1. Introduction

Organic pollutions originated from anthropogenic chemicals including textile production, printing, plastics, electroplating, and dyeing industries, etc. have become serious environmental issues, possessing toxic or lethal effects on our ecosystem. Especially, some complex aromatic moleculars such as methyl blue (MB) molecules are persistent organic pollutants since they are generally stable to light, heat or oxidization agents [1–6]. The project of using nanomaterials as a versatile tool for removing organic contaminants has garnered immense scientific interest in the water treatment areas. Based on visible light-driven photodegradation, many interesting hybrid nanostructures including zinc oxide based photo-catalysts [7], titanium dioxide based nanocomposites Ag₃PO₄@TiO₂ [8], graphene-based semiconductors (CdS, Bi₂WO₄, BiOCl, WO₃, etc.) photo-catalysts [9], have been well developed in recent years. Meanwhile, it should be noted that there are still various challenges (low solar sensitivity, recombination of generated redox environment, etc.) to get superior photo-catalytic activity. Therefore, some novel nanocomposites with well adsorption performances have been demonstrated to remove organic pollutants from the water via simple adsorbing process. It is well established that various adsorbents including h-XG/SiO₂ nanocomposites [2], flower-like sodium titanate [6], hierarchical MoS₂ microspheres [10], Fe₃O₄@polydopamine-Ag core-shell microspheres [5], porous BiOBr/Bi₂MoO₆ heterostructures [11] are widely used for the efficient decontamination of MB dye pollutants. There is no doubt that the previous works exhibit well adsorption performances for the removal of MB molecules. However, it has been recognized that most of adsorbents have been synthesized by standard chemical fabrication via the reduction of a precursor in complicated stabilizers and soft directing agents. Despite of the complicated purification procedures, the adsorbents with inevitable residual reagents will bring some unwanted anthropogenic chemicals into the wastewater treatment, resulting in secondary pollution once again. Thus, the efficient adsorption of MB dye pollutants via non-toxic and eco-friendly adsorbents has garnered considerable attention.

Increasing evidence has shown that laser-induced fabrication in liquid is a novel green approach to the synthesis of novel nanomaterials [3,4,12–16]. Most recently, our group has confirmed that hybrid Ag₂S@Ag nano-particles can be removed from solutions easily by filters after adsorbing MB molecules. Yang’ group has also demonstrated that the amorphous Fe₂O₃, CoO and NiO nanoparticles fabricated by laser irradiation in liquid possess super adsorption capabilities for MB solutions [4]. The mixed solution (metal oxide and MB molecules) required to be continuously stirred for 12 h before the adsorption experiments. On the other hand, compared with solid nanoparticles or core-shell nanostructures, the hierarchical nanomaterials with porous structures are very useful building blocks for the adsorption of dye molecules [6,9,10]. Up to now, it is still unknown whether the novel strategy can be extended to the fabrication of porous metal nanocomposites in the absence of any additives. Hence, it is very necessary to explore pure and porous nanostructures by the laser induced fabrication approach.

Herein, for the first time, we report on the one-step synthesis of porous Cu₂O@CuO nanocomposites with overall size of 60~100 nm by laser irradiation of CuO powders in distilled water. Without using any chemical reagents, the pure hybrid nanostructures with porous-size of 4~8 nm can be obtained by 60 min laser irradiation. Compared with the original CuO powders, the as-prepared porous Cu₂O@CuO nanocomposites exhibit excellent adsorption performances for the removal of MB molecules from wastewater. Our measurements indicate that 99.69% MB molecules (80mg/L, 3mL) can be removed from the solution via the 1.8 mg Cu₂O@CuO nanocomposites after 1 min, while 95.35% via the 200 mg original CuO powders in the same reaction time. Meanwhile, based on the porous structures and the abundant hydroxyl (-OH) groups formed on the surfaces, a detailed discussion of the relevant mechanism is addressed. Our results have opened up a novel green paradigm to obtain pure and porous nanocomposites, demonstrating excellent adsorption performance in the removal of MB molecules from wastewater.

2. Experimental section

The experimental apparatus based on laser irradiation in liquid is very similar to that described in previous studies [3,4,6–16]. The CuO powders with a purity of 99% as the original metal oxides were purchased from Aladdin Industrial Corporation (China). In a typical experiment, the 32 mg CuO powders was placed on the bottom of a rotating glass dish with a speed of ~600 rpm that was filled with 6mL distilled water. A Q-switched Nd-YAG (Yttrium Aluminum Garnet) laser (Quanta Ray, Spectra Physics) beam operating at wavelength of 532 nm with pulse duration of 10 ns, 10 Hz repetition rate, energy density of 350 mJ/pulse was used to irradiate the CuO solution. The average spot size of the laser beam at the solution was about 9 mm in diameter. After laser irradiation, the products were collected in a glass dish. The products were centrifuged at 18000 rpm for 10 min in an ultracentrifuge. The morphological and chemical compositions of the sediments and the original CuO were investigated by field emission scanning electron microscope (SEM, Hitachi, S-4800) equipped with energy-dispersive-x-ray spectroscopy (EDS). The crystallographic investigations of the products and the original CuO powders were acquired by X-ray diffraction (XRD). The detailed sample surface compositions of the products were studied by X-ray photoelectron spectra (XPS) on a PHI Quantera SXM with an Al Kα = 280.00 eV excitation source. Then, the sediments were dropped on a copper mesh for observation by transmission electron microscopy (JEOL-JEM-2100F). The Fourier transforms infrared spectrums (FTIR) of the nanomaterials were measured by a UV-Vis-NIR spectrometer (370~7800cm⁻¹, ALPHA-T, Bruker). In a typical adsorption experiment, the decontamination of MB pollutants was carried out by simply adding the as-prepared Cu₂O@CuO nanocomposites into MB solution under constant stirring (200 rpm) at room temperature. All adsorption experiments are performed in PH~7.0 condition. It should be noted that the mixing solution were not stirred for hours to reach equilibrium before the adsorption experiments. After 1~60min adsorption, the organic material concentrations (centrifuged at 18000 rpm for 10 min) were separately measured by the absorbance spectrums, which were carried out by a UV-Vis-IR spectrometer (UV-1800, Shimadzu).

3. Result and discussion

The morphologies of the original CuO powders are shown in Fig. 1(a) and 1(b). The typical low- and enlarged scanning electron microscopy (SEM) images illustrate that the morphologies of original CuO powders are irregular rod-like nanostructures with average sizes of about 120~200 nm. The relatively strong peaks of the EDS pattern (inset in Fig. 1(b)) only contain the Cu and O species. The corresponding ratio of Cu and O elements is about 50.15:49.85, which is consistent with the CuO chemical composition. After laser irradiation of the irregular shaped CuO powders in distilled water for 60 min, the obtained product is shown in Fig. 1(c). The morphologies of the final nanomaterials are rice-shaped architectures with length around 100 nm and diameter about 50 nm, which become smaller than the original CuO powders. Compared with the smooth CuO powders in Fig. 1(b), closer view of these final nanostructures indicates that the surfaces are very rough and rugged structures. The unique nanostructures will be illustrated in the next sections. The inset in Fig. 1(c) shows that the EDS pattern of the obtained nanomaterials is also composed of Cu and O elements, and the relative ratio is 56.44:43.56. The concentration of O species in the final nanomaterials by laser irradiation is slightly low than the original powders. In addition, the crystallographic investigations of the nanostructures before and after laser irradiation were established by X-ray diffraction (XRD) in Fig. 1(d). Three diffraction peaks are observed at 35.49°, 38.73° and 67.94° in original powders, corresponding to (002), (111) and (113) lattice planes of CuO structure (JCPDS, no.45-0937).

 

Fig. 1 (a-b) The representative low-magnification and enlarged SEM images of the original CuO powders. The inset shows the EDS result. (c) The typical SEM image of the final products after laser irradiation of CuO powders for 60 min. The inset shows the EDS result. (d) The XRD patterns of the products and the original CuO powders.

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After laser irradiation, besides the CuO nanocrystals, a series of (111), (200), (220) and (311) Cu₂O diffraction peaks (JCPDS, no.34-1354) at 37.01°, 42.61°, 62.44° and 74.40° are also indeed found in the final products. It can be deduced that the final products obtained by laser irradiation of CuO powders are the hybrid Cu₂O@CuO nanocomposites. Because of the relative higher peak at 37.01° in XRD pattern, the preferential alignment of the Cu₂O(111) orientation should be formed in final nanocomposites.

Moreover, the morphologies of the hybrid Cu₂O@CuO nanocomposites are illustrated by transmission electron microscopy (TEM) in Fig. 2. The low- and enlarged magnification images in Fig. 2(a-c) clearly show that the obtained Cu₂O@CuO nanocomposites are indeed nanoporous structures. The pores in products are shown as contrasting lighter imager with their walls as darker ones, due to different penetration depths of the incident electron beam. The porous size is about 4~8 nm by measuring the diameters of more than 300 nanocomposites in sight on the TEM images. The HRTEM image in Fig. 2(d) provides a typical structural detail of the cross points. The region marked by red lines with a d-spacing of 0.252 nm is indexed as the (002) plane in the CuO structure. On the other hand, the area marked by violet color lines with a periodicity corresponding to a d-spacing of 0.242 nm could be indexed with reference to the Cu₂O plane structure. The above results are the best evidences for the formation of porous Cu₂O@CuO nanocomposites by laser irradiation of CuO powders in distilled water. In order to further verify the purity and the surface composition of the as-prepared nanocomposites, the X-ray photoelectron spectroscopy (XPS) results are illustrated in Fig. 3. The Cu and O as well as C elements were detected in the XPS spectrum. The binding energies were calibrated by referencing the C1s peak at 284.8eV to reduce the sample charge effect [3,17–21]. The peaks of O1s, O2s, Cu3s, Cu2p, Cu3p as well as O(A) and Cu(A)(from Auger electrons) are illustrated in Fig. 3(a). By using a Gaussian-Lorentzian fitting method, the Cu2p_3/2 spectrum (Fig. 3(b)) is best fitted to two spin-orbit doublets characteristic of Cu²⁺ and Cu¹⁺, suggesting the co-existence of Cu₂O and CuO in the hybrid porous nanocomposites. Based on the Cu²⁺ and Cu¹⁺ peak areas, the relative ration of Cu₂O and CuO species is about 2:5. The possible growth of porous Cu₂O@CuO nanocomposites will be described in the following section. When laser beam arrives at the CuO powders in liquid, the laser energy absorbed by the nanomaterials is eventually transformed into heat, which enables the exposed surfaces of the initial CuO structures to be local-melted in solution. Compared to Cu metal elements, the O species will be more easily to be removed/escaped from original structures. Meanwhile, the recrystallizations of residual Cu and O species in original region will take place during pulse laser irradiation, resulting in the formation of hybrid Cu₂O@CuO nanocomposites. The laser density used in this paper is relatively lower than that in previous works [4,22]. The whole structures of the initial powders will not be significantly destroyed by the moderate laser irradiation, since the ultra-small nanomaterials were not detected in this work. Moreover, the single-crystalline nature can be well kept during the recrystallization process (XRD patterns in Fig. 1(d)). As for the relative low laser density adopted in this work, a type of local-vaporization instead of complete-explosion will play a critical role in the laser irradiation of original CuO powders. It is the main reason for the formation of nanoporous structures in the final products. On the other hand, the abundant hydroxyl (-OH) groups can also be generated on the nanoporous surface, which will be discussed by Fourier Transform Infrared Spectroscopy (FTIR) in the next section.

 

Fig. 2 (a) The typical low-magnification TEM image of the products obtained by laser irradiation of CuO powder for 60min. (c-d) The corresponding enlarged TEM images of the obtained hybrid Cu₂O@CuO nanocomposites.

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Fig. 3 XPS spectra of the porous hybrid products. (a) Survey structure (b) Cu2p_3/2 of the nanocomposites.

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Finally, the unique adsorption performances for the removal of MB from wastewater are illustrated in Fig. 4. In the presence of 4 mg original CuO powders, the adsorption behavior of MB molecules in wastewater (80 mg/L, 3 mL) is illustrated in Fig. 4(a). The main absorption band of MB molecules at about 588 nm drops from about 2.25 a.u to 0.83 a.u with the adsorption time prolonging from 0 to 1 min, and then slightly decreases to 0.41 a.u after 60 min adsorption process. The adsorption nearly 81.77% MB molecules can be obtained in 60 min. By using the some amount of MB solution, 4 mg porous Cu₂O@CuO nanocomposites obtained by 30 min laser irradiation exhibit excellent adsorption performance in Fig. 4(b). Compared to the adsorption by CuO powders, the corresponding adsorption of MB molecules drastically drops from about 2.25 a.u to 0.025 a.u in 1 min, and slightly further decreases to 0.019 a.u in next 9 min (inset in Fig. 4(b)). The adsorption nearly 99.11% of MB molecules can be achieved in 1 min. In order to get the similar adsorption performance (>95% MB in 1 min), 200 mg original CuO powders should be required, as shown in Fig. 4(c). Moreover, the porous Cu₂O@CuO nanocomposites fabricated by 60 min laser irradiation can further lead to an improved adsorption performance, as shown in Fig. 4(d). In the presence of 1.8mg as-prepared Cu₂O@CuO nanocomposites, the adsorption nearly 99.69% of MB molecules can be reached in 1min. It is also agreed with the color change of the solution from dark blue to colorless in 1min (inset in Fig. 4(d)). In summary, the obvious comparison among the above results clearly demonstrates that the as-prepared porous Cu₂O@CuO nanocomposites possess superior adsorption performance in the reduction of MB solution. The fascinating feature is highly related to the unique nanoporous structures and the abundant –OH groups formed on the porous surfaces by laser irradiation. The porous architectures with larger surface areas will improve the adsorption performance, which is well demonstrated in many previous works [6,9,10]. The detailed mechanisms are also coincident with our case.

 

Fig. 4 The reduction performance of the removal of MB molecules from the water solution (80mg/L, 3mL). (a-b) In the presence of 4mg original CuO powders and Cu₂O@CuO nanocomposites obtained by 30 min laser irradiation, respectively. (c) With 200mg original CuO powders. (d) In the presence of porous Cu₂O@CuO nanocomposites.

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On the other hand, the –OH groups can also provide much more active sites and act as adsorption sites [4,23]. To verify the formation of –OH groups on the Cu₂O@CuO nanoporous structures, the FTIR spectrums of the original CuO, the products by 30 and 60 min laser irradiation, respectively, are shown in Fig. 5(a). The obtained results clearly show that the spectrum of –OH groups at about 3464 cm⁻¹ can be significantly enhanced as the laser irradiation time increases from 0 to 60 min. The porous Cu₂O@CuO nanostructures with enriched –OH groups will be very suitable for the reduction of MB molecules in wastewater. As illustrated in previous works [4,23], the hydrogen bonding interaction will be formed between –OH groups and the oxygen groups in MB molecules. After adsorbing MB molecules, several enhanced FTIR spectrum originated from MB structures can be clearly found in porous Cu₂O@CuO absorbents, as shown in Fig. 5(b). The C-N stretching vibration at 1337 cm⁻¹, the aromatic ring vibrations at 1575 and 1495 cm⁻¹ and the –SO₃Na functional groups at 1169, 1121, 1032 and 1005 cm⁻¹ have been observed in the FTIR spectrum, which is consistent with previous works [3,4]. The result is the evidence for the chemical bonds between the adsorbates and adsorbents. Therefore, the abundant –OH groups generated on the porous Cu₂O@CuO surfaces will facilitate the formation of hydrogen bonding during MB adsorption process. In summary, the pure Cu₂O@CuO nanoporous structures with excellent MB adsorption properties will be very suitable for efficient removal of other organic dyes in the future.

 

Fig. 5 (a)The FTIR spectrums of the original CuO powders, products obtained by 30min and 60 min laser irradiation. (b) The FTIR spectrum of the absorbent by 60 min laser irradiation after adsorbing MB molecules.

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4. Conclusions

In conclusion, we have reported on the successful synthesis of porous Cu₂O@CuO nanocomposites by laser irradiation of CuO powders in distilled water without any surfactants or additives. The moderate laser irradiation will lead to local-evaporation generated on the original powders, resulting in numerous porous structures formed in the final products. Compared with original CuO powders, the as-prepared porous Cu₂O@CuO nanocomposites with enriched –OH groups exhibit excellent adsorption performance for the removal of MB molecules from wastewater. Clearly, it can be seen that MB can be removed 99.69% via 1.8mg Cu₂O@CuO nanocomposites after 1 min reaction time, while 95.35% via 200mg original raw CuO powders. The results suggest that the pure and porous Cu₂O@CuO nanocomposite is a promising adsorbent for organic pollutants adsorption. The work will also offer a green and convenient way to fabricate pure other complex nanoporous structures in distilled water. In our future work, it is worthwhile to produce nanocomposite films with controlled porous structures, which will provide a good opportunity for the new methods to be used in wide range of applications.