ABSTRACT

Zn_xCd_1−xS solid solutions with controlled morphology have been successfully synthesized by a facile solution-phase method. The prepared samples were characterized by X-ray powder diffraction (XRD), UV-vis diffuse reflectance spectra, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photocatalytic activity of Zn_xCd_1−xS was evaluated in the 2,4,6-trichlorophenol (TCP) degradation and mineralization in aqueous solution under direct solar light illumination. The experiment demonstrated that TCP was effectively degraded by more than 95% with 120 min. The results show that ZnS with Cd doping (Zn_xCd_1−xS) exhibits the much stronger visible light adsorption than that of pure ZnS, the light adsorption increasing as the Cd²⁺ doping amount. These results indicate that Cd doping into a ZnS crystal lattice can result in the shift of the valence band of ZnS to a positive direction. It may lead to its higher oxidative ability than pure ZnS, which is important for organic pollutant degradation under solar light irradiation. Furthermore, the photocatalytic activity studies reveal that the prepared Zn_xCd_1−xS nanostructures exhibit an excellent photocatalytic performance, degrading rapidly the aqueous 2,4,6-trichlorophenol solution under solar light irradiation. These results suggest that Zn_xCd_1−xS nanostructure will be a promising candidate of photocatalyst working in solar light range.

Keywords:

Photocatalyst, Nanostructures, Solution Phase, 2,4,6-Trichlorophenol, Solar Light, Photocatalytic Activity

1. Introduction

Among the water contaminants, chlorophenols are the most hazardous class of water pollutants [1] . Chlorophenols (CPs) are used in many industries such as paint, pharmaceutical, pesticide, solvent, wood, paper, and pulp industries [2] . However, these CPs are very harmful to humans because they are toxic, mutagenic and carcinogenic. They affect the nervous system and cause respiratory problems [3] . They must be decomposed before discharging to the rivers or lakes. Under this background, it is of importance to find an easy and effective method to completely remove these chlorinated organic pollutants [4] .

During recent decades, semiconductor based photocatalysis has been extensively studied and has become one of the most promising methods in environmental protection procedures such as air and water purification and hazardous waste remediation [5] . TiO₂ nanomaterials have often been used as photocatalysts because of their efficiency, stability, non-toxicity, and low cost [6] . However, it has some drawbacks such as the relatively high value of the band gap energy (~3.2 eV), which limits its absorption to the UV spectral region, and the high recombination rate of photo-induced electrons and holes, which decreases its photocatalytic activity [7] . Therefore, many efforts have focused on investigating high-efficiency visible-light photocatalysts because of their potential for various applications, including environmental pollution control and solar energy utilization.

Metal sulfides have been intensively studied as active photocatalysts because of their high light harvesting for promoting charge separation [8] - [12] . For example, CdS shows a high photocatalytic activity in water splitting for hydrogen production [13] . However, there remain some drawbacks including fast recombination of photogenerated electron-hole pairs and suffer from severe photocorrosion, both of which prohibit the wide application of CdS. To solve these problems, forming CdS/ZnS heterojunction structure is an effective method, because the same coordination mode between CdS and ZnS will promote interfacial charge transfer and inhibit the photo-corrosion of CdS via dispersing ZnS on the surface as well [14] . The key point of this CdS/ZnS heterojunction is to achieve a strong join between CdS and ZnS intersurface and the uniform distribution of each kind of atoms inside bulk material. Moreover, the electronic structure of the Zn_xCd_1−xS solid solution can be easily controlled by tuning the molar ratio of Zn²⁺ and Cd²⁺ [15] . Therefore, the synthesis of Zn_1−xCd_xS solid solutions is of immense interest and increasing technological importance for future applications.

The controlled synthesis of inorganic nanomaterials with desired morphology has received considerable attention due to the physical and chemical properties of sold nanomaterials specially depend on their phase, size, shape, and organization [16] - [25] . Over the past few years, many methods have been developed to synthesize Zn_xCd_1−xS solid solutions with controlled morphology, such as solid state reactions [26] , solution- phase process [27] , cation exchange reactions [28] , and microwave synthesis [29] . However, it is still a big challenge to find an easy way to prepare Zn_xCd_1−xS solid solutions with controlled morphology. Also unfortunately, there has been no report available for the degradation of 2,4,6-trichlorophenol using Zn_xCd_1−xS solid solutions. In this paper, we report for the first time on the photocatalytic degradation of 2,4,6-trichlo- rophenol (TCP). The result shows that, with the increase of Cd molar fraction, the character of UV-visible absorption of Zn_xCd_1−xS crystal gradually evolves from ZnS to CdS, and this Zn_xCd_1−xS sample under solar irradiation demonstrates a considerable photocatalytic degradation of aqueous 2,4,6?trichlorophenol (TCP).

2. Experimental

2.1. Synthesis of Zn_xCd_1−xS Samples

The Zn_xCd_1−xS solid solutions with different Cd molar ratios were prepared with Cd(NO₃)₂∙4H₂O, Zn(NO₃)₂∙6H₂O and thioacetamide. The Zn_xCd_1−xS solid solutions were prepared according to the previous reported methods [26] [27] . All chemicals were purchased as guaranteed regents and used without further purification. The stock solution was prepared by dissolving amount of Cd(NO₃)₂∙4H₂O, Zn(NO₃)₂∙6H₂O and thioacetamide in a 250 mL round bottom flask and by filling with 100 mL of distilled water. Afterwards, the stock solution was heated to reflux at 105˚C. After the mixture was refluxed for 45 min, a precipitate was obtained, which was filtered then washed with absolute ethanol and distilled water several times. After being dried in oven at 60˚C for 24 hours, the products were collected for characterization.

2.2. Characterization

The crystalline properties of Cd doping ZnS nanocrystals were studied by X-ray diffraction (XRD) using a Bruker (D5005) X-ray diffractometer equipped with graphite monochromatized CuK_α radiation (λ = 1.54056 Å). The crystal morphology was examined by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The FESEM images were obtained from Hitachi S-4800, which is equipped with an energy dispersive X-ray spectrometer (EDX). The lattice spacing of the products was also taken by high resolution TEM (HRTEM) (JEOL JEM-3010). The chemical states and relative compositions of the samples were studied by X-ray photoelectron spectroscopy (XPS) (ThermoVG, U.K) with a monochromatized Al Kα irradiation (1.4867 eV) and a pass energy of 20 eV. All XPS spectra were obtained with an energy step of 0.1 eV and a dwell time of 50 ms. An Avantage Thermo VG software was used to analyze the XPS data. The absorption spectra of the as-prepared samples were recorded using a UV-Vis diffuse reflectance spectroscopy (DRS) (Jasco V670) in the wavelength range of 200 - 1000 nm, with BaSO₄ as a reference.

2.3. Photocatalytic Degradation of 2,4,6-Trichlorophenol

Photocatalytic reactions were performed in a cylindrical borosilicate glass reactor vessel with the volume of 250 mL, with a simulated solar light irradiation for the degradation of 2,4,6-trichlorophenol and a recycling water jacket to keep cooling. The photocatalytic tests were performed with 100 mg of catalyst suspended in 2,4,6-trichlorophenol aqueous solution (250 mL, 5mg/L). The concentration of 2,4,6-trichlorophenol left in the centrifuged aqueous solution was determined by LC/MS spectrophotometer (Model: Agilent Technologies?6460 Triple quad LC/MS), in which Water Symmetry C-8 column (3.5 micro Mt-2.1 × 150?Part No.: WAT106011) was used and mobile phase of methanol/water (95:5 v/v) with 5 mmol ammonium acetate was employed at a flow rate of 0.5 mL/min.

3. Results and Discussion

3.1. Phase Structures

Figure 1 shows the XRD patterns of the as prepared Zn_xCd_1−xS (x = 0, 0.2, 0.5, 0.8 and 1) samples. These samples show the similar pattern, and all the diffraction peaks can be well indexed to the cubic ZnS (JCPDS Card No. 75 - 1547). When a small amount of Cd was doped into the ZnS crystal, (for example, Zn_0.8Cd_0.2S), the diffraction peaks of ZnS exhibited an obvious shift toward the lower angle. This implies that Cd²⁺ incorporates into the ZnS crystal lattice and increases the fringe lattice distance of the ZnS crystal due to the larger radius of Cd²⁺ (0.97 Å) than that of the Zn²⁺ ion (0.74 Å) [30] . With

increasing Cd²⁺ content in the Zn_xCd_1−xS solid solution, the XRD peak positions continuously shift to low angle side, and the crystal phase of the Zn_xCd_1−xS solid solution gradually changes from the cubic to hexagonal phase and finally becomes the hexagonal wurtzite CdS phase for the CdS sample (JCPDS Card No. 42-1411). This implies that the Cd²⁺ in the ZnS crystal influence the positions of Zn²⁺ and consequently change the lattice structure of ZnS during the formation of the Zn_xCd_1−xS solid solutions.

3.2. UV-Visible Diffuse Reflection Spectra

The optical properties of Zn_xCd_1−xS nanostrucures are examined by using UV-vis DRS spectrum, as shown in Figure 2(a). With increasing the molar ratio of Cd²⁺/Zn²⁺, the absorption edge of Zn_xCd_1−xS shows an obvious red shift. This red shift indicates that the electronic structure of Zn_xCd_1−xS nanostrucures can be easily turned by the changing the Zn²⁺/Cd²⁺ molar ratios in precursors. The samples are changed from white to yellow.

The optical absorption follows the equation áhv = A(hí−E_g)^n/2, where á, í, E_g and A are the absorption coefficient, the light frequency, the band gap and a constant, respectively. Among them, n determines the characteristics of the transition in a semiconductor [31] . The band gaps are measured to be about 2.22, 2.14 and 2.05 eV for x = 0.2, 0.5 and 0.8 Zn_xCd_1−xS, respectively (Figure 2(b)), which incorporated Cd²⁺ into the ZnS crystal lattice and induced the band gap narrowing.

3.3. XPS Study

In order to study the compositions on Cd_0.5Zn_0.5S surface, XPS measurements are carried out. Figure 3(a) shows the wide spectrum of the Zn_0.5Cd_0.5S sample, which confirms the presence of Zn, Cd, S, O and C in the sample. The observed C and O peaks is due to the carbon supporting film used for measurement and the adsorbed oxygen molecules on the sample surface, respectively.

Figures 3(b)-(d) shows that the high-resolution XPS spectra of Zn_0.5Cd_0.5S. The peaks at 404.9 (Cd 3d_5/2), 411.7 (Cd 3d_3/2), 1045.5 (Zn 2p_1/2), 1022.3 (Zn 2p_3/2), 163.0 (S 2p_1/2) and 162.0 (S 2p_3/2) were attributed to the Cd_0.5Zn_0.5S surface condition, which

agree well with the results reported by Yang et al. [32] Compare to ZnS, Zn peak of the Cd_0.5Zn_0.5S show a shift toward low degree (Figure 3(b)). Compare to CdS, Cd peak of the Cd_0.5Zn_0.5S show a shift toward low degree (Figure 3(c)). Compare to ZnS and CdS, S peak of the Cd_0.5Zn_0.5S is in the middle (Figure 3(d)).

3.4. SEM Morphology Analysis

SEM measurement was used to investigate the influence of Cd²⁺ doping on the morphology of ZnS nanocrystals, as shown in Figure 4. The ZnS sample is composed of the sphere-like nanostructures about 2.5 - 3.0 μm in diameter, the sphere is composed of small nanoparticles (Figure 4(a)). When molar ratio of Zn²⁺ to Cd²⁺ is 0.8:0.2, the product is composed of the sphere-like nanostructures illustrated in Figure 4(b), the diameter is about 250 - 300 nm the surface is a little rough. When the molar ratio is 0.5:0.5, the diameters of the spheres are 200 - 300 nm and 80-100 nm, the size is not uniform (Figure 4(c)). When the molar ratio is decreased to 0.2:0.8, the diameters of the spheres are in the range of 250 - 270 nm with uniform size (Figure 4(d)). The CdS sample is composed of the sphere-like nanostructures about 1.5 - 3.0 μm in diameter, the sphere surface is smooth (Figure 4(e)). These SEM results indicate that the general morphology of these Zn_xCd_1−xS solid solutions is almost the same, small particles assemble for big particles. However, it is apparent that the diameter of the individual Zn_xCd_1−xS spherical particle can be controlled by turning the ratio of Zn²⁺ to Cd²⁺.

3.5. TEM Test for Samples

The morphologies of as prepared ZnS, Zn_xCd_1−xS and CdS samples are investigated using TEM and HRTEM analysis, respectively (Figure 5). From the TEM images of as synthesized ZnS samples, it can be seen that the product was present in a large scare spherical with the diameter in 100 - 120 nm (Figure 5(a)). There is also spherical particle shape but no uniform size for the as-synthesized Cd_xZn_1−xS sample (choose Zn_0.5Cd_0.5S as an example). Some size is around 50 nm some size is around 300 nm (Figure 5(b)). For the CdS sample, make up some irregular nanoparticles with the size of 20-40 nm (Figure 5(c)). The diameter of this nanoparticle is consistent with the result from the XRD data according to the calculation of Scherrer Formula. The compo- nents of the products were further confirmed by EDX, which indicates that the ratio of Zn, Cd and S is 0.5:0.5:1 for Zn_0.5Cd_0.5S (Figures 5(a1)-(c1)), which is consistent with the XPS result above (Figure 3).

3.6. Photocatalytic Activity Test

In order to evaluate the photocatalytic activity of Zn_xCd_1−xS samples, photodegradation of TCP was carried out in aqueous suspension using pure ZnS and Zn_xCd_1−xS catalysts with a different Cd content between 0.2 and 0.8 mole ratio, and the experimental results are shown in Figure 6. Figure 7 represents the time-dependent LC/MS peak intensity of the TCP solution during photo degradation in the presence of CdS microspheres. The peak intensity at around 0.9 min decreased gradually with irradiation time. The experiment demonstrated that TCP was effectively degraded by more than 95% with 120 min. At the end of the degradation after 240 min, no peak could be detected, implying that complete oxidation of TCP occurred due to the presence of CdS microsphere under solar light irradiation. A bigger doping amount of Cd on ZnS can be detrimental to the TCP photodegradation efficiency.

Figure 6 shows that the photocatalytic activity of Zn_0.5Cd_0.5S for degradation of TCP is much higher than that of pure ZnS. This result indicates that doping Cd ions promotes the charge pair separation efficiency for ZnS catalysts. The electron activates from the conduction band of ZnS to Cd atoms is thermodynamically due to the Fermi level of ZnS is higher than that of Cd metal [33] [34] . This results in the formation of a Schottky barrier at metal-semiconductor contact region and improves the efficiency of charge separation of ZnS [35] [36] . Contrarily, at the high content of Cd doping, Cd atoms also will act as a recombination center and decrease the photocatalytic activity of ZnS. From this work, the as prepared Zn_xCd_1−xS samples demonstrated an excellent performance in photocatalytic degradation of aqueous TCP solution under solar irradiation.

4. Conclusion

In the present study, the feasibility of employing Zn_xCd_1−xS solid solutions for the degradation of 2,4,6-trichlorophenol has been demonstrated successfully. Zn_xCd_1−xS solid solutions with tunable chemical composition and controlled morphology have been successfully synthesized by a facile solution-phase method without the use of any surfactants. The Zn_xCd_1−xS products demonstrate well-dispersed spheres with different diameters. With the increase of Cd molar fractions, the UV-visible absorption of Zn_xCd_1−xS crystal gradually evolves from ZnS to CdS, which leads to that the Zn_xCd_1−xS sample under solar irradiation demonstrates a considerable photocatalytic degradation of 2,4,6-Trichlorophenol in aqueous solution.

Acknowledgements

The authors (RS and MAM) are grateful to Sultan Qaboos University for providing financial support to carry out this work under SQU-UAEU joint collaborative research grant (CL/SQU-UAEU/16/04).

Declare

The authors declare no competing financial interest.

Cite this paper

Selvaraj, R., Qi, K.Z., Pandian, S.B.S., Meetani, M.A., Al Wahaibi, B., Al Lawati, H., Al Kindy, S.M.Z., Kim, Y. and Sillanpää, M. (2016) Morphology-Controlled Synthesis of Zn_xCd_1-xS Solid Solutions: An Efficient Solar Light Active Photocatalyst for the Degradation of 2,4,6- Trichlorophenol. Journal of Environmental Protection, 7, 1605-1617. http://dx.doi.org/10.4236/jep.2016.711132

References