Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/NanoLett
Disorder-to-Order Transition Mediated by Size Refocusing: A Route toward Monodisperse Intermetallic Nanoparticles Hannah M. Ashberry,† Jocelyn T. L. Gamler,† Raymond R. Unocic,‡ and Sara E. Skrabalak*,† †
Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, One Bethel Valley Road, Oak Ridge, Tennessee 37831, United States
‡
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S Supporting Information *
ABSTRACT: Intermetallic nanoparticles are remarkable due to their often enhanced catalytic, magnetic, and optical properties, which arise from their ordered crystal structures and high structural stability. Typical syntheses of intermetallic nanoparticles include thermal annealing of the disordered counterpart in atmosphere (or vacuum) or colloidal syntheses, where the phase transformation is achieved in solution. Although both methods can produce intermetallic nanoparticles, there is difficulty in achieving monodisperse nanoparticles, which is critical to exploiting their properties for various applications. Here, we show that overgrowth on random alloy AuCu nanoparticles mediated by size refocusing yields monodisperse intermetallic AuCu nanoparticles. Size refocusing has been used in syntheses of semiconductor and upconverting nanocrystals to achieve monodisperse samples, but now we demonstrate size refocusing as a mechanism to achieve the disorder-to-order phase transformation in multimetallic nanoparticles. The phase transformation was monitored by time evolution experiments, where analysis of reaction aliquots with transmission electron microscopy and powder X-ray diffraction revealed the generation and dissolution of small nanoparticles coupled with an increase in the average size of the nanoparticles and conversion to the ordered phase. This demonstration advances the understanding of intermetallic nanoparticle formation in colloidal syntheses, which can expedite the development of electrocatalysts and magnetic storage materials. KEYWORDS: Size refocusing, nanoparticles, disorder-to-order transition, intermetallic
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size and shape is feasible; however, the synthetic conditions which will induce the disorder-to-order transition while promoting monodispersity are often unclear. In our group, monodisperse intermetallic PdCu nanoparticles were achieved by seed-mediated co-reduction (SMCR), in which the disordered-to-ordered phase transformation was achieved under mild conditions by gradual overgrowth of random alloy PdCu nanoparticles from simultaneous deposition of Pd and Cu onto the seeds.20 In contrast, annealing the same random alloy PdCu nanoparticles in solvent without growth did not facilitate the disorder-toorder transition at the same temperature. Also, when supported on carbon, the same nanoparticles required high temperatures and heating time to achieve the intermetallic phase, and this phase transformation was accompanied by an increase in particle size due to sintering and coalescence.20 Such a size increase after achieving the intermetallic phase with thermal annealing is common in other intermetallic compositions, as well.8,21−25 This observation suggests that the random alloy nanoparticles may need to overcome a size-
ntermetallic nanoparticles exhibit long-range atomic ordering with a defined stoichiometry, leading often to enhancement of catalytic, magnetic, and optical properties.1−17 Although intermetallic nanoparticles are desirable for a variety of applications, the synthesis of monodisperse intermetallic nanoparticles is challenging.1,2 Two major approaches used to achieve intermetallic nanoparticles are thermal annealing of preformed nanoparticles of the disordered phase in atmosphere (or vacuum) and colloidal syntheses where the phase transformation is achieved directly in solution.2 Thermal annealing is a two-step process, in which random alloy or core@shell nanoparticles are synthesized and then heated at a high temperature (≥500 °C) in atmosphere or vacuum to facilitate the phase transition to the ordered phase; the nanoparticles are often dispersed on a support for thermal treatment.2 Unfortunately, the high temperature and long annealing times required for the phase conversion lead to sintering, causing sample polydispersity. However, monodispersity can be achieved with thermal annealing by confining bimetallic nanoparticles in an inorganic shell during the annealing process.18,19 Colloidal syntheses employ the use of surfactants and reducing agents to induce the disorder-to-order transition in solution.2 As these syntheses are often at low or moderate temperatures (≤300 °C), control of nanoparticle © XXXX American Chemical Society
Received: June 27, 2019 Revised: August 1, 2019
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DOI: 10.1021/acs.nanolett.9b02610 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. (A) TEM image of monodisperse random alloy AuCu NPs of A1 structure; (B) HRTEM image of individual random alloy AuCu NP with inset of the corresponding selected area FFT; (C) TEM image of monodisperse AuCu NPs obtained with overgrowth; (D) high-angle annular dark-field/STEM image along the [110] zone axis (confirmed by the FFT in the inset); (E) histogram of particle distribution of random alloy AuCu NPs (red) and AuCu NPs resulting from overgrowth (blue); (F) PXRD patterns of AuCu nanoparticles (NP) with face-centered cubic random alloy configuration (red) and face-centered tetragonal intermetallic configuration (blue).
ripening), with an overall increase in the average size of the CdSe nanocrystals.33 Interestingly, the size distribution could then be narrowed again by an additional precursor injection, which reverses the Ostwald ripening process by increasing the supersaturation level to enable the smallest nanoparticles to “catch up” to the larger nanoparticles.33 Johnson and coworkers implemented size refocusing for upconverting nanoparticles by deliberately defocusing the ensemble of nanoparticles by introducing small sacrificial ones, which shifted the critical radius to a smaller size.34 The small sacrificial nanoparticles rapidly dissolved, leading to deposition of their material onto the larger nanoparticles; this process refocused the ensemble to a narrow size distribution.34,35 Our work demonstrates that the disorder-to-order transition in AuCu nanoparticles can be mediated by size refocusing. Overgrowth was implemented on random alloy AuCu nanoparticles to increase the size of the nanoparticles, which is coupled with the phase transformation from the random alloy A1 phase to the intermetallic L10 phase of AuCu. This process resulted in monodisperse intermetallic AuCu nanoparticles facilely. Specifically, random alloy AuCu nanoparticles were prepared and used as seeds to deposit a uniform overgrowth, with the initial seeds and resulting nanoparticles shown in Figure 1. The seed synthesis was by hot-injection of a Au/Cu precursor solution into a preheated solution of 1octadecene (ODE), oleic acid (OA), and 1,2-dodecanediol (see Supporting Information for full details). Monodisperse random alloy AuCu nanoparticles (Figure 1A) with an average size of 6.5 ± 1.2 nm (Figure 1E, red) were achieved. The lattice fringes shown in the high-resolution transmission electron microscopy (HRTEM) image (Figure 1B) and the corresponding selected area fast Fourier transform (FFT) of the HRTEM image (Figure 1B inset) indicate single crystallinity. The powder X-ray diffraction (PXRD; Figure 1F, red) matches the reference for the random alloy A1 AuCu phase. The (111) reflection features slight asymmetry, which indicates there is some variation in the Au-to-Cu ratio from particle to particle. Scanning transmission electron microscopy (STEM)/energy-dispersive X-ray (EDX) analysis, shown in
dependent activation barrier to achieve ordering. Both thermodynamic and kinetic parameters influence the origin of this size dependency.2 First, the change in free energy when going from the disordered to the ordered phase is usually negative, providing the main driving force for the phase transition; however, as the total surface free energy of particles increases with decreasing size, this driving force toward ordering is reduced.2 Second, atomic diffusion facilitates phase transitions, with smaller nanoparticles exhibiting faster atom transport as the barrier to diffusion at the nanoscale is lower than that of the bulk.2 Taken together, smaller nanoparticles can facilitate faster atom transport but restrict the formation of a new phase, which suggests there is an optimal size in which the intermetallic phase is achieved at a given temperature. For example, Takahashi and co-workers observed that FePt particles smaller than 4 nm could not be ordered to the L10 phase after long annealing times at 600 °C, whereas larger, 7 nm, nanoparticles were completely ordered.26 From a Helmholtz energy calculation using the Lennard-Jones potential, the thermodynamic order−disorder temperature (Tc) was determined to be lower than the annealing temperature for nanoparticles below 4 nm.26,27 Ordering cannot proceed because the disordered state is more stable than the ordered state at the annealing temperature, which is attributed to an increase in interfacial energy with respect to volume free energy.26,27 Here, overgrowth conditions were implemented on random alloy AuCu nanoparticles to achieve monodisperse intermetallic AuCu nanoparticles in solution; however, our analysis revealed size refocusing as an alternative growth mechanism to seed-mediated deposition being responsible for formation of monodisperse intermetallic AuCu nanoparticles. Size refocusing has been observed primarily in the synthesis of monodisperse quantum dots.28−32 For example, Alivisatos and co-workers studied the growth kinetics of CdSe nanocrystals via hot-injection and observed that there was a narrowing of the size distribution with rapid growth of the nanocrystals after the initial injection of the precursor.33 Over time, however, the size distribution broadened (Ostwald B
DOI: 10.1021/acs.nanolett.9b02610 Nano Lett. XXXX, XXX, XXX−XXX
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the nanoparticles grew from 6.6 ± 0.8 to 9.7 ± 1.5 nm as the reaction reached 280 °C. Over the course of the incubation period at 280 °C, the overall change in the size of the nanoparticles was minimal, with the diameter remaining at an average of 9.7 nm throughout the 60 min. Although most of the nanoparticles were similar in size during the incubation period, nanoparticles smaller than the initial seeds, approximately 5 nm, were observed. The ultimate preservation of sample monodispersity is consistent with SMCR, wherein the monodispersity of the initial seeds is transferred to the final structure by simultaneous reduction of Au and Cu metal precursors and metal deposition directly onto the seeds. However, the appearance of nanoparticles smaller than the initial seeds suggests that overgrowth is proceeding by an alternative mechanism, known as size refocusing. In the case of size refocusing, smaller nanoparticles are unstable relative to larger nanoparticles and dissolve in solution, with the metal redepositing on the larger nanoparticles to increase their size further.34,35,38 Monodispersity is preserved when there is complete dissolution of all of the smaller nanoparticles.34 Corresponding PXRD patterns were collected at the various time points to monitor the change in crystal phase over the period of the reaction (Figure 2A). Initially, the as-synthesized
Figure S1A−D, displays a uniform distribution of Au and Cu at the single-particle level. A line scan (Figure S1E) of the particle shown in Figure S1A shows there is approximately equal amounts of Au and Cu. The ratio of the line intensity is consistent with results from SEM-EDX, as well (Table S1), which shows there is a 55:45 mol ratio of Au-to-Cu. As heating is typically required to promote ordering, an initial experiment was to heat the as-synthesized random alloy AuCu nanoparticles in ODE at 280 °C for 1 h. The PXRD pattern (Figure S2A) of the resulting particles matches the reference peaks corresponding to the intermetallic L10 phase, indicating that ordering has occurred. However, although the disordered-to-ordered phase transition was achieved, the monodispersity of the nanoparticles was not preserved due to coalescence of the random alloy nanoparticles (Figure S2B). Such a size increase and loss of sample monodispersity is common with the synthesis of intermetallics and suggests there is a size-dependent transformation barrier to achieve ordering in bimetallic nanoparticles.8,20−25 Thus, co-reduction overgrowth methods were employed to increase the size of the random alloy AuCu nanoparticles in order to observe whether or not the disorder-to-order transformation could be facilitated while maintaining sample monodispersity, as demonstrated previously in our work with the PdCu system.20 Specifically, the random alloy AuCu nanoparticles were added to a solution of ODE, oleylamine (OLA), 1,2-dodecanediol, Au(ac)3, and copper(I) bromide (CuBr). Control experiments (Figure S3) show that CuBr must be used instead of copper(II) acetate to achieve substantial overgrowth on the random alloy seeds. Our previous work suggested that Br− may catalyze the deposition of growth monomers.20 The solution was gradually heated to 280 °C and incubated at this temperature for 60 min. Monodisperse intermetallic AuCu nanoparticles (Figure 1C) were obtained, and the size of the nanoparticles increased to 10.3 ± 1.0 nm (Figure 1E, blue). A high-angle annular darkfield STEM (HAADF-STEM) image (Figure 1D) shows continuous lattice fringes, indicating single crystallinity. Selected area FFT of the HAADF-STEM image support the single-crystalline nature of the nanoparticles. The observed lattice fringes (0.297 nm) correspond to the AuCu {110} plane, which is the superlattice plane indicative of an intermetallic phase. The PXRD pattern (Figure 1F, blue) matches the ICSD pattern for face-centered tetragonal AuCu L10 phase, also indicating that ordering is present, with the superlattice (001) and (110) peaks at 24.2 and 31.9°, respectively.6 Figure S4 shows the STEM-EDX mapping of an intermetallic nanoparticle. The mapping of Cu content of the nanoparticle (Figure S4D) appears less than the Au mapping (Figure S4C), which may indicate surface enrichment with Au. Additionally, the line scan (Figure S4E) shows an intensity spike for Au on the edge of the particle, which is indicative of a Au-rich exterior, common in the synthesis of AuCu intermetallic nanoparticles.6 Skrabalak and co-workers also observed a noble-metal-rich exterior for the B2 PdCu nanoparticles, which was attributed to surface segregation that occurs at high temperatures.20,36,37 A time-evolution study was carried out to monitor the conversion of the random alloy AuCu nanoparticles to the ordered L10 phase, along with the corresponding changes in nanoparticle size. Reaction aliquots were removed from the reaction vessel at different time points, and the samples were characterized. TEM images in Figure S5 show that the size of
Figure 2. (A) Time-evolution XRD patterns of AuCu NPs sampled during the overgrowth process, with the dashed lines indicating the superlattice peaks of intermetallic AuCu (blue) and for the facecentered cubic Au fundamental peak (green); (B) STEM image of selected AuCu NPs after 0 min at 280 °C for STEM-EDX analysis, and yellow boxes highlight the nanoparticles composed of mostly Au. Elemental mapping of selected area (B) denotes the presence of (C) Cu (blue) and (D) Au (yellow).
random alloy nanoparticles are in the A1 phase, as there is no appearance of superlattice peaks at 24.2 and 31.9°. The transition to the ordered face-centered tetragonal phase is fully observed after 60 min at 280 °C, where superlattice peaks are observed and the splitting of the (220) reflection into the (220) and the (202) is evident also. An interesting thing to note is that as the temperature increased to 240 °C the (111) reflection is observed to shift to lower 2θ, indicative of increased gold content. When the temperature is increased to 280 °C, a peak at 38° emerges, which corresponds to the position of the (111) reflection of pure Au, with this peak also observed after 30 min at 280 °C. Rietveld refinement of the PXRD patterns was performed to determine the percent of each phase present at each of the time points, and the results are shown in Figure S6. As the reaction heats up to 280 °C, the C
DOI: 10.1021/acs.nanolett.9b02610 Nano Lett. XXXX, XXX, XXX−XXX
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particles, which results in the growth of the larger particles at the expense of the smaller particles.35 This size refocusing is accompanied by the transition from the random alloy to the intermetallic phase, as the increase in the size of the nanoparticles lowers the interfacial energy while maintaining fast atom transport to facilitate ordering. The proposed size refocusing mechanism was verified further by heating a solution of small random alloy AuCu sacrificial nanoparticles with large random alloy AuCu seeds. The random alloy AuCu sacrificial nanoparticles had an average size of 5.4 ± 1.1 nm, and the large random alloy AuCu seeds had average size of 8.9 ± 1.4 nm (Figure 4A). The two different size populations of random alloy AuCu nanoparticles were heated at 280 °C for 60 min in a solution of ODE and OLA to achieve the monodisperse nanoparticles with an average size of 9.1 ± 0.7 nm shown in Figure 4B. The histogram in Figure 4C shows the bimodal distribution of the nanoparticle size before heating, and the size distribution becomes unimodal after heating with the resulting nanoparticles being larger than the initial nanoparticles. Figure S9 shows the UV−visible spectrum of the initial random alloy AuCu seeds and sacrificial AuCu nanoparticles together in solution and is compared with the UV−visible spectrum of the resulting intermetallic AuCu nanoparticles. The features are consistent with prior reports of similar particles.39,40 The initial solution has one broad peak, which is consistent with the bimodal distribution of size that arises from the large seeds and smaller sacrificial nanoparticles. After heating and intermetallic nanoparticle formation, a slight narrowing of the peak is evident, supporting the narrowing of the size distribution. The PXRD patterns show that the AuCu seeds (Figure 4D, red) and the sacrificial nanoparticles (Figure 4D, green) are random alloys, as they match the calculated random alloy AuCu reference and superlattice peaks are not present. The PXRD pattern of the resulting nanoparticles (Figure 4D, blue) after heating match the reference pattern for the ordered L10 phase, signifying that monodisperse intermetallic nanoparticles are achieved by heating two different size populations of nanoparticles. As determined previously, heating the random alloy AuCu seeds on their own resulted in polydisperse intermetallic nanoparticles (Figure S1). Heating the random alloy AuCu sacrificial nanoparticles on their own preserved the monodisperse sample, but the intermetallic phase was not achieved (Figure S10). This experiment confirms size refocusing is occurring when there are two populations, due to the observation of the dissolution of the sacrificial nanoparticles and the increase in size of the resulting nanoparticles; this experiment also is consistent with the phase transition temperature likely being dependent on particle size. Size refocusing increases the size of the nanoparticles to lower the interfacial energy and preserve fast atom transport necessary to achieve the disorder-to-order transformation. Significantly, the deliberate use of size refocusing to facilitate the disorder-to-order phase transformation is anticipated to be generalizable to other bimetallic systems. There are a few key points to consider when optimizing the reaction toward other bimetallic systems. The ratio between the seeds and the sacrificial nanoparticles should be optimized, as a large population of sacrificial nanoparticles will shift the critical radius size and prevent dissolution. Moreover, the size of the sacrificial nanoparticles should be considered as they need to be much smaller than the seeds to guarantee dissolution, as small size increases their solubility according to the Gibbs−
percent of the pure Au phase increases. As the reaction is incubated at 280 °C, a decrease in the pure Au phase is observed with minimal amount of pure Au being present at the end of the reaction, where we see the highest percent for the intermetallic AuCu phase. This analysis supports that Au nanoparticles are forming and dissolving over the course of the reaction, which is consistent with the size refocusing mechanism. A STEM image (Figure 2B) shows that the nanoparticles at 0 min at 280 °C are of various sizes. STEM-EDX mapping of the nanoparticles (Figure 2C,D) shows that the two nanoparticles highlighted with a yellow box are mostly composed of Au, which likely accounts for the observed pure (or nearly pure) Au phase in the PXRD pattern. Nanoparticles mostly composed of Au that were also smaller than the initial AuCu seeds were also observed with STEM-EDX mapping of samples at 240 °C and 30 min at 280 °C (Figures S7 and S8A,B). There is evidence of small Cu nanoparticles, as well, indicated by the yellow box in the STEM image (Figure S8C), where STEM-EDX mapping (Figure S8Di-ii) of the particle shows that it is mostly composed of Cu. The absence of the Cu face-centered cubic phase in the PXRD patterns may be attributed to the small amount of Cu nanoparticles present and the overlap between the (111) reflection of Cu with the (200) reflection of the intermetallic AuCu phase, which hinders the observation of reflections corresponding to Cu. The appearance of small Au and Cu nanoparticles during the early stages of the reaction suggests that a size refocusing mechanism is occurring, and the proposed mechanism is shown in Figure 3. First, random alloy seeds are synthesized
Figure 3. Scheme depicting the proposed mechanism for the formation of monodisperse intermetallic NPs.
and put into a solution with Au and Cu metal precursors, reducing agent, and surfactant. The metal precursors are reduced and generate Au, Cu, or even AuCu nanoparticles that are smaller than the initial random alloy AuCu seeds. The generation of the small nanoparticles defocuses the nanoparticle ensemble. The small Au, Cu, and AuCu nanoparticles observed during the reaction, however, dissolve and redeposit onto the larger random alloy AuCu nanoparticles to achieve overgrowth. The dissolution of all the small nanoparticles by the end of the reaction period refocuses the ensemble to a narrow size distribution. Size refocusing has been used to achieve monodisperse samples of semiconductor and upconverting nanoparticles.33,34 The mechanism is synthetically possible due to larger nanoparticles with a smaller surface-tovolume ratio being favored over energetically less stable smaller D
DOI: 10.1021/acs.nanolett.9b02610 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 4. (A) TEM image of the synthesized the combined random alloy AuCu NP seeds and sacrificial nanoparticles; (B) TEM image of the resulting nanoparticles after heating in solution of ODE and OLA; (C) histogram of the random alloy AuCu NPs (red), sacrificial nanoparticles (green), and resulting nanoparticles (blue); (D) corresponding XRD patterns of the nanoparticles.
Thomson equation.41 Peng and co-workers established with syntheses of semiconductor nanoparticles in which rapid dissolution of sacrificial nanoparticles within the time frame of the reaction was necessary to avoid defocusing of the size distribution by Ostwald ripening.35,38,41 Moreover, temperature can be manipulated to change the rate of diffusion to control the effect of size refocusing,41 with both the reaction time and temperature being important considerations to ensure complete dissolution along with the disorder-to-order transformation. This work shows the generality of achieving monodisperse intermetallic nanoparticles by depositing uniform overgrowths on random alloy seeds. Overgrowth was determined to proceed via a size refocusing mechanism through TEM and PXRD aliquot analysis over the course of the reaction period. Our work demonstrates size refocusing as a mechanism to facilitate the disorder-to-order phase transformation in multimetallic nanoparticles and achieve sample monodispersity. This study provides a strategy toward synthesizing monodisperse intermetallic nanoparticles and provides insight into the disorder-to-order transformation. Excitingly, our work can expedite the development of electrocatalysts and magnetic storages materials by advancing the understanding of intermetallic nanoparticle formation in colloidal syntheses.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.E.S., H.M.A., and J.T.L.G. acknowledge financial support from Indiana University and U.S. DOE BES Award DESC0018961. Access to the powder diffractometer was provided by NSF CHE CRIF 1048613. We also thank the IU Electron Microscopy Center and Nanoscale Characterization Facility for access to the necessary instrumentation. A portion of the electron microscopy characterization was conducted as part of a user proposal at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences, a U.S. Department of Energy Office of Science User Facility.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.9b02610.
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REFERENCES
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Materials and experimental methods, additional characterization, control experiments (PDF)
AUTHOR INFORMATION
Corresponding Author
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[email protected]. ORCID
Raymond R. Unocic: 0000-0002-1777-8228 Sara E. Skrabalak: 0000-0002-1873-100X E
DOI: 10.1021/acs.nanolett.9b02610 Nano Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.nanolett.9b02610 Nano Lett. XXXX, XXX, XXX−XXX
