A ZnO hollow nanotube-wheat biochar heterojunction composite catalyst, its preparation method and application

Abstract

The present invention belongs to the fields of materials science and photocatalysis technology, and relates to a ZnO hollow nanotube-wheat biochar heterojunction composite catalyst, a preparation method thereof and an application thereof. The heterojunction composite catalyst uses wheat biochar particles as a carrier, and zinc oxide hollow nanotubes are dispersed on the wheat biochar particles to form a heterojunction structure; the mass ratio of the wheat biochar particles to the zinc oxide hollow nanotubes is 9 to 1:1. The present invention is synthesized by a one-step hydrothermal method, using polyvinylpyrrolidone as a structure-directing agent and hexamethylenetetramine as a base source, and at the same time using wheat biochar as a carrier to stabilize PVP micelles and ZnO crystal nuclei. The present invention utilizes biochar derived from waste wheat straw as a co-catalyst and at the same time acts as a carrier to reasonably construct a heterojunction composite catalyst system of ZnO hollow nanotubes and wheat biochar, and this system exhibits excellent photocatalytic application potential.

Description

Technical Field

[0001] The present invention belongs to the fields of materials science and photocatalysis technology, and mainly relates to a ZnO hollow nanotube-wheat biochar heterojunction composite catalyst, a preparation method thereof, and photocatalytic applications. Background Art

[0002] The wide application of antibiotics has brought serious environmental problems while significantly reducing the mortality rate of diseases caused by bacterial infections. Many antibiotics have high chemical stability and are difficult to be completely degraded in the natural environment. These antibiotics and their metabolites enter water bodies after being metabolized by organisms and are difficult to be effectively removed by traditional water treatment technologies. Therefore, it is necessary to develop efficient degradation technologies to eliminate the potential risks brought by antibiotics and their intermediates in water bodies.

[0003] Advanced oxidation technologies based on peroxymonosulfate (PMS) have been widely studied and reported for the degradation of antibiotics in water due to their characteristics of deep mineralization of organic pollutants. PMS can be activated by various energies, such as light, electricity, sound, and the redox reaction of transition metals. Among them, solar energy is a green and renewable energy source that will not cause secondary pollution at all and has strong application potential. If light is used to activate PMS, it is necessary to reasonably design and construct semiconductor materials that can generate a light response. Among them, ZnO has natural advantages in photoinduced activation of PMS in terms of surface electronic structure, atomic arrangement, energy band structure, etc., which has been confirmed by many literatures. However, pure ZnO particles still face serious problems such as the recombination of photo-generated carriers during photocatalysis, and the utilization rate of photo-electrons is low. Therefore, it is necessary to develop heterojunction materials to improve the separation and transfer of photo-generated carriers, and then improve the activation efficiency of PMS.

[0004] Although common noble metal materials have strong conductivity and can achieve the above-mentioned effects of carrier separation and transfer by forming a Schottky junction with zinc oxide, the cost is too high. And traditional carbon materials, such as graphene, also have the problem of high cost. Biochar is a kind of carbon material obtained by pyrolyzing biomass resources. It can efficiently recycle waste biomass resources and has a low cost. At present, there is no photocatalyst material that combines wheat biochar with zinc oxide to degrade antibiotics in water. This can not only achieve the purpose of environmental governance but also realize the recycling of waste biomass resources, which has strong practical significance. Summary of the Invention

[0005] The object of the present invention is to overcome the above-mentioned deficiencies and high costs of the prior art, and to provide a simple preparation method and application of a ZnO hollow nanotube and wheat biochar heterojunction composite catalyst, which will be used in the reaction of photocatalytic activation of PMS to degrade CIP. The present invention proposes a preparation method for directly synthesizing a ZnO hollow nanotube-modified wheat biochar heterojunction composite material by a soft-template-mediated one-step hydrothermal method. Compared with common hard-template methods (such as electrospinning method, etching method), chemical vapor deposition method and electrochemical deposition method, this method has the advantages of simple operation, low cost, controllable conditions, etc. Although there have been reports on the synthesis of hollow tubular ZnO by hydrothermal methods, these methods generally indirectly obtain hollow tubular ZnO by first synthesizing a precipitation precursor of Zn. This method not only avoids the increase of synthesis steps, but also prepares a novel heterojunction composite material with photocatalytic function by using waste wheat biochar as a co-catalyst and carrier.

[0006] To achieve the above-mentioned invention object, the present invention adopts the following technical solutions:

[0007] The present invention provides a ZnO hollow nanotube-wheat biochar heterojunction composite catalyst, which uses wheat biochar particles as a carrier, and zinc oxide hollow nanotubes are dispersed on the wheat biochar particles to form a heterojunction structure; the mass ratio of the wheat biochar particles to the zinc oxide hollow nanotubes is 9~1:1.

[0008] In the above technical solution, further, the mass ratio of the wheat biochar particles to the zinc oxide hollow nanotubes is 4~1:1.

[0009] In the above technical solution, further, the average length of the ZnO hollow nanotubes is 2.0~3.5 μm, and the diameter is 500~700 nm; the particle size of the wheat biochar particles is <200 μm.

[0010] The present invention also provides a preparation method for a ZnO hollow nanotube-wheat biochar heterojunction composite catalyst, which uses wheat biochar as a carrier, polyvinylpyrrolidone PVP as a structure-directing agent, and hexamethylenetetramine HMTA as a base source, and controllably synthesizes a ZnO hollow nanotube and wheat biochar heterojunction composite catalyst by a one-step hydrothermal method, and realizes the activation of PMS under light and the degradation of CIP in an aqueous solution; the preparation method includes the following steps:

[0011] (1) Dissolve a zinc source, a base source, and a structure-directing agent in water and mix them evenly to obtain a mixed solution A; the zinc source is Zn(NO3)2‧6H2O, the base source is hexamethylenetetramine, and the structure-directing agent is polyvinylpyrrolidone; the mass ratio of the zinc source, the base source, the structure-directing agent to water is 1:1~1.5:2~3:80~112;

[0012] (2) Add wheat biochar powder to the mixed solution A in step (1), and ultrasonically mix evenly to obtain a mixed solution B; the mass ratio of the mixed solution A to the wheat biochar powder is 1000:1 - 3;

[0013] (3) The mixed solution B is hydrothermally reacted at 100 - 130 °C for 10 - 15 h, cooled to room temperature, centrifuged, washed, and dried.

[0014] In the above technical solution, further, the mixing in step (1) is carried out at room temperature and stirred, and the stirring time is not less than 1 h.

[0015] In the above technical solution, further, the wheat biochar powder is prepared by pyrolyzing wheat straw; the pyrolysis temperature is 500 - 750 °C, the heating rate is 5 - 10 °C / min, the pyrolysis time is 1 - 4 h, and the N2 atmosphere; the wheat biochar powder passes through an 80 - 100 mesh sieve.

[0016] In the above technical solution, further, in step (3), it is heated to 100 - 130 °C at a heating rate of 5 - 6 °C / min.

[0017] In the above technical solution, further, the drying temperature in step (3) is 65 - 90 °C.

[0018] The present invention also provides the application of the aforementioned ZnO hollow nanotube - wheat biochar heterojunction composite catalyst or the preparation method of the aforementioned zinc oxide hollow nanotube and wheat biochar heterojunction composite catalyst in the reaction of photocatalytic activation of PMS to degrade CIP.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] (1) The present invention provides a new method for synthesizing a hollow nanotube - shaped ZnO - modified wheat biochar heterojunction composite material. During the hydrothermal process, the addition of two reagents is involved to assist the ZnO hollow nanotubes to grow with uniform size and randomly disperse on the wheat biochar particles. One is PVP, which is a common surfactant and can direct the growth of nanocrystals in a certain direction by forming micelles; the other is HMTA, which is an organic base and its hydrolysis process in the hydrothermal system is controllable. In addition, wheat biochar stabilizes the ZnO crystal nuclei and PVP micelles. Therefore, the growth rate of ZnO hollow nanotubes can be controlled by controlling the hydrothermal temperature to obtain nanoparticles with uniform size, thus avoiding the step of first synthesizing a Zn salt precipitate as a precursor in general synthesis methods.

[0021] (2) In the present invention, wheat biochar is used as a carrier. When wheat biochar is introduced as a component of the heterojunction, it can effectively play the role of dispersing ZnO due to its porous property and large specific surface area.

[0022] (3) The novel ZnO hollow nanotube and wheat biochar heterojunction composite material of the present invention exhibits superior performance in photocatalytic activation of PMS for CIP degradation under light compared to the individual ZnO material and wheat biochar. This fully demonstrates that the heterojunction formed by ZnO hollow nanotubes and wheat biochar plays a role in accelerating the separation and transfer of charge carriers. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 are the SEM characterizations of the ZnO hollow nanotube - wheat biochar heterojunction composite catalyst, ZnO, and wheat straw biochar; Figure 1 a~ Figure 1 d are the SEM images of the ZnO hollow nanotube - wheat biochar heterojunction composite catalyst at different magnifications; Figure 1 e is the SEM image of pure ZnO; Figure 1 f is the SEM image of wheat straw biochar.

[0024] Figure 2 are the XRD and FTIR characterizations of ZnO hollow nanotubes, wheat biochar, and the ZnO hollow nanotube - wheat biochar heterojunction composite catalyst; Figure 2 a is the XRD characterization, Figure 2 b is the FTIR characterization.

[0025] Figure 3 are the photocatalytic activation of PMS for CIP degradation reaction performance and optoelectronic properties of ZnO hollow nanotubes, wheat biochar, and the ZnO hollow nanotube - wheat biochar heterojunction composite catalyst. Figure 3 a is the CIP degradation performance graph, Figure 3 b is the pseudo - second - order reaction kinetics fitting graph, Figure 3 c is the reaction rate constant bar graph, Figure 3 d is the cyclic stability experimental graph of ZC - 30, Figure 3 e is the EIS spectrum graph, Figure 3 f is the transient photocurrent response curves of ZnO and ZC - 30.

[0026] Figure 4 are the Uv - vis DRS characterizations, Tauc plots, and Mott - Schottky curves of ZnO hollow nanotubes, wheat biochar, and the ZnO hollow nanotube - wheat biochar heterojunction composite catalyst; Figure 4 a is the Uv - vis DRS characterization, Figure 4 b is the Tauc plot, Figure 4c is the Mott - Schottky curve of ZnO, Figure 4 d is the Mott - Schottky curve of wheat biochar.

[0027] Figure 5 is a schematic diagram of the mechanism for the photocatalytic activation of PMS by the ZnO hollow nanotube - wheat biochar heterojunction composite catalyst to degrade CIP. Specific implementation manners

[0028] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited in any way.

[0029] To enable those skilled in the art to better understand the technical solutions of the present invention, the preparation method and application of a ZnO hollow nanotube and wheat biochar heterojunction composite catalyst provided by the present invention will be described in detail below in conjunction with embodiments. The following embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. Wheat straw can be directly purchased commercially.

[0030] Example 1 Preparation of ZnO hollow nanotube and wheat biochar heterojunction composite catalyst

[0031] The preparation method includes the following steps:

[0032] (1) Wheat straw is rinsed with clear water; after being washed, it is thoroughly crushed with the help of a crusher; 2 g of the crushed wheat straw residue is placed in a porcelain boat - shaped container and placed in the center of a tubular furnace. Before heating, N2 is continuously passed for 30 min. At a heating rate of 5 °C / min, it is pyrolyzed at a high temperature of 650 °C for 3 hours. The obtained black solid is evenly ground and passed through an 80 - mesh sieve to obtain wheat biochar powder.

[0033] (2) At room temperature, 30 mL of deionized water is added to a 100 - mL beaker, and 0.8 g of PVP, 0.3449 g of HMTA, and 0.3133 g of Zn(NO3)2‧6H2O are respectively added under stirring and stirred for 1 h. Then 0.2 g of wheat biochar powder is added to the above - mentioned solution and the solution is ultrasonically mixed for 30 min, and then stirred for another 30 min. After being mixed evenly, the mixture is transferred and sealed into a high - pressure autoclave with a 50 - mL polytetrafluoroethylene inner liner, hydrothermally treated at 120 °C for 12 h, naturally cooled to room temperature, the obtained product is centrifuged and filtered, the precipitate is washed with deionized water, and the obtained black precipitate is dried in an oven at 70 °C to obtain a ZnO hollow nanotube - wheat biochar heterojunction composite catalyst.

[0034] Meanwhile, according to the above - mentioned preparation method, the only difference is that no wheat biochar powder is added, and a single ZnO material is prepared. The prepared ZnO mainly presents a flaky morphology.

[0035] The Zn(NO3)2‧6H2O used in this invention is of analytical purity, with a purity ≥ 99.0%, HMTA is of analytical purity, with a purity ≥ 99.0%, PVP is of analytical purity, with a purity ≥ 99% and an average molecular weight of 58,000, and wheat straw is purchased by oneself. The ZnO hollow nanotube and wheat biochar heterojunction composite catalyst, pure ZnO catalyst, and wheat biochar catalyst prepared in this invention are respectively labeled as ZC-30, ZnO, and Wheat biochar.

[0036] Figure 1 The SEM images of the ZnO hollow nanotube / wheat biochar composite material are shown. Obviously, ZnO is distributed in large quantities and uniformly on the surface of wheat biochar, as Figure 1 shown in Figure 1 a and Figure 1 b. Further obtaining high-magnification SEM images, ZnO is obviously in a hollow tube structure with uniform size, with a diameter between 500 - 700 nm and a length concentrated at about 2.5 μm ( Figure 1 c and Figure 1 d). Figure 1 e shows pure ZnO, Figure 1 and Figure 1 f is wheat straw biochar. In

[0037] Figure 2 e, ZnO presents a flaky structure; while for the wheat straw biochar in

[0038] Figure 2 f, its surface has porous characteristics. The above results fully show that wheat biochar plays an important role in the formation process of ZnO hollow nanotubes, strongly promoting the generation of this structure. -1The vibration peak of the C-O bond appears, indicating the presence of oxygen-containing functional groups in wheat biochar. The infrared absorption signals of ZnO appear at 702.6 cm -1 , 826.3 cm -1 and 1271.6 - 1632.1 cm -1 respectively, which are related to the vibrations of Zn-O and O-H. The absorption peaks related to monomers simultaneously appear in the FTIR of ZC-30, indicating that the heterojunction composite material composed of ZnO hollow nanotubes and wheat biochar has been successfully prepared.

[0039] Example 2: Photocatalytic activation of PMS for the degradation of CIP in water

[0040] The photocatalytic activation of PMS for the degradation of CIP in water was carried out in a jacketed glass reactor. Tap water was passed through the jacket to ensure that the temperature of the whole reaction process was at room temperature and constant. For each degradation experiment, 100 mL of 5 mg / L CIP solution was added to the reactor. After adding 10 mg of the catalyst, the timing started. The first 30 min was the adsorption stage. Every 10 minutes, 3 mL of the reaction solution was sucked out with a dropper, and the reaction solution was filtered through a 0.22 μm water-washed filter membrane into a quartz cuvette. The cuvette was placed in a double-beam spectrophotometer (TU-1901) to measure the absorbance of the solution at a wavelength of 251 nm to determine the relative content of CIP in the solution. After sucking out the solution adsorbed for 30 min, 20 mg of PMS was added to the system and a 300 W Xe light source (without a cut-off filter) was turned on. During the photoreaction process, 3 mL of the solution was sucked out every 5 min and the absorbance was measured. During the whole reaction process, the rotation speed of the rotor was kept at 500 rpm. For the cyclic stability experiment, the catalyst after the photocatalytic reaction was centrifuged, washed with deionized water and dried at 70 °C for the next experiment. If the recovered amount of the catalyst was not enough, multiple parallel degradation experiments were needed.

[0041] The catalysts used were the ZnO hollow nanotube-wheat biochar heterojunction composite catalyst (ZC-30) prepared in Example 1, ZnO material and wheat biochar.

[0042] Figure 3 a shows the performance of the synthesized catalysts in degrading CIP. Due to its porous structure and specific surface area, the performance of wheat biochar is mainly reflected in the adsorption stage. It adsorbed 30.6% of CIP within half an hour, which is higher than 10.8% of ZnO and 14.4% of ZC-30. The obvious decrease in the adsorption rate of the composite material means that the ZnO hollow nanotubes are uniformly dispersed on the surface of wheat biochar, hindering the adsorption of CIP by wheat biochar, which is consistent with that in Figure 1The results are consistent with those of the SEM of ZC-30 observed. After the light illumination started, only 54.5% of CIP was degraded within 20 min by wheat biochar, which was much lower than 82.6% of ZC-30 and 75.2% of ZnO, indicating that it is difficult for wheat biochar to generate useful photogenerated electrons under light to activate PMS, while ZnO is the main component for activating PMS. Due to the introduction of wheat biochar to separate photogenerated carriers, the removal rate of CIP by ZC-30 was further improved. Then we fitted the degradation reaction rate constant through the integral formula of the pseudo-second-order reaction kinetic differential equation, which is shown in Figure 3 b and Figure 3 c. Among them, the reaction rate constant of ZC-30 is the largest, which is 0.0481 mg -1 ‧L‧min -1 , which is 1.61 times and 6.59 times that of ZnO (0.0298 mg -1 ‧L‧min -1 ) and Wheat biochar (0.0073 mg -1 ‧L‧min -1 ), respectively. Figure 3 d shows the cyclic stability experiment of ZC-30. After 4 consecutive reactions, the activity of ZC-30 can still be maintained at 74.6%, indicating that ZC-30 has certain catalytic stability.

[0043] Example 3 Photoelectric properties of the synthesized catalyst

[0044] The photoelectric properties of the synthesized catalyst were tested on an electrochemical workstation (CHI760D). This test involves the assembly of a three-electrode system. The counter electrode is a Pt electrode, the reference electrode is a silver chloride electrode, and the working electrode was prepared as follows: 50 mg of the catalyst was uniformly dispersed in 2 mL of ethanol and 30 μL of perfluorinated resin aqueous solution, and ground in a mortar until the solution became thick. Then, the viscous liquid was evenly coated on the conductive side of the FTO conductive glass with a pipette, and the conductive glass coated with the catalyst was further dried in an infrared oven for 20 min to prevent the catalyst coating from falling off during the test. The electrolyte solution was 0.5 M Na2SO4 solution. In the test of photocurrent, a 300 W xenon lamp was still used as the light source.

[0045] The catalysts used were the ZnO hollow nanotube-wheat biochar heterojunction composite catalyst (ZC-30) prepared in Example 1 and ZnO material, respectively.

[0046] Figure 3e is the EIS spectrum of the synthesized catalyst. Among them, the Nyquist arc radius of wheat biochar is the smallest, indicating that wheat biochar has the best electrical conductivity; the Nyquist arc radius of ZnO is the largest, indicating that the photogenerated carriers of ZnO are most prone to recombination. The Nyquist arc radius of the composite material ZC-30 decreases significantly, which can improve the separation effect of photogenerated carriers. Figure 3 f is the transient photocurrent response curves of ZnO and ZC-30. The transient photocurrent intensity of ZC-30 is higher than that of ZnO, indicating that after the separation effect of photogenerated carriers in ZC-30 is enhanced, more photoelectrons are transferred to the photocatalytic surface to participate in the activation of PMS.

[0047] Example 4 Band structure of the synthesized catalyst

[0048] The band structure of the synthesized catalyst was determined by ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) and Mott-Schottky curve (MS curve), and Lambda 355 and CHI760D were used respectively.

[0049] Figure 4 a is the UV-Vis DRS of the synthesized catalyst. Wheat biochar has absorption in the full spectral range and no absorption edge, which is a typical conductor material. Therefore, it cannot effectively generate photogenerated carriers by itself. However, its light absorption ability is very strong, and the absorbed photons can only be released in the form of heat through the ways of molecular vibration and exciton recombination finally, and the utilization rate of photons is very low. The absorption edge of ZnO is located at 399.50 nm, indicating that ZnO can only absorb ultraviolet light, has a relatively wide band gap, and has a band structure. In ZC-30, its absorption edge extends to 490.82 nm, which is exactly the effect produced by the heterojunction. After the ZnO hollow nanotubes and wheat biochar are in close contact, the lattice atoms at the ZnO interface are more disordered, which leads to more dispersed energy levels in the energy band at the ZnO interface, reduces the band gap and enhances the light absorption range. Then the band gaps of ZnO and ZC-30 were determined by the Tauc plot method ( Figure 4 b), in which the band gap value of ZnO and the band gap value of ZC-30 were determined to be 3.20 eV and 2.90 eV respectively.

[0050] Figure 4 c and Figure 4 d are the MS curves of ZnO and wheat biochar respectively. The slope of the MS curve represents the type of semiconductor. At this time, ZnO is an n-type semiconductor and conducts electricity by electrons. The intersection value of the tangent line of the straight line segment of the MS curve on the X-axis represents the flat band potential (E f ) of the material. Therefore, E f (ZnO) = -0.52 V vs. SSC, E f(Wheat biochar) = -0.37 V vs. SSC. Since the conduction band potential of n-type semiconductors is generally 0.2 V less than the flat band potential, thus E CB (ZnO) = -0.72 V vs. SSC. Then convert the potential of the saturated silver chloride electrode (SSC) to the potential of the standard hydrogen electrode (NHE), that is E CB (ZnO) = -0.52 V vs. NHE, E f (Wheat biochar) = -0.17 V vs. NHE. Finally, calculate the valence band potential of ZnO based on the band gap value of ZnO, that is E VB (ZnO) = 2.68 V vs. NHE.

[0051] Example 5 Mechanism of photocatalytic activation of PMS by ZnO hollow nanotube and wheat biochar heterojunction composite for the degradation of CIP

[0052] According to the above characterization results and analysis, we roughly described the mechanism of photocatalytic activation of PMS by ZnO hollow nanotube and wheat biochar heterojunction composite for the degradation of CIP, as Figure 5 shown. After ZnO is photoexcited, the electrons in the valence band will transition to the conduction band to form photogenerated electrons. A part of the photogenerated electrons directly activate PMS in the conduction band of ZnO to generate ‧SO4 - to degrade CIP, and another part of the photogenerated electrons are separated and transferred to wheat biochar to activate PMS to generate ‧SO4 - to degrade CIP adsorbed on wheat biochar.

[0053] By constructing a heterojunction between a semiconductor and a conductor, not only the defects of a single semiconductor itself are made up for, which is beneficial to the target reaction, but also the working principle of the heterojunction can be deeply understood to guide the development of heterojunction catalysts. In this invention, a wheat biochar heterojunction composite catalyst modified with ZnO hollow nanotubes was prepared by a one-step hydrothermal method. By using the unique hollow tube structure and the synergistic effect of the formed semiconductor / conductor heterojunction, the separation of photogenerated electrons and holes is maximized, further improving the performance of photocatalytic activation of PMS for the degradation of CIP in aqueous solution.

[0054] The above examples of the present invention have been described in detail in combination with the embodiments, but the present invention is not limited to the above examples. Within the scope of knowledge possessed by those of ordinary skill in the art, various changes can be made without departing from the purpose of the present invention, and it should also be regarded as the protection scope of the present invention.

Claims

1. A ZnO hollow nanotube-wheat biochar heterojunction composite catalyst, characterized in that, Using wheat biochar particles as a carrier, zinc oxide hollow nanotubes are dispersed on the wheat biochar particles to form a heterojunction structure; the mass ratio of the wheat biochar particles to the zinc oxide hollow nanotubes is 9 to 1:

1.

2. The ZnO hollow nanotube-wheat biochar heterojunction composite catalyst according to claim 1, wherein The average length of the zinc oxide hollow nanotubes is 2.0 to 3.5 μm, and the diameter is 500 to 700 nm; the particle size of the wheat biochar particles is <200 μm.

3. The preparation method of a ZnO hollow nanotube-wheat biochar heterojunction composite catalyst according to claim 1, characterized in that, It includes the following steps: (1) Dissolve a zinc source, an alkali source, and a structure-directing agent in water and mix them evenly to obtain a mixed solution A; the zinc source is Zn(NO3)2‧6H2O, the alkali source is hexamethylenetetramine, and the structure-directing agent is polyvinylpyrrolidone; the mass ratio of the zinc source, the alkali source, the structure-directing agent to water is 1:1 to 1.5:2 to 3:80 to 112; (2) Add wheat biochar powder to the mixed solution A in step (1), and ultrasonically mix them evenly to obtain a mixed solution B; the mass ratio of the mixed solution A to the wheat biochar powder is 1000:1 to 3; (3) The mixed solution B is subjected to a hydrothermal reaction at 100 to 130 °C for 10 to 15 h, cooled to room temperature, centrifuged and washed, and dried.

4. The preparation method of the ZnO hollow nanotube-wheat biochar heterojunction composite catalyst according to claim 3, characterized in that, In step (1), mix at room temperature and stir, and the stirring time is not less than 1 h.

5. The preparation method of the ZnO hollow nanotube-wheat biochar heterojunction composite catalyst according to claim 3, characterized in that, The wheat biochar powder is prepared by pyrolyzing wheat straw; the pyrolysis temperature is 500 - 750 °C, the heating rate is 5 - 10 °C / min, the pyrolysis time is 1 - 4 h, and the N2 atmosphere; the wheat biochar powder passes through an 80 - 100 mesh sieve.

6. The preparation method of the ZnO hollow nanotube-wheat biochar heterojunction composite catalyst according to claim 3, characterized in that, In step (3), it is heated to 100 - 130 °C at a heating rate of 5 - 6 °C / min.

7. The preparation method of the ZnO hollow nanotube-wheat biochar heterojunction composite catalyst according to claim 3, characterized in that, In step (3), the drying temperature is 65 - 90 °C.

8. Application of the ZnO hollow nanotube-wheat biochar heterojunction composite catalyst according to any one of claims 1 to 2 or the preparation method of the ZnO hollow nanotube-wheat biochar heterojunction composite catalyst according to any one of claims 3 to 7 in the reaction of photocatalytic activation of PMS to degrade CIP.

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