rus
Issue #6/2021
V.E.Polkovnikov, A.V.Strogonov, T.G.Menshikova, A.A.Karionova
Synthesis of Zn2SNO4 films deposited using spray pyrolysis technology and their application in NO2 gas sensors for a bronchial asthma diagnostic device
Synthesis of Zn2SNO4 films deposited using spray pyrolysis technology and their application in NO2 gas sensors for a bronchial asthma diagnostic device
DOI: 10.22184/1993-8578.2021.14.6.350.360
This paper describes the spray pyrolysis technology of deposition, from aqueous solutions of metal salts, polycrystalline film Zn2SnO4, with a grain size of 9 nm calculated by the Scherrer formula. The film is applied by spraying a mixture of metal salt solutions in the form of an aerosol onto a glass substrate heated to 420 °C. The optimal deposition mode was demonstrated and the composition of the resulting structure was monitored using X-ray diffraction analysis. The morphology of the film surface was studied by AFM. The electrical parameters of the film were measured by the Van der Pauw method and the Hall effect. The resistivity of the film, the type of conductivity, concentration and mobility of charge carriers were measured. The band gap was determined from the light absorption spectra. With the help of the obtained information on the composition and morphology in an analytical way, based on the literature and calculated data, it was concluded that this material can be used for a supersensitive NO2 express control sensor for a bronchial asthma diagnostic device.
This paper describes the spray pyrolysis technology of deposition, from aqueous solutions of metal salts, polycrystalline film Zn2SnO4, with a grain size of 9 nm calculated by the Scherrer formula. The film is applied by spraying a mixture of metal salt solutions in the form of an aerosol onto a glass substrate heated to 420 °C. The optimal deposition mode was demonstrated and the composition of the resulting structure was monitored using X-ray diffraction analysis. The morphology of the film surface was studied by AFM. The electrical parameters of the film were measured by the Van der Pauw method and the Hall effect. The resistivity of the film, the type of conductivity, concentration and mobility of charge carriers were measured. The band gap was determined from the light absorption spectra. With the help of the obtained information on the composition and morphology in an analytical way, based on the literature and calculated data, it was concluded that this material can be used for a supersensitive NO2 express control sensor for a bronchial asthma diagnostic device.
Теги: application modes electrophysical parameters gas sensor metal oxide films microrelief nitrogen oxide spray pyrolysis zinc stannate zto газовый датчик микрорельеф оксид азота пленки оксида металла режимы применения спрей-пиролиз станнат электрофизические параметры
Synthesis of Zn2SNO4 films deposited using spray pyrolysis technology and their application in NO2 gas sensors for a bronchial asthma diagnostic device
This paper describes the spray pyrolysis technology of deposition, from aqueous solutions of metal salts (Zn, Sn), polycrystalline film Zn2SnO4, with a grain size of 9 nm calculated by the Scherrer formula. The film is applied by spraying a mixture of metal salt solutions in the form of an aerosol onto a glass substrate heated to 420 °C. The optimal deposition mode was demonstrated and the composition of the resulting structure was monitored using X-ray diffraction analysis. The morphology of the film surface was studied by atomic force microscopy. The electrical parameters of the film were measured by the Van der Pauw method and the Hall effect. The resistivity of the film, the type of conductivity, concentration and mobility of charge carriers were measured. The obtained metal oxide film Zn2SnO4 has a thickness of 0.45 μm, a charge carrier concentration of 1.5 ∙ 1018 cm–3, a charge carrier mobility of 4.86 cm2 / V∙s and a resistivity of 0.49 Ohm cm. The band gap was determined from the light absorption spectra and amounted to 3.5 eV. With the help of the obtained information on the composition and morphology in an analytical way, based on the literature and calculated data, it was concluded that this semiconductor material can be used for a supersensitive NO2 express control sensor for a bronchial asthma diagnostic device.
INTRODUCTION
Zinc orthostannate Zn2SnO4 (zinc tin oxide – ZTO) is a wide band semiconductor (Eg > 3 eV) of n-type conductivity. This metal oxide has different electrophysical and morphological characteristics depending on the synthesis method and regime.
The spray pyrolysis method is an inexpensive, vacuum-free process for the synthesis of materials in the form of powders and films. In the case of films, they are usually deposited on a wide range of substrates that can be readily adapted for large area deposition and industrial production processes. This method is a chemical synthesis of materials at atmospheric pressure, in which a solution of chemical compounds in a suitable solvent is sprayed, through the nozzle of a spraying system (airbrush) onto a hot substrate to deposit films, where a pyrolysis reaction is achieved, and metal oxides being the preferred compound to be obtained by this method. It should be emphasized that Zn2SnO4 consists of non-toxic and widely distributed elements in the Earth crust that have a low cost of extraction [1].
According to the Global Strategy for Asthma Management and Prevention (GINA), there are currently 300 million people worldwide who have asthma. The prevalence of asthma is higher than such diseases as coronary heart disease (300 times), lung cancer (33 times) and stroke (15 times). The fact that more than 10% of children have bronchial asthma is of great concern. New methods of medical diagnosis, including the use of various gas analysers, are becoming widespread. Nitric oxide is produced in the body to a greater extent in inflammatory processes such as asthma. Thus, this circumstance can be used to detect and influence such processes.
Healthy and sick people have different nitric oxide levels: a healthy person has 5 ppb and a sick person has more than 20 ppb. Nitric oxide fraction measurement is not the only and uncontroversial diagnostic method, it should be used combining with other known techniques [2, 3]. Therefore, the analysis of a patient’s exhaled air composition can be used as a non-invasive method of diagnosing bronchial asthma, with the possibility of detecting the disease at an early stage. This is also useful when studying the response of a body to pharmacological treatment. In air, NO is easily oxidised to nitrogen dioxide NO2, a mixture of NO where NO2 makes the greater part is referred to as NOx. However, the main problem in measuring NOx in exhaled air is a low concentration of the monitored gas – at a few ppb level. Measuring such ultra-low NOx concentrations in air requires special techniques for manufacturing low-temperature metallic oxide sensor elements. Gas sensitivity of metal oxides will be the higher the ratio of the surface to the volume ratio of the microcrystal (aspect ratio) is. As the grain size of the polycrystal decreases, the aspect ratio increases, and the sensory properties of the metal oxide improve.
It is possible to reduce the size of microcrystals by selecting the heat treatment modes of the synthesized amorphous metal oxides. Mixing different oxides gives good results: increases the grain sizes and defectiveness of the sensor. Nanostructured metal oxide semiconductors have been used quite successfully as gas sensor layers for monitoring nitrogen oxides in air [4]. Their gas sensitivity is due to a change in electrical resistivity of the semiconductor surface as the molecules of the gas under control interact with the surface states [5]. Nanostructured metal oxide sensors successfully measure NOx in air at units or even fractions of ppm, making it possible to monitor the MPS of NOx in air at low and even room temperatures [4]. Chemically, NO is a reducing gas, and NO2 is a strong oxidising agent. The effect of these gases on electrical properties of metal oxide semiconductors varies essentially. For example, n-type semiconductors will increase their electrical conductivity when interacting with NO, and the electrical resistance of the gas sensor will increase when interacting with NO2. A particular gas sensor (n- Zn2SnO4) may, therefore, have a different sensitivity to NO and NO2 gases. Since NO2 is a very strong oxidising agent, the gas sensitivity of metal oxide semiconductors to NO2 is generally higher than to NO.
Because of the high selectivity and excellent sensitivity of Zn2SnO4 to both oxidising and reducing gases [6–9], zinc spinel stannate is often studied as a sensor for NO and NO2 gases [10]. There are studies in the foreign literature on the synthesis by spray pyrolysis of Zn2SnO4, which has a polycrystalline cubic inverse spinel structure. These synthesised Zn2SnO4 structures have the best selectivity for NO2 and all that at operating temperatures (around 200 °C), making zinc stannate the best metal oxide gas-sensitive material for NO2 sensors [11].
The aim of this work is to determine the optimum parameters for fabrication of zinc orthostannate Zn2SnO4 prepared by spray pyrolysis, which has a polycrystalline cubic inverse spinel structure. The other goal is to study parameters of the fabricated Zn2SnO4 film to evaluate its use as a semiconductor material for an ultra-sensitive NO2 express control sensor to be used in a bronchial asthma diagnostic device.
EXPERIMENTAL METHODS AND EQUIPMENT
The reaction mechanism of the synthesis of multicomponent ternary metal oxide thin films using spray pyrolysis is a complex problem, as the growth of individual oxides, formation of composite materials or formation of two phases are more likely [12]. For both components to be deposited simultaneously on the substrate, optimum deposition parameters are required. The reaction involved in the synthesis of the Zn2SnO4 oxide system is as follows: when the atomised precursor solution passes through a temperature gradient, the solution decomposes into precursors, and the solvent evaporates. Nucleation centre formation and subsequent film growth begins when parts of the solution reach the hot substrate and a crystallisation reaction of the precursors is initiated, leading to deposition of the Zn2SnO4 film after the precursor solution has been completely decomposed.
The spray pyrolysis technique can deposit different structures on different substrate materials. In the current study, a glass substrate in the form of a slide for micro-preparations (GOST 9284-75) of 26 × 76 × 1 mm size was selected. The substrate was heated to the desired temperature using a 245 × 60 mm IR ceramic heating element. The substrate was laid out along the centre of the heater in order to achieve maximum temperature maintenance and uniform application. A HoldPeak hp-1500 pyrometer was used for temperature control. In the experiment for the synthesis of Zn2SnO4 structure different temperature modes in the range of 300 to 500 °C were tried, resulting in an optimum temperature of 420 °C being selected. To apply the solution onto a hot substrate by spray pyrolysis, a spray system was used in which the aerosol was created using an OPHIR AC004A airbrush with a nozzle dia. 0.3 mm. Air pressure to the airbrush was supplied by an AS186 oil-free piston compressor. A schematic diagram of the spray pyrolysis unit is shown in Fig.1.
The synthesis of multicomponent metal oxide film Zn2SnO4 was carried out from two metal salts zinc acetate [Zn(CH3COO)2 · 2Н2О] and tin chloride [SnCl2 · 2H2O]. These precursors were used as a source of chemical elements (Zn, Sn) in zinc stannate compound. According to the chemical formula of zinc stannate, the molar ratio of the selected precursors in the solution was 2:1. Then the corresponding amounts of chemical reagents were dissolved in deionized water separately from each other with stirring for 30 minutes. Concentrated hydrochloric acid HCl was added to the tin chloride solution to prevent hydrolysis. Then, to prepare the final sprayable composition, the previous solutions were mixed into one in which a residue precipitated, hydrochloric acid was added and stirred for 2 hours. Stirring of each solution at room temperature was carried out using an IKA RH basic 2 magnetic stirrer. A simplified flow diagram for the fabrication of Zn2SnO4 film by spray pyrolysis is shown in Fig.2.
The multilayer film application mode consisted of 1.5 min cycles of continuous aerosol application to the surface of the heated substrate with a pause of 40 seconds to ensure complete recovery of temperature of the substrate surface. The distance to the substrate was 85 cm. As a result, one layer was formed on the surface of the substrate in one cycle and a total of seven layers of Zn2SnO4 film were formed.
RESULTS AND DISCUSSION
The synthesized samples were subjected to X-ray phase analysis to determine a composition of the precipitated material. X-ray studies of Zn2SnO4 were carried out using a Bruker DIFFRAC EVA3.0 X-ray spectrometer. X-ray spectra were taken in bands from 2Θ (25) to 2Θ (50) with a step width of 2Θ (0.02), using an X-ray wavelength of 1.54056 Å. The X-ray spectra were analysed by comparison with the ICDD PDF2012 database and, according to the database, the peak positions were illustrated. The XRD data of the Zn2SnO4 film are shown in Fig.3.
The X-ray diffraction spectrum of the Zn2SnO4 thin film deposited at substrate temperature (420 °C) shows that the X-ray diffraction spectrum is in good agreement with the ICDD PDF2012 database and corresponds to the polycrystalline structure of the cubic inverse spinel Zn2SnO4.
The polycrystalline nature of the films is evidenced by the prominent major broad reflex from the (3,1,1) plane and the position of the 2Θ peak (34,52). Other weakly pronounced diffraction peaks can be attributed to SnO2, ZnO and Zn2SnO4, indicating the presence of amorphous and mixed film composition characteristic of insufficiently annealed zinc stannate samples. The grain size of the resulting sample was estimated using the Scherrer formula, with the XRD peak from the plane (3,1,1) and with the position of the 2Θ peak (34,52). The grain size was 9.15 nm.
The surface scanning by a FemtoScan-001 atomic force microscope provided an insight into the microrelief of the thin film under study. The microrelief of the Zn2SnO4 thin film is uniform, with moderately packed grains as shown in Fig.4.
As can be seen in Fig.5, the spray pyrolysis results in a film structure made up of large spherical grains, with the result that the actual surface area is larger than that of the investigated film surface. This contributes to increasing the gas sensitivity of sensors based on this semiconductor material. The average surface roughness of the film is Sa = 8.07309 nm and the average surface roughness equals Sq = 11.3253 nm.
Electrophysical parameters such as film resistivity and charge carrier mobility of Zn2SnO4 were measured by Van der Pauw method and Hall effect in 0.63 T magnetic field. The obtained data are recorded in Table 1. The conductivity type of the semiconductor film was evaluated using the thermal probe and the Hall effect, and was determined to be n-type. In order to calculate resistivity of the films under study, it was necessary to measure their thickness. Determination of the thickness of metal-oxide films was carried out using a MII-4 interference microscope. The transition boundary between glass and film was created with heat-resistant Kapton Tape, and then the film thickness was estimated by interference fringes. The Zn2SnO4 film thickness was 0.45 µm.
Table 1 shows that Zn2SnO4 film has high electrical conductivity and concentration of free carriers (n = 1.5 ∙ 1018 cm–3) which is due to peculiarities of the current transfer in Zn2SnO4 film [13].
Spectral examination of the metal oxides optical properties has several advantages, it is contactless and non-destructive. Transmission spectra are the main type of optical investigation. This type of measurement is highly accurate and easy to perform, and the results can be quickly processed. Transmission coefficient studies were taken by a SPEX SSP-715-M spectrophotometer.
The SPEX SSP-715-M spectrophotometer is used to take optical transmission spectra of liquid and solid samples in various spectral ranges. The optical scheme makes use of a two-beam lamp. The spectrum produced by the lamp is in the range of 190–1100 nm. Transmission spectrum readout ranges from 0.1 to 100%. It is used to measure the transmittance spectrum of samples and process the results. Transmission spectrum of Zn2SnO4 film is shown on Fig.5.
The transmission spectrum for Zn2SnO4 film has a transparency of over 80% in the visible and infrared part of the spectrum. The transparency threshold is in the ultraviolet range. This makes them suitable for use in transparent electronics and solar energy.
Based on the absorption thresholds it is possible to determine the absorption coefficient and the band gap width of the film in the coordinates (αhν)2 = f(hν). The data for calculating the band gap width is shown in Fig.6.
The band gap width was determined from the plot straightening (αħν)2 = f(ħν) and was 3.5 eV for Zn2SnO4.
CONCLUSIONS
In this work zinc orthostannate film Zn2SnO4, which has the structure of a cubic inverse spinel, was synthesized. For this film the optimum parameters of spray pyrolysis synthesis were determined, allowing reproducibility of the result. Immediately after deposition the film had satisfactory electrophysical parameters, which makes it suitable for practical use immediately after production, reduces the number of technological operations and makes further production of articles based on it cheaper. The given synthesized semiconductor material has a thickness of 0.45 μm, charge carrier concentration of 1.5 ∙ 1018 cm–3, carrier mobility of 4.86 cm2 /∙s and resistivity of 0.49 Ohm∙cm, bandgap of 3.5 eV that corresponds to the literature data. In order to determine composition of the synthesised structures an X-ray phase analysis was carried out, which showed that the obtained X-ray patterns agreed well with the ICDD PDF2012 database and corresponded to the polycrystalline structure of the cubic inverse spinel Zn2SnO4. The polycrystalline nature of the films is evidenced by the prominent major broad reflex from the (3,1,1) plane position of the 2Θ peak (34,52). According to the X-ray diffraction analysis, Zn2SnO4 films have the similar parameters – the same structure, grain size of 9.15 nm and electrophysical parameters that have been reported in numerous studies. These are capable of detecting small concentrations of NO2 at low operating temperatures, being an indispensable material for semiconductor NO2 gas sensors. Based on the obtained information about the composition and morphology, it is possible to analyse the literature and computational data and conclude on a possibility to use this promising semiconductor material for a supersensitive express-control NO2 sensor for bronchial asthma diagnostic device.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This paper describes the spray pyrolysis technology of deposition, from aqueous solutions of metal salts (Zn, Sn), polycrystalline film Zn2SnO4, with a grain size of 9 nm calculated by the Scherrer formula. The film is applied by spraying a mixture of metal salt solutions in the form of an aerosol onto a glass substrate heated to 420 °C. The optimal deposition mode was demonstrated and the composition of the resulting structure was monitored using X-ray diffraction analysis. The morphology of the film surface was studied by atomic force microscopy. The electrical parameters of the film were measured by the Van der Pauw method and the Hall effect. The resistivity of the film, the type of conductivity, concentration and mobility of charge carriers were measured. The obtained metal oxide film Zn2SnO4 has a thickness of 0.45 μm, a charge carrier concentration of 1.5 ∙ 1018 cm–3, a charge carrier mobility of 4.86 cm2 / V∙s and a resistivity of 0.49 Ohm cm. The band gap was determined from the light absorption spectra and amounted to 3.5 eV. With the help of the obtained information on the composition and morphology in an analytical way, based on the literature and calculated data, it was concluded that this semiconductor material can be used for a supersensitive NO2 express control sensor for a bronchial asthma diagnostic device.
INTRODUCTION
Zinc orthostannate Zn2SnO4 (zinc tin oxide – ZTO) is a wide band semiconductor (Eg > 3 eV) of n-type conductivity. This metal oxide has different electrophysical and morphological characteristics depending on the synthesis method and regime.
The spray pyrolysis method is an inexpensive, vacuum-free process for the synthesis of materials in the form of powders and films. In the case of films, they are usually deposited on a wide range of substrates that can be readily adapted for large area deposition and industrial production processes. This method is a chemical synthesis of materials at atmospheric pressure, in which a solution of chemical compounds in a suitable solvent is sprayed, through the nozzle of a spraying system (airbrush) onto a hot substrate to deposit films, where a pyrolysis reaction is achieved, and metal oxides being the preferred compound to be obtained by this method. It should be emphasized that Zn2SnO4 consists of non-toxic and widely distributed elements in the Earth crust that have a low cost of extraction [1].
According to the Global Strategy for Asthma Management and Prevention (GINA), there are currently 300 million people worldwide who have asthma. The prevalence of asthma is higher than such diseases as coronary heart disease (300 times), lung cancer (33 times) and stroke (15 times). The fact that more than 10% of children have bronchial asthma is of great concern. New methods of medical diagnosis, including the use of various gas analysers, are becoming widespread. Nitric oxide is produced in the body to a greater extent in inflammatory processes such as asthma. Thus, this circumstance can be used to detect and influence such processes.
Healthy and sick people have different nitric oxide levels: a healthy person has 5 ppb and a sick person has more than 20 ppb. Nitric oxide fraction measurement is not the only and uncontroversial diagnostic method, it should be used combining with other known techniques [2, 3]. Therefore, the analysis of a patient’s exhaled air composition can be used as a non-invasive method of diagnosing bronchial asthma, with the possibility of detecting the disease at an early stage. This is also useful when studying the response of a body to pharmacological treatment. In air, NO is easily oxidised to nitrogen dioxide NO2, a mixture of NO where NO2 makes the greater part is referred to as NOx. However, the main problem in measuring NOx in exhaled air is a low concentration of the monitored gas – at a few ppb level. Measuring such ultra-low NOx concentrations in air requires special techniques for manufacturing low-temperature metallic oxide sensor elements. Gas sensitivity of metal oxides will be the higher the ratio of the surface to the volume ratio of the microcrystal (aspect ratio) is. As the grain size of the polycrystal decreases, the aspect ratio increases, and the sensory properties of the metal oxide improve.
It is possible to reduce the size of microcrystals by selecting the heat treatment modes of the synthesized amorphous metal oxides. Mixing different oxides gives good results: increases the grain sizes and defectiveness of the sensor. Nanostructured metal oxide semiconductors have been used quite successfully as gas sensor layers for monitoring nitrogen oxides in air [4]. Their gas sensitivity is due to a change in electrical resistivity of the semiconductor surface as the molecules of the gas under control interact with the surface states [5]. Nanostructured metal oxide sensors successfully measure NOx in air at units or even fractions of ppm, making it possible to monitor the MPS of NOx in air at low and even room temperatures [4]. Chemically, NO is a reducing gas, and NO2 is a strong oxidising agent. The effect of these gases on electrical properties of metal oxide semiconductors varies essentially. For example, n-type semiconductors will increase their electrical conductivity when interacting with NO, and the electrical resistance of the gas sensor will increase when interacting with NO2. A particular gas sensor (n- Zn2SnO4) may, therefore, have a different sensitivity to NO and NO2 gases. Since NO2 is a very strong oxidising agent, the gas sensitivity of metal oxide semiconductors to NO2 is generally higher than to NO.
Because of the high selectivity and excellent sensitivity of Zn2SnO4 to both oxidising and reducing gases [6–9], zinc spinel stannate is often studied as a sensor for NO and NO2 gases [10]. There are studies in the foreign literature on the synthesis by spray pyrolysis of Zn2SnO4, which has a polycrystalline cubic inverse spinel structure. These synthesised Zn2SnO4 structures have the best selectivity for NO2 and all that at operating temperatures (around 200 °C), making zinc stannate the best metal oxide gas-sensitive material for NO2 sensors [11].
The aim of this work is to determine the optimum parameters for fabrication of zinc orthostannate Zn2SnO4 prepared by spray pyrolysis, which has a polycrystalline cubic inverse spinel structure. The other goal is to study parameters of the fabricated Zn2SnO4 film to evaluate its use as a semiconductor material for an ultra-sensitive NO2 express control sensor to be used in a bronchial asthma diagnostic device.
EXPERIMENTAL METHODS AND EQUIPMENT
The reaction mechanism of the synthesis of multicomponent ternary metal oxide thin films using spray pyrolysis is a complex problem, as the growth of individual oxides, formation of composite materials or formation of two phases are more likely [12]. For both components to be deposited simultaneously on the substrate, optimum deposition parameters are required. The reaction involved in the synthesis of the Zn2SnO4 oxide system is as follows: when the atomised precursor solution passes through a temperature gradient, the solution decomposes into precursors, and the solvent evaporates. Nucleation centre formation and subsequent film growth begins when parts of the solution reach the hot substrate and a crystallisation reaction of the precursors is initiated, leading to deposition of the Zn2SnO4 film after the precursor solution has been completely decomposed.
The spray pyrolysis technique can deposit different structures on different substrate materials. In the current study, a glass substrate in the form of a slide for micro-preparations (GOST 9284-75) of 26 × 76 × 1 mm size was selected. The substrate was heated to the desired temperature using a 245 × 60 mm IR ceramic heating element. The substrate was laid out along the centre of the heater in order to achieve maximum temperature maintenance and uniform application. A HoldPeak hp-1500 pyrometer was used for temperature control. In the experiment for the synthesis of Zn2SnO4 structure different temperature modes in the range of 300 to 500 °C were tried, resulting in an optimum temperature of 420 °C being selected. To apply the solution onto a hot substrate by spray pyrolysis, a spray system was used in which the aerosol was created using an OPHIR AC004A airbrush with a nozzle dia. 0.3 mm. Air pressure to the airbrush was supplied by an AS186 oil-free piston compressor. A schematic diagram of the spray pyrolysis unit is shown in Fig.1.
The synthesis of multicomponent metal oxide film Zn2SnO4 was carried out from two metal salts zinc acetate [Zn(CH3COO)2 · 2Н2О] and tin chloride [SnCl2 · 2H2O]. These precursors were used as a source of chemical elements (Zn, Sn) in zinc stannate compound. According to the chemical formula of zinc stannate, the molar ratio of the selected precursors in the solution was 2:1. Then the corresponding amounts of chemical reagents were dissolved in deionized water separately from each other with stirring for 30 minutes. Concentrated hydrochloric acid HCl was added to the tin chloride solution to prevent hydrolysis. Then, to prepare the final sprayable composition, the previous solutions were mixed into one in which a residue precipitated, hydrochloric acid was added and stirred for 2 hours. Stirring of each solution at room temperature was carried out using an IKA RH basic 2 magnetic stirrer. A simplified flow diagram for the fabrication of Zn2SnO4 film by spray pyrolysis is shown in Fig.2.
The multilayer film application mode consisted of 1.5 min cycles of continuous aerosol application to the surface of the heated substrate with a pause of 40 seconds to ensure complete recovery of temperature of the substrate surface. The distance to the substrate was 85 cm. As a result, one layer was formed on the surface of the substrate in one cycle and a total of seven layers of Zn2SnO4 film were formed.
RESULTS AND DISCUSSION
The synthesized samples were subjected to X-ray phase analysis to determine a composition of the precipitated material. X-ray studies of Zn2SnO4 were carried out using a Bruker DIFFRAC EVA3.0 X-ray spectrometer. X-ray spectra were taken in bands from 2Θ (25) to 2Θ (50) with a step width of 2Θ (0.02), using an X-ray wavelength of 1.54056 Å. The X-ray spectra were analysed by comparison with the ICDD PDF2012 database and, according to the database, the peak positions were illustrated. The XRD data of the Zn2SnO4 film are shown in Fig.3.
The X-ray diffraction spectrum of the Zn2SnO4 thin film deposited at substrate temperature (420 °C) shows that the X-ray diffraction spectrum is in good agreement with the ICDD PDF2012 database and corresponds to the polycrystalline structure of the cubic inverse spinel Zn2SnO4.
The polycrystalline nature of the films is evidenced by the prominent major broad reflex from the (3,1,1) plane and the position of the 2Θ peak (34,52). Other weakly pronounced diffraction peaks can be attributed to SnO2, ZnO and Zn2SnO4, indicating the presence of amorphous and mixed film composition characteristic of insufficiently annealed zinc stannate samples. The grain size of the resulting sample was estimated using the Scherrer formula, with the XRD peak from the plane (3,1,1) and with the position of the 2Θ peak (34,52). The grain size was 9.15 nm.
The surface scanning by a FemtoScan-001 atomic force microscope provided an insight into the microrelief of the thin film under study. The microrelief of the Zn2SnO4 thin film is uniform, with moderately packed grains as shown in Fig.4.
As can be seen in Fig.5, the spray pyrolysis results in a film structure made up of large spherical grains, with the result that the actual surface area is larger than that of the investigated film surface. This contributes to increasing the gas sensitivity of sensors based on this semiconductor material. The average surface roughness of the film is Sa = 8.07309 nm and the average surface roughness equals Sq = 11.3253 nm.
Electrophysical parameters such as film resistivity and charge carrier mobility of Zn2SnO4 were measured by Van der Pauw method and Hall effect in 0.63 T magnetic field. The obtained data are recorded in Table 1. The conductivity type of the semiconductor film was evaluated using the thermal probe and the Hall effect, and was determined to be n-type. In order to calculate resistivity of the films under study, it was necessary to measure their thickness. Determination of the thickness of metal-oxide films was carried out using a MII-4 interference microscope. The transition boundary between glass and film was created with heat-resistant Kapton Tape, and then the film thickness was estimated by interference fringes. The Zn2SnO4 film thickness was 0.45 µm.
Table 1 shows that Zn2SnO4 film has high electrical conductivity and concentration of free carriers (n = 1.5 ∙ 1018 cm–3) which is due to peculiarities of the current transfer in Zn2SnO4 film [13].
Spectral examination of the metal oxides optical properties has several advantages, it is contactless and non-destructive. Transmission spectra are the main type of optical investigation. This type of measurement is highly accurate and easy to perform, and the results can be quickly processed. Transmission coefficient studies were taken by a SPEX SSP-715-M spectrophotometer.
The SPEX SSP-715-M spectrophotometer is used to take optical transmission spectra of liquid and solid samples in various spectral ranges. The optical scheme makes use of a two-beam lamp. The spectrum produced by the lamp is in the range of 190–1100 nm. Transmission spectrum readout ranges from 0.1 to 100%. It is used to measure the transmittance spectrum of samples and process the results. Transmission spectrum of Zn2SnO4 film is shown on Fig.5.
The transmission spectrum for Zn2SnO4 film has a transparency of over 80% in the visible and infrared part of the spectrum. The transparency threshold is in the ultraviolet range. This makes them suitable for use in transparent electronics and solar energy.
Based on the absorption thresholds it is possible to determine the absorption coefficient and the band gap width of the film in the coordinates (αhν)2 = f(hν). The data for calculating the band gap width is shown in Fig.6.
The band gap width was determined from the plot straightening (αħν)2 = f(ħν) and was 3.5 eV for Zn2SnO4.
CONCLUSIONS
In this work zinc orthostannate film Zn2SnO4, which has the structure of a cubic inverse spinel, was synthesized. For this film the optimum parameters of spray pyrolysis synthesis were determined, allowing reproducibility of the result. Immediately after deposition the film had satisfactory electrophysical parameters, which makes it suitable for practical use immediately after production, reduces the number of technological operations and makes further production of articles based on it cheaper. The given synthesized semiconductor material has a thickness of 0.45 μm, charge carrier concentration of 1.5 ∙ 1018 cm–3, carrier mobility of 4.86 cm2 /∙s and resistivity of 0.49 Ohm∙cm, bandgap of 3.5 eV that corresponds to the literature data. In order to determine composition of the synthesised structures an X-ray phase analysis was carried out, which showed that the obtained X-ray patterns agreed well with the ICDD PDF2012 database and corresponded to the polycrystalline structure of the cubic inverse spinel Zn2SnO4. The polycrystalline nature of the films is evidenced by the prominent major broad reflex from the (3,1,1) plane position of the 2Θ peak (34,52). According to the X-ray diffraction analysis, Zn2SnO4 films have the similar parameters – the same structure, grain size of 9.15 nm and electrophysical parameters that have been reported in numerous studies. These are capable of detecting small concentrations of NO2 at low operating temperatures, being an indispensable material for semiconductor NO2 gas sensors. Based on the obtained information about the composition and morphology, it is possible to analyse the literature and computational data and conclude on a possibility to use this promising semiconductor material for a supersensitive express-control NO2 sensor for bronchial asthma diagnostic device.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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