ARTICLE pubs.acs.org/JPCC
Cu2O Nanocrystals: Surfactant-Free Room-Temperature Morphology-Modulated Synthesis and Shape-Dependent Heterogeneous Organic Catalytic Activities You Xu, Huan Wang, Yifu Yu, Lei Tian, Weiwei Zhao, and Bin Zhang* Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, P. R. China.
bS Supporting Information ABSTRACT: A facile room-temperature surfactant-free solutionchemical route has been developed to fabricate Cu2O nanocubes, octahedrons, spheres, plates, and polyhedrons by varying the reaction atmosphere (air or Ar) and reducing agent. It is believed that the oxygen adsorption and reaction on the surface of Cu2O may be able to slow down the reduction of Cu(OH)2 and the nucleation of Cu2O and further exert a noticeable influence on the following growth. We find that reducing agents are also important in determining the samples’ final morphology, structure, and composition. It has been demonstrated that the size of Cu2O nanocrystals can be modulated by changing the pH value of the reacting solution or the initial concentration of reacting agents or by adding solvents with high viscosity. In addition, the organic catalytic activity has been demonstrated to be susceptible to the shapes of the synthesized Cu2O materials. {111} planes of Cu2O are found to show higher catalytic activity (two times at 30 °C) than that of {100} facets on the N-arylation reaction of iodobenzene with imidazole. The difference among various crystal planes in catalytic activity may be related to crystal facets’ densities of surface Cu atoms, surface energies, turnover frequency, and electronic surface properties. Furthermore, O2-assisted selective etching is proposed to improve the activity of {100} facets by increasing the active sites. This fundamental understanding shows that morphological control of transition-metal oxides allows selective exposure of catalytically active planes and will most probably be applicable in the development of the next generation of highly efficient heterogeneous catalysts. The selective etching of crystallographic planes may be developed to a general technique to improve catalytic activity of noble metal, oxide nanocrystals, as well as porous catalysts.
1. INTRODUCTION During the past few decades, intensive attention has been focused on the synthesis of inorganic nanocrystals with tailored shapes owing to their intrinsic shape-dependent properties.1 Recently, a great deal of effort has been devoted to the morphology effect of metal materials on the organic catalytic activity.2 For instance, the shape of platinum nanoparticles has been found to exert a noticeable effect on benzene hydrogenation selectivity.2b However, these thought-provoking works mainly focused on the selectivity of noble metal nanocatalysts in organic reaction. To the best of our knowledge, there are few reports on crystal-facet effects of nanoscale metal oxides on heterogeneous catalytic activities. Cuprous oxide (Cu2O) has been considered to be an efficient and reusable catalyst to generate primary aromatic amines and promote CN and CS cross coupling reaction.3 In these cases, Cu2O is usually in bulk form or particle without regular shapes. Although great advance has been made on the synthesis of Cu2O nanorods, cubes, octahedrons, pyramids, and hollow structures,4 polymers or surfactants had often been adopted in these techniques, which restricts these particles’ applications in organic catalysis.5 Recently, capping-agent-free methods were successful r 2011 American Chemical Society
in the controlled synthesis of one or two kinds of Cu2O microcrystals and Cu@Cu2O coreshell nanocrystals,4gi but a one-step solutionchemical morphology-controlled method to produce Cu2O nanocrystals with abundant and regular shapes still remains challenging. Therefore, it is significant for exploring the effect of crystallographic planes on catalytic properties to design a simple, more inexpensive and convenient solution-phase approach to produce nanoscale Cu2O cubes with {100} planes, octahedrons with {111} facets, nanocuboctahedrons, and nanospheres in the absence of polymers or surfactants. The atmosphere, especially O2, has been found to be critical in controlling the morphology of nanocrystals.68 Papadimitrakopoulos and his coworkers had revealed that oxygen could passivate the nonpolar facets of CdSe seeds and thus control their following 1D or 3D growths through changing the concentration of oxygen.6 Oxygen partial pressure was also demonstrated to play a key role in modulating the rodlike and sheetlike morphology of ZnO in carbothermal reduction process.7 Received: May 28, 2011 Revised: July 6, 2011 Published: July 07, 2011 15288
dx.doi.org/10.1021/jp204982q | J. Phys. Chem. C 2011, 115, 15288–15296
The Journal of Physical Chemistry C Xia found that oxygen from air can combine etching agents to etch seeds in the early stage of the synthesis and modulate the morphology of noble metal nanocrystals.8 These interesting works inspire us to introduce different atmospheres into reaction system to modulate the shapes of Cu2O crystals. It is widely accepted that the terraces, steps, and corner sites of solid catalyst are active sites for breaking chemical bonds in heterogeneous reaction.2b,9 For improvement of the catalytic activity, one effective route is to increase the density of active sites.2b,9 It is reasonably expected that the catalytic activity of Cu2O can be greatly improved through increasing the surface active sites of cuprous oxide nanocrystals. This drives us to search for a simple and general method to increase active sites and improve catalytic activity of Cu2O. Herein, we develop a surfactant (or polymer)-free roomtemperature approach to the synthesis of Cu2O nanocrystals with controlled shapes and planes. It has been found that Cu2O nanocubes, octahedrons, cuboctahedrons, and spheres can be successfully fabricated, respectively, through reducing newprepared Cu(OH)2 using hydrazine hydrate or sodium ascorbate. Oxygen has been thought to be significant in the morphologycontrolled synthesis of Cu2O materials. In addition, we have demonstrated that the catalytic activity of {111} planes in Cu2O is much better than that of {100} facets or irregular particles in selected model organic reaction. Finally, it has been shown that the catalytic ability of {100} planes could be improved by oxygen-assisted selective etching of Cu2O. The improved activities are considered to be related to the increase in the corner and edge sites on the particles.
2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were of analytical grade and were used as received without further purification. Aqueous solutions were prepared using Milli-Q water. 2.2. Synthesis of Cu2O Nanocubes, Octahedrons, Cuboctahedrons, Spheres, and Facet-Etched Cubes. In a typical procedure (experimental parameters are shown in Figure 1), under constant strong stirring, 1 mL of 0.35 mol 3 L1 NaOH solution is quickly added to CuCl2 aqueous solution (30 mL, 0.0032 mol 3 L1) under an argon (or air) atmosphere; blue Cu(OH)2 colloids are immediately produced. Then, reducing agent (sodium ascorbate or N2H4 3 H2O) is quickly dropped in the above solution at room temperature, and the solution became brick-red, indicating the formation of Cu2O. The brick-red precipitates were quickly collected through centrifugation and washed three times with distilled water and absolute ethanol, respectively. The final samples were stored in an Ar atmosphere after being dried in vacuum at 45 °C for 6 h. 2.3. Characterization. The scanning electron microscopy (SEM) images and energy-dispersive X-ray spectra (EDX) were taken with a Hitachi S-4800 scanning electron microscope (FESEM, 15 kV) equipped with the Thermo Scientific energydispersion X-ray fluorescence analyzer. Transmission electron microscopy (TEM) images were obtained with a JEOL-2100F system at 200 kV. The specimens of TEM measurements were prepared via spreading a droplet of ethanol suspension onto a copper grid, coated with a thin layer of amorphous carbon film, and allowed to dry in air. The X-ray diffraction patterns (XRD) of the products were recorded with a Bruker D8 focus diffraction system using a Co KR source (λ = 0.179 nm). FTIR spectrum was recorded on a MAGNA-IR 750 (Nicolet Instrument) FTIR
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Figure 1. Scheme illustrating five different conditions to control the reduction of new-prepared Cu(OH)2 colloids and the corresponding SEM images of different morphologies. This suggests that the controlled synthesis of Cu2O cubes, octahedrons, cuboctahedrons, spheres, and facet-etched cubes can be fulfilled by changing the atmosphere and reducing agent.
spectrometer. The as-prepared product-containing solution was directly dried in vacuum at 50 °C to get rid of water; then, a thick film of sample was prepared by employing a mixture of KBr and the sample. The surface area of the synthesized materials was determined by nitrogen physisorption using Quadrasorb SII Quantachrome Instrument. The surface area was calculated using the BrunauerEmmettTeller method. 2.4. Evaluation of Catalytic Performance. The N-arylation reaction of iodobenzene with imidazole to synthesize 1-phenylimidazole was selected to investigate the catalytic activities of the samples. In the above CN coupling reaction, the mole of imidazole is excessive for iodobenzene adopted (Table 1). The conversion of iodobenzene is defined as the ratio of moles of iodobenzene consumed in the Cu-catalyzed reaction to the total moles of iodobenzene initially added (eq 1). The selectivity to 1-Phenylimidazole is defined as the ratio of number of moles of 1-phenylimidazole generated to the total number of moles of all the products (eq 2). When the conversion and selectivity are 100%, the mole of 1-phenylimidazole produced is equal to the mole of iodobenzene initially added. The GC yield of 1-phenylimidazole can be defined as the ratio of number of moles of 1-phenylimidazole generated (measured from GC trace quantitative analysis) to the total number of the total moles of iodobenzene initially added. conversion of iodobenzene ¼
moles of iodobenzene consumed moles of iodobenzene initially added
ð1Þ selectivity to 1 phenylimidazole ¼
moles of 1-phenylimidazole moles of all the products
ð2Þ 15289
dx.doi.org/10.1021/jp204982q |J. Phys. Chem. C 2011, 115, 15288–15296
The Journal of Physical Chemistry C
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Table 1. Screening Reaction Conditions for CopperCatalyzed N-Arylation of Imidazole with Iodobenzenea
entry 1
Cu2O catalyst
time (h)
yield (%)b
110
5
70.1
2
110
10
80.6
3
110
15
94.8
4
5
10
5.6
5
30
10
24.4
6
nanocubes
T (°C)
nanospheres
7 8 9
5
43.1
110 110
10 15
78.2 88.7
110
5
84.9
10
110
10
96.7
11
110
15
98.5
12
5
10
9.3
13
30
10
53.0
110
10
99.9
14
nano-octahedrons
110
facet-etched nanocubes
a
Reaction condition: iodobenzene (1 mmol), imidazole (1.5 mmol), Cu2O catalyst (0.1 mmol), KOH (2 mmol), and DMSO (2 mL) under an Ar atmosphere. b GC yield.
GC yield ¼
moles of 1-phenylimidazole measured by GC moles of iodobenzene initially added
ð3Þ
Gas chromatography (GC) trace was utilized to analyze the composition of the reaction mixture, which can be determined by the typical equation and method.3c,10 GC trace was carried out with a Agilent 6890 GC (USA) with an HP-INNOWAX polyethylene glycol column (30 m 320 μm 0.25 μm) equipped with a flame ionization detector. The injector and the detector temperatures were both set at 280 °C. The column temperature was held at 80 °C for 2.0 min, then raised to 240 at 40 °C/min and kept for 6 min. Nitrogen was used as the carrier gas at 1.5 mL/min. Split ratio was 30:1. The average error for this determination was
