In Situ Observation of the Growth of ZnO Nanostructures Using Liquid

Dec 8, 2017 - Understanding the growth mechanisms and associated kinetics is a fundamental issue towards the specific function-oriented controlled syn...
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In Situ Observation of the Growth of ZnO Nanostructures Using Liquid Cell Electron Microscopy Yubo Wang, Shuai Wang,* and Xing Lu* School of Chemistry and Chemical Engineering & School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, China S Supporting Information *

ABSTRACT: Understanding the growth mechanisms and associated kinetics is a fundamental issue toward the specific function-oriented controlled synthesis of nanostructures. In this work, the growth of zinc oxide nanostructures with different sizes and morphologies are directly observed by in situ liquid-cell transmission electron microscopy (TEM). Real-time observation and quantitative analysis reveal that the concentration ratios of the precursors are responsible for the different growth kinetics, resulting in different morphology and size of the synthesized ZnO nanostructures.

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study of electrochemical reactions,23,24 and imaging of biological samples in liquid.25,26 Hence, the in situ liquid cell TEM is a powerful tool in material science, chemistry, physics, and biology. Real-time imaging of the growth of ZnO NPs and nanowires in aqueous solution has been realized using this technique.27,28 However, the influence of the concentration ratio of precursors on the growth kinetics, size, and morphology of the nanostructures has not been discussed. In this work, we studied the growth and dissolution of ZnO nanostructures using an ultrathin liquid sample inside a liquid TEM cell, and the associated mechanisms were demonstrated in detail. The growth kinetics were revealed by comparing to the classical Lifshitz−Slyozov−Wagner (LSW) growth model under different concentration ratios.

INTRODUCTION As a II−VI semiconductor with diverse morphologies and a large exciton binding energy (60 meV), the zinc oxide (ZnO) nanomaterial has gained substantial research interest, due to its excellent chemical and thermal stability and its special mechanical, electrical, and optical properties.1−5 Using the conventional synthesis techniques such as chemical-vapor deposition,6 thermal sublimation,7 and solution growth methods,5 nanorods,5 nanobelts,8 nanowires,5,9 and nanorings10 of ZnO have been synthesized under special conditions. Considering key factors such as the cost and scalability, the solution growth method, especially the low-temperature aqueous synthesis, is most commonly used for synthesizing ZnO nanostructures.5,11 This method typically employs zinc salt and amine as sources of Zn2+ ions and hydroxide ions (OH−), respectively. For example, in the zinc nitrate and hexamethylenetetramine (HMTA) reaction system, ZnO precipitation occurs upon heating the solution (>60 °C).12,13 Even though various ZnO nanostructures with different morphologies have been synthesized by the solution growth methods, little is known about the growth dynamics and mechanism of ZnO nanostructures. Understanding the fundamental mechanism is important for controlling the size, structure, and morphology of ZnO nanostructures, thereby tuning their physical and chemical properties that strongly affect the applications of these nanodevices.9,14 The direct observation of the chemical and physical events in liquid is therefore urgently needed to truly understand the fundamental growth mechanism of ZnO nanostructures. Recently, as a new breakthrough in electron microscopy and nanofabrication, the emergence of liquid cell transmission electron microscopy (TEM) has made it possible to directly investigate the dynamics of material transformations in liquid environments with high spatial and temporal resolution.15−18 This technique has found many applications in the observation of nanoparticle (NP) formation,19,20 manipulation of NPs,21,22 © XXXX American Chemical Society

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EXPERIMENTAL SECTION The aqueous solutions of Zn(NO3)2·6H2O (99.9% purity, Sigma-Aldrich) and HMTA (99.9% purity, Sigma-Aldrich) with different concentration ratios were prepared by dissolving the precursors in deionized water. A fluid holder (Hummingbird scientific,U.S.A.) and two sorts of silion microchips (Square spacer chip and Square window chip, Hummingbird scientific, U.S.A.) with electron transparent window (Si3N4, 50 nm) for the in situ experiments were used to encapsulate the liquid. The chips need to be cleaned with oxygen plasma for 5 min so as to remove organic contamination and make the surfaces hydrophilic. All of the in situ experiments and ex situ TEM characterizations were performed using a FEI Tecnai G20 F20 microscope operated at 200 kV. The dwell time was set at 4 μs and the beam current is about 500 pA during the in situ experiments. Received: October 11, 2017 Revised: December 8, 2017 Published: December 8, 2017 A

DOI: 10.1021/acs.jpcc.7b10064 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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RESULTS AND DISCUSSION Figure 1 shows a time sequence of high-angle annular dark field (HAADF) images of growing ZnO NPs in the aqueous solution

between the Si3N4 membrane and the thin liquid layer, may hinder the precursor diffusion and therefore significantly impact the nucleation and growth of ZnO NPs.29,30 On the other hand, the growth trend is quite different when the two ZnO NPs are very close, as indicated by the red and blue arrows in Figure 1 A−F. Their growth curves, depicted in Figure 1H, more resemble an exponential function than a power function: with a fast initial growth rate that drops to almost zero later. It is not entirely clear why two contiguous NPs grow differently compared to separate NPs, although the interaction between them may play an important role in the entire growth process.31 When the concentration of HMTA increases to 50 mM (the concentration ratio is 1:5), the growth of the NPs changes dramatically as shown in Figure 2 (see Movie S2 in the

Figure 1. (A−F) Time-lapse series of high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images, showing the growth of ZnO NPs from the aqueous solution of Zn(NO3)2 and HMTA. The concentration ratio is 1:3. The scale bar is 500 nm. The beam current is about 500 pA. (G) Average radius of two relatively noninteracting ZnO NPs (labeled with green and magenta arrows in A−F) versus growth time. (H) Average radii for two adjacent ZnO NPs (labeled with red and blue arrows) versus growth time.

containing 10 mM Zn(NO3)2·6H2O and 30 mM HMTA (see Movie S1, Supporting Information (SI)) and plots the size of NPs as a function of growth time. The growth of NPs with different sizes reveals that the nucleation and growth are inhomogeneous under the irradiation of electron beam, which leads to the decomposition of HMTA to form OH−.27,28 Four representative nuclei, labeled with arrows of different colors in Figure 1A, begin to grow as the HMTA decomposes. During the following growth process, the NPs do not coalesce or become mobile, suggesting that they grow by monomer addition on the Si3N4 membrane (i.e., electron-transparent window of liquid cell). After 70 s, almost all of the ZnO NPs in Figure 1F are larger than 200 nm in diameter, which is the width of the spacer of liquid cell. In other words, the longitudinal growth of the NPs is limited when they reach the size constraint of the liquid cell. Figure 1G depicts the time-dependent evolution of the mean radii of two ZnO NPs (labeled with green and magenta arrows) that are relatively independent because of the large interval between them (see Figure 1A−F). By fitting the two growth curves, the increase of the average radius was found to be t1/6, which is different from t1/3 obtained from the pure diffusionlimited LSW model by a factor of 2 in the power. A previous study showed that the effective radius during the growth process of silver nanocrystals scales as t1/8, and this power is almost three times smaller than 1/3.29 Multiple effects, such as the finite size of the ultrathin liquid layer and the proximity

Figure 2. (A−F) Selected image sequences showing the growth and dissolution processes of individual ZnO NPs from the aqueous solution of Zn(NO3)2 and HMTA. The concentration ratio is 1:5. The scale bar is 10 nm. The electron dose used here is 10.7 e/Å2·s. (G−J) Time-dependent evolution of the diameters of NPs labeled 1−4 in panels A and B. (K, L) Average diameters of NPs labeled 1−4 during the whole process. (M) Size-dependent etching rate of ZnO NPs.

Supporting Information). The NPs are only 1−10% the size of those grown at the concentration ratio of 1:3 (Figure 1). Moreover, the grown ZnO NPs are gradually etched instead of staying at the same size. After 74 s, all separate NPs and some large clusters disappear, as shown in Figure 2F. The large clusters dissolve faster than individual NPs due to their larger contact area with OH− ions and the uneven concentration of OH− resulting from the irradiation of electron beam. To further understand the growth and dissolution of these ZnO NPs, we plotted the time-dependent evolution of the mean diameter during the whole process (Figure 2K,L). Four selected individual ZnO NPs were analyzed. Particles 1 and 2 (Figure 2K) appear on the image at the same time, which is about 5 s earlier than that for particles 3 and 4 (Figure 2L). By fitting these data, we found the following growth and dissolution phenomena. First, the growth time is obviously less than that of dissolution, which is associated with the decreasing OH− concentration with the increasing reaction B

DOI: 10.1021/acs.jpcc.7b10064 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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(1:8), the grown nanostructures become dendrite-like with sizes between 200 and 250 nm. Their detailed growth process can be described as follows. First, the nuclei are formed on the Si3N4 membrane at the liquid−solid interface in the liquid cell after a few scan cycles. Seconds later, these nuclei grow into dendrite-like clusters with multiple branches at their edges. The clusters then continue to grow, developing obviously dendritic characteristics under the continuous exposure to electron beam. The ZnO dendritic nanostructures are not dissolved under these two conditions, which indicated that the HMTA in the liquid cell is sufficient to be absorbed on the surface to prevent the hydrolysis reactions.33 The sizes (mean diameter) of ZnO dendritic nanostructures (DNs) with growth time are shown in Figure 3C,D. Data for the concentration ratio of 1:6 could be approximately fitted to t1/2 based on the LSW model, while the fit for 1:8 is much better. These results clearly suggest that the growth of DNs is reaction-limited. The growth of nanostructures in the liquid cell is generally considered to be diffusion-limited, because both the finite size of the ultrathin liquid layer and the proximity of the Si3N4 membrane will hinder the diffusion of precursors.29,34 However, if there are several kinds of precursors at very different concentrations, the growth will be controlled by the precursor with the lowest concentration, rather than the diffusion of precursors. The growth of ZnO nanostructures in this work gradually transitions from the diffusion-limited model at Zn(NO3)2:HMTA concentration ratios of 1:3 and 1:5 (spherical nanostructures) to the reaction-limited model at 1:6 and 1:8 (dendrite-like nanostructures). A previous study showed that the electron beam current plays an important role in controlling the growth mechanism of nanocrystals, with low beam currents encouraging reaction-limited growth and high currents for diffusion-limited growth.29 However, the beam currents are nearly constant (about 500 pA) during our in situ experiments, so their effect on the growth of nanostructures can be neglected. Instead, the concentration ratio of precursors has a key role in determining the growth kinetics and morphology of the nanostructures. From these experimental findings, the entire growth and dissolution mechanism of ZnO NPs can be summarized in Figure 4. (1) HMTA decomposes under the irradiation of highenergy electron beam to form formaldehyde and ammonia in the liquid cell (eq 1, Supporting Information). Then, the

time. Second, NPs of different sizes have different average dissolution rates (Figure 2M), which suggests the dissolution rate is probably dependent on the NP size.32 Specifically, the dependence is exponential within a certain size range, as shown in the inset of Figure 2M. In order to reveal the growth kinetics of individual ZnO NPs under this condition, we plotted the variation of NPs diameter versus growth time. The results (Figure 2G−J) suggest that the growth is controlled by the precursor diffusion, since the curves scale with t1/3. When the concentration of HMTA further increases to 60 and 80 mM (the concentration ratios being 1:6 and 1:8, respectively), both the morphology and size of ZnO nanostructures change greatly (see Movies S3 and S4, Supporting Information). As shown in Figure 3A (1:6) and B

Figure 3. (A, B) Video images acquired at different times, showing the growth process of ZnO dendritic nanostructures (DNs) in the liquid cell. (A) Concentration ratio of 1:6, time interval of 5 s. (B) Concentration ratio of 1:8, time interval of 10 s. The scale bar is 50 nm. Note that t = 0 is not the starting time of exposure to the electron beam. (C) Time-dependent evolution of the diameter of the single ZnO DN shown in (A). (D) Diameter versus growth time for the two ZnO DNs shown in (B). Black lines in C and D are the fitting curves based on the LSW model. The beam current is about 500 pA.

Figure 4. Schematic illustration of the growth and dissolution of ZnO nanostructures in a liquid cell under the irradiation of high-energy electron beam in the TEM. C

DOI: 10.1021/acs.jpcc.7b10064 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C formed ammonia reacts with water to produce OH− (eqs 2 and 3, Supporting Information), which drives the precipitation of Zn(OH)2 (eq 4, Supporting Information). This unstable hydroxide decomposes to form the ZnO monomer under the irradiation of electron beam (eq 5, Supporting Information). (2) These ZnO monomers aggregate to form irregularly shaped tiny clusters, which will continue to grow until their radii exceed the critical radius rc. After that, these nuclei will gradually grow by monomer addition. If the concentration of HMTA is low, the formed ZnO nanostructures are spherical due to the diffusion-limited growth. On the other hand, a high HMTA concentration will result in dendrite-like nanostructures from the reaction-limited growth. (3) The surface of mature ZnO NPs will be hydrolyzed due to the excess OH− from the steady decomposition of HMTA (eq 6, Supporting Information). Then, the hydrolyzate reacts with OH− to produce Zn(OH)42−, which loses protons to generate zinc acid radical ions (ZnO22−; eqs 7−9, Supporting Information). In other words, the layer-by-layer hydrolysis of the surface of ZnO NPs and the loss of the protons lead to the complete dissolution of the NPs, namely a surface-dissolved mechanism (SDM). The growth and etching rates of the ZnO NPs (eqs 10 and 11, Supporting Information) are qualitatively compared to show their sum effects. At the beginning, the growth reaction dominates because the concentration of Zn2+ is far higher than that of Zn(OH)2. Only when the Zn2+ ions are almost exhausted does the dissolution reaction control the whole process, until the NPs are completely dissolved.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shuai Wang: 0000-0001-9328-0408 Xing Lu: 0000-0003-2741-8733 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.W. acknowledges the Center of Electron Microscopy, School of Materials Science and Engineering Zhejiang University for the support on the exchange program. We thank Dr. Chuanhong Jin for fruitful discussion and suggestions. This work was financially supported by the National Thousand Talents Program of China, the National Natural Science Foundation of China (Nos. 51472095, 51672093, 51602112, 51602097, and 21103224), and Program for Changjiang Scholars and Innovative Research Team in University (IRT1014), and the Technological Innovation Fund of Innovation Institute at HUST (0118013077).

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CONCLUSIONS In summary, we have directly investigated the precipitation and subsequent etching of ZnO nanostructures in aqueous solution using the liquid cell TEM technique. Nanostructures with different sizes and morphologies were synthesized in situ, by adjusting the concentration ratio between the two precursors under the irradiation of electron beam that directly leads to the heterogeneous decomposition of HMTA. We find that the nanostructures grow mainly by means of monomer addition, although coalescence events occasionally happen. With increasing HMTA concentration, the diffusion-limited growth kinetics will switch to reaction-limited kinetics. This change in kinetics may explain the different morphology and size of the synthesized ZnO nanostructures. This study suggests that liquid cell TEM can be used to investigate precipitation reactions in detail. The obtained knowledge will contribute to the controlled synthesis of nanomaterials in real solution environments, and weakening the electron beam effect in the liquid cell due to the zero charge transfer in the reaction system.

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Movie S4 showing the in situ growth of ZnO nanostructures in liquid cell under a concentration ratio of 1:8. (AVI) The temperature rise calculation and supplementary figures. Figure S1 showing the schematic illustrations of liquid cell. Figure S2 showing more examples of the growth of ZnO particles under the concentration ratio of 1:3. Figure S3 showing more examples of the growth and dissolution of ZnO nanoparticles under the concentration ratio of 1:5. Figure S4 showing the growth of ZnO nanoparticle by interparticle coalescence. Figure S5 showing the high resolution (HRTEM) and selected area electron diffraction (SAED) analysis. Figure S6 showing more examples of the growth of ZnO dendritic nanostructures. (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b10064. Movie S1 showing the in situ growth of ZnO nanostructures in liquid cell under a concentration ratio of 1:3. (AVI) Movie S2 showing the in situ growth of ZnO nanostructures in liquid cell under a concentration ratio of 1:5. (AVI) Movie S3 showing the in situ growth of ZnO nanostructures in liquid cell under a concentration ratio of 1:6. (AVI) D

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DOI: 10.1021/acs.jpcc.7b10064 J. Phys. Chem. C XXXX, XXX, XXX−XXX