CN111729680B

CN111729680B - High-efficiency difunctional oxygen electrocatalyst with heterostructure and preparation and application thereof

Info

Publication number: CN111729680B
Application number: CN202010558431.5A
Authority: CN
Inventors: 陈作锋; 牛艳丽; 滕雪; 巩帅奇
Original assignee: Tongji University
Current assignee: Tongji University
Priority date: 2020-06-18
Filing date: 2020-06-18
Publication date: 2023-05-05
Anticipated expiration: 2040-06-18
Also published as: CN111729680A

Abstract

The invention relates to a high-efficiency difunctional oxygen electrocatalyst with a heterostructure and preparation and application thereof, wherein cobalt nitrate and p-phenylenediamine are firstly dissolved in methanol, stirred and mixed uniformly, and then placed in a reaction kettle for reaction for 8 hours to obtain a flower-shaped amorphous cobalt-based metal organic compound precursor; secondly, the inclusion of the ligand p-phenylenediamine on different metals is utilized to realize the introduction of iron element; finally, carbonizing the obtained product to obtain the target product. The flower-like nano-sheet structure increases the contact area between the electrolyte and the catalyst, and the coating of the graphitized carbon layer enhances the conductivity and stability of the catalyst. The related method has low cost, easy operation, good oxygen reduction (ORR) and Oxygen Evolution (OER) electrocatalytic activity in alkaline electrolyte, and potential application value in the fields of energy conversion and storage.

Description

High-efficiency difunctional oxygen electrocatalyst with heterostructure and preparation and application thereof

Technical Field

The invention belongs to the technical field of catalysts, and relates to a high-efficiency difunctional oxygen electrocatalyst with a heterostructure, and preparation and application thereof.

Background

In recent years, energy crisis and environmental pollution have driven and accelerated the inevitable transition from fossil fuels to clean renewable energy sources, and the development of renewable energy storage and conversion devices has been curtailed, with fuel cells and metal-air cells playing an important role in such energy conversion. It has been found that the Oxygen Reduction Reaction (ORR) during oxygen electrode discharge and the Oxygen Evolution (OER) reaction during charging involve multiple electron/proton transfer processes with very slow kinetics, largely impeding the commercialization and large-scale application of these new devices, often requiring large amounts of catalyst to accelerate the reaction. At present, noble metal platinum and ruthenium/iridium based oxides are regarded as the best ORR and OER catalysts, respectively, but noble metal catalysts are scarce in reserves, high in cost, poor in durability and single in function, and cannot catalyze both reactions at the same time. Therefore, it is necessary to design and develop efficient, inexpensive non-noble metal bifunctional oxygen electrocatalysts.

The heteroatom doped carbon loaded bimetallic alloy (CoFe, niCo and FeNi) composite material has certain selectivity for different catalytic reactions due to unique double active sites and rich valence state conversion, and the interaction between metals effectively modifies the electronic structure of the composite material, so that the surface energy of the catalyst is regulated, and the binding energy of hydroxyl free radicals and the surface of the catalytic material is optimized. The existing synthesis method of the bimetallic alloy catalyst mainly comprises the following steps: (i) The metal salt and the nitrogen-rich small molecule precursor are pyrolyzed on the carbon matrix. Dicyandiamide and melamine are commonly used carbon-nitrogen precursors for preparing a bimetal nanoparticle oxygen electrocatalyst coated by nitrogen-doped graphitized carbon, and the one-step solid state pyrolysis method is simple and easy to implement, but the uncontrollability of the size and the structure of the nanoparticles limits the catalytic activity of the material to a great extent. (ii) pyrolyzing the metal organic framework. Conventional MOF-derived materials primarily use simple MOF crystals as a single precursor, however, most simple MOFs are single core metals, and the integration of two or more metals into a single MOF has been a challenge. Currently, bimetallic alloy composites are typically prepared by a dual MOF assisted pyrolysis process. For example, fe-based MIL-101 and Co-based ZIF-67 were chosen by Zhang et al as precursors for the synthesis of Co-Fe alloy/NC catalyst (S.L.Zhang, B.Y.Guan and X.W.D. Lou, small,2019,15,1805324.) adding to the complexity of the experiment. In addition, in order to maintain the porosity and integrity of the MOF structure, pyrolysis is usually performed at a lower temperature, but the synthesized catalytic material often has the problems of low graphitization degree, weakened interaction between the active metal nano particles and the conductive carbon layer, and the like, so that the activity and stability of the material are greatly reduced.

Therefore, it is particularly necessary to further improve the stability and catalytic activity of the catalyst. The present invention has been made in view of the above-described problems.

Disclosure of Invention

The invention aims to provide a high-efficiency difunctional oxygen electrocatalyst with a heterostructure, and preparation and application thereof. The p-phenylenediamine with better inclusion is adopted as a ligand, and bimetal is introduced into an organic compound by means of the double-solvent effect of n-hexane and methanol. After high-temperature carbonization, the target product still maintains the original lamellar structure of the precursor and generates a thin graphitized carbon layer, so that the stability of the catalyst is improved. More interestingly, in the ion exchange process, some porous structures are generated on the nano-sheets, so that the specific surface area of the material is increased, more active sites are exposed, and the catalytic activity of the material is facilitated.

The aim of the invention can be achieved by the following technical scheme:

one of the technical schemes of the invention provides a high-efficiency difunctional oxygen electrocatalyst with a heterostructure, which is formed by loading cobalt/cobalt-iron alloy heterogeneous nano particles on a nitrogen doped porous carbon nano sheet.

The second technical scheme of the invention provides a preparation method of a high-efficiency difunctional oxygen electrocatalyst with a heterostructure, which is characterized by comprising the following steps of:

(1) Dispersing soluble cobalt salt and p-phenylenediamine serving as raw materials in a solvent, uniformly mixing, heating for reaction, cooling, separating and drying to obtain a cobalt-based metal organic compound precursor (namely a Co-PPD precursor);

(2) Dispersing the cobalt-based metal organic compound precursor in normal hexane, adding a soluble ferric salt methanol solution, stirring to introduce iron ions into the cobalt-based metal organic compound precursor, centrifugally drying the obtained product, and carbonizing to obtain the target product, namely the high-efficiency bifunctional oxygen electrocatalyst Co/CoFe@NC.

Further, in the step (1), the soluble cobalt salt is cobalt nitrate and the solvent is methanol.

Further, in the step (1), the mass ratio of the soluble cobalt salt to the p-phenylenediamine is (0.3-1.2): (0.8-1.2), corresponding to each 0.8-1.2g of p-phenylenediamine, and the volume of methanol is 5-20mL.

Further, in the step (1), the temperature of the heating reaction is 90-140 ℃ and the reaction time is 6-10h.

Further, in step (2), the soluble iron salt used is ferric nitrate.

Further, in the step (2), the mass of Co-PPD is 30-60mg, the corresponding volume of n-hexane added is 10-20mL, the concentration of ferric nitrate is 10-25mg/L, and the dropwise adding volume is 140-560 mu L.

Further, in the step (2), the stirring time is 0.5-1.5h.

Further, in the step (2), the carbonization temperature is 750-950 ℃ and the carbonization time is 2-5h. Further, the temperature rising rate is 1-5 ℃/min.

Further, the soluble ferric salt methanol solution was added to n-hexane in a dropwise manner.

In the invention, cobalt nitrate and p-phenylenediamine are respectively used as a metal source and a ligand, and Co-PPD is formed by utilizing the strong binding capacity of an amino functional group at a special position in the p-phenylenediamine to metal ions in the hydrothermal synthesis process. If the amount of p-phenylenediamine is too large in this step, polymerization into spheres will occur by itself, and if too little, the metal ions will not coordinate to form a complex. Then, hydrogen ions are generated by utilizing iron ions through hydrolysis, a part of cobalt is dissolved out for ion exchange, and the iron ions are successfully introduced into the precursor complex. In the process, the preparation of ferric nitrate solution is carried out by using methanol, and Co-PPD is agglomerated when other solvents are added into n-hexane in a dropwise manner; the iron ions can be uniformly dispersed in the solution by dropwise adding and ultrasonic dispersion in the dripping process. Finally, the precursor is converted into the nitrogen-doped porous carbon nano-sheet modified by cobalt/cobalt-iron alloy heterogeneous nano-particles through high-temperature carbonization, the carbonization temperature is too high, the nano-particles are agglomerated, and the active sites are reduced; when the temperature is too low, the graphitization degree of the catalyst is not strong, resulting in poor conductivity of the catalyst.

The third technical scheme of the invention provides application of the high-efficiency difunctional oxygen electrocatalyst with the heterostructure, which is characterized in that the high-efficiency difunctional oxygen electrocatalyst is used in ORR and OER under alkaline conditions.

The flower-like nano-sheet structure increases the contact area between the electrolyte and the catalyst, and the coating of the graphitized carbon layer enhances the conductivity and stability of the catalyst. The related method has low cost, easy operation, good oxygen reduction (ORR) and Oxygen Evolution (OER) electrocatalytic activity in alkaline electrolyte, and potential application value in the fields of energy conversion and storage.

Compared with the prior art, the invention has the following advantages:

(1) The structure is stable. Conventional simple MOFs choose high temperature pyrolysis in order to enhance the conductivity of the catalyst, but typically result in structural collapse. The Co/CoFe@NC catalyst prepared by the method maintains a precursor Co-PPD nanoflower structure, increases the contact area of a catalytic material and electrolyte, possibly generates cation vacancies in the process of ion exchange, leaves a large number of mesopores on a nanosheet after high-temperature pyrolysis, not only increases the specific surface area of the material, but also is beneficial to the permeation of the electrolyte and the transmission of ions in the reaction process, and releases more catalytic active sites.

(2) And (5) constructing an interface. The Co/CoFe heterostructure constructed by the method effectively promotes charge transfer, and the interface of the heterostructure can serve as a main catalytic active site to accelerate the dynamics of ORR and OER.

Drawings

FIG. 1-1 is an infrared spectrum of a precursor Co-PPD prepared in example 1 of the present invention;

FIGS. 1-2 are X-ray diffraction (XRD) patterns of the precursor Co-PPD prepared in example 1 of the present invention;

FIGS. 1-3 are scanning electron microscope images (SEM, c-d) of the precursor Co-PPD prepared in example 1 of the present invention;

FIGS. 1 to 4 are element distribution diagrams (e-h) of the precursor Co-PPD prepared in example 1 of the present invention;

FIG. 2-1 is an X-ray diffraction (XRD) spectrum (a) of the catalyst Co/CoFe@NC prepared in example 2 of the present invention;

FIG. 2-2 is a Raman spectrum of the catalyst Co/CoFe@NC prepared in example 2 of the present invention;

FIGS. 2-3 are scanning electron microscope images (SEM, c-d) of the catalyst Co/CoFe@NC prepared in example 2 of the present invention;

FIGS. 2-4 are transmission electron microscope images (TEM, e-f) of the catalyst Co/CoFe@NC prepared in example 2 of the present invention;

FIGS. 2-5 are elemental profiles (g-k) of the catalyst Co/CoFe@NC prepared in example 2 of the present invention;

FIG. 3 is an X-ray photoelectron spectrum (XPS) of the catalyst Co/CoFe@NC prepared in example 2 of the present invention;

FIG. 4-1 shows the catalyst Co/CoFe@NC prepared in example 2 of the present invention and a commercial catalyst RuO ₂ A linear sweep voltammogram (a) and a corresponding tafel slope value graph (b) for a 1M potassium hydroxide electrolyte;

FIG. 4-2 is a cyclic voltammogram of the Co/CoFe@NC catalyst prepared in example 2 of the present invention at different sweep rates;

FIGS. 4-3 are graphs of current density difference versus different sweep rates for Co/CoFe@NC catalyst at a relative hydrogen reduction potential of 0.13V;

FIGS. 4-4 are impedance maps of different electrodes;

FIGS. 4-5 are stability diagrams of Co/CoFe@NC electrodes;

FIG. 5-1 is a graph of linear sweep voltammograms (a) and corresponding Tariff slope values (b) for the catalyst Co/CoFe@NC prepared in example 2 of the present invention versus a commercial catalyst Pt/C (20%) in 0.1M potassium hydroxide electrolyte;

FIG. 5-2 is a graph of linear sweep voltammograms at different rotational speeds and K-L at different potentials for the catalyst Co/CoFe@NC prepared in example 2 of the present invention;

FIGS. 5-3 are stability of Co/CoFe@NC catalyst prepared in example 2 of the present invention;

FIGS. 5-4 are dual function LSV curves for different electrodes;

FIGS. 5-5 are bifunctional OER and ORR activities of the catalyst Co/CoFe@NC prepared in example 2 of the present invention with other catalysts previously reported;

FIG. 6 is a flow chart of the preparation process of the invention.

Detailed Description

The invention will now be described in detail with reference to the drawings and specific examples. The present embodiment is implemented on the premise of the technical scheme of the present invention, and a detailed implementation manner and a specific operation process are given, but the protection scope of the present invention is not limited to the following examples.

In the following embodiments or examples, potassium hydroxide, n-hexane and methanol were purchased from Shanghai Michelia Biochemical technologies Co., ltd, p-phenylenediamine, nonahydrate and ferric nitrate and cobalt nitrate hexahydrate were purchased from Aba Ding Shiji (Shanghai) Co., ltd. The remainder of the raw material products or processing techniques not specifically described are all indicated as conventional commercial products or conventional processing techniques in the art.

Electrochemical data were collected by CHI760E (Shanghai cinnabar) and a rotating disc test system.

Example 1:

preparation of precursor Co-PPD, as shown in FIG. 6:

(1) Weighing p-phenylenediamine and cobalt nitrate, respectively dissolving in methanol with the same volume, stirring and dissolving, wherein the mass of the p-phenylenediamine is 1.0g, the mass of the cobalt nitrate is 0.9g, and the volume of the methanol is 20mL;

(2) And (3) rapidly mixing and stirring the two solutions in the step (1), and then transferring the mixed solution into a polytetrafluoroethylene liner for solvothermal pretreatment, wherein the reaction temperature is 120 ℃ and the reaction time is 8 hours. And after the reaction is finished, cooling to room temperature, centrifuging to remove impurities, collecting the impurities, and finally, drying the mixture in a vacuum drying oven at 80 ℃ for 10 hours to obtain the Co-PPD precursor.

Example 2:

preparation of Co/CoFe@NC bifunctional catalyst as shown in FIG. 6:

(1) Co-PPD in example 1 was weighed, dispersed in n-hexane by ultrasonic, then ferric nitrate methanol solution was prepared, dropwise added to the above solution, stirring was continued for 1 hour after the dropwise addition, wherein the mass of Co-PPD was 50mg, the volume of n-hexane was 12mL, the ultrasonic time was 1 hour, the concentration of ferric nitrate was 25mg/L, and the dropwise addition volume was 280. Mu.L.

(2) Carbonizing the product in the step (1) under the protection of argon to obtain the Co/CoFe@NC difunctional oxygen electrocatalyst, wherein the carbonization temperature is 850 ℃, the carbonization time is 3h, and the heating rate is 3 ℃/min.

FIG. 1-1 shows the IR spectra of ligand PPD and precursor Co-PPD, from which the decrease in the stretching vibration intensity of the-N-H functional group in Co-PPD can be seen, indicating successful chelation of metal ions on the amino functional group of the ligand. FIGS. 1-2 are XRD representations of Co-PPD with no distinct peaks, indicating that it is an amorphous structure. FIGS. 1 to 3 and 1 to 4 show SEM and element distribution diagrams of Co-PPD, the precursor takes the shape of nanoflower, and cobalt nitrogen element is uniformly dispersed on the surface of the precursor.

FIGS. 2-1 and 2-2 are XRD and Raman diagrams of bifunctional oxygen electrocatalysts prepared when different volumes of ferric nitrate solutions were added dropwise, respectively, and it can be seen that catalysts of different components can be obtained through effective regulation and control of ferric ions, wherein the target product of a heterostructure is obtained when the added dropwise volume is 280. Mu.L, and the graphitization degree is highest. FIGS. 2-3 and 2-4 are morphological characterizations of Co/CoFe@NC, which can be seen that the precursor has a nanoflower structure, the nanoparticles are uniformly dispersed on the porous nitrogen-doped carbon nanoplatelets after carbonization, and the cobalt simple substance and the cobalt-iron alloy form a heterogeneous interface. The elemental profile of the product of fig. 2-5 shows that the product consists essentially of the four elements C, N, fe and Co, and is formed from a cobalt-iron alloy.

FIG. 3 is an XPS characterization of the elements in the product Co/CoFe@NC, and in the overall diagram, the successful doping of nitrogen into the carbon lattice can be seen, and the XPS spectrum of the metal shows the 0 valence state, confirming the presence of the alloy and the simple substance.

Test method for preparing difunctional oxygen electrocatalyst by Co/CoFe@NC

(1) The final product catalyst obtained in example 2 was taken as OER catalyst: the reaction system is a three-electrode system, the graphite rod is a counter electrode, and the mercury/oxidized mercury is a reference potential. 5mg of catalyst was weighed and dispersed ultrasonically in 0.49mL of isopropyl alcohol and 10. Mu.L of 5wt% Nafion to form a uniform suspension, and after that 5. Mu.L of the suspension was pipetted by a pipette and dropped onto a 5mm clean glassy carbon electrode, and the suspension was dried naturally for later use. The electrolyte used for the test was 1M potassium hydroxide. The electrodes were subjected to 50 a prior to testingActivating by cyclic voltammetry scanning, and scanning speed of linear scanning voltammetry curve is 5mV s ^-1 . Electrochemical impedance spectra measured at different overpotential were measured in the range of 10khz to 0.1hz with an amplitude of 5mV. Stability is carried out at 10mA cm by adopting a constant current timing potential method ^-2 The test was continued for 12h. The results of the examples are shown in FIGS. 4-1 to 4-4.

As shown in fig. 4-1, compared to the commercial catalyst RuO ₂ The synthesized Co/CoFe@NC shows a larger current density and a lower overpotential when the current density is 10mA cm ^-2 When the overpotential is only 0.3V versus the reversible hydrogen electrode; FIG. b shows the Tafil slope of the two, from which it can be seen that the Tafil slope of Co/CoFe@NC has a value of only 49mV dec ^-1 Far lower than the catalyst RuO ₂ This demonstrates that the oxygen production kinetics rate of the prepared catalyst is superior to that of the noble metal RuO ₂ 。

Fig. 4-2 is a cyclic voltammogram of the catalyst at different scan rates, and half the difference between the oxidation current and the reduction current at 1.05V is selected as the capacitive current. The scanning rate is taken as an abscissa, the capacitance current under different scanning rates is taken as an ordinate, the capacitance current is in direct proportion to the scanning rate, the slope of the straight line is the double-layer capacitance of the material, and the electrochemical active area is in direct proportion to the double-layer capacitance.

It can be seen from FIGS. 4-3 and 4-4 that the electric double layer capacitance value of Co/CoFe@NC was 22.8mF cm ^-2 Indicating that it has a large number of oxygen-generating active sites. Notably, co/cofe@nc has a minimum semicircular diameter (rct=8.9Ω) and a steeper slope, confirming its faster charge transfer rate, lower electrode/electrolyte interface resistance, and faster mass diffusion. FIGS. 4-5 show that this catalyst also has good stability.

(2) The final product catalyst obtained in example 2 was taken as OER catalyst: the reaction system is a three-electrode system, the graphite rod is a counter electrode, and the mercury/oxidized mercury is a reference potential. 5mg of catalyst was weighed and sonicated in 0.49mL of isopropyl alcohol and 10. Mu.L of 5wt% Nafion to form a uniform suspension, and 12. Mu.L of the suspension was pipetted onto a 5mm clean RDE and dried naturally for use. The electrolyte used for the test was 0.1MPotassium hydroxide. Before testing, the electrode is activated by 50 circles of cyclic voltammetry scanning, and the scanning speed of the linear scanning voltammetry curve is 5mV s ^-1 。

The graph a in FIG. 5-1 shows the linear sweep voltammogram of 1600rpm for different catalysts on a rotating disk electrode, from which it can be seen that both the onset and half-wave potentials are relatively similar, and the Tafil slope of graph b shows that Co/CoFe@NC has faster ORR reaction kinetics.

FIG. 5-2 shows a good linear relationship for K-L curves at different potentials, indicating that the first order kinetics of ORR is consistent with the concentration of dissolved oxygen, and that the reaction path for catalytic oxygen reduction is four-electron based.

FIGS. 5-3 show that the catalyst has good stability. The dual function linear sweep voltammograms of the different electrodes in fig. 5-4, as well as the comparison of fig. 5-5 with the previously reported catalysts, demonstrate that Co/cofe@nc can be used as an effective dual function oxygen electrocatalyst with potential application value in the energy conversion and storage fields.

Example 3:

compared with example 1, the preparation method is the same in most parts except that the addition amounts of the raw materials are respectively changed into: the mass of p-phenylenediamine is 0.8g, the mass of cobalt nitrate is 1.2g, and the volume of methanol is 10mL.

Example 4:

compared with example 1, the preparation method is the same in most parts except that the addition amounts of the raw materials are respectively changed into: the mass of the p-phenylenediamine is 1.2g, the mass of the cobalt nitrate is 0.3g, and the volume of the methanol is 5mL.

Example 5:

in comparison with example 1, the process conditions were largely identical except that in step (1), the process conditions were respectively changed to: the reaction temperature is 90 ℃ and the reaction time is 10 hours; drying in a vacuum drying oven at 60 ℃ for 12 hours.

Example 6:

in comparison with example 1, the process conditions were largely identical except that in step (1), the process conditions were respectively changed to: the reaction temperature is 140 ℃ and the reaction time is 6 hours; drying in a vacuum drying oven at 120 ℃ for 8 hours.

Example 7:

compared with example 1, the preparation method is the same in most parts except that the addition amounts of the raw materials are respectively changed into: the mass of Co-PPD is 30mg, the volume of n-hexane is 20mL, the ultrasonic time is 1h, the concentration of ferric nitrate is 20mg/L, and the dropwise adding volume is 140 mu L.

Example 8:

compared with example 1, the preparation method is the same in most parts except that the addition amounts of the raw materials are respectively changed into: the mass of Co-PPD is 60mg, the volume of n-hexane is 10mL, the ultrasonic time is 1h, the concentration of ferric nitrate is 10mg/L, and the dropwise adding volume is 560 mu L.

Example 9:

compared with example 1, the method is the same in most parts, except that in the step (2), each process condition is respectively changed into: the carbonization temperature is 750 ℃, the carbonization time is 5 hours, and the heating rate is 1 ℃/min.

Example 10:

compared with example 1, the method is the same in most parts, except that in the step (2), each process condition is respectively changed into: the carbonization temperature is 950 ℃, the carbonization time is 2 hours, and the heating rate is 5 ℃/min.

The previous description of the embodiments is provided to facilitate a person of ordinary skill in the art in order to make and use the present invention. It will be apparent to those skilled in the art that various modifications can be readily made to these embodiments and the generic principles described herein may be applied to other embodiments without the use of the inventive faculty. Therefore, the present invention is not limited to the above-described embodiments, and those skilled in the art, based on the present disclosure, should make improvements and modifications without departing from the scope of the present invention.

Claims

1. The application of the high-efficiency difunctional oxygen electrocatalyst with the heterostructure is characterized in that the high-efficiency difunctional oxygen electrocatalyst is used in ORR and OER under alkaline conditions;

the high-efficiency bifunctional oxygen electrocatalyst is formed by loading cobalt/cobalt-iron alloy heterogeneous nano particles on a nitrogen doped porous carbon nano sheet, and the specific preparation process comprises the following steps:

(1) Dispersing soluble cobalt salt and p-phenylenediamine serving as raw materials in a solvent, uniformly mixing, heating for reaction, cooling, separating and drying to obtain a cobalt-based metal organic compound precursor;

(2) Dispersing a cobalt-based metal organic compound precursor in n-hexane, adding a soluble ferric salt methanol solution, stirring to introduce iron ions into the cobalt-based metal organic compound precursor, centrifugally drying the obtained product, and carbonizing to obtain the target product high-efficiency bifunctional oxygen electrocatalyst;

in the step (1), the mass ratio of the soluble cobalt salt to the p-phenylenediamine is (0.3-1.2): (0.8-1.2);

in the step (2), the carbonization temperature is 750-950 ℃ and the carbonization time is 2-5h;

in the step (1), the soluble cobalt salt is cobalt nitrate and the solvent is methanol;

in the step (1), the temperature of the heating reaction is 90-140 ℃ and the reaction time is 6-10h;

in the step (2), the soluble ferric salt is ferric nitrate;

in the step (2), the mass of the cobalt-based metal organic compound precursor is 30-60mg, the corresponding volume of n-hexane added is 10-20mL, the concentration of ferric nitrate is 10-25mg/L, and the dropwise adding volume is 140-560 mu L.

2. The use of a high efficiency bifunctional oxygen electrocatalyst having a heterostructure according to claim 1, wherein in step (2) the stirring time is from 0.5 to 1.5 hours.