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Cite this: DOI: 10.1039/c4nr00412d
A one-step green route to synthesize copper nanocrystals and their applications in catalysis and surface enhanced Raman scattering† Pinhua Zhang,ab Yongming Sui,*a Chao Wang,a Yingnan Wang,a Guangliang Cui,b Chunzhong Wang,c Bingbing Liua and Bo Zou*a A nontoxic, simple, inexpensive, and reproducible strategy, which meets the standard of green chemistry, is introduced for the synthesis of copper nanocrystals (Cu NCs) with olive oil as both reducing agent and capping agent. By changing the reaction parameters, the shape, size and surface structure of the Cu NCs can be well controlled. The obtained Cu nanocubes show excellent catalytic properties for the catalytic
Received 22nd January 2014 Accepted 1st March 2014
reduction of dyes and CO oxidation. Moreover, the prepared Cu nanocubes as substrates exhibit surface enhanced Raman scattering (SERS) activity for 4-mercaptopyridine (4-Mpy). Therefore, this facile route
DOI: 10.1039/c4nr00412d
provides a useful platform for the fabrication of Cu NCs which have the potential to replace noble
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metals for certain applications.
Introduction Over the past decade there has been increasing emphasis on the topic of “green” chemistry and chemical processes.1–5 Green chemistry or sustainable chemistry is dened by the United States Environmental Protection Agency6 as “the design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances.” In recent years, there is a great societal expectation that chemists and chemical engineers should produce greener and more sustainable chemical processes. More efforts have been made towards the total elimination or at least the minimization of generated waste and the implementation of sustainable processes through adoption of 12 fundamental principles.1 Utilization of nontoxic chemicals, environmentally benign solvents, and renewable materials is the key issue that deserves important consideration in a green synthesis strategy. Thus, it is desirable to design feasible green chemical processes to synthesize nanocrystals (NCs) with desired planes and novel geometrical shapes. In terms of the perspective of green chemistry, the three main steps in the preparation of nanoparticles are the choice of the solvent medium used for the synthesis, the choice of an environmentally benign reducing agent, and the choice of a
a
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China. E-mail: [email protected]; [email protected]
b
Institute of Condensed Matter Physics, Linyi University, Linyi, Shandong, 276000, China
c Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun, 130012, China
† Electronic supplementary 10.1039/c4nr00412d
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nontoxic material for the stabilization of nanoparticles.7 Olive oil is mainly composed of mixed fatty acids of oleic acid and palmitic acid and of other fatty acids, which contains about 75% of its fat in the form of oleic acid. It is commonly used in cooking, cosmetics, pharmaceuticals, and soaps and as a fuel for traditional oil lamps. The monounsaturated fat content of olive oil has been linked not only to cholesterol reduction, but also to reduction of blood pressure. Furthermore, olive oil is oen used as a coordinating solvent to synthesize NCs. Recently, our group has synthesized ZnS and ZnSe NCs using olive oil as a solvent.8,9 It may be a suitable “green” alternative to organic solvents in applications. Copper (Cu) NCs have attracted much attention owing to their potential applications in the elds of catalysis, photovoltaics, electronics, optics and electrocatalysis.10–16 They have been explored as a possible substitute for noble metals on account of their unique properties. Cu NCs are also widely used as catalysts for various reactions including water–gas shi,17 gas detoxication reactions,18 and as electrocatalysts in solid oxide fuel cells.19 The basic properties of Cu NCs are mainly determined by their size, shape, composition, crystallinity and structure. Studies have also shown that the catalytic property of metal NCs is closely associated with the exposed crystallographic facets.20–22 Metal NCs with different sizes, shapes, and aspect ratios also exhibit distinct localized surface plasmon resonance (LSPR) and surface-enhanced Raman spectra (SERS) properties.23–25 Therefore, the preparation of monodispersed Cu NCs with controllable size and shape continues to be of considerable interest. In light of fast and growing applications of Cu NCs, the design and exploration of a facile and environmentally friendly strategy is highly desirable for the increasing demand. So far, the preparations of metal nanoparticles have been carried out through
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different methods, such as radiation methods, microemulsion techniques, electrode discharge and wet chemical reduction.26–29 Among the above methods, the wet chemical reduction method is one of the most common methods to produce metal nanoparticles, and has advantages of easy control of the reaction process and production rate. However, most of the wet chemical reduction methods need several key factors in the reducing process, including the solvent medium, the reducing agent, and the capping agent.30 Most of the wet chemical reduction methods reported to date rely heavily on organic solvents and environmentally or biologically hazardous reducing agents (i.e., hydrazine, sodium borohydride or hypophosphite, etc.).29,31,32 Although these methods could successfully produce pure and well-dened Cu NCs, the consequences of these methods are higher energy consumption and heavier pollution. Consequently, there is an unequivocal need to develop a more cost-effective and environmentally benign alternative to these existing methods. In this paper, we present a simple, environmentally friendly and cost-effective method for preparing monodisperse Cu NCs through the thermal reduction of copper oxide in olive oil. By varying the reaction time, the morphology and average diameter can be greatly adjusted. This approach eliminates the need for any air-sensitive, toxic, and expensive starting material. Olive oil is used in the reaction as both reducing and capping agents to prevent the aggregation of Cu NCs. The obtained Cu nanocubes show excellent catalytic properties and SERS properties. This inexpensive, environmentally friendly method for the synthesis of Cu NCs may be extended to prepare many other noble metallic nanostructures.
Experimental section
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powder diffraction (XRD, Shimadzu, XRD-6000) operating at 40 kV and 40 mA with Cu Ka radiation. Data were collected from 20 to 80 with a sampling interval of 0.02 per step and a scan speed of 4 per minute. Samples for TEM characterization were prepared by adding several drops of Cu NC solution onto 300mesh copper grids with a carbon support lm. TEM and highresolution transmission electron microscopy (HRTEM) images were obtained via a JEM-2200FS transmission electron microscope at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) analysis was performed during TEM measurements. Ultravioletvisible absorption (UV-vis) spectra were obtained using an Ocean Optics QE65000 spectrometer. Catalytic reduction of dyes To study the catalytic properties of the as-synthesized Cu nanocubes, we evaluated their catalytic activities for the catalytic reduction of dyes by NaBH4 in water. In a typical experiment, 1.7 mL of methylene blue (0.1 mM, aqueous solution) was mixed with 0.7 mL of NaBH4 (0.04 M, aqueous solution) in a quartz cell (3.0 mL). Then the aqueous solution of Cu nanocubes obtained at 10 min (0.1 mL, 15 mM) was added to the mixture of methylene blue and NaBH4, and the changes of the solution were monitored with a UV-vis spectrophotometer at different time intervals. Similar procedures were used to catalyze the reduction of rhodamine B, methyl orange and congo red. For each catalytic reaction, the added amounts of dyes, NaBH4 and Cu nanocubes were equal. All the reactions were conducted at room temperature. For the recycling experiment, the catalysts were collected by centrifugation (12 000 rpm, 5 min, room temperature), washed with deionized water, and reused in the next reaction.
Materials CuO (99.9%) was purchased from Aldrich. Olive oil (produced by Shandong Luhua Group Co., Ltd.; product code: GB23347) was purchased from a supermarket. Methanol was purchased from Beijing Chemical Company. All chemicals were used in the experiments without further purication. Synthesis of Cu NCs In a typical preparation process, a mixture of CuO powder (0.016 g) and olive oil (4.0 mL) was added into a 50 mL three-neck ask. Then the ask was connected to a Schlenk line equipped with a thermocouple and mantle. This mixture was heated to 270 C under the nitrogen protection at a heating rate of 5 C min1, until the black CuO mixture was completely turned to dark red. At different reaction moments, aliquots were taken from the reaction ask and naturally cooled to room temperature. Aliquots were puried with methanol by centrifuging and ultrasonication. These NCs were puried three times. The puried aliquot was dispersed in toluene and dropped on copper grids for characterization using transmission electron microscopy (TEM). Characterization All measurements were performed at room temperature. The phase purity of the obtained samples was characterized by X-ray
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Catalytic oxidation of CO To study the catalytic activity of CO oxidation, the as-prepared Cu nanocubes were deposited on ferric hydroxide. Under stirring at room temperature, NaOH solution was added to an aqueous mixture of Fe(NO3)3$9H2O and Cu nanocubes. Aer 2 h stirring and aging, the resulting precipitate was centrifuged, washed with distilled water several times, and then dried at 200 C for 6 h. The catalytic performances of CO oxidation were monitored in a continuous ow xed-bed reactor. About 0.10 g of solid catalyst (particle size within 40- to 60-mesh) was loaded in each run. The gas mixture consisted of 1% CO, 10% O2, balanced with Ar, and the total ow rate was 100 mL min1.
Results and discussion Fig. 1 exhibits the TEM and HRTEM images of the as-synthesized Cu NCs obtained at 10 min. As shown in Fig. 1a, the obtained samples are Cu nanocubes, with purity approaching 100%. The average side length of the Cu cubes is 18.61 nm, which have a uniform size distribution with a standard deviation (Std Dev) of 1.38 nm (Fig. 1b). The HRTEM image of an individual nanocube is given in Fig. 1c, and we can see that Cu nanocubes are single crystalline in nature, indicating high crystallinity. Moreover, the lattice spacing of 0.181 nm agrees
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Fig. 1 (a) The typical TEM image of the as-prepared Cu nanocubes taken at 10 min; (b) size distribution of the Cu nanocubes; (c) and (d) shows HRTEM images of an individual Cu nanocube. The area marked with a white frame in (c) is displayed in (d).
well with the d value of the (200) plane of the face-centered cubic (fcc) Cu, which further suggests that the nanocubes are enclosed by {100} facets (Fig. 1d).33 The obtained Cu nanocubes were further examined by EDS, and the result is shown in Fig. 2. The EDS spectrum only exhibits the characteristic peaks of Cu, indicating that pure Cu NCs were prepared under the present experimental conditions. The morphology and size of the product depended strongly on the reaction time. Aiming at investigating the reaction time effect, XRD and TEM were used to characterize the products taken at different reaction stages in the experiment. Fig. 3 depicts the corresponding temporal XRD patterns for 10 min, 1 h, 2 h, 4 h and 6 h, respectively. The diffraction peaks at 2q ¼ 43.5, 50.6,
Fig. 2
EDS spectrum of the Cu nanocubes.
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Fig. 3
Temporal XRD pattern evolution of the as-prepared Cu NCs.
and 74.3 were indexed as the (111), (200), and (220) planes of face-centered cubic (fcc) structured copper (JCPDS no. 85-1326), respectively. No impurity diffraction peak was detected, such as copper oxide, further indicating the purity of the obtained Cu NCs. For the Cu nanocubes, the (200) diffraction peak showed the strongest intensity in the XRD pattern (Fig.3a and b), and its intensity was nearly twice that of the commonly observed strongest (111) diffraction peak, whereas the (220) peak was not observed. It suggests that the nanocubes are perfectly terminated with {100} facets, which is consistent with the HRTEM results. With prolonging reaction time, the (111) diffraction peaks became more clear-cut and the intensities increased gradually, while the intensities of the (200) diffraction peaks weakened gradually. It has been reported that the relative intensity of the XRD peaks depended on the ratio of the exposed facets. A high intensity in the XRD peaks meant that the corresponding facets were highly exposed.34,35 Therefore, such evolution indicates that the (111) facets are the dominantly exposed facets when prolonging the reaction time to 6 h. In order to further examine the evolution of these samples, TEM images are recorded and presented in Fig. 4, and their corresponding size distributions are shown in Fig. 5. Fig. 4a shows the TEM image of the sample taken at 1 h. The dominant products are nanocubes with the average side length of 27.8 1.5 nm (Fig. 5a). With the reaction time increasing from 1 h to 6 h, the nanocubes tend to grow and form the thermodynamically favorable shape in an effort to minimize the surface energy. Meanwhile, there is a slight size growth accompanied by the mophology change, as shown in Fig. 4b and c. And their corresponding size distributions are uniform (Fig. 5b and c). The schematic illustration (Fig. 4e) reveals the shape revolution of Cu NCs clearly. We have carried out a series of experiments to
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Fig. 4 The shape evolution process of the as-prepared Cu NCs after being maintained for a certain time at the temperature of 270 C: (a) 1 h; (b) 2 h; (c) 4 h and (d) 6 h. (e) Schematic illustration of the growth process of Cu NCs.
The histograms for the size distribution of the obtained Cu NCs; (a) 1 h; (b) 2 h; (c) 4 h and (d) 6 h.
Fig. 5
tune the size and morphology of the Cu NCs by varying the temperature and the amount of olive oil (Fig. S1†). These results indicate that the size and morphology of the samples both
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changed greatly when we modied the reaction conditions. Thus, by carefully changing the reaction time, it is easy to achieve a shape-controlled synthesis of monodisperse Cu NCs. Herein, olive oil could be used as both reducing and capping agents. Oleic acid is known to have the ability of reduction. Various reducing agents, such as C, CO, and H2, which were generated from the thermolysis of oleic acid, seemed to be responsible for the formation of Cu NCs.36 It has also been demonstrated that oleic acid could behave as surfactant protecting metal NCs.37–40 We conducted an experiment to conrm the reduction ability of oleic acid. The procedure was similar to the experiment in the report, just employing oleic acid to replace olive oil. The obtained products are Cu nanoparticles, as shown in Fig. S2.† Therefore, oleic acid can act as a reducing agent to reduce the Cu(II) salt to Cu NCs. However, the Cu nanocrystals synthesized using pure oleic acid appear in the shape of the spheroid. In this paper, the formation of Cu nanocubes is the result of the combination of oleic acid and other fatty acids. Due to the complicated composition of olive oil, we cannot give a precise reaction mechanism. However, we believe that oleic acid in olive oil played a role of reduction in the reaction process. To conrm that this synthesis method is more general and reproducible, we perform the control experiment using olive oil of the same brand but different batch. The TEM image of the obtained Cu sample is shown in Fig. S3,† which indicates that the shape and size of Cu NCs do not change.
Catalytic reduction of dyes The catalytic reductions of dyes with an excess amount of NaBH4 were chosen as model reactions to evaluate the catalytic activity of Cu NCs. It is well documented that the reductions of dyes can be catalyzed by metal NCs, and the color fading and eventual bleaching involved in the reduction also provide a simple way to monitor the reaction kinetics based on spectroscopic measurements.41–44 It is worth noting that the activity of metallic catalysts depends on not only the surface to volume ratio, but also the exposed facets.45–47 A previous report demonstrated that the Cu(100) facets have higher catalytic activity than the Cu(111) facets in the catalytic reduction reaction.20 In order to verify this statement, we perform a controlled experiment using the Cu nanoparticles obtained at 6 h, as shown in Fig. S4.† The results indicate that the {100} planes are more active than {111} planes. Therefore, Cu nanocubes, which are ideally bound by six (100) facets, were chosen to be the desirable catalyst for the reduction of dyes. Because the amount of Cu nanocubes added was very small, the absorption spectra of dyes are hardly affected by the Cu nanocubes. In this work, we have studied the catalytic ability of Cu nanocubes towards the reduction of rhodamine B, methylene blue, methyl orange and congo red, with the maximal absorption peak (lmax) at 555, 675, 465 and 495 nm, respectively. The reductions of dyes by NaBH4 in aqueous solution take place very slowly without any catalysts but proceed very rapidly in the presence of Cu nanocubes. The catalytic reductions of dyes were monitored by UV-vis absorption spectroscopy of the reaction mixture aer the addition of the catalysts. As can be
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The evolution of the UV-vis spectra of dye aqueous solution in the presence of Cu nanocubes. (a) Rhodamine B; (b) methylene blue; (c) methyl orange; (d) congo red. Fig. 6
seen in Fig. 6, the maximum absorption peak gradually dropped in intensity as the reduction reaction proceeded. Aer several minutes, the maximal absorption peak disappeared aer the addition of Cu nanocubes, which indicated that the catalytic reduction of dyes had proceeded completely. The reaction processes can also be observed from the fading and ultimate bleaching of the color of dyes aer the addition of the catalysts (insets of Fig. 6). In the catalytic reactions, the Cu nanocubes can act as an electron relay system.48 The Cu nanocubes start the catalytic reduction by relaying electrons from the donor BH4 to the acceptor dye molecules, where the Cu nanocubes accept electrons from BH4 ions and conveys them to the dyes. Reduction of dyes by NaBH4 is oen used as a standard for determining the catalytic activity of the metallic nanoparticles, such as silver, palladium and alloys.49–51 From the above analysis the following mechanism for the catalytic reaction can be proposed: MB + Cu 4 MB–Cuads MB–Cuads + NaBH4 / LMB
The reaction forms the colorless leucomethylene blue (LMB), which can be transformed again to blue MB just by allowing the oxygen to enter into the reaction media. Aer the adsorption equilibrium between MB and Cu nanocubes forms an intermediate MB–Cuads, a posterior collision between this intermediate and the reduction agent, NaBH4, allows the electrons to be transferred to MB, mediated by the Cu nanocubes. Herein, Cu nanocubes also exhibit high catalytic activity, which can be accounted for the following reasons. Firstly, the planes with higher surface energy are more reactive.23 Therefore, the Cu nanocubes enclosed by the less stable {100} planes exhibited high catalytic activity.20 In addition, there are many atoms located at the exposed edges and corners of the nanocubes which are more
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Fig. 7 Plots of ln(C/C0) of methylene blue against time.
active than those on the surface of their spherical counterparts. Additionally, the reusability of Cu nanocubes was evaluated for the reduction of methylene blue, as shown in Fig. 7. Since the concentration ratio of NaBH4 to dyes was kept high during the reaction to ensure the reaction followed pseudo rst order kinetics, the same catalyst was utilized repeatedly up to six times for the reduction reaction. As illustrated in Fig. 7, it is found that Cu catalysts retained their catalytic activity even aer six cycles. The kinetic reaction rate constant is calculated to be 0.020 s1 for the rst cycle, and decreases to 0.015 s1 for the sixth cycle. The decrease in catalytic efficiency may be a result of the loss of smaller Cu nanoparticles during centrifugation and purication processes of catalysts. The high catalytic efficiency and excellent durability of Cu catalysts involved in reduction of dyes indicate that such catalysts will nd promising applications in catalysis. Catalytic performance for CO oxidation A metal modied-support is used to obtain a high dispersion of metal catalyst or an improvement in the number of active sites for the catalytic reaction. Such surface modication signicantly enhances the dispersion, the interaction between the metal nanoparticles and the support, and the thermal stability of metal supported on the support, which are crucial factors affecting the catalytic activity. Among the various supports, ferric hydroxide has been proved to be effective for O2 activation and has been used extensively as an additive to metal-based catalysts.52 The results showed that the large number of OH containing groups played an important role in determining the catalytic performance. Herein, we investigated the oxidation of CO on Cu supported on ferric hydroxide with different Cu loadings. Prior to the activity test, all the catalysts were prereduced in H2 for 30 min. For oxidation of CO, pure ferric hydroxide itself possessed lower activity, as shown in Fig. S5.† Also, Fig. S6† shows the TEM and HRTEM images of the assynthesized Cu/ferric hydroxide obtained by drying at 200 C for 6 h, which indicates that the shape and size of Cu NCs do not
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Fig. 8 CO conversion vs. reaction temperature over Cu/ferric hydroxide with different Cu loadings.
change. However, initial oxidation of the Cu was found to be largely dominated by the formation of a thin CuO layer at the particle/support interface, which will be substantially stabilized by the support. As a result, the supported Cu system shows a pronounced maximum of the oxygen-storage capacity at interfacial layers. A similar oxygen-storage mechanism has been conrmed in the Pd/Fe3O4 system.53 To investigate the effect of Cu loading on the catalytic activity, we prepared a series of samples with the Cu loading of 1%, 2%, 3% and 4%, respectively. The catalytic properties of Cu/ferric hydroxide for CO oxidation were determined by the temperature-programmed method. At low temperature, the reaction rate is limited by the transport of reactants to the catalyst active sites.54,55 In the intermediate light-off region, the reaction rate can be enhanced during the heating ramp by the evolved reaction heat, which results in high conversion.56 Fig. 8 illustrates the proles of CO conversion as a function of reaction temperature of the ferric hydroxide supported catalysts with different Cu contents. It is seen that the conversion curves are displaced to lower temperatures with increasing Cu loading, indicating that the activity of these catalysts increases with Cu loading between 1% and 3%. And in the 4% case, at temperature range from 80–150 C, the catalytic activity is increased compared with 3% Cu loaded, while there is a shi of the conversion curve to higher temperature when the temperature was higher than 150 C. For 3% Cu/ferric hydroxide, the temperature for 100% CO conversion was 180 C. Among the as-prepared catalysts, 3% Cu/ferric hydroxide provides the lowest temperatures of total conversion of CO. In this catalytic reaction, we know that the oxygen adsorbed on the support dissociates immediately, which can subsequently react with absorbed CO. When the Cu loading is 3%, we proposed that there is a balance between the absorbed CO and oxygen. The excess loading of metal particles may cover active sites on the ferric hydroxide surface thereby reducing catalytic efficiency. Beyond this value, the oxygen supply is rate limiting.
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(a and c) Raman spectrum of 0.1 and 0.01 M 4-MPy aqueous solutions; (b and d) SERS spectrum of 0.1 and 0.01 M 4-MPy absorbed on the Cu substrate. Data acquisition was the result of a single 30 s accumulation.
Fig. 9
SERS property Recent studies revealed that nanocrystals with shaper edges or corners would be more active in SERS applications due to the stronger eld enhancement near the edges or corners.57–59 Therefore, the Cu nanocubes were chosen to evaluate the SERS properties. The applications of cubic Cu NCs in SERS were investigated by using 4-mercaptopyridine (4-Mpy) as probe molecules. As shown in Fig. 9, curves a and c show Raman spectra of 0.1 M and 0.01 M 4-MPy aqueous solutions, respectively. Curves b and d are SERS spectra measured aer immersing Cu layers in 0.1 M and 0.01 M 4-MPy aqueous solutions for 100 min, respectively. Notably, the Cu SERS substrates (curve b) show a strong and distinctive Raman signal for 4-MPy, indicating the remarkable enhancement effect. When the amount of 4-MPy on the Cu substrate was reduced to 0.01 M, peaks of 4-MPy molecules were still observed as shown in curve d, which contains the same characteristic Raman peaks of 4-MPy as curve b. The results indicate that the Cu substrate possessed high surface enhancement efficiency and showed promise for application as SERS substrates.
Conclusions An environmentally benign approach is developed for the synthesis of monodisperse Cu NCs by the thermal reduction of copper oxide in olive oil. Only one reagent (olive oil) is required to obtain the monodisperse Cu NCs, indicating the advantages of simplicity and environmental friendliness. The morphology and average diameter can be greatly adjusted by varying the reaction time. The obtained Cu nanocubes show excellent catalytic properties for the catalytic reduction of dyes with good
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reusability. Cu nanocubes supported on ferric hydroxide are also very active for CO oxidation. Moreover, the prepared Cu nanocubes demonstrate a stark SERS activity for probe molecules of 4-MPy. Since the reagents used in the reaction medium are completely nontoxic and environmentally friendly, this green method can be encouraged for a broader range of applications.
Acknowledgements This work was supported by NSFC (no. 91227202 and 51202086), RFDP (no. 20120061130006 and 20110061120014), the National Basic Research Program of China (no. 2011CB808200), and the China Postdoctoral Science Foundation (no. 2013T60325).
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PAPER
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Cite this: DOI: 10.1039/c4nr00412d
A one-step green route to synthesize copper nanocrystals and their applications in catalysis and surface enhanced Raman scattering† Pinhua Zhang,ab Yongming Sui,*a Chao Wang,a Yingnan Wang,a Guangliang Cui,b Chunzhong Wang,c Bingbing Liua and Bo Zou*a A nontoxic, simple, inexpensive, and reproducible strategy, which meets the standard of green chemistry, is introduced for the synthesis of copper nanocrystals (Cu NCs) with olive oil as both reducing agent and capping agent. By changing the reaction parameters, the shape, size and surface structure of the Cu NCs can be well controlled. The obtained Cu nanocubes show excellent catalytic properties for the catalytic
Received 22nd January 2014 Accepted 1st March 2014
reduction of dyes and CO oxidation. Moreover, the prepared Cu nanocubes as substrates exhibit surface enhanced Raman scattering (SERS) activity for 4-mercaptopyridine (4-Mpy). Therefore, this facile route
DOI: 10.1039/c4nr00412d
provides a useful platform for the fabrication of Cu NCs which have the potential to replace noble
www.rsc.org/nanoscale
metals for certain applications.
Introduction Over the past decade there has been increasing emphasis on the topic of “green” chemistry and chemical processes.1–5 Green chemistry or sustainable chemistry is dened by the United States Environmental Protection Agency6 as “the design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances.” In recent years, there is a great societal expectation that chemists and chemical engineers should produce greener and more sustainable chemical processes. More efforts have been made towards the total elimination or at least the minimization of generated waste and the implementation of sustainable processes through adoption of 12 fundamental principles.1 Utilization of nontoxic chemicals, environmentally benign solvents, and renewable materials is the key issue that deserves important consideration in a green synthesis strategy. Thus, it is desirable to design feasible green chemical processes to synthesize nanocrystals (NCs) with desired planes and novel geometrical shapes. In terms of the perspective of green chemistry, the three main steps in the preparation of nanoparticles are the choice of the solvent medium used for the synthesis, the choice of an environmentally benign reducing agent, and the choice of a
a
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China. E-mail: [email protected]; [email protected]
b
Institute of Condensed Matter Physics, Linyi University, Linyi, Shandong, 276000, China
c Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun, 130012, China
† Electronic supplementary 10.1039/c4nr00412d
information
(ESI)
available.
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See
DOI:
nontoxic material for the stabilization of nanoparticles.7 Olive oil is mainly composed of mixed fatty acids of oleic acid and palmitic acid and of other fatty acids, which contains about 75% of its fat in the form of oleic acid. It is commonly used in cooking, cosmetics, pharmaceuticals, and soaps and as a fuel for traditional oil lamps. The monounsaturated fat content of olive oil has been linked not only to cholesterol reduction, but also to reduction of blood pressure. Furthermore, olive oil is oen used as a coordinating solvent to synthesize NCs. Recently, our group has synthesized ZnS and ZnSe NCs using olive oil as a solvent.8,9 It may be a suitable “green” alternative to organic solvents in applications. Copper (Cu) NCs have attracted much attention owing to their potential applications in the elds of catalysis, photovoltaics, electronics, optics and electrocatalysis.10–16 They have been explored as a possible substitute for noble metals on account of their unique properties. Cu NCs are also widely used as catalysts for various reactions including water–gas shi,17 gas detoxication reactions,18 and as electrocatalysts in solid oxide fuel cells.19 The basic properties of Cu NCs are mainly determined by their size, shape, composition, crystallinity and structure. Studies have also shown that the catalytic property of metal NCs is closely associated with the exposed crystallographic facets.20–22 Metal NCs with different sizes, shapes, and aspect ratios also exhibit distinct localized surface plasmon resonance (LSPR) and surface-enhanced Raman spectra (SERS) properties.23–25 Therefore, the preparation of monodispersed Cu NCs with controllable size and shape continues to be of considerable interest. In light of fast and growing applications of Cu NCs, the design and exploration of a facile and environmentally friendly strategy is highly desirable for the increasing demand. So far, the preparations of metal nanoparticles have been carried out through
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different methods, such as radiation methods, microemulsion techniques, electrode discharge and wet chemical reduction.26–29 Among the above methods, the wet chemical reduction method is one of the most common methods to produce metal nanoparticles, and has advantages of easy control of the reaction process and production rate. However, most of the wet chemical reduction methods need several key factors in the reducing process, including the solvent medium, the reducing agent, and the capping agent.30 Most of the wet chemical reduction methods reported to date rely heavily on organic solvents and environmentally or biologically hazardous reducing agents (i.e., hydrazine, sodium borohydride or hypophosphite, etc.).29,31,32 Although these methods could successfully produce pure and well-dened Cu NCs, the consequences of these methods are higher energy consumption and heavier pollution. Consequently, there is an unequivocal need to develop a more cost-effective and environmentally benign alternative to these existing methods. In this paper, we present a simple, environmentally friendly and cost-effective method for preparing monodisperse Cu NCs through the thermal reduction of copper oxide in olive oil. By varying the reaction time, the morphology and average diameter can be greatly adjusted. This approach eliminates the need for any air-sensitive, toxic, and expensive starting material. Olive oil is used in the reaction as both reducing and capping agents to prevent the aggregation of Cu NCs. The obtained Cu nanocubes show excellent catalytic properties and SERS properties. This inexpensive, environmentally friendly method for the synthesis of Cu NCs may be extended to prepare many other noble metallic nanostructures.
Experimental section
Paper
powder diffraction (XRD, Shimadzu, XRD-6000) operating at 40 kV and 40 mA with Cu Ka radiation. Data were collected from 20 to 80 with a sampling interval of 0.02 per step and a scan speed of 4 per minute. Samples for TEM characterization were prepared by adding several drops of Cu NC solution onto 300mesh copper grids with a carbon support lm. TEM and highresolution transmission electron microscopy (HRTEM) images were obtained via a JEM-2200FS transmission electron microscope at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) analysis was performed during TEM measurements. Ultravioletvisible absorption (UV-vis) spectra were obtained using an Ocean Optics QE65000 spectrometer. Catalytic reduction of dyes To study the catalytic properties of the as-synthesized Cu nanocubes, we evaluated their catalytic activities for the catalytic reduction of dyes by NaBH4 in water. In a typical experiment, 1.7 mL of methylene blue (0.1 mM, aqueous solution) was mixed with 0.7 mL of NaBH4 (0.04 M, aqueous solution) in a quartz cell (3.0 mL). Then the aqueous solution of Cu nanocubes obtained at 10 min (0.1 mL, 15 mM) was added to the mixture of methylene blue and NaBH4, and the changes of the solution were monitored with a UV-vis spectrophotometer at different time intervals. Similar procedures were used to catalyze the reduction of rhodamine B, methyl orange and congo red. For each catalytic reaction, the added amounts of dyes, NaBH4 and Cu nanocubes were equal. All the reactions were conducted at room temperature. For the recycling experiment, the catalysts were collected by centrifugation (12 000 rpm, 5 min, room temperature), washed with deionized water, and reused in the next reaction.
Materials CuO (99.9%) was purchased from Aldrich. Olive oil (produced by Shandong Luhua Group Co., Ltd.; product code: GB23347) was purchased from a supermarket. Methanol was purchased from Beijing Chemical Company. All chemicals were used in the experiments without further purication. Synthesis of Cu NCs In a typical preparation process, a mixture of CuO powder (0.016 g) and olive oil (4.0 mL) was added into a 50 mL three-neck ask. Then the ask was connected to a Schlenk line equipped with a thermocouple and mantle. This mixture was heated to 270 C under the nitrogen protection at a heating rate of 5 C min1, until the black CuO mixture was completely turned to dark red. At different reaction moments, aliquots were taken from the reaction ask and naturally cooled to room temperature. Aliquots were puried with methanol by centrifuging and ultrasonication. These NCs were puried three times. The puried aliquot was dispersed in toluene and dropped on copper grids for characterization using transmission electron microscopy (TEM). Characterization All measurements were performed at room temperature. The phase purity of the obtained samples was characterized by X-ray
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Catalytic oxidation of CO To study the catalytic activity of CO oxidation, the as-prepared Cu nanocubes were deposited on ferric hydroxide. Under stirring at room temperature, NaOH solution was added to an aqueous mixture of Fe(NO3)3$9H2O and Cu nanocubes. Aer 2 h stirring and aging, the resulting precipitate was centrifuged, washed with distilled water several times, and then dried at 200 C for 6 h. The catalytic performances of CO oxidation were monitored in a continuous ow xed-bed reactor. About 0.10 g of solid catalyst (particle size within 40- to 60-mesh) was loaded in each run. The gas mixture consisted of 1% CO, 10% O2, balanced with Ar, and the total ow rate was 100 mL min1.
Results and discussion Fig. 1 exhibits the TEM and HRTEM images of the as-synthesized Cu NCs obtained at 10 min. As shown in Fig. 1a, the obtained samples are Cu nanocubes, with purity approaching 100%. The average side length of the Cu cubes is 18.61 nm, which have a uniform size distribution with a standard deviation (Std Dev) of 1.38 nm (Fig. 1b). The HRTEM image of an individual nanocube is given in Fig. 1c, and we can see that Cu nanocubes are single crystalline in nature, indicating high crystallinity. Moreover, the lattice spacing of 0.181 nm agrees
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Fig. 1 (a) The typical TEM image of the as-prepared Cu nanocubes taken at 10 min; (b) size distribution of the Cu nanocubes; (c) and (d) shows HRTEM images of an individual Cu nanocube. The area marked with a white frame in (c) is displayed in (d).
well with the d value of the (200) plane of the face-centered cubic (fcc) Cu, which further suggests that the nanocubes are enclosed by {100} facets (Fig. 1d).33 The obtained Cu nanocubes were further examined by EDS, and the result is shown in Fig. 2. The EDS spectrum only exhibits the characteristic peaks of Cu, indicating that pure Cu NCs were prepared under the present experimental conditions. The morphology and size of the product depended strongly on the reaction time. Aiming at investigating the reaction time effect, XRD and TEM were used to characterize the products taken at different reaction stages in the experiment. Fig. 3 depicts the corresponding temporal XRD patterns for 10 min, 1 h, 2 h, 4 h and 6 h, respectively. The diffraction peaks at 2q ¼ 43.5, 50.6,
Fig. 2
EDS spectrum of the Cu nanocubes.
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Fig. 3
Temporal XRD pattern evolution of the as-prepared Cu NCs.
and 74.3 were indexed as the (111), (200), and (220) planes of face-centered cubic (fcc) structured copper (JCPDS no. 85-1326), respectively. No impurity diffraction peak was detected, such as copper oxide, further indicating the purity of the obtained Cu NCs. For the Cu nanocubes, the (200) diffraction peak showed the strongest intensity in the XRD pattern (Fig.3a and b), and its intensity was nearly twice that of the commonly observed strongest (111) diffraction peak, whereas the (220) peak was not observed. It suggests that the nanocubes are perfectly terminated with {100} facets, which is consistent with the HRTEM results. With prolonging reaction time, the (111) diffraction peaks became more clear-cut and the intensities increased gradually, while the intensities of the (200) diffraction peaks weakened gradually. It has been reported that the relative intensity of the XRD peaks depended on the ratio of the exposed facets. A high intensity in the XRD peaks meant that the corresponding facets were highly exposed.34,35 Therefore, such evolution indicates that the (111) facets are the dominantly exposed facets when prolonging the reaction time to 6 h. In order to further examine the evolution of these samples, TEM images are recorded and presented in Fig. 4, and their corresponding size distributions are shown in Fig. 5. Fig. 4a shows the TEM image of the sample taken at 1 h. The dominant products are nanocubes with the average side length of 27.8 1.5 nm (Fig. 5a). With the reaction time increasing from 1 h to 6 h, the nanocubes tend to grow and form the thermodynamically favorable shape in an effort to minimize the surface energy. Meanwhile, there is a slight size growth accompanied by the mophology change, as shown in Fig. 4b and c. And their corresponding size distributions are uniform (Fig. 5b and c). The schematic illustration (Fig. 4e) reveals the shape revolution of Cu NCs clearly. We have carried out a series of experiments to
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Fig. 4 The shape evolution process of the as-prepared Cu NCs after being maintained for a certain time at the temperature of 270 C: (a) 1 h; (b) 2 h; (c) 4 h and (d) 6 h. (e) Schematic illustration of the growth process of Cu NCs.
The histograms for the size distribution of the obtained Cu NCs; (a) 1 h; (b) 2 h; (c) 4 h and (d) 6 h.
Fig. 5
tune the size and morphology of the Cu NCs by varying the temperature and the amount of olive oil (Fig. S1†). These results indicate that the size and morphology of the samples both
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changed greatly when we modied the reaction conditions. Thus, by carefully changing the reaction time, it is easy to achieve a shape-controlled synthesis of monodisperse Cu NCs. Herein, olive oil could be used as both reducing and capping agents. Oleic acid is known to have the ability of reduction. Various reducing agents, such as C, CO, and H2, which were generated from the thermolysis of oleic acid, seemed to be responsible for the formation of Cu NCs.36 It has also been demonstrated that oleic acid could behave as surfactant protecting metal NCs.37–40 We conducted an experiment to conrm the reduction ability of oleic acid. The procedure was similar to the experiment in the report, just employing oleic acid to replace olive oil. The obtained products are Cu nanoparticles, as shown in Fig. S2.† Therefore, oleic acid can act as a reducing agent to reduce the Cu(II) salt to Cu NCs. However, the Cu nanocrystals synthesized using pure oleic acid appear in the shape of the spheroid. In this paper, the formation of Cu nanocubes is the result of the combination of oleic acid and other fatty acids. Due to the complicated composition of olive oil, we cannot give a precise reaction mechanism. However, we believe that oleic acid in olive oil played a role of reduction in the reaction process. To conrm that this synthesis method is more general and reproducible, we perform the control experiment using olive oil of the same brand but different batch. The TEM image of the obtained Cu sample is shown in Fig. S3,† which indicates that the shape and size of Cu NCs do not change.
Catalytic reduction of dyes The catalytic reductions of dyes with an excess amount of NaBH4 were chosen as model reactions to evaluate the catalytic activity of Cu NCs. It is well documented that the reductions of dyes can be catalyzed by metal NCs, and the color fading and eventual bleaching involved in the reduction also provide a simple way to monitor the reaction kinetics based on spectroscopic measurements.41–44 It is worth noting that the activity of metallic catalysts depends on not only the surface to volume ratio, but also the exposed facets.45–47 A previous report demonstrated that the Cu(100) facets have higher catalytic activity than the Cu(111) facets in the catalytic reduction reaction.20 In order to verify this statement, we perform a controlled experiment using the Cu nanoparticles obtained at 6 h, as shown in Fig. S4.† The results indicate that the {100} planes are more active than {111} planes. Therefore, Cu nanocubes, which are ideally bound by six (100) facets, were chosen to be the desirable catalyst for the reduction of dyes. Because the amount of Cu nanocubes added was very small, the absorption spectra of dyes are hardly affected by the Cu nanocubes. In this work, we have studied the catalytic ability of Cu nanocubes towards the reduction of rhodamine B, methylene blue, methyl orange and congo red, with the maximal absorption peak (lmax) at 555, 675, 465 and 495 nm, respectively. The reductions of dyes by NaBH4 in aqueous solution take place very slowly without any catalysts but proceed very rapidly in the presence of Cu nanocubes. The catalytic reductions of dyes were monitored by UV-vis absorption spectroscopy of the reaction mixture aer the addition of the catalysts. As can be
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The evolution of the UV-vis spectra of dye aqueous solution in the presence of Cu nanocubes. (a) Rhodamine B; (b) methylene blue; (c) methyl orange; (d) congo red. Fig. 6
seen in Fig. 6, the maximum absorption peak gradually dropped in intensity as the reduction reaction proceeded. Aer several minutes, the maximal absorption peak disappeared aer the addition of Cu nanocubes, which indicated that the catalytic reduction of dyes had proceeded completely. The reaction processes can also be observed from the fading and ultimate bleaching of the color of dyes aer the addition of the catalysts (insets of Fig. 6). In the catalytic reactions, the Cu nanocubes can act as an electron relay system.48 The Cu nanocubes start the catalytic reduction by relaying electrons from the donor BH4 to the acceptor dye molecules, where the Cu nanocubes accept electrons from BH4 ions and conveys them to the dyes. Reduction of dyes by NaBH4 is oen used as a standard for determining the catalytic activity of the metallic nanoparticles, such as silver, palladium and alloys.49–51 From the above analysis the following mechanism for the catalytic reaction can be proposed: MB + Cu 4 MB–Cuads MB–Cuads + NaBH4 / LMB
The reaction forms the colorless leucomethylene blue (LMB), which can be transformed again to blue MB just by allowing the oxygen to enter into the reaction media. Aer the adsorption equilibrium between MB and Cu nanocubes forms an intermediate MB–Cuads, a posterior collision between this intermediate and the reduction agent, NaBH4, allows the electrons to be transferred to MB, mediated by the Cu nanocubes. Herein, Cu nanocubes also exhibit high catalytic activity, which can be accounted for the following reasons. Firstly, the planes with higher surface energy are more reactive.23 Therefore, the Cu nanocubes enclosed by the less stable {100} planes exhibited high catalytic activity.20 In addition, there are many atoms located at the exposed edges and corners of the nanocubes which are more
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Fig. 7 Plots of ln(C/C0) of methylene blue against time.
active than those on the surface of their spherical counterparts. Additionally, the reusability of Cu nanocubes was evaluated for the reduction of methylene blue, as shown in Fig. 7. Since the concentration ratio of NaBH4 to dyes was kept high during the reaction to ensure the reaction followed pseudo rst order kinetics, the same catalyst was utilized repeatedly up to six times for the reduction reaction. As illustrated in Fig. 7, it is found that Cu catalysts retained their catalytic activity even aer six cycles. The kinetic reaction rate constant is calculated to be 0.020 s1 for the rst cycle, and decreases to 0.015 s1 for the sixth cycle. The decrease in catalytic efficiency may be a result of the loss of smaller Cu nanoparticles during centrifugation and purication processes of catalysts. The high catalytic efficiency and excellent durability of Cu catalysts involved in reduction of dyes indicate that such catalysts will nd promising applications in catalysis. Catalytic performance for CO oxidation A metal modied-support is used to obtain a high dispersion of metal catalyst or an improvement in the number of active sites for the catalytic reaction. Such surface modication signicantly enhances the dispersion, the interaction between the metal nanoparticles and the support, and the thermal stability of metal supported on the support, which are crucial factors affecting the catalytic activity. Among the various supports, ferric hydroxide has been proved to be effective for O2 activation and has been used extensively as an additive to metal-based catalysts.52 The results showed that the large number of OH containing groups played an important role in determining the catalytic performance. Herein, we investigated the oxidation of CO on Cu supported on ferric hydroxide with different Cu loadings. Prior to the activity test, all the catalysts were prereduced in H2 for 30 min. For oxidation of CO, pure ferric hydroxide itself possessed lower activity, as shown in Fig. S5.† Also, Fig. S6† shows the TEM and HRTEM images of the assynthesized Cu/ferric hydroxide obtained by drying at 200 C for 6 h, which indicates that the shape and size of Cu NCs do not
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Fig. 8 CO conversion vs. reaction temperature over Cu/ferric hydroxide with different Cu loadings.
change. However, initial oxidation of the Cu was found to be largely dominated by the formation of a thin CuO layer at the particle/support interface, which will be substantially stabilized by the support. As a result, the supported Cu system shows a pronounced maximum of the oxygen-storage capacity at interfacial layers. A similar oxygen-storage mechanism has been conrmed in the Pd/Fe3O4 system.53 To investigate the effect of Cu loading on the catalytic activity, we prepared a series of samples with the Cu loading of 1%, 2%, 3% and 4%, respectively. The catalytic properties of Cu/ferric hydroxide for CO oxidation were determined by the temperature-programmed method. At low temperature, the reaction rate is limited by the transport of reactants to the catalyst active sites.54,55 In the intermediate light-off region, the reaction rate can be enhanced during the heating ramp by the evolved reaction heat, which results in high conversion.56 Fig. 8 illustrates the proles of CO conversion as a function of reaction temperature of the ferric hydroxide supported catalysts with different Cu contents. It is seen that the conversion curves are displaced to lower temperatures with increasing Cu loading, indicating that the activity of these catalysts increases with Cu loading between 1% and 3%. And in the 4% case, at temperature range from 80–150 C, the catalytic activity is increased compared with 3% Cu loaded, while there is a shi of the conversion curve to higher temperature when the temperature was higher than 150 C. For 3% Cu/ferric hydroxide, the temperature for 100% CO conversion was 180 C. Among the as-prepared catalysts, 3% Cu/ferric hydroxide provides the lowest temperatures of total conversion of CO. In this catalytic reaction, we know that the oxygen adsorbed on the support dissociates immediately, which can subsequently react with absorbed CO. When the Cu loading is 3%, we proposed that there is a balance between the absorbed CO and oxygen. The excess loading of metal particles may cover active sites on the ferric hydroxide surface thereby reducing catalytic efficiency. Beyond this value, the oxygen supply is rate limiting.
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(a and c) Raman spectrum of 0.1 and 0.01 M 4-MPy aqueous solutions; (b and d) SERS spectrum of 0.1 and 0.01 M 4-MPy absorbed on the Cu substrate. Data acquisition was the result of a single 30 s accumulation.
Fig. 9
SERS property Recent studies revealed that nanocrystals with shaper edges or corners would be more active in SERS applications due to the stronger eld enhancement near the edges or corners.57–59 Therefore, the Cu nanocubes were chosen to evaluate the SERS properties. The applications of cubic Cu NCs in SERS were investigated by using 4-mercaptopyridine (4-Mpy) as probe molecules. As shown in Fig. 9, curves a and c show Raman spectra of 0.1 M and 0.01 M 4-MPy aqueous solutions, respectively. Curves b and d are SERS spectra measured aer immersing Cu layers in 0.1 M and 0.01 M 4-MPy aqueous solutions for 100 min, respectively. Notably, the Cu SERS substrates (curve b) show a strong and distinctive Raman signal for 4-MPy, indicating the remarkable enhancement effect. When the amount of 4-MPy on the Cu substrate was reduced to 0.01 M, peaks of 4-MPy molecules were still observed as shown in curve d, which contains the same characteristic Raman peaks of 4-MPy as curve b. The results indicate that the Cu substrate possessed high surface enhancement efficiency and showed promise for application as SERS substrates.
Conclusions An environmentally benign approach is developed for the synthesis of monodisperse Cu NCs by the thermal reduction of copper oxide in olive oil. Only one reagent (olive oil) is required to obtain the monodisperse Cu NCs, indicating the advantages of simplicity and environmental friendliness. The morphology and average diameter can be greatly adjusted by varying the reaction time. The obtained Cu nanocubes show excellent catalytic properties for the catalytic reduction of dyes with good
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reusability. Cu nanocubes supported on ferric hydroxide are also very active for CO oxidation. Moreover, the prepared Cu nanocubes demonstrate a stark SERS activity for probe molecules of 4-MPy. Since the reagents used in the reaction medium are completely nontoxic and environmentally friendly, this green method can be encouraged for a broader range of applications.
Acknowledgements This work was supported by NSFC (no. 91227202 and 51202086), RFDP (no. 20120061130006 and 20110061120014), the National Basic Research Program of China (no. 2011CB808200), and the China Postdoctoral Science Foundation (no. 2013T60325).
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