Vanadate is the general name of pentavalent vanadium oxy acid salt.Similar to phosphate, vanadate also has orthovanadate MVOfourPyrvanadate MVtwoOsevenAnd metavanadate MVOthreeEtc.These salts can be extracted fromsolutionThe colorless sodium vanadate solution can be prepared by dissolving vanadium pentoxide in concentrated sodium hydroxide solution, in which vanadium exists in the form of orthovanadate VO43 -.In aqueous solution, with the increase of solution acidity, vanadate will condense to different degrees to form polyanions with different compositions.
Photocatalytic degradation using TiO2Organic pollutantsIt is the current research in the field of photocatalysishotspotone of.However, due to its wide band gap (32 eV), which needs ultraviolet light to excite, largely limits the application of TiO2.In order to overcome this shortcoming, on the one hand, people improve TiO2's response to visible spectrum through various doping technologies; on the other hand, they actively research and develop new types of photocatalysts with photocatalytic activity in the visible light rangePhotocatalyst。
Metal vanadates are a kind of excellent functional materials.In addition to being widely used as a good matrix material in the field of fluorescence and laser materials, it can also be used as a cathode material for lithium ion batteries.Recent research shows that some vanadates have great potential applications in the field of photocatalysis, and they are a new type of highly active photocatalysts.The synthesis methods, modification technologies and research progress in photocatalytic activity of vanadate photocatalysts, such as bismuth vanadate, indium vanadate, iron vanadate and silver vanadate, are reviewed[1]。
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Bismuth vanadate (BiVO4)
B iVO4 is a non TiO2 based visible lightSemiconductor photocatalyst。At the same time, it is also a functional material with ferroelectricity and thermochromism, which may be used asSolid oxide fuel cellAlternatives to electrolyte or cathode materials in.There are three main crystal structures of BiVO4: tetragonal scheelite type (high temperature phase), tetragonal zirconium silicate type and monoclinic deformed scheelite type (limonite type).
Monocline BiVO4 is generally prepared by high-temperature solid state reaction, hydrothermal reaction or metal alkoxide hydrolysis.becauseSolid state reaction methodGenerally, it is controlled by the diffusion process between reaction components under solid state conditions, which usually requires higher reaction temperature and longer reaction time;The prepared products have irregular morphology and large particle size, usually containing impurities.Although the hydrothermal method can significantly improve the crystallinity and purity of the product compared with the solid phase method, special synthesis equipment is required, and the yield of general products is not high.Metal alkoxide hydrolysis has the advantages of simple preparation process and accurate control of chemical composition, but the cost is high and more organic additives are needed.
Tetragonal zirconium silicate BiVO4 can be prepared by aqueous precipitation method, but it is difficult to obtain monoclinic or tetragonal scheelite structure by this precipitation method.Although BiVO4 materials with different crystal structures can be obtained at lower temperatures by selecting special reaction materials (such as K3V5O14, KV3O8), it generally requires a long reaction time.The results show that the BiVO4 band gap of monoclinic system is 23 ～ 2.4 eV, under visible light irradiation (λ>420 nm), it shows higher photocatalytic activity than tetragonal system.
Kudo et al. used the liquid phase method to prepare tetragonal and monoclinic bismuth vanadate materials at room temperature for 3 days.The absorption edge of the prepared tetragonal phase BiVO4 falls in the UV region, and the band gap is about 29 eV;The absorption edge of monoclinic phase can be extended to the visible region, and the band gap is correspondingly reduced to 2.3~24 eV。The subsequent experiment of decomposing AgNO3 aqueous solution to produce O2 under ultraviolet light showed that BiVO4 in monoclinic phase had higher catalytic activity than that in tetragonal phase B iVO4;Compared with the high-temperature solid state reaction method, the monoclinic phase BiVO4 prepared by the liquid phase method has higher photocatalytic activity in the visible light region.Shigeru et al. also showed that the ability of BiVO4 to degrade alkylphenol in wastewater under sunlight was better than TiO2.Zhang et al. synthesized high purity and good dispersivity BiVO4 nanosheets by hydrothermal method, with a thickness of about 10~40 nm.It was pointed out that the morphology of the particles could be effectively controlled by adding sodium dodecyl benzene sulfonate (SDBS). In the hydrothermal process, tetragonal BiVO4 nanoparticles were first obtained;With the extension of reaction time, the nanocrystals aggregate and undergo phase transformation under the action of SDBS, and finally form monoclinic BiVO4 nanosheets.The results of photocatalytic experiments show that BiVO4 nanosheets have stronger catalytic activity than bulk materials under visible light, which may be due to the larger specific surface area of BiVO4 nanosheets.
Starting from catalyst immobilization and practical application, Luo et al. prepared scheelite monoclinic BiVO4 thin films by polymer deposition method. The light absorption edge is 400 ～ 500 nm, which has good response ability in the visible light region. BiVO4 thin films have larger specific surface area[2]。
Indium vanadate (InVO4)
InVO4 has two crystal structures: orthorhombic crystal system and monoclinic crystal system.Because its band gap is only 2.0 eV, it has a response in the wavelength range of λ<650 nm.In recent years, InVO4 with orthorhombic crystal system has attracted increasing attention in the fields of solar energy utilization, environmental protection, etc.
Ye et al. have prepared a new type of tungsten manganese iron type photocatalyst InMO4 (M=V, Nb, Ta) by high temperature solid state reaction (850 ℃). It can directly decompose water into H2 and O2 under visible light, and is an excellent photocatalyst with great potential.However, theSpecific surface area of catalystSmaller (<05 m2 /g) 。Zhang et al. [23] successfully synthesized single-phase nano InVO4 catalyst by low-temperature calcination, and investigated the effects of calcination temperature and time on the crystallinity and size of InVO4 nanoparticles.Compared with the high-temperature solid-state reaction method, the low-temperature synthesis can effectively prevent the agglomeration of InVO4 nanoparticles, increase the specific surface area and improve the photocatalytic performance.The results show that the products prepared at 500 ℃ are amorphous;When the temperature rises to 550 ℃, an obvious characteristic peak of InVO4 appears, accompanied by some impurity phases.Pure InVO4 can be obtained when the reaction temperature is higher than 600 ℃. Moreover, with the further increase of the reaction temperature, the XRD diffraction peak becomes sharper and sharper, indicating that the crystallinity of InVO4 is getting better and better.In addition, increasing the reaction temperature or prolonging the reaction time will also promote the growth of InVO4 particle size, which ranges from 40 to 200 nm.The relevant photocatalytic activity tests showed that InVO4 prepared at 600 ℃ for 8 h had the best photocatalytic activity, mainly due to its smaller particle size and larger specific surface area.
Xu et al. used template oriented self-assembly method to synthesize InVO4 nanoparticles with a particle size of about 30-40 nm.Later experiments of decomposing aquatic hydrogen showed that the hydrogen production rate was 1836 μ mol/(g · h), which was higher than TiO2 and InVO4 synthesized by high-temperature solid phase method under the same conditions.Chen et al. also synthesized InVO4 nanoparticles by hydrothermal method at 200 ℃ for 24 hours, and compared the morphology changes of InVO4 product particles with the help of different organic additives. Without any additives, the size of the product particles is different, and the microscopic morphology is very irregular;After adding sodium dodecyl sulfate, the photocatalytic performance of the product was improved, and the micro morphology was basically regular nanorods, with the length of 200~400 nm and the diameter of 100~140 nm;When cetyltrimethylamine bromide was used, the morphology of InVO4 particles changed again from nanorod to square sheet.Instead, useEDTAWhen used as an additive, the micro morphology of particles is mainly brick like, and the size is also significantly larger (750 nm long, 550 nm in diameter)[2]。
Silver vanadate (Ag3 VO4)
The low-energy valence band of Ag3VO4 consists of the hybridization of 4d10 orbits of Ag and 2p6 orbits of O, while its high-energy conduction band consists of the hybridization of 5s orbits of Ag and 3d orbits of V.The hybrid valence band structure has more active energy levels than the single O 2p6, resulting in a narrower band gap.The structural feature of Ag3VO4 extends its light response range to the visible light region, making it another promising visible light responsive photocatalyst.
Konta et al. synthesized α 2AgVO3, β 2AgVO3, Ag4V2O7 and Ag3VO4 by chemical precipitation followed by solid state reaction.The absorption edge of the product falls in the visible light region, but the subsequent visible light catalytic decomposition experiment of aquatic O2 shows that only Ag3VO4 has good visible light catalytic activity.Hu et al. used V2O5 and AgNO3 as starting reactants, first dissolved V2O5 in sodium hydroxide solution, and then mixed AgNO3 solution with it under magnetic stirring to obtain yellow precipitate.Ag3VO4 microcrystals can be obtained by hydrothermal treatment of the mixture at different temperatures.
The results show that the product is not pure Ag3VO4, in which Ag is also generated, and the size of the synthesized Ag3VO4 particles is between 1 and 4 μ m.At 140 ℃ and 180 ℃, the crystallinity of the product is poor, and it contains more heterophase Ag;At the hydrothermal temperature of 160 ℃, the crystallinity of the product is good, the surface is smooth, and the heterophase Ag is obviously suppressed.It was reported that the highest photocatalytic activity of Ag3VO4 was obtained under the condition of adding excessive vanadium salt and holding at 160 ℃ for 48 h.Excess vanadium salt is beneficial to increase the crystallinity of the product, and can inhibit the emergence of heterophase Ag.The photocatalytic degradation of acid red B (ARB) under visible light shows that the degradation rate exceeds 70% in 100 minutes, while TiO2 can only degrade about 40% ARB under the same conditions.
At present, micron Ag3VO4 has been synthesized by different methods, and its photocatalytic experiments have also proved that it has excellent photocatalytic performance.Due to the high quantum efficiency and large specific surface area of nanoparticles, it is expected that they should have higher photocatalytic activity, so it is necessary to strengthen the research on the synthesis of nanoscale Ag3VO4.
Iron vanadate (FeVO4)
Iron vanadate belongs to ABO4 type photocatalyst and has four crystal structures, namely, triclinic, orthorhombic (I), orthorhombic (II) and monoclinic.At present, there are few reports on the preparation of FeVO4, mainly including high-temperature solid state reaction, hydrothermal method and liquid phase synthesis.
Hayashibara et al. mixed Fe2O3 and V2O5 in the mass ratio of 1 ∶ 1 by high-temperature solid state reaction, and pressed them into small balls under the pressure of 200 MPa.Then the pellets were kept at 650 ℃ for 6 h to produce triclinic FeVO4.However, the FeVO4 powder synthesized by this method has coarse particle size, uneven distribution, easy agglomeration and small specific surface area, which is not conducive to improving the performance of photocatalyst[3]。
Oka [30] used hydrothermal method to mix FeCl3 and VOCl2, put them in a hydrothermal kettle, and reacted at 280 ℃ for 40 hours to prepare orthogonal (I) FeVO4 with particle size of about 100~300 μ m.Deng et al. used the wet chemical method to reduce 0The 26 mol/L ferric nitrate solution was quickly compared with the pre prepared 427 × 1022 mol/L ammonium metavanadate solution was mixed and stirred continuously for 1 h at 75 ℃;Then, wash the sediment with water and acetone respectively
The precursor of FeVO4 can be obtained by removing impurities and drying in natural state.The precursor was then calcined at 100~600 ℃ for 2h to obtain the final product.The results show that the weak FeVO4 characteristic peak appears only when the calcination temperature reaches 500 ℃.The subsequent photocatalytic degradation experiment of orange II showed that FeVO4 had better photocatalytic activity than α 2Fe2O3, Fe3O4 and γ 2FeOOH.
At present, the research on FeVO4 photocatalyst is still in the initial stage.It is still a challenging research work whether we can use a simpler method to synthesize FeVO4 nanoparticles under more mild conditions.
Doping modification of vanadate photocatalyst
Compared with the pure phase photocatalyst, doping in the matrix phase will acceleratePhotogenerated carrierThe effective separation of electrons and holes can be realized by the migration of electrons and holes, thus improving the photocatalytic performance of the materials.At the same time, doping technology may extend the absorption wavelength range of photocatalyst to the visible light region.Therefore, the research on doping technology of photocatalyst is undoubtedly of great significance.At present, there have been some research reports on the doping modification of vanadate photocatalysts.
Xu et al. studied the photocatalytic performance of Fe, Co and Cu deposited on BiVO4.The particle size of BiVO4, Fe/B iVO4, Co/BiVO4 and Cu/B iVO4 prepared are all about 30 nm, and the band gap width is 24 eV、2. 35 eV、2. 29 eV and 233 eV。The sample pairs were examined under visible and ultraviolet light irradiationMethylene blue(MB). The results show that the transition metal deposition can effectively improve the degradation ability of BiVO4 to MB. Among them, the efficiency of BiVO4 doped with (4%) Cu2 is the highest.
Ge successfully synthesized Pt doped BiVO4 photocatalyst. The optical absorption test showed that the absorption edge of pure phase BiVO4 sample was about 553 nm, while the absorption edge of Pt/BiVO4 reached 700 nm.Pt doping can greatly expand the absorption edge wavelength of BiVO4.Subsequent experiments on catalytic degradation of methyl orange under visible light showed that the photocatalytic activity of 1% Pd doped BiVO4 was the highest[2]。
Study on Complex Vanadate Photocatalyst
Another effective measure to improve the photocatalytic activity of the catalyst is to reduce the recombination probability of photogenerated electrons and photogenerated holes.Research shows that binary or multi-component semiconductor composite technology can achieve this goal.When two or more semiconductor materials are combined, the photocatalytic activity of the materials may be significantly improved.
Lin et al. compounded NiO with InVO4 to synthesize NiO/InVO4 composite photocatalyst with a particle size of about 5 μ m.The experiment of decomposing aquatic hydrogen showed that the composite product prepared at 500 ℃ had the highest photocatalytic activity, and the hydrogen production rate reached 896 μ mol/(g · h).
Jiang et al. synthesized V2O5/BiVO4 composite photocatalyst by self propagating combustion at 500 ℃ for 4 h.SEM photos show that the surface of pure phase BiVO4 is rough, and the particle size ranges from hundreds of nanometers to 1 μ m.However, the V2O5/BiVO4 particles prepared by the self propagating combustion method are spherical (V2O5 doping<10%) with smooth surface and size of 150~500 nm.When the V2O5 doping amount reaches 10%, the particle size of V2O5/BiVO4 increases to about 2 μ m.Through the comparative experiment of MB degradation, it was found that the V2O5/BiVO4 composite photocatalyst with 9% V2O5 doping had the best photocatalytic performance.The report points out that the improvement of photocatalysis ability of composite photocatalyst is mainly due to multi-component recombination, which makes the rapid transfer of photoelectrons and reduces the recombination probability of photogenerated electron 2 holes[3]。
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As the core component of photocatalysis technology, the research, development and application of new photocatalysts will be the eternal theme pursued by people.Although vanadate is a new type of photocatalyst with great potential, the reported bismuth vanadate, indium vanadate and silver vanadate have very narrow band gaps, which can make full use of solar energy to degrade pollutants.However, there are still many problems to be solved in the actual application:
(1) The problem of mechanism.The in-depth study of photocatalytic mechanism is particularly important for improving the photocatalytic efficiency and practical application of vanadate catalysts.In view of the complexity of photocatalysis process and the difference in the crystal structure of vanadates, the related photocatalysis reaction mechanism needs to be further studied;
(2) Immobilization of photocatalyst.The fixation and regeneration of photocatalyst is a common problem in photocatalysis technology.For vanadate photocatalysts, there are still some technical problems such as the reasonable selection of catalyst carrier and the coating of photocatalyst.Through relevant research, it is expected to realize the effective fixation of photocatalyst, at the same time, it is conducive to the progress of photocatalytic reaction and improve the photocatalytic activity;
(3) The preparation method of photocatalyst.At present, most of the reported vanadate synthesis methods are carried out under high temperature or harsh reaction conditions. Due to the limitations of the methods themselves, the photocatalytic activity of photocatalysts and their wide application in the future will be directly affected.In contrast, wet chemical synthesis process can better prevent the agglomeration of product particles, increase the specific surface area of the catalyst, and improve the performance of the catalyst.Therefore, the search for simple, practical and batch photocatalyst preparation methods is still the concern of the majority of researchers.
In a word, with the research and development of a series of new photocatalysts including vanadates, people will make full use of solar energy to photolysis pollutants.The implementation of more effective prevention and control of environmental pollution is in line with the national policy of "energy conservation and emission reduction".It is of great significance to the sustainable development of China's national economy[3]。
