GAS SENSOR

Abstract

The present invention refers to a gas sensor comprising a hybrid material of perovskite and graphene, to the method for obtaining said sensor and to the gas detection method using said sensor.

Claims

1. Gas sensor comprising: a hybrid material comprising formula ABX.sub.3 metal halide perovskite nanocrystals, and graphene deposited on a substrate with electrodes where the perovskite nanocrystals are embedded in the graphene.

2. Gas sensor according to claim 1, where the graphene has less than 10% of oxygen functional groups.

3. Gas sensor according to claim 1, where A in the formula ABX.sub.3 is a cation selected from: methylammonium, formamidinium and cesium.

4. Sensor according to claim 1, where the perovskite nanocrystals are comprised between 6 and 8 nanometers in size.

5. Sensor according to claim 1, the substrate of which is made of alumina.

6. Sensor according to claim 1, where A in the formula ABX.sub.3 is methylammonium and X is bromine.

7. Sensor according to claim 1, where the gas to be detected is benzene and toluene, and A in the formula ABX.sub.3 is methylammonium.

8. Sensor according to claim 1, where the gas to be detected is NO.sub.2, and A in the formula ABX.sub.3 is formamidinium.

9. Sensor according to claim 1 where the gas to be detected is NH.sub.3, A in the formula ABX.sub.3 of the perovskite is methylammonium and X is a chlorine anion.

10. Gas sensor according to claim 1, where the halide of the perovskite is selected from chlorine and/or bromine and/or iodine.

11. Method for obtaining a sensor defined according to claim 1 comprising the following steps: a) preparing a graphene dispersion; b) exfoliating the graphene from the previous dispersion; c) adding and mixing the perovskite nanocrystals to the exfoliated graphene solution to obtain a hybrid material of graphene and perovskite; d) depositing the hybrid material of graphene and perovskite on a substrate containing electrodes.

12. Method according to claim 11, where the solvent of the solution in step a) is toluene or hexane and the graphene consists of sheets of graphene.

13. Method according to claim 11, the graphene is exfoliated in step b) by pulsed sonication.

14. Method according to claim 11, where in step d) the substrate is alumina containing screen-printed platinum interdigitated electrodes.

15. Gas detection method comprising the stages of: a) placing the sensor defined in claim 1 in a chamber through which the gas flow passes, b) measuring the variation of the resistance after the passage of gas.

16. Method according to claim 15, characterized in that stage b) is carried out at room temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 AB shows the responses obtained for the detection of benzene (1A) and toluene (1B) using graphene decorated with different NCs of MAPbBr.sub.3.

[0026] FIG. 2 AB shows the responses obtained for the detection of benzene (2A) and toluene (2B) using graphene decorated with different NCs of CsPbBr.sub.3.

[0027] FIG. 3AB shows the responses obtained for the detection of benzene (3A) and toluene (3B) using graphene decorated with different NCs of FAPbBr.sub.3.

[0028] FIG. 4 AB shows the responses obtained for the detection of benzene (4A) and toluene (4B) using graphene decorated with different NCs of MAPbCl.sub.3.

[0029] FIG. 5AB shows the responses obtained for the detection of benzene (5A) and toluene (5B) using graphene decorated with different NCs of MAPbBr.sub.2.5 I.sub.0.5.

[0030] FIG. 6 is an example response and recovery curve for a lead halide perovskite decorated graphene sensor operating at room temperature.

[0031] FIG. 7 shows a reproducibility analysis using the MAPbBr.sub.3 perovskite.

[0032] FIG. 8 shows an example of an electrical response to NO.sub.2 using FAPbBr.sub.3.

[0033] FIG. 9 shows exposure to different NH.sub.3 concentrations by using FAPbBr.sub.3 NC-decorated graphene.

[0034] FIG. 10 shows a comparison of electrical responses to NH.sub.3 using MAPbCl.sub.3 (black line) and MAPbBr.sub.3 (grey line) NCs.

[0035] FIGS. 11A and 11B show calibration curves obtained for the detection of benzene using different cation (11A) and anion (11B) in lead halide perovskites.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0036] As stated above, the first aspect of the invention refers to a gas sensor comprising: a hybrid material comprising metal halide perovskite nanocrystals of formula ABX.sub.3 and graphene deposited on a substrate with electrodes where the perovskite nanocrystals are embedded in the graphene.

[0037] Preferably the sensor has less than 10% oxygen functional groups. This feature gives it a better transport of carriers (hollows). This is due to shifting the Fermi level towards the valence band and increasing the work function. In addition, the limited oxygen content maintains the high hydrophobicity of graphene, thus continuing to confer high stability to gas sensors composed of perovskite and graphene nanocrystals.

[0038] Preferably in the ABX.sub.3 configuration, B is selected from divalent metals, more preferably from: Pb.sup.+2, Sn.sup.+2, Cu.sup.+2, Mn.sup.+2, Fe.sup.+2, Ge.sup.+2, Bi.sup.+2, Sb.sup.+2 or a mixture thereof.

[0039] Preferably A in the formula ABX.sub.3 is an organic and/or inorganic monovalent cation. More preferably, it is selected from: methylammonium, MA (CH.sub.3NH.sub.3.sup.+), formamidinium, FA ((NH.sub.2).sub.2CH.sup.+) and cesium (Cs.sup.+), guanidinium, phenylethylammonium, K.sup.+ and Rb.sup.+ or combinations thereof.

[0040] Preferably X.sub.3 represents the anion, selected from Cl.sup.−, Br.sup.− and I.sup.−.

[0041] Preferably perovskite nanocrystals are comprised between 6 to 8 nanometers in size.

[0042] Preferably the substrate is alumina.

[0043] The monitoring of benzene and toluene is of great interest due to its danger, for example, benzene is considered a carcinogen. For all these reasons, the detection of trace levels in the environment is highly relevant.

[0044] Preferably the gas to be detected in the sensor is benzene and toluene, and the cation of the perovskite is methylammonium.

[0045] Preferably the cation of the perovskite is methylammonium and the anion is bromine.

[0046] Preferably the gas to be detected is NO.sub.2, and the cation of the perovskite is formamidine.

[0047] Preferably the gas to be detected is NH.sub.3, the cation of the perovskite is methylammonium and the anion of the perovskite is a chlorine anion.

[0048] Preferably halogen of the perovskite is selected from an anion of chlorine and/or bromine and/or iodine.

[0049] As has been stated before, the second aspect of the invention refers to a method for obtaining a sensor defined above that comprises the following stages:

[0050] a) preparing a graphene dispersion;

[0051] b) exfoliating the graphene from the previous dispersion;

[0052] c) adding and mixing the perovskite nanocrystals to the exfoliated graphene solution to obtain a hybrid material of graphene and perovskite;

[0053] d) depositing the hybrid material of graphene and perovskite on a substrate containing electrodes.

[0054] Preferably the solvent of the solution of step a) is toluene or hexane and the graphene consists of sheets of graphene.

[0055] Preferably the graphene is exfoliated in step b) by pulsed sonication.

[0056] Preferably in stage d) the substrate is alumina containing screen-printed platinum interdigitated electrodes.

[0057] Preferably, the gas detection method described above as the third aspect of the invention is carried out at room temperature. Despite the fact that operation at room temperature usually implies a weak recovery of the sensor baseline, due to the low desorption rate of adsorbed molecules, room temperature is used for several reasons: low power consumption, to preserve the NC of perovskite from its degradation, thus improving the useful life of the sensor.

EXAMPLES

[0058] The following examples are only for illustrative purposes of this invention, and should not be construed as limiting the same.

[0059] In all the examples the methods to quantify the color transmittance in the prepared samples are as follows.

Example 1

[0060] Synthesis of perovskite nanocrystals (NC).

[0061] The synthesis of MA cation perovskite NC was adapted from the method proposed by L. Schmidt et al. First, a stock solution was prepared by adding 85 mg of oleic acid (OA) to 2 ml of 1-octadene (ODE). The solution was stirred and heated to 80° C. Subsequently, 33.5 mg of octylammonium bromide (OABr) was added.

[0062] Next, other specific solutions for each perovskite anion were prepared using different precursors. In the case of MAPbBr.sub.3NCs, 26.4 mg and 18.3 mg of methylammonium bromide (MABr) and lead (II) bromide (PbBr.sub.2), respectively, were dissolved in 200 μL of dimethylformamide (DMF). Meanwhile, 3.37 mg and 13.9 mg of methyl ammonium chloride (MACI) and lead (II) chloride (PbCl.sub.2), respectively, were dissolved in 200 μL of dimethyl sulfoxide (DMSO) to form MAPbCl.sub.3. Finally, to prepare the NCs of MAPbBr.sub.2.5I.sub.0.5, 2.7 mg, 3 mg, and 18.5 mg of methylammonium iodide (MAI), methylammonium bromide (MABr), and lead (II) bromide (PbBr.sub.2) were added, respectively, to 300 μL of DMF. The solutions were shaken until completely dissolved.

[0063] Finally, each of the solutions with the specific precursors was added to the base solution. Subsequently, the solutions were cooled to 60° C. and 5 ml of acetone was added, causing the immediate precipitation of the different nanocrystals. Yellow, white and yellow-orange precipitates were obtained for MAPbBr.sub.3, MAPbCl.sub.3 and MAPbBr.sub.2.5I.sub.0.5 respectively. Subsequently, the solutions were centrifuged at 6000 rpm for 10 minutes to extract the precipitates, and finally they were dispersed in toluene.

[0064] CsPbBr.sub.3: for the synthesis of this type of nanocrystals, the method proposed by L. Protesescu et al. To prepare the Cs oleate, Cs.sub.2CO.sub.3 (814 mg), ODE (40 mL) and OA (2.5 mL) were added to a 3-necked flask. Subsequently, the solution was mixed under stirring and heated at 120° C. for 1 hour. Finally, the temperature was increased to 150° C. under a nitrogen atmosphere to ensure the complete reaction of Cs.sub.2CO.sub.3 with oleic acid. The solution was cooled to room temperature obtaining a precipitate of Cs oleate.

[0065] Subsequently, another solution was prepared by mixing 69 mg of PbBr.sub.2 and 5 ml of ODE in a 3-necked flask. The solution was then dried under vacuum at 120° C. for 1 hour. Next, 0.5 ml of dry oleylamine (OLA) and OA was injected while creating a nitrogen atmosphere. After complete solubilization, the temperature was raised to 140° C. and the Cs oleate solution (0.4 ml, prewarmed to 100° C. before injection) was rapidly injected. Five seconds later, the final solution was cooled using an ice water bath. Finally, 5 ml of tert-butyl alcohol (tBuOH) were added to favor the complete precipitation of the NCs. After the centrifugation step explained in the previous point, the CsPbBr.sub.3 NCs were dispersed in hexane.

[0066] FAPbBr.sub.3: NCs containing FA cation were carried out following the method proposed by L. Protesescu et al. First, the FA oleate precursor was prepared, where 521 mg of formamidinium acetate (FA(CH.sub.3COO)) and 20 ml of OA were added to a 3-necked flask. Next, similar to the previous synthesis, the solution was heated at 120° C. for 1 hour. Subsequently, the temperature was increased to 130° C. until complete reaction. Finally, the FA oleate was dried for 30 minutes at 50° C. under vacuum and cooled to room temperature.

[0067] Subsequently, another solution was prepared by mixing ODE (5 ml) and PbBr.sub.2 (69 mg) in a 3-necked flask. The solution was dried in vacuo for 1 hour at 120° C. Next, 0.5 ml of OLA and 1 ml of OA were injected at 120° C. under nitrogen flow. After complete solubilization of the PbBr.sub.2 salt, the temperature was lowered to 100° C. Next, 2.5 mL of FA oleate solution was rapidly injected, and 5 seconds later, the reaction mixture was cooled using an ice-water bath. Finally, 10 ml of toluene and 5 ml of acetonitrile were added to favor the complete precipitation of the NCs. Finally, the solution was centrifuged and the FAPbBr3 NCs were dispersed in hexane.

TABLE-US-00001 TABLE 1 Synthesis summary Solvent Perovskite Precursors Reagents Solvents stabilizer MAPbBr.sub.3 26.4 mg MABr 85 mg AO 5 mL Acetone Toluene 18.3 mg PbBr.sub.2 2 mL ODE 200 μL DMF 33.5 mg OABr MAPBCl.sub.3 3.37 mg MACl 85 mg AO 5 mL Acetone Toluene 13.9 mg PbCl.sub.2 2 mL ODE 200 μL DMSO 33.5 mg OABr MAPbBr.sub.2.5I.sub.0.5 2.7 mg MAl 85 mg AO 5 mL Acetone Toluene 3 mg MABr 2 mL ODE 300 μL DMF 18.5 mg PbBr.sub.2 33.5 mg OABr CsPbBr.sub.3 814 mg Cs.sub.2CO.sub.3 45 ml ODE 5 mL tBuOH Hexane 69 mg PbBr.sub.2 2.5 mL OA 0.5 mL OLA FAPbBr.sub.3 521 mg FA(CH.sub.3COO) 21 mL OA 10 mL Toluene Hexane 5 mL ODE 5 mL Acetonitrile 0.5 mL OLA

TABLE-US-00002 TABLE 2 Mean size of the crystals. Data extracted from HRTEM images Perovskite Mean size (nm) Interplanar distance (Å) MAPbBr.sub.3 7.2 ± 2.2 2.8 CsPbBr.sub.3 8.7 ± 1.1 5.8 FAPbBr.sub.3 6.9 ± 1.2 23 MAPbCl.sub.3 5.6 ± 1.5 2.6 MAPbBr.sub.2.5I.sub.0.5 6.3 ± 0.6 3.0

[0068] Graphene Decoration with Perovskite NC and Configuration of Gas Measurements.

[0069] Once the different perovskite NCs were synthesized, a solution of graphene in toluene or hexane (0.5 mg/ml) was prepared using commercial graphene nanosheets from Strem Chemicals, Inc. (USA). Subsequently, the solution was placed in an ultrasonic tip to apply pulsed sonication (1s on/2s off) at 280 W for 90 minutes. Once the graphene is properly exfoliated, the perovskite NCs (5% by weight) were added to the solution. The nanomaterials were mixed in an ultrasonic bath for 1 hour. Finally, perovskite NC decorated graphene was deposited by a spray-coating technique onto alumina substrates containing screen-printed platinum interdigitated electrodes.

[0070] The developed sensors were placed in a Teflon chamber with a volume of 35 cm3, which was connected to gas cylinders calibrated with synthetic air.

[0071] To study the detection of different gases, different dilutions were made in order to expose the sensors to variable concentrations. Sensors are stabilized under synthetic air for 5 minutes prior to application of the target gas concentration during 1 minute exposure. Total flow was set to 100 mL/min using a set of Bronkhorst High-Tech BV (Ruurlo, The Netherlands) flow controllers, while resistance changes were recorded using an Agilent HP 34972A multimeter connected to the measurement chamber. Responses are defined as (ΔR/R.sub.0) expressed as a percentage. Where ΔR is the resistance change during one minute of exposure to the gas, while R.sub.0 corresponds to the reference resistance.

[0072] FIGS. 1 to 5 show the responses obtained for the detection of benzene (A) and toluene using graphene decorated with different perovskite NCs. For both gases, 2, 4, 6 and 8 ppm were applied in three consecutive cycles.

TABLE-US-00003 TABLE 3 Example of the average responses and relative error for the detection of benzene with graphene decorated with the different perovskite NCs. C.sub.6H.sub.6 (ppm) MAPbBr.sub.3 FAPbBr.sub.3 MAPbBr.sub.2.5I.sub.0.5 MAPbCl.sub.3 CsPbBr.sub.3 2 0.202 ± 0.007 0.079 ± 0.004 0.096 ± 0.004 0.080 ± 0.005 0.0472 ± 0.0001 4 0.283 ± 0.008 0.117 ± 0.004 0.139 ± 0.006 0.119 ± 0.006 0.070 ± 0.001 6 0.345 ± 0.012 0.147 ± 0.006 0.172 ± 0.006 0.149 ± 0.004 0.0904 ± 0.0005 8 0.402 ± 0.014 0.167 ± 0.007 0.197 ± 0.007 0.169 ± 0.005 0.104 ± 0.001

[0073] The measurement methodology used in this work results in highly reproducible (less than 5% error), reversible (absence of significant baseline drift) and rapid (1 minute exposure) responses at room temperature.

[0074] Since the behavior of the sensor during the exposure to the gas until the stabilization of the response is important, FIG. 6 shows the saturation of the sensor and its initial recovery. Response and recovery times (t90) are approximately 30 minutes for a flow rate of 400 mL/min. Once the resistance baseline was stable in synthetic air, 10 ppm toluene was applied until saturation of the sensor response was reached. Basal recovery was achieved in fresh air.

[0075] FIG. 7 shows a reproducibility analysis using the MAPbBr.sub.3perovskite. Resistance changes (solid line) were recorded under exposure to 10 ppm benzene (dashed line) for long periods (30 minutes).

[0076] FIG. 8 shows examples of electrical response to NO.sub.2 using FAPbBr.sub.3. Three consecutive cycles with four concentrations (250, 500, 750 and 1000 ppb) were applied for a 1 minute exposure. Synthetic air was used for a five minute wash between the different concentrations measured. FIG. 9 shows exposure to different NH.sub.3 concentrations by using FAPbBr.sub.3 NC decorated graphene. No sensitivity to ammonia was obtained because the resistance changes recorded are practically identical for the different analyte concentrations. FIG. 10 shows a comparison of electrical responses to NH.sub.3 using MAPbCl.sub.3 (black line) and MAPbBr.sub.3 (red line) NCs. Three consecutive cycles with four concentrations (25, 50, 75 and 100 ppm) were applied for one minute. Again, there were applied five minute cleanings in synthetic air periods.

[0077] FIGS. 11A and 11B show the calibration curves for benzene detection using the different cations (a) and halide anions (b). Regarding the effect of the cation, MA shows a clear improvement in responses (up to 3 times greater) and in sensitivity (slope of the curve) compared to FA and Cs. Meanwhile, responses obtained by using different halide anions reveal that Br anions offer a higher response and sensitivity than Cl.sup.− and I.sup.− anions. Equivalent behavior was observed for toluene vapors. Aromatic molecules such as benzene and toluene can act as electron donating groups due to their delocalized electrons. A significant effect of Cs, MA or FA cations is clearly observed in graphene decorated with perovskite NC when exposed to these gases. In fact, higher electrical responses to both gases are recorded when MA is present in the perovskite structure. These better electrical properties are due to the positions of the energy levels (band structure) and the concentration of defects (traps).