Various nanomaterials have been established as versatile adsorbents for the removal of organic or heavy metal pollutants from wastewater. Unfortunately, the subsequent cleaning of the suspended nano-adsorbents is very difficult and costly. Herein, we report a novel synthetic route to fabricate fibrous-shaped CuO@CuS nanoporous structures by the laser irradiation of CuO powders in sodium sulfide (Na2S) liquid. Superior to conventional chemical approaches, 532 nm laser beam irradiation will lead to the formation of rich- O and S vacancy defect states in the final products. Zeta-potential experiments confirm that the positive surface charges of CuO@CuS nanomaterials significantly increase from 2 to 70 mV with an increase of laser irradiation time (0~30 min). As for the methyl blue (MB) and hexavalent chromium Cr(VI) solution, the CuO@CuS nanocomposites adsorbed with these adsorbates can be significantly self-agglomerated, due to the positive electrostatic adsorption effect on negatively charged pollutants. After the adsorption of MB molecules and Cr(IV) ions, the absorption spectra of two supernatant liquids confirm that 99.6% and 98.9% CuO@CuS nano-adsorbents can be deposited on the bottom of the pool, respectively. Without the aid of the centrifugation process or an external magnetic field in complicated nano-adsorbent separation procedures, the unique adsorption-deposition process is a significant breakthrough in the wastewater purification.
© 2017 Optical Society of America
Water pollution originated from organic contaminants and heavy-metal pollutants have been a serious environmental hazard to aquatic bio-systems, and then severely endanger animals and human health [1–8]. It has been recognized that some stable dye compounds with complex aromatic molecular structures, such as Methyl blue (MB) molecules, have potential carcinogenic and mutagenic effects to humans and aquatic organisms [1–6]. On the other hand, because of high acute bio-toxicity, heavy-metal pollutants can also induce severe environmental problems in human beings. For example, it has been demonstrated that the Minamata disease event in Japan were caused by mercury wastewater, which damaged the public health seriously . Because of the increasing anthropogenic pollutions, therefore, the water purification has become as urgent global environmental issue and has garnered immense scientific interest [9–29]. Among all available treatments for these pollutants such as chemical oxidation, biodegradation, photo-catalytic degradation, etc. the prospect of using nanomaterials as a versatile adsorbent represents a simple, low-cost, yet effective strategy for the removal of contaminants from the liquid [2, 5, 6, 11–15, 17–20, 22, 23, 25–30]. For example, compared with the mature photo-catalytic technology [7, 8, 10, 16, 21, 24], there is no need any additional assists (ultra-or visible light beam irradiation) in adsorption process. Until now, it is no doubt that the developing of nano-adsorbent is a wise choice for removing dye molecules and heavy-metal pollutants from water. Unfortunately, there is an obvious drawback in the adsorption treatment. In most cases, after adsorption process, the suspended nano-adsorbents in the solution will inevitably result in secondary unwanted anthropogenic pollution once again. However, the subsequent removal and cleaning of these nano-adsorbents from solution is very difficult and costly, which is usually carried out by centrifugation process or external magnetic field [4–6, 11–15, 22, 23, 25–29]. Although a great deal of nano-structures with excellent adsorption properties (high removal efficiencies and adsorption capacity, etc.) has been illustrated as promising adsorbents, the subsequent complicated separation of nano-adsorbents remains a serious challenge for wastewater purification. Therefore, it is highly desirable to explore novel nano-adsorbent that can reduce or even avoid the subsequent adsorbent-separation process, which has promising high-applicability in practical wastewater treatment.
Alternatively, some novel multifunctional composites with three-dimensional (3D) foams, such as polyurethane (PU) sponge, graphene foam (GF), melamine foam (MF) and cellulose-based materials, have been illustrated as promising adsorbents for treatment of oily or organic solvents [6, 13, 17]. For example, Yan et al. reported free-standing poly (vinyl alcohol)/poly (acrylic acid) membranes with polydopamine coating (PVA/PAA@PDA) prepared by the combination of electrospinning and self-polymerization of dopamine for the removal of organic MB dyes . Compared with traditional nano-adsorbents, these multifunctional composites are easy to retrieve and elute after adsorption process, which can overcome the subsequent adsorbent-separation limitation. However, the scope of these 3D foams with porous structures remains restricted solely to the adsorption of oils or organic wastewater by van der Waals forces. There are few reports on the adsorption of heavy-metal ions through these 3D foams. More importantly, these polymer skeletons with different functional groups on their surfaces will be destroyed and dissolved in solution during repeated applications, which also bring some unwanted secondary pollution. Recently, our group has reported an effective Ag2S@Ag nano-adsorbent for removing MB molecules from solution . The strong ionic bounds between positive charged adsorbent and negative charged dye molecules enable them to be deposited on the bottom of the solution after adsorption process . There is no need of complicated adsorbent separation after wastewater treatment. However, the only disadvantage is that the intrinsic high-cost of the noble Ag metal will also severely limit its wide application in the treatment of wastewater. So, extensive research efforts should be devoted toward to the development of low-cost semiconductor nano-adsorbents. Up to now, the inexpensive nano-adsorbent with unique adsorption-deposition property has not been explored so far.
Herein, for the first time, we report on the successful synthesis of a novel low-cost fibrous-like CuO@CuS nanoporous structures with plentiful small-sized (~2 nm) surface pores through laser irradiation of CuO powders in Na2S solution. Different from traditional fabrications, the laser irradiation-induced photochemical process will give rise to the generation of rich-oxygen and sulfur vacancy defects in CuO@CuS nano-structures, then result in the formation of positive charged adsorbents. The novel CuO@CuS nanomaterials with pronounced positive surface charges can provide enhanced positively charged-electrostatic adsorption effect on negatively charged contaminants. After adsorbing MB solution and Cr(VI) liquid, respectively, the distinctive advantage of as-prepared adsorbent is its ability to be self-agglomerated and then completely deposited on the bottom of pool. Compared with the noble Ag2S@Ag nanocomposites , the semiconductor CuO@CuS adsorbent with unique adsorption-deposition feature is cost-effective, possessing high applicability for water purification.
2. Experimental section
The experiment used laser irradiation in liquid condition has been widely illustrated in our previous works [30–32]. Briefly, the 5 mg CuO powders with a purity of 99% were placed on the bottom of a rotating glass dish with a speed of ~1000 rpm that was filled with 5 mL liquid solution containing 2 mg sodium sulfide (Na2S) and 10μL hydrochloric acid (HCl). The Na2S provides the sulfur source for the construction of CuS composition. The acid condition will lead to porous nanostructure during the nucleation process. The 532 nm laser beam with pulse duration of 10 ns, 10 Hz repetition rate and ~400 mJ/pulse was used to irradiate the solution. After 30 min laser irradiation, the products were investigated by transmission electron microscopy (TEM, JEOL-JEM-2100F), X-ray photoelectron spectra (XPS, PHI Quantera SXM), scanning electron microscope (SEM, Hitachi, S-4800), X-ray diffraction pattern (XRD, Rigaku, RINT-2500VHF), and Fourier transforms infrared spectrums (FTIR, ALPHA-T, Bruker). The Zeta potential is carried out by SZ-100 nanopartica series instrument. The adsorption experiments were carried out via adding 5 mg products into 10 mL, 50 mg/L Rhodamine 6G (R6G), methyl orange (MO) and MB dye solutions, respectively. All the adsorption experiments are performed in dark environment, in order to avoid the interference with visible light irradiation. The solutions were adjusted at pH ~6.0 condition. The optimum pH in this paper is about 6.0, which is very beneficial to the adsorption performance. As illustrated in previous work , the structure of CuO@CuS adsorbent will be destroyed at low pH condition. On the other hand, the adsorption property decreases with alkaline pH solution, because of the completion of OH with dye molecules on the nanocomposites. Another heavy-metal solution contains 10 mL, 5 × 10−4 mol/L K2Cr2O7. The concentrations of these contaminants were separately measured by the absorbance spectrums (UV-1800, Shimadzu).
3. Result and discussion
The morphologies of the products are shown in Fig. 1. The typical TEM and SEM images in Figs. 1(a) and 1(b) clearly reveal that the numerous nanochains coupled with some nanoparticles are interconnected and accreted with each other, forming fibrous-shaped architectures. These fibrous networks in Fig. 1(b) show that the nanoparticles were displayed as white points in cross sections. The average diameter of nanochains is about 6 nm, which is apparently different from the original CuO powders with irregular rod-like nanostructures . The enlarged TEM results in Fig. 1(c) further confirm that the nano-nets are characterized by plentiful porous surface structures. The pores in the obtained nanomaterials are shown as contrasting light imager with their frameworks as darker ones, due to different penetration depths of the incident electron beam. In sight on the TEM images, the average porous size is about 2 nm by measuring the diameters of more than 300 pores. The frameworks are found to be well crystalline according to the clear lattice fringes in Fig. 1(d). The regions indicated by red and magenta arrows with a d-spacing of 0.25 nm and 0.19 nm are separately indexed as the (002) plane in CuO structure and the (107) plane in CuS (Covellite), respectively. The HRTEM image in Fig. 1(d) also shows that the CuO and CuS quantum dots are mixed with each other, forming ultra-sized framework structures.
The chemical composition and crystallographic feature of the products were investigated by energy-dispersive-x-ray spectroscopy (EDS) and X-ray diffraction (XRD), respectively, as shown in Figs. 2(a) and 2(b). The relatively strong peaks of the EDS in Fig. 2(a) show that the obtained products are only composed of Cu, O, and S elements, and the ratio is about 2.2:1:1.2. The XRD pattern in Fig. 2(b) reveals two obvious diffraction peaks at 35.47°, 38.72°, which are originated from (002) and (111) lattice planes of CuO structure (JCPDS, no. 45-0937). On the other hand, three diffraction peaks at 29.26°, 47.76° and 59.33° were indeed detected in XRD pattern, which should be attributed to (102), (107) and (116) of CuS plane structure (JCPDS, no. 06-0464). The results demonstrate the products should be CuO@CuS nanocomposites. Moreover, the element valence and surface purity of the CuO@CuS nanocomposites were illustrated by X-ray photoelectron spectroscopy (XPS) spectra in Fig. 2(c). Generally, the binding energies were calibrated by referencing the C1s peak at 284.7eV to reduce the sample charge effect [30, 32]. Then, the peaks of Cu2p, S2s and O1s as well as Cu(A) (from Auger electrons) can be clearly detected in XPS spectra. The peaks of 932.4 eV (Cu2p3/2) and 952.3 eV (Cu2p1/2) reveal that the oxidation state of Cu2+ formed in the nanocomposites. In addition, the FTIR result in Fig. 2(d) provides more information of the CuO@CuS nanocomposites. A serious of H-O-H bounds in H2O modes was observed in FTIR spectrum. Meanwhile, the pronounced spectrum at about 3470 cm−1 implies the enriched hydroxyl (-OH) groups formed on the porous structures.
The growth mechanism of fibrous-shaped CuO@CuS nanoporous structures is described in this section. At the moment of pulsed laser beam reaching at colloidal suspension, laser induced nanowelding of melted CuO nanoparticles with the formed CuS nanomaterials will dominate the subsequent irradiation process. Laser irradiation caused sintering or sticking together of metal-based nanoparticles has been verified in previous works [33, 34], which is also coincident with our case. It should be the most likely reason for the formation of fibrous-like CuO@CuS nanocomposites. The adsorption spectrum of CuO colloidal solution (Fig. 3(a)) shows that the broadened surface plasmon resonance (SPR) can be clearly detected at about 500~600 nm in visible region. The 532 nm laser beam in our experiment ensures that the incident excitation wavelength is commensurate with the SPR of CuO colloidal solution. Therefore, the laser energy will be significantly adsorbed by CuO solution, giving rise to the superheating of CuO nanomaterials . Then, the absorbed photon energy of 532 nm laser beam enables the exposed CuO surfaces to be local-melted in solution. Some unstable CuO nanomaterials will be dissolved into copper and oxygen ions by laser fragmentation. As illustrated in previous work , in contrast to metallic Cu elements, the O species in CuO powder will be more easily to be escaped from original structure. Moreover, because of plasmon resonance absorption of CuO nanostructure, the incident 532 nm laser beam can efficiently separate photo-excited electron-hole pairs, leading to the formation of excited Cu species on the surface of precursor. Meanwhile, the laser-induced superheating process can also promote the temperatures of residual Cu species and surrounding Na2S solution temperature. In this way, the re-nucleation of Cu and S (from Na2S) ions will take place during pulsed laser irradiation process. The corresponding localized surface plasmon resonance (LSPR) spectrum of nanocomposites (Fig. 3(b)) illustrates that the adsorption peak distinctly red-shift to 1000~1060 nm in near-infrared region (NIR), which is mainly originated from CuS composition. The longer irradiation time results in larger percentage of O species escaping from the CuO particles, which subsequently form structures with CuS nanomaterials. The O deficient due to O species having escaped from original CuO particles plays an important role in the laser-induced nanowelding process. Then, the subsequent laser irradiation will result in the formation of fibrous-shaped CuO@CuS nanostructures by sticking together of CuO nanoparticles with as-prepared CuS nanomaterials. Based on the EDS results of eight groups of CuO@CuS nanocomposites, the variation of O species content as a function of irradiation time is displayed in Fig. 3(c). The result shows that the relative ratio of O species nonlinear decreases from about 50% in original CuO precursor to about 24.5% after 30 min laser irradiation. On the contrary, the S species content exponential growths to about 25.4% during the same laser irradiation process (Fig. 3(d)). During laser irradiation process, the additive HCl in activated solution will also lead to ultra-rapid acid etching process, making some Cu, O and S species to be dissolved and removed from the nanocomposites. The acid etching in the early stage of laser-induced recrystallization has been verified in our previous report . It is suitable for the generation of surface pores on the nanocomposites. Therefore, the fragmentation of CuO precursor coupled with complicated nucleation of Cu and S species as well as ultra-rapid acid etching processes result in the formation of fibrous-shaped CuO@CuS architectures. During pulsed laser irradiation (duration of 10 ns), the unique ultra-rapid fabrication process enable many Cu, O and S species to deviate from original positions, resulting in the formation of abundant disorderly arranged vacancy-defects on the surface of products. The rich-vacancies defect states can be determined by electron transition among energy levels in the nanocomposites, which were carried out by photoluminescence (PL) emission spectrum. As shown in Fig. 4(a), The NIR luminescence peak at ~1000 nm were well indexed to O, and S vacancies defect states [36–38]. During the laser-induced growth of CuO@CuS nanocomposites, the corresponding NIR PL intensity significantly increases from about 120 a.u to 2200 a.u as the irradiation time increases from 5 min to 30 min. It is the best evidence for the formation of rich- O and S vacancies defect states after laser-induced fabrication. The enriched defects can also change the surface charges of the final products. The surface charges of CuO@CuS nanoporous structures were determined by the zeta-potential experiment in Fig. 4(b). During 30 min laser irradiation, the positive surface charges of CuO@CuS nanocomposites significantly increase from about 2.1 mV at initial structures to 70.2 mV at final products. The pronounced positive vacancies-induced electrostatic states will play a critical role in the following adsorption of negatively charged contaminants from the liquid solution.
Finally, in the presence of 5 mg fibrous-shaped CuO@CuS nanoporous materials, the adsorption performances for removing the R6G, MO and MB molecules from liquid solutions are demonstrated in Fig. 5. After 7 hours adsorption processes, the color changes of the three different dye solutions (Fig. 5(a)) clearly show that R6G and MO dye solutions change into brown color suspensions. The two stable suspensions could still be sustained for three months, which must be centrifuged at 18000 rpm for 15 min in order to separate the adsorbents from the dye solutions (right photographs in Fig. 5(a)). In contrast to these stable suspensions, the CuO@CuS nanocomposites adsorbed with MB molecules significantly self-aggregated/agglomerated together and completely deposited on the bottom of the solution after 7 hours adsorption process. The distinctive advantage is that there is no need for the subsequent separation of adsorbent from the solution by centrifuge process or external magnetic field. The clear water (supernatant liquid in Fig. 5(a)) can be easily obtained by simply adsorption process. Moreover, the corresponding reduction performances of the removal of three dye solutions are illustrated by absorption spectra in Figs. 5(b)-5(d), respectively. As for the positively charged R6G molecules (Fig. 5(b)), the CuO@CuS nanocomposites exhibit very low adsorption performance, since the main absorption band at about 520 nm slightly changes from 2.82 a.u to 2.76 a.u. If the negatively charged dye molecules (MO and MB) were used in this paper, the adsorption properties of the CuO@CuS nanocompossites significantly improve with an increase of negative charges of the dye molecules (Figs. 5(c) and 5(d)). Thus, the CuO@CuS nanocomposites exhibit charge-selective adsorption performance for different charged dye molecules. The adsorption property can be significantly enhanced as increasing negative charges of the dye molecules. The strong ionic bonding between the positively charged CuO@CuS nanocomposites and negatively charged functional groups of MB(-SO3-) is the main reason for the formation of agglomerated adsorbents. Moreover, compared with solid or core-shell structures, the unique fibrous-shaped nanoporous architectures with plentiful ultra-small-sized surface pores will also further improve the adsorption performance, which can provide much more positively charged active sites.
Figure 6 shows the detailed evolution of MB molecules in the presence of fibrous-like CuO@CuS nanocomposites. After 2 hours adsorption, the direct photographs of the MB solution clearly show that the novel absorbents coupled with MB molecules became significantly aggregated/agglomerated together and developed in to larger-sized material, forming muddy-like water in Fig. 6(a). Then, they are completely self-deposited on the bottom of the solution within 7 hours reaction. Finally, the clear water was successfully obtained by this simply adsorption process. Correspondingly, the main absorption bands of the MB molecules (Figs. 6(c) and 6(d)) exponentially decrease with an increase of adsorption time. For example, the absorption peak at about 585 nm drops from 1.5 a.u. to 0.008 a.u. during 7 h adsorption process (Fig. 6(d)). The adsorption of 99.4% MB molecules can be achieved in this paper. After adsorption process, the SEM image (inset in Fig. 6(d)) shows the adsorbents are completely covered with abundant organic compounds. It is obviously different from the as-prepared fibrous-like CuO@CuS nanoporous structures. After adsorption process, the FTIR spectrum (Fig. 6(e)) of the sediment shows that several enhanced signals originated from MB structures can be clearly detected in the adsorbents. The -SO3- functional groups at 1167, 1121 and 1024 cm−1 as well as the aromatic ring vibrations at 1501 and 1589 cm−1 have been determined in FTIR spectrum. It is best evidence for the formation of strong chemical bonds between positive charged adsorbents and negative charged MB molecules. In addition to the adsorption of organic MB molecules, the novel adsorbent can also exhibit pronounced adsorption performance for removing the heavy metal Cr(IV) pollutants from the solution. As shown in Figs. 7(a) and 7(b), the adsorbents coupled with Cr(IV) are accreted with each other and agglomerated/aggregated together after 8 min adsorption, and also completely deposited on the bottom of solution within 20 min reaction. Different from the yellow color of the original K2Cr2O7 solution, the supernatant in Fig. 7(b) turns into colorless after 20 min adsorption. The quantitative results in Figs. 7(c) and 7(d) illustrate that the concentration of Cr(IV) exponential decay with an increase of adsorption time. The 99.3% Cr(IV) ions can be adsorbed by CuO@CuS nano-adsorbents within 20 min reaction. Compared with functional groups of MB, the inorganic negative Cr2O72- ions can be more easier adsorbed on the positive charged adsorbent, since the self-deposition time is much less than that of adsorption of MB molecules.
In order to confirm that the novel adsorbent can be completely self-deposited on the bottom of the solutions, the absorption spectra of 5 mg CuO@CuS nanocomposites in 10 mL solution (before adsorption process) and two supernatant liquids after Cr(IV) ions and MB molecules adsorptions are illustrated in Fig. 8. After agglomeration and deposition process, the absorption peak of CuO@CuS nano-adsorbents sharply dropped from about 1.06 a.u to 0.011 a.u and 0.004 a.u for Cr(IV) and MB adsorption reactions, respectively. After adsorption of Cr(IV) ions and MB molecules, it is reasonable to deduce that the 98.9% and 99.6% CuO@CuS nano-adsorbents can be deposited on the bottom of the solutions, respectively. There is no doubt that the inexpensive semiconductor CuO@CuS nanocomposites should be established as low-cost advanced adsorbents for self-deposition of organic and heavy metal pollutants in wastewater.
In summary, we have reported on the one-step synthesis of fibrous-shaped CuO@CuS nanoporous structures with abundant small-sized (~2 nm) surface pores by laser ablation of CuO powder in Na2S solution. The fibrous-like CuO@CuS architectures should be attributed to the laser-induced nanowelding of melted CuO nanoparticles with formed CuS nanomaterials. The subsequent ultra-rapid acid etching process during laser irradiation plays an important role in the formation of nanoporous structures. The obtained CuO@CuS nanoporous materials have been developed as unique adsorbents for removal of negatively charged dye contaminant and heavy-metal pollutant. The adsorption of 99.4% MB molecules and 99.3% Cr(IV) ions can be achieved by using CuO@CuS nano-adsorbents. Interestingly, after adsorption of MB molecules and Cr(IV) in solution, the novel CuO@CuS adsorbents can be self-aggregated/agglomerated together, and then completely deposited on the bottom of the solutions. The experimental results show that the 98.9% and 99.6% CuO@CuS nano-adsorbents can be easily deposited after adsorption of Cr(IV) ions and MB molecules, respectively. The fascinating property is originated from the strong ionic bonding between positively charged adsorbents and negatively charged adsorbates. Without any additional process for the adsorbents separation after adsorption treatment, it is of great significance for the wastewater purification.
National Natural Science Foundation of China (Nos.11575102 and 11105085); Fundamental Research Funds of Shandong University (No.2015JC007).
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