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Observing Growth of Nanostructured ZnO in Liquid Ting-Huan Hsieh, Jui-Yuan Chen, Chun-Wei Huang, and Wen-Wei Wu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02040 • Publication Date (Web): 31 May 2016 Downloaded from http://pubs.acs.org on June 4, 2016
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Observing Growth of Nanostructured ZnO in Liquid Ting-Huan Hsieh†, Jui-Yuan Chen†, Chun-Wei Huang, and Wen-Wei Wu* Department of Materials Science and Engineering,National Chiao Tung University,Hsinchu 300, Taiwan ABSTRACT: Hydrothermal synthesis is commonly used for producing large area of ZnO nanowires due to its simple and low-cost process. However, the mechanism of hydrothermal synthesis remains unknown. In this work, zinc acetate and HMTA dissolved in DI water as precursor solution was sealed in a liquid cell for observation by in-situ TEM. The growth of ZnO nanowires was classified into two steps. The first stepwas the nucleation and growth of ZnO nanoparticles. The ZnO nanoparticles grew as a result of either isotropic monomer attachment on the2110 and0110 surfaces or coalescence of nanoparticles in the same crystal arrangement. The second step was that the anisotropic growth of ZnO nanoparticles grew into nanowires anisotropically on the (0001) surface. Because the (0001) surface is Zn-terminated with positive charges that can attract the negatively charged monomers,i.e.,[ ] ,the monomers tended to deposit on the (0001) surface,resulting in ZnO nanowires growing along the [0001] direction.Moreover, the growth of ZnO nanowires was identified to be a reaction-controlled system. The direct observation of the dynamic process sheds light on the hydrothermal synthesis method.
Zinc oxide(ZnO) is an important II-VI compound semiconductor material with a direct wide band gap of 3.37eV and large bonding energy of 60meV. Among various type of nanostructures, for example nanobeltsnanorods and nanowires, one-dimensional (1D) ZnO nanowires have attracted significant attention due to their unique physicochemical properties, such as semi-conductivity, photoelectricity, and piezoelectricity. ZnO nanowires have been used in many nanodevices, including nanogenerators1, solar cells2, LEDs3 and photo sensors4. Many techniques have been used to synthesize ZnO nanowires, such as thermal evaporation5, metal organic chemical-vapor deposition6 (MOCVD), pulsed laser deposition7 (PLD) and hydrothermal synthesis8. Among these techniques, hydrothermal synthesis is the most promising because it is able to synthesize ZnO nanowires in a simple, low-cost process and large scale9. However, the mechanism of hydrothermal synthesis is not well understood. Because different morphologies of nanomaterials will have different physical properties that can strongly affect the functions of nanodevices10-13, understanding the mechanism of the synthetic process will improve the applicability and controllability of hydrothermal synthesis. Existing literature has only reported the before/after states of the hydrothermal method. Thus, information on the chemical formation and atomic dynamics has not been provided and requires a detailed investigation. Therefore, the in-situ liquid TEM regarded as the most directly technique to verify the hydrothermal synthesis.
Recently, the development of the microfabricated liquid cell14-17 offers an opportunity to interpret the mechanism of a synthetic process under a high vacuum environment. This new technique enables transmission electron microscope (TEM) sample to encapsulate a small amount of liquid, which allows for in-situ observation of the liquid phase. In-situ TEM has been shown to be a powerful tool for studying electrochemical reactions and growth kinetics18-24. In previous research, liquid cells have been used to study nanocrystal synthesis. For example, the in-situ observations of facet development25-26, kinetics27-28, and the surfactant effect29 in nanocrystals have provided useful information to establish the growth mechanism. However, these reports focused on nanocrystals synthesized via sol-gel method30, which utilized an organic solvent. The mechanism of the hydrothermal method utilizing an aqueous solvent remains elusive. For this purpose, we investigated the evolution of ZnO synthesized from the hydrothermal method to establish its growth mechanism. The concentration of the precursor solution used in the in-situ liquid cell was the same as that used in the normal synthesis. The difference between the normal process and the liquid cell was that a seed layer was not present in the liquid cell. Although the process did not use a seed layer, ZnO nanowires could still be synthesized; details are included in the Supplementary Information (Figure S1). Thus, we can apply this new technique to study the growth mechanism of ZnO nanowires synthesized by the hydrothermal method. Understanding the mechanism
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JEOL JEM-ARM 200F TEM was used for analyzing the structures of the products. ■RESULTSAND DISCUSSION
Figure 1. The growth of ZnO nanoparticles by monomer attachment. The series of images was taken from Movie S1. During the growth process, the nanoparticles grew isotropically with regular hexagonal morphology. The growth planes of ZnO nanoparticles were identified as 2110 and 0110.
would be beneficial for the development of the bottomup processing of ZnO nanowires.
■MATERIALSANDMETHODS The precursor solution was prepared by dissolving zinc acetate and hexamethylenetetramine (HMTA) in deionized water. The precursor solution was then mixed under vigorous stirring for half an hour. The concentrations for zinc acetate and HMTA were both 1 mM, and the solution was used without further purification. The liquid cell for the in-situ experiment contained a bottom chip and a top chip. Silicon nitride membranes in the middle of both chips acted as windows for observations, as shown in Figure S2. The thickness of the silicon nitride membrane was 60 nm, which allowed the passage of incident electrons. The bottom chip contained a gold spacer with a thickness of approximately 200 nm. The gold spacer controlled the volume of the liquid layer when the bottom chip and the top chip were sealed together. The liquid cell was loaded into the Protochips Poseidon 510 holder, which is available for most JEOL TEM models. The detailed process of solution loading is included in the Supplementary Information (Figure S3). Once the loading process was finished, the solution was sealed inside using both chips and o-rings in the holder and subsequently loaded into a TEM (JEOL JEM-2100F TEM). The TEM operated at 200 kV, and the beam current density was 10 A/m . The heating effect of the electron beam acted as a driving force for hydrothermal synthesis20. The calculated temperature of solution under electron beam was about 85°C, and the pressure was 760 torr31. The details of the estimation of electron beaminduced temperature rise were included in the supporting information. The construction of the process is shown in Figure S4. This setting enabled the in-situ observation of the nucleation and growth process of ZnO nanowires.
According to the in-situ TEM experimental results, the evolution of ZnO nanowires can be divided into two steps: 1) nucleation and growth of nanoparticles and 2) growth into nanowires. Figures 1 to 2 illustrate the first step, i.e., the nucleation and growth of ZnO nanoparticles. Here, we observed two mechanisms of the growth process in ZnO nanoparticles. The first is shown in Figure 1, which was taken from Movie S1. Here, a nanoparticle with a regular hexagonal shape, marked by the red arrow, increases in size during the experiment. The regular hexagonal structure remained the same shape during the whole process. The size of the nanoparticle increased from 23.8 nm to 29.8 nm (as shown in Figures 1B-E), where the size refers to the distance between the diagonal planes. The growth rate of each plane was calculated to be 0.107 nm/s. We deduced that isotropic growth of nanoparticles resulted from monomer attachment. Because the structure of ZnO was wurtzite structure, the hexagonal shape was more stable for the ZnO nanoparticles. The planes of the regular hexagonal shape in the nanoparticle were identified as 2110and 0110, as shown in Figure S5. The second growth mechanism is shown in Figure 2. As shown in Figures 2A-E, the two nanoparticles marked by the red circle grew via monomer attachment, which is similar to the case in Figure 1. In Figures 2E and 2F, the
Figure 2. Series of TEM images, showing that ZnO nanoparticles grew by coalescence. (A-E) Two nanoparticles grew by monomer attachment. (F-H) The coalescence process took place as two particles attached to each other. (I) Schematic of the coalescence process showing the way building blocks filled the kink sites. The blue parallelograms represent the building blocks of ZnO due to the wurtzite structure of ZnO.
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coalescence process started when the two nanoparticles became attached to each other. The smaller nanoparticle merged into the larger one, as shown in Figures 2F and 2G. The monomers first filled the neck during the coalescence process and then filled the corner, thereby maintaining the hexagonal shape of the particle, as shown in Figure 2H. In Figure 2G, the morphology of the coalesced nanoparticle became an irregular hexagon. Subsequently, the bottom left plane of the nanoparticle grew faster, and finally, the nanoparticle turned into a regular hexagon. The attachment behavior of the monomer will be discussed later. The growth rate of the bottom left plane was calculated to be 1.57 nm/s. From Figure 2, it was obvious that not all the adjacent nanoparticles would grow by the coalescence process. The coalescence process takes place only when the adjacent nanoparticles have the same crystal construction and the same arrangement of the plane direction. In our work, these was no surfactants in our experiment, the solution was prepared by 1mM zinc acetate and 1mM HMTA in D.I. water. The nanoparticles tended to become a regular hexagon in shape, which can be explained by the Wulff construction16,26,32. The Wulff construction is used to determine the equilibrium shape of crystals by minimizing the Gibbs free energy. It can be interpreted that the different growth rates in different planes led to ZnO aggregation into a hexagonal shape. Furthermore, the Kinetic Wulff construction may be more appropriate for liquid cell TEM. The model of Kinetic Wulff construction (h t λ t v ) is similar to that of the Wulff construction(h λγ ), which differed from the shape, determined by growth velocities (v) instead of the surface free energies (γ). In our cases, the attachment energies for hexagonal ZnO are calculated and the 2110 and 0110 planes are with the lower attached energy (1.126 x 10-19 J/atom).The predicted shape of ZnO crystal was the same both in Wulff construction and kinetic Wulff construction models. To simplify the description, the Wulff construction model has been adopted since it is direct and general. Figure 2I shows a schematic of the coalescence process and the magnification of the bottom left plane in Figure 2G. Due to the wurtzite crystal structure of ZnO (α = β = 90°, γ = 120°), we used parallelograms to illustrate the building blocks of ZnO nanoparticles. When ZnO nanoparticles are shaped as hexagons, steps exist on the planes. From the kinetic view point, steps lead to an increase in dangling bonds. Dangling bonds cause an increase in the surface energy. To reduce the surface energy, building blocks tend to fill these steps, as illustrated in Figure 2I. The building blocks attached themselves at suitable sites, called kink sites, which are incomplete rows of atoms. The building blocks would quickly fill into the kink sites existed at the bottom left plane. In the end, the plane became smooth, and the morphology became hexagonal shape. Also, the growth rate of the planes became uniform that all planes grew by monomer attachment.
Figure 3. Anisotropic growth of ZnO nanowires. (A) Series of TEM images taken from Movie S2, where a ZnO nanowire grows at one end in the axial direction, as marked by the red arrow. (B) The crystal structure of ZnO. and represent the dipole moment induced by the separation of and .
From the two growth mechanisms of ZnO nanoparticles, we noticed that the nanoparticles ultimately turn into regular hexagonal shapes. In addition, the growth rate of the coalescence process was 15 times faster than monomer attachment. These phenomena demonstrated that the nanoparticles would reach the lowest surface energy during growth. In the monomer attachment process, the nanoparticles would maintain the regular hexagonal shape. The process of monomer attachment required more energy becauseincreasing the surface area resulted in an increase in the surface energy, while in the coalescence process, the surface area did not increase as the building blocks filled the kink sites from a kinetic viewpoint. As a result, the coalescence process needed less time to take place. We explored the mechanism of nanoparticle growth and identified that the growth satisfied the Wulff construction theory. The second step in the formation of ZnO nanowires was the anisotropic growth of ZnO nanoparticles. The process is shown in Figure 3A. From these sequential images, we observed that a nanoparticle grew along a specific direction and transformed its morphology to a nanowire. For ZnO crystal, 0001 is terminated by and 0001 is terminated by , as shown schematically in Figure 3B. This structure led to a positive charge at the 0001 surface and a negative charge at the 0001 surface, which resulted in a spontaneous dipole moment along the caxis. As a basic characteristic of the dipole moment, the
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positive end of the surface would attract the negatively charged ions in solution and vice versa. The chemical reactions in the solution are the following: ! 4 ↔ [ ] [ ] ↔ $ ! ! 2 where[ ] is the monomer in hydrothermal synthesis. The monomers tended to attach onto the 0001 surface due to electrostatic attractions between the positive end of the dipole and the ions. In contrast, the monomers had difficulty attaching onto the 0001 surface due to electrostatic repulsion between the negative end of the dipole and the ions. Hence, ZnO grew anisotropically along one end of the c-axis. Here, we did not directly exhibit the nanoparticle nucleation and growth into nanowire at the same case for the following reasons: Firstly, on the requirement of in-situ observation, the “diameter and shape” change of nanoparticles grew from monomer attachment or coalescence should be observed from the top view(zone axis [0001]); on the other hand, the “length” increase of nanowire should be observed from the side view. Secondly, the nanoparticle lying on membrane (c axis is parallel to electron beam) may be suppressed as it grew to nanowire due to the limited space in z direction. (The nucleation and growth processes at the same case was shown in Figure S6) Figure 4A is a high-resolution (HR) image of nanowires formed in-situ in the liquid cell. The enlarged image of the nanowires and the corresponding diffraction pattern is shown in Figure 4B. The high-resolution image revealed that the ZnO nanowire was single crystalline with a smooth surface and further verified that the nanowire is a wurtzite-structured ZnO. The growth direction was along the c-axis [0001] in agreement with the dipole theory mentioned above. Moreover, using data acquired from Movie S2, we plotted the length and diameter against time to analyze the formation behavior of a ZnO nanowire. In Figure 5A, the length of a ZnO nanowire grew at a steady rate. Thegrowth rate in the axial direction was 4.3 nm/s, which was
Figure 4.TEM analysis of ZnO nanowires. (A) HRTEM image of ZnO nanowires in the liquid cell (B) Enlarged image and the corresponding diffraction pattern of the ZnO nanowire.
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even higher than the rate in the radial direction via monomer attachment and the coalescence process. This meant that the driving force induced by the dipole moment was higher than monomer attachment and the coalescence process, leading to the formation of ZnO nanowires. In addition, the monomers move fast in liquid, which could result from large diffusivity encountered in Fick’s law of diffusion. The diffusion rate was not the main control factor of the growth process; instead, the growth rate is determined by the chemical reaction rate. The axial growth rate was roughly linear, which meant that the growth of ZnO nanowires was a reaction controlled system. In Figure 5B, the diameter of the ZnO nanowire did not change significantly during observed time period. The nucleation process exhibited the nanoparticle growing its diameter, whenever in monomer attachment or in the coalescence process. While there is no obviously change in radial direction during nanowire growing. This results demonstrated that the diameter of the ZnO nanowire was mainly determined by the original dimensions of the ZnO nanoparticles before they transformed into a nanowire. The whole nucleation and growth processes in a same case was shown in Figure S6. The diameter increased in radial direction during nucleation and it increased in axial during growth. As mentioned above, ZnO nanoparticles are regular hexagonal in shape to reduce the surface energies from the viewpoint of kinetics. Adding building blocks onto the surface increased the dimensions, which led to an increase in the surface area and the surface energy. To maintain the equilibrium state of nanoparticles, more time and energy were needed to increase the diameter. In addition, the dipole moment increased because the sum of the overall polarity increased as the length of the nanowires increased. The monomers preferred to attach onto the [0001] surface instead of the {2110} or the {0110} surfaces. We also observed and calculated the growth rates in the length and diameter of other ZnO nanowires and found that the change of diameter as a function of time in these ZnO nanowires was insignificant. Note that the diameter of the nanowires may become larger after a longer period of growth time. Because the observation time in our experiment was limited, an increase in the diameter during ZnO nanowire growth was not observed. Furthermore, during
Figure 5. Plot of the growth rate of a ZnO nanowires. (A) the length and (B) the diameter as a function of time.
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our observation process, the evolution of the ZnO structure was often hindered by adjacent nanostructures and bubbles formed by the irradiation of the electron beam.
■CONCLUSIONS Using a liquid cell combined with in-situ TEM is a promising technique for studying reactions in the liquid phase. In this work, we successfully observed the evolution of ZnO nanowires by hydrothermal synthesis using liquid cell TEM. The growth process was divided into two steps. The first step was the nucleation and growth of ZnO nanoparticles. The growth of ZnO nanoparticles mainly resulted from monomers attachment and coalescence. During the coalescence process, the monomers filled the kink sites to reduce the surface energy. This phenomenon can be explained using the theory of Wulff construction. The second step was the anisotropic growth of ZnO nanowires from ZnO nanoparticles. The negatively charged monomers [ ] easily attached onto the 0001 surface, which was terminated. In contrast, the negatively charged monomers had difficulties attaching onto the 0001 surface, which was terminated by . Therefore, ZnO nanowires grew anisotropically along the [0001] direction. From the growth rate analysis, we demonstrated that the growth process in liquid is a reaction controlled system, and the diameter of ZnO nanowires was determined by the original size of the nanoparticles.
■ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. SEM, TEM, XRD of normal hydrothermal synthesis ZnO nanowires, schematic of liquid cell and loading process, HRTEM of ZnO nanoparticles on liquid cell. In addition, three in-situ TEM videos (AVI) are included.
■AUTHOR INFORMATION Corresponding Author * Email:
[email protected] Author Contributions †
These authors T.H. Hsieh and J.Y. Chen contributed equally.
Notes The authors declare no competing financial interest.
■ACKNOWLEDGMENT W.W.W. acknowledges the support by Ministry of Science and Technology through grants 103-2221-E-009-056-MY2, 103-2221-E-009-222-MY3, and 104-2221-E-009 -050 -MY4.
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