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In situ liquid cell TEM reveals bridge-induced contact and fusion of Au nanocrystals in aqueous solution Biao Jin, Maria L. Sushko, Zhaoming Liu, Chuanhong Jin, and Ruikang Tang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03139 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018
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In situ liquid cell TEM reveals bridge-induced contact and fusion of Au nanocrystals in aqueous solution Biao Jin, † Maria L. Sushko,§ Zhaoming Liu,† Chuanhong Jin,*‡ and Ruikang Tang*†‡ †
Department of Chemistry, ‡State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China
§
Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States
ABSTRACT: During nanoparticle coalescence in aqueous solution, dehydration and initial contact of particles are critically important but poorly understood processes. In this work, we used in situ liquid-cell transmission electron microscopy to directly visualize the coalescence process of Au nanocrystals. It is found that the Au atomic nanobridge forms between adjacent nanocrystals that are separated by a ~0.5 nm hydration layer. The nanobridge structure first induces initial contact of Au nanocrystals over their hydration layers and then surface diffusion and grain boundary migration to rearrange into a single nanocrystal. Classical density functional theory calculations and ab initio molecular dynamics simulations suggest that the formation of
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the nanobridge can be attributed to the accumulation of auric ions and a higher local supersaturation in the gap, which can promote dehydration, contact, and fusion of Au nanocrystals. The discovery of this multistep process advances our understanding of the nanoparticle coalescence mechanism in aqueous solutions. KEYWORDS: Au nanocrystals, liquid cell TEM, nanobridge, coalescence, dehydration
Coalescence plays a fundamental role in both crystallization processes and particle assembly.1,2 It is widely used to design and synthesize diverse nanomaterials with controllable structures for various applications.3-6 Owing to its wide utility and interest, extensive research efforts have focused on understanding the nanoparticle coalescence mechanism in aqueous solution.7-15 Although thermodynamic understanding of the surface energy reduction has been accepted,16 the nanoscale dynamics of the coalescence process still are poorly understood. It has been suggested that coalescence begins with an effective collision17 or “jump-to-contact” event.8,18,19 It is worth noting that interacting nanoparticles can contact and undergo coalescence only after their contact surfaces are fully dehydrated or surface ligands are expulsed,8,20 because the repulsive solvation force due to interfacial solvent structuring and adsorption prevents their close approach and contact.8,19,21-23 Aside from the solvation layer effect, other factors need to be considered in multicomponent solutions. For example, solvent and ions at the solution-solid interface are believed to play an important role in nanoscale physics and chemistry by controlling alignments and attachments.24-28 Assumptions about the coalescence process exist, however, firm understanding of the mechanism that connects the structure of solvents and ions at an interface to the ensemble coalescence process is still lacking.1 Therefore, discerning solution structure and
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establishing initial contact at solid-solution interfaces are pivotal to understand the coalescence mechanism of nanomaterials. In the work described in this paper, we used liquid cell transmission electron microscopy (LCTEM),29 an emerging advanced method for investigating nanoparticle growth dynamics and morphology evolution,7-9,12,13,15 to investigate the coalescence mechanism of Au nanocrystals in aqueous solution. The formation of Au nanocrystals was initiated in 400 nL 1.0 mM HAuCl4 aqueous solution by electron beam irradiation.8 The growth dynamics of nanocrystals were recorded with a field emission FEI Tecnai G2 F20 microscopy operated at 200 kV with electron dose rates ranging from ~1675 to 1836 electrons/Å2s. Low-resolution TEM images showed that these nanocrystals grew through both monomer addition and coalescence (Figure S1), as had been observed previously in Pt and oxyhydroxide nanoparticles.7,18 It is difficult to observe the initial nucleation process of nanoparticles in a precursor solution because of limited temporal resolution and strong solvent reduction by the electron beam.30 However, using LC-TEM, it is possible to directly observe very important features of coalescence process that could not be investigated by ex situ imaging methods. The movies of Au nanocrystals growth in a liquid cell (Movie S1 and Movie S2) reveal a remarkable coalescence process. The time-resolved TEM images in Figure 1a (see Movie S1 for details) show the approach, contact, and fusion process of Au nanocrystals in aqueous solution. Initially, adjacent nanocrystals are separated by ~1.5 nm. The nanocrystals get closer (Stage I in Figure 1b) because of the attractive van der Waals force (vdW, Figure S2),8,19 and then pause for about 3 s when separated by ~0.5 nm (Figure 1b). Tracking multiple coalescence events shows that the average gap distance is ~0.5 nm prior to contact (Figure S3). This separation corresponds to a typical thickness of the two hydration layers reported by Mirsaidov et al due to interactions between the
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Figure 1. Coalescence dynamics of Au nanocrystals. (a) Time-series TEM images showing the coalescence process of Au nanocrystals, indicating the approach of nanocrystals and establishment of a sterically stabilized transient nanocrystals dimer at ~0.5 nm gap, followed by nanobridge formation (marked by two black triangles at 39.6 s) and subsequently fusion of the nanocrystals. The dark protrusions are seen at point of the white arrow at 39.4 s. The graphics below each image are corresponding schematic illustrations. Scale bar is 2 nm. (b) Gap distance (red symbols) and normalized intensity (black symbols) changes during the coalescence process, showing the approach (I), metastable dimer (II), contact and fusion (III) of Au nanocrystals. (c) The profile of gray values in the gap region (white frame) at 39.4 s. (d) The length of nanocrystals dimer as a function of time during nanobridge formation. Au surface and water molecules.8 At this separation, a sterically stabilized transient nanocrystal dimer with hydration layers forms, indicating that the repulsive hydration force balances the attractive vdW force.8 The existence of the metastable dimer enables nanoscale bodies to find a suitable coalescence pathway. As shown in Figure 1a, dark protrusions appear in the gap at 39.4 s, which is confirmed
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by the higher contrast in the gap region compared to the background (Figure 1c). It is consistent with the result that the normalized intensity (Figure S4a) decreases slightly (Stage II in Figure 1b). Subsequently, the protrusions connect the particles (marked by black triangles at 39.6 s in Figure 1a) to produce a dumbbell-shaped nanocrystal. The connected protrusion structure is called a nanobridge. Previous publications reported similar processes in which atomic bridges between closely spaced Au nanoparticles on carbon film initiated subsequent coalescence.31,32 One possible mechanism for bridging between two nanoparticles involves continued attachment of adatoms, leading to the formation of an atomic bridge,31 whereas another possibility is a bridge formed by the free and isolated atoms located on carbon film.32 Distinguishing between these two mechanisms requires a more detailed mechanistic study. Neither of these models can be used to interpret our observations because our experimental and imaging conditions are completely different. On one hand, nanoparticles in our experiment were formed in aqueous solution (See the Supporting Information and Figure S5) so rich coalescence dynamics were captured (Figure S1b). On the other hand, imaging was performed using 200 kV accelerated voltage of TEM, which is 100 kV lower than used in previous experiments,31,32 possibly leading to a weaker electron knock-on effect.33 Consequently, the two adjacent ~5 nm Au nanoparticles loaded on carbon film failed to coalesce under otherwise similar LC-TEM experimental imaging conditions except that the electron dose rate was higher (Figure S6). These results imply that the bridge-forming mechanism in solution is probably different from that previously suggested.31,32 Our results point to a novel mechanism of the coalescence process involving nanobridgeinduced contact rather than a direct “jump-to-contact” process.18 That mechanism is manifested here via the unchanged particle edge-to-edge distance (L, along the center-to-center direction) between two adjacent nanocrystals during nanobridge formation (Figure 1d). It is also
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noteworthy that the mechanism differs from the conventional understanding of neck formation upon contact and adhesion34,35 in which initial contact triggers neck growth in a sequential process during coalescence.36,37 In contrast, the Au atomic nanobridge forms between adjacent but solvent-separated nanocrystals, thereby accelerating the fusion of the dumbbell-shaped nanocrystal (Stage III in Figure 1b). Finally, the dumbbell-shaped nanocrystal fuses and relaxes into a single crystal (at 57.4 s in Figure S4b). To reveal the details of the nanobridge formation process, trajectories of nanocrystals coalescence were recorded at lattice fringe resolution, enabling us to examine nanobridge evolution dynamics (Figure 2a and Movie S2). Notably, the high resolution TEM images can be acquired in liquid film with a smaller effective thickness due to electron beam induced bubbles formation.38 The interacting nanocrystals with different orientations identified by lattice fringes are located in close proximity at 4.2 s. During the first time period, they approach closer, rotate slightly, and pause for about 3 seconds at a separation of ~0.5 nm (Figure S7a). Then, the nanobridge without obvious lattice fringes forms in the gap (marked by white triangles at 32.0 s in Figure 2a) before contact, which is consistent with previous work in which the Au atoms nucleated between closing nanocrystals in a disordered non-crystalline (amorphous) manner.39 Remarkably, the nanoparticle lattice orientation is mismatched before contact (Figure S7b), possibly accounting for the distinguished contact mechanism under the present experimental conditions relative to previous reports in which “jump-to-contact” was considered to be the main mechanism in oriented attachment18 driven by orientation specific vdW, ion correlation, and chemical forces.19,27,40 In fact, the “jump-to-contact” process also was observed in our experiments when nanocrystals became aligned before contact (Figure S8).
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Analysis of time-series lattice fringe images provides direct insight into the dynamics of the process that leads to nanobridge formation and nanocrystal rearrangement. As shown in Figure 2b, the power law shows that the nanobridge width (W, defined in Figure S7c-d) scales as t8.9 during the very early stages of coalescence, which significantly deviates from the relationship dneck ~ t0.16 predicted by classical continuum theory based on the surface diffusion process.41 After the nanobridge forms, the high number of coordination sites for diffusion and redistribution of atoms can be attributed to the highly curved nanobridge region, which facilitates evolution of the nanobridge. It shows that the nanobridge forms to promote nanocrystal fusion. At a later stage, the relationship W ~ t0.17 is close to the power law dneck ~ t0.16. Based on the very similar value, we speculate that surface diffusion should play a key role in subsequent rearrangement.41 Slightly lattice mismatched nanoparticle coalescence, resulting in defect formation at the contact interface, appears to explain the formation mechanism at the grain boundary, as shown at 34.8 s in Figure 2a. Although the interface energy of the grain boundary is known to be thermodynamically significant,42 the relative angle of lattice orientations in the nanocrystal
Figure 2. Trajectories of nanocrystals and dynamics of a nanobridge during coalescence. (a) Time-series high resolution TEM images of two Au nanocrystals undergoing coalescence. The nanobridge is marked by two white triangles. Scale bar is 2 nm. (b) Logarithmic relationship between the nanobridge width (W, as show in inset) and time.
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interior decreases in our case (Figure S7b), indicating the growth of one lattice orientation at the expense of the other.15 Afterward, the connected nanocrystals rearrange into a single crystal at 84.2 s. These results indicate that in our cases, structural rearrangement of the nanocrystals occurs by both grain boundary migration and surface rearrangements.9,15 Given the complexity of the solution environment, additional factors should be considered to elucidate the solvent-mediated coalescence mechanism. Considering that solution species (i.e., water, ions, or atoms) have significantly higher diffusivities than surface atoms, they may contribute to or even drive the nanocrystal coalescence process.26 The resulting coalescence mechanism is likely to be more complicated in the conventional solution-based nanoparticle preparation process. The low number density of free Au atoms (~1 × 10-12 Au atoms/nm3) in a saturated tetrachloroauric acid solution suggests that they may not nucleate in solution.43 In -
contrast, AuCl4 anions have a high diffusion coefficient (~0.5 × 109 nm2/s)44 and strongly
Figure 3. cDFT calculations and AIMD simulation results. (a) The general set-up of the simulation cell. (b) The excess chemical potential (µ) of AuCl4 as a function of gap distance. (c) -
-
The normalized density of AuCl4 anions and water molecules in the gap as a function of gap distance. Density is calculated along the line normal to (x,y) plane and crossing the plane in the middle and normalized by the corresponding bulk value. (d) AIMD simulations for two Au surfaces separated by 0.5 nm without (left) and with (right) additional Au atoms in the gap. ACS Paragon Plus Environment
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interact with Au nanocrystal surfaces,45 causing them to migrate toward the surfaces and possibly accumulate in the gap.46 Meanwhile, we found that the area of the nanobridge increases after 31.2 s, whereas the areas of nanocrystals hardly change during nanobridge formation (Figure S9). This finding implies that surface atom diffusion may be not responsible for the formation of the nanobridge. Hence, we hypothesize that nanobridge formation can be attributed to the higher -
local supersaturation of Au atoms being reduced by accumulation of AuCl4 anions in the gap rather than surface atom diffusion (see the Supporting Information for details). To validate our hypotheses, we performed classical density functional theory (cDFT) calculations and ab initio molecular dynamics (AIMD) simulations (see Supporting Information for details, Figure 3 and Figure S10). The cDFT (known as density functional theory of complex fluids) approach used in this work encompasses explicit description of the solvent. The free energy functional comprises entropic and enthalpic solvation interactions, microscopic and interparticle vdW interactions, and electrostatic forces that include Coulombic, image, and ion correlation interactions. Although the model (Figure 3a) does not consider electron beam-
induced reduction of AuCl4 anions, it shows that interfacial interactions create the conditions for -
nanobridge formation. The excess chemical potential (µ) of AuCl4 anions in the gap region is -
negative (Figure 3b), indicating the attractive driving force, which induces AuCl4 anions to migrate into the gap region. The excess chemical potential is dominated by attractive electrostatic correlation and ion-surface interactions and by repulsive entropic hydration and electrostatic image interactions (Figure S10a-d). cDFT calculations predict a significant increase -
in the concentration of AuCl4 anions in the gap as the gap distance decreases (schematically illustrated in Figure 4a), whereas the concentration of water molecules hardly changes if gap
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distances are larger than 0.75 nm (Figure 3c). Noticeably, electrostatic image interactions become weakly attractive at 1.25 nm separation (Figure S10c), which creates an additional -
driving force for accumulation AuCl4 anions in the gap. The total interparticle attractive force dominated by vdW and ion correlation interactions becomes independent of the gap distance in the 0.5 to 0.75 nm region (Figure S2), marking the formation of a metastable water-bonded -
transient nanocrystal pair as schematically shown in Figure 4b. AuCl4 anions accumulation in the gap induces the competition for hydration water with the surfaces, thereby providing the mechanism for partial water displacement out of the gap and a reduction in hydration repulsion (Figure S10d). -
To complete the nanobridge formation process, AuCl4 anions in the gap must be reduced by the electron beam (Figure 4b). The reduction process can be described by the following
Figure 4. Schematic illustration of the nanobridge formation process. (a) Interacting -
nanocrystals at a small gap distance, accompanying the accumulation of AuCl4 anions. (b) -
Formation of a metastable water-bonded transient nanocrystal pair at ~0.5 nm. AuCl4 anions in the gap are reduced into atoms, resulting in the increase of local atom supersaturation. (c)
Nucleation of Au atoms in the gap to form a nanobridge. (d) Nanocrystals connected by a nanobridge. ACS Paragon Plus Environment
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reversible chemical reaction equilibrium:47
+ 3 ⇋ 0 + 4
(1)
Le Chatelier's principle dictates that the concentration of Au0 increases when the concentration -
-
of AuCl4 anions increases. The concentration of AuCl4 anions also affects reactivity because the formal potential (E) for the reaction depends on the concentration. The potential can be calculated using the Nernst equation, which combined with equation (1), has the following form: = +
(2)
where Eθ is the standard potential; R is the gas constant; T is the temperature; n is the number of electrons involved in the reaction; and F is the Faraday constant. Equation (2) shows that E would be higher in the gap because the concentration of the AuCl4- anions would be higher, which indicates that the reaction described by equation (1) would proceed more readily in the -
gap. In contrast, the lower concentration of AuCl4 anions in the bulk solution results in the lower E. Although Au0 is still favored under standard conditions, the decrease in redox potential -
marginally changes the reaction equilibrium toward AuCl4 anions.47 In other words, the local -
AuCl4 anion accumulation creates conditions for nucleation by increasing local supersaturation of Au0 and thus decreasing the heterogeneous nucleation barrier, which is manifested in Au nanobridge formation (Figure 4c). Finally, the interacting nanocrystals are connected by the nanobridge (Figure 4d). Furthermore, AIMD simulation for two flat surfaces (Figure 3d, left) or stepped surfaces (Figure S11a) separated by 0.5 nm without additional Au atoms in the gap shows that the two surfaces are stable. This finding rules out the possibility of bridge formation via surface Au atom diffusion in the absence of solution Au species in the gap between
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nanoparticles at experimental temperatures. Similarly, previously reported classical molecular dynamics simulations predicted that the bridge cannot form via surface diffusion at temperatures below 800 K.48 Instead, if additional Au atoms accumulate in the gap, they readily form bonds with both surfaces and develop an amorphous nanobridge structure (Figure 3d, right), even with a smaller number of atoms (Figure S11b), which agrees with the experimental observations (32.0 s in Figure 2a). An important conclusion that can be drawn from these cDFT and AIMD -
simulations is that AuCl4 anions accumulation and the increase of local supersaturation in the gap are crucial for local dehydration and initial contact. Data from calculations and simulations along with experimental results provide a complete picture of nanoparticle coalescence mechanism. Although the electron beam plays a critical role during nanoparticle growth in the LC-TEM experiments, the initial contact of nanocrystals via the nanobridge reported in this work may be independent of electron beam irradiation. The electron beam may affect coalescence through several mechanisms, which are discussed now. Estimated temperature increases due to the electron heat effect are negligible,49 which excludes electron beam-induced melting of Au nanocrystals.50 In addition, the pH change in the solution is so small because the initial pH is
