A method for growing high-quality MoBiVO4 single crystal material

Abstract

The application relates to a growth method of high-quality MoBiVO4 single crystal materials. Specifically, the Mo:BiVO4 single crystal material is prepared by using an optical floating zone growth method. By controlling the growth process including a growth speed, an oxygen partial pressure and a relative rotating speed, the high-quality Mo:BiVO4 crystal is obtained, and the carrier transport and separation efficiency in the photoelectrode and the photoelectric conversion efficiency are further improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a growth method of a photoelectric functional Mo:BiVO4 single crystal material, which can be used for obtaining photoelectrocatalytic decomposition of water to produce hydrogen, and belongs to the field of crystal growth. BACKGROUND

[0002] With the rapid development of human society and economy, the large use of fossil fuels has caused a serious energy crisis. Photoelectrochemical decomposition of water to produce hydrogen technology has the advantages of high theoretical solar energy conversion efficiency (the theoretical value of photoelectric conversion efficiency is as high as 30%) and easy separation and collection of hydrogen and oxygen produced by water splitting, and has become one of the most ideal solutions to solve future energy problems.

[0003] Since the 1970s, Japanese scientists have used TiO2 single crystal photoelectrode to realize photoelectrochemical decomposition of water to produce hydrogen and oxygen, in order to improve the photoelectric conversion efficiency of the photoelectrode, people have developed many new photoelectrode materials, such as layered compounds, metal oxides, nitrogen oxides, sulfides, etc. Among them, the valence band position of monoclinic BiVO4 in metal oxide is positive (about 2.5V vs. RHE) and meets the potential required for oxygen production reaction (1.23V vs. RHE); and its band gap is moderate (about 2.4-2.5eV), which can be excited by visible light, and can absorb 11% of standard AM1.5 solar spectrum, so the maximum photocurrent density of BiVO4 can be generated in theory 7.6mA / cm 2 , and the conversion rate of solar energy to H2 is 9.3%. Therefore, monoclinic BiVO4 is one of the best choices for the most widely studied high-efficiency photoelectrochemical photoelectrode. However, the actual energy conversion rate of BiVO4 is far lower than the theoretical value. This is mainly due to the poor photoelectric charge separation efficiency, poor electron transport ability, slow water oxidation kinetics, and low charge mobility of BiVO4 nanomaterials, which leads to poor photoelectrochemical activity, greatly limiting its development and practical application.

[0004] Based on the fact that single crystal materials have few internal defects, few interfaces, and good orientation, the transport and separation of carriers can be greatly optimized, the recombination of carriers at the interface is reduced, and the energy conversion efficiency of the photoelectrode is improved. The traditional single crystal preparation method of pulling method or crucible lowering method needs to use noble metal Pt crucible, and the strong corrosiveness of Bi2O3 can easily cause the loss of noble metal, increasing the cost of crystal growth. SUMMARY

[0005] In view of the above problems, the application provides a growth method of high-quality Mo:BiVO4 crystal. The high-quality Mo:BiVO4 crystal is obtained by controlling the growth process including the growth speed, oxygen partial pressure and relative rotation speed through the optical floating zone method, and the carrier transport separation efficiency and photoelectric conversion efficiency in the photoelectrode are further improved.

[0006] In one aspect, the application provides a growth method of Mo:BiVO4 single crystal material, which adopts the optical floating zone growth method to prepare the Mo:BiVO4 single crystal material, preferably including:

[0007] (1) mixing Bi2O3 powder and V2O5 powder, then adding MoO3 powder, and then calcining at 600-800 DEG C for 8-12 hours (for example, 10 hours) to obtain Mo:BiVO4 polycrystal powder;

[0008] (2) forming the obtained Mo:BiVO4 polycrystal powder into a rod, and sintering at 700-900 DEG C (for example, 800 DEG C) for 12-24 hours to obtain a Mo:BiVO4 polycrystal raw material rod;

[0009] (3) taking the obtained Mo:BiVO4 polycrystal raw material rod as an upper feeding rod and a lower feeding rod, wherein the lower feeding rod is used as a seed crystal; then in the optical floating zone furnace, the upper feeding rod is arranged at the top and the lower feeding rod is arranged at the bottom, so that the upper feeding rod and the lower feeding rod are butted to form a stable melting zone; oxygen atmosphere is introduced, and the temperature is raised by irradiation of a halogen lamp light source until the upper feeding rod is melted, then the rotation speed of the upper feeding rod and the lower feeding rod is adjusted, and after the melting zone is stable, the single crystal is grown, and finally the Mo:BiVO4 single crystal material is obtained.

[0010] In the previous research of the present inventors, it is found that the technical difficulties in the growth of Mo:BiVO4 crystal by the optical floating zone growth method include: (1) V2O3 is easy to volatilize in the growth process; (2) the melting zone is unstable and easy to collapse in the growth process; and (3) the stoichiometric ratio of Bi and V needs to be continuously optimized, and this non-stoichiometric ratio is very important. Therefore, the present inventors creatively control: 1) the excess stoichiometric ratio of V; and 2) the sufficient oxygen partial pressure in the growth process. Finally, the growth of Mo:BiVO4 single crystal material is realized. Moreover, the optical floating zone method is adopted in the application, and the cost is low and the growth parameters can be easily and timely adjusted in the growth process.

[0011] Preferably, in step (1), the purity of the Bi2O3 powder is at least 99.99%, and the purity of the V2O5 powder is at least 99.99%;

[0012] The molar ratio of the Bi2O3 powder to the V2O5 powder is 1:(1.01-1.2), preferably 48:52;

[0013] The MoO3 powder is added in an amount of 0.1-2 at% of the total moles of the Bi2O3 powder and the V2O5 powder.

[0014] Preferably, in step (1), the mixing is performed by ball milling, and the ball milling is performed at a rotation speed of 200-800 rpm for 8-12 hours.

[0015] Preferably, in step (1), the raw material powder obtained after mixing the MoO3 powder is poured into a polytetrafluoroethylene mold and is pressed into a disc under a pressure of 50-100 MPa, and the disc has a diameter of 20-40 mm and a thickness of 3-8 mm; preferably, the disc has a diameter of 30 mm and a thickness of 5 mm, and the Mo:BiVO4 polycrystalline powder is a single pure phase.

[0016] Preferably, in step (2), the Mo:BiVO4 polycrystalline raw material rod has a diameter of 6-8 mm and a length of 50-80 mm.

[0017] Preferably, the Mo:BiVO4 polycrystalline powder is sealed in a latex sleeve and is pressed into a rod by hydrostatic pressure for 15-20 minutes, and the hydrostatic pressure has a pressure of 80-120 MPa, preferably 100 MPa.

[0018] Preferably, in step (3), the growth temperature is in the range of 800-1000°C, and the light source has a heating power of 1-2 kW, and the irradiation time required for heating to the melting point of the rod is 0.5-1 hour.

[0019] The protective atmosphere for growing the single crystal is oxygen, and the oxygen has a pressure of 1 bar-5 bar, preferably 2 bar-3 bar.

[0020] Preferably, in step (3), the rotation speed of the upper rod and the lower rod is controlled to be the same and opposite in direction; preferably, the relative rotation speed of the upper rod and the lower rod is 16-20 rpm.

[0021] Preferably, in step (3), the descending speed of the upper rod and the lower rod is set according to the mass conservation and the diameter of the upper rod and the lower rod; the descending speed of the upper rod is 3.0-3.2 mm / h, and the descending speed of the lower rod is 2.8-3.0 mm / h.

[0022] Preferably, in step (3), after a stable melting zone is obtained and maintained for 30-50 minutes, the upper rod is kept stationary, the seed crystal is lowered at the programmed descending speed, and the necking process is completed to reduce the self-orientation of the crystal grains after the melting zone becomes thin.

[0023] Preferably, in step (3), after the growth of the Mo:BiVO4 single crystal material enters the final stage, the melting zone is slowly pulled off, and the heating power of the light source is adjusted to 0.1-0.5 kW until the temperature of the melting zone is reduced to 50-120°C (for example, 100°C).

[0024] In another aspect, the present application provides a Mo:BiVO4 single crystal material prepared according to the above growth method, wherein the diameter of the Mo:BiVO4 single crystal material is 3-10 mm, and the length is 10-60 mm.

[0025] In still another aspect, the present application provides an application of the above Mo:BiVO4 single crystal material in photoelectrocatalytic decomposition of water to produce hydrogen.

[0026] Advantages:

[0027] The present application uses a pure-phase polycrystalline raw material prepared by a solid-phase method as a precursor, and grows through an optical floating zone method in an oxygen atmosphere. By continuously adjusting the growth process and the ratio of the raw material, a single crystal material of high quality is obtained, and can be applied to a photoelectrode water splitting reaction to produce hydrogen. In the present application, oxygen is used as the growth atmosphere, and the growth temperature is controlled at 960-980°C, thereby improving the stability of the crystal growth and obtaining a high-quality Mo:BiVO4 single crystal material. The use of the container-free optical floating zone method greatly reduces the growth cost. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The XRD pattern of the polycrystalline raw material powder prepared by the solid-phase reaction method in Example 1 shows that a pure-phase BiVO4 is obtained, and there is no other impurity phase;

[0029] Figure 2 The polycrystalline raw material rod obtained after high-temperature sintering in Example 1 is relatively dense and uniform.

[0030] Figure 3 The growth interface parameter adjustment during the crystal growth process in Example 1 shows that the melting zone is unstable and needs to be adjusted in time.

[0031] Figure 4 The Laue pattern (lower panel B) and the double-crystal rocking curve (upper panel A) of the 0.3% Mo:BiVO4 synthesized in Example 1 show that the single crystal has good crystalline quality.

[0032] Figure 5 The photoelectrochemical performance (A) and the stability (B) of the 0.3% Mo:BiVO4 photoelectrode in Example 1 show that it has good photoelectrocatalytic performance.

[0033] Figure 6The crystal photo obtained by growing the example 2. DETAILED DESCRIPTION

[0034] The present application is further illustrated by the following examples, which should not be construed as limiting the present application.

[0035] In the present application, the optical floating zone method is used to grow Mo:BiVO4 single crystal material. The growth method has a shorter growth cycle, lower growth cost, and simple synthesis steps; oxygen is used as the growth atmosphere to improve the stability of crystal growth, and high-quality single crystals can be obtained. The growth method of the optical floating zone method for growing Mo:BiVO4 single crystal material is described illustratively below.

[0036] Bi2O3 and V2O5 are mixed in a molar ratio of 1:1, and MoO3 is added in a proportion of 0.1-2at%. As an example, Bi2O3 and V2O5 with a purity of 99.99% are uniformly mixed in a molar ratio of 1:1, and 0.1% to 2at% MoO3 is added. The three mixtures are uniformly mixed by ball milling at 300 rpm / min for 24 h using a ball mill to obtain the raw material powder.

[0037] The raw material powder is calcined at a temperature of 600-800°C for 10h to obtain Mo:BiVO4 polycrystalline powder. As an example, the raw material powder is poured into a polytetrafluoroethylene mold and pressed into a circular sheet with a diameter of 30mm and a thickness of 5mm under a pressure of 50-100MPa, and then placed in a muffle furnace for calcination at 600-800°C for 10h to obtain Mo:BiVO4 polycrystalline powder with a single pure phase composition.

[0038] The Mo:BiVO4 polycrystalline powder is made into a rod with a diameter of 6-8mm and a length of 50-80mm, and then sintered at 800°C for 12-24h to obtain Mo:BiVO4 polycrystalline rods. Specifically, the polycrystalline powder is sealed in a latex sleeve and pressed into a rod shape using 100MPa hydrostatic pressure for 15-20min. The rod is removed from the latex sleeve and subjected to sintering treatment at 800°C for 12-24h to further improve the densification of the rod and give it a certain hardness and regular shape for single crystal growth.

[0039] The Mo:BiVO4 polycrystalline rod is used as the raw material rod (referred to as the upper rod) and the seed crystal (referred to as the lower rod) for growth in the optical floating zone furnace, with the upper rod on top and the seed crystal on the bottom, and the two are butted together at the melting zone.

[0040] The oxygen atmosphere is introduced to keep the internal pressure at 2-3 bar; the temperature is raised to the growth temperature within 0.5-1 hour to make the melt zone appear liquid phase, and then the relative rotation speed of the raw material rod is adjusted in the range of 16-20 rpm. The rotation speeds of the upper and lower raw material rods are the same or in opposite directions. According to the mass conservation and the diameter size of the upper and lower raw material rods, the descending speed of the upper and lower raw material rods is set. After the stable melt zone is determined and stabilized for 30-50 min, the upper raw material rod is kept stationary, and the lower raw material rod is slowly lowered according to the program setting. When the melt zone becomes thin, the necking process is completed to reduce the self-orientation process of the crystal grains.

[0041] After the melt zone shape is stabilized for 0.5-1 hour, the Mo:BiVO4 single crystal is grown, the descending speed of the upper raw material rod is controlled at 3.0-3.2 mm / h, and the descending speed of the lower raw material rod is controlled at 2.8-3.0 mm / h (i.e., the growth rate is controlled at 2.8-3.0 mm / h). The diameter of the grown crystal is in the range of 6 mm-7.5 mm, and the length is 15 mm-40 mm. After the crystal growth enters the ending stage, the melt zone is slowly pulled off, and the heating power of the light source is adjusted to reduce the melt zone temperature to 100°C. It is noted that the liquid drop is avoided as much as possible during the pulling-off process.

[0042] The crystal is cut into a wafer in a specific direction for photoelectrochemical experiments.

[0043] The following further examples are used to illustrate the present application in detail. It should also be understood that the following examples are only used to further illustrate the present application, and cannot be understood as limiting the scope of the present application. Some non-essential improvements and adjustments made by those skilled in the art according to the above content of the present application are within the protection scope of the present application. The specific process parameters in the following examples are only one example in the appropriate range, i.e., those skilled in the art can make appropriate selection within the range according to the description herein, and are not limited to the specific values in the following examples.

[0044] Example 1:

[0045] At room temperature, 93.09 g of Bi2O3, 36.27 g of V2O5 and 0.173 g of MoO3 were added into a ball mill jar, and about 8 ml of absolute ethanol was added as a ball milling aid and ball milled for 24 h. Subsequently, the obtained raw materials were pressed into a disc with a diameter of 30 mm and a thickness of 5 mm in a mold of polytetrafluoroethylene at a pressure of 10 MPa and calcined at 600 °C for 10 h in a muffle furnace. After the calcination, the obtained disc was again ball milled for 24 h to obtain a pure phase of the polycrystalline raw material powder, which was made into a polycrystalline rod. The powder was filled into a rubber sleeve through a funnel, and then the rubber sleeve was placed between two flat glass plates and rolled to make the powder into a rod. The rod (diameter 6 mm, length 60 mm) was placed in a hydrostatic press and pressed at a pressure of 100 MPa for 15 min. The rod was taken out of the rubber sleeve and sintered again at 600 °C for 10 h to further improve the densification of the rod and to have a certain hardness and regular shape for the growth of 0.3% Mo:BiVO4 single crystals.

[0046] The obtained rod was bound with a nickel wire and hung on the upper shaft, and the lower rod was fixed on the lower shaft as a seed crystal for growth by spontaneous nucleation. The control panel was opened to make the upper and lower shafts rotate in opposite directions, and the position of the upper rod was observed and adjusted to ensure that the rotation centers of the upper and lower rods were in the same vertical direction. After the rod was installed, a quartz tube was installed outside the growth part to isolate the growth part from the external heating device, and an oxygen atmosphere was introduced to maintain an internal pressure of 2 bar. Then the power of the optical floating zone furnace was turned on, and the light source was raised to 960 °C by pre-setting the program, at which time the voltage was 35 V, until the bottom of the upper and lower rods began to melt and switched to manual mode, keeping the upper rod rotating clockwise and the lower rod rotating counterclockwise at a relative rotation speed of 16 revolutions per minute. The display window of the optical floating zone furnace was observed at all times, and the lifting device was manually controlled to bring the upper and lower rods closer together. After complete melting, a stable melt zone was formed. The lowering speed of the upper rod was set to 3.5 mm / h, and the lowering speed of the lower rod was set to 3 mm / h. After the melt zone was determined to be stable for 30 min, the upper rod was kept stationary, and the lower rod was slowly lowered according to the program setting. When the melt zone became thin, the necking process was completed. Then the normal growth was carried out according to the set program, and small adjustments were made according to the changes in the growth state. The entire stable growth process lasted about 16 h. When the crystal growth entered the final stage, the upper rod was kept stationary, and the lower rod was lowered at a normal speed of 3 mm / h. After the melt zone was pulled off, the heating power of the light source was adjusted to reduce the temperature over 1 h. Finally, the furnace chamber was opened, and high-quality 0.3% Mo:BiVO4 single crystal material was obtained.

[0047] The high quality single crystal obtained is oriented by Laue to obtain the wafer in C direction, and the photoelectrochemical performance and stability of the material are tested by an electrochemical workstation to obtain the results as shown in Figure 5

[0048] Example 2

[0049] The growth process of the Mo:BiVO4 single crystal material in this example 2 is referred to example 1, and the only difference is that the oxygen atmosphere partial pressure in step (3) is 1 bar.

[0050] The single crystal obtained is oriented by Laue to obtain the wafer in C direction, and the photoelectrochemical performance and stability of the material are tested by an electrochemical workstation. The crystal quality obtained in this example 2 is poor as shown in Figure 6 .​

Claims

1. A method for growing a Mo:BiVO4 single crystal material, characterized by, The application discloses a method for preparing Mo:BiVO4 single crystal material by using an optical floating zone growth method, and belongs to the technical field of crystal growth. The method comprises the following steps: (1) mixing Bi2O3 powder and V2O5 powder, then adding MoO3 powder, and then calcining at 600-800 DEG C for 8-12 hours to obtain Mo:BiVO4 polycrystal powder; the molar ratio of the Bi2O3 powder and the V2O5 powder is 1:(1.01-1.2); (2) making the obtained Mo:BiVO4 polycrystal powder into a rod shape, and then sintering at 700-900 DEG C for 12-24 hours to obtain a Mo:BiVO4 polycrystal raw material rod; the diameter of the Mo:BiVO4 polycrystal raw material rod is 6-8 mm, and the length is 50-80 mm; 2. The growing method of claim 1, wherein, (3) taking the obtained Mo:BiVO4 polycrystal raw material rod as an upper feeding rod and a lower feeding rod, wherein the lower feeding rod is used as a seed crystal; then in an optical floating zone furnace, the upper feeding rod is arranged at the upper position, the lower feeding rod is arranged at the lower position, the upper feeding rod and the lower feeding rod are butt-jointed to form a stable melting zone; an oxygen atmosphere is introduced, and the upper feeding rod is heated until melting by irradiation of a halogen lamp light source, then the rotating speeds of the upper feeding rod and the lower feeding rod are adjusted, and after the melting zone is stable, single crystal growth is started, and finally the Mo:BiVO4 single crystal material is obtained. In step (1), the purity of the Bi2O3 powder is at least 99.99%, and the purity of the V2O5 powder is at least 99.99%.

3. The growth method of claim 1, wherein, The adding amount of the MoO3 powder is 0.1-2 at% of the total molar amount of the Bi2O3 powder and the V2O5 powder.

4. The growth method of claim 1, wherein, In step (1), the mixing mode is ball milling mixing, the rotating speed of the ball milling mixing is 200-800 revolutions / minute, and the time is 8-12 hours.

5. The growing method of claim 4, wherein, In step (1), the raw material powder obtained after the MoO3 powder is mixed is poured into a mold of polytetrafluoroethylene, and is pressed into a round sheet under a pressure of 50-100 MPa; the diameter of the round sheet is 20-40 mm, and the thickness is 3-8 mm; and the Mo:BiVO4 polycrystal powder is a single pure phase.

6. The growth method of claim 1, wherein, The diameter of the round sheet is 30 mm, and the thickness is 5 mm.

7. The growth method of claim 6, wherein, In step (2), the Mo:BiVO4 polycrystal powder is sealed in a latex sleeve, and is pressed into a rod shape by using hydrostatic pressure for 15-20 minutes; the pressure of the hydrostatic pressure is 80-120 MPa.

8. The growth method of claim 1, wherein, The pressure of the hydrostatic pressure is 100 MPa. In step (3), the growth temperature ranges from 800 DEG C to 1000 DEG C; the heating power of the light source is 1-2 kW, and the irradiation time required for heating until the upper feeding rod is melted is 0.5-1 hour; 9. The growing method of claim 8, wherein, The protective atmosphere for growing the single crystal is oxygen, and the oxygen gas pressure is stabilized at 1 bar-5 bar.

10. The growth method of claim 1, wherein, The oxygen gas pressure is stabilized at 2 bar-3 bar.

11. The growth method of claim 10, wherein, In step (3), the rotating speeds of the upper feeding rod and the lower feeding rod are controlled to be the same and opposite. In step (3), the relative rotating speed of the upper feeding rod and the lower feeding rod is controlled to be 16-20 revolutions / minute.

12. The growing method of claim 1, wherein, In step (3), the descending speed of the upper / lower feeding rod is set according to the mass conservation and the diameter size of the upper / lower feeding rod; the descending speed of the upper feeding rod is 3.0-3.2 mm / h; and the descending speed of the lower feeding rod is 2.8-3.0 mm / h.

13. The growth method of claim 1, wherein, In step (3), after the stable melting zone is determined and is stable for 30-50 min, the upper feeding rod is kept stationary, the seed crystal is caused to descend at the descending speed set by the program, and after the melting zone is thinned, the necking process is completed to reduce the self-orientation process of the crystal grains.

14. The growing method of any one of claims 1-13, wherein, In step (3), after the growth of the Mo:BiVO4 single crystal material enters the ending stage, the melting zone is slowly pulled off, and meanwhile the heating power of the light source is adjusted to 0.1-0.5 kW until the temperature of the melting zone is reduced to 50-120 ℃.

15. A Mo:BiVO4 single crystal material prepared according to the growth method of any one of claims 1-14, characterized in that, The Mo:BiVO4 single crystal material has a diameter of 3-10 mm and a length of 10-60 mm.

16. Application of the Mo:BiVO4 single crystal material in claim 15 to photoelectrocatalytic decomposition of water to produce hydrogen.

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