Atypical I-type heterojunction h-BN / CdS photocatalyst as well as preparation method and application thereof

Abstract

The invention discloses an atypical I-type heterojunction h-BN / CdS photocatalyst, a preparation method thereof and an application of the atypical I-type heterojunction h-BN / CdS photocatalyst in photocatalytic degradation of tetracycline, a broad-band gap material (h-BN) and a narrow-band gap material (CdS) are used as composite objects, and a precursor is rapidly thermally decomposed through a Joule heating technology; the h-BN / CdS heterojunction photocatalyst with the atypical I-type heterojunction structure can be prepared only in several seconds, and the h-BN / CdS heterojunction photocatalyst is used for photocatalytic degradation of tetracycline and is 20 times of that of h-BN and 1.1 times of that of CdS.

Description

Technical Field

[0001] The invention belongs to the technical field of photocatalysis, and in particular relates to an atypical type I heterojunction h-BN / CdS photocatalyst and a preparation method thereof, and application thereof in photocatalytic degradation of tetracycline. Background Art

[0002] Tetracycline residues are highly toxic, long-term residual, semi-volatile and highly lipid-soluble. They can be enriched and transmitted in the food chain, and can be migrated and distributed globally through a variety of transmission pathways, causing serious harm to human health and the ecological environment. In order to eliminate the hazards of tetracycline residues, photocatalytic technology with the advantages of green sustainability, high-efficiency oxidation, and mild reaction conditions has attracted everyone's attention. However, photocatalytic technology still has limitations in practical applications. The main limiting factor is the separation efficiency of photogenerated carriers. The rapid recombination of photogenerated electrons and holes on and on the surface of photocatalysts seriously hinders the photocatalytic efficiency. In 1951, Gubanov first proposed the concept of heterojunction, which is a special PN junction formed by two or more layers of different semiconductor material films deposited in sequence on the same substrate. It can achieve effective interface transfer and separation of photogenerated carriers under light excitation.

[0003] At present, the traditional heterojunction is composed of two or more narrow bandgap semiconductor materials, using a wider spectral range to improve photocatalytic activity and light energy utilization. Among the many narrow bandgap photocatalysts driven by visible light, CdS has become an excellent choice for constructing heterojunctions because of its suitable bandgap and high photoelectric conversion efficiency. In addition, wide bandgap semiconductors have a large bandgap width, high thermal stability and high electron mobility, and can be used as components of heterojunctions. Wide bandgap semiconductors are currently widely used in the fields of physics and devices, but few studies have explored the reaction mechanism and photocatalytic activity after the combination of wide bandgap semiconductors and narrow bandgap semiconductors. This is because the combination of wide and narrow bandgaps has a long electron transmission path and electron-hole confinement zone, which is easy to construct a type I heterojunction, which is not conducive to the improvement of photocatalytic performance. Summary of the invention

[0004] In order to overcome the shortcomings of the prior art, the purpose of the present invention is to provide an atypical type I heterojunction h-BN / CdS photocatalyst and a preparation method thereof and an application in the photocatalytic degradation of tetracycline. A wide bandgap material (h-BN) and a narrow bandgap material (CdS) are used as composite objects, and the precursor is quickly thermally decomposed by Joule heating technology. It only takes a few seconds to prepare an h-BN / CdS heterojunction photocatalyst with an atypical type I heterojunction structure. The photocatalyst is used for photocatalytic degradation of tetracycline, which is 20 times that of h-BN and 1.1 times that of CdS.

[0005] In order to achieve the above technical objectives, the present invention adopts the following technical solutions:

[0006] A method for preparing an atypical type I heterojunction h-BN / CdS photocatalyst comprises the following steps:

[0007] (1) adding cadmium chloride and thiourea to anhydrous ethanol in sequence, heating and stirring until fully mixed, to obtain a cadmium sulfide precursor salt;

[0008] (2) mixing a cadmium sulfide precursor salt and h-BN to obtain a mixed powder; placing the mixed powder in a Joule heating apparatus under a protective atmosphere for reaction, thereby obtaining an atypical type I heterojunction h-BN / CdS photocatalyst.

[0009] Preferably, in step (1), the molar ratio of cadmium chloride to thiourea is 1:2-3, and the heating temperature is 50-60°C.

[0010] Preferably, in step (2), the mass ratio of the cadmium sulfide precursor salt to h-BN is 1:1 to 1.5.

[0011] Preferably, in step (2), the parameters of the Joule heater are set as follows: voltage 30-40 V, current 300-350 A, reaction time 15-20 s, reaction temperature 1000-1500° C.

[0012] The present invention also provides an atypical I-type heterojunction h-BN / CdS photocatalyst prepared by the above preparation method.

[0013] The present invention also provides the application of the atypical I-type heterojunction h-BN / CdS photocatalyst, which is used for photocatalytic degradation of tetracycline.

[0014] The present invention uses wide bandgap material (h-BN) and narrow bandgap material (CdS) as composite objects, and quickly thermally decomposes the precursors through Joule heating technology. It only takes a few seconds to prepare h-BN / CdS heterojunction photocatalyst with an atypical I-type heterojunction structure. Unlike the conventional I-type heterojunction, photogenerated electrons are transferred from h-BN to CdS, while photogenerated holes move in the opposite direction, which significantly improves the photocatalytic efficiency of the heterojunction.

[0015] The advantages of the present invention are:

[0016] (1) The h-BN / CdS photocatalyst of the present invention is composed of a wide bandgap material (h-BN) and a narrow bandgap material (CdS). Density functional theory calculation analysis and experimental verification show that it belongs to an atypical type I heterojunction structure. Its built-in electric field and bandgap arrangement improve the effective separation of photogenerated electrons and holes in the heterojunction, showing excellent photoelectrochemical performance.

[0017] (2) The preparation process of the h-BN / CdS photocatalyst of the present invention adopts Joule heating technology, which is fast and efficient and can be prepared in just a few seconds. Compared with the catalysts prepared by direct physical mixing and tubular furnace, it has a heterogeneous interface and has more excellent photoelectrochemical performance.

[0018] (3) The h-BN / CdS photocatalyst of the present invention has higher photocatalytic activity for the degradation of tetracycline than single h-BN or CdS and catalysts prepared by direct physical mixing and tubular furnace. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 XRD spectra of different samples, a is CdS (Comparative Example 2), b is h-BN (Comparative Example 1), and c is h-BN / CdS (Example 1).

[0020] Figure 2 are Raman spectra of different samples, a is CdS (Comparative Example 2), b is h-BN (Comparative Example 1), and c is h-BN / CdS (Example 1).

[0021] Figure 3 HRTEM spectrum (a) and element mapping (b) of h-BN / CdS (Example 1).

[0022] Figure 4 These are the EPR spectra of different samples, a is CdS (Comparative Example 2), b is h-BN / CdS (Example 1), c is h-BN+CdS (Comparative Example 3), and d is h-BN-CdS (Comparative Example 4).

[0023] Figure 5 These are the transient photocurrent diagrams of different samples, a is CdS (Comparative Example 2), b is h-BN (Comparative Example 1), and c is h-BN / CdS (Example 1).

[0024] Figure 6 The photocatalytic degradation diagram of tetracycline by different samples, a is h-BN (Comparative Example 1), b is h-BN / CdS (Example 1), c is CdS (Comparative Example 2), d is h-BN+CdS (Comparative Example 3), e is h-BN-CdS (Comparative Example 4)

[0025] Figure 7 This is a stability investigation diagram of h-BN / CdS (Example 1). DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with specific implementation examples.

[0027] The h-BN in the present invention can be purchased commercially or prepared in-house, for example:

[0028] Sodium amide (NaNH2) and sodium borohydride (NaBH4) of equal mass were mixed and stirred evenly and then placed in a nickel crucible; then, in a flowing N2 atmosphere, the mixture was placed in a tube furnace and heated to 500°C at a heating rate of 5°C / min and maintained for 1 hour, and then heated to 850°C and maintained for 2 hours; finally, the obtained sample was repeatedly washed with deionized water several times and vacuum dried to obtain h-BN.

[0029] Photocatalytic activity evaluation: In a DW-01 photochemical reactor, 100 mL of a 20 mg / L tetracycline solution was added to the reactor and its initial value was determined, then 0.02 g of a catalyst was added, the light source was not turned on, the temperature was set to 30°C, the light was not turned on, air was passed (the aeration rate was 2 mL / min), the magnetic stirring was turned on (the speed was 600 rpm / min), and after reaching adsorption equilibrium, visible light was irradiated again, the magnetic stirring was turned on (the speed was 600 rpm / min) and the aeration device was turned on to pass air (the flow rate was 2 mL / min), the temperature was set to 30°C, and samples were taken and analyzed at intervals of 10 min during the illumination process. The concentration was determined by a UV-visible spectrophotometer, and the photodegradation rate Dr was calculated by the formula: Dr = (C0-C) × 100 / C0, where C0 is the concentration of tetracycline when adsorption equilibrium is reached, C is the concentration of the tetracycline solution measured at time t, and t is the reaction time.

[0030] Comparative Example 1

[0031] First, NaNH2 (3 g) and NaBH4 (3 g) were mixed and stirred evenly and then put into a nickel crucible; then, they were placed in a tubular furnace for reaction in a flowing N2 gas atmosphere with a heating rate of 5°C / min. The specific heating program was as follows: heating to 500°C and maintaining for 1 h, and then heating to 850°C and maintaining for 2 h; finally, the obtained sample was washed three times with deionized water and vacuum dried at 50°C for 12 h, recorded as h-BN.

[0032] Comparative Example 2

[0033] First, CdCl2 (0.46 g) and thiourea (0.38 g) were added to anhydrous ethanol (50 mL) solution in sequence, and stirred continuously for 6 h in a 60°C oil bath at a stirring speed of 500 rpm / min; subsequently, the sample after stirring and drying was collected, which was the cadmium sulfide precursor salt; thereafter, the cadmium sulfide precursor salt was placed in a graphite sheet and transferred to a Joule heater in an Ar atmosphere for reaction, and the program setting parameters in the Joule heater were a rapid heating mode, a voltage of 40 V, a current of 350 A, a reaction time of 20 s, and a reaction temperature of 1127°C; after the reaction, the product was collected and washed repeatedly with deionized water three times to remove residual salt; finally, the obtained sample was vacuum dried at 50°C for 12 h, and recorded as CdS.

[0034] Example 1

[0035] First, CdCl2 (0.46 g) and thiourea (0.38 g) were added to anhydrous ethanol (50 mL) solution in sequence, and stirred continuously for 6 h in a 60°C oil bath at a stirring speed of 500 rpm / min; subsequently, the sample after stirring and drying was collected, which was the cadmium sulfide precursor salt; thereafter, the cadmium sulfide precursor salt and h-BN (Comparative Example 1) were mixed in equal amounts, the mixed sample was placed in a graphite sheet, and transferred to a Joule heater in an Ar atmosphere for reaction, the program setting parameters in the Joule heater were a rapid heating mode, a voltage of 40 V, a current of 350 A, a reaction time of 20 s, and a reaction temperature range of 1127°C; after the reaction, the product was collected and washed repeatedly with deionized water 3 times to remove residual salt; finally, the obtained sample was vacuum dried at 50°C for 12 h, recorded as h-BN / CdS, in which the actual proportion of CdS was approximately 26 wt%.

[0036] Comparative Example 3

[0037] h-BN (Comparative Example 1) and CdS (Comparative Example 2) were directly mixed in an equal weight ratio and ground evenly in a mortar, which was recorded as h-BN+CdS.

[0038] Comparative Example 4

[0039] First, CdCl2 (0.46 g) and thiourea (0.38 g) were added to anhydrous ethanol (50 mL) solution in sequence, and stirred continuously for 6 h in a 60°C oil bath at a stirring speed of 500 rpm / min; then, the sample after stirring and drying was collected, which was the cadmium sulfide precursor salt; thereafter, the cadmium sulfide precursor salt and h-BN (Comparative Example 1) were mixed in equal amounts, and the mixed sample was placed in a nickel crucible; then, in a flowing inert gas Ar atmosphere, it was placed in a tubular furnace for heating reaction, the heating rate was set to 5°C / min, and the target temperature was set to 1127°C; the obtained sample was repeatedly washed 3 times with deionized water to remove residual salt; finally, the obtained sample was vacuum dried at 50°C for 12 h, and was recorded as h-BN-CdS.

[0040] like Figure 1 As shown, the characteristic diffraction peaks of h-BN / CdS at 2θ = 24.84°, 28.22°, 36.66°, 43.74°, 47.89°, 50.95°, 51.89°, 52.87°, 58.35°, 66.86°, 69.36°, 70.96°, 72.47° and 75.57° correspond to the diffraction peaks of CdS, and the diffraction peaks at 26.18° and 41.79° correspond to the (002) and (110) crystal planes of h-BN, respectively.

[0041] like Figure 2 As shown, the Raman peak of h-BN / CdS is at 300.54 cm -1 and 597.74cm -1 There are strong LO phonon and overtone 2LO peaks at 1376.67 cm-1, which are attributed to the formation of CdS. -1 The G band frequency peak belonging to h-BN is found at ; in addition, the Raman peak of h-BN / CdS shifts compared with CdS and h-BN. This change is mainly due to the change of chemical bonds or interactions between atoms and the migration of electron clouds, which indirectly proves the formation of heterojunction.

[0042] like Figure 3 As shown, in the HRTEM spectrum of h-BN / CdS, the lattice spacing between CdS and h-BN was seen, indicating a good interface contact between h-BN and CdS; in addition, the uniform distribution of B, N, Cd and S elements in h-BN / CdS was observed through the element mapping diagram.

[0043] like Figure 4 As shown, h-BN / CdS has an obvious EPR signal at g=2.003. Compared with the EPR signals of CdS, h-BN+CdS and h-BN-CdS, the EPR intensity of h-BN / CdS is stronger, indicating that more unpaired electrons are generated. This proves that a heterogeneous interface exists in the h-BN / CdS prepared by Joule heating, while the catalyst prepared by physical mixing and conventional heating does not form a heterogeneous interface.

[0044] like Figure 5 As shown, in Example 1, the actual proportion of CdS is about 26wt%, but its photocurrent response is close to the CdS curve and is much higher than the photocurrent density of h-BN, which proves that the constructed heterojunction can effectively accelerate the rapid separation and transfer of photogenerated carriers and inhibit the recombination of photogenerated electrons and holes, proving the excellent photoelectrochemical performance of the atypical type I heterojunction photocatalyst.

[0045] like Figure 6 As shown, at the same catalyst dosage, the photodegradation rate of tetracycline by h-BN / CdS reached 68.67%, which was 20 times and 1.1 times that of h-BN and CdS, respectively, showing better photocatalytic activity. At the same time, the photodegradation performance of different catalyst preparation methods was investigated. The photodegradation rate of tetracycline by the atypical type I heterojunction photocatalyst reached 68.67%, while the degradation rate of the h-BN+CdS physical mixed catalyst was only 27.88%, and the h-BN-CdS tubular furnace prepared catalyst only reached 9.14%.

[0046] Figure 7To investigate the stability of the atypical type I heterojunction photocatalyst, in the stability experiment, after each cycle experiment, the catalyst after the reaction was collected by centrifugation, washed repeatedly with deionized water several times, and then placed in a vacuum oven to dry for the next cycle. As can be seen from the figure, five experiments on the photodegradation of tetracycline were carried out, and the difference in degradation performance after five cycles was less than 1%, indicating that the atypical type I heterojunction photocatalyst prepared by Joule heating synthesis has good stability and can be used multiple times.

Claims

1. A method for preparing an atypical type I heterojunction h-BN / CdS photocatalyst, characterized in that: The steps include: (1) adding cadmium chloride and thiourea to anhydrous ethanol in sequence, heating and stirring until fully mixed, to obtain a cadmium sulfide precursor salt; (2) mixing a cadmium sulfide precursor salt and h-BN to obtain a mixed powder; placing the mixed powder in a Joule heating apparatus under a protective atmosphere for reaction, thereby obtaining an atypical type I heterojunction h-BN / CdS photocatalyst.

2. The preparation method according to claim 1, characterized in that: In step (1), the molar ratio of cadmium chloride to thiourea is 1:2-3, and the heating temperature is 50-60°C.

3. The preparation method according to claim 1, characterized in that: In step (2), the mass ratio of the cadmium sulfide precursor salt to h-BN is 1:1 to 1.

5.

4. The preparation method according to claim 1, characterized in that: In step (2), the parameters of the Joule heating device are set as follows: voltage 30-40 V, current 300-350 A, reaction time 15-20 s, reaction temperature 1000-1500° C.

5. The atypical type I heterojunction h-BN / CdS photocatalyst prepared by the preparation method according to any one of claims 1 to 4.

6. The use of the atypical type I heterojunction h-BN / CdS photocatalyst according to claim 5, characterized in that: It was used for photocatalytic degradation of tetracycline.

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