harmful gas, for instance, H2 S [15], the reducing VOCs [16] and oxidizing gas [17]. To our knowledge, little data are available pertaining to the detection of NO2 . The charge separation behavior of heterojunctional systems is widely accepted in the field of photocatalysis, which can
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
reasonably elaborate the improvement of photoelectric property or photocatalytic degradation efficiency [18,19]. With regards to the configuration of heterostructured system, the laminated architecture is considered as an attractive candidate option because the appropriate energy band matching always has a remarkable positive contribution to facilitate the charge separation. Especially the carrier separation will be more efficient in the aid of external bias electric field. Similar opinion is approved by the photocatalytic investigations [18,20,21]. On the whole, in the field of gas sensor, the cognition of heterojunctional function is confined to an opinion that the conductivity change only results from the variation of the space charge region at heterointerface, or is restricted to a simple viewpoint that the heterojunction only provides more active sites for the surface adsorption and chemical reactions. Importantly, as for the gas sensing heterojunctional system, the charge separation behavior has not yet received attention. Here, in order to fill the research gap, we construct a laminated CuO/ZnO p–n heterostructured configuration for application of detecting noxious NO2 . In this work, the system consists of the parallel Au electrode, CuO layer and ZnO layer, and the three functional layers are laminated from bottom to top using the screen printing technology. Combining the features of material system and testing results, a plausible mechanism is proposed in detail from the perspective of charge separation by the p–n heterointerfaces. The presented mechanism focuses on the carrier transport behavior with the aid of thermodynamic energy band matching and external circuit. Meanwhile, an abnormal response phenomenon is observed and its formation mechanism is also elucidated based on the defect chemistry theory. We expect this new discovering and mechanism interpretation of CuO/ZnO heterostructured system could provide a novel perspective for the investigation of sensing material and mechanism, which is also applicable to other heterostructured systems. In turn, it may open a new avenue for the design of gas sensing material systems and devices with excellent performance.
2. Experimental 2.1. Fabrication of material chips ZnO (30 nm), CuO (70 nm) were obtained from Beijing Dekedaojin Sci&Tech. Co. Ltd., China, and all other chemicals used were analytical pure grade. All of them were used as received without any pretreatment. In this work, the preparation of Au electrode–CuO–ZnO heterostrucured configuration was based on the screen printing technique, which was widely applied in gas sensor industry. To reduce the test error in the process of gas response, as shown in Fig. 1a, all the sensing layers were loaded on a material chip with the coplanar 36 independent electrodes, which suitable for the self-designed high throughput measurement platform [13]. The distribution of the Au electrodes and sensing films were schematically shown in Fig. 1b. As to the laminated configurations, such as [electrode/ZnO/CuO] and [electrode/CuO/ZnO], we called them as T-CuO and T-ZnO for short, respectively. Their corresponding schematic diagrams were depicted in Fig. 1c Firstly, the chemical powder (CuO or ZnO) and a certain amount of organic solvent (composed of terpineol, butyl carbitol, ethylcellulose, span 85 and di-n-butyl phthalate) were mixed in a proper ratio. All of them were put into the agate ball milling tank by means of ball-milling with 5 h at the speed of 300 rpm, and then the screen printed pastes with suitable viscosity were obtained. About the detailed information of paste preparation were reported in our previous work [22]. Secondly, the CuO paste was printed onto the Au electrode the sensing layer which had been preprinted on a planar Al2 O3 substrate (79 × 79 × 2.5 mm). After drying in an oven at 80 ◦ C for
501
30 min, the ZnO paste was deposited onto the CuO layer. The size and thickness of each layer were 0.6 mm × 0.8 mm and 10 m, respectively, which were decided by the selected screen printing process. Finally, the as-obtained material chips were allowed to dry for 12 h at 80 ◦ C in an oven, and then the chips were subjected to a heat treatment at 200 ◦ C for 15 min to eliminate the organic carrier. The final annealing was performed in a muffle furnace with temperature programming at 550 ◦ C for 2 h to ensure that the electrodes and sensing layer had established good contacts. Repeating the aforementioned procedures, the [electrode/ZnO/CuO] heterostructured layer and the parallel samples with pure CuO and ZnO could be obtained on the same material chip simultaneously. Thus far, all of desired heterostructured samples were integrated into a material chip to guarantee high throughput measurement in target gaseous atmosphere. 2.2. Characteristic method and detection technique The cross section morphology of the CuO/ZnO heterostructured was observed using Field-emission Scanning Electron Microscopy (FE–SEM, Sirion 200, FEI) equipped with an energy dispersive spectroscopy (EDS) system. Meanwhile, the information of metal elemental analysis for the heterointerface was also obtained by EDS line scanning technology. NO2 gas sensing properties of the fabricated sensors were tested in a self-designed high throughout measurement platform [13], which could real-time record the variation of resistance (R) for all sensors with the test time (t). The response tests were performed under the diluted NO2 gas at a temperature range from 250 to 400 ◦ C with the interval of 50 ◦ C. The diluted NO2 (29 ppm) was obtained by mixing 58 ppm standard gas (Shanghai Weichuang Standard Gas Analytical Technology Co., Ltd., China) and dry air at a volume ratio of 1:1, and NO2 passed over the sensor at a total flow rate of 1000 SCCM. In our measurement platform, the gas flow was controlled by several mass flow controllers (MFCs, DZ-7A, Beijing Seven-Star Co. Ltd., China). The experimental platform was used no fewer than 30 min at every operating temperature to stabilize the sensing films before every measurement, and the repeated R–t test was allowed to at least 3 times at every experimental condition. The data acquisition using PCI-6225 (National Instruments, USA) combined with a LabVIEW® program on a PC computer. Further detailed information was described in our previous work [13,23]. 3. Results and discussion 3.1. FE–SEM characteristics For simplicity, four laminated samples from bottom to top: [electrode/CuO/ZnO], [electrode/ZnO/CuO], [electrode/CuO/CuO] and [electrode/ZnO/ZnO] are designated as T-ZnO, T-CuO, CuO and ZnO, respectively. The schematic diagram is shown in Fig. 2. The typical surface and cross section morphology of T-CuO configuration are shown in Fig. 2a. The printed film is c.a. 20 m in thickness, in which the ZnO and CuO layers are both c.a. 10 m. An apparent interface between ZnO and CuO layers is observed, which means an ordered heterointerface is fabricated. With respect to our laminated configuration samples, the unidirectional built-in field at the p–n interface implies that our heterostructure is different from the disordered mixture in the other composite system [24,25]. From SEM image of T-CuO configuration in Fig. 2a, we can clearly observe the surface morphology of the top layer (CuO) and the profile morphology of the sandwich layer (ZnO). According to the inset of Fig. 2a, we notice that the thick films have pronounced
502
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
Fig. 1. Schematic diagram of the material chip for high throughput testing platform (a), the distribution of sensing films and electrodes (b) and the laminated heterostructured configurations (c).
micro-holes, which come from the volatilization of organic solvents during the sintering process. The porous frame provides many convenient channels for the interactive reaction between the target gas and sensing materials. Fig. 2b exhibits the typical EDS line-scanning diagram of the straight line AB on the cross section of T-CuO. The remarkable variation of the element distribution curves occurs at
the CuO/ZnO interface, which implies there is no obvious penetrative diffusion on the printed CuO and ZnO layers and also the ordered heterointerface is retained after sintering at 550 ◦ C. 3.2. NO2 sensing properties of the plain CuO and ZnO The NO2 sensing properties of the pure CuO and ZnO sensors are measured at various sensing operating temperatures. Herein, for simplicity, the symbol Tw represents for the operating temperature. The sensing results are shown in Fig. 3. Diluted NO2 gas (29 ppm) is injected to check the gas response and the sample is purged with air after successive injections. From Fig. 3, at a temperature range
Fig. 2. (a) A typical cross-sectional FE–SEM image of the as-prepared T-CuO, the inset represent the morphology of top surface CuO layer and sandwich ZnO layer; (b) EDS line scanning spectrum of line AB in the cross-sectional of T-CuO.
Fig. 3. Response curves of pure ZnO (a) and pure CuO (b) on exposure to 29 ppm NO2 at different operating temperatures.
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
from 250 to 400 ◦ C, the sensing results show stable responses and fast recovery after NO2 gas exposure and the air purge, respectively. For an oxidizing gas, such as NO2 , the gas sensing response (S) can be defined as S = Ra /Rg for p-typed materials and S = Rg /Ra for ntyped materials, in which Ra and Rg represent the resistance when a sensor on exposure to dry air and NO2 gas, respectively. As is evident in Fig. 3, the resistance in dry air, hereafter referred to as the baseline resistance of the sensors, decreases with increasing Tw from 250 to 400 ◦ C. We believe the baseline resistance change result from conductive behavior dominating by impurity ionization at this temperature region [26]. After the injection of oxidizing NO2 gas, the electric resistance decreases for CuO and on the contrary that increases for ZnO at each experimented temperature. In general, this response behavior in accordance with the typical response model of p- and n-typed sensing material, respectively [27,28]. That is to say, the gas sensing response mode is also one of estimated approach to judge the conductive types for MOS gas sensing material. Interestingly, we also observed a distinct mutational phenomenon once the removal of NO2 . For convenience, this special response mode is called as “off overshoot” phenomenon. It describes an abnormal experimental phenomenon that the resistance reaches to an extreme value abruptly once the NO2 gas is cut off, and then returns to a stable amplitude. As manifested by Fig. 3a, the tendency of overshoot behavior weakens with Tw from 250 to 350 ◦ C and eventually disappears at 400 ◦ C. In fact, to our knowledge, the similar overshoot response mode was also reported previously in others research work [29,30]. However, the underlying regularity has not been further investigated yet. As to the detailed formation mechanism of the “off overshoot” phenomenon will be argued later in this paper. In the progress of the gas sensing, the environmental atmosphere, such as O2 or NO2 , dominates the surface resistance by mean of regulating the width of the surface space charge layer due to the exchange of the electrons between gas and MOS [27,28]. Therefore, the density of surface charge and its distribution of spatial position would influence the performance of a gas sensor. In that case, an intriguing hypothesis is beginning to emerge in the mind: whether one can strengthen the charge separation by introduction of a heterointerface to avail the charge exchange between the gas and sensing material? In order to seek the solution, we notice a fact that the laminated heterostructured configuration was proven as an efficient strategy to strengthen the charge separation [18,31]. Therefore, we predict that the experimental results of the laminated heterostructures will be different from the plain samples. To validate this hypothesis, let us go on with R–t tests for the sensors with laminated heterostructured configuration. 3.3. NO2 sensing properties and mechanism of the CuO/ZnO laminated heterostructured configuration Fig. 4 shows the response curves of T-ZnO and T-CuO laminated heterostructure configurations. Interestingly, both of response features are similar to the plain p-type CuO. In other words, this p-type response behavior is independent of the screen printing sequence of the CuO and ZnO layers. When the laminated heterostructured sensors are exposed to oxidizing NO2 , the electrons on MOS surface are captured by NO2 molecules, which indicate the surface resistance is no longer same as the bulk resistance. It is well known, the carrier is always apt to transport via a low resistance channel. That is to say, as depicted in Fig. 5, the carrier transport from one grain to another by overcoming high surface potential barriers for ZnO layer, but on the contrary the carrier can directly transport via the low surface resistance for CuO layer. Moreover, Fig. 6 describes the pathway of carrier transport in T-ZnO and T-CuO laminated heterostructured systems. We can deduce that the carrier flows
503
Fig. 4. Response curves of T-CuO and T-ZnO with laminated heterostructured configurations upon exposure to 29 ppm NO2 at different operating temperatures.
through the bottled CuO layer for T-ZnO sample. Unlike T-CuO system, the carrier will traverse the CuO/ZnO heterointerfaces and the top of CuO layer. Otherwise, it will overcome lots of high energy barriers if the bottled ZnO layer as a conductive shortcut. Meanwhile, the carrier transport process is depicted intuitively as the corresponding equivalent circuit in Fig. 6b and d. The validity of description in Fig. 6 can be confirmed by the resistance comparison of four samples. And the sequence of the baseline resistance is: RZnO > RT-ZnO > RT-CuO > RCuO at every experimental temperature. Thus far, we believe that the conduction of laminated CuO/ZnO system is dominated by the low-resistance CuO layer, which is the reason for two laminated configurations show p-type response feature to the oxidizing NO2 gas. Therefore, the contribution of CuO/ZnO heterointerfaces can be extracted by the comparison of performance for bare CuO, T-CuO and T-ZnO samples and the relevant response results are shown in Fig. 7. It is found that all of the gas sensing performance has been improved for the laminated T-CuO and T-ZnO vs. bare CuO at Tw from 250 to 400 ◦ C. To further quantitatively evaluate the influence of heterostructure for T-CuO, herein, we define an appraisement coefficient as k1 = ST-CuO /SCuO , and k1 value has a specifically physical significance. If k1 > 1, it reveals the accelerating effect of heterointerfaces due to the additional top CuO layer. By contrast, k1 < 1 means the additional top CuO layer goes against the gas sensing performance. Similarly, the definition of k2 = ST-ZnO /SCuO is used to estimate the effect of T-ZnO heterointefaces. All the relevant data are shown in Table 1. From Table 1 we notice the performance of all heterojunctional samples is enhanced once the introduction of CuO/ZnO heterointefaces. Especially, the best gas sensing performance enhancement comes from T-ZnO sample and its response value increases 3.58 times in comparison with the plain CuO at 350 ◦ C. We believe the increase in the gas sensing performance can be ascribed to the charge separation between CuO and ZnO interfaces. On the basis of the available data regarding the band diagrams of ZnO and CuO, a tentative energy band diagram of a CuO/ZnO p–n heterojunction can Table 1 Response of the plain CuO and laminated heterostructures upon exposure to 29 ppm NO2 at different operating temperature (Tw ). Tw (◦ C)
250
300
350
400
S(CuO) S(T-CuO) S(T-ZnO) k1 = S(T-CuO) /S(CuO) k2 = S(T-ZnO) /S(CuO)
2.48 2.83 3.72 1.141 1.500
2.53 3.95 5.57 1.561 2.202
2.57 3.43 9.20 1.334 3.580
1.83 1.88 2.17 1.027 1.186
504
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
Fig. 5. Schematic of the resistance distribution in crystalline grains and the corresponding energy band structures in dry air and NO2 for the plain CuO (a–b) and plain ZnO (c–d).
be constructed in accordance with the Anderson model [32–35]. Herein, CuO is p-type sensing film, the holes of the valance band participate the conductive process. Dissimilarly for the n-typed ZnO, the excited electrons are considered as the majority carrier in the process of conduction. When the p-typed material contacts the n-typed material, the majority carrier diffuses into each other. As a result of the build-in electric field (Ei ) is constructed at the interfaces and its direction points to p-typed materials [29]. Specific to CuO/ZnO p–n heterostructured system in this work, a mechanistic scheme of the charge separation for the CuO/ZnO system is shown in Fig. 8. As can be seen, the conduction band (CB) edge of ZnO and CuO are situated at −4.35 and −4.07 eV vs. vacuum level. The valence band (VB) edge of ZnO (−7.67 eV) is much lower
than that of CuO (−5.42 eV). The thermodynamic equilibrium state of CuO/ZnO p–n heterojunction is depicted in Fig. 8a. With regards to T-ZnO sample, as exhibited in Fig. 8b, on the one hand, in according with the thermodynamic principal, holes of the top layer ZnO can flow toward VB of the bottom layer CuO, result in a thicker surface hole accumulation layer. And then, the transferred holes via thermal excitation are drawn by the external circuit to enhance the conductivity of CuO layer. On the other hand, benefiting from the removal holes on the side of ZnO, the amount of electron–hole recombination decreases and more ZnO surface electrons are captured by O2 and NO2 , which are contributing to the improvement of gas sensor. Therefore, we can draw a conclusion that the synergistic effect of three influencing factors, including
Fig. 6. Schematic diagrams of the possible conductive channels and the equivalent circuits for the laminated configuration T-CuO (a–b) and T-ZnO (c–d).
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
Fig. 7. The response of the plain CuO and the laminated heterostructures upon exposure to NO2 at different operating temperatures.
the direction of Ei , large valance band offset (Ev = 1.69 eV) and the electrode position of external circuit, is in favor of the charge separation smoothly. With regards to T-CuO sensor, as shown in Fig. 8c, the holes in VB of ZnO can still transfer into CuO due to the appropriate energy band matching, which broadens the hole accumulation layer on the side of CuO and result in the decrease of resistance. Nevertheless, as depicted in Fig. 6a, the Au electrodes are covered by the bottom ZnO layer with high resistance. Under the strong drawing force of external electric field, the carriers are obliged to pass across the p–n interfaces and eventually flow to the external circuit. Comparing
505
with the T-ZnO system, the lower efficiency of charge separation is obtained from T-CuO by reason of the hindering effect of p–n space charge layer. But even so, for T-CuO sensor, the feeble effect of the charge separation has been also appeared and the results of enhanced performance are observed in Table 1. Besides, for T-CuO porous architecture, the gas molecules can diffuse into the under layered ZnO film, but the micro-hole structure still has an obvious hindering effect of the gas–solid interactions and further decreases the amount of generating holes on ZnO surface. This is the other reason for the result of k2 < k1 in Table 1. From what has been argued above, we can conclude that the charge separation behavior stems from the energy band matching of the laminated CuO/ZnO heterostructure with the assistance of the external circuit, and the relative position of the Au electrodes decides the efficiency of charge separation. Although the channels of carrier transport are dominated by low-resistance CuO layer, the effect of high-resistance ZnO layer should not be ignored since it can contribute abundant holes and then transfer into CuO layer to achieve the unidirectional charge separation in CuO/ZnO heterostructured configuration. Therefore, it seems that the design of T-ZnO is a desired configuration to enhance the gas sensing performance of detecting NO2 gas. In brief, on the basis of the experimental results, in order to further understand the features of laminated heterostructured configuration, three functional layers are summarized as follows: (1) n-Typed ZnO layer as a hole donor by means of the interaction with the oxidizing O2 and NO2 gas molecules. It provides extra holes in VB of ZnO and the holes can be separated in time into p-typed CuO layer. (2) p-Typed CuO layer as the hole generator and acceptor, which dominates the conduction process under different gas
Fig. 8. Schematic diagrams illustrate the constructed energy band of the CuO/ZnO p–n heterojunction under thermal equilibrium condition (a) and the charge separation effect of the laminated T-CuO (b) and T-ZnO (c).
506
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
atmosphere. It is an important medium to transfer smoothly the holes into the external circuit. (3) Au electrode as an impeller provides strong external drawing force to accelerate the unidirectional separation of holes. The relative position can influence the efficiency of charge separation. 3.4. “Off overshoot” phenomenon and its possible mechanism
typed ZnO results from the oxygen vacancy VO•• as a donor defect level, which lead to the deviation from the stoichiometry [36]. Just because of this, the hole for CuO and the electron for ZnO are the so-called majority carrier, which dominates the conductivity of semiconductor materials. When the sensors are exposed to dry air, the oxygen molecular adsorbs on the surface of ZnO and CuO with the exchange of electrons. The process can be described by the defect equations as the following. For ZnO sensing materials, the gaseous O2 prefers to adsorb at the site of oxygen vacancy, which can be described as
VO•• +
1 − + e− → VO•• − O(ads) 2 O2(g)
(1)
wherein “g” and “ads” refer, respectively, to gas and adsorbate, VO•• represents the oxygen vacancy with two positive charges. Consequently, for ZnO surface, an electron depletion layer is built, which gives rise to the decrease of the electron density of conduction band, and further to the result of high surface resistance than the bulk, the schematic diagram is shown in Fig. 5c. Similarly, for CuO sensing layer, the gaseous O2 also prefers to adsorb at the site of copper vacancy and the relevant defect equation can be written as
VCu +
1 − → VCu − O(ads) 2 O2
− VO•• + NO2(g) + e− → VO•• − NO2(ads)
(3)
− VO•• − O(ads) + VO•• − NO2(gas) + e−
From what has been described above, an inevitable question is posed: what is causing the “off overshoot” phenomenon? To address the question, we attempt to propose a plausible mechanism on the basis of defeat equations. It is generally considered that the defects and interfaces with high activity for the adsorption/desorption or chemical reaction, and the different active sites provide an opportunity for different gas species, which means the active sites has a special selectivity for the adsorbed gas [14,34,35]. In this paper, the sensing process of the sensors towards 29 ppm NO2 is divided into three stages: (1) initial period in dry air, (2) response period with NO2 injection and (3) recovery period with NO2 cutting off. The detailed interpretations are as follows. (1) Initial period in dry air. In this work, the conductive types are confirmed as p-type for CuO and n-type for ZnO in the light of the response mode. Generally speaking, p-typed CuO origins from the copper vacancy VCu as an acceptor defect level and the n-
species, leaving an active defect. Afterwards, the exposed defect site is occupied by adsorbed O2 again with an electron transferring from MOS to adsorbed O2 . More specifically speaking, on the one hand, for ZnO sensing films, the interaction processes can be depicted as
(2)
wherein VCu , h+ represents, respectively, the copper vacancy with two negative charges and hole. According to Eq. (2), a hole accumulation space charge layer exists on the surface of CuO, which indicates the increase of the hole density of valence band and ultimately result of low surface resistance, the schematic diagram is shown in Fig. 5a. (2) Response period with NO2 injection. NO2 is an oxidizing gas and therefore traps free electrons from the MOS surface resulting in a decrease (n-typed ZnO) or increase (p-typed CuO) in its conductivity. As discussed in other work, we believe that NO2 gas molecules interact preferentially with the available defect sites instead of reacting with the adsorbed O− [34,37]. Thereby, NO2 gas molecules capture electrons from MOS unoccupied defects result in a variation of sensor resistance. After all of the effective defects are occupied by NO2 gas molecules, there will be valid for a reaction with − − − the adsorbed NO− 2 and neighboring O to produce O − NO2(ads)
→ O− − VO•• − NO2−(ads) + VO•• +
1 2 O2(gas)
(4)
It is found that the thickness of electron depletion layer has further become wider with the electron transference from the conduction band to adsorbates, and in turn the electric resistance is escalated. This coincides with the gas sensing response experimental results shown in Fig. 3a. On the other hand, for the CuO sensing films, the NO2 gas will also attack preferentially the copper vacancy defeats and then interact with the adsorbed O− . However, the electron affinity CuO (−0.13 eV) > ZnO (−4.35 eV) will easily obtain the nitrite specie on the CuO surface [34,36]. The corresponding reaction equations are expressed by
− VCu + NO2(g) → VCu − NO2(ads) + h+
(5)
− − VCu − O(ads) + VCu − NO2(ads)
− → O− − VCu − NO2(ads) + VCu +
1 + 2 h+ 2 O2
− VCu + NO2 → VCu − ONO(ads) + h+
(6) (7)
As can be seen, Eqs. ((5)–(7)) increase the thickness of the hole accumulation layer and result in the decrease of the resistance, which also agrees well with the result in Fig. 3b. (3) Recovery period with NO2 off. In this stage, the gas atmosphere is switched from 29 ppm NO2 to dry air by means of MFC. Therefore, the adsorbed NO2 or its evolutive species on the surface will be desorbed. For ZnO sensing layer, the NO2 desorption reactions can be written as − O− − VO•• − NO2(ads) → VO•• + NO2(gas) + − VO•• − NO2(ads) → VO•• + NO2(g) + e−
1 + 2 e− 2 O2(gas)
(8) (9)
We can see that two electrons are released back to the conduction band (CB) by Eq. (8), simultaneously, a single electron is also released by Eq. (9). According to the limitation condition of Eq. (4), the limited quantity of O− − VO•• − NO− 2 exists on the ZnO surface, which decides the double electrons releasing is an unendurable process. Just because of this, we believe that the combining effect of Eqs. (8) and (9) are responsible for the “off overshoot” when NO2 is cut off. Once the limited O− − VO•• − NO− 2 species are consumed completely, Eq. (8) is invalid, which result in the disappearance of overshoot phenomenon. Afterwards, as observed in Fig. 3a, the stable resistance is only dominated by Eq. (9). Perhaps the observed off-overshoot phenomenon is attributed to a continuous conversion from double electrons to single electron. Similar analysis approach can apply to CuO sensing film, the obtained − − and VCu − NO2(ads) species in Eqs. (5) and (6) O− − VCu − NO2(ads) will desorb by adopting the methods, which is similar to Eqs. (8) and (9). The desorption process decreases the width of the hole accumulation layer and finally results in the off-overshoot behavior. the However, other important transformation from the nitroso VCu − ONO− to the nitrosyl O− − VCu − NO+ should not ignore [34,38], as shown in Eq. (10). In addition, as manifested by
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
Eqs. ((11)–(12)), because of a weak bond of nitrosyl, –NO+ escapes easily from CuO surface and finally reacts with the adsorbed O− . Here, we notice that the electron release to the CuO conduction band via Eq. (10) and the positive charges escape from the CuO surface via Eq. (11), which contributes to the diminution of the hole accumulation layer. The equations are written as the followings.
− + VCu − ONO(ads) → O− − VCu − NO(ads) + e−
O
−
+ − VCu − NO(ads)
→
− VCu − O(ads)
+ NO
(10)
+
(11)
NO+ + O− → NO2 (g)
[8]
[9] [10]
(12) [11]
So after, the “off overshoot” phenomenon of CuO origins from − , the following three processes: desorption of O− − VCu − NO2(ads)
[7]
− desorption of VCu − NO2(ads) and the decomposition of VCu − ONO− . In summary, it is plausible to conclude that the underlying mechanism of the “off overshoot” for n-type ZnO and p-type CuO. With regards to ZnO, the “off overshoot” phenomenon can be ascribed to the predominated transformation reactions from the double electrons to the single electron. As regards to CuO, however, apart from the aforementioned similar reactions, the evolution of VCu − ONO− species has an important contribution by means of adjusting the width of the hole accumulation layer.
4. Conclusion In the presented work, we reported a novel perspective for the investigation of heterostructured configuration in the field of gas sensor. An ordered laminated CuO/ZnO was designed for detection of NO2 gas. The laminated configuration remarkably enhanced NO2 gas responses compared to the plain samples. A plausible carrier transport mechanism was firstly proposed from the perspective of charge separation with the assistance of external circuit. The experimental results suggested the T-ZnO heterostructrue was a desired design because the charge separation was operated smoothly. Furthermore, we noticed the abnormal “off overshoot” phenomenon and further argued its formation mechanism by using defect chemistry theory. It was found that the abnormal response behaviors resulted from the transformation of the double electrons to single electron reactions once the NO2 gas was cut off. The analysis approach could apply to the laminated p–n heterostructure configuration.
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
Acknowledgments [25]
This work was supported by Nature Science Foundation of China (Nos. 51204072 and Nos. 50927201), the National Basic Research Program of China (Grant Nos. 2009CB939705 and 2009CB939702). The authors are also grateful to Analytical and Testing Center of Huazhong University of Science and Technology.
[26] [27]
[28]
References [29] [1] M.T. Lerdau, J.W. Munger, D.J. Jacob, The NO2 flux conundrum, Science 289 (2000) 2291–2293. [2] G.V. Zyryanov, Y. Kang, D.M. Rudkevich, Sensing and fixation of NO2 /N2 O4 by calix[4]arenes, Journal of the American Chemical Society 125 (2003) 2997–3007. [3] A. Geupel, D. Schönauer, U. Röder-Roith, D.J. Kubinski, S. Mulla, T.H. Ballinger, H. Chen, J.H. Visser, R. Moos, Integrating nitrogen oxide sensor: a novel concept for measuring low concentrations in the exhaust gas, Sensors and Actuators B: Chemical 145 (2010) 756–761. [4] A. Groß, M. Kremling, I. Marr, D.J. Kubinski, J.H. Visser, R. Tuller, H.L. Moos, Dosimeter-type NOx sensing properties of KMnO4 and its electrical conductivity during temperature programmed desorption, Sensors 13 (2013) 4428–4449. [5] A. Groß, G. Beulertz, I. Marr, D.J. Kubinski, R. Visser, J.H. Moos, Dual mode NOx sensor: measuring both the accumulated amount and instantaneous level at low concentrations, Sensors 12 (2012) 2831–2850. [6] S. Fischer, R. Pohle, B. Farber, R. Proch, J. Kaniuk, M. Fleischer, R. Moos, Method for detection of NOx in exhaust gases by pulsed discharge measurements using
[30]
[31]
[32]
[33] [34]
[35]
507
standard zirconia-based lambda sensors, Sensors and Actuators B: Chemical 147 (2010) 780–785. C. Li, S.P. Zhang, M.L. Hu, C.S. Xie, Nanostructural ZnO based coplanar gas sensor arrays from the injection of metal chloride solutions: Device processing, gassensing properties and selectivity in liquors applications, Sensors and Actuators B: Chemical 153 (2011) 415–420. C.S. Xie, L.Q. Xiao, M.L. Hu, Z.K. Bai, X.P. Xia, D.W. Zeng, Fabrication and formaldehyde gas-sensing property of ZnO–MnO2 coplanar gas sensor arrays, Sensors and Actuators B: Chemical 151 (2010) 457–463. J. Kim, K. Yong, Mechanism study of ZnO nanorod-bundle sensors for H2 S gas sensing, Journal of Physical Chemistry C 115 (2011) 7218–7224. G.X. Zhu, H. Xu, Y.Y. Xiao, Y.J. Liu, A.H. Yuan, X.P. Shen, Facile fabrication and enhanced sensing properties of hierarchically porous CuO architectures, ACS Applied Materials & Interfaces 4 (2012) 744–751. Y.V. Kaneti, J. Yue, X.C. Jiang, A.B. Yu, Controllable synthesis of ZnO nanoflakes with exposed (1 0 1 0) for enhanced gas sensing performance, Journal of Physical Chemistry C 117 (2013) 13153–13162. C.Q. Ge, C.S. Xie, S.Z. Cai, Preparation and gas-sensing properties of Ce-doped ZnO thin-film sensors by dip-coating, Materials Science and Engineering B 137 (2007) 53–58. G.Z. Zhang, S.P. Zhang, L. Yang, Z.J. Zou, D.W. Zeng, C.S. Xie, La2 O3 -sensitized SnO2 nanocrystalline porous film gas sensors and sensing mechanism toward formaldehyde, Sensors and Actuators B Chemical 188 (2013) 137–146. M.R. Yu, R.J. Wu, M. Chavali, Effect of “Pt” loading in ZnO–CuO hetero-junction material sensing carbon monoxide at room temperature, Sensors and Actuators B: Chemical 153 (2011) 321–328. S.J. Kim, C.W. Na, I.S. Hwang, J.H. Lee, One-pot hydrothermal synthesis of CuO–ZnO composite hollow spheres for selective H2 S detection, Sensors and Actuators B: Chemical 168 (2012) 83–89. J.R. Huang, Y.J. Dai, C.P. Gu, Y.F. Sun, J.H. Liu, Preparation of porous flower-like CuO/ZnO nanostructures and analysis of their gas-sensing property, Journal of Alloys and Compounds 575 (2013) 115–122. Q. Simon, D. Barreca, A. Gasparotto, C. Maccato, E. Tondello, C. Sada, E. Comini, G. Sberveglieri, M. Banerjee, K. Xu, CuO/ZnO nanocomposite gas sensors developed by a plasma-assisted route, ChemPhysChem 13 (2012) 2342–2348. Y. Liu, C.S. Xie, H.Y. Li, H. Chen, T. Zou, D.W. Zeng, Improvement of gaseous pollutant photocatalysis with WO3 /TiO2 heterojunctional-electrical layered system, Journal of Hazardous Materials 196 (2011) 52–58. L.G. Kumar, S.G. Devi, Review on modified TiO2 photocatalysis under UV/visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics, Journal of Physical Chemistry A 115 (2011) 13211–13241. S.S. Zhang, C.S. Xie, Z.J. Zou, L. Yang, H.Y. Li, S.P. Zhang, High photoconductive response of gas-sensitized porous nanocrystalline TiO2 film in formaldehyde ambience and carrier transport kinetics, Journal of Physical Chemistry C 116 (2012) 19673–19681. L. Liu, S.C. Li, L.Y. Wang, C.C. Guo, Q.Y. Dong, W. Li, Enhancement ethanol sensing properties of NiO-SnO2 nanofibers, Journal of the American Ceramic Society 94 (2011) 771–775. L. Yang, S.P. Zhang, G.Z. Zhang, S.S. Zhang, H.Y. Li, C.S. Xie, Specially environmental responses induced by multi-field coupling for nanocrystalline SnO2 porous film as gas sensor, Sensors and Actuators B: Chemical 182 (2013) 239–249. L. Yang, S.P. Zhang, H.Y. Li, G.Z. Zhang, C.Y. Zhan, C.S. Xie, Conduction model of coupled domination by bias and neck for porous films as gas sensor, Sensors and Actuators B: Chemical 176 (2013) 217–224. A. Zainelabdin, G. Amin, S. Zaman, O. Nur, J. Lu, L. Hultman, M. Willander, CuO/ZnO nanocorals synthesis via hydrothermal technique: growth mechanism and their application as humidity sensor, Journal of Materials Chemistry 22 (2012) 11583–11590. M.R. Yu, G. Suyambrakasam, R.J. Wu, M. Chavali, Performance evaluation of ZnO–CuO hetero junction solid state room temperature ethanol sensor, Materials Research Bulletin 47 (2012) 1713–1718. E.K. Liu, B.S. Zhu, J.J. Luo, Semiconductor Physics, seventh ed., Electronics Industry Publication, Beijing, China, 2009. N. Barsan, C. Simion, T. Heine, S. Pokhrel, U. Weimar, Modeling of sensing and transduction for p-type semiconducting metal oxide based gas sensors, Journal of Electroceramics 25 (2010) 11–19. N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, Journal of Electroceramics 7 (2001) 143–167. Y. Yamada, M. Ogita, Transient response of resistive-type NO2 sensor on temperature change, Sensors and Actuators B: Chemical 93 (2003) 546–551. P. Rai, W.K. Kwak, Y.T. Yu, Solvothermal synthesis of ZnO nanostructures and their morphology-dependent gas-sensing properties, ACS Applied Materials & Interfaces 5 (2013) 3026–3032. Y. Liu, C.S. Xie, H.Y. Li, H. Chen, Y.C. Liao, D.W. Zeng, Low bias photoelectrocatalytic (PEC) performance for organic vapour degradation using TiO2 /WO3 nanocomposite, Applied Catalysis B: Environmental 102 (2011) 157–162. A. Zainelabdin, S. Zaman, G. Amin, O. NurM. Willander, Optical and current transport properties of CuO/ZnO nanocoral p–n heterostructure hydrothermally synthesized at low temperature, Applied Physics A 108 (2012) 921–928. P.S. Nayar, Properties of zinc phosphide/zinc oxide heterojunctions, Journal of Applied Physics 53 (1982) 1069–1075. A. Sharma, M. TomarV. Gupta, WO3 nanoclusters–SnO2 film gas sensor heterostructure with enhanced response for NO2 , Sensors and Actuators B: Chemical 176 (2013) 675–684. R.I. Masel, Principles of adsorption and reaction on solid surfaces, first ed., John Wiley & Sons, Inc., New York, NY, 1996, pp. 438–476.
508
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
[36] R. Dong, D.S. Lee, W.F. Xiang, S.J. Oh, D.J. Seong, S.H. Heo, H.J. Choi, M.J. Kwon, S.N. Seo, M.B. Pyun, Reproducible hysteresis and resistive switching in metal–CuO–metal heterostructures, Applied Physics Letters 90 (2007) 42107. [37] B. Ruhland, T. Becker, G. Müller, Gas-kinetic interactions of nitrous oxides with SnO2 surfaces, Sensors and Actuators B: Chemical 50 (1998) 85–94. [38] L.G. Teoh, Y.M. Hon, J. Shieh, W.H. Lai, M.H. Hon, Sensitivity properties of a novel NO2 gas sensor based on mesoporous WO3 thin film, Sensors and Actuators B: Chemical 96 (2003) 219–225.
Biographies Li Yang received his M.Sc. from China University of Geosciences in 2010 on material science and chemistry. Currently he is working on gas sensors as a Ph.D. student in Huazhong University of Science and Technology. Changsheng Xie received his Ph.D. from Huazhong University of Science and Technology in 1985 on material science and engineering. He is Professor of Material Science and Engineering at Huazhong University of Science and Technology. His research interests include the synthesizing of nanomaterials and electronic noses.
Guozhu Zhang received his B.Sc. from Wuhan Institute of Technology in 2010 on material science and engineering. Currently he is working on gas sensors as a Ph.D. student in Huazhong University of Science and Technology. Jianwei Zhao received his B.Sc. from Liaocheng University in 2012 on material science and engineering. Currently he is working on gas sensors as a master student in Huazhong University of Science and Technology. Xueli Yu received his M.Sc. from Huazhong Normal University in 2006 on condensed matter physics. Currently he is working on gas sensors as a Ph.D. student in Huazhong University of Science and Technology. Dawen Zeng received his Ph.D. from Huazhong University of Science and Technology in 1998 on materials science and engineering. He is a professor of Materials Science and Engineering at the same university. His research interests include the synthesizing, characterizing of inorganic functional materials. Shunping Zhang received his Ph.D. from Huazhong University of Science and Technology in 2009 on material science and engineering. He is a lecturer of Materials Science and Engineering at Huazhong University of Science and Technology. His research is on the electronic noses.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Enhanced response to NO2 with CuO/ZnO laminated heterostructured configuration Li Yang a , Changsheng Xie a,b , Guozhu Zhang a , Jianwei Zhao a , Xueli Yu a , Dawen Zeng a,b , Shunping Zhang a,b,∗ a
State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China Nanomaterials and Smart Sensors Research Laboratory, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b
a r t i c l e
i n f o
Article history: Received 22 September 2013 Received in revised form 9 November 2013 Accepted 8 January 2014 Available online 22 January 2014 Keywords: CuO/ZnO Laminated heterostructured configration Charge separation Gas sensor
a b s t r a c t The charge separation behavior is easily ignored in the field of gas sensor. In this work, a novel perspective focuses on the charge separation theory for the exploration of the gas sensing mechanism. For this target, the novel laminated CuO/ZnO p–n heterostructured systems are introduced to achieve this attempt. The desired system is composed of the parallel Au electrodes, CuO and ZnO functional layers. It is worth noting that the synergistic effect of the laminated heterostructure and external electric field can strengthen the unidirectional charge separation in both CuO and ZnO layers. Comparing with the plain CuO, the optimal response is enhanced 3.58 times by utilizing the [electrode/CuO/ZnO] heterostructured configuration at 350 ◦ C. Interestingly, we observe that the resistance changed remarkably once the NO2 gas is cut off, and its detailed formation mechanism is discussed. We believe that the transformation process from double electrons to single electron is responsible for the abnormal results. The analysis method for gas sensing mechanism has a certain guiding significant for the design of gas sensor with high performance by using the laminated heterostructured configuration. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Primarily known for its presence in atmospheric pollution as a toxic gas, nitric oxide (NOx ) is an important kind of target molecule for MOS gas sensors. Nitrogen dioxide (NO2 ) is a major component of so-called NOx gases [1], which is involved in the formation of ground level ozone, participates in global warming, and also forms toxic chemicals, nitrate particles and acid rain [2]. Many relevant efforts are tried to reduce the environmental pollution, for example, developing the sensors to detect NOx gas and adopting the novel catalysts to degrade NOx into the harmless gaseous species [3–6]. Metal oxide semiconductors (MOS) with chemo-resistance have shown good gas sensing properties, such as extensive response, short response-recovery time [7–10]. For this reason, MOS sensors are used as an effective tool to detect various gaseous volatile organic compounds (VOCs), nitric oxide and sulfide fumes [5,8,9]. Recently, CuO and ZnO semiconductors exhibit great potential for broad applications in detection of low concentration NO2 [10,11]. Nevertheless, to improve the performance of gas sensor, some
∗ Corresponding author at: Huazhong University of Science and Technology, Department of Materials Science and Engineering, 1037 Luoyu Road, Wuhan, China. Tel.: +86 27 87556544/+86 27 87543778; fax: +86 27 87543776. E-mail address: [email protected] (S. Zhang). 0925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2014.01.030
widely accepted strategies have been utilized via regulating the content and distribution of foreign chemical elements in sensing films. For example, the modification of nanostructures by the introduction of functional activators, such as noble metal elements (Au, Pt, Pd, etc.) or the rare earth elements onto the surface of MOS, has been confirmed as an effective surface modification technology for enhancing the performance of gas sensor [12–14]. Aside from noble metals modification, MOS also has been extensively designed as the laminated heterostructured configuration by resorting to the diversified deposition technologies. As a rule, the heterostructure can extend the space charge region and regulate the carrier transportation among the grains to improve functional properties [10]. Among these heterostructured configurations applied in a range of gas sensor, CuO/ZnO has become a focus of intense research because the p–n heterostructured interface can provide electric junction properties under an applied external electric field. Unfortunately, the basic surface reaction underlying the sensing mechanism of CuO/ZnO is not fully understood. Until now, many relative papers have revealed that CuO/ZnO heterostructured system is suitable for the application of detecting various harmful gas, for instance, H2 S [15], the reducing VOCs [16] and oxidizing gas [17]. To our knowledge, little data are available pertaining to the detection of NO2 . The charge separation behavior of heterojunctional systems is widely accepted in the field of photocatalysis, which can
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
reasonably elaborate the improvement of photoelectric property or photocatalytic degradation efficiency [18,19]. With regards to the configuration of heterostructured system, the laminated architecture is considered as an attractive candidate option because the appropriate energy band matching always has a remarkable positive contribution to facilitate the charge separation. Especially the carrier separation will be more efficient in the aid of external bias electric field. Similar opinion is approved by the photocatalytic investigations [18,20,21]. On the whole, in the field of gas sensor, the cognition of heterojunctional function is confined to an opinion that the conductivity change only results from the variation of the space charge region at heterointerface, or is restricted to a simple viewpoint that the heterojunction only provides more active sites for the surface adsorption and chemical reactions. Importantly, as for the gas sensing heterojunctional system, the charge separation behavior has not yet received attention. Here, in order to fill the research gap, we construct a laminated CuO/ZnO p–n heterostructured configuration for application of detecting noxious NO2 . In this work, the system consists of the parallel Au electrode, CuO layer and ZnO layer, and the three functional layers are laminated from bottom to top using the screen printing technology. Combining the features of material system and testing results, a plausible mechanism is proposed in detail from the perspective of charge separation by the p–n heterointerfaces. The presented mechanism focuses on the carrier transport behavior with the aid of thermodynamic energy band matching and external circuit. Meanwhile, an abnormal response phenomenon is observed and its formation mechanism is also elucidated based on the defect chemistry theory. We expect this new discovering and mechanism interpretation of CuO/ZnO heterostructured system could provide a novel perspective for the investigation of sensing material and mechanism, which is also applicable to other heterostructured systems. In turn, it may open a new avenue for the design of gas sensing material systems and devices with excellent performance.
2. Experimental 2.1. Fabrication of material chips ZnO (30 nm), CuO (70 nm) were obtained from Beijing Dekedaojin Sci&Tech. Co. Ltd., China, and all other chemicals used were analytical pure grade. All of them were used as received without any pretreatment. In this work, the preparation of Au electrode–CuO–ZnO heterostrucured configuration was based on the screen printing technique, which was widely applied in gas sensor industry. To reduce the test error in the process of gas response, as shown in Fig. 1a, all the sensing layers were loaded on a material chip with the coplanar 36 independent electrodes, which suitable for the self-designed high throughput measurement platform [13]. The distribution of the Au electrodes and sensing films were schematically shown in Fig. 1b. As to the laminated configurations, such as [electrode/ZnO/CuO] and [electrode/CuO/ZnO], we called them as T-CuO and T-ZnO for short, respectively. Their corresponding schematic diagrams were depicted in Fig. 1c Firstly, the chemical powder (CuO or ZnO) and a certain amount of organic solvent (composed of terpineol, butyl carbitol, ethylcellulose, span 85 and di-n-butyl phthalate) were mixed in a proper ratio. All of them were put into the agate ball milling tank by means of ball-milling with 5 h at the speed of 300 rpm, and then the screen printed pastes with suitable viscosity were obtained. About the detailed information of paste preparation were reported in our previous work [22]. Secondly, the CuO paste was printed onto the Au electrode the sensing layer which had been preprinted on a planar Al2 O3 substrate (79 × 79 × 2.5 mm). After drying in an oven at 80 ◦ C for
501
30 min, the ZnO paste was deposited onto the CuO layer. The size and thickness of each layer were 0.6 mm × 0.8 mm and 10 m, respectively, which were decided by the selected screen printing process. Finally, the as-obtained material chips were allowed to dry for 12 h at 80 ◦ C in an oven, and then the chips were subjected to a heat treatment at 200 ◦ C for 15 min to eliminate the organic carrier. The final annealing was performed in a muffle furnace with temperature programming at 550 ◦ C for 2 h to ensure that the electrodes and sensing layer had established good contacts. Repeating the aforementioned procedures, the [electrode/ZnO/CuO] heterostructured layer and the parallel samples with pure CuO and ZnO could be obtained on the same material chip simultaneously. Thus far, all of desired heterostructured samples were integrated into a material chip to guarantee high throughput measurement in target gaseous atmosphere. 2.2. Characteristic method and detection technique The cross section morphology of the CuO/ZnO heterostructured was observed using Field-emission Scanning Electron Microscopy (FE–SEM, Sirion 200, FEI) equipped with an energy dispersive spectroscopy (EDS) system. Meanwhile, the information of metal elemental analysis for the heterointerface was also obtained by EDS line scanning technology. NO2 gas sensing properties of the fabricated sensors were tested in a self-designed high throughout measurement platform [13], which could real-time record the variation of resistance (R) for all sensors with the test time (t). The response tests were performed under the diluted NO2 gas at a temperature range from 250 to 400 ◦ C with the interval of 50 ◦ C. The diluted NO2 (29 ppm) was obtained by mixing 58 ppm standard gas (Shanghai Weichuang Standard Gas Analytical Technology Co., Ltd., China) and dry air at a volume ratio of 1:1, and NO2 passed over the sensor at a total flow rate of 1000 SCCM. In our measurement platform, the gas flow was controlled by several mass flow controllers (MFCs, DZ-7A, Beijing Seven-Star Co. Ltd., China). The experimental platform was used no fewer than 30 min at every operating temperature to stabilize the sensing films before every measurement, and the repeated R–t test was allowed to at least 3 times at every experimental condition. The data acquisition using PCI-6225 (National Instruments, USA) combined with a LabVIEW® program on a PC computer. Further detailed information was described in our previous work [13,23]. 3. Results and discussion 3.1. FE–SEM characteristics For simplicity, four laminated samples from bottom to top: [electrode/CuO/ZnO], [electrode/ZnO/CuO], [electrode/CuO/CuO] and [electrode/ZnO/ZnO] are designated as T-ZnO, T-CuO, CuO and ZnO, respectively. The schematic diagram is shown in Fig. 2. The typical surface and cross section morphology of T-CuO configuration are shown in Fig. 2a. The printed film is c.a. 20 m in thickness, in which the ZnO and CuO layers are both c.a. 10 m. An apparent interface between ZnO and CuO layers is observed, which means an ordered heterointerface is fabricated. With respect to our laminated configuration samples, the unidirectional built-in field at the p–n interface implies that our heterostructure is different from the disordered mixture in the other composite system [24,25]. From SEM image of T-CuO configuration in Fig. 2a, we can clearly observe the surface morphology of the top layer (CuO) and the profile morphology of the sandwich layer (ZnO). According to the inset of Fig. 2a, we notice that the thick films have pronounced
502
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
Fig. 1. Schematic diagram of the material chip for high throughput testing platform (a), the distribution of sensing films and electrodes (b) and the laminated heterostructured configurations (c).
micro-holes, which come from the volatilization of organic solvents during the sintering process. The porous frame provides many convenient channels for the interactive reaction between the target gas and sensing materials. Fig. 2b exhibits the typical EDS line-scanning diagram of the straight line AB on the cross section of T-CuO. The remarkable variation of the element distribution curves occurs at
the CuO/ZnO interface, which implies there is no obvious penetrative diffusion on the printed CuO and ZnO layers and also the ordered heterointerface is retained after sintering at 550 ◦ C. 3.2. NO2 sensing properties of the plain CuO and ZnO The NO2 sensing properties of the pure CuO and ZnO sensors are measured at various sensing operating temperatures. Herein, for simplicity, the symbol Tw represents for the operating temperature. The sensing results are shown in Fig. 3. Diluted NO2 gas (29 ppm) is injected to check the gas response and the sample is purged with air after successive injections. From Fig. 3, at a temperature range
Fig. 2. (a) A typical cross-sectional FE–SEM image of the as-prepared T-CuO, the inset represent the morphology of top surface CuO layer and sandwich ZnO layer; (b) EDS line scanning spectrum of line AB in the cross-sectional of T-CuO.
Fig. 3. Response curves of pure ZnO (a) and pure CuO (b) on exposure to 29 ppm NO2 at different operating temperatures.
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
from 250 to 400 ◦ C, the sensing results show stable responses and fast recovery after NO2 gas exposure and the air purge, respectively. For an oxidizing gas, such as NO2 , the gas sensing response (S) can be defined as S = Ra /Rg for p-typed materials and S = Rg /Ra for ntyped materials, in which Ra and Rg represent the resistance when a sensor on exposure to dry air and NO2 gas, respectively. As is evident in Fig. 3, the resistance in dry air, hereafter referred to as the baseline resistance of the sensors, decreases with increasing Tw from 250 to 400 ◦ C. We believe the baseline resistance change result from conductive behavior dominating by impurity ionization at this temperature region [26]. After the injection of oxidizing NO2 gas, the electric resistance decreases for CuO and on the contrary that increases for ZnO at each experimented temperature. In general, this response behavior in accordance with the typical response model of p- and n-typed sensing material, respectively [27,28]. That is to say, the gas sensing response mode is also one of estimated approach to judge the conductive types for MOS gas sensing material. Interestingly, we also observed a distinct mutational phenomenon once the removal of NO2 . For convenience, this special response mode is called as “off overshoot” phenomenon. It describes an abnormal experimental phenomenon that the resistance reaches to an extreme value abruptly once the NO2 gas is cut off, and then returns to a stable amplitude. As manifested by Fig. 3a, the tendency of overshoot behavior weakens with Tw from 250 to 350 ◦ C and eventually disappears at 400 ◦ C. In fact, to our knowledge, the similar overshoot response mode was also reported previously in others research work [29,30]. However, the underlying regularity has not been further investigated yet. As to the detailed formation mechanism of the “off overshoot” phenomenon will be argued later in this paper. In the progress of the gas sensing, the environmental atmosphere, such as O2 or NO2 , dominates the surface resistance by mean of regulating the width of the surface space charge layer due to the exchange of the electrons between gas and MOS [27,28]. Therefore, the density of surface charge and its distribution of spatial position would influence the performance of a gas sensor. In that case, an intriguing hypothesis is beginning to emerge in the mind: whether one can strengthen the charge separation by introduction of a heterointerface to avail the charge exchange between the gas and sensing material? In order to seek the solution, we notice a fact that the laminated heterostructured configuration was proven as an efficient strategy to strengthen the charge separation [18,31]. Therefore, we predict that the experimental results of the laminated heterostructures will be different from the plain samples. To validate this hypothesis, let us go on with R–t tests for the sensors with laminated heterostructured configuration. 3.3. NO2 sensing properties and mechanism of the CuO/ZnO laminated heterostructured configuration Fig. 4 shows the response curves of T-ZnO and T-CuO laminated heterostructure configurations. Interestingly, both of response features are similar to the plain p-type CuO. In other words, this p-type response behavior is independent of the screen printing sequence of the CuO and ZnO layers. When the laminated heterostructured sensors are exposed to oxidizing NO2 , the electrons on MOS surface are captured by NO2 molecules, which indicate the surface resistance is no longer same as the bulk resistance. It is well known, the carrier is always apt to transport via a low resistance channel. That is to say, as depicted in Fig. 5, the carrier transport from one grain to another by overcoming high surface potential barriers for ZnO layer, but on the contrary the carrier can directly transport via the low surface resistance for CuO layer. Moreover, Fig. 6 describes the pathway of carrier transport in T-ZnO and T-CuO laminated heterostructured systems. We can deduce that the carrier flows
503
Fig. 4. Response curves of T-CuO and T-ZnO with laminated heterostructured configurations upon exposure to 29 ppm NO2 at different operating temperatures.
through the bottled CuO layer for T-ZnO sample. Unlike T-CuO system, the carrier will traverse the CuO/ZnO heterointerfaces and the top of CuO layer. Otherwise, it will overcome lots of high energy barriers if the bottled ZnO layer as a conductive shortcut. Meanwhile, the carrier transport process is depicted intuitively as the corresponding equivalent circuit in Fig. 6b and d. The validity of description in Fig. 6 can be confirmed by the resistance comparison of four samples. And the sequence of the baseline resistance is: RZnO > RT-ZnO > RT-CuO > RCuO at every experimental temperature. Thus far, we believe that the conduction of laminated CuO/ZnO system is dominated by the low-resistance CuO layer, which is the reason for two laminated configurations show p-type response feature to the oxidizing NO2 gas. Therefore, the contribution of CuO/ZnO heterointerfaces can be extracted by the comparison of performance for bare CuO, T-CuO and T-ZnO samples and the relevant response results are shown in Fig. 7. It is found that all of the gas sensing performance has been improved for the laminated T-CuO and T-ZnO vs. bare CuO at Tw from 250 to 400 ◦ C. To further quantitatively evaluate the influence of heterostructure for T-CuO, herein, we define an appraisement coefficient as k1 = ST-CuO /SCuO , and k1 value has a specifically physical significance. If k1 > 1, it reveals the accelerating effect of heterointerfaces due to the additional top CuO layer. By contrast, k1 < 1 means the additional top CuO layer goes against the gas sensing performance. Similarly, the definition of k2 = ST-ZnO /SCuO is used to estimate the effect of T-ZnO heterointefaces. All the relevant data are shown in Table 1. From Table 1 we notice the performance of all heterojunctional samples is enhanced once the introduction of CuO/ZnO heterointefaces. Especially, the best gas sensing performance enhancement comes from T-ZnO sample and its response value increases 3.58 times in comparison with the plain CuO at 350 ◦ C. We believe the increase in the gas sensing performance can be ascribed to the charge separation between CuO and ZnO interfaces. On the basis of the available data regarding the band diagrams of ZnO and CuO, a tentative energy band diagram of a CuO/ZnO p–n heterojunction can Table 1 Response of the plain CuO and laminated heterostructures upon exposure to 29 ppm NO2 at different operating temperature (Tw ). Tw (◦ C)
250
300
350
400
S(CuO) S(T-CuO) S(T-ZnO) k1 = S(T-CuO) /S(CuO) k2 = S(T-ZnO) /S(CuO)
2.48 2.83 3.72 1.141 1.500
2.53 3.95 5.57 1.561 2.202
2.57 3.43 9.20 1.334 3.580
1.83 1.88 2.17 1.027 1.186
504
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
Fig. 5. Schematic of the resistance distribution in crystalline grains and the corresponding energy band structures in dry air and NO2 for the plain CuO (a–b) and plain ZnO (c–d).
be constructed in accordance with the Anderson model [32–35]. Herein, CuO is p-type sensing film, the holes of the valance band participate the conductive process. Dissimilarly for the n-typed ZnO, the excited electrons are considered as the majority carrier in the process of conduction. When the p-typed material contacts the n-typed material, the majority carrier diffuses into each other. As a result of the build-in electric field (Ei ) is constructed at the interfaces and its direction points to p-typed materials [29]. Specific to CuO/ZnO p–n heterostructured system in this work, a mechanistic scheme of the charge separation for the CuO/ZnO system is shown in Fig. 8. As can be seen, the conduction band (CB) edge of ZnO and CuO are situated at −4.35 and −4.07 eV vs. vacuum level. The valence band (VB) edge of ZnO (−7.67 eV) is much lower
than that of CuO (−5.42 eV). The thermodynamic equilibrium state of CuO/ZnO p–n heterojunction is depicted in Fig. 8a. With regards to T-ZnO sample, as exhibited in Fig. 8b, on the one hand, in according with the thermodynamic principal, holes of the top layer ZnO can flow toward VB of the bottom layer CuO, result in a thicker surface hole accumulation layer. And then, the transferred holes via thermal excitation are drawn by the external circuit to enhance the conductivity of CuO layer. On the other hand, benefiting from the removal holes on the side of ZnO, the amount of electron–hole recombination decreases and more ZnO surface electrons are captured by O2 and NO2 , which are contributing to the improvement of gas sensor. Therefore, we can draw a conclusion that the synergistic effect of three influencing factors, including
Fig. 6. Schematic diagrams of the possible conductive channels and the equivalent circuits for the laminated configuration T-CuO (a–b) and T-ZnO (c–d).
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
Fig. 7. The response of the plain CuO and the laminated heterostructures upon exposure to NO2 at different operating temperatures.
the direction of Ei , large valance band offset (Ev = 1.69 eV) and the electrode position of external circuit, is in favor of the charge separation smoothly. With regards to T-CuO sensor, as shown in Fig. 8c, the holes in VB of ZnO can still transfer into CuO due to the appropriate energy band matching, which broadens the hole accumulation layer on the side of CuO and result in the decrease of resistance. Nevertheless, as depicted in Fig. 6a, the Au electrodes are covered by the bottom ZnO layer with high resistance. Under the strong drawing force of external electric field, the carriers are obliged to pass across the p–n interfaces and eventually flow to the external circuit. Comparing
505
with the T-ZnO system, the lower efficiency of charge separation is obtained from T-CuO by reason of the hindering effect of p–n space charge layer. But even so, for T-CuO sensor, the feeble effect of the charge separation has been also appeared and the results of enhanced performance are observed in Table 1. Besides, for T-CuO porous architecture, the gas molecules can diffuse into the under layered ZnO film, but the micro-hole structure still has an obvious hindering effect of the gas–solid interactions and further decreases the amount of generating holes on ZnO surface. This is the other reason for the result of k2 < k1 in Table 1. From what has been argued above, we can conclude that the charge separation behavior stems from the energy band matching of the laminated CuO/ZnO heterostructure with the assistance of the external circuit, and the relative position of the Au electrodes decides the efficiency of charge separation. Although the channels of carrier transport are dominated by low-resistance CuO layer, the effect of high-resistance ZnO layer should not be ignored since it can contribute abundant holes and then transfer into CuO layer to achieve the unidirectional charge separation in CuO/ZnO heterostructured configuration. Therefore, it seems that the design of T-ZnO is a desired configuration to enhance the gas sensing performance of detecting NO2 gas. In brief, on the basis of the experimental results, in order to further understand the features of laminated heterostructured configuration, three functional layers are summarized as follows: (1) n-Typed ZnO layer as a hole donor by means of the interaction with the oxidizing O2 and NO2 gas molecules. It provides extra holes in VB of ZnO and the holes can be separated in time into p-typed CuO layer. (2) p-Typed CuO layer as the hole generator and acceptor, which dominates the conduction process under different gas
Fig. 8. Schematic diagrams illustrate the constructed energy band of the CuO/ZnO p–n heterojunction under thermal equilibrium condition (a) and the charge separation effect of the laminated T-CuO (b) and T-ZnO (c).
506
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
atmosphere. It is an important medium to transfer smoothly the holes into the external circuit. (3) Au electrode as an impeller provides strong external drawing force to accelerate the unidirectional separation of holes. The relative position can influence the efficiency of charge separation. 3.4. “Off overshoot” phenomenon and its possible mechanism
typed ZnO results from the oxygen vacancy VO•• as a donor defect level, which lead to the deviation from the stoichiometry [36]. Just because of this, the hole for CuO and the electron for ZnO are the so-called majority carrier, which dominates the conductivity of semiconductor materials. When the sensors are exposed to dry air, the oxygen molecular adsorbs on the surface of ZnO and CuO with the exchange of electrons. The process can be described by the defect equations as the following. For ZnO sensing materials, the gaseous O2 prefers to adsorb at the site of oxygen vacancy, which can be described as
VO•• +
1 − + e− → VO•• − O(ads) 2 O2(g)
(1)
wherein “g” and “ads” refer, respectively, to gas and adsorbate, VO•• represents the oxygen vacancy with two positive charges. Consequently, for ZnO surface, an electron depletion layer is built, which gives rise to the decrease of the electron density of conduction band, and further to the result of high surface resistance than the bulk, the schematic diagram is shown in Fig. 5c. Similarly, for CuO sensing layer, the gaseous O2 also prefers to adsorb at the site of copper vacancy and the relevant defect equation can be written as
VCu +
1 − → VCu − O(ads) 2 O2
− VO•• + NO2(g) + e− → VO•• − NO2(ads)
(3)
− VO•• − O(ads) + VO•• − NO2(gas) + e−
From what has been described above, an inevitable question is posed: what is causing the “off overshoot” phenomenon? To address the question, we attempt to propose a plausible mechanism on the basis of defeat equations. It is generally considered that the defects and interfaces with high activity for the adsorption/desorption or chemical reaction, and the different active sites provide an opportunity for different gas species, which means the active sites has a special selectivity for the adsorbed gas [14,34,35]. In this paper, the sensing process of the sensors towards 29 ppm NO2 is divided into three stages: (1) initial period in dry air, (2) response period with NO2 injection and (3) recovery period with NO2 cutting off. The detailed interpretations are as follows. (1) Initial period in dry air. In this work, the conductive types are confirmed as p-type for CuO and n-type for ZnO in the light of the response mode. Generally speaking, p-typed CuO origins from the copper vacancy VCu as an acceptor defect level and the n-
species, leaving an active defect. Afterwards, the exposed defect site is occupied by adsorbed O2 again with an electron transferring from MOS to adsorbed O2 . More specifically speaking, on the one hand, for ZnO sensing films, the interaction processes can be depicted as
(2)
wherein VCu , h+ represents, respectively, the copper vacancy with two negative charges and hole. According to Eq. (2), a hole accumulation space charge layer exists on the surface of CuO, which indicates the increase of the hole density of valence band and ultimately result of low surface resistance, the schematic diagram is shown in Fig. 5a. (2) Response period with NO2 injection. NO2 is an oxidizing gas and therefore traps free electrons from the MOS surface resulting in a decrease (n-typed ZnO) or increase (p-typed CuO) in its conductivity. As discussed in other work, we believe that NO2 gas molecules interact preferentially with the available defect sites instead of reacting with the adsorbed O− [34,37]. Thereby, NO2 gas molecules capture electrons from MOS unoccupied defects result in a variation of sensor resistance. After all of the effective defects are occupied by NO2 gas molecules, there will be valid for a reaction with − − − the adsorbed NO− 2 and neighboring O to produce O − NO2(ads)
→ O− − VO•• − NO2−(ads) + VO•• +
1 2 O2(gas)
(4)
It is found that the thickness of electron depletion layer has further become wider with the electron transference from the conduction band to adsorbates, and in turn the electric resistance is escalated. This coincides with the gas sensing response experimental results shown in Fig. 3a. On the other hand, for the CuO sensing films, the NO2 gas will also attack preferentially the copper vacancy defeats and then interact with the adsorbed O− . However, the electron affinity CuO (−0.13 eV) > ZnO (−4.35 eV) will easily obtain the nitrite specie on the CuO surface [34,36]. The corresponding reaction equations are expressed by
− VCu + NO2(g) → VCu − NO2(ads) + h+
(5)
− − VCu − O(ads) + VCu − NO2(ads)
− → O− − VCu − NO2(ads) + VCu +
1 + 2 h+ 2 O2
− VCu + NO2 → VCu − ONO(ads) + h+
(6) (7)
As can be seen, Eqs. ((5)–(7)) increase the thickness of the hole accumulation layer and result in the decrease of the resistance, which also agrees well with the result in Fig. 3b. (3) Recovery period with NO2 off. In this stage, the gas atmosphere is switched from 29 ppm NO2 to dry air by means of MFC. Therefore, the adsorbed NO2 or its evolutive species on the surface will be desorbed. For ZnO sensing layer, the NO2 desorption reactions can be written as − O− − VO•• − NO2(ads) → VO•• + NO2(gas) + − VO•• − NO2(ads) → VO•• + NO2(g) + e−
1 + 2 e− 2 O2(gas)
(8) (9)
We can see that two electrons are released back to the conduction band (CB) by Eq. (8), simultaneously, a single electron is also released by Eq. (9). According to the limitation condition of Eq. (4), the limited quantity of O− − VO•• − NO− 2 exists on the ZnO surface, which decides the double electrons releasing is an unendurable process. Just because of this, we believe that the combining effect of Eqs. (8) and (9) are responsible for the “off overshoot” when NO2 is cut off. Once the limited O− − VO•• − NO− 2 species are consumed completely, Eq. (8) is invalid, which result in the disappearance of overshoot phenomenon. Afterwards, as observed in Fig. 3a, the stable resistance is only dominated by Eq. (9). Perhaps the observed off-overshoot phenomenon is attributed to a continuous conversion from double electrons to single electron. Similar analysis approach can apply to CuO sensing film, the obtained − − and VCu − NO2(ads) species in Eqs. (5) and (6) O− − VCu − NO2(ads) will desorb by adopting the methods, which is similar to Eqs. (8) and (9). The desorption process decreases the width of the hole accumulation layer and finally results in the off-overshoot behavior. the However, other important transformation from the nitroso VCu − ONO− to the nitrosyl O− − VCu − NO+ should not ignore [34,38], as shown in Eq. (10). In addition, as manifested by
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
Eqs. ((11)–(12)), because of a weak bond of nitrosyl, –NO+ escapes easily from CuO surface and finally reacts with the adsorbed O− . Here, we notice that the electron release to the CuO conduction band via Eq. (10) and the positive charges escape from the CuO surface via Eq. (11), which contributes to the diminution of the hole accumulation layer. The equations are written as the followings.
− + VCu − ONO(ads) → O− − VCu − NO(ads) + e−
O
−
+ − VCu − NO(ads)
→
− VCu − O(ads)
+ NO
(10)
+
(11)
NO+ + O− → NO2 (g)
[8]
[9] [10]
(12) [11]
So after, the “off overshoot” phenomenon of CuO origins from − , the following three processes: desorption of O− − VCu − NO2(ads)
[7]
− desorption of VCu − NO2(ads) and the decomposition of VCu − ONO− . In summary, it is plausible to conclude that the underlying mechanism of the “off overshoot” for n-type ZnO and p-type CuO. With regards to ZnO, the “off overshoot” phenomenon can be ascribed to the predominated transformation reactions from the double electrons to the single electron. As regards to CuO, however, apart from the aforementioned similar reactions, the evolution of VCu − ONO− species has an important contribution by means of adjusting the width of the hole accumulation layer.
4. Conclusion In the presented work, we reported a novel perspective for the investigation of heterostructured configuration in the field of gas sensor. An ordered laminated CuO/ZnO was designed for detection of NO2 gas. The laminated configuration remarkably enhanced NO2 gas responses compared to the plain samples. A plausible carrier transport mechanism was firstly proposed from the perspective of charge separation with the assistance of external circuit. The experimental results suggested the T-ZnO heterostructrue was a desired design because the charge separation was operated smoothly. Furthermore, we noticed the abnormal “off overshoot” phenomenon and further argued its formation mechanism by using defect chemistry theory. It was found that the abnormal response behaviors resulted from the transformation of the double electrons to single electron reactions once the NO2 gas was cut off. The analysis approach could apply to the laminated p–n heterostructure configuration.
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
Acknowledgments [25]
This work was supported by Nature Science Foundation of China (Nos. 51204072 and Nos. 50927201), the National Basic Research Program of China (Grant Nos. 2009CB939705 and 2009CB939702). The authors are also grateful to Analytical and Testing Center of Huazhong University of Science and Technology.
[26] [27]
[28]
References [29] [1] M.T. Lerdau, J.W. Munger, D.J. Jacob, The NO2 flux conundrum, Science 289 (2000) 2291–2293. [2] G.V. Zyryanov, Y. Kang, D.M. Rudkevich, Sensing and fixation of NO2 /N2 O4 by calix[4]arenes, Journal of the American Chemical Society 125 (2003) 2997–3007. [3] A. Geupel, D. Schönauer, U. Röder-Roith, D.J. Kubinski, S. Mulla, T.H. Ballinger, H. Chen, J.H. Visser, R. Moos, Integrating nitrogen oxide sensor: a novel concept for measuring low concentrations in the exhaust gas, Sensors and Actuators B: Chemical 145 (2010) 756–761. [4] A. Groß, M. Kremling, I. Marr, D.J. Kubinski, J.H. Visser, R. Tuller, H.L. Moos, Dosimeter-type NOx sensing properties of KMnO4 and its electrical conductivity during temperature programmed desorption, Sensors 13 (2013) 4428–4449. [5] A. Groß, G. Beulertz, I. Marr, D.J. Kubinski, R. Visser, J.H. Moos, Dual mode NOx sensor: measuring both the accumulated amount and instantaneous level at low concentrations, Sensors 12 (2012) 2831–2850. [6] S. Fischer, R. Pohle, B. Farber, R. Proch, J. Kaniuk, M. Fleischer, R. Moos, Method for detection of NOx in exhaust gases by pulsed discharge measurements using
[30]
[31]
[32]
[33] [34]
[35]
507
standard zirconia-based lambda sensors, Sensors and Actuators B: Chemical 147 (2010) 780–785. C. Li, S.P. Zhang, M.L. Hu, C.S. Xie, Nanostructural ZnO based coplanar gas sensor arrays from the injection of metal chloride solutions: Device processing, gassensing properties and selectivity in liquors applications, Sensors and Actuators B: Chemical 153 (2011) 415–420. C.S. Xie, L.Q. Xiao, M.L. Hu, Z.K. Bai, X.P. Xia, D.W. Zeng, Fabrication and formaldehyde gas-sensing property of ZnO–MnO2 coplanar gas sensor arrays, Sensors and Actuators B: Chemical 151 (2010) 457–463. J. Kim, K. Yong, Mechanism study of ZnO nanorod-bundle sensors for H2 S gas sensing, Journal of Physical Chemistry C 115 (2011) 7218–7224. G.X. Zhu, H. Xu, Y.Y. Xiao, Y.J. Liu, A.H. Yuan, X.P. Shen, Facile fabrication and enhanced sensing properties of hierarchically porous CuO architectures, ACS Applied Materials & Interfaces 4 (2012) 744–751. Y.V. Kaneti, J. Yue, X.C. Jiang, A.B. Yu, Controllable synthesis of ZnO nanoflakes with exposed (1 0 1 0) for enhanced gas sensing performance, Journal of Physical Chemistry C 117 (2013) 13153–13162. C.Q. Ge, C.S. Xie, S.Z. Cai, Preparation and gas-sensing properties of Ce-doped ZnO thin-film sensors by dip-coating, Materials Science and Engineering B 137 (2007) 53–58. G.Z. Zhang, S.P. Zhang, L. Yang, Z.J. Zou, D.W. Zeng, C.S. Xie, La2 O3 -sensitized SnO2 nanocrystalline porous film gas sensors and sensing mechanism toward formaldehyde, Sensors and Actuators B Chemical 188 (2013) 137–146. M.R. Yu, R.J. Wu, M. Chavali, Effect of “Pt” loading in ZnO–CuO hetero-junction material sensing carbon monoxide at room temperature, Sensors and Actuators B: Chemical 153 (2011) 321–328. S.J. Kim, C.W. Na, I.S. Hwang, J.H. Lee, One-pot hydrothermal synthesis of CuO–ZnO composite hollow spheres for selective H2 S detection, Sensors and Actuators B: Chemical 168 (2012) 83–89. J.R. Huang, Y.J. Dai, C.P. Gu, Y.F. Sun, J.H. Liu, Preparation of porous flower-like CuO/ZnO nanostructures and analysis of their gas-sensing property, Journal of Alloys and Compounds 575 (2013) 115–122. Q. Simon, D. Barreca, A. Gasparotto, C. Maccato, E. Tondello, C. Sada, E. Comini, G. Sberveglieri, M. Banerjee, K. Xu, CuO/ZnO nanocomposite gas sensors developed by a plasma-assisted route, ChemPhysChem 13 (2012) 2342–2348. Y. Liu, C.S. Xie, H.Y. Li, H. Chen, T. Zou, D.W. Zeng, Improvement of gaseous pollutant photocatalysis with WO3 /TiO2 heterojunctional-electrical layered system, Journal of Hazardous Materials 196 (2011) 52–58. L.G. Kumar, S.G. Devi, Review on modified TiO2 photocatalysis under UV/visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics, Journal of Physical Chemistry A 115 (2011) 13211–13241. S.S. Zhang, C.S. Xie, Z.J. Zou, L. Yang, H.Y. Li, S.P. Zhang, High photoconductive response of gas-sensitized porous nanocrystalline TiO2 film in formaldehyde ambience and carrier transport kinetics, Journal of Physical Chemistry C 116 (2012) 19673–19681. L. Liu, S.C. Li, L.Y. Wang, C.C. Guo, Q.Y. Dong, W. Li, Enhancement ethanol sensing properties of NiO-SnO2 nanofibers, Journal of the American Ceramic Society 94 (2011) 771–775. L. Yang, S.P. Zhang, G.Z. Zhang, S.S. Zhang, H.Y. Li, C.S. Xie, Specially environmental responses induced by multi-field coupling for nanocrystalline SnO2 porous film as gas sensor, Sensors and Actuators B: Chemical 182 (2013) 239–249. L. Yang, S.P. Zhang, H.Y. Li, G.Z. Zhang, C.Y. Zhan, C.S. Xie, Conduction model of coupled domination by bias and neck for porous films as gas sensor, Sensors and Actuators B: Chemical 176 (2013) 217–224. A. Zainelabdin, G. Amin, S. Zaman, O. Nur, J. Lu, L. Hultman, M. Willander, CuO/ZnO nanocorals synthesis via hydrothermal technique: growth mechanism and their application as humidity sensor, Journal of Materials Chemistry 22 (2012) 11583–11590. M.R. Yu, G. Suyambrakasam, R.J. Wu, M. Chavali, Performance evaluation of ZnO–CuO hetero junction solid state room temperature ethanol sensor, Materials Research Bulletin 47 (2012) 1713–1718. E.K. Liu, B.S. Zhu, J.J. Luo, Semiconductor Physics, seventh ed., Electronics Industry Publication, Beijing, China, 2009. N. Barsan, C. Simion, T. Heine, S. Pokhrel, U. Weimar, Modeling of sensing and transduction for p-type semiconducting metal oxide based gas sensors, Journal of Electroceramics 25 (2010) 11–19. N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, Journal of Electroceramics 7 (2001) 143–167. Y. Yamada, M. Ogita, Transient response of resistive-type NO2 sensor on temperature change, Sensors and Actuators B: Chemical 93 (2003) 546–551. P. Rai, W.K. Kwak, Y.T. Yu, Solvothermal synthesis of ZnO nanostructures and their morphology-dependent gas-sensing properties, ACS Applied Materials & Interfaces 5 (2013) 3026–3032. Y. Liu, C.S. Xie, H.Y. Li, H. Chen, Y.C. Liao, D.W. Zeng, Low bias photoelectrocatalytic (PEC) performance for organic vapour degradation using TiO2 /WO3 nanocomposite, Applied Catalysis B: Environmental 102 (2011) 157–162. A. Zainelabdin, S. Zaman, G. Amin, O. NurM. Willander, Optical and current transport properties of CuO/ZnO nanocoral p–n heterostructure hydrothermally synthesized at low temperature, Applied Physics A 108 (2012) 921–928. P.S. Nayar, Properties of zinc phosphide/zinc oxide heterojunctions, Journal of Applied Physics 53 (1982) 1069–1075. A. Sharma, M. TomarV. Gupta, WO3 nanoclusters–SnO2 film gas sensor heterostructure with enhanced response for NO2 , Sensors and Actuators B: Chemical 176 (2013) 675–684. R.I. Masel, Principles of adsorption and reaction on solid surfaces, first ed., John Wiley & Sons, Inc., New York, NY, 1996, pp. 438–476.
508
L. Yang et al. / Sensors and Actuators B 195 (2014) 500–508
[36] R. Dong, D.S. Lee, W.F. Xiang, S.J. Oh, D.J. Seong, S.H. Heo, H.J. Choi, M.J. Kwon, S.N. Seo, M.B. Pyun, Reproducible hysteresis and resistive switching in metal–CuO–metal heterostructures, Applied Physics Letters 90 (2007) 42107. [37] B. Ruhland, T. Becker, G. Müller, Gas-kinetic interactions of nitrous oxides with SnO2 surfaces, Sensors and Actuators B: Chemical 50 (1998) 85–94. [38] L.G. Teoh, Y.M. Hon, J. Shieh, W.H. Lai, M.H. Hon, Sensitivity properties of a novel NO2 gas sensor based on mesoporous WO3 thin film, Sensors and Actuators B: Chemical 96 (2003) 219–225.
Biographies Li Yang received his M.Sc. from China University of Geosciences in 2010 on material science and chemistry. Currently he is working on gas sensors as a Ph.D. student in Huazhong University of Science and Technology. Changsheng Xie received his Ph.D. from Huazhong University of Science and Technology in 1985 on material science and engineering. He is Professor of Material Science and Engineering at Huazhong University of Science and Technology. His research interests include the synthesizing of nanomaterials and electronic noses.
Guozhu Zhang received his B.Sc. from Wuhan Institute of Technology in 2010 on material science and engineering. Currently he is working on gas sensors as a Ph.D. student in Huazhong University of Science and Technology. Jianwei Zhao received his B.Sc. from Liaocheng University in 2012 on material science and engineering. Currently he is working on gas sensors as a master student in Huazhong University of Science and Technology. Xueli Yu received his M.Sc. from Huazhong Normal University in 2006 on condensed matter physics. Currently he is working on gas sensors as a Ph.D. student in Huazhong University of Science and Technology. Dawen Zeng received his Ph.D. from Huazhong University of Science and Technology in 1998 on materials science and engineering. He is a professor of Materials Science and Engineering at the same university. His research interests include the synthesizing, characterizing of inorganic functional materials. Shunping Zhang received his Ph.D. from Huazhong University of Science and Technology in 2009 on material science and engineering. He is a lecturer of Materials Science and Engineering at Huazhong University of Science and Technology. His research is on the electronic noses.