Article pubs.acs.org/JPCC
Synthesis of Copper Hydroxide Branched Nanocages and Their Transformation to Copper Oxide Alec P. LaGrow,† Lutfan Sinatra,† Ahmed Elshewy,‡ Kuo-Wei Huang,‡ Khabiboulakh Katsiev,† Ahmad R. Kirmani,† Aram Amassian,† Dalaver H. Anjum,§ and Osman M. Bakr*,†
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Division of Physical Sciences and Engineering, Solar and Photovoltaics Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ‡ Division of Physical Sciences and Engineering, KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia § Imaging and Characterization Lab, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia S Supporting Information *
ABSTRACT: Copper oxide nanostructures have been explored in the literature for their great promise in the areas of energy storage and catalysis, which can be controlled based on their shape. Herein we describe the synthesis of complex branched nanocages of copper hydroxide with an alternating stacked morphology. The size of the nanocages’ core and the length of the branches can be controlled by the temperature and ratio of surfactant used, varying the length from 85 to 232 nm long, and varying the core size from 240 to 19 nm. The nanostructures’ unique morphology forms by controlling the growth of an initial spherical seed, and the crystallization of the anisotropic arms. The Cu(OH)2 nanostructures can be converted to polycrystalline CuO branched nanocages and Cu2O nanoframes. We show that the branched nanocage morphology of CuO has markedly superior catalytic properties to previous reports with CuO nanomaterials, resulting in a rapid and efficient catalyst for C−S coupling.
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INTRODUCTION Metal oxide nanostructures are of great technological importance due to their applications in energy storage,1−3 biomedicine,1 and catalysis.2 One particularly important metal oxide system is the copper oxides, cupric oxide (CuO) and cuprous oxide (Cu2O), which are p-type semiconductors used in energy storage4,5 and catalytic2,6 applications. Recent promising reports on copper oxide have shown its potential as a replacement for expensive noble metal catalysts in the oxidation of carbon monoxide, nitric oxide, and volatile organics,7−9 as well as C−N and C−S cross-coupling reactions.6,10 Despite these advances, a significant performance gap remains before copper oxides may achieve parity with noble metal catalysts, and hence fundamentally alter the economics of many technologically important reactions. A promising route to close this gap is by tailoring the materials morphology by controlling the exposed crystal facets,11 forming complex threedimensional shapes12 and forming hollow architectures.3 These morphologies have shown enhanced catalytic performance versus standard spherical copper oxide particles.13 Synthetic strategies to form complex 3-dimensional or hollow structures of CuO have been studied by using Cu2O and copper hydroxide [Cu(OH)2] as templates.2,14,15 Copper hydroxide is the best candidate as a template as it is an intermediate product in the most common synthetic strategy, the hydrothermal method, to form CuO nanostructures.2,16,17 One strategy to fully utilize the underlying structure of the © 2014 American Chemical Society
Cu(OH)2 intermediate is by isolating it from solution and dehydrating it in the solid state.18−20 However, currently the shape control of CuO via both of these methods is limited to nanoparticles, one-dimensional nanostructures,16,21 two-dimensional nanostructures,7 and three-dimensional urchin-like nanomaterials.9,22 The reason for these limitations is that the shape-controlled syntheses of the Cu(OH)2 template is equally as limited.23,24 Herein we describe a facile and scalable room temperature synthesis of complex branched Cu(OH)2 nanocages and their conversion into CuO branched nanocages and Cu2O nanoframes. The use of Cu(OH)2 as a template is shown to allow the synthetic control over the proportion and length of the branches, as well as the size of the nanocage core. The reaction was observed to grow from a spherical particle that then facets and allows the 1-dimensional growth of the Cu(OH)2 crystal structure to extrude into branches. Electron tomography was used to elucidate that the structure is formed from alternating stacks of branches running through the material surrounding a hollow core. This alternating branched structure, when converted to CuO, is highly selective for C−S cross-coupling reactions and has an increase in the reaction rate of more than Received: April 13, 2014 Revised: July 26, 2014 Published: July 29, 2014 19374
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RESULTS AND DISCUSSION In a typical reaction, the Cu(OH)2 branched nanocages are formed by the reduction of CuCl2 with NaBH4 in ethanol with 10 equiv of PVP to the metal precursor. The bright-field TEM (BF-TEM) micrograph in Figure 1A shows branched
double the previously reported copper oxide nanostructures.4,5,7,10
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Article
EXPERIMENTAL DETAILS
Copper Hydroxide Synthesis. In a typical synthesis the copper(II) chloride (Aldrich, anhydrous, 99.995%) salt was weighed out to be 0.01 mM in ethanol (absolute, AnalaR NORMAPUR, VWR Chemicals). The CuCl2 salt was dissolved in 80% of the total volume of ethanol and then 1−20 mol equiv of poly(vinylpyrrolidone) (PVP) 55 000 MW (Aldrich) was added to the solution, and the equivalents were calculated based on the repeating unit C6H9NO, with an average molecular weight of 111 g/mol per unit. The mixture was placed in a sonicator until it was completely dissolved. The reaction was stirred for 1 h, and then sodium borohydride (Reagent Plus, 99%) was dissolved in 20% of the total volume of ethanol and injected into the solution. The total volume of the solution filled just under half the reaction flask used. The solution instantaneously turned brown, and once the bubbling had stopped, the reaction flask was sealed. The solution turned bright blue in color after 8 h, which is indicative of Cu(OH)2 (Figure S1 in the Supporting Information (SI)).23 At the end of the reaction, the nanostructures were purified by centrifugation at 7500 rpm for 15 min. The supernatant was discarded and the blue nanomaterials were redispersed in ethanol and cleaned by centrifugation twice more. For the reactions carried out at 65 °C the reaction vial was suspended in a bath of silicon oil that was kept stirred. The reaction produced the same nanostructures when done with a total volume of between 10 and 500 mL. Dehydration To Form CuO. To form CuO from the Cu(OH)2, the Cu(OH)2 was dried under nitrogen and then put into a vacuum oven at 200 °C and 1 mbar and kept there for 24 h. After 24 h the oven was cooled to room temperature under vacuum, before it was released to air. Reduction to Form Cu2O. To form Cu2O, the Cu(OH)2 was dried under nitrogen and then put into a desiccator with a 20 mL vial of hydrazine under reduced pressure for 2 weeks. Transmission Electron Microscopy (TEM). TEM was carried out on the Titan G2 80-300CT, FEI Co., the Titan G2 80-300ST, FEI Co., Super Twin, x-FEG, by operating at 300 kV, and the Technai G212 BT, FEI Co., by operating at 120 kV. The cryoTEM was done on the Titan Krios by operating at 300 kV. The size distributions were created by counting 500 particles. The body lengths were measured across the main structure of the nanoparticle, and the branches were measured from the edge of the nanostructures body to reduce uncertainty as they appeared to run through the whole structure of the material. Catalysis. A 5 mL sealed microwave tube equipped with a magnetic stir bar was charged with base (0.75 mmol), CuOH nanocrystals (3.6 × 10−3 mmol), 1-octanethiol (1.2 mmol), iodotoluene (1.00 mmol), and DMSO (2.0 mL) were added into the sealed tube, and the reaction vessel was heated to the indicated temperature. After stirring at that temperature for the indicated time, the heterogeneous mixture was cooled to room temperature and diluted with dicloromethane (20 mL). The resulting solution was filtered through a pad of Celite and then washed with water (10 mL × 3) and concentrated to give the crude material which was then purified by column chromatography to yield the pure product.
Figure 1. (A) Bright field (BF) TEM micrograph of the Cu(OH)2 branched nanocage. (B) A single Cu(OH)2 nanostructure showing the branches extruding form the core. (C) Tomogram of (B) showing the stacked structure of the branches running straight through the material and stacking in an oscillating fashion.
nanocages with an average diameter of 221 ± 28 nm and branches with a length of 74 ± 24 nm and width of 17 ± 4 nm protruding from the nanostructure. Each particle has a lighter contrast in the middle which was due to a cavity in the center of the particle (Figure 1A and Figure S2 in the SI). The branches were clearly resolved by performing the electron tomography (ET) analysis of the nanocage shown in Figure 1B. The tomogram shows that the arms stretch all the way through the material and display an oscillating stacked structure (Figure 1C, and Figures S3 and S4 in the SI). The Cu(OH)2 phase is unstable when exposed to electron radiation (Figure S5 in the SI). The beam sensitivity of the material is consistent with previous reports,18 where Wang et al. showed that the decomposition of Cu(OH)2 under TEM conditions is almost instantaneous.25 The phase of the material was investigated by X-ray photoelectron spectroscopy (XPS). The XPS spectrum of Cu (2p) is shown in Figure 2A. The Cu (2p3/2) and Cu (2p1/2) are at 934.6 and 954.5 eV with a 19.9 eV splitting. This is attributed to the Cu2+ oxidation state.26−28 Also the appearance of shakeup satellites centered at 943.5 and 962.8 eV confirms that the material is an oxidized form of copper.28 The peak at 934.6 eV is in the center of the reported literature values for CuO (934.2 eV) and Cu(OH)2 (935.0 eV) and therefore is likely composed of a mixture of the two structures.27 The position of the Auger electron at 917.2 eV (Figure 2B) is consistent with Cu2+ in the literature.28−30 To study the binding between the oxygen and the copper, the O 1s core level was probed. The broad peak in 19375
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Figure 2. XPS spectra of (A) Cu 2p core level, (B) Auger electron transition, and (C) O 1s core level of the branched nanostructures.
Figure 2C can be fit with three components, at 532.5, 531.0, and 529.5 eV which correspond to adsorbed water, the oxygen in bound hydroxyl groups, Cu(OH)2, and the oxygen in lattice oxide sites, CuO, respectively.31 The observed results indicate that the nanostructures are Cu(OH)2 with some surface CuO from dehydration. To investigate the growth mechanism of the branched nanostructures, aliquots of the reaction were taken after 1, 2, 4, 6, 8, and 12 h and characterized by TEM (Figure 3, and Figure S6 in the SI). The particles after 1 h were seen to be roughly spherical in shape with a size of 113 ± 24 nm (Figure 3A). By 2 h the particles are 153 ± 26 nm, and some degree of faceting is seen (Figure 3B). After 4 h the particles are noticeably faceted with a size of 162 ± 22 nm, and they have small growths from the facets of 14 ± 5 nm (Figure 3C). After 6 h the particles are 167 ± 20 nm and larger growths from the facets are from short extrusions from the core with a length of 24 ± 9 nm. By 8 h the particles are 201 ± 22 nm, and short branches of 46 ± 18 nm can be obviously resolved. These branches are seen to continue to extend and by 12 h they have a length of 51 ± 18 nm and a body size of 204 ± 22 nm. The particles are formed from an almost spherical nanostructure, and at times above 4 h the growth occurs predominantly along the arms. The stable form of Cu(OH)2 is anisotropic in nature. The original formation of the pseudospherical seeds could occur due to fast kinetic aggregation based growth of the initial oxidized copper clusters formed immediately after the injection of NaBH4. The copper clusters would be oxidized to form Cu(OH)2 nanostructures under the basic environment created by the sodium borohydride reduction. After the seeds are formed from the oxidized clusters the particles begin to facet and the arms grow in their favored direction as anisotropic rods.
Figure 3. BF-TEM micrographs of the nanostructures formed after (A) 1 h, (B) 2 h, (C) 4 h, (D) 6 h, (E) 8 h, and (F) 12 h.
The initial morphology of the nanomaterials was studied while frozen in ethanol by CryoTEM (Figure 4A), while in
Figure 4. BF-TEM micrographs acquired with a 14 day reaction time containing (A) Cu(OH)2 frozen in ethanol acquired with cryoTEM technique of a sample, (B) dried Cu(OH)2, (C) CuO, and (D) Cu2O nanoframes. 19376
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hydrazine vapor at room temperature for 2 weeks. The reduction to Cu2O fully exposed the cavity of the particle, and the branches disintegrated to formed a 207 ± 24 nm nanoframe of Cu2O made up of 7 ± 3 nm crystallites (Figure 5D and Figure S8 in the SI). It was hypothesized that during the drying period of the solvated Cu(OH)2 the strain caused by the contraction bent the branches as well as opened up pores inside the structure. Once the lattice compression is too large, as in the conversion to Cu2O, the overall morphology breaks down into small crystallites, leaving only the frame of the template. This mechanism is markedly different from other syntheses of hollow/cagelike nanostructures that are formed via galvanic exchange,32 the Kirkendall effect,33 or the use of sacrificial template.34 The mechanism reported here forms the hollow by the volume loss during the drying process of the solvated form of Cu(OH)2. Further studies were carried out to control the morphology of the branched nanocages by altering the ratio of PVP to CuCl2. At a ratio of PVP:CuCl2 of 1, a similar shape is formed showing an increase in the size distribution of the sample with a body size of 126 ± 65 nm, and branch lengths of 67 ± 25 nm (Figure 5A). When the PVP:CuCl2 ratio is increased to 5, the branched nanocages have 243 ± 45 nm bodies and 83 ± 23 nm long arms (Figure 5C). This is increased from the standard conditions with a PVP:CuCl2 ratio of 10 (Figure 5E). Upon increasing the PVP:CuCl2 ratio to 20 the main body size is 135 ± 20 nm, and the arm length is 65 ± 15 nm (Figure 5G). At higher concentrations of PVP, above 20 equiv, there is no further decrease in particle size, and by 50 equiv, particles are not formed during the reaction. Finally, without PVP, Cu(OH)2 is not formed, and therefore the PVP is necessary to form particles of Cu(OH)2. The same reactions were carried out at 65 °C to study the effect of temperature. With a PVP:CuCl2 ratio of 1 the structure is seen to be polydisperse with arm lengths of 509 ± 321 nm long and body sizes 131 ± 75 nm (Figure 5B). With a ratio of 5, the main body was 80 ± 10 nm, and the arms were 232 ± 38 nm in length and the average structure composed of four linear rods with an angle of ∼105° between them giving a diamondshaped body (Figure 5D). With a ratio of 10 equiv, the particles retained the same shape as at 5 equiv except the main body was 75 ± 16 nm and an arm length of 228 ± 37 (Figure 5F). When the PVP ratio was increased to 20, the arm length of the particles was 145 ± 19 nm, and its body was 19 ± 6 nm (Figure 5H). It should be noted that the cavity of the nanocage is retained until 20 equiv of PVP where only a joint remains (Figure S9 in the SI). The synthetic studies show three major ways to control the size of the nanoparticles body and branch lengths: these are time, PVP:CuCl2 ratio, and temperature. Increasing the temperature to 65 °C increases the length of the branches and reduces the size of the bodies. It should be noted that using a temperature between 25 and 65 °C resulted in a reduced size of the body of the particles and a secondary smaller branched species that is similar to the nanostructures formed at 65 °C (Figure S10 in the SI). By increasing the PVP:CuCl2 ratio, the body size and the branch sizes are both seen to decrease. Finally, by tuning the reaction time, both arm length and body size increase. The arm length grows with time whereas the body plateaus. The continued growth of the arms after the precursor has been used up is attributed to an etching mechanism, where the less stable bodies of the particles are etched as the arms grow. This behavior would explain the lengthening arms with
solution the arms of the particles were straight and there was no evidence of a void at the center of the particle (Figure 4A). The particles were formed with a reaction time of 14 days had a body size of 281 ± 31 nm and an arm length of 149 ± 33 nm (Figure 4A). Upon drying, the particles underwent a size reduction of 29% in the core and a negligible contraction for the branch length and a 43% decrease in the branch width, giving a final size of 198 ± 21 nm for the core, and 141 ± 31 nm for the branches (Figure 4B). After 24 h at 200 °C, the main body of the particles was 189 ± 21 nm, and made up of CuO (Figure 5C and Figure S7 in the SI). The size reduction
Figure 5. BF-TEM micrographs of the nanostructures made with PVP:CuCl2 ratios of (A) 1:1, (C) 5:1, (E) 10:1, and (G) 20:1 PVP. Parts B, D, F, and H have the same PVP:CuCl2 ratios, respectively, at 65 °C.
between the dried nanoparticles and the CuO was only 5%, and the morphology was retained, which is consistent with the literature.18−20 The Cu(OH)2 was reduced to Cu2O by 19377
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g−1 (Figure S14 in the SI), compared to the bundles, 39.3 m2 g−1. These values are also enhanced compared to literature reports for similar catalytic reactions with a maximum of 39.6 m2 g−1.7,9 The branched nanocages were efficient catalysts for C−S coupling reactions and catalyzed the reaction in under half the time of other CuO nanoparticles,4,5,7,10,38,39 or at a seventh of the loading of the randomly branched hollow CuO catalyst.40 In summary, a novel complex structure of Cu(OH)2 was formed of alternating layers of arms extruding from a central body. The arm length, body size, and number of arms could be controlled with temperature, time, and PVP:CuCl2 ratio by controlling the kinetics of the aggregation versus the crystallization of the Cu(OH)2. The branched nanostructures of solvated Cu(OH)2 formed a hollow material due to their volume contraction upon drying. The branched nanocages could be converted into CuO branched nanocages and Cu2O nanoframes. The polycrystalline CuO branched nanocage is reported to be a highly efficient catalyst, as shown with the C− S coupling system. The branched nanocages could be extended into the catalysis of other systems where copper based catalysts are being tested to replace expensive noble metals, such as poisonous gas oxidation.7−9
time and the small joints seen in the particles grown with 20 equiv PVP at 65 °C. The reactions carried out at 65 °C are hypothesized to decrease the induction period before crystallization and thus reduce the body size. The solvent and precursor were crucial in the reaction. Without an alcohol solvent and a chlorine based precursor, Cu(OH)2 was not formed. When the reaction was done in methanol bundles of rods were formed (Figure S11 in the SI), which is the thermodynamically favored form of Cu(OH)2. The Cu(OH)2 phase and the lower solubility of CuCl2 in ethanol are hypothesized to favor the initial random aggregation ofclusters forming the spherical particles. The Cl− from CuCl2 could form a O2/Cl− pair, and oxidatively etch the initially formed Cu0 clusters to form Cu(OH)2, as has been suggested with palladium and silver nanoparticles by Xia et al.35−37 The OH− ions would come from the reaction between NaBH4 and the H2O in the ethanol. The growth of the nanostructure occurs by a complex interplay of the surfactant, CuCl2 precursor, and solvent present. The initial reaction occurs by the reduction of CuCl2 to Cu0 by NaBH4. The Cl− from the precursor would then undergo an oxidative etching process in the presence of O2 which would create the initial spherical nuclei. These nuclei would then begin to facet as the thermodynamics of the system took over and favored the crystallization of Cu(OH)2 and thus the growth of branches would predominate. The CuO branched nanocages were tested for their catalytic properties. CuO nanoparticles have been reported to be efficient catalysts for the C−S cross-coupling of a thiol and an aryl iodide.7,10 Herein we examined the catalytic performance of our branched nanocages of polycrystalline CuO. The catalytic cross-coupling reaction was carried out at 130 °C under microwave with dimethyl sulfoxide as the solvent and cesium carbonate as a base. The reaction reached a conversion efficiency of 91% in 2 h (Figure 6 and Figure S12 in the SI). Without the catalyst only 3% of the product was formed. When the same reaction was carried out with CuO prepared from Cu(OH)2 bundle formed in methanol (Figure S13 in the SI), the conversion was much lower, only 75% under the optimized conditions. The enhanced catalytic properties of the branched nanocages were attributed to their higher surface area, 67.5 m2
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ASSOCIATED CONTENT
S Supporting Information *
EDS, UV/VIS, and TEM images of beam damaged materials, additional low-resolution TEM images, and XRD data of the converted material. 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support of KAUST’s University Research Fund.
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REFERENCES
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Figure 6. Catalytic thiol etherification of benzyl iodide over time with and without the CuO branched nanocages form in 20 h with 10 equiv of PVP at room temperature. 19378
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dx.doi.org/10.1021/jp503612k | J. Phys. Chem. C 2014, 118, 19374−19379
