CN107413387B - A kind of preparation method of manganese-doped titanium dioxide nanofiber material - Google Patents

Abstract

The invention provides a preparation method of a manganese-doped titanium dioxide nanofiber material, which belongs to the technical field of photocatalytic materials and has high photocatalytic degradation efficiency for organic pollutants in the environment. The preparation method includes: dispersing the amphiphilic cationic short peptide in water, dispersing by ultrasonic, adjusting the pH value of the solution, and placing it at room temperature for more than a week to obtain a short peptide self-assembly solution; dispersing a titanium dioxide precursor, potassium permanganate The solution and the manganese nitrate solution are sequentially added to the amphiphilic short peptide self-assembly solution, vortexed and mixed, and after the reaction at room temperature, centrifugation is performed to obtain a gray-brown precipitate; the gray-brown precipitate is washed and heated to remove the short peptide template to obtain Manganese-doped titanium dioxide nanofibrous material. The invention can be used for preparing manganese-doped titanium dioxide nanofiber material.

Description

A kind of preparation method of manganese-doped titanium dioxide nanofiber material

technical field

The invention relates to the technical field of photocatalytic materials, in particular to a preparation method of a manganese-doped titanium dioxide nanofiber material.

Background technique

Titanium dioxide is non-toxic, inexpensive, highly active, resistant to ultraviolet light radiation, strong oxidants, and acid and alkali resistant. application prospects. However, the inherent forbidden band width of titanium oxide is too wide (3.0 eV for anatase and 3.2 eV for rutile), so that it can only be used by ultraviolet light with a wavelength less than 387 nm (the proportion of sunlight in sunlight). less than 5%) excitation, but not sensitive to visible light accounting for more than 50%. The low light utilization rate severely limits the application effect of titanium oxide. To this end, researchers have carried out a lot of research work to adjust the forbidden band width of titanium oxide and expand its photoresponse range to the visible light region. At present, the main modification technologies for titanium oxide photocatalysts at home and abroad include ion doping (see Mao Y. et al., "German Applied Chemistry", 2014, 53: 10485), surface noble metal deposition (Yun J. et al., "American Chemistry"). will apply materials to interfaces", 2015, 7: 2055), semiconductor composites (Thapa A. et al., "Nano Research", 2014, 7: 1154) and metal complexes and dye photosensitization (Altin I. et al., "Dilute" and Water Treatment, 2016, 57:16196). Among them, doping other elements into the titanium oxide phase to expand the absorption boundary to the visible light region is considered to be an efficient and simple method, which has attracted the attention of many researchers.

Doping modifications of titanium oxide include non-metal ion doping (eg, carbon, nitrogen, sulfur, fluorine, etc.) and metal ion doping (eg, iron, cobalt, copper, vanadium, gallium, etc.). Among them, non-metal ions generally exist as anions in the main phase, while metal ions exist as cations in titanium oxide. Compared with non-metal doping, which requires complex process control (such as anion doping by pyrotechnic reaction), transition metal cation doping is relatively simple and is considered to be a very promising doping method. Among the many transition metal elements, multivalent metals such as manganese and iron have been proved to enhance the absorption of visible light by titanium oxide. The intermediate energy band can constitute a significant change in the electronic energy band structure. This curved intermediate band (IB) will act as a "stepping stone" to efficiently participate in the electron transport of the valence band during light absorption at different wavelengths, resulting in a significant red shift in the material's light absorption.

As a photocatalytic material, the photocatalytic performance is not only related to its light absorption ability, but also controlled by its photogenerated electron and hole separation and transport properties. One-dimensional nanomaterials have a certain quantum size effect, which can improve the quantum yield of photocatalytic reactions; in terms of optical properties, one-dimensional nanomaterials have a large aspect ratio, which can significantly enhance the scattering and absorption of light; in terms of microstructure , One-dimensional nanomaterials have a small electron migration path, which is conducive to the transmission of electrons and delays the destruction of electrons. Therefore, by preparing manganese-doped TiO2 nanofibers with controllable structure, it is expected to obtain higher solar light utilization efficiency and photocatalytic conversion effect, and be applied in the treatment of organic pollutants, photoelectric/photochemical conversion and other fields.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a preparation method of manganese-doped titanium dioxide nanofiber material, which can improve the utilization rate and photocatalytic efficiency of titanium dioxide for sunlight, thereby improving the photocatalytic conversion efficiency and the degradation effect of organic pollutants.

The invention provides a preparation method of manganese-doped titanium dioxide nanofiber material, comprising:

The amphiphilic cationic short peptide molecules are dispersed in water by ultrasonic wave, the pH value of the solution is adjusted, and the solution is placed at room temperature for more than 1 week to obtain a self-assembly solution of the amphiphilic short peptide;

The titanium dioxide precursor, the potassium permanganate solution and the manganese nitrate solution were added to the self-assembly solution of the amphiphilic short peptide in turn, vortexed and mixed, and after reacting at room temperature for 10-24 hours, centrifuged to obtain a gray-brown precipitate, and washed Precipitation and heating at 300-500°C for 2-10 hours to remove short peptide templates to obtain manganese-doped titanium dioxide nanofiber materials;

Wherein, the hydrophobic unit of the amphiphilic cationic short peptide molecule is composed of 3-6 leucine, isoleucine, glycine and valine, and the hydrophilic head group is composed of 1-2 lysine, histidine It is composed of acid and arginine, and can form nanofibrous structure in aqueous solution.

Optionally, the titanium dioxide precursor is selected from at least one of n-butyl titanate, isopropyl titanate, and diammonium bis(2-hydroxypropionic acid)dihydroxide.

Preferably, the concentration of the amphiphilic short peptide is 0.5-10 mmol L ^-1 , the concentration of the titanium dioxide precursor is 1-10 mmol L -1 , the concentration of potassium permanganate is 0.001-0.1 mmol L ^-1 , the concentration of manganese nitrate is 0.001-0.1 mmol L ^-1 , The concentration is 0.0015-0.15 mmol L ^-1 .

Preferably, after ultrasonically dispersing the amphiphilic short peptide in water, the pH value of the solution is adjusted to a range of 5-9.

Optionally, the washing treatment specifically includes alternately using pure water and ethanol to wash the gray-brown precipitate.

Preferably, the rotational speed of the centrifugal treatment is 5000-10000 rpm, and the time is 20-60 minutes.

Another aspect of the present invention provides a manganese-doped titanium dioxide nanofiber material prepared by the method for preparing a manganese-doped titanium dioxide nanofiber material according to any one of the above technical solutions.

Another aspect of the present invention provides an application of the manganese-doped titanium dioxide nanofiber material as described in the above technical solution in photocatalytic degradation of organic pollutants.

The invention provides a preparation method of manganese-doped titanium dioxide nanofiber material. Compared with the prior art, the method can recognize and catalyze the molecular recognition and catalysis of organic templates in the presence of amphiphilic short peptide self-assembled nanofibers. The action induces the nucleation and growth of titanium dioxide on the surface of the template, and at the same time, manganese ions are introduced into the titanium dioxide through the redox reaction of potassium permanganate and manganese nitrate regulated by the peptide template, and finally the manganese-doped titanium dioxide nanofiber material is formed. Since the doping is guided by molecular recognition, it has obvious advantages in the doping site, doping structure and composition of the material. The manganese-doped titanium dioxide nanofiber material prepared by the method has a forbidden band width of 1.55 eV, and the absorption in the visible light region is significantly enhanced. At the same time, the doping of manganese ions and the existence of one-dimensional nanostructures greatly improve the separation and transport efficiency of carriers, and its photoelectric conversion and transport efficiency are greatly increased. The degradation experiments of model organic pollutants show that, compared with the commercial nano-TiO2 photocatalyst P25, the photocatalytic performance of the manganese-doped TiO2 nanofiber material prepared by the present invention is obviously improved, and the catalytic sites caused by aggregation are avoided. Point reduction and catalytic efficiency are reduced, and the stability of recycling is greatly improved. The method prepares the manganese-doped titania nanofiber material under the condition of room temperature and near-neutral aqueous solution, and the whole synthesis process has the advantages of simplicity, energy saving and environmental protection.

Description of drawings

Fig. 1 is the transmission electron microscope photograph of the manganese-doped titanium dioxide nanofiber material prepared by the embodiment of the present invention;

Fig. 2 is the ultraviolet-visible absorption spectrum of manganese-doped titania nanofiber material and pure titania nanofiber material prepared in the embodiment of the present invention;

Fig. 3A Ti 2p X-ray photoelectron spectrum of manganese-doped titania nanofiber material and pure titania nanofiber material prepared in the embodiment of the present invention;

3B Mn 2p X-ray photoelectron spectrum of the manganese-doped titania nanofiber material prepared in the embodiment of the present invention;

Fig. 4 is the photocurrent response curve of manganese-doped titania nanofiber material and pure titania nanofiber material prepared in the embodiment of the present invention;

5A is the photocatalytic methylene blue degradation curve of the manganese-doped titania nanofiber material and the commercial titania nanomaterial prepared in the embodiment of the present invention;

5B is the photocatalytic Rhodamine B degradation curve of the manganese-doped titania nanofiber material and the commercial titania nanomaterial prepared in the embodiment of the present invention;

6A is a concentration-reaction time curve of cyclic photocatalytic degradation of methylene blue by manganese-doped titania nanofiber materials;

Figure 6B is the concentration-reaction time curve of cyclic photocatalytic degradation of methylene blue for commercial titanium dioxide (P25).

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

An embodiment of the present invention provides a method for preparing a manganese-doped titanium dioxide nanofiber material, comprising:

S1: ultrasonically disperse the amphiphilic cationic short peptide molecule in water, adjust the pH value of the solution, and place it at room temperature for more than 1 week to obtain the self-assembly template of the amphiphilic cationic short peptide

In this step, a one-dimensional organic template is constructed by self-assembly of amphiphilic cationic short peptides. Specifically, after ultrasonically dispersing the amphiphilic cationic short peptide in water, the pH value of the system is adjusted to near neutral, and it is left to stand for more than a week to obtain a self-assembly solution of the amphiphilic short peptide. In this step, since an amphiphilic cationic short peptide is used, the near-neutral pH value can protonate the side chain of the cationic amino acid residue, provide electrostatic effect and stabilize the self-assembled structure formed, and the near-neutral pH value can stabilize the self-assembled structure formed. Neutral pH environment is also beneficial to obtain suitable titanium dioxide formation rate.

S2: adding titanium dioxide precursor, potassium permanganate solution and manganese nitrate solution to the amphiphilic short peptide self-assembly solution in sequence, vortexing and mixing, reacting at room temperature for 10-24 hours, and centrifuging to obtain a gray-brown precipitate , washed the precipitate, and heated at 300-500 °C for 2-10 hours to remove the short peptide template to obtain a manganese-doped titanium dioxide nanofiber material.

In this step, in order to obtain manganese-doped titanium dioxide nanofiber material, titanium dioxide precursor, potassium permanganate solution and manganese nitrate solution are sequentially added to the amphiphilic short peptide assembly solution, so that the formation of titanium dioxide and high manganese The redox reactions of potassium phosphate and manganese nitrate proceed under the regulation of amphiphilic short peptide self-assembly templates. It can be understood that the reaction time depends on the sufficiency of the reaction, and can be 10, 12, 14, 16, 18, 20, 22, 24 hours or any other value within the above range. The resulting precipitate is washed to remove unreacted titanium dioxide precursor, potassium permanganate and manganese nitrate. The purpose of heating is to remove the short peptide template and form the corresponding crystal structure. The heating temperature and heating time are interdependent, and those skilled in the art can adjust it within the above range. If the temperature is high, the time can be short, but the heating temperature is too high. The structure and morphology of the material are changed, and the temperature is too low to achieve the purpose of removing the short peptide template.

In an embodiment of the present invention, the hydrophobic unit of the amphiphilic cationic short peptide is composed of 3-6 leucine, isoleucine, glycine, and valine, and the hydrophilic head group is composed of 1-2 It is composed of lysine, histidine and arginine, and can form nanofibrous structure in aqueous solution. In this example, amphiphilic cationic short peptides are composed of hydrophilic amino acid residues and hydrophobic amino acid residues. The type, quantity and position of amino acids determine the shape of the short peptide self-assembly, that is, the shape of the template. and structure, while having a significant impact on the structure and properties of the formed manganese-doped titania nanofibrous materials. In addition to attracting the oppositely charged inorganic precursors to aggregate in the vicinity of the hydrophilic head group on the surface of the short peptide template, it has an important catalytic effect on the formation of titanium dioxide, as well as on the subsequent structure of manganese ions and titanium dioxide in nanofiber materials. The row plays an important guiding role.

In an embodiment of the present invention, the titanium dioxide precursor is selected from at least one of n-butyl titanate, isopropyl titanate, and diammonium bis(2-hydroxypropionic acid)dihydroxide. In this example, due to the different hydrolysis rates of titania precursors, selecting different precursors can adjust the generation rate of titania, so as to obtain a reaction system with suitable nucleation rate and growth rate to prepare manganese-doped manganese with a designed structure Titanium dioxide nanofiber material.

In an embodiment of the present invention, the concentration of the amphiphilic short peptide is 0.5-10 mmol L ^-1 , the concentration of the titanium dioxide precursor is 1-10 mmol L ^-1 , and the concentration of potassium permanganate is 0.001-0.1 mmol L -1 ^-1 , the concentration of manganese nitrate is 0.0015-0.15 mmol L ^-1 . In this embodiment, in order to obtain a manganese-doped titania nanofiber material with higher light absorption efficiency and photocatalytic ability to degrade organic matter, it is necessary to control the generation rate of titania and the doping rate of manganese ions (mainly Mn ⁴⁺ and Mn ^{3 +} ) , the concentration of titanium dioxide precursor and manganese-containing compound (potassium permanganate solution and manganese nitrate solution) must be controlled within the above-mentioned ratio range, those skilled in the art can understand that within the above-mentioned concentration range, titanium dioxide precursor, high manganese The solution prepared by potassium acid and manganese nitrate can meet the needs of subsequent preparation of manganese-doped titanium dioxide nanofiber materials that meet the needs, so it can be adjusted within the above range as needed. For example, the concentration of the titanium dioxide precursor is 1, 2, 4, 6, 8, 10 mmol L ^-1 , etc., or any value that meets the above conditions is acceptable. Similarly, the concentrations of potassium permanganate and manganese nitrate are any values satisfying 0.001-0.1 mmol L ^-1 and 0.0015-0.15 mmol L ^-1 , respectively. In this embodiment, the amphiphilic short peptide mainly provides a template for the preparation of manganese-doped titania nanofiber materials. In addition to providing template guidance, it also provides catalysis for the hydrolysis and polycondensation reaction of titania. Therefore, the concentration of amphiphilic short peptides is not only related to their ability to form stable one-dimensional assemblies, but also affects the reaction rate of titanium dioxide. It can be understood that the amphiphilic short peptide within the above concentration range can meet the requirements for preparing manganese-doped titania nanofiber materials, and those skilled in the art can choose according to needs.

In an embodiment of the present invention, after dispersing the amphiphilic cationic short peptide in water, the pH value is adjusted to a range of 5-9. In the present embodiment, adjusting the pH value within the range of 5-9 can ensure that the entire reaction system reacts in a near-neutral environment, so as to ensure that the entire synthesis process is environmentally friendly. It can be understood that the pH value can be adjusted to 6, 7, 8, or any other value within the above range.

In this step, the gray-brown precipitate is washed with pure water and ethanol in turn, in order to remove unreacted titanium dioxide precursor, potassium permanganate, manganese nitrate and other compounds.

In an embodiment of the present invention, the rotational speed of the centrifugal treatment is 5000-10000 rpm, and the time is 20-60 minutes. In this embodiment, in order to separate and obtain the manganese-doped titanium dioxide nanofiber material, centrifugation is used to separate it. It can be understood that when the rotational speed and time of the centrifugal treatment are limited within the above ranges, the separation effect of nanomaterials can be better, and those skilled in the art can adjust within the above ranges according to the actual situation. Specifically, the rotational speed of the centrifugal treatment can be 4000, 5000, 6000, 7000, 8000, 9000, 10000 rpm, and the time can be 20, 30, 40, 50, 60 minutes, etc. In addition, as long as the rotational speed of the centrifugal treatment and Any value within the above range may be used for the time.

Another embodiment of the present invention provides a manganese-doped titanium dioxide nanofiber material prepared by the method for preparing a manganese-doped titanium dioxide nanofiber material according to any one of the above embodiments. In the manganese-doped titanium dioxide nanofiber material prepared by the embodiment of the present invention, the manganese element mainly exists in the titanium dioxide in the state of Mn ³⁺ and Mn ⁴⁺ ions, and replaces part of the Ti ⁴⁺ ions into the lattice structure of titanium dioxide, resulting in Lattice shrinkage. Multivalent manganese ion doping provides multiple intermediate energy levels for TiO2, which broadens the photoresponse range of the material. The doped TiO2 has enhanced visible light absorption capability, and doping can effectively inhibit photogenerated electron-hole. composite, so that it exhibits good photocatalytic degradation properties for organic pollutants. On the other hand, the diameter of the manganese-doped titania nanofiber material prepared by the embodiment of the present invention is between 10 and 20 nanometers, has a large specific surface area and surface binding sites, and a small electron migration path reduces the photogenerated The probability of electron and hole recombination further improves the photocatalytic degradation efficiency of the material.

The manganese-doped titanium dioxide nanofiber material prepared by the embodiment of the present invention has high specific surface area, good visible light response properties and photoelectric transmission behavior, and thus has good photocatalytic properties. When used as a photocatalyst for the degradation of organic pollutants, it has more excellent photocatalytic activity than commercial nano-titania, and has good reusability, so it is suitable for use as a photocatalytic reagent for organic pollutants in the environment.

In order to introduce the preparation method of the manganese-doped titania nanofiber material provided by the embodiments of the present invention more clearly and in detail, the following will be described with reference to specific embodiments.

Example 1

Preparation of Manganese-Doped Titanium Dioxide Nanofibrous Materials

(1) Preparation of amphiphilic cationic short peptide self-assembly solution

Dispersing a certain mass of amphiphilic cationic short peptides in water, ultrasonically dispersing, adjusting the pH of the solution to 5-9, and placing at room temperature for 7-14 days to obtain an amphiphilic cationic short peptide self-assembly solution;

(2) Synthesis of manganese-doped titania nanofibers

A) Adding titanium dioxide precursor, potassium permanganate solution and manganese nitrate solution to the self-assembly solution of amphiphilic short peptides in sequence, in the solution, the amphiphilic cationic short peptide, titanium dioxide precursor, potassium permanganate, nitric acid The concentrations of manganese were 0.5-10 mmol L ^-1 , 1-10 mmol L ^-1 , 0.001-0.1 mmol L ^-1 , 0.0015-0.15 mmol L ^-1 , mixed by vortex, reacted at room temperature for 10-24 hours, and then centrifuged. (5000-10000rpm, 20-60 minutes) to obtain gray-brown precipitate;

B) The obtained gray-brown precipitate is washed alternately with pure water and ethanol, and then heated at 300-500° C. for 2-10 hours to obtain a manganese-doped titanium dioxide nanofiber material.

Example 2

Morphology and structural characterization of manganese-doped titania nanofibers

High-resolution transmission electron microscope, model: JEM-2100UHR, instrument manufacturer: JEOL, accelerating voltage: 200kV.

In this example, the morphology and structure of the manganese-doped titanium dioxide nanofiber material were observed with high-resolution transmission electron microscopy. Specifically, the sample was dispersed in ethanol, dropped on a copper mesh coated with carbon film, and dried using a special sample rod. Put it into the sample chamber, evacuate it, adjust the appropriate resolution and focal length, choose the appropriate exposure time, and take an image.

It was found that the obtained sample had a one-dimensional nanostructure with a diameter of 10-20 nanometers and a length of several hundred nanometers or even several micrometers, as shown in Figure 1. Outside the nanofibers, there are less random depositions, indicating that the amphiphilic cationic short peptide template has a good template-directing effect on inorganic minerals and their precursors. The reaction is carried out under the control of an organic template.

Example 3

Characterization of light-responsive properties of manganese-doped titania nanofibers

Using a UV-Vis spectrophotometer (with a diffuse reflectance measuring device-integrating sphere, produced by Shimadzu Corporation, UV-1700PharmaSpec), the scanning speed is medium speed, the slit width is 1nm, and the measurement range is 200-800nm. For reference, the sample was coated on a barium sulfate sheet and pressed for measurement.

As shown in Figure 2, the light absorption of pure titanium dioxide is all in the ultraviolet region, and there is basically no absorption in the visible light region greater than 400 nm. After modification by manganese doping, the response of the material in the visible light region is significantly enhanced. According to the Kubelak-Munk formula It is calculated that the forbidden band width can reach 1.55 eV, which can be adjusted by changing the doping content and doping method.

Example 4

Composition and chemical state analysis of manganese-doped titania nanofibers

ESCALAB 250 X-ray electron energy spectrometer of ThermoFisher SCIENTIFIC Company was used, Al target and Kα rays were selected. Spread the powder sample evenly on the aluminum foil, cover it with a piece of aluminum foil, flatten it with a hydraulic press, open it up, stick the pressed sheet-like sample on the sample holder with conductive tape, put it into the sample chamber of the instrument and vacuumize it for 10 hours before testing. The principle is to use X-rays to irradiate the sample, so that the inner electrons or valence electrons of atoms or molecules are excited and emitted, and the electrons excited by photons become photoelectrons, and the energy of photoelectrons can be measured. When the intensity is the ordinate, a photoelectron spectrum can be drawn to obtain the composition of the substance to be measured.

It can be seen from Fig. 3A that the binding energy peak positions of Ti 2p _1/2 and Ti 2p _3/2 in the pure TiO sample are 459.2 eV and 464.9 eV, respectively, and the spin energy interval is 5.7 eV, corresponding to the Ti ⁴ of TiO ₂ ⁺ ions, which are in good agreement with the standard titania spin energy interval. When doped with manganese, the binding energy peak position of Ti 2p characteristic peak shifts to the direction of lower binding energy, but the spin energy interval does not change, indicating that the Ti element in TiO2 still exists as Ti ⁴⁺ . In order to understand the valence state of manganese in the system, the Mn 2p characteristic peak spectrum was analyzed by deconvolution integration. Figure 3B shows the deconvolution spectrum of the Mn2p characteristic peak of the manganese-doped titanium dioxide sample. It can be seen that the Mn 2p _3/2 peak can be divided into two peaks through the peak separation, and the peak positions fall at 641.9eV and 643.1eV, respectively. The peaks correspond to the Mn 2p _3/2 peaks of Mn ³⁺ and Mn ⁴⁺ respectively. The relative content of manganese ions in different valence states can be calculated by the ratio of Mn ³⁺ /Mn ⁴⁺ . The binding energy of Mn ⁴⁺ ions is increased by 0.7 eV compared with manganese dioxide, which is caused by the chemical interaction of manganese ions with Ti ⁴⁺ and O ^2- present in the main phase. The above results show that in the manganese-doped titanium dioxide nanofiber material system, manganese exists in the form of Mn ³⁺ ions and Mn ⁴⁺ ions, which replace part of Ti ⁴⁺ into the titanium dioxide lattice, indicating that the system has multiple doped ions. Heterogeneous energy level, which will be more beneficial to broaden the photoresponse range of TiO2.

Example 5

Characterization of Photoelectric Response Properties of Manganese-Doped Titanium Dioxide Nanofibers

The test was performed using an electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., CHI660D). A three-electrode system was adopted, the prepared sample was used as the working electrode, and the working area was 1 cm ² . The counter electrode is a platinum sheet electrode, the reference electrode is a saturated dry mercury electrode (SCE), and the electrolyte is a 1 Mol L ^-1 Na ₂ SO ₄ solution. Use a 300W xenon lamp as a light source, add a filter when using, and the wavelength range is >420nm. The preparation process of the working electrode is as follows: take 0.2 g of the sample and mix it with 0.06 g of polyethylene glycol (PEG, molecular weight 4000), add 1 mL of ethanol and fully stir and grind to make a slurry. A blank area of 1 cm × 1 cm was glued on the conductive glass (ITO) with transparent glue, the prepared slurry was coated on this area, and the coating was dried at room temperature, and the prepared electrode was placed in a muffle furnace for 400 calcined at ℃ for 30 min.

First, the open circuit potential of the sample is tested in the open circuit potential-time mode under dark conditions. Due to the difference in the morphology and composition of the sample, the surface charge distribution is different, and the test time of the open circuit potential is not fixed. In the experiment, when the open circuit potential is 30s When the internal change is less than 0.0001V, the system is considered to reach a stable state, and the open circuit potential value at this time is recorded. The voltage of the open circuit potential is applied to the working electrode to prevent the interference of the potential of the sample itself to the photocurrent, to ensure that the current after the lamp is turned on is all generated by the illumination, and the current changes with time are recorded. During the test, the light source was first stabilized for 100s and then turned on. With 30s as a time interval, the lights were turned on and off in turn. This cycle was repeated 5 times, and the curve of the current changing with the switch light source was recorded.

As can be seen from Figure 4, a strong current signal was detected at the moment of turning on the light, indicating that the sample was excited to generate a large number of photoelectrons. With the increase of illumination time, the photocurrent showed a downward trend, but the amplitude was not large. This is because the photo-generated electrons and holes are in continuous rapid recombination during the illumination period, and the recombination rate is slightly faster than the rate at which the sample is excited by the illumination to generate new photo-generated electrons, thus showing a state of decreasing photocurrent on a macroscopic scale. The faster the recombination rate, the steeper the current curve and the shorter the time to reach equilibrium. After turning on the light for 30s, turn off the light and avoid light. Since the photo-generated electrons and holes are rapidly recombined, and no new photo-generated electrons are generated, the photocurrent drops to zero almost instantaneously. After being protected from light for 30s, the light was turned on again, and the sample was excited by light to generate a large number of photoelectrons, and the photocurrent surged instantly. By comparison, it is found that the photocurrent intensity of the pure titanium dioxide sample is the lowest during the turn-on period, while the photocurrent intensity increases rapidly after doping with manganese ions, which can reach more than 4 times that of the undoped sample. This is because the doping of metal ions can act as a shallow trap for electrons (or holes), thereby reducing the recombination efficiency of photogenerated electrons and holes.

Example 6

Characterization of photocatalytic degradation of organic pollutants by manganese-doped titania nanofibers

A UV-Vis spectrophotometer (produced by Shimadzu Corporation, UV-1700PharmaSpec) was used, the slit width was 1 nm, and the scanning range was 200-800 nm. The samples were placed under a xenon lamp and irradiated with visible light with a wavelength greater than 420 nm, and samples with different irradiation times were taken for testing.

Figure 5A shows the methylene blue (MB) catalytic degradation experiments of manganese-doped titania samples and commercial titania (P25) under visible light (>420 nm). MB reached the maximum absorbance at 663 nm, and the absorbance of the dye decreased with the illumination time, indicating that the dye was decomposed. In the blank experiment with no catalyst added, the absorbance of MB showed a very slight decrease under visible light irradiation, indicating that its aqueous solution is relatively stable under visible light, and the self-decomposition rate is slow, which will not affect the experimental results. Compared with commercialized nano-TiO2 (P25), manganese ion-doped TiO2 has better photocatalytic efficiency. Figure 5B shows the catalytic degradation experiments of rhodamine B (RhB) under visible light (>420 nm) for manganese-doped titanium dioxide samples and commercial titanium dioxide (P25). Similar to the degradation of MB, manganese-doped titanium dioxide nanofibers showed better performance photocatalytic degradation effect.

The cycling stability of photocatalysts is another very important evaluation criterion besides photocatalytic activity. Taking commercial nano-titania (P25) as a reference, the cycling stability of the manganese-doped titania nanofiber material was investigated, as shown in Figure 6A. After 5 cycles, the photocatalytic efficiency of the manganese-doped titania nanofiber material did not decrease significantly, and the degradation rate of methylene blue remained above 93%. After 5 cycles of P25, the degradation rate of methylene blue was reduced from 76% to 59%, as shown in Figure 6B, indicating that the manganese-doped titania nanofiber material has better cycling stability than P25. The obvious difference in the cycle stability of the two materials is mainly due to the difference in the morphology and structure of the materials. The photocatalytic degradation of organic compounds in titanium dioxide mainly occurs on the surface of the material. The morphology of P25 is nanoparticles, and the aggregation of nanoparticles is easy to occur during the photocatalysis process, which leads to the decrease of the specific surface area. The nanofiber material has a one-dimensional structure, which can not only effectively shorten the migration path of electrons, improve the transmission efficiency of photogenerated carriers, and enhance the photocatalytic activity of the material itself, but also can avoid the aggregation phenomenon during the cycle process, and further improve the material's performance. Cyclic stability.

Claims (5)

1. A preparation method of a manganese-doped titanium dioxide nanofiber material is characterized by comprising the following steps:

ultrasonically dispersing amphiphilic cationic short peptide molecules into water, adjusting the pH value of the solution to be within the range of 5-9, and placing at room temperature for more than 1 week to obtain a self-assembly solution of the amphiphilic short peptide;

sequentially adding a titanium dioxide precursor, a potassium permanganate solution and a manganese nitrate solution into the amphiphilic short peptide self-assembly solution, uniformly mixing by vortex, reacting at room temperature for 10-24 hours, centrifuging to obtain a grey brown precipitate, washing the precipitate, heating at the temperature of 300 ℃ and 500 ℃ for 2-10 hours to remove a short peptide template to obtain a manganese-doped titanium dioxide nanofiber material;

the hydrophobic unit of the amphiphilic cationic short peptide consists of 3-6 leucine, isoleucine, glycine and valine, the hydrophilic head group consists of 1-2 lysine, histidine and arginine, and a nanofiber structure can be formed in an aqueous solution;

the titanium dioxide precursor is selected from at least one of n-butyl titanate, isopropyl titanate and bis (2-hydroxypropionic acid) diammonium dihydroxide titanium;

the concentration of the amphiphilic short peptide is 0.5-10 mmol.L^-1The concentration of the titanium dioxide precursor is 1-10 mmol.L^-1, the concentration of potassium permanganate is 0.001-0.1 mmol.L^-1The concentration of manganese nitrate is 0.0015-0.15 mmol.L^-1。

2. The method according to claim 1, wherein the washing treatment comprises washing the grayish brown precipitate with pure water and ethanol alternately.

3. The method as claimed in claim 1, wherein the rotation speed of the centrifugation treatment is 5000-10000rpm for 20-60 minutes.

4. A manganese-doped titanium dioxide nanofiber material prepared by the method for preparing a manganese-doped titanium dioxide nanofiber material as claimed in any one of claims 1 to 3.

5. Use of the manganese-doped titanium dioxide nanofiber material of claim 4 in photocatalytic degradation of organic pollutants.

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