Cu2O-at-Ag / g-C3N4 nano composite material as well as preparation method and application thereof

Abstract

The invention discloses a Cu2O (at) Ag / g-C3N4 nano composite material, which is characterized in that silver nanocrystals are uniformly distributed and coated on the surface of the Cu2O nanocrystals to form Cu2O (at) Ag nanocrystals, and the Cu2O (at) Ag nanocrystals are uniformly loaded on the surface of a g-C3N4 nanosheet. The invention also discloses a preparation method of the Cu2O-coated Ag / g-C3N4 nano composite material, which comprises the following steps: uniformly mixing a copper source, a silver source, a g-C3N4 nanosheet and a solvent, adding an alkaline substance, carrying out primary reaction, then adding a reducing agent, carrying out secondary reaction, and carrying out solid-liquid separation to obtain the Cu2O-coated Ag / g-C3N4 nano composite material. The invention also discloses an application of the Cu2O / Ag / g-C3N4 nano composite material as an SERS (Surface Enhanced Raman Scattering) substrate and a photocatalytic degradation agent. Meanwhile, the photocatalytic degradation performance is good, and the SERS detection sensitivity is high.

Description

Technical Field

[0001] The present invention relates to the technical field of SERS, and in particular to a Cu2O@Ag / g-C3N4 nanocomposite and its preparation method and application. Background Art

[0002] Antibiotics have been widely used in medical treatment, agriculture, aquaculture and animal husbandry due to their powerful bactericidal and bacteriostatic effects. However, due to insufficient intestinal absorption or incomplete digestion, antibiotics may be excreted and released into the natural ecosystem in their original form or as bioactive metabolites after being used by humans and animals, which has been proven to have various adverse effects on the environment and thus threaten human health. Antibiotics have become an emerging and ubiquitous environmental pollutant and have attracted much attention. For example, tetracycline (TC) is one of the most commonly used broad-spectrum antibiotics in the world and is widely used to treat various diseases such as acne, cholera, brucellosis, plague and malaria. Due to the stable molecular structure of the aromatic ring and functional groups of tetracycline, it is difficult to remove under natural conditions, resulting in its continuous accumulation in various water environments including surface water, groundwater and even drinking water. More seriously, studies have shown that even low doses of tetracycline can cause significant harm to the human body. Therefore, the need for ultrasensitive detection and efficient treatment of tetracycline is extremely urgent.

[0003] Currently, there are various detection techniques for determining antibiotics, including high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), enzyme-linked immunosorbent assay (ELISA), and high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS / MS), etc. Although these traditional methods have their own advantages, they generally have defects such as limited detection sensitivity, poor analysis selectivity, and cumbersome and complex operations.

[0004] As an emerging and convenient analytical technique, surface-enhanced Raman spectroscopy (SERS) has been proven to be an excellent tool for detecting trace tetracycline because it can quickly and non-destructively provide molecular fingerprint vibration information at the single-molecule level. And while achieving surface-enhanced Raman scattering (SERS) detection of antibiotics, the efficient removal of antibiotics is also the focus of attention of researchers. Photocatalytic degradation technology is considered to be the most promising technology for treating antibiotics because of its simplicity, environmental friendliness and higher efficiency compared with traditional treatment methods. Rational design of photocatalysts for efficient degradation of antibiotics is crucial and quite challenging. Summary of the Invention

[0005] Based on the technical problems existing in the background art, the present invention proposes a Cu2O@Ag / g-C3N4 nanocomposite material, its preparation method and application. The Cu2O@Ag / g-C3N4 nanocomposite material of the present invention has good photocatalytic degradation performance, and at the same time, it can be used as a SERS substrate for SERS detection with high sensitivity.

[0006] The present invention proposes a Cu2O@Ag / g-C3N4 nanocomposite material. Silver nanocrystals are uniformly distributed and coated on the surface of Cu2O nanocrystals to form Cu2O@Ag nanocrystals, and the Cu2O@Ag nanocrystals are uniformly loaded on the surface of g-C3N4 nanosheets.

[0007] Preferably, the Cu2O@Ag / g-C3N4 nanocomposite material has a ternary heterojunction structure.

[0008] Preferably, Ag serves as the electron transport medium in the ternary heterojunction.

[0009] The above-mentioned Cu2O nanocrystals are Cu2O cubic crystals; the above-mentioned ternary heterojunction structure is a fully solid-state Z-type ternary heterojunction structure.

[0010] Preferably, the particle size of the Cu2O nanocrystals is 500-700nm.

[0011] Preferably, the particle size of the silver nanocrystals is 10-100nm.

[0012] Preferably, the wrapping thickness of the g-C3N4 nanosheets is 2-4nm.

[0013] The present invention also proposes a preparation method of the above-mentioned Cu2O@Ag / g-C3N4 nanocomposite material, which includes the following steps: mixing a copper source, a silver source, g-C3N4 nanosheets and a solvent, adding an alkaline substance, performing a first reaction, then adding a reducing agent, performing a second reaction, and separating the solid from the liquid to obtain the Cu2O@Ag / g-C3N4 nanocomposite material.

[0014] Preferably, the copper source is copper chloride.

[0015] Preferably, the silver source is silver nitrate.

[0016] Preferably, the solvent is water.

[0017] Preferably, the alkaline substance is sodium hydroxide.

[0018] Preferably, the reducing agent is ascorbic acid.

[0019] Preferably, the temperature of the first reaction is 50-60°C and the time is 20-40min.

[0020] Preferably, the temperature of the secondary reaction is 50-60 °C and the time is 20-40 min.

[0021] After the above-mentioned copper source, silver source, g-C3N4 nanosheets and solvent are mixed evenly, the concentration of Cu is 8-12 mmol / L, preferably 10 mmol / L.

[0022] Preferably, the molar ratio of Cu in the copper source to Ag in the silver source is 1:0.25-1.

[0023] Preferably, the dosage ratio of Cu in the copper source to g-C3N4 nanosheets is 1 mmol: 0.8-1.2 g.

[0024] Preferably, the molar ratio of Cu in the copper source to the alkaline substance is 1:20-40.

[0025] Preferably, the molar ratio of Cu in the copper source to the reducing agent is 1:5-7.

[0026] Preferably, after solid-liquid separation, the precipitate is washed and dried to obtain the Cu2O@Ag / g-C3N4 nanocomposite.

[0027] The above-mentioned water is all deionized water; the above-mentioned g-C3N4 nanosheets refer to graphitic carbon nitride nanosheets.

[0028] The present invention also proposes the application of the above-mentioned Cu2O@Ag / g-C3N4 nanocomposite as a SERS substrate and a photocatalyst.

[0029] Preferably, the Cu2O@Ag / g-C3N4 nanocomposite can photocatalytically degrade antibiotics.

[0030] Preferably, the antibiotics are at least one of tetracycline, oxytetracycline, and chlortetracycline.

[0031] The present invention has successfully prepared a Cu2O@Ag / g-C3N4 nanocomposite with a specific structure, and used the finite-difference time-domain (FDTD) method, supplemented by relevant band gap calculations, to clarify the potential mechanism of SERS enhancement of the Cu2O@Ag / g-C3N4 nanocomposite; the photocatalytic performance and its degradation mechanism of the Cu2O@Ag / g-C3N4 nanocomposite have also been studied.

[0032] Although Cu2O@Ag nanocrystals also have photocatalytic degradation performance, their photostability will be greatly reduced because silver and cuprous oxide nanocrystals are easily oxidized. In the present invention, introducing g-C3N4 nanosheets can provide a protective layer for Cu2O@Ag nanocrystals to prevent them from photocorrosion, thereby effectively extending the storage time and service life of the Cu2O@Ag / g-C3N4 nanocomposite; in addition, g-C3N4 nanosheets have a high specific surface area, providing a regular arrangement platform for nanocrystals, enabling them to expose more active sites; more importantly, the band gap energy level of g-C3N4 can be well matched with that of Cu2O. The Ag nanocrystals located between the Cu2O nanocrystals and the g-C3N4 nanosheets can form a Z-type ternary heterojunction structure as a medium. The Ag nanocrystals can serve as an electron mediator and a charge transfer bridge, thus accelerating the electron transfer rate and further enhancing the photocatalytic activity.

[0033] In the present invention, g-C3N4 is incorporated into Cu2O@Ag nanocrystals, making the Cu2O@Ag / g-C3N4 nanocomposite have a Z-type heterojunction structure, promoting effective charge transfer, improving photocatalytic activity, and further enhancing the surface-enhanced Raman scattering (SERS) signal of analyte molecules; on the one hand, g-C3N4 nanosheets can enrich target molecules through electrostatic adsorption and π-π conjugation, thereby shortening the distance between the target molecules and silver nanocrystals, and on the other hand, the heterojunction formed between the Cu2O nanocrystals and the g-C3N4 nanosheets can generate additional charge transfer. Therefore, the Cu2O@Ag / g-C3N4 nanocomposite can realize the dual-functional applications of SERS detection and photocatalytic degradation.

[0034] This makes it possible to achieve self-cleaning of the SERS substrate using visible light, thus enabling the recycling of the SERS substrate; it can promote the development of self-cleaning SERS substrates and promote the creation of sustainable environmental remediation technologies. The present invention provides a new strategy for the selective detection and treatment of environmental pollutants.

[0035] The Cu2O@Ag / g-C3N4 nanocomposite described in the present invention has ultra-sensitive SERS detection performance and ultra-high visible light photocatalytic degradation performance for tetracycline. Description of the Drawings

[0036] Figure 1 SEM pictures of Cu2O nanocrystals, g-C3N4 nanosheets, Cu2O@Ag nanocrystals, and Cu2O@Ag / g-C3N4 nanocomposites, where a is the Cu2O nanocrystals of Comparative Example 4, b is the g-C3N4 nanosheets of Example 1, c-e are the Cu2O@Ag nanocrystals of Comparative Examples 1-3 in sequence, and f is the Cu2O@Ag / g-C3N4 nanocomposite of Example 1.

[0037] Figure 2 Corresponding elemental mapping images of the Cu2O@Ag / g-C3N4 nanocomposite of Example 1.

[0038] Figure 3 TEM images and HRTEM images of the Cu2O@Ag nanocrystals of Comparative Example 2 and the Cu2O@Ag / g-C3N4 nanocomposite of Example 1, where a is the TEM image of the Cu2O@Ag nanocrystals, b-c are the HRTEM images of the Cu2O@Ag nanocrystals, d is the TEM image of the Cu2O@Ag / g-C3N4 nanocomposite, and e-f are the HRTEM images of the Cu2O@Ag / g-C3N4 nanocomposite.

[0039] Figure 4 XPS measurement spectra and XPS high-resolution spectra of the Cu2O@Ag / g-C3N4 nanocomposite, where a is the Cu2O@Ag / g-C3N4 nanocomposite, b is N1s, c is C1s, d is Cu 2p, e is Ag 3d, and f is O1s.

[0040] Figure 5 XRD patterns and FT-IR spectra of Cu2O nanocrystals, g-C3N4 nanosheets, Cu2O@Ag nanocrystals, and Cu2O@Ag / g-C3N4 nanocomposites, where a is the XRD pattern, b is the FT-IR spectrum, and AC-1 to AC-3 are the Cu2O@Ag nanocrystals of Comparative Examples 1-3 in sequence.

[0041] Figure 6 SERS detection results of Cu2O nanocrystals, g-C3N4 nanosheets, Cu2O@Ag nanocrystals, and Cu2O@Ag / g-C3N4 nanocomposites, where a is the SERS signal of each material for 10 -4 M tetracycline solution, and AC-1 to AC-3 are the Cu2O@Ag nanocrystals of Comparative Examples 1-3 in sequence; b is the SERS spectrum of different concentrations of tetracycline solution on the Cu2O@Ag / g-C3N4 nanocomposite; c is the linear relationship diagram of the SERS signal intensity at 1626 cm -1 of the Cu2O@Ag / g-C3N4 nanocomposite versus the logarithm of the tetracycline concentration); d is the SERS spectrum of 10 -4 M tetracycline solution recorded at 20 random points on the Cu2O@Ag / g-C3N4 nanocomposite; e is the corresponding SERS intensity at 1626 cm -1 of the Cu2O@Ag / g-C3N4 nanocomposite; f is the 10 -4SERS spectrum of M tetracycline solution.

[0042] Figure 7 FDTD models of Cu2O@Ag nanocrystals for Comparative Examples 1-3 and their corresponding electric field spatial distributions, where a is Comparative Example 1, b is Comparative Example 2, and c is Comparative Example 3.

[0043] Figure 8 a is the Tauc curve of Cu2O nanocrystals, g-C3N4 nanosheets, Cu2O@Ag nanocrystals, and Cu2O@Ag / g-C3N4 nanocomposites, where CA-1 to CA-3 are the Cu2O@Ag nanocrystals of Comparative Examples 1-3 in sequence.

[0044] Figure 8 b is the UPS spectrum of Cu2O nanocrystals and g-C3N4 nanosheets.

[0045] Figure 8 c is the M-S curve of Cu2O nanocrystals at different frequencies (1.0, 1.5, and 2.0 kHz).

[0046] Figure 8 d is the M-S curve of g-C3N4 nanosheets at different frequencies (1.0, 1.5, and 2.0 kHz).

[0047] Figure 9 a is the schematic diagram of the energy band structure of Cu2O and Ag; Figure 9 b is the schematic diagram of the electron transfer path and the internal electric field formation process of Ag and Cu2O on Cu2O@Ag nanocrystals without laser irradiation; Figure 9 c is the schematic diagram of the energy band structure of Cu2O, Ag, and g-C3N4, Figure 9 d is the electron transfer path on Cu2O@Ag / g-C3N4 nanocomposites without laser irradiation.

[0048] Figure 10 Schematic diagram of charge transfer of Cu2O@Ag nanocrystals and Cu2O@Ag / g-C3N4 nanocomposites under 532 nm laser irradiation, where a is the Cu2O@Ag nanocrystal of Comparative Example 2 and b is the Cu2O@Ag / g-C3N4 nanocomposite.

[0049] Figure 11 a is the UV-visible absorption spectrum of the degradation of tetracycline by Cu2O@Ag / g-C3N4 nanocomposites under visible light; Figure 11b is the relationship diagram of C / C0 and reaction time for the catalytic degradation of tetracycline by Cu2O nanocrystals, Cu2O@Ag, and Cu2O@Ag / g-C3N4 nanocomposites of Comparative Examples 1-3; where CA-1 is Comparative Example 1, CA-2 is Comparative Example 2, and CA-3 is Comparative Example 3. Figure 11 c is the first-order kinetic diagram of the catalytic degradation of tetracycline by Cu2O nanocrystals, Cu2O@Ag, and Cu2O@Ag / g-C3N4 nanocomposites of Comparative Examples 1-3, where the inset is the corresponding kinetic rate constant. Figure 11 d is the cyclic test result of the photocatalytic degradation of tetracycline by the Cu2O@Ag / g-C3N4 nanocomposite.

[0050] Figure 12 are the (a) ultraviolet-visible diffuse reflectance spectra (UV-vis DRS), (b) photoluminescence spectra (PL), (c) transient photocurrent responses, and (d) electrochemical impedance spectra (EIS) of Cu2O nanocrystals, g-C3N4 nanosheets, Cu2O@Ag, and Cu2O@Ag / g-C3N4 nanocomposites of Comparative Examples 1-3, where CA-1 is Comparative Example 1, CA-2 is Comparative Example 2, and CA-3 is Comparative Example 3.

[0051] Figure 13 are the degradation efficiency results of tetracycline by the Cu2O@Ag / g-C3N4 nanocomposite after adding different scavengers, where No scavenger means no scavenger is added, EDTA-2Na is disodium ethylenediaminetetraacetate, IPA is isopropanol, and BQ is benzoquinone.

[0052] Figure 14 are three pathways for the catalytic degradation of tetracycline by the Cu2O@Ag / g-C3N4 nanocomposite.

[0053] Figure 15 is the mechanism model for the degradation of tetracycline by the Cu2O@Ag / g-C3N4 nanocomposite under visible light irradiation, where a is the traditional type-II heterojunction model and b is the Z-scheme heterojunction model. Detailed implementation mode

[0054] Next, the technical solutions of the present invention will be described in detail through specific examples.

[0055] Example 1

[0056] A preparation method of a Cu2O@Ag / g-C3N4 nanocomposite includes the following steps:

[0057] Put 5 g of melamine into a crucible, heat it to 350 °C at a rate of 5 °C / min, calcine it in an air atmosphere for 2 h, cool it naturally to room temperature, and grind it into fine powder to obtain the precursor powder; then take 2 g of the precursor powder, heat it to 550 °C at a rate of 5 °C / min and calcine it for 2 h. After washing it three times with ethanol and deionized water, dry it overnight in a vacuum oven at 80 °C to obtain g-C3N4 nanosheets;

[0058] Take 100 mL of 10 mmol / L copper chloride aqueous solution, add silver nitrate aqueous solution (total amount of silver nitrate added is 0.5 mmol) and 1 g of g-C3N4 nanosheets and mix well. Then add 10 mL of 3 mol / L sodium hydroxide aqueous solution dropwise. In a water bath at 55 °C, stir at a speed of 160 r / min for the first reaction for 30 min. Then add 10 mL of 0.6 mol / L L-ascorbic acid aqueous solution dropwise, and continue to keep warm and stir for the second reaction for 30 min. Centrifuge, take the precipitate and wash it three times with ethanol and deionized water respectively, and then dry it in a vacuum oven at 60 °C for 5 h to obtain Cu2O@Ag / g-C3N4 nanocomposite.

[0059] Example 2

[0060] A preparation method of Cu2O@Ag / g-C3N4 nanocomposite includes the following steps:

[0061] The total amount of silver nitrate added is 0.25 mmol, the dosage of g-C3N4 nanosheets is 0.8 g, and the others are the same as in Example 1, to obtain Cu2O@Ag / g-C3N4 nanocomposite.

[0062] Example 3

[0063] A preparation method of Cu2O@Ag / g-C3N4 nanocomposite includes the following steps:

[0064] The total amount of silver nitrate added is 1 mmol, the dosage of g-C3N4 nanosheets is 1.2 g, and the others are the same as in Example 1, to obtain Cu2O@Ag / g-C3N4 nanocomposite.

[0065] Comparative Example 1

[0066] Without adding g-C3N4 nanosheets, and the total amount of silver nitrate added is 0.25 mmol, and the others are the same as in Example 1, to obtain Cu2O@Ag nanocrystals.

[0067] Comparative Example 2

[0068] Without adding g-C3N4 nanosheets, and the total amount of silver nitrate added is 0.5 mmol, and the others are the same as in Example 1, to obtain Cu2O@Ag nanocrystals.

[0069] Comparative Example 3

[0070] Without adding g-C3N4 nanosheets and with a total addition amount of silver nitrate of 1 mmol, and the other conditions being the same as in Example 1, Cu2O@Ag nanocrystals were prepared.

[0071] Comparative Example 4

[0072] Without adding silver nitrate aqueous solution and g-C3N4 nanosheets, and the other conditions being the same as in Example 1, Cu2O nanocrystals were prepared.

[0073] The g-C3N4 nanosheets and Cu2O@Ag / g-C3N4 nanocomposites prepared in Example 1, the Cu2O@Ag nanocrystals prepared in Comparative Examples 1-3, and the Cu2O nanocrystals prepared in Comparative Example 4 were tested, and the results are as Figures 1 - 15 shown.

[0074] Figure 1 SEM images of Cu2O nanocrystals, g-C3N4 nanosheets, Cu2O@Ag nanocrystals, and Cu2O@Ag / g-C3N4 nanocomposites are shown. Among them, a is the Cu2O nanocrystals of Comparative Example 4, b is the g-C3N4 nanosheets of Example 1, c-e are the Cu2O@Ag nanocrystals of Comparative Examples 1-3 in sequence, and f is the Cu2O@Ag / g-C3N4 nanocomposite of Example 1.

[0075] It can be seen from Figure 1 that: The Cu2O nanocrystals are in a cubic shape, with a smooth surface and uniform particle size, about 600 nm;

[0076] g-C3N4 presents a clear two-dimensional (2D) layered structure with a rough surface. During the calcination process, when the g-C3N4 nanosheets are exposed to air, they will condense, forming a large number of irregular pore structures on the surface of the g-C3N4 nanosheets, and the pore diameter is between 10-150 nm;

[0077] The Cu2O@Ag nanocrystals are in sharp contrast to the smooth Cu2O nanocrystals. Ag nanocrystals with a diameter of about 10-100 nm are deposited on the surface of the Cu2O nanocrystals, and its surface becomes rough; and it can be seen from Figure 1 c-e that with the increase in the addition amount of the AgNO3 solution, more Ag nanocrystals are deposited on the surface of the Cu2O nanocrystals; however, when the addition amount of the AgNO3 solution is further increased, the Ag nanocrystals on the surface of the Cu2O nanocrystals will aggregate to form Ag nanosheets and wrap the Cu2O nanocrystals;

[0078] In the Cu2O@Ag / g-C3N4 nanocomposite, both the Cu2O@Ag nanocrystals and the g-C3N4 nanosheets are clearly visible.

[0079] Figure 2The corresponding elemental mapping images of the Cu2O@Ag / g-C3N4 nanocomposite of Example 1.

[0080] It can be seen from Figure 2 that the C, N, O, Cu, and Ag elements are uniformly distributed throughout the Cu2O@Ag / g-C3N4 nanocomposite, Figures 1 - 2 preliminarily indicating the successful construction of the Cu2O@Ag / g-C3N4 nanocomposite.

[0081] Figure 3 TEM images and HRTEM images of the Cu2O@Ag nanocrystals of Comparative Example 2 and the Cu2O@Ag / g-C3N4 nanocomposite of Example 1, where a is the TEM image of the Cu2O@Ag nanocrystals, b-c are the HRTEM images of the Cu2O@Ag nanocrystals, d is the TEM image of the Cu2O@Ag / g-C3N4 nanocomposite, and e-f are the HRTEM images of the Cu2O@Ag / g-C3N4 nanocomposite.

[0082] It can be seen from Figure 3 a that silver nanocrystals are uniformly adsorbed on the surface of cubic Cu2O nanocrystals with a diameter of about 600 nm; Figure 3 The enlarged area of the yellow dashed box in a is as shown in Figure 3 b, and it can be observed that the diameter of the silver nanocrystals on the surface of the Cu2O nanocrystals is about 15 nm, which is consistent with the Figure 1 results; Figure 3 The enlarged area of the dashed box in b is as shown in Figure 3 c, and it can be seen that the interplanar spacings of the fringes are 0.24 and 0.25 nm, corresponding to the (111) crystal planes of silver and Cu2O, respectively. This result further proves the successful formation of the Cu2O@Ag heterojunction.

[0083] It can be seen from Figure 3 d that in the Cu2O@Ag / g-C3N4 nanocomposite, g-C3N4 nanosheets and Cu2O nanocrystals coexist and are in good contact; Figure 3 The enlarged view of the dashed box in d ( Figure 3e) The presence of silver nanocrystals was confirmed; upon careful observation, it was found that Cu2O@Ag nanocrystals were loaded on the surface of g-C3N4 nanosheets with a thickness of about 2.5 nm (since the g-C3N4 nanosheets have a large area and are prone to bending, sometimes a state similar to the g-C3N4 nanosheets wrapping the Cu2O@Ag nanocrystals can be seen in the enlarged images of SEM, TEM, and HRTEM. In fact, the Cu2O@Ag nanocrystals are uniformly loaded on the surface of the g-C3N4 nanosheets, rather than in a wrapped state). A large number of uniformly distributed silver nanocrystals act as a medium between Cu2O and the g-C3N4 nanosheets, thus forming a ternary heterojunction structure of Cu2O@Ag / g-C3N4 nanocrystals, which is beneficial to improving the adsorption efficiency of visible light; Figure 3 The enlarged image within the yellow dashed box in e ( Figure 3 f) shows that the interlayer spacings of 0.25 nm and 0.24 nm correspond to the (111) crystal planes of Cu2O and silver nanocrystals, respectively; this indicates that the introduction of g-C3N4 nanosheets does not change the structures of Cu2O and silver nanocrystals.

[0084] Figure 4 are the XPS measurement spectra and XPS high-resolution spectra of the Cu2O@Ag / g-C3N4 nanocomposite. Among them, a is the Cu2O@Ag / g-C3N4 nanocomposite, b is N1s, c is C1s, d is Cu 2p, e is Ag 3d, and f is O1s.

[0085] From Figure 4 it can be seen that XPS was used to analyze the oxidation states of the elements in the Cu2O@Ag / g-C3N4 nanocomposite. The external C1s peak at 284.8 eV was used as a reference point to correct the possible deviation caused by the charging effect; as Figure 4 shown in a, the XPS full spectrum fully indicates the presence of five different elements, C, Ag, N, O, and Cu, which indirectly confirms the successful preparation of the Cu2O@Ag / g-C3N4 nanocomposite; as Figure 4 shown in b, three peaks are shown at 401.97 eV, 399.77 eV, and 398.9 eV, corresponding to N-H x , N-(C)3, and C-N=C, respectively; as Figure 4 shown in c, the electron binding energies of the C 1s spectrum can be fitted to three different peaks: 284.8 eV, 286.13 eV, and 288.14 eV. These peaks belong to C-C in the conjugated system of the g-C3N4 molecule, C-NH x at the edge of the heptazine ring, and the sp 2 bond C in N-C=N, respectively; as Figure 4 shown in d, the peaks with binding energies of 953.5 eV and 933.6 eV correspond to the Cu 2p of Cu2O 1 / 2and Cu 2p 3 / 2 , in addition, surface oxidation is responsible for the additional weak satellite peaks appearing at 944.2 eV; as Figure 4 shown in e, two peaks are shown in the Ag 3d spectrum, namely the 3d 3 / 2 peak and the Ag 3d 5 / 2 peak (resulting from spin - orbit splitting), located at 374.55 eV and 368.54 eV respectively, which can be attributed to metallic silver; as Figure 4 shown in f, the O 1s XPS spectrum can be fitted to two binding - energy peaks, located at 531.79 eV and 532.82 eV respectively. The former originates from Cu2O, while the latter is related to chemisorbed oxygen. The high - resolution XPS spectra of O 1s and Cu 2p indicate that the copper in the Cu2O@Ag / g - C3N4 nanocomposite exists in the form of Cu + , rather than Cu 2+ . The XPS results further confirm the successful synthesis of the Cu2O@Ag / g - C3N4 nanocomposite.

[0086] Figure 5 XRD patterns and FT - IR spectra of Cu2O nanocrystals, g - C3N4 nanosheets, Cu2O@Ag nanocrystals, and Cu2O@Ag / g - C3N4 nanocomposites are shown, where a is the XRD pattern and b is the FT - IR spectrum. AC - 1 to AC - 3 are Cu2O@Ag nanocrystals of Comparative Examples 1 - 3 in sequence.

[0087] As Figure 5As shown in a, the diffraction peaks of Cu2O nanocrystals at 29.55°, 36.42°, 42.30°, 52.46°, 61.34°, 73.53° and 77.32° correspond to the (110), (111), (200), (211), (220), (311) and (222) crystal planes of Cu2O (JCPDS: 05-0667), respectively; the (002) diffraction peak of g-C3N4 nanosheets appears at about 27.7°; for the Cu2O@Ag nanocrystals of Comparative Examples 1-3, the typical silver diffraction peaks at 2θ values of 38.12°, 44.28°, 64.43° and 77.47° correspond to the (111), (200), (220) and (311) crystal planes of silver (JCPDS: 04-0783), respectively. In addition, as the silver loading amount increases from 0.25 to 1 mmol, the intensity of the characteristic peaks of silver gradually increases, which confirms the successful synthesis of Cu2O@Ag nanocrystals with different silver loading amounts; in the Cu2O@Ag / g-C3N4 nanocomposite, the main diffraction peaks of Cu2O, Ag and g-C3N4 still exist, and the peaks related to Cu2O, Ag and g-C3N4 show obvious and sharp characteristics, indicating that the structure of Cu2O has not changed after adding Ag and g-C3N4;

[0088] As Figure 5 shown in b, for Cu2O nanocrystals, the peak at 632 cm -1 corresponds to the stretching vibration of Cu-O; compared with Cu2O nanocrystals, the stretching band of Cu-O in Cu2O@Ag nanocrystals shows a slight red shift, indicating that electrons are transferred from the electron-donating Ag to the electron-accepting Cu2O; the characteristic absorption peaks of g-C3N4 nanosheets, Cu2O@Ag nanocrystals of Comparative Examples 1-3 and Cu2O@Ag / g-C3N4 nanocomposites at 809, 1200-1700 and 2800-3500 cm -1 are attributed to the contraction of triazine rings, the stretching of C-N heterocycles in triangular prisms and bridging units, and the stretching of -NH x groups in g-C3N4, respectively; in particular, g-C3N4 and Cu2O often interact strongly, which is proved by the good matching of the energy gap energy levels between g-C3N4 nanosheets and Cu2O nanocrystals; therefore, compared with Cu2O@Ag nanocrystals, the absorption peak of the stretching vibration of the Cu-O bond in the Cu2O@Ag / g-C3N4 nanocomposite shows a red shift; in addition, the absorption peak of the triazine ring shifts from 807 cm -1 in g-C3N4 to 809 cm -1 in the Cu2O@Ag / g-C3N4 nanocomposite; therefore, the above results indicate that a heterojunction rather than a physical mixture is formed in the Cu2O@Ag / g-C3N4 nanocomposite.

[0089] Figure 6 SERS detection results of Cu2O nanocrystals, g-C3N4 nanosheets, Cu2O@Ag nanocrystals, and Cu2O@Ag / g-C3N4 nanocomposites. Among them, a is the SERS signal of each material for 10 -4 M tetracycline solution. AC-1 to AC-3 are the Cu2O@Ag nanocrystals of Comparative Examples 1-3 in sequence; b is the SERS spectrum of tetracycline solutions with different concentrations on the Cu2O@Ag / g-C3N4 nanocomposite; c is the linear relationship diagram of the SERS signal intensity at 1626 cm -1 of the Cu2O@Ag / g-C3N4 nanocomposite and the logarithm of the tetracycline concentration); d is the SERS spectrum of 10 -4 M tetracycline solution recorded at 20 random points on the Cu2O@Ag / g-C3N4 nanocomposite; e is the corresponding SERS intensity at 1626 cm -1 of the Cu2O@Ag / g-C3N4 nanocomposite; f is the SERS spectrum of 10 -4 M tetracycline solution stored for different days on the Cu2O@Ag / g-C3N4 nanocomposite.

[0090] As Figure 6 shown in a, when using Cu2O nanocrystals and g-C3N4 nanosheets as the SERS substrate, no SERS signal of tetracycline was detected; when using the Cu2O@Ag nanocrystals of Comparative Examples 1-3 as the SERS substrate, significant SERS signals of tetracycline were observed. Among them, the Cu2O@Ag nanocrystals of Comparative Example 2 showed the most obvious SERS enhancement effect, which is consistent with the Figure 7 FDTD simulation results; the characteristic peaks located at 1416 cm -1 and 1626 cm -1 are attributed to the stretching vibrations of the furan ring and C=O respectively; while the Cu2O@Ag / g-C3N4 nanocomposite has the strongest SERS signal;

[0091] As Figure 6 shown in b, as the concentration of tetracycline decreases, the SERS signal gradually weakens. However, even at a low concentration of 10 -12 M, the Cu2O@Ag / g-C3N4 nanocomposite can still generate an obvious SERS signal;

[0092] As Figure 6 shown in c, there is a strong linear relationship between the Raman signal intensity and the logarithmic concentration of tetracycline. The Cu2O@Ag / g-C3N4 nanocomposite has the ability to quantitatively detect the target analyte and has an extremely low detection limit (LOD) of 10 -12M;

[0093] like Figure 6 As shown in de, 20 points were randomly selected on the surface of Cu2O@Ag / g-C3N4 nanocomposite to detect 10 - 4 The SERS signal of tetracycline solution was measured and it was found that the SERS spectrum intensity of tetracycline remained almost unchanged, and the relative standard deviation (RSD) value was less than 10%, indicating that the Cu2O@Ag / g-C3N4 nanocomposite material had excellent uniformity as a SERS substrate.

[0094] like Figure 6 f), the Cu2O@Ag / g-C3N4 nanocomposite was stored at room temperature for one month and the 10 -4 The SERS intensity of tetracycline solution was measured and it was found that the SERS intensity hardly changed within one month, indicating that the Cu2O@Ag / g-C3N4 nanocomposite material has excellent stability as a SERS substrate.

[0095] Figure 7 FDTD models of Cu2O@Ag nanocrystals of Comparative Examples 1-3 and their corresponding electric field spatial distributions, wherein a is Comparative Example 1, b is Comparative Example 2, and c is Comparative Example 3.

[0096] like Figure 7 As shown: the Cu2O@Ag nanocrystals in comparative example 1 only form a small number of hot spots due to the small number of silver nanocrystals loaded on the Cu2O surface, resulting in low SERS activity; when the addition amount of silver nitrate solution is increased to 0.5 mmol, the number of silver nanocrystals on the Cu2O surface increases significantly, and a large number of hot spots appear between adjacent silver nanocrystals, which makes it easier to produce a strong LSPR effect, thereby enhancing the Raman signal; however, excessive silver nanocrystals will lead to a decrease in the number and intensity of hot spots because they overlap with each other, which is not conducive to the enhancement of SERS signals; time-domain finite-difference (FDTD) simulation shows that the Cu2O@Ag nanocrystals in comparative example 2 exhibit the best SERS performance.

[0097] Figure 8 a is the Tauc curve of Cu2O nanocrystals, g-C3N4 nanosheets, Cu2O@Ag nanocrystals, and Cu2O@Ag / g-C3N4 nanocomposites, wherein CA-1 to CA-3 are the Cu2O@Ag nanocrystals of Comparative Examples 1-3, respectively.

[0098] Figure 8 b is the UPS spectrum of Cu2O nanocrystals and g-C3N4 nanosheets.

[0099] Figure 8c is the M-S curve of Cu2O nanocrystals at different frequencies (1.0, 1.5, and 2.0 kHz).

[0100] Figure 8 d is the M-S curve of g-C3N4 nanosheets at different frequencies (1.0, 1.5, and 2.0 kHz).

[0101] Carrier migration (CM) caused by photoinduced charge transfer between adsorbed molecules and the SERS substrate cannot be ignored. To gain a deeper understanding of the carrier migration mechanism of the Cu2O@Ag / g-C3N4 nanocomposite as a substrate, the band alignment between g-C3N4 nanosheets, Cu2O, and Ag nanocrystals was investigated through a series of measurements of the band structure.

[0102] As Figure 8 shown in g a, the E

[0103] values of Cu2O nanocrystals, g-C3N4 nanosheets, Cu2O@Ag of Comparative Examples 1-3, and Cu2O@Ag / g-C3N4 nanocomposites are 2.06, 2.78, 2.04, 2.03, 2.02, and 1.97 eV, respectively. Figure 8 As F shown in F b, the Ecutoff potentials of Cu2O nanocrystals and g-C3N4 nanosheets are 16.08 and 16.64 eV, respectively; their Φ values are calculated to be 5.14 and 4.58 eV, and their E

[0104] (vs. vacuum) are -5.14 and -4.58 eV, respectively, and their E Figure 8 (vs. NHE) are 0.70 and 0.14 eV, respectively. fb As fb shown in fb c-d, Cu2O nanocrystals have a negative slope, while g-C3N4 nanosheets show a positive slope, indicating that Cu2O has p-type semiconductor properties, while g-C3N4 exhibits n-type semiconductor properties; the E fbAt approximately 0.10 eV below, it can thus be inferred that the VB potential of the Cu2O nanocrystals is 0.9 eV, while the CB potential of the g-C3N4 nanosheets is -0.29 eV; meanwhile, according to Mulliken electronegativity theory (VB = CB + Eg), the CB of the Cu2O nanocrystals and the VB of the g-C3N4 nanosheets are calculated to be -1.16 and 2.49 eV (relative to NHE), respectively.

[0105] Figure 9 a is a schematic diagram of the energy band structures of Cu2O and Ag; Figure 9 b is a schematic diagram of the electron transfer path and the formation process of the internal electric field between Ag and Cu2O on the Cu2O@Ag nanocrystals without laser irradiation; Figure 9 c is a schematic diagram of the energy band structures of Cu2O, Ag, and g-C3N4, Figure 9 d is the electron transfer path on the Cu2O@Ag / g-C3N4 nanocomposite without laser irradiation.

[0106] Figure 10 Figure 14 is a schematic diagram of charge transfer of Cu2O@Ag nanocrystals and Cu2O@Ag / g-C3N4 nanocomposites under 532 nm laser irradiation, where a is the Cu2O@Ag nanocrystals of Comparative Example 2 and b is the Cu2O@Ag / g-C3N4 nanocomposite.

[0107] As Figure 9 shown in a, the work function, E F (vs. NHE), CB, and VB values of the Cu2O nanocrystals are measured to be 5.14, 0.70 (E F1 ), -1.16, and 0.9 eV, respectively; the work function and E F (vs. NHE) of the silver nanocrystals are 4.26 and -0.18 eV (E F2 ), respectively; as Figure 9 shown in b, once the silver nanocrystals are fixed on the surface of the Cu2O nanocrystals, a Schottky junction will form at the interface between the Cu2O nanocrystals and the silver nanocrystals, which will prompt electrons to spontaneously migrate from silver to the Cu2O nanocrystals until the E F of Cu2O and silver nanocrystals reaches equilibrium and a new E F is formed; during this process, the Cu2O nanocrystals that obtain electrons are negatively charged, while the silver nanocrystals are positively charged, which will lead to the formation of an internal electric field and the bending of the energy band edges of the Cu2O nanocrystals; as Figure 10As shown in Fig. a, after irradiation with 532 nm laser, since the excitation energy (2.33 eV) is higher than the bandgap (2.06 eV) of Cu2O nanocrystals, electrons in the valence band of Cu2O nanocrystals can jump to the conduction band; in addition, due to the local surface plasmon resonance effect, the photo-generated electrons in Ag nanocrystals can also be injected into the conduction band of Cu2O. Subsequently, the electrons in the conduction band of Cu2O will jump to the lowest unoccupied molecular orbital (LUMO) of tetracycline and finally transfer to the highest unoccupied molecular orbital (HUMO); therefore, the double enhancement of electromagnetic (EM) and charge transfer (CE) effects significantly improves the surface-enhanced Raman scattering (SERS) performance; it should be noted that the amount of Ag also plays a key role. On the one hand, insufficient Ag nanocrystals cannot provide enough photo-generated electrons to support effective charge transfer. On the other hand, too many Ag nanocrystals will completely cover the surface of Cu2O, hindering light absorption and ultimately resulting in a decrease in the number of photo-generated electrons;

[0108] As shown in Figure 9 Fig. c, the work function, E F (vs. NHE), CB and VB values of g-C3N4 nanosheets are 4.58, 0.14, -0.29 and 2.49 eV respectively; as shown in Figure 9 Fig. d, when g-C3N4 nanosheets are further assembled onto Cu2O@Ag nanocrystals, electrons can spontaneously migrate from g-C3N4 nanosheets to the interface of Cu2O@Ag nanocrystals. Subsequently, the E F in the interface region gradually moves until a new stable equilibrium is reached; during this process, electrons can accumulate on Cu2O@Ag nanocrystals while g-C3N4 nanosheets lose electrons; an internal electric field can be formed at the heterojunction interface; as shown in Figure 10 Fig. b, according to the above calculation results, the energy barriers between the VB of g-C3N4 and the HOMO of tetracycline and between the VB of g-C3N4 and the valence band of Cu2O are 0.88 eV and 1.59 eV respectively, and both of these energy barrier values are less than the excitation energy (2.33 eV, 532 nm) of the laser source; however, it should also be noted that the bandgap value (2.78 eV) of g-C3N4 nanosheets is higher than the excitation energy of the incident laser. Therefore, compared with Cu2O@Ag nanocrystals, the introduction of g-C3N4 provides two additional charge transfer pathways; these two pathways are the transitions of photo-generated electrons in the valence band (VB) of g-C3N4 to the HUMO of tetracycline and the valence band of cuprous oxide respectively; in addition, the electrons in the LUMO of tetracycline will first migrate to the conduction band (CB) of g-C3N4 and finally transfer to the highest occupied molecular orbital of tetracycline; the above charge transfer process provides a reasonable explanation for the excellent surface-enhanced Raman scattering (SERS) performance observed in Cu2O@Ag / g-C3N4 nanocomposites from the perspective of charge transfer.

[0109] To evaluate the photocatalytic performance of different substances, photocatalytic degradation experiments were carried out under visible light irradiation. Before the photocatalytic reaction, dark adsorption was carried out for 30 min, and the results are as Figure 11 shown.

[0110] Figure 11 a is the UV-visible absorption spectrum of the degradation of tetracycline by Cu2O@Ag / g-C3N4 nanocomposite under visible light; Figure 11 b is the relationship diagram of C / C0 and reaction time for the catalytic degradation of tetracycline by Cu2O nanocrystals, Cu2O@Ag of Comparative Examples 1-3, and Cu2O@Ag / g-C3N4 nanocomposite; where CA-1 is Comparative Example 1, CA-2 is Comparative Example 2, and CA-3 is Comparative Example 3; Figure 11 c is the first-order kinetic diagram of the catalytic degradation of tetracycline by Cu2O nanocrystals, Cu2O@Ag of Comparative Examples 1-3, and Cu2O@Ag / g-C3N4 nanocomposite. The inset is the corresponding kinetic rate constant; Figure 11 d is the cyclic test result of the photocatalytic degradation of tetracycline by Cu2O@Ag / g-C3N4 nanocomposite.

[0111] As Figure 11 shown in a, the adsorption equilibrium of the Cu2O@Ag / g-C3N4 nanocomposite can be reached within 30 min. Then, with the increase of the photocatalytic reaction time, the typical absorption peak of tetracycline decreases rapidly and almost completely disappears after 60 min, indicating that the Cu2O@Ag / g-C3N4 nanocomposite exhibits excellent photocatalytic performance under visible light conditions.

[0112] As Figure 11 shown in b, the degradation efficiency of Cu2O nanocrystals is only 59%; after modifying the surface of Cu2O nanocrystals with silver nanocrystals, its photocatalytic activity is significantly enhanced because silver nanocrystals can promote the effective migration of electrons and reduce the recombination of photo-generated carriers, thus improving the photocatalytic activity; it should be noted that with the increase of silver loading, the photocatalytic degradation performance of Cu2O@Ag nanocrystals first increases and then decreases; the Cu2O@Ag nanocrystals of Comparative Example 2 show the best photocatalytic performance, achieving a degradation efficiency of 80% within 60 min; however, the degradation efficiency of the Cu2O@Ag nanocrystals of Comparative Example 3 abnormally decreases to 60%, which can be attributed to the excessive silver loading on the surface of Cu2O nanocrystals. This is because excessive silver nanocrystals can reduce the available active sites and light absorption ability; after introducing g-C3N4, due to the formation of a Z-type ternary heterojunction, the degradation efficiency of the Cu2O@Ag / g-C3N4 nanocomposite is further increased to 94%.

[0113] As Figure 11As shown in c, the natural logarithm of the ratio of the tetracycline concentration (C t ) to the initial concentration (C0) with respect to time t shows a linear relationship with time. It is worth noting that the Cu2O@Ag / g-C3N4 nanocomposite exhibits the highest kinetic rate constant, with a value of 33.7×10 -3 min -1 , which is approximately 2.37 times that of the Cu2O nanocrystals and approximately 1.6 times that of the Cu2O@Ag nanocrystals in Comparative Example 2.

[0114] Reusability is another important consideration for evaluating the performance of photocatalysts. In this invention, six consecutive catalytic cycle experiments were conducted to investigate the reusability of the Cu2O@Ag / g-C3N4 nanocomposite. After each photocatalytic reaction, the catalyst was centrifuged for recovery, rinsed with deionized water, and dried in an oven, thus making it ready for subsequent cycles.

[0115] As Figure 11 shown in d, even after six cycles, the degradation efficiency of the Cu2O@Ag / g-C3N4 nanocomposite for tetracycline still remains at 90% within 60 min, indicating its high stability and reusability in photocatalytic reactions.

[0116] Figure 12 Are the (a) ultraviolet-visible diffuse reflectance spectra (UV-vis DRS), (b) photoluminescence spectra (PL), (c) transient photocurrent responses, and (d) electrochemical impedance spectra (EIS) of Cu2O nanocrystals, g-C3N4 nanosheets, Cu2O@Ag in Comparative Examples 1-3, and the Cu2O@Ag / g-C3N4 nanocomposite, where CA-1 is Comparative Example 1, CA-2 is Comparative Example 2, and CA-3 is Comparative Example 3.

[0117] As Figure 12 shown in a, the g-C3N4 nanosheets exhibit stable light absorption in the range of 300 - 466 nm. The absorption edge of the Cu2O nanocrystals is located at approximately 652 nm. The introduction of silver causes the absorption edge of these binary catalysts to redshift and the absorption intensity to increase. With the introduction of the g-C3N4 nanosheets, the absorption edge of the Cu2O@Ag / g-C3N4 nanocomposite further redshifts to 670 nm, and at the same time, the absorption intensity further increases. This phenomenon indicates that the visible light response region of the Cu2O@Ag / g-C3N4 nanocomposite expands, thereby improving the light absorption ability.

[0118] As Figure 12As shown in Fig. b, the photoluminescence (PL) spectra of the six samples all exhibited broad emissions in the range of 400 - 600 nm. Among them, the PL intensity of the Cu2O@Ag / g-C3N4 nanocomposite was the lowest. Generally, a higher separation efficiency of photo-generated electron-hole pairs is related to a lower PL intensity, indicating that the Z-scheme Cu2O@Ag / g-C3N4 heterojunction can effectively reduce electron-hole recombination and increase the migration of photo-generated carriers.

[0119] As Figure 12 shown in Fig. c, all six samples showed repeatable and rapid photocurrent responses under visible light irradiation. In particular, the Cu2O@Ag / g-C3N4 nanocomposite exhibited the highest photocurrent density, indicating that the Z-scheme heterojunction can effectively accelerate the separation and migration of photo-generated charges.

[0120] As Figure 12 shown in Fig. d, the Cu2O@Ag / g-C3N4 nanocomposite had the smallest arc radius, indicating the lowest interfacial charge transfer resistance and the fastest transfer ability of photo-generated charges compared with the other five samples. These findings confirmed that the successful construction of the Z-scheme heterojunction contributed to the separation and migration of photo-generated charges, reduced the possibility of electron and hole recombination, and thus provided great potential for enhancing photocatalytic activity.

[0121] To determine the types of active radicals generated during the photocatalytic degradation of tetracycline by the Cu2O@Ag / g-C3N4 nanocomposite and thus reveal the photocatalytic reaction mechanism, radical trapping experiments were carried out. In the radical trapping experiments, isopropyl alcohol (IPA), disodium ethylenediaminetetraacetate (EDTA-2Na), and benzoquinone (BQ) with a concentration of 0.1 mmol / L were used as scavengers to trap photo-generated hydroxyl radicals (·OH), photo-generated holes (h + ) and superoxide radicals (·O 2- ). The results are as Figure 13 shown.

[0122] Figure 13 Fig. shows the degradation efficiency results of tetracycline by the Cu2O@Ag / g-C3N4 nanocomposite after adding different scavengers. Among them, No scavenger means without adding scavengers, EDTA-2Na is disodium ethylenediaminetetraacetate, IPA is isopropyl alcohol, and BQ is benzoquinone.

[0123] As Figure 13 shown, in the absence of scavengers, the degradation efficiency of tetracycline catalyzed by the Cu2O@Ag / g-C3N4 nanocomposite was as high as 94%; in contrast, in the presence of IPA, EDTA-2Na, and BQ, the degradation efficiencies of the photocatalytic reaction system decreased to 75%, 48%, and 68% respectively; this indicates that ·OH, h + and ·O2- participated in the photodegradation process of tetracycline, in which h + played a key role in the degradation of tetracycline.

[0124] To study the mechanism of photocatalytic degradation of tetracycline by Cu2O@Ag / g-C3N4 nanocomposites, the present invention used an LC-MS system to detect the intermediates generated during the photocatalytic reaction. The chemical structures of 14 degradation intermediates detected are shown in Table 1. Based on the LC-MS spectral data, the present invention proposed three possible degradation pathways, as Figure 14 shown.

[0125] Table 1 Intermediates detected during the catalytic degradation of tetracycline by Cu2O@Ag / g-C3N4 nanocomposites

[0126]

[0127]

[0128] Figure 14 are three pathways for the catalytic degradation of tetracycline by Cu2O@Ag / g-C3N4 nanocomposites.

[0129] As Figure 14 shown, in Pathway 1, the amide group and C=C double bond of tetracycline are extremely vulnerable to the attack of active free radicals. Specifically, tetracycline is attacked by highly active ·OH and h + free radicals, and hydroxylation and demethylation reactions occur successively to generate P1; this process is accompanied by the oxidative cleavage of the C=C double bond and the loss of two methyl groups, and then the product P1 is further converted to the product P2 by eliminating the amide group; under the action of h + , ·O 2- and ·OH free radicals, deamination, dehydroxylation and ring-opening reactions generate P3;

[0130] In Pathway 2, due to the attack of ·O 2- free radicals, tetracycline loses two methyl groups located on the N atom to form P4, and then P5 is obtained through the cleavage and ring-opening reactions of amino acids, formamide, and hydroxyl groups, and is further converted to P6;

[0131] In Pathway 3, the attack of h + and ·O 2- free radicals promotes the ring cleavage reaction. As the degradation reaction continues, the further attack of ·OH leads to the loss of amino groups, and finally P7 is formed. Subsequently, the decarboxylation process of P7 forms P8, and then the ROS-induced ring-opening and dehydroxylation reactions further convert P8 to P9.

[0132] As the degradation reaction proceeds, the above-mentioned intermediates gradually decompose into low-molecular-weight organic compounds, including P10, P11, P12, P13, and P14. Eventually, these intermediates can be completely decomposed to form harmless small-molecule organic substances such as CO2 and H2O.

[0133] Two possible photocatalytic mechanisms for the degradation of tetracycline by Cu2O@Ag / g-C3N4 nanocomposites were proposed through band structure analysis and free radical verification experiments. As Figure 15 shown.

[0134] Figure 15 Figure 9 is the mechanism model for the degradation of tetracycline by Cu2O@Ag / g-C3N4 nanocomposites under visible light irradiation. Among them, a is the traditional type-II heterojunction model, and b is the Z-scheme heterojunction model.

[0135] As Figure 15 shown in a, if the Cu2O@Ag / g-C3N4 nanocomposite follows the type-II heterojunction mechanism, electrons first transfer from the CB of Cu2O (-1.16 eV) to Ag, and then migrate to the CB of g-C3N4 (-0.29 eV). However, since the redox potential of O2 / ·O 2- (-0.33 eV vs NHE) is more negative than the edge potential of the CB of g-C3N4, the electrons accumulated on the CB of g-C3N4 cannot reduce oxygen to the active ·O 2- radical. Similarly, since the redox potential of H2O / ·OH (2.40 eV vs NHE) is greater than the VB potential of Cu2O (2.40 eV vs NHE), the holes accumulated on Cu2O also cannot oxidize H2O to the active ·OH radical, which contradicts the results of the free radical capture experiment.

[0136] On the contrary, as Figure 15 shown in b, if the Cu2O@Ag / g-C3N4 nanocomposite follows the Z-scheme heterojunction mechanism, under visible light irradiation, the electrons in the VB of Cu2O and g-C3N4 can be excited to their respective CBs. The Ag nanocrystals can act as a CT bridge between Cu2O and g-C3N4, and the electrons located on the CB of g-C3N4 can migrate to the metal Ag and recombine with the holes generated in the VB of Cu2O. This process effectively inhibits the direct recombination of electrons and holes between Cu2O and g-C3N4. In addition, considering that the VB level of g-C3N4 is greater than the redox potential of H2O / ·OH, and the CB level of Cu2O is greater than the redox potential of O2 / ·O 2- the Cu2O@Ag / g-C3N4 nanocomposite has the ability to generate ·O 2-The ability to scavenge ·OH radicals is consistent with the results of the radical capture experiment. Therefore, the above results indicate that the Cu2O@Ag / g-C3N4 nanocomposite follows the Z-scheme heterojunction mechanism.

[0137] The present invention successfully synthesized a Cu2O@Ag / g-C3N4 nanocomposite with a Z-scheme heterojunction, which has sensitive SERS detection performance and efficient photocatalytic degradation performance. It exhibits good adsorption and photocatalytic ability under visible light irradiation; the optimal Cu2O@Ag nanocrystals are selected by adjusting the doping amount of Ag. Under visible light irradiation, the degradation rate of tetracycline can reach 80% within 60 min. To further improve the performance of the catalyst, g-C3N4 with a large specific surface area is further introduced, providing good adsorption ability and laying a foundation for more effective synergistic removal of tetracycline in water.

[0138] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes, should be covered within the protection scope of the present invention.

Claims

1. A Cu2O@Ag / g-C3N4 nanocomposite, characterized in that, Silver nanocrystals are uniformly distributed and coated on the surface of Cu2O nanocrystals to form Cu2O@Ag nanocrystals, and the Cu2O@Ag nanocrystals are uniformly loaded on the surface of g-C3N4 nanosheets.

2. The Cu2O@Ag / g-C3N4 nanocomposite according to claim 1, wherein The Cu2O@Ag / g-C3N4 nanocomposite has a ternary heterojunction structure; preferably, the ternary heterojunction uses Ag as the electron transport medium.

3. The Cu2O@Ag / g-C3N4 nanocomposite according to claim 1 or 2, characterized in that, The particle size of the Cu2O nanocrystals is 500 - 700 nm; preferably, the particle size of the silver nanocrystals is 10 - 100 nm; preferably, the wrapping thickness of the g-C3N4 nanosheets is 2 - 4 nm.

4. A method for preparing the Cu2O@Ag / g-C3N4 nanocomposite according to any one of claims 1-3, characterized in that, It includes the following steps: mixing a copper source, a silver source, g-C3N4 nanosheets and a solvent, adding an alkaline substance, performing a first reaction, then adding a reducing agent, performing a second reaction, and performing solid-liquid separation to obtain the Cu2O@Ag / g-C3N4 nanocomposite.

5. The preparation method of the Cu2O@Ag / g-C3N4 nanocomposite according to claim 4, characterized in that, The copper source is copper chloride; preferably, the silver source is silver nitrate; preferably, the solvent is water; preferably, the alkaline substance is sodium hydroxide; preferably, the reducing agent is ascorbic acid.

6. The preparation method of the Cu2O@Ag / g-C3N4 nanocomposite according to claim 4 or 5, characterized in that, The temperature of the first reaction is 50 - 60 °C and the time is 20 - 40 min; preferably, the temperature of the second reaction is 50 - 60 °C and the time is 20 - 40 min.

7. The preparation method of the Cu2O@Ag / g-C3N4 nanocomposite according to any one of claims 4-6, characterized in that, The molar ratio of Cu in the copper source to Ag in the silver source is 1:0.25 - 1; preferably, the dosage ratio of Cu in the copper source to g-C3N4 nanosheets is 1 mmol:0.8 - 1.2 g; preferably, the molar ratio of Cu in the copper source to the alkaline substance is 1:20 - 40; preferably, the molar ratio of Cu in the copper source to the reducing agent is 1:5 - 7.

8. The preparation method of the Cu2O@Ag / g-C3N4 nanocomposite material according to any one of claims 4-7, characterized in that, After solid-liquid separation, wash the precipitate and dry it to obtain the Cu2O@Ag / g-C3N4 nanocomposite.

9. The application of the Cu2O@Ag / g-C3N4 nanocomposite according to any one of claims 1 - 3 as a SERS substrate and a photocatalyst.

10. The application according to claim 9, wherein The Cu2O@Ag / g-C3N4 nanocomposite can photocatalytically degrade antibiotics; preferably, the antibiotics are at least one of tetracycline, oxytetracycline, and chlortetracycline.

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