Zero-dimensional cobalt-nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst and preparation method and application thereof

Abstract

The application discloses a zero-dimensional cobalt-nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst as well as a preparation method and application thereof, and belongs to the technical field of photocatalysis. The application provides a preparation method of the zero-dimensional cobalt-nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst, which comprises the following steps: dispersing CNNTs in an organic solvent to obtain a CNNTs dispersion liquid through ultrasonic treatment; mixing the CNNTs dispersion liquid, metal salt and a precipitating agent to obtain a mixed dispersion liquid; centrifuging and drying the mixed dispersion liquid to obtain a precursor, and then performing calcination, cooling and grinding to obtain the photocatalyst. The metal oxide quantum dots and g-C3N4 in the catalyst have zero-dimensional and one-dimensional structures respectively, the metal oxide quantum dots are distributed on the surface of the g-C3N4, the migration path of photo-generated charges is shortened, the recombination probability of photo-generated carriers is reduced, more active free radicals can be generated, and the performance of the photocatalyst is significantly improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of photocatalysis, and particularly relates to a zero-dimensional cobalt-nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst as well as a preparation method and application thereof. BACKGROUND

[0002] With the rapid development of industry, the amount of industrial wastewater discharge is increasing, and the toxic inorganic substances, organic substances and heavy metal ions contained in the wastewater will not only pollute water resources and the environment, but also pose a serious threat to human health and aquatic organisms. Among various water pollutants, dyes are a common source of pollution, which have complex organic molecular structures and are widely used in textile, leather and other industrial production. Dyes have the characteristics of high colority, high organic matter concentration, complex composition and high toxicity. After being discharged into water bodies, the following hazards will occur: (1) easy to form colored layers, increase the turbidity of water, hinder the photosynthesis or respiration of underwater organisms, and make them unable to survive; (2) strong acidity and alkalinity, high organic matter content, which seriously interfere with the oxygen transfer mechanism and self-purification process of water bodies; (3) strong irritant to the skin, contact with the eyes can cause eye burns and even permanent blindness, and inhaling the chemical substances evaporated from dye wastewater can cause symptoms such as rapid breathing, nausea and vomiting; (4) triphenylmethane dyes, azo dyes, anthraquinone dyes, etc. are also common carcinogens, which can cause serious damage to the reproductive system, nervous system, liver, brain, etc.; (5) there are heavy metal ions such as copper, cadmium and nickel, which are highly toxic and cannot be biodegraded, and have a serious impact on the ecosystem. In addition, dyes have outstanding anti-photolysis and anti-oxidation abilities, which makes it difficult to treat dye wastewater.

[0003] Therefore, removing toxic dyes from water bodies is a great challenge, especially for industrial and municipal wastewater. In the late 1990s, the treatment of dye wastewater only had simple purification processes such as sedimentation. With the continuous progress of science and technology, research on dye wastewater treatment has increased, and current treatment methods include three categories, namely biological method, physical method and chemical method. Due to the difficulty of degrading dye molecules, traditional physical and biological methods cannot meet the needs of dye degradation. Chemical method is mainly advanced oxidation method (AOPs), mainly including Fenton method, ozone oxidation method, electrochemical oxidation method, photocatalytic oxidation method, persulfate advanced oxidation method, etc. Photocatalytic oxidation degradation of dyes has attracted widespread attention because when light is irradiated on a semiconductor catalyst, a hole and an electron pair are generated, which are used for oxidation and reduction processes respectively, and can produce active free radicals. Active free radicals have strong oxidizing ability and can ultimately oxidize and degrade dye molecules into CO2 and water, achieving complete degradation.

[0004] A composite material of double metal oxide quantum dots and carbon nitride nanosheets is disclosed in CN112619682A, which includes continuous phase carbon nitride nanosheets and dispersed phase copper oxide quantum dots and iron oxide quantum dots (zero-dimensional / two-dimensional). In the composite material of double metal oxide quantum dots and carbon nitride nanosheets, the ultra-thin carbon nitride nanosheets have a large number of nitrogen coordination, and the carbon nitride nanosheets as carriers can effectively anchor copper and iron oxide quantum dots, prevent the agglomeration of metal nanoparticles, promote the transformation of Fe(III) / Fe(II), and at the same time, Fe 3+ As an electron acceptor, the recombination of photo-generated electron-hole pairs in the carbon nitride nanosheet can be inhibited, thereby improving the efficiency of photo-Fenton oxidation degradation of organic pollutants tetracycline.

[0005] CN109304204A discloses a phosphorus-doped tricobalt tetraoxide quantum dot modified graphite phase carbon nitride composite material, a preparation method and application thereof. The preparation method uses vitamin B12 and melamine as raw materials, a small molecule alcohol as a solvent, drying, heating to 500-600℃, and keeping warm for 2-6h, and then cooling and grinding to obtain the target product. The graphite phase carbon nitride composite material prepared by the method shows excellent catalytic activity in photocatalytic degradation of AO7 (dye-orange II).

[0006] CN111584834A discloses a preparation method of metal oxide quantum dots embedded in three-dimensional carbon nanomaterials, which designs and synthesizes a new type of metal oxide quantum dot composite three-dimensional carbon nanomaterial by in-situ growth method to embed metal oxide quantum dots in the inner wall of three-dimensional carbon material. The metal oxide quantum dots are one or more of tin dioxide, iron oxide, sodium oxide and other metal oxides; the three-dimensional carbon nanomaterial is one or more of carbon nanotube array, carbon bubble, 3D interconnected carbon nanobubble, 3D single-connected carbon nanobubble and other three-dimensional carbon nanomaterials.

[0007] However, the main shortcomings of photocatalytic oxidation applied to degrade organic pollutants in wastewater at present are: (1) most commercial catalysts of TiO2 can only be excited by ultraviolet light to generate photo-generated carriers and then generate free radicals, the absorption range of ultraviolet light is narrow, the utilization rate of light energy is low, and its efficiency is also limited by the properties of the catalyst, the wavelength of ultraviolet light and the reactor; (2) the electron-hole pairs generated by light are easy to recombine and deactivate; (3) the removal rate of pollutants by photocatalysis is low.

[0008] The main factors affecting the oxidation performance of semiconductor photocatalysts are: (1) the energy band structure and energy level position of the photocatalyst; (2) the separation efficiency of photo-generated electrons and holes of the photocatalyst; (3) the surface properties of the photocatalyst, including surface reaction overpotential, surface reactive sites, morphology and spatial arrangement of the photocatalyst.

[0009] Based on the above factors, existing catalysts used in photocatalytic oxidation still have some problems: (1) Narrow spectral response range; due to the limitation of their own band structure, most photocatalysts can only absorb ultraviolet light, which accounts for less than 5% of the solar spectrum energy, and cannot utilize the large proportion of visible light; (2) Low quantum efficiency; for most semiconductor photocatalysts, the probability of recombination of photogenerated electrons and holes on the surface and within the structure is very high and cannot be avoided; (3) Low surface reaction efficiency; the surface reaction potential of most semiconductor photocatalysts is too high, and there are no suitable reactive sites or few reactive sites, resulting in the above photocatalysts exhibiting a low surface reaction rate, which affects the photocatalytic degradation efficiency. (4) The cost of the co-catalyst is too high and the particle size is too large. Most of the co-catalysts used are precious metals, which are scarce and expensive. Moreover, they are almost all micro- and nano-structured materials, and the particle size can be further optimized to shorten the carrier transport distance. (5) The degradation process is complicated. The existing technology uses photocatalyst materials (MOs QDs / g-C3N4) composed of metal oxide quantum dots (MOs QDs) and carbon nitride (g-C3N4) to perform photocatalytic oxidation. Instead, it is combined with other advanced oxidation methods such as persulfate oxidation, H2O2 oxidation, Fenton oxidation and other methods. Summary of the Invention

[0010] To address the problems of existing technologies, this invention provides a novel zero-dimensional cobalt-nickel oxide / one-dimensional carbon nitride nanotube heterojunction (CNNTs@CoNiO QDs, CNNTs@NiO QDs, and CNNTs@Co3O4 QDs) photocatalyst and its preparation method. This heterojunction material has advantages such as visible light response, high carrier separation, non-toxicity, and low cost. Compared with bulk g-C3N4 and CNNTs, it has improved light utilization, enhanced photo-oxidation ability, and significantly improved degradation efficiency for organic pollutants, making it a photocatalyst material with promising applications in photocatalytic oxidation.

[0011] This invention first provides a method for preparing a zero-dimensional cobalt-nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst, which includes the following steps:

[0012] A. Disperse CNNTs in an organic solvent and sonicate to obtain a CNNTs dispersion;

[0013] B. Mix the CNNTs dispersion obtained in step A with a metal salt and a precipitant, and stir continuously to obtain a mixed dispersion; the metal salt is at least one selected from CoCl2·6H2O, NiCl2·6H2O, Co(CH3COO)2·4H2O, and Ni(CH3COO)2·4H2O; the precipitant is at least one selected from ammonium bicarbonate, ammonium carbonate, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, sodium hydroxide, and potassium hydroxide.

[0014] C. Centrifuge the mixed dispersion obtained in step B, collect the lower substrate and dry it to obtain the precursor of CNNTs supported by metal oxide quantum dots; calcine the precursor at a heating rate of 2-8℃ / min to 350-400℃, then cool it to room temperature and grind it to obtain the zero-dimensional cobalt nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst.

[0015] In step A, the CNNTs are prepared by the following method:

[0016] a. Melamine is dissolved in water and subjected to a hydrothermal reaction. After the hydrothermal reaction, the system is cooled and centrifuged. The centrifuged product is washed and dried sequentially to obtain nanotube precursor powder. The precursor powder is calcined at a heating rate of 2-5℃ / min to 520-550℃, then cooled to room temperature and ground to obtain g-C3N4 nanotube powder CNNTs.

[0017] In the above preparation method, in step A, the organic solvent is at least one of anhydrous ethanol, isopropanol, ethylene glycol, acetone, and dimethylformamide.

[0018] Preferably, in the above preparation method, in step A, the organic solvent is anhydrous ethanol.

[0019] In the above preparation method, in step A, the ultrasound time is 15-30 minutes.

[0020] In the above preparation method, in step A, the concentration of the CNNTs dispersion is 5–20 mg / mL.

[0021] In the above preparation method, in step B, the mass ratio of CNNTs, metal salt and precipitant in the CNNTs dispersion is 0.1:0.0157~0.0329:0.0079~0.0420.

[0022] In the above preparation method, in step B, the metal salt is at least one of CoCl2·6H2O or Co(CH3COO)2·4H2O, and a combination of at least one of NiCl2·6H2O or Ni(CH3COO)2·4H2O.

[0023] Preferably, in the above preparation method, in step B, the metal salt is CoCl2·6H2O and NiCl2·6H2O, or Co(CH3COO)2·4H2O and Ni(CH3COO)2·4H2O.

[0024] More preferably, in the above preparation method, in step B, the molar ratio of Co salt to Ni salt in the metal salt is 1:0.5-3.

[0025] In the most preferred embodiment of the above preparation method, in step B, the molar ratio of Co salt to Ni salt in the metal salt is 1:1.5 to 2.5.

[0026] Preferably, in the above preparation method, in step B, the precipitant is ammonium bicarbonate, sodium carbonate, or sodium hydroxide.

[0027] In the above preparation method, in step B, the continuous stirring time is 7-9 hours.

[0028] In the above preparation method, in step C, the centrifugation speed is 8000-11000 r / min.

[0029] In the above preparation method, in step C, the centrifugation time is 3 to 5 minutes.

[0030] In the above preparation method, in step C, the drying temperature is 60-80℃.

[0031] In the above preparation method, in step C, the drying time is 10-14 hours.

[0032] In the above preparation method, in step C, the calcination time is 60-120 min.

[0033] In the above preparation method, in step a, the mass ratio of melamine to water is 1:15-30.

[0034] In the above preparation method, in step a, the temperature of the hydrothermal reaction is 200-220℃.

[0035] In the above preparation method, in step a, the hydrothermal reaction time is 10-14 hours.

[0036] In the above preparation method, in step a, the centrifugation speed of the obtained system is 8000-10000 r / min.

[0037] In the above preparation method, in step a, the centrifugation time of the obtained system is 3 to 6 minutes.

[0038] In the above preparation method, step a involves washing by centrifugation with water and anhydrous ethanol 3 to 5 times respectively.

[0039] In the above preparation method, in step a, the drying temperature is 60-80℃.

[0040] In the above preparation method, in step a, the drying time is 10-16 hours.

[0041] In the above preparation method, in step a, the calcination time is 180-240 min.

[0042] The present invention also provides a zero-dimensional cobalt-nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst, which is prepared by the above method.

[0043] This invention also provides the application of the above-mentioned zero-dimensional cobalt nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst in photocatalytic degradation of organic pollutants and / or photocatalytic water splitting to produce hydrogen.

[0044] Terminology Explanation:

[0045] Quantum dots (QDs) are an important type of low-dimensional semiconductor material, whose dimensions in all three dimensions are no greater than twice the exciton Bohr radius of their corresponding semiconductor materials. Quantum dots are generally spherical or near-spherical, with diameters typically between 2 and 20 nm.

[0046] Zero-dimensional nanomaterials: Nanomaterials with dimensions (x, y, z) between 0.1 and 100 nm in each dimension; One-dimensional nanomaterials: Materials with dimensions (x, y, z) between 0.1 and 100 nm in two dimensions; Two-dimensional nanomaterials: Materials with dimensions (x, y, z) between 0.1 and 100 nm in only one dimension; Three-dimensional materials: Materials with dimensions (x, y, z) between 0.1 and 100 nm in no dimension.

[0047] The beneficial effects of this invention are:

[0048] This invention uses stable, non-toxic, pollution-free, and simple non-metallic g-C3N4 as the main catalyst, which has advantages such as short process flow, fewer equipment required, low equipment requirements, and short preparation time. Furthermore, this invention nano-scales g-C3N4 to form a special morphology with one-dimensional nanotubes, giving it the advantage of directional electron transport. Compared with bulk g-C3N4, the valence band edge of g-C3N4 nanotubes is more negative, which is more conducive to oxidative degradation reactions.

[0049] This invention uses inexpensive transition metals cobalt and nickel, as well as carbon and nitrogen materials, to replace precious metals and reduce the cost of catalysts. Furthermore, by nano-sizing the structure of transition metal oxide cocatalysts, zero-dimensional quantum dot materials are prepared, which have the advantages of multi-exciton generation, easy control of charge carriers, and abundant surface sites. Through the individual or synergistic effects of cobalt and nickel, the catalyst performance is significantly improved.

[0050] In this invention, the metal oxide quantum dots (CoNiO, NiO, Co3O4) and g-C3N4 nanotubes in the catalyst have zero-dimensional and one-dimensional structures, respectively. The metal oxide quantum dots are distributed on the surface of the g-C3N4 nanotubes, which can shorten the migration path of photogenerated charges, enabling them to migrate rapidly to the material surface and be quickly transported to the metal oxide surface. This reduces the recombination probability of photogenerated charge carriers, allowing more charge carriers to participate in the oxidation reaction, thereby generating more active free radicals to attack target pollutant molecules, thus achieving the purpose of rapid degradation of pollutants and photocatalytic hydrogen production from water. Attached Figure Description

[0051] Figure 1 The XRD patterns are of comparative example 2 CNNTs, bimetallic oxide Co1Ni1O and example 3 CNNTs@Co1Ni1O QDs.

[0052] Figure 2 The UV-Vis diffuse reflectance spectra of Comparative Example 1 BCN, Comparative Example 2 CNNTs, and Example 3 CNNTs@Co1Ni1O QDs are shown.

[0053] Figure 3 For Comparative Example 1 BCN, Comparative Example 2 CNNTs, and Example 3 CNNTs@Co1Ni1O QDs (αhν) 1 / 2 The relationship between the band gap energy hν and the band gap energy.

[0054] Figure 4 SEM images of Comparative Example 1 BCN and Comparative Example 2 CNNTs.

[0055] Figure 5 This is a SEM image of the bimetallic oxide Co1Ni1O.

[0056] Figure 6 This is a SEM image of CNNTs@Co1Ni1O QDs from Example 3.

[0057] Figure 7 The graph shows the photocatalytic degradation rate of RhB in Comparative Examples 1-2 and Examples 1-5. Detailed Implementation

[0058] Specifically, a method for preparing a zero-dimensional cobalt-nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst includes the following steps:

[0059] A. Disperse CNNTs in an organic solvent and sonicate to obtain a CNNTs dispersion;

[0060] B. Mix the CNNTs dispersion obtained in step A with a metal salt and a precipitant, and stir continuously to obtain a mixed dispersion; the metal salt is at least one selected from CoCl2·6H2O, NiCl2·6H2O, Co(CH3COO)2·4H2O, and Ni(CH3COO)2·4H2O; the precipitant is at least one selected from ammonium bicarbonate, ammonium carbonate, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, sodium hydroxide, and potassium hydroxide.

[0061] C. Centrifuge the mixed dispersion obtained in step B, collect the lower substrate and dry it to obtain the precursor of CNNTs supported by metal oxide quantum dots; calcine the precursor at a heating rate of 2-8℃ / min to 350-400℃, then cool it to room temperature and grind it (until there is no obvious particle feel) to obtain the zero-dimensional cobalt nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst.

[0062] In step A, the CNNTs are prepared by the following method:

[0063] a. Melamine is dissolved in water (generally, stirring continuously at room temperature for 2 hours can completely dissolve melamine), and a hydrothermal reaction is carried out. After the hydrothermal reaction, the system is cooled and centrifuged. The centrifuged product is washed and dried sequentially to obtain nanotube precursor powder. The precursor powder is calcined at a heating rate of 2-5℃ / min to 520-550℃, then cooled to room temperature and ground (until there is no obvious particle feel) to obtain g-C3N4 nanotube powder CNNTs.

[0064] Non-metallic g-C3N4 nanotube powder (graphitized carbonitride) has the following advantages: it is a non-metallic semiconductor photocatalytic material that responds to visible light and is stable at high temperatures; however, it also has disadvantages: low visible light quantum utilization, small specific surface area, and easy recombination of photogenerated carriers. Furthermore, existing MOs QDs / g-C3N4 preparations are mainly zero-dimensional / two-dimensional and zero-dimensional / three-dimensional composite materials. Two-dimensional materials are prone to developing defect structures on their surfaces during preparation and storage. These defects form localized states that bind electrons, reducing electron mobility within the two-dimensional material and leading to a decline in its various properties. One-dimensional nanomaterials, due to their orientation characteristics along a certain direction, are considered ideal materials for directional electron transport and are the smallest dimensional structures that can be used for effective electron and photoexciton transport. One-dimensional nanomaterials are ideal systems for studying electron transport behavior and the relationship between optical, magnetic, and other physical properties and size / dimension; their specific geometric morphology plays an important role in constructing nanoelectronic and optical devices.

[0065] Therefore, this invention modulates the microstructure of CNNTs to prepare them into nanotubes, reducing their lateral dimensions and making them one-dimensional nanomaterials. This increases their specific surface area and reactive sites, while also adjusting the band structure and raising the valence band position, resulting in high charge mobility, which is more conducive to oxidation reactions and effectively solves the problem of easy carrier recombination. Furthermore, by combining them with transition metal-based cobalt / nickel cocatalysts, this composite photocatalyst brings some unique advantages, such as enhanced visible light absorption and scattering ability, rapid and long-distance electron transfer, more active sites, larger specific surface area, and lower photo-regenerated electron-hole recombination.

[0066] Based on the structural characteristics of CNNTs in this invention, the dispersibility of CNNTs in various solvents was tested to facilitate the loading of metal oxide quantum dots into CNNTs. The tests showed that deionized water has a higher viscosity than organic solvents, which may affect the flowability of the powder particles and thus the dispersion effect; therefore, water is not suitable as a dispersion solvent. Organic solvents have lower viscosity and surface tension, allowing the powder particles to flow more easily and forming a uniform suspension. They also have higher volatility; therefore, in step A of this invention, at least one organic solvent selected from anhydrous ethanol, isopropanol, ethylene glycol, acetone, and dimethylformamide is used as the dispersion solvent. Furthermore, anhydrous ethanol can disperse CNNTs more uniformly, which is more conducive to the uniform distribution of metal oxide quantum dots in CNNTs. It also has a suitable boiling point, is non-toxic, odorless, and has lower cost, making it the preferred solvent in this invention.

[0067] In step A of this invention, the acoustic flow, cavitation, and wetting effects generated during ultrasonic oscillation are utilized to alter the structure and morphology of CNNTs, making the CNNT particles smaller and more dispersed. This increases the surface area of ​​the dispersed phase and makes it easier for it to interact with the dispersion medium, improving the contact area between the dispersion and the solvent. This is beneficial for the uniform distribution of metal oxide quantum dots in the CNNTs and for enhancing the activity of the catalyst. Experiments have shown that if the ultrasonic treatment time is too short, the catalyst particles exhibit high agglomeration, which is detrimental to increasing the specific surface area and reducing the exposure of active sites. If the ultrasonic treatment time is too long, the morphology of the catalyst particles may be damaged by the instantaneous burst pressure of bubbles formed by ultrasonic cavitation, affecting the material properties. Therefore, in step A, the ultrasonic treatment time is controlled to be 15–30 minutes.

[0068] In step A of this invention, the concentration of the CNNTs dispersion is controlled to be 5–20 mg / mL; simultaneously, in step B, the mass ratio of CNNTs, metal salt, and precipitant in the CNNTs dispersion is controlled to be 0.1:0.0157–0.0329:0.0079–0.0420; thereby ensuring that the mass fraction of metal (monometallic and / or bimetallic) oxide quantum dots in the quantum dot-loaded CNNTs (CNNTs@MO QDs) is 5%–9%.

[0069] In step B of this invention, the cocatalyst is selected from transition metal oxides cobalt and / or nickel. Nickel oxide and cobalt tetroxide are the most stable forms of nickel and cobalt, with significant electron transfer capabilities. Compared with Co3O4 and NiO nanoparticles, QDs have more catalytic active sites with exposed edges, making them one of the cocatalyst choices in photocatalysis. Loading bimetallic oxide quantum dots (CoNiO QDs) on g-C3N4 has the following advantages: (1) smaller lateral size, larger band gap, and stronger redox ability; (2) simultaneously, smaller size means shorter time for electron-hole pairs to reach the catalyst surface, enabling faster reaction with substances adsorbed on the catalyst surface; (3) lack of long-range atomic order; (4) abundant surface defects, allowing for a rich supply of active sites for catalysis.

[0070] Therefore, in step B of this invention, the metal oxide is regulated to be in the form of quantum dots, which makes its surface smoothness worse and forms uneven atomic steps, thereby increasing the contact surface for chemical reactions, which is conducive to the transport of charge carriers with g-C3N4 and enhances its catalytic activity. In addition, forming a zero-dimensional / one-dimensional heterojunction between g-C3N4 nanotubes (CNNTs) and metal oxide quantum dots, so that the quantum dots are distributed on the surface of g-C3N4 nanotubes, has the following advantages: (1) it expands the light absorption range and makes full use of solar energy; (2) it shortens the migration path of photogenerated charges, so that the photogenerated charges can quickly migrate to the material surface and be quickly transported to the surface of metal oxide, thereby improving the charge separation and transfer ability, reducing the recombination probability of photogenerated charge carriers, and allowing more charge carriers to participate in the oxidation reaction; (3) the high specific surface area and high stability of g-C3N4 make it an ideal carrier for loading transition metals, dispersing transition metal oxides and preventing agglomeration; (4) it promotes the transfer of charge carriers to generate more active free radicals, thereby achieving the purpose of rapid degradation of pollutants and photocatalytic hydrogen production from water. Therefore, in step B of this invention, at least one of CoCl2·6H2O, NiCl2·6H2O, Co(CH3COO)2·4H2O, and Ni(CH3COO)2·4H2O is used as a metal salt to form cobalt-nickel monometallic or bimetallic oxide quantum dots.

[0071] Experiments have shown that the synergistic effect between Ni and Co bimetals facilitates carrier transport with g-C3N4, enhancing catalytic activity. Therefore, in step B of this invention, the metal salt is at least one of CoCl2·6H2O or Co(CH3COO)2·4H2O, combined with at least one of NiCl2·6H2O or Ni(CH3COO)2·4H2O to produce cobalt-nickel bimetallic oxide quantum dots. Preferably, CoCl2·6H2O and NiCl2·6H2O, or Co(CH3COO)2·4H2O and Ni(CH3COO)2·4H2O, are used as the metal salts. The amount of metal elements presents differences in material properties, which have a certain impact on the final performance. Therefore, this invention preferably controls the molar ratio of Co salt to Ni salt in the metal salt to be 1:0.5 to 3. In particular, when the molar ratio of Co salt to Ni salt in the metal salt is controlled to be 1:1.5 to 2.5, the photocatalyst performance is significantly improved. Through experimentation, it was found that this invention requires the use of hydrated metal salts because the solubility of hydrated precursor salts is greater than that of anhydrous substances, making it easier for them to dissolve and form quantum dots. Therefore, anhydrous substances are not suitable.

[0072] Furthermore, this invention uses Ni and Co as transition metal oxides, which have the advantages of low cost and abundant reserves, effectively solving the problem of using precious metals in traditional co-catalysts.

[0073] In step B of this invention, a precipitate is generated by reacting a precipitant with a metal salt. The precipitate is further decomposed by heating (step C) into Co3O4 and / or NiO. Preferably, the precipitant is ammonium bicarbonate, sodium carbonate, or sodium hydroxide. In step B of this invention, the stirring time is controlled to be 7-9 hours to ensure that the precipitant and the metal salt are fully dissolved and mixed evenly in the ethanol dispersion of CNNTs, allowing the product to precipitate fully.

[0074] In step C of this invention, the centrifugation speed is 8000-11000 r / min, and the centrifugation time is 3-5 min.

[0075] In step C of this invention, the centrifuged product is obtained by centrifugation in an organic solvent. Therefore, the product also contains organic solvent and a small amount of water from the metal salt. If the centrifuged product is directly calcined, the residual solvent and water may cause the metal oxide particles to agglomerate, thereby affecting the particle size of the metal oxide and the formation of quantum dots. Therefore, in this invention, the centrifuged product is first dried at 60–80°C, generally for 10–14 hours.

[0076] In step C of this invention, the particle size initially decreases and then increases with increasing calcination temperature. At lower calcination temperatures, the particle size decreases as the temperature rises because the particle size gradually decreases due to the gradual removal of moisture and crystal transformation. However, at excessively high combustion temperatures, the surface atoms of the particles obtained from the high-temperature decomposition of the precursor are highly reactive, easily diffusing to adjacent surfaces and bonding with their corresponding atoms to form stable chemical bonds, thus forming permanent hard agglomerates. This alters the particle shape, causing the particle diameter to increase again. To avoid secondary particle growth, the calcination temperature should be kept as low as possible while forming the metal oxide. Simultaneously, a slow heating rate prolongs the quantum dot growth time and increases the crystal size; a rapid heating rate results in uneven quantum dot growth and a wide size distribution. Similarly, a short heating time makes it difficult for quantum dot crystals to grow uniformly, resulting in small particle sizes; a long heating time results in large quantum dot crystals that are prone to agglomeration. Therefore, in step C of this invention, the precursor is heated to 350-400°C at a heating rate of 2-8°C / min and calcined for 60-120 minutes.

[0077] In preparing CNNTs, the mass ratio of melamine to water (generally deionized water) is controlled at 1:15-30; the hydrothermal reaction temperature is controlled at 200-220℃, and the hydrothermal reaction time is generally 10-14h; the obtained hydrothermal reaction product is centrifuged at 8000-10000r / min for 3-6min, and then washed 3-5 times by centrifugation with water (generally deionized water) and anhydrous ethanol respectively; after washing, the material is first dried at 60-80℃ for 10-16h, and then calcined, generally for 180-240min.

[0078] The present invention also provides a zero-dimensional cobalt-nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst, which is prepared by the above method.

[0079] This invention also provides the application of the above-mentioned zero-dimensional cobalt nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst in photocatalytic degradation of organic pollutants and / or photocatalytic water splitting to produce hydrogen.

[0080] The present invention will be further described in detail below through embodiments, but the scope of protection of the present invention is not limited to the embodiments described herein.

[0081] Comparative Example 1

[0082] Weigh 4g of melamine into a covered crucible, place the crucible in a muffle furnace, set the temperature to 550℃, and heat for 4 hours at a heating rate of 2℃·min. -1After the muffle furnace has completely cooled to room temperature, open the muffle furnace and grind the resulting yellow agglomerate into powder to obtain g-C3N4 bulk (BCN) powder.

[0083] Comparative Example 2

[0084] Weigh 4g of melamine and dissolve it in 80mL of deionized water, stirring continuously at room temperature for 2h. Transfer the solution to a 100mL polytetrafluoroethylene reactor and place it in an oven for hydrothermal reaction at 200℃ for 12h. After the reaction is complete and the solution has cooled to room temperature, open the reactor and transfer it to a centrifuge tube for centrifugation. Centrifuge three times each with deionized water and anhydrous ethanol, at a speed of 8000r / min for 6min. Dry the centrifuged product at 60℃ for 12h to obtain g-C3N4 nanotube precursor powder. Place the g-C3N4 nanotube precursor powder in a covered crucible and place the crucible in a muffle furnace at 550℃ for 240min at a heating rate of 2℃ / min. After the muffle furnace has completely cooled to room temperature, open the furnace, collect the product in the crucible, and grind it thoroughly to obtain g-C3N4 nanotube powder (CNNTs).

[0085] Example 1

[0086] 0.1 g of CNNTs (prepared from Comparative Example 2) was weighed and dispersed in 10 mL of anhydrous ethanol. The mixture was sonicated for 15 min to form a 10 mg / mL CNNTs dispersion. 0.0238 g of NiCl2·6H2O and 0.0237 g of NH4HCO3 were added sequentially to the dispersion, and the mixture was stirred continuously at room temperature for 7 h. The dispersion was then centrifuged at 10000 r / min for 5 min, and the lower substrate was collected and dried at 60 °C for 12 h to obtain the NiO quantum dot-loaded CNNTs (CNNTs@NiO QDs) precursor. Finally, the precursor was calcined in a muffle furnace at 350 °C for 120 min at a heating rate of 5 °C / min. After cooling to room temperature, the product was collected and thoroughly ground to obtain CNNTs@NiO QDs.

[0087] Example 2

[0088] 0.1 g of CNNTs were weighed and dispersed in 10 mL of anhydrous ethanol, and sonicated for 15 min to form a 10 mg / mL CNNTs dispersion. 0.0238 g of CoCl₂·6H₂O and 0.0237 g of NH₄HCO₃ were added sequentially to the dispersion, and the mixture was stirred continuously at room temperature for 7 h. The dispersion was then centrifuged at 10000 r / min for 5 min, and the lower substrate was collected and dried at 60 °C for 12 h to obtain the Co₃O₄ quantum dot-loaded CNNTs (CNNTs@Co₃O₄ QDs) precursor. Finally, the precursor was calcined in a muffle furnace at 350 °C for 120 min at a heating rate of 5 °C / min. After cooling to room temperature, the product was collected and thoroughly ground to obtain CNNTs@Co₃O₄ QDs.

[0089] Example 3

[0090] Weigh 0.1 g CNNTs and disperse them in 10 mL of anhydrous ethanol. Sonicate for 15 min to form a 10 mg / mL CNNTs dispersion. Add 0.0119 g CoCl2·6H2O, 0.0119 g NiCl2·6H2O and 0.0237 g NH4HCO3 to the dispersion in sequence and stir continuously at room temperature for 7 h. Then, centrifuge the mixture at 10000 r / min for 5 min, collect the lower substrate and dry it at 60 °C for 12 h to obtain the CoNiO quantum dot-loaded CNNTs (CNNTs@Co1Ni1O QDs) precursor. Finally, calcine the precursor in a muffle furnace at 350 °C for 120 min at a heating rate of 5 °C / min. After cooling to room temperature, collect the product and grind it thoroughly to obtain CNNTs@Co1Ni1O QDs (Co1Ni1O represents n(CoCl2·6H2O):n(NiCl2·6H2O)=1:1).

[0091] Example 4

[0092] 0.1 g CNNTs were weighed and dispersed in 10 mL of anhydrous ethanol, and sonicated for 20 min to form a 10 mg / mL CNNTs dispersion. 0.0081 g CoCl2·6H2O, 0.0157 g NiCl2·6H2O, and 0.0237 g NH4HCO3 were added sequentially to the dispersion, and the mixture was stirred at room temperature for 8 h. The mixture was then centrifuged at 10000 r / min for 5 min, and the lower substrate was collected and dried at 60 °C for 12 h to obtain the CoNiO quantum dot-loaded CNNTs (CNNTs@Co1Ni2O QDs) precursor. Finally, the precursor was calcined in a muffle furnace at 350 °C for 120 min at a heating rate of 5 °C / min. After cooling to room temperature, collect the product and grind it thoroughly to obtain CNNTs@Co1Ni2O QDs (Co1Ni2O represents n(CoCl2·6H2O):n(NiCl2·6H2O)=1:2).

[0093] Example 5

[0094] Weigh 0.1 g CNNTs and disperse them in 10 mL of anhydrous ethanol. Sonicate for 30 min to form a 10 mg / mL CNNTs dispersion. Add 0.0157 g CoCl2·6H2O, 0.0081 g NiCl2·6H2O and 0.0237 g NH4HCO3 to the dispersion in sequence and stir continuously at room temperature for 9 h. Then, centrifuge the mixture at 10000 r / min for 5 min, collect the lower substrate and dry it at 60 °C for 12 h to obtain the CoNiO quantum dot-loaded CNNTs (CNNTs@Co2Ni1O QDs) precursor. Finally, calcine the precursor in a muffle furnace at 350 °C for 120 min at a heating rate of 5 °C / min. After cooling to room temperature, collect the product and grind it thoroughly to obtain CNNTs@Co2Ni1O QDs (Co2Ni1O represents n(CoCl2·6H2O):n(NiCl2·6H2O)=2:1).

[0095] Table 1. Process Parameters for Examples

[0096]

[0097]

[0098] In this invention, regarding the dispersion of CNNTs in anhydrous ethanol, the ultrasonic dispersion time of CNNTs in anhydrous ethanol is controlled to be 15-30 min. Within this range, CNNTs can be dispersed, but there are slight differences in the degree of agglomeration of some particles. The stirring time of 7-9 h reflects the degree of precipitation reaction between CNNTs and metal salts, and complete precipitation is achieved after 7 h. The different molar ratios are due to the difference in the content of Ni and Co. Compared with the influence of the degree of agglomeration of some particles, the difference in material properties reflected by the content of the metal elements themselves has a more significant impact on the final performance.

[0099] The experimental conditions for the photocatalytic degradation of Rhodamine B (RhB) dye in this invention were as follows: catalyst dosage: 0.5 g / L (40 mg catalyst dispersed in 80 mL RhB solution); solution pH: 7; RhB concentration: 10 mg / L; visible light irradiation time: 90 min; stirring in the dark before irradiation: 40 min. The photocatalytic degradation results are shown in Table 2.

[0100] Table 2 Comparison of photocatalytic degradation rates of RhB

[0101] Case Catalyst Degradation rate within 30 min Degradation rate within 90 min Example 1 CNNTs@NiO 85.24％ 99.80％ Example 2 CNNTs@Co3O4 84.46％ 99.75％ Example 3 CNNTs@Co1Ni1O 85.88％ 99.82％ Example 4 CNNTs@Co1Ni2O 98.75％ 99.85％ Example 5 CNNTs@Co2Ni1O 88.79％ 99.91％ Comparative Example 1 BCN 12.40％ 33.49％ Comparative Example 2 CNNTs 83.13％ 99.73％

[0102] Table 2 shows that pure RhB solution exhibits almost no self-degradation ability under light irradiation without the addition of a catalyst; the degradation effect of one-dimensional CNNTs is far superior to that of bulk g-C3N4; the degradation ability of CNNTs supported by bimetallic CoNiO quantum dots is better than that of monometallic quantum dots; and the degradation ability of CNNTs@Co1Ni2O QDs, i.e., Example 4, is the best. The same results can be obtained in… Figure 7 get.

[0103] Table 3 Comparison of photocatalytic oxidation degradation performance between the present invention and existing technologies

[0104]

[0105]

[0106] Note: Reference 1: M. Liu et al. Applied Catalysis B: Environmental 301(2022)120765; Reference 2: H. Gao et al. Small 2018,14,1801353.

[0107] As shown in Table 3, compared with the prior art, the present invention has the following advantages: 1. The oxidation technology of the present invention is simple. References 1 and 2, and CN112619682A all use a combination of photocatalytic oxidation and other oxidation technologies; the present invention only uses photocatalytic oxidation technology; 2. The morphology of the g-C3N4 catalyst material of the present invention is more special. The g-C3N4 used in References 1 and 2 is a two-dimensional nanosheet array structure; the present invention is a one-dimensional nanotube structure with stronger directional electron transport capability; 3. The selection of the metal oxide material for the co-catalyst in the present invention improves the catalyst performance by combining the synergistic effect of Ni and Co; 4. The degradation ability of the present invention is outstanding. The degradation ability of the same dye (RhB) using photocatalytic oxidation and other oxidation technologies in References 1 and 2, and CN112619682A is comparable to that of the present invention using only photocatalytic oxidation technology, which fully demonstrates that the catalyst used in the present invention has a superior photocatalytic degradation ability.

[0108] In summary, the CNNTs@CoNiO QDs photocatalyst material obtained by the method of this invention exhibits excellent photocatalytic oxidation performance due to the short-distance charge transport and electronic interaction between one-dimensional CNNTs and zero-dimensional CoNiO QDs. This provides an effective design approach for developing low-cost, safe, reliable, and highly active g-C3N4-based photocatalysts with high solar energy utilization.

[0109] Figure 1 The XRD patterns of CNNTs obtained in Comparative Example 2, the bimetallic oxide Co1Ni1O, and CNNTs@Co1Ni1OQDs obtained in Example 3 show that CoNiO is composed of Co3O4 and NiO. CNNTs (Comparative Example 2) did not change the phase composition of g-C3N4. In Example 3 (CNNTs@Co1Ni1O QDs) after loading CoNiO QDs, the main component is still g-C3N4. Although the loading of Co3O4 and NiO is very small, the catalytic test results show that Co3O4 and NiO QDs were successfully loaded.

[0110] Preparation method of bimetallic oxide (Co1Ni1O) (Ni:Co molar ratio = 1:1):

[0111] Weigh out 5 mmol NiCl2·6H2O (or 5 mmol Ni(CH3COO)2·4H2O), 5 mmol CoCl2·6H2O (or 5 mmol Co(CH3COO)2·4H2O), and 15 mmol NH4HCO3 (or 15 mmol Na2CO3, or 15 mmol NaOH), respectively, and add them to 5–20 mL of anhydrous ethanol. Stir continuously at room temperature for 7 h. Then, centrifuge the mixture at 8000–11000 r / min for 3–5 min, collect the lower substrate, and dry it at 60℃–80℃ for 10–14 h to obtain the Co1Ni1O precursor. Calcinate the precursor at 350–400℃ for 60–120 min at a heating rate of 2–8℃ / min. After cooling to room temperature, collect the product and grind it thoroughly to obtain the Co1Ni1O zero-dimensional nanomaterial.

[0112] Figure 2 The UV-Vis diffuse reflectance spectra of BCN obtained in Comparative Example 1, CNNTs obtained in Comparative Example 2, and CNNTs@Co1Ni1O QDs obtained in Example 3 are shown. It can be seen that compared with bulk g-C3N4 (Comparative Example 1), the absorption edge of one-dimensional CNNTs (Comparative Example 2) is blue-shifted, the band gap is increased, the positions of the conduction band and valence band change, and its redox ability also changes accordingly. Compared with Comparative Example 2 (CNNTs), the absorption edge of Example 3 (CNNTs@Co1Ni1O QDs) is red-shifted, the band gap is decreased, and the light absorption ability is enhanced.

[0113] Figure 3 The (αhν) values ​​are calculated for the BCN obtained in Comparative Example 1, the CNNTs obtained in Comparative Example 2, and the CNNTs@Co1Ni1O QDs obtained in Example 3. 1 / 2 The relationship between the band gap energy (hν) and the band gap energy shows that the band gap of one-dimensional CNNTs widens, but the band gap decreases slightly after loading bimetallic oxide quantum dots, indicating the successful synthesis of the composite.

[0114] Figure 4 The SEM images of BCN obtained in Comparative Example 1 and CNNTs obtained in Comparative Example 2 show that their morphologies are completely different, indicating that the successful synthesis of one-dimensional CNNTs is more conducive to loading metal oxide quantum dots.

[0115] Figure 5The image shows a SEM image of CoNiO nanomaterials. Without the support of CNNTs, the particle size is as small as 50 nm or less, making it a zero-dimensional nanomaterial. When loaded with CNNTs, the particle size is further limited to the nanometer scale, forming CoNiO quantum dots (the relationship between zero-dimensional materials and quantum dots: zero-dimensional materials include quantum dots). In the bimetallic case, zero-dimensional nanomaterials of Co3O4 and NiO are formed; in the monometallic case, it is either Co3O4 or NiO, therefore, monometallic Ni or Co are also zero-dimensional materials.

[0116] Figure 6 The image shows the SEM image of CNNTs@Co1Ni1O QDs obtained in Example 3. The morphology of the one-dimensional CNNTs remains unchanged, but the quantum dots are too small to be observed by the scanning electron microscope.

Claims

1. The application of zero-dimensional cobalt-nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst in the photocatalytic degradation of organic pollutants, characterized in that: The preparation method of the zero-dimensional cobalt-nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst includes the following steps: A. Disperse CNNTs in an organic solvent and sonicate to obtain a CNNTs dispersion; B. Mix the CNNTs dispersion obtained in step A with a metal salt and a precipitant, and stir continuously to obtain a mixed dispersion; the precipitant is at least one of ammonium bicarbonate, ammonium carbonate, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, sodium hydroxide, and potassium hydroxide. C. Centrifuge the mixed dispersion obtained in step B, collect the lower substrate and dry it to obtain the precursor of CNNTs supported by metal oxide quantum dots; calcine the precursor at a heating rate of 2~8℃ / min to 350~400℃, then cool it to room temperature and grind it to obtain the zero-dimensional cobalt nickel oxide quantum dot / one-dimensional carbon nitride nanotube composite photocatalyst. In step A, the CNNTs are prepared by the following method: a. Melamine is dissolved in water and subjected to a hydrothermal reaction. After the hydrothermal reaction, the system is cooled and centrifuged. The centrifuged product is washed and dried to obtain nanotube precursor powder. The precursor powder is calcined at 520-550℃ with a heating rate of 2-5℃ / min, then cooled to room temperature and ground to obtain g-C3N4 nanotube powder CNNTs. In step A, the organic solvent is at least one of anhydrous ethanol, isopropanol, ethylene glycol, acetone, and dimethylformamide; In step A, the ultrasound duration is 15-30 minutes; In step A, the concentration of the CNNTs dispersion is 5~20 mg / mL; In step B, the mass ratio of CNNTs, metal salt, and precipitant in the CNNTs dispersion is 0.1:0.0157~0.0329:0.0079~0.0420; In step B, the metal salt is at least one of CoCl2·6H2O or Co(CH3COO)2·4H2O, and a combination of at least one of NiCl2·6H2O or Ni(CH3COO)2·4H2O; In step B, the molar ratio of Co salt to Ni salt in the metal salt is 1:1.5~2; In step a, the temperature of the hydrothermal reaction is 200~220℃.

2. The application according to claim 1, characterized in that: In step A, the organic solvent is anhydrous ethanol.

3. The application according to claim 1, characterized in that: In step B, the metal salt is CoCl2·6H2O and NiCl2·6H2O, or Co(CH3COO)2·4H2O and Ni(CH3COO)2·4H2O.

4. The application according to claim 1, characterized in that: In step B, the precipitant is ammonium bicarbonate, sodium carbonate, or sodium hydroxide.

5. The application according to claim 1, characterized in that: In step B, the continuous stirring time is 7-9 hours.

6. The application according to claim 1, characterized in that: In step C, the centrifugation speed is 8000~11000 r / min.

7. The application according to claim 1, characterized in that: In step C, the centrifugation time is 3-5 minutes.

8. The application according to claim 1, characterized in that: In step C, the drying temperature is 60~80℃.

9. The application according to claim 1, characterized in that: In step C, the drying time is 10-14 hours.

10. The application according to claim 1, characterized in that: In step C, the calcination time is 60~120 min.

11. The application according to any one of claims 1 to 10, characterized in that: In step a, at least one of the following must be satisfied: The mass ratio of melamine to water is 1:15~30; The hydrothermal reaction time is 10-14 hours; The resulting system was centrifuged at a speed of 8000~10000 r / min; The resulting system was centrifuged for 3-6 minutes. The washing process involves centrifuging and washing with water and anhydrous ethanol 3 to 5 times respectively. The drying temperature is 60~80℃; The drying time is 10-16 hours; The calcination time is 180~240 min.

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