rus
Issue #5/2020
M.D.Andreev, V.A.Kovalev, V.V.Amelichev, S.S.Generalov, A.V.Nikolaeva, S.A.Polomoshnov, A.M.Gaskov, V.V. Krivetskiy
Synthesis of ultrafine tin dioxide by flame spray pyrolysis for inkjet printing of sensitive elements of gas sensors
Synthesis of ultrafine tin dioxide by flame spray pyrolysis for inkjet printing of sensitive elements of gas sensors
DOI: 10.22184/1993-8578.2020.13.5.276.283
Inkjet printing of stable suspensions on top of the MEMS structures with micro-heater has been used for thick film layer formation on the basis of nanocrystalline SnO2 with uniform porous structure. Gas sensitivity of the obtained materials is described.
Inkjet printing of stable suspensions on top of the MEMS structures with micro-heater has been used for thick film layer formation on the basis of nanocrystalline SnO2 with uniform porous structure. Gas sensitivity of the obtained materials is described.
Теги: gas sensitivity inkjet printing mems porous structure газовая чувствительность мэмс пористая структура струйная печать
INTRODUCTION
Metal oxide semiconductor gas sensors attract great interest due to the combination of their characteristics – high sensitivity, the possibility of long-term continuous operation. The use of MEMS structures in the design of such sensors provides low power consumption, miniaturization and low cost, which opens up prospects for wide application in mass consumer devices [1]. The key issue in the manufacture of such sensors is the process of assembly of a high surface area gas-sensitive metal oxide layer with an integrated structures manufactured using planar integrated circuit technologies [2]. One possible solution is the deposition of thick-film elements through a shadow mask directly during the synthesis of oxides [3]. However, the disadvantages of this approach include a sophisticated procedure of the substrate and the mask alignment, which makes it economically feasible to manufacture only large batches of sensors. In addition, it is worth noting the relatively high losses of the active substance during deposition and possible limitations of post-synthetic processing of gas-sensitive material associated with the design of the MEMS crystal.
In recent years, the use of the inkjet printing method in the manufacture of semiconductor gas sensors has been actively considered as an alternative approach [4, 5]. Some scientific groups are searching for approaches to manufacture all the functional elements of sensors in this way [6], but the greatest interest is observed particularly in the formation of a gas-sensitive element on the surface of MEMS crystal with a microheater manufactured via microelectronic technology [7–10]. The distinct feature of this approach is the versatility in terms of the applied semiconductor material – its chemical composition and structure, the possibility to use it in the manufacture of small batches and even single sensors, and the scalability of the approach for mass production. It is also worth noting the absence of losses of the active substance during its deposition.
The greatest challenge of metal oxide semiconductor layers inkjet printing is the production of the inks – stable suspensions of semiconductor material particles with the necessary parameters of viscosity and surface tension. The stability of such suspensions is determined both by the size of the grains of semiconductor oxides and the size of their agglomerates formed during heat treatment at the stage of material synthesis. Often, especially in the case of chemically modified, doped oxide materials synthesis for gas sensors, it is necessary to anneal such materials at temperatures of the order of 500 oC or higher, which leads to the growth of particles and consolidation of agglomerates. In this case long-term high-energy ball milling is often used [11, 12], which is aimed to reduce the size of agglomerates. However, this process is associated with the losses of the active substance and the risk of undesirable impurities introduction. An alternative approach consists in obtaining of highly dispersed materials during synthesis with the use of expendable templates and the use of polymer dispersants to stabilize suspensions [13], but this significantly increases the cost of obtaining gas-sensitive materials and complicates the printing process.
In this regard, the metal oxide materials synthesis for gas sensor application by flame spray pyrolysis is promising [14]. This approach allows to obtain metal oxides in the nanocrystalline form with a low degree of grain agglomeration, provides a homogeneous distribution of doping elements and a uniform distribution of modifiers over the grain surface [15]. The present paper demonstrates the manufacturing and testing of properties of a semiconductor gas sensor based on a MEMS crystal with micro-heating structure with a thick-film porous sensing element made of nanocrystalline SnO2 deposited by the method of inkjet micro-printing from a stable suspension. The suspension was obtained by suspending of SnO2 obtained by flame spray pyrolysis in ethylene glycol using a simple ultrasonic treatment.
METHODS AND MATERIALS
Ultrafine nanocrystalline SnO2 has been synthesized by flame spray pyrolysis [16]. The resulting powder after synthesis was annealed at 500 oC for 24 hours in air. The obtained material was studied by Х-Ray diffraction (XRD) with the use of Rigaku d/MAX-2500 diffractometer (Rigaku, Japan). The grain size was calculated using the Scherrer formula. The size and morphology of nanocrystals were studied by transmission electron microscopy (TEM) using a Libra 200 mc microscope (Zeiss, Germany) at an accelerating voltage of 200 kV with an Ultra Scan 4000 CCD camera (Gatan, USA). Image analysis was performed using the ImageJ software package. The sizes of particle agglomerates were estimated by dynamic light scattering technique on the Malvern Zetasizer Nano ZS analyzer (Malvern Analytical, great Britain). The specific surface area was determined by the method of low-temperature nitrogen adsorption and the BET model calculation using the Chemisorb 2750 device (Micromeritics, USA). The suspension for printing of gas sensitive layers was prepared by ultrasonic treatment in an Elmasonic s15h bath (Elma, Germany) for 1 hour at room temperature. To prepare the suspension, 50 mg of annealed SnO2 powder were suspended in a mixture of ethylene glycol and water in a 9:1. The sensitive layer was deposited using a piezoelectric micro-dispenser NanoTip-HV (Gesim, Germany). The frequency, duration, and amplitude of the pulses are 20 Hz, 40μs, and 60 V, respectively.
The calculated mass of the applied dry matter was 100 ng. The deposition was made on top of the manufactured square 2 × 2 mm MEMS crystals. MEMS crystal structure has thin (~1.5 μm) dielectric membrane, which bears electrically insulated from each other thin film metallic micro-heating element and Pt electrodes. After deposition, the binder was removed by ramp-heating of the micro-heater to a temperature of 400 oC. To form a stable thick porous film, the entire MEMS crystal was subsequently annealed at a temperature of 400 °C in the air. The morphology of the deposited sensitive layer was investigated by scanning electron microscopy (SEM) with the use of NVision 40 microscope (Zeiss, Germany). After annealing, the crystals were fixed for sensor measurements in TO-5 cases (Mars, Russia) using ultrasonic welding. The gas sensitivity of the obtained materials was studied in relation to a set of gases – CO, H2; NH3, NO2, H2S, acetone, toluene and methanol. Constant streams of gases with a given concentration in the air were fed into a PTFE sealed sensor chamber. Certified gas cylinders were used to set a fixed gas concentration (Monitoring, Russia). Clean air from the clean air generator GCHV 1.2 (Himelektronika, Russia) was used to dilute the gases down to the desired concentration. Dilution was performed using precision mass gas flow controllers (Bronkhorst, Netherlands). The measurements were carried out at a fixed humidity set generator humidity Cellkraft P-2 (Cellkraft, Sweden) To calculate the magnitude of the sensor response the ratio of the difference in electrical conductance of the sensitive layer in a current of pure air and air with gas admixture in relation to the conductance in the clean air was used.
RESULTS
According to the XRD (Fig.1b), SnO2 with a rutile structure and an average grain size of about 14 nm was obtained during synthesis. This size corresponds to the average size of SnO2 nanoparticles (Fig.1b), which have a shape close to spherical. The value of the specific surface area of the obtained nanocrystalline SnO2 according to BET data is 27 m2/g. The size of agglomerates of nanocrystals in the powder is about 120–140 nm.
The substance deposited as a suspension is distributed over the surface of the circular heated area of the membrane over the measuring contacts (Fig.2a). After the binder removal, the film of the sensitive element represents an even, uniform layer without formation of a so-called "coffee spot" (Fig.2a). The formed sensitive layer has a submicron thickness and has a developed pore structure (Fig.2b).
The resulting sensor element is most sensitive to acetone vapors and nitrogen dioxide gas (Fig. 3a). In this case, the response to NO2 has a negative value due to the oxidative chemical nature of this gas (Fig.3b) [17]. Also, a significant response is observed in relation to other gases and vapors of volatile organic compounds with pronounced reducing properties – hydrogen, hydrogen sulfide, and methanol. The response to CO, ammonia, and toluene is weak. The observed absolute values of sensory signals do not exceed the values described in the literature to the date [16]. It is worth noting that the concentration dependence of the values of the sensory response in relation to acetone and nitrogen dioxide has a small slope (Fig.3d-e).
The obtained sensing element has a high ratio of useful signal to the background response.
DISCUSSION
The resulting nanocrystalline SnO2 after high-temperature annealing at 500 oC has a relatively low dispersiveness, which differs from the materials obtained by flame spray pyrolysis [14]. Nevertheless, the uniform highly porous morphology of the formed film reflects the perspectivity of this approach for obtaining suspensions of metal oxide materials for further printing of functional layers and elements based on them. Despite the relatively low absolute values of the sensor response values, the resulting sensor element has a wide range of detectable concentrations due to the small angle of the sensor response concentration dependence slope. Moreover, the high signal to noise ratio allows reliably detect acetone at a concentration of less than 1 ppm, and NO2 at a concentration of less than 20 ppb.
CONCLUSIONS
The perspectivity of the flame spray pyrolysis method for the ultrafine metal oxides synthesis for functional elements and layers inkjet printing suspensions fabrication is demonstrated. The resulting layers based on nanocrystalline SnO2 have a well-controlled geometry and uniform porous morphology, which opens up the possibilities for scalable manufacturing of semiconductor sensors with high reproducibility of characteristics. The resulting gas sensor based on a micro-heating MEMS crystal has a high signal to noise ratio and allows to detect a wide range of gases in concentrations less than 1 ppm.
ACKNOWLEDGEMENTS
The reported study was funded by RFBR according to the research project № 18-33-20220. The facilities in the present work were used within the framework of the M.V. Lomonosov Moscow State University Program of Development. SEM measurements were performed using the equipment of the JRC PMR IGIC RAS. The study was performed using the equipment of core facility "Functional control and diagnostics of micro- and nanosystem technics" of Scientific-Manufacturing Complex "Technological Centre". ■
Metal oxide semiconductor gas sensors attract great interest due to the combination of their characteristics – high sensitivity, the possibility of long-term continuous operation. The use of MEMS structures in the design of such sensors provides low power consumption, miniaturization and low cost, which opens up prospects for wide application in mass consumer devices [1]. The key issue in the manufacture of such sensors is the process of assembly of a high surface area gas-sensitive metal oxide layer with an integrated structures manufactured using planar integrated circuit technologies [2]. One possible solution is the deposition of thick-film elements through a shadow mask directly during the synthesis of oxides [3]. However, the disadvantages of this approach include a sophisticated procedure of the substrate and the mask alignment, which makes it economically feasible to manufacture only large batches of sensors. In addition, it is worth noting the relatively high losses of the active substance during deposition and possible limitations of post-synthetic processing of gas-sensitive material associated with the design of the MEMS crystal.
In recent years, the use of the inkjet printing method in the manufacture of semiconductor gas sensors has been actively considered as an alternative approach [4, 5]. Some scientific groups are searching for approaches to manufacture all the functional elements of sensors in this way [6], but the greatest interest is observed particularly in the formation of a gas-sensitive element on the surface of MEMS crystal with a microheater manufactured via microelectronic technology [7–10]. The distinct feature of this approach is the versatility in terms of the applied semiconductor material – its chemical composition and structure, the possibility to use it in the manufacture of small batches and even single sensors, and the scalability of the approach for mass production. It is also worth noting the absence of losses of the active substance during its deposition.
The greatest challenge of metal oxide semiconductor layers inkjet printing is the production of the inks – stable suspensions of semiconductor material particles with the necessary parameters of viscosity and surface tension. The stability of such suspensions is determined both by the size of the grains of semiconductor oxides and the size of their agglomerates formed during heat treatment at the stage of material synthesis. Often, especially in the case of chemically modified, doped oxide materials synthesis for gas sensors, it is necessary to anneal such materials at temperatures of the order of 500 oC or higher, which leads to the growth of particles and consolidation of agglomerates. In this case long-term high-energy ball milling is often used [11, 12], which is aimed to reduce the size of agglomerates. However, this process is associated with the losses of the active substance and the risk of undesirable impurities introduction. An alternative approach consists in obtaining of highly dispersed materials during synthesis with the use of expendable templates and the use of polymer dispersants to stabilize suspensions [13], but this significantly increases the cost of obtaining gas-sensitive materials and complicates the printing process.
In this regard, the metal oxide materials synthesis for gas sensor application by flame spray pyrolysis is promising [14]. This approach allows to obtain metal oxides in the nanocrystalline form with a low degree of grain agglomeration, provides a homogeneous distribution of doping elements and a uniform distribution of modifiers over the grain surface [15]. The present paper demonstrates the manufacturing and testing of properties of a semiconductor gas sensor based on a MEMS crystal with micro-heating structure with a thick-film porous sensing element made of nanocrystalline SnO2 deposited by the method of inkjet micro-printing from a stable suspension. The suspension was obtained by suspending of SnO2 obtained by flame spray pyrolysis in ethylene glycol using a simple ultrasonic treatment.
METHODS AND MATERIALS
Ultrafine nanocrystalline SnO2 has been synthesized by flame spray pyrolysis [16]. The resulting powder after synthesis was annealed at 500 oC for 24 hours in air. The obtained material was studied by Х-Ray diffraction (XRD) with the use of Rigaku d/MAX-2500 diffractometer (Rigaku, Japan). The grain size was calculated using the Scherrer formula. The size and morphology of nanocrystals were studied by transmission electron microscopy (TEM) using a Libra 200 mc microscope (Zeiss, Germany) at an accelerating voltage of 200 kV with an Ultra Scan 4000 CCD camera (Gatan, USA). Image analysis was performed using the ImageJ software package. The sizes of particle agglomerates were estimated by dynamic light scattering technique on the Malvern Zetasizer Nano ZS analyzer (Malvern Analytical, great Britain). The specific surface area was determined by the method of low-temperature nitrogen adsorption and the BET model calculation using the Chemisorb 2750 device (Micromeritics, USA). The suspension for printing of gas sensitive layers was prepared by ultrasonic treatment in an Elmasonic s15h bath (Elma, Germany) for 1 hour at room temperature. To prepare the suspension, 50 mg of annealed SnO2 powder were suspended in a mixture of ethylene glycol and water in a 9:1. The sensitive layer was deposited using a piezoelectric micro-dispenser NanoTip-HV (Gesim, Germany). The frequency, duration, and amplitude of the pulses are 20 Hz, 40μs, and 60 V, respectively.
The calculated mass of the applied dry matter was 100 ng. The deposition was made on top of the manufactured square 2 × 2 mm MEMS crystals. MEMS crystal structure has thin (~1.5 μm) dielectric membrane, which bears electrically insulated from each other thin film metallic micro-heating element and Pt electrodes. After deposition, the binder was removed by ramp-heating of the micro-heater to a temperature of 400 oC. To form a stable thick porous film, the entire MEMS crystal was subsequently annealed at a temperature of 400 °C in the air. The morphology of the deposited sensitive layer was investigated by scanning electron microscopy (SEM) with the use of NVision 40 microscope (Zeiss, Germany). After annealing, the crystals were fixed for sensor measurements in TO-5 cases (Mars, Russia) using ultrasonic welding. The gas sensitivity of the obtained materials was studied in relation to a set of gases – CO, H2; NH3, NO2, H2S, acetone, toluene and methanol. Constant streams of gases with a given concentration in the air were fed into a PTFE sealed sensor chamber. Certified gas cylinders were used to set a fixed gas concentration (Monitoring, Russia). Clean air from the clean air generator GCHV 1.2 (Himelektronika, Russia) was used to dilute the gases down to the desired concentration. Dilution was performed using precision mass gas flow controllers (Bronkhorst, Netherlands). The measurements were carried out at a fixed humidity set generator humidity Cellkraft P-2 (Cellkraft, Sweden) To calculate the magnitude of the sensor response the ratio of the difference in electrical conductance of the sensitive layer in a current of pure air and air with gas admixture in relation to the conductance in the clean air was used.
RESULTS
According to the XRD (Fig.1b), SnO2 with a rutile structure and an average grain size of about 14 nm was obtained during synthesis. This size corresponds to the average size of SnO2 nanoparticles (Fig.1b), which have a shape close to spherical. The value of the specific surface area of the obtained nanocrystalline SnO2 according to BET data is 27 m2/g. The size of agglomerates of nanocrystals in the powder is about 120–140 nm.
The substance deposited as a suspension is distributed over the surface of the circular heated area of the membrane over the measuring contacts (Fig.2a). After the binder removal, the film of the sensitive element represents an even, uniform layer without formation of a so-called "coffee spot" (Fig.2a). The formed sensitive layer has a submicron thickness and has a developed pore structure (Fig.2b).
The resulting sensor element is most sensitive to acetone vapors and nitrogen dioxide gas (Fig. 3a). In this case, the response to NO2 has a negative value due to the oxidative chemical nature of this gas (Fig.3b) [17]. Also, a significant response is observed in relation to other gases and vapors of volatile organic compounds with pronounced reducing properties – hydrogen, hydrogen sulfide, and methanol. The response to CO, ammonia, and toluene is weak. The observed absolute values of sensory signals do not exceed the values described in the literature to the date [16]. It is worth noting that the concentration dependence of the values of the sensory response in relation to acetone and nitrogen dioxide has a small slope (Fig.3d-e).
The obtained sensing element has a high ratio of useful signal to the background response.
DISCUSSION
The resulting nanocrystalline SnO2 after high-temperature annealing at 500 oC has a relatively low dispersiveness, which differs from the materials obtained by flame spray pyrolysis [14]. Nevertheless, the uniform highly porous morphology of the formed film reflects the perspectivity of this approach for obtaining suspensions of metal oxide materials for further printing of functional layers and elements based on them. Despite the relatively low absolute values of the sensor response values, the resulting sensor element has a wide range of detectable concentrations due to the small angle of the sensor response concentration dependence slope. Moreover, the high signal to noise ratio allows reliably detect acetone at a concentration of less than 1 ppm, and NO2 at a concentration of less than 20 ppb.
CONCLUSIONS
The perspectivity of the flame spray pyrolysis method for the ultrafine metal oxides synthesis for functional elements and layers inkjet printing suspensions fabrication is demonstrated. The resulting layers based on nanocrystalline SnO2 have a well-controlled geometry and uniform porous morphology, which opens up the possibilities for scalable manufacturing of semiconductor sensors with high reproducibility of characteristics. The resulting gas sensor based on a micro-heating MEMS crystal has a high signal to noise ratio and allows to detect a wide range of gases in concentrations less than 1 ppm.
ACKNOWLEDGEMENTS
The reported study was funded by RFBR according to the research project № 18-33-20220. The facilities in the present work were used within the framework of the M.V. Lomonosov Moscow State University Program of Development. SEM measurements were performed using the equipment of the JRC PMR IGIC RAS. The study was performed using the equipment of core facility "Functional control and diagnostics of micro- and nanosystem technics" of Scientific-Manufacturing Complex "Technological Centre". ■
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