CN113155910B - Preparation method and application of carbon quantum dot-cobalt tetracyanide nickelate composite material ammonia gas sensor - Google Patents
Preparation method and application of carbon quantum dot-cobalt tetracyanide nickelate composite material ammonia gas sensor Download PDFInfo
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Abstract
The invention discloses a preparation method of an ammonia gas sensor made of a carbon quantum dot-cobalt tetracyanide nickelate composite material, which relates to the technical field of nano gas sensors, and adopts polyvinylpyrrolidone as a surfactant to prepare two-dimensional cobalt tetracyanide nickelate nanosheets, synthesizes carbon quantum dots through one-step pyrolysis, and prepares CQDs @ Co [ Ni (CN)4]Material and NH based on composite film thereof3A sensor. The obtained sensor has good response sensitivity, high ammonia sensing response speed, high response conductivity and good humidity interference resistance.
Description
Technical Field
The invention relates to the technical field of nano gas sensors, in particular to a preparation method of an ammonia gas sensor made of a carbon quantum dot-cobalt tetracyanide nickelate composite material and application of the ammonia gas sensor in ammonia sensitive film sensing.
Background
Ammonia gas (NH)3) Is a colorless gas with pungent odor, and is a common toxic and harmful gas. It is widely used in the production and life of people. Leakage and exhaust emissions from industrial processes, automobile exhaust emissions, waste decay, the use of large quantities of fertilizers in agriculture and the excretion of animals in animal husbandry are all responsible for the increasing NH content3And (4) pollution problem. Due to NH3Highly soluble in water and therefore easily irritates and damages the human eye and respiratory mucosa. Therefore, there is an urgent need to develop cost-effective, sensitive, and highly selective NH for human health and environmental protection3A gas sensor.
Two-dimensional Coordination Polymers (CPs), which are typical materials in the metal-organic frameworks (MOFs), have unique properties due to their ultra-thin thickness, large surface area, and highly accessible active sites, and have attracted considerable attention in the prior art in recent years.
Most coordination polymers are poor electrical conductors due to the weak degree of overlap between the p orbitals of the insulating ligands and the d orbitals of the metal ions. Particularly for the film sensor of the two-dimensional coordination polymer, the resistance is often in the G omega level when the relative humidity is low, and the film sensor cannot be directly used in production practice. Thus to NH3The sensing performance test of the sensor can only be carried out under relatively high humidity, which greatly limits the application of the sensor in real life and industry. The sensing sensitivity is easily lost by simply selecting a high-conductivity ligand.
Disclosure of Invention
In order to overcome the problems in the prior art, the invention aims to adjust the conductivity of two-dimensional cyano-bridged Co-Ni heterometal nano-sheets and keep the conductivity of the two-dimensional cyano-bridged Co-Ni heterometal nano-sheets to NH3The high sensitivity of the sensor makes the sensor adapt to more various working environments. Provides a preparation method of an ammonia gas sensor made of a carbon quantum dot-cobalt tetracyanide nickelate composite material, and the obtained sensor has good performanceGood response sensitivity, high ammonia sensing response speed, high response conductivity and good humidity interference resistance.
The invention provides a preparation method of an ammonia gas sensor of a carbon quantum dot-cobalt tetracyanide nickelate composite material,
preparing two-dimensional cyano-bridged Co-Ni hybrid metal nanosheets by using cobalt nitrate hexahydrate and potassium tetracyanide nickelate as raw materials, polyvinylpyrrolidone as a surfactant and sodium citrate as a control agent under the combined action of the cobalt nitrate hexahydrate and the potassium tetracyanide nickelate;
② disodium ethylene diamine tetraacetate (EDTA-2 Na for short) is used as precursor, in N2Or calcining at 200-300 ℃ under inert atmosphere at the heating rate of 1-5 ℃/min for 1-3 h; dispersing the product in water, performing ultrasonic treatment, centrifuging, filtering, dialyzing, and removing impurities to obtain Carbon Quantum Dots (CQDs);
③ mixing Co [ Ni (CN)4]The solution was mixed with the synthetic CQDs solution with constant stirring, and the resulting CQDs @ Co [ Ni (CN)4]Nanosheets, using CH3OH is washed and dispersed for standby.
The preparation process of the carbon quantum dots preferably comprises the following steps:
placing EDTA-2Na in a tube furnace in N2Calcining for 1-2h in a tubular furnace at 200-250 ℃ at the heating rate of 3-5 ℃/min under the atmosphere;
secondly, grinding and dispersing the product in water, carrying out ultrasonic treatment on the suspension at room temperature, and centrifuging at a high speed of 8000-10000 rpm; filtering the upper layer solution with filter paper of 0.3 μm or less to remove deposited Na salt;
and thirdly, dialyzing the solution by using a dialysis tube, removing residual salt and fragments, drying to obtain carbon quantum dot powder, and dispersing the carbon quantum dot powder in water for later use.
The two-dimensional cyano-bridged Co-Ni hetero-metal nanosheet is cobalt tetracyanium nickelate (Co [ Ni (CN))4]) The preparation process preferably comprises:
firstly, cobalt nitrate hexahydrate (Co (NO)3)2·6H2O) and polyvinylpyrrolidone (PVP) in methanol (CH)3OH);
② potassium tetracyanid nickelate (K)2[Ni(CN)4]) Hening lemonSodium citrate (Na)3C6H5O7·2H2O) is dissolved in a methanol-water mixed solvent (10mL, the volume ratio is 1: 1);
mixing the two solutions obtained in the first step and the second step uniformly and carrying out ultrasonic oscillation; then oscillating using a vortex oscillator; then carrying out ultrasonic oscillation to obtain a mixed uniform solution;
fourthly, after the uniform solution is stood at room temperature, repeating the step three at least once;
placing the solution at room temperature until the solution becomes turbid; collecting Co [ Ni (CN)4]And (3) repeatedly centrifuging and washing the nanosheets with a methanol solution for a plurality of times.
Sixthly, obtaining Co [ Ni (CN)4]The nanosheets were redispersed in a methanol solution for use.
Preferably, in the step (i), the mass ratio of the cobalt nitrate hexahydrate to the polyvinylpyrrolidone is 1 (10), and the mass ratio of the cobalt nitrate hexahydrate to the potassium tetracyanide nickelate is 1 (0.5-4).
The invention also provides an application of the quantum dot-cobalt tetracyanide nickelate nanosheet prepared by the preparation method in ammonia gas detection. CQDs @ Co [ Ni (CN) ] are preferably prepared using spin-coating4]Thin film sensor, gas sensitive test platform, and NH treatment of the obtained sensor3The gas-sensitive characteristic test of (1).
Preferably, the cobalt tetracyanide nickelate film sensor is used for detecting electron conductivity, ammonia load response value, dynamic response characteristic, stability and response recovery characteristic, selectivity and humidity characteristic.
Further, at CQDs @ Co [ Ni (CN)4]In thin film sensors, CQDs are reported in CQDs @ Co [ Ni (CN)4]The loading on the sensor is 1.25 wt% -30 wt%, when the loading on the CQDs is preferably 14.01 wt%, the sensor response value under 67% RH environment is 241%, and the sensor is pure Co [ Ni (CN) ] under the same condition4]2.5 times of the nano sheet.
Preferably, in CQDs @ Co [ Ni (CN)4]Real-time dynamic response NH of thin film sensors3In gas detection, CQDs @ Co [ Ni (CN)4]The thin film sensor is exposed to a gas concentration of NH in the range of 0.1-30ppm3And switching measurements were performed in air with a time interval of 200s for each switching.
Preferably, in CQDs @ Co [ Ni (CN)4]Real-time dynamic response NH of thin film sensors3In gas detection, CQDs @ Co [ Ni (CN)4]The response of the film sensor and the fitting curve graph of the gas concentration are calculated through a detection limit formula to obtain the theoretical detection limit LOD of the sensor, and the detection limit formula of the sensor is as follows:
LOD＝3σ/S
wherein, σ is the standard deviation of the response value within a certain time after the sensor reaches the ventilation equilibrium state, and S is the sensitivity.
Preferably, in CQDs @ Co [ Ni (CN)4]NH for stabilization of thin film sensors3In gas detection, CQDs @ Co [ Ni (CN)4]Thin film sensors were exposed to four concentrations of NH at room temperature, 0.5, 0.75, 1 and 10ppm3Performing a repeatability test, wherein each concentration is repeatedly tested for 3 times; by exposing the sensor to 0.25, 0.5, 1, 5, 10 and 20ppm NH at room temperature3In the evaluation of Co [ Ni (CN) ], the response value of the sensor was measured every 5 days for one month4]Long term stability of the thin film sensor.
Further, CQDs @ Co [ Ni (CN)4]The film sensor is used for detecting ammonia gas under different relative humidity environments, and different saturated salt solutions are adopted to simulate the humidity testing environment of the gas sensor; the humidity testing environment is preferably configured by the following process: lithium chloride (LiCl) and potassium acetate (CH)3COOK), magnesium chloride (MgCl)2) Potassium carbonate (K)2CO3) Magnesium nitrate (Mg (NO)3)2) Copper chloride (CuCl)2) Sodium chloride (NaCl), potassium chloride (KCl) and potassium sulfate (K)2SO4) Saturated salt solution with humidity of 11%, 23%, 33%, 43%, 52%, 67%, 75%, 85% and 97% RH, respectively, and phosphorus pentoxide (P)2O5) The powder acts as a desiccant to provide a dry detection environment (0% RH) for the moisture sensitive sensor. CQDs @ Co [ Ni (CN)4]The humidity environment of the thin film sensor is preferably 43% RH or more.
Further, tetracyanidelenickelateNH in detection of ammonia gas by cobalt nanosheet3After adsorption to CQDs @ Co [ Ni (CN)4]Oxidation-reduction reaction is carried out after the surface of the film to obtain NH in an ionic state4+Exists in the form of (a); and NH3Selective adsorption to Co2+Metal site at Co2+Interlayer water molecule and NH on site3Hydrogen bonds are formed between molecules; by reaction of NH3Molecules with CQDs @ Co [ Ni (CN)4]Stronger electron hole activity is provided among the nanosheet frameworks.
Compared with the prior art, the invention has the following beneficial effects:
the invention synthesizes CQDs with the size of about 2nm and good dispersibility by a one-step pyrolysis method from bottom to top, and simply synthesizes CQDs @ Co [ Ni (CN)4]Nanosheets, the addition of CQDs found by TEM, AFM and EDX characterization techniques to be unchanged by Co [ Ni (CN)4]The nano-sheet 2D has ultrathin basic structure and chemical property, and the CQDs are uniformly loaded. Subsequently, the actual loading of CQDs was quantitatively studied by FS characterization technique using the fluorescence characteristics of CQDs, and the influence of the loading on the ammonia sensitive performance of the sensor was analyzed. CQDs @ Co [ Ni (CN)4]Film sensor pair NH3And the gas-sensitive and moisture-sensitive properties of (1), with pure Co [ Ni (CN)4]The test results of the thin film sensors were compared. Modification of CQDs could be observed on pure Co [ Ni (CN)4]The response and detection limit of the film sensor are obviously improved, and the anti-interference performance to humidity is enhanced. Furthermore, a series of characterization techniques based on PXRD, FTIR and UPS prove CQDs @ Co [ Ni (CN)4]Thin film enhanced NH3Sensitive mechanism, CQDs @ Co [ Ni (CN)4]The thin film sensor can be used for quantitative analysis of NH3The concentration of (2).
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention.
FIG. 1(a) CQDs @ Co [ Ni (CN)4]50nm TEM TE of nanosheetM represents a graph.
FIG. 1(b) CQDs @ Co [ Ni (CN)4]Transmission electron microscope TEM representation picture of 10nm nanosheet.
FIG. 1(c) CQDs @ Co [ Ni (CN)4]A transmission electron microscope HRTEM representation picture of 2nm nanosheets.
FIG. 1(d) CQDs @ Co [ Ni (CN)4]And (4) a nanosheet AFM characterization map and a thickness statistical distribution map.
FIG. 1(e) CQDs @ Co [ Ni (CN)4]And (5) an energy dispersive X-ray spectrum EDX mapping image corresponding to the nano-sheet transmission electron microscope TEM.
FIG. 2(a) is a fluorescence spectrum of a CQDs standard solution.
FIG. 2(b) CQDs @ Co [ Ni (CN) loaded with CQDs of different contents4]Fluorescence spectra of the nanosheets.
FIG. 2(c) fitting curves for standard solutions and deposits on CQDs @ Co [ Ni (CN)4]Actual loading of CQDs on nanosheets estimated figures.
FIG. 3(a) CQDs @ Co [ Ni (CN) loaded with CQDs of different contents4]Conductivity of the nanoplatelets.
FIG. 3(b) CQDs @ Co [ Ni (CN) loaded with different CQDs content4]Resistance drop curve of sensor and for 1ppm NH3Response rising curve of (2).
FIG. 4(a) is a graph showing the dynamic response of CQDs @ Co [ Ni (CN)4] thin film sensors to ammonia.
FIG. 4(b) CQDs @ Co [ Ni (CN)4]The thin film sensor was fitted to the ammonia curve.
FIG. 5(a) CQDs @ Co [ Ni (CN)4]And (3) ammonia gas repeatability detection graph of the film sensor.
FIG. 5(b) CQDs @ Co [ Ni (CN)4]And (3) detecting the long-term stability of the ammonia gas by using a film sensor.
FIG. 6(a) CQDs @ Co [ Ni (CN)4]Response and recovery time point plots of the thin film sensor.
FIG. 6(b) CQDs @ Co [ Ni (CN)4]Selectivity profile of the thin film sensor for 5ppm different reducing gases.
FIG. 7(a) humidity vs. CQDs @ Co [ Ni (CN)4]Line graph of the effect of the base resistance of the thin film sensor.
FIG. 7(b) CQDs @ Co [ Ni (CN)4]Response of thin film sensor with humidity and NH3Three-dimensional scatter plots of concentration.
FIG. 7(c) CQDs @ Co [ Ni (CN)4]Response of thin film sensor with humidity and NH3Linear dependence of concentration.
FIG. 8 CQDs @ Co [ Ni (CN)4]Nanosheets having bound thereto NH3PXRD patterns before and after contact.
FIG. 9 CQDs @ Co [ Ni (CN)4]Nanosheets having bound thereto NH3Fourier infrared spectrograms before and after touch.
FIG. 10 CQDs @ Co [ Ni (CN)4]Nanosheets having bound thereto NH3UPS patterns of uv-electron spectra before and after contact.
Detailed Description
The preferred embodiments of the present invention will be described in conjunction with the accompanying drawings, and it will be understood that they are described herein for the purpose of illustration and explanation and not limitation.
Compositional tailoring is a means to compensate for the insufficient conductivity of coordination polymer sensing materials. However, the composite conductive material does not necessarily improve the conductivity property of the two-dimensional hetero-metal film. On the contrary, the conductive material merely physically doped is poor in the degree of bonding with the surface of the two-dimensional coordination polymer and the active site thereof, and is liable to affect the charge transfer between the two-dimensional nanosheet layer and the active site according to the barrel effect.
Carbon Quantum Dots (CQDs) are quasi-spherical zero-dimensional carbon-based materials with extremely small size (2-10nm), and are mostly applied to the technical fields of biomedical imaging, sensors, photoelectric elements and the like. CQDs have great application prospects in the sensing field due to their large surface area and fast electron transfer performance. CQDs can be used to enhance gas sensing response by compounding with sensitive materials.
EDTA-2Na with flexible structure is used as a precursor raw material for preparing CQDs and contains relatively stable carboxylate anion (COO)-) The CQDs prepared by the method have good water solubility and are easy to combine with active sites on two-dimensional hetero-metal nano-sheets so as to further retain the CQDs after pyrolysis by a specific process at low temperatureCatalyzing the ammonia adsorption reaction of the cobalt sites.
The present invention can synthesize CQDs with uniform size by a simple one-step pyrolysis method to produce CQDs @ Co [ Ni (CN)4]Material and NH based on composite film thereof3The sensor is further applied to ammonia gas detection to characterize the sensing performance. In addition, pure Co [ Ni (CN)4]Thin film sensors and CQDs/Co [ Ni (CN)4]The performance of the composite film sensor was compared and a reasonable explanation was given from the intrinsic mechanistic point of view.
Examples
In this example, first, Carbon Quantum Dots (CQDs) having a uniform size were prepared by a one-step bottom-up pyrolysis method,
then preparing 2D ultrathin cyano-bridged Co-Ni hetero-metal nanosheet-cobalt tetracyani nickelate (Co [ Ni (CN))4]) The nano-sheet is prepared by the steps of,
and made based on CQDs @ Co [ Ni (CN)4]NH of composite film3A sensor.
A preparation method of an ammonia gas sensor made of a carbon quantum dot-cobalt tetracyanide nickelate composite material comprises the following steps:
preparing two-dimensional cyano-bridged Co-Ni hybrid metal nanosheets by using cobalt nitrate hexahydrate and potassium tetracyanide nickelate as raw materials, polyvinylpyrrolidone as a surfactant and sodium citrate as a control agent under the combined action of the cobalt nitrate hexahydrate and the potassium tetracyanide nickelate;
② disodium ethylene diamine tetraacetate (EDTA-2 Na for short) is used as precursor, in N2Or calcining at 200-300 ℃ under inert atmosphere at the heating rate of 1-5 ℃/min for 1-3 h; dispersing the product in water, performing ultrasonic treatment, centrifuging, filtering, dialyzing, and removing impurities to obtain Carbon Quantum Dots (CQDs);
③ mixing Co [ Ni (CN)4]The solution was mixed with the synthetic CQDs solution with constant stirring and the resulting CQDs @ Co [ Ni (CN)4]Nanosheets, using CH3OH is washed and dispersed for standby.
The preparation process of the carbon quantum dots specifically comprises the following steps:
put quartz boat of 1.6g EDTA-2Na in tube typeFurnace central to quartz tube and in N2Calcining for 2h in a tube furnace at 250 ℃ at the heating rate of 5 ℃/min under the atmosphere;
secondly, grinding and dispersing the product into 100ml of water, carrying out ultrasonic treatment on the suspension for 15min at the room temperature of 300W and 40kHz, and centrifuging at the high speed of 10000rpm for 20 min; filtering the upper brown solution using a 0.25 μm slow quantitative microporous filter paper to remove the deposited Na salt;
and thirdly, dialyzing the solution for 48 hours by using a dialysis tube, removing residual salt and fragments, drying for 24 hours at the temperature of 60 ℃ to obtain carbon quantum dot powder, and dispersing the carbon quantum dot powder in water for later use. The quantum dot powder can also be dissolved in common solvents, such as polar organic solvents and benzene-based non-polar organic solvents.
The two-dimensional cyano-bridged Co-Ni hetero-metal nanosheet is specifically cobalt tetracyanonickelate (Co [ Ni (CN))4]) The preparation process comprises the following steps:
[ 26mg of cobalt nitrate hexahydrate (Co (NO))3)2·6H2O) and 100mg polyvinylpyrrolidone (PVP) in methanol (CH)3OH), stirring to obtain a uniform solution;
② 24mg of potassium tetracyanid nickelate (K)2[Ni(CN)4]) And 21.5mg sodium citrate (Na)3C6H5O7·2H2O) is dissolved in a methanol-water mixed solvent (10mL, the volume ratio is 1: 1);
mixing the two solutions obtained in the first step and the second step uniformly and performing ultrasonic oscillation for 1 min; then oscillating for 5min by using a vortex oscillator; then carrying out ultrasonic oscillation for 1min to obtain a mixed uniform solution;
fourthly, after the uniform solution is kept stand for 10min at room temperature, repeating the step three at least once;
placing the solution at room temperature until the solution becomes turbid; centrifuging at 8000rpm for 10min, collecting Co [ Ni (CN)4]And repeatedly centrifuging and washing the nanosheets for more than 5 times by using a methanol solution.
Sixthly, obtaining Co [ Ni (CN)4]The nanosheets were redispersed in a methanol solution for use.
To verify the synergy of loaded CQDs with two-dimensional heterometal nanoplates, in pure Co [ Ni (CN)4]Nano-scale sheet loads massCQDs in percentages of 1.25%, 3.12%, 5.30%, 14.01%, 17.93%, and 27.22%, respectively, were also targeted to select the best performing preferred sensors. Therefore, the number of the first and second electrodes is increased,
the compounding process of the carbon quantum dots and the cobalt tetracyanium nickelate specifically comprises the following steps:
10mL of Co [ Ni (CN)4]To these cells, 1mL (example 1), 2.5mL (example 2), 4.5mL (example 3), 10mL (example 4), 15mL (example 5) and 30mL (example 6) of a 0.1mg/mL aqueous solution of CQDs synthesized were added, and the mixture was stirred at room temperature for 3 hours to mix well. Subsequently, the resulting product CQDs @ Co [ Ni (CN) ] was collected by centrifugation at 8000rpm for 10 minutes4]Nanosheets, combined with CH3OH washes were performed twice. Finally, the CQDs @ Co [ Ni (CN)4]Redispersion in 10mL CH3In OH for preparing and detecting NH3The thin film sensing layer of (1).
CQDs @ Co [ Ni (CN) ] prepared by the method4]Nanoplatelets, each electron microscopic microstructure analyzed using transmission electron microscopy, TEM, and atomic force microscopy, AFM, fluorescence spectroscopy on CQDs @ Co [ Ni (CN)4]And (4) carrying out quantitative analysis on the actual loading quantity of the CQDs in the nanosheet composite material. The results are shown in FIGS. 1(a) to 1 (e).
From FIG. 1(a), it can be seen that Co [ Ni (CN) ]after deposition of CQDs4]The nanosheet surface becomes rough. As shown in FIG. 1(b), CQDs @ Co [ Ni (CN)4]The composite morphology of the nano-sheet still presents a regular sheet structure and CQDs nano-particles are attached to the surface of the nano-sheet. Furthermore, after compounding CQDs, the whole Co [ Ni (CN)4]The 2D ultrathin structure of the nanosheet is not obviously changed.
FIG. 1(c) shows CQDs @ Co [ Ni (CN)4]High resolution tem (hrtem) images of the nanoplates in which CQDs exhibited uniform and dispersed spherical particles, approximately 2nm in size. In addition, it is shown that the lattice spacing of a single carbon dot having a crystal structure is 0.216nm, corresponding to the lattice fringes of the (100) plane of carbon. As shown in FIG. 1(d), the AFM image clearly shows CQDs @ Co [ Ni (CN)4]The nanosheets maintained an ultrathin structure, with a thickness of about 3 nm. FIG. 1(e) is the energy dispersion of the resulting compositeEDX-ray Spectroscopy, indicating CQDs @ Co [ Ni (CN)4]The nanosheets contain only Co, Ni, C and N elements. All the above results confirm that the 2D ultra-thin CQDs @ Co [ Ni (CN)4]The specific structure of the nano-sheet composite material lays a foundation for the subsequent sensing performance.
To characterize CQDs @ Co [ Ni (CN)4]Fluorescence analysis of the prepared products was performed for the actual loading weight percentage of CQDs on the nanoplates.
The fluorescence analysis method comprises the following steps: first, a CQDs standard solution with a known concentration was prepared: the solution was fixed to 2mL during the experiment, and the masses of CQDs were 0.0025mg, 0.005mg, 0.02mg, 0.05mg, 0.1mg and 0.15mg, respectively, to thereby obtain experimental standard samples. As shown in FIG. 2(a), the fitting equation of the fluorescence intensity (Y) and the mass (X) of CQDs is expressed as Y-200.1422 +47229.9877X, and the regression coefficient R of the fitting is2Is 0.9883. Then, as shown in FIG. 2(b), for Co [ Ni (CN) ] loaded with different weight percentages of CQDs4]The nanoplatelets were subjected to fluorescence testing.
FIG. 2(c) shows Co [ Ni (CN)4]The actual loading of the deposited CQDs on the nanoplatelets was 1.25%, 3.12%, 5.30%, 14.01%, 17.93%, and 27.22%, respectively. The above calculation results show that in Co [ Ni (CN)4]The actual content of CQDs deposited on the nanoplatelets was less than the added amount, which is caused by the high solubility of CQDs in water and methanol, and it was also confirmed from the side that a higher density of carboxyl groups remained on CQDs.
CQDs @ Co [ Ni (CN)4 obtained by the preparation method]Application of nanosheet in ammonia gas detection, CQDs @ Co [ Ni (CN)4]A film sensor, a gas sensitive test platform is set up, and NH is carried out on the obtained sensor at room temperature3The gas-sensitive characteristic test of (1).
In this example, the preparation of the gas-sensitive film was carried out by spin coating. The spin-coating method selected in the embodiment has the advantages of controllable film forming thickness, simplicity and convenience in operation and the like. After a long time of development, the spin coating technology has been widely applied to the fields of biomedicine, microelectronics and the like.
The preparation steps of the spin coating method comprise:
firstly, fixing an interdigital electrode in the center of a base by using a double-sided adhesive tape, and dripping a fixed amount of two-dimensional cobalt tetracyanide nickelate nanosheet dispersion liquid; then, starting a spin coating instrument, instantly throwing out redundant dispersion liquid on the interdigital electrode due to centrifugal force, and uniformly coating the rest part on the surface of the electrode; finally, the electrode was dried to obtain a thin film with uniform adhesion.
The thickness of the formed film is comprehensively controlled by controlling parameters such as rotating speed, time, dropping amount, solution concentration and viscosity in the spin coating process. The spin coater is WS-650MZ-8NPPB type from Laurell, USA, the spin rate is 800r/min, and the spin time is 30 s.
And (3) building a gas-sensitive test platform:
to realize sensor device pair NH3In this embodiment, the existing laboratory apparatus and equipment are used to build the gas testing device shown in fig. 2. In view of cost and operability, the present example employs a 750mL Erlenmeyer flask as the closed test chamber, sealed with a rubber stopper. Firstly, welding a prepared sensing device with a contact pin and connecting the sensing device with a lead, then penetrating the lead through a rubber plug in a punching mode and fixing the lead by using sealant so as to lead the lead to the outside of a bottle to be connected with a test lead of a Keysight 34470A digital multimeter to acquire resistance information of the sensor in real time.
To test the sensor pair NH3In the operation, a static gas distribution method is adopted, a certain amount of ammonia water is injected into a conical dilution bottle through a micropipettor and is volatilized to obtain NH with a certain concentration3A gas cylinder. Then, specific NH is obtained through proportion calculation3The gas volume required for concentration, the corresponding volume of NH is withdrawn from the dilution flask3And (4) injecting the mixture into a test bottle, wherein the digital multimeter can acquire a resistance response signal of the sensor at the moment and transmit data to the PC end through a Universal Serial Bus (USB). The ammonia water injected into the dilution bottle can be completely volatilized and evenly filled in the bottle, so that NH in the bottle3Can be calculated by the following formula:
wherein ρ represents the density of ammonia water (0.91 g/cm)3) (ii) a T is the working temperature (unit: K) of the sensor; the whole experiment is carried out at room temperature (25 ℃), so that 298K is taken for T; vsThe volume of ammonia water (unit: μ L) injected into the test bottle; m is NH3(ii) a molar mass of (17 g/mol); v represents the volume of the test erlenmeyer flask (750 mL).
The CQDs @ Co [ Ni (CN)4] thin film sensor is used for detecting electron conductivity, ammonia gas dynamic response characteristic, stability and response recovery characteristic, selectivity and humidity characteristic.
Loading different content CQDs Co [ Ni (CN)4]The nanoplates were drop-coated onto glass substrates with ITO interdigitated electrodes using a Keysight B2910A precision power supply/measurement unit with integrated DC power supply to Co [ Ni (CN) loaded with varying amounts of CQDs4]The nanosheets were subjected to measurement of electronic conductivity.
The interdigital array electrode consists of 10 pairs of ITO electrodes deposited on a glass substrate, and the size of the interdigital array electrode is as follows: the pitch was 75 μm, the overlap length was 5850 μm, and the electrode thickness was 20 nm. The conductivity σ is calculated by the following formula:
wherein d is the inter-electrode distance, I is the current, n is the number of digits of the number of electrodes, L is the overlapping length of the electrodes, and h is the thickness of the film deposition electrode.
As shown in FIG. 3(a), 2D Co [ Ni (CN) ]was improved by modification of CQDs4]Conductivity of the nanosheet is from 2.347X 10-4The S/cm is increased to 4.162X 10-3S/cm。
2D ultrathin CQDs @ Co [ Ni (CN)4]Film on NH3Sensing capability detection of (2). To react with pure Co [ Ni (CN)4]The thin film sensors were compared and the experiments were all performed in an environment with a relative humidity of 67% RH. As shown in FIG. 3(b), the resistance and response of the sensor has a large relationship with the loading of the CQDs. CQDs @ Co [ Ni (CN) ]with increasing CQDs content4]The resistance of the nanosheets decreased and gradually stabilized to 1ppm NH3The response of (c) is shown as a trend of "increase-max-decrease". When CQDs loading was 14.01 wt%, the highest response value was obtained at 241% which was approximately pure Co [ Ni (CN)4]2.7 times that of the nanosheet. Therefore, subsequent experiments were conducted using devices loaded with 14.01 wt% CQDs for gas sensing experiments.
FIG. 4(a) shows CQDs @ Co [ Ni (CN)4]Composite film sensor for different NH concentrations at room temperature3Continuous dynamic response test. Mixing CQDs @ Co [ Ni (CN)4]Thin film sensor exposure to NH in the range of 0.1-30ppm3And switching in air, wherein the switching time interval is 200s, and the response of the sensor along with NH can be obviously observed3The increase in concentration increased with a response that reached a maximum at 30ppm, approximately 87.7, for various concentrations of NH3Have good response recovery characteristics. The response curve of the sensor at 0.1-1ppm is embedded in the graph because the response value at 0.1-1ppm is greatly different from the response value at 30ppm of high concentration, so that the low-concentration dynamic response curve is not obvious.
In particular, CQDs @ Co [ Ni (CN)4]The response of the composite film sensor is slightly larger than that of pure Co [ Ni (CN)4]Film sensor, about 80.8 at 30ppm and capable of detecting practically 100ppb of NH3. Thereby embodying the modification of CQDs not only to Co [ Ni (CN)4]The response of the thin film sensor is facilitated and the detection limit of the sensor is also successfully reduced. Furthermore, it was found during the experiment that when the sensor was exposed to NH3Thereafter, the resistance still showed a tendency to drop significantly, indicating that doping CQDs did not change the type of the semiconductor.
FIG. 4(b) shows CQDs @ Co [ Ni (CN)4]Response of thin film sensor and NH3Fitted graph of concentration and error bar. And Co [ Ni (CN)4]Thin film sensor identity, following NH3The response of the sensor increases linearly with the increase of the concentration, and the fitting equation is-1.3820 +2.8700X, and the regression coefficient is R20.9865. Due to the large difference between the concentration intervals and the response values in the range of ppb level and ppm levelThe fit curve is not apparent in the ppb range, so the fit curve of the sensor at 0.1-1ppm and an error bar are embedded in the figure. Response of the sensor with NH in the ppb range3The increase in concentration was still linear with the fitting equation of-27.1450 +0.2612X with a regression coefficient R20.9698. Notably, by comparison of CQDs @ Co [ Ni (CN)4]Film sensor and Co [ Ni (CN)4]The fitting coefficients of the thin film sensor in the ppm scale range, 2.8700 and 2.6429, can find CQDs @ Co [ Ni (CN)4]Thin film sensor pair NH3And is more sensitive. The standard deviation of the triplicate measurements was calculated to be about 6%, indicating that the fit equation correlates well with the data points. Thus, CQDs @ Co [ Ni (CN)4]The thin film sensor can be used for quantitative analysis of NH3The concentration of (c). In addition, the theoretical detection limit of the sensor is calculated to be as low as 8ppb by using a detection limit formula, compared with pure Co [ Ni (CN)4]The performance of the film sensor is greatly improved.
FIG. 5(a) is a graph of CQDs @ Co [ Ni (CN) ]under the same experimental conditions4]Thin film sensor at NH3The reproducibility of the sensors at concentrations of 0.5, 0.75, 1 and 10ppm, respectively, was repeated 3 times for each concentration. The experimental result shows that the response recovery of the gas sensor has good consistency and reproducibility. In addition, by targeting CQDs @ Co [ Ni (CN).)4]The thin film sensor was tested for long term stability to further illustrate the stability of the sensor. As shown in FIG. 5(b), 6 NH species were detected every 5 days during the 30-day test period3The long term stability plots for the sensors at 0.25ppm, 0.5ppm and 1ppm are embedded in the figure, since the concentration intervals and response values in the ppb range are significantly different from those in the ppm range, causing the long term stability curves in the ppb range to be compressed without being significant. As can be seen from the graph, the response value of the device fluctuates little, indicating CQDs @ Co [ Ni (CN)4]The thin film sensor has excellent long-term stability characteristics.
FIG. 6(a) shows CQDs @ Co [ Ni (CN)4]Thin film sensor inExposure to various concentrations of NH at room temperature3Later response and recovery time point plots. The response and recovery time point plots of the sensor in the ppb level range, i.e., 100, 150, 200, 250, 500, and 750ppb, are embedded in the plot because the concentration intervals in the ppb level range are significantly different from those in the ppm level range, resulting in compression without significant degradation of the response and recovery time data in the ppb level range. It can be seen from the figure that the response time of the sensor in the ppb range does not change significantly compared to the ppm range, about 40s, while the recovery time drops from greater than 20s to about 10 s. Overall, compared to pure Co [ Ni (CN)4]Film sensor, CQDs @ Co [ Ni (CN)4]The response/recovery time of the thin film sensor is not significantly improved.
To detect CQDs @ Co [ Ni (CN)4]Cross-sensitivity of thin film sensors to common reducing gases CQDs @ Co [ Ni (CN)4]And Co [ Ni (CN)4]The selectivity of the thin film sensor was compared and experiments were performed with the same reducing gas at the same concentration, i.e., 5ppm concentration, and a three-dimensional histogram as shown in fig. 7 was made. CQDs @ Co [ Ni (CN)4]The thin film sensor has a thin film made of a material having a chemical structure similar to that of Co [ Ni (CN)4]Thin film sensor pair NH3Similar specific detection capability, the response value to most reducing gases is much lower than that of NH3. For example, the response values for highly reactive gas molecules acetone and methanol are 2.60 and 2.41, respectively, which are much lower than for NH3Response value of 10.2.
Influence of humidity on detection of ammonia by carbon quantum dot-cobalt tetracyanide composite film sensor
Mixing CQDs @ Co [ Ni (CN)4]The film sensor is used for detecting ammonia gas under different relative humidity environments, and different saturated salt solutions are adopted to simulate the humidity testing environment of the gas sensor; the humidity testing environment configuration process comprises the following steps: lithium chloride (LiCl) and potassium acetate (CH)3COOK), magnesium chloride (MgCl)2) Potassium carbonate (K)2CO3) Magnesium nitrate (Mg (NO)3)2) Copper chloride (CuCl)2) Sodium chloride (NaCl), potassium chloride (KCl) and potassium sulfate (K)2SO4) Pair of saturated salt solutionsThe humidity should be 11%, 23%, 33%, 43%, 52%, 67%, 75%, 85% and 97% RH, respectively, using phosphorus pentoxide (P)2O5) The powder acts as a desiccant to provide a dry detection environment (0% RH) for the moisture sensitive sensor. .
FIG. 7 is CQDs @ Co [ Ni (CN)4]Baseline resistance point plots for the thin film sensor in air at 43%, 52%, 67%, 75%, 85%, and 97% relative humidity. Co [ Ni (CN)4]Film sensor consistency, when CQDs @ Co [ Ni (CN)4]The thin film sensor is placed in a humid environment and its base resistance is significantly reduced. This may be due to adsorption of water molecules in the test environment on the surface of the sensitive layer, water or H produced by ionization thereof3O+Ion pair CQDs @ Co [ Ni (CN)4]The film has a further protonation doping function, so that the carrier transmission speed of the film is accelerated, and the base resistance of the whole composite film is reduced. At a relative humidity of 33%, the resistance of the sensor is greater than 1 G.OMEGA.3The sensing performance test of (1) was performed in an environment having a relative humidity of 43%, 52%, 67%, 75%, 85%, and 97%, respectively. FIG. 7(b) illustrates the sensor at these 6 humidities for different concentrations of NH3The response value data point diagram and the three-dimensional plane fitting diagram thereof have the fitting equation: z is-6.53X +2.49Y +4.75, and the regression coefficient is R20.9702. Response value of sensor to relative humidity and NH3The relationship between concentrations is shown by a linear fit curve as shown in FIG. 7(c), and CQDs @ Co [ Ni (CN)4]The film sensor has negative alpha and positive beta values, overall trend and pure Co [ Ni (CN)4]Thin film sensors are uniform and humidity can also have an effect on the response of the sensor.
Apparently, CQDs @ Co [ Ni (CN)4]The alpha absolute value of the film sensor is less than that of pure Co [ Ni (CN)4]Absolute value of alpha for thin film sensors, which means that loading CQDs can significantly improve humidity versus pure Co [ Ni (CN)4]The influence of the thin film sensor and the high response value and the low detection limit are simultaneously obtained, and the sensing performance is provided.
Mechanism for sensing carbon quantum dot/cobalt tetracyanide nickelate composite film sensor on ammonia gas
The invention proves CQDs @ Co [ Ni (CN)4]Thin film sensor pair NH3The sensitive mechanism of (2).
FIG. 8 shows CQDs @ Co [ Ni (CN)4]Adsorption of nano-sheet on NH3Front and back powder X-ray diffraction PXRD spectrograms from which it can be seen that its crystal structure is exposed to saturated NH3Then remains unchanged, NH3The molecule does not have CQDs @ Co [ Ni (CN)4]The structure of the nanoplatelets causes irreversible destruction.
As shown in FIG. 9, CQDs @ Co [ Ni (CN)4]Adsorption of NH by nanosheets3Front and back Fourier infrared spectrograms. First, at about 1660cm-1The absorption peak is the characteristic peak of deformation vibration of water molecules, which means that the peak is at CQDs @ Co [ Ni (CN)4]Water molecules exist between layers of the nano-sheet, and the characterization result is compared with pure Co [ Ni (CN)4]The nanosheets are in a consistent mechanism, with water molecules providing potential hydrogen bonding channels to enhance CQDs @ Co [ Ni (CN)4]Conductivity of the composite film. In particular, at 1662cm-1At the position of NH adsorbed on the Bronsted acid site4+Shows NH by the symmetrical bending vibration absorption peak3Adsorption to CQDs @ Co [ Ni (CN)4]The film surface will lose electrons and form NH4+Exist in the form of (1). This further confirms NH3Molecular adsorption on CQDs @ Co [ Ni (CN)4]O of film surface2 -A redox reaction occurs, which is also consistent with the gas sensing experimental phenomenon of the present invention. Notably, in 2168cm-1The characteristic peak of C ≡ N stretching vibration observed in the method is used for adsorbing NH3The molecules then undergo a red-shift. Generally, when the structure of a compound is changed or influenced by external conditions, the absorption peak is shifted, and when the wavelength of the absorption peak is shifted to a low frequency, the red shift is obtained. The formation of hydrogen bonds tends to shift the stretching vibration frequency in the low frequency direction, and the absorption intensity is increased, and the peak shape is widened. Therefore, the reason is that it is located in Co2+Interlayer water molecule and NH on site3Hydrogen bond is formed between moleculesResulting in a red shift of the absorption peak to cyano in Co-C.ident.N. In addition, generally, the cyano group absorption peak is characterized by a sharp needle-like peak shape and a strong strength, but it can be observed from the figure that NH is adsorbed3The shape of the cyano absorption peak then changes from a peak to a shoulder, which is NH3Selective adsorption to Co2+The metal sites provide theoretical support. The above is in pure Co [ Ni (CN)4]There was no observed phenomenon in the characterization results of the nanosheets. Finally, it is located at 937cm-1Absorption peak at (1) is gas phase NH3Or weakly adsorbed NH3A molecule. The above results show that compared to pure Co [ Ni (CN)4]Nanosheets, NH3Molecules with CQDs @ Co [ Ni (CN)4]The skeletons of the nano sheets have stronger interaction. In particular, the carbon quantum dots loaded on the surface of the material not only play a role in catalyzing NH3The oxidation-reduction reaction of adsorption and Co promotion2+Water-ammonia hydrogen bond channel formation on active site, thereby serving as a node of electron transition, solving pure Co [ Ni (CN)4]Sensor water molecules can occupy partial active sites on the surface of the sensor, so that the response performance is influenced, and the humidity resistance is improved.
In addition, CQDs @ Co [ Ni (CN)4]Adsorption of NH by nanosheets3The results of UPS characterization of ultraviolet photoelectron spectroscopy found CQDs @ Co [ Ni (CN)4]The work function of the nanoplatelets was reduced by 0.96eV as shown in figure 10. And pure Co [ Ni (CN)4]The nano sheets have consistent characterization results and adsorb NH3Thereafter, the work function of the sample is decreased. In particular, in the adsorption of NH3Thereafter, CQDs @ Co [ Ni (CN)4]The work function change of the nano sheet is larger than that of pure Co [ Ni (CN)4]Nanosheets due to CQDs @ Co [ Ni (CN)4]Nanosheet and NH3There is a more intense electron-hole activity between the molecules, which explains why CQDs @ Co [ Ni (CN)4]Thin film sensor pair NH3With higher response values.
In summary, a series of characterization results are not only CQDs @ Co [ Ni (CN)) 4]Film sensor pair NH3The sensing performance of (2) is explained, andevidence of the present invention CQDs @ Co [ Ni (CN)4]Film sensor to NH compared to existing sensors3With higher response and lower detection limit.
Finally, it should be noted that: although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that changes may be made in the embodiments and/or equivalents thereof without departing from the spirit and scope of the invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (13)
1. CQDs @ Co [ Ni (CN)4]The preparation method of the nanosheet is characterized by comprising the following steps:
1) preparing two-dimensional cyano-bridged Co-Ni hybrid metal nanosheets by using cobalt nitrate hexahydrate and potassium tetracyanide nickelate as raw materials, polyvinylpyrrolidone as a surfactant and sodium citrate as a control agent under the combined action of the cobalt nitrate hexahydrate and the potassium tetracyanide nickelate;
2) disodium ethylene diamine tetraacetate EDTA-2Na is taken as a precursor in N2Or calcining at 200-300 ℃ under inert atmosphere at the heating rate of 1-5 ℃/min for 1-3 h; dispersing the product in water and performing ultrasonic treatment, and performing centrifugal filtration and dialysis to remove impurities to obtain CQDs (CQDs for short) as carbon quantum dots;
3) mixing Co [ Ni (CN)4]The solution was mixed with the synthetic CQDs solution with constant stirring and the resulting CQDs @ Co [ Ni (CN)4]Nanosheets, using CH3OH washing and dispersing for later use;
the two-dimensional cyano-bridged Co-Ni hetero-metal nanosheet is specifically Co [ Ni (CN)4]Nanoplatelets prepared by a process comprising:
firstly, cobalt nitrate hexahydrate Co (NO)3)2·6H2O and polyvinylpyrrolidone PVP dissolved in methanol CH3In OH;
② potassium tetracyanium nickelate K2[Ni(CN)4]And sodium citrate Na3C6H5O7·2H2Dissolving O in 10mL of methanol-water mixed solvent with the volume ratio of 1: 1;
mixing the two solutions obtained in the first step and the second step uniformly and performing ultrasonic oscillation; then oscillating using a vortex oscillator; then ultrasonic oscillation is carried out to obtain a mixed uniform solution;
fourthly, after the uniform solution is stood at room temperature, repeating the step three at least once;
placing the solution at room temperature until the solution becomes turbid; collecting Co [ Ni (CN)4]Washing the nano-sheet by repeated centrifugation with methanol solution for more than two times;
sixthly, obtaining Co [ Ni (CN)4]The nanosheets were redispersed in a methanol solution for use.
2. The CQDs @ Co [ Ni (CN) ] according to claim 14]The preparation method of the nanosheet is characterized by comprising the following steps: the preparation process of the carbon quantum dot comprises the following steps:
a. EDTA-2Na was placed in a tube furnace under N2Calcining for 1-2h in a tubular furnace at 200-250 ℃ at the heating rate of 3-5 ℃/min under the atmosphere;
b. grinding and dispersing the product in water, carrying out ultrasonic treatment on the suspension at room temperature, and carrying out high-speed centrifugation at 8000-10000 rpm; filtering the upper layer solution with filter paper of 0.3 μm or less to remove deposited Na salt;
c. dialyzing the solution with a dialysis tube, removing residual salt and fragments, drying to obtain carbon quantum dot powder, and dispersing in water for later use.
3. The CQDs @ Co [ Ni (CN) ] according to claim 24]A method of preparing a nanoplate, comprising: in the step I, the mass ratio of the cobalt nitrate hexahydrate to the polyvinylpyrrolidone is 1 (10), and the mass ratio of the cobalt nitrate hexahydrate to the potassium tetracyanide nickelate in the step 1 is 1 (0.5-4).
4. CQDs @ Co [ Ni (CN)4]The application of the nano-sheet in ammonia gas detection is characterized in that: CQDs @ Co [ Ni (CN)4]Nanosheets being produced using the production method as defined in any one of claims 1 to 3, C being produced using a spin-coating methodQDs@Co[Ni(CN)4]Thin film sensor, gas sensitive test platform, and NH treatment of the obtained sensor3The gas-sensitive characteristic test of (1).
5. Use according to claim 4, characterized in that: mixing CQDs @ Co [ Ni (CN)4]The film sensor is used for detecting electron conductivity, load ammonia response value, dynamic response characteristic, stability and response recovery characteristic, selectivity and humidity characteristic.
6. Use according to claim 4, characterized in that: at CQDs @ Co [ Ni (CN)4]In thin film sensors, CQDs are reported in CQDs @ Co [ Ni (CN)4]The loading amount on the catalyst is 1.25-30 wt%.
7. Use according to claim 6, characterized in that: when the CQDs loading is 14.01 wt%, the sensor response value is 241% under 67% RH environment, and the CQDs loading is pure Co [ Ni (CN)4]2.5 times of the nano sheet.
8. Use according to claim 4, characterized in that: at CQDs @ Co [ Ni (CN)4]Real-time dynamic response NH for thin film sensors3In gas detection, CQDs @ Co [ Ni (CN)4]The thin film sensor is exposed to NH with a gas concentration in the range of 0.1-30ppm3And switching measurements were performed in air, with a time interval of 200s for each switching.
9. Use according to claim 8, characterized in that: at CQDs @ Co [ Ni (CN)4]NH for stabilization of thin film sensors3In the gas detection, CQDs @ Co [ Ni (CN)4]Thin film sensors were exposed to four concentrations of NH at room temperature, 0.5, 0.75, 1 and 10ppm3Performing a repeatability test, wherein each concentration is repeatedly tested for 3 times; by exposing the sensor to 0.25, 0.5, 1, 5, 10 and 20ppm NH at room temperature3In (1), the response value of the sensor was measured every 5 days for one month, and CQDs @ Co [ Ni (CN)4]Of thin-film sensorsLong term stability.
10. Use according to claim 4, characterized in that: mixing CQDs @ Co [ Ni (CN)4]The film sensor is used for ammonia detection in different relative humidity environments, and different saturated salt solutions are adopted to simulate the humidity testing environment of the gas sensor.
11. Use according to claim 10, characterized in that: the humidity testing environment configuration process comprises the following steps: lithium chloride LiCl and potassium acetate CH3COOK, MgCl chloride2Potassium carbonate K2CO3Magnesium nitrate Mg (NO)3)2Copper chloride CuCl2Sodium chloride NaCl, potassium chloride KCl and potassium sulfate K2SO4Saturated salt solution with humidity of 11%, 23%, 33%, 43%, 52%, 67%, 75%, 85% and 97% RH, respectively, and phosphorus pentoxide P2O5The powder is used as a drying agent and is CQDs @ Co [ Ni (CN)4]The thin film sensor provides a dry detection environment of 0% RH.
12. Use according to claim 10, characterized in that: CQDs @ Co [ Ni (CN)4]The humidity environment of the film sensor is greater than or equal to 43% RH.
13. Use according to claim 4, characterized in that: CQDs @ Co [ Ni (CN)4]Nano-plate in ammonia gas detection, NH3After adsorption to CQDs @ Co [ Ni (CN)4]Oxidation-reduction reaction is carried out after the surface of the film to obtain NH in an ionic state4+Exists in the form of (1); and NH3Selective adsorption to Co2+Metal site at Co2+Interlayer water molecules and NH on the sites3Hydrogen bonds are formed among molecules; make NH3Molecules with CQDs @ Co [ Ni (CN)4]Stronger electron-hole activity is provided among the nanosheet skeletons.
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