Zinc oxide whisker and preparation method thereof

Abstract

The invention discloses a zinc oxide whisker and a preparation method thereof. The preparation method comprises the following steps: providing a precursor mixture containing zinc oxide and carbon black; heating the precursor mixture to a first temperature zone in a vacuum environment to generate zinc vapor; transporting the zinc vapor to a second temperature zone, wherein the temperature of the second temperature zone is lower than that of the first temperature zone; introducing oxygen by stages, firstly introducing oxygen at a first oxygen flow rate to promote nucleation of zinc steam, and then introducing oxygen at a second oxygen flow rate to promote nucleus growth to obtain zinc oxide whiskers; wherein the second oxygen flow rate is lower than the first oxygen flow rate. According to the invention, cheap ZnO and carbon black are used for replacing high-purity Zn powder, and an Au catalyst is completely omitted, so that the cost index is far lower than that of a PVT method and a CVD method. The method shows high robustness, the allowable fluctuation range of the temperature and the oxygen flow is larger than that of a comparison method, and particularly, the control requirement for the oxygen flow is far lower than that of a CVD method. Therefore, the process is easier to control, the consistency between batches is better, and the method is more suitable for industrial production.

Description

A zinc oxide whisker and its preparation method Technical Field

[0001] This invention relates to the field of zinc oxide whisker preparation technology, and in particular to a zinc oxide whisker and its preparation method. Background Technology

[0002] In existing technologies, tetraneedle-shaped zinc oxide (T-ZnO) whiskers have significant application prospects in composite material reinforcement, optoelectronic devices, and sensors due to their excellent semiconductor, piezoelectric, and ultraviolet shielding properties. Currently, the mainstream preparation methods mainly rely on physical vapor transport (PVT) by directly evaporating high-purity zinc powder or chemical vapor deposition (CVD) using noble metal catalysts (such as gold (Au)). However, these methods have the following significant drawbacks: First, during direct evaporation of zinc powder, the violent reaction and concentrated exothermic reaction easily generate vapor pulses, leading to large fluctuations in zinc vapor concentration and making it difficult to obtain products with uniform morphology and controllable size. Second, CVD methods typically require the introduction of noble metals such as gold (Au) as catalysts to promote nucleation, which not only significantly increases raw material costs but may also introduce difficult-to-remove metallic impurities into the final product, affecting its purity and electrical properties. In addition, existing processes generally lack effective synergistic control over zinc vapor condensation, droplet nucleation and subsequent oxidation growth processes, resulting in products often containing non-target structures such as needle-like and cluster-like structures, with low regularity and poor batch repeatability.

[0003] Therefore, developing a method that requires no external catalyst, has stable processing, is low cost, and can achieve high-purity, high-regularity, and large-scale controllable growth of four-needle zinc oxide whiskers has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for preparing zinc oxide whiskers. This method enables the low-cost, high-purity, high-regularity, and large-scale controllable preparation of tetrapter-shaped and multipter-shaped zinc oxide whiskers. Specifically: In a first aspect, embodiments of this invention provide a method for preparing zinc oxide whiskers, comprising: providing a precursor mixture containing zinc oxide and carbon black; heating the precursor mixture to a first temperature zone in a vacuum environment to generate zinc vapor; transporting the zinc vapor to a second temperature zone, the second temperature zone being lower than the first temperature zone; introducing oxygen in stages, first introducing oxygen at a first oxygen flow rate to promote zinc vapor nucleation, and then introducing oxygen at a second oxygen flow rate to promote nucleus growth, thereby obtaining zinc oxide whiskers; wherein the second oxygen flow rate is lower than the first oxygen flow rate.

[0005] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The purpose and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0006] As a preferred technical solution, in the method for preparing zinc oxide whiskers, the mass ratio of zinc oxide to carbon black is (0.8:1) to (1.2:1).

[0007] As a preferred technical solution, in the preparation method of the zinc oxide whiskers, the first temperature zone is 1000~1100℃ and the second temperature zone is 800~900℃.

[0008] As a preferred technical solution, in the method for preparing zinc oxide whiskers, the system pressure of the vacuum environment is 100~5000 Pa.

[0009] As a preferred technical solution, in the method for preparing zinc oxide whiskers, the first oxygen flow rate is 4~8 sccm and the introduction time is 5~10 minutes; the second oxygen flow rate is 1~2 sccm and the introduction time is 20~40 minutes.

[0010] As a preferred technical solution, the method for preparing zinc oxide whiskers includes zinc oxide whiskers with four needle-like structures and zinc oxide whiskers with multiple needle-like structures.

[0011] As a preferred technical solution, the method for preparing zinc oxide whiskers, wherein the carbon black is a porous carbon black.

[0012] As a preferred technical solution, in the method for preparing zinc oxide whiskers, the diameter of the root of the four needle-like zinc oxide whiskers is 100 nm to 2 μm, and the morphological regularity is greater than 90%.

[0013] As a preferred technical solution, in the method for preparing zinc oxide whiskers, the zinc vapor condenses during transport to form nano- to submicron-sized zinc droplets, which serve as autocatalytic centers.

[0014] Secondly, a zinc oxide whisker, wherein the zinc oxide whisker is prepared by the preparation method described above.

[0015] Beneficial effects: Compared with the prior art, the embodiments of the present invention have the following advantages: By using commercial zinc oxide powder and inexpensive carbon black as precursors and completely eliminating expensive ultra-high purity zinc powder and precious metal catalysts, the cost of raw materials is reduced, while the catalyst pretreatment process is eliminated, simplifying the process and reducing the overall production cost.

[0016] High product morphology uniformity and purity: The continuous and stable release of zinc vapor was achieved through a carbon black activation system, effectively solving the vapor pulse problem in traditional methods. Combined with the synergistic effect of gradient temperature field and low-pressure transport, the uniformity of the nucleation process was ensured. X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) results confirmed that the final product contained no residual foreign catalyst elements and exhibited excellent crystal quality. By optimizing the process window, the morphological regularity of the product could reach over 90%, and the needle root diameter could be precisely controlled within the range of 100 nm to 2 μm.

[0017] Process robustness and scalability: The process parameters determined in this invention have a wide tolerance range (e.g., temperature ±50℃, oxygen flow rate ±3 sccm), and low equipment requirements, requiring only a conventional tube furnace. Gram-scale product can be collected in a single experiment, demonstrating its good repeatability and potential for large-scale production, laying a solid technical foundation for continuous industrial production.

[0018] Decoupled and Controllable Growth Mechanism: This invention proposes a strategy of "precisely controlling oxygen in stages," decoupling the two key processes of "nucleation" and "growth" by dynamically changing the oxygen partial pressure. A higher oxygen flow rate in the early stage facilitates the rapid formation of numerous nucleation sites, while an extremely low oxygen flow rate in the later stage maintains an oxygen-deficient environment in the growth region. This not only protects the Zn droplets, which act as autocatalytic centers, from excessive oxidation and deactivation but also promotes the preferential growth of ZnO crystals along the 0002 crystal orientation, thereby directionally forming a tetrap-like structure. The establishment of this mechanism provides an important methodological reference for the controllable synthesis of other metal oxide nanostructures. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 is a schematic diagram of the preparation method of zinc oxide whiskers provided by the present invention.

[0021] Figure 2 is a schematic diagram of the gas phase transport system provided in the embodiment of the present invention; Figure 3 is a SEM image of the four needle-like ZnO nanostructure in Example 1; Figure 4 is a TEM image of the needle-like part of the four needle-like ZnO nanostructure in Example 1; Figure 5 is an XRD pattern of the four needle-like ZnO nanostructure in Example 1; Figure 6 is an EDS pattern of the four needle-like ZnO nanostructure in Example 1. Detailed Implementation

[0022] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] As shown in Figure 1, this embodiment of the invention provides a method for preparing zinc oxide whiskers, which relies on a gas-phase transport system as shown in Figure 2. The gas-phase transport system mainly includes: a horizontally placed quartz tube reactor 100, a three-zone tube furnace 200, a vacuum pump system 300 with an ultimate vacuum of 0.1 Pa, a multi-channel gas mass flow controller 400, and a product collection unit 500. The quartz tube reactor 100 is made of high-purity fused quartz, possessing excellent high-temperature resistance and chemical corrosion resistance. The three-zone tube furnace 200 is located outside the quartz tube reactor 100, and its interior has two sets of independently temperature-controlled heating wires (unlabeled), corresponding to the upstream first temperature zone and the downstream second temperature zone, respectively. This design ensures a stable and precise axial temperature gradient between the two zones. The vacuum pump system 300 is connected to the tail outlet of the quartz tube reactor 100 via a stainless steel corrugated pipe, used for evacuation and adjusting the static pressure within the system. The multi-channel gas mass flow controller 400 is connected to high-purity argon (Ar, 99.999%) and high-purity oxygen (O2, 99.999%) gas sources. Its output end is connected to the gas inlet at the front end of the quartz tube reactor 100 through a polytetrafluoroethylene (PTFE) pipe for precise control of the flow rate of carrier gas and reaction gas.

[0024] The precursor mixture was placed in a high-purity alumina ceramic boat, which possesses good thermal stability and chemical inertness, preventing side reactions with the reactants. The precursor mixture was a homogeneous blend of commercially available zinc oxide powder (ZnO, average particle size 10 μm) and non-graphite porous carbon black (CB) at a mass ratio of 1:1. The selection of carbon black instead of graphite is one of the core innovations of this invention, based on the fact that carbon black (such as acetylene black or furnace black) has a significantly higher specific surface area (>50 m²) than graphite. 2With its abundant microporous / mesoporous structure, this porous characteristic provides more active reaction sites during carbothermic reduction reactions, enabling efficient reduction of ZnO to Zn vapor at lower temperatures (compared to the use of graphite or H2 reducing agents). More importantly, its porous network structure acts as a miniature "buffer pool," absorbing and slowly releasing the zinc vapor generated in the reaction. This fundamentally solves the problem of vapor pulses and concentration fluctuations caused by the instantaneous and violent reaction in traditional direct evaporation methods of zinc powder, providing the prerequisites for subsequent stable transport and uniform nucleation. The alumina ceramic boat is placed upstream of the quartz tube reactor 100, precisely at the center of the first temperature zone.

[0025] The entire preparation process follows the flow chart shown in Figure 1. First, the weighed precursor mixture is placed in an alumina ceramic boat and carefully pushed into the designated position of the quartz tube reactor 100. Then, both ends of the reactor are sealed, and the vacuum pump system 300 is started to evacuate the reaction chamber to an ultimate vacuum (≤0.1 Pa) to completely remove air and moisture and prevent interference from impurities. Next, high-purity argon gas is introduced as a carrier gas at a flow rate of 100 sccm, and at least three vacuum-purging cycles are performed to further purify the system environment. After completing the vacuuming and purging procedures, the argon gas is turned off, and the vacuum is evacuated again to 100 Pa as the initial pressure of the process.

[0026] Subsequently, the heating program of the three-zone tube furnace 200 was initiated. The first temperature zone was heated from room temperature to 1050°C at a rate of 10°C / min and held at this temperature for 30 minutes. Simultaneously, the second temperature zone was heated at the same rate to 850°C and held at a constant temperature. The temperature settings of the two zones were optimal ranges verified through extensive experiments. 1050°C is sufficient to ensure a complete carbothermic reduction reaction between ZnO and carbon black, generating sufficient zinc vapor; while 850°C is slightly lower than the melting point of ZnO but high enough to ensure controlled condensation of Zn vapor when transported to this zone, and also provides the necessary activation energy for the subsequent oxidation reaction. At this time, the static pressure within the system was precisely controlled at 3000 Pa by the vacuum pump system 300. The choice of this moderate vacuum level (100-5000 Pa) is crucial, as it synergizes with the upstream and downstream temperature gradients (ΔT ≈ 200℃): on the one hand, a moderate negative pressure environment facilitates the directional transport of zinc vapor from upstream to downstream; on the other hand, this pressure range is precisely near the critical point for homogeneous nucleation of zinc vapor, prompting transported zinc vapor molecules to preferentially condense heterogeneously on existing condensation nuclei, thereby forming uniform nano- to submicron-sized liquid zinc droplets within the second temperature zone, rather than random gas-phase nucleation or disordered solid particles. These in-situ formed Zn droplets serve as autocatalytic centers for subsequent growth, realizing a VLS (vapor-liquid-solid) growth mechanism without an external catalyst.

[0027] Once the temperatures in both temperature zones stabilize at their set values, high-purity oxygen is introduced via a multi-channel mass flow controller 400. The first oxygen flow rate is set to 6 sccm and continuously supplied for 8 minutes. During this stage, the higher oxygen flow rate rapidly increases the oxygen partial pressure in the second temperature zone. Sufficient oxygen molecules react violently with the zinc vapor and the surface of newly formed Zn droplets arriving in this region, forming numerous ZnO nuclei on the surface of the Zn droplets. Due to the intense reaction, a large number of nucleation sites are generated instantaneously, laying the foundation for subsequent multidirectional growth. This "high-oxygen nucleation" stage is the crucial first step in achieving high-density, high-regularity products. Next, the "low-oxygen growth" stage begins. The oxygen flow rate is rapidly reduced from 6 sccm to 1.5 sccm and maintained at this low flow rate for 30 minutes. This significant reduction in oxygen flow rate causes a sharp drop in the oxygen partial pressure in the growth zone (second temperature zone), creating a relatively oxygen-deficient environment. Under this environment, the oxidation rate of ZnO is inhibited, while the supply of Zn vapor remains sufficient. According to the VLS growth mechanism, Zn vapor continuously dissolves into the Zn droplet, and once supersaturation is reached, ZnO precipitates at the droplet / solid interface. Since the 0002 crystal orientation is the fastest-growing polar face for ZnO crystals, its growth advantage is further amplified under oxygen-deficient conditions. Simultaneously, the low-oxygen environment also acts as a "protective" element, preventing the Zn droplet from being completely oxidized and losing its catalytic activity. Driven by both surface energy minimization and anisotropic growth, ZnO crystals tend to preferentially extend from four specific directions of the Zn droplet, ultimately forming a stable tetrap-like structure. By precisely controlling the duration of the low-oxygen flow (20-40 minutes), the length and diameter of the needles can be effectively controlled.

[0028] After growth, oxygen supply was stopped, and the entire system was cooled to room temperature at a rate of 5°C / min under an argon atmosphere. After cooling, the tube furnace and vacuum system were shut off, the reactor was opened, and the product was removed. The SEM image shown in Figure 3 clearly shows that the obtained product is a typical tetrap-shaped zinc oxide whisker, with four needles radiating outwards from the central spherical node at a 109° angle, exhibiting a highly regular morphology. Measurements showed that the diameter at the needle root was concentrated around 500 nm, within the controllable range of 100 nm to 2 μm. Using porous carbon black, the morphological regularity was statistically over 92%. XRD patterns indicated that the product was a hexagonal wurtzite ZnO with good crystallinity. EDX analysis (Figure 6) showed that the product contained only Zn and O elements, with no other significant impurities (the C element signal came from the conductive adhesive).

[0029] Based on the same inventive concept, this invention also provides zinc oxide whiskers, which are prepared using the preparation method described above. The specific preparation process has already been explained in detail above and will not be repeated here.

[0030] The technical solutions provided by the present invention will be further explained and illustrated below through specific embodiments.

[0031] Example 1 This example uses the integrated gas phase transport system shown in Figures 1 and 2. The precursor mixture is a homogeneous mixture of commercial zinc oxide powder (average particle size 10 μm) and porous carbon black (acetylene black, specific surface area >50 m² / g) at a mass ratio of 1:1.

[0032] An alumina ceramic boat containing the precursor was placed in the center of the first temperature zone of the quartz tube reactor. After sealing the system, a vacuum was drawn to below 0.1 Pa, and the system was repeatedly purged with high-purity argon. Subsequently, the system pressure was maintained at 1000 Pa. The dual-temperature zone tube furnace was turned on, and the first temperature zone was raised to 1050°C at a rate of 10°C / min and held at that temperature, while the second temperature zone was simultaneously raised to 850°C and held at that temperature.

[0033] Once the temperature stabilizes, implement phased oxygen supply: first, introduce oxygen at a flow rate of 6 sccm for 8 minutes (high-oxygen nucleation stage); then reduce the oxygen flow rate to 1.5 sccm and maintain it for 30 minutes (low-oxygen growth stage).

[0034] After the reaction was completed, the mixture was cooled to room temperature under an argon atmosphere. The product was collected and observed by SEM. It was found to be mainly composed of tetraneedle-shaped zinc oxide whiskers with a morphological regularity of approximately 92%. The diameter of the needle roots was concentrated in the range of 400-600 nm. XRD analysis showed that it was a pure wurtzite structure.

[0035] Example 2 The main difference between this example and Example 1 is that the key process parameters are taken as the lower limit of the range.

[0036] The mass ratio of zinc oxide to carbon black in the precursor is 0.8:1.

[0037] The system pressure is controlled at 100 Pa.

[0038] The temperature in the first temperature zone is 1000℃, and the temperature in the second temperature zone is 800℃.

[0039] The first oxygen flow rate is 4 sccm, lasting for 10 minutes; the second oxygen flow rate is 1 sccm, lasting for 40 minutes.

[0040] The product contains a high proportion of tetraneedle-shaped zinc oxide whiskers, with a morphological regularity of approximately 85%. The needles are relatively slender, and the root diameter is mainly distributed between 100-300 nm. The results indicate that even at the lower limit of the parameter range, the method of this invention can still effectively prepare tetraneedle-shaped zinc oxide whiskers.

[0041] Example 3 The main difference between this example and Example 1 is that the key process parameters are taken at the upper limit of the range.

[0042] The mass ratio of zinc oxide to carbon black in the precursor is 1.2:1.

[0043] The system pressure is controlled at 5000 Pa.

[0044] The temperature in the first temperature zone is 1100℃, and the temperature in the second temperature zone is 900℃.

[0045] The first oxygen flow rate is 8 sccm, lasting for 5 minutes; the second oxygen flow rate is 2 sccm, lasting for 20 minutes.

[0046] The yield of the obtained product is high, with approximately 90% regularity in the morphology of the tetra-needle-shaped zinc oxide whiskers. The needles are relatively robust, and the root diameter is mainly distributed between 1 and 2 μm. The results show that the method of the present invention is also effective at the upper limit of the parameter range and helps to obtain whiskers of larger size.

[0047] In summary, this invention provides a method for preparing zinc oxide whiskers, comprising: providing a precursor mixture containing zinc oxide and carbon black; heating the precursor mixture to a first temperature zone in a vacuum environment to generate zinc vapor; transporting the zinc vapor to a second temperature zone, the second temperature zone being lower than the first temperature zone; introducing oxygen in stages, first introducing oxygen at a first oxygen flow rate to promote zinc vapor nucleation, and then introducing oxygen at a second oxygen flow rate to promote nucleus growth, thereby obtaining zinc oxide whiskers; wherein the second oxygen flow rate is lower than the first oxygen flow rate. By using commercially available zinc oxide powder and inexpensive carbon black as precursors, and completely eliminating expensive ultra-high purity zinc powder and precious metal catalysts, the raw material cost is reduced by more than 60%, while the catalyst pretreatment process is eliminated, simplifying the process and reducing overall production costs. The continuous and stable release of zinc vapor is achieved through a carbon black activation system, effectively solving the vapor pulse problem in traditional methods. Combined with the synergistic effect of a gradient temperature field and low-pressure transport, the uniformity of the nucleation process is ensured. X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) results confirmed that the final product contained no residual foreign catalyst elements and exhibited excellent crystal quality. By optimizing the process window, the morphological regularity of the product could reach over 90%, and the diameter of the needle root could be precisely controlled within the range of 100 nm to 2 μm. This invention proposes a strategy of "stage-wise precise control of oxygen," which decouples the two key processes of "nucleation" and "growth" by dynamically changing the oxygen partial pressure. A higher oxygen flow rate in the early stage facilitates the rapid formation of numerous nucleation sites, while an extremely low oxygen flow rate in the later stage maintains an oxygen-deficient environment in the growth region. This not only protects the Zn droplets, which serve as self-catalytic centers, from excessive oxidation and deactivation but also promotes the preferential growth of ZnO crystals along the 0002 crystal orientation, thereby directionally forming a tetraneedle-like structure. The establishment of this mechanism provides an important methodological reference for the controllable synthesis of other metal oxide nanostructures.

[0048] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

[0049] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing zinc oxide whiskers, characterized in that, include: A precursor mixture comprising zinc oxide and carbon black is provided; the precursor mixture is heated to a first temperature zone in a vacuum environment to generate zinc vapor; The zinc vapor is transported to a second temperature zone, the temperature of which is lower than that of the first temperature zone; oxygen is introduced in stages, first at a first oxygen flow rate to promote zinc vapor nucleation, and then at a second oxygen flow rate to promote nucleus growth, to obtain zinc oxide whiskers; wherein the second oxygen flow rate is lower than the first oxygen flow rate.

2. The method for preparing zinc oxide whiskers according to claim 1, characterized in that, The mass ratio of zinc oxide to carbon black is (0.8:1) to (1.2:1).

3. The preparation method of zinc oxide whiskers according to claim 1, characterized in that, The first temperature zone is 1000~1100℃, and the second temperature zone is 800~900℃.

4. The method for preparing zinc oxide whiskers according to claim 1, characterized in that, The system pressure of the vacuum environment is 100~5000 Pa.

5. The method for preparing zinc oxide whiskers according to claim 1, characterized in that, The first oxygen flow rate is 4-8 sccm, and the introduction time is 5-10 minutes; the second oxygen flow rate is 1-2 sccm, and the introduction time is 20-40 minutes.

6. The method for preparing zinc oxide whiskers according to claim 1, characterized in that, The zinc oxide whiskers include tetraneedle zinc oxide whiskers and multineedle zinc oxide whiskers.

7. The preparation method of zinc oxide whiskers according to claim 1, characterized in that, The carbon black is a porous carbon black.

8. The method for preparing zinc oxide whiskers according to claim 7, characterized in that, The diameter of the needle root of the four needle-like zinc oxide whiskers is 100 nm to 2 μm, and the morphological regularity is greater than 90%.

9. The method for preparing zinc oxide whiskers according to claim 1, characterized in that, The zinc vapor condenses during transport to form nano- to submicron-sized zinc droplets, which serve as autocatalytic centers.

10. A zinc oxide whisker, characterized in that, The zinc oxide whiskers are prepared by the preparation method described in any one of claims 1-9.

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