Compositions, Structures, and Catalytic Activities of CeO2@Cu2O

Article pubs.acs.org/Langmuir

Compositions, Structures, and Catalytic Activities of CeO2@Cu2O Nanocomposites Prepared by the Template-Assisted Method Huizhi Bao, Zhenhua Zhang, Qing Hua, and Weixin Huang* Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, China S Supporting Information *

ABSTRACT: CeO2@Cu2O nanocomposites were prepared from Cu2O cubes and octahedra by the template-assisted method involving the liquid (Ce(IV))−solid (Cu2O) interfacial reaction. Their compositions, structures, and catalytic activities in CO oxidation were studied in detail. Under the same reaction conditions, CeO2@Cu2O nanocomposites prepared from cubic and octahedral Cu2O templates exhibit different compositions and structures. With an increasing amount of Ce(IV) reactant, a smooth CeO2−CuOx shell develops on the surface of Cu2O cubes and eventually void cubic core/multishell Cu2O/CeO2−CuOx nanocomposites form; however, a rough CeO2−CuOx shell develops on the surface of Cu2O octahedra, and eventually hollow octahedral CeO2−CuOx nanocages form. The formation of different compositions and structures of CeO2@Cu2O nanocomposites was correlated with the different exposed crystal planes and surface reactivities of Cu2O cubes and octahedra. The catalytic activity of CeO2@Cu2O nanocomposites in CO oxidation depends on their compositions and structures. The most active CeO2@Cu2O nanocomposites become active at 70 °C and achieve a 100% CO conversion at 170 °C. These results broaden the versatility of Cu2O nanocrystals as the sacrificial template for the fabrication of novel nanocomposites with core/shell and hollow nanostructures and exemplify the morphology effect of Cu2O nanocrystals in liquid−solid interfacial reactions with respect to the composition, structure, and properties of nanocomposites prepared by the template-assisted method. composites with various morphologies. Zhang et al.20 first reported the synthesis of hollow octahedral polyanilline micro/ nanostructures by using Cu2O octahedra as a template in the presence of H3PO4. Jiao et al.21 demonstrated the synthesis of uniform nonspherical Cu2S mesocages with single-crystalline shells by using shape-controlled Cu2O crystals as sacrificial templates. Qin et al.22 fabricated Au and Au-CuO cubic microcages using Cu2O cubes as the sacrificial template. Liu23,24 synthesized Au−Cu2O nanocomposites and porous Au nanocages with various morphologies by the template-assisted method. Kuo et al.25 employed cubic and octahedral Cu2O nanocrystals and Au@Cu2O core−shell heterostructures as sacrificial templates to synthesize cubic Cu2S nanocages and Au−Cu2S core−cage structures with thin walls. Lou’s group26,27 systematically demonstrated the controlled synthesis of various uniform core/shell and hollow nanocomposites including Cu2O@Fe(OH)x nanorattles, Fe(OH)x nanocages, Cu2O@Au nanocomposites, Cu2O@MnOx nanorattles, SnO2 nanoboxes, and ZrO2 nanocages by template-engaged redox etching of shape-controlled Cu2O nanocrystals. Sohn et al.28 reported the

1. INTRODUCTION Multicomponent nanocomposites with controlled composition and structure have potential applications in photonic crystals, catalysis, drug delivery, sensing, and rechargeable batteries and thus have generated considerable interest.1−6 A diverse range of methods have been explored for the controlled synthesis of nanocomposites with novel structures. The template-assisted method, in which solid product forms on a sacrificial solid template via the liquid−solid interfacial reaction, is capable of taking advantage of the great success achieved in the structurally controlled synthesis of single-component nanostructures and thus has been demonstrated to be a powerful strategy in the controlled synthesis of core/shell or hollow nanocomposites. For example, Pt−Ag, Pd−Ag, and Au−Ag nanoboxes were successfully synthesized by a galvanic replacement reaction using a Ag nanostructure as the template.7,8 In the template-assisted method, the sacrificial solid template acts as the structure-directing scaffold for the solid product, and thus its size and morphology are generally inherited by the resulting solid product. Due to their rich and well-controlled morphologies, including cubes,9−11 octahedra,12 truncated octahedra,13,14 nanowires,15−18nanoplates,19 and appropriate reactivity, Cu2O nanocrystals have been much explored as sacrificial templates for the controlled growth of core/shell and hollow nano© XXXX American Chemical Society

Received: February 4, 2014 Revised: May 11, 2014

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dx.doi.org/10.1021/la501406w | Langmuir XXXX, XXX, XXX−XXX

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CeO2@Cu2O nanocomposites were synthesized by the templateassisted method employing Cu2O cubes and octahedra as the sacrificial templates. In a typical procedure, 2 mL of a NaCl aqueous solution (0.855 mol/L) was mixed with a suspension of 20 mg of the desired Cu2O nanocrystals in 80 mL of ethanol. The mixture was adequately dispersed under ultrasonication for 30 min and then transferred to an oil bath at 40 °C. Twenty milliliters of a (NH4)2Ce(NO3)6 ethanol solution (0.1 mmol/L) was added dropwise to the above mixture under vigorous stirring. After an additional 1 h of reaction, the solid product was collected by several rinse−centrifugation cycles and then dried in vacuum at room temperature. We changed the volumes of added (NH4)2Ce(NO3)6 ethanol solution and NaCl aqueous solution to change the amounts of added (NH4)4Ce(NO3)6 and NaCl, respectively. The elemental composition of the CeO2@Cu2O nanocomposites was analyzed with an Optima 7300 DV inductively coupled plasma atomic emission spectrometer (ICP-AES). Powder X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert PROS diffractometer using a nickel-filtered Cu Kα (wavelength = 0.15418 nm) radiation source with the operation voltage and operation current being 40 kV and 50 mA, respectively. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 high-performance electron spectrometer using monochromatized Al Kα (hν = 1486.7 eV) as the excitation source. The likely charging of samples was corrected by setting the binding energy of the adventitious carbon (C 1s) to 284.5 eV. Scanning electron microscope (SEM) experiments were performed on a JEOL JSM-6700 field emission scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) experiments were performed on a JEM2100F high-resolution transmission electron microscope, and elemental mapping images were acquired on an Oxford X MAX accessory. The catalytic activity in CO oxidation was evaluated with a fixed-bed flow reactor. As-synthesized CeO2@Cu2O nanocomposites were annealed in pure Ar at 300 °C for 1 h (heating rate = 2 °C/min, flow rate = 20 mL/min) to prepare CeO2@Cu2O nanocomposite catalysts. The reaction gas consisting of 1% CO and 99% dry air was fed into 50 mg of the CeO2@Cu2O nanocomposite catalyst diluted with 50 mg of SiO2 at a rate of 20 mL/min. The catalyst was heated to the desired reaction temperatures at a rate of 2 °C/min and then kept for 35 min until the catalytic reaction reached the steady state. The composition of effluent gas was analyzed with an online GC-14 gas chromatograph. The conversion of CO was calculated from the change in CO concentration in the inlet and outlet gases.

fabrication of hollow metal oxide nanocrystals by etching Cu2O nanocrystal templates with metal(II) ions. In the template-assisted method, the liquid−solid interfacial reaction is always initiated on the surface of the sacrificial template. The morphology of oxide nanocrystals determines their exposed crystal planes and subsequently their surface composition/structure and thus strongly affects their reactivity.29 In the case of Cu2O nanocrystals, we have previously reported the morphology-dependent reactivity of Cu2O nanocrystals in the gas−solid reaction,30,31 the oxidative dissolution reaction,32 and the coordinative dissolution reaction33 as well as morphology-dependent catalytic performances in CO oxidation34,35 and propylene oxidation with molecular O2.36 It is thus expected that the morphology of sacrificial Cu2O nanocrystals can affect the compositional and structural evolution of the resulting nanocomposites. This issue is interesting because on one hand it can advance our fundamental understanding of solid reactivity; on the other hand, it might provide a facile method for tuning the composition, structure, and thus properties of resulting nanocomposites prepared by the template-assisted method. CeO2-based catalysts have wide application in heterogeneous catalysis,37 and their catalytic performance has been demonstrated to be composition- and morphology-dependent.29,38−46 CuO-CeO2 catalysts are active in catalyzing the oxidation of CO.38,47,48 In this article, we have studied the fabrication of CeO2@Cu2O nanocomposites by the template-assisted method and their catalytic activity in CO oxidation with an emphasis on establishing and understanding the morphology effect of the sacrificial Cu2O nanocrystals. Uniform Cu2O cubes and octahedra respectively exposing well-defined (100) and (111) crystal planes will greatly facilitate the correlations between their morphologies and the compositional/structural evolution of the resulting CeO2@Cu2O nanocomposites and thus were employed as the sacrificial templates in our studies. The composition and structure of synthesized CeO2@Cu2O nanocomposites from cubic and octahedral Cu2O templates under the same reaction conditions were comparatively studied in detail in order to establish the morphology effect of sacrificial Cu2O nanocrystals and understand the underlying microscopic mechanism.

3. RESULTS AND DISCUSSION Figure 1 shows typical SEM and TEM images of the assynthesized Cu2O nanocrystals. Uniform cubic and octahedral Cu2O nanocrystals with edge lengths of 500−1000 nm were successfully synthesized. TEM and ED patterns confirm that cubic and octahedral Cu2O nanocrystals expose {100} and {111} crystal planes, respectively. Our previous XPS results demonstrated that the surfaces of as-synthesized Cu2O nanocrystals remain as Cu2O.30 As-synthesized Cu2O cubes and octahedra were then employed as sacrificial templates to prepare CeO2@Cu2O nanocomposites by the template-assisted method. Ce(IV) (0.0125, 0.025, 0.05, and 0.1 mmol ) was added during the experiments, giving calculated Ce/Cu atomic ratios of 0.045, 0.09, 0.18, 0.36, respectively. We herein denote CeO2@Cu2O nanocomposites as CeO2@c/o-Cu2O-x, in which c and o correspond to cubic and octahedral Cu2O nanocrystal templates and x is the calculated Ce/Cu atomic ratio. The actual compositions of various CeO2@Cu2O nanocomposites determined by ICP-AES are summarized in Table 1. The compositional evolutions of CeO2@c-Cu2O and CeO2@oCu2O nanocomposites are different, and under the same

2. EXPERIMENTAL SECTION All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without further purification. Uniform cubic and octahedral Cu2O nanocrystals were synthesized following Zhang et al.’s recipe.13 To synthesize cubic Cu2O nanocrystals, 10 mL of a NaOH aqueous solution (2.0 mol/L) was added dropwise to 100 mL of a CuCl2 aqueous solution (0.01 mol/L). After the mixture was adequately stirred for 0.5 h, 10 mL of an ascorbic acid aqueous solution (0.6 mol/L) was added dropwise to the solution. The mixed solution was adequately stirred for 5 h at 55 °C. The resulting precipitate was collected by centrifugation, then washed with distilled water and absolute ethanol, and finally dried in vacuum at room temperature for 12 h. To synthesize octahedral Cu2O nanocrystals, 10 mL of a NaOH aqueous solution (2.0 mol/L) was added dropwise to 100 mL of a CuCl2 aqueous solution (0.01 mol/L) containing 4.44 g of poly(vinylpyrrolidone) (PVP, Mw = 30 000) at 55 °C. After the mixture was adequately stirred for 0.5 h, 10 mL of an ascorbic acid aqueous solution (0.6 mol/L) was added dropwise to the solution. The mixed solution was adequately stirred for 3 h at 55 °C. The resulting precipitate was collected by centrifugation, then washed with distilled water and absolute ethanol, and finally dried in vacuum at room temperature for 12 h. B


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2D,F). In [email protected], which has the highest Ce content, besides the smooth shell initially formed on the original sacrificial Cu2O cubes, an additional cubic shell develops on the surface of the shrinking cubic core, forming void core/double-shell nanostructures (Figure 2H). Figure 3 shows TEM and HRTEM images of a representative [email protected] core/shell nanostructure. The smooth shell is polycrystalline and exhibits lattice fringes of 0.19 and 0.31 nm which correspond to the (220) and (111) planes of CeO2 (JCPDS card 81-0792), respectively. We further measured the elemental distribution of a representative [email protected] core/shell nanostructure by an elemental mapping technique. As shown in Figure 4A1−A3, the core region is Cu2O and the shell region is CeO2, but weak Cu signals were also mapped in the shell region. Figure 5 shows typical SEM and TEM images of various CeO2@o-Cu2O nanocomposites. Similar to the case of CeO2@ c-Cu2O nanocomposites, the SEM images demonstrate that CeO2@o-Cu2O nanocomposites inherit the octahedral morphology of sacrificial Cu2O octahedra, but their surfaces are quite rough. Moreover, as indicated in their SEM image (Figure 5G), [email protected] nanocomposites have a hollow octahedral nanostructure. The structural evolution of CeO2@ o-Cu2O nanocomposites is clearly demonstrated by TEM images. In [email protected], small adparticles could be observed to form on the surface of Cu2O octahedra (Figure 5B); meanwhile, many nanoparticles form away from Cu2O octahedra (Figure 5A,B). The corresponding HRTEM images of adparticles on the surface of Cu2O octahedra (Figure S1A and S1B) and nanoparticles away from Cu2O octahedra (Figure S1C and S1D) demonstrate that they are all CeO2. This indicates that CeO2 nucleates and grows both on the surface of Cu2O octahedra and in the solution. With the increase in Ce content, rough shells develop and void octahedral core/shell nanostructures form (Figure 5D,F). In [email protected], which has the greatest Ce content, hollow octahedral nanostructures with rough shells form (Figure 5H). Figure 6 shows TEM and HRTEM images of a representative [email protected] hollow octahedral nanostructure. The rough shell is polycrystalline and consists of irregularly stacked nanosized grains. The observed lattice fringe of 0.31 nm arises from the (111) planes of CeO2. The elemental distribution of a representative [email protected] core/shell nanostructure was measured by an elemental mapping technique. As shown in Figure 4B1−B3, the core region is Cu2O and the shell region is CeO2, but weak Cu signals were also mapped in the shell region. The microscopy results above demonstrate the successful synthesis of CeO2@Cu2O nanocomposites by employing the template-assisted method with Cu2O nanocrystals as the sacrificial template. CeO2@Cu2O nanocomposites well reproduce the morphology of the employed Cu2O nanocrystal template and exhibit core/shell, void core/shell, and hollow nanostructures. These results add examples to demonstrate Cu2O nanocrystals as versatile sacrificial templates for the fabrication of novel nanocomposites with core/shell and hollow nanostructures.20−28 Additional spectroscopic characterizations including XRD and XPS were performed to further probe the compositions and structures of CeO2@Cu2O nanocomposites. [email protected] exhibits strong diffraction peaks of Cu2O and very weak and diffuse diffraction peaks of CeO2 (Figure 7A), in agreement with its core (Cu2O)/shell (CeO2) nanostructure. [email protected] exhibits broad diffraction

Figure 1. Representative SEM and TEM images of as-synthesized cubic (A, C) and octahedral (B, D) Cu2O nanocrystals. The insets show the corresponding electron diffraction patterns.

Table 1. Compositions and Surface Compositions of Various CeO2@Cu2O Nanocomposites Respectively Determined by ICP and XPS sample

Ce/Cu atomic ratio

surface Ce/Cu atomic ratio

[email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected]

0.025 0.051 0.090 0.19 0.053 0.11 0.25 0.90

0.11 0.20 0.38 0.49 0.50 0.22 0.23 0.70

reaction conditions, the actual Ce/Cu atomic ratio of CeO2@cCu2O nanocomposite is smaller than that of the corresponding CeO2@o-Cu2O nanocomposite. This demonstrates that the Cu2O−Ce(IV) solid−liquid reaction proceeds much faster on Cu2O octahedra than on Cu2O cubes. Figure 2 shows typical SEM and TEM images of various CeO2@c-Cu2O nanocomposites. The SEM images demonstrate that CeO2@c-Cu2O nanocomposites exhibit uniform cubic nanostructures with their edge lengths similar to those of sacrificial Cu2O cubes. The surfaces of cubic CeO2@c-Cu2O nanocomposites are rather smooth. The TEM images demonstrate that CeO2@c-Cu2O nanocomposites actually have a cubic core/shell nanostructure, and the smooth shell well duplicates the cubic morphology of sacrificial Cu2O cubes. The core/shell nanostructure of CeO2@c-Cu2O nanocomposites is also evidenced by SEM images in which parts of the shells flake away and the cores become visible. The core/shell nanostructure of CeO2@c-Cu2O nanocomposites evolves with their composition. In [email protected], a smooth and continuous thin shell develops on the surfaces of Cu2O cubes (Figure 2B). With the increase in Ce content, the cubic core keeps shrinking and the space between the shell and the core becomes large, forming void core/shell nanostructures (Figure C


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Figure 2. SEM and TEM images of (A, B) [email protected], (C, D) [email protected], (E, F) [email protected], and (G, H) [email protected].

spectra of [email protected], [email protected], and CeO2@ Cu2O-0.18, but both Cu2O and CuO (Cu 2p3/2 binding energy at ∼933.6 eV and the associated satellite peak)49 features were observed in the Cu 2p XPS spectrum of [email protected]. Thus, CuO was formed during the reactions between Cu2O nanocrystals and Ce(IV) with large amounts of Ce(IV), but its formation was not detected by XRD, indicating that CuO is a minor product. The XPS results also demonstrate that the surfaces of CeO2@Cu2O nanocomposites within the detection range of XPS (∼5 nm) consist of both CeO2 and CuOx. The surface Ce/Cu atomic ratios of CeO2@Cu2O nanocomposites were evaluated from XPS results and are summarized in Table 1. For CeO2@c-Cu2O nanocomposites, the surface Ce/Cu atomic ratio is always larger than the corresponding bulk Ce/ Cu atomic ratio and it increases almost linearly with the corresponding bulk Ce/Cu atomic ratio until [email protected] is reached. Thus, CeO2 is always enriched in the surface region of CeO2@c-Cu2O nanocomposites, indicating the occurrence of the chemical reactions between Ce(IV) and Cu2O cubes dominantly within the surface region of Cu2O cubes. The observation that the surface enrichment of CeO2 is less in [email protected] than in other CeO2@c-Cu2O composites could be related to the formation of CuO on the surfaces of [email protected]. The surface compositional evolution of CeO2@o-Cu2O nanocomposites differs greatly from that of CeO2@c-Cu2O nanocomposites. The surface Ce/ Cu atomic ratio is much larger than the bulk Ce/Cu atomic ratio for the [email protected] nanocomposite, demonstrating an obvious surface enrichment of CeO2; however, the surface Ce/Cu atomic ratio greatly decreases for [email protected] although its bulk Ce/Cu atomic ratio increases, and the surface Ce/Cu atomic ratio approaches the bulk Ce/Cu atomic ratio for the [email protected] nanocomposite. These observations demonstrate that the formed CeO2 is initially enriched in the surface region for CeO2@o-Cu2O nanocomposites with small bulk Ce/Cu atomic ratios and then distributed more and more homogeneously with Cu2O with the increase in the bulk Ce/Cu atomic ratio. This indicates that the

Figure 3. (A) TEM image of a typical [email protected] core/shell cubic nanocomposite, (B) TEM image of its shell regions, (C) HRTEM image of the region indicated by the dashed white square in (B), and (D) enlarged HRTEM image of the area indicated by the dashed white square in (C).

peaks of CeO2 but no diffraction peaks of Cu2O (Figure 7B), in agreement with its hollow CeO2 nanostructure. These XRD results show that CeO2 in as-synthesized CeO2@Cu2O nanocomposites has poor crystallinity. Figure 8 shows the Ce 3d and Cu 2p XPS spectra of CeO2@ Cu2O nanocomposites. The Ce 3d XPS spectra of all of the nanocomposites are dominated by the Ce(IV) features, but the Ce 3d5/2/3d3/2 peak pair at ∼885.0/903.2 eV arising from Ce(III)49 is also visible. Only the Cu2O feature with a Cu 2p3/2 binding energy at ∼932.2 eV49 was observed in the Cu 2p XPS D


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Figure 4. TEM images (A1, B1), Cu Kα1 elemental mapping images (A2, B2), and (C) Ce Lα1 elemental mapping images (A3, B3) of typical [email protected] (A1−A3) and [email protected] (B1−B3) nanocomposites.

Figure 5. SEM and TEM images of (A, B) [email protected], (C, D) [email protected], (E , F) [email protected], and (G, H) [email protected].

Fe(OH)x,26 and the reactions involved in the system of Cu2O nanocrystals and SnCl4 were proposed to be the reaction between Cu2O and SnCl4 to form CuCl and SnO2, followed by the coordinative dissolution of CuCl with Cl− to [CuClx]1−x.27 In our case, CeO2 is the dominant solid product, but minor CuO forms when the added amount of Ce(IV) is large. We proposed that the etching of Cu2O nanocrystals is initiated by the redox reaction between Cu2O and Ce(IV). The involved chemical reactions are listed as the following:

chemical reactions between Ce(IV) and Cu2O octahedra initially occur mainly within the surface region of Cu2O octahedra but then extend to the bulk as they proceed. The smaller surface Ce/Cu atomic ratio of [email protected] compared to the bulk Ce/Cu atomic ratio could be related to the formation of CuO on their surfaces. The formation of void cubic and octahedral core/shell Cu2O/CeO2 nanostructures and hollow octahedral CeO2 nanostructures could be qualitatively explained by the template-engaged etching of shape-controlled Cu2O nanocrystals proposed by Wang et al.26,27 The involved chemical reactions differ in different systems. The reactions involved in the system of Cu2O nanocrystals and FeCl3 were proposed to be the redox reaction between Cu2O and Fe(III) to form Cu(II) and Fe(II) followed by the nucleation and growth of

Cu 2O(s) + 2Ce 4 + + H 2O → 2Cu 2 + + 2Ce3 + + 2OH− (1)

Ce 4 + + 4OH− → Ce(OH)4 (s) → CeO2 (s) + 2H 2O (2) E


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Figure 6. (A) TEM image of a typical [email protected] hollow octahedral nanocomposite, (B) TEM image of its shell regions, (C) HRTEM image of the region indicated by the dashed white square in (B), and (D) enlarged HRTEM image of the area indicated by the dashed white square in (C).

Figure 8. (A) Ce 3d and (B) Cu 2p XPS spectra of various CeO2@ Cu2O nanocomposites.

and the precipitation reaction of Ce(IV) to form nanostructures well reproducing the morphology of Cu2O templates. Ethanol with NaCl aqueous solution first developed by Wang et al.26,27 is an appropriate reaction medium. We studied the synthesis processes at different volumes of added NaCl aqueous solution (Figure S2). Without NaCl aqueous solution, no obvious reaction could be observed to occur. Thus, the reactions proceed very slowly in ethanol with low ionizability. Ethanol was also reported to exert a stabilizing effect on the Cu2O surface that prevents the Cu2O surface from undergoing chemical reactions.50 With increasing amounts of NaCl aqueous solution, CeO2 nanostructures appear and evolve into those well reproducing the morphologies of cubic and octahedral Cu2O nanocrystal templates. However, the aggregation of irregular CeO2 nanoparticle was obtained when the reaction proceeded in water instead of in ethanol with NaCl aqueous solution, which is likely due to the fast precipitation reaction rate of Ce(IV). Therefore, in the medium of ethanol with NaCl aqueous solution, the rates of reactions involved in the template-assisted method are finely controlled by the controllable addition of NaCl aqueous solution. Although the involved chemical reactions are same, we observed obvious differences between the liquid−solid reactions of Ce(IV) with Cu2O octahedra and cubes. In [email protected] nanocomposites, CeO2 forms a smooth and continuous thin shell on the surface of Cu2O cube

Figure 7. XRD patterns of the (A) [email protected] nanocomposite and [email protected]−573 K nanocomposite catalyst and the (B) [email protected] composite and [email protected]−573 K nanocomposite catalyst.

Ce3 + + 3OH− → Ce(OH)3 (s)

(3)

4Ce(OH)3 (s) + O2 → 4CeO2 (s) + 6H 2O

(4)

Cu 2 + + 2OH− → Cu(OH)2 (s) → CuO(s) + H 2O

(5)

Among the hydroxide precipitates, the solubility product constant (Ksp) of Ce(OH)4 (Ksp = 2 × 10−48) is far smaller than that of Ce(OH)3 (Ksp = 1.6 × 10−20) and Cu(OH)2 (Ksp = 2.2 × 10−20), thus reaction 2 is the dominant precipitation reaction. Therefore, CeO2 is the dominant solid product, and minor CuO forms only with large Ce(IV) amounts, producing high concentrations of OH−. In the Cu2O nanocrystal template-assisted method for the synthesis of CeO2@Cu2O nanocomposites, it is crucial to appropriately control the reaction rates of both the redox reaction between the Cu2O nanocrystal template and Ce(IV) F


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75% two-coordinate saturated Cu (CuCSA) and 25% onecoordinate Cu (CuCUS) (Figure 9). Reasonably, CuCUS is more reactive than CuCSA; meanwhile, the redox reaction between CuCSA and Ce(IV) is accompanied by the hydrolysis reaction of OCUS in the topmost layer to produce OH− within the neighboring region of Cu2O(111) surface, but the redox reaction between CuCUS and Ce(IV) reduces only the coordination number of OCSA in the third layer of Cu2O(111) bonded to CuCUS and does not release any free O anions for the hydrolysis reaction. Thus, under the same reaction conditions, the liquid (Ce(IV))−solid (Cu2O) interfacial reaction proceeding on the Cu2O(111) surface is not as homogeneous as on the Cu2O(100) surface, and the local OH− concentration within the region neighboring the Cu2O(111) surface is not as high as within the region neighboring Cu2O(100). As a result, the nucleation of CeO2 on the Cu2O(111) surface in the subsequent precipitation reaction is inhomogeneous, forming loosely aggregated CeO2 nanoparticles. Therefore, the morphology and exposed crystal plane of the Cu2O nanocrystal template control their reactivity in the liquid (Ce(IV))−solid (Cu2O) interfacial reaction occurring in the template-assisted method and thus affect the compositional and structural evolution of resulting nanocomposites. As evidenced by the ICP, XPS, and microscopy results, the chemical reactions between Ce(IV) and Cu2O cubes proceed within the surface region of Cu2O cubes, forming cubic Cu2O/CeO2−CuOx core/ shell nanocomposites with the surface enrichment of CeO2; however, the chemical reactions between Ce(IV) and Cu2O octahedra proceed quickly and extend from the surface to the bulk of Cu2O octahedra, eventually forming hollow octahedral CeO2−CuOx nanocages. These results, together with our previous results,30−33 comprehensively exemplify the concept of the crystal-plane-controlled surface reactivity of oxide nanocrystals.29 The above results clearly demonstrate that under the same reaction conditions CeO2@Cu2O nanocomposites prepared from cubic and octahedral Cu2O templates exhibit different compositions and structures. It is thus expected that they will exhibit different properties. CuO-CeO2 catalysts are active in catalyzing the oxidation of CO.38,47,48 Therefore, we evaluated the catalytic activity of CeO2@Cu2O nanocomposites in CO oxidation. Prior to the activity evaluation, as-synthesized CeO2@Cu2O nanocomposites were annealed in pure Ar at 300 °C for 1 h to stabilize their structures, and the acquired nanocomposite catalysts were denoted as CeO2@c/o-Cu2O-x573 K. As shown by XRD patterns (Figure 7) and SEM, TEM, and HRTEM images (Figure S3), [email protected]−573 K nanocomposite catalysts well preserve the morphologies of corresponding [email protected] nanocomposites, and the crystallinity of CeO2 in [email protected]−573 K nanocomposite catalysts is better than that in corresponding [email protected] nanocomposites. Figure 10 shows the catalytic activity of various catalysts. The catalytic performances of pure o-Cu2O and c-Cu2O agree with our previous results.34 Their surfaces get oxidized to more-active CuO during the first run of activity evaluation, thus CO conversion acquired during the second run of activity evaluation is higher than that during the first run of activity evaluation; meanwhile, the CuO thin film formed on o-Cu2O is more catalytically active than that formed on c-Cu2O. Similar to pure Cu2O nanocrystals, CeO2@ Cu2O-573 K nanocomposite catalysts during the second run of activity evaluation are more active than during the first run of activity evaluation. The Cu 2p XPS spectra of nanocomposite

nanocrystals, but in [email protected] nanocomposites, CeO2 aggregated loosely as nanoparticles on the surface of the Cu2O octahedra template and also forms away from the Cu2O octahedra template. Thus, the subsequent redox etching rate of Cu2O nanocrystals proceeds much slower for Cu2O cubes enclosed in a smooth and continuous CeO2 thin shell than for Cu2 O octahedra covered with loosely aggregated CeO2 nanoparticles; therefore, the cubic Cu2O core remains in CeO2@c-Cu2O nanocomposites and [email protected] exhibits a void cubic core/shell Cu2O/CeO2 nanostructure, but the octahedral Cu2O core can be reacted off and [email protected] exhibits a hollow octahedral CeO2 nanostructure. These observations indicate that the nucleation of CeO2 on the Cu2O nanocrystal template in the template-assisted method is much more uniform on Cu2O cubes than on Cu2O octahedra. Since the reaction conditions employing cubic and octahedral Cu2O nanocrystals as the templates were kept same, the different nucleation behaviors of CeO2 on cubic and octahedral Cu2O nanocrystals should originate from the different structures of cubic and octahedral Cu2O nanocrystals. The redox reaction between Ce(IV) and Cu2O nanocrystals is always initiated on the surface of Cu2O nanocrystals, and thus the surface composition/structure of Cu2O nanocrystals plays an important role. Reasonably, the redox etching of Cu2O nanocrystals by Ce(IV) is likely initiated by the redox reaction between Cu(I) in Cu2O and Ce(IV) to produce Cu(II) and Ce(III) followed by the hydrolysis of the O anion bonded to Cu(I) with H2O to produce OH−. Cubic and octahedral Cu2O nanocrystals selectively expose {100} and {111} crystal planes, respectively. As shown in Figure 9,30 the topmost layer consists of two-coordinate O

Figure 9. Optimized surface structures of Cu2O(100) and (111) surfaces respectively exposed on Cu2O cubes and octahedra. Red, pink, and green balls represent O atoms and coordination-saturated and coordination-unsaturated Cu atoms, respectively.

(OCUS) and the second layer consists of two-coordinate Cu (CuCSA) on the Cu2O(100) surface. Therefore, CuCSA cations in the second layer have the same coordination environment and exhibit the same reactivity; meanwhile, OCUS in the topmost layer gets released as soon as the redox reaction between Cu(I) and Ce(IV) occurs and facilely reacts with H2O to produce OH− within the region of the neighboring Cu2O(100) surface. As a result, the liquid (Ce(IV))−solid (Cu2O) interfacial reaction proceeds homogeneously on the Cu2O(100) surface and the local OH− concentration within the neighboring-region Cu2O(100) surface is high, facilitating the uniform nucleation of CeO2 on the Cu2O(100) surface in the subsequent precipitation reaction and the formation of the CeO2 thin film. On the Cu2O(111) surface, the topmost layer consists of three-coordinate O (OCUS) and the second layer consists of G


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Figure 10. Catalytic performances of various samples in two consecutive runs of activity evaluation in CO oxidation: (A) Cu2O cubes, CeO2@cCu2O-573 K-0.045 (a), CeO2@c-Cu2O-573 K-0.09 (b), CeO2@c-Cu2O-573 K-0.18 (c), and CeO2@c-Cu2O-573 K-0.36 (d) nanocomposite catalysts; (B) Cu2O octahedra, CeO2@o-Cu2O-573 K-0.045 (a), CeO2@o-Cu2O-573 K-0.09 (b), CeO2@o-Cu2O-573 K-0.18 (c), and CeO2@oCu2O-573 K-0.36 (d) nanocomposite catalysts.

with large Ce/Cu atomic ratios and subsequently small surface CuO concentrations exhibit low catalytic activities. On the other hand, [email protected]−573 K, [email protected]−573 K, [email protected]−573 K, and [email protected]−573 K nanocomposite catalysts with small Ce/ Cu atomic ratios and subsequently large surface CuO concentrations exhibit catalytic activities that increase with the surface CeO2 concentration. These observations suggest that the CeO2@Cu2O-573 K composite catalyst with the largest exposed CuO-CeO2 interface is the most catalytically active in CO oxidation, providing additional evidence to support the CuO-CeO2 interface as the active structure in CuOx-CeO2 catalysts in CO oxidation.51 By employing Cu2O cubes and octahedra, we have solidly demonstrated the morphology effect of the sacrificial Cu2O template on the composition/structure/catalytic activity of CeO2@Cu2O nanocomposites prepared by the templateassisted method. The underlying mechanism is that the (111) crystal plane exposed on Cu2O octahedra and the (100) crystal plane exposed on Cu2O cubes exhibit different surface composition/structure and thus different reactivity in the liquid (Ce(IV))−solid (Cu2O) interfacial reaction occurring in the template-assisted synthesis. Cu2O nanocrystals with various morphologies are mostly enclosed by low-index (111), (100), and (110) crystal planes, thus the similar morphology effect could be expected in the template-assisted synthesis of nanocomposites employing Cu2O templates with other morphologies. Meanwhile, the morphology effect could also be utilized to tune the composition, structure, and properties of nanocomposites prepared by the template-assisted method.

catalysts after the catalytic activity evaluation (Figure S4) demonstrate the partial oxidation of Cu2O into more active CuO during the first run of activity evaluation. CeO2@Cu2O573 K nanocomposite catalysts become stable after the first run of activity evaluation (Figure S5), and their morphologies are quite well preserved after the catalytic activity evaluation (Figure S6). The catalytic activity of stable CeO2@c-Cu2O-573 K nanocomposite catalysts follows the order [email protected]−573 K > [email protected]−573 K > [email protected]−573 K ≫ [email protected]−573 K, and the catalytic activity of stable CeO2@o-Cu2O-573 K nanocomposite catalysts follows the order [email protected]−573 K ≫ [email protected]−573 K > [email protected]−573 K ≫ [email protected]−573 K. The most active [email protected]−573 K and [email protected]−573 K nanocomposite catalysts exhibit similar catalytic activity, becoming active at 70 °C and achieving 100% CO conversion at 170 °C. It is noteworthy that the catalytic activities of [email protected]− 573 K and [email protected]−573 K nanocomposite catalysts in CO oxidation are similar to those of CeO2supported CuO catalysts prepared by routine catalyst preparation methods.51 These results suggest that large nanocrystals can be used to fabricate active nanocomposite catalysts. The apparent activation energies of CO oxidation catalyzed by various stable nanocomposite catalysts were calculated from the Arrhenius plots (Figure S7) and summarized in Table S1. All stable nanocomposite catalysts exhibit similar apparent activation energies of 50−60 kJ/mol, demonstrating that their active sites for CO oxidation are similar. Thus, their different catalytic activities should arise from the number of active sites. Our nanocomposite catalysts were prepared from solid Cu2O nanocrystals with very small specific surface areas and no pore structure,36 so it was thus expected that they should not exhibit much difference on their specific surface areas and pore structures. We found that the different catalytic activities of CeO2@Cu2O-573 K nanocomposite catalysts could be correlated with their different surface compositions. In our nanocomposite catalysts, Cu2O is mainly located in the core region. On one hand, [email protected]− 573 K, [email protected]−573 K, [email protected]−573 K, and [email protected]−573 K nanocomposite catalysts

4. CONCLUSIONS We have successfully fabricated CeO2@Cu2O nanocomposites from Cu2O cubes and octahedra by the template-assisted method involving the liquid (Ce(IV))−solid (Cu2O) interfacial reaction. Under the same reaction conditions, CeO2@Cu2O nanocomposites prepared from cubic and octahedral Cu2O templates exhibit different compositions and structures. With increasing amounts of Ce(IV) reactant, a smooth CeO2−CuOx shell develops on the surface of Cu2O cubes and eventually void cubic core/multishell Cu2O/CeO2−CuOx nanocompoH


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sites form; however, a rough CeO2−CuOx shell develops on the surface of Cu2O octahedra, and eventually hollow octahedral CeO2−CuOx nanocages form. The different compositions and structures of CeO2@Cu2O nanocomposites prepared from Cu2O cubes and octahedra were correlated with the different exposed crystal planes and surface reactivities of Cu2O cubes and octahedra. The catalytic activities of CeO2@Cu2O nanocomposites in CO oxidation depend on their compositions and structures. These results exemplify the morphology effect of Cu2O nanocrystals in liquid−solid interfacial reactions on the composition, structure, and properties of nanocomposites prepared by the template-assisted method.

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ASSOCIATED CONTENT

S Supporting Information *

HRTEM images of [email protected]. SEM images of nanostructures obtained with different amounts of NaCl. Microscopy images of [email protected]−573 K and [email protected]−573 K nanocomposite catalysts before and after the catalytic activity evaluation. Cu 2p XPS spectra of nanocomposite catalysts after the catalytic activity evaluation. Catalytic stability of [email protected]−573 K nanocomposite catalysts. Arrhenius plots and calculated apparent activity energies of CO oxidation catalyzed by stable CeO2@Cu2O573 K nanocomposite catalysts. 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]. Present Address

H.B.: Center for Green Chemistry and Catalysis, Department of Chemistry, McGill University, 801 Sherbrooke St. West, Montreal, QC, H3A 0B8, Canada. Author Contributions

H.B. and Z.Z. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2013CB933104, 2010CB923301) and the National Natural Science Foundation of China (21173204, U1332113).

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Compositions, Structures, and Catalytic Activities of CeO2@Cu2O

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