Nanoparticle-aggregated octahedral copper hierarchical nanostructures

Abstract

A facile solution route has been developed to synthesize nanoparticle-aggregated octahedral copper hierarchical nanostructures. The formation of these novel octahedral copper hierarchical nanostructures can be attributed to a gas bubbles assisted aggregation mechanism. Based on the shape evolution and corresponding growth mechanism, it is believed that more and more copper crystals with novel hierarchical nanostructures could be obtained.

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The morphological control of crystals has been an exciting challenge for the current material synthesis because novel shapes often exhibit unique chemico-physical properties, which would develop their widespread potential applications and significant scientific value.^1–3 As one type of attractive structures with large surface areas and good conductivity for charge and mass (gas) transport, tremendous interests in hierarchical three-dimensional (3D) superstructures have been aroused because of their potential applications in sensors, dye-sensitized solar cells, and supercapacitors, etc.^4–7 Currently, hierarchical 3D superstructures, with different building blocks, can be achieved by precisely controlling the nucleation and growthvia bottom-up self-assembly route, and many recent efforts have been employed to the synthesis of hierarchical architectures. So far, the reports on hierarchical 3D superstructures have mainly focused on oxide and sulfide,⁸ but novel hierarchical architectures of pure metals are less reported. Especially, the recent reports on hierarchical dendritic gold and plate-like silver mesostructures,^9,10 are highly desirable to prepare other pure metals with hierarchical 3D superstructures. Hence, it is still a challenge for material scientists to develop new facile methods to fabricate pure metal hierarchical superstructures with well-defined non-spherical morphologies.

Copper (Cu) is chosen in this work because of its high electrical and thermal conductivity, low price and stability at high frequencies.^4,11 To date, copper nanostructures with different shapes, such as nanowalls,⁴dendrites,¹¹ nanocubes,¹²nanowires,¹³ and 3D foams,¹⁴ have been synthesized by various bottom-up technologies. Herein, we report a facile solution phase route to fabricate novel hierarchical octahedral copper nanostructures, and the growth mechanism is discussed in detail.

In a typical procedure, 0.426 g of CuCl₂ and 0.85 g of PVP (K 30, MW = 40 000) were dissolved in deionized water (65 mL) under constant stirring for 10 min at 25 °C. A blue precipitate was produced when 5 mL of NaOH (6 M) solution was added dropwise to the above solution. After being stirred for 10 min, 3.0 mL of N₂H₄·H₂O (35 wt%) was added dropwise to the mixed solution under constantly stirring for 5 min. It was found that the colour of the precipitate in solution turned from blue to yellow along with the reaction time increase. Afterward, the obtained mixture was heated up at 80 °C for 5 min, and then allowed to cool to room temperature naturally. Finally, the dark red products were separated and cleaned by repeated centrifugation with water and ethanol, and stored in a water-hydrazine solution to prevent oxidation. The crystal phase of as-prepared products was characterized by an X-ray diffractometer (Bruker-AXS D8 ADVANCE) using CuKα radiation (λ = 1.54 Å) in the range (20–80°). The morphology of the powders was investigated by field-emission scanning electron microscope (FESEM) using JEOL (JSM-7000F) at an accelerating voltage of 20 kV. EDX analysis was obtained with an Oxford INCA energy dispersive X-ray detector installed on the JEOL JSM-7000F. The transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) analysis as well as selected-area electron diffraction (SAED) pattern analysis were performed on a JEOL JEM-2100 transmission electron microscope operating at an accelerating voltage of 200 kV.

The low-magnification FESEM image (Fig. 1a) clearly displays that the as-prepared products are well-defined octahedral shapes, and high-magnification FESEM image (Fig. 1b) shows that each particle is made of many nanoparticles, namely, nanoparticle-aggregated hierarchical nanostructures with octahedral morphologies are achieved. The average size is about 530 nm, and the size distribution diagram is shown in Fig. S1 (see ESI†). Fig. 1c shows the XRD patterns of the as-prepared products, all of the diffraction peaks are well indexed to (111), (200), (220) peaks of standard face-centered cubic (fcc) copper (JCPDS file no. 04-0836). No peaks of impurities such as cuprous or cupric oxide are detected, suggesting the high purity of the as-obtained products. The ratio between the intensity of the (111) and (200) peak is higher than the standard value (2.87 vs. 2.04). The facets with a slower growth rate are exposed more on the crystal surface,¹⁵ and exhibit relatively stronger diffraction intensity in the corresponding XRD pattern.¹⁶ Hence, this observation indicates that the octahedral copper hierarchical nanostructures are primarily dominated by (111) facets. Fig. 1d gives the electron energy dispersive X-ray (EDX) spectrum. It can be found that only copper element is detected in the spectrum, further confirming that the obtained products are pure copper. Further structural characterization of the octahedral copper hierarchical nanostructures was performed on TEM and HRTEM investigations. Fig. 1e displays a typical TEM image of an individual as-prepared product. From it we can find that the particle is made of nanoparticles and vacancies, suggesting the formation of porous nanostructures. Fig. 1f shows the SAED pattern, indicating that the octahedral copper hierarchical nanostructures are polycrystalline. The interplanar spacings, obtained from the SAED pattern, can be indexed to the (111), (200), (220), and (311) facets of fcc copper, respectively, which is in agreement with the results above from the XRD data. Fig. 1g is the HRTEM image taken from the area marked with red circle in Fig. 1e, and the lattice fringes are marked by white lines and arrows, and the corresponding lattice spacing are 0.2050 nm, 0.2090 nm and 0.2096 nm, respectively, which are in good agreement with the d value of the (111) facets of fcc copper crystal (0.2088 nm), and it is corresponding to the XRD analysis. Moreover, it can be found that the growth directions of these (111) facets are not homogeneous. Fig. 1h and Fig. 1i show the Inverse Fourier transform of the HRTEM images taken from the areas marked with red squares (I and II) in Fig. 1g, respectively. We can clearly see that the lattice fringes of these nanoparticles have been attached. It has a misorientation of 16° between the two (111) facets as shown in red square I, and it also has a misorientation of 19° between the two (111) facets as shown in red square II, suggesting that the formation of octahedral copper hierarchical nanostructures is attributed to the oriented attachment of copper nanoparticles.

Fig. 1 (a) Low- and (b) high-magnification FESEM images, (c) XRD pattern, (d) EDX spectrum, (e) TEM image and (f) SAED pattern of the octahedral copper hierarchical nanostructures. (g) HRTEM image of the area marked with red circle in (e). Inverse Fourier transform of the HRTEM images (h) and (i) taken from the areas marked with red squares (I and II) in (g), respectively.

In order to shed light on the formation mechanism of these octahedral copper hierarchical nanostructures, the growth process has been followed by examining the products obtained at different reaction conditions. To obtain the morphology, structure and composition of the products synthesized in the initial stage, SEM and XRD characterizations were performed immediately after the formation of products. Fig. 2a gives the FESEM image of the products harvested at 5 min without heating. From it we can see that the products are smooth octahedral shapes, and the surfaces are in absence of hierarchical nanostructures. Fig. 2b is the corresponding XRD pattern of these products, and it can be found that all of the diffraction peaks are well indexed to (110), (111), (200), (211), (220), (311), (222) peaks of standard cubic structure Cu₂O (space group: Pn3m, lattice constant a = 0.427 nm, JCPDS file no. 05-0667). No peaks of impurities such as copper or cupric oxide were detected, suggesting that the Cu₂O were formed in the initial stage. According to the experimental results of Fig. 1 and Fig. 2, it is believed that octahedral Cu₂O nanocrystals were formed initially, and subsequently, Cu₂O were reduced to Cu as the reaction temperature and time increase. Hence, the formation of metal copper in our experiment is based on the following reactions:

Cu²⁺ + 2OH⁻ → Cu(OH)₂↓ (1)

Cu(OH)₂ + 2OH⁻ → [Cu(OH)₄]²⁻ (2)

4[Cu(OH)₄]²⁻ + N₂H₄ → Cu₂O↓ + 6H₂O + N₂↑ + 8OH⁻ (3)

2Cu₂O + N₂H₄ → 4Cu + N₂↑ + 2H₂O (4)

Fig. 2 (a) FESEM image and (b) XRD pattern of the products obtained at 5 min without heating.

The reaction temperature is a main factor affecting the shape-evolution process. Fig. S2 shows the FESEM images of the products obtained at different reaction time without heating (see ESI†). It can be found that the products were still smooth octahedral Cu₂O crystals as the reaction time was 20 min and 40 min, respectively (ESI, Fig. S2a–b).† A little of octahedral copper hierarchical nanostructures was formed as the reaction time was 60 min (ESI, Fig. S2c).† Further prolong the reaction time to 95 min, it can be seen that the products were ill-defined hierarchical nanostructures (ESI, Fig. S2d).† However, when the smooth Cu₂O crystals were heated at 80 °C, octahedral copper hierarchical nanostructures can be achieved (Fig. 2). To investigate the shape-evolution process along with the reaction temperature increase, SEM characterization was performed on the products obtained at different reaction temperature (see ESI†). Fig. S3a is the FESEM image of the products produced at 45 °C, it can be seen that many irregular nanoparticles and octahedral particles were in the presence of the products. The sizes of octahedral particles decreased, and many pits on their surfaces were formed (the area marked with pink circle), which may suggest that the Cu₂O crystals gradually transform to Cu with the reaction temperature increasing. Fig. S3b is the FESEM image of the products produced at 60 °C, the hierarchical nanostructures with octahedral morphologies were generated, which is similar to the shapes of the as-prepared octahedral copper hierarchical nanostructures. Therefore, it indicates that the higher reaction temperature is in favour of the formation of copper hierarchical nanostructures with well-defined octahedral shapes. The chemical interaction of capping agent in solution to different crystal facets could determine the formation of various morphologies by controlling the growth direction.^17,18 Our experimental results showed that PVP molecules were also significant for the formation of octahedral copper hierarchical nanostructures. It was found that only irregular copper nanoparticles were prepared in the absence of PVP molecules (Fig. S4, see ESI†). In the presence of PVP molecules, PVP molecules absorbed on the surfaces of the octahedral Cu₂O crystals can prevent the reduced Cu nanoparticles from irregular aggregation and ensure the formation of octahedral profiles. It is believed that PVP molecules contribute to the oriented attachment of nanoparticles to octahedral copper hierarchical nanostructures.

Based on the above observation and analysis, it is proposed that the gas bubbles assist in the oriented aggregation of nanoparticles to form octahedral copper hierarchical nanostructures, which is similar to the growth mechanism of Co nanospherical architectures¹⁹ and ZnSe hollow microspheres.²⁰ In our reactions, Cu²⁺ ions in the solution react with excess OH⁻ ions to generate blue [Cu(OH)₄]²⁻ complexes, and the [Cu(OH)₄]²⁻ complexes are reduced first by hydrazine to fabricate octahedral Cu₂O crystals (yellow), which can be reduced further by hydrazine to transform to Cu (dark red) as the reaction temperature increase. The whole process is illustrated in Fig. S5 (see ESI†). Meantime, microbubbles of N₂ can be formed in the reaction, which provide the aggregation centers of Cu nanoparticles. In order to minimize the interfacial energy, small Cu nanoparticles might aggregate to octahedral profiles with the aid of PVP molecules. Finally, the formation of octahedral copper hierarchical nanostructures can be attributed to the release of N₂ from different directions.

In summary, we have developed a facile solution route to synthesize novel nanoparticle-aggregated octahedral copper hierarchical nanostructures. Octahedral Cu₂O nanocrystals were formed initially, and subsequently, Cu₂O were reduced further by hydrazine to transform to octahedral copper hierarchical nanostructuresvia a gas bubbles assisted aggregation mechanism. Based on the understanding of shape evolution and corresponding growth mechanism, it is believed that more and more copper crystals with novel hierarchical nanostructures could be obtained.

Acknowledgements

Financial support from the National Basic Research Program of China (no. 2010CB635101), National Science Foundation of China (NSFC no. 51071116, 50871080 and 50901056) and National High Technology Research and Development Program of China (863 Program, no. 2009AA03Z320).

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Footnote

† Electronic supplementary information (ESI) available: FE-SEM images of the products obtained at different reaction conditions (Fig. S1–S5). See DOI: 10.1039/c0ce00565g

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