Mechanistic Insight into the Conversion Chemistry between Au-CuO

Feb 15, 2017 - Mechanistic Insight into the Conversion Chemistry between Au-CuO ... as a major product in most of the cases at relatively high heat tr...
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Mechanistic Insight into the Conversion Chemistry between Au-CuO Heterostructured Nanocrystals Confined inside SiO2 Nanospheres Ki-Wan Jeon,†,‡ Dong-Gyu Lee,†,‡ You Kyung Kim,§,∥ Kangkyun Baek,⊥ Kimoon Kim,‡,⊥ Taewon Jin,‡ Ji Hoon Shim,‡ Jeong Young Park,*,§,∥ and In Su Lee*,†,‡ †

National Creative Research Initiative Center for Nanospace-confined Chemical Reactions (NCCRs) and ‡Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea § Center for Nanomaterials and Chemical Reactions, Institute for Basic Science and ∥Graduate School of EEWS, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea ⊥ Center for Self-assembly and Complexity, Institute for Basic Science, Pohang 37673, Korea S Supporting Information *

ABSTRACT: By taking advantage of a nanospace-confined nanocrystal conversion protocol via high-temperature solid-state reaction within the SiO2 nanosphere, an in-depth study was conducted into the unique transformation behavior of the Au-CuO heterostructured nanocrystals (HNCs), which was discovered during the oxidative annealing of the embedded AuCu alloy nanocrystal (NC). The type of heterojuction structure of the oxidized AuCu NCs, between core@shell and heterodimer, could be determined by modulating either the annealing temperature (Tann) or Cu contents (Pcu) in AuCu NCs; Au@CuO was generated only either at low temperature (Tann ≤ 250 °C) or with very low Cu contents (Pcu = 2.1), whereas the Au/CuO heterodimer was obtained as a major product in most of the cases at relatively high heat treatment (>250 °C). The systematic investigation of the conversion between HNCs could elucidate the distinct evolution pathway of the Au/CuO heterodimer via the kinetically accessed Au@CuO, revealing the escaping motion of the encapsulated Au core, which is more facilitated through a thicker CuO shell. This also demonstrated the high thermal stability of the Au@CuO with a very thin shell thickness due to the insufficient compressive lattice stain on the CuO shell to drive the morphological transformation into the heterodimer. Moreover, the higher operational stability could be detected for the Au@CuO with the lowest Cu content during catalytic CO oxidation, which correlates with its resistance against the thermal deformation.

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INTRODUCTION Heterostructured nanocrystals (HNCs), which combine two or more dissimilar materials at nanoscale heterojunction areas,

and growth of a secondary material on top of the preformed colloidal nanocrystal, which is often influenced by the degree of lattice matching across the heterojunction interface.5−9 Very recently, an alternative synthetic approach was suggested to exploit phase segregation of the metallic alloys during solutionmediated chalcogenization, thus producing several Au-containing HNCs of mostly heterodimeric structures, such as Au/ Fe3O4, Au/Cu2S, Au/In2O3, Au/Ag2S, and Au/MnO.8,10−14 However, despite significant advances of synthetic techniques, understanding and controlling the conversion between the different HNC structures, which is imperative to ensure reliable and durable operation in any applications, remains a significant challenge.15−18 Given this context, our research has focused on studying the conversion chemistry of HNCs in a rarely explored solidstate environment. This environment employs a nanometersized reaction medium, allowing us to systematically investigate the phase and morphological transformations of an embedded

Scheme 1. Formation and Transformation of Au-CuO HNC during Oxidative Phase Segregation of AuCu@SiO2

represent a promising class of functional materials that not only integrate the distinct functionalities of individual domains but also can induce novel and synergistically enhanced properties.1 Given that the properties of HNCs depend strongly on the mutual arrangement of components at the interface, the precise and deliberate regulation of the heterojunction structure must be addressed to realize any practical applications.2−4 So far, much effort has been devoted to controlling the HNC morphology in solution-based synthesis by modulating the nucleation © XXXX American Chemical Society

Received: December 20, 2016 Revised: January 19, 2017

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was washed with ethanol 3 times by repeated centrifugation and redispersion in ethanol by sonication. The washed product was dried in an oven at 70 °C for further solid-state reaction. Thermal Annealing Process. To fully reduce both the Au and Cu cations within the SiO2 shell to form the AuCu random alloy nanocrystal, the (Au/Cu2+)@SiO2 product was ground in an agate mortar and pestle to obtain a fine (Au/Cu2+)@SiO2 powder, which was then transferred to an alumina tray. The alumina tray containing the fine (Au/Cu2+)@SiO2 powder was then placed in a tube furnace and gradually heated at 5 °C/min to 500 °C, where it was kept for 12 h under a flow of a mixed Ar + 4% H2 gas, and radiatively cooled to room temperature, producing AuCu1.2 @SiO2, AuCu2.1 @SiO2, AuCu3.4@SiO2, and AuCu6.1@SiO2, depending on the different concentrations of Cu precursor solution. To study the thermal oxidative transformation of the AuCu nanocrystal into either Au@CuO core@shell-type or Au/CuO heterodimeric structures, the AuCu@ SiO2 powder was ground in an agate mortar and pestle again. The finely ground AuCu@SiO2 powder was then transferred to an alumina tray and was placed in a box-type furnace and gradually heated at5 °C/min to a target temperature ranging from 250 to 600 °C under the air, where it was kept for 10 h and thereafter radiatively cooled to room temperature, thus obtaining either Au@ CuO or Au/CuO, depending on the Cu content and heat-treatment temperature. Calculation of Off-Center Distance in Three Dimensional Space. The 2D migration distance of Au in the CuO shell was estimated from TEM images showing 2D-projected migration of the Au nanoparticle. However, it cannot reflect the real migration distance of Au in 3D as well as 2D migration distance cannot quantitatively explain the migration distance in a real system. In Figure S1, for instance, although particle A, whose migration direction is nearly outof-plane, diffuses farther than particle B; particle B looks like that it diffuses farther than A in the view of the 2D-projected image. Suppose that we put rdiff as the migration distance of the particle in 3D and there is no preferred migration direction of the particle in SiO2, then its distribution of the migration distance in 3D and 2D as a function of θ can be described as in Figure S2a. By inverting l = r sin θ to θ =

HNC while protecting the isolated state from sintering even at high temperature.19−21 This paper highlights our findings on the unique thermal transformation behavior of the Au-CuO HNC from core@shell to heterodimeric structures,9,22,23 which was discovered during oxidative phase segregation of the AuCu alloy (AuCu) within a SiO2 nanosphere. This finding allowed us to elucidate the distinct evolution pathway of the heterodimeric HNC (Au/CuO) via the kinetically trapped core@shell structure (Au@CuO), revealing that the nanocrystalline Au can escape from the core, which is largely influenced by the Cu content in the starting AuCu (Scheme 1). Along with providing in-depth knowledge on the conversion chemistry of HNCs, this finding offers the possibility for optimizing the structural stability and resultant operational durability of HNC-based materials by modulating the compositional and morphological parameters.24,25 In this study, we were able to investigate and interpret the effect of Cu content on the catalytic properties of Au-CuO HNCs by comparatively examining their performance in CO oxidation facilitated by a synergistic coupling of the catalytic properties at the Au-CuO heterojunction.26,27 We report here the operational stability detected for Au@CuO with the lowest Cu content during catalytic CO oxidation, which correlates with its resistance to thermal deformation.

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EXPERIMENTAL SECTION

General Consideration. All reagents, including HAuCl4·xH2O (Strem Chemicals), Cu(NO3)2·3H2O (Strem Chemicals), NH4OH (Samchun Chemical), tetraethoxyorthosilane (TEOS) (Acros), cyclohexane (Samchun Chemical), NaOH (Samchun Chemical), and IGEAL-CO-520 (Aldrich), were used as purchased. Transmission electron microscopy (TEM) analyses were performed using a JEOL JEM-2100, JEOL JEM-2200, and an aberration-corrected FEI Titan Themis 60−300 electron microscope operated at 80 kV. A highvoltage electron microscope (HVEM, JEOL Ltd., JEM ARM 1300S) operated at 1250 kV was used for monitoring Au core migration in the CuO shell. All samples for TEM analysis were prepared by dispersing the powder in ethanol (98%) by sonication, and 20 μL of the suspension was dripped on carbon-coated Cu and Ni TEM grids for TEM and STEM analysis, respectively, and dried under air. Powder X-ray diffraction patterns were obtained using an X-ray diffractometer (18 kW, Rigaku, Japan). The metal contents were determined using inductively coupled plasma-atomic emission spectrometry (ICP-AES) by means of a Direct Reading Echelle ICP (Leeman Laboratories, Inc., NH, USA). The oxidation state of the samples was determined using an X-ray photoelectron spectroscope (XPS) with an X-ray source of monochromated Al Kα radiation (1486.6 eV) using a theta probe AR-XPS System (Thermo Fisher, U.K.). Synthesis of the (AuCu2+)@SiO2 Nanospheres. A series of (Au/Cu2+)@SiO2 nanospheres were used as the starting material in this study; these nanospheres were synthesized using the modified reverse microemulsion technique previously reported by our group.28 Generally, the synthetic conditions were as follows: 2 mL of IGEPAL CO-520 was dispersed in 34 mL of cyclohexane in a 50 mL glass vial and stirred with a magnetic bar for 10 min at room temperature. 62.5 μL of aqueous HAuCl4·xH2O (16 mg/mL) and Cu(NO3)2·3H2O (25−100 mg/mL) were added dropwise to the mixture during stirring, forming a transparent blue suspension; the higher the concentration of the Cu precursor solution in the mixture, the more intense the bluecolored suspension. After 10 min, 200 μL of NH4OH (30%) was added dropwise to the reaction vial, and the color of the mixture became an intense blue and was stirred for an additional 10 min. Finally, 400 μL of TEOS was added dropwise to the mixture, and was then stirred for 16 h at room temperature. To collect the (Au/Cu2+)@ SiO2 nanospheres, methanol was added to the reaction suspension and vortexed to break the reverse micelle system, precipitating a bluecolored (Au/Cu2+)@SiO2 solid on the bottom, which was collected by carefully decanting the supernatant solution. The resulting product

sin−1

l r

( ) (Figure S2b), θ(l = b) − θ(l = a) can be obtained, which is

proportional to the population of the migration distance in 2D. Therefore, the distributed migration distance in 2D can be reproduced from the distributed migration distance in 3D with the assumption that particles migrate randomly. However, in a real system, distributed migration distance in 3D and 2D is more complex, shown in Figure S3a,b. Then, the 2D distribution can be expressed by the combination of sin−1

l r

( ). For example, the population of the migration distance in 2D

from 1 to 2 nm can be expressed as follows:

⎡ ⎡ ⎛2⎞ ⎛ 1 ⎞⎤ ⎛2⎞ ⎛ 1 ⎞⎤ W1∗ ⎢sin−1⎜ ⎟ − sin−1⎜ ⎟⎥ + W2∗ ⎢sin−1⎜ ⎟ − sin−1⎜ ⎟⎥ ⎢⎣ ⎢⎣ ⎝ r1 ⎠ ⎝ r1 ⎠⎥⎦ ⎝ r2 ⎠ ⎝ r2 ⎠⎥⎦ ⎡ ⎛2⎞ ⎛ 1 ⎞⎤ + W3∗ ⎢sin−1⎜ ⎟ − sin−1⎜ ⎟⎥ + ··· ⎢⎣ ⎝ r3 ⎠ ⎝ r3 ⎠⎥⎦ Wi means the population of the migration distance in 3D at ri. In this calculation, our purpose is finding a distribution of the migration distance in 3D based on the distribution of migration distance in 2D, which can be estimated from the TEM image. Therefore, using genetic algorithm in MATLAB code with the constraint, Wi ≥ 0 to avoid negative Wi which is unphysical, the population of the migration distance in 3D was arbitrarily selected, which allows us to reproduce the distribution of the migration distance in 2D. After that, the reproduced distribution of the migration distance in 2D was compared with the distribution of the migration distance in 2D obtained from the TEM image and an error between reproduced data and experimental data was calculated. Finally, the population of the 3D distribution showing the minimum error from the experimental data was used. Repeated Cycles of Catalytic CO Oxidation over [email protected] and Au/CuO6.1. CO oxidation was performed with the nanocatalyst B

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was used for the in situ heating TEM. The in situ heating experiment was performed in a high-voltage electron microscope (HVEM, JEOL Ltd., JEM ARM 1300S) operated at 1250 kV. The vacuum level of the specimen chamber was maintained at ∼2 × 10−6 Pa during the in situ heating process. The specimen was first heated from room temperature to 180 °C at a constant heating rate of 10 °C min−1 while turning off the electron beam to avoid detrimental electron beam damage. Real-time movies were recorded during heating to 400 °C at a constant heating rate of 10 °C min−1.

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RESULTS AND DISCUSSION Preparation of (Au/Cu2+)@SiO2 and Reductive Transformation into AuCu@SiO2. As the starting material, (Au/ Cu2+)@SiO2, which incorporates a Au nanocrystal (NC) and Cu2+ ions together in a SiO2 nanosphere, was prepared through a sol−gel reaction with additional injection of aqueous solutions of HAuCl4·3H2O (62.6 μL, 0.04 M) and Cu(NO3)2·3H2O (62.6 μL, various conc. from 0.17−0.66 M) into a microemulsion system in cyclohexane.28 Transmission electron microscopy (TEM), element mapping using scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) analyses of the isolated (Au/Cu2+) @SiO2 revealed a 2.3−4.1 nm (±0.4 nm) sized Au/Cu2+ nanoparticle, most likely composed of a Au core and Cu2+-based shell, was embedded at the center with additional Cu2+ ions dispersed over a 25.0 (±0.7) nm SiO2 nanosphere (Figures 1a, S4, S6). When the (Au/Cu2+)@SiO2 powders were heated at 500 °C for 12 h under Ar + 4% H2, the core nanoparticle became more crystalline and increased in size to 3.0−5.3 nm (±0.5 nm) (Figures 1b,1c, S5). XRD and XPS analyses of the resulting powder confirmed the reduction of the ionic species and the growth of a single face-centered cubic metallic phase of the AuCu alloy (Figures 1d, S7). The element mapping and line profiling analyses using STEM-EDS validated the centralization of all the metallic species inside the SiO2 nanosphere into a single AuCu NC of random-alloy phase (Figure 1b). The Cu/Au ratio in the alloy was determined to increase from 1.2 to 6.1 as the size of the AuCu increased (3.0−5.3 nm), which is also consistent with a higher-angle shift of the XRD peaks (41.3−42.7°) for the larger AuCu (Figure 1d).31 Accordingly, the AuCu could be labeled according to its size and the fraction of Cu in the alloy (PCu): AuCu1.2

Figure 1. TEM, HRTEM (inset), STEM-EDS element mapping (right lower) and line profiling (right upper) images of (a) [Au/(Cu2+)6.1]@ SiO2 and (b) AuCu6.1@SiO2. (c) HRTEM images and (d) XRD patterns of AuCu@SiO2 with various PCu. samples in a flow reactor.29,30 The reactant gas composition was 4% CO, 10% O2, and 86% He (balance). The total gas flow rate was 50 mL min−1, controlled by a mass flow controller (Brooks Instrument). The amount of Au-CuO catalysts used for CO oxidation was 40 mg. The reaction was repeated three times while ramping the temperature from 80 to 250 °C at 5 °C/10 min without any pretreatment. The typical pretreatment under hydrogen gas led to the reduction of Au-CuO catalysts and morphological changes of the catalysts. The gas mixture passing through the catalyst powder was analyzed using gas chromatography (DS Science). Sample Preparation for the ICP Measurements. A 10 mg portion of AuCu1.2@SiO2, AuCu2.1@SiO2, AuCu3.4@SiO2, or AuCu6.1@SiO2 powder was added to a 40 mL glass vial containing a 3 M NaOH aqueous solution (5 mL), whereafter the sample was kept for 24 h at room temperature without disturbance to etch out the SiO2 shell; 10 mL of aqua regia was then added to the glass vial and kept for 24 h at room temperature without disturbance to completely dissolve the AuCu nanocrystals, generating both Au and Cu cations. Finally, 5 mL of deionized water was added to the glass vial to make a total of 20 mL of the solution; the solution was then filtered with a membrane and the collected filtered solution was used for the ICP measurement. In Situ Heating TEM. An amorphous carbon thin film coated upon a 300 mesh Mo grid (Pacific Grid-Tech, Mo-300CN), which is typically used for in situ heating TEM experiments, was used as an experimental specimen. A single heating holder (Gatan Inc., 628TA)

Figure 2. HRTEM images and STEM-EDS line profiling (inset) of 250 °C-annealed (a) [email protected]@SiO2 and (b) [email protected]@SiO2 and 600 °Cannealed (d) [email protected]@SiO2 and (e) [email protected]@SiO2. HRTEM images of (c) and (f) 250 °C- and 600 °C-annealed products of AuCu@SiO2 with various PCu. C

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Figure 3. TEM images of annealed products of (a) AuCu1.2@SiO2, (b) AuCu2.1@SiO2, (c) AuCu3.4@SiO2, (d) AuCu6.1@SiO2 at various Tann and (e) histogram of the population ratio of core@shell and heterodimer structures. Blue color: core@shell and red color: heterodimer.

Alloy-to-HNC Transformation of AuCu@SiO2 during Oxidative Annealing. A series of AuCu@SiO2 samples with different PCu were then treated at high temperature (Tann) in

[3.0 (±0.4) nm, PCu = 1.2], AuCu2.1 [3.7 (±0.6) nm, PCu = 2.1], AuCu3.4 [4.5 (±0.5) nm, PCu = 3.4], and AuCu6.1 [5.3 (±0.4) nm, PCu = 6.1] (Table S1). D

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to 600 °C to test the interchangeability between HNC morphologies, the complete conversion of Au@(CuO)6.1 into Au/ (CuO)6.1 was induced (Figures 4b, S11). In contrast, the conversion experiment of treating the 600 °C-annealed [Au/ (CuO)6.1]@SiO2 at Tann = 250 °C did not affect the heterodimeric morphology, implying that the morphological conversion only proceeded in one direction, (i.e., from core@shell to heterodimer). Moreover, when [Au/(CuO)6.1]@SiO2 was retreated under Ar + 4% H2 at Tann = 500 °C, the two grains, Au and CuO, merged back into AuCu6.1. This AuCu6.1 reverted to Au@(CuO)6.1 upon retreating at Tann = 250 °C in air, implying that the pseudo-reversible transformation of core@ shell → heterodimer → alloy → core@shell can be cycled by switching the gas environment and Tann (Figures 4a,b, S11). Mechanistic Investigation of Conversion between Core@Shell and Heterodimer. A closer examination of the TEM images of the displacement of the Au domain in the phasesegregated Au-CuO HNC revealed that, as Tann increased, the Au core of Au@(CuO) containing PCu ≥ 2.1 moved outward, reached the CuO and SiO2 interface, and finally protruded from the CuO sphere, giving rise to a snowman-like structure of the Au/CuO (Figure 3b−d). For instance, the migration distances of the Au NC from the center of the Au@(CuO)6.1@SiO2, which were individually estimated from the distribution of their off-center displacement in the plane-projected TEM images, continuously increased from 1.0 (±0.3) nm to 3.1 (±0.1) nm to 4.5 (±0.2) nm as Tann was raised from 250 to 450 to 600 °C, respectively, while the CuO sphere remained unchanged (Figures 3, 5). On the basis of these observations, the Au@ CuO-to-Au/CuO transformation can be depicted as the nanocrystalline Au core escaping through the surrounding CuO shell. This core-migration mechanism is distinctly different from a mechanism based on atomic diffusion suggested previously for the core@shell-to-heterodimer transformation.9,22 The core-migration mechanism was definitively monitored using in situ heating TEM of [Au@(CuO)6.1]SiO2, which confirmed that the Au core maintained its initial ∼3.0 nm size and spherical shape as it moved out of the surrounding CuO shell (Figure 6; 32× time-lapse movie is presented in the Supporting Information). Taking this mechanism into account allowed us to deduce that the evolution of the heterodimeric Au/CuO from AuCu proceeds via the pathway involving the formation of the core@shell-type Au@CuO first, which is kinetically accessed during oxidative phase segregation, followed by the migration of the Au core through the CuO shell, thus converting to the heterodimeric morphology. On the other hand, in the case of [email protected], which had the thinnest CuO shell, the Au core did not show any discernible displacement from the center of the SiO2 sphere even at the highest Tann, demonstrating a high stability against thermal deformation. The observed tendency for higher convertibility of the Au@ CuO with increasing PCu can be interpreted as a facilitated migration of the Au core through a thicker CuO shell. For further elucidation of this tendency, we conducted a comparative examination of the HRTEM images and histogram of the Au/ CuO population changes of the Au-CuO HNCs from AuCu1.2 to AuCu6.1 (with the lowest and highest PCu, respectively) (Figure 3e). For the 250 °C-annealed [email protected] and Au@ CuO1.2, their CuO shells both appeared to be amorphous or polycrystalline in nature, as indicated by the absence of lattice fringes in their HRTEM images or diffraction peaks in the XRD pattern (Figures 2a,b, S8a, S10). In comparison, when they

Figure 4. (a) Illustration of the pseudo-transformation between the core@shell and heterodimer structure and (b) TEM images showing the pseudo-reversible transformation of [email protected] → Au/CuO6.1 → AuCu6.1.

the presence of air, which tended to induce the separation of the elements according to their susceptibility to oxidation. Upon air annealing at Tann = 600 °C for 10 h, the resulting products showed separation of the oxidized Cu phase from the oxidation-resistant Au, confirming the intended alloy-to-HNC transformation, as revealed by TEM, high-resolution TEM (HRTEM), XRD, and XPS analyses (Figures S8, S9, S10). A rather unexpected, but noteworthy, finding was that the resulting HNC morphology varied depending on the PCu of the AuCu. AuCu1.2 with the lowest PCu afforded Au@CuO with a core@shell structure, consisting of a Au core with a diameter of 2.6 (±0.4) nm and a CuO shell with a thickness of 0.70 (±0.04) nm. All other AuCu with PCu ≥ 2.1, on the other hand, were converted into snowman-like heterodimeric structures of Au/CuO, in which the Au NC with a diameter of 3.0 (±0.5) nm was bound to the phase-segregated CuO spheres with diameters of 2.5 (±0.3) nm, 5.1 (±0.8) nm, and 5.9 (±0.5) nm, respectively. In contrast to the observation at Tann = 600 °C, air annealing at Tann = 250 °C produced isomorphic core@shell structures (Au@CuO) independent of PCu; the thickness of the CuO shell around a 3.0 (±0.5) nm Au core increased gradually from 0.70 (±0.04) nm to 1.8 (±0.4) nm as PCu increased (Figures 2c, S8a). Intrigued by these disparate results at the two different annealing temperatures, further investigation was conducted by varying Tann between 250 and 600 °C, which revealed an apparent trend for the preferred formation of the heterodimeric structure with increasing Tann and PCu of the AuCu (Figure 3b−d). Air annealing of AuCu1.2 of PCu = 1.2 yielded Au@(CuO)1.2 as a transformation product over the entire Tann range from 250 to 600 °C (Figure 3a). In the case of AuCu2.1 (PCu = 2.1), heterodimeric Au/(CuO)2.1 began to emerge along with core@shell Au@(CuO)2.1 in the Tann range from 450 to 500 °C; the proportion of the former structure increased from 23% to 58% in the mixed morphology, eventually becoming a sole product at Tann = 600 °C. In comparison, AuCu3.4 and AuCu6.1 (both PCu ≥ 3.4) showed dominant transformation into the heterodimer even at Tann = 350 °C and the exclusive formation of Au/(CuO)3.4 and Au/ (CuO)6.1, respectively, at Tann ≥ 450 °C. More interestingly, when the 250 °C-annealed [Au@(CuO)6.1]@SiO2 was heated E

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Figure 5. (a) Migration distances of the Au core in the CuO shell estimated from TEM images of air-annealed Au-CuO@SiO2 and (b) histogram of the off-centered distance of the Au core in the CuO shell, depending on PCu and Tann in air and calculated average migration distance of Au core in the CuO shell.

were annealed at 600 °C, the amorphous CuO was found to change into the crystalline phase, thus developing singlecrystalline CuO domains both in the spherical grain of the Au/ CuO6.1 and in the shell of the [email protected] (Figures 2d,e, S8b). The HRTEM image of the Au/CuO6.1 revealed the formation of a heterojunction interface between the (200) plane of Au and the (200) facets of CuO, which have lattice parameters of 2.04 and 2.51 Å, respectively, forming a lattice mismatch of 23% (Figure S8). From this data, the amorphous-to-crystalline

transformation of the CuO phase, which generated a high interfacial energy at the lattice-mismatched heterojunction, was determined to be the main impetus for the migration of the Au core from the center of the CuO shell, causing a transformation of the [email protected] into Au/CuO6.1 to minimize the interfacial area.9,15,16,22 In contrast, in the HRTEM images of [email protected], which remained core@shell even after annealing at 600 °C, the lattice parameters of Au (200) and CuO (200) were detected to be 2.04 and 2.42 Å, respectively, which deviate somewhat F

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thermally resistant materials show better durability when operating at high reaction temperatures.

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CONCLUSION In summary, by taking advantage of a nanospace-confined nanocrystal conversion protocol via a high-temperature solidstate reaction within the SiO2 nanosphere, an in-depth study of the formation and transformation behavior of the Au-CuO HNCs was conducted, which was revealed to be largely influenced by Tann and Pcu. The observed core@shell-to-heterodimer conversion could be elucidated based on the Au core-migration mechanism, which was driven by the high interfacial energy at the lattice-mismatched heterojunction of the crystalline domains. These findings suggest that optimization of the structural stability and the resultant operational durability of HNCbased materials are possible by adjusting the compositional and morphological parameters.

Figure 6. Snapshots of the recorded video on [email protected]@SiO2 using in situ heating TEM.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 7. (a) Change of CO conversion measured at 250 °C after each cycle of temperature ramping. TEM images of (b) [email protected] and (c) Au/CuO6.1 after CO oxidation.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05380. Additional analysis data (PDF) Video file of in situ heating TEM (AVI)

from those reported for the bulk phases. This indicates that the atomic dislocation at the heterojunction of the [email protected] does not induce a high enough compressive lattice strain on the CuO shell to drive the movement of the Au core, presumably due to the very thin shell of just a few atomic layers.32 This, therefore, suggests that the thermal stability and the operational durability of the core@shell-type HNC can be highly enhanced by minimizing the shell thickness. Operational Stability of Au@CuO in the CO Oxidation. To verify the expected durability enhancement with a thin shell, a comparative examination between the performance of Au@ CuO1.2@SiO2 and [email protected]@SiO2 at Tann = 250 °C under CO oxidation was carried out (Figure S12).29,30 Figure 7 shows the change in CO conversion for the [email protected] and Au@ CuO6.1 measured at 250 °C after each temperature ramping cycle. While the conversion on [email protected] remains almost constant after repeated cycles, [email protected] exhibits a significant reduction in conversion from 50% to 15%. This result clearly demonstrates the differences in durability of the catalysts between [email protected] and [email protected]. [email protected] shows that the temperature with maximum CO conversion increased after the second cycle, resulting in the transformation from the core@shell to the heterodimer structure, which was confirmed by TEM (Figure 7c), whereas the morphology of the Au@ CuO1.2 was unchanged after CO oxidation (Figure 7b). We envisaged that decreasing CO oxidation activity by transformation from core@shell to heterodimer could be ascribed to reducing interface area between Au and CuO. It is known that the interface between Au and CuO can give rise to the synergistic effect due to the strong metal−support interaction.33 Additionally, the CO oxidation experiment with the AuCu1.2 alloy, which was regenerated by treating [email protected]@SiO2 at a hydrogen gas environment (250 °C, 30 min), gave the declined catalytic activity (cycles 1 and 2 in Figure S13), proving a better reactivity of the Au-CuO HNC. Moreover, from the third cycle, the AuCu1.2 alloy exhibited the similar CO conversion profile to that of the [email protected], most likely due to the oxidation of the AuCu during the catalytic operation, which signifies the superior durability of the [email protected] during catalytic CO oxidation. This result directly indicates that higher

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Y.P.). *E-mail: [email protected] (I.S.L.). ORCID

Kimoon Kim: 0000-0001-9418-3909 In Su Lee: 0000-0002-2588-1444 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (NRF-2016R1A3B1907559) (I.S.L.), and J.Y.P. acknowledges support by IBS-R004-G4. STEM-EDS line profiling and mapping images were obtained at the TEM facility in the Center for Self-assembly and Complexity, Institute for Basic Science.

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REFERENCES

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DOI: 10.1021/acs.chemmater.6b05380 Chem. Mater. XXXX, XXX, XXX−XXX