Facile synthesis of NiS/CdS nanocomposites for photocatalytic degradation of quinoline under visible-light irradiation

Abstract

NiS/CdS nanocomposites with good visible-light-induced photocatalytic activity were successfully prepared via a facile two-step process. In the composites, well-crystalline NiS nanoparticles grew on the surfaces of CdS particles prepared by a hydrothermal method or by microwave method. The structural features were investigated by powder X-ray diffraction and transmission electron microscopy. The as-synthesized NiS/CdS nanocomposites exhibited good photocatalytic performance in the degradation of quinoline under visible light irradiation. The amount of NiS loading and the phase of CdS affected the photocatalytic performance of NiS/CdS nanocomposites. In addition, the effects of operational parameters such as the amount of photocatalyst and initial pH on the photodegradation of quinoline have been analyzed.

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1. Introduction

Quinoline is widely used for the synthesis of pharmaceuticals, dyes, pesticides and various chemical additives. Due to the presence of a nitrogen-atom in the ring system, quinoline has high water solubility and is easily accumulated in natural water environments. Consequently, it can cause environmental pollution due to its recalcitrant, persistent, toxic, cancerogenic, and teratogenic nature. As a typical nitrogenous heterocyclic compound, quinoline is recalcitrant to biodegradation.^1,2 Therefore, it has become a challenge to achieve effective removal of quinoline from wastewaters to minimize its risk.

Photocatalytic oxidation is a promising alternative to the biological methods for the complete mineralization of quinoline, because it can be carried out under ambient conditions and complete mineralization of pollutants to CO₂, water and mineral acids.^3,4 Among many photocatalysts, titanium dioxide (TiO₂) is the most popular photocatalyst due to its chemical stability, low-cost, and nontoxicity. However, TiO₂ can only be excited under ultraviolet irradiation due to its wide band gap. For the sake of efficient utilization of sunlight, the development of visible light-driven photocatalysts is necessary. Cadmium sulfide (CdS), with a large direct band gap of 2.4 eV, has been recognized as one of the most promising semiconductors that can utilize visible light for photocatalytic reactions.^5,6 However, the low separation efficiency of photogenerated electron–hole pairs is one critical issue that results in a poor efficiency of photocatalytic reactions of pure CdS and consequently limits the practical application in the fields of environmental protection. To improve the quantum yield and the photocatalytic activity of CdS, many researchers have been focused on the composites combining CdS with other components, such as ZnS,⁷ TiO₂,⁸ ZnO,⁹ and so on. Nickel sulfide (NiS), with a band gap of 0.5 eV, is reported to be a good photocatalyst in the degradation of pollutants.^10,11 As a p-type semiconductor, NiS has been used to couple with n-type semiconductors to form p–n heterojunctions, such as NiS/TiO₂ (ref. 12) and NiS/ZnS composites,¹³ and these p–n heterojunctions exhibit enhanced photocatalytic performance under visible light irradiation, because the p–n heterojunction with an internal electric field can significantly enhance the separation of photogenerated charge carriers.^14,15 Thus, combining p-type NiS with n-type CdS may be an ideal system to achieve an enhanced photocatalytic performance. In fact, NiS/CdS heterojunctions have been reported as good catalysts for photocatalytic H₂-production,^16,17 but the NiS/CdS as photocatalysts for degradation of pollutants has not received prior investigation.

In this work, a series of NiS/CdS nanocomposites with different amounts of NiS loadings were prepared using a simple and low-cost chemical deposition method. The obtained composite was used as a photocatalyst in the photodegradation of quinoline. And the effects of different experimental parameters, including amount of the catalyst and solution initial pH, were studied on the degradation of quinoline.

2. Experimental

2.1. Sample preparation

All the reagents were of analytical grade and were used without further purification. Distilled water was used in all experiments.

2.1.1. Preparation of CdS. In a typical synthesis, 2.01 g of CdCl₂·4H₂O and 2.4 g of Na₂S·9H₂O were dissolved in distilled water (30 mL), respectively. After all the CdCl₂·4H₂O and Na₂S·9H₂O have dissolved, the two solutions were mixed together. After stirring for 24 h, the mixture were added into an autoclave with an inner Teflon lining and maintained at 180 °C for 24 h (hydrothermal method), or heated by microwave irradiation in the microwave reactor for 30 min (microwave method). After that, the yellow precipitate was collected, washed with distilled water and ethanol for more than three times, and then dried in an oven at 80 °C for 5 h. The sample prepared by hydrothermal method was assigned as CdS-h, the catalyst prepared by microwave method was assigned as CdS-m, and the mixture of CdS-h and CdS-m (1 : 1) was assigned as CdS-mix.

2.1.2. Preparation of NiS/CdS composites. The prepared CdS (1.086 g) were ultrasonically dispersed in water, and then a certain volume of an aqueous solution containing C₄H₆NiO₄·4H₂O and Na₂S·9H₂O was quickly added. The mixed solution volume was adjusted to 60 mL with deionized water and stirred for 30 min at room temperature. After that, the suspension was transferred to a 100 mL Teflon-lined autoclave and maintained at 150 °C for 6 h. The final products were washed with distilled water and ethanol for more than three times, and dried at 80 °C for 5 h. A series of NiS/CdS catalysts with 1.2, 1.8, 3, 4 and 5 wt% of NiS were prepared via the above procedure by controlling the amount of C₄H₆NiO₄·4H₂O and Na₂S·9H₂O. For comparison, pure NiS nanoparticles were synthesized under the same conditions without adding CdS.

2.2. Characterization of catalysts

The structure and the morphology of the catalyst powders were observed by transmission electron microscope (TEM, JEM-100CX, JEOL, Japan). The crystal phase and crystallinity of samples were characterized by powder X-ray diffraction (XRD, D/max-2200/PC, Rigaku Corporation, Japan) with Cu Kα radiation, operating at 40 kV and 30 mA, where λ = 0.15418 nm for the Cu Kα line.

2.3. Photocatalytic degradation of quinoline

Quinoline was selected as a model chemical to evaluate the activity of the catalysts. A 500 W Xe lamp was used as the light source of a homemade photoreactor. The short wavelength components (<420 nm) of the light were cut off using a glass optical filter. For a typical photocatalytic experiment, 0.02 g NiS/CdS powders were added to 100 mL quinoline solution (50 mg L⁻¹) in the beaker with stirring. Prior to irradiation, the suspensions were magnetically stirred in the dark for 1 h to ensure the establishment of an adsorption/desorption equilibrium. For a given duration, the concentration of quinoline (after the removal of photocatalysts) was monitored at 313 nm using a UV-vis spectrophotometer (UV-2100).

3. Results and discussion

3.1. Property of the NiS/CdS nanoparticles

Fig. 1 shows the TEM image of the 4 wt% NiS/CdS-mix nanoparticles with size ranging from 20 to 40 nm. The crystal structure and phase composition of NiS/CdS samples were investigated by XRD measurements. As shown in Fig. 2b, the diffraction peaks of NiS/CdS-h can be easily indexed to the hexagonal phase of CdS, which are in good agreement with the data in the standard card (JCPDS 41-1049). The major peaks at 24.9°, 26.4°, 28.2°, 36.6°, 43.7°, 47.9° and 51.9° can be indexed as (100), (002), (101), (102), (110), (103) and (112) planes.^6,18 And the diffraction peaks of NiS/CdS-m (Fig. 2c) can be indexed to the cubic phase of CdS, which are in good agreement with the data in the standard card (JCPDS 42-1411).¹⁹ The major peaks at 26.5°, 43.8°, and 51.8° can be indexed as (111), (220) and (311) planes. X-ray diffraction patterns of NiS/CdS-mix (Fig. 2a) indicated that both cubic and hexagonal phases of CdS are present in NiS/CdS-mix sample. Meanwhile, the NiS phase is not detected in the composite sample, which may be attributed to the weak crystallization and high dispersion of NiS particles deposited on the surface of CdS. As a matter of fact, NiS particles were crystallized completely, and the diffraction peaks are in good agreement with the data in the standard card (JCPDS 2-1280) (Fig. 3).¹²

Fig. 1 TEM images of the 4 wt% NiS/CdS-mix nanoparticles.

Fig. 2 XRD patterns of NiS/CdS samples.

Fig. 3 XRD pattern of prepared NiS particles (a) and the standard XRD pattern of NiS (JCPDS2-1280) (b).

3.2. Photocatalytic reactions

Fig. 4 shows the degradation rate of quinoline on NiS/CdS-m catalysts with different amounts of NiS loadings. From Fig. 4, we know that photocatalytic degradation rate of quinoline is increased with the increase of the reaction time, because more electron–hole pairs will be generated with the increase of the reaction time. The degradation of quinoline is negligible when no catalyst was added, suggesting that quinoline is very stable under visible light irradiation. CdS-m alone shows activity in photocatalytic degradation of quinoline, but the degradation rate of quinoline is low (ca. 20%) after 10 hours. After loading NiS on CdS-m, the photocatalytic activity is increased. With the increase of the amount of NiS loaded on CdS-m, the degradation rate of quinoline on NiS/CdS-m is increased further and achieves a maximum when the loading amount of NiS on CdS-m is about 4 wt%, the degradation rate of quinoline is about 48% after 10 hours. This behavior should be attributed to the formation of the p-NiS/n-CdS-m heterojunction, which can provide a potential driving force to reduce the recombination of electron–hole pairs,¹⁰ resulting in high quantum efficiency and excellent photocatalytic properties. However, the photocatalytic activity decreased when the NiS ratio was more than 4 wt%, because NiS particles with a narrowband gap may act as recombination centers. In fact, the appearance of a maximum in activity with an optimum loading of cocatalyst has also been observed for other photocatalysts loaded with MoS₂,²⁰ Cu₂O (ref. 21) and RuO₂.²² Fig. 5 shows that NiS/CdS-mix has higher degradation efficiency than NiS/CdS-h and NiS/CdS-m. In NiS/CdS-mix photocatalysts, CdS was composed of hexagonal and cubic phase of CdS, and the mixed-phase CdS possessed synergistic effect. In fact, the enhanced activity of mixed phases relative to pure phases has been repeatedly reported.^23,24 According to the above results, we chose 4 wt% NiS/CdS-mix for further studies.

Fig. 4 Photodegradation performances of the as-synthesized nanoparticles, (a) 4 wt% NiS/CdS-m, (b) 3 wt% NiS/CdS-m, (c) 5 wt% NiS/CdS-m, (d) 1.8 wt% NiS/CdS-m, (e) 1.2 wt% NiS/CdS-m, (f) CdS-m and (g) no catalyst. Reaction conditions: catalyst dosage: 0.2 g L⁻¹ quinoline concentration: 50 mg L⁻¹; pH: 5.5.

Fig. 5 Photodegradation performances of the different catalysts, (a) 4 wt% NiS/CdS-mix, (b) 4 wt% NiS/CdS-h and (c) 4 wt% NiS/CdS-m. Reaction conditions: catalyst dosage: 0.2 g L⁻¹ quinoline concentration: 50 mg L⁻¹; pH: 5.5.

3.3. Effect of catalyst dosage on the photocatalytic performance

The variation in the photocatalyst dosage covering the range from 0.1 to 0.5 g L⁻¹ was studied and its effect on the degradation rate of quinoline aqueous solution was followed. Observed form Fig. 6, the photodegradation rate of quinoline increased from 27.8% to 68.2% with increasing 4 wt% NiS/CdS-mix dosage from 0.1 to 0.2 g L⁻¹, and further increase in photocatalyst loading had a negative effect on the photodegradation rate of quinoline. When catalyst dosage increased from 0.2 to 0.3 g L⁻¹, the photodegradation rate of quinoline decreased from 68.2% to 44.4% after 10 hours. Many studies have demonstrated that photodegradation efficiency of organic pollutants is strongly affected by the number of active sites and the photon-absorption ability of the photocatalyst.^25–28 The increasing 4 wt% NiS/CdS-mix dosage will increase the overall available surface area and enhance the light absorption. Consequently, the number of active sites and photogenerated carriers will be improved, which finally resulted in enhanced photodegradation rate of quinoline. However, further increasing of 4 wt% NiS/CdS-mix loading may cause the aggregation of free catalyst and also increased opacity and a decrease in light penetration as a result of increased scattering effect,^25–27 which eventually reduced the photodegradation efficiency.

Fig. 6 Effect of catalyst concentration on photocatalytic degradation of quinoline. Reaction conditions: quinoline concentration: 50 mg L⁻¹; pH: 5.5.

3.4. Effect of solution initial pH on the photocatalytic performance

Solution initial pH is an important parameter influencing organic pollutants photocatalytic degradation.^29,30 Thus, the effect of solution initial pH on the photocatalytic degradation of quinoline was studied. During these experiments, pH of the solution was adjusted before light irradiation by concentrated solutions of sodium hydroxide or hydrochloric acid, and it was not controlled during the course of the reaction. Fig. 7 shows photocatalytic degradation rate of quinoline by 4 wt% NiS/CdS-mix particles at different initial pH values under visible-light irradiation. From the curves, it can be seen that degradation rate of quinoline increased with the pH increase. The removal rate of quinoline at pH 3.5 is only about 44% after 10 hours. Meanwhile, the quinoline removal efficiency is up to 68% at pH 5.5, which is the natural pH value of quinoline in water, without pH adjustment. Quinoline is a weak organic tertiary amine base with a pK_a of 4.5.³¹ The state of quinoline in the solution is affected by the pH. Quinoline tends to exist in a protonated form with a positive charge at lower pH values,^26,29 which not easily adsorb on the surface of 4 wt% NiS/CdS-mix particles. At alkaline conditions, quinoline is primarily in its nonionic form,²⁹ which easily adsorbs on the surface of 4 wt% NiS/CdS-mix particles, resulting in the increase of photocatalytic degradation efficiency. Meanwhile, OH⁻ ions are easily adsorbed on the semiconductor surface at alkaline conditions, which promotes the generation of hydroxyl free radicals and subsequently a probable higher pollutant oxidation.^26,29 Thus, the photocatalytic degradation rate of quinoline increased with the increase of pH, exhibiting maximum efficiency (81%) at pH 11.

Fig. 7 Influence of pH on photocatalytic degradation of quinoline. Reaction conditions: quinoline concentration: 50 mg L⁻¹; catalyst dosage: 0.2 g L⁻¹.

4. Conclusions

Pure CdS powders were synthesized by hydrothermal method or by microwave method, NiS nanoparticles grew on the surfaces of CdS particles prepared by hydrothermal method. Compared with pure CdS, NiS/CdS heterojunctions showed improved photocatalytic degradation of quinoline under visible light irradiation. The maximum degradation rate of quinoline was realized when the mixture of hexagonal and cubic phase of CdS and 4 wt% loading amount of NiS were adopted. In addition, the best catalyst concentration was 0.2 g L⁻¹ for photocatalytic degradation of quinoline, and degradation efficiency of quinoline increased with the increase of solution initial pH.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51408204), the Natural Science Foundation of Jiangsu Province of China (No. BK20141350, No. BK20150692), the Fundamental Research Funds for the Central Universities of China (No. ZJ13071, No. ZJ15013, No. ZJ15002).

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