Controllable green synthesis of Cu₂O nanocrystals with shape evolution from octahedra to truncated octahedra

Abstract

A simple controlled synthesis of Cu₂O nanocrystals from octahedra to their different truncated forms was successfully achieved by using an ultrasonic method. The crystals were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FESEM), UV-vis spectroscopy (UV-vis) and transmission electron microscopy (TEM) as well as high-resolution transmission electron microscopy (HRTEM). The growth processes of cuprous oxide nanocrystals were analyzed and possible growth mechanisms were discussed. The optical properties of the obtained crystals were investigated.

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1. Introduction

The properties of materials can be tuned by manipulating their morphology, resulting in the rapid growth in nanotechnology over many years.^1,2 Therefore, the synthesis of nanoparticles with well-controlled shape has become a focus for material researchers.³ Cu₂O, as a p-type semiconductor with a direct band gap of 2.0–2.17 eV,^4,5 has unique electrical,⁶ magnetic,⁷ and negative thermal expansion properties,⁸ which gives it potential applications in catalysis,^9,10 gas sensors,¹¹ templates,¹² electrochromism,¹³ solar-driven water splitting,¹⁴ and antibacterial activity.¹⁵ Up to now, much effort has been devoted to synthesizing novel nanomaterials with various morphologies, such as spheres,^16,17 multipods,¹⁸ hollow cages,¹⁹ cubes,²⁰ nanowires,⁶ octahedra.²¹ However, there are still many problems to be solved. Many methods used either complex instrument or can't control the morphology of the products exactly. It is highly desirable to develop a simple route to prepare cuprous oxide which can control the shape of the product exactly.

The ultrasonic method can not only raise the reaction rate, but also make the reaction occur at low temperature just because of cavitation phenomenon. The transmission of high-frequency ultrasound in liquid could cause the formation, growth, and collapse of bubbles, and the rapid collapse of bubbles creates high temperature (about 5000 K), high pressure (about 20 MPa), and high cooling rate (about 1010 K s⁻¹).²²

Herein, we have developed a simple method to synthesize cuprous oxide by ultrasonic irradiation. Their shape is systematically controlled from octahedron to truncated octahedron by simply coordinating the amount of ethylene glycol and de-ionized water in the reaction solution. Their structures have been extensively characterized and confirmed. The growth processes of them have been studied by examining the particles formed at the early growth stage. Finally, the optical performance of the product was studied.

2. Experimental

2.1. Chemicals

Copper(II) acetate (M = 181.63, 98%), D-fructose (M = 180.16, ≥99%), ethylene glycol and ethanol were purchased from Sigma-Aldrich. All the reagents used in this experiment were of analytical grade and used without any further purification. De-ionized water was secondary water.

2.2. Synthesis of cuprous oxide

In a typical procedure, 0.20 g copper acetate was dissolved in 10 ml de-ionized water until a clear blue solution was obtained, which was named solution A. Separately, solution B was prepared by dissolving 0.45 g D-fructose in 10 ml de-ionized water. Then these solutions of copper acetate and fructose were simultaneously added into a glass beaker containing 15 ml solvent (Label C; component of the solvents are summarized in Table 1). After stirring with a magnetic blender for about 30 s, the beaker C was placed in the reaction stage. Ultrasonic waves were generated from a titanium alloy which was directly immersed into solution C and the output power injected into the solution was 300 W (SONIC & MATERIALS INC VCX750). In order to avoid rapid increase of temperature in the reaction medium, the generated ultrasonic is triggered following a cycled periodic pulse. The width of the pulse in a period was 2 s. The ultrasound time was 1 s and the resting time was 1 s. Several minutes later, the reaction system gradually became green, yellow green, yellow and finally turned reddish brown. The duration of ultrasound was 15 min, and then the precipitate was collected by centrifugation, washed several times with absolute ethanol and de-ionized water. Finally, the products were dried in a vacuum at 50 °C for 6 h.

Table 1 List of various amounts of ethylene glycol and de-ionized water. Note that the rate of ethylene glycol and de-ionized water is only variable

Sample code	Ethylene glycol (ml)	De-ionized water (ml)
S1	0	15
S2	10	5
S3	15	0

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2.3. Characterization

The X-ray powder diffraction (XRD) characterization was carried out on a X-ray diffractometer (XD-3 system, Beijing Purkinje General Instrument Co., Ltd.) with Cu Kα radiation (λ = 0.154178 nm) at a scanning rate of 0.02° s⁻¹ in the 2θ range from 5° to 80°. The morphology and size of the final products were determined by field emission scanning electron microscopy (FESEM). Energy dispersive X-ray spectroscopy (EDX) was measured with a FEI Quanta 250 emission scanning electron microscopy equipped with an energy spectrum analytical system. The transmission electron microscopy (TEM), including high-resolution transmission electron microscopy (HRTEM) image and the selected area electron diffraction (SAED) spectrum were taken on a JEM-2100 transmission electron microscope operated at an accelerating voltage of 200 kV with the samples on copper grid. UV-vis absorption spectra were recorded with a Hitachi U-3310 spectrophotometer.

3. Results and discussion

The phase and purity of the products were first examined by XRD, and the results show in Fig. 1 (samples S1–S3). All the diffraction peaks of these samples are indexed according to the standard cubic structure of Cu₂O (space group: Pn3̄m (224), a = 0.427 nm, JCPDS no. 05-0667). No peaks of impurities were detected, suggesting the high purity of the as-prepared products. Besides, the strong intensity and sharpness of the peaks indicate that the products are highly crystalline.

Fig. 1 XRD patterns of samples S1–S3 and bulk Cu₂O.

The chemical composition and purity of the products were further examined by energy dispersive X-ray spectroscopy (EDX). Fig. 2 shows the EDX spectra of the as-obtained products (S1–S3). It can be seen that only Cu and O signals were found in the spectrum. The Si signal came from the substrate which was used to support the products. It is obvious that Cu and O atomic ratio is close to 2 : 1, confirming that the obtained products are Cu₂O crystals.

Fig. 2 EDX spectra of samples (S1–S3).

The morphology and size of the obtained Cu₂O crystals were observed by FESEM, TEM and HRTEM. Fig. 3(a) was the FESEM, TEM and HRTEM images of the sample S1. It can be found that S1 was regular octahedron with the edge about 500 nm. We can know that the products were well-crystalline from Fig. 3(a3). Fig. 3(b) was the images of the sample S2, which was obtained with ethylene glycol 10 ml and de-ionized water 5 ml. The products were truncated octahedral morphologies, and each particle has three pairs of squares and four pairs of hexagons. The edges of squares are about 120 nm in sizes, while the long edges of hexagons are about 400 nm. Fig. 3(b3) showed that the products were well-crystalline. When ethylene glycol was increased to 15 ml without de-ionized water, the morphology of the products was showed in Fig. 3(c). It was obvious that the products were also truncated octahedra. However, the sections were squares and regular hexagons and the edge of them was about 250 nm. So we can name the sample S2 with small truncated octahedron, while the sample S3 with large truncated octahedron. Similar to Fig. 3(a3) and Fig. 3(b3), Fig. 3(c3) revealed that the sample S3 was also well-crystalline.

Fig. 3 FESEM images, TEM images, high-resolution TEM images and SAED patterns of the obtained Cu₂O crystals. (a) S1 (octahedron); (b) S2 (small truncated octahedron); (c) S3 (large truncated octahedron). Inset in (a3–c3) were SAED patterns.

To shed light on the growth process of these crystals with different morphologies, a series of time-dependent experiments were carried out. So far, this method was widely used to study the morphological formation of various kinds of crystals under different reaction conditions.¹⁷ Fig. 4 showed the FESEM images of the products obtained at different reaction time. Fig. 4(a) was the product with the shape of octahedral (S1) while Fig. 4(b) was the small truncated octahedron (S2) and Fig. 4(c) was the large truncated octahedron. At the beginning of the reaction, the spherical Cu₂O particles nucleated (Fig. 4(a1–c1)). This was consistent with the idea that the large crystals grew from smaller particles. Then small particles dissolved again and large particles grew larger via a process known as Ostwald Ripening.²³ 11 min later, the particles grew along the different directions with different growth rates due to their different surface energies and became turn to different shapes (Fig. 4(a3–c3)). After ultrasound for 15 min, the shape and size of the products were shown in Fig. 4(a4–c4). They were octahedron and truncated octahedron with smooth surface. So it can be found that the formation of crystals was mainly achieved through two stages: nucleation and growth. The possible formation mechanism of the samples was shown in Fig. 5. Murphy had reported that the preferential absorption of molecules and ions in solution to different crystal faces directs the growth of nanoparticles into various shapes by controlling the growth rates along different crystal axes.²⁴ Wang reported that the shape of an fcc nanocrystal was mainly determined by the ratio (R) between the growth rates along the 〈100〉 and 〈111〉 directions.²⁵ For octahedra bounded by eight equivalent {111} planes (S1), R equalled 1.73. For the truncated octahedra with regular hexagons and squares (large truncated octahedron, S3), R was a value close to 1.15. It can be found that R of the product with small truncated octahedron (S2) fell in between 1.15 and 1.73. During these experiments, the shape of the final products evolved from octahedron to truncated octahedron with the rate of ethylene glycol and de-ionized water variable. When we added ethylene glycol into reaction system, it was believed that ethylene glycol tended to suppress the growth rate of the {100} facets more than that of the {111} facets. When there was no ethylene glycol added into the reaction system, ethylene glycol had no effect on Cu₂O particles and the products with perfect octahedron were obtained (R = 1.73). When we added 10 ml ethylene glycol into the reaction system, the effect of ethylene glycol toward Cu₂O particles became enhanced, and it could block the growth on the {100} facets and facilitated the growth on the {111} facets. The obviously change of the growth rate on the {100} and {111} facets induced the ratio R decrease (1.15 < R < 1.73). Further increasing the volume of ethylene glycol to 15 ml, large truncated octahedron with squares and irregular hexagons were obtained and the R decreased to 1.15. It can be concluded that ethylene glycol played an important role on the morphologies of the resulting products and the formation of the cuprous oxide crystals can be rationally expressed as a kinetically controlled process.²⁶ Based on above experiments, the growth mechanism was proposed (Fig. 5).

Fig. 4 FESEM images of the products obtained at different reaction time. (a) S1 (octahedron); (b) S2 (small truncated octahedron); (c) S3 (large truncated octahedron); (a1–c1) 5 min; (a2–c2) 9 min; (a3–c3) 11 min; (a4–c4) 15 min.

Fig. 5 Schematic illustration for the formation process of Cu₂O crystals.

Optical absorption behavior is one of the key properties of Cu₂O crystals. The UV-vis spectra of as-prepared Cu₂O products were in Fig. 6. It can be found that the optical absorption peak center of the octahedral Cu₂O crystals (S1) was at about 530 nm. According to the equation αE_p = K(E_p − E_g)⁻², in which α is the absorption coefficient, K represents a constant, E_p stands for the discrete photo energy, and E_g is the band gap energy.²⁷ In accordance with the formula, the band gap energy of S1 is calculated to be 2.34 eV. The optical absorption peak center of the small truncated octahedral Cu₂O crystals (S2) was at 588 nm and the evaluated band gap is 2.11 eV, which is close to the bulk value of Cu₂O. Similarly, the optical absorption peak center of the large truncated octahedral cuprous oxide is 565 nm, corresponding to the band gap energy of 2.19 eV. The evaluated band gap energy of S1 and S3 are larger than bulk Cu₂O (2.0–2.17 eV). The increasing of the band gap may due to the quantum confinement effect.²⁸ What is more, the optical absorption was influenced by the morphology and crystallinity.²⁹

Fig. 6 UV-vis spectra of samples S1 (octahedron), S2 (small truncated octahedron) and S3 (large truncated octahedron).

4. Conclusions

In this paper, we present an effective and green route for the shape-controlled synthesis of Cu₂O crystals via ultrasonic method. Cu₂O crystals from octahedral to its different truncated forms are successfully achieved. It is obvious that the rate of ethylene glycol and de-ionized water plays a crucial role to design the shape of products. Based on the relatively clear understanding of shape evolution and growth mechanism, we believe that this method can be easily scaled up and also can be applied to other oxide materials.

Acknowledgements

This work is supported by the Chongqing Key Natural Science Foundation (cstc2012jjB50011), the Fundamental Research Key Funds for the Central Universities (XDJK2013B017), and Chongqing Fundamental and Advanced Science Research Projects (cstc2013jcyjA50015).

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