Journal of Nanomaterials & Molecular NanotechnologyISSN: 2324-8777

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Research Article, J Nanomater Mol Nanotechnol Vol: 3 Issue: 3

Fabrication of Cu Hollow Microspheres by Liquid Reduction Method

Liye Xu1, Da Kuang2*, Wenbin Hu1 and Yida Deng1*
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China
Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, AB, Canada T2N 1N4
Corresponding authors : Dr. Da Kuang
Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, AB Canada T2N 1N4
Tel: +1 587 889 0190; Fax: +1 403 220 7664
E-mail: [email protected]
Dr. Yida Deng
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China
E-mail: [email protected]
Received: March 08, 2014 Accepted: April 18, 2014 Published: April 23, 2014
Citation: Liye X, Kuang D, Wenbin H, Deng Y (2014) Fabrication of Cu Hollow Microspheres by Liquid Reduction Method. J Nanomater Mol Nanotechnol 3:3. doi:10.4172/2324-8777.1000147

Abstract

Fabrication of Cu Hollow Microspheres by Liquid Reduction Method

Cu hollow microspheres have been successfully synthesized through a two-step liquid reaction method. Firstly, Cu(OH)2 precipitates were formed by adding CuSO4•5H2O into NaOH and they were reduced to Cu2O solid microspheres by glucose. Then, PVP and allylthiourea were added to the Cu2O suspension and they were further reduced to Cu hollow microspheres by hydrazine hydrate. The sample was characterized by X-ray diffraction, Field-emission scanning electron microscopy, and transmission electron microscopy. The results showed the Cu hollow microspheres with average diameter about 800nm. The growth process of Cu hollow microspheres was proposed and the photocatalytic property in degradation reaction of methylene blue (MB) was also explored. It was found that the morphology and hollow structure of the Cu microspheres are highly dependent on the amount of allylthiourea added. Compared to the Cu microparticles with irregular morphology, Cu hollow microshperes show much higher photocatalytic activity in the degradation reaction of MB.

Keywords: Hollow structures; Cu microspheres; Hydrazine hydrate; Allylthiourea; Photocatalytic degradation

Keywords

Hollow structures; Cu microspheres; Hydrazine hydrate; Allylthiourea; Photocatalytic degradation

Introduction

In the past few years, hollow metal micro- and nanostructures have received intensive attention because of their specific structure, interesting properties which apparently superior to their solid counterparts [1,2], and widespread potential applications in catalysis [3], drug delivery [4], optical sensing [5], and other areas [6,7]. Among many metallic materials, Cu has attracted intense attention because of its high electrical and thermal conductivity as well as important role in electronics, catalysis, and thermal conduction [8]. Until now, the synthesized Cu structures include nanocubes [9], nanowires [10], nanotubes [11], and nanodisks [12].
However, to the best of our knowledge, there are very few reports of the synthesis of hollow Cu nano/microshperes [11,13]. For example, Chu Y et al reported a surfactant-assisted hydrothermal process to fabricate hollow Cu nano/microstructures (Cu nanotubes, microparticles assembled from hollow Cu nanoparticles, and hollow Cu microparticles) by reduction of CuSO4•5H2O with glucose. It is indicated that the concentration of sodium dodecyl sulfate (CSDS), glucose (Cglucose), and NaOH (CNaOH) play important roles in the formation of the hollow Cu nano/microstructures [11]. Xia LX et al designed another method to prepare micron-sized hollow copper spheres by using ZSM-5 molecular sieve as the removable templates. The formation of the hollow copper spheres demonstrates that the ZSM-5 molecular sieve is an effective template owing to the presence of many active negatively charged oxygen sites and also the presence of holes on its surface [13].
Meanwhile, it is well known that methylene blue (MB), which has dark color and toxicity [14], is widely used as dye for printing and fluorescent pigments. The direct release of wastewater containing MB causes serious environmental problems. However, the biological methods and traditional techniques to deal with superfluous MB, such as adsorption on activated carbon, are ineffective at the decolorization of MB, because of the stability and complex aromatic structures of the dyes [15,16].
In this paper, we used a simple liquid reduction method for fabricating Cu hollow microspheres, in which Cu2O microspheres were used as the sacrificial templates as well as the reagent to provide Cu+ and hydrazine hydrate was used as the reductant. Our process highlights this unique in the needless additional templates and hydrothermal condition, which can not only avoids the introduction of impurities, but also decreases the preparation temperature and simplifies the synthesis procedure. The photocatalytic property of the as-prepared Cu hollow microspheres in the degradation of MB also was investigated. And the products show good photocatalytic activities in the degradation reaction because of their special structures.

Experimental

Chemicals and materials
Copper sulfate (CuSO4•5H2O), sodium hydroxide (NaOH), glucose, Polyvinyl Pyrrolidone (PVP), allylthiourea and hydrazine hydrate (N2H4•H2O) were obtained from National Chemical Reagent Ltd., Shanghai, China. All of the chemicals were analytical grade and used without further purification. In this work, Polyvinyl Pyrrolidone (PVP) was added as a surfactant to improve the surface activity for the reduction of Cu2O and an additional preventive measure against particle agglomeration.
Preparation of CuO precursors
In a typical procedure, CuSO4 solution (100 ml, 1 M) was first added into NaOH solution (100 ml, 3 M) at 30°C under constant magnetic stirring. Then the blue Cu(OH)2 precipitates appeared immediately. After the solution was stirred for about 5 min, glucose solution (150 ml, 1 M) was added into the suspension and it was hold at 30°C for 1 h with constant stirring. The solution gradually turned from blue to bright yellow, yellow, until to brick-red, which implied the formation of Cu2O. After that, the precipitates were filtered and washed with de-ionized water for four times.
Preparation of Cu hollow microspheres
100 ml de-ionized water was added to the as-prepared Cu2O precipitates and heated to 45°C. 2 g PVP was added and the solution was kept heating to 70°C, then 3 mg allylthiourea and N2H4•H2O solution (50 ml, 2 M) were added sequentially. The mixed solution was maintained at 70°C for 30 min and kept stirring through the whole process. The solution gradually turned into black-red, which implied the formation of Cu. The precipitates were washed with deionized water and acetone for several times. The final products were dried in a vacuum furnace at 70°C for 5h.
Photocatalytic activity test
In the photocatalytic degradation experiment, the Cu hollow microspheres were used as catalyst for the oxidation and decomposing of the methylene blue (MB) dye with the assistance of hydrogen peroxide (H2O2) at room temperature. The photocatalytic effect of copper is based on oxidative reactions initiated by hydroxyl radicals (•OH), which are often generated by the photolysis of hydrogen peroxide. The original solution was prepared by adding 1.3 ml H2O2 (30%, w/w) to 50 ml MB solution (20 mg/l). Then, 30mg Cu hollow microspheres were added to the solution. At once, the dispersion was magnetically stirred in the dark for 30 min to establish an adsorption/ desorption equilibrium of MB. Afterward, and the dispersion was allowed to react at room temperature under the light of xenon lamp. At given time intervals, the dispersion was sampled for 1 ml and diluted to 5 ml, and centrifuged to separate the catalyst.
Characterization
Structural characterization was performed by X-ray diffraction (XRD) using a Rigaku Ultima IV diffractometer, which was operated at 40 kV/30mA with Cu-Kα radiation (λ = 1.54056 Å). Transmission electron microscopy (TEM) images were recorded on a JEOL 2100 microscope operating at 200 kV. Field-emission scanning electron microscopy (FESEM) measurements were performed with a Philips XL30FEG operated at an accelerating voltage of 20 kV. The ultraviolet–visible (UV–vis) absorption spectra were measured at room temperature on a spectrophotometer (Spectrumlab PC22).

Results and Discussion

Figure 1a presents the FESEM image of the Cu2O microspheres with average diameter about 800nm. Figure 1b shows the FESEM image of the Cu microspheres with the same diameter, which were reduced by the hydrazine hydrate. The inset of Figure 1b exhibits two broken microspheres with the hollow inner, which indicates that the Cu microspheres were assembled with a large amount of nanoparticles. Figure 1c shows the TEM image of the hollow spheres, a strong contrast between the dark ring and the pale center further confirms the hollow nature of the sphere. The selected-area electron diffraction (SAED) pattern shown in the inset of figure 1c indicates that the hollow Cu microspheres are polycrystalline and the diffuse rings corresponding to d(111)=0.203 nm, d(200)=0.176 nm and d(222)=0.124 nm are clearly seen in it. Figure 1d shows the powder XRD patterns of samples in figures 1a and b respectively. The XRD diffraction peaks of sample (a) can be indexed to the cubic phase of Cu2O (JCPDS No. 65-3288). The main peaks of sample (b), which correspond to the crystallographic plane of Cu fcc lattice (JCPDS No. 65-9743), suggest that this sample mainly comprises of Cu. However, some weak Cu2O peaks also presented in the XRD pattern of sample (b), which may be attributed to the slight oxidation of the Cu particles during the process of drying at 70°C for 5 h.
Figure 1: (a) FESEM image of Cu2O microspheres (b)FESEM image of Cu microspheres which were reduced by hydrazine hydrate (inset: the FESEM image of the broken Cu microspheres with the hollow inner), (c) TEM image of the Cu hollow spheres (inset: SAED pattern), (d) XRD pattern of samples shown in (a) and (b).
In our study, it was found that the hollow structure of the Cu microspheres highly dependents on the amount of allylthiourea. To understand the formation mechanism of the Cu hollow microspheres, the amount of allythiourea was varied while keeping all other experimental parameters constant (Table 1).
Table 1: Amount of allylthiourea Used in the Preparation of Cu hollow microshperes.
Figures 2a-c display the FESEM images of sample S0 extracted at 5 min, 15 min and 30 min in the reaction process. Figures 2d-f and Figures 2g-i respectively represent the FESEM images of samples S1 and S3, and they also have the same extracting rules as S0 from left to right. From Figures 2a, it can be seen that at 5 min, the small particles of sample S0 have already dispersed and grown into irregular shape. These particles grew bigger as the reaction time was prolonged to 15 min or 30 min, as showed in Figures 2b-c. For sample S1, which are showed in Figures 2d-f, although there were still a lot of dispersed particles, some microspheres have already appeared. As shown in Figures 2g, the sample S3 exhibits the spherical structure when the reaction time was 5 min. When the reaction time was prolonged to 15 min or 30 min, as shown in Figures 2h-i, the overall morphology still kept as spheres. It is noted that a few spheres with broken surface present when the reaction time is 30 min (Figures 2i). Energydispersive X-ray spectroscopy (EDS) and XRD patterns shown in (Figures 2j and k) can both demonstrate that the as-prepared samples are metallic Cu and has fcc crystallographic lattice.
Figure 2: (a)-(c), (d)-(f), (g)-(i) are FESEM images of samples S0, S1, S3 in 5 min, 15 min, 30min respectively, (j) and (k) are XRD and EDS pattern of the samples S0, S1, S3 in 30 min.
Based on the above results, the formation process of hollow Cu microspheres is proposed in Scheme 1, which is a two-step growth model. At the first step, the CuSO4 solution and NaOH solution react together to form Cu(OH)2 and then they are reduced by glucose to form Cu2O nanoparticles, followed by the self-assembly of Cu2O nanoparticles to form solid spherical Cu2O at the 30° (eq. 1 and 2).
Scheme 1: Schematic representation of reaction process when the allythiourea is (a) sufficient, (b) insufficient.
At the second step, the Cu2O can be reduced to metal Cu by hydrazine hydrate (eq. 3) and the Cu2O spheres can continually decompose due to the production of the H+ ions (eq. 4). The allythiourea is the key to fabricate the Cu hollow spheres. It was reported that the Cu+ complexes with allylthiourea exhibit trigonal or tetrahedral coordination [17]. The surfaces of the as-formed Cu2O are surrounded by allylthiourea. When the amount of allythiourea is sufficient, it can form a protective layer on the surface of the precursor and help to keep its spherical shape, as shown in the direction (a) of Scheme 1.
On the other hand, the decomposed Cu2O nanoparticles also supply a large amount of Cu+ to the solution. Then, the inner side of the Cu2O could gradually decompose and reduced metal Cu will deposit on the surface of the spherical Cu2O. Then, the hollow structures of Cu are fabricated. Whereas, when the amount of allylthiourea is insufficient, there are no protect layer formed in the surface of the precursor, the Cu2O spheres are reduced and decomposed by hydrazine hydrate directly. This will create many dispersed particles, as shown in the direction (b) of Scheme 1.
CuSO4 + 2NaOH → Cu(OH)2 + NaSO4 (1)
2Cu(OH)2 + C6H12O6 + NaOH → Cu2O↓ + C6H11O7Na + 3H2O (2)
4Cu+ + N2H4 → 4Cu + N2 + 4H+ (3)
Cu2O + 2H+ → 2Cu+ + H2O (4)
To investigate their photocatalytic activity, the hollow copper spheres and the irregular copper particles were employed as catalysts in the photocatalytic degradation of MB. The progression of the photocatalytic degradation of MB can be easily monitored by the decrease in optical density at the wavelength at 665 nm for the absorbance maximum of MB. Figure 3 shows the absorbency of the degradation of MB in the presence of different structures of Cu.
Figure 3: Degradation rates of MB in the presence of different structures of Cu under the same light of xenon lamp: (a) in the presence of H2O2, (b) in the presence of irregular Cu and H2O2, (c) in the presence of Cu hollow microspheres and H2O2.
It can be seen that when there was no catalyst, the degradation rate of MB was only 30% after 80 min. And that of MB was 80% when irregular Cu microparticles were added. In the contrast, when the hollow Cu microspheres were added, the color of the mixture vanished rapidly. The degradation rate of MB in the presence of hollow Cu microspheres and H2O2 was 93% after the same time. This result indicates that the hollow copper sphere catalyst has higher activity in degradation of MB compared to irregular Cu microparticles. The highly catalytic activity of hollow copper spheres is attributed to their hollow structure and the rough surface, which comprise of many nanoparticles.

Conclusion

In conclusion, we have demonstrated a facile liquid reduction method to prepare Cu hollow microspheres with a size in the range of 700-900 nm. The amount of allylthiourea is the key factor of controlling the formation of the Cu hollow spheres. The photocatalytic properties of these hollow structures in the degradation of MB exhibit the outstanding catalytic activity. This work added a new example of fabricating Cu hollow microspheres. We can use the Cu2O with different diameters as precursor to fabricate Cu hollow microspheres with other sizes. And this work would be helpful in developing novel hollow copper catalysts as well.

Acknowledgments

This work was supported by the National Science Foundation for Distinguished Young Scholars of China (51125016), the National Science Foundation of China (51001075, 51371119), Program of Shanghai Subject Chief Science (11XD1402700) and Following Research Project supported by the “Dawn” program of Shanghai Education Commission (10GG06).

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