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Outstanding gas sensing performance of CuO-CNTs nanocomposite based on asymmetrical schottky junctions
Applied Surface Science 428 (2018) 415–421
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Outstanding gas sensing performance of CuO-CNTs nanocomposite based on asymmetrical schottky junctions Yiming Zhao a , Muhammad Ikram a , Jiawei Zhang b,∗ , Kan Kan a , Hongyuan Wu a , Wanzhen Song a , Li Li a , Keying Shi a,∗ a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Key Laboratory of Physical Chemistry, School of Chemistry and Material Science, Heilongjiang University, Harbin, 150080, PR China b Key Laboratory of Chemical Engineering Process & Technology for High-efficiency Conversion, School of Chemistry and Material Science, Heilongjiang University, Harbin 150080, PR China
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Article history: Received 1 July 2017 Received in revised form 13 September 2017 Accepted 20 September 2017 Keywords: 1D CuO-CNTs composite Carbon nanotubes (CNTs) Nitrogen dioxide (NO2 ) Gas sensor
a b s t r a c t To fabricate a high-performance material for sensor devices at room temperature and further improve the synthetic approach of sensing materials, one dimensional (1D) CuO-CNTs nanocomposites were prepared with CNTs and CuO nanorods (NRs) via a facile reflux method. The 1D composite with the molar ratio of CuO and CNTs at 2.4:1 displays excellent gas sensing performance, i.e. the lowest detectable limit of 970 ppb and the short response time of 6 s–97.0 ppm NO2 at room temperature. In the 1D composite, the CNTs part provides a channel to enable effective and fast carrier transport, while the CuO NRs fabricates an asymmetrical schottky contact at the interface between the composites and the Au electrode. The advantage of the synergy of CNTs and CuO which possesses superior conductivity benefits the sensing of our 1D CuO-CNTs composite by providing affluent electrons. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Industrial development in the past decades has enormously increased the amount of toxic gases in the atmosphere, which results in dramatic consequences for both the environment and public health [1,2]. Among various air pollutants, the noxious gases such as NO and NO2 (formulated as NOx ) are highly toxic. When the levels of NOx are above 1 ppm, it causes serious diseases in humans body such as damaging respiratory system, lung tissues and heart disease [3]. The NO2 gas also plays a major role in photochemical smog, ozone depletion and acid rain, causing pollution and demaging water sources [4–6]. Consequently, developing senor devices that detect NOx gas at room temperature is a popular research direction in the filed of gas sensing [7]. Generally, good sensing performance can be achieved while materials have merits including uniform distribution, excellent morphology and nanostructures with large surface area to volume ratio. The manipulatable carrier in charge transport channel is also one of the greatest promises for gas sensing device (GSD). Most of GSDs reported are consisting of only single one dimensional
∗ Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (K. Shi). http://dx.doi.org/10.1016/j.apsusc.2017.09.173 0169-4332/© 2017 Elsevier B.V. All rights reserved.
(1D) functional material mounted at each two fingers of an electrode. The two contact ends are usually designed to have ohmic characteristic to enhance conductance and minimize the resistance of carriers that flow across the contacts; however, it is not good enough for commercial applications due to the low response of gas sensors. Wang et al. reported a non-symmetrical schottky contact of GSD with one end in ohmic contact and the other end in schottky contact. This featured structure can greatly enhance the GSD response through adjusting the electrical conductivity [8]. CNTs possess a large surface area to volume ratio, which would benefit to improve the gas response. Moreover, the proportion of 1D structure radii and Debye length offers greater potential in sensing performance [9–12]. It, however, is difficult for a single CNTs-based gas sensor to form a non-symmetrical schottky contact of GSD. To overcome this drawback, CNTs composite sensors have been explored for 1D well-designed nanostructures. In recent years, many efforts have been devoted to modifying functional CNTs with semiconductor metal oxides (SMOs) such as ZnO, In2 O3 , SnO2 and CuO [13–16]. These composites have been successfully used for NOx detection [17–20]. Among SMOs, CuO is increasingly attractive for gas-sensing application because of its p-type semiconductivity, high catalytic activity, narrow band gap (1.2 eV in bulk), fast redox ability [21], high-rate capability [22], abundant availability, non-toxic nature and low production cost
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[23–26]. The 1D CuO nanostructures can serve as significant building blocks for nanodevices and integrated nanosystems [17]. The electroconductive limitation of the nanostructure may attribute to intrisic defects/vacanies [27]. Therefore, it is desirable to use 1D CuO nanostructures as an additive for appropriate electrical conductivity to improve gas sensing properties. So far, the CuO NRs/nanowires have been investigated for sensing performance in the range of 200–400 ◦ C as a working temperature [28–32]. A CuOCNTs thin film has been synthesized to sense ethanol, showing high working temperature (400 ◦ C) and low response time [33]. Given above difficulties, it may be more convenient by introducing a conductive material or an electronic channel network, which would fasten response time and lower working temperature. Herein, the 1D CuO-CNTs composites have been synthesized by a facile reflux technique. The small CuO NRs were modified on the exterior surface of CNTs. And then, electronic channel network was fabricated by the connection of the high conductive channel of CNTs with the low conductive channel of CuO NRs. The Ohmic/Schottky contacts between the 1D CuO-CNT composites and the Au electrode were adjusted by thermal treatment (based on the device current-voltage characterization (Ids -Vds )). The 1D CuO-CNTs in gas sensing may show some interesting characteristics: (i) the highly conductive CNTs part may improve the conductivity of whole composite; (ii) the diameters of CuO NRs which fall within Debye length La (3.74 nm) and 2La may facilitating the defects/vacancies creation; (iii) the CuO NRs may modify the microstructures of the 1D CuO-CNTs composite and alter the electrical conductivity, which together might benefit for the formation of asymmetrical schottky contacts with Au electrode. Therefore, the 1D CuO-CNTs sensor to NO2 gas displays excellent sensing properties with a high response and a short response time at room temparature.
2. Experimental section 2.1. Hydroxylation of CNTs All the chemicals were of analytical grade and used as-received without further purification. (Multi-walled) CNTs were purchased from Shenzhen Nanotech Port Co. Ltd, with purity of >97.0%. Specific amount of CNTs was functionalized by 100.0 mL of mixed acids (HNO3 :H2 SO4 in 1:3 (v/v) ratio) under constant stirring for 30 min and then ultrasonication for 4 h and finally filtered and washed with deionized water until the pH reached 7. The washed sample were dried at 60 ◦ C for 12 h and the final product of hydroxylated CNTs were obtained.
2.2. Synthesis of 1D CuO-CNTs composites The hydroxylated CNTs of different concentrations i.e. (0.005 g, 0.010 g, 0.015 g) were dispersed in 49.0 mL N,N-Dimethyl Formamide (DMF) and 21.0 mL deionized water. The solution was ultrasonic for 20 min, and heated up to 60 ◦ C, and then 0.40 g Cu(CH3 COO)2 ·H2 O and 1.46 g Cetyltrimethylammonium Bromide (CTAB) were added and stirred vigorously. Later, 0.40 g NaOH was added to the solution, being retained at 60 ◦ C and refluxed for 15 min. Finally, the solution was cooled down to RT for filtration. After washing with ethanol and deionized water, the samples were dried at 50 ◦ C in a vacuum oven. The as-prepared 1D CuO-CNTs composites with (theoretical) CuO:CNTs molar ratio of 4.8:1, 2.4:1 and 1.6:1 were named as CuO-CNT-1, CuO-CNT-2 and CuO-CNT-3, respectively. The components of the samples were listed in Table S1.
2.3. Apparatus Thermo gravimetric-differential scanning calorimetry (TG-DSC) analysis of the samples was performed by TA-SDTQ600. The crystal phase of the samples was characterized by X-ray powder diffraction (XRD, D/MAX-III-B-40 KV, Japan, Cu-K␣ radiation, = 1.5406 Å). The Brunauer-Emmett-Teller (BET) surface area of the products were measured by N2 adsorption-desorption (TriStar II 3020). The surface morphology of the composites was characterized by scanning electron microscopy (SEM, HITACHI S-4800). Transmission electron microscopy (TEM, JEOL-2100) was performed to inspect the structures of the composites. X-ray photoelectron spectroscopy (XPS) was carried out using AXIS ULPRA DLD (Shimadzu Corporation) system to analyze the adsorbed oxygen species in the samples. Electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) plot measurements were carried out by using an electrochemical working station (CHI660C, Shanghai, China) in a half-cell setup configuration at RT. In the EIS measurement, the range of frequency was 0.01 Hz–100 kHz, and the excitation amplitude was 4 mV. Current-voltage characteristics (Ids -Vds ) was performed on 1D CuO-CNT device using a Keithley 2602 source meter. Ids -Vds curves of the 1D CuO-CNT device was measured (the gate voltage Vg = 0.1 eV).
2.4. Gas sensing tests 1.0 mg for each set of 1D CuO-CNT composite was evenly dispersed in 1.0 mL of deionized water to prepare a suspension. A 0.05 mL suspension was dropped onto the interdigitated Au electrode (the width of Au fingers was about 2 m). The sensor was treated at 250 ◦ C (or 300 ◦ C and 400 ◦ C) in Ar for 1 h and a CNT based gas sensor was obtained. The sensor was mounted into a test chamber with an inlet and an outlet. The chamber was flushed with air for 2 min to remove any contaminants from the flask and also to stabilize the film before testing. Syringe was used to inject the required volume of NO2 gas (99.8%, NO < 0.2%) into the test chamber. Then, NO2 as the detected gas would come into contact with the surface of the samples. The changes in the electrical resistance of the samples were recorded by a home-made automatic resistance apparatus over time. Finally, the sensor resietance was recovered by purging the chamber with air. The electrical resistance measurements of the sensor were carried out at RT with a relative humidity of (RH) around 26.0%. The sensor response was defined as the ratio (R0 -RN )/R0 , where R0 and RN were the resistances of the sensor in air and NO2 gas, respectively. The response time was defined as the time required for the variation in resistance to reach 90% of the equilibrium value after a test gas was injected. The material was tested to respond to various detecting gases such as NO2 , H2 S, NH3 , CO or H2 , and the highest response (to a kind of gas) signified a good sensing selectivity of the material.
3. Results and discussion 3.1. Composition and morphology TG-DSC analysis was displayed in Fig. 1. As seen from the TG-DSC curves of CNTs (Fig. 1a), the mass ratio of CNTs begins to decrease at about 460 ◦ C and sharply decreases in going from 570 ◦ C to 685 ◦ C. For CuO-CNT-1, CuO-CNT-2, and CuO-CNT-3 (the calculated molar ratios of CuO:CNTs are 4.8:1, 2.4:1 and 1.6:1, respectively), the decreased mass was attributed to consumption of CNTs in the range of temperatures from 240 ◦ C to 550 ◦ C (Fig. 1b). The change indicated that the interaction between CNT and CuO was stronger.
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Fig. 1. TG-DSC Analysis at 10 ◦ C min−1 in air: (a) CNTs; (b) CuO-CNTs composites.
The XRD patterns of CNTs and pure CuO were shown in Fig. S1. For CNTs, two significantly intense peaks at 26.0◦ and 42.2◦ were observed, which were assigned to the graphite structure (002) and (100) planes, respectively (JCPDS card no.75-1621). In addition, the reflections along (110), (002), (111), (-202) and (-311) etc. arose from the monoclinic phase CuO, in agreement with the standard data of CuO (JCPDS card no. 89-2530) [34]. Fig. 2a illustrated the powder XRD patterns of the CuO-CNTs composites. The peaks observed at 32.2◦ , 35.5◦ , 38.5◦ , 48.7◦ and 66.1◦ were well matched with the (110), (002), (111), (−202) and (−311) crystal planes of CuO. For CuO-CNT-1 and CuO-CNT-2 composites, no CNTs diffraction peaks were observed due to its low amount. A weak diffraction peak of CNTs was shown in CuO-CNT-3 composite. Peaks of metallic Cu and Cu2 O were not found, indicating that the as-synthesized CuO-CNTs nanocomposites are highly pure. XRD results illustrated that the composites consisted of CNTs and well crystallized CuO. Fig. 2b presented N2 adsorption-desorption isotherms and BET pore-size distribution of CuO-CNT-2 composite. The curves exhibited a IV type isotherm, which is regarded as the typical characteristic of mesoporous materials. The BET surface area was determined to be 94.2 m2 g−1 . The pores were distributed uniformly and the dominant mesopore in size was about 9 nm. SEM and TEM images of pristine CuO NRs were shown in Fig. S2. The CuO NRs were about 100–200 nm in length and 3–7 nm in width. Four batches of 1D CuO-CNTs composites were prepared under the same condition and characterized in Fig. S3. For the CuOCNT-1 composite, the low magnification TEM images showed that the flowerlike CuO architectures were aggregated on the surface of CNTs. There were relatively high quantity of CuO NRs that covered on the outside surface of CNTs, while only a few CuO NRs were found for the CuO-CNT-3 composite. The composite morphologies were relatively stable in different batches. The insert image in Fig. 3a was the low magnification TEM image of the CuO-CNT-2 composite. One can see that CuO NRs modify the exterior surface of CNTs. The inner and outer diameters of CNTs about 8 nm and 20 nm, respectively, were clearly observed
Fig. 2. (a) XRD diffraction patterns of CuO-CNT-1, CuO-CNT-2 and CuO-CNT-3 composites; (b) Nitrogen adsorption-desorption isotherms and pore-size distribution curve of CuO-CNT-2 composite.
in Fig. 3a. The CNTs owns seventeen walls and CuO NRs adhere to the exterior surface of the CNTs. High magnification TEM (HRTEM) images in Fig. 3(b–e) suggested that CuO NRs around 3 nm in size were obtained. The interplanar spacings of CuO NRs were measured to be about 2.78, 2.52 (2.53), 2.33 (2.36) and 2.27 (2.30) Å, corresponding to the (110), (002), (111) and (200) planes of CuO, respectively. In addition, the (002) plane of CNTs could be observed, reflected by the lattice spacing of 3.52 (3.75) Å. Moreover, it was the blue line that separated CuO NRs from CNTs. To study the electrical characteristics of the 1D CuO-CNTs composites, Mott-Schottky (MS) and Electrochemical Impedance Spectroscopy (EIS) measurements were carried out. The conduction types of the samples were determined using MS measurements. The obtained results were shown in Fig. S4. The MS curves of the samples were approximately linear. The negative slopes of MS plots indicated a p-type semiconducting behavior for all the samples. This was in accordance with the results of the dynamic responserecovery curves of sensors. As expected, the resistance and electron transportation of materials plays an important role in gas sensing response. Thus, the electron transportation ability of samples was studied. The impedance parameters (R and Rct ) and carrier density (Na ) of samples were listed in Table S2 and Table S3, respectively. The results revealed that CNTs (Na = 1.06 × 1019 ) had much higher carrier (positive hole) density than CuO (Na = 7.77 × 1017 ). Hence, CNTs could easily capture electrons that migrated from the conduction band of CuO [20]. Moreover, the function slope of the ImZ (the imaginary part of the CuO-CNT-2 impedance) and (angular frequency) was 0.52, which was close to −1/2 (see the inset of lower right corner in Fig. S4b). This confirmed that the CuO-CNT-2 sample indeed possessed highly abundant porous structures [35], which could improve gas transmission channel from its interior the surface. In addition, Debye length (La ) of the sample was 3.74 nm, and the related detailed information is given in Supporting Information.
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Fig. 3. TEM images of CuO-CNTs composite. (a) TEM image of the open end of CNT, the inset is low magnification TEM image of the CuO-CNT-2 composite; (b) HRTEM image of (a); (c), (d), (e) HRTEM image of the inset (a), many defects existed in the multijunctions/interfaces between CuO NRs and CNT.
Fig. 4. XPS spectra of O 1s: (a) CuO, (b) CuO-CNT-2, (c) CuO-CNT-3 and C 1s: (d) CuO-CNT-2, (e) CuO-CNT-3; (f) Valence band of CuO and CuO-CNT-2.
The XPS spectra of CNTs, CuO, CuO-CNT-1, CuO-CNT-2 and CuOCNT-3 were shown in Fig. 4 and Fig. S5, together with the binding energy of O peaks and C peaks in Table S4. As seen in Fig. 4(a–c) and Fig. S5c, the high-resolution O 1s spectra of all the samples revealed three separating peaks at ∼529.5 ± 0.1, ∼531.1 ± 0.3 and ∼532.5 ± 0.4 eV, which were attributed to crystal lattice O2− (Peak 1), oxygen defects/vacancies (Peak 2), and the near-surface oxygen surface hydroxylation, adsorbed H2 O, or O2 (Peak 3), respectively [36,37]. The calculated oxygen defects/vacancies peak area ratios were about 36.8% for CuO, 37.1% for CuO-CNT-1, 39.0% for CuO-CNT-2 and 38.3% for CuO-CNT-3, respectively. Obviously, the oxygen defects/vacancies peak area of the CuO-CNT-2 sample was the largest. Because the oxygen defects/vacancies were taken as shallow donors, more electrons were suggested be available in the CuO NRs of the CuO-CNT-2 composite. The C 1s specta of the CNTs (Fig. S5b) presented a main peak at 284.5 eV arising from the graphite-like carbon atoms of CNTs [38]. The peak at 285.8 eV was attributed to the defects in CNTs structure [39], while ones at 288.9 eV corresponded to O C O [40]. Fig. 4(d-e) and Fig. S5d showed the C 1s spectra of CuO-CNT-2, CuOCNT-3 and CuO-CNT-1 composites. Among them, the carbon defect
peak area ratio of the CuO-CNT-2 sample was the smallest, implying the minimum free electrons in its CNTs part. Hence, CNTs captured electrons from the conduction band of CuO, giving rise to highly reduced CNTs. The valence-band spectra were shown in Fig. 4f. The top peak of the valence band of CuO-CNT-2 shifted to the lower binding-energy side (0.7 eV) compared to the pristine CuO. This resulted from the interaction CuO NRs with CNTs. The intense peak positioned at 3.6 eV was also related to the strong interaction between the Cu 3d valence electrons and the O 2p valence electrons. It can be seen that the hydroxyl peak increased in the 8–13 eV region [41]. For CuO-CNT-2, the component of the O 2p orbital (at ∼5 eV) decreased in intensity due to the Cu 3d orbital shifting to the lower bindingenergy side compared with the pure CuO, which demonstrated the enhancement of O atoms deficiency of CuO-CNT-2. Furthermore, the valence band maximum (VBM) of each sample was determined. The VBM raised a high binding energy from 0.71 to 1.11 because of the interaction of CuO with CNTs, which illustrated that the VBM position of CNTs was higher than that of CuO. The resistance of materials in air was known to play a key role in their gas sensing responses. The proper resistance caused by a
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Fig. 5. (a-b) I–V curves measured for CuO-CNT-2 and CuO-CNT-3; (c-d) SEM image of CuO-CNT-2 and CuO-CNT-3 devices.
better electron transport would induce a higher response. There were two styles of contacts between 1D CuO-CNTs nanocomposite and Au electrode. The contact of one end in ohmic contact and the other end in schottky contact was the most promising candidate to show high response. Therefore, the electron transport ability of the device was studied. As shown in Fig. S6, the devices were composed of the 1D CuO-CNTs nanocomposites and Au electrode, which was treated at 250 ◦ C, 300 ◦ C and 400 ◦ C in Ar for 1 h, respectively. The I–V curves of pristine CuO NRs devices displayed a typical schottky contact characteristics (see Fig. S7a). On the contrary, the I–V plots of CNTs appear to be linear, showing an ohmic contact characteristic (see Fig. S7b). As shown in Fig. 5(a, c), the I–V plots of the CuOCNT-2 device declared the asymmetrical schottky contact, which contained a schottky barrier contact (electrode contacts with CuO NRs) at one side of the device, and an ohmic contact at the other side (electrode contacts with CNTs). The I–V curves of the CuOCNT-2 device presented the decreasing junction resistance with the increasing different treatment temperatures. Unquestionably, the shorter response time and the high response were achieved by using the asymmetrical schottky contacted device of CuO-CNT-2, agreeing with Heiu’s study [42]. For the CuO-CNT-3 device, the I–V plot exhibited a line in Fig. 5b, implying ohmic contacts and the corresponding SEM image was in Fig. 5d.
RT. When the NO2 gas was injected into the sensing chamber, the resistance of the CuO-CNT-2 sensor declined rapidly and reached a minimum resistance value in a short period of time. Obviously, the CuO-CNT-2 sensor exhibited a rapid and reversible response signal to NO2 gas even for the 0.97 ppm gas. The response time was also shortened with increasing NO2 concentration. When NO2 concentration is 97.0 ppm, the response time was only 6.0 s, and the response reached 96.4%. Response time of CNTs, CuO, CuO-CNT-1, CuO-CNT-2 and CuO-CNT-3 sensors was listed in Table S5. In the inset of Fig. 6a, CuO-CNT-2 sensor displayed that the value of the linear correlation coefficient R2 formed by plotting log S versus the log [CNO2 ] was 0.995 (R ≈ 0.997). The response of the CuO-CNT-2 sensor was 8 fold larger than that of CNTs sensor while the response time of CNTs sensor was 3.2 fold slower than that of the CuO-CNT-2 sensor. The CuO-CNT-2 sensor exhibited a relatively higher response and faster response time than the other sensors at RT. In addition, the stability of the CuO-CNT-2 sensor was measured at different NO2 concentrations for 35 days (Fig. 6d). Its sensor response and response time to 9.70, 4.85 and 2.91 ppm NO2 remained nearly unchanged within 35 days at RT, which confirmed its good stability. The CuO-CNT-2 sensor was expected to be attractive for practical applications; this further study indicated the combination of CuO NRs and CNTs could pave the way to highly sensitive sensors for NO2 gas.
3.2. Sensing performance 3.3. Sensing mechanism The gas sensing properties of 1D CuO-CNTs sensors were investigated at room temparature. In order to study the gas sensing properties, the selectivity of the CuO-CNT-2 sensor was tested for different concentrations of various gases including NO2 , H2 S, NH3 , CO and H2 . As shown in Fig. 6a, the responses were 96.4% for 97.0 ppm NO2 gas, 89.7% for 970.0 ppm H2 S gas and about 3% for 9700 ppm NH3 , H2 , and CO gases. The sensor showed high response to NO2 gas at as low-concentration as 97.0 ppm, which might be due to the high electron affinity of NO2 molecule. Fig. 6b exhibited the response curve of the CuO-CNT-2 sensor for 970.0 ppm H2 S at RT in air. However, no variation was observed when the gas was removed (gas out), which might contribute to the irreversible reaction of CuO and H2 S. Fig. 6c and Fig. S8 depicted dynamic responses of thin film sensors with NO2 gas concentration from 97.0 ppm to 0.97 ppm at
As a resistance-type sensor, the gas sensing mechanism of the CuO-CNT-2 sensor could be explained by the change in resistance of the sensor, while being exposed to different gas atmospheres. The resistance of the sensor consisted of two parts: the resistance of 1D CuO-CNTs and the resistance between 1D CuO-CNTs and Au electrode. It is known that electronic transmission ability of CNTs was superior to that of CuO. Therefore, CNTs was behaved as a high conductive channel and CuO as a low conductive channel. Furthermore, due to the lack of electrons, the hole-accumulation layer was served as poor conductance channel. So, the resistance of 1D CuOCNTs largely depended on the resistance of CuO NRs. The schematic for the mechanism was shown in Fig. 7. According to the results of MS, we calculated the carrier density and the Debye length. For CuO-CNT-2, the carrier density was
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Fig. 6. (a) Response of CuO-CNT-2 sensor to different gases; (b-c) Response-recovery curves for CuO-CNT-2 sensor to H2 S and NO2 gas, respectively; (d) The stability curves of the CuO-CNT-2 sensor.
NRs, established the channels of electrons transmission (Fig. 7b). The electron transfer between CuO and CNTs was shown in Fig. S9. Here, CNTs in the CuO-CNT-2 composite enhanced the conductivity of the sensor; simultaneously, the CuO NRs created an asymmetrical schottky contact with the Au elctrode, and acted as electron donor. Therefore, the CuO-CNT-2 sensor had a greatly short response time of 6.0 s and a high response of 96.4%. In addition, the irregular edges of the CuO NRs consisting of about 3 nm CuO grains should also favor the existence of large quantities of defects. The defects, which were mainly contributed by the vacancies of copper ion, accelerated the surface adsorption rate of O2 molecules. Moreover, the defects were beneficial to the diffusion of ions and atoms, which consequently enhanced the catalytic activity and gas sensing properties [44]. Thus, the higher was amount of charge carriers, the better would be the gas response [45]. When a 1D CuO-CNTs sensor was exposed to air, oxygen molecules adsorbed on the surface of the sample tended to trap the electrons from the CuO conduction band and generate O2 − adsorbates (Eq. (1)) [46,47]. O2 (ads) + e− → O2 − (ads)
Fig. 7. Sensing mechanism of CuO NR-CNTs composites based sensor. (a) the model of CuO NR-CNTs composite on electrode; (b) the surface state of CuO NR in NO2 atmosphere.
(1)
In the case that the sensor was exposed to NO2 gas, the gas molecules could attract electrons from 1D CuO-CNTs because of the high electron affinity of NO2 molecules. As shown in Fig. 7b, NO2 molecules reacted with the preadsorbed O2 − species and further extracted electrons from the conduction band of sensing material (Eqs. (2), (3)) [48]. Both processes could result in a decrease of electron density, an increase of the hole’s density on the surface of the semiconductor and a rapid decrease of the resistance of the sensor. NO2 (gas) + e− → NO2 − (ads) −
−
−
(2) 2−
NO2 (ads) + O (ads) + 2e → NO(gas) + 2O about 2.72 × 1018 , and Debye length La was 3.74 nm at RT. Here, the diameter of CuO NRs was 3–7 nm, which was located between La and 2La . In this case, the electron density in the CuO NRs had an effect on the hole-accumulation layer. The fully depleted cylinders occurred [43] and meanwhile formed a low conductance channel at the center region of CuO NRs. The high conductance channel of CNTs, combining with many low conductance channels of the CuO
(ads)
(3)
4. Conclusions In summary, we have successfully synthesized 1D CuO-CNTs composites by a facile reflux method. The 1D CuO-CNTs composites provide many electronic transmission channels, which could contribute to gas diffusion and lead to the gas sensing enhancement.
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In the sensors/devices, CNTs boost their conductivity, while CuO NRs build interfacial asymmetrical schottky contacts with the Au electrode and adjust the electrical conductivity. The synergy of the superior conductive CNTs and defects-rich CuO NRs allows for the outstanding NO2 gas sensing properties. As a result, the CuO-CNT-2 exhibits a low detection limit of 0.97 ppm, a high sensor response of 96.4% to 97 ppm NO2 and the response time of 6.0 s at room temperature. Our strategy that combines metal oxides and excellent conductive CNTs to fabricate an integrated structure will open up new opportunities for the application of multifunctional sensors. Acknowledgements This work was supported by the Program for Innovative Research Team in Chinese Universities (IRT1237); the National Natural Science Foundation of China (No.2167010747; 21671060); International Cooperation in Science and Technology Projects of China (2014DFR40480); Applied Technology Research and Development Program Foreign Cooperation Project of Heilongjiang Province (WB15C101). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.09. 173. References [1] Y. Yong, H. Jiang, X. Li, S. Lv, J. Cao, Phys. Chem. Chem. Phys. 18 (2016) 21431–21441. [2] B.O. Okesola, D.K. Smith, Chem. Soc. Rev. 45 (2016) 4226–4251. [3] A.W. Choi, V.M. Yim, H. Liu, K.K. Lo, Chem. Eur. J. 20 (2014) 9633–9642. [4] L. Zhao, C. Li, Y. Wang, H. Wu, L. Gao, J. Zhang, G. Zeng, Catal. Sci. Technol. 6 (2016) 6076–6086. [5] T. Samerjai, D. Channei, C. Khanta, K. Inyawilert, C. Liewhiran, A. Wisitsoraat, D. Phokharatkul, S. Phanichphant, J. Alloys Compd. 680 (2016) 711–721. [6] L. Gan, Q. Zhong, Y. Song, L.D. Li, X.L. Zhao, J. Alloys Compd. 628 (2015) 390–395. [7] T.J. Wallington, J.E. Anderson, E.M. Kurtz, P.J. Tennison, Faraday Discuss. 189 (2016) 121–136. [8] T.Y. Wei, P.H. Yeh, S.Y. Lu, Z.L. Wang, J. Am. Chem. Soc. 131 (2009) 17690–17695. [9] B. Show, N. Mukherjee, RSC Adv. 6 (2016) 75347–75358. [10] K. Saetia, J.M. Schnorr, M.M. Mannarino, S.Y. Kim, G.C. Rutledge, T.M. Swager, P.T. Hammond, Adv. Funct. Mater. 24 (2014) 492–502. [11] Y. Li, H. Ban, M. Jiao, M. Yang, RSC Adv. 6 (2016) 74944–74956. [12] S. Mao, G.H. Lu, J.H. Chen, J. Mater. Chem. A 2 (2014) 5573–5579. [13] M. Bao, Y.J. Chen, F. Li, J.M. Ma, T. Lv, Y.J. Tang, L.B. Chen, Z. Xu, T.H. Wang, Nanoscale 6 (2014) 4063–4066.
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Outstanding gas sensing performance of CuO-CNTs nanocomposite based on asymmetrical schottky junctions Yiming Zhao a , Muhammad Ikram a , Jiawei Zhang b,∗ , Kan Kan a , Hongyuan Wu a , Wanzhen Song a , Li Li a , Keying Shi a,∗ a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Key Laboratory of Physical Chemistry, School of Chemistry and Material Science, Heilongjiang University, Harbin, 150080, PR China b Key Laboratory of Chemical Engineering Process & Technology for High-efficiency Conversion, School of Chemistry and Material Science, Heilongjiang University, Harbin 150080, PR China
a r t i c l e
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Article history: Received 1 July 2017 Received in revised form 13 September 2017 Accepted 20 September 2017 Keywords: 1D CuO-CNTs composite Carbon nanotubes (CNTs) Nitrogen dioxide (NO2 ) Gas sensor
a b s t r a c t To fabricate a high-performance material for sensor devices at room temperature and further improve the synthetic approach of sensing materials, one dimensional (1D) CuO-CNTs nanocomposites were prepared with CNTs and CuO nanorods (NRs) via a facile reflux method. The 1D composite with the molar ratio of CuO and CNTs at 2.4:1 displays excellent gas sensing performance, i.e. the lowest detectable limit of 970 ppb and the short response time of 6 s–97.0 ppm NO2 at room temperature. In the 1D composite, the CNTs part provides a channel to enable effective and fast carrier transport, while the CuO NRs fabricates an asymmetrical schottky contact at the interface between the composites and the Au electrode. The advantage of the synergy of CNTs and CuO which possesses superior conductivity benefits the sensing of our 1D CuO-CNTs composite by providing affluent electrons. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Industrial development in the past decades has enormously increased the amount of toxic gases in the atmosphere, which results in dramatic consequences for both the environment and public health [1,2]. Among various air pollutants, the noxious gases such as NO and NO2 (formulated as NOx ) are highly toxic. When the levels of NOx are above 1 ppm, it causes serious diseases in humans body such as damaging respiratory system, lung tissues and heart disease [3]. The NO2 gas also plays a major role in photochemical smog, ozone depletion and acid rain, causing pollution and demaging water sources [4–6]. Consequently, developing senor devices that detect NOx gas at room temperature is a popular research direction in the filed of gas sensing [7]. Generally, good sensing performance can be achieved while materials have merits including uniform distribution, excellent morphology and nanostructures with large surface area to volume ratio. The manipulatable carrier in charge transport channel is also one of the greatest promises for gas sensing device (GSD). Most of GSDs reported are consisting of only single one dimensional
∗ Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (K. Shi). http://dx.doi.org/10.1016/j.apsusc.2017.09.173 0169-4332/© 2017 Elsevier B.V. All rights reserved.
(1D) functional material mounted at each two fingers of an electrode. The two contact ends are usually designed to have ohmic characteristic to enhance conductance and minimize the resistance of carriers that flow across the contacts; however, it is not good enough for commercial applications due to the low response of gas sensors. Wang et al. reported a non-symmetrical schottky contact of GSD with one end in ohmic contact and the other end in schottky contact. This featured structure can greatly enhance the GSD response through adjusting the electrical conductivity [8]. CNTs possess a large surface area to volume ratio, which would benefit to improve the gas response. Moreover, the proportion of 1D structure radii and Debye length offers greater potential in sensing performance [9–12]. It, however, is difficult for a single CNTs-based gas sensor to form a non-symmetrical schottky contact of GSD. To overcome this drawback, CNTs composite sensors have been explored for 1D well-designed nanostructures. In recent years, many efforts have been devoted to modifying functional CNTs with semiconductor metal oxides (SMOs) such as ZnO, In2 O3 , SnO2 and CuO [13–16]. These composites have been successfully used for NOx detection [17–20]. Among SMOs, CuO is increasingly attractive for gas-sensing application because of its p-type semiconductivity, high catalytic activity, narrow band gap (1.2 eV in bulk), fast redox ability [21], high-rate capability [22], abundant availability, non-toxic nature and low production cost
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[23–26]. The 1D CuO nanostructures can serve as significant building blocks for nanodevices and integrated nanosystems [17]. The electroconductive limitation of the nanostructure may attribute to intrisic defects/vacanies [27]. Therefore, it is desirable to use 1D CuO nanostructures as an additive for appropriate electrical conductivity to improve gas sensing properties. So far, the CuO NRs/nanowires have been investigated for sensing performance in the range of 200–400 ◦ C as a working temperature [28–32]. A CuOCNTs thin film has been synthesized to sense ethanol, showing high working temperature (400 ◦ C) and low response time [33]. Given above difficulties, it may be more convenient by introducing a conductive material or an electronic channel network, which would fasten response time and lower working temperature. Herein, the 1D CuO-CNTs composites have been synthesized by a facile reflux technique. The small CuO NRs were modified on the exterior surface of CNTs. And then, electronic channel network was fabricated by the connection of the high conductive channel of CNTs with the low conductive channel of CuO NRs. The Ohmic/Schottky contacts between the 1D CuO-CNT composites and the Au electrode were adjusted by thermal treatment (based on the device current-voltage characterization (Ids -Vds )). The 1D CuO-CNTs in gas sensing may show some interesting characteristics: (i) the highly conductive CNTs part may improve the conductivity of whole composite; (ii) the diameters of CuO NRs which fall within Debye length La (3.74 nm) and 2La may facilitating the defects/vacancies creation; (iii) the CuO NRs may modify the microstructures of the 1D CuO-CNTs composite and alter the electrical conductivity, which together might benefit for the formation of asymmetrical schottky contacts with Au electrode. Therefore, the 1D CuO-CNTs sensor to NO2 gas displays excellent sensing properties with a high response and a short response time at room temparature.
2. Experimental section 2.1. Hydroxylation of CNTs All the chemicals were of analytical grade and used as-received without further purification. (Multi-walled) CNTs were purchased from Shenzhen Nanotech Port Co. Ltd, with purity of >97.0%. Specific amount of CNTs was functionalized by 100.0 mL of mixed acids (HNO3 :H2 SO4 in 1:3 (v/v) ratio) under constant stirring for 30 min and then ultrasonication for 4 h and finally filtered and washed with deionized water until the pH reached 7. The washed sample were dried at 60 ◦ C for 12 h and the final product of hydroxylated CNTs were obtained.
2.2. Synthesis of 1D CuO-CNTs composites The hydroxylated CNTs of different concentrations i.e. (0.005 g, 0.010 g, 0.015 g) were dispersed in 49.0 mL N,N-Dimethyl Formamide (DMF) and 21.0 mL deionized water. The solution was ultrasonic for 20 min, and heated up to 60 ◦ C, and then 0.40 g Cu(CH3 COO)2 ·H2 O and 1.46 g Cetyltrimethylammonium Bromide (CTAB) were added and stirred vigorously. Later, 0.40 g NaOH was added to the solution, being retained at 60 ◦ C and refluxed for 15 min. Finally, the solution was cooled down to RT for filtration. After washing with ethanol and deionized water, the samples were dried at 50 ◦ C in a vacuum oven. The as-prepared 1D CuO-CNTs composites with (theoretical) CuO:CNTs molar ratio of 4.8:1, 2.4:1 and 1.6:1 were named as CuO-CNT-1, CuO-CNT-2 and CuO-CNT-3, respectively. The components of the samples were listed in Table S1.
2.3. Apparatus Thermo gravimetric-differential scanning calorimetry (TG-DSC) analysis of the samples was performed by TA-SDTQ600. The crystal phase of the samples was characterized by X-ray powder diffraction (XRD, D/MAX-III-B-40 KV, Japan, Cu-K␣ radiation, = 1.5406 Å). The Brunauer-Emmett-Teller (BET) surface area of the products were measured by N2 adsorption-desorption (TriStar II 3020). The surface morphology of the composites was characterized by scanning electron microscopy (SEM, HITACHI S-4800). Transmission electron microscopy (TEM, JEOL-2100) was performed to inspect the structures of the composites. X-ray photoelectron spectroscopy (XPS) was carried out using AXIS ULPRA DLD (Shimadzu Corporation) system to analyze the adsorbed oxygen species in the samples. Electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) plot measurements were carried out by using an electrochemical working station (CHI660C, Shanghai, China) in a half-cell setup configuration at RT. In the EIS measurement, the range of frequency was 0.01 Hz–100 kHz, and the excitation amplitude was 4 mV. Current-voltage characteristics (Ids -Vds ) was performed on 1D CuO-CNT device using a Keithley 2602 source meter. Ids -Vds curves of the 1D CuO-CNT device was measured (the gate voltage Vg = 0.1 eV).
2.4. Gas sensing tests 1.0 mg for each set of 1D CuO-CNT composite was evenly dispersed in 1.0 mL of deionized water to prepare a suspension. A 0.05 mL suspension was dropped onto the interdigitated Au electrode (the width of Au fingers was about 2 m). The sensor was treated at 250 ◦ C (or 300 ◦ C and 400 ◦ C) in Ar for 1 h and a CNT based gas sensor was obtained. The sensor was mounted into a test chamber with an inlet and an outlet. The chamber was flushed with air for 2 min to remove any contaminants from the flask and also to stabilize the film before testing. Syringe was used to inject the required volume of NO2 gas (99.8%, NO < 0.2%) into the test chamber. Then, NO2 as the detected gas would come into contact with the surface of the samples. The changes in the electrical resistance of the samples were recorded by a home-made automatic resistance apparatus over time. Finally, the sensor resietance was recovered by purging the chamber with air. The electrical resistance measurements of the sensor were carried out at RT with a relative humidity of (RH) around 26.0%. The sensor response was defined as the ratio (R0 -RN )/R0 , where R0 and RN were the resistances of the sensor in air and NO2 gas, respectively. The response time was defined as the time required for the variation in resistance to reach 90% of the equilibrium value after a test gas was injected. The material was tested to respond to various detecting gases such as NO2 , H2 S, NH3 , CO or H2 , and the highest response (to a kind of gas) signified a good sensing selectivity of the material.
3. Results and discussion 3.1. Composition and morphology TG-DSC analysis was displayed in Fig. 1. As seen from the TG-DSC curves of CNTs (Fig. 1a), the mass ratio of CNTs begins to decrease at about 460 ◦ C and sharply decreases in going from 570 ◦ C to 685 ◦ C. For CuO-CNT-1, CuO-CNT-2, and CuO-CNT-3 (the calculated molar ratios of CuO:CNTs are 4.8:1, 2.4:1 and 1.6:1, respectively), the decreased mass was attributed to consumption of CNTs in the range of temperatures from 240 ◦ C to 550 ◦ C (Fig. 1b). The change indicated that the interaction between CNT and CuO was stronger.
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Fig. 1. TG-DSC Analysis at 10 ◦ C min−1 in air: (a) CNTs; (b) CuO-CNTs composites.
The XRD patterns of CNTs and pure CuO were shown in Fig. S1. For CNTs, two significantly intense peaks at 26.0◦ and 42.2◦ were observed, which were assigned to the graphite structure (002) and (100) planes, respectively (JCPDS card no.75-1621). In addition, the reflections along (110), (002), (111), (-202) and (-311) etc. arose from the monoclinic phase CuO, in agreement with the standard data of CuO (JCPDS card no. 89-2530) [34]. Fig. 2a illustrated the powder XRD patterns of the CuO-CNTs composites. The peaks observed at 32.2◦ , 35.5◦ , 38.5◦ , 48.7◦ and 66.1◦ were well matched with the (110), (002), (111), (−202) and (−311) crystal planes of CuO. For CuO-CNT-1 and CuO-CNT-2 composites, no CNTs diffraction peaks were observed due to its low amount. A weak diffraction peak of CNTs was shown in CuO-CNT-3 composite. Peaks of metallic Cu and Cu2 O were not found, indicating that the as-synthesized CuO-CNTs nanocomposites are highly pure. XRD results illustrated that the composites consisted of CNTs and well crystallized CuO. Fig. 2b presented N2 adsorption-desorption isotherms and BET pore-size distribution of CuO-CNT-2 composite. The curves exhibited a IV type isotherm, which is regarded as the typical characteristic of mesoporous materials. The BET surface area was determined to be 94.2 m2 g−1 . The pores were distributed uniformly and the dominant mesopore in size was about 9 nm. SEM and TEM images of pristine CuO NRs were shown in Fig. S2. The CuO NRs were about 100–200 nm in length and 3–7 nm in width. Four batches of 1D CuO-CNTs composites were prepared under the same condition and characterized in Fig. S3. For the CuOCNT-1 composite, the low magnification TEM images showed that the flowerlike CuO architectures were aggregated on the surface of CNTs. There were relatively high quantity of CuO NRs that covered on the outside surface of CNTs, while only a few CuO NRs were found for the CuO-CNT-3 composite. The composite morphologies were relatively stable in different batches. The insert image in Fig. 3a was the low magnification TEM image of the CuO-CNT-2 composite. One can see that CuO NRs modify the exterior surface of CNTs. The inner and outer diameters of CNTs about 8 nm and 20 nm, respectively, were clearly observed
Fig. 2. (a) XRD diffraction patterns of CuO-CNT-1, CuO-CNT-2 and CuO-CNT-3 composites; (b) Nitrogen adsorption-desorption isotherms and pore-size distribution curve of CuO-CNT-2 composite.
in Fig. 3a. The CNTs owns seventeen walls and CuO NRs adhere to the exterior surface of the CNTs. High magnification TEM (HRTEM) images in Fig. 3(b–e) suggested that CuO NRs around 3 nm in size were obtained. The interplanar spacings of CuO NRs were measured to be about 2.78, 2.52 (2.53), 2.33 (2.36) and 2.27 (2.30) Å, corresponding to the (110), (002), (111) and (200) planes of CuO, respectively. In addition, the (002) plane of CNTs could be observed, reflected by the lattice spacing of 3.52 (3.75) Å. Moreover, it was the blue line that separated CuO NRs from CNTs. To study the electrical characteristics of the 1D CuO-CNTs composites, Mott-Schottky (MS) and Electrochemical Impedance Spectroscopy (EIS) measurements were carried out. The conduction types of the samples were determined using MS measurements. The obtained results were shown in Fig. S4. The MS curves of the samples were approximately linear. The negative slopes of MS plots indicated a p-type semiconducting behavior for all the samples. This was in accordance with the results of the dynamic responserecovery curves of sensors. As expected, the resistance and electron transportation of materials plays an important role in gas sensing response. Thus, the electron transportation ability of samples was studied. The impedance parameters (R and Rct ) and carrier density (Na ) of samples were listed in Table S2 and Table S3, respectively. The results revealed that CNTs (Na = 1.06 × 1019 ) had much higher carrier (positive hole) density than CuO (Na = 7.77 × 1017 ). Hence, CNTs could easily capture electrons that migrated from the conduction band of CuO [20]. Moreover, the function slope of the ImZ (the imaginary part of the CuO-CNT-2 impedance) and (angular frequency) was 0.52, which was close to −1/2 (see the inset of lower right corner in Fig. S4b). This confirmed that the CuO-CNT-2 sample indeed possessed highly abundant porous structures [35], which could improve gas transmission channel from its interior the surface. In addition, Debye length (La ) of the sample was 3.74 nm, and the related detailed information is given in Supporting Information.
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Fig. 3. TEM images of CuO-CNTs composite. (a) TEM image of the open end of CNT, the inset is low magnification TEM image of the CuO-CNT-2 composite; (b) HRTEM image of (a); (c), (d), (e) HRTEM image of the inset (a), many defects existed in the multijunctions/interfaces between CuO NRs and CNT.
Fig. 4. XPS spectra of O 1s: (a) CuO, (b) CuO-CNT-2, (c) CuO-CNT-3 and C 1s: (d) CuO-CNT-2, (e) CuO-CNT-3; (f) Valence band of CuO and CuO-CNT-2.
The XPS spectra of CNTs, CuO, CuO-CNT-1, CuO-CNT-2 and CuOCNT-3 were shown in Fig. 4 and Fig. S5, together with the binding energy of O peaks and C peaks in Table S4. As seen in Fig. 4(a–c) and Fig. S5c, the high-resolution O 1s spectra of all the samples revealed three separating peaks at ∼529.5 ± 0.1, ∼531.1 ± 0.3 and ∼532.5 ± 0.4 eV, which were attributed to crystal lattice O2− (Peak 1), oxygen defects/vacancies (Peak 2), and the near-surface oxygen surface hydroxylation, adsorbed H2 O, or O2 (Peak 3), respectively [36,37]. The calculated oxygen defects/vacancies peak area ratios were about 36.8% for CuO, 37.1% for CuO-CNT-1, 39.0% for CuO-CNT-2 and 38.3% for CuO-CNT-3, respectively. Obviously, the oxygen defects/vacancies peak area of the CuO-CNT-2 sample was the largest. Because the oxygen defects/vacancies were taken as shallow donors, more electrons were suggested be available in the CuO NRs of the CuO-CNT-2 composite. The C 1s specta of the CNTs (Fig. S5b) presented a main peak at 284.5 eV arising from the graphite-like carbon atoms of CNTs [38]. The peak at 285.8 eV was attributed to the defects in CNTs structure [39], while ones at 288.9 eV corresponded to O C O [40]. Fig. 4(d-e) and Fig. S5d showed the C 1s spectra of CuO-CNT-2, CuOCNT-3 and CuO-CNT-1 composites. Among them, the carbon defect
peak area ratio of the CuO-CNT-2 sample was the smallest, implying the minimum free electrons in its CNTs part. Hence, CNTs captured electrons from the conduction band of CuO, giving rise to highly reduced CNTs. The valence-band spectra were shown in Fig. 4f. The top peak of the valence band of CuO-CNT-2 shifted to the lower binding-energy side (0.7 eV) compared to the pristine CuO. This resulted from the interaction CuO NRs with CNTs. The intense peak positioned at 3.6 eV was also related to the strong interaction between the Cu 3d valence electrons and the O 2p valence electrons. It can be seen that the hydroxyl peak increased in the 8–13 eV region [41]. For CuO-CNT-2, the component of the O 2p orbital (at ∼5 eV) decreased in intensity due to the Cu 3d orbital shifting to the lower bindingenergy side compared with the pure CuO, which demonstrated the enhancement of O atoms deficiency of CuO-CNT-2. Furthermore, the valence band maximum (VBM) of each sample was determined. The VBM raised a high binding energy from 0.71 to 1.11 because of the interaction of CuO with CNTs, which illustrated that the VBM position of CNTs was higher than that of CuO. The resistance of materials in air was known to play a key role in their gas sensing responses. The proper resistance caused by a
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Fig. 5. (a-b) I–V curves measured for CuO-CNT-2 and CuO-CNT-3; (c-d) SEM image of CuO-CNT-2 and CuO-CNT-3 devices.
better electron transport would induce a higher response. There were two styles of contacts between 1D CuO-CNTs nanocomposite and Au electrode. The contact of one end in ohmic contact and the other end in schottky contact was the most promising candidate to show high response. Therefore, the electron transport ability of the device was studied. As shown in Fig. S6, the devices were composed of the 1D CuO-CNTs nanocomposites and Au electrode, which was treated at 250 ◦ C, 300 ◦ C and 400 ◦ C in Ar for 1 h, respectively. The I–V curves of pristine CuO NRs devices displayed a typical schottky contact characteristics (see Fig. S7a). On the contrary, the I–V plots of CNTs appear to be linear, showing an ohmic contact characteristic (see Fig. S7b). As shown in Fig. 5(a, c), the I–V plots of the CuOCNT-2 device declared the asymmetrical schottky contact, which contained a schottky barrier contact (electrode contacts with CuO NRs) at one side of the device, and an ohmic contact at the other side (electrode contacts with CNTs). The I–V curves of the CuOCNT-2 device presented the decreasing junction resistance with the increasing different treatment temperatures. Unquestionably, the shorter response time and the high response were achieved by using the asymmetrical schottky contacted device of CuO-CNT-2, agreeing with Heiu’s study [42]. For the CuO-CNT-3 device, the I–V plot exhibited a line in Fig. 5b, implying ohmic contacts and the corresponding SEM image was in Fig. 5d.
RT. When the NO2 gas was injected into the sensing chamber, the resistance of the CuO-CNT-2 sensor declined rapidly and reached a minimum resistance value in a short period of time. Obviously, the CuO-CNT-2 sensor exhibited a rapid and reversible response signal to NO2 gas even for the 0.97 ppm gas. The response time was also shortened with increasing NO2 concentration. When NO2 concentration is 97.0 ppm, the response time was only 6.0 s, and the response reached 96.4%. Response time of CNTs, CuO, CuO-CNT-1, CuO-CNT-2 and CuO-CNT-3 sensors was listed in Table S5. In the inset of Fig. 6a, CuO-CNT-2 sensor displayed that the value of the linear correlation coefficient R2 formed by plotting log S versus the log [CNO2 ] was 0.995 (R ≈ 0.997). The response of the CuO-CNT-2 sensor was 8 fold larger than that of CNTs sensor while the response time of CNTs sensor was 3.2 fold slower than that of the CuO-CNT-2 sensor. The CuO-CNT-2 sensor exhibited a relatively higher response and faster response time than the other sensors at RT. In addition, the stability of the CuO-CNT-2 sensor was measured at different NO2 concentrations for 35 days (Fig. 6d). Its sensor response and response time to 9.70, 4.85 and 2.91 ppm NO2 remained nearly unchanged within 35 days at RT, which confirmed its good stability. The CuO-CNT-2 sensor was expected to be attractive for practical applications; this further study indicated the combination of CuO NRs and CNTs could pave the way to highly sensitive sensors for NO2 gas.
3.2. Sensing performance 3.3. Sensing mechanism The gas sensing properties of 1D CuO-CNTs sensors were investigated at room temparature. In order to study the gas sensing properties, the selectivity of the CuO-CNT-2 sensor was tested for different concentrations of various gases including NO2 , H2 S, NH3 , CO and H2 . As shown in Fig. 6a, the responses were 96.4% for 97.0 ppm NO2 gas, 89.7% for 970.0 ppm H2 S gas and about 3% for 9700 ppm NH3 , H2 , and CO gases. The sensor showed high response to NO2 gas at as low-concentration as 97.0 ppm, which might be due to the high electron affinity of NO2 molecule. Fig. 6b exhibited the response curve of the CuO-CNT-2 sensor for 970.0 ppm H2 S at RT in air. However, no variation was observed when the gas was removed (gas out), which might contribute to the irreversible reaction of CuO and H2 S. Fig. 6c and Fig. S8 depicted dynamic responses of thin film sensors with NO2 gas concentration from 97.0 ppm to 0.97 ppm at
As a resistance-type sensor, the gas sensing mechanism of the CuO-CNT-2 sensor could be explained by the change in resistance of the sensor, while being exposed to different gas atmospheres. The resistance of the sensor consisted of two parts: the resistance of 1D CuO-CNTs and the resistance between 1D CuO-CNTs and Au electrode. It is known that electronic transmission ability of CNTs was superior to that of CuO. Therefore, CNTs was behaved as a high conductive channel and CuO as a low conductive channel. Furthermore, due to the lack of electrons, the hole-accumulation layer was served as poor conductance channel. So, the resistance of 1D CuOCNTs largely depended on the resistance of CuO NRs. The schematic for the mechanism was shown in Fig. 7. According to the results of MS, we calculated the carrier density and the Debye length. For CuO-CNT-2, the carrier density was
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Fig. 6. (a) Response of CuO-CNT-2 sensor to different gases; (b-c) Response-recovery curves for CuO-CNT-2 sensor to H2 S and NO2 gas, respectively; (d) The stability curves of the CuO-CNT-2 sensor.
NRs, established the channels of electrons transmission (Fig. 7b). The electron transfer between CuO and CNTs was shown in Fig. S9. Here, CNTs in the CuO-CNT-2 composite enhanced the conductivity of the sensor; simultaneously, the CuO NRs created an asymmetrical schottky contact with the Au elctrode, and acted as electron donor. Therefore, the CuO-CNT-2 sensor had a greatly short response time of 6.0 s and a high response of 96.4%. In addition, the irregular edges of the CuO NRs consisting of about 3 nm CuO grains should also favor the existence of large quantities of defects. The defects, which were mainly contributed by the vacancies of copper ion, accelerated the surface adsorption rate of O2 molecules. Moreover, the defects were beneficial to the diffusion of ions and atoms, which consequently enhanced the catalytic activity and gas sensing properties [44]. Thus, the higher was amount of charge carriers, the better would be the gas response [45]. When a 1D CuO-CNTs sensor was exposed to air, oxygen molecules adsorbed on the surface of the sample tended to trap the electrons from the CuO conduction band and generate O2 − adsorbates (Eq. (1)) [46,47]. O2 (ads) + e− → O2 − (ads)
Fig. 7. Sensing mechanism of CuO NR-CNTs composites based sensor. (a) the model of CuO NR-CNTs composite on electrode; (b) the surface state of CuO NR in NO2 atmosphere.
(1)
In the case that the sensor was exposed to NO2 gas, the gas molecules could attract electrons from 1D CuO-CNTs because of the high electron affinity of NO2 molecules. As shown in Fig. 7b, NO2 molecules reacted with the preadsorbed O2 − species and further extracted electrons from the conduction band of sensing material (Eqs. (2), (3)) [48]. Both processes could result in a decrease of electron density, an increase of the hole’s density on the surface of the semiconductor and a rapid decrease of the resistance of the sensor. NO2 (gas) + e− → NO2 − (ads) −
−
−
(2) 2−
NO2 (ads) + O (ads) + 2e → NO(gas) + 2O about 2.72 × 1018 , and Debye length La was 3.74 nm at RT. Here, the diameter of CuO NRs was 3–7 nm, which was located between La and 2La . In this case, the electron density in the CuO NRs had an effect on the hole-accumulation layer. The fully depleted cylinders occurred [43] and meanwhile formed a low conductance channel at the center region of CuO NRs. The high conductance channel of CNTs, combining with many low conductance channels of the CuO
(ads)
(3)
4. Conclusions In summary, we have successfully synthesized 1D CuO-CNTs composites by a facile reflux method. The 1D CuO-CNTs composites provide many electronic transmission channels, which could contribute to gas diffusion and lead to the gas sensing enhancement.
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In the sensors/devices, CNTs boost their conductivity, while CuO NRs build interfacial asymmetrical schottky contacts with the Au electrode and adjust the electrical conductivity. The synergy of the superior conductive CNTs and defects-rich CuO NRs allows for the outstanding NO2 gas sensing properties. As a result, the CuO-CNT-2 exhibits a low detection limit of 0.97 ppm, a high sensor response of 96.4% to 97 ppm NO2 and the response time of 6.0 s at room temperature. Our strategy that combines metal oxides and excellent conductive CNTs to fabricate an integrated structure will open up new opportunities for the application of multifunctional sensors. Acknowledgements This work was supported by the Program for Innovative Research Team in Chinese Universities (IRT1237); the National Natural Science Foundation of China (No.2167010747; 21671060); International Cooperation in Science and Technology Projects of China (2014DFR40480); Applied Technology Research and Development Program Foreign Cooperation Project of Heilongjiang Province (WB15C101). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.09. 173. References [1] Y. Yong, H. Jiang, X. Li, S. Lv, J. Cao, Phys. Chem. Chem. Phys. 18 (2016) 21431–21441. [2] B.O. Okesola, D.K. Smith, Chem. Soc. Rev. 45 (2016) 4226–4251. [3] A.W. Choi, V.M. Yim, H. Liu, K.K. Lo, Chem. Eur. J. 20 (2014) 9633–9642. [4] L. Zhao, C. Li, Y. Wang, H. Wu, L. Gao, J. Zhang, G. Zeng, Catal. Sci. Technol. 6 (2016) 6076–6086. [5] T. Samerjai, D. Channei, C. Khanta, K. Inyawilert, C. Liewhiran, A. Wisitsoraat, D. Phokharatkul, S. Phanichphant, J. Alloys Compd. 680 (2016) 711–721. [6] L. Gan, Q. Zhong, Y. Song, L.D. Li, X.L. Zhao, J. Alloys Compd. 628 (2015) 390–395. [7] T.J. Wallington, J.E. Anderson, E.M. Kurtz, P.J. Tennison, Faraday Discuss. 189 (2016) 121–136. [8] T.Y. Wei, P.H. Yeh, S.Y. Lu, Z.L. Wang, J. Am. Chem. Soc. 131 (2009) 17690–17695. [9] B. Show, N. Mukherjee, RSC Adv. 6 (2016) 75347–75358. [10] K. Saetia, J.M. Schnorr, M.M. Mannarino, S.Y. Kim, G.C. Rutledge, T.M. Swager, P.T. Hammond, Adv. Funct. Mater. 24 (2014) 492–502. [11] Y. Li, H. Ban, M. Jiao, M. Yang, RSC Adv. 6 (2016) 74944–74956. [12] S. Mao, G.H. Lu, J.H. Chen, J. Mater. Chem. A 2 (2014) 5573–5579. [13] M. Bao, Y.J. Chen, F. Li, J.M. Ma, T. Lv, Y.J. Tang, L.B. Chen, Z. Xu, T.H. Wang, Nanoscale 6 (2014) 4063–4066.
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