Dandelion-like ZnS/carbon quantum dots hybrid materials with enhanced photocatalytic activity toward organic pollutants

Abstract

In this paper, dandelion-like ZnS was synthesized via a facile hydrothermal method, and then as the support to further synthesize dandelion-like ZnS/carbon quantum dot hybrid materials. Multiple techniques were applied to investigate the structures, morphologies, electronic and optical properties of the samples. It can be observed that the carbon quantum dots were distributed uniformly on the surface of the ZnS. The photocatalytic activities of the as-prepared materials were investigated by the photodegradation of methylene blue, Rhodamine B and the colorless antibiotic ciprofloxacin hydrochloride, respectively. The as-synthesized hybrid materials exhibit higher photocatalytic activity than that of the pure ZnS under simulated sunlight (λ > 380 nm), indicating a broad-spectrum of photocatalytic degradation activity. The most beneficial amount of carbon quantum dots to improve the photocatalytic activity of the ZnS is 2.0 wt%.

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1. Introduction

ZnS, as a vital wide-band gap semiconductor material, has been widely used for the manufacture of catalysis, sensors, photoluminescence etc.^1–3 Recently, many efforts have been devoted to enhance the photocatalytic performance of ZnS, including hybridizing with noble metals and compositing with other carbon materials such as graphene oxide (GO), reduced graphene oxide (RGO) and carbon nanotubes (CNTs).^4–7 However, the contact interfaces of these hybrid systems are insufficient and unconsolidated, therefore they can't form perfect interfaces on account of the large-size materials.⁸ Thus, the electron–hole pairs recombine preferentially, which further limit the improving of the photocatalytic performance. Herein, carbon quantum dots (CQDs) as the co-catalyst were introduced into the ZnS in order to eliminate the limitation.

The CQDs has been wildly used in the applications such as light-emitting devices,⁹ bioimaging,^10,11 catalysis¹² and sensor,^13,14 due to its low toxicity, low cost and high resistance to photo-bleaching. The CQDs can form sufficient contracted sufficiently and distributed uniformly on the surface of other materials because of its small size. What's more, it can construct the bulk-to-surface channels for the electrons owing to the excellent ability for charge transport. Therefore, such a semiconductor-conductively quantum dots system is worthy of expectation. Recently, the CQDs have been introduced into the semiconductors (such as Ag₃PO₄,¹⁵ ZnO,¹⁶ TiO₂ (ref. 17) and Fe₂O₃ (ref. 18)) to improve the photocatalytic performances. However, most of them evaluated the photocatalytic activity by single organic dye and the relationships between the structure and photocatalytic activity have not been studied.

In this work, dandelion-like ZnS was synthesized and serviced as the support to fabricate the CQDs/ZnS hybrid materials. The structure, morphologies, electronic and optical properties were investigated in detail. The unique nanostructure is assembled by the plenty of nanowires, which makes it possesses the mesopores structure. Additionally, the porous morphology endows it has a large specific surface areas and good adsorption properties which are of great importance for a catalyst.¹⁹ The photocatalytic activities of the CQDs/ZnS were evaluated by the degradations of organic dyes (methylene blue (MB), Rhodamine B (RhB)) and ciprofloxacin hydrochloride (CIP). To the best of our knowledge, there are no existing reports on the preparation and investigation of photocatalytic performance of such a CQDs/ZnS system and no reports about the degradation of CIP by ZnS system.

2. Experimental section

2.1 Preparation of dandelion-like ZnS and CQDs

All the materials are commercially available and used as received without further purification. In a typical procedure, 1 mmol of Zn(NO₃)₂·6H₂O was dissolved in 16 mL ethylenediamine–water mixture solvent (1 : 1 in volume ratio). After stirring for 1 h, 0.1 mmol of CTAB was added into them. Then the solution was stirred until the CTAB was fully dissolved. After that, 3 mmol of thiourea was added into the mixture solution and stirred for 2 h. Then the colloid solution was heat treated in a Teflon-lined autoclave at 180 °C for 24 h. After the reaction, the products were collected and washed by ethanol and deionized water several times, respectively.

The CQDs were synthesized according to the reported literature.²⁰ In a typical procedure, citric acid (1.0507 g) and ethylenediamine (335 μL) was dissolved in DI-water (10 mL). Then the solution was transferred to a poly (tetrafluoroethylene) (Teflon)-lined autoclave (30 mL) and heated at 200 °C for 5 h. After the reaction, the reactors were cooled to room temperature by water or naturally. The product, which was brown-black and transparent, was subjected to dialysis in order to obtain the CQDs.

2.2 Preparation of dandelion-like ZnS/CQDs hybrid materials

To synthesize the hybrid materials, 100 mg dandelion-like ZnS was added into 30 mL deionized water which contains a certain amount of CQDs, then the hybrid systems were obtained via an oil bath reflux at 90 °C for 3 h. The obtained samples were washed with deionized water, and then dried in an oven at 60 °C for 12 h. The added contents of CQDs in the CQDs/ZnS hybrid materials were 1 wt%, 2 wt%, 4 wt%, respectively.

2.3 Characterization

The phase and morphology of the products were characterized by X-ray diffraction (XRD, Rigaku, Japan) with Cu Ka radiation (λ = 1.54056 Å), scanning electron microscopy (SEM, ZEISS SIGMA, Germany), and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100, Japan). The BET-surface areas of the ZnS samples were measured by N₂ adsorption using the single point method with TriStar II 3020 (U.S.). X-ray photoelectron spectroscopy (XPS) measurement was performed on PHI Quantum-2000 XPS (U.S.). The UV-vis diffuse reflectance spectra were measured in the solid state, and BaSO₄ powder was used as the substrate (Cary-5000, Agilent, U.S.). The photoluminescence spectroscopy of the samples was measured by the fluorescence spectrophotometer (F-7000, Hitachi, Japan). The electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical system (CHI-660D, China).

2.4 Photocatalytic activity measurement

The photocatalytic activities were evaluated by the decomposition of MB, RhB and CIP under simulated sunlight irradiation. The pH value was not adjusted when the reaction was conducted. Experiments were carried out at ambient temperature with a circulating water system to prevent thermal catalytic effects. Simulated sunlight irradiation was provided by a 300 W Xe lamp (NBET HSX-F300, China). 30 mg of the photocatalyst was totally dispersed in an aqueous solution of MB (50 mL, 20 mg L⁻¹), RhB (50 mL, 20 mg L⁻¹) and CIP (50 mL, 10 mg L⁻¹), respectively. Before irradiation, the suspensions were magnetically stirred in the dark for 30 min to obtain the absorption–desorption equilibrium. At certain time intervals, 3 mL aliquots were sampled and centrifuged to remove the particles. The concentrations of MB, RhB and CIP were analyzed by recording the absorbance at the characteristic band of 664 nm, 557 nm and 276 nm using an UV-vis spectrophotometer (UV-2550, Shimadzu), respectively.

2.5 High performance liquid chromatography (HPLC)

The chromatograph was Agilent 1100 Infinity (USA) equipped with a UV detector and a binary gradient pump. The chromatographic column was a Agilent HC-C18 column (250 mm 4.6 mm i.d., 5 μm). The mobile phase was water and acetonitrile containing 0.1% formic acid (v/v), at a ratio of 78 : 22 (v/v). The mobile phase flow rate was 1.0 mL min⁻¹ and the column was kept at 30 °C. The injection volume was 20 μL and the CIP residual were monitored at a wavelength of 270 nm.

2.6 EIS measurement

For the preparation of working electrodes, 10 mg of sample, 1 mL of ethanol and 1 mL of ethylene glycol were mixed to produce a suspension and then 80 μL of the suspension was spread on an ITO glass electrode (1 cm × 2 cm). Electrochemical measurements were taken by using an electrochemical analyzer in a standard three electrode system with a platinum plate as the counter electrode, and saturated calomel electrode as the reference electrode. 1 M KOH aqueous solution was utilized as the electrolyte for the measurement.

3. Results and discussion

3.1 Phase structure and morphology

The XRD patterns of the pure ZnS and hybrid CQDs/ZnS are presented in Fig. 1(a). The diffractogram of the pure ZnS indicates that all diffraction peaks can be well indexed as a hexagonal wurtzite phase structure (JCPDS no. 36-1450). For the CQDs modified ZnS samples, no characteristic peaks of the CQDs can be detected, which may be attributed to its low content in the samples. Fig. 1(b) and (c) show the SEM images of the typical dandelion-like ZnS. TEM images of the CQDs/ZnS hybrid materials are shown in Fig. 1(d) and (e). These observations indicate that the dandelion-like ZnS are assembled by plenty of nanowires and the products retain the morphology and structure after hybridizing with CQDs. In addition, it can be found that the diameter of ZnS nanowires is ∼10 nm and some dark dots are distributed on the ZnS (Fig. 1(e) and (f)), implying that the CQDs are deposited on the surface of the dandelion-like ZnS. The HRTEM image revealed in Fig. 1(f) shows the lattice structure of an individual ZnS nanowire. The (002) plane of the ZnS which illustrates an interplanar space of 0.311 nm can be observed. Moreover, the SAED (Fig. 1(g)) pattern further demonstrates the polycrystalline structure of the products. The morphology and lattice structure of the CQDs are shown in Fig. 2(a) and (b). It can be seen that the average particle sizes of the CQDs are in the range of 2–4 nm (Fig. 2(c)). The lattice spacing of CQDs was determined to be 0.32 nm, which corresponding to the (002) crystal plane.¹⁵

Fig. 1 (a) XRD patterns of the pure ZnS and the CQDs/ZnS hybrid materials; (b) and (c) SEM images of the dandelion-like ZnS; (d) and (e) TEM images of CQDs/ZnS hybrid materials; (f) HRTEM image of the CQDs/ZnS hybrid materials and (g) SAED of the dandelion-like ZnS.

Fig. 2 (a) TEM image of CQDs, (b) HRTEM image of CQDs and (c) the CQDs size distribution histograms.

The N₂ adsorption–desorption isotherm measurements were performed to calculate the specific surface areas and pore size distributions of the samples. As shown in Fig. 3, the hysteresis loops (inset) vary in the range of 0.5–1.0 P/P₀, indicating the formation of mesopores in the dandelion-like ZnS. The BET surface areas of the dandelion-like ZnS are calculated to be 98.4 m² g⁻¹.

Fig. 3 N₂ adsorption–desorption isotherms of the dandelion-like ZnS. The inset shows the pore size distributions calculated by the BJH method from the desorption branch of the isotherm.

3.2 Compositional and surface properties

XPS measurements were carried out in order to study the chemical bonding states of the CQDs modified ZnS samples. Fig. 4(a) shows full scan spectrum of 2 wt% CQDs/ZnS, confirming the presence of Zn 2p, S 2p and C 1s.⁷ The high resolution XPS spectrum of the Zn and S element in Fig. 4(b) and (c) show peaks at 1020 and 161.5 eV which correspond to the Zn 2p3/2 and S 2p3/2 peaks of ZnS, demonstrating that Zn and S exist in the form of ±2 valence state.²¹ It is noted that the characteristic peak of the S 2p exhibits obvious right shift, whereas the peaks of the Zn 2p become broaden in the 2 wt% CQDs/ZnS materials compared with the pure ZnS. It implies that there exist interactions between the CQDs and ZnS. As can be observed in Fig. 4(d), the high resolution XPS spectrum of the C 1s of the 2 wt% CQDs/ZnS can be divided into three peaks at 284.8, 286.6 and 288.4 eV, corresponding to C–C bond with the sp² orbital, C–O–C bond and C=O bond, respectively.²² The results of XPS analysis indicate that CQDs and ZnS have been coupled together successfully.

Fig. 4 XPS spectra of the pure ZnS and 2 wt% CQDs/ZnS hybrid materials. (a) Survey of the sample; (b) Zn 2p and (c) S 2p; (d) C 1s of 2 wt% CQDs/ZnS hybrid materials.

Energy Dispersive Spectrometer (EDS) measurements are employed to determine the elements ratio and the real contents of the CQDs in the ZnS and the hybrid materials. The results are shown in Fig. S1 and Table S1.† As expected, there is no signal of the C element in the pure ZnS. The real contents of the CQDs in the 1 wt%, 2 wt% and 4 wt% CQDs/ZnS are 0.86 wt%, 1.72 wt% and 3.39 wt%, respectively. It should be noted that we use 1 wt% CQDs/ZnS, 2 wt% CQDs/ZnS and 4 wt% CQDs/ZnS to define the hybrid materials in the following paragraphs for convenience.

3.3 Photocatalytic test

Photocatalytic activity of as-prepared samples was first evaluated by the degradation of MB and RhB under simulated sunlight. Fig. 5(a) and (c) show the photocatalytic activities of the CQDs/ZnS hybrid materials with respect to the CQDs content under simulated sunlight for MB and RhB. It can be seen that the introduction of the CQDs enhances the photocatalytic performance of the ZnS. The most beneficial amount of the CQDs to improve the photocatalytic activity of the ZnS coating is 2 wt%. However, an excessive addition of CQDs produces detrimental effects due to the excess CQDs may cover the active sites of the ZnS. Meanwhile, the photocatalytic degradation kinetics of the MB and the RhB were investigated, the results are shown in Fig. 5(b) and (d). The kinetic model is expressed as −ln(C/C₀) = kt, where k is the apparent reaction constant, C₀ is the initial concentration of the dye and C is the concentration of the dye at different reaction times. The k value can be determined by a linear fit of the plot of −ln(C/C₀) versus reaction time t, and the R² values are given to determine the confidence intervals. The determined k values for MB of the pure ZnS, 1 wt% CQDs/ZnS, 2 wt% CQDs/ZnS and 4 wt% CQDs/ZnS are 0.0183 min⁻¹, 0.0278 min⁻¹, 0.0306 min⁻¹ and 0.0238 min⁻¹, respectively. Additionally, the determined k values for RhB of the pure ZnS, 1 wt% CQDs/ZnS, 2 wt% CQDs/ZnS and 4 wt% CQDs/ZnS are 0.0054 min⁻¹, 0.0087 min⁻¹, 0.0114 min⁻¹ and 0.0075 min⁻¹, respectively. Obviously, the photocatalytic degradation rate is improved by the introduction of CQDs. The 2 wt% CQDs content hybrid sample exhibits the highest degradation rate which is about 1.67 and 2.11 times higher than that of the pure ZnS sample for MB and RhB, respectively. The specific data are listed in Tables 1 and 2.

Fig. 5 (a), (c) and (e) Photocatalytic degradation of MB, RhB and CIP, respectively, with the catalysts under simulated sunlight; (b) and (d) kinetic fit for the degradation of MB and RhB, respectively.

Table 1 Pseudo-first-order rate constant for MB photocatalytic degradation under different photocatalysts

Series	Photocatalyst	The first order kinetic equation	Rate constant k (min^−1)	R^2
1	ZnS	−ln(C/C0) = 0.0183t	0.0183	0.9784
2	1 wt% CQDs/ZnS	−ln(C/C0) = 0.02779t	0.0278	0.9765
3	2 wt% CQDs/ZnS	−ln(C/C0) = 0.03058t	0.0306	0.9901
4	4 wt% CQDs/ZnS	−ln(C/C0) = 0.02381t	0.0238	0.9899

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Table 2 Pseudo-first-order rate constant for RhB photocatalytic degradation under different photocatalysts

Series	Photocatalyst	The first order kinetic equation	Rate constant k (min^−1)	R^2
1	ZnS	−ln(C/C0) = 0.00539t	0.0054	0.9521
2	1 wt% CQDs/ZnS	−ln(C/C0) = 0.00866t	0.0087	0.9674
3	2 wt% CQDs/ZnS	−ln(C/C0) = 0.01138t	0.0114	0.9875
4	4 wt% CQDs/ZnS	−ln(C/C0) = 0.00748t	0.0075	0.9553

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Furthermore, nowadays, the widespread use of antibiotics may cause various adverse effects on the ecosystem proliferation of bacterial drug resistance. Herein, CIP, the broad-spectrum antibiotic agent, was degradation by the as-prepared samples. As shown in Fig. 5(e), the results reveal that the 2 wt% CQDs/ZnS hybrid materials exhibit an improved photocatalytic activity for the CIP degradation than that of the pure ZnS. The temporal UV-vis absorption spectral changes during the photocatalytic degradation of the MB, RhB and CIP with the presence of 2 wt% CQDs/ZnS are shown in Fig. 6(a–c), the intensities of the characteristic peaks become weaker with the increase of irradiation time, revealing the degradation of the pollutants. The shifts of the characteristic peaks indicate the presence of decomposition products of the pollutants. The above results imply that the introduction of the CQDs improves the photocatalytic efficiency of the ZnS effectively.

Fig. 6 Temporal UV-vis absorption spectral changes during the photocatalytic degradation of (a) MB, (b) RhB and (c) CIP, respectively, in aqueous solution in the presence of 2 wt% CQDs/ZnS materials.

The analysis of CIP decomposition has been performed using the HPLC technique. The results are shown in the Fig. 7. The peak at retention time of 4.82 min responses to the CIP, and the peaks at other retention times are the byproducts of the degradation of CIP. As shown in Fig. 7, the intensity of the peak directed to the CIP decreases with the increase of the irradiation time, indicating the degradation of CIP. It can be found that the peaks of the degradation products emerge immediately when the reaction occurs. A representative chromatogram after 30 min irradiation was shown in Fig. S2,† and the relative kinetic evolution profile of the degradation products was displayed in Fig. S3.† As reported in other systems,^23,24 the dominant product detected during the degradation of CIP was desethylene ciprofloxacin (T1) at the retention time of 4 min, which is formed by a net loss of C₂H₂ at the piperazinyl substituent of CIP.²² Therefore, the possible reaction pathway of the CIP in this work is that parts of the CIP transformed to mineral with the release of C₂H₂ and CO molecule, forming CO₂ and H₂O eventually. In addition, the remaining CIP transformed to the organic intermediates. The degradation of the CIP requires a meticulous investigation in future work.

Fig. 7 HPLC chromatograms of the CIP degradation of 2 wt% CQDs/ZnS under simulated sunlight irradiation.

Furthermore, the maximum absorption wavelength shifts from 664 to 637 nm owing to the demethylation process of MB.^25–27 The shifts from 557 to 530 nm caused by the deethylation of the RhB was attributed to the attack of the active oxygen species on the N-ethyl group.^28,29 The intermediates of MB and RhB transformed into CO₂, H₂O and mineral acids eventually.

3.4 Optical and electrochemical properties

In order to explore the mechanisms behind the enhanced photocatalytic activity, the optical and electrochemical properties have been investigated by UV-vis diffuse reflectance spectra, PL (photoluminescence) spectra and EIS, respectively. The UV-vis diffuse reflectance spectra are shown in Fig. 8. The pure ZnS exhibits the absorption in the region ranging from 200 nm to 400 nm. This absorption originates from the charge transfer response of ZnS from the valence band to the conduction band upon excitation by light. The absorption regions get broaden and the absorption intensity increase with the introduction of CQDs, corresponding to the optical absorption of CQDs. Red shifting for CQDs/ZnS hybrid materials samples probably promote the formation of electron–hole pairs, resulting in the improved photocatalytic activity. In addition, the PL spectra of the CQDs/ZnS were studied to reveal the charge transfer, migration and recombination processes in photocatalytic. It is well known that the weaker intensity of the PL represents lower recombination probability of photoexcited charge carriers.³⁰ Fig. 9 provides the PL spectra of the pure ZnS and the hybrid materials. It is obvious that the intensity of the emission band decreases significantly when the CQDs is anchored on the ZnS surface compared with the pure ZnS, and the 2 wt% CQDs/ZnS possesses the lowest intensity, suggesting an efficient transfer of photoexcited electrons from ZnS to CQDs.

Fig. 8 UV-vis diffuse reflectance spectra of pure ZnS and CQDs/ZnS hybrid materials.

Fig. 9 PL spectra of pure ZnS and CQDs/ZnS hybrid materials.

Based on the above results, one can draw a conclusion that the introduction of the CQDs reduces the electrons and holes recombination rate, which promotes the effective charge separation of ZnS and thus improves the photocatalytic activities. For further illustrating this assumption, EIS measurements for pure ZnS and CQDs/ZnS hybrid material samples were carried out to study the process of electron transfer, and the results are shown in Fig. 10. The semicircle in the high frequency region can be ascribed to charge-transfer resistance, showing the charge transfer through the electron/electrolyte interface.³¹ Obviously, the diameters of the Nyquist circle at the high frequency region decrease with the increase of the CQDs content due to the good conductivity of the CQDs, indicating the lower electron transfer resistance. It means the occurrence of a faster interfacial charge transfer to the electron acceptor (CQDs), which contributes to the separation of electron–hole pairs.^32,33 This result shows good agreement with the PL analysis, demonstrating that the introduction of the CQDs is a resultful way to improve the photocatalytic activity of the ZnS.

Fig. 10 Nyquist plots of pure ZnS and CQDs/ZnS hybrid materials.

To sum up, a schematic illustration of the photocatalytic process of the CQDs/ZnS hybrid materials is shown in Fig. 11. When the ZnS is irradiated by the simulated sunlight, the electrons can be excited from the valence band (VB) to the conduction band (CB) of the ZnS (eqn (1)), producing the holes on the VB. Generally, these photo-generated electrons and holes recombine rapidly and only few charges participate in the photocatalytic process. Nevertheless, when the CQDs are introduced to form hybrid materials, the electrons on the CB of the ZnS tend to transfer to CQDs owing to their excellent electronic conductivity (eqn (2)), resulting in an effective separation of the photo-generated electron–hole pairs. Thus, the photocatalytic activity of the hybrid materials is enhanced. Eventually, the electrons react with absorbed molecular oxygen forming the superoxide radical anion ˙O₂⁻ (eqn (3)).^8,34 In aqueous medium, the photogenerated holes in the VB of ZnS can be easily trapped by the hydroxyl ions in the solution, producing extremely strong oxidant radicals ˙OH (eqn (4)). The hydroxyl radicals will recombine to form H₂O₂ (eqn (5)), and the H₂O₂ may react with the superoxide radical anions to regenerate hydroxyl radicals (eqn (6)). These photo-generated active factors can react with the organic pollutants (MB, RhB, CIP), degrading into carbon dioxide and water eventually (eqn (7)).⁴

ZnS + hν → e_ZnS⁻ + h_ZnS⁺ (1)

e_ZnS⁻ → e_CQDs⁻ (2)

e_CQDs⁻ + O₂ → ˙O₂⁻ (3)

h_ZnS⁺ + H₂O → ˙OH (4)

2˙OH → H₂O₂ (5)

H₂O₂ + ˙O₂⁻ → OH⁻ + ˙OH + O₂ (6)

˙OH + dye → H₂O + CO₂ + intermediates (7)

Fig. 11 Schematic of the separation and transfer of photogenerated charges in the CQDs/ZnS hybrid material combined with the possible reaction mechanism of photocatalytic procedure.

4. Conclusions

In summary, the CQDs/ZnS hybrid materials were fabricated via a simple and efficient hydrothermal and bath reflux strategy. Importantly, the as-prepared products exhibit enhanced photocatalytic decomposition activities of both organic dyes (MB and RhB) and colorless antibiotic (CIP) under simulated sunlight irradiation. The 2 wt% CQDs/ZnS hybrid materials possess the optimal photocatalytic performance. The optical and electrochemical properties of the products are investigated and prove that the enhancement of photocatalytic activities are ascribed to the improved charge separation efficiency which caused by CQDs. The reaction mechanism has been discussed as well. This work may provide inspirations in improving the photocatalytic activity of semiconductor materials.

Acknowledgements

The authors thank the National Natural Science Foundation of China (51372212).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02840c

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