Crystal-Plane-Dependent Etching of Cuprous Oxide Nanoparticles of

Article pubs.acs.org/JPCC

Crystal-Plane-Dependent Etching of Cuprous Oxide Nanoparticles of Varied Shapes and Their Application in Visible Light Photocatalysis Jaya Pal,† Mainak Ganguly,† Chanchal Mondal,† Anindita Roy,† Yuichi Negishi,‡ and Tarasankar Pal*,† †

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, West Bengal, India Department of Applied Chemistry, Tokyo University of Science, Tokyo 1628601, Japan

‡

S Supporting Information *

ABSTRACT: We report a simple, facile, surfactant-free chemical route to fabricate morphologically different Cu2O nanoparticles such as octahedron, truncated octahedron, hollow octahedron, cube, and sphere by varying the hydrolyzing agents, complexing agent, and reducing agents. Then the componential and morphological evolution of Cu2O nanoparticles have been studied independently, employing different etching agents such as aqueous NaOH, triethylamine (TEA), and oxalic acid solution. Particles of varied shapes and compositions resulted from the etching, and those particles were characterized by different physical methods. The oxidative dissolution of morphologically different Cu2O nanoparticles with different etching agents depends on the exposed crystal planes. During oxidative dissolution in aqueous oxalic acid solution, it is realized that the stability of the (100) crystal plane is higher than that of the (111) crystal plane. Among all the etching reagents used, only oxalic acid exhibits shape transformation of the as-prepared Cu2O nanoparticles. Oxalic acid etching causes the formation of cubes and hollow cubes as etching products with a 50% reduction of edge length compared to that of octahedral, truncated octahedral, and hollow octahedral Cu2O nanoparticles. But ill-defined cubes are always obtained as the etching products with a 40% reduction of size compared to that of Cu2O cubes and spheres. As-prepared Cu2O nanoparticles and chemically etched products exhibit facet-dependent photocatalytic activity under visible light irradiation where mineralization of congo red takes place. Experimentally it has been concluded that photocatalytic activity of different particles bears a close relationship with exposed crystal planes, surface area, and particle size for congo red degradation. Interestingly, NaOH-etched product with hollow octahedral morphology bearing many (111) facets demonstrates the highest photocatalytic activity.

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degradation,18 etc. Cu2O is a p-type semiconductor with a band gap value of 2.2 eV.18 Various types of Cu2O nanoparticles such as nanooctahedron,19 nanotruncated octahedron,19 nanocube,19 nanotruncated cube,19 nanosphere,18 nanowire,20 nanocages,21 nanomultipod,22 nanoflower,23 and various hollow24 structures have been synthesized by different synthetic protocols such as the hydrothermal method, microwave irradiation method, surfactant assisted route, wet chemical method, etc.25−28 To date, much effort has been devoted to synthesize Cu2O nanoparticles with hollow structures because of their unique physical and chemical properties, including low density, an interior void, high surface-to-volume ratio, excellent permeability, and high chemical reactivity.29 A common synthetic procedure to obtain hollow Cu2O nanoparticle is a hardtemplate-based strategy.30 There exist only a few reports relating to the template-free synthesis of hollow Cu2O nanoparticles.18 Thus, the design and fabrication of hollow Cu2O nanoparticles have become a key focus for low-cost materials. The morphology of as-prepared Cu2O nanoparticles has been found to develop with their local chemical

INTRODUCTION In recent years, chemical reactions of solids in solutions have received considerable interest owing to the formation of new solid surfaces whose structures (composition and morphology) and subsequent reactivity are dynamically changed. The reactivity of solid surfaces in solution depends on the surface structure and composition of the solid particle. Etching of materials refers to the spontaneous dissolution of surface atoms from the material matrix by chemical and physical processes. It is of interest and important to understand the etching process for the design of materials and realization of their useful functionality. It is reported that most of the etching processes have taken place in water and this water-based chemical etching process causes size and shape evolution of materials.1−3 Wet chemical etching is also advantageous for introducing greater surface area. To date, there exist a number of different etchants such as NaOH,4 KOH,5 NH4OH,6,7 H2SO4,8 HNO3,9 HF,10 HCl,11 and acetic acid12 which have been widely used in varied etching processes. The fabrication of cuprous oxide (Cu2O) nanoparticles has become increasingly important because of the nanoparticles’ unique properties and prospective applications in solar energy conversion,13 as catalysts for organic reactions,14electrodes for lithium ion batteries,15 gas sensors,16 CO oxidation,17 dye © 2013 American Chemical Society

Received: September 17, 2013 Revised: October 28, 2013 Published: October 28, 2013 24640

dx.doi.org/10.1021/jp409271r | J. Phys. Chem. C 2013, 117, 24640−24653

The Journal of Physical Chemistry C

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environment. Hua et al. reported the crystal-plane-dependent oxidative dissolution of cubic, octahedral, and rhombic dodecahedral Cu2O nanocrystals in a weak acid solution12 and in aqueous ammonia solution.7 Sui et al. reported the morphologies of as-synthesized Cu2O nanoframes and nanocages with single-crystal walls by selective oxidative etching at room temperature.31 Sun et al. reported the branching growth of Cu2O nanocrystal via selective oxidative etching with ethanol solution.32 A variety of uniform hollow structures having Cu2O@Fe(OH)x nanorattles and Fe(OH)x cages by template-engaged redox etching of shape-controlled Cu2O crystals have been reported by Wang et al.33 Qui et al. investigated the structural evolution of Cu2O microcrystals via chemical etching with ammonia.34 Kuo and Huang successfully synthesized Cu2O nanocages and nanoframes from truncated rhombic dodecahedral Cu2O particles in an aqueous HCl− ethanol mixture.24 Different organic dyes have versatile applicability in our daily life, such as in paper, leather, clothing, printing, cosmetics, plastics, drugs, and electronics.35 But because of their toxicity, long persistence, and nonbiodegradable nature, they are hazardous to our environment and cause great damage.36 So the mineralization of dyes from wastewater is a priority for ensuring a safe and clean environment. Nowadays, different advanced processes have been developed for the remediation of dye molecules, e.g., adsorption tactics, Fenton-like reactions, biological degradation, and photocatalysis, without the formation of hazardous byproducts.37−40 Among these techniques, photocatalytic degradation of dye molecules has become rewarding as this method is eco-friendly. Here we have demonstrated a surfactant-free approach to synthesize different morphologies of Cu2O nanoparticles, such as octahedral and hollow octahedral Cu2O exposing (111) crystal planes, truncated octahedral Cu2O exposing (111) and (100) crystal planes, cubic Cu2O exposing (100) crystal planes, and spherical Cu2O. All these Cu2O nanoparticles of different shapes were chemically etched selectively by NaOH, TEA, and oxalic acid solution. During etching, the morphology and composition of Cu2O nanoparticles were altered and their photocatalytic activity was also widely tuned. It has been established that the stability of the (100) crystal plane is greater than that of the (111) crystal plane,7,12 which is revealed from oxalic acid dependent morphology evolution of different Cu2O nanoparticles. Finally, Cu2O nanoparticles and their chemically etched products have been exploited to study the photodegradation of congo red solution under visible light irradiation. The chemical etching, in general, improves photodegradation efficiency of the catalyst because of a greater number of exposed crystal planes, increased surface area, and decreased particle size.

X-ray photoelectron spectroscopy (XPS) analysis was performed with VG Scientific ESCALAB MK II spectrometer equipped with a Mg Kα excitation source (1253.6 eV) and a five-channeltron detection system to analyze the elemental state. Field emission scanning electron microscopy (FESEM) was performed with a Supra 40 (Carl Zeiss Pvt. Ltd) instrument. Transmission electron microscopy (TEM) was carried out on a Hitachi H-9000 NAR instrument using an accelerating voltage of 300 kV. All UV−vis absorption spectra were recorded on a SPECTRASCAN UV 2600 digital spectrophotometer (Chemito, India). Synthesis of Cu2O Octahedrons, Truncated Octahedrons, Cubes, Hollow Octahedrons and Spheres. In a typical synthesis, 5 mL of 0.1 M and 10 mL of 0.02 M pyridine solution were added to a CuSO4·5H2O solution (5 mL of 0.05 M and 10 mL of 0.02 M) under constant stirring. After the addition of pyridine, the color of the solution turned deep blue and in a few minutes blue Cu(OH)2 colloid was produced. In the next step, heating in a water bath (WB) for 5 min with 5 mL of 0.1 M glucose solution under alkaline condition (10 mL of 0.1 M NaOH), we obtained red-brown Cu2O nanoparticles with octahedral shape (Scheme 1). In contrast, from the wellstirred reaction mixture with hydrazine hydrate (40 μL, 99− 100%) at room temperature, we obtained spherical Cu2O nanoparticles (Scheme 1) in 5 min.

EXPERIMENTAL SECTION Chemicals. All the chemicals were of AR grade. Doubledistilled water was used throughout the experiment. CuSO4· 5H2O, pyridine (py), 2-methylpyridine, ethylenediamine (en), glucose, NaOH, NH2NH2·H2O (99−100%), triethylamine (TEA), and oxalic acid were purchased from E-Merck. Congo red (CR) dye was received from S.D. Fine Chemicals, India. All the glassware was properly cleaned and dried well before use. Analytical Instruments. Powder X-ray diffraction (XRD) was carried out with a PW1710 diffractometer (Philips, The Netherlands) instrument. The XRD data were analyzed using JCPDS software.

Syntheses of truncated octahedral and cubic Cu2O nanoparticles were done with 2-methylpyridine and CuSO4·5H2O solution. In both cases, alkaline (10 mL of 0.1 M NaOH) 5 mL of 0.1 M glucose solution was introduced as the common reducing agent and the reaction mixture was heated in a water bath for 5 min at ∼80 °C. Specifically for truncated octahedron, 5 mL of 0.05 M CuSO4·5H2O solution and 5 mL of 0.01 M 2methylpyridine solution were used (Scheme 1). But for cubes, 5 mL of 0.01 M CuSO4·5H2O solution and 5 mL of 0.1 M 2methylpyridine solution were needed (Scheme 1). During heating, the solution became red-brown, indicating the formation of Cu2O nanoparticles of varied shapes.

Scheme 1. Schematic Representation of Five Different Conditions for Synthesis of Octahedral, Truncated Octahedral, Cubic, Hollow Octahedral, and SphericalShaped Cu2O Nanoparticles by Using Different Hydrolyzing or Complexing Agents and Reducing Agents and the Corresponding FESEM Images of Different Morphologies

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spectra of the supernatant solution were recorded using a UV− visible spectrophotometer.

If we use ethylenediamine (en) instead of pyridine or 2methylpyridine, then we get the hollow octahedral morphology of Cu2O nanoparticles. Here 5 mL of 0.05 M CuSO4·5H2O solution was taken in a beaker, and to it 5 mL of 0.1 M ethylenediamine (en) solution was added under stirring condition. The solution color changed from light blue to deep blue, indicating the formation of [Cu(en)2]+2 complex in solution. Then 5 mL of 0.1 M glucose solution and 10 mL of 0.1 M NaOH were added to that solution and heated in a water bath for 15 min at ∼80 °C. The solution became red-brown, indicating the formation of Cu2O nanoparticles with hollow octahedral morphology (Scheme 1). Finally, all five types of Cu2O nanoparticles were carefully washed, first with distilled water and then with absolute ethanol, and dried in vacuum. Etching of Cu2O Nanoparticles by NaOH, Triethylamine (TEA), and Oxalic acid. A 20 mg sample of red-brown solid Cu2O nanoparticle was separately mixed with 40 mL of 0.1 M NaOH (pH ∼ 12), 0.1 M triethylamine (TEA) (pH ∼ 10), or 0.01 M oxalic acid (pH ∼ 1) solution. Then the mixture was stirred with a magnetic stirrer at 400 rpm for 20, 30, or 10 min for NaOH, TEA, or oxalic acid respectively (Scheme 2).

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RESULTS AND DISCUSSION X-ray Diffraction Analysis. The phase structure and purity of as-synthesized Cu2O nanoparticles and products obtained by etching with NaOH, TEA, and oxalic acid were analyzed by Xray diffraction study. The (a) curves in Figure 1A−E show the expected (110), (111), (200), (220), and (311) reflection peaks for as-prepared octahedral, truncated octahedral, cubic, hollow octahedral, and spherical Cu2O nanoparticles (JCPDS file 050667) confirming that all the Cu2O nanoparticles are phase pure. Cubic Cu2O nanoparticles exhibit a (200) reflection peak that is stronger than the other diffraction peaks, indicating that cubic Cu2O exclusively exposes (100) planes. Whereas in the case of octahedral and hollow octahedral Cu2O, the (111) diffraction peak is much more intense than other peaks in the XRD pattern, representing exclusively (111) plane exposure. From the 3D model, it is observed that cubic Cu2O is bound by six (100) facets, octahedral Cu2O by eight (111) facets, and truncated octahedral Cu2O by six square (100) facets and eight triangle (111) facets (Figure 2). As shown in Figure 2, the diffraction intensity ratios of (111)/(200) progressively increase according to the following order: cubes < truncated octahedrons < octahedrons as a result of decreasing (100) facets. The (b) curves in Figure 1A−E display the XRD patterns of NaOH-etched products obtained from assynthesized Cu2O nanoparticles. Here the diffraction peaks at 2θ = 32.5°, 35.6°, 38.4°, 48.8°, 53.7°, 56.5°, 58.1°, 61.7°, 66.1°, 68.1°, and 74.8° correspond to (110), (1-11) and (002), (111) and (200), (2−02), (020), (021), (202), (1-13), (1-13), (220), and (004) planes for CuO nanoparticle (JCPDS file 48−1548). Thus, the experimental result suggests that Cu2O nanoparticles are completely converted to CuO nanoparticles after the etching reaction proceeds in aqueous NaOH solution. Curves (c) and (d) in Figure 1A−E show the XRD patterns of the products obtained after as-synthesized Cu2O nanoparticles are etched in TEA and oxalic acid solution. Here all the diffraction peaks in XRD patterns can be indexed as Cu2O nanoparticles. These experimental results imply that there is no compositional change of Cu2O nanoparticles. XPS Analysis. The chemical state and surface atomic composition of both Cu2O nanomaterials and chemically etched products of Cu2O were further investigated by XPS analysis. Figure 3a illustrates the peaks of Cu2p3/2 and Cu2p1/2 at 932.5 and 952.5 eV, respectively, arising from octahedral Cu2O nanoparticles.41 A few shakeup satellite peaks are also observed at the higher energy side in the spectra, 940.4 and 943.1 eV, which can be attributed to Cu(II) states.41 The presence of shakeup satellite peaks of as-prepared Cu2O nanoparticle suggest the presence of a thin layer of CuO on the Cu2O surface due to surface oxidation. There is no change in peak positions of Cu2p XPS spectra after these octahedral Cu2O nanoparticles are etched in TEA and oxalic acid solution (Figure 3c,d). However, the Cu2p XPS spectrum shows significant changes when the etching reaction of octahedral Cu2O proceeds in NaOH solution with pH ∼ 12 for 20 min (Figure 3b). Figure 3b shows that the XPS spectra of Cu2p3/2 and Cu2p1/2 appeared at 933.5 and 953.5 eV, respectively, corresponding to the presence of CuO42,43 and not any Cu2O. Thus, the XPS spectrum suggests the change in chemical composition from Cu2O nanoparticles to CuO nanoparticles after etching in NaOH solution. Similar results are also

Scheme 2. Schematic Representation of Chemical Etching of Cu2O Nanoparticles Having Varied Shapes; Same Color of the Etched Products Obtained from Cu2O Nanoparticle of a Particular Shape

The as-obtained solids were washed, first with distilled water and then with absolute ethanol. Finally, products were dried in vacuum and reserved for further characterizations. Photocatalytic Activity. To evaluate the shape-dependent and composition-dependent photocatalytic activity of Cu2O nanoparticles as well as their chemically etched products, congo red solution was used. In a typical process, 0.01 g of Cu2O nanoparticles bearing different shapes as well as the chemically etched products were dispersed into the aqueous solution of congo red (20 mL of 5 × 10−5 M); the solution was irradiated under visible light (100 W electric bulb). Before light illumination, the suspension was magnetically stirred in the dark for 2 h to reach adsorption−desorption equilibrium of the corresponding dye. Then 3 mL of the reaction mixture was removed at different time intervals and centrifuged to avoid the scattering due to suspended catalyst particles. Then absorption 24642



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Figure 1. XRD patterns of (A) octahedral, (B) truncated octahedral, (C) cubic, (D) hollow octahedral, and (E) spherical Cu2O nanoparticles: (a) asprepared Cu2O nanoparticles, (b) NaOH-etched products, (c) TEA-etched products, and (d) oxalic acid etched products.

that the edges but not the faces of octahedral Cu2O nanoparticles exclusively exposing (111) planes become jagged. Here the overall morphology of octahedral Cu2O remains the same. This result is also confirmed by TEM analysis (a3 in Figure 5 and Figure S2 of the Supporting Information). Both FESEM images (a4 in Figure 4 and Figure S1 of the Supporting Information) and TEM images (a4 in Figure 5 and Figure S2 of the Supporting Information) reveal that the octahedral Cu2O exposing (111) planes transform to cubic Cu2O with rough surfaces having (100) exposed planes after etching in oxalic acid solution. This observation demonstrates that the Cu2O (100) planes are more stable than Cu2O (111) planes. FESEM (b1, c1, d1, and e1 in Figure 4 and Figure S1 of the Supporting Information) and TEM images (b1, c1, d1, and e1 in Figure 5 and Figure S2 of the Supporting Information) show the well-defined, smooth, and uniform morphology of truncated octahedral, cubic, hollow octahedral, and spherical Cu2O nanoparticles, respectively. After 20 min of etching in NaOH solution, FESEM (b2, c2, d2, and e2 in Figure 4 and Figure S1 of the Supporting Information) and TEM images (b2, c2, d2, and e2 in Figure 5 and Figure S2 of the Supporting

obtained from the XPS spectra of truncated octahedral, cubic, hollow octahedral, and spherical Cu2O nanoparticles before and after etching reactions carried out with NaOH, TEA, and oxalic acid solutions. FESEM, TEM, and HRTEM Analysis. The morphology and structure of Cu2O nanoparticles as well as the products obtained after etching with NaOH, TEA, and oxalic acid solutions were studied by both FESEM and TEM images. FESEM images (a1 in Figure 4 and Figure S1 of the Supporting Information) display the well-defined, smooth, and uniform octahedral shape of Cu2O nanoparticles. The result is further ascertained by TEM analysis (a1 in Figure 5 and Figure S2 of the Supporting Information). FESEM images (a2 in Figure 4 and Figure S1 of the Supporting Information) clearly show the appearance of sheetlike nanostructures bearing octahedral morphology after the etching reaction proceeds in NaOH solution for 20 min. The presence of sheetlike nanostructures is also observed by TEM images (a2 in Figure 5 and Figure S2 of theSupporting Information). After the etching reaction proceeds in TEA solution, the FESEM images (a3 in Figure 4 and Figure S1 of the Supporting Information) demonstrate 24643



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in Figure 5 and Figure S2 of the Supporting Information) reveal that the shape evolution from cube and sphere to ill-defined cube with rough surface takes place during the course of the etching process in oxalic acid solution. Among all the etching reagents used, only oxalic acid demonstrates shape transformation of the as-prepared Cu2O nanoparticles. Oxalic acid etching causes the formation of cubes and hollow cubes as etching products with a 50% reduction of edge length compared to that of octahedron, truncated octahedron, and hollow octahedron Cu2O nanoparticles. But ill-defined cubes are always obtained as the etching products with a 40% reduction in size compared to that of Cu2O cubes and spheres. The SAED patterns of both original octahedral Cu2O and Cu2O nanoparticles obtained after etching with TEA solution expose (222) crystal planes (Figure 6a,c). These results authenticate the single-crystalline nature of both nanostructures. When the etching reaction proceeds in NaOH solution, the SAED pattern of the sheetlike CuO nanostructures depicts a ringlike pattern, indicating the etched product acquires polycrystallinity (Figure 6b). The formation of CuO nanoparticles has also been demonstrated by XRD and XPS analysis. The SAED pattern (Figure 6d) shows that oxalic acid etched cubic Cu2O products expose (200) crystal planes which indicates the single-crystalline character. As a continuation of the SAED pattern analysis of octahedral Cu2O mentioned in the preceding section, the SAED patterns (b1, c1, and d1 in Figure S3 of the Supporting Information) of the surface region of truncated octahedral, cubic, and hollow octahedral Cu2O indicate that all are single-crystalline in nature. Only spherical Cu2O exhibits ringlike SAED pattern, which authenticates its polycrystalline nature (e1 in Figure S3 of the Supporting Information). The SAED patterns (b2, c2, d2, and e2 in Figure S3 of the Supporting Information) of all NaOHetched products of all four Cu2O nanostructures give ringlike patterns, which means all are polycrystalline. Although the edges of TEA-etched product of truncated octahedron are zigzag, it exhibits a single-crystalline SAED pattern (b3 in Figure S3 of the Supporting Information). From FESEM (c3, d3, and e3 in Figure 4) and TEM images (c3, d3, and e3 in Figure 5) it can be seen that many small particles are present on the surface of the other three TEA-etched products of cubic, hollow octahedral, and spherical Cu2O nanoparticles and that these surfaces are also very rough. The ringlike SAED patterns (c3, d3, and e3 in Figure S3 of the Supporting Information) of these three TEA-etched products of cubic, hollow octahedral, and spherical Cu2O demonstrate that they are polycrystalline. The rough surface of the material with randomly orientated small particles confirms the polycrystalline character.44 The SAED patterns (b4, c4, d4, and e4 in Figure S3 of the Supporting Information) of the surface region of all oxalic acid etched products of truncated octahedral, cubic, hollow octahedral, and spherical Cu2O nanoparticles show singlecrystalline character. HRTEM images (a and c in Figure 7 and Figure S4 of the Supporting Information) reveal that the original octahedral and truncated octahedral Cu2O and their TEA-etched products show lattice fringe of 0.25 nm, indicating (111) crystal planes of Cu2O. The d spacing of NaOH-etched products of octahedral and truncated octahedral Cu2O are 0.27 nm which coincides with the lattice distance of (110) plane of monoclinic CuO (b in Figure 7 and Figure S4 of the Supporting Information). Again, the lattice spacing of oxalic acid etched products of the above two nanostructures are 0.30 nm, which is in good

Figure 2. FESEM images (a, b, and c) and 3-D geometrical models (e, f, and g) of octahedral Cu2O exposing (111) facets, truncated octahedral Cu2O exposing (111) and (100) facets, and cubic Cu2O exposing (100) facets.

Figure 3. Cu2p XPS spectra of (a) as-prepared Cu2O octahedral nanoparticles, (b) NaOH-etched products, (c) TEA-etched products, and (d) oxalic acid etched products of Cu2O octahedral nanoparticles.

Information) clearly display the appearance of sheetlike nanostructures bearing the original shapes of truncated octahedral, cubic, hollow octahedral, and spherical, respectively. The edges of the truncated octahedral Cu2O nanoparticles become jagged during etching with TEA solution, shown in FESEM (b3 in Figure 4 and Figure S1 of the Supporting Information) and TEM images (b3 in Figure 5 and Figure S2 of the Supporting Information). FESEM (c3, d3, and e3 in Figure 4 and Figure S1 of the Supporting Information) and TEM images (c3, d3, and e3 in Figure 5 and Figure S2 of the Supporting Information) indicate that the surfaces of cubic, hollow octahedral, and spherical Cu2O become rough after the etching reaction proceeds in TEA solution. FESEM (b4 and d4 in Figure 4 and Figure S1 of the Supporting Information) and TEM analysis (b4 and d4 in Figure 5 and Figure S2 of the Supporting Information) reveal that the shape evolution from truncated octahedron and hollow octahedron to cube and hollow cube with rough surface takes place during the course of the etching process in oxalic acid solution. This result also demonstrates that Cu2O (100) planes are more stable than Cu2O (111) planes. FESEM (c4 and e4 in Figure 4 and Figure S1 of the Supporting Information) and TEM images (c4 and e4 24644



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Figure 4. FESEM images of (a1) as-synthesized octahedral, (b1) truncated octahedral, (c1) cubic, (d1) hollow octahedral, and (e1) spherical Cu2O nanoparticles. FESEM images showing the etched product from Cu2O: (a2, b2, c2, d2, and e2) NaOH-etched products; (a3, b3, c3, d3, and e3) TEA-etched products; and (a4, b4, c4, d4, and e4) oxalic acid etched products of octahedral, truncated octahedral, cubic, hollow octhedral and spherical Cu2O nanoparticles, respectively.

pyridine or 2-methylpyridine as hydrolyzing agents and ethylenediamine as complexing agent. In the second step, the as-obtained Cu(OH)2 or [Cu(en)2]+2 are reduced to Cu2O nanoparticles by specific reducing agents like glucose/NaOH or by N2H4·H2O solution. For octahedral and spherical Cu2O nanoparticle preparation, pyridine has been used as hydrolyzing agent. As illustrated in FESEM images (a1 in Figure 4 and Figure S1 of the Supporting Information), octahedral Cu2O was obtained when glucose was used as reducing agent under alkaline condition, whereas spherical Cu2O nanoparticles were formed when N2H4·H2O was used as reducing agent under room temperature condition (e1 in Figure 4 and Figure S1 of the Supporting Information).

agreement with (110) directional growth of Cu2O nanoparticles (d in Figure 7 and Figure S4 of the Supporting Information). Formation Mechanism of Cu2O Nanoparticles of Different Shapes and Their Chemically Etched Products. Although the exact mechanism of formation of differently shaped Cu2O nanoparticles under simple wet chemical condition is still uncertain, the role of hydrolyzing agents, complexing agent, and different kinds of reducing agents are definitely important. Here, Cu2O nanoparticles of varied shapes have been prepared in two step reactions (Scheme 1). In all cases CuSO4·5H2O is used as metal ion precursor. The first step involves the formation of Cu(OH)2 and [Cu(en)2]+2 with 24645



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Figure 5. TEM images of (a1) as-synthesized octahedral, (b1) truncated octahedral, (c1) cubic, (d1) hollow octahedral, and (e1) spherical Cu2O nanoparticles. TEM images showing the etched product from Cu2O: (a2, b2, c2, d2 and e2) NaOH-etched products, (a3, b3, c3, d3 and e3) TEAetched products and (a4, b4, c4, d4 and e4) oxalic acid etched products of octahedral, truncated octahedral, cubic, hollow octahedral, and spherical Cu2O nanoparticles respectively.

thermodynamically stable octahedral Cu2O nanoparticles exposing (111) crystal planes. As shown in FESEM images (c1 in Figure 4 and Figure S1 of the Supporting Information), cubic Cu2O was formed when the concentration of the CuSO4· 5H2O solution was 0.01 M and that of 2-methylpyridine was 0.1 M. The truncated octahedral Cu2O morphology was obtained when the concentration of CuSO4·5H2O solution was 0.05 M and that of 2-methylpyridine was 0.01 M (b1 in Figure 4 and Figure S1 of the Supporting Information). When the concentration of CuSO4·5H2O was decreased and concen-

In this study, octahedral and spherical Cu2O nanoparticles have been obtained by varying only the reducing agents. The formation of spherical Cu2O nanoparticles may be ascribed to the rapid reduction of Cu(OH)2 and nucleation of Cu2O due to thte strong reducing ability of hydrazine hydrate over glucose. The spherical Cu2O nanoparticles are produced very quickly and coalesced to form larger spherical Cu2O with lower surface free energy. In the case of glucose (under alkali condition), the reduction rate of Cu(OH)2 and the nucleation of Cu2O become much slower which caused the formation of 24646



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2-methylpyridine is a stronger hydrolyzing agent than pyridine. When a weak hydrolyzing agent (pyridine) is employed, then the product becomes octahedral Cu2O nanoparticles exposing (111) crystal facets. Stronger hydrolyzing agent (2-methylpyridine) causes the formation of truncated octahedral Cu2O nanoparticles with exposed (111) and (100) facets. If we increase the concentration of 2-methylpyridine, a stronger hydrolyzing agent, then we obtained cubic Cu2O nanoparticles with (100) facets. A stronger hydrolyzing agent is able to etch the high-energy corners, evolving truncated species which finally gives rise to cubes. This result corroborates the dominance of (111) crystal facet of Cu2O with weak hydrolyzing agent. Finally, keeping the glucose (under alkaline condition) as the reducing agent, the effect of a complexing agent has been tested, and the as-prepared product was hollow octahedral Cu2O if ethylenediamine (en) was used as complexing agent instead of hydrolyzing agent (pyridine or 2methylpyridine) under heating conditions (∼80 °C for 15 min). Though the formation mechanism of hollow octahedral Cu2O nanoparticles is still uncertain, the role of en and aging time undoubtedly have bearing on particle formation. The above experimental results exhibited that five morphologically different Cu2O nanoparticles can be etched with aqueous NaOH, TEA, and oxalic acid solutions (Scheme 2). Etching reaction with aqueous NaOH solution (pH ∼ 12.0) can cause both structural and compositional changes of differently shaped Cu2O nanoparticles. Upon addition of aqueous NaOH solution, Cu2O is oxidized to produce Cu(OH)2 in the presence of dissolved O2. Now the tetrahydroxocuprate(II) anion [Cu(OH)4]2− is formed under strong basic conditions, and ultimately thermodynamically stable black product CuO is formed and precipitated as solid particles. The chemical reaction involves for the formation of CuO nanoparticles in the presence of aqueous NaOH solution are as follows:

Figure 6. SAED patterns of (a) as-synthesized octahedral Cu2O nanoparticles, (b) NaOH-etched products, (c) TEA-etched products, and (d) oxalic acid etched products of octahedral Cu2O nanoparticles.

2Cu 2O +

1 O2 + 3H 2O + 2NaOH → 4Cu(OH)2 + 2Na + 2

Cu(OH)2 + 2OH− → [Cu(OH)4 ]2 − [Cu(OH)4 ]2 − ↔ CuO(S) + H 2O + 2OH−

During the etching process with triethylamine solution (TEA), Cu2O reacts with TEA to produce [Cu(Et3N)4]+ complex which is oxidized by dissolved O2 to form copper(II)-triethylamine complex [Cu(Et3N)4]+2 Cu 2O(S) + 8Et3N + H 2O → 2[Cu(Et3N)4 ]+ + 2OH−

Figure 7. HRTEM images of (a) as-synthesized octahedral Cu2O nanoparticles, (b) NaOH-etched products, (c) TEA-etched products, and (d) oxalic acid etched products of octahedral Cu2O nanoparticles.

4[Cu(Et3N)4 ]+ + O2 + 2H 2O → 4[Cu(Et3N)4 ]+2 + 4OH−

The presence of Cu2+ ion clearly indicates that oxidative dissolution of Cu2O nanoparticle has taken place in the presence of oxalic acid solution. The presence of Cu2+ ion in solution was confirmed by potassium ferrocyanide test

tration of 2-methylpyridine was increased, keeping all other reaction conditions the same, we obtained cubic Cu2O nanoparticle with six (100) facets instead of the truncated octahedral shape. High concentration of 2-methylpyridine is very helpful in growing Cu2O (100) planes rather than (111) planes. Therefore, concentrations of both CuSO4·5H2O and 2methylpyridine solution play an important role in shape evolution of Cu2O nanoparticles from Cu(OH)2. If we consider Cu2O nanoparticles with octahedral and truncated octahedral morphology, then it is observed that varying only the hydrolyzing (pyridine or 2-methylpyridine) agent caused the evolution of two different morphologies. It is known to us that

2Cu 2O + O2 + 8H+ → 4Cu+2 + 4H 2O

Mineralization of Dye Molecules. To investigate the shape-dependent visible light photocatalytic activity of the assynthesized Cu2O nanoparticles and chemically etched products, we have selected congo red as a model pollutant. Congo red is a water-soluble anionic dye having two −NN− groups. The aqueous solution of congo red shows maximum absorption at 498 nm, which can be assigned to the conjugate 24647



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Figure 8. Photodegradation of congo red solution in the presence of visible light using 0.01 g of (a) octahedral, (b) truncated octahedral, (c) cubic, (d) hollow octahedral, and (e) spherical Cu2O nanoparticles. (f) represents the corresponding ln A vs time plot.

system exhibited by the −NN− bonds. In a typical process, 0.01 g of as-synthesized differently shaped Cu2O nanoparticles were added to 20 mL of 5 × 10−5 M aqueous solution of congo red. At first, the mixture was stirred in the dark for 2 h before the photocatalytic measurement (in the presence of visible light) to establish adsorption−desorption equilibrium. Then 3 mL of the reaction mixture was taken out at different time intervals, and after centrifugation, absorbance spectra of the supernatant were measured by a UV−visible spectrophotometer. Figure 8a−e shows the extent of photodegradation of congo red solution as a function of irradiation time for octahedral, truncated octahedral, cubic, hollow octahedral and spherical Cu2O nanoparticles respectively. Adsorption ability as well as photocatalytic decomposition of congo red by differently shaped Cu2O nanoparticles follow the order hollow octahedron (d) > octahedron (a) > truncated octahedron (b) > cube (c) >

sphere (e). It can be seen that Cu2O hollow octahedron (d) and octahedron (a) are much more photocatalytically active than the other three, implying that (111) facets of Cu2O are responsible for the higher reactivity compared to other particles with (100) facets. Both hollow octahedral Cu2O and octahedral Cu 2O are bound by eight (111) facets. The highest photocatalytic activity of hollow octahedral Cu2O nanoparticles can be explained by considering its larger surface area and presence of many (111) facets. Truncated octahedral Cu2O is composed of six square (100) facets and eight triangular (111) facets, and cubic Cu2O is bound by six (100) facets. As (100) facets are less reactive than (111) facets, the extent of photodegradation is lower with cubes than with the truncated octahedron. The lowest photodegradation efficiency by spherical particles may be attributed to the lowest surface area. The photocatalytic reactivity of different shaped Cu2O nanoparticles demonstrates good surface-dependent activities. 24648



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Figure 9. Photodegradation of congo red solution in the presence of visible light using 0.01 g of (a) hollow octahedral Cu2O nanoparticles, (b) NaOH-etched products, (c) TEA-etched products, and (d) oxalic acid etched products of hollow octahedral Cu2O nanoparticles. (e) represents the corresponding ln A vs time plot.

of octahedral, truncated octahedral, and hollow octahedral Cu2O nanoparticles, it has been noticed that the adsorption ability as well as photocatalytic activity follow the order NaOHetched product > TEA-etched product > as-synthesized Cu2O > oxalic acid etched product. But in the case of chemically etched products of cubic and spherical Cu2O nanoparticles, it has been observed that the adsorption ability as well as photocatalytic activity follows the sequence NaOH-etched product > TEA-etched product > oxalic acid etched product > as-synthesized Cu2O nanoparticles. Here, in all the cases when the etching reaction is carried out in NaOH solution, stacked sheetlike nanostructures are formed with increased surface area. Edges and corners of both octahedral and truncated octahedral Cu2O nanoparticles become jagged after etching in TEA solution. The faces of cubic, hollow octahedral, and spherical Cu2O nanoparticles are roughened and become corrugated

Complete mineralization of congo red required about 70, 120, 180, 50, and 210 min for octahedral, truncated octahedral, cubic, hollow octahedral, and spherical Cu2O nanoparticles respectively (Figure 8a−e). The plots ln A vs time (min) for different Cu2O nanoparticle shapes exhibit a straight line having negative slopes (Figure 8f). So here the photocatalysis of congo red follows first-order kinetics obeying the equation ln A = −kt, where A is the absorbance, t the time, and k the reaction rate constant. The values of the rate constant for octahedral, truncated octahedral, cubic, hollow octahedral, and spherical Cu2O nanoparticles (0.01 g) are 0.0345, 0.0189, 0.0161, 0.0510, and 0.0146 min−1 respectively. Now the visible light photodegradation of congo red solution was performed with different chemically etched products of the as-synthesized Cu2O nanoparticles, keeping all other reaction conditions unaltered. In the case of chemically etched products 24649



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Figure 10. Photodegradation of congo red solution in the presence of visible light using 0.01 g of (a) cubic Cu2O nanoparticles, (b) NaOH-etched products, (c) TEA-etched products, and (d) oxalic acid etched products of cubic Cu2O nanoparticles. (e) represents the corresponding ln A vs time plot.

after etching in TEA solution. So in all the cases it has been found that photocatalytic activity of NaOH-etched products is higher than that of TEA-etched products. It has also been found that octahedral, truncated octahedral, and hollow octahedral Cu2O nanoparticles transformed to cubic and hollow cubic Cu2O nanoparticles after the etching reaction proceeds in oxalic acid solution. In all the above three cases, the reactivity of the as-synthesized Cu2O nanoparticles is higher than that of oxalic acid etched products because Cu2O (100) facets are less reactive than (111) facets. But the order is reversed in the case of oxalic acid etched products obtained from the cubic and spherical Cu2O nanoparticles, and it can be explained by the particle size and surface area. In both cases (cubic and spherical Cu2O), ill-defined cubic nanoparticles with rough surfaces are formed and their particle size becomes

smaller than that of the original cubic and spherical Cu2O nanoparticle. Decrease in particle size causes increase in surface area, which is highly advantageous for photocatalysis, as is also reflected in the order of photocatalytic reactivity. Figure 9a−d displays the extent of photodegradation of congo red solution as a function of irradiation time for the as-synthesized hollow octahedral Cu2O and NaOH, TEA, and oxalic acid etched products of Cu2O hollow octahedral nanoparticles. Complete mineralization of congo red required about 50, 20, 35, and 72 min for as-synthesized hollow octahedral Cu2O and NaOH, TEA, and oxalic acid etched products of Cu2O hollow octahedral nanoparticles, respectively. The plots ln A versus time (min) for different chemically etched products of hollow octahedral Cu2O nanoparticles exhibit a straight line having negative slope (Figure 9e). The values of the rate constant for 24650



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hollow octahedral Cu2O and NaOH, TEA, and oxalic acid etched products of hollow octahedral Cu2O nanoparticles (0.01 g) are 0.0510, 0.1183, 0.0871, and 0.0333 min−1, respectively. Figure 10a−d displays the extent of photodegradation of congo red solution versus time for cubic Cu2O nanoparticles and NaOH, TEA, and oxalic acid etched products of Cu2O cubic nanoparticles. The corresponding rate constant values obtained from the plots of ln A versus time (min) are 0.0161, 0.0268, 0.0186, and 0.0178 min−1 (Figure 10e). A slight decrease in photocatalytic activity was observed after the fifth consecutive cycle of photodegradation studies for congo red mineralization under visible light irradiation. The catalyst can be reutilized efficiently even after the fifth cycle. After completion of the photocatalytic reaction, the catalyst was removed from the reaction mixture by centrifugation. Then the samples were analyzed by X-ray diffraction and morphology was characterized again by TEM analysis. However, there were no changes in the XRD patterns or TEM images after the photocatalytic reaction. The above analysis demonstrates that our as-synthesized Cu2O nanoparticles and different chemically etched products have high photostability under the above experimental conditions, which is very important in practical applications. During semiconductor-initiated photocatalytic reaction, electron and hole pairs are created upon visible light excitation. The dissolved O2 reacts with photogenerated electrons to prepare intermediate superoxide radicals (O2·−), which upon protonation produce hydroperoxyl radicals (HO2·). Ultimately, hydrogen peroxide (H2O2) is formed from the hydroperoxyl radicals (HO2·). Then the photogenerated holes (h+) react with hydroxyl groups to form OH· radicals. The OH· radicals are known to be a very strong oxidant which then reacts with congo red dye, and eventually mineralization takes place (Scheme 3).

Scheme 3. Schematic Representation for the Photogeneration of Holes and Electrons in Different Types of Cu2O and Chemically Etched Products upon Visible Light Irradiation for Successive Mineralization of Congo Red

the (111) crystal plane. In addition, as-prepared Cu2O nanoparticles and chemically etched products exhibit facetdependent visible light photodegradation of congo red, a wellknown water pollutant. The differences in the rate of photocatalytic activity have been related to the exposed crystal planes, surface area, and particle size. The result demonstrates that the NaOH-etched product obtained from Cu2O hollow octahedron shows higher photocatalytic activity due to the presence of many (111) facets.

H 2O → OH− + H+

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O2 + e− → O2·−

FESEM, TEM, SAED, and HRTEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

ASSOCIATED CONTENT

S Supporting Information *

HO2· + HO2· ↔ H 2O2 + O2

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OH− + h+ → OH·

Notes

O2·− + H+ → HO2·

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. The authors declare no competing financial interest.

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In summary, photocatalytic activity of the as-synthesized Cu2O nanoparticles and different chemically etched products has a close relationship with the exposed crystal planes, surface area, and particle size. It can be clearly seen that NaOH-etched products of Cu2O hollow octahedron demonstrate the highest photocatalytic activity in comparison to other particles because of larger surface area and increased number of (111) facets.

ACKNOWLEDGMENTS The authors are thankful to the UGC, DST, BRNS, and CSIR New Delhi for financial assistance and IIT Kharagpur for research facilities. The authors are also thankful to Miss Isozaki of Tokyo University of Science, Tokyo, Japan, for XPS measurement.

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■

CONCLUSIONS In summary, we demonstrate a simple, facile, surfactant-free approach for obtaining Cu2O nanoparticles of different shapes. The role of hydrolyzing agent, complexing agent, and reducing agent are very important to achieve shape-dependent nanoparticles. All these parameters have been accounted for in this report for the syntheses of octahedral, truncated octahedral, hollow octahedral, cubic, and spherical Cu2O nanoparticles. Then we introduced NaOH, TEA, and oxalic acid as independent etching agent for new morphology evolution of Cu2O nanoparticles in aqueous solution. It has been observed that the stability of the (100) crystal plane is higher than that of

REFERENCES

(1) Liu, L.; Peng, Q.; Li, Y. Preparation of CdSe Quantum Dots with Full Color Emission Based on a Room Temperature Injection Technique. Inorg. Chem. 2008, 47, 5022−5028. (2) Liu, J.; Yang, X.; Wang, K.; Wang, D.; Zhang, P. Chemical Etching with Tetrafluoroborate: A Facile Method for Resizing of CdTe Nanocrystals under Mild Conditions. Chem. Commun. 2009, 40, 6080−6082. (3) Liu, J.; Aruguete, D. M.; Jinschek, J. R.; Rimstidt, J. D.; Hochella, M. F. The Non-Oxidative Dissolution of Galena Nanocrystals: Insights into Mineral Dissolution Rates as a Function of Grain Size, Shape and Aggregation State. Geochim. Cosmochim. Acta 2008, 72, 5984−5996.

24651



Article

Aggregation and Acidic Etching. J. Am. Chem. Soc. 2008, 130, 12815−12820. (25) Wu, S.; Liu, T.; Zeng, W.; He, J.; Yu, W.; Gou, Z. Rose-Like Cu2O Synthesized by Assisted PVP K30 Hydrothermal Method. J. Mater. Sci.: Mater. Electron 2013, 24, 2404−2409. (26) Basu, M.; Sinha, A. K.; Pradhan, M.; Sarkar, S.; Pal, A.; Mondal, C.; Pal, T. Methylene Blue-Cu2O Reaction Made Easy in Acidic Medium. J. Phys. Chem. C 2012, 116, 25741−25747. (27) Wang, W.; Tu, Y.; Zhang, P.; Zhang, G. Surfactant-Assisted Synthesis of Double-Wall Cu2O Hollow Spheres. CrystEngComm 2011, 13, 1838−1842. (28) Ahmed, A.; Gajbhiye, N. S.; Joshi, A. G. Low Cost, SurfactantLess, One Pot Synthesis of Cu2O Nanooctahedra at Room Temperature. J. Solid State Chem. 2011, 184, 2209−2214. (29) Kong, C.; Sun, S.; Zhang, X.; Song, X.; Yang, Z. NanoparticleAggregated Hollow Copper Microcages and Their Surface-Enhanced Raman Scattering Activity. CrystEngComm 2013, 15, 6136−6139. (30) Yu, Y.; Zhang, L.; Wang, J.; Yang, Z.; Long, M.; Hu, N.; Zhang, Y. Preparation of Hollow Porous Cu2O Microspheres and Photocatalytic Activity under Visible Light Irradiation. Nanoscale Res. Lett. 2012, 7, 347/1−347/6. (31) Sui, Y.; Fu, W.; Zeng, Y.; Yang, H.; Zhang, Y.; Chen, H.; Li, Y.; Li, M.; Zou, G. Synthesis of Cu2O Nanoframes and Nanocages by Selective Oxidative Etching at Room Temperature. Angew. Chem., Int. Ed. 2010, 49, 4282−4285. (32) Sun, S.; You, H.; Kong, C.; Song, X.; Ding, B.; Yang, Z. EtchingLimited Branching Growth of Cuprous Oxide During Ethanol-Assisted Solution Synthesis. CrystEngComm 2011, 13, 2837−2840. (33) Wang, Z.; Luan, D.; Li, C. M.; Su, F.; Madhavi, S.; Boey, F. Y. C.; Lou, X. W. Engineering Nonspherical Hollow Structures with Complex Interiors by Template-Engaged Redox Etching. J. Am. Chem. Soc. 2010, 132, 16271−16277. (34) Qiu, C.; Bao, Y.; Netzer, N. L.; Jiang, C. Structure Evolution and SERS Activation of Cuprous Oxide Microcrystals via Chemical Etching. J. Mater. Chem. A 2013, 1, 8790−8797. (35) Kar, A.; Smith, Y. R.; Subramanian, V. Improved Photocatalytic Degradation of Textile Dye Using Titanium Dioxide Nanotubes Formed over Titanium Wires. Environ. Sci. Technol. 2009, 43, 3260− 3265. (36) Ma, W.; Li, J.; Tao, X.; He, J.; Xu, Y.; Yu, J. C.; Zhao, J. Efficient Degradation of Organic Pollutants by Using Dioxygen Activated by Resin-Exchanged Iron(II) Bipyridine under Visible Irradiation. Angew. Chem., Int. Ed. 2003, 42, 1029−1032. (37) Zhuang, X.; Wan, Y.; Feng, C.; Shen, Y.; Zhao, D. Highly Efficient Adsorption of Bulky Dye Molecules in Wastewater on Ordered Mesoporous Carbons. Chem. Mater. 2009, 21, 706−716. (38) Georgi, A.; Schierz, A.; Trommler, U.; Horwitz, C. P.; Collins, T. J.; Kopinke, F.-D. Humic Acid Modified Fenton Reagent for Enhancement of the Working pH Range. Appl. Catal. B: Environ. 2007, 72, 26−36. (39) Pazdzior, K.; Klepacz, S. A.; Ledakowicz, S.; Sojka, L. J.; Mrozinska, Z.; Zylla, R. Integration of Nanofiltration and Biological Degradation of Textile Wastewater Containing Azo Dye. Chemosphere 2009, 75, 250−255. (40) Mondal, C.; Ganguly, M.; Sinha, A. K.; Pal, J.; Sahoo, R.; Pal, T. Robust Cubooctahedron Zn3V2O8 in Gram Quantity: A Material for Photocatalytic Dye Degradation in Water. CrystEngComm 2013, 15, 6745−6751. (41) Liu, X.−W.; Wang, F.−Y.; Zhen, F.; Huang, J.−R. In Situ Growth of Au Nanoparticles on the Surfaces of Cu2O Nanocubes for Chemical Sensors with Enhanced Performance. RSC Adv. 2012, 2, 7647−7651. (42) Liu, Y.; Liao, L.; Li, J.; Pan, C. From Copper Nanocrystalline to CuO Nanoneedle Array. Synthesis, Growth Mechanism, and Properties. J. Phys. Chem. C 2007, 111, 5050−5056. (43) Wu, C.−K.; Yin, M.; O’Brien, S.; Koberstein, J. T. Quantitative Analysis of Copper Oxide Nanoparticle Composition and Structure by X-ray Photoelectron Spectroscopy. Chem. Mater. 2006, 18, 6054− 6058.

(4) Xiong, J.; Das, S. N.; Kar, J. P.; Choi, J. H.; Myoung, J. M. A Multifunctional Nanoporous Layer Created on Glass through a Simple Alkali Corrosion Process. J. Mater. Chem. 2010, 20, 10246−10252. (5) Fraunhofer, J. Joseph von Fraunhofer Gesammelte Schriften, Munich, Germany, 1888. (6) Minot, M. J. Single-Layer, Gradient Refractive Index Antireflection Films Effective from 0.35 to 2.5 μ. J. Opt. Soc. Am. 1976, 66, 515−519. (7) Hua, Q.; Chen, K.; Chang, S.; Ma, Y.; Huang, W. Crystal PlaneDependent Compositional and Structural Evolution of Uniform Cu2O Nanocrystals in Aqueous Ammonia Solutions. J. Phys. Chem. C 2011, 115, 20618−20627. (8) Chinyama, G.; Roos, A.; Karlsson, B. Stability of Antireflection Coatings for Large Area Glazing. Sol. Energy 1993, 50, 105−111. (9) Spierings, G. Wet Chemical Etching of Silicate Glasses in Hydrofluoric Acid Based Solutions. J. Mater. Sci. 1993, 28, 6261−6273. (10) Grosse, A.; Grewe, M.; Fouckhardt, H. Deep Wet Etching of Fused Silica Glass for Hollow Capillary Optical Leaky Waveguides in Microfluidic Devices. J. Micromech. Microeng. 2001, 11, 257−262. (11) Liu, M.; Zheng, Y.; Zhang, L.; Guo, L.; Xia, Y. Transformation of Pd Nanocubes into Octahedra with Controlled Sizes by Maneuvering the Rates of Etching and Regrowth. J. Am. Chem. Soc. 2013, 135, 11752−11755. (12) Hua, Q.; Shang, D.; Zhang, W.; Chen, K.; Chang, S.; Ma, Y.; Jiang, Z.; Yang, J.; Huang, W. Morphological Evolution of Cu2O Nanocrystals in an Acid Solution: Stability of Different Crystal Planes. Langmuir 2011, 27, 665−671. (13) Briskman, R. N. A Study of Electrodeposited Cuprous Oxide Photovoltaic Cells. Sol. Energy Mater. Sol. Cells 1992, 27, 361−368. (14) Li, J. H.; Tang, B. X.; Tao, L. M.; Xie, Y. X.; Zhang, M. B. Reusable Copper-Catalyzed Cross-Coupling Reactions of Aryl Halides with Organotins in Inexpensive Ionic Liquids. J. Org. Chem. 2006, 71, 7488−7490. (15) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Taracon, J. M. Nano-Sized Transition-Metal Oxides as Negative-Electrode Materials for Lithium-ion Batteries. Nature 2000, 407, 496−499. (16) Zhang, J.; Liu, J.; Peng, Q.; Wang, X.; Li, Y. Nearly Monodisperse Cu2O and CuO Nanospheres: Preparation and Applications for Sensitive Gas Sensors. Chem. Mater. 2006, 18, 867− 871. (17) White, B.; Yin, M.; Hall, A.; Le, D.; Stolbov, S.; Rahman, T.; Turro, N.; O’Brien, S. Complete CO Oxidation over Cu2O Nanoparticles Supported on Silica Gel. Nano Lett. 2006, 6, 2095− 2098. (18) Feng, L.; Zhang, C.; Gao, G.; Cui, D. Facile Synthesis of Hollow Cu2O Octahedral and Spherical Nanocrystals and Their MorphologyDependent Photocatalytic Properties. Nanoscale Res. Lett. 2012, 7, 276/1−276/10. (19) Zhang, D.-F.; Zhang, H.; Guo, L.; Zheng, K.; Han, X.-D.; Zhang, Z. Delicate Control of Crystallographic Facet-Oriented Cu2O Nanocrystals and the Correlated Adsorption Ability. J. Mater. Chem. 2009, 19, 5220−5225. (20) Wang, W. Z.; Wang, G. H.; Wang, X. S.; Zhan, Y. J.; Liu, Y. K.; Zheng, C. L. Synthesis and Characterization of Cu2O Nanowires by a Novel Reduction Route. Adv. Mater. 2002, 14, 67−69. (21) Sui, Y. M.; Fu, W. Y.; Zeng, Y.; Yang, H. B.; Zhang, Y. Y.; Chen, H.; Li, Y. X.; Li, M. H.; Zou, G. T. Synthesis of Cu2O Nanoframes and Nanocages by Selective Oxidative Etching at Room Temperature. Angew. Chem., Int. Ed. 2010, 49, 4282−4285. (22) Ho, J.-Y.; Huang, M. H. Synthesis of Submicrometer-Sized Cu2O Crystals with Morphological Evolution from Cubic to Hexapod Structures and Their Comparative Photocatalytic Activity. J. Phys. Chem. C 2009, 113, 14159−14164. (23) Li, S.-K.; Guo, X.; Wang, Y.; Huang, F.-Z.; Shen, Y.-H.; Wang, X.-M.; Xie, A.-J. Rapid Synthesis of Flower-Like Cu2O Architectures in Ionic Liquids by the Assistance of Microwave Irradiation with High Photochemical Activity. Dalton Trans. 2011, 40, 6745−6750. (24) Kuo, C.-H.; Huang, M. H. Fabrication of Truncated Rhombic Dodecahedral Cu2O Nanocages and Nanoframes by Particle 24652



Article

(44) Xiang, G.; Zhuang, J.; Wang, X. Morphology-Controlled Synthesis of Inorganic Nanocrystals via Surface Reconstruction of Nuclei. Inorg. Chem. 2009, 48, 10222−10230.

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Crystal-Plane-Dependent Etching of Cuprous Oxide Nanoparticles of

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