In situ wet-cell TEM observation of gold nanoparticle motion in an aqueous solution
© Chen and Wen; licensee Springer. 2012
Received: 30 August 2012
Accepted: 16 October 2012
Published: 29 October 2012
In situ wet-cell transmission electron microscopy (TEM) technology enables direct observation of nanomaterials in a fully hydrated environment with high spatial and temporal resolution, which can be used to address a wide range of scientific problems. In this paper, the motions of approximately 5-nm sized gold nanoparticles in an aqueous solution are studied using the wet-cell TEM technology. It is observed that gold nanoparticles can be either in a single particle or cluster forms, and dynamic displacement and rotation motions are observed for both forms in the solution. Under electron beam irradiation, nanoparticles in some clusters gradually fused together; sometimes they also showed dramatic growth behavior. Mechanisms for the motion and growth of the particles/clusters are discussed.
KeywordsIn situ transmission electron microscopy Gold Nanoparticles Wet cell
Nanoparticle assemblies are often achieved involving liquids. Real-time observation of nanoparticle assembly and dynamics is thus of great importance[1, 2]. In situ transmission electron microscopy (TEM) techniques provide a local probe of structure and dynamics that other techniques cannot observe readily. In situ observation provides dynamic information about nanosystems, which is difficult to obtain by other techniques. Conventional TEM requires drying of samples in order to be compatible with vacuum. The structural features of the sample can change significantly during the process. Thus, for samples prepared in liquids, it would be ideal if it can be observed directly with TEM. With the development of robust silicon nitride (Si3N4) membrane windows for the in situ cell, the construction of wet-cell and in situ observation of liquids becomes readily possible inside TEM. Applications using the wet-cell technology are upsurging, and further exciting development is expected in the future. For examples, this technique has been applied to the observation of electrochemical dynamic procedure of Cu and Ni nanoclusters, electron beam-induced growth of Pt and lead sulfide nanocrystals in liquid, semiconductor nanorod embedded in liquid crystal cells for optoelectronic applications, Al2O3 nanoparticles and carbon nanotubes in water, and biological cells. Earlier liquid cell TEM reactor yielded a spatial resolution of only 5nm, but recent development has improved the resolution to the sub-nanometer range. The development in Berkeley using graphene sheet to replace Si3N4 even pushed the wet-cell TEM imaging resolution to the atomic level.
Zheng et al. recently made an analysis on gold nanoparticle diffusion during liquid evaporation. In addition, Grogan and Bau reported observation with in situ STEM on gold clusters in an aqueous solution using a lower electron beam energy of 20 keV to test the liquid cell hermeticity. However, the image resolution is poorer due to the lower electron energy.
High spatial resolution TEM requires both high voltage and relatively high electron beam flux. Local heating and structural transformations may occur during observation due to the electron beam irradiation. Previous reports have shown that electron beam can initiate nanoparticle nucleation and growth in a liquid. Such beam effect needs to be carefully addressed in wet-cell TEM experiments.
In this paper, we report an in situ observation of gold nanoparticles in aqueous water solution using the wet-cell TEM technology. Sub-nanometer resolution images were obtained. Dynamic motion and dramatic growth of clusters of gold nanoparticles have been observed. These observations allow a discussion of electron beam effect on the growth of nanoparticle clusters.
Results and discussion
Figure3d is the enlargement of one section of the center cluster, in which we see individual gold nanoparticles in the cluster are merging with time, confirming that the particle agglomeration behavior is happening in the liquid environment. The cluster looks like individual beads that are attached to each other in the beginning but became more like a web made up of nanowires after 3 min of electron beam irradiation. It is significant that no bubbling in the liquid is seen during the observation, which indicates that the welding procedure is happening even below the boiling temperature of water. It has been calculated that the temperature change in a wet-cell under electron beam irradiation is generally much smaller than 1 K. Dramatic changes in gold nanoparticles such as dissolution had been previously observed under relatively high electron doses of 8 × 105 e/nm2. In our experiment, with the longer observation time of several minutes, the electron dose may accumulate to beyond a limit that affects the observable gold crystal morphology.
Although a longer beam exposure time (12 min in Figure4 vs. 3 to 4 min in Figures2 and3) can be used to partially explain such a dramatic shape change, it is unusual to see that the cluster increases in size greatly with time. Chemical reactions could have occurred that transported materials into the cluster from regions away from the electron beam. Although a noble metal, gold can form many diverse compounds. One possibility is that gold reacted with water under the high energy electron bombardment and formed gold oxide or gold hydroxide; however, this growth mechanism is not fully sustainable. After all gold atoms in the cluster are oxidized, the volume increase will stop. As the cluster in Figure4 is fixed in location without being able to move freely around in the liquid, it must be closely attached to the silicon nitride window, and wetting of the gold on the window might also account for the morphology change. Arrow 4 in Figure4d points to a black line in the growing cluster, suggesting a trace for material migration during the growth. Compared with Figure4c, it appears that the round dark region near the top center (region 5 in Figure4d) is the source region for the mass redistribution, and the growing branches to the left are the regions where the materials are transported toward. The nanowires are in much lighter color in these TEM bright field images than the earlier gold particles, suggesting that they are much thinner, thus results in a relatively large area increase. This supports the idea that the gold wets the window under electron beam irradiation. In comparison, we irradiated gold nanoclusters on a dry grid for the same amount of time. Besides the simpler merging behavior like in Figure2, no such dramatic growth behavior was observed, suggesting that water might have played a role in helping catalyze the nanowire growth. Recently, Zheng et al. reported in situ TEM observation of Pt3Fe nanorod growth in solution, in which Pt3Fe nanoparticles attach and coalesce into nanoparticle chains. The chains were winding and markedly flexible, and gradually turned into nanowires through mass redistribution procedure. Most of the nanowires remain polystalline and twisted for an extended period of time. Yuk et al. further reported in situ TEM observation of Pt nanoparticles coalescing in liquid. These observations are similar to our results here. It is not fully excluded that there is the possibility that gold dissolution happened somewhere in the liquid, which got redeposited onto the growing cluster under the beam interaction. Further study on the gold morphology change under electron beam irradiation in solution is still needed.
In summary, we report the observation of gold nanoparticles in water solution using in situ wet-cell TEM technology. The gold nanoparticle system showed a variety of dynamic behaviors in the aqueous solution involving single particles, particle clusters, and nanowires, which include dynamic displacement and rotation motions, fusion of particles, and even dramatic size growth behavior under the electron beam. The fusion and dramatic growth of the particles happened at temperatures much lower than the gold melting temperature. Random motions of the gold particles and clusters are greatly suppressed by the drag from the silicon nitride windows.
XC is currently a professor at the School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, China. JW is a materials scientist at Electron Microscopy Center and Materials Science Division, Argonne National Laboratory, Argonne, IL, USA
The TEM experiment was carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois, which are partially supported by the US Department of Energy under grants DE-FG02-07ER46453 and DE-FG02-07ER46471. The authors thank SJ Dillon, JM Zuo, CH Lei, W Swiech, and B Sankaran for the kind support, and Dr. L Martin for the valuable discussions. The support from Shanghai Leading Academic Discipline Project (B502), Shanghai Key Laboratory Project (08DZ2230500), and Science and Technology Commission of Shanghai Municipality Project (11nm0507000) is highly acknowledged. The research was partially accomplished at the Electron Microscopy Center at Argonne National Laboratory, a US Department of Energy Office of Science Laboratory operated under contract no. DE-AC02-06CH11357 by UChicago Argonne, LLC.
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