Fabrication of Diverse Cu2O Nanoframes through Face-Selective

Nov 7, 2013 - the fabrication of symmetrical nanocages and nanoframes with certain empty faces.14,18 Ag2O possesses the same cuprite crystal structure...
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Fabrication of Diverse Cu2O Nanoframes through Face-Selective Etching Ya-Huei Tsai, Chun-Ya Chiu, and Michael H. Huang* Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: Two approaches have been employed to generate Cu2O nanoframes. Novel edge-truncated cubic nanoframes with empty {110} edges can be obtained directly by growing Cu2O nanocrystals in the presence of HCl etchant. After 1 h of reaction for particle growth, introduction of ethanol and sonication of the mixture effectively removes surface-adsorbed sodium dodecyl sulfate (SDS) surfactant to facilitate HCl etching. Crystal structures of the nanoframes have been examined. The entire nanoframe formation process was captured by recording the complete solution color changes. By injecting precise volumes of HCl solution to a solution of presynthesized {100}-truncated and all-cornertruncated Cu2O rhombic dodecahedra, nanoframes with etched {110} faces and hollow interior were produced in 10 min. Observations made on some partially etched particles suggest etching rapidly proceeds from the surface {110} faces into the interior regions of the rhombic dodecahedra. The strong light scattering feature observed for solid rhombic dodecahedra extending from the visible to the near-infrared regions largely disappears after their transformation into nanoframes.

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INTRODUCTION Hollow nanostructures are useful because they have high surface-to-volume ratio and sufficient interior space for potential applications in areas such as materials and molecular encapsulation and release, Li ion batteries, and catalysis.1−5 Various spherical and nonspherical metal, metal oxide, chalcogenide, and nitride hollow structures have been prepared.5−13 Remarkably, Cu2O hollow nanostructures can be generated directly without the use of templates, or they can be obtained by etching preformed Cu2O nanocrystals.14−20 An unusual hollow-shell-refilled growth mechanism has been observed in the preparation of Au−Cu 2 O core−shell heterostructures, further suggesting the formation of hollow structures is often favorable for Cu2O.21 Because of these unique crystal growth and etching properties, an interesting development in the synthesis of hollow Cu2O nanostructures is the fabrication of symmetrical nanocages and nanoframes with certain empty faces.14,18 Ag2O possesses the same cuprite crystal structure as that of Cu2O, so polyhedral Ag2O crystals have also been face-selectively etched to yield exotic structures.22 Truncated rhombic dodecahedral Cu2O nanoframes with exclusively empty {100} or {110} faces were first prepared while growing Cu2O nanocrystals in the presence of HCl etchant.14 Another way to generate highly symmetrical Cu2O hollow nanostructures may be achieved by etching preformed nanocrystals on certain faces. Previously, we have reported the synthesis of Cu2O nanocrystals with systematic shape evolution from cubic to rhombic dodecahedral structures.23,24 {100}-truncated rhombic dodecahedral and all© 2013 American Chemical Society

corner-truncated rhombic dodecahedral Cu2O nanocrystals were also synthesized. Since it is still quite challenging to produce structurally well-defined hollow nanostructures for all materials, it should be interesting to turn these new particles into nanoframes to expand the variety of Cu2O hollow structures. In this study, we have prepared novel cubic Cu2O nanoframes with empty {110} faces directly by introducing HCl solution during particle synthesis. Cu2O nanoframes derived from {100}-truncated rhombic dodecahedral and allcorner-truncated rhombic dodecahedral Cu2O nanocrystals were also produced by synthesizing these particles first and carefully injecting precise amounts of HCl solution to etch only the {110} faces. All of these new nanoframes have been extensively characterized by electron microscopy. The cubic nanoframe formation process has been monitored by following the solution color changes during particle growth. The solid Cu2O nanocrystals and their corresponding nanoframes show markedly different optical characteristics.

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EXPERIMENTAL SECTION Materials. Anhydrous copper(II) chloride (CuCl2, 97%), hydroxylamine hydrochloride (NH2OH·HCl, 99%), hydrogen chloride (HCl, 37%), and sodium hydroxide (NaOH, 98%) were acquired from Aldrich. Sodium dodecyl sulfate (SDS, Received: September 3, 2013 Revised: October 16, 2013 Published: November 7, 2013 24611

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RESULTS AND DISCUSSION The synthetic procedure for making cubic Cu2O nanoframes employs the condition previously used for the formation of edge- and corner-truncated octahedra and the idea of acid etching by adding HCl as ethant.14,23 Aqueous CuCl2 solution, SDS surfactant, NaOH, NH2OH·HCl, and HCl solution were introduced in the order listed. HCl concentration has been carefully tuned. The solution pH is 3.15. After nanoparticle growth for 1 h, ethanol was added. The solution was left undisturbed for 1 h and sonicated for 1 min to yield the final nanoframe product. The initially formed Cu(OH)2 is reduced to form Cu2O crystals, which are subsequently face-selectively etched to yield nanoframes with the following reactions:

100%) was purchased from J.T. Baker. All chemicals were used without further purification. Ultrapure distilled and deionized water (18.2 MΩ) was used for all solution preparations. Synthesis of Cubic Cu2O Nanoframes with Hollow Edges. First, 8.32 mL of deionized water, 0.50 mL of 0.1 M CuCl2, and 0.0870 g of SDS (3.0 × 10−2 M final concentration) were mixed with vigorous stirring. The sample vial was placed in a water bath set at 32−34 °C. After the complete dissolution of the SDS powder, 0.18 mL of 1.0 M NaOH was added. The resulting solution immediately turned light blue, indicating the formation of Cu(OH)2 precipitate. Next, 0.92 mL of 0.1 M NH2OH·HCl was quickly injected into the sample vial (generally in 1 s), and the vial was stirred for 20 s. The solution color turned from light blue to green within seconds after the addition of NH2OH·HCl. Finally, 0.08 mL of 1 M HCl was introduced, and the vial was stirred for an additional 20 s. The solution color turned from green to yellow within seconds after the addition of HCl. The total solution volume is 10 mL. The solution was aged in the water bath for 1 h for nanocrystal growth. To make the cubic Cu2O nanoframes with empty {110} faces, 3 mL of 95% ethanol was added into the solution, left undisturbed for 1 h, and sonicated the solution for 1 min. The clear yellowish solution became cloudy presumably due to the suspension of SDS molecules. The sample was centrifuged at 5000 rpm for 3 min. The top solution was removed, and the precipitate was washed with 10 mL of a 1:1 volume ratio of water and ethanol. The precipitate was centrifuged and washed again using the same water/ethanol mixture to remove unreacted chemicals and SDS surfactant. The final washing step used 5 mL of ethanol, and the precipitate was dispersed in 0.5 mL of ethanol for storage and analysis. A drop of the solution was transferred to either a clean silicon substrate or a carbon-coated copper grid for electron microscopy analysis. Fabrication of Cu2O Nanoframes from {100}-Truncated Rhombic Dodecahedral and All-Corner-Truncated Rhombic Dodecahedral Cu2O Nanocrystals through Face-Selective Etching. Procedures for the syntheses of {100}-truncated rhombic dodecahedral and all-corner-truncated rhombic dodecahedral Cu2O nanocrystals have been described previously.23 To generate nanoframes from these nanocrystals via face-selective acid etching, 4.75 mL of deionized water, 0.05 mL of 1.0 M NaOH, and 0.2 mL of Cu2O nanocrystal solution were mixed with sonication for 10 s. The mixture was then kept in a water bath set at 35 °C for 5 min. Different volumes of 0.02 M HCl solution were injected into the mixture under stirring at a rate of 18 mL/h by the use of a syringe pump (see Scheme S1, Supporting Information). The reaction was stopped after HCl injection was completed. The sample was centrifuged at 5000 rpm for 3 min. The top solution was removed, and the precipitate was washed with 6 mL of a 1:1 volume ratio of water and ethanol. The precipitate was centrifuged and washed again using the same water/ ethanol mixture. The final washing step used 2 mL of ethanol, and the precipitate was dispersed in 0.2 mL of ethanol for analysis. Instrumentation. TEM characterization was performed on a JEOL JEM-2100 electron microscope with an operating voltage of 200 kV. SEM images of the products were taken using a JEOL JSM-7000F electron microscope. XRD patterns were collected using a Shimadzu XRD-6000 diffractometer with Cu Kα radiation (λ = 1.5418 Å). UV−vis−NIR extinction spectra were acquired with the use of a JASCO V-670 spectrophotometer.

2Cu(OH)2 + 2NH 2OH → Cu 2O + N2 + 5H 2O

(1)

Cu 2O + 4HCl → 2HCuCl 2 + H 2O

(2)

Figure 1 shows SEM images of the synthesized edge-truncated cubic Cu2O nanoframes with empty {110} faces. Single nanoframes viewed along the ⟨100⟩, ⟨110⟩, and ⟨111⟩ directions are also provided. The cubic nanoframes are quite uniform with sizes of 340−370 nm. While the majority of the particles are hollow cubes, some unetched particles are still observed. The {100} faces are stable under the etching

Figure 1. (a, b) SEM images of cubic Cu2O nanoframes with empty {110} edges. (c−f) SEM images of single cubic nanoframes viewed along the (c, f) ⟨100⟩, (d, g) ⟨110⟩, and (e, h) ⟨111⟩ directions and their corresponding drawings. Scale bars in panels c−e are equal to 100 nm. 24612

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Figure 2. (a, d) TEM images and (b, e) the corresponding SAED patterns of a single cubic Cu2O nanoframe viewed along the ⟨100⟩ and ⟨110⟩ directions. The viewing zone axes are indicated. (c, f) High-resolution TEM images of the square regions in panels a and d.

condition employed. This is a new Cu2O hollow structure not reported before. The hollow interior space of these cubic nanoframes can be confirmed through TEM characterization. Figure 2 gives TEM and selected-area electron diffraction (SAED) patterns of single cubic nanoframes viewed along the ⟨100⟩ and ⟨110⟩ directions. The interior space of a nanoframe is indeed empty. The wall thickness is 20−22 nm. The recorded SAED patterns suggest the nanoframe is single-crystalline, and the diffraction spots are consistent with the orientation of the examined nanoframes. The elliptical hollow regions are the etched {110} faces, and the framework structures are constructed of {100} and {111} faces. High-resolution TEM images taken show clear lattice fringes with d-spacing of 2.1, 2.4, and 3.0 Å that can be assigned to the respective (200), (111), and (110) planes of Cu2O. The lattice fringe directions also match the corresponding spots seen in the SAED patterns. The XRD pattern of the cubic nanoframes gives weak (111), (200), and (220) peaks of Cu2O (Figure S1, Supporting Information). The thin walls of the nanoframes cause a substantial loss of peak intensities in the XRD pattern. Figure 3 displays the UV−vis−NIR extinction spectrum of cubic Cu2O nanoframes. Two characteristic Cu2O nanocrystal absorption bands at 370 and 450 nm are present. At longer wavelengths beyond 470 nm to the near-infrared region, the nanoframes show a continuously descending extinction trend due to their hollow structure. The solution color changes during the cubic Cu2O nanoframe formation process have been recorded. Figure 4 offers photographs of the solution vial to illustrate the color evolution. After mixing CuCl2 and NaOH, Cu(OH)2 formed initially giving a blue solution color. When NH2OH·HCl was quickly added, the solution turned from light blue to green within seconds, indicating the reduction of Cu(II) species to Cu2O. After HCl was added, the green solution color gradually turned into yellow and then orange. This color changes results from the growth of Cu2O particles. It appears that the particle growth is practically complete within 10 min of reaction. After the solution has been aged for 1 h, 3 mL of ethanol was added, and the mixture was sonicated for 1 min. Remarkably, introduction of ethanol led to the formation of a yellow

Figure 3. UV−vis−NIR extinction spectra of cubic Cu2O nanoframes and {100}-truncated rhombic dodecahedral nanocrystals and nanoframes.

solution with observable orange precipitate at the bottom of the vial. The solution appears clear after sonication, but the upper portion is cloudy for minutes, presumably due to the suspension of SDS surfactant. Introduction of ethanol is believed to disturb the adsorption of SDS molecules from the particle surfaces, and sonication further helps to temporarily remove the surfactant and facilitate face-selective etching by HCl, forming HCuCl2.14 The faster etching rate on the {110} faces than those on the {100} and {111} faces leads to the formation of edge-truncated cubic frames with empty {110} faces. Cu2O cubes, octahedra, and rhombic dodecahedra with submicrometer sizes have been used to examine the relative stability of different surfaces in a weak acetic acid solution environment.25 Oxidative dissolution of Cu2O in the acidic solution turned the less stable {110} and {111} faces into the most stable {100} faces. Surface-etched particles, rather than hollow structures, were obtained, presumably because the Cu2O crystals employed are much larger than those used in this 24613

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Figure 4. Photographs showing the solution color changes during the course of cubic Cu2O nanoframe formation process. (a) After mixing CuCl2 and NaOH, Cu(OH)2 formed initially. (b) Immediately after NH2OH·HCl was added. (c) 2 s, (d) 30 s, (e) 1.5 min, (f) 3 min, (g) 5 min, (h) 10 min, (i) 30 min, and (j) 60 min after adding HCl. (k) 3 mL of 95% ethanol was added into the solution. (l) After the solution was sonicated for 1 min.

particles were synthesized in a weakly acidic solution condition.23 A large-area SEM image of the {100}-truncated Cu2O rhombic dodecahedral nanoframes is also provided (Figure S2), showing extensive production of nanoframes. Injection of 2.90 (pH = 5.65) to 3.45 mL (pH = 5.49) of 0.02 M HCl through a syringe pump at a rate of 18 mL/h was found to yield the best nanoframes. SEM images of the {100}truncated rhombic dodecahedral nanoframes viewed along the ⟨110⟩, ⟨100⟩, and ⟨111⟩ directions are also displayed. The majority of the rhombic dodecahedra have turned into hollow structures, although some unetched particles are still observed. Use of lesser HCl volumes led to insufficient degree of etching. Introduction of higher HCl volumes can destroy the particles forming broken framework pieces. Various degrees of partially etched nanocrystals can be seen in Figure 5b when only 1.90 mL of HCl solution (pH = 6.14) was injected. A large-area SEM image showing more of these partially etched nanocrystals is also provided (Figure S3). For some particles, even though the {110} faces have been significantly removed, the solid interior Cu2O still remains. Inspection of these intermediate structures indicates that the hollow structures are formed by gradual etching from the surfaces to the interior regions of the particles. It is remarkable that solid Cu2O crystals with sizes of 500−550 nm can be converted into nanoframes with thin walls of 20−30 nm in just 10 min by HCl etching, demonstrating the effectiveness of this etching strategy. TEM characterization of single {100}-truncated rhombic dodecahedral Cu2O nanoframes viewed along the ⟨100⟩ and ⟨110⟩ directions has been performed (Figure S4). The nanoframes clearly show hollow interior space. The SAED patterns recorded match the corresponding orientations of the nanoframes. High-resolution TEM images of the nanoframes reveal lattice fringes aligning in directions consistent with the SAED patterns obtained. The XRD pattern of the {100}truncated rhombic dodecahedral nanoframes is available in Figure S1. Again, the peak intensities are weak due to the hollow nature of the nanoframes. Figure 3 also displays UV− vis−NIR extinction spectra of the {100}-truncated rhombic

study, and acetic acid is a less powerful etchant to Cu2O than HCl. The order of relative facet stability was found to be {100} > {111} > {110}. The Cu−O bond on the {100} face was determined to be the shortest among the three surfaces and has the strongest bond strength, so the {100} face is the most stable face under a weakly acidic solution condition.25 The Cu2O crystal stability in a basic ammonia solution was also found to follow the same order.26 In another study, cornertruncated Cu2O octahedra were oxidatively etched in the presence of sodium citrate.27 The {100} corners are stable, while the {111} faces have been converted to jagged {100} faces. We have previously shown that the relative facet stability of Ag2O crystals to chemical etching in an ammonia solution follows the order of {111} > {110} > {100}, but in a weakly acidic HNO3 solution condition the {100} faces are the most stable faces.22 Thus, the formation of edge-truncated cubic Cu2O nanoframes with empty {110} faces is consistent with these previous experimental observations. However, it is important to recognize that hollow structures are produced only in this work, so face-selective etching by HCl should be critical to the formation of cages and frames. To make {100}-truncated rhombic dodecahedral Cu2O nanoframes with empty {110} faces by HCl etching, solid {100}-truncated rhombic dodecahedra were synthesized first following a reported procedure.23 This approach differs from our previous method to make Cu2O nanoframes having the same structure, in which particles were produced directly during synthesis (i.e., the same method as that employed to make the edge-truncated cubic nanoframes).14 To better control the extent of face-selective etching, NaOH was first mixed with the {100}-truncated Cu2O rhombic dodecahedra to stabilize the crystals before continuous injection of HCl solution. Cu2O crystals are more stable in a basic solution. Figure 5 gives SEM images of the synthesized {100}-truncated Cu2O rhombic dodecahedra and the resulting nanoframes after injecting 1.90, 2.90, and 3.45 mL of 0.02 M HCl solution. Some of the as-prepared truncated rhombic dodecahedra exhibit slight etching or depression over the {110} faces because the 24614

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Figure 5. (a) SEM image of {100}-truncated Cu2O rhombic dodecahedra. Some particles have slightly etched or depressed areas over the {110} faces possibly because the solution is weakly acidic. (b−d) SEM images of {100}-truncated rhombic dodecahedral Cu2O nanoframes after adding (b) 1.90, (c) 2.90, and (d) 3.45 mL of 0.02 M HCl solution. Inset image of panel b shows a partially etched particle. (e) Model of a {100}-truncated rhombic dodecahedron. (f−k) SEM images of single {100}-truncated rhombic dodecahedral Cu2O nanoframes viewed along the (f) ⟨110⟩, (g) ⟨100⟩, (h) ⟨111⟩ directions and the corresponding drawings of the particles.

removed interior Cu2O can also be observed. UV−vis−NIR spectra of the all-corner-truncated rhombic dodecahedra before and after acid etching to form nanoframes were taken (Figure S5). The solid rhombic dodecahedra exhibit strong light scattering feature from the visible to the near-infrared regions. After HCl etching to form nanoframes, a steady decrease in the extinction values of light scattering feature was observed from 550 nm to the near-infrared region.

dodecahedra and nanoframes formed after adding 2.90 mL of 0.02 M HCl solution. For the {100}-truncated rhombic dodecahedra, a characteristic optical profile of Cu2O crystals with sizes of several hundreds of nanometers was recorded, showing a distinct absorption band at 485 nm and strong light scattering feature in the near-infrared region. After conversion into nanoframes, the large light scattering feature has largely been eliminated despite the similar sizes of the solid crystals and their framework structures. Such dramatic optical changes can be used to evaluate the extent of nanoframe formation. We have also used all-corner-truncated Cu2O rhombic dodecahedra for the formation of nanoframes by selectively etching the {110} faces. Figure 6a gives SEM image of the synthesized all-corner-truncated Cu2O rhombic dodecahedra with sizes of largely 300−350 nm. They look very similar to the {100}-truncated rhombic dodecahedra, but their {111} corners are also truncated. Injection of 3.30 (pH = 5.92) to 3.60 mL (pH = 5.58) of 0.02 M HCl solution gives the best all-cornertruncated rhombic dodecahedral nanoframes (Figure 5c−f). Again, the majority of the particles have become nanoframes with empty {110} faces. Some nanoframes containing partially

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CONCLUSIONS In this study, novel edge-truncated cubic Cu2O nanoframes with empty {110} faces have been prepared directly by adding HCl in the growth of Cu2O nanocrystals, followed by the introduction of ethanol and sonication to remove surfaceadsorbed SDS surfactant for facile acid etching. The entire nanoframe formation process can be monitored by following the solution color changes. Alternatively, we have used presynthesized {100}-truncated and all-corner-truncated Cu2O rhombic dodecahedra for face-selective etching by injecting specific volumes of HCl solution to the nanocrystal solution. The {110} faces are least stable and preferentially 24615

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Figure 6. (a, b) SEM image of all-corner-truncated Cu2O rhombic dodecahedra and its drawing. The circled particle shows a clear {111} facet. (c−f) SEM images of all-corner-truncated rhombic dodecahedral Cu2O nanoframes synthesized after adding (c−e) 3.30 mL and (f) 3.60 mL of 0.02 M HCl solution.

etched. Etching proceeds rapidly until the whole interior space becomes hollow. The strong light scattering feature seen in the solid particles disappears after conversion into nanoframes. The simple fabrication of Cu2O nanoframes with excellent morphology control means their use as catalysts and hosts for nanostructure encapsulation can be considered.

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank National Science Council of Taiwan for the financial support of this work (Grant NSC 101-2113-M-007018-MY3).

ASSOCIATED CONTENT

(1) Liu, X. W.; Archer, L. A.; Yang, Z. Hollow Micro-/ Nanostructures: Synthesis and Applications. Adv. Mater. 2008, 20, 3987−4019. (2) Kuo, C.-H.; Huang, M. H. Morphologically Controlled Synthesis of Cu2O Nanocrystals and Their Properties. Nano Today 2010, 5, 106−116. (3) Wang, Z.; Zhou, L.; Lou, X. W. Metal Oxide Hollow Nanostructures for Lithium-Ion Batteries. Adv. Mater. 2012, 24, 1903−1911. (4) Wang, W.; Dahl, M.; Yin, Y. Hollow Nanocrystals through the Nanoscale Kirkendall Effect. Chem. Mater. 2013, 25, 1179−1189. (5) Kuo, C.-H.; Chu, Y.-T.; Song, Y.-F.; Huang, M. H. Cu2O Nanocrystal-Templated Growth of Cu2S Nanocages with Encapsulated Au Nanoparticles and In-Situ Transmission X-ray Microscopy Study. Adv. Funct. Mater. 2011, 21, 792−797.

S Supporting Information *

Schematic drawing of the synthetic procedure used for making rhombic dodecahedral nanoframes, XRD patterns, additional SEM and TEM images of nanoframes, and UV−vis−NIR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (M.H.H.). Notes

The authors declare no competing financial interest. 24616

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