Simplified Fabrication Strategy of Graphene Liquid Cells for In-Situ

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C: Physical Processes in Nanomaterials and Nanostructures

Simplified Fabrication Strategy of Graphene Liquid Cells for In-Situ TEM Study of Au Nanoparticles Wenbo Xin, Igor M. De Rosa, Peiyi Ye, Li Zheng, Yang Cao, Chezheng Cao, Larry Carlson, and Jenn-Ming Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11704 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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The Journal of Physical Chemistry

Simplified Fabrication Strategy of Graphene Liquid Cells For In-situ TEM Study of Au Nanoparticles Wenbo Xin,a* Igor M. De Rosa,a,b Peiyi Ye, a Li Zheng, c Yang Cao, Carlson b and Jenn-Ming Yang a

d

Chezheng Cao, a Larry

aDepartment

of Materials Science and Engineering, University of California, Los Angeles, 410 Westwood Plaza, Los Angeles, California 90095 USA bInstitute

for Technology Advancement, University of California, Los Angeles, 410 Westwood Plaza, Los Angeles, California 90095 USA cShanghai

Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai, P. R. China dAtmospheric

Data Solutions, LLC., 1992 Racquet Hill, Santa Ana, California 92705 USA

*Corresponding Author: [email protected]

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Abstract Graphene liquid cells (GLCs) have recently attracted increasing attention in multiple research fields because this technique has provided unique opportunities of studying liquid phase reactions. Here, we report a simplified approach for the fabrication of GLCs using ultrasonication assistance. We demonstrate morphologies of fabricated GLCs depend on the number of graphene layers. With the appropriate ultrasound, few-layer graphene (flG) assemblies to graphene shells and multilayer graphene (mlG) transforms to graphene scrolls that preserve the liquid pocket inside. We reveal liquid cells made from mlG offer superior spatial resolution to achieve sub-nanometer detection. More importantly, we investigate the in-situ coalescence processes of Au nanoparticles both in the liquid cell and on solid graphene support. From the direct comparison, we show a unique jump to contact mode associated with the coalescence of Au nanoparticles in water. Our study provides a simplified route to prepare GLCs and enhance our understanding of Au nanoparticle’s coalescence in the different media.


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INTRODUCTION Transmission electron microscopy (TEM) has proven to be an indispensable characterization tool to investigate morphologies, structures, interfaces, and defects of materials on the nanoscale. Recent advances in the electron microscopy bring cutting-edge technologies that allows one to image objects down to sub-angstrom resolution,1 which have significantly boosted the development of the nano-world with a large number of novel findings.2-5 In particular, liquid-cell TEM has made groundbreaking contributions to improve the understanding of the nucleation, growth, and coalescence processes of nanomaterials in liquid.6-8 Other dynamic activities of nanomaterials in the hydrated state including interfacial interaction,9 facets growth,10 structural11 and morphological12 evolution, self-assembly,13 and so forth have also been extensively investigated by liquidcell TEM with satisfying temporal and spatial resolutions. In the majority of liquid cell TEM studies, silicon nitride (SiNx) membrane is the most commonly used window material.7,8 Liquid cells constructed with SiNx windows have provided certain spatial resolution to reveal a number of interesting phenomena,9-13 which indeed benefits researchers in chemical, nanomaterial, biological, and other fields. However, with all of these advantages, there are a few drawbacks from SiNx liquid cells. Their window thickness is usually restricted (10-100 nm), which is undesirable for researchers who need to pursue improved spatial resolution and decrease the window thickness.14 Relatively high atomic number of this window material may cause high degree of electron scattering, which also limits the spatial resolution that is needed for accurate imaging and analysis. Moreover, because of their special design, shadowing effects arising from window edges reduce their capabilities of chemical analysis and 3-D studies.15



To overcome these limitations, researchers have developed a variety of strategies that not only focus on the improvement of SiNx windows but also seek for alternative materials.15-19 For instance, Hutzler et al. designed an advanced hybrid liquid cell by combining one SiNx membrane with one graphene film.15 Liao et al. employed the homemade silicon nitride membranes to fabricate liquid cells, which offer a better control over the liquid and window thickness.17,18 To obtain nanometer-resolution elemental mapping, Kelly et al. demonstrated a new design of the liquid cell consisting of hollow boron nitride nanocrystal encapsulated with graphene windows on both sides.19Among these approaches, liquid cells made from graphene20 and its derivatives graphene oxide21 attract increasing attention. Graphene owns low atomic number, atomic-thin thickness, and high thermal and electrical conductivity. Besides, graphene sheets can be adhered by relatively strong van der Waals interaction, preserving liquid pockets in the high-vacuum TEM chamber.14,20 Therefore, graphene is an ideal window material for the fabrication of liquid cells. As a consequence, researchers have successfully made a number of robust progresses towards a better understanding of the dynamic behaviors of nanomaterials by using GLCs.22-25 Nevertheless, albeit conceptually simple, the actual operation of preparing GLCs usually involves multiple steps, including cleaning, etching, drying, stamping, peeling, annealing, and so forth.16,20,22-24 These multi-step treatments are time-consuming and difficult to control. Moreover, the superior sensitivity of atomically thin graphene sheets to the multistep processes, particularly, to the transfer process, may cause poor reproducibility as well as the difficulty in the steady fabrication of GLCs. Whereas continuous efforts have been devoted to improving the performance of GLCs, there is surprising rare attention on the simplification of GCLs’ design.


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In addition, dynamic processes, such as nucleation, growth and coalescence of nanoparticles in the hydrated state have been comprehensively investigated,26-31 thanks to the robust advances of liquid-cell TEM. Particularly, the coarsening of nanoparticles shows similar dynamic behaviors including the migration of nanoparticles followed by the neck growth and fusion. Liquid medium plays an important role in these processes. For instance, Au nanoparticles are poised to a stabilized state in water by virtue of hydration shells when the two nanoparticles separate at a distance within two water molecules.31 On other hand, our groups recently report that graphene template in the aqueous solution can induce the growth of Au nanocrystals with controllable shapes and dimensions.32-34 However, individual effects of different media, e.g. water versus dry graphene template, on the coalescence dynamics of nanoparticles still remains vague. To explore such effects, direct comparisons between nanoparticles’ activities in the liquid and those on solid substrates are highly desirable. Herein, we demonstrate a fast and simple fabrication route for preparing GLCs that only includes two steps (1) encapsulating aqueous precursor by sonicating graphene in the water and (2) evaporating the extra liquid on the TEM grid. We show that two types of GCLs can be created by this processing method including graphene envelopes and graphene scrolls, both of which are able to preserve liquid pockets in the vacuum environment. We demonstrate that the structures of obtained GLCs depends on the number of graphene layers, i.e. window thickness. More importantly, the simply designed GCL enables the in-situ TEM imaging of dynamic behaviors of Au nanoparticles at the atomicscale. Ultimately, we investigate coalescence processes of Au nanoparticles both in GLCs



and on graphene substrates, and show that the process proceeds differently in the different media. EXPERIMENTAL Chemicals and Materials. Gold chloride hydrate (HAuCl4 ∙xH2O, 99.999%) was purchased from Sigma-Aldrich, USA. Few-layer graphene nanopowder (product number: AO-1, short for flG) and multilayer graphene nanopowder (product number: AO-2, short for mlG) were purchased from Graphene Supermarket (Calverton, New York). Materials and chemicals were used as received without further purification. Fabrication of graphene liquid cells. The same procedure is applied to prepare graphene liquid cells using both mlG and flG window materials. First, 0.5 mg graphene powder was mixed with 10 mL deionized water, followed by the addition of 80 µL aqueous gold precursor, HAuCl4 (40 mM). After that, the suspension was ultrasonicated by a vibracell (VCX-130 with VC-50 mirco-tip, Sonics & Materials, Inc.) at 25% amplification with the on and off frequencies of 0.5 Hz and 0.1 Hz, respectively for 300 s. When the ultrasonication was done, one droplet of the obtained mixture (~ 40 µL) was casted onto a TEM grid (Ted Pella, Inc., Product # 01840-F) by using a micro-pipette. TEM grid supported liquid sample was dried in the fume hood at the room temperature for 24 hours. The sample was then loaded on the TEM holder and transferred to the TEM chamber for in-situ characterizations. Characterizations. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were acquired using a FEI Titan S/TEM system. We employed the accelerating voltage of 300 keV for all images capture. Dose rate in this study was related to the magnification, spot size, and condenser aperture. 6 ACS Paragon Plus Environment

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Generally, the electron dose rate in our experiments ranged from 2 × 103 e-/Å2∙s to 6 × 104 e-/Å2∙s. Particularly, all the high-resolution images for studying coalescence processes of Au nanoparticles were obtained with the dose rate of 2 × 103 e-/Å2∙s at the magnification of 350, 000. All images (2048 pixels × 2048 pixels) were acquired using BM-UltraScan CCD camera built in the Gatan Digital Micrograph Program (Gatan Inc.) at the exposure time of 0.2 s. Raman spectroscopy analysis was obtained with a Renishaw In-Via Raman system with the laser length of 633 nm and laser power of 1 mW at objective lens magnification of 50 × and a grating spacing of 1200 l/mm. Typical acquisition time per scan is 10 s and a total of 3 scans are integrated to obtain the data. Spectral resolution in our data is 1 cm-1. RESULTS AND DISCUSSIONS Two types of graphene nanopowders are utilized in this study including few-layered (flG) and multi-layered graphene (mlG). The technical datasheet from the vendor shows the average thicknesses of flG and mlG are 1.6 nm (~3 layers) and 8.0 nm (~20-30 layers), respectively. TEM images (Figure S1(a), (d)) show that pristine flG has more surface wrinkles, while mlG demonstrates a smoother surface with more straightened edges. In addition, their thickness differences, i.e. 4.1 nm of flG versus 10.9 nm of mlG, can be identified from the edges, which were arbitrarily selected from the high-resolution TEM images in Figure S1(b) and (e). Raman spectra of flG and mlG are recorded and presented in Figures S1(c) and (f) respectively, where relatively large ID/IG ratio from flG indicates a defective nature of the surfaces.



We first inspect the structure and performance of the liquid cell made from flG. In Figure 1(a), we observe a few nanodroplets with the average diameter around 150 nm on the flG support. There is an obvious contrast difference between the droplets and graphene background. We select one of these pockets to have a more detailed study. With a short time of electron irradiation (20 seconds with electron dose J = 2 × 103 e-/Å2∙s), nanobubbles start to present in the pocket, as shown in Figure 1(b). Continuous irradiation (30 s to 120 s) leads to the changes in their sizes and shapes as shown in Figure S2, indicative of dynamic interactions between incident electrons and the substance entrapped in the cell. It is well established that gas bubbles can be originated from the water radiolysis by the electron beam.35-38 Therefore, the presence of gas bubbles with dynamic behaviors is considered the convincing evidence that designates the liquid is preserved in the cell. Here, such bubbles are clearly observed, which supports the successful encapsulation of the liquid in the flG cell. Moreover, the flG cell continuously shrink over the irradiation time as presented in Figure 1(c)-(e). Graphene defects are possibly induced by the electron bombardment here. It has been reported electron irradiation can generate surface defects such as vacancies and holes on graphene sheet.39 It is plausible that the liquid evaporates through the defective sites on graphene in the ultrahigh-vacuum TEM chamber. We do not observe any forms of solid clusters or nanoparticles until the final stage at 450 s. However, nanoparticles show up when the cell is completely dried, as shown in Figure 1(e). A polycrystalline nature can be identified from randomly orientated lattice fringes (Figure 1(f)). Its corresponding fast-Fourier transformation (FFT) pattern in Figure 1(g) confirms a polycrystalline FCC gold structure with Au (111), Au (200), and Au (220) planes indexed.40 Graphene spots corresponding to (0110) planes can be identified at the 8 ACS Paragon Plus Environment

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same time, which suggests graphene sheet is the support for the liquid pocket. More examples of flG liquid cells can be found in Figure S3, showing good reproducibility from this simple GLC design. To explore the effect of graphene layers on the structure of liquid cells, we utilize multilayer graphene (mlG) to fabricate GLCs with the same approach. Interestingly, mlG cell is featured as a nanoscroll, as shown in Figure 2(a), which is possibly rolled up by the graphene edge assisted through the ultrasonication process. In Figure 2(b), we observe a seamlessly sealed end of the scroll. This is a critical sign of a well-functional liquid cell since the liquid can be only preserved in the well-sealed vessel. The measured window thickness of the cell is ~13.3 nm, suggesting approximately 40 graphene sheets are included. In the highlighted area of Figure 2(a), we also observe a number of nanobubbles (Figure 2(c)), indicating the native liquid is preserved in the cell. In contrast to flG cell, a few nanoparticles present in the mlG cell as shown in Figure 2(c) and (d). As is known, the incident electron beam causes radiolysis of the water, likely generating highly reactive species, such as solvated electrons (eaq−) and free radicals, which can reduce the gold precursor (HAuCl4) to Au0.29,41 As a matter of fact, we confirm the nanoparticle is actually Au from a higher magnification image (Figure 2(e)), where interplanar distances, as measured by the two red parallel lines is ~2.35 Å, the spacing between Au {111} planes. Moreover, the lattice distance of the background material is ~ 2.12 Å, which equals to the interplanar distances of graphene {0110} planes. Correspondingly, Au (111) and graphene (0110) spots can be identified from the FFT pattern of this selected area (Figure 2(f)). The result convincingly shows that atomicresolution can be achieved using this simple mlG cell. 9 ACS Paragon Plus Environment


The mechanism of fabricating flG envelopes and mlG scrolls using ultrasonication is readily comprehensible. It has been reported ultrasound, by introducing high-shear force to the system, could deform graphene/graphene oxide films and transform them into different carbon nanostructures such as graphene quantum dots,42 graphene nanoscrolls,43 and even carbon nanotubes44,45 under appropriate conditions. Here, it is ultrasonication that induces self-assembly of few-layer graphene shells and rolling-up edges of multi-layer graphene, both of which demonstrate the capability of encapsulating and preserving the liquid. Quite interestingly, we only obtain envelope-cells from flG and scroll-cells from mlG, which likely originates from differences in their thickness and properties. Moreover, we arbitrarily checked 20 mlG scrolls, and found 9 of them had liquid encapsulated (~45 %). To clarify a detailed function of ultrasonication, we utilize different ultrasonication times (0 s to 900 s) to prepare mlG cells. Besides 300 s (the one shown in Figure 2), four more representative stages are studied, i.e. 0 s, 100 s, 450 s and 900 s, as presented in Figure S4. mlG scroll liquid cells can be only obtained with the ultrasonication time between 300 s to 450 s, as demonstrated in Figure S4(c) and (g). We speculate that the force induced from deficient ultrasound (< 300 s) is not capable of rolling up graphene edges. On the other hand, too much ultrasound may cause serious damages to the sheet, which is detrimental to the formation of nanoscrolls (Figure S4(d) and (h)). The result shows that we can reproducibly fabricate mlG liquid cells using the proper ultrasonication condition. More specific studies regarding the influence of ultrasonication time and intensity on the formation of graphene nanoscrolls can be found somewhere else,43 which are out the scope of this work. In order to demonstrate a well-functional mlG liquid cell, we investigate the in-situ dynamic processes of Au nanoparticles in the same cell. In particular, we arbitrarily selected an area, 10 ACS Paragon Plus Environment

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where it shows two Au nanoparticles separated with an inter-particle distance of ~ 4 nm (Figure 3(a)). The two nanoparticles follow the conventional coalescence steps. They first migrate towards each other, resulting in a steady reduction of the gap distance (Figure 3(b)(d)). The migration stops at a point where they physically get contacted and the neck forms (Figure 3(f)). To pursue a more comprehensive understanding of the kinetics of AuNPs coalescence in water, we investigate more Au nanoparticles’ activities in another scroll cell, where the nanobubbles can be detected from both side and top views (Figure S5).38 This suggests a pristine liquid state preserved in the cell. We first identify these nanoparticles are Au from the FFT pattern in the inset of Figure 4(a), where spots characterizing Au (111) and graphene (0002) can be identified. From Figure 4(b) to (f), we capture the coalescence processes of Au dimers 1-3. Detailed analysis of the dynamic processes of Au nanoparticles in Figures 3 and 4 will be discussed in the following section. In order to improve the understanding of nanoparticles performance in different media, we conduct an investigation of similar-size Au nanoparticles on solid graphene support. Under the same dose rate (J = 2 × 103 e-/Å2∙s), Au nanoparticles approach to each other, get physically contact and then start to coalescence on the dry graphene substrate. Time-lapsed TEM images of the processes are recorded in Figure 5. It is worthy to note that kinetics of Au nanoparticle’s coalescence on dry graphene are different from those in the liquid. To better reveal such differences, we select three Au dimers in both cases and plot the change of pairwise distance d as a function of the time t, as demonstrated in Figure 6(a). Noticeably, there are two differences in the relation of d ~ t between the two cases. First, in the liquid, Au nanoparticles move towards each other and got into contact within 175 s, with the initial gap in a range of 3.0 nm to 4.5 nm. However, on the graphene support, 11 ACS Paragon Plus Environment


only Au dimers with the gap less than 1.5 nm can get attached within 250 s. For example, the gap between nanoparticles highlighted in Figure 5(c)-(h) by red circles deceases from 4.5 nm to 3.4 nm under the electron irradiation for 250 s, suggesting more irradiation time is needed to trigger their physical attachment. Second, we observe a “jump-to-contact” mode from all three cases of Au nanoparticles in water but none of the nanoparticle pairs on dried graphene follows this mode. The approach of jump to contact is considered the main mechanism to initiate the physical connection of nanoparticles, which usually favors the oriented attachment,27,46 an important pathway for crystal growth. Another mechanism, namely, nanobridge induced contact has been reported very recently, which could also realize their contact in aqueous solution.26 Misaligned lattices between the adjacent particles possibly account for the occurrence of nanobridge induction mechanism, quite distinct from the jump-to-contact mode. In our study, Au nanoparticles in water go through a typical jump to contact process, where they suddenly jump to contact when their distance decreases to ~ 5 Å. The dimers are stabilized by hydration layers formed by water molecules, and the stabilization becomes unbalanced when the distance is less than 5 Å, leading to the jump-to-contact.31 Due to the observation of the jump-to-contact mode, it is likely that nanoparticles in this study also have the oriented attachment. Overall, the result confirms water is encapsulated in the graphene nanoscroll, which also explains the absence of such a sudden jump from Au nanoparticles on graphene support, as elucidated by red dotted curves in Figure 6(a). Moreover, to systematically reveal the coalescence kinetics, we further study their neck growth after the contact and record the growth of neck width D as the function of time t (Figure 6(b)). We find that the neck growth of Au nanoparticles on dried graphene is 12 ACS Paragon Plus Environment

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generally faster than those in the liquid. From the classical continuum theory, the power law relation, D ~ t a is given with the exponent a between 0.14 and 0.17.47 Accordingly, we fit the curves in Figure 6(b) by using the above relation, and obtain exponent values from the cases of nanoparticles on graphene template and in water are 0.35, 0.33, 0.33 (average = 0.34) and 0.29, 0.28, 0.23 (average = 0.27), respectively. It has been reported neck growth is driven by the reduction of chemical potential at the contact point of two nanoparticles.48 To realize that, surface diffusion is considered to be the most possible mass transport mechanism in FCC metals. The exponent a can be largely affected by the diffusion condition, such as temperature, particle geometries, surface diffusivity, and surface energy. The calculation result indicates accumulated heat on the nanoparticles due to the electron irradiation is negligible (lower than 1 K),49 thus the temperature rise can be ignored in this case. On the other hand, both experimental and simulation work show that two decahedral Au nanoparticles fused with Au (100) facets give rise to an exponent ~ 0.32 in the dry state.50 Moreover, the exponent value can be varied significantly from 0.17 to 8.9 even in the same coalescence case in water.20 Therefore, it is hardly to conclude the exponent differences in the two cases (0.34 versus 0.27) are attributed to the environmental medium.

CONCLUSIONS To summarize, we report a simple and fast approach to fabricate GLCs for in-situ investigation of Au nanoparticles in the liquid phase. The fabrication relies on the employment of ultrasound, which induces self-assembly of graphene sheets and form liquid cells with different structures. GLCs obtained by our simple deign are reproducible using the



proper ultrasonication time. We evidently show that GLCs in this study are well functional with the spatial resolution down to sub-nanometer scale. Accordingly, we investigate the coalescence process of Au nanoparticles using mlG liquid cell and elucidate the substantial difference in the pre-contact mode between nanoparticles in water and those on dried graphene. This study shines the light on the fast preparation of GLCs and may also bring benefits to the fabrication approaches that rely on the ultrasonication.

Supporting Information Characterization of flG and mlG nanopowders; Nanobubbles presented in the liquid pocket entrapped by flG; More examples of flG liquid cell; Influence of ultrasonication time on mlG liquid cells; Nanobubbles generated in a mlG liquid cell; Conflicts of interest There are no conflicts of interest to declare. Acknowledgements This work was financially supported by the U.S. Department of Defense (fund # 000-16C-0081). We thank California NanoSystems Institute (CNSI) at University of California, Los Angeles for the assistance of TEM imaging.


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FIGURES AND CAPTIONS

Figure 1. Liquid cell made from flG. (a) A representative TEM image of a few flG liquid cells featured as graphene nanodroplets. (b)-(e) Time-lapsed TEM images of a selected cell in (a) pointed by the red arrow. (f) High magnification image of the area highlighted in (e). (g) FastFourier transformation (FFT) pattern of the section in (f) presenting polycrystalline FFC gold.



Figure 2. Liquid cell made of mlG. (a) TEM image of mlG liquid cell featured as a graphene nanoscroll. (b) Closed end of the graphene nanoscroll. (c) HRTEM image showing the entrapped liquid and the window thickness. (d) A selected area in the nanoscroll containing a few Au nanoparticles in the liquid phase. (e) Atomic structure of the selected Au nanoparticle with graphene background resolved. (f) FFT pattern of (e).


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Figure 3. Time-lapsed TEM images of the first stage in coalescence process—Au nanoparticles approaching to each in the mlG liquid cell.



Figure 4. Time-lapsed TEM images of the coalescence process of Au nanoparticles in mlG liquid cell. It includes the complete two-step coalescence process of Au dimer 1, and the secondstep process, i.e. neck growth of Au dimer 2 and 3.


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Figure 5. (a)-(h) Time-lapsed TEM images of Au nanoparticles coalesced on solid graphene substrate from t = 0 s to t = 390 s.



Figure 6. Coalescence process of Au nanoparticles captured in mlG liquid cell and on solid graphene support. (a) Prior to contact process. (b) Neck growth over time.


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TOC Graphic


Simplified Fabrication Strategy of Graphene Liquid Cells for In-Situ

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