RU2801392C1 - Method for producing ferromagnetic oxygen-deficient titanium dioxide in the bronze phase - Google Patents

Abstract

FIELD: chemical synthesis.

SUBSTANCE: invention relates to the production of nanocrystalline ferromagnetic oxygen-deficient titanium dioxide with a monoclinic bronze structure, promising from the point of view of photocatalysis and solar energy. The method is based on alkaline hydrothermal treatment of titanium dioxide powder at a temperature of 170°C for 3 days, protonation of the precipitate in a solution of 0.05 M hydrochloric acid for 72 h with a solution change every 24 h, drying, followed by heat treatment at 350°C for 3 hours under vacuum or under nitrogen or argon.

EFFECT: improved electronic properties and optical activity.

1 cl, 3 dwg, 4 ex

Description

The invention relates to the field of chemical synthesis, in particular to the production of nanocrystalline ferromagnetic oxygen-deficient titanium dioxide in the bronze phase with a monoclinic structure, promising for use in the fields of photocatalysis and solar energy, for creating dilute magnetic semiconductors and electrochemical energy storage devices.

Titanium oxide semiconductors are used as photocatalysts, sensor elements, photoelectric conversion elements, and photoconductor electrodes due to high refraction of light and absorption of UV radiation. Nanocrystalline materials based on polymorphic modifications of TiO2 of a nonstoichiometric composition containing intrinsic and/or impurity structural defects are of interest due to enhanced optical and catalytic activity, improved electronic properties, and the presence of ferromagnetism at room temperature.

In nature, titanium dioxide is presented in the form of several modifications: brookite, rutile, anatase, and TiO2(B) with a monoclinic structure (metastable β-phase of TiO2). Each of the crystalline forms of TiO2 has a unique structural arrangement of TiO6 octahedra, which causes a difference in physicochemical properties, determining the direction of practical application. For example, nanosized anatase/rutile heterophase systems are preferred for use in the field of photocatalysis, however, since the TiO2 band gap is 3.0–3.3 eV, the anatase/rutile TiO2 nanocomposite as a photocatalyst is only capable of absorbing ultraviolet light (i.e. less than 5% solar radiation). TiO2(B) attracts the attention of researchers working in the field of metal-ion batteries, however, due to the wide band gap, TiO2(B) has insufficient electronic conductivity to be used as an electrode material for batteries. The creation of nonstoichiometric modifications based on TiO2 with intrinsic and/or impurity structural defects (so-called color centers) makes it possible to improve its optical activity and electronic properties. Also, defective forms of TiO2, due to the presence of ferromagnetism at room temperature, have attracted the interest of developers of semiconductor magnets in recent years.

Doping with impurities of the cationic or anionic type is today one of the main ways to create defects in TiO2. At the same time, this approach is not without drawbacks and limitations. For example, the presence of impurities can change the conditions of structural-phase transformations in TiO2 during heating (lower the temperature of such transitions), provoke particle enlargement, and lead to a decrease in the specific surface area. On the other hand, the formation of structural defects in TiO2 can be achieved in a pure way without changing the chemical composition, for example, due to heat treatment under specific conditions, in particular, in a gaseous medium. In most cases, a reducing atmosphere is used for these purposes, which leads to the production of self-doped TiO ₂ containing Ti3+ centers.

Thus, a method is known for producing black anatase TiO2 nanoparticles in an H2 atmosphere at 20 bar and a temperature of 200°C for five days [X. Chen, et al. "Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals" // Science, 2011, V.331, pp.746-50]. First, TiO2 nanoparticles were obtained by heating a reaction solution containing titanium tetraisopropoxide, Pluronic F127 block copolymer, hydrochloric acid, deionized water, and ethanol in a molar ratio of 1:0.005:0.5:15:40 at 40°C for 24 h. followed by evaporation, drying at 110°C for 24 h and calcination at 500°C. Hydrogenation was then carried out at high pressure for a long time (20 bar H2, 200°C, 5 days; 1 bar = 0.1 MPa) to form black TiO2 in the anatase modification. The resulting TiO2 nanoparticles have a core/shell structure, where the core retains an ordered structure, and the shell contains nonstoichiometric regions, which leads to increased optical absorption and a significant decrease in the band gap energy (up to 1.54 eV) of titanium dioxide.

The disadvantages of the method, in addition to the fact that they did not obtain defective TiO2 in the modification of bronzes, include energy consumption due to prolonged heat treatment and high temperatures. This approach has another significant drawback - insecurity, since the use of hydrogen is a factor of serious danger. It is also a disadvantage that the hydrogenation process must be tightly controlled, since hydrogen treatment can lead to the following consequences: 1) structural changes with the transformation of the crystalline phase into a disordered one, without changing the valence state of titanium; 2) partial reduction of titanium, but to a lower form of valency; 3) deep reduction with the separation of titanium into the metal phase; 4) reduction with the formation of metallic titanium, the subsequent oxidation of which by air in contact with the environment leads to the formation of an oxide layer with a disordered structure on the surface.

In [Wei Zhou et al. “Ordered mesoporous black TiO2 as highly efficient hydrogen evolution photocatalyst” // J. Am. Chem. Soc., 2014, V.136, pp.9280-9283], mesoporous TiO2 in the form of anatase with black color was obtained by hydrogenation at 500°C for 3 h under normal pressure. In the first step, 1 g of Pluronic P123 triblock copolymer dissolved in 15 ml of ethanol was added to 4 g of tetrabutyl titanate solution and 3.2 ml of concentrated hydrochloric acid. The resulting sol solution was gelled at room temperature over several weeks. Then the gel was annealed at 200°C for 4 hours, boiled with an aqueous solution of ethylenediamine for 48 hours at 90-100°C at pH=11-12, washed and dried at 60°C overnight. After that, the material was calcined in a nitrogen atmosphere at 350°С for 3 h, followed by annealing in air at 700°С for 2 h. The resulting black mesoporous TiO2 demonstrated an increase in the optical absorption of electromagnetic radiation up to the infrared region.

The main disadvantage of this method is the inability to obtain oxygen-deficient titanium dioxide in the structural modification of bronzes. Also, a significant disadvantage of the method is the duration of the process, which takes more than a week.

Another approach to the creation of intrinsic defects in TiO2 is thermal treatment in an inert gas environment - argon, nitrogen, or in vacuum. However, it is important that in this case the type of medium in which heat treatment takes place determines not only the degree of defectiveness of TiO2, but also the methodology of the procedure (temperature, time, etc.), which in turn determines the energy consumption of the process as a whole.

A known method for producing black-to-blue colored TiO2 having high photocatalytic activity [Huaqiao Tan, et al. “A facile and versatile method for preparation of colored TiO2 with enhanced solar-driven photocatalytic activity” // Nanoscale, 2014, V.6, pp.10216-10223]. For this, 4 g of nanosized TiO2 (P25; a mixture of anatase and rutile in a ratio of 3:1) was mixed with 1.5 g of NaBH4 by trituration for 30 min at room temperature. Then the mixture was placed in a tube furnace and heated to 300-400°C in an argon atmosphere at a heating rate of 10°C/min, holding at a given temperature for 5-60 minutes. After natural cooling to room temperature, colored TiO2 was obtained, which was washed several times with deionized water and ethanol to remove unreacted NaBH4 and dried at 70°C. By the described method, a series of defective TiO2 samples of different colors, from light blue to black, was obtained by controlling the temperature and reaction time.

The disadvantage is that the phase composition of the obtained products does not correspond to TiO2 in the form of bronzes, as well as the use of hazardous sodium borohydride as a reducing agent. In addition, there is no information about the ferromagnetic properties of the resulting product.

The possibility of obtaining deficient TiO2 in the structural modification of bronzes was demonstrated in the method described in [Yan Zhang et al. "Oxygen vacancies evoked blue TiO2(B) nanobelts with efficiency enhancement in sodium storage behaviors" // Adv. Funct. Matter, 2017, V.27, No. 27, pp.170856-1700868]. The process included several stages. At the first stage, white TiO2(B) nanoribbons were obtained. To do this, NaOH (18 g, 11.25 M) was dissolved in 40 ml of deionized water, followed by the addition of commercial titanium dioxide P25 (0.3 g) with vigorous stirring on a magnetic stirrer. The freshly prepared slurry was transferred to a 50 ml Teflon-lined stainless steel autoclave and hydrothermally treated with an oil bath for 48 hours at 160° C. with a stirring speed of 500 rpm. After the reaction, the autoclave was removed from the oil bath and cooled to room temperature. The white product was separated from the mother liquor by centrifugation and washed with deionized water until pH=9. After that, ion exchange was carried out in a dilute solution of HNO3 (0.1 M) to obtain a protonated form, which was separated by centrifugation and washed with deionized water until a neutral solution was obtained. After drying at 100°C for 12 hours in a vacuum, the product was annealed at 500°C for 2 hours at a heating rate of 3°C/min in air to obtain white nanoribbon TiO2(B). In the second step, 0.15 g of cetyltrimethylammonium bromide was dissolved in 40 ml of 0.1 M HCl in an ice bath. After intensive stirring for 15 min, 200 mg of the resulting TiO2(B) and 0.26 g of ammonium peroxydisulphate were added to the freshly prepared solution with constant stirring for 15 min. Then, 25 µl of pyrrole was added dropwise to the reaction solution of the above mixture with stirring at a temperature of 0-5°C for 24 hours. The resulting blue precipitate was separated by centrifugation and washed with deionized water. Then, after drying in a vacuum, the deposit was annealed at 500°C for 2 h at a heating rate of 3°C/min in an argon flow to form blue colored TiO2(B).

Significant disadvantages of the method include many technological methods and the use of additional reagents, which, in the form of impurities, can affect the properties of the obtained blue TiO2(B). In addition, ferromagnetic properties have not been described for the known deficient TiO2(B).

The prototype of the claimed invention under the terms of the synthesis of TiO2 in the structural modification of bronzes is the method proposed in [Ping Li et al. "Synthesis of mesoporous TiO2-B nanobelts with highly crystallized walls towards efficient H2 evolution" // Nanomaterials (Basel), 2019, V.9, No. 7, pp. 919-925]. The source of information describes the production of mesoporous TiO2(B) nanoribbons by hydrothermal synthesis. To do this, 0.5 g of P25 titanium dioxide powder was dispersed in 20 ml of 10 M NaOH solution with magnetic stirring for 30 min at room temperature, and then transferred to a 25 ml stainless steel autoclave with a polytetrafluoroethylene cell and heated at 180 °C within 48 hours. The product was washed three times with a solution of 0.1 M HCl, separated by centrifugation and dried at 80°C overnight. Mesoporous TiO2(B) nanoribbons were obtained during further heat treatment at a temperature of 450°С for 2 h in air.

In the specified source of information, evidence of the formation of oxygen-deficient TiO2(B), as well as the presence of magnetic properties, is not given, which is the main drawback of the prototype.

Thus, the technical result of the claimed invention is a method for producing oxygen-deficient titanium dioxide in the modification of bronzes, TiO2(B) with the properties of a dilute magnetic semiconductor, based on a combination of hydrothermal treatment, ion exchange and heat treatment in vacuum or in an atmosphere of nitrogen or argon. The temperature and duration of hydrothermal treatment declared in the method, the duration and mode of ion exchange, the use of vacuum or an atmosphere of nitrogen or argon in combination with heat treatment lead to the formation of color centers in the form of oxygen donor vacancies in the anionic sublattice of the bronze polymorph, which causes its coloration in dark gray color. The presence of anion vacancies increases the intensity of absorption in the visible and near infrared ranges, leading to a decrease in the band gap from 3.23 to 3.03 eV. The resulting nonstoichiometric TiO2(B) belongs to the class of dilute magnetic oxide semiconductors, since it can exhibit ferromagnetic properties at room temperature.

The technical result is achieved by a method for producing ferromagnetic oxygen-deficient TiO2(B) with a bronze structure based on alkaline hydrothermal treatment of titanium dioxide powder at a temperature of 170°C for 3 days, protonation of the resulting product in a 0.05M hydrochloric acid solution for 72 hours with by changing the solution every 24 hours, drying and subsequent heat treatment at 350°C for 3 hours in a vacuum or in an atmosphere of nitrogen or argon.

The technical result is confirmed by a number of methods and is illustrated by the following figures:

Fig.1. X-ray diffraction patterns (a) and EPR spectra (b) of TiO2(B) samples with bronze structure obtained in Example 1 (1) and Example 2 (2).

Fig.2. Diffuse reflectance spectra in the ultraviolet, visible and near infrared regions recorded in the absorption mode (a) and the corresponding Tauc dependences (b) for TiO2(B) samples with the structure of bronzes obtained in example 1 (1), according to example 2 (2) , according to example 3 (3), according to example 4 (4).

Fig.3. The dependence of magnetization on an external magnetic field at room temperature for a sample of oxygen-deficient TiO2(B) with a bronze structure, obtained according to example 1.

The phase composition of ferromagnetic oxygen-deficient TiO2(B) with a bronze structure was determined on a Bruker D8-Advance X-ray powder diffractometer in CuKα radiation in the Bragg-Brentano focal geometry. The diffraction patterns were analyzed according to the PDF-2 reference database (2015). The surface morphology was studied by scanning electron microscopy. The specific surface area, volume, and pore size distribution were estimated by the gas adsorption method using nitrogen at 77 K with data processing using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BDH) algorithms.

The nature and properties of paramagnetic centers were determined by the method of electron paramagnetic resonance (EPR). EPR spectra were recorded on a JEOL JES-X330 instrument in the X-range of operating frequencies with the following settings: microwave radiation power 2 mW, modulation amplitude 0.2 gauss, modulation frequency 100 kHz. Temperature-dependent measurements were performed in a continuous flow of nitrogen gas using a JEOL ES-13060 DVT5 variable temperature standard unit. The ultraviolet irradiation of the samples was carried out using a standard JEOL ES-USH500 ultraviolet irradiation unit. The integral intensities and values of the g-factors of the EPR lines were calibrated, respectively, according to the integral intensity and the value g=2.002293±0.000003 of the spin resonance signal on the conduction electrons of Li nanoparticles in the LiF:Li reference sample, which are in the range from 2 to 400 K do not change.

Optical properties and electronic band structure were confirmed using spectroscopy in the ultraviolet, visible and near infrared regions with registration of spectra in coordinates A (optical density of the analyzed medium) from λ (wavelength of electromagnetic radiation) on a spectrophotometer with an attachment for operating in the diffuse reflection mode at room temperature. temperature in the wavelength range of 200-1400 nm with a spectral slit width of 1 nm and a scanning step of 1 nm. Barium sulfate was used as a non-absorbing standard.

The field dependences of the samples were studied at two temperatures, 300 and 4 K, on a VSM vibromagnetometer, which is part of the PPMS 9T setup for studying the physical properties of matter. The range of variation of the external field was ±30 kOe.

Experimental data indicate that the samples obtained by the claimed method consist of nanoribbons 30-60 nm wide, 6-8 nm thick and several micrometers long with cylindrical mesopores with open ends 2-50 nm in size. The specific surface area of dark gray TiO2(B) calculated from the BET model is 36.1 m2/g. The total pore volume of the samples determined by the BDC is ~0.19 cm3/g. The shape of the pore size distribution curves confirms the presence of mesopores (about 0.12 cm3/g total volume).

X-ray phase analysis indicates the formation of a bronze phase of titanium dioxide (JCPDS 74-1940) of a monoclinic system (sp. gr. C2m) with unit cell parameters a=12.208Å, b=3.749Å, c=6.535Å and β=107.36°.

According to the data of electron paramagnetic resonance, the value of the g-factor (g=2.0037(1)) of the main component of the EPR spectra is close to the value of the g-factor of electrons localized in oxygen vacancies of titanium dioxide (the so-called F-centers related to the centers colors).

According to optical spectroscopy, the presence of oxygen vacancies leads to a "narrowing" of the band gap from 3.23 to 3.03 eV and a noticeable change in optical absorption in the visible and near infrared ranges (Fig.2).

An increase in the magnetization of TiO2(B) as a result of thermal treatment in vacuum indicates that the imperfection of the crystal structure (the presence of vacancies in the anionic sublattice) plays the only significant role in the appearance of magnetic properties in the obtained oxygen-deficient TiO2(B), which appear already at room temperature. temperature.

The essence of the invention is confirmed by the following examples.

Example 1. First, 0.26 g of commercial titanium dioxide in the anatase modification with an average particle size of ~100 nm was dispersed in 40 ml of an aqueous solution of NaOH (14 M) with continuous stirring on a magnetic stirrer for 1-2 hours. For 3 days at a temperature of 170°C, they were kept in a steel autoclave with a Teflon liner with a volume of 50 ml. After completion of the process, the reaction mixture was cooled to room temperature, the precipitate was separated by centrifugation. By treating the filtered product in a solution of 0.05 M HCl, taken in a 5-fold excess, Na+ was exchanged for H+. The ion exchange process was carried out for 72 h, every 24 h the HCl solution was replaced. The resulting protonated form was separated from the mother liquor, washed with deionized water to a neutral level (pH=7-8), dried at a temperature of 80°C. At the final stage of the synthesis, heat treatment was carried out in a tube furnace at 350°C for 3 h under vacuum, which led to the formation of a dark gray sample. The resulting product belongs to oxygen-deficient TiO2(B) (Figure 1), has a band gap of 3.03 eV (Figure 2) and belongs to the class of dilute magnetic oxide semiconductors since it exhibits ferromagnetic properties at room temperature (Figure 3) .

Example 2 Alkaline hydrothermal treatment and ion exchange were carried out as described in Example 1. Temperature treatment was carried out in a tube furnace at 350° C. for 3 hours in ambient air. As a result, a titanium dioxide sample was obtained with the structural organization of white bronzes, without ferromagnetic properties.

Example 3. The stages of autoclaving in an alkaline environment and the displacement of Na + cations from the structure by protons were carried out as described in example 1. Temperature treatment was carried out in an argon atmosphere using a tube furnace. Thus obtained TiO2(B) with nanoribbon morphology - had a light gray color and has a high optical activity (Figure 2), as well as magnetic properties. The band gap of such TiO2(B) was 3.22 eV.

Example 4 Hydrothermal synthesis and the subsequent ion exchange step with separation and drying were carried out in the same way as in example 1. The final heat treatment was carried out under nitrogen in a quartz tube furnace. The synthesized material has a composition represented by the oxygen-deficient TiO2(B) phase. The color of the product is grey. Morphology - ribbon-like nanoparticles. The material is characterized by good optical activity in the visible and near infrared ranges (Figure 2). The band gap energy is 3.20 eV.

Claims (1)

A method for producing oxygen-deficient TiO ₂ (B) with a bronze structure based on alkaline hydrothermal treatment of titanium dioxide powder, ion exchange, drying and subsequent thermal treatment, characterized in that hydrothermal synthesis is carried out at a temperature of 170°C for 3 days, sediment protonated in a solution of 0.05 M hydrochloric acid for 72 h with a change of solution every 24 h, and heat treatment at 350°C for 3 h is carried out in a vacuum or in an atmosphere of nitrogen or argon.