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Microstructure and sensing properties of CdS-ZnO1−x coatings deposited by liquid plasma spray and treated with hydrogen peroxide solution for nitrogen dioxide detection at room temperature
Journal of Alloys and Compounds 687 (2016) 286e293
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Microstructure and sensing properties of CdS-ZnO1x coatings deposited by liquid plasma spray and treated with hydrogen peroxide solution for nitrogen dioxide detection at room temperature Xin Geng a, b, Jiajun You a, Chao Zhang a, * a b
College of Mechanical Engineering, Yangzhou University, Yangzhou 225127, PR China College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 April 2016 Received in revised form 29 May 2016 Accepted 9 June 2016 Available online 14 June 2016
Metal oxide semiconductor gas sensors usually respond slowly and weakly to target gas at room temperature. In this paper, we report a nitrogen dioxide gas sensor based on porous CdS-ZnO1x coatings using a combination of sensitization, surface modification and visible light illumination methods to improve sensing characteristics. Porous CdS-ZnO coatings were deposited by liquid plasma spraying process. Then the coating surface was modified by immersing in a hydrogen peroxide solution and annealing to generate oxygen vacancies and finally obtain CdS-ZnO1x coatings. Photoluminescence spectroscopy, electron paramagnetic resonance and UVeVis diffuse reflectance spectrophotometer were utilized to characterize the oxygen vacancies and optical properties of the obtained coatings. The UVeVis results revealed that the absorption of the obtained coatings was extended to the whole visible light region after the treatment. Nitrogen dioxide gas sensing properties of the CdS-ZnO and CdS-ZnO1x coatings were measured and compared. The gas sensing results showed that the surface modification (surface oxygen vacancies) can greatly enhanced sensor responses and significantly shortened the response time and recovery time. The sensing mechanism of the obtained CdS-ZnO1x coatings was discussed in terms of the effect of oxygen vacancies. © 2016 Elsevier B.V. All rights reserved.
Keywords: CdS-ZnO1x sensor Oxygen vacancy Liquid plasma spray Nitrogen dioxide Room temperature
1. Introduction Nitrogen oxides (NOx: NO and NO2) derived from vehicle emission and power generation exhaust will lead to serious environmental pollution, and thus have a negative influence on the health of human beings [1]. For the protection of people’s health and promotion of sustainable development of society, it is imperative to enhance the monitoring of toxic gases, especially to nitrogen dioxide (NO2) [2]. Many researchers have reported that zinc oxide is one of the most promising NO2 gas sensing materials [2,3]. ZnO based gas sensors exhibit excellent response to NO2 even at low concentration. However, working temperature is a big problem due to traditional ZnO based gas sensors are generally operated at elevated temperature ranging from 200 to 600 C [4]. Decreasing working temperature can not only effectively reduce power
* Corresponding author. College of Mechanical Engineering, Yangzhou University, 196 West Huayang Road, Yangzhou, Jiangsu, PR China. E-mail addresses: [email protected], [email protected] (C. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.06.079 0925-8388/© 2016 Elsevier B.V. All rights reserved.
consumption, but also enhance the security in the complex atmosphere, particularly under the conditions of flammable gases [5]. Apart from improving economic efficiency and safety, developing low temperature or even room-temperature semiconductor sensors also raises the long-term stability of sensors [6]. Photogenerated electron-hole pairs are generated only when semiconductors are illuminated under light with photon energy higher than its band-gap energy [7e10]. That is to say, the wavelength of excitation light should be less than 380 nm because the intrinsic wide band-gap of ZnO is ca. 3.2 eV. Incorporating narrow band-gap semiconductors like CdS [7,8], PbS [9], Bi2S3 [10] into ZnO is an effective way to enable the composite coatings work at low temperature. When narrow band gap semiconductor, like CdS, PbS and Bi2O3, are exposed to visible light illumination, electron-hole pairs will be generated and then electrons will be transferred to ZnO. In the previous study [8], we have reported that 1 ppm NO2 can be detected at room temperature by CdS-ZnO sensor under visible light illumination [8]. However, the response time and recovery time of this sensor are long. Therefore, one of the big challenges for CdS-ZnO sensors is to increase the reaction rate. It has
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been reported that semiconductor gas sensors based on metal oxides will be more sensitive to oxidizing gases with assistance of oxygen vacancies [11e13]. Therefore, the sensing characteristics may be greatly enhanced by introducing oxygen vacancies into sensing materials. The coating microstructure is well-known to greatly influence the gas sensing characteristics [13]. The sensing materials with nanosized and porous structure are expected to obtain excellent sensing performance [14]. Liquid plasma spray (LPS) is such a method to deposit nanostructured ceramic coatings in which a liquid feedstock is injected into a plasma flame. In contact with hot plasma, after evaporation and precipitation of the liquid droplets, produced solid particles are heated, accelerated and impacted on the substrate to form coatings. The LPS coatings are easily tailored to be porous with a large surface-to-volume ratio. In this article, CdS-ZnO composite coatings were deposited by LPS process. A post-treatment process with H2O2 treatment and annealing is utilized to obtain the oxygen-vacant CdS-ZnO1x coatings. The phase constitutions, surface morphologies and electrical resistances of the obtained coatings were characterized. The effects of light wavelength on the sensor characteristics were investigated. The sensing mechanism of the oxygen vacant CdSZnO1x sensors under visible light illumination was discussed. 2. Experimental methods 2.1. Coating preparation CdS-ZnO composite coatings were deposited on alumina substrates equipped with a pair of interdigitated gold electrodes (CMAC Micro Technology Company, Belgium) by LPS with a F4MB-XL plasma spray torch (Oerlikon Metco, Switzerland) attached to a sixaxis ABB robot arm (ABB, Sweden). In this work, the liquid used was an aqueous solution containing 0.2 M zinc acetate and 0.02 M cadmium sulfide, and injected into the plasma jet by a nozzle with an inner diameter of 0.26 mm through a peristaltic pump. The spraying parameters were listed in Table 1. CdS-ZnO1x coatings were obtained by dipping the as-sprayed coatings into 20 ml H2O2 (30 vol%) at 80 C for 1 h. Then, the coatings were annealed in a furnace at 400 C for 1 h. 2.2. Coating characterization
287
the presence of oxygen vacancies in the CdS-ZnO1x coatings was confirmed by the advanced techniques, including photoluminescence spectroscopy (PL, with He-Cd 325 nm laser) and electron paramagnetic resonance (EPR, A300-10/12, Bruker, Germany). 2.3. Gas sensor testing A 12 W LED lamp was installed in a sealed chamber and put in front of the as-sprayed CdS-ZnO and CdS-ZnO1x sensors. The chamber was in steel and a Teflon film was coated in the inner wall of the chamber. The lamps with a wavelength of 400, 480, 530, 580 and 640 nm (purple, blue, green yellow and red light) were used to study the effect of light wavelength on the sensing properties. The distance between the LED lamp and the sensors was controlled at 30 mm. Visible light illumination and electrical resistance measurement were carried out in the chamber. The sensors were connected to an Agilent data acquisition instrument to record the electrical resistances. Prior to the NO2 tests, synthetic air with a flow rate of 500 mL/min was introduced into the chamber until the electrical resistance was stable. Then, 10 ppm NO2 with controlled flow rates was injected into the chamber to obtain a desired NO2 concentration. The sensor response was defined as S ¼ RNO2/Rair where RNO2 and Rair were the resistances of sensors in presence of NO2 gas and synthetic air, respectively. Response time is defined as the time required for the electrical resistance starting from Rair to reach 90% of RNO2 while recovery time represents the time needed for resistance decreasing from RNO2 to 110% of Rair. 3. Results and discussion 3.1. Coating characterization Fig. 1 illustrates the XRD patterns of the coatings in different steps with a scanning rate of 5 $min1. There is Al2O3 peaks emitted from the alumina substrate because the as-sprayed coatings are thin. The result demonstrates that the as-sprayed coating is well in agreement with hexagonal wurtzite ZnO (PDF 36e1451). In addition, it can also be easily seen that the (111) diffraction peak of cubic CdS appeared according to PDF 65e2887, which reveals that the CdS particles are deposited in the composite coatings. Similar
Surface morphology of the obtained coatings were inspected using field-emission scanning electron microscopy (FE-SEM, S4800 II, Hitachi, Japan). The crystal structure of the coatings was determined by X-ray diffractometer (XRD, D8 Advance, Bruker, Germany) using a CuKa radiation. 2q range of XRD diffraction angle was 20e70 with a scanning rate of 5 $min1 and 30e40 with 0.1 $min1 was used in this test. UVeVis diffuse reflectance spectrophotometer (Cary5000, Varian, USA) using BaSO4 as the reference was used to record UVeVis absorption spectra of the coatings within light wavelength ranging from 300 to 800 nm. Additionally,
Table 1 Liquid plasma spraying parameters. Parameters
Value
Arc current Torch power Spraying distance Argon volume flow rate Hydrogen volume flow rate Nozzle inner diameter Liquid flow rate
513 A 25 kW 100 mm 40 L/min 2 L/min 0.26 mm 20 mL/min
Fig. 1. X-ray diffraction patterns of the as-sprayed CdS-ZnO coatings, H2O2-treated CdS-ZnO coatings, and annealed CdS-ZnO coatings after H2O2 treatment.
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results have been observed in the previously published paper [8]. It can be found from the brown pattern that ZnO2 is produced when the coatings are treated by H2O2 solution. After the annealing, ZnO2 phase disappeared. Fig. 2 presents the XRD pattern of the as-sprayed and annealed after H2O2 treatment coatings with a scanning rate of 0.1 $min1 in the range of 30e40 . The crystallite size in the coatings is calculated through Scherrer formula:
1=2 D ¼ k$l=cosq b2 b2
(1)
where D is the average crystallite size, k is a shape factor of 0.89, l is the wavelength of X-ray (CuKa ¼ 0.15418 nm), b is the full width at half maximum, b is the instrumental broadening equal to 0.075, and q is the Bragg diffraction angle. The crystallite size at the three main peaks of the as-sprayed coatings is 28 ± 5 nm, while that of the annealed with H2O2 treated coatings is 31 ± 5 nm. The observed slight grain growth may be attributed to the annealing treatment at 400 C for 1 h. Fig. 3(aed) depict the surface morphology with low and high magnifications of the as-sprayed and the annealed with H2O2 treatment coatings. It can be observed that the coatings are nanostructured with crystallite size of ca. 30 nm, which is consistent with the XRD results. The LPS method is a rapid cooling process with high cooling speed over 105 K/s, resulting in small crystallite size of the as-sprayed coatings [1]. Meanwhile, due to LPS process adopting liquid instead of powder as feedstock and the water in liquid consuming a part of plasma enthalpy, less energy is applied on individual droplets and nanostructured CdS-ZnO coatings are produced [3,13]. Moreover, it could also be found that the coating is porous. The size of pores existing in the as-sprayed coatings are about microns, which is ascribed to the entrapped gases, cracking and premature solidification of some splats [1]. Furthermore, it can be found from Fig. 3 (c) and (d) that the annealed coatings are smoother compared with the as-sprayed coatings, which can be ascribed to the immersion in hydrogen peroxide solution for 1 h. Some badly-bonded particles on the surface are dissolved in the H2O2 solution and the well-bonded ones are retained. Fig. 4 presents the UVeVis absorption spectra of the as-sprayed CdS-ZnO and annealed CdS-ZnO1x coatings recording under light illumination with a wavelength of 300e800 nm. It can be observed
Fig. 2. X-ray diffraction patterns of the as-sprayed and annealed CdS-ZnO coatings after H2O2 treatment coatings with slow scan.
that there are two absorption edges locating at 400 and 545 nm, respectively. The 400 nm absorption edge was originated from band to band excitation of ZnO, while the 545 nm was arose from interband electronic transition of CdS. This result confirmed that CdS was successfully deposited in the CdS-ZnO and CdS-ZnO1x coatings. The absorption range of the two coatings was extended to 545 nm and more. Furthermore, it can be found that the absorption of the CdS-ZnO1x coating was significantly enhanced compared with that of the CdS-ZnO coating. In the previous paper [8], we have reported that the light intensity can shorten the response time and recovery time, so high intensity LED lamp (12 W) is used in this study. Fig. 5 shows the electrical resistance responses of the CdS-ZnO sensors to 1 ppm NO2 gas with different light wavelength at room temperature, and relevant data are listed in Table 2. It can be observed that the base resistance decreased with reduction in light wavelength. In addition, the reaction rate was slightly enhanced. However, the response time and recovery time were still long, which limited its practical application. Moreover, the sensor responses under all lights were lower than 2 and significantly decreased compared with that operated at lower light intensity of 3 W [8]. It should be noticed that the sensor response was the lowest illuminated under red light. This can be ascribed to that CdS cannot be excited illuminated under the light with wavelength less than 545 nm (red light) and electron number in the conduction band of ZnO is not enough. Therefore, less electrons existed on the surface and less NO2 molecules can be captured. In order to further improve the reaction rate, another attempt was taken in this study. Oxygen vacancies are strong adsorption sites for oxidizing molecules, and reported to be beneficial for NO2 adsorption on the metal oxide surface [15e18]. The surface reactivity of metal oxide is supposed to be increased by introducing oxygen vacancies into the sensing materials [19]. Therefore, the gas sensing characteristics would be greatly improved by increasing the amount of oxygen vacancies in sensing materials. According to [12,20], oxygen vacancies can be produced by H2O2 treatment with annealing. In this work, the as-sprayed CdS-ZnO sensors were treated by dipping into 20 mL H2O2 (30 vol%) at 80 C for 1 h, then annealing at 400 C for 1 h. XRD analysis was performed on the coatings in different steps to study the phase transformations during the H2O2 treatment and annealing. The as-sprayed CdS-ZnO and annealed CdS-ZnO1x after H2O2 treatment were well in accordance with hexagonal ZnO (PDF 36e1451), whereas the H2O2-treated CdS-ZnO consisted of ZnO2 phase (PDF 13e0311) apart from the ZnO phase. These results indicated that after H2O2 treatment, a part of ZnO was transformed into ZnO2. The XRD results also showed that the ZnO2 recovered to ZnO after annealing. It should be noticed that the CdS phase was unchanged during the H2O2 treatment and annealing, which means that H2O2 treatment and annealing have no obvious effect on CdS. However, as for whether oxygen vacancies were generated or not, other characterization techniques should be utilized. PL spectroscopy was a useful method to examine the existence of oxygen vacancies in CdS-ZnO1x coatings. Fig. 6 shows the PL spectrum of the coatings excited by a 325 nm He-Cd laser at room temperature. There are two peaks in the PL spectrum. One is located at 379 nm and the other is in the visible light region ranging from green to red. The 379 nm called near-band-edge (NBE) emission is arisen from free-exciton emission, which is ascribed to the electronic transitions from the conduction band to the valence band. The peak located at visible light region called broad deeplevel (DL) emission is attributed to the presence of oxygen vacancies [21e23]. According to [15e18], due to oxygen vacancies are dynamically unstable, they mainly exist in three kinds of forms, i.e., neutral charge state (Vxo , two electrons), singly (V$o , one electron)
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Fig. 3. Low magnification surface morphology of (a) the as-sprayed coatings and (b) the annealed CdS-ZnO coatings after H2O2 treatment, high magnification surface morphology of (c) the as-sprayed coatings and (d) the annealed CdS-ZnO coatings after H2O2 treatment.
Fig. 5. The electrical resistance responses of the as-sprayed CdS-ZnO sensors to 1 ppm NO2 gas with different light wavelength at room temperature. Fig. 4. UVeVis absorption spectra of the as-sprayed CdS-ZnO coatings and the CdSZnO1x coatings.
and doubly (Vo€) charged oxygen vacancies in metal oxide semiconductors. In addition, it can be seen that the intensity of DL emission is higher than that of NBE emission, which reveals that there is a large amount of oxygen vacancies in the CdS-ZnO1x coatings. Apart from the PL spectrum, EPR spectroscopy is another useful technique to identify paramagnetic vacancies. The EPR signal of the coatings exhibits two peaks corresponding to g-factors of 1.9610 and 2.0048, respectively (as shown in Fig. 7). The EPR peak at gz1.9610 is attributed to V$o , while the peak at gz2.0048 is ascribed to Vxo [23e25]. The EPR spectrum further confirmed that there is a large amount of oxygen vacancies in the CdS-ZnO1x
coatings. Moreover, it can be found that the EPR intensity of V$o is higher than that of Vxo , which means that V$o is the dominant surface defect in the coatings. Combined with the XRD, PL and EPR results, phase transformation and the presence of oxygen vacancies were demonstrated in the CdS-ZnO1x coatings. The H2O2 treatment with annealing modified the CdS-ZnO surface, generating highly concentrated oxygen vacancies. In addition, V$o is the dominant surface oxygen defect on the coating surface. The following reactions can be used to depict these changes during H2O2 treatment and annealing:
ZnO þ H2 O2 /ZnO2 þ H2 O
(2)
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Table 2 Sensing characteristics of the sensor based on LPS CdS-ZnO coating to 1 ppm NO2 illuminated by different wavelength light. Wavelength (nm)
Light color
Rair (U)
640 580 530 480 400
Red Yellow Green Blue Purple
3.72 9.74 6.03 8.55 5.29
104 103 103 102 102
Sensor response (RNO2/Rair)
Response time (min)
Recovery time (min)
1.45 1.93 1.78 1.60 1.76
25.7 23.6 23.1 15.7 22.7
>60 >60 >60 44.4 >60
further confirms that CdS is not affected during the post-treatment. The gas sensing characteristics of CdS-ZnO1x sensors are performed in the testing chamber. The electrical resistances of sensor based on CdS-ZnO1x coatings are plotted against time towards 1 ppm NO2 illuminated by lights with wavelengths ranging from 400 to 640 nm. It can be noted that light wavelength plays an important role in the sensor performance. The main sensor characteristics are drawn from Fig. 8 and listed in Table 3. Some results can be drawn: Firstly, the base resistance Rair diminished when light wavelength decreased from 640 to 480 nm. The base resistance decreased from 3.41 105 under red light to 5.91 104 U illuminated by purple light. The photon energy increases with the decrease in light wavelength according to the formula [26]: E ¼ hc/ l, where E is the light energy, h is the Planck’s constant, c is the velocity of light and l is the light wavelength. Hence, more electron-hole pairs are generated on CdS-ZnO1x surface as the light wavelength decreases, which results in a reduction of electrical resistance. Secondly, the CdS-ZnO1x sensor exhibits significant responses to NO2. The sensor response decreases when short wavelength light is used (except purple light). This is attributed to the narrow band gap of CdS and the presence of oxygen vacancies. The reason for the variation in sensor response will be discussed in the Section of Sensing mechanism. Thirdly, the response time and recovery time of CdS-ZnO1x sensors significantly decreased compared with the CdS-ZnO sensor. The response and recovery time are 5.7 and 9.9 min when blue light is employed. In addition, the response and recovery time diminishes when using short wavelength lights (except purple light). The role of photon energy on response and recovery rates is similar to heating [26,27]. When the light wavelength decreases, the photon energy increases, and thus the rates of gas diffusion as well as reaction on CdS-ZnO1x surface increased. It greatly enhances
Fig. 6. PL spectra of the CdS-ZnO1x coatings.
Fig. 7. EPR spectra of the CdS-ZnO1x coatings.
annealing
2ZnO2 ! 2ZnO þ Vxo þ O2
(3)
Vxo HV$o þ e
(4)
V$o HV$$ o þe
(5)
In order to elucidate the effect of H2O2 treatment with annealing on the as-sprayed CdS-ZnO coatings, UVeVis characterization was carried out. The pink line in Fig. 4 illustrates the UVeVis spectrum of CdS-ZnO1x. From that, it can be observed that the coating has absorption in the visible light range, even in red light region, which is attributed to the ionization of oxygen vacancies. Moreover, the spectrum also has an absorption edge suited at 545 nm, which
Fig. 8. The electrical resistances of sensor based on CdS-ZnO1x coatings are plotted against time towards 1 ppm NO2 illuminated by lights with wavelength ranging from 400 to 640 nm.
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Table 3 Sensing characteristics of the sensor based on CdS-ZnO1x coating to 1 ppm NO2 illuminated by different wavelength light. Wavelength (nm)
Light color
Rair (U)
640 580 530 480 400
Red Yellow Green Blue Purple
3.41 2.33 1.92 9.15 5.91
105 105 105 104 104
Sensor response (RNO2/Rair)
Response time (min)
Recovery time (min)
9.22 8.16 7.42 5.62 7.01
8.7 7.9 7.3 5.7 6.7
>20 >20 >20 9.9 15.3
the reaction rate between the sensing material and target gas. With the decrease in light wavelength (except purple light), the sensor response lowered whereas the response time and recovery time were shortened. Therefore, a compromise between sensor response and response time should be made. To sum up, blue light would be the best choice for CdS-ZnO1x sensor to get short response time and recovery time while maintaining an acceptable sensor response. Although the sensor response under blue light is not as high as that under other lights, a value of 5.62 is already enough for gas sensing applications. Similar compromise exists in sensors using heating, i.e., the higher the temperature, the lower the response time and recovery time. Compared with the results between the CdS-ZnO sensors (Table 2) and CdS-ZnO1x sensors (Table 3), three different points can be observed. First, the base resistance of CdS-ZnO1x sensors was higher than that of CdS-ZnO sensor. Due to oxygen vacancies acting as active sites for oxidizing gases, more oxygen molecules will be adsorbed on the CdS-ZnO1x surface. As well known, the adsorbed oxygen molecules would extract electrons from the conduction band of the sensitive material, which resulted in high base resistance in the CdS-ZnO1x sensor. Second, the response of CdS-ZnO1x sensor was higher than that of CdS-ZnO. All the sensor responses of CdS-ZnO were lower than 2.0 whereas those of CdSZnO1x were more than 5.6. The reason for this phenomenon was also attributed to the presence of more oxygen vacancies. Oxygen vacancies were the preferential sites for oxidizing gases, so more NO2 molecules would be adsorbed on the sensitive coatings upon exposing to NO2 gases, which led to higher sensor response. Third, CdS-ZnO1x sensors took less time to respond and recover compared with CdS-ZnO (it will be addressed in the Section of Sensing mechanism). 3.2. Sensing mechanism Surface depletion model and double Schottky barrier model are adopted to explain the sensing process of CdS-ZnO1x sensors in this study [8,26,27]. When the CdS-ZnO1x is exposed to air, the adsorbed oxygen ions O 2 are formed on ZnO1x surface by capturing electrons from the conduction band of ZnO1x at room temperature. A depletion region is then formed on the surface of ZnO1x and a barrier at the grain boundary is created, which results in an increase of the CdS-ZnO1x resistance. These reactions occurred via:
O2 ðgÞHO2 ðadÞ
(6)
O2 ðadÞ þ e H O 2 ðadÞ
(7)
Due to oxygen vacancies forming a deep donor state between the valence and conduction band of ZnO (abbreviated as Ed) [12], Vxo is easily ionized into V$o and V$$ o , as confirmed in PL and EPR spectra, like Reaction (4) and (5). When light is on, two factors, i.e., the ionization of oxygen vacancies and interband electronic transition in CdS, lead to the generation of electron-hole pairs. For CdSZnO coatings, owing to red light lay outside of visible light
absorption region (as displayed in the UVeVis spectrum), the photon energy of red light is not sufficient to excite CdS. However, in CdS-ZnO1x coatings, due to oxygen vacancies are unstable and easily positively charged [28], the electron-hole pairs will be formed when illuminated by red light. Once exposure to light with photon energy higher than red light, CdS will be excited and then more photogenerated electron-hole pairs are formed. On account of the energy band-gap matching and the effect of heterojunction between CdS and ZnO, the photogenerated electrons and holes could be effectively separated [29,30]. Then, the electrons from the excited CdS would be transferred to the conduction band of ZnO, which results in a rapid decrease in the electrical resistance and a thin depleted layer. Fig. 9 depicts the schematic of the excited process of the electrons the in CdS-ZnO1x coatings when exposing to visible light illumination. However, it can be observed that the base resistance of CdSZnO1x coatings is higher than that of CdS-ZnO coatings. As mentioned above, oxygen vacancies are strong adsorption sites for oxygen molecules [15e18], more oxygen species O 2 will be adsorbed on the CdS-ZnO1x coatings by taking electrons from the conduction band of ZnO, which significantly reduce the number of free electrons. The following expressions can be used to describe this: $ O2 ðadÞ þ Vxo HO 2 ðadÞ þ Vo
(8)
$$ O2 ðadÞ þ V$o HO 2 ðadÞ þ Vo
(9)
When NO2 is introduced into the chamber, due to higher electron affinity of NO2 (2.27 eV) than that of O2 (0.44 eV) [31], NO2 molecules would capture electrons from oxygen species adsorbed on CdS-ZnO1x coatings (Reaction (10)). Besides, NO2 can react with the adsorbed O 2 and facilitate the oxygen species to extract more electrons from ZnO conduction band (Reaction (11)). These reactions occur through: NO2 ðadÞ þ O 2 ðadÞHNO2 ðadÞ þ O2 ðgÞ
(10)
NO2 ðadÞ þ O 2 ðadÞ þ 2e HNO2 ðadÞ þ 2O ðadÞ
(11)
It has been reported that the surface acceptor level created by NO 2 is deeper than that of O2 , which increases the width of depletion layer and the height of grain boundary barrier [8,26,27]. It means that the electron concentration in ZnO1x conduction band decreases rapidly resulting in a dramatic increase of the electrical resistance. In addition to capturing the electrons from adsorbed ionized oxygen, the physisorbed NO2 can also extract electrons from the conduction band of ZnO1x surface creating NO 2 by:
NO2 ðadÞ þ e H NO 2 ðadÞ
(12)
It could be seen that the responses of CdS-ZnO1x sensors are far higher than those of CdS-ZnO sensors. The reason for this phenomenon is attributed to the presence of oxygen vacancies. Oxygen vacancy is reported to be the preferential and strong sites for
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Fig. 9. Schematic of the process of carriers formed in oxygen-vacant CdS-ZnO1x coating under visible light.
oxidizing gases [15e18], so more NO2 molecules would adsorb on the CdS-ZnO1x upon exposing to NO2 gases. More NO2 molecules involved in the reaction between the sensitive materials and target gases result in higher sensor response. The response is 5.62 under blue light illumination when NO2 concentration is 1 ppm, indicating that the CdS-ZnO1x sensors developed in this study have a high detectability. The following reactions can be utilized to describe this: $ NO2 ðadÞ þ Vxo HNO 2 ðadÞ þ Vo
(13)
$$ NO2 ðadÞ þ V$o HNO 2 ðadÞ þ Vo
(14)
In addition, it should be noticed that the response of CdS-ZnO in red light is the lowest (1.45) whereas the response of CdS-ZnO1x is the highest (9.22). The reason for this result is also attributed to the presence of oxygen vacancies. On account of the ionization of oxygen vacancies, many electron-hole pairs are generated under red light illumination, which increases the electron concentration in CdS-ZnO1x coatings. It will accelerate the Reactions (10)e(14) to form more NO 2 , which increases the sensor response. Furthermore, it can be observed that the biggest difference between the CdS-ZnO and CdS-ZnO1x sensors is the response time and recovery time. The response time and recovery time for CdSZnO1x sensor are only 5.7 and 9.9 min whereas those of the CdSZnO sensor are 15.7 and 44.4 min when blue light is used. The reaction rate is significantly enhanced for CdS-ZnO1x sensors. As oxygen vacancies are preferential and strong adsorption sites for NO2 molecules, the reaction between CdS-ZnO1x and NO2 gas is rapid once introducing NO2 gas. In addition, due to the photon energy of blue light is high, the surface reaction rate will be further enhanced, which increases the response and desorption rate of CdS-ZnO1x sensor. The effect of light-generated oxygen ions O 2 (hn) on the response and recovery rate should also be taken into account. Fan et al. [32] proposed that photogenerated electron-hole pairs released the O 2 ion and formed O2 (hn) via:
As aforementioned, more O 2 species exist in CdS-ZnO1x coatings, which facilitates the Reaction (16). Therefore, there are more O 2 (hn) on CdS-ZnO1x coating surface. O2 (hn) is reported to be weakly bound to metal oxide [33], so the adsorption and desorption of NO2 molecules could proceed facilely on CdS-ZnO1x surface. It greatly accelerates the surface reaction rate, which decreases the response and recovery time. After removal of NO2, the Reactions (10)e(14) go in the opposite direction. The electrons will transfer from NO 2 to oxygen molecules reforming ionized oxygen again or be moved to the conduction band of CdS-ZnO1x, which decreases electrical resistance. In comparison with blue light, although the photon energy of purple light is higher, however, its response time and recovery time increase, which may be ascribed to the additional reactions of NO2 under purple light [26]. 4. Conclusions In order to improve the response time and recovery time of the sensors based on liquid plasma-sprayed CdS-ZnO coatings, surface modification was applied on CdS-ZnO surface to obtain rich oxygen vacancies. The CdS-ZnO1x coatings were prepared by H2O2 treatment with annealing of the CdS-ZnO ones. The production of oxygen vacancies was confirmed by PL and EPR analysis and found to extend light response range of the sensors towards longer wavelength. It was revealed that V$o was the dominant surface defect in the CdS-ZnO1x coatings. The CdS-ZnO1x sensors showed better sensing characteristics, from sensor response to response time and recovery time, compared with CdS-ZnO sensors. These results indicated that the oxygen vacancy on CdS-ZnO1x was beneficial to enhance NO2 sensing performance and the reaction rate between sensing material and target gas. A simple model based on ionization energy of oxygen vacancy and interband electronic transition of CdS was used to qualitatively explain the improvement in sensing characteristics of CdS-ZnO1x coatings. Acknowledgements
hn/e þhþ
(15)
hþ þ O 2 /O2
(16)
O2 þ e /O 2 ðhnÞ
(17)
This work is supported by the funding of the Natural Science Foundation of China under Grant No.51402255, the Jiangsu Natural Science Foundation of China under Grant No.BK20140487, the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Testing Center of Yangzhou University.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure and sensing properties of CdS-ZnO1x coatings deposited by liquid plasma spray and treated with hydrogen peroxide solution for nitrogen dioxide detection at room temperature Xin Geng a, b, Jiajun You a, Chao Zhang a, * a b
College of Mechanical Engineering, Yangzhou University, Yangzhou 225127, PR China College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 April 2016 Received in revised form 29 May 2016 Accepted 9 June 2016 Available online 14 June 2016
Metal oxide semiconductor gas sensors usually respond slowly and weakly to target gas at room temperature. In this paper, we report a nitrogen dioxide gas sensor based on porous CdS-ZnO1x coatings using a combination of sensitization, surface modification and visible light illumination methods to improve sensing characteristics. Porous CdS-ZnO coatings were deposited by liquid plasma spraying process. Then the coating surface was modified by immersing in a hydrogen peroxide solution and annealing to generate oxygen vacancies and finally obtain CdS-ZnO1x coatings. Photoluminescence spectroscopy, electron paramagnetic resonance and UVeVis diffuse reflectance spectrophotometer were utilized to characterize the oxygen vacancies and optical properties of the obtained coatings. The UVeVis results revealed that the absorption of the obtained coatings was extended to the whole visible light region after the treatment. Nitrogen dioxide gas sensing properties of the CdS-ZnO and CdS-ZnO1x coatings were measured and compared. The gas sensing results showed that the surface modification (surface oxygen vacancies) can greatly enhanced sensor responses and significantly shortened the response time and recovery time. The sensing mechanism of the obtained CdS-ZnO1x coatings was discussed in terms of the effect of oxygen vacancies. © 2016 Elsevier B.V. All rights reserved.
Keywords: CdS-ZnO1x sensor Oxygen vacancy Liquid plasma spray Nitrogen dioxide Room temperature
1. Introduction Nitrogen oxides (NOx: NO and NO2) derived from vehicle emission and power generation exhaust will lead to serious environmental pollution, and thus have a negative influence on the health of human beings [1]. For the protection of people’s health and promotion of sustainable development of society, it is imperative to enhance the monitoring of toxic gases, especially to nitrogen dioxide (NO2) [2]. Many researchers have reported that zinc oxide is one of the most promising NO2 gas sensing materials [2,3]. ZnO based gas sensors exhibit excellent response to NO2 even at low concentration. However, working temperature is a big problem due to traditional ZnO based gas sensors are generally operated at elevated temperature ranging from 200 to 600 C [4]. Decreasing working temperature can not only effectively reduce power
* Corresponding author. College of Mechanical Engineering, Yangzhou University, 196 West Huayang Road, Yangzhou, Jiangsu, PR China. E-mail addresses: [email protected], [email protected] (C. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.06.079 0925-8388/© 2016 Elsevier B.V. All rights reserved.
consumption, but also enhance the security in the complex atmosphere, particularly under the conditions of flammable gases [5]. Apart from improving economic efficiency and safety, developing low temperature or even room-temperature semiconductor sensors also raises the long-term stability of sensors [6]. Photogenerated electron-hole pairs are generated only when semiconductors are illuminated under light with photon energy higher than its band-gap energy [7e10]. That is to say, the wavelength of excitation light should be less than 380 nm because the intrinsic wide band-gap of ZnO is ca. 3.2 eV. Incorporating narrow band-gap semiconductors like CdS [7,8], PbS [9], Bi2S3 [10] into ZnO is an effective way to enable the composite coatings work at low temperature. When narrow band gap semiconductor, like CdS, PbS and Bi2O3, are exposed to visible light illumination, electron-hole pairs will be generated and then electrons will be transferred to ZnO. In the previous study [8], we have reported that 1 ppm NO2 can be detected at room temperature by CdS-ZnO sensor under visible light illumination [8]. However, the response time and recovery time of this sensor are long. Therefore, one of the big challenges for CdS-ZnO sensors is to increase the reaction rate. It has
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been reported that semiconductor gas sensors based on metal oxides will be more sensitive to oxidizing gases with assistance of oxygen vacancies [11e13]. Therefore, the sensing characteristics may be greatly enhanced by introducing oxygen vacancies into sensing materials. The coating microstructure is well-known to greatly influence the gas sensing characteristics [13]. The sensing materials with nanosized and porous structure are expected to obtain excellent sensing performance [14]. Liquid plasma spray (LPS) is such a method to deposit nanostructured ceramic coatings in which a liquid feedstock is injected into a plasma flame. In contact with hot plasma, after evaporation and precipitation of the liquid droplets, produced solid particles are heated, accelerated and impacted on the substrate to form coatings. The LPS coatings are easily tailored to be porous with a large surface-to-volume ratio. In this article, CdS-ZnO composite coatings were deposited by LPS process. A post-treatment process with H2O2 treatment and annealing is utilized to obtain the oxygen-vacant CdS-ZnO1x coatings. The phase constitutions, surface morphologies and electrical resistances of the obtained coatings were characterized. The effects of light wavelength on the sensor characteristics were investigated. The sensing mechanism of the oxygen vacant CdSZnO1x sensors under visible light illumination was discussed. 2. Experimental methods 2.1. Coating preparation CdS-ZnO composite coatings were deposited on alumina substrates equipped with a pair of interdigitated gold electrodes (CMAC Micro Technology Company, Belgium) by LPS with a F4MB-XL plasma spray torch (Oerlikon Metco, Switzerland) attached to a sixaxis ABB robot arm (ABB, Sweden). In this work, the liquid used was an aqueous solution containing 0.2 M zinc acetate and 0.02 M cadmium sulfide, and injected into the plasma jet by a nozzle with an inner diameter of 0.26 mm through a peristaltic pump. The spraying parameters were listed in Table 1. CdS-ZnO1x coatings were obtained by dipping the as-sprayed coatings into 20 ml H2O2 (30 vol%) at 80 C for 1 h. Then, the coatings were annealed in a furnace at 400 C for 1 h. 2.2. Coating characterization
287
the presence of oxygen vacancies in the CdS-ZnO1x coatings was confirmed by the advanced techniques, including photoluminescence spectroscopy (PL, with He-Cd 325 nm laser) and electron paramagnetic resonance (EPR, A300-10/12, Bruker, Germany). 2.3. Gas sensor testing A 12 W LED lamp was installed in a sealed chamber and put in front of the as-sprayed CdS-ZnO and CdS-ZnO1x sensors. The chamber was in steel and a Teflon film was coated in the inner wall of the chamber. The lamps with a wavelength of 400, 480, 530, 580 and 640 nm (purple, blue, green yellow and red light) were used to study the effect of light wavelength on the sensing properties. The distance between the LED lamp and the sensors was controlled at 30 mm. Visible light illumination and electrical resistance measurement were carried out in the chamber. The sensors were connected to an Agilent data acquisition instrument to record the electrical resistances. Prior to the NO2 tests, synthetic air with a flow rate of 500 mL/min was introduced into the chamber until the electrical resistance was stable. Then, 10 ppm NO2 with controlled flow rates was injected into the chamber to obtain a desired NO2 concentration. The sensor response was defined as S ¼ RNO2/Rair where RNO2 and Rair were the resistances of sensors in presence of NO2 gas and synthetic air, respectively. Response time is defined as the time required for the electrical resistance starting from Rair to reach 90% of RNO2 while recovery time represents the time needed for resistance decreasing from RNO2 to 110% of Rair. 3. Results and discussion 3.1. Coating characterization Fig. 1 illustrates the XRD patterns of the coatings in different steps with a scanning rate of 5 $min1. There is Al2O3 peaks emitted from the alumina substrate because the as-sprayed coatings are thin. The result demonstrates that the as-sprayed coating is well in agreement with hexagonal wurtzite ZnO (PDF 36e1451). In addition, it can also be easily seen that the (111) diffraction peak of cubic CdS appeared according to PDF 65e2887, which reveals that the CdS particles are deposited in the composite coatings. Similar
Surface morphology of the obtained coatings were inspected using field-emission scanning electron microscopy (FE-SEM, S4800 II, Hitachi, Japan). The crystal structure of the coatings was determined by X-ray diffractometer (XRD, D8 Advance, Bruker, Germany) using a CuKa radiation. 2q range of XRD diffraction angle was 20e70 with a scanning rate of 5 $min1 and 30e40 with 0.1 $min1 was used in this test. UVeVis diffuse reflectance spectrophotometer (Cary5000, Varian, USA) using BaSO4 as the reference was used to record UVeVis absorption spectra of the coatings within light wavelength ranging from 300 to 800 nm. Additionally,
Table 1 Liquid plasma spraying parameters. Parameters
Value
Arc current Torch power Spraying distance Argon volume flow rate Hydrogen volume flow rate Nozzle inner diameter Liquid flow rate
513 A 25 kW 100 mm 40 L/min 2 L/min 0.26 mm 20 mL/min
Fig. 1. X-ray diffraction patterns of the as-sprayed CdS-ZnO coatings, H2O2-treated CdS-ZnO coatings, and annealed CdS-ZnO coatings after H2O2 treatment.
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results have been observed in the previously published paper [8]. It can be found from the brown pattern that ZnO2 is produced when the coatings are treated by H2O2 solution. After the annealing, ZnO2 phase disappeared. Fig. 2 presents the XRD pattern of the as-sprayed and annealed after H2O2 treatment coatings with a scanning rate of 0.1 $min1 in the range of 30e40 . The crystallite size in the coatings is calculated through Scherrer formula:
1=2 D ¼ k$l=cosq b2 b2
(1)
where D is the average crystallite size, k is a shape factor of 0.89, l is the wavelength of X-ray (CuKa ¼ 0.15418 nm), b is the full width at half maximum, b is the instrumental broadening equal to 0.075, and q is the Bragg diffraction angle. The crystallite size at the three main peaks of the as-sprayed coatings is 28 ± 5 nm, while that of the annealed with H2O2 treated coatings is 31 ± 5 nm. The observed slight grain growth may be attributed to the annealing treatment at 400 C for 1 h. Fig. 3(aed) depict the surface morphology with low and high magnifications of the as-sprayed and the annealed with H2O2 treatment coatings. It can be observed that the coatings are nanostructured with crystallite size of ca. 30 nm, which is consistent with the XRD results. The LPS method is a rapid cooling process with high cooling speed over 105 K/s, resulting in small crystallite size of the as-sprayed coatings [1]. Meanwhile, due to LPS process adopting liquid instead of powder as feedstock and the water in liquid consuming a part of plasma enthalpy, less energy is applied on individual droplets and nanostructured CdS-ZnO coatings are produced [3,13]. Moreover, it could also be found that the coating is porous. The size of pores existing in the as-sprayed coatings are about microns, which is ascribed to the entrapped gases, cracking and premature solidification of some splats [1]. Furthermore, it can be found from Fig. 3 (c) and (d) that the annealed coatings are smoother compared with the as-sprayed coatings, which can be ascribed to the immersion in hydrogen peroxide solution for 1 h. Some badly-bonded particles on the surface are dissolved in the H2O2 solution and the well-bonded ones are retained. Fig. 4 presents the UVeVis absorption spectra of the as-sprayed CdS-ZnO and annealed CdS-ZnO1x coatings recording under light illumination with a wavelength of 300e800 nm. It can be observed
Fig. 2. X-ray diffraction patterns of the as-sprayed and annealed CdS-ZnO coatings after H2O2 treatment coatings with slow scan.
that there are two absorption edges locating at 400 and 545 nm, respectively. The 400 nm absorption edge was originated from band to band excitation of ZnO, while the 545 nm was arose from interband electronic transition of CdS. This result confirmed that CdS was successfully deposited in the CdS-ZnO and CdS-ZnO1x coatings. The absorption range of the two coatings was extended to 545 nm and more. Furthermore, it can be found that the absorption of the CdS-ZnO1x coating was significantly enhanced compared with that of the CdS-ZnO coating. In the previous paper [8], we have reported that the light intensity can shorten the response time and recovery time, so high intensity LED lamp (12 W) is used in this study. Fig. 5 shows the electrical resistance responses of the CdS-ZnO sensors to 1 ppm NO2 gas with different light wavelength at room temperature, and relevant data are listed in Table 2. It can be observed that the base resistance decreased with reduction in light wavelength. In addition, the reaction rate was slightly enhanced. However, the response time and recovery time were still long, which limited its practical application. Moreover, the sensor responses under all lights were lower than 2 and significantly decreased compared with that operated at lower light intensity of 3 W [8]. It should be noticed that the sensor response was the lowest illuminated under red light. This can be ascribed to that CdS cannot be excited illuminated under the light with wavelength less than 545 nm (red light) and electron number in the conduction band of ZnO is not enough. Therefore, less electrons existed on the surface and less NO2 molecules can be captured. In order to further improve the reaction rate, another attempt was taken in this study. Oxygen vacancies are strong adsorption sites for oxidizing molecules, and reported to be beneficial for NO2 adsorption on the metal oxide surface [15e18]. The surface reactivity of metal oxide is supposed to be increased by introducing oxygen vacancies into the sensing materials [19]. Therefore, the gas sensing characteristics would be greatly improved by increasing the amount of oxygen vacancies in sensing materials. According to [12,20], oxygen vacancies can be produced by H2O2 treatment with annealing. In this work, the as-sprayed CdS-ZnO sensors were treated by dipping into 20 mL H2O2 (30 vol%) at 80 C for 1 h, then annealing at 400 C for 1 h. XRD analysis was performed on the coatings in different steps to study the phase transformations during the H2O2 treatment and annealing. The as-sprayed CdS-ZnO and annealed CdS-ZnO1x after H2O2 treatment were well in accordance with hexagonal ZnO (PDF 36e1451), whereas the H2O2-treated CdS-ZnO consisted of ZnO2 phase (PDF 13e0311) apart from the ZnO phase. These results indicated that after H2O2 treatment, a part of ZnO was transformed into ZnO2. The XRD results also showed that the ZnO2 recovered to ZnO after annealing. It should be noticed that the CdS phase was unchanged during the H2O2 treatment and annealing, which means that H2O2 treatment and annealing have no obvious effect on CdS. However, as for whether oxygen vacancies were generated or not, other characterization techniques should be utilized. PL spectroscopy was a useful method to examine the existence of oxygen vacancies in CdS-ZnO1x coatings. Fig. 6 shows the PL spectrum of the coatings excited by a 325 nm He-Cd laser at room temperature. There are two peaks in the PL spectrum. One is located at 379 nm and the other is in the visible light region ranging from green to red. The 379 nm called near-band-edge (NBE) emission is arisen from free-exciton emission, which is ascribed to the electronic transitions from the conduction band to the valence band. The peak located at visible light region called broad deeplevel (DL) emission is attributed to the presence of oxygen vacancies [21e23]. According to [15e18], due to oxygen vacancies are dynamically unstable, they mainly exist in three kinds of forms, i.e., neutral charge state (Vxo , two electrons), singly (V$o , one electron)
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289
Fig. 3. Low magnification surface morphology of (a) the as-sprayed coatings and (b) the annealed CdS-ZnO coatings after H2O2 treatment, high magnification surface morphology of (c) the as-sprayed coatings and (d) the annealed CdS-ZnO coatings after H2O2 treatment.
Fig. 5. The electrical resistance responses of the as-sprayed CdS-ZnO sensors to 1 ppm NO2 gas with different light wavelength at room temperature. Fig. 4. UVeVis absorption spectra of the as-sprayed CdS-ZnO coatings and the CdSZnO1x coatings.
and doubly (Vo€) charged oxygen vacancies in metal oxide semiconductors. In addition, it can be seen that the intensity of DL emission is higher than that of NBE emission, which reveals that there is a large amount of oxygen vacancies in the CdS-ZnO1x coatings. Apart from the PL spectrum, EPR spectroscopy is another useful technique to identify paramagnetic vacancies. The EPR signal of the coatings exhibits two peaks corresponding to g-factors of 1.9610 and 2.0048, respectively (as shown in Fig. 7). The EPR peak at gz1.9610 is attributed to V$o , while the peak at gz2.0048 is ascribed to Vxo [23e25]. The EPR spectrum further confirmed that there is a large amount of oxygen vacancies in the CdS-ZnO1x
coatings. Moreover, it can be found that the EPR intensity of V$o is higher than that of Vxo , which means that V$o is the dominant surface defect in the coatings. Combined with the XRD, PL and EPR results, phase transformation and the presence of oxygen vacancies were demonstrated in the CdS-ZnO1x coatings. The H2O2 treatment with annealing modified the CdS-ZnO surface, generating highly concentrated oxygen vacancies. In addition, V$o is the dominant surface oxygen defect on the coating surface. The following reactions can be used to depict these changes during H2O2 treatment and annealing:
ZnO þ H2 O2 /ZnO2 þ H2 O
(2)
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Table 2 Sensing characteristics of the sensor based on LPS CdS-ZnO coating to 1 ppm NO2 illuminated by different wavelength light. Wavelength (nm)
Light color
Rair (U)
640 580 530 480 400
Red Yellow Green Blue Purple
3.72 9.74 6.03 8.55 5.29
104 103 103 102 102
Sensor response (RNO2/Rair)
Response time (min)
Recovery time (min)
1.45 1.93 1.78 1.60 1.76
25.7 23.6 23.1 15.7 22.7
>60 >60 >60 44.4 >60
further confirms that CdS is not affected during the post-treatment. The gas sensing characteristics of CdS-ZnO1x sensors are performed in the testing chamber. The electrical resistances of sensor based on CdS-ZnO1x coatings are plotted against time towards 1 ppm NO2 illuminated by lights with wavelengths ranging from 400 to 640 nm. It can be noted that light wavelength plays an important role in the sensor performance. The main sensor characteristics are drawn from Fig. 8 and listed in Table 3. Some results can be drawn: Firstly, the base resistance Rair diminished when light wavelength decreased from 640 to 480 nm. The base resistance decreased from 3.41 105 under red light to 5.91 104 U illuminated by purple light. The photon energy increases with the decrease in light wavelength according to the formula [26]: E ¼ hc/ l, where E is the light energy, h is the Planck’s constant, c is the velocity of light and l is the light wavelength. Hence, more electron-hole pairs are generated on CdS-ZnO1x surface as the light wavelength decreases, which results in a reduction of electrical resistance. Secondly, the CdS-ZnO1x sensor exhibits significant responses to NO2. The sensor response decreases when short wavelength light is used (except purple light). This is attributed to the narrow band gap of CdS and the presence of oxygen vacancies. The reason for the variation in sensor response will be discussed in the Section of Sensing mechanism. Thirdly, the response time and recovery time of CdS-ZnO1x sensors significantly decreased compared with the CdS-ZnO sensor. The response and recovery time are 5.7 and 9.9 min when blue light is employed. In addition, the response and recovery time diminishes when using short wavelength lights (except purple light). The role of photon energy on response and recovery rates is similar to heating [26,27]. When the light wavelength decreases, the photon energy increases, and thus the rates of gas diffusion as well as reaction on CdS-ZnO1x surface increased. It greatly enhances
Fig. 6. PL spectra of the CdS-ZnO1x coatings.
Fig. 7. EPR spectra of the CdS-ZnO1x coatings.
annealing
2ZnO2 ! 2ZnO þ Vxo þ O2
(3)
Vxo HV$o þ e
(4)
V$o HV$$ o þe
(5)
In order to elucidate the effect of H2O2 treatment with annealing on the as-sprayed CdS-ZnO coatings, UVeVis characterization was carried out. The pink line in Fig. 4 illustrates the UVeVis spectrum of CdS-ZnO1x. From that, it can be observed that the coating has absorption in the visible light range, even in red light region, which is attributed to the ionization of oxygen vacancies. Moreover, the spectrum also has an absorption edge suited at 545 nm, which
Fig. 8. The electrical resistances of sensor based on CdS-ZnO1x coatings are plotted against time towards 1 ppm NO2 illuminated by lights with wavelength ranging from 400 to 640 nm.
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Table 3 Sensing characteristics of the sensor based on CdS-ZnO1x coating to 1 ppm NO2 illuminated by different wavelength light. Wavelength (nm)
Light color
Rair (U)
640 580 530 480 400
Red Yellow Green Blue Purple
3.41 2.33 1.92 9.15 5.91
105 105 105 104 104
Sensor response (RNO2/Rair)
Response time (min)
Recovery time (min)
9.22 8.16 7.42 5.62 7.01
8.7 7.9 7.3 5.7 6.7
>20 >20 >20 9.9 15.3
the reaction rate between the sensing material and target gas. With the decrease in light wavelength (except purple light), the sensor response lowered whereas the response time and recovery time were shortened. Therefore, a compromise between sensor response and response time should be made. To sum up, blue light would be the best choice for CdS-ZnO1x sensor to get short response time and recovery time while maintaining an acceptable sensor response. Although the sensor response under blue light is not as high as that under other lights, a value of 5.62 is already enough for gas sensing applications. Similar compromise exists in sensors using heating, i.e., the higher the temperature, the lower the response time and recovery time. Compared with the results between the CdS-ZnO sensors (Table 2) and CdS-ZnO1x sensors (Table 3), three different points can be observed. First, the base resistance of CdS-ZnO1x sensors was higher than that of CdS-ZnO sensor. Due to oxygen vacancies acting as active sites for oxidizing gases, more oxygen molecules will be adsorbed on the CdS-ZnO1x surface. As well known, the adsorbed oxygen molecules would extract electrons from the conduction band of the sensitive material, which resulted in high base resistance in the CdS-ZnO1x sensor. Second, the response of CdS-ZnO1x sensor was higher than that of CdS-ZnO. All the sensor responses of CdS-ZnO were lower than 2.0 whereas those of CdSZnO1x were more than 5.6. The reason for this phenomenon was also attributed to the presence of more oxygen vacancies. Oxygen vacancies were the preferential sites for oxidizing gases, so more NO2 molecules would be adsorbed on the sensitive coatings upon exposing to NO2 gases, which led to higher sensor response. Third, CdS-ZnO1x sensors took less time to respond and recover compared with CdS-ZnO (it will be addressed in the Section of Sensing mechanism). 3.2. Sensing mechanism Surface depletion model and double Schottky barrier model are adopted to explain the sensing process of CdS-ZnO1x sensors in this study [8,26,27]. When the CdS-ZnO1x is exposed to air, the adsorbed oxygen ions O 2 are formed on ZnO1x surface by capturing electrons from the conduction band of ZnO1x at room temperature. A depletion region is then formed on the surface of ZnO1x and a barrier at the grain boundary is created, which results in an increase of the CdS-ZnO1x resistance. These reactions occurred via:
O2 ðgÞHO2 ðadÞ
(6)
O2 ðadÞ þ e H O 2 ðadÞ
(7)
Due to oxygen vacancies forming a deep donor state between the valence and conduction band of ZnO (abbreviated as Ed) [12], Vxo is easily ionized into V$o and V$$ o , as confirmed in PL and EPR spectra, like Reaction (4) and (5). When light is on, two factors, i.e., the ionization of oxygen vacancies and interband electronic transition in CdS, lead to the generation of electron-hole pairs. For CdSZnO coatings, owing to red light lay outside of visible light
absorption region (as displayed in the UVeVis spectrum), the photon energy of red light is not sufficient to excite CdS. However, in CdS-ZnO1x coatings, due to oxygen vacancies are unstable and easily positively charged [28], the electron-hole pairs will be formed when illuminated by red light. Once exposure to light with photon energy higher than red light, CdS will be excited and then more photogenerated electron-hole pairs are formed. On account of the energy band-gap matching and the effect of heterojunction between CdS and ZnO, the photogenerated electrons and holes could be effectively separated [29,30]. Then, the electrons from the excited CdS would be transferred to the conduction band of ZnO, which results in a rapid decrease in the electrical resistance and a thin depleted layer. Fig. 9 depicts the schematic of the excited process of the electrons the in CdS-ZnO1x coatings when exposing to visible light illumination. However, it can be observed that the base resistance of CdSZnO1x coatings is higher than that of CdS-ZnO coatings. As mentioned above, oxygen vacancies are strong adsorption sites for oxygen molecules [15e18], more oxygen species O 2 will be adsorbed on the CdS-ZnO1x coatings by taking electrons from the conduction band of ZnO, which significantly reduce the number of free electrons. The following expressions can be used to describe this: $ O2 ðadÞ þ Vxo HO 2 ðadÞ þ Vo
(8)
$$ O2 ðadÞ þ V$o HO 2 ðadÞ þ Vo
(9)
When NO2 is introduced into the chamber, due to higher electron affinity of NO2 (2.27 eV) than that of O2 (0.44 eV) [31], NO2 molecules would capture electrons from oxygen species adsorbed on CdS-ZnO1x coatings (Reaction (10)). Besides, NO2 can react with the adsorbed O 2 and facilitate the oxygen species to extract more electrons from ZnO conduction band (Reaction (11)). These reactions occur through: NO2 ðadÞ þ O 2 ðadÞHNO2 ðadÞ þ O2 ðgÞ
(10)
NO2 ðadÞ þ O 2 ðadÞ þ 2e HNO2 ðadÞ þ 2O ðadÞ
(11)
It has been reported that the surface acceptor level created by NO 2 is deeper than that of O2 , which increases the width of depletion layer and the height of grain boundary barrier [8,26,27]. It means that the electron concentration in ZnO1x conduction band decreases rapidly resulting in a dramatic increase of the electrical resistance. In addition to capturing the electrons from adsorbed ionized oxygen, the physisorbed NO2 can also extract electrons from the conduction band of ZnO1x surface creating NO 2 by:
NO2 ðadÞ þ e H NO 2 ðadÞ
(12)
It could be seen that the responses of CdS-ZnO1x sensors are far higher than those of CdS-ZnO sensors. The reason for this phenomenon is attributed to the presence of oxygen vacancies. Oxygen vacancy is reported to be the preferential and strong sites for
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Fig. 9. Schematic of the process of carriers formed in oxygen-vacant CdS-ZnO1x coating under visible light.
oxidizing gases [15e18], so more NO2 molecules would adsorb on the CdS-ZnO1x upon exposing to NO2 gases. More NO2 molecules involved in the reaction between the sensitive materials and target gases result in higher sensor response. The response is 5.62 under blue light illumination when NO2 concentration is 1 ppm, indicating that the CdS-ZnO1x sensors developed in this study have a high detectability. The following reactions can be utilized to describe this: $ NO2 ðadÞ þ Vxo HNO 2 ðadÞ þ Vo
(13)
$$ NO2 ðadÞ þ V$o HNO 2 ðadÞ þ Vo
(14)
In addition, it should be noticed that the response of CdS-ZnO in red light is the lowest (1.45) whereas the response of CdS-ZnO1x is the highest (9.22). The reason for this result is also attributed to the presence of oxygen vacancies. On account of the ionization of oxygen vacancies, many electron-hole pairs are generated under red light illumination, which increases the electron concentration in CdS-ZnO1x coatings. It will accelerate the Reactions (10)e(14) to form more NO 2 , which increases the sensor response. Furthermore, it can be observed that the biggest difference between the CdS-ZnO and CdS-ZnO1x sensors is the response time and recovery time. The response time and recovery time for CdSZnO1x sensor are only 5.7 and 9.9 min whereas those of the CdSZnO sensor are 15.7 and 44.4 min when blue light is used. The reaction rate is significantly enhanced for CdS-ZnO1x sensors. As oxygen vacancies are preferential and strong adsorption sites for NO2 molecules, the reaction between CdS-ZnO1x and NO2 gas is rapid once introducing NO2 gas. In addition, due to the photon energy of blue light is high, the surface reaction rate will be further enhanced, which increases the response and desorption rate of CdS-ZnO1x sensor. The effect of light-generated oxygen ions O 2 (hn) on the response and recovery rate should also be taken into account. Fan et al. [32] proposed that photogenerated electron-hole pairs released the O 2 ion and formed O2 (hn) via:
As aforementioned, more O 2 species exist in CdS-ZnO1x coatings, which facilitates the Reaction (16). Therefore, there are more O 2 (hn) on CdS-ZnO1x coating surface. O2 (hn) is reported to be weakly bound to metal oxide [33], so the adsorption and desorption of NO2 molecules could proceed facilely on CdS-ZnO1x surface. It greatly accelerates the surface reaction rate, which decreases the response and recovery time. After removal of NO2, the Reactions (10)e(14) go in the opposite direction. The electrons will transfer from NO 2 to oxygen molecules reforming ionized oxygen again or be moved to the conduction band of CdS-ZnO1x, which decreases electrical resistance. In comparison with blue light, although the photon energy of purple light is higher, however, its response time and recovery time increase, which may be ascribed to the additional reactions of NO2 under purple light [26]. 4. Conclusions In order to improve the response time and recovery time of the sensors based on liquid plasma-sprayed CdS-ZnO coatings, surface modification was applied on CdS-ZnO surface to obtain rich oxygen vacancies. The CdS-ZnO1x coatings were prepared by H2O2 treatment with annealing of the CdS-ZnO ones. The production of oxygen vacancies was confirmed by PL and EPR analysis and found to extend light response range of the sensors towards longer wavelength. It was revealed that V$o was the dominant surface defect in the CdS-ZnO1x coatings. The CdS-ZnO1x sensors showed better sensing characteristics, from sensor response to response time and recovery time, compared with CdS-ZnO sensors. These results indicated that the oxygen vacancy on CdS-ZnO1x was beneficial to enhance NO2 sensing performance and the reaction rate between sensing material and target gas. A simple model based on ionization energy of oxygen vacancy and interband electronic transition of CdS was used to qualitatively explain the improvement in sensing characteristics of CdS-ZnO1x coatings. Acknowledgements
hn/e þhþ
(15)
hþ þ O 2 /O2
(16)
O2 þ e /O 2 ðhnÞ
(17)
This work is supported by the funding of the Natural Science Foundation of China under Grant No.51402255, the Jiangsu Natural Science Foundation of China under Grant No.BK20140487, the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Testing Center of Yangzhou University.
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