CN110208168B - Transmission electron microscope technology for in-situ research of three-dimensional distribution structure of nanoparticles - Google Patents
Transmission electron microscope technology for in-situ research of three-dimensional distribution structure of nanoparticles Download PDFInfo
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- CN110208168B CN110208168B CN201910571773.8A CN201910571773A CN110208168B CN 110208168 B CN110208168 B CN 110208168B CN 201910571773 A CN201910571773 A CN 201910571773A CN 110208168 B CN110208168 B CN 110208168B
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Abstract
The invention discloses a transmission electron microscope technology for in-situ research of a three-dimensional distribution structure of alloy nanoparticles in a sintering process, which is used for obtaining carbonized nano carbon fibers through an electrostatic spinning technology and heat treatment; precursor salt of the alloy is loaded on the carbon nanofibers; a metal needle tip is arranged at the fixed end of a sample rod of a transmission electron microscope, a nano carbon fiber is provided with a gold needle platform, and a gold needle is arranged at the movable end of the sample rod; inserting a sample rod loaded with a metal needle point and a gold needle into a transmission electron microscope, adjusting the height and the position of the gold needle at the movable end of the sample rod to enable the nano carbon fiber on a gold needle platform to be in contact with the metal needle point, applying a certain voltage within a certain instant time to enable precursor salt loaded on the nano carbon fiber on the gold needle platform to perform instant carbothermic reaction to form alloy nano particles, and observing the distribution change and sintering condition of the alloy nano particles under the action of a current field; and (4) shooting a two-dimensional projection transmission electron microscope photo, and carrying out alignment, three-dimensional reconstruction and visualization processing on the electron microscope photo through software.
Description
Technical Field
The invention relates to a transmission electron microscope technology, in particular to a method for in-situ research on three-dimensional distribution condition of multi-component alloy nanoparticles under the action of a current field under a transmission electron microscope.
Technical Field
Fossil energy has been the foundation on which human beings live, supporting the rapid development of modern society. However, as is well known, fossil energy is a non-renewable energy, and as the demand of human society for energy is getting larger and larger, the international energy situation is becoming more and more severe, and the exhaustion of energy and the problem of environmental pollution caused by the use of fossil energy have attracted extensive attention of countries in the world. In order to solve the dual problems of non-renewable property and environmental pollution caused by the traditional fossil energy, the demand of human beings for new energy is increasing day by day, and renewable and sustainable energy such as solar energy, nuclear energy, wind energy, tidal energy and the like are paid attention to and researched by energy researchers. Among them, the fuel cell is a high-efficiency energy conversion device that directly converts chemical energy into electrical energy, has high cleanliness and environmental friendliness, and has a wide, environmentally friendly, and sustainable source of fuel, so the fuel cell is an attractive alternative energy source.
At present, the performance of a catalyst in a fuel cell becomes a key link for restricting the development of the fuel cell, and the core for promoting the development of the fuel cell lies in preparing the catalyst with high electrocatalytic performance, high durability and low cost. Among them, the control of the size and composition of the catalyst is an important means for obtaining a catalyst having an ideal catalytic performance. Many researches show that the nano catalyst has higher catalytic potential than the traditional large-size catalyst, and particularly, when the size of the catalyst reaches the nano level, the catalytic activity of the catalyst is improved to a large extent. This benefits from the large specific surface area and quantum size effects due to the nanoscale catalyst. Compared with the traditional single-component metal catalyst, the multi-component alloy catalyst is prepared by introducing other alloy components in a proper proportion, the synergistic effect among different metal elements can be effectively utilized, the multi-component alloy catalyst with higher catalytic performance can be obtained, the use amount of noble metal can be reduced to a certain extent, and the catalyst cost can be reduced. In addition, the high-quality carrier material can also effectively improve the activity and stability of the catalyst, and simultaneously can reduce the use cost and increase the catalytic efficiency of the catalyst. At present, advanced carbon materials such as carbon nanotubes, carbon nanofibers and mesoporous carbon become carrier materials which are widely researched by virtue of high corrosion resistance, high specific surface area, proper porosity and the like.
The existing methods for preparing the supported catalyst mainly comprise a dipping-liquid phase reduction method, a gas phase reduction method, an electrochemical deposition method, a gel-sol method and the like, but the methods all have the defects of overlarge particle size of the metal catalyst, difficulty in controlling the size, poor dispersibility and the like. Therefore, a teaching team of a junior in the university of maryland creatively provides the preparation of the multicomponent alloy nano catalyst particles loaded on the carbon nano fibers by using an instant joule heating method. The method can prepare the alloy nano catalyst particles with the size of about 10 nm, has the obvious advantages that the components of the alloy nano catalyst particles are uniformly distributed, the alloy particles containing eight components can be prepared at most, the catalytic performance is further improved by fully utilizing the synergistic effect among metal elements, and the bottleneck that the traditional preparation method can only prepare three-component alloy particle catalysts at most is broken through. Based on the basic principle of an instant joule heating method, the in-situ transmission electron microscope technology is utilized to realize in-situ observation of the forming process of metal catalyst particles, study the influence of current, voltage, component components and the like on the forming mechanism and distribution of the metal catalyst particles, simulate the current field environment of a catalyst in a fuel cell, study the sintering behavior of the particles and the three-dimensional distribution condition of the particles under the action of the current field in situ, and improve the theoretical support for preparing alloy nano catalyst particles with higher catalytic performance.
Disclosure of Invention
The invention aims to provide a method for in-situ research on sintering phenomenon and three-dimensional distribution condition of multi-component alloy nanoparticles loaded on carbon nanofibers under the action of a current field under a transmission electron microscope.
In order to achieve the purpose of the invention, the technical scheme is as follows.
A transmission electron microscope technology for in-situ research of sintering phenomenon and three-dimensional distribution condition of particles of multi-component alloy nanoparticles loaded on carbon nanofibers under the action of a current field comprises the following steps:
the method for in-situ preparation of the carbon fiber attached multicomponent alloy nanoparticles in the transmission electron microscope comprises the following steps:
step 1: obtaining the carbon nanofibers with the diameter range of 100-200nm after carbonization treatment by an electrostatic spinning technology and early-stage heat treatment;
step 2: preparing a metal precursor salt solution, and soaking the carbon nanofibers obtained in the step (1) in the precursor salt solution to obtain carbon nanofibers loaded with precursor salt;
and step 3: taking another nickel-chromium metal needle with the diameter of 0.2-0.25 mm, preparing a metal needle point by an electrochemical polishing method, shearing the metal needle point into the length of 3-5 mm, and then placing the metal needle point into the fixed end of the sample rod to enable the metal needle point to face outwards;
and 4, step 4: taking 2mm and 1mm long and wide carbon nanofibers loaded with precursor salt, adhering the carbon nanofibers on a clamped gold needle platform with the diameter of about 0.25mm by using conductive silver adhesive, and placing gold needles at the movable end of a sample rod;
and 5: inserting the sample rod loaded with the metal needle tip and the gold needle into a transmission electron microscope, and adjusting the height and the position of the gold needle at the movable end of the sample rod to enable the metal needle tip and the gold needle to be positioned at the same height and at the opposite position;
step 6: enabling the nano carbon fibers on the golden needle platform to be in contact with the metal needle tip, and applying a certain voltage within a certain instant time to enable precursor salt loaded on the nano carbon fibers on the golden needle platform to perform instant carbon thermal reaction to form alloy nanoparticles;
and 7: after the alloy nanoparticles are formed, the metal needle tip is continuously kept in contact with the carbon nanofibers, continuous small current is applied, and the distribution change and sintering conditions of the alloy nanoparticles under the action of a current field are observed;
and 8: carrying out 180-degree rotation on the carbon nanofibers loaded with the alloy nanoparticles, and shooting central bright field images of 1 carbon nanofiber attached with the alloy nanoparticles at intervals of 2 degrees so as to obtain a series of two-dimensional projection transmission electron microscope photos at different angles;
and step 9: and (4) carrying out alignment, three-dimensional reconstruction and visualization processing on the two-dimensional projection transmission electron microscope photos under a series of different angles obtained in the step (8) by using reconstruction software.
Further, step 1 is implemented by the following method:
step 1.1: mixing Polyacrylonitrile (PAN) and Dimethylformamide (DMF) according to a certain proportion, and then filling into an injector;
step 1.2: placing a substrate bearing the carbon nanofibers on the receiving screen, preferably using copper foil or carbon paper as the substrate; experience shows that the copper foil has relatively high strength and low bonding property with the nano carbon fiber, so that the nano carbon fiber is easy to peel or curl in the spinning and heat treatment processes.
Step 1.3: adjusting the relative position of the receiving screen and the ejector to start the spinning process; after spinning is finished, a piece of PAN nanofiber is formed on the substrate;
step 1.4: and (3) carrying out heat treatment on the PAN nanofiber obtained by spinning: firstly, placing PAN nano-fiber at 260 ℃ in an air environment for 5 hours of pre-oxidation treatment, and then placing the PAN nano-fiber at 800 ℃ for 2 hours of carbonization treatment under the protection of argon gas, so as to obtain the nano-carbon fiber.
Further, in the step 2, precursor salt with alloy components is mixed in the ethanol solution to obtain a precursor ethanol salt solution; and soaking the carbonized carbon nanofibers in a precursor ethanol salt solution to fully soak the carbon nanofibers, taking out the carbon nanofibers, placing the carbon nanofibers in a glass dish, and naturally drying the carbon nanofibers to obtain the carbon nanofibers loaded with the precursor salt.
Further, the process of preparing the metal tip by the electrochemical polishing method in the step 3 is as follows:
step 3.1: preparing corrosive liquid, putting the corrosive liquid into a glass container, and placing a plastic bracket beside the glass container;
step 3.2: connecting two leads with the tail ends connected with the copper sheet with two ends connected with two ends of a power supply, immersing the copper sheet connected with the negative electrode into corrosive liquid of a glass container, and fixing the copper sheet connected with the positive electrode on a plastic bracket and placing the copper sheet above the corrosive liquid;
step 3.3: sleeving a metal round bar to be corroded on two plastic sleeves, namely an upper plastic sleeve and a lower plastic sleeve, wherein one end of the round bar is connected with a copper sheet fixed on a plastic bracket; a small gap of 0.5-1.0 mm is exposed between the upper plastic sleeve and the lower plastic sleeve; the upper end of the upper plastic sleeve is positioned above the liquid level, and the lower end of the lower plastic sleeve is positioned below the lower end of the round rod;
step 3.4: after the arrangement is finished, turning on a power supply, adjusting the voltage value to be 5-20V, and carrying out corrosion reaction;
step 3.5: and (3) breaking the small seam between the two plastic sleeves, turning off the power supply after the lower end of the round rod falls into the corrosive liquid, taking out the falling round rod from the corrosive liquid, and taking out the plastic sleeves from the tail part of the round rod by using tweezers to obtain the nanoscale metal needle tip.
Further, step 4 is realized by the following method:
step 4.1: and tearing off small pieces with the length and the width of 2mm and 1mm respectively from the precursor salt-loaded carbon nanofiber by using a pair of tweezers, and forming a small tip in the tearing process so as to easily realize the contact with the metal needle point at the fixed end of the sample rod in the transmission electron microscope.
Step 4.2: and (3) taking a gold needle with the length of about 3mm and the diameter of about 0.25mm, and using flat nose pliers to clamp a section of the gold needle flat by utilizing the advantage of good ductility of metal to obtain a small platform with the thickness of about 0.05mm, so that the nano carbon fiber torn off in the step (4.1) can be firmly fixed on the platform through the conductive silver adhesive, and the tip of the nano carbon fiber faces outwards.
Step 4.3: and (4) placing the gold needle adhered with the carbon nanofibers into the movable end of the sample rod.
Further, step 5 is implemented by the following method:
step 5.1: and inserting the sample rod loaded with the metal needle tip and the gold needle into an electron microscope, and adjusting the Z-axis height of the electron microscope to enable the metal needle tip at the fixed end of the sample rod to be in a positive focal state.
Further, in step 6:
step 6.1: the height of the movable end is adjusted to enable the part of the nano carbon fiber on the gold needle needing instantaneous carbon thermal reaction to be in a positive focal state, and the front and back and the left and right of the movable end of the sample rod are adjusted to enable the part of the nano carbon fiber in the positive focal state to be in contact with the metal needle point at the fixed end of the sample rod;
step 6.2: applying a certain instantaneous voltage between the fixed end and the movable end within a certain instantaneous time, observing the carbon thermal reaction process of the nano-fibers at the contact part of the gold needle and the metal needle tip in situ, and observing the formation process of alloy nano-particles;
step 6.3: and (5) moving the nano-fibers at other parts to contact with the metal needle tip, and observing and researching the influence of different time and voltage on the formation of the alloy nano-particles by changing the instantaneous time and the instantaneous voltage.
Further, in step 7, after alloy nanoparticles are formed by precursor salt under the action of instantaneous carbon heat, the contact state of the carbon nanofibers and the metal needle tip is continuously maintained, a small current is applied between the fixed end and the movable end of the sample rod, the form and the size of the current are changed according to the situation, and the distribution and the sintering condition of the alloy nanoparticles under the continuous action of a current field are observed in situ by using pulse current or constant current and the like.
Further, step 8 is implemented by the following method:
step 8.1: in order to prevent two ends from colliding due to unstable movement in the subsequent selection process, the movable end of the sample rod needs to move backwards to be far away from the fixed end of the sample rod;
step 8.2: before the carbon nanofibers with alloy nanoparticles are subjected to rotary shooting, the carbon nanofibers are moved to an axis by using displacement compensation through the axis aligning operation of a sample rod, so that the carbon nanofibers do not generate large drift in the rotating process and deviate from the visual field;
step 8.3: after the shaft is aligned, the height Z of the electron microscope is adjusted to enable the carbon nanofibers to be in a positive focal state, then 180-degree rotation is carried out, and a central bright field image of the carbon nanofibers attached with the alloy nanoparticles is shot at intervals of 2 degrees, so that a series of two-dimensional projection transmission electron microscope photos under different angles are obtained.
Further, in step 9, a proper three-dimensional reconstruction software is selected, and the same carbon nanofiber before and after the action of the current field is subjected to image preprocessing, axis aligning, reconstruction and visualization processing to obtain the real distribution condition of the carbon nanofiber particles before and after the action of the current field.
In addition, the three-dimensional reconstruction analysis of the particle distribution of the alloy nanoparticles before and after the action of the current field can be realized by combining the steps 7, 8 and 9, so that the influence result of the current field on the particle space distribution is obtained in more detail.
The invention has the advantages that:
1. the formation mechanism of the alloy nanoparticles is fully understood, which is beneficial to preparing the alloy nano catalyst with higher performance, and the process of forming the alloy nanoparticles by the precursor salt under the instant joule heat can be observed in situ in the transmission electron microscope by the method.
2. By changing the instantaneous voltage and the instantaneous electrifying time applied to the carbon fiber loaded with the precursor salt, the influence of different voltages and electrifying times on the precursor salt in the process of forming the alloy nanoparticles can be researched, so that the alloy nanoparticles with ideal size and shape can be obtained by selecting proper voltage and electrifying time.
3. The high vacuum electron microscope environment can provide an oxygen-free vacuum environment for the in-situ instantaneous carbothermic reaction, and the oxidation of precursor salt in the process of forming alloy nano particles is avoided.
4. The sintering condition of the alloy nanoparticles on the carrier and the three-dimensional distribution condition of the alloy nanoparticles on the carbon nanofibers under the action of a current field are studied in situ, and the real positions of the alloy nanoparticles on the carrier are more intuitively revealed.
Drawings
1. FIG. 1 is a schematic illustration of electrospinning.
2. FIG. 2 is a schematic diagram of a metal precursor salt loading process.
3. FIG. 3 is a schematic illustration of the electrochemical etching of a metal tip.
4. FIG. 4 is a schematic view of a gold needle adhered with a carbon nanofiber loaded with precursor salt
5. FIG. 5 is a transmission electron micrograph of alloy nanoparticles obtained after in situ energization.
6. FIG. 6 is a transmission electron micrograph of alloy nanoparticles obtained at different transient voltages.
7. FIG. 7 is a transmission electron micrograph of the alloy nanoparticles before and after the electric field.
8. Fig. 8 is a transmission electron micrograph and three-dimensional reconstruction of alloy nanoparticles.
In the above figures: the device comprises a metering pump 1, an ejector 2, a polymer solution 3, a jet orifice 4, a carbon nanofiber 5, a receiving screen 6, a high-voltage power supply 7, a glassware 8, a plastic support 9, a copper sheet 10, a metal round rod 11, an upper plastic sleeve 12, a lower plastic sleeve 13, a lead 14, a corrosive liquid 15, a glass container 16, a gold needle platform 17 and a gold needle 18.
Detailed Description
Example 1
The method for in-situ research on sintering phenomenon and three-dimensional distribution condition of multi-component alloy nano particles loaded on carbon nano fibers under the action of a current field under a transmission electron microscope comprises the following steps:
1. in order to obtain the nano carbon fiber, an electrostatic spinning method is adopted. Before spinning according to the electrospinning scheme shown in fig. 1, Polyacrylonitrile (PAN) and Dimethylformamide (DMF) are mixed according to a certain proportion to form a polymer solution, and then the polymer solution is filled into an injector. A substrate for carrying the filamentous nanocarbon is then placed in an appropriate position on the receiving screen directly opposite the ejection opening. Copper foil or carbon paper was selected as a substrate, but experimental experience shows that carbon nanofibers are prepared by selecting carbon paper as a substrate in this example because copper foil has relatively high strength and weak bonding with carbon fibers, and thus peeling and curling of the carbon nanofibers easily occur during spinning and post heat treatment, compared to carbon paper. After the carbon paper is fixed, the relative position of the receiving screen and the ejector is adjusted, spinning can be started after experiment parameters such as proper voltage are set, the injection amount of polymer solution in the ejector is controlled through the metering pump, and voltage is applied through the high-voltage power supply to carry out spinning. After spinning, a layer of white PAN nanofibers is formed on the carbon paper.
And carrying out later-stage heat treatment on the PAN nanofiber obtained by electrostatic spinning to obtain the carbon nanofiber. The heat treatment is mainly divided into two parts, the first part is a pre-oxidation treatment at 260 ℃ for 5 hours in an air environment. The second part is carbonized at 800 ℃ for 2 hours under the protection of argon. And obtaining the carbon nanofibers after the heat treatment is finished.
2. In this example, gold-nickel alloy nanoparticles need to be prepared, so the ratio of 1: 1 ratio AuCl2And NiCl2Dissolving in ethanol solution to prepare the gold-nickel precursor ethoxide solution. As shown in FIG. 2, carbonizingAnd soaking the obtained carbon nanofibers in the prepared gold-nickel precursor ethanol salt solution, taking out the soaked carbon nanofibers after the carbon nanofibers are fully soaked, placing the soaked carbon nanofibers in a glassware, and waiting for natural drying to obtain the carbon nanofibers loaded with precursor salt.
3. The metal needle point etching apparatus shown in FIG. 3 was arranged to etch the needle point with a 0.25mm diameter nichrome rod. As shown in fig. 3, a glass container is taken, a proper amount of prepared corrosive liquid is poured into the glass container, and a plastic bracket is placed beside the glass container; different materials adopt different corrosive liquids. For the nickel-chromium alloy, a mixed solution of 10% of perchloric acid and 90% of alcohol is adopted. Two leads with the tail ends connected with the copper sheets are connected with two ends of a power supply, the copper sheet connected with the negative electrode is immersed into the corrosive liquid of the glass container, the copper sheet connected with the positive electrode is fixed on a bracket and is arranged above the corrosive liquid, and attention is paid to the fact that the copper sheet is not in contact with the corrosive liquid. Two plastic sleeves (an upper plastic sleeve and a lower plastic sleeve) are sleeved on the nickel-chromium alloy round bar, and one end of the metal round bar is connected with a copper sheet fixed on the bracket. A small gap of about 0.5mm is exposed between the upper plastic sleeve and the lower plastic sleeve, the upper end of the upper plastic sleeve is positioned above the liquid level, and the lower end of the lower plastic sleeve is positioned below the lower end of the metal round rod, as shown in fig. 3. The metal round bar is protected by the plastic sleeve, so that only the middle small seam of the metal round bar is subjected to electrochemical corrosion reaction, and a longer and thinner metal needle point can be obtained.
And turning on a power supply, and adjusting to a proper voltage value, wherein in the embodiment, the adjusted voltage value is 5-20V. And (3) after the small seam between the two plastic sleeves is fractured, the lower end of the metal round rod falls into the corrosive liquid, the power supply is turned off, the dropped metal round rod is taken out from the corrosive liquid, and the plastic protective sleeve is gently taken out from the tail of the metal round rod by using tweezers (the fracture part of the metal round rod is called as the head, and the end opposite to the fracture part of the metal round rod is called as the tail). The metal round bar is polished to be a needle point with the length of 10-100 micrometers by an electrochemical polishing method, and the diameter of the tip of the needle point is 5-500 nm, so that the metal round bar can be in contact with carbon nanofibers at the movable end of a lens sample rod for electrifying and other operations.
Because the upper part and the lower part of the metal round bar are protected by the plastic sleeves, only the middle small seam is in direct contact with the corrosive liquid to generate electrochemical corrosion reaction; along with the reaction, the metal at the middle small seam is slowly corroded, and the radius of the metal gradually becomes smaller; under the action of the gravity of the lower part of the metal round bar, the corroded area at the middle small seam is subjected to tension; when the radius of the corrosion area is small to a certain value, namely the borne tension is larger than the bearable maximum tension, the middle corrosion area is pulled apart to form a metal needle point; in addition, the corrosion condition should be paid attention to all the time in the corrosion process, after the metal round bar is disconnected from the middle small gap, the power supply is immediately turned off, and the dropped metal round bar is taken out from the corrosion liquid.
Shearing the corroded metal round bar into a proper length, wherein in the embodiment, the length of the corroded metal round bar is 3-5 mm; the metal needle point faces outwards, the other end of the metal needle point is plugged into the movable end of the transmission electron microscope sample rod, the screw is screwed, the metal needle point is slightly pulled outwards by the tweezers, and the fact that the sample is clamped firmly is confirmed
4. In order to make the carbon nanofibers in the partial region contact with the metal needle tip in the electron microscope, it is necessary to lightly tear off pieces with length and width of 2mm and 1mm respectively on the carbon nanofibers loaded with the precursor salt by using tweezers, and care is taken to make the tearing process as small as possible to form a small tip as shown in fig. 4.
A gold needle with a length of about 3mm and a diameter of about 0.25mm as shown in FIG. 4 was used, and a section of the gold needle was clamped flat by using flat nose pliers, using the advantage of good ductility of metal, to obtain a gold needle platform with a thickness of about 0.05mm, so that the carbon nanofibers torn off in the above step could be firmly fixed on the gold needle platform by conductive silver paste, taking care that the tips of the carbon nanofibers would face outward when sticking.
And (4) placing the gold needle adhered with the carbon nanofibers into the movable end of the sample rod.
5. The sample rod loaded with the two ends is inserted into an electron microscope, and the fixed end does not have the degree of freedom, so that the height of the Z axis of the electron microscope needs to be adjusted, and the metal needle point at the fixed end is in a positive focal state.
Then selecting a proper area for carrying out instantaneous carbothermic reaction in the large nanometer carbon fiber loaded with precursor salt at the movable end. The height of the movable end is adjusted by the freedom degree of the movable end to enable the movable end to be in a positive focal state, and the front and back and the left and right of the movable end are adjusted to enable the movable end to be in contact with the metal needle point of the fixed end.
When a transient voltage is applied between the fixed end and the movable end for a certain time, a phenomenon shown in fig. 5 can be observed, and precursor salts loaded on the carbon fibers undergo a transient carbothermic reaction to form a large amount of alloy nanoparticles.
6. In this example, the precursor salt-loaded carbon nanofibers in different regions can be brought into contact with the metal tip by moving the active end, and different voltages are applied at different instantaneous times, so that instantaneous carbothermic reactions under different conditions can be performed in different regions, and the influence of different times and voltages on the formation of the alloy nanoparticles can be observed and studied, for example, fig. 6 is a transmission electron microscope image of the nano alloy particles obtained at different instantaneous voltages.
7. In this example, after the alloy nanoparticles have been formed by the in-situ instantaneous carbothermic reaction, the contact state is maintained, and a 0.5V pulse voltage of 1000 s is applied to both ends, as shown in fig. 7, the alloy particles on the carbon nanofibers undergo a sintering phenomenon in which the size of the large particles gradually increases and the size of the small particles gradually decreases under the action of the current field. Research has shown that the alloy nano-catalyst particles can generate sintering phenomena under long-term work, the sintering phenomena are related to the stability of the catalyst, and the sintering phenomena can cause the reduction of the activity of the catalyst.
8. In this example, after gold-nickel alloy nanoparticles are prepared on carbon fibers loaded with precursor salts through in-situ transient carbothermic reaction, the movable end of the sample rod is driven to be far away from the fixed end of the sample rod, so as to prevent the two ends from colliding due to instability of movement in the later rotation process.
Through the axis-aligning operation of the sample rod, the carbon nano-wire to be researched is moved to the rotating axis by utilizing the displacement compensation function, so that the phenomenon of deviating from the visual field due to larger drift in the rotating process is avoided.
After the shaft is aligned, the carbon nanofiber is in a positive focal state by adjusting the height Z of an electron microscope. To obtain better contrast, a three-stage objective aperture was used to cover the central spot in this example, and a central bright field image was taken. By operating the rotation operation software, the carbon nanofiber is rotated by 180 degrees in an electron microscope, and a central bright field image of the carbon nanofiber attached with the alloy nanoparticles is shot at intervals of 2 degrees, so that a series of two-dimensional projection transmission electron microscope photos at different angles are obtained. The left image of fig. 8 is a transmission electron microscope image of a carbon fiber loaded with alloy nanoparticles at an angle.
9. In this example, IMOD software was selected to preprocess, axis and reconstruct a series of captured transmission electron microscope photographs of dimensional projection at different angles. The reconstruction algorithm selected in this example is a relatively basic, convenient weighted back projection method. Subsequently, the reconstructed file is visualized by tommiz software to obtain a three-dimensional reconstruction structure as shown in the right-hand diagram of fig. 8. The result can be used for determining the real three-dimensional space distribution condition of the alloy nanoparticles on the nano fibers, and the defect that the traditional two-dimensional transmission electron microscope projection result lacks thickness information is overcome.
Claims (10)
1. A transmission electron microscope technology for in-situ research of three-dimensional distribution structure of nano particles is characterized in that: the nano particles are multi-component alloy nano particles loaded on carbon nanofibers, the three-dimensional distribution structure refers to a three-dimensional distribution structure of the nano particles in a sintering process under the action of a current field, and the method comprises the following steps:
step 1: obtaining the nano carbon fiber with the diameter range of 100-200nm after carbonization treatment by an electrostatic spinning technology and heat treatment;
step 2: preparing a metal precursor salt solution, and soaking the carbon nanofibers obtained in the step (1) in the precursor salt solution to obtain carbon nanofibers loaded with precursor salt;
and step 3: taking a nickel-chromium metal needle with the diameter of 0.2-0.25 mm, preparing a metal needle point by an electrochemical polishing method, shearing the metal needle point into a length of 3-5 mm, and then placing the metal needle point into the fixed end of a sample rod of the transmission electron microscope to enable the metal needle point to face outwards;
and 4, step 4: taking 2mm and 1mm long and wide carbon nanofibers loaded with precursor salt, adhering the carbon nanofibers on a clamped gold needle platform with the diameter of about 0.25mm by using conductive silver adhesive, and placing gold needles at the movable end of a sample rod of the transmission electron microscope;
and 5: inserting the sample rod loaded with the metal needle tip and the gold needle into a transmission electron microscope, and adjusting the height and the position of the gold needle at the movable end of the sample rod to enable the metal needle tip and the gold needle to be positioned at the same height and at the opposite position;
step 6: enabling the nano carbon fibers on the golden needle platform to be in contact with the metal needle tip, and applying a certain voltage within a certain instant time to enable precursor salt loaded on the nano carbon fibers on the golden needle platform to perform instant carbon thermal reaction to form alloy nanoparticles;
and 7: after the alloy nanoparticles are formed, continuously keeping the metal needle tip in contact with the carbon nanofibers, applying a continuous small current, and observing the distribution change and sintering condition of the alloy nanoparticles under the action of a current field;
and 8: carrying out 180-degree rotation on the carbon nanofibers loaded with the alloy nanoparticles, and shooting central bright field images of 1 carbon nanofiber attached with the alloy nanoparticles at intervals of 2 degrees so as to obtain a series of two-dimensional projection transmission electron microscope photos at different angles;
and step 9: and (4) carrying out alignment, three-dimensional reconstruction and visualization processing on the two-dimensional projection transmission electron microscope photos under a series of different angles obtained in the step (8) by using reconstruction software.
2. The transmission electron microscope technology for in-situ research on the three-dimensional distribution structure of nanoparticles according to claim 1, wherein the step 1 is realized by the following method:
step 1.1: mixing PAN and Dimethylformamide (DMF) according to a certain proportion, and then filling the mixture into an injector;
step 1.2: placing a substrate bearing carbon nanofibers on a receiving screen, and using copper foil or carbon paper as the substrate;
step 1.3: adjusting the relative position of the receiving screen and the ejector to start the spinning process; after spinning is finished, a piece of PAN nanofiber is formed on the substrate;
step 1.4: and (3) carrying out heat treatment on the PAN nanofiber obtained by spinning: firstly, placing PAN nano-fiber at 260 ℃ in an air environment for 5 hours of pre-oxidation treatment, and then placing the PAN nano-fiber at 800 ℃ for 2 hours of carbonization treatment under the protection of argon gas, so as to obtain the nano-carbon fiber.
3. The transmission electron microscope technology for in-situ research on the three-dimensional distribution structure of nanoparticles according to claim 1, wherein in step 2, precursor salt with alloy components is mixed in ethanol solution to obtain precursor ethoxide solution; and soaking the carbonized carbon nanofibers in a precursor ethanol salt solution to fully soak the carbon nanofibers, taking out the carbon nanofibers, placing the carbon nanofibers in a glass dish, and naturally drying the carbon nanofibers to obtain the carbon nanofibers loaded with precursor salts.
4. The transmission electron microscope technology for in-situ research on the three-dimensional distribution structure of nanoparticles according to claim 1, wherein the process of preparing the metal needle tip by the electrochemical polishing method in the step 3 is as follows:
step 3.1: preparing corrosive liquid, putting the corrosive liquid into a glass container, and placing a plastic bracket beside the glass container;
step 3.2: connecting two leads with the tail ends connected with the copper sheet with two ends connected with two ends of a power supply, immersing the copper sheet connected with the negative electrode into corrosive liquid of a glass container, and fixing the copper sheet connected with the positive electrode on a plastic bracket and placing the copper sheet above the corrosive liquid;
step 3.3: sleeving a metal round bar to be corroded on two plastic sleeves, namely an upper plastic sleeve and a lower plastic sleeve, wherein one end of the round bar is connected with a copper sheet fixed on a plastic bracket; a small gap of 0.5-1.0 mm is exposed between the upper plastic sleeve and the lower plastic sleeve; the upper end of the upper plastic sleeve is positioned above the liquid level, and the lower end of the lower plastic sleeve is positioned below the lower end of the round rod;
step 3.4: after the arrangement is finished, turning on a power supply, adjusting the voltage value to be 5-20V, and carrying out corrosion reaction;
step 3.5: and (3) breaking the small seam between the two plastic sleeves, turning off the power supply after the lower end of the round rod falls into the corrosive liquid, taking out the falling round rod from the corrosive liquid, and taking out the plastic sleeves from the tail part of the round rod by using tweezers to obtain the nanoscale metal needle tip.
5. The transmission electron microscope technology for in-situ research on the three-dimensional distribution structure of the nano particles as claimed in claim 1, wherein the step 4 is realized by the following method:
step 4.1: tearing off small pieces with the length and the width of 2mm and 1mm respectively on the carbon nanofiber loaded with the precursor salt by using a pair of tweezers, and forming a small tip in the tearing process so as to easily realize the contact with the metal needle point at the fixed end of the sample rod in the transmission electron microscope;
step 4.2: taking a gold needle with the length of about 3mm and the diameter of about 0.25mm, and using the advantage of good metal ductility to clamp a section of the gold needle flat, so as to obtain a small platform with the thickness of about 0.05mm, so that the nano carbon fiber torn down in the step 4.1 can be firmly fixed on the platform through conductive silver adhesive, and the tip of the nano carbon fiber faces outwards;
step 4.3: and (4) placing the gold needle adhered with the carbon nanofibers into the movable end of the sample rod.
6. The transmission electron microscope technology for in-situ research on the three-dimensional distribution structure of nanoparticles according to claim 1, wherein the step 5 is realized by the following method:
step 5.1: and inserting the sample rod loaded with the metal needle tip and the gold needle into an electron microscope, and adjusting the Z-axis height of the electron microscope to enable the metal needle tip at the fixed end of the sample rod to be in a positive focal state.
7. The transmission electron microscope technology for in-situ research on the three-dimensional distribution structure of nanoparticles according to claim 1, wherein the step 6 is realized by the following method:
step 6.1: the height of the movable end is adjusted to enable the part of the nano carbon fiber on the gold needle needing instantaneous carbon thermal reaction to be in a positive focal state, and the front and back and the left and right of the movable end of the sample rod are adjusted to enable the part of the nano carbon fiber in the positive focal state to be in contact with the metal needle point at the fixed end of the sample rod;
step 6.2: applying a certain instantaneous voltage between the fixed end and the movable end within a certain instantaneous time, observing the carbon thermal reaction process of the nano-fibers at the contact part of the gold needle and the metal needle tip in situ, and observing the formation process of alloy nano-particles;
step 6.3: and (5) moving the nano-fibers at other parts to contact with the metal needle tip, and observing and researching the influence of different time and voltage on the formation of the alloy nano-particles by changing the instantaneous time and the instantaneous voltage.
8. The transmission electron microscope technology for in-situ research on the three-dimensional distribution structure of nanoparticles according to claim 1, wherein the step 7 is realized by the following method:
after alloy nanoparticles are formed by precursor salt under the action of instantaneous carbon heat, the contact state of the carbon nanofibers and the metal needle point is continuously kept, small current is applied between the fixed end and the movable end of the sample rod, the current form and the current size are changed, pulse current or constant current is used, and the distribution and sintering conditions of the alloy nanoparticles under the continuous action of a current field are observed in situ.
9. The transmission electron microscope technology for in-situ research on the three-dimensional distribution structure of nanoparticles according to claim 1, wherein the step 8 is realized by the following method:
step 8.1: in order to prevent two ends from colliding due to unstable movement in the subsequent selection process, the movable end of the sample rod needs to move backwards to be far away from the fixed end of the sample rod;
step 8.2: before the carbon nanofibers with alloy nanoparticles are subjected to rotary shooting, the carbon nanofibers are moved to an axis by using displacement compensation through the axis aligning operation of a sample rod, so that the carbon nanofibers do not generate large drift in the rotating process and deviate from the visual field;
step 8.3: after the shaft is aligned, the height Z of the electron microscope is adjusted to enable the carbon nanofibers to be in a positive focal state, then 180-degree rotation is carried out, and a central bright field image of the carbon nanofibers attached with the alloy nanoparticles is shot at intervals of 2 degrees, so that a series of two-dimensional projection transmission electron microscope photos under different angles are obtained.
10. The transmission electron microscope technology for in-situ research on the three-dimensional distribution structure of nanoparticles according to claim 1, wherein the step 9 is realized by the following method:
and (3) using three-dimensional reconstruction software to carry out image preprocessing, axis aligning, reconstruction and visualization on the same carbon nanofiber before and after the action of the current field to obtain the real distribution condition of the carbon nanofiber particles before and after the action of the current field.
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