Despite gallium-based liquid metal alloys attracting extensive attention for various applications, their phase behavior at the nanoscale is still underexplored. Understanding the impact of phase separation in nanoparticles can be extremely useful for developing new structures and exploring suitable applications. Here, we report on the investigation of phase behavior for spherical nanoparticles made from gallium-based liquid metal alloys upon cooling. We discover the thermally stable coexistence of solid cores in spherical liquid metal shells without the support of a crystalline substrate at room temperature. Given the unique properties of liquid metal, together with the facile process for producing core-shell and Janus nanoparticles, this work will encourage further investigation of the properties of such nanoparticles for developing applications in the fields of electronics, catalysis, nanomedicine, and beyond.

Nanoparticles produced from gallium-based liquid metal alloys have been explored for developing applications in the fields of electronics, catalysis, and biomedicine. Nonetheless, physical properties, such as phase behavior at micro-/nanosize scale, are still significantly underexplored for such nanoparticles. Here, we conduct an in situ investigation of phase behavior for gallium-based liquid metal nanoparticles and discover the unprecedented coexistence of solid particles in spherical liquid metal shells without the support of a crystalline substrate. The particles can also transform into solid Janus nanoparticles after temperature cycling. In addition, we investigate the optical properties of the nanoparticles before and after phase separation using in situ electron energy-loss spectroscopy. Most importantly, we discover that increasing the content of indium within the nanoparticle can stabilize the solid-core/liquid-shell structure at room temperature. This study provides a foundation to engineer liquid metal nanoparticles for developing new applications in nanoscale optical platforms and shape-configurable transformers.

Herein, we report on the in situ observation of solid/liquid coexistence in spherical LM NPs without the support of a crystalline substrate. Upon lowering the temperature, phase separation occurs for the NPs produced from Ga-based alloys, forming a solid-core/liquid-shell structure. We investigated the effect of the content of dissolved metal (In) in Ga on the stability of the core-shell structure at room temperature. We also report on the transformation of LM NPs into Janus NPs after cooling and subsequent heating. In addition, using in situ electron energy-loss spectroscopy (EELS), we studied the surface plasmon resonance (SPR) and bulk plasmon for the LM NPs before and after phase separation. Finally, we demonstrated that in addition to binary NPs, phase separation can also occur in ternary Ga-In-Sn NPs.

In recent years, NPs of Ga-based liquid metal (LM) alloys, such as eutectic gallium-indium (EGaIn, 75 WT % Ga and 25 WT % In) and gallium-indium-tin (Galinstan, 68.5 WT % Ga, 21.5 WT % In, and 10 WT % Sn), are increasingly being studied for candidate applications in the fields of soft electronics,catalysts,and biomedicines.These applications are facilitated by the unique properties of the LM alloys, such as high electrical and thermal conductivities, negligible vapor pressure, and relatively low toxicity in comparison with mercury. Most importantly, the presence of a functional native gallium oxide layer on the surface of the LM allows for the formation of stable NPs after comminuting the bulk materials using various methods, such as sonication,liquid-based nebulization,and rotor-induced shear.The fluid nature of LM also allows for the production of its microdroplets using platforms enabled by microfluidic chips,injection needles,and piezoelectric transducers.Nonetheless, despite the recent exploration of applications for the LM NPs, their physical properties, such as phase behaviors at micro- to nanosize, are still significantly underexplored.

It has been well established for inorganic NPs that their melting temperature decreases linearly with R, where R is the particle radius.This can be understood by considering the Gibbs energy of the system to have an additional term accounting for the free energy at the NP surface with respect to the bulk. However, metals such as gallium (Ga), tin (Sn), bismuth, and lead are exceptions to this scaling relationship.For instance, Ga exhibits an extreme supercooling (as low as 34 K before freezing) effect,and also exhibits superheating at certain sizes.An interesting discovery shows that when in contact with a sapphire substrate, Ga (bulk melting point 303 K) forms stable solid particles within a liquid shell (hemispherical-shaped) over a wide range of temperature (from 180 to 800 K).In this case, the sapphire substrate is crucial as the lattice structures of the solid γ-Ga core and sapphire are near coincidence, and this enables the formation of a stable solid-core/liquid-shell structure. However, there are no reports of stable solid particles within a spherical liquid shell (without the support of a crystalline substrate) at room temperature.

The high surface-to-volume ratio of nanoparticles (NPs) can lead to significantly altered behaviors relative to the corresponding bulk materials.Phase transitions may take place when changing temperature or chemical composition of the NPs, and thus nuclei of a new phase may occur and the NPs will consist of a core phase surrounded by a shell phase.Understanding the impact of phase separation in NPs can be extremely useful for developing new structures and exploring novel properties; as an example, phase separation in NPs has enabled various innovative applications in catalysis and energy storage/conversion.

Results and Discussion

24 Tang S.-Y.

Qiao R.

Yan S.

Yuan D.

Zhao Q.

Yun G.

Davis T.P.

Li W. Microfluidic mass production of stabilized and stealthy liquid metal nanoparticles. , 25 Lin Y.

Genzer J.

Li W.

Qiao R.

Dickey M.D.

Tang S.-Y. Sonication-enabled rapid production of stable liquid metal nanoparticles grafted with poly (1-octadecene-alt-maleic anhydride) in aqueous solutions. Figure 1 Phase Separation of EGaIn NPs Show full caption (A and B) HAADF images and EDS elemental maps of Ga and In at a temperature of (A) 297 K and (B) 206 K. The enlargement of the boxed particle in (B) shows a time-lapse series highlighting the movement of the solid In core (bright) within the liquid Ga shell (dark). (C and D) HAADF image and EDS elemental maps of Ga and In at a temperature of 173 K (C; all phases are solid); the boxed particle is shown enlarged in (D) along with EDS maps at the solid Ga-In interface. (E) HRSTEM bright-field image of the Ga-In interface at a temperature of 173 K. (F) Restructuring of the phase boundary on heating from 173 to 236 K to form Janus particles (all phases are solid). LM NPs were grafted with trisodium citrate by sonicating 50 μL of EGaIn in 5 mL of trisodium citrate (1 mM) solution for 5 min. We have previously shown that trisodium citrate can act as an antioxidant and grafting molecule and is suitable for obtaining a stable LM NP suspension.We separated large particles using a centrifuge (∼270 × g for 3 min), and the hydrodynamic size distribution of the NPs had a peak at ∼180 nm (see Figure S1 ). We next characterized the NPs using scanning transmission electron microscopy (STEM). Figure 1 A shows the high-angle annular dark field (HAADF) and bright-field (BF) images, as well as the energy-dispersive X-ray spectroscopy (EDS) maps of the LM NPs at room temperature (297 K) on an amorphous carbon support film. No periodic diffraction pattern was observed from the convergent-beam electron diffraction (CBED, data not shown) measurement, indicating that the NPs were in a liquid state. The EDS map indicates a uniform distribution of Ga and In within the NPs. Note that the contrast in the HAADF images varies as a function of thickness and atomic number. For a given thickness, bright regions have higher mean atomic number.

9 Di Cicco A.

Filipponi A. Local correlations in liquid and supercooled gallium probed by X-ray absorption spectroscopy. , 34 Li X.F.

Fei G.T.

Chen X.M.

Zhang Y.

Zheng K.

Liu X.L.

De Zhang L. Size-temperature phase diagram of gallium. 35 Balamurugan B.

Kruis F.

Shivaprasad S.

Dmitrieva O.

Zähres H. Size-induced stability and structural transition in monodispersed indium nanoparticles. 36 Yin X.Y.

Collins G.S. The solubility of indium in liquid gallium supercooled to 12 K. Next, we gradually lowered the temperature of the specimen. A bulk eutectic material, such as EGaIn (bulk melting point ∼290 K), should solidify completely at a single temperature (in the case of EGaIn, it should form solid Ga and solid In). Instead, we observed a phase separation within the NPs when the temperature reached 206 K (81°C of supercooling), forming stable core-shell-structured NPs, as shown in Figure 1 B. The EDS map clearly indicates the formation of an In core and a Ga shell within an NP, and the CBED and image contrast showed that the In core was solid while the Ga shell remained liquid (see Figure S2 ). The liquid state of Ga at almost 100°C below the bulk melting point can be attributed to the supercooling effect, as the probability of having an extrinsic nucleating center within such a small volume is negligible for the NPs.We observed that the In core is able to move freely within the liquid Ga (detail of boxed region in Figure 1 B, see also Video S1 ), and the selected area electron diffraction (SAED) pattern obtained for the NP cluster reveals that the In core has a face-centered cubic (FCC) structure (cf. Figure S3 A and Table S1 ).The occurrence of this phase-separation phenomenon can be attributed to the fact that the solubility of In within Ga is very low (<3.5%) at such a low temperature and, therefore, precipitation of In occurs to form a solid particle.Due to the instant and nearly full separation between In and Ga phases, the size of the In core is independent of temperature and is determined by the concentration of In within the alloy only.

eyJraWQiOiI4ZjUxYWNhY2IzYjhiNjNlNzFlYmIzYWFmYTU5NmZmYyIsImFsZyI6IlJTMjU2In0.eyJzdWIiOiI4NmI5Y2M0Y2Q0ZGNlODcxOTVlMmI4NGRjNzVhOGU3ZiIsImtpZCI6IjhmNTFhY2FjYjNiOGI2M2U3MWViYjNhYWZhNTk2ZmZjIiwiZXhwIjoxNjAwOTU3NDk3fQ.sbbXzkWAp_YnJxqiGPV7bMKZfSdbPich66BGHh_aPZ9qTWVOlikV1GV1LN1xpm3CTFamZoQbp0fRuJ7iLTMQaXpT5ubV9q5tFxelcbPd020eE7BBhDDNBXCAaYw3R9vmkJjgviII1rWv4qXB0-4N2SfeHpUCrYm9NXSHcaC1nxMwADef-GIzPBI51Uv7-CwKaSKkpB9z8DmdndT5sRkrHkQ_Yax2NeCRxhs2hAwQtYdmbiKz3IVGxQw2g_kBQ0rRlNACjZGmXvnQ-vRiRcyB1QLF9LB7aixYwIOiWN66_Tfk88MQfIvF3XFmWOhMZWjQXz1gmIrD5ZiR_ErZloLd_A

2 O 3 shell on the NP, with the thickness of this oxide layer being ∼2.2 nm (see We found that the Ga phase eventually solidified after reducing the temperature to 173 K, as shown in Figure 1 C. The In phase was pushed toward the edge of the NPs and became non-spherical. The boxed particle in Figure 1 C has been enlarged in Figure 1 D to show the interface between these two solid phases along with the corresponding EDS maps. This shows that almost no In can be detected within the Ga phase. Some Ga is detected within the In phase due to the presence of a α-Gashell on the NP, with the thickness of this oxide layer being ∼2.2 nm (see Figure S4 for details). The high-resolution STEM (HRSTEM) BF image ( Figure 1 E), together with the SAED studies (see Figure S3 B and Table S2 ) show the formation of FCC-structured In and γ-Ga within the NP. The diffractogram ( Figure 1 E inset) obtained from fast Fourier processing of the image of the In phase is consistent with an FCC structure with a = 0.48 nm.

34 Li X.F.

Fei G.T.

Chen X.M.

Zhang Y.

Zheng K.

Liu X.L.

De Zhang L. Size-temperature phase diagram of gallium. Interestingly, we observed the rearrangement of the Ga and In phases and the formation of Ga-In Janus NPs when increasing the temperature from 173 K to 236 K, as shown in Figure 1 F. This phenomenon is probably due to the transformation of γ-Ga into β-Ga during the heating process.When we further increased the temperature to ∼273 K, the Ga phase became liquid and the Janus NPs transformed into a solid-In-core/liquid-Ga-shell structure. However, we could only capture indistinct images of the solid In cores due to their rapid movement within the liquid Ga phase, as shown in Figure S5 . The In core eventually disappeared after ∼10 min when holding the temperature to 297 K due to dissolution into the Ga. The phase-separation process is highly repeatable and we did not observe any change of the phase behaviors during different thermal cycles, as shown in Figure S6

13 Losurdo M.

Suvorova A.

Rubanov S.

Hingerl K.

Brown A.S. Thermally stable coexistence of liquid and solid phases in gallium nanoparticles. Figure 2 Plasmon Measurements of EGaIn NPs at Room Temperature Show full caption (A and B) HAADF image (A) and EDS elemental map (B) of the EGaIn NPs. (C) EELS spectrum obtained from area 1 in (A) showing a surface plasmon peak at 6.5 eV. The inset shows how the surface plasmon peak position shifts to higher energies for larger particles (particle size indicated). (D) EELS map showing the distribution of the SPR for the energy range spanning 4.5 to 9.5 eV. (E) EELS spectrum obtained from area 2 in (A). (F) EELS map showing the bulk plasmon distribution for the energy range spanning 12 to 15 eV. Since it has been shown that the optical properties of liquid and solid Ga NPs are different,we investigated the optical properties of the EGaIn NPs with different structures at different temperatures using in situ EELS. At room temperature (297 K), the EGaIn NPs are in a liquid state and the distribution of Ga and In is uniform, as shown in Figures 2 A and 2B . Interestingly, we discovered that the EGaIn NPs exhibit a broad SPR with the peak at ∼6.5 eV ( Figure 2 C, obtained for the area 1 of Figure 2 A). The SPR is size dependent, the energy of which increases with NP size. Spectra obtained for NPs with the diameters of ∼27, 53, 72, and 87 nm are shown in inset of Figure 2 C. These spectra, together with the SPR intensity map of the NPs within the energy band between 4.5 and 9.5 eV ( Figure 2 D), show that the SPR becomes stronger for NPs of larger size. We obtained EELS spectra from a gallium oxide nanorod and detected a broad bulk plasmon with a peak at 22.9 eV; meanwhile we did not detect SPR within the energy band between 4.5 and 9.5 eV for the nanorod, as shown in Figure S7 . This result indicates that gallium oxide has a low free electron density and the detected SPR for the EGaIn NPs is not due to the surface or bulk plasmon induced by the oxide layer.

37 Yarema M.

Wörle M.

Rossell M.D.

Erni R.

Caputo R.

Protesescu L.

Kravchyk K.V.

Dirin D.N.

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von Rohr F. Monodisperse colloidal gallium nanoparticles: synthesis, low temperature crystallization, surface plasmon resonance and Li-ion storage. , 38 Albella P.

Garcia-Cueto B.

González F.

Moreno F.

Wu P.C.

Kim T.-H.

Brown A.

Yang Y.

Everitt H.O.

Videen G. Shape matters: plasmonic nanoparticle shape enhances interaction with dielectric substrate. 39 George A.

Choudhary H.K.

Satpati B.

Mandal S. Synthesis, characterization and optical properties of ligand-protected indium nanoparticles. 40 Scholl J.A.

Koh A.L.

Dionne J.A. Quantum plasmon resonances of individual metallic nanoparticles. , 41 Peng S.

McMahon J.M.

Schatz G.C.

Gray S.K.

Sun Y. Reversing the size-dependence of surface plasmon resonances. 42 Catalán-Gómez S.

Redondo-Cubero A.

Palomares F.

Nucciarelli F.

Pau J. Tunable plasmonic resonance of gallium nanoparticles by thermal oxidation at low temperatures. 42 Catalán-Gómez S.

Redondo-Cubero A.

Palomares F.

Nucciarelli F.

Pau J. Tunable plasmonic resonance of gallium nanoparticles by thermal oxidation at low temperatures. 42 Catalán-Gómez S.

Redondo-Cubero A.

Palomares F.

Nucciarelli F.

Pau J. Tunable plasmonic resonance of gallium nanoparticles by thermal oxidation at low temperatures. , 43 Sanz J.

Ortiz D.

Alcaraz De La Osa R.

Saiz J.

González F.

Brown A.

Losurdo M.

Everitt H.

Moreno F. UV plasmonic behavior of various metal nanoparticles in the near-and far-field regimes: geometry and substrate effects. 42 Catalán-Gómez S.

Redondo-Cubero A.

Palomares F.

Nucciarelli F.

Pau J. Tunable plasmonic resonance of gallium nanoparticles by thermal oxidation at low temperatures. These SPR results are interesting, as previous studies show a redshift (lower energy) of the SPR peak, instead of blueshift (higher energy), for Ga NPs of larger size.However, the blueshift of the SPR peak for In NPs with increasing size is also reported.In our case, we can rule out the quantum size effect since the size of our NPs are relatively large (>25 nm).We believe the blueshift of the SPR for larger EGaIn NPs can be attributed to the effect of the gallium oxide layer.Previous studies show the redshift of the SPR peak for Ga NPs in the presence of an oxide layer, and the peak further redshifts to longer wavelengths as the thickness of the oxide layer increases.The presence of the oxide layer induces two effects on the SPR: on the one hand, the oxide shell produces a redshift due to the large difference in the dielectric constant between Ga and gallium oxide; on the other hand, the reduction of the size of Ga core would blueshift the SPR to a shorter wavelength.However, the effect of the oxide layer due to the difference in dielectric constant dominates the spectral behavior of the SPR.Our measurements show that the thickness of the oxide layer on the EGaIn NPs is very similar (∼2.2 nm) regardless of particle size, as shown in Figure S8 . Therefore, the oxide layer on a smaller EGaIn NP will represent a larger volume fraction, and this would amplify the effect of the oxide layer to induce more redshift of the SPR in comparison with that of larger NPs. This explains the observation of the redshift of the SPR for NPs with a smaller size.

We also observed the bulk plasmon with the peak at 13.3 eV for the Ga-In alloy, as shown in the EELS spectrum given in Figure 2 E (obtained for the area 2 of Figure 2 A). Figure 2 F shows the intensity map of this bulk plasmon for the NPs within the energy band between 12 and 15 eV. We detected multiple plasmon peaks at 13.9, 27.9, and 41.7 eV for Ga NPs (see Figure S7 B). These peaks arise from multiple scattering as a result of the particle being so large (ca. 400 nm diameter), and occur at energies an integer multiple of the bulk plasmon energy. We hypothesize that the shift (13.9 to 13.3 eV) of the bulk plasmon peak for the Ga-In alloy can be attributed to the presence of In within the NPs.

Figure 3 Plasmon Maps of EGaIn NPs after Phase Separation Show full caption (A) BF and HAADF images of the NPs. (B) EDS map of the NPs at the temperature of 173 K. (C and D) EELS spectra obtained for area 1 (C) and area 2 (D) in (A). (E and F) EELS spectra obtained for the area 3 in (A) at the temperatures of 206 K (E) and 173 K (F). (G–I) EELS maps for the energy bands of 4.5–9.5 (G), 10–13 (H), and 12.5–16 eV (I). To examine our hypothesis, we obtained EELS spectra for the NPs after inducing phase separation by gradually lowering the temperature. Figure 3 A shows the STEM images of the completely solidified NPs at a temperature of 173 K, and the EDS map given in Figure 3 B shows the distribution of Ga and In phases. Figure 3 C presents the EELS spectrum obtained from the edge of the particle (labeled 1 in Figure 3 A) and shows a pronounced surface plasmon peak at 6.8 eV. The spectrum ( Figure 3 D) from the solid In phase (area 2 of Figure 3 A at 173 K) shows a strong bulk plasmon of In with a peak at 11.6 eV.

13 Losurdo M.

Suvorova A.

Rubanov S.

Hingerl K.

Brown A.S. Thermally stable coexistence of liquid and solid phases in gallium nanoparticles. During the cooling process, we also obtained the EELS spectrum ( Figure 3 E) for the Ga phase in the liquid state (area 3 of Figure 3 A at 206 K), and from the same region in the solid state ( Figure 3 F at 173 K). For the liquid Ga phase (206 K) we measured a bulk plasmon at the energy of 13.9 eV. Due to particle thickness causing multiple scattering, a second plasmon at twice this energy was also found. We obtained a similar bulk plasmon energy peak from pure Ga NPs (see Figure S7 C), indicating that the lower bulk plasmon energy (13.3 eV) observed for Ga-In NPs is due to the presence of In. After solidifying the Ga phase at 173 K to form γ-Ga, the bulk plasmon shifted to a higher energy (from 13.9 to 14.3 eV) ( Figure 3 F). The second (multiply scattered) plasmon necessarily shifted by an equivalent amount. This blueshift is due to the increased free electron density in solid γ-Ga, as its density is higher than that of liquid Ga.The plasmon maps for different energy bands ( Figures 3 G–3I) clearly show the SPR and the corresponding bulk plasmon for Ga and In phases on the NPs.

2 Shirinyan A.S.

Wautelet M. Phase separation in nanoparticles. , 13 Losurdo M.

Suvorova A.

Rubanov S.

Hingerl K.

Brown A.S. Thermally stable coexistence of liquid and solid phases in gallium nanoparticles. Δ G = G s − G m ≈ NTk [ ( 1 − C In ) ln ( 1 − C In ) + C In ln C In ] + 4 π ( 3 N M In C In 4 π N A ρ In ) 2 / 3 σ In - Ga , (Equation 1)

where G m and G s are the total Gibbs free energy of a Ga-In NP before and after phase separation, respectively; T is the temperature; k is the Boltzmann constant; C In is the atomic fraction of In within the NP; M In and ρ In are the molecular mass and density of In, respectively; N A is the Avogadro constant; and σ In-Ga is the interfacial tension between the In and Ga phases. We only observed the formation of single cores within the NPs during the phase-separation process, and this is also true for particles with larger sizes (diameters of a few hundred nanometers to ∼1.5 μm), as shown in Figure S9 . The formation of a single-core structure is due to the minimization of Gibbs free energy within the NPs.This can be interpreted as follows: with the fixed amount of In within an NP, a single spherical In core minimizes surface area and thus generates the minimum amount of interfacial energy after phase separation. Ignoring the effect of the gallium oxide layer, we can estimate the change of the Gibbs free energy ΔG for an NP with N atoms before and after the formation of solid-In-core/liquid-Ga-shell structure, as expressed by (see Supplemental Information 10 for the detailed deduction of this equation):where Gand Gare the total Gibbs free energy of a Ga-In NP before and after phase separation, respectively; T is the temperature; k is the Boltzmann constant; Cis the atomic fraction of In within the NP; Mand ρare the molecular mass and density of In, respectively; Nis the Avogadro constant; and σis the interfacial tension between the In and Ga phases.

In ) within the NP. We calculate the value of ΔG with respect to different In content for an NP (R = ∼50 nm) when T = 297 K, as shown in 30 In 70 ) for producing the LM NPs. 30 In 70 NPs at 297 K. The EDS map indicates a uniform distribution of Ga and In within the NPs, and no periodic diffraction pattern was observed from the CBED measurement (data not shown), indicating that the NPs are in the liquid state. This is probably due to the supercooling effect, as well as the increased solubility of In in nanosized particles. 9 Di Cicco A.

Filipponi A. Local correlations in liquid and supercooled gallium probed by X-ray absorption spectroscopy. , 13 Losurdo M.

Suvorova A.

Rubanov S.

Hingerl K.

Brown A.S. Thermally stable coexistence of liquid and solid phases in gallium nanoparticles. , 25 Lin Y.

Genzer J.

Li W.

Qiao R.

Dickey M.D.

Tang S.-Y. Sonication-enabled rapid production of stable liquid metal nanoparticles grafted with poly (1-octadecene-alt-maleic anhydride) in aqueous solutions. , 44 Porter D.A.

Easterling K.E.

Sherif M. Phase Transformations in Metals and Alloys (Revised Reprint). Figure 4 Phase Separation of the Ga 30 In 70 NPs Show full caption (A–C) HAADF images and EDS elemental maps of Ga and In at temperatures of (A) 297 K, (B) 241 K, and (C) 173 K. (D) Redistribution of Ga and In phases after increasing the temperature to 236 and 297 K. Scale bars, 50 nm. From Equation 1 we can see that at a constant T, ΔG is a function of the atomic fraction of In (C) within the NP. We calculate the value of ΔG with respect to different In content for an NP (R = ∼50 nm) when T = 297 K, as shown in Figure S10 . We obtained a positive value of ΔG for the case of EGaIn (25 WT % of In), indicating that its core-shell structure is not thermodynamically favorable at 297 K. This explains the disappearance of In cores within the NPs when increasing the temperature to 297 K (cf. Figure S5 ). Interestingly, our calculation shows that ΔG gradually decreases and could become negative when increasing the content of In above 65 WT % (see Figure S10 ), and we therefore hypothesize that the core-shell structure can be stable for NPs with a high In content. To verify our hypothesis, we utilized an alloy which contains 70 WT % of In and 30 WT % of Ga (GaIn) for producing the LM NPs. Figure 4 A shows the HAADF images and EDS maps of the GaInNPs at 297 K. The EDS map indicates a uniform distribution of Ga and In within the NPs, and no periodic diffraction pattern was observed from the CBED measurement (data not shown), indicating that the NPs are in the liquid state. This is probably due to the supercooling effect, as well as the increased solubility of In in nanosized particles.

Similar to the case of EGaIn, we observed the phase separation and the formation of solid-In-core/liquid-Ga-shell structure when reducing the temperature to 241 K, as shown in Figure 4 B. Due to the high In content, a much larger In core was formed within the NP in comparison with the case of EGaIn. We further observed the solidification of Ga phase and the formation of Janus particles after decreasing the temperature to 173 K, as shown in Figure 4 C. When increasing the temperature to 236 K, we observed the rearrangement of the Ga and In phases and the NPs remained solid and Janus, as shown in Figure 4 D. After increasing the temperature to 297 K, we found that the Ga phase became liquid while the In phase became spherical and remained solid, forming stable core-shell-structured NPs during the course of our experiment ( Figure 4 D). This result is consistent with our hypothesis that NPs with a high In content could be stable (i.e., ΔG < 0) taking the form of solid-In-core/liquid-Ga-shell structures at room temperature. The In core gradually disappeared after increasing the temperature to 331 K, as shown in Figure S11

13 K/s. 45 Pan K.

Li Y.

Zhao Q.

Zhang S. Simulation of solidification process of metallic gallium and its application in preparing 99.99999% pure gallium. Since the cooling rate used in our experiment is slow (19 K/min), we did not observe the formation of amorphous solid Ga within the NPs. It seems that a slow cooling rate is favorable for crystallization of liquid Ga, and amorphous solid Ga may form by using an ultra-high cooling rate of 10K/s.In addition to the investigation of NPs made from Ga-In alloys, we also examined the phase behaviors of pure Ga and In NPs, as shown in Figures S12 and S13 and Tables S3 and S4 . The Ga NPs were liquid at room temperature and transformed into solid γ-Ga when lowering the temperature to 130 K. We observed the transformation of γ-Ga into β-Ga during the heating process. On the other hand, the In NPs were solid at room temperature and we did not observe any phase transformation when lowering the temperature.

Figure 5 Phase Separation of Ternary NPs Composed of Ga, In, and Sn Show full caption (A–C) HAADF images and EDS elemental maps of Ga, In, and Sn at the temperature of (A) 297 K, (B) 206 K, and (C) 128 K. (D) Formation of solid bicrystal In-Sn cores within the liquid Ga shell after increasing the temperature to 297 K. We also showed that in addition to NPs made from Ga-In alloy, phase separation and the formation of core-shell structure can also be achieved in NPs made from other Ga-based alloys such as Ga-Sn, as shown in Figure S14 . Moreover, apart from binary NPs, we further studied the formation of core-shell structure within ternary NPs. In doing so, we produced the LM NPs using an alloy which contains ∼20 WT % of Ga, ∼45 WT % of In, and ∼35 WT % of Sn. Figure 5 A shows the HAADF images and EDS elemental maps of the ternary NPs at 297 K. The EDS maps indicate a uniform distribution of Ga, In, and Sn within the NPs, and no periodic diffraction pattern was observed from the CBED measurement (data not shown), indicating that the NPs are in the liquid state. Similar to the case of binary NPs, we observed the phase separation and the formation of solid-core/liquid-shell structure when reducing the temperature to 206 K ( Figure 5 B). The EDS maps show that the core is an alloy of In and Sn. Solidification of the Ga phase occurred within the NPs after decreasing the temperature to 128 K, as shown in Figure 5 C, in which no phase separation was observed between In and Sn phases. Surprisingly, we observed the formation of solid bicrystal cores with a sharp interface within the NPs when increasing the temperature from 128 to 297 K ( Figure 5 D). The EDS map shows that the bicrystal is composed of In-Sn alloy ( Figure 5 D). The formation of a solid bicrystal occurred in most of the NPs regardless of their size, as shown in Figure S15 . The cores remained stable during the course of experiment and eventually disappeared after the temperature was increased to 332 K.

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Müller A.H. Janus particles: synthesis, self-assembly, physical properties, and applications. 18 Zhang W.

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Li W.

Qiao R.

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Tang S.-Y. Sonication-enabled rapid production of stable liquid metal nanoparticles grafted with poly (1-octadecene-alt-maleic anhydride) in aqueous solutions. , 27 Tang S.-Y.

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Wang C.

Sun W.

Ligler F.S.

Dickey M.D.

Gu Z. Transformable liquid-metal nanomedicine. , 24 Tang S.-Y.

Qiao R.

Yan S.

Yuan D.

Zhao Q.

Yun G.

Davis T.P.

Li W. Microfluidic mass production of stabilized and stealthy liquid metal nanoparticles. , 25 Lin Y.

Genzer J.

Li W.

Qiao R.

Dickey M.D.

Tang S.-Y. Sonication-enabled rapid production of stable liquid metal nanoparticles grafted with poly (1-octadecene-alt-maleic anhydride) in aqueous solutions. Conventionally, various “bottom-up” methods utilizing processes such as chemical reduction, sol-gel, hydrolysis, precipitation, hydrothermal decomposition, wet chemical reaction, electrochemical deposition, controlled growth, self-assembly, and desymmetrization at interfaces have been demonstrated for producing inorganic core-shell- or Janus-structured NPs.In contrast, our approach provides an unconventional method for constructing such NPs, and we believe the stable core-shell-structured NPs made from LM may find applications in catalysis. Although photocatalytic activity has been demonstrated for micro- to nanosized Ga-based LM particles,the wide band gap (∼4.8 eV) of the gallium oxide layer on the surface of the LM significantly hinders the generation of sufficient electron-hole pairs upon illumination by visible light.Advantageously, the liquid Ga shell of the particles presented in this work may allow for the controlled growth of gold or silver noble metal NPs on the surface simply using the galvanic replacement method.This noble metal NP coating could induce the SPR effect to increase visible light absorption, thus enhancing the photocatalytic activity of the NPs.In addition, it has been demonstrated that polymers with trithiocarbonate, thiol, or carboxyl groups can be used for grafting the surface of Ga to electrostatically or sterically stabilize the NPs.Consequently, the polymer-stabilized NPs may prevent aggregation in reagents for enhancing the catalytic activity.