Brønsted-Lewis acidic ionic liquid-derived ZnS quantum dots: synthesis, characterization, and multifunctional applications in pollutant degradation and iodine sorption

Abstract

A pair of Brønsted-Lewis acidic chlorozincate ionic liquids based on 2-alkyl-1,3-disulfoimidazolium cations (1a & 1b) was developed with complex anionic speciation [Zn₂Cl₆]²⁻/[ZnCl₄]²⁻. These ionic liquids were further used as templates for fabricating ZnS quantum dots (QDs) via a grinding method. The ZnS QDs were characterized using various techniques. The use of ionic liquids (ILs) containing complex metal chloride anions resulted in small size and porous nature of the QDs. The presence of various types of defects was verified through XPS, EPR and photoluminescence spectroscopic analyses. These two QDs were used as reusable and recyclable catalysts for the degradation of a broad spectrum of pollutants such as crystal violet (CV), methylene blue (MB), malachite green (MG), morin hydrate, and oxytetracycline (OTC) under UV light irradiation. Free radical scavenging experiments showed that ˙OH and ˙O₂⁻ acted as primary reactive species during the degradation process. These QDs were further employed for iodine sorption experiments in water and hexane solutions. The XPS analysis revealed that the adsorption process occurred in molecular (I₂) and polyiodide (I₃⁻) forms. The recyclability study of the iodine sorption revealed that the QDs could retain 90.6% and 89.4% of their initial efficiency after the 5th cycle in water and hexane solution, respectively. No such reports regarding the use of Brønsted-Lewis acidic chlorozincate ionic liquids for the synthesis of mesoporous defective ZnS QDs has been published. Moreover, the utilization of the pristine ZnS QDs for iodine capture experiments is reported for the first time.

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Introduction

The direct release of industrial effluents consisting of poisonous and non-biodegradable compounds into water bodies causes water pollution, posing a major global threat to living beings.¹ Among various pollutants, dangerous organic dye compounds that find significant uses in textiles, food, pulp, paints, paper, etc., account for 17%–20% of water pollution and make the sustainability of various ecological systems in the environment challenging due to their high thermal stability against natural degradation and persistence in the environment with harmful effects.^2–4 Similarly, pharmaceuticals are another major source of water pollutants, which can easily affect aquatic life and food webs. The continuous deposition of organic pollutants on the surface of water does not allow sunlight to penetrate the water bodies, resulting in eutrophication and an increase in chemical oxygen demand.⁵

Along with these organic contaminants, radioactive iodine (I⁻) also becomes a serious threat to the environment as it readily dissolves in water. The radioactive isotopes of iodine (¹²⁹I, ¹³¹I) are commonly released into natural waterbodies as waste materials during nuclear accidents and from nuclear power plants in addition to other radioactive isotopes (such as ²²⁰Rn, ⁹⁰Sr, and ¹³⁷Cs).^6,7 Such hazardous chemicals cause serious environmental damage and worldwide health menaces to living organisms. Therefore, the eradication of these organic pollutants and hazardous radiochemical materials from the environment is of foremost interest. To address these issues, one endeavor is to prepare a cheap single material based on ionic liquid-assisted metal nanoparticle synthesis under environmentally benign conditions, which is capable of removing organic pollutants and radiochemical materials very efficiently.

Out of numerous techniques of water treatment such as coagulation-flocculation, precipitation, and adsorption, advanced oxidation processes (AOP) have attracted overwhelming attraction from the researchers due to their promising removal efficiency towards various organic contaminants.^8–10 Moreover, the AOPs are environmentally more benign than many other approaches, as they neither release masses of pernicious residues nor divert pollutants from one phase to another. The magnificent activity and versatility of such methods originate from the release of reactive oxygen species such as ˙OH, SO₄˙⁻, ˙O₂⁻, and ¹O₂ that react with the pollutants and lead to their degradation to innocuous byproducts.¹¹

As functional materials, ionic liquids (ILs) are mostly described as molten organic salts below 100 °C with unique physicochemical properties, which often contain a range of inorganic or organic anions coupled with organic cations such as imidazolium, tetraalkylammonium, pyrrolidinium, and pyridinium.¹² Their physicochemical properties can be varied based on the nature of the ion-pair, which is very useful for the designing of task-specific ionic liquids. For example, the presence of –COOH and –SO₃H groups in the ions of ILs provides Brønsted acidic properties. Due to the characteristic physicochemical properties of ILs, their applications cover a wide range including catalysis, reaction medium, electrochemistry, analytical chemistry, nanoparticle synthesis, biowaste conversion, and surfactant studies.^13–15 Incorporating metal-based complex anions into the ionic liquid endow many metal-specific properties to the ILs containing metal ions, which are termed metal-containing ionic liquids (MILs) such as halometallate materials.^16,17 They have the potential to stabilize and facilitate the formation of metal nanoparticles under simple reaction conditions as compared to various traditional methods involving a longer reaction time in a high-temperature and -pressure reaction vessel.^18–20 It was reported in the literature that using an ionic liquid as the medium or template, the size of metal nanoparticles can be manipulated, and thus help in improving the catalytic properties of the nanoparticles.²¹

Among all the reported II–VI types of semiconductor materials, ZnS bears distinct physico-chemical characteristics such as chemical stability and eco-friendliness. Numerous articles have been published on the synthesis of ZnS nanomaterials using traditional methods for potential applications such as adsorbent materials, degradation of organic pollutants, and bioactivity studies.^22–26 However, much experimental evidence revealed some drawbacks of the prepared ZnS nanoparticles for wide applicability because of their structural limitations arose from the synthetic methods. In this context, various methods have been developed to fabricate the ZnS nanoparticles containing improved structural and optical properties, which include hydrothermal, wet chemical method, co-precipitation method, microwave irradiation, and biological methods.^27–30 However, only limited numbers of reports have been found regarding the preparation of ZnS QDs using ionic liquids as templates or reaction media.^31–35

The influence of functionalized ILs or MILs bearing other functional groups on the structural and physicochemical properties of nanomaterials has not been fully explored yet. This work is aimed to study the use of two Brønsted-Lewis acidic chlorozincate ionic liquids of 2-alkyl-1,3-disulfoimidazolium cations (Fig. 1a and Fig. S1, S2†) with different compositions of chlorozincate anions, namely, [Zn₂Cl₆]²⁻/[ZnCl₄]²⁻ for potential application as the template/precursor for the synthesis of ZnS QDs by an environmentally benign method under atmospheric conditions. By following the characterization methods of MIL and the ZnS nanoparticles using various analytical tools, the optical properties of the synthesized ZnS QDs were analyzed as recyclable photocatalysts for the degradation of various organic pollutants (Fig. 1b) in aqueous solutions under UV light using H₂O₂ as an oxidant.^36,37 Furthermore, the porous nature of these nanomaterials was utilized for the sorption study of iodine solution in water and hexane solvents.

Fig. 1 (a) Schematic of the synthesis procedure of ZnS QDs. (b) Overall pictorial representation of the current work.

Results and discussion

Characterizations of the synthesized Zn containing ILs and ZnS QDs

The anionic composition of two precursors of chlorozincates (1a) and (1b) (Fig. 1a) used for the preparation of nano QDs of ZnS was known from their HRMS data (Fig. S3 and S4†), included in the ESI.† The peaks at m/z 717.4111, 701.4398 and base peak around 679.4618 could be assigned to the fragmentation of [MDSIM]₂[Zn₂Cl₆] composition. This hybrid also showed another intermediate peak at 276.7038 related to the anionic speciation [Zn₂Cl₆]²⁻ after the loss of two chloride atoms. The HRMS spectrum of chlorozincate of [BDSIM]⁺ cation displayed the precursor as a mixture of [Zn₂Cl₆]²⁻ and [ZnCl₄]²⁻ anions. The peaks at m/z 800.6183, 787.5544 (base peak), 773.3010, 760.6852 could be attributed to the fragmentation of [BDSIM]₂[Zn₂Cl₆] composite in addition to another peak assigned to anionic speciation [Zn₂Cl₆]²⁻ at 347.0987. The spectrum also displayed fragmentation of the [BDSIM]⁺ cation at m/z of 286.1338, 205.0645, and 125.1112 (base peak) after the loss of two sulfonic groups as well as a n-butyl chain. Furthermore, the precursor spectrum showed another mass peak at m/z of 776.9974 correspond to anionic speciation [ZnCl₄]²⁻.

Powder X-ray diffraction patterns (Fig. 2) of the synthesized samples of ZnS QDs labelled as ZnS-1 and ZnS-2 showed three diffraction peaks at 2θ values of 28.7°, 47.9°, and 56.5° corresponding to the (111), (220) and (311) diffraction planes (JCPDS card number: 80-0020), respectively, for a cubic zinc blende structure. The sharpness of these peaks indicated the highly crystalline nature of the QDs. The most intense peak of diffraction plane (111) expressed that most of the nanoparticles are formed along this plane. Debye–Scherrer's equation (eqn (1)) was used to calculate the average crystallite size of the QDs for the (111) plane:

L = Kλ/B cosθ (1)

where L represents the coherent length, λ is the wavelength of the X-ray radiation of the instrument (0.15406), K is the shape factor having values ranging between 0.62 and 2.08 and taken as 0.89, B indicates the line broadening at half the intensity of the most intense XRD peak, and θ is Bragg's angle. The diameter of a nanoparticle (D) can be calculated using eqn (2):³⁸

D = 4L/3 (2)

Fig. 2 Powder XRD pattern of ZnS-1 (a) and ZnS-2 (b) QDs. FT-IR plots of ZnS-1 (c) and ZnS-2 (d) QDs.

The calculated values of diameters were found to be 4.18 nm for ZnS-1 and 4.10 nm for ZnS-2 QDs, respectively. The FTIR spectra of both the ZnS samples showed the O–H stretching and bending vibrations of adsorbed water molecules at 3404–3406 cm⁻¹ and 1619–1620 cm⁻¹, respectively (Fig. 2). The peaks at 615–620 cm⁻¹ was assigned to the characteristic vibrational motion of ZnS, while the peak at 1106–1110 cm⁻¹ could be attributed to the ZnS lattice that arises due to resonance interaction between vibrational modes of sulphide ions in the crystal.^39,40

The UV-Vis DRS spectra of ZnS QDs in Fig. S5† displayed three absorbance verges within 200–600 nm. The sharp edge around 340 nm could be assigned to the main band gap of the ZnS QDs along with two other defective states observed around 400 nm and 550 nm in the visible regions. The calculated band gap E_g values were determined using Tauc equation (3) (Fig. S5†) by plotting hν and (αhν)², which disclosed the direct band gap energy of ZnS QDs.

[F(R_∞) hν]^1/n = A(hν − E_g) (3)

where F(R_∞) represents the Kubelka–Munk function, h represents Planck's constant, ν is the vibrational frequency, E_g denotes the band gap energy and A is a constant. The exponent n represents the nature of optical transition (n = ½ and 2 denotes direct and indirect allowed transition, respectively). The direct and indirect band gap energies could be calculated from the [F(R_∞) hν]^1/nvs. hν plot. Thus, the so-obtained hν-intercept values are considered as band gap values of both the ZnS-1 and ZnS-2 QDs.⁴¹ The band gap (E_g) values were found to be 3.4 eV for both the QDs, and are lower than those of the reported bulk ZnS nano, which might be due to the generation of some defective sites in the QDs for change in the preparation procedure involving the metal-containing ionic liquid as the template.^42–44 The band gap energy of ZnS QDs was found to be 3.4 eV, which is much higher than expected for such tiny particles. This discrepancy could be described from quantum confinement of electrons in discrete energy levels in the nanoparticles within a limited space like QDs.^45,46 This quantization of energy levels results in a larger band gap between the valence and conduction levels of the nanoparticles.

To assess the morphology, average size, and elemental composition of the ZnS nanomaterials, a comprehensive study was carried out. The high-resolution SEM images of ZnS-1 and ZnS-2 revealed their porous nature with a hollow agglomerated form to attain the stable states of smaller sized nanoparticles. The TEM images depicted the spherical shape of nanoparticles that are homogenously dispersed with a size range of 2–7 nm. The low surface tension values of ionic liquids could increase the nucleation rates to prepare smaller sized nanoparticles^47,48 Furthermore, the metal-based chlorozincate ionic liquid templates may facilitate various types of inter/intra-molecular electrostatic as well as H-bonding interactions for the controlled generation of nanoparticles within a network of interactions, as shown in Fig. 3.⁴⁹

Fig. 3 Various types of interactions involved during the synthesis process.

Heteroatoms such as O, N and Cl present at the cations and anions of the ILs would participate in the H-bonding network. Moreover, the O and –OH group present in the –SO₃H group available in the organic part of the MIL in association with the complex chlorozincate anions may involve in the assembly of all the metal precursors through extended H-bonding networks that increase the stability of generated ZnS QDs. The mutual repulsive interactions of bulkier alkyl groups of the imidazolium cation of metal precursors could not allow the nanoparticles to grow. Similarly, the π–π interactions generated by the electron clouds of the imidazolium moieties aligned in parallel position and the nanomaterials placed in between could cause the size-diminishing effect on the nanomaterials.⁵⁰ From the TEM images (Fig. 4), the average diameters of ZnS-1 and ZnS-2 nanoparticles were found to be 4.22 nm and 4.04 nm, respectively, which are very close to the values calculated using Debye-Scherrer's equation. The selected area electron diffraction (SEAD) pattern indicated the polycrystallinity of the ZnS QDs. The lattice fringes with a distance of 0.311 nm were found for both the QDs that were well indexed to the (111) plane. These results were in good agreement with the PXRD data. The energy-dispersive X-ray (EDX) spectroscopy with elemental mapping was performed to check the purity of the ZnS QDs. The EDX spectra of ZnS QDs showed (Fig. S6†) the presence of all the elements in the nanoparticles.

Fig. 4 FESEM images of (a) ZnS-1 and (b) ZnS-1. HRTEM images of (c) ZnS-1 and (d) ZnS-2. SAED pattern of (e) ZnS-1 and (f) ZnS-2. d-spacing values of (g) ZnS-1 and (h) ZnS-2. Average particle size images of (i) ZnS-1 and (j) ZnS-2; particle size distribution plots of (k) ZnS-1 and (l) ZnS-2. EDX elemental mapping images of (m) Zn and (n) S.

Moreover, XPS analysis data (Fig. 5) of the ZnS QDs also revealed the presence of Zn and S in the samples. The survey spectra of the QDs exhibited peaks for Zn and S along with small quantities of oxygen and adventitious carbon. Binding energies of Zn 2p displayed two peaks at 1044 eV and 1021 eV corresponding to Zn(2p_3/2) and Zn (2p_1/2), respectively, in the XPS spectra of ZnS QDs.⁵¹ Two Gauss-Laurentian deconvoluted peaks appeared for S²⁻ state of the ZnS-1 sample at S 2p_3/2 (162.6 eV) and S 2p_½ (161.6 eV), while for the ZnS-2 sample at S 2p_3/2 (162.7 eV) and S 2p_½ (161.3 eV) in the XPS spectra. A weak peak around 163.3 eV could be attributed to the presence of less negative oxidation state of S, which was identified as elemental sulfur defect (S⁰).⁵² No peak was found related to SO₄²⁻ form around 169 eV in these spectra.⁵³ To get more information about the types of defects, the ZnS QDs were subjected to electron paramagnetic resonance (EPR) and photoluminescence (PL) analysis.

Fig. 5 XPS survey spectra of (i) (a) ZnS-1 and (ii) (a) ZnS-2 QDs; (b) and (c) of (i) and (ii) represent the XPS spectra of Zn and S.

Fig. S7† shows the BET N₂ gas sorption isotherm of the synthesized ZnS QDs. The measured specific surface areas of ZnS-1 and ZnS-2 were found to be 60.7 and 62.5 m² g⁻¹, respectively. The slight differences of surface area of ZnS QDs could be expected due to differences in the networks of electrostatic as well as H-bonding interactions related to the sizes of alkyl chain length and the variations in anionic compositions of chlorozincate speciation in the respective precursor MIL during the synthesis of nanoparticles. They showed Type IV isotherms with hysteresis loops that signified the mesoporous nature of the nanomaterials.⁵⁴ The average BJH pore diameters of the ZnS QDs were found to be 3.82 nm and 3.81 nm for the ZnS-1 and ZnS-2, respectively. This porosity of the ZnS QDs was also evident from the HRTEM images (Fig. 4), as discussed earlier. The average surface area and pore size of the prepared ZnS were observed to be higher than earlier reported ZnS nanomaterials.^55,56

The X-band electron paramagnetic resonance (EPR) spectra, shown in Fig. S8,† for both the ZnS-QDs confirmed the availability of unpaired electrons by displaying paramagnetic absorption signals at a Lande g-factor value of 2.008 with slight differences in signal intensities. The more intense peak of ZnS-2 could be expected as an outcome of the presence of a greater number of defects in this sample. To further support the defect-rich nature of ZnS QDs, the photoluminescence (PL) analysis of ZnS-2 QD was carried out at room temperature with an excitation energy of 300 nm. A broad peak for the PL spectrum of QD was observed around 400 nm and extended up to 550 nm, which indicated the existence of more defective sites in the nanoparticles corresponding to the emission peaks in the higher wavelength region. The deconvolution of this broad peak in the Gaussian mode exhibited four small peaks (Fig. S9†) and displayed various defects of the ZnS QDs, including zinc vacancy (V_Zn), sulfur vacancy (V_S), interstitial sulfur atoms (I_S) and interstitial Zn atoms (I_Zn). The two peaks at 400.4 nm and 420.2 nm could be assigned to the ‘S’ vacancy and the other peaks near 446.4 nm and 473.6 nm could be attributed to the ‘Zn’ defects.^57,58

Photocatalytic degradation reactions of organic pollutants

To examine the photocatalytic performance of the synthesized catalyst, various organic pollutants such as CV, MG, OTC, Morin and MB were chosen.

For detailed analysis and to search for optimization conditions for the degradation processes, the CV dye was chosen as the model dye. The photocatalytic activity of the prepared ZnS QDs was monitored by examining the absorbance peak of CV at λ_max 589. To select the suitable advanced oxidation process (AOP) conditions for the degradation of the model dye using H₂O₂, UV light, H₂O₂/UV and H₂O₂/UV/ZnS QD methods, the photodegradation was optimized by varying the dosages of catalyst amount and H₂O₂ concentrations at initial pH 7.8 for 90 min at 298 K. Among the studied methods, the system of H₂O₂/UV/ZnS QDs showed efficient degradation at degradation rates of 92.3% and 98.1%, respectively, for the ZnS-1 and ZnS-2 samples. The percent degradation was found to be maximum for the large surface area ZnS-2 as compared to the lower surface area ZnS-1 QDs, as shown by BET analysis. The negligible degradations of the dye in UV light or H₂O₂ conditions were expected for the production of low amounts of hydroxy radicals in the absence of the photocatalyst. The effect of several impact factors such as catalyst dosage, H₂O₂ concentration, and initial pH of the solutions were systematically examined and discussed below various sub-units. Fig. 6 and Fig. S10† indicate that the reaction kinetics for the CV removal followed the pseudo-first-order kinetics with equation ln (C_t/C₀) = −kt, where k denotes the first-order rate constant and t denotes the time with a k value equal to 0.02972 min⁻¹.

Fig. 6 Kinetic study plots of CV degradation under different conditions.

Effect of H₂O₂ dosage

The effect of H₂O₂ dosage on the catalytic efficiency of the ZnS QDs in the CV degradation in 90 min is shown in Fig. 7a. The degradation efficiency of CV increased from 81.7% to 98.1% with the increase in the concentration of [H₂O₂] from 10 mM to 20 mM with increasing production of more ˙OH radicals. However, with a further increase in [H₂O₂] from 20 mM to 25 mM, the degradation efficiency decreased. This is because excessive H₂O₂ started to scavenge the active hydroxy radicals by generation of perhydroxy radicals.⁵⁹ The scavenging effects of the perhydroxy radicals impact on the degradation process to a considerable extent.⁶⁰ The scavenging effects of excess H₂O₂ can be understood from eqn ((m) and (n)):⁶¹

H₂O₂ + ˙OH → HO₂˙ + H₂O (m)

HO₂˙ + ˙OH → H₂O + O₂ (n)

Fig. 7 (a) Effect of H₂O₂ dosage, (b) catalyst dosage, (c) CV dosage, (d) pH effect, (e) catalyst reusability plot, (f) and (g) scavenger effect plots, and (h) and (i) water source plots during the CV degradation process.

Moreover, a higher amount of H₂O₂ enhances competitive adsorption of H₂O₂ and CV on the surface catalyst.⁶² Thus, the optimal [H₂O₂] was taken as 20 mM to optimize the dosages of catalyst amount.

Effect of catalyst dosage

The amount of ZnS QDs is another crucial parameter influencing the degradation efficiency of CV. The CV degradation with different dosages of ZnS from 0.250 g L⁻¹ to 1.25 g L⁻¹ was performed (Fig. 7b). The increase in ZnS dosages (0.250 g L⁻¹ to 1 g L⁻¹) accelerated the rate of degradation from 77.6% to 98.1% with increasing active sites on the catalyst surface for the generation of the ˙OH radical.^63–65 However, a further increase in the catalyst dosage to 1.25 g L⁻¹ decreased the degradation rate to 93.7%, which could be attributed to the light scattering effect, non-transparency of the suspension and agglomeration of the catalyst particles. Thus, the excess nano catalysts may envelope some of the photo active surfaces of catalysts and block the light pathway to reach the dye molecules, thereby reducing the production of hydroxy radicals and consequently diminishing the decolorization of CV.^66,67 Due to the above-mentioned reasons and reaction costs, 1 g L⁻¹ catalyst was considered as the optimum dosage and used for further studies.

Effect of initial dye concentrations

To investigate the influence of initial concentration of the CV dye on the degradation process, four different solutions of the CV concentrations (15, 20, and 25 mg L⁻¹) were taken for controlled experiments. As shown in Fig. 7c, the increase in the CV concentrations declined the degradation rate from 98.1% using 15 mg L⁻¹ dose to 78.8% with 25 mg L⁻¹ of the CV due to reduction in available active sites of the catalyst for the production of reactive oxidative species. Additionally, a higher dye concentration increases the opaqueness of the solution that leads to difficulty in the diffusion of UV light that diminishes the formation of hydroxyl radicals.^68,69 From this observation, it was decided to consider 15 mg L⁻¹ dye solution as the optimized amount for further analysis.

Effect of pH

The zero point charge (P_ZC) of the ZnS QDs was determined⁷⁰ prior to the investigation of the pH effect on the degradation process and found to be 6.1 (Fig. S11†). It implies that at lower pH (pH < pH P_ZC), the catalyst surface is positively charged and at higher pH (pH > pH P_ZC), it acquires more –OH groups on the surface and becomes negatively charged. The influence of pH values on the degradation of the CV dye was investigated within the pH range of 2.8–9.1 by keeping the other parameters constant. The experimental results clearly depicted a substantial (Fig. 7d) effect of the pH values on the degradation of CV. It has been seen that the degradation was more pronounced under moderate basic conditions than at acidic pH. The % degradation of CV was calculated to be 18.25 at pH 2.8 after 90 min of UV irradiation. The % degradation increased to 98.1 as the pH increased from 2.8 to 7.6 and revealed preferential existence of positively charged surfaces of the nano catalyst at acidic pH having lower tendency to adsorb the cationic dye CV before degradation.⁷¹ Additionally, at lower pH, the available H⁺ ions may transform the H₂O₂ molecules into less reactive H₃O₂⁺ ions, as shown in the following equations:⁷²

H₂O₂ + H⁺ → H₃O₂⁺ (o)

˙OH + H⁺ + e⁻ → H₂O (p)

As the pH changes from neutral (pH = 7) to slightly alkaline (pH = 7.8), the % degradation increased from 68.4 to 98.1, which reflects the favorable formation of negatively charged active sites on the catalyst surface for effective adsorption of the cationic dyes to increase the rate of degradation. Again, a further increase in the basicity of the solution caused a reduction in the degradation efficiency of the CV dye. This is because at higher basicity, the decomposition of H₂O₂ into H₂O and O₂ occurs instead of the generation of ˙OH.⁷³

Study of the photodegradation of other organic pollutants

Subsequently, the photocatalytic activity of ZnS-2 QD was studied for the complete degradation of other organic pollutants (Fig. S12†) such as malachite green (MG), methylene blue (MB), oxytetracycline (OTC) and morin hydrate using 1 g L⁻¹ amount of the catalyst at different time points and pH within 7.6–8 by addition of 20 mM solution of hydrogen peroxide under UV light irradiation, as shown in Fig. S11.† All the pollutants were successfully degraded by the same catalyst in 4 h under the same condition. It depicted the versatility of our synthesized catalyst.

Catalyst recovery and reusability study of the ZnS QDs

The reusability and stability of the catalyst were evaluated by four consecutive cycles for 90 min under the optimal conditions. Fig. 7e indicates the excellent stability and reusability of the catalyst in the degradation operations. After each cycle of the reaction, the used catalyst was filtered, washed several times with alcohol and deionized (DI) water and dried for subsequent tests. This study depicted slow reduction in the degradation efficiency of the photocatalyst with increasing numbers of reuses, from 98.1% to 84.7% in the 1st and 4th cycles, respectively. This might be due to the deactivation of the active sites and change in pore structures of the catalyst along with adsorption of reaction intermediates of the photodegradation reaction in the active sites. Although the reuse of the catalyst was less effective than the original ZnS QDs, the stability and reusability of the catalyst were relatively magnificent. The PXRD, FT-IR spectroscopy and FESEM analysis of the used catalyst showed no obvious change after the 4th catalytic cycle (Fig. S13†). They displayed the retention of the original crystal planes and the morphology of the used ZnS QDs. The metal leaching test of the photocatalyst into the dye solution was studied by ICP-OES analysis, and it showed excellent recoverability and stability of ZnS QDs with a value of 0.38 mg L⁻¹ of Zn ion in the dye solution.

Possible mechanism

To identify the actual reactive species involved in the dye degradation process, different free radical scavenging experiments were conducted. EDTA, AgNO₃, isopropyl alcohol, and p-benzoquinone were chosen as scavengers for holes (h⁺), electrons (e⁻), hydroxyl radicals (OH), and superoxide radicals (O₂⁻), respectively. All the radicals influence the CV degradation process to different extents, as displayed in Fig. 7f and g. However, the inhibition was more pronounced in the presence of isopropyl alcohol and p-benzoquinone (from 98.1% to 30.3% and 43.9%, respectively), indicating ˙OH and ˙O₂⁻ as the main reactive species in the photodegradation reaction. Based on the above analysis, the possible mechanism of the production of ˙OH radicals in the presence of ZnS/UV/H₂O₂ can be evaluated (Fig. 8). Upon irradiation of the UV light, electrons in the VB got excited to the CB, generating e⁻/h⁺ pairs. These CB electrons reacted with electron acceptors such as adsorbed O₂ molecules to produce ˙O₂⁻. The photo-induced holes in the VB reacted with water molecules or got trapped by the electron donors such as OH⁻ to generate ˙OH.^74,75 The H₂O₂ present in the system can generate enormous ˙OH radicals either by accepting photogenerated electrons from the CB of ZnS QDs or by combining with the ˙O₂⁻ anions (Fig. 8). All these so-formed photogenerated active species are involved in the degradation process of CV dyes into small molecules such as CO₂ and water. The process of using photogenerated electrons slowed down the e⁻/h⁺ recombination rate that improved the degradation efficiency. Moreover, the photolysis of H₂O₂ under UV light can produce ˙OH radicals, resulting in the enhancement of the CV degradation.

Fig. 8 Plausible photocatalytic degradation mechanism.

Photodegradation of the CV in different water matrixes

To explore the practical relevance of photodegradation reaction using the ZnS-2/UV/H₂O₂ system in a real environment, various water samples including tap water, river water and pond water were employed to evaluate the effects of various water matrices on the catalytic degradation of CV. The experimental results shown in Fig. 7(h) and (i) revealed that the % degradation of CV was highest in deionized water (98.1%) followed by tap water (91.3%), river water (86.9%) and pond water (79.9%). This may be attributed to the pH effects exhibited by these water systems. Tap water having an initial pH value of 7.1 showed little delay in the degradation process followed by the river (pH 6.5) and pond water (pH 6.2). Moreover, this degradation process also suffered from the interference caused by the various inorganic anions, dissolved organic matters (DOMs), and natural organic matters (NOMs) present in the natural water environments, which delayed the process.^76–78

Iodine sorption study by ZnS QD

Both the samples of ZnS QDs were also studied as sorbent materials for the sorption of iodine from molecular solvents at room temperature due to the presence of a considerable amount of porosity, surface area and stable crystalline nature. The crystalline powder of ZnS QDs was activated at 80 °C under vacuum for 12 h before use as sorbent materials. Before use, the adsorbent materials (ZnS QDs) were heated under vacuum for 12 h at 80 °C. This process is performed to open the pores and maximize the surface areas, which is known as the “activation process”. It increases their ability to capture iodine onto their surfaces by eliminating any moisture and impurities, which might block the active sites present on the ZnS QDs, thus making the QDs more functional during the iodine sorption process.

Water and hexane were chosen as polar and non-polar media for the iodine solution to conduct the experiment at ambient temperature. The I₃⁻ solution of iodine in water was prepared by mixing equal proportions of I₂ and KI, which were then treated with the ZnS QDs according to the experimental procedure, as mentioned in the Experimental section for specific time. It was seen that after a few minutes, the yellow color of I₃⁻ solution became colorless in the water medium (Fig. 9). The hexane solution of iodine did not require any additive for the solubilization of the iodine molecules. To obtain a colorless solution through the sorption of iodine, the ZnS QDs required around 45 min in the hexane solution.

Fig. 9 Iodine sorption plots: (a) in water and (b) in hexane using ZnS-2 at room temperature.

To establish the interactions between the ZnS QDs and the sorbed iodine molecules, the iodine sequestration mechanism was further prospected employing XPS and EDX mapping investigations. Guo et al. reported that the average chain size of iodine molecule is 3.35 Å, which was observed to be much smaller than the pore size of the synthesized ZnS QDs (∼3.8 nm).⁷⁹ With this smaller size, there was a high probability of entering the iodine molecules into the pores.

The XPS spectra (Fig. 10) of ZnS complexes with I₂ labelled as H for the ZnS-1 and W for the ZnS-2 disclosed the sorption of iodine molecules on the active sites of ZnS QDs. The comparative XPS survey plots of the two ZnS-I₂ complexes with ZnS clearly indicated the iodine peaks in the 617–638 eV region. The peak positions at 619.6 eV, 632.1 eV for the ZnS-I₂ H and at 620.7 eV, 632.3 eV for the ZnS-I₂ W displayed partial sorption of the molecular I₂via the physisorption process, whereas the peaks at 620.7 eV, 632.3 eV for the H complex and at 621.8 eV, 633.3 eV for the W complex signified the existence of polyiodide anions (I₃⁻ and I₅⁻) involving the chemisorption process.⁷⁹ Therefore, the overall sorption processes are a mixture of both the physisorption and chemisorption phenomena. Apart from these interactions, there is a strong possibility of the generation of small amount of ZnI₂.⁸⁰ In the sorption complexes of ZnS-I₂, the binding energies of Zn and S seemed to be shifted to higher energy states, which could be ascribed to the change in local chemical environment as well a charge redistribution of the constituent ions of ZnS QDs involving strong interactions between the Zn²⁺ and the electronegative S²⁻ states. The lone pairs of electrons of S-atoms augmented the generation of CT complexes between the adsorbates and the adsorbents.⁸¹ EDX-mapping images showed the uniform distribution of iodine over the surface of ZnS in both the solutions (Fig. 11).

Fig. 10 (a) XPS survey plots depicting the iodine sorption under both the conditions, and sorbent iodine peak in water (b) and hexane (c).

Fig. 11 Elemental mapping images of sorbed iodine on ZnS-2: (a) water and (b) hexane solution.

For practical uses of ZnS-QDs as sorbent materials for the iodine sorption process, the recyclability study was conducted by soaking the iodine-trapped ZnS-QD complexes in organic solvents such as ethanol and tetrahydrofuran at room temperature to release the sorbed iodine in solutions. The change of colourless organic solution to yellow colour with time indicated the desorption of iodine from the QDs (Fig. S14†), which was further proved by examining the UV-Vis desorption intensity plots with time (Fig. S15†). This process was repeated several times until the complete release of iodine followed by drying of the sorbents for further use. Fig. S16† demonstrates that the QDs could retain 90.6% and 89.4% of their initial efficiency after five cycles in water and hexane solution, respectively. These observations identified that the ZnS-QDs represent better sorbent materials for iodine sorption in terms of faster sorption-release, efficient reusability and low-cost nature of the materials than the literature data.^82–85

Experimental section

Materials

2-Methylimidazole, 2-butyl imidazole, zinc chloride (ZnCl₂), sodium sulfide (Na₂S), crystal violet (CV), methylene blue (MB), malachite green (MG), morin hydrate, and oxytetracycline (OTC) were purchased from suppliers like Merck, TCI, Alfa-Aesar and SRL. They were used without any further purification.

Synthesis of metal-containing ionic liquids (MILs) and ZnS QDs

In the 1st step, two chloride-based ILs, namely, 2-methyl-1,3-disulfoimidazolium chloride and 2-butyl-1,3-disulfoimidazolium chloride were prepared according to standard method reported in the literature,³³ mentioned in the supporting file. By following this, 1.36 g (10 mmol) of ZnCl₂ was added to 10 mmol of the prepared 2-alkyl-1,3-disulfoimidazolium chloride in a 100 mL round-bottomed flask and the mixture was heated to 75 °C for around 3 h. The metal salts thus formed were labelled as metal-containing ionic liquids (MILs). The above-mentioned in situ generated metal salts were mixed with 2.4 g (10 mmol) of Na₂S and ground in a mortar for few minutes. The reaction mixture was filtered using distilled water to remove the unreacted MILs and NaCl. The residue was kept in an oven at 80 °C to dry for 24 h to obtain the pure ZnS quantum dots.

Methods

¹H and ¹³C spectra of the ILs were recorded using a JEOL ECS spectrophotometer (400 MHz) in DMSO-d₆ as the solvent. Electrospray Ionization-Mass Spectrometry (ESI-MS) analysis of the metal-containing ILs was performed using a SYNAPT-G2S instrument with acetonitrile as the solvent. Powder X-Ray diffraction analysis of the nanoparticles was performed using an X-ray diffractometer (Bruker, Germany) equipped with a Ni-filtered Cu K radiation source. The surface morphology analysis of NPs was performed using a field emission scanning electron microscope (FESEM with EDS detector; model: Zeiss Gemini-500) and a transmission electron microscope (HRTEM; model: JEOL-2100F). The samples were diluted and sonicated followed by dropping onto a carbon-coated copper grid for TEM. The average particle size was evaluated by combining TEM images using the ImageJ software. X-ray photo-electron spectroscopic (XPS) analysis was performed using a Thermoscientific NEXA surface analyzer. FT-IR spectra were recorded using a PerkinElmer FIR FT-IR spectrophotometer (model: IMPACT-410). A UV-Diffuse reflectance spectroscopic (UV-DRS) study was conducted using a UV-2600i spectrophotometer (Shimadzu, Japan). The N₂ sorption behavior was studied using the Brunauer–Emmett–Teller (BET) method to calculate the average surface area and average pore volume of the QDs (Quantachrome; model: Autosorb-IQ MP). Electron paramagnetic resonance spectroscopy was performed using an electron paramagnetic spectrophotometer (EPR) (Model: Jeol, JES X 320). The photoluminescence spectra were recorded using a photoluminescence spectrophotometer (model: Cary Eclipse Agilent, with an 80 Hz xenon lamp) to determine the types of defects present in the materials. The metal leaching test of the photocatalyst was examined using an inductively coupled plasma-optical emission spectrometer (Thermo Fisher Scientific, model: iCAP Pro X Duo) (ICP-OES analysis) with a con. HCl solution. The pH of the pollutant solutions was measured using a PC 700 pH/mV/conductivity/°C/°F Bench Meter. Three pH buffer capsules were utilized at pH 4.01, 7 and 10.01 for calibration purposes. All these pH measurements were repeated at least three times to confirm the results. The zero point charge (P_ZC) of the photocatalyst (ZnS QDs) was measured by a batch equilibrium method.⁷⁰ For this, each of the 10 mg of the ZnS QDs was mixed with 10 ml aq. KNO₃ solution (0.1 mol L⁻¹) in a set of six different conical flaks. The pH of these solutions was adjusted to the range of 2–10 by adding different amounts of aq. HCl (0.01 M) or aq. NaOH (0.01 M) solutions. These suspensions were stirred for 8 h at room temperature and the final pH was measured (pH_f). The value of (ΔpH = pH_f − pH_i) was plotted against the initial pH (pH_i) of the solutions, and the P_ZC value of the photocatalyst was represented by the x-intercept.

Photocatalytic degradation of dyes and antibiotics using ZnS QDs

Various dyes (CV, MB and MG) and pharmaceuticals (Morin and Oxytetracycline) were taken as target contaminants to examine the catalytic degradation. In a 100 mL beaker, 20 mL aqueous target solutions were stirred in the presence of 1 g L⁻¹ of ZnS QDs under dark conditions for 60 minutes to attain equilibrium between adsorption and desorption before its irradiation. To the mixture, 200 μL of 30% H₂O₂ solution was added to conduct oxidative degradation of the dye molecules under irradiation of a UV lamp (11 W). The intensity of the absorption peak of the dyes was monitored using a UV-visible spectrophotometer from the sample of deteriorating reaction mixture collected at regular intervals of time. To detect the repeat activity of the catalyst, the catalyst was filtered, washed with deionized water followed by drying at 80 °C and subjected to further recycling experiments.

Iodine uptake in water and hexane solution

The time-dependent solution-phase experiment of iodine sorption was conducted using 500 mg L⁻¹ of iodine in water as well as hexane solutions (10 mL) containing 10 mg dosages of nanoparticles (NPs) with continuous stirring. Since iodine is poorly soluble in water, some amount of solid KI was added to the prepared solution for complete solubility. The sorption efficiency of NPs was determined from the calculation of change in the concentration of the aliquots pipetted out at different time intervals with the aid of UV-Vis spectroscopy.

Conclusion

In this study, two acidic ionic liquids of complex chlorozincate anions containing 2-alkyl-1,3-disulfoimidazolium cations were employed as templates/precursors for the preparation of ZnS-QDs by a solvent-free grinding method under ambient conditions in less reaction time. Various analytical techniques such as PXRD, FT-IR spectroscopy, FESEM, HRTEM, Raman spectroscopy, BET, XPS, UV-Vis spectroscopy, and UV-DRS analysis were used to identify the structural and optical data of the ZnS-QDs. The analytical data of UV-DRS, EPR and photoluminescence revealed the existence of various types of defective sites in the QDs for which they worked efficiently as recyclable heterogeneous photocatalysts for the degradation of organic pollutants, particularly with the model cationic dye CV. The scavenging studies displayed ˙OH and ˙O₂⁻ as the primary reactive species in the degradation process. The porous nature of ZnS-QDs as observed from the BET analysis provided information about their applications as recyclable efficient sorbent materials for sorption of iodine from solution of water as well as hexane.

Author contributions

Debanga Bhusan Bora: methodology; data curation; investigation; formal analysis; and writing – original draft. Sukanya Das: investigation and formal analysis. Abhilekha Phukan: data curation and investigation. Sangeeta Kalita: validation. Prapti Priyam Handique: investigation. Ruli Borah: conceptualization; methodology; supervision; and writing – review & editing.

Data availability

The data supporting the findings of this study are available in the ESI† of this article.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors are thankful to Sophisticated Analytical Instrumentation Centre, Tezpur University for analytical support and the Department of Chemistry, Shiv Nadar Institute of Eminence, Delhi NCR for EPR analysis. The co-author, Debanga Bhusan Bora, is grateful to the University Grants Commission, Government of India, New Delhi, for providing Junior Research Fellowship with Award Number: 16.9(June 2018). 2019(NET.CSIR).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nr00043b

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