Z-scheme type composite photocatalytic material and preparation method thereof

Abstract

This invention discloses a Z-Scheme type composite photocatalytic material and its preparation method. Specifically, a series of Cu2O / rGO / BiOBr composite materials were successfully prepared using a two-step method, and the amount of Cu2O in the composite material was optimized by photoreduction of Cr(VI) under visible light irradiation. By transferring and separating electrons and charges among Cu2O, rGO, and BiOBr, the light energy utilization rate and photogenerated electron-hole separation efficiency are improved, thereby enhancing the photocatalytic efficiency.

Description

Technical Field

[0001] This invention belongs to the field of photocatalysis technology, specifically relating to a Z-Scheme type composite photocatalytic material and its preparation method. Background Technology

[0002] With industrial development, water resources have been polluted to varying degrees. Hexavalent chromium (Cr(VI)) is a common pollutant in wastewater, posing a threat to humans and wildlife. Currently, methods for removing Cr(VI) include adsorption, chemical reduction, precipitation, and photocatalysis. Adsorption cannot completely remove Cr(VI), and chemical reduction requires acidic solutions and stringent conditions. Therefore, photocatalysis is considered a simple and mild method. In this technology, the band structure of the semiconductor plays a crucial role in the photocatalytic process. Cu2O is a low-cost visible-light-responsive semiconductor with a suitable CB potential, making it an ideal candidate material for the photoreduction of Cr(VI). However, the application of Cu2O is greatly limited by its photocorrosion properties, particularly Cu... + Photogenerated pores are easily oxidized. To overcome this problem, many methods have been explored, including adding sacrificial agents, constructing nanostructures, and coupling with non-metallic semiconductors. This invention discovers that BiOBr, a type of BiOX compound, has its CB edge located between the CB and VB potentials of Cu2O, allowing it to combine with Cu2O to form a Z-type heterojunction photocatalyst. In this heterojunction, photoelectrons can transfer from the CB of BiOBr to the VB of Cu2O, thereby improving the reduction ability of Cu2O, while the CB potential becomes more negative. Furthermore, this composite material can effectively segment the photogenerated support while suppressing photocorrosion of Cu2O. Simultaneously, reduced graphene oxide (rGO), due to its more adsorption sites and conjugated π structure, can increase the interaction between rGO and the semiconductor, superior to the most commonly used noble metals.

[0003] Therefore, this invention develops a Z-Scheme-type composite photocatalytic material based on extensive research, aiming to improve the photocatalytic efficiency of the material. Summary of the Invention

[0004] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide a Z-Scheme type composite photocatalytic material and its preparation method.

[0005] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0006] The first aspect of the present invention provides a Z-Scheme type composite photocatalytic material, characterized in that Cu2O particles are adhered to the surface of rGO and combined with BiOBr microspheres to form a Z-Scheme type composite photocatalytic material, namely Cu2O / rGO / BiOBr;

[0007] Furthermore, the conduction band and valence band potentials of Cu2O are both higher than the corresponding potentials of BiOBr;

[0008] Furthermore, a heterojunction is formed at the interface between Cu2O, rGO, and BiOBr.

[0009] A second aspect of the present invention provides a method for preparing a Z-Scheme type composite photocatalytic material, characterized in that the Cu2O / rGO / BiOBr composite material is prepared by an in-situ growth method;

[0010] Further steps include the following:

[0011] (1) Preparation of BiOBr: KBr and a certain amount of PVP were dissolved in distilled water and stirred evenly to obtain solution A; Bi(NO3)3 was added to ultrapure water and sonicated to obtain solution B; solution A was added to solution B and the pH was adjusted to about 6. Then the mixed solution was transferred to an autoclave and reacted at a certain temperature. After cooling to room temperature, the product was collected, washed several times with ultrapure water and ethanol, and dried.

[0012] (2) Preparation of Cu2O / rGO / BiOBr: A certain amount of BiOBr and rGO were dispersed in deionized water and ultrasonically treated. Then, Cu(AC)2•H2O, CTAB and EDTA were dissolved in the above mixture. NaOH solution and ascorbic acid solution were added. The resulting mixture was stirred, centrifuged to separate the precipitate, washed with deionized water and ethanol, and then vacuum dried to obtain the target product.

[0013] Furthermore, in step (1), the molar ratio of KBr to Bi(NO3)3 is 1~1.1:1~1.1, preferably 1:1;

[0014] Furthermore, in step (1), the reaction temperature is 90~110℃, preferably 100℃; the reaction time is 1~2h, preferably 1h; the drying temperature is 40~50℃, preferably 40℃; and the drying time is 10~15h, preferably 12h.

[0015] Furthermore, in step (2), the mass ratio of Cu(AC)2•H2O, CTAB and EDTA is 2~4:4~6:4~6, preferably 3:5:5;

[0016] Furthermore, in step (2), the concentration of the NaOH solution is 0.4~0.5 mol / L, preferably 0.45 mol / L; the concentration of the ascorbic acid solution is 0.2~0.4 mol / L, preferably 0.3 mol / L; and the volume ratio of the NaOH solution to the ascorbic acid solution is 1~1.1:1~1.1, preferably 1:1.

[0017] Furthermore, in step (2), the drying temperature is 60~70℃, preferably 60℃; the drying time is 10~12h, preferably 10h.

[0018] In this invention, if there is a conflict between the Chinese name and the structural formula of a compound, the structural formula shall prevail, unless the structural formula is obviously incorrect.

[0019] The beneficial effects of this invention are as follows:

[0020] This invention successfully prepared a series of Z-Scheme-type composite photocatalyst materials Cu2O / rGO / BiOBr using a two-step method and optimized the amount of Cu2O in the composite material through photoreduction of Cr(VI) under visible light irradiation. By transferring and separating electrons and charges among Cu2O, rGO, and BiOBr, the light energy utilization rate and photogenerated electron-hole separation efficiency are improved, thereby enhancing the photocatalytic efficiency. Attached Figure Description

[0021] Figure 1 : Schematic diagram of charge transfer performance between Cu2O, rGO and BiOBr designed in this invention;

[0022] Figure 2 Scanning electron microscope images of BiOBr and Cu2O materials synthesized by hydrothermal synthesis;

[0023] Figure 3 Scanning electron microscope image of rCB-20 composite material;

[0024] Figure 4 XRD patterns of pure BiOBr, pure Cu2O and rCB-x composite materials;

[0025] Figure 5 Photoluminescence spectrum of pure BiOBr and rCB-20 composite material;

[0026] Figure 6 Electrochemical impedance spectroscopy of pure BiOBr, pure Cu2O and rCB-x composite materials;

[0027] Figure 7 Photocatalytic reduction activity of different samples for Cr(VI) under visible light irradiation (pH=4);

[0028] Figure 8Photocatalytic reduction activity of rCB-20 on Cr(VI) at different pH values. Detailed Implementation

[0029] The present invention is illustrated below with reference to examples, but is not intended to limit the invention. Any simple substitutions or modifications made to the present invention by those skilled in the art are within the scope of the technical solutions protected by this invention.

[0030] Example 1:

[0031] 1. Preparation of BiOBr:

[0032] In beaker A, 1 mmol of KBr and a certain amount of PVP were dissolved in 20 mL of distilled water and stirred for 45 min to ensure uniform dispersion. In beaker B, 1 mmol of Bi(NO3)3 and 20 mL of ultrapure water were added and sonicated for 15 min. The solution from beaker A was then added to beaker B, and the pH of the solution was adjusted to 6. The solution was transferred to a 60 mL polytetrafluoroethylene autoclave and incubated at 100 °C for 1 h. After cooling to room temperature, the solution was collected, washed repeatedly with ultrapure water and ethanol, and dried at 40 °C for 12 h.

[0033] 2. Preparation of BiOBr / rGO / Cu2O:

[0034] A certain amount of BiOBr and 6 mg of reduced graphene oxide were dispersed in 50 mL of deionized water and sonicated for 30 min to obtain a homogeneous mixture. Then 60 mg of Cu(AC)2 was added. . H₂O, 0.1 g CTAB, and 0.1 g EDTA are dissolved in the above mixture. Then, 20 mL of 0.45 mol•L⁻¹ solution is added. -1 NaOH and 20 mL of 0.3 mol•L -1 Ascorbic acid was obtained. The mixture was stirred for 30 min, centrifuged to separate the precipitate, washed three times with deionized water and ethanol, and dried under vacuum at 60 °C for 10 h to obtain the target product. The mass ratios of Cu₂O in the composite materials were 10%, 20%, and 30%, respectively. The composite material was labeled rCB-x, where x is the mass ratio of Cu₂O. For comparison, pure Cu₂O was obtained without the addition of BiOBr under the same conditions. Furthermore, rCB-20 with the same composition of BiOBr / rGO, rGO / Cu₂O, and BiOBr / Cu₂O were synthesized using the same method.

[0035] Experimental example:

[0036] A schematic diagram of the charge transfer performance between Cu2O and BiOBr is shown below. Figure 1As shown above, the CB and VB potentials of Cu2O are higher than those of BiOBr. This means the electron Fermi level in Cu2O is higher than that in BiOBr. Therefore, in the Cu2O / rGO / BiOBr composite material, a heterojunction is formed between Cu2O, rGO, and BiOBr, and electrons transfer from the Fermi level of Cu2O to the Fermi level of BiOBr until equilibrium is reached (rGO is used as the electron transfer medium). During electron transfer from Cu2O to BiOBr, a built-in electric field (E) can be established in the space charge region near the interface. Based on the band gaps of Cu2O and BiOBr, they can both be excited by visible light and generate electron-hole pairs. Subsequently, driven by the electric field, photogenerated electrons at the CB of BiOBr move towards rGO and are captured by holes at the VB of Cu2O. Simultaneously, photoinduced electrons at the CB of Cu2O migrate to the surface and participate in the reduction of Cr(VI). In short, on the one hand, the combination of Cu2O, rGO, and BiOBr effectively separates the photocarrier; on the other hand, the photo-induced electrons with good redox capabilities at the conduction band of Cu2O can be effectively utilized. Therefore, by constructing a Cu2O / rGO / BiOBr “Z-Scheme” composite material, the photoreduction efficiency of Cr(VI) can be improved.

[0037] from Figure 2 It can be seen that the BiOBr morphology consists of layered microspheres with a diameter of approximately 800 nm. The Cu2O sample is composed of a large number of particles with a particle size of approximately 80-100 nm. Figure 3 The images show that Cu2O particles, combined with BiOBr microspheres, adhere to the rGO surface. This indicates that the Cu2O / rGO / BiOBr composite material was successfully prepared.

[0038] like Figure 4 XRD analysis of the sample phase structure revealed that all diffraction peaks of BiOBr and Cu2O corresponded to JCPDS NO. 09-0393 and JCPDS NO. 05-0667, indicating the purity of these samples. Notably, all characteristic diffraction peaks of BiOBr were detectable in the composite material, suggesting that the deposition of Cu2O and rGO did not alter the phase structure of BiOBr. Interestingly, the diffraction peak intensity of BiOBr gradually decreased with increasing Cu2O / rGO content. This is attributed to the Cu2O / rGO coating on the BiOBr surface. Furthermore, no Cu2O signal was observed on the surface of the composite sample, indicating that Cu2O is highly dispersed on the surfaces of rGO and BiOBr. Additionally, due to the low crystallinity of rGO, no rGO diffraction peaks were observed in the composite material.

[0039] Figure 5Photoluminescence (PL) spectra of Cu₂O, BiOBr, and rCB-20 were measured to understand the charge separation performance of the samples. Higher photoluminescence intensity indicates a higher recombination rate of photogenerated electrons and holes. Figure 5 As shown, the emission intensity of rCB-20 is lower than that of pure BiOBr, indicating that the “Z-Scheme” structure effectively separates photoinduced electron-hole pairs.

[0040] Figure 6 The charge transfer characteristics of Cu2O, BiOBr, and rCB-20 samples were investigated, and electrochemical impedance spectroscopy (EIS) was performed on the samples. In the figure, the arc radius of the EIS-Nyquist plot of rCB-20 is smaller than that of Cu2O and BiOBr, indicating that the charge transfer efficiency of the rCB-20 sample at the solid-liquid interface is higher than that of the Cu2O or BiOBr samples.

[0041] like Figure 7 As shown, the photocatalytic performance of the synthesized sample was evaluated by reducing Cr(VI). In this process, the initial concentration of Cr(VI) was 20 mg / L, and the sample dosage was 50 mg. Figure 7 As shown in Figure a, the adsorption process reached equilibrium after the suspension was stirred in the dark for 50 min. The composite sample exhibited better adsorption performance for Cr(VI) than pure Cu2O and BiOBr. This may be due to the introduction of reduced graphene oxide with a higher specific surface area. Under photocatalytic conditions, the rCB-20 sample showed the best photocatalytic activity for Cr(VI) reduction. After 40 min of reaction at pH=4, all Cr(VI) was removed, significantly higher than the catalytic activities of Cu2O (60%) and BiOBr (32%). Furthermore, the Cu2O content in the composite material also affected the Cr(VI) reduction efficiency. For the composite sample, a Cu2O content below or above 20% led to a decrease in photocatalytic activity. The variation in the Cr(VI) removal efficiency of the composite sample may be due to the following reasons: First, when the Cu2O content is insufficient (less than 20%), the photogenerated carrier cannot undergo effective transfer and separation; second, when the Cu2O content exceeds 20%, some photocarriers may rapidly recombine in Cu2O, thus limiting the photocatalytic activity.

[0042] from Figure 7As shown in b, the Cr(VI) concentration hardly decreased when the solution was irradiated without photocatalysis. Furthermore, the adsorption equilibrium Cr(VI) concentration remained unchanged without light irradiation (rCB-20 was used as the adsorbent here). The decrease in Cr(VI) concentration under visible light irradiation is related to the photocatalytic process, but not to chemical reduction. Under the same experimental conditions, using rGO / Cu2O, rGO / BiOBr, and Cu2O / BiOBr with the same mass composition as rCB-20 as photoreduction catalysts, the effect of different sample compositions on the reaction was investigated. Figure 7 As shown in b, the Cr(VI) removal efficiency of the rCB-20 sample is higher than that of rGO / Cu2O and rGO / BiOBr, indicating that the combination of rGO and a single semiconductor cannot achieve good separation of photogenerated carriers. Furthermore, compared to rCB-20, the photocatalytic activity of the Cu2O / BiOBr sample is relatively low because there is no rGO as an electron transport medium, and there is no effective electron transport between Cu2O and BiOBr.

[0043] like Figure 8 As shown, the photoreduction of Cr(VI) in rCB-20 was carried out at different pH values. Although the reduction efficiency of Cr(VI) gradually decreased with increasing pH value, it could be completely removed within 90 minutes at pH 7. This is because the photoelectrons at the conduction band of Cu2O have strong reducing properties.

[0044] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. An application of a Z-Scheme type composite photocatalytic material for the photoreduction of Cr(VI), characterized in that, A Z-Scheme-type composite photocatalytic material, namely Cu2O / rGO / BiOBr, is formed by Cu2O particles adhering to the surface of rGO and combining with BiOBr microspheres; the Cu2O particle size is 80-100nm; the mass ratio of Cu2O in the composite material is 20%; the conduction band and valence band potentials of Cu2O are higher than the corresponding potentials of BiOBr; a heterojunction is formed at the interface between Cu2O, rGO and BiOBr.

2. A method for preparing a Z-Scheme-type composite photocatalyst material for the photoreduction of Cr(VI) according to claim 1, characterized in that, The Cu2O / rGO / BiOBr composite material was prepared by in-situ growth.

3. The preparation method according to claim 2, characterized in that, Includes the following steps: (1) Preparation of BiOBr: KBr and a certain amount of PVP were dissolved in distilled water and stirred evenly to obtain solution A; Bi(NO3)3 was added to ultrapure water and ultrasonically treated to obtain solution B; solution A was added to solution B and the pH value was adjusted to about 6. Then the mixed solution was transferred to an autoclave and reacted at a certain temperature. After cooling to room temperature, the product was collected, washed several times with ultrapure water and ethanol and dried. (2) Preparation of Cu2O / rGO / BiOBr: A certain amount of BiOBr and rGO were dispersed in deionized water and ultrasonically treated. Then, Cu(AC)2•H2O, CTAB and EDTA were dissolved in the above mixture. NaOH solution and ascorbic acid solution were added. The resulting mixture was stirred, centrifuged to separate the precipitate, washed with deionized water and ethanol, and then vacuum dried to obtain the target product.

4. The preparation method according to claim 3, characterized in that, In step (1), the molar ratio of KBr to Bi(NO3)3 is 1~1.1:1~1.

1.

5. The preparation method according to claim 3, characterized in that, In step (1), the reaction temperature is 90~110℃ and the reaction time is 1~2h; the drying temperature is 40~50℃ and the drying time is 10~15h.

6. The preparation method according to claim 3, characterized in that, In step (2), the mass ratio of Cu(AC)2•H2O, CTAB and EDTA is 2~4:4~6:4~6.

7. The preparation method according to claim 3, characterized in that, In step (2), the concentration of NaOH solution is 0.4~0.5 mol / L and the concentration of ascorbic acid solution is 0.2~0.4 mol / L; the volume ratio of NaOH solution to ascorbic acid solution is 1~1.1:1~1.

1.

8. The preparation method according to claim 3, characterized in that, In step (2), the drying temperature is 60~70℃ and the drying time is 10~12h.

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