CN117737887B - Preparation method and application of composite nanofiber gas-sensitive material - Google Patents
Preparation method and application of composite nanofiber gas-sensitive material Download PDFInfo
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- 239000013154 zeolitic imidazolate framework-8 Substances 0.000 claims abstract description 37
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- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 claims abstract description 31
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- Investigating Or Analyzing Materials By The Use Of Fluid Adsorption Or Reactions (AREA)
Abstract
The invention provides a preparation method and application of a composite nanofiber gas-sensitive material, comprising the following steps: s1: preparing a ZIF-67 crystal material; s2: preparing a ZIF-67 nanofiber membrane from the ZIF-67 crystal material prepared in the step S1 by an electrostatic spinning method; s3: preparing ZIF-67 nanofiber membranes and ZIF-8 growth solution from the ZIF-67 nanofiber membranes prepared in the step S2 to prepare ZIF-67@ZIF-8 core-shell nanofibers; s4: and (3) annealing the ZIF-67@ZIF-8 core-shell nanofiber prepared in the step (S3) to obtain the Co 3O4/ZnO composite nanofiber. The invention has the beneficial effects that: the composite nanofiber gas-sensitive material has low detection limit, high sensitivity, strong selectivity, good repeatability and long-term stability, and provides a feasible method for detecting the concentration of hydrogen sulfide gas in air.
Description
Technical Field
The invention belongs to the field of gas sensitive material synthesis and gas sensor manufacturing, and particularly relates to a preparation method of a hydrogen sulfide gas sensor based on a ZIF-67@ZIF-8 derived Co 3O4/ZnO composite nanofiber gas sensitive material.
Background
The gas sensor is an electronic device for converting the type and the concentration of target gas into electric signals according to a certain rule, and has wide application in the fields of atmosphere pollution monitoring, petrochemical industry, intelligent medical treatment, agricultural production and the like. Hydrogen sulfide is used as a highly toxic gas and is mainly inhaled and poisoned by respiratory tracts, the central nervous system, the respiratory system and the cardiovascular system are all affected, and under high concentration, H 2 S can immediately cause death. The gas sensor mainly utilizes the sensitivity characteristic of a gas-sensitive material to target gas to realize the identification and concentration detection of the target gas, and the semiconductor material in the prior art is often poor in selectivity, insufficient in anti-interference capability and low in detection precision, so that the hydrogen sulfide gas sensor is low in sensitivity and low in selectivity.
Disclosure of Invention
In view of the above, the invention aims to provide a preparation method and application of a composite nanofiber gas-sensitive material, so as to solve the problems of low sensitivity and low selectivity of a hydrogen sulfide gas sensor.
In order to achieve the above purpose, the technical scheme of the invention is realized as follows:
A preparation method of a composite nanofiber gas-sensitive material comprises the following steps:
s1: preparing a ZIF-67 crystal material;
S2: preparing a ZIF-67 nanofiber membrane from the ZIF-67 crystal material prepared in the step S1 by an electrostatic spinning method;
S3: preparing ZIF-67 nanofiber membranes and ZIF-8 growth solution from the ZIF-67 nanofiber membranes prepared in the step S2 to prepare ZIF-67@ZIF-8 core-shell nanofibers;
S4: and (3) annealing the ZIF-67@ZIF-8 core-shell nanofiber prepared in the step (S3) to obtain the Co 3O4/ZnO composite nanofiber.
Further, the preparation method of the ZIF-67 crystal material in the step S1 comprises the following steps: mixing the solution containing cobalt ions with the dimethyl imidazole solution, precipitating after the reaction is finished, washing the obtained precipitate, centrifuging and drying to obtain the ZIF-67 crystal material.
Further, the solution containing cobalt ions is a cobalt nitrate solution;
And/or the solvent of the cobalt ion-containing solution and the dimethyl imidazole solution is methanol;
And/or the mole ratio of cobalt ions to dimethylimidazole is 1:4, a step of;
and/or precipitating after the reaction is finished, wherein the precipitation comprises the steps of precipitating the obtained solution after the reaction at room temperature by at least 24 h, pouring out supernatant, pouring the rest suspension into a centrifuge tube, and centrifuging in the centrifuge to obtain precipitate;
And/or washing with absolute ethanol during washing;
And/or the temperature of drying is 60 ℃, and the drying time is at least 24 h.
Further, in the step S2, the step of electrostatic spinning comprises the steps of adding the ZIF-67 crystal material and the polymer prepared in the step S1 into an organic solvent, uniformly mixing to obtain a precursor solution, and spinning into a ZIF-67 nanofiber membrane by an electrostatic spinning method;
the mass ratio of ZIF-67 to polymer is 12.5-37.5%.
For the mass ratio of ZIF67 to PAN, the variables are set at the same parameters as DMF/THF, the mass ratio is 12.5-37.5, more preferably, the mass ratios are 12.5, 25 and 37.5. Different mass ratios can affect the morphology of the fibers and also affect the gas-sensitive properties of the material.
Further, the polymer comprises one of polyacrylonitrile, polyvinyl alcohol and polyvinylpyrrolidone; the organic solvent is N, N dimethylformamide and tetrahydrofuran;
the volume ratio of N, N dimethylformamide to tetrahydrofuran is 1:3-5.N, N dimethylformamide and tetrahydrofuran are used in a matching way, so that the spinning stability can be improved, and the obtained fiber is more uniform.
And/or the advancing speed of the needle tube in the electrostatic spinning is 1.0 mL/h, the distance between the needle tip and the collector is 10-20 cm, the external voltage of the electrostatic spinning is 18-kV, and the relative humidity of the environment in the electrostatic spinning is 40% -45%.
Further, the preparation of the ZIF-67@ZIF-8 core-shell nanofiber in the step S3 comprises the steps of putting the ZIF-67 nanofiber membrane obtained in the step S2 into a ZIF-8 growth solution, stirring at constant temperature, taking out the treated nanofiber membrane, and washing to obtain the ZIF-67@ZIF-8 nanofiber with a core-shell structure.
Further, the preparation of the ZIF-8 growth solution in the step S3 comprises the steps of putting zinc nitrate into methanol to obtain a mixed solution A, putting dimethyl imidazole into methanol to obtain a mixed solution B, mixing and stirring the mixed solution A and the mixed solution B, and controlling different masses to form different molar ratios.
The molar ratio of zinc ions to dimethylimidazole in the ZIF-8 growth solution is 1:2-6, limiting the molar ratio to control the nucleation size of ZIF-8, thereby affecting the morphology of the core-shell fiber.
In the step S4, the annealing temperature is 450-550 ℃, the heating rate is set to 2 ℃/min, and the heat preservation time is set to 125-175 min. The calcining temperature directly affects the morphology of the obtained metal oxide, and the heating rate and the heat preservation time indirectly affect the hollow porous structure and the specific surface area of the material.
The application of the composite nanofiber gas-sensitive material prepared by the preparation method of the composite nanofiber gas-sensitive material as the gas-sensitive material.
A hydrogen sulfide gas sensor uses a composite nanofiber gas-sensitive material prepared by a preparation method of the composite nanofiber gas-sensitive material.
The preparation method of the hydrogen sulfide gas sensor comprises the steps of grinding the prepared Co 3O4/ZnO composite nanofiber into powder, dripping absolute ethyl alcohol to form a uniformly dispersed suspension, dripping the suspension onto the surface of an interdigital electrode, standing at room temperature, and aging after the ethanol is completely volatilized to obtain the hydrogen sulfide gas sensor.
In the aspect of molecular structure design of the gas-sensitive material, the invention adopts a Metal Organic Framework (MOF) derived method to prepare the Co 3O4/ZnO composite nanofiber, and has the following three main advantages: first, the semiconductor metal oxide derived by using MOF as a self-sacrifice template has regular internal pores, large specific surface area and open metal sites, has remarkable enhancement on gas-sensitive performance, and has unique advantages in gas sensing. Secondly, the invention combines two different MOFs together to form a novel heterogeneous tuberculosis shell fiber crystal structure ZIF-67@ZIF-8 (MOF-ON-MOF), solves the problem of poor gas-sensitive performance of single material, and enhances the sensitivity of the derived Co 3O4/ZnO gas-sensitive material to H 2 S. Thirdly, the Co 3O4/ZnO composite nanofiber formed by the two MOF materials has a p-n heterojunction, and the existence of the p-n heterojunction can improve the electron transfer efficiency and enlarge the electron depletion layer, so that the sensitivity of the material to H 2 S is improved.
In the aspect of a gas sensitive material synthesis technical route, the invention takes ZIF-67@ZIF-8 core-shell fiber as a template, and the Co 3O4/ZnO nanofiber with a p-n heterojunction is obtained through calcination. ZIF-67 and ZIF-8 are composed of inorganic metal cations (mainly Co or Zn) and imidazole ligands, and have excellent thermal stability and chemical stability. And ZIF-67 and ZIF-8 have the same topological structure, similar lattice parameters and the same organic ligand, and the two-phase interface is completely in a coherent matching relationship, so that epitaxial growth at the two-phase interface is easy to realize. Thus, by utilizing the attractive forces between MOFs, the ZIF-67/PAN film is attached to the nucleated ZIF-8 in the ZIF-8 growth solution to form a ZIF-67@ZIF-8 core-shell fiber structure.
In the aspect of a specific preparation method of the gas-sensitive material, the invention adopts an electrostatic spinning method and an in-situ growth technology to prepare ZIF-67@ZIF-8 core-shell fiber, and Co 3O4/ZnO obtained by annealing is a one-dimensional hollow nano composite fiber under different thermal stresses by using a Kendall effect.
According to the invention, a Metal Organic Framework (MOF) ZIF-67 is firstly prepared, an electrostatic spinning technology is used for preparing a ZIF-67 nanofiber membrane, ZIF-67 and ZIF-67 are used for compounding ZIF-67 and ZIF-8 to form an MOF-on-MOF structure by utilizing mutual attraction between in-situ growth and MOF, ZIF-67@ZIF-8 core-shell nanofiber is prepared, and a Co 3O4/ZnO composite nanofiber derived from ZIF-67@ZIF-8 is obtained after high-temperature annealing and is used as a sensitive material for preparing a hydrogen sulfide gas sensor. According to tests, the sensitivity of the metal oxide Co 3O4/ZnO nanofiber material derived from the MOF is improved compared with that of single traditional metal oxide Co 3O4, znO and traditional structure Co 3O4/ZnO nanofiber materials.
Compared with the prior art, the preparation method and application of the composite nanofiber gas-sensitive material have the following advantages:
1. The invention establishes a preparation technology for preparing Co 3O4/ZnO composite nano-fiber by using a MOF material-derived method, and compared with the traditional metal oxide nano-material synthesis method, the synthesized Co 3O4/ZnO composite nano-fiber has a unique porous tubular structure, shows higher porosity and larger specific surface area, and improves the sensitivity to hydrogen sulfide gas.
2. The invention realizes the controllable preparation of the novel MOF-on-MOF nano material by simple and feasible technical means such as electrostatic spinning, and solves the limitation of poor gas-sensitive performance of a single material by constructing a heterojunction to improve the gas-sensitive performance of the material. The sensitivity of the sensor formed by the Co 3O4/ZnO composite nano-fiber is about 10 times higher than that of the sensor formed by ZnO and about 50 times higher than that of the sensor formed by Co 3O4.
3. The gas-sensitive material of the invention has a fiber structure in microstructure, and has obviously enhanced gas-sensitive performance compared with the nano materials with structures such as ZnO/Co 3O4 polyhedron, co 3O4 @ZnO core-shell and the like in the prior study.
4. The gas sensor has low detection limit, good response to 200 ppb low-concentration hydrogen sulfide gas, response time of 88.7 s and recovery time of 110.6 s.
5. The gas sensor has strong selectivity, and the sensitivity to hydrogen sulfide gas is far higher than that of interference gases such as ammonia gas (NH 3), ethanol (ethanol), xylene (xylene) nitrogen dioxide (NO 2), sulfur dioxide (SO 2) formaldehyde (Formaldehyde) and the like under the conditions of 325 ℃ and 200 ppb gas concentration.
6. The gas sensor has good repeatability, and the sensitivity to 200 ppb H 2 S gas under the working condition is always stable to about 45V/g.
7. The gas sensor has good long-term stability, and the sensitivity of the sensor to hydrogen sulfide of 5 ppm is stabilized to be about 1000V/g within 30 days.
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 invention. In the drawings:
FIG. 1 is a scanning electron microscope image of ZIF-67 crystal in example 1 of the present invention;
FIG. 2 is a graph of electrostatic spinning needles of different volume ratios of N, N-dimethylformamide to amide tetrahydrofuran according to the present invention (a: DMF/THF volume ratio of 1:3, b: DMF/THF volume ratio of 1:4, c: DMF/THF volume ratio of 1:5, d: DMF/THF mass ratio of 1:0);
FIG. 3 is a scanning electron micrograph of ZIF-67/PAN nanofibers of example 1 (a is 12.5 wt% ZIF-67/PAN nanofibers without tetrahydrofuran added; b-d is nanofibers of ZIF-67/PAN with DMF/THF 1:4, where b is 12.5 wt% ZIF-67/PAN, c is 25wt% ZIF-67/PAN, d is 37.5wt% ZIF-67/PAN);
FIG. 4 is a Co 3O4/ZnO electron microscope image of different mole ratios of zinc ions to dimethylimidazole growth of the present invention (the mole ratio of zinc ions to dimethylimidazole in a is 1:6, the mole ratio of zinc ions to dimethylimidazole in b is 1:4, and the mole ratio of zinc ions to dimethylimidazole in c is 1:2);
FIG. 5 shows the variation of the thermal decomposition temperature versus morphology of the Co 3O4/ZnO nanofiber of the present invention (a is a graph of Co 3O4/ZnO nanofiber at 450 ℃, b is a graph of Co 3O4/ZnO nanofiber at 500 ℃, c is a graph of Co 3O4/ZnO nanofiber at 550 ℃, d is a schematic view of the thermal decomposition process
FIG. 6 is a schematic diagram of a Co 3O4/ZnO composite nanofiber of the present invention (a is a TEM image of a Co 3O4/ZnO composite nanofiber, b is an electron diffraction image of a Co 3O4/ZnO composite nanofiber, c is a Co 3O4 and ZnO lattice spacing diffraction image (10 nm) of a Co 3O4/ZnO composite nanofiber, d is a Co 3O4 and ZnO lattice spacing diffraction image (2 μm) of a Co 3O4/ZnO composite nanofiber, e is an element profile (O) of a Co 3O4/ZnO composite nanofiber, f is an element profile (Co) of a Co 3O4/ZnO composite nanofiber, g is an element profile (Zn) of a Co 3O4/ZnO composite nanofiber);
FIG. 7 is a schematic diagram of the hydrogen sulfide gas sensor of the present invention;
FIG. 8 is a graph showing the response of the assembled sensor of Co 3O4/ZnO、Co3O4 fiber, znO fiber, etc. of the present invention to 5 ppm H 2 S at different temperatures;
FIG. 9 is a graph showing the response of materials prepared with different molar ratios of zinc ions and dimethylimidazole of the present invention to 5 ppm H 2 S at different temperatures;
FIG. 10 is a graph of the Co 3O4/ZnO response of the present invention to 5 ppm H 2 S at a calcination temperature of 450-550 ℃;
FIG. 11 is a graph of the dynamic response of the Co 3O4/ZnO sensor of the present invention to 200 ppb H 2 S gas at 325 ℃;
FIG. 12 is a graph of the dynamic response of the Co 3O4/ZnO sensor of the present invention to H 2 S at 325℃over a concentration range of 200 ppb-5 ppm;
FIG. 13 is a graph showing the response of a Co 3O4/ZnO sensor of the present invention to various gases having a concentration of 200 ppb at 325 ℃;
FIG. 14 is a graph of the dynamic cyclic response of a Co 3O4/ZnO sensor of the present invention to 200 ppb H 2 S gas at 325 ℃;
FIG. 15 is a graph of the long term stability of the Co 3O4/ZnO sensor of the present invention to 5 ppm H 2 S at 250 ℃;
FIG. 16 is a graph showing the specific surface area and pore size analysis of Co 3O4/ZnO、Co3O4 fiber and ZnO fiber according to the present invention (a is a graph showing the specific surface area and pore size analysis of Co 3O4/ZnO nanofiber, b is a graph showing the specific surface area and pore size analysis of Co 3O4 fiber, and c is a graph showing the specific surface area and pore size analysis of ZnO fiber).
Detailed Description
It should be noted that, without conflict, the embodiments of the present invention and features of the embodiments may be combined with each other.
The invention will be described in detail below with reference to the drawings in connection with embodiments.
Example 1
The preparation method of the hydrogen sulfide gas sensor based on the ZIF-67/ZIF-8 derived Co 3O4/ZnO composite nanofiber gas-sensitive material comprises the following steps:
And step 1, preparing the ZIF-67 nanomaterial.
The molar ratio of cobalt ions to dimethylimidazole was controlled to 1:4. weighing 5.82 g cobalt nitrate hexahydrate, and putting the cobalt nitrate hexahydrate into a beaker A containing 250 mL methanol for stirring and dissolving; 6.56 g dimethylimidazole was weighed into beaker B containing 250 mL methanol and dissolved with stirring. The solutions in beakers a and B were mixed and vigorously stirred for 15 minutes, after which the reaction was completed a purple opaque solution was finally obtained and precipitated at room temperature for 24 hours. Pouring out supernatant, pouring the rest suspension into a centrifuge tube, and centrifuging in a centrifuge for 5min (the rotation speed of the centrifuge is set to 10000 revolutions per minute) to obtain precipitate. Washing the precipitate with absolute ethyl alcohol for 6 times, repeating the centrifugation process, and finally, placing the obtained precipitate in a 60 ℃ oven for drying 24h to obtain the ZIF-67 nanomaterial. The nano ZIF-67 obtained by the method is in a dodecahedron shape, has uniform and regular size and is about 350nm in size, and a Scanning Electron Microscope (SEM) diagram is shown in figure 1.
And 2, preparing the ZIF-67 nanofiber membrane by an electrostatic spinning method.
The mass ratio of the uniform ZIF-67 to the PAN is controlled to be 1:4. 0.4g ZIF-67 and 1.6g Polyacrylonitrile (PAN) are weighed, placed in a 50 mL beaker, DMF/THF solvents with different volume ratios are added, magnetically stirred until the solvents are dissolved in a water bath environment at 60 ℃, and moved to a normal temperature environment to be stirred for 24 hours, so that a purple precursor solution is obtained. And (3) after the solution is subjected to ultrasonic dispersion for 30min, pouring the solution into a needle tube of an electrostatic spinning device, carrying out electrostatic spinning under a direct-current high-voltage power supply, taking off the aluminum foil paper on the receiving plate after the spinning process is finished, and removing the nanofiber membrane on the aluminum foil by using tweezers to obtain the ZIF-67 nanofiber membrane. In the electrostatic spinning process, under different DMF/THF ratios, the electrostatic spinning needle head shows different spinning states. It can be seen from fig. 2a-d that, with the other parameters unchanged, the addition of solvent THF significantly changed the droplet size at spinning, because the solvent formulation was different resulting in a change in solvent volatility, and in fig. 2d it can be seen that DMF, due to its higher boiling point, may evaporate at a slower rate at normal temperature and pressure, and thus the solution may accumulate at the needle, with the increase of the droplet, the charge distribution at the needle being unstable and a small portion of the fibers may be drawn from the sides of the droplet into filaments. After addition of THF, the volume ratio 1 in panel c was as follows: 5, the improvement is made, but the liquid drops are still present. In figure a, when the ratio of DMF to THF reaches 1:3, it can be observed that droplets form at the needle, at which time the droplets are due to too fast solvent evaporation, PAN is not stretched into filaments in time, resulting in gradual solidification and coagulation of the polymer, and as the spinning process proceeds, the droplets gradually increase until they drop off and accumulate again. Therefore, when the ratio of DMF to THF reaches 1:4, the spinning parameters are balanced, and the stable spinning can be realized without aggregation of liquid drops. FIG. 3a shows ZIF-67/PAN nanofibers without THF, FIG. 3b-d shows ZIF-67/PAN nanofibers of different mass with THF, FIG. 3b shows that the fiber diameter is thickened to about 1 μm, the thickness is significantly uniform, the spinning solution is sufficiently stretched, the surface has raised ZIF-67, and the aggregation is significantly reduced, and ZIF-67 is mostly coated in the fiber because it is much smaller than the fiber diameter, and a small amount of aggregation forms raised portions. This shows that the addition of THF with high volatility to adjust the surface tension and volatilization rate can well solve the problem of insufficient solvent volatility. The effect of varying ZIF-67 content on the appearance of the fiber was followed when the ratio of DMF to THF was 1:4, the stable spinning is obtained as shown in FIG. 3 c. When ZIF-67 is increased to 0.4g, i.e., 25% by mass, the ZIF-67 grains begin to agglomerate in small amounts, but are largely encapsulated in the fiber. So that intuitively, 25wt% ZIF-67/PAN is the ideal ratio. FIGS. 3c-d show that with increasing ZIF-67 mass, the aggregation of the fibers, ZIF-67, increases, and when ZIF-67 increases to 0.6 g, ZIF-67 grains form a cluster as shown in FIG. 3d, the dispersibility is poor, and the subsequent coating of ZIF-8 and maintenance of the morphology of the fibers after calcination are not favored, thereby affecting the gas-sensitive performance.
And 3, preparing the ZIF-67/ZIF-8 composite nanofiber.
And (3) putting the ZIF-67 nanofiber membrane obtained in the step (2) into a ZIF-8 growth solution. Preparation of ZIF-8 growth solution: 1.0g of zinc nitrate is placed in 40mL methanol, 1.0g of dimethyl imidazole is placed in 20 mL methanol, the two are mixed and stirred, and different masses are controlled to form different molar ratios, and the mixture is stirred for 3 hours in a water bath at the constant temperature of 25 ℃. And taking out the treated nanofiber membrane, washing the nanofiber membrane with methanol solution for 6 times to clean the residual ZIF-8 in the pores of the nanofiber membrane, and finally obtaining the ZIF-67/ZIF-8 nanofiber with the core-shell structure. FIG. 4 a-c is a drawing of the resulting ZIF-67/ZIF-8 nanofiber electron microscope at different molar ratios. FIG. 4a shows that when the molar ratio of zinc ions to dimethylimidazole is 1: and 2, ZIF-8 grains are smaller. FIG. 4b when the molar ratio of zinc ions to dimethylimidazole is 1: and 4, the grains are medium and surround the fiber to form a core-shell structure. When the molar ratio is further increased by 1:6, the grains are further enlarged, so that the fiber diameter is enlarged. The reason for this phenomenon is that as the molar ratio of imidazole increases, the more reactive the zinc ions, the more ZIF-8 grows, the easier the ZIF-8 grains nucleate, and the smaller the grains become.
And 4, preparing the Co 3O4/ZnO composite nanofiber.
Placing the ZIF-67/ZIF-8 nanofiber obtained in the step 3 into an alumina crucible, placing the crucible into a muffle furnace, annealing in an air atmosphere, setting the heating rate to be 2 ℃/min, (setting the annealing temperature to be 450 ℃,500 ℃,550 ℃ and the heat preservation time to be 150 min), thus obtaining the Co 3O4/ZnO composite nanofiber, as shown in fig. 5 a-c, wherein the thermal decomposition temperature directly influences the morphology of the fiber, when the calcining temperature is 500 ℃, the morphology of the fiber is optimal, and when the calcining temperature is 450 ℃ and 550 ℃, different fiber morphologies are caused due to different shrinkage forces.
The schematic diagram of the principle of thermal decomposition is shown in fig. 5 d, and the Co 3O4/ZnO nanofibers exhibit a highly porous hollow nanostructure, which can promote the diffusion of gas molecules, provide a large number of reactive sites, increase the specific surface, and facilitate the improvement of the gas sensitivity of the sensor. The hollow structure is formed because the Kendall effect (KIRKENDALL EFFECT) occurs in the heat treatment process, when the temperature in the muffle furnace is increased, the temperature gradient from the outer surface of the ZIF-67/ZIF-8 core-shell fiber to the inner core of the ZIF-67/ZIF-8 core-shell fiber is different, the ZIF-8 on the outermost layer is rapidly oxidized and crystallized to form a thermally stable ZnO crystal thin shell, and the PAN/ZIF-67 on the innermost layer is not volatilized in time due to being coated to form a molten state, so that internal ZIF-67 ions permeate into the surface by taking the molten PAN as a carrier. Specifically, two opposite forces, i.e., a contraction force (Fc) and a bonding force (Fa), occur at the interface between the crystal plane and the inside of the amorphous state. As PAN continues to carbonize, ZIF-8 tends to shrink inward, resulting in an increasing Fc. At the same time, the opposite Fa prevents the inward shrinkage of the fibers. With further increases in annealing temperature and incubation time, C, H and other elements have almost evaporated and disappeared, thus decreasing Fc. The precursor diffuses out at a much faster rate than air diffuses in, creating internal voids. When Fc is smaller than Fa, the internal ions further diffuse and shrink outwards from the center, and finally the hollow Co 3O4/ZnO nanofiber with stable skeleton structure and compact interdiffusion of Co 3O4 is formed.
To verify whether the target product was synthesized in the above synthesis step, the microstructure of the Co 3O4/ZnO composite nanofiber was further studied by TEM and HRTEM. Fig. 6a discloses a hollow structure of Co 3O4/ZnO composite nanofibres, with an average diameter of about 1 μm, a shell thickness of 40 to 80 nm. FIG. 6b shows a complex concentric multi-ring pattern of Co 3O4/ZnO samples by Selective Area Electron Diffraction (SAED). The presence of a large number of diffraction rings indicates the polycrystalline nature of the nanofibers. These diffraction rings correspond to crystal planes (220) (511) and (101) (102) of Co 3O4 and ZnO, respectively. Fig. 6c shows the apparent lattice fringes of Co 3O4 and ZnO lattice spacing (d). The measured lattice spacings of 0.260 nm and 0.462 nm correspond to the (111) crystal plane of Co 3O4 and the (002) plane of ZnO, respectively, indicating that heterojunction is formed between ZnO grains and Co 3O4 crystals. The corresponding EDS element profiles (fig. 6e, 6f, 6 g) verify the presence of O, co and Zn elements. In addition, the uniform distribution of Zn and Co elements on the surface of the fiber provides evidence for successful synthesis of Co 3O4/ZnO composite nanofibers.
And 5, preparing the hydrogen sulfide gas sensor.
And (3) grinding the Co 3O4/ZnO composite nanofiber obtained in the step (4) into powder in a mortar, and dripping absolute ethyl alcohol (serving as an adhesive) to form a uniformly dispersed suspension. And (3) transferring the suspension by using a pipetting gun, dripping the suspension onto the surfaces of the interdigital electrodes, standing at room temperature for 30min, and after the ethanol is completely volatilized, placing the sensor into an aging tester for aging and stabilizing for 72h to obtain the hydrogen sulfide gas sensor, as shown in fig. 7.
In order to verify the performance of the sensor, the prepared hydrogen sulfide gas sensor is subjected to gas-sensitive performance test.
Fig. 8 is a graph showing the response of a sensor such as Co 3O4/ZnO、Co3O4 or ZnO to 5 ppm H 2 S at different temperatures. As shown, the sensitivity of the sensor made of Co 3O4/ZnO composite nanofiber was about 10 times higher than that of the sensor made of ZnO fiber and about 50 times higher than that of the sensor made of Co 3O4 fiber at 325 ℃ and 5 ppm H 2 S. The method is characterized in that the P-N heterojunction is constructed, the electric fields at the two ends of the depletion layer transfer most carriers to different directions, the density is increased, the service life is prolonged, and the sensitivity of the sensor is improved, so that the excellent performance of the nano material designed by the method is verified.
FIG. 9 is a graph showing the response of materials prepared according to the present invention at different molar ratios of Zn to ligand to 5 ppm H 2 S at different temperatures. Different molar ratios exhibit different optimum operating temperatures and responses because different crystalline forms of ZIF-8 exhibit different properties and exhibit different gas-sensitive properties. As can be seen from the figure, the molar ratio is 1:4, the best gas-sensitive performance is exhibited.
FIG. 10 is a graph of the gas sensitive properties of Co 3O4/ZnO of the present invention at different calcination temperatures. 25 The mole ratio of the ZIF-67/PAN fiber in the growth solution is 1:4, calcining at different temperatures to obtain Co 3O4/ZnO nano hollow fiber, and exhibiting optimal response at 325 ℃ under 5 ppm H 2 S. Wherein, when the calcination temperature is 500 ℃, the material has the optimal sensitivity, which is consistent with the obtained morphology of the electron microscope image, and the influence of the fiber morphology on the gas-sensitive performance is proved.
FIG. 11 is a graph of the dynamic response of a Co 3O4/ZnO sensor to 200ppb H 2 S gas at 325 ℃. As shown, the nanofibers fully adsorb oxygen in air and the resistance reaches about 2.5×10 7 Ω. The response time of the sensor was 88.7 s when 200ppb of hydrogen sulfide gas was in contact with the sensor, and the sensor was re-exposed to air after the resistance value of the sensor in the hydrogen sulfide gas had substantially equilibrated, at which time the recovery time of the sensor was 110.6s, indicating that the sensor had a lower detection limit and had a good response to 200ppb of hydrogen sulfide gas.
FIG. 12 is a graph of the dynamic response of a Co 3O4/ZnO sensor of the present invention to H 2 S at 325℃over a concentration range of 200 ppb-5 ppm. As shown in the figure, when the material is in the air, the sensitivity of the material can still return to the vicinity of the initial value after the hydrogen sulfide gas with different concentrations is sequentially adsorbed and desorbed. As the concentration of the hydrogen sulfide gas increases, the sensitivity of the sensor increases, and the sensitivity in the hydrogen sulfide gas of 200 ppb, 500ppb, 1ppm, 2ppm and 5ppm is respectively 48.25, 100.12, 223.26, 425.37 and 1058.23, and the reaction of the sensor and the hydrogen sulfide gas gradually approaches saturation in the process. The response time of the sensor was longer at 200 ppb, 500ppb, 1ppm concentrations, 90.7s, 87.6s, 84.2 s respectively, and when the gas concentration reached 5ppm, the response time of the material was shortened because the gas molecules were more likely to contact the material surface and participate in the reaction, the reaction was more nearly saturated, when the gas concentration was higher. The resistance value change caused by the gas reaction in a short time increases, and thus the response time decreases.
FIG. 13 is a graph showing the response of a Co 3O4/ZnO sensor of the present invention to various gases having a concentration of 200 ppb at 325 ℃. The selectivity of the sensor to the gas is an important index of the sensor, and under the condition of 325 ℃, the sensitivity of the sensor to 200 ppb ammonia (NH 3), ethanol (ethanol), xylene (xylene) nitrogen dioxide (NO 2), sulfur dioxide (SO 2) formaldehyde (Formaldehyde) and other interference gases is tested respectively, and the sensitivity of the sensor to hydrogen sulfide is far higher than that of other gases, SO that the Co 3O4/ZnO sensor has very strong selectivity to hydrogen sulfide gas under low concentration and has good popularization and application prospects.
FIG. 14 is a graph of the dynamic cyclic response of a Co 3O4/ZnO sensor of the present invention to 200 ppb H 2 S gas at 325 ℃. As shown, the sensitivity of the sensor to 200 ppb H 2 S gas was always stable around 45, indicating that the sensor had good repeatability.
FIG. 15 is a graph of the long term stability of the Co 3O4/ZnO nanofiber sensor of the present invention against 5ppm H 2 S at 250 ℃. As shown in the figure, the sensitivity of the sensor to 5ppm hydrogen sulfide is stabilized at about 1000 in 30 days, indicating that the sensor has good long-term stability.
FIG. 16 is a graph showing the specific surface area and porosity of Co 3O4/ZnO nanofibers, znO fibers, and Co 3O4 fibers of the present invention, by N 2 adsorption-desorption isotherms. Specific surface area BET calculations were 31.892, 29.575 and 18.718 m 2/g, respectively. The nanostructure with larger surface area can improve the contact area between the sensor and the target gas, and rich active sites are obtained so as to improve the gas-sensitive performance. The specific surface area of the Co 3O4/ZnO nanofiber after compounding is higher than that of single oxide ZnO fiber and Co 3O4 fiber. In addition, according to the IUPAC classification, all adsorption and desorption curves of Co 3O4/ZnO nanofiber, pure ZnO and Co 3O4 have H1 type hysteresis loops, and belong to the IV type curve, which proves that a large amount of mesoporous structures exist in the sample.
Comparative example
To further illustrate the performance advantages of the gas sensitive materials of the present invention, the gas sensitive performance of different nanomaterial sensors at 325 ℃ for different concentrations of H 2 S was compared, as shown in table 1.
TABLE 1 gas sensitivity properties of different nanomaterial sensors to different concentrations of H 2 S at 325℃
ZIF-67 is derivatized to produce Co 3O4 and ZIF-8 is derivatized to produce ZnO. As is clear from the table, first, the sensor composed of pure ZIF-8 and ZIF-67 had very low sensitivity, and showed no resistance change for H 2 S gas at 200ppb concentration (the sensitivity was 1, which showed no change in resistance), and H 2 S gas at 200ppb concentration could not be detected. The sensitivity of Co 3O4 generated after ZIF-67 derivatization to 200ppb H 2 S was 2, and the sensitivity of ZnO generated after ZIF-8 derivatization to 200ppb H 2 S was 5, which demonstrates that the sensitivity of the MOF-derivatized material is greater than that of the material before derivatization, and that this characteristic is exhibited for each concentration H 2 S.
Second, when ZIF-67 and ZIF-8 are compounded, the sensitivity to each concentration H 2 S is higher than that of single ZIF-67 and ZIF-8, and when Co 3O4 and ZnO are compounded, the sensitivity to each concentration H 2 S is higher than that of single Co 3O4 and ZnO, which proves that the existence of the p-n heterojunction after the material is compounded enhances the gas sensitivity.
Thirdly, the gas sensitivity of Co 3O4/ZnO materials with different microstructures is known, under the condition of 200ppb H 2 S, the sensitivity of Co 3O4/ZnO generated by MOF derivatization is more than 1 time higher than that of Co 3O4/ZnO with a polyhedral structure, and is 4.5 times higher than that of Co 3O4 @ZnO materials with a core-shell structure. This shows that the sensitivity of the gas-sensitive material of the Co 3O4/ZnO fiber structure synthesized by the invention is obviously higher than that of Co 3O4/ZnO gas-sensitive materials of other microstructures, and the gas of H 2 S with low concentration (200 ppb) can be well detected.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, alternatives, and improvements that fall within the spirit and scope of the invention.
Claims (6)
1. A preparation method of a composite nanofiber gas-sensitive material is characterized by comprising the following steps: the method comprises the following steps:
s1: preparing a ZIF-67 crystal material;
S2: preparing a ZIF-67 nanofiber membrane from the ZIF-67 crystal material prepared in the step S1 by an electrostatic spinning method;
S3: preparing ZIF-67 nanofiber membranes and ZIF-8 growth solution from the ZIF-67 nanofiber membranes prepared in the step S2 to prepare ZIF-67@ZIF-8 core-shell nanofibers;
S4: annealing the ZIF-67@ZIF-8 core-shell nanofiber prepared in the step S3 to obtain a Co 3O4/ZnO composite nanofiber;
The step S2 of preparing the ZIF-67 nanofiber membrane by an electrostatic spinning method comprises the steps of adding the ZIF-67 crystal material prepared in the step S1 and a polymer into an organic solvent, uniformly mixing to obtain a precursor solution, and spinning the precursor solution into the ZIF-67 nanofiber membrane by the electrostatic spinning method;
the mass ratio of ZIF-67 to polymer is 12.5-37.5%;
the polymer is one of polyacrylonitrile, polyvinyl alcohol and polyvinylpyrrolidone; the organic solvent is N, N dimethylformamide and tetrahydrofuran;
the volume ratio of N, N dimethylformamide to tetrahydrofuran is 1:3-5;
And/or the pushing rate of the needle tube in the electrostatic spinning is 1.0 mL/h, the distance between the needle tip and the collector is 10-20 cm, the external voltage of the electrostatic spinning is 18-kV, and the relative humidity of the environment in the electrostatic spinning is 40% -45%;
the preparation of the ZIF-67@ZIF-8 core-shell nanofiber in the step S3 comprises the steps of putting the ZIF-67 nanofiber membrane obtained in the step S2 into a ZIF-8 growth solution, stirring at constant temperature, taking out the treated nanofiber membrane, and washing to obtain the ZIF-67@ZIF-8 nanofiber with a core-shell structure;
The preparation of the ZIF-8 growth solution in the step S3 comprises the steps of putting zinc nitrate into methanol to obtain a mixed solution A, putting dimethyl imidazole into the methanol to obtain a mixed solution B, and mixing and stirring the mixed solution A and the mixed solution B;
the molar ratio of zinc ions to dimethylimidazole in the ZIF-8 growth solution is 1:2-6;
In the step S4, the annealing temperature is 450-550 ℃, the heating rate is set to 2 ℃/min, and the heat preservation time is set to 125-175 min.
2. The method for preparing the composite nanofiber gas-sensitive material according to claim 1, wherein the method comprises the following steps: the preparation method of the ZIF-67 crystal material in the step S1 comprises the following steps: mixing the solution containing cobalt ions with the dimethyl imidazole solution, precipitating after the reaction is finished, washing the obtained precipitate, centrifuging and drying to obtain the ZIF-67 crystal material.
3. The method for preparing the composite nanofiber gas-sensitive material according to claim 2, wherein the method comprises the following steps: the solution containing cobalt ions is cobalt nitrate solution;
And/or the solvent of the cobalt ion-containing solution and the dimethyl imidazole solution is methanol;
And/or the mole ratio of cobalt ions to dimethylimidazole is 1:4, a step of;
and/or precipitating after the reaction is finished, wherein the precipitation comprises the steps of precipitating the obtained solution after the reaction for at least 24 hours at room temperature, pouring out supernatant, pouring the rest suspension into a centrifuge tube, and centrifuging in the centrifuge to obtain precipitate;
And/or washing with absolute ethanol during washing;
and/or the temperature of drying is 55-65 ℃, and the drying time is at least 24 hours.
4. Use of a composite nanofiber gas-sensitive material prepared by the preparation method of the composite nanofiber gas-sensitive material according to any one of claims 1-3 as a gas-sensitive material.
5. A hydrogen sulfide gas sensor, a composite nanofiber gas-sensitive material prepared by the method for preparing a composite nanofiber gas-sensitive material according to any one of claims 1 to 3.
6. A method of making a hydrogen sulfide gas sensor as defined in claim 5, wherein: grinding the prepared Co 3O4/ZnO composite nanofiber into powder, dripping absolute ethyl alcohol to form uniformly dispersed suspension, dripping the suspension on the surface of an interdigital electrode, standing at room temperature, and aging after the ethanol is completely volatilized to obtain the hydrogen sulfide gas sensor.
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