Preparation and piezoelectric-photocatalytic application of oxygen vacancy-containing bismuth ferrite nano material

Abstract

The invention belongs to the field of nanomaterial piezoelectric-photocatalysis, and relates to preparation and piezoelectric-photocatalysis application of an oxygen vacancy-containing bismuth ferrite nanomaterial. The oxygen vacancy-containing bismuth ferrite nano-material is successfully prepared through a hydrogen calcination method, on one hand, the light absorption range and the light absorption intensity of the bismuth ferrite nano-material are remarkably improved due to the existence of oxygen vacancies, the absorption and utilization efficiency of sunlight is higher, and on the other hand, the oxygen vacancies serve as active sites on the surface of a photocatalyst, so that the photocatalytic activity of the photocatalyst is improved. The activity of the catalyst can be effectively improved, so that the piezoelectric photocatalytic performance is remarkably improved. The efficiency of 2-hour piezoelectric-optical synergistic catalytic degradation of organic pollutants (rhodamine B and tetracycline hydrochloride) is 96.5% and 72.17% respectively, and is 4.1 times and 1.7 times that of a pure-phase bismuth ferrite nano material. Meanwhile, the prepared oxygen vacancy-containing bismuth ferrite has relatively strong magnetism, is convenient to recycle in application, and is strong in operability, simple and convenient in process flow and wide in application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of piezoelectric-photocatalysis, and specifically relates to the preparation of a bismuth ferrite containing oxygen vacancies (BFO-OVs) nanomaterial and the application of the material in piezoelectric-photosynergistic catalytic degradation of organic pollutants. Background Art

[0002] The treatment of environmental pollution has long been a common concern of all sectors of society. Effective treatment of environmental pollution will effectively reduce the impact of industrial production waste, especially some toxic substances, organic dyes, heavy metal ions and some non-degradable substances on people's lives. Therefore, strict management of industrial wastewater is an effective means of controlling water pollution. Effective prevention and control of industrial water use can not only make full use of resources, but also reduce or avoid environmental pollution, and continuously improve the water environment on which people depend for survival.

[0003] As water pollution becomes increasingly serious, the development of new water pollution treatment methods and technologies has been valued by countries around the world. Photocatalytic technology is a green and environmentally friendly method for water pollution treatment, but the conversion efficiency of catalysts is often limited by light utilization, low charge separation and diffusion efficiency, and poor photostability. In order to optimize charge separation and light utilization, many scholars have proposed using the built-in electric field generated by the light, piezoelectric and ferroelectric effects of semiconductor catalysts to enhance carrier transport and separation. Piezoelectric-photosynergistic catalytic technology can simultaneously utilize solar energy and mechanical energy to degrade organic pollutants into non-toxic inorganic substances and is therefore quite popular.

[0004] Photocatalytic activity is usually affected by the rapid recombination of photogenerated carriers. The piezoelectric effect generates an internal electric field that can be used to more effectively separate positive and negative charges. Therefore, piezoelectric-photocatalysts have both piezoelectric and photocatalytic properties. The internal electric field induced by the piezoelectric effect can be used to separate photogenerated electron-hole pairs, thereby further improving the catalytic performance. Generally, carriers are generated under the irradiation of light, and the photogenerated electron-hole pairs are separated and migrate to the catalyst surface. At the same time, external mechanical strain causes the positive and negative charges to deform, generating a piezoelectric photocatalyst center with an internal electric field. Under the action of the internal electric field, electrons and holes are attracted to the catalyst surface by positive and negative charges, thereby improving the separation efficiency of carriers.

[0005] In recent years, researchers have reported semiconductor materials with low cost and suitable band gap width. Among them, p-type bismuth ferrite (BiFeO3, abbreviated as BFO) semiconductor has the advantages of narrow band gap (~2.2eV) and good carrier transport capacity, and is an ideal photocatalytic semiconductor material. At the same time, oxygen vacancies (OV) can expand the photoresponse area of ​​photocatalysts, and due to the charge separation and promotion of localized electrons in OV, oxygen vacancies (OV) are the most active active sites on the surface of photocatalysts. The development of bismuth-based catalysts with oxygen vacancies will effectively improve the catalytic performance.

[0006] Therefore, the present invention adopts a hydrogen calcination method to prepare bismuth ferrite containing oxygen vacancies (BFO-OVs) nanomaterials, and further studies the degradation efficiency of the composite material on organic pollutants under piezoelectric catalysis, photocatalysis, and piezoelectric-photocatalysis conditions. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide an oxygen vacancy nano-piezoelectric photocatalytic material for efficient piezoelectric photocatalytic degradation of organic pollutants and convenient magnetic recovery, overcoming the limitation of low catalytic efficiency caused by the energy band structure and piezoelectric properties or optical properties of BiFeO3. At the same time, the prepared oxygen vacancy bismuth ferrite nano-material has strong magnetism and is easy to recover in the application of organic pollutant degradation.

[0008] The present invention is a preparation and piezoelectric photocatalytic application of a bismuth ferrite nanomaterial containing oxygen vacancies, which mainly involves: the preparation and characterization of bismuth ferrite nanomaterials containing oxygen vacancies, and the study of the performance of the nanomaterial in degrading organic pollutants, which is specifically carried out through the following steps:

[0009] Step 1, dissolving bismuth nitrate and ferric nitrate in a dilute nitric acid solution, forming a uniform transparent colorless solution by ultrasonic treatment, then adjusting the pH value of the solution to make it alkaline, and then centrifuging to obtain a precipitate and drying it, and finally obtaining a certain amount of solid;

[0010] Step 2: Grind the solid obtained in step 1, and put the ground powder into a tube furnace, and heat it from room temperature at a certain heating rate until it reaches the calcination temperature, and finally obtain a calcined solid powder;

[0011] Step 3, the calcined solid powder obtained in step 2 is acid-washed and water-washed, and then centrifuged, alternately three times, and finally washed twice with ethanol, and dried under vacuum conditions to finally obtain bismuth ferrite nanoparticles;

[0012] The molar ratio of bismuth nitrate to ferric nitrate in step 1 is 1:1 to 1:1.2;

[0013] The dilute nitric acid solution described in step 1 is prepared by mixing concentrated nitric acid with water in a volume ratio of 1:30 to 40;

[0014] The pH value of step 1 is alkaline and is 9 to 10;

[0015] The drying conditions in step 1 are: oven temperature of 40 to 80° C., preferably 60° C.; drying time of 6 to 24 hours, preferably 12 hours;

[0016] The heating rate in step 2 is 3-7°C / min, preferably 5°C / min;

[0017] The calcination temperature in step 2 is 350-550°C, preferably 500°C;

[0018] The calcination time in step 2 is 20 to 80 minutes, more preferably 60 minutes;

[0019] The centrifugal speed in step 3 is 9000-11000 r / min;

[0020] The vacuum drying conditions in step 3 are as follows: temperature of 40 to 80° C., preferably 60° C.; vacuum degree of -20 to -40 kPa, preferably -30 kPa; time of 6 to 24 h, preferably 12 h;

[0021] Step 4: Place the bismuth ferrite obtained in step 3 into a tube furnace, evacuate the tube furnace tube and introduce N2, repeat three times to completely remove the air in the tube furnace;

[0022] Step 5, heating the tubular furnace in step 4 from room temperature to a calcination temperature at a certain heating rate under the protection of N2, introducing H2 for a certain period of time for calcination when the calcination temperature is reached, and naturally cooling the calcined solid powder to room temperature under the protection of N2, and the obtained solid powder is the oxygen-vacancy bismuth ferrite nanomaterial;

[0023] The heating rate in step 5 is 4-8°C / min, preferably 5°C / min;

[0024] The calcination temperature in step 5 is 200-500°C, preferably 300°C;

[0025] The H2 introduced in step 5 is provided by a hydrogen generator;

[0026] The time of introducing H2 in step 5 is 15 to 60 minutes, preferably 30 minutes;

[0027] The flow rate of H2 introduced in step 5 is 200-400 mL / min, preferably 330 mL / min;

[0028] Step 6, application of oxygen-vacancy bismuth ferrite nanomaterials in piezoelectric-photocatalytic degradation of organic pollutants, the specific steps are as follows: taking oxygen-vacancy bismuth ferrite nanomaterials and placing them in rhodamine B (RhB) or tetracycline hydrochloride (TCH) solution, stirring in the dark to reach adsorption and desorption equilibrium, and then applying mechanical force, visible light or their synergistic effect, taking a certain amount of solution at a given time interval, collecting the supernatant after high-speed centrifugation, and using a UV-visible spectrophotometer to analyze the concentration change of the RhB solution or the TCH solution. At the same time, in order to avoid the influence of thermal effects during the reaction process, the reactor is cooled by circulating water throughout the process to keep the temperature of the reactor at room temperature;

[0029] The concentration of the RhB solution in step 6 is 5 mg / L, and the concentration of the TCH solution is 5-20 mg / L;

[0030] The mechanical force and visible light described in step six are provided by an ultrasonic cleaning machine (300 W, 40 kHz) and a xenon lamp (300 W, filter: λ>420 nm), respectively.

[0031] The present invention characterizes the oxygen vacancy-containing bismuth ferrite nanomaterial and performs piezoelectric-photocatalytic performance testing by using instruments such as an X-ray powder diffractometer, a scanning electron microscope, an electron paramagnetic resonance instrument, and an ultraviolet-visible spectrophotometer.

[0032] Advantages and effects of the present invention:

[0033] The invention is simple to operate. Bismuth ferrite nanomaterials containing oxygen vacancies are prepared by a hydrogen calcination method. Bismuth ferrite is modified by introducing oxygen vacancies. The presence of oxygen vacancies significantly improves the light absorption range and light absorption intensity of the bismuth ferrite nanomaterials on the one hand. Therefore, the bismuth ferrite nanomaterials with oxygen vacancies have higher absorption and utilization efficiency for sunlight. On the other hand, oxygen vacancies serve as active sites on the surface of photocatalysts, which can effectively improve the activity of the catalyst, thereby significantly improving the piezoelectric-photocatalytic performance.

[0034] Under the action of piezoelectric photocatalysis, the oxygen-vacancy bismuth ferrite nanomaterials achieved effective degradation of RhB and TCH in 2 hours, with degradation efficiencies of 96.5% and 72.17%, respectively. At the same time, the prepared oxygen-vacancy bismuth ferrite has high magnetic properties and is easy to recycle in the application of organic pollutant degradation.

[0035] The present invention solves the problems of poor separation efficiency of photogenerated carriers and limited light absorption capacity of pure bismuth ferrite nanomaterials, effectively improves the efficiency of piezoelectric photocatalysis, effectively degrades organic pollutants, broadens the practical application of sewage treatment, and has broad application prospects in the field of piezoelectric-photoelectronics. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 X-ray powder diffraction (XRD) patterns of pure bismuth ferrite and bismuth ferrite nanomaterials containing oxygen vacancies;

[0037] Figure 2 This is a scanning electron microscope (SEM) image of bismuth ferrite nanomaterials containing oxygen vacancies;

[0038] Figure 3 The electron paramagnetic resonance spectra (EPR) of pure bismuth ferrite and bismuth ferrite nanomaterials containing oxygen vacancies are shown;

[0039] Figure 4 The fluorescence spectra (PL) of pure bismuth ferrite, bismuth ferrite containing oxygen vacancies and pure phase bismuth ferrite nanomaterials are shown;

[0040] Figure 5 The ultraviolet visible diffuse reflectance spectra (UV-vis DRS) of pure bismuth ferrite and bismuth ferrite nanomaterials containing oxygen vacancies;

[0041] Figure 6 The degradation curves of pure bismuth ferrite and bismuth ferrite nanomaterials containing oxygen vacancies on RhB solution under piezoelectric (U), light (L), and piezoelectric-light synergistic (U+L) catalytic conditions respectively;

[0042] Figure 7 The degradation curves of pure bismuth ferrite and bismuth ferrite nanomaterials containing oxygen vacancies on TCH solution under piezoelectric (U), light (L), and piezoelectric-light synergistic (U+L) catalytic conditions respectively;

[0043] Figure 8 The piezoelectric-photosynergistic (U+L) degradation curve of oxygen vacancy bismuth ferrite nanomaterials to different concentrations of TCH;

[0044] Fig. 9 Schematic diagram of magnetic recovery of bismuth ferrite nanomaterials containing oxygen vacancies. DETAILED DESCRIPTION

[0045] The present invention is described in detail below in conjunction with specific implementation modes.

[0046] The degradation efficiency is calculated according to the following formula:

[0047] D=(C0-C) / C0*100%

[0048] D: Degradation efficiency

[0049] C0: initial concentration

[0050] C: Concentration after degradation reaction

[0051] Implementation 1:

[0052] Step 1: Dissolve bismuth nitrate and ferric nitrate in a molar ratio of 1:1.1 in a dilute nitric acid solution (the volume ratio of concentrated nitric acid to water is 1:30), and form a uniform, transparent, colorless solution by ultrasonic treatment. Then adjust the pH value of the solution to 10, and then centrifuge to obtain a precipitate, which is dried in an oven at 60°C for 12 hours to finally obtain a certain amount of solid;

[0053] Step 2: Grind the solid obtained in step 1, and put the ground powder into a tube furnace, heat it from room temperature to 500° C. at a heating rate of 5° C. / min, and calcine for 60 min to finally obtain a calcined solid powder;

[0054] Step 3: The calcined solid powder obtained in step 2 is acid-washed and water-washed, and then centrifuged, alternately three times, and finally washed twice with ethanol, and dried at 60° C. for 12 h under vacuum conditions to finally obtain pure phase bismuth ferrite nanoparticles;

[0055] Step 4: Place the bismuth ferrite obtained in step 3 into a tube furnace, evacuate the tube furnace tube and introduce N2, repeat three times to completely remove the air in the tube furnace;

[0056] Step 5, heating the tubular furnace in step 4 to 300°C at a heating rate of 5°C / min under nitrogen protection, and when reaching the calcination temperature, introducing 330mL / min of H2 for calcination for 30min, and the solid powder after calcination is the oxygen vacancy bismuth ferrite nanomaterial;

[0057] Step six, take 25 mg of the oxygen vacancy nanomaterial obtained above and place it in 25 mL of RhB solution (5 mg / L) or TCH solution (10 mg / L), stir in the dark to reach adsorption and desorption equilibrium, and then degrade organic pollutants for 2 hours under the action of ultrasound (ultrasonic cleaning machine, 300 W, 40 kHz), light (xenon lamp, 300 W, filter: λ>420 nm), and ultrasound-assisted light (piezoelectric-light). Sampling is performed every 20 minutes, and the degradation efficiency is calculated by measuring the absorbance of the samples taken.

[0058] The following test is used to verify the effect of the invention:

[0059] 1. Preparation and characterization of bismuth ferrite nanomaterials containing oxygen vacancies

[0060] The crystal phase structure of the oxygen vacancy-containing bismuth ferrite nanomaterial and the pure phase bismuth ferrite nanomaterial prepared in Example 1 was characterized by X-ray powder diffraction (XRD). Figure 1The XRD patterns of oxygen vacancy bismuth ferrite nanomaterials and pure phase bismuth ferrite nanomaterials are shown in the figure. As shown in the figure, the diffraction peaks 2θ=22.4°, 31.8°, 32.0°, 39.5°, 45.8° and 51.3° correspond to the (012), (104), (110), (202), (024) and (116) crystal planes of bismuth ferrite, respectively. The XRD diffraction pattern of bismuth ferrite nanomaterials is consistent with the standard PDF card (JCPDS: 71-2494), indicating that it has a rhombus structure and a space group of R3c. However, after hydrogen reduction, the characteristic peaks of elemental bismuth appear, and the diffraction peaks 2θ=27.2° and 37.9° correspond to the (012) and (104) crystal planes of elemental bismuth, respectively. The diffraction peaks of bismuth ferrite after hydrogen calcination are similar to those of pure phase BFO, indicating that the crystal structure of bismuth ferrite is relatively stable during hydrogen calcination.

[0061] The morphology of the oxygen vacancy bismuth ferrite nanomaterial prepared in Example 1 was observed by scanning electron microscopy (SEM). Figure 2 As shown, the prepared oxygen-vacancy-containing bismuth ferrite nanomaterial has an irregular particle morphology and an average particle size of about 40 to 160 nm.

[0062] The oxygen vacancy content of the bismuth ferrite and the bismuth ferrite nanomaterial containing oxygen vacancies prepared in Example 1 was analyzed by electron paramagnetic resonance spectroscopy (EPR). Figure 3 As shown in the figure, a pair of sharp symmetrical peaks are related to the trapped electrons at the defect position, indicating the presence of oxygen vacancies. Compared with pure bismuth ferrite, the EPR signal of bismuth ferrite containing oxygen vacancies is more obvious, indicating a higher concentration of oxygen vacancies.

[0063] The optical properties of the bismuth ferrite and bismuth ferrite nanomaterials containing oxygen vacancies prepared in Example 1 were compared and studied by fluorescence spectroscopy (PL) and solid ultraviolet visible absorption spectroscopy (UV-Vis DRS). Figure 4 As shown in the figure, the PL intensity of the oxygen vacancy bismuth ferrite nanomaterial is significantly lower than that of bismuth ferrite. This fluorescence quenching phenomenon indicates that the prepared oxygen vacancy bismuth ferrite nanomaterial effectively improves the separation efficiency of photogenerated electrons and holes. Figure 5 As shown in Figure 2, bismuth ferrite nanomaterials exhibit an absorption edge at 660nm due to their inherent narrow band gap. However, compared with pure bismuth ferrite nanomaterials, the absorption edge of bismuth ferrite nanomaterials containing oxygen vacancies shifts to longer wavelengths, and the absorption spectrum gradually red-shifts. The red-shifted optical absorption edge indicates that the utilization efficiency of solar energy is higher than that of pure bismuth ferrite.

[0064] 2. Study on the degradation performance of organic pollutants

[0065] The degradation performance of bismuth ferrite and bismuth ferrite nanomaterials containing oxygen vacancies prepared in Example 1 under the action of ultrasound, light, and ultrasound-assisted light was determined by ultraviolet spectrophotometry.

[0066] like Figure 6 As shown, the degradation efficiency of RhB by oxygen vacancy bismuth ferrite under the conditions of piezoelectric (U), light (L), and piezoelectric-light (U+L) synergistic catalysis is 44.7%, 74.2%, and 96.5%, respectively, which are 2.6 times, 3.4 times, and 4.1 times the degradation efficiency of pure bismuth ferrite under the same catalytic conditions.

[0067] like Figure 7 As shown, the efficiency of TCH degradation of oxygen vacancy bismuth ferrite under the conditions of piezoelectric (U), light (L), and piezoelectric-light (U+L) synergistic catalysis was 29.32%, 50.49% and 72.17%, respectively. Compared with pure bismuth ferrite, the degradation efficiency was increased by 68%, 21% and 42% under the same catalytic conditions.

[0068] like Figure 8 As shown, oxygen vacancy bismuth ferrite has good degradation performance for TCH solutions of different concentrations under piezoelectric photocatalytic conditions. Within 2 hours, the degradation efficiency of 20 mg / L, 15 mg / L, and 10 mg / L TCH solutions is 53.92%, 63.67%, and 72.17%, respectively.

[0069] like Fig. 9 As shown, the left picture is a schematic diagram of oxygen vacancy bismuth ferrite dispersed in the solution after degrading organic pollutants. A magnet is placed outside the beaker. After standing for a period of time, the material is attracted by the magnet and the solution gradually clarifies (as shown in the right picture). Therefore, the material can be recovered by magnet attraction in subsequent practical applications.

[0070] In summary, the present invention discloses a method for preparing a nanomaterial for piezoelectric synergistic photocatalytic degradation of organic pollutants and its effective removal of organic pollutants (RhB solution and TCH solution). Oxygen-vacancy bismuth ferrite nanomaterials were successfully prepared by hydrogen calcination. At the same time, the prepared oxygen-vacancy bismuth ferrite has strong magnetism and can be easily recovered in subsequent applications. This good coupling of mechanical energy and light energy broadens the application of piezoelectric materials in photocatalysis, creates a new way to design efficient catalysts and make full use of relatively mild low-frequency mechanical energy in nature, and helps to promote the practical application of piezoelectric photocatalysis.

Claims

1. Preparation of oxygen vacancy bismuth ferrite nanomaterials and their piezoelectric photocatalytic applications, Features The preparation and application method are as follows: Step 1, dissolving bismuth nitrate and ferric nitrate in a dilute nitric acid solution, forming a uniform, transparent and colorless solution by ultrasonic treatment, then adjusting the pH value of the solution to make it alkaline, and then centrifuging to obtain a precipitate and drying it, and finally obtaining a certain amount of solid; Step 2: Grind the solid obtained in step 1, and put the ground powder into a tube furnace, and heat it from room temperature at a certain heating rate until it reaches the calcination temperature, and finally obtain a calcined solid powder; Step 3, the calcined solid powder obtained in step 2 is acid-washed and water-washed, and then centrifuged, alternately three times, and finally washed twice with ethanol, and dried under vacuum conditions to finally obtain bismuth ferrite nanoparticles; Step 4: Place the bismuth ferrite obtained in step 3 into a tube furnace, evacuate the tube furnace tube, and introduce N 2 , repeated three times to completely remove the air in the tube furnace; Step 5: Heat the tubular furnace in step 4 from room temperature to calcination temperature at a certain heating rate under nitrogen protection, and introduce H for a certain period of time after reaching the calcination temperature. 2 The solid powder after calcination is heated to N 2 The solid powder obtained is bismuth ferrite nanomaterial containing oxygen vacancies. Step 6: placing the material obtained in step 5 in a rhodamine B (RhB) or tetracycline hydrochloride (TCH) solution, stirring in the dark to reach adsorption and desorption equilibrium, then applying mechanical force, visible light or their synergistic effect, taking a certain amount of solution at a given time interval, collecting the supernatant after high-speed centrifugation, and analyzing the concentration change of the RhB solution or the TCH solution using a UV-visible spectrophotometer. At the same time, in order to avoid the influence of thermal effects during the reaction process, the reactor is cooled by circulating water throughout the process to keep the temperature of the reactor at room temperature.

2. The preparation method according to claim 1, Features: The molar ratio of bismuth nitrate to ferric nitrate in step 1 is 1:1 to 1:1.2; The dilute nitric acid solution described in step 1 is prepared by mixing concentrated nitric acid with water in a volume ratio of 1:30 to 40; The pH value of step 1 is alkaline and is 9 to 10; The drying conditions described in step 1 are: oven temperature of 40 to 80° C. and time of 6 to 24 hours.

3. The preparation method according to claim 1, Features: The heating rate in step 2 is 3-7°C / min; The calcination temperature in step 2 is 350-550°C; The calcination time in step 2 is 20 to 80 minutes.

4. The preparation method according to claim 1, Features: The centrifugal speed in step 3 is 9000-11000 r / min; The vacuum drying conditions in step 3 are as follows: temperature of 40 to 80° C.; vacuum degree of -20 to -40 kPa; and time of 6 to 24 h.

5. The method according to claim 1, Features: The heating rate in step 5 is 4-8°C / min; The calcination temperature in step 5 is 200-500°C; The H introduced in step 5 2 Supplied by hydrogen generator; Step 5: 2 The time is 15 to 60 minutes; Step 5: 2 The flow rate is 200~400mL / min.

6. The application method according to claim 1, Features: The concentration of the RhB solution in step 6 is 5 mg / L, and the concentration of the TCH solution is 5-20 mg / L; The mechanical force and visible light described in step six are provided by an ultrasonic cleaning machine (300 W, 40 kHz) and a xenon lamp (300 W, filter: λ>420 nm), respectively.

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