CN113117717B - Bi modified BiOBr-g-C 3 N 4 Heterojunction photocatalyst and preparation method and application thereof - Google Patents

Bi modified BiOBr-g-C 3 N 4 Heterojunction photocatalyst and preparation method and application thereof Download PDF

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CN113117717B
CN113117717B CN202110230397.3A CN202110230397A CN113117717B CN 113117717 B CN113117717 B CN 113117717B CN 202110230397 A CN202110230397 A CN 202110230397A CN 113117717 B CN113117717 B CN 113117717B
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heterojunction photocatalyst
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CN113117717A (en
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李冬梅
黄毅
梁奕聪
邓玥
曾庆洋
尤炜弘
谢震宇
陈海强
陈锦妹
林志奇
陈志炫
李俊添
杨思睿
方文钦
陈梓浩
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Guangdong University of Technology
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Abstract

The invention relates to Bi modified BiOBr-g-C 3 N 4 A heterojunction photocatalyst, a preparation method and application thereof. Bi-modified BiOBr-g-C of the invention 3 N 4 Bi modified BiOBr and g-C of heterojunction photocatalyst 3 N 4 Compounding the sheets; the Bi modified BiOBr comprises a spherical structure formed by BiOBr nanosheets and a Bi simple substance modified on the surfaces of the BiOBr nanosheets; wherein Bi, BiOBr and g-C 3 N 4 The mass ratio of (A) to (B) is 0.01-2.1: 0.4-7.2: 1. Bi modified BiOBr-g-C prepared by the invention 3 N 4 Under the irradiation of visible light, the photocatalytic degradation rate of the heterojunction photocatalyst on RhB can reach 99.8 percent, is improved by 41.16 percent compared with BiOBr, and is compared with BiOBr-g-C 3 N 4 Compared with the prior art, the improvement is 23.21%; the reaction rate constant of the reaction on RhB can reach 0.097/min, is 16.44 times of that of BiOBr, and is BiOBr-g-C 3 N 4 3.73 times of the total weight of the powder.

Description

Bi modified BiOBr-g-C 3 N 4 Heterojunction photocatalyst and preparation method and application thereof
Technical Field
The invention relates to the technical field of photocatalysis, in particular to Bi modified BiOBr-g-C 3 N 4 A heterojunction photocatalyst, a preparation method and application thereof.
Background
In recent years, bismuth oxybromide (BiOBr) has attracted much attention in the field of development of visible-light-responsive photocatalysts due to its characteristics of being inexpensive, non-toxic and excellent in optical activity. BiOBr is a tetragonal layered structure, and its crystal form is PbFCl type, [ Bi ] 2 O 2 ] 2+ Layer and double Br - The unique layered structure can improve the separation efficiency of photo-generated electrons and holes, thereby improving the photocatalytic activity of BiOBr. However, the forbidden band width of BiOBr is about 2.64eV to 2.91eV, the utilization rate of visible light is low, and the visible light catalytic activity is weak.
In order to improve the visible light catalytic activity of the BiOBr, researchers modify the BiOBr, and the modification methods mainly comprise the following steps: A) control of morphology (e.g. Ma H C, Zhao M, Xing H M, et al 2015 Synthesis and enhanced phosphor compositions with associated oxygen sources J H]Mater Electron,26: 10002-10011); b) Element doping, such as chinese patent CN111744505A discloses a niobium doped bismuth oxybromide catalyst; c) Depositing metal; D) constructing heterojunctions, e.g. Zhang Wendon (BiOBr and C) 3 N 4 Preparation and characterization of (1) and research on visible light catalytic oxidation rhodamine B performance [ D]) The two-dimensional nano heterojunction g-C is researched 3 N 4 And visible light degradation of rhodamine B by BiOBr.
At the present time, it is known that,the construction of the heterojunction has better effect in improving the visible light catalytic activity of the BiOBr, which is caused by the graphite carbonitride (g-C) 3 N 4 ) Is a novel polymer material without metal, and has good electronic performance and chemical stability at high temperature. g-C 3 N 4 Has a conduction band (-0.98eV) higher than that of BiOBr (0.32eV), and a valence band (3.05eV) higher than that of g-C 3 N 4 Has a low valence band (1.44eV), holes in the valence band of BiOBr are transferred to g-C 3 N 4 While the photo-generated electrons in the conduction band of BiOBr are transferred to g-C 3 N 4 So that the composite heterojunction can effectively separate the photo-generated electron pair.
Construction of g-C 3 N 4 Although the BiOBr heterojunction can effectively reduce the recombination of electron-hole pairs and improve the visible light photocatalytic activity of BiOBr, the improvement of the BiOBr forbidden band width is limited (the BiOBr forbidden band width is 2.98eV, and g-C is 3 N 4 The forbidden band width of BiOBr: 2.76 eV-2.92 eV), the visible light absorption is insufficient, and the visible light photocatalytic efficiency of BiOBr still needs to be further improved.
Therefore, it is necessary to provide a method capable of effectively reducing the forbidden band width of the BiOBr catalyst, so as to further improve the visible light catalysis performance of the BiOBr catalyst.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a Bi modified BiOBr-g-C with lower forbidden bandwidth 3 N 4 A heterojunction photocatalyst. Bi modified BiOBr-g-C prepared by the invention 3 N 4 Under the irradiation of visible light, the photocatalytic degradation rate of the heterojunction photocatalyst to RhB can reach 99.8%, and is improved by 41.16% compared with BiOBr, and compared with BiOBr-g-C 3 N 4 Compared with the prior art, the improvement is 23.21%; the reaction rate constant on RhB can reach 0.097/min, which is 16.44 times of that of BiOBr and is BiOBr-g-C 3 N 4 3.73 times higher.
Another object of the present invention is to provide the Bi-modified BiOBr-g-C 3 N 4 A preparation method of a heterojunction photocatalyst.
Another object of the present invention is toIn providing the Bi-modified BiOBr-g-C 3 N 4 Application of the heterojunction photocatalyst in visible light photocatalytic degradation of organic pollutants.
In order to realize the purpose, the invention adopts the following technical scheme:
bi modified BiOBr-g-C 3 N 4 Heterojunction photocatalyst with Bi-modified BiOBr and g-C 3 N 4 Compounding the sheets; the Bi modified BiOBr comprises a spherical structure consisting of BiOBr nanosheets and a Bi simple substance modified on the surfaces of the BiOBr nanosheets; wherein Bi, BiOBr and g-C 3 N 4 The mass ratio of (A) to (B) is 0.01-2.1: 0.4-7.2: 1.
The forbidden band width of BiOBr is about 2.64 eV-2.91 eV, the utilization rate of visible light is low, and the visible light catalytic activity is weak.
The invention discovers that the BiOBr-g-C with specific morphology 3 N 4 The heterojunction photocatalyst can improve the visible light photocatalytic activity of the catalyst. Wherein, the BiOBr nanosheets form a spherical structure, which can effectively reduce the recombination of electron hole pairs and improve the visible light photocatalytic activity of the BiOBr, but the BiOBr-g-C of the structure 3 N 4 The heterojunction photocatalyst has limited improvement on the forbidden band width, which is about 2.8 eV.
The invention further researches and discovers that Bi modified BiOBr-g-C obtained by modifying Bi metal simple substance on a BiOBr nano sheet 3 N 4 The heterojunction photocatalyst can obviously reduce the forbidden bandwidth of the heterojunction photocatalyst, and the reason is probably due to the synergistic effect of the plasma effect of the Bi metal simple substance and the heterojunction structure, and can obviously reduce Bi modified BiOBr-g-C 3 N 4 The heterojunction photocatalyst has the advantages of improving the forbidden bandwidth of the heterojunction photocatalyst, improving the utilization rate of the heterojunction photocatalyst to visible light, obviously improving the photocatalytic rate, having good catalytic stability and being reusable.
The inventors further investigated and found that the modification amount of the elementary metal Bi and BiOBr and g-C 3 N 4 The ratio of (a) has a great influence on the reduction of the forbidden band width: the modification amount of the Bi simple substance is too small, and the reduction effect of the photocatalyst is not obvious; when the amount of the modified Bi simple substance is too large, it is necessary to introduce moreThe reducing agent of (3) is used for reducing the Bi simple substance, and the use amount of the reducing agent is too much, so that the formation of heterojunction is not facilitated. In the proportion of the invention, the synergistic effect of the plasma effect of the Bi metal simple substance and the heterojunction structure can be effectively exerted, and Bi modified BiOBr-g-C is obviously reduced 3 N 4 The forbidden bandwidth of the heterojunction photocatalyst improves the utilization rate of the heterojunction photocatalyst on visible light, and obviously improves the photocatalytic rate.
Preferably, Bi modifies BiOBr-g-C 3 N 4 Heterojunction photocatalyst species of Bi, BiOBr and g-C 3 N 4 The mass ratio of (A) to (B) is 0.1-0.5: 0.8-1.4: 1.
The Bi modified BiOBr-g-C 3 N 4 The preparation method of the heterojunction photocatalyst comprises the following steps:
s1, mixing g-C 3 N 4 The bismuth precursor and the morphology directing agent are added into a solvent to obtain a solution A;
s2, dissolving soluble bromine salt and a reducing agent in a solvent to obtain a solution B;
s3, dripping the solution A into the solution B to form a suspension under the condition of continuous stirring, continuously stirring for 5-180 min, reacting for 1-24 h at 50-250 ℃, cooling to 25-30 ℃, centrifuging to obtain a precipitate, washing the obtained precipitate, and freeze-drying to obtain the Bi-modified BiOBr-g-C 3 N 4 A heterojunction photocatalyst.
Preferably, the bismuth precursor in step S1 is Bi (NO3) 3 Or Bi 2 O 3 One or two of them.
Further preferably, the bismuth precursor in the step S1 is Bi (NO3) 3 。
Preferably, in step S1, the morphology directing agent is one or a combination of PVP, hydrochloric acid or sulfuric acid. Wherein the hydrochloric acid is dilute hydrochloric acid, and the concentration of the hydrochloric acid is 0.8-1.5 mol/L; the sulfuric acid is dilute sulfuric acid, and the concentration of the dilute sulfuric acid is 0.8-1.5 mol/L.
Further preferably, the morphology directing agent in step S1 is PVP.
Preferably, the amount of the morphology directing agent used in step S1 is g-C 3 N 4 1.5-3 times of the mass.
The addition of the morphology directing agent can prevent g-C 3 N 4 Stacking, thinning the catalyst and improving the light absorption capability.
Preferably, in step S1, Bi in the bismuth precursor 3+ And g-C 3 N 4 The molar mass ratio of (a) to (b) is 0.5 to 50 mmol/g.
Further preferably, in step S1, Bi in the bismuth precursor 3+ And g-C 3 N 4 The molar mass ratio of (b) is 1 to 15 mmol/g.
Further preferably, in step S1, Bi in the bismuth precursor 3+ And g-C 3 N 4 The molar mass ratio of (a) to (b) is 5 mmol/g.
Preferably, the reducing agent in step S2 is one or both of citric acid and ascorbic acid.
Further preferably, the reducing agent in step S2 is citric acid.
The reducing agent acts to react Bi 3+ Reducing the solution into a Bi simple substance.
Preferably, the soluble bromine salt in step S2 is one or both of KBr or NaBr.
Further preferably, the soluble bromine salt in step S2 is KBr.
Preferably, Br in the soluble bromine salt - With Bi in the bismuth precursor 3+ The molar ratio of (a) to (b) is 1-1.2: 1.
Preferably, the reducing agent is reacted with Bi in the bismuth precursor 3+ The molar ratio of (A) to (B) is 0.5-6: 1.
Further preferably, in step S2, the reducing agent and Bi in the bismuth precursor 3+ The molar ratio of (a) to (b) is 1-3: 1.
Further preferably, in step S2, the reducing agent and Bi in the bismuth precursor 3+ In a molar ratio of 2: 1.
The amount of the reducing agent can affect the growth of the BiOBr flaky crystal, thereby affecting the morphology of the catalyst and further affecting the photocatalytic performance of the photocatalyst. Wherein, if the dosage of the reducing agent is too small, the reduced Bi simple substance is less, and the photocatalytic performance of the photocatalyst is not obviously improved; the BiOBr flaky crystal grows when the dosage of the reducing agent is too muchToo long and dense for g-C 3 N 4 The insertion of (2) to form a heterojunction structure, the photocatalytic performance of the photocatalyst is reduced.
Preferably, the solvent is one or a combination of more of isopropanol, ethylene glycol or tert-butanol.
Further preferably, the solvent is ethylene glycol.
Preferably, the stirring time in step S3 is 15 to 120min, and more preferably 30 min.
Preferably, the reaction temperature in step S3 is 100 to 200 ℃, and more preferably 180 ℃.
Preferably, the reaction time in step S3 is 3-12 h, and more preferably 6 h.
Preferably, the washing in step S3 is performed by respectively using ethanol and water.
Preferably, the temperature of the freeze-drying in step S3 is-40 ℃ to-80 ℃.
Preferably, the freeze-drying time in step S3 is 20h to 48 h.
Incidentally, conventionally commercially available graphitized carbon nitride (g-C) has been proposed 3 N 4 ) Can be used in the present invention. However, the present invention also provides a specific porous g-C 3 N 4 Selecting the porous g-C 3 N 4 The prepared heterojunction photocatalyst has good photocatalytic performance effect.
Preferably, the porous g-C 3 N 4 The preparation method comprises the following steps:
sequentially calcining urea at 150-450 ℃ for 0.5-5 h for the first time, calcining urea at 480-700 ℃ for the second time for 0.5-10 h, cooling to 25-30 ℃, grinding, washing and drying to obtain porous g-C 3 N 4 。
Preferably, in the primary calcination, the heating rate of heating to 150-450 ℃ is 2-15 ℃/min, and more preferably 5 ℃/min.
Preferably, the temperature rise rate between the first calcination and the second calcination is 2-15 ℃/min, and more preferably 5 ℃/min.
Preferably, the temperature of the primary calcination is 200-400 ℃, and further preferably 350 ℃.
Preferably, the time of the primary calcination is 0.5-3 h, and more preferably 1 h.
Preferably, the temperature of the secondary calcination is 500-650 ℃, and more preferably 550 ℃.
Preferably, the time of the secondary calcination is 1-5 h, and further preferably 3 h.
Preferably, the washing is washing with ethanol and water respectively.
Preferably, the drying temperature is 60-120 ℃.
Preferably, the drying time is 8-36 h.
The Bi-modified BiOBr-g-C 3 N 4 The application of the heterojunction photocatalyst in the visible light catalytic degradation of organic pollutants is also within the protection scope of the invention.
Compared with the prior art, the invention has the following beneficial effects:
the invention constructs BiOBr-g-C 3 N 4 On the basis of heterojunction, Bi metal is introduced to the surface of BiOBr, so that the recombination of electron-hole pairs can be effectively reduced, the forbidden bandwidth can be remarkably reduced, and Bi modifies BiOBr-g-C 3 N 4 The forbidden band width of the material is 2.43-2.47 eV, and is reduced by at least 17.11% compared with the forbidden band width of BiOBr (the forbidden band width is 2.98eV), and compared with BiOBr-g-C 3 N 4 (the forbidden band width is 2.8eV), the forbidden band width is reduced by at least 11.79%; correspondingly, the Bi modified BiOBr-g-C prepared by the invention 3 N 4 Under the irradiation of visible light, the photocatalytic degradation rate of the heterojunction photocatalyst on RhB can reach 99.8 percent, is improved by 41.16 percent compared with BiOBr (the photocatalytic degradation rate is 70.7 percent), and is compared with BiOBr-g-C 3 N 4 The photocatalytic degradation rate is increased by 23.21 percent compared with that of the catalyst (the photocatalytic degradation rate is 81 percent); the reaction rate constant for RhB can be as high as 0.097/min, which is 16.44 times that of BiOBr (reaction rate constant is 0.0059/min), which is BiOBr-g-C 3 N 4 (reaction rate constant 0.026/min) 3.73 times.
Drawings
FIG. 1 shows Bi-modified BiOBr-g-C 3 N 4 A schematic structural diagram of a heterojunction photocatalyst;
FIG. 2 is an XRD spectrum of the photocatalysts prepared in examples 1-3 and comparative example 1 with different CA addition amounts;
FIG. 3 is a SEM topographic structure diagram of photocatalysts prepared in examples 1-3 and comparative example 1 under different CA addition amounts;
FIG. 4 is a regional high resolution XPS spectrum of Bi 4f (a), O1s (b), Br 3d (C), C1s (d), N1s (e) of the photocatalyst BBC-2 prepared in example 2;
FIG. 5 shows the UV-diffuse reflectance-visible light absorption spectra of the photocatalysts prepared in examples 1 to 3 and comparative example 1 at different CA addition amounts, wherein a is the absorption spectrum of the photocatalyst at different CA addition amounts, and b is (ahv) 1/2 A relationship of variation to light energy;
FIG. 6 is a PL (photoluminescence spectrum) spectrum of the photocatalysts prepared in examples 1-3 and comparative example 1 with different CA addition amounts;
fig. 7 is a graph of visible light photocatalytic degradation performance of the photocatalysts prepared in examples 1 to 3 and comparative example 1 under different CA addition amounts to RhB, where a is a photodegradation effect of the different photocatalysts to RhB, and b is a reaction rate constant of the different photocatalysts to RhB;
FIG. 8 is a graph showing the results of a radical trapping experiment for the photocatalyst prepared in example 2;
FIG. 9 shows Bi-modified BiOBr-g-C of the present invention 3 N 4 A photocatalytic mechanism diagram of a heterojunction photocatalyst;
fig. 10 shows the repeated photocatalytic degradation effect of the photocatalyst prepared in example 2 under irradiation of visible light.
Detailed Description
The present invention will be further described with reference to the following specific examples and drawings, which are not intended to limit the invention in any manner. Reagents, methods and apparatus used in the present invention are conventional in the art unless otherwise indicated.
Unless otherwise indicated, reagents and materials used in the present invention are commercially available.
Example 1
This example provides a Bi-modified BiOBr-g-C 3 N 4 The preparation method of the heterojunction photocatalyst comprises the following steps:
s1. preparation of g-C 3 N 4
Weighing 10g of urea in a ceramic crucible with a cover, placing the crucible in a muffle furnace, heating to 350 ℃ at a heating rate of 5 ℃/min, carrying out primary calcination, and keeping for 1 h; then heating to 550 ℃ at the heating rate of 5 ℃/min, carrying out secondary calcination, and keeping for 3 h; cooling to room temperature (25-30 ℃), grinding into powder, washing with water and ethanol respectively, and drying in an oven at 60 ℃ for 36h to obtain porous g-C 3 N 4 ；
S2, preparing Bi modified BiOBr-g-C 3 N 4 Heterojunction photocatalyst
S21, preparing g-C from S1 3 N 4 0.2g、1mmol Bi(NO3) 3 And 0.3g PVP in a beaker containing 20mL Ethylene Glycol (EG) to give solution A;
s22, dissolving 1mmol of KBr and 1mmol of Citric Acid (CA) in a beaker filled with 20mL of Ethylene Glycol (EG) to obtain a solution B;
s23, under the condition of continuous stirring, dripping the solution A into the solution B to form a suspension, continuously stirring for 30min, reacting for 6h at 180 ℃, cooling to room temperature (25-30 ℃), centrifuging to obtain a precipitate, washing the obtained precipitate with water and ethanol respectively, and freeze-drying at-40 ℃ for 48h to obtain the Bi modified BiOBr-g-C 3 N 4 Heterojunction photocatalyst, noted BBC-1, wherein Bi, BiOBr and g-C 3 N 4 The mass ratio of (A) was obtained by EDS elemental analysis and was 0.14:1.27: 1.
Example 2
This example provides a Bi-modified BiOBr-g-C 3 N 4 A heterojunction photocatalyst, differing from example 1 in that the amount of citric acid added in step S22 was 2mmol, and Bi-modified BiOBr-g-C prepared finally 3 N 4 Heterojunction photocatalyst, noteIs BBC-2, wherein Bi, BiOBr and g-C 3 N 4 The mass ratio of (a) to (b) is 0.27:1.14: 1.
Example 3
This example provides a Bi-modified BiOBr-g-C 3 N 4 A heterojunction photocatalyst, differing from example 1 in that the amount of citric acid added in step S22 was 3mmol, and Bi-modified BiOBr-g-C prepared finally 3 N 4 Heterojunction photocatalyst, noted BBC-3, where Bi, BiOBr and g-C 3 N 4 The mass ratio of (a) to (b) is 0.41:1: 1.
Example 4
This example provides a Bi-modified BiOBr-g-C 3 N 4 The preparation method of the heterojunction photocatalyst comprises the following steps:
s1, preparation of g-C 3 N 4
Weighing 10g of urea in a ceramic crucible with a cover, placing the crucible in a muffle furnace, heating to 200 ℃ at a heating rate of 10 ℃/min, carrying out primary calcination, and keeping for 3 hours; then heating to 500 ℃ at the heating rate of 10 ℃/min, carrying out secondary calcination, and keeping for 5 h; cooling to room temperature (25-30 ℃), grinding into powder, washing with water and ethanol respectively, and drying in a drying oven at 60 ℃ for 36h to obtain porous g-C 3 N 4 ；
S2, preparing Bi modified BiOBr-g-C 3 N 4 Heterojunction photocatalyst
S21, preparing g-C from S1 3 N 4 1g、1mmol Bi(NO3) 3 And 1.5g PVP in a beaker containing 60mL Ethylene Glycol (EG) to give solution A;
s22, dissolving 1.2mmol of KBr and 6mmol of Citric Acid (CA) in a beaker filled with 60mL of Ethylene Glycol (EG) to obtain a solution B;
s23, under the condition of continuous stirring, dripping the solution A into the solution B to form a suspension, continuously stirring for 15min, reacting for 3h at 200 ℃, cooling to room temperature (25-30 ℃), centrifuging to obtain a precipitate, washing the obtained precipitate with water and ethanol respectively, and freeze-drying at-50 ℃ for 36h to obtain the Bi modified BiOBr-g-C 3 N 4 Heterojunction photocatalyst of Bi, BiOBr and g-C 3 N 4 The mass ratio of (A) to (B) is 0.05:1.1: 1.
Example 5
This example provides a Bi-modified BiOBr-g-C 3 N 4 The preparation method of the heterojunction photocatalyst comprises the following steps:
s1. preparation of g-C 3 N 4
Weighing 10g of urea in a ceramic crucible with a cover, placing the crucible in a muffle furnace, heating to 400 ℃ at a heating rate of 15 ℃/min, carrying out primary calcination, and keeping for 0.5 h; then heating to 650 ℃ at the heating rate of 15 ℃/min, carrying out secondary calcination, and keeping for 1 h; cooling to room temperature (25-30 ℃), grinding into powder, washing with water and ethanol respectively, and drying in an oven at 60 ℃ for 36h to obtain porous g-C 3 N 4 ；
S2, preparing Bi modified BiOBr-g-C 3 N 4 Heterojunction photocatalyst
S21, preparing g-C from S1 3 N 4 1g、15mmol Bi(NO3) 3 And 3g PVP was dissolved in a beaker with 60mL of isopropanol to give solution A;
s22, dissolving 15mmol of NaBr and 9mmol of ascorbic acid in a beaker filled with 60mL of isopropanol to obtain a solution B;
s23, under the condition of continuous stirring, dripping the solution A into the solution B to form a suspension, continuously stirring for 120min, reacting for 24h at 100 ℃, cooling to room temperature (25-30 ℃), centrifuging to obtain a precipitate, washing the obtained precipitate with water and ethanol respectively, and freeze-drying at-80 ℃ for 20h to obtain the Bi modified BiOBr-g-C 3 N 4 Heterojunction photocatalyst of Bi, BiOBr and g-C 3 N 4 The mass ratio of (A) to (B) is 2.1:7.2: 1.
Comparative example 1
This comparative example provides a BiOBr-g-C unmodified by Bi 3 N 4 The heterojunction photocatalyst is different from that of example 1 in that citric acid is not added in step S22, and the obtained photocatalyst is denoted as BC.
Comparative example 2
This comparative example provides a Bi-modified BiOBr photocatalyst, which differs from example 1 in that step S1 is eliminated and no g-C is added in step S21 3 N 4 The resulting photocatalyst is denoted as BB.
The photocatalysts prepared in the above examples and comparative examples are subjected to performance characterization, and specific test items and test methods are as follows:
1. phase structure: the phase structure was analysed by X-ray diffractometry (model Bruke-D8, Germany, 2018) and the results are shown in FIG. 2.
As can be seen from FIG. 2, diffraction peaks of the simple Bi metal are shown in photocatalysts BBC-1, BBC-2 and BBC-3, and the content of the reduced simple Bi metal increases with the increase of the CA addition amount. In addition, no diffraction peak of carbon was detected in the photocatalysts BBC-1, BBC-2 and BBC-3, indicating that carbon-containing by-products were not produced as the amount of CA added was increased. Comparing the (110) plane diffraction peaks of the 4 photocatalysts shows that: the height of a diffraction peak of BBC-1 is weaker than that of BC, and the addition of 1mmol of CA is not beneficial to the growth of BiOBr crystals; when the addition amount of CA reaches 2mmol, the diffraction peak of BBC-2 is sharp, and the crystallinity is the highest; compared with BBC-2, the diffraction peak of BBC-3 is reduced, and the 110-plane crystallinity is reduced. The average grain sizes of BC, BBC-1, BBC-2 and BBC-3 are respectively calculated by a 110 crystal face according to a Scherrer formula to be 17.42nm, 14.31nm, 12.98nm and 13.04 nm. This indicates that the addition of CA can also regulate the BiOBr morphology. When the addition amount of CA is 2mmol (namely, the reducing agent and Bi in the bismuth precursor) 3+ In a molar ratio of 2:1, Bi, BiOBr and g-C in the prepared photocatalyst 3 N 4 The mass ratio of (2) to (1) is 0.25:1:1), the crystallinity of the nano BiOBr crystal is the highest, and the average thickness is the smallest.
The test results of examples 4-5 were similar to those of example 1.
The phase structure analysis result shows that the embodiment of the invention successfully synthesizes Bi modified BiOBr-g-C 3 N 4 A heterojunction photocatalyst.
2. The appearance structure is as follows: the surface morphology of the catalyst was observed using a field emission scanning electron microscope (LYRA 3 XMU, Czech Tescan, 2018), and the results are shown in FIG. 3.
FIG. 3 shows the structure diagram of the morphology of the composite photocatalyst under different CA addition amounts. The shape of the catalyst is regulated and controlled, which is beneficial to improving the catalytic performance. As can be seen from figure 3, the spherical structure is formed by growing a large number of BiOBr nano-sheets, the diameter of the BiOBr nano-sheets is 2-4 um, and g-C 3 N 4 Lamellar intercalation into BiOBr microspheres to attenuate g-C 3 N 4 Thinning it (see fig. 3a and the markings); when the adding amount of CA is 1mmol, the diameter of the microsphere is about 1-2 um, the BiOBr flaky crystal grows incompletely (see figure 3b), and the 110 crystal face diffraction peak is low (corresponding to BBC-1 in figure 3); when the addition amount of CA is 2mmol, the diameter of the microsphere is increased by 2-3 um, and at the moment, g-C 3 N 4 The BiOBr microspheres are embedded into the surfaces of the BiOBr microspheres and are folded into a plate shape (see figure 3c), so that the utilization rate of visible light is improved; when the addition amount of CA is 3mmol, the diameter of the BiOBr microsphere is 3-4 um, the BiOBr lamella on the microsphere grows more compactly, and BiOBr and g-C are reduced 3 N 4 The active site of the binding, is not conducive to heterojunction formation (see figure 3 d).
The test results of examples 4-5 were similar to those of example 1.
Bi modified BiOBr-g-C can be constructed by the phase and SEM morphology analysis 3 N 4 The structural schematic diagram of the heterojunction photocatalyst is shown in figure 1, and the structural diagram is shown in figure 1, the catalyst is in a spherical structure consisting of BiOBr nano sheets, and g-C 3 N 4 The lamella is embedded into the BiOBr microsphere, and Bi is modified on the surface of the BiOBr nanosheet.
3. Elemental composition and chemical valence: an X-ray photoelectron spectrometer was selected for analysis of elemental composition and chemical valence (Thermo Fisher, usa, 2018), and the results are shown in fig. 4.
The elemental composition and valence state characteristics may reflect the composition and chemical characteristics of the photocatalyst.
As can be seen in FIG. 4, the Bi 4f to which the peaks at 163.9eV and 158.6eV of the Bi element 4f orbital binding energy belong 5/2 And Bi 4f 7/2 Of Bi 3+ (see FIG. 4a), the characteristic peaks at 161.2eV and 155.9eV correspond to the binding energy generated by Bi metal.
XPS spectrum of O1s shows that O element has three forms, namely Bi modified BiOBr-g-C at 532eV of binding energy 3 N 4 The O-H bond of the surface water molecule of the heterojunction photocatalyst; the binding energy is 531.1eV, which belongs to the oxygen vacancy on the surface of the sample; and a characteristic peak corresponding to the Bi-O bond of the BiOBr lattice at a binding energy of 529.5eV (see FIG. 4 b).
Peak 3d of Br 3d 5/2 And 3d 3/2 The binding energies were 67.6eV and 68.7eV, respectively, which indicates that Br is in the negative valence state (see FIG. 4 c).
By peak-splitting fitting, C1s exhibited a peak at 288.1eV for N-C — N cyclic sp2 hybridized C in carbon nitride, while the peak at 284.8eV corresponded to a special sp in carbon nitride 2 C-C bond (see FIG. 4d), and the C peak at 284.8eV corresponds to a C-C bond, which is derived from g-C 3 N 4 Amorphous carbon adsorbed to surface carbon atoms (see fig. 4 d). No other C contamination peaks were detected at element C, indicating that the sample was pure.
By peak fitting, the N1s region was fitted to four peaks, exhibiting a C ═ N-C bond sp in carbon nitride at 398.3eV 2 Peaks of hybridized aromatic rings, N- (C) in carbon nitride at 399.8eV 3 The bond, the N peak at 401.0eV is attributed to the C-N-H bond in carbon nitride and the N peak at 404.0eV is attributed to the large π bond in carbon nitride (see FIG. 4 e).
Analysis by XPS revealed not only the presence of oxygen vacancies on BBC-2, but also the presence of g-C 3 N 4 Valence states in Bi metal and BiOBr element, thus further demonstrating Bi/BiOBr/g-C 3 N 4 The heterojunction composite photocatalyst is successfully prepared.
The test results of the other examples were similar to those of example 2.
4. Visible light absorption capacity: the absorption capacity of the catalyst for visible light was analyzed by UV-vis UV-visible diffuse reflectance spectroscopy (UV-3600(DRS), Shimadzu corporation, Japan, 2018), and the test results are shown in FIG. 5.
The visible light absorption capacity may reflect the optical properties of the catalyst. As can be seen from FIG. 5, BBC-1, BBC-2 and BBC-3 absorb light in the wavelength range of 430nm to 900nm in comparison with BCThe force is improved significantly. However, when the amount of CA added was 3mmol, the light absorption property of BBC-3 was significantly weaker than that of BBC-2. Further, when the amount of CA added was changed from 0 to 3mmol, the forbidden band widths of BC, BBC-1, BBC-2 and BBC-3 were 2.8eV, 2.47eV, 2.43eV and 2.46eV, respectively. Among them, BBC-2 has the lowest forbidden band width, which is attributed to the plasma effect of Bi metal and the heterojunction structure after being compounded with carbon nitride. Thus, 2mmol (i.e., reducing agent and Bi from the bismuth precursor) 3+ In a molar ratio of 2:1, Bi, BiOBr and g-C in the prepared photocatalyst 3 N 4 In a mass ratio of 0.25:1:1) is the optimum amount of CA to be added, in which case the modification of Bi metal results in BiOBr/g-C 3 N 4 The catalyst activity is strongest.
In comparative example 2, the forbidden bandwidth of the prepared catalyst is 2.96 only by modifying the Bi simple substance, so that the influence of only selecting the Bi simple substance for modification on the forbidden bandwidth of the catalyst is small.
5. Photocatalytic activity: the photocatalytic activity of the catalyst (comparison of the rate of recombination of photo-generated electrons with holes) was analyzed by photoluminescence spectroscopy (Fluorolog-3, HORIBA, usa, 2018), and the results of the test are shown in fig. 6.
Generally, the lower the emission peak intensity, the lower the recombination rate of the photo-generated electron-hole pairs. FIG. 6 shows that BBC-2 composite photocatalyst has the weakest emission peak intensity and the lowest photoelectron and hole recombination rate. In the wavelength range of 400-600nm, the emission peaks of different photocatalysts BC, BBC-1, BBC-2 and BBC-3 have similar shapes and all appear at 460 nm. Wherein, the emission peak intensity of the BBC-2 composite photocatalytic material is the weakest. The analysis of the intensity sequence of the emission peak of the fluorescence spectrum and the thickness sequence of the BiOBr nanosheet in XRD characterization shows that the smaller the average grain thickness of BC, BBC-1, BBC-2 and BBC-3 is, the lower the fluorescence spectrum intensity is. BBC-2 with the weakest emission peak intensity has the lowest photoelectron and hole recombination rate, which is beneficial to improving the photocatalytic performance.
6. The photocatalytic degradation performance of the photocatalyst on RhB is as follows:
using 500W xenon lamp as light source, adding 0.05g of the prepared photocatalyst solid powder into a solution containing 50mL 10mg/L rhodamine B (RhB)Quartz reactor (kept at room temperature). Selecting a cationic dye RhB as a target object, and researching Bi modified BiOBr-g-C under two conditions of a dark state and illumination 3 N 4 Photocatalytic activity of the heterojunction photocatalyst. Under the condition of visible light irradiation, 5mL of suspension after reaction is taken every 15 minutes, after high-speed centrifugation, the absorbance of RhB is detected by an ultraviolet visible spectrophotometer, the photocatalytic degradation effect is analyzed, and the test result is shown in figure 7 in detail.
The photocatalytic degradation performance of different photocatalysts on RhB is shown in fig. 7. From fig. 7a, RhB is not degraded without adding photocatalyst, indicating that RhB is stable under visible light irradiation. Under the condition of stirring in a dark state, the four photocatalysts BC, BBC-1, BBC-2 and BBC-3 show adsorption characteristics, and the adsorption of RhB reaches balance within 30 min. The adsorption removal rates of the BBC-1, BBC-2 and BBC-3 composite photocatalysts are respectively 20%, 31% and 25%, and are higher than that of BC. When the irradiation time of the visible light is 60min, the photocatalytic degradation rates of the BC, the BBC-1, the BBC-2 and the BBC-3 on the RhB are 81%, 98.7%, 99.8% and 99.5% respectively. The composite photocatalyst modified by Bi has improved adsorption performance, strong visible light catalytic degradation capability and reaction rate constants of 0.069min -1 、0.097min -1 And 0.083min -1 (see FIG. 7 b). Wherein BBC-2 (Bi/BiOBr/g-C) 3 N 4 ) The visible light catalytic degradation performance of the composite is strongest.
Comparative example 2 the catalyst prepared by modifying only Bi simple substance had a reaction rate constant of 0.0074min -1 . Therefore, the influence of only selecting Bi simple substance for modification on the catalytic effect of the catalyst is small.
7. The mechanism of photocatalytic degradation of RhB by the photocatalyst is as follows:
in order to explore the photocatalytic mechanism of BBC-2, an active group capture experiment was performed. In experiments for the photocatalytic degradation of Rhb by BBC-2, ethylenediamine tetraacetic acid (EDTA, 2mmol/L), p-benzoquinone (BQ, 1mmol/L), and isopropanol (IPA, 10mmol/L) were added as cavities (h) at the beginning of the dark reaction, respectively + ) Superoxide radical (. O) and hydroxyl radical (. OH) trapping agent, and then carrying out photocatalytic reaction for 60 min.
Free radical trapping experiments show that O 2- And H + Is the main active species for degradation of RhB by BBC-2 (see FIG. 8). As can be seen from FIG. 8, the photodegradation rate was greatly reduced after the capture agents quinone BQ and disodium ethylenediaminetetraacetate EDTA-2NA were introduced into the reaction system, while the removal rate remained high after the isopropanol IPA was added. This indicates that,. O 2- And H + Are all active species that cause photodegradation of RhB, of which hole H + Plays a major role in BBC-2 catalysts, while OH plays only a minor role.
In combination with the mechanism diagram of photocatalytic degradation RhB (see fig. 9), the visible photocatalytic degradation performance of BBC-2 heterojunction photocatalyst is enhanced for four main reasons:
firstly, BBC-2 can provide more reactive active sites to participate in a photocatalytic reaction in a degradation process; secondly, the Bi metal simple substance reduced on BBC-2 has a plasma resonance effect, which is beneficial to the rapid migration of photo-generated electron flow; III is g-C 3 N 4 And/or the heterojunction structure of BiOBr is favorable for inhibiting the recombination of photo-generated electron-hole pairs; fourthly, BiOBr/g-C 3 N 4 The surface of the catalyst introduces a proper amount of oxygen vacancy in the process of reducing Bi metal by CA and EG, which is beneficial to capturing oxygen and accelerating the photodegradation process of the composite photocatalyst to RhB.
8. And (3) testing the photocatalytic performance cycle:
in order to explore the recycling capability of the photocatalyst, a photocatalytic stability experiment is carried out on BBC-2, and the photocatalytic quality and the dye concentration are not changed in the experiment. In the circulation experiment, dark reaction adsorption is carried out for 30min, the BBC-2 is filtered, washed and dried after photocatalytic reaction is carried out for 60min, dark reaction is carried out for 30min, photocatalytic reaction is carried out for 60min, and the rest is done in this way, and circulation is carried out for four times.
As can be seen from fig. 10, by repeating the catalytic degradation experiment 4 times, the photocatalytic degradation efficiency of BBC-2 on RhB is still as high as 96.4%, which is only 3.4% lower than the photocatalytic degradation efficiency (99.8%) when it is used for the first time. Thus, the Bi modified BiOBr-g-C prepared by the invention 3 N 4 The heterojunction photocatalyst has good catalytic stability and can be repeatedly used.
The above-mentioned embodiments, objects, technical solutions and advantages of the present invention are further described in detail, it should be understood that the above-mentioned embodiments are only examples of the present invention, and are not intended to limit the scope of the present invention, and any modifications, equivalent substitutions, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (7)

1. Bi modified BiOBr-g-C 3 N 4 A heterojunction photocatalyst characterized in that BiOBr and g-C are modified with Bi 3 N 4 Compounding the layers; wherein the Bi modified BiOBr comprises a spherical structure consisting of BiOBr nano sheets and a Bi simple substance g-C modified on the surfaces of the BiOBr nano sheets 3 N 4 The sheet layer is embedded into a spherical structure; the Bi modified BiOBr-g-C 3 N 4 Oxygen vacancies exist on the surface of the heterojunction photocatalyst; wherein Bi, BiOBr and g-C 3 N 4 The mass ratio of (A) to (B) is 0.1-0.5: 0.8-1.4: 1; the Bi modified BiOBr-g-C 3 N 4 The preparation method of the heterojunction photocatalyst comprises the following steps:
S1.g-C 3 N 4 the bismuth precursor and the morphology directing agent are added into a solvent to obtain a solution A;
s2, dissolving soluble bromine salt and a reducing agent in a solvent to obtain a solution B;
s3, dripping the solution A into the solution B to form a suspension under the condition of continuous stirring, continuously stirring for 5-180 min, reacting for 1-24 h at 50-250 ℃, cooling to 25-30 ℃, centrifuging to obtain a precipitate, washing the obtained precipitate, and freeze-drying to obtain the Bi-modified BiOBr-g-C 3 N 4 A heterojunction photocatalyst;
wherein, the shape directing agent in the step S1 is one or a combination of more of PVP, hydrochloric acid or sulfuric acid.
2. The Bi-modified BiOBr-g-C of claim 1 3 N 4 The heterojunction photocatalyst is characterized in that in step S1, the bismuth precursor is Bi (NO) 3 ) 3 Or Bi 2 O 3 One or two of them.
3. The Bi-modified BiOBr-g-C of claim 1 3 N 4 The heterojunction photocatalyst is characterized in that the reducing agent in the step S2 is one or two of citric acid and ascorbic acid.
4. The Bi-modified BiOBr-g-C of claim 1 3 N 4 The heterojunction photocatalyst is characterized in that the soluble bromine salt in the step S2 is one or two of KBr or NaBr.
5. The Bi-modified BiOBr-g-C of claim 1 3 N 4 The heterojunction photocatalyst is characterized in that the solvent is one or a combination of more of isopropanol, ethylene glycol or tertiary butanol.
6. The Bi-modified BiOBr-g-C of claim 1 3 N 4 The heterojunction photocatalyst is characterized in that the temperature of the freeze drying in the step S3 is-40 ℃ to-80 ℃; the freeze drying time is 20-48 h.
7. The Bi-modified BiOBr-g-C as claimed in claim 1 3 N 4 The application of the heterojunction photocatalyst in visible light catalytic degradation of organic pollutants.
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