A versatile method to prepare size- and shape-controlled copper nanocubes using an aqueous phase green synthesis

Abstract

We demonstrate a versatile, simple, and environmentally friendly method for preparing copper nanocubes with controlled morphology in aqueous solution at room temperature. Copper(II)chloride is used as a precursor and the reduction is performed under an argon atmosphere with milder and non-toxic sodium borohydride in de-ionized water. The molar ratios of the precursor and the reducing agent, and sodium borohydride addition time play a key role in the preparation of copper nanocubes with an average edge length in the range of 100 ± 35 nm. With the addition of 20 w% poly(vinylpyrrolidone) prior to the addition of the reducing agent, well-dispersed PVP-capped copper nanocubes were also prepared in considerably good yield. The UV-visible absorption traces and transmission electron microscopy analysis were used to monitor the formation of copper nanocubes. The powder X-ray diffraction (XRD) and selected area electron diffraction (SAED) reveals the packing of copper nanocrystals to cubic 3D structures.

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Introduction

The development of novel synthetic methodologies to prepare metallic nanostructures has been advancing in past decades due to their size- and shape-related properties.^1–5 For example, copper nanoparticles exhibit enhanced nonlinear optical properties due to the unique optical property known as localized surface plasma resonance (LSPR).⁶ As such properties are determined from a direct combination of their sizes and shapes, controlling the morphologies into having desired sizes and shapes is a critical need. With a variety of shape-controlled synthesis methods, metal nanostructures of spherical particles,⁷ cubes,^5,8–11 wires,^5,8,12–15 prisms,^16–20 rods,^21,22 polyhedra,^10,23,24 and plates²⁵ have been synthesized to date. As evidenced by previous studies, cube shapes of metal nanostructures have many special properties due to the rather polar surface features of {100} faces compared to nonpolar surfaces of {111} faces.²⁶ For example, platinum (Pt) nanocubes, which featured the dominance of {100} faces on the surface show higher catalytic activity for oxygen reduction compared to commercial Pt catalysts.²⁷ The ratio of the growth rate in {100} to that in {111} determines the shape of a face-centered cubic (FCC) crystal. Recent studies reported by Murphy et al. showed that the different crystal phases have preferential absorption of molecules and ions present in the solution, resulting in various shapes of nanoparticles by controlling the growth rates along the two crystal axes of {100} and {111} faces.²⁸

The formation of ordered metal nanostructures requires nanocrystals with well-dispersed particle size and shape, compatibility with surface ligands, and van der Waals interactions between nanocrystals.^29–32 Typically, cubic or hexagonal packing nanostructures can form via self-assembly of spherical nanocrystals through van der Waals interaction to achieve the highest packing efficiency.^33–36 The self-assembly of nanocrystals into different morphologies possesses attractive physical and optical properties due to collective interaction between these nanocrystals. For example, gold (Au) nanorods show a red shift and a blue shift in the plasmon band of the absorption spectra depending upon the orientation of self-assembled nanorods.³⁷ Nonetheless, such ordered metal nanostructures with perfect symmetry of 2D or 3D can serve as excellent flat forms for the formation of highly ordered nano- or micro-super lattices.^10,38

Copper (Cu) possesses the second highest electrical conductivity among all the metals³⁹ and makes highly promising as an alternative for high-cost silver (Ag) and gold. Great efforts have made to prepare Cu nanostructures with different morphologies. However, shape-controlled synthesis of Cu nanostructures is still need a significant improvement compared to the progress made for silver and gold. Much of the synthesis methodologies developed for making Cu nanomaterials involve high temperatures,³⁹ harsh and toxic reducing agents,⁴⁰ capping ligands,^41–43 electrochemical processes,^44,45 and high vacuum vapour depositions.⁴⁶ The synthesis of copper nanowires and nanocubes has been achieved by a number of methods including the chemical reduction methods in water⁴⁷ or organic solvents in the presence of variety of capping agents.⁴⁸ In all these synthesis methods, the capping agent plays an important role in controlling the shapes and sizes of nanostructures. Wang et al. reported a non-aqueous solution-phase synthesis method to make Cu nanocubes with the edge lengths of 100 ± 25 nm using ascorbic acid as a reducing agent and poly(vinylpyrrolidone) (PVP) as a capping agent.⁴⁹ Recently, Yang et al. prepared monodisperse Cu nanocubes with an average edge length of 75.3 nm from copper chloride precursor in the presence of trioctylphosphine and octadecylamine as shape control agents.³⁹ However, these excellent works involved high temperatures, longer reaction time, and non-aqueous solvents. Therefore, it remains a great challenge to develop a “green synthesis” method where we can utilize environmentally benign substances, including solvents and energy efficient chemical processes for preparing copper nanocubes with controlled morphologies.

A facile chemical reduction approach in the presence of sodium borohydride (NaBH₄) as a reducing agent at room temperature has not been widely explore for making copper nanocubes with controlled morphology. In the past, a few reports described the preparation of spherical copper nanoparticles using NaBH₄ under ambient conditions.⁴² Herein, we demonstrate a well-established aqueous-phase green synthesis method to make size- and shape-controlled copper nanocubes and nanoparticles without a capping agent in the presence of NaBH₄ as a reducing agent. The method developed here is a “green synthesis” where we utilized environmentally friendly and benign materials with energy efficient chemical approach. The nucleation and growth of nanocubes were controlled upon changing the addition rate of NaBH₄ while maintaining the molar ratio of precursor to NaBH₄ at 1 : 2. With the optimized procedure, we also synthesized Cu nanocubes in the presence of PVP, which used as a stabilizer and shape controller for making copper nanocubes in a previously reported work.⁴⁹

Results and discussion

Preparation and characterization of non-capped copper nanostructures

Copper nanocubes and copper nanoparticles were synthesized by the chemical reduction of copper(II)chloride (CuCl₂) with NaBH₄ in de-ionized water under argon atmosphere. In a typical procedure, the reaction was performed on a scale of 0.087 mol L⁻¹ concentration of the precursor at room temperature under a short reaction time. The molar ratio of the precursor to NaBH₄, addition time of NaBH₄, and reaction completion time was evaluated in order to produce either copper nanocubes (Cu-NCs) or copper nanoparticles (Cu-NPs) with controlled size and shape. Under all these different parameters, the particle formation was monitored by observing the colour changes of the reaction mixture and acquiring UV-visible spectra and TEM images.

Copper nanocubes were prepared with the optimized conditions of 1 : 2 molar ratios of CuCl₂ to NaBH₄ by maintaining the NaBH₄ addition rate at 0.1 mL min⁻¹ over the total addition time of 50 minutes. The progress of the reaction was observed from the initial colour change from blue solution to brown suspension to final colour of brownish black solid as shown in Fig. 1a. The UV-visible absorption spectra of the reaction mixture taken at six different time intervals during the addition of NaBH₄ revealed the progress of particle formation and the reaction completion time. All the spectra were recorded in solution as either a clear solution or a suspension. As depicted in Fig. 1b, initial absorption maxima from Cu²⁺ solution was observed at 800 nm. With the addition of NaBH₄ progresses, the absorption band at 800 nm shifted to a shorter wavelength region and a new characteristic absorption band arises in the range of 460–600 nm. The disappearance of absorption band at 800 nm and the appearance of an absorption peaks at 515 and 590 nm confirmed the successful formation of Cu-NCs. As evidenced by other published research work, the copper nanostructures exhibits an absorption maximum in the range of 460–600 nm due to the surface plasma resonance effect.⁵⁰ With the increase of NaBH₄ concentration in the reaction mixture, the plasmon absorption at 590 nm pronounced and clearly recognizable after 30 min of NaBH₄ addition time.

Fig. 1 (a) An image of reaction vials at different time intervals, and (b) their respective UV-visible absorption spectra in solution.

The particle formation and the morphology changes during the addition of NaBH₄ were also monitored from transmission electron microscopy (TEM). As shown in Fig. 2, the clusters of Cu²⁺ ion crystals turned to highly populated small particles with the order of average size of 5 nm and clusters of spherical particles after the addition of 1.5 mL of NaBH₄ over 15 minutes addition time. As reveals from the TEM images taken after 40 min addition time of NaBH₄ solution (4 mL), well-defined cubes were visible. As the concentration of NaBH₄ increases in the reaction mixture, the nucleation and growth rate for the formation of Cu-NCs pronounced. After the addition time of 50 minutes with the addition rate of 0.1 mL min⁻¹, the reaction showed well-dispersed nanocubes with the average edge length of 100 ± 35 nm. The nanocubes prepared in this manner were isolated after completing the 50 min addition time of NaBH₄. The particles were collected by repeated centrifugation and repeated washing with water and dried in a vacuum oven to yield dark brownish black powder (0.133 g, yield = 38% w/w).

Fig. 2 TEM images taken at different time intervals over 50 minutes addition time of NaBH₄ solution; (a) initial Cu²⁺ solution, (b) 5 minutes, (c) 15 minutes, (d) 30 minutes, (e) 40 minutes, (f) 50 minutes.

The TEM images of isolated nanocubes exhibit sharp edged cubic morphology with much of which are arranged either as face-centered cubes or hexagons on the TEM grid as shown in Fig. 3 TEM images taken from different areas of the grid. The statistical dimension analysis indicates that an average edge length of most of nanocubes is in the range of 80–120 nm and 80% of the main product is cubic in shape and the reminder of particles are either spherical or mostly truncated cubic or rectangular structures. The truncated cubic structures mostly aligned on the substrate as either cubes or hexagons.

Fig. 3 Selected TEM images of isolated Cu-NCs taken from different areas of the TEM grid under two different magnifications (isolated NPs were re-dispersed in water).

We performed a series of multiple trial reactions for reproducibility using the reaction conditions of 1 : 2 molar ratio of CuCl₂ : NaBH₄ at addition rate of 0.1 mL min⁻¹ over 50 minutes addition time (Table S1†). Maintaining the addition rate of NaBH₄ at 0.1 mL min⁻¹ is critical to make nanocubes with controlled morphology. The reducing agent addition time influence the size and shape of nanocubes as the reducing agent initiates the nucleation growth of copper nanocrystals. We have also found out that there is no effect of additional reaction time on either the particle formation nor particle morphology initially formed in the absence of additional NaBH₄.

Typically, depending upon the concentration of nuclei at a given time, the size of particles varies. For example, at higher nuclei concentration, the particle size decreases since smaller metal nuclei grow and consume metal ions at the same time.⁴² In order to understand the effect of NaBH₄ addition rate on particle formation, a series of reactions was performed at addition rate of 0.33, 0.25, 0.16, 0.12, 0.10, and 0.08 mL min⁻¹ over 15, 20, 30, 40, 50, and 60 minutes of NaBH₄ addition time and particle formation was monitored under TEM just after the completion of NaBH₄ addition (see Fig. 4). Table 1 summarized the particle morphologies with respect to the addition rate of NaBH₄. All the trail reactions were conducted following the procedure described for the preparation of Cu nanocubes with the only variable being the addition time.

Fig. 4 TEM images taken just after the complete addition of NaBH₄ solution for different addition rate of the NaBH₄ solution (mL min⁻¹); (a) 0.33, (b) 0.25, (c) 0.16, (d) 0.12, (e) 0.10, and (f) 0.08.

Table 1 Morphologies of Cu nanostructures formed with respect to different addition rate of NaBH₄

Trial #	NaBH4 addition rate (mL min^−1)	Morphologies of nanostructures
1	0.33	Cubes and distorted spherical particles
		Average cube length 50 ± 15 nm
2	0.25	Distorted spherical particles and truncated cubes
		Average particle size 60 ± 25 nm
3	0.16	Truncated cubic particles and nanoclusters
		Average cube length ∼80 ± 25 nm
4	0.12	Truncated cubes and particles
		Average cube length ∼40 ± 10 nm
5	0.10	Mostly cubes with well-defined edges
		Average cube length ∼100 ± 35 nm
6	0.08	Highly aggregated spherical particles with very few cubes

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The addition rate of 0.12 mL min⁻¹ yields a mixture of nanocubes and spherical particles with less aggregated nanostructures. The addition rate of 0.1 mL min⁻¹ is found to be the best suited condition to make rather well defined nanocubes. Surprisingly, the lowest addition rate of 0.08 mL min⁻¹ yielded unorganized nanostructures. We speculated that there might be not enough concentration of the reducing agent present at the early stage of nucleation to initiate the nanocrystal growth.

The crystallinity and packing pattern of Cu nanocubes were analyzed from the wide-angle X-ray powder diffraction of a powdered sample and the selected area electron diffraction (SAED) of a single nanocube taken from the TEM under dark field diffraction mode by directing the electron beam perpendicular to one of a nanocube's square faces and is depicted in Fig. 5.

Fig. 5 (a) Powder XRD pattern of isolated Cu nanocubes; (b) SAED pattern of (b) a nanocube and (c) a hexagon taken by aligning the electron beam perpendicular to the face of a nanocube and a hexagon; (d) TEM image of a nanocube orient parallel to the supporting surface (cubic view); (e) TEM image of a nanocube orient from the side on the substrate (hexagon view).

The XRD pattern recorded from a powder sample of Cu nanocubes reflects diffraction of [111], [200], and [220] planes (Fig. 6a). The ratio between the intensities of [200] and [111] diffraction peaks is 0.30, which is lower than the conventional powder sample (∼0.46) of Cu nanocubes and nanowires.^51,52 The results indicate that the preferential orientation of cubes are along the [111] diffraction planes and less preference to orient parallel to the supporting substrates resulting in low intensity from [200] planes.³⁹ The SAED pattern taken from a single Cu nanocube (Fig. 6b) viewed along the [001] axis confirms that the nanocube is a single crystal with surfaces bounded by (100) facets and are consistent by previously published reports.^39,49 The SAED pattern taken on a hexagon face exhibits both first order and second order of (100) facets which further evidence hexagons are side-oriented nanocubes. In order to understand the mechanism of preferential formation of nanocubes with respect to different time intervals during the addition of NaBH₄, we have performed SAED analysis of the reaction mixture by taking little aliquot at 5, 15, 30, 40, and 50 min time intervals and followed by drop casting on the formvar coated copper grid. The SAED diffraction patterns obtained in this manner are depicted in Fig. S1 in the ESI.† As reaction proceeds, the SAED analysis reflects crystallinity changes from poly crystallinity to more ordered crystalline pattern. After 40 minutes of time interval, the crystalline pattern shows clear facets of (100) along the [001] axis confirming the formation of nanocubes compare to the initial stages of the reaction.

Fig. 6 Arrangement of PVP coated Cu nanocubes on a formvar coated substrate; (a) visualized from TEM, (b) visualized from SEM at low resolution and (c) visualized from SEM at higher resolution.

Preparation and characterization of PVP-capped copper nanocubes

In order to investigate the influence of the capping agent for the formation of nanocubes using the optimized synthesis procedure, we performed a series of reactions in the presence of PVP (M_w = 40 kDa) as a capping agent. It is known that PVP can acts as both stabilizer and shape controller for the preparation of Cu nanocubes as evidenced by other published research reports.⁴⁹ However, there is no prior work reported making PVP-capped nanocubes upon reduction with NaBH₄ in an aqueous medium.

PVP-capped Cu nanocubes were prepared by following the procedure described in the experimental section. Prior to the addition of NaBH₄, a solution of PVP (20% by weight to the precursor) was added to the CuCl₂ solution. The addition rate of NaBH₄ and molar ratio between CuCl₂ and NaBH₄ were maintained at 0.1 mL min⁻¹ and 1 : 2 molar ratio respectively. After the total addition time of 50 min, the suspension was subjected to centrifugation to separate PVP-grafted copper nanocubes, which remained in the solution leaving un-grafted copper particles in the centrifuged solid. The TEM analysis was performed prior to the separation to ensure that copper nanocubes formation was completed over the total reaction time of 50 minutes.

As depicted in Fig. 6, TEM and SEM images evidenced that the grafting with PVP affected to make Cu nanocubes with controlled sizes and shapes. In general, PVP acts as a “wrap” during the growth process of nanocubes while selectively interacting with nanocrystals to promote the formation of well-defined nanocubes. When no PVP was added, we observed wide distribution of size range with rather highly aggregated nanocubes. Therefore, these results show indirect selective interaction of the particles with PVP during the nanocube formation leading to homogeneous nanocubes.

Experimental

Materials

Sodium borohydride (99.99% purity), Copper(II) Chloride (97% purity), Ethyl alcohol (99.5% purity), and poly(vinylpyrrolidone) (PVP, M_w = 40 000 g mol⁻¹) were obtained from Aldrich Chemicals. All the chemicals and solvents are used as received without any purification otherwise specified.

Characterization

Transmission electron microscopy (TEM) observations were performed on a JEOL 1400 Plus at 80 kV. These images were recorded using an AMT XR81 digital camera. Scanning electron microscopy images were obtained from a JEOL JSM 6510LV with a lanthanum hexaboride gun. Images and energy dispersive X-ray spectra were captured using IXRF system detectors and software. The UV-visible absorption spectra were recorded from UV-visible spectrometer (Perkin Elmer, Lambda 35). X-ray powder diffraction patter was recorded by using a Thermo ARL X’TRA Powder diffractometer with wide-angle scattering mode.

Typical procedure for the preparation of Cu nanocubes (Cu-NCs)

To a 100 mL round bottom flask charged with a magnetic stirrer, copper(II)chloride (0.350 g, 2.6 mmol) were dissolved in 30.00 mL deionized water and the flask was degassed about 10–15 min prior to leave the reaction mixture under inert atmosphere with argon gas. To this solution, sodium borohydride (0.200 g, 5.28 mmol) dissolved in 5.00 mL of deionized water were added through a syringe with the drop rate of 0.1 mL min⁻¹ in dropwise over 50 minutes time period. After 50 minutes of total addition time, the dark brown solution was centrifuged and solid and supernatant were separated. Solid was washed once with deionized water and dried in the vacuum oven. TEM grids were prepared from the solutions prior to the centrifugation to confirm the successful preparation of Cu nanocubes. The nanocubes prepared in this manner were re-dispersed in water and image under TEM.

Typical procedure for the preparation of PVP-capped Cu nanocubes (PVP-capped Cu-NCs)

In a 100 mL round bottom flask with a magnetic stirrer, copper(II)chloride (0.350 g, 2.60 mmol) were dissolved in deionized water (30.00 mL) and the flask was degassed about 10–15 min prior to leave the reaction mixture under inert atmosphere with argon gas. The solution was degassed with the aid of two needles where one acted as a gas argon gas inlet and the other acted as a gas outlet. The gas out let needles was removed after 10–15 min and left the flask under continuous argon gas flow. To this solution, polyvinylpyrrolidone (PVP) (0.070 g, 20% by weight to copper(II)chloride) was added. In a separate vial, sodium borohydride (NaBH₄, 0.200 g, 5.28 mmol) was freshly prepared by dissolving in deionized water (5.00 mL). The slow drop feeding of NaBH₄ solution through a syringe with the rate of 0.1 mL min⁻¹ yielded a dark brown solution with the total of 50 minutes addition time. Then, just right after the completion of NaBH₄ addition, the resulted solution was imaged under TEM to confirm the formation of Cu nanocubes. The reaction mixture was centrifuged and the solid and the supernatant were separated. Solid was collected after washed once with ethanol and followed by centrifugation. The re-dispersed solid in ethanol and the supernatant were imaged under TEM.

TEM grids were prepared by taking a liquid sample from the reaction mixture using a pipette and putting one drop on a formvar coated copper grid and dried under the hood. For the centrifuged solid, a sample from the centrifuged washed solid was re-dispersed in deionized water and followed the same procedure to make the TEM grid.

Conclusions

In summary, the present study demonstrates the preparation of well-defined copper nanocubes from an aqueous phase green synthesis method at ambient temperature. Cu nanocubes with sizes of 100 ± 35 nm were synthesized upon reduction of copper(II)chloride in the presence of sodium borohydride. The addition rate of the reducing agent and molar ratio of the precursor to NaBH₄ play an important role in forming seed crystals stabilizing face-centred crystal planes to achieve monodisperse nanocubes. The nanocubes prepared with the addition rate of NaBH₄ at 0.1 mL min⁻¹ over 50 minute addition time of NaBH₄ while maintaining the molar ratio of precursor : NaBH₄ at 1 : 2 tend to arrange on the surface either as cubes or hexagons. The XRD pattern suggests that the nanocubes dominate orientation on a substrate along their [111] planes. PVP-capped nanocubes prepared using the similar experimental conditions can be dispersed in water-based solutions. Hence the synthesis method provides opportunities to fabricate conductive copper nanocubes over large surface area using a simple spray coating method allowing the fabrication of conductive copper patterns on flexible substrates.

Acknowledgements

Financial support for this work is provided, in part, from NASA EPSCoR RA (KY-14-NASA-001), and from the National Science Foundation (CHE-MRI 1338072, and MRI-1429563). The authors gratefully acknowledge Dr John Andersland for TEM and SEM support.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures. See DOI: 10.1039/c6ra17037d

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