Ultrasmall Cu₂O nanocrystals: facile synthesis, controllable assembly and photocatalytic properties

Abstract

In this work, we have successfully synthesized ultrasmall Cu₂O nanocrystals with uniform sizes through a solution-based method. By virtue of the ideal Cu₂O building blocks, 2D or 3D Cu₂O superstructures can be easily achieved by tailoring the assembly process. The results indicate that diethyl carbonate (DEC) plays a crucial role for controlled self-assembly of Cu₂O nanoparticles into a range of superstructures. Interestingly, the aggregative growths of Cu₂O nanocrystals in the self-assembly process are accompanied by oriented attachment and Ostwald ripening. Furthermore, Cu₂O and Cu₂O–Au 3D colloidal spheres can be also obtained by an emulsion-based bottom-up self-assembly strategy. The results show that, compared with pure Cu₂O nanospheres, the as-obtained Cu₂O–Au nanospheres exhibit remarkable photocatalytic activities in the photodegradation of methyl orange (MO).

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Introduction

Ultrasmall nanocrystals (USNCs) with sizes down to 5 nm, or even 2 nm usually possess unique optical, electrical, magnetic and catalytic properties owing to their high ratio of surface atoms and extraordinary quantum-confinement effects.^1–6 Meanwhile, USNCs can be utilized to construct diverse superstructures with advanced functions. Therefore, the controlled synthesis and assembly of USNCs have attracted special research interest during the past decade. In recent years, solution-based bottom-up assembly strategies have been employed to synthesize superstructures with individual USNCs. For example, Li and coworkers have developed a general emulsion-based bottom-up self-assembly (EBS) for assembling ligand-stabilized nanoparticles into three-dimensional (3D) superstructures with controllable sizes and compositions.⁷ More recently, uniform ZnO nanocrystals as artificial atoms can be directly assembled into 3D mesoporous ellipsoids in a methanol/diethyl carbonate/chloroform (MDC) system.⁸

Cu₂O, as an important p-type semiconductor, has attracted much interest due to its potential applications in gas sensing, photocatalysis and solar energy conversion. A variety of synthetic protocols have been put forward to tailor the exposed crystal facets of Cu₂O microstructures.^9–27 The synthesis and self-assembly of uniform Cu₂O nanocubes or Cu₂O rhombic dodecahedra with sizes of 20–70 nm has been achieved.²⁸ Yang et al. have reported the solution-based synthesis of environmentally benign oxide materials for photovoltaic devices.²⁹ Huang's group has developed a series of synthetic strategies for fabricating various Cu₂O microstructures.¹⁴ In addition, the ultralong Cu₂O nanowires have also been synthesized in solution-based synthetic method.³⁰ Ultrasmall Cu₂O nanostructures may exhibit the fascinating applications in photocatalysis, low-cost electro-optical devices and high-performance batteries due to their high surface area/volume ratio.^1,31 However, the controllable synthesis and self-assembly of ultrasmall Cu₂O with high purity have not yet made much progress. Though, Yang et al.³² and Saunders et al.³³ have respectively reported two-step methods to prepare Cu₂O nanocrystals base on the oxidation of Cu nanoparticles. The strategies for one-step controllable synthesis and self-assembly of ultrasmall Cu₂O nanocrystals with high purity still need to be further explored.

Here, we are motivated to develop a one-step method for controllable synthesis of ultrasmall Cu₂O nanocrystals with uniform sizes in the presence of oleic acid (OA), polyvinyl pyrrolidone (PVP) and KBH₄. Furthermore, based on the DEC-mediated and EBS strategies, two-dimensional (2D) or 3D Cu₂O superstructures will be achieved by tailoring the assembly process. Especially, the as-obtained Cu₂O–Au 3D colloidal spheres exhibit remarkable photocatalytic activity in the photodegradation of methyl orange (MO).

Experimental section

Reagents and materials

All the chemicals were analytical grade reagents and without further purification. Cu(NO₃)₂·3H₂O, ethanol, ethylene glycol, cyclohexane, chloroform, polyvinyl pyrrolidone (PVP), oleic acid (OA) and cetyltrimethyl ammonium bromide (CTAB) were obtained from Shanghai Sinopharm Chemical Reagent Co., Ltd. KBH₄ was obtained from Changzhou Xinhua active material research institute. Diethyl carbonate (DEC) and HAuCl₄·4H₂O were purchased from Aladdin reagent Co., Ltd.

Synthesis of Cu₂O USNCs

Typically, 1 mmol Cu(NO₃)₂·3H₂O was dissolved in 20 mL ethylene glycol (EG, 99%). Then, 0.5 g PVP and 0.5 mL OA were dissolved in the above solution with ultrasonic assistance.

0.2 g KBH₄, as the reducing agent, dissolved in 5 mL deionized water, was quickly poured into the EG solution under vigorous stirring at the room temperature. The reaction solution immediately became dark red color from light blue once the KBH₄ solution joined. The resulting samples were washed with ethanol by centrifugation for several times. Finally, the product was redispersed in cyclohexane for further investigation and characterization.

Self-assembly of Cu₂O USNCs

Cu₂O USNCs was dispersed in 10 mL cyclohexane or chloroform. Then, different amounts of DEC (0.5–3 mL) were injected into above solution, respectively. After shaking the mixture to form a homogeneous solution, the resulting solution was further aged for 3 days at room temperature. Finally, the samples were washed with ethanol and redispersed in cyclohexane.

Controlled assembly of Cu₂O and Cu₂O–Au 3D colloidal spheres

Cu₂O USNCs (∼50 mg) were dispersed in 5 mL cyclohexane and poured into 50 mL deionized water containing 0.15 g CTAB. The mixture was vigorous stirred (∼3000 rpm) under the paddle at room temperature for 1 h. After that, the products were rotary vacuum evaporation at the 40 °C and −0.065 to −0.075 MPa for 3 h. Finally, the colloidal spheres were obtained by centrifuging and dried in vacuum at 70 °C.

To prepare Cu₂O–Au colloidal spheres, 1 mL KBH₄ (10 mg mL⁻¹) was added to the as-prepared Cu₂O colloidal solution under the magnetic stirring. After a few minutes, different volumes of HAuCl₄ solution (1 mg mL⁻¹) were dropped and the reaction solution was further stirred for 1 h. The resulting Cu₂O–Au samples were obtained by centrifuging and dried in vacuum at 70 °C.

Characterization

The composition of the as-prepared samples were measured Powder X-ray diffraction (XRD) on a XD-3 (Puxi, Beijing, China) with a Cu Ka X-ray source (λ = 1.54 Å) at 2θ ranging from 20° to 80°. The sizes and shapes were observed by transmission electron microscope (TEM, JEOLJEM-2100), high-resolution transmission electron microscope (HRTEM) and scanning electron microscopy (SEM, Hitachi S-4800).

Photocatalytic activity of the Cu₂O colloidal spheres

To observe the photocatalytic activity of Cu₂O colloidal spheres, methyl orange (MO, 2 g L⁻¹) was chosen as a dye molecule solution. First, 0.02 g of the Cu₂O or Cu₂O–Au was dispersed into 100 mL deionized water with ultrasonic assistance for 5 min. Then, 7 mL MO solution was added to the mixing solution and stirred in dark for 1 h. Then, the above solution was illuminated by a 300 W Xe lamp (PLS-SXE300/300UV, Philae technology Co. Ltd., Beijing, China). 3 mL aliquots of the mixing solution were withdrawn every 10 min and centrifuged for measuring. UV spectra were carried out on an Agilent 8453UV-vis spectrophotometer.

Results and discussion

Synthesis of the Cu₂O USNCs

Fig. 1 shows the typical transmission electron microscopy (TEM) image of Cu₂O USNCs. When 0.5 g PVP and 0.5 mL OA were used as surfactants, monodisperse Cu₂O nanoparticles were easily prepared. The as-prepared Cu₂O nanocrystals present uniform and ultrasmall sizes with the average diameter of ca. 2.2 nm (Fig. 1a and S2a†). The corresponding selected area electron diffraction (SAED) was shown in Fig. S3.† The high-resolution TEM (HRTEM) image (Fig. 1b) indicates that the lattice fringes of the as-prepared Cu₂O USNCs with d-spacing of 1.95 Å are corresponded to the (200) planes of Cu₂O. The fast Fourier transform (FFT) pattern (Fig. 1c) is also consistent with the results. Moreover, the phase purity of the as-prepared Cu₂O USNCs was further characterized using X-ray diffraction (XRD) (see Fig. S1(c) in the ESI†). It can be observed that all the diffraction peaks of the sample are indexed to the pure phase Cu₂O (JCPDS card no. 78-2076).

Fig. 1 TEM (a), HRTEM (b) and FFT (c) images of Cu₂O USNCs.

The sizes of Cu₂O USNCs can be easily adjusted by changing the amounts of PVP and OA. With increasing of OA (1 mL), the Cu₂O nanoparticles still show ultrasmall and uniform sizes with the average diameter of ca. 2.5 nm (Fig. S2b†). However, in the presence of OA (0.2 mL) and PVP (0.2 g), the Cu₂O nanoparticles become larger sizes and they are irregular due to the lower amounts of the capping surfactants. The TEM image shows the Cu₂O nanoparticles present an average size of 4.5 nm (Fig. S2c†). Increasing OA to 1 mL (Fig. S2d†), the sizes of Cu₂O nanoparticles smaller and regular, and the average size is 3.4 nm. The corresponding diffraction peaks in the XRD patterns of the Cu₂O USNCs at 2θ values of 29.7°, 36.7°, 42.7°, 61.6° and 73.5° are observed, which correspond to the lattice plane of (110), (111), (200), (220), (311) and (222), respectively (Fig. S1†).

Self-assembly of Cu₂O superstructures

A facile method was initially developed to assembly the Cu₂O USNCs into various 2D or 3D superlattices (Fig. 2). By tuning the addition amounts of diethyl carbonate (DEC) in the Cu₂O solutions, the Cu₂O superlattices with tunable morphologies can be obtained. In the DEC–cyclohexane system, when 0.5 mL DEC was added, 3D Cu₂O nanospheres were formed by self-assembly of the Cu₂O USNCs as building blocks in cyclohexane. Fig. 3a indicates that the as-obtained 3D nanostructures are composed of ultrasmall Cu₂O building blocks and present uniform sizes with the mean diameter of 65 nm. Notably, in our work, the Cu₂O USNCs can be easily dispersed in cyclohexane or chloroform, but not in DEC. The experiment results showed that DEC plays a key role in directing the self-assembly of the Cu₂O USNCs into tunable superstructures. For instance, 3 mL DEC was introduced to the cyclohexane solution of Cu₂O USNCs, yielding the 2D Cu₂O nanocubes superlattices. Interestingly, the 2D superstructures of Cu₂O nanocubes show relatively uniform shapes and sizes with the average size of ca. 20 nm (Fig. 3b and 4a). The SEM image (Fig. S4†) further indicates that the as-obtained Cu₂O particles possess uniform cubic-like morphology. It is noting that, after this self-assembly process, the initial ultrasmall Cu₂O building blocks have tended to larger Cu₂O nanocubes. To further verify the size enlargement, HRTEM was employed to characterize the assembly of Cu₂O nanocubes. As shown in Fig. 4b and c, the corresponding HRTEM analyses reveal that individual nanocube apparently present good crystalline nature. The lattice fringe spacing of ca. 2.8 Å corresponds to the (110) plane of Cu₂O nanoparticles. These results indicate that the Cu₂O nanocubes may originate from the Cu₂O USNCs through oriented attachment and Ostwald ripening during the self-assembly process. However, when chloroform was as solvent, a very different phenomenon occurs compared with the above system. With an appropriate amount of DEC (1 mL) added, the quasi-nanocubes with the average diameter of 15 nm were obtained (Fig. 3c). In contrast, the typical nanocube superlattices with the average diameter of 25 nm were also produced by adding 0.5 mL DEC (Fig. 3d). However, as shown in Fig. S5,† the increased amounts of DEC (1.5 mL or 3.0 mL) lead to the formation of irregular Cu₂O aggregates.

Fig. 2 Scheme illustration of Cu₂O superstructures.

Fig. 3 TEM images of the self-assembly of Cu₂O superlattices: (a) 0.5 mL DEC, cyclohexane (b) 3 mL DEC, cyclohexane (c) 1.0 mL DEC, chloroform (d) 0.5 mL DEC, chloroform.

Fig. 4 TEM (a) and HRTEM (b and c) images of the Cu₂O nanocubes.

To better understand the formation process, different DEC contents were employed for the self-assembly of Cu₂O USNCs in cyclohexane. As shown in Fig. S6,† at low concentrations of DEC (0.5 mL), uniform 3D Cu₂O nanospheres can be produced (Fig. S6a†). However, when DEC was 1 mL (Fig. S6b†), the Cu₂O nanospheres were compactly assembled with irregular sizes. However, the continued increase of the volume of DEC (1.5 mL) could obtain the Cu₂O nanocubes with uneven morphologies (Fig. S6c†). Higher concentrations of DEC (3 mL) will lead to the formation of Cu₂O nanocubes. Obviously, by varying the addition of DEC from 0.5 mL to 3 mL, the 3D Cu₂O nanospheres are gradually evolved into the resulting Cu₂O nanocubes. This change can be attributable to the addition of DEC. In our system, the suitable potential barrier and viscosity are important factors.^8,34 The addition of DEC could supply the self-assembly of Cu₂O nanocrystals with suitable viscosity and potential barrier. The potential barrier will arise in our system once the diethyl carbonate employed, which affects remarkable the collision process of Cu₂O USNCs according to the collision theory. Moreover, the viscosity of DEC (0.75) is obviously higher than those of cyclohexane (0.31) and chloroform (0.57). With decreasing solvent viscosity, the Cu₂O USNCs tend to aggregate. The largest aggregate is produced by dozens of Cu₂O nanoparticles in the suitable viscosity solvent. Noticeable, we found the viscosity of the solvent and the sizes of the particle aggregation have an inverse relationship. The assembling dynamics might be mainly dominated by the viscosity of solvents. With the continuous increase of viscosity, the sizes of the aggregate are decreasing. As shown in Fig. S6,† with the increased amounts of DEC from 0.5 mL to 3 mL, the corresponding sizes of Cu₂O aggregate decreased from 65 nm to 15 nm. On the other hand, oriented attachment has been considered to be one of most important mechanism that controls the crystal growth in the self-assembly.^35,36 As described in Fig. 5a, the oriented attachment among the Cu₂O USNCs may be responsible for the self-assembly of Cu₂O superlattices. In this work, cyclohexane and chloroform are good solvents for the Cu₂O USNCs capped by oleic acid. To our knowledge, free OA molecules in cyclohexane or chloroform are in dynamic equilibrium with those adsorbing on the surfaces of Cu₂O USNCs. However, DEC is a poor solvent in this system. With the addition of DEC, the reversible dissolution–deposition balance was broken and led to the irreversible aggregation of the Cu₂O USNCs in a highly oriented manner, which was driven by the reduction of the surface energy in our assembly system.^37,38 It was found that the Cu₂O building blocks were attached to each other along the different axis with extended reaction such as Lou et al.³⁹ Furthermore, the weakly protected Cu₂O nanoparticles undergo entropy-driven aggregation through strong interactions between the Cu₂O USNCs themselves. With aging time prolonged, the nuclei grew gradually by the oriented attachment due to the impact of the viscosity and potential barrier. Ostwald ripening and recrystallization may also happen in the final process. As shown in Fig. 3, after the self-assembly process, the Cu₂O superlattices have increased their sizes compared to the ultrasmall Cu₂O building blocks. During the self-assembly process, the particle–particle interactions among the Cu₂O USNCs were enhanced and the Cu₂O aggregates were fused into larger Cu₂O nanocubes. From the HRTEM analysis (Fig. 4b), the Cu₂O cubes present good crystallinity, which is consistent with our analysis. Thus, due to the well-defined crystal facets of Cu₂O nanocubes and presence of OA with long alkyl chains as capping ligands, the self-assembled Cu₂O superlattices have been achieved.

Fig. 5 The assembly process for Cu₂O nanoparticles.

Controlled assembly of Cu₂O and Cu₂O–Au 3D colloidal spheres

To construct Cu₂O 3D colloidal assemblies, we employed the emulsion-based bottom-up self-assembly strategy.⁷ As illustrated in Fig. 5b, an O/W microemulsion system was initially designed. The Cu₂O USNCs in cyclohexane were used as the oil phase and the water phase contains cetyltrimethyl ammonium bromide (CTAB). The stable O/W microemulsion system was finally formed after the vigorous stirring, indicating the Cu₂O USNCs confined in the oil droplets. Subsequently, a suitable method was chosen to evaporate the low-boiling solvent (cyclohexane) from the microemulsion system for fabricating 3D Cu₂O colloidal spheres. Herein, rotary vacuum evaporation was firstly used to assemble Cu₂O superlattices. Fig. 6a shows the SEM image of the resulting Cu₂O nanostructures. It can be seen that the as-obtained Cu₂O colloidal spheres present uniform sizes with an average diameter was ca. 120 nm. Interestingly, when the microemulsion solution was exposed in air and evaporated at 70 °C, the Cu₂O USNCs was gradually oxidized into CuO ellipsoids during the assembly process. Fig. S7† shows the corresponding XRD pattern, which is in agreement with the phase of CuO. The resulting CuO ellipsoids possess uniform shape with a width of ca. 70 nm and a length of ca. 230 nm (Fig. 6b). Meanwhile, by adding KBH₄ and HAuCl₄ solution, the Cu₂O–Au 3D colloidal spheres were also successfully obtained. Notably, after loaded Au onto Cu₂O spheres, the Cu₂O–Au colloidal spheres show no obvious change (Fig. S8a†). Simultaneously, the Au nanoparticles can be observed loading on the Cu₂O spheres and the corresponding of HRTEM images are shown in Fig. S8b and c.†

Fig. 6 SEM of the Assembly Cu₂O: (a) evaporated in the rotary vacuum (b) evaporated in the beaker.

Photocatalytic activities of Cu₂O and Cu₂O–Au colloidal spheres

We have also investigated the photocatalytic properties of the as-obtained Cu₂O and Cu₂O–Au nanospheres. By adding different amounts of HAuCl₄, Cu₂O–Au nanocatalysts with various Au content, denoted as Cu₂O–Au(1, 2, 3, 4), were firstly prepared. In this work, the photocatalytic degradation of MO dye was chosen as a model system for different catalysts. As shown in Fig. 7, the degradation efficiency was calculated as C/C₀, where C₀ and C represent the initial concentration of MO dye and the concentrations with different irradiation time, respectively. It can be seen that the pure Cu₂O nanospheres showed weak photocatalytic ability for MO dye with only 13% degradation rate after 1 h. In comparison, the Cu₂O–Au samples exhibit remarkable photocatalytic performance for the MO dye. When the adding amount of HAuCl₄ solution was 1 mL (Cu₂O–Au(1)), the degradation rate quickly reach 74% in 30 minutes. If the HAuCl₄ solution increased to 3 mL (Cu₂O–Au(2)), the degradation rate eventually reach amazing 90%. If we continued to increase the HAuCl₄ to 5 mL (Cu₂O–Au(3)), the degradation rate was fast in the initial 20 minutes. However, too much content of Au present, such as Cu₂O–Au(4) with 10 mL HAuCl₄ added, the photocatalytic efficiency would be decreased. The compositions of the as-prepared Cu₂O–Au samples were also examined by inductive couple plasma atomic emission spectrometer (ICP-AES). The results of ICP-AES indicate that the Au contents for Cu₂O–Au(1, 2, 3, 4) samples are 0.22%, 0.61%, 1.5% and 2.7%, respectively. To our knowledge, the addition of Au can effectively accelerate the segregation efficiency of electron–hole pairs. The UV-vis diffuse reflectance spectra of Cu₂O and Cu₂O–Au were tested to calculate the band gap (Fig. S9†). According to Kubelka–Munk function, the absorption range of the Cu₂O–Au was enlarged. But, the overloading of Au content could also occupy most of the surface of Cu₂O, block the hollow lattices and decrease the specific surface areas of the as-prepared catalysts.⁴⁰ Moreover, excessive Au in the composite increases the opportunity for the collision of electrons and holes and promotes the recombination of the photo-generated electron–hole pairs.⁴¹ In our system, Cu₂O–Au(2) exhibits the excellent photocatalytic efficiency. Fig. S10† shows the corresponding UV-vis absorption spectra of MO aqueous solution.

Fig. 7 Photocatalytic activities of Cu₂O–Au nanocatalysts (Cu₂O–Au(1): 0.22 wt% Au; Cu₂O–Au(2): 0.61% wt% Au; Cu₂O–Au(3): 1.5% wt% Au; Cu₂O–Au(4): 2.7% wt% Au).

Conclusion

We report a solution-based method for fabricating the ultrasmall Cu₂O nanocrystals. Furthermore, based on the EBS and DEC-mediated self-assembly strategies, various 2D or 3D Cu₂O superlattices can be successfully produced from the ideal Cu₂O building blocks. It is notably that DEC plays a crucial factor for controllable self-assembly of Cu₂O USNCs. Significantly, the aggregative growths of Cu₂O nanocrystals in the self-assembly process are accompanied via oriented attachment and Ostwald ripening. Meanwhile, Cu₂O and Cu₂O–Au 3D colloidal spheres can be also achieved by EBS strategy. The photocatalytic performance show that, compared to pure Cu₂O nanospheres, the as-obtained Cu₂O–Au hybrid nanospheres exhibit excellent photocatalytic efficiency in the photodegradation of MO dye. Our results may be expected to provide an effective method to synthesize and assemble other ultrasmall oxides.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant no. 51472001, 21201001), Anhui Provincial Natural Science Foundation (grant no. 1208085QB25), Ph.D. Start-up Fund and 211 Project of Anhui University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06141e

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