CN1308070C - Hydrothermal method for preparing nano crystal Bi2WO6 powder photocatalytic material with visible light activity - Google Patents

Abstract

The invention proposes a hydrothermal-thermal treatment method for preparing nanocrystalline Bi ₂ WO ₆ powder photocatalytic material with visible light activity. In this method, bismuth nitrate and sodium tungstate are used as raw materials, which are prepared into a mixed solution, and then put into a reaction kettle and subjected to hydrothermal treatment at a temperature of 100-200°C for 10-48 hours, then the reaction kettle is taken out, and the system is cooled to room temperature Finally, the obtained product is repeatedly washed with deionized water until neutral, then dried under vacuum, and finally heat-treated at 100-1000°C for 0.5-10 hours to obtain a Bi ₂ WO ₆ nanocrystalline photocatalytic material with visible light activity. This method has the advantages of low synthesis temperature, large specific surface area and small particle size of the obtained sample. The prepared Bi ₂ WO ₆ nanocrystalline photocatalytic material can not only show photocatalytic activity in the ultraviolet region. Moreover, it shows photocatalytic activity in the visible light region such as sunlight or indoor lighting.

Description

Hydrothermal-thermal treatment method for preparing nanocrystalline Bi2WO6 powder photocatalytic material with visible light activity

technical field

The invention relates to a preparation method of a nanocrystalline Bi ₂ WO ₆ powder photocatalyst material with large specific surface area, small particle size and high visible light activity, and also relates to the technical field of low-temperature preparation of visible light photocatalytically active photocatalysts.

technical background

Since Fujishima and Honda published a paper on photocatalytic water splitting on _TiO2 electrodes in Nature in 1972, photocatalytic technology has received widespread attention in environmental governance and energy development. The research and development of highly active photocatalytic materials has gradually become a hot spot in current domestic and foreign research. However, most of the photocatalysts reported now have low efficiency and wide band gaps, and can only show photocatalytic activity in the ultraviolet region. In the solar spectrum, ultraviolet light (below 400nm) is less than 5%, while visible light with a wavelength of 400-750nm accounts for 43%. Therefore, in order to effectively utilize sunlight or indoor light, it is of great significance to study photocatalytic materials with high photocatalytic activity under visible light irradiation; the preparation of photocatalytic materials with high visible light activity will be a step forward for the development of photocatalytic technology to be practical. inevitable trend of globalization. In recent years, a series of single-crystal solid-state photocatalysts have emerged for photocatalytic water splitting. Among them, layered oxides, such as K ₄ Nb ₆ O ₁₇ , BiInNbO ₇ , Sr ₂ Nb ₂ O ₇ and In _1-x Ni _x TaO ₄ have higher photocatalytic water splitting than TiO ₂ and SrTiO ₃ The photocatalytic activity has attracted extensive attention. Bi ₂ WO ₆ is a visible light photocatalyst with a narrow band gap, which can be used for visible light photocatalytic degradation of organic pollutants. However, such catalysts are generally prepared by high-temperature solid-state reaction methods. However, samples prepared by high-temperature solid-state reaction methods not only lack homogeneity, but also cause a large decrease in specific surface area, which is a fatal shortcoming of solid-state reaction methods. In order to prepare nanocrystalline _Bi2WO6 powder photocatalysts with high visible-light photocatalytic activity, large specific surface area, and small particle _size , a simple and effective preparation method needs to be investigated.

Contents of the invention

The object of the present invention is to overcome the shortcoming of existing method, propose a kind of simple and effective preparation method, the nanocrystalline Bi ₂ WO ₆ powder photocatalyst prepared by this method has high visible light photocatalytic activity, has ratio by solid phase reaction The nanocrystalline Bi ₂ WO ₆ photocatalyst prepared by this method has higher specific surface area and smaller particle size.

According to the current research status at home and abroad and the feasibility of preparing nanocrystalline Bi ₂ WO ₆ powder photocatalyst by hydrothermal method, the idea of the present invention is to prepare nanocrystalline Bi ₂ WO ₆ photocatalyst with high specific surface area by hydrothermal-heat treatment method. As we all know, changing the preparation method and the doping of catalysts are two important means to improve the catalytic activity of photocatalysts. The idea of the present invention is to reduce the crystallization temperature of the nanocrystalline Bi ₂ WO ₆ photocatalyst and increase the specific surface area by changing the preparation method, thereby achieving the purpose of improving its photocatalytic activity.

According to the above idea, the object of the present invention can be achieved through the following solutions.

A method for preparing nanocrystalline Bi ₂ WO ₆ powder photocatalytic material with visible light activity. It is characterized by a hydrothermal-heat treatment method. This hydrothermal-heat treatment method uses bismuth nitrate and sodium tungstate as raw materials, and a precipitation reaction occurs under hydrothermal conditions. After the system is cooled to room temperature, the resulting product is washed with deionized water. Repeated washing until neutral, then vacuum drying, and finally heat treatment, wherein the molar concentration of bismuth nitrate is 0.01-0.5M, the molar concentration of sodium tungstate is 0.01-1M, the molar ratio of bismuth nitrate and sodium tungstate It is 1:3～1:2. The pH value of the solution is 1-12; the hydrothermal temperature is 100-200°C, and the hydrothermal time is 10-48 hours; the heat treatment temperature is 100°C-1000°C, and the heat treatment time is 0.5-10 hours.

The method for preparing nanocrystalline Bi ₂ WO ₆ powder photocatalytic material with visible light activity of the present invention, wherein the molar concentration of bismuth nitrate is preferably 0.04-0.1M, the molar concentration of sodium tungstate is 0.08-0.2M, bismuth nitrate and tungstic acid The molar ratio of sodium is 1:3-1:2.

Preferably, the pH value of the solution is 6-8.

Preferably, the hydrothermal temperature is 140°C to 160°C.

The preferred hydrothermal time is 20 to 26 hours.

Preferably, the heat treatment temperature is 500°C to 600°C.

Preferably, the heat treatment time is 1 to 3 hours.

The hydrothermal-heat treatment method of the present invention can be used not only to prepare Bi ₂ WO ₆ nanocrystalline photocatalyst with visible light activity, but also to prepare other semiconductor powders, such as titanium dioxide, silicon dioxide, zinc oxide, zirconium oxide and the like.

The photocatalytic activity of the nanocrystalline Bi ₂ WO ₆ powder photocatalyst of the present invention is characterized by decomposing formaldehyde gas in the air by illuminating the nanocrystalline Bi ₂ WO ₆ coating. Formaldehyde (HCHO) is a common compound widely used in various industrial and civil products, so we choose it as the simulated polluting organic compound. The photocatalytic oxidative decomposition of formaldehyde is based on the following chemical reactions:

(1)

The light source for the photocatalysis experiment was a 15W fluorescent lamp. The photocatalytic degradation of formaldehyde was carried out in a 15L airtight rectangular plexiglass container. The preparation process of the photocatalyst sample is to apply _{the Bi2WO6} suspension evenly to three petri dishes with a diameter of 9 cm, and the mass of the photocatalyst for each test is 0.5 g. The petri dishes are dried at 100 ° C for 3 h and cooled to Take it out at room temperature for photocatalytic experiments. During the experiment, the petri dish was put into the photocatalytic reactor, which was directly connected with a desiccator containing calcium chloride in order to control the initial humidity in the reactor. The distance between the petri dish and the ultraviolet lamp is kept at about 5 cm, and the effective radiation area of the petri dish is 190 cm ² . Then drop a certain amount of formaldehyde into the reactor. After the formaldehyde is completely volatilized, the initial concentration of formaldehyde in the reactor is always controlled at (180±10)ppm before the start of each photocatalytic reaction. The concentration of formaldehyde, carbon dioxide and water vapor in the reactor was detected and analyzed online with a photoacoustic IR multigas monitor (INNOVA air tech instruments model 1312), and a set of data was read every minute. The initial temperature in the reactor was about 25°C, and the initial concentration of water vapor was 1.20±0.01 vol%. During the photocatalytic reaction, the concentration ratio of formaldehyde degradation and carbon dioxide generation is almost maintained at a ratio of 1:1. The experiment of photocatalytic degradation of formaldehyde was carried out at room temperature for 20 h with fluorescent lamp as the light source.

The photocatalytic activity of the samples can be quantitatively estimated by the degradation rate of formaldehyde (R(%)). The degradation rate R (%) can be calculated by formula 4-1:

Degradation rate $R(\%)=\frac{{\left[\mathrm{gas}\right]}_0-{\left[\mathrm{gas}\right]}_t}{{\left[\mathrm{gas}\right]}_0}\×100\%---\left(2\right)$

[gas] ₀ and [gas] _t denote the initial equilibrium concentration of formaldehyde gas and the concentration in the reaction, respectively.

The characterization of the physical properties of the nanocrystalline Bi ₂ WO ₆ powder photocatalyst includes: X-ray diffraction (XRD) characterizes the phase structure and grain size of the Bi ₂ WO ₆ photocatalyst. The particle size and morphology of nanocrystalline Bi ₂ WO ₆ photocatalyst were observed by scanning electron microscope (SEM). The infrared absorption spectrum of the sample was tested on an infrared spectrometer of the model Nialet-60SXB produced in the United States. UV-Vis diffuse reflectance spectrometer (UV2505, Shimadzu, Japan) was used to estimate the bandgap width of nanocrystalline Bi ₂ WO ₆ photocatalyst. The specific surface area of the nanocrystalline Bi ₂ WO ₆ powder photocatalyst was characterized by a nitrogen adsorption instrument model AUTOSORB-1 (Quantachrome Instruments, USA).

Description of drawings

Figure 1 XRD pattern of Bi ₂ WO ₆ nanocrystalline powder prepared at different heat treatment temperatures:

Figure 2 SEM photos of nanocrystalline Bi ₂ WO ₆ powder photocatalysts prepared at different heat treatment temperatures:

Figure 3 UV-Vis slow reflectance spectra of nanocrystalline Bi ₂ WO ₆ powder photocatalysts prepared at different heat treatment temperatures

Fig.4 Effect of heat treatment temperature on the photocatalytic activity of Bi ₂ WO ₆

In the figure, A1, A2, A3, A4, A5 and A6 are XRD patterns of Bi ₂ WO ₆ powders heat-treated at 100, 300, 400, 500, 600 and 700°C, respectively; B1 and B2 are prepared by heat-treating at 500 and 700°C, respectively. SEM photographs of nanocrystalline Bi ₂ WO ₆ powder photocatalysts; UV-visible slow reflection spectra of nanocrystalline Bi ₂ WO ₆ powder photocatalysts prepared by heat treatment at 400, 500, 600 and 700 °C for C1, C2, C3 and C4, respectively.

Detailed ways

Example 1:

The method for preparing nanocrystalline Bi ₂ WO ₆ powder photocatalyst by hydrothermal method is as follows: first weigh 9.8g Na ₂ WO ₄ 2H ₂ O and dissolve it in 150ml distilled water, stirring continuously; then weigh 7.2g Bi(NO ₃ ) ₃ ·5H ₂ O was added to the above solution and stirred continuously. After stirring for about 15 minutes, the above reaction solution was put into a 200ml reaction kettle, and hydrothermally treated at 150° C. for 24 hours. The reactor was taken out, and after the reactor was cooled to room temperature, the precipitate was separated, and the obtained precipitate was repeatedly washed with distilled water until neutral, and finally dried in vacuum at 80°C to obtain Bi ₂ WO ₆ powder. The obtained Bi ₂ WO ₆ powder was heat-treated at 500 °C for 2 h. That is, the nanocrystalline Bi ₂ WO ₆ photocatalyst is obtained. This sample exhibited the best photocatalytic activity, and its visible light photocatalytic degradation rate reached 68.7%.

Example 2:

In order to test the effect of hydrothermal treatment temperature on the catalytic activity of nanocrystalline Bi ₂ WO ₆ photocatalyst, except for the different hydrothermal temperature, other reaction conditions such as: reactant concentration, amount of solvent water, hydrothermal time, heat treatment temperature, etc. Example 1 is exactly the same. It was found that when the hydrothermal treatment temperature was 100, 150 and 180 °C, the visible light photocatalytic degradation rates were 13.7%, 68.7% and 69.7%, respectively. The reason may be that when the hydrothermal temperature is too low, the reaction is incomplete, the yield is low, there are many impurities, and the photocatalytic activity is weak; when the hydrothermal temperature _rises to 150 °C, the nanocrystalline _Bi2WO6 photocatalyst shows Good photocatalytic activity; when the temperature is further increased, the photocatalytic activity of the sample does not change significantly, which causes a waste of energy. Therefore, the optimum hydrothermal temperature for preparing nanocrystalline Bi ₂ WO ₆ photocatalyst by hydrothermal method is 150-160℃.

Example 3:

In order to test the effect of hydrothermal time on the catalytic activity of nanocrystalline Bi ₂ WO ₆ photocatalyst, except for the different hydrothermal time, other reaction conditions such as: reactant concentration, amount of solvent water, hydrothermal temperature, heat treatment temperature, etc. Example 1 is exactly the same. It was found that when the hydrothermal treatment time was 10, 20, 24 and 40, the visible light photocatalytic degradation rates were 23.5%, 56.9%, 68.7% and 70.2%, respectively. The reason may be that when the hydrothermal time is too short, the reaction is incomplete, the yield is low, there are many impurities, and the photocatalytic activity is weak; when the hydrothermal _time is 24 hours, the nanocrystalline _Bi2WO6 photocatalyst shows good Photocatalytic activity; while further prolonging the hydrothermal time, the photocatalytic activity of the sample did not increase significantly. Therefore, the optimal hydrothermal time for preparing nanocrystalline Bi ₂ WO ₆ photocatalyst by hydrothermal method is about 24 hours.

Example 4:

In order to test the effect of heat treatment temperature on the catalytic activity of nanocrystalline Bi ₂ WO ₆ photocatalyst, in addition to different heat treatment temperatures, other reaction conditions such as: reactant concentration, amount of solvent water, heat treatment time, hydrothermal temperature, hydrothermal time, etc. Exactly the same as Example 1. The results are shown in Figure 1, Figure 2, Figure 3, Figure 4 and Table 1.

Fig. 1 is the XRD patterns of Bi ₂ WO ₆ powders heat-treated at different temperatures (100, 300, 400, 500, 600 and 700°C). It can be seen from the figure that the unheated Bi ₂ WO ₆ powder prepared by the hydrothermal method shows a certain degree of crystallization; as the heat treatment temperature increases, the diffraction peak of the Bi ₂ WO ₆ powder becomes stronger, and the diffraction peak The half maximum width of the peak is narrowed, which is caused by the increased crystallinity of the sample. XRD results show that when the heat treatment temperature rises to 400°C, the sample exhibits a well-crystallized square structure (JCPDS Card: 26-1044). At the same time, with the increase of heat treatment temperature, the average grain size of Bi ₂ WO ₆ powder decreased (as shown in Table 1).

Figure 2 (B1) and (B2) are the SEM pictures of the nanocrystalline Bi ₂ WO ₆ photocatalysts obtained by heat treatment at 500°C and 700°C for 2 hours, respectively. It can be seen from the figure that the particle size of the sample obtained by heat treatment at 500°C is smaller; however, when the heat treatment temperature rises to 700°C, the particle size of the sample grows significantly. This also shows that as the heat treatment temperature increases, the particles of the sample grow up and the specific surface area decreases (as shown in Table 1).

Fig. 3 is the ultraviolet-visible slow reflection spectrum of the nanocrystalline Bi ₂ WO ₆ powder photocatalyst prepared at different heat treatment temperatures. It can be estimated from the figure that the bandgap widths of nanocrystalline Bi ₂ WO ₆ photocatalysts obtained by heat treatment at 400, 500, 600 and 700°C are 2.77, 2.64eV, 2.52 and 2.50eV, respectively. This indicates _{that Bi2WO6} is suitable for visible light photocatalysis.

Figure 4 shows the effect of heat treatment temperature on the photocatalytic activity of nanocrystalline Bi ₂ WO ₆ . It can be seen from the figure that the newly prepared sample without heat treatment exhibits certain photocatalytic activity (degradation rate R%=8.3%). Then, the photocatalytic activity of the nanocrystalline _Bi2WO6 photocatalyst _was enhanced with the increase of heat treatment temperature. When the heat treatment temperature rose to 500 °C, the sample showed the best photocatalytic activity, and its photocatalytic degradation rate reached 68.7%. However, the photocatalytic activity of the Bi ₂ WO ₆ sample began to decrease with the further increase of the heat treatment temperature. This may be caused by the following reasons: Because the hydrothermal preparation of Bi ₂ WO ₆ powder was prepared at 150°C, drying at 100°C has little effect on its crystallization degree. As shown in Fig. 1, the degree of crystallization of Bi ₂ WO ₆ samples prepared by hydrothermal treatment without heat treatment is very weak. Therefore, although it has a high specific surface area (21.1 g/m ² ), it still exhibits weak photocatalytic activity. With the increase of heat treatment temperature, the degree of crystallization of Bi ₂ WO ₆ samples increases, which leads to the enhancement of the photocatalytic activity of the samples. The Bi ₂ WO ₆ sample prepared by heat treatment at 500°C showed a better degree of crystallization, and at the same time it had a higher specific surface area, so it showed the best photocatalytic activity. With the further increase of heat treatment temperature, although the crystallinity of the sample is enhanced to some extent, this enhancement is not obvious; at the same time, its specific surface area is greatly reduced (from 10.2m ² /g to 2.2m ² /g) . Therefore, the photocatalytic activity of Bi ₂ WO ₆ nanocrystals decreased continuously.

Example 5:

In order to test the effect of heat treatment time on the catalytic activity of nanocrystalline Bi ₂ WO ₆ photocatalyst, in addition to different heat treatment time, other reaction conditions such as: reactant concentration, amount of solvent water, hydrothermal temperature, hydrothermal time, heat treatment temperature, etc. Exactly the same as Example 1. It was found that when the heat treatment time was 0.5, 2, 4 and 10 hours, the visible light photocatalytic degradation rates were 33.5%, 68.7%, 70.6% and 65.2%, respectively. At 500℃, when the heat treatment time is 2 hours, the nanocrystalline Bi ₂ WO ₆ photocatalyst exhibits good photocatalytic activity. This may be due to the poor crystallinity of the samples when the heat treatment time was too short (0.5 h), while the specific surface area decreased when _the heat treatment time was too long (10 h) due to the increased grain size of _Bi2WO6 , so the photocatalysis decreases slightly.

Embodiment 6:

In order to examine the effect of the molar ratio of reactants in the precursor liquid on the catalytic activity of nanocrystalline Bi ₂ WO ₆ photocatalyst, in addition to the different molar ratios of reactants, other reaction conditions such as: bismuth nitrate concentration, amount of solvent water, hydrothermal temperature, water Thermal time, heat treatment temperature etc. are all identical with embodiment 1. It was found that when the molar ratio of bismuth nitrate and sodium tungstate is 1:5, 1:3, 1:2, 2:1. , the visible light photocatalytic degradation rates were 23.1%, 66.3%, 68.7% and 46.2%, respectively. When the molar ratio of reactants is 1:2, the nanocrystalline Bi ₂ WO ₆ photocatalyst exhibits the best photocatalytic activity.

Embodiment 7:

In order to examine the effect of the pH value of the precursor solution on the photocatalytic activity of the nanocrystalline _Bi2WO6 photocatalyst, in addition to the different pH values, other reaction conditions such as the concentration of the reactant, the amount of solvent water, the hydrothermal temperature, the hydrothermal time, and the heat treatment _temperature Etc. are identical with embodiment 1. It was found that when the pH values of the predisplacing solution were 1, 5, 7 and 10, the visible light photocatalytic degradation rates were 21.5%, 56.6%, 68.7% and 45.2%, respectively. When pH=7, the nanocrystalline Bi ₂ WO ₆ photocatalyst exhibited the best photocatalytic activity.

Table 1 Effect of heat treatment temperature on specific surface area and grain size of Bi ₂ WO ₆ powder Heat treatment temperature (℃) 100 400 500 600 700 BET specific surface area (m ² /g) 21.14 13.79 10.21 5.458 2.197 Average grain size (nm) 9.1 27.9 28.5 52.0 72.9

Claims (3)

1, a kind of preparation has visible light activity and receives brilliant Bi ₂WO ₆The method of powder photocatalytic material, it is characterized in that hydro-thermal-heat treatment method, said hydro-thermal-heat treatment method is to be raw material with bismuth nitrate and sodium tungstate, precipitation reaction takes place under hydrothermal condition, after the system for the treatment of is cooled to room temperature, the product of gained is extremely neutral with the deionized water cyclic washing, vacuum drying then, last heat treatment makes, the molar concentration of wherein said bismuth nitrate is 0.04～0.1M, the molar concentration of sodium tungstate is 0.08～0.2M, and the mol ratio of bismuth nitrate and sodium tungstate is 1: 3～1: 2, and the pH value of solution is 6～8, hydrothermal temperature is 140 ℃～160 ℃, the hydro-thermal time is 20～26 hours, and heat treatment temperature is 100 ℃～1000 ℃, and heat treatment time is 0.5～10 hour.

2, the method for claim 1 is characterized in that described heat treatment temperature is 500 ℃～600 ℃.

3, the method for claim 1 is characterized in that described heat treatment time is 1～3 hour.

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