Method for in situ dynamic characterization of sn02 nanocrystal growth process

Abstract

The application discloses a method for in-situ dynamic characterization of SnO2 nanocrystal growth process, comprising the following steps: collecting in-situ data of the sample by using a transmission electron microscope; changing the external liquid phase environment of the sample, and recording in-situ data of the sample under different influencing factors; dividing the growth stage of the SnO2 nanocrystal according to the in-situ data; measuring and calculating size, area and reaction rate data in the growth process of the SnO2; fitting the relationship between the size and time to obtain the LSW growth kinetics model to which the SnO2 nanocrystal belongs; calculating the surface energy of the exposed high-energy surface in the growth process of the SnO2 to obtain the relationship between the surface energy and the preferred growth direction of the nanocrystal; measuring and calculating the rotation speed and translation speed between the SnO2 nanocrystals, and analyzing the driving force which plays a leading role. The application clarifies the growth mechanism by constructing a growth kinetics model, and provides a reliable theoretical basis for efficient regulation of the microstructure of SnO2.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of material characterization and analysis, and particularly relates to a method for in-situ dynamic characterization of SnO2 nanocrystal growth process. BACKGROUND

[0002] At present, people have tried to design and process SnO2, realizing the regulation of SnO2 morphology and structure, which not only enriches the diversity of SnO2, but also improves the performance of SnO2 to some extent. In fact, the essence of regulating the morphology and structure of SnO2 is to regulate the surface / interface of the material. People pay attention to the regulation of the surface / interface because a series of chemical reactions are carried out on the surface / interface of SnO2, and there is a spatial difference in the crystal structure of SnO2, and the atomic stacking and arrangement rules are different in different crystal faces, thus showing different surface electronic structures and surface defect states. These different surface structures and surface properties greatly affect the chemical reaction process, and finally make SnO2 different crystal faces show great performance difference. Experiments have confirmed that different exposed crystal faces of SnO2 have a great impact on its various performances. The high-energy crystal face of SnO2 has more surface dangling bonds and higher surface energy, which not only can provide more active sites in the reaction process, but also can reduce the reaction activation energy, showing obvious advantages and application potential. It is found that the essence of designing the structure and morphology of SnO2 is to regulate the surface / interface, and the purpose is to preferentially expose the high-energy crystal face.

[0003] The preparation methods of SnO2 nanomaterials can be divided into liquid phase method, gas phase method and solid phase method. Among them, the liquid phase method is the most widely used. By controlling experimental parameters such as reaction temperature, reaction time, precursor solution concentration and the like, the nucleation and growth of nanocrystals are adjusted to synthesize SnO2 nanocrystals with specific size, morphology and structure, but this process needs a large number of pre-experiments to summarize the rules. In addition, due to the interaction of kinetics and thermodynamics, in the natural nucleation and growth process of SnO2 nanocrystals, the main exposed crystal faces of SnO2 are (110), (101) and (100) faces with low surface energy, while (111), (221) and (332) high-energy faces will eventually disappear because of too fast growth speed. It can be seen that the optimization in the liquid phase synthesis does not start from the mechanism level of SnO2 itself, and fundamentally regulates its structure. This is because the physical nature behind the structure evolution of SnO2 nanocrystals and the understanding of its growth mechanism are not deep enough. However, due to the limitation of conditions, the understanding of the growth mechanism of nanocrystals is mostly limited to the stage of derivation or theoretical simulation. At present, this information is lagging and non-in-situ, which cannot reveal the truth and is difficult to track the in-situ dynamic process of the structure evolution and growth of SnO2. Therefore, it fundamentally hinders the effective regulation of the surface / interface structure. Only by real-time dynamic characterization of the growth process of the surface / interface structure of SnO2 nanocrystals and in-depth study of its growth mechanism, can the properties of SnO2 be better grasped and designed, and SnO2 devices with specific functions can be constructed. SUMMARY

[0004] The technical problem solved by the present application is that the present application provides a method for in-situ dynamic characterization of the growth process of SnO2 nanocrystals, which clarifies the growth mechanism by constructing a growth kinetics model, and provides a reliable theoretical basis for efficient regulation of the microstructure of SnO2.

[0005] Technical scheme: The method for in-situ dynamic characterization of the growth process of SnO2 nanocrystals comprises the following steps:

[0006] Step 1. Collecting in-situ data of the sample by using a transmission electron microscope;

[0007] Step 2. Changing the external liquid phase environment of the sample and recording the in-situ data of the sample under different influencing factors;

[0008] Step 3. Dividing the growth stage of SnO2 nanocrystals according to the in-situ data;

[0009] Step 4. Measuring and calculating the size, area and reaction rate data in the growth process of SnO2;

[0010] Step 5. Fitting the relationship between size and time to obtain the LSW growth kinetics model to which the SnO2 nanocrystals belong;

[0011] Step 6. Calculate the surface energy of the exposed high-energy surface in the growth process of SnO2, and obtain the relationship between the surface energy and the preferred growth direction of the nanocrystal;

[0012] Step 7. Measure and calculate the rotation speed and translation speed between SnO2 nanocrystals, obtain the motion state of the nanocrystal, and analyze the dominant role of the driving force accordingly.

[0013] Preferably, the sample is a liquid cavity formed by a double-layer ultra-thin carbon film covering the reaction solution.

[0014] Preferably, the in-situ data includes: (1) the nucleation and growth initiation time of SnO2 nanocrystals and the crystal face structure state, (2) the solid-liquid interface phenomenon of SnO2 nanocrystals in contact with the reaction solution, (3) the change of the structure and morphology of SnO2 nanocrystals in the growth process, and the preferred growth direction of SnO2 nanocrystals.

[0015] Preferably, the growth stage includes: atom formation period, nucleation period and growth period.

[0016] Preferably, in step 4, Gatan Digital Micrograph software is used to analyze the transmission electron microscope image, and Image J software is used to measure and calculate the size, area and reaction rate data in the growth process of SnO2.

[0017] Preferably, the size is represented by the effective diameter d of the nanocrystal, and the calculation formula is Where A represents the projected area of the nanocrystal.

[0018] Preferably, the calculation formula of the surface energy is Where, is the total energy after template structure optimization, E bulk is the total energy after unit cell optimization, n is the number of cell units contained in the template, and a is the surface area on both sides of the template.

[0019] Preferably, the driving force includes: electrostatic force, van der Waals force, hydration force and magnetic force.

[0020] Beneficial effects: The application fills the understanding of the physical nature behind the structural evolution of SnO2 nanocrystals, and pushes the understanding of the original derivation or theoretical simulation stage to in-situ dynamic characterization, which has important guiding significance for material design and development. The application uses liquid transmission electron microscopy technology to carry out in-situ experiment of SnO2 nanocrystal growth, dynamically characterizes the growth process of SnO2 nanocrystals, constructs a growth kinetics model, and illustrates the growth mechanism, thereby providing a solid foundation for efficient regulation of SnO2 microstructure. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a schematic diagram of SnO2 nanocrystal growth stages under LaMer model;

[0022] Figure 2 is a diagram of SnO2 nanocrystal effective diameter change over time;

[0023] Figure 3 is a diagram of rotation speed and translation speed between nanocrystals over time. DETAILED DESCRIPTION

[0024] The present application is further described below in conjunction with the accompanying drawings and specific examples.

[0025] Example 1

[0026] The method for in-situ dynamic characterization of SnO2 nanocrystal growth process comprises the following steps:

[0027] Step 1. Collect in-situ data of the sample by using transmission electron microscopy;

[0028] Step 2. Change the external liquid phase environment of the sample, and record in-situ data of the sample under different influencing factors;

[0029] Step 3. Divide the growth stages of SnO2 nanocrystals according to the in-situ data;

[0030] Step 4. Measure and calculate the size, area and reaction rate data in the growth process of SnO2;

[0031] Step 5. Fit the relationship between size and time to obtain the LSW growth kinetics model to which the SnO2 nanocrystals belong;

[0032] Step 6. Calculate the surface energy of the exposed high-energy surface in the growth process of SnO2 to obtain the relationship between the surface energy and the preferred growth direction of the nanocrystals;

[0033] Step 7. Measure and calculate the rotation speed and translation speed between SnO2 nanocrystals to obtain the motion state of the nanocrystals, and analyze the dominant role of the driving force accordingly. The rotation speed calculation formula is ω = Ф / t, where Ф is the rotation angle of any exposed crystal face of the nanocrystal, and t is the motion time. The translation speed calculation formula is v = s / t, where s is the distance moved by the nanocrystal, and t is the motion time.

[0034] Specifically, in step 1, the sample is constructed by the following way:

[0035] To obtain high-resolution imaging, in this experiment, the reaction solution was coated with double-layer carbon film for in-situ transmission electron microscopy observation. The specific method is as follows: SnCl2.2H2O aqueous solution was used as the precursor solution. A piece of ultrathin carbon film was clamped with tweezers with the front face upward, 2.5 μL of the precursor solution was sucked with a pipette and directly dropped onto the center of the ultrathin carbon film. Then, another piece of the same ultrathin carbon film was clamped with the front face downward to cover the precursor solution. At this time, the liquid cavity is similar to a sandwich structure, with the upper and lower ultrathin carbon films sandwiching the liquid layer. After the excess solution evaporates, the van der Waals force between the two layers of ultrathin carbon film can encapsulate a small amount of liquid, and the liquid layer will become a plurality of nanoscale island-shaped droplets. It is calculated that the thickness of the island-shaped droplets in the experiment is several nanometers to several tens of nanometers. The island-shaped droplets will flow slowly under electron beam irradiation and are easy to identify. Compared with some widely used SiNx / Si liquid chips, the double-layer carbon film-coated liquid cavity has the advantages of simple process, low cost and high resolution, and can realize the observation of SnO2 nanocrystals at the atomic scale.

[0036] The way to collect in-situ data is as follows:

[0037] After successfully building the double-layer carbon film-coated liquid cavity, it is matched with a single tilting sample rod, and then the sample rod is placed into a transmission electron microscope for in-situ characterization. The SnO2 surface / interface dynamic characterization experiment will be carried out based on the transmission electron microscopes FEI Tecnai G 20 (accelerating voltage 200 kV) and FEI Titan 80-300 (accelerating voltage 300 kV), wherein the resolution of the transmission electron microscope FEI Titan 80-300 can be as high as 80 picometers. The complete growth process is recorded by the CCD installed in the transmission electron microscope, and the video rate is 10 frames per second. In-situ data of the following three aspects are collected: (1) the nucleation and growth initiation time of SnO2 nanocrystals and the crystal face structure state, (2) the solid-liquid interface phenomenon of SnO2 nanocrystals in contact with the reaction solution, (3) the change of SnO2 nanocrystal structure and morphology during growth, and the preferred growth direction of SnO2 nanocrystals. For the low-contrast transmission electron microscope pictures taken, digital image processing technology can be developed to improve the contrast of the pictures.

[0038] The growth mechanism of SnO2 monomers is analyzed by steps 3-6:

[0039] (a) Characterization of the influence of different liquid environments on SnO2 growth. By changing the external liquid environment (surfactant CTAB, concentration and pH value of the precursor solution SnCl2.2H2O, etc.), in-situ data of the influence of the above factors on the microstructure of SnO2 are recorded.

[0040] (b) Establishing the growth kinetics model to reveal the SnO2 growth mechanism. According to the SnO2 growth kinetics process recorded in situ by liquid transmission electron microscopy, the transmission electron microscopy images were analyzed using Gatan Digital Micrograph software, the size, area and growth rate during the SnO2 growth process were measured and calculated using ImageJ software, and the obtained data were analyzed in detail.

[0041] Firstly, according to LaMer growth theory, SnO2 is divided into the following growth stages (see Figure 1 ): (I) Atomic formation period, SnCl2·2H2O precursor liquid generates SnO2 under the action of electron beam, SnO2 is free in the reaction liquid before the SnO2 concentration reaches the nucleation saturation, and no nanoparticles are generated in this process; (II) Nucleation period, when the SnO2 concentration increases to the nucleation supersaturation, SnO2 starts to spontaneously nucleate in large quantities, and as the nucleation proceeds, the SnO2 concentration starts to decrease, and when it decreases to the nucleation threshold, nucleation no longer occurs, and the number of nucleated nanoparticles is determined by the number of nucleated nanoparticles in this stage; (III) Growth period, this stage is mainly the growth process of monomers around the crystal nucleus, forming spherical nanoparticles.

[0042] Then, in order to further reveal the SnO2 nanocrystal growth kinetics, the size change with time was quantitatively analyzed, and the effective diameter of the nanocrystal was used to represent the size, and the calculation formula is where A represents the projected area of the nanocrystal. The classical Lifshitz-Slyozov-Wagner (LSW) growth kinetics theory model is used as the theoretical basis for quantitative analysis, when the size of the nanocrystal and the time relationship follows D∝t 1 / 2 , the growth of the nanocrystal in the solution is limited by the system reaction itself, that is, the reaction-controlled growth (Reaction-limited growth). When the size of the nanocrystal and the time relationship follows D∝t 1 / 3 , the growth of the nanocrystal is limited by the monomer diffusion, that is, the diffusion-controlled growth (Diffusion-limited growth). By fitting the measured data points, the size change of SnO2 nanocrystal in the growth process is approximately proportional to t 1 / 3 , see Figure 2 . Therefore, the SnO2 nanocrystal in the solution is a diffusion-controlled growth.

[0043] Finally, the growth and evolution process of the high-energy crystal face of the SnO2 nanocrystal is focused on, the preferred growth direction of the SnO2 nanocrystal is determined, and the surface energy calculation formula is: where, is the total energy after template structure optimization, E bulkEtot is the total energy of the optimized unit cell, n is the number of unit cells contained in the template, and a is the surface area of the template. The surface energy of the exposed high-energy facets is calculated, and the nanocrystals preferentially grow along the direction of high surface energy.

[0044] The Sn02 aggregation growth mechanism is analyzed by step 7:

[0045] A large number of in-situ experiments show that, in addition to monomer attachment growth, there is also aggregation growth between nanocrystals in the crystal growth process. It is found that there is a strong driving force when nanocrystals are directionally aggregated, and these driving forces are directional. In the growth process of Sn02, the exploration of driving force is the key to the study of the growth mechanism of Sn02. These driving forces include electrostatic force, van der Waals force (vdW), hydration force and magnetic force, etc. The specific driving force that plays a leading role can be obtained according to the experimental results and the solution environment.

[0046] Through the collected in-situ data, the rotation angle of any exposed (200) crystal plane and the moving distance of the nanoparticles are measured using Gatan Digital Micrograph, and the rotation speed and translation speed between nanocrystals are calculated (see Figure 3 ). In the first growth stage (5-25s), the angular velocity is basically zero. If we assume that a nanocrystal is fixed, then another nanocrystal will translate in a direction. This shows that the surface of the nanocrystal will produce electrostatic repulsion, which hinders the aggregation growth of Sn02 nanocrystals. Within a short distance, the Coulomb interaction is greater than the vdW interaction, which makes the nanocrystals continue to move, rather than directly attaching to another nanocrystal. According to the calculation formula of Coulomb's law: where ε0 is the dielectric constant of vacuum (8.854 x 10 -12 C 2 J -1 m), ε r is the relative dielectric constant of the medium (vacuum = 1), q i and q j represent the charges of nanocrystals i and j, and r ij is the distance between nanocrystals i and j. From the above formula, it can be seen that the Coulomb force is inversely proportional to the distance between nanocrystals. Therefore, as the distance between nanocrystals increases, the Coulomb force gradually decays, and the translation speed also decreases from 0.5 to 0.04 nm s -1Subsequently, in the second growth stage (25-38 s), the SnO2 nanocrystals transform into another growth mode, the aggregation growth through instantaneous jumps. In this growth stage, the translation velocity and angular velocity of the nanocrystals increase significantly. This indicates that when the separation reaches a certain distance, the Coulomb interaction between the nanocrystals will be lower than the vdW interaction. At this time, the vdW interaction acts as an attractive force, which is the dominant driving force, and can make the nanocrystals close to each other, resulting in the aggregation growth of the nanoparticles. Once the aggregation growth occurs, the nanocrystal surfaces quickly fuse and co-crystallize, and the total energy of the nanocrystals suddenly decreases, thereby reaching an equilibrium state.

Claims

1. A method for in-situ dynamic characterization of the growth process of SnO2 nanocrystals, characterized in that, The steps include the following: Step 1. Collect in-situ data of the sample using transmission electron microscopy; the sample is a liquid cavity formed by a reaction solution coated with a double-layer ultrathin carbon film; Step 2. Change the external liquid environment of the sample and record the in-situ data of the sample under different influencing factors; the in-situ data includes: (1) the nucleation and growth start time and crystal plane structure state of SnO2 nanocrystals, (2) the solid-liquid interface phenomenon of SnO2 nanocrystals in contact with the reaction solution, (3) the changes in the structure and morphology of SnO2 nanocrystals during the growth process, and the crystal orientation of SnO2 nanocrystals for preferential growth. Step 3. Determine the growth stages of SnO2 nanocrystals based on in-situ data; Step 4. Measure and calculate the size, area, and reaction rate data during the SnO2 growth process; Step 5. Fit the relationship between size and time to obtain the LSW growth kinetic model of SnO2 nanocrystals; Step 6. Calculate the surface energy of the high-energy surface exposed during SnO2 growth to obtain the relationship between surface energy and the crystal orientation of the preferred growth of nanocrystals; Step 7. Measure and calculate the rotational and translational velocities between SnO2 nanocrystals to obtain the motion state of the nanocrystals, and analyze the dominant role of the driving force accordingly; the driving force includes: electrostatic force, van der Waals force, hydration force and magnetic force.

2. The method for in-situ dynamic characterization of the SnO2 nanocrystal growth process according to claim 1, characterized in that, The growth stages include: the atom formation stage, the nucleation stage, and the growth stage.

3. The method for in-situ dynamic characterization of the SnO2 nanocrystal growth process according to claim 1, characterized in that, In step 4, Gatan Digital Micrograph software is used to analyze transmission electron microscopy images, and ImageJ software is used to measure and calculate the size, area, and reaction rate data during the SnO2 growth process.

4. The method for in-situ dynamic characterization of the SnO2 nanocrystal growth process according to claim 1, characterized in that, The size is represented by the effective diameter d of the nanocrystal, and the calculation formula is as follows: ,in A This represents the projected area of ​​the nanocrystal.

5. The method for in-situ dynamic characterization of the SnO2 nanocrystal growth process according to claim 1, characterized in that, The formula for calculating the surface energy is as follows: ,in, The total energy after template structure optimization. It is the total energy after optimization per unit cell. n The number of unit cells contained in the template. a The surface area on both sides of the template.

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