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Nanocrystals

Facet-Dependent Optical Properties Revealed through Investigation of Polyhedral Au–Cu2O and Bimetallic Core–Shell Nanocrystals Michael H. Huang,* Sourav Rej, and Chun-Ya Chiu

The ability to prepare Au–Cu2O core–shell nanocrystals From the Contents 1. Introduction .............................................. 2 2. Metal-Cu2O Core-Shell Nanocrystals............2 3. Bimetallic Core-Shell Nanocrystals ............. 7 4. Conclusion and Perspectives ................... 10

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with precise control over particle size and shape has led to the discovery of facet-dependent optical properties in cuprous oxide crystals. The use of Au cores not only allows the successful formation of Au–Cu2O core–shell nanocrystals with tunable sizes, but also enables the observation of facet-dependent optical properties in these crystals through the Au localized surface plasmon resonance (LSPR) absorption band. By tuning the Cu2O shell morphology from rhombic dodecahedral to octahedral and cubic structures, and thus the exposed facets, the Au LSPR band position can be widely tuned. Such facet-dependent optical effects are not observed in bimetallic Au–Ag and Au–Pd core–shell nanocrystals with the same precisely tuned particle sizes and shapes. It is believed that similar facet-dependent optical properties could be observed in other ionic solids and other metal– metal oxide systems. The unusually large degree of plasmonic band tuning covering from the visible to the near-infrared region in this type of nanostructure should be quite useful for a range of plasmonic applications.

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1. Introduction

properties of Cu2O crystals and possibly other ionic crystals being facet-dependent.

Preparation of bimetallic and metal–semiconductor coreshell nanocrystals with well-defined shapes can enrich material properties and functionality, and the particles may also be suitable for the examination of facet-dependent physical and chemical properties.[1–4] For example, Au-Pd coreshell tetrahexahedra, octahedra, and cubes with tunable shell thicknesses have been demonstrated to act as highly responsive and reusable plasmonic hydrogen sensors.[5] For metal-semiconductor heterostructures with polyhedral shell morphologies, the metal-Cu2O system is likely the most explored combination because of the remarkable ability of Cu2O to accommodate various metal nanostructures despite their large lattice mismatches.[6–8] Previously, Au-Cu2O coreshell octahedra bound by the {111} faces exhibited a greatly enhanced photocatalytic activity toward the photodegradation of methyl orange compared to that of the pristine Cu2O octahedra, while both Cu2O cubes and Au-Cu2O core-shell cubes were inactive for this reaction.[9] The photocatalytic enhancement should come from the more efficient separation of photogenerated electrons and holes and possibly plasmonic effects.[10] However, photocatalytic enhancement can only be achieved with Cu2O shells possessing proper surface facets. In another study examining the electrical conductivities of single Cu2O and Au-Cu2O core-shell cubes and octahedra, it was found that both pristine and core-shell cubes were barely conductive below an applied voltage of 3 V, while both pristine and core-shell octahedra were highly conductive with Au-Cu2O octahedron being most conductive.[11] Again, the presence of a metal core could enhance electrical conductivity in a Cu2O crystal, but enhancement came mainly from particles having {111} surface facets. These results are relevant to our understanding of the observed facet-dependent optical properties of Au-Cu2O crystals. In the past, synthesized Cu2O and Au-Cu2O polyhedra frequently had sizes on the order of hundreds of nanometers or larger, so their absorption spectra were dominated by strong light-scattering bands.[6,9,12–14] The LSPR absorption band from the gold cores cannot be unambiguously located. In the quest to make much smaller Cu2O nanocrystals for the examination of their optical properties without much lightscattering interference, it turned out that the use of a metal core is quite effective at achieving this goal while preserving the particle shape. By synthesizing smaller Au-Cu2O crystals with tunable sizes and shapes and the evolution from cubic to octahedral structures, their facet-dependent optical properties were discovered.[15] Au-Cu2O rhombic dodecahedra bound by the {110} facets were subsequently synthesized to complete the investigation.[16] The next interesting question to ask is whether the same facet-dependent optical properties can also be observed in bimetallic core-shell nanocrystals. Toward this grand goal, Au-Ag and Au-Pd core-shell nanocrystals with tunable sizes and shapes were synthesized to scrutinize their optical properties for evidence of such peculiar optical phenomena.[17,18] The conclusion was that metals practically do not display facet-dependent optical properties. This Review gives an historical account of our recent efforts to address the never-considered issue of optical

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2. Metal–Cu2O Core–Shell Nanocrystals Several groups have reported the formation of metal-Cu2O core-shell nanocrystals and examined their optical properties as a function of increasing shell thickness.[7,8,19–23] Au, Ag, Pd, and Pt cores have been used. Wang and co-workers synthesized roughly spherical Au-Cu2O core-shell particles with tunable shell thickness.[19,20] However, the shells were composed of tiny Cu2O particles and may have contained many pores, therefore, water can potentially penetrate into the shell layer and change the overall surrounding refractive index. As the Cu2O shell thickness increases, the Au LSPR absorption band, located in the near-infrared (NIR) region, is progressively red-shifted (Figure 1a). Very large red-shifts of over 200 nm from the starting LSPR band position of Au cores have been observed. Increasing the refractive index of the medium surrounding plasmonic nanoparticles is known to red-shift the LSPR band.[5,24] Since the refractive index of the medium is related to its dielectric constant via εm = n2, a shell material with a high dielectric constant also leads to a significant LSPR band red-shift.[24] On the other hand, silica formed via the sol–gel process has a low refractive index, so the LSPR band shift for Ag-SiO2 nanoparticles with thick silica shells is only 30 nm.[25] The high refractive index of Cu2O (2.7 at 800 nm)[26] and its high dielectric constant (ε = 7.2)[19,20,27] causes huge red-shifts. Recently, the optical properties of AgCu2O nanocrystals synthesized using 40 nm Ag nanocube cores with different shell thicknesses have been studied.[23] The extent of the plasmonic band red-shift from the Ag cores slows down with increasing shell thickness. However, these particles do not have a consistent shell morphology. Tian and co-workers made similar Au-Cu2O nanoparticles with more crystalline shells.[21] The shell morphology was irregular, possibly because of the use of Au cores without a well-defined shape (Figure 1b). The shells have been determined to be pinhole-free. By increasing the Cu2O shell thickness from 2 to 21 nm, the Au LSPR band shifts from about 545 to 630 nm. The LSPR band extinction also increases for particles with thicker shells, possibly due to some light-scattering effects. Mokari and co-workers made Au nanorod-Cu2O nanocrystals with different shell thicknesses.[7] When Cu2O shell thickness increases from 17 to 65 nm, the longitudinal LSPR absorption band of the Au nanorods shifts significantly (by 130 nm) from 1240 to 1370 nm. It is worthy to note that a large spectral red-shift of 80 nm was recorded with the shell thickness increasing by only 10 nm, from 17 to 27 nm. A more moderate red-shift of just 50 nm was observed with a further

Prof. M. H. Huang, S. Rej, Dr. C.-Y. Chiu Department of Chemistry National Tsing Hua University Hsinchu 30013, Taiwan E-mail: [email protected] DOI: 10.1002/smll.201403542

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Michael H. Huang obtained his BA in chemistry from Queens College in 1994, and his PhD from the Department of Chemistry and Biochemistry at UCLA in 1999. After postdoctoral research at UC Berkeley and UCLA, he joined the Department of Chemistry at NTHU in 2002. He was promoted to Associate Professor in 2006, and then to Professor in 2010. His current research focus is on the shape-controlled synthesis of nanocrystals and the examination of their facet-dependent properties.

Sourav Rej received a BSc degree from Burdwan University, India, in 2009 and a MSc from the Indian Institute of Technology Roorkee, India, in 2011. He is currently pursuing his PhD at the National Tsing Hua University, Taiwan, under the supervision of Prof. M. H. Huang. His research interests include the shape-controlled synthesis of nanocrystals and their catalytic applications.

Chun-Ya Chia received a BS degree from the Department of Materials Science at the National University of Tainan in 2007 and an MA from the Institute of Biopharmaceutical Sciences at National Yang-Ming University in 2009. She received her PhD in 2014 from the National Tsing Hua University under

Figure 1. a,b) Photographs of collodial suspensions of Au-Cu2O coreshell nanoparticles with average Cu2O shell thicknesses of 35, 47, 67, 97, and 120 nm (from top to bottom) and measured extinction spectra of the five collodial samples. c) Photograph, TEM image, and UV-vis spectra of Au-Cu2O nanoparticles with increasing shell thickness. d) TEM images and UV-vis spectra of Au nanorods and Au nanorod-Cu2O nanocrystals with different shell thicknesses. Their TEM images are also shown: I, Au nanorods; II-IV, Au-Cu2O nanocrystals with II) 17, III) 27, and IV) 65 nm shells. Reproduced with permission.[7,19,21] Copyright 2011 and 2012, American Chemical Society; Copyright 2013, Royal Society of Chemistry.

shell thickness increase of 38 nm. So the rate of the spectral red-shift slows down with increasing shell thickness. Facetdependent optical properties cannot be studied using these heterostructures without well-defined surfaces. Due to significant light scattering effects from large Cu2O crystals with sizes of hundreds of nanometers, a series of AuCu2O nanocrystals with cubic, cuboctahedral, and octahedral structures and sizes of 90-220 nm were synthesized.[15] Light scattering interference is greatly reduced when particle sizes are less than 200 nm. In such cases, the LSPR absorption band from the gold cores emerges clearly. In fact, excellent control of Cu2O crystal size and shape cannot be achieved without the use of a metal particle core. By gradually increasing the amount of octahedral gold nanocrystal seeds used for Cu2O shell growth, the dimensions of the synthesized Au-Cu2O nanocrystals were reduced progressively. With this high small 2015, DOI: 10.1002/smll.201403542

the supervision of Prof. M. H. Huang on the synthesis, assembly, and hydrogen-sensing applications of Au-Pd nanocrystals. She is currently working as a researcher at the United Microelectronics Corporation, Tainan, Taiwan.

degree of particle morphology and size tunability, it was then possible to systematically examine their optical properties. In the first study, octahedral gold cores with sizes of 50, 60, and 70 nm and plasmonic bands in the range of 550 to 560 nm were used to grow Cu2O core-shell nanocrystals (Figure 2a). Transmission electron microscopy (TEM) characterization on a single Au-Cu2O cube, in combination with its high-resolution TEM image and selected-area electron diffraction (SAED) pattern, reveals an exact lattice orientation relationship between the core and the shell (Figure 2b-d). That is, the (200) planes of Au are aligned parallel to the (200) planes of Cu2O. The SAED pattern generated shows that diffraction spots for the same set of lattice planes of Au and Cu2O are positioned on the same straight line. The clean diffraction spot pattern indicates single-crystalline nature of the core-shell cube. Such structural analysis is important, as it shows no detectable lattice defects present in the crystal and at the Au-Cu2O interface. There is also consistently no evidence of lattice defects present in these core-shell nanocrystals from our previous TEM analyses, so the Au

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Figure 2. a) UV–vis absorption spectra of octahedral Au nanocrystal solutions. Average particle size increases from 50 to 70 nm. b) TEM analysis of a single Au–Cu2O core–shell nanocube viewed along the [100] direction. The dotted line indicates the outline of the octahedral Au nanocrystal core. c) SAED pattern of this nanocube, giving both Au and Cu2O diffraction spots. d) High-resolution TEM image of the square region. The dotted line marks the interfacial region between Au and Cu2O. The Au (200) and Cu2O (200) lattice planes are aligned along the same direction and parallel to the {100} faces of the Cu2O shell. Inset gives a schematic drawing the the particle viewed along the [001] zone axis. Reproduced with permission.[15] Copyright 2014, Royal Society of Chemistry.

LSPR band shifts observed in these nanocrystals cannot be attributed to defects.[9,11] UV-vis spectra of these Au-Cu2O core-shell nanocubes, cuboctahedra, and octahedra with different sizes prepared using 50, 60, and 70 nm octahedral gold cores are presented in Figure 3.[28] The band at around 490 nm arising from the Cu2O shells red-shifts progressively with increasing particle size, as expected. The light scattering features begin to emerge for large particles with sizes near and beyond 200 nm. For all these particles, the Au LSPR absorption band can be easily identified. The Au peaks are widely red-shifted from their original positions by over 200 nm because of the Cu2O shells. Remarkably, the Au plasmonic band remains fixed in position despite significant changes in the shell thickness, but can vary substantially depending on the Cu2O shell morphology and hence the exposed surface facets. For particles with 50 nm gold cores, the Au LSPR band can vary by as much as 26 nm between octahedra and cubes. It is still not clear why cubes are most red-shifted. The same facet-dependent optical behavior can also be observed in particles with 60 nm Au cores. For Au-Cu2O cubes and octahedra with 70 nm octahedral gold cores, a progressively red-shifted LSPR band with

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increasing particle size can indeed be observed. However, the magnitude of this shift is relatively moderate (≈60 nm red-shift with the shell thickness increasing from 17.5 nm to 62 nm for the core-shell octahedra), compared to the extent of shift seen in the Au-Cu2O nanorods discussed earlier. The Au-Cu2O nanocrystals with 70 nm gold cores should also display the same facet-dependent optical properties, as the cubes are similarly more red-shifted than the octahedra. However, to reach the state with a fixed Au LSPR band, the shell thickness needs to be larger. Such large particles were not synthesized due to the strong light scattering interference. It can initially be difficult to comprehend why AuCu2O nanocrystals with 50 and 60 nm gold cores and those with 70 nm gold cores all have the same optical behavior. A slight red-shift of the gold plasmonic band for Au-Cu2O nanocubes with 50 nm gold cores and overall sizes of less than 90 nm was also observed. Thus, below a certain shell thickness threshold, the typical progressive plasmonic band red-shift with increasing particle size can be observed in all Au-Cu2O nanocrystals. To clearly show that all these particles give the same optical responses, it is highly useful to synthesize Au-Cu2O octahedra with octahedral gold core sizes in

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Figure 3. a-c) UV–vis absorption spectra of the octahedral, cuboctahedral, and cubic Au–Cu2O core–shell nanocrystal solutions with different particle sizes. 50-nm octahedral Au cores were used. d,e) UV–vis absorption spectra of Au–Cu2O octahedra and nanocubes with 60-nm Au cores. f,g) UV–vis absorption spectra of Au–Cu2O octahedra and nanocubes with 70-nm Au cores. Reproduced with permission.[15] Copyright 2014, Royal Society of Chemistry.

the range of 55 to 75 nm to see the transition from a shell thickness-independent plasmonic state to progressive spectral red-shift as the gold core size increases. Octahedral gold nanocrystals with sizes of 58, 65, 68, and 73 nm were prepared to make Au-Cu2O octahedra with different particle sizes for spectral measurements. Figure 4a gives UV-vis absorption spectra of the gold cores. UV-vis absorption spectra of all the Au-Cu2O samples are also provided in Figure 4. When AuCu2O octahedra with 58 nm Au cores were used, the gold LSPR peak position remained fixed at 773 nm, as expected. For Au-Cu2O octahedra with 65 and 68 nm Au cores, only a slight LSPR band red-shift of 15 nm was recorded with particle sizes increasing by more than 130 nm. Nicely, their peak positions also become fixed when the shell thickness reaches 73.5 nm, so the expected transition has been observed. When 73 nm gold cores are used, a progressive but moderate shift of 32 nm was measured with Cu2O shell thickness increasing from 13 nm to 110 nm. Spherical Ag-SiO2 and Au-SiO2 nanoparticles have also been shown to display thickness-independent plasmonic behavior after reaching certain silica shell thicknesses.[25,29]

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When gold nanocubes were used to generate Au-Cu2O core-shell cubes and octahedra, the same facet-dependent and shell thickness-independent optical properties were also observed (Figure 5). The core and shell also have an exact lattice orientation relationship. Finite-difference time-domain (FDTD) calculations were used to produce simulated spectra for the Au-Cu2O core-shell nanocrystals dispersed in an aqueous solution. The same particle dimensions were used for the calculations. Figure 5d shows the simulation spectra for cubic Au–Cu2O core–shell nanocrystals with different sizes. The simulated resonance positions red-shift continuously with the shell thickness, which is inconsistent with experimental results showing a fixed band position. In the sudy of Ag-Cu2O core-shell nanocrystals using 40 nm Ag nanocube cores with increasing shell thicknesses, a large discrepancy between the experimentally measured plasmonic band positions and the FDTD simulations for these samples exists.[23] This problem occurs because the idea of facet effects on plasmonic band shifts has not been considered in the traditional simulation model. It demonstrates the importance of recognizing the presence of facet effects on the optical spectra of plasmonic

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Figure 4. a) Normalized UV-vis spectra of octahedral gold cores with four different average sizes. b-e) UV-vis spectra of octahedral Au-Cu2O coreshell nanocrystals synthesized using b) 58, c) 65, d) 68, and e) 73 nm octahedral gold cores. The particle sizes are provided. f) Summary of extents of gold LSPR wavelength shifts as a function of Au-Cu2O particle size. Drawing of a Au-Cu2O octahedron is also provided. The extent of spectral red-shift is based on the LSPR band position of the octahedral gold cores used. Reproduced with permission.[16] Copyright 2014, Wiley-VCH.

metal nanocrystals coated with a shell of crystalline ionic solid such as metal oxide. The traditonal spherical particle model adopted to generate simulated spectra without regard to the clear presence of some surface facets is insufficient and can lead to incorrect intepretation of the spectra obtained. Since the observed optical phenomenon is related to the facet-dependent electrical conductivity and photocatalytic activity of Cu2O crystals, the Cu2O shells may also exhibit facet-dependent optical properties. Figure 5 shows TEM images of 104 nm Au-Cu2O cubes and 118 nm corner-truncated octahedra. The octahedra are somewhat larger than the cubes. Interestingly, the 104 nm cubes give a Cu2O absorption band at 488 nm, while the Cu2O absorption band for the 118 nm truncated octahedra is at 477 nm. In contrast to the normal behavior of larger particles showing a more redshifted absorption band, here, the smaller cubes are actually more red-shifted compared to the slightly larger octahedra. This unusual optical behavior can only be explained by

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the facet effect. The analysis demonstrates that the facetdependent optical properties observed in these Au-Cu2O nanocrystals really come from the Cu2O shells. The same more red-shifted Au LSPR band feature can also be observed in larger particles with cubic gold cores (Figure 5c). Figure 3 also shows 92 nm Au-Cu2O cubes with a Cu2O absorption band at 494 nm, while the 110 nm octahedra absorbs at 486 nm. The magnitude of such a peculair optical response is not so large, so the effect can be easily missed if particles with different shapes but similar sizes were not prepared. To complete the investigation of facet-dependent optical properties of Au-Cu2O nanocrystals, it is necessary to make particles with a rhombic dodecahedral shape exposing the {110} facets. Figure 6 presents UV-vis absorption spectra of Au-Cu2O core-shell rhombic dodecahedra, octahedra, and cubes with three different average particle sizes synthesized using 35 nm octahedral gold cores. Because smaller gold cores were used, the final Au LSPR band positions for the octahedra

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Figure 5. a) TEM image of Au–Cu2O core–shell cubes viewed from the [100] direction. b) TEM image of Au–Cu2O core–shell truncated octahedra. The dotted lines give the outline of the cubic Au cores. c) UV–vis absorption spectra of the Au–Cu2O core–shell cubes and truncated octahedra with cubic Au cores. d) Simulation results for UV–vis absorption spectra of the cubic Au–Cu2O core–shell nanocrystal solutions with different particle sizes. Reproduced with permission.[15] Copyright 2014, Royal Society of Chemistry.

and cubes are at 721 and 741 nm, respectively. The same shell thickness-independent optical response was also recorded for the rhombic dodecahedra. Remarkably, tuning the oxide shell surface facets can produce a band position difference as large as 47 nm from rhombic dodecahedral to cubic shell structures. Such huge LSPR band tuning is unprecedented. Intermediate particles with shapes between rhombic dodecahedral and octahedral structures yield a band at ≈710 nm. The Au LSPR band adjusts its position depending on the relative

proportions of different Cu2O facets exposed, so its position can be roughly predicted by considering the particle shape. The origin of the facet-dependent optical properties can be explained by considering the results obtained from the facet-dependent electrical conductivity measurements performed on a single Cu2O cube and octahedron, in which two tungsten probes were brought into contact with a single Cu2O crystal for the measurements.[11] Previously, we have shown that a single Cu2O octahedron is highly electrically conductive, while a single cube is barely conductive below an applied voltage of 3 V. To explain the results, one can treat the different surface facets as having different degrees of valence and conduction band bending, such that charge carrier transport across the {111} interface of Cu2O is much easier with a relatively low barrier, but the barrier at the {100} interface is much higher. Surfaces with different band structures can be considered somewhat different materials with different refractive indices. When the plasmonic field reaches particle surfaces, it encounters a thin layer with different refractive indices. The plasmonic band adjusts its position accordingly to give the observed facet-dependent optical effects.

Figure 6. UV-vis absorption spectra of Au-Cu2O core-shell a) rhombic dodecahedra, b) octahedra, and c) cubes with three different average particle sizes. Reproduced with permission.[16] Copyright 2014, Wiley-VCH.

3. Bimetallic Core–Shell Nanocrystals

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After the discovery of the facet-dependent optical properties of Cu2O crystals through the investigation of optical spectra

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Figure 7. a,b) UV-vis absorption spectra of Au-Ag core-shell a) cubes and b) octahedra with different body diagonals. c) UV-vis spectra of Au-Ag core-shell nanocubes, trunacted cubes, cuboctahedra, truncated octahedra, and octahedra. Their body diagonal lengths are given. The gold cores have a rhombic dodecahedral shape. Reproduced with permission.[17] Copyright 2014, American Chemical Society.

of Au-Cu2O nanocrystals, the next question to ask is whether bimetallic core-shell nanocrystals exhibit similar facetdependent optical properties. Obviously, this is a rather challenging question to address, because the LSPR absorption band for bimetallic core-shell nanoparticles should red-shift progressively with increasing particle size. There have been some reports describing the plasmonic resonance properties of bimetallic core-shell nanocrystals such as the Au-Ag and Ag-Cu particle systems.[30–33] Tan and co-workers used gold cubes, octahedra, and rhombic dodecahedra as cores to make Au-Ag nanocubes and studied their optical properties.[34] Because the particles have sizes of 70-100 nm, the most prominent band is located at 550−580 nm, showing the influence of the gold cores to strongly red-shift the plasmonic band of the Au-Ag nanocrystals. Xia and co-workers used 11 nm spherical gold seeds to prepare Au-Ag nanocubes with edge lengths of 13.4-50 nm and shell thicknesses of 1.2-20 nm.[35] Beyond a shell thickness of 3 nm, the Au LSPR band is completely screened by the Ag shell. Because only core-shell cubes were synthesized, the possible presence of facet-dependent optical properties of Au-Ag nanocrystals cannot be studied. To address this fundamental question, we synthesized Au-Ag core-shell nanocrystals with systematic shape evolution

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from cubic to octahedral structures using 77 nm rhombic dodecahedral gold cores as measured by the opposite corner-to-corner distance.[17] In addition, Au–Ag core–shell cubes and octahedra with tunable sizes were obtained to observe any size-independent LSPR feature from the gold cores as seen in Au-Cu2O nanocrystals. Figure 7 presents the UV-vis absorption spectra collected from these samples. Au–Ag cubes with body diagonals of 130, 144, and 161 nm and octahedra with body diagonals of 113, 126, and 143 nm have been synthesized. Au–Ag octahedra with thinner Ag shells (12 to 16.5 nm) show a blue-shifted major LSPR band relative to the LSPR band of the gold cores at 538 nm. For Au–Ag octahedra and cubes with thicker shells (22.5 to 37 nm), the major LSPR band is progressively redshifted from that of the gold cores with increasing particle size. Although the major LSPR band of the cubes are more red-shifted than that for the octahedra as similarly observed in Au-Cu2O nanocrystals, the cubes may also be larger than the octahedra. Figure 7c further shows that bimetallic Au-Ag nanocrystals do not possess recognizable facet-dependent optical properties, because there is no logical trend for the major LSPR band positions with respect to the relative fractions of exposed facets that can be identified. The major SPR peak position seems to be more sensitive to the particle size.

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Figure 8. UV−vis absorption spectra of the Au–Pd core−shell a) cubes, b) truncated cubes, c) cuboctahedra, d) truncated octahedra, and e) octahedra with different sizes. The Au core size is 35 nm. f) Combined UV−vis absorption spectra of Au–Pd nanocrystals with thinnest shell thicknesses. Reproduced with permission.[18] Copyright 2014, Royal Society of Chemistry.

We next considered the examination of Au-Pd core-shell nanocrystals for the possible presence of facet-dependent optical properties, because polyhedral Au-Pd nanocrystals with an exact lattice orientation relationship between the cores and the shells can be readily prepared.[36,37] The formation of Au-Pd nanocrystals with systematic shape evolution was also demonstrated.[37] Previously Park and co-workers synthesized Au-Pd cubes using spherical gold seeds as the cores.[38] The Au-Pd particle size can be tuned from 11 to 41 nm. For particles with a Pd shell thickness of just 0.7 nm, the Au LSPR band blue-shifts significantly. The band disappears as the Pd shell thickness reaches 2.7 nm. This shows that a Pd shell needs to be extremely thin for the Au LSPR band to be observed. For the most complete comparison of the plasmonic properties between Au-Pd and Au-Cu2O nanocrystals, Au-Pd core-shell nanocrystals with systematic shape evolution and small 2015, DOI: 10.1002/smll.201403542

tunable size need to be synthesized. Furthermore, octahedral gold cores with different sizes should be used to generate the same series of Au-Pd nanocrystals with shape evolution to evaluate the core size effect on the plasmonic band shift. This is truly a challenging synthetic task, and requires a simple and reliable method to afford the formation of so many products with excellent size and shape control. Fortunately, a facile synthetic method has been developed for the formation of Au– Pd core–shell nanocrystals in aqueous solution in 0.5–2 h at 50 °C with systematic shape evolution from cubic to truncated cubic, cuboctahedral, truncated octahedral, and octahedral structures using octahedral gold cores.[18] Gold cores of three different sizes (35, 45, and 74 nm in opposite corner distance) were used to make Au-Pd nanocrystals with the same series of particle shapes for a most complete examination of their plasmonic properties. Figure 8 shows UV-vis absorption spectra of the Au–Pd core−shell nanocrystals with various shapes

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Figure 9. UV−vis absorption spectra of the Au–Pd core−shell a) cuboctahedra, b) truncated octahedra, and c) octahedra with different sizes. The Au core size is 74 nm. d) Combined UV−vis absorption spectra of selected Au–Pd nanocrystals to illustrate the spectral band shift. Reproduced with permission.[18] Copyright 2014, Royal Society of Chemistry.

synthesized using 35 nm Au cores. For the smaller cubes, cuboctahedra, and truncated octahedra prepared using 35 and 45 nm Au cores, the LSPR band from the gold cores appears only when the Pd shell thickness is just 1 nm at the thinnest points of the particles. For the Au–Pd octahedra, this band is already observable at a shell thickness of 5 nm because of the uniform shell thickness of core-shell octahedra. The plasmonic band shift from Au–Pd cubes to octahedra is only 7 nm. With such a small shift, it is not possible to conclude the presence of any facet-dependent optical properties in these nanocrystals. This minute spectral shift would not be recognizable under most synthetic conditions. Figure 9 shows UV-vis spectra of Au–Pd core−shell cuboctahedra, truncated octahedra, and octahedra with different sizes synthesized using 74 nm Au cores. The Au LSPR band is actually easier to see with the use of larger gold cores. Again, the very small Au LSPR band shift of 8 nm from 506 to 514 nm is insufficient to conclude the presence of any facet-dependent optical properties. Even if this minor spectral shift signifies some facet dependence, the ultrasmall shift is not useful for practical applications. These studies clearly show that bimetallic core-shell nanocrystals do not possess facet-dependent optical properties.

4. Conclusion and Perspectives The discovery of facet-dependent optical properties in AuCu2O core-shell nanocrystals is truly exciting, because this is another facet-dependent property revealed by Cu2O

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crystals. Previously, we demonstrated that Cu2O crystals display facet-dependent electrical conductivity and photocatalytic activity, which involves charge carrier transport into and out of a Cu2O crystal through different facets or interfaces. Facet-dependent organocatalytic properties of Cu2O crystals, on the other hand, are considered to be mainly surface-related properties, because all surface facets exhibit some degrees of catalytic activity.[39–42] The facet-dependent optical responses arise in Au-Cu2O crystals because of different degrees of band bending for different surface facets of Cu2O, such that different facets can be considered to consist of a thin surface layer with varying refractive indices to tune the plasmonic band positions of the gold cores. Au-Ag and Au-Pd core-shell nanocrystals were found not to display any facet-dependent plasmonic properties. So this unusual optical behavior does not extend to bimetallic core-shell systems. Facet-dependent optical and other properties should be prevalent in other inorganic solids. However, significant synthetic challenges to making semiconductor nanocrystals with a series of well-defined shapes limit such investigation. It can be even more difficult to grow core-shell semiconductor particles with shape control of both components. However, the revelation of several different physical and chemical properties being facet-dependent means that such effects need to be considered and explored. The simplest synthetic extension is to make metal-Cu2O nanocrystals with other plasmonic metal cores for facet-dependent optical property examination. The fact a plasmonic band position can be tuned over such a wide range should be highly valuable. For example,

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excitation of the plasmonic cores with a particular wavelength of light can be selected by changing the Cu2O shell morphology and possibly the metal composition to cover a wide range from visible to near-infrared regime. The high level of plasmonic band tunability achieved through Cu2O shell facet control is unsurpassed particularly for NIR excitation applications.[43–45] Studies demonstrating the utility of facet-dependent optical effects should bring advances to plasmonic research.

Acknowledgements We thank the Ministry of Science and Technology of Taiwan for the support of this work (NSC 101–2113-M-007–018-MY3 and NSC102–2633-M-007–002).

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Received: November 28, 2014 Revised: January 7, 2015 Published online:

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Facet-Dependent Optical Properties Revealed through Investigation of Polyhedral Au-Cu₂O and Bimetallic Core-Shell Nanocrystals.

The ability to prepare Au-Cu2O core-shell nanocrystals with precise control over particle size and shape has led to the discovery of facet-dependent o...
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