In Situ Atomic-Scale Observation of Droplet Coalescence Driven

In Situ Atomic-Scale Observation of Droplet Coalescence Driven Nucleation and Growth at Liquid/Solid Interfaces Junjie Li,† Zhongchang Wang,‡ and Francis Leonard Deepak*,† †

Department of Advanced Electron Microscopy, Imaging and Spectroscopy, International Iberian Nanotechnology Laboratory (INL), Avenida Mestre Jose Veiga, Braga 4715-330, Portugal ‡ Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *

ABSTRACT: Unraveling dynamical processes of liquid droplets at liquid/solid interfaces and the interfacial ordering is critical to understanding solidification, liquid-phase epitaxial growth, wetting, liquid-phase joining, crystal growth, and lubrication processes, all of which are linked to different important applications in material science. In this work, we observe direct in situ atomic-scale behavior of Bi droplets segregated on SrBi2Ta2O9 by using aberrationcorrected transmission electron microscopy and demonstrate ordered interface and surface structures for the droplets on the oxide at the atomic scale and unravel a nucleation mechanism involving droplet coalescence at the liquid/solid interface. We identify a critical diameter of the formed nanocrystal in stabilizing the crystalline phase and reveal lattice-induced fast crystallization of the droplet at the initial stage of the coalescence of the nanocrystal with the droplet. Further sequential observations show the stepped coalescence and growth mechanism of the nanocrystals at the atomic scale. These results offer insights into the dynamic process at liquid/solid interfaces, which may have implications for many functionalities of materials and their applications. KEYWORDS: liquid/solid interface, nucleation, crystal growth, phase transition, in situ observation, aberration-corrected transmission electron microscopy

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other hand, Matsudaira et al. reported in situ observation of stick−slip movements of water nanodroplets on a hydrophilic Si3N4 membrane using a TEM liquid cell.18 However, due to the lack of direct experimental observations at the atomic scale and also the intricacies of tackling such challenging systems, much confusion still exists regarding the atomistic understanding of the dynamic processes at liquid/solid interfaces. Bismuth bulk metal has a low melting point of 544.4 K, and the melting temperature of its nanoparticles can be as low as room temperature due to size effects.19 As such, Bi is an ideal model material to track electron-beam-induced nucleation and growth.20−24 Recently, Wang et al. applied conventional TEM to observe the nucleation and growth trajectory of interfacial metal nanoparticles on an oxide support25 and reported that Bi segregates out of SrBi2Ta2O9 and nucleates on the oxide under electron beam irradiation. Nevertheless, due to the limit of spatial resolution of a conventional TEM, many critical details,

ynamic processes at the liquid/solid interfaces are of key significance across broad areas of technological interest, such as solidification, liquid-phase epitaxial growth, wetting, liquid-phase joining, crystal growth, and lubrication.1−6 Many studies have been reported with the indirect evidence of density fluctuations at liquid/solid interfaces on the basis of X-ray scattering methods7−9 and atomic force microscopy (AFM)10,11 and with the support of atomistic simulations.12−15 Transmission electron microscopy (TEM) can, in principle, allow us to observe dynamic processes directly, yet to date such investigations are scarce due to the exceptionally high need of an elegant microscope and a suitable system that enables seeing liquids, solids, and their junctions simultaneously. For instance, Oh et al.1 conducted the real-time high-temperature observation of an aluminum/alumina liquid/ solid interface using a ultrahigh-voltage TEM and provided evidence of the ordering of liquid atoms adjacent to its interface with a crystal. Luo et al.16,17 observed directly the growth of bismuth (Bi)-enriched clusters on the surface of Bi-enriched amorphous films of ∼2 nm supported on a Bi2O3-doped oxide under electron beam irradiation inside the microscope. On the © 2017 American Chemical Society

Received: February 10, 2017 Accepted: May 24, 2017 Published: May 24, 2017 5590

DOI: 10.1021/acsnano.7b00943 ACS Nano 2017, 11, 5590−5597

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Figure 1. Image and line profile for a Bi liquid droplet on SrBi2Ta2O9. (a) Typical HRTEM image of a Bi droplet on the support of SrBi2Ta2O9. (b) Line profile showing image intensity along the arrow in (a), illustrating local ordering.

Figure 2. Coalescence-induced crystallization at the Bi/SrBi2Ta2O9 interface. (a−f) HRTEM images showing the coalescence of two droplets 1 and 2 with temporal evolution. (g−l) HRTEM images showing rearrangement of the formed nanoparticles. The electron dose rate is 4.22 × 104 e−/Å2·s.

manipulating electron doses and demonstrate droplet coalescence driven interfacial liquid−solid phase transformation and a stepped liquid−solid coalescence and growth mechanism. We provide definitive evidence for the droplet coalescence driven crystallization and identify the critical diameter of the formed crystal in stabilizing crystalline phases. Interestingly, in contrast to the rapid coalescence of two liquid droplets, the coalescence of a nanocrystal with a liquid droplet takes place via a clear step-migration mechanism.

such as atomic-scale structures of the Bi/SrBi2Ta2O9 liquid− solid interface, initial liquid/solid phase transformation of Bi droplets on the support, and the coalescence mechanism between Bi droplets and the nanocrystal on the support, are still not fully understood and clearly established. With the development of aberration-corrected electron microscopes, the atomic resolution can be achieved even at low voltage, allowing us to directly resolve extremely small nanostructures that are not clearly visible by conventional TEM.26,27 Here, we report in situ atomic-scale observation of segregated Bi droplets on an oxide SrBi2Ta2O9 support under irradiation by 5591


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Figure 3. Sequential HRTEM images showing detailed coalescence-induced crystallization. (a, b) HRTEM images showing the nucleation of the initial Bi droplet on the substrate. (c−k) HRTEM images showing the coalescence-induced crystallization of the Bi droplet on the substrate. (l) HRTEM image showing rearrangement of the formed nanoparticles in order to reduce surface energy. The electron dose rate is 2.15 × 104 e−/Å2·s.

RESULTS AND DISCUSSION The molten salt method was used to synthesize the SrBi2Ta2O9 (SBTO) platelets.28 X-ray diffraction (XRD) analysis (Supplementary Figure 1) revealed the formation of SrBi2Ta2O9 with an orthorhombic structure (JCPDS Card No. 49-0609). Further low-magnification TEM and scanning TEM (STEM) characterization confirm the thin platelet morphology for the obtained SrBi2Ta2O9 nanoparticle (Supplementary Figure 2). It is known that Bi tends to diffuse out of SrBi2Ta2O9 platelets under electron irradiation (Supplementary Figure 3). Hence we take this system comprising the formed Bi droplets on a crystalline SrBi2Ta2O9 support to probe such a liquid/solid interface and study the following aspects: (i) the atomic structure of the Bi liquid droplet formed at the interface with a crystalline SrBi2Ta2O9 support, (ii) coalescence of liquid droplets, (iii) the link between coalescence and initial liquid− solid phase transformation of droplets at the interface, and (iv) the coalescence mechanism between a Bi droplet and a crystalline Bi nanoparticle on the oxide support. Figure 1a shows a typical high-resolution TEM (HRTEM) image of a segregated Bi droplet on SrBi2Ta2O9 viewed along the [001] direction of the oxide support. Interestingly, ordered interfacial structures emerge at the liquid/solid interface and

present a shrinkage of lattice spacing in one direction toward the interior of the droplet from d1 = 3.5 ± 0.1 Å to d6 = 3.0 ± 0.1 Å (Figure 1a), as observed for the case of interfacial Al layers of Al droplets on an Al2O3 support.1,29 Such ordered Bi interfacial layers are possibly due to the template effect, and the change in the lattice spacing may arise from the transient phase caused by lattice misfit induced strain.30−32 Surface ordering also occurs in the Bi liquid droplet, which has never been imaged directly at the atomic scale.6,33−36 On the basis of surface tension measurements, the formation of a crystalline structure at the surface is typically characterized by a slope change in the surface tension versus temperature curve (γ(T)) from dγ/dT > 0 for a crystalline surface to dγ/dT < 0 for a liquid surface.5,37 Indeed, in the present case, the slope is positive for the Bi droplet on an oxide support,36,38 which accounts for the surface crystallization of the Bi droplet. Such structural ordering at both the interface and surface of the Bi droplet can be further confirmed by an intensity profile line scanned across the Bi droplet (red arrow in Figure 1a), revealing periodic image contrast perturbations at the interface and surface region (Figure 1b), while random ones occur in the interior of the droplet (see box in Figure 1b).1,5,6,29 The contrast perturbations are also identified in other droplets that 5592


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structure until the size of the nanocrystal reaches a critical value, dc (∼14 nm). The observed phenomena can be understood in light of the critical size governed by the excess free energy, which can be expressed as follows for a spherical cap embryo formed on a support:46,47

we have observed, consolidating the structural ordering at the interface and surface of the Bi droplet. To uncover the coalescence process of droplets on the support, we show in Figure 2 sequential TEM images taken under an electron dose rate of 4.22 × 104 e−/Å2·s revealing the droplets coalescence induced liquid−solid transformation and atomic rearrangements of the newly formed nanocrystal (see also Supplementary Video 1). The small Bi droplets (labeled 1 and 2 in Figure 2b) first prefer to migrate on the surface of the support under electron irradiation (Figure 2a−e) and then undergo coalescence, which completes in a very short time (Figure 2f). A crystalline structure appears immediately after coalescence, induced by the liquid−solid phase transformation at the interface. The phase transformation should be related to the change in the melting/solidification point of a nanostructure with size. Although energy is increasingly accumulated on a particle with increasing irradiation time, it consequently attains a balance between energy gain from the electron beam and energy loss to the environment.39,40 As a result, owing to the heat transformation between the support and Bi nanoparticles, the temperature of the Bi nanoparticles in a local area could be consistent when the system reaches an energy balance.41 When two droplets coalesce, the size (volume) of the newly formed nanostructure increases rapidly, causing the melting/solidification point of the nanostructure to increase based on Shi’s model and Liu’s experimental observation revealing an increase of the melting/solidification point of a nanostructure with size.42,43 If the two droplets before coalescence are in an equilibrium state (melting at their size-dependent melting point), the newly formed much larger nanoparticle is rapidly undercooled at this temperature, causing transformation from liquid to solid to take place. An epitaxial orientation relation of [5̅51̅]Bi//[001]SrBi2Ta2O9 is observed initially at the Bi/ SrBi2Ta2O9 interface. However, to minimize their total energy,42,44,45 the formed nanocrystal subsequently shows a series of atomic rearrangements, leading to a range of changes in orientation relationship between the Bi nanocrystal and the SrBi2Ta2O9 (SBTO), e.g., [421̅]Bi//[001]SBTO (Figure 2g), [121̅]Bi//[001]SBTO (Figure 2h), [421̅]Bi//[001]SBTO (Figure 2i), [5̅51̅]Bi//[001]SBTO (Figure 2j), and [421̅]Bi// [001]SBTO (Figure 2k,l). To further probe the coalescence dynamics, we plot the separation between droplet centers (s) as a function of coalescence time (Δt = tc − t, where tc is the time taken when two particles collide) for a typical pair of droplets (marked in Figure 2b), as shown in Supplementary Figure 4. One can see a nonlinear correlation between s and Δt, indicating that coalescence of two droplets in contact is completed swiftly, in contrast to the linear dynamic correlation observed in domain coalescence of a polymer forming a neck structure.36 It is worth noting that the droplets formed at an early stage, i.e., during initial liquid nucleation, are too small to be tracked precisely by using this high electron dose. To further investigate coalescence-induced nucleation and gain insights on the interfacial phase transformation, we reduced the electron dose rate to 2.15 × 104 e−/Å2·s and conducted in situ observations of the coalescence process, as shown in Figure 3 (see also Supplementary Video 2). Interestingly, the segregated Bi forms a Bi layer with a thickness of ∼2 nm, upon which the liquid Bi droplets nucleate and further grow. As seen in Figure 3d−l, the droplets go through a periodic evolution of liquid−coalescence−crystallization and develop into a nanocrystal with a stable facet

⎛ 1 ⎞ ΔG = ⎜ − πd3ΔG V + πd 2γSL⎟(2 + cos θ )(1 − cos θ )2 /4 ⎝ 6 ⎠ (1)

where ΔGV is the free energy change per volume, γSL is the surface energy between the solid and liquid, d is the diameter of a spherical cap, and θ is the contact angle on the support. One can note that the interfacial energy increases as d2, whereas the volume free energy decreases as d3, giving rise to a critical diameter of the nucleus, dc, and the corresponding maximum excess free energy, ΔG*.48 If d < dc, it is unfavorable for a formed transient nanocrystal embryo to grow since ΔG increases, which is more favorable for a solid particle to dissolve back into its liquid state. On the other hand, when d ≥ dc, it is favorable for a solid nucleus to grow since ΔG decreases, giving rise to a transformation from liquid to a stable crystalline state. This provides an explanation for the periodic evolution of liquid−coalescence−crystallization and the eventual formation of a stable facet structure when the size of the nanocrystal reaches a critical value, dc, as shown in Figure 3. These results offer a strong support for the coalescence-induced crystallization mechanism for the supported Bi droplets and demonstrate the existence of a series of transient phases before a stable crystalline state is reached. We further use the fast Fourier transformation (FFT) of Figure 3d−l (Supplementary Figure 5) to show the periodic phase transformation between liquid and solid for the Bi droplets as well as to reveal that the nanocrystals formed during the liquid−solid fluctuations indeed have a rhombohedral structure with different orientations, [44̅1̅], [4̅2̅1], and [01̅0] (JCPDS card no. 05-0519). To shed further light on the structural evolution of Bi nanoparticles, we have carried out statistical analysis of the change in phase, length, and contact angle during the entire coalescence process, as shown in Figure 4. At 20 s, the droplets follow a linear growth mode and exhibit a length jump from 8 to 12 nm due to the coalescence of two droplets (Figure 3b,c). Such coalescence increases the melting point of the newly combined droplets, leading to the fast transformation of the droplets from liquid to crystalline structure. The nanocrystals gain energy from the electron beam and melt again, following a periodic cycle of liquid−coalescence−crystalline, which is finally halted when the diameter of the nanoparticles exceeds the critical size of ∼14 nm, forming a stable crystalline structure. Likewise, the contact angle for Bi droplets on the SrBi2Ta2O9 support shows a similar tendency in the whole process (Figure 4b). It first increases in a linear way driven by the reduction of the interfacial energy49,50 and then reaches the highest value of ∼90°. We also find that the contact angle shows jumps ahead of the length, which is attributed to the occurrence of migration before coalescence.51−53 Figure 5 shows an atomic-resolution imaging of the coalescence between a nanocrystal and a droplet (Figure 5a− e) and the step-growth procedure (Figure 5f−p) (see also Supplementary Video 3). Interestingly, in the beginning of the coalescence, the already existing stable nanocrystal can induce the droplet in contact to crystallize quickly, and the newly formed nanocrystal experiences a fast atomic rearrangement, forming a stepped structure on the (003) surface (marked by 5593


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to the upper terrace, in line with the simulated classical step growth on the reconstructed (001) surface of diamond54,55 and the experimental observation of graphene growth starting from the step-edge of the bilayer graphene substrate.56 The growth source of the monomer (Bi atoms) at this stage should come from the residual free atoms or clusters on the surface of the Bi nanocrystal (marked by blue arrows in Figure 5m,n). The change of step shape and number with time in the stepped Bi structure is sketched in Figure S6, where one can note that, accompanied by the atomic migration to a lower step, the number of steps is reduced and the upper step grows (Figure S6a). Consequently, the formed large step in Figure 5l is displaced out of the surface of the nanocrystal by adatom migration to fill the terrace (it takes time, as shown in Figure S6b), and the nanocrystal is eventually transformed to a perfect faceted structure. Further FFT analyses reveal that the orientation relationship between the Bi nanocrystal and the SrBi2Ta 2O9 substrate in Figure 5p is [010]Bi//[001]SrBi2Ta2O9 (Supplementary Figure 7). The lattice-induced growth can reduce the nucleation energy barrier, facilitating the growth of large single crystals, which is similar to the classical spontaneous nucleation driven by undercooling.57−59 Figure 6 sketches the solid Bi nucleation and stepped coalescence and growth mechanism for a Bi droplet on the SrBi2Ta2O9 support. The Bi segregates initially from its bulk oxide, forming thin Bi layers, upon which the Bi droplet is further developed. Driven by reducing interfacial energy, the droplet increases its contact angle with the support and grows, forming an ordered structure. Such nucleation from the droplet is triggered by the coalescence of droplets and is completed in a

Figure 4. Length, phase, and contact angle analysis of the Bi nanoparticles. (a) Fluctuation of length and liquid−solid phase with temporal evolution obtained from video 2. The liquid phase is represented in green, and the solid one in red. (b) Fluctuation of the contact angle obtained from video 2.

yellow arrows in Figure 5c−e). Subsequently, the nanocrystal undergoes a series of step growth (Figure 5f−p) from the lower

Figure 5. Coalescence of a nanocrystal with a droplet on the support and the step-growth mechanism. (a−e) HRTEM images showing the coalescence of a nanocrystal with a droplet on the support at the atomic scale. (f−p) HRTEM images showing the atomic-scale observation of the step growth on the supported Bi nanocrystal. The electron dose rate is 2.15 × 104 e−/Å2·s. 5594


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Figure 6. Solid nucleation and growth mechanism on a support. (a) Schematic illustration of the Bi droplet coalescence induced solid nucleation mechanism on a SrBi2Ta2O9 support. (b) Schematic illustration of the stepped coalescence and growth mechanism between a Bi nanocrystal and a Bi droplet on the support.

EXPERIMENTAL SECTION

very short time. Once its size reaches a threshold value, the stable nanocrystal shows coalescence with the droplet to grow; that is, the migrated Bi droplet first contacts the nanocrystal and is partially crystallized, induced by the Bi nanocrystal lattice, and subsequently the whole droplet transforms to a crystalline structure with a stepped atomic arrangement to reduce the surface energy.

Sample Synthesis. The molten salt method was used to synthesize SrBi2Ta2O9 platelets.28 In a typical synthesis process, 0.2089 g of strontium nitrate (Sr(NO3)2, 99.5%), 0.4608 g of bismuth oxide (Bi2O3, 99.9%), and 0.4377 g of tantalum oxide (Ta2O5, 99.9%) were first mixed with 1.2400 g of potassium chloride and 0.9740 g of sodium chloride, which were adopted as solvent when melted. The reaction mixtures were then ground for 0.5 h in an agate mortar and loaded into an alumina crucible followed by heating at 850 °C in air for 3 h in a tube furnace and natural cooling to room temperature. The resultant products were washed with deionized water several times to remove the solvent and dried at 60 °C for 2 h. The phases of the final products were subsequently identified by X-ray diffraction (X’Pert PRO diffractometer, PANalytical). In Situ TEM Observation and Chemical Analysis. The obtained samples were dispersed in ethanol under ultrasonication, and a drop of this dispersion was placed onto a holey carbon grid, which was subsequently dried for TEM experiments. TEM imaging was conducted using a Titan Themis TEM equipped with both probe and image Cs correctors operated at 200 kV, which offered an opportunity to probe structures with sub-angstrom resolution. Series images were acquired using the Tecnai Imaging and Analysis software. The electron dose was tuned by manipulating the spot size. EDS mapping was performed on a FEI Titan ChemiSTEM equipped with a probe corrector, a Super-X EDS detector, and a Gatan EELS spectrometer.

CONCLUSIONS The capability of performing in situ atomic-scale observations of dynamic processes of liquid droplets at a liquid/solid interface using advanced transmission electron microscopy represents a significant step forward in understanding liquids, solids, and their interactions at the atomic scale. We have shown that the Bi droplets formed on a SrBi2Ta2O9 support have ordered interface and surface structures and demonstrate a nucleation mechanism involving droplet coalescence for the Bi/ SrBi2Ta2O9 liquid/crystalline phase transformation at the interface. A critical diameter is identified in stabilizing the crystalline structure, and the stable nanocrystal induces the droplet in contact to experience a quick crystallization. Such a newly formed nanocrystal undergoes a fast atomic rearrangement, forming epitaxial structures on the support. The findings provide detailed dynamic information at the Bi/SrBi2Ta2O9 liquid/solid interface at the atomic scale and should help to advance our general understanding of dynamic processes in other liquid/solid interfaces.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00943. 5595


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XRD results of the obtained SrBi2Ta2O9 powder, lowmagnification TEM and STEM images, EDS mapping and spectrum of segregated Bi nanocrystals, coalescence dynamics of Bi droplets, FFT patterns obtained based upon the images in Figure 3d−l, step growth in the stepped Bi structure, orientation relationship between faceted Bi nanoparticles and a SrBi2Ta2O9 substrate (PDF) Video S1 (AVI) Video S2 (AVI) Video S3 (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail (F. L. Deepak): [email protected]. ORCID

Francis Leonard Deepak: 0000-0002-3833-1775 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank M. Gonzalez-Debs for help with XRD analysis. The authors also appreciate the helpful discussions with E. A. Anumol and Q. Li. J.J.L. and F.L.D. acknowledge the financial support from the N2020 program: Nanotechnology based functional solutions (NORTE-45-2015-02). REFERENCES (1) Oh, S. H.; Kauffmann, Y.; Scheu, C.; Kaplan, W. D.; Rühle, M. Ordered Liquid Aluminum at the Interface with Sapphire. Science 2005, 310, 661−663. (2) Li, X.; Fautrelle, Y.; Ren, Z. Influence of Thermoelectric Effects on the Solid-Liquid Interface Shape and Cellular Morphology in the Mushy Zone during the Directional Solidification of Al-Cu Alloys under a Magnetic Field. Acta Mater. 2007, 55, 3803−3813. (3) Shekhah, O.; Wang, H.; Paradinas, M.; Ocal, C.; Schüpbach, B.; Terfort, A.; Zacher, D.; Fischer, R. A.; Wö ll, C. Controlling Interpenetration in Metal−Organic Frameworks by Liquid-Phase Epitaxy. Nat. Mater. 2009, 8, 481−484. (4) Zhou, Y.; Gale, W.; North, T. Modelling of Transient Liquid Phase Bonding. Int. Mater. Rev. 1995, 40, 181−196. (5) Shpyrko, O. G.; Streitel, R.; Balagurusamy, V. S.; Grigoriev, A. Y.; Deutsch, M.; Ocko, B. M.; Meron, M.; Lin, B.; Pershan, P. S. Surface Crystallization in a Liquid AuSi Alloy. Science 2006, 313, 77−80. (6) Wang, H.; Zepeda-Ruiz, L. A.; Gilmer, G. H.; Upmanyu, M. Atomistics of Vapour-Liquid-Solid Nanowire Growth. Nat. Commun. 2013, 4, 1956. (7) Mezger, M.; Schröder, H.; Reichert, H.; Schramm, S.; Okasinski, J. S.; Schöder, S.; Honkimäki, V.; Deutsch, M.; Ocko, B. M.; Ralston, J. Molecular Layering of Fluorinated Ionic Liquids at a Charged Sapphire (0001). Science 2008, 322, 424−428. (8) Reichert, H.; Klein, O.; Dosch, H.; Denk, M.; Honkimäki, V.; Lippmann, T.; Reiter, G. Observation of Five-Fold Local Symmetry in Liquid Lead. Nature 2000, 408, 839−841. (9) Huisman, W. J.; Peters, J. F.; Zwanenburg, M. J.; de Vries, S. A.; Derry, T. E.; Abernathy, D.; van der Veen, J. F. Layering of a Liquid Metal in Contact with a Hard Wall. Nature 1997, 390, 379−381. (10) Biggs, S.; Mulvaney, P. Measurement of the Forces Between Gold Surfaces in Water by Atomic Force Microscopy. J. Chem. Phys. 1994, 100, 8501−8505. (11) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press, 2011. (12) Sosso, G. C.; Chen, J.; Cox, S. J.; Fitzner, M.; Pedevilla, P.; Zen, A.; Michaelides, A. Crystal Nucleation in Liquids: Open Questions and 5596


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