ARTICLE pubs.acs.org/JPCC
Interior Structural Tailoring of Cu2O Shell-in-Shell Nanostructures through Multistep Ostwald Ripening Li Zhang and Hui Wang* Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208, United States
bS Supporting Information ABSTRACT: We report a multistep Ostwald ripening approach through which the interior multilayer structures of a Cu2O shell-in-shell nanoparticle can be controllably tailored. Using this method, we can fine-control several important geometrical parameters of Cu2O multilayer nanoshells, such as the number of layers, the thickness of each shell, and the intershell spacing, to systematically fine-tune the synergistic optical properties of the particles over a broad spectral range in the visible and near-infrared regions. We have performed Mie scattering theory calculations to interpret the origin of the complex, multipeaked extinction spectral line shapes and geometry-dependent optical tunability of the Cu2O multilayer nanoshells.
’ INTRODUCTION Hollow micro/nanostructures, also known as nanoshells or nanocapsules, have attracted tremendous attention because of their interesting geometry-dependent optical, electronic, and surface properties.1 By judiciously tailoring the shell dimensions and structures, desired properties can be selectively implemented into the nanoshell-based material systems to optimize a series of important physical and chemical processes, such as light matter interactions,2 drug delivery,3 catalysis,4 molecular sensing,5 and energy storage and conversion.6 Over the past two decades, significant progress has been made in the geometry-controlled fabrication of hollow micro/ nanostructures with the goal of achieving highly tunable properties without changing the material composition.1,7 A more recent trend in this field is to construct hollow micro/nanostructures with increasing structural complexity by creating multilevel interior architectures,8 such as yolk shell nanoparticles,9 multilayer nanoshells,2,10 and particles with interior nanochamber structures.11 These multilevelarchitectured hollow nanostructures possess multiple geometrical parameters that one can adjust, either individually or simultaneously, to further fine-tune the synergistic properties of the materials. Here, we report a multistep Ostwald ripening approach through which the interior multilayer structures of a Cu2O shell-in-shell nanoparticle can be controllably tailored. Cu2O is an important p-type semiconductor with interesting optical, electronic, and surface properties appealing for photovoltaic12 and photocatalytic applications.13 Because of the fact that Cu2O nanostructures may exhibit geometrically tunable properties, immense efforts have been devoted to the shape control of Cu2O nanostructures, such as spheres,14 wires,15 cubes,16 and polyhedra.17 In comparison to nanoparticles with solid interior structures, Cu2O hollow nanostructures may exhibit superior properties and further enhanced tunability. However, the fabrication of Cu2O nanoparticles with well-tailored r 2011 American Chemical Society
hollow interior structures has been significantly more challenging. Although Cu2O single-layer and multilayer nanoshell structures have been fabricated using various hard or soft templates,18 methods for the fine-control over geometrical parameters are still needed in order to develop a detailed, quantitative understanding of the structure property relationship of Cu2O nanoshell structures. Ostwald ripening has been demonstrated to be an important chemical approach to the creation of various symmetric and asymmetric hollow nanostructures.6,19 Recently, we reported that polycrystalline Cu2O nanospheres underwent a symmetric hollowing process at room temperature due to Ostwald ripening through which solid spherical particles were controllably converted into thick nanoshells, thin nanoshells, and eventually collapsed shell structures.20 This one-step Ostwald ripening process provides a unique approach for us to fine-control the shell thickness and thereby to fine-tune the optical properties of Cu2O single-layer nanoshells20 (also see Figure S1 in the Supporting Information). Inspired by this interesting phenomenon, we have been able to develop a multistep Ostwald ripening approach for the geometry-controlled fabrication of Cu2O particles with multilayered shell-in-shell interior structures. Using this method, we can fine-control several important geometrical parameters of Cu2O multilayer nanoshells, such as the number of layers, the thickness of each shell, and the intershell spacing, to systematically fine-tune the synergistic optical properties of the particles over a broad spectral range in the visible and near-infrared regions. We have performed Mie scattering theory calculations to theoretically interpret the origin of the extinction spectral line-shape complexity and geometry-dependent optical tunability of the Cu2O multilayer nanoshells that we have experimentally observed. Received: June 24, 2011 Published: August 02, 2011 18479
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Figure 1. (A) Schematics illustrating the formation of Cu2O double-layer nanoshells through a two-step Ostwald ripening process. TEM images of (B) single-layer nanoshells obtained at 40 min during first-step Ostwald ripening and double-layer nanoshells obtained at various reaction times during the second-step Ostwald ripening: (C) 1, (D) 40, (E) 60, (F) 80, and (G) 150 min. High-magnification TEM images and SAED patterns (insets) taken on individual particles obtained at (H) 40 min during the first-step Ostwald ripening and (I) 1, (J) 40, (K) 60, (L) 80, and (M) 150 min during the second-step Ostwald ripening. TEM images in the middle and bottom rows share the same scale bars as those in panels B and H, respectively.
’ EXPERIMENTAL SECTION Materials. Polyvinylpyrrolidone (PVP) (average MW = 58 000) and Cu(NO3)2 were purchased from Alfa Aesar. Hydrazine solution (N2H4 3 H2O solution, 35 wt %) was purchased from Sigma-Aldrich. Ethanol (200 proof) was purchased from Fisher Scientific. Ultrapure water (18.2 MΩ resistivity, Barnstead EasyPure II 7138) was used in these experiments. Fabrication of Cu2O Single-Layer Nanoshells. A 1.0 g portion of PVP was added to 50 mL of 0.01 M Cu(NO3)2 aqueous solution under rapid magnetic stirring (300 rpm). The mixture was kept stirring for several minutes until the powders were completely dissolved. N2H4 solution (17 μL) was then introduced into the mixture solution. A colloidal suspension of Cu2O particles, which was orange in color, formed immediately after the introduction of N2H4 within ∼15 s. The resulting colloidal solution was kept stirring under ambient air at room temperature for Ostwald ripening. During the Ostwald ripening process, we typically withdrew aliquots of colloidal solutions at certain reaction times from the reaction mixture, then immediately centrifuged the particles, washed them three times with water and ethanol, and finally redispersed them in ethanol. The asfabricated Cu2O nanoshells had average outer diameters around 400 nm and were observed to be stable without any structural or compositional change over several months if they were stored in ethanol either at room temperature or at 4 °C in a refrigerator. Fabrication of Cu2O Double-Layer Nanoshells. Cu2O singlelayer nanoshells were fabricated using the method described above. After the Cu2O single-layer nanoshells underwent Ostwald ripening for a certain period of time, for example, 20, 40, or 70 min, another 17 μL of N2H4 solution was added into the mixture to initiate a shell growth process, which gave rise to the formation of thicker nanoshells with an increased overall outer dimension. The shell growth was observed to be a fast reaction process that typically took less than 10 s. The particles were then kept in this reaction mixture under magnetic stirring (300 rpm) at room temperature for a second-step Ostwald ripening, during which both the inner and the outer shells would become thinner gradually, leading to the formation of doublelayer nanoshell structures. The Cu2O double-layer nanoshells were separated from the reaction mixture at different times during the second-step Ostwald ripening process through multiple cycles of centrifugation and final redispersion in ethanol.
Fabrication of Triple-Layer and Quadruple-Layer Nanoshells. By repeating the shell growth and Ostwald ripening
processes multiple times, multilayer nanoshells with more than two layers could be obtained. We introduced 17 μL of N2H4 solution into the reaction mixture at a reaction time of 30 min during the secondstep Ostwald ripening process to initiate a shell growth process on the Cu2O double-layer nanoshells. After the shell growth, the particles were kept in the reaction mixture under magnetic stirring (300 rpm) for a third-step Ostwald ripening. In this way, Cu2O triple-layer nanoshells with varying shell thicknesses were controllably fabricated. To obtain Cu2O quadruple-layer nanoshells, 28.3 μL of N2H4 solution was added at a reaction time of 20 min during the thirdstep Ostwald ripening. After the shell growth, the particles were kept in the reaction mixture for a fourth-step Ostwald ripening through which Cu2O quadruple-layer nanoshells with controllable shell thicknesses were obtained. The Cu2O triple-layer and quadruplelayer nanoshells were separated from the reaction mixtures through multiple cycles of centrifugation and redispersion in ethanol. Characterizations. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were taken using a Hitachi H-8000 transmission electron microscope operated at an accelerating voltage of 200 kV. For TEM and SAED measurements, all samples were dispersed in ethanol, drop-cast, and dried on 200-mesh Formvar/carbon-coated Cu grids. Powder X-ray diffraction (PXRD) measurements were performed at room temperature using a Rigaku D/Max 2100 Powder X-ray Diffractometer (Cu KR1 radiation) with a diffracted beam graphite monochromator. The extinction spectra of Cu2O particles (colloidal particles dispersed in water) were measured using a Beckman Coulter Du 640 spectrophotometer at room temperature.
’ RESULTS AND DISCUSSION As schematically illustrated in Figure 1A, we started from Cu2O single-layer nanoshells obtained from the first-step Ostwald ripening process. Upon the introduction of additional reactants into the reaction mixture, more Cu2O nanocrystallites were produced and we could further assemble and pack these newly generated nanocrystallites both inside and outside the first nanoshell layer to form a thicker nanoshell with an increased overall outer dimension. When a second Ostwald ripening process occurred, both the inner 18480
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Figure 2. Powder XRD patterns of Cu2O single-layer nanoshells obtained at (a) 10 and (b) 40 min during the first-step Ostwald ripening and double-layer nanoshells obtained at (c) 10 and (d) 40 min during the second-step Ostwald ripening process. The standard diffraction pattern for cubic-phase Cu2O with the lattice constant a = 4.258 Å (JCPDS card no. 05-0667) is also included at the bottom for comparison.
and the outer shells would become thinner gradually, leading to the formation of double-layer nanoshell structures. The inner shell might also be movable inside the outer shell, giving rise to the formation of nonconcentric double-shell structures when the intershell spacing further increased. Because the particles underwent an inside-out hollowing process during Ostwald ripening, the outer diameters of both the inner and the outer shell remained unchanged during Ostwald ripening while the inner diameters of the inner and outer shells both increased progressively until the outer shell began to collapse on the outer surface of the inner shell when a thin shell limit was reached. Although the Ostwald ripening of Cu2O particles under the current experimental conditions was observed to be a continuous process, it could be effectively inhibited once the particles were separated from the reaction mixture through centrifugation and redispersion in ethanol. In this way, we could stop the Ostwald ripening process at any time spot during the whole process to obtain Cu2O double-layer nanoshells with fine-controlled inner and outer shell thicknesses. The TEM images shown in Figure 1B M clearly reveal the structural evolution of Cu2O double-layer nanoshells during Ostwald ripening. The SAED patterns obtained on individual particles (insets in Figure 1H M) indicate that these nanoshells are polycrystalline in nature and consist of cubic-phase Cu2O nanocrystals with the lattice constant a = 4.258 Å (JCPDS card no. 05-0667). The cubic-phase crystalline structures of Cu2O have also been verified by PXRD measurements, as shown in Figure 2. No detectable diffraction peaks corresponding to either metallic Cu or cupric oxide (CuO) were observed. The PXRD peaks are significantly broadened largely due to the small size of the primary Cu2O nanocrystals in the shells and the sharpening of the PXRD peaks as Ostwald ripening proceeds indicates the ripening of the Cu2O nanocrystals in the shells. In addition to the control over reaction time, the relative inner and outer shell thicknesses of the double-layer nanoshells can be further adjusted by starting the second-step Ostwald ripening process from Cu2O single-layer nanoshells with different shell thicknesses (see Figures S2 and S3 in the Supporting Information). Through fine-control over both the reaction time and the shell dimensions of
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Figure 3. (A) Extinction spectra of Cu2O particles obtained at reaction times of 1 and 40 min during the first-step Ostwald ripening process. The Cu2O single-layer nanoshells obtained at 40 min were used to initiate the second-step Ostwald ripening for the fabrication of doublelayer nanoshells. (B) Extinction spectra of Cu2O particles obtained at different reaction times during the second-step Ostwald ripening.
the starting single-layer nanoshells, we could fine-tune the dimensions of both the inner and the outer shells over a broad size regime, enabling the fine-tuning of the synergistic optical properties of the particles over a broad spectral range. We have previously demonstrated,20 in great detail, that the light absorption and scattering properties of Cu2O single-layer nanoshells can be systematically tuned in the visible region by tailoring the shell thickness (also see Figure S1 in the Supporting Information). As shown in Figure 3, the as-fabricated Cu2O doublelayer nanoshells exhibit even more complicated optical signatures in their extinction (absorption + scattering) spectra and further enhanced optical tunability in comparison to the Cu2O single-layer nanoshells. During the first-step Ostwald ripening process, the extinction spectral features of Cu2O single-layer nanoshells progressively blue shifted as the shell thickness decreased. Upon the introduction of additional hydrazine into the reaction mixture at a reaction time of 40 min, the extinction maximum red shifted back toward longer wavelengths with additional multipeaked spectral features showing up in the visible and near-infrared regions due to the increase in both the shell thickness and the overall particle size. During the second-step Ostwald ripening, all the spectral features progressively blue shifted as the thicknesses of both the inner and the outer shell decreased. The multipeaked features in the extinction spectra arise from the appearance of higher-order modes at Mie scattering resonances due to phase-retardation effects when the particle size becomes comparable to the wavelength of incident light.21 Both the peak positions and the overall line shapes of the extinction spectra were observed to be sensitively dependent upon both the inner and the outer shell dimensions (see Figure 3B and Figures S2 and S3 in the Supporting Information), providing unique spectral fingerprints for the correlation between the optical characteristics and the geometrical parameters of Cu2O double-layer nanoshells. The observed multipeaked extinction line shapes and 18481
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Figure 4. (A) Geometry of double-layer Cu2O nanoshells employed for Mie scattering theory calculations. (B) Calculated extinction, absorption, and scattering spectra of a Cu2O double shell with geometrical parameters of [R1, R2, R3, R4] = [140, 200, 260, 300] nm. (C) Calculated dipolar (l = 1), quadrupolar (l = 2), octopolar (l = 3), and higher-order multipolar (l = 4, 5, 6, 7, 8) scattering (upper panel) and absorption (lower panel) modes of a Cu2O double-shelled particle with geometrical parameters of [R1, R2, R3, R4] = [140, 200, 260, 300] nm. (D) Calculated extinction spectra of a concentric Cu2O double-shelled particle with (upper panel) [R2, R3, R4] = [200, 260, 300] nm and varying R1 and (lower panel) [R1, R2, R4] = [140, 200, 300] nm and varying R3 as indicated in the figure.
geometry-dependent tunability are apparently the synergistic optical responses from the Cu2O double-layer nanoshells rather than from the primary Cu2O nanocrystals in the shells. For Cu2O nanocrystals smaller than 20 nm, their extinction is dominated by absorption rather than scattering and only an absorption edge below ∼450 nm is typically displayed in their extinction spectra.20,22 To gain further insights into the origin of the spectral complexity and tunability of Cu2O double-layer nanoshells, we have calculated the light absorption, scattering, and extinction properties of individual double-layer nanoshells with varying geometrical parameters using Mie scattering theory.23 Figure 4A shows the geometry of the particle we calculated, which is a double-shell particle with spherically concentric, alternating dielectric and Cu2O layers. We used the experimentally measured frequency-dependent, complex empirical dielectric function for bulk Cu2O24 and the dielectric media inside the core, between the Cu2O shell layers, and the surrounding outer surfaces of the particles were all assumed to be water (refractive index of 1.33). The calculated extinction was expressed as an efficiency, which was the ratio of the energy scattered or absorbed by the particle to the energy incident on its physical cross section. Figure 4B shows the calculated extinction, absorption, and scattering spectra of a double-layer Cu2O nanoshell with geometrical parameters of [R1, R2, R3, R4] = [140, 200, 260, 300] nm. In qualitative agreement with the experimental data, the calculated extinction spectrum also shows multipeaked spectral features in the visible and near-infrared. The extinction of this submicrometer-sized multilayer sphere is dominated by scattering rather
than absorption in visible and near-infrared regions. Only within the spectral region where Cu2O’s interband transitions occur (λ < 700 nm) can a significant increase in the absorption efficiency be observed. As shown in Figure 4C, the complex, multipeaked extinction line shape can be further decomposed into scattering and absorption of dipolar (l = 1), quadrupolar (l = 2), octupolar (l = 3), and even higher-order multipolar resonance modes (l > 3), each of which has its own characteristic resonance frequencies and line shapes and, accordingly, contributes to the complexity of the overall extinction line shape. Mie scattering theory calculations clearly show that the synergistic optical responses of Cu2O double-layer nanoshells are sensitively dependent on the dimensions of both the inner and the outer shells. The upper panel of Figure 4D shows the calculated extinction spectra of Cu2O double-layer nanoshells with varying R1 and fixed R2, R3, and R4. While the extinction efficiency does not change much as the thickness of the inner shell varies, the frequencies of the multipolar resonances progressively red shift as the inner shell thickness increases, introducing significant modifications to the peak positions and overall line shape of the extinction spectra. The lower panel of Figure 4D shows the calculated extinction spectra of Cu2O double-layer nanoshells with varying R3 and fixed R1, R2, and R4. As the thickness of the outer shell increases, all the extinction peaks red shift and the extinction efficiencies increase progressively. The results of Mie scattering theory calculations provide a clear picture of the correlation between the particle geometry and the optical signatures of the Cu2O double-layer nanoshells. 18482
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Figure 5. TEM images of (A) Cu2O triple-layer nanoshells obtained at a reaction time of 40 min during the third-step Ostwald ripening process and (B) Cu2O quadruple-layer nanoshells obtained at a reaction time of 40 min during the fourth-step Ostwald ripening process. The insets show high-magnification TEM images and SAED patterns taken on individual particles.
Both the experimentally observed multipeaked line shape and geometry dependence have been well-reproduced in the calculated extinction spectra. Some discrepancies between the calculated and the experimental spectra, however, have also been observed largely due to several possible reasons. The geometries of Cu2O employed in the Mie theory calculations are perfectly smooth, homogeneous, and spherically symmetric double-layer nanoshells; however, the as-fabricated Cu2O nanoshells are inhomogeneous and polycrystalline in nature with a large number of Cu2O nanocrystallites randomly oriented and packed inside each nanoshell layer. Such structural discontinuity and nonideality may introduce modifications to the dielectric functions of the Cu2O nanoshells in comparison to that of the single-crystalline bulk Cu2O that we used for these calculations. In addition, inner and outer shells are apparently nonconcentric and the shell offset becomes especially pronounced when the intershell spacing is relatively large. The inner and outer shells may even be in direct contact when the surfaces of the shells touch. Furthermore, inhomogeneous broadening due to polydispersity of the Cu2O particles may introduce further modifications to the overall line shape of the spectral features. We have further fabricated Cu2O triple-layer and quadruplelayer nanoshell structures through three-step and four-step Ostwald ripening processes, respectively. The multilayered interior structures of the particles can be resolved by TEM images, as shown in Figure 5. TEM images revealing the detailed structural evolution of the triple-layer and quadruple-layer nanoshells during Ostwald ripening are shown in Figures S4 and S5, respectively, in the Supporting Information. In both cases, we observed that, as Ostwald ripening proceeded, the thickness of each nanoshell layer progressively decreased and the number of layers decreased by one when the outermost shell collapsed on the outer surface of the inner shell. The triple-layer and quadruple-layer nanoshells exhibited a further increased line-shape complexity and further red shifted spectral features in their extinction spectra in comparison to the double-layer nanoshells.
Figure 6. Histograms showing the size distribution (left column) and the fraction of single-shell, double-shell, triple-shell, and quadruple-shell (right column) of Cu2O particles obtained at a reaction time of 40 min during (A, B) first-step, (C, D) second-step, (E, F) third-step, and (G, H) fourth-step Ostwald ripening processes.
Despite spectral complexity, the progressive blue shift of the extinction peaks of triple-layer and quadruple-layer nanoshells during Ostwald ripening could be clearly observed (see Figures S4 and S5 in the Supporting Information). In principle, multilayer nanoshells with an arbitrary number of layers can be obtained by simply repeating multiple steps of shell growth and Ostwald ripening. However, practically, the maximum number of nanoshell layers we could controllably fabricate was limited by the propagation of particle polydispersity. As shown Figure 6A, the single-layer nanoshells obtained at 40 min during the first-step Ostwald ripening exhibit a bimodal size distribution with a major peak at ∼400 nm accounting for >90% of the total population and a minor peak at ∼250 nm. All the particles in this sample were determined to be single-layer nanoshells based on TEM measurements (Figure 6B). After the shell growth and the second-step Ostwald ripening occurred, the two subpopulations grew into double-layer nanoshells with two different overall sizes of ∼630 and ∼450 nm, respectively. At the same time, a new subpopulation of single-layer nanoshells with smaller overall sizes (∼200 nm) also formed during this process, as shown in Figure 6C,D. Repeating the shell growth and Ostwald ripening processes for multiple times would lead to further increased polydispersity in both particle size and number of layers of the nanoshells (see Figure 6E H). Despite the particle polydispersity, the ensemble extinction line shapes of these multilayer nanoshell samples are mostly determined by the subpopulations with the largest overall size and number of layers simply because these larger particles typically have larger extinction cross sections than the smaller particles. 18483
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’ CONCLUSIONS In summary, we have demonstrated, using Cu2O as a model system, that the interior structures of multilayer nanoshells can be fine-controlled through multistep Ostwald ripening processes. Using this method, we could fine-control the number of layers, thickness of each shell, and intershell spacing of Cu2O multilayer nanoshells and thereby systematically fine-tune the synergistic optical characteristics of the particles. Because the Ostwald ripening-based symmetric hollowing of other oxide and chalcogenide materials6,19 has also been observed under various conditions, we anticipate that this approach, with some modifications of experimental conditions, may be extended to the construction of multilayer nanoshell structures of other functional materials. The multistep Ostwald ripening process reported here provides a unique approach to tailoring the multilevel interior architectures of individual nanoparticles, further enhancing and expanding the property tunability of the nanoparticles. ’ ASSOCIATED CONTENT
bS
Supporting Information. Extinction spectra and TEM images of Cu2O nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT H.W. would like to acknowledge the College of Arts and Sciences of the University of South Carolina for generous Start-up support and the USC Electron Microscopy Center for instrument use and scientific and technical assistance. The authors would like to thank Prof. Naomi Halas of Rice University for providing access to Mie scattering theory codes (written in C2+) based on which the optical properties of Cu2O multilayer nanoshells were calculated. ’ REFERENCES (1) (a) Lou, X. W.; Archer, L. A.; Yang, Z. C. Hollow micro-/ nanostructures: Synthesis and applications. Adv. Mater. 2008, 20 (21), 3987–4019. (b) Johnston, A. P. R.; Cortez, C.; Angelatos, A. S.; Caruso, F. Layer-by-layer engineered capsules and their applications. Curr. Opin. Colloid Interface Sci. 2006, 11 (4), 203–209. (c) Skrabalak, S. E.; Chen, J. Y.; Sun, Y. G.; Lu, X. M.; Au, L.; Cobley, C. M.; Xia, Y. N. Gold nanocages: Synthesis, properties, and applications. Acc. Chem. Res. 2008, 41 (12), 1587–1595. (2) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A hybridization model for the plasmon response of complex nanostructures. Science 2003, 302 (5644), 419–422. (3) Zhu, Y. F.; Shi, J. L.; Shen, W. H.; Dong, X. P.; Feng, J. W.; Ruan, M. L.; Li, Y. S. Stimuli-responsive controlled drug release from a hollow mesoporous silica sphere/polyelectrolyte multilayer core shell structure. Angew. Chem., Int. Ed. 2005, 44 (32), 5083–5087. (4) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. Fabrication of hollow palladium spheres and their successful application to the recyclable heterogeneous catalyst for Suzuki coupling reactions. J. Am. Chem. Soc. 2002, 124 (26), 7642–7643. (5) (a) Schwartzberg, A. M.; Oshiro, T. Y.; Zhang, J. Z.; Huser, T.; Talley, C. E. Improving nanoprobes using surface-enhanced Raman scattering from 30-nm hollow gold particles. Anal. Chem. 2006, 78 (13), 4732–4736. (b) Zhang, H. G.; Zhu, Q. S.; Zhang, Y.; Wang, Y.; Zhao, L.; Yu, B. One-pot synthesis and hierarchical assembly of hollow Cu2O
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