Resistive Low-Temperature Sensor Based on the SiO2ZrO2 ... - MDPI

chemosensors Article

Resistive Low-Temperature Sensor Based on the SiO2ZrO2 Film for Detection of High Concentrations of NO2 Gas Tatiana N. Myasoedova 1, *, Tatiana S. Mikhailova 1 , Galina E. Yalovega 2 and Nina K. Plugotarenko 1 1 2

*

Institute of Nanotechnologies, Electronics and Equipment Engineering, Southern Federal University, Chekhov str. 2, 347928 Taganrog, Russia; [email protected] (T.S.M.); [email protected] (N.K.P.) Faculty of Physics, Southern Federal University, Zorge str. 5, 344090 Rostov-on-Don, Russia; [email protected] Correspondence: [email protected]; Tel.: +7-918-523-3488

Received: 14 November 2018; Accepted: 14 December 2018; Published: 19 December 2018







Abstract: The SiO2 ZrO2 composite films were prepared by means of sol-gel technology and characterized by scanning electron microscopy, energy dispersive X-ray (EDX) analysis, and X-ray diffraction. The presence of the stable monoclinic ZrO2 with an impurity of tetragonal phases is shown. The film surface is characterized by the presence of ZrOCl2 ·6H2 O or ZrCl(OH)/ZrCl(OH)2 grains. The crystallite size negligibly depends on the annealing temperature of the film and amount to 10–12 nm and 9–12 nm for the films thermally treated at 200 ◦ C and 500 ◦ C, respectively. The film’s resistance is rather sensitive to the presence of NO2 impurities in the air at a low operating temperature (25 ◦ C). Accelerated stability tests of the initial resistance showed high stability and reproducibility of the sensor based on the SiO2 ZrO2 film thermally treated at 500 ◦ C. Keywords: gas sensor; zirconia; composite film; nitrogen dioxide

1. Introduction As a low-cost selective oxidizer for the industry, nitrogen dioxide (or nitrogen tetroxide) is currently gaining recognition with a resultant increase of production in tank car quantities. The toxic level presence of this gas in any working area is an industrial hygiene problem, and its unwanted presence in any area may present an air-pollution problem. In the workplace, exposures to NO2 have been reported in such occupations as electroplating, acetylene welding, agriculture, space exploration, the detonation of explosives, certain military activities, and burning of nitrogen-containing propellants [1]. In such situations, exposure concentrations can be very high. For example, in armored vehicles during live-fire tests, peak concentrations of NO2 have been measured at over 2000 ppm. That decreases to about 500 ppm after one minute and decreases to about 20 ppm within five minutes [2]. So, the reliable sensors are in demand. At present, the well-known and popular sensors are the semiconducting sensors. Such advantages as high sensitivity, low cost, and simplicity of fabrication allow for the use of semiconducting oxides for gas sensor production. The significant disadvantages of semiconducting oxides are low selectivity and stability, mainly caused by the recrystallization processes and surface degradation during the exploitation. The way to improve the sensor’s characteristics is the synthesis of mixed oxide systems by doping the high dispersed matrix with different catalysts. Once formed, the mixed oxides display acidic sites due to the excess negative or positive charge or due to defects in the lattice that positively affect the catalytic properties. Therefore, research activities have turned towards the development of mixed oxide gas sensitive materials that can be stable in harsh environments and can be used for a long time. In this context, Chemosensors 2018, 6, 67; doi:10.3390/chemosensors6040067

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silica-supported zirconia oxide can be used because of its thermal and chemical stability and its higher resistance to degradation compared to the isolated oxides [3,4]. Silica is the basic material of microelectronics—it finds an application as a matrix for the future production of nanosized gas sensitive materials or for improving gas sensitivity and stability of sensors [5,6]. Also, silica application in mixed oxide systems minimizes the buildup of thermal stresses during sensor fabrication and, therefore, improves adhesion. Zirconia (ZrO2 ) is a material of great technological interest, having high thermal and chemical stability, low thermal conductivity, excellent corrosion resisting properties, as well as chemical and microbial resistance [7,8]. Also, ZrO2 is a wide band gap p-type semiconductor that exhibits abundant oxygen vacancies on its surface. So, zirconia-based electrochemical and semiconducting sensors are attracting much attention [9–11]. Zirconia-based sensors are mainly used as O2 and O3 sensors as they can rapidly respond to changes in the exhaust gas composition. A few works have been devoted to the elaboration of NOx zirconia-based sensors [12–15]. Micro-sensors of this type has been tested to monitor ozone (O3 ) in ambient air at low concentrations (10 ppb) since the end of the 1980s [12–14]. However, due to reliability problems, there is a hesitancy to apply these sensors for air pollution monitoring, especially in harsh environments in the industry. Almost all the zirconia-based sensors work at high operating temperatures (over 300 ◦ C) [16]. A line of research, which has attracted attention in the past few years, is the low-temperature sensitivity of metal–oxide gas sensors. Some researchers have shown that metal–oxides can also work at room temperature and slightly above [17–21]. Our previous studies showed the sensitivity of SiO2 ZrO2 composite films, fabricated using a Zr(ONO3 )2 precursor, to small concentrations of NO2 and O3 gases (1 ppm and 0.02 ppm, respectively) under low working temperatures, and showed the high stability of the initial resistance [22]. In this work, the studies on SiO2 ZrO2 thin films, fabricated using a ZrOCl2 ·8H2 O precursor, for low-temperature NO2 sensor applications are presented. 2. Materials and Methods The SiO2 ZrO2 composite films were synthesized by the sol-gel technique. ZrOCl2 ·8H2 O was preliminarily dissolved in distilled water at room temperature and then poured into an alcoholic solution of tetraethoxysilane (TEOS), prepared 24 h before. TEOS was dissolved in absolute isopropanol at a volume ratio of 1:7. The zirconium concentration in the TEOS-based solution was 1 wt.%. The final mixture was spin-coated onto the silicon substrate. Prior to the spin-coating procedure, a 2 µm-thick silicon dioxide insulating layer was first grown on a Si (111) substrate by wet oxidation. The samples prepared by the technique as a result of this procedure were first dried at 100 ◦ C for 2 h and then thermally treated at 500 ◦ C for 2 h in air. Film morphology was characterized by SEM (scanning electron microscopy) (microscope LEO 1560, ZEISS, Oberkochen, Germany) with an operating voltage of 5 kV. The chemical analysis of the films was investigated using an energy dispersive X-ray (EDX) method. All measurements were carried out using a complex analytic device based on the both scanning electron microscope VEGA II LMU with an operating voltage of 20 kV and a microanalysis system INCA ENERGY450/XT with a silicon drift detector (X-Act SDD) (Tescan/OXFORD Instruments Analytical, Great Britain, UK). EDX and wavelength-dispersive X-ray (WDX) detectors were used for greater accuracy in the determination of chlorine. The thickness of the film treated at 500 ◦ C estimated from SEM images is ~155 nm. X-ray diffraction (XRD) phase analysis was performed with an ARL X’TRA diffractometer equipped with a solid-state Si(Li) detector that registered selectively Cu Ka radiation (λ = 1.5418 A◦ ). Due to the very small thickness of the layer, the first conventional scan in the Bragg–Brentano mode only revealed the single-crystalline Si substrate in the (001) orientation. Then, all other scans were made in the grazing incidence mode, with incidence angles (ω) of 3◦ . The band gap energy was determined from the optical absorption spectra recorded using a commercial spectrophotometer LEKI SS1207 (Leki Instruments, Helsinki, Finland) and analyzed by

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the empirical relation: αhν ∞(hν-Eg)m, where h- is the photon energy, Eg is the band gap energy and m is an exponent determined by the nature of the electron transition during the absorption process, Chemosensors 2018, 6, 67 3 of 14 i.e., m = 1/22018, for6,direct transition and m = 2 for indirect transition. Chemosensors x 3 of 14 To fabricate the SiO2ZrO2 gas sensors, V/Cu/Cr layers were deposited via an interdigitated mm shadow maskrelation: onto theαhν SiO∞(hν-Eg) 2ZrO2/SiO /Si templates tophoton serve asenergy, the contact (Figure 1).and theempirical empirical relation: αhν ∞(hν-Eg) where hvisisthe the photon energy, Eg is the energy the , 2,where hiselectrodes the band bandgap gap energy andm nature ofof thethe electron transition during thethe absorption process, i.e., misisananexponent exponentdetermined determinedbybythe the nature electron transition during absorption process, m =m1/2 forfor direct transition and mm = 2= for indirect transition. i.e., = 1/2 direct transition and 2 for indirect transition. To fabricate fabricate the the SiO SiO22ZrO ZrO22gas gassensors, sensors, V/Cu/Cr V/Cu/Cr layerswere weredeposited depositedvia viaan aninterdigitated interdigitated To layers shadow mask onto the SiO ZrO /SiO /Si templates to serve as the contact electrodes (Figure shadow mask onto the SiO2ZrO 2/SiO 2/Si 2 2 2 templates to serve as the contact electrodes (Figure 1). 1).

Figure 1. Schematic diagram of the fabricated SiO2ZrO2-based sensor.

The measurement setup used for the measurement of gas sensor characteristics and electronic parameters in controlled environments is the shown in Figure 2. 2The setup consists of: (i) a test Figure 1.gas Schematic diagram of fabricated SiO2 ZrO -based sensor. chamber with theFigure substrate holder, heating element and Pt-thermocouple, and 1. Schematic diagram of the fabricated SiO2ZrO2-based sensor. (ii) control, supply, The measurement setup(voltmeter used for the measurement gas sensor characteristics and electronic and measurement electronics and temperature of controller). The operating temperature and parameters in controlled gasused environments isare shown inofFigure 2. The consists of: (i) a The test electronic control of thesetup measurement system automated means ofsetup a personal computer. The measurement for the measurement gasbysensor characteristics and electronic chamber the substrate holder, heating Pt-thermocouple, (ii) control, gas inlet iswith controlled by the block andiselement rotameters. parameters in controlled gasvalve environments shown and in Figure 2. The setupand consists of: (i) supply, a test and measurement electronics (voltmeter and temperature controller). operating temperature Thewith gas-sensitive properties of the films were to NOThe 2 inputs variedsupply, in and the chamber the substrate holder, heating element andtested Pt-thermocouple, and to (ii)be control, electronic control of the measurement are automated by means of aoperating personal computer. The gas concentration range of 40–1060 ppm. system The were heated fromThe room temperature to 200 °C to and measurement electronics (voltmeter andsensors temperature controller). temperature and inletthe is dependence controlled valve block and rotameters. get ofthe resistance on temperature the sensor operating electronic control ofbythe measurement system are and automated by means of atemperature. personal computer. The gas inlet is controlled by the valve block and rotameters. The gas-sensitive properties of the films were tested to NO2 inputs to be varied in the concentration range of 40–1060 ppm. The sensors were heated from room temperature to 200 °C to get the dependence of resistance on temperature and the sensor operating temperature.

Figure 2. Schematic Schematic diagram of the measurement setup. LED == light-emitting light-emitting diode. diode.

The gas-sensitive properties of the films were tested to NO2 inputs to be varied in the concentration range of 40–1060 ppm. The sensors were heated from room temperature to 200 ◦ C to get the dependence of resistanceFigure on temperature the sensor operating temperature. 2. Schematicand diagram of the measurement setup. LED = light-emitting diode.

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3. Results and Discussion 3.1. Characterization SEM micrographs (Figure 3a) reveal the formation of 1 µm-thick crystallites with a length range of 1–4 µm on the surface of the film. It seems appropriate to consider that the particles which appear in the SEM image have a rectangular shape and crystalline structure. The EDX study was carried out to clarify the chemical composition of different points of SiO2 ZrO2 films, namely: crystallites (Figure 3b, point 1) and the film surface itself (Figure 3b, point 2). Note that the minimum electron beam spot size achievable for the EDX analysis was approximately 500 nm, which allows for the accurate determination of the elemental composition of the crystallites. As can be seen, crystallites contain zirconium and chlorine (point 1), while on the surface of the films (point 2), there is a significant decrease in the amount of zirconium and the complete absence of chlorine. The relative content of elements in terms of 100% (semi-quantitative analysis) is presented in Table 1. The thickness of the films excludes quantitative analysis—the depth of generation of the analytical signal (~2 µm) is greater than the thickness of the film. Table 1. The relative content of elements in both points of SiO2 ZrO2 films. Units = wt.%.

1 2

O

Si

Cl

Zr

Total

41.67 47.67

10.70 47.13

0.87

46.76 5.19

100.00 100.00

The used synthesis technique leads to the formation of a complex, multiphase system in SiO2 ZrO2 films, which is clearly illustrated by the diffraction pattern (Figure 4). The X-ray diffraction patterns of the thermally treated samples at 500 ◦ C and 200 ◦ C have the same peaks and contain both low-intensity and high-intensity wide peaks that may indicate the presence of small size crystallites or non-stoichiometric phases in the samples. Lower intensity peaks at 200◦ can be explained by a smaller number and the size of the crystallites. A sufficiently high background, hindering the identification of weak peaks of the crystalline phase, indicates the presence of amorphous structures. Since the peaks are wide enough, it is reasonable to identify them as groups of reflections from different crystalline phases, since overlapping or similar peaks cannot be separated in this case. As shown in [23,24], the weakest peaks around 17◦ , 24.26◦ and 28.3◦ (Figure 4, insert) are due to the (110), (011), (−111) planes and can be attributed to the most intense reflections of a stable monoclinic ZrO2 with an impurity of tetragonal phases. The weak intensity of the peaks can be explained by the fact that the temperature of 500 ◦ C is the initial temperature of the formation of the zirconia crystal structure in the films [25]. A wide peak at 25◦ can include a reflex from the (011) plane of SiO2 . However, these phases do not explain the presence of high-intensity bright peaks around 14.22◦ , 17.0◦ and 18.6◦ on the diffractogram. As the literature analysis showed, a peak of 14.22◦ is a characteristic of ZrCl4 [26]. However, the presence of peaks at 17.0◦ and 18.6◦ , which are not characteristic of this compound, does not allow one to unequivocally approve the full decomposition of the precursor ZrOCl2 ·8H2 O into ZrO2 and ZrCl4 . The partial peaks at 14.22◦ , 17.0◦ and 18.6◦ are closest to an underdeveloped ZrOCl2 ·8H2 O with less water, for example, ZrOCl2 ·6H2 O or ZrCl(OH)/ZrCl(OH)2 as it was shown for CaCl(OH)/CaCl(OH)2 [27]. It is possible that the introduction of silicon prevents the complete decomposition of ZrOCl2 ·8H2 O and the removal of chlorine.

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Figure Figure 3. 3. Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) micrograph micrograph of of the the typical typical surface surface morphology morphology (a); (a); combined energy dispersive X-ray (EDX) and WDX spectra of crystallites (point 1), and EDX analysis combined energy dispersive X-ray (EDX) and WDX spectra of crystallites (point 1), and EDX analysis ◦ °C; SEM micrograph and of surface (point (point2)2)(b); (b);the the SiO 2ZrO2 films thermally treated at 500 of films films surface SiO 2 ZrO2 films thermally treated at 500 C; SEM micrograph and EDX ◦ EDX spectrum (insert) of the SiO 2 ZrO 2 films thermally treated at 200 °C spectrum (insert) of the SiO2 ZrO2 films thermally treated at 200 C (c). (c).

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Figure 4. X-ray diffraction (XRD) patterns of the SiO2 ZrO2 films thermally treated at 500 ◦ C and 200 ◦ C.

Thus, based on the combined analysis of EDX and XRD data, we suppose that crystallites on the surface of the films can be formed by ZrO2 and residual ZrCl4 /ZrOCl2 ·6H2 O or ZrCl(OH)/ZrCl(OH)2 phases in the presence of silicon dioxide. We assume that small zirconium oxide nanoparticles and zirconium ions are embedded in a silicon matrix, as was observed for the SiO2 CuOx films studied by us earlier [28]. The average size of the crystallites in the film estimated from the Scherrer equation was 10–12 nm and 9–12 nm for the films thermally treated at 200 ◦ C and 500 ◦ C, respectively. So, we can assume that large grains on the film surface (Figure 3) are really the crystallite agglomerates. The authors [29] produced the pure ZrO2 material by use of the sol-gel technique. The average particle size of ZrO2 nanoparticles is found to be 25 nm. In the present work, hierarchical pore structures were prepared by the sol-gel method in alkoxy-derived silica systems [30]. The silica particles cover the zirconia crystallites and prevent aggregation during the thermal treatment. Such a structure allows gas adsorption onto zirconia crystallites through the silica pores. 3.2. Gas Sensitivity Measurements The film resistance was reversibly changed under NO2 exposure (Figure 5) with the characteristic response time to be found as 3, 5, 6, 8 and 10 s for 40 ppm, 62 ppm, 110 ppm, 534 ppm and 1060 ppm concentrations, respectively (Figure 6). The reaction was observed immediately after the gas flow in the test chamber and characterized by high stability and reproducibility. After purging the test chamber with air, the film resistance returned to the initial value. The quick response may be due to the faster oxidation of gas. The average recovery time depended on the gas concentration and was increased from 60 s to 960 s with gas concentration changing from 40 ppm to 1060 ppm (Figure 6b). So, we observed good recovery characteristics (16 min) even after the sensor was exposed to a large concentration of NO2 (1060 ppm).

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Figure5.5.Dependences Dependencesofofsensor sensorresponse response NO 2 concentration (for the samples thermally treated at Figure ononNO 2 concentration (for the samples thermally treated at Figure 5.(1) Dependences of sensor response on NO2of concentration (for the samples thermally treated at ◦ ◦ 200 °C and 500 °C (2); operating temperature °C). 200 C (1) and 500 C (2); operating temperature of 2525◦ C). 200 °C (1) and 500 °C (2); operating temperature of 25 °C).

Figure 6. Dependences of response (a) and recovery (b) time on the concentration of NO2 . Figure 6. Dependences of response (a) and recovery (b) time on the concentration of NO2. Figure 6. Dependences of response (a) and recovery (b) time on the concentration of NO2.

The minimum NO2 concentration that was detected by the SiO2 ZrO2 -based sensor is 40 ppm, but The minimum NO2 concentration was detected by5.the SiOFigure 2ZrO2-based sensor is 40 ppm, but the maximum is not limited by 1060 ppmthat as shown in Figure Also, 5 reveals that the response The minimum NO 2 concentration that was detected by the SiO2ZrO2-based sensor is 40 ppm, but the maximum is not limited by 1060 ppm as shown in Figure 5. Also, Figure 5 reveals that at the ◦ of the sensor based on the SiO2 ZrO2 film thermally treated at 200 C is worse than that treated the maximum is sensor not limited by 1060 ppm as shown in Figure 5. Also, Figure 5 reveals that the response of the based on the SiO 2ZrO2 film thermally ◦treated at 200 °C is worse than that ◦ 500 C. This is due to the fact that at low temperatures (100–200 C), Zr(OH)2 and/or ZrOCl2 ·4H2 O response the °C. sensor based ontothe 2ZrO2 film thermally treated at 200 °C is worse than that treated atof500 This due theSiO fact that low temperatures (100–200 °C), 2 and/or are more likely to form andisthe film structure is notatstable. This fact also was proved byZr(OH) XRD analysis. treated at 500 °C. This is due to the fact that at low temperatures (100–200 °C), Zr(OH) 2 and/or ZrOCl2·4H are the more likely to form and of thezirconium film structure is not stable. According to2O [29], thermolysis process oxochloride is: This fact also was proved by ZrOCl 2·4H2O are more likely to form and the film structure is not stable. This fact also was proved by XRD analysis. According to [29], the thermolysis process of zirconium oxochloride is: ◦C XRD analysis. According to° [29], the80thermolysis process110 of−zirconium oxochloride 40−80 ◦ C −110 ◦ C 130 ◦ C 400is: −480 ° ZrOCl2 ·8HZrOCl ⇒ 5H2 0 ∙ 5H⇒ ZrOCl 0 0 ⇒ ° Zr 0.7H200 ⇒° Zr0Zr02 (1) (OH )2∙·0.7H 20 2 ·ZrOCl 2 ·4H∙24H (1) ∙ 8H 0 ZrOCl 0 ZrOCl Zr OH ° ° ° ° (1) ZrOCl ∙ 8H 0 ZrOCl ∙ 5H 0 ZrOCl ∙ 4H 0 Zr OH ∙ 0.7H 0 Zr0 Further investigations were done for for thethe sample thermally treated at 500 °C.◦ C. Further investigations were done sample thermally treated at 500 Further investigations were done for the sample thermally treated to at various 500 °C. gases, namely NO2, NH3 To evaluate the selective nature of the sensor, it was exposed To evaluate the selective nature of the sensor, it was exposed to various gases, namely NO2 , NH3 To evaluate the selectiveresponse nature of the sensor, it was exposed to various gases, namely NO2,sensor NH3 reveals that SiO 2ZrO 2-based andCO, CO, andthe theobserved observed shown Figure and and response isisshown inin Figure 7. 7It. It reveals that thethe SiO sensor 2 ZrO 2 -based 7 . It reveals that the SiO 2ZrO2-based sensor and CO, and the observed response is shown in Figure washighly highlyselective selectivetotoNO NO 2 gas. was 2 gas. was highly selective to NO2 gas.

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Figure 7. Selectivity of the sensor. Figure 7. Selectivity of the sensor.

Figure 8 depicts the variation of the gas response of with operating temperature of the SiO2ZrO2 Figure 7. Selectivity the an sensor. sensor for 1060 ppm. It is clear from the figure that the largest response is observed °C. The Figure 8 depicts the variation of the gas response with an operating temperature of at the25 SiO 2 ZrO2 ◦ C. Figure 8would depicts of the the gas with operating oftemperatures the SiO 2ZrO 2 material then adsorb theresponse oxygen species attemperature higher sensor for 1060 ppm.the Itvariation is clear from figure that thean largest response is observed at 25 2observed − − at 25 2°C. − The sensor for 1060 ppm. It is clear from the figure that the largest response is (𝑂 → 2𝑂 → 𝑂 ) and this process is competitive with NO 2 adsorption. The material would then adsorb the oxygen species at higher temperatures ( O → 2O → O ) and material would thenwith adsorb species mechanism. at higher temperatures Sensitivity under low temperatures isthe due tooxygen the gas sensitivity this process is competitive NO2 adsorption. (𝑂 → 2𝑂 → 𝑂 ) and this process is competitive with NO2 adsorption. Sensitivity under low temperatures is due to the gas sensitivity mechanism.

Figure 8. Sensor response to 1060 ppm of NO2 as a function of operating temperature. Figure 8. Sensor response to 1060 ppm of NO2 as a function of operating temperature.

Sensitivity under low temperatures is due to the gas sensitivity mechanism. 3.3. Gas Sensitivity Mechanism Figure 8. Sensor response to 1060 ppm of NO2 as a function of operating temperature. 3.3. Gas Mechanism TheSensitivity gas sensitivity mechanism of semiconductor sensors is based on the interaction of the 3.3. Gas Sensitivity Mechanism surface with gas. The interaction on the grainsensors boundaries of metal–oxides and of may The gas sensitivity mechanismoccurs of semiconductor is based on the interaction the include surface The gas sensitivity mechanism of semiconductor sensors is based on the interaction the such processes as oxidation–reduction, gas adsorption and surface chemical reactions [31]. with gas. The interaction occurs on the grain boundaries of metal–oxides and may includeofsuch surface with gas. NO The2 interaction occurs on the grain boundaries of metal–oxides and the mayelectrons include In our gas adsorbsgas to the surface of thesurface p-typechemical semiconductor and[31]. takes processes aswork, oxidation–reduction, adsorption and reactions such processes as oxidation–reduction, gas adsorption and surface chemical reactions [31]. to theIn near-surface layer. our work, NO2 gas adsorbs to the surface of the p-type semiconductor and takes the electrons our work, NO 2 gas adsorbs to the surface of the p-type semiconductor and takes the electrons to theInnear-surface layer. (2) NO g + e → NO ads to the near-surface layer. NO2 (g) + e → NO2− (ads) (2)

(2) + e →of NOthe ads So, a layer of L size is formed nearNO the gsurface semiconductor, comparable to the Debye length with ofnear “holes” (HAL) [32]. this case, for small grain sizes (D)Debye such So,(LD) a layer ofaLhigh size content is formed the surface of theInsemiconductor, comparable to the that D (LD) < 2 L,with the entire nanocrystallite process presence length a highvolume content of the “holes” (HAL) [32].participates In this case, in forthis small grain [33]. sizes The (D) such that So, a layer of L size is formed near the surface of the semiconductor, comparable to the Debye length (LD) with a high content of “holes” (HAL) [32]. In this case, for small grain sizes (D) such that

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D < 2 L, the entire volume of the nanocrystallite participates in this process [33]. The presence of a negative surface charge causes the zone to bend, which creates a potential barrier on the surface. Its of a negative surface charge causes the zone to bend, which creates a potential barrier on the surface. height and depth depend on the surface charge, which in turn depends on the number and type of Its height and depth depend on the surface charge, which in turn depends on the number and type of adsorbed molecules [34]. The smaller the crystallite size, the smaller the potential barrier. However, adsorbed molecules [34]. The smaller the crystallite size, the smaller the potential barrier. However, on on crystallites that are too small, the barrier practically does not arise. The response of the sensor crystallites that are too small, the barrier practically does not arise. The response of the sensor becomes becomes unstable, as was observed in films annealed at 200 °C. unstable, as was observed in films annealed at 200 ◦ C. An increase in the content of “holes” during adsorption leads to an increase in conductivity, An increase in the content of “holes” during adsorption leads to an increase in conductivity, which which is observed during the experiment. At operating sensor temperatures below 100 °C, there are is observed during the experiment. At operating sensor temperatures below 100 ◦ C, there are a number a number of features at absorption. of features at absorption. Surface morphology significantly affects gas-sensitive properties [35]. In our case, a thin film of Surface morphology significantly affects gas-sensitive properties [35]. In our case, a thin film of friable amorphous silicon dioxide and unformed zirconia promotes the penetration of gas molecules friable amorphous silicon dioxide and unformed zirconia promotes the penetration of gas molecules to the grain boundary of the nanocrystallite. Also, SEM and XRD investigations reveal the presence to the grain boundary of the nanocrystallite. Also, SEM and XRD investigations reveal the presence of of ZrOCl2·6H2O grains with OH groups and adsorbed H2O molecules (H2O (ads)) on the film surface. ZrOCl2 ·6H2 O grains with OH groups and adsorbed H2 O molecules (H2 O (ads)) on the film surface. The authors [36] showed oxygen ionization as О2− (ads) at temperatures lower than 147 °С. The authors [36] showed oxygen ionization as O2− (ads) at temperatures lower than 147 ◦ C. According to [37], adsorbed oxygen occupies about 5% of the surface. With increasing temperature, According to [37], adsorbed oxygen occupies about 5% of the surface. With increasing temperature, the occupied area will grow and as stated earlier, this process is competitive with NO2 adsorption. the occupied area will grow and as stated earlier, this process is competitive with NO2 adsorption. According to the above, the following processes occur: According to the above, the following processes occur: (3) 4NO g + 3e + 2H O ads + O ads → 4NO − OH ads (3) 4NO2 (g) + 3e + 2H2 O(ads) + O2− (ads) → 4NO2 − OH− (ads) (4) NO g + e + OH → NO − OH ads NO2 (g) + e + OH → NO2 − OH− (ads)

(4)

3.4. Electrophysical Measurements 3.4. Electrophysical Measurements The literature has widely stated that pure ZrO2 is an indirect band gap semiconductor having The literature has widely stated that pure ZrO is an indirect band gap semiconductor having refractive index r = 2 [38] and with high indicators of2 band gap energy width 5.0–5.2 eV [39] or refractive index r = 2 [38] and with high indicators of band gap energy width 5.0–5.2 eV [39] or 4.28–4.46 eV [38]. In our case, the estimation of the double-oxide film effective band gap energy from 4.28–4.46 eV [38]. In our case, the estimation of the double-oxide film effective band gap energy from the optical spectra (Figure 9) reveals the value of 0.2–0.3 eV. the optical spectra (Figure 9) reveals the value of 0.2–0.3 eV.

Figure 9. The plot of (ahw)2 versus photon energy of the film thermally treated at 500 ◦ C. Figure 9. The plot of (ahw)2 versus photon energy of the film thermally treated at 500 °C.

The semiconducting nature of the SiO2 ZrO2 film is observed from the measurements of resistance The semiconducting nature of the SiO2ZrO2 film is observed from the measurements of with operating temperature (Figure 10). The semiconductivity of the film is due to the presence of resistance with operating temperature (Figure 10). The semiconductivity of the film is due to the ZrO2 and its large oxygen deficiency. presence of ZrO2 and its large oxygen deficiency.

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Figure 10. 10. Dependence of the the sensor resistance on operating temperature (for the the sample thermally Figure 10. Dependence Dependence of of the sensor sensor resistance resistance on on operating operating temperature temperature (for (for the sample sample thermally thermally Figure ◦ C). treated at 500 treated at at 500 500 °C). °C). treated

The current voltage characteristics of a sensor tell us much about its operation and can be a very The current current voltage voltage characteristics characteristics of of aa sensor sensor tell tell us us much much about about its its operation operation and and can can be be aa very very The useful tool in determining the operating characteristics by showing its possible combinations of current useful tool in determining the operating characteristics by showing its possible combinations of useful tool in determining the operating characteristics by showing its possible combinations of and voltage. Figure 11 depicts the I-V characteristics of the SiO ZrO sensor. The asymmetrical nature 2 2 currentand andvoltage. voltage.Figure Figure11 11depicts depictsthe theI-V I-Vcharacteristics characteristicsof ofthe theSiO SiO22ZrO ZrO22sensor. sensor.The Theasymmetrical asymmetrical current of the I-V characteristics shows that the that contacts are non-ohmic in nature.inItnature. is observed from Figure 11 nature of the the I-V characteristics characteristics shows the contacts contacts are non-ohmic non-ohmic is observed observed from nature of I-V shows that the are in nature. ItIt is from that the resistivity of the film is high enough because zirconia is an ionic conductor. Figure 11 11 that that the the resistivity resistivity of of the the film film is is high high enough enough because because zirconia zirconia is is an an ionic ionic conductor. conductor. Figure

Figure 11. Current voltage characteristics of the SiO2 ZrO2 film thermally treated at 500 ◦ C.

3.5. Stability Tests Figure 11. Current Current voltage voltage characteristics characteristics of of the the SiO SiO22ZrO ZrO22 film film thermally thermally treated treated at at 500 500 °C. °C. Figure 11. It is known that one of the main reasons for parameter instability of gas sensors based on 3.5. Stability Stability Tests 3.5. metal–oxideTests semiconductors is their high sensitivity to changes in ambient humidity. So, the effect of air humidity in the range on sensor was investigated 12). The based influence is known known that oneofof of40–90% the main main reasonsresponse for parameter parameter instability(Figure of gas gas sensors sensors based on ItIt is that one the reasons for instability of on of air humidity on the sensor resistance was negligible with a drift of about 3%. In fact, ionic-type metal–oxide semiconductors is their high sensitivity to changes in ambient humidity. So, the effect of metal–oxide semiconductors is their high sensitivity to changes in ambient humidity. So, the effect of metal–oxides such pureof mostresponse commonwas patterns for humidity gas sensors, but the air humidity in in the as range ofzirconia 40–90 % %are onthe sensor response was investigated (Figure 12). The influence influence air humidity the range 40–90 on sensor investigated (Figure 12). The sensor is stable to exposure even at negligible room temperature. This be3%. dueIn to fact, the of of air humidity humidity onhumidity the sensor sensor resistance was negligible with aa drift drift ofmay about 3%. In fact,presence ionic-type of air on the resistance was with of about ionic-type the silica matrix in the composite film [40]. metal–oxides such such as as pure pure zirconia zirconia are are the the most most common common patterns patterns for for humidity humidity gas gas sensors, sensors, but but the the metal–oxides sensor is is stable stable to to humidity humidity exposure exposure even even at at room room temperature. temperature. This This may may be be due due to to the the presence presence of of sensor the silica silica matrix matrix in in the the composite composite film film [40]. [40]. the

Chemosensors Chemosensors2018, 2018,6,6,67x Chemosensors 2018, 6, x

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Figure 12. Dependence of sensor resistance on air humidity for the sample thermally treated◦ at Figure Dependenceofofsensor sensorresistance resistance on air sample thermally treated at 500 Figure 12.12. Dependence air humidity humidityfor forthe the sample thermally treated at C 500 °C (operating temperature = 25 °C). temperature = 25 =◦ C). 500(operating °C (operating temperature 25 °C).

The stability of semiconductor materials for gas sensor fabrication is one of the problem stability semiconductor materials sensor fabrication is one problem TheThe stability of of semiconductor materials forfor gasgas sensor fabrication is one of of thethe problem characteristics. Sensor devices are continuously in contact with aggressive substances. The characteristics. Sensor devices are continuously in contact with aggressive substances. The accelerated characteristics. Sensor devices are continuously in contact with aggressive substances. The accelerated stability tests of the initial resistance were performed by periodically exposing the stability tests of thetests initialofresistance were performed by periodically the fabricated sensors accelerated stability the initial resistance were performed exposing by periodically exposing the to fabricated sensors to NO2 at the operating temperature for nine months (Figure 13). The stability tests NO2 at the operating for nine months (Figure 13).months The stability tests high stability fabricated sensors to NOtemperature 2 at the operating temperature for nine (Figure 13).showed The stability tests showed high stability and reproducibility of the sensors. The initial resistance drift is about 2–5%. and reproducibility of the sensors. The initial resistance drift is about 2–5%. showed high stability and reproducibility of the sensors. The initial resistance drift is about 2–5%.

Figure 13. Dependence of the surface resistance of thin film sensors on time. Figure 13. Dependence of the surface resistance of thin film sensors on time. Figure 13. Dependence of the surface resistance of thin film sensors on time.

The comparative study of the resistive zirconia-based sensors showed that their working The comparative study of the resistive zirconia-based sensors2) showed thatoftheir working temperature is in thestudy range temperature to 400 ◦sensors C (Table and the gases The comparative offrom the room resistive zirconia-based showed thatlist their target working temperature is in the range from room temperature to 400 °C (Table 2) and the list of target gases ◦ includes CO , O , NO , O , and NH . The NO sensor operating at 60 C was produced in 2 range 2 from 3 3 2 to 400 °C (Table 2) and the list of target gasesour temperature is 2in the room temperature includeswork CO2, O2, NO 2, O3, and NH3. The NO2 sensor operating at 60 °C was produced in our previous previous zirconium sources asoperating Zr(ONO3 )at was characterized the mixture 2 and includes CO2, O2, [22] NO2from , O3, and NH3. The NO2 such sensor 60 °C was produced inwith our previous work [22] from zirconium sources such as Zr(ONO 3)2 and was characterized with the mixture of of tetragonal, monoclinic,sources and cubic ZrO phases. 3The Zr5was Si3 phase was also with observed. The average work [22] from zirconium such as2 Zr(ONO )2 and characterized the mixture of tetragonal, monoclinic, and cubicWe ZrO 2 phases. The Zr5Si3 phase was also observed. The average crystallite size was about 35 nm. reported the sensitivity to a small concentration NO2 gas tetragonal, monoclinic, and cubic ZrO2 phases. The Zr5Si3 phase was also observed. Theofaverage was about 35 nm. We reported the sensitivity to a small concentration of NO2 gas (1crystallite ppm).sizesize crystallite was about 35 nm. We reported the sensitivity to a small concentration of NO2 gas (1 ppm). (1 ppm). SiO2ZrO2 sensors are promising candidates for low-temperature NO2 gas sensors. Variation in SiO2ZrO2 sensors are promising candidates for low-temperature NO2 gas sensors. Variation in the zirconium source allows for the production of sensors with different structures, morphologies, the zirconium source allows for the production of sensors with different structures, morphologies, and crystallite size.] and crystallite size.] Table 2. Comparative characteristics of resistive zirconia-based gas sensors. Table 2. Comparative characteristics of resistive zirconia-based gas sensors.

Material Material

Production Production Technique Technique

Target Gas Target Gas

Operating Concentration Source Operating Concentration Temperature Range Source Temperature Range

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Table 2. Comparative characteristics of resistive zirconia-based gas sensors. Material ZrO2 5%Y/ZrO2 Ag–5%Y/ZrO2 CeO2 -xZrO2 ZrO2 Zirconia-doped ceria ZrO2 SiO2 ZrO2

Production Technique

Target Gas

Operating Temperature

Concentration Range

Source

Hydrothermal synthesis

CO2

300–400 ◦ C

600–1000 ppm

[40]

Sol-gel Commercial ZrO2 powder with an organic binder (thick film resistor) Thick film from the ceria–zirconia solid solution powder Sol-gel assisted by microwave

O2

400 ◦ C

0.4–20%

[9]

NH3

250–350 ◦ C

100 ppm

[41]

10−5 –1 atm

[42]

Sol-gel

O2 H2

Room temperature

50 ppm

[29]

NO2 O3

60 ◦ C 30 ◦ C

1 ppm 0.02 ppm

[22]

SiO2 ZrO2 sensors are promising candidates for low-temperature NO2 gas sensors. Variation in the zirconium source allows for the production of sensors with different structures, morphologies, and crystallite size. 4. Conclusions The SiO2 ZrO2 films were fabricated using the sol-gel technique on silicon substrates and characterized by SEM, EDAX and XRD techniques. The synthesis technique used leads to the formation of a complex, multiphase system in SiO2 ZrO2 films. So, a stable monoclinic ZrO2 with an impurity of tetragonal phases was observed. Also, the film surface can be characterized by the presence of ZrOCl2 ·6H2 O or ZrCl(OH)/ZrCl(OH)2 grains due to the introduction of silicon, which prevents the complete decomposition of ZrOCl2 ·8H2 O and the removal of chlorine. The described structure determines the gas sensitivity mechanism at low operating temperatures. The electrophysical studies showed the semiconducting nature of the SiO2 ZrO2 film. Effective band gap energy evaluated from the optical spectra reveals the value of 0.2–0.3 eV, which is lower than for ZrO2 and may be responsible for the fast sensor response towards NO2 . The SiO2 ZrO2 -based sensor showed promising gas-sensitive properties to NO2 gas with fast response times at a low operating temperature (25 ◦ C) and high stability of the initial resistance during a long-term period. The designed sensor may be used for the detection of high NO2 concentrations as a result of accidents in industrial enterprises. Author Contributions: T.S.M. contributed to the design of the experiments and the data analysis. T.N.M. contributed data interpretation and wrote the manuscript. G.E.Y. and N.K.P provided a contribution to the discussion. All authors contributed to analyses of the results in the manuscript. Funding: This work is financially supported by the Ministry of Education of Russia, under Contract No. 14.575.21.0126 (unique identifier of the contract is RFMEFI57517X0126). Acknowledgments: Authors acknowledge the Center for collective use of "Microsystem Technology and Integrated Sensorics" (Southern Federal University (Russia)), on the basis of which the experiments were performed for work. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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