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Light assisted room-temperature NO2 sensors with enhanced performance based on black SnO1-α@ZnO1-β@SnO2-γ nanocomposite coatings deposited by solution precursor plasma spray
Author’s Accepted Manuscript Light assisted room-temperature NO2 sensors with enhanced performance based on black SnO1nanocomposite coatings α@ZnO1-β@SnO2-γ deposited by solution precursor plasma spray Xin Geng, Yifan Luo, Hanlin Liao, Chao Zhang, Marc Debliquy
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S0272-8842(17)30160-8 http://dx.doi.org/10.1016/j.ceramint.2017.01.136 CERI14598
To appear in: Ceramics International Received date: 6 November 2016 Revised date: 22 January 2017 Accepted date: 27 January 2017 Cite this article as: Xin Geng, Yifan Luo, Hanlin Liao, Chao Zhang and Marc Debliquy, Light assisted room-temperature NO 2 sensors with enhanced performance based on black SnO1-α@ZnO1-β@SnO2-γ nanocomposite coatings deposited by solution precursor plasma spray, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.01.136 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Light assisted room-temperature NO2 sensors with enhanced performance based on black SnO1-α@ZnO1-β@SnO2-γ nanocomposite coatings deposited by solution precursor plasma spray Xin Genga,b,c, Yifan Luoa, Hanlin Liaod, Chao Zhanga,*, Marc Debliquyc a College of Mechanical Engineering, Yangzhou University, Yangzhou 225127, PR China b College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, PR China c Service de Science des Matériaux, Faculté Polytechnique, Université de Mons, Mons 7000, Belgium d IRTES-LERMPS, Université de technologie Belfort-Montbéliard, Belfort 90000, France * Corresponding author Prof. Chao Zhang College of Mechanical Engineering Yangzhou University Huayang West Road 196 Yangzhou 225127, Jiangsu Province P.R. China Tel/Fax: +86-514-87436008 Email: [email protected] [email protected]
Abstract The development of room-temperature gas sensors with high performance for nitrogen dioxide gases is of great importance and appealing. Our previous works show that visible light illumination is an effective method to replace heating. In this paper, solution precursor plasma spray (SPPS) process is used to prepare black SnO1-α@ZnO1-β@SnO2-γ sensitive coatings. Stoichiometric ZnO, SnO2 and SnO are in white color. The introduced highly concentrated donor defects provided by SPPS process make them turn to black. The light absorption range of the black coatings was extended from ultraviolet area to yellow light region. Raman spectroscopy, photoluminescence spectroscopy and X-ray photoelectron spectroscopy were used to characterize the chemical compositions and oxidation states of SPPS coatings. Donor defects formed deep donor levels between the valence band and conduction band, which can be excited under visible light illumination. FE-SEM and HRTEM images exhibited a highly porous nanostructure, and n-n heterojunctions were formed between SnO1-α, ZnO1-β and SnO2-γ. The SPPS coatings exhibited an obvious response towards 1.0 ppm NO2 gas under visible light illumination at room temperature. The enhanced sensing performance was mainly attributed to the presence of donor defects and homogenous heterojunctions. Keywords: Solution precursor plasma spray; Homo-heterojunction; Donor defects; Gas sensors 1. Introduction Metal oxide semiconductors are the most widely studied sensing materials for detecting NO2 gas,
owing to their high sensitivity, cost effectiveness, small size, etc. [1-2]. However, pristine metal oxide exhibits relatively weak sensing response to NO2. It has been reported that constructing homogenous semiconductor hybrids, e.g., ZnO@SnO2 [3], ZnO@TiO2 [4], TiO2@WO3 [5], can effectively suppress the recombination of electron-hole pairs by transferring the electrons and holes in the heterojunction areas, and thus improving the gas sensing characteristics. Moreover, the synergistic effects between the binary components can also enhance the gas sensing properties. Currently, most wide bandgap semiconductors operate at elevated temperature (>200oC) so as to obtain a rapid response and recovery speed [6]. Nevertheless, working at high temperature may induce many problems, especially in presence of flammable atmosphere which is likely to trigger an explosion [7]. In addition, the high-temperature gas sensors need special heat elements made of Pt or other noble metals and substrates that can sustain the elevated temperatures for a long time, which increases the fabrication cost and design complexity. Therefore, developing room-temperature gas sensors is urgent and important in the field of gas sensors [7]. In order to obtain an acceptable room-temperature gas sensing performance comparable to a counterpart at high temperature, other approaches need to be adopted. Visible light irradiation is a feasible method to accelerate surface sensing kinetics [8]. However, it is not appropriate for homogenous semiconductor hybrids, especially for n-type semiconductors, due to their relatively wide bandgaps. For example, in the case of ZnO@SnO2 hybrids, the bandgaps of ZnO and SnO2 are 3.37 and 3.5 eV, respectively and only ultraviolet light is able to excite the electrons. So less gas molecules species (O2-, NO2-) are chemisorbed on the hybrid surface upon exposure to NO2 gas because they cannot capture free electrons from the material surface. It is expected that stoichiometric ZnO@SnO2 hybrids would have no or weak response to NO2 without heating. Our previous works show that incorporating large amounts of donor defects, e.g., oxygen vacancies for n-type semiconductors, is an interesting method to reduce their bandgaps, and thus enables the sub-stoichiometric metal oxides to respond under visible light [9]. Several strategies have been reported to prepare deficient metal oxides. Putting the metal oxide materials into reducing atmosphere at high temperature can produce a large amount of donor defects. For example, black TiO2 containing highly concentrated oxygen vacancies can be prepared by hydrogenation process, and its light absorption range is extended from ultraviolet (UV) area to the whole visible light region [10]. Immersing wide bandgap semiconductors into hydrogen peroxide (H2O2) solution at 80oC and then annealed at 400oC in a furnace is another method to get donor-deficient oxide semiconductors [11]. The obtained deficient materials have a broader visible light absorption range. Moreover, a process that first is subjected to high-temperature heating and then cooled down rapidly also can produce many donor defects. G. Ou and co-authors adopted arc-melting strategy to fabricate diverse metal oxide semiconductors rich in oxygen vacancies, and their bandgaps were significantly reduced [12]. The heating temperature and cooling rate of arc-melting process were ca. 3000oC and 103 K/s, respectively. Solution precursor plasma spray (SPPS) is a relatively novel nanostructured coating deposition technique being used to prepare a variety of materials, e.g., metal, ceramic, composite [13]. Similar to atmospheric plasma spray (APS), SPPS is a process with ultra-high heating temperature (>10000oC) and cooling speed (>105 K/s) [14]. Extremely fast heating and cooling are beneficial to retain the produced metastable phases and large amounts of donor defects in the materials, especially for metal oxide semiconductors. Furthermore, SPPS technique utilized argon (Ar) and hydrogen (H2) as working gases, so a strong reducing atmosphere would be generated during the
spray process [15]. Therefore, when the solution precursor is injected into the plasma plume, coatings with rich defects are expected. Unlike typical APS using powder as feedstock, aqueous or organic solution is used as the feedstock in SPPS. Moreover, the solution precursor in plasma flame should undergo processes including evaporation, precipitation, pyrolysis and melting. Partial plasma energy is used to evaporate the solution. As a result, less energy is applied on the formed particles and a coating with smaller particle size would be obtained. In this study, black nanocomposite coatings were prepared by SPPS in presence of hydrogen in order to get rich donor defects. ZnO, SnO2, and SnO phases were present in the black coatings while donor defects were confirmed by the characterization results. Moreover, the coatings were porous and nanostructured and many n-n heterojunctions could be observed. A large amount of unpaired electrons were produced arising from the excitation of donor defects and the materials presented a strong absorption in the whole visible spectrum. In these conditions, interactions with NO2 were favoured and the SPPS material used as gas sensing layer should exhibited a good response under illumination with visible light. 2. Experimental method 2.1 Coating preparation The mixed precursor solution was prepared by adding 1.75 g stannic chloride (SnCl4·5H2O) and 14.87 g zinc nitrate (Zn(NO3)2·6H2O) (Sinopharm Chemical Reagent, China) into 250 mL deionized water according to the molar ratio Sn/Zn = 1:10. The prepared solution was stirred in a high-speed homogenizer (WiseTis@HG-15D, Korea) for 10 min until a uniformly dispersed solution was obtained. Commercial sensor substrates (C-MAC Micro Technology Company, Belgium) made of alumina (Al2O3) equipped with a pair of interdigitated gold electrodes were adopted in this study, as displayed in the insert picture of Figure 1. Prior to spraying, an ultrasonic cleaning machine was utilized to wash the substrates in an anhydrous ethanol for 5 min. The prepared mixture solution was then injected to the plasma flame through a peristaltic pump. A six-axis robotic arm (ABB, Sweden) was used to hold the plasma torch (Oerlikon Metco, Switzerland) to deposit uniform coatings. The schematic set-up of SPPS was shown in Figure 1, and the spray parameters were listed in Table 1. 2.2 Coating characterization X-ray diffractometer (XRD, D8 Advance, Bruker, Germany) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, USA) were used to characterize the phase constitutions and chemical structure of the SPPS coatings. The detection of donor defects and electronic properties was performed using XPS, photoluminescence spectroscopy (PL, Renishaw, England) and Raman spectroscopy (In via, Renishaw, England). The laser wavelengths used in PL and Raman were 325 and 532 nm. The optical properties of electron state of the coatings were identified by ultraviolet-visible spectrophotometer (UV-Vis, Cary5000, Varian, USA) and electron paramagnetic resonance (EPR, A300-10/12, Bruker, Germany). Field-emission scanning electron microscopy (FE-SEM, S4800Ⅱ, Hitachi, Japan) and high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F30 S-TWIN, FEI, USA) were utilized to examine the microstructure of the SPPS coatings. 2.3 Gas sensing test A Teflon-sealed stainless steel box was used as a home-made gas sensing test chamber and 0.15 W/cm2 LED lamps with different light wavelengths were set on the top of the chamber. Five color
lights, i.e., red, yellow, green, blue and purple, were used to investigate the effect of light wavelength on the sensing properties. Additionally, in order to compare the sensing characteristics with or without visible light illumination, the gas sensing test was also performed in the dark. The obtained gas sensor based on the SPPS coatings was placed just below the LED lamp providing external photon energy to accelerate the reaction rates. Prior to test, 500 mL/min reference air was injected into the chamber to stabilize the base resistance. And then 50 mL/min 10 ppm NO2 was injected into the chamber for 30 min to get the desired concentration of 1.0 ppm. All the gas flow rates in this work were controlled by mass flowrate controller (MFC, Bronkhorst High-Tech, Netherlands). The detailed gas sensing test process has been reported in [13-15]. The sensor response of oxidizing gases was defined as S=(Roxidizing gas-Rair)/Rair, where Roxidizing gas and Rair were the resistance in presence of oxidizing gases and synthetic air, respectively. The sensor response of reducing gases represented S=(Rair-Rreducing gas)/Rreducing gas, in which Rreducing gas and Rair were the resistance when exposed to reducing gases and synthetic air, respectively. 3. Results and discussion The XRD diffraction pattern of the SPPS coatings was displayed in Figure 2. The characteristic peaks were clearly observed, which were attributed to hexagonal ZnO with dominant planes of (100), (002) and (101) located at 31.7, 34.4 and 36.2o. The result was well in accordance with the PDF 36-1451. The peaks suited at 26.6 and 33.8o belonging to SnO2 match well with PDF 70-4177. In addition, the peaks centered at 29.9 and 37.3o attributed to SnO were also observed, which was in good agreement with PDF 85-0712. From these results, it can be concluded that ZnO, SnO2 and SnO phases were present in the obtained coatings. It can also be observed that the peak intensity of ZnO was much stronger, compared with the SnO2 and SnO peak intensity, meaning that ZnO was the dominant phase in the SPPS coatings. Due to thin thickness of the SPPS coatings, there were strong Al2O3 peaks (PDF 46-1212) emitted from the alumina sensor substrates. A similar phenomenon has been observed in [13-15]. From the insert picture in Figure 1, it can be seen that the coatings exhibited a dark color. It is well known that the stoichiometric ZnO, SnO and SnO2 are in white. Similar phenomenon was also found in black TiO2 produced by hydrogenation [10], and the authors attributed the black TiO2 to the formation of large amounts of defects. Therefore, some changes in phase constitutions must be present in the SPPS coatings. To provide more insights of the phase structures of the SPPS coatings, several characterizations were performed. Raman spectra exhibited many peaks in the range of 150-1000 cm-1, as depicted in Figure 3. The signals at 330, 384 and 437 cm-1 assigned to the 2-E2(M), A1(TO) and E2(h) vibrational photon mode of wurtzite ZnO were obviously observed [16]. In addition, a typical A1g vibration mode located at 218.6 cm-1 corresponded to SnO while a broad peak ranging from 628 to 774 cm-1 was related to the A1g and B2g modes of SnO2 [17]. It was interesting to find that an additional peak suited at 585 cm-1 was found in the spectrum, which mainly resulted from the presence of donor defects [18]. The peak intensity of 585 cm-1 was comparable to the main ZnO peak, implying that the concentration in donor defects introduced into the SPPS coatings was quite large. Apart from Raman, PL spectroscopy was another technique to analyze the defects. From Figure 4, three peaks centered at ca. 340, 380 and 400-750 nm can be observed. The 340 nm signal was assigned to the intrinsic peak of the mixture of SnO2 and SnO, while the 380 one was the intrinsic peak of ZnO, indicating
the presence of ZnO, SnO2 and SnO. The wide peak located at the visible light region in the range of 400-750 nm was attributed to the donor defects of zinc interstitials and oxygen vacancies [19]. The peak intensity of donor defects was higher than those of intrinsic peaks, revealing a big concentration of donor defects in the coatings. In order to further confirm the production of defects, the chemical compositions and oxidation states of the SPPS coatings on the outmost surface were analyzed in detail by XPS. Figure 5(a) illustrated the XPS survey of the coatings, where all the expected elements were probed comprising the C 1s, Zn2p, Sn 3d and O 1s. In this work, all the core levels were calibrated by C 1s at 284.8 eV. Zn 2p exhibited a doublet pair suited at 1021.3 and 1044.5 eV, as depicted in Figure 5(b). The peak suited at ca. 1021.3 eV was stemmed from the binding energy of Zn 2p3/2, and the 1044.5 eV peak was derived from the binding energy of Zn 2p1/2, hinting the presence of ZnO [20]. To further determine the chemical state of Zn element, the ZnLMM spectrum was also probed. Figure 5(c) showed the ZnLMM signal and it was deconvoluted into two peaks, including the main and minor peak at 988.1 and 992.1 eV. The main peak was assigned to Zn2+, and the minor one was originated from metallic zinc (Zn0) [21]. The intensity of Zn2+ was much stronger than that of Zn0, suggesting that Zn2+ was the dominant phase and zinc interstitials were present in the SPPS coatings. The Zn 2p and ZnLMM spectra revealed that ZnO was present in the SPPS coatings, while it was not a stoichiometric form. The Sn 3d spectrum was similarly deconvoluted into two peaks (Figure 5(d)): a low binding energy peak located at 487.3 eV was associated with Sn2+, whereas a high binding energy peak lied at 486.3 eV was arose from Sn4+, indicating the presence of SnO2 and SnO [18]. Moreover, the peak intensity of Sn4+ was much higher than that of Sn2+, which showed that Sn element mainly existed in the form of SnO2. Since ZnO, SnO2 and SnO were oxides, the investigation of O1s core level was very necessary. The O1s core level consisted of three contributions located at ca. 530.1, 531.4 and 532.7 eV, as displayed in Figure 5(e). The contribution of 530.1 eV was derived from the lattice oxygen in ordered metal oxides, while the 531.4 eV one was related to the oxygen-defective regions of metal oxides [22]. The 532.7 eV was attributed to the hydroxyl groups (OH-) from the adsorbed water molecules or the C=O bonds from the carbon oxides in the air. The strong peak intensity of 531.4 eV showed that there were large numbers of oxygen vacancies in the SPPS coatings. From the XPS characterization, it can be concluded that oxygen-defective ZnO1-β, SnO1-α and SnO2-γ phases were all present in the coatings. In addition, highly concentrated donor defects were demonstrated to be incorporated into the coatings, such as zinc interstitials and oxygen vacancies. Combined with the XRD, Raman, PL and XPS measurements, it can be concluded that black SnO1-α@ZnO1-β@SnO2-γ hybrids have been successfully synthesized. In a typical SPPS process, H2 was used as a second plasma forming gas to provide enthalpy and increase thermal conductivity of plasma flame [13-15]. When the mixture solution precursor of Zn(NO3)2 and SnCl4 was injected into the plasma plume with a certain kinetics provided by a peristaltic pump, the mixture droplets were firstly submitted to evaporation and precipitation. And then the precipitated particles would be subjected to pyrolysis to produce ZnO and SnO2 according to the reactions below: Zn(NO3)2 + H2O → ZnO + 2HNO3 (1) SnCl4 + 2H2O → SnO2 + 4HCl (2) In addition, some SnO2 were reduced to SnO followed the equations by: SnO2 + H2 → SnO + H2O (3)
An extremely high reaction temperature and active radicals provided by ionization greatly increase the driving force of the chemical reaction and particle growth. The dwelling time of the particles in the plasma flame was in the order of milliseconds, so the formed particles still would be in nanoscale. Furthermore, the particles tended to produce a large number of defects in the rapid nucleation on account of the transient retention in the high-temperature region and the reducing atmosphere [12]. ZnO + βH2 → ZnO1-β + βH2O (4) SnO + αH2 → SnO1-α + αH2O (5) SnO2 + γH2 → SnO2-γ + γH2O (6) By quenching to freeze, the particles can be obtained at different stages. Therefore, surface defects inside the particles can be kept and regulation, ensuring a uniform distribution of defects and an effective control of extrinsic defects. Features including transient retention in the high-temperature region and rapid cooling, can fully avoid the difference of the matrix precipitation. Since visible light will be used as the activation source in this work, the optical absorption properties of the SPPS coatings should be detected. In order to study the light absorption of the SnO1-α@ZnO1-β@SnO2-γ coatings, the optical absorption of pristine ZnO and SnO2 samples was also measured. Pristine SnO had a similar bandgap and light absorption to SnO2. Therefore, the light absorption of SnO was not measured herein. The pristine ZnO and SnO2 samples were prepared by annealing the commercial powder at 1000oC in a furnace for 4h to get stoichiometric samples. Figure 6(a) exhibited the UV-Vis absorption spectra of the coatings, pristine ZnO and SnO2 samples. Pristine ZnO sample only absorbed light below 380 nm, and pure SnO2 sample had an absorption edge below 340nm. However, the absorption edge of the SPPS coatings was red-shifted to ca. 600 nm, located at the yellow light region. The result revealed that the light absorption of the SPPS coatings in the visible light region was significantly enhanced compared with the bare ZnO and SnO2 samples. As well known, the ability of light absorption is directly associated with the bandgaps of materials [8-9,20], therefore, it is imperative to investigate the bandgaps of three samples. The bandgaps were estimated from the graph of (αhν)2 versus hν (Tauc plot) in Figure 6(b) according to the formula [23]: (αhν)2 = k (hν- Eg) (7) where α was the absorption coefficient, hν was the energy of incident light and k was a constant. The bandgaps of the pristine SnO2, ZnO and SPPS coatings were estimated to be 3.47, 3.22 and 2.51 eV, suggesting that the bandgap of the coatings was greatly narrowed down. The reduction of bandgaps of the SPPS coatings was mainly attributed to the presence of rich donor defects. Donor defects will produce additional deep donor levels between the valence and conduction band, giving rise to a decrease in bandgaps [24]. In addition to optical properties, electrical properties were also very important to the gas sensing performance, because the sensing process was dependent on the variation in electron concentration in the conduction band through the electron transfer between the materials and target gas molecules. EPR technique is used to detect the unpaired electrons. Figure 7 depicted the EPR spectrum of the SPPS SnO1-α@ZnO1-β@SnO2-γ coatings, where a strong peak corresponding to lange factor (g-factor) equal to 1.9610 was observed. It indicated that there were abundant unpaired electrons in SPPS coatings, being beneficial to the chemisorption of testing gases. According to [25], the 1.9610 signal was originated from the presence of oxygen
vacancies (Vox ) and zinc interstitials (Znxi ), which significantly increased the electron concentration in the detected materials. In SPPS coatings, a large number of electrons in the conduction band resulted from two aspects: the presence of donor defects, and the formation of homogenous heterojunctions between SnO1-α, ZnO1-β and SnO2-γ. Vox and Znxi were thermodynamically unstable and easily ionized into single and double electropositive oxygen vacancies (Vo• ,Vo•• ) and zinc interstitials (Zn•i ,Zn•• i ) by [26-27]: x • x • Vo (Zni ) ⇌ Vo (Zni ) + e- ⇌ Vo•• (Zn•• (8) i )+e Then the excited electrons were diverted from the donor levels to the conduction band of metal oxides. As a consequence of the difference in Fermi levels among ZnO, SnO and SnO2, the produced electrons would be transferred from the high-energy levels to low-energy levels. Simultaneously, the generated holes were moved in the opposite direction to electrons, which resulted in the effective separation of electron-hole pairs [28]. Then an energy bandgap bending would occur at the interface of the hybrids, forming a potential barrier at the heterojunctions. In the case of the gas sensors, the surface microstructure played an important role in the final gas sensing properties [7]. The sensor sensitivity is highly determined by the particle size in the sensitive materials [7]. When the particle is comparable to twice Debye length of semiconductors, the particles would be partially or completely depleted. Then the surface adsorption and desorption process would noticeably change the electrical properties, leading to a high sensitivity. As the particle size increases, the surface sensing behavior cannot affect the sensitivity very much. Moreover, a sensing material with high porosity was beneficial for gas diffusion, giving rise to a quicker response. Thus FE-SEM and HRTEM characterization were conducted. Figure 8 illustrates the FE-SEM images of the SPPS coatings with different magnifications. The low magnification image indicated that the coatings were highly porous. Besides, the high magnification revealed that the coatings were nanostructured, and its particle size was in the range of 15-40 nm. The Debye length of metal oxides was reported to be typically in a few nanometers [29]. Therefore, it can be expected that the SPPS coatings may exhibit nanostructure with a size comparable to twice Debye length, being favorable for obtaining a high sensitivity. To get an overview of the detailed structure of the SPPS coatings, some images were acquired by HRTEM. Figure 9 depicts the HRTEM images of the SnO1-α@ZnO1-β@SnO2-γ coatings with different magnifications. As can be seen in Figure 9(a), the SPPS coatings were composed of many blended nanoparticles and the nanoparticles were interconnected with each other. The particle size was in the range of 15-40 nm, being compatible with the FE-SEM results. The fringe spacing in Figure 9(c) was measured to be 0.26, 0.28, 0.29 and 0.34 nm, respectively. 0.26 and 0.28 nm spacing was attributed to the (002) and (100) planes of ZnO [30]. 0.34 nm spacing corresponded to the interplanar spacing of SnO2 (110) plane [31], and that of 0.29 nm was assigned to the (101) plane of SnO [32]. It can be deduced that many n-n heterojunctions between SnO1-α, ZnO1-β and SnO2-γ were formed in the SPPS coatings. Figure 10 shows the plot of the sensor response of the black SnO1-α@ZnO1-β@SnO2-γ coatings as a function of 1.0 ppm performed in the dark and visible light illumination. The relevant values were listed in Table 2. The SPPS coatings had a weak response in the dark, whose sensor response was only 0.35. Furthermore, its response time was very long, more than 30 min. The desorption process was almost negligible, indicating that the NO2 species cannot be desorbed from the coating surface in the dark. With respect to the dark condition, the NO2 sensing performance illuminated by visible light including the sensor response, response and recovery time, was
considerably enhanced. In addition, it also can be observed that the light wavelength also had a significant effect on the sensor properties. The response time and recovery rates increased with the light wavelength decrease (from red light to blue light) except for purple light. As a result of the high photon energy of purple light, some additional reactions between the sensing materials and testing gases occurred, prolonging the response and recovery time [33]. The effect of light wavelength on the sensor response was complex for the SPPS coatings. The sensor response of red light was still very little (1.11), whereas it greatly increased when other color lights were used. It may be ascribed to the fact that the light absorption range of SPPS coatings was only extended to ca. 600 nm, not including the red light. Consequently, when the red light was used as an incident light resource, its photon energy was so low that it cannot excite the electrons in the sensitive materials, resulting in a weak response and slow response process. Except the red light, the sensor response decreased when the light wavelength was lowered. The photon energy of the incident light was inversely proportional to the light wavelength [8-9]. Our previous work demonstrated that the role of photon energy on the sensing properties was similar to that of heating [33]. A higher photon energy can obviously enhance the surface desorption rate, less target gas molecules were adsorbed on the surface, leading to a reduction of sensor response. In general, the blue light was the best choice for practical applications because it had the best response and recovery rate while possessing an acceptable sensor response. The electrical resistance response of the black SnO1-α@ZnO1-β@SnO2-γ coatings submitted to NO2 with concentrations ranging from 1 to 10 ppm with blue light stimulation at room temperature was displayed in Figure 11(a). It can be seen in Figure 11(b) that the sensor response of the SPPS coatings was always increased as the sensor was exposed to increased NO2 concentration, moreover, the sensor response varied linearly with NO2 concentration. The results indicated that the SnO1-α@ZnO1-β@SnO2-γ sensors possessed an excellent detectability to different concentrations of NO2. Figure 12 depicts the stability test results of the presented sensors towars 1.0 ppm NO2 under blue light illumination at room temperature during 15 days. It can be found that the value of sensor response was always around 2.36, exhibiting a good stability performance. Figure 13 illustrates the selectivity of the SPPS SnO1-α@ZnO1-β@SnO2-γ coatings to 1.0 ppm NO, 15 ppm HCHO, 100 ppm SO2, 100 ppm H2, 100 ppm NH3 and 100 ppm CO. All these experiments were performed under blue light irradiation at room temperature. From that, the prepared SnO1-α@ZnO1-β@SnO2-γ sensors exhibited a higher response towards NO2 when compared with those to other gases. Therefore, it can be concluded that the SPPS SnO1-α@ZnO1-β@SnO2-γ sensor possessed a good selectivity to NO2. Stoichiometric ZnO, SnO2 and SnO were all wide bandgap semiconductors (>3.3 eV). Under normal circumstances, and they had no electrons in their conduction band even if visible light sources were employed to irradiate them. Therefore, the chemisorption of gas molecules cannot be reacted on their surface, leading to no or weak response to NO2 gas at room temperature. In this work, SPPS approach was adopted to deposit black SnO1-α@ZnO1-β@SnO2-γ coatings with large numbers of donor defects. Donor defects can be ionized under visible light irradiation to form light-generated electron-hole pairs, as verified by the EPR results. On account of the presence of n-n heterojunctions, the photogenerated electrons will be transferred from high-energy to low-energy levels due to the Fermi-level mediated charge transfer effect, suppressing the recombination of electron-hole pairs. Effective charge separation greatly increased the lifetime of photogenerated charge carriers, and more electrons can be participated
in the reaction with the testing gases, leading to a higher response. The electron excitation and transfer process were exhibited in Figure 14(a). The deep donor levels formed by donor defects were abbreviated as Ed in the schematic. Donor defects, i.e., zinc interstitials and oxygen vacancies, produced in the black coatings were preferential sites for oxidizing gases [34]. Therefore, more surface active adsorption sites existed in the defective coatings with respect to those of the stoichiometric ones, forming a large surface coverage of chemisorbed oxygen species. Furthermore, the adsorption energy of oxidizing gas on the donor-defective metal oxides was much smaller than that on stoichiometric counterparts [35]. When the coatings were exposed in reference air, oxygen molecules would be facilely chemisorbed on the material surface, especially on the defect sites, to form O2- species. Vox + O2 ⇌ Vo• + O2(9) x • Zni + O2 ⇌ Zni + O2 (10) It would cause the formation of a depletion layer at the heterojunction area, leading to a high potential barrier and electrical resistance. As the reaction time going, the adsorption and desorption kinetics of O2- species reached an equilibrium state, exhibiting a stable electrical resistance. When NO2 gas was injected, because the electron affinity of NO2 is greater than O2 [13-15,36-37], NO2 molecules can capture electrons from the adsorbed O2- species. O2- + NO2 ⇌ NO2- +O2 (11) x • Vo + NO2 ⇌ Vo + NO2 (12) Znxi + NO2 ⇌ Zn•i + NO2(13) The surface acceptor levels formed by NO2- species was deeper than that of O2- [8-9,33], which resulted in a wider surface depletion layer and a higher potential barrier at the heterojunction areas. It greatly increased the electrical resistance of the coatings. Owing to chemisorbed oxygen species and rich donor defects presented on the SPPS coatings, a large number of NO2 molecules would be adsorbed on the coating surface, exhibiting excellent NO2 sensing properties. The gas sensing process of SPPS coatings was illustrated in Figure 14(b). 4. Conclusions The solution precursor plasma spray process was used to prepare black SnO1-α@ZnO1-β@SnO2-γ coatings. The nonstoichiometry of the SPPS coatings was investigated by various spectroscopic techniques, including Raman, PL and XPS spectra. The results indicated that large amounts of donor defects (zinc interstitial and oxygen vacancy) were found in the coatings as a result of their process features, i.e., ultra-high temperature heating, rapid cooling, and reducing atmosphere. XRD analysis revealed that the deficient SnO1-α@ZnO1-β@SnO2-γ coatings were successfully prepared without changing their crystal structure. UV-Vis spectra showed that a broader light absorption range of the SPPS coatings was obtained, extending to the yellow light, on account of the introduction of highly concentrated donor defects. A large number of unpaired electrons existed in the coatings, as verified by the EPR characterization. FE-SEM and HRTEM images exhibited a highly porous nanostructure, and n-n heterojunctions were formed between SnO1-α, ZnO1-β and SnO2-γ. The gas sensing test results indicated that visible light illumination can greatly enhance the gas sensing performance and the blue light illumination endowed the SPPS coatings possessing the best NO2 sensing properties in terms of the sensor response and response and recovery rates. The composite also shows a good selectivity for NO2 towards other possible interfering gases.
Acknowledgements This work is supported by the Jiangsu Natural Science Foundation of China under Grant No. BK20140487, the Natural Science Foundation of China under Grant No. 51402255, State Key Laboratory for Mechanical Behavior of Materials under Grant No. 20161808 and the Funding of Jiangsu Innovation Program for Graduate Education under Grant No. KYZZ16_0490. The authors thank Ms. Pascale Hoog of University of Technology of Belfort Montbéliard for her English help on this paper. Marc Debliquy also thanks the Waloon Region for its support in the framework of the Captindoor project. References [1] C. Zhang, M. Debliquy, A. Boudiba, H. Liao, C. Coddet. Sensing properties of atmospheric plasma-sprayed WO3 coating for sub-ppm NO2 detection. Sensors and Actuators B: Chemical, 144 (2010) 280-288. [2] M.E. Franke, T.J. Koplin, U. Simon. Metal and metal oxide nanoparticles in chemiresistors: Does the Nanoscale Matter? Small, 2 (2006) 36-50. [3] W. Guo, Z. Wang. Composite of ZnO spheres and functionalized SnO2 nanofibers with an enhanced ethanol gas sensing properties. Materials Letters, 169 (2016) 246-249. [4] C.W. Zou, J. Wang, W. Xie. Synthesis and enhanced NO2 gas sensing properties of ZnO nanorods/TiO2 nanoparticles heterojunction composites. Journal of Colloid and Interface Science, 478 (2016) 22-28. [5] W. Meng, L. Dai, J. Zhu, Y. Li, W. Meng, H. Zhou, L. Wang. A novel mixed potential NH3 sensor based on TiO2@WO3 core–shell composite sensing electrode. Electrochimica Acta, 193 (2016) 302-310. [6] C. Zhang, J. Wang, X. Geng. Tungsten oxide coatings deposited by plasma spray using powder and solution precursor for detection of nitrogen dioxide gas. Journal of Alloys and Compounds, 668 (2016) 128-136. [7] J. Zhang, X. Liu, G. Neri, N. Pinna. Nanostructured materials for room temperature gas sensors. Advanced materials, 28 (2016) 795-831. [8] X. Geng, C. Zhang, M. Debliquy. Cadmium sulfide activated zinc oxide coatings deposited by liquid plasma spray for room temperature nitrogen dioxide detection under visible light illumination. Ceramics International, 42 (2016) 4845-4852. [9] X. Geng, J. You, C. Zhang. 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) 286-293. [10] X. Chen, L. Liu, P.Y. Yu, S.S. Mao. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science, 331 (2011) 746-750. [11] J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang, Y. Dai. Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO. ACS Applied Material & Interfaces, 4 (2012) 4024-4030. [12] G. Ou, D. Li, W. Pan, Q. Zhang, B. Xu, L. Gu, C. Nan, H. Wu. Arc- Melting to Narrow the Bandgap of Oxide Semiconductors. Advanced Materials, 27 (2015) 2589–2594. [13] C. Zhang, X. Geng, M. Olivier, H. Liao, M. Debliquy. Solution precursor plasma-sprayed tungsten oxide coatings for nitrogen dioxide detection. Ceramics International, 40 (2014)
11427-11431. [14] C. Zhang, M. Debliquy, H. Liao. Deposition and microstructure characterization of atmospheric plasma-sprayed ZnO coatings for NO2 detection. Applied Surface Science, 256 (2010) 5905-5910. [15] C. Zhang, X. Geng, H. Li, P. He, M. Planche, H. Liao, M. Olivier, M. Debliquy. Microstructure and gas sensing properties of solution precursor plasma-sprayed zinc oxide coatings. Materials Research Bulletin, 63 (2015) 67-71. [16] E. Manikandan, G. Kavitha, J. Kennedy.Epitaxial zinc oxide, graphene oxide composite thin-films by laser technique for micro-Raman and enhanced field emission study. Ceramics International, 40 (2014) 16065-16070. [17] N. Salah, W.M. AL-Shawafi, S.S. Habib, A. Azam, A. Alshahrie. Growth-controlled from SnO2 nanoparticles to SnO nanosheets with tunable properties. Materials & Design, 103 (2016) 339-347. [18] A. Shanmugasundaram, P. Basak, L. Satyanarayana, S.V. Manorama. Hierarchical SnO/SnO2 nanocomposites: Formation of in situ p–n junctions and enhanced H2 sensing. Sensors and Actuators B: Chemical, 185 (2013) 265-273. [19] D.E. Motaung, P.R. Makgwane, S.S. Ray. Induced ferromagnetic and gas sensing properties in ZnO-nanostructures by altering defect concentration of oxygen and zinc vacancies. Materials Letters, 139 (2015) 475-479. [20] C. Zhang, J. Wang, M. Olivier, M. Debliquy. Room temperature nitrogen dioxide sensors based on N719-dye sensitized amorphous zinc oxide sensors performed under visible-light illumination. Sensors and Actuators B: Chemical, 209 (2015) 69-77. [21] J. Winiarski, W. Tylus, B. Szczygieł. EIS and XPS investigations on the corrosion mechanism of ternary Zn–Co–Mo alloy coatings in NaCl solution. Applied surface science, 364 (2016) 455-466. [22] S. Kabcum, D. Channei, A. Tuantranont, A. Wisitsoraat, C. Liewhiran, S. Phanichphant. Ultra-responsive hydrogen gas sensors based on PdO nanoparticle-decorated WO3 nanorods synthesized by precipitation and impregnation methods. Sensors and Actuators B: Chemical, 226 (2016) 76-89. [23] M.H. Habibi, M.H. Rahmati. Fabrication and characterization of ZnO@CdS core-shell nanostructure using acetate precursors: XRD, FESEM, DRS, FTIR studies and effects of cadmium ion concentration on band gap. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 133 (2014) 13-18. [24] W. Kim, M. Choi, K. Yong. Generation of oxygen vacancies in ZnO nanorods/films and their effects on gas sensing properties. Sensors and Actuators B: Chemical, 209 (2015) 989-996. [25] W. Xia, C. Mei, X. Zeng, G. Fan, J. Lu, X. Meng, X. Shen. Nanoplate-Built ZnO Hollow Microspheres Decorated with Gold Nanoparticles and Their Enhanced Photocatalytic and Gas-Sensing Properties. ACS Applied Material & Interfaces, 7 (2015) 11824-11832. [26] N. Han, L. Chai, Q. Wang, Y. Tian, P. Deng, Y. Chen. Evaluating the doping effect of Fe, Ti and Sn on gas sensing property of ZnO. Sensors and Actuators B: Chemical, 147 (2010) 525-530. [27] N. Han, X. Wu, L. Chai, H. Liu, Y. Chen. Counterintuitive sensing mechanism of ZnO nanoparticle based gas sensors. Sensors and Actuators B: Chemical, 150 (2010) 230-238. [28] G. Zhou, X. Xu, T. Ding, B. Feng, Z. Bao, J. Hu. Well−steered charge−carrier transfer in 3D branched CuxO/ZnO@Au heterostructures for efficient photocatalytic hydrogen evolution. ACS Applied Material & Interfaces, 7 (2015) 26819-26827.
[29] Y. Li, T. Liu, H. Zhang. New insight into gas sensing performance of nanoneedle-assembled and nanosheet-assembled hierarchical SnO2 structures. Materials Letters, 176 (2016) 9-12. [30] V.V. Ganbavle, S.I. Inamdar, G.L. Agawane, J.H. Kim, K.Y. Rajpure. Synthesis of fast response, highly sensitive and selective Ni: ZnO based NO2 sensor. Chemical Engineering Journal, 286 (2016) 36-47. [31] L.F. da Silva, J.C. M’Peko, A.C. Catto, S. Bernardini, V.R. Mastelaro, K. Aguir, C. Ribeiro, E. Longo. UV-enhanced ozone gas sensing response of ZnO-SnO2 heterojunctions at room temperature. Sensors and Actuators B: Chemical, 240 (2017) 573-579. [32] Y. Li, W. Zhou, Y. Yang, P. Wu. Fabrication, characterization and physical properties of Co-doped SnO particles via hydrothermal method. Journal of Alloys and Compounds, 685 (2016) 448-453. [33] C. Zhang, A. Boudiba, P. Marco, R. Snyders, M. Olivier, M. Debliquy. Room temperature responses of visible-light illuminated WO3 sensors to NO2 in sub-ppm range. Sensors and Actuators B: Chemical, 181 (2013) 395-401. [34] M. Chen, Z. Wang, D. Han, F. Gu, G. Guo.High-sensitivity NO2 gas sensors based on flower-like and tube-like ZnO nanomaterials. Sensors and Actuators B: Chemical, 157 (2011) 565-574. [35] Y. Qin, Z. Ye. DFT study on interaction of NO2 with the vacancy-defected WO3 nanowires for gas-sensing. Sensors and Actuators B: Chemical, 222 (2016) 499-507. [36] C. Zhang, X. Geng, H. Liao, C. Li, M. Debliquy. Room-temperature nitrogen-dioxide sensors based on ZnO1−x coatings deposited by solution precursor plasma spray. Sensors and Actuators B: Chemical, 242 (2017) 102-111. [37] X. Geng, C. Zhang, Y. Luo, M.Debliquy. Preparation and characterization of CuxO1-y@ZnO1-α nanocomposites for enhanced room-temperature NO2 sensing applications. Applied Surface Science, 401 (2017) 248-255.
Figure captions Figure 1. Schematic set-up for SPPS, insert: the photo of the sensor substrate and the sensor based on SPPS coatings. Figure 2. X-ray diffraction patterns of SPPS coatings. Figure 3. Raman spectra of SPPS coatings. Figure 4. PL spectrum of SPPS coatings. Figure 5. XPS spectra of SPPS coatings, (a) survey, (b) Zn2p, (c) ZnLMM, (d) Cu2p (e) O1s, the green lines represent the measured curve, while the lines in other colors are the fitted curves. Figure 6. (a) UV-Vis absorption spectra (b) Tauc plot of the ZnO, SnO2 samples and SPPS coatings. Figure 7. EPR spectrum of SPPS coatings. Figure 8. FE-SEM images of SPPS coatings with different magnifications. Figure 9. HRTEM images of SPPS coatings with different magnifications.
Figure 10. Electrical resistance of SPPS coatings plotted against time towards 1.0 ppm NO2 illuminated under visible light with different wavelengths at room temperature. Figure 11. The electrical resistance response of the SPPS coatings towards NO2 with concentrations ranging from 1 to 10 ppm with blue light stimulation at room temperature Figure 12. The stability test results of the SPPS coatings to 1.0 ppm NO2 under blue light illumination at room temperature. Figure 13. Selectivity of the SPPS coatings towards 1.0 ppm NO, 15 ppm HCHO, 100 ppm SO2, 100 ppm H2, 100 ppm NH3 and 100 ppm CO under blue light illumination at room temperature. Figure 14. Schematic of the (a) electron excited process (b) gas sensing process of SPPS coatings.
Table 1. Solution precursor plasma spraying (SPPS) parameters. Parameters
Value
Argon volume flow rate (L/min) Hydrogen volume flow rate (L/min) Spraying distance (mm) Arc current (A) Torch power (kW) Nozzle inner diameter (mm) Liquid flow rate (mL/min)
40 2 100 513 25 0.26 20
Table 2. Sensing characteristics of the sensor based on SPPS coatings to 1.0 ppm NO2 performed in the dark and under visible light illumination with different light wavelength. Wavelength (nm)
Light color In the dark
640 580 530 480 400
Red Yellow Green Blue Purple
Rair (Ω)
Sensor response
Response time
Recovery time
((RNO2-Rair)/Rair)
(min)
(min)
3.68×10
7
0.35
>30
>60
6.51×10
6
1.11
26.6
>60
2.70×10
6
4.57
23.4
58.9
1.21×10
6
3.35
21.8
37.6
6.47×10
5
2.36
17.6
23.4
2.73×10
5
2.09
20.3
28.8
PII: DOI: Reference:
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S0272-8842(17)30160-8 http://dx.doi.org/10.1016/j.ceramint.2017.01.136 CERI14598
To appear in: Ceramics International Received date: 6 November 2016 Revised date: 22 January 2017 Accepted date: 27 January 2017 Cite this article as: Xin Geng, Yifan Luo, Hanlin Liao, Chao Zhang and Marc Debliquy, Light assisted room-temperature NO 2 sensors with enhanced performance based on black SnO1-α@ZnO1-β@SnO2-γ nanocomposite coatings deposited by solution precursor plasma spray, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.01.136 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Light assisted room-temperature NO2 sensors with enhanced performance based on black SnO1-α@ZnO1-β@SnO2-γ nanocomposite coatings deposited by solution precursor plasma spray Xin Genga,b,c, Yifan Luoa, Hanlin Liaod, Chao Zhanga,*, Marc Debliquyc a College of Mechanical Engineering, Yangzhou University, Yangzhou 225127, PR China b College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, PR China c Service de Science des Matériaux, Faculté Polytechnique, Université de Mons, Mons 7000, Belgium d IRTES-LERMPS, Université de technologie Belfort-Montbéliard, Belfort 90000, France * Corresponding author Prof. Chao Zhang College of Mechanical Engineering Yangzhou University Huayang West Road 196 Yangzhou 225127, Jiangsu Province P.R. China Tel/Fax: +86-514-87436008 Email: [email protected] [email protected]
Abstract The development of room-temperature gas sensors with high performance for nitrogen dioxide gases is of great importance and appealing. Our previous works show that visible light illumination is an effective method to replace heating. In this paper, solution precursor plasma spray (SPPS) process is used to prepare black SnO1-α@ZnO1-β@SnO2-γ sensitive coatings. Stoichiometric ZnO, SnO2 and SnO are in white color. The introduced highly concentrated donor defects provided by SPPS process make them turn to black. The light absorption range of the black coatings was extended from ultraviolet area to yellow light region. Raman spectroscopy, photoluminescence spectroscopy and X-ray photoelectron spectroscopy were used to characterize the chemical compositions and oxidation states of SPPS coatings. Donor defects formed deep donor levels between the valence band and conduction band, which can be excited under visible light illumination. FE-SEM and HRTEM images exhibited a highly porous nanostructure, and n-n heterojunctions were formed between SnO1-α, ZnO1-β and SnO2-γ. The SPPS coatings exhibited an obvious response towards 1.0 ppm NO2 gas under visible light illumination at room temperature. The enhanced sensing performance was mainly attributed to the presence of donor defects and homogenous heterojunctions. Keywords: Solution precursor plasma spray; Homo-heterojunction; Donor defects; Gas sensors 1. Introduction Metal oxide semiconductors are the most widely studied sensing materials for detecting NO2 gas,
owing to their high sensitivity, cost effectiveness, small size, etc. [1-2]. However, pristine metal oxide exhibits relatively weak sensing response to NO2. It has been reported that constructing homogenous semiconductor hybrids, e.g., ZnO@SnO2 [3], ZnO@TiO2 [4], TiO2@WO3 [5], can effectively suppress the recombination of electron-hole pairs by transferring the electrons and holes in the heterojunction areas, and thus improving the gas sensing characteristics. Moreover, the synergistic effects between the binary components can also enhance the gas sensing properties. Currently, most wide bandgap semiconductors operate at elevated temperature (>200oC) so as to obtain a rapid response and recovery speed [6]. Nevertheless, working at high temperature may induce many problems, especially in presence of flammable atmosphere which is likely to trigger an explosion [7]. In addition, the high-temperature gas sensors need special heat elements made of Pt or other noble metals and substrates that can sustain the elevated temperatures for a long time, which increases the fabrication cost and design complexity. Therefore, developing room-temperature gas sensors is urgent and important in the field of gas sensors [7]. In order to obtain an acceptable room-temperature gas sensing performance comparable to a counterpart at high temperature, other approaches need to be adopted. Visible light irradiation is a feasible method to accelerate surface sensing kinetics [8]. However, it is not appropriate for homogenous semiconductor hybrids, especially for n-type semiconductors, due to their relatively wide bandgaps. For example, in the case of ZnO@SnO2 hybrids, the bandgaps of ZnO and SnO2 are 3.37 and 3.5 eV, respectively and only ultraviolet light is able to excite the electrons. So less gas molecules species (O2-, NO2-) are chemisorbed on the hybrid surface upon exposure to NO2 gas because they cannot capture free electrons from the material surface. It is expected that stoichiometric ZnO@SnO2 hybrids would have no or weak response to NO2 without heating. Our previous works show that incorporating large amounts of donor defects, e.g., oxygen vacancies for n-type semiconductors, is an interesting method to reduce their bandgaps, and thus enables the sub-stoichiometric metal oxides to respond under visible light [9]. Several strategies have been reported to prepare deficient metal oxides. Putting the metal oxide materials into reducing atmosphere at high temperature can produce a large amount of donor defects. For example, black TiO2 containing highly concentrated oxygen vacancies can be prepared by hydrogenation process, and its light absorption range is extended from ultraviolet (UV) area to the whole visible light region [10]. Immersing wide bandgap semiconductors into hydrogen peroxide (H2O2) solution at 80oC and then annealed at 400oC in a furnace is another method to get donor-deficient oxide semiconductors [11]. The obtained deficient materials have a broader visible light absorption range. Moreover, a process that first is subjected to high-temperature heating and then cooled down rapidly also can produce many donor defects. G. Ou and co-authors adopted arc-melting strategy to fabricate diverse metal oxide semiconductors rich in oxygen vacancies, and their bandgaps were significantly reduced [12]. The heating temperature and cooling rate of arc-melting process were ca. 3000oC and 103 K/s, respectively. Solution precursor plasma spray (SPPS) is a relatively novel nanostructured coating deposition technique being used to prepare a variety of materials, e.g., metal, ceramic, composite [13]. Similar to atmospheric plasma spray (APS), SPPS is a process with ultra-high heating temperature (>10000oC) and cooling speed (>105 K/s) [14]. Extremely fast heating and cooling are beneficial to retain the produced metastable phases and large amounts of donor defects in the materials, especially for metal oxide semiconductors. Furthermore, SPPS technique utilized argon (Ar) and hydrogen (H2) as working gases, so a strong reducing atmosphere would be generated during the
spray process [15]. Therefore, when the solution precursor is injected into the plasma plume, coatings with rich defects are expected. Unlike typical APS using powder as feedstock, aqueous or organic solution is used as the feedstock in SPPS. Moreover, the solution precursor in plasma flame should undergo processes including evaporation, precipitation, pyrolysis and melting. Partial plasma energy is used to evaporate the solution. As a result, less energy is applied on the formed particles and a coating with smaller particle size would be obtained. In this study, black nanocomposite coatings were prepared by SPPS in presence of hydrogen in order to get rich donor defects. ZnO, SnO2, and SnO phases were present in the black coatings while donor defects were confirmed by the characterization results. Moreover, the coatings were porous and nanostructured and many n-n heterojunctions could be observed. A large amount of unpaired electrons were produced arising from the excitation of donor defects and the materials presented a strong absorption in the whole visible spectrum. In these conditions, interactions with NO2 were favoured and the SPPS material used as gas sensing layer should exhibited a good response under illumination with visible light. 2. Experimental method 2.1 Coating preparation The mixed precursor solution was prepared by adding 1.75 g stannic chloride (SnCl4·5H2O) and 14.87 g zinc nitrate (Zn(NO3)2·6H2O) (Sinopharm Chemical Reagent, China) into 250 mL deionized water according to the molar ratio Sn/Zn = 1:10. The prepared solution was stirred in a high-speed homogenizer (WiseTis@HG-15D, Korea) for 10 min until a uniformly dispersed solution was obtained. Commercial sensor substrates (C-MAC Micro Technology Company, Belgium) made of alumina (Al2O3) equipped with a pair of interdigitated gold electrodes were adopted in this study, as displayed in the insert picture of Figure 1. Prior to spraying, an ultrasonic cleaning machine was utilized to wash the substrates in an anhydrous ethanol for 5 min. The prepared mixture solution was then injected to the plasma flame through a peristaltic pump. A six-axis robotic arm (ABB, Sweden) was used to hold the plasma torch (Oerlikon Metco, Switzerland) to deposit uniform coatings. The schematic set-up of SPPS was shown in Figure 1, and the spray parameters were listed in Table 1. 2.2 Coating characterization X-ray diffractometer (XRD, D8 Advance, Bruker, Germany) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, USA) were used to characterize the phase constitutions and chemical structure of the SPPS coatings. The detection of donor defects and electronic properties was performed using XPS, photoluminescence spectroscopy (PL, Renishaw, England) and Raman spectroscopy (In via, Renishaw, England). The laser wavelengths used in PL and Raman were 325 and 532 nm. The optical properties of electron state of the coatings were identified by ultraviolet-visible spectrophotometer (UV-Vis, Cary5000, Varian, USA) and electron paramagnetic resonance (EPR, A300-10/12, Bruker, Germany). Field-emission scanning electron microscopy (FE-SEM, S4800Ⅱ, Hitachi, Japan) and high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F30 S-TWIN, FEI, USA) were utilized to examine the microstructure of the SPPS coatings. 2.3 Gas sensing test A Teflon-sealed stainless steel box was used as a home-made gas sensing test chamber and 0.15 W/cm2 LED lamps with different light wavelengths were set on the top of the chamber. Five color
lights, i.e., red, yellow, green, blue and purple, were used to investigate the effect of light wavelength on the sensing properties. Additionally, in order to compare the sensing characteristics with or without visible light illumination, the gas sensing test was also performed in the dark. The obtained gas sensor based on the SPPS coatings was placed just below the LED lamp providing external photon energy to accelerate the reaction rates. Prior to test, 500 mL/min reference air was injected into the chamber to stabilize the base resistance. And then 50 mL/min 10 ppm NO2 was injected into the chamber for 30 min to get the desired concentration of 1.0 ppm. All the gas flow rates in this work were controlled by mass flowrate controller (MFC, Bronkhorst High-Tech, Netherlands). The detailed gas sensing test process has been reported in [13-15]. The sensor response of oxidizing gases was defined as S=(Roxidizing gas-Rair)/Rair, where Roxidizing gas and Rair were the resistance in presence of oxidizing gases and synthetic air, respectively. The sensor response of reducing gases represented S=(Rair-Rreducing gas)/Rreducing gas, in which Rreducing gas and Rair were the resistance when exposed to reducing gases and synthetic air, respectively. 3. Results and discussion The XRD diffraction pattern of the SPPS coatings was displayed in Figure 2. The characteristic peaks were clearly observed, which were attributed to hexagonal ZnO with dominant planes of (100), (002) and (101) located at 31.7, 34.4 and 36.2o. The result was well in accordance with the PDF 36-1451. The peaks suited at 26.6 and 33.8o belonging to SnO2 match well with PDF 70-4177. In addition, the peaks centered at 29.9 and 37.3o attributed to SnO were also observed, which was in good agreement with PDF 85-0712. From these results, it can be concluded that ZnO, SnO2 and SnO phases were present in the obtained coatings. It can also be observed that the peak intensity of ZnO was much stronger, compared with the SnO2 and SnO peak intensity, meaning that ZnO was the dominant phase in the SPPS coatings. Due to thin thickness of the SPPS coatings, there were strong Al2O3 peaks (PDF 46-1212) emitted from the alumina sensor substrates. A similar phenomenon has been observed in [13-15]. From the insert picture in Figure 1, it can be seen that the coatings exhibited a dark color. It is well known that the stoichiometric ZnO, SnO and SnO2 are in white. Similar phenomenon was also found in black TiO2 produced by hydrogenation [10], and the authors attributed the black TiO2 to the formation of large amounts of defects. Therefore, some changes in phase constitutions must be present in the SPPS coatings. To provide more insights of the phase structures of the SPPS coatings, several characterizations were performed. Raman spectra exhibited many peaks in the range of 150-1000 cm-1, as depicted in Figure 3. The signals at 330, 384 and 437 cm-1 assigned to the 2-E2(M), A1(TO) and E2(h) vibrational photon mode of wurtzite ZnO were obviously observed [16]. In addition, a typical A1g vibration mode located at 218.6 cm-1 corresponded to SnO while a broad peak ranging from 628 to 774 cm-1 was related to the A1g and B2g modes of SnO2 [17]. It was interesting to find that an additional peak suited at 585 cm-1 was found in the spectrum, which mainly resulted from the presence of donor defects [18]. The peak intensity of 585 cm-1 was comparable to the main ZnO peak, implying that the concentration in donor defects introduced into the SPPS coatings was quite large. Apart from Raman, PL spectroscopy was another technique to analyze the defects. From Figure 4, three peaks centered at ca. 340, 380 and 400-750 nm can be observed. The 340 nm signal was assigned to the intrinsic peak of the mixture of SnO2 and SnO, while the 380 one was the intrinsic peak of ZnO, indicating
the presence of ZnO, SnO2 and SnO. The wide peak located at the visible light region in the range of 400-750 nm was attributed to the donor defects of zinc interstitials and oxygen vacancies [19]. The peak intensity of donor defects was higher than those of intrinsic peaks, revealing a big concentration of donor defects in the coatings. In order to further confirm the production of defects, the chemical compositions and oxidation states of the SPPS coatings on the outmost surface were analyzed in detail by XPS. Figure 5(a) illustrated the XPS survey of the coatings, where all the expected elements were probed comprising the C 1s, Zn2p, Sn 3d and O 1s. In this work, all the core levels were calibrated by C 1s at 284.8 eV. Zn 2p exhibited a doublet pair suited at 1021.3 and 1044.5 eV, as depicted in Figure 5(b). The peak suited at ca. 1021.3 eV was stemmed from the binding energy of Zn 2p3/2, and the 1044.5 eV peak was derived from the binding energy of Zn 2p1/2, hinting the presence of ZnO [20]. To further determine the chemical state of Zn element, the ZnLMM spectrum was also probed. Figure 5(c) showed the ZnLMM signal and it was deconvoluted into two peaks, including the main and minor peak at 988.1 and 992.1 eV. The main peak was assigned to Zn2+, and the minor one was originated from metallic zinc (Zn0) [21]. The intensity of Zn2+ was much stronger than that of Zn0, suggesting that Zn2+ was the dominant phase and zinc interstitials were present in the SPPS coatings. The Zn 2p and ZnLMM spectra revealed that ZnO was present in the SPPS coatings, while it was not a stoichiometric form. The Sn 3d spectrum was similarly deconvoluted into two peaks (Figure 5(d)): a low binding energy peak located at 487.3 eV was associated with Sn2+, whereas a high binding energy peak lied at 486.3 eV was arose from Sn4+, indicating the presence of SnO2 and SnO [18]. Moreover, the peak intensity of Sn4+ was much higher than that of Sn2+, which showed that Sn element mainly existed in the form of SnO2. Since ZnO, SnO2 and SnO were oxides, the investigation of O1s core level was very necessary. The O1s core level consisted of three contributions located at ca. 530.1, 531.4 and 532.7 eV, as displayed in Figure 5(e). The contribution of 530.1 eV was derived from the lattice oxygen in ordered metal oxides, while the 531.4 eV one was related to the oxygen-defective regions of metal oxides [22]. The 532.7 eV was attributed to the hydroxyl groups (OH-) from the adsorbed water molecules or the C=O bonds from the carbon oxides in the air. The strong peak intensity of 531.4 eV showed that there were large numbers of oxygen vacancies in the SPPS coatings. From the XPS characterization, it can be concluded that oxygen-defective ZnO1-β, SnO1-α and SnO2-γ phases were all present in the coatings. In addition, highly concentrated donor defects were demonstrated to be incorporated into the coatings, such as zinc interstitials and oxygen vacancies. Combined with the XRD, Raman, PL and XPS measurements, it can be concluded that black SnO1-α@ZnO1-β@SnO2-γ hybrids have been successfully synthesized. In a typical SPPS process, H2 was used as a second plasma forming gas to provide enthalpy and increase thermal conductivity of plasma flame [13-15]. When the mixture solution precursor of Zn(NO3)2 and SnCl4 was injected into the plasma plume with a certain kinetics provided by a peristaltic pump, the mixture droplets were firstly submitted to evaporation and precipitation. And then the precipitated particles would be subjected to pyrolysis to produce ZnO and SnO2 according to the reactions below: Zn(NO3)2 + H2O → ZnO + 2HNO3 (1) SnCl4 + 2H2O → SnO2 + 4HCl (2) In addition, some SnO2 were reduced to SnO followed the equations by: SnO2 + H2 → SnO + H2O (3)
An extremely high reaction temperature and active radicals provided by ionization greatly increase the driving force of the chemical reaction and particle growth. The dwelling time of the particles in the plasma flame was in the order of milliseconds, so the formed particles still would be in nanoscale. Furthermore, the particles tended to produce a large number of defects in the rapid nucleation on account of the transient retention in the high-temperature region and the reducing atmosphere [12]. ZnO + βH2 → ZnO1-β + βH2O (4) SnO + αH2 → SnO1-α + αH2O (5) SnO2 + γH2 → SnO2-γ + γH2O (6) By quenching to freeze, the particles can be obtained at different stages. Therefore, surface defects inside the particles can be kept and regulation, ensuring a uniform distribution of defects and an effective control of extrinsic defects. Features including transient retention in the high-temperature region and rapid cooling, can fully avoid the difference of the matrix precipitation. Since visible light will be used as the activation source in this work, the optical absorption properties of the SPPS coatings should be detected. In order to study the light absorption of the SnO1-α@ZnO1-β@SnO2-γ coatings, the optical absorption of pristine ZnO and SnO2 samples was also measured. Pristine SnO had a similar bandgap and light absorption to SnO2. Therefore, the light absorption of SnO was not measured herein. The pristine ZnO and SnO2 samples were prepared by annealing the commercial powder at 1000oC in a furnace for 4h to get stoichiometric samples. Figure 6(a) exhibited the UV-Vis absorption spectra of the coatings, pristine ZnO and SnO2 samples. Pristine ZnO sample only absorbed light below 380 nm, and pure SnO2 sample had an absorption edge below 340nm. However, the absorption edge of the SPPS coatings was red-shifted to ca. 600 nm, located at the yellow light region. The result revealed that the light absorption of the SPPS coatings in the visible light region was significantly enhanced compared with the bare ZnO and SnO2 samples. As well known, the ability of light absorption is directly associated with the bandgaps of materials [8-9,20], therefore, it is imperative to investigate the bandgaps of three samples. The bandgaps were estimated from the graph of (αhν)2 versus hν (Tauc plot) in Figure 6(b) according to the formula [23]: (αhν)2 = k (hν- Eg) (7) where α was the absorption coefficient, hν was the energy of incident light and k was a constant. The bandgaps of the pristine SnO2, ZnO and SPPS coatings were estimated to be 3.47, 3.22 and 2.51 eV, suggesting that the bandgap of the coatings was greatly narrowed down. The reduction of bandgaps of the SPPS coatings was mainly attributed to the presence of rich donor defects. Donor defects will produce additional deep donor levels between the valence and conduction band, giving rise to a decrease in bandgaps [24]. In addition to optical properties, electrical properties were also very important to the gas sensing performance, because the sensing process was dependent on the variation in electron concentration in the conduction band through the electron transfer between the materials and target gas molecules. EPR technique is used to detect the unpaired electrons. Figure 7 depicted the EPR spectrum of the SPPS SnO1-α@ZnO1-β@SnO2-γ coatings, where a strong peak corresponding to lange factor (g-factor) equal to 1.9610 was observed. It indicated that there were abundant unpaired electrons in SPPS coatings, being beneficial to the chemisorption of testing gases. According to [25], the 1.9610 signal was originated from the presence of oxygen
vacancies (Vox ) and zinc interstitials (Znxi ), which significantly increased the electron concentration in the detected materials. In SPPS coatings, a large number of electrons in the conduction band resulted from two aspects: the presence of donor defects, and the formation of homogenous heterojunctions between SnO1-α, ZnO1-β and SnO2-γ. Vox and Znxi were thermodynamically unstable and easily ionized into single and double electropositive oxygen vacancies (Vo• ,Vo•• ) and zinc interstitials (Zn•i ,Zn•• i ) by [26-27]: x • x • Vo (Zni ) ⇌ Vo (Zni ) + e- ⇌ Vo•• (Zn•• (8) i )+e Then the excited electrons were diverted from the donor levels to the conduction band of metal oxides. As a consequence of the difference in Fermi levels among ZnO, SnO and SnO2, the produced electrons would be transferred from the high-energy levels to low-energy levels. Simultaneously, the generated holes were moved in the opposite direction to electrons, which resulted in the effective separation of electron-hole pairs [28]. Then an energy bandgap bending would occur at the interface of the hybrids, forming a potential barrier at the heterojunctions. In the case of the gas sensors, the surface microstructure played an important role in the final gas sensing properties [7]. The sensor sensitivity is highly determined by the particle size in the sensitive materials [7]. When the particle is comparable to twice Debye length of semiconductors, the particles would be partially or completely depleted. Then the surface adsorption and desorption process would noticeably change the electrical properties, leading to a high sensitivity. As the particle size increases, the surface sensing behavior cannot affect the sensitivity very much. Moreover, a sensing material with high porosity was beneficial for gas diffusion, giving rise to a quicker response. Thus FE-SEM and HRTEM characterization were conducted. Figure 8 illustrates the FE-SEM images of the SPPS coatings with different magnifications. The low magnification image indicated that the coatings were highly porous. Besides, the high magnification revealed that the coatings were nanostructured, and its particle size was in the range of 15-40 nm. The Debye length of metal oxides was reported to be typically in a few nanometers [29]. Therefore, it can be expected that the SPPS coatings may exhibit nanostructure with a size comparable to twice Debye length, being favorable for obtaining a high sensitivity. To get an overview of the detailed structure of the SPPS coatings, some images were acquired by HRTEM. Figure 9 depicts the HRTEM images of the SnO1-α@ZnO1-β@SnO2-γ coatings with different magnifications. As can be seen in Figure 9(a), the SPPS coatings were composed of many blended nanoparticles and the nanoparticles were interconnected with each other. The particle size was in the range of 15-40 nm, being compatible with the FE-SEM results. The fringe spacing in Figure 9(c) was measured to be 0.26, 0.28, 0.29 and 0.34 nm, respectively. 0.26 and 0.28 nm spacing was attributed to the (002) and (100) planes of ZnO [30]. 0.34 nm spacing corresponded to the interplanar spacing of SnO2 (110) plane [31], and that of 0.29 nm was assigned to the (101) plane of SnO [32]. It can be deduced that many n-n heterojunctions between SnO1-α, ZnO1-β and SnO2-γ were formed in the SPPS coatings. Figure 10 shows the plot of the sensor response of the black SnO1-α@ZnO1-β@SnO2-γ coatings as a function of 1.0 ppm performed in the dark and visible light illumination. The relevant values were listed in Table 2. The SPPS coatings had a weak response in the dark, whose sensor response was only 0.35. Furthermore, its response time was very long, more than 30 min. The desorption process was almost negligible, indicating that the NO2 species cannot be desorbed from the coating surface in the dark. With respect to the dark condition, the NO2 sensing performance illuminated by visible light including the sensor response, response and recovery time, was
considerably enhanced. In addition, it also can be observed that the light wavelength also had a significant effect on the sensor properties. The response time and recovery rates increased with the light wavelength decrease (from red light to blue light) except for purple light. As a result of the high photon energy of purple light, some additional reactions between the sensing materials and testing gases occurred, prolonging the response and recovery time [33]. The effect of light wavelength on the sensor response was complex for the SPPS coatings. The sensor response of red light was still very little (1.11), whereas it greatly increased when other color lights were used. It may be ascribed to the fact that the light absorption range of SPPS coatings was only extended to ca. 600 nm, not including the red light. Consequently, when the red light was used as an incident light resource, its photon energy was so low that it cannot excite the electrons in the sensitive materials, resulting in a weak response and slow response process. Except the red light, the sensor response decreased when the light wavelength was lowered. The photon energy of the incident light was inversely proportional to the light wavelength [8-9]. Our previous work demonstrated that the role of photon energy on the sensing properties was similar to that of heating [33]. A higher photon energy can obviously enhance the surface desorption rate, less target gas molecules were adsorbed on the surface, leading to a reduction of sensor response. In general, the blue light was the best choice for practical applications because it had the best response and recovery rate while possessing an acceptable sensor response. The electrical resistance response of the black SnO1-α@ZnO1-β@SnO2-γ coatings submitted to NO2 with concentrations ranging from 1 to 10 ppm with blue light stimulation at room temperature was displayed in Figure 11(a). It can be seen in Figure 11(b) that the sensor response of the SPPS coatings was always increased as the sensor was exposed to increased NO2 concentration, moreover, the sensor response varied linearly with NO2 concentration. The results indicated that the SnO1-α@ZnO1-β@SnO2-γ sensors possessed an excellent detectability to different concentrations of NO2. Figure 12 depicts the stability test results of the presented sensors towars 1.0 ppm NO2 under blue light illumination at room temperature during 15 days. It can be found that the value of sensor response was always around 2.36, exhibiting a good stability performance. Figure 13 illustrates the selectivity of the SPPS SnO1-α@ZnO1-β@SnO2-γ coatings to 1.0 ppm NO, 15 ppm HCHO, 100 ppm SO2, 100 ppm H2, 100 ppm NH3 and 100 ppm CO. All these experiments were performed under blue light irradiation at room temperature. From that, the prepared SnO1-α@ZnO1-β@SnO2-γ sensors exhibited a higher response towards NO2 when compared with those to other gases. Therefore, it can be concluded that the SPPS SnO1-α@ZnO1-β@SnO2-γ sensor possessed a good selectivity to NO2. Stoichiometric ZnO, SnO2 and SnO were all wide bandgap semiconductors (>3.3 eV). Under normal circumstances, and they had no electrons in their conduction band even if visible light sources were employed to irradiate them. Therefore, the chemisorption of gas molecules cannot be reacted on their surface, leading to no or weak response to NO2 gas at room temperature. In this work, SPPS approach was adopted to deposit black SnO1-α@ZnO1-β@SnO2-γ coatings with large numbers of donor defects. Donor defects can be ionized under visible light irradiation to form light-generated electron-hole pairs, as verified by the EPR results. On account of the presence of n-n heterojunctions, the photogenerated electrons will be transferred from high-energy to low-energy levels due to the Fermi-level mediated charge transfer effect, suppressing the recombination of electron-hole pairs. Effective charge separation greatly increased the lifetime of photogenerated charge carriers, and more electrons can be participated
in the reaction with the testing gases, leading to a higher response. The electron excitation and transfer process were exhibited in Figure 14(a). The deep donor levels formed by donor defects were abbreviated as Ed in the schematic. Donor defects, i.e., zinc interstitials and oxygen vacancies, produced in the black coatings were preferential sites for oxidizing gases [34]. Therefore, more surface active adsorption sites existed in the defective coatings with respect to those of the stoichiometric ones, forming a large surface coverage of chemisorbed oxygen species. Furthermore, the adsorption energy of oxidizing gas on the donor-defective metal oxides was much smaller than that on stoichiometric counterparts [35]. When the coatings were exposed in reference air, oxygen molecules would be facilely chemisorbed on the material surface, especially on the defect sites, to form O2- species. Vox + O2 ⇌ Vo• + O2(9) x • Zni + O2 ⇌ Zni + O2 (10) It would cause the formation of a depletion layer at the heterojunction area, leading to a high potential barrier and electrical resistance. As the reaction time going, the adsorption and desorption kinetics of O2- species reached an equilibrium state, exhibiting a stable electrical resistance. When NO2 gas was injected, because the electron affinity of NO2 is greater than O2 [13-15,36-37], NO2 molecules can capture electrons from the adsorbed O2- species. O2- + NO2 ⇌ NO2- +O2 (11) x • Vo + NO2 ⇌ Vo + NO2 (12) Znxi + NO2 ⇌ Zn•i + NO2(13) The surface acceptor levels formed by NO2- species was deeper than that of O2- [8-9,33], which resulted in a wider surface depletion layer and a higher potential barrier at the heterojunction areas. It greatly increased the electrical resistance of the coatings. Owing to chemisorbed oxygen species and rich donor defects presented on the SPPS coatings, a large number of NO2 molecules would be adsorbed on the coating surface, exhibiting excellent NO2 sensing properties. The gas sensing process of SPPS coatings was illustrated in Figure 14(b). 4. Conclusions The solution precursor plasma spray process was used to prepare black SnO1-α@ZnO1-β@SnO2-γ coatings. The nonstoichiometry of the SPPS coatings was investigated by various spectroscopic techniques, including Raman, PL and XPS spectra. The results indicated that large amounts of donor defects (zinc interstitial and oxygen vacancy) were found in the coatings as a result of their process features, i.e., ultra-high temperature heating, rapid cooling, and reducing atmosphere. XRD analysis revealed that the deficient SnO1-α@ZnO1-β@SnO2-γ coatings were successfully prepared without changing their crystal structure. UV-Vis spectra showed that a broader light absorption range of the SPPS coatings was obtained, extending to the yellow light, on account of the introduction of highly concentrated donor defects. A large number of unpaired electrons existed in the coatings, as verified by the EPR characterization. FE-SEM and HRTEM images exhibited a highly porous nanostructure, and n-n heterojunctions were formed between SnO1-α, ZnO1-β and SnO2-γ. The gas sensing test results indicated that visible light illumination can greatly enhance the gas sensing performance and the blue light illumination endowed the SPPS coatings possessing the best NO2 sensing properties in terms of the sensor response and response and recovery rates. The composite also shows a good selectivity for NO2 towards other possible interfering gases.
Acknowledgements This work is supported by the Jiangsu Natural Science Foundation of China under Grant No. BK20140487, the Natural Science Foundation of China under Grant No. 51402255, State Key Laboratory for Mechanical Behavior of Materials under Grant No. 20161808 and the Funding of Jiangsu Innovation Program for Graduate Education under Grant No. KYZZ16_0490. The authors thank Ms. Pascale Hoog of University of Technology of Belfort Montbéliard for her English help on this paper. Marc Debliquy also thanks the Waloon Region for its support in the framework of the Captindoor project. References [1] C. Zhang, M. Debliquy, A. Boudiba, H. Liao, C. Coddet. Sensing properties of atmospheric plasma-sprayed WO3 coating for sub-ppm NO2 detection. Sensors and Actuators B: Chemical, 144 (2010) 280-288. [2] M.E. Franke, T.J. Koplin, U. Simon. Metal and metal oxide nanoparticles in chemiresistors: Does the Nanoscale Matter? Small, 2 (2006) 36-50. [3] W. Guo, Z. Wang. Composite of ZnO spheres and functionalized SnO2 nanofibers with an enhanced ethanol gas sensing properties. Materials Letters, 169 (2016) 246-249. [4] C.W. Zou, J. Wang, W. Xie. Synthesis and enhanced NO2 gas sensing properties of ZnO nanorods/TiO2 nanoparticles heterojunction composites. Journal of Colloid and Interface Science, 478 (2016) 22-28. [5] W. Meng, L. Dai, J. Zhu, Y. Li, W. Meng, H. Zhou, L. Wang. A novel mixed potential NH3 sensor based on TiO2@WO3 core–shell composite sensing electrode. Electrochimica Acta, 193 (2016) 302-310. [6] C. Zhang, J. Wang, X. Geng. Tungsten oxide coatings deposited by plasma spray using powder and solution precursor for detection of nitrogen dioxide gas. Journal of Alloys and Compounds, 668 (2016) 128-136. [7] J. Zhang, X. Liu, G. Neri, N. Pinna. Nanostructured materials for room temperature gas sensors. Advanced materials, 28 (2016) 795-831. [8] X. Geng, C. Zhang, M. Debliquy. Cadmium sulfide activated zinc oxide coatings deposited by liquid plasma spray for room temperature nitrogen dioxide detection under visible light illumination. Ceramics International, 42 (2016) 4845-4852. [9] X. Geng, J. You, C. Zhang. 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) 286-293. [10] X. Chen, L. Liu, P.Y. Yu, S.S. Mao. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science, 331 (2011) 746-750. [11] J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang, Y. Dai. Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO. ACS Applied Material & Interfaces, 4 (2012) 4024-4030. [12] G. Ou, D. Li, W. Pan, Q. Zhang, B. Xu, L. Gu, C. Nan, H. Wu. Arc- Melting to Narrow the Bandgap of Oxide Semiconductors. Advanced Materials, 27 (2015) 2589–2594. [13] C. Zhang, X. Geng, M. Olivier, H. Liao, M. Debliquy. Solution precursor plasma-sprayed tungsten oxide coatings for nitrogen dioxide detection. Ceramics International, 40 (2014)
11427-11431. [14] C. Zhang, M. Debliquy, H. Liao. Deposition and microstructure characterization of atmospheric plasma-sprayed ZnO coatings for NO2 detection. Applied Surface Science, 256 (2010) 5905-5910. [15] C. Zhang, X. Geng, H. Li, P. He, M. Planche, H. Liao, M. Olivier, M. Debliquy. Microstructure and gas sensing properties of solution precursor plasma-sprayed zinc oxide coatings. Materials Research Bulletin, 63 (2015) 67-71. [16] E. Manikandan, G. Kavitha, J. Kennedy.Epitaxial zinc oxide, graphene oxide composite thin-films by laser technique for micro-Raman and enhanced field emission study. Ceramics International, 40 (2014) 16065-16070. [17] N. Salah, W.M. AL-Shawafi, S.S. Habib, A. Azam, A. Alshahrie. Growth-controlled from SnO2 nanoparticles to SnO nanosheets with tunable properties. Materials & Design, 103 (2016) 339-347. [18] A. Shanmugasundaram, P. Basak, L. Satyanarayana, S.V. Manorama. Hierarchical SnO/SnO2 nanocomposites: Formation of in situ p–n junctions and enhanced H2 sensing. Sensors and Actuators B: Chemical, 185 (2013) 265-273. [19] D.E. Motaung, P.R. Makgwane, S.S. Ray. Induced ferromagnetic and gas sensing properties in ZnO-nanostructures by altering defect concentration of oxygen and zinc vacancies. Materials Letters, 139 (2015) 475-479. [20] C. Zhang, J. Wang, M. Olivier, M. Debliquy. Room temperature nitrogen dioxide sensors based on N719-dye sensitized amorphous zinc oxide sensors performed under visible-light illumination. Sensors and Actuators B: Chemical, 209 (2015) 69-77. [21] J. Winiarski, W. Tylus, B. Szczygieł. EIS and XPS investigations on the corrosion mechanism of ternary Zn–Co–Mo alloy coatings in NaCl solution. Applied surface science, 364 (2016) 455-466. [22] S. Kabcum, D. Channei, A. Tuantranont, A. Wisitsoraat, C. Liewhiran, S. Phanichphant. Ultra-responsive hydrogen gas sensors based on PdO nanoparticle-decorated WO3 nanorods synthesized by precipitation and impregnation methods. Sensors and Actuators B: Chemical, 226 (2016) 76-89. [23] M.H. Habibi, M.H. Rahmati. Fabrication and characterization of ZnO@CdS core-shell nanostructure using acetate precursors: XRD, FESEM, DRS, FTIR studies and effects of cadmium ion concentration on band gap. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 133 (2014) 13-18. [24] W. Kim, M. Choi, K. Yong. Generation of oxygen vacancies in ZnO nanorods/films and their effects on gas sensing properties. Sensors and Actuators B: Chemical, 209 (2015) 989-996. [25] W. Xia, C. Mei, X. Zeng, G. Fan, J. Lu, X. Meng, X. Shen. Nanoplate-Built ZnO Hollow Microspheres Decorated with Gold Nanoparticles and Their Enhanced Photocatalytic and Gas-Sensing Properties. ACS Applied Material & Interfaces, 7 (2015) 11824-11832. [26] N. Han, L. Chai, Q. Wang, Y. Tian, P. Deng, Y. Chen. Evaluating the doping effect of Fe, Ti and Sn on gas sensing property of ZnO. Sensors and Actuators B: Chemical, 147 (2010) 525-530. [27] N. Han, X. Wu, L. Chai, H. Liu, Y. Chen. Counterintuitive sensing mechanism of ZnO nanoparticle based gas sensors. Sensors and Actuators B: Chemical, 150 (2010) 230-238. [28] G. Zhou, X. Xu, T. Ding, B. Feng, Z. Bao, J. Hu. Well−steered charge−carrier transfer in 3D branched CuxO/ZnO@Au heterostructures for efficient photocatalytic hydrogen evolution. ACS Applied Material & Interfaces, 7 (2015) 26819-26827.
[29] Y. Li, T. Liu, H. Zhang. New insight into gas sensing performance of nanoneedle-assembled and nanosheet-assembled hierarchical SnO2 structures. Materials Letters, 176 (2016) 9-12. [30] V.V. Ganbavle, S.I. Inamdar, G.L. Agawane, J.H. Kim, K.Y. Rajpure. Synthesis of fast response, highly sensitive and selective Ni: ZnO based NO2 sensor. Chemical Engineering Journal, 286 (2016) 36-47. [31] L.F. da Silva, J.C. M’Peko, A.C. Catto, S. Bernardini, V.R. Mastelaro, K. Aguir, C. Ribeiro, E. Longo. UV-enhanced ozone gas sensing response of ZnO-SnO2 heterojunctions at room temperature. Sensors and Actuators B: Chemical, 240 (2017) 573-579. [32] Y. Li, W. Zhou, Y. Yang, P. Wu. Fabrication, characterization and physical properties of Co-doped SnO particles via hydrothermal method. Journal of Alloys and Compounds, 685 (2016) 448-453. [33] C. Zhang, A. Boudiba, P. Marco, R. Snyders, M. Olivier, M. Debliquy. Room temperature responses of visible-light illuminated WO3 sensors to NO2 in sub-ppm range. Sensors and Actuators B: Chemical, 181 (2013) 395-401. [34] M. Chen, Z. Wang, D. Han, F. Gu, G. Guo.High-sensitivity NO2 gas sensors based on flower-like and tube-like ZnO nanomaterials. Sensors and Actuators B: Chemical, 157 (2011) 565-574. [35] Y. Qin, Z. Ye. DFT study on interaction of NO2 with the vacancy-defected WO3 nanowires for gas-sensing. Sensors and Actuators B: Chemical, 222 (2016) 499-507. [36] C. Zhang, X. Geng, H. Liao, C. Li, M. Debliquy. Room-temperature nitrogen-dioxide sensors based on ZnO1−x coatings deposited by solution precursor plasma spray. Sensors and Actuators B: Chemical, 242 (2017) 102-111. [37] X. Geng, C. Zhang, Y. Luo, M.Debliquy. Preparation and characterization of CuxO1-y@ZnO1-α nanocomposites for enhanced room-temperature NO2 sensing applications. Applied Surface Science, 401 (2017) 248-255.
Figure captions Figure 1. Schematic set-up for SPPS, insert: the photo of the sensor substrate and the sensor based on SPPS coatings. Figure 2. X-ray diffraction patterns of SPPS coatings. Figure 3. Raman spectra of SPPS coatings. Figure 4. PL spectrum of SPPS coatings. Figure 5. XPS spectra of SPPS coatings, (a) survey, (b) Zn2p, (c) ZnLMM, (d) Cu2p (e) O1s, the green lines represent the measured curve, while the lines in other colors are the fitted curves. Figure 6. (a) UV-Vis absorption spectra (b) Tauc plot of the ZnO, SnO2 samples and SPPS coatings. Figure 7. EPR spectrum of SPPS coatings. Figure 8. FE-SEM images of SPPS coatings with different magnifications. Figure 9. HRTEM images of SPPS coatings with different magnifications.
Figure 10. Electrical resistance of SPPS coatings plotted against time towards 1.0 ppm NO2 illuminated under visible light with different wavelengths at room temperature. Figure 11. The electrical resistance response of the SPPS coatings towards NO2 with concentrations ranging from 1 to 10 ppm with blue light stimulation at room temperature Figure 12. The stability test results of the SPPS coatings to 1.0 ppm NO2 under blue light illumination at room temperature. Figure 13. Selectivity of the SPPS coatings towards 1.0 ppm NO, 15 ppm HCHO, 100 ppm SO2, 100 ppm H2, 100 ppm NH3 and 100 ppm CO under blue light illumination at room temperature. Figure 14. Schematic of the (a) electron excited process (b) gas sensing process of SPPS coatings.
Table 1. Solution precursor plasma spraying (SPPS) parameters. Parameters
Value
Argon volume flow rate (L/min) Hydrogen volume flow rate (L/min) Spraying distance (mm) Arc current (A) Torch power (kW) Nozzle inner diameter (mm) Liquid flow rate (mL/min)
40 2 100 513 25 0.26 20
Table 2. Sensing characteristics of the sensor based on SPPS coatings to 1.0 ppm NO2 performed in the dark and under visible light illumination with different light wavelength. Wavelength (nm)
Light color In the dark
640 580 530 480 400
Red Yellow Green Blue Purple
Rair (Ω)
Sensor response
Response time
Recovery time
((RNO2-Rair)/Rair)
(min)
(min)
3.68×10
7
0.35
>30
>60
6.51×10
6
1.11
26.6
>60
2.70×10
6
4.57
23.4
58.9
1.21×10
6
3.35
21.8
37.6
6.47×10
5
2.36
17.6
23.4
2.73×10
5
2.09
20.3
28.8