1. Introduction

It is well known that the organic compounds in underground water have serious toxicity or lethal effect on aquatic living and terrestrial life including humans and animals [1–5]. Besides metal pollution in drinking water, increasing evidence has shown that the organic contaminants discharged from electroplating, textile production, cosmetics, pharmaceuticals, etc. are the main reasons for the higher morbidities of kidney, liver, and bladder cancers, etc [3–5]. Organic contaminants, especially Methyl blue (MB) and methyl orange (MO) as the common organic contaminants have complex aromatic molecular structures, which are usually stable to light, heat or oxidizing agents, and very difficult to segregated/degraded in wastewater. Conventional wastewater treatment techniques including chemical precipitation, biological oxidation or ozonation, and photo-catalysis, have been developed to remove the dyes from water [3–6]. Because of the relative poor removal efficiency and complex procedures, most research and development have been focused on the new strategy with environment friendly adsorption, which is becoming more and more important issue to remove organic pollutions in water via simply adsorbing. For example, Das et al. reported an enhanced adsorption of the dyes with hydroxyl (-OH) groups based on iron oxide nanoparticles [4]. Most recently, the advanced nano-materials with well adsorption performance, such as transitional metal oxide (Fe, Co, and Ni oxides) nanoparticles, have been demonstrated to remove MB from wastewater [2]. However, up to date, the subsequent purification procedures are inevitable after water treatment, such as these adsorbent materials or degradation/reduction agents should be removed from the solution by centrifugal process [2] or external magnetic field [4]. These are often complicated and not suitable for practical water treatment. In practical wastewater treatment plant, as one of many challenges in the design and synthesis of nano-objects, the advanced materials require to be deposited on the bottom of pool without any centrifugal process or external magnetic field after water treatment.

Laser fabrication in liquid is characterized by high non-equilibrium processing with high temperature and pressure, which is a new green approach to the synthesis of novel metal-stable phases of material [2,7–17]. Recently, many interesting works such as electronic reconstruction of α-Ag₂WO₄ nanorods [7], fabrication of carbon micro- and nanocubes with bcc structure [9], MIT effect in silicon nano-sphere oligomers [10], the new phase formation of nanocrystals [8] and formation of Ag/AgCl hetero-structured cubes [11] have been reported by Yang et al. In the field of laser fabrication in liquid (LAL), Yang’ group has significantly contributed to the related application in nanoparticles fabrication. Herein, we design a rapid, simple and versatile route to synthesize mono-dispersed Ag₂S@Ag hybrid nano-particles directly from a bulk Ag target based on laser ablation in liquid solution containing thioacetamide (TAA) and hexadecyl trimethyl ammonium bromide (CTAB). The as-prepared Ag₂S@Ag hybrid nano-particles exhibit excellent adsorption performance for the removal of MB and MO organic contaminants from wastewater. After adsorbing MB or MO molecules the Ag₂S@Ag nano-particles will be agglomerated/aggregated together with the presence of SO₃Na functional groups and deposited on the bottom of the water. Meanwhile, a detailed discussion of the relevant mechanism is addressed. This is a breakthrough in the subsequent purification procedures after water treatment, which is of great importance for the practical application. The aim of this work is to extend a new method of synthesizing the superior adsorbent materials in adsorption of organic contaminants.

2. Experimental section

This experimental setup used laser ablation of Ag target in liquid is similar to that described in our previous studies [13–16]. In a typical experiment, a well-polished Ag metal as the target was placed on the bottom of a rotating glass dish with speed of ~350 rpm that was filled with 4.5 mm depth of liquid solution containing 0.2M TAA, 0.05M CTAB and 10 mL distilled water. The TAA will provide the sulfur sources for the fabrication of Ag₂S structure, and CTAB is used as surfactant for the mono-dispersed nano-particles. A Q-switched Nd-YAG (Yttrium Aluminum Garnet) laser (Quanta Ray, Spectra Physics) beam operating at wavelength of 1064 nm with a pulse duration of 10 ns and 10 Hz repetition rate was focused on the Ag target surface. The laser beam was focused on the target by a quartz lens with 65 mm focal length. The experiments were carried out in laser power densities range about 6 ~9 GW/cm², and the average spot size of the laser beam at the target surface was about 370 μm. After laser ablation of 30 minutes, the products were carefully washed in distilled water, and centrifuged at 10000 rpm for 10 minutes in an ultracentrifuge. The sediments were dropped on a copper mesh and dried in an oven at room temperature for observation by transmission electron microscopy (JEOL-JEM-2100F). In addition, the morphological and chemical composition investigations were measured by field emission scanning electron microscope (SEM, Hitachi, S-4800) equipped with energy-dispersive-x-ray spectroscopy (EDS). The crystallographic investigation of the products were analyzed by X-ray diffraction (XRD) patterns (Rigaku, RINT-2500VHF) using Cu Kα radiation (λ = 0.15406 nm). The detailed sample compositions were studied by X-ray photoelectron spectra (XPS) on a PHI Quantera SXM with an Al Kα = 280.00 eV excitation source. In a typical adsorption experiment, the removal of dyes molecules from wastewater was carried out by simply adding the as-prepared Ag₂S@Ag nano-particles into MB, MO and Rhodamine B (RhB) solutions, respectively. The reaction vessel is a common beaker. Because of the superior adsorption activity of Ag₂S@Ag nano-particles, it should be noted that the adsorption experiments in this work were not stirred to reach adsorption equilibrium. Moreover, all the adsorption experiments carried out in dark environment, in order to avoid the interference with visible light irradiation. At the end of adsorption, the adsorbent materials will be deposited on the bottom of the reaction vessel, and easily removed from the solution by filters (Millipore, 0.22 μm). The organic material concentrations were measured by the absorbance spectrums, which were carried out by a UV-Vis-IR spectrometer (UV-1800, Shimadzu). The Fourier transforms infrared spectrums (FTIR) of the nano-particles were measured by a UV-Vis-NIR spectrometer (370~7800 cm⁻¹, ALPHA-T, Bruker).

3. Result and discussion

After cumulative pulse laser ablation of pure Ag target in activated solution containing 0.2M TAA, 0.05M CTAB and 10 mL distilled water, the detailed structures of Ag₂S@Ag nano-particles were analyzed in Fig. 1. The typical low-magnification TEM and SEM images of the nano-particles in Fig. 1(a) and 1(b) clearly show that numerous quasi-cubical and spherical nano-particles with sizes varying from about 35 nm to 55 nm are likely to be fabricated one by one separately, and are almost not hinge joined. With the liquid solution containing CTAB, the surfactant acts as a dispersing and stabilizing agent, which plays a critical role in the formation of mono-dispersed nanostructures. The HRTEM image in Fig. 1(c) provides a representative structural detail of the nano-particles. The typical area marked by yellow lines with a periodicity corresponding to a d-spacing of 0.283 nm could be indexed to the Ag₂S (−112) plane structure. Meanwhile, the out region marked by cyan lines with a d-spacing of 0.238 nm should be indexed with reference to (111) plane in the Ag structure. The result of EDS in Fig. 1(b) demonstrates that the nano-particles are composed of Ag and S elements. The ratio of Ag and S is calculated about 3:1. The element ratio is completely fit to stoichiometry of Ag₂S:Ag~1:1, supporting the formation of Ag₂S@Ag hybrid nano-particles. In addition, the crystallographic investigation of Ag₂S@Ag hybrid nano-particles was established by X-ray diffraction (XRD) in Fig. 1(d). The XRD patterns clearly reveal that a series of monoclinic α-Ag₂S diffraction peaks (JCPDS no.14-0072) and four typical peaks from Ag structures at higher angles (JCPDS no.65-8428) were indeed detected.

 

Fig. 1 (a-b) The representative low-magnification and enlarged TEM images of the nano-particles by laser ablation of Ag target in TAA solution. (c) The SEM morphology of the products, and the inset shows the result of the EDS. (d) XRD pattern of the as-prepared nano-particles.

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In order to further verify the hybrid nano-particles composition, we examined XPS images of the nano-structures. The Ag and S as well as C and O impurities were detected in the XPS spectrum, as shown in Fig. 2(a). The oxygen impurity in the spectra was attributed to the surface oxide after keeping them in an oven. In the XPS spectra, the binding energies were calibrated by referencing the C1s peak (284.8 eV) to reduce the sample charge effect [18–22]. As shown in Fig. 2(b), the spectra of C1s was composed of two peaks located at 284.7 and 285.6 eV, which could be attributed to the -C-C and -C-N structures, respectively. The two structures exist in the TAA molecule, which suggests that some inevitable TAA molecules adsorbed on the surface of the as-synthesized hybrid nano-particles. The doublet feature of Ag3d_5/2 and Ag3d_3/2 in Fig. 2(c) is attributed to the spin-orbit separation. Moreover, the spin-orbit separation is applied to the double feature of S2p_3/2 and S2p_1/2 in Fig. 2(d) with peaks at 160.7 and 161.9 eV, respectively. The doublet features in Fig. 2(c-d) are consistent with previous report [18]. Based on the Ag and S peaks areas, the actual Ag/S atomic ratio on the surfaces of hybrid nano-particles is about 3.2:1, which is slightly higher than the EDS result ~3:1 in Fig. 1(c). It is reasonable to deduce that the outside regions of hybrid nano-particles are composed of abundant Ag species. The above results confirm that there is regularity of orientation between the Ag₂S and Ag domains in hybrid nano-particles. The possible growth of Ag₂S@Ag hybrid nano-particles will be described in the following section. At the moment of laser beam arriving at Ag target, rapid boiling and vaporization of Ag element will take place, resulting in the formation of explosive Ag plasma with much high temperature (~thousands Celsius) on the irradiated spot [16]. The nucleation of Ag and S (from TAA hydrolyzing reactions) will take place in the early stage of rapid condensation of the Ag plasma, since the hot explosive Ag plasma in active solution should significantly improve the surrounded TAA hydrolyzing degrees. The nucleation will sharply terminate due to expiration of the pulse and exhaustive expansion of the Ag vapor. The Ag shell structure will be generated through laser sintering of the assembled particles. In the following subsequent laser beam penetration, the exposed surface of initial Ag₂S nano-crystals will be local-melted in solution. In this way, compared to Ag element, the S element is more easily to be removed from original particle, resulting in more Ag structure left on the surface and then the formation of Ag₂S@Ag hybrid nano-particles.

 

Fig. 2 XPS spectra of (a) over structure, (b) C1s, (c) Ag3d and (d) S2p of Ag₂S@Ag hybrid nano-particles.

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Following this mechanism, a higher laser density should result in more S elements deviating from original hybrid nano-particles, and then lead to a higher Ag content in the final materials. The variation of laser density will result in different Ag content in the hybrid nano-particles. To verify this assumption, the average Ag content with an error bar of 5% as function of laser density is displayed in Fig. 3. The Ag content in the hybrid Ag₂S@Ag nano-material increases linearly with an increase of laser density (4~9 GW/cm²). The insets (left-right) show the typical HRTEM images of Ag₂S/Ag hybrid nano-particles that the relative molar ratios of Ag to Ag₂S are 0.4:1, 1:1 and 1.4:1, respectively. According to the HRTEM images, a higher laser density (~9 GW/cm²) will also lead to local amorphous Ag nano-structure formed in hybrid Ag₂S@Ag nano-particles. By using higher-power laser irradiation, the nano-materials in the liquid are always in a chaotic shape, which is also described in previous work [7]. The recrystallization process of Ag will occur rapidly and defects can be frozen. The amorphous Ag structure should be introduced into the crystal lattice. Yang et al. and Zeng et al. illustrated that the origination of defects in the laser ablation process should be related to the high non-thermodynamic equilibrium conditions, rapid recrystallization rate and improved reactant environment, which is agree with in our case [7,17].

 

Fig. 3 The average Ag content with an error bar of 5% versus the laser density (4~9 GW/cm²). The insets (from left to right) show the HRTEM images of the Ag₂S@Ag nano-materials that the relative ratios of Ag to Ag₂S are 0.4:1, 1:1 and 1.4:1, respectively.

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In the presence of 7.58 mg as-prepared Ag₂S@Ag hybrid nano-particles, the unique adsorption behaviors of MB molecules in wastewater (60 mg/L, 10 mL) are illustrated in Fig. 4(a). As shown in Fig. 4(a), the main absorption band of MB molecules at about 588 nm drastically decreases with the reaction time prolonging from 0 to 5 min. In detail, the absorption peak of MB molecules sharply drops from 2.474 to 0.004 a.u (the adsorption of nearly 99.83% MB molecules) as the reaction time increases to 5 min. The UV-visible absorption spectra of 7.58 mg Ag₂S@Ag nano-materials solution is shown in Fig. 4(b). After adding MB molecules, the direct photographs of the solutions in the Fig. 4(b) clearly show that the Ag₂S@Ag nano-materials adsorbed MB molecules became aggregated/agglomerated together and developed into numerous larger-sized clusters, then deposited on the bottom of the solution. Meanwhile, the aggregation and deposition lead a significant change in the solution color from blue to colorless. Without any centrifugal process, the adsorbent can be easily removed from the solution by filters. After filtration process, the UV-visible absorption spectra of the solution is illustrated in Fig. 4(b). As shown in Fig. 4(b), the absorption peak of Ag₂S@Ag nano-materials sharply dropped from about 3.97 to 0.001 a.u after removing the adsorbent from the solution by filters. After MB adsorption reaction, it is reasonable to deduce that the aggregated Ag₂S@Ag nano-materials (99.999%) can be removed from the solution by filters. Moreover, we further reveal the variation of MB adsorption capacity versus the ratio of Ag to Ag₂S in Ag₂S@Ag hybrid nano-particles, and the results are illustrated in Fig. 4(c). The chart clearly shows that the removal degree of MB increases from about 30 to 60 mg/L as the Ag fraction rises from 0.4/1 to 1/1, and then decreases rapidly at higher Ag content. It is obvious that the Ag content in the adsorbent Ag₂S@Ag nano-particles plays a significant role in the active sites of nano-adsorbents during the MB adsorption reaction. The optimum Ag fraction in hybrid nanoparticles (Ag:Ag₂S) is about 1:1 in this paper, since the adsorption capacity of MB decreases at higher Ag content. The relevant mechanism of the unique MB adsorption features will be addressed in the following section.

 

Fig. 4 (a) The reduction performance of the remove of MB molecules from the solution (60 mg/L, 10 mL) in the presence of 7.58 mg as-prepared Ag₂S@Ag hybrid nano-particles. (b) The UV-visible absorption spectra of the solution in the presence of 7.58 mg Ag₂S@Ag nano-materials and the MB solution after removing the adsorbent from the solution by filters. The inset shows the direct photographs of gradual color change of the solution with reaction time. (c) The curve of the MB adsorption capacity versus the ratio of Ag to Ag₂S in Ag₂S@Ag hybrid nano-particles. The dosage of the adsorbents in each MB solution is about 7.58 mg, and the reduction time is 5 min in each experiment.

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Then, the morphologies of the absorbents after adsorbing MB molecules are examined by transmission electron microscopy, as shown in Fig. 5(a-b). Compared with the mono-dispersed Ag₂S@Ag nano-structures in Fig. 1(a), the low-magnification TEM image in Fig. 5(a) shows that the adsorbents are accreted with each other and agglomerated/aggregated together. Moreover, the HRTEM image in Fig. 4(b) illustrates that the outside-surfaces of the adsorbents turned into amorphous structure without clear lattices fringes.

 

Fig. 5 (a-b) The low-magnification and enlarged TEM images of the Ag₂S@Ag absorbents after MB absorption. (c-d) XRD and FTIR spectrums of the Ag₂S@Ag nano-particles before and after adsorbing MB molecules.

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The corresponding XRD patterns reveal the detailed structural evolution of the Ag₂S@Ag adsorbents before and after MB adsorption, as shown in Fig. 5(c). After MB adsorption, the adsorbents also contain a series of diffraction peaks from monoclinic α-Ag₂S structure, as the overlap signals shown in Fig. 5(c). Meanwhile, the four typical peaks (111), (200), (220) and (311) from Ag structures completely disappear in the adsorbents after MB adsorption. Before and after MB solution treatment, the obvious differences in the surfaces of the adsorbents imply that the Ag crystallites in Ag₂S@Ag nano-structures should be confirmed as the active sites of nano-adsorbents during the adsorption reaction. Because the experiment of the removal of MB from solution by Ag₂S@Ag nanoparticles was carried out in dark environment, the photo-catalytic-reduction of MB molecules can be ignored in this paper. To get more information of the MB adsorption by Ag₂S@Ag nano-structures, then we compare the surface stages of the adsorbents before and after adsorbing MB by Fourier Transform Infrared Spectroscopy (FTIR), as shown in Fig. 5(d). The FTIR spectra of the starting Ag₂S@Ag nano-materials has not obvious absorption peak, except some hydroxyl (OH) groups at about 3465 nm, which is consistent with previous studies [2]. After adsorbing MB molecules, several improved FTIR spectra originated from organic material can be detected in Fig. 5(d). Clearly, the peaks located at 2925 and 2857 cm⁻¹ can be attributed to the –CH₃ and –CH₂- functional groups, and peaks located at 1575, 1495 and 1449 cm⁻¹ should be originated from the adsorption of the aromatic ring vibrations. Moreover, the C-N stretching vibration at 1337 cm⁻¹ and the –SO₃^- functional groups at 1169, 1121 and 1035 cm⁻¹ have been found in the spectra of the adsorbent after adsorbing MB molecules. The corresponding result in Fig. 5(d) is the best evidence that chemical bonds between Ag₂S@Ag nanoparticles and MB molecules are indeed formed after adsorption process. Inspired by previous works [2,3,23], we deduce that the possible mechanism of MB adsorption should be considered as the strong electron-static interaction between the positive active site of the Ag₂S@Ag nanoparticles and the negative charge of MB molecules. After adsorbing MB, the main reason for the formation of agglomerated absorbents is based on the ionic bonding between the positively charged region of Ag₂S@Ag nanoparticles and negatively charged functional groups of MB (–SO₃^-). In this way, different amount of –SO₃^- functional groups in dyes will influence the adsorption performance by using same Ag₂S@Ag absorbent materials.

To verify these deductions, thus, we compare the adsorption performances of the as-prepared Ag₂S@Ag hybrid nano-particles for MB, MO and Rhodamine B (RhB) dyes solution. MB, MO and RhB molecules have three, one and zero SO₃Na functional groups, respectively. As shown in Fig. 6(a), the peak intensity of MO molecules at about 464 nm decreases rapidly from 2.1 to 0.211 a.u with reaction time prolonging from 0 to 15 min. As for the RhB solution in Fig. 6(b), the main absorption at about 553 nm slightly decreases from 2.2 to 2.1 a.u after 15 min, implying relatively poor adsorption performance. Figure. 6(c) shows the reduction time dependence of the relative concentration C/C₀ of MB, MO and RhB molecules. Where C and C₀ are the concentration of dyes after adsorption and initial solutions, respectively. Compared with the adsorption performance for MB, the results of MO and RhB indicate that the adsorption performance is highly related and proportional to the amount of negatively charged functional groups in dyes solution. Therefore, these evidences indicate that the agglomerated Ag₂S@Ag nano-structures after adsorbing MB or MO molecules should be considered as chemical bonds between positive active sites of the adsorbent and the negative charges of -SO₃^- functional groups. The schematic agglomeration process of the Ag₂S@Ag nano-particles adsorbed with MB is provided in Fig. 6(d). As illustrated in the previous works [2,3,23], the Ag₂S nano-structures enable the electron distribution of Ag crystallite to be polarized, resulting in the negative and positive charge regions that are far from and close to the Ag₂S@Ag hybrid nano-particles. Moreover, during the fabrication of Ag₂S@Ag nanoparticles, the non-equilibrium condition created by subsequent laser ablation can also lead to various defects, resulting in the present of abundant disorderly arranged Ag species on the surface of nano-particles. After laser fabrication, the rapid quenching process enable the disorder degree of Ag species to become more and more intensive, which makes these species at a highly excited state [14]. The unique excited Ag species combined with the positive charge regions enable the Ag₂S@Ag hybrid nano-particles to have much more active sites as adsorption sites. As shown in Fig. 6(d), the enhanced adsorption sites can hang more negatively charged functional groups of MB (three –SO₃^-) and then enable MB dye molecules to be adsorbed on the surface of the hybrid nano-particles. Meanwhile, the excess amount of Ag crystallite in the hybrid nanoparticles will hinder polarized Ag species to emerge on the outsider of nano-particles.

 

Fig. 6 (a-b) The adsorption performance of the as-prepared Ag₂S@Ag hybrid nano-particles for MO and Rhodamine B (RhB) solution, respectively. (c) The reduction-time dependence of the relative concentration C/C₀ of MB, MO and RhB. (d) The schematic agglomeration process of the adsorbent Ag₂S@Ag adsorbed with MB molecules.

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It will decrease the number of positively charged sites on the surface of the hybrid nano-particles, and reduce the attraction between adsorbent surface and the dye. For this reason, it is reasonable to believe that the removal degree of MB molecules decreases as the further increase of Ag fraction beyond the optimum content (Ag:Ag₂S~1:1) in this paper. It is the main reason for the reduced MB adsorption performance at higher Ag content, as observed in Fig. 4(b). On the other hand, it should be noted that the dye molecules are not easily desorbed from the agglomerated adsorbents by standard heat or thermal treatment, owing to the strong ionic bounds between the adsorbents and dye molecules. Considering mature acid and alkali treatment in chemical field, our further studies should investigate desorption of dye molecules from the Ag₂S@Ag adsorbents, which will be very suitable for recycling in the future. In summary, based on the novel Ag₂S@Ag hybrid nano-particles fabricated by laser fabrication in liquid, the agglomerated and deposited absorbents after adsorbing MB or MO molecules are very beneficial to the practical wastewater treatment plant.

4. Conclusions

In conclusion, we have reported the successful synthesis of mono-dispersed Ag₂S@Ag hybrid nano-particles by pulses laser ablation of Ag metal in activated liquid containing TAA, CTAB and distilled water. The hybrid nano-particles growth process is highly related to the subsequent laser irradiation, which will result in the formation of local-melted region on the primal Ag₂S nanoparticles, then enable S element on the surface to be removed from original structure. The as-prepared Ag₂S@Ag nano-particles exhibit excellent adsorption performance for the removal of MB and MO from wastewater. Interestingly, the novel adsorbents will be accreted with each other and aggregated/agglomerated together, then deposited on the bottom of the solution, because of the strong chemical bands between positive active sites of the adsorbents and the negative charges of –SO₃^- functional groups in dyes. In this way, the adsorbents can be easily removed from the solution by simple filters after adsorbing MB or MO molecules. This finding will be able to avoid the subsequent adsorbents purification, which promotes the application of Ag₂S@Ag nano-particles as advanced adsorbent materials for practical wastewater treatment in the further.