CN116102085B - Method for preparing iron spinel heterojunction array material in situ - Google Patents

Abstract

The invention discloses a method for preparing an iron-based spinel heterojunction array material in situ, and relates to a method for preparing an iron-based spinel heterojunction array material. The method solves the problems that the prior art can only prepare the metal oxide heterojunction array structure by a two-step method and has no broad spectrum. The method comprises the following steps: and covering a seed layer on the surface of the substrate by using a room temperature dip-coating and high-temperature calcination method, and then growing the Fe ₂O₃/RFe₂O₄ iron spinel heterojunction array material on the surface of the seed layer in situ by using a one-step hydrothermal and high-temperature calcination method. The method is used for preparing the iron spinel heterojunction array material in situ.

Description

A method for in-situ preparation of iron-based spinel heterojunction array material

Technical Field

The invention relates to a method for preparing an iron-based spinel heterojunction array material.

Background Art

As people pay more and more attention to the environment and health, the real-time detection of toxic and harmful gases has been concerned by various fields. Among them, VOCs gas is extremely harmful to the environment and human body. It is easy to form PM _2.5 and may produce ozone in an oxidizing atmospheric environment; a certain concentration of VOCs can cause human olfactory failure, damage vision and organs, and even cause cancer in a short time. Therefore, how to achieve real-time detection of low-concentration VOCs in the atmospheric environment is of great significance to protecting human health and environmental safety. At the same time, with the development of modernization, human survival and social activities are closely related to gases, and it is difficult to find a field that is not related to gases. Traditional methods for detecting gases include ion mobility spectroscopy, mass spectrometry, liquid chromatography, etc., but these detection methods are expensive and cannot be detected in real time. In contrast, gas sensors have attracted much attention in gas detection due to their advantages such as high portability, simple use, and low cost. They are widely used in the food industry, industrial pollution, medical diagnosis, pesticide residues, chemical reagent detection, drug storage, tobacco industry, etc. Therefore, gas sensors can detect and analyze various gases in real time, and have the advantages of high sensitivity and short response time. In addition, the rapid development of microelectronics, micromachining technology, automation, and intelligent technology has made gas sensors smaller, cheaper, and easier to use. Therefore, they have been widely used in military, medical, transportation, environmental protection, quality inspection, anti-counterfeiting, home furnishing and other fields. However, there are still some problems with the gas sensors currently on the market, such as poor selectivity and stability. The performance indicators of gas sensors need to be further improved, and the development of new gas-sensitive materials and new gas sensors is receiving increasing attention. Countries around the world have invested heavily in this field. Therefore, in the face of more complex environments, it is necessary to develop integrated gas detection methods. By preparing the developed sensing materials into integrated sensor devices, the sensing materials will respond differently to different gases, so that people can respond to leaked gases in a timely manner.

Metal oxide nanomaterials, due to their unique nano-size, exhibit outstanding physical and chemical properties such as high specific surface area, electron mobility, thermal stability, mechanical strength and surface defects, which give them excellent optical, electrical, magnetic, catalytic and other properties, and are widely used in adsorption materials, catalytic materials, sensors, high-mobility transistors, energy storage devices and other fields. In addition, they are also indispensable raw materials for high-tech industries such as advanced equipment manufacturing, new energy, and emerging industries. They are valuable and key strategic resources and have attracted widespread attention from scientific researchers. In recent years, the research on metal oxide nanomaterials has expanded from traditional low-dimensional nanoparticles, nanorods, nanofibers and nanosheets to three-dimensional metal oxide nanomaterials with continuous porous network frameworks, and many strategies are used to improve the performance of materials, such as the design of multi-level structures, the doping of single atoms, the construction of appropriate heterojunctions and in-situ growth arrays.

As we all know, the construction of metal oxide heterojunction interface is an effective strategy to enhance the performance of functional materials, and it has been a hot topic of research in recent years. One-step hydrothermal in-situ growth array materials can prepare uniformly arranged structured materials on various substrates. Thanks to its structural characteristics, it can further improve the material's electronic transmission ability, structural stability, specific surface area, increase the loading amount of active substances and the number of exposed active sites, and accelerate the transmission rate of substances or electrons during the reaction process. Therefore, this structured nanoarray material has excellent application prospects in many fields and is very competitive in the in-situ preparation of devices. At present, most of the prepared metal oxide heterojunctions are powder materials of different dimensions or the existing seed layer needs to be prepared into an array structure first, and then a multi-metal oxide is further grown on the basis of the array. For example, the existing technology uses a two-step _hydrothermal method to prepare a ZnO/ _ZnFe2O4 array, which is to first prepare a ZnO array on the array structure _seed layer, and then further grow a _ZnFe2O4 array on the basis of the ZnO array. However, there are very few metal oxide heterostructure array materials that can be controllably constructed in situ by hydrothermal in one step on the device, especially there are almost no multi-metal oxide heterojunction array composite materials prepared in one step. For example, the existing synthesis reports of iron-based _spinel heterojunction array materials are not broad-spectrum, and most elements have not yet been reported for synthesis. Only a few single metal oxides _{ZnO, Fe2O3 ,} NiO, and _Co3O4 are first synthesized and prepared in an array by electrochemical or deposition methods, and then ferrites are prepared to form iron-based spinel heterojunction array materials. Therefore, how to construct multi-metal oxide heterojunction array composite materials in situ in one step will be a huge challenge.

Summary of the invention

The present invention aims to solve the problem that the prior art can only prepare metal oxide heterojunction array structures by a two-step method and has no broad spectrum, and provides a method for in-situ preparation of iron-based spinel heterojunction array materials.

A method for in-situ preparation of an iron-based spinel heterojunction array material is carried out according to the following steps:

A seed layer is covered on the surface of the substrate by room temperature immersion pulling and high temperature calcination method, and then _{a Fe2O3} / _RFe2O4 iron-based _spinel heterojunction array material is in situ grown on the surface of the seed layer by a one-step hydrothermal and high temperature calcination method, wherein R is Cd, Ca, Mn, Co, Ni, Cu, Zn or Mg.

The present invention constructs a heterojunction array in situ in one step based on only a seed layer. Compared with the previous multi-step method for preparing heterojunction arrays, the method is not only simple, but also can prepare uniformly dispersed heterojunctions with abundant active sites. In addition, the heterojunction array prepared by hydrothermal method in one step has strong electronic coupling, which provides powerful energy for accelerating electron transfer and provides a strong theoretical basis for in situ preparation of heterojunction arrays on devices in the future. It is a huge breakthrough in in situ preparation.

The beneficial effects of the present invention are:

(1) This synthesis method can be widely used in the synthesis of most iron-based spinel heterojunction metal oxides and has excellent universality. Different materials can detect different VOCs gases and are used in the fields of medical diagnosis of chemical reagent residues, pesticide residues, etc.

(2) This synthesis method can be used to in-situ prepare heterojunction arrays on a variety of substrates and has excellent development and application prospects.

(3) The synthesis method of this material is simple and low-cost. The solvent used is alcohol, which is environmentally friendly and more suitable for large-scale production.

(4) The material has excellent gas sensitivity and can be used for detection in different environments based on its excellent performance.

(5) Through testing, it can be found that using a gas sensor with high sensitivity and ultra-fast response recovery capability can accurately identify a single VOCs gas.

The invention is used for a method for in-situ preparation of an iron-based spinel heterojunction array material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a SEM image of the Fe ₂ O ₃ /CdFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 1, a is magnified 1k times, and b is magnified 5k times;

FIG2 is an XRD test diagram of the Fe ₂ O ₃ /CdFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 1;

FIG3 is a TEM image of the Fe ₂ O ₃ /CdFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 1, a and b are TEM images, c is HRTEM image, and d is a SAED image;

FIG4 is a SEM image of the Fe ₂ O ₃ /CaFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 2, a is magnified 2k times, and b is magnified 20k times;

FIG5 is an XRD test diagram of the Fe ₂ O ₃ /CaFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 2;

FIG6 is a TEM image of the Fe ₂ O ₃ /CaFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 2, a and b are TEM images, c is HRTEM image, and d is a SAED image;

FIG. 7 is a SEM image of the Fe ₂ O ₃ /MnFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 3, a is magnified 2k times, and b is magnified 20k times;

FIG8 is an XRD test diagram of the Fe ₂ O ₃ /MnFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 3;

FIG9 is a TEM image of the Fe ₂ O ₃ /MnFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 3, a and b are TEM images, c is HRTEM image, and d is a SAED image;

FIG. 10 is a SEM image of the Fe ₂ O ₃ /CoFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 4, a is magnified 2k times, and b is magnified 20k times;

FIG11 is an XRD test diagram of the Fe ₂ O ₃ /CoFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 4;

FIG12 is a TEM image of the Fe ₂ O ₃ /CoFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 4, a and b are TEM images, c is HRTEM image, and d is a SAED image;

FIG. 13 is a SEM image of the Fe ₂ O ₃ /NiFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 5, a is magnified 2k times, and b is magnified 20k times;

FIG. 14 is an XRD test diagram of the Fe ₂ O ₃ /NiFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 5;

FIG. 15 is a TEM image of the Fe ₂ O ₃ /NiFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 5, a and b are TEM images, c is HRTEM image, and d is a SAED image;

FIG. 16 is a SEM image of the Fe ₂ O ₃ /CuFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 6, a is magnified 2k times, and b is magnified 20k times;

FIG. 17 is an XRD test diagram of the Fe ₂ O ₃ /CuFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 6;

FIG. 18 is a TEM image of the Fe ₂ O ₃ /CuFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 6, a and b are TEM images, c is HRTEM image, and d is a SAED image;

FIG. 19 is a SEM image of the Fe ₂ O ₃ /ZnFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 7, a is magnified 2k times, and b is magnified 20k times;

FIG20 is an XRD test diagram of the Fe ₂ O ₃ /ZnFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 7;

FIG21 is a TEM image of the Fe ₂ O ₃ /ZnFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 7, a and b are TEM images, c is HRTEM image, and d is a SAED image;

FIG22 is a SEM image of the Fe ₂ O ₃ /MgFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 8, a is magnified 2k times, and b is magnified 20k times;

FIG23 is an XRD test diagram of the Fe ₂ O ₃ /MgFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 8;

FIG24 is a TEM image of the Fe ₂ O ₃ /MgFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 8, a and b are TEM images, c is HRTEM, and d is a SAED image;

FIG25 is a SEM image of the Fe ₂ O ₃ array material prepared in the comparative experiment, a is magnified 2k times, and b is magnified 20k times;

FIG26 is an XRD test diagram of Fe ₂ O ₃ array material prepared in a comparative experiment;

FIG27 is a scanning electron micrograph of the seed layer prepared in Example 1, a is magnified 1k times, and b is magnified 30k times;

FIG28 is a schematic structural diagram of a VOCs indirectly heated gas sensor based on the iron-based spinel heterojunction array material of Examples 1 to 8, wherein 1 is an Al ₂ O ₃ ceramic tube, 2 is a nickel-chromium alloy heating coil, 3 is a platinum wire, 4 is a gold electrode, and 5 is an Al ₂ O ₃ ceramic tube loaded with the iron-based spinel heterojunction array material;

Figure 29 is a graph showing the response of the sensor to different gases with a concentration of 100 ppm at 170°C, where a is a FO sensor, b is a FCdFO sensor, c is a FCaFO sensor, d is a FMnFO sensor, e is a FCoFO sensor, f is a FNiFO sensor, g is a FCuFO sensor, h is a FZnFO sensor, and i is a FMgFO sensor;

Figure 30 is the response recovery curve of the sensor to different gases with a concentration of 10 ppm at 170°C, a is the FZnFO sensor detecting acetone gas, b is the FMgFO sensor detecting di-n-butylamine gas, and c is the FNiFO sensor detecting sulfur gas;

Figure 31 shows the response recovery curves of the sensor to different gases with concentrations ranging from 0.01ppm to 100ppm at 170°C, where a is the FZnFO sensor detecting acetone gas, b is the FMgFO sensor detecting di-n-butylamine gas, and c is the FNiFO sensor detecting sulfur gas.

DETAILED DESCRIPTION

Specific implementation method 1: This implementation method is a method for in-situ preparation of iron-based spinel heterojunction array materials, which is carried out according to the following steps:

A seed layer is covered on the surface of the substrate by room temperature immersion pulling and high temperature calcination method, and then _{a Fe2O3} / _RFe2O4 iron-based _spinel heterojunction array material is in situ grown on the surface of the seed layer by a one-step hydrothermal and high temperature calcination method, wherein R is Cd, Ca, Mn, Co, Ni, Cu, Zn or Mg.

The present specific embodiment provides a universal method, through room temperature immersion pulling, so that the pre-prepared seed layer substrate does not realize array, and thereby directly induces the directional growth of ferric acetylacetonate and acetylacetonate salt (cadmium acetylacetonate, calcium acetylacetonate, manganese acetylacetonate, iron acetylacetonate, cobalt acetylacetonate, nickel acetylacetonate _, copper acetylacetonate, zinc acetylacetonate or magnesium acetylacetonate), and then by changing the components in the raw materials and adding a suitable amount of solvent, iron atoms are stripped from the basis of _Fe2O3 to form a _ferrite structure _RFe2O4 , thereby realizing a one-step solvothermal in-situ construction of a multi-metal oxide heterojunction array composite material, which can be generally applied to the in-situ synthesis of iron-based spinel metal oxides _Fe2O3 /(Cd, Ca, Mn, Co, Ni, Cu, Zn, Mg) _-Fe2O4 with a heterojunction tube array _{structure .} This method has not been reported yet. This method utilizes an unarrayed seed layer and directly grows an array and a heterojunction simultaneously in a one-step hydrothermal process without a previous array, thus overcoming the problem that the prior art cannot construct a heterojunction array structure without a previous array.

Due to the excellent structural characteristics of the heterojunction array material, the gas sensor prepared by it can have extremely high sensitivity and extremely fast response recovery speed. Different iron-based spinel heterojunction array materials can detect different single toxic and harmful gases. Therefore, this specific embodiment also involves a method for detecting a single toxic and harmful gas using a high-precision and high-sensitivity gas sensor. By integrating different iron-based spinel heterojunction array material sensors together, when there is a target gas leak, the sensor corresponding to the target gas will generate different electrical signals. Finally, the electrical signal data is analyzed and processed to obtain the gas type and leakage concentration, etc.

The array material synthesized in this specific embodiment can effectively reduce the response recovery time of the gas sensor and improve the sensing performance. At the same time, relying on the excellent performance of the array material, a new multi-channel integrated single-selective detection of toxic and harmful gases can be developed, which is more conducive to reducing energy consumption, improving performance, and coping with complex gas environments.

The beneficial effects of this specific implementation are:

(1) This synthesis method can be widely used in the synthesis of most iron-based spinel heterojunction metal oxides and has excellent universality. Different materials can detect different VOCs gases and are used in the fields of medical diagnosis of chemical reagent residues, pesticide residues, etc.

(2) This synthesis method can be used to in-situ prepare heterojunction arrays on a variety of substrates and has excellent development and application prospects.

(3) The synthesis method of this material is simple and low-cost. The solvent used is alcohol, which is environmentally friendly and more suitable for large-scale production.

(4) The material has excellent gas sensitivity and can be used for detection in different environments based on its excellent performance.

(5) Through testing, it can be found that using a gas sensor with high sensitivity and ultra-fast response recovery capability can accurately identify a single VOCs gas.

Specific embodiment 2: This embodiment is different from specific embodiment 1 in that: the use of room temperature immersion pulling and high temperature calcination method to cover the seed layer on the surface of the substrate is specifically carried out in the following steps: at room temperature and an immersion speed of 0.1mm/s to 10mm/s, the substrate is immersed in the sol solution for 30s to 120s by immersion pulling method, and then at room temperature and a pulling speed of 0.1mm/s to 10mm/s, the immersed substrate is taken out and calcined for 0.5h to 5h at a temperature of 100℃ to 700℃ to obtain a substrate loaded with a seed layer. The rest is the same as specific embodiment 1.

Specific implementation method 3: This implementation method is different from specific implementation method 1 or 2 in that the substrate is ceramic, glass or quartz. The rest is the same as specific implementation method 1 or 2.

Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that the sol solution is a titanium oxide sol solution or a zinc oxide sol solution with a concentration of 1 mol/L to 10 mol/L. Other aspects are the same as specific embodiments 1 to 3.

Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that: the one-step hydrothermal and high-temperature calcination method is used to in-situ grow Fe ₂ O ₃ /RFe ₂ O ₄ iron-based spinel heterojunction array material on the surface of the seed layer, specifically in the following steps:

The substrate loaded with the seed layer is immersed in the precursor solution, and then placed in a stainless steel reactor, the stainless steel reactor is sealed, and the reaction is carried out at a temperature of 70°C to 220°C for 1h to 24h, and then cooled to room temperature, the substrate is taken out and washed and dried to obtain a substrate after hydrothermal reaction, and the substrate after hydrothermal reaction is heated to 300°C to 800°C in an air atmosphere and a heating rate of 1°C/min to 20°C/min, and then calcined at a temperature of 300°C to 800°C for _1h to 4h, and _Fe2O3 / _RFe2O4 iron-based _spinel heterojunction array material is obtained on the surface of the substrate, and the R is Cd, Ca, Mn, Co, Ni, Cu, Zn or Mg. The rest is the same as the specific embodiments one, two to four.

Specific embodiment 6: This embodiment is different from specific embodiments 1 to 5 in that the precursor solution is specifically prepared by mixing ferric acetylacetonate, acetylacetonate, urea and a solvent. The rest is the same as specific embodiments 1 to 5.

Specific embodiment 7: This embodiment is different from Specific embodiments 1 to 6 in that: the acetylacetonate salt is cadmium acetylacetonate, calcium acetylacetonate, manganese acetylacetonate, iron acetylacetonate, cobalt acetylacetonate, nickel acetylacetonate, copper acetylacetonate, zinc acetylacetonate or magnesium acetylacetonate; the solvent is composed of methanol and polyethylene glycol 400, and the volume ratio of methanol to polyethylene glycol 400 is 1: (0.1-10). Others are the same as Specific embodiments 1 to 6.

Specific embodiment 8: This embodiment is different from specific embodiments 1 to 7 in that the molar ratio of the ferric acetylacetonate to the acetylacetonate salt is 1:(0.1-0.5). The rest is the same as specific embodiments 1 to 7.

Specific embodiment 9: This embodiment is different from specific embodiments 1 to 8 in that the molar ratio of the total mole of acetylacetonate iron and acetylacetonate to urea is 1:(0.1-10). Other aspects are the same as specific embodiments 1 to 8.

Specific embodiment 10: This embodiment differs from Specific embodiments 1 to 9 in that the volume ratio of the total mole of ferric acetylacetonate, acetylacetonate and urea to the solvent is 1 mmol: (1 to 20) mL. Other aspects are the same as Specific embodiments 1 to 9.

The following examples are used to verify the beneficial effects of the present invention:

Embodiment 1:

A method for in-situ preparation of an iron-based spinel heterojunction array material is carried out according to the following steps:

At room temperature and an immersion speed of 2 mm/s, the substrate was immersed in the sol solution for 60 seconds by an immersion pulling method, and then at room temperature and a pulling speed of 2 mm/s, the immersed substrate was taken out, and calcined at a temperature of 500°C for 2 hours to obtain a substrate loaded with a seed layer, the substrate loaded with the seed layer was immersed in a precursor solution, and then placed in a 50 mL polytetrafluoroethylene-lined stainless steel reactor, the stainless steel reactor was sealed, and the reaction was carried out at a temperature of 140°C for 15 hours, and then cooled to room temperature, the substrate was taken out and washed, and dried at a temperature of 70°C for 12 hours to obtain a substrate after a hydrothermal reaction, and the substrate after the hydrothermal reaction was heated to 500°C in an air atmosphere at a heating rate of 2°C/min, and then calcined at a temperature of 500°C for 2 hours, _{and Fe2O3} / _CdFe2O4 iron-based _spinel heterojunction array material was obtained on the surface of the substrate;

The substrate is an Al ₂ O ₃ ceramic tube;

The sol solution is a zinc oxide sol solution with a concentration of 4 mol/L;

The precursor solution is specifically obtained by magnetically stirring 1 mmol of ferric acetylacetonate, 0.2 mmol of acetylacetonate, 7 mmol of urea and a solvent for 1 hour.

The acetylacetonate is cadmium acetylacetonate; and the solvent is composed of 24 mL of methanol and 6 mL of polyethylene glycol 400.

Figure 1 is a SEM image of the _Fe2O3 / _CdFe2O4 iron-based _spinel heterojunction array material prepared in Example 1, a is magnified 1k times, b is magnified _5k times. It can be seen from the figure that the array is an array of nanorods (length 8μm-12μm, diameter 3μm-5μm).

Figure 2 is an XRD test graph of the _Fe2O3 / _CdFe2O4 iron-based _spinel heterojunction array material prepared in Example 1. As can be seen from the figure, after the substrate is treated, the crystallization is _complete , and the XRD diffraction peaks of the composite material array structure have a good correspondence with the _{hexagonal Fe2O3 standard} spectrum JCPDSCardNo.33-0664 and the _{cubic CdFe2O4} standard spectrum JCPDSCardNo.22-1063 structure.

Figure 3 is a TEM image of the Fe ₂ O ₃ /CdFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 1, a and b are TEM, c is HRTEM, and d is SAED image. It can be seen from the figure that the Fe ₂ O ₃ /CdFe ₂ O ₄ heterojunction rod array is orderly assembled by small nanorods (length 200nm ~ 500nm) and nanoparticles (diameter 10nm ~ 50nm), and the lattice fringes corresponding to Fe ₂ O ₃ and CdFe ₂ O ₄ are observed in Figure c, indicating the successful construction of the heterostructure.

Embodiment 2: This embodiment is different from Embodiment 1 in that: the acetylacetonate is calcium acetylacetonate; and the Fe ₂ O ₃ /CaFe ₂ O ₄ iron-based spinel heterojunction array material is obtained on the substrate surface. Other aspects are the same as Embodiment 1.

Figure 4 _{is a SEM image of the Fe2O3 / CaFe2O4} iron-based _spinel heterojunction array material prepared in Example 2, a is 2k times magnified, b is 20k times magnified. It can be seen from the figure that the array is a nanorod array (length 20μm-25μm, diameter 5μm-8μm).

Figure 5 is an XRD test graph of the _Fe2O3 / _CaFe2O4 iron-based _spinel heterojunction array material prepared in Example 2. As can be seen from the figure, after the substrate is treated, the crystallization is _complete , and the XRD diffraction peaks of the composite material array structure have a good correspondence with the _{hexagonal Fe2O3 standard} spectrum JCPDSCardNo.33-0664 and the _{cubic CaFe2O4 standard} spectrum JCPDSCardNo.32-0168 structure.

Figure 6 is a TEM image of the Fe ₂ O ₃ /CaFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 2, a and b are TEM, c is HRTEM, and d is SAED image. It can be seen from the figure that the Fe ₂ O ₃ /CaFe ₂ O ₄ heterojunction rod array is orderly assembled by nanoparticles (diameter 10nm to 20nm) and small nanorods (length about 100nm), and the lattice fringes corresponding to Fe ₂ O ₃ and CaFe ₂ O ₄ are observed in Figure c, indicating the successful construction of the heterostructure.

Embodiment 3: This embodiment is different from Embodiment 1 in that: the acetylacetonate is manganese acetylacetonate; and Fe ₂ O ₃ /MnFe ₂ O ₄ iron-based spinel heterojunction array material is obtained on the substrate surface. Others are the same as Embodiment 1.

Figure 7 is a SEM image of the _Fe2O3 / _MnFe2O4 iron-based _spinel heterojunction array material prepared in Example 3, a is a 2k-fold magnification, _and b is a 20k-fold magnification. It can be seen from the figure that the array is a rectangular array (side length is 5μm to 20μm).

Figure 8 is an XRD test graph of the _Fe2O3 / _MnFe2O4 iron-based _spinel heterojunction array material prepared in Example 3. As can be seen from the figure, after the substrate is treated, the crystallization is _complete , and the XRD diffraction peaks of the composite material array structure have a good correspondence with the _{hexagonal Fe2O3 standard} spectrum JCPDSCardNo.33-0664 and the _{cubic MnFe2O4} standard spectrum JCPDSCardNo.10-0319 structure.

Figure 9 is a TEM image of the Fe ₂ O ₃ /MnFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 3, a and b are TEM images, c is HRTEM image, and d is SAED image. It can be seen from the figure that the Fe ₂ O ₃ /MnFe ₂ O ₄ heterojunction rectangular array is orderly assembled by nanoparticles (10nm to 20nm), and the lattice fringes corresponding to Fe ₂ O ₃ and MnFe ₂ O ₄ are observed in Figure c, indicating the successful construction of the heterostructure.

Embodiment 4: This embodiment is different from Embodiment 1 in that: the acetylacetonate is cobalt acetylacetonate; and the Fe ₂ O ₃ /CoFe ₂ O ₄ iron-based spinel heterojunction array material is obtained on the substrate surface. Others are the same as Embodiment 1.

Figure 10 _is a SEM image of the _Fe2O3 / _CoFe2O4 iron-based _spinel heterojunction array material prepared in Example 4, a is a 2k-fold magnification, b is a 20k-fold magnification. It can be seen from the figure that the array is a nanorod array (20μm-30μm in length and 5μm-10μm in diameter).

Figure 11 is an XRD test graph of the _Fe2O3 / _CoFe2O4 iron-based _spinel heterojunction array material prepared in Example 4. As can be seen from the figure, after the substrate is treated, the crystallization is _complete , and the XRD diffraction peaks of the composite material array structure have a good correspondence with the _{hexagonal Fe2O3 standard} spectrum JCPDSCardNo.33-0664 and the _{cubic CoFe2O4 standard} spectrum JCPDSCardNo.03-0864 structure.

Figure 12 is a TEM image of the Fe ₂ O ₃ /CoFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 4, a and b are TEM, c is HRTEM, and d is SAED image. It can be seen from the figure that the Fe ₂ O ₃ /CoFe ₂ O ₄ heterojunction rod array is orderly assembled by nanoparticles (diameter 10nm-20nm) and small nanorods (length 50nm-100nm), and the lattice fringes corresponding to Fe ₂ O ₃ and CoFe ₂ O ₄ are observed in Figure c, respectively, indicating the successful construction of the heterostructure.

Embodiment 5: This embodiment is different from Embodiment 1 in that: the acetylacetonate is nickel acetylacetonate; and the Fe ₂ O ₃ /NiFe ₂ O ₄ iron-based spinel heterojunction array material is obtained on the substrate surface. Other aspects are the same as Embodiment 1.

Figure 13 _is a SEM image of the _Fe2O3 / _NiFe2O4 iron-based _spinel heterojunction array material prepared in Example 5, a is a 2k-fold magnification, b is a 20k-fold magnification. It can be seen from the figure that the array is a nanorod array (10μm-20μm in length and 5μm-10μm in diameter).

Figure 14 is an XRD test graph of the _Fe2O3 / _NiFe2O4 iron-based _spinel heterojunction array material prepared in Example 5. As can be seen from the figure, after the substrate is treated, the crystallization is _complete , and the XRD diffraction peaks of the composite material array structure have a good correspondence with the _{hexagonal Fe2O3 standard} spectrum JCPDSCardNo.33-0664 and the _{cubic NiFe2O4 standard} spectrum JCPDSCardNo.10-0325 structure.

Figure 15 is a TEM image of the Fe ₂ O ₃ /NiFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 5, a and b are TEM images, c is HRTEM, and d is SAED image. It can be seen from the figure that the Fe ₂ O ₃ /NiFe ₂ O ₄ heterojunction rod array is orderly assembled by nanoparticles (diameter 10nm-50nm) and small nanorods (length 100nm-150nm), and the lattice fringes corresponding to Fe ₂ O ₃ and NiFe ₂ O ₄ are observed in Figure c, indicating the successful construction of the heterostructure.

Embodiment 6: This embodiment is different from Embodiment 1 in that: the acetylacetonate is copper acetylacetonate; and the Fe ₂ O ₃ /CuFe ₂ O ₄ iron-based spinel heterojunction array material is obtained on the substrate surface. Others are the same as Embodiment 1.

Figure 16 is a SEM image _{of the Fe2O3 / CuFe2O4} iron-based _spinel heterojunction array material prepared in Example 6, a is a 2k-fold magnification, b is a 20k-fold magnification. It can be seen from the figure that the array is a nanorod array (10μm-20μm in length, 3μm-8μm in diameter).

Figure 17 is an XRD test graph of the _Fe2O3 / _CuFe2O4 iron-based _spinel heterojunction array material prepared in Example 6. As can be seen from the figure, after the substrate is treated, the crystallization is _complete , and the XRD diffraction peaks of the composite material array structure have a good correspondence with the _{hexagonal Fe2O3 standard} spectrum JCPDSCardNo.33-0664 and the _{cubic CuFe2O4 standard} spectrum JCPDSCardNo.25-0283 structure.

Figure 18 is a TEM image of the Fe ₂ O ₃ /CuFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 6, a and b are TEM images, c is HRTEM, and d is SAED image. It can be seen from the figure that the Fe ₂ O ₃ /CuFe ₂ O ₄ heterojunction rod array is orderly assembled by nanoparticles (50nm to 100nm), and the lattice fringes corresponding to Fe ₂ O ₃ and CuFe ₂ O ₄ are observed in Figure c, indicating the successful construction of the heterostructure.

Embodiment 7: This embodiment is different from Embodiment 1 in that: the acetylacetonate is zinc acetylacetonate; and the Fe ₂ O ₃ /ZnFe ₂ O ₄ iron-based spinel heterojunction array material is obtained on the substrate surface. Other aspects are the same as Embodiment 1.

Figure 19 is a SEM image _of the _Fe2O3 / _ZnFe2O4 iron-based _spinel heterojunction array material prepared in Example 7, a is a 2k-fold magnification, b is a 20k-fold magnification. It can be seen from the figure that the array is a nanotube array (10μm-20μm in length, 3μm-5μm in diameter).

Figure 20 is an XRD test graph of the _Fe2O3 / _ZnFe2O4 iron-based _spinel heterojunction array material prepared in Example 7. As can be seen from the figure, after the substrate is treated, the crystallization is _complete , and the XRD diffraction peaks of the composite material array structure have a good correspondence with the _{hexagonal Fe2O3} standard spectrum JCPDSCardNo.33-0664 and the _{cubic ZnFe2O4} standard spectrum JCPDSCardNo.22-1012 structure.

Figure 21 is a TEM image of the Fe ₂ O ₃ /ZnFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 7, a, b are TEM, c is HRTEM, and d is SAED image. It can be seen from the figure that the Fe ₂ O ₃ /ZnFe ₂ O ₄ heterojunction tubular array is orderly assembled by nanoparticles (diameter 10nm ~ 20nm) and small nanorods (length 50nm ~ 100nm), and the lattice fringes corresponding to Fe ₂ O ₃ and ZnFe ₂ O ₄ are observed in Figure c, respectively, indicating the successful construction of the heterostructure.

Embodiment 8: This embodiment is different from Embodiment 1 in that: the acetylacetonate is magnesium acetylacetonate; and the Fe ₂ O ₃ /MgFe ₂ O ₄ iron-based spinel heterojunction array material is obtained on the substrate surface. Other aspects are the same as Embodiment 1.

Figure 22 is a SEM image of the _Fe2O3 / _MgFe2O4 iron-based _spinel heterojunction array material prepared in Example 8, a is 2k times magnified, b is _20k times magnified. It can be seen from the figure that the array is a nanotube array (length 10μm-20μm, diameter 5μm-8μm).

Figure 23 is an XRD test graph of the _Fe2O3 / _MgFe2O4 iron-based _spinel heterojunction array material prepared in Example 8. As can be seen from the figure, after the substrate is treated, the crystallization is _complete , and the XRD diffraction peaks of the composite material array structure have a good correspondence with the _{hexagonal Fe2O3} standard spectrum JCPDSCardNo.33-0664 and the _{cubic MgFe2O4} standard spectrum JCPDSCardNo.17-0464 structure.

Figure 24 is a TEM image of the Fe ₂ O ₃ /MgFe ₂ O ₄ iron-based spinel heterojunction array material prepared in Example 8, a and b are TEM images, c is HRTEM image, and d is SAED image. It can be seen from the figure that the Fe ₂ O ₃ /MgFe ₂ O ₄ heterojunction tubular array is orderly assembled by nanoparticles (diameter 30nm ~ 50nm) and small nanorods (length 50nm ~ 100nm), and the lattice fringes corresponding to Fe ₂ O ₃ and MgFe ₂ O ₄ are observed in Figure c, respectively, indicating the successful construction of the heterostructure.

Comparative experiment: The difference between this embodiment and the first embodiment is that the addition of acetylacetonate is eliminated and Fe ₂ O ₃ array material is obtained on the surface of the substrate. The rest is the same as the first embodiment.

Figure 25 is a SEM image of the Fe ₂ O ₃ array material prepared in the comparative experiment, a is a 2k-fold magnification, and b is a 20k-fold magnification. It can be seen from the figure that the array is a nanosheet array assembled in an orderly manner from nanoparticles.

Figure 26 is an XRD test diagram of the Fe ₂ O ₃ array material prepared in the comparative experiment. It can be seen from the figure that after the substrate is treated, the crystallization is complete, and the XRD diffraction peak of the composite material array structure corresponds well to the hexagonal Fe ₂ O ₃ standard spectrum JCPDS Card No. 33-0664 structure.

Figure 27 is a scanning electrochemical image of the seed layer prepared in Example 1, a is a 1k-fold magnification, and b is a 30k-fold magnification. It can be seen from the figure that, firstly, when the immersion time is 60s and the temperature is room temperature, the amount of seed layer caused by immersing the ceramic tube can be ignored, and secondly, the morphology does not form an array form.

VOCs indirectly heated gas sensors were assembled using the iron-based spinel heterojunction array materials prepared in situ on the Al ₂ O ₃ ceramic tube in Examples 1 to 8 and the Fe ₂ O ₃ array materials prepared in situ on the Al ₂ O ₃ ceramic tube in a comparative experiment, and were named FCdFO sensor (Example 1), FCaFO sensor (Example 2), FMnFO sensor (Example 3), FCoFO sensor (Example 4), FNiFO sensor (Example 5), FCuFO sensor (Example 6), FZnFO sensor (Example 7), FMgFO sensor (Example 8) and FO sensor (Comparative experiment) respectively;

28 is a schematic structural diagram of a VOCs indirectly heated gas sensor based on the iron-based spinel heterojunction array material of Examples 1 to 8, wherein 1 is an Al ₂ O ₃ ceramic tube, 2 is a nickel-chromium alloy heating coil, 3 is a platinum wire conductor, 4 is a gold electrode, and 5 is an Al ₂ O ₃ ceramic tube loaded with the iron-based spinel heterojunction array material.

Figure 29 is a graph showing the response of the sensor to different gases at a concentration of 100 ppm at 170°C, where a is a FO sensor, b is a FCdFO sensor, c is a FCaFO sensor, d is a FMnFO sensor, e is a FCoFO sensor, f is a FNiFO sensor, g is a FCuFO sensor, h is a FZnFO sensor, and i is a FMgFO sensor. It can be seen from the figure that the FO sensor (comparative experiment) has poor selectivity and poor sensitivity to VOCs gas; the FCdFO sensor responds to 100 ppm ethanol gas by 17.2. The FCaFO sensor responds to 100ppm triethylamine gas at 12.3; the FMnFO sensor responds to 100ppm sulfur gas at 14.2; the FCoFO sensor responds to 100ppm sulfur gas at 18.9; the FNiFO sensor responds to 100ppm sulfur gas at 62.3; the FCuFO sensor responds to 100ppm sulfur gas at 14.2; the FZnFO sensor responds to 100ppm acetone gas at 60.2; and the FMgFO sensor responds to 100ppm di-n-butylamine gas at 60.3.

Figure 30 shows the response recovery curves of the sensor to different gases with a concentration of 10 ppm at 170°C, where a is the FZnFO sensor detecting acetone gas, b is the FMgFO sensor detecting di-n-butylamine gas, and c is the FNiFO sensor detecting sulfur mystery gas. It can be seen from the figure that the response time of the ZnFO sensor to 10 ppm acetone is 6.8 s, and the recovery time is 146.2 s; the response time of the FMgFO sensor to 10 ppm di-n-butylamine gas is 6.3 s, and the recovery time is 172.3 s; the response time of the FNiFO sensor to 10 ppm sulfur mystery gas is 3.2 s, and the recovery time is 175.3 s.

Figure 31 shows the response recovery curves of the sensor to different gases with concentrations of 0.01ppm to 100ppm at 170°C, where a is the FZnFO sensor detecting acetone gas, b is the FMgFO sensor detecting di-n-butylamine gas, and c is the FNiFO sensor detecting sulfur gas. It can be seen from the figure that the minimum detection limit of the FZnFO sensor for detecting acetone gas is 0.02ppm, the minimum detection limit of the FMgFO sensor for detecting di-n-butylamine gas is 0.01ppm, and the minimum detection limit of the FNiFO sensor for detecting sulfur gas is 0.01ppm. The three sensors, FZnFO sensor, FNiFO sensor and FMgFO sensor, all have good response recovery and the response increases with the increase of concentration.

Claims (5)

1. A method for in-situ preparation of an iron-based spinel heterojunction array material, characterized in that it is carried out according to the following steps: A seed layer is covered on the surface of a substrate by room temperature immersion pulling and high temperature calcination method to obtain a substrate loaded with the seed layer, the substrate loaded with the seed layer is immersed in a precursor solution, and then a _Fe2O3 / _RFe2O4 iron-based _spinel heterojunction array material is in situ grown on the surface of the seed layer by a one-step hydrothermal _and high temperature calcination method, wherein R is Cd, Ca, Mn, Co, Ni, Cu, Zn or Mg; The one-step hydrothermal and high-temperature calcination method has a hydrothermal temperature of 70°C to 220°C and a hydrothermal reaction time of 1h to 24h. In the one-step hydrothermal and high-temperature calcination method, the high-temperature calcination temperature is 300°C to 800°C, and the high-temperature calcination time is 1h to 4h; The precursor solution is specifically prepared by mixing iron acetylacetonate, urea, a solvent and other acetylacetonate salts, wherein the other acetylacetonate salts are selected from one of cadmium acetylacetonate, calcium acetylacetonate, manganese acetylacetonate, cobalt acetylacetonate, nickel acetylacetonate, copper acetylacetonate, zinc acetylacetonate and magnesium acetylacetonate; The solvent is composed of methanol and polyethylene glycol 400, and the volume ratio of methanol to polyethylene glycol 400 is 1:(0.1-10); The molar ratio of the ferric acetylacetonate to the remaining acetylacetonate salts is 1:(0.1-0.5); The molar ratio of the total mole of the ferric acetylacetonate and the remaining acetylacetonate salts to urea is 1:(0.1-10); The volume ratio of the total mole number of the ferric acetylacetonate, the remaining acetylacetonate salts and urea to the solvent is 1 mmol: (1-20) mL. 2. According to the method for in-situ preparation of iron-based spinel heterojunction array materials according to claim 1, it is characterized in that the seed layer is covered on the surface of the substrate by using room temperature immersion pulling and high temperature calcination method, and is specifically carried out according to the following steps: at room temperature and an immersion speed of 0.1mm/s to 10mm/s, the substrate is immersed in the sol solution for 30s to 120s by immersion pulling method, and then at room temperature and a pulling speed of 0.1mm/s to 10mm/s, the immersed substrate is taken out, and calcined at a temperature of 100℃ to 700℃ for 0.5h to 5h to obtain a substrate loaded with a seed layer. 3. The method for in-situ preparation of iron-based spinel heterojunction array materials according to claim 2, characterized in that the substrate is ceramic, glass or quartz. 4. The method for in-situ preparation of an iron-based spinel heterojunction array material according to claim 2, characterized in that the sol solution is a titanium oxide sol solution or a zinc oxide sol solution with a concentration of 1 mol/L to 10 mol/L. 5. The method for in-situ preparation of an iron-based spinel heterojunction array material according to claim 1, characterized in that the one-step _hydrothermal and high-temperature calcination method is used to in-situ grow the _Fe2O3 / _RFe2O4 iron-based _spinel heterojunction array material on the surface of the seed layer, specifically in the following steps: The substrate loaded with the seed layer is immersed in the precursor solution, and then placed in a stainless steel reactor, the stainless steel reactor is sealed, and the reaction is carried out at a temperature of 70°C to 220°C for 1h to 24h, and then cooled to room temperature, the substrate is taken out and washed and dried to obtain a substrate after hydrothermal reaction, and the substrate is heated in an air atmosphere at a heating rate of 1°C/min to 20°C/min. The substrate after the hydrothermal reaction is heated to 300°C to 800°C, and then calcined at a temperature of 300°C to 800°C for 1h to 4h. A Fe ₂ O ₃ /RFe ₂ O ₄ iron-based spinel heterojunction array material is obtained on the substrate surface.