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Room-temperature nitrogen-dioxide sensors based on ZnO1−x coatings deposited by solution precursor plasma spray
Sensors and Actuators B 242 (2017) 102–111
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Room-temperature nitrogen-dioxide sensors based on ZnO1−x coatings deposited by solution precursor plasma spray Chao Zhang a,∗ , Xin Geng a,b , Hanlin Liao c , Chang-Jiu Li d , Marc Debliquy e a
College of Mechanical Engineering, Yangzhou University, Yangzhou 25127, PR China College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, PR China IRTES-LERMPS, Université de Technologie de Belfort-Montbeliard, Belfort 90000, France d State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xian 700149, PR China e Service de Science des Matériaux, Faculté Polytechnique, Université de Mons, Mons 7000, Belgium b c
a r t i c l e
i n f o
Article history: Received 13 July 2016 Received in revised form 26 October 2016 Accepted 5 November 2016 Available online 9 November 2016 Keywords: ZnO Gas sensors Oxygen vacancy Porous nanostructure Solution precursor plasma spray
a b s t r a c t The stoichiometric ZnO has a bandgap greater than 2.6 eV and has no or weak response to visible light, which greatly suppresses its applications. We report here that a process of solution precursor plasma spray (SPPS) can effectively narrow the bandgap and extend the absorption of ZnO to visible light region, which mainly results from highly concentrated oxygen vacancies generated during SPPS process. By photoluminescence spectroscopy, electron paramagnetic resonance and X-ray photoelectron spectroscopy, we found that a large number of oxygen vacancies were implanted into ZnO1−x prepared by SPPS. The generation of highly concentrated oxygen vacancies were mainly attributed to reducing atmosphere as well as fast heating and cooling process inherently provided by SPPS. The findings of this work create a new way for developing narrow bandgap ZnO1−x . We present that the SPPS ZnO1−x coatings can be utilized directly as sensitive materials under visible-light illumination. The oxygen vacancies have considerable influence on its optical and electrical properties. The ZnO1−x coatings exhibited an obvious absorption covering the whole visible-light region and its bandgap was calculated to be 2.15 eV which was much narrower than that of stoichiometric ZnO (3.37 eV). The sensors based on ZnO1−x coatings showed significant responses to NO2 at room temperature. In addition, the sensor response increased linearly with NO2 concentration. The enhanced sensor properties were attributed to the rich oxygen vacancies and special coating microstructure provided by SPPS. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In the last decades, increasing attention has been paid on the wide band gap semiconductors owing to their extensive application in solar cells, photocatalysts, optoelectronic devices and gas sensors [1–4]. In order to further widen the application in the visible light range and enhance the properties, it is necessary to narrow their bandgaps [5]. The bandgap is a key character for metal oxide semiconductors and a pure oxide semiconductor material usually has a constant value. Metal or nonmetal element doping is a conventional approach to adjust the bandgap by adding additional donor and acceptor level into the materials, such as Fe, Al, Mn, C and N [6,7]. However, these strategies are often limited by various aspects, including tedious experimental steps, harsh preparation
∗ Corresponding author at: College of Mechanical Engineering, Yangzhou University, Huayang West Road 196, Yangzhou 225127, Jiangsu Province, PR China. E-mail addresses: [email protected], zhangchao [email protected] (C. Zhang). http://dx.doi.org/10.1016/j.snb.2016.11.024 0925-4005/© 2016 Elsevier B.V. All rights reserved.
conditions, inhomogeneous doping and expensive facility. Different from that, self-doping with oxygen vacancies is an emerging method to narrow the bandgap efficiently [8–10]. As a dopant-free method, introducing oxygen vacancies into oxide semiconductors not only can reduce their bandgaps, but also is favorable in preserving the intrinsic crystal structure [10]. Therefore, fabricating wide bandgap semiconductors with large amount of oxygen vacancies may be an efficient way. Several papers have been reported to generate oxygen vacancies on wide bandgap semiconductors surface, including hydrogen peroxide solution modification, arc melting or reducing atmosphere treatment [8–12]. However, the number of oxygen vacancies produced during these processes is very little and their bandgap decreased limitedly. The concentration of oxygen vacancies is too low to expand the visible light responding obviously. Therefore, preparing wide bandgap semiconductors with high concentration of oxygen vacancies remains a great challenge at present. Ou and co-authors postulated that arc melting with high tempera-
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ture (∼3000 ◦ C) and cooling rate (103 K/s) would form rich oxygen defects in ZnO [12]. Other researchers founded that heating TiO2 under vacuum or reducing atmosphere (e.g., H2 ) also can generate highly concentrated oxygen vacancies [5,8,9]. In addition, nano-size effect is also found to have considerable influence on the oxygen vacancies and its content will be increased by decreasing the size of oxide semiconductors [13]. Therefore, we believe that a nanomaterial preparation process combining ultrahightemperature heating and rapid cooling with reducing atmosphere may produce large amount of oxygen vacancies on wide bandgap semiconductors surface. Solution precursor plasma spray (SPPS) is an emerging coating deposition technique for preparing nanostructured ceramic coatings [14–16]. Similar to traditional plasma spray process (PS) [17], a reducing Ar-H2 mixed atmosphere is often used as the plasma forming gas, which can generate ultrahigh temperature plasma flame (>10,000 ◦ C). Injected materials will be heated up to their melting point quickly, and cooled down in a few seconds with a rapid cooling rate (105 K/s) during SPPS process. Unlike PS, the solution precursor rather than powders is utilized as the feedstock in SPPS. After injection into the plasma plume, the injected droplets are subjected to the evaporation, precipitation and pyrolysis. Finally, metal oxide semiconductor coatings with rich oxygen vacancies were deposited thanks to ultrahigh-temperature heating and rapid cooling with reducing atmosphere. Furthermore, the microstructure of SPPS coatings is generally reported to be porous and nanostructured possessing huge surface-to-volume ratio due to less enthalpy is applied on individual particles. Nitrogen oxides mainly arising from industrial waste gases and automotive emissions, especially nitrogen dioxide (NO2 ), is a typical air pollutant and may give rise to acid rain and photochemical fog [14,15]. Therefore, gas sensors which can detect NO2 rapidly and accurately are urgently required. Resistive gas sensors based on semiconducting metal oxide widely utilized for the monitoring of toxic, flammable and explosive gases have played important roles in gas sensing devices due to its lightweight, good-sensitivity, low power consumption and easy-integration compared with gas analyzer as well as other sensing elements [16]. Among the sensitive materials, ZnO is a promising candidate [17]. As well known, traditional ZnO based sensors are generally required to be operated at elevated temperatures of 150–500 ◦ C to enhance the desorption, which brings about many problems, such as excessive power consumption, obvious growth of the grain size, and decreasing the safety in flammable atmosphere and deterioration of sensor components [18]. Therefore, one of the problems needed to be solved is to reduce the working temperature. Adopting light illumination instead of heater accelerating the reaction rate between the sensing materials and target gases on the metal oxide surface is an alternative method [19]. Owing to wide bandgap of stoichiometric ZnO (3.37 eV in theoretically), it requires UV illumination whose photon energy exceeds 3.37 eV, thus practically ruling out the use of visible light as an energy source [20,21]. However, UV light is not easy to obtain and dangerous to the operator, such as the eyes and skin. Besides, some sensor components will be failure and the target gases may decompose into other gases due to its high photon energy. Therefore, bandgap narrowing is needed if we want to use ZnO as the sensing material under visible light illumination. Herein, we successfully prepared nanostructured ZnO1−x sensitive coatings with highly concentrated oxygen vacancies through SPPS process, which simplifies the production of highly oxygenvacant ZnO. The advanced spectroscopy confirmed that large amounts of oxygen vacancies were introduced into the ZnO and the bandgap was significantly reduced. Apart from the bandgap narrowing, the amount of adsorption sites is another important factor for gas sensors. The sensor response will be improved with
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Table 1 SPPS parameters. Parameters
Value
Argon volume flow rate Hydrogen volume flow rate Arc current Torch power Liquid flow rate Spray distance Nozzle inner diameter Traverse velocity
40 L/min 2 L/min 513 A 25 kW 20 mL/min 100 mm 0.26 mm 200 mm/s
increasing gas adsorption sites. On the basis of the density functional theory (DFT) and experimental results, the sensing materials with highly concentrated oxygen vacancies are favorable for oxidizing molecules adsorption [22–25], due to their strong adsorption sites provided by oxygen vacancies. Bruno and co-authors reported that the surface reactivity of metal oxide surface is also associated with oxygen vacancies [26]. Therefore, sensing properties would be greatly enhanced by introducing highly concentrated oxygen vacancies onto ZnO surface. Moreover, the number of adsorption sites is also related to the specific surface area of the sensing materials, and nanostructured materials are known to having a large surface-to-volume ratio [27]. As a result, porous nanostructured ZnO coatings with highly concentrated oxygen vacancies would have excellent gas sensing properties under visible light illumination at room temperature.
2. Experimental method 2.1. Oxygen vacant coating preparation ZnO was deposited on alumina (Al2 O3 ) substrates via a F4MBXL plasma spray torch (Oerlikon Metco, Switzerland). The torch was connected to a six-axis robot arm (ABB, Sweden). A pair of interdigitated gold electrodes was screen-printed on the Al2 O3 substrates on one side, as shown in Fig. 1(a). Uniform and reproducible coatings were obtained by moving the spray torch with a constant velocity. Before spraying, the substrates were cleaned in absolute ethanol for 5 min by ultrasonic cleaner. Herein, 0.2 M aqueous zinc acetate solution (Sinopharm Chemical Reagent, China) was used as liquid feedstock. Subsequently, a nozzle with an inner diameter of 0.26 mm was used to inject the liquid feedstock into the plasma jet driven through a peristaltic pump. In this study, the mixed gas of Ar + H2 was utilized as the plasma forming gases. The detailed plasma spray parameters were listed in Table 1.
2.2. Oxygen vacant coating characterization X-ray diffraction analyzer (XRD, D8 Advance, Bruker, Germany) was utilized to identify the crystal structure of the obtained coatings with CuK␣ radiation. Several advanced techniques, including X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, USA), photoluminescence spectroscopy (PL, with He-Cd 325 nm laser) and electron paramagnetic resonance (EPR, A30010/12, Bruker, Germany) were used to detect the oxygen vacancies in the as-sprayed coatings. Besides, in order to examine the effect of oxygen vacancies on the optical properties, UV–vis diffuse reflectance spectrophotometer (Cary5000, Varian, USA) was adopted. Surface and cross-sectional morphology were observed by field-emission scanning electron microscopy (FE-SEM, S4800 II, Hitachi, Japan).
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Fig. 1. Schematic of (a) the sensor based on oxygen vacant coatings; (b) gas sensing setup.
2.3. Gas sensor testing We directly used the as-sprayed ZnO1−x as the sensitive materials and tested its room temperature NO2 sensing properties under visible light irradiation. The testing chamber used in this work was made of stainless steel with a Teflon liner container. Five colors LED lights (red, yellow, green, blue and purple) were connected to the chamber and set them in front of the sensors, as shown in Fig. 1(b). Before introducing NO2 gas, 500 mL/min commercial synthetic air was injected into the chamber. After the electrical resistance was stable, NO2 was injected to get a concentration of 1 ppm. All the testing gases were purchased from Nanjing Special Gas Co., Ltd. In this study, the sensor response represents S = RNO2 /Rair , in which RNO2 and Rair were the resistances upon exposure to NO2 gas and synthetic air, respectively. More gas sensing test details have been reported in [16]. 3. Results and discussion Fig. 2(a) shows the XRD pattern of the SPPS ZnO1−x coatings. The selected scanning 2 section is 20–70◦ with a scanning rate of 5◦ min−1 . Three main ZnO peaks assigned to (110), (002) and (101) planes are clearly observed. The phase composition of as-sprayed coatings is well in line with hexagonal wurtzite ZnO, corresponding to PDF 36-1451. It means that the introduced oxygen vacancies do not change intrinsic crystal structure of ZnO, which is consistent with references [10] and [12]. In view of the thin thickness of the coatings, alumina peaks from the substrate also exist in the XRD pattern. Fig. 2(b) illustrates the slow scan (0.1◦ min−1 ) pattern of the as-sprayed coatings ranging from 30◦ to 40◦ . Based on the Scherrer formula, the average crystallite size (D) can be calculated as follows: D = k·/cos·(2 − b2 )1/2
(1)
Where  is the full width at half maximum of the three main peaks, b is the instrumental broadening of 0.075, k is a constant of 0.89, is the X-ray wavelength (CuK␣ = 0.15418 nm), and is the measured X-ray diffraction angle. The average crystallite size of the as-sprayed coatings is estimated to be 14 ± 3 nm. Fig. 3(a) and (b) illustrates the surface morphology with different magnifications. The as-sprayed coatings are nanostructured with crystallite size in the range of 10–20 nm, which is in accordance with the slow-scanning XRD results. Plasma spraying is a typical approach with ultra-high-temperature heating (>10,000 ◦ C) and rapid cooling rate (>105 K/s) [14]. Adopting aqueous solution as feedstock rather than powder is another reason to form nanostructured coatings. The water would consume parts of heat, subsequently less energy is applied on individual droplets [15]. Moreover, the dwelling time of the injected droplets is in the order
of milliseconds, so the melting particles have no sufficient time to grow up even if the plasma temperature is high [16]. All these factors lead to the formation of the small crystallite size in the assprayed coatings. According to [13], more oxygen vacancies will be generated as the particle size decreasing. As a result, the obtained nanostructure in SPPS ZnO1−x coatings is beneficial for increasing the oxygen vacancy content. Moreover, the as-sprayed coatings are highly porous and the size of pores is in micrometer size, which is also observed in [17]. As seen from Fig. 4(a), the coating is ca. 15 m in thickness, which results in the appearance of Al2 O3 peak in the XRD pattern. Fig. 4(b) displays the cross-sectional morphology with a high magnification. It also indicates that the as-sprayed coating is porous, which is well in accordance with the surface FE-SEM images. The color of the as-sprayed ZnO1−x coatings is darkened, whereas the stoichiometric ZnO is well-known to be white. Similar findings are observed in reduced TiO2−x [28]. The reason for this phenomenon is attributed to the nanostructured particles, extremely fast heating and cooling, as well as the reducing atmosphere in the growth of ZnO particles. When feedstock solution is injected into the plasma plume, the zinc acetate liquid droplets are firstly subjected to evaporation, precipitation and pyrolysis. Then, produced ZnO particles are heated, accelerated, and finally impacted on the substrate to form ZnO1−x coating, as depicted in Fig. 5. During this period, melting particles are already far from the plasma nozzle exit where the reducibility is strongest and would be cooled down rapidly [15]. So highly concentrated oxygen vacancies are generated in the as-sprayed coatings. The formation of ZnO1−x coatings in the plasma jet can be proposed as follows: Zn(CH3 COO)2 + 1 − xH2 + xH2 → ZnO1−x + 2CH3 COOH + Vxo
(2)
Photoluminescence (PL) measurement is performed on the coating surface to detect oxygen vacancies in the as-sprayed coatings. Fig. 6 illustrates the PL spectra recorded at room temperature excited by a 325 nm He-Cd laser. A near-band-edge (NBE) emission located at 379 nm as well as a broad deep-level (DL) emission are exhibited in the spectrum. The NBE emission is originated from free-exciton emission on account of the interband electronic transitions, whereas the broad DL emission is mainly related to the contribution of oxygen vacancies [13,29]. The peak intensity and area of DL emission at the visible light region ranging from green to red is far larger than that of NBE emission, which means that rich oxygen vacancies generated in the as-sprayed ZnO1−x coatings. Apart from PL spectra, the use of X-ray photoelectron spectroscopy (XPS) characterization is another effective way to detect the content of oxygen vacancies, as shown in Fig. 7. The binding energy is calibrated by taking C1s peak at 284.8 eV. The O1s spectra are fitted by three sub spectral components centered at 530.2, 531.8
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Fig. 2. X-ray diffraction patterns of the as-sprayed coatings: (a) rapid scan; and (b) slow scan.
Fig. 3. Surface morphology of the as-sprayed coatings with different magnifications.
Fig. 4. Cross-section morphology of as-sprayed coatings with different magnifications.
and 533.4 eV, respectively [10,11]. The peak located at 530.2 eV is associated with the lattice oxygen in the stoichiometric ZnO, while that suited at 531.8 eV is mainly related to the contribution of oxygen vacant ZnO1−x [11,12]. The 533.4 eV peak is assigned to the water vapor component on ZnO surface [30]. In this paper, the concentration of ZnO1−x is evaluated using the following equation: Oxygen
vacant
ZnO1−x
concentration (%) =
Aii × 100% Ai + Aii
where Ai is the area of the 530.2 eV peak and Aii is the area of the 531.8 eV peak in the binding energy range of 526–538 eV. The curve fitting with the Gaussian and Lorentzian composite function is utilized to calculate the peak areas, and their relative errors are controlled within ±1%. The calculated value is 86.84% in this study, which demonstrates that highly concentrated oxygen vacancies are implanted into the as-sprayed coatings.
To gain more insight into the ionized states of the oxygen vacancies in the as-sprayed coatings, electron paramagnetic resonance (EPR) spectroscopy characterization is utilized. EPR spectroscopy is a useful technique to detect the unpaired electrons, which has been widely used to study defects in oxide semiconductors. Two EPR peaks corresponding to g-factors of 1.9610 and 2.0034 are observed in the EPR signal of the as-sprayed coatings, as displayed in Fig. 8. On the basis of [31–33], the EPR peak at g ≈ 1.9610 is assigned to singly positively charged oxygen vacancy Vo· , and the peak at g ≈ 2.0034 is also ascribed to positively charged oxygen vacancies. It demonstrates that there are a lot of unpaired electrons in the as-sprayed coatings due to the presence of rich oxygen vacancies. Moreover, the intensity of 1.9610 peak is extremely strong, which indicates that Vo· is the main surface defect in the as-sprayed coatings.
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Fig. 5. Schematic of SPPS process.
Fig. 6. PL spectra of the as-sprayed coatings. Fig. 8. EPR spectra of the as-sprayed coatings.
vacancies on the optical and electrical properties, some as-sprayed ZnO1−x coatings were annealed at 500 ◦ C for 4 h to reduce the content of oxygen vacancies. The UV–vis absorption spectroscopy is used to evaluate the optical absorption of as-sprayed and annealed coatings, respectively. For studying the effect of oxygen vacancies on the optical properties, UV–vis characterization is used. Fig. 9(a) depicts the UV–vis absorption spectra of the as-sprayed coatings. It can be observed that the as-sprayed ZnO1−x coating has absorption in the whole visible light region whereas the annealed ZnO coatings can only absorb UV light. Optical band gap of ZnO can be calculated using the Tauc model through [34]: ␣h = A(h − Eg)n
Fig. 7. XPS spectra of O1s peak in the as-sprayed coatings.
Wang and co-authors reported that the amount of oxygen vacancies significantly decreased once annealing in air at a temperature more than 400 ◦ C [10]. To investigate the effect of oxygen
(3)
where h is the Planck constant, ␣ is the absorption coefficient, is the photon frequency, A is a constant, Eg is the optical band gap and n = 1/2 is used for ZnO. Based on this equation, the band gap of the as-sprayed ZnO1−x coatings is estimated to be 2.15 eV from the Tauc plot of (␣h)2 versus h, as illustrated in Fig. 9(b). However, the band gap of heat-treated ZnO is 3.12 eV (theoretical bulk value is 3.37 eV). The significant reduction of band gap is mainly attributed to the presence of oxygen vacancies in high concentrations. Many
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Fig. 9. (a) UV–vis absorption spectra (b) Tauc plot of the as-sprayed and annealed coatings.
Fig. 11. Electrical resistances of sensors based on the SPPS ZnO1−x coatings with 1 ppm NO2 under different light conditions at room temperature. Fig. 10. Schematic of the process of carriers formed in the SPPS ZnO1−x coatings under visible light.
mid-gap states formed between the conduction and valence bands acting as deep donor defect levels due to the oxygen vacancies [8–12]. According to [11,35], oxygen vacancies mainly exist in three kinds of forms, i.e., neutral charge state (Vxo , two electrons electrons), singly (Vo· , one electron) and doubly (Vo· · ) positively charged states. Vlasenko et al. [35] proposed that Vxo is dynamically unstable which is easily ionized into Vo· and Vo· · under the excitation energy such as heating and light illumination. When the light is on, high energy photons are absorbed by Vxo and Vo· . The photogenerated electrons are created as follows: Vxo Vo· + e−
(4)
Vo· Vo· · + e−
(5)
The electronic excitation process in ZnO1−x coatings is illustrated in Fig. 10. As confirmed in the EPR results, Vo· is the main existing oxygen vacancy, which means that most Vxo have been singly ionized into Vo· at room temperature. This is proved by the electrical resistance measurement. The electrical resistance of the annealed ZnO coatings was above 109 Ohm in the dark or under visible light illumination, while that of the ZnO1−x coatings is less than 105 Ohm. This is attributed to less oxygen vacancies in the annealed ZnO coatings acting as the electron donors. In conclusion, the presence of oxygen vacancies in ZnO can greatly narrow the
bandgap and decrease its electrical resistance by forming a donor level between the valence and conduction band. Due to stoichiometric ZnO is a wide bandgap semiconductor, which strongly limits its further applications. Herein, we successfully prepare visible light responsive ZnO1−x by introducing large amounts of oxygen vacancies. In this study, we find that SPPS ZnO1−x coatings have excellent sensing performance under visible light illumination at room temperature. Fig. 11 presents the electrical resistances of the SPPS ZnO19x coatings plotted against time towards 1 ppm NO2 illuminated by lights with a wavelength ranging from 400 to 640 nm. The bulbs having different light wavelengths play an important role in the sensor properties. The main sensor characteristics drawn from Fig. 11 are listed in Table 2. Several points can be drawn as follows: Firstly, the base resistance Rair diminished as the light wavelength decreasing. The baseline resistance was 5.63 × 104 irradiated by red light, while under purple illumination, it reduced to 5.48 × 103 . The photon energy increases with the decrease of light wavelength according to the formula: E = hc/, where E is the light energy, h represents the Planck constant, c is the light velocity, and is the wavelength of light. Hence, more electron-hole pairs on ZnO1−x surface are generated as the light wavelength decreases, which results in a reduction of electrical resistance. Secondly, the sensor response increases when short wavelength light is used (except purple light). Upon exposure to light illumination with short wavelength, more oxygen vacancies are excited
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Fig. 12. The sensing response of the SPPS ZnO1−x sensors to 0.4–1 ppm NO2 in air under blue light illumination: (a) electrical resistance response; (b) sensor response. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 Sensing characteristics of the sensor based on SPPS ZnO(1−x ) coatings to 1 ppm NO2 illuminated by different wavelength light. Wavelength (nm)
Light color
Rair ()
Sensor response (RNO2 /Rair )
Response time (min)
640 580 530 480 400
Red Yellow Green Blue Purple
5.63 × 104 3.36 × 104 1.90 × 104 7.78 × 103 5.48 × 103
1.27 1.56 2.31 3.59 3.47
29.3 25.7 23.3 13.7 17.7
and then more electrons are produced, which promotes the generation of O2 − on the surface. When NO2 is introduced into the testing chamber, more NO2 − species are formed by interacting with the excited oxygen vacancies and adsorbed O2 − species, which greatly enhance the sensor response. Thirdly, the response time and recovery time diminish when using short wavelength light (except purple light). The role of photon energy on surface reaction rates between the gas molecules and the sensing materials is similar to heating [16,36]. When the light wavelength decreases, the photon energy increases, which enhances the response and recovery rates. Compared with blue light, although the purple light has higher photon energy, the response time and recovery time were not improved. It may be ascribed to the additional reactions of NO2 when the purple light is used. According to [36], NO2 molecules will transform into NO under light illumination with the wavelength lower than 420 nm. With the decrease in light wavelength, the sensor response lowers whereas the response 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 ZnO1−x sensor to get rapid response and recovery rate while maintaining an acceptable sensor response. Fig. 12(a) illustrates the electrical resistance responses of the oxygen-vacant ZnO1−x sensors towards 0.4–1 ppm NO2 gas irradiated under blue light. The oxygen-vacant ZnO1−x sensors exhibit significant response to NO2 . The response is 2.8 even the NO2 concentration was as low as 0.4 ppm, indicating that the gas sensors developed in this study have a high detectability. The sensor response based on the SPPS ZnO1−x coatings increases linearly with NO2 concentration (Fig. 12(b)). The performance of the ZnO1−x sensors are compared with the previously reported ZnO based room-temperature NO2 gas sensors, as shown in Table 3. It can be concluded that the ZnO1−x illuminated under blue light showed good response to NO2 whereas the response time needed to be improved. It is well known that metal oxide gas sensors are often sensitive to humidity. Therefore, the humidity, being one of the
Fig. 13. The electrical resistance response of SPPS ZnO1−x sensors to 1 ppm NO2 in air under blue light illumination with R.H. from 0 to 100%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
main influencing agent, should be considered in the gas sensor application. Fig. 13 presents the electrical resistance response of the oxygen-vacant ZnO1-x sensors to 1 ppm with R.H. (relative humidity) ranging from 0 to 100%. Several results can be obtained from this figure. Firstly, the electrical resistance decreased when R.H. increased. Further to observe Fig. 13, it can be found that the base resistance decreases from 7.78 × 103 to 4.33 × 103 . When the R.H. was further increased above 25%, there was no obvious change in base resistance. Secondly, the sensor response increased with R.H., especially higher than 50%. Thirdly, the recovery time is significantly shortened when higher R.H. (>50%) is used. Similar results were found in [30]. All these behaviors should be attributed to the presence of adsorbed water molecules on the metal oxide surface. Stability is another key parameter for practical application, so relative studies were performed on the SPPS ZnO1-x sensors. Fig. 14
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Table 3 The room temperature performance of the SPPS ZnO1−x sensors in comparison with the ZnO based sensors in literature. Material
Preparation method
NO2 concentration (ppm)
Response (RNO2 /Rair )
Response Time(min)
Light illumination
Reference
ZnO ZnO ZnO ZnO@SnO2 ZnO@SnO2 ZnO@PS (Porous Silicon) ZnO@PS ZnO@GO ZnO@rGO ZnO1−x
Soft e-beam lithography Facile solution Drop-cast Wet chemical Thermal evaporation Electrochemical deposition Hydrothermal Solvothermal In-situ production Solution precursor plasma spray
20 5 5 0.5 1 1 50 50 5 1
2.2 2.5 1.1 10.1 2.4 1.5 1.35 1.21 1.25 3.59
15 0.5 1.5 12 5 10 17 2.5 3 13.7
UV UV UV UV UV – – – – Blue light
[37] [38] [39] [40] [41] [42] [43] [44] [45] This work
Fig. 14. Long-term stability of SPPS ZnO1−x sensors to 1 ppm NO2 in air under blue light illumination. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
shows the long-term stability results during 15 days. The results indicated that the response of ZnO1-x coatings to 1 ppm NO2 was fluctuated around 3.2, which demonstrated that the obtained sensors possess excellent long-term stability. In addition to stability, selectivity was another important factor for gas sensors. Chemiresistive gas sensors with high sensitivity towards specific gas were expected for many applications. In this study, 1 ppm NO2 , 100 ppm SO2 , 4000 ppm H2 , 3000 ppm acetone, 3000 ppm ethanol and 3000 ppm methanol were used as tested gases illuminated under blue light. It can be found that the response of SPPS ZnO1-x sensors to NO2 was much higher than other gases (as depicted in Fig. 15). Therefore, the obtained sensors have very good selectivity for NO2 . The oxygen vacancies are proven to be the strongly preferential adsorption sites for oxidizing gases, such as NO2 [22–25]. Therefore, the chemisorption of NO2 molecules facilely proceeds on ZnO1−x surface and the depletion layer is created. Besides, the sensor response is highly dependent on the crystallite size (D). For D > Debye length (LD ), the thickness of electron depletion layer is far smaller than D, meaning that the surface reaction does not influence the sensing process. When D is comparable to or less than 2LD , the whole grain is depleted of electrons, thus making the sensor highly sensitive to the target gases. The LD of ZnO is reported to be about 7.5 nm [46]. From the results of XRD and FE-SEM, the particle size in the as-sprayed ZnO1−x coatings is ca. 14 nm, which is close to twice LD . Furthermore, the ZnO1−x coatings also have porous structure, which is favorable for gas diffusion from outside to internal and make them effectively react with gas molecules. So more NO2 species will be adsorbed on the ZnO1−x surface and the sensor exhibits superior NO2 sensing performance.
Fig. 15. Selectivity of SPPS ZnO1−x sensors to 1 ppm NO2 , 100 ppm SO2 , 4000 ppm H2 , 3000 ppm acetone, 3000 ppm ethanol and 3000 ppm methanol under blue light illumination. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Conclusions In this study, we use SPPS to deposit narrow bandgap ZnO1−x . The bandgap reduction can be ascribed to large amounts of oxygen vacancies due to the nanostructured coatings, reducing atmosphere as well as extremely rapid heating and cooling rate during SPPS. The presence of oxygen vacancies in the as-sprayed coatings were confirmed by the PL, EPR and XPS characterization. The visible light adsorption range for SPPS wide bandgap semiconductor coatings was extended to visible light range which should be attributed to the formation of a donor level between the conduction and valence band. In addition, The SPPS ZnO1−x coatings showed significant responses to NO2 with concentration even as low as 0.4 ppm gas sensing properties illuminated under visible light. Moreover, the sensing response increased linearly with NO2 concentration. The enhanced sensor properties were related to the highly concentrated oxygen vacancies and the porous nanostructure provided by SPPS. Finally, we found that the light wavelength played a crucial role in the sensor performance. The coating resistance decreased in reference air with the light wavelength reducing. The response time and recovery time of the coating firstly decreased and then increased with the light wavelength. The best sensing characteristics were obtained when a blue light was used. Acknowledgements This work is supported by State Key Laboratory for Mechanical Behavior of Materials under Grant No. 20161808, the Natural Science Foundation of China under Grant No. 51402255, the Jiangsu Natural Science Foundation of China under Grant No. BK20140487 and the Funding of Jiangsu Innovation Program for Graduate Education under Grant No. KYZZ16 0490. This is a project funded by the
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Priority Academic Program Development of Jiangsu Higher Education Institutions and the Testing Center of Yangzhou University and the Testing Center of Yangzhou University.
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Biographies Chao Zhang received a B.S. degree from the Chongqing University (China) in 2003 and a joint Ph.D. degree from Technology University of Belfort-Montbéliard (France) and Xi’an Jiaotong University (China) in June 2008. From September 2007 to January 2009, he worked as a teaching-research assistant in Technology University of Belfort-Montbéliard. Since Feb 2009, he is a postdoctoral researcher, and then a senior researcher in Materials Science Department of Engineering School of University of Mons (Belgium). In 2014, he joined Yangzhou University (China) as professor where he is leading a research group on thermal spray coatings and gas sensors. Since August 2016, he is Vice Dean in charge of research in College of Mechanical Engineering. His research interests include thermal-sprayed techniques and coatings, especially gas sensing and wear-resistant coatings. Xin Geng received his B.S. degree in 2013 at Yangzhou University. He is currently pursuing his PhD degree at the same university and takes an interest in smart materials for room temperature gas sensors. Hanlin Liao received his Ph.D. degree in materials science in 1994 at Technology University of Belfort-Montbeliard. After then, he has worked as an associate professor in Laboratoire d’Etudes et de Recherches sur les Matériaux, les Procédés et les Surfaces (LERMPS). From 2004, he is professor at Technology University of BelfortMontbeliard. Since 1985, he has been working in the field of process development of thermal spraying and industrial applications. He published more than 200 peer reviewed papers in thermal sprayed coatings.
C. Zhang et al. / Sensors and Actuators B 242 (2017) 102–111 Chang-Jiu Li received his B.Sc. from the Department of Mechanical Engineering, Xian Jiaotong University in 1982. He received his M.S. and Ph.D. in welding engineering from Osaka University in 1986 and 1989, respectively. From 1989 to 1992, he was a postdoctoral research fellow at the Advanced Materials Processing Institute, Kinki, Japan. During this period, he also worked with Tocalo Company on projects involving HVOF cermet coatings and splat formation in plasma spraying. In September 1992, he became a lecturer at Xian Jiaotong University and was promoted to full professor at the end of 1992. Prof. Li leads the thermal spray group at State Key Laboratory for Mechanical Behavior of Materials. He has published over 350 academic papers.
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Marc Debliquy received his Ph.D. at Faculty of Engineering in Mons (Belgium) in 1999 in the field of organic semiconductors for fire detection. He joined the Sochinor Company in 2000. He left in 2003 for joining Materia Nova (Research Center in the field of Materials in Mons). He was responsible for the research activities in the field of gas sensors. Since October 2008, he joined in Material Science Department of Faculty of Engineering of University of Mons and worked as a team leader of semiconductor and sensor group. He was promoted as professor in June 2013. His main research interest is smart coatings for chemical detection.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Room-temperature nitrogen-dioxide sensors based on ZnO1−x coatings deposited by solution precursor plasma spray Chao Zhang a,∗ , Xin Geng a,b , Hanlin Liao c , Chang-Jiu Li d , Marc Debliquy e a
College of Mechanical Engineering, Yangzhou University, Yangzhou 25127, PR China College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, PR China IRTES-LERMPS, Université de Technologie de Belfort-Montbeliard, Belfort 90000, France d State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xian 700149, PR China e Service de Science des Matériaux, Faculté Polytechnique, Université de Mons, Mons 7000, Belgium b c
a r t i c l e
i n f o
Article history: Received 13 July 2016 Received in revised form 26 October 2016 Accepted 5 November 2016 Available online 9 November 2016 Keywords: ZnO Gas sensors Oxygen vacancy Porous nanostructure Solution precursor plasma spray
a b s t r a c t The stoichiometric ZnO has a bandgap greater than 2.6 eV and has no or weak response to visible light, which greatly suppresses its applications. We report here that a process of solution precursor plasma spray (SPPS) can effectively narrow the bandgap and extend the absorption of ZnO to visible light region, which mainly results from highly concentrated oxygen vacancies generated during SPPS process. By photoluminescence spectroscopy, electron paramagnetic resonance and X-ray photoelectron spectroscopy, we found that a large number of oxygen vacancies were implanted into ZnO1−x prepared by SPPS. The generation of highly concentrated oxygen vacancies were mainly attributed to reducing atmosphere as well as fast heating and cooling process inherently provided by SPPS. The findings of this work create a new way for developing narrow bandgap ZnO1−x . We present that the SPPS ZnO1−x coatings can be utilized directly as sensitive materials under visible-light illumination. The oxygen vacancies have considerable influence on its optical and electrical properties. The ZnO1−x coatings exhibited an obvious absorption covering the whole visible-light region and its bandgap was calculated to be 2.15 eV which was much narrower than that of stoichiometric ZnO (3.37 eV). The sensors based on ZnO1−x coatings showed significant responses to NO2 at room temperature. In addition, the sensor response increased linearly with NO2 concentration. The enhanced sensor properties were attributed to the rich oxygen vacancies and special coating microstructure provided by SPPS. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In the last decades, increasing attention has been paid on the wide band gap semiconductors owing to their extensive application in solar cells, photocatalysts, optoelectronic devices and gas sensors [1–4]. In order to further widen the application in the visible light range and enhance the properties, it is necessary to narrow their bandgaps [5]. The bandgap is a key character for metal oxide semiconductors and a pure oxide semiconductor material usually has a constant value. Metal or nonmetal element doping is a conventional approach to adjust the bandgap by adding additional donor and acceptor level into the materials, such as Fe, Al, Mn, C and N [6,7]. However, these strategies are often limited by various aspects, including tedious experimental steps, harsh preparation
∗ Corresponding author at: College of Mechanical Engineering, Yangzhou University, Huayang West Road 196, Yangzhou 225127, Jiangsu Province, PR China. E-mail addresses: [email protected], zhangchao [email protected] (C. Zhang). http://dx.doi.org/10.1016/j.snb.2016.11.024 0925-4005/© 2016 Elsevier B.V. All rights reserved.
conditions, inhomogeneous doping and expensive facility. Different from that, self-doping with oxygen vacancies is an emerging method to narrow the bandgap efficiently [8–10]. As a dopant-free method, introducing oxygen vacancies into oxide semiconductors not only can reduce their bandgaps, but also is favorable in preserving the intrinsic crystal structure [10]. Therefore, fabricating wide bandgap semiconductors with large amount of oxygen vacancies may be an efficient way. Several papers have been reported to generate oxygen vacancies on wide bandgap semiconductors surface, including hydrogen peroxide solution modification, arc melting or reducing atmosphere treatment [8–12]. However, the number of oxygen vacancies produced during these processes is very little and their bandgap decreased limitedly. The concentration of oxygen vacancies is too low to expand the visible light responding obviously. Therefore, preparing wide bandgap semiconductors with high concentration of oxygen vacancies remains a great challenge at present. Ou and co-authors postulated that arc melting with high tempera-
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ture (∼3000 ◦ C) and cooling rate (103 K/s) would form rich oxygen defects in ZnO [12]. Other researchers founded that heating TiO2 under vacuum or reducing atmosphere (e.g., H2 ) also can generate highly concentrated oxygen vacancies [5,8,9]. In addition, nano-size effect is also found to have considerable influence on the oxygen vacancies and its content will be increased by decreasing the size of oxide semiconductors [13]. Therefore, we believe that a nanomaterial preparation process combining ultrahightemperature heating and rapid cooling with reducing atmosphere may produce large amount of oxygen vacancies on wide bandgap semiconductors surface. Solution precursor plasma spray (SPPS) is an emerging coating deposition technique for preparing nanostructured ceramic coatings [14–16]. Similar to traditional plasma spray process (PS) [17], a reducing Ar-H2 mixed atmosphere is often used as the plasma forming gas, which can generate ultrahigh temperature plasma flame (>10,000 ◦ C). Injected materials will be heated up to their melting point quickly, and cooled down in a few seconds with a rapid cooling rate (105 K/s) during SPPS process. Unlike PS, the solution precursor rather than powders is utilized as the feedstock in SPPS. After injection into the plasma plume, the injected droplets are subjected to the evaporation, precipitation and pyrolysis. Finally, metal oxide semiconductor coatings with rich oxygen vacancies were deposited thanks to ultrahigh-temperature heating and rapid cooling with reducing atmosphere. Furthermore, the microstructure of SPPS coatings is generally reported to be porous and nanostructured possessing huge surface-to-volume ratio due to less enthalpy is applied on individual particles. Nitrogen oxides mainly arising from industrial waste gases and automotive emissions, especially nitrogen dioxide (NO2 ), is a typical air pollutant and may give rise to acid rain and photochemical fog [14,15]. Therefore, gas sensors which can detect NO2 rapidly and accurately are urgently required. Resistive gas sensors based on semiconducting metal oxide widely utilized for the monitoring of toxic, flammable and explosive gases have played important roles in gas sensing devices due to its lightweight, good-sensitivity, low power consumption and easy-integration compared with gas analyzer as well as other sensing elements [16]. Among the sensitive materials, ZnO is a promising candidate [17]. As well known, traditional ZnO based sensors are generally required to be operated at elevated temperatures of 150–500 ◦ C to enhance the desorption, which brings about many problems, such as excessive power consumption, obvious growth of the grain size, and decreasing the safety in flammable atmosphere and deterioration of sensor components [18]. Therefore, one of the problems needed to be solved is to reduce the working temperature. Adopting light illumination instead of heater accelerating the reaction rate between the sensing materials and target gases on the metal oxide surface is an alternative method [19]. Owing to wide bandgap of stoichiometric ZnO (3.37 eV in theoretically), it requires UV illumination whose photon energy exceeds 3.37 eV, thus practically ruling out the use of visible light as an energy source [20,21]. However, UV light is not easy to obtain and dangerous to the operator, such as the eyes and skin. Besides, some sensor components will be failure and the target gases may decompose into other gases due to its high photon energy. Therefore, bandgap narrowing is needed if we want to use ZnO as the sensing material under visible light illumination. Herein, we successfully prepared nanostructured ZnO1−x sensitive coatings with highly concentrated oxygen vacancies through SPPS process, which simplifies the production of highly oxygenvacant ZnO. The advanced spectroscopy confirmed that large amounts of oxygen vacancies were introduced into the ZnO and the bandgap was significantly reduced. Apart from the bandgap narrowing, the amount of adsorption sites is another important factor for gas sensors. The sensor response will be improved with
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Table 1 SPPS parameters. Parameters
Value
Argon volume flow rate Hydrogen volume flow rate Arc current Torch power Liquid flow rate Spray distance Nozzle inner diameter Traverse velocity
40 L/min 2 L/min 513 A 25 kW 20 mL/min 100 mm 0.26 mm 200 mm/s
increasing gas adsorption sites. On the basis of the density functional theory (DFT) and experimental results, the sensing materials with highly concentrated oxygen vacancies are favorable for oxidizing molecules adsorption [22–25], due to their strong adsorption sites provided by oxygen vacancies. Bruno and co-authors reported that the surface reactivity of metal oxide surface is also associated with oxygen vacancies [26]. Therefore, sensing properties would be greatly enhanced by introducing highly concentrated oxygen vacancies onto ZnO surface. Moreover, the number of adsorption sites is also related to the specific surface area of the sensing materials, and nanostructured materials are known to having a large surface-to-volume ratio [27]. As a result, porous nanostructured ZnO coatings with highly concentrated oxygen vacancies would have excellent gas sensing properties under visible light illumination at room temperature.
2. Experimental method 2.1. Oxygen vacant coating preparation ZnO was deposited on alumina (Al2 O3 ) substrates via a F4MBXL plasma spray torch (Oerlikon Metco, Switzerland). The torch was connected to a six-axis robot arm (ABB, Sweden). A pair of interdigitated gold electrodes was screen-printed on the Al2 O3 substrates on one side, as shown in Fig. 1(a). Uniform and reproducible coatings were obtained by moving the spray torch with a constant velocity. Before spraying, the substrates were cleaned in absolute ethanol for 5 min by ultrasonic cleaner. Herein, 0.2 M aqueous zinc acetate solution (Sinopharm Chemical Reagent, China) was used as liquid feedstock. Subsequently, a nozzle with an inner diameter of 0.26 mm was used to inject the liquid feedstock into the plasma jet driven through a peristaltic pump. In this study, the mixed gas of Ar + H2 was utilized as the plasma forming gases. The detailed plasma spray parameters were listed in Table 1.
2.2. Oxygen vacant coating characterization X-ray diffraction analyzer (XRD, D8 Advance, Bruker, Germany) was utilized to identify the crystal structure of the obtained coatings with CuK␣ radiation. Several advanced techniques, including X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, USA), photoluminescence spectroscopy (PL, with He-Cd 325 nm laser) and electron paramagnetic resonance (EPR, A30010/12, Bruker, Germany) were used to detect the oxygen vacancies in the as-sprayed coatings. Besides, in order to examine the effect of oxygen vacancies on the optical properties, UV–vis diffuse reflectance spectrophotometer (Cary5000, Varian, USA) was adopted. Surface and cross-sectional morphology were observed by field-emission scanning electron microscopy (FE-SEM, S4800 II, Hitachi, Japan).
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Fig. 1. Schematic of (a) the sensor based on oxygen vacant coatings; (b) gas sensing setup.
2.3. Gas sensor testing We directly used the as-sprayed ZnO1−x as the sensitive materials and tested its room temperature NO2 sensing properties under visible light irradiation. The testing chamber used in this work was made of stainless steel with a Teflon liner container. Five colors LED lights (red, yellow, green, blue and purple) were connected to the chamber and set them in front of the sensors, as shown in Fig. 1(b). Before introducing NO2 gas, 500 mL/min commercial synthetic air was injected into the chamber. After the electrical resistance was stable, NO2 was injected to get a concentration of 1 ppm. All the testing gases were purchased from Nanjing Special Gas Co., Ltd. In this study, the sensor response represents S = RNO2 /Rair , in which RNO2 and Rair were the resistances upon exposure to NO2 gas and synthetic air, respectively. More gas sensing test details have been reported in [16]. 3. Results and discussion Fig. 2(a) shows the XRD pattern of the SPPS ZnO1−x coatings. The selected scanning 2 section is 20–70◦ with a scanning rate of 5◦ min−1 . Three main ZnO peaks assigned to (110), (002) and (101) planes are clearly observed. The phase composition of as-sprayed coatings is well in line with hexagonal wurtzite ZnO, corresponding to PDF 36-1451. It means that the introduced oxygen vacancies do not change intrinsic crystal structure of ZnO, which is consistent with references [10] and [12]. In view of the thin thickness of the coatings, alumina peaks from the substrate also exist in the XRD pattern. Fig. 2(b) illustrates the slow scan (0.1◦ min−1 ) pattern of the as-sprayed coatings ranging from 30◦ to 40◦ . Based on the Scherrer formula, the average crystallite size (D) can be calculated as follows: D = k·/cos·(2 − b2 )1/2
(1)
Where  is the full width at half maximum of the three main peaks, b is the instrumental broadening of 0.075, k is a constant of 0.89, is the X-ray wavelength (CuK␣ = 0.15418 nm), and is the measured X-ray diffraction angle. The average crystallite size of the as-sprayed coatings is estimated to be 14 ± 3 nm. Fig. 3(a) and (b) illustrates the surface morphology with different magnifications. The as-sprayed coatings are nanostructured with crystallite size in the range of 10–20 nm, which is in accordance with the slow-scanning XRD results. Plasma spraying is a typical approach with ultra-high-temperature heating (>10,000 ◦ C) and rapid cooling rate (>105 K/s) [14]. Adopting aqueous solution as feedstock rather than powder is another reason to form nanostructured coatings. The water would consume parts of heat, subsequently less energy is applied on individual droplets [15]. Moreover, the dwelling time of the injected droplets is in the order
of milliseconds, so the melting particles have no sufficient time to grow up even if the plasma temperature is high [16]. All these factors lead to the formation of the small crystallite size in the assprayed coatings. According to [13], more oxygen vacancies will be generated as the particle size decreasing. As a result, the obtained nanostructure in SPPS ZnO1−x coatings is beneficial for increasing the oxygen vacancy content. Moreover, the as-sprayed coatings are highly porous and the size of pores is in micrometer size, which is also observed in [17]. As seen from Fig. 4(a), the coating is ca. 15 m in thickness, which results in the appearance of Al2 O3 peak in the XRD pattern. Fig. 4(b) displays the cross-sectional morphology with a high magnification. It also indicates that the as-sprayed coating is porous, which is well in accordance with the surface FE-SEM images. The color of the as-sprayed ZnO1−x coatings is darkened, whereas the stoichiometric ZnO is well-known to be white. Similar findings are observed in reduced TiO2−x [28]. The reason for this phenomenon is attributed to the nanostructured particles, extremely fast heating and cooling, as well as the reducing atmosphere in the growth of ZnO particles. When feedstock solution is injected into the plasma plume, the zinc acetate liquid droplets are firstly subjected to evaporation, precipitation and pyrolysis. Then, produced ZnO particles are heated, accelerated, and finally impacted on the substrate to form ZnO1−x coating, as depicted in Fig. 5. During this period, melting particles are already far from the plasma nozzle exit where the reducibility is strongest and would be cooled down rapidly [15]. So highly concentrated oxygen vacancies are generated in the as-sprayed coatings. The formation of ZnO1−x coatings in the plasma jet can be proposed as follows: Zn(CH3 COO)2 + 1 − xH2 + xH2 → ZnO1−x + 2CH3 COOH + Vxo
(2)
Photoluminescence (PL) measurement is performed on the coating surface to detect oxygen vacancies in the as-sprayed coatings. Fig. 6 illustrates the PL spectra recorded at room temperature excited by a 325 nm He-Cd laser. A near-band-edge (NBE) emission located at 379 nm as well as a broad deep-level (DL) emission are exhibited in the spectrum. The NBE emission is originated from free-exciton emission on account of the interband electronic transitions, whereas the broad DL emission is mainly related to the contribution of oxygen vacancies [13,29]. The peak intensity and area of DL emission at the visible light region ranging from green to red is far larger than that of NBE emission, which means that rich oxygen vacancies generated in the as-sprayed ZnO1−x coatings. Apart from PL spectra, the use of X-ray photoelectron spectroscopy (XPS) characterization is another effective way to detect the content of oxygen vacancies, as shown in Fig. 7. The binding energy is calibrated by taking C1s peak at 284.8 eV. The O1s spectra are fitted by three sub spectral components centered at 530.2, 531.8
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Fig. 2. X-ray diffraction patterns of the as-sprayed coatings: (a) rapid scan; and (b) slow scan.
Fig. 3. Surface morphology of the as-sprayed coatings with different magnifications.
Fig. 4. Cross-section morphology of as-sprayed coatings with different magnifications.
and 533.4 eV, respectively [10,11]. The peak located at 530.2 eV is associated with the lattice oxygen in the stoichiometric ZnO, while that suited at 531.8 eV is mainly related to the contribution of oxygen vacant ZnO1−x [11,12]. The 533.4 eV peak is assigned to the water vapor component on ZnO surface [30]. In this paper, the concentration of ZnO1−x is evaluated using the following equation: Oxygen
vacant
ZnO1−x
concentration (%) =
Aii × 100% Ai + Aii
where Ai is the area of the 530.2 eV peak and Aii is the area of the 531.8 eV peak in the binding energy range of 526–538 eV. The curve fitting with the Gaussian and Lorentzian composite function is utilized to calculate the peak areas, and their relative errors are controlled within ±1%. The calculated value is 86.84% in this study, which demonstrates that highly concentrated oxygen vacancies are implanted into the as-sprayed coatings.
To gain more insight into the ionized states of the oxygen vacancies in the as-sprayed coatings, electron paramagnetic resonance (EPR) spectroscopy characterization is utilized. EPR spectroscopy is a useful technique to detect the unpaired electrons, which has been widely used to study defects in oxide semiconductors. Two EPR peaks corresponding to g-factors of 1.9610 and 2.0034 are observed in the EPR signal of the as-sprayed coatings, as displayed in Fig. 8. On the basis of [31–33], the EPR peak at g ≈ 1.9610 is assigned to singly positively charged oxygen vacancy Vo· , and the peak at g ≈ 2.0034 is also ascribed to positively charged oxygen vacancies. It demonstrates that there are a lot of unpaired electrons in the as-sprayed coatings due to the presence of rich oxygen vacancies. Moreover, the intensity of 1.9610 peak is extremely strong, which indicates that Vo· is the main surface defect in the as-sprayed coatings.
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Fig. 5. Schematic of SPPS process.
Fig. 6. PL spectra of the as-sprayed coatings. Fig. 8. EPR spectra of the as-sprayed coatings.
vacancies on the optical and electrical properties, some as-sprayed ZnO1−x coatings were annealed at 500 ◦ C for 4 h to reduce the content of oxygen vacancies. The UV–vis absorption spectroscopy is used to evaluate the optical absorption of as-sprayed and annealed coatings, respectively. For studying the effect of oxygen vacancies on the optical properties, UV–vis characterization is used. Fig. 9(a) depicts the UV–vis absorption spectra of the as-sprayed coatings. It can be observed that the as-sprayed ZnO1−x coating has absorption in the whole visible light region whereas the annealed ZnO coatings can only absorb UV light. Optical band gap of ZnO can be calculated using the Tauc model through [34]: ␣h = A(h − Eg)n
Fig. 7. XPS spectra of O1s peak in the as-sprayed coatings.
Wang and co-authors reported that the amount of oxygen vacancies significantly decreased once annealing in air at a temperature more than 400 ◦ C [10]. To investigate the effect of oxygen
(3)
where h is the Planck constant, ␣ is the absorption coefficient, is the photon frequency, A is a constant, Eg is the optical band gap and n = 1/2 is used for ZnO. Based on this equation, the band gap of the as-sprayed ZnO1−x coatings is estimated to be 2.15 eV from the Tauc plot of (␣h)2 versus h, as illustrated in Fig. 9(b). However, the band gap of heat-treated ZnO is 3.12 eV (theoretical bulk value is 3.37 eV). The significant reduction of band gap is mainly attributed to the presence of oxygen vacancies in high concentrations. Many
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Fig. 9. (a) UV–vis absorption spectra (b) Tauc plot of the as-sprayed and annealed coatings.
Fig. 11. Electrical resistances of sensors based on the SPPS ZnO1−x coatings with 1 ppm NO2 under different light conditions at room temperature. Fig. 10. Schematic of the process of carriers formed in the SPPS ZnO1−x coatings under visible light.
mid-gap states formed between the conduction and valence bands acting as deep donor defect levels due to the oxygen vacancies [8–12]. According to [11,35], oxygen vacancies mainly exist in three kinds of forms, i.e., neutral charge state (Vxo , two electrons electrons), singly (Vo· , one electron) and doubly (Vo· · ) positively charged states. Vlasenko et al. [35] proposed that Vxo is dynamically unstable which is easily ionized into Vo· and Vo· · under the excitation energy such as heating and light illumination. When the light is on, high energy photons are absorbed by Vxo and Vo· . The photogenerated electrons are created as follows: Vxo Vo· + e−
(4)
Vo· Vo· · + e−
(5)
The electronic excitation process in ZnO1−x coatings is illustrated in Fig. 10. As confirmed in the EPR results, Vo· is the main existing oxygen vacancy, which means that most Vxo have been singly ionized into Vo· at room temperature. This is proved by the electrical resistance measurement. The electrical resistance of the annealed ZnO coatings was above 109 Ohm in the dark or under visible light illumination, while that of the ZnO1−x coatings is less than 105 Ohm. This is attributed to less oxygen vacancies in the annealed ZnO coatings acting as the electron donors. In conclusion, the presence of oxygen vacancies in ZnO can greatly narrow the
bandgap and decrease its electrical resistance by forming a donor level between the valence and conduction band. Due to stoichiometric ZnO is a wide bandgap semiconductor, which strongly limits its further applications. Herein, we successfully prepare visible light responsive ZnO1−x by introducing large amounts of oxygen vacancies. In this study, we find that SPPS ZnO1−x coatings have excellent sensing performance under visible light illumination at room temperature. Fig. 11 presents the electrical resistances of the SPPS ZnO19x coatings plotted against time towards 1 ppm NO2 illuminated by lights with a wavelength ranging from 400 to 640 nm. The bulbs having different light wavelengths play an important role in the sensor properties. The main sensor characteristics drawn from Fig. 11 are listed in Table 2. Several points can be drawn as follows: Firstly, the base resistance Rair diminished as the light wavelength decreasing. The baseline resistance was 5.63 × 104 irradiated by red light, while under purple illumination, it reduced to 5.48 × 103 . The photon energy increases with the decrease of light wavelength according to the formula: E = hc/, where E is the light energy, h represents the Planck constant, c is the light velocity, and is the wavelength of light. Hence, more electron-hole pairs on ZnO1−x surface are generated as the light wavelength decreases, which results in a reduction of electrical resistance. Secondly, the sensor response increases when short wavelength light is used (except purple light). Upon exposure to light illumination with short wavelength, more oxygen vacancies are excited
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Fig. 12. The sensing response of the SPPS ZnO1−x sensors to 0.4–1 ppm NO2 in air under blue light illumination: (a) electrical resistance response; (b) sensor response. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 Sensing characteristics of the sensor based on SPPS ZnO(1−x ) coatings to 1 ppm NO2 illuminated by different wavelength light. Wavelength (nm)
Light color
Rair ()
Sensor response (RNO2 /Rair )
Response time (min)
640 580 530 480 400
Red Yellow Green Blue Purple
5.63 × 104 3.36 × 104 1.90 × 104 7.78 × 103 5.48 × 103
1.27 1.56 2.31 3.59 3.47
29.3 25.7 23.3 13.7 17.7
and then more electrons are produced, which promotes the generation of O2 − on the surface. When NO2 is introduced into the testing chamber, more NO2 − species are formed by interacting with the excited oxygen vacancies and adsorbed O2 − species, which greatly enhance the sensor response. Thirdly, the response time and recovery time diminish when using short wavelength light (except purple light). The role of photon energy on surface reaction rates between the gas molecules and the sensing materials is similar to heating [16,36]. When the light wavelength decreases, the photon energy increases, which enhances the response and recovery rates. Compared with blue light, although the purple light has higher photon energy, the response time and recovery time were not improved. It may be ascribed to the additional reactions of NO2 when the purple light is used. According to [36], NO2 molecules will transform into NO under light illumination with the wavelength lower than 420 nm. With the decrease in light wavelength, the sensor response lowers whereas the response 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 ZnO1−x sensor to get rapid response and recovery rate while maintaining an acceptable sensor response. Fig. 12(a) illustrates the electrical resistance responses of the oxygen-vacant ZnO1−x sensors towards 0.4–1 ppm NO2 gas irradiated under blue light. The oxygen-vacant ZnO1−x sensors exhibit significant response to NO2 . The response is 2.8 even the NO2 concentration was as low as 0.4 ppm, indicating that the gas sensors developed in this study have a high detectability. The sensor response based on the SPPS ZnO1−x coatings increases linearly with NO2 concentration (Fig. 12(b)). The performance of the ZnO1−x sensors are compared with the previously reported ZnO based room-temperature NO2 gas sensors, as shown in Table 3. It can be concluded that the ZnO1−x illuminated under blue light showed good response to NO2 whereas the response time needed to be improved. It is well known that metal oxide gas sensors are often sensitive to humidity. Therefore, the humidity, being one of the
Fig. 13. The electrical resistance response of SPPS ZnO1−x sensors to 1 ppm NO2 in air under blue light illumination with R.H. from 0 to 100%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
main influencing agent, should be considered in the gas sensor application. Fig. 13 presents the electrical resistance response of the oxygen-vacant ZnO1-x sensors to 1 ppm with R.H. (relative humidity) ranging from 0 to 100%. Several results can be obtained from this figure. Firstly, the electrical resistance decreased when R.H. increased. Further to observe Fig. 13, it can be found that the base resistance decreases from 7.78 × 103 to 4.33 × 103 . When the R.H. was further increased above 25%, there was no obvious change in base resistance. Secondly, the sensor response increased with R.H., especially higher than 50%. Thirdly, the recovery time is significantly shortened when higher R.H. (>50%) is used. Similar results were found in [30]. All these behaviors should be attributed to the presence of adsorbed water molecules on the metal oxide surface. Stability is another key parameter for practical application, so relative studies were performed on the SPPS ZnO1-x sensors. Fig. 14
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Table 3 The room temperature performance of the SPPS ZnO1−x sensors in comparison with the ZnO based sensors in literature. Material
Preparation method
NO2 concentration (ppm)
Response (RNO2 /Rair )
Response Time(min)
Light illumination
Reference
ZnO ZnO ZnO ZnO@SnO2 ZnO@SnO2 ZnO@PS (Porous Silicon) ZnO@PS ZnO@GO ZnO@rGO ZnO1−x
Soft e-beam lithography Facile solution Drop-cast Wet chemical Thermal evaporation Electrochemical deposition Hydrothermal Solvothermal In-situ production Solution precursor plasma spray
20 5 5 0.5 1 1 50 50 5 1
2.2 2.5 1.1 10.1 2.4 1.5 1.35 1.21 1.25 3.59
15 0.5 1.5 12 5 10 17 2.5 3 13.7
UV UV UV UV UV – – – – Blue light
[37] [38] [39] [40] [41] [42] [43] [44] [45] This work
Fig. 14. Long-term stability of SPPS ZnO1−x sensors to 1 ppm NO2 in air under blue light illumination. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
shows the long-term stability results during 15 days. The results indicated that the response of ZnO1-x coatings to 1 ppm NO2 was fluctuated around 3.2, which demonstrated that the obtained sensors possess excellent long-term stability. In addition to stability, selectivity was another important factor for gas sensors. Chemiresistive gas sensors with high sensitivity towards specific gas were expected for many applications. In this study, 1 ppm NO2 , 100 ppm SO2 , 4000 ppm H2 , 3000 ppm acetone, 3000 ppm ethanol and 3000 ppm methanol were used as tested gases illuminated under blue light. It can be found that the response of SPPS ZnO1-x sensors to NO2 was much higher than other gases (as depicted in Fig. 15). Therefore, the obtained sensors have very good selectivity for NO2 . The oxygen vacancies are proven to be the strongly preferential adsorption sites for oxidizing gases, such as NO2 [22–25]. Therefore, the chemisorption of NO2 molecules facilely proceeds on ZnO1−x surface and the depletion layer is created. Besides, the sensor response is highly dependent on the crystallite size (D). For D > Debye length (LD ), the thickness of electron depletion layer is far smaller than D, meaning that the surface reaction does not influence the sensing process. When D is comparable to or less than 2LD , the whole grain is depleted of electrons, thus making the sensor highly sensitive to the target gases. The LD of ZnO is reported to be about 7.5 nm [46]. From the results of XRD and FE-SEM, the particle size in the as-sprayed ZnO1−x coatings is ca. 14 nm, which is close to twice LD . Furthermore, the ZnO1−x coatings also have porous structure, which is favorable for gas diffusion from outside to internal and make them effectively react with gas molecules. So more NO2 species will be adsorbed on the ZnO1−x surface and the sensor exhibits superior NO2 sensing performance.
Fig. 15. Selectivity of SPPS ZnO1−x sensors to 1 ppm NO2 , 100 ppm SO2 , 4000 ppm H2 , 3000 ppm acetone, 3000 ppm ethanol and 3000 ppm methanol under blue light illumination. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Conclusions In this study, we use SPPS to deposit narrow bandgap ZnO1−x . The bandgap reduction can be ascribed to large amounts of oxygen vacancies due to the nanostructured coatings, reducing atmosphere as well as extremely rapid heating and cooling rate during SPPS. The presence of oxygen vacancies in the as-sprayed coatings were confirmed by the PL, EPR and XPS characterization. The visible light adsorption range for SPPS wide bandgap semiconductor coatings was extended to visible light range which should be attributed to the formation of a donor level between the conduction and valence band. In addition, The SPPS ZnO1−x coatings showed significant responses to NO2 with concentration even as low as 0.4 ppm gas sensing properties illuminated under visible light. Moreover, the sensing response increased linearly with NO2 concentration. The enhanced sensor properties were related to the highly concentrated oxygen vacancies and the porous nanostructure provided by SPPS. Finally, we found that the light wavelength played a crucial role in the sensor performance. The coating resistance decreased in reference air with the light wavelength reducing. The response time and recovery time of the coating firstly decreased and then increased with the light wavelength. The best sensing characteristics were obtained when a blue light was used. Acknowledgements This work is supported by State Key Laboratory for Mechanical Behavior of Materials under Grant No. 20161808, the Natural Science Foundation of China under Grant No. 51402255, the Jiangsu Natural Science Foundation of China under Grant No. BK20140487 and the Funding of Jiangsu Innovation Program for Graduate Education under Grant No. KYZZ16 0490. This is a project funded by the
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Priority Academic Program Development of Jiangsu Higher Education Institutions and the Testing Center of Yangzhou University and the Testing Center of Yangzhou University.
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Biographies Chao Zhang received a B.S. degree from the Chongqing University (China) in 2003 and a joint Ph.D. degree from Technology University of Belfort-Montbéliard (France) and Xi’an Jiaotong University (China) in June 2008. From September 2007 to January 2009, he worked as a teaching-research assistant in Technology University of Belfort-Montbéliard. Since Feb 2009, he is a postdoctoral researcher, and then a senior researcher in Materials Science Department of Engineering School of University of Mons (Belgium). In 2014, he joined Yangzhou University (China) as professor where he is leading a research group on thermal spray coatings and gas sensors. Since August 2016, he is Vice Dean in charge of research in College of Mechanical Engineering. His research interests include thermal-sprayed techniques and coatings, especially gas sensing and wear-resistant coatings. Xin Geng received his B.S. degree in 2013 at Yangzhou University. He is currently pursuing his PhD degree at the same university and takes an interest in smart materials for room temperature gas sensors. Hanlin Liao received his Ph.D. degree in materials science in 1994 at Technology University of Belfort-Montbeliard. After then, he has worked as an associate professor in Laboratoire d’Etudes et de Recherches sur les Matériaux, les Procédés et les Surfaces (LERMPS). From 2004, he is professor at Technology University of BelfortMontbeliard. Since 1985, he has been working in the field of process development of thermal spraying and industrial applications. He published more than 200 peer reviewed papers in thermal sprayed coatings.
C. Zhang et al. / Sensors and Actuators B 242 (2017) 102–111 Chang-Jiu Li received his B.Sc. from the Department of Mechanical Engineering, Xian Jiaotong University in 1982. He received his M.S. and Ph.D. in welding engineering from Osaka University in 1986 and 1989, respectively. From 1989 to 1992, he was a postdoctoral research fellow at the Advanced Materials Processing Institute, Kinki, Japan. During this period, he also worked with Tocalo Company on projects involving HVOF cermet coatings and splat formation in plasma spraying. In September 1992, he became a lecturer at Xian Jiaotong University and was promoted to full professor at the end of 1992. Prof. Li leads the thermal spray group at State Key Laboratory for Mechanical Behavior of Materials. He has published over 350 academic papers.
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Marc Debliquy received his Ph.D. at Faculty of Engineering in Mons (Belgium) in 1999 in the field of organic semiconductors for fire detection. He joined the Sochinor Company in 2000. He left in 2003 for joining Materia Nova (Research Center in the field of Materials in Mons). He was responsible for the research activities in the field of gas sensors. Since October 2008, he joined in Material Science Department of Faculty of Engineering of University of Mons and worked as a team leader of semiconductor and sensor group. He was promoted as professor in June 2013. His main research interest is smart coatings for chemical detection.