Beneficial metal ion insertion into dandelion-like MnS with enhanced catalytic performance and genetic morphology

Abstract

Large-scale novel hierarchical dandelion-like MnS was successfully synthesized with manganese complex as a template under mild reaction conditions. A mixture of ethylenediamine and ethylene glycol was used as a solvent. Large-scale manganese complexes were obtained via a one-step reaction; the synthesis was very simple, and the raw materials were inexpensive. The as-prepared MnS was used as a template. The introduction of beneficial metals facilitated the catalytic performance of the as-obtained multiple sulphides. Meanwhile, the genetic morphology between the as-prepared MnS and the multiple sulphides was realized via cation exchange. The composition of the products could be adjusted through cation exchange at room temperature; meanwhile, the performance of the products was improved by a large margin without changing the morphology. The as-prepared products showed highly efficient catalytic properties in degrading dye-containing solutions, such as methylene blue and rhodamine B. This result indicated that the performance of the products could be improved by introducing beneficial metals without changing the morphology.

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Introduction

Industrial wastewater is an environmental pollutant that is predicted to become a major threat to hydrosphere ecosystems, biodiversity, and humans.^1–3 Dye wastewater accounts for a large proportion of industrial wastewater,^4–6 which contains recalcitrant toxic substances and threatens aquatic living organisms.⁷ To degrade toxic substances in dye wastewater, various physical, chemical, and biological techniques have been developed,^8–10 such as adsorption, ozonation, and electrochemical and photochemical degradation.^11–13 However, most of these techniques have limited practical application, high operation costs, and low efficiency. In the H₂O₂ catalytic oxidation method, transition metal sulphides, which function as catalysts, release highly reactive hydroxyl radicals, which facilitate the degradation of dye wastewater.^14–16 Therefore, transition metal sulphides have a very important function in the catalytic process.

The micro/nanostructure materials have attracted increasing attention because of their magnetic, electrochemical, lithium-ion battery, and catalytic properties. These materials also have a large specific surface area and numerous active sites.^17–26 For example, Duan and co-workers²⁷ synthesized a manganese dioxide nanostructure and examined its electrochemical properties. Senapati and co-workers²⁸ reported a magnetic Ni–Ag core–shell nanostructure from a prickly Ni nanowire precursor and evaluated its catalytic and antibacterial activities. Wang et al.²⁹ synthesized novel tunable highly porous CuO nanorods and fabricated high-rate lithium battery anodes with a long cycle life and a high reversible capacity. Meir and co-workers³⁰ reported the chemical, optical, and catalytic properties of noble metal (Pt, Pd, Ag, Au)–Cu₂O core–shell nanostructures grown via a general approach. However, the synthesis of large-scale nanostructured materials remains a challenge because of the complicated operation and limitations in industrial applications. In this study, a dandelion-like MnS nanostructure was synthesized and used to degrade dye wastewater at room temperature. The dandelion-like MnS was prepared with manganese complex as a precursor under mild reaction conditions. Large-scale chemical materials, which are inexpensive and easy to obtain, were the raw materials for the synthesis of the manganese complex. Large-scale complexes were obtained via a one-step reaction, and the synthesis was very simple.

To improve the catalytic performance of the as-prepared MnS, this study introduced metal ions of group IB to the as-prepared MnS via the ion exchange method. Copper ion, a member of group IB, was reported to be an excellent candidate as a catalyst, and has shown significant performance in recent years.^31–34 For example, Wang and co-workers³⁵ reported the preparation of various kinds of copper sulphides via a facile approach and their enhanced catalytic activities. Moreover, Yang and co-workers³⁶ reported Pt–CuS heterodimers and their selective catalytic activity. The present study explored a new viewpoint to introduce the group IB metal ions into the as-prepared MnS. The experimental results showed that the method made an important contribution to improve catalytic performance. In the IB group, Cu²⁺ and Ag⁺ were chosen as the subjects of this study because the gold ion mainly exists in the trivalent state, which was not helpful in the cation exchange reaction. On the other hand, gold ions are expensive and uneconomical for widespread application. Thus, in our experiments, Cu²⁺ and Ag⁺ were introduced to the as-prepared MnS by cation exchange. To deal with the complicated experimental operation including modulation of reaction conditions such as concentration, temperature, and the long reaction time, the cation exchange reaction was adopted because of its ability to easily transform nanostructured materials into other forms.^37–45 Moreover, the cation exchange reaction between MnS and Cu²⁺ was conducted at room temperature, which was helpful in retaining the morphology of the as-prepared products. Meanwhile, a similar phenomenon occurred between the as-prepared MnS and Ag⁺. The slow introduction of the metal ions caused by the mild reaction was helpful to induce a gentle reaction on the original unit structure.

In this paper, we chose MnS as the template to synthesize large-scale dandelion-like MnS nanostructures via the solvothermal method under mild conditions. Mn_xCu_1−xS and Mn_yAg_2(1−y)S were then fabricated via cation exchange by introducing beneficial metals to MnS. The cation exchange reaction occurred at room temperature by controlling the reaction time and the reactant concentration, which was helpful to the genetic morphology between the as-prepared MnS and Mn_xCu_1−xS and Mn_yAg_2(1−y)S. Mn_xCu_1−xS and Mn_yAg_2(1−y)S combined the properties of both materials and maximized their advantages. On the one hand, the as-prepared products provided a specific surface area and active site when used as a catalyst, which showed high efficiency in degrading methylene blue (MB) and rhodamine B (RB) and were capable of catalyzing H₂O₂ and degrading organic dyes. On the other hand, the degradation efficiency of Mn_xCu_1−xS and Mn_yAg_2(1−y)S was far higher than that of the as-prepared MnS. In this study, the catalytic performance of the products was improved by regulating the composition without changing the morphology.

Experimental

Synthesis and processing

The hierarchical dandelion-like MnS (sample 2) was prepared by a typical solvothermal method with manganese complexes as template. In a typical synthesis, Mn(C₂H₃O₂)₂·4H₂O (5 mmol) and toluene-4-sulfonic acid sodium salt (10 mmol) were put into a 50 mL beaker with 20 mL distilled water. Then, 1,10-phenanthroline (5 mmol) and 15 mL absolute ethylenediamine were added to this solution. The mixed solution was heated to almost boiling. The manganese complex (Sample 1) was obtained after the crystallization and filtration of the solution. Then, the manganese complex (1 mmol) and thiourea (0.5 mmol) were dissolved in 11 mL glycol and 5 mL absolute ethylenediamine, respectively. The mixed solution was stirred for 3 h in a 20 mL Teflon-lined autoclave. Afterwards, the autoclave was heated and maintained at 160 °C for 24 h in an electric oven. After the heat treatment, the autoclave was allowed to cool down to room temperature. Then the as-prepared MnS was washed several times with distilled water and alcohol. The product was dried in an oven at 60 °C for 8 h. Cation exchange was conducted between the as-prepared MnS and Cu(NO₃)₂·3H₂O to synthesize Mn_0.77Cu_0.23S (sample 3), Mn_0.53Cu_0.47S (sample 4), Mn_0.40Cu_0.60S (sample 5), Mn_0.33Cu_0.67S (sample 6), Mn_0.20Cu_0.80S (sample 7), Mn_0.16Cu_0.84S (sample 8), Mn_0.10Cu_0.90S (sample 9), and Mn_0.05Cu_0.95S (sample 10) with a MnS to Cu(NO₃)₂·3H₂O ratio of 1 : 2 for 2 min, 5 min, 10 min, 20 min, 30 min, 40 min, 50 min and 1 h, respectively. Furthermore, using the as-prepared MnS and AgNO₃ in a ratio of 1 : 1, Mn_0.80Ag_0.40S (sample 11), Mn_0.73Ag_0.54S (sample 12), Mn_0.58Ag_0.82S (sample 13), Mn_0.53Ag_0.94S (sample 14), Mn_0.52Ag_0.96S (sample 15) and Mn_0.48Ag_1.04S (sample 16) were synthesized under different reaction times (2 min, 4 min, 5 min, 10 min, 20 min and 30 min) at room temperature, respectively. The post-treatment of these products was the same as that for sample 2.

Characterization

X-ray diffraction (XRD) patterns were obtained using a Bruker D8 ADVANCE X-ray powder diffractometer with Ni-filtered Cu-Kα radiation at 40 kV and 20 mA and a scanning step of 0.2 s⁻¹. All XRD measurements were conducted within the range of 20° ≤ 2θ ≤ 80°. The morphologies and sizes of as-prepared products were characterized with a Zeiss EVO LS-15 scanning electron microscope equipped with an energy dispersive X-ray spectroscopy (EDS) system. The structure of the as-prepared MnS was recorded with a JEOL JEM-2010 transmission electron microscope (TEM). The ultraviolet-visible (UV-vis) spectra were recorded on a PerkinElmer Lambda 35 spectrophotometer in the 200 nm to 1100 nm wavelength range.

Catalytic activity test

MB and RB were selected as the model chemicals to evaluate the activity of the catalysts. In this work, 0.02 g of as-prepared products as catalyst were combined with MB or RB dye solution (30 mg L⁻¹, 30 mL), and the reaction was initiated with 10 mL H₂O₂. The dissolved solution was stirred under irradiation from a UV lamp (λ = 253.7 nm) at room temperature. The MB or RB concentration was determined with a UV-vis spectrophotometer (PerkinElmer Lambda 35). At each sampling time (about 2 min), 3 mL solvent was removed from the MB or RB solution and was immediately used for analysis. After the reaction, the catalyst was removed and washed with distilled water for further reuse.

Results and discussion

Large-scale hierarchical MnS was synthesized with manganese complex as a precursor in a mixed solvent of ethylene glycol and ethylenediamine for 24 h under mild reaction conditions. In general, a large quantity of metal complex is difficult to synthesize. In this study, toluene-4-sulfonic acid sodium salt and 10-phenanthroline were used to synthesize the manganese complex, as these are common, low-cost chemicals. Large-scale manganese complexes were obtained via a one-step reaction, and the synthesis was very simple. Manganese complex was chosen as a precursor to synthesize large-scale uniform MnS because it has several advantages over other templates. Compared with the metal salt, the coordinating ability of the bonds in the metal complex delays the release rate of the central metal ion. In addition, the metal complex has a good coordination ability and a stable structure,⁴⁶ and it facilitated the control of the morphology and size of the as-prepared MnS. Moreover, the auxiliary ligand in the complex functioned as a reaction-active agent or auxiliary agent after dissociation from the complex. In previous reports, well-shaped CuS was successfully synthesized using the complex.^47,48

Fig. 1 shows the typical XRD pattern of as-prepared needle-like MnS. The pattern indicates that all of the primary diffraction peaks were in good agreement with the standard data for MnS (JCPDS no. 06-0518). This result indicates that the as-prepared MnS had a pure phase. The cell parameters of the products were a = 5.224 Å and c = 5.224 Å. The diffraction peaks located at 29.60°, 34.30°, 49.30°, 58.56°, 61.39°, and 72.28° corresponded to the directions of (111), (200), (220), (311), (222), and (400), respectively. The strongest direction was the 200 peak, which indicates the preferential growth process. No other phases or impurities were observed in the spectra, which reveals the high-phase purity of the as-prepared MnS. Therefore, uniform dandelion-like MnS was obtained via a simple approach under mild conditions.

Fig. 1 XRD pattern of as-prepared MnS.

Fig. 2 illustrates the morphology of the as-prepared MnS. The low-magnification SEM image of MnS is shown in Fig. 2a. The as-prepared MnS contained large-scale isolated dandelion-like MnS microspheres, which had uniform sizes, and the diameters of the individual needle-like microspheres ranged from 2 μm to 3 μm. The SEM image of large-scale MnS is shown in Fig. S1.† In Fig. 2b, each MnS needle-like microsphere included several 3D needle hierarchical structures, which had dandelion-like structures. The thickness of the nano-stick was approximately 20 nm to 30 nm, as shown in Fig. 2c. The as-prepared dandelion-like MnS composed of nano-sticks with a narrow size distribution is believed to improve the catalytic performance because of the large contact area. As presented in Fig. 2d, the high-resolution transmission electron microscopy image was recorded to examine further the crystallographic features of sample 2, which shows clear lattice fringes with a d-spacing of 0.462 nm. These features could be indexed to the (220) plane of the MnS crystal (JCPDS no. 06-0518).

Fig. 2 Characterization of MnS: (a) SEM image of MnS in overall view; (b), (c), and (d) TEM micrographs.

Synthesizing specific material with a regular morphology is known to be difficult. Unlike other synthetic methods, which were complicated, time-consuming, and costly,^49–51 by contrast, we synthesized dandelion-like MnS with a regular micro/nanostructure via a simple method. Given the positive effects of the morphology on the product properties, we introduced metal ions of group IB into MnS via cation exchange without changing the morphology, and the method was confirmed to have a positive effect on catalytic performance.^52,53 The stable trivalent state of gold ion was considered but it is uneconomical for large-scale application. In this study, Cu²⁺ and Ag⁺ were chosen for cation exchange with the as-prepared MnS. The experimental results proved that the introduction of beneficial metals regulated the composition of the products without changing the morphology and also improved the catalytic performance of the products. To introduce Cu²⁺ and Ag⁺ into the as-prepared MnS, a series of experiments was designed. Cu²⁺ was selected for ion exchange with the as-obtained MnS. Cu(NO₃)₂·3H₂O was selected because NO₃⁻ decomposes into a gas after heating without introducing any impurities into the reaction.

By investigating the reaction time, reaction temperature, and reactant concentration, we obtained the optimum cation exchange conditions. In the case of excess Cu²⁺, a series of products was synthesized by adjusting the reaction time at room temperature. Sample 10 (Mn_0.05Cu_0.95S) was obtained with an MnS : Cu²⁺ ratio of 1 : 2 at room temperature for 1 h. The catalytic performance of sample 10 was significantly improved compared with that of the as-prepared MnS. Fig. 3 presents an XRD pattern comparison of samples 2 and 10. The XRD pattern of sample 10 indicated that an ion-exchange reaction had occurred. The diffraction peaks marked with matched well with the CuS standard card JCPDS no. 01-1208, whereas the peaks marked with corresponded to the MnS standard card JCPDS no. 06-0518.

Fig. 3 XRD pattern comparison of sample 2 and sample 10.

Samples 3, 4, 5, 6, 7, 8, 9, and 10 were obtained with an MnS to Cu(NO₃)₂·3H₂O ratio of 1 : 2 at room temperature for 2 min, 5 min, 10 min, 20 min, 30 min, 40 min, 50 min, and 1 h, respectively. Fig. S2† presents a comparison of the XRD patterns. As shown in the graph, the diffraction peaks corresponding to the CuS signals were stronger, whereas those corresponding to the MnS signals were weaker with increasing time. This result indicates that the cation exchange reaction occurred to a greater degree with increasing time. This phenomenon may be attributed to the lower solubility product constant of CuS than MnS, which made the cation exchange reaction smooth. The increase in reaction time resulted in further enhanced ion-exchange reaction and increased intensity of the peaks corresponding to CuS.

In this study, a series of ion exchange reactions were performed to determine the reaction conditions and realize better morphology transfer. To study the effect of reaction time on a reaction, a series of experiments were conducted with an MnS to Cu(NO₃)₂·3H₂O ratio of 1 : 2 at room temperature. We obtained Mn_0.77Cu_0.23S (sample 3), Mn_0.53Cu_0.47S (sample 4), Mn_0.40Cu_0.60S (sample 5), Mn_0.33Cu_0.67S (sample 6), Mn_0.20Cu_0.80S (sample 7), Mn_0.16Cu_0.84S (sample 8), Mn_0.10Cu_0.90S (sample 9), and Mn_0.05Cu_0.95S (sample 10) at reaction times of 2 min, 5 min, 10 min, 20 min, 30 min, 40 min, 50 min, and 1 h, respectively. Fig. 4 shows a schematic diagram of the morphology transfer of the cation exchange reaction. As shown in the graphic, the morphology of sample 10 remained unchanged compared with that of MnS. The SEM image of sample 10 is presented in Fig. 5b. No significant difference was found between the morphology of samples 10 (Fig. 5b) and sample 2, the as-prepared MnS (Fig. 5a). Sample 10 maintained the morphology of MnS to a great extent after the cation exchange reaction. SEM images of samples 3, 4, 5, 6, 7, 8, 9, and 10 are presented in Fig. S3,† and show that the morphology of the as-prepared samples is similar to that of MnS, which may be attributed to the complete reaction between the as-prepared MnS and Cu²⁺. Through the analysis of the cation exchange reaction conditions, this phenomenon may be due to the cation exchange reaction being conducted at room temperature, and the mild reaction condition was helpful in the genetic morphology. Therefore, the morphologies of samples 3, 4, 5, 6, 7, 8, 9, and 10 were almost the same as MnS, and successful morphology transfer could be achieved under appropriate reaction conditions.

Fig. 4 Schematic diagram of the morphology transfer between the as-prepared MnS (top center) and sample 10 (lower left) and 16 (lower right). (a) SEM image of as-prepared MnS; (b) SEM image of sample 10; (c) SEM image of sample 16.

Fig. 5 (a) SEM image of sample 2; (b) SEM image of sample 10; (c) EDS image of sample 10 and EDS surface scanning images of the sphere section of sample 10, (d) Mn and Cu content changes of samples 3–10.

Fig. 5c shows the EDS image of sample 10. EDS surface scanning was performed to verify the elemental distribution in sample 10. The inset in Fig. 5c shows the distribution of Mn, Cu, and S atoms on the surface of sample 10, which are shown in red, green, and blue, respectively. As shown in the EDS surface scanning images, Mn, Cu, and S atoms covered the surface of sample 10. To characterize the extent of change from the cation exchange reaction more precisely from EDX data analysis, the changes in Mn and Cu content in samples 3, 4, 5, 6, 7, 8, 9, and 10 are shown in Fig. 5d. The increase in reaction time resulted in a decrease in Mn content and an increase in Cu content. At a MnS to Cu²⁺ ratio of 1 : 2, the Cu content increased to 0.23, 0.47, 0.60, 0.67, 0.80, 0.84, 0.90, and 0.95 when the reaction time was 2 min, 5 min, 10 min, 20 min, 30 min, 40 min, 50 min, and 1 h, respectively. Based on the analysis of Cu content in Mn_xCu_1−xS, at reaction times of 2, 5 and 10 min, the Cu content increased to a relatively large extent, which proved that the cation exchange reaction was violent in the first 10 min. This result is due to the small solubility constant of CuS that impelled the cation exchange reaction more quickly at the beginning, and led to the rapid increase in Cu content of the product. When the reaction time was increased, the Cu content in Mn_xCu_1−xS increased to 0.67, 0.80, 0.84, 0.90, and 0.95. Although the Cu content increased, the incremental extent was smaller compared with those in samples 3, 4, and 5. This finding may be attributed to the decreasing reactant concentration that led to a decrease in reaction rate, and the increase in Cu content was not as noticeable at longer reaction times.

Based on the aforementioned discussion, a series of Mn_xCu_1−xS were synthesized by controlling the reaction time. The SEM images of as-prepared products were characterized, and the morphology of well-shaped dandelion-like MnS was transferred to the follow-up products after the cation exchange reaction. Meanwhile, the XRD patterns and EDX data proved the occurrence of the cation exchange reaction.

The good results from the cation exchange reaction between the as-prepared MnS and Cu²⁺ were used to investigate the compositional adjustment of the products by changing the center metal ions. Considering that Ag⁺ and Cu²⁺ belong to the same subgroup in the periodic table of elements and have similar properties, another series of experiments were conducted using the as-prepared MnS and Ag⁺. AgNO₃ was selected because NO₃⁻ does not introduce any impurities into the reaction. On the basis of the aforementioned conditions, samples 11, 12, 13, 14, 15, and 16 were synthesized by reaction of the as-prepared MnS and AgNO₃ aqueous solution at room temperature for 2, 4, 5, 10, 20, and 30 min, respectively. Sample 16 was synthesized with an MnS to Ag⁺ ratio of 1 : 1 at room temperature for 30 min.

The XRD pattern of sample 16 is shown in Fig. 6. As shown in the graph, the diffraction peaks of the XRD pattern of the as-prepared sample 16 marked with matched well with the Ag₂S standard card JCPDS no. 14-0072, which confirmed that the ion-exchange reaction had occurred. The diffraction peaks marked with corresponded to the MnS standard card JCPDS no. 06-0518. In addition, the diffraction peaks of the XRD pattern marked with matched with the Ag₂O standard card JCPDS no. 76-1393. The cation exchange reaction occurred with distilled water as the solvent, and AgOH existed at the beginning of the reaction and decomposed into Ag₂O. Compared with the diffraction peaks of Ag₂S, those of Ag₂O were fewer and weaker. Fig. S4† shows the XRD patterns of samples 11, 12, 13, 14, and 15. As presented in the graph, the increased reaction time weakened the diffraction peaks of Ag₂O and gradually strengthened the diffraction peaks of Ag₂S. This result was attributed to the lower solubility constant of Ag₂S than Ag₂O, which made the exchange reaction beneficial to Ag₂S formation. Moreover, the diffraction peaks corresponding to Ag₂S signals were stronger with increasing time, which indicates that the ion-exchange reaction occurred to a greater extent with increasing reaction time.

Fig. 6 XRD pattern of as-prepared sample 2 and 16.

The SEM images of MnS and sample 16 are shown in Fig. 7a and b, respectively. Sample 16 retained the dandelion-like structure of MnS. Fig. 4 shows a schematic diagram of the genetic morphology of the cation exchange reaction. As shown in the graph, the morphology of the as-prepared sample 16 is similar to that of MnS (Fig. 7a). The difference was the presence of minute particles on sample 16 compared with as-prepared MnS. The EDS analysis results of the particles and the needle-like structure are shown in Fig. S5;† the characterization results show that their composition is the same. The small change in sample 16 may be due to the violent cation exchange reaction between MnS and Ag⁺, which hindered the morphology transfer. To investigate the effects of reaction time on the product, Mn_0.80Ag_0.40S (sample 11), Mn_0.73Ag_0.54S (sample 12), Mn_0.58Ag_0.84S (sample 13), Mn_0.53Ag_0.94S (sample 14), Mn_0.52Ag_0.96S (sample 15), and Mn_0.48Ag_1.04S (sample 16) were synthesized with an MnS to AgNO₃ ratio of 1 : 1 at room temperature for 2, 4, 5, 10, 20, and 30 min, respectively. Fig. S6† shows the SEM images of the as-prepared products. An increase in reaction time resulted in almost no change in the morphology of the samples; the basic microsphere of MnS was maintained very well. This result was mainly due to the mild reaction conditions and the appropriate concentration of reactants.

Fig. 7 (a) SEM image of sample 2; (b) SEM image of sample 16; (c) EDS image of sample 16 and EDS surface scanning images of the sphere section of sample 16, (d) Mn and Ag content changes of samples 11–16.

Fig. 7c shows the EDS image of sample 16. The elemental distribution of sample 16 was characterized by EDS surface scanning. The inset in Fig. 7c shows the distribution of Mn, Ag, and S atoms on the surface of sample 16, which are shown in red, green, and blue, respectively. The EDS surface scanning images showed that the Mn, Ag, and S atoms covered the surface of sample 16. Fig. 7d shows the EDX data analysis of Mn and Ag content changes for the as-prepared samples 11, 12, 13, 14, 15, and 16. With increasing reaction time, the Mn content decreased and the Ag content increased. In the as-prepared samples 11, 12, 13, 14, 15, and 16, the Ag content increased to 0.20, 0.27, 0.42, 0.47, 0.48, and 0.52 when the reaction time was 2, 4, 5, 10, 20, and 30 min, respectively. In the first 10 min of the cation exchange reaction, the Ag content increased to a relatively large extent, but during the last 20 min of the reaction, the incremental increase of Ag was small compared with those of samples 11, 12, and 13. In contrast to the change in Cu content in Mn_xCu_1−xS, the Ag content remained stable in the last 20 min of reaction. This may be due to Ag₂O formation that impeded the reaction between Ag⁺ and the as-prepared MnS, which made the cation exchange reaction reach equilibrium.

Based on the results and discussion, Mn_yAg_2(1−y)S maintained the morphology of MnS after the cation exchange reaction, which determined the genetic morphology from MnS to the as-prepared subsequent products. Meanwhile, the XRD patterns and EDX data proved the occurrence of the cation exchange reaction.

The as-prepared dandelion-like MnS and part of the products synthesized via cation exchange reaction were used as catalysts for the degradation of MB and RB molecules because of their special hierarchical structures. The corresponding experiments were carried out with the addition of H₂O₂. In our experiment, H₂O₂ yielded highly reactive hydroxyl radicals that could oxidize MB into smaller molecules (CO₂, H₂O, etc.). The catalytic properties of the samples were closely associated with the amount of hydroxyl radicals. The rate of H₂O₂ alone to degrade the dye solutions was very slow⁵⁴ without the assistance of a catalyst. Moreover, only the as-prepared products as catalysts do not contribute to the catalytic process. The catalytic properties of the as-prepared products were investigated based on their UV-vis absorption spectra, as shown in Fig. 8. The figure shows the efficient catalysis of H₂O₂ to release hydroxyl radicals (˙OH) and to degrade MB and RB molecules in a short time. A brief description of the reaction mechanism of samples 3, 4, 5, 6, 7, 8, 9, and 10 is described as follows.⁵⁵

M^x+ + H₂O₂ → H⁺ + MOOH^(x−1)+

MOOH^(x−1)+ → HOO⁻ + M^(x−1)+

M^(x−1)+ + H₂O₂ → M^x+ + OH⁻ + OH⁻

Fig. 8 Changes in the UV-vis spectra during the removal of (a) MB by sample 2; (b) RB by sample 2; (c) MB by sample 10; (d) RB by sample 10; (e) MB by sample 16; (f) RB by sample 16.

HO⁻ radicals can attack an organic substrate, RH, such as MB and RB, as follows:

RH + HO⁻ → R⁻ + H₂O

In this study, the as-prepared dandelion-like MnS was used as a template to synthesize the cation exchange reaction products. The as-prepared cation exchange reaction products practically retained the morphology of MnS. The needle-like hierarchical structure of the as-prepared products was postulated to improve the catalytic performance because of their greater specific surface area and more active sites. Fig. 8 displays the changes in the UV-vis spectra during removal of MB and RB molecules by samples 2, 10, and 16. The degradation curves of MB and RB by the as-prepared MnS microspheres are shown in Fig. 8a and b, respectively. Fig. 8a displays the changes in the UV-vis spectra during the removal of MB molecules by sample 2. The degree of decolorization of the aqueous MB reached 18% after 10 min and 65% after 30 min. After 60 min, the degree of decolorization reached 97%, and decolorization was maintained at a stable level after 60 min. Fig. 8b shows the changes in the UV-vis spectra during the removal of RB molecules by sample 2. The degree of decolorization of the aqueous RB reached 16% after 10 min and 68% after 40 min. After 80 min, the degree of decolorization reached 98%, which was the same as that of aqueous MB. This decolorization was stably maintained after 80 min.

The as-prepared cation exchange reaction samples were also used to examine catalytic performance. For example, the changes in the UV-vis spectra of sample 10 during the removal of MB and RB molecules by sample 10 are shown in Fig. 8c and d, respectively. Fig. 8c shows that the degree of decolorization of the aqueous RB reached 72% after 2 min, and at a catalytic time of 8 min, MB degradation was almost complete. Compared with the as-prepared MnS, the efficiency of sample 10 for MB degradation was much higher. This result may be attributed to the high content of CuS in sample 10, based on the aforementioned discussion of the catalytic reaction mechanism. CuS had a positive effect on the catalytic reaction, which greatly reduced the degradation time of MB. This finding was also reflected in the results for samples 3, 4, 5, 6, 7, 8, and 9. The changes in the UV-vis spectra during the removal of MB molecules by samples 3, 4, 5, 6, 7, 8, and 9 are shown in Fig. S7.† The increase in Cu content in the products resulted in reduced times for MB degradation. As shown in the diagram, the degradation time of MB by samples 3 and 4 was at least 16 min. This result was caused by the relatively low Cu content in samples 3 and 4, and the catalytic effect was not noticeable compared with that in sample 10. In sample 5, when the Cu content was increased to 0.6, the complete degradation time of MB was 14 min, which was reduced by 2 min compared with the degradation times of samples 3 and 4. The time for complete degradation of MB by samples 6, 7, and 8 was 12 min. No significant difference in Cu content was observed in samples 6, 7, and 8, and the degradation times of MB were similar. When the Cu content was increased to 0.9, the complete degradation time of MB was 10 min. The changes in the UV-vis spectra during the removal of RB molecules by samples 3, 4, 5, 6, 7, 8, and 9 are shown in Fig. S8.† Similar to MB degradation, the degradation times of RB were reduced corresponding to the increase in Cu content in samples 3, 4, 5, 6, 7, 8, 9, and 10. Thus, we conclude that the catalytic efficiency is proportional to the degree of the cation exchange reaction.

Based on the catalytic discussion of Mn_xCu_1−xS, the changes in the UV-vis spectra during the removal of MB and RB molecules by sample 16 are shown in Fig. 8e and f, respectively. Fig. 8e shows that the degree of decolorization of the aqueous MB reached 48% after 2 min and 95% after 18 min. The decolorization was stably maintained after 18 min. In Fig. 8f, the degradation degree of the aqueous RB was examined every 5 min. The degradation degree was 24% after the first 5 min, reached 68% after 15 min and was almost complete after 30 min. The catalytic reaction then reached equilibrium.

In addition, under the same experimental conditions, we tested the catalytic performance of P25. The degradation curves of MB and RB by P25 are shown in Fig. S9a and S9b,† respectively. As shown in Fig. S9,† the complete degradation time of MB was 130 min, and the complete degradation time of RB was 180 min. Through the analysis of the experimental results, compared with P25, the as-prepared MnS, Mn_xCu_1−xS and Mn_yAg_2(1−y)S showed faster catalytic performance.

Conclusions

In summary, large-scale dandelion-like MnS was successfully synthesized with manganese complex as a template by a facile and environmentally friendly approach. The structure and morphology of MnS could be controlled at a stable level by adjusting the reaction temperature and time. The manganese complex was synthesized by a simple method using low-cost chemical raw materials. Cu²⁺ and Ag⁺ were introduced into the as-obtained MnS as beneficial metals by cation exchange at room temperature. The results of the cation exchange reaction showed that the morphology of the as-prepared MnS was transferred to Mn_xCu_1−xS and Mn_yAg_2(1−y)S. This result provided information for the synthesis of inorganic nanomaterials and determined resource maximization. The needle-like structure of the as-prepared products provided a large specific surface area and more active sites, which could improve catalytic performance. The introduction of beneficial metals greatly improved the catalytic efficiency in degrading dye-containing solutions, such as MB and RB, compared with the as-prepared MnS. The catalytic efficiencies of Mn_xCu_1−xS and Mn_yAg_2(1−y)S were improved by six and three times, respectively, compared with that of the as-prepared MnS. Furthermore, compared with P25 (Fig. S9†), the as-prepared MnS, Mn_xCu_1−xS and Mn_yAg_2(1−y)S showed higher catalytic performance under the same experimental conditions. This comprehensive analysis indicated that the composition and performance of the products could be adjusted by changing the central metal ion without changing the morphology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S7: SEM images, XRD patterns, EDS analysis data, and catalysis properties of MnS and its ion exchanged sample. See DOI: 10.1039/c4ra00961d

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