EP4437334A1 - Gas sensor - Google Patents

Gas sensor

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Publication number
EP4437334A1
EP4437334A1 EP22826410.7A EP22826410A EP4437334A1 EP 4437334 A1 EP4437334 A1 EP 4437334A1 EP 22826410 A EP22826410 A EP 22826410A EP 4437334 A1 EP4437334 A1 EP 4437334A1
Authority
EP
European Patent Office
Prior art keywords
gas sensor
sol
solution
insulating substrate
stage
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP22826410.7A
Other languages
German (de)
French (fr)
Inventor
Roberto Bernasconi
Luca Magagnin
Laura Maria Teresa CAPELLI
Carmen BAX
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Politecnico di Milano
Original Assignee
Politecnico di Milano
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Politecnico di Milano filed Critical Politecnico di Milano
Publication of EP4437334A1 publication Critical patent/EP4437334A1/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
    • G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
    • G01N27/125—Composition of the body, e.g. the composition of its sensitive layer

Definitions

  • the present invention relates to a gas sensor configured to detect gas, to identify chemical species dispersed in an environment.
  • the present invention is usefully employed to realize electronic noses for the recognition of simple or complex odors in certain environments, for example to detect the presence of dangerous gases in industrial environments, such as refineries, or more generally to detect pollutants for air quality evaluation.
  • Gas sensors based on different operating principles are known in the background art.
  • electrochemical, optical, catalytic or chemoresi stive gas sensors are known, for example, electrochemical, optical, catalytic or chemoresi stive gas sensors.
  • the latter envisage the presence of an at least partly conductive, active layer, bridged between two electrodes.
  • active layer modifies its electrical properties upon contacting with certain substances, leading to a change in resistance, which generally decreases.
  • the materials employed to realize the active layers belong to several categories as, for example, the semiconductor metal oxides, such as ZnO, SnCh, TiCh and CuO.
  • the active layers are deposited in film form on ceramic substrates by means of techniques such as screen printing, sputtering or drop casting.
  • Patent Document CN 103675028 A describes a gas sensor based on such principle, comprising an active layer deposited on an insulating substrate.
  • the insulating substrate is inkjet printed. Heating electrodes and conductive electrodes made of gold, silver, platinum, or so, are deposited on such substrate.
  • the active material is given by a nickel oxide film functionalized with carbon nanotubes and electrically connects the conductive electrodes each other.
  • a further object of the present invention is to make available a gas sensor proving to be stable and maintaining its sensitivity over time, in order to provide an electronic nose, whose values are reliable over time.
  • Object of the present invention is also to make available a gas sensor which is simply and inexpensively produced.
  • a gas sensor comprising:
  • - a pattern of conductive lines deposited on the insulating substrate and defining at least two electrodes.
  • the at least one functional layer deposited on the insulating substrate and on at least part of each of both electrodes comprises a semiconductor metal oxide selected from zinc oxide, titanium dioxide and tin dioxide and combinations thereof.
  • This/These layer(s) is/are obtained by a process comprising inkjet printing of an ink comprising semiconductor metal oxide precursors in sol -gel or nanoparticle dispersion form, and subsequent heat treatment by annealing.
  • this annealing treatment is especially characterized in that the cooling method is common to both types, this being a gradual cooling taking place partly in the furnace and partly in the air.
  • the annealing treatment comprises the following steps: i) when the precursor is a nanoparticle dispersion, said annealing comprises the following steps: a first step envisaging heating from room temperature to 200 °C with a first temperature rise ramp of 10 °C/min, a second step at constant temperature of 200 °C for a time period of 60 minutes, a final step of slow cooling in the furnace up to 75 °C, thereafter the sample being left in the air until reaching the room temperature, ii) when the precursor is a sol-gel, said annealing comprises the following steps: a first step envisaging heating from room temperature to 200 °C with a first temperature rise ramp of 10 °C/min, an intermediate step at constant temperature for a time period of 15 minutes at 200 °C, a second step of heating from 200 to 400°C with a second temperature rise ramp of 10 °C/min, a further intermediate step at constant temperature for a time period of 60 minutes at 400 °C, a final step of furnace slow cooling
  • FIG. 1 schematically shows a possible gas sensor architecture according to the present invention
  • FIGS. 2a-2c graphically show how the resistance changes, in a predetermined time range, in an embodiment of the sensor gas according to the present invention, when put into contact, respectively, with n-butanol, dimethyl disulphide and trimethylamine at different concentrations;
  • FIGS. 3a-3c graphically show the R/Ro pattern for different embodiments of a gas sensor according to the present invention when exposed to increasing amounts of n- butanol;
  • FIG. 4 graphically shows the temperature pattern over time in the final cooling step in annealing treatments intended for gas sensors according to the present invention, which are obtained through sol-gel precursors;
  • FIG. 5 graphically shows the temperature pattern over time in the first heating step and in the second step at constant temperature in annealing treatments intended for gas sensors according to the present invention, which are obtained through nanoparticle dispersions;
  • FIG. 6 graphically shows the temperature pattern over time in the first heating step, followed by the step at constant temperature, by the second heating step and by the further step at constant temperature, in annealing treatments intended for gas sensors according to the present invention, which are obtained through sol-gel precursors;
  • FIG. 7 graphically shows the desorption isotherms of the sensors as a function of the relative pressure P/Po (Po represents the saturation pressure of 759,339 mmHg).
  • the bottom curve identified by small square markers represents the desorption isotherm for the sensor according to the present invention, in which the functional layer is ZnO deposited by inkjet printing;
  • the middle curve identified by small circle markers represents the desorption isotherm for the commercial sensor Figaro TGS2600;
  • the top curve identified by small square markers represents the desorption isotherm for an alumina substrate (AI2O3) without active material;
  • FIGS. 8a and 8b show the microstructure of the commercial sensor Figaro TGS2600, obtained through scanning electron microscopy (SEM), at 100,000x and 400,000x magnification, respectively;
  • FIGS. 9a and 9b show the microstructure of the sensor according to the present invention, in which the functional layer is ZnO deposited by inkjet printing, obtained through scanning electron microscopy (SEM), at 100,000x and 500,000x magnification, respectively;
  • the subject matter of the present invention is a gas sensor hereinafter designated by reference number 1.
  • the gas sensor 1 according to the present invention is configured to detect a gas and is especially useful to realize electronic noses. Accordingly, also encompassed in the scope of the present invention is an electronic nose comprising the gas sensor described below.
  • the gas sensor 1 according to the present invention has an architecture corresponding to that of known gas sensor, as shown for example in Figure 1.
  • the gas sensor 1 comprises an insulating substrate 2.
  • the insulating substrate 2 comprises a first surface 21 and an opposite second surface 22.
  • the insulating substrate 2 is given by an inorganic material. Still preferably, the insulating substrate 2 is given by an inert material capable of sustaining high temperatures. Accordingly, the insulating substrate 2 is preferably made of ceramic material. For example, the insulating substrate 2 is made of alumina.
  • the gas sensor 1 comprises a pattern of conductive lines 3 deposited on the insulating substrate 2. Such conductive lines 3 are deposited in particular on the first surface 21. Preferably, the conductive lines 3 have a symmetrical arrangement.
  • the conductive lines 3 define at least two conductive electrodes 31.
  • Such conductive electrodes 31 are made of noble metals, such as gold, platinum, palladium, and the like.
  • the conductive electrodes 31 are spaced apart one from the other by a strip of insulating substrate 2.
  • the conductive lines 3 define also a heating electrode 32 acting as a resistance and configured in order to provide a heat field. Accordingly, the gas sensor 1 comprises such heating electrode 32.
  • the heating electrode 32 is also made of noble metals, such as gold, platinum, palladium, and the like.
  • the heating electrode 32 has, for example, a coil arrangement.
  • the conductive electrodes 31 are at least partly enclosed by the heating electrode 32.
  • the gas sensor 1 comprises further at least one functional layer 4.
  • the functional layer 4, or active layer is deposited on the insulating substrate 2 and on at least part of each of both conductive electrodes 31.
  • the functional layer 4 is bridged between the conductive electrodes 31 and is arranged to electrically connect both conductive electrodes 31.
  • the conductive electrodes 31 are interposed between the functional layer 4 and the insulating substrate 2 at places in which the functional layer 4 is placed on the conductive electrodes 31. Otherwise, the functional layer 4 contacts the first surface 21 of the insulating substrate 2.
  • the functional layer 4 comprises a metal oxide which is a semiconductor.
  • the semiconductor metal oxide is selected from zinc oxide, titanium dioxide and tin dioxide and combinations thereof.
  • the gas sensor 1 may comprise one functional layer 4 or more functional layers 4 overlapped with each other. In case the gas sensor 1 comprises one functional layer 4, this may be a monomaterial or a mixed layer. In case the gas sensor 1 comprises instead more functional layers 4 overlapped with each other, these may be same or different one from another.
  • the at least one functional layer 4 is obtained, in particular, by inkjet printing of an ink comprising semiconductor metal oxide precursors.
  • the ink consists of such precursors.
  • the gas detection occurs by means of the conductive electrodes 31 made of noble metals deposited on the insulating substrate 2, on which the functional layer 4 comprising a semiconductor metal oxide has been deposited by inkjet printing, by detecting the resistance change when the current flows between the two conductive electrodes 31.
  • the precursors contained in the ink are a nanoparticle dispersion or sol-gel precursors.
  • a first embodiment of the present invention envisages obtaining the metal oxide/metal oxides of the functional layer 4 from sol-gel precursors.
  • a second embodiment of the present invention envisages instead obtaining the metal oxide/metal oxides from a nanoparticle dispersion.
  • sol-gel precursors the control on the crystallinity and on the final properties of the functional layer 4 is better, the printability is good and a possible doping is simpler.
  • the layers deposited with metal oxide are characterized by a higher purity, while the temperatures needed for heat treatments are lower.
  • the process to obtain the gas sensor according to the present invention comprises a first stage of a) preparing the sol-gel precursors or the nanoparticle dispersion.
  • the process comprises also a step of b) depositing by inkjet printing the nanoparticle dispersion or the sol-gel precursor(s) on the insulating substrate 2 and on at least part of the conductive electrodes 31.
  • Such stage b) may be repeated as many times as the number of functional layers 4 to be obtained.
  • the process envisages the annealing heat treatment according to the present invention.
  • the sol-gel precursors of zinc oxide and/or tin dioxide are prepared from a zinc and/or tin salt solution in an amount ranging between 10 and 30% by weight, preferably of 20%, in a mixture of a C2-C5 alcohol and C2-C4-alkylene glycol by a process comprising the steps described below.
  • a first step comprises a-1-1) dissolving the salt in said C2-C4-alkylene glycol.
  • a second step envisages a-1-2) adding the C2-C5 alcohol to the solution obtained in a- 1-1).
  • the process envisages a step of a- 1-3) sonicating, for a time period ranging between 10 and 50 minutes, the mixture from a-1-2), preferably for 30 minutes.
  • a subsequent step comprises a-1-4) removing the residue formed in stage a-1-3).
  • Such step is preferably performed by filtration with a paper filter.
  • the final solution obtained according to such steps is uniform and clear.
  • Such solution represents the sol-gel precursor which may be used as inkjet printing ink, in order to deposit a homogeneous metal oxide film, i.e. a functional layer 4.
  • the salt is preferably a zinc and/or tin chloride.
  • the salt is still preferably a zinc acetate and/or tin acetate. These acetates may be anhydrous or hydrates.
  • the solvent is a mixture of ethylene glycol and ethanol.
  • the C2-C4-alkylene glycol is ethylene glycol and the C2-C5 alcohol is ethanol.
  • the C2-C4-alkylene glycol exists in an amount of 30% by weight, based on the final weight of the solution.
  • the C2-C5 alcohol exists in an amount of 50% by weight, based on the final weight of the solution.
  • a step in which the mixture is stirred at temperature envisaged.
  • such step envisages stirring the mixture for a time period ranging between 20 and 40 minutes, preferably for 30 minutes, at temperatures ranging between 120 and 160 °C, preferably at 140 °C.
  • the sol-gel precursors of titanium dioxide are preferably prepared from a solution containing A) a C2-C5 titanium alkoxide at concentrations ranging between 30 and 70% by weight, based on the total weight of the solution.
  • the titanium alkoxide concentration is of 50% by weight, based on the total weight of the solution.
  • the solution contains also B) between 3 and 6% by weight of lactic acid, based on the total weight of the solution.
  • the solution comprises 4.3% by weight of lactic acid, based on the total weight of the solution.
  • the solution comprises further C) a mixture of C2-C4 alcohol and water.
  • the alkoxide is titanium isopropoxide and tert-butoxide.
  • the alcohol is isopropanol.
  • the water is demineralized water.
  • such titanium dioxide precursor solution is obtained by a process comprising the following steps.
  • a-2-1 mixing the C2-C5 titanium alkoxide with a C2-C4 alcohol, in order to form a first solution.
  • the process comprises then a second step of a-2-2) pouring the solution in water under stirring and stirring for some minutes.
  • the water is comprised in an amount ranging between 2 and 3 times the weight of C2-C5 titanium alkoxide.
  • the water exists in an amount of 2.5 times the weight of C2-C5 titanium alkoxide.
  • Such step envisages the solution being stirred for a time period ranging between 3 and 6 minutes, preferably for 4 or 5 minutes.
  • the solution is maintained under stirring until it becomes transparent.
  • lactic acid is added.
  • it is envisaged of a-2-3) stirring for 24 hours at a temperature ranging between 60 and 100 °C, preferably at 80 °C.
  • the titanium dioxide sol-gel precursors are prepared from a solution obtainable through the so-called “reverse micelles” approach.
  • a solution obtainable through the so-called “reverse micelles” approach.
  • Such solution comprises titanium alkoxides as well, however it envisages the use of a different solvent, such as xylene.
  • the xylene is preferably mixed with a surfactant, preferably a nonionic one.
  • the non-ionic surfactant is Triton-X.
  • Such solution comprises also deionized water.
  • such titanium dioxide precursor solution is obtained by a process comprising the following steps.
  • a-3-1) mixing the xylene and the surfactant under stirring, in order to form a first solution.
  • the process comprises then a second step of a-3-2) adding deionized water to the solution formed in a-3-1) and stirring for some minutes.
  • a step a-3-3) follows, in which titanium alkoxide is added dropwise to the solution formed in a-3-2).
  • the sol-gel precursors contain transition metal ions, preferably selected from iron ions, copper ions or nickel ions.
  • the transition metal ions are preferably added in salt form selected from chlorides, nitrates, acetates or sulphates.
  • the transition metal ions are added to the sol-gel precursors at concentrations ranging between 1 and 4 g/1.
  • the concentration of the transition metal ions in the solution is 2 g/1.
  • Such transition metal ions are added to the solution prior to step b) of depositing the precursor.
  • the nanoparticle dispersion is prepared by dispersing zinc oxide and/or tin dioxide and/or titanium dioxide nanoparticles in a mixture comprising water and a polyhydric alcohol or the C1-C4 monoalkyl ether thereof.
  • the nanoparticles have a diameter ranging between 1 and 100 nm.
  • the polyhydric alcohol is ethylene glycol.
  • the C1-C4 monoalkyl ether thereof is triethylene glycol monomethyl ether.
  • the mixture of polyhydric alcohol or the C1-C4 monoalkyl ether thereof and water comprises between 10 and 30% by weight of polyhydric alcohol or C1-C4 monoalkyl ether thereof, more preferably 20%.
  • the nanoparticles are contained in an amount ranging between 2% and 4% by weight based on the total weight of the dispersion.
  • the nanoparticles are contained in an amount of 3% by weight, based on the total weight of the dispersion.
  • the mixture comprises further a surfactant.
  • a surfactant exists in an amount ranging between 0.5% and 1.5%, preferably of 1% by weight, based on the total weight of the dispersion.
  • the surfactant is an anionic or neutral organic surfactant and is preferably selected from polyacrylic acid sodium salt, polyethylene glycol, polyvinyl alcohol.
  • Step a) of preparing the precursors i.e. the ink
  • the use of a piezoeletric or thermal print head can be envisaged.
  • the insulating substrate 2 can be subjected to treatments intended to improve its wettability, such as corona treatment.
  • the insulating substrate 2 may be maintained heated at a temperature ranging between 40 °C and 80 °C, preferably at 60 °C.
  • the precursors prepared according to the above described stages and steps exhibit a good stability over time, resulting in a high print quality.
  • the method comprises a step of performing a heat treatment, i.e. stage c) of annealing according to the present invention.
  • Figures 5-6 depict the individual heating stages using two different metal oxide precursors and Figure 4 depicts the cooling curve for the stage c) of annealing for functional layers 4 obtained through sol-gel precursors.
  • the semiconductor metal oxide is surface-functionalized with noble metal or transition metal nanoparticles. Such surface-functionalization is performed in order to improve the selectivity of the gas sensor 1 toward given substances and/or to improve its stability.
  • the method then optionally comprises, prior to the treatment of c) annealing and subsequent to step b) of depositing, a stage d) of functionalizing.
  • stage is performed dipping the gas sensor 1 in a solution containing nanoparticles of the noble metal or the transition metal with which the functional layer 4 is intended to be functionalized.
  • stage d) of functionalizing is performed depositing metal nanoparticles on the functional layer 4 which has been deposited on the insulating substrate 2.
  • Such nanoparticles may be deposited by various methods, such as sputtering or evaporation.
  • the gas sensors 1 realized according to the present invention exhibit a good sensitivity even at low gas concentrations.
  • Figures 2a-2c show how the resistance changes, in a predetermined time range, at different gas concentrations, in a gas sensor 1 according to the present invention, in which the functional layer 4 is put into contact with gases, such as n-butanol, dimethyl disulphide and trimethylamine, respectively.
  • the functional layer 4 has three TiCh layers obtained through sol-gel precursors according to Example 7, hereinafter described.
  • the printer comprises, in particular, a support for the substrate, which may be heated. Such support was heated until it reached 60 °C, before placing the substrate thereon.
  • the substrate was also previously corona treated for 4 minutes.
  • the substrate used is made of alumina comprising platinum conductive lines, among which two platinum electrodes.
  • the precursor was deposited on a surface of the substrate measuring 0.5 mm x 0.7 mm, in order to form the active layer.
  • a single metal oxide layer was formed.
  • a heat treatment in a furnace was performed as follows: a first heating ramp at 10 °C/min until reaching 200 °C, a step at constant temperature of 200 °C for 15 minutes, a second heating ramp at 10 °C/min until reaching 400 °C, a further step at constant temperature of 400 °C for 60 minutes, and a slow cooling inside the furnace until reaching 75 °C and subsequent cooling in the air until reaching room temperature.
  • Example 1 variations the above procedure, however with the deposition of three or six metal oxide layers.
  • Example 4 The solution was prepared similarly to Example 2. Printing and substrate similar to Example 1. The number of functional layers being deposited is six. After deposition, the active part thereby realized was dipped in a 0.05 M PdCh solution in ethanol. The heat treatment was performed after this step similarly to what described in Example 1.
  • Example 4
  • Gas sensor in which the active layer is SnCh obtained from sol-gel precursors starting from 90.25 g (i.e. 0.4 moles) of tin(II) acetate dihydrate dissolved in 120 ml of ethylene glycol. Such mixture was stirred at 140 °C for 30 minutes. Later, the mixture was cooled at room temperature and, upon completion of the cooling, 280 ml of ethanol were added. Then, the solution was sonicated for 30 minutes and filtered with a paper filter. 400 ml of solution, i.e. of precursor ink, were obtained overall. Such precursor was charged in a 10 pl cartridge of a Dimatix materials printer DMP-2850 (by Fujifilm).
  • the printer comprises, in particular, a support for the substrate, which may be heated. Such support was heated until it reached 60 °C, before placing the substrate thereon.
  • the substrate was also previously corona treated for 4 minutes.
  • the substrate used is made of alumina comprising platinum conductive lines, among which two platinum electrodes.
  • the precursor was deposited on a surface of the substrate measuring 0.5 mm x 0.7 mm, in order to form the active layer.
  • a heat treatment in a furnace was performed as follows: a first heating ramp at 10 °C/min until reaching 200 °C, a step at constant temperature of 200 °C for 15 minutes, a second heating ramp at 10 °C/min until reaching 400 °C, a further step at constant temperature of 400 °C for 60 minutes, and a slow cooling inside the furnace until reaching 75 °C and subsequent cooling in the air until reaching room temperature.
  • the mixed precursor was obtained from sol-gel precursors starting from 3.66 g (i.e. 0.017 moles) of zinc acetate dihydrate dissolved with 11.28 g (i.e. 0.05 moles) of tin chloride dihydrate in 30 ml of ethylene glycol. Such mixture was stirred at 140 °C for 30 minutes. Later, the mixture was cooled at room temperature and, upon completion of the cooling, 70 ml of ethanol were added. Then, the solution was sonicated for 30 minutes and filtered with a paper filter. 60 ml of solution, i.e. precursor ink, were obtained overall.
  • Gas sensor in which the active layer is ZnO obtained from nanoparticle suspension precursors.
  • the nanoparticle suspension was prepared from a mixture of 20% by weight ethylene glycol in water, to which 3% by weight ZnO nanoparticles and 1% by weight polyacrylic acid sodium salt were added. The suspension was stirred for 30 minutes and thereafter sonicated for 15 minutes. The suspension thus obtained was printed, printing and substrate being similar to those of the previous Examples.
  • the active material layers being deposited are one or two.
  • a heat treatment in a furnace was performed as follows: a first heating ramp at 10 °C/min until reaching 200 °C, a step at constant temperature of 200 °C for 60 minutes, and a slow cooling inside the furnace until reaching 75 °C and subsequent cooling in the air until reaching room temperature.
  • Gas sensor in which the active is TiCh obtained from sol-gel precursors starting from a mixture of 5.1 ml of xylene and 2.5 ml of Triton-X under stirring. Thereafter, to the mixture were added 60 pl of water. Such mixture was stirred for some minutes until it becomes transparent. Thereafter, 1 ml of titanium isopropoxide was added to the mixture, forming a yellow solution. Such solution represents the precursor ink, which was charged in a 10 pl cartridge of a Dimatix materials printer DMP-2850 (by Fujifilm).
  • Figures 3a-3c show graphs representing the R/Ro pattern for several examples of gas sensors according to the present invention when exposed to increasing amounts of n- butanol.
  • the sensors referred to in the following tables are all zinc oxide based.
  • thin metallic Zn films were deposited through RF magnetron sputtering (13.56 MHz) on 2 mm square alumina and silicon substrates. The thickness of the Zn thin films was 600 nm.
  • the Zn thin films were anodized in a two electrodes system at room temperature. A platinum sheet was used as counter electrode. The anodization was performed in a 2 M concentrated electrolytic solution of oxalic acid dihydrate (C2H2O4 2H2O) containing ethanol. The applied voltage was 30 V and the anodization time was 20 minutes.
  • the prepared samples were zinc oxalate dihydrates (ZnC2C>4 2H2O).
  • the samples were annealed in a tubular furnace at 400 °C in an atmosphere of 50% O2 and 50% Ar, for 5 h. 1 It should be noted that the efficiency is unchanged even after 10 months for the gas sensors realized according to the present invention. On the contrary, the commercial sensors are subject to a remarkable efficiency loss after few months.
  • the sensor according to the present invention is capable of desorbing a significantly higher volume (see Figure 7, bottom curve with small square markers) of material (mainly moisture) in relation to commercial sensors (middle graph with small circle markers), thus indicating a higher porosity level of the first one.
  • the measurement carried out on the alumina substrate (used as substrate for the sensors according to the present invention - top figure with small square markers) was performed as control test.

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Abstract

Described is a gas sensor comprising: - an insulating substrate (2); - a pattern of conductive lines (3) deposited on the insulating substrate (2) and defining at least two conductive electrodes (31); - at least one functional layer (4) deposited on said insulating substrate (2) and on at least part of each of both conductive electrodes (31), wherein said functional layer (4) comprises a semiconductor metal oxide selected from zinc oxide, titanium dioxide and tin dioxide and combinations thereof, wherein the at least one functional layer (4) is obtained by inkjet printing of an ink comprising precursors of said semiconductor metal oxide.

Description

TITLE: “Gas sensor”
DESCRIPTION
Technical field
The present invention relates to a gas sensor configured to detect gas, to identify chemical species dispersed in an environment. The present invention is usefully employed to realize electronic noses for the recognition of simple or complex odors in certain environments, for example to detect the presence of dangerous gases in industrial environments, such as refineries, or more generally to detect pollutants for air quality evaluation.
Background art
Gas sensors based on different operating principles are known in the background art. Among these are known, for example, electrochemical, optical, catalytic or chemoresi stive gas sensors. The latter, in particular, envisage the presence of an at least partly conductive, active layer, bridged between two electrodes. Such active layer modifies its electrical properties upon contacting with certain substances, leading to a change in resistance, which generally decreases. The materials employed to realize the active layers belong to several categories as, for example, the semiconductor metal oxides, such as ZnO, SnCh, TiCh and CuO.
The active layers are deposited in film form on ceramic substrates by means of techniques such as screen printing, sputtering or drop casting.
Patent Document CN 103675028 A describes a gas sensor based on such principle, comprising an active layer deposited on an insulating substrate. The insulating substrate is inkjet printed. Heating electrodes and conductive electrodes made of gold, silver, platinum, or so, are deposited on such substrate. The active material is given by a nickel oxide film functionalized with carbon nanotubes and electrically connects the conductive electrodes each other.
Problem of the known technique
Disadvantageously, the semiconductors employed in the above document are not suitable for realizing particularly stable and accurate gas sensors. Indeed, the sensors obtained from such materials exhibit detection limits in case the gas is present at very small, but anyway critical amounts.
Disadvantageously, the aforementioned deposition techniques, which are commonly used to realize the active layer, are very complex and expensive.
Summary of the invention
In this context, the technical task on which the present invention is based is to provide a gas sensor which overcomes the aforementioned drawbacks of the known art.
In particular, it is an object of the present invention to make available a gas sensor exhibiting a better sensitivity and a good selectivity to predetermined gases.
A further object of the present invention is to make available a gas sensor proving to be stable and maintaining its sensitivity over time, in order to provide an electronic nose, whose values are reliable over time.
Object of the present invention is also to make available a gas sensor which is simply and inexpensively produced.
The technical task and the specified objects are essentially attained by a gas sensor comprising:
- an insulating substrate;
- a pattern of conductive lines deposited on the insulating substrate and defining at least two electrodes. - The at least one functional layer deposited on the insulating substrate and on at least part of each of both electrodes comprises a semiconductor metal oxide selected from zinc oxide, titanium dioxide and tin dioxide and combinations thereof.
This/These layer(s) is/are obtained by a process comprising inkjet printing of an ink comprising semiconductor metal oxide precursors in sol -gel or nanoparticle dispersion form, and subsequent heat treatment by annealing.
Beside having specific heating operating conditions, this annealing treatment is especially characterized in that the cooling method is common to both types, this being a gradual cooling taking place partly in the furnace and partly in the air.
The annealing treatment comprises the following steps: i) when the precursor is a nanoparticle dispersion, said annealing comprises the following steps: a first step envisaging heating from room temperature to 200 °C with a first temperature rise ramp of 10 °C/min, a second step at constant temperature of 200 °C for a time period of 60 minutes, a final step of slow cooling in the furnace up to 75 °C, thereafter the sample being left in the air until reaching the room temperature, ii) when the precursor is a sol-gel, said annealing comprises the following steps: a first step envisaging heating from room temperature to 200 °C with a first temperature rise ramp of 10 °C/min, an intermediate step at constant temperature for a time period of 15 minutes at 200 °C, a second step of heating from 200 to 400°C with a second temperature rise ramp of 10 °C/min, a further intermediate step at constant temperature for a time period of 60 minutes at 400 °C, a final step of furnace slow cooling to 75 °C in the furnace and thereafter in the air to room temperature.
Description of the drawings
- FIG. 1 schematically shows a possible gas sensor architecture according to the present invention;
- FIGS. 2a-2c graphically show how the resistance changes, in a predetermined time range, in an embodiment of the sensor gas according to the present invention, when put into contact, respectively, with n-butanol, dimethyl disulphide and trimethylamine at different concentrations;
- FIGS. 3a-3c graphically show the R/Ro pattern for different embodiments of a gas sensor according to the present invention when exposed to increasing amounts of n- butanol;
- FIG. 4 graphically shows the temperature pattern over time in the final cooling step in annealing treatments intended for gas sensors according to the present invention, which are obtained through sol-gel precursors;
- FIG. 5 graphically shows the temperature pattern over time in the first heating step and in the second step at constant temperature in annealing treatments intended for gas sensors according to the present invention, which are obtained through nanoparticle dispersions;
- FIG. 6 graphically shows the temperature pattern over time in the first heating step, followed by the step at constant temperature, by the second heating step and by the further step at constant temperature, in annealing treatments intended for gas sensors according to the present invention, which are obtained through sol-gel precursors;
- FIG. 7 graphically shows the desorption isotherms of the sensors as a function of the relative pressure P/Po (Po represents the saturation pressure of 759,339 mmHg). The bottom curve identified by small square markers represents the desorption isotherm for the sensor according to the present invention, in which the functional layer is ZnO deposited by inkjet printing; the middle curve identified by small circle markers represents the desorption isotherm for the commercial sensor Figaro TGS2600; the top curve identified by small square markers represents the desorption isotherm for an alumina substrate (AI2O3) without active material;
- FIGS. 8a and 8b show the microstructure of the commercial sensor Figaro TGS2600, obtained through scanning electron microscopy (SEM), at 100,000x and 400,000x magnification, respectively;
- FIGS. 9a and 9b show the microstructure of the sensor according to the present invention, in which the functional layer is ZnO deposited by inkjet printing, obtained through scanning electron microscopy (SEM), at 100,000x and 500,000x magnification, respectively;
Detailed description of the invention
For the purposes of the present invention, the terms “comprising” and “containing” do not exclude the presence of further components in addition to those specifically listed after such term, while the terms “composed of’ and “consisting of’ exclude the presence of further components in addition to those listed after such term.
The subject matter of the present invention is a gas sensor hereinafter designated by reference number 1. The gas sensor 1 according to the present invention is configured to detect a gas and is especially useful to realize electronic noses. Accordingly, also encompassed in the scope of the present invention is an electronic nose comprising the gas sensor described below.
The gas sensor 1 according to the present invention has an architecture corresponding to that of known gas sensor, as shown for example in Figure 1.
The gas sensor 1 comprises an insulating substrate 2. The insulating substrate 2 comprises a first surface 21 and an opposite second surface 22.
Preferably, the insulating substrate 2 is given by an inorganic material. Still preferably, the insulating substrate 2 is given by an inert material capable of sustaining high temperatures. Accordingly, the insulating substrate 2 is preferably made of ceramic material. For example, the insulating substrate 2 is made of alumina.
The gas sensor 1 comprises a pattern of conductive lines 3 deposited on the insulating substrate 2. Such conductive lines 3 are deposited in particular on the first surface 21. Preferably, the conductive lines 3 have a symmetrical arrangement.
In particular, the conductive lines 3 define at least two conductive electrodes 31. Such conductive electrodes 31 are made of noble metals, such as gold, platinum, palladium, and the like. The conductive electrodes 31 are spaced apart one from the other by a strip of insulating substrate 2.
The conductive lines 3 define also a heating electrode 32 acting as a resistance and configured in order to provide a heat field. Accordingly, the gas sensor 1 comprises such heating electrode 32. Preferably, the heating electrode 32 is also made of noble metals, such as gold, platinum, palladium, and the like. The heating electrode 32 has, for example, a coil arrangement.
The conductive electrodes 31 are at least partly enclosed by the heating electrode 32. The gas sensor 1 comprises further at least one functional layer 4. The functional layer 4, or active layer, is deposited on the insulating substrate 2 and on at least part of each of both conductive electrodes 31. In particular, the functional layer 4 is bridged between the conductive electrodes 31 and is arranged to electrically connect both conductive electrodes 31. More in detail, the conductive electrodes 31 are interposed between the functional layer 4 and the insulating substrate 2 at places in which the functional layer 4 is placed on the conductive electrodes 31. Otherwise, the functional layer 4 contacts the first surface 21 of the insulating substrate 2.
The functional layer 4 comprises a metal oxide which is a semiconductor. According to the present invention, the semiconductor metal oxide is selected from zinc oxide, titanium dioxide and tin dioxide and combinations thereof. In particular, the gas sensor 1 may comprise one functional layer 4 or more functional layers 4 overlapped with each other. In case the gas sensor 1 comprises one functional layer 4, this may be a monomaterial or a mixed layer. In case the gas sensor 1 comprises instead more functional layers 4 overlapped with each other, these may be same or different one from another.
The at least one functional layer 4 is obtained, in particular, by inkjet printing of an ink comprising semiconductor metal oxide precursors. Preferably, the ink consists of such precursors.
In the gas sensor 1, the gas detection occurs by means of the conductive electrodes 31 made of noble metals deposited on the insulating substrate 2, on which the functional layer 4 comprising a semiconductor metal oxide has been deposited by inkjet printing, by detecting the resistance change when the current flows between the two conductive electrodes 31. More in detail, according to the present invention, the precursors contained in the ink are a nanoparticle dispersion or sol-gel precursors.
Such precursors are alternative one to the other, therefore, a first embodiment of the present invention envisages obtaining the metal oxide/metal oxides of the functional layer 4 from sol-gel precursors.
A second embodiment of the present invention envisages instead obtaining the metal oxide/metal oxides from a nanoparticle dispersion.
These two precursors provide different benefits; for example, in case of sol-gel precursors, the control on the crystallinity and on the final properties of the functional layer 4 is better, the printability is good and a possible doping is simpler. On the other hand, in case of nanoparticles, the layers deposited with metal oxide are characterized by a higher purity, while the temperatures needed for heat treatments are lower.
Preferably, the process to obtain the gas sensor according to the present invention comprises a first stage of a) preparing the sol-gel precursors or the nanoparticle dispersion.
Still preferably, the process comprises also a step of b) depositing by inkjet printing the nanoparticle dispersion or the sol-gel precursor(s) on the insulating substrate 2 and on at least part of the conductive electrodes 31. Such stage b) may be repeated as many times as the number of functional layers 4 to be obtained.
Then, the process envisages the annealing heat treatment according to the present invention.
According to the first embodiment, preferably the sol-gel precursors of zinc oxide and/or tin dioxide are prepared from a zinc and/or tin salt solution in an amount ranging between 10 and 30% by weight, preferably of 20%, in a mixture of a C2-C5 alcohol and C2-C4-alkylene glycol by a process comprising the steps described below. A first step comprises a-1-1) dissolving the salt in said C2-C4-alkylene glycol. Then, a second step envisages a-1-2) adding the C2-C5 alcohol to the solution obtained in a- 1-1).
Hereinafter, the process envisages a step of a- 1-3) sonicating, for a time period ranging between 10 and 50 minutes, the mixture from a-1-2), preferably for 30 minutes.
A subsequent step comprises a-1-4) removing the residue formed in stage a-1-3). Such step is preferably performed by filtration with a paper filter.
The final solution obtained according to such steps is uniform and clear. Such solution represents the sol-gel precursor which may be used as inkjet printing ink, in order to deposit a homogeneous metal oxide film, i.e. a functional layer 4.
In particular, the salt is preferably a zinc and/or tin chloride. Alternatively, the salt is still preferably a zinc acetate and/or tin acetate. These acetates may be anhydrous or hydrates.
According to the present invention, it is envisaged that different chlorides or acetates are also mixed together, giving mixed solutions.
Preferably, the solvent is a mixture of ethylene glycol and ethanol.
Accordingly, the C2-C4-alkylene glycol is ethylene glycol and the C2-C5 alcohol is ethanol.
Yet preferably, the C2-C4-alkylene glycol exists in an amount of 30% by weight, based on the final weight of the solution. Still preferably, the C2-C5 alcohol exists in an amount of 50% by weight, based on the final weight of the solution.
Optionally, between step a-1-1) and step a-1-2), a step in which the mixture is stirred at temperature is envisaged. Preferably, such step envisages stirring the mixture for a time period ranging between 20 and 40 minutes, preferably for 30 minutes, at temperatures ranging between 120 and 160 °C, preferably at 140 °C. Still according to the first embodiment, the sol-gel precursors of titanium dioxide are preferably prepared from a solution containing A) a C2-C5 titanium alkoxide at concentrations ranging between 30 and 70% by weight, based on the total weight of the solution. Preferably, the titanium alkoxide concentration is of 50% by weight, based on the total weight of the solution.
Optionally, the solution contains also B) between 3 and 6% by weight of lactic acid, based on the total weight of the solution. Preferably, the solution comprises 4.3% by weight of lactic acid, based on the total weight of the solution.
The solution comprises further C) a mixture of C2-C4 alcohol and water.
Preferably, the alkoxide is titanium isopropoxide and tert-butoxide. Still preferably, the alcohol is isopropanol. Yet preferably, the water is demineralized water.
Preferably, such titanium dioxide precursor solution is obtained by a process comprising the following steps.
According to a first step, it is envisaged of a-2-1) mixing the C2-C5 titanium alkoxide with a C2-C4 alcohol, in order to form a first solution.
The process comprises then a second step of a-2-2) pouring the solution in water under stirring and stirring for some minutes. In particular, the water is comprised in an amount ranging between 2 and 3 times the weight of C2-C5 titanium alkoxide. Preferably, the water exists in an amount of 2.5 times the weight of C2-C5 titanium alkoxide. Such step envisages the solution being stirred for a time period ranging between 3 and 6 minutes, preferably for 4 or 5 minutes. In particular, the solution is maintained under stirring until it becomes transparent. Optionally, lactic acid is added. According to a third step, it is envisaged of a-2-3) stirring for 24 hours at a temperature ranging between 60 and 100 °C, preferably at 80 °C. Alternatively, the titanium dioxide sol-gel precursors are prepared from a solution obtainable through the so-called “reverse micelles” approach. Such solution comprises titanium alkoxides as well, however it envisages the use of a different solvent, such as xylene. Moreover, the xylene is preferably mixed with a surfactant, preferably a nonionic one. For example, the non-ionic surfactant is Triton-X. Such solution comprises also deionized water.
Preferably, such titanium dioxide precursor solution is obtained by a process comprising the following steps.
According to a first step, it is envisaged of a-3-1) mixing the xylene and the surfactant under stirring, in order to form a first solution.
The process comprises then a second step of a-3-2) adding deionized water to the solution formed in a-3-1) and stirring for some minutes.
A step a-3-3) follows, in which titanium alkoxide is added dropwise to the solution formed in a-3-2).
Still according to the first embodiment envisaging sol-gel precursors, the sol-gel precursors contain transition metal ions, preferably selected from iron ions, copper ions or nickel ions. The transition metal ions are preferably added in salt form selected from chlorides, nitrates, acetates or sulphates. Specifically, the transition metal ions are added to the sol-gel precursors at concentrations ranging between 1 and 4 g/1. Preferably, the concentration of the transition metal ions in the solution is 2 g/1. Such transition metal ions are added to the solution prior to step b) of depositing the precursor.
Advantageously, it is possible to dope the solution, obtaining a final doped oxide, whose electrical features are better in respect to the same oxide, undoped. According to the second embodiment, preferably in the stage a) the nanoparticle dispersion is prepared by dispersing zinc oxide and/or tin dioxide and/or titanium dioxide nanoparticles in a mixture comprising water and a polyhydric alcohol or the C1-C4 monoalkyl ether thereof.
Preferably, the nanoparticles have a diameter ranging between 1 and 100 nm.
Still preferably, the polyhydric alcohol is ethylene glycol. Alternatively, the C1-C4 monoalkyl ether thereof is triethylene glycol monomethyl ether.
Preferably, the mixture of polyhydric alcohol or the C1-C4 monoalkyl ether thereof and water comprises between 10 and 30% by weight of polyhydric alcohol or C1-C4 monoalkyl ether thereof, more preferably 20%.
According to a preferred embodiment, the nanoparticles are contained in an amount ranging between 2% and 4% by weight based on the total weight of the dispersion. Preferably, the nanoparticles are contained in an amount of 3% by weight, based on the total weight of the dispersion.
The mixture comprises further a surfactant. Preferably, such surfactant exists in an amount ranging between 0.5% and 1.5%, preferably of 1% by weight, based on the total weight of the dispersion.
Still preferably, the surfactant is an anionic or neutral organic surfactant and is preferably selected from polyacrylic acid sodium salt, polyethylene glycol, polyvinyl alcohol.
Step a) of preparing the precursors, i.e. the ink, is followed by loading the ink in an inkjet printing set-up, in order to perform step b) of depositing by inkjet printing the sol-gel solution or the nanoparticle dispersion to realize a homogeneous film representing the functional layer 4.
For printing, the use of a piezoeletric or thermal print head can be envisaged. Optionally, prior to printing, the insulating substrate 2 can be subjected to treatments intended to improve its wettability, such as corona treatment.
Still preferably, during step b) of depositing the precursor by inkjet printing, the insulating substrate 2 may be maintained heated at a temperature ranging between 40 °C and 80 °C, preferably at 60 °C.
The precursors prepared according to the above described stages and steps exhibit a good stability over time, resulting in a high print quality.
After the deposition of the metal oxide film, the method comprises a step of performing a heat treatment, i.e. stage c) of annealing according to the present invention.
Figures 5-6 depict the individual heating stages using two different metal oxide precursors and Figure 4 depicts the cooling curve for the stage c) of annealing for functional layers 4 obtained through sol-gel precursors.
According to embodiments, the semiconductor metal oxide is surface-functionalized with noble metal or transition metal nanoparticles. Such surface-functionalization is performed in order to improve the selectivity of the gas sensor 1 toward given substances and/or to improve its stability.
Therefore, the method then optionally comprises, prior to the treatment of c) annealing and subsequent to step b) of depositing, a stage d) of functionalizing. Such stage is performed dipping the gas sensor 1 in a solution containing nanoparticles of the noble metal or the transition metal with which the functional layer 4 is intended to be functionalized. Alternatively, stage d) of functionalizing is performed depositing metal nanoparticles on the functional layer 4 which has been deposited on the insulating substrate 2. Such nanoparticles may be deposited by various methods, such as sputtering or evaporation. The gas sensors 1 realized according to the present invention exhibit a good sensitivity even at low gas concentrations. Figures 2a-2c show how the resistance changes, in a predetermined time range, at different gas concentrations, in a gas sensor 1 according to the present invention, in which the functional layer 4 is put into contact with gases, such as n-butanol, dimethyl disulphide and trimethylamine, respectively. In this specific case, the functional layer 4 has three TiCh layers obtained through sol-gel precursors according to Example 7, hereinafter described.
Practical realization examples
Example 1
Gas sensor in which the active layer is ZnO, obtained from sol-gel precursors starting from 0.4 moles of zinc acetate dihydrate dissolved in 120 ml of ethylene glycol. Such mixture was stirred at 140 °C for 30 minutes. Later, the mixture was cooled at room temperature and, upon completion of the cooling, 280 ml of ethanol were added. Then, the solution was sonicated for 30 minutes and filtered with a paper filter. 400 ml of solution, i.e. of precursor ink, were obtained overall. Such precursor was charged in a 10 pl cartridge of a Dimatix materials printer DMP-2850 (by Fujifilm).
The printer comprises, in particular, a support for the substrate, which may be heated. Such support was heated until it reached 60 °C, before placing the substrate thereon. The substrate was also previously corona treated for 4 minutes. The substrate used is made of alumina comprising platinum conductive lines, among which two platinum electrodes.
The precursor was deposited on a surface of the substrate measuring 0.5 mm x 0.7 mm, in order to form the active layer. In this case, a single metal oxide layer was formed. After printing, a heat treatment in a furnace was performed as follows: a first heating ramp at 10 °C/min until reaching 200 °C, a step at constant temperature of 200 °C for 15 minutes, a second heating ramp at 10 °C/min until reaching 400 °C, a further step at constant temperature of 400 °C for 60 minutes, and a slow cooling inside the furnace until reaching 75 °C and subsequent cooling in the air until reaching room temperature. Example 1 variations: the above procedure, however with the deposition of three or six metal oxide layers.
Example 2
Similar to Example 1 relative to solution preparation. 2 g/1 of copper chloride dihydrate were added to the final solution, obtaining the precursor ink.
Printing, substrate and heat treatment are similar to Example 1.
Example 3
The solution was prepared similarly to Example 2. Printing and substrate similar to Example 1. The number of functional layers being deposited is six. After deposition, the active part thereby realized was dipped in a 0.05 M PdCh solution in ethanol. The heat treatment was performed after this step similarly to what described in Example 1. Example 4
Gas sensor in which the active layer is SnCh, obtained from sol-gel precursors starting from 90.25 g (i.e. 0.4 moles) of tin(II) acetate dihydrate dissolved in 120 ml of ethylene glycol. Such mixture was stirred at 140 °C for 30 minutes. Later, the mixture was cooled at room temperature and, upon completion of the cooling, 280 ml of ethanol were added. Then, the solution was sonicated for 30 minutes and filtered with a paper filter. 400 ml of solution, i.e. of precursor ink, were obtained overall. Such precursor was charged in a 10 pl cartridge of a Dimatix materials printer DMP-2850 (by Fujifilm).
The printer comprises, in particular, a support for the substrate, which may be heated. Such support was heated until it reached 60 °C, before placing the substrate thereon. The substrate was also previously corona treated for 4 minutes. The substrate used is made of alumina comprising platinum conductive lines, among which two platinum electrodes.
The precursor was deposited on a surface of the substrate measuring 0.5 mm x 0.7 mm, in order to form the active layer.
After printing, a heat treatment in a furnace was performed as follows: a first heating ramp at 10 °C/min until reaching 200 °C, a step at constant temperature of 200 °C for 15 minutes, a second heating ramp at 10 °C/min until reaching 400 °C, a further step at constant temperature of 400 °C for 60 minutes, and a slow cooling inside the furnace until reaching 75 °C and subsequent cooling in the air until reaching room temperature.
Example 5
Gas sensor in which the active layer is a mixed layer ZnO/SnCh. The mixed precursor was obtained from sol-gel precursors starting from 3.66 g (i.e. 0.017 moles) of zinc acetate dihydrate dissolved with 11.28 g (i.e. 0.05 moles) of tin chloride dihydrate in 30 ml of ethylene glycol. Such mixture was stirred at 140 °C for 30 minutes. Later, the mixture was cooled at room temperature and, upon completion of the cooling, 70 ml of ethanol were added. Then, the solution was sonicated for 30 minutes and filtered with a paper filter. 60 ml of solution, i.e. precursor ink, were obtained overall.
Printing and substrate, as well as heat treatment, are similar to Example 1.
Example 6
Gas sensor in which the active layer is ZnO, obtained from nanoparticle suspension precursors. The nanoparticle suspension was prepared from a mixture of 20% by weight ethylene glycol in water, to which 3% by weight ZnO nanoparticles and 1% by weight polyacrylic acid sodium salt were added. The suspension was stirred for 30 minutes and thereafter sonicated for 15 minutes. The suspension thus obtained was printed, printing and substrate being similar to those of the previous Examples. The active material layers being deposited are one or two.
After printing, a heat treatment in a furnace was performed as follows: a first heating ramp at 10 °C/min until reaching 200 °C, a step at constant temperature of 200 °C for 60 minutes, and a slow cooling inside the furnace until reaching 75 °C and subsequent cooling in the air until reaching room temperature.
Example 7
Gas sensor in which the active is TiCh, obtained from sol-gel precursors starting from a mixture of 5.1 ml of xylene and 2.5 ml of Triton-X under stirring. Thereafter, to the mixture were added 60 pl of water. Such mixture was stirred for some minutes until it becomes transparent. Thereafter, 1 ml of titanium isopropoxide was added to the mixture, forming a yellow solution. Such solution represents the precursor ink, which was charged in a 10 pl cartridge of a Dimatix materials printer DMP-2850 (by Fujifilm).
Substrate and printing were similar to the previous Examples. The heat treatment performed after the deposition of the active layer is similar to Examples 1-5.
Figures 3a-3c show graphs representing the R/Ro pattern for several examples of gas sensors according to the present invention when exposed to increasing amounts of n- butanol.
Collected data
Hereinafter, tables representing how the efficiency varies over time will be reported for several gas sensors realized according to the present invention and for two commercial sensors, along with the drift times.
It should be pointed out that the sensors referred to in the following tables are all zinc oxide based. In order to prepare the two commercial sensors, thin metallic Zn films were deposited through RF magnetron sputtering (13.56 MHz) on 2 mm square alumina and silicon substrates. The thickness of the Zn thin films was 600 nm. The Zn thin films were anodized in a two electrodes system at room temperature. A platinum sheet was used as counter electrode. The anodization was performed in a 2 M concentrated electrolytic solution of oxalic acid dihydrate (C2H2O4 2H2O) containing ethanol. The applied voltage was 30 V and the anodization time was 20 minutes. The prepared samples were zinc oxalate dihydrates (ZnC2C>4 2H2O). Next, the samples were annealed in a tubular furnace at 400 °C in an atmosphere of 50% O2 and 50% Ar, for 5 h.1 It should be noted that the efficiency is unchanged even after 10 months for the gas sensors realized according to the present invention. On the contrary, the commercial sensors are subject to a remarkable efficiency loss after few months.
1Galstyan, V., Comini, E., Baratto, C., Ponzoni, A., Bontempi, E., Brisotto, M., Faglia, G., & Sberveglieri, G. (2013). Synthesis of self-assembled chain-like ZnO nanostructures on stiff and flexible substrates. CrystEngComm, 75(15), 2881-2887. https : //doi.org/10.1039/c3ce27011d
Sensor characterization
Desorption isotherms
Comparing the desorption isotherms depicted in the graph of Figure 7, it is apparent that the sensor according to the present invention is capable of desorbing a significantly higher volume (see Figure 7, bottom curve with small square markers) of material (mainly moisture) in relation to commercial sensors (middle graph with small circle markers), thus indicating a higher porosity level of the first one. The measurement carried out on the alumina substrate (used as substrate for the sensors according to the present invention - top figure with small square markers) was performed as control test.
Microscopic analysis (SEM)
Comparing Figures 8a-8b vs 9a-9b, characterized by similar and entirely comparable magnification levels, it is apparent that the sensor according to the present invention is characterized by nanometer-sized pores, small enough for not being resolved through SEM microscopy. Such nanometric porosity proves also to be uniform in the sample being analyzed. On the contrary, the commercial sensor seems to be characterized by pores which are significantly larger and less dimensionally uniform.
The image comparison indicates that the sensor according to the present invention is characterized by a higher and dimensionally finer porosity. This observation entirely confirms the previously discussed results obtained with the desorption isotherms (Figure 7).

Claims

1. Gas sensor (1) configured to detect a gas, comprising:
- an insulating substrate (2);
- a pattern of conductive lines (3) deposited on said insulating substrate (2) and defining at least two conductive electrodes (31);
- at least one functional layer (4) deposited on said insulating substrate (2) and on at least part of each of said at least two conductive electrodes (31), said functional layer (4) comprising a semiconductor metal oxide selected from zinc oxide, titanium dioxide and tin dioxide and combinations thereof, wherein said at least one functional layer (4) is obtained by a process comprising:
• inkjet printing with an ink comprising precursors of said semiconductor metal oxide in sol-gel or nanoparticle dispersion form, and
• subsequent heat treatment by annealing, wherein i) when the precursor is a nanoparticle dispersion, said annealing comprises the following steps: a first step envisaging heating from room temperature to 200 °C with a first temperature rise ramp of 10 °C/min, a second step at constant temperature of 200 °C for a time period of 60 minutes, a final step of slow cooling in the furnace to 75 °C, then in the air back to room temperature; or ii) when the precursor is a sol-gel, said annealing comprises the following steps: a first step envisaging heating from room temperature to 200 °C with a first temperature rise ramp of 10 °C/min, an intermediate step at constant temperature for a time period of 15 minutes at 200 °C, a second step of heating from 200 to 400°C with a second temperature rise ramp of 10 °C/min, a further intermediate step at constant temperature for a time period of 60 minutes at 400 °C, a final step of furnace slow cooling to 75 °C in the furnace and thereafter in the air to room temperature.
2. Gas sensor (1) according to claim 1, wherein said process comprises the following stages: a) preparing the sol-gel precursors or the nanoparticle dispersion; b) depositing by inkjet printing the nanoparticle dispersion or the sol -gel precursor(s) on the insulating substrate (2) and on at least part of said conductive electrodes (31); this stage being performed as many times as the number of functional layers (4) to be obtained, c) annealing the material from stage b).
3. Gas sensor (1) according to claim 2, wherein in stage a) of the process the sol -gel precursors of zinc oxide and/or tin dioxide comprising a zinc and/or tin salt solution in an amount ranging between 10 and 30% by weight, preferably of 20%, in a mixture of a C2-C5 alcohol and a C2-C4 alkylene glycol, are prepared by a process comprising the following steps: a-1-1) dissolving the salt in said C2-C4-alkylene glycol; a- 1-2) adding the C2-C5 alcohol to the solution obtained in a-1-1); a-1-3) sonicating for 30 minutes the mixture from a-1-2); a-1-4) removing the residue formed in stage a-1-3).
4. Gas sensor (1) according to claim 3, wherein the salt is a zinc and/or tin chloride and/or a zinc and/or tin acetate and the solvent is a mixture of ethylene glycol and ethanol.
5. Gas sensor (1) according to claim 2, wherein in stage a) sol-gel precursors of tin dioxide consisting of a solution containing:
A) a C2-C5 titanium alkoxide at concentrations ranging between 30 and 70% by weight, preferably of 50% by weight, based on the total weight of the solution,
B) between 3 and 6%, preferably 4.3% by weight, of lactic acid based on the total weight of the solution,
C) a mixture of C2-C4 alcohol and water, are prepared, said precursor being obtained by a process comprising the following steps: a-2-1) mixing said C2-C5 titanium alkoxide and C2-C4 alcohol to form a first solution, a-2-2) pouring the solution in water under stirring and stirring for 5 minutes, adding lactic acid, a-2-3) stirring for 24 hours at 80 °C.
6. Gas sensor (1) according to claim 5, wherein said alkoxide is titanium isopropoxide and tert-butoxide and said alcohol is isopropanol.
7. Gas sensor (1) according to any one of claims 1-6, wherein the sol-gel precursors contain transition metal ions, preferably selected from iron ions, copper ions or nickel ions, said transition metal ions being in the form of salts selected from chlorides, nitrates, acetates or sulphates; said transition metal ions being added to the sol-gel precursors at concentrations ranging between 1 and 4 g/1, preferably of 2 g/1.
8. Gas sensor (1) according to claim 2, wherein in stage a) said nanoparticle dispersion is prepared by dispersing zinc oxide and/or tin dioxide and/or titanium dioxide nanoparticles having a diameter preferably ranging between 1 and 100 nm in a mixture comprising water and a polyhydric alcohol or the C1-C4 monoalkyl ether thereof.
9. Gas sensor (1) according to claim 8, wherein the polyhydric alcohol is ethylene glycol or the C1-C4 monoalkyl ether thereof is ethylene glycol monomethyl ether.
10. Gas sensor (1) according to claim 8 or 9, wherein the nanoparticles are contained in an amount ranging between 2% and 4%, preferably of 3% by weight based on the total weight of the dispersion, said mixture comprising also a surfactant in an amount ranging between 0.5% and 1.5%, preferably of 1% by weight based on the total weight of the dispersion.
11. Gas sensor (1) according to claim 10, wherein said surfactant is an anionic or neutral organic surfactant and is preferably selected from polyacrylic acid sodium salt, polyethylene glycol, polyvinyl alcohol.
12. Gas sensor (1) according to any one of claims 1-11, wherein the semiconductor metal oxide is functionalized with noble metal or transition metal nanoparticles in a stage preceding stage c) of annealing.
13. Gas sensor (1) according to any one of claims 1-12, wherein the gas detection occurs through said conductive electrodes (31) made of noble metals deposited on said insulating substrate (2) and on which at least a functional layer (4) comprising a semiconductor oxide selected from the group consisting of zinc oxide, titanium dioxide and tin dioxide and combinations thereof has been deposited by inkjet printing.
14. Electronic nose comprising the gas sensor (1) according to any one of claims 1-13.
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