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Facet-dependent optical properties of polyhedral Au–Cu2O core–shell nanocrystals† Yu-Chen Yang,a Hsiang-Ju Wang,b Jennifer Whang,b Jer-Shing Huang,b Lian-Ming Lyu,b Po-Heng Lin,b Shangjr Gwoa and Michael H. Huang*b We fabricated Au–Cu2O core–shell octahedra, cuboctahedra, and nanocubes having sizes of 90–220 nm using 50 nm octahedral cores. The smaller particle sizes minimize the strong light scattering features from the Cu2O shells and enable the surface plasmon resonance (SPR) absorption band of the gold cores to be clearly identified. Beyond a lower shell thickness limit, the SPR band positions of the gold cores are independent of the shell thickness, but are strongly dependent on the exposed particle surfaces. The plasmonic band red-shifts from Au–Cu2O octahedra to cuboctahedra and nanocubes, and differs by as much as 26 nm between the octahedra and the nanocubes. The same facet-dependent optical effects were observed using larger octahedral gold cores and cubic gold cores. In contrast, simulation spectra show progressively red-shifted SPR band positions with increasing shell thickness. The Cu2O shells are
Received 26th November 2013 Accepted 4th February 2014
also found to exhibit facet-dependent optical behavior. These nanocrystals can respond to changes in the solvent environment such as solvents with different refractive indices, indicating that the plasmonic
DOI: 10.1039/c3nr06293g
field of the gold cores can extend beyond the particle surfaces despite the presence of thick shells.
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Plane-selective spectral responses to low concentrations of surfactants were also recorded.
Introduction Recent developments in the syntheses of polyhedral inorganic nanocrystals with excellent shape control have enabled the examination of their facet-dependent catalytic, electrical, and molecular adsorption properties.1–12 Core–shell metal and semiconductor nanocrystals with excellent control of core size and shell thickness are interesting due to their tunable optical properties.13–19 In particular, polyhedral bimetallic and metal– semiconductor core–shell heterostructures can be generated using metallic nanocrystal cores, allowing the investigation of the plane-selective properties of the shell materials in the presence of the metal cores.4,20,21 For example, the Au coreenhanced photodegradation of Au–Cu2O core–shell nanocrystals was only observed for particles exposing suitable Cu2O surface facets.7 The Au cores also enhance the electrical conductivity of single Au–Cu2O core–shell octahedral nanocrystals, but the core–shell nanocubes remain non-conductive.5 Facet-dependent optical properties of polyhedral core–shell nanocrystals are interesting materials properties to examine, but have never been demonstrated or even considered. This is presumably due to the availability of structurally well-dened
a
Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
b
core–shell heterostructures until recently. Previously the Au– Cu2O core–shell nanocrystals synthesized were relatively large in the range of hundreds of nanometers in size that the localized surface plasmon resonance (SPR) absorption band of the gold cores cannot be observed or easily identied. By reducing the nanocrystal sizes to minimize the light scattering effects arising from the Cu2O shells, the SPR band of the gold cores appears and reveals surprising facet-dependent optical responses to the shell morphology and external environment surrounding the particles for the rst time. Here we show that the localized SPR absorption band positions of the Au cores in the Au–Cu2O core–shell heterostructures are highly dependent on the surface facets of the single-crystalline Cu2O shells. Within a considerably large shell thickness range, the SPR band positions are independent of the shell thickness. The results are in sharp contrast to the simulation spectra showing continuously red-shied SPR band positions with increasing particle size. Remarkably, the Au cores can sense changes in the external environment including solvents and surfactants despite the presence of thick shells, and respond with SPR band shis. The facet-dependent SPR responses should be general and observable in other metal–metal oxide systems.
Experimental section
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan. E-mail: [email protected]
Synthesis of Au–Cu2O core–shell nanocrystals
† Electronic supplementary 10.1039/c3nr06293g
The synthesis procedure adopts our previously reported method with slight modications.13,14 H2O, CuCl2 solution, sodium
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dodecyl sulfate (SDS) surfactant, Au nanocrystals, NaOH solution, and NH2OH$HCl solution were added in the order listed. The concentrations and amounts of reagents used to prepare the various Au–Cu2O core–shell nanocrystals are summarized in Scheme S1, ESI.† Aer stirring the solution to dissolve SDS completely, the concentrated Au nanocrystal solution was introduced, and the solution was shaken for 10 s. Following the addition of NaOH and NH2OH$HCl solution, the reaction mixture was shaken for 15 s and kept at room temperature for 2 h for shell growth. To remove the surfactant, the solution was washed with deionized water and centrifuged four times at 5000 rpm for 3 min. Finally, the collected products were dispersed in 1.0 mL of absolute ethanol for direct spectral analysis and long-term storage. Sample preparation for spectral analysis To take UV-vis absorption spectra of the core–shell nanocrystals in another solvent, 0.1 mL of the particle-containing ethanol solution was withdrawn and centrifuged. Ethanol was removed and 1.0 mL of the solvent of interest was added. Aer sonication, to disperse the particles, the solution was transferred to a cuvette for spectral measurements. To test the surfactant effect, 0.1 mL of the particle-containing ethanol solution was centrifuged to remove ethanol. Then 1.0 mL of water was introduced, followed by the addition of 0.005 mL of 10 mM surfactant solution. The surfactant concentration is 0.05 mM in this nanocrystal solution. Aer sonication, the solution was transferred to a cuvette for analysis. Instrumentation Scanning electron microscopy (SEM) images of the nanocrystals were obtained using a Zeiss Ultra 55 eld-emission scanning electron microscope. Transmission electron microscopy (TEM) characterization was performed on a JEOL JEM-2100 electron microscope. X-ray diffraction (XRD) patterns were collected using a Shimadzu XRD-6000 diffractometer with Cu Ka radiation. UV-vis absorption spectra were acquired with the use of a JASCO V-570 spectrophotometer. Zeta potential measurements were carried out on a Malvern Zetasizer Nano S90 zeta potential analyzer. Numerical methods We employed a nite-difference time-domain method (FDTD solutions, Lumerical, Canada) to simulate the extinction spectra of our core–shell nanoparticles. Refractive indices of gold and Cu2O are obtained from experimental data.22 The refractive index of the surrounding medium is set to be 1.36 in order to mimic the aqueous solution environment. Boundaries of the simulation area are set to be at least 2000 nm away from the outer surface of the shell such that spurious absorption of optical near elds is avoided. We used a uniform mesh size with discretization of 1 1 1 nm3 to cover the whole core–shell nanoparticles. To simulate the real structures, the edges and corners of the octahedral cores are rounded with cylinders or spheres with a radius of 12 nm. An about 3 femtosecond broadband plane wave source centered at 800 nm was used as a
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source, and the polarization was aligned to the diagonal direction of the octahedral core. All particle dimensions are obtained from the SEM images.
Results and discussion To observe the facet-dependent SPR absorption effects and their invariance to the shell thickness, the Au–Cu2O core–shell nanocrystals need to be synthesized with excellent morphology and size control. Previous studies on the optical properties of Au–Cu2O and metal oxide–Au core–shell nanoparticles all used roughly spherical particles for measurements and modeling, so their facet-dependent SPR properties cannot be observed.23–26 Furthermore, material imperfections such as the polycrystalline nature of the core and/or shell materials and particle shape uniformity may broaden the Au and Cu2O absorption bands. The general observation was that the SPR band of the core shis progressively to red with increasing shell thickness.23,24 In this study, we examined the effects of particle morphology and shell thickness on the optical properties of the Au–Cu2O core–shell nanocubes, cuboctahedra, and octahedra with sizes of less than 200 nm to avoid the strong light scattering features appearing from the visible to the near-infrared region in larger Cu2O particles.3,4,20,21,27 The octahedral Au nanocrystal cores were produced by a hydrothermal synthesis approach.28 By varying the reaction time from 12 to 16 and 20 h, octahedral Au nanocrystals with respective average sizes of 50, 60, and 70 nm were obtained (Fig. S1, ESI†). Their UV-vis absorption spectra show a single SPR absorption band, which is slightly red-shied from 550 to 553 and 560 nm with increasing particle dimensions (Fig. S2, ESI†). The Au–Cu2O core–shell nanocrystals were synthesized by mixing an aqueous solution of CuCl2, SDS, Au nanocrystals, NaOH, and NH2OH$HCl.20,21 By increasing the volume of NH2OH$HCl added, systematic shape evolution of the Cu2O shells from cubic to octahedral structures was achieved. Furthermore, increasing the volume of the concentrated gold nanocrystal solution added to the reaction mixture can gradually reduce the sizes of the Au–Cu2O nanocrystals (see Scheme S1, ESI† for the exact reagent amounts used and Fig. 1 for the determination of particle sizes). Similar size tuning of the Au–Cu2O octahedra prepared using short Au nanorod cores has been reported through the adjustment of the Au core amount.29 Fig. 2 shows SEM images of the Au–Cu2O core–shell nanocubes, cuboctahedra, and octahedra synthesized using 50 nm
Fig. 1 Schematic drawing of the Au–Cu2O core–shell octahedron, cuboctahedron, and cube. The arrows indicate the particle sizes determined in this study.
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SEM images of the synthesized Au–Cu2O core–shell nanocrystals. The particle morphologies are (a–e) cubic, (f–j) cuboctahedral, and (k–o) octahedral in shape. Particle sizes decrease from left to right. The octahedral gold cores have an average size of 50 nm. Insets show enlarged views of the Au–Cu2O core–shell nanocrystals. Scale bar in each inset is equal to 100 nm.
Fig. 2
octahedral Au cores. Particles with ve different size ranges have been prepared for each of these shapes (see Table S1† for the particle sizes and their standard deviations). The nanocrystals have sizes in the range of 90–220 nm. With the exception of broader size distributions for the larger octahedra, the nanocrystals are highly uniform in size and shape, which is critically important for their facet-dependent optical property characterization. The octahedral Au cores are visible due to the
Fig. 3
relatively small Au–Cu2O particle sizes. XRD patterns of the nanocrystals show both Au and Cu2O reection peaks (Fig. 3). Intensities of the Au peaks are weaker than those of the Cu2O peaks because they are located inside the particles. Intensities of the Au and Cu2O (200) peaks are stronger than those of the (111) peaks for the core–shell cubes because their cubic faces are preferentially oriented parallel to the substrate surface. On the other hand, the Au–Cu2O octahedra exhibit a stronger (111)
XRD patterns of the Au–Cu2O core–shell cubes, cuboctahedra, and octahedra.
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peak intensity because of the exposed {111} facets. Uniformly thinner shells for the Au–Cu2O octahedra lead to stronger reection peaks for the Au cores. Energy-dispersive X-ray spectroscopy (EDS) line-scan on a single core–shell nanocube conrms the presence of the gold core (Fig. S3, ESI†). TEM characterization of a single core–shell nanocube, cuboctahedron, and octahedron further veries the exact lattice orientation relationship between the gold cores and the Cu2O shells (see Fig. 4, S4 and S5, ESI†). For example, the selected-area electron diffraction (SAED) pattern of a core–shell cube shows that the Au (200) and Cu2O (200) spots are aligned on the same line, conrming their exact lattice orientation relationship. Furthermore, a high-resolution TEM image of one such particle indicates that the (200) lattice planes of its Au core are aligned parallel to the (200) planes of its Cu2O shell. With decreasing average particle sizes from 218 nm to 110 nm, the dark orange solution color gradually changes to light green and nally dark green or greenish blue (Fig. 5a). This vivid solution color change is mainly associated with the absorption prole evolution of the Cu2O shells. Fig. 5b–d present the UV-vis absorption spectra of the Au–Cu2O core–shell nanocubes, cuboctahedra, and octahedra with the ve different average sizes for each particle shape as shown in Fig. 2. For particle stability reason, spectral analysis was performed in ethanol unless noted. For all the particle shapes, the Cu2O absorption band centered at 490–500 nm for the smallest particles shows a slight red-shi with increasing particle dimensions, accompanied by gradual appearance of the light scattering feature moving toward the infrared region.20,27 This size-dependent spectral prole is characteristic of Cu2O nanocrystals even without the gold cores.27 Because the core–shell nanocrystals are mostly less than 200 nm in size, the interference from the light scattering feature of the Cu2O shells is not so severe, so the SPR absorption band of the gold cores can be easily identied. The high refractive index of Cu2O (2.7 at 800 nm)30 and its high dielectric constant (3 ¼ 7.2)23,25 causes a huge red-shi of the SPR absorption band from 550 nm for the octahedral gold nanocrystals to 752–778 nm for the Au–Cu2O nanocrystals. By comparison, the longitudinal SPR band of the Au nanorod cores in Au–Cu2O nanorods can red-shi by
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470–600 nm depending on the shell thickness.31 Specically, the Au–Cu2O nanocubes, cuboctahedra, and octahedra have xed SPR band positions respectively at 752, 768, and 778 nm. Remarkably, the band positions remain xed for these Au–Cu2O nanocrystals despite signicant changes in their shell thickness. Shell thicknesses vary from 30 to 84 nm for the Au–Cu2O octahedra, 32–64 nm for the cuboctahedra, and 21–66 nm for the nanocubes (or 65–143 nm when measured from the diagonal direction of the cube). The results are in sharp contrast to the previous observations showing that the SPR absorption band shis steadily to red with increasing Cu2O shell thickness.23,24 We found that a minor spectral red-shi can indeed be observed in smaller Au–Cu2O nanocubes with the SPR band shiing from 769 nm for the 81 nm nanocubes to 778 nm for nanocubes larger than 90 nm (Fig. S6†). Thus, beyond a lower shell thickness limit of 20–30 nm, Au–Cu2O polyhedra with thicker shells should exhibit xed SPR absorption band positions. Because of the large refractive index of Cu2O, increasing shell thickness means that the Au cores experience a surrounding medium with increasing refractive index and respond by shiing the SPR band to longer wavelengths.32 When the shell thickness is sufficiently thick, the Au cores will see the surrounding medium as the same despite changes in the shell thickness, so the SPR band remains xed in position. One should note that for the SPR band position to be invariant with shell thickness of just beyond 20–30 nm, the shells need to shield the cores completely; the solvent can come into contact with the cores if the cores are not completely insulated and change the effective refractive index of the surroundings. Similar shell thickness-independent SPR wavelength shis have been observed for silica-coated silver nanoparticles with shell thicknesses beyond 30 nm.33 In contrast to the amorphous silica shells, here we show that the same phenomenon can also be observed for particles with single-crystalline oxide shells. The most striking feature these spectra reveal is of course that the gold SPR wavelength shis widely from octahedra to nanocubes by as much as 26 nm. Cuboctahedra displayed an intermediate band position. Since all three particle shapes have overlapping shell thicknesses, shell thickness differences cannot explain the observed large SPR band shis. The results clearly indicate that
Fig. 4 TEM analysis of a single Au–Cu2O core–shell nanocube. (a) TEM image of a Au–Cu2O nanocube viewed along the [100] direction. The white dotted line indicates the outline of the octahedral Au nanocrystal core. (b) SAED pattern of this nanocube giving both Au and Cu2O diffraction spots. (c) High-resolution TEM image of the red square region. The red 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. Insets show a schematic drawing of the particle viewed along the [001] zone axis.
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Fig. 5 (a) Photograph of the octahedral Au–Cu2O core–shell nanocrystal solutions with decreasing particle sizes. (b–d) UV-vis absorption spectra of the (b) octahedral, (c) cuboctahedral, and (d) cubic Au–Cu2O core–shell nanocrystal solutions with different particle sizes. 50 nm octahedral Au cores were used.
the SPR band positions are dependent on the morphology, and hence the surface facets, of the Cu2O shells. Because cuboctahedra have both the {100} and {111} faces with similar surface areas, it makes sense that these particles show an intermediate SPR band position. Amazingly, in addition to the drastic facetdependent photocatalytic and electrical properties of Au–Cu2O core–shell nanocubes and octahedra, their optical properties are also strongly facet-dependent.5,20 The same facet-dependent SPR absorption phenomenon was also observed in Au–Cu2O core–shell cubes and octahedra synthesized using 60 nm octahedral gold nanocrystal cores (Fig. 6a and b). Particle size distributions are also narrow for these nanocrystals (Table S2, ESI†). With larger gold cores, the SPR band positions of the Au–Cu2O octahedra and nanocubes red-shi to 777 and 800 nm, respectively. However, when 70 nm octahedral gold cores were used, the SPR absorption bands of the Au–Cu2O octahedra and nanocubes show a progressive redshi with increasing particle sizes (Fig. 6c and d). For nanocubes and octahedra with similar dimensions, the nanocubes also display more red-shied SPR bands than the octahedra do. The band to the shorter wavelength side of the SPR band (i.e., the band at 626 nm for the 105 nm Au–Cu2O octahedra sample) is probably part of the light scattering feature from the Cu2O shells. The spectral results obtained are not necessarily in conict with those for the Au–Cu2O nanocrystals synthesized using smaller gold cores. The magnitudes of spectral red-shi become smaller with increasing particle sizes, suggesting that xed SPR band positions can still be observed for these Au– Cu2O nanocrystals with sufficiently large particle sizes (Fig. 6e and f). Larger particles were not prepared due to consideration of strong interference from the light scattering bands of Cu2O. It is important to recognize that the magnitude of the SPR band red-shi is relatively mild (50 nm red-shi with the shell thickness increasing from 17.5 nm to 62 nm for the octahedral
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shell case) compared to that seen in Au–Cu2O nanorods with different Cu2O shell thicknesses (135 nm red-shi with the shell thickness increasing from 17 nm to 65 nm).31 We tried to make
Fig. 6 (a and b) UV-vis absorption spectra of the Au–Cu2O octahedra (a) and nanocubes (b) with 60 nm Au cores. (c and d) UV-vis absorption spectra of the Au–Cu2O octahedra (c) and nanocubes (d) with 70 nm Au cores. (e and f) Plots summarizing the SPR band shifts of (e) Au– Cu2O octahedra and (f) nanocubes with respect to the particle sizes. The error bars give standard deviations of the particle sizes in the samples. The dash lines are the best-fit lines of the data points.
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Au–Cu2O nanocrystals with octahedral Au core sizes between 60 and 70 nm and see when the Au SPR band started to show a slight red-shi. To show that the same facet-dependent SPR properties can still be seen by using other Au cores, gold nanocubes were used to form Au–Cu2O nanocubes and octahedra (Fig. S7, ESI†). The gold nanocubes were synthesized following a seed-mediated growth approach.34 The SPR band of the 47 nm gold nanocubes was at 541 nm. The SPR band of the Au–Cu2O corner-truncated octahedra was xed at 705 nm with varying shell thickness, while that of the Au–Cu2O nanocubes was at 736 nm with different particle sizes. These results demonstrate that the facet-dependent and shell thicknessindependent optical properties of Au–Cu2O nanocrystals, and likely other metal–Cu2O core–shell nanoparticles, can be observed in other polyhedral samples with different core sizes and shapes, as long as shells with a sufficiently large thickness can be synthesized. However, the experimental data suggest that use of smaller cores makes it easier to observe these novel plasmonic properties. We also performed rigorous numerical simulations in order to gain insight into the localized SPR behavior of the core–shell nanocrystals under purely electromagnetic consideration. We employed the nite-difference time-domain (FDTD) method to simulate the extinction spectra of the Au–Cu2O core–shell nanocrystals with 50 nm octahedral gold cores and exactly the same shell thicknesses of the synthesized particles. Fig. 7a gives
Fig. 7 (a) Simulation results for UV-vis absorption spectra of the octahedral Au–Cu2O core–shell nanocrystal solutions with different particle sizes. (b) Simulated peak positions of localized surface plasmon resonance of Au–Cu2O core–shell nanoparticles with cubic (black squares), cuboctahedral (red dots), and octahedral (green triangles) shells. The black, red and green dashed lines indicate the localized SPR peak positions of the corresponding particles obtained from the experiment.
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the simulation results for the octahedral Au–Cu2O nanocrystal solutions with different particle sizes. The simulation spectra for the octahedral gold cores, as well as the spectra for the cuboctahedral and cubic nanocrystal solutions are shown in Fig. S8, ESI.† The SPR band position at 550 nm for the gold cores is in good agreement with the experimental data. In addition, Fig. S9, ESI† displays the simulated electric near-eld intensity distributions of the cubic, cuboctahedral, and octahedral Au–Cu2O particles, showing the dipolar resonance feature of the surface plasmon resonance of the gold core. Having conrmed the localized SPR of the gold cores, we further simulated the spectra of various Au–Cu2O nanocrystals. Real dimensions obtained from the SEM images of the whole particles were used in the simulations. In purely linear electromagnetic FDTD simulations, we do not take into account any chemical properties but the refractive index of the bulk material. The SPR band positions of all Au–Cu2O core–shell nanocrystals show distinct dependence on the shell thickness, as summarized in Fig. 7b. The reason that cubes are more redshied than cuboctahedra and octahedra of similar sizes (for example, comparing 113 nm cubes, 115 nm cuboctahedra, and 110 nm octahedra) is because cubes have a much larger shell thickness as measured from the diagonal distance. While the experimental resonance position stays constant beyond a particle size of 90 nm, simulated resonance positions of all particles red-shi continuously with the shell thickness. The discrepancy between the simulated and experimental data suggests that the shell does not behave merely like a thin layer of the bulk dielectric material of Cu2O. As we have observed, the localized SPR of the core can “feel” the difference in the surrounding shell surfaces, which is the only difference between the three particle shapes. Clearly, the simulations cannot yield reasonably matched spectra, because the effects of surrounding solid medium with different surfaces have not been taken into account in conventional simulation methods. The results further show that the observed facet-dependent optical properties are truly new phenomena, and simulations need to be modied in order to generate correct and useful spectra. The reason facet-dependent optical properties are observed is because the {100} and {111} faces of Cu2O have very different surface properties.4,5,20 Such surface properties have not been considered previously in the explanation and modeling of SPR band positions of Au–Cu2O core–shell nanocrystals. Cu2O has a body-centered cubic crystal structure with copper atoms occupying half of the tetrahedral sites in the unit cell. The {100} faces of Cu2O are oxygen atom-terminated and more negatively charged, while the {111} faces have more surface copper atoms exposed with unsaturated coordination and are more positively charged (Fig. 8). As a demonstration, Au–Cu2O nanocubes and octahedra immersed in a negatively charged methyl orange solution showed very different behaviors. Octahedra can be well dispersed in this solution, but nanocubes were found to oat to the top of the solution aer stirring (Fig. S10, ESI†). Electrostatic repulsive interactions between the negatively charged surfaces of the Au–Cu2O core–shell nanocubes and anionic methyl orange can explain this phenomenon. The measured
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Fig. 8 Crystal models of the different planes of Cu2O. The models show (a) (100) and (b) (111) surfaces of Cu2O. The fully exposed surface copper atoms are highlighted with orange circles.
zeta potentials for the Au–Cu2O octahedra and nanocubes are +3.37 mV and 8.23 mV, respectively, supporting the different surface charge states of the {111} and {100} faces of Cu2O (Fig. S11, ESI†). In addition to the surface charges, we have previously shown that the {100} facets of Cu2O nanocrystals are not electrically conductive, while the {111} facets are highly electrically conductive.5 This may be due to different ways of band bending near different surfaces of Cu2O crystals, such that electrons can easily pass through the {111} faces when they come into contact with tungsten probes, but a large potential barrier exists at the edge of the {100} face, such that electron transport is much more difficult. This difference in the band structure at the particle surfaces can be considered as band structure tuning, and effectively makes different crystal faces act like different materials with different refractive indices. The plasmonic eld of the Au cores traveling to particle edges meets a boundary with different refractive indices, and responds to these differences by shiing the SPR band positions. This explanation is new but should be considered to account for the facet-dependent electrical and optical properties recorded. That is, facets can affect both electrical and optical properties of a material, but this possibility has never been considered because of the lack of a system to demonstrate this. Since the facet-dependent plasmonic effects are derived from Cu2O, Cu2O nanocrystals themselves should exhibit this intrinsic facet-dependent optical property. Upon close examination of the Cu2O band positions shown in Fig. 5, 6 and S7, ESI,† cubes are generally more red-shied than octahedra for particles of comparable sizes. TEM images presented in Fig. S7, ESI† show that the Au–Cu2O truncated octahedra are appreciably larger than cubes, but the Cu2O absorption band at 488 nm for the 104 nm cubes are still more red-shied than the 118 nm truncated octahedra at 477 nm. The size effect cannot explain the observed spectral results, because larger particles are expected to show a more red-shied extinction band. Thus pristine Cu2O nanocrystals should also possess facet-dependent optical properties. It is reasonable to conclude that surfacerelated refractive index differences give rise to the observed facet-dependent SPR band shis. A more red-shied SPR band of Au–Cu2O nanocrystals is accompanied by a more red-shied Cu2O absorption band, and vice versa. The fact that the Cu2O absorption band appears at 460 nm in the simulation spectra for the smallest cubes, cuboctahedra, and octahedra, as compared to their recorded positions at 486, 489, and 494 nm (Fig. 5), also supports the proposition that surface facets, in
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addition to the particle size, shape, and composition, should be considered to accurately present UV-vis spectra of Cu2O nanocrystals. The facet-dependent optical behavior of Cu2O crystals can also explain why their spectral band positions are changing and not so predictable. Our examination of the effect of external environment on the optical properties of the Au–Cu2O nanocrystals reveals that these particles can respond to solvent changes by shiing the SPR band positions. Fig. 9 gives UV-vis absorption spectra of the 110 nm Au–Cu2O core–shell octahedra dispersed in ethanol, dichloromethane, and toluene with very different polarity values. With increasing solvent refractive index, the SPR band shis progressively to red. Ethanol and water (n ¼ 1.33) have similar refractive indices, so almost identical UV-vis absorption spectra were recorded in these solvents (Fig. S12, ESI†). The magnitude of the spectral shi is about 77 nm per RIU (refractive index unit). Although the refractive index sensitivity of the Au–Cu2O nanocrystals is far less than gold nanostructures of different shapes such as nanoprisms,35 the measurable band shis show that these nanocrystals can still sense solvent refractive index changes despite the presence of thick shells. Alternatively, the SPR band becomes red-shied when dispersing the core–shell particles in solvents with lower dielectric constant values (24.5, 8.93, and 2.38 respectively for ethanol, dichloromethane, and toluene). The fact that facetdependent SPR properties are maintained for both polar and non-polar solvents eliminates the possibility of particle surface charge-induced solvent reorganization, and supports the proposition that the Cu2O surface facets can tune the SPR band positions. The exhibited solvent sensitivity suggests that the surface plasmon elds of the gold cores can actually extend beyond the boundary of the shell surfaces into the immediate external region surrounding the particles. Thus, the SPR band locations of these Au–Cu2O nanocrystals are determined by a combination of the shell thickness, its refractive index (or dielectric constant), exposed facets, and the external solution environment. Remarkably, the SPR sensitivity to environmental conditions can reach beyond a shell thickness of 60 nm and likely more than 100 nm. The discovery of changing the solution environment to tune SPR band positions offer opportunities to use these Au–Cu2O nanocrystals for plane-selective molecular sensing and
Fig. 9 UV-vis absorption spectra of the Au–Cu2O core–shell octahedra dispersed in different solvents. (a) UV-vis absorption spectra of 110 nm Au–Cu2O core–shell octahedra in ethanol, dichloromethane, and toluene. (b) SPR band positions of Au–Cu2O core–shell octahedra and cubes with respect to the refractive indices of the solvents. The nanocubes have sizes of 90 nm.
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101-2113-M-007-002-MY2 and NSC 101-2628-M-007-006). JSH thanks the support from the Center for Nanotechnology, Materials Sciences, and Microsystems at National Tsing Hua University.
References Fig. 10 UV-vis absorption spectra of (a) the Au–Cu2O core–shell octahedra and (b) nanocubes dispersed in an aqueous solution containing 0.05 mM of SDS, CTAB, or SDBS surfactant. At this surfactant concentration, the octahedra only respond to the presence of SDBS, while nanocubes only respond to the presence of CTAB. The octahedra have an average size of 110 nm, while the average nanocube size is 92 nm. The Au cores are 50 nm in size.
adsorption. The analysis is similar to the spectral shi of gold nanoparticles to the dielectric environment changes aer molecular adsorption or bonding,36 but here a thick Cu2O shell separates the analytes and the interior gold nanocrystal. Since SDS has been shown to adsorb preferentially onto the {111} planes of Cu2O possibly through electrostatic interactions,6 different surfactants including cetyltrimethylammonium bromide (CTAB) and sodium dodecyl benzene sulfonate (SDBS) have been added to investigate their ability to shi the SPR band of the Au–Cu2O nanocrystals (Fig. 10). At a low surfactant concentration of 0.05 mM, octahedra only respond to the presence of SDBS (6 nm blue-shi), while nanocubes only respond to the presence of CTAB (4 nm blue-shi). The blueshi actually results from instability of Cu2O crystals in large quantities of the surfactant; shell etching occurs at high surfactant concentrations as seen for SDS (Fig. S13, ESI†). Nevertheless, the observed SPR band shis show that surfactant adsorption may be detected.
Conclusion In summary, the facet-dependent SPR absorption properties of Au–Cu2O core–shell nanocrystals have been demonstrated for the rst time. Beyond a lower shell thickness limit, the SPR band positions of the core–shell nanocrystals are independent of the shell thickness, but shi progressively to the long wavelength side from octahedra to cuboctahedra and then cubes. In contrast, simulations yield continuously red-shied SPR band positions with increasing shell thickness. The Cu2O shells should also exhibit facet-dependent optical behavior. The plasmonic eld of the interior gold cores extends beyond the surfaces of the nanocrystals and can respond to changes in the solvent environment. This work raises the possibility that such facet-dependent plasmonic effects can be observed in other materials.
Acknowledgements We thank the National Science Council of Taiwan for the support of this work (NSC 98-2113-M-007-005-MY3, NSC
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1 K. M. Bratlie, H. Lee, K. Komvopoulos, P. Yang and G. A. Somorjai, Nano Lett., 2007, 7, 3097–3101. 2 A.-X. Yin, X.-Q. Min, Y.-W. Zhang and C.-H. Yan, J. Am. Chem. Soc., 2011, 133, 3816–3819. 3 W.-C. Huang, L.-M. Lyu, Y.-C. Yang and M. H. Huang, J. Am. Chem. Soc., 2012, 134, 1261–1267. 4 J.-Y. Ho and M. H. Huang, J. Phys. Chem. C, 2009, 113, 14159– 14164. 5 C.-H. Kuo, Y.-C. Yang, S. Gwo and M. H. Huang, J. Am. Chem. Soc., 2011, 133, 1052–1057. 6 C. G. Read, E. M. P. Steinmiller and K.-S. Choi, J. Am. Chem. Soc., 2009, 131, 12040–12041. 7 H.-L. Wu, H.-R. Tsai, Y.-T. Hung, K.-U. Lao, C.-W. Liao, P.-J. Chung, J.-S. Huang, I-C. Chen and M. H. Huang, Inorg. Chem., 2011, 50, 8106–8111. 8 Y. Bi, S. Ouyang, N. Umezawa, J. Cao and J. Ye, J. Am. Chem. Soc., 2011, 133, 6490–6492. 9 M. Leng, C. Yu and C. Wang, CrystEngComm, 2012, 14, 8454– 8461. 10 M. H. Huang and P.-H. Lin, Adv. Funct. Mater., 2012, 22, 14– 24. 11 K. Chanda, S. Rej and M. H. Huang, Chem.–Eur. J., 2013, 18, 16036–16043. 12 K. Chanda, S. Rej and M. H. Huang, Nanoscale, 2013, 5, 12494–12501. 13 J. van Embden, J. Jasieniak and P. Mulvaney, J. Am. Chem. Soc., 2009, 131, 14299–14309. 14 J. Li, S. K. Cushing, J. Bright, F. Meng, T. R. Senty, P. Zheng, A. D. Bristow and N. Wu, ACS Catal., 2013, 3, 47–51. 15 R. T. Tom, A. S. Nair, N. Singh, M. Aslam, C. L. Nagendra, R. Philip, K. Vijayamohanan and T. Pradeep, Langmuir, 2003, 19, 3439–3445. 16 Y. Ma, W. Li, E. C. Cho, Z. Li, T. Yu, J. Zeng, Z. Xie and Y. Xia, ACS Nano, 2010, 4, 6725–6734. 17 D. Y. Kim, K. W. Choi, X.-L. Zhong, Z.-Y. Li, S. H. Im and O. O. Park, CrystEngComm, 2013, 15, 3385–3391. 18 Y.-C. Tsao, S. Rej, C.-Y. Chiu and M. H. Huang, J. Am. Chem. Soc., 2014, 136, 396–404. 19 C.-Y. Chiu and M. H. Huang, Angew. Chem., Int. Ed., 2013, 52, 12709–12713. 20 W.-C. Wang, L.-M. Lyu and M. H. Huang, Chem. Mater., 2011, 23, 2677–2684. 21 C.-H. Kuo, T.-E. Hua and M. H. Huang, J. Am. Chem. Soc., 2009, 131, 17871–17878. 22 Gold: P. B. Johnson and R. W. Christy, Phys. Rev. B: Condens. Matter Mater. Phys., 1972, 6, 4370–4379; . Cu2O: Handbook of Optical Constants of Solids II, ed. E. D. Palik, Academic Press, Waltham, 1991. 23 L. Zhang, D. A. Blom and H. Wang, Chem. Mater., 2011, 23, 4587–4598.
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Facet-dependent optical properties of polyhedral Au–Cu2O core–shell nanocrystals† Yu-Chen Yang,a Hsiang-Ju Wang,b Jennifer Whang,b Jer-Shing Huang,b Lian-Ming Lyu,b Po-Heng Lin,b Shangjr Gwoa and Michael H. Huang*b We fabricated Au–Cu2O core–shell octahedra, cuboctahedra, and nanocubes having sizes of 90–220 nm using 50 nm octahedral cores. The smaller particle sizes minimize the strong light scattering features from the Cu2O shells and enable the surface plasmon resonance (SPR) absorption band of the gold cores to be clearly identified. Beyond a lower shell thickness limit, the SPR band positions of the gold cores are independent of the shell thickness, but are strongly dependent on the exposed particle surfaces. The plasmonic band red-shifts from Au–Cu2O octahedra to cuboctahedra and nanocubes, and differs by as much as 26 nm between the octahedra and the nanocubes. The same facet-dependent optical effects were observed using larger octahedral gold cores and cubic gold cores. In contrast, simulation spectra show progressively red-shifted SPR band positions with increasing shell thickness. The Cu2O shells are
Received 26th November 2013 Accepted 4th February 2014
also found to exhibit facet-dependent optical behavior. These nanocrystals can respond to changes in the solvent environment such as solvents with different refractive indices, indicating that the plasmonic
DOI: 10.1039/c3nr06293g
field of the gold cores can extend beyond the particle surfaces despite the presence of thick shells.
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Plane-selective spectral responses to low concentrations of surfactants were also recorded.
Introduction Recent developments in the syntheses of polyhedral inorganic nanocrystals with excellent shape control have enabled the examination of their facet-dependent catalytic, electrical, and molecular adsorption properties.1–12 Core–shell metal and semiconductor nanocrystals with excellent control of core size and shell thickness are interesting due to their tunable optical properties.13–19 In particular, polyhedral bimetallic and metal– semiconductor core–shell heterostructures can be generated using metallic nanocrystal cores, allowing the investigation of the plane-selective properties of the shell materials in the presence of the metal cores.4,20,21 For example, the Au coreenhanced photodegradation of Au–Cu2O core–shell nanocrystals was only observed for particles exposing suitable Cu2O surface facets.7 The Au cores also enhance the electrical conductivity of single Au–Cu2O core–shell octahedral nanocrystals, but the core–shell nanocubes remain non-conductive.5 Facet-dependent optical properties of polyhedral core–shell nanocrystals are interesting materials properties to examine, but have never been demonstrated or even considered. This is presumably due to the availability of structurally well-dened
a
Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
b
core–shell heterostructures until recently. Previously the Au– Cu2O core–shell nanocrystals synthesized were relatively large in the range of hundreds of nanometers in size that the localized surface plasmon resonance (SPR) absorption band of the gold cores cannot be observed or easily identied. By reducing the nanocrystal sizes to minimize the light scattering effects arising from the Cu2O shells, the SPR band of the gold cores appears and reveals surprising facet-dependent optical responses to the shell morphology and external environment surrounding the particles for the rst time. Here we show that the localized SPR absorption band positions of the Au cores in the Au–Cu2O core–shell heterostructures are highly dependent on the surface facets of the single-crystalline Cu2O shells. Within a considerably large shell thickness range, the SPR band positions are independent of the shell thickness. The results are in sharp contrast to the simulation spectra showing continuously red-shied SPR band positions with increasing particle size. Remarkably, the Au cores can sense changes in the external environment including solvents and surfactants despite the presence of thick shells, and respond with SPR band shis. The facet-dependent SPR responses should be general and observable in other metal–metal oxide systems.
Experimental section
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan. E-mail: [email protected]
Synthesis of Au–Cu2O core–shell nanocrystals
† Electronic supplementary 10.1039/c3nr06293g
The synthesis procedure adopts our previously reported method with slight modications.13,14 H2O, CuCl2 solution, sodium
information
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(ESI)
available.
See
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dodecyl sulfate (SDS) surfactant, Au nanocrystals, NaOH solution, and NH2OH$HCl solution were added in the order listed. The concentrations and amounts of reagents used to prepare the various Au–Cu2O core–shell nanocrystals are summarized in Scheme S1, ESI.† Aer stirring the solution to dissolve SDS completely, the concentrated Au nanocrystal solution was introduced, and the solution was shaken for 10 s. Following the addition of NaOH and NH2OH$HCl solution, the reaction mixture was shaken for 15 s and kept at room temperature for 2 h for shell growth. To remove the surfactant, the solution was washed with deionized water and centrifuged four times at 5000 rpm for 3 min. Finally, the collected products were dispersed in 1.0 mL of absolute ethanol for direct spectral analysis and long-term storage. Sample preparation for spectral analysis To take UV-vis absorption spectra of the core–shell nanocrystals in another solvent, 0.1 mL of the particle-containing ethanol solution was withdrawn and centrifuged. Ethanol was removed and 1.0 mL of the solvent of interest was added. Aer sonication, to disperse the particles, the solution was transferred to a cuvette for spectral measurements. To test the surfactant effect, 0.1 mL of the particle-containing ethanol solution was centrifuged to remove ethanol. Then 1.0 mL of water was introduced, followed by the addition of 0.005 mL of 10 mM surfactant solution. The surfactant concentration is 0.05 mM in this nanocrystal solution. Aer sonication, the solution was transferred to a cuvette for analysis. Instrumentation Scanning electron microscopy (SEM) images of the nanocrystals were obtained using a Zeiss Ultra 55 eld-emission scanning electron microscope. Transmission electron microscopy (TEM) characterization was performed on a JEOL JEM-2100 electron microscope. X-ray diffraction (XRD) patterns were collected using a Shimadzu XRD-6000 diffractometer with Cu Ka radiation. UV-vis absorption spectra were acquired with the use of a JASCO V-570 spectrophotometer. Zeta potential measurements were carried out on a Malvern Zetasizer Nano S90 zeta potential analyzer. Numerical methods We employed a nite-difference time-domain method (FDTD solutions, Lumerical, Canada) to simulate the extinction spectra of our core–shell nanoparticles. Refractive indices of gold and Cu2O are obtained from experimental data.22 The refractive index of the surrounding medium is set to be 1.36 in order to mimic the aqueous solution environment. Boundaries of the simulation area are set to be at least 2000 nm away from the outer surface of the shell such that spurious absorption of optical near elds is avoided. We used a uniform mesh size with discretization of 1 1 1 nm3 to cover the whole core–shell nanoparticles. To simulate the real structures, the edges and corners of the octahedral cores are rounded with cylinders or spheres with a radius of 12 nm. An about 3 femtosecond broadband plane wave source centered at 800 nm was used as a
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source, and the polarization was aligned to the diagonal direction of the octahedral core. All particle dimensions are obtained from the SEM images.
Results and discussion To observe the facet-dependent SPR absorption effects and their invariance to the shell thickness, the Au–Cu2O core–shell nanocrystals need to be synthesized with excellent morphology and size control. Previous studies on the optical properties of Au–Cu2O and metal oxide–Au core–shell nanoparticles all used roughly spherical particles for measurements and modeling, so their facet-dependent SPR properties cannot be observed.23–26 Furthermore, material imperfections such as the polycrystalline nature of the core and/or shell materials and particle shape uniformity may broaden the Au and Cu2O absorption bands. The general observation was that the SPR band of the core shis progressively to red with increasing shell thickness.23,24 In this study, we examined the effects of particle morphology and shell thickness on the optical properties of the Au–Cu2O core–shell nanocubes, cuboctahedra, and octahedra with sizes of less than 200 nm to avoid the strong light scattering features appearing from the visible to the near-infrared region in larger Cu2O particles.3,4,20,21,27 The octahedral Au nanocrystal cores were produced by a hydrothermal synthesis approach.28 By varying the reaction time from 12 to 16 and 20 h, octahedral Au nanocrystals with respective average sizes of 50, 60, and 70 nm were obtained (Fig. S1, ESI†). Their UV-vis absorption spectra show a single SPR absorption band, which is slightly red-shied from 550 to 553 and 560 nm with increasing particle dimensions (Fig. S2, ESI†). The Au–Cu2O core–shell nanocrystals were synthesized by mixing an aqueous solution of CuCl2, SDS, Au nanocrystals, NaOH, and NH2OH$HCl.20,21 By increasing the volume of NH2OH$HCl added, systematic shape evolution of the Cu2O shells from cubic to octahedral structures was achieved. Furthermore, increasing the volume of the concentrated gold nanocrystal solution added to the reaction mixture can gradually reduce the sizes of the Au–Cu2O nanocrystals (see Scheme S1, ESI† for the exact reagent amounts used and Fig. 1 for the determination of particle sizes). Similar size tuning of the Au–Cu2O octahedra prepared using short Au nanorod cores has been reported through the adjustment of the Au core amount.29 Fig. 2 shows SEM images of the Au–Cu2O core–shell nanocubes, cuboctahedra, and octahedra synthesized using 50 nm
Fig. 1 Schematic drawing of the Au–Cu2O core–shell octahedron, cuboctahedron, and cube. The arrows indicate the particle sizes determined in this study.
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SEM images of the synthesized Au–Cu2O core–shell nanocrystals. The particle morphologies are (a–e) cubic, (f–j) cuboctahedral, and (k–o) octahedral in shape. Particle sizes decrease from left to right. The octahedral gold cores have an average size of 50 nm. Insets show enlarged views of the Au–Cu2O core–shell nanocrystals. Scale bar in each inset is equal to 100 nm.
Fig. 2
octahedral Au cores. Particles with ve different size ranges have been prepared for each of these shapes (see Table S1† for the particle sizes and their standard deviations). The nanocrystals have sizes in the range of 90–220 nm. With the exception of broader size distributions for the larger octahedra, the nanocrystals are highly uniform in size and shape, which is critically important for their facet-dependent optical property characterization. The octahedral Au cores are visible due to the
Fig. 3
relatively small Au–Cu2O particle sizes. XRD patterns of the nanocrystals show both Au and Cu2O reection peaks (Fig. 3). Intensities of the Au peaks are weaker than those of the Cu2O peaks because they are located inside the particles. Intensities of the Au and Cu2O (200) peaks are stronger than those of the (111) peaks for the core–shell cubes because their cubic faces are preferentially oriented parallel to the substrate surface. On the other hand, the Au–Cu2O octahedra exhibit a stronger (111)
XRD patterns of the Au–Cu2O core–shell cubes, cuboctahedra, and octahedra.
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peak intensity because of the exposed {111} facets. Uniformly thinner shells for the Au–Cu2O octahedra lead to stronger reection peaks for the Au cores. Energy-dispersive X-ray spectroscopy (EDS) line-scan on a single core–shell nanocube conrms the presence of the gold core (Fig. S3, ESI†). TEM characterization of a single core–shell nanocube, cuboctahedron, and octahedron further veries the exact lattice orientation relationship between the gold cores and the Cu2O shells (see Fig. 4, S4 and S5, ESI†). For example, the selected-area electron diffraction (SAED) pattern of a core–shell cube shows that the Au (200) and Cu2O (200) spots are aligned on the same line, conrming their exact lattice orientation relationship. Furthermore, a high-resolution TEM image of one such particle indicates that the (200) lattice planes of its Au core are aligned parallel to the (200) planes of its Cu2O shell. With decreasing average particle sizes from 218 nm to 110 nm, the dark orange solution color gradually changes to light green and nally dark green or greenish blue (Fig. 5a). This vivid solution color change is mainly associated with the absorption prole evolution of the Cu2O shells. Fig. 5b–d present the UV-vis absorption spectra of the Au–Cu2O core–shell nanocubes, cuboctahedra, and octahedra with the ve different average sizes for each particle shape as shown in Fig. 2. For particle stability reason, spectral analysis was performed in ethanol unless noted. For all the particle shapes, the Cu2O absorption band centered at 490–500 nm for the smallest particles shows a slight red-shi with increasing particle dimensions, accompanied by gradual appearance of the light scattering feature moving toward the infrared region.20,27 This size-dependent spectral prole is characteristic of Cu2O nanocrystals even without the gold cores.27 Because the core–shell nanocrystals are mostly less than 200 nm in size, the interference from the light scattering feature of the Cu2O shells is not so severe, so the SPR absorption band of the gold cores can be easily identied. The high refractive index of Cu2O (2.7 at 800 nm)30 and its high dielectric constant (3 ¼ 7.2)23,25 causes a huge red-shi of the SPR absorption band from 550 nm for the octahedral gold nanocrystals to 752–778 nm for the Au–Cu2O nanocrystals. By comparison, the longitudinal SPR band of the Au nanorod cores in Au–Cu2O nanorods can red-shi by
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470–600 nm depending on the shell thickness.31 Specically, the Au–Cu2O nanocubes, cuboctahedra, and octahedra have xed SPR band positions respectively at 752, 768, and 778 nm. Remarkably, the band positions remain xed for these Au–Cu2O nanocrystals despite signicant changes in their shell thickness. Shell thicknesses vary from 30 to 84 nm for the Au–Cu2O octahedra, 32–64 nm for the cuboctahedra, and 21–66 nm for the nanocubes (or 65–143 nm when measured from the diagonal direction of the cube). The results are in sharp contrast to the previous observations showing that the SPR absorption band shis steadily to red with increasing Cu2O shell thickness.23,24 We found that a minor spectral red-shi can indeed be observed in smaller Au–Cu2O nanocubes with the SPR band shiing from 769 nm for the 81 nm nanocubes to 778 nm for nanocubes larger than 90 nm (Fig. S6†). Thus, beyond a lower shell thickness limit of 20–30 nm, Au–Cu2O polyhedra with thicker shells should exhibit xed SPR absorption band positions. Because of the large refractive index of Cu2O, increasing shell thickness means that the Au cores experience a surrounding medium with increasing refractive index and respond by shiing the SPR band to longer wavelengths.32 When the shell thickness is sufficiently thick, the Au cores will see the surrounding medium as the same despite changes in the shell thickness, so the SPR band remains xed in position. One should note that for the SPR band position to be invariant with shell thickness of just beyond 20–30 nm, the shells need to shield the cores completely; the solvent can come into contact with the cores if the cores are not completely insulated and change the effective refractive index of the surroundings. Similar shell thickness-independent SPR wavelength shis have been observed for silica-coated silver nanoparticles with shell thicknesses beyond 30 nm.33 In contrast to the amorphous silica shells, here we show that the same phenomenon can also be observed for particles with single-crystalline oxide shells. The most striking feature these spectra reveal is of course that the gold SPR wavelength shis widely from octahedra to nanocubes by as much as 26 nm. Cuboctahedra displayed an intermediate band position. Since all three particle shapes have overlapping shell thicknesses, shell thickness differences cannot explain the observed large SPR band shis. The results clearly indicate that
Fig. 4 TEM analysis of a single Au–Cu2O core–shell nanocube. (a) TEM image of a Au–Cu2O nanocube viewed along the [100] direction. The white dotted line indicates the outline of the octahedral Au nanocrystal core. (b) SAED pattern of this nanocube giving both Au and Cu2O diffraction spots. (c) High-resolution TEM image of the red square region. The red 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. Insets show a schematic drawing of the particle viewed along the [001] zone axis.
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Fig. 5 (a) Photograph of the octahedral Au–Cu2O core–shell nanocrystal solutions with decreasing particle sizes. (b–d) UV-vis absorption spectra of the (b) octahedral, (c) cuboctahedral, and (d) cubic Au–Cu2O core–shell nanocrystal solutions with different particle sizes. 50 nm octahedral Au cores were used.
the SPR band positions are dependent on the morphology, and hence the surface facets, of the Cu2O shells. Because cuboctahedra have both the {100} and {111} faces with similar surface areas, it makes sense that these particles show an intermediate SPR band position. Amazingly, in addition to the drastic facetdependent photocatalytic and electrical properties of Au–Cu2O core–shell nanocubes and octahedra, their optical properties are also strongly facet-dependent.5,20 The same facet-dependent SPR absorption phenomenon was also observed in Au–Cu2O core–shell cubes and octahedra synthesized using 60 nm octahedral gold nanocrystal cores (Fig. 6a and b). Particle size distributions are also narrow for these nanocrystals (Table S2, ESI†). With larger gold cores, the SPR band positions of the Au–Cu2O octahedra and nanocubes red-shi to 777 and 800 nm, respectively. However, when 70 nm octahedral gold cores were used, the SPR absorption bands of the Au–Cu2O octahedra and nanocubes show a progressive redshi with increasing particle sizes (Fig. 6c and d). For nanocubes and octahedra with similar dimensions, the nanocubes also display more red-shied SPR bands than the octahedra do. The band to the shorter wavelength side of the SPR band (i.e., the band at 626 nm for the 105 nm Au–Cu2O octahedra sample) is probably part of the light scattering feature from the Cu2O shells. The spectral results obtained are not necessarily in conict with those for the Au–Cu2O nanocrystals synthesized using smaller gold cores. The magnitudes of spectral red-shi become smaller with increasing particle sizes, suggesting that xed SPR band positions can still be observed for these Au– Cu2O nanocrystals with sufficiently large particle sizes (Fig. 6e and f). Larger particles were not prepared due to consideration of strong interference from the light scattering bands of Cu2O. It is important to recognize that the magnitude of the SPR band red-shi is relatively mild (50 nm red-shi with the shell thickness increasing from 17.5 nm to 62 nm for the octahedral
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shell case) compared to that seen in Au–Cu2O nanorods with different Cu2O shell thicknesses (135 nm red-shi with the shell thickness increasing from 17 nm to 65 nm).31 We tried to make
Fig. 6 (a and b) UV-vis absorption spectra of the Au–Cu2O octahedra (a) and nanocubes (b) with 60 nm Au cores. (c and d) UV-vis absorption spectra of the Au–Cu2O octahedra (c) and nanocubes (d) with 70 nm Au cores. (e and f) Plots summarizing the SPR band shifts of (e) Au– Cu2O octahedra and (f) nanocubes with respect to the particle sizes. The error bars give standard deviations of the particle sizes in the samples. The dash lines are the best-fit lines of the data points.
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Au–Cu2O nanocrystals with octahedral Au core sizes between 60 and 70 nm and see when the Au SPR band started to show a slight red-shi. To show that the same facet-dependent SPR properties can still be seen by using other Au cores, gold nanocubes were used to form Au–Cu2O nanocubes and octahedra (Fig. S7, ESI†). The gold nanocubes were synthesized following a seed-mediated growth approach.34 The SPR band of the 47 nm gold nanocubes was at 541 nm. The SPR band of the Au–Cu2O corner-truncated octahedra was xed at 705 nm with varying shell thickness, while that of the Au–Cu2O nanocubes was at 736 nm with different particle sizes. These results demonstrate that the facet-dependent and shell thicknessindependent optical properties of Au–Cu2O nanocrystals, and likely other metal–Cu2O core–shell nanoparticles, can be observed in other polyhedral samples with different core sizes and shapes, as long as shells with a sufficiently large thickness can be synthesized. However, the experimental data suggest that use of smaller cores makes it easier to observe these novel plasmonic properties. We also performed rigorous numerical simulations in order to gain insight into the localized SPR behavior of the core–shell nanocrystals under purely electromagnetic consideration. We employed the nite-difference time-domain (FDTD) method to simulate the extinction spectra of the Au–Cu2O core–shell nanocrystals with 50 nm octahedral gold cores and exactly the same shell thicknesses of the synthesized particles. Fig. 7a gives
Fig. 7 (a) Simulation results for UV-vis absorption spectra of the octahedral Au–Cu2O core–shell nanocrystal solutions with different particle sizes. (b) Simulated peak positions of localized surface plasmon resonance of Au–Cu2O core–shell nanoparticles with cubic (black squares), cuboctahedral (red dots), and octahedral (green triangles) shells. The black, red and green dashed lines indicate the localized SPR peak positions of the corresponding particles obtained from the experiment.
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the simulation results for the octahedral Au–Cu2O nanocrystal solutions with different particle sizes. The simulation spectra for the octahedral gold cores, as well as the spectra for the cuboctahedral and cubic nanocrystal solutions are shown in Fig. S8, ESI.† The SPR band position at 550 nm for the gold cores is in good agreement with the experimental data. In addition, Fig. S9, ESI† displays the simulated electric near-eld intensity distributions of the cubic, cuboctahedral, and octahedral Au–Cu2O particles, showing the dipolar resonance feature of the surface plasmon resonance of the gold core. Having conrmed the localized SPR of the gold cores, we further simulated the spectra of various Au–Cu2O nanocrystals. Real dimensions obtained from the SEM images of the whole particles were used in the simulations. In purely linear electromagnetic FDTD simulations, we do not take into account any chemical properties but the refractive index of the bulk material. The SPR band positions of all Au–Cu2O core–shell nanocrystals show distinct dependence on the shell thickness, as summarized in Fig. 7b. The reason that cubes are more redshied than cuboctahedra and octahedra of similar sizes (for example, comparing 113 nm cubes, 115 nm cuboctahedra, and 110 nm octahedra) is because cubes have a much larger shell thickness as measured from the diagonal distance. While the experimental resonance position stays constant beyond a particle size of 90 nm, simulated resonance positions of all particles red-shi continuously with the shell thickness. The discrepancy between the simulated and experimental data suggests that the shell does not behave merely like a thin layer of the bulk dielectric material of Cu2O. As we have observed, the localized SPR of the core can “feel” the difference in the surrounding shell surfaces, which is the only difference between the three particle shapes. Clearly, the simulations cannot yield reasonably matched spectra, because the effects of surrounding solid medium with different surfaces have not been taken into account in conventional simulation methods. The results further show that the observed facet-dependent optical properties are truly new phenomena, and simulations need to be modied in order to generate correct and useful spectra. The reason facet-dependent optical properties are observed is because the {100} and {111} faces of Cu2O have very different surface properties.4,5,20 Such surface properties have not been considered previously in the explanation and modeling of SPR band positions of Au–Cu2O core–shell nanocrystals. Cu2O has a body-centered cubic crystal structure with copper atoms occupying half of the tetrahedral sites in the unit cell. The {100} faces of Cu2O are oxygen atom-terminated and more negatively charged, while the {111} faces have more surface copper atoms exposed with unsaturated coordination and are more positively charged (Fig. 8). As a demonstration, Au–Cu2O nanocubes and octahedra immersed in a negatively charged methyl orange solution showed very different behaviors. Octahedra can be well dispersed in this solution, but nanocubes were found to oat to the top of the solution aer stirring (Fig. S10, ESI†). Electrostatic repulsive interactions between the negatively charged surfaces of the Au–Cu2O core–shell nanocubes and anionic methyl orange can explain this phenomenon. The measured
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Fig. 8 Crystal models of the different planes of Cu2O. The models show (a) (100) and (b) (111) surfaces of Cu2O. The fully exposed surface copper atoms are highlighted with orange circles.
zeta potentials for the Au–Cu2O octahedra and nanocubes are +3.37 mV and 8.23 mV, respectively, supporting the different surface charge states of the {111} and {100} faces of Cu2O (Fig. S11, ESI†). In addition to the surface charges, we have previously shown that the {100} facets of Cu2O nanocrystals are not electrically conductive, while the {111} facets are highly electrically conductive.5 This may be due to different ways of band bending near different surfaces of Cu2O crystals, such that electrons can easily pass through the {111} faces when they come into contact with tungsten probes, but a large potential barrier exists at the edge of the {100} face, such that electron transport is much more difficult. This difference in the band structure at the particle surfaces can be considered as band structure tuning, and effectively makes different crystal faces act like different materials with different refractive indices. The plasmonic eld of the Au cores traveling to particle edges meets a boundary with different refractive indices, and responds to these differences by shiing the SPR band positions. This explanation is new but should be considered to account for the facet-dependent electrical and optical properties recorded. That is, facets can affect both electrical and optical properties of a material, but this possibility has never been considered because of the lack of a system to demonstrate this. Since the facet-dependent plasmonic effects are derived from Cu2O, Cu2O nanocrystals themselves should exhibit this intrinsic facet-dependent optical property. Upon close examination of the Cu2O band positions shown in Fig. 5, 6 and S7, ESI,† cubes are generally more red-shied than octahedra for particles of comparable sizes. TEM images presented in Fig. S7, ESI† show that the Au–Cu2O truncated octahedra are appreciably larger than cubes, but the Cu2O absorption band at 488 nm for the 104 nm cubes are still more red-shied than the 118 nm truncated octahedra at 477 nm. The size effect cannot explain the observed spectral results, because larger particles are expected to show a more red-shied extinction band. Thus pristine Cu2O nanocrystals should also possess facet-dependent optical properties. It is reasonable to conclude that surfacerelated refractive index differences give rise to the observed facet-dependent SPR band shis. A more red-shied SPR band of Au–Cu2O nanocrystals is accompanied by a more red-shied Cu2O absorption band, and vice versa. The fact that the Cu2O absorption band appears at 460 nm in the simulation spectra for the smallest cubes, cuboctahedra, and octahedra, as compared to their recorded positions at 486, 489, and 494 nm (Fig. 5), also supports the proposition that surface facets, in
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addition to the particle size, shape, and composition, should be considered to accurately present UV-vis spectra of Cu2O nanocrystals. The facet-dependent optical behavior of Cu2O crystals can also explain why their spectral band positions are changing and not so predictable. Our examination of the effect of external environment on the optical properties of the Au–Cu2O nanocrystals reveals that these particles can respond to solvent changes by shiing the SPR band positions. Fig. 9 gives UV-vis absorption spectra of the 110 nm Au–Cu2O core–shell octahedra dispersed in ethanol, dichloromethane, and toluene with very different polarity values. With increasing solvent refractive index, the SPR band shis progressively to red. Ethanol and water (n ¼ 1.33) have similar refractive indices, so almost identical UV-vis absorption spectra were recorded in these solvents (Fig. S12, ESI†). The magnitude of the spectral shi is about 77 nm per RIU (refractive index unit). Although the refractive index sensitivity of the Au–Cu2O nanocrystals is far less than gold nanostructures of different shapes such as nanoprisms,35 the measurable band shis show that these nanocrystals can still sense solvent refractive index changes despite the presence of thick shells. Alternatively, the SPR band becomes red-shied when dispersing the core–shell particles in solvents with lower dielectric constant values (24.5, 8.93, and 2.38 respectively for ethanol, dichloromethane, and toluene). The fact that facetdependent SPR properties are maintained for both polar and non-polar solvents eliminates the possibility of particle surface charge-induced solvent reorganization, and supports the proposition that the Cu2O surface facets can tune the SPR band positions. The exhibited solvent sensitivity suggests that the surface plasmon elds of the gold cores can actually extend beyond the boundary of the shell surfaces into the immediate external region surrounding the particles. Thus, the SPR band locations of these Au–Cu2O nanocrystals are determined by a combination of the shell thickness, its refractive index (or dielectric constant), exposed facets, and the external solution environment. Remarkably, the SPR sensitivity to environmental conditions can reach beyond a shell thickness of 60 nm and likely more than 100 nm. The discovery of changing the solution environment to tune SPR band positions offer opportunities to use these Au–Cu2O nanocrystals for plane-selective molecular sensing and
Fig. 9 UV-vis absorption spectra of the Au–Cu2O core–shell octahedra dispersed in different solvents. (a) UV-vis absorption spectra of 110 nm Au–Cu2O core–shell octahedra in ethanol, dichloromethane, and toluene. (b) SPR band positions of Au–Cu2O core–shell octahedra and cubes with respect to the refractive indices of the solvents. The nanocubes have sizes of 90 nm.
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101-2113-M-007-002-MY2 and NSC 101-2628-M-007-006). JSH thanks the support from the Center for Nanotechnology, Materials Sciences, and Microsystems at National Tsing Hua University.
References Fig. 10 UV-vis absorption spectra of (a) the Au–Cu2O core–shell octahedra and (b) nanocubes dispersed in an aqueous solution containing 0.05 mM of SDS, CTAB, or SDBS surfactant. At this surfactant concentration, the octahedra only respond to the presence of SDBS, while nanocubes only respond to the presence of CTAB. The octahedra have an average size of 110 nm, while the average nanocube size is 92 nm. The Au cores are 50 nm in size.
adsorption. The analysis is similar to the spectral shi of gold nanoparticles to the dielectric environment changes aer molecular adsorption or bonding,36 but here a thick Cu2O shell separates the analytes and the interior gold nanocrystal. Since SDS has been shown to adsorb preferentially onto the {111} planes of Cu2O possibly through electrostatic interactions,6 different surfactants including cetyltrimethylammonium bromide (CTAB) and sodium dodecyl benzene sulfonate (SDBS) have been added to investigate their ability to shi the SPR band of the Au–Cu2O nanocrystals (Fig. 10). At a low surfactant concentration of 0.05 mM, octahedra only respond to the presence of SDBS (6 nm blue-shi), while nanocubes only respond to the presence of CTAB (4 nm blue-shi). The blueshi actually results from instability of Cu2O crystals in large quantities of the surfactant; shell etching occurs at high surfactant concentrations as seen for SDS (Fig. S13, ESI†). Nevertheless, the observed SPR band shis show that surfactant adsorption may be detected.
Conclusion In summary, the facet-dependent SPR absorption properties of Au–Cu2O core–shell nanocrystals have been demonstrated for the rst time. Beyond a lower shell thickness limit, the SPR band positions of the core–shell nanocrystals are independent of the shell thickness, but shi progressively to the long wavelength side from octahedra to cuboctahedra and then cubes. In contrast, simulations yield continuously red-shied SPR band positions with increasing shell thickness. The Cu2O shells should also exhibit facet-dependent optical behavior. The plasmonic eld of the interior gold cores extends beyond the surfaces of the nanocrystals and can respond to changes in the solvent environment. This work raises the possibility that such facet-dependent plasmonic effects can be observed in other materials.
Acknowledgements We thank the National Science Council of Taiwan for the support of this work (NSC 98-2113-M-007-005-MY3, NSC
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